Studies in Surface Science and Catalysis 144 CHARACTERIZATION OF POROUS SOLIDS VI
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Studies
in S u r f a c e
Science
and Catalysis
Advisory Editors: B. Delmon and J.T. Yates Vol. 144
CHARACTERIZATION OF POROUS SOLIDS VI Proceedings of the 6th International Symposium on the Characterization of Porous Solids (COPS-VI), Alicante, Spain, May 8-11, 2002
Edited by F. R o d r i g u e z - R e i n o s o
1, B. M c E n a n e y
2, J. R o u q u e r o l
3 a n d K. U n g e r
University of Alicante Alicante, Spain 2 University of Bath, Bath, UK -~CNRS Marseille, France Johannes Guttenberg-Universitat Mainz, Germany
0
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Contents
Preface Committees Financial support
XV XVII XVII
Scanning probe microscopies for the characterization of porous solids: strengths and limitations J.I. Paredes, A. Martinez-Alonso, J.M.D. Tasc6n Role of gas adsorption in nanopore characterization K. Kaneko, T. Ohba, Y. Hattori, M. Sunaga, H. Tanaka, H. Kanoh
11
Reconstruction method for the characterization of porous carbons J. Pikunic, C. Clinard, N. Cohaut, K.E. Gubbins, J.-M. Guet, R.J.-M. Pellenq, I. Rannou, J.-N. Rouzaud
19
A new method for microporosity detection based on the use of the corrugated pore structure model (CPSM) C.E. Salmas, V.N. Stathopoulos, A.K. Ladavos, P-J. Pomonis, G.P. Androutsopoulos
27
Physisorption in nanopores of various sizes and shapes: a Grand Canonical Monte Carlo simulation study B. Coasne, A. Grosman, C. Ortega, R.J.M. Pellenq
35
Induced porosity in cross-linked polymer networks: Mean field theory and simulations S. Srebnik
43
Microbean small angle X-ray scattering (ttSAXS): a novel technique for the characterization of activated carbon fibers D. Lozano-Castell6, E. Raymundo-Pifiero, D. Cazorla-Amor6s, A. Linares-Solano, M. Miiller, C. Riekel
51
"Real time" determination of porosity development in carbons: a combined SAXS/TGA approach J.M. Calo, P.J. Hall, S. Houtmann, D. Lozano-Castell6, R.E. Winans, S. Seifert
59
SANS investigations of adsorption mechanisms in model porous silicas S. Kallus, A. Hahn, J.D.F. Ramsay
67
Preparation and surface characterization of novel ceria-copper and ceria-manganese mixed oxides M. Christophidou, C.R. Theocharis
75
About the exclusive mesoporous character of MCM-41 A. Berenguer-Murcia, J. Garcia-Martinez, D. Cazorla-Amor6s, A. Martinez-Alonso, J.M.D. Tasc6n, A. Linares-Solano
83
Incorporation of appropriate contact characterization by mercury porosimetry J.C. Groen, L.A.A. Peffer, J. P~rez-Ramirez
angles
in
textural
A two-stage Horvath-Kawazoe adsorption distribution analysis R.J. Dombrowski, C.M. Lastoskie
model for pore size
91
99
Textural and chemical characterization of NaX zeolite exchanged with Zn(ll) ions J. Silvestre-Albero, A. Sepfilveda-Escribano, F. Rodriguez-Reinoso
107
Evaluation and comparison of the pore structure and related properties of particulate and monolithic silicas for liquid phase separation processes K.K. Unger, B. Bilingmaier, C. du Fresne von Hohenesche, D. Lubda
115
Percolation phenomena in micropores: influence on single and multicomponent adsorption equilibria S. Ismadji, S.K. Bhatia
123
Characterization of the surface chemistry of activated carbon by molecular simulation of water adsorption M. Jorge, N.A. Seaton
131
Comparison of porous carbons developed via templating approaches P.M. Barata-Rodrigues, T.J. Mays, N.A. Seaton, G.D. Moggridge
139
Simulation of adsorption in 3-D reconstructed mesoporous materials by a simulated annealing algorithm M.E. Kainourgiakis, E.S. Kikkinides, G.Ch. Charalambopoulou, A.K. Stubos
147
Understanding adsorption hysteresis in porous glasses and other mesoporous materials H.-J. Woo, L. Sarkisov, P.A. Monson
155
vii Structural studies of mesoporous alumina membranes by small angle X-ray scattering J.C. Dore, D. Grandjean, R.E. Benfield, M. Kroll, D. Le Bolloc'h
163
Assessing microporosity by immersion microcalorimetry into liquid nitrogen or liquid argon J. Rouquerol, P. Llewellyn, R. Navarrete, F. Rouquerol, R. Denoyel
171
The lower closure point of the adsorption hysteresis loop of fluids in mesoporous silica materials A. Schreiber, S. Reinhardt, G.H. Findenegg
177
New metodologies in mercury porosimetry S.P. Rigby
185
Theoretical calculation of high micropore volumes on activated carbons J. Alcafiiz-Monge
193
Assessment of ultramicroporosity on carbon molecular sieves by water adsorption J. Alcafiiz-Monge, D. Lozano-Castell6
201
Active surface area of carbon materials determined by different methods A. Arenillas, F. Rubiera, J.B. Parra, J.J. Pis
209
Heterogeneity of sewage sludge derived materials as a factor governing their performance as adsorbents of acidic gases A. Bagreev, S. Bashkova, B. Reznik, V. Zibat, T.J. Bandosz
217
Evaluation of microporous structure of carbon molecular sieves using the pycnometric method M. Balys, B. Buczek, E. Vogt
225
Characterization of porous carbonaceous sorbents using adsorption data in wide temperature and pressure ranges G. De Weireld, M. Fr6re
231
Ammonia accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry M. Domingo-Garcia, A.J. Groszek, F.J. L6pez-Garz6n, M. P6rez-Mendoza
239
An IGC and TA study of acetaldehyde adsorption on activated carbons Y. E1-Sayed, T.J. Bandosz
247
viii A novel approach for characterizing carbon catalysts by TAP experiments V. Fierro, M.T. Izquierdo, Y. Schuurman, B. Rubio, C. Mirodatos
255
Preparation of activated carbons with controlled pore size M.M.A. Freitas, J.L. Figueiredo
261
Characterization of porous solids using low pressure VOC adsorption data J.Nokerman, S. Dutour, M. Fr~re, S. Limborg-Noetinger, S. Jullian
267
Energetics and mechanism of physical sorption by carbonaceous solids: Evaluation of surface area and porosity factors E.L. Fuller, Jr.
275
Phenanthrene adsorption on a carbonaceous material: moisture and CO2 influence A.M. Mastral, T. Garcia, R. Murillo, M.S. Call6n, J.M. L6pez, M.V. Navarro
283
Water adsorption on micro and mesoporous silicas J. Alcafiiz-Monge, D. Lozano-Castell6
291
The effect of surface functionalization of mesoporous silicas with propylimidazol on porosity, pore connectivity and tortuosity G.S. An~atas, C.E. Salmas, G.P. Androutsopoulos, P.J. Pomonis
299
Preparation of porous silica by acid activation of metakaolins C. Belver, M.A. Bafiares, M.A. Vicente
307
The effect of particle shape on the filtration rate and shear strength of quartz and dolomite mineral filter cakes B. Benli G6ntil, O. Ozcan
315
Characterisation of silica low-density xerogels in presence of additives by image analysis and nitrogen adsorption-desorption S. Blacher, C. Alia, C. Gommes, P. Lodewyckx, R. Pirard, J.-P. Pirard
323
Texture characterization of ultramacroporous materials using nondestructive methods S. Blather, V. Maquet, A. L~onard, G. Chapelle, M. Crine, R. J~rome, J.-P. Pirard Formation of hierarchically ordered silicas prepared by spray drying of nanosized spheres C. du Fresne von Hohenesche, V. Stathopoulos, K.K. Unger, A. Lind, M. Lind6n
331
339
Ultrathin porous glass membranes with controlled texture properties D. Enke, F. Friedel, F. Janowski, T. Hahn, W. Gille, R. Mtiller, H. Kaden
347
Reconstruction of mesoporous silica glass Gelsil| 50 N. Eschricht, E. Hoinkis, F. M/adler, P. Schubert-Bischoff
355
Pore structural characteristics of mesostructured materials prepared under different conditions A.E. Candeias, M.M.L. Ribeiro-Carrott, P.J.M. Carrott, K. Schumacher, M. Grtin, K.K. Unger
363
A grand canonical Monte Carlo simulation study of water adsorption in a Vycor-like disordered mesoporous material at 300K J. Puibasset, R.J.-M. Pellenq
371
A review of the application of the BET equation to experimental data: the C parameter F. Salvador, C. Sfinchez-Jim~nez, M.J. Sfinchez-Montero, A. Salvador
379
Thermogravimetric and sorption measurement techniques/instruments J.U. Keller, E. Robens, C. du Fresne von Hohenesche
387
Influence of method of washcoat preparation on hydrothermal stability of alumina support M. Kulazynski, J. Trawczynski, J. Walendziewski
395
Limestone influence on PAH emissions from coal AFBC. Catalytic or/and adsorption effect? A.M. Mastral, T. Garcia, M.S. Call6n, J.M. L6pez, R. Murillo, M.V. Navarro
403
Freezing point elevation in nanospace detected directly by atomic force microscopy M. Miyahara, M. Sakamoto, H. Kanda, K. Higashitani
411
Is it possible to obtain a coherent image of the texture of a porous material? F. Noville, C. Gommes, C. Doneux, A. Brasseur, R. Pirard, J.-P. Pirard
419
Preparation and characterisation of Cr-Co spinels for methane oxidation in presence of sulphur compounds J.R. Paredes, S. Ord6fiez, F.V. Diez, H. Sastre
427
Characterising the porous structure of Egyptian mortars using thermoporometry, mercury intrusion porometry and gas adsorption manometry J. Raga'f, T. Poyet, I. Beurroies, F. Rouquerol, P. Llewellyn
435
Monitoring fast pressure changes in gas transport and sorption analysis G. Reichenauer, H.-J. Fella, J. Fricke
443
Effect of coke deposition on catalyst texture during catalytic cracking reaction J.P. Reymond, C. Delattre, M. Forissier
451
Precision of porosity measurements on cementitious mortars K. Rtibner, Th. Fritz, F. Jacobs
459
Freezing in mesopores: aniline in silica glasses and MCM-41 M. Sliwinska-Bartkowiak, G. Dudziak, R. Radhakrishnan, K.E. Gubbins
467
Transport characteristics of porous solids derived from chromatographic measurements O. Solcovfi, P. Schneider
475
Effects of porous solid structures on the electrical behaviour: prediction key of transport properties in sedimentary reservoir rock A. Cerepi, R. Burlot, S. Galaup, J-P. Barde, C. Loisy, L. Humbert
483
Pore and surface characteristics of porous melamine- and phenolicformaldehyde polymers by sorption and XPS measurements A. Derylo-Marczewska, J. Goworek
491
Porous structure of multifunctional mineral-carbon and zeolite-carbon sorbents E.B. Drag, M. Kulazynski, J. Kaczmarczyk
499
Waste air cleaning using activated carbon fibre cloths regenerable by direct electric heating E.Schippert, C. M6hner, B. Pannwitt, H. Chmiel
507
Adsorption of inflammatory cytokines and endotoxin by mesoporous polymers and activated carbons M.C. Murphy, S. Patel, G.J. Phillips, J.G. Davies, A.W. Lloyd, V.M. Gun'ko, S.V. Mikhalovsky
515
Separation of adsorption isotherms of N2 in internal and interstitial nanopores of single-walled carbon nanohorn. A comparative study with experiment and simulation T. Ohba, H. Kanoh, K. Murata, M. Yudasaka, S. Iijima, K. Kaneko
521
Visualizing the porous structure of different carbon materials: a scanning tunneling microscopy study J.I. Paredes, A. Mart/nez-Alonso, J.M.D. Tasc6n
529
Textural characterisation of activated carbons obtained from poly(ethylene terephthalate) by carbon dioxide activation J.B. Parra, C.O. Ania, A. Arenillas, J.J. Pis
537
A Monte Carlo study on the structure of carbon dioxide adsorbed in
microporous carbons Th.A. Steriotis, G.K. Papadopoulos, A.K. Stubos, N. Kanellopoulos
545
Preloading of GAC by natural organic matter: effect of surface
chemistry on TCE uptake J.E. Kilduff, R. Srivastava, T. Karanfil
553
Structural studies of saccharose- and anthracene-based carbons by high-energy X-ray scattering A. Szczygielska, A. Burian, S. Duber, J.C. Dore, V. Honkimaki
561
The dynamic adsorption behaviour of volatile organic compounds on activated carbon honeycomb monoliths M. Yates, J. Blanco, M.A. Martin-Luengo
569
Structural properties of Cu-MCM-41 and Cu-AI-MCM-41 (Si/AI=30) catalysts G. Ferraris, G. Moretti, G. Fierro, M. Lo Jacono
577
Comparative study of the textural properties of alumina-pillared saponites synthesised from the intercalation with various aluminium oligomers L.M. Gandfa, M.A. Vicente, A. Gil
585
Chord length distribution and pore size distribution of porous VYCOR glass W. Gille, D. Enke, F. Janowski
593
A structural study of dehydration/rehydration of tobermorite, a model
cement compound A. Gmira, R.J.-M. Pellenq, I. Rannou, L. Duclaux, C. Clinard, T. Cacciaguerra, N. Lequeux, H. Van Damme
601
Support mesoporosity: a tool for better control of catalytic behavior of cobalt supported Fischer Tropsch catalysts A. Griboval-Constant, A.Y. Khodakov, R. Bechara, V.L. Zholobenko
609
Textural characterization of montmoriHonite pillared with aluminum/lanthanum polyoxycations J.A. Morante, C. Pesquera, C. Blanco, F. Gonzdlez
617
xii Influence of pH in mesoporous silica aluminas (MSA) synthesis C. Rizzo, A. Carati, C. Barabino, C. Perego, G. Bellussi
625
The adsorption of l-hexene and 3,3-dimethyl-l-butene on Ru-MCM 41 U. Singh, R.T. Williams, I.D. Salter
633
XPS studies of MCM 41 postmodified by a Schiff base copper complex U. Singh, R.T. Williams, I.D. Salter, K.R. Hallam
639
Influence of synthesis conditions on surface heterogeneity of M41 type materials studied with lattice Monte Carlo F.R. Siperstein, K.E. Gubbins
647
Porosity of chemically modified silica gels by nitrogen adsorption, positron annihilation and small angle X-ray scattering J. Goworek, A. Bor6wka, S. Pikus, J. Wawryszczuk
655
Preparation and characterization of porous sorbents for hot gas desulfurization F. ToroAs-Alonso, I. Orenes-Femfindez, J.M. Palacios-Latasa
663
Synthesis and characterization of MWW zeolites L.M. Vtjurina, S.S. Khvoshchev
671
Structural analysis of oxyanion-cation complexes anchored by organic group in mesoporous silicas H. Yoshitake, T. Yokoi, T. Tatsumi
677
Adsorption equilibrium of polar/non-polar mixtures on MCM-41: experiments and Monte Carlo simulation J.-H. Yun, Y. He, M. Otero, T. Dtiren, N.A. Seaton
685
Nondestructive characterization of porous ceramics by X-ray refraction K.-W. Harbich, F.E. Fensch-Kleemann, B. R6hl-Kuhn, P. Klobes
693
lon-exchange properties of a novel layered titanium(IV) phosphate O.A. Khainakova, A. Espina, C. Trobajo, S.A. Khainakov, J.R. Garcia, A.I. Bortun
701
The anomalous sorptive behaviour of ZSM-5 and Silicalite-l: observation of low pressure hysteresis in nitrogen adsorption G. Kyriakou, C.R. Theocharis
709
Characterization of the textural properties of chemically dealuminated Y zeolites R. L6pez-Fonseca, B. de Rivas, J.I. Guti~rrez-Ortiz, J.R. Gonz~ilez-Velasco
717
xiii Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar S. Motet, T. Poyet, D. Bigot, B. Polster, S. Crispel, J. Rouquerol, P. Llewellyn
723
The applicability of the Dubinin-Radushkevich equation to the very low pressure region of isotherms of various microporous solids P. Lodewyckx, L. Verhoeven
731
Controlled porosity and surface area titania gels as novel photocatalytic washcoats M. Yates, E. Garcia
737
The effect of geometry and axial orientation of spheroidal particles on the adsorption rate in a granular porous medium F.A. Coutelieris
745
The effect of Peclet on the Sherwood number in high porosity granular media F.A. Coutelieris, M.E. Kainourgiakis, A.K. Stubos
753
The application of J~intti's method for the fast calculation of equilibrium in case of multilayer adsorption J.A. Poulis, C.H. Massen, E. Robens, G. Reichenauer
761
Characterization of controlled pore glasses by small angle X-ray scattering and evaluation of the scattering data by the indirect Fourier transformation method A. Christoforides, N. Kanellopoulos, A. Mitropoulos, K.L. Stefanopoulos, K. Tarchanides
769
Author Index
775
Other volumes in the series
781
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XV
PREFACE
The Sixth International Symposium on the Characterization of Porous Solids (COPS-VI) was held at Alicante, Spain, on May 8-11, 2002 with about 220 participants from 29 countries. There were three Keynote lectures, 29 lectures and 154 poster presentations, with several sessions devoted to general discussion. The intensive three-day programme provided a stimulating forum for the exchange of information on novel research findings, concepts, techniques and materials. This volume contains 99 of the papers which were selected for publication. Taken together, the papers collected in this volume represent an up-to-date and authoritative account of recent developments around the world in the major methods used to characterize porous solids. Most of the leading international specialists in the subject are contributors to the book. I hope that the book will prove to be a useful work of reference for anyone interested in characterizing porous solids. Also included in the bookare papers giving accounts of new developments in the synthesis of porous solids, such as MCM-41 mesoporous materials, pillared clays and porous carbons. As usual at this series of meetings, papers on pore structure determination using gas adsorption feature strongly, together with papers on small angle scattering methods, mercury porosimetry, microcalorimetry, scanning probe microscopies, and image analysis. Papers reporting recent developments in molecular simulations of adsorption of gases and vapours in pores are also included. In addition to theoretical papers there are a number of papers dealing with practical applications of porous solids such as activated carbons, mineral, cements and polymers. I would like to express my special thanks to the members of the Scientific Committee (B. McEnaney, J. Rouquerol, K.K. Unger, and the Honorary member K.S.W. Sing) and to the Local Committee (M. Molina-Sabio, A. Septllveda-Escribano, F.J. Narciso-Romero) for their efforts in composing a scientific programme of great quality and in running a smooth and interesting symposium. The generous financial support of Bancaja, University of Alicante, Ministerio de Ciencia y Tecnologia, and OCIT-Generalitat Valenciana is gratefully acknowledged. It has been decided that COPS-VI will be held at Aix at Provence, France in 2005. Francisco Rodriguez-Reinoso Chairman of the Scientific Committee
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xvii Scientific Committee
F. Rodriguez-Reinoso, Universidad de Alicante, Spain (Chairman) B. McEnaney, University of Bath, Bath, United Kingdom J. Rouquerol, CTM du CNRS, Marseille, France K.K. Unger, Johannes Gutenberg-Universit~it, Mainz, Germany
Local Committee
F. Rodriguez-Reinoso, Universidad de Alicante, Spain M. Molina-Sabio, Universidad de Alicante, Spain A. Sept~lveda-Escribano, Universidad de Alicante, Spain F.J. Narciso-Romero, Universidad de Alicante, Spain
Financial Support The organizers gratefully acknowledge the financial support of the following sponsors: Fundaci6n Bancaja-Universidad de Alicante Ministerio de Ciencia y Tecnologia OCIT. Generalitat Valenciana
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Scanning probe microscopies for the characterization of porous solids: strengths and limitations J.I. Paredes, A. Martinez-Alonso, J.M.D. Tasc6n Instituto Nacional del Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain
Due to their ultrahigh resolution capabilities, scanning probe microscopies (SPM), and particularly scanning tunneling and atomic force microscopy (STM/AFM) are, a priori, ideally suited techniques for the direct visualization of porosity in solids of interest. In the work presented here, the application of STM/AFM for the mentioned purpose is reviewed and discussed. In general terms, the main factors which limit the ability of SPM to resolve small pores are the tip size (i.e., a blunt tip vs. a sharp one) and the degree of sample corrugation (rough vs. smooth surfaces). The examples presented are divided into three main groups: carbon materials (e.g., activated carbon fibres), inorganic materials (such as silica, alumina or zeolites) and organic/biological materials (polymeric membranes, dentin, nacre). 1. INTRODUCTION The term scanning probe microscopy (SPM) comprises a whole class of techniques that measure surface properties of materials on an extremely local scale. Although at the very beginning only topography could be examined, further advances extended the measurement capabilities to other surface properties, such as friction, adhesion, electric and magnetic forces or specific interactions between molecules. The scanning tunneling microscope (STM) was the first instrument of this family to be conceived and put into practice, some 20 years ago [ 1], for the study of conducting surfaces in vacuum. As a logical sequel to this invention, a few years later the atomic force microscope (AFM) was developed [2], expanding the realm of SPM application to nonconducting materials. Since then, the number of SPM-based techniques has been growing at a rapid and steady pace and currently amounts to a few tens of variants [3,4]. Nevertheless, the bulk of SPM work performed to date has been (and still is) carried out using either STM or AFM in any of its operation modes. The unique resolution capabilities of SPM are based on the combined use of two elements: an ultrasharp tip, which probes the sample surface and enables their mutual interaction (and therefore the measured property) to be highly localized, and a piezoceramic scanner, which controls the tip-sample relative position in the three spatial directions with subangstrom precision. This approach has allowed the imaging of surfaces with atomic resolution in STM/AFM, as many examples have demonstrated [5]. To attain such resolution level, the sample under study is mainly required to be atomically fiat and highly crystalline.
As a result of the development of probe microscopy, many areas of research have experienced a remarkable progress and the variety of materials that have been visualized and characterized by STM, AFM and/or related techniques is correspondingly broad. These include metals [6], semiconductors [7] and superconductors [8], layered inorganic materials [9] and self-assembled monolayers [10] or polymers [11] and macromolecules (including biomacromolecules) [ 12,13]. In this context, the SPM techniques (and especially STM and AFM) appear, a priori, ideally suited for the direct visualization of the porous structure of materials at scales which are not so readily accessible by other means (e.g., scanning and transmission electron microscopies). However, the performance of such a task is confronted with two major limitations. The first one arises from the fact that detection with SPM is exclusively restricted to the outermost surface of the sample. Accordingly, this implies that only the most external porosity of the material can be probed, whereas no information on the bulk (inner) porosity, which might not be identical to the former, is revealed. The second drawback is related to the finite dimensions of the probing tip, which limits the size of the voids (pores) physically accessible (and thus detectable) by the tip on the sample surface. Obviously, pores significantly smaller than the tip diameter will pass unnoticed to the instrument when the surface is scanned. As a specific example, the tips normally employed in AFM are not sharp enough to provide access to the whole mesopore range (between 2 and 50 nm). In spite of the mentioned disadvantages, useful information has been obtained from SPM imaging of a number of porous materials. To illustrate such point, the present review examines the research that has addressed the visualization of the porous structure of solids by SPM. A wide variety of materials is covered, such as porous silicon, activated carbon materials, aluminas, synthetic membranes or biological materials. 2. OVERVIEW OF THE SCANNING PROBE MICROSCOPY TECHNIQUE All SPMs are based on the same principle of operation: a sharp tip and the sample surface of interest are brought to a close proximity (several A or nm), or to mechanical contact. As a result, a physical interaction is established between tip and sample, which, in turn, gives rise to a measurable signal. The signal usually has a sharp dependence on the tip-sample distance, so it can be employed to accurately control such separation and track the surface topography of the sample under study. This is accomplished by a feedback system, which, as the tip scans the sample, adjusts the tip-sample distance in order to keep the measured signal constant. The relative motion between tip and sample in the three spatial directions is realized by a piezoceramic scanner, whose position can be controlled with subangstrom precision, enabling atomic-scale imaging. The main differences between the different SPM techniques lie in the type of interaction that is used to control the tip-sample distance. Although the SPM offspring are remarkably numerous [3,4], here we will focus only on STM and AFM, as they remain the most widely used and the best suited for high resolution imaging of surface structures.
2.1. Scanning tunneling microscopy The STM was the first scanning probe microscope to be developed [ 1]. Although the original aim was to image the surface topography of metals on the atomic scale and in
ultrahigh vacuum, it was later applied for the study of other (conducting) materials [5] and in additional environments (air and liquids). In STM, a bias voltage is applied between the sample and a metallic tip. On approaching both electrodes to just a few A, a tunneling current is established. This current decreases exponentially with tip-sample separation and is used as the signal to be controlled (i.e., to be kept constant) by the feedback system. Provided that the surface to be studied is flat enough, STM can render images with atomic resolution [5,14]. However, in general such images contain both topographic and electronic information and their interpretation is frequently a difficult task. By contrast, on large distance scales topographic effects usually dominate over electronic ones and the features observed in the images are more straightforward to elucidate [15]. 2.2. Atomic force microscopy The AFM was developed to overcome one of the main limitations of the STM, namely, the necessity of electrically conducting samples [2], and is based on the interaction forces (short- or long-ranged, attractive or repulsive) that exist universally between atoms and molecules. To measure the tip-sample interaction force, the former is attached to one end of a flexible lever, referred to as cantilever [5]. The forces exerted on the tip (~10 "9 N [16]) induce a measurable deflection of the cantilever, which is, essentially, the signal to be controlled by the feedback system. Basically, an atomic force microscope can be operated in three different modes: contact, intermittent contact (also referred to as tapping) and noncontact mode [5]. In contact mode, the tip is brought into physical contact with the sample and the repulsive force that originates thereof is used to track the sample topography. In the intermittent contact mode, the cantilever is vibrated at its resonance frequency and the tip allowed to intermittently contact the sample surface. Such interaction reduces the cantilever oscillation amplitude (compared to the noninteracting situation), which is then used as feedback signal [ 17]. In the noncontact mode, the instrument is operated in the net attractive force regime and the frequency shift arising from the interaction is employed as feedback [ 18]. 2.3. Resolution in STM and AFM Provided that the sample to be imaged is reasonably rigid, resolution in STM/AFM at the nano- and micrometer scale is fundamentally limited by the shape of the probing tip [19,20]. Thus, a certain degree of distortion will always be present on the images [21,22], so that prominent features on the surface will appear wider than they really are (the blunter the tip, the wider the apparent size). By contrast, depressions in the topography, such as pores, will appear narrower (again, the blunter the tip, the narrower the pore will appear) or will not be detected at all. Such effect has important implications for imaging the porous structure of surfaces by these techniques. Considering the tip radii of curvature of commercial AFM
cantilevers (a few tens of nm), the micropore and small mesopore (< 10 nm) range will not be accessible by AFM, unless the surface is molecularly fiat and highly ordered. This is the most serious constraint for the imaging of pores on corrugated and nonconducting samples, where only AFM can be employed. In the case of STM, it is possible to prepare probes with very sharp protrusions or minitips [23], one of which (the closest to the surface) will act as the tunneling tip, enabling the detection of smaller pores (even micropores).
3. APPLICATION OF SCANNING PROBE CHARACTERIZATION OF POROSITY IN SOLIDS
MICROSCOPY
TO
THE
3.1. Carbon materials
Shortly after its invention, the STM was employed for the study of highly-ordered carbons, particularly highly oriented pyrolytic graphite (HOPG) [24]. Due to the chemical inertness and atomic flatness of this material, atomic resolution images could be easily obtained in air, which made HOPG one of the most thoroughly investigated materials by STM. Such intensive research on graphite provided a solid background of knowledge which enabled subsequently the STM study of other (less ordered) carbon materials, such as carbon fibers [25]. In this context, the porous structure of carbonaceous materials has been studied by STM (and, to a much lesser extent, by AFM) since the early 90"s, when the first reports on the topic began to emerge [26,27]. Since porous carbons possess a highly disordered structure, their surface tends to be very rough on the atomic scale and, insome cases, also on the nanometer scale. As a result, the resolution attained for such systems will be in general lower than that achieved on atomically fiat, highly crystalline surfaces (e.g., HOPG) [15], which, in turn, may complicate the visualization of very small pores (i.e., micropores). For example, on studying by STM active carbons prepared from poly(ethylene terephthalate) and cellulose, B6ta et al. [28] failed to attain a resolution comparable to the size of the regions governing the microporous behavior of the specimens. Therefore, their micropore structure could not be visualized. Such negative result was attributed by the authors to the amorphous and very rough surface structure of these samples. A similar example can be found when trying to image the ultramicroporous structure (pore size < 0.7 nm) of nonactivated carbon materials prepared by pyrolysis of a polymeric precursor [ 15]. In this case, it was concluded that such tiny features cannot be probed by the STM tip with sufficient accuracy so as to completely rule out that they are artifactual. Thus, both examples underline the necessity of probing considerably flat surfaces if micropores are to be detected with STM. By choosing relatively fiat areas, Paredes et al. were able to image the mesoporous and microporous structure of activated carbon fibers (AFCs) prepared from Kevlar pulp [ 15]. A highly spongy mesoporous texture (pore sizes between 4 and 16 nm) was noticed on specific areas of the sample. The presence of such type of porosity was also deduced from the observation of hysteresis in the N2 adsorption (77 K) isotherms of the sample and was attributed as developing from low crystallinity regions of the polymer precursor. Concerning smaller pores, a large number of slit-shaped or elongated micropores (-1 nm wide) forming irregular and interconnected networks were observed, as shown in Fig. 1, consistent with the data obtained from the N2 adsorption measurements. The porosity of ACFs prepared from phenolic precursors was also visualized with STM by Economy et al. [29] and Daley et al. [30]. Again, elongated micropores (-1-2 nm wide) were detected, but in this case not only on the surface but also at cross-sections of the fibers, i.e, the inner porosity was also probed. Further, the whole range of mesopore sizes was ascertained in the different samples studied. A quantitative evaluation of the shape and size of pores in activated carbon spheres prepared from phenolic resin was attempted by Vignal et al. [31]. To this end, the authors proposed a numerical method based on contour maps from the STM images. Although
reasonable agreement was found between the STM results and the adsorption capacity (N2 and CHC13) of the samples, some inconsistencies arose and were ascribed to the chemical nature of the pore walls, which cannot be determined by STM. We will mention now two very recently published examples of STM on activated carbons. In one of them, Shi and Shiu [32] studied glassy carbon electrodes before and after electrochemical activation. They reported the development of pores following activation and a change in the electrochemical behavior of the sample, which was related to these structural changes. In the second example, Pfeifer et al. [33] examined a series of activated carbons which displayed an extended fractal network of channels. As expected from such structure, only sparse entrances (-1.3 nm wide) were observed by STM on the surface of the samples. Finally, we note that, although most of the SPM work on active carbons has been performed with STM, there exist a few examples in which AFM was used as well [34,35]. However, in this case the relatively large radius of curvature of the tips employed compromised the resolution and only macropores/large mesopores could be detected (Fig. 2).
Figure 1. STM image of the microporous structure of activated carbon fiber prepared from Kevlar pulp. Reprinted with permission from ref. 15. Copyright 2001 American Chemical Society.
Figure 2. AFM image of activated carbon fiber prepared from rayon. Macropores and large mesopores can be observed. Reprinted with permission from ref. 35. Copyright 2000 Elsevier Science.
3.2. Inorganic materials Due to their high electrical conductivity, metals constitute the most typical class of materials that can be studied by STM. In the context of porosity, silicon has been, by far, the most frequently studied metal using STM. Parkhutik et al. [36] made a rather pioneering application of STM to study the effect of silicon electrochemical anodization regime on the resulting porosity. Closely packed cylindrical mesopores were shown to form at low current densities, whereas branched, fibrous-like mesopores were obtained at high current densities. A simulation model was used to justify the formation of these two different types of pores. Enzel et al. [37] applied AFM to nanoporous semiconductors based on tin (IV) sulfide and tin (IV) selenide. The surface structure of these materials displayed regular pattems of micropores, in agreement with expectations from the crystallographic projections of their bulk structures. AFM studies of silica have dealt with, e.g., amorphous silica gels [38] or
monolithic mesoporous silica [39]. As an illustration, Fig. 3 shows AFM images of monolithic silica where mesopores ca. 4.5 nm in diameter can be easily identified, the pore size being in good agreement with data deduced from N2 adsorption. Xu et al. [40] have very recently prepared alumina with U-shaped pores of a uniform diameter (40-50 nm) whose bottoms were opened in a controlled way by Ar§ milling. Fig. 4 shows an AFM image of the resulting pores after short-term ion milling, which produced pores 8-14 nm wide. By contrast, long-term ion milling led to a hole diameter equal to nanopore diameter, indicating that the U-shaped bottom cap was etched away completely.
Figure 3. Three-dimensional AFM image of the surface of monolithic mesoporous silica. Taken from ref. 39 - Reproduced by permission of The Royal Society of Chemistry.
Figure 4. AFM image of Ar+-milled (9 min) hexagonally ordered nanoporous alumina. Reprinted with permission from ref. 40. Copyright 2002 American Chemical Society.
Many applications of AFM to pillared clays or zeolites have not specifically addressed the porosity characteristics, but rather the occurrence of adsorbed surface A1 species in, e.g., pillared montmorillonite [41 ], or the crystal growth processes, adsorption on porous surfaces and the surface structure of natural zeolites [42]. Sugiyama et al. [43] succeeded to reveal the ordered pore structure of the (001) surface of mordenite after removal of impurities that clogged the pores. The authors indicated that resolution in AFM imaging of zeolites is significantly affected by the magnitude of the periodical corrugation on the crystal surface, so that if the surface contains deep pores only the pore structure, but not the atomic structure, can be resolved. AFM has also been applied to characterizing the porosity of various oxide-based materials such as vanadia-silica glasses, where AFM provided an explanation for the increased microporosity when vanadium was incorporated to the xerogel [44]: while pure silica exhibited a mesopore structure consisting of voids between pillowlike structures, filling of voids between these globular structures took place when vanadium was present, leading to the creation of micropores. Inorganic membranes have also been studied. Thus, AFM has been used to probe the surface morphology and pore structure of micro- and ultrafiltration membranes, both in contact and noncontact mode, the latter being very suitable for soft and delicate materials. One of the first reports concerned alumina microfiltration membranes (Anapore) [45] and the authors performed statistical analysis to obtain the pore size distribution from the AFM
images. Alumina microfiltration membranes have been studied by Bailey et al. [46] and Jones et al. [47]. Differences in pore morphology were observed depending on the precursor material used to prepare the membrane. Oyama et al. [48] have inspected glass membranes employing AFM (Fig. 5), as well as silica membranes produced by chemical vapor deposition of a silica precursor onto the former substrate. Morphological differences between both samples were observed, which helped to explain their different permeability properties. Also, nanoporous silica membranes prepared by sol-gel methods have been characterized by Fujii et al. [49]. Void nanospaces between particles in such membranes, consistent with nitrogen adsorption measurements, were evidenced in their AFM images. Finally, macroporous silicate films fabricated by the sol-gel method have very recently been examined by Kramov et al. using AFM [50]. Uniform pores of several tens of nm were visualized.
3.3. Organic and biological materials Much work on AFM of polymer membranes has been carried out by Bowen et al. [45,51,52]. An example is provided in Fig. 6. Membranes'made of regenerated cellulose, polysulfone or poly(co-(acrylonitrile-vinyl chloride)), among others, were investigated by these authors. It was concluded that pore dimensions could be best determined when the membrane roughness was low, as tip-sample convolution effects were reduced. Polyethersulfone ultrafiltration membranes were studied by Ochoa et al. [53]. Again in this case, the resolution of the pore structure was deeply conditioned by the sample surface roughness. Stamatialis et al. [54,55] applied AFM to the characterization of the active layer of a series of cellulose ester membranes. The permeation performance of such membranes was improved when their surfaces displayed lower roughness as measured by AFM. Other applications of AFM to polymeric membranes addressed changes in morphology following plasma etching to improve their hydrophilicity [56] or the quantification of interactions between the membranes and other materials of interest (e.g., cellulose) [57]. A new class of artificial membranes, based on lipid bilayers supported on porous alumina, were recently studied by Hennesthal and Steinem using AFM [58]. In this case, tipsample convolution effects affecting the detection of pores (see Section 2.3) were clearly demonstrated by the authors on comparing the average pore size of the alumina support as obtained by SEM (60 nm) and by AFM (50 nm). STM has been successfully applied to image nanopores produced in a self-assembled monolayer (SAM) of alkanethiol deposited on a silver electrode [59]. Pore diameters of 5-7 nm and submicrometric pore depth (corresponding to the tickness of the SAM) could be measured, confirming the good resolution of STM. Marshall et al. [60] studied the effect of treatment with NaC10 (a typical deproteinizing agent used in endodontic treatment) on dentin, and evidenced by contact mode AFM a certain development of channels and canaliculi (i.e., macropores) following removal of collagen fibrils when pre-etching with citric acid had been performed. Jandt [61 ] has recently reviewed in depth the applications of AFM to biomaterials surfaces and interfaces; interestingly, pore creation by nanoindentation, using AFM tips made from diamond, has become a useful tool in characterizing the mechanical properties of tooth tissues [61,62]. Due probably to the characteristics of the materials themselves, most results shown for this type
of materials were obtained at low resolution; the main advantage of AFM in this case would seem to be relevance to living conditions and lack of sample damage thanks to the simplicity of preparation for microscopic examination. This also seems to apply in a study of abalone nacre growth, where AFM coupled with scanning ion conductance microscopy showed the existence of pores, 5-50 nm in diameter in the interlamellar organic sheets, coming in support of a new model of nacre growth by mineral bridge formation through these sheets [63].
Figure 5. AFM image of Vycor glass membrane showing pore mouths about 4 nm wide. Reprinted with permission from ref. 48. Copyright 2001 Kluwer Academic Publishers.
Figure 6. Three-dimensional AFM image of a Desal GN polymeric membrane. Reprinted with permission from ref. 52. Copyright 2000 John Wiley & Sons.
4. CONCLUSIONS This brief review has shown the possibilities of the scanning probe microscopy techniques, particularly scanning tunneling and atomic force microscopy (STM/AFM), for the characterization of porous materials and the work that has been undertaken to date on the topic. Regarding the resolution of pore structures, and due to the higher resolution capabilities of STM compared to those of AFM, conducting materials (the only class of samples that STM can be applied to) have benefited from these techniques to a far greater extent than insulating materials. Particularly, carbon materials (e.g., activated carbon fibers) have been studied down to the micropore level using STM, thus providing direct visual information about their porosity at scales not readily accessible by means of other techniques. By contrast, in the case of AFM and, therefore, of nonconducting materials, the visualization of pore structures has been much less successful (restricted to the macropore and large mesopore range). The origin of this limitation can mainly be traced to the relatively large curvature radius of the tips available in AFM. It is believed that improvements in this respect (for example, by using the recently developed carbon nanotube AFM tips) will alleviate the problem to some extent. ACKNOWLEDGEMENT The authors are grateful to DGICYT and CICYT for financial support (projects PB98-0492 and 1FD 1997-1915, respectively).
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Role of Gas Adsorption in Nanopore Characterization K . K a n e k o * ' t, T. O h b a t ,y. Hattori*, M. S u n a g a t ,H. T a n a k a * , and H. K a n o h , *) Department of Chemistry, Faculty of Science, *) Center for Frontier Electronics and Photonics, Chiba University, 1-33 Yayoi, Inage, Chiba 263--8522, Japan Characterization of nanopores must be carried out with integrated information from different levels and angles. It is shown that gas adsorption can provide a key information in the integrated characterization of nanopores. In particular, further understanding of pore filling is necessary; an evidence on quantum effect in pore filling and a new method of adsorption isotherm from P/Po = 10 -9 are introduced with the relevance to the nanopore characterization. 1.
INTRODUCTION
Recently a variety of new nanoporous materials have been developed, stimulating fundamental sciences and technologies. These nanoporous materials have an intensive feature that their nanoporosity can be controlled. Templating method using the molecular assemblies of surfactant molecules provides a variety of geometrical structures of pores, as shown in MCM-41 series.Ill The organic--inorganic hybrid nanoporous solids have a great possibility in the structure designing. [2,3] Also assembly of single wall carbon nanotube (SWNT)s which are composed of a single graphene sheet have gathered a great attraction from H2 storage and electronic devices; several modification forms have been prepared. [4,5] Hence, a better characterization of nanoporous solids has been requested more in wide fields such as pharmacy, organic chemistry, and medicine in addition to adsorption, catalysis, and separation sciences. Although the routine analytical method of porosity such as BET analysis is established for mesoporous and macroporous solids, nanoporosity evaluation method is not necessarily established. As the term of nanoporosity is not recommended by IUPAC, we need to define the nanoporosity here. The classical capillary condensation theory using the Kelvin relation has a catastrophe for the pores whose pore width w is less than about 4 nm in the N2 adsorption isotherm at 77 K; N2 adsorption isotherms even on cylinderical mesopores of w < about 4 nm and open both ends at 77 K disappear, which cannot be described by the classical theory. [6,7] Hence, it is quite convenient to use the nanopores in this articles for the pores whose pore width is less than about 5 nm. Generally speaking new nanoporous solids have well-defined pore structures.
However, a better structure designing requests a finer characterization of nanopores. We need to know structural features of nanopores as accurate as possible in order to develop the best nanostructured materials for the specific function. Nevertheless, nanopores are hidden in the bulk of solids. Consequently, established surface science tools cannot be directly applied to the nanopore characterization, leading to necessity of an inherent characterization method for nanopores on the basis of gas adsorption. This paper summarizes main characterization methods, which can be applied to nanopore systems, and essential roles of gas adsorption will be described. 2.
R O L E OF GAS ADSORPTION
Basically gas adsorption provides average information on the nanoporosity on the basis of thermodynamics. The typical nanoporosity description can be done by pore volume, surface area, and nanopore width distribution. If we use a specific molecule other than N2, the nanoporosity inherent to the probe molecule can be obtained. This is very important even if the crystallographic data can be applied to derive the nanoporosity. The X-ray crystallographic data cannot provide the effective nanoporosity for specific molecules or ions. This is because imperfect pore wall structures such as roughness in the sub--nanometer scale cannot be neglected in the nano-range performance of nanoporous solids. Then the effective nanoporosity evaluation should be recommended for newly developed nanoporous solids. The temperature dependence of the adsorption isotherm gives the isosteric heat of adsorption, which is a scale of the interaction. The hydrophobic or hydrophilic nature of the nanopore can be quantitatively evaluated by gas adsorption using water and organic vapor. If we use the supercritical gases for nanopore characterization, subnanometer structures (ultramicropores: w < 0.7 nm) can be elucidated. However, the difference in the molecular size of probes is not necessarily the pore width difference. At the same time, supercritical gas adsorption itself is not sufficiently understood. Better understanding of the supercritical gas adsorption is desired for application to nanopore characterization. Murata and Kaneko proposed the determination method of the absolute adsorption isotherm in addition to the surface excess adsorption isotherm; their analysis provides the information on the interaction energy and adsorbed state transformation, giving further understanding of supercritical gas adsorption. [8,9] Adsorption using different probe molecules is available for determination of the pore entrance structure. N2 molecules adsorbed near the pore entrance at 77 K often block further adsorption, indicating the presence of ultramicropores and pore-neck structures. The preadsorption technique is also effective for elucidation of the pore entrance structure. Fractal analysis using adsorption is helpful to understand the fine structure of nanopore walls. [10,11] However, the probe molecules must be carefully chosen and the monolayer capacity must be evaluated precisely, because the BET monolayer should not be used. Thus gas adsorption provides essentially important information on the nanopores from the sub--nanometer level, which is often inherent to a specific molecule. Also the density measurement is necessary for the exact porosity evaluation. The accurate particle and true densities give close porosity. Even He molecules are adsorbed in small nanopores at room temperature and thereby the He replacement method is not fit for determination of the particle density. Authors developed the high pressure He buoyancy method until 10
MPa for determining the particle density.[12] The accurate particle density is indispensable to evaluate the high pressure adsorption amount for supercritical gas, in particular, in hydrogen storage problem. Thus, the accurate determination of density is still not easy. 3.
I N T E G R A T E D C H A R A C T E R I Z A T I O N OF N A N O P O R E S
We need to use electromagnetic waves which can penetrate solids to elucidate the nanopores. At the same time, characterization of nanoporous solids must be examined from different angles and levels using optimum methods. The necessary levels are electronic, atomic, and morphological structures.
3.1 Electronic Level Characterization of Nanoporous Systems The electronic structures of porous solids have been examined by X-ray photoelectron spectroscopy (XPS). However, the penetration depth of electrons is 1 nm at best and XPS cannot examine electronic structures of inner pore-walls. XPS has been often used for the determination of surface chemical structures such as surface functional groups in activated carbon. Ar etching leads to the depth profile of electronic structures. This depth profile is often effective to evidence the presence of nanoporosity. Most of porous solids are diamagnetic and their bulk is transparent from the magnetic field. Activated carbon of relatively high purity is diamagnetic at room temperature and shows paramagnetism below 20 K. Then the magnetic susceptibility measurement over the wide temperature from 2 K to an ambient temperature is effective to elucidate spin structural changes of activated carbon. [13] If we use the magnetic probe such as an oxygen molecule which shows paramagnetism, nanopore structures can be estimated. [14] This is because the magnetism of oxygen confined in nanopores is sensitive to the nanopore structure. The electronic conductivity measurement can provide the information on the electronic structure of nanoporous solids. The electronic carriers transport through highly conductive paths in nanoporous solids and thereby the electronic conductivity measurement gives information on the intergranular contact. 3.2 Atomic-Level Characterization of Nanoporous Systems X-ray and neutron are quite useful and thereby their diffraction and scattering techniques have been used to analyze the pore structures from the atomic level. The diffraction techniques can determine the regular structures of long--range order. Regular mesoporous silica is a good example that the diffraction is effective for determination of the nanopore structures regardless of noncrystalline pore-wall structures. The radial distribution function analysis of the diffraction patterns can provide important structure information even for less-crystalline solids. The radial distribution function of activated carbon depends on the precursor and activation conditions. At the same time, in-situ diffraction measurements are very useful to determine the molecular state of adsorbate molecules in nanopores. Iiyama et al showed that water adsorbed in micropores of activated carbon fiber (ACF) at 303 K has a highly ordered structure and the structure does not vary even at 150 K. [15] Ohkubo et al showed that alcohol molecules form an oriented structure along the pore--walls of ACF at 303 K. [16] Thus, the diffraction technique is useful even for less-crystalline solids as well as well-crystalline solids. If
14 neutron facility is available, the comparative application of X-ray and neutron is quite powerful. The small angle scattering of X-ray 2.5 (SAXS) or neutrons can provide inhomogeneous structures of 0.3 nm to 100 nm.[17] In case of nanoporous solids, pores 2 @ are highly populated and thereby the Guinier analysis should not be applied routinely. The Debye analysis is more useful to determine both of the correlation length which is associated with the pore or pore-wall unit ~3 structures and the surface area of all interfaces in solids. The difference of the surface areas O. from SAXS and N2 adsorption at 77 K leads to the surface area of inaccessible pores. If the 0 I I , I 0 0.2 0.4 0.6 0.8 1 SAXS, N2 adsorption, particle density, and Fra ctiona l Filling true density are examined, the average size Figure 1. The zero angle scattering and surface area of inaccessible pores can be intensity ratio vs. fractional filling for derived. [18] In situ SAXS is also useful to water adsorption on pitch-based activated understand the adsorbed state of molecules in carbon fiber (average pore width = 12 nm) nanopores. Iiyama et al measured the water at different temperatures. Solid and adsorption and SAXS at the same time using broken lines denote adsorption and ACE They compared with the scattering desorption branches, respectively. O :303 profiles on the courses of adsorption and desorption at the same adsorption amount. K, /k: 313 K, and D: 323 K. [19] The scattering profiles were analyzed by the following Ornstein--Zernike (OZ) plot [20], which is composed of the scattering intensity I(s) at the scattering parameter s (s = 2~sine/)~), the scattering intensity at the zero angle I(0), and the OZ correlation length ,~. Here 2.0 and )~ are the scattering angle and X-ray wave length. I(0) is directly associated with the density fluctuation of the system, which provides the distribution state of molecules adsorbed in nanopores. The relationship between I(0) and the fractional filling on the courses of adsorption and desorption indicates that molecular cluster on adsorption is larger than that on desorption. Ohba et al measured the I(0) and fractional filling at 303 K, 313K, and 323 K, as shown in Fig. 1. The I(0) vs. fractional filling on the course of adsorption at 313 K is close to that at 303 K, while the relation on the course of desorption at 313 K resembles to that at 323 K. The relation on adsorption at 323 K is similar to that on desorption at 303 K. These results suggest that the molecular cluster sizes on adsorption and desorption are different from each other and the stability of the large clusters changes above 313 K on adsorption, giving rise to smaller clusters. Thus, diffraction and scattering techniques give the complementary structural information from atomic and molecular levels. Another new technique from the atomic structure is X-ray absorption spectroscopy (XAFS) which needs high quality X--ray from synchrotron. XAFS can focus a specific atom to give the local structure of the coordination number and the coordination distance. XAFS consists of XANES and EXAFS. XAFS has been actively applied to characterize the surfaces of catalysts. However, there are not so many studies on application of XAFS
to characterization of nanopores. Recently nanoporous Ni metal was characterized with XAFS. [22] Nanoporous Ni metal was produced by heating Ni(OH)2 in the texture of polyvinylalcohol (PVA) in air at different temperatures. Figure 2 shows Ni K-edge EXAFS spectra of Ni(OH)z-PVA heated at 673 273 K ~] K to 923 K. The EXAFS feature varies with the heating temperature. The products heated at 773 K and 673K j ' 923 K were close to Ni metal. The Ni-Ni distance of product at 923 K is 0.250 nm which agrees with that of metallic Ni (0.249 nm). Although the basic structure of this sample cannot be determined by X--ray diffraction, EXAFS analysis is effective for this 82'50' 83'00 83'5o 84'00 ' 8& Photon energy / eV sample. XPS of this sample coincides with that of Ni foil. N2 adsorption isotherm has a clear hysteresis Figure 2. Ni K-edge EXAFS giving the average pore width of 4rim and surface area spectra of PVA-Ni(OH)2 o f 120 m2/g. heated at high temperatures. NMR should be powerful to characterize nanopore structures. Fraissard et al proposed 129Xe NMR technique. [23] Although the chemical shift of 129Xe is associated with the pore width, its application to unknown porous system is not established yet. '
i
,
i
,
i
,
i
3.3 Morphological-Level Characterization of Nanoporous Systems There are two kinds of pores according to the origin. Pores come from intraparticle and interparticle structures. In case of nanoparticles, the interparticle pores play an essential role. The predominant nanopores in the above nanoporous Ni stem from the intergranular gaps. The morphological characterization using microscopes with different ranges of resolution is especially important in interparticle pore systems. Basically statistical analyses of images measured by the microscope give an average information on the nanoporosity and thereby the comparison with information from gas adsorption provides quantitatively the contribution by the intraparticle porosity. High resolution transmission electron microscopy (HR-TEM) has contributed to determine the subnano-structures of nanoporous materials. If nanoporous solids have a periodical structure, HR-TEM is more efficient for determination of three dimensional pore structures than X-ray diffraction. [24] The effectiveness of HR-TEM has been shown in studies on SWNT. Iijima et al showed the presence of nano-scale windows on the pore-wall of the single graphene sheet of single wall carbon nanohorn (SWNH) particle and the essential role of nano-scale windows in gas adsorption properties of SWNH is evidenced. [25,26] Another powerful technique is scanning probe microscopy (SPM) such as STM and AFM. This subject was well reviewed in the keynote lecture given by Tasc6n et al. [27]
3.4 Inter-Level Characterization of Nanoporous Systems The above information from electronic, atomic, and morphological levels must be integrated together with gas adsorption data to understand structures and functions of nanoporous systems. We must stress the importance of calorimetry and molecular
16
simulation. Calorimetry provides the interaction energy, being indispensable to understand gas adsorption in nanopores. Recent progresses in heat of immersion are quite powerful to analyze nanopore structures. [28,29] The fundamental information from calorimetry is necessary for molecular simulation and DFT studies which accelerate to elucidate adsorption mechanism of gas in nanopores. The detailed paper on these techniques was given by Neimark in COPS IV as the Keynote lecture.J30] Molecular simulation is available for development of the optimum analysis of gas adsorption data. Setoyama et al showed the effectiveness and limitation of as-plot for slit-shaped nanopores with GCMC simulation. [31] Recently Ohba and Kaneko pointed the necessity of inherent as--analysis for cylindrical nanopores such as SWNT with GCMC simulation. [32] The reverse Monte Carlo method is now. giving an insight to nanopore structures using the information from different levels together with gas adsorption data. [33] 4
New Directions in Nanopore Characterization with Gas Adsorption
It is stressed that gas adsorption should play a key role in nanopore characterization. However, understanding of gas adsorption mechanism and gas adsorption technique are not enough to elucidate nanopore structures exactly. The main mechanism of gas adsorption in nanopores is pore filling. The basic feature of pore filling for vapor can be understood using molecular simulation and DFT except for water adsorption, as shown above. However, an efficient gas adsorption technique for subnano--range pores must be developed. Kaneko et al attempted to apply He adsorption at 4.2 K for evaluation of ultramicropores in activated carbon. [34] Although He adsorption at 4.2 K is efficient for detection of presence of ultramicropores, quantitative evaluation of ultramicroporosity is still difficult; a kind of quantum effect is speculated. Johnson et al pointed that quantum effect is predominant in hydrogen adsorption in SWNT from their theoretical studies. [35] We need a small probe molecule for ultramicropore characterization. At the same time, contribution by quantum effect must be understood in order to establish nanopore characterization using the small probe molecule. 80 < Tanaka et al measured Ne adsorption on well-crystalline A1PO4-5 at 27 K to 33 K near -~ boiling temperature of Ne and calculated the ~ DFT isotherm, suggesting the presence of Z /,, quantum effect.[36] Thus, even Ne molecules 40 @ show quantum effect. Small probe molecules o~ such as He and H2 should show a pronounced quantum effect. Recently, N2 adsorption isotherm of 0 ' nanoporous solids is measured at 77 K over the -8 -6 -4 -2 0 wide relative pressure range of 10-6 to 1. Such (LogP/Po) a wide pressure range adsorption isotherm leads Figure 3. N2 Adsorption isotherms of to high resolution (xs-plot which provides carbon black at 77 K with SWPA (C)) accurate surface area and pore volume even for and WPA (A) methods
17 nanopores after subtraction of the effect of enhanced adsorption by nanopores. Nevertheless, we have a question whether the low relative pressure range until 10 -6 is enough to evaluate nanoporosity. Sunaga et al constructed a new gravimetric adsorption apparatus covering the wider relative pressure range from 10-9 to 1. This new method is named super-wide pressure range adsorption (SWPA) technique. [37] The background pressure is less than 10-Tpa. The N2 adsorption isotherms of nonporous carbon black samples were measured with the SWPA and WPA methods. Both of WPA and SWPA isotherms are representatives of type ]I of IUPAC classification in the ordinary expression of the relative pressure. The isotherm feature is indicative of nonporosity. However, the SWPA isotherm shifts upward. The BET plot of the WPA isotherms gives a complete straight line, which gives the surface area of 56 m2g-1, being close to that evaluated by TEM observation. Accordingly, carbon black sample may be presumed to be a nonporous solid from the conventional WPA analysis. Why does the SWPA isotherm shift upward ? The detailed information is shown in Figure 3, in which the abscissa is expressed by the logarithm of the relative pressure. The SWPA isotherm has a clear step at P/P0 = 2>( 10 -8 , accompanying with a wide plateau region until P/P0=10 "4. This low pressure uptake must be caused by the presence of open ultramicroporosity which cannot be evaluated by the WPA method. The pore width is estimated to be 0.58 nm with the aid of GCMC simulation. If we subtract the additional adsorption detected by the SWPA method, the resultant isotherm coincides with the observed isotherm by the WPA method. Consequently, even the WPA method using N2 cannot evaluate precisely the ultramicropores due to a predominant entrance blocking effect. Active studies on nanoporous materials request further development of fine characterization with gas adsorption. New methods should be challenged more.
Acknowledgement These works were funded by a Grant-in-Aid for Scientific Research on Priority Areas (No.288) and General B from the Japanese Government.
REFRENCES M. Kruk, M. Jaroniec, and A. Sayari, J. Phys. Chem. B, 101 (1997) 583. M. Kondo, T. Okubo, A.Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, and K. Seki, Angew. Chem. Int. Ed., 38 (1999) 140. 3. D. Li and K. Kaneko, J. Phys. Chem. B, 104 (2000) 8940. 4. S. Iijima, Nature, 354 (1991) 56. 5. K. Tanaka, T. Yamabe, K. Fukui Eds., The Science and Technology of Carbon Nanobubes, Elsevier, Amsterdam (1999). 6. P.I. Ravikovitch, S. C. 6 Domhnaill, A. V. Neimark, E Schfith, and K. K. Unger, Langmuir, 11 (1995)4765. 7. A.V. Neimark, P. I. Ravikovitch, and A. Vishnyakov, Phys. Rev. E, 62 (2000) R1493. 8. K. Murata, M. EI-Merraoui, and K. Kaneko, J. Chem. Phys., 114 (2001) 4196. 9. K. Murata and K. Kaneko, J. Phys. Chem. B, 105 (2001) 8498. 10. P. Pfeifer and D. Avnir, J. Chem. Phys., 79 (1983) 3558. 11. M. Sato, T. Sukegawa, T. Suzuki, K. Kaneko, J. Phys. Chem., 101 (1997) 1845. 1. 2.
18 12. K. Murata, K. Kaneko, E Kokai, K. Takahashi, M. Yudasaka, and S. Iijima, Chem. Phys. Lett., 331 (2000) 14. 13. K. Kaneko, C. Ishii, H. Kanoh, Y. Hanzawa, N. Setoyama, and T. Suzuki, Adv. Colloid Interface Sci., 76 / 77 (1998) 295. 14. A. Tohdoh and K. Kaneko, Chem. Phys. Lett., 340 (2001) 33. 15. T. Iiyama, K. Nishikawa, T. Otowa, and K. Kaneko, J. Phys. Chem., 99 (1995) 10075. 16. T. Ohkubo, T. Iiyama, K. Nishikawa, T. Suzuki, and K. Kaneko, J. Phys. Chem. B, 103 (1999) 1859. 17. M. C. Fairbanks, A. N. North, and R. J. Newpore eds., Newtron and X-ray Scattering: Complementary and Techniques, Institute of Physics, Bristol (1990). 18. M. Ruike, T. Kasu, N. Setoyama, T. Suzuki, and K. Kaneko, J. Phys. Chem., 98 (1994) 9594. 19. T. Iiyama, K. Nishikawa, T. Suzuki, and K. Kaneko, Chem. Phys. Lett., 274 (1997) 152. 20. L. S. Ornstein and F. Zernike, Proc. Sect. Sci. K. Med. Akad. Wet., 17 (1914) 793. 21. T. Ohba, T. Iiyama, H. Kanoh, and K. Kaneko, J. Phys. Chem., to be submitted. 22. Y. Hattori, H. Kanoh, and K. Kaneko, Adv. Mater., in press. 23. M. A. Springuel-Huet, J. L. Bonardet, and J. Fraissard, Appl. Magn. Reson., 8 (1995) 427. 24. S. H. Joo, S. J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, and R. Ryoo, Nature, 412 (2001) 169. 25. S. Bandow, M. Takizawa, K. Hirahara, M. Yudasaka, and S. Iijima, Chem. Phys. Lett., 337 (2001) 48. 26. K. Murata, K. Kaneko, W. A. Steele, E Kokai, K. Takahashi, D. Kasuya, K. Hirahara, M. Yudasaka, and S. Iijima, J. Phys. Chem., 105 (2001) 10210. 27. J. I. Paredes, A. Martfnez--Alonso, and J. M. D. Tasc6n, Abstract of 6 th International Symposium on the Characterization of Porous Solids, Alicante, Spain (2002) pp. 9. 28. A. V. Neimark, P. I. Ravikovitch, A. Vishnyakov, Abstract of 6 th International Symposium on the Characterization of Porous Solids, Alicante, Spain (2002) pp. 8. 29. F. R.ouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press (1999) pp. 227. 30. E Rodriguez-Reinoso and M. Molina-Sabio, Adv. Colloid Interface Sci., 76 / 77 (1998) 271. 31. N. Setoyama, T. Suzuki, and K. Kaneko, Carbon, 36, (1998) 1459. 32. T. Ohba and K. Kaneko, J. Phys. Chem., in press. 33. E R. Siperstein and K. E. Gubbins, Fundamentals of Adsorption 7, K. Kaneko, H. Kanoh, and Y. Hanzawa eds., International Adsorption Soc., Chiba, (2002) pp. 434. 34. N. Setoyama, K. Kaneko, and E Rodribuez--Reinoso, J. Phys. Chem., 100 (1996) 10331. 35. S. R. Challa, D. S. Sholl, and J. K. Johnson, J. Chem. Phys., 116 (2002) 814 36. H. Tanaka, M. E1-Merraoui, T. Kodaira, and K. Kaneko, Chem. Phys. Lett., 351 (2002) 417. 37. M. Sunaga, T. Ohba, T. Suzuki, H. Kanoh, and K. Kaneko, J. Phys. Chem., to be submitted.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Reconstruction
M e t h o d for the C h a r a c t e r i z a t i o n
19
of Porous Carbons
J. Pikunic a, C. Clinard b, N. Cohaut b, K.E. Gubbins a, J.-M. Guet b, R.J.-M. Pellenq b, I. Rannou b and J.-N. Rouzaud b a Department of Chemical Engineering, North Carolina State University, 113 Riddick Labs, Raleigh, NC 27695-7905, USA
b Centre de Recherche sur la Mati6re Divis6e (UMR 131 CNRS), 1B rue de la Ferollerie, 45071 Orl6ans C6dex 02, France We implement a modified version of the reconstruction method developed in a previous work to model two porous carbons produced by the pyrolysis of saccharose and subsequent heat treatment at two different temperatures. We use the Monte Carlo g(r) method to obtain the pair correlation functions of the two materials. We then use the resulting pair correlation functions as target functions in our reconstruction method. Our models present structural features that are missing in the slit-pore model. Structural analyses of our resulting configurations are useful to characterize the materials that we model.
1. INTRODUCTION In most simulation studies of fluids confined in porous carbons, the pore volume is modeled as the space between two parallel infinite graphite walls forming slit-shaped pores of a specified width. After performing molecular simulations at several different pore widths, any physical property is calculated by averaging that property over the pore sizes. If the experimental adsorption isotherm is available, it is possible to obtain the pore size distribution. This approach relies on an important assumption: the heterogeneity of the carbon can be described with a distribution function that depends only on the pore size. In other words, the material is described as a collection of graphite-like slit pores that are independent and unconnected. However, in most real carbons of industrial interest, the pore walls are nongraphitic and the pores are highly connected. These sources of heterogeneity must be included in the model to accurately predict the equilibrium properties and especially the dynamics of fluids confined in porous carbons. More realistic models have been proposed in the last few years and they have been recently reviewed [1 ]. Most of these models are based on very simple and qualitative reconstruction of experimental structure data. Our aim is to develop a method to generate atomic configurations of carbon atoms that quantitatively match the structural properties of real porous carbons. Thomson and Gubbins [2] applied Reverse Monte Carlo (RMC) to develop a model composed of rigid and perfect graphene segments for an activated carbon. The RMC model of Thomson and Gubbins is reasonable for many graphitisable carbons. However, the use of graphene segments as the basic structural units fails to allow for the formation of defects (i.e., rings of 5 or 7 carbon atoms) that are important sources of heterogeneity of many porous carbons [1,3]. In a more recent work [3], we presented an atomistic approach, also based in
20 RMC, that allows for the formation of non-aromatic rings which are responsible for the curvature and cross-linking in carbon plates. We combined our atomistic approach with Simulated Annealing (SA) to develop a reconstruction method [4] that gives better fits to the experimental data than the original RMC technique. In this work, we explore the use of the pair correlation function as the target function in our reconstruction method [4]. This speeds up the simulations, allowing us to construct models in much larger simulation boxes. We build models for two saccharose-based carbons treated at different temperatures. We compare the exact pore size distributions and perform Grand Canonical Monte Carlo (GCMC) simulations of nitrogen at 77 K in the resulting models. 2. M E T H O D S 2.1.
Reconstruction Method
We use a reconstruction method based on Reverse Monte Carlo (RMC) [5] and Simulated Annealing (SA) [6] developed in our previous work [4]. The goal is to produce a three dimensional array of carbon atoms that is consistent with a set of experimental data. The method consists of changing the atomic positions of some initial atomic configuration through a stochastic procedure. Changes in the atomic configuration, or moves, are accepted or rejected based on the agreement between some simulated structural property and the corresponding target structural property, which is usually obtained experimentally. Throughout the simulation, the differences between the simulated and the target functions are minimized. In this work, we use the pair correlation function, g(r), as the target function. The quantity to be minimized is: neap
X 2 =Z[gs,m(r,)-gexr(r~)]
2
(1)
l=l
where n~xp is the number of experimental points, g~m(r~) is the simulated g(r) and g~p(r~) is the experimental g(r) evaluated at ri. The answer to whether or not a configuration obtained by RMC is unique is implicit in the uniqueness theorem of statistical mechanics [7]. For systems in which the potential is pairwise additive, we can show that for a given pair correlation function there is one and only one pairwise potential, and this potential determines all the higher-order correlation functions [8,9]. However, the forces acting on carbon atoms in porous carbons do not correspond to pairwise potentials. There are angular contributions to the interaction potential that are, by definition, many-body interactions. When three-body forces are present, we can show that for a given set of a pair correlation function and a three-body correlation function, all the higherorder correlation functions are determined. Therefore, assuming that only two- and three-body forces are important in disordered porous carbons, we use the pair correlation function along with a set of simple expressions to describe the three-body correlation to completely specify the structure of the system [4]. We assume that carbon atoms in our model h a v e sp 2 hybridization. Each carbon atom is bonded to one, two or three other carbon atoms. We further assume that the angular contribution to the potential is proportional to the sum over all bond angles of the squared difference between the cosine of the actual bond angle and the cosine of the equilibrium bond angle (120 degrees). In our previous work, we used a harmonic approximation that is valid only for small angles. The expression used in this work is a better model of angular contributions for all bond angles. The fraction of carbon atoms with three neighbors may be estimated from the experimental composition (hydrogen:carbon or H/C and oxygen:carbon or
21 O/C ratios). The goal of our reconstruction method is thus to minimize three quantities; the usual 22, given by Eq. (1), along with: ~ 2 = ~ - , c o s ( ~ ) - cos l=l
d2
E/
N3
]2
and
(2)
(3)
where ~ are the different bond angles in radians, no is the total number of bond angles, N3 is the number of C atoms with three neighbors and N is the total number of C atoms. As in our previous work [4], we minimize a linear combination of the cost functions, 2 2, and oa using Simulated Annealing (SA) [6]. The procedure is as follows. We fix the parameters T r z and T J Tz (see Eq.(4)) and place a number of carbon atoms at random positions in a simulation box. The density of the system is equal to the density of the real material at the length scale of the simulation box (excluding macropores), obtained from Hg porosimetry results. The simulation starts at a high temperature Tz. The functions 2A, ~ and oa are calculated. An atom is then randomly selected and displaced to a new position and the functions 2 2, ~ and oa are calculated for the new configuration. The move is accepted with a probability:
Pace = min 1, exp -
2"n~w- 2"otJ) +
1
(2
2
1
(4)
The system runs at this temperature for a number of random moves. This is called an annealing step. At the end of each annealing step, the temperature Tz is multiplied by a constant between zero and one and the simulation proceeds to the next annealing step at this new temperature. The initial temperature is given a high enough value so that nearly all the moves are accepted. The simulation is completed when no significant changes in the three functions to be minimized are observed.
2.2. Obtaining the pair correlation function from diffraction experiments In the Reverse Monte Carlo (RMC) method [5], the pair correlation function or the structure factor is calculated after each random move (Ssim(q) or gsim(r)) and compared to the respective target function obtained from experimental diffraction data (Sexp(q) or gexp(r)). It is possible to calculate Ssim(q) with full periodicity from the atomic positions. This method is best in principle [10], but the computational cost is much greater than for any of the other available methods. It is also possible to obtain Ssim(q) by first calculating gsim(F) from the atomic positions and then Fourier transform this function and calculate Ssim(q). The disadvantage of this approach is that there is an additional computational cost associated with the Fourier transform of gsim(F) after each move. An alternative approach is to u s e gexp(r) as the target function in the RMC simulations. The analysis of the experimental diffraction data to obtain Sexp(q) involves a sequence of corrections that are generally well understood. However, in order to obtain gexp(r), it is necessary to Fourier transform Sexp(q). This operation is particularly vulnerable to the limitations of the experimental data [10-12]. For example, Sew(q) is obtained up to a maximum value of q, at which there may still be oscillations. Since the Fourier transform involves an integral from q equals zero to infinity, this limitation yields to truncation errors
22 that are reflected in the resulting gexp(r). Therefore, direct Fourier transform of Sexp(q) to obtain gexp(r) is not desirable. A way round this problem is the so-called Mote Carlo g(r) (MCGR) method [ 11,12]. MCGR is an inverse method, rather than a direct Fourier transform. The idea is to randomly modify a numerical g(r) until its Fourier transform is consistent with S~xp(q). The procedure is analogous to a one-dimensional Reverse Monte Carlo [10]. Since the numerical g(r) can be generated for arbitrarily large r-values (limited only by the experimental q resolution), truncation errors are avoided in the Fourier transform. The resulting g(r) may be used a s gexp(r) in the RMC procedure (instead of S~w(q)), significantly reducing the computational cost and thus allowing the study of larger systems. The MCGR method has been successfully applied in other RMC studies [ 13]. 3. RESULTS We prepared two carbons by pyrolysis of saccharose followed by heat treatment at two different temperatures: 400~ (CS400) and 1000~ (CS1000). We performed X-ray diffraction and SAXS on each of these porous materials and obtained the structure factors, S(q), following the procedure described by Franklin [14]. The resulting S(q)'s are shown in Figure 1 (bold line). We performed Hg porosimetry to obtain the density of both carbons, accounting for the volume occupied by C atoms, closed pores and smaller open pores. We also measured the H/C and the O/C ratios by combustion experiments. The results are summarized in Table 1. a b
21
I 0
V 1
2
3
4
5
q O/A)
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
q O/A)
Figure 1. Structure factors of saccharose-based carbons obtained from diffraction experiments (bold line) and MCGR fit (thin line), a) CS400, b) CS 1000 We applied MCGR to extract the pair correlation functions from the experimental structure factors. The resulting pair correlation functions and the corresponding structure factors are shown in Figure 2 (solid line) and Figure 1 (thin line), respectively. The MCGR structure factors are in very good agreement with the experimental data. The deviations can be attributed to statistical errors in the structure factor data and in other experimental measurements that directly determine the coordination constraint used in the MCGR procedure, e.g. density, H/C and O/C ratios. The peaks are in the same positions as the experimental peaks and the deviations are not systematic. The peaks in the pair correlation functions (Figure 2) are in similar positions to those observed in the pair correlation function of graphite. The peaks of the pair correlation function of CS 1000 are more pronounced than those for CS400, indicating that higher pyrolysis temperatures produce more ordered structures. We observed that when the structure factor data is included up to only 7.5 ~-1 in the MCGR procedure, important structural features are lost in the pair correlation function.
23 Table 1. Experimental densities and compositions H/C 0.53 0.15
CS400 CS 1000
O/C 0.123 0.041
Hg p (g/ml) 1.275 1.584
We used the pair correlation functions obtained by the MCGR method shown in Figure 2 (solid line) as the target function in our reconstruction method (explained in section 2.1). The simulation boxes are 50 A long and the density of C atoms was determined using the experimental results summarized in Table 1. Each annealing step consisted of 32 MC moves / atom, for a total of 4800 MC moves / atom. We fixed the temperature of an annealing step as 90% of the temperature of the previous annealing step. We changed the maximum atomic displacement to set the acceptance ratio to 40%. The pair correlation functions and snapshots of the resulting structures are shown in Figure 2 (dashed line) and Figure 3, respectively. a b
A
4
2
0
2
4
6
8 r (A,)
10
12
14
0
2
4
6
8 r
10
12
14
(&)
Figure 2. Pair correlation functions of saccharose-based carbons obtained with MCGR (solid line) and reconstruction method (dashed line), a) CS400, b) CS 1000 The pair correlation functions of the resulting models are in excellent agreement with the target functions. The fact that the agreement is better for CS400 seems to indicate that the minimization method is more effective for more disordered materials. The region of configurational space in which the global minimum is encountered narrows down as the material becomes more ordered. Therefore, the use of a more sophisticated minimization technique, such as simulated tempering, should improve the results. A comparison between Figure 3a and b (look at the left side for clarity) reveals how the size of the graphene segments increase with the pyrolysis temperature, as expected. Most C atoms in CS400 seem to be part of chains that link the small graphene segments. On the other hand, most C atoms in CS 1000 are placed in defective graphene segments. To further characterize these models, we calculated the pore size distributions of the resulting structural models following the procedure described in ref. [ 15]. We define the pore size in terms of the reentrant surface in the volume accessible to nitrogen. In other words, it is equivalent to the size probed by a nitrogen molecule. We define the pore size distribution in the following way: p(H)dH is the fraction of pore volume corresponding to pore sizes in the range H to H + dH. The resulting pores size distributions are shown in Figure 4. As expected, the pore size distribution becomes narrower with pyrolysis temperature. In fact, there is a mode in the distribution for CS400 at larger pore sizes that is not observed in the distribution for CS 1000. Also, the density of smaller pores is larger for CS 1000.
24
Figure 3. Snapshot slabs of the models resulting from the reconstruction method. The rods represent C-C bonds, a) CS400, b) CS 1000. We performed Grand Canonical Monte Carlo (GCMC) simulations of nitrogen at 77 K in the resulting structural models at different nitrogen chemical potentials to obtain the nitrogen adsorption isotherms at 77 K. The acceptance criteria and other details of the Monte Carlo simulations in this ensemble are given in [16,17]. We modeled each nitrogen molecule and each carbon atom as a Lennard-Jones sphere, with the same interaction parameters used in a previous work [2]. The oxygen and hydrogen atoms were not included. We related the chemical potential with the bulk pressure using the ideal gas equation. The excess amount adsorbed is presented as a function of the relative bulk pressure for both carbons in Figure 5. Snapshots of the simulations at a relative pressure of 1 are shown in Figure 6. The resulting isotherms are type I, characteristic of microporous solids. The adsorption capacity of CS400 is higher than that of CS1000, due to the higher porosity of CS400. The filling process starts at much lower (approximately 2 orders of magnitude) relative pressures in CS 1000 than in CS400. This is because CS 1000 has a higher fraction of smaller pores (see Figure 4), which produces regions in the pore volume with higher attractive potential energy. 0.8 0.7-
:~.
0.6-
!
6 4..
5-
0.5-
s
0.40.3-
2" 0.2 1-
0.1" 0 0
,
,
,
,
,
,
!
2
3
4
5
6
H
(A)
0 I 7
8
9
10
1.0E-10
. 1.0E-08
.
.
.
1.0E-06
1.0E-04
1.0E-02
1.0E+00
P/Po
Figure 4. Pore size distribution of the volume Figure 5. Nitrogen adsorption isotherms at 77 K accessible to nitrogen in the resulting models of obtained from GCMC simulations in the resulting CS400 (solid line) and CS 1000 (dashed line), models of CS400 (squares) and CS 1000 (circles). Although many nitrogen molecules are adsorbed in pores that have slit shape (see Figure 6), most of them are in pores with shapes that significantly differ from a slit geometry. It is interesting to note that a significant fraction of the pores are quasi-one-dimensional, and the
25 nitrogen molecules form chain-like structures in these pores, similar to those formed by adsorbate molecules in carbon nanotubes. a
Figure 6. Snapshots of GCMC simulations of nitrogen at 77 K in the resulting models at bulk pressure of P/Po = 1. The rods and the spheres represent C-C bonds and nitrogen molecules, respectively, a) CS400, b) CS 1000 4. CONCLUSIONS We have used a modified version of the reconstruction method developed in a previous work [4] to model two porous carbons produced by the pyrolysis of saccharose and subsequent heat treatment at two different temperatures. We used the Monte Carlo g(r) method to obtain the pair correlation functions of the two materials. We used the resulting pair correlation functions as target functions in our reconstruction procedure. Our models evidence that the size of the defective graphene segments increases with the pyrolysis temperature. We calculated the geometric pores size distribution of the resulting models. The carbon heated at higher temperatures has a narrower pore size distribution and smaller pores. We also performed GCMC simulations to obtain the simulated nitrogen adsorption isotherms at 77 K. The filling process in the carbon heated at a higher temperature starts at relative bulk pressures that are about 2 orders of magnitude lower than those in the carbon heated at the lower temperature. A significant fraction of the pore volume has shapes that significantly differ from a slit geometry. Some of the pores in both models provide quasi-one-dimensional confinement environments. 5. ACKNOLEDGEMENTS We thank the Department of Energy for support of this research under grant no. DE-FG02-98ER14847. Supercomputer time was provided under a NSF/NRAC grant (no. MCA 93 SO11). We also thank Henry Bock for helpful discussions. REFERENCES
[1]
T.J. Bandosz, M.J. Biggs, K.E. Gubbins, Y. Hattori, T. Iiyama, K. Kaneko, J. Pikunic and K.T. Thomson, in: L.R. Radovic (Ed.), Chemistry and Physics of Carbon, Marcel Dekker, New York, 2002 (in press).
26
[2] [3] [41
[5] [6] [7] [s] [9] [10] [11] [12] [13] [ 14] [ 15] [16] [17]
K.T. Thomson, K.E. Gubbins, Langmuir 16 (2000) 5761. J. Pikunic, R. J.-M. Pellenq, K.T. Thomson, J.-N. Rouzaud, P. Levitz, K.E. Gubbins, Studies in Surface Science and Catalysis 132 (2001) 647. J. Pikunic, R.J.-M. Pellenq, K.E. Gubbins, in: K. Kaneko, H. Kanoh, Y. Hanzawa (Eds.), Proceedings of Fundamentals of Adsorption 7, IK International, Chiba, 2002. p. 377. R.L. McGreevy, L. Pusztai, Mol. Simul. 1 (1988) 359. S. Kirkpatrick, C.D. Gelatt, Jr., M.P. Vecchi, Science 220 (1983) 671. R. Evans, Mol. Simul. 4 (1990) 409. R.L. Henderson, Phys. Lett. 49A (1974) 197. C.G. Gray and K.E. Gubbins, Theory of Molecular Fluids, Clarendon Press, Oxford, 1984, p. 178. R.L. McGreevy, in: C.R.A. Catlow, ed. Computer Modeling in Inorganic Crystallography. Academic Press, San Diego, 1997. p 151. A.K. Soper, in: M. Davidovic, A.K. Soper, eds. Springer Proceedings in Physics, vol. 40. Springer-Verlag Berlin, Heidelberg, 1989. p 189. A.K. Soper, Institute of Physics Conference Series 107 (1990) 57. R.L. McGreevy, Materials Science Forum 166-169 (1994) 45. R.E. Franklin, Acta Cryst. 3 (1950), 107. L.D. Gelb, K.E. Gubbins, Langmuir 15 (1998) 305. M.P. Allen, D.J. Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987. D. Frenkel, B. Smit, Understanding Molecular Simulation: From Algorithms to Applications, second edition, Academic Press, San Diego, 2002.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
27
A New Method for Microporosity Detection Based on the Use of the Corrugated Pore Structure Model (CPSM). C.E. Salmas a, V.N. Stathopoulos b A.K. Ladavos b, P.J. Pomonis b and G.P. Androutsopoulos a
a Department of Chemical Engineering, National Technical University of Athens, Athens 15 780, Greece. b Department of Chemistry, University ofloannina, Ioannina 45 100, Greece. This paper deals with the application of the CPSM and a,. methods to determine microporosities of silica gels and microporous clays pillared with All.xFexOy oxidic species. A reasonably good agreement between the two methods was ascertained. However, the CPSM method evaluates slightly higher relative microporosities in the case of pillared clays. By raising the Fe content a decrease of the relative microporosity of pillared clays was observed. The CPSM method is most appropriate for the tracing of supermicropores i.e. pore sizes Dp=l-2 nm, as it is based on the assumption that an adsorbed gas monolayer exists. The CPSM model through the simulation of gas sorption hysteresis, enables a unified analysis of porous materials exhibiting mixed meso-micropore structures, involving evaluation of mesomicro pore size distribution, determination of pore surface area in good agreement with the corresponding values from a Langmuir equivalent model, estimation of tortuosity factors rCPSM and detection of relative microporosity in satisfactory agreement with relevant data obtained by the as method. 1.
INTRODUCTION
The characterization of porous materials exhibiting a composite pore structure encompassing micro-meso-and perhaps macro-pore sizes, is of particular significance for the development of separation and reaction processes. Among the characterization methods for materials exhibiting ultramicropore structures, Dp
28 of the PSDs over the micropore range and the results with be compared with the pertinent as-
plot data. The tests will involve two groups of materials i.e. a set of microporous silica gels, that do not exhibit hysteresis, and a family of pillared clays showing wide hysteresis loops whose size depends on the Fe content. 2. THEORY 2.1.
Corrugated Pore Structure Model (CPSM) The CPSM model is a unified theory enabling the simulation of Nitrogen Sorption [8,9] and Mercury Porosimetry Hysteresis [10-12] phenomena as well as the prediction of pore structure Tortuosity Factors. CPSM-Nitrogen, is a probabilistic model, which simulates gas sorption hysteresis phenomena and evaluates a single PSD. The mathematical analysis is based on a corrugated pore structure configuration. The latter is envisaged to be composed of a sequence of N~. cylindrical pore segments of constant length and distributed diameter. N~. stands for a statistical Nominal Pore Length. The development of the CPSM-Nitrogen model is based on a number of assumptions: The Physical Multi-layer Adsorption on the surface of the corrugated pore as described by a modified Halsey Equation and the Capillary Condensation according to the Kelvin Equation. The condensation is assumed to originally occur preferably on a cylindrical interface at the narrowest segment and then proceeds through a hemispherical one, whereas evaporation proceeds preferably through hemispherical interface geometry. Following a propabilistic analysis of the envisaged physical steps taking place within the corrugated pore, the pertinent formulae have been developed, [8]. Various applications covering the entire spectrum of hysteresis loop types according to the IUPAC classification have been worked out, [9, 19-23]. CPSM-Tortuosity. This model consists of an empirical correlation (Appendix 1, eq. 1-1) that is based on CPSM-Nitrogen predictions of intrinsic pore size distribution and nominal pore length (i.e. Ns) data. The CPSM-Tortuosity Model [13] enables realistic predictions of pore structure Tortuosity Factors in satisfactory agreement with relevant literature data, [18]. Normally, high tortuosity factors correspond to low pore structure connectivities and usually wide hysteresis loops. 2.2.
Kelvin Equation The Kelvin Equation for a general fiquid nitrogen miniscus geometry reads: D r ( n m ) = 4?VL cosO _ 1.908 R T ln(P o / P ) - ln(Po/p---~cosO
(1)
for constant, y-8.85.10 7 J/cm 2, Vr=34.71 cma/mol and T=77.5 K. For a hemispherical miniscus cos0=l, while for a cylindrical cos0=0.5. Values of cos0<0.5 were reported [8,9] in simulations of N2 isotherms of materials possessing microporosity, assuming an arbitrary vapor-liquid interface geometry and y and VL values that are considered constant and typical of purely mesoporous materials. 2.3.
Thickness of Adsorbed Gas Layer The application of the CPSM model in the interpratation a variety of nitrogen sorption hysteresis loops validated a slightly modified Halsey correlation for the statistical adsorbed gas layer thickness [8,9]. Thus:
t(nm)-n
I ln(Po/p) 5 ]l/S(eo/P)m
(2)
29
where n is usually taken equal to n=0.35 and m varies in the range m=O-0.2. For m=0 eq. 2 reduces to the conventional Halsey equation. Pore Diameter D(nm) 2
~
3so.oBhambham , et al. (D) 2
8
~~
(3 ~
'hi et. al. (E)
2
1
2
0
3
B r ~ a u e r et. al. D-03
P
~
~"
!15
-~o.
r
~
""
I=
"~
200-
lO
~
,
~
5o,'
,~,
o5
O-
,
0.0
0.2
04
0.6
0.8
--
I.O
0.2
(a)
0.4
0.6
0.8
(~JB
,,0.0
0.2
0.4
0.6
0.8
1.0
0~
t0
I ,~
20
2s
Relative Pressure (P/Po)
Pore Diameter D(nm) oo
05
fo
is
20
a6
os
AI/Fe 100/0-z
~0
15
20
es
AI/Fe 95/5-z
AI/Fe 8~115-z
2
o
=
.g
,~ AI/Fe 7 5 / 2 5 - z
A
I
/
F
f
-= =
~)
AI/Fe
i
oo
(b)
02
04
os
oe
AI/Fe25/75-z
r
It__ 9
I00/0-z
~
'
02
"
'
o,
"
'
06
'
'
oe
02 '
Q~
o '2
o '4
o 'e
o8
,
lO
Relative Pressure (P/Po)
Fig. 1. CPSM simulation of Nitrogen-Sorption (77.5 K) data and the related predictions of the intrinsic PSDs. (a) Literature data for Microporous Silicas: (al), (az) Bhambhani et al, Fig. 2 curve E and D respectively [15], (as): Mikhail et al. Fig. 1, [7], (b) Pillared Clays as indicated on the plots, [ 16]. 3. RESULTS AND DISCUSSION 3.1. Materials Literature N2-sorption data of two categories of microporous materials were studied. The first category includes three microporous silicas (Fig. 1, a) possessing a different percentage of microporosity and exhibit a low-pressure adsorption step but zero hysteresis,
30 [7,15]. The second set of data (Fig. 1, b) concerns a family of pillared clays Al~_~Fe~Oywith systematically varied AI and Fe contents [ 16]. The sorption data of the pillared clays samples exhibit at low pressure a high adsorption step and at higher relative pressure an H4-type (1UPAC classification) hysteresis loop that is transformed into an H2-type loop as the Fe content is being increased. Details of the preparation procedure and the results from the application of other characterization tests are provided in [ 16]. 3.2. Simulation of N2-Sorption Data. The Corrugated Pore Structure Model (CPSM), [8,9] was used to simulate the sorption data of both types of materials under consideration and the results are presented in Fig. 1 (continuous lines through the points). It is obvious from the plots of Fig. 1 that in all cases an excellent fit was achieved for the set of CPSM parameters reported in Table 1. The predicted intrinsic pore volume distributions, PSDs, extend over the full meso- micropore range and are of bimodal type as the number of the required (b~) parameters of the BSD (Bell Shape Distribution) indicates. The method of combining two or more BSD distributions is described elsewhere [8]. In Fig. 1, is drawn only the peak of the PSDs, which corresponds to the micropore or lower mesopore sizes. In all simulation situations the equality cosOc=cosOh=cosO holds and indicates the absence of metastability effects appearing between capillary condensation and evaporation processes. Plausibly, therefore the observed hysteresis phenomena should be attributed to the pore structure networking effects. CPSM is a onedimensional model of constant connectivity that equals to two(c=2) (i.e. one segment is connected to two neighboring segments) but networking in terms of the CPSM theory depends also on the value of Tortuosity Factor [ 13 ]. The latter combines the effect of N~. (i.e. nominal pore length) and the characteristics of the intrinsic pore volume distribution as being calculated from the CPSM theory (Appendix I). For most of the materials studied in this work cos0<0.13, a fact indicating the strong presence of microporosity. In only two pillared clay samples with the highest Fe content microporosity was reduced and the simulation of hysteresis was achieved for cos0=0.25 in one case and cos0=0.29 in the second. It should be emphasized, however, that the CPSM simulations presented in this work, were implemented for G=(4Vr y cosO/RT), the Kelvin parameter, varied over the range G=0.052-0.553. These values are well below those for either a hemispherical interface geometry (i.e. G=1.908) or a cylindrical one (i.e. G=0.954) and may be attributed to the change of either Vr and/or ycosO due to the presence of microporosity. As regards the modified Halsey correlation, in all simulations parameter n is equal to n=0.35 while parameter m varies over the range m=O-O.15 (Table 1). These values are indicative of the minor deviation from the conventional Halsey correlation (i.e. n=0.35 and m=0). The nominal pore length varies in the range Ns=2-9. It is worth noting that N~.=2 for the three silicas, which do not exhibit sorption hysteresis phenomena and hence their pore structure characteristics are free of networking and capillarity metastability effects. 3.3. Surface Area In the formulation of the CPSM model the formation of an adsorbed gas layer was assumed, which allows the evaluation of cumulative surface areas and their comparison with the corresponding BET and Langmuir equivalent data. In the present study, CPSM cumulative surface areas were found to agree well with those deduced from a Langmuir equivalent model (i.e. BETmolayer, restricted adsorption [17]) for the corresponding P/Po linearity region (Table 2) and deviate substantially from surface areas determined using the BET multilayer
Table 1 , CPSM-Nitrogen Parameters for the Simulation of N2-SorptionHysteresis Data
Kelvin Parameters
F(D) Distribution CharacterizationParameters Material Designation
vgmm cm3N2
N,'
6,
62
63
w1
w2
w3
PJP0
2 2 2
-2500 -2300 -10000
-10 -130 -10
-
13 20 4
1.0 1.0 1.1
-
0.2 0.2 0.4
4.7 6.0 6.0 8.0 7.0 9.0 7.5
-11000 -11000 -11000 -6000 -11000 -8000 -8000
-170 -300 -200 -130 -500 -1500 -1500
-
1.1 1.2 1.1 1.0 1.0 1.0 1.0
1.0 1.0 0.7 1.0 1.5 2.1 2.1
-
0.470 0.455 0.470 0.465 0.465 0.420 0.430
t (nm)3
cos0,
cos&
m
n
0.9995 0.9995 0.9999
0.115 0.130 0.040
0.115 0.130 0.040
0.10 0.10 0.15
0.35 0.35 0.35
0.99980 0.99980 0.99946 0.99920 0.99946 0.99946 0.99946
0.035 0.062 0.027 0.045 0.080 0.290 0.250
0.035 0.062 0.027 0.045 0.080 0.290 0.250
0.00 0.00 0.02 0.07 0.10 0.15 0.14
0.35 0.35 0.35 0.35 0.35 0.35 0.35
(P4'J-
(STP)g
Microporous Silicas BhambhaniD [15] 187.30 BhambhaniE [15] 225.40 BrunauerD-03 [7] 331.03 Pillared Clays (AIl,Fe,OJ AI/Fe:100/0-z 99 Al/Fe:95/05-z 99 AI/Fe: 85/15-z 99 Al/Fe:75/25-z 106 Al/Fe:50/50-z 120 Al/Fe:25/75-z 104 Al/Fe:0/100-z 104
-
-150 -300 -250
0.8 1.3 1.3
1. Nominal Pore Length (Frequency of Corrugated Pore Cross Sectional Area Variation, [S]) 2. BSD: Bell Shape Distribution Function [8,11]. 3. Parameters of the Modified Halsey Equation (eq. 2).
32
adsorption model [17]. Indeed, CPSM surface areas are higher by 15-23% for pillared clays and 28-34% for the silicas. The C values for a Langmuir equivalent model vary within acceptable limits (i.e.C-19-219, recommended range C=20-200 [1]) while those of the BET multilayer method are much higher (i.e. C-64-3039). From a number of published cases [e.g. 9, 20-21, 23], CPSM surface areas tend to agree better with the pertinent BET data for purely mesoporous materials, whereas they are in closer agreement with Langmuir equivalent surface areas in materials exhibiting microporosity. Furthermore, addition of Fe in percentages equal or greater than x-50% results in surface area reduction. The latter observation is valid independently of the method of surface area determination. 3.4. Microporosity The relative micropore volume (Vm/V) and surface area (Sm/S), for all materials studied here, were calculated by the as-plot [ 16] and the CPSM methods. In the case of the CPSA/I model the evaluation of microporosity involves the integration of the respective intrinsic PSDs over the micropore range i.e. from Drain to D=2 nm. Comparisons of microporosities deduced by the as and CPSM methods are shown in Fig. 2 for both pore volume and surface area. The micropore volume comparison plot for all materials indicates clearly a satisfactory agreement between the predictions of the two methods since the data points approach closely the diagonal line. However, the CPSM method predicts slightly higher relative microporosities, which become more pronounced in the micropore surface area comparison graph of Fig. 2. The latter deviations may be attributed to the observed differences between the BET and CPSM surface area evaluations. The effect of Fe content of the pillared clay samples on the relative microporosity is illustrated in the relevant graphs of Fig. 3. The addition of an appreciable amount of Fe decreases the microporosity possibly through the blocking of micropore regions. 100~
O
B h a m b h a n i et. al.
90 1
9
Pillared Clays AlvzFexOy
80t 9 n r u n a u e r 60 5O
-~ 9oi
loo
/ O ~/"
O 03
0
10
20
-
60-
4o
-"
3o
-
20a
i
j
~" 5 0 -
lO ,
Pillared Clays AI~ F e Oy
7o -~
1 ./. 9 !
Bhambhani et. al.
9
80 J
olf
0
O
,
i
30
,
CPSM
!
40
,
!
50
,
!
60
( V ~ / V p crsM)
,
!
70
,
'|
80
,
!
90
10,
1O0
0 0
10
20
30
40
CPSM
50
60
70
80
90
100
( S ~ / S crsM)
Fig. 2. Comparison of Relative Microporosities deduced from the as plots and the corresponding CPSM simulations. For the pillared clay materials, average deviations between relative micropoprosity predictions obtained by the two methods, of 4.5% and 6% were observed for pore volume and surface area respectively, (Fig. 3).
3.5. Pore Structure Tortuosity The silica samples show a tortuosity factor rcesM =1 (i.e. Ns-2) indicating that these materials possess a regular pore structure that does not allow for networking and metastability effects.
33 Table 2. Specific Surface Area Data, deduced from BET, Langmuir and CPSM Methods.
Sample
P~o
Notation
m/g
Microp. Silicas Bhambhani D [15] Bhambhani E [15] Brunauer D03 [7] Pillared Clays A1/Fe:100/0-z A1/Fe:95/05-z A1/Fe:85/15-z A1/Fe:75/25-z A1/Fe:50/50-z A1/Fe:25/75-z A1/Fe:0/100-z
P/Po
S BETtmo)
(Langmuir equivalenO
SBE~(m.) CBEr(m~) linearity limits
ScPsM
CBEr(mo) linearity
me/g
mZ/g
limits
531 627 803
218 190 118
0.05-0.15 0.05-0.15 0.06-0.16
758 874 1214
27 31 19
0.10-0.30 0.09-0.23 0.15-0.29
761 871 1215
217 216 233 248 232 146 155
2867 3039 1352 317 165 64 73
0-0.12 0-0.13 0-0.15 0.05-0.16 0.01-0.20 0.05-0.20 0.05-0.20
260 254 283 324 298 179 190
137 219 123 52 50 40 42
0.03-0.15 0.00-0.15 0.01-0.16 0.04-0.19 0.04-0.15 0.04-0.12 0.04-0.13
260 254 283 324 298 179 190
The pillared clay materials exhibit a Tortuosity Factor rcPsM =4.69-8.41. Such values are typical of many porous catalytic supports, [18]. The variation of rcPsM with respect to Fe percentage is illustrated in Fig. 3. For Fe content higher than x=25% a substantial increase rcPsM is observed and reaches a maximum value of 8.4 for x=0.75%. The high Fe content apart from blocking microporosity regions it probably intensifies the irregularities of the corrugated pore 11.
Ioo
PsM
9
---O'--CPSM --..ira a
lo-
~
/~
o-
6~i 4o
2o.
y
a 9
, 20
9
40
..
6~)
%Fe Content
80
.
100
o I 0
.
, 20
.
,
40
9
, 60
%Fe Content
.
, 80
b
c
loo
2;
,; 6; %Fe Content
.'0
,oo
Fig. 3. Variation of, (a) Tortuosity Factors, (b) Relative Micropore Volume and (c), Micropore Surface Area with respect to Fe Content of Pillared Clays, All.xFexOy structural elements. This can be also readily seen from the wider hysteresis loop for sample AI/Fe 25/75-z (Fig. 1), which is indicative of higher tortuosity in view of the absence of capillarity metastability effects. 4. C O N C L U S I O N S From the results presented in this study it seems that the CPSM model is a suitable method for a unified analysis of porous materials exhibiting mixed meso-micropore structures. With regard to the assessement of microporosity, the CPSM method compares satisfactorily with a~. plot method and it is most appropriate for the tracing of supermicropores i.e. pore sizes in the range Dp=l-2 nm, as it involves the assumption that an adsorbed gas layer exists and the determination of the monolayer capacity becomes possible. The CPSM method, through the simulation of gas sorption hysteresis, enables the evaluation of meso-micro intrinsic pore volume and surface area distributions, the determination of cumulative pore surface areas in good agreement with those predicted by a Langmuir equivalent model, the evaluation of
34
tortuosity factors and the detection of relative microporosity in satisfactory agreement with relevant data obtained by the as method. It is of particular importance that in the case of the CPSM there is no need for the selection of a standard material which constitutes the main source of error of the as method for microporosity detection. REFERENCES
1 2. 3 4. 5 6. 7. 8 9. 10. 11. 12 13 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
F. Rouquerol, J. Rouquerol, K Sing, Adsorption by Powders and Porous Sofids, Acad. Press, 1999. M.M. Dubinin, L.V. Radushkevich, Proc. Acad. Sci. 55 (1947) 331. M.M. Dubinin, V.A. Astakhonv, Adv. Chem. Series, 102 (1970) 69. M.M. Dubinin, H.F. Stoeckli, J. Colloid Interface Sci. 75 (1980) 34. G. Horvath, K. Kawazoe, J. Chem. Eng. Japan, 16 (1983) 470. L.P. Barrett, L.G. Joyner, P.P. Halenda, J. Amer. Chem. Soc., 73 (1951) 373-380. R.Sh. Mikhail, S. Brunauer, E.E. Bodor, J. Colloid Interface. Sci., 26 (1968) 45-53. G.P. Androutsopoulos, C.E. Salmas, Ind. Eng. Chem. Res., 39 (2000), 3747-3763. G.P. Androutsopoulos, C.E. Salmas, Ind. Eng. Chem. Res, 39 (2000) 3764-3777. G.P. Androutsopoulos, C.E. Salmas, Chem. Eng. Commun., 176 (1999) 1-42. G.P. Androutsopoulos, C.E. Salmas, Chem. Eng. Commun., 181 (2000) 137-177. G.P. Androutsopoulos, C.E. Salmas, Chem. Eng. Commun., 181 (2000) 179-202. C.E. Salmas, G.P. Androutsopoulos, Ind. Eng. Chem. Res., 40 (2001) 721-730. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic press: 2nd; 1982. M.R. Bhambhani, P.A. Cutting, K.S.W. Sing, D.H. Turk, J. Colloid Interface. Sci., 38 (1972) 109-117. V.N. Stathopoulos, A.K. Ladavos, K.M. Kolonia, S.P. Skaribas, D.E. Petrakis, P.J. Pomonis, Microporous and Mesoporous Materials 31 (1999) 111-121. S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc. 60 (1938) 309. C.N. Satterfield, Mass Transfer in Heterogeneous Catalysis, M.I.T. Press, London, 1970 (pp 37, Fig. 1.5). C.E Salmas, A.H. Tsetsekou, K.S. Hatzilyberis, G.P. Androutsopoulos, Drying Technology, 19(1) (2001) 35-64. C.E. Salmas, G.P. Androutsopoulos, Appl. CatalA:General, 210 (2001) 329-338. C.E. Salmas, V.N. Stathopoulos, P.J. Pomonis, H. Rahiala, J.B. Rosenholm, G.P. Androutsopoulos, Appl. Catal.A: General, 216 (2001) 23-39. C.E. Salmas, G.P. Androutsopoulos, J. Colloid Interface Sci., 239 (2001) 178-189. C.E. Salmas, V.N. Stathopoulos, P.J. Pomonis, G.P. Androutsopoulos, Langmuir, 18 (2002) 423-432.
APPENDIX I CPSM-Tortuosity Model
The CPSM-Nitrogen theory has been utilized in the formulation of an empirical correlation that predicts tortuosity factors, thus,
rCPSM = l + 0.69 (Dmax'eff - Dmin'eff ; ( Ns - 2 )
(I-1)
where Dmax, eff and Drain, eft are trancated exteem pore diameters at 2.5% vol. on either end of the intrinsic pore volume distribution. Details of the CPSM-Tortuosity theory are given in [13].
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso,B. McEnaney,J. Rouqueroland K. Unger(Editors) 9 2002 ElsevierScience B.V. All rightsreserved.
35
Physisorption in nanopores of various sizes and shapes" A Grand Canonical Monte Carlo simulation study B. C o a s n e a, A. G r o s m a n a, C. Ortega a and R. J. M. Pellenq b Groupe de Physique des Solides, UMR 7588, Universit6s Paris 6 & 7, 2 place Jussieu, 75251 Paris, Cedex 05, France.
a
b Centre de Recherche sur la Mati~re Divisde, UMR 6619, CNRS et Universit6 d'Orl6ans, l b rue de la F6rollerie, 45071 Orldans, Cedex 02, France.
Abstract. We have performed atomistic Grand Canonical Monte Carlo simulations (GCMC) of adsorption of argon at 77 K in silica nanopores of different size and shape in order to assess the concept of t-plot (thickness of the adsorbed film with increasing pressure) used in phenomenological models for capillary condensation and mesoporous solids characterisation. Results obtained for cylindrical pores of different sizes are compared to the case of a non porous substrate. The film adsorbed in pore of diameter 10 nm has the same thickness than that obtained on the planar substrate. By contrast, as pores get smaller, we show the existence of a confinement effect on the t-plot. A FHH type of law (with a pore-size depending exponent) can be used to model this confinement effect. We have also investigated the effect of surface roughness. Results compared to the case of a smooth cylindrical pore indicates that surface roughness increases significantly the adsorbed amount and leads to an apparent film thickness thicker than for the smooth substrate. Roughness at larger length scale (pore constriction) is shown to have also a strong influence on capillary condensation which occurs at lower pressure than for the equivalent unconstricted pore; the corresponding apparent t-plot is largely affected by the presence of the constriction. Finally, simulations performed in two ellipsoidal and a hexagonal pores are reported. For the largest ellipsoidal pore and the hexagonal pore, we show that the gas/adsorbate interface keeps memory of the pore morphology at low pressures and, then, adopts a cylindrical shape as the pressure is increased. By contrast, in the smallest ellipsoidal pore, which also presents the most asymmetrical shape, the interface remains asymmetrical over the entire pressure range prior to the capillary condensation. For some of the pore geometries studied in this work, we discuss the validity of the BET model. 1. I N T R O D U C T I O N O v e r the years, vapour a d s o r p t i o n a n d c o n d e n s a t i o n in p o r o u s materials c o n t i n u e to attract a great deal of attention b e c a u s e of (i) the f u n d a m e n t a l p h y s i c s of l o w - d i m e n s i o n s y s t e m s due to c o n f i n e m e n t a n d (ii) the practical applications in the field of porous solids characterisation. Particularly, the specific surface area, as in the w e l l - k n o w n B E T model [ 1], is obtained f r o m an a d s o r b e d a m o u n t o f fluid that is a s s u m e d to c o v e r u n i f o r m l y the p o r e wall of the p o r o u s material. F r o m a m o r e f u n d a m e n t a l viewpoint, the interest in studying the thickness o f the a d s o r b e d film as a function o f the p r e s s u r e (i.e. t = f (P/P0) the so-called t-plot) is linked to the effort in d e s c r i b i n g the capillary c o n d e n s a t i o n p h e n o m e n o n i.e. the g a s + a d s o r b e d film to liquid transition of the c o n f i n e d fluid. Indeed, m i c r o s c o p i c and m e s o s c o p i c a p p r o a c h e s u n d e r l i n e the i m p o r t a n c e of the stability o f such a film on the t h e r m o d y n a m i c a l e q u i l i b r i u m of the c o n f i n e d fluid [2-3]. In s i m p l e pore g e o m e t r y (slit or cylinder), n u m e r o u s s i m u l a t i o n w o r k s and theoretical studies ( m a i n l y D e n s i t y F u n c t i o n a l T h e o r y ) h a v e s h o w n that the ( e q u i l i b r i u m ) pressure for the gas/liquid p h a s e transition in pores greater than 8 n m is correctly p r e d i c t e d by the Kelvin e q u a t i o n p r o v i d e d the pore radius R0 is r e p l a c e d by the core radius of the gas p h a s e i.e. ( R 0 - t) [4]. Thirty year ago, S a a m and C o l e [5] p r o p o s e d that the capillary c o n d e n s a t i o n transition is driven by the instability of the a d s o r b e d film at the surface of an infinite
36 cylindrical pore. Reliability of the theoretical predictions with respect to experiment is however limited because of the assumptions made about the pore geometry (slit or cylindrical shapes, description of the substrate as a continuum, infinitely smooth surface .... ). The work presented in this paper has been initiated from adsorption experiments of simple fluids in p+ type porous silicon [6]. This material is a good candidate to study gas adsorption since it consists in an assembly of unconnected parallel tubular pores which can be well characterized. Pores have a non cylindrical (polygonal) section which can be in addition asymmetrical (figure 1(a)). The perimeter and the area of each pore section of the porous layer has been extracted from an image analysis of the TEM picture [7]. Section perimeters allow the estimation of the geometrical surface of the porous layer. The PSD corresponding to pores having a section area equal to that of the pore has a mean diameter of 25 nm and exhibits an important dispersion (+/- 12 nm). Two thicknesses of the adsorbed film have been estimated from nitrogen adsorption experiments (77 K) up to the onset of the capillary condensation. First, using the PSD determined from the TEM picture, we convert the adsorbed amount into the thickness of the equivalent homogeneous film covering the pore walls (assuming the bulk density for the adsorbed phase and t independent of the pore size). The second film thickness has been determined as the ratio of the adsorbed amount with the geometrical surface of the porous layer, which has been estimated from the TEM picture. Those two estimations of the film thickness are compared in figure 1(b) to the thickness of the N2 film adsorbed in a M C M 41 sample having a diameter of 5.5 nm. M C M 41 is made of unconnected pores which are almost cylindrical and almost monodisperse. The film thickness for this sample has been extracted from adsorption measurements (77 K) made by Kruk and Sayari [8]. We have also reported a t-curve obtained by the semi-empirical FHH law and currently proposed to describe adsorption on a non porous substrate. The first result is that the thickness of the film adsorbed in porous silicon layer from the estimation of the geometrical surface is found similar to that extracted from nitrogen adsorption experiments in M C M 41 material. These two t-curves are thicker than that proposed to describe adsorption on a non porous substrate (FHH equation). The second result is that we observe that the use of the PSD of cylindrical pores having the same section area than the real pores leads obviously to an apparent t-curve thicker than that obtained on the basis of the geometrical surface (perimeters). This shows that the knowledge of the mean dimension of the pore is not sufficient to determine t-curve from adsorption isotherm.
Figure 1 : (a) Bright field TEM image in plane view of a porous Si layer with 70 % porosity prepared from p+ type (- 3.10 .3 ff2.cm) [100] Si substrate. Pores (in white) are separated by Si walls (in black). (b) Film thickness derived from N2 adsorption isotherm at 77 K for a porous Si layer: (n) extracted from the pore size distribution of cylindrical pores having the same section area as real pores, (o) from the geometrical surface. (A) are film thickness for MCM 41 (5.5 nm). Solid line shows a t-curve obtained by the semi-empirical law FHH and currently proposed to describe adsorption on a non porous substrate.
37 Those different aspects (pore size and pore geometry) have been considered in this paper in which we present a study of gas adsorption (Ar, 77 K) in silica pores of different size and shape by atomistic Monte Carlo simulations in the Grand Canonical ensemble (GCMC). Section 2 is devoted to a brief description of the simulation procedure. Results are presented and discussed in section 3. In the first part, we present simulation results for smooth cylindrical pores of different size and consider the confinement effect on adsorption and t-plot prior to capillary condensation. Adsorption on a planar silica surface is taken as a reference system. In the second part, we present some results concerning the effect of the surface roughness on adsorption in a cylindrical pore by considering surface defects (i.e. a pore wall having a microporous texture) and pore defects (i.e. pore constrictions. In the third part of section 3, we have studied the effect of the pore geometry by comparing simulation results of adsorption in non cylindrical pores (pores having an ellipsoidal section and a hexagonal section) with that for the corresponding cylindrical pore. Finally, the last part of section 3 presents a discussion of the validity of the model BET in the case of the different pore morphologies studied in this work. 2. S I M U L A T I O N P R O C E D U R E Pores used for simulation were prepared from a 15 3 unit cell cristoballite cube (cristoballite is a siliceous non porous solid). This allows us to cut out portions of this initial volume in order to obtain different geometries or different sizes. In order to model a realistic silica surface and ensure electroneutrality, all oxygen dangling bonds are saturated with hydrogen atoms (all silicon atoms in an incomplete tetrahedral environment are removed). Hydrogen atoms have been placed in the pore void, perpendicularly to the interface at 1 ,~ from the closest unsaturated O atom. Periodic boundary conditions are applied in x, y and z directions to simplify the GCMC adsorption procedure. Interactions between Ar atom and substrate atoms are calculated using the PN-TrAZ potential as reported initially for rare gas adsorption in zeolite (microporous solid) [9-10] and mesoporous silica (vycor-like) glass [11-12]. Adsorption intermolecular energy is written as the sum of dispersion interaction with the repulsive short range contribution and an induction term due to the interaction of the adsorbate atoms with the local field created by the partial charge carried by substrate atoms (qo = -le-, qsi = +2e- and qH= +0.5e-). The AffAr interaction was calculated on the basis of a Lennard-Jones potential (~ = 120 K and (y = 0.34 nm) which correctly reproduces properties of the Ar 3D-liquid. 3. R E S U L T S AND D I S C U S S I O N
3. 1 Adsorption in pores of different diameters We have studied the confinement effect on adsorption prior to capillary condensation by simulating Ar (adsorption) isotherms at 77 K in cylindrical pores of different diameter (4.0, 6.0 and 10.0 nm) and on a plane substrate. Density profiles of the adsorbed phase show that the fluid density and structure is very close to that of the 3D-liquid (p = 1.49 g/cm3). By integrating the density profile, the thickness of the Ar adsorbed film is determined as the thickness of an homogeneous film formed by the same number of adatoms and having the bulk density value. Figure 2(a) presents the different film thickness (t-curves) obtained as a function of the relative pressure of the gas. These simulations indicate that for pores larger than 10.0 nm, the t-curve equals that obtained for the non porous substrate. By contrast, for smaller pores, one observes that the film thickness increases as the pore size decreases.
38
2.0,--
2.0
1.5
1.5
1.0 A b-'
0.5
eXt~cA:~
(b) 0
o
9
02:
9
0;4
o2e P/Po
.' . . . . . . . .
oJz
~'
1
On
o
9
i
0.2
9
,
0.4
oi~
6.8
i
P/po
Figure 2 : (a) Film thickness obtained by GCMC simulations of Ar adsorption at 77 K in cylindrical pores of different diameter: (o): 4.0 nm, (o): 6.0 nm, (~): 10,0 rim. (o) indicates the thickness of the film adsorbed obtained on a silica plane substrate. (b) Thickness of the experimental Ar film adsorbed at 87 K on MCM 41 pores of different diameter: ([]): 3.6 nm, (o): 5.1 nm, (A): 6.5 nm. This result is in a qualitative agreement with the experimental t-plot of Ar adsorption at 87 K on M C M 41 samples (see Figure 2(b)) using the data given in reference [13]. As for simulation data, we assume that the density of the adsorbate equals that of the 3D-liquid and we have determined the thickness of the adsorbed film as the ratio of the adsorbed volume with the surface of the sample. Assuming pores of M C M 41 are cylinders, the specific surface S of each sample was determined via the relation between the porous volume V (given by the adsorbed amount after capillary condensation) and the diameter d of the pores: S = 4V/d. Comparison of the different t-curves indicates that there is a pore size (5.1 nm) above which no confinement effect occurs on multilayer adsorption. Below this value, the thickness of the adsorbed film increases as the pore diameter decreases, t-curves are often analysed with the Frenkel-HalseyHill equation [ 14] 9 ..]l/n
f.:_
'=
~ Lkr (P/P0)j
where o is the kinetic diameter of the adsorbed atom and (a; n) are two parameters to be adjusted to fit available data. We first determine the set of parameters for the t-curve obtained for the plane substrate. Exponent n was found equal to 2.41, which is lower than the theoretically expected value of 3. This result, which is in agreement with numerous adsorption experiments [15], is due to the use of a mean field approach in the FHH theory. Since o~ is an energy parameter which to a first approximation is expected to depend only on the substrate surface chemistry, we fix its value to that found for the plane substrate (6.17 kT). Then we determined the value of the n exponent for cylindrical pores of different diameters (table 1) : Pore Exponent n
Plane substrate 2.41
Cylinder 10.0 nm I Cylinder 6.0 nm I Cylinder 4.0 nm 2.40 2.00 1.47
Table 1 : Value of the exponent n in the FHH equation for adsorption GCMC simulations in cylindrical pores and on a plane substrate. n decreases with increasing confinement. Therefore, it is possible to use the FHH equation provided that the pore-size dependence of the exponent is taken into account. 3.2 Effect of the surface r o u g h n e s s In this section we consider the effect of the surface texture on gas adsorption. We have prepared a rough pore which can be defined as an assembly of strips of the same thickness (1.07 nm) along the pore axis but of random depth. The mean diameter is 9.4 nm and the dispersion is
39 about 0.5 nm. Snapshots obtained at different pressures by GCMC simulation of Ar adsorption at 77 K in this rough pore are shown in figure 3. We have separated atoms which are trapped in the troughs defined by the roughness and atoms which are adsorbed on the remaining surface. 6060~=lO-
(c)
(b)
(a)
020-
20.
oo
! -20-40
-60
,i
,,,
,J
t
0-
-20
-20.
-40
-40.
-60
.......
, ..... ,
-60
~0 0 20 40 60 0 zb 4b 6b Figure 3 " 2D-representation of Ar adsorbed at 77 K in a 9.4 nm rough pore: abscissa indicates distance of a given atom to the pore axis (in/~) and ordinate the z-position of the atom along the pore axis (A). (o) are trapped atoms, (o) are adsorbed atoms (see text). (a) P = 10.4 P0, (b) P = 0.4 Po, (c) P = 0.7 P0. 0
'20
40
Adsorption isotherms obtained in this rough pore and in a 10.0 nm smooth cylindrical pore are shown in figure 4(a) (for comparison curves have been normalised to the porous volume). Due to the trapped atoms, one observes that surface roughness obviously increases the adsorbed amount and thus leads to an apparent t-curve thicker than in the case of the smooth pore. This difference appears for very low pressures (P - 10 -2 P0) and remains constant on all the pressure scale. Considering only " a d s o r b e d " atoms separately from the trapped atoms, we have determined for each strip the thickness of the Ar film. The corresponding t-curve obtained which has been averaged on the 10 strips is compared with that for the smooth 10.0 nm cylindrical pore (figure 4(b)). This last result shows that it is then possible, knowing the diameter of the pores, to determine the surface roughness since the adsorption isotherm can be described as the sum of the adsorbed atoms which define a t-curve, identical to that obtained for a smooth substrate, and trapped atoms which correspond to a constant adsorbed amount. Now let us turn to a larger length scale roughness : a pore constriction. We prepared a pore having a diameter 6 nm with a constriction of a diameter 4 nm. The film thickness in this pore has been determined as the ratio of the adsorbed amount with the geometrical surface. Such a t-curve is close to that obtained for the 6 nm pore. Describing the constricted pore as a regular cylindrical pore having the same pore volume, the equivalent diameter is found equal to 5.14 rim. Interestingly, the t-curve obtained as the thickness of the film covering the surface of this equivalent cylindrical pore is .,.-?
1
2.0 ~
(a)
0.8
~"
(b)
1.5-
jo
~ 0.6 1.0.~
0.4
"~ <
0.2 G
/f /o
0
0 ~'~~
o-l-o'1
0.2
0 -I"-0"-"~0 ~ 0 ,___,_--0
0.4
.-~.M~-.x:: 0.5. ['-'
0.6
P/Po
0.8
1
0
f y o~" 0.2
0.4
0.6
P/Po
O.8
I
Figure 4" (a) Ar adsorption isotherm (77 K) obtained by GCMC simulations in a 9.4 nm pore with surface roughness (o) and in a smooth pore (o). (b) Thickness of the Ar film adsorbed corresponding to the adsorption isotherm shown in (a).
40 significantly increased compared to the t-curve which was determined considering the whole surface of the constricted pore (figure 5(a)).
Figure 5 : (a) Thickness of the Ar film adsorbed in the constricted pore : (n) considering the whole surface of the pore, (i) describing the pore as a regular cylindrical pore having the same pore volume. (b) Ar adsorption isotherm (77 K) : (e) the regular 6 nm pore, (o) the regular 4 nm pore, (n) the pore with constriction. (c) equilibrium snapshot with increasing pressure (from the left to the right: P/P0 = 0.27; 0.36; 0.40; 0.49). More interesting is the effect of the constriction on the capillary condensation. Figure 5a shows the adsorption isotherm; Clearly, due to the constriction, the condensation pressure is shifted toward the low pressure end. The mechanism of adsorption is also greatly affected by the constriction indeed as can be seen in figure 5b, once the (metastable) condensation has taken place in the 4 nm constriction, filling of the remaining porosity occurs at equilibrium with the displacement of two hemispherical gas/liquid interfaces (menisci). This process lasts until a nearly spherical gas bubble is formed (due to periodic boundary condition). A final (metastable) condensation then occurs giving rise to the last increase in the adsorbed amount at P/P0 =0.35. Considering the adsorption isotherm of the pore with a constriction, the propagation of hemispherical menisci from the constriction can be misleadingly interpreted as being the consequence of a strong adsorption potential field of the substrate. This can be seen as an overestimated t-plot.
3.3 Effect of the pore morphology Real porous materials exhibit pores with irregular shape. We have studied the effect of the pore shape on adsorption considering pores with a dissymmetrical or an angular section. We first consider adsorption on two model pores with smooth walls, infinite length and an ellipsoidal section. The size of the sections are 6.4 • 2.5 nm and 8.1 x 5.8 nm. To minimize the surface term in the grand potential, we expect that the gas/adsorbate interface tends to a cylindrical shape, leading to an important increase of the adsorbed amount compared to the symmetrical case of a cylindrical pore. G C M C configurations of Ar atoms adsorbed in ellipsoidal pores are shown in figure 6. In each ellipsoidal pore, we observe that at low pressure the interface between the gas and the liquid does not adopt a cylindrical shape but keeps memory of the pore geometry. As the pressure increases, the interface in the ellipsoidal pores becomes less asymmetrical even if the shape remains ellipsoidal. The exact shape of the interface at the condensation pressure awaits further calculations, t-curves may be defined in such asymmetrical pores as the average value of the angular profile of the film thickness. For each ellipsoidal pore, t-curve thus obtained is compared in figure 7 to the t-curve obtained in the case of a cylindrical pore having the same "section area" (4 nm for the smallest ellipse and 6 nm for the largest). These comparisons indicate that films adsorbed in ellipsoidal pores are always thicker than those adsorbed in cylindrical pores having an equivalent section area. It is observed that this effect strengthens as the asymmetry and/or the confinement increase.
41
Figure 6 9Configurations of Ar atoms adsorbed at 77 K in an ellipsoidal pore (6.4 x 2.5 nm) at (a) P = 10 -3 P0, (b) P = 10.2 P0, (c) P = 5.10 .2 P0 and in an ellipsoidal pore (8.1 • 5.8 nm) at (d) P = 10-4 P0, (e) P = 0.44 P0, (f) P = 0.77 P0. Black atoms correspond to the hydrogen atoms which delimitate the pore surface, the white spheres are argon atoms. 2.0 2.0
(b)
Q
~15
1.5. tA
~o
o o. ~
].0
N 05
0,5
( o
61
0.'2
PIPo
o13
. o14
oo
o'.~
o14
o'.6
o'.s
P/'Po
Figure 7 9(a) Film thickness obtained by GCMC simulations of Ar adsorption at 77 K in the ellipsoidal pores (o) and in cylindrical pore having the same section area (.). (a) The ellipsoidal pore (6.4 x 2.5 nm) compared to the cylindrical pore of a diameter 4.0 nm and (b) the ellipsoidal pore (8.1 x 5.8 nm) compared to the cylindrical pore of diameter 6.0 nm. W e h a v e also studied A r a d s o r p t i o n in a pore exhibiting an a n g u l a r section. W e p r e p a r e d a h e x a g o n a l p o r e h a v i n g a largest d i m e n s i o n of 10 nm. F i g u r e 8 presents snapshots o b t a i n e d at different pressures. As in the case o f the largest ellipsoidal pore c o n s i d e r e d p r e v i o u s l y , w e o b s e r v e that the g a s / a d s o r b a t e interface keeps m e m o r y o f the p o r e m o r p h o l o g y at low p r e s s u r e s and tends to a cylindrical s h a p e as the p r e s s u r e is increased. To define the thickness of the film adsorbed, w e first c o n v e r t the n u m b e r of A r atoms in an a d s o r b e d v o l u m e using the d e n s i t y o f the 3D-liquid. Then, the film t h i c k n e s s was d e t e r m i n e d as the ratio of this quantity ( n m 3) with
Figure 8 : Configurations of Ar atoms adsorbed at 77 K in a hexagonal pore having a largest dimension of 10 nm at (a) P = 0.1 P0, (b) P = P = 0.22 P0, (c) P = 0.44 P0. Black spheres correspond to the hydrogen atoms which delimitate the pore surface, the white spheres are argon atoms. the g e o m e t r i c a l surface o f the p o r e (nm2). C o m p a r i s o n o f the t curve for the h e x a g o n a l p o r e and the t-curve for a cylindrical p o r e h a v i n g a d i a m e t e r of 10 n m indicates that the film is quite identical in both pore g e o m e t r i e s . H o w e v e r , we note preferential a d s o r p t i o n in the pore angles. Results o b t a i n e d for the largest ellipsoidal pore and the h e x a g o n a l p o r e are in a g r e e m e n t with e x p e r i m e n t a l d e t e r m i n a t i o n of t-curves in porous silicon w h i c h s h o w that the film t h i c k n e s s
42 extracted on the basis of the section perimeter is closer to that determined in MCM 41 than that extracted using the PSD of cylindrical pores having the same section area. 3.4 D i s c u s s i o n on the validity of the B E T m o d e l For each pore presented in this study, we have determined the geometrical surface of adsorption and the surface determined via the BET model. Comparison of both specific surfaces are reported in the table 2. We used the value of 0.138 nm 2 for the Ar atomic area.
Rough pore Cylinder Cylinder Ellipse Hexagone 6.0 nm 4.0 nm 9.5 nm 8.1 • 5.8 nm . . .10.0 nm . 286 / 202 231 / 134 517 / 388 358 / 237 446 / 321 SBET / Sgeo Ratio 1.42 1.51 1.39 . . . . 1.72 . . . . 1.33. . Table 2 : Comparison between geometrical surface and surface extracted from adsorption via the BET model. Pore
Cylinder 10.0 nm 421 / 336 1.25
The BET analysis of the adsorption isotherm systematically leads an overestimated value of the specific surface when compared to the intrinsic geometrical value. Gelb and Gubbins have also tested the BET equation for a model of silica glasses and obtained similar results [16]. 4. C O N C L U S I O N We have performed atomistic grand canonical Monte Carlo (GCMC) simulations of adsorption of Ar in silica pores of different sizes and morphologies. For pores smaller than 10 nm, confinement leads to an increase the film thickness. By describing the t-curves in the FHH equation, we have shown that the exponent n varies with the pore size. Simulations in a pore with nanometer-scale surface roughness indicates that the resulting adsorption isotherm leads to an apparent t-curve thicker than in the case of smooth cylindrical pore. The case of a nanopore with a nanometric constriction was also considered. The adsorption mechanism is strongly affected by such a large scale defect leading to an overestimated t-plot. Results for ellipsoidal and hexagonal pores outline the effect of pore asymmetry. Finally, we have found that the BET method for surface area determination always overestimates the "true" geometrical surface area. Dr. J. Puibasset is gratefully acknowledged. We also thank IDRIS (CNRS) for the computing grant 1t427. REFERENCES
[1] S. Brunauer, P. H. Emmett ad E. Teller, J. Am. Chem. Soc., 60, 309 (1938). [2] F. Celestini, Phys. Lett. A 28, 84 (1997). [3] R. J-M. Pellenq and R. Denoyel, Langmuir, 18, 2710 (2002). [4] L. D. Gelb et at., Rep. Prog. Phys. 62, 1573 (1999). [5] W. F. Saam and M. W. Cole, Phys. Rev. B 11, 1086 (1975). [6] B. Coasne, A. Grosman, C. Ortega and M. Simon, Phys. Rev. Lett., 88, 256102 (2002). [7] B. Coasne et al., Phys. Chem. Chem. Phys., 3, 1196 (2001). [8] M. Kruk and A. Sayari, Langmuir, 13, 6267 (1997). [9] R. J.-M. Pel|enq, and D. Nicholson, J. Phys. Chem. 98, 13339 (1994). [10] D. Nicholson, R. J.-M. Pellenq, Adv. in Colloid and Interface Science, 76-77, 179 (1998). [11] R. J.-M. Pellenq, S. Rodts, V. Pasquier, A. Delville, P. Levitz, Adsorption, 6, 241 (2000). [12] R. J.-M. Pellenq, B. Rousseau, P. Levitz, Phys. Chem. Chem. Phys., 3, 1207 (2001) [13] M. Kruk, M. Jaroniec, Chem. Mater., 12, 222 (2000). [14] G. Halsey, J. Chem. Phys. 16 (10), 931 (1948). [15] S. J. Gregg and K. S. W. Sing in "Adsorption, Surface area and Porosity ", Academic press, London (1982). [16] L. D. Gelb and K. E. Gubbins, Langmuir, 14, 2097 (1998).
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. MeEnaney. J. Rouquerol and K K. Unger (Editors) 9 2002 Elsevxer Science B.V. All rights reserved
43
Induced porosity in cross-linked polymer networks: Mean field theory and simulations Simcha Srebnik a "~Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel Molecularly imprinted polymers are formed through the self-assembly of functional monomers around a template or imprinting agent, followed by cross-linking of the monomers to form a firm gel. Subsequently, the templates are removed to leave behind pores that are complementary in size, shape and functionality. Successful molecular imprinting of organic and inorganic gels requires the formation of a stable gel having a sufficiently large number of fully functional pores. We study the imprinting efficiency of gels made up of tetra-functional monomers using a simple statistical model that accounts for reaction kinetics. The equilibrium fraction and functionality of the imprinted pores are predicted as a function of the preparation conditions. The induced porous structure on the macromolecular level is investigated using molecular dynamics simulations of gelation of tetra-functional monomers in the presence of spacer molecules. 1. INTRODUCTION In the past two decades, methods for tailoring the structure and chemical affinity of gels have advanced significantly. Perhaps the most successful approach, termed molecular imprinting, involves the self-assembly of organic or inorganic precursors to form stable ordered (and functional) gels [e.g., 1-8]. Molecular imprinting is achieved through polymerization and cross-linking of functional monomers in the presence of an imprinting agent, or template, which is subsequently removed leaves cavities within the gel that are complementary to the size, shape, and/or functionality of the template. Several general guidelines towards fabricating stable imprinted gels have been established over time. For instance, it is known that highly cross-linked (70-90%) rigid polymers and a controlled porosity are elements necessary for achieving specificity sufficient for molecular recognition [9,10]. On the other hand, the polymer matrix must be flexible enough to allow for rapid diffusion of the target molecules [11]. In general, it has been observed that the difficulty of making the imprinted materials increases with increasing concentration and size [12,13]. Several additional features must be realized in order to achieve recognition between the imprinted gel and target molecules [14-16]: 1) Molecules of more rigid structures make for better imprinting agents. 2) Affinity towards the imprinting molecule and the site is enhanced by stronger interactions between the complementary functional sites. 3) The bond between the imprint agent and the polymer should be strong but reversible. 4) The polymer should be highly cross-linked to maximize pore integrity yet flexible to allow for rebinding of the imprinting agent.
44 Very recently, a number of theoretical efforts have emerged that investigate the interplay of the various parameters influencing the molecular imprinting process. Lu et al. [15] approximated the volume occupied by a propyl methacrylate molecule, the imprint agent. By comparison with experimental porosity measurements they determined the percolation threshold volume fraction needed for accessibility of the pores created by removal of the agents. Enoki et al. [ 17] have studied the effects of frustration on molecular imprinting using a non-conventional imprinting technique. The experimental approach involved copolymerization of functional ligands (that interact with the target molecule, or imprint agent) and backbone monomers, followed by induced post cross-linking. They found that post cross-linking in the presence of the target molecule minimizes frustration, and thus allows the imprinted copolymer gel to approach its free energy minimum. They have found that the affinity towards the target molecule increases exponentially with increasing concentrations of post cross-linking agents, and modeled this behavior using simple probabilistic arguments. Using mean-field theory, Srebnik et. al. have investigated the induced porosity of polymer networks obtained through various approaches [18-20]. In this work, we concentrate on characterization of the imprinted cavities and porous gel as a function of the size and functionality of the templates. 2. THEORY
We develop our model following the approach by Corrales and Keefer [21], where the formation of various reacted tetra-functional monomers corresponding, e.g., to the functionality of carbon or silica, are calculated based on equilibrium reaction probabilities. We let Qi (i = {0,1 ..... 4} be a monomer having i cross-linked bonds. Defining AfT as the total number of imprinting agents of functionality f, then the total possible number of bonds between the monomers and the imprinting agents, or templates, is fNT. The different species are "placed" one by one on a tetrahedral lattice, where the volume of each site is equivalent to the volume occupied by a monomer. The size of the templates, M, and thus the size of the cavities that remain in the network, is defined in terms of the number of lattice sites that they occupy, r. Thus, r is the volume ratio of the template to a monomer. We consider a dilute concentration of the templates and assume that no network defects exist. In other words, all templates are solvated (though not all of their functional sites are necessarily associated) and of the monomers, only Q4 and Q3 species exist. (Q4 corresponds to a fully cross-linked monomer). Such a model allows for the prediction of the efficiency of the imprinting process as well as the volume fraction that can be occupied by the imprinted cavities under equilibrium conditions. Assuming all equilibrium constants are equal, the reactions considered can be written as K
04 + M i r Q~M i+1
"~" leaving
group
( 1)
where Mi refers to a template with i associated sites. In the model, the leaving group is assumed to be small. The equilibrium constants are related to the thermodynamic Gibb's free energy in the regular manner: ksTlnK,, = AG,,~ = AH,,~ - TASn~ where k8 is Boltzmann's constant and T is the temperature, and ideal mixing (no volume change) is assumed. The chemical equilibrium constants can also be written in terms of the activities of the different species, defined in terms of the chemical potential,/t~,, of specie k, as ak=exp(ttdk~T). Thus, at equilibrium
45 K
(2)
aM,+'
=
a4aM,
Again, assuming no free imprinting agents, such that at equilibrium, i# 0, and denoting the f
total number of agents as n M - ~ n M ,
where riM, denotes the number of templates that have
i=l
i associated functional sites, the partition function describing the number of possible states of the system can be written as Z - s
4 ,.~Ml ~M2 "" "aMZ
, M
n
where ~ is the is configurational contribution, determined from the number of ways of placing a mixture of the different species on a lattice of Ns sites, given that they must be arranged in such a way as to form combinations of specific sites, f~ is calculated in the manner of Flory's lattice models [22,23]. In the following derivation we consider the templates to be rigid rods made up of r units (each of size of the monomer) and having four functional sites. Assuming no non-associated templates, ~'-~=~'-~1~"~211~'-~312,1~'-'~413,2,1, where
~.2k=~rtM,M,~=i,~HVkil.and vi+~k = (N s - ( r
+ 1)j)z(1- fj )r fc0k , Z is the coordination number of the
lattice and ~ is the number of distinguishable ways of placing the imprinting agent having k associated functional sites. Solving for ~2 we obtain o.)nM~ f# }?'/M4
..0.=
N," !2 2"M('0~M'(02M~ 3 ~'4
(4)
HM 1 11, 9 ZM 2 .I ]~M3.I RM4 ! H 4 I. N .N,-n4-nM
The Gibb's free energy is obtained by combining the logarithms of Eqs. 3 and 4. Using Stirling's approximation, we obtain
COn
--
"w"
x4 - /J4
,+ /,nXl / riM'
- ' ~ 1 - /'lM,
+ nv:
ln4-~ -/'tv:
/
+ nv'-
1n4-~3-/av'
) (5)
- ~UM, + rnM, + (r + 1)n M + (r + 2)riM, + (r + 3)riM, + Zvn3 + ZFrnM (1 -- rx M )
where x/= ni/N,~. The last two terms have been added to account for the energetic contribution of the imprinting agent-monomer associating non-specific and specific interactions, respectively. The isothermal differential of the free energy can be written as dG - f l 4 d n 4 + ,llM d n M , + itlM2drtM2 +/.lM3drtM3
+ II.IM4d~M4
(6)
At chemical equilibrium the partial derivatives of the free energy with respect to the different species equals zero, and the following resulting equations for the number fractions of the different species need to be solved simultaneously: r+l -Zv-r-ZFr(l-2rx M )
XMl = 4 0 ) l X 4 e
(7)
x,, e = (o,:lo,,
<s>
=
(9)
46
x~,~ = (co~/co, )x~e-3<~+" x~,
(10)
x 4 - 1 - ( r + 1)XM, --(r+2)XM2 --(r+3)XM ' --(r+4)XM~
(11)
X M -- XMI "Jv XM2 -Jv XM~ -Jr-XM4
(12)
3. S I M U L A T I O N S The mean-field model developed above allows us to predict the number of fully functional pores formed by the imprinting process. However, to learn about the ordering of the porous structure induced by the templates, we employ atomistic simulations. We generate a crosslinked polymer networks by first equilibrating a concentrated solution of oligomers using Gen_Pol [24], an off-lattice molecular dynamics simulation. Gen_Pol provides an efficient way to generate concentrated polymer solutions using a Monte Carlo procedure based on the canonical ensemble (N, V,T). The solution is then equilibrated using a microcanonical (N, V,E) molecular dynamics routine that integrates Newton's equations of motion using the Verlet algorithm [25]. Ref. 24 provides a detailed description of the general simulation algorithm. Prior to the equilibration process, we introduce the templates. As our primary interest is to understand the induced pore structure and network ordering by molecular imprinting, as a first approximation, we consider the templates to be excluded volume spheres of LennardJones radius crt that interact through short-range forces with the oligomers (see below). Once equilibrated, cross-linking constraints are enforced according to the desired degree of crosslinking, f.t. Monomers of quadruple functionality are considered, again corresponding to the functionality of carbon or silica. The cross-linking sites are randomly chosen between monomers that are of a chosen minimum distance apart. The system once again is allowed to equilibrate according to the conditions specified by Gen_Pol. Although the memory requirements are hardly increased with the addition of templates and cross-linking (-0.06 MB), the simulation time increases approximately ten-fold from less than a minute (on a Compaq ES40 Alpha 667 MHz CPU processor) for 1,000 particles to about 10 minutes. This is mainly due to the additional forces imposed by the cross-linking constraints. The equilibration time increases even further with increasing size of the templates. For simplicity, only short-ranged two-body Lennard-Jones (LJ) interactions are considered, truncated at a short distance to impose mainly repulsive interactions. Average LJ parameters are used where appropriate in accordance with the conventional averaging scheme: cr=~-(o- 0 +o',) and E=(~C'0~g't)1/2 . A shifted potential is used to account for the truncation error
U(r)=4e
-
-Uu(rc"')'
-- O,
r
(13)
r > rc.t
In addition, the bonds between neighboring monomers as well as cross-linked monomers are assumed to obey the Fene potential and harmonic cosine bending potential, respectively:
Ubond ( r ) - - - ~ kR~ ln(1-(r/Ro)21 =oo gbend
(r)= ~-~(cos O - c o s 00 )2
r < Ro r> R o
(14)
47 All parameters are provided in LJ units, such that distances are normalized by the LJ parameter corresponding to monomer size, or0 (e.g. CH2 or SiO2), and energies are normalized by the LJ parameter e0. Molecular masses are normalized by the monomer's atomic mass. The MD simulation proceeds towards minimizing the free energy and the simulation terminates when a specified minimum distance between monomers is achieved. 4. RESULTS AND DISCUSSION 4.1. Theory The equilibrium composition of the imprinted gel, assuming that there are no network defects apart for incomplete saturation of the functional sites of the templates, is obtained by solving Eqs. 7-12. Different materials and preparation conditions are modeled through the interactions parameters Zv, which is related to the excluded volume of the monomers, and ZF, which is a measure of the difference in the interactions between the different materials (monomers and templates) and is proportional to the inverse of the temperature [26]. ZF vanishes when the interactions are similar in magnitude, it is negative when the interactions are unfavorable, and it is positive when the interactions are favorable. Thus, different values of the interaction parameters effectively correspond to different preparation conditions defined by the temperature and solvent. The formation of an imprinted porous material requires that the templates be removed after gelation. Consequently, since we assume highly cross-linked and rigid gels, the resulting cavities can be assumed to retain characteristics complementary to the templates. Fig. l a shows that under various solution conditions, the fraction of agents that can be accommodated by the gel decreases rapidly with increasing pore size. For most sizes, the theory suggests that accommodate a larger number of templates compared with ideal solution conditions (ZV=ZF=0). Conditions favoring phase separation or aggregation of the different molecule (2"F>0) also result in higher number of imprinted cavities. However, such conditions are in many cases unfavorable since the specific imprint of a single agent is lost [ 1]. On the other hand, mixing conditions favor very small templates. The results are seen more clearly by a plot of the volume fraction of imprint cavities versus their size in Fig. lb. It is seen that the size of the resulting pores can be increased substantially using solvents that increase the effective excluded volume of the monomers.
0.6 0.5
x~ 0.8
0.4
0.6
rX M
0.3 0.4
0.2
0.2
0.1 0 0
1
2
3 r
4
5
0
1
2
3
4
5
r
Fig. 1. (a) Number fraction and (b) volume fraction of imprinted pores as a function of size Curves correspond to ( + ) ZV=ZF=O; (---~) Zv=2, ZF=0; ( ~ ) Zv=2, ZF=5" (---X--)Zv=8, ZF=5 9(---~) Zv=2, ZF=-5; (---~) Zv=2, ZF=-10; (-+-) ZV=ZF=5.
48 0.4
-
-,,!
1
(b)
+
0.3
0.75 0.2
+
m
0.5
0.1 0
0.25 0
1
2
T"
3
4
5
I
0
1
I
2
:
r
T
3
"
I
4
5
Fig. 2. (a) Volume fraction of fully functional pores as a function of pore size for various preparation conditions. (b) Order parameter indicating the fraction of imprinted pores that are fully functional. Curves correspond to the same preparation conditions as in Fig. 1. The volume fractions of fully functional pores as a function of template size for the different preparation conditions are plotted in Fig. 2a. In contrast to the total number of imprinted pores, it appears as if ideal solution conditions hardly favor solvation of the functional sites on the templates, and any deviation from such conditions increases the imprinting effect. However, not only do optimum conditions exist for a given size of the templates (typified by a peak at a particular r), optimum conditions that determine the excluded volume of the monomers as well as the phase segregation tendency appear to exist. Comparison of curves in Fig. 2a show that for r >2, Zv=5 proves to be more efficient than both Zv=2 and Zv=8. In addition, mixing interactions (ZF<0) are favorable for small templates, while segregating interactions are favorable for larger templates. It is convenient to define a measure of the imprinting efficiency through use of an order parameter, m, which measures the degree of solvation of the imprinting agents: m = n 3 / f n M < 1,
- l,
partially functional imprint cavities fully functional imprint cavities
(16)
The change of the order parameter with pore size for different preparation condition is plotted in Fig. 2b. It is seen that a plateau is reached in the imprinting efficiency, and that the plateau value differs for different conditions. Mixing conditions, which are favorable for imprinting due to the avoidance of aggregation of the imprinting agents, result in somewhat over 50% of fully imprinted cavities, yet by comparison with Figs. 1 or 2, it is clear that such condition lead to an overall low fraction if imprinted pores. It is seen that sharp transitions reminiscent of phase transitions is observed for conditions where interactions between different species are favored over interactions between the same species. When the imprinting agents are small, the imprinting efficiency is sterically hindered, that is, f monomers must be in close vicinity and attach to the same small molecule. On the other hand, for agents greater than a critical size (of the order of the size of a monomer) the lengthscale of repulsive interactions between the monomers attached to the same template during preparation is smaller than the distance between the monomers, and further increase in molecular size does not alter this, so we observe a plateau. Excluded volume interactions in effect increase the monomer size and delay the transition to larger values of r. An interesting case is when ZF=5 and Zv=8. Here, the imprinting efficiency of is relatively high and c o n s t a n t for all imprinting agent sizes.
49 150 .............................................................................
140
u
em
=
130 120
9
110 100
. . . . . . . : ............
-i
0
' : ...............
1
-
:r
.........
" .... : ....... i
2 time
-
: ............
3
4
Fig. 3. Average bond angle as a function of simulated reduced time for a polymer network (no templates present).
4.2. S i m u l a t i o n s While theory was able to predict microscopic order on the level of the imprint cavities, simulations can be used to predict macroscopic ordering of the functional cavities within the gel. The following discussion analyzes simulated cross-linking of 100 oligomers consisting of 10 monomers each, in the presence of 0-250 templates of different sizes. The degree of crosslinking is taken to be 50% for quadruple monomer functionality ~.t=4). The average density is 0.80o ~3 in all the simulations. The oligomers are first allowed to equilibrate in presence of the templates, followed by cross-linking and equilibration. First we note that cross-linking itself induces structural order typified by bond angles of about 109 ~ in accordance with a tetrahedral lattice, as can be observed from Fig. 3. In further support of the observed ordering, comparison of the structure factor, S(k) where k is the scattering wavevector, for a noncross-linked versus a cross-linked system, plotted in Fig. 4a, shows ordering that extends over much greater values of r for the latter system. When templates are introduced prior to cross-linking, increased order is anticipated. Fig. 4b indicates that larger imprinting agents induce ordering over greater lengthscales, or smaller wavevectors. Clearly, a larger fraction of templates results in more ordered pores, as the imprinting agents themselves begin to exhibit an ordered liquid structure (results not shown).
2.5
9non-crosslinked
(a)
0 crosslinked
s(k)
2.5
i
-f
| (b) 2-1
i
qb ',
9 0
~=1 i ~=1.25_ !
S(k)
1.5
1.5 1
0.5 _____..A
L
j
1 0
0
5
15
10
k
20
25
0
I
5
10
L
I
15
20
i 25
k
Fig. 5. Structure factor of (a) noncross-linked versus cross-linked polymer; (b) an imprinted network in presence of 250 imprinting agents of different sizes.
50 5. CONCLUSIONS Ordered porous structures having molecule-specific functionality can be induced in gels through polymerization and cross-linking of monomers in the presence of functional templates. Using a simple statistical model it was shown that the fraction of template-induced pores could be maximized for a particular template size using appropriate preparation conditions. Further, using molecular dynamics simulations it was shown that larger templates present in greater concentrations result in more ordered pore structures, although at the expense of material stability. ACKNOWLEDGEMENT
The P. and E. Nathan Research Fund and Kobner-Klein Fund are acknowledged for financial support. REFERENCES
[1 ] [2] [3] [4]
B. Boury, R.J.P. Corriu and V. Le Strat, Chem. Mater., 11 (1999) 2796. B. Boury, R.J.P. Corriu, V. Le Strat and P. Delord, New J. Chem., 23 (1999) 531. M.M. Collinson, Crit. Rev. Anal. Chem., 29 (1999) 289. J. Matsui, T. Kato, T. Takeuchi, M. Suzuki, K. Yokoyama, E. Tamiya and I. Karube, Anal. Chem., 65 (1993) 2223. [5] M. Kempe and K. Mosbach, J. Chromatogr. A, 664 (1994) 276. [6] M. Kempe and K. Mosbach, J. Chromatogr. A, 694 (1995) 3. [7] Y. Kondo and M. Yoshikawa, Analyst, 126 (2001) 781. [8] M. Yoshikawa, A. Shimada and J.-I. Izumi, Analyst, 126 (2001) 775. [9! G. Wulff, R. Kemmerer, J. Vietmeier and H.-G. Poll, Nouv. J. Chim., 6 (1982) 681. [10] K. Haupt, Analyst, 126 (2001) 747. [11] G. Wulff, Angew. Chem. Int. Ed. Engl., 34 (1995) 1812; and references therein. [12] K. Hau?t and K. Mosbach, Chem. Rev., 100 (2000) 2495. [13] O. Ramstr6m and R.J. Ansell, Chirality, 10 (1998) 195; and references therein. [ 14] I. A. Nicholls, K. Adbo, H.S. Andersson, P.O. Andersson, J. Ankarloo, J. HedinDahlstrom, P. Jokela, J.G. Karlsson, L. Olofsson, J. Rosengren, S. Shoravi, J. Svenson and S. Wikman, Anal. Chim. Acta, 435 (2001) 9. [ 15] Y. Lu, G. Cao, R.P. Kale, S. Prabakar, G.P. L6pez and C.J. Brinker, Chem. Mater., 11 (19o9) 1223. [16] S.A. Piletsky, T.L. Panasyuk, E.V. Piletskaya, I.A. Nicholls and M. Ulbricht, J. Mem. Sci., 157 (1999) 263. [ 17] T. Enoki, K. Tanaka, T. Watanabe, T. Oya, T. Sakiyama, Y. Takeoka, K. Ito, G. Wang, M. Annaka, K. Hara, R. Du, J. Chuang, K. Wasserman, A. Yu. Grosberg, S. Masamune and T. Tanaka, Phys. Rev. Lett., 85 (2000) 5000. [18] S. Srebnik, O. Lev, and D. Avnir, Chem. Mater., 13 (2001) 811. [ 19] S. Srebnik and O. Lev, J. Chem. Phys., 116 (2002) 10967. [20] S. Srebnik and O. Lev, J. Sol-Gel Sci. Tech., accepted. [21 ] L.R. Corrales and K.D. Keefer, J. Chem. Phys., 106 (1997) 6460. [22] P.J. Flory, J. Chem. Phys., 10 (1942) 51. [23] P.J. Flory, J. Chem. Phys., 12 (1944) 426. [24] M. Kroger, Comp. Phys. Comm., 118 (1999) 278. [25] L. Verlet, J. Phys. Rev., 159 (1967) 98. [26] T.A. Orofino and P.J. Flory, J. Chem. Phys., 26 (1957) 1067.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
51
Microbeam small angle X-ray scattering (~tSAXS): a novel technique for the characterization of activated carbon fibers. D. Lozano-Castell6, E. Raymundo-Pifiero, D. Cazorla-Amor6s, A. Linares-Solano M. Miiiler~, C. Riekel b Departamento de Quimica Inorg~inica. Universidad de Alicante. Alicante. Spain aUniversit~it Kiel, Germany; bEuropean Synchrotron Radiation Facility. Grenoble. France
Small angle X-ray scattering (SAXS) technique offers a number of advantages for the characterization of porous materials. At the microfocus beamline (ID13) in the "European Synchrotron Radiation Facility" (Grenoble) there are available X-ray microbeams with sizes down to 2 pm and a position-resolved scattering set-up (Microfocus Beamline, ID13). This experimental technique (pSAXS) has allowed to study single activated carbon fibers by Small angle X-ray scattering, instead of fiber bundles, and to analyze the porosity development across the fiber diameter. In addition, the use of an area detector (MAR-CCD) permits to obtain the two-dimensional scattering patterns, which have been used to detect anisotropic features related to the porosity development. In the present work, some examples corresponding to the first pSAXS experiments done with single activated carbon fibers (ACFs) are presented to show the usefulness of this technique. 1. INTRODUCTION The porous textural characterization of activated carbons is a very important subject due to the growing interest in the preparation of materials with well-defined pore structures and high adsorption capacities. Porosity characterization is an essential task to foresee their behavior in a given use and requires a combination of different techniques. Gas adsorption techniques constitute the most common approach to the characterization of the pore structure of porous materials. However these techniques have some limitations. Small Angle Scattering (SAS) techniques represent an alternative to gas adsorption methods. The two most common subatomic scattering particles used in SAS are X-rays (Small Angle X-Ray Scattering, SAXS) and neutrons (Small Angle Neutron Scattering, SANS). Both SAXS and SANS can provide stru~tu,-al details of porous materials on a scale covering a range from a few A to about 2000 A. This scale ranges from lengths slightly larger than the diameters of single atoms to distances almost large enough to be resolved in an ordinary optical microscope. Since much of the structure in porous solids lies in this interval, smallangle scattering can play an important role in the study of porosity. These techniques present some advantages, such as they are sensitive to both closed and open porosity. Another important advantage is that SAS intensity profiles are sensitive to shape and orientation of the scattering objects so that additional information can be obtained in anisotropic studies in oriented samples such as fibers.
52 Our research group has dedicated an important effort to analyze the development of porosity in isotropic pitch-based carbon fibers. For this purpose, different techniques have been used: N2 and CO2 adsorption data at 77 K and 273 K, respectively [1,2], measurements of tensile strength, Positron Annihilation Lifetime Spectroscopy [3] and SAXS [4]. In these SAXS experiments [4] a bundle of about 20 parallel fibers was held in a sample frame, and each sample was therefore measured at different angles to the vertical in the plane normal to the direction of the incident beam. Depending on the experimental system and the beam diameter, SAXS experiments in fibers are carried out in this way, or packing the sample in a cell. The availability at ESRF of X-ray microbeams with sizes down to less than 2 l,tm (Microfocus Beamline, ID13), together with a position-resolved X-ray scattering method makes this technique suitable for analyzing single fibers [5]. Experiments demonstrating the successful use of this technique in fibrous materials were carried out e.g. on cellulose [6-9], and carbon fibers [10,11]. In the work carried out with carbon fibers [10,11] the internal structure of single carbon fibers, from different precursors (PAN-based fiber and mesophase-pitch based fibers) was investigated. However, in the literature there is no report about the use of this technique for the characterization of activated carbon fibers. In this sense, the main objective of this work is to show the usefulness of the Microfocus Beamline (ID13) in the characterization of activated carbon fibers. Thus, examples with activated carbon fibers with different burn-off degree obtained with different activating agents, and different fiber diameter will be presented. 2. M I C R O B E A M SMALL ANGLE X-RAY SCATTERING The scattering experiments were carried out at the Microfocus Beamline (ID13) in the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The most remarkable characteristics of this beamline are: -Monochromatic beams of 2, 5 or 10 l,tm in diameter can be used for small angle scattering experiments. -An area detector (MAR-CCD) with an active diameter of 130 mm is available. The use of an area detector has the advantage of having a better angular resolution than a quadrant detector, where the measurements have to be carried out by rotating the sample in the plane normal to the direction of the incident beam. Thus, two-dimensional scattering patterns can be obtained. - It has the so-called "scanning set-up", with a Huber goniometer and a high-resolution microscope located at a calibrated distance from the beam position, so that a specific point can be selected on the sample and automatically transferred into the beam.
A scheme of the experimental set-up is shown in Figure 1. In the experiments presented in this work, the beam selected has 2 ~m diameter and it is produced by a tapered glass capillary (wavelength X= 0.948 A). The distance from the samples to the detector was 190 mm and the fibers were examined with the fibers axis perpendicular to the X-ray beam. The carbon fibers were mounted on an aluminium frame. Several fibers of a given sample were glued in each frame to repeat the measurements and to check the reproducibility. All measured data was corrected for background. Data evaluation was done using the soitware package FIT2D. The basic theory of small-angle scattering has been reviewed elsewhere [12].
53
Figure 1.- Scheme of the experimental set-up in the pSAXS system. 3. RESULTS AND DISCUSSION In order to show the suitability of the IaSAXS technique to characterize activated carbon fibers, the examples have been divided in two sections: i) experiments at the center of the fiber, and ii) experiments across the fiber diameter. The materials used for these examples include a commercial petroleum pitch based carbon fiber (Kureha Chemical Industry Co.), which is named as CF, and three activated carbon fibers (ACFs), prepared from sample CF by activation in CO2 up to 29% and 50% bum-off (CFC29 and CFC50) and in steam up to 48% bum-off (CFS48). In addition, the examples also include the results corresponding to an activated carbon fiber prepared from a coal tar pitch based carbon fiber activated with CO2 up to a 63% burn-off (CFBC63). Table 1 contains the micropore volumes obtained by applying the Dubinin Radushkevich equation to the N2 and CO2 isotherms at 77 K and 273 K, respectively. The fiber diameter for each sample is also included in Table 1. It must be remarked that the original carbon fiber does not present N2 adsorption, because this sample has a very narrow microporosity, where N2 at 77 K has diffusional problems. For the ACFs, it can be observed that, in general, the higher the burnoff the higher the total micropore volume (VDR(N2))and the higher the difference between VDR(N2)and VDR(CO2),indicating a wider micropore size distribution [1 ].
Table 1.- Fiber diameter and porous texture characterization results obtained by physical adsorption of N2 at 77 K and CO2 at 273 K Sample CF
CFC29 CFC50 CFS48 CFBC63
VDR3(N2)
VDR(CO2) (cm3/g)
Fiber diameter (~tm)
0.44 0.72 0.59 0.90
0.18 0.42 0.33 0.32 0.47
28 28 28 26 56
(cm/g)
54 3.1. ~tSAXS experiments at the center of the carbon fibers
In the present section, the suitability of pSAXS technique to characterize single carbon fibers is shown. Figure 2 presents the two-dimensional scattering patterns corresponding to the center of each sample, after background and sample volume correction. These twodimensional scattering patterns have been plotted for a maximum value of scattering vector (q) of 10 nm -1. These figures show that, in all the cases (carbon fiber, and activated carbon fibers prepared from different precursor and using different activating agents) the scattering is the same in all the directions. These results indicate that the porosity is isotropically created along the fiber axis, which confirms previous experiments [4,13 ].
Figure 2.- Two-dimensional scattering patterns after sample volume and background corrections corresponding to: a) CF, b) CFC29, c) CFC50, d) CFS48, and e) CFBC63.
In Figure 2a it can be also observed that the original carbon fiber (CF) has a low scattering, as it is expected according to its low micropore volume. The comparison of the scattering pattern
55 of the original carbon fiber (Figure 2a) and the ACFs prepared with CO2 (Figures 2b and 2c) shows than the scattering intensity increases with the burn-off degree, as a consequence of the increase in the micropore volume (see Table 1). The other two samples (CFS48 and CFBC63, see Figures 2d and 2e) present a medium and high scattering intensity, respectively, according to their micropore volumes. These examples show that ~tSAXS experiments are an easy way to analyze the isotropy of the porosity in activated carbon fibers and to observe the development of porosity with the activation process.
3.2. lxSAXS experiments across the fiber diameter Once the suitability of taSAXS experiments to characterize a single ACF has been demonstrated, the technique has been used to analyze single fibers, as it has been shown in previous section, but also for analyzing different regions of the same fiber to obtain a map of pore distribution across the fiber diameter. Thus, the objective of this section is to show the usefulness of the scanning set-up (Huber goniometer and a high-resolution microscope) of the I,tSAXS technique to characterize the porosity development across the fiber diameter. For this purpose, the results obtained with activated carbon fibers prepared using different activating agents and different precursors are presented.
3. 2.1. Effect of the activating agent According to previous results of our research group regarding the use of CO2 and steam as the activating agents in activated carbon fibers preparation [1-4], there are differences between CO2 and steam carbon gasification: steam apparently focuses the activation in the outer parts of the fibers, whereas CO2 activation takes place, predominantly, within the fibers. The confirmation of the structural changes produced in the activation process using beth activating agents, could be carried out analyzing the porosity development across the fiber diameter. Thus, in this section the results corresponding to two activated carbon fibers prepared with each activating agent and similar burn-off (CFC50 and CFS48) are included. The experiments consist in scanning the fiber across its diameter with an accuracy of 0.5 ktm. In these experiments the scans were of 40 ~tm width, and were divided in 10 steps. Measurement times were 20 s/step. For a better understanding of the experiments carried out in this section, Figure 3 includes a scheme of the different beam positions respect to the carbon fiber in the first six measurements (from the left to the center of the fiber). The next measurements (711, from the center to the fight of the fiber) were taken in the same way on the right hand side of the fiber.
Figure 3.- Scheme of the different beam positions respect to the carbon fiber in the first six measurements of IaSAXS experiments.
56 The scattering measurements have been corrected considering the volume fraction, due to the cylindrical shape of the fibers. From the corrected scattering results, the Porod Invariant (PI) has been estimated. PI is related to the void fraction of the material under investigation, and gives a useful comparison on how the void fraction of materials changes with the treatment. Figure 4 includes the estimated relative Porod Invariant (PI) values (PI values relative to the maximum PI value for each sample) versus the beam position. In the X-axis of this figure, the beam position equal to zero corresponds to the center of the fiber. Then, this figure can be considered as a map of the distribution of porosity across the fiber diameter. 1 0.8
~
I~
-e- CFCSO
~. 0.6
.>_
_m n, 0.4
0.2
0
-16
J
-12
i
~
l
-8
.4
0
F
4 Beam position (rim)
~
i
8
12
16
Figure 4.- Relative Porod Invariant values (calculated relative to the maximum Porod Invariant value for each sample) corresponding to the measurement carried out across the fiber diameter for the samples CFC50 and CFS48. In this figure, it can be observed that, for both samples, the maximum scattering corresponds to the measurement carried out in the external zone of the fibers, indicating a higher concentration of pores in the outer zone of the fiber. In addition, it can be observed that the PI profiles, as a function of the position of the fibers, are different for CO2 and steam activated materials. In the case of steam, the scattering from all the internal zones is very similar and much lower than from the external part of the fiber. On the other hand, in the case of CO2, the porosity is more developed in the inner regions of the fibers compared to the steam activated carbon fibers. These results constitute a direct proof of the different behavior of CO2 and steam as activating agents, and show the suitability of this novel experiments for characterizing different regions of activated carbon fibers.
3. 2. 2. Effect of the fiber diameter. Another example to show the usefulness of this technique is to study the effect of the fiber diameter in the porosity development. In doing that, two activated carbon fibers prepared
57
from different precursors and using the same activating agent have been selected: sample CFC50 and sample CFBC63. In both samples the scans were divided in 10 steps. However, due to the difference in fiber diameter, the widths of the scans were different: 40 ~tm in the case of the thinnest sample (CFC50) and 80 lam in the case of the sample (CFBC63). Similarly to the previous section, the relative Porod Invariant corresponding to each measurement for the two carbon fibers has been calculated and plotted versus beam position in Figure 5.
1
l
- ~ - CFCSO
0.8
-A- FCBC63
~.. 0.6
m
~ 0.4
0.2 & 0
-28
I
I
I
I
i
1
I
I
I
i
i
i
I
-24
-20
-16
-12
-8
-4
0
4
8
12
16
20
24
~8
Beam position (rLm)
Figure 5.- Relative Porod Invariant values (calculated relative to the maximum Porod Invariant value for each sample) corresponding to the measurement across the fiber diameter. In this figure it can be observed that the porosity development across the fiber diameter is different depending on the diameter of the carbon fiber used as precursor. In the case of the thick fiber (CFBC63), the CO2 molecules do not reach the internal zone of the fiber, while in the case of the thinnest activated carbon fiber (CFC50), the porosity is much more developed in the center compared to the thick fiber. These results indicate that the starting carbon fiber does not contain transport pores (i.e. macro and mesopores) that permit an easy diffusion and a homogeneous distribution of CO: molecules. Thus, for a similar burn-off degree, the higher the diameter of the fiber, the activation process focuses mainly in the extemal zones, where most of the porosity will be located. 4. CONCLUSIONS The examples presented in this work illustrate the suitability of IaSAXS technique to characterize activated carbon fibers. It has been shown the isotropy features in activated carbon fibers prepared from different precursors and using different activating agents. In addition, this technique is able to obtain scattering measurements across the fiber diameter, which has allows us to obtain maps of pores distribution. The present results show that
58 interesting research can be carried out for porous materials science by using ~SAXS technique. Acknowledgements
The authors thank ESRF (Experiment number ME-93) for the facilities and financial support. Also we would like to thank MCYT (Project MAT-2000-0621) for financial support. 5. REFERENCES
[1] Alcafiiz-Monge J, Cazorla-Amor6s D, Linares-Solano A, Yoshida S, Oya A. Carbon 1994; 32(7): 1277-1283. [2] Alcaftiz-Monge J, Cazorla-Amoros D, Linares-Solano A. Carbon 1997; 35(10-11):16651668. [3] Lozano-Castell6 D, Cazorla-Amor6s D, Linares-Solano A, Hall, PJ, Fernandez JJ. In Unger KK, et al. Editors. Studies in Surface Science and Catalysis Vol. 128, Characterisation of Porous Solids V, The Netherlands: Elsevier Science, 2000. p.523532. [4] Cazorla-Amor6s D, Salinas-Martinez de Lecea C, Alcaftiz-Monge J, Gardner M, North A, Dore J. Carbon 1998; 36(4), 309-312. [5] Riekel, C. Rep.Prog.Phys. 2000, 63,233. [6] MOller M, Czihak C, Vogl,G, Fratzl P, Schober H, Riekel C. Macromolecules 1998; 31: 3953-3957. [7] MOller M, Burghammer M, Riekel C. ESRFNewsletter 1999; 33: 12-13. [8] MOller M, Riekel C, Vuong R, Chanzy H. Polymer 2000; 41:2627-2632. [9] MOller M, Czihak C, Burghammer M, Riekel C. Journal of Applied Crystallography 2000; 33: 817-819. [10]Paris O, Loidl D, Peterlik H, Mtiller M, Lichtenegger H, Fratzl P. Journal of Applied Crystallography 2000: 33: 695-699. [ll]Paris, O.; Loidl, D.; Mtiller, M.; Lichtenegger, H.; Peterlik, H. Journal of Applied Crystallography 2001; 34: 473-479. [12]Guinier A, Fournet G. Small Angle Scattering of X-Ray, John Wiley&Sons Inc., New York, 1995. [13] Ridgen JS, Dore JC, North AN. In Rouquerol J. et al. Editors. Studies in Surface Science and Catalysis Vol. 87, Characterisation of Porous Solids III, The Netherlands: Elsevier Science, 1994. p.263-271.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
59
"Real time" d e t e r m i n a t i o n of porosity d e v e l o p m e n t in carbons: a c o m b i n e d SAXS/TGA approach J.M. Caloa, p.j. Hall b, S. Houtmannb, D. Lozano Castello c, R.E. Winans(l, and S. Seifert d aDivision of Engineering, Brown University, Providence, RI, USA bDepartment of Chemical Engineering, Strathclyde University, Glasgow, Scotland, UK ~Dpto. de Quimica Inorgfinica, Universidad de Alicante, Alicante, Spain dChemistry Division, Argonne National Laboratory, Argonne, 1L, [JSA Time-resolved X-ray scattering data are presented for three different carbons that were activated in sit~t in the beamline at the Advanced Photon Source at the Argonne National Laboratory in Argonne, IL, USA. The resultant data exhibit varied behavior, depending on the carbon type. The data suggest a dynamic porosity development mechanism. A net population balance model of pores within a particular size range may explain these observations. 1. INTRODUCTION Small angle scattering (SAS) techniques offer a number of advantages for the investigation of the nature and behavior of porous materials. In particular, with respect to carbons, the nonintrusive nature of SAS means that, in principle, characterization can be performed on carbons in situ during activation processes, allowing real time resolution of porosity development. These types of studies have not been practical nor possible heretofore, primarily due to the prohibitively long counting times required to span the requisite wave vector (q) range over tile entire burn-off process. However, the availability of new instruments, such as the small angle X-ray scattering (SAXS) facility at the Advanced Photon Source (APS) at the Argonne National Laboratory (ANI,), has now made these types of experiments possible. Some preliminary data from these efforts have been reported previously [1]. Here we propose to present results incorporating a thermogravimetric analyzer in the beamline to directly correlate burn-off with small angle X-ray scattering (SAXS) data. Results on the behavior of some different carbons during activation in oxygen are presented, as well as the effect of a catalytic agent, calcium.
60 2. E X P E R I M E N T A L
The SAXS instrument was constructed at ANL and used on the Basic Energy Sciences Synchrotron Radiation Center (BESSRC-CAT) undulator beamline ID-12 at the APS. Monochromatic X-rays (8.98 keV here) were scattered offthe samples and collected on a 15 cm x 15 cm Mosaic CCD array (,1538 x 1538 pixels). This system is capable of a resolution of ,-~0.5 s. The scattered intensities were corrected for absorption, empty cell scattering, and instrument background. A Cahn TG-121 therlnogravimetric apparatus (TGA) was adapted for use directly in the beamline. Two holes, approximately 2 cm in diameter, located 180 ~ apart, were drilled through the walls of the TGA furnace to accommodate the X-ray beam. Two corresponding holes were also made in the quartz hangdown tube of the TGA to admit the X-ray beam. Char samples, on the order of tens of nag, contained in aluminum foil packets with multiple perforations (to allow for adequate transport of oxygen through the sample), were suspended from the microbalance on a platinum wire. The experimental arrangement and procedures were gradually improved with increasing experience with the system. Since the TGA system was operated in an "open" mode, the reaction gas (oxygen) was used to purge the quartz hangdown tube to maintain uniform cornposition and temperature. With the flow rate set too high, the sample tended to oscillate on the platinum wire. This resulted in some observable oscillation in scattering intensity at high time resolutions. This problem was solved by lowering the oxygen flow rate to the point where sample oscillation vanished. Also, the location of the sample thermocouple, necessitated by the experimental arrangement, resulted in a systematic offset in the temperature measurements. This problem was rectified via calibrations of the sample temperature in a laboratory TGA. 10 g samples of saran and microgranular cellulose, obtained fi'om the Aldrich Chenlical Co., were heated in a tube furnace at 10Ir to 900~ with a l h heat soak time, under flowing nitrogen. Calcium-loaded cellulose was prepared by mixing with calcium acetate solution, followed by drying. The resultant calcium content was approximately 3% tbllowing carbonization. 3. R E S U L T S AND DISCUSSION 3.1. Saran (.'hat"
The TGA data presented in Figure 1 were obtained under nonisothermal conditions in an atmosphere of oxygen. As shown, the sample was first rapidly heated to 200~ held at that temperature for 2 lnin, and then heated to 560~ at 6K/rain, followed by an isothermal period at the final temperature. The bulk of the sample burnt out at about 52 rain, prior to attaining the final temperature. From this point on, the mass signal appears to be due to slow burnout of residual, presumably more unreactive, material. SAXS curves corresponding to selected points during the activation history are presented in Figure 2. In the absence of other effects, scattering at high wave vector (q) values
61 is primarily due to small microporosity, while that at low values is due to meso- and larger porosity [2]. The plateau at high q is indicative of a significant population of microporosity. At low-burn-offs there is a notable decrease in the mesoporosity, accompanied by microporosity development at high q. Therealter, scattering in the microporosity region falls off markedly as the carbon in the micropores is burned off. Somewhat similar results were obtained for saran char under near-isothermal conditions in oxygen. The sample temperature was increased at 60K/rain to 435~ and then held there. The sample mass actually began to decrease prior to attaining isothermal conditions. This illustrates the difficulty of resolving the porosity development history under isothermal conditions where the burn-off behavior typically varies widely with the nature and preparmion history of the sample. Nonisothermal temperature programs appear to be more efficient in this regard. The corresponding scattering data are presented in Figure 3. The vm'iation in scattering intensity with activation appears to be somewhat complex. It is important to note that only net porosity differences are observed. Thus, the predominant behavior in the low burn-off regime (0-7.5 rain) appears to be net microporosity development, while scattering from the larger pores remains relatively more constant. However, with continuation of the burn-off history in Figure 3, the process becomes inverted such that the predominant behavior appears to become net consumption of mesoporosily, while the net microporosity remains relatively constant.
Figure 1. Saran char sample mass history during nonisothermal activation in oxygen. "l'he symbols correspond to the scattering curves in Figure 2.
Figure 2. SAXS curves lbr saran char during nonisothermal activation in oxygen to 565~ to the indicated mass losses corresponding to the symbols in Figure 1.
62 The apparent behavior of porosity with activation obtained from raw scattering data is somewhat distorted by the variation of the solid volume fraction. Os. in the following manner. The total scattering intensity is given by:
I(q) = l),,2 V ~(I- ~,,) ./'o'~y[sin(qr)/'qr] 4nv':dr
(])
where I(q) is the scattering intensity at a value of the scattering wave vector, q = 4a sin(0)/Z, 0 is the scattering halt-angle, )~ is the neutron wavelength, "/the correlation function, by is the contrast thctor between solid and voids, and V is the scattering volulne. The mean square of the fluctuations of the scattering length density is bv:~s(l- qb~),which varies with ~s, and thus acts as a scaling factor for the scattering intensity. One way to account for this effect is to correct the scattering intensities with the Porod invariant (PI) [3]: (2) Since all tile parameters on the righthand side of this expression should be constant with activation, except for ~s. the value of tile Porod invariant should follow the behavior of ~s(1- ~0. Thus, division of the scattering intensities by this factor can be used to correct for the variation of qbs. It is emphasized, however, that this is only an approximation for a number of reasons, including the thct that when I(q) varies with q to an exponent less than --2 (as at high q for the current data), the integral diverges [3], and accurate values of the Porod invariant cannot be obtained.
Figure 3. SAXS curves for saran char activated at 435~ in oxygen to the indicated mass losses.
Figure 4. SAXS curves tbr saran char from Figure 3), divided by the corresponding Porod invariants.
63
Figure 5. "Corrected" scattering data from Figure 4 tbr saran char activated at 435~ in oxygen to the indicated mass losses, amplifying the behavior of the smaller pores. The Porod invariants calculated from the scattering curves presented in Figure 3 were found to increase initially up to about 10.29 nag mass loss, and then decrease monotonically thereafter. The scattering curves in Figure 3, corrected by the Porod invariant, are presented in Figure 4. 'I'he principal effect of the correction is to draw the curves much closer together, especially for the smallest pores (i.e., at high q). As shown in Figure 4, the curves all seem to intersect in the vicinity of q ~ 0.045~x~-~. The population ol" the larger micro- and mesopores at lower q decrease monotonically with activation up to about 29.59 mg mass loss, and then increase with activation beyond this point. As shown in Figure 5, the population of the smaller micropores (q > 0.045 A -1) develops slowly with activation until about 10.29 mg mass loss. It then increases more rapidly to 32.47 mg mass loss, and decreases thereafter. It is noted that the decrease in the smallest pores coincides with the point at which tile population of the larger pores begins to increase. 'l'his behavior is consistent with pore wall collapse among the smallest pores. These data suggest a dynamic porosity development mechanism beginning with depletion of the larger micro- and mesoporosity and the development of tile smaller microporosity. At higher burn-offs the process changes to one of net loss of the smallest microporosity while the net population of the larger micro- and mesopores increases. Of course, as indicated previously, only net porosity differences are observed. Thus, tbr example. when tile porosity in one particular size range is relatively constant, this could be due either to no appreciable reaction occurring in those pores at that particular stage in the activation, or that the rates of consumption and development are in relative balance over this burn-off range. Since it would be difficult to reconcile the former mechanism with the nature of the activation process, it appears that the net population balance of pores in a particular size range is responsible for these observations.
64
Figure 6. SAXS curves for cellulose char activated at 425~ in oxygen to the indicated mass losses.
Figure 7. SAXS curves for cellulose char activated at 425~ in oxygen (q/. Figure 6) to the indicated mass losses, divided by the corresponding Porod invariants.
3.2. Cellulose C h a r Scattering data for a cellulose char sample activated isothermally at 425~ are presented in Figure 6. "l'hese data show that this carbon, like the saran char, initially exhibits a significant amount of microporosity, as indicated by the plateau centered at about q = 0.1A l. The Porod invariants lbr these data initially increase and then decrease with activation, also like the saran char. The scattering data, corrected by the PI values, are presented in Figure 7, which shows somewhat different behavior than for the saran char. As shown in Figure 7, initially the population of the smallest micropores centered c a . 0.1A -~ increases much more markedly than for the saran char. This is accompanied by relatively little development of the larger porosity up to 9.42 mg mass loss, following a significant initial loss at low q values, l towever, continued net microporosity development with activation then slows significantly beyond 13.00 mg mass loss, and finally begins to decrease al 14.05 mg mass loss, while simultaneously the population of the larger pores begins to increase at an accelerating rate. Once again, .just as for the saran char, this change ill behavior coincides with a decreasing population of the smallest pores, consistent with pore wall collapse among the smallest pores.
65
Figure 8. SAXS curves for Ca-loaded cellulose char during isothermal activation at 340~ in oxygen as a function of time. l'he total burn-off was 73%.
Figure 9. SAXS curves tbr Ca-loaded cellulose char during isothermal activation at 340~ in oxygen (c/. Figure 8), divided by the corresponding Porod invariants.
Differences in the behavior of the microporosity in the saran and cellulose chars may be related to :'closed porosity." It is well known that "glassy" carbons can exhibit a considerable amount of microporosity that is initially "'blocked" from access to the activating agent by more reactive, less ordered carbon. Early activation of these types of carbons primarily involves preferential burn-off of this carbon, exposing the intrinsic underlying microporous structure. For example, using contrast-matching small angle neutron scattering (SANS), no new microporosity development was observed upon activation of a char produced from a highly crosslinked phenol-formaldehyde resin up to 21% burn-off [21. The only mechanism observed was unblocking of initially "closed" microporosity. Unfortunately, SAXS is not amenable to contrast-matching as is SANS. Consequently, this approach could not be used in the current experiments. I lowever, a similar mechanism may be at least partially responsible for the relatively slower and less marked evolution of microporosity with activation in the saran char than in the cellulose char. 3.3. Calcium-Loaded Cellulose Char Mass loss data tbr the Ca-loaded char are unavailable since these data were obtained prior to the TGA being incorporated into the experimental system. A 1 mm diameter quartz capillary was used as the sample holder. The scattering data for this sample are presented in Figure 8. The Porod invariants ['or the data in Figure 8 increase monotonically with activation. No maximum was observed as for tile saran char and unpromoted cellulose char samples. The
66 scattering data corrected for the PI values are presented in Figure 9. As shown, the behavior of the Ca-loaded cellulose char differs significantly from that of the unpromoted cellulose (of Figure 7). Initially. the scattering intensity remains relatively constant and actually decreases slightly with activation over the entire q range. This behavior is similar to that observed fbr phenolic resin char at low q at low burn-offs, that was attributed primarily to preferential gasification of disordered carbon clusters ot" characteristically large sizes [2]. This is followed by a pattern of rapidly increasing scattering intensity at lower q values (q < 0.045 A-~) than for the unpromoted cellulose char, which continues to progressively shift to lower q with activation. As shown, there appears to be little or no small microporosity development (q > 0.045 A ~) over the entire activation history. The preferential development of mesoporosity rather than microporosity in the Ca-loaded char may be due to the catalytic effect of calcium, which may not have been well dispersed in the smallest pores. 4. CONCLUSIONS The SAXS/TGA approach has been demonstrated to be a usethl technique tbr timeresolution of porosity development in carbons during activation processes. Qualitative interpretation of the data obtained thus )hr suggests that a population balance approach focusing on the rates of production and consumption of pores as a fiJnction of size may be a fi'uittul approach to the development of quantitative models of activation processes. These then could become uselhl tools tbr the optimization of pore size distributions for particular applications by providing descriptions and predictions of how various activating agents and time-temperature histories affect resultant pore size distributions. ACKNOWLEDGEMENTS
The authors acknowledge EPSRC support under grant number GR/N03006. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract No. W-31-109-ENG-38. REFERENCES
[1] [21 [3]
J.M. Calo, P.J. Hall, S.D. Brown, R.E. Winans, and S. Seifert, Proc. Eurocarbon 2000, Berlin, Germany, 113-114 (2()00). M.M. Antxustegi, P.J. Hall, P.J., and J.M. Calo, J. Coll. Intf. Sci., 202 (1998), 490-498. J.S. Higgins and H. C. Benoit, Polymers and Neutron Scattering, Oxlbrd University Press, 1994.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
67
SANS Investigations of Adsorption Mechanisms in Model Porous Silicas S.Kallus a*, A.Hahn b and .J.D.F.Ramsaya a
Institut Europ6en des Membranes, CNRS, 1919 Route de Mende, Montpellier 34293, France
h Institute for Inorg. Chem., Johannes Gutenberg University, Mainz, Germany present address: Grace Davison, 67545 Worms, Germany A recent method for the characterisation of the structure o'f porous materials involves the measurement of small angle neutron scattering (SANS) performed in situ during isothermal gas adsorption. In this technique the coherent scattering length of the condensed gas or vapour is selected to match that of the porous solid. This method is illustrated by investigations on several mesoporous silica gels, having a highly ordered structure (MCM-41 and MCM-48), and pore sizes in the range from 1.9 to 6.4 nm. From measurements, made in situ with contrast matched benzene, adsorbed at 31 OK, and cyclohexane at 298K, it has been possible to differentiate this ordered mesoporosity from that due to the larger pores which arise from the micron-sized grain texture in these materials. The role of these two different types of porosity on kinetics of adsorption of hydrocarbon gases in mesoporous silica have also been investigated by the contrast matching technique. Results for n-hexane and cyclohexane show that the latter is adsorbed more slowly in MCM41 silica (pore diameter 2.3 nm) possibly because of hindered diffusion, due to the larger kinetic diameter of the molecule. Further kinetic investigations with hexane isomer mixtures, having different scattering lengths, have also provided an insight into the rate of exchange and partitioning of gas mixtures in MCM-41 silica. 1. INTRODUCTION An important and recent development of the technique of small angle neutron scattering (SANS) in the characterisation of porous solids has been the application of the contrast variation technique (1-7). More detailed insight into surface and porous properties, are obtained with this method by performing SANS measurements on samples in which the pores are filled or partially filled with an adsorbed vapour. In general, the scattering length density, Pb, of the condensed liquid adsorbate, which occupies the pore space, is selected to be the same as that of the solid matrix. Under such conditions "contrast matching" occurs. Such contrast matching is possible with SANS for a wide range of adsorbates (eg. water, hydrocarbons) because Pb can be varied readily by an isotopic substitution of H for D in the molecule (see Table 1). Here we illustrate two applications of the contrast matching technique applied to model mesoporous silicas (MCM-41 and MCM-48), using different adsorbates (benzene, n-hexane, cyclohexane). In the first, the larger inter-granular pores of the silica is highlighted and differentiated from the ordered cylindrical pores within the grains. These measurements have been made under
68 equilibrium isothermal conditions, at different pressures. In the second, kinetic measurements of n-hexane and cyclohexane in a smaller pore diameter MCM-41 silica are illustrated (under isothermal and isobaric conditions). Table 1
Molecular scattering lengths 5',bcohfor different liquid sorbates and corresponding scattering length densities, Pb, for mass densities, ~5, compared with amorphous silica. Dmol*, corresponds to kinetic diameter of molecule in vapour phase. Material Ebcoh/10-12cm ~5/ g cm-3 Pb/ 101~cm-2 Drool* (293K) 0.88 1.75 1.18 5.9 benzene -h6 0.95 5.44 7.99 -d6 -0.50 0.78 -0.28 6.0 cyclohexane -hi2 12.00 0.89 6.70 -d12 -hi4 - 1.25 0.66 -0.57 4.3 n-hexane -d14 13.33 0.77 6.18 water -h2 -0.17 1.00 -0.56 4.3 -d2 1.91 1.11 6.36 nitrogen (77K) 1.88 0.81 3.27 3.6 silica 1.58 2.20 3.47 _
Such kinetic investigations are feasible at high flux sources, such as ILL, Grenoble, where measurements having good statistics can be obtained in time frames of minutes. From these measurements differences in the rate of uptake are evident. It is furthermore possible to distinguish two distinct stages in the adsorption process 9A rapid initial adsorption is ascribed to uptake in the inter-granular texture; subsequent adsorption then occurs within the ordered mesoporous intra-granular structure.
2. MATERIALS AND EXPERIMENTAL PROCEDURES
The mesoporous silica powders were kindly provided by Dr Ulla Junges. The MCM-41 sample (S1M), containing the largest mesopores, was prepared as previously described by Stucky and coworkers (8). A commercial copolymer (Pluronic-123) was used in the synthesis mixture during the hydrolysis of tetra ethyl orthosilicate (TEOS). High resolution field emission SEM (Hitachi $4200) showed the powder consisted of small small rhombohedral grains having a uniform shape and size (width-~ 0.25 ~tm and length 0.8 to 1.2 ~tm). Within the grains, the aligned cylindrical mesopores (diameter 6 nm), oriented parallel to the facetted surfaces of the grains, had a periodicity of-~ 12 nm. The other MCM-41 and MCM-48 samples were prepared using cationic n-alkyl-trimethyl ammonium surfactant templates (ClaTAB and CI6TAB), and contained pores of smaller diameter in the range 1.9 to 3 nm (see Table 2). Adsorption isotherms isotherms of nitrogen (77K) and benzene (310K) were measured as previously (9). For the S1 sample, the isotherms were of type IV, showing hysteresis loops, corresponding to capillary condensation in the larger mesopores. All the other samples (S2M, S3M and S4M) contained smaller mesopores, and the nitrogen adsorption/desorption isotherms were reversible and sigmoidal in shape. Surface and porous properties of different silica samples (Table 2) were derived from nitrogen adsorption data.
69 SANS measurements were made at HMI, Berlin, LLB, Saclay and ILL, Grenoble, using the V4, PAXE and D22 spectrometers respectively. In situ measurements were made on outgassed samples after exposure to different sorbates using two different procedures. In the first, the sample, at a temperature of 31 OK, was exposed to controlled relative vapour pressures, p/p0, which allowed the hysteresis loop to be scanned, as previously described (10). In the second, controlled doses of liquid sorbate were injected with a calibrated Hamilton micro-syringe through a silicone rubber septum into rectangular quartz (Hellma) cells (pathlength 2mm), containing precisely weighed amounts of the outgassed samples at 298K. Prior to dosing the samples were outgassed in-situ in the quartz cells at 398K for approximately 8 hours in a vacuum oven (- 104 torr) and closed-off with the septum cap before exposure to ambient atmosphere. By using the same mass and packing density in the cells for a given series of samples it was thus possible to make quantitative comparisons of SANS data as previously demonstrated (1). Table 2 Properties of Mesoporous Silica Samples. Sample S 1M MCM-41 Structure 6.4 Pore Size / dp* nm Pore Volume / 1.01 cm 3 g-1
S2M MCM-41 2.3
S3M MCM-41 3.0
S4M MCM-48 1.9
0.63
0.46
0.38
610
630
0.14 (0.25) 45
0.19 (0.36) 33
830 920 Specific Surface Area / m 2 0.065 Diffraction Peak, 0.17 Qmax/ A "I (0.13) (0.33) d-spacing / A 96 36 * Derived from N2 adsorption isotherms at 77K.
g-1
In the experiments described here the volume of liquid adsorbate which was injected was sufficient to saturate the intra-granular mesopores. This volume was calculated from a knowledge of the sample mass in the cell and the mesopore volume, derived from the nitrogen adsorption isotherms. (eg for the kinetic measurements with the S2M silica, this mass was 42mg for each of the two samples and the injected volume was 40 gl.) This volume was injected via the hypodermic needle, to the base of the cell which was below the zone of the incident neutron beam. The transmissions for the different samples was in a range of 0.7 to 0.9, being typical for such porous silicas in these standard quartz neutron scattering cells.
3. RESULTS AND DISCUSSION
The SANS curve for the evacuated S 1M sample shown in figure 1 is typical of that observed for the other ordered mesoporous silicas. This features Porod scattering ( viz. I(Q) oc Q-4 ) in the low Q range (where Q = 4nsin0/~,) and small angle diffraction features at higher Q, corresponding to the cylindrical pores ordered in a hexagonal array. Thus the peaks at 6.5 x 10 "2 and ~ 1.3 x 10"l A l can be ascribed to the (100) and (200) reflections associated with a hexagonal pore structure (space group p6mm) as described previously by Stucky et al (6) from XRD analysis. Here the (100) peak corresponds to a d-spacing of-~ 96 A "~ indicating a large
70 unit-cell parameter (a0---111 A). The positions of the first maximum and corresponding dspacings for the other silica samples are given in Table 2. We will consider the SANS in these two different Q regions separately. POROD SCATTERING REGION
[ 1000
DIFFRACTION REGION
100 I(Q) 10 1
o.i
O v
xd-"-
0.01 . . . . . . . . . . . . . . . . . . . 0.01
r
t..~ 0.1
0
0.0
015
1.0
t9
Q I A "1
Figure 1. SANS of evacuated mesoporous silica, S1M (dp - 6nm). SANS contains two components: (a) at low Q - Porod Scattering; (b) at high Q - Diffraction.
Figure 2. Dependence of I(Q) for sample S1M in the Porod Scattering region (at Q = 10-2A-l) on fractional volume filling of mesopores, O, with matched benzene.
3.1. Porod scattering The Porod scattering has been ascribed tentatively to the larger-scale granular texture of the samples (9). Further evidence for this comes from the evolution of the SANS during progressive isothermal adsorption (310K) of contrast matched benzene. Under these conditions the Porod scattering remains, but increases progressively as the relative pressure, p/P0, is increased. Such a feature corresponds with the progressive filling of the mesopores in the grains. We can consider this to occur as follows: Thus for Porod scattering from a two phase system we have (11, 12): I(Q) ~ S/V. Q-4 [pl.p212
(1)
Where Pl and 192are respectively the scattering length densities of the two phases and S/V is the surface to volume ratio of the system. In this case S/V corresponds to the surface to volume ratio of the MCM silica grains, which will remain unchanged during the progressive filling of the mesopores. Furthermore pl will correspond to the "effective" scattering density of the grains and P2 the large inter-granular pores ( p2 will be zero as these pores are unfilled with sorbate at this stage ). pl will be determined by that of the solid phase (Psilica) and the intra-granular pore space (pps) respectively, and will be given by Pl = PsilicaCsilica + Psorbatet~meso .l~
(2)
where (~silica and d~meso a r e the respective volume fractions of the solid and mesopores in the silica grains and O is the fractional filling of the mesopores with liquid sorbate at a particular p/p0. For the case where Psiliea = Psorbate , ( v i z . contrast matching) we then have for a fixed value of Q (in the Porod region) I(Q) oc [~silica+~mesoO] 2
(3)
71 This relationship is represented in figure 2, which shows the relative intensity of scattering, I(Q)rel compared with the evacuated sample ( as measured at Q = 10.2 A "1) for three successive pressures (p/p0 of 0.24, 0.54 and 0.80 respectively). The linearity is satisfactory over a range during which the scattered intensity increases by a factor of 6.3.
3.2. Low-angle diffraction Changes in the low-angle diffraction region during progressive uptake of sorbate were examined in detail with three different samples: S1M (benzene); S3M (cyclohexane); S4M (cyclohexane), having different MCM structures and pore sizes (cf. Table 2). For these three samples, the main diffraction peaks were observed at 6.5 x 10"2, 1.4 x 10"1 and 1.9 x 10"l A 1 respectively, and changes in the peak intensity were noted during the process of mesopore filling with matched benzene and cyclohexane respectively. Changes in the relative intensity compared to that of the evacuated sample, I(Q)rel, showed the same trend when determined as function of increasing volume fraction fillings of the mesopores, t9.: Firstly there was a significant, although modest, increase in intensity on initial adsorption which reached a plateau when the mesopore filling was in the region of 50%. This was followed after approximately 70% filling by a marked reduction in intensity which effectively approached zero at mesopore saturation. Other SANS measurements were made on a series of identical S3M samples to investigate the changes in the diffraction region resulting from a variation in the contrast of the cyclohexane filling the mesopores (viz for 19 = 1). These, measurements made with different volume fraction mixtures of C6D12 and C6nl2, are illustrated in figure 3. (The same sorbate injections were made in all cases, so as to saturate the mesopore volume). The intensity reaches zero at a composition of 50% C6D12, corresponding to effectively the contrast matching condition. The scattering length density of this composition is 3.21 x 10 l~ cm "2 and is close, although slightly less, to that calculated for amorphous silica (Table 2). Such a small negative difference could arise from a slightly lower silica density but might also occur if the cyclohexane molecule was inaccessible to the total porous structure (eg. micropores). Such differences have been noted previously with microporous oxides (1) although for mesoporous solids good agreement has generally been reported (1,3). More quantitative comparison is complicated by changes in the scattering in the Porod region (see discussion 3.1 above) which occur with the highly deuterated compositions. These changes can be ascribed to the differences in the effective contrast of the grains of MCM-41 when filled with different cyclohexane compositions. 4
3
(a)
(b)
(c)
(d)
(e)
(f)
~
.
.
(g)
~ 0.0
0,1
0.2 Q I A "~
0.3
0.4
0.1
0.2
0.3
Q / A "~
.
.
.
. QIA
O. "~
-
9 QIA
"~
,4
.
.
.
Q I A "1
.
.
.
. Q/A
. "~
Q I A "~
Figure 3. Contrast variation for S3M mesoporous silica samples. The mesopores in the different samples are filled with cyclohexane having the following isotopic compositions, % C6D12: (a) 0%; (b) 25%; (c) 45%; (d) 50%; (e) 60%; (f) 75%; (g) 100%.
72
3.2.1. Kinetics of adsorption Another application of the contrast variation technique concerns the kinetics of adsorption of different adsorbate molecules in ordered mesoporous silica. In particular we compare the uptake of n-hexane and cyclohexane in the silica S2M. S2M has the same MCM-41 structure as S1M and S3M (cf. Table 2) but a much smaller pore diameter (2.3 nm). The kinetic evolution of the SANS in the high Q diffraction region of S2M is shown in figure 4 after exposure to the saturated vapour pressure of matching cyclohexane (50% C6DI2) [sample A]. (Here a measured injection was again made so as to saturate the mesopore volume). Firstly we note that the first diffraction peak and the weak secondary maximum are now observed at much higher Q (0.17 and 0.31 A "l respectively) than for the S1M silica (figure 1). This indicates a correspondingly smaller unit-cell parameter (a0 - 42 A), which is in accord with the smaller pore diameter of the hexagonal structure. On exposure to matching cyclohexane the intensity of the diffraction peak is progressively reduced (figure 4). There is an apparent induction period of-~ 9 minutes and then a rapid reduction, which is virtually complete after 25 minutes. Also of note, particularly at higher Q, is the increase in the flat background. This is due to the incoherent scattering of the adsorbed cyclohexane arising from the protons in the molecule. [N.B. These have an extremely large incoherent scattering cross-section (C~incoh(H) 80 barns) compared with other nuclei (t~incohfor SiO2 is zero).] This incoherent background reaches a limiting value somewhat before the elimination of the diffraction feature. This can be ascribed to a process in which the cyclohexane is firstly adsorbed in the inter-granular space before subsequently diffusing into the ordered mesoporous structure- a process which depresses the diffraction feature.
Figure 4. Kinetic evolution of SANS at high Figure 5. Kinetic changes in the intensity of Q of Silica S2M after exposure to matched the diffraction peak of silica S2M samples after exposure to cyclohexane and n-hexane. cyclohexane (50% C6D12) at 289 K. (a.l) Sample A (evacuated) exposed to 50% D cyclohexane; (b.l) Sample B (evacuated) exposed to 50% D n-hexane; (a.2) Sample A (after (a. 1)) exposed to 100% D n-hexane; (b.2) Sample B (after(b. 1) exposed to 100% D cyclohexane. A similar SANS experiment was also made with n-hexane (50% C6D14) [sample B]. Here the rate of adsorption, as determined from the suppression of the diffraction peak, was markedly faster. This is illustrated in figure 5, by the time dependence of changes in intensity of the diffraction peaks for both samples A and B. Thus for sample B there was no apparent
73 induction period, and the uptake appears to have reached completion between 7 and 11 minutes. (N.B. For sample B a constant residual diffraction feature remains after 11 minutes because total contrast matching was not achieved at the 50% D isotopic composition with nhexane as compared to cyclohexane. This is due to a small, but significant, difference in the scattering length densities of the two alkanes: 3.21 and 2.81 x 10 ~~ cm "2 for 50% D cyclohexane and n-hexane respectively; cf. Table 2.) The smaller kinetic diameter of n-hexane (4.3 A "l compared to 6.0 A "l for cyclohexane) may partly account for the faster diffusion into the cylindrical mesopores of the MCM-41 structure, although other effects due to differences in the confinement and specific adsorption of the hexane isomers may play a role. 3.3.2. Kinetics of counter-diffusion and displacement Further SANS measurements were also conducted on these two SM2 samples, loaded with 50% D cyclohexane and 50% D n-hexane ( Samples A and B ). These measurements were designed so as to compare the rates of displacement of the two isomers when further exposed to 100% D n-hexane and 100% D cyclohexane respectively. By comparing the kinetic changes in the diffraction peaks, after exposure of equivalent volumes of the two different isomers, insight into the counter-diffusion and mixing processes was obtained. It can be shown that for the condition of complete mixing (viz. equivolumes of (a) 50% D cyclohexane + 100% D nhexane [Sample A] and (b) 50% D n-hexane + 100% D cyclohexane [Sample B]) the scattering length density in both cases will be almost identical, viz. 4.70 and 4.75 x 101~ cm 2 respectively. This would imply that the intensity of the diffraction peaks in the two experiments would be the same, if there was complete mixing, and the partitioning of the two isomers was equivalent in the mesopore space of the two identical MCM samples. The results of this further experiment are also illustrated in figure 5. For Sample A there is only a slow and small increase in the diffraction intensity on exposure to 100% D n-hexane. This implies that there is little displacement of the cyclohexane adsorbed in the mesopores by n-hexane in the vapour phase. In contrast, when Sample B is exposed to 100% D cyclohexane, there is initialy a rapid reduction in intensity to zero, followed by a steady increase, which then exceeds that of Sample A. This implies that the nhexane in the mesopores exchanges more rapidly with the cyclohexane in the vapour phase. The initial reduction in intensity is caused by the increase in the scattering length density of the sorbed phase as the 100% D cyclohexane is exchanged. At the beginning, the scattering density of the sorbed n-hexane is not completely matched, being less than that of the silica matrix. However after approximately 20 minutes of exposure, the matching condition is reached and thereafter the intensity begins to increase. These conclusions can only be tentative with the limited data here. However the potential scope for future studies of the kinetics of adsorption and partitioning of mixed sorbates is evident. This is of particular interest with microporous sorbents, such as carbons, especially where molecular size effects may be predominant. Such an aspect is indeed particularly relevant in gas separation processes which may involve membranes and pressure swing adsorption.
4. REFERENCES 1. 2. 3. 4.
J.D.Ramsay and G.Wing, J.Coll. Int. Sci., 141 (1991) 475 E.Hoinkis and A.J.Allen, J. Coll. Int. Sci., 145 (1991) 540 J.C.Li, D.K.Ross, M.J.Benham, J.Appl. Cryst., 24 (1991) 794 D.W.Hua, J.V.D.Souza, P.W.Schmidt, D.Smith, Stud. Surf. Sci. Catalysis, 87 (1994) 255
74 5.
J.D.F.Ramsay and E.Hoinkis, in <
>, Royal Society of Chemistry, London, 1997, p.33 6. J.D.F.Ramsay and E.Hoinkis, Physica B 248 (1998) 322 7. See eg. J.D.F.Ramsay in "Handbook of Porous Solids", F.Schtith, K.S.W.Sing and J.Weitkamp (eds.), Wiley-VCH, pp. 135-181 (in press) 8. D.Zhao,. J.Feng, Q.Huo, N.Melosh, G.H.Fredrikson, B.F.Chemelka, G.D.Stucky, Science, 279 1998) 548 9. J.D.F.Ramsay, S.Kallus, E.Hoinkis, Studies in Surface Science and Catalysis, 128, K.K.Unger et al. (eds.), 2000, p.439 10. E.Hoinkis, Langmuir, 12 (1996) 4299 11. G.Porod, Kolloidn. Zh., 124 12. A.Guinier and A.Fournet, "Small Angle Scattering of X-rays", Wiley, New York, 1955
5. A C K N O W L E D G E M E N T S We are indebted to Dr. U. Junges for kindly providing the mesoporous silica samples, to Mr. G. Nabias for the SEM measurements and Drs. Th. Steriotis and E.S. Kikkinides for benzene isotherm data. Technical support at the neutron scattering facilities at ILL, Grenoble, LLB, Saclay and BENSC, Berlin, together with the help and co operation of Drs. P. Timmins, L. Auvray and E. Hoinkis are gratefully acknowledged.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
75
Preparation And Surface Characterisation Of Novel Ceria-Copper And Ceria-Manganese Mixed Oxides Mafia Chdstophidou, and Chaffs R. Theochafis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus The paper presents surface texture and catalysis results for a series of copper and manganese-containing ceria samples. These samples, especially the 20% manganese containing one, had significantly higher surface area than the pristine ceria. The NO denox performance of the 20% copper containing sample indicated high selectivity towards nitrogen formation. 1. INTRODUCTION There has been much recent interest in the preparation of inorganic solids with controlled porosity in the microporous and mesoporous range, or with other desirable surface properties. The interest has somewhat shifted in the recent few years away from microporous towards mesoporous materials not least because of the increased interest in the use of porous materials in air or water pollution control. Ceria (CeO2), in particular, has attracted much interest because of its use as an additive in the socalled triodic automobile exhaust catalyst [1,2], but also because of its use as a catalyst in its own right, or as a catalyst support. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]. In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of alkali and alkaline earth metal doped ceria
76 catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [13]. Zhang and Baerns explained the observed dependence of C2 selectivity on the Ca content in terms of oxygen-ion conductivity [13]. Because of the promoting effects of ceria in many catalytic reactions, the preparation of high surface area and thermally stable ceria phases as well as the study of the parameters which control the structural, textural and redox properties of the material are of particular interest [14-15].
Ceria or metal supported on ceria have
been found to catalyse such diverse reactions as methanol oxidation to CO and HE [ 16], CO oxidation to CO2 [ 17], and reduction of NO and N20 to N2 [ 18]. In addition, several reports have appeared on the catalytic activity of mixed metal-cerium oxides.
At the University of Cyprus there is a long-standing interest in the synthesis and study of porous ceria [ 19-21 ]. Work so far has concentrated studying ways of enhancing and stabilising higher porosity, as well as studying the chemistry of the surface. Two strategies have been used, the insertion of controlled amounts of dopants on the one hand, and the use of organic matrices on the other. The mesoporosity of these solids is not intra-crystalline in the sense that the porosity of zeolites is, but depends upon aggregation of primary particles, and as such is susceptible to synthesis conditions such as pH and concentration of the cerium and other cations precursors used. In the present paper we present the results of an investigation into the use of copper and manganese ions as dopants. Copper was used because of reports in the literature about the catalytic potency of copper bearing ceria [22,23], and manganese because of it possesses a number of stable oxidation states. 2. EXPERIMENTAL
The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purification. The chemicals were in the form of the nitrate salts and were obtained commercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing 0.01M Ce 4+ by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a 0.1M NaOH solution as base. Mixed oxide samples were prepared by the co-precipitation from aquatic solutions of the cations ([Ce(IV)] = 0.01 M) by adding equal volume of 1 M NH3 solution under
77 controlled conditions. The surface properties were investigated by nitrogen adsorption isotherm analysis and FTIR spectroscopy. The nitrogen adsorption isotherms were measured at 77 K using an ASAP 2000 analyser (Micromeritics) after outgassing the samples under vacuum (0.15Pa) at 423K. FTIR spectroscopy was carried out with a Shimadzu spectrophotometer (FTIR-8501) using both KBr and the DRIFTS method. The total pore volume referred to below, corresponds to the pore volume measured from the nitrogen isotherm, at p/p~
Thermogravimetric recordings were
obtained using a Shimadzu TGA apparatus in flowing air. The catalytic efficacy of ceria samples was measured vis ~ vis to oxidation of NO to N2. A continuous flow method was used, and measurements were carried out at various catalyst temperatures. The gaseous mixture used was 0.25% NO, 1% H2, 5% 02 and the rest helium. Mass flow controllers were used to control concentrations. The effluent gas synthesis was determined by quadrupole mass spectrometry. A Baker mass spectrometer was employed. 3. RESULTS Table 1 shows the adsorption isotherm data and acidity measurements for the samples under examination. Figures 1 and 2 show the nitrogen adsorption isotherms for the copper-containing and manganese-containing samples respectively. TABLE 1 Adsorption and Surface Acidity Measurements Pore Surface Acid Real Cu Content by Volume Sites Concn ICP (%) (cm3g"1) (mol/100g)
Sample
SBET (m2g-1)
CeO2 2% Cu 8% Cu 20% Cu 2% Mn 20% Mn
128 88 63 54 113 232
0.08 0.13 0.12 0.11 0.19 0.31
1.73 1.604 1.662 3.845 3.974
0 0.34 1.34 2.97 0 0
Figure 3 contains a graph showing the variation in N2 selectivity during catalysis runs for a variety of CuxCel_xO4 solids, and Figure 4 the corresponding percent conversions, as a function of temperature. Figures 5 and 6 contain the FTIR spectra for CuxCel_xO4and MnxCel.xO4 solids respectively.
78 4. Discussion
Examination of Figure 1 reveals that the FTIR spectrum of ceria and mixed metal oxide-ceria solids has three main features: first a manifold of broad bands centred 2%C u, B%C u & 2 0%C u
['
.
(,,)(' ' [ )2o.(c,,8" 2.,,c. . oc,,)
'
(c) I.-
(b) (a) ,
4000
3100
,
.
,
2 200
I
llcm
3'o0
'
' 400
Figure 1 FTIR spectrum of CeO2 containing (a) 2%, (b) 8% and (c) 20% Cu 2+ Around 3100 cm-1 second a band at 1630 crnl which is attributable to H-O-H bending mode and is indicative of the presence of molecular water in the sample, and thirdly a sharp peak at 1383 cm-1 which has been attributed to Ce-OH stretching and superimposed to higher and lower wavenumbers by a broad band, attributable to
2%Mn(d1) & 20%Mn(cl)
~ I-.-
~
(b)
f \ (a)
4000
I
I 31 O0
I
I 2200
I
I 1300
llcm Figure 2 FTIR spectrum of CeO2 containing 2%, and 20% Mn 2§
I 400
79 Ce-O-Ce modes which are characteristic of the fluorite structure of ceria [20]. The FTIR spectrum of pure CeO2 is characterised by a broad band at 1383 cm-1 which is narrower than that found in mixed oxide spectra, as that shown in Figure 1. This suggests that inclusion of the hetero-atoms in the ceria structure leads to a distortion of the lattice. No extra bands were observed in the FTIR spectra on inclusion of the hetero-atoms. Comparison of X-ray dit~actogrammes for pure and doped ceria, has indicated that there is a single phase, with cell parameters which are very similar to those of the pure phase. This apparent lack of distortion can be explained by the fact that the copper ion content found by ICP (see Table 1) was significantly lower than that in the mother liquor, and reached a maximum of only 2.97%. The significant differences between the spectra in Figure 1 were in the OH stretching frequencies: in the spectrum of pure ceria, only one band is observed [20] at 3400 cm-1, whereas the mixed copper-cerium oxide samples had three distinct peaks, at 3020, 3130 and 3400 cm-1. The fact that the intensity of the H-O-H bend band at 1640 cm-1 was very similar in the mixed oxide and pure ceria spectra, indicated that the differences between the pure and mixed oxides in the OH stretching bands, was due to differences in surface OH bands, attributable to Cu-OH groups, and may indicate that the heteroatoms are present as a dispersed microphase within the pores of the ceria phase.
In similar fashion, the FTIR spectra for Mn 2+ containing samples in Figure 2, showed that the samples showed two OH stretching bands at 3200 and 3400 cm-~, the latter band being attributable to OH stretch in molecular water. The differences between the spectra for CeO2, CuxCel.xO2 and MnxCel.,,O2 suggests that the extra bands are due to OH stretch of groups which are linked to the hetero-atom, suggesting the presence of the dispersed microphase. The differences in the surface acid sites concentration between the two mixed oxides (Table 1) confirms the presence of these surface groups. It is noteworthy that cation concentration had no effect on the acid site concentration, corroborating the probability o f a microphase. Figures 3 and 4 s u ~ i s e
the catalysis experiments results for the mixed copper-
cerium oxide and pristine ceria samples for the NO denox reaction. It can be seen from Figure 3 that the selectivity towards N2 was very high, and for one of the samples higher than for pristine ceria. However, as it can be seen from Figure 4 the
80 conversion rate achieved for these samples was not high, and significantly lower than those achieved by pristine ceria. lee r o
,m,
m
=em
4-W
u o m o (/)
Z
4=
.=..
60
o c
~J 2o
31
o
eM
(.I
ze=
.e
=o=
.o
Temperature
=o
do
see
I=
o Z
(~
e
'~ee
25e
lIQ
Temperature
350
415
4SO
see
(OC)
Figure 3 Figure 4 Nitrogen selectivity in NO denox NO conversion for CuxCel_xO2samples reaction for CuxCe].xO2 samples (In both diagrams: O: 2% Cu, A 8%, square: 15% and inverted triangle, 20%)
100
, , = -=----
80
E6o
Vads-2%Cu Vdes-2%Cu Vads-8%Cu Vdes-8%Cu Vads-20%Cu Vdes-20%Cu
|
J
............
rt
e~
E
i
I
..........................
'i
o40
>o20
0
0 p/po0 dg
Figure 5 Nitrogen adsorption isotherms for CuxCel.xO2samples Figure 5 contains the nitrogen adsorption isotherms for the 20% Cu containing sample, whilst the surface properties of all samples are summarised in Table 1. The isotherms are of Type IV with hysteresis loops of the H4 type. There are no significant differences in the shape of the isotherm or the hysteresis loop, but as observed for other mixed oxides, there was a decrease in the SBET values for the
81
mixed oxide compared with the pure oxide, whilst there is a significant decrease in
SBET values with increasing copper ion inclusion. On the other hand, there is an increase in apparent pore volume for the mixed oxides compared with the pristine oxide. Significantly, there is no change with increasing hetero-atom concentration.
250
Vads-2%Mn Vdes-2%Mn Vads-20%Mn Vdes-20%Mn
200
! i t !
!
i
i
] l
>o
50
o
Figure 6 Nitrogen adsorption isotherms for
Mno.20Ceo.8oO2
samples
Figure 6 shows the nitrogen adsorption isotherm for the 20% manganese containing samples. It is noteworthy that these mixed oxides have a much higher apparent pore volume than the pristine sample. The solid with a 2% manganese content has a BET surface area comparable with the pristine sample, but the sample with 20% manganese has a significantly higher BET surface area (Table 1). These observations combined with those from the FTIR spectra suggest that the presence of the heteroatoms leads to a distortion of the lattice. The significantly higher value of surface acid sites concentration for the MnxCel.xO2 samples compared to the CuxCel.xO2 ones may be partly due to the higher apparent surface area for the former compare to the latter, but presumably is also linked to the differences in polarisation of the Mn-OH and CuOH groups, as well as the effect of the presence of the hetero-atoms to the properties of the Ce-OH groups. The significant increase in apparent surface area achieved for ceria with the controlled incorporation of hetero-atoms, as well as the interesting
82 catalytic activity exhibited by these solids, augurs well for the development of novel ceria-based solids with interesting properties. Acknowledgement We thank Dr I. Pashalidis and Ms G. Kyriakou for useful discussions, and the University of Cyprus and the European Union for financial assistance.
References 1. G.J.K. Acres, in "Perspectives in Catalysis", (Eds J.M. Thomas and K.I.Zamaraev), Blackwell Scientific Publications, London, (1992) 2. J.G. Nunan, H.J. Robota, M.J.Cohn and S.A. Bradley, J. Catal., 133 (1992) 309 3. D. Terribile, A. Trovarelli, C. de Leitenburg and G. Dolcetti, Chem. Mater., 9 (1997) 2676 4. T. Bunluesin, R.J. Gorte and G.W. Graham, Applied. Catal. B-Environmental, 15 (1998) 107 5. V.P. Zhdanov and B. Kasemo, Applied Surface Science, 135 (1998) 297 6. Se H. Oh and C.C. Eickel, J. Catal., 115 (1988) 543 7. Thi X.T. Sayle, S.C. Parker and C.Richard A. Catlow, J. Phys. Chem., 98 (1994) 13625 8. A. Takami, T. Takemoto, H. Iwakuni, K. Yamada, M. Shigetsu, and K. Komatsu, Catal. Today, 35 (1997) 75 9. P.G. Harrison, W. Azelee, A.T. Mubarak, C. Bailey, W. Daniell, Studies in Surface Science and Catalysis, 116, 495, (1998) 10. W. Liu, C. Wadia, and M. Flytzani-Stephanopoulos, Catal. Today, 28 (1996) 391 11. P.K. Rao, K.S.R. Rao, S.K. Masthan, K.V. Narayana, T. Rajiah, and V.V. Rao, Applied. Catal. A-General, 163 (1997) 123 12. A. Naydenov, B. Stoyanova, and D. Mehandjiev, Journal of Molecular Catalysis A-Chemical, 98 (1995) 9 13. Z.-L. Zhang and M. Baems, J. Catal., 135 (1992) 317 14. J.Z. Shyu, W.H.Weber and H.S.Gandhi, J. Phys. Chem., 92 (1988) 4964 15. D. Terribile, A. Trovarelli and G. Dolcetti, J of Catalysis., 178 (1998) 299 16. W.J. Shen and Y. Matsumura, Phys Chem Chem Phys 2,1519, (2000) 17. S.H. Overbury, D.R. Mullins, and D.R. Huntley, J Catal 186, 296, (1999) 18. E.S. Putna, R.J. Gone, J.M. Vohs, J Catal 178, 598, (1998) 19. I. Pashalidis and C.R. Theocharis, Second European East West Workshop on Chemistry and Energy, Sintra, Portugal, March 1995 20. I. Pashalidis, and C. R. Theocharis, in (Eds K.K. Unger, G. Kreysa and J.P. Baselt), "Studies in Surface Science and Catalysis", Elsevier Science Publishers, 128, 643- 652, (2000) 21. G. Kyriakou, I. Paschalidis and C.R. Theocharis, Fundamentals of Adsorption, 7, In the Press 22. A.Martinez-Arias, M. Femandez-Garcia, J. Sofia, J Catal 182, 367, (1999) 23. J. Soria, J.C. Conesa, A. Martinezarias, Solid State Ionics 63-5, 755, (1993)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
83
A b o u t the exclusive m e s o p o r o u s c h a r a c t e r of M C M - 4 1 A. Berenguer-Murcia 0), j. Garcia-Martinez (1), D. Cazorla-Amor6s Aionso <2)J.M.D. Tasc6n <2), and ~i,. Linares-Solano O) (1)
(.1), A. Martinez-
Departamento de Quimica Inorg/mica, Universidad de Alicante, Apartado 99 - 03080 Alicante, Spain.
(2) Instituto Nacional del Carbrn, La Corredoria s/n, Ap. 73, 33080 Oviedo, Spain. Physical gas adsorption was used to study the porous texture of two silicas prepared in this work. These two silicas are the ordered mesoporous material named MCM-41, and a precipitated silica that was prepared under identical conditions but without the use of a templating agent as in the case of MCM-41. Physical gas adsorption of N2 at 77 K (coveting the relative pressure range of 10-6 to 1) and CO2 at 273 K (subatmospheric and high pressures, coveting the relative fugacity range of 105-0.7) were carried out on the samples. From the analysis of the adsorption data, that includes a deep comparative study of these two silicas, we wonder about the exclusive mesoporous character of MCM-41. Contrarily to what is commonly accepted, we present evidences that our MCM-41 has, in addition to mesoporosity, roughness or heterogeneities that do not differ in behavior from classical microporosity. Surprisingly, the characteristics of our MCM-41 do not differ from others reported as only mesoporous. I. INTRODUCTION Ordered Mesoporous from the moment of their novel potential synthesized by Mobil
Materials (OMMs) have been known for more than 10 years [ 1-2] and their discovery, they have gained increasing importance as a result of catalytic properties [3,4]. Among the OMMs, the M41S family, first Oil Corporation [2, 5-6] has focused much of the attention. The main
feature of this type of materials is a hexagonal array of uniform uni-dimensional parallel mesoporous channels with a narrow pore size distribution. The pore size of these materials can be tailored in the range of 1.4 to 10 nm by varying the synthesis conditions [7] making them extremely versatile for different applications, such as adsorbents, as catalysts or as catalyst supports and as part of host-guest reactions [3]. This has caused a dramatic increase in research related to these materials together with the appearance of numerous reviews (see for instance Refs. [3-4,8-10]). M41S materials have been extensively characterized using different techniques such as gas and vapor adsorption [ 11-14], X-Ray diffraction [ 15-16] and
84 spectroscopic techniques [17]. After all these studies, some authors have concluded that MCM-41, which belongs to the M41S family, is an exclusively mesoporous material [ 11, 12, 14]. However, the presence of surface heterogeneities inside the pore walls has been already pointed out from SAS studies [18] and from simulation calculations [19-21]. Furthermore, recent studies [22] have noted the presence of microporosity in MCM-41 from n-nonane and gas adsorption studies. Therefore, the aim of the present work is to deepen into the accepted exclusive mesoporous character of MCM-41 by carefully analyzing the physical adsorption data of N2 at 77 K and CO2 at 273 K, coveting in both cases a wide range of relative pressures (from 10-5 to near unity) and comparing the adsorption data of a sample of MCM-41 with a precipitated silica, prepared under identical conditions but without the templating agent. 2. EXPERIMENTAL MCM-41 was prepared by the following synthesis [23]. 4.8 g of Cetyltrimethyl Ammonium Bromide (CTA-Br) were dissolved in 440 ml of distilled water. This mixture was stirred and heated gently until complete dissolution. Afterwards, 16 ml of NH4OH (30% wt.) were added and the resulting mixture was stirred at 380 r.p.m, for 5 minutes; 20 ml of TEOS (tetraethoxysilane) were added and the solution stirred at 380 r.p.m, for one hour. The mixture was filtered and dried overnight at 323 K. The synthesis of the precipitated silica (PS) was carried out in a similar way, slightly modifying the synthesis in Ref [23] without adding surfactant and adding ethanol to favor the precipitation of the silica. Finally the template removal of MCM-41 was carried out in N2 at a temperature of 823 K with a heating rate of 5 K-min-1 keeping the temperature constant for one hour. The nitrogen flow (flow rate: 100 ml.min1) was switched to oxygen for a further five hours at 823 K before cooling. X-ray diffraction analysis of the samples was performed in a Seifert 2002 powder diffractometer using Cu K~t radiation (0.154 nm) from a 20 of 1.5 to 7.5 ~ Nitrogen adsorption studies were carried out in a Quantachrome Autosorb-6 adsorption equipment at 77 K. Nitrogen adsorption studies at 77 K at low relative pressures were carried out in a Micromeritics ASAP 2010 equipment. Carbon dioxide adsorption at 273 K was carried out in a Coulter Omnisorp 610 adsorption equipment (<1 bar), and high-pressure carbon dioxide studies were carried out using a gravimetric equipment (Sartorius 4406 DMT high-pressure microbalance). In all cases, the samples were degassed at 523 K at a pressure of 7.5x10 -7 Pa overnight. The experiments have been corrected for buoyancy effects due to sample holder, pan and adsorbed phase [24].
85
3. RESULTS AND DISCUSSION 3.1. PS.
Figure 1a shows the N2 adsorption isotherm at 77 K for PS. We observe it is a type I isotherm, with a steep uptake at low relative pressures, which is typical of microporous solids. From the isotherm we can obtain the pore volume for this sample, which is 0.39 cm3-g-1 for a density of liquid nitrogen of 0.808 g. cm3. In order to gain better knowledge of our sample PS, the adsorption of CO2 at 273K at subatmospheric pressures was performed. This is a well-known method for characterizing microporous carbons [24-26] that has been recently extended to zeolites [27], which are well-known microporous solids. When working at pressures below atmospheric, the relative pressure of CO2 can be brought down to values ranging from 10-5 to 10-2. Working under these conditions, we can obtain valuable information on the microporosity of the material that is being analyzed. Figure l b shows the CO2 adsorption isotherm of PS at 273 K. We observe from the isotherm, that although its shoulder is quite wide, as it corresponds to a heterogeneous pore size distribution, the CO2 uptake is quite noticeable. The micropore volume obtained by applying the DR equation to the isotherm is 0.14 cm3-g1. 0.45 0.4 0.35
0.07 0.06
0.3
~ o.o5
oC 0.25
~ 0.04
6
6
m : E 0.2
:::~0.03
>-~ 0.15
:~ 0.02
0.1 0.05
/
.-~MCM-41
0.01 ,
,
,
,
,
0.2
0.4
0.6
0.8
1
P/Po Figure la. N2 adsorption isotherm at 77 K for PS.
0
,
0
,
,
,
|
,
0.005 0.01 0.015 0.02 0.025 0.03 P/Po
Figure lb.CO2 adsorption isotherm at 273K for PS and MCM-41
3.2. MCM-41.
The XRD pattern of the sample of MCM-41 showed that the material was high quality MCM41. This sample was characterized in the same way as PS. Figures 2a and l b show the N2 adsorption isotherm at 77 K and the CO2 adsorption isotherm at 273 K at subatmospheric pressures, respectively, for MCM-41. Figure 2b also presents the complete CO2 adsorption isotherm obtained at high pressures. The shape of the N2 adsorption isotherm is typical for M41S materials and the pore volume obtained from the isotherm is 0.76 cm3g-1. The micropore volume obtained by applying the DR equation to the CO2 adsorption isotherm (Figure l b) for MCM-41 is 0.14 cm3g'l, which is the same as the one for PS.
86
o
~ e E
0.8
0.8
0.7
0.7
0.6
0.6
0.5
d 0.5 d
,,_..
9 0.4
0.4
E -= 0.3
-= 0.3 >o 0.2
0.2
0.1
0.1
0
0
,
|
|
,
0.2
0.4
0.6
0.8
P/Po
....
0
,
1
0
l
I
I
l
I
I
I
|
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
P/Po
Figure 2a. N2 adsorptionisothermat 77 K for MCM- Figure2b. CO2 adsorptionisothermat 273 K for MCM41 41 at subatmosphericpressures (full dots) and at high pressures(emptydots) In the case of Figure 2b, the relative fugacity of CO2 was brought up to 0.7 so as to obtain the whole isotherm. It must be noted that the pore volumes obtained from both the N2 and CO2 adsorption isotherms are very similar to each other (~0.76 cm3.g1) and that the pore radii obtained from both isotherms at the inflection point are the same. 3.3. MCM-41 vs. PS. From Figure 1b and from the micropore volume values obtained by applying the DR equation to the CO2 adsorption isotherms of both materials there seems to be a remarkable coincidence between MCM-41 and PS. This similarity might arise from the fact that the two materials were prepared in the same way being the only difference that MCM-41 was prepared by firstly adding a surfactant to the reaction mixture. With the aim to deepen into this similarity we compared the N2 adsorption isotherms of both materials. Figure 3 shows the two nitrogen adsorption isotherms plotted together. From the figure it is clear that both materials behave very similarly at low relative pressures, which suggests that both materials might have the same type of pores that adsorb N2 at relative pressures <0.1. 0.8
=
A
~
~
j
0.8
_
0.7
0.7-
0.6
.._,0.6 -
d 0.5
d 0.5o
|
E >=
0.4 ~
0.3
9 0.4
F
E :~
-= 0.3
PS
0.2
0
~
~
, PS,
0.1
0.10
0.2
|
|
|
|
!
0.2
0.4
0.6
0.8
1
P/Po Figure 3. N2 adsorption isotherms at 77 K for both MCM-41 and PS
0 1E-07 1E-06 1E-05 1E-04 0.001 0.01 0.1 1 P/Po Figure 4. N2 adsorption isotherms at 77 K for both MCM-
41 a n d PS at low relative pressures
87 In order to corroborate this similarity, the N2 adsorption isotherms at 77K at low relative pressures for both materials were obtained. In both cases, the relative pressure of N2 was brought down t o 10 -6 . The isotherms obtained for the two materials at low pressures are shown in Figure 4. Both MCM-41 and PS begin to adsorb at P/P0 slightly lower than 10-6 and they practically overlap up to a relative pressure of 0.1. It is clear that the two materials adsorb equal quantities of N2 in the range 106-10 "1, indicating that both silicas (synthesised following the same route with the exception of the addition of a surfactant to the synthesis solution in the case of MCM-41) have similar narrow porosity and that they only differ in the adsorption behaviour for P/P0>0.1-0.2, which corresponds to the mesoporosity region. These results show that our MCM-41 contains a microporosity contribution similar to that of PS sample. In fact, the shape of the isotherm of MCM-41 can be thought to be formed by the contribution of a type I isotherm (up to P/P0 of 0.1, as PS), and then the mesoporosity contribution corresponding to a type IV isotherm. It must be'emphasised that other MCM-41 with lower surface area might have no microporosity but the characteristics of our MCM-41 is similar to others reported as "only mesoporous", which have about 1000 m2/g of BET surface area. This aspect could be the subject for future research. Maddox et al. [ 19] tried to explain the unusual shape of the N2 adsorption isotherm for MCM41 assuming an ordered hexagonal mesoporous structure, nevertheless models in which the solid-fluid interaction potential is homogeneous (cylindrical pores with smooth walls) cannot reproduce the form of the adsorption isotherm, particularly at low pressures, and the presence of heterogeneities inside the mesopores with strong solid-fluid interactions needs to be considered. The work by Maddox et al. clearly states that "the nature of the surface heterogeneities is not clear at present"; our suggestion is that those heterogeneities derive from the precipitation of silica around a hexagonal array of surfactant micellar rods, and are most probably micropores. References [20-21 ] have pointed out another possible source for the presence of micropores in the MCM-41 porous array. The authors have simulated the formation of micelles in a surfactant solution. The resulting micelles have a highly rough surface and thus, upon removal of the surfactant the walls of the resulting material will not be smooth. Furthermore, Sonwane et al. [18] have noted the presence of constrictions in the mesopore channels using small angle scattering techniques, which again points out the possible presence of micropores in the MCM-41 pore array.
3.4. Characteristic curves.
Adsorption isotherms of
N2 at
77 K and CO2 at 273 K can be fairly compared by plotting
them as a characteristic curve. Figures 5 and 6 show the characteristic curves for N2 and CO2 for both PS and MCM-41.
88 '
"
200
400
|
l
600 '|
800
1000
|
0
|
200
0
I,
400 ,
I
-1_2~ i ~ l ~ -2 >-3
~9 ~
~
~
--..,,,,
CO
2
~4
,
800
1000
I
I
I
)2(kJim~ ~ ~ .
>-3
600
c02
E J - 4
-5 -6
Figure 5. Characteristic curves for N2 and CO2 on PS
-7
Figure 6. Characteristic curves for N2 and CO2 on MCM-41.
If we draw the characteristic curve for N2 at 77 K and CO2 at 273 K (at subatmospheric and high pressures), both adsorptives fit the same curve for both materials for (A/13)2 values lower than 150 KJ/mol, clearly indicating that both adsorptives follow the same adsorption mechanism. If the N2 adsorption data obtained at low relative pressures (10 -6 to 10"2) are plotted it is clear that the resulting characteristic curve falls below the one corresponding to CO2. This observed diIfusional problems of N2 molecules to enter a part of the porosity of MCM-41 at 77 K must be due to the presence of narrow micropores (<0.7 nm) which are accessed by CO2 at 273 K (0.14 cm3/g micropore volume from the DR equation). Considering that working with N2 at very low relative pressures needs the use of relatively expensive equipment and extreme conditions, and that N2 at 77 K presents diffusional problems, it is clear that CO2 works better for characterizing micropores.
3.5. a-plot studies. This well accepted method [28] has been used extensively in the characterization of M41S materials [11-12,14]. From the application of this method to MCM-41, it has been concluded that this material contains no significant amounts of microporosity. This is the main evidence presented so far in order to conclude that MCM-41 is
exclusively
mesoporous. As it happens
with any good method its limitations need to be considered in order to avoid misinformation. In the case of the ot method the choice of the reference isotherm is crucial. All the reference silicas should be nonporous in order to allow a reliable analysis of MCM-41. Unfortunately, we observed that most of them have a steep rise in their N2 adsorption isotherms at 77 K at low relative pressures and BET surface areas varying from 40 [29] to 400 m2/g [30]. For this reason, our sample of MCM-41 was heat-treated so as to sinter the silica particles and thus obtain a nonporous silica (BET surface area 1.5 m2/g) and as similar as possible to our MCM41. The N2 adsorption isotherms for a reference silica [29] and our sintered MCM-41 are shown in Figure 7.
89 0.06-
0.9 0.8
0.05 -
f o v
o
E _=
0.7
j
0.04
0.03
Reference Silica
0.02
~0.6 o
~o.5 (D
A~'A"
E 0.4
Sintered MCM-41
> 0.3 0
0.2
0.01 '
.
/ 0.1 / 0
.
.
0
0.2
''
0.4
~ P/Po
0.6
0.8
Figure 7. N2 adsorption isotherms of two reference at 77K
1
!
0.5
!
i
|
1
1.5
2
~-S
Figure 8. o~-plotsfor MCM-41 using the two reference silicas in Figure 7.
Figure 8 shows the or-plots obtained for our sample MCM-41 using as standards our sintered MCM-41 and the reference silica [29] whose N2 adsorption isotherms are shown in Figure 7. From Figure 8 it is clear that depending on the standard that is used, different conclusions can be reached, because depending on it we can conclude that MCM-41 is either exclusively mesoporous or that it contains a significant amount of microporosity. Therefore, it is clear that the Gt-method can reach contradicting conclusions when using different reference materials, which makes the characterisation of the material more complex, moving the problem from the material which is being characterised to the reference material itself 4. CONCLUSIONS
Two silicas have been prepared in this study-an ordered mesoporous material named MCM41 and a precipitated silica- in order to analyze the generally accepted exclusive mesoporous character of MCM-41. This study includes a careful analysis carried out by physical adsorption data obtained with N2 at 77 K and CO2 at 273 K, comparing the results for a sample of MCM-41 with those obtained with a precipitated silica, prepared under identical conditions but without the templating agent. From these adsorption data, that cover a similar pressure range (10 -6 to 1) for N2 at 77 K and and relative fugacities 10-5 to 0.7 for CO2 at 273 K, we conclude that our MCM-41 contains microporosity. Considering that the characteristics of our MCM-41 do not differ from other reported as "only mesoporous" we wonder about the
exclusive mesoporous character of MCM-41. We feel that our sample has a non-negligible microporosity contribution of which about 0.14 cm3/g correspond to pores smaller than 0.7 nm in size. This observation can be in agreement with previous results that comment about the presence of roughness or heterogeneities [19], constrictions in the mesopores [18] and/or micropores [22].
90 Acknowledgements. The authors would like to thank MCYT (MAT-2000-0621) and CICYT (PB98-0983). A.B.M. would also like to thank MECD for his PhD thesis fellowship. REFERENCES [ 1] T. Yaganisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn., 1990, 63, 988. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 1992, 359, 710. [3] U. Ciesla, F. Schtith, Microp. Mesop. Mater., 1999, 27, 131. [4] A. Corma, Chem. Rev., 1997, 97, 2373. [5] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Sot., 1992, 114, 10835. [6] J.S. Beck, J.C. Vartuli, Curr. Opin. Solid State Mater. Sci., 1996, 1, 76. [7] R. Ryoo, J.M. Kim, J. Chem. Soc., Chem. Commun., 1995, 7, 711. [8] F. Sch0th, Stud. Surf. Sci. Cat., 135, 1-12 (2001). [9] T. Maschmeyer, Curr. Opin. Solid State Mater. Sci., 1998, 3, 71. [10] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater., 1999, 27, 329. [11] P.J. Branton, P.G. Hall, K.S.W. Sing, J. Chem. Sot., Chem. Commun., 1993, 16, 1257. [12] M. Kruk, M. Jaroniec, J.M. Kim, R. Ryoo, Langmuir, 1999, 15, 5279. [13] C.G. Sonwane, S.K. Bhatia, N. Calos, Ind. Eng. Chem. Res., 1998, 37, 2271. [14] M.M.L. Ribeiro Carrott, A.J.E. Candeias, P.J.M. Carrott, P.I. Ravikovitch, A.V. Neimark, A.D. Sequeira, Microp. Mesop. Mater., 2001, 47, 323-337. [ 15] C.G. Sonwane, S.K. Bhatia, Langmuir, 1999, 15, 2809. [ 16] M. Kruk, M. Jaroniec, Y. Yang, A. Sayari, J. Phys. Chem. B, 2000, 104, 1581. [17] K.J. Edler, P.A. Reynolds, P.J. Branton, F.R. Trouw, J. W. White, J. Chem. Soc., Faraday Trans., 1997, 93, 1667. [18] C.G. Sonwane, S.K. Bhatia, N.J. Calos, Langmuir, 1999, 15, 4603-4612. [19] M.W. Maddox,, J.P. Olivier, K.E. Gubbins, Langmuir, 1997, 13, 1737-1745. [20] B.J. Palmer, J. Liu, Langmuir 1996, 12, 746-753. [21 ] S.Bandyopadhyay, M Tarek, M.L. Lynch, M.L. Klein, Langmuir, 2000, 16, 942-946. [22]/i. Berenguer-Murcia, J. Garcia-Martinez, D. Cazorla-Amor6s,/~. Linares-Solano, A.J. Fletcher, K.M. Thomas, submitted [23] M. Grtin, K.K. Unger, A. Matsumoto, K. Tsutsumi, Microp. and Mesop. Mater., 1999, 27, 207. [24] D. Cazorla-Amor6s, J. Alcafiiz-Monge,/i,. Linares-Solano, Langmuir, 1996, 12, 2820. [25] /i,. Linares-Solano, C. Salinas-Martinez de Lecea, J. Alcafiiz-Monge, D. CazorlaAmor6s, Tanso, 1998, 185, 316. [26] D. Cazorla-Amor6s, J. Alcafiiz-Monge, M.A. de la Casa-Lillo, A. Linares-Solano, Langmuir, 1998, 14, 4589. [27] J. Garcia-Martinez, D. Cazorla-Amoros, /~. Linares-Solano, Stud. Surf. Sci. Catal., 2000, 128, 485. [28] S.J. Gregg, K.S.W. Sing in "Adsorption, Surface Area and Porosity" 2nd Ed., Academic Press, London 1982, pp. 104-105. [29] S.R. Bhambhani, P.A. Cutting, K.S.W. Sing, D.H. Turk, J. Coll. Interface Sci., 38, 1. [30] M. Kruk, M. Jaroniec, A. Sayari, Langmuir, 1997, 13, 6267.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Incorporation of appropriate contact angles characterization by mercury porosimetry
91
in
textural
J.C. Groen,a, * L.A.A. Peffer, a and J. P~rez-Ramlrez, b Applied Catalyst Characterization, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands. 9email: [email protected]
a
b Hydrocarbon Processes and Catalysis, Research Centre, Norsk Hydro, P.O. Box 2560, N-3907, Porsgrunn, Norway. In this paper the application of the measured contact angle in textural characterization of porous solids by mercury intrusion porosimetry (MIP) has been investigated. The contact angle of mercury on various materials was measured by the advancing angle and static angle methods. Both methods led to very similar and reproducible results. Most oxidic materials, including A1203, SiOe, and TiO2, and supported catalysts present a contact angle close to 140 ~ which is typically presumed in pore size calculations. However, the contact angle of different carbon samples (_>150~) and cement-like materials (_<130~) strongly differs from this value. For such materials and for porous solids in general, incorporation of the measured contact angle is essential for a proper textural characterization by MIP. This has been supported by a good correlation between the pore size distribution obtained from MIP and N2 adsorption measurements over different samples, after incorporation of the appropriate (measured) contact angle.
1. I N T R O D U C T I O N Physico-chemical techniques are widely used for characterization of catalysts and porous materials in general. Well-known methods based on physical adsorption of inert gases (N2 and CO2) and penetration of mercury at elevated pressures provide information on the total surface area, pore volume, and pore size distribution (PSD) of the sample [1,2]. Gas adsorption and mercury porosimetry are often compared since they generate data of similar nature in the pore size range 4 - 100 nm. In the field of N2 adsorption, the development of model mesoporous materials like MCM-41 and SBA-15, showing a uniform mesopore size distribution in the range of 2 - 20 nm, has led to numerous studies to determine the PSD accurately. Since it is widely known that the BJH model underestimates the mesopore size, these studies resulted in adaptation of existing models and development of new
92
models [3-5]. In mercury intrusion porosimetry (MIP), major attention has been paid to the modeling of pore network and percolation effects [6,7], while the influence of more standard variables like the contact angle 0 between mercury and the porous sample has often been ignored. The contact angle has traditionally been taken as a standard value of 140 ~ independent of the material under investigation. The last value may lead to deviations (6) in the 0 value up to 20 ~ resulting in an error (~) of 35% in the calculated pore size (Fig. 1). This can be derived from the W a s h b u r n equation (eq. (1)), which relates the diameter of a cylindrical pore dpo,e, the measured pressure p and the surface tension of mercury Y.
dpo,.e =
-4 y cos 0
(1)
P Different methods have been developed to determine the contact angle of mercury on a solid material [8]. The static angle method uses an optical microscope combined with a goniometer to quantify the non-wetting behavior of a mercury droplet on a solid 5O surface. The advancing angle method measures the 40 pressure needed to force a o ~ 30 mercury droplet of known volume through a hole with -9 20 cO known diameter. The ",.~ 10 Washburn equation can then > 9 0 be used to determine the "0 (i.) experimental value of 0. > -10 J In this contribution we -20 n," have evaluated the -30 ' ' importance of incorporating 110 120 170 130 140 150 160 the measured contact angle 0 / degrees (by advancing and static angle methods) in the textural Fig. 1. Relative deviation in the calculated characterization of porous porediameter dporeas a function of the contact angle materials by MIP. 0 (reference value 140~ i
,
J
I t
i
,
i
,
,
2. E X P E R I M E N T A L 2.1. M a t e r i a l s Different porous materials, including oxides (a-AleO3, SiOe, TiOe), zeolites (NaZSM-5, H-ZSM-5, H-beta, Na-Y, and H-USY), carbon (activated carbon, Norit, and mesoporous carbon (Novacarb from MAST Carbon Ltd.)), cement-like materials (enci and tras, with low and high lime content, respectively), and catalysts (1 wt.% Pt/A1203, 5 wt.% Pt/A1203, and 16 wt.% Ni/A1203) were used, as well as home-made samples (A1203, MCM-41, and SBA-15).
93
2.2. C h a r a c t e r i z a t i o n
2.2.1. Contact angle measurement The contact angle of mercury on the various materials was m e a s u r e d using two different methods. Prior to the pellet preparation, the samples were milled to obtain a particle size < 10 pm. 9 Advancing angle method, using a Quantachrome contact anglometer (Fig. 2a) [9]. Approximately 30-60 mg of powder was brought into the sample holder and pelletized (pressure applied is 12 MPa) around a pin having a well-defined diameter. In this way a hole of known diameter (0.813 mm) is obtained in the pellet. Subsequently, the pressure difference needed to force a mercury droplet (50 pl) through the hole is measured. From this result the advancing contact angle is calculated by application of eq. (1), since the pressure, pore diameter and surface tension are known and the remaining variable cos 0is obtained. 9 Static method, using a Krfiss G-1 optical microscope equipped with a goniometer (Fig. 2b) [10]. Approximately 100 mg of the powder is compressed into a pellet at a pressure of 1000 MPa. Subsequently a mercury droplet of 26 ~1 is placed on the pellet and a goniometer in the ocular of the microscope is then used to visually determine the specific contact angle (so-called static contact angle).
Fig. 2. Schematic representation of the (a) advancing angle and (b) static angle method for determination of the contact angle 0.
2.2.2. Mercury intrusion porosimetry (MIP) MIP experiments were performed on CE I n s t r u m e n t s PASCAL 140 and 440 porosimeters, which operate in the pressure range of vacuum to 400 kPa and 100 kPa to 400 MPa, respectively. Prior to the intrusion experiments the samples were degassed in vacuum at 625 K for 16 h. The PSD was determined from the W a s h b u r n equation, taking the surface tension of mercury being 480 N.m -1. The contact angle was experimentally determined as described above.
94
2.2.3. N2 physisorption N2 a d s o r p t i o n m e a s u r e m e n t s a t 77 K w e r e p e r f o r m e d on a Q u a n t a c h r o m e A u t o s o r b - 6 B gas a d s o r p t i o n a n a l y z e r . P r i o r to t h e a d s o r p t i o n m e a s u r e m e n t s t h e s a m p l e s w e r e d e g a s s e d in v a c u u m a t 625 K for 16 h. A full ad- a n d d e s o r p t i o n i s o t h e r m w a s m e a s u r e d in t h e r e l a t i v e p r e s s u r e (p.p0 -1) r a n g e of 0.1-1. T h e P S D w a s d e t e r m i n e d from t h e B r o e k h o f f - d e B o e r (BdB) m o d e l a p p l i e d to t h e d e s o r p t i o n b r a n c h of t h e i s o t h e r m [3].
3. R E S U L T S AND D I S C U S S I O N 3.1. D e t e r m i n a t i o n of the c o n t a c t angle T h e c o n t a c t a n g l e m e a s u r e m e n t s on t h e v a r i o u s m a t e r i a l s a r e p r e s e n t e d in T a b l e 1. T h e r e s u l t s of t h e a d v a n c i n g a n g l e m e t h o d a r e v e r y s i m i l a r to t h o s e determined with the static angle measurement.
Table 1. Average contact angles of the samples investigated, as m e a s u r e d with the advancing angle and static angle method. ........................................
Method ~
Advancing angle 0 +SD 1
Materia 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Static angle 0 +SD
a-A120~
degre e 138 + 0.2
. . . . degree 137 + 0.3
A120~
141 + 0.2
141 + 0.7
Amorphous Si02
141 + 0.2
140 + 0.9
n.d. 2
140 + 0.3
TiO2 1 wt.% Pt/A1203
142 + 0.4
143 + 0.8
5 wt.% Pt/A1203
136 + 0.4
138 + 0.3
16 wt.% Ni/A1203
139 + 0.2
139 + 1.3
H-ZSM-5, Si/Al=15
138 + 0.8
n.d.
Na-ZSM-5, Si/AI=40
140 + 0.7
140 + 0.7
H-beta, Si/Al=12.5
128 + 0.4
n.d.
H-beta, Si/Al=150
127 + 0.2
n.d.
Na-Y, Si/AI=2.5
132 + 0.2
n.d.
H-USY, Si/Al=16
138 + 0.5
n.d.
C e m e n t enci (low lime content)
128 + 0.9
129 + 0.7
C e m e n t tras (high lime content)
129 + 0.4
n.d.
Novacarb mesoporous carbon
150 + 0.4
151 + 0.7
Activated carbon
155 + 0.4
155 + 0.6
Norit
163 + 1.1
n.d.
1 Values for 0 and S~ ~(stan~ measurements. 2 n.d." not determined.
on three r e p e t i t i v e
95 The p r i m a r y particle size of the powdered samples is to a certain extent of minor importance on the measured contact angle. The high pressure applied (101000 MPa) upon pelletizing causes the original particle size and shape to be distorted in such a way t h a t a very smooth surface and a well-defined hole (advancing angle method) is obtained. This has been supported by Scanning Electron Microscopy m e a s u r e m e n t s of the powders and pellets (see e.g. Fig. 3 for a Ni/A1203 catalyst).
Fig. 3. SEM micrographs of a 16 wt.% Ni/A1203 (a) powder and (b) pellet. The contact angle for most metal oxides, including A1203 and SiO2, as well as for supported catalysts is close to the presumed 8= 140 ~ In zeolites contact angles range from 127 ~ to 140 ~, and assuming 140 ~ may lead to erroneous interpretation if meso- or macroporosity determinations of these materials are required [11]. The zeolite framework type seems to be the most i m p o r t a n t variable influencing the interaction between mercury and the solid material. The most striking results are found for cement-like materials and carbonaceous materials, which show a relatively low (< 130 ~ and high (> 150 ~ contact angle, respectively. From this perspective, a more detailed study on a well-defined mesoporous carbon (MAST Novacarb) and metal oxide (A1203) is performed to verify the influence of the measured contact angle on the calculated pore size and finally the correlation between the MIP and N2 physisorption techniques.
3.2. Mercury intrusion porosimetry The intrusion curves on both A1203 and Novacarb show a well-defined step as a result of mercury filling the pores in a narrow pressure range (Figs. 4a and 4b). The inflection point in the intrusion curve is found around 200 MPa for A1203 and 100 MPa for the mesoporous carbon, indicating a smaller average pore size is found in A1203 t h a n in the carbon. The intrusion at pressures < 1 MPa in Fig. 4b is due to filling of interparticle voids between the carbon particles and results in their redispersion. Both materials exhibit an extrusion curve, parallel to the
95 0.5
0.45
(
(b)
0.4 "T 03
,~.030.30
,4 0.3
E 0
,-0.2 - 0.15 0.1
1
10
100
0.00 0.01
1000
p / MPa
0.1
1 10 p / MPa
100
1000
F i g . 4. M e r c u r y i n t r u s i o n a n d e x t r u s i o n curve on (a) A120:~ a n d (b) N o v a c a r b . O p e n s y m b o l s : i n t r u s i o n curve, solid symbols: e x t r u s i o n curve.
intrusion curve and a final volume decrease close to the total i n t r u d e d volume, indicating t h a t most of the mercury in the porous structure is released upon pressure decrease. Re-intrusion experiments on the same samples give similar results, suggesting no significant modification of the porous structure upon pressurizing and depressurizing.
3.3. N2 physisorption N2 adsorption m e a s u r e m e n t s at 77 K on both the A1203 and Novacarb samples show a type IV isotherm with a type A hysteresis loop, indicative of the presence of mesopores with cylindrical pore geometry (Figs. 5a and 5b) [1]. The almost vertical capillary condensation step indicates a relatively narrow distribution of mesopores. Application of the Broekhoff-de Boer (BdB) model leads to a n a r r o w PSD centered around 8 and 15 nm for A1203 and carbon, respectively. 450
350
J
(a)
300
a..
n
I-- 250 0o
300
V
03 200
03
I
`4 E
co
E ~ 150 -8
0
.8 150
~'~ lOO 50 0
0.0
0.2
0.4
0.6
p.po -1/-
0.8
1.0
0
0.0
0.2
0.4
0.6
p.po -1 I-
Fig. 5. N2 physisorption isotherm at 77 K on (a) A1203 and (b) Novacarb. Open symbols: adsorption isotherm, solid symbols: desorption isotherm.
0.8
1.0
97
3.4. Comparison of t e c h n i q u e s and evaluation Fig. 6a shows the good correlation between the PSD derived from MIP and N2 adsorption on A1203. No incorporation of the m e a s u r e d contact angle has to be implemented, since the contact angle of this sample is very close to the oftenp r e s u m e d 140 ~ However, the carbon sample (Fig. 6b) initially shows a discrepancy between the PSDs of both techniques, where MIP results predict a ca. 15% smaller pore size t h a n the N2 adsorption results. Incorporation of the appropriate contact angle for this sample (8= 150 ~ shifts the PSD towards the N2 adsorption results and both PSDs show an excellent agreement.
(b)
(a)
r
0
5
10 dpore /
15 nm
20
5
10
.
15
dpore /
.
.
.
.
20
_-,
25
nm
Fig. 6. Comparison of the pore size distribution on (a) A1203 and (b) Novacarb: (A) MIP, with 8 = 140 ~ (o) MIP, with 8 = 150 ~ (m) N2 physisorption, using BdB model. For cement-like m a t e r i a l s an overestimation of the pore size is expected, since the contact angle of _< 130 ~ is much lower t h a n the s t a n d a r d value of 140 ~ However, the availability of cement-based materials with an ordered pore structure in the appropriate pore size range is limited and as a consequence these results cannot be discussed here. The presence of pore network effects and interconnectivity can of course strongly influence the reliability of the data obtained via both techniques. Therefore development of new models describing these effects combined with the m e a s u r e m e n t of the contact angle will strongly contribute to assess the PSD from MIP m e a s u r e m e n t s . Additionally one should realize t h a t pore geometry is an i m p o r t a n t a s s u m p t i o n in the underlying models used in both techniques. A pore geometry different from the p r e s u m e d cylindrical shape (in W a s h b u r n equation, eq. (1)) would result in severe deviations in the calculated pore diameter. From this point of view, model materials like MCM-41, and SBA-15, showing a hexagonal a r r a n g e m e n t of almost cylindrical pores, should be interesting to evaluate MIP results. However, upon pressurizing the porous structure collapse, as can be derived from a relatively flattened intrusion curve and a relatively broad PSD (not shown here).
98 The extrusion curve for these materials hardly shows any release of mercury and reintrusion experiments exhibit a negligible intrusion, indicating the original hexagonal porous structure is damaged and as a consequence these materials are inappropriate as model materials in MIP.
4. C O N C L U S I O N S Incorporation of the measured contact angle in mercury intrusion porosimetry data is essential for an accurate determination of the pore size distribution. Both the advancing and static angle methods are suitable to carry out this measurement, leading to very similar results. For most oxidic materials and supported oxides, the contact angle is -140 ~ and incorporation of the actual contact angle is less critical in the pore size determination. However, important deviations are observed in carbon and cement-like materials, with contact angles of__ 150 ~ and __ 130 ~ respectively. This has been shown by comparison of the pore size distribution obtained from mercury porosimetry and N2 adsorption measurements.
Acknowledgements The authors t h a n k MAST Carbon Ltd. for providing the Novacarb mesoporous carbon. V. Butselaar and R. Pavan are gratefully acknowledged for performing the SEM and contact angle measurements, respectively. S. Brouwer is t h a n k e d for fruitful discussions.
REFERENCES
[1]
[2] [3]
[4] [5] [6] [7] is] [9]
S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2 nd ed., Academic Press, London, 1982. J. van Brakel, S. Mod:~ and M. Svatfi, Powder Technol., 29 (1981) 1. J.C. Groen, M.C. Doorn and L.A.A. Peffer, in D.D. Do (Ed.), Adsorption Science and Technology, World Scientific, Singapore, 2000, p. 229. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. P.I. Ravikovitch, G.L. Haller and A.V. Neimark, Adv. Colloid Interface Sci., 76-77 (1998) 203. K.L. Murray, N.A. Seaton and M.A. Day, Langmuir 15 (1999) 8155. S.P. Rigby, J. Colloid Interface. Sci., 224 (2000) 382. A.W. N e u m a n n and R.J. Good, Surface and Colloid Science, Vol. 11, Plenum Press, New York, 1979, chapter 2. S. Lowell and J.E. Shields, Powder Surface Area and Porosity, 3 ~d ed., C h a p m a n and Hall, London, 1991, p. 225.
[10] http://www.erma.co.jp/ENG/pageO2e/O204e/O204e.htm. [11] J.C. Groen, J. P~rez-Ramirez and L.A.A. Peffer, Chem. Lett., 2002, 94.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
99
A Two-Stage Horvath-Kawazoe Adsorption Model for Pore Size Distribution Analysis Robert J. Dombrowski and Christian M. Lastoskie* Department of Chemical Engineering Michigan State University East Lansing, MI 48824-1226 USA The Horvath-Kawazoe (HK) method is capable of generating model isotherms more efficiently than either molecular simulation (MS) or density functional theory (DFT) to characterize the pore size distribution (PSD) of microporous solids. A two-stage HK method is introduced that accounts for monolayer adsorption in mesopores prior to capillary condensation. PSD analysis results from the original and two-stage HK models are evaluated. 1. INTRODUCTION Porosity and pore structure are properties that control diffusive transport, selective reaction, and sorption-based separations of gases in adsorbents and catalysts [1,2]. Sorption porosimetry may be used to characterize the porosity of both mesoporous and microporous solids. The pore size distribution F(H) is obtained from the experimental isotherm/-(P) F(P) = .~ F(P, H)F(H)dH
(1)
using an assumed thermodynamic pore filling model F(P,H) that relates the specific excess adsorption at bulk pressure P in pores of physical pore width H. The PSD obtained depends on the choice of the thermodynamic model used to solve equation (1). Pore filling models based upon the Kelvin equation are not generally applicable for adsorbents with micropores or small mesopores, where continuum thermodynamic models do not accurately represent the properties of the confined fluid [1,3,4]. For example, the Barrett-Joyner-Hallenda (BJH) method becomes inaccurate for pores smaller than 20 nm, while the Derjaguin-Broekhoff-de Boer (DBdB) method yields significant errors for pores smaller than 7 nm [5]. The HK method yields an analytic pore filling correlation calculated semi-empirially from an estimated mean free energy change of adsorption that accounts for adsorbateadsorbent interactions. In the original HK approach [6], the pores of the adsorbent are assumed to be either completely filled or completely empty, depending on whether a pore is above or below its respective filling pressure. The HK model also assumes that filling occurs in a single step and that no pore wall wetting or compression of the condensed phase occurs as the bulk pressure increases. Model isotherms constructed per the original HK method do not exhibit wetting or film growth, features observed in DFT- and MS-derived isotherms. In spite of such limitations, it has been shown that the HK method reproduces the DFT pore filling pressure correlation with surprising accuracy when the HK and DFT models are compared using the same gas-solid potential and the same dispersion parameter values [7]. Various modifications of the original HK method have been proposed. These involve corrections to either the shape of the pore [7,8,9] or the method of calculating the mean *Author to whom correspondence should be addressed. Current address: Department of Civil & Environmental Engineering, Universityof Michigan, Ann Arbor, M148109-2125 USA. Email: [email protected].
100 adsorption free energy change [3,6,10]. Other analytic adsorption models have been suggested that derive a relationship between the pore filling pressure and the Gibbs energy of adsorption [11,12]. As with the variations of the HK method, however, these adsorption models do not account for the multiple filling events that occur in mesopores. To improve the realism and the range of applicability of the HK method, it would be useful to modify the original HK model so that the significant effect of pore wall wetting is accounted for in some manner. A new adsorption model based upon this concept, referred to as the two-stage HK pore filling model, is presented in this paper. The two-stage HK model is described in Section 2.2, and a comparison of the PSD predictions of this model with results obtained for the predecessor "single-stage" HK method are presented in Section 3. Original DFT calculations are reported in Section 3 for comparison with the two-stage HK model results. 2. DESCRIPTION OF TWO-STAGE HORVATH-KAWAZOE METHOD 2.1. Review of the Original HK Model
The premise of the HK method is that the pressure at which pore filling occurs may be obtained from the mean free energy change of the adsorbate molecule as it is transferred from the bulk gas phase to the adsorbed phase, through the equation kTIn(PHK leo)= ~,K (H) (2) where r H~H) is the mean heat of adsorption, k is the Boltzmann constant, T is the temperature, PHX is the condensation pressure, and P0 is the bulk saturation pressure. In the original HK method, the mean free energy change for adsorption in a slit pore is calculated as
e(H,z)dz -
~HK (H) =
"~'~f
(H,z)dz =
~'sf
(3)
f-o'sf dz
H - 2O'sf
where z is the spatial coordinate across the width of a pore in which the surface layer nuclei of the walls are located at z=0 and z=H; and ~sf is the arithmetic mean of the adsorbent and adsorbate diameters. The gas-solid potential ~ selected for the original HK model is a 10-4 potential for an adsorbate molecule between two infinite parallel planes of adsorbent atoms
~(H,z)=INsA~+NfAf/II(-~)I~ 2d 4
(-~L)4 -
( d~ )1~ / d~ + H-z - H-z
)41
(4)
In equation (4), N is the number density of atoms per unit surface area; A is the dispersion constant; the subscripts s and f refer to the adsorbent and adsorbate, respectively; and do 0.85~f is the z-coordinate at which the 10-4 potential for a single planar surface passes through its zero-point value. The 10-4 potential is obtained by integration of the LennardJones 12-6 potential over an infinite planar surface. The dispersion constants AsS and Aff represent the adsorbate-adsorbent and adsorbate-adsorbate interactions, respectively; these coefficients are calculated from the Kirkwood-Muller equations in the original HK paper [6]. Combining equations (2-4) yields an equation that relates filling pressure to pore width:
In(PHKI ~[ - f ~ I N {Hs A ~ ( +i N . ) f| A y ] I I ( d ~ 1 7 6 ~, Po J = L k Td3 __ - 2 o'~., , JL-9 ~ ~, j - -3 k crss )
do
/ 9+31/H--o'sf do 131
(5)
H - osr
In the original HK model, the density of the condensed phase in a pore is set equal to the liquid adsorbate density ,ol if the bulk gas pressure is greater than the filling pressure given by
101
equation (2). Otherwise, the pore is assumed to be empty. The model isotherms/-(P,H) generated in the original HK method can thus be represented as Heaviside step functions /-(P,H) = pt O[P- P/-/r(H)] (6) where O[x] - 0 for x < 0, and | - 1 for x > 0. Gas uptake at bulk pressure P in the experimental isotherm can be uniquely assigned to condensation in pores of width H as given by equation (5). The HK pore filling correlation of equation (2) can therefore be used to expediently determine the PSD of an adsorbent, by substituting equation (6) into equation (1). Several modifications of the original HK method have been suggested. These involve the use of different potential parameters [7] or alternative methods of calculating the mean heat of adsorption [3,7] so as to bring the HK pore filling correlation into closer agreement with MS and DFT results. In the original HK method, the adsorbate-adsorbate interaction energy is calculated in a physically inconsistent manner. However, for nitrogen adsorption on a planar carbon surface, adsorbate-adsorbate interactions contribute modestly (approximately 7%) to the total adsorption energy. It is therefore reasonable to assign the free energy change on adsorption to that arising solely from the adsorbate-adsorbent interaction potential. Improved HK pore filling correlations have been obtained using this approach [7,10]. 2.2. Description of the Two-Stage HK Pore Filling Model One of the principal shortcomings of the original HK method is that it is unable to portray the full sequence of filling events that occur in mesopores. The original HK method does not generate Type IV isotherms for mesoporous adsorbents, in which pore wall wetting precedes capillary condensation. A two-stage HK method is now introduced that incorporates two steps in the model isotherm, rather than just a single step as in the original method. The first step corresponds to formation of a monolayer, and the second step represents capillary condensation. In this new model, the basic methodology of the original HK model is retained, with the addition of new features that describe wetting of a monolayer. The twostage HK model is generalized so that it applies for any adsorbate/adsorbent pair at any subcritical temperature. The adsorbate-adsorbate and adsorbate-adsorbent interactions are modeled using Lennard-Jones potentials with respective well-depth parameters eft andesf. The 2 - - - I ~ ----
k~ c
.R Q. L.
0 "0
<
u) 4) o
X LU
3
5
1
Relative P r e s s u r e P I P o
Figure 1" Schematic of two-stage HK pore filling model showing the principal features of the model mesopore isotherm: (1) capillary condensation pressure; (2) condensed fluid interval; (3) film wetting pressure; (4) empty pore interval; and (5) monolayer interval.
102
adsorbate and adsorbent molecular diameters are assumed identical, so that o-p~,=~pcr. The two-stage HK model isotherms are of the general form illustrated in Figure 1. The transition pressures and adsorbed fluid densities are calculated using the following procedure. (1). The pressure Pc at which capillary condensation occurs in the two-stage HK model is calculated in the same manner as in the original HK method:
~/2~(H,z)dz ~/2(p(H,z)dz
kTln(Pc/PO)=~c(H ) = "-'0
~
t
/2 dz
= "-'0
(7)
H/2 - d o
where the gas-solid potential (I~(H,z) is symmetric about the pore centerline at H/2 and includes potential contributions from both pore walls. Equation (7) is the same as that given by equations (2) and (3), except that the lower integration endpoint has been changed to do, the distance from the surface at which the gas-solid potential equals zero (d0-0.85o" for the 10-4 and 10-4-3 potentials). The implicit assumption is that the accessible pore volume is the region where the gas-solid potential is negative and adsorption is exothermic. In this paper, results are reported for the 10-4-3 potential between two infinite graphitic carbon slabs [ 13]: ~ ( H , z) = 2xo%fp,
9
"
--
+
0"3
O" H-z
-
-
Cr
H'z
-
3(z + 0.61o') 3
(8)
1
3(H - z + 0.61o') 3 The adsorbent density ,o~*=,o~o) is set equal to the density of graphite, for which ,o~*=5.2. (2). For bulk gas pressure P>P~, the adsorbed fluid density/-(P,H) is assumed to be equal to a constant value Pl calculated from the temperature T*=kT/~y and the gas-solid potential well depth E*= e~/'e~as pf(T*,E*) = PO0 - co ln(T*))* (1 + c 1 ln(E*)) (9) where ,oo=0.834, c0-0.600 and C1=0.0375. This correlation was fitted to DFT results of a Lennard-Jones fluid adsorbed in slit pores of width H=20cr [7]. The condensate density is a strong function of temperature and a weak function of the gas-solid well depth. (3). The pressure P,, at which the monolayer film adsorbs is given as
~~' +cr~(H,z)dz kVln(Pm/Po)=~m(H)=
~o' + ~ ( H , z ) d z =
o~+~ dz
(10)
o"
Equation (10) is applicable to slits of width greater than H,,=2(d0+cr); for the potential used in this work, Hm=3.70cr. Thus for pore widths H>Hm, the model isotherm has two filling transitions, one for monolayer wetting and one for capillary condensation, and an isotherm is obtained similar to that shown in Figure 1. For pore widths H
103
limiting value that corresponds to the wetting pressure on a planar nonporous surface. Equation (10) is therefore both mathematically well-defined and physically consistent with the wetting behavior observed in DFT model calculations, as shown in Figure 3. (4). For bulk gas pressure P
A series of model isotherms were generated for nitrogen adsorption in carbon slit pores at 77 K using the original HK, two-stage HK and DFT methods for pore widths between 7 and 70 A. The well-depth potential parameters c~k---93.98 K and 6s/k=-53.22 K are selected from fitted DFT adsorption model results [4]. Figure 2 illustrates that the two-stage HK method is capable of modeling both single-step filling in micropores and two-step adsorption in mesopores with reasonable accuracy. As the pore size increases, the monolayer filling pressure in the two-stage HK model correctly approaches an asymptotic limit, as shown in 40 E o 35
1.E+00
-,,,. ,,,.,.,
0
o
eL 1.E-02
E 30 E k, 25
Qo
-~ 1.E-04
c
W ffl
.2 20
.= a. 1.N-06
o 15 < ffl
Q o x
w
01 c
10
, m
~.
5 0
1.E-08
1 .E-10
0.000001
0.0001
0.01
Relative Pressure
P/Po
Figure 2: Comparison of isotherms generated by the two-stage HK method (solid lines) and DFT (diamonds) for nitrogen adsorption at 77 K in carbon slit pores of width (from letI to right) 10, 15 and 67 A.
5
10
15
20
25
30
Pore Width H (A)
Figure 3: Comparison of predicted transition pressures for capillary condensation (upper lines) and monolayer wetting (lower lines) obtained from the two-stage HK method (open triangles) and DFT (filled triangles) for nitrogen adsorption at 77 K in carbon slit pores. The capillary condensation pressure given by the original HK method is also shown (diamonds).
104 Figure 3. Also in accordance with expectations, the contribution of the monolayer film to the total adsorption within the pore decreases in larger pores, as evidenced by Figure 2. Figure 3 shows that the original HK model overestimates the filling pressures of pores smaller than 25 ,~. The condensation pressure of the two-stage HK method, by contrast, is in good agreement with DFT for pores smaller than 15 A, although the two-stage model does underestimate the filling pressures of larger pores. The condensation pressure in the twostage method is calculated using the same equation as in the original HK approach, except that the 10-4-3 potential, equation (8), is used instead of the 10-4 potential. In addition, the well-depth parameters of the two-stage and single-stage HK methods are different, since it has been established that the nitrogen-carbon well depth is too small in the original HK model [7]. The two-stage HK method predicts monolayer formation at pressures higher than that predicted by DFT (see Figure 3), and this can cause errors as noted later. The shapes of the mesopore isotherms (Figure 2) generated by the two-stage model are nonetheless closer in appearance to the DFT isotherms than those generated by the original HK method. A nitrogen isotherm measured at 77 K on a sample of granular activated carbon (GAC) is shown in Figure 4. The GAC isotherm was analyzed with the original HK method, twostage HK model and DFT method [4] using a least-squares algorithm to find the PSD for each model that best fits the isotherm. The PSD for each model was fitted to a trimodal gamma distribution. An excellent fit to the experimental isotherm is obtained for all three models. The PSDs that correspond to the fitted isotherms of Figure 4 are shown in Figure 5 for each model. All three models yield a PSD that is principally microporous, with some mesoporosity in the 20 to 30 A range. The pore size maxima computed for the original HK method and two-stage HK model are nearly identical. Both HK models slightly underestimate the PSD maximum relative to DFT. However, the specific pore volume obtained by integrating the area under the PSD curve is closer to the DFT result for the two-stage HK method than for the original HK method. Whether a single filling stage or two filling stages are used, the HK method characteristically yields more structured PSDs than the DFT approach, as can be seen ..90
F
~
1.2
m 80 O
A
E 70 E u
L, 6O e~
U.
.2 5O
E
0 w
~
Ib,,
o O.
lO
0.000001
0.0001
9 0.6
>o 0.4
-o 3o < w 20 Io oX
0.8
0.01
Relative Pressure PIPo
Figure 4" Theoretical model isotherms for the original HK method (diamonds), the two-stage HK method (triangles), and DFT (squares) fitted to the experimental isotherm (circles) for nitrogen adsorption at 77 K on a granular activated carbon.
0.2
0
10
20
30
40
Pore Width H (A)
Figure 5" Comparison of PSDs obtained from the original HK method (diamonds), the two-stage HK method (triangles), and DFT (squares) for the granular activated carbon isotherm shown in Figure 4.
105
in the bimodal nature of the PSDs obtained from application of the HK methods versus the unimodal distributions returned from DFT analysis. A fictitious isotherm is shown in Figure 6 for a material composed entirely of mesopores, with a PSD as given by the curve marked with squares in Figure 7. The DFT isotherm in Figure 6 was generated from DFT model isotherms for nitrogen adsorption in carbon slit pores at 77 K [4], weighted with respect to the PSD shown in Figure 7. The DFT isotherm was then analyzed using the original and two-stage HK methods to determine the extent to which the preselected pore size distribution could be recovered. As seen in Figure 7, both HK methods have difficulty reproducing the original PSD used to generate the DFT isotherm. In the case of the original HK method, which does not account for pore wall wetting in mesopores, uptake that occurs at relative pressures near P/P0=I 0 -4 is erroneously attributed to adsorption in micropores. Consequently, the original HK method represents the mesopore region of the PSD reasonably well, but it generates a fictitious peak at 11 A. The PSD recovered using the two-stage HK model likewise does not resemble the original PSD of Figure 7. In the two-stage HK method, pore wall wetting is explicitly incorporated into the mesopore isotherm. However, the two-stage model presented in this paper predicts that mesopore wetting occurs at pressures near P/P0=I 0-3 (see Figure 3); this is too large relative to the actual DFT wetting pressure. As a result, the two-stage HK method gives a poor fit to the isotherm of Figure 6 in the region around P/P0=10 -3. To compensate for the misplaced wetting transition, the PSD obtained from the two-stage HK method includes a peak in the micropore size distribution to account for uptake that occurs below P/P0=10 3. As in the case of the original HK model, this peak is fictitious and arises from an incorrect representation of filling events that occur in the low pressure region of the isotherm (i.e. relative pressures near P/P0=I 0-4). The mesopore peak in the PSD for the two-stage HK model is also shifted relative to the original PSD on account of the difficulties encountered in 0.40
70 eo
m 0.30 E
E 50
E
} 3o
0.20
o
~ 2o o
lO . . . . . . . . . . . . . . . .
0.000001
0.0001
0.00 0
0.01
Relative Pressure
0.10
PIP o
Figure 6: Theoretical model isotherms for the original HK method (triangles) and the two-stage HK method (diamonds) fitted to a fictitious mesoporous carbon isotherm generated from the DFT model (squares) for nitrogen adsorption at 77 K.
10
20
30
40
50
Pore Size H (A)
Figure 7: Comparison of PSDs obtained from the original HK method (triangles) and the two-stage HK method (diamonds) for the fictitious mesoporous carbon isotherm shown in Figure 6. The original PSD used to generate the DFT isotherm is also shown (squares).
106 fitting the isotherm at P/P0= 103. In order to correct these inaccuracies, the procedure used to calculate the monolayer wetting pressure in the two-stage HK method must be further modified so as to yield wetting transitions at relative pressures closer to those observed for DFT; e.g. at P/P0=I 0-4 or thereabouts for nitrogen adsorption on carbon surfaces at 77 K. 4. CONCLUSION The original HK method overestimates the filling pressures of micropores and therefore underestimates the sizes of pores that are present in micro- and mesoporous solids. By altering the potential function used in the HK method and by adding a monolayer filling step to the adsorption model, a two-stage HK pore filling model has been introduced that attempts to address these deficiencies, yielding a more robust adsorption model that still retains an analytic form that is suitable for rapid PSD analysis using a simple spreadsheet program. For slit-shaped micropores less than 12 A in width, this two-stage HK model reduces to the single-step HK adsorption model reported in a recent publication [7]. Thus, the two-stage model can be applied to modeling the PSDs of adsorbents that are exclusively microporous, exclusively mesoporous, or possess a combination of micro- and mesoporosity. In tests carried out using the original and two-stage HK models, it was found that both HK methods tend to overstate the population of micropores in the adsorbent PSD. This results fi'om overestimation of the condensation pressure in the original HK method, and overestimation of the monolayer filling pressure in the two-stage HK model. In some cases, the original HK method gives a better fit to the adsorption isotherm than the two-stage HK method. This is because the original HK model, with its single filling transition and monotonic pore filling correlation, is mathematically unconstrained in comparison to the twostage HK model, which has two filling transitions and may therefore encounter difficulties in simultaneously fitting the low- and high-pressure regions of the adsorbent isotherm. In spite of such difficulties, the two-stage HK model is conceptually sounder in its basis than the original HK method, and it was determined that the two-stage HK approach yields a more accurate value for the specific pore volume than the original HK model. Additional modifications of the two-stage HK method are under consideration that will improve the correlation for the monolayer wetting transition (i.e. equation 10). These adjustments, when complete, will significantly imp'rove the accuracy of the two-stage HK adsorption model. ACKNOWLEDGEMENTS Support of this research by the National Science Foundation through a CAREER award (CTS-9733086) is gratefully acknowledged. The authors thank the Beckman Coulter Corporation for providing an Omnisorb gas sorption analyzer for adsorption measurements. REFERENCES [1] S. Storck, H. Bretinger and W. Maier, Applied Catalysis A, 174 (1998) 137. [2] M.W. Maddox, S.L. Sowers and K.E. Gubbins, Adsorption, 2 (1996) 23. [3] S. Rege and R.T. Yang, AIChE Journal, 46 (2000) 734. [4] C.M. Lastoskie, K.E. Gubbins and N. Quirke, Langmuir, 9 (1993) 2693. [5] P.I. Ravikovitch and A.V. Neimark, Colloids & Surfaces A, 187 (200 l) 11. [6] G. Horvath and K. Kawazoe, Journal of Chemical Engineering of Japan, 16 (1983) 470. [7] R.J. Dombrowski, D.R. Hyduke and C.M. Lastoskie, Coll. & Surf. A, 187 (2001) 23. [8] A. Satio and H.C. Foley, AIChE Journal, 37 (1991) 429. [9] L.S. Cheng and R.T. Yang, Chemical Engineering Science, 49 (1994) 2599. [ 10] C.M. Lastoskie, Studies in Surface Science and Catalysis, 128 (2000) 475. [ 11] M. Jaroniec, K.P. Gadkaree and J. Choma, Coll. & Surf. A, 118 (1996) 203. [12] R.H. Nilson and S.K. Griffiths, Journal of Chemical Physics, 111 (1999) 4281. [13] W.A.Steele, Surface Science 36 (1973) 317.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. McEnaney, J. Rouquerol and K.K. Unger ~ditors) 9 2002 Elsevier Science B.V. All rights reserved
107
Textural and chemical characterization of NaX zeolite exchanged with Zn(ll) ions J. Silvestre-Albero, A. Sep61veda-Escribano and F. Rodriguez-Reinoso Departamento de Quimica Inorg~ica, Universidad de Alicante, Aptdo.99, E-03080, Alicante, Spain A series of Zn(II)-containing NaX zeolites has been prepared by ion exchange with different exchange degrees. The samples have been characterized by several techniques: N2 adsorption at 77K, scanning electronic microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and immersion calorimetry. XRD analysis shows a decrease in the crystallinity with the Zn(II) exchange degree, which is associated to an enhanced roughness of the external surface of the zeolite crystals, as observed by SEM. The enthalpy of immersion into hydrocarbons and chlorinated molecules decreases as the amount of Zn(II) in the zeolite increases, and this can be attributed to some endothermic reaction processes associated with the immersion experiment. Calorimetric measurements into water and n-butylamine allows for the estimation of the hydrophilic and acid-base character of the exchanged zeolites.
1. INTRODUCTION Zeolitic materials have been widely used in the last decades in the chemical and petrochemical industries. This increasing interest on these materials is based in their unique properties" a uniform intra-crystalline mieroporosity that provides access to a large and well-defined surface, the molecular sieve effect, and the electrostatic field centered at zeolite cations. Furthermore, some properties of zeolites can be tailored by changing the nature of the compensating cation located in the inner part of the cavities by means of their ion-exchange capability. In this way, the pore accessibility of some zeolites used in gas separation processes, as well as the adsorbent-adsorbate interactions, can be tailored by the introduction of cations with different size and chemical nature. Similarly, different cations can be used to introduce new chemical properties (acid-base, redox, etc.), which are needed for a given application in catalytic processes. Zinc containing zeolites exhibit potential applications in optics, electronics, sensors and photocatalysis [ 1]. In catalytic applications, the addition of Zn produces an increase in both the catalytic activity and the selectivity towards aromatization of small alkanes in catalytic reforming reactions [2,3]. Zinc species promote dehydrogenation reactions avoiding the preferential removal of hydrogen by transfer to hydrocarbon
108 fragments to form light alkanes. In the same way, the presence of Zn in Pt~aX catalysts has been shown to favor the selective hydrogenation of the carbonyl double bond in (~,]3unsaturated aldehydes [4]. A number of studies have been addressed to define the location and nature of the active species [2,5]. Likewise, the assessment of textural and chemical properties induced by the exchanged cations becomes crucial to the understanding of the applicability of these materials in a given process. A large variety of methods can be used to evaluate these surface properties. Among them, N2 adsorption, IR spectroscopy and adsorption of probe molecules are widely used. Immersion calorimetry is a very useful technique for the surface characterization of solids. It has been widely used with for the characterization of microporous solids, mainly microporous carbons [6]. The heat evolved when a given liquid wets a solid can be used to estimate the surface area available for the liquid molecules. Furthermore, specific interactions between the solid surface and the immersion liquid can also be analyzed. The appropriate selection of the immersion liquid can be used to characterize both the textural and the surface chemical properties of porous solids. Additionally, in the case zeolites, the enthalpy of immersion can also be related to the nature of the zeolite framework structure, the type, valence, chemistry and accessibility of the cation, and the extent of ion exchange. This information can be used, together with that provided by other techniques, to have a more complete knowledge of the textural and chemical properties of these materials. This communication reports the characterization of a series of Zn(II) exchanged NaX zeolites with the help of several techniques, with the aim of gaining some insight into the textural and chemical modifications induced by the presence of exchanged Zn(II) species. 2. EXPERIMENTAL SECTION A series of 13X(Zn) zeolites have been prepared by ion exchange. The raw material was a NaX zeolite (from Aldrich) with a Si/A1 ratio of 1.5, a BET surface area of 508 m2/g and a pore volume of 0.22 cm3/g (N2, 77K). Ion exchange has been carried out, at 333 K, with aqueous solutions of Zn(NO3)2 9 6H20 of the appropriate concentrations to obtain the desired exchange degrees (50, 60, 75, 100 %). After three days under continuous stirring, the zeolites were washed with deionized water and dried overnight at 373 K. Finally, all Zn-exchanged zeolites were heat treated in air at 673 K for 4h. Exchanged zeolites were characterized by N2 adsorption at 77K, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), immersion calorimetry and NH3-temperature programmed desorption (NHa-TPD). X-ray diffraction patterns (XRD) were obtained with a JSO Debye-Flex 2002 system, from Seifert, fitted with a Cu cathode and a Ni filter, using CuK~ radiation (~.-1.5419) and 2~ l of scanning rate. X-ray photoelectron spectroscopy (XPS) spectra were acquired with a VG-Microtech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer and Mg K(~ (1253.6 eV) 300W X-ray source.
109 N2 adsorption isotherms at 77K have been measured with an Autosorb-6 Quantachrome equipment. Previous to adsorption measurements, samples have been outgassed at 523K during 4h to a final pressure of ~10 4 Torr. NHa-TPD spectra were carried out in a U-shaped quartz cell using a 50 cmamin"l helium flow and 200 mg of sample. Before the measurement, the zeolite samples were heat treated in a 50 cm3min-1 helium flow during 3h at 573 K to remove surface impurities. After cooling down to 373 K, a NHa/He (1:1) gas flow was allowed to contact the samples for 5 minutes. After complete removal of weakly adsorbed NH3 at 373 K, NHa-TPD was recorded with a quadrupole mass spectrometer, following the signal at m/e - 16. Immersion calorimetry measurements were carried out with a C80 calorimeter from SETARAM. Before the measurement, the samples were outgassed at 523 K during 4h to a final pressure of 105-10 .6 Torr. Calorimetric experiments were performed at 297.6 K. 3. RESULTS AND DISCUSSION 3.1. Characterization Nitrogen adsorption at 77K has been studied on the exchanged zeolites to assess for the textural changes produced upon the incorporation of Zn. The N2 isotherms are shown in Figure 1. As it can be seen, there is an increase in the amount of N2 adsorbed (mmol-g l ) on zeolite samples with the amount of Zn(II) exchanged. This higher adsorption capacity results into an increase in the equivalent BET surface area with Zn incorporation (SBET: 508 m2/g for NaX zeolite and SBEV: 563 m2/gfor 100%ZnX zeolite). The smaller size of Zn 2+ cation (0.745 A) in comparison with the parent Na + cation (1.02 A), together with the 2:1 stoichiometry of the ion exchange process, favor a higher adsorption capacity in the intra-crystalline zeolite space. However, as the amount of
mmol-g a
-,Oh- l a X 5 0 % Z n
~
9
0
~/'
J
T
,
,
,
0.2
0.4
0.6
0.8
P~o
Figure 1. N2 isotherms for Zn(II) exchanged zeolites at 77K.
110
Zn(II) increases, the N2 isotherms exhibit a more pronounced hysteresis loop, which is typical of mesoporous materials. In order to analyze the origin of the hysteresis loops, the external texture of the samples have been studied by SEM. SEM images of zeolite NaX particles show perfect cubic crystal structures of around 2~tm size. However, as the amount of Zn(II) increases, the external surface of exchanged zeolites exhibits some important changes. SEM images shows that the external surface of the zeolite particles becomes more and more rough as the amount of exchanged Zn(II) increases, roughness being really important for samples with 75 % and 100 % exchange degree. It seems likely that the exchange process of Na + ions by Zn(II) in the NaX zeolite produces the partial destruction of the zeolitic structure, producing an amorphous structure with cavities in the range of mesoporosity which are responsible of the hysteresis loops observed in N2 isotherms. These results correlated with IR studies reported by Wolf et al. on NaA zeolites exchanged with different mono- and divalent cations [7]. These authors found a decrease of the signal intensity, together with a broadening of the chain frequency at 565 cm l for Zn 2+, Sr2+, Ba 2+, Co 2+ and Ni 2+ ion exchanged NaA zeolite, at very high exchange levels, and after calcination at 723 K. Based on these observation, and taking into account that this band is relatively insensitive to the cation form, the authors concluded that a crystallographic degradation of the exchanged zeolites had taken place upon the exchange process. In order to confirm the loss of crystallinity of ZnNaX zeolites upon ion exchange, the samples were characterized by X-ray diffraction. XRD spectra show a reduction on the peak intensity for the main X zeolite peaks, together with a broadening of the diffraction peaks, which can be related to the reduction of crystal size or to a partial destruction on the crystalline structure. X-ray photoelectron spectroscopy (XPS) analysis of NaX zeolite and ZnNaX exchanged zeolites have been carried out with the calcined samples after a vacuum treatment at room temperature to a final pressure of around 10-9 Torr. Table 1 shows the Si/A1 atomic ratio, the Na/Zn atomic ratio and the %Zn for the different samples. Table 1 shows that the Si/A1 atomic ratio obtained by XPS is close to the 1-1.5 range estimated for X zeolites. However, it seems that this ratio increases for Zn exchanges lower than 50% and it decreases thereafter. As the Si/AI ratio of the parent NaX zeolite and the most exchanged zeolite are very similar, the occurrence of some dealumination processes as the origin of the loss of crystallinity can be discarded. Logically, the Na/Zn atomic ratio must decrease as the amount of Zn increases, given that the Na + ions are being replaced by Zn(II),. This exchange produces an increase in the atomic percentage of Zn detected by XPS in the zeolites up to a maximum of 4.54%, which corresponds to an exchange degree of around 90%. XP spectra do not allow to discern the oxidation state of Zn species, due to the proximity of the binding energies corresponding to Zn 2+ and to metallic Zn. For this reason, analysis of Zn LMM Auger transition have been proposed in order to asses for the electronic nature of the Zn species (reference values are kinetic energy = 993eV for metallic Zn and kinetic energy - 988eV for Zn 2+ species from ZnO). Unfortunately, in the case of the Na form of zeolites, the Na KLL Auger transition appears very close to the Zn LMM transition (988 eV for Na KLL and 986 eV for Zn LMM). However, an
111 Table 1 Si/A1 and Na/Zn atomic ratios, atomic %Zn and Zn (LMN), Na (KLL) binding energies for ZnNaX exchanged zeolite Sample NaX ZnNaX (50) ZnNaX (60) ZnNaX (75) ZnNaX (100)
Si/AI 1.37 1.82 1.72 1.56 1.34
Na/Zn ..... 0.4 0.16 0.10 0.08
Atomic % Zn
Zn (LMN)(eV)
2.65 2.96 3.76 4.54
985.8 986.0 986.3 986.5
Na (KLL)(eV) 988.4 988.6 988.9 989.4 989.7
accurate deconvolution of the spectra into two contributions can allow to analyze separately both species. Table 1 reports the kinetic energies ascribed to Na KLL and Zn LMM Auger transitions for all the Zn exchanged zeolites. As it can be seen, the parent NaX zeolite exhibits a single contribution at 988.4 eV, which corresponds to Na + cations present in cation exchange zeolite positions. After Zn(II) addition there is a second contribution at about 986 eV, which corresponds to the presence of Zn(II) species. However, this kinetic energy value is shifted when compared with the energy value attributed to Zn 2§ species from a ZnO particle. These results clearly manifest that Zn species in the ZnNaX exchange zeolites are present in an oxidized state as Zn(II). However, the shift observed in the kinetic energy can be attributed to a different electronic environment for Zn(II) species in the X zeolite. As described above, immersion calorimetry constitutes a powerful technique for the textural and chemical characterization of porous solids. In the absence of specific adsorbate-adsorbent interactions, heats of immersion can be related to the surface area available for the molecules of the liquid. However, the use of polar molecules or molecules with functional groups produces specific adsorbent-adsorbate interactions related to the surface chemical properties of the solid. An adequate selection of the immersion liquid can be used to study hydrophilicity, acid-base character, etc. Table 2 reports the enthalpies of immersion (J/g) into different lineal and branched hydrocarbons (n-hexane, 2-methyl-pentane and 2,2-dimethyl-butane) for Zn exchanged NaX zeolites. Surprisingly, the enthalpy of immersion into the hydrocarbons decreases as the degree of Zn(II) exchange increases. In the absence of specific interactions, the heat of immersion is related to the surface area available and, if the general trend observed with the equivalent SBET obtained from N2 is assumed to be correct, then an increased value with the Zn(II) content would be expected. Furthermore, this decrease in the enthalpy of immersion is more pronounced for highly branched hydrocarbons. It is interesting to point out that, after the calorimetric experiments, there is a slight change on the sample color (it becomes yellow), which is more evident for samples with the highest exchange degrees when using di-branched hydrocarbons. This feature may be due to the fact that the hydrocarbon is suffering some kind of reaction in the zeolite cavities, of endothermic type, during the calorimetric measurement. The reactive behavior of Zn exchanged zeolites in hydrocarbon reforming reactions have been previously described by Iglesia [2] and Ono [3], who found that Zn(II) cations are highly active and selective for aromatization reactions carded out at 773 K. This promoting effect was attributed to the increased rate of hydrogen removal (rate limited step in aromatization reactions), by the
112
Table 2 Enthalpy of immersion, -AHim m (j.g-l), for Zn exchange NaX zeolites into different hydrocarbons and clorinated compounds Liquid \ Sample n-hexane 2-methyl-pentane 2,2-dimethyl-butane
NaX 74.5 77.5 80.4
CH2C12 CHC13
132.3 154.1 196.6
CC14
ZnNaX (50) ZnNaX (60) ZnNaX (75) ZnNaX (100) 67.8 67.8 60.4 55.4 77.8 .... 45.8 ..... 42.6 54.1 53.1 50.7 113.9 156.4 .....
103.8 143.1 151.8
94.3 150.4 87.7
93.3 ..... 68.8
recombinative desorption of H atoms as H2 over Zn(II) cations. Additionally, Ono et al. found that this catalytic activity and selectivity was higher for branched hydrocarbons due to the easier formation of a carbonium cation [3]. Under this assumption, a higher reactivity with 2,2-dimethyl-butane (tertiary carbonium ion) will be expected, a little lower for 2-methyl-butane (secondary carbonium ion) and finally, a lower reactivity for lineal hydrocarbons. This agrees with the calorimetric results. Probably, the heat evolved during the immersion process inside the zeolite channels is high enough to overcome the activation barrier of 53.4 Kcal/mol [8] calculated for the hydrocarbon activation over Znexchanged zeolites. Table 2 also reports the immersion enthalpies of these exchanged zeolites into different chlorinated molecules. For the parent zeolite, NaX, the heat of immersion is related to the number of chlorine functional groups present in the immersion liquid, as corresponds to the high specific interactions exhibited by these groups with the solid surface. As described for hydrocarbons, the presence of Zn(II) produces a decrease in the enthalpies of immersion for all chlorinated molecules, this reduction being higher as the chlorine content increases. These results also correlate with the reactive nature of branched hydrocarbons over the Zn exchanged zeolites. Similarly, there is a color change in the zeolite sample (it become brown), which increases with Zn content. Water has been widely used as a probe molecule for the characterization of zeolites, especially of those with a high aluminium content [9]. Water adsorption on hydrophilic zeolites has been used to measure their pore volume, and it has been shown that the amount of water adsorbed is a linear function of the alttminitun content [10]. Additionally, water adsorption is also highly sensitive to the nature, valence and accessibility of extra-framework cations [11]. Immersion calorimetry allows for the measurement of the degree of interaction between the zeolite and water, and this can be compared with the interaction between the zeolite and other molecules with different polarity. In this way, the polar character of the zeolite surface can be assessed. Table 3 reports the enthalpies of immersion (mJ.m 2) of Zn(II) exchanged zeolites into H20. As it can be seen, these values are higher than those obtained with the other molecules used in this study. The reduced size of the water molecules (2.65 A) allows them to access not only to the supercages of X zeolite as the other molecules, but also to
113 Table 3 Enthalpies of immersion, -AHimm(mJ'm2), for different Zn 2+ exchange NaX zeolites into H20 and n-butylamine. Liquid \ Sample H20 N-butilamine
NaX 704.1 413.1
ZnNaX (50) 610.7 504.1
ZnNaX (60) 605.4 506.5
ZnNaX (75) 531.3 459.6
ZnNaX (100) 519.0 415.4
the sodalite cages, this confirming the restricted accessibility through the sodalite windows reported by Breck et al. [12] for molecules larger than 3 A. The enthalpy of immersion into water decreases with the Zn(II) exchange degree, from 704 mJ.m -2 for the parent zeolite to 519 mJ-m 2 for the zeolite with the highest Zn content. Previous immersion calorimetry studies into water carried out with NaX zeolites exchanged with different mono- and divalent cations have shown a decrease in the enthalpy of immersion with the cation radii [ 11 ]. This correlation is in accordance to the Born ion-solvation theory [13], which postulates the change in enthalpy when an ion is transferred from the vacuum into a solvent as inversely proportional to the ion radius re. In principle, the results reported in Table 3 do not agree with this theory, as the Zn 2+ cation is smaller than Na + and, accordingly, a higher enthalpy of immersion in the presence of Zn(II) would be expected. However, this exceptional behavior exhibited by Zn(II) exchanged zeolites has been previously described by Coughlan et al., and it has been attributed to the formation of some Zn(OH) + species during the ion-exchange process [ 11 ]. Characterization of the acidic properties of zeolites is essential in order to understand the catalytic behavior in acid based reactions. Temperature programmed desorption of ammonia (NH3-TPD) is probably the most frequently used technique to assess the acidic properties of zeolites [2,14]. The use of ammonia as a probe molecule is due to the size of this molecule, which is smaller than that of the zeolite pores. However, the TPD experiments contain complex factors like re-adsorption and diffusion of ammonia in the micropore of the zeolite, which can lead to the misinterpretation of the spectra. A complementary technique to the TPD experiments is the measurement of the enthalpy of immersion of a giving zeolite in a small basic molecule such as n-butylamine. The enthalpies obtained from this interaction can be related to the number and strength of the acid sites present in the zeolite. Table 3 reports the enthalpies of immersion per surface area (mJ-m-2) into n-butylamine for the different Zn(II) exchanged NaX zeolites. The results show a maximum enthalpy value for the immersion for ZnNaX zeolites with an exchange degree of about 60%. Based in the higher Lewis acid character of Zn 2+ ions with respect to Na § ions, an increase in the enthalpy of immersion into n-butylamine with the Zn content would be expected. However, as it has been described above, the exchange process is accompanied by a crystallographic degradation of the zeolite. Previous studies on the acidic properties of zeolites have shown that the acid strength in zeolites is mainly controlled by the crystal structure and it is less affected by the zeolite composition [ 14]. This assumption involves a decrease in the acid character with the loss of crystallinity. Based in these findings, the loss of crystallinity associated with a higher
114 Zn(II) content would be responsible for the decrease in the acid character observed by immersion calorimetry into n-butylamine. This trend in the acid character has also been confirmed by the NH3-TPD experiments. The amount of NH3 desorbed increases from the parent NaX zeolite up to a maximum for the zeolite ZnNaX (60), and decreases thereafter for higher Zn loadings. This correlation between both techniques verifies the applicability of immersion calorimetry as a useful and fast technique for acid strength characterization, mainly if liquids with different basicity are used. 4. CONCLUSIONS The ion exchange process in NaX zeolites with Zn(II) produces several effects that have been characterized by different techniques. The exchange process causes a crystallographic and textural degradation of the zeolite particles, as detected by nitrogen adsorption, XRD and SEM. The BET surface area of the exchanged zeolites increases with the Zn loading, and incipient mesoporous porosity is developed. Linear and branched hydrocarbons and Cl-containing molecules seem to react in the cavities of the exchanged zeolites, yielding coloured products. The reactivity also increases with the amount of zinc, and it has been related to the dehydrogenating properties of the samples. Finally, NH3-TPD and immersion calorimetry into n-butylamine have shown that the acidity of the exchanged zeolites passes through a maximum for an exchange degree of about 60%, and decreases for higher Zn loadings. REFERENCES
[1] V.B. Kazansky, V. Borovkov, A.I. Serykh, R.A. Van Santen and P.J. Stobbelaar, Phys. Chem. Chem. Phys., 1 (1999) 2881. [2] J.A. Biscardi, G.D. Meitzner, and E. Iglesia, E., J. Catal., 179 (1998) 192. [3] Y. Ono and K. Kanae, J. Chem. Soc. Faraday Trans., 87 (1991) 669. [4] J. Silvestre-Albero, F. Coloma, A. Sep61veda-Escribano and F. Rodriguez-Reinoso, Stud. Surf. Sci. Catal., 135 (2001) 29-P- 15. [5] L.M. Kustov and V.B. Kazansky, J. Chem. Soc. Faraday Trans., 87 (1991) 2675. [6] J. Silvestre-Albero, C. G6mez de Salazar, A. Sep61veda-Escribano, and F. RodriguezReinoso, Colloids & Surfaces A, 187-188 (2001) 151. [7] F. Wolf, and H. Fuerting, Tonind. Ztg. Keram. Rundschau, 90 (1966) 310. [8] M.V. Frash and R.A.van Santen, Phys. Chem. Chem. Phys. 2 (2000) 1085. [9] D.H. Olson, W.O. Haag and W.S. Borghard, Micr. Mes. Mater. 35-36 (2000) 435. [ 10] T. Sano, K. Takeda, T. Kasuno, S. Arasaki and Y. Kawakawi, Stud. Surf. Sci. Catal. 105 (1997) 1771. [ 11 ] B. Coughlan and W.M. Carroll, J. Chem. Soc. Faraday I, 72 (1976) 2016. [12] D.W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. [ 13] J.O'M. Bockris and A.K.N. Reddy, Modem Electrochemistry, vol. 1, Plenum Press (1970). [ 14] M. Niwa and N. Katada, N., Catal. Surveysfrom Japan 1 (1997) 215.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouqueroi and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
115
Evaluation and comparison of the pore structure and related properties of particulate and monolithic silicas for liquid phase separation processes K.K. Ungera, B. Bidlingmaier b, C. du Fresne von Hohenesche a, D. Lubda * Institut ftir Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universit~it, Duesbergweg 10-14, D-55128 Mainz, Germany b Steigerwald Arzneimittelwerk GmbH, Havelstrasse 5, D-64295 Darmstadt, Germany c Merck KGaA, LSP R&D MDA, Frankfurter LandstraBe 250, D-64271 Darmstadt, Germany a
Two types of silicas, highly porous beads of 10 ~tm average particle diameter and particle porosity between 48 to 86 % and monolithic silicas with constant mesopore diameter of 12 nrn, constant total porosity of 80 % at variable macropore diameter between 2 and 6 ~tm were subjected to pore structural characterisation and tests in high performance liquid chromatography (HPLC). The column permeability KF of the columns packed with highly porous beads was twice as much as for the monolithic silica columns. The differences in theoretical plate height - linear velocity curves could be explained on the basis of the pore connectivity values nr of the materials which were computed from the nitrogen sorption isotherms using the pore network model of Meyers and Liapis. 1. INTRODUCTION Although adsorbent and column technology in high-performance liquid chromatography (HPLC) have now reached a high standard and reproducibility, thorough attempts are still undertaken to further optimise the performance of liquid phase separation processes. One major focus is directed to the tailored synthesis of adsorbents with control of morphology and pore structural parameters, as these are the most relevant parameters affecting the hydrodynamic properties, mass transfer kinetics and thermodynamic properties of the separation columns. Monolithic columns with cominuous beds based on silica and polymers have been fully exploited [1,2] and provide an alternative to packed particulate columns. The porous adsorbents are usually characterised by nitrogen adsorption and mercury intrusion measurements. The standard parameters calculated from these data, however, give an incomplete description of the pore structure with respect to the application in liquid phase adsorption processes. To minimise pore diffusion the degree of pore connectivity should be as high as possible. There should be a sufficient number of large macropores present which enable an intra-particle flow. The surface of the mesopores should be accessible to allow fast adsorption and desorption of the compounds to be separated. This paper presents a study on selected particulate and monolithic silicas with a high porosity designed for preparative liquid phase separation processes. The aim is to elucidate the decisive pore structural properties of the bulk materials and to correlate these data with those obtained at column operation, i.e. column pressure drop and column performance.
116 2. EXPERIMENTAL SECTION 2.1
Synthesis of porous silica beads
The particulate silicas were prepared by hydrolysis and condensation of a partially condensed tetraethoxysilane (TES40, Wacker Chemie, Burghausen) in a two step procedure described elsewhere [3]. The first step comprised the hydrolysis and condensation of TES40 under acidic conditions to a polyethoxysiloxane (PES) with a molecular mass of approximately 1000 g mol"1. The purified PES was then converted in an emulsion under rigorous stirring to silica hydrogel beads under basic conditions. The graded porosity of the particles was achieved by adding amounts of cyclohexane as a porogen in the emulsion polymerisation step (see Tab 1). The beads were subjected to sedimentation to remove the fines, dried at 150 ~ for 12 h and calcined at 550 ~ for 6 h. The batch was size-classified into a narrow fraction of an average particle diameter of dp = 10 ~tm using a zig-zag siever 100 MZR (Alpine AG, Augsburg). The sized batches were subjected to a treatment with hydrochloric acid (reagent grade) of 17.5 % w/w for 12 h. This procedure served to activate the surface of silica. The material was packed into stainless steel columns (125 x 4 mm) by the well-known slurry technique. 2.2
Monolithic silica columns
Five monolithic columns (100 x 4.6 mm) with a graded macropore diameter between 1.9 and 6.0 ~tm at constant mesopore diameter of 12 nm were obtained as research samples by Merck KGaA, Darmstadt They were synthesised according to a procedure published elsewhere [ 1,2]. The silica monoliths are cladded with polyetheretherketone (PEEK). 2.3
Characterisation
Adsorption and desorption isotherms wore measured at 77.4 K on a Quantachrome Autosorb 6B (Quantachrome Corporation, Boynton Beach, FL, USA) using nitrogen. The samples were out-gassed at 423 K and 1 mPa for 14 h before adsorption measurements. Scanning electron micrographs were obtained using a Zeiss DSM 962 scanning electron microscope (Zeiss, Jena, Germany). Samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. Mercury porosimetry was conducted using a Porosimeter 2000 (Carlo Erba Instruments). The inverse size exclusion chromatographic measurements were performed on an HP 1100 (Agilent Technologies) using polystyrene standards with molecular masses between 200 and 3,000,000gmol 1 and tetrahydrofuran as solvent. Sample concentration was 1 mg mr 1 and injection volume 10 ~tl. The flow rate was kept at 1 ml minl. Detection was performed by UV at 254 nm wavelength. The pore size distribution of the particulate silicas were computed from the calibration plots logarithm of molecular weight vs. the elution volume, using a soitware (Porocheck) supplied by Polymer Standards Service, Mainz, Germany. The pore connectivity values of the mesopores of both types of silicas were calculated from the nitrogen isotherms by soitware supplied by Prof. Liapis, Department of Chemical Engineering, University ofMissouri-Rolla, Rolla, MO, USA [4,5].
117 The column pressure drop as a function of the flow rate was measured on each column packed with the particulate (G1-G - G5-G) and monolithic silicas ( P K l l l - P K l l 8 ) with nheptane/ethylacetate (80/20, v/v). The flow rate was increased by 0.5 ml min l intervals from 0.5 to 4.0 ml min i and the corresponding pressure recorded. The same measurement was performed in the reversed order. The measured pressure was corrected by the pressure drop caused by the connecting tubings. This measurement was done without the column. The column permeability KF was calculated according to equation (1): fv-rI-L K~ = - Ap.n.r 2
(1)
where f~ is the flow rate, r/the viscosity of the eluent, L the column length, Ap the column pressure drop and r the column diameter. The dependency of the theoretical plate height of a retained solute (dibutylphthalate with a retention coefficient k = 5) on the linear velocity of the eluent was assessed on a HPLC equipment with minimum extra-column dead volume. The eluent was n-heptane/ethylacetate of 90/10 (v/v) at the columns with particulate columns and heptane/ethylacetate of 80/20 (v/v) at the monolithic column to achieve the same retention coefficient of 5 for the test solute. 10 ~tl of the test analyte at a concentration of 1 mg ml1 were injected. Detection was performed by UV at 254 nm. The flow rate was stepwise altered between 0.25 and 5.0 ml min-l. Injection was repeated twice. 3. RESULTS AND DISCUSSION 3.1
Pore structural parameters
3.1.1
Highly porous silica beads
The pore structural parameters derived from nitrogen sorption were calculated according to BET (as, specific surface area), Gurvitsch (Vp, specific pore volume) and BJH (Pal, mean pore diameter). The particle size distribution was adjusted via the viscosity of the PES and via the stirring speed at the second synthesis step. The pore size and porosity tuning was conducted via the addition of cyclohexane which merely acts as a porogen. In this study, the viscosity of the PES and the stirring speed were kept constant at all batches prepared in order to investigate the effect of the porogen on the pore structure of the products. The pore structural parameters as a function of the porogen/polyethoxysiloxane ratio are listed in Tab. 1. Fig. 1 shows the pore diameter distribution of the particulate silicas. The mean pore diameter is proportional to the added amount ofporogen in the synthesis, as can also be seen from Tab. 1.
118
porogen to PES ratio ,,. . . . . . (v/v) . .
silica
a, (BET) I mZg1
vp(G)/ml g-i Pd (BJH) pore (particle porosity / %) of mesopores/nm connectivity/IT
0 316 ' 0.42 (48) .... 4.4 6.2 0.27 359 0.79 (64) 8.1 8.0 0.55 363 1.!0 (71) 13.3 17.4 0.82 365 1.99(82) 25.0 >18 1.09 429 2.74 (86) 35.8 > 18 Table 1" Effect of amount of porogen on the pore structural parameters of particulate silicas. Synthesis conditions: 500 ml of PES (viscosity 75 mP s), Cyclohexane: 0, 150, 300, 450 and 600 ml, 3,000 ml of 2-propanol, 250 ml of ammonia, stirring speed 1,500 rpm. G1-G G2-G G3-G ....Ga-G
7-
GIG
6 "7
G2G
.
G5G
o'J
"o r o "~"
4 3
s r'O r
o a
2 1 0 1
'
'
'
'
"
'
|
'
110
'
'
'
'
'
'
'
~
I
100
'
m e a n p o r e d i a m e t e r p~ ( B J H ) / n m
Figure 1" Apparent pore volume distributions of particulate silicas G 1-G to G5-G according to BJH from the nitrogen desorption isotherms. Although the particle porosity of the materials increases from 48 to 86 % (specific pore volume: 0.4 ml g-i to 2.7 ml g-l), the specific surface area remains nearly unchanged within + 50 m2g1. The average pore diameter increases significantly with the amount of porogen added and spans approximately one order of magnitude. The last two silicas G4-G and G5-G exhibit a large portion of macropores which is seen in the pore volume distribution in Fig. 1. It was also seen from mercury intrusion measurements on these samples (results not shown). The value of the pore connectivity nr of the first two silicas (G1-G and G2-G) is similar to those values found for commercial HPLC silicas such as Kromasil (Eka Chemicals, Bohus, Sweden), Purospher and LiChrospher 100 (Merck KGaA, Darmstadt, Germany) which exhibit similar pore structural parameters. The n r values of the three remaining silicas reach values up to 18 which is the maximum according to the underlying pore network model. 3.1.2
Monolithic silicas
The results of the characterisation of the monolithic silicas are summarised in Tab. 2. The investigated materials possess very similar mesopore structural parameters as the particulate silicas in Tab. 1. The as (BET) values are approximately 300 m2g"l, the mesopore volumes are all about 0.9 ml g-l, the average mesopore diameters scatter around 12 nm. The specific macropore volume is slightly increasing from PK111 to PK118 with the exception of PK114.
119 The macropore diameter decreases by a factor of three from PK111 to PK118 (see Fig. 2). Notably, the n r values decrease from PK111 to PK118 with PK114 as an exception. This means that the pore connectivity diminishes with increasing macropore diameter.
pore pd (BJH) Pd (mercury) connectivity Sample (BET) mesopore macropore of mesopores /pm / nm / mZga
vp mesopore (vp macro~ore) /mlg-
as
nT
17.2 0.97(2.67) 1.9 13.0 PKlll 299 10.8 0.96 (2.85) 2.9 12.2 PKll4 314 0.92 (2.69) 15.3 3.6 12.2 PKll5 303 13.0 0.93 (2.73) 4.6 12.2 PK117 306 0.88 (2.89) 10.0 6.0 12.0 PKll8 294 Table 2" Pore structural parameters of monolithic silicas derived from nitrogen mercury intrusion experiments.
"7
E~
4
3
.
.
.
.
.
.
"9
.
.
.
. ,
.
.
P K l 1 8 (6.0 pro)
~
~
.-
~'=--": . . . . . ;.. ~-.:..--.. t , , , . . - ~
E o >
.
------......~..~;..:
~
.......
2
3.64 (81) 3.81 (80) 3.61 (79) 3.66 (78) 3.77 (79) sorption and
-- PK111 (19 pm)
.......... !--?- [-?; ~.T~...... T-7--!--i-? ! !;i .......... ?-!-~-? !-!Si . . . . . . . .
.
vp (total) /mi g-I (porosity / %)
~.-L"
.
.
.
.
.
. n
E
1
' I1""
._L%..'=
'
'
'
'
' ' " I
'
10
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IO0
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'
'
'
'
' " I
'
1000
"~-'
L ' ' " I
10000
'
'
'
'
' ' "
10O000
mean pore diameter Pa/nm
Figure 2- Mercury intrusion measurements of monolithic silicas PK111 to PK118. Characterisation methods based on gas sorption have the drawback of using small tracer molecules like nitrogen or argon to evaluate the surface and pore structural parameters of porous materials. The solutes to be resolved and purified in liquid phase separation processes are much larger in size and volume and might interact with a smaller portion of the BET surface area. The same considerations can be put to the pore size distribution. Thus, we prefer to assess the mesopore size distribution from size exclusion chromatographic measurements and using inverse size exclusion chromatography (ISEC) to assess the distribution curves. Such curves are shown for the monolithic silicas in Fig. 3. The average pore diameter at the maximum of the five curves is approximately between 10 to 12 nm and is slightly shiiting to larger values with increasing macropore diameter.
120
1,5 L
:
:
:
',:
:::
;
:
', ', ' , . ' , i l l
:
:
;
:',
',:',
(1)
E :3 O
~> r O
....... ,~,,,,.,,-,
1,0
. a
:3 .Q L_
.__.
--
p.117(4.6~)
.
i : :~:i:
:,
"o
0~ 0,5
N
,
,
,
,
,
. m
(1) o ca.
i i l ii:~-a;
i
,
,
i
J ?~l???
,
,
i lilill "~!
, i~,.:,|
:
i ? K ilil
, , ,
,
,
:
: ::::':
! ) :?!:"
~;I~,,,:: i::ii
, , ,
,
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,
,
,
,
!
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.
(]1
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: : ~ . .
.
.
:
.
.
.
1
.
.
.
.
.
.
.
.
10
'
:
i
; 1111
......
leo
transverse pore diameter R / nm
Figure 3" Pore size distributions o f monolithic silicas PK111 to PK118 according to I S E C data. 3.1.3
Scanning electron microscopy (SEM)
121
Figure 4: SEM micro graphs of the silica monoliths PK111 (a), PK114 (b), PK115 (c), PK117 (d), PK118 (e) at a constant magnification level of 5,000.
SEM provides a direct image of the macropore size and allows one to estimate the skeleton size of the silicas. It is clearly seen from the SEM magnifications of Fig. 4 a - 4e that with increasing macropore diameter the skeleton size also increases, i.e. the number of macropores per unit of pore volume decreases. 3.2
Chromatographic properties
3.2.1
Hydrodynamic characteristics
It is interesting to compare the flow characteristics of both types of silica columns with respect to the column pressure drop and column permeability. The column pressure drop / flow rate curves of both types indicate a much lower slope of the monolithic columns (not shown) than the particulate columns. In other words, the column pressure drop of the monolithic columns corresponds to that of a column packed with 15 ~tm silica particles [6]. The column permeability values are listed in Table 3. Silica
P a r t i c u l a t e silica
type
Gi-G
G2-G
G3-G
/ 10Kr -14 m z
5.1
4.2
4.7
.
.
.
.
M o n o l i t h i c silica
G4-G G5-G P K l i l 3.0
3.2
2.1
PKll4 PKll5 PKll7 PKll8 2.6
2.6
2.7
2.9
.
Table 3: Column permeability KF of particulate and monolithic silica columns. The KF values for the particulate silica columns indicate a decrease of the permeability with increasing average mesopore diameter, increasing total porosity and increasing pore connectivity at constant average particle diameter of 10 ~tm. The monolithic column show a slight increase of the permeability with increasing macropore diameter at constant total porosity which is to be expected. 3.2.2
Column performance characteristics
The plate height-linear velocity dependencies are depicted in Fig. 5 and Fig. 6. At constant particle diameter the plate height of the particulate silica columns drops with increasing particle porosity and increasing average mesopore diameter and increasing pore connectivity. There is a marked difference between silicas G1-G and G2-G and the group of G3-G, G4-G and G5-G. The latter show similar behaviour: the theoretical plate height corresponds to approximately one particle diameter and only slightly enhances at higher linear velocities.
122
In contrast, the monolithic columns together exhibit a more common feature. The scatter in plate height is less pronounced. However, a clear trend can be recognised: the columns with the average macropore diameter of 1.9, 2.9, 3.6 and 4.6 pm have a similar performance as compared to the column with 6.0 lam macropore diameter which is worse. In general, the slope of the H vs. u curves is twice as higher than those of the columns with silicas G3-G, G4G and G5-G, respectively. ~0-
[
.......... ~ ......... Ga-~3 .
[ 60
I
i
.
.
.
G4-G
GSG
.
[.
...........
/ ...-"
.....
o-
...., "'"
-'-
...........
~_~
PK118 PKl17 PKl15 PKl14 PK111
. /
...,..
,
-
"-
'
411 ...,'""" 9
. ...........
....
............
=:: = ::; =-.= = - : - - = -
~__-.::-.~'~~ 0 0
1
2
3
linear flow velocity u~ m m s
H vs. Figure 5: particulate materials.
I\
.............. ,,"" .......
u
plot
o 0-
4
5
; ....... 0
"1
of
..... .---..--.--"
1
2
3
4
5
l i n e a r f l o w v e l o c i t y u / n T n s -~
the
Figure 6: materials.
H vs. u plot of the monolithic
4. CONCLUSIONS The pore connectivity nr of two types of silica (highly porous beads, monolithic silicas) was calculated according to the pore network model proposed by Meyers and Liapis. Nr was proportional to the particle porosity in the case of highly porous beads. The differences in the pore connectivity for both types of silica were reflected in the mass transfer kinetics in liquid phase separation processes by measuring the theoretical plate height-linear velocity dependencies. In a future study, monolithic silicas possessing different macro- as well as mesopores will be investigated and compared with the presented results. 5. ACKNOWLEDGEMENT The authors thank Prof. A.I. Liapis and B. Grimes for supplying the software for calculating the pore connectivity parameter and for helpful discussions in the interpretation of the results. We thank the Stiflung Innovation, Rheinland-Pfalz for financial support. 6. LITERATURE K. Nakanishi, Journal of Porous Materials, 1997, 4, 67-112. K. Nakanishi, H. Minakuchi, N. Soga and N. Tanaka, Journal of Sol-Gel Science and Technology, 1997, 8, 547-552. K.K. Unger, J. Schick-Kalb, and K.F. Krebs, J. Chromatogr., 1973, 83, 5-9. J.J. Meyers, A.I. Liapis, Journal of Chromatography A, 1998, 827, 197-213. J.J. Meyers, A.I. Liapis, Journal of Chromatography A, 1999, 852, 3-23. F.C. Leinweber, D. Lubda, K. Cabrera and U. Tallarek, Anal. Chem., 2002, 74, 24702477.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. McEnaney, J. Rouquerol and K.K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved
123
Percolation Phenomena in Micropore Adsorption: Influence on Single and Multicomponent Adsorption Equilibria S. Ismadji and S.K. Bhatia Department of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia A novel and simple method for determination of micropore network connectivity of activated carbon using liquid phase adsorption is presented in this paper. The method is applied to three different commercial carbons with eight different liquid phase adsorptives as probes. The effect of the pore network connectivity on the prediction of multicomponent adsorption equilibria was also studied. For this purpose, the Ideal Adsorbed Solution Theory (lAST) was used in conjuction with the modified DR single component isotherm. The results of comparison with experimental data show that incorporation of the connectivity, and consideration of percolation processes associated with the different molecular sizes of the adsorptives in the mixture, can improve the performance of the lAST in predicting multicomponent adsorption equilibria.
I. Introduction Activated carbons are materials having complex porous structures with associated energetic as well as chemical inhomogeneities. Their structural heterogeneity is a result of the existence of micropores, mesopores, and macropores of different sizes and shapes. Characterization of the pore structure of the activated carbons is crucially important to adsorption and separation processes. The latter is generally made in the terms of pore size distribution. Quantitation of the pore size distribution is often performed by employing the generalized adsorption isotherm (GAI) [1] in conjunction with a local isotherm model. Another important characteristic of the structure of porous materials, which governs their reaction and transport properties is the connectivity of the pore network. The most common method of quantifying this is in the terms of a coordination number Z, defined as the number of pores intersecting at a junction or node. The pore network connectivity is usually determined by gas sorption analysis [2-4] or mercury intrusion [5] based on percolation theory. Recently, Ismadji and Bhatia [6] have successfully employed the liquid phase adsorption isotherms to determine the pore network connectivity and the pore size distribution of three commercial activated carbons. In our recent study [7], the pore network connectivity of three commercial activated carbons was characterized using liquid phase adsorption isotherms of eight different compounds. In that study we used ester molecules with complex structure, as probe molecules. The structural heterogeneity of activated carbon is a result of the existence of micropores, mesopores, and macropores of different sizes and shapes, randomly connected in a pore network. In a pore network, some of the pores, that are large enough to accommodate the probe molecules, may be accessible only through smaller pores that only permit the passage of molecules having sufficiently small size. Therefore only a part of the available pores is
124
actually accessible to a given probe molecule. The fraction of actually accessible pores is a function of the network topology, pore network connectivity, and the fraction of available pores. In the adsorption process, especially involving large and complex probe molecules, the pore network connectivity is very important, and governs the transport and reaction properties of the pores. In this paper, we present the influence of incorporation of the pore network connectivity on the prediction of binary liquid phase adsorption of large molecules using the Ideal Adsorbed Solution Theory (lAST).
2. Theory For randomly connected pore networks, some of the supercritical pores (those larger than the molecular size) are connected only through subcritical pores (those smaller than the molecular size) and are therefore not accessible. In a given network, in which not all the pores can accommodate the probe molecules, the number fraction of the pores that are available is
~[f (H) dH (1)
~f (-g ) ag where f(H) is the normalized pore volume distribution function, H is the pore width, and q~ is the fraction of available pores. A slit pore model is assumed, as is now well established for carbons. In randomly connected pore networks the fraction of the pores that are actually accessible to the probe molecule ~ a , is less than the number fraction of the pores that are available. Based on the simple expression obtained by Zhang and Seaton [8] upon fitting simulation results for a cubic lattice (Z=6), subsequently generalized by Lopez-Ramon et al. [3], the actual accessible pore fraction for an arbitrary coordination number Z can be written as follows: 9 '~ = O, 9 < (1.494 / Z)
(2)
q~a [1.314(~Z - 1.494) TM + 3.153(q~Z - 1 . 4 9 4 ) - 3.48(q~Z- 1.494)2 + 1.433(q~Z- 1.494)3]/Z,
(1.494 / Z) < 9 < (2.7 / Z) ~a = ~ , ~ > (2.7 / Z)
(3) (4)
The coordination number Z can be obtained using nonlinear least-square fitting by comparing the theoretical value of V~,cc (i) a
V,,~c= ~
oo
V,o, ~f (H)dH
(5)
de
with actual experimental accessible pore volume, Vfxp, obtained from the DubininRadushkevich characteristic plots. In eq. (5) Vtotis the total specific pore volume.
125 The differences in critical molecular sizes of the different components involved in the multicomponent adsorption of large molecules give additional complexities in the problem of the pore network accessibility. Here we will briefly describe the influence of percolation phenomena and accessibility on binary adsorption. Further details of this are presented elsewhere [9,10]. Here, we consider the binary adsorption of two components, component 1 has a critical molecular size, d~l, and component 2, dc2. The critical molecular size of component 1 is smaller than component 2. Figure 1 depicts the adsorption behaviour of the binary mixture in the pores.
Figure 1. Schematic diagram of binary adsorption process in micropore network. We envisage that there are four different zones of pore space available to the mixture components in activated carbon. 9 Zone I In the pore size range between dc~ and dc2 (dcl
C~al dc2 [qf.~e ( H ) f (H)dH
(6)
126
where qpure is the local single component adsorption isotherm, here given the DR form tIpure = QO -It
max, t
[ (RTln(a;,,_./a,)]/] exp[6, (TO- T)]exp oo Z. [~,.,2(n,, Zmi n
(7)
~,-,~oo (Zmi n
Here QO max, l is the actual maximum capacity at reference temperature To and f a constant parameter representing the temperature coefficient of expansion of the adsorbate. The terms of activities (as./as) account the bulk liquid phase non-ideality. The activity coefficient for each species is estimated through the UNIFAC group contribution model. The characteristic energy term r, [ff2(H', Zmin ) -- ~,~oo (Zmi n )], which includes the repulsive interaction in small pores, was calculated based on 10-4-3 potential model of Steele [11 ]. Details of eq. (7) are available elsewhere [1,7, 12]. Zone II The sizes of the pores in this zone lie between dc2 and oo. Some of these pores are accessible to both compounds, so that a mixture of components 1 and 2 will be present within these latter pores. Zone III The sizes of the pores in this zone are also between dc2 and o0. However, some of the connecting pores have size between dct and de2, and are accessible only to component 1. Zone IV This zone also comprises pores having size between dc2 and o0, but the connecting pores have dimension smaller than dr Consequently, pores in this zone are inaccessible to both compounds. Therefore, the amount of component 1 adsorbed in zones II-IV can be written as
q l b = Oa2 (I)----2dc
r
all)a2
q(ure ( H ) f ( H ) d H
q~,,x(H)f(H)dH + [_.O~----] 1 02 Idc2
(8)
Here q l ~ is determined from the lAST model based on the single component model in eq. (7). The total mount adsorbed of component 1 will be equal to qt~ + qib, leading to
miX
~
ql,tot -_ (~al ~-----~d, q(ure(H)f(H)dH
+ CI)a2S[q'~=(H)-q~e(H)~(H)dH ~2. a.
(9)
and for component 2 the amount adsorbed is given by oo moc
(I)a2
q2,,o, = ~
q2'~x( H ) f ( H l d H de2
(10)
Equations (9) and (10) are valid for any single and binary component isotherm. The ideal adsorbed solution theory of Myers and Prausnitz [13] has been used for developing a multicomponent isotherm model based on the single component isotherm at any pore size. The following result is obtained for the multicomponent isotherm for each component studied in a pore of physical width H (-H'-0.3354)
127
[
[RTln(as"l]] ~, a ~ )JJ (11)
Ol exp[6, (TO- T)]--~ 1 - erf - ~
B
I"I1 --- 1-'I2 =
where B = r, [f~(H', Zmi n ) - - ~ " ~ | a pore,
(2~ni. n )],
n ' is the centre to centre distance between the walls of
a, =a~
(12)
N
~--'x, =1
(13)
i=1
and the total amount adsorbed, qr, can be expressed as qr =
(14) l=l
where qO
=
qpure(a o ),
i.e.
qpureis evaluated at
a, = a ~ 9The amount adsorbed for each species
now can be calculated by
q~,'~ =x, qr
(15)
3. Results and Discussion Commercially available coal based activated carbons Filtrasorb-400, Norit ROW 0.8 and Norit ROX 0.8 were chosen as the adsorbents. The adsorbates used for determination of the pore network connectivity of the carbons were ethyl propionate, ethyl butyrate, ethyl isovalerate, isobutyl acetate, benzaldehyde, hexyl acetate, methyl salicylate and 2-ethyl hexyl acetate. For the evaluation of the pore network connectivity, we used the fitted maximum capacity obtained from isotherm fitting to calculate the accessible pore volume [7]. The pore size distribution used in the present study has the bimodal gamma function form
f ( H ) = b~
w~' +ill a, e-W,i4
+ (1 - b1)
w2a, +lHa2 e-W214
(16) F(a~ + 1) F(a 2 + 1) where F(a, + 1)is the gamma function, while w~ and ag are pore structural parameters and b~ is the fraction of the pore volume associated with the first peak. These parameters were obtained from fitting of the single component isotherms. As mentioned above the pore network connectivity was calculated using the fitted maximum capacity Q~axby application of the following equation QO
= -o-ap , V , o ,
(17) O where Vtotis the total pore volume of the carbon. The fitted parameters in this calculation were Z and Vtot. The mean coordination number Z obtained by this method is 4.03 for Filtrasorb400, 4.84 for Norit ROW 0.8, and 4.89 for Norit ROX 0.8, while the Vtot values are 0.445 cma/g for Filtrasorb-400, 0.408 cma/g for Norit ROW 0.8, and 0.475 cma/g for Norit. These values are slightly lower than those obtained from argon adsorption (0.468 cm3/g for Filtrasorb-400, 0.443 cma/g for Norit ROW 0.8, and 0.467 cm3/g for Norit ROX 0.8). Figure 2 shows the correlation between estimated accessible pore volume and molecular size.
128 To study the influence of the pore network connectivity on multicomponent adsorption equilibria, we used ethyl propionate, ethyl butyrate, and ethyl isovalerate as the adsorbates and Filtrasorb-400 and Norit ROW 0.8 activated carbon as the adsorbents. For predicting binary isotherms we used known parameters for single component adsorption of these compounds as previously determined. The effect of the pore network connectivity was taken into account in 0.4
0.4
0.3
0.3
o~ E o E _= o>
o n
o o
i
0.2
0.1
J z= 4.0aI gamma
0.0 0 48
0.50
0 52
u> 0.1 o o
(a)
9
0 54
0.56
0.2
m
0.58
O0 0.48
0 60
I
- -
9 gamma Z = 4.84
0.50
0 52
(b)
0.54
0 56
0.58
0 60
molecular size, nm
molecular size, nm 0.5
o~ E 0.4 o a; E :3 0 3
o
@
L. ~-
0.2
~, ~ o
0.1
0.0 0 48
9 Z gamma = 4.89 i
0.50
0.52
0.54
0.56
0.58
0.60
molecular size, nm
Figure 2. Correlation between accessible pore volume and fitted molecular size, estimated based on bimodal gamma function fitting. (a) Filtrsorb-400, (b) Norit ROW 0.8, and (c) Norit ROX 0.8. the prediction of the binary adsorption isotherms using the lAST method. Figure 3 depicts the adsorption isotherm of ethyl butyrate in the presence various starting concentrations of ethyl isovalerate on Filtrasorb-400 activated carbon. In this figure, the experimental data are indicated by symbols, while the solid lines represent the predicted values based on single component isotherm fits using bimodal gamma function distribution. No fitting parameters are used in the binary isotherm predictions. In general, the model can represent the experimental data fairly well as seen in Figure 3. The adsorption isotherms for the other binary systems on Filtrasorb-400, and Norit ROW 0.8 are available elsewhere [9]. In order to test the importance of the incorporation of the pore network connectivity concept, the ideal adsorbed solution theory was also applied without considering the connectivity of the pore network. For this purpose, we also used the binary
129
adsorption experimental data of the ethyl butyrate-ethyl isovalerate system. The overall isotherm for each component follows oo
(18)
q,,,ot = q,m,~( H ) f ( H ) d H
300
~ ~
300
(a)
250
-
(b)
250
00~ 150 =
100
m
~
a,
~ 100
9
50 0
~ 0
200
400
60o
, 800
equilibrium c o n c e n ~ o n ,
,
50
o
lOOO
0
2o0
400
600
800
1000
equilibnum c o n c e n k a t ~ n , m ~ L
m~L
3OO t~
250
~
150
~
50 o
(c)
--
0
200
400
600
800
| = z~ 9
Co Co Co Co
El = 0 rng/L El = 150 mglL El = 300 rng/L El = 450 mglL
Model
1000
equilibrium concentration, mg/L
Figure 3. Adsorption isotherms of ethyl butyrate in the presence of ethyl isovalerate on Filtrasorb-400, and model predictions with gamma function distribution. The predicted values obtained from above equation for ethyl butyrate-ethyl isovalerate system is depicted in Figure 4. The predicted values obtained from eq. (18) are somewhat lower than actual adsorbed amounts obtained from experiments. This due to the fact that actually pure component adsorption of ethyl butyrate occurs in those pores not accessible to ethyl isovalerate, an effect that enhances ethyl butyrate adsorption. The error is also higher with the increase of concentration of the larger component (ethyl isovalerate) as expected. On the other hand when percolation effect and accessibility factor was appropriately considered the predictions were more accurate as seen in Figure 3.
4. Conclusions The effect of the pore network connectivity on the prediction of binary adsorption equilibria is studied. The Ideal Adsorbed solution theory (IAST) is used in conjunction with modified DR single component isotherm, and it is found that incorporation of the connectivity
130
can improve the performance of multicomponent isotherm model in predicting experimental binary adsorption equilibria. 300
300 . (a)
250
(b)
E 15 200 Q
.~
200
.Q
o -o
150
" o
100
~
100
50
m
50
m
0.0
200
400
600
800
equilibriume~ncen~ion,
1000
0
m g/L
200
400
600
800
1000
equilibrium c o n e e n ~ a t i o n , mg/L
3~176 l 250
(c)
e = ,, ,~
~oo ~
150
Co Co Co Co
El El El El
= = = =
Model
0 mg/L 150 mglL 300 mglL
450 mglL
100
m
50 0 0
200
400
600
800
1000
equilibrium concentration, mg/L
Figure 4. Adsorption isotherm of ethyl butyrate in the presence of ethyl isovalerate on Filtrasorb-400 activated carbon and predicted values neglecting percolation effects at (a) 303.15 K, (b) 308.15 K, and (c) 313.15 K.
References 1. S. Ismadji and S.K. Bhatia, Langmuir, 17 (2001) 1488. 2. N.A. Seaton, Chem. Eng. Sci., 46 (1991) 1895. 3. M.V. Lopez-Ramon, J. Jagiello, T.J. Bandoz and N.A. Seaton, Langmuir, 13 (1997) 4435. 4. K.L. Murray, N.A. Seaton and M.A. Day, Langmuir, 14 (1998) 4953. 5. R.G. Larson and N.R. Morrow, Powder Technol., 30 (1981) 123. 6. S. Ismadji and S.K. Bhatia, Langmuir, 16 (2000) 9303. 7. S. Ismadji and S.K. Bhatia, J. Colloid Interface Sci., 244 (2001) 319. 8. L. Zhang and N.A. Seaton, Chem. Eng. Sci., 51 (1996) 3257. 9. S. Ismadji and S.K. Bhatia, AIChE J., in press (2002) 10. S. Ismadji and S.K. Bhatia, Applied Surf. Sci., in press (2002) 11. W.A. Steele, Surface Sci., 36 (1973) 317. 12. S. Ismadji and S.K. Bhatia, Carbon, 39 (2001) 1237. 13. A.L. Myers and J.M. Prausnitz, AIChE J., 11 (1965) 121.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
131
Characterisation of the Surface Chemistry of Activated Carbon by Molecular Simulation of Water Adsorption M. Jorge and N. A. Seaton* School of Chemical Engineering, University of Edinburgh King's Buildings, Mayfield Road, Edinburgh, EH9 3JL, UK We propose a model for activated carbon incorporating both structural and chemical heterogeneity. Structural heterogeneity is represented by an array of slit-shaped pores, characterised by a pore size distribution. This distribution was calculated from experimental data on pure-component adsorption of ethane, using Grand Canonical Monte Carlo simulations to describe adsorption at the individual pore level. Chemical heterogeneity is represented in our model by including oxygen-containing surface sites. The results from molecular simulation in the model carbon are compared to the experimental pure-water adsorption isotherm on the same sample. From such a comparison, one is able to draw conclusions regarding the concentration and distribution of surface sites in pores of different width. 1.
INTRODUCTION
Activated carbons are extensively used for adsorption processes in industrial applications. A number of these applications (such as air purification) involves the separation of organic molecules from mixtures containing trace amounts of water. For an efficient design of these processes, it is essential to develop a model for the activated carbon adsorbents that possesses predictive capabilities. Such a model must take into account the fundamental characteristics of the adsorbent that affect the adsorption of the molecular species of interest. Activated carbons are amorphous materials composed of graphite plates stacked together in a more or less irregular way [ 1]. Adsorption occurs mainly in the interstices of the plates. As a consequence of the activation process, chemically heterogeneous groups of several types can be present on the edges of the graphitic plates or on defect sites. The most common types are oxygen-containing groups, such as carboxyl, carbonyl and hydroxyl [2]. Structural heterogeneity has a strong effect on adsorption of all chemical species, while chemical heterogeneity affects mainly the adsorption of polar species. In the case of water/organic mixtures, both types of heterogeneity must be taken into account for the development of a suitable model. The most widely used model of the structure of activated carbon is an array of slitshaped pores, characterised by a pore size distribution (PSD). This approach has been used successfully to model the adsorption of non-polar species [see, e.g. 3-6]. Although the PSD model neglects a number of aspects of the carbon structure, such as network connectivity and surface defects, the more complex models proposed (see e.g. [7,8]) have not yet advanced to the stage where they can quantitatively match adsorption in real solids, even of non-polar compounds. Attempts to include chemical heterogeneity in model carbons have not been so prolific. To our knowledge, the first of these attempts was undertaken by Segarra and Glandt [9]. Their model consisted of a disordered stacking of disk-shaped graphitic platelets. Surface
132 polarity was represented by dipole moments smeared along the edges of the plates. Agreement between the model and experimental data was only qualitative. A later model, by McCallum et al. [ 10], included a representation of the discrete nature of the surface sites. The basis of the model was a PSD, determined by density functional theory and nitrogen adsorption, and surface sites were randomly distributed on the surface of the pores. Water molecules and surface oxygen atoms were represented by simplified molecular models, based on square-well potentials to mimic hydrogen bonding. The total concentration of active sites was determined experimentally by titration methods and the interaction potential between water molecules and surface sites was obtained from a fit to low-pressure water adsorption data. The simulations were performed in only a few pore widths and then interpolated, using the semi-empirical equation of Talu and Meunier [11], to represent the continuous PSD. Although agreement with experiment was reasonable, the potential models ignore the longrange nature of the electrostatic interactions among water molecules, and between the water molecules and polar surface groups. This makes it difficult to extent the model to more general cases (e.g. other temperatures, multicomponent adsorption). The most recent model of activated carbon to include chemical heterogeneity was proposed by Brennan et al. [12]. In this model, the structure of the adsorbent is represented by a stacking of graphitic plates of several sizes, which is optimised to reproduce experimentally determined carbon-carbon radial distribution functions by a procedure termed reverse Monte Carlo simulation [7]. Polar carbonyl sites were placed randomly on the edges of the plates. Although it is a more realistic representation of activated carbon, this model is also extremely complex, which carries disadvantages such as very computationally demanding simulations and difficulty of application to a wide range of adsorbents. No comparison of this model with experiment has yet been presented. In this paper, we present a model for activated carbon that takes into account the characteristics of the adsorbent that affect the adsorption of both polar and non-polar species. The structure of the carbon is represented by a PSD, obtained from the analysis of pure-ethane adsorption, and chemical heterogeneity is included by placing regularly distributed carbonyl sites on the surface of the pores. The single-pore isotherms for water and ethane are calculated by grand canonical Monte Carlo (GCMC) simulation. 2.
GCMC SIMULATION IN SINGLE PORES
This method is particularly suited for adsorption studies, since the temperature (T), volume (V) and chemical potential (/2) are kept constant, while the total number of molecules is allowed to fluctuate. By averaging the number of adsorbed molecules at equilibrium and calculating the pressure (P) from the chemical potential, one can obtain adsorption isotherms in model pores. The Peng-Robinson equation of state [13] was used for the pressure calculations. We have used the general methodology for GCMC simulations [14,15]. For each point of the isotherm the system was allowed to equilibrate for at least 107 steps (each step consisting of a displacement, creation or destruction trial) and the sampling period was divided into 20 blocks of at least 105 steps each. Ethane molecules are represented by two Lennard-Jones (L-J) centres, one for each CH3 group. Geometrical and potential parameters were taken from Cracknell and Nicholson [ 16]. Water is represented by the SPC/E potential. This model is composed of a single L-J site, at a location corresponding to the oxygen atom in the water molecule, and three point charges, two positive ones located at the positions of the hydrogen atoms and a negative charge at the centre of the oxygen atom. Geometric and potential parameters were taken from Berendsen et al. [17]. The pores are slit-shaped, bounded by two parallel walls comprising an infinite
133 number of graphite layers. Each layer is composed of L-J atoms, which are uniformly smeared out to form smooth walls. The potential between an L-J centre in an adsorbate molecule and each of the pore walls is given by the 10-4-3 potential of Steele [ 18]. The polar surface sites are carbonyl groups, represented by the OPLS potential for C=O groups in amino acids [ 19]. These include an L-J centre and a negative point charge for the oxygen atom and a positive point charge on the carbon atom in the basal plane of graphite, as described in a previous publication [20]. The simulation cell is rectangular, bounded in the z direction by the pore walls and replicated in the x and y directions by using periodic boundary conditions. The length of the simulation cell in both directions parallel to the wall was chosen to be 3 nm since this was shown to be sufficient to eliminate any finite-size effects [21]. The calculation of the potential between two point charges requires the use of special techniques, due to its long-ranged character. We have compared several of these techniques for the slab geometry [22] and concluded that the method proposed by Heyes and van Swol [23] provided the best compromise between accuracy and speed of computation. This method performs a summation of the Coulomb potential over a small number of periodic boxes and then includes a correction term for the remaining boxes up to infinity. Cross-species Lennard-Jones parameters were calculated using the Lorentz-Berthelot combining rules.
3.
MODEL FOR ACTIVATED CARBON
The structure of the activated carbon is represented by a distribution of slit-shaped pores. In a PSD model, the total adsorption is given by the adsorption integral equation (ALE)
[4]:
N(P) - IW~ f (w)p(P, w)dw
(1)
where N(P) is the total number of moles adsorbed at a pressure P, W m i n and Wma x are the widths of the smallest and largest pores present, p(P,w) is the molar density of the adsorbent at a pressure P in a pore of width w, and f(w) is the PSD. In order to calculate the PSD of a given adsorbent, Eq. (1) must be solved for f(w), which presents an ill-posed problem. To solve the AIE, we have used the method of Davies et al. [24]. Essentially, the PSD is discretised and a representative result is obtained by incorporating a regularisation factor in the minimisation procedure. The optimum factor is determined by an analysis of the error of the fit. This method has been successful in predicting pure-component and binary adsorption of methane and ethane in several activated carbons from molecular simulation [25]. We have calculated a PSD for BPL carbon based on the experimental ethane adsorption of Russell and LeVan [26] at three different temperatures (273, 298 and 323 K). This set of experimental data was chosen since these workers present adsorption of pure water and pure ethane, as well as their mixtures, in BPL activated carbon over a wide range of temperatures. After the single pore isotherms for ethane were obtained from GCMC simulations, the AIE was solved using a non-linear minimisation routine [27] interfaced through the GAMS mathematical programming package [28]. The distribution with the optimum regularisation factor is shown in Figure 1 and the fit to the experimental data in Figure 2. The calculated PSD fits the experimental data well over the entire pressure range.
134
Figure 1: Pore size distribution of BPL activated carbon determined from the analysis of pure ethane adsorption at 273, 298 and 323 K. 5 4.5 4 =m 3.5 0
I=
E
,>, "~ t,,,,, cl
3
2,5 2 1.5 1 0.5 T
i
I
I
I
I
0
0.2
0.4
0.6
0.8
1
Pressure (bar) Figure 2: Fit to the pure ethane experimental data of Russell and LeVan [26] using the PSD of Figure 1. Diamonds are for data at 273 K, open squares are for 298 K and triangles are for 323 K. After determining the structure of our activated carbon, we have incorporated a degree of chemical heterogeneity in the model by making some assumptions: the sites were all of the same type (carbonyl groups); they were regularly distributed on the surface of the carbon; and their concentration was considered to be the same in every pore, regardless of its width. The first assumption is justified on the basis that both experimental [29] and molecular simulation [20] results suggest that the amount of water adsorbed in activated carbon depends on the
135
concentration of oxygen on the surface, regardless of its functionality. As for the distribution of the polar sites on the surface, we have observed from molecular simulation that the local density of active sites has a strong impact on low-pressure adsorption but almost no effect on capillary condensation [20]. Since most of the experimental water adsorption data was obtained at high pressure, in the region of condensation (see Figure 4), the assumption of a regular distribution of isolated sites will have little effect on the comparison. Finally, the concentration of surface sites was set to be the same in pores of all sizes, in the absence of a priori information to the contrary. These assumptions reduce the process of the characterisation of the chemical heterogeneity to the determination of a single variable - the overall density of carbonyl sites in the carbon. This concentration was estimated by analysing the experimental pure water adsorption isotherm at 298 K as follows. First the modal pore width was determined based on the PSD obtained from pure ethane adsorption. Then a series of GCMC simulations in a pore of that width (1.24 nm) for different concentrations of active sites was performed. The chosen concentration was such that the pressure of spontaneous condensation in the single-pore isotherm corresponded roughly to the inflexion point of the experimental isotherm. After this procedure, GCMC isotherms for 13 pore widths containing the chosen concentration of active sites (1.33 sites/nm 2) were calculated. These pore widths span the whole range of the PSD of Figure 1 and retain the characteristics of the distribution. Some of these isotherms are shown in Figure 3.
3o1
O3
25
E "~ 20 ._.e 0
E 15 ~9 10
c
cl
5
0
0.2
0.4
0.6
0.8
1
P/Psat Figure 3: GCMC adsorption isotherms for pure water at 298 K in slit-shaped pores with a concentration of surface sites of 1.33 sites/nm 2 and different widths: filled squares - 0.68 nm; open diamonds -0.76 nm; crosses -0.84 nm; open triangles - 1.04 nm; filled circles - 1.24 nm; open squares - 1.44 nm; filled triangles - 2.04 nm. The pressure is reduced by the saturation pressure at the same temperature (0.03167 bar). At low pressure, there is hardly any water adsorbed in the pores. At a certain characteristic pressure (specific to each pore width), there is a step-like increase in the amount adsorbed, until the pores are completely filled. All the isotherms, down to the smallest pore width, exhibit this discontinuous transition, which is evidence of capillary condensation. It is
136
worth noting that the pressure of spontaneous condensation in a GCMC simulation does not represent the equilibrium vapour-liquid transition pressure in the pore. Rather, during adsorption calculations, the vapour-like phase remains metastable up to pressures well above equilibrium [20] (the inverse is true during desorption runs). The fact that GCMC isotherms are used in our calculations is justified by the fact that metastability is also expected to occur in porous solids during adsorption experiments (see e.g. [30,31]). Even though the two types of metastability (simulated and experimental) are distinct, and represent different processes [31,32], recent studies in well characterised porous materials [33] show good agreement between simulation and experiment and suggest that the experimental adsorption branch is closer to the point of spontaneous condensation in GCMC simulation than to the equilibrium transition. It is worth noting that we have recently calculated the equilibrium phase transition pressure of water in both graphitic [21] and activated [20] carbon pores by molecular simulation using the gauge-cell method [34]. Our results confirm the occurrence of capillary condensation even in very small pores, as well as the presence of metastable vapour and liquid phases during GCMC adsorption and desorption runs, respectively. Integration of the single-pore isotherms over the PSD produced the total adsorption isotherm for the model carbon, which can be compared with the experimental data. This comparison is shown in Figure 4.
2520-
A
o'/
0
l::: 1 5 l::
.,.1
~9 1 O -
J
r.
I::l
~
5-
0
J
4," I
I
1
I
I
0.2
0.4
0.6
0.8
1
P/Psat
Figure 4: Comparison between the experimental data of Russell and LeVan [26] for pure water at 298 K (diamonds) and the isotherms obtained from the model carbon at two different concentrations of carbonyl sites: 1.333 sites/nm2 (solid line) and 0.889 sites/nm2 (dashed line). The dotted line connecting the experimental data points is a guide to the eye. As we can see from Figure 4, the simulated isotherm (solid line) is qualitatively different from the experimental isotherm (dotted line). The bimodal character of the PSD is reflected in the simulated isotherm, producing two "steps". On the other hand, the experimental isotherm has a typical S shape. Furthermore, the simulation results lie almost entirely at lower pressures than the experiment, which suggests that the concentration of surface sites is too high. As an attempt to improve the model, we have performed simulations
137 for a surface site concentration of 0.889 sites/nm 2. The results are shown as the dashed line in Figure 4. The improvement in the agreement with experiment is only slight, as the simulated isotherm still exhibits two steps. Any other concentration of surface sites would shift the isotherm on the pressure axis, but would retain the bimodal character of the isotherm. The choice of a different local distribution of sites would have a significant impact on adsorption at low pressure, but not on the condensation pressure [20]. Thus, we can conclude that our model, in which the concentration of surface sites is the same for pores of all sizes, is incapable of matching the experimental data for water. (The excellent fit of the model to ethane adsorption over a wide range of temperature and pressure demonstrates that the description of structural heterogeneity by a PSD is sound.) It follows that a possible improvement to the model would be to allow the concentration of active sites to differ from pore width to pore width. A model in which the pores of width above 1.24 nm contain a slightly higher amount of active sites and those between 0.84 and 1.24 nm are less activated would result in a closer match with the experimental isotherm. Regardless of this fact, the overall concentration of oxygen.on the surface of the carbon is likely to be of around 0.889 sites/nm 2. By using the PSD and the geometrical definition of the model pores, we are able to calculate a value for the surface area of our model carbon. This value was 925 mZ/g, which compares well with the BET surface area of 1200 mZ/g determined experimentally on the same sample [3] (if one bears in mind that the BET method is likely to overestimate the real specific area of the carbon, since it does not take into account the enhancement of the adsorption energy in small pores). By using this value, one is able to determine the concentration of oxygen per gram of carbon. This value, 1.365 mmol/g, is within the range of 1.14 - 1.40 mmol/g determined experimentally by MacDonald and coworkers using temperature-programmed desorption on BPL carbon [35-37]. 4.
CONCLUSIONS
In this paper we have presented preliminary results of an attempt to model both structural and chemical heterogeneity of activated carbon adsorbents, in order to quantitatively predict water adsorption. We have represented the structure of the carbon by a distribution of smooth slit-shaped pores, with regularly distributed polar sites as a measure of chemical heterogeneity. The concentration of surface oxygen that provides the best agreement with the experimental data is within the range of experimentally determined values. From a comparison between simulation and experiment we are able to draw conclusions with respect to the distribution of surface sites along pores of different width. Thus we have observed that the small micropores in BPL carbon are likely to exhibit a lower concentration of sites than the larger pores. An attempt to improve our model by simulating adsorption in pores with different concentrations of surface sites is underway. The predictive capabilities of the model will be assessed based on a comparison with experimental binary adsorption of water and ethane. ACKNOWLEDGEMENTS M. Jorge wishes to acknowledge financial support from Fundaq~o para a Ci~ncia e Tecnologia- Programa PRAXIS XXI.
REFERENCES 1.
B. McEnaney, Carbon, 26 (1988), 267.
138 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
H.P. Boehm, Carbon, 32 (1994), 759. B.P. Russell and M. D. LeVan, Carbon, 32 (1994), 845. N.A. Seaton, J. P. R. B. Walton and N. Quirke, Carbon, 27 (1989), 853. C. Nguyen and D. D. Do, Langmuir, 16 (2000), 1319. P.I. Ravikovitch, A. Vishnyakov, R. Russo and A. V. Neimark, Langmuir, 16 (2000), 2311. K.T. Thomson and K. E. Gubbins, Langmuir, 16 (2000), 5761. J.R. Dahn, W. Xing and Y. Gao, Carbon, 35 (1997), 825. E.I. Segarra and E. D. Glandt, Chem. Eng. Sci, 49 (1994), 2953. C. L. McCallum, T. J. Bandosz, S. C. McGrother, E. A. Maller and K. E. Gubbins, Langmuir, 15 (1999), 533. O. Talu and F. Meunier, AIChE J., 42 (1996), 809. J. K. Brennan, T. J. Bandosz, K. T. Thomson and K. E. Gubbins, Colloids and Surfaces A: Physicochem. Eng. Aspects, 187-188 (2001), 539. S.I. Sandler, Chemical and Engineering Thermodynamics, Wiley, New York, 1989. M. P. Allen and D. J. Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1989. D. Frenkel and B. Smit, Understanding Molecular Simulation, Academic Press, London, 1996. R.F. Cracknell and D. Nicholson, J. Chem. Soc., Faraday Trans., 90 (1994), 1487. H.J.C. Berendsen, J. R. Grigera and T. P. Straatsma, J. Phys. Chem., 91 (1987), 6269. W. A. Steele, The Interaction of Gases with Solid Surfaces, Pergamon Press, Oxford, 1974. W.L. Jorgensen and J. Tirado-Rives, J. Am. Chem. Soc., 110 (1988), 1657. M. Jorge, C. Schumacher and N. A. Seaton, submitted to Langmuir. M. Jorge and N. A. Seaton, submitted to Mol. Phys.. M. Jorge and N. A. Seaton, Mol. Phys., 100 (2002), 2017. D.M. Heyes and F. van Swol, J. Chem. Phys., 75 (1981), 5051. G.M. Davies, N. A. Seaton and V. S. Vassiliadis, Langmuir, 15 (1999), 8235. G.M. Davies and N. A. Seaton, Langmuir, 15 (1999), 6263. B.P. Russell and M. D. LeVan, Ind. Eng. Chem. Res., 36 (1997), 2380. MINOS 5.0, Systems Optimization Laboratory, Department of Operations Research, Stanford University, CA, 1988. GAMS Development Corporation, 1217 Potomac Street, NW, Washington, DC 20007, 1998. S.S. Barton, M. J. B. Evans and J. A. F. MacDonald, Langmuir, 10 (1994), 4250. G.S. Heffelfinger, F. van Swol and K. E. Gubbins, J. Chem. Phys., 89 (1988), 5202. L. Sarkisov and P. A. Monson, Langmuir, 16 (2000), 9857. M. Schoen, C. L. Rhykerd, J. H. Cushman and D. J. Diestler, Mol. Phys., 66 (1989), 1171. P.I. Ravikovitch, A. Vishnyakov and A. V. Neimark, Phys. Rev. E, 64 (2001), 011602. A.V. Neimark and A. Vishnyakov, Phys. Rev. E, 62 (2000), 4611. S. S. Barton, M. J. B. Evans, S. Liang and J. A. F. MacDonald, Carbon, 34 (1996), 975. S. S. Barton, M. J. B. Evans, E. Halliop and J. A. F. MacDonald, Carbon, 35 (1997), 1361. J. A. F. MacDonald, M. J. B. Evans, S. Liang, S. E. Meech, P. R. Norman and L. Pears, Carbon, 38 (2000), 1825.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
139
Comparison of Porous Carbons Developed via Templating Approaches P. M. Barata-Rodrigues t, T. J. Mays ~, N. A. Seaton 82 and G.D. Moggridgef* f Department of Chemical Engineering, Cambridge University, Cambridge CB2 3RA, UK Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK 82 School of Chemical Engineering, University of Edinburgh, Edinburgh EH9 3JL, UK Porous carbons synthesised from templates have been characterised and compared. The synthesis consists of the impregnation of a carbon precursor within the voids of an inorganic template, followed by low temperature carbonisation and demineralisation of the mineral-carbon composite. The templating systems studied were Mg-AI layered double hydroxides (Clay Templated Carbon R o u t e CTC), zeolites (Zeolite Templated Carbon R o u t e - ZTC) and mesoporous molecular sieves (Mesoporous Molecular Sieve Templated Carbon R o u t e - MTC). The carbon materials have been characterised and compared using scanning and transmission electron microscopy, elemental analysis and powder x-ray diffraction. Nitrogen adsorption at 77 K was used to find the templated carbons' pore structure, using 'classical' methods as well as the more recent Density Functional Theory approach for quantification of adsorption properties and a systematic determination of pore size distributions. Results showed different micro/mesoporosities and high adsorption capacities for the carbons, showing potential for applications in gas and liquid-phase adsorptive applications. 1. I N T R O D U C T I O N Over the past decade the engineering of porosity of materials, and in particular of carbons, has provoked great scientific interest. These materials can f'md application where molecular recognition is required, such as in selective adsorption. The objective of this work is to develop porous carbon materials, synthesised via molecular templates, and to investigate through a systematic comparative method their suitability as adsorbents. The performance as an adsorbent is intimately related to pore structure. The ideal manufacturing process should yield carbons with controllable mean pore sizes and narrow pore size distribution. The authors aim to produce new adsorbents with the desirable attributes of already existing carbons chemical inertness, high surface area, resistance to high temperatures and relative hydr0phobicitywhilst gaining something of the ability to control the pore structure afforded by the templates used. In this paper, three nanoporous carbon routes are compared, synthesised using Mg-Al layered double hydroxide (an anionic clay [1]), NaX zeolite and HMS (mesoporous molecular sieve silica [2]) as templates. Previous studies of these types of carbons [3-6] suggest that the manufacturing process sometimes succeeds in replication, though a more detailed and comparative study of the materials is required, particularly with regards to their adsorption characteristics. The carbons synthesised via these three templating routes present different adsorption properties and these were therefore studied in relation to the corresponding host frameworks. A comparison has also been made with the carbons obtained under the same conditions but without any template for further insight. Clays, with their layered structure, are very interesting as carbon templates, as the resulting carbons are thought to have truly slit-shaped pores. In particular, anionic clays are lamellar mixed hydroxides with interlayer spaces containing exchangeable anions, allowing the intercalation of a diverse range of organic anions. These clays, known as hydrotalcite-type compounds or layered double hydroxides (LDHs), are based on the Brucite structure (Mg(OH)2). These units produce infinite two-dimensional layers (of ca. 4.9 A thickness) which stack on top of each other to form a threedimensional structure. Applying a similar method to that of Bandosz e t al. [3], a Mg-AI layered double hydroxide was used in this study as carbon template. The voids between the ordered inorganic clay Corresponding author: gdm [email protected].
140 layers are saturated with an aromatic monomer (4-styrenesulfonate), afterwards polymerised, carbonised and demineralised. With a similar concept and a method first developed by Kyotani et al. [4], a commercial NaX zeolite was used as a template and impregnated with furfuryl alcohol, as the carbon precursor, with additional propylene carbon vapour deposition in some of the samples. Zeolite X features a three-dimensional interconnected channel system, with pore openings (7.4 A) framed by twelve Si/AIO4 units. The channels in zeolite X (with 8 A average thickness) zigzag and intersect at tetrahedral angles. In an analogous carbon synthesis route to that of the zeolite, an aluminium-HMS template was used. This template was synthesised as described by Tanev and Pinnavaia [2]. Originally described as disordered hexagonal molecular silica and denoted HMS, this neutral framework silica has more recently been shown to possess wormhole-like framework structures [7]. HMS has wide pores (>30 A) and walls thought to be ca. 10 A thick. 2. E X P E R I M E N T A L Names of precursors, intermediates and carbons are denoted by a mixture of the starting material, the procedure this starting material has undergone and important experimental conditions. For example, LSS1C673DC means 'Layered double hydroxide of StyreneSulfonate batch 1, Carbonised at 670 ~ for 3 hours, subsequently Demineralised in HCI'. In the clay templating route (CTC), the carbon was derived from 4-styrenesulfonate intercalated within Mg-Al (2:1) layered double hydroxide. The synthesis of the layered double hydroxide (samples LSS1 and LSS2, of different batches but otherwise similar) followed Drezdon's direct synthesis [8]. Part of LSS 1 was further mixed with K2S208 to check for the effect of pre-polymerisation of the intercalated styrenesulfonate (LSS1P). LSS samples were then subjected to slow pyrolysis (15 ~ min-~), in a flow of pure N2 in a horizontal-tube furnace, dwelling for 3 hours at either 600 ~ (LSS1C63, LSS2C63) or 670 ~ (LSS1C673, LSS1PC673). The clay matrix was removed by treatment with ca. 100 ml of 1 N HCI per gram of composite, mixing for 3 days. The resulting carbons were further washed and dried (LSS1C63DC, LSS2C63DC, LSS1C673DC, LSS1PC673DC). For comparison, a blank experiment was done on sodium styrenesulfonate (NSSC673DC). In the zeolite templating route (ZTC), a NaX zeolite (Tosoh), SiOdA1203 molar ratio of 1.3 (NX2 sample), was used as substrate for the carbon framework growth. Excess furfuryl alcohol (FA) was mixed with the dried zeolite (ca. 10 ml of FA per g of zeolite) under reduced pressure, at room temperature for 5 days. The mixture was then filtered and washed with mesitylene, to remove the majority of FA adsorbed on the external surface of the zeolite particles. The FA impregnated zeolite was transferred into a quartz boat for carbonisation in a horizontal furnace. In order to polymerise the FA, the impregnated zeolite was heated at 80 ~ for 24 hours, after which the temperature was raised to 150 ~ for 8 hours to promote FA cross-linking. The furnace temperature was increased to 700 ~ dwelling for 3 hours to ensure complete carbonisation (NX2FC73). Part of the carbonised composite was also subjected to further treatments with propylene (P) carbon vapour deposition (CVD) at 800 ~ for 4 hours (NX2FC73P84) or 700 ~ for 4 hours and further heating at 900 ~ for 3 hours (NX2FC73P74N93). All ramps in the heating program were set to 5 ~ rain -~. The zeolite-carbon composites were twice mixed with cone. HF for ca. 6 hours and boiled with cone. HCI. The resulting carbons were filtered, washed and dried (NX2FC73 DFC, NX2FC 73 P84DFC, NX2FC73P74N93DFC). A blank experiment was done on FA alone (FC73DF). The mesoporous molecular sieve templating route (MTC) was similar to ZTC, with the template being an HMS aluminosilicate synthesised according to the procedure described in [2] (samples AHS1 and AHS3, different batches but otherwise similar). Demineralisation was carried out either with an aqueous solution of 3M NaOH (AHS 1FC73DN) or cone. HF (AHS3FC73DF, AHS3FC73P74N93DF). As routine characterisation, elemental analysis (via combustion of the sample and Energy Dispersive X-Rays) was done on all carbons confirming the successful and complete removal of template. Powder x-ray diffraction (PXRD) was measured on a Philips XPG3000 powder diffractometer, using Cu-K~ radiation (2=1.5418 A). Scanning electron microscopy (SEM) was performed in a Philips XL30 FEG SEM. Imaging was done at 2-5 kV accelerating voltage, using the secondary electron imaging technique. Transmission electron microscopy (TEM) was performed on a Philips CM 100 at an accelerating voltage of 100 keV and a JEOL 2000 (200 keV), using the dark field
141 imaging mode. Powder samples were briefly sonicated in ethanol and dried onto holey carbon film nickel grids. The clay sample was prepared by ultramicrotomy. Adsorption was investigated using N2 (at 77 K) as the adsorbate in an Accelerated Surface Area Porosimeter equipped with 1, 10 and 1000 torr transducers (ASAP 2010, Micromeritics). Samples (approx. 100 mg) were degassed in the analysis port itself, at 200 ~ for 10 hours in vacuum. During the micropore analysis, minimum equilibration step-times were set to 45 s. Surface area and micropore volume were estimated by the BET [9] and Dubinin-Ashtakov [ 10] equations, respectively. The mesopore volume was obtained from the BJH equation [ 11] and total pore volumes were estimated at relative pressures c a . 0.99. Pore size distributions (PSDs) were determined using the non-local density functional theory (DFT) included in the ASAP's software, for slit-like and cylindrical pore geometries (S- DFT for slit geometry [12], TTarazona-based model [13] and H- the recent Hybrid model [14], as supplied by Dr. Olivier, U.S. Micromeritics). A note in the legend of the PSD plots states which model resulted in the best fit. 3. R E S U L T S AND D I S C U S S I O N 3.1. Clay Templated Carbon (CTC) Route Figs. 1-2 show SEM images of the starting clay and the finished carbon product. The carbon shows a clear replication from the micron to the nanometre level of the layered morphology seen in the template. TEM images give an enhanced resolution of the clay particles, and with ultramicrotomy preparation of the sample it is possible to distinguish a layered intra-particle structure from the nanometre to the angstrom scale (Fig 3). However, no contrast was achieved with similar preparation of the carbon sample. Fig. 4 shows the templated carbon prepared in a suspension, and no layered structure is evident.
The PXRD patterns confirm these findings with the clays showing a polycrystalline type of structure and the carbons an amorphous nature (at least as regards long range order). The loss of long range order takes place before leaching away the inorganic matrix and is not recovered once the process is completed (Fig. 5). From nitrogen adsorption experiments (Fig. 6), it is clear that all the clay templated carbons have more than double the surface area compared with the non-templated one. The BET surface area of LSS2C63DC is 1279 m 2 g~, while that of the NSSC673DC is only 528 m 2 g-~. The porosity of the non-templated carbon arises in the HCI step, as the carbonised precursor NSSC673 showed no measurable porosity. It is therefore likely that a significant contribution to the total porosity of the templated carbon will stem not only from the removal of the template, but also from the interaction between the precursor and the acid. The PSDs obtained with DFT slit -pore model (Fig. 7) show the lower microporosity (pore size<10 A) to be common to all carbons derived from styrenesulfonate, while the upper microporosity (>10 A) arises only from the templating procedure. Differences in carbonisation temperatures and template batch do not cause a significant change in PSD. It should be noted that the observed minima at 9-10 A can be an artefact of the DFT model itself, as described by Olivier [15]. Only a small fraction of the pore volume of these templated carbons falls into the mesoporous region.
142
TEM pictures (Figs. 11-13) provide further information, in particular through the diffraction patterns obtained for thin edges of imaged particles. The crystalline morphology of the zeolite is obvious from the spots of the diffraction pattern in Fig. 11 and orientation of the propylene-furfuryl alcohol derived carbon is also evident in Fig. 13. Fringes are easily found in the P-FA carbon (Fig. 13), while the FA carbon's fringes (Fig. 12) are not. PXRD scans confirm the long-range order in the zeolite and its preservation through the impregnation and heat treatment steps of the synthesis route (Fig. 14). The FA carbon does not show replication of the main zeolite peak (6.08 o 20), and this particular sample was still contaminated (with 6 wt.% fluoride as found in the EDX analysis), indicating an incomplete process during demineralisation. Additional carbon vapour deposition with propylene during carbonisation introduces a repeating distance that corresponds to the characteristic { 111 } peak of the zeolite template (14.5 A), indicating replication at the molecular level. This feature had previously been reported with Y-zeolite
143
Nitrogen adsorption isotherms (Fig. 15) allow a hypothesis to be formed which may explain why the FA carbon does not show long range order. The zeolite-FA-carbon composite (NX2FC73) had a surface area of 384 m 2 g-l, which could imply incomplete impregnation of the zeolite; on the other hand, once propylene was added there was no porosity left in subsequent composite (NX2FC73P84), indicating that the small molecules of propylene were able to fill in the gaps between the FA derived carbon molecules and the inorganic framework. The voids present in the NX2FC73 composite probably cause a partial collapse of the carbon structure after leaching the zeolite, while propylene treatment makes the interconnections between the carbon channels strong enough to resist the removal of the templating matrix. The zeolite NX2 has a surface area of 724 m 2 g-1 and a total pore volume of 0.28 cm 3 g-i (P/P0-0.99), while these figures for the FA templated carbon (NX2FC73DFC) are 829 m 2 g-1 and 0.44 cm 3 g-l, respectively. The P-FA carbons have much higher values with 1802m2g 1 and 1.23 cm3g "1 for NX2FC73P84DFC and 1600 m 2 g-1 and 0.952 cm 3 g-i for NX2FC73P74N93DFC. Most of the porosity in the latter two carbons falls into the upper microporous and lower mesoporous region, as seen from the isotherm plots and PSDs. The non-templated carbon (FC73DF) showed no measurable porosity, indicating that the acid leaching step does not influence the carbon porosity, apart from the removal of the template. This result is desirable because it increases the likelihood of replicating template structure rather than creating porosity with random pore structure, as would be the case if porosity were increased through acid treatment (c.f CTC routte). More generally, experiments not included in this paper have shown that treating the carbonised FA with HF or HCI does not add porosity to the carbon, contrary to the clay route case, where styrenesulfonate is the precursor. The choice of precursor is therefore imperative if pore structure is more important than merely creating a high porosity material. In order to evaluate the PSDs of the materials, the three DFT models available in the ASAP 2010 were applied (Figs. 16-19). NX2 shows
144 only lower microporosity, as expected from a faujasite zeolite structure, while the templated carbons have all got both lower and upper microporosity, besides mesoporosity to varying extents. Normally, carbon materials are characterised using the DFT which assumes a slit-like pore geometry [12]. However, these zeolite templated carbons are best described by the DFT-Hybrid
3.3. Mesoporous Molecular Sieve Templated Carbon (MTC) Route As in the other two routes, the mesoporous molecular sieve templating route produced carbons with the same morphology as their templates (SEMs, Figs. 20-21). TEM imaging was easier in this route because the channels of molecular sieve templates are in the mesoporous range (i.e. > 20 A) and therefore clearly visible with the resolution of the microscopes used (Fig. 22). The carbons produced are expected to have pores with walls of similar dimensions to the pore diameters of the template. This worm-like carbon structure is also easily distinguished by the TEM technique applied. All these materials are however quite unstable to the electron beam.
145 PXRD plots (Fig. 24) show that most of the produced carbons in this routte are molecular replicas of the templates AHS 1 and AHS3, confirming TEM results. This is seen by the retention in the upper plot of the template diffraction peak at 2 ~ 2 0. It is however difficult to resolve as low as this angle in a wide-angle powder x-ray diffractometer such as the one used. Therefore, if the peaks are not of sufficient intensity, they can show up as ,~o~-~ shoulders of the characteristic high background [ ~ 2.1. . . . . . . . . AHSIFC73DN 800~ ~42.50A Tomplated&Non~AHS3FC73DF scatter for porous carbons below 3 ~ 20, as seen t'"'" . . . . . . . . X " Tcmplatcd Carbons . . . . . AHS3FC73P74DF 600~. .... _..... k .... --FC73DF in Fig. 24 (top) for the propylene treated carbon ":t,..t.'~"'-.- . . . . . . . . . . . AHS3FC73P74DF. In addition to these findings, 40O .......~.,~ the carbon demineralised with NaOH shows a ~0o~. ......~,__~_. . -... ... .. . .. . .. . .. . .. . . . . . . . . .,, ~--.................. " - ~ Z S " : : : - - 7 : ? " .............. L~ broad peak at 25 ~ more intense than the ~ ~ ? . . . . . ".--~ other templated carbons. This peak has previously been related to a stacking structure of carbon layers and ascribed to deposits of the ~,sc~ _ _ ~c~,~ precursor on the external surface of the template particles [4]. ~ The carbons in Fig. 24 are also compared with SNU2, which is a carbon produced by a Korean research group [6] using a similar 5 template, but a different carbon precursor H f ..... ... '.~A "--~ "\ MMS AHSl (phenol formaldehyde) intercalated in the gas 60o I \ Templates - AHS3 phase. The comparison between these templated carbons indicates that the choice of precursor ~ 3 ~~ does not significantly alter the replication in this 3~ 3.98 A route. Fig. 25 shows the adsorption isotherms for the AHS3 route. The template is mesoporous Diffraeti~a angle 20 (o) with 714 m 2 g-1 surface area and 0.93 cm 3 g-i Fig. 24 PXRD: Templating routes via AHS 1 and AHS3 (included pattern for the Korean carbon SNU2, [6]) pore volume. Templated carbons AHS3FC73DF and AHS3FC73P74N93DF were very similar, with 1467 m 2 g-1 surface area and 1.3 c m 3 g-I pore volume, twice that of the SNU2's. Both DFT and BJH models were used for PSDs (Figs. 26-30). For AHS3, the hybrid DFT model yielded a narrow PSD centred at 45 A, and the BJH analysis gave a single peak at 34 A. The templated carbon PSDs are expected to reflect the dimension of the pore walls of the template rather than the template pore diameters. AHS materials have been reported to have pore walls 10 A thick [2,7]. However, Hybrid DFT PSDs show pores of 20-30 A for the AHS3 templated carbons, indicating that the pore walls of our template might be thicker than the reported estimate or that, more likely, the carbon shrunk during carbonisation. In this route, the addition of propylene was found not to make much difference to the porosity of the resulting carbon. 22
146
4. C O N C L U S I O N S Three different routes of templated carbons were successfully prepared, and a range of different directing structures was used as inorganic matrices, allowing the analysis of template effects on the synthesised carbons. A clear replication of the template morphology is seen in scanning electron micrographs in all the synthesis routes. Nitrogen adsorption, powder x-ray diffraction and transmission electron microscopy permitted evaluation of how far the replication extended to the molecular level. The clay route produced microporous carbons, with high surface areas (>1000 m 2 g-l) but no long-range order. It was found that the demineralisation step in the synthesis procedure produced part of the porosity in its own, and that the layered structure of the carbon-mineral composite collapsed on heating. The zeolite-based route resulted in microporous carbons, also with high surface areas (ca. 1000 m 2 gl) which were doubled by additional propylene CVD treatment. CVD treatment resulted in a more precise structure replication of the starting template, which is thought to be due to more complete filling of the mineral pores during impregnation and subsequent carbonisation. Mesoporous molecular sieve-templated carbons have larger pores than the previous two routes, but equally high surface areas, and were found to retain the repeating distance characteristic of the template. This is probably due to the larger pores of the template resulting in a more robust carbon framework which better withstands the chemical treatment necessary for demineralisation. The synthesis routes studied provide the tools to produce carbons with different nanoscale properties. This is potentially useful for various adsorption applications given the carbons' high surface areas and pore size distributions which vary according to the chosen template. ACKNOWLEDGMENTS P.M. Barata-Rodrigues would like to thank the Portuguese Foundation of Science and Technology (PRAXIS XXI/BD/18002/98). Acknowledgmentsare due to the EPSRC and MoD for sponsoring this research project and to all those who contributed to this work, in particular T.Bustnes, T. Kyotani, J. Lee, R. Ryoo and J. Olivier. REFERENCES [1] Reichle, W.T., Solid State Ionics, 22 (1986) 135. [2] Tanev, P.T. and Pinnavaia, T.J., Science, 267 (1995) 865. [3] Bandosz,T., Putyera, K., Jagiello, J., Schwarz, J.A., Applied Clay Science, 10 (1995) 177. [4] Kyotani,T., Ma, Z. and Tomita. A., Chemical Communications,23 (2000) 2365. [5] Ryoo, R., Joo, S.H., Jun, S., Journal of Physical Chemistry, 103 (1999) 7743. [6] Lee, J., Yoon, S., Oh, S.M., Shin, C.-H. and Hyeon, T., Advanced Materials, 12 (2000) 359. [7] Zhang, W., Pauly, T.R. and Pinnavaia, T.J., Chemistry of Materials, 9 (1997) 249 I. [8] Drezdon, M. A., 'Pillared hydrotalcites', US Patent 4,774,212 (1988). [9] Brunauer, S., Emmett, P.J. and Teller, E., Journal of the American Chemical Society, 60 (1938) 309. [ 10] Dubinin, M.M. and Ashtakov, B.A., Advanced Chemical Series, 102 (1971) 69. [ 11] Barrett, E.P., Joyner, L.S., Halenda, P.P., Journal of the American Chemical Society, 73 (1951) 373. [ 12] Olivier, J. P. and Conklin, W. B., The DFT Plus Models Library (1996) Appendix A. [13] Tarazona, P., Phys. Rev. A, 31 (1985) 2672; Tarazona, P., Phys. Rev. A, 32 (1985) 3148; [ 14] Jaroniec, M., Kruk, M., Olivier, J.P. and Koch, S., Proceedings of COPS-V, Heidelberg, Germany (1999). [15] Olivier, J.P., Carbon, 36 (1998) 1469.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
147
Simulation of Adsorption in 3-D Reconstructed M e s o p o r o u s Materials by a Simulated Annealing Algorithm M.E.Kainourgiakis a, E.S. Kikkinides b, G. Ch. Charalambopoulou a and A.K. Stubos a a
National Center for Scientific Research "Demokritos", 15310 Ag. Paraskevi Attikis, Greece
b Centre for Research and Technology Hellas, Chemical Process Engineering Research Institute, 6th km. Charilaou- Thermi Road, 57001 Thermi, Thessaloniki, Greece Aim of the present work is the determination of the spatial distribution of a condensable adsorbate in reconstructed porous domains of Vycor membranes. The digital representation of the dry porous structure is achieved by a stochastic method. The produced binary domains have the same statistical content as the actual materials. The distribution of the condensable adsorbate in the reconstructed porous structure, for a given degree of saturation, is found assuming that in equilibrium the total interfacial free energy reaches a global minimum value. The configuration leading to the minimisation of this objective function is determined through a simulated annealing method. In a further step, the effective diffusivity of an inert gas such as He, in the reconstructed Vycor porous glass, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is then calculated by normalising the computed effective diffusivity at each degree of saturation by that computed for the dry material. The comparison of the simulation results with experimental relative permeability data exhibits good agreement, supporting the validity of the suggested adsorption simulation process.
1. Introduction The spatial distribution of an adsorbate in mesoporous domains is of primary importance for a wide range of applications such as separations, catalytic reactions etc. The study, in particular, of the flow of a non-adsorbable gas through such a porous structure partially blocked by a condensed phase, can provide a useful tool for either its structural characterization [1 ], or the evaluation of its performance in various industrial processes [2,3]. What is sought after in such an effort, is to determine the effect of pore blocking on the overall transport process by combining dynamic (permeability) with static measurements (adsorption). This can be achieved by introducing a stationary condensed phase of an adsorbed vapour into the porous material and subsequently measuring the permeability of a second non-adsorbable gas, which does not condense in the pores, at least under the conditions of the experiment. The above type of experiment is known as gas relative permeability [4], and since it is usually performed at low pressures, intermolecular collisions are rare and the transport process is determined primarily from the collisions between the molecules and the solid walls. This mechanism is within the Knudsen regime and thus permeability reduces to Knudsen diffusivity.
148 Eventhough there is a well-established theoretical description of both adsorption and transport mechanisms in mesoporous media [5], the adequate representation of the three-phase system (vapour- condensed adsorbate - solid matrix) occurring during adsorption and transport processes, is extremely difficult due to the complexity of the porous structure. Although considerable progress has been made in the efficient representation of the internal structure of porous media using pore networks [6], there still exists a strong need for a direct quantitative description of the complex microstructure. The exponential growth of computational resources has recently led to advanced methods for the binary representation of biphasic media. These can be divided in two major categories: (a) the statistical reconstruction methods [7,8,9,10,11], where the binary array respects a number of statistical properties of the actual biphasic medium, and (b) the process based methods [12,13,14], where the computational procedure tries to account the physical processes underlying the formation of the medium. The binary domains resulting from either approach can be used for the simulation of either dynamic processes, such as diffusion [15,16] and flow [8] or equilibrium properties such as sorption [17,18]. In previous studies [15,19], we adopted the stochastic reconstruction technique proposed by Crossley et al. [ 11 ] to generate 3D images of Vycor glass, a model mesoporous material, and showed that this particular method not only reproduces the actual porous structure accurately, but also constitutes a kind of process-based approach [19]. In the present work, the same methodology is employed for the digital representation of dry Vycor geometry and an attempt is made to determine the concentration profile of a condensable adsorbate in the reconstructed porous domains. The distribution of the condensable adsorbate in the reconstructed porous structure, for a given degree of saturation Vs (fraction of pore space occupied by the adsorbate), is defined assuming that in equilibrium the total interfacial free energy is minimal. A simulated annealing method is employed for the optimisation procedure. In a further step, the effective diffusivity of an inert gas such as He, in the reconstructed Vycor porous glass, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is calculated by dividing the computed effective diffusivity at each degree of saturation by that computed for the dry material. The simulation results are compared with experimental relative permeability curves. 2. Representation of porous structure - Dry Vycor geometry
The spatial distribution of matter in a porous medium can be typically represented by the phase function Z(x), defined as follows: if x belongs to the pore space (1) otherwise where x is the position vector from an arbitrary origin. Due to the disordered nature of porous media, Z(x) can be considered as a stochastic process, characterized by its statistical properties. The porosity, e, and the auto-correlation function Rz(u) can be defined by the statistical averages [8,20]:
149
Rz (u):
(z<x
2b)
2
Note that <. > indicates spatial average. For an isotropic medium, Rz(u) becomes onedimensional as it is only a function of u=]u[. Ideally, a representative reconstruction of a porous medium in three dimensions should have the same correlation properties as those measured on a single two-dimensional section, expressed properly by the various moments of the phase function. In the present work the first-two moments (porosity and auto-correlation function) are considered. In general, information on the porosity is obtained through sorption experiments, while information on the correlation function is obtained either directly from 2D images of sections of the material (assuming isotropy in the third dimension) or indirectly from the scattering spectrum measured by Small Angle Scattering techniques. The basic idea in most reconstruction techniques is to convolute a purely random process to a correlated one through the use of an appropriate kernel. The resulting correlated process is transformed into a binary phase function by thresholding it according to the porosity of the medium. The final 2D or 3D binary medium possesses the same porosity and correlation function with the actual porous medium. In the present work, the reconstruction method proposed by Crossley et al. [11 ] has been selected, since it is relatively simple, while it gives an excellent fit of the whole SANS spectrum and thus the correlation function of the material. According to this method, the reconstructed Vycor image is generated by starting with an initial three-dimensional random from a uniform probability distribution in [0,1]. The initial random number array image becomes correlated through a convolution with a Laplacian-Gaussian Kernel, which introduces a correlation length 09:
Io(x,y,z)
K(x,y,z;co)
+oo+oo+~
I(x,y,z): ~ ~ ~Km(x-x/,y- y/,z-z/)'lo(x/,Y',z')dx'dy'dz' where Km is given by:
(3a)
Km(x,y,z)=(_6+4.(x2+
(3b)
-oo..-ot>-oo
y2+
zZ)/wZ).exp(_(x:+ y2+ zZ)/w2)
I(x,y,z)
Finally the correlated array is binarised by thresholding with the porosity of the material. The resulting three-dimensional array represents a binary medium with properties similar to those of actual Vycor porous glass. This is demonstrated in Figure 1, where the auto-correlation function of the reconstructed domain is compared with the corresponding curve for the real dry-state material, derived from SAXS experiments and TEM images as described in earlier works [15,21,22]. A 2-D image of the reconstructed dry Vycor at porosity e=0.28 is presented in Figure 2a.
3. Determination of the spatial distribution of adsorbate in reconstructed Vycor Consider a sorption experiment, where a mesoporous solid, denoted hereafter as S, is progressively loaded by a condensable gas or vapour. Initially, a layer of adsorbate L is building up on the walls of the pores. When condensation occurs, all the pores with radii smaller than a critical value, given by the Kelvin equation, are progressively blocked, and the adsorbate is in equilibrium with its vapour, V. The distribution of the condensed phase in a reconstructed Vycor structure for a given degree of pore filling (saturation), is determined by
150 1.0 0.8
TEM Image [10]
0.6
Stochastic reconstruction
~-~ O.4
0.2 0.0 -0.2
100 6-'--- ~
200
ai.
u(~
300
-
~
I
I
400
Fig. 1. Autocorrelation function of actual and reconstructed dry Vycor assuming that in equilibrium the total interfacial free energy, Gs, associated with the multiphase system, is minimal [23]. Gs is defined as: Gs
=ZA, Yi
(4)
i
where Ai is the interfacial area i and ~i is the interfacial free energy of area i. In the present case, the total interfacial energy has contributions from three different interfaces: (a) condensed phase L - solid S, (b) vapour V - solid S, and (c) condensed phase L - vapour V. The corresponding ~i components always obtain positive values and for complete wetting of the solid surface (contact angle equal to 0 ~ cancel according to Young-Dupr6 equation: YsL = Ysv - 7"Lv
(5)
The actual distribution of phase L is represented by the arrangement of the fluid pixels that minimizes the total interfacial energy. This Gs minimum is determined through a simulated annealing algorithm (SA). This method was introduced by Kirkpatrick et al. [24], who identified the analogy between the annealing process in solids, the behaviour of systems with many degrees of freedom in thermal equilibrium at a finite temperature and the optimisation problem of finding the global minimum of a multi-parameter objective function. SA is based on the Metropolis algorithm at every step of which the configuration of the system under study, changes randomly causing a variation of the corresponding objective function by AE. If at a certain step AE<0, the new configuration is unconditionally accepted while if AE>0 the new configuration is accepted with a probability given by:
P(AE)= exp(- ~-~T 1
(6)
where k8 is Boltzmann constant and T is the temperature (or an arbitrary analogue of it, used only to symbolically represent the degree of randomness in the spatial distribution of the system phases). This step prevents the system being trapped in a local lowest-energy state. After a sufficient number of iterations, the system approaches the equilibrium state where the free energy reaches its minimum value. Subsequently the procedure is repeated for several T values lower than the initial one (using every time as initial configuration the one found as
151 equilibrium state for the previous T value). The process ends when despite the change in T, the number of accepted changes in different configurations becomes lower than a prespecified value. In our case the objective function to be minimized, is the total interfacial energy, Gs. In order to find the equilibrium distribution of liquid and vapour for a certain Vs, the required number
Fig. 2.2D sections of the corresponding 3D reconstructed images: (a) dry, (b) wet (Vs=0.5) of liquid elements, L, are first randomly selected and thus an initial state of the system is defined corresponding to a reference total energy value, Go. Progressively, the positions of one site in the pore space occupied by L phase and one V element are swapped, preserving the specified saturation. Let us consider the n-th trial of this process. If AG=(Gtrial- Gn_l)<0, the position interchange is allowed and Gn=Gtrial. In case that Gtrial > Gn-l, the new configuration is accepted with a probability given by exp(-AG/Gref), where Greyis a control parameter analogous to temperature T, having the same units as the objective function Gs. The calculations are repeated until no change in Gn is observed. A 2D section of the reconstructed 3D images for saturation equal to 0.3 is presented in Figure 2b.
4. Simulation of Knudsen Diffusion in Reconstructed Vycor The orientationally averaged effective diffusivity of He in the reconstructed Vycor structure (dry or wet), is calculated from the mean-square displacement <~2>, of a statistically sufficient number of identical particles injected in the void space of the medium, according to: D = l i m (42) t---~ 6t
(7)
where t is the travel time of the particles. The displacement is monitored throughout the distance, s, traveled by the particles assuming that they move at a constant speed equal to the mean thermal speed, "- ( 8 R T [ ~ M ) 1/2 [15,25,26]. The travel time has to be large enough to ensure that the molecules feel the effect of all the structural details of the porous medium. In this sense, the material can be considered as macroscopically homogeneous in terms of its structural and diffusion characteristics. At first, a random position in the pore space is defined as the initial position of the molecule to travel within the porous medium. Subsequently, direction angles are randomly assigned to the molecule, which starts its random walk moving from voxel to voxel along this direction. At each step it is checked whether the molecule hits a solid wall, and if this happens it undergoes a diffuse reflection according to the cosine law.
152 At all times a test is made to determine whether the molecule reaches the boundaries of the 3D medium. Periodic boundary conditions are employed in this case and two sets of coordinates have been used: local coordinates of the molecule inside the medium and the global ones used for the computations of the total displacements. Using the exact solution for an infinitely long tube of diameter le, in the Knudsen regime, D, -- ~3 le and substituting t=S/, Equation (7) obtains the dimensionless form:
D I D k = lim
(42/le2)
(8)
2ST(e
In addition, the incorporation of porosity in diffuSivity yields effective diffusivity D e# = D" c , also known as permeability, P, for the case of inert gases. Computer simulations have been performed in 3-D images of recons~ucted dry Vycor porous glass, with pixel size/e=30/~ and sample size of 100xl00xl00. A total number of 3000 test molecules was used in the majority of the simulations, while the total number of time steps was greater than 105. The effective Knudsen diffusivity, for the case of a Vycor domain with porosity e=0.28, has been found equal to 0.073xDk [15]. Since the value of Dk for He at 298 K, is 0.0126 cm2/s, it follows that the computed Knudsen diffusivity of He in dry Vycor porous glass is ~9x 10"4 cm2/s, in excellent agreement with experimental results [27,28]. The same computational procedure was repeated for the case of binary domains representing the wet Vycor structure for different degrees of saturation, Vs varying from 0.1 to 0.48. In order to evaluate the effect of the adsorbate nature on the adsorbate distribution and therefore on the transport properties of the system, three sets of interfacial energies 7i, were examined. For each of the three systems gas relative permeability, PR, was determined by normalising the effective diffusivity computed for a certain Vs value by the one for the dry material. The simulation results are compared against experimental data obtained from the measurement of He permeability in Vycor preadsorbed with CH2Br/[28], in Figure 3. A good agreement between simulation and experiment is observed, demonstrating that the suggested approach can reproduce the concentration profile of a condensable adsorbate in reconstructed Vycor domains. Most importantly, this distribution does not seem to be affected by the actual values of Yi. Eventhough ~i components are significantly varied, always preserving the complete 1.0~ []
0.8
0
0.6
Experiment [28] 1,s~22, 1,LV=72,1,9_--350 7s-v=lO0, YLV=30,7SL=70 YS~, YLV=1, ~'S=1 L
ct'~: 0.4-
0.2 .0
o.o
'
~w
ola
'
o14
i
o16
o18
|
1'.o
Fig. 3. Comparison between simulated and experimental gas relative permeability of He on Vycor porous glass partially filled with a sorbed phase
153 wetting condition (7sv>YsD, the computed relative permeability values fall upon a single curve. One can therefore claim that the spatial distribution of an adsorbate in a mesoporous medium depends mainly on the morphology of the porous structure and not on the absolute values of the set of the interfacial energies, given that proper physical conditions are satisfied. 5. Conclusions
In the present work the spatial distribution of a condensable adsorbate in reconstructed porous domains of Vycor membranes is determined. A stochastic method is employed to generate digital pictures of the dry mesoporous structure. The distribution of a condensable adsorbate in the reconstructed domains, for a given degree of saturation, is found assuming that in equilibrium the total interfacial free energy reaches a global minimum value. The adsorbate concentration profile leading to the minimisation of this objective function is identified through a simulated annealing algorithm. In addition, the effective diffusivity of He in the reconstructed Vycor, is determined by a combination of the mean square displacement method with a standard random walk process. The gas relative permeability, PR, is then calculated by normalising the computed effective diffusivity at each degree of saturation by that computed for the dry material. The comparison of the simulation results with experimental relative permeability curves exhibits good agreement, proving that this methodology can reproduce the concentration profile of a condensable adsorbate in reconstructed Vycor domains. In addition, this distribution is not affected by the nature of the solid-adsorbate interaction. References
[1] A.K. Stubos, Th. A. Steriotis, A.Ch. Mitropoulos, G.E. Romanos and N.K. Kanellopoulos, "Inorganic membranes: pore structure characterisation. In: Physical Adsorption Experiments, Theory and Applications, J. Fraisard (eds.), Kluwer Academic Publishers, The Netherlands, 1997 [2] R.J.R. Uhlhorn, K. Keizer and A.J. Burggraaf, J. Membrane Sci., 66, 259 (1992) [3] D.P. Sperry, J.L. Falconer and R.D. Noble, J. Membrane Sci., 60, 185 (1991) [4] R. Ash, R.M. Barrer and C.G. Pope, Proc. Roy. Soc., A271, 1 (1963) [5] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd Ed., Academic Press, London, 1982 [6] D. Nicholson and J.H. Petropoulos, J. Chem. Soc. Farad. Trans. I., 80, 1069 (1984) [7] J. A. Quiblier, J. Colloid Interface Sci., 98, 84 (1986) [8] P.M. Adler, 'Porous Media: Geometry and Transports', Butterworth, London, 1992 [9] C.L.Y. Yeong and S. Torquato, Phys. Rev. E, 57, 495 (1998) [10] P. Levitz, V. Pasquier and I. Cousin, in Characterization of Porous Solids IV, edited by B. Mc Enaney, T. J. Mays, J. Rouq6rol, F. Rodriguez-Reinoso, K. S. W. Sing and K. K. Unger (Bath, UK, 1996), pp. 135-140 [11] P.A. Crossley, L.M. Schwartz and J.R. Banavar, Appl. Phys. Lett., 59, 3553 (1991) [12] S.L. Bryant, C.A. Cade and D.W. Mellor, AAPG Bulletin, 77, 1338 (1993) [13] S. Bakke and P.E. t~ren, SPE Journal, 2, 136 (1997) [14] P.E. Oren, S. Bakke, and O.J. Arntzen SPE Journal, 3, 324 (1998) [15] M.E. Kainourgiakis, E.S. Kikkinides, A.K. Stubos, N.K. Kanellopoulos, J. Chem. Phys., 111, 2735 (1999) [16] M.E. Kainourgiakis, E.S. Kikkinides, Th.A. Steriotis, A.K. Stubos, K.P. Tzevelekos, N.K. Kanellopoulos, J. Colloid Interface Sci., 231, 158 (2000)
154 [17] L. D. Gelb and K. E. Gubbins, Langmuir, 14, 2097 (1998) [18] H.J. Woo, L. Sarkisov and P.A. Monson, Langmuir, 17, 7472 (2001) [19] M.E. Kainourgiakis, E.S. Kikkinides and A.K. Stubos, "Diffusion and flow in porous domains constructed using process-based and stochastic techniques", J. Porous Mat., in press [20] J.G. Berryman, J. Appl. Phys., 57, 2374 (1985) [21] P. Levitz, G. Ehret, S.K. Sinha and J.M. Drake, J. Chem. Phys., 95, 6151 (1991) [22] P. Levitz and D. Tchoubar, J. Phys., 1 2, 771 (1992) [23] R. Knight, A. Chapman and M. Knoll, J. Appl. Phys., 68, 994 (1990) [24] S. Kirkpatrick, C.D. Gelatt, M.P. Vecchi, Science, 200, 671 (1983) [25] M.M. Tomadakis and S.V. Sotirchos, AIChE J., 39, 397 (1993) [26] V.N. Burganos, J. Chem. Phys., 109, 6772 (1998) [27] C.N. Satterfield and T.K. Sherwood, The Role of Diffusion in Catalysis, AddisonWesley, Reading, 1963 [28] P.K. Makri, G.E. Romanos, Th.A. Steriotis, N.K. Kanellopoulos, A.Ch. Mitropoulos, J. Colloid Interface Sci., 206, 605 (1998)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Understanding Adsorption Hysteresis and Other Mesoporous Materials
155
in Porous
Glasses
H.-J. Woo, L. Sarkisov, and P. A. Monson Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, USA The origin of adsorption hysteresis in porous glasses is investigated using a mean field theory and Monte Carlo simulations, utilizing a coarse-grained lattice model. Results in the hysteresis region closely agree with the behavior seen in the experiments of xenon adsorption in porous glass by Everett and co-workers (Everett, 1982). Hysteresis and scanning behavior occurs due to the presence of a very large number of metastable states. Adsorption hysteresis is dynamic in origin, but the long time dynamics is so slow that on accessible time scales, the system appears equilibrated. 1. I N T R O D U C T I O N Structural and energetic heterogeneity in various types of porous materials often profoundly influence fluid behavior such as adsorption properties. Among other developments, recent progresses in molecular modeling research are beginning to have impact on our view on the microstructure characterization. Use of molecular models (Gelb and Gubbins, 1998; Page and Monson, 1996; Sarkisov and Monson, 2000) as well as more coarse-grained lattice models (Kierlik et al, 2001; Woo et al, 2001) allows us to examine many of the phenomenologies observed in experiments within the statistical mechanical perspective. In the current paper, we discuss some of the new approaches and results that have been developed and obtained recently within the context of such molecular modeling research, and in particular with the mean field and Monte Carlo studies of a lattice model. The next section describes the Gaussian random field method (Woo et al, 2001), which provides a computationally efficient route to generate realistic representations of the disordered mesoporous glasses. Application of the mean field theory, and Monte Carlo simulations are described in Secs. 3 and 4, respectively.
156 2. M E S O P O R O U S
GLASSES
Vycor or controlled pore glasses are synthesized by quenching a binary mixture of silica and boron oxide to low temperatures, which induces spinodal decomposition and a glass transition, resulting in the disordered network of pores interconnected throughout the space (Levitz et al, 1991; Gelb et al, 1999). The Gaussian random field method considers a stochastic random field in the threedimensional space, which is constrained to have a number of statistical properties corresponding to the physical material. For Vycor glasses, the porosity and the structure factor measured in Figure 1" Two-dimensional crossthe small-angle neutron scattering exper- section of a Vycor glass realization geniments serve as the minimal set of con- erated by the Gaussian random field straints (Woo et al, 2001). The three- method. The cubic grid size is 10/~. dimensional space is discretized into a cubic grid. Random fields are generated on the grid points in the Fourier space with correlations designed to match the experimental structure factor of the Vycor glass, and inverse Fourier-transformed into the real space via fast Fourier transform (Press et al, 1992). A level-cut of the resulting field (Cahn, 1965) yields a realization of the solid density distribution in the real space. In contrast to the approach of simulating the physical synthesis of the material, the computational cost involved in the Gaussian random field method is minimal. Statistically independent realizations with the desired porosity and spatial correlations can be repeatedly generated simply by changing the random number seed of the Gaussian random field. 3. M E A N F I E L D T H E O R Y A lattice model Hamiltonian that has been successfully used to model fluids in mesoporous glasses reads H - - J ~_, n i t i n j t j - # ~ niti - y J ~
i
[niti(1 - tj) + n j t j ( 1 - ti)],
(1)
157
-p
1.0
-0.170
0.8
-0.172
0.6
~co
--0.176
0.2
-0.178
,
--4.5 I
,
I -3.5
,
i
,
I
,
o
l
-2.5
i
-0.174
0.4
0"05.5
i
-0"1800.2
I
0.4
,
I
0.6
|
0.8
p
Figure 2: Left- Adsorption (solid lines) and desorption (dotted lines) isotherms from mean field theory for a = 10 A, y = 0.9, and (a) T* = 0.75, (b) T* = 0.6, and (c) T* = 0.5. Right: Distribution of the mean field grand potential minima as a function of density for T* - 0.6 and ~# -- -3.555.
where # is the chemical potential, ni = 0,1 and ti = 1, 0 represent the fluid and matrix occupation of the lattice site i (ti = 0 if the site is blocked by the matrix and 1 otherwise), the interactions are only between nearest-neighbor sites, and J and y J are the fluid-fluid and fluid-solid interaction parameters, respectively. W i t h realizations of the Vycor glass as shown in Fig. 1, mean field theory applied to the lattice model (1) provides a simple and sufficiently realistic method to examine fluid adsorption behavior on a coarse-grained level. In particular, the local density on each site pi = (niti) is self-consistently determined by ti (2) Pi 1 + exp [ - ~ # - ~ J ~ j e i ( P j + Y - ytj)]' where the sum in the exponent is for nearest-neighbors of the site i. Equation (2) can be easily solved numerically by iteration with a given set of matrix configurations {ti}. As shown in Fig. 2, the hysteresis region enveloped by the adsorption and desorption curves in fact contain many local minima of free energy (Kierlik et al, 2001), which makes the nature of hysteresis in such systems fundamentally different from the van der Waals-type capillary condensations in simple pore systems (Kierlik et al, 2001; Woo et al, 2001). In contrast to the conventional first-order phase transitions where there exist two global minima
158
in free energy each corresponding to the gas and liquid phases, the fluid in mesoporous glasses possesses a very large number of metastable states, each of which can be reached depending on the history of the control parameter scanning in experiments. Such features also provide a natural explanation (Woo et al, 2001) for the 'scanning behavior' (Everett, 1982). 4. M O N T E C A R L O S I M U L A T I O N S A N D D Y N A M I C S Density functional theories, or more generally, mean field theories neglect thermal fluctuations, and Monte Carlo simulations are necessary to address the relative importance of barrier crossings in relaxation dynamics within the many-valley free energy landscape. In general, grand canonical simulations parallel real dynamics qualitatively (Sarkisov and Monson, 2000), and slow relaxations of real fluids in the hysteresis region (which are expected 0.8 to be so slow that the system would appear to be in "equilibrium" within p 0.6 the time-scale of experiments) are 0.4 closely correlated with the sluggish equilibration in the grand canonical 0.2 Monte Carlo. A set of isotherms anali I , 0.0 ogous to the mean field result can be -6.0 -5.5 -5.0 obtained by gradually changing the chemical potential in either direction and allowing the system to equilibrate F i g u r e 3: Isotherms with g r a n d canonfor a limited amount of time, as shown ical Monte Carlo simulations in periodic in Fig. 3. b o u n d a r y conditions, y - 0.9 and T* -
/
0.5.
To examine the dynamical aspect of the hysteretic behavior, we consider the system geometry shown in Fig. 4. The porous material of length L in the z-direction is bounded by the gas reservoirs at z - 0 and z = L. Periodic boundary conditions are imposed on the x and y-directions. In typical experimental situations, starting from a (quasi)-equilibrium state, the external vapor pressure of the gas reservoir is instantaneously changed by a small amount, which induces gradual relaxations of the system into a new state. This geometry was used in recent work on dynamics of off-lattice models of adsorption (Sarkisov and Monson,
159
Figure 4: Simulation box of adsorption experiments.
20o0). Typical results of the grand canonical Monte Carlo simulation of the geometry of Fig. 4 are shown in Fig. 5 (a). Starting with an initial configuration, the bulk density inside the system initially shows a relatively rapid relaxation to a certain level, and subsequently enters a different regime where the system appears essentially unchanged over time scales many orders of magnitude larger. Although one might conclude from one of such simulations (or experiments) that the system has reached equilibrium as the latter regime is established, prior discussions based on the mean field theory implies that the system would have been in fact stuck in one of the extensively many metastable states. In Fig. 5, the effects of thermal fluctuations, necessary for barrier crossings between multiple minima, are seen to be negligible. Although dynamical interpretation of the grand canonical simulation can be given a precise meaning (Glauber, 1963), relaxations of fluids in mesoporous glasses are further constrained by the mass conservation. Diffusion thus serves as the primary means of density relaxation, and the dynamical behavior of the fluids is expected to be influenced both by the diffusive and activated barrier-crossing dynamics. To simulate such behavior, the geometry shown in Fig. 4 can be used with fluid occupation variables in the reservoirs and the system evolving with the grand canonical, and Kawasaki spin-exchange (Kawasaki, 1972) dynamics, respectively (Luzar and Leung, 2000; Woo and Monson, 2002). In Kawasaki dynamics, pairs of nearestneighbor lattice variables are swapped based on the Metropolis criterion, thus conserving the total density. Relaxations in the bulk density of the system becomes only possible with the mass transfer via the interfaces in contact with the grand canonical reservoirs.
160 a)
0.15 p'
0.160.14 ........ I ........ I ........ I ........ I ........
P
Ol,
O.I0 I
, " )
)
" ~.........................
~
L
p
o.,o
"
, 0.05 '
IZ1
.......
tO
0.08
0.0610o
10 ~
102 103 t (gmcs)
104
105
0.00 0
le+06
l (mcs)
2~+06
Figure 5: Density relaxations in Monte Carlo simulations of the geometry shown in Fig. 4 with conditions same as in Fig. 3 (~# = -5.5); (a) Grand canonical simulations. (b) Simulation with mass conservation. The solid line, dotted line, and the open circles are the Kawasaki dynamics, ideal diffusion, and the grand canonical result shown in (a) with TO= 2 gmcs. The inset shows the initial diffusion-limited regime in rescaled by the logarithmic scale. TO
Mass conservation severely restricts the already slow density relaxations (Fig. 5), and furthermore the extent of the expansion of time scale depends on the system size. It is in fact possible to quantify the relationship between the grand canonical and mass-conserved simulation results by considering the diffusion process of the geometry of Fig. 4 (Woo and Monson, 2002). Specifically, the density relaxation in Kawasaki dynamics can initially be modeled accurately by the ideal diffusion with a diffusion constant D, but at a later stage shows deviations. The latter dynamical regime closely follows the effective grand canonical dynamics rescaled by the characteristic diffusive relaxation time T O (Fig. 5). The rescaling (dilation) of the original time-scale tg of the grand canonical simulation to obtain the conserved dynamics can be written as tg --+ TDtg/TO, where TO is a microscopic unit of time, and T D --- L2/4~2D is the diffusive relaxation time where L is the system size. 5. C O N C L U S I O N S The theoretical treatments sketched in this paper provide a comprehensive
161 understanding of the origin of hysteresis in the adsorption/desorption of fluids in mesoporous glasses. The simple lattice model, combined with the realistic representation of the glass matrices generated by the Gaussian random field method, successfully describes the essential physics of the complex fluid behavior in such systems. The mean field theory of the model reveals that the hysteresis and scanning behavior arise due to the appearance of very many local minima in the free energy landscape. Such features show that the fluids in mesoporous glasses are fundamentally affected by the effects of disorder. The grand canonical simulations of the model confirm that the effect of thermal fluctuations ignored in the mean field treatment is in fact negligible for systems in the strongly hysteretic regime. It has been observed that the grand canonical simulations produce behavior qualitatively similar to experimental trends in the adsorption experiments. The hysteresis observed in such simulations has been argued as an artifact of the simulation method failing to equilibrate the system (Schoen, 1989). However, the qualitative picture described in this paper implies that attempts to find the global equilibrium in the rugged free energy landscape would not only be difficult, but also practically irrelevant to experiments, since the real system would never reach the equilibrium state. Instead, the dynamical interpretation of the grand canonical simulation (Glauber dynamics) and the concept of time-rescaling described in Sec. 4 provide a concrete foundation for understanding the dynamical aspects of hysteresis in terms of the statistical mechanical perspective. Our overall conclusion, therefore, is that for mesoporous glasses adsorption, hysteresis is a dynamic phenomenon that is not simply related to a capillary vapor-liquid phase transition. Slow dynamics for long times makes the states accessible in experiments in the hysteresis loop appear equilibrated and quite reproducible. Mean field theory and Monte Carlo simulations in the grand ensemble provide a physically realistic description of these phenomena. 6. A C K N O W L E D G M E N T S The research has been funded by a grant from the National Science Foundation (CTS-9906794).
162 REFERENCES
Cahn, J. W., 1965, J. Chem. Phys., 42, 93. Everett, D. H., 1982, in The Solid Gas Interface, 2, pp. 1055, E. A. Flood, editor, Academic, New York. Gelb, L. D. and Gubbins, K. E., 1998, Langmuir, 14, 2097. Gelb., L. D. and Gubbins, K. E., Radhakrishnan, R., and SliwinskaBartkowiak, M., 1999, Rep. Prog. Phys. 62, 1573. Glauber, R. J., 1963, J. Math. Phys., 4, 294. Kawasaki, K., 1972, in Phase Transitions and Critical Phenomena, 2, pp. 443, C. Domb and M. S. Green, editor, Academic, New York. Kierlik, E. Monson, P. A., Rosinberg, M. L., Sarkisov L., and Tarjus, G., 2001, Phys. Rev. Lett., 87, 055701. Levitz, P., Ehret, G., Sinha, S. K., and Drake, J. M., 1991, J. Chem. Phys., 95, 6151. Luzar, A. and Leung, K., 2000, J. Chem. Phys., 113, 5836. Page, K. S. and Monson, P. A., 1996, Phys. Rev. E 54, R29. Pitard, E., Rosinberg, M. L., Stell, G., and Tarjus, G., 1995, Phys. Rev. Lett., 74, 4361. Press, W. H., Teukolsky, S. A., Vertterling, W. T., and Flannery, B. P., 1992, Numerical Recipes in C, Cambridge, New York. Sarkisov, L. and Monson, P. A., 2000, Langmuir, 16, 9857. Shoen, M., Rhykerd, C. L., Cushman, J. H., and Diestler, D. J., 1989, Mol. Phys., 66, 71. Woo, H.-J. and Monson, P. A., 2002, to be published.
Woo, H.-J., Sarkisov, L., and Monson, P. A., 2001, Langmuir, 17, 7472.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouqueroi and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
163
Structural studies of mesoporous alumina m e m b r a n e s by small angle X-ray scattering John C. Dore a, Robert E. Benfield a, Didier Grandjean a, Giinter Schmid b, Michael KrSll b'd and David Le Bolloc'h c'e aCentre for Materials Research, School of Physical Sciences, University of Kent, Canterbury CT2 7NR, U.K. blnstitut Rir Anorganische Chemie, Universit~it GH Essen, Universit~itsstr. 5-7, D-45117 Essen, Germany. CEuropean Synchrotron Radiation Facility, 38043 Grenoble Cedex, France. dpresent address: Physics Department, Trinity College Dublin, Dublin 2, Ireland. epresent address: Laboratoire de Physique des Solides, Bat. 510, Univ. Paris Sud, 91405 Orsay, France. Small-angle X-ray scattering (SAXS) has been used to study the structures of mesoporous alumina membranes. These membranes, produced by anodic deposition, have an arrangement of parallel cylindrical pores centred on a disordered hexagonal lattice. The SAXS intensity profile varies over five orders of magnitude, and is a convolution of a structure factor for the 2D distribution of pore axes with a cylinder form factor for the pores. Rotation of the plane of the membrane changes the pattern from a ring structure for channels parallel to the X-ray beam to a set of vertical spots for channels perpendicular to the beam. The oscillatory pattern changes systematically with the anodic deposition voltage, confirming a linear relationship between voltage and pore separation. Initial results are also reported for alumina membranes containing cobalt nanowires in the pore volume. The SAXS technique complements and extends direct-imaging methods such as electron microscopy, which view only the surface structure for a limited area of the sample. The results indicate that the membranes show greater disorder than normally deduced from other techniques. 1. INTRODUCTION Mesoporous alumina membranes ("anodic almninium oxide", or AAO) are prepared by anodic oxidation of aluminium metal [1,2]. The cylindrical pores, perpendicular to the membrane surface, form hexagonal arrays of straight non-intersecting channels with pore densities up to 1011/cm2. Their diameters are controllable within the range 5 - 100 nm as a linear function of anodisation voltage. These membranes are used as molecular sieves, and have also found application as templates for metallic nanowires [3,4,5,6], metal clusters and colloids [7,8], and carbon nanotubes [9,10]. Structural characterisation of AAO membranes has been based mainly on direct imaging methods such as scanning or transmission electron microscopy [3,4,11]. These techniques image only a small area or volume of the material, which may not be representative of the overall structure or long-range order. To understand and control the properties of these membranes as host materials, a more complete structural characterisation is required.
164 Although small-angle X-ray scattering (SAXS) has been widely used to study porous solids, there have been few previous applications to membranes containing regular arrays of parallel pores. A preliminary USAXS (ultra-small angle X-ray scattering) study of AAO membranes has given a measurement of inter-pore distance [12]. A small-angle neutron scattering (SANS) study placed a limiting value on the pore size [13]. SANS has also been used to characterise a model of a biological membrane, supported in AAO membranes [14]. All these studies were restricted to AAO films marketed commercially as filters, with two different pore openings (20nm and 200 nm) based on a main pore diameter of 200nm. We have now used SAXS to make a systematic study of AAO membranes with a range of different pore diameters. Our aim was to evaluate the extent to which SAXS can more fully characterise the structure of these materials on several different length scales: pore diameter, pore separation, the long-range regularity of pore separation, pore length and aspect ratio, pore continuity and variation of pore diameter along the length [15]. For membranes containing metal nanowires, important additional parameters are the proportion of pores containing metal, the regularity of distribution of full and empty pores, and the degree to which individual pores are filled with metal. 2. EXPERIMENTAL Mesoporous alumina membranes were prepared by anodising electropolished high-purity aluminium foils in a polyprotic acid such as sulfuric or oxalic acid, using a lead plate cathode, followed by removal of the membrane from the AI backing. Details are described elsewhere [5,15]. Anodising voltages from 4 V to 60 V were used, giving membranes with pore diameters between 5 nm and 72 nm as confirmed by SEM measurements. Fig. 1 shows a typical AFM image of a membrane prepared using a voltage of 40 V. Membrane thickness was controlled by varying the anodising time, and ranged between a few hundred nanometres and several hundred microns. The 40 V membranes discussed in this paper had a thickness of 35 microns. Cobalt nanowires were deposited within the porous membranes, before their removal from the A1 backing, by electrochemical plating from an aqueous COSO4 solution using H3BO3 as a supporting electrolyte [6].
Fig I Structural features of the mesoporous alumina membranes: Schematic cross section (before removal of A1 backing) and schematic top view, together with an atomic force microscope AFM (Voltage) image for a membrane produced by anodisation at 40 V.
165 The SAXS experiments were conducted on beamline ID01 at the ESRF (Grenoble, France) using an incident wavelength of 1.54,~. A three-circle sample mount enabled the membrane orientation to be precisely varied by remote control. The beam size of 3.0 mm x 0.2 mm provided 1012 photons/sec at the sample position. SAXS intensity profiles were measured with a 2D gas-filled wire detector, consisting of an active array 230 mm x 230 mm with resolution of (179 lam)2. The central area was masked by a 6 mm diameter beamstop. The detector was positioned 4.5 m from the sample, giving a range of scattering vector k = (4n/~,)sin(O/2) from 0.005 to 1.2 nm-1. Extremely high forward scattering intensity was observed from the membrane samples, so an attenuator was used in the X-ray beam. Similar intensity enhancement when the orientation of the array of pores is parallel to the beam has also been observed in small-angle neutron scattering [14]. Counting times were typically 5 minutes. Background counts, recorded with an empty sample holder, were very low and there was no significant difference between background-subtracted and uncorrected datasets. Data analysis was carried out using the program FIT2D [ 16]. 3. EMPTY MEMBRANES
Samples with the face of the membrane perpendicular to the incident X-ray beam (i.e. with pores parallel to the beam) gave ring patterns [15] as shown in Fig. 2a for the 40V membrane. Rotation of the sample position about two axes to optimise this pattern allowed the sample orientation to be defined as perpendicular to the beam to within better than 0.2 ~. Rotation of the membranes about the vertical axis covered all possible orientations, as there is cylindrical symmetry around the beam axis. When the membranes were rotated about a vertical axis, the SAXS patterns changed systematically through a complex sequence, developing an anisotropy that was dependent on the membrane orientation. For membranes edge-on to the incident X-ray beam, with pore axes effectively in a horizontal direction perpendicular to the beam, the anisotropy increased to the limiting case where the SAXS intensity was confined to a set of vertical spots (Fig. 2b). In this geometry the coherent scattering is restricted to the vertical direction if the cylinder axes are accurately parallel. Fig. 2b shows that deviations from the spot pattern are very small, proving that the pore axes in the AAO membranes are parallel to within _+0.2~
Fig. 2 SAXS pattems observed from 40 V alumina membrane. (a, left) with the face of the membrane perpendicular to the incident X-ray beam (b, right) with the membrane edge-on to the beam.
166
The SAXS intensity profile is a product of the cylinder form-factor for the individual pores and a two-dimensional structure factor for the disordered array of parallel channels. The general intensity profile for an assembly of parallel pores oriented at an angle to the incident beam is very complex, and no general analytic expression for the intensity distribution for any orientation of a defined cylinder array has yet been derived. Consequently, a full analytical interpretation of the data is not yet possible. Quantitative data can best conveniently be extracted by studying radial summations of the two-dimensional SAXS data, summing the intensity values around the ring [ 15]. Fig. 3 shows typical results for 20V, 40V and 60V membranes face-on to the beam. The intensity varies over five orders of magnitude and is presented as a log-log plot. Similar profiles were obtained from membranes edge-on to the beam, and from membranes of different thicknesses. The general shape is similar for all three samples. A regular oscillatory pattern is clearly defined over four periods, covering a k-range of 0.02 to 0.5 nm-1, but displaced to different kvalues for each sample. These features arise from Bragg peaks in the structure factor from the two-dimensional hexagonal array of pores. The first peak position, k0, is related to a spatial dimension, d, given by d = 2n/k0 and corresponding to the mean separation of the pore axes. An increased membrane growth voltage gives a larger pore size and separation distance, so the first peak is displaced to lower k-values. The results, combined with those from membranes grown at other voltages, confirm the linear relationship between anodic growth voltage and pore separation, with a slope of 2.1 nm V-1. The Bragg peaks are broad, and do not give the full range of sharp peaks expected for a perfect hexagonal lattice. This shows that the AAO membranes have pore distributions that are more disordered than apparent from images of their surface structure (Fig. 1), which cover only a limited area. The variation in magnitude of the oscillations in the curves indicate that the 20 V membranes have the most regular arrangement of pore axes. 10000 40V Empty membrane
20V Empty membrane
\_
1000 100
lO ~ o~ r~
1
~
0.1 60V Empty membrane
0.01 0.001 0.01
0.1
1
k lnm-q Fig 3. Experimental SAXS intensity profile (log-log plot) for mesoporous alumina membranes grown at 20 V, 40 V and 60 V. The membranes were face-on to the X-ray beam.
167
1000 ~
k
40V
Empty
membrane
Face-on to the beam
100
./
10
'~
1
,~
0.1
~
CFace-on to the beam
~
tl
(R=24nm)
,o.., ............................. ~.............. q........ 4. . . . q 0.01
~I~.N
il ~ A
ll:t[
0.01 0.001
~=-~-+=4 -=~. . . . . . . . . . . . . . . . . . ~............. 1........ ~ ......... I...... ~..... ,--T-,-0.1
1
k [nm "l] Fig. 4 SAXS intensity profile from a 40V cobalt-filled AAO membrane in the face-on position, compared with that from an empty 40V membrane. The Bessel function dependence of the cylinder form-factor with R = 24 nrn, scaled for ease of comparison with the intensity curves, is also shown. Clearly, the structure factor dominates the SAXS patterns. It is relevant to ask whether the cylinder form factor, depending on the pore radius, also plays a significant role in the scattering distribution. The calculated cylinder form factor is defined by a Bessel function [12,15,17] which has zeroes at specific k-values. As shown in Fig 4, the experimental profiles for 40 V membranes (pore diameter 48nm) do not display a clear link to this pattern. The predicted first minimum is close to the broad third-order structure factor peak. It is consequently impossible to derive a value for the pore radius directly from the results without a more detailed analytic treatment. This is disappointing, as the pore size is fundamentally important in the use of AAO membranes in filtration or as templates. Electron microscopy studies show that for the synthetic conditions employed, pore diameters above 12nm are linearly related to anode voltage (1.2 nm/V) and so are approximately half the mean pore separation [7,15]. The SAXS patterns at higher k-values display asymptotic linear slopes indicating a power law relation. For a sharp interface with a smooth geometry, the intensity is expected to follow a k--4 power law in this Porod region. The observed slope varies systematically with pore size, membrane orientation and thickness. In some cases, exponents more negative than 4 are observed. This may arise from either angulosity or a non-sharp interface, and is discussed in more detail elsewhere [15]. 4. MEMBRANES FILLED WITH COBALT NANOWIRES In SAXS, the amplitude function of the coherently-scattered monochromatic X-ray beam is proportional to the electron density distribution pe(r) integrated over the sample volume. In a two-phase porous medium such as the AAO membranes, the electron density is defined by two possible values, and we may write the spatial distribution as:
168 p,(r) = 6 p r(r)
with
6 p = ps - p~
where Ps and pv are the electron densities for the solid matrix and the void volume respectively; f(r) is a form-factor which is defined by the geometry of the pores. If the pores are empty, pv = 0, but if they are filled with another material, pv is finite and the magnitude of AO depends on the contrast between the two values of the electron density. If the pore contents have the same electron density as the host matrix, then Ap = 0, and the scattering intensity (proportional to the square of the amplitude function) will be zero. This technique is known as contrast matching. Any observed scattering will then give detailed information on the degree and regularity of pore-filling, and on inhomogeneities in the interface region, as voids in the filling medium convert the assembly to a three-phase system. Table 1 shows electron density values pe for porous solids (Al203, SiO2), a n d some molecular liquids and metals relevant in various applications of AAO membranes. Several liquid halogenated hydrocarbons with pe in the same range as SiOz have been used for contrast matching in SAXS studies of porous silicas [18]. However, A1203 has strong ionic bonding leading to a high density. Alumina membranes therefore have a higher electron density than SiO2 and cannot be X-ray contrast matched with these molecular liquids, even allowing for the fact that AAO consists of poorly crystalline gamma-alumina rather than corundum. Conversely, the metals deposited as nanowires in AAO membranes [5,6] have pe values too high to give a contrast match with the alumina host matrix. Instead of attempting to contrast match, we chose cobalt as a pore-filling material. The contrast values Ap for alumina/void and alumina/cobalt, from Table 1, are of opposite sign but close in magnitude. If the cobalt completely occupies the pore cavities, the intensity profile should therefore have the same shape as that for the empty membrane. Structural studies (high energy X-ray diffraction and cobalt K-edge EXAFS) [6], together with NMR studies by others [19], have shown that cobalt nanowires within the pores of AAO membranes are a mixture of fee and hop structures. In bulk cobalt, the fcc phase is stable only at high temperatures. The hcp phase has the same lattice structure as bulk Co, but the orientation of the c-axis and the magnetic axis within the pores are important issues in understanding the unusual ferromagnetic properties of these nanoscale materials [6,20]. The regularity of pore filling is also of importance. Table I Relative total electron density values pe for several materials
Material
Densi~, (pro) (g cm- )
Electron density (pc) (103 electrons nm-3)
Al203 (corundum) A1203 (gamma) SiO2 (quartz) SiO2 (cristoballite) CBr4 CHBr3 CHzBr2 H20 Fe Co Ag Au Sn
3.96 3.5 - 3.9 2.64 2.32 3.42 2.89 2.49 1.00 7.87 8.80 10.50 19.32 7.28
1.17 1.03 - 1.15 0.79 0.70 0.91 0.77 0.67 0.33 2.20 2.45 2.75 4.67 1.85
,,,
,
169
Fig. 4 illustrates the general features of the SAXS patterns obtained from AAO membranes containing cobalt nanowires. The intensity profile from a 40V cobalt-filled AAO membrane (pore diameter 48 nm) in the face-on position is compared with that from an empty 40V membrane. There is a much more substantial modification of the SAXS pattern than expected. The peaks from the cobalt-filled membrane are displaced to slightly higher kvalues; the oscillatory amplitude is reduced; and there is an additional broad subsidiary peak at lower k-values (k=0.033 nm-~). This behaviour in the intensity patterns is unexpected. We have observed similar effects in cobalt-filled membranes with different pore sizes, and also in initial experiments on membranes with pores filled by other metals such as silver [21]. This suggests that the distribution of the metal in the pores does not correspond to the void volume of the empty pores, leading to increased disorder in the SAXS profile. Some nanowires may be discontinuous, or there may be some empty pores. More studies are needed to ensure that the electrochemical nanowire deposition process does not change the membrane mesostructure. Absorption effects, which are usually neglected in the SAXS formalism, may also be important for this geometry and an attenuation factor might be required in the theoretical treatment of the coherently-scattered waves. This also raises the interesting possibility of using anomalous scattering methods, such as ASAXS, near an absorption edge for the metallic substrate to study the behaviour in more detail. 5. CONCLUSIONS Structural characterisation of AAO membranes by SAXS complements and extends direct-imaging methods such as electron microscopy, which view only the surface structure for a limited area of the sample. The ability to measure anisotropic X-ray scattering with a two-dimensional detector is particularly significant, allowing study of changes in the SAXS patterns caused by rotation of the membranes. The dominant characteristic in the intensity profile is the distribution of the 2D hexagonal pore array, and the results indicate that there is greater disorder than normally deduced from other techniques. Cobalt-filled membranes show significant changes in the SAXS profile, which can be attributed to incomplete filling of the pore volume and/or X-ray attenuation. The initial studies presented here have opened up a new range of possibilities for quantitative determination of pore characteristics in AAO membranes using synchrotron X-ray techniques. ACKNOWLEDGEMENTS For financial support we thank the EU TMR programme [22] contract number FMRXCT98-0177. MK acknowledges a grant funded by Deutsche Forschungsgemeinschafi (DFG). The X-ray studies at the ESRF were supported by the EPSRC programme for work on international facilities.
REFERENCES [ 1] [2] [3] [4]
J. W. Diggle, T. C. Downie & C. W. Goulding, Chem. Rev., 69 (1969) 365. J. P. O'Sullivan & G. C. Wood, Proc. Roy. Soc. Lond., A 317 (1970) 511. J. C. Hulteen & C. R. Martin, J. Mater. Chem., 7 (1997) 1075. T.-A. Hanaoka, A. Heilmann, M. Kr6U, H.-P. Kormann, T. Sawitowski, G. Schmid, P. Jutzi, A. Klipp, U. Kreibig & R. Neuendorf, Appl. Organometal. Chem., 12 (1998) 367.
170
[5] R. E. Benfield, D. Grandjean, R. Pugin, T. Sawitowski, M. Kr611& G. Schmid, J. Phys. Chem. B, 105 (2001) 1960. [6] R. E. Benfield, D. Grandjean, J. C. Dore, Z. Wu, T. Sawitowski, M. Kr611& G. Schmid, Eur. Phys. J. D, 16 (2001) 399. [7] G. Homyak, M. Kr611, R. Pugin, T. Sawitowski, G. Schmid, J.-O. Bovin, G. Karsson, H. Hofmeister & S. Hopfe, Chem. Eur. J., 3 (1997) 1951. [8] T. Hanaoka, H-P. Kormann, M. Kr611, T. Sawitowski & G. Schmid, Eur. J. Inorg. Chem., (1998) 807. [9] T. Kyotani, B. K. Pradhan & A. Tomita, Bull. Chem. Soc. Japan, 72 (1999) 1957. [ 10] J. C. Dore, A. Burian, T. Kyotani and V. Honkimaki, in preparation; J. C. Dore, A. Burian, T. Kyotani & V. Honkimaki, ESRF Sciemific Highlights (2001) 23. [ 11] A. Heilmann, F. Altmann, D. Katzer, F. Mtiller, T. Sawitowski & G. Schmid, App. Surf. Sci., 144-145 (1999) 682. [12] J. S. Rigden, J. C. Dore & A. N. North, Stud. Surf. Sci. Catal., 87 (1994) 263. [13] L. Auvray, S. Kallus, G. Golemme, G. Nabias & J. D. F. Ramsay, Stud. Surf. Sci. Catal., 128 (2000) 459. [ 14] D. Marchal, C. Bourdillon & B. Dem6, Langmuir, 17 (2001) 8313. [ 15] D. Grandjean, J. C. Dore, R. E. Benfield, M. KrSll, G. Schmid & D. Le Bolloc'h, submitted for publication. [ 16] A. P. Hammersley, ESRF Internal Report ESRF97HA02T, "FIT2D: An Introduction and Overview", (1997). [ 17] I. Livsey, J. Chem. Soc., Faraday Trans 2, 83 (1987) 1445. [18] D. W. Hua, J. V. D'Souza, P. W. Schmidt & D. M. Smith, Stud. Surf. Sci. Catal., 87 (1994) 255. [ 19] G.J. Strijkers, J.H.J. Dalderop, M.A.A. Broeksteeg, H.J.M. Swagten & W.J.M. de Jonge, J. Appl. Phys., 86 (1999) 5141. [20] P. Paulus, F. Luis, M. KrSll, G. Schmid, & L. J. de Jongh, J. Magn. Magn. Mater., 224 (2001) 180. [21 ] R. E. Benfield, D. Grandjean, J. C. Dore, M. Kr611& G. Schmid, work in progress. [22] EU TMR project "CLUPOS". Homepage: www.elupos.lth.se
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Assessing microporosity nitrogen or liquid argon
by
immersion
171
microcalorimetry
into
liquid
Jean Rouquerol, Philip Llewellyn, Ricardo Navarrete, Franqoise Rouquerol and Renaud Denoyel MADIREL, (UMR6121) CNRSAJniversit6 de Provence, 26 rue du 141 e RIA, 13331 Marseille cedex 3, France This study highlights the use of low temperature immersion microcalorimetry into liquid argon and nitrogen with respect to various carbon and silica samples. We suggest that this method gives a closer estimation of the "real" surface area in the case of microporous samples. 1. I N T R O D U C T I O N We have previously shown that a most interesting information provided by immersion microcalorimetry into organic liquids was, in the case of carbons, a direct assessment of the internal surface area of the micropores [1]. This conclusion was based on calculations of adsorption potentials in micropores, on geometrical considerations and on microcalorimetric measurements on a number of activated carbons. It was only validated for carbons. More polar adsorbents (such as most oxides) are not easily amenable to a similar procedure because in polar liquids they give rise to specific interactions contributing to the enthalpy of immersion and m o d i f y i n g - in an a-priori unknown manner-its relationship with the surface area. Thus specific interactions can not be the same with the two walls of a slit shaped micropore containing just one molecule. This is why we thought it worthwhile to switch to immersion microcalorimetry into either liquid nitrogen or - even better - liquid argon, by making use of an isothermal, heat-flux, microcalorimeter, initially designed and built in our laboratory for the sake of gas adsorption experiments at 77 or 87 K. The experimental procedure includes the achievement of a highly reproducible initial state of the sample (through the use of Controlled Rate Thermal Outgassing) and also manages to exclude the ill-controlled heat of bulb-breaking out of the calorimetric measurement proper. We applied it to a set of six samples : either carbons or oxides, either porous or non-porous, either microporous or mesoporous. Combining gas adsorption experiments with immersion microcalorimetry shows the specific interest of the latter approach to assess the area of the surface wetted by the liquid.
172
2. Experimental 2.1 Samples Three carbon samples and three silica samples were chosen for the present study. Within each of these sets, one sample is non-porous, one sample mesoporous and one sample microporous. The following table gives some information concerning these samples. Table 1 : Description of the Designation C-np C-gp C-lap+mp S-np S-mp S-lap
samples used in the present study Sample name Origin Carbon black Vulcan 3, Cabot Pine wood charcoal CECA, coked to 900~ Pine wood charcoal CECA, steamed to 1000~ Precipitated silica XO75 LS, Biosepra Silica glass Vycor, Coming Silica gel GASIL 200, Unilever
The samples were outgassed prior to each exiperiment using Controlled Rate Thermal Analysis or CRTA [2]. A constant pressure of 10 mbar was used for the thermal preparation up to a final temperature of 250~
2.2 Adsorption manometry The isotherms were obtained with a commercial manometric device (ASAP 2010, Micromeritics) and using a point by point procedure of adsorptive introduction. The nitrogen isotherms were obtained at 77 K in liquid nitrogen and the argon isotherms were obtained at 87 K in liquid argon. The adsorptives used in the present study were obtained from Air Liquide (Alphagaz, purity greater than 99.999 %). The quantities of sample used for these experiments vary according to their nature. Typically around 50 m 2 of sample was placed into the cell and outgassed using the CRTA procedure mentioned above.
2.3 Immersion microcalorimetry The apparatus used for the low temperature immersion microcalorimetry is the same as that developed previously in house for gas adsorption experiments [3]. The glass sample cell used for these experiments is slightly modified with respect to those used for adsorption. A fragile capillary is glass blown to the cell outside the calorimeter thermopile but well into the immersion liquid (nitrogen or argon). Typically around 200 mg of sample was used for each experiment. The samples were outgassed using the CRTA procedure mentioned above. The sample was then placed into the calorimeter and thermal equilibrium was obtained after around 2 hours. The actual experiment consists of breaking the fragile capillary and measuring the heat effect. This heat effect typically lasts 15 minutes for the carbon samples and 25 minutes for the poorer conducting silica samples. Blank experiments carried out using cells without sample gave an effect of around 0.8 J that is small with respect to the values measured. This blank effect can be considered to be due to both the wetting of the cell and a compression of the system. As the fragile point is not in the
173 thermopile, the breaking of the capillary not measured at all. This is in complete contrast to the immersion calorimetry experiments that are carried out at room temperature [4]. Thus the blank effect is subtracted from the heat effect measured which is then divided by sample weight. To obtain a surface area from this immersion data, a reference BET area for the non-porous sample was taken. The heat effect is then divided by the heat effect obtained with the non-porous sample and multiplied by the BET surface area of the non-porous sample.
3. RESULTS AND DISCUSSION
3.1 Immersion into nitrogen and argon : direct comparison
Prior to any further reasoning, we can directly compare the enthalpies of immersion of our six samples into liquid nitrogen (Table 2) and into liquid argon (Table 3). The ratios
Nz/IAo HI, Ar
IAo sHI,
are given in the 2nd column of Table 4.
Table 2 : Results obtained with nitrogen (gas or liquid) at 77 K : immersion calorimetry and gas adsorption Sample
_AimmH / j.g-1 aimrn/ m2.g l aBET/ m2.g -i (aBET-aimm). 100/aimm (%)
C-np
13.5
81.7
81.7
0.0
C-~tp
87.8
530.5
563.0
6.1
C-lap+mp 225.0
1359.8
1187.0
-12.7
S-np
16.2
113.4
113.4
0.0
S-mp
23.7
165.6
172.0
3.9
S-lap
84.5
591.2
658.0
11.3
It can be seen that, for the three carbon samples, the enthalpies of immersion are similar in nitrogen and in argon, within + o r - 3%. Now, the situation is rather different for the silica samples, where IA,dsH[, N2 is continuously higher than [A,dsH ] , Ar. This can be explained by the specific interaction which is known to exist, on the surface of silicas, between the nitrogen molecule and the surface hydroxyls and which is negligible in the case of argon : this is what we could demonstrate, in an earlier work, by direct calorimetric measurements on adsorption from the gas phase, for a set of silica surfaces with different surface hydroxyl content [5].
174
Table 3 : Results obtained with ar__rg~ (gas or liquid) at 87 K : immersion calorimetry and gas adsorption Sample
-AimmH/ J.g-~ aimm/ m2.g -I aBET/ m2.g 1 (aBET-aimm).100/aimm (%)
C-np
13.1
72.9
72.9
0.0
C-gp
88.6
494.7
520.0
5.1
C-gp+mp
232.9
1300.9
1114.0
-14.4
S-np
14.4
89.0
89.0
0.0
S-mp
22.0
136.1
136.6
0.4
S-gp
81.0
501.0
591.0
18.0
Table 4 : Comparison between nitrogen and argon : enthalpes of immersion and BET surface areas Sample -AimmH N2 / -AimmH Ar aBETN2 / aBET mr C-np
1.04
1.12
C-gp
0.99
1.08
C-gp+mp
0.97
1.07
S-np
1.13
1.27
S-mp
1.08
1.17
S-gp
1.00
1.11
3.2 Comparing the BET areas obtained with nitrogen at 77 K and argon at 87 K
The BET areas reported in the 4 th column of Table 2 were calculated using the conventional area of 0.162 nm 2 per nitrogen molecule, whereas, for the argon molecule, an area of 0.138 nm 2 was used to provide the values listed in the 4 th column of Table 3. The ratios aBET,N2/ aBET,Ar are given in the 3rd column of Table 4. We can observe that, these ratios are all above 1 (from 7. to 27%). This is indeed what one expects from the possible orientation of the nitrogen molecule, whose effect is to increase the packing of this molecule on the surface and to lead to a monolayer content which is higher than that expected with a freely rotating molecule able to cover an area of 0.162 nm 2. 3.3 Comparing BET areas with those derived from immersion calorimetry
So far as the validity of the BET method is concerned, the role of the pore-size can, in first approximation, be explained as follows: -In ultra-micropores, accommodating less than two molecules in width, each molecule is partly or completely surrounded by the internal surface of the pore and therefore covers an area up to four times larger than that covered by the same molecule on a fiat surface.
175
-ln broader super-micropores, accommodating more than three molecules in width, a number of molecules located in the centre of the pore do not even touch the surface, so that, in reality, the surface area they are covering is zero. - In mesopores, the monolayer formation- at least statistical - now has a physical reality. It therefore follows that the apparent BET surface area, derived with a molecular crosssectional area suited for adsorption on a fiat surface, is, by principle, smaller than the reality in the case of ultra-micropores, larger than the reality in the case of broader super-micropores and satisfactory in the case of mesopores and, of course, macropores. In the light of the above remarks, let us now have a look to the ratios 100 (aBET - aimm) / almm which are reported in the last column of Tables 2 and 3. Their values, together with their algebraic sign, can be interpreted relatively easily: for the microporous/mesoporous carbon, the BET surface areas, for both nitrogen and argon, are in excess,which is consistent with the existence of broad micropores which were actually shown to also exist in this sample [ 1]. for the microporous carbon, the BET surface areas, for both nitrogen and argon, are larger than reality, as expected for super-micropores for the microporous Vycor glass, the BET surface areas, for both nitrogen and argon, are in excess, which corresponds to the existence of broader micropores; this is again a reasonable conclusion, since the BET specific surface areas of this sample are nearly twice smaller than those of the microporous carbon: it makes sense that the micropores are here somewhat larger -
-
3.4 Areal enthalpies of immersion The prerequisite for determining meaningful areal enthalpies of immersion is of course an independent assessment of the surface area really wetted by the immersion liquid. This is why, in Table 5, we only give them for the two reference samples which are known to be neither micro- nor meso-porous, ie for which the BET surface area can be expected to be reliable. Table 5 : Areal enthalpies of immersion (-AimmH / mJ.m -2) in liquid nitrogen or liquid argon Reference to BET, N2 surface area
Reference to BET, Ar surface area
Sample
In liquid nitrogen
In liquid argon
In liquid nitrogen
In liquid argon
C-np
165.5
159.7
185.5
179.0
S-np
198.3
176.1
222.2
197.4
Now, we still have the choice between the BET surface areas measured by nitrogen adsorption at 77 K and those measured by argon adsorption at 87 K: which are the "safest" ? Although there are good reasons to consider that the cross-sectional area of the spherical argon molecule is less debatable than the cross-sectional area of the diatomic nitrogen molecule, which depends on its orientation over the adsorbing surface, we prefer to keep in Table 5 the two types of results, since the nitrogen-BET remains the standard method.
176 4. Conclusion
The above work reminds us of the limited meaning of the BET surface area of a microporous material and of the absolute need of referring to an "Apparent BET surface area" or to an "Equivalent BET surface area" as soon as the sample is known to be microporous. It seems that immersion calorimetry into liquid nitrogen or liquid argon allows to go one step further in the determination of the internal surface area of micropores. These experiments requested a specially designed calorimeter operating at 77 or 87 K, with the special feature that the brittle end broken to start the immersion is located out of the calorimeter proper and therefore has no effect on the calorimetric measurement. Liquid nitrogen and liquid argon provide a very similar areal enthalpy of immersion of carbons : for instance, 165 and 160 mJ. m -2 in nitrogen and argon, respectively, if the surface area of the reference material is measured by the BET method with nitrogen at 77 K. Now, the enthalpy of immersion of the silica samples into liquid nitrogen is systematically higher than into liquid argon but this should not influence the derivation of the "immersion surface area" provided the reference sample is correctly selected. We must therefore rule out the simplifying asumption (initially made by the pioneers of the method, Chessick et al [6] and Taylor [7]) that the areal enthalpy of immersion is somewhat independent from the chemical nature of the adsorbent. This means that a calibration with a non-porous sample is needed for each type of surface. This is not too much of a problem since the duration of a complete calorimetric experiment (after preliminary weighing and outgassing of the sample) is ca 2 hours. We think that the next step will be to associate the as method with this immersion calorimetry method to evaluate the size of the micropores.
References
[ 1] R. Denoyel, J. Fernandez-Colinas, Y. Grillet and J. Rouquerol, Langmuir, 9 (1993) 515-8 [2] J.Rouquerol, .Thermochim. Acta, 144, (1989) 209-24. [3] J. Rouquerol, in Thermochimie, CNRS Ed., Paris, 1971, p.537. [4] S. Partyka, F. Rouquerol, J. Rouquerol, J. Coll. Interf. Sci., 68 (1979) 21. [5] J. Rouquerol, F. Rouquerol, C. Peres, Y. Grillet, M. Boudellal, in "Characterisation of Porous Solids" (S. J. Gregg, K. S. W. Sing, H. F. Stoeckli Eds.) Soc. Chem. Ind., London, 1979, p.107. [6] J.J. Chessick,G.J. Young and A.C. Zettlemoyer, Trans. Far. Soc., 50(1954) 587-92 [7] A.G. Taylor, Chemistry and Industry, (1965) 2003.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
177
The lower closure point of the adsorption hysteresis loop of fluids in mesoporous silica materials A. Schreiber, S. Reinhardt and G.H. Findenegg Stranski Laboratory of Physical and Theoretical Chemistry, Technical University Berlin, Strasse des 17. Juni 112, 10623 Berlin, Germany The pore condensation hysteresis of two fluids (cI-iF3 and C2F6) in mesoporous silicas with open cylindrical pores of uniform size (MCM-41 and SBA-15), and in a silica with large cellular mesopores which are accessible only via micropores or narrow mesopores, has been studied over a wide temperature range up to the critical point of the fluids. From the sorption isotherms in MCM-41 and SBA-15 the hysteresis onset-temperapore TH and the corresponding pore condensation pressure (P/P0)H was determined for several materials of different pore radius R. For the material with cellular pores the temperature dependence of the lower closure point pressure (P/P0)L was measured over a wide temperature range. It is found that in a hysteresis phase diagram (p/po vs. T/Tc)the locus of (P/p0)H and of the closure point-pressure (P/p0)L almost coincide. Thus it appears that this locus represents a universal border line between sorption isotherms with and without hysteresis loop. 1. INTRODUCTION It has long been recognized that the lower closure point of the hysteresis loop of nitrogen adsorption isotherms (77 K) is frequently situated at a relative pressure p/po close to 0.4 but never below that value, irrespective of the nature of the porous material. 1 A possible explanation for such a lower limit of the closure point was proposed more than 50 years ago by Schofield 2 on the basis of the limit of metastability of the capillary-condensed liquid. Later, this so-called tensile strength theory was analysed by Burgess and Everett 3 and by Sonwane and Bhatia 4, but its status is still not settled. A major point of uncertainty remained the influence of the pore morphology on the hysteresis. In addition, there has been a lack of experimental pore condensation data covering a sufficiently wide temperature range. In order to overcome these obstacles, we have studied the pore condensation of fluids in two types of mesoporous silica of strongly different pore morphology: (a) MCM-41 and SBA-15 with open cylindrical pores of uniform pore diameter, and (b) silica materials with large cellular mesopores that are accessible via narrow mesopores or micropores only. Sorption isotherms of a nonpolar fluid (C2F6) and a polar fluid (CHF3) in these two types of mesoporous silica
178 were measured in a wide temperature range, from the normal liquid region up to nearly the critical temperature To. Our study indicates that the locus of lower closure point pressures (P/p0)L of fluids in cellular silicas is closely related to the so-called hysteresis phase diagram of fluids in a set of materials with open cylindrical pores of uniform pore width. We will sketch briefly a tentative explanation for this observation. 2. EXPERIMENTAL 2.1. Materials MCM-41 silicas were synthesized according to the prescription by GrOn et al. 5, using alkyl trimethylammomium bromides (CnTAB) with alkyl chain length n =14, 16 and 18 as the structure-directing agent, and tetraethoxysilane (TEOS) as the silicate precursor. SBA-15 silicas of different pore diameters (5-10 nm) were synthesized as reported by Zhao et al. 6, using technical-grade PEO-PPO-PEO triblock copolymers (Pluronic P103 and P123) as the template, and TEOS as the silicate precursor. Small-angle XRD patterns in a range of 20 angles from 0.5 ~ to 6 ~ were recorded by a Kratky compact camera. All samples exhibit the Bragg pattern of a 2D hexagonal lattice, with five well-resolved peaks for the MCM-41 samples, and at least three well-resolved peaks for the SBA-15 samples. Nitrogen adsorption isotherms were obtained with a Gemini 2375 volumetric gas adsorption apparatus (Micromeritics) at 77 K. The specific pore volume Vp was calculated from the adsorbed amount in the plateau region above the pore condensation pressure and the density of liquid nitrogen at 77.4 K (0.808 gcm'3). The mean pore diameter D was derived from the adsorption branch of the nitrogen isotherms using the prescription by Kruk et al. 7 For the materials $2 and $3 with pore widths D > 8 nm (pore condensation pressures p/po > 0.7), for which the Kruk prescription overestimates the true pore size, a corrected value of D was derived from an in-situ SANS nitrogen adsorption study. Details of the synthesis and characterisation of the materials are given elsewhere. 8'9 Some properties of the materials are summarized in Table 1. A disordered mesoporous silica material (sample C1) was obtained by the route outlined for SBA-15, when the Pluronic P123 used as the template was replaced with an equal-weight mixture of Pluronic and trimethylbenzene. SEM micrographs indicate that this material constitutes a system of spherical pores of wide size distribution, connected and accessible by small mesopores and/or micropores only. The nitrogen adsorption isotherm indicates a wide pore size distribution and a H2 type hysteresis loop. Some properties of this material are included in Table 1. The experimental fluids, trifluoromethane (CHF3) and hexafluoroethane (C2F6) were obtained from Linde (purity 99.95%) and were used as received. 2.2. Adsorption measurements Adsorption measurements were made by a gravimetric method using two ultra-microbalances (Sartorius S3D), suitable for measurements in the low-pressure region up to 1 bar (isotherms up to 188 K for CHF3), and the region of elevated pressures (isotherms up to the critical temperature), respectively. Due to the symmetrical two-pan construction of these balances, buoyancy effects of the sample can be compensated by using combinations of taring materials. In this way it is possible to determine the mass of the fluid in the pore space in a direct way. Details of the experimental setup and procedure are given in ref. 9.
179
Table 1 Properties of materials used in this study: Ordered mesoporous silicas MCM-41 (M1 - M3) and SBA-15 (S1 - $3), and a cellular silica material (C1).
p/Po p/po 'Sampie ao nm Ads. Des. M1 4.04 0.280 0.280 M2 4.63 0.354 0.354 M3 4.92 0.418 0.411 S1 9.33 0.66 0.59 $2 11.1 0.75 0.68 $3 12.3 0.81 0.74 C1 0.88 0.49
D
Vp
nm
cm 3 g-1 m 2~1
3.4 3.8 4.2 7.2 8.9 9.5 ~18
0.79 0.90 0.99 0.87 1.06 1.42 0.57
as
1060 1040 870 760 750 910 540
4.0
9 ~t ,~ 1 ~'E
0.o
~'~
0.0
,,,K / r ~ - - - t I"K ~ ~ ~ -
0.2
0.4
1~/~233
0.6 P/P0
9
l
0.8
9 -
K
?
1.0
~.,]
/ ~ 4
I''K
~'oE 1.S
0.0
0.0
2G3 K
0.2
0.4
0.6 PIP0
0.8
1.0
Fig. 1. Sorption isotherms o f CHF3 in the SBA-15 sample $2 (Fig. 1A, lett) and in the cellular material C1 (Fig. 1B, right) for 8 different temperatures in the range 168 K to 293 K, as indicated in the graphs. The adsorbed amount is expressed by the mean density p of the pore fluid. For clarity the isotherms are displaced along the ordinate by increments of 0.3 gcm "3 (Fig. 1A) or by 0.2 gcm -3 (Fig. 1B). The critical temperature of CHF3 in the bulk fluid state, To, is 299.1 K.
180 3. RESULTS 3.1. Sorption hysteresis in MCM-41 and SBA-15 Sorption isotherms of C H F 3 and C2F6 in the MCM-41 and SBA-15 samples listed in Table 1 were recorded over the entire pressure range (0 < p/p0 <1) in a temperature range from 168 K to 293 K, i.e. from just below the triple point of C2F6 (Tt = 173.1 K) to the critical temperature of the fluids (To = 292.9 K for C2F6 and 299.1 K for CHF3). Qualitatively, a similar temperature evolution of the sorption isotherms was found for all systems. Figure 1A illustrates this evolution for CHF3 in the sample $2. The adsorbed amount is expressed as a mean density of the pore fluid, p = m/Vp, where m is the specific mass of adsorbed gas and Vp the specific pore volume of the sample (Table 1). The isotherms cover a range of reduced temperatures T/Tc from 0.60 to 0.98. Up to 233 K (T/T~ = 0.78) the isotherms exhibit pronounced H1 type hysteresis loops, but the width of the loop decreases with increasing temperature. While at 253 K (T/T~ = 0.85) there is still a narrow loop, the isotherm at 273 K (T/T~ = 0.91) clearly exhibits pore condensation without hysteresis. The temperature at which hysteresis disappears is called the hysteresis temperature TH of the given system. From Fig. 1A we deduce that TH = 263 _+ 5 K for the CHF3/S2 system. The isotherm for 293 K exhibits no pore condensation step but a continuous pore filling, which implies that the pore fluid is in a supercritical state at this temperature, although the bulk fluid is still below its critical temperature. The maximum density of the pore fluid at this temperature (at p/po = 1) is nearly 1 g/cm 3, i.e. significantly higher than the critical density of the bulk fluid (pc = 0.525 g/cm3). 1~ Figure 2 shows the temperature dependence of the pore condensation and pore evaporation pressures p/po (defined operationally as the point of inflection on the ascending or descenting steps of the isotherms) for CHF3 in three ordered mesoporous silicas (M3, S 1, and $3). In all cases the pore condensation and pore evaporation pressures coincide above a specific temperature, i.e. the hysteresis temperature TH. The uncertainty in the determination of TH is rather large (ca. + 5 K), mainly due to the limited number of measured isotherms for the relevant temperature range. Qualitatively, the behaviour illustrated in Figure 2 is found for both fluids, CHF3 and C2F6, in all SBA-15 and MCM-41 materials except M1. Generally, the hysteresis temperature decreases with decreasing pore radius R, and all data conform to a relation ATH/T~ - (Tc - TH)/T~ = a/R, where a is a parameter proportional to the molecular diameter of the fluid. For the sample of smallest pore size (M1), the present fluids exhibit pore condensation without hysteresis down to the lowest experimental temperature (178 K), indicating that TH/T~ < 0.60 for this material, in agreement with the above relation. An analysis of the hysteresis temperatures TH in the pristine and surface modified SBA-15 and MCM-41 materials will be presented elsewhere. ~1 Since the hysteresis temperature of fluids in open cylindrical pores is a singular function of the pore radius, which in turn is a function of the pore condensation pressure, one also expects a singular relation between the hysteresis temperature TH and the respective pore condensation pressure (p/p0)H.12 Such a relation is indeed observed for the present systems, as shown in Figure 3A, where the results of the present study are shown together with results taken from the work ofMorishige et al. 13 for several other gases (argon, nitrogen, oxygen and ethylene) in MCM-41. Within the limits of experimental error all data can be represented by a common curve which is almost linear in the range of reduced temperatures T/T~ from 0.5 to 0.9. This curve marks the border line between the region of pore condensation hysteresis (at
181
low reduced temperatures T/Tc and high relative pressures p/po) and the region of pore condensation without hysteresis (high temperatures T/Tr low pressures p/po), in materials with open-ended cylindrical pores.
0.9 0.8
0.7
7.3 nm~-'"'~
0.6
3.7 nm
O.S
0.4 0.3
160
180
,
21)0
,
22:0 TIK
2'10
260
280
Fig. 2. Temperature dependence of the pore condensation and pore evaporation pressures p/po for CI-/F3 in MCM-41 (sample M2) and SBA-15 (samples S1 and $3). Open symbols: pore condensation; full symbols: pore evaporation. Arrows indicate the hysteresis temperature TH. 1.0
9
u
i
"
9
n
u
9
1.0
ill,
9
i i
0.8
[]
9 CHF3 9 C=F~
0.9
/
,'.:J CHF3 9 CzF6
0.8
Morish
0.6
./
: / ~
a. 0.7
n.
0.6
0.4
rn rn
Drl
/"
0.5
[]
0.2 0.4 D
0.4 0.5 016 017 018 0.9 1.0 t/'l"c I
'
!
.
.
.
.
I
'
0.5
0:6
017
t/t
018
0.9 !
1.0
Fig. 3A (le~): Hysteresis phase diagram for fluids in materials with open-ended cylindrical pores (MCM-41 and SBA-15): Hysteresis pressure (P/p0)H VS. hysteresis temperature TH/Tc for materials of different pore radius. Results of this study (triangles) and data for argon, nitrogen, oxygen and ethylene from Morishige et al. ~3(open squares). Fig. 3B (right): Temperature dependence of the lower closure point pressure (P/p0)L of the hysteresis loop of CHF3 and C2F6 in the closed-pore material C 1.
182 3.2. Sorption hysteresis in the cellular material The effect of "pore blocking" on the sorption hysteresis was assessed by studying the pore condensation of CHF3 and C2F6 in sample C1. Figure 1B shows a series of adsorption isotherms for CHF3 in a temperature range from 168 K to 283 K, corresponding to reduced temperatures T/T~ from 0.56 to 0.95. The isotherms exhibit pronounced H2 type hysteresis loops" Adsorption along the ascending pressure scan increases in a continuous way, with just a weak point of inflection at a relative pressures that increases from p/po ~ 0.85 at low temperatures to p/po ~ 1 at high temperatures. The desorption branch of the sorption isotherms exhibits a pronounced plateau region and a sharp decrease caused by pore evaporation. The pore evaporation pressure, again defined as the inflection point of the descending branch, exhibits a strong temperature dependence, increasing from p/po = 0.42 at T/T~ = 0.56 to p/po = 0.96 at T/T~ = 0.95. Similar results were obtained for C2F6 in this material. Figure 3B shows the temperature dependence of the pore evaporation pressures(p/P0)L for CHF3 and C2F6 in the pore-blocking material C1. For both fluids a nearly linear temperature dependence of (P/P0)L over almost the entire range of reduced temperatures (0.55 <_ T/T~ <_ 0.95) is observed. Since the pore evaporation pressure is a measure of the lower closure point of the hysteresis loop, the locus of (P/p0)L also represents the temperature dependence of the lower closure point and thus of the lower limit of hysteresis in materials with "pore blocking". Our results are in broad agreement with published results for the lower closure point of the hysteresis loop. Specifically, our results agree within + 0.05 in p/po with results for Xe in Vycor 3 and mesoporous silica ~4 as well as SF6 in porous graphitic carbon. ~5 4. DISCUSSION The primary aim of this work has been to compare the limits of pore condensation hysteresis of fluids in silica materials of grossly different pore morphologies. For materials with open ended cylindrical pores of uniform pore width (MCM-41 and SBA-15) we have determined the hysteresis temperatures TH of the two fluids in samples with pore widths from 4 to 10 nm. Our results yield a linear dependence of the hysteresis temperature increment, ATn = Tc-TH, on the inverse pore radius, 1/R, in agreement with similar results reported in the literature. 13'~6 In addition, we have found (Figure 3) that the pore condensation pressure at the hysteresis temperature, (P/p0)H, in samples of different pore radius is a nearly linear function of the respective reduced hysteresis temperature (T/Tc)H. The locus of these points in a p/po vs. T/To diagram marks the border line between pore condensation with and without hysteresis (hysteresis phase diagram) for fluids in materials with open-ended cylindrical pores. Figure 4 shows the hysteresis phase diagram of the present fluids, and of argon, nitrogen, oxygen and ethylene in MCM-41 and SBA-15 (Figure 3A) and, in the same graph, the temperature dependence of the lower closure point - expressed by the line of pore evaporation pressures (P/p0)L- of fluids in materials with pore blocking (Figure 3B). It is seen that the experimental points for the two phenomena fall broadly on a common line which appears to terminate at the critical point the bulk fluid. Although further studies are necessary to confirm this remarkable observation, the present findings hint at a common origin of the two hysteresis phenomena. Below we sketch a tentative explanation of the observed lower closure points of the hysteresis in cellular materials in terms of the hysteresis phase diagram of the open-pore systems.
183
1.0
. '
0.8
CHF 3
$
C2F 6
A
Xenon
T
SF 6
o no
9
0.6
.
I
[]
. 9
.
. I
9
I
lit T d, @
pore blocking A~
[3
A
CHF 3 C2F 6
cylinder
J' till
pores
various gases
0
'
I []
C) 0
,Ip
0.4 A
0o2
,
0.3
A
0.4 '
.
I
0.5
.
9
I
.
9
0.6
I
0.7
.
,,
0.8 '
0 i9
1.0
Tff e
Fig. 4. A comparison of the hysteresis phase diagram of fluids in silicas with open-ended pores (MCM-41 and SBA-15) with the temperature dependence of the lower closure point pressure (P/p0)L of fluids in materials with closed-end pores (see text).
As explained above, the silica C1 contains large cavities which are accessible through more narrow channels. The classical view of hysteresis in such materials is that on desorption the liquid in the large cavities cannot evaporate until the narrow channels empty ("pore blocking"). However, a recent molecular dynamics study of adsorption and desorption in model pores of ink-bottle geometry ~7 has indicated that true pore blocking by the liquid in the narrow channels does not occur, as the large cavities can be emptied while the narrow channels remain filled. This process was shown to occur by diffusional mass transport of the fluid from the large cavity to the narrow channel and evaporation from the channel into the gas phase. We conjecture that this process will depend on whether the fluid in the connecting channel is below or above the respective hysteresis temperature. We consider a model in which the pore space of the cellular material constitutes a network of mesopores (or constrictions) of different radii ri (rmin < ri < rmax), and we postulate that pore segments of radius ri in which the pore fluid is above the respective hysteresis temperature TH(ri) c a n n o t cause delayed evaporation. Accordingly, at some temperature T3 = TH(r3) only the pore segments with radii ri > r3 can cause delayed evaporation of the pore liquid, and the height of the desorption step at this temperature corresponds to the mass of fluid contained in that part of the pore volume that has pore radii ri > r3. At a lower temperature T2 = TH(r2) pores of radius r2 < r3 can cause delayed evaporation, and thus the lower closure point will be shifted to a lower pressure p/po corresponding to the hysteresis pressure (P/po)H for the pores of radius r2. Accordingly, in this simple model the locus of hysteresis pressures (p/po)H as determined from materials with open-ended pores will dictate the locus of lower closure points (P/Po)Lof the cellular material.
184 The essence of the above model is the assumption that pore segments of radius ri in which the liquid is above the hysteresis temperature TH(ri) cannot cause delayed desorption. This assumption is immediately plausible if TH were to coincide with the pore critical temperature, as in this case the pore fluid is in a supercritical state above TH, and thus the mass transport is not retarded by a gas/liquid meniscus. Some aspects of our model are remeniscent of the tensile strength hypothesis, although the concept of la pore critical temperature was not discussed at the time when that hypothesis was proposed. On the other hand, the present picture does not imply that the locus of lower closure points (P/p0)L should be independent of the nature of the porous matrix. We conjecture that (P/p0)L is given by the locus of pore hysteresis points of the fluid in open-pore systems of uniform pore size. A more comprehensive discussion of this model will be presented elsewhere. 11 Acknowledgement. The authors thank I. Ketelsen for help with the experimental measurements. This work was supported by the Deutsche Forschunsgemeinschafi in the framework of the Sonderforschungsbereich 448 (Project B 1).
REFERENCES
1 S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, pp. 154. 2 R.K. Schofield, Discuss. Faraday Soc., 1948, 3, 105. 3 C.G.V. Burgess and D.H. Everett, J. Colloidlnterface ScL, 1970, 33, 611. 4 C . G . Sonwane and S.K. Bhatia, Chem. Eng. ScL, 1998, 17, 3143; Langmuir, 1999, 15, 5347. 5 M. Gr0n, K.K. Unger, A. Matsumoto, and K. Tsutsumi, Characterization of Porous Solids, Vol. IV, ed. B. McEnaney et. al., Royal Society of Chemistry ,Cambridge, UK, 1997, p. 81. 6 D. Zhao, J. Feng, Q. Huo, N. Melosh ,G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 1998, 279, 548 7 M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 1997, 13, 6267; J. Phys. Chem. B, 1997, 101,583; M. Kruk, M. Jaroniec, C.H. Ko and R. Ryoo, Chem. Mater., 2000, 12, 1961. 8 A. Schreiber, I. Ketelsen and G.H. Findenegg, Phys. Chem. Chem. Phys., 2001, 3, 1185. 9 A. Schreiber, Ph.D. Thesis, Technische Universitat Berlin, 2002. 10 T.E. Morsy, J. Chem. Eng. Data, 1970, 15, 256. 11 A. Schreiber and G.H. Findenegg, manuscript in preparation 12 R. Evans, U.M.B. Marconi and P. Tarazona, J. Chem. Phys., 1986, 84, 2376. 13 K. Morishige, H. Fujii, M. Uga and D. Kinukawa, Langmuir, 1997, 13, 3494; K. Morishige and K. Shikimi, J. Chem. Phys., 1998, 108, 7821. 14 W.D. Machin, J. Chem. Soc. Faraday Trans., 1992, 88, 729; Langmuir, 1994, 10, 1235. 15 T. Michalski, A. Benini and G.H. Findenegg, Langmuir, 1991, 7, 185. 16 E.G. Sonwane, S.K. Bhatia and N. Calos, Ind. Eng. Chem. Res., 1998, 37, 2271. 17 L. Sarkisov and P.A. Monson, Langmuir, 2001, 17, 7600.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
185
New methodologies in mercury porosimetry Sean P. Rigby Department of Chemical Engineering, University of Bath, Claverton Down, Bath, BA2 7AY, U.K.
This paper describes a methodology that can be used to determine the macroscopic (>-30 gm) spatial arrangement of characteristic pore sizes within the macropore network of a bidisperse porous medium. INTRODUCTION Many reactions taking place within catalyst or absorbent pellets in industrial plants are diffusion-limited. Under the typical operating conditions for many absorbents, diffusion of gases into the porous solid occurs in the Knudsen regime. In such circumstances the rate of gas pick-up of these materials is strongly dependent on the pore structure. The pore structure for absorbent pellets that will deliver the most efficient operation of an absorbent bed requires a pervasive system of macropores which provide rapid transport of the gas flux into the centre of the pellet. A network of ramified mesopores branching off the macropores then provides extensive surface area for absorption of gas molecules. Therefore, when manufacturing an absorbent it is necessary to be able to determine the spatial distribution of the macropore network in a product to ensure that the pore structure is the most appropriate for the particular duty for which it is intended. In previous work [1] a combination of mercury porosimetry, nitrogen sorption and magnetic resonance imaging (MRI) experimental techniques was used to determine the spatial distribution of the macropore network in a bidisperse alumina pellet. The methodology [ 1] also allowed the determination of the pattern of the spatial distribution of pore neck sizes that caused "pore-shielding" within the macropore network. However, since this methodology required the use of conventional 1H MRI techniques, it was unsuitable for use with more chemically heterogeneous materials such as supported metal catalysts, coked catalysts or mixed oxide absorbents. There is therefore a requirement for a technique which is able to determine the same sort of information on the spatial distribution of the macropore network for these types of materials. It is possible to examine the structure of the void space of porous media directly using micro-focus x-ray (MFX) imaging techniques. However, even utilising the latest x-ray sources, MFX imaging systems can only directly image samples of a few mm thickness down to -1 ~tm, or samples of a few microns in size down to -50-100 nm. These resolution limits are still much larger than the size of mesopores (pore sizes 2-50 nm) and smaller macropores, and thus these techniques are unsuitable for examining the pore space of these materials. 3D-
186 TEM is capable of directly examining the pore space of mesoporous samples at resolutions o f - l - 1 0 nm. However, in order to achieve this resolution it is necessary to have a sample size of-~500 nm or less. Unfortunately, in order to fully understand the performance of a typical heterogeneous catalyst pellet or absorbent, it is necessary to, both, possess the detailed local (for small domains of the sample) information concerning regional pore structure (porosities, pore sizes etc.) that could be provided by 3D-TEM, but also be able to map the variations in these properties and their juxtaposition across the whole macroscopic pellet (of size >1 mm). Obtaining all of this information by 3D-TEM would be both laborious and impractical, since it would necessitate the examination o f - 1 0 ~ separate microscopic samples from a single macroscopic (-1-10 mm) catalyst pellet, while maintaining a detailed knowledge of the former locations of all of these samples within the original pellet! It is the purpose of this paper to present a methodology which is able to obtain information about the spatial distribution of the macropore network for a bidisperse alumina sample using mercury porosimetry. THEORY
Mercury porosimetry is based on the physical principle that mercury is a non-wetting fluid for most materials, and therefore requires an externally imposed pressure in order to force it into the pores of a porous solid. An increasing pressure of mercury is required to force it into pores of decreasing size. The basic mercury porosimetry experiment, therefore, consists of increasing the imposed pressure in small increments and measuring the volume of mercury entering the sample during each pressure increment. The imposed pressure is generally related to a pore size via the Washbum [2] equation: 27 cos 0 p =- ~ ,
(1)
where p is the imposed pressure, y is the surface tension, 0 is the contact angle and r is the radius of the pore. When using this equation to interpret raw mercury porosimetry data it is generally assumed that the parameters y and 0 are constants. In addition, it is also common practice to assume arbitrary values of 0 or the value of 0 appropriate to a single large pore size as determined using the method of Shields and Lowell [3]. As a consequence of this type of assumption, a comparison of the pore size distributions for different samples would then only be on a relative basis. A large amount of work described in the literature has demonstrated that the values of the surface tension and contact angle may vary with pore size [4], the nature of the surface [5], or whether the mercury is advancing or retreating [4,5]. Liabastre and Orr [6] made a study of the morphology of controlled pore glasses using electron microscopy and mercury porosimetry. These workers obtained values for the diameters of the pores in the glasses by direct observation from microscopy. They compared these values with the corresponding values obtained from mercury intrusion and extrusion porosimetry via Eq. (1), assuming fixed values of contact angle and surface tension. Kloubek [4] used these data to determine the relationships for the variation of the product 7.cosO as a function of pore radius, for both advancing and retreating mercury menisci. Kloubek [4] obtained expressions of the form:
187
Table 1. A
.
Intrusion Retraction
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
B
.
7" c o s 0 = A + - - ,
.
.
.
.
-302.533 -68.366 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
B
Range of validity (nm) ,,,
-0.739 -235.561
6-99.75 4-68.5 .............
,
(2)
F
where A and B are constants whose values are dependent on whether the mercury meniscus is advancing or retreating. The values of these constants found for the controlled pore glasses, and their pore size ranges of application, are given in Table 1. The values of the constants A and B can be found by the substitution of Eq. (2) into Eq. (1) and the calibration of the pressure at which the intrusion or extrusion of mercury occurs for a particular porous solid against an independent measure of pore size. For the controlled pore glasses studied by Liabastre and Orr [6] this independent technique was electron microscopy but other techniques may also be suitable. Many authors [7] propose that mercury intrusion and nitrogen desorption are analogous invasion percolation processes. Therefore nitrogen desorption is a way of independently fixing the pore size at which mercury intrusion occurs and hence may be a different way of determining the values of the constants A and B for mercury intrusion. The raw mercury intrusion data for a new material can be analysed using the values of A and B determined for intrusion of controlled pore glasses using microscopy. If this analysis gives rise to an apparent pore size distribution in agreement with that obtained from nitrogen desorption then the values of A and B for controlled pore glasses must also be appropriate for the new material. Similarly the values of A and B determined for mercury retraction in controlled pore glasses can also be used to analyse the raw extrusion porosimetry data for the new sample. If an a p r i o r i superposition of the intrusion and retraction curves for the new material is subsequently achieved, then the values of A and B determined for retraction from controlled pore glasses must also be appropriate for the new material. If no such superposition is obtained, then, either the aspect of a surface that controls the value of y cos 0 for retraction differs between the new material and the controlled pore glasses in some way (in which case the values of A and B will differ), or there is some residual structural hysteresis. Hysteresis between mercury intrusion and retraction occurs for two reasons. First, as discussed above, it may arise due to contact angle hysteresis, where the values of the product y cosO differ for advancing or retreating menisci. Additionally, it may arise due to structural hysteresis, where the pressures at which intrusion and retraction processes occur are controlled by different aspects of the pore structure. For example, for void spaces consisting of large pore bodies interspersed with narrow pore necks, intrusion may be controlled by pore neck sizes, whereas extrusion is controlled by pore body sizes. Eqs. (1) and (2), together with the values of A and B given in Table 1, have been used to analyse the raw mercury porosimetry data for both whole pellet and fragmented samples of several different batches of sol-gel silica spheres [8]. For whole pellet samples the new analysis
188
lead to a partial superposition of the intrusion and retraction curves at smaller pore sizes, but hysteresis and entrapment were still observed at larger pore sizes. This result was interpreted as being due to the complete deconvolution of contact angle hysteresis to leave the intrusion and extrusion curves solely influenced by structural hysteresis. However, when samples of the sol-gel spheres were fragmented into powders with a particle size of -30-40 ~tm, it was found that the Kloubek correlations a priori lead to a complete superposition of the respective intra-particle intrusion and retraction curves. This finding meant that, for mercury intrusion and retraction in these powdered samples, the contact angle hysteresis had been completely removed using the Kloubek correlations, showing that no structural hysteresis or mercury entrapment was now taking place either. A complete lack of hysteresis suggests that both intrusion and extrusion must occur at pressures corresponding to the same critical pore size (percolation threshold) in a given domain of the porous medium. This result suggests that, for a new material with a similar type of pore structure to the sol-gel silica spheres, where the values of A and B for mercury intrusion given in Table 1 have been shown independently to be correct, then the unknown values of A and B for mercury extrusion may also be determined. The determination of the unknown values of A and B for mercury extrusion for a new material, denoted I, is only possible if a complete superposition of its intra-particle intrusion and extrusion curves can be obtained. The pore structure for material I with the unknown values of A and B for mercury extrusion must be similar to that of the sol-gel silica spheres because of the necessary requirement to eliminate structural hysteresis to be able to determine A and B for extrusion. The proposed method for determining the values of A and B for mercury extrusion is as follows. The raw mercury intrusion data is analysed using Eqs. (1) and (2), and the values of A and B shown in Table 1. The raw mercury extrusion data are also analysed using Eqs. (1) and (2), but the values of A and B in Eq. (2) are then adjusted until it is possible to obtain a complete superposition of the intrusion and retraction curves, within the experimental error present in the values of A and B for mercury intrusion. If this complete superposition is possible, then the values of A and B for mercury extrusion used to obtain that superposition are taken to be correct. The new values of A and B for extrusion determined by this method for material I (that is not the controlled pore glasses or sol-gel silicas) can then be applied to the analysis of raw mercury porosimetry data for a further, different new sample (with a similar surface), denoted II. For the new values of A and B for extrusion to be applicable to material II, the values of A and B for intrusion from Table 1 must still give rise to an apparent pore size distribution for material II that is the same as that obtained from nitrogen desorption. If a complete superposition of the mercury intrusion and retraction curves for material II is achieved a priori using the new values for A and B for extrusion, then this finding will confirm that the original residual hysteresis in the curves for material I, analysed using both sets of values in Table 1, was due to differences between the natures of the surfaces of the controlled pore glasses and the new types of material, and thus not residual structural hysteresis. If materials I and II are amorphous, it is highly unlikely that the pore structures of materials I and II will be sufficiently similar to have exactly the same ratio of the two characteristic dimensions that control mercury intrusion and extrusion for the whole range of values of these dimensions present in these materials. Hence, if hysteresis between the intrusion and retraction curves for material II is eliminated using the values of A and B for both intrusion and retraction appropriate for material I, then it is likely that the hysteresis eliminated was purely contact angle hysteresis.
189 0.8
"~ 1.9 1.8
20.7
~1.7 = 1.6 1.5 1.4 1.3
~,~ 0.6 ~0.5 0.4 =~ 9
~
%
0.3
o
9
0.2
~0.1 0.0
0~
(a)
. . . . . . . . . .
"' 9 j
. . . . .
A,,-J,,
10
% ,,,*
~
. . . .
Q
9 1 4 9 9 1 4 9. . ,...
,
100
Pore radius/(nm)
,
,,
,., ,l
1000
gO
%0 all
O
;o
]2
= 1.1 1.0
(b) ,
,
,
,
,
a ,~....~..,~q,~.o o . ,
,,I
,
10 Pore radius/(nm)
.
.
.
.
.
100
Figure 1. Apparent pore size distributions obtained for (a) whole and (b) fragmented samples from batch E2. (O, mercury intrusion and Kloubek correlation; o, mercury extrusion and Kloubek correlation; n, mercury extrusion and new correlation; O, nitrogen adsorption; ,,, nitrogen desorption).
A type of pore structure where it is possible to eliminate structural hysteresis by fragmentation of the sample is a macroscopically heterogenous pore structure, where long range correlations in pore size are present. For highly correlated pore structures mercury floods or retracts from the whole of large, macroscopic domains of void space at the same pressure. Experiments [9] in glass micro-models have shown that "pore shielding" is a necessary requirement for entrapment to occur. These same experiments have also shown that non-random structural heterogeneity is a source of entrapment. For highly correlated pore structures, the "pore shielding" of the individual heterogeneity domains necessary to give rise to mercury entrapment is provided by surrounding domains, within whose boundaries local pore sizes are smaller than in the shielded domain. If these domains are macroscopic then fragmentation into particles smaller than the characteristic domain size will eliminate the shielding effect and thus the source of entrapment. Long range correlations in pore size may be detected using relaxation time-contrasted magnetic resonance imaging. A model suitable to represent materials with a similar pore structure to that described above is given as follows. The model consists of a square grid of sites. Each site contains the same specific volume as each other site but the characteristic pore size may differ between sites. Within each site the void structure consists of a network of cylindrical pore elements each with the same characteristic size that has been allocated to the site containing them. Pore sizes may be allocated to the grid sites from a given probability density function according to various algorithms which give rise to different patterns in the spatial arrangement of pore sizes. The pattern of the spatial distribution of pore sizes is characterised by four characteristic statistical parameters: fractal dimension, percolation threshold, degree of correlation and correlation length -'he nature and calculation of these statistics is described in detail elsewhere [10] and will not be repeated here. Once the model has been constructed mercury intrusion and retraction may be simulated. The algorithms used in the simulations are in accordance with the results of the experiments on glass micro-models described above. The algorithms are described in great detail elsewhere [10].
190 ,-" 0.5
1.2 ....
"~ 0.4
1.0
!
0.3
9
~
9
_~
0.2
go nm
0.8 0.6
~0.1 0.0
(a) ~J'4~l'r~'daPkaluL,iL. ql, IP , . n o
. . . . . . . .
t
. . . . . . . .
10
j
1O0
Pore radius/(nm)
oN
m
.
eD 9 . . . . . . . .
.
o o
9
0.4 = 0.2
Ill
9 OO
(b)
".
9
1000
0.0
. . . . . . . .
!
10
. . . . . . . .
i
. . . . . . . .
100
!
1000
. . . . . . . .
10000
Pore radius/(nm)
Figure 2. Apparent pore size distributions obtained for (a) whole and (b) fragmented samples from batch E3. (O, mercury intrusion and Kloubek correlation; n, mercury extrusion and new correlation; O, nitrogen adsorption; A, nitrogen desorption).
RESULTS The materials studied in this work are a cylindrical alumina extrudate, denoted E2, and a cylindrical alumina tablet, denoted E3, both of a diameter of 3 mm. These materials were studied as samples of both whole pellets and powder fragments. The raw mercury porosimetry and nitrogen sorption data for these samples have been published elsewhere [11] and therefore will not reported here. Figure 1(a) shows a comparison of the apparent pore size distributions for whole pellets from batch E2 obtained from the mercury intrusion curve, analysed using the appropriate Kloubek correlation, and the nitrogen desorption isotherm, analysed using the Kelvin equation for a hemispherical meniscus and the BJH method. It can be seen that these two pore size distributions are very similar. This close similarity would suggest that the values of A and B in the Kloubek correlation for mercury intrusion are appropriate for analysing the data from batch E2. Figure l(b) shows the apparent pore size distributions obtained from the raw mercury intrusion and retraction data for a fragmented sample (with a particle size of 30-40 gm, determined using laser diffraction) from batch E2 using the Kloubek correlations for advancing and retreating mercury menisci, respectively. It can be seen that some residual hysteresis is present between these two curves. Figure l(b) also shows the raw mercury retraction data for a fragmented sample from batch E2 analysed using Eqs. (1) and (2) but where the values of the constants A and B have values of-40 and -240, respectively. It can be seen that this new correlation for mercury retraction data leads to a complete superposition of the intrusion and retraction curves, within the uncertainty (~4-5 %) due to the experimental error in the constants A and B for intrusion. Figure 1(a) also shows the mercury retraction data for the whole pellet sample analysed using the new correlation. It is noted that the use of the new correlation leads to a superposition of the mercury intrusion and retraction curves at low pore sizes. However, some hysteresis and entrapment is still evident at larger pore sizes. A comparison of the mercury intrusion and retraction data for whole and fragmented samples from batch E2, shown in Figures l(a) and l(b), respectively, shows that the mercury entrapment present in the whole pellet data is not present in the curves for the fragmented sample. This finding would suggest that mercury entrapment in batch E2 is caused by the macroscopic (>40 gm) properties of the pellets.
191
Figure 2(a) shows a comparison of the apparent pore size distributions for whole pellet samples from batch E3 obtained from mercury intrusion data, analysed using the Kloubek intrusion correlation, mercury extrusion data, analysed using Eq. (2) with the constants A and B taking values of-40 and -240, respectively, and nitrogen adsorption and desorption data, analysed using the Kelvin equation appropriate to a hemispherical meniscus and the BJH method. Since capillary condensation of nitrogen only occurs in mesopores the ultimate cumulative pore volumes in the nitrogen sorption pore size distributions have been renormalised to the ultimate intrusion volume in the mercury intrusion curve. It is noted that the apparent mesopore size distribution obtained from mercury intrusion and the Kloubek correlation matches the mesopore size distribution obtained from nitrogen desorption. It is also noted that the new correlation for y.cosO for analysing mercury retraction data leads to a complete superposition of the intrusion and retraction curves for batch E3 in the mesopore region. It can be seen that batch E3 has a bidisperse pore size distribution containing both macropores and mesopores. However, the total specific pore volume for the mesopore network obtained from nitrogen sorption is the same as the total volume of apparent mesoporosity in the mercury intrusion curve. This observation suggests that no part of the mesopore network is pore shielding any part of the macropore network. The whole macropore network is fully directly accessible from the surface of the pellet. The mercury curves in Figure 2(a) also suggest t h a t - 8 0 % of the mercury entering the macropore ( > - 6 0 nm) network becomes entrapped on retraction of the mercury front, whereas none of the mercury entering the mesopore network is entrapped. Figure 2(b) shows the apparent pore size distributions obtained from the mercury porosimetry data for a fragmented sample from batch E3 (with particle sizes in the range 45-53 pm by sieve analysis). The mercury intrusion data was analysed using the Kloubek intrusion correlation, whereas the mercury extrusion data was analysed using Eqs. (1) and (2) with the constants A and B taking values of-40 and -240, respectively. While, due to the powder particle size, there is some overlap between inter-particle and intra-particle intrusion regions, it can be seen that fragmentation of the sample has eliminated the entrapment occurring during mercury retraction in the whole pellet sample. This would suggest that the mercury entrapment in the macropore network is caused by the macroscopic (>---50 pm) structure of the pellet. The above results suggest that there is no shielding of the macropore network by the mesopore network in batch E3. It is, therefore, proposed that mercury intrusion and retraction processes within the macropore network in batch E3 can be studied independently of the influence of the mesopore network and, hence, the macroporous structure can be represented by the type of model pore structure described in the theory section. Simulations of mercury intrusion and retraction within a model of the general type described in the Theory section have indicated that a model with an entrapment o f - 8 0 % has a pattern in the spatial distribution of pore size characterised by the values of 1.67, 0.918, 0.785, and 0.36 for the fractal dimension, percolation threshold, degree of correlation and (reduced) correlation length, respectively, irrespective of the form of the probability density function for pore sizes. It is therefore proposed that the pattern of the spatial distribution of macropores within pellets from batch E3 is characterised by these same four statistical parameters.
192 DISCUSSION
Previous NMR imaging studies [10,12] have shown that the sol-gel silicas studied in previous work [8] and pellets from batches E2 and E3 possess macroscopic (>10 pm) heterogeneities in the spatial distribution of local average pore size. The typical length scales of the regions of similar pore size have been found to be -0.1-1 mm, which is much larger than the size of the particles in the fragmented samples studied in this work. It has also been found [10] that anomalously low lattice sizes are obtained from a percolation analysis [7] of the nitrogen sorption data of the sol-gel silicas, E2 and E3. This finding is also consistent with highly spatially correlated pore sizes. Since, when the relevant mercury porosimetry data is analysed in the way proposed here mercury entrapment only arises for whole pellet samples, it is suggested that the non-random, macroscopic heterogeneity gives rise to mercury entrapment. These findings collectively suggest that all of the different materials (sol-gel silicas, E2 and E3) have similar macroscopic structures. When a correlation for y.cosO for retreating mercury menisci, developed using batch E2, is used to analyse the retraction data for batch E3 the apparent mesopore size distributions for mercury extrusion and nitrogen desorption are the same, and the complete superposition of the curves for mercury intrusion and extrusion within batch E3 in the mesopore region is achieved. As described in the Theory section these findings suggest that the new values of A and B for mercury extrusion obtained using batch E2 do eliminate only contact angle hysteresis. CONCLUSIONS
A new correlation for the product ?'.cos0 for retreating mercury menisci in alumina samples has been developed. This correlation has been used, together with simulations of mercury porosimetry on suitable models, to obtain statistics that characterise the pattern of the spatial distribution of macropore sizes in a bidisperse alumina tablet. REFERENCES
1. S.P. Rigby, R.S. Fletcher and S.N.Riley, J. Colloid Interface Sci. 240 (2001) 190. 2. E.W. Washburn, Phys. Rev. 17 (1921) 273. 3. J.E. Shields and S. Lowell, Powder Technol. 31 (1982) 227. 4. J. Kloubek, Powder Technol. 29 (1981) 63. 5. L. Moscou and S. Lub, Powder Technol. 29 (1981) 45. 6. A.A. Liabstre and C. Orr, J. Colloid Interface Sci. 64 (1978) 1. 7. K.L. Murray, N.A. Seaton and M.A. Day, Langmuir 15 (1999) 8155. 8. S.P. Rigby and K.J. Edler, J. Colloid Interface Sci. 250 (2002) 175. 9. N.C. Wardlaw and M. McKellar, Powder Technol. 29 (1981) 127. 10. S.P. Rigby, J. Colloid Interface Sci. 224 (2000) 382. 11. S.P. Rigby, D. Barwick, R.S. Fletcher and S.N. Riley, Appl. Catal. A., in press. 12. M.P. Hollewand and L.F. Gladden, Chem. Engng Sci. 50 (1995) 327.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
193
T h e o r e t i c a l c a l c u l a t i o n of high m i c r o p o r e v o l u m e s on a c t i v a t e d c a r b o n s
J. Alcafiiz-Monge Departamento Quimiea Inorgfinica. Universidad de Alicante. Alicante. Spain. One of the most important characteristics of carbonaceous porous solids is their specific surface area and their micropore volume. The aim of this work is to answer the following questions: What is the maximum adsorption capacity of a carbonaceous microporous solid? And, Why carbon molecular sieves (CMS) can not be prepared with a micropore volume higher than 0.3 cc/g?. Micropore volume has been calculated using a series of very simple microporous carbon models, where porous carbons are described as assemblages of slit-shaped pores, and where parameters of graphitic structure (interlayer spacing and density) and parameters of pore structure (pore width and micropore walls thickness) are used. From these simple approaches, it is shown that it is possible to obtain microporous carbons with micropore volume higher than 2 cc/g and,
that CMS with
micropore size around (5-6 A) and with a micropore volume higher than 0.35 cc/g can not be obtained. 1. INTRODUCTION One of the most important characteristics of carbonaceous porous solids is their specific micropore volume and surface area. For gas storage applications, in principle, the preparation of activated carbons (ACs) is focussed on the development of high micropore volumes. Thus, during the last years, the manufacture of "superactivated carbons" (SACs) with high micropore volume is a very important research area. Several researches have produced SAC with high micropore volume, close to 2 cc/g, even some of these SAC have been commercialised, i.e. Maxsorb and PX21[1]. On the other hand, for gas separation applications, carbon molecular sieves (CMS) with high micropore volume of a narrow pore size (3-7 angstroms) are needed. Several methods have been proposed for producing CMS, obtaining successfully CMS with tailored pore size. However, a major difficulty in such methods is obtaining a CMS with high micropore volume. In fact, most CMS reported on literature showed a micropore volume around 0.25-0.3 cc/g.
194 The characterisation of these materials in terms of surface area and pore volume is critical in assessing their applicability. Since 64 years ago, BET analysis is being used to characterised the microporosity and to evaluate the specific surface area [2]. It was shown that there is a theoretical upper limit of the specific surface area (2630 m2/g), which was calculated considering that all carbon atoms formed an unique graphitic layer [3]. However, carbonaceous materials with exceptionally high surface area, exceeding considerably the theoretical upper limit of surface area have been prepared [1 ]. In this case, it can be explain on the theoretical calculation including the edge contribution [4]. It is important to point out that, pores with different pore width can be found in the range of microporosity: Narrow micropores (pore size < 7 A ) and supermicropores (7 A < pore size < 20 A). In the case of supermicropores, the value of specific surface area could be overestimated from the BET analysis. The IUPAC recommends do not use the specific surface area forthe characterisation of microporous solids. Theoretical methods have been developed to calculate the micropore volumes. The most widely used method is the Dubinin-Radushkevic method. It is reasonable to think that there is a theoretical upper value of micropore volume, which must correspond to a carbonaceous solid formed by graphite layers separated at a distance of 20 A. In spite of the importance of this subject, there are not so many papers about the upper limit of the micropore volume. The aim of this work is to answer the follow questions: (i) What is the maximum adsorption capacity of carbonaceous microporous solids? and, (ii) Why CMS can not be prepared with a micropore volume higher than 0.3 cc/g?. In doing that, micropore volume has been calculated in a series of very simple microporous carbon models, described as assemblages of slit-shaped pores, and using the parameters of graphitic structure (interlayer spacing and density) and the pore structure (pore width and micropore walls thickness). 2. ACTIVATED CARBON MODELS AND CALCULATION PROCEDURES In order to develop a microporous carbon model, we will accept the following characteristics of adsorption process on active carbon described by different authors: * The adsorption in micropores is due to overlap of the adsorption potentials from opposite walls until 12 A, and by co-operative process until a width of 20 A [3], which is the micropore width limit generally established [5]. * The micropores are slit-shaped [6]. * The micropore walls consist mainly on flat bases of graphitic layers [7].
195
~,
Interlayer spacing I (3.35 A) ~"" 9 .~
n (n ~ layers)
t~ ~ ~ ( ( / ~
! ]Y
Figure 1 a. Microcrystalline structure
Taking into account all these experimental facts, Figure l a shows the scheme of a graphitic microcrystal and Figure 1b the different models used. The parameter considered for the development of the model has been the number of the graphitic layers forming the micropore walls (thickness).
Figure I b. Activated carbon Model used.
In Figure 1b, each figure is considered as a unique structure, which is repeated to form the porous solid. In each model, the variation of the micropore volume with the pore width (w) has been calculated. (It must be noted that w is the effective pore width, according to definition ofKaneko et al [5]).
196
Firstly, we consider a graphitic microcrystal, as shown in Figure 1a, formed by n graphite layers. Table I shows the equations relating the volume and the mass of microcrystal with its dimensions. As it is deduced from these equations, the volume (eq.4) depends on the interlayer spacing (D and on the lamella dimensions (yz = S), meanwhile the mass (eq.6) is a function of the interlayer spacing (/), of the lamella dimensions (S) and of the density (p). For calculations, we will consider the theoretical values of the interlayer spacing and the density of graphite: 3.35 A and 2.269 g/cc, respectively. TeLble I. Volume and mass of the mierocrystal I
V = x.y.z
(1) I
Z = (n-l). I
(2)
as = x.y
(3) I
M = v.bp
(5)
1
IV = (n-l)d.S
(4)
T .
.
.
.
.
] M = (n-1)./.S.p
(6)
1
.
From the mass equation applied to this graphitic microcrystal, we will develop the different models showed in Figure 1. For this purpose, depending on the model applied, an increase in the separation between certain number of graphite layers has to be considered Thus, for the simplest model (model E), we would separate every graphitic layers to a distance of w. On the other hand, in model A we would increase the interlayer space until a value of w every set of three graphite layers. This procedure originates a micropore volume in our graphitic microcrystal (its mass does not vary with the separation of the layers). The calculation of this micropore volume is easy, because the dimensions of the generated pores (separation between the layers (w) and dimensions of the lamella (S)) and the number of pores per structural unit (f(n)) (which depends on the model) are known. Table II. M i c r o p o r e V o l u m e model E Vmicro--(n-l). w.S (cc)
(7)
Vesp= Vmicro/M (cc.g "1) (8)
M = (n-I).LS.o
Vesp= w/L 13 (cc.g "1)
(6)
Table II contains the micropore volume equations for model E. The first equation shows the value of the micropore volume for this structural unit, which, logically, will be equal to the volmne of one micropore multiplied by the number of micropores in the structural trait (eq. 7). The mass of this unit is known (eq. 6) and, consequently, the specific micropore volume can be calculated. It can be observed that the specific micropore volume only depends on the pore width (w) and on the graphitic crystallite parameters (J and p), being independent of the assumed shape of the lamellas, consequently it can be accept a more real shape (discotic).
197
Table III. Specific micropore volume obtained from the different models. T tModel Layers per pore Mieropore volume [
E I1
i~ :C
j
B
iA
i~2 ii32.5
V~p= w / I.o
l
Ves__p= w / L P. 1.5
! 1
_Vesp= w / Lp. 2
1
' V ~ = . w / I . O. 2.5
I
Vesp=.W / Lp. 3
I
Table III presents the equations obtained applying the other models, following the abovedescribed procedure. It is observed that the different equations could be expressed as a function of the number of graphite layers forming the micropore walls. 3. RESULTS.
MICROPORE
VOLUME
EVOLUTION
ON
FUNCTION
OF
MICROPORE WIDTH AND PORE WALL THICKNESS In Figure 2 the value of micropore volume obtained by applying different models is plotted as a function of the pore width. The maximum value obtained is around 2.7 cc/g, which decreases quickly as the number of layers forming the wall increases. Note that, as the number of layers forming the wall increases, the difference between the volume obtained from different models is greatly reduced. So, big differences exist between the model E and D, while the differences between the rest of the models are less pronounced. Other interesting fact which can be deduced from Figure 2, is that the maximum calculated volume corresponding to the narrowest microporosity (size < 7 A), is close to 0.9 cc/g. This value is close to the maximum volume of narrow microporosity found in the literature, which correspond to an ACF prepared by physical activation with CO2 ( 0.77 cc/g) [8] and to a KOH activated coal (0.85 cc/g) [9]. That indicates that the model seems to be suitable for this calculations. On the other hand, CMS used in gas separation (air separation, natural gas..) must contain narrow micropores (between 36 A). These materials with so well-defined pore structure are prepared by a mild activation process of carbonised samples, or by chemical vapour deposition (CVD) over an activated carbon. As a consequence of the preparation process, the micropore structure of CMS cannot be assembled by the models E or D. Thus, in Figure 2 it can be seen that, a CMS with a pore width of 6 A, could show a maximum micropore volume of 0.35 cc/g (model C).
198
Figure 2. Micropore volume as a function of pore width for each model.
In order to check these models, Figure 3 includes the micropore volume and pore width corresponding to some samples reported in the literature. These samples include high performance AC: Activated carbon fibers (ACF1 [10], ACF2 [11], ACF3 [12]), and activated mesophase microbeads (a-MCMB1 [13], a-MCMB2 [14], a-MCMB3 [15]); and conventional activated carbon: AC from lignocellulosic charcoal (C-87 [16]), AC from coke (Supersorb [ 1]), and AC from coal (Coal-AC [ 17]). In all the cases, the pore width value has been estimated from the gas adsorption data and, so they can be erroneous. Figure 3 also includes the sample with the maximum value of microporosity reported (a-MCMB1, 1.97 cc/g) [12]. It must be noted that this sample is located above the line of the model E. In order to analyse if it is possible that a real carbonaceous microporous solid shows a structure as proposed by model E, two aspects have been considered: a) mechanism of activation, and b) graphitic structure of precursors. The above-described sample has been prepared by KOH activation (so that sample Supersorb). As it is known, the activation process with KOH consists mainly on the exfoliation of graphitic layers due to the intercalation of the K between them [17]. After KOH is removed by washing, the lamellas are randomly ordered leaving between them an considerable interlaminar space called microporosity. Considering this activation mechanism, it seems reasonable that in this sample the activating agent is able to separate all the layers. Thus, the microporous solid obtained has all its layers separated by a distance higher than the distance in a graphitic structure (model E).
199
Figure 3. Micropore volume versus pore width (w) for selected AC. In the analysis considering the graphitic structure of precursors, it must said that ACFs and a-MCMBs have high micropore volume while conventional AC present low micropore volume. This different behaviour is a consequence of the different graphitic structure of the precursors. CF and MCMB present a very homogeneous structure of graphitic microcrystal with small size. This characteristic makes the activation process to be very homogeneous. In fact, some authors have observed by X-ray diffraction (XRD) that the walls of these microcrystals are formed by 1 to 2 lamellas [13, 14]. These results agree with the proximity of the reported samples to the line corresponding to the models C, D and E. Finally, conventional AC obtained by activation of lignocellulosic chars or coals, which present a disorganised graphitic structure, usually present micropore values in the range of 0.50.9 cc/g, which are located in the line of the model A (3 layers). In fact, the first micropore model developed for carbonaceous microporous solids, proposed that the walls of this solids are formed by 3-4 layers, which agrees with our model [18].
200 4. CONCLUSIONS. A good correlation between the reported experimental pore volume and the values obtained by applying the models proposed in this work has been found. From this simple approach, it has been shown that it is possible to obtain microporous carbons with a micropore volume higher than 2 cc/g, and that a volume higher than 2.5 cc/g can be achieved. On the other hand, it has been explained why CMS with micropore size around (5-6 A) can not present a micropore volume higher than 0.35 cc/g, as a consequence of their pore wall structure. References
[1] Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powder and Porous Solids: Principles, Methodology and Applications. Academic Press Inc: Cornwall, 1999. [2] Brunauer, S.; Emmet, P.H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 209. [3] Gregg, S. J.; Sing, K.S.W. Adsorption, Surface Area and Porosiy; Academic Press: London, 1982. [4] Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H.; Carbon 1992, 30, 1075. [5] Kaneko, K.; Cracknell, R.; Nicholson, D. Langmuir 1994, 10, 4606. [6] Stoechi; H.F. Carbon 1990, 28, 1. [7] Ehrburger, P.; Pusset, N.; Dziedzinl, P. Carbon 1992, 30, 1105. [8] Alcafiiz-Monge, J.; Cazorla-Amor6s, D.; Linares-Solano, A.; Yoshida, S.; Oya, A. Carbon 1993, 34, 1277. [9] Lozano-Castell6, D.; Lillo-Rodenas, M.A.; Cazorla-Amor6s, D.; Linares-Solano, A.; Carbon 2001, 39, 741. [ 10] Kaneko, K.; Shimizu, K.; Suzuki, T. J. Chem. Phys 1992, 97, 8705. [ 11] Kaneko, K.; Setoyama, N.; Suzuki, T.; Kuwabara, H. 4th Int. Conf. on Fundamentals of Adsorption, Kyoto, 1992; p 315. [12] Kaneko, K.; Ishii, C.; Rybolt, T. Characterization of Porous Solids III; Rouquerol, J.; Rodriguez-Reinoso, F.; Sing, K.S.W.; Unger, K.K., Eds.; Elservier Sci., 1994; p 583. [13] Ishii, C.; Shindo, N.; Kaneko, K. Chemical Physics Letters 1995, 242, 196. [ 14] Kaneko, K.; Ishii, C. Colloids and Surfaces 1992, 67, 203. [15] Stoeckli, F.; Ballerini, L. Fuel 1991, 70, 557. [16] Ahmadpour, A.; Do, D.D. Carbon 1996, 34, 471. [17] Marsh, H.; Crawfork, D.; O'Grady, T.M.; Wennerberg, A. Carbon 1982, 20, 419. [18] Masters, K.J.; McEnaney, B. Carbon 1984, 22, 595.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Remoso, B. McEnaney, J. Rouquerol and K.K. Unger (Editors) 9 2002 Elsevier Science B.V. All fights reserved
201
Assessment of ultramicroporosity on carbon molecular sieves by water adsorption J. Alcafiiz-Monge, D. Lozano-Castell6 Dpto. Quimica Inorgfinica. Universidad de Alicante. Apdo. 99. E-03080 Alicante. Spain
This work focuses on the application of water adsorption to the characterisation of narrow microporosity (ultramicroporosity) on carbon molecular sieves (CMS). In addition, the mechanism of water adsorption in carbonaceous solids is analysed. Despite of the presence of surface groups in these materials, they present low adsorption at low relative pressures (P/Po <0.2), indicating the significant role of micropore size in the starting of water adsorption. Interestingly, it has been seen that water adsorbs even on samples, which do not present CO2 adsorption.
1. INTRODUCTION
Molecular sieves (MS) are solids with very narrow and tmiform porosity, which separate gas mixtures formed by molecules of different sizes. Therefore, in order to optimise the performance of these materials, a good characterisation in terms of pore size and pore size distribution is required. Physical adsorption of gases is the most widely used technique for the characterisation of porous solids [1,2]. Among the different gases, N2 adsorption at 77 K is the most common gas used and, usually, has a special status of recommended adsorptive [3]. The advantage of N2 adsorption is that it covers relative pressures from 10-8 to 1, what results in adsorption in the whole range of porosity. The main disadvantage of N2 adsorption at 77 K is that when it is used for the characterisation of microporous solids, as it is the case of MS, diffusional problems of the molecules inside the narrow porosity (size < 0.7 nm) occur [4]. To overcome this problem, the use of other adsorptives has been proposed. CO2 adsorption, either at 273 K or 298 K [4-6], and He adsorption at 4.2 K [7] are two alternatives to N2 adsorption for the assessment of the narrow microporosity. In spite of the interesting results obtained with He, the
202 experimental conditions used (adsorption at 4.2 K) makes this technique not to be as available as CO2 adsorption technique. In the case of CO2 adsorption, though the critical dimension of the CO2 molecule is similar to that of N2, the higher temperature of adsorption used for CO2 results in a larger kinetic energy of the molecules, which are able to enter into the narrow porosity. In this way, the study on CO2 adsorption has demonstrated that it is an appropriate complementary technique for the analysis of the porous texture [4-6]. It can be used to assess the narrow microporosity (size <0.7 nm) where N2 adsorption can be kinetically restricted.
Similarly, there is a great potential in the use of water vapour for the analysis of the porous texture, because it has considerable potential due to both the easy experimental conditions (at room temperature the whole range of relative pressures can be covered) and the characteristics of the molecule itself (polar molecule and small kinetic diameter-0.28 nm). This vapour is widely used in the characterisation of inorganic porous solids, such as zeolites, silicas, and clays.
However, its interaction with carbon materials (microporous carbons:
coals, activated carbon fibres, carbon molecular sieves; and porous carbons: activated carbons), is more complex than the interaction of non-polar molecules [8].
In previous studies, we have investigated water adsorption in activated carbon fibres (ACFs), which are mainly microporous solids with a well-defined pore structure, and some activated carbons (AC) containing micro, meso and macroporosity [9,10].
On all these
materials, we were able to show that water adsorption in micropores depends on the micropore size, taking place gradually according to the pore size. It was concluded that adsorbed water in microporosity is in a solid, ice-like phase, with a density around 0.92 g.cn, 3. The present paper presents the results corresponding to the extension of this research to CMS, studying both the mechanism of water adsorption in carbonaceous solids and the suitability of water adsorption to characterise CMS.
2. EXPERIMENTAL
Water adsorption was studied in a series of CMS: Two commercial CMS (Takeda 3A and Takeda 5A), two carbon fibres obtained in the laboratory from different coal tar pitches (CF A, CF C) [11], three heat treated CF from Kureka, (at 1273, 1473 and 1673 K; CFK12, CFK14, CFK16, respectively), and a CMS prepared in the laboratory from an anthracite-based AC by
203
carbon deposition at 973 K (KUAB) [12]. Porous texture analysis was carried out by
N2
adsorption at 77 K and CO2 adsorption at 273 K and 298 K (Autosorb 6- Quantachrome). The samples were degassed at 623 K under vacuum, up to 10"3 torr. Water adsorption studies were carded out at 298 K (Belsorp 18). In this case, the samples were degassed at 623 K under high vacuum (10 -6 torr).
3. RESULTS AND DISCUSSION
3.1. Micropore characterisation from
N2
and CO2 adsorption.
Figure 1 shows CO2 adsorption isotherms and Table I contains the micropore volume calculated from both nitrogen and carbon dioxide adsorption isotherms. 90
, - T5A
80 9 T3A
70
~,
o KUAB
o o,t
60
a CFK12
50
Z
,m
40
o CFA
@ @
30
-
n
O
"s
O
" CFC-298
<>
ii
20
"e O Ill
10
am 9
0,
9
~m
O 9
9t 9 0
0.005
9 CFC
9
9o 9 9 CFK14
9
-
9
i
9
0.01
t -
A --i
0.015
A
0.02
A
A
0.025
A
--.
0.03
P/P 9
Figure 1. CO2 adsorption isotherm on CMS.
In Table I it can be observed that any sample present N2 adsorption, except from sample T5A, but some of them present CO2 adsorption. Thus, nitrogen adsorption is not suitable for the characterisation of the porosity of these samples, as it presents diffusional problems to enter into the microporosity. These results indicate the presence of a very narrow
204 microporosity in these samples, highlighting their sieve properties. In addition, it can be observed that some of these samples do not present CO2 adsorption either (heat treated CF), or they adsorb a scarce amount of CO2 (CF C). This behaviour could be due to the nonexistence of porosity, to the blockage of the porosity by the carbon deposition or as a consequence of the growing of graphitic layers. Considering the results corresponding to CO2 adsorption at 298 K, it can be observed that, at 298 K the sample CF C adsorbs a similar amount of CO2 than CF A. This indicates the existence of a very narrow microporosity, which causes diffusional problems even for CO2 at 273 K.
Table I. Micropore volumes deduced from N2, CO2 and H20 adsorption I
II
I
cm3.g"l
I
III
I
II
I
VN2
III
II
I
Vc02
VH20 i
T3A
0
0.21
0.19
T5A
0.27
0.29
0.28
CF A
0
0.17
0.12
CF C
0
0.05 (0.10)*
0.11
CFK12
0
0.13
0.13
CFK14
0
0
0.08
CFK16
0
0
0.04
KUA1B
0
0.20
0.20
I
I
* the number between brackets correspond to the micropore volume calculated from the CO2 adsorption data at 298 K.
3.2. Water adsorption isotherms on CMS.
Figure 2 contains water adsorption isotherms corresponding to some of the samples. Figure 3 presents the water adsorption isotherms for the series of heat treated CF. In Figure 2 it can be seen that water isotherms are type V, characteristic of water adsorption in microporous AC [8]. It must be indicated that, contrary to CO2 adsorption
and to the
adsorption of many other gases and vapours, where adsorption in the microporosity takes place at low relative pressures (see Figure 1), in the case of water, adsorption takes place in
205 the range of relative pressure (P/Po) 0.2-0.8, as a consequence of the low interaction of water molecules with graphitic surface [1,8].
- T5A
350
300
o KUAlb8 bis
250 "
O
-
Z
~
"
150
-O
_
.J
P
I00
-
_or
9
O
9
9T3A
O O
r
O
,
; - g... ',2 e-
O
9
OO
-
u
O
-
200
OO
o
o"
9
~-
9 OsklO00
"
•
a
oFCA
9J , a'~o 50
tFCC 0
i
t
t
i
0.2
0.4
0.6
0.8
1
P/Po
Figure 2. Water adsorption isotherms at 298 K.
Comparing CO2 adsorption isotherms (at 273 K and 298 K) and water adsorption isotherms (Figure 1 and 2, respectively), it can be observed that, considering the total adsorption capacity, CO2 and water adsorption capacity follow a similar trend for all the samples.
In Figure 2 it can be seen that the shape of the isotherms (the start and the knee of the isotherms) are similar for all the samples, which agree with their similar micropore size, as it has been seen previously. It must be noted that, although the amount of surface groups in these materials is relatively low (most of the samples are non-activated samples and they were treated at 1273 K), water adsorption takes place at low relative pressure (0.2).
Figure 3 contains water adsorption isotherms corresponding to three heat-treated Kureha carbon fibres. For comparison purpose, this figure also includes the water isotherm
206 corresponding to an ACF described in a previous work [9]. This sample (CFC20) showed water adsorption at the lowest relative pressure compared with the rest of the samples of the series [9]. As it can be observed, the non-activated CF (CFK12) adsorbs water on a similar range of relative pressure than sample CFC20, even though CFK12 presents a lower amount of surface groups than the activated sample CFC20. These results corroborate those obtained on a previous work with ACF, where it was shown the significant role of micropore size in the starting of water adsorption. This is reasonable, since the micropore adsorption potential increases as the micropore size decreases, favouring water adsorption at the lowest relative pressures. These results agree with recent molecular simulation results of water adsorption on activated carbons, where it was shown that pore filling pressure depends strongly on pore width [ 13]. 160
CFC20
., 3 0 0
"
mO m O B
0
o
[]
250
al
0
120 -
9
/
0 0
0
oo
O
-CFK12
0 0
O
9
0
Z~ tim e~
e~
0 0 0
ooO
9
0
80-
CFK14
9
9
O
9
0
9
0
0
9 9 9 0
o"
i
150
o
0 n
.
~
i i
o
0 m
0
o
9
0"
40-
- 200 Oq
0
100
0 0
50
CFK16
oo
@o"
0.2
0.4
0.6
0.8
P/Po
Figure 3. Water adsorption isotherm at 298 K on heat-treated carbon fibers.
From Figures 2 and 3 it can be seen that all the samples studied showed water adsorption. It must be remarked that samples CFK14 and CFK16, which do not adsorb CO2, even at room temperature, they adsorb water. These results are very interesting, because they seems to indicate that water molecule (molecule diameter of 0.28 nm) can assess the narrow
207 microporosity, which can not be assessed by CO2 (molecule diameter 0.33 nm). Since CO2 adsorption is also done at room temperature, water and CO2 kinetic energy must be similar. Thus, the different behaviour must be related to the steric impediment for the CO2 molecules. From these results it can be said that water adsorption provides a better understanding of the development of microporosity by heat-treatment than CO2 adsorption. Thus, by just using CO/adsorption, it is deduced that a heat treatment at 1473 K closes the porosity. However, from the water adsorption results, this conclusion seems not to be completely correct. It seems that the heat treatment produces a reduction on the micropore volume (see Figure 3), but still some microporosity accessible to water molecules exist. Additionally, the shift of the knee of water isotherm for the sample CFK14 and CFK16 (compared to sample CFK12), suggests that the heat treatment produces a increase on the micropore size [14]. These results agree with those reported by Kaneko et al. related to the change of the porosity of ACF with heat treatment [ 14]
Table I contains micropore volumes estimated from water adsorption. These values have been obtained using a value of 0.92 g.cm "3 for the adsorbed water density, which was demonstrated to be the most suitable value for microporous carbons [9,10]. The comparison of micropore volume obtained from water and CO2 adsorption shows that for most of the samples both values are very similar. Thus, considering these results and those obtained in previous works, where a wide variety of ACs were studied, it can be said that adsorbed water in micropores (including both supermicroporosity and very narrow microporosity) has a density around 0.92 g.cm 3.
4. CONCLUSIONS
This study presents new data supporting that the mechanism of water adsorption on the microporosity is governed by physical adsorption. According to the results presented in this paper, the use of water adsorption seems to be a very interesting tool for the characterisation of CMS. Water adsorption allows to complete the characterisation of microporosity, because it is adsorbed in the range of ultramicroporosity, which is not accessible to CO2. Pore volumes obtained from water adsorption data using the density of water in solid phase (0.92 g/cc) are quite similar to the corresponding micropore volumes obtained from CO2 adsorption data.
208
Acknowledgements J.Alcafiiz would like to thank The Leverhulme Trust for the award of a visiting fellowship to the Department of Materials at the University of Leeds, UK, where part of this collaborative study was carded out.
References 1) Gregg, S.J.; Sing, S.K.W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. 2) Rouquerol, F.;
Rouquerol, J.; Sing, K. Adsorption by Powder and Porous Solids:
Principles, Methodology and Applications. Academic Press Inc: Cornwall, 1999. 3) Rouquerol, J. et al. Characterization of Porous Solids III; Rouqurrol, J., RodriguezReinoso, F., Sing, K.S.W., Unger, K.K., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1994, p 1. 4) Rodriguez-Reinoso, F.; Linares-Solano, A. Chemistry and Physics of Carbon 21 (1998) 2146. H. Thorwer, Ed., Marcell Dekker Inc., New York. 5) Cazorla-Amor6s, D.; Alcar~z-Monge, J.; Linares-Solano, A. Langmuir 12 (1996) 2820. 6) Cazorla-Amorrs, D.; Alcafiiz-Monge, J.; De la Casa-Lillo, M.A.; Linares-Solano, A. Langmuir 14 (1998) 4589. 7) Setoyama,N.; Ruike, M.; Kasu, T.; Suzuki, T.; Kaneko, K. Langmuir 1993, 9, 2612. 8) Dubinin, M.M. Carbon 1980, 18, 355. 9) Alcafiiz-Monge, J.; Linares-Solano, A.; Rand, B. J. Phy. Chem. B 2002, 106, 3209.
10) Alcafiiz-Monge, J.; Linares-Solano, A.; Rand, B. J. Phy. Chem. B 2001, 105, 7998. 11) De la Casa-Lillo, M. A.; Alcafiiz-Monge, J.; Raymundo-Pifiero, E.; Cazorla-Amorrs, D.; Linares-Solano, A. Carbon 1998, 36, 1353. 12) Lozano-Castello, D. Master Thesis. Universidad Alicante. 2001. 13) McCallum, C.L.; Bandosz, T.J.; McGrother, S.C.; Mfiller, E.A.; Gubbins, K.E. Langmuir 1999,15, 533. 14) Kaneko, K.; Ishii, C.; Kanoh, H.; Hanzawa, Y.; Setoyama, N.; Suzuki, T. Advances in Colloid and Interface Science 1998, 76- 77, 295.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
209
Active surface area of c a r b o n materials d e t e r m i n e d by different methods A. Arenillas, F. Rubiera, J.B. Parra and J.J. Pis Instituto Nacional del Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain ABSTRACT In carbon materials, active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gas-solid reactions. However, although ASA reveals interesting information about the sample, there is a need for a reliable and standardised method for its estimation. The aim of this work is to compare ASA determination by the usual methods (i.e., gravimetry, TPD) with another method, which is based on pressure measurements in order to perform an oxygen chemisorption isotherm (OCI). The results showed that the OCI method seems to be a valuable and alternative method for ASA determination, as it avoids the main potential source of errors inherent in the usual methods.
Keywords: ASA, oxygen chemisorption, active sites, TPD, TGA 1. INTRODUCTION In the heterogeneous reactions between gases and solids, including carbons, the rate of reaction is generally assumed to be proportional to the accessible surface area of the solid. However, it has long been recognized that the total surface area derived from physical adsorption does not reveal the actual number of reactive sites, because there are many sites that are reactive only under extreme conditions. Active surface sites only represent a fraction of the total surface area, called active surface area (ASA). Knowledge of the nature and concentration of the active sites is of paramount importance for a better understanding of the kinetics involved in heterogeneous gassolid reactions [1]. However, although ASA seems to reveal interesting information about the sample, there is a need for a reliable and standardised method for its determination. The evaluation of chemisorbed oxygen and ASA depends on the method used [2-4], and until now a direct comparison of ASA values between different research groups has not been possible, due to the wide variety of conditions (time, temperature, oxygen partial pressure) employed. Furthermore, the usual methods of ASA
210 determination (i.e., gravimetry and temperature programmed desorption-TPD-) present inherent problems that may lead to errors in ASA determination. The aim of this work is to compare ASA determination by the usual methods with another valuable method, which is based on pressure measurements. 2. E X P E R I M E N T A L
An activated carbon, CM, was treated at several temperatures under vacuum (1400, 1800, 2000, 2200 and 2400 ~ in order to obtain a series of samples with different properties. The samples were denoted as CM-1400, CM-1800, CM-2000, CM2200 and CM-2400, respectively. Ultimate analysis of these samples is presented in Table 1. Textural characterisation was carried out by physical adsorption of N2 at-196 ~ The apparent surface areas of the samples were obtained by using the BET equation [5]. Three methods of ASA determination were evaluated and compared: gravimetric, temperature-programmed desorption (TPD) and oxygen chemisorption isotherms (OCI). The gravimetric and TPD methods were performed in a thermogravimetric analyser (TGA). In order to minimise diffusion problems and secondary reactions of the CO and/or CO2 with the solid material, 5 mg of sample and a gas flow rate of 75 mL min ~ were used. For the determination of the optimum chemisorption temperature, the samples were outgassed at 15 ~ rain ~ up to 1000 ~ and were maintained at this final temperature for 5 hours. After cooling down to room temperature, the samples were heated at 1 ~ min -~ up to 500 ~ under 75 mL min l of oxygen. Fig. 1 shows, as an example, the mass loss of the sample CM-1400 with increasing temperature. It can be seen in this figure that there is a mass increase at low temperatures due to oxygen chemisorption on the sample surface, which reaches a maximum at around 320 ~ Two processes (chemisorption and gasification) take place simultaneously, but at low temperatures the main process is chemisorption resulting in a mass increase, while at high temperatures the main process is gasification, which produces the rapid mass loss presented in Fig. 1. Table 1 Analysis of the samples studied SAMPLE CM-1400 CM- 1800 CM-2000 CM-2200 CM-2400
Ultimate Analysis (wt%) 0 0.3 0.2 0.1 0.1 0.1
C 99.6 99.7 99.8 99.8 99.8
H 0.1 O.1 0.1 0.1 0.1
211
1.0
-
0,5
-
A V
..m,_,
mO.O
mmmm
0
[]
,mm,
m,
m m
iuinnn ,
I
I
I
mmm
mm m
I
I
I
9[]
[
[]
I
I
I
I
I
~-0.5 -
-I .0
400
200
300
Temperature (~
400
500
Fig. 1. Mass variation of CM-1400 with temperature under oxygen atmosphere. In order to establish the optimum chemisorption temperature, a series of isothermal chemisorption experiments were performed at different temperatures between 200 and 300 ~ The sample was first outgassed in Ar (15 ~ min 1, 1000 ~ 5 hours). The temperature was then lowered to the chemisorption temperature, and a flow rate of 75 mL min 1 of oxygen was introduced to the TGA. In this way, an optimised chemisorption temperature of 250 ~ was found, so that equilibrium could be achieved in a reasonable period of time, and simultaneous carbon gasification could be avoided. Once the chemisorption temperature was optimised, the ASA evaluation by the gravimetric method could be performed, and the mass increase was assigned to the chemisorbed oxygen. After the chemisorption step the oxygen was swept by flowing Ar, and temperature-programmed desorption-TPD method- was performed in the TGA by heating the sample at 15 ~ min 1 from room temperature to 1000 ~ The desorbed gases (CO and CO2) were followed by means of a mass spectrometer (MS). The optimisation of the coupling system and the parameters used have been described elsewhere [6]. The amount of oxygen desorbed during TPD as CO and CO2 can be related to the amount of active sites. In order to quantify the evolved CO and CO2, a previous calibration with calcium oxalate was performed [7]. Fig. 2 shows the relationships between the theoretical amount of CO and CO2 produced from calcium oxalate decomposition and the experimental peak area values from the CO and CO2 evolution profiles during the calibration tests.
212
7 -
Y = 46268x+ 0.0375
6AC
5
, = =
E
~r '
4
,~
9' .a
-
.
.6
,*
@-.6
,,=l: 3
O'
.
4'
~5 . ,o
~
0C:5 0.E+00
,
,
,
5.E-05
I.E-04
2.E-04
2.E-04
C 0 2 (mol-g)
y = 4 4 0 2 5 x - O. 1521
_
_
AC ,
.,~
5
m
E m
~
4-
~
3-
9'
~
_
_
~ 9
O.E+O0 5.E-05
1.E-04
2.E-04
2.E-04
C O (mol-g)
Fig. 2. Calibration plots representing the number of moles of CO and CO2 against the area under the peaks from the decomposition of calcium oxalate. The peak area values obtained during the TPD experiments of the samples studied were related to the amount of CO and C02, using the equations obtained from calibration and shown in Fig. 2.
213
1st isotherm ( P h y s i s o r _
_
%
6-
o
"o ..Q
0
t~ "o
5
-
~
4-
. . . . . . . . . .
Difference
< 3E -~ o
2_
0 1O0
200
300 400 500 Pressure (ram Hg)
600
700
800
Fig. 3. OCI method for ASA evaluation. For the third ASA determination method (OCI), a volumetric apparatus was used. After the initial "in situ" cleaning step under vacuum at 1000 ~ the temperature was reduced to 250 ~ and oxygen pulses were introduced to perform the oxygen isotherm. The process was followed by measuring variations in pressure after each oxygen injection. The first isotherm represents the total amount (chemical and physical) of oxygen adsorbed. The sample was then degassed under vacuum at 250 ~ and the oxygen isotherm was conducted again. The second oxygen isotherm corresponds only to physically adsorbed oxygen. The chemisorbed oxygen was defined as the amount that was adsorbed and could not be removed by evacuation at the adsorption temperature (250 ~ as can be seen in Fig. 3. A similar methodology has already been employed to characterise catalysts with several adsorbates (i.e., H2, alkanes, etc.), but not to determine the so-called active surface area of different carbon materials. The three methods presented in this work evaluate the total active surface area. No distinction is made, at this point, between reactive surface area (RSA) and stable oxygen complexes [8]. The nomenclature of ASA instead of TASA will be used for simplification. The ASA values (m 2 g-l) were calculated using the equation first proposed by Laine et al. [9]. 3. RESULTS AND DISCUSSION The ultimate analysis of the samples shows significant low oxygen and hydrogen contents (cf. Table 1). No other heteroatoms (i.e., nitrogen or sulphur) were detected in
214 the analyses. Thus, it can be stated that the samples are materials that are composed mainly of carbon. The adsorption isotherms of nitrogen at -196 ~ for the samples studied are shown in Fig. 4. All the isotherms are type II in the BDDT classification [ 10]. The BET apparent surface areas values are in Fig. 4. The increase in the temperature of treatment produces a decrease in the amount of nitrogen adsorbed, with an important decrease in the BET areas due to an ordering in the sample structure. The great differences in the BET surface areas of the samples make it possible to measure a wide range of active surface areas. A comparison of the ASA values obtained from the three methods can be obtained from the data presented in Table 2. It can be observed that the gravimetric method gives the highest ASA values. This could be due to the contribution of the chemisorbed and physisorbed oxygen to the mass uptake in the gravimetric method, resulting in the overestimation of the surface area of the active sites. The lower ASA values obtained by the TPD method are probably due to the formation of C-O complexes which are stable even at 1000 ~ [11], and thus not included in the quantification of the CO and CO2 desorbed during TPD.
1.0 ~0.9 I ~E
0.8 I _.] 0.7 / O.6 --I
SAMPLE
BET Area (m 2 g-l)
CM-1400 CM-1800 CM-2000 CM-2200
761 534 385 212
~
~
~
. ~ _.~"
E= 0.5 "~ O.4 0.3
o 0.2 ~
0.1 O.OT
0
,
,
,
,
0.2
0.4
0.6
0.8
PIPo
1
I--~-CM-1400 --,,-CM-1800 - - CM-2000 -o- CM-2200 ~ CM-24001
Fig. 4. N2 adsorption isotherms at-196 ~ for the samples studied and their BET apparent surface area values.
215 Table 2 Comparison between the gravimetric, TPD and OCI methods for ASA evaluation in carbon materials SAMPLE
ASA-TG (m 2 g-l)
ASA-TPD (m 2 g-l)
ASA-OCI (m 2 g-l)
CM-1400 CM-1800 CM-2000 CM-2200 CM-2400
60 56 48 42 37
52 40 36 28 23
19 13 5 2 0.4
It is probable that some of the physisorbed oxygen was also removed during the sweeping step after oxygen chemisorption. However, whether all the physisorbed oxygen was removed from the solid cannot be ascertained [12]. The remaining physisorbed oxygen, which is desorbed at temperatures higher than 250 ~ may lead to an overestimation of the amount of CO and/or CO2 during TPD, due to the gasification of the sample. On the other hand, the TPD process involves the diffusion of CO and CO2 throughout the solid pores with the possibility that secondary reactions may occur. Thus the ASA values obtained by the TPD method might entail some experimental errors that should be taken into account. The third method (OCI) avoids these errors, since the physisorbed oxygen is quantified and eliminated from the final ASA calculation. Moreover, the evaluation of the chemisorbed oxygen does not require the oxygen surface groups formed to evolve, since the chemisorbed oxygen is evaluated from the direct measurement of the variations in oxygen pressures during the chemisorption process. 4. CONCLUSIONS The usual methods of ASA evaluation (gravimetric and TPD) present some inherent problems due to the uncertainty regarding the total elimination of the physisorbed oxygen from the ASA calculation, or to the errors arising from the quantification of surface oxygen complexes evolved. The OCI method seems to be an interesting and alternative method for ASA determination, as it avoids the main potential source of errors that the usual methods present. REFERENCES [1] [2] [3] [4]
P. Ehrburger, F. Louys, J. Lahaye, Carbon, 27 (1989) 389. B.J. Waters, R.G. Squires, N.M. Laurendeau, Carbon, 24 (1986) 217. H. Teng, C.T. Hsieh, Ind. Eng. Chem. Res. 38 (1999) 292. J. Lahaye, J. Dentzer, P. Soulard, P. Ehrburger, Fundamental Issues in Control of Carbon Gasification Reactivity, Kluwer Academic Publishers, Netherlands, 1991, 143.
216 [5] J.B. Parra, J.C. de Sousa, R.C. Bansal, J.J. Pis, J.A. Pajares, Adsorption Sci. Yechnol., 12 (1995) 51. [6] A. Arenillas, F. Rubiera, J.J. Pis, J. Anal. Appl. Pyrolysis, 50 (1999) 31. [7] P. Causton, B. McEnaney, Fuel 64 (1985) 1447. [8] L.R. Radovic, A.A. Lizzio, H. Jiang, Fundamental Issues in Control of Carbon Gasification Reactivity, Kluwer Academic Publishers, Netherlands, 1991, 235. [9] N.R. Laine, J. Vastola, P.L. Walker Jr., J. Phys. Chem., 67 (1963) 2030. [ 10] S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Am. Chem. Soc., 62 (1940) 1723. [11] B. McEnaney, Fundamental Issues in Control of Carbon Gasification Reactivity, Kluwer Academic Publishers, Netherlands, 1991, 175. [12] K. Skokova, L.R. Radovic, Prep. Am. Chem. Soc., Div. Fuel Chem., 41 (1996) 143.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
217
Heterogeneity of sewage sludge derived materials as a factor governing their performance as adsorbents of acidic gases A. Bagreev, S. Bashkova ~, B. Reznik 2, V. Zibat2, and T. J. Bandosz* Department of Chemistry and 1The Graduate School of CUNY, The City College of New York; New York, N Y 10031;* E-mail: [email protected]; Tel: (212) 650-6017; Fax: (212) 650-6107 :Laboratorium fur Elektronenmikroskopie, Universit~t Kaflsruhe, Kaiserstr. 12, Posffach 6980, D-76128 Karlsruhe, Germany Sewage sludge derived materials obtained by carbonization of organic fertilizer Terrene| at 800 and 950 ~ were used as adsorbents of hydrogen sulfide and sulfur dioxide from moist air. Since the performance of materials significantly differs depending on carbonization temperature an attempt was made to identify the surface features that govern H2S and SO 2 removal. The adsorbents were characterized using sorption of nitrogen, thermal analysis, SEM, and EDX. The results obtained demonstrate that the chemical form of catalytically active metals and their surface dispersion affect the adsorption/oxidation processes. 1. INTRODUCTION One of municipal wastes produced in abundant quantity is sewage sludge [1]. Various methods have been used to dispose of or utilize it [2], including incineration, landfilling, road surfacing, conversion to fertilizer, compression into building blocks, and carbonization [1-4]. Carbonization of sludge [4-6] in the presence of chemical activating agents such as zinc chloride and sulfuric acid produces new sorbents, with patented applications in such processes as removal of organics in the final stages of water cleaning [5] and removal of chlorinated organics [6]. The process of carbonization of biosolids has been studied in detail using different chemical agents and various conditions [8-11]. Materials obtained as a result of the treatment have surface areas between 100 and 500 m2/g [10, 11]. So far the most promising results have been obtained in our laboratory [12-14]. The performance of new materials as H2S adsorbents is comparable to that of caustic impregnated activated carbons. Besides excellent capacity for hydrogen sulfide removal the sewage sludge derived materials proved to work well as adsorbents of sulfur dioxide. In the cases of both sulfur containing pollutants, the inorganic constituents of adsorbents such as iron, zinc, copper, or calcium along with the presence of fine micropores within carbonaceous deposit are likely crucial for adsorption/oxidation process. The objective of this paper is to demonstrate that unique surface heterogeneity of sludge derived materials is a factor governing P- performance as desulfurization adsorbents. Complex chemical nature of inorganic matrix provides active centers for adsorption/oxidation. On the other hand, carbonaceous matter, is responsible for existence of pores of small diameters where oxidation process occurs and where catalysts can be highly dispersed. All of these provide active centers and space for storage of the surface oxidation products.
218
2. EXPERIMENTAL 2.1. Materials Terrene | was obtained from the New York Organic Fertilizer Company, (Bronx, New York) in the form of 3 mm diameter granules with about 5% water content. The detailed chemical composition is presented in elsewhere [12, 15]. It contains around 35% inorganic matter mainly in the form of silicon (10-12 %), iron (2.7%), aluminium (0.7%) calcium (2%) oxides and carbonates and 60% of organic matter. The adsorbents studied were prepared by pyrolysis of Terrene | at temperatures at 800 and 950~ in a nitrogen atmosphere in a fixed bed (horizontal furnace). The heating rate was 10 ~ and holding time 60 min. The samples are referred to as SC-1 and SC-2, respectively. The prepared materials were studied in subsequent order as hydrogen sulfide and sulfur dioxide adsorbents in the dynamic tests described below. After exhaustion of its adsorbent capacity, each sample is identified by adding the letter H or S referring to hydrogen sulfide or sulfur dioxide adsorption, respectively. The order of the letters reflects the order of the experiments. For instance, SC-1HS is an exhausted sample on which first the hydrogen sulfide breakthrough test was carried out. After exhaustion, the SO2 breakthrough test was performed on the same sample. 2.2. Methods
Breakthrough capacity The dynamic tests were carried out at room temperature to evaluate the capacity of sorbents for H:S or SO: removal. Adsorbent samples were packed into a column (height 80 mm, diameter 9 mm, bed volume 6 cm3) and prehumidified with moist air (relative humidity 80 % at 25 ~ for an hour. The amount of adsorbed water was estimated from the increase in the sample weight after prehumidification (the sorbents were removed from the column and weighted). Moist air (relative humidity 80 % at 25 ~ containing 0.3 % (3000 ppm) HzS or SO2 was then passed through the column of adsorbent at 0.5 L/min. The elution of H:S was monitored using an Interscan LD-17 H2S continuous monitor system interfaced with a computer data acquisition program. The elution of SO: was monitored using Lumidor electrochemical sensor. The tests were stopped at the breakthrough concentration of 500 and 350 ppm for H2S and SO2, respectively. The adsorption capacities of each sorbent in terms of g of H:S or SO: per gram of carbon were calculated by integration of the area above the breakthrough curves, and from the H:S or SO: concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. For the second adsorption run the prehumidfication was done at the same conditions as for the initial run.
Nitrogen adsorption Nitrogen adsorption isotherms were measured using an ASAP 2010 analyzer (Micromeritics, Norcross, GA, USA) a t - 1 9 6 ~ Before the experiment the samples were degassed at 120 ~ to constant pressure of 10.5 torr. The isotherms were used to calculate the specific surface area, Spa-r; micropore volume, Vmic;average size of micropores, Lmic, ; and pore size distribution using Density Functional Theory (DFT) [16, 17]. The total pore volume, Vt was evaluated from the last point of the isotherm.
219
pH A 0.4 g sample of dry adsorbent was added to 20 mL of deionized water and the suspension stirred overnight to reach equilibrium. The sample was filtered and the pH of solution was measured using an Accumet Basic pH meter (Fisher Scientific, Springfield, NJ, USA). Thermal analysis Thermal analysis was carried out using TA Instruments Thermal Analyzer (New Castle, DE, USA). The heating rate was 10 ~ in a nitrogen atmosphere at 100 mldmin flow rate. SEM Freshly fractured surfaces were examined by SEM in a LEO 1530 microscope with a Schottky field-emission gun without evaporating conductive layers prior to the investigation. EDX & WDX Polished embedded in epoxy samples were analyzed by Camebax SX-50 electron probe microanalyzer. Element mapping were recorded by means of WDX-spectrometer with the Ep = 15 kV from the area 512 x 512/~m. EDX analyses were done with Voyager Analyzer (Noran). 3. RESULTS AND DISCUSSION The results from the breakthrough experiments are collected in Table 1. As expected, the samples differ in H2S adsorption whereas the adsorption of SO2 is comparable [12, 13]. Much higher H2S breakthrough capacity is obtained for SC-2 than for SC-1 (three fold difference). It is worth to mention that the obtained capacities are slightly smaller than those reported previously [12, 13]. This is likely the result of fertilizer oxidation during its exposure to air. After exhaustion in SO2 breakthrough tests still some capacity for H2S exists and, especially on the SC2 sample. Another interesting observation is almost identical behavior of samples exposed for SO2 adsorption. Regardless the carbonization temperature the same amounts of SO2 are adsorbed on the samples exhausted in the H2S breakthrough test. This suggests that after exhaustion of all active centers responsible for H2S adsorption the differences in surface chemistry, which play a role in sulfur dioxide adsorption/oxidation on fresh surfaces, seem to be somehow screened by H2S adsorption products. Nevertheless, it is interesting that still some capacity exists. The total amount of sulfur adsorbed in mmol/g clearly shows that the SC-2 surface favors adsorption of sulfur containing species, especially hydrogen sulfide. Only on this sample, and only when H2S is involved, the sulfur retained on the surface reaches over 2 mmol/g. Since the amount adsorbed is few times greater than for the other samples the catalytic effect, probably on iron oxides, seems to govern the performance of this material. Differences are also noticed in the values of the surface pH and amounts of preadsorbed water. The pH values for the exhausted samples after subsequent SO2 and H2S adsorption runs are much lower than those after H2S adsorption followed by SO2 adsorption. This suggests differences in the surface reaction products. These differences are also reflected in the amount of water adsorbed after the first runs in the breakthrough tests. After SO2 adsorption much more water is preadsorbed before the next run than after adsorption of H2S. This once again indicates differences in the chemistry of inorganic phase. After SO2 adsorption it is likely that still some oxides able to adsorb water are present (hydrophilic surface) whereas reactions with H2S and deposition of sulfur [12, 14] almost totally "screen" active centers for water adsorption.
220 Table 1. H2S and SO2 breakthrough capacities [mg/g/ mmol/g], total amount of sulfur adsorbed [mmol(~], surface pH values, and the amounts of preadsorbed water [m~/~J, , Sample pH/pHE H2S.brth. Cap. SO2 brth. Cap. Sulfur total Water adsorbed SC-1H --19.5/ 0.57 --0.57 45.1 SC-1SH 10.3/8.9 8.9/ 0.26 --0.59 12.6 SC-2H 72.3/ 2.13 --2.13 65.1 SC-2SH 9.3/7.7 18.9/ 0.56 --0.90 18.7 SC-1S --20.9/ 0.33 0.33 42.1 SC-1HS 10.3/9.3 --6.6/ 0.10 0.67 3.3 SC-2S --21.9/ 0.34 0.34 42.1 SC-2HS 9.3/9.6 --6.5/ 0.10 2.23 3.8 I
I
I
I
I
II
i
i
iiii
The parameters of porous structure of our adsorbents, the initial ones and after the subsequent breakthrough tests, are collected in Table 2. They were calculated from the nitrogen adsorption isotherms. As reported previously, SC-2 sample has slightly higher surface area and micropore volume than SC-1 (around 10 %). Microporosity consists of about 20 % of the total pore volume and it is located likely within the carbonaceous deposit or on the interface between inorganic phase and carbonaceous material. After subsequent adsorption runs micropores almost disappear. It is interesting that for all samples the decrease in the volume of micropores is equal, even though different amounts of species were adsorbed (the total capacity of SC-2 is higher than that of SC-1). When the decrease in the total pore volume is considered slight differences are noticed. The decrease is greater for SC-2, which is consistent with higher adsorption. This suggests that in the case of this material not only micropores are active in the adsorption process. This supports our hypothesis that surface chemistry and its complex nature is an important factor governing the performance of these materials [12, 13]. It is interesting that either for SC-1 or SC2 after subsequent SO2 and H2S adsorption average sizes of micropores are greater than for those samples with the reversed order of adsorption. Once again, it suggests that active centers for hydrogen sulfide are located not only in very small pores comparable in size to the adsorbate molecule, as suggested for adsorption of SO2 on activated carbons [18]. Indeed, in the case of SO2 adsorption porosity is likely to play a role. Since the volume of micropores is similar for both SC-1 and SC-2 the adsorption of sulfur dioxide is also similar. Table 2. Structural Sampl e SC- 1 SC-1HS SC-1SH SC-2 SC-2HS SC-2SH
parameters calculated from nitrogen adsorption isotherms Vt [cm3/g] S [m2/g] V~ic [cm3/g]. 79 0.025 0.117 8 0.002 0.063 9 0.002 0.071 90 0.026 0.136 22 0.003 0.072 19 0.003 0.071 I IIII
I
Lmi~[]k] 9.2 13.3 14.9 9.3 13.9 14.2 II
I
Examples of changes in the pore size distributions for SC- 2 samples are presented in Figure 1. For both subsequent adsorption orders similar changes are observed and almost all
221 pores are affected. Although, this suggests that whole surface become active in the catalytic processes, the capacity lasts until all micropores are filled with the adsorption/oxidation products.
Fig. 1. PSD for SC-2 samples.
Fig. 2. DTG curves for SC-2 samples.
The results of thermal analysis can be used to demonstrate changes in the products of surface reactions depending on the order of the adsorption run, SH or HS. The DTG curves for the SC-2 series of samples is presented in Figure 2. For SC-1, owing to small adsorption, the peaks on the DTG curves are not well pronounced. As described elsewhere, the first sharp peak represents the removal of water and weakly adsorbed SO 2 or H2S. Even after adsorption of either SO2 or H2S differences on the curves can be noticed. For the former process the peaks centered at about 270 ~ 480 ~ and 820 ~ can be assigned to the removal of sulfuric acid, decomposition of iron sulfate, and mixed salts containing calcium and sulfur, respectively [12, 13, 19]. When this sample was exposed to H2S adsorption the shape of DTG curve changed. The peak located between 200 and 350 ~ became much broader and complex (with a shoulder) whereas the other two mentioned previously peaks significantly decreased their intensity. This suggests that less iron and calcium sulfates is on the surface. The possible reason for this is reaction of H2S and sulfates leading to formation of elemental sulfur and metal oxides. This would explain the broad shoulder at about 300 ~ which may be assigned to the presence of elemental sulfur. Since this sulfur, if present in small pores can be oxidized to SO2, the intensity of the peak centered at about 250 ~ also increased. Supporting for this hypothesis are the results obtained from the sample exposed to H2S adsorption in the first run. The positions of peaks are almost the same as for SC-SH, however definitely more sulfur is present (broad peak at 400 ~ owing to bigger adsorption. The peak centered at about 500 ~ can be related to iron sulfate. When that sample was exposed to SO2 the DTG curves slightly changed indicating formation of more sulfur dioxide likely from decomposition of sulfuric acid. These results indicate that different centers are involved in both adsorption processes and calcium oxide or hydroxide is definitely more active in SO2 adsorption then H2S. Iron oxide participates in both reactions, however, it acts as a catalysts for sulfur formation only in reaction with H2S.
222 Since the performance of adsorbents, especially, for H2S removal differs depending on the pyrolysis temperature, the very active form of iron has to be created by heating at temperature higher than 800 ~ in reducing atmosphere. The SEM study showed that heat treatment has an effect not only on the amount of hydrogen sulfide adsorbed but also on the physical form of species present on the surface after adsorption. The SEM micrographs for SC-1, SC-2 and their exhausted counterparts are presented in Figure 3. The surfaces of both initial samples look similar (SC-2 is with magnification10 K, whereas SC-1 15 K). The white, dispersed dots are likely catalytic centers for growing sulfur crystals. This conclusion is formulated based on the micrographs for the exhausted sample. Close look on them reveals that new entities grew on those centers and, what is very interesting, they differ significantly in shape depending on the pyrolysis temperature. Since both samples were exposed to H2S adsorption the new species must be elemental sulfur in different crystallographic forms. For some reason on SC-2H rhombic sulfur is present, whereas on SC-1H - the monoclinic one. This is an unexpected finding since the physical conditions for formation of a and 13 sulfur are quiet different [20]. In the light of the fact that experiments were carded out at exactly the same conditions the only possible reason for these differences is in surface chemistry and its catalytic effects.
MAG" 1,5.00 K X
wo= 5ram SlgtmlA=lnLens D I ~ ' 1 9 T ~ . 2001 B-~r~ ~ooow FI~ NIItl~ = NY 03b. 7.~
MAG= 15.00KX o~ro-t~dPs, w
wo= 4ram SJgn~A=rLeJlS DIm:lilSq~2001 r=HT:IoI~w FbName:NY_(13E_3:lif
i! t
o,~ r , - t ~
wD= 4rnm S~gnatA=W,.~ DaM -19 ,,~p 2001 SH'r= looow Fie Name = NY_04b_~,tif
M A G - 15,00KX olJq~ ra-l]e(adl Prlrmr
wo= 5m'n Sl~jl~llA=lrlLens ~ IgSe~_2001 EHT= looow File Name = NY_O4e..4~
Figure 3. SEM images. To study the surface chemistry the EDX analysis was done. The comparison of the scanned areas (512 x 512 ptm) is presented as BSE (back scanning electron) maps of elements in
223
Figure 4. Intensity is calibrated according to the intensity of sulfur in SC-2H, which should have the highest content of this element. Besides sulfur the content of oxygen, phosphorus and calcium is tested. The data shows sulfur "spots" dispersed in both initial materials (sulfur content ~ 0.7% wt. in initial non-carbonized samples) and the degree of dispersion seems to be higher in SC-1. It is interesting that there is an agreement in localization of sulfur and calcium. As mentioned before, some form of mixed oxide salts containing calcium and sulfur must be present in the matrix. However, spots of calcium are similar in initial and exhausted SC-1, much less calcium is seen on surface of SC-2H than on its initial counterpart. At this stage of our study we are not able to explain this transformation. BSE
S
O
SC-1
SC-1H
....
~! ~
SC-2
.
.
SC-2H
Figure 4. X-ray maps of elements Fa ~..,-
.... ~ : 2 L s
~
.:2~.,
,
222s163
~. . . . . . . .
2:2s163
Figure 5. Iron distribution in SC-2H
P
Ca
224 Since iron seems to be important as a catalyst for hydrogen sulfide oxidation on SC-2 sample [21], its distribution on the exhausted sample was evaluated using EDX (Figure 5). It was found that some bright spots of iron exactly match the bright spots of phosphorus with no link to oxygen. This suggests the iron phosphide is present on the surface in the form of large crystallites. The lack of easy distinguished crystals of iron oxide suggests very fine dispersion of this crucial for H2S oxidation compound. It has to be mentioned here that the area scanned in EDX experiments is four orders of magnitude larger than micropores in our materials. It follows that the chemical constituents and their distribution in small pores, which in fact are filled with the oxidation products can be quite different than on the "fiat" surface which is seen on EDX images. 4. CONCLUSIONS Results presented in this paper show that differences in the chemical composition of sewage sludge derived adsorbents lead to differences in their performance as adsorbents of acidic gases. It has been demonstrated that however some adsorption centers can be common for both gases, there are surface features on the sample pyrolized at 950 ~ which favor oxidation of hydrogen sulfide to elemental sulfur. This is likely due to the catalytic action of the iron species. When adsorption of S O 2 takes part calcium species play a crucial role. Surface chemistry has also its effect on the physical form of sulfur deposited on the surface. It is either rhombic or monoclinic depending on the pyrolysis temperature and chemical changes imposed by heat treatment. REFERENCES [1 ] Manahan, S.E. Environmental Chemistry, 6th ed.; CRC Press: Boca Raton, FL, 1994. [2] Biosolid Generation, Use, and Disposal in The United States: EPA530-R-99-009, Sept. 1999. [3] Sutherland, J. US Patent 3,998,757 (1976). [4] Nickerson, R.D.; Messman, H. C., US Patent. 3,887,461 (1975) [5] Lewis, F.M. US Patent 4,122,036 (1977). [6] Kemmer, F.N.; Robertson, R.S.; Mattix, R.D. US Patent 3,619,420 (1971) [7] Abe, H.; Kondoh, T.; Fukuda, H." Takahashi, M; Aoyama, T.; Miyake, M. US Patent 5,338,462 (1994). [8] Chiang, P.C.; You, J.H. Can. J. Chem. Eng. 65 (1987) 922. [9] Lu, G.Q; Low J.C.F.; Liu, C.Y.; Lau A.C. Fuel 74 (1995) 3A.4A.. [10] Lu, G.Q.; Lau, D.D. Gas Sep. Purif. 10(1996) 103. [11] Lu, G.Q. Environ. Tech. 16 (1995) 495. [12] Bagreev, A.;Bashkova, S.;Locke, D.C;Bandosz, T.J. Environ. Sci. Technol. 35 (2001) 1537. [13]Bashkova, S.;Bagreev, A.;Locke, D.C;Bandosz, T. J. Environ. Sci. Technol. 35 (2001) 3263. [14] Bagreev, A.; Bandosz, T. J. Ind. Eng. Chem. Res. 40 (2002) 3502. [15] Bagreev,A.; Locke, D.C.; Bandosz, T.J. Carbon 39 (2001) 1971. [16] Lastoskie, C.M.; Gubbins, K.E.; Quirke, N. J. Phys. Chem. 97 (1993) 4786. [17] Olivier, J.P.J. Porous Materials 2 (1995) 9. [18] Bagreev, A.; Bashkova, S; Bandosz, T. J. Langmuir 18 (2002)1257. [ 19] Handbook of Chemistry and Physics, 67th ed.,Weast, R.C. Ed. CRC, Boca Raton, FI, 1986. [20] Donohue, J. in Elemental Sulfur; Meyer, B. Ed.; Wiley, New York, 1965. [21] Davydov, A.; Chuang, K. I.; Sanger, A. R. J. Phys. Chem. B 102 (1998) 4745.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
225
Evaluation of microporous structure of carbon molecular sieves using the pycnometric method M. Balys, B. Buczek, E. Vogt Faculty of Fuels and Energy, University of Mining and Metallurgy, 30-059 Krakow, al. Mickiewicza 30, Poland Two kinetic (CMS-K1, CMS-K2) and one equilibrium (CMS-R) carbon molecular sieves, used originally for separation of gaseous mixtures, were investigated. The adsorption N2 isotherms at 77 K, in static conditions where obtained. In the case of the two first sieves the adsorption was so low that the calculation of parameters characterizing the texture was impossible. The volume of nitrogen adsorbed on the sieve CMS-R is remarkable From obtained results parameters characterizing micropore structure according to DubininRadushkevich equation and Horvath- Kawazoe method were determined. The diameter and volume of the micropores were also determined by the measurement of the density using as displacement molecules with different sizes, e.g., helium, water, benzene, decaline. It was found that in the case of CMS-K1 and CMS-K2 sieves, the micropores with the pore size within the range 0.255-0.528 are dominated and that the used measurements enable characterisation of the structure of carbon molecular sieves. For equilibrium sieve the analysis of the micropores volume with the use of the pycnometric technique does not give proper results. 1. INTRODUCTION Carbon molecular sieves belong to the group of microporous materials characterized by the relatively high adsorption capacity and selectivity towards a wide spectrum of gases. They posses amorphous structure with well developed surface area and the pore size comparable to the effective diameter of small molecules. For example, the molecular sieves used for separation of nitrogen from air are materials having the pore size within the range 0.3 - 0.5 nm. This feature differentiates them from typical carbonaceous adsorbents, featuring by wide pore size distribution [ 1].
226 Specific properties of molecular sieves are the reason for which these materials found application as adsorbents for separation of gas mixtures, especially air [2]. In fact, one can say that carbon-based molecular sieves played fundamental role in commercialisation of the pressure swing adsorption process (PSA) used for separation of nitrogen from air [3]. The hydrophobic character of molecular sieves surface and high resistance to acidic and basic media increased their competitiveness for use as adsorbents for separation of gases. They proved themselves even in such processes as separation of gaseous mixtures where the critical size of molecules differed only by 0.02 nm. The possibility to control molecular sieve properties during their preparation is the next reason for the interest in these materials as industrial adsorbents [4]. The mechanism of gas mixture separation depends on the type of adsorbent. Two different mechanisms are distinguished. The first mechanism is based on the kinetically controlled gas diffusion, caused by constrictions of the pore apertures. Here the diameters of pores are in the same range as those of the gas molecules. The particles having smaller diameter can penetrate much quicker into the pores than larger molecules. In the second separation mechanism, the pore system is sufficiently wide to enable fast diffusion, while the separation is caused by the selective adsorption dependent upon different van der Waals forces of the gas species [5]. If we define in that way the phenomena taking place on the molecular sieves surface, the first step to characterize the type of separation mechanism of a specific gas mixture should be determination of the pore sizes. However, selection of the measurement method for characterization of porous structures also has to take into account these mechanisms. For description of textural properties of carbonaceous adsorbents, adsorption/desorption isotherms of vapours and gases in static conditions as well as mercury porosimetry are used. The latter method often leads to destruction of porous structure of investigated materials while the usage of the former one is affected by the specific properties of molecular sieves described above. Taking into account these limitations, in this work the authors have made an attempt of determination of porous structure of carbon molecular sieves with the used of the pycnometric technique.
2. EXPERIMENTAL AND RESULTS Materials. Three different carbon molecular sieves, CMS-K1, CMS-K2, CMS-R, used
originally for separation of gaseous mixtures were investigated. For the first two adsorbents the size of pores was comparable to the size of the separated molecules and the process was
227 based on the kinetics effect.
The CMS-R contained pores large enough for the gaseous
molecules to diffuse quickly to the inside pores. In this case the separation was possible on the basis of different adsorption equilibrium. Adsorption measurements. The adsorption isotherms of N2 at 77 K were obtained in the same range of the time for all CMSs studied. The elongation of adsorption time for CMS - R sieve did not advisable for the sake of the equilibrium is not reached in real time. The adsorption-desorption isotherms are shown in Figure 1.
a)
200.0
b)
6.0
CMS-K2 160.0 ,.-,
4.0
~
2.0
1
40.0 0.0
0.0 0.0
0.2
0.4
0.6
0.8
1.0
~ 0.0
P/Po
,
~
0.4
0.8
.
, 1.2
plPo
Fig. 1. Adsorption-desorption isotherm o f nitrogen: a) CMS - R, b) C M S - K2, CMS - K I .
The volume of nitrogen adsorbed on the sieve CMS - R is remarkable. In the case of the other two sieves (CMS-K1, CMS-K2), the adsorption was so low that, the calculation of parameters characterizing the texture, such as pore size distribution, pore volume, etc., was impossible. In this way it was found that the adsorption of nitrogen at 77 K on the kinetic sieves did not allow to characterize their porous structure. From obtained isotherm were determined parameters characterizing micropore structure according to Dubinin - Radushkevich equation [6] and H o r v a t h - Kawazoe method [7] which are presented below: Micropore volume Wo, cm 3/g
0.28
Characteristic adsorption energy Eo, kJ/mol
21.7
Surface area of micropores Smir m2/g
509
Average micropore size XHK,nm
0.6
228
Effective density. The diameter and volume of the micropores were determined by the measurement of the density using as displacement molecules with different sizes of effective diameter, e.g., helium (0.25 nm), water (0.264 nm), benzene (0.370 x 0.528 nm), and decaline (0.472 • 1.01 • 0.624 nm). All pycnometric fluids are non-polar, except water. This adsorbate was used for the sake of the little diameter of its molecule. In the case of CMSs studied - not including of oxygen surface groups [8] - water molecule is good molecular probe. The measurements were carried out with the use of special construction pycnometers [9]. The effective density (pc) of the CMSs was calculated from Eq. 1 Pe-[(ml-m)'Ppl/[V'pp-(m:-ml)
]
(1)
where: m, ml, m: - weight of empty pycnometer, pycnometer with pycnometric liquid, and, pycnometer with sample and pycnometric liquid, respectively, V - volume of pycnometer, 9p-density of pycnometric liquid. The results of the densymetric measurements are shown in Table 1.
Table 1 Effective density
(g/cm3) of carbon molecular sieves investigated CMS - R 2.061
Effective density 9e CMS - K1 1.844
CMS - K2 1.93 7
water (H20)
1.890
1.773
1.797
benzene (C6H6)
1.992
1.536
1.513
decaline (C10H18)
1.867
1.508
1.478
Pycnometric fluid helium (He)
The micropore volume within the range corresponding to the size of molecules of the applied pycnometric fluids was calculated from: 1
1
PH20
PIle
V0.255 +0.264 = - -
The results are collected in Table 2 and presented in Figure 2.
(2)
229 Table 2 Micropore volumes (cm3/g) within the ranges of molecular size (nm) of pycnometric fluids Pore volume Vmic The range of the size CMS - R CMS - K1 CMS - K2 0.255 + 0.264 0.044 0.022 0.040 0.264 + 0.528 -0.027 0.087 0.104 0.528 + 0.624 0.033 0.012 0.015 0.264 + 0.624 0.006 0.099 0.120 0.255 + 0.624 0.050 0.121 0.160 3. C O N C L U S I O N We have found that in the case of CMS - K1 and CMS - K2 sieves, the micropores with the pore size within the range 0.255-0.528 nm are dominated (Fig. 2).
0.16 ~,~ ~ 0.14 ~
o.12 -J~ I]]]1CMS-R DCMS-K1 m CMS-K2
0.1 ~ ~ o.o
o.oz~~~
o
0.0~r
C).255 + 0.528 ~.8 + 0.624
CM~ CMS-R
Fig. 2. Micropore volume for the pore size within the range of the diameters of pycnometic fluid molecules.
Such result is consistent with the literature [1]. In this way it was found that the measurements based on displacement of fluids with different molecular sizes enable characterization of the micropore structure of carbon molecular sieves. However for CMS- R sieves a some amount of micropores with the size of 0.255 + 0.528 nm was found, but its volume is smaller than in the case of CMSs-K. The pycnometric technique makes possible the calculation of the pore volume with any size (Tab. 2), but remarkable values were obtained only for kinetic sieves. Hence, this analysis does not give proper results for equilibrium carbon molecular sieves.
230 Results obtained by the above method could be presented in due form of summary volume curves
or histograms. Such curves with adsorption or mercury porosimetry results
are used together to calculate of volume distribution curves. More advanced analysis of density measurements results enable us determination of shape of pores. The authors are grateful to the State Committee of Scientific Research (KBN) for its financial support of this work. REFERENCES
[ 1] N.O. Lemcoff, Adsorption and its Applications in Industry and Enviromental Protection Studies in Science and catalysis, 120, (1998), 347. [2] C. R. Reid, K.M. Thomas Abstract and Programme Poster Presentation, Eurocarbon 9-13 July, Berlin, 691, (2000). [3] A.J. Shirley, N.O. Lemcoff, AIChE Journal, 43, 2, (1997) 419. [4] B. Laciak, L. Czepirski, PrzemysI chemiczny, 72, 3, (1993) 135. [5] H. Jtintgen, K. Knoblauch, K. Harder, Fuel, 60, (1981) 817. [6] M.M.Dubinin, Izw. Ak. Nauk SSSR, Seria Chim., 9, (1991) 1. [7] G. Horvath, K. Kawazoe, J.Engn. Jpn., 16, (1983) 6. [8] M. Batys, B. Buczek, J. Zi~tkiewicz, Abstract and Programme Poster Presentation, ISSHAC-4, 27-31 August, Krakow, 105 (200 I). [9] Polish Standard PN-81/C-04307.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
231
Characterization of porous carbonaceous sorbents using adsorption data in wide temperature and pressure ranges Guy De Weireld, Marc Fr~re Facult6 Polytechnique de Mons, Thermodynamics Division, 31 bd Dolez, 7000 Mons, Belgium, E-mail: [email protected]
In this paper we present a new characterisation method for porous carbonaceous materials. It is based on a theoretical treatment of adsorption isotherms measured in wide temperature (303 to 383 K) and pressure ranges (0 to 10000 kPa) and for different adsorbates (N2, CH4, Ar, C3H8 and n-C4H10). The theoretical treatment relies on the Integral Adsorption Equation concept. We developed a local adsorption isotherm model based on the extension of the Redlich-Kwong equation of state to surface phenomena and we improved it to take into account the multilayer formation. The pore size distribution function is assumed to be a bimodal gaussian. By a minimisation procedure, it is possible to determine the bi-modal pore size distribution function witch can be used for purely characterisation purposes or to predict adsorption isotherms. I.
INTRODUCTION
The determination of the structural properties of porous adsorbents is of prime importance in the field of gas and liquid adsorption technology. Indeed, the pore shape, the pore dimensions and the chemical structure of its walls are the main characteristics to be considered when studying the potentiality of a given adsorbent to fit a given application. As far as the microporosity of heterogeneous solids is concerned, one must note that the different methods now available are not satisfactory. These methods are based on the theoretical treatment of experimental adsorption data, with generally consist of a nitrogen adsorption isotherm at 77 K. The methods depend on the theoretical treatment which is used. A majority of them are based on the Generalised Adsorption Isotherm (GAI) also called the Integral Adsorption Equation (IAE). The more recent approaches use the Monte Carlo simulations or the density functional theory to calculate the local adsorption isotherm. The analytical form of the pore size distribution function (PSD) is not a priori assumed. It is determined using the regularization method [1,2,3]. Older methods use the Dubinin-Radushkevich or the DubininAstakhov models as kernel with a gaussian or a gamma-type function for the pore size distribution. In some cases, the generalised adsorption equation can be solved analytically and the parameters of the PSD appear directly in the isotherm equation [4,5,6]. Other methods which do not rely on the GAI concept are sometimes used: the MP and the Horvath-Kawazoe (H-K) methods are the most well known [7,8].
232 Few studies have been devoted to the comparison of all these methods [9,10] so that it is quite difficult to dispose of satisfying conclusions on their efficiency. On a theoretical point of view, the methods based on the GAI concept and on molecular simulation calculations should be the most efficient ones. However a recent study has shown that the Stoeckli method (based on the DubininAstakhov theory) [6] gave results similar to those obtained from the molecular simulation methods [9]. On the other hand, the H-K and the MP methods are known to be rather inconsistent. In the near future, the development of the molecular simulation methods and the availability of results of comparison studies for a wide range of microporous sorbents should make the situation clearer. However, these methods are always based on the same kind of experimental data: a N2 adsorption isotherm at 77 K. These experimental conditions are very often far from those prevailing in the industrial applications. The use of a single adsorption isotherm within standard conditions could be considered as an advantage as it simplifies the experimental part of the characterization procedure. On the other hand, the possibility of using adsorption data in a wider domain of temperature and pressure conditions and for a large range of adsorbates should be helpful to prove or to invalidate the efficiency of the theoretical treatments. Actually, the development of specific characterization methods based on adsorption data obtained in experimental conditions similar to those prevailing in the final industrial application should allow a better understanding of the influence of the structure of the adsorbent on the behaviour of the industrial processes. Consequently, the synthesis of new adsorbents specifically developed for given industrial applications would become easier. Such characterization methods could also be used to interpolate or extrapolate adsorption data for process design purpose. It is the aim of this paper to present a new characterization method using adsorption isotherms in wide temperature and pressure ranges and for different adsorbates. We used adsorption isotherms of N2, Ar, CH4, C3H8 and n-C4H10 on an activated carbon (F30-470, chemviron carbon) at five different temperatures (303, 323, 343, 363 and 383 K) and for pressure ranging from 0 to the vapour pressure of the adsorbate at the experimental temperature for subcritical conditions and up to 10000 kPa for supercritical temperatures. We developed a model based on the Integral Adsorption Equation concept. The local adsorption isotherm model is an extension of the Redlich-Kwong equation of state to surface phenomena. It was adapted to take into account the multilayer formation. The pore size distribution function is assumed to be a bi-modal gaussian. 2.
EXPERIMENTAL
We used excess adsorption isotherms for N2, CH4, Ar, C3Hs and n-C4Hl0 on an activated carbon (F30-470 from Chemviron Carbon) at 303, 323,343, 363 and 383 K. These data have been measured using a magnetic suspension balance specially instrumented to perform simultaneous high pressure and high temperature adsorption isotherms measurements. The experimental apparatus and the data are presented elsewhere [ 11, 12, 13]. 3.
THEORETICAL DEVELOPMENTS
The model is based the Integral Adsorption Equation (IAE) concept [14, 15, 16]. We have developed it for our specific experimental conditions, i.e., for the treatment of excess
233 adsorption isotherms for both supercritical and subcritical compounds on an activated carbon in the field of high temperatures and high pressures. Equation 1 gives the classical IAE formulation: oo
m(T,P)= ~M I"(T,P,H) FA(H)dH
(I)
0
in which:
re(T, P)
is the real adsorbed mass (kg kg "l) at temperature T (K) and at pressure P (kPa) M is the molar mass of the adsorbate (kg mol "l) I"(T,P,H)is the surface concentration (mol m 2) in a pore of diameter H (m)
-
-
-
FA(H)
-
is the surface distribution function (m2 kg-' m").
Given we are working at high pressure with supercritical fluids, the experimental results which are used for the characterization are not real but excess adsorbed masses. Equation 2 gives the corresponding formulation for the excess adsorbed mass: oo
oo
mexc(T,P)= ~M F(T,P,H)FA(H)dH-p i ~2r, Ep,(H)FA(H)dH 0
(2)
0
in which:
P) is the excess adsorbed mass (kg kg l ) at temperature T (K) and at pressure P (kPa) pCisthe gas density (kg m -3) ri is the half-thickness of an adsorbed layer (m); published in the literature Ep,(H) is the number of adsorbed layers in a pore of diameter H.
"
mexc(Z,
_
-
-
The calculation of m and
mexc
by equations 1 and 2 requires the knowledge of pC, ri,
E.,(/4),F.(H) and r(v, e, H). / is calculated by a classical equation of state (RedlichKwong-Soave type). Epi is calculated from geometric considerations involving ri and H. FA(H) can be related to the pore volume distribution function Fv(H) if the pore shape is known. In this work, we consider classical slit-like pores to describe the porous structure of the activated carbon and we take a bimodal gaussian for the mathematical expression of Fv(H). As a consequence:
(H)= x(H) F (H)
(3)
H and
Fv (H)= qr2 x cr,H
2o'1. 2
~r2 ~r o'214
exp/-2-2. -m2" ' /
(4)
234 in which: H is the pore diameter being here the distance between the two parallel carbon surfaces - x(H) is a function ranging between 1 (if only one layer can be located between the two surfaces) and 2 (if two or more layers can be located between the surfaces): it takes into account the steric effect of the first adsorbed layer on each surface of the pore; it does not take into account the multilayer formation on each surface. Its expression is based on purely geometric considerations " Vlpores, V2pores, mlH, m2H, O'IH and cr2H are respectively, the pore volumes (m 3 kgl), the average diameters (m) and the standard deviations (m) of each contribution of the bimodal distribution; they are unknown parameters. -
1-'(T,P,H)is called the local adsorption model. It must be efficient for high temperature and high pressure calculations to deal with high adsorbed phase densities. We developed a specific local model based on a Redlich-Kwong behaviour of both the adsorbed phase and the gas phase:
p
E III IOvl bv v exp v-b~ v'b,
RT"b~
exp
av - R T l 5 ( v + b v)
I1
RT (2 n" M R T) 89 -~v = R T N Oh
qe. rotqe, v,bqge 1 F I Fb,,,ot ! qsrot qsvib -q-~se qsv,bz O-- 1-'bmo,) exp 1 - 1-'bmol O+l-'bm~
exp/-
a,,ol R
T
1 +
1-" 1-" b mot
exp
RT
,,.o, Rrbm~ ~-~
(5)
In this expression: v is the molar volume of the adsorbate in the gas phase (m 3 moll); it is calculated as a function of temperature and pressure by the Redlich-Kwong-Soave equation of state av (J m 3 K ~ mol 2) and by (m 3 mol l ) are the parameters of the Redlich-Kwong-Soave equation of state; they are easily calculated from the critical temperature and pressure and the acentric factor of the adsorbate R is the ideal gas constant (=8.314 J mol -I K -1) - No is the Avogadro constant (=6.022 1023 mol "l) - h is the Planck constant (=6.626 10-34 J S) -qgrot, qgvib, qge, qsrot, q~ib and qse are the rotational, vibrational and electronic contributions to the partition function of a molecule in the gas phase (index g) and in the adsorbed phase (index s); the term (qgrot qg~ib qge)/(qsrot q~vib q~e) Can be taken equal to 1 if we assume that the adsorption does not affect the rotational, vibrational and electronic behaviour of the molecule - a,,ot (J m 2 mol 2) and b,not (m 2 mol 1) are the Redlich-Kwong parameters of the adsorbate in the adsorbed phase; they must be considered as unknown parameters - q~vibz is the molecular vibrational contribution to the partition function of the molecule (vibration perpendicularly to the adsorption surface) - Tc (K) is supposed to be the two-dimension critical temperature; in fact it can be integrated in the amot parameter - Uomot (J mol -~) is the molar adsorption energy of the adsorbate in the pore. -
-
-
235
qs,,~bz and UOmol C a n be calculated from the potential energy function of the system composed of an adsorbate molecule located at a given distance z of the carbon surface of a slit-like pore Upot(z). Indeed:
I
exp
qsv,bz=
hNovz) 2RT (6)
( hN~ ) 1-exp RT
Uomol --NoUpot(Zeq
)
(7)
in which: - vz is the vibration frequency of the molecule perpendicularly to the surface (s ~) l
v~ :~--x
(8) Z -- Zeq
- mu is the mass of the molecule (kg)
- Zeq is the value of z for which: Oz )~e
(9)
We will not enter the details of the calculation of epot(Z). It is based on the works by Crowell, Steele and Kiselev devoted to the interactions between molecules or groups of molecules and a carbon surface [ 17]. All the parameters required for the calculation of Upot(Z) are given in the literature (mainly polarisabilities and magnetic susceptibilities). Equations 1 to 9 makes possible the calculations of m~, P) and mexc~, 19) if we consider that only one layer can be formed on the surfaces of the pores. To allow the calculations in the case a of multilayer formation, the procedure must be repeated several times. The second layer is supposed to be formed on a surface composed of the first layer of adsorbate the surface concentration of which is known thanks to the first step of the calculation. The potential energy function must be recalculated on such a basis (and thus Uomotand Vz). The pore diameter is taken equal to H-2z~q. The surface concentration of the second layer can then be calculated by equation 5. The procedure is repeated for each new layer. 4.
RESULTS
AND COMMENTS
Using equations 2 to 9 and the excess adsorption data we have determined the unknown parameters of our model that is to say amol and bmol for each compounds and the parameters of the pore volume distribution function Vlpores, V2pores,mill, m2n, Gin and Cr2n.This optimisation procedure is done using the whole set of data: Vlpo,.es, V2po,.es,ram, m2m, CrlH and Cr2nare characteristics of the adsorbent and should not be determined separately for each adsorbate. Tables I to III show these values of the parameters and the average deviation between the recalculated excess results and the experimental excess data for each adsorbate.
236 Table I:
Parameters of the pore volume distribution function
mln (A) crm(A ) V1pore(m 3 kg "1) Table II: Adsorbate Ar N2 CH4 C3H8 n-Call10
8.812 0.425 1.133 10-4
m2H(A)
16.383 0.662 V2pore(m 3 kg -1) 1.584 10-4
O'2H(t~)
Redlich-Kwong parameters of the adsorbates in the adsorbed phase
amol(J m 2 mol "2) bmol ( m 2 mo1-1) 2.02 1.45 9.33 1.65 3.76
108 108 108 108 108
47640 95462 67606 43366 56478
Table III: Average deviation between calculated and experimental results (excess data) Adsorbate
Deviation (%)
Ar N2 CH4 C3H8 n-Call10
2.94 5.66 4.78 3.71 3.96
Figures 1 to 4 show the pore size distribution functions (obtained by the H-K and IAE) methods and the comparison between the experimental results and the recalculated isotherms for three of the five adsorbates. The highest mean deviation is 5.66% for nitrogen. Consequently, our characterization method appears to be an efficient modelling tool as it allows to simulate adsorption isotherms of five different adsorbates in wide temperature and pressure conditions (subcritical and supercritical isotherm temperatures) using a unique pore size distribution function of the adsorbent. The parameters of the pore size distribution function given in Table I are related to the microporosity of the activated carbon. The total pore volume is 0.272 c m 3 g-l. We compared these results to those provided by the classical characterization methods based on the treatment of a nitrogen isotherm at 77 K. (t-plot, D-R, H-K). The micropore volume provided by the t-plot method is 0.413 c m 3 g-l. The macropore and mesopore surface area is 62 m E g-1. The t-plot method shows that the main part of the microporosity is located in the very low pore diameter area (H<7.08 A). For such pore diameters, the t-plot method should be considered with care as it is valid for a layer thickness higher than 3.54 A. The H-K method as well as the D-R method provide a total micropore volume of 0.414 c m 3 g-1. The maximum of the pore size distribution function provided by the H-K method is located at H=9.2 A. The total micropore volumes calculated by the classical characterization methods are in good agreement which is quite normal as they are based on the use of the same experimental results. The value of the micropore volume provided by our method is somewhat lower. This is due to the fact that we used both supercritical and subcritical data to perform our characterization calculations. As to the average pore diameter given by the different methods,
237
the results must be considered with care as they depend on the analytical form of the pore size distribution function. We may just state that the different methods give coherent results. 1.2E-04 ,~ 1.0E-04 u08.0E-05 + + +
~N 6.0E-05 ~4.0E-05 ;r~2.0E-05 0.0E+00
0
i
i
5
10
+ +++~
15
20
Pore diameter (H) / .~ Figure 1" Pore size distribution functions obtained by the IAE (-)and H-K (+)methods
0.20
,~0.15
. *
It 9 9 tt -9, ._- .' 6"6* 6 6 6 6 6 0 t t tt~ "'A "'''' ..,,,,,~
o.o
. .
t
.~
t ~" .~ : ~ 6
0.05 j
ills i 6i i = it
i I i
* t 9tt t 9
|
66t
t t
,-, 4000 6000 Pressure / kPa
0.00 0
Figure 2:
::- :-"'"':7.:'
2000
8000
10000
Experimental ( 9 and re-calculated ( A ) excess adsorption Ar isotherms 0.30 -,0.25 ~0.20 9 9 41,000 0
i
~0.15 r~ ~0.10
9
g
"< 0.05 0.00
T
1000
Figure 3" Exper aemal (r
m
2000 Pressure I kPa
I
3000
4000
re-calculated (A) excess adsorption C3H8 isotherms
238 0.40 0.35 ] i
o
it
'i::
9 9 9 ""
-
_
~. o.151
'm 0.10 < 0.05 0.00
0
r 500
, 1000
I
1500
Pressure/ kPa Figure 4: Experimental (#)and re-calculated (A) excess adsorption n-C4H10 isotherms
REFERENCES [1]
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17]
T.J. Mays, Proceedings of the 5 th international conference on Fundamentals of Adsorption, M.D. Le Van (Ed.), Kluwer- Dordrecht (1996) 603. N.A. Seaton, J.P.R.B. Walton and N. Quirke, Carbon, 27 (1989) 853. C. Lastoskie, K-E. Gubbins and N. Quirke, J. phys. Chem., 97 (1993) 4786. M.M. Dubinin and H.F. Stoeckli, J. Coll. Int. Sci., 75 (1980) 34. M.M. Dubinin, Carbon, 23 (1985) 373. H.F. Stoeckli, Carbon, 27 (1989) 962. J.H. de Boer, B.G. Linsen, Th. Vanderplas and G.J. Zandervan, J. Catal., 4 (1965) 469. Q. Horvath and K. Kawazoe, J. Chem. Eng. Jap., 16 (1983) 470. P.J.M. Carott, M.M.L. Ribeiro Carott and T.J.Mays, Proceedings of the 6th international conference on Fudamentals of adsorption, F. Meunier (Ed.), Elsecier Paris (1998) 677. M. Kruk, M. Jaroniec and J. Choma, Adsorption, 3 (1997) 209. M. Frdre, G. De Weireld and R. Jadot, Proceedings of the 6 th Conference on Fundamemals of Adsorption, F. Meunier (Ed.), Elsevier- Paris (1998) 279. G. De Weireld, M. Frdre and R. Jadot, Meas. Sci. Technol., 10 (1999) 117. M. Fr~re and G. De Weireld, High pressure and high temperature excess adsorption isotherms of N2, CH4 and C3H8 on activated carbon, J. Chem & Eng. Data, accepted for publication in Chem. Eng. Data. S. Ross, J.P. Olivier, On physical adsorption, Interscience - New York (1964). M. Fr~re, J. Bougard and R. Jadot, Adsorption, 3 (1996) 55. M. Fr6re, M. Zinque, K. Berlier and R. Jadot, Adsorption, 4 (1998) 239. W.A. Steele, The interactions of gases with solid surfaces, Pergamon - Oxford (1974).
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
239
A m m o n i a accessibility to the porosity of several activated carbons measured by flow adsorption microcalorimetry M. Domingo-Garcia a, A.J. Groszek b, F.J. L6pez-Garz6n a and M. P6rez-Mendoza c aDpto, de Qufmica Inorgfinica. Fac. de Ciencias. 18071 Granada. Spain bMicroscal Limited, 79 Southern Row, London W10 5AL, UK CDepartment of Chemical Engineering. The University of Edinburgh. The King's Buildings. Mayfield Road. Edinburgh EH9 3JL. UK
Ammonia adsorption was studied on several activated carbons with different textural and chemical characteristics by flow adsorption microcalorimeter. The textural and chemical nature of the samples was measured by N2 and CO2 adsorption and temperature programmed desorption (TPD-MS) respectively. The ammonia adsorption consists in reversible (related to physisorption) and irreversible (related to chemisorption on chemical groups) components. From the molar heats of adsorption it can be concluded that the samples have a wide distribution of acidic sites some of which are very strong. However, they are not always easily accessible to ammonia because constrictions in the pore-network hinder the access, forcing the adsorbed molecules to re-arrange.
1. INTRODUCTION
Most of the catalytic applications of carbon materials deal with their use in processes carried out under dynamic conditions which are very different to those of static systems. In such processes the accessibility to the catalytically active sites or chemical functionalities is especially important because of the short contact times between the reactives and the carbon catalyst and the polymodal distribution of pores that they present [1]. A large proportion of their surface area and chemical surface groups resides in micropores [2] of diameters very close to the molecular dimension of the reactives and/or the products and so the reaction kinetics may be strongly diffusion controlled. Then characterisation of the active sites under such conditions is of great importance and has been successfully studied by flow methods. The advantages of these methods were recently discussed by Fadoni and Lucarelli [3]. In the present work, several activated carbons
240 with different chemical and textural characteristics have been used to study the adsorption of ammonia. The base probe adsorption was measured under dynamic conditions by flow adsorption microcalorimetry following methodologies developed by one of the authors [4].
2. EXPERIMENTAL
Three commercial activated carbons were used: BPL, CAL and GAe, manufactured by Chemviron, Calgon and CECA respectively. In addition, sample GAe-oxl was prepared by oxidation of GAe in aqueous solution of (NH4)28208 and further pyrolysis in N2 flow at 773 K [5]. The specific surface areas were obtained applying the BET and Dubinin-Asthakov equations to the adsorption of N2 at 77 K and CO2 at 273 K respectively. Moreover, the CO2 adsorption data permitted the evaluation of the micropore size distributions and the mean value of pore width using the Dubinin-Stoeckli equation [6] which supposes a gaussian distribution of pore sizes. The chemical nature of the surface was analysed by Temperature Programmed Desorption followed by Mass Spectrometry (TPD-MS). The samples (0.1 g) were heated up to 1273 K in He flow at a rate of 20 K.min-1. The desorbed amounts of CO2, CO, H20 and H2 were monitored. The ammonia dynamic adsorption was studied using a Microscal Flow Adsorption Microcalorimeter fitted with a thermal conductivity detector. Details of the experimental procedure are given elsewhere [4,7,8]. Before the measurements the samples were purged with N2 at 423 K for 20 hours. Then He was changed to a 5% NH3/N2 mixture flow. Heats of adsorption and the corresponding amount of adsorbed ammonia were measured. The adsorption cycle was followed by a desorption one by switching the NH3/N2 mixture flow. Differential molar heats of adsorption and desorption were calculated dividing the rates of heat evolution by the corresponding adsorption quantities.
3. RESULTS AND DISCUSSION
The results of the textural characterisation are collected in Table 1. Although the methods used to determine the surface areas have limited value when applied to microporous solids, here they have been used as convenient methods to compare the different samples. The results show that samples have large surface areas and a well developed microporosity.
241 Table 1 Textural characteristics of the samples ,,,
SN2
a
sco 2
(mZ/g)
Sample
Lo a
Wo b
(nm)
(cm3/g)
CAL
868
970
1.47
0.368
BPL
1017
1148
1.67
0.436
GAe
1026
1185
1.61
0.449
GAe-ox 1
1129
1403
1.77
0.494
L0: mean value of the micropore width
b W0: micropore volume from CO2 adsorption
These parameters are very close for samples BPL and GAe and they are larger for sample GAe-ox 1 as a consequence of the chemical treatment with (NH4)2S208. CAL sample has the lowest values for surface areas and micropore volume. An important feature of these samples is that they present larger CO2 than N2 surface areas which suggests that there are slit-shaped constrictions which partially hinder the N2 adsorption at 77 K [9]. These constrictions seem to be more important in percentage in sample GAe-oxl which means that the chemical treatment has developed the microporosity of the parent sample, although a part of it is hardly reached by the N2 molecules. The micropore size distribution curves obtained by applying the Dubinin-Stoeckli equation to the CO2 adsorption data are shown in Fig. 1. CAL has the narrowest micropore size distribution, BPL and GAe show very similar curves and GAe-oxl has the most open distribution that extends beyond the microporosity limit (2 nm).
The mean value of the
micropore size distribution ranges between 1.47 and 1.77 nm for CAL and GAe-oxl respectively.
Figure 1: Micropore size distributions obtained from Dubinin-Stoeckli equation
242 Table 2: Ammonia adsorption parameters Percentage of Amount of adsorption
Amountof desorption
irreversible adsorption
Sample CAL BPL GAe GAe-oxl
(gmol. g-l) 18 136 192 270
(gmol. g-l) 9 41 76 72
% 50 70 60 73
The study of ammonia adsorption was carried out in a flow adsorption microcalorimeter under dynamic conditions. Some of the parameters from these experiments are collected in Table 2.. In all cases the amount desorbed was much smaller than the adsorbed one, and so was the absolute value of the heat evolved. This clearly points out that ammonia adsorption consists of two different components. One is related to chemisorption (irreversible adsorption) and the other one (more labile or reversible) to physisorption in the pores. Another important feature about these experiments is that heat was still released long afterwards the NH3 uptake was negligible. This heat evolution, already reported in other experimental systems [4,7,8], is due to diffusion of adsorbed ammonia from low energy sites to higher energy sites having low accessibility. The lack of further uptake of NH3 may be due to irreversibly adsorbed molecules on the borders of micropores blocking NH3 towards the end of the adsorption process.
Figure2: ammonia adsorption vs time
243 Table 3: Amounts of CO and CO2 desorbed measured by TPD-MS CO Sample CAL BPL GAe GAe-oxl
C02 (lamol/g)
3524 1294 1535 2833
1682 429 405 387
%0 11.02 3.44 3.75 5.77
The amount adsorbed versus time is plotted for all samples in Fig. 2. The trend found for CAL is rather unexpected considering the textural characteristics of this sample. This behaviour can be explained if chemical characteristics are also considered. The amounts of chemical functionalities desorbed as CO2 and CO in the TPD-MS experiments and the percentage of oxygen content are collected in Table 3. CAL has the largest amount of chemical surface groups and oxygen content by far, but it also has the narrowest microporosity (Fig. 1). It is known [2,10,11] that chemical functionalities are in the most energetic parts of the carbon materials, i.e. the entrances of the micropores. Therefore, the concentration of chemical groups in the micropore borders of this sample should be very high and it is likely that they hinder the access of ammonia to other chemical groups inside the sample. The behaviour of GAe-oxl is opposite to this of CAL as, although it has less oxygen groups (but large amount of them), it shows the widest micropore size distribution in such a way that it adsorbs the largest amount of ammonia. Interesting conclusions about the accessibility of ammonia to the chemical groups can be drawn from the plots of the molar heat of adsorption versus cumulative adsorption (in mmol'g -1) (Fig. 3a and b).
Figure 3 (a and b): variation of the molar heat with the cumulative adsorption
244 The trend found for CAL is noticeable. At small uptake, the evolution of heat per mole of adsorbed ammonia is not large in absolute value, but it rapidly increases almost asymptotically. This evolution of adsorption heat with a small rise in the uptake is a consequence of re-arrangement of adsorbed molecules to more thermodynamically favoured acidic groups [4,7,12,13]. This means that the most energetic sites are not reached in the early stages of adsorption due to the large amount of chemical groups present in this sample. Heat values as large a s - 1 5 0 kJ-mo1-1 are evolved in the re-arrangement process. This value is twofold the initial one and similar or larger to those reported for ammonia adsorption on several zeolites [4, 14-16]. The behaviour of BPL and GAe is almost the same except for low cumulative adsorption where the former presents much higher heats of adsortption. This is in agreement with the very similar textural and chemical surface characteristics of both samples, although it points out that while GAe has higher amount of accessible acidic groups (Fig. 2), the accessibility to the strongest ones is easier in BPL. Both samples also present rearrangement for large values of cumulative adsorption. GAe-oxl doesn't present any rearrangement and evolves nearly the same molar heat of adsorption in the whole range. This is due to this sample has the widest micropore size distribution, and therefore the most stable acidic groups in this sample are reached easily since the early stages of adsorption. Furthermore, from a comparison between the data collected in Tables 2 and 3 it can be deduced that most chemical groups are not accessible to the adsorbent, assuming that one ammonia molecule is adsorbed per chemical surface group. Thus, the textural parameters of activated carbons control the diffusion of the adsorptives and may be crucial to understand the adsorption and catalytic behaviour of these materials. This relationship can be clearly seen if the irreversible adsorption is plotted versus the mean size of the micropore distribution, Lo (Fig. 4). The irreversible adsorption is related to the textural characteristics in terms of accessibility of the adsorbate to the most active sites, i.e. the possibility of reaching the chemical groups in pores is larger as the micropore distribution is more open.
4. C O N C L U S I O N S
Accessibility to the chemical surface groups is a very important factor in order to understand the behaviour of activated carbons as adsorbents and catalysts. This factor becomes particularly important if constrictions in the pore-network are present. So dynamic techniques should be used in order to characterise these kinds of materials.
245 85 ~ 75 '~ ~ ~ = 65 ~
R2- 0-9641
.~..""~
55 45 1.4
,
,
,
1.5
1.6
1.7
!
1.8
t~ (na~ Figure 4" percentage of irreversible adsorption vs. mean pore size
Among them, flow adsorption microcalorimetry has been successfully used for this purpose proving to be very useful to obtain adsorption and thermodynamic data under experimental conditions close to those operating in industrial practice. The activated carbons used in this work present a wide distribution of acid groups and some of which exceed in strength those existing on acidic zeolites and pillared clays, although they are not always accessible due to the presence of constrictions in the pore network. As a consequence of this, kinetic re-arrangements are produced. Thus, not only a large number of chemical surface groups is desirable, but also a proper textural structure that facilitates the access to them. In all cases, ammonia adsorption consists in two components: irreversible and reversible adsorption. The former one is a consequence of chemisorption on chemical groups while the latter is related to physisorption.
AKNOWLEDGEMENTS
M.P.M. acknowledges the financial support of the Ministerio de Educacion Ciencia y Deporte.
The
acknowledged.
support
of
DGESIC
under
project
BQU2001-2936-CO2-01
is
also
246 REFERENCES
1. R.C. Bansal, J.-B. Donnet, Active Carbon, Marcel Dekker, New York, 1988. 2. F. Rodriguez-Reinoso, H. Marsh, E.A. Heintz, F. Rodriguez-Reinoso (eds.), Introduction to Carbon Technologies, Secretariado de Publicaciones, Universidad de Alicante, 1997. 3. M. Fadoni and L. Lucarelli, A. Dabrowski (ed.), Adsorption and its Application in Industry and Environmental Protection in Surface Science and Catalysis, vol. 120A, Elsevier, Amsterdam, 1999, pp 177-225 4. R. Brown and A.J. Groszek, Langmuir 16 (2000) 4207. 5. C. Moreno-Castilla, F. Carrasco-Maffn and A. Mueden, Carbon 35 (1997) 1619. 6. M.A. Salas-Peregrfn, F. Carrasco-Maffn, F.J. L6pez-Garz6n and C. MorenoCastilla, Energy & Fuels 8 (1994) 239. 7. A.J. Groszek and C. Aharoni, Langmuir 15 (1999) 5956. 8. A.J. Groszek, A. Dabrowski (ed.), Adsorption and its Applications in Industry and Environmental Protection, vol. 120A, Elsevier, Amsterdam, 1999, p. 143. 9. F. Rodriguez-Reinoso and A. Linares-Solano, P.A. Thrower (ed.), Chemistry and Physics of Carbon, vol 21, Marcel Dekker, New York, 1989, p.1. 10. F. Rodriguez-Reinoso, Carbon 36 (1998) 159. 11. M. P6rez-Mendoza, M. Domingo-Garcfa and F.J. L6pez-Garz6n, Carbon 37 (1999) 1463. 12. F. Xie, J. Phillips, I.F. Silva, M.C. Palma and J.A. Men6ndez, Carbon 38 (2000) 691. 13. A.S. Gow and J. Phillips, Ind. Eng. Chem. Res. 31 (1992) 193. 14. C.M. Ilao, J. Yamamoto and K. Segawa, Journal of Catalysis 161 (1996) 20. 15. K. Segawa and H. Tachibana, Guczi et al. (eds), New Frontiers in Catalysis, Proceedings of the 10th International Congress on Catalysis, Budapest, 1992. 16. D.T. Cheng, L. Zhang, C. Yi and J.A. Dumesic, Journal of Catalysis 146 (1994) 257.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
247
An IGC and TA study of acetaldehyde adsorption on activated carbons Y. El-Sayed and T. J. Bandosz Department of Chemistry, the City College of New York and the Graduate School of City University of New York, New York, NY 10031; USA. Tel: (212) 650-6017; fax: (212) 6506107, E-mail: [email protected]; http://www.sci.ccny.cuny.edu/~tbandosz Two samples of activated carbons of various origins were modified using oxidation with nitric acid and impregnation with urea followed by thermal treatment. The surface of carbons was characterized using sorption of nitrogen, Boehm titration,--and thermal analysis. Inverse gas chromatography was used to study the adsorption of acetaldehyde at infinite dilution. From the retention volumes the heats of acetaldehyde adsorption were calculated. The results showed that on materials with very small pores interactions of acetaldehyde are strongest. This is due to dispersive interactions of hydrocarbon moiety with the activated carbon pore walls. Introduction of functional groups containing oxygen or nitrogen results in creation of additional adsorption centers which start to play a role when all high energy centers, small pores, are filled with acetaldehyde molecules. After modification of carbon surfaces the isosteric heat of acetaldehyde adsorption decreased. This is due to changes in the pore structure and an increase in the density of surface groups that results in partial blocking of the pore entrances and thus decreases the contact of hydrocarbon moiety with the carbon surface. 1. INTRODUCTION Activated carbons have a wide application for adsorption of traces of gases or vapors. This is due to their large surface area, pore volume, and microporosity where dispersive interactions are strongly enhanced [1, 2]. Another important factor contributing to their wide application, especially for removal and separation of polar compounds, is surface chemistry [39]. It is related to the presence of heteroatoms such as oxygen, nitrogen, hydrogen, sulfur, chlorine and phosphorus within the carbon matrix bonded to the edges of the graphite-like layers [3]. They have a significant influence on the chemical properties of the carbon surface. One of the activated carbons' applications is purification of air and removal of odoriferous substances, hazardous to human health, from the atmosphere. Acetaldehyde can be considered as such a compound. Adsorption of formaldehyde on activated carbons was studied by Domingo-Garcia et al. [ 10]. They found that it is strongly adsorbed and the retention volumes obtained in that study increased with an increase in the surface areas of activated carbons. The objective of this study is to evaluate which features of activated carbon surfaces are important for adsorption of acetaldehyde. The evaluation is based on the values of isosteric heats of adsorption, which reflect the strength of molecule interactions with the sorbent surface. After oxidation and urea modification the changes in surface chemistry and porous structure occurred. Those changes are expected to affect the dispersive and specific interactions of acetaldehyde with the activated carbon surfaces.
248 2. EXPERIMENTAL 2.1. Materials Two activated carbons were chosen for this study. They are as follows: BPL (Calgon, bituminous coal origin), and BAX (Westvaco-wood origin, chemical activation with phosphoric acid). Before experiments, the initial carbons were washed in a Soxhlet apparatus to remove water-soluble species. Three samples of each carbon were modified. One was oxidized with nitric acid [ 11 ]. The other two were impregnated with urea (saturated solution) and heated at 723 or 1223 K in nitrogen at the rate of 10 deg/min for 1 hour in order to introduce nitrogen groups [12]. The three samples were then washed in a Soxhlet apparatus to remove excess oxidizing agent, excess of urea decomposition products and water-soluble compounds. After modifications the samples are referred to as BPL-O, BAX-O, BPL-N 1, BPL-N2, BAX-N 1,and BAX-N2 where O and N refer to the presence of oxygen or nitrogen functionality, respectively, and 1 and 2 represent the temperatures of heat treatment: 723 and 1223 K, respectively.
2.2. Methods
Boehm titration One gram of carbon sample was placed in 50 mL of the following 0.05N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate and hydrochloric acid. The vials were sealed and shaken for 24 h and then 10 mL of each filtrate was pipetted and the excess of base or acid was titrated with HC1 or NaOH. The numbers of acidic sites were calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups; Na2CO 3 - carboxyl and lactonic; and NaHCO 3 only carboxyl groups [3, 4]. The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon.
Sorption of nitrogen Nitrogen isotherms were measured using an ASAP 2010 (Micromeritics) at 77 K. Before the experiment the samples were heated at 473K and then outgassed at this temperature under a vacuum of 10 -5 torr to constant pressure. The isotherms were used to calculate the specific surface area, SN2, micropore volume, Vmic, and total pore volume, V t. All of the above parameters were calculated using Density Functional Theory (DFr) [ 12, 13].
Thermal analysis Thermal analysis was carried out using TA Instrument Thermal Analyzer. The instrument settings were: heating rate 10 K/min and a nitrogen atmosphere with 100 mL/min flow rate. For each measurement about 25 mg of a ground carbon sample were used.
Adsorption of acetaldehyde at saturation conditions A beaker with 10 mL of acetaldehyde was placed in a dessicator along with several 10 mL weighting dishes containing 1g of a powdered carbon samples. The samples were kept in the atmosphere saturated with acetaldehyde vapors for 13 days (vapor pressure of acetaldehyde at 293 K is equal to 760 torr). It was assumed that the adsorption equilibrium was reached [ 11 ].
Elemental analysis The content of nitrogen for virgin and urea modified samples was determined by Huffman Laboratories, Golden CO.
Inverse gas chromatography The chromatographic experiments were performed with an SRI gas chromatograph equipped with a flame ionization detector. The injector temperature was 473 K. The stainless steel columns (20 cm long, 2.17 mm in diameter) were filled with carbon particles of size
249 ranging from 0.2 to 0.4 mm. Helium was used as a carrier gas with a flow rate 30 - 180 cm3/min (the flow rate over 100 ml/min were used for urea modified samples) and methane as a nonretained species. The range of experimental temperatures was: 313-473K (20 K step). Gas chromatography is called inverse gas chromatography when it is applied to study the adsorption on a solid stationary phase [15-18]. When the experiment is carried at infinite dilution the amounts of solutes injected are close to the detection limit and Henry's law governs the sorption process [15]. Under these conditions, only interactions between the adsorbate molecules and the surface are considered, whereas the interactions between the adsorbed molecules are neglected. From the values of retention volumes,VN, calculated at various temperatures, the isosteric heat of adsorption Qst can be calculated using the following expression 31n VN
Qst - R
1T O-T
3. RESULTS AND DISCUSSION
Table 1. Structural ~arameters calculate d from nitrogen adsorption isotherna, s
Sample ......... BAX BAX-O BAX-N1 BAX-N2 BPL BPL-O B PL- N 1 BPL-N2 iJll
iiii
i
iii
I
I
I
SDFT
Vt(DFT)
V<20A
[m2/g] 1370 873 975 1116 764 810 693 791
[cm3/g] 1.339 0.521 1.033 0.852 0.376 0.404 0.3 70 0.388
[cm3!gl 0.528 0.362 0.380 0.387 0.319 0.337 0.293 0.320
I I
II
I
I
V
L [A] 12.9 10.9 12.7 11.0 10.4 10.9 10.8 9.9 I
II
E~ [kJ/mol] 15.6 19.5 16.4 18.5 21.1 20.8 20.2 21.2 i
II
Adsorption of small molecule compounds such as acetaldehyde is sensitive to the pore structure of carbon. It is well known that at infinite dilution small molecules will adsorb in the smallest pores, similar in sizes to their diameter [ 19]. The values of the surface areas and pore volumes calculated from the isotherms are collected in Table 1. We list there the surface area, So~-r, micropore volume,V<2o• , volume of pores smaller than 10 A (V
250 revealed by Boehm titration did not change significantly. For BAX and BPL samples the number of acidic groups is basically the same. For urea modified carbons after heat treatment at 723 K, nitrogen incorporated into the surface is expected to be in the form of-NH and -NH2, amides or NH4 species [26]. These surface groups decompose at around 900K and some nitrogen is likely converted to aromatic nitrogen, pyridine - like configurations [27], built into carbon matrix. Because nitrogen containing organic groups after protonation, especially the amine-like ones, may have the pK a similar to those for carboxylic acids [28] they cannot be classified according to the categories specified by Boehm [3]. It is worth to mention that the amount of basic groups increased for both carbons, which suggests the alteration in the chemistry of the carbon matrix after heating at 723 and 1223 K. Table 2. Boehm titration results [mmol/g] sampl e Carbox. Lacton. Phenolic BAX 0.255 0.140 0.367 BAX-O 1.616 0.448 0.328 BAX-N1 . . . . . . . . . BAX-N2 . . . . . . . . . BPL 0.000 0.025 0.163 BPL-O 0.255 0.166 0.292 BPL-N1 . . . . . . . . . BPL-N2 . . . . . . . . . i
i
Basic 0.363 0.000 0.718 0.595 0.450 0.363 0.507 0.675
Acidic 0.763 2.392 0.563 0.678 0.188 0.713 0.167 0.178
i
Total 1.125 2.392 1.281 1.272 0.638 1.076 0.674 0.853
i
Changes in surface chemistry after modification are also observed on DTG curves measured in nitrogen presented in Figures 1 and 2. The peaks on the curves represent the weight loss due to the decomposition of surface groups. As well established using temperature programmed desorption (TPD), carboxylic acid decompose as CO 2 between 473 and 873 K whereas at temperature higher than 873 K weak acids, phenols and basic groups decompose as CO [29, 30]. After oxidation an increase in the weight loss due to the decomposition of oxygen groups is noticed for both samples. Differences in thermal stability are also found for the initial samples. As seen from the curves, the effects of oxidation are the most pronounced for BAX carbon where a significant amount of oxygen containing groups was introduced into the carbon matrix (Figure 1). A Significant increase in weight loss between 673 and 873K is found for both urea modified samples treated at 723K (Figure 2). The effect of modification is also manifested between 873 and 1073K; however, the intensity of these peaks is much smaller. Since the "NI" samples were heat treated at 723 K the peaks on DTG curves likely represent the decomposition of amine type groups (NH, and NH2) incorporated to the carbon matrices [ 11 ]. In the case of the BPL-N1 sample well-defined peak is found between 1023 and 1223 K, which suggests high degree of heterogeneity of nitrogen containing surface groups. After treatment at 1223K the surface of this carbon is almost featureless and resembles the initial material. On the other hand, the surface of BAX seems to be different. The broad peak between 873 and 1173 K observed from the initial sample disappears and surface seems to be more stable. It is a result of carbonization at 1223K, which caused an increase in the degree of aromatization and likely incorporation of nitrogen containing species. Indeed the elemental analysis showed that nitrogen content increases form 0.44 % to 3.65 % and 1.44 % for BPL carbons, N1 and N2 respectively, and from 0.19 to 9.30 % and 3.05 % for BAX carbons, N1 and N2 respectively.
251
Fig. 1. DTG curves for BAX samples
Fig. 2. DTG curves for BPL samples.
All of the above-described differences in the carbons studied, and changes after modification should be reflected on the adsorption of acetaldehyde, its strength and amount. Acetaldehyde is a small molecule with chemical formula CH3CH = O. Its Van der Waals diameter is around 3.8 A [ 11 ]. The heat of acetaldehyde adsorption on activated carbon has not been reported in the literature. It should be greater than the heat of ethane adsorption and smaller than that of acetone. The heats for these compounds on nonporous carbon blacks were reported by Avgul and Kiselev and their values are 15 kJ/mol and 35 kJ/mol, respectively [31]. On the porous materials the heats of adsorption should be about twice bigger than on a flat surface due to the overlapping of adsorption potentials in small pores [19]. According to this, the heats are expected to reach about 30 kJ/mol for ethane and 70 kJ/mol for acetone [19]. For comparison, the heat of formaldehyde adsorption on various activated carbons was found to be between 15-33 kJ/mol [10]. Following this, and Avgul's and Kiselev's evaluation of the heat of o n e C H 3 moiety on nonporous carbon black to be equal to 9 kJ/mol [31], the heat of acetaldehyde obtained on microporous carbons should be in the range of 33-51 kJ/mol. Indeed, the values reported in Table 3 are in the expected range. When contribution of hydrogen bonding exists, the heat should be even greater of about 10 kJ/mol [ 11 ]. The presence of dispersive and specific (hydrogen bonding) interactions should be seen in the values of isosteric heats of adsorption calculated from inverse gas chromatography data, from the slope of the dependence of the free energy of adsorption on temperature (Figure 3) [10, 15, 32]. Comparison of the data obtained for two significantly different samples, BPL and BAX, indicate the differences in the mechanism of adsorption (Table 3). The heat of acetaldehyde adsorption on BAX is about 8 kJ greater than that for BPL. In fact the comparison of PSDs, described elsewhere [ 11, 33], indicates that the smaller pores, and in a higher volume, are present in the BPL carbon, which leads to the higher heat of adsorption than that for BAX. The reverse results, however unexpected, suggest that in the case of the wood based carbon the oxygen containing groups present on the surface contribute to the energetic of the process. In such a situation, hydrogen bonding between aldehyde group and functional groups on the carbon surface and aldehyde-aldehyde functional groups can be formed. To justify the values obtained we can
252
compared them to those obtained on graphitized carbon black. We found the heat of acetaldehyde adsorption be equal to 25.8 kJ/mol which suggests that on microporous activated carbons the heat should be around 52 kJ/mol [31 ]. ii
-0.5
I
OBPL
-1
9BPL-O
-1.5
~
i
,&BPL-N1 II BPL-N2
, -2.5 -3 -3.5 -4 -4.5 0.00205
0.00225
0.00245
0.00265
in
Table 3. Isosteric heats of acetaldehyde adsorption on the carbons studied [kJ/mol]. Sample Qst BAX 69.4 BAX-O 32.4 BAX-N1 49.9 BAX-N2 56.4 BPL 61.6 BPL-O 50.3 BPL-N1 51.9 BPL-N2 63.7 II
III
I
I
I
I
I
I
Fig. 3. Dependence of In(VN/(IlT)) on 1/T for BPL carbons. After oxidation and urea modification at 723K the heats decreased for both carbons [32]. Although the number of functional groups significantly increased, the porous structure was affected to the greater extent and it seems to be more important factor than surface functionality. As shown in Table 1 the volume in micropores decreased. Another factor which may contribute to the decrease of adsorption strength is the high density of surface groups (amines and acidic groups). This density increased compared to the initial materials. As a result of this, the surface became more hydrophilic leading to the weaker interactions of hydrocarbon moiety of acetaldehyde with the sorbent matrix. Supporting for this is the fact that, the hydrogen bond energy is weak compared to dispersive forces in small pores. Another reason for the decrease may be in inaccessibility of very small pores to acetaldehyde molecule caused by the presence of groups at the edges of garphene layers. Their high density may result in blocking the smallest pores for adsorptive molecules. The observed decrease in the heat of acetaldehyde adsorption for BPL carbon after oxidation occurred is spite of the fact that the pore structure is left almost intact after oxidation. This support the above mentioned hypothesis. After urea treatment at 1223K an increase in the heat is found again, however the initial value is reached only for BPL carbon. As indicated from other analysis this carbon is the least resistant for heat treatment in the presence of urea [32]. For other samples, the similar effects as after oxidation is found due to 1) an increase in the density of groups, and 2) blocking the entrances to the small pores, as suggested above. Figures 4 and 5 present the difference between the DTG curves obtained for samples after adsorption of acetaldehyde at saturation conditions and the initial ones. Analysis of the shapes of the curves indicates that at least two types of interactions exist. They are represented by two peaks, which partially overlap. The first one, centered about 373 K likely represents weak interactions of acetaldehyde with functional groups via hydrogen bonding. The presence of mesopores on the surface results also in such a weak adsorption since these interactions are not energetically favorable. Moreover, the surface acidic sites present mainly on the unmodified and
253 oxidized samples may play a role of catalysts in the polymerization of acetaldehyde. New species, due to their bigger size are excluded from the very small pores and likely desorb at low temperature. On the other hand, the second peak, centered at about 423 K likely represents desorption of acetaldehyde from micropores. As shown previously, the amount of acetaldehyde adsorbed is mainly govern by the volume of carbon micropores [32]. It is worth to mention that the peaks obtained for BAX carbons are much broader than those for BPL, which may be related to heterogeneity in the pore sizes of this group of carbons.
Fig. 4. Difference in DTG curves after and before acetaldehyde adsorption for BAX carbons.
Fig.5. Difference in DTG curves after and before acetaldehyde adsorption for BPL carbons.
4. CONCLUSIONS The results obtained in this study show the strong dependence of acetaldehyde adsorption on the pore size distributions and the surface chemistry of activated carbons. When very small pores, close to the size to acetaldehyde molecule, and surface functional groups containing oxygen or nitrogen are present (to certain extent), the adsorption is highly energetic. It was found that small density of surface groups can enhance the heat of adsorption whereas high density leads to a decrease in the strength of adsorption forces. This occurs due to the blocking of the pore entrances with functional groups and a decrease in the accessibility of hydrophobic surface where the dispersive interactions of hydrocarbon moiety with small pore walls can be enhanced. Acetaldehyde adsorption at saturation conditions depends strongly on the volume of micropores. ACKNOWLEGMENT: This study was partially supported by PSC-CUNY (62381-0031). Help of Dr. Andrey Bagreev and Ms. Anna Kleyman is appreciated. REFERENCES [1 ] Bansal, R. C., Donnet, J. B., Stoeckli, F. Active Carbon, Marcel Dekker, New York, 1988. [2] Dubinin, M. M. Carbon 18 (1980) 355. [3] Boehm, H. P. In Advances in Catalysis, Academic Press, New York, 1966. [4] Boehm, H. P. Carbon 32 (1994) 759.
254 [5] Leon y Leon, C. A., Radovic, L. R. In Chemistry and Physics of Carbon, Thrower, P. A. Ed.; Dekker, New York, 1992. [6] Puff, B. R. In Chemistry and Physics of Carbon, Thrower, P. A., Ed.; Dekker, New York, Vol.6, 1970. [7] Zawadzki, J. In Chemistry and Physics of Carbon, Thrower, P. A., Ed.; Marcel Dekker, New York, vo121,1989. [8] Bandosz, T. J., Jagiello, J., Contescu, C., Schwarz, J. A. Carbon 31 (1993) 1193. [9] Jagiello, J., Bandosz, T. J., Schwarz, J. A. Carbon 32 (1994) 1026. [ 10] Domingo-Garcia, M., Fernandez-Morales, F.J., Lopez-Garzon, F.J., Moreno-Castilla, C., Perez-Mendoza, M. Langmuirl5 (1999) 3226. [11] EI-Sayed,Y.; Bandosz, T.J.J. Colloid and Interface Sci. 242 (2001) 44. [ 12] Adib, F.; Bagreev, A.; Bandosz, T. J. Langmuir 16 (2000) 1980. [13] Olivier, J. P., Conklin, W. B. presented at 7th Int. Conf. on Surf. and Coll. Sci., Compiegne, France, 1991. [ 14] Lastokie, Ch. M.; Gubbins, K. E.;Quirke, N., J. Phys. Chem. 97 (1993) 4786. [15] Conder J. R.; Young C. L. Physicochemical Measurement by Gas Chromatography. Wiley: New York, 1979. [16] Tijburg, I., Jagiello, J., Vidal, A., Papirer, E. Langmuir 7 (1991) 2243. [17] Jagiello, J., Bandosz, T. J., Schwarz J. A. J. Colloid Interface Sci. 151 (1991) 433. [18] Voelkel, A. Inverse gas chromatography: characterization of polymers, fibers, modified silicas, and surfactants. Crit.Rev. Anal. Chem. 22, (1991) 411. [ 19] Everett, D.H., Powl, J.C.J. Chem. Soc. Faraday Trans. 1 72 (1976) 619. [20] Bandosz, T. J., Jagiello, J., Schwarz, J. A. Anal. Chem. 64 (1992) 891. [21] Moreno-Castilla, C., Carraso-Marin, F., Mueden, A. Carbon 35 (1997) 1619. [22] Bandosz, T. J., Jagiello, J., Schwarz, J. A., Krzyzanowski, A. Langmiur 12 (1996) 6480. [23] Salame, I. I., Bandosz, T. J. J. Colloid Interface Sci. 210 (1999) 36. [24] Salame, I. I., Bandosz, T. J. Langmuir 15 (1999) 587. [25] Salame, I. I., Bandosz, T. J. Ind. Chem. Eng. Res. 39 (2000) 301. [26] Salame, I. I., Bandosz, T. J. J. Colloid Interface Sci., 240 (2001) 252. [27] Bagreev A.; Lahaye, J.; Nanse, G.; Strelko, V. Carbon 37 (1999) 585. [28] Kortum, G.; Vogel,W.; and Andrussow, K., Dissociation constants of organic acids in aqueous solutions, Butterworth, London,1961. [29] Papirer, E., Bantzer, J., Sheng, L., Donnet, J. B. Carbon 29 (1991) 69. [30] Otake, Y., Jenkins, R. G. Carbon 31 (1993) 109. [31] Avgul, N. N., Kiselev, A. V. In Chemistry and Physics of Carbon, Walker, P. J., Jr., Ed.; Dekker, New York, 1970. [32] Jagiello, J., Bandosz, T.J., Schwarz, J.A.J. Colloid Interface Sci. 151 (1992) 433. [33] E1-Sayed,Y.; Bandosz, T.J. Langmuir 18 (2002) 3213.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
255
A novel approach for characterising carbon catalysts by TAP experiments V. Fierro a, M.T. Izquierdo b, Y. Schuurman ~, B. Rubio b and C. Mirodatos ~
a Institut de Recherches sur la Catalyse, CNRS, 2 avenue A. Einstein, F-69626 Villeurbanne Cedex, France. b Instituto de Carboquimica, CSIC, c/Miguel Luesma Castfin, 4, 50015 Zaragoza, Spain.
Two samples were used in this study: a coal based carbon obtained by carbonisation and a gas-phase oxidized sample from the raw carbon. These samples were extensively characterized and their NO removal capacity was also determined. TAP-2 (Temporal Analysis of Products) reactor was used to obtain adsorption parameters and diffusion coefficients of the system NO-carbon catalyst. The diffusivity of NO as well as the enthalpy of adsorption does not change after increasing the number of functional groups by an oxidative pretreatment of the carbon. INTRODUCTION Combustion of fossil fuels in power plants contributes to acid rain and photochemical smog due to NO emissions [ 1]. The selective catalytic reduction (SCR) is the only flue gas denitrification technique so far that has proven to be very effective. It has been extensively studied, successfully commercialised and applied on a large scale [2]. Carbon can be used as catalyst in the SCR unit at lower temperatures than those used with conventional SCR catalysts. The adsorbent and catalytic properties of carbons are usually dependent on the physicochemical features such as surface areas, pore size distributions and surface chemistry. Especially, surface oxygen functional groups are closely related to the catalytic activity of carbons. These groups are expected to change the interaction between the carbon surface and the reactants through a variation of adsorption and reaction characteristics [3]. Besides traditional characterization techniques (temperature programmed desorption, acid-base titration, gas physisorption) transient experimental techniques are a powerful tool in the investigation of adsorption and catalytic reaction mechanisms [4]. Thus, the measurement of the concentrations of reacting species in transient conditions gives direct information on the surface processes. Moreover, modelling of the transient responses allows extraction of the adsorption parameters and the diffusion coefficients in the micropores. This work shows the ability of the temporal analysis of products technique to obtain the transport and sorption parameters for the NO-carbon system. These parameters are directly related to the nature and number of functional groups on the carbon.
256 EXPERIMENTAL
Preparation and characterization of carbons Carbon labelled SC900 was obtained from carbonisation of a low rank coal from NW of Spain in inert atmosphere at 900~ of temperature. Experimental installation for preparation of carbons is described elsewhere [5]. Sample SC900 was treated with a flow containing 1000 ppmv SO2, 6% v/v 02, 10% v/v H20 and N2 to obtain an amount of 400 mg sulphuric acid equivalent that is removed thereafter by heating up to 400~ and maintained at this temperature until SO2 evolution is negligible. Sample thus obtained was labelled as SC900R. Both samples were sieved to a particle size 0.2-0.3 mm. Elemental analysis of the samples was carried out in a Carlo Erba CHNSO 1108 apparatus. Ash content was determined according to ASTM standards for coal and coke. Surface areas were determined with N2 (BET equation) and CO2 (DR equation) physisorption at 77K and 273K, respectively, in an Autosorb-1 apparatus from Quantachrome. Outgassing conditions were a temperature of 250~ and a pressure of 10.6 mm Hg. Temperature programmed desorption (TPD) experiments were carried out to determine the relative amounts of CO and CO2 evolving from each sample. An amount of 0.6 g of sample was placed in a quartz tube under a stream of argon flowing at a rate of 30 ml min 1. Gas evolved was analysed by gas chromatography. Boehm's procedure [6] was used to titrate carboxyls, lactones, phenols and carbonyls functional surface groups. NO removal The NO removal capacity of the carbons was determined by passing a flow of 0.5 1/min containing 1000 ppmv of NO, 6% 02, 1000 ppmv NH3 and nitrogen as a balance through a fixed bed of 15 g of carbon at 150~ The NO concentration was measured before starting an experiment by bypassing the reactor. The effluent concentration was monitored continuously during the reaction. More details of the experimental installation and the operational procedure is described elsewhere [7]. TAP experiments The TAP-2 reactor system [4] was used to perform transient response experiments under vacuum and at temperatures ranging from 300 to 400~ A carbon loading of 100 mg was placed between two layers of quartz particles (0.2-0.3 mm particle size). Neon was used as an internal standard for calibration and as a reference for diffusion. Nitric oxide and neon were introduced by pulses in the microreactor (25.4 mm in length and 4 mm in diameter) in a volume ratio of 1:1. The reactor was continuously evacuated and the response of the pulses as a function of time was analysed on-line by a quadrupole mass spectrometer. RESULTS AND DISCUSION Table 1 reports the elemental analysis and the ash content of the samples studied. The ash content is high for each sample due to the low grade coal precursor. Table 1 also reports surface areas of both samples. There is an increase of surface area and pore volume after treatment of raw sample. This fact can be explained in terms of the following reaction: 2H2SO4 + C -) 2SO2 + CO2 + 2H20
257
Table 1. Elemental analysis, ash content and surface areas of the studied samples. SC900 SC900R Ash, % 32 32 Carbon, % a 94.8 95.0 Hydrogen, % 0.7 0.7 Nitrogen, % 0.3 0.3 Total sulphur, % 4.1 4.1 SBET(N2), m2/g 52 112 Pore volume (HK), cm3/g 0.025 0.055 Pore volume (BJH), cm3/g 0.061 0.082 SDR (CO2), m2/g 389 350 Pore volume (DR), cm3/g 0.170 0.156 a dry and ash free basis The treatment with a flow containing SO2+H20+O2 gives an amount of 400 mg of sulphuric acid. Heating up this sample, sulphuric acid is removed from the surface by reduction that leads to carbon consumption. This mild gasification can produce either an opening of the microporosity to mesoporosity and/or the creation of new microporosity. This can be followed by the increase of pore volume calculated by Horvath-Kawazoe (HK, for micropores) and Barret-Joyner-Halenda (BJH, for mesopores) methods. The treatment of the raw sample produces an increase in oxygen functionalities as can be deduced from Table 2 where amounts of CO and CO2 evolved during a TPD experiment are reported. The surface chemical structures that evolve CO2 are less stable and are related to carboxylic, anhydride or lactonic functions. The evolution of CO is attributable to fairly stable structures and could be postulated as phenols, quinones, ethers and carbonyls (3, 8). CO is more abundant than CO2 during decomposition of surface oxides for both samples, indicating the presence of more stable groups on the samples surface. Moreover, sample SC900R was heated up to 400~ after chemical treatment, so an important part of CO2-evolving structures had disappeared before TPD experiments. Table 2. Total amount of CO and CO2 evolved from TPD experiments and Boehm's [6] titration. SC900R SC900 0.65 0.36 CO2, mmol/g 0.49 1.03 CO, mmol/g O,O~ a 1.94 3.73 0.153 0.102 Carboxyls, meq/g 0.152 0.111 Lactones, meq/g 0.048 0.042 Phenols, meq/g 0.855 Carbonyls, meq/g 0.320 a from the amount of CO and CO2
Figure 1 shows TPD profiles for both samples. The TPD patterns indicate that surface groups yielding oxygen evolved almost completely below 1100~ The CO profiles for sample SC900 show a single and quite symmetric peak centered at about 800~ probably due to carbonyl and/or quinone-like groups that are thermally more stable than phenolic or hydroquinone groups. The chemical treatment does not shift this peak meaning that surface functional groups introduced are of the same type that exist on the raw sample. This can be confirmed with the results obtained by Boehm's titration. The most important change is observed for the carbonyl groups, increasing its value almost three times.
258
The CO2 profile exhibits a peak at about 400~ and 700~ for sample SC900R, indicating the presence of two different surface functionalities with different thermal stability. The first peak is attributable to carboxyl groups that are present on the sample SC900 in a low extent. This can be confirmed by the results from Boehm's titration shown in Table 2. ................... SC900 SC900R
1
a
0
200
400
600
800
1000
1200
Temperature,-~
Figure 1. Profiles of CO (1) and CO2 (2) evolution during a TPD experiment. Once the samples were characterized by conventional methods, the NO removal capacity was tested. The curves corresponding to NO conversion and amount of NO removed versus time are shown in Figure 2. 100
600
80
.~
500 E
400 "~
60
300
8
40
200 ~
o
0 0
240
480
720
960
1200
time, min
Figure 2. NO removal capacity of samples SC900 and SC900R versus time. The breakthrough of NO does not occur before 10 min, and it needs more than 10 h until the steady state is reached. The fact that the breakthrough of the gas occurs so late indicates that he surface reaction takes place between species adsorbed in a large amount in a preceding step. The gas-phase treatment enhances the catalytic activity of the carbon. However, it is difficult to attribute this enhancement to only one feature of the treated carbon, i.e., chemical or physical characteristics. The activation of the raw sample leads to an increase in surface area and pore volume as well as to an increase in the surface functionalities. The literature shows no consensus on the correlation between the NO conversion and the carbon surface area [8, 9]. In this case, the surface area of sample SC900R is about twice as large as that of sample SC900 but the NO conversion increases approximately four times. The contradictory findings in the literature and the results showed here lead to believe that the chemical characteristics of the functional groups are an important element for the NO removal performance of the carbons. The enhanced reduction of NO due to the introduction of new oxygen functional groups can
259 be understood in terms of the production of active sites. In the present case, the structures that increase to a larger extent after the treatment are the carbonyls. The creation of stable C-O complexes was found beneficial as also reported in the literature [ 10, 11 ] and an optimum for the amount of active sites was found. 12 10
10
8
8
"~
6
"~
6
~
4
~
4
.I
SC900R
o
2
2 0
0 0
0.2
0.4
06
0.8
1
0
0.2
time, s
0.4
0.6
0.8
1
time, s
Figure 3. Experimental (points) and model (lines) responses from TAP-2 experiments. The adsorption characteristics of the samples SC900 and SC900R were studied with transient adsorption experiments using NO as adsorbate. Figure 3 shows the experimental (points) and model (lines) responses from TAP-2 experiments for samples SC900 and SC900R at 300, 350 and 400~ of temperature. The results of the nitric oxide pulse experiments demonstrate that NO removal occurs in a bigger extent over the gas-phase oxidized sample, where the surface area as well as the number of surface functional groups have increased as reported in Tables 1 and 2. The model fits adequately the experimental data allowing the extraction of the adsorption parameters and the diffusion coefficients for the transport inside the micropores. Table 3 reports these parameters for both samples. Similar diffusion parameters were found for both samples. Despite the difference in volume of the micropores as calculated by the HK method (Table 1), a big difference between SBET and SDR for each sample is found indicating problems of accessibility for nitrogen molecules at the low temperature at which the adsorption process is carried out (77 K). The micropore volume according to the DR method gives similar values for both samples (Table 1) The values of the diffusion parameters found by modelling of the transient responses for both samples are very close. Adsorption enthalpies are also very similar but quite different pre-exponential factors are obtained. This indicates that the same sites are involved for the adsorption of NO but their number has increased. The increase of the number of active sites can be confirmed by TPD experiments obtaining that the total amount of CO and CO2 increased considerably after treatment of the raw sample (Table 2). On the other hand, the amount of different type of surface groups varies in a low extent after treatment of the raw sample, except for the carbonyls that increases in concentration by approximately a factor of three. So, these structures could be the responsible of the enhancement of the NO conversion once determined that diffusion coefficients are similar for both samples. Table 3. Estimated parameters for NO on carbon samples. Sample D~ Ediff H~ -AH mZs"1 kJ mol -I kJ mol l SC900 3.4 10-9 15 2.4 105 72+3 SC900R 2.8 10.9 15 8.5 10.5 73+_3 .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
260 In conclusion, the transient technique TAP has been shown to be a powerful tool for the fast characterization of carbon catalysts and the results are in accordance with those inferred by conventional methods and also allowed to obtain assess the transport and sorption parameters for the NO-carbon system. Further research will be carried out to gain more understanding on the catalytic reduction of NO over carbon catalysts. REFERENCES
1. I. Smith. Nitrogen oxides from coal combustion-environmental effects, IEA Coal Research, London, 1990. 2. I. Aarna, E.M. Suuberg. Fuel 76(1997)475. 3. F. Rodriguez-Reinoso in: H. Marsh, E.A. Heintz, F. Rodriguez-Reinoso (Eds.) Introduction to Carbon Technologies. Univ. of Alicante, 1997 p. 35. 4. J.T. Gleaves, G.S. Yablonskii, P. Phanawadee, Y. Shuurman. Appl. Catal. A: General 60(1997)55. 5. R. Moliner, J.V. Ibarra, M.J. Lazaro. Fuel 73 (1994)1214. 6. H.P. Boehm. Carbon 32(1994)759. 7. M.T. Izquierdo, B. Rubio. Environ. Sci Technol. 32(1998)4017. 8. M.J. Illan-Gomez, A. Linares-Solano, C. Salinas-Martinez de Lecea, J.M. Calo. Energy Fuels 7(1993) 146. 9. S.N. Ahmed, J.M. Stencel, F.J. Derbyshire, R.M. Baldwin. Fuel Proc. Technol. 34(1993)123. 10. M.T. Izquierdo, B. Rubio, C. Mayoral, J.M. Andres. Appl. Catal. B: Environ. 33(2001)315. 11. L. Singoredjo, F. Kapteijn, J.A. Mouljin, J.M. Martin-Martinez, H.P. Boehm. Carbon 31(1993)213.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
261
Preparation o f activated carbons with controlled pore size M.M.A. Freitas a,b and J.L. Figueiredo a aLaborat6rio de Cathlise e Materiais, Departamento de Engenharia Quimica, Faculdade de Engenharia da Universidade do Porto, 4200-465 Porto, Portugal bDepartamento de Engenharia Quimica, Instituto Superior de Engenharia do Porto, Instituto Polit6cnico do Porto, 4200-072 Porto, Portugal
In this work we developed a methodology to produce activated carbons of high adsorption capacity and with controlled pore size, based on the KOH catalysed gasification with CO2 of a coconut shell char. To study the effect of potassium in the activation step two series of activated carbons were prepared, with (CK1173) and without KOH (DC1173). The results obtained show that the size of the micropores is much smaller in the series CK1173.
1. INTRODUCTION The porous texture of activated carbons depends on the method of activation and the nature of the precursor. Lignocellulosic materials are excellent precursors, and they offer the advantage of small ash content. Activated carbons with a large volume of micropores of small and uniform dimensions can be used as precursors in the preparation of carbon molecular sieves [1 ]. Materials with an extremely high adsorption capacity can be prepared by chemical activation with KOH, where advantage is taken of the intercalation properties of potassium [2-8]. In the present work we used KOH in a different way, namely as a carbon gasification catalyst. Various gasification catalysts have been used to control the pore size of carbon adsorbents upon activation. Transition metals like Ni or Co lead to the development of mesopores [9-11 ], while K has been shown to lead mainly to the formation of micropores of small dimensions [11,12]. Based on this backgrotmd, we attempted to develop a methodology to produce activated carbons of high adsorption capacity, which could be subsequently converted into carbon molecular sieves. For this purpose, two series of activated carbons were prepared from the same precursor by CO2 gasification to different extents, with and without catalyst. The textural properties of the materials obtained in each series were compared, particular attention being paid to the development of micropores of controlled dimensions.
2. EXPERIMENTAL As raw-material, we used a char obtained by carbonisation of Babagu coconut shell, which is abundant in Brazil (Orbygnia Phalerata) with a particle size of 1.40-1.65 mm,
262 further heat treated at 1173 K for 2 hours under inert atmosphere. This char was impregnated with a KOH solution of defined concentration in order to obtain the KOH load of 7%. The char was first degassed under vacuum to remove the air inside the pores, then the KOH solution was added and the mixture was stirred during 3 hours. The KOH impregnated char was dried overnight in an oven at 383 K. This material was activated by gasification with carbon dioxide in a vertical furnace under a CO/flow of 100 mL.min 1 at 1173 K to different bum-offs (series CK1173). After activation, the materials were washed with HC1 0.1 M and then with water until pH=6, and then dried overnight. Another series of activated carbons was prepared under the same conditions but in the absence of KOH (series DC1173). Each sample in a series is identified by the bum-off obtained (e. g., CK1173-31 is a sample activated to 31% B.O.). C1173-0 corresponds to the original char. The textural characterization of the materials was based on the adsorption isotherms of nitrogen at 77 K and CO2 at 273 K determined with a COULTER OMNISORP 100CX apparatus (adso.Ttion data obtained in the relative pressure ranges of 10 -6 - 1 for nitrogen at 77 K, and 5x10 - 3x10 2 for carbon dioxide at 273 K). The samples were degassed at 623 K up to a vacuum better than 8xl 0 4 Pa prior to the adsorption measurements. The isotherms were analyzed by the t-plot with reference to an appropriate standard isotherm [13], yielding the micropore volume (W0) and the mesopore surface area (Smo). The data were also fitted to the Dubinin-Radushkevich equation (DR). When the DR plot showed a type IV deviation [14], the adsorption data were considered as the sum of two microporous structures, characterized by different values of the parameters. The volumes of the smaller micropores (W01) and of the larger micropores (W02) were calculated in this case. The average micropore width (L) of the first structure was determined by the Stoeckli equation [15]. To calaculate the micropore volumes from the CO2 adsorption data, the CO2 density was taken as 0.936 g.cm 3. 3. RESULTS AND DISCUSSION The nitrogen adsorption isotherms obtained with both series of activated carbons were of type I, corresponding to microporous materials. The results of the textural characterization are shown in tables 1 and 2. The last column in both tables was calculated from the CO2 adsorption data, the remaining parameters being determined from the nitrogen adsorption data.
Table 1 Results of the textural characterization of the DC1173 series.
t Sample C1173-0 DCl173-12 DC1173-22 DC1173-36 DC1173-40 DCl173-47 DCl173-63 DCl173-71
Wo
DR Sine
W01
W02
L
Wc02
(cm3.g -1)
(m2.g-1)
(cm3.g -1)
(cm3.g -1)
(nm)
(cm3.g -1)
0.170 0.307 0.382 0.488 0.559 0.599 0.742 0.830
6 14 29 63 44 59 43 60
0.156 0.306 0.371 0.474 0.532 0.561 0.646 0.664
0.019 0.010 0.009 0.017 0.064 0.041 0.100 0.169
1.10 0.60 0.71 0.83 0.93 1.10 1.30 1.51
0.254 0.367 0.453 0.522 0.528 0.545 0.554 0.588
263 Table 2 Results of the textural characterization of the CK1173 series.
t
DR
sample
Wo (cm3.g "1)
. .m( .2 g-I)
Cl173-0 CKl173-31 CK1173-54 CKl173-68 CKl173-74
0.170 0.389 0.561 0.607 0.688
6 3 30 37 45
Sme
(cm3.g "1)
W02
(cm3.g -1)
L (nm)
We02 (cm3.g -1)
0.156 0.358 0.496 0.530 0.576
0.019 0.033 0.083 0.102 0.120
1.10 0.82 0.91 1.05 1.10
0.254 0.350 0.369 0.499 0.539
W01
The results obtained in the DC1173 series (table 1) show that the micropore volumes (Wo, or Wol+Wo2) increase with the extent of activation, and that up to 40% B.O. there is a simultaneous development of mesopores, as shown by the increase in Smo. Fig. 1 shows the evolution of Wol and W02 with the extent of activation. It can be observed that both W01 and W02 increase with B.O., but that initially there is only the formation of small micropores, W02 being negligible up to about 30 % B.O.. Further activation leads to the formation of new micropores, but also to the enlargement of the micropores formed initially, as shown by the increase in Wo2. In addition, as the micropore volume increases, so does the average pore width. In the original char Wco2 > Wo~, showing that there are very small micropores which are not reached by N2 adsorption at 77 K. As the extent of activation increases, these parameters tend to a common value at about 40-50% B.O..
0.8-
D Wol W
0.7 0.6
Y
0 . 5 -
0.4 0.3 0.2
7
/S
0.1 0.0
<
0
'
I
1;
'
20
'
'
30
I
'
40
I
50
'
'
gO
'
40
I
80
B.o.(% ) Fig. 1. Evolution of Wol and Wo2 with the extent of activation to the DC1173 series.
264 1.81.6 1.4 1.2
0
AI.O E
~0.8
/
..I
0.6 0.4 0.2 0.0
o
CK1173
o
DCl 173
'
1'0 '
2'0 '
3'0 '
4}) '
5'0 '
6'0
'
70
I
80
B.O.(%) Fig. 2. Micropore width (L)
versus
bum-off for DC1173 and CK1173 series.
The results obtained in the CKl173 series (table 2) show very similar trends. However, the size of the micropores at large extents of activation (B.O. >50%) is much smaller in this series, as shown in fig. 2. It may be observed that in the KOH catalysed gasification the enlargement of the micropores occurs at a much slower rate. Even for larger extents of activation (> 50%) the average micropore width does not exceed 1.1 nm, while in the non-catalysed activation (series DC1173) the micropores with are in the range 1.1-1.5 nm. So, the micropore volumes obtained m series DC1173 are higher than those of series CK1173, but the later originates materials with narrower microporosity. There are two possible mechanisms, which may account for the increase in micropore volume, which occurs during activation, namely pore enlargement and pore deepening [9]. The results obtained here show that the KOH catalysed gasification of carbon proceeds mainly through a process of pore deepening, which does not strongly affect the micropore size. This is no doubt a consequence of the particular mechanism of the potassium catalysed gasification of carbon, which may be due to the intercalation of potassium atoms between the graphene layers [ 11 ]. 4. CONCLUSIONS By using potassium as a carbon gasification catalyst, it is possible to obtain activated carbons of large adsorption capacity (large micropore volume), but with micropores of small dimensions. Nevertheless, these materials could not be converted into carbon molecular sieves by carbon deposition from benzene pyrolysis. Success was achieved with chars which were activated only to a limited extent [ 16].
265 REFERENCES
[ 1]- F. Rodriguez-Reinoso, 1cr Taller Iberoamericano sobre Tamices Moleculares, (1991) 1. [2]- H. Marsh, D. Crawford, T. M. O'Grady, and A. Wennerberg, Carbon, No.20 (1982) 419. [3]- H. Marsh, D. S. Yan, T. M. O'Grady, and A. Wennerberg, Carbon, No.22 (1984) 603. [4]- T. M. O'Grady, and A. Wennerberg. In, J. D. Bacha, J.W. Newman and J.L.White ed(s), Petroleum Derived Carbons, ACS Symposium Series, 1986, 302-309. [5] - Z. Hu, M.P. Srinivasan, Microporous and Mesoporous Materials, No.27 (1999) 11. [6]- S. u
H. Hu, G. Chen, Carbon, No. 40 (2002) 277.
[7]- C. Moreno-Castilla, F. Carrasco-Marin, M. V. L6pez-Ram6n, M. A. Alvarez-Merino, Carbon, No. 39 (2001) 1415. [8]- D. Lozano-Castell6, M. A. Lillo-R6deras, D. Cazorla-Amor6s, A. Linares-Solano, Carbon, No. 39 (2001) 741. [9] T. Wigmans. In J.L. Figueiredo and J.A. Moulijn (eds.), Carbon and Coal Gasification, Martinus Nijhoff Publishers, Dordrecht, 1986, 559-599. [10]- J. L. Figueiredo, J.J- M. Orf~o, M. C. A. Ferraz, Fuel, No. 63 (1984) 1054. [11]- T. Wigmans, A. Hoogland, P. Tromp and J. A. Moulijn, Carbon, No. 21 (1983) 13. [12]- J. Laine and A. Calafat, Carbon, No. 29 (1991) 949. [13]- F. Rodriguez-Reinoso, J. M. Martin-Martinez, C. Prado-Burguete and B. McEnaney, Journal of Physical Chemistry, No. 91 (1987) 515. [14]- A. Linares-Solano. In J.L. Figueiredo and J.A. Moulijn (eds.), Carbon and Coal Gasification, Martinus NijhoffPublishers, Dordrecht, 1986, 137-178. [15]- H. F. Stoeckli, Carbon, No. 28 (1990) 1. [16] M. M. A. Freitas, J. L. Figueiredo, Fuel, No. 80 (2001) 1.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
267
Characterization of porous solids using low pressure V O C adsorption data Joi~lle Nokerman, S6bastien Dutour and Marc Fr~re a Sophie Limborg-Noetinger b and Sophie Jullian c a Facultd Polytechnique de Mons - Thermodynamics Division, 31 bd Dolez - 7000 Mons Belgium (E-mail [email protected]) b Institut Fransais du Petrole (IFP), 1-4 avenue de Bois Pr6au - 92852 Rueil-Malmaison France c Institut Fransais'du P6trole (IFP)-Lyon, CEDI - BP3 - 69390 Vernaison- France In this paper, we present a new experimental procedure which aims at measuring adsorption isotherms and heats of adsorption of VOCs on porous media in the low relative pressure range for temperatures ranging from 30 to 300~ The results provide the Henry constant and the isosteric heat of adsorption at low coverage. The experimental set up and the experimental procedure are presented as well as a validation of the results in the saturation area of the isotherm on the NaY/Toluene system at 150~ The first result obtained in the Henry area is presented. This work is a part of a more general study aiming at characterizing efficient adsorbents used for the reduction of the VOCs emissions. This characterization method is based on the determination of the Henry constants at industrial operating temperatures and on the direct measurement of the isosteric heats of adsorption at low coverage. 1. INTRODUCTION Volatile organic compounds (VOCs) are atmospheric pollutants which provoke the depletion of the ozone layer. Some of them are toxic or carcinogenic. VOCs are released by industrial activities such as industrial cleaning, printing, painting, etc. The waste gas flow generated by a given industrial activity can be charaterized by the mass flow rate, the kind of VOC (hydrocarbons, alcohols, ketones, aromatic compounds, halogenated compounds, etc.) and the VOC concentration in the gas flow. The reduction of the VOCs emissions can be achieved by different techniques. The most well known are thermal oxidation, catalytic oxidation and adsorption. Adsorption is generally preferred as far as low VOCs concentrations are concerned. The wide range of adsorbents offers interesting possibilities for each kind of VOC. The efficiency of a given adsorbent to adsorb a given compound is characterized by the quantity adsorbed at a given adsorbate pressure. The treatment of a very low VOC concentration gas flow requires adsorbents which are able to fix very large quantities of VOC at very low partial pressure and at ambient temperature. The adsorption isotherm of the given adsorbent/VOC system is then of type I, with a very high initial slope and a saturation plateau covering a large relative pressure area. The desorption occurs in the same pressure range as
268 adsorption but at higher temperature (up to 300~ The design of industrial VOCs adsorption units requires basic adsorption data in such pressure and temperature conditions. Up to now it is not possible to obtain reliable data in such experimental conditions, and more especially for low temperatures and low relative pressures. The common procedure consists in measuring adsorption data at high temperatures (150-300~ Given the high value of the vapour pressure of the compound at such temperatures, it is easier to reach low relative pressures. These data are used for the calculation of the Henry constant at high temperature. The Henry constant at ambient temperature is determined by extrapolation using the van 't Hoff law. The high temperature adsorption data are generally measured using the perturbation chromatographic method or the static volumetric or gravimetric methods. The pertubation chromatographic method [1, 2, 3] consists in studying the dynamic response of an adsorption column to a pulse of VOC concentration in a pure helium flow. Such a method requires several assumptions on the thermodynamic and kinetic behaviour of the system. Namely, the adsorption equilibrium is described by the Henry law. The retention time of the injected compound allows to calculate the Henry constant; the peak broadening is due to the mass transfer resistances. The performances of such a method depend on the validity of the assumptions. For example, the exactness of the Henry law requires to work at high temperature. The perturbation chromatographic method provides the Henry constant without any possibility to check the validity of the Henry law hypothesis. The static methods consist in injecting small quantities of VOC in the adsorption chamber of the apparatus. The adsorbed mass is determined by a gravimetric method [4-8] or by a mass balance on the vapour phase [9]. The pressure of the adsorbate is measured by transducers working at very low pressures. Given the accuracy of such pressure transducers, it is difficult to obtain reliable data for pressures below 50 Pa. For systems exhibiting a very high Henry constant (that is to say for systems of industrial interest), it is not possible to reach the Henry zone of the isotherm : the Henry constant is calculated assuming a linear behaviour between the origin and the lowest pressure data point. These isotherms measurements are generally performed at high temperatures. Very high temperature isotherms are characterized by a progressive decrease of the slope with increasing pressure. For such isotherms, it is difficult to separate the Henry zone and the plateau so that the Henry constants determined from the chromatographic or static methods must be carefully considered. The extrapolation procedure of the Henry constant from the high temperatures range to the ambient temperatures range remains a difficult task due to the steep variation of the Henry constant with temperature. The purpose of this paper is to present a new method for the simultaneous measurement of adsorption isotherms and heats of adsorption for VOCs on porous materials in the Henry zone of the isotherms for temperatures ranging from ambient to 300~ A dynamic method using a thermobalance coupled to a calorimeter is used. The balance allows a direct and accurate measurement of the mass. The dynamic method prevents from the necessity to measure low total pressures but requires the generation of low VOC concentrations in a helium flow. The measurement of the adsorption heat allows to check the thermodynamic consistency between the massic and calorimetric data. 2.
EXPERIMENTAL SECTION
2.1 Experimental installation The experimental device (Fig. 1), is based on a dynamic gravimetric method. It is composed of three main parts. The thermoanalyzer (TG-DSC111 Setaram, France) provides
269 simultaneous massic and calorimetric adsorption data. A saturation system is used to generate an inert gas (helium) flow with a given VOC concentration. A FID gas analyser (Signal Model 3000AM Analytic Systems, England) allows an accurate measurement of the VOC concentration in the He flow.
Fig. 1 "Experimental device
A. Thermoanalyzer TG-DSC111 The thermoanalyzer TG-DSC111 is a thermobalance (B111) coupled to a calorimeter (DSC111). The B111 thermobalance (measurement accuracy: l~tg) provides the direct measurement of the mass uptake as a function of time (TG signal). The TG signal is used to determine the total amount of VOC adsorbed during an adsorption experiment. It is also used to measure the outgassed sample mass. The maximal sample mass is about 70 mg. The calorimeter allows the measurement of the heat flow (HF signal) released by the sample during the experiment as a function of time. Its measurement accuracy is 5 ~tW. A thermocouple connected to the calorimeter measuring cells provides the temperature of the sample as a function of time (T signal).
B. Saturation System The saturation system consists in two glass-made saturators in a thermostated bath (cryostat). Both saturators are half-filled with the liquid VOC. Saturation is performed by bubbling helium through the liquid. The vapour pressure of the VOC in helium depends on the saturation temperature (Ts). The cryostat can operate between -50~ and + 50~ The temperature of the thermostated bath (Tr is measured by a thermocouple with an accuracy of 0.3~ The temperature of the liquid VOC (Ts) in the second saturator is measured with an accuracy of 0.1~ by a Ptl00. A 0-2 bar pressure transducer is used to measure the total
270
pressure (Pto~) at the outlet of the saturators during the adsorption measurements. Its accuracy is 0.2% FSD.
C. Signal Model 3000AM Gas Analyzer This apparatus is based on the principle of the detection by flame ionisation. It is especially dedicated to the study of gaseous organic compounds (except formaldehyde, formic acid and carbon sulfur). It allows the measurement of the VOC molar ratio in the helium flow (y). The gas analyser is able to perform VOC molar ratio measurements up to 10000 ppm THC. The measuring range may be selected (minimum end of scale value : 4 ppm THC). Accuracy and repeatability on each range are better than 1% FSD. 2.2 Materials We studied the NaY/toluene system. NaY zeolite was provided by the Institut frangais du P6trole (IFP, France) in the form of both crystalline powder and pellets containing 19.9% of a clay binder. The purity of the HPLC-grade toluene is 99.8%.
2.3 Experimental procedure Before each adsorption experiment, a zeolite sample (_+ 30 mg) is introduced in the crucible of the balance. The sample is outgassed. During outgassing, the adsorbent is first heated from the ambient temperature to 400~ with a heating rate of 10~ under a dynamic vacuum of 103 mm Hg. The adsorbent is then kept at 400~ during 15 h. Such a long time is necessary to avoid temperature gradients inside the sample, which would lead to an outgassing heterogeneity. The adsorbent is then cooled down to the experimental temperature with a cooling rate of 10~ This cooling step is performed with a 2 l/h helium flow rate. Helium flowing prevents from a mass uptake during the cooling due to the porosity of the calorimeter cells. Once the zeolite is returned to the experimental temperature, the helium flow is maintained during two additional hours. In this way, the homogeneity of the temperature is ensured. The mass signal recorded at that moment corresponds to the outgassed sample mass (TGms). The TG signal in the same temperature and helium flow rate conditions with empty crucibles is recorded before or after the experiment (TGe). The difference between TG,~s and TGe provides the mass of the outgassed sample (mo~). Repeatability on this value is 0.3%. The entire outgassing procedure lasts 17h50. A few hours before the end of the outgassing procedure, the cryostat temperature is set to a given value required to generate the correct VOC molar ratio. Helium is then sent through the saturators to generate the VOC-He flow (> 10 l/h). The flow rate bypasses the TG-DSC and is released to the hood until the end of the outgassing procedure. Once the outgassing is completed, we switch from the pure helium flow to the VOC-He flow in the TG-DSC. The total flow rate passing through the saturators is split in two parts: the first one (2 l/h) goes to the TG-DSC; the second part (> 8 l/h) goes to the FID gas analyzer to perform the concentration measurements. During the adsorption step, the sample is then theoretically submitted to an instantaneous concentration step. In fact, a finite time is required to reach the equilibrium concentration. The mass uptake (TG signal) and the heat flow (HF signal) versus time as well a s Ts, Ptot and y are recorded during the entire adsorption experiment. The adsorption equilibrium is reached when the TG signal is constant (TGeq). The difference between TGeq and TG,,s gives the adsorbed mass at equilibrium (meq).
271 3.
T R E A T M E N T OF T H E R E S U L T S
3.1 Determination of the V O C partial pressure in the V O C - H e flow
The VOC partial pressure (Pvoc) in the helium flow passing through the calorimeter is given by: Pvoc = Y eatm
(1)
In this equation" - Patmis the atmospheric pressure (TG-DSC operating pressure), - y is the VOC molar ratio in the gas flow. y and thus Pvoc depend on the VOC temperature in the saturators (T~). y may be determined by two differem methods : A. By direct measurement from the gas analyser
The VOC molar ratio is recorded as a function of time with a frequency of one value per 10s. B. By use o f a vapor pressure law
The VOC molar ratio is given by: y=
P,,,,-voc(Ts)
(2)
Ptot
in which" Ptot is the total pressure at the outlet of the saturators, - Psat-vocis the VOC vapour pressure at temperature Ts. This equation is based on the assumption of a helium flow saturated by the COV. P, at-VOC are calculated from a correlation based on the DIPPR database . The validity of this correlation in the low pressure domain has been checked by comparison with experimental data from Mokbel et al [ 10]. Discrepancies between the two series are lower than 2%. -
3.2 Treatment of the HF signal
A. Thermodynamical meaning o f the H F signal
The HF signal is given by" HF = -fads "qst
in which" - f~dsis the adsorption flow rate, qst is the isosteric heat of adsorption. -
(3)
272
The integration of the HF signal gives the heat released during adsorption for the corresponding adsorbed quantity on the basis of the isosteric heat. Eq. 3 shows that this heat corresponds to a global variation of enthalpy (AH). B. Calculation o f the experimental and &osteric heats o f adsorption
The experimental heats presented in the "results and comments" part of this paper are calculated by dividing AH (result of the integration of the HF signal) by the outgassed sample mass (mos) "
AH Qoxp = ~
(4)
m os
Those experimental heats are pseudo integral heats of adsorption. They are indeed related, as shown by Eq. 3, to the isosteric heat and not to the differential heat of adsorption. Accuracy on Qexp is 5%. The isosteric heat is calculated from the derivation of Qexp versus meq (adsorbed mass at equilibrium) 9
0Qo~
(5)
q st = m os " C~m e q
Qexp(meq) is obtained by performing several adsorption experiments for different VOC concentrations. The correct calculation of the isosteric heat requires the derivation of Qexp(meq). Very otten, Ameq between two experiments. This procedure provides the average isosteric heat of adsorption q~t~. q~t~ is equal to q~t if the mass uptake between two adsorption experiments remains low. This assumption is difficult to check for systems exhibiting type I isotherms with a high initial slope. That is the reason why we present the calorimetric results on the basis of Qexp as a function of meq. qst is calculated by dividing Qexp by the mass uptake
3.3 T r e a t m e n t o f the T G signal
The difference between the TGms and TGe signals gives the outgassed sample mass mos. Such a procedure does not take into account the buoyancy effect on the volume of the sample. The real sample m a s s (mosr) Can be calculated from the measured sample mass by: mos = mosr - p'Vaa ~
(6)
in whichV,,dsis the volume of the sample, - p is the gas density. -
The pVads term is the buoyancy effect on the volume of the adsorbent. It can be neglected as it leads to an error on the sample mass of about 0.04%.
273
The difference between TGeqand TGms gives the adsorbed mass at equilibrium (meq).The buoyancy effect on the adsorbed phase is neglected. The coverage ratio is calculated by: | =
m eq
(7)
mo,M
in which: M is the molar mass of the adsorbate. -
4.
RESULTS AND COMMENTS We studied the NaY/toluene system at 150~
and for high partial pressure of toluene The adsorption isotherm and the experimental heats of adsorption are shown on Fig. 2 and 3. Such data correspond to the end of the Henry area and to the saturation plateau of the isotherm. The average isosteric heat calculated for a coverage ratio of 2.081 mmol/g is 97.8 kJ/mol. In this area it is possible to compare our values to existing data. The discrepancies appear to be less than 5% on the adsorption isotherm. We provide a first result in the Henry area (Pcov=l 1Pa) for which it is not possible to achieve a comparison with other data. Going down to the Henry area leads to very long experimental times due to the low VOC concentration in the He flow. Such long experimental times are not a problem for the adsorbed mass measurements but could lead to important errors (up to 10%) on the determination of the heat of adsorption due to the long integration time.
(Pcov>30 Pa) corresponding to saturation temperatures (Ts) higher than -40~
As a conclusion, we provide a new experimental procedure which allows to reach the Henry area for VOC/adsorbent systems. It provides both the adsorption isotherms and the heat of adsorption. Comparison studies with existing data at high temperature should be performed prior to the data acquisition at lower temperatures.
~._m.2"5
f
i2 m 90
......,i~
.......
3E............... T
& ....................
&
1
u 0.5
0
0.2
0.4
0.6
0.8
1 1.2 P lkPa
Fig. 2 "NaY/Toluene 9Isotherm/150~
1.4
1.6
1.8
2
2.2
274
300 250 200
8-
d
~5o
100 50
0.5
1
1.5
Coverage ratio I m m o l . g "1
2
2.5
Fig.3 9NaY/Toluene 9Experimental Heats REFERENCES
D.M. Ruthven, Principles of adsorption and adsorption processes, John Wiley, New York (1984) [2] J.F.M. Denayer, G.V. Baron, Adsorption 3 (1997) 251-265 [3] J.R. HuRon, D.M. Ruthven, R.P. Danner, Microporous Materials 5 (1995) 39-52 [4] D.M. Ruthven, B.K. Kaul, Ind. Eng. Chem. Res. 35 (1996) 2060-2064 [51 S. Palmas, A.M. Polcaro, R. Carta, G. Tola, J. Chem. Eng. Data 36 (1991) 1-4 [6] J.C. Moise, J.P. Bellat, A. M6thivier, Microporous and Mesoporous Materials 43 (2001) 91-1001 [7] Z. Chen, J. Lu, X. Liu, T. Ding, Chinese J. of Chem. Eng. 8(4) (2000) 283-286 [8] D.M. Ruthven, I.H. Doetsch, AIChE journal 22(5) (1976) 882-885 [9] J.-H. Yun, D.K. Choi, S.-H. Kim, AIChE journal 44(6) (1998) 1344-1350 [10] I. Mokbel, E. Rauzy, J.P. Meille, J. Jose, Fluid Phase Equilibria 147 (1998) 271-284
[1]
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
275
Energetics and Mechanism of Physical Sorption by Carbonaceous Solids" Evaluation of surface area and porosity factors. E. Loren Fuller, Jr. Lorela Enterprises, PO Box 355 Stanton NE 68779-0355, USA The physical adsorption of inert gases (nitrogen, argon, etc.) is the most informative method for evaluation of surface area and porosity of finely divided materials. Small (in the molecular size range) micropores interact very strongly -- sorbing at very low pressures (higher sorption potential). Nitrogen adsorption was analyzed in terms of the AutoShielding Potential (ASP) theory for a series of nonmicroporous carbonaceous chars, microporous (activated) chars and nonmicroporous deactivated chars.
i. Introduction Current industrial and environmental processes are increasing the need for storage, separation, purification, sorbents and techniques for characterizing sorbent systems. Physical sorption techniques I traditionally involve the measurement of the amount of sorbate associated with a sorbent at a fixed (isotherm) temperature with volumetric or gravimetric techiques. 2 There are few, if any, techniques that can probe in great detail into and around composites, molecular sieves, agglomerates, etc. Granted the resulting isotherm, expressing sorption, F, as a function of pressure, P:
r=r(#)
(1)
is inherently complex and requires statistical and thermodynamic interpretation with respect to the relevant properties of interest (surface area, pore volume, pore sizes, sorp~ion energy, etc.) 3 ' This work examines physical sorption (physisorption) as the result of attraction of gaseous species into the force field (sorption potential) that exists at the heterogenic boundary at the interface between solid and fluid (vacuum) phases. Modeling of the physisorption processes involves the considerations: m Low energy, less than that noted for chemical bond formation (chemisorption) and more than that for condensation of the liquid. The sorbed entity is statistically associated with the substrate with the loss of only a few degrees of freedom. In the limit, there is only one degree of translational motion lost in the physisorption process. 9 Loose association with the surface with free exchange with the gas phase. Even for crystalline materials, the periodic potential fields cannot be very great or else the
276
9
9
9
9
9
9
process of binding would be in the classical realm of chemisorption (and undoubtedly irreversible at the temperature of the isotherm experiment). There is no association of sorbed entities with any given site. This precludes analyses of the Langmuir and the related BET a theory for physisorption isotherm analyses. Each sorbed molecule is free to move across the surface unhindered with completely elastic collisions and/or escape to the headspace. There is no way to specifically identify any molecule except as a statistical approximation as to its location. Rapid equilibration limited to the transport to the surface. The only rate effects to be noted are enthalpic equilibration (exothermic heat transfer) and/or diffusional over very short distances. The exception would be for small micropores if they exist in the inner reaches of a bulk form, such as monolithic zeolites. Liquid like properties with the concept of density meaningless in the classic sense of a three dimensional definition. The density of the sorbed species can have only statistical meaning and achieves physical meaning as the liquid film state is formed. Energetic trends are continuous in, and through, the monolayer and multilayer formation stages -- ever decreasing as the surface concentration increases. This in contrast to the assumptions inherent in the BET "multilayer' theory 4 where all layers beyond the first are assumed to be formed with energy equal to that of liquid condensation. In reality this condition occurs only at the equilibrium vapor pressure of the sorbate liquid at the temperature of the isothermal experiment. Wide pressure range for data acquisition to allow more complete description of the system of interest. It all to easy to overlook important trends and construct an incomplete or inaccurate paradigm. One should avoid limiting the analyses to restricted data regimes such as the traditional "BET region" of the sorption isotherm. Maximum number of data points should be acquired to further elucidate trends or show the absence of thermodynamic and/or kinetic trends. Modem automated computer driven equipment has permitted the efficient use of operator time and equipment. 5 Low pressure data should be included to reveal the limiting pressure below which there is no physisorption. In the limit, the first physisorbed molecules must have a finite vapor pressure below which there is no physisorption per se, There is no perfect vacuum limit for physisorption isotherms!
Inherent in all of these concepts is that a monolayer (or any other multilayer) does not exist as a discrete entity, but is only described (exists) in a statistical sense. This is not to detract from the importance and utility of such a parameter as a means of describing a substrate in terms of surface area, porosity, etc. The AutoShielding Potential (ASP) theory 6 has shown an exponential relationship for the Polyani (Verb. Dtsch. Phys. Ges. (1914) 16 1012) sorption potential -- defined as the reversible work required to achieve the equilibrium pressure, P, in a closed system. This is conveniently expressed for the single vapor component with respect to the liquid sorbate reference pressure, P(0), at a given temperature, T, E = -RTIn[P/P(0)]
(2)
277 with respect to the monolayer/multilayer sorption. In essence this is the amount of reversible work required to be done on a system to achieve the given state with respect to the liquid phase at the temperature of the isotherm. This is a minimum amount of work that must be done on the system in industrial processing. The ASP theory casts the adsorption isotherm into a rectilinear coordinate format with respect to the initial sorption potential, E*: E = [E*]exp(-0)
(3)
where 0 = fractional monolayer coverage = F/F(m)
(4)
with F expressed in the desired sorption units [cc(STP), mg/g, etc.] relative to the monolayer capacity. The first order rectilinear form r = r ( m ) {In(E*/RT)- In(E/RT)}
(5)
This can be represented in rectilinear form: F = b(0) - b(1) In(E/RT)
(5a)
a simple mathematical relationship with only two adjustable parameters that accurately defines physical sorption isotherms for numerous organic7 and inorganic8 substrates with only two (2) adjustable parameters over a wide pressure range. The relationships of Equations 5 and 2 are unquestionably valid for unlimited surface coverage on ideal external open (flat, planar, accessible) surfaces ranging from nil at E* to infinity at E=0. All of the inherent assumptions (tabulated above) are equally valid as models for physical adsorption in internal constricted regions. These are classically denoted as ultramicropores (<<2 nm), micropores(<2 nm), mesopores (2<1000nm) and macropores (very large and difficult to define with adsorption isotherm). 1~ In these instances there are finite concentration limits corresponding to the volume (space, void) size domain(s). Although caution is needed to deduce models from thermodynamic data, we can expect to observe linear relationships over the respective domains. The results will be consistent with, albeit not absolute proof of the models. Existing literature and data will be used to avoid personal or institutional bias. One system involves relatively inert (nitrogen) physisorption on chars of interest as sorbents, catalysts, molecular sieves, separation substrates, storage media, etc. All of the sorption data presented here exist in tabular form, such that verification of this work and development of alternate theories, verification, and methodology is facilitated. 2. E x p e r i m e n t a l
The experimental portion of this work was carried out in two separate laboratories on two very different and important solid substrate materials, carbon chars and natural minerals of geochemical interest. The first involves excellent work performed by Victor Dietz et. al. at the National Bureau of Standards (NBS), now the National Institute of Standards and Testing (NIST). 11 This research was meticulously carried out before the advent of
278 automated, digital, computer controlled equipment. The work is documented in tabular form, comprehensively covering the range from 10 -6 P(0) to >0.999 P(0) using manometers, McLeod gauges, etc. and hours of tedious data aquisition and processing. This is a very nice piece of work retained in a format useful for comparative purposes and developing analytical technologies.
3. Results and Discussion The adsorption isotherms for the chars of interest are shown in Figure 1 in the classical pressure format. The activated Char 27 shows a very enhanced sorption that is due to volume filling of the voids generated in the activation of the original carbonized Char. that often is erroneously attributed to additional surface area if one focuses on the limited data in the region near the "knee" of the isotherm to the exclusion of all of the other features of the isotherm. One can be mislead by evaluating the surface area in terms of the absolute magnitude to the near complete disregard of the trends apparent in this pressure regime. This problem is circumvented by the ASP analyses of the sorption data as presented in Figure 2. The slopes of the rectilinear portions of the curves is directly related to the monolayer capacity and the respective volume capacity terms. There is a striking resemblance of the activated char data to the n, ms t, etc. comparative plots currently employed for discerning porosity for materials. These relative techniques measure sorption by a given sample with respect to that measured for a reference nonporous sample assumed to be of the same chemical composition. In this instance. adsorption on both of the unactivated char and the deactivated char is excellently defined in terms of the ASP parameters. If needed, either could serve as reference nonporous materials for comparative purposes. This would involve unwarranted operations, interpolation, and processing problems. The ASP plots could serve as reference isotherm(s) for analyses of the activated char. Alternatively, the direct analyses of the rectilinear trends permit one to come to the same conclusions. Why should we compare one unto the other when they both are valid in their own right? The ASP theory well defines the data, as witnessed by the least squares regression fit that is excellent for the wide range of sorption far beyond the very limited regimes of the BET and other existing theories. The ASP parameters can be used to evaluate the continuous trends in the classical type II adsorption isotherms as noted in Figure 1. Relevant points: 1) there exists a cutoff (finite) energy above which there is no significant physical sorption; 2) both of the nonporous materials give rise to adsorption isotherms that are each fitted well with two adjustable parameters, tabulated in Table 1; 3) There are three distinctly separate rectilinear sorption trends noted for the porous activated char. Each of the parameters were evaluated statistically and the corresponding standard deviation variation of fit are given in Table 1. (r2 = 1.00 for perfect fit to the experimental data). A SigmaPlot TM data processing package was used for graphics and statistical analyses. In each case,
In(E*/RT)= -b(O)/b(1)
(6)
279 500
! 400
o
S
13_ [] I-- 300--[~- ......... O3
0 1:1. 0 200 -[7............
L_
[]
100
I
0.0
0.2
0.4
Relative
0.6
0.8
1.0
Pressure, PIP(O)
F i g u r e 1. S o r p t i o n I s o t h e r m : N i t r o g e n on C h a r s at 77.2 K (open circles u n a c t i v a t e d p r e c u r s o r char, open s q u a r e s - activated char, filled circles deactivated char. In each case the lines are a p p r o x i m a t i o n s calculated on the bases of the ASP p a r a m e t e r s of F i g u r e 2 a n d T a b l e 1.
280 500
400
12. I'-- 300 03 V
E 0
,,~,=
L_
0 200
100
-2
0
2
4
6
8
-In(E/RT) Figure 2. ASP Plot for Nitrogen on Chars at 77.2 K (see Figure 1 for data designation).
281 and
(7)
F(m)= b(1).
The additional features for the activated chars has been conventionally separated into the categories of ultramicropores and micropores for convenience. The amount of material required to fill these voids is readily assessed as the difference in the intensive property, F, evaluated graphically and/or mathematically as the intersection of the linear relationships. A more meaningful measure would result by mathematically converting to liquid volume from the extensive sorption measure , ml(STP)/g, used in the original tabulation {cc(STP)/g by more recent convention} For topographical definition of the activated char the ratio of physisorption by each of the modes is an important factor:
F(physisortion) 9F(micropores) 9 F(ultramicropores)= 12.4" 165" 238
(8)
for this specific sample in terms of the standard temperature and pressure measure {cc(STP)/g(char)} of sorption. It would be more straight forward and less ambiguous to express the capacities as mass or molar concentrations (i.e. mg/g, mmol/g, pounds/pound, percent, etc.). Even the use of equivalent liquid volume measure requires assumptions as to the density of the respective phases. The amount and range for each term will vary with the extent and nature of the activation processes. Such analyses of this and other molecular sieve materials provide important energy and capacity information for modeling and industrial processing that is difficult to obtain by other means.
Table 1. ASP Parameters for Nitrogen Sorption by Chars at 77.2 K Statistical parameters evaluated with SigmaPlot TM Sorption Regime Adsorption Micropores Ultramicropores Char Designation Char 27 (Unactivated)
Nonexistent
Nonexistent
Char 1
b[0] b[1]
b[O] b[1]
(Activated)
Char 2 (Deactivated)
724.7 258.9 r 2 = 0.9906 ln(E*/RT) = 2.799 Nonexistent
443.2 107.1 r 2 = 0.9975
Nonexistent
b[0] = 9.512 ml(STP)/g b[1] = 3.151 ml(STP)/g r 2 = 0.9958 ln(E*/RT) = 2.993 b[0] 373.2 ml(STP)/g b[ 1] 12.41 ml(STP)/g r 2 = 0.9935
b[0] b[ 1]
41.89 ml(STP)/g 13.97ml(STP)/g r 2 = 0.9901 ln(E*/RT) = 3.019
This report quantitatively delineates the subdivision of micropores and external surfaces without the "determination of isotherms on a wider range of typical substances -__.,12 Ref. 12 presents excellent qualitative depictions of the contributions of micropores
282 and external surfaces where there are "three processes which, in principle, may occur in succession: primary, occurring in very narrow pores (w/~ < ~2); secondary, occurring in wider pores (~2 < w/~ < ~5); and tertiary, reversible capillary condensation (w = width of pore; ~ = molecular diameter = 3.54 A ~ for N2)." In the present instance the ASP treatment of the physisorption isotherm data for unactivated chars, activated chars and deactivated chars provides an equation defining the physisorption paradigm with the significant features: 1. Two adjustable parameters with firm theoretical basis for each of the sorption regimes. 2. Excellent agreement between the sorption data and the model. 3. Simplicity, based on economy of postulates and thermodynamic considerations. 4. Provision of quantitative measures of interest to research and applied engineering. 5. Extensive applicability over a wide range of pressures { 10-6 P(0) to P(0)}.Intuitively correct in the low pressure (finite, non-zero, vapor pressure for initially sorbed molecules) and high pressure (infinite sorption as the phase transformation to the liquid phase is observed at saturation pressure, P/P(0) = 1).
References 1. S.J. Greg and K. S. W. Sing, "Adsorption, Surface Area, and Porosity." WileyInterscience (1967). 2. E. Loren Fuller, Jr..E. Robens, et. al. "Volumetric and Gravimetric Methods for Adsorption Isotherms Evaluation," Thermochim Acta. 2___99(1979) 315. 3.A. Adamson, "Physical Chemistry of Surfaces," Wiley-Interscience, New York, 1976. 4. P. H. Emmett and S. Brunauer, J. Am. Chem. Soc. 59 (1937)1553 5. Computer Controlled Vacuum Microbalance Techniques Of Surface Area And Porosity Measurements. K. A. Thompson and E. L. Fuller, Langmuir 3 :(5) (1987) 699-703 6. Statistical Mechanical Evaluation Of Surface Area From Physical Adsorption Of Gases. .E.L. Fuller and J. B. Condon, Colloids and Surfaces 171 (1989) 37 7. E. L. Fuller, Jr. "Morphology of Carbons Deduced from Physisorption Isotherms. I. Nuclear Grade Graphite," Proc 24th Conference on Carbon pp 14-15 (1999) and "II. Amorphous and Activated Carbon," ibid. pp 16-17. 8. E. L. Fuller Jr. and K.A. Thompson, "Chemical and Geometric Factors in Physical Adsorption/Desorption of Gases on Solids," Langmuir, 3 (1987) 171. 9. J. B. Condon, "Equivalency of the Dubinin-Polyani Equations and the QM Based Isotherm Equation: Part A. A Mathematical Derivation," Microporous and Mesoporous Materials, 38 (2000) 359 and "Part B. Simulation of Heterogeneous Surfaces," ibid. p 377. 10, IUPAC Compendium of Chemical Terminology, Second edition (1997), (1972), 31, 385; (1976) 46, 79 11. Victor Dietz, et.al. J. Res. Nat. Bur. Stds. 55 (1955) 11 12. Adsorption of gases - Tool for the Study of the Texture of Solid.s, S.J. Gregg, Adsorption at the Gas - Solid and Liquid - Solid Interface, J. Rouquerol and K.S.W. Sing (Editors), Elsivier, 1982
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
P h e n a n t h r e n e A s o r p t i o n on a C a r b o n a c e o u s Material: M o i s t u r e and CO2 Influence A M Mastral*, T Garcia, R Murillo, M S Call6n, J M L6pez and M V Navarro Instituto de Carboquimica, CSIC, M Luesma Castan 4, 50015-Zaragoza, Spain Phone: 34 976 733977, Fax: 34 976 733318, e-mail: [email protected] Considering that Phenanthrene (Phe) is one of the main pollutants in the waste flue gases from organic material combustion, the aim of this paper is to examine the use of carbon materials to remove low concentrations of Phe from a hot exhaust gas stream as a function of its moisture and CO2 content. Adsorption isotherms were measured with steam, CO2 and Phe concentrations in the ranges of 0-20%, 0-30% and 0.02-3.8 ppmv, respectively searching for operating conditions close to the ones observed in energy generation systems. Two classic models [Freundlich and Dubinin-Radushkevich (DR)] were applied and their parameters were then determined by regression analysis. It was found that those isotherms obtained when steam was present in the inlet gas stream fitted the DR model. The determined parameters showed that, for a carbonaceous material, the higher the steam percentage (in volume) in the gas stream, the lower its Phe adsorption capacity. Regarding to the CO2 influence on the Phe adsorption process, a negative influence of CO2 in Phe adsorption capacity was observed, probably due to a competitive effect between CO2 and Phe molecules for the adsorption sites within the carbonaceous material. However, it was found that high CO2 concentrations did not affect the Phenanthrene adsorption more negatively than low CO2 concentrations. In addition, isotherms obtained when CO2 is present were fitted to the Freundlich equation in all the concentration range.
Keywords: PAH, Phe, adsorption, moisture, hot gas cleaning. 1.
INTRODUCTION
Combustion processes of fossil fuels are currently the main source of energy generation. It is well known that, in these processes, pollutants -such as SO2, NOx and CO2 and COare emitted to the atmosphere. Organic emissions, whose harmful nature has become the cause of growing concern, are another important source of pollution released in these processes. In fact, Polycyclic Aromatic Hydrocarbons (PAH) have been widely studied and their carcinogenic and mutagenic potential has been a matter of considerable concern [ 1,2]. These emissions can be minimized by combustion optimization but not completely
283
284 avoided [3-5] and therefore, it is necessary to design new processes able to reduce emissions under future legislation limits. Two different technologies seem to be the most promising alternatives to reduce gaseous PAH emissions: catalytic PAH destruction [6, 7] and PAH adsorption on carbonaceous materials [8, 9]. Historically, carbonaceous materials have been used for the removal of vapor phase organic compounds from about 100 ppmv to 10,000 ppmv concentrations in industrial waste gas streams [10]. Recently, it has been shown that dioxins, furans and PAH, at ppbv or lower concentrations, can be effectively removed from waste incinerator combustion gases by using carbon injection or carbon bed technology [11 ]. Previous works have shown that the main process variables affecting PAH adsorption on carbonaceous materials are: temperature, adsorbate [9], adsorbent characteristics [12, 13] and inlet gas composition, where humidity and CO2 might be specially influencing the PAH adsorption capacity of carbonaceous materials. In this paper, and with the aim of being closer to the real conditions found in combustion processes flue gases, the influence on Phe adsorption of another important components present in these waste streams, steam and CO2, is studied within the concentration ranges normally emitted. 2. EXPERIMENTAL
A carbonaceous material (CA-3) was supplied by the company RWE-Rheinbraun. It is a coke from German Rhenish lignite and it is well known all over Europe for its interesting applications in the field of waste gas and waste water cleaning. However, no information was given about its carbonization process. The textural parameters of this carbonaceous material are compiled in Table 1. Table 1. CA-3 sample textural parameters. DR equation
VN2 (cma/g) 0.11
DA equation
TPD
Vc02 (cma/g)
n
L (nm)
CO2 (lamol/g)
CO (ktmol/g)
0.11
2.1
1.3
499
542
BJH method
VBJH (cm3/g) 0.11
The pore volumes and the micropore distribution parameters of the carbon samples were analyzed in an automatic volumetric sorption analyzer (model ASAP 2000, Micromeritics Instrument Co. Norcross, GA, USA) using N2 and CO2 adsorption at 77 and 273 K, respectively. Five different experiments were carried out to determine the experimental error with sample CA-3 finding a value around 3%. Phe adsorption experiments were carried out using the laboratory scale rig shown in Figure 1, where a Phe vapor stream with pure Helium as carrier at different steam percentages (0%, 5%, 10% and 20%) and C02 percentages (0%, 10%, 20% and 30%) was generated. In this system, which was described in detail elsewhere [9], a water vapor generator was added to generate a gas stream with a constant moisture concentration and another mass flow controller was included to ensure a constant CO2 flow. The CA-3
285 adsorbing bed, composed of 25 mg of adsorbent (100-200 ktm average particle size diameter) was mixed with 1.0 g of sand, inert material with the same particle size, to provide enough bed length (11 cm) assessing in this way a plug flow throughout the reactor. Blank tests were carried out to check the inert material adsorption capacity. The temperature of the adsorption reactor was fixed at 150~ (+1 ~ accuracy) in the inside of a chromatographic furnace. More details about the experimental procedure can be found elsewhere [9].
Fig. 1.
Experimental system used for Phe adsorption Measurement
Adsorption capacities, w* (mg Phe/mg CA), were calculated as tb*Co*Q/W,where tb was the breakthrough time for 2% of penetration time (min); Co was the Phe inlet stream concentration (ppmv), Q was the flow (ml/min) and W was the adsorbent weight (mg). The PAH concentration (C) in the outlet gas stream was directly measured by a previously calibrated FID. The reproducibility analysis of Phe adsorption capacity was carried out at an influent concentration of 2.0 + 0.1 ppmv (mean _+standard deviation). A value of(0.11 + 0.01 mg Phe/mg CA-3) was obtained for five experimental runs. 3. RESULTS AND DISCUSSION
The aim of this paper is to study the behavior of a carbonaceous material in PAH abatement in hot gas cleaning in real conditions, e.g. power stations where moisture and CO2 are present. Therefore, the Phe adsorption isotherms containing different steam and CO2 concentrations were obtained and fitted to different theoretical models. 3.1. Steam influence
Figure 2 shows the Phe adsorption isotherms in terms of the steam percentage in the inlet gas. The adsorption isotherms are close to type I of the BDDT classification. Thus, the initial part of the adsorption of the isotherm represents micropore filling and the slope of the plateau at higher concentrations is due to multilayer adsorption on the non-
286 microporous surface (mesopores, macropores and the external surface). So, the Phe adsorption is mainly accomplished in the molecular size pores. The isotherms shape suggested that the presence of moisture in the gas stream seems to avoid the multilayer adsorption because lower slopes in the isotherms plateau are found when steam is present in the inlet gas stream. The presence of steam in the inlet gas stream is severely reducing the Phe adsorption capacity of the carbonaceous material and the higher the steam percentage, the lower the Phe adsorption capacity. The Dubinin-Radushkevich (DR) equation is usually applied to describe the physical adsorption of organic vapors on microporous adsorbents. It is based on the micropore volume-filling theory and the Polanyi concept of adsorption potential. The DR equation can be expressed as
(1)
Fig. 2.
Phe Adsorption Isotherms on the CA-3 sample at different steam percentages, (T=150~ 0.02
Where W is the equilibrium adsorption capacity, W0 is the total volume of the micropores accessible to the given adsorbate, ~cis a characteristic constant related to the pore structure of the adsorbent, 13is an affinity coefficient, Csat is the saturation concentration in the gas phase of liquid adsorbate at the adsorption temperature T, and C is the concentration of adsorbate vapor in equilibrium. Plotting In(W) versus [RTln(Csat/C)] 2, the parameters ~132 and W0 in the DR equation were determined by the slope and the intercept of the linear lines respectively. The obtained results and correlation coefficients are compiled in Table 2. This Table shows the DR equation parameters and the
287 correlation coefficients obtained by the DR model in the studied experimental conditions. The good correlation coefficients show that the DR model, both in humid and dry gas streams, fits the Phe adsorption capacities on carbonaceous materials, within the experimental conditions. It is observed that the total volume of the micropores, W0, accessible to the Phe and calculated from DR equation, decreases when the steam relative pressure increases in the gas stream. This fact agrees with what was mentioned above, the higher the humidity, the higher the amount of adsorbed water, and so, the more difficult the Phe diffusion into the micropores and the lower the Phe adsorption capacity.
Table 2
Parameters of Dubinin-Radushkevich Isotherm for Phe Adsorption on CA-3 Sample at Different Steam Percentages.
% steam
[Phe] Range (ppmv) Wo(cm3/g CA-3)
KoR/]~
r2
0
0.024-3.4
0.19
1.0•
-9
0.97
5
0.027-3.1
0.13
6.2x10 l~
0.99
10
0.031-3.1
0.13
6.9x10 l~
0.96
20
0.032-3.1
0.12
6.8•
0.96
-l~
3.2. CO2 influence Adsorption isotherm of Phe vapors on CA-3 sample were obtained at different C02 percentages, 0%, 10%, 15% and 30% at 150~ (see Fig 3). The adsorption isotherms are close to type I of the BDDT classification. It is observed that the adsorption is mainly produced in the molecular size pores of Phe and the presence of CO2 in the gas stream seems not to avoid the multilayer adsorption. The decrease on the Phe adsorption capacity when CO2 is added to the gas stream is probably due to a competitive effect between both molecules for the adsorption sites. It is also observed that the increase of the CO2 percentage from 10% to 30% does not seem to affect to Phe adsorption capacities. This fact is congruent with the results obtained by Yong et a114who reported that the CO2 adsorption at high temperature on non-chemically modified carbonaceous materials, follows Henry's law with a negligible influence of CO2 partial pressure on its adsorption capacity. From the adsorption isotherm data, it could be deduced that the incorporation of C02 to the gas stream does not change the Phe adsorption mechanism on the carbonaceous material surface. This fact was not observed when steam is involved in the Phe adsorption.
288
Fig. 3.
Phe Adsorption Isotherms on the CA-3 sample at different C02 percentage (Ta=150~ 0.02
Isotherm experimental results were fitted to the Freundlich adsorption model in order to predict the adsorbent behavior in energy generation systems where concentrations are ranging from low ppbv up to several ppmv. The modeling of the obtained isotherms was carried out by applying the Freundlich equation. This is an empirical relationship based on the assumption of a logarithmic decrease in adsorption heat with adsorption surface coverage. The Freundlich equation is commonly used in the form:
w , =KjC ~ 1/,,f
(2
where w* is the equilibrium adsorption capacity, Co is the inlet Phe concentration, and KU and nf are empirical constants which also depend on the temperature. If the equilibrium experimental data are used (w* and Co), the isotherm parameters (Kfand nf) in Eq. (2) can be determined by plotting ln(w*) against ln(C0). Generally, the parameter nfhas a value greater than unity, showing a favorable adsorption onto the microporous adsorbent. The magnitude of KUis related to the adsorbent capacity. Table 3 shows the parameters KUand nf and correlation coefficients obtained by applying the Freundlich model to the Phe isotherm data. The Freundlich model is able to fit the experimental data at the studied conditions. This is due to the fact that the Freundlich equation allows for the averaging of differing multilayer adsorption energies and differing surface adsorption groups which are both affected by the size and distribution of pores in an adsorbent [ 15]. Therefore, this equation is taking account the multilayer process involved on the Phe adsorption. All the nf values shown in the Table 3 were greater than unity, indicating a favorable adsorption process on the CA-3 sample. According to the above mentioned, all the KUvalues were
289 identical when C02 was present in the gas stream and lower than the obtained in the case of CO2 absence when only Phe is processed.
Table 3
Parameters of Freundlich Isotherm for Phe Adsorption on CA-3 Sample at Different CO2 Percentages.
% COs
[Phe] Range (ppmv) Kf (mg/mg)/(mg/l)
1]nf
r2
0
0.024-3.4
0.13
0.15
0.98
10
0.032-3.8
0.11
0.10
0.99
15
0.031-3.5
0.11
0.12
0.97
30
0.027-3.5
0.11
0.13
0.99
It is worth commenting that in this paper it has been shown that the presence of C02 and H20 in the inlet gas stream affects negatively to the Phe adsorption capacity. Although no experiments have been performed including CO2, H20 and Phe at the same time in the inlet gas stream it could be expected that the Phe adsorption capacity would be even more negatively affected in this conditions. Probably, both negative effects would work in the same direction decreasing the Phe adsorption capacity and therefore, the closer the real conditions in energy generation systems, the lower the adsorbent efficiency in PAH removal from exhaust gas. 4. CONCLUSIONS
From the bulk of the experimental results obtained, it is concluded that: For a carbonaceous material, the higher the steam percentage (in volume) in the gas stream, the lower its Phe adsorption capacity. The isotherms shape suggests that the presence of moisture in the gas stream seems to avoid the multilayer adsorption. The best model to fit the Phe adsorption capacities on carbonaceous materials is the DubininRadushkevich model. A negative influence of C02 in Phenanthrene adsorption capacity was observed, probably due to a competitive effect between CO2 and Phenanthrene for the adsorption sites within the carbonaceous material. The Freundlich model was successfully applied to the Phe isotherms at any CO2 concentration suggesting the formation of multilayer during Phe adsorption. ACKNOWLEDGEMENTS
The authors wish to thank the ECSC for their financial support (Project 7220/EC/089) and the Government of Arag6n (DGA, Spain) for the fellowships awarded to T. Garcia, and J.M. Lopez. M.S. Callrn and R. Murillo would like to thank to the Spanish Ministry
290 of Science and Technology for their "Ram6n y Cajal" Program contracts. Thanks are also due to RWE-Rheinbraun for supplying the CA-3 sample. REFERENCES
[ 1]. P.T. Williams, J. Ins. Energy, 63 (1990) 22. [2] M.L. Lee, M. Novotny, K.D. Bartle, Analytical Chemistry of Polycyclic Aromatic Compounds; Academic Press: New York, 1981 [3] K.L. Liu, W.J. Han, W.P. Pan, J.T. Riley, J Hazard Mater, 84 (2001) 175. [4] A.M. Mastral, M.S. Call6n, R. Murillo, T. Garcia, Fuel, 77 (1998) 1513. [5] A.M. Mastral, M.S. Call6n, R. Murillo, T. Garcia, Fuel, 78 (1999) 1553. [6] P. Liljelind, J. Unsworth, O. Maaskant, S. Marklund, Chemosphere, 42 (2001) 615. [7] R. Weber, M. Plinnke, Z. Xu, M. Wilken, Appl. Catal. B: Environ. 31 (2001) 195. [8] J.J. Cudahy, R.W. Helsel, Waste Management, 20 (2000) 339. [9] A.M. Mastral, T. Garcia, M.S. Call6n, M.V. Navarro, Energy and Fuels, 15 (2001) 1. [10] C.L. Mantell Chemical & Metallurgical Engineering 47 (1940) 305. [ 11] J.J. Cudahy, R.W. Helsel, Waste Management, 20 (2000) 339. [ 12] A.M. Mastral, T. Garcia, M.S. Call6n, J.M. L6pez, M.V. Navarro and J. Galb~in, Environ. Sci. Technol., 35 (2001) 2395. [13] USEPA, EPA-456/R-95-003, Office of Air Quality Planning and Standards. Research Triangle Park. NC. (May 1995). [ 14] Z. Yong, V.G. Mata, A.E. Rodriguez, Adsorption, 7 (2001) 41. [15] P.D. Paulsen, F.S. Cannon, Carbon, 37 (1999) 249.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
291
Water adsorption on micro and mesoporous silicas J. Alcafiiz-Monge, D. Lozano-Castell6 Dpto. Quimica Inorg~nica. Universidad de Alicante. Apdo. 99. E-03080 Alicante. Spain In this work, the influence of pore size distribution on water adsorption is analysed for a series of partially dehydroxylated silicas. The different pore volume of silica gels is evaluated from the analysis of nitrogen and CO2 isotherms. Some silica gels show water adsorption mainly in the first range of relative pressures (P/Po from 0-0.6), while other silica gels also show water adsorption in a second region at higher relative pressures. From the comparative analysis between the N2 and water isotherms, it can be said that only silica containing mesoporosity show water adsorption on the range of high relative pressures. On the other hand, a good correlation between the initial part of water isotherms (0-0.3) and CO2 isotherms exists (the higher the CO2 adsorption capacity, the higher the water adsorption). 1. INTRODUCTION The synthesis and structural characterisation of inorganic porous solids, like AC, Silicas and Zeolites, have received a growing and intensive investigation during the last decades. In this work, we are interested in water adsorption on amorphous silica (Silica gels). Water adsorption on amorphous silica has been investigated by numerous researchers due to the importance that water interaction with the surface of silica has on many of the uses of this material as a drying agent, catalyst support, chromatographic and filler [1,4]. The mechanism of water adsorption on porous solids has been the subject matter of numerous investigations during the last several decades. However, at the present, several ambiguities still remain unresolved, which make this subject very complex [3,4]. 2. EXPERIMENTAL A series of silica gels were synthesised from TEOS, selecting the experimental conditions (pH and water/TEOS ratio) in order to obtain samples with different porosity. Porous texture characterisation of these samples was done by gas adsorption (N2 and CO2 adsorption at 77 K and 273 K, respectively) (Quantachrome. Autosorb 6). The samples were degassed at 623 K under vacuum, until 10"3 torr. Water adsorption studies were carded out at 298 K in an automatic volumetric gas adsorption instrument (Belsorb 18). Experimental data was corrected for adsorption on inner wall of apparatus. Additionally, a blank experiment on all bulbs used showed that water adsorption on the inner surface of glass was negligible. Previous to water adsorption
292 experiments, the samples were degassed at 623 K under high vacuum (10 -6 torr). Diffuse reflectance FTIR characterisation of silica was realised in a spectrometer Mattson Infinities (MI60). Powdered samples were located in a controlled environmental chamber that disposal this equipment. Experiments were carried out on N2 flow and heater up to 623 K. 3. RESULTS AND DISCUSSION 3.1. Porous texture characterisation of Silica Gels
Figure 1 presents the nitrogen adsorption isotherms corresponding to all the samples. The considerable nitrogen uptake at low relative pressure is characteristic of Type I isotherms (according to the IUPAC classification), which reflects the presence of microporosity. On the other hand, nitrogen uptake at relative pressures P/Po>0.2, approximately, indicates the existence of mesoporosity. The different shape of the isotherms indicates the different pore size distribution on this series of samples. Thus, in this series of samples there are essentially microporous samples, without mesoporosity, which is reflected by the plateau of the isotherms (Series A), and samples containing both microporosity and Figure 1. Nitrogen isotherms on serie of silicas. mesoporosity (Series B). Figure 2 contains the CO2 isotherms at 273 K for all the samples. It is known that CO2 adsorption at subatmospheric pressure is a useful complementary technique for the characterisation of the narrow microporosity, due to the higher adsorption temperature (273 K) and the smaller kinetic diameter of CO2 [5,6]. As it can be seen in Figure 2, all the silica gels adsorbed CO2, indicating that they all contain narrow micropores. Figure 2. CO2 adsorption isotherms
293
Values of the micropore volume obtained from N2 and C02 Dubinin-Radushkevich plots are shown in Table I.
3.2. Water isotherms on silica gels: Influence of pore size distribution. Figure 3 presents water adsorption isotherms corresponding to all silica gels. In this figure it can be observed that, similarly to the N2 adsorption isotherms, silica gels of Series A (essentially microporous) adsorb water in a unique range, from low to medium relative pressures (0-0.6), while silica gels of Series B (micro- and mesoporous silica gels) adsorb water over two ranges of relative pressure: the first one up to medium relative pressures (0.5), and the second one at higher relative pressures. 900
000C 3B
800
4B
5B
oO.++~ 0 "
700
2B 600 50O
/ o ~
~
**~162
400
~
0
.~vo
.
"~-g_~o~"x=
9
,,~~ .~o
o i~ol 1B 2A X X X x 3A is A
9
9 9
,, "Xoe.: 9a,~'o-
300
1A
9 . . . . . . .
.
I00 1I ~ 0
0
~,,,'A. p~ o~ u o 0.2
o
0.4
0.6
0.8
1
P/Po
Figure 3. Water adsorption isotherms on silica gels From the comparative analysis between N2 and water isotherms, it can be deduced that only silica containing mesoporosity show water adsorption on the range of high relative pressures. These results seems to indicate that water adsorption at low relative pressures occurs mainly on the microporosity, while the adsorption at higher P/Po seems to be due to the presence of mesoporosity. These conclusions have been also obtained from water adsorption on AC [7,8]. Next step in this research work is to analyse the way water adsorbs in each type of porosity. Starting the analysis from the region of low relative pressures, in Figure 4 it is evident that the water uptake was quite small at low relative pressures. This is an indication of the weak water-silica interaction in this type of solids [9]. The non-specific contributions are weak in the case of water and silica pore wall, as consequence of the low polarizability of
294 water molecule. Hence water adsorption at low relative pressures must be very low according with this interaction.
Figure 4. Water adsorption at very low relative pressures. It is known that surface of silica is covered with silanol or siloxane groups [1,2]. The SiOH shows a specific interaction with water (hydrophilic), by means of the localised electron density of water molecule and the positive charge of OH groups. On the other hand, Si-O-Si are hydrophobic due to the non-polarity of these bonding [10]. Thus, it is reasonable to think that depending on the amount of silanol groups in the surface of silica, the amount of water adsorbed at very low relative pressures would be different. Considering the results reported by other authors saying that a heat treatment up to 573 K on silica gels removes a major part of silanol groups [3,11 ], silica gels of the present study must suffer an important dehydroxylation during the degassing process at 623 K, previous to water adsorption experiments. This fact could explain the low water uptake of these silica gels at low relative pressures. These results agree with the study done by Baker and Sing [11 ], where it was demonstrated that all dehydroxylated silica presented a very small water adsorption at P/Po < 0.3, due to the hydrophobicity of these samples. In order to check whether there is a relationship between the number of hidroxy groups on silicas and water adsorption capacity, the characterisation of surface groups was investigated by infrared spectroscopy (DR-FTIR). Figure 5 contains the FTIR spectra corresponding to some silicas. The signal related with OH groups appears in the region between 3800-3000 cm" I. Each sample shows a sharp peak around 3750 cm "l attributed to free OH groups, and a broad band between 3700-3000 cm l , related to hydrogen-bonded OH groups. Comparing the intensity of these bands and the water uptake at low relative pressure (Figure 4), it can be observed that a correlation between the amount of total OH groups and the amount of water
295 adsorbed at low relative pressure does not exist. It can be seen that the sample with the highest and lowest water adsorption capacity (2A and 3B, respectively), present similar amounts of OH groups. Moreover, sample 1A, which contains a low amount of OH groups, has a quite high adsorption of water at low relative pressure. These results seem to indicate the small effect of surface chemistry in water adsorption of these samples. Thus, the isotherm shape must be a function of the micropore size distribution. The rest of the present study is focussed in this aspect.
Figure 5. DR-FTIR spectra of selected silicas. In Figure 4 it can be appreciated that sample 2A presents the highest water adsorption up to relative pressures of 0.4. This behaviour can be related with the highest N2 adsorption on the initial part of isotherm and the highest CO2 adsorption capacity (see Figure 1 and 2, respectively) of this sample, which is indicating the presence of narrow microporosity. The same correlation can be appreciated for the samples with the next highest water adsorption, samples 3A and lB. In fact, except from sample 1A, a good correlation between the initial part of water isotherms (0-0.3) and their CO2 isotherms and the initial part of N2 isotherms exists. The sample 1A presents a similar water adsorption than sample 1B on the range of relative pressures 0-0.2, while their CO2 isotherm and the initial part of N2 isotherm is comparable to other samples (2A, 4B, 5B). According to these results, sample 1A adsorbs more water at low relative pressures than expected, which could be due to the presence of micropores with very narrow size. In the very narrow pores the adsorption potential is higher than in wider pores [12], favouring the water adsorption at low relative pressures. Thus, from these results it can be said that in a first step water fills the narrow microporosity. Samples 1A and 1B, which have similar narrow micropore volume (Vco2; see Table 1 and Figure 2) show similar water adsorption in the range of P/Po <0.3. The adsorption of water presented by these samples at higher pressures must be related with the presence of wider micropores (supermicropores; supermicropore volume=VN2 - Vco2). As microporosity gets wider, water
296 adsorbs at higher P/Po (i.e. water adsorption in sample 2A ends at P/Po around 0.5, while 3A ends at 0.6) The existence of mesoporosity makes complex the analysis of the transition from micropores to mesopores. Sample 2A, which does not present mesoporosity, as it can be deduced from the small slope of the plateau in its N2 isotherm, adsorbs water mainly up to P/Po around 0.6. On the other hand, sample 2B, which has an important mesoporosity, shows a knee for water isotherm around 0.7. Considering that the micropore volume for both samples is quite similar (VN2, Table 1), the higher water adsorption from P/Po around 0.6, for the sample 2B compared to 2A, must be a consequence of water adsorption on mesoporosity, which indicates that both processes overlap. Thus, when a clear transition does not exist between micro and mesoporosity, as it is the case of these samples (Figure 1), both the final filling of micropores and the start of mesopore filling could be taken place in the same range of relative pressures. Hence, they overlap given a single adsorption curve. The rest of mesoporous silica (3B, 4B, 5B) present the knee of their water isotherm at higher relative pressures than the 2B. 5B shows the knee around P/Po 0.8, 3B around 0.9, and the knee for sample 4B is less defined. These trends can be related with the mesopore size distribution. From N2 isotherms, it can be deduced that these samples present different mesopore size distribution. 2B contains mesopores with narrow size, because it only adsorbs N2 until P/Po 0.6, while the others samples have wider mesoporosity because they present adsorption at higher P/Po. From these results it can be said that the shape of water isotherm up to relative pressures around 0.6 is mainly related to the micropore size distribution. On the other hand, the adsorption at higher P/Po is due to the presence of mesoporosity. Depending on the pore size distribution and the contribution of the different pore volumes, the shape of water isotherms can be more similar to the sample 1A or 3B. 3.3. Water isotherms on silica gels: physical state of adsorbed phase.
Another interesting result that can be extracted from water adsorption data is pore volume. Table 1 includes these values considering that the adsorptive is in the liquid state (liquid density for water 1.0 g. cm'3). According to the Gurvitsch rule both volume values should be similar [3]. However, it is normally observed that water volume is lower than the nitrogen saturation volume [13,14]. From this discrepancy it has been suggested that the density of water in the adsorbed phase is lower than the normal liquid at the corresponding temperature [3,15]. Several studies have been carried out to understand the behaviour of water confined on pores. These include X-ray and neutron diffraction studies, calorimetric (DTA, DSC) techniques applied to the analysis of the freezing behaviour of water adsorbed [16-18], nuclear magnetic resonance (NMR) spectroscopy, dielectric and so on. All they showed that adsorbed water has different structures depending on the pore size where it is adsorbed. On
297 narrow pores (micropores) has a solid-like structure even at room temperature, as a consequence of the strong wall interaction. On the other hand on mesopores, water is adsorbed as a liquid [ 19]. Experimental density of water adsorbed in the microporosity of AC has been obtained, giving a value of 0.92 g. cm "3, which is quite similar to the ordinary solid phase of water [7,8]. This value has been used to estimate the micropore volume from water adsorbed up to P/Po around 0.6, while the liquid density has been used to calculate the water volume adsorbed on the range of P/Po around 0.6-0.95. Table I contains these calculated values. Table I. Pore volumes estimated from CO2, N2 and water adsorption. VH20 L VH20 s Cm 3. S total . . [)'m2.g-l" V~tCO2 V~tN2 Vmeso Vtotal (0-0.95) (0- 0.6) G-I
VH20 L (0.6-0.95)
a
420
0.16
0.20
0.00
0.20
0.19
0.20
0.02
1B
720
0.16
0.30
0.07
0.37
0.36
0.30
0.07
2A
640
0.23
0.34
0.00
0.34
0.32
0.34
0.02
2B
905
0.15
0.37
0.10
0.47
0.45
0.37
0.10
3a
643
0.21
0.32
0.00
0.32
0.30
0.31
0
3B
1362
0.11
0.34
0.34
0.68
0.66
0.34
0.34
4B
1210
0.13
0.32
0.31
0.63
0.61
0.33
0.30
5B
1160
0.14
0.32
0.25
0.57
0.55
0.32
0.26
l
9 V~t CO2 = narrow micropore volume (pore< 0.7 nm); VIa N2 = micropore volume ( pore < 2 nm). P Liquid water: 0.99 g.cm3; P Solid water: 0.92 g.cm"3 As it can be observed in Table I, the volumes obtained from water adsorption are quite similar to the calculated micropore and mesopore volume. These results indicate that the approach considered in this study is suitable. Water adsorbs initially on microporosity (up to P/Po 0.6), as a solid-like structure. In these pores the density of adsorbed water can be considered to be similar to the ice density. On the other hand, the similar values of Vmeso and Vmo 0.6-0.95 indicate that water adsorbs on mesopores as a liquid. 4. C O N C L U S I O N S From the comparative analysis of N2, CO2 and water isotherms, it has been concluded that water adsorption at low relative pressures occurs mainly in the microporosity, and only silica gels containing mesoporosity show water adsorption on the range of high relative pressures. It has been seen that a good correlation between the initial part of water isotherms (P/Po from 0 to 0.3) and the CO2 isotherms exists. Thus, water fills in the narrow microporosity, and as the microporosity gets wider, water needs higher relative pressure to adsorb.
298 Pore volumes have been obtained from water adsorption considering two different water densities. The density of water in solid phase (0.92 g/cc) has been used to estimate the micropore volume from the amount of water adsorbed until relative pressure around 0.6. In the other hand, the liquid water density has been used to calculate the water volume adsorbed on the relative pressure range around 0.6-0.95. The pore volumes obtained from water adsorption data are quite similar to the corresponding micropore and mesopore volumes obtained by nitrogen adsorption, which seems to corroborate that water adsorbs in the microporosity as solid ice, while it adsorbs in the mesoporosity as liquid.
Acknowledgment. J.Alcafiiz would like to thank The Leverhulme Trust for the award of a visiting fellowship to the Department of Materials at the University of Leeds, UK, where part of this collaborative study was carded out. J.A. also wish to thanks to Prof. H.P. Boehm for helpful discussions and for his suggestion for improvements this work.
References 1) Iler, R. K.; The chemistry of Silica, Wiley, N. Y., 1979. 2) Unker, K.K. Porous Silica. Elsevier, Amsterdam, 1979. 3) Gregg, S.J.; Sing, K.S.W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. 4) Rouquerol, F.; Rouquerol, J.; Sing, K. Adsorption by Powder and Porous Solids: Principles, Methodology and Applications. Academic Press Inc: Cornwall, 1999. 5) Rodriguez-Reinoso, F.; Linares-Solano, A. Chemistry and Physics of Carbon 1988, 21, 2146. Thrower, P, Ed.; Marcell Dekker Inc: New York, 1988. 6) Cazorla-Amorrs, D.; Alcafiiz-Monge, J.; Linares-Solano, A. Langmuir 1996, 12, 2820. 7) Alcafiiz-Monge, J.; Linares-Solano, A.; Rand, B. J. Phy. Chem. B 2001, 105, 7998. 8) Alcafiiz-Monge, J.; Linares-Solano, A.; Rand, B. J. Phy. Chem. B 2002, 106, 3209. 9) Kiselev, A.V.; Yashin, Y.I. Gas-Adsorption Chromatography; Plenum Press: New York, 1969. 10) Hair, M.L.; Hertl, W. J. Chem. Phys 1969, 73, 4269. 11) Baker, F.S.; Sing, K.S.W.J. Coll. Interface Sci. 1976, 55, 605. 12) Young, D.H.; Crowell, A.O. Physical Adsorption of Gases; Butterworth: London, 1962. 13) Bambrough, C.M., Slade, R.C.T., Williams, R.T., Burkett, S.L., Sims, S.D., Mann, S.; J. Coll. Interf. Sci. 1998, 201,220. 14) Bradley, R.H.; Rand, B. Carbon 1991, 29, 1165. 15) Steytler, D.C.; Dore, J.C. Mol. Phys. 1985, 56, 1O01.. 16) Bellissent-funel, M.C.; Sridi-Dorbez, R.; Bosio, L..i.. Chem. Phys. 1996,104,10023. 17) Watanabe, A.; Liyama, T.; Kaneko, K. Internt. Symp. Carbon; Tokyo, 1998, p 624. 18)Kaneko, K.; Miyawaki, J.; Watanabe, A.; Suzuki, T. Fundamentals of adsorption; Mnetmier, F., Ed.; Elsevier: Amsterdam, 1998, p 51. 19) Morishige, K.; Kawano, K. J. Chem. Phys. 1999, 110, 10.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
299
The Effect of Surface Functionalization of Mesoporous Silicas with Propylimidazol on Porosity, Pore Connectivity and Tortuosity. G.S. Armatas a, C.E. Salmas b, G.P. Androutsopoulos b and P.J. Pomonis a
~Department of Chemistry, University of loannina, Ioannina 45 110, Greece. bDepartment of Chemical Engineering, National Technical University of Athens, Athens 15 780, Greece. Mesoporous silica was gradually functionalized with silano-(trimethoxy)-propylimidazole (STPI) groups. The degree of surface functionalization n=0, 0.23, 0.30, 0.40, 0.52, 0.60 and 0.85 was controlled by previous knowledge of surface acidity, determined by TPD/NH3. From N2 adsorption/desorption measurements the specific surface area Sp (mE g-l), the specific pore volume Vp (cm 3 g-l) and the corresponding pore size distributions PSD were determined while the connectivity c of the solids was calculated according to the method of Seaton. The increase of functionalization resulted in a linear drop of Sp and Vp while the maximum Dmax of the PSD and the Full Width at Half Maximum, FWHM-~2c~ of distribution drop in a regular way whereas the ratio (Dmax/2a) remains practically constant. The connectivity c also decreases from c=12.5 at n=0 to c=3 at n=0.60-0.85 presumably because of blocking of channels connecting various pores. Next, the co-called Corrugated Pore Structure Model, CPSM, was employed for the estimation of tortuosity r of the porous solids and the simulation of the experimental adsorption/desorption isotherms. From those CPSM simulations the corresponding specific surface areas ScPsM (m2 g-l), specific pore volume VCPSM (cm3 g-l) and the corresponding pore size distribution PSDcPsM were estimated. The tortuosity r of the system drops with the degree of functionalization from r=4.22 at n=0 to r=3.37 at functionalization n=0.23 while subsequently remains practically constant at about r=3.40+0.10, for the same reasons which affect the connectivity i.e. blocking of various pore channels. The comparison between the parameters Vp on one hand and VcPsM, is quite satisfactory while the ScPsM values appear systematically higher by almost 8-23% compared to the Sp ones. Similarities and discrepancies between the results obtained by the two methods are discussed. 1.
INTRODUCTION The most common kind of porous materials are the ones possessing a disordered or random porous network [1,2]. In this class of solids belong various silicas [3] aluminas [4] aluminophosphates [5] as well as a large number of aluminosilicate solids encountered in geological forms [6]. The pore size distribution of such disordered or random porous materials is usually characterized on a routine basis by N2 adsorption at 77K. From such measurerrient the following characteristic parameters of porous solids can be calculated: 9 The specific surface area Sp (m2 g-l) and the specific pore volume Vp (cm3 g-l) [1,2, 7]. Those parameters are the most usual ones giving a fair idea about the kind of porous materials at hand but also help in the calculation of surface reaction rates per m 2 and for the estimation of adsorption capacity per g of solid. 9 The maximum of the pore size distribution (PSD) curve, Dm~ (nm) as well as the variance of distribution 2a (nm). These parameters are important since they express the maximum and
300 the narrowness of PSD which can be related to the sieving capacity at the porous materials. The ratio of those two terms Dmax/2Ois also very informative about the relative narrowness of distribution which is extremely useful in chromatographic separations. 9 The pore connectivity c. This parameter, although might be critical in understanding and describing porous systems, is not commonly scrutinized by workers in the field. One of the reasons is that until the last decade of the last century there was not available a tractable and acceptable method to find c. Today, thanks to the work of Seaton [8] there is such a methodology, while Liapis and co-workers have also provided an alternative method [9]. The practical use of c is that it can be helpful in deciding about the choice of porous materials in separation processes [9]. We also mentioned that the connectivity c can be parallelized to the branching ratio - b of trees [10]. A comparison between those parameters c and b reveals some very interesting topological hetero-similarities between the pore network in a solid and the branching of trees. 9 The tortuosity of pores r. This is a difficult property to estimate and it is often considered as a correction factor to various model deficiencies. According to the words of Aris [11] "When models are made of the configuration of pore structure, ~, can be related to some other geometrical parameters but in general it is a fudge factor of greater or less sophistication". Tortuosity is traditionally [12] regarded as a function of a pore length (or angle)factor 'L' and a pore shape 'S' factor: (1) r =f('L','S') Factor 'L' accounts for the longer distance that a diffusing species has to travel through a real pore channel connecting two fixed points within the particle, compared to the straight distance between the points. The shape f a c t o r ' S ' reflects the effect of pore cross sectional area variation along the pore length upon the diffusion process [12]. Tortuosity factors are normally greater than unity. The experimental determination of 9 necessitates measurements of effective diffusivities D~rrin a Wicke-Kallenbach cell and then use of the formula:
D.::
Db =
~
T
(2)
where Db stands for the binary diffusion coefficient and e is the voidage fraction. Details on how eq. (2) can be employed are cited in [ 13]. Numerous x values for a variety of materials are reported in [12]. The first attempt to estimate tortuosity factors without the execution of diffusion measurements was made by Carniglia [ 14] who used data from Hg porosimetry. But a drawback of that method is that it does not take into account the mesopore and micropore regions, which are not traced by mercury porosimetry. Nevertheless, recently Androutsopoulos and Salmas [15] proposed a method, which enables the prediction of tortuosities 9 in porous solids from N2 adsorption-desorption measurements. This method is based on the so-called Corrugated Pore Structure Model (CPSM), suitable for the simulation of all types of adsorption/desorption hysteresis loops [ 16]. Now, an important question is: How, those parameters Sp, Vp, (Dmax/2C), c, and ~ are affected by the gradual narrowing of pores due to some kind of surface functionalization? A second question is which of, and how, the parameters r, c and (Drnax/2~) are interrelated. The question becomes more interesting, and perhaps intriguing, since all the above quantities are calculated just from one kind of measurement, namely the N2 adsorption/desorption data. A partly answer to the above question was attempted in a previous work [10] in which sixteen mesoporous vanado-phoshoro-aluminates solids were tested and some relationships between c and (Dmax/2~) were established. A first target of this paper is to extend the search for such possible inter-relations to a class of mesoporous silicas, with a random pore size distribution whose porosity has been systematically and gradually modified by surface functionalization
301 using silano-(trimethoxy)-propyl-imidazole (STPI) as surface modification agents. A second target is to compare the parameters ScPsM and VcPsM calculated via the CPSM simulations to the corresponding ones Sp and Vp found by the more standard methods. The materials used for testing were seven (7) samples of commercial SiO2 with a degree of surface functionalization equal to n = 0, 0.23, 0.30, 0.40, 0.52, 0.60 and 0.85.
2. EXPERIMENTAL 2.1 Preparation ofsamples Seven samples, based of a commercial SiO2, were prepared with gradual functionalization of its acid sites with a silano-(trimethoxy)-propyl-imidazole groups (STPI in the next). The extent of functionalization was controlled to n=0.00, 0.23, 0.30, 0.40, 0.52, 0.60 and 0.85 as follows in the preparation procedures. Commercial silica of 60-80 mesh, (Davison grade 59) was dried in the oven at 150~ and refluxed in toluene with the estimated amount of imidazole and redistilled 3-chloropropyl-trimethoxy-silane (C1PTMS). Different degrees of coverage were obtained by varying the concentration of reagents and the time of reflux from 2 to 12h. The relevant reactions, which took place, are shown schematically in Figure 1. The obtained solids, with some of their properties are listed in Table 1.
Figure 1. Schematic reaction pathways for the functionalization of the acid sites of SiO2 with silano(trimethoxy)-propyl-imidazole(STPI) groups.
2.2 Surface acidity measurements To enable the controlled addition of STPI groups on the acid sites of the silica surface, its total (Lewis+Br6nsted) acidity was determined using NH3 adsorption in a standard Temperature Program Desorption apparatus (TPD) similar to that described [ 17]. Briefly, the procedure was the following. The sample weighing about 200 mg was put on the perforated bed of a silica tube with a diameter of 1 cm. The tube was connected to a gas chromatograph (Shimadzu GC-8A) with a TC detector. Each sample was initially heated at 500~ in a flow of helium for 2h, whereupon the temperature was reduced to the working level of 100~ Then the sample was saturated with dried ammonia by replacing, for 30 min, the flow of helium through the sample by a flow of dried ammonia. The weakly adsorbed ammonia was flushed out with helium flow (50 ml min 1) for 2h at 110~ After stripping of the weakly adsorbed ammonia the sample was cooled to 100~ and heating begun at a rate of 10o C min 1 till 500 o C where it remained for 2h. The amount of ammonia eluted was trapped in a bubble bottle,
302 which contained an excess of 0.01 N hydrochloric acid solution. The trapped a m m o n i a was determined by volumetric titration of the excess HC1 using 0.05 N NaOH. K n o w i n g the exact n u m b e r o f acid sites per gram o f SiO2, it was possible to control the addition o f STPI groups by the corresponding amount added in the preparation beaker. Table 1. Samples of functionalized Si02 prepared and their specific surface areas (Sp), and specific pore volume (Vp). The maximum Dmax of the PS, the variance 20 of distribution, the ratio Dmax/2O as well as the connectivity c are also cited. Sample
Sp (BET) SCPSM Vp (m2 ~-1) (m2 ga) (cc g-l)
Connect.
VcPsM Dmax FWHM (cc g-l) (nm) =2~ (nm)
(C)
Dmax/2~
Tort.
x
SiO2(~)
279
305
1.14
1.15
14.25
8.30
1.72
12.4 +0.4
4.22
SiO2(STPI)o.23
255
327
0.92
0.96
12.41
6.45
1.92
7.9 +0.5
3.37
SiO2(STPI)o.3o
242
317
0.95
0.90
12.44
5.90
2.11
7.6 +0.6
3.36
SiO2(STPI)0.40
244
301
0.82
0.91
12.47
6.90
1.81
6.2 +0.3
3.30
SiO2(STPI)o.52
232
283
0.81
0.88
14.88
2.60
5.72
3.2 _+0.4
3.74
SiO2(STPI)o.6o
227
297
0.88
0.82
12.07
7.25
1.66
3.1 _+0.2
3.17
SiO2(STPI)o.85
209
267
0.77
0.77
12.78
6.75
1.89
3.0 _+0.5
3.53
(a) Total surface acidity (NH3 TPD) = 12.81x102~acid sites/g n = 0.23, 0.30, 0.40, 0.52, 0.60, 0.85 n = mmoles (STPI) added/mmoles of acid sites (NH3 TPD)
Pore diameter D (nm) 10
100
,
10
,
.
SIO
.
.
.
.
.
.
.
100
,
.;
.
0 10
:,.
600.
.
~ 2
o~o
0.08 0.06
400.
('i
0.04 0.02
200,
0.130 t
~
'
'
,
I
'
I
,
I
'
I
O.lO
~ 600.
SIO2(STPI)o.~
0.08 0.06
~ 400.
0.04
200.~
0.02
0
,
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
.
,
0.8
.
0.00
1.0
Relative pressure(P/P) Figure 2. Typical Nitrogen adsorption-desorption isotherms at 77K and the corresponding pore size distribution PSD curves (dotted line ---). The points (o) and (e) are the experimental adsorptiondesorption data connected by a light line. The darker line corresponds to the CPSM simulation. The PSD derived according th the CPSM model are also given by the continuous line (-). 2.3 Surface area and p o r o s i t y A Fisons Sorptomatic 1900 instrument was used to carry out pore size distribution measurements. The characterization techniques included the determination o f nitrogen
303
adsorption-desorption isotherm at 77K. Prior to each experiment the samples were degassed at 250~ in a vacuum of 5 x 10-2 mbar for 12h. The desorption branch of the isotherms was used for the calculation of the pore size distribution PSD and shown in Figure 2. The specific surface area of the sample was calculated by applying the BET equation using the linear part (0.05
RESULTS In Table 1 the specific surface areas Sp (mE g-l) calculated according to the BET method and the specific pore volume Vp (cm 3 gq) calculated from the total nitrogen volume adsorbed, are tabulated. In the same table the maximum Dmax of the pore size distribution and the variance (2a) of distribution is shown. We mention the (2G) corresponds roughly to the Full Width at Half Maximum (FWHM) of PSD, expressing its narrowness. Finally the ratio (Dmax/2~) is also cited, which expresses the maximum of PSD, reduced over the value of 2~, or otherwise measured with a "stick" equal to 2g. The surface acidity of the parent SiO2 was estimated from the TPD/NH3 experiments to the equal to 12.81x102~ sites/g or 4.58x1018 sites/m2. The last values corresponds to an average distance between two acid sites equal to -~5.2A (see fig. 1). This is a typical value for acidic silicas. Using the above acid density as basis of calculation, the proper amount of functionalizing agent(s) was used as to achieve the various degrees (n) of functionalization (see above, Preparation of samples). 3.1 The calculation of connectivities c.
The procedure for the calculation of connectivities c, following closely Seaton [8], can be summarized as follows: The bond occupation probability f was obtained as a function of percolation probability F from the adsorption isotherms (Figure 1) using the pore size distribution as follows f =
I. nrdr r
f
-- :
n r dr
(3)
/
Vflatmax - Vde
(4)
F Vflat max - Wads where n r - the corresponding PSD using the BJH method for cylindrical pores, //flatm a x is the part of desorption curve before the start of desorption and Vdes, Vads - the corresponding volumes in the desorption and adsorption curves. Then the best c and L values are obtained by fitting the experimental scaling data (F, J) obtained above to generalized scaling relation (5) between F a n d f [ 18]. LIS/VcF : G[(c/- 3/2)L l/v] (5) where c - connectivity and L - the so-called characteristic size of particles that expresses the number of pore lengths. In our analysis the generalized scaling relation (5) was constructed using the critical exponents ,8=0.41 and v=0.88 given by Kirkpatrick [18]. 3.2
Simulation of Nitrogen Sorption Data Using the Corrugated Pore Structure Model CPSM
Additionally to the above, the so-called Corrugated Pore Structure Model - CPSM [ 13,15,16] was employed to simulate the N2 adsorption - desorption experimental data. The intention is to work out true PSD curves based on a simulation of the entire hysteresis loop and to a single PSD prediction and thus to overcome the difficulty of choosing either one or the other branch of hysteresis loops, which result in di-similar predictions. The PSD predicted by the CPSM model (i.e. solid line PSDs in Fig. 2) indicate Omax values that approach those
304 deduced by the BJH method. Cumulative pore volumes calculated by the CPSM predicted PSDs which are in satisfactory agreement with those evaluated directly from the adsorption isotherm data. CPSM simulation also helps in extracting additional pore structure information e.g. the nominal pore length Ns that enables the estimation of the pore structure tortuosity factors [ 15]. A detail description of the CPSM model and how it is applied for the simulation of N2 adsorption-desorption loop, the estimation of the PSD and the calculation of empirical tortuosity factors r are far too lengthy to be describe here in detail. But the interested reader can find details in the original papers [13, 15, 16]. The PSDcPsM data were then used to evaluate the mean pore size Dmean,the effective minimum and maximum of the pore sizes i.e. Dmi~eff and Dmax,eff which were then utilize to calculate Tortuosity factors r via the empirical correlation
TcPsM - l + 0.69 (Dm~'e/y -Dm~n"Y/ ) ( Ns - 2 ) ~
(6)
D mean
The various parameters calculated either via classical/standard methods as well as those found via the CPSM method are presented in Table 1. CPSM surface areas are in close agreement with the respective values calculated from the Langmuir equivalent method [16(a) eq. 34] over the linearity region P/P0=0.02-0.12. 4.
DISCUSSION
The gradual functionalization of the surface of SiO2 with the STPI groups results in a gradual decrease of surface areas Sp and the pore volume Vp of the solids as shown in Table 1. In the same Table the variation of other parameters like connectivity c, the Dmax and the variance 2~ of PSD, the ratio (Dmax/2a) as well as the tortuosity z are shown as a function of degree of functionalization n. Various interesting observation can be made from this data. Perhaps the most trivial and rather highly expectable is the almost linear decrease of surface area Sp and pore volume Vp with the degree of functionalization. Clearly the STPI groups having a length of around -8A (see fig. 1) block various microporous, in a total or partial extend, so the remaining area and the volume available for N2 adsorption is substantially lower. Since n=l actually corresponds to the introduction of STPI groups to the total number of acid sites N=12.8x102~ it can be easily estimated for the data of Table 1 that
and
Loss of S p (ASp)
90 (me~g)
for n : l (An)
12. 8x102o (sites~g)
Loss of Vp (d Vp ) for n : l (An)
~
0.45 (cclg)
= 7. 0
~t 2
/STPI group
3 : 350 ~4 /STPI group
(9)
(10)
12. 8x102o (sites~g)
This last value indicates the more dramatic effect of functionalization in blocking the previously accessible pore volume compared to the effect on the surface area. Clearly the STPI groups block the mouths of pores and this affects also the connectivity of the system. The effect of functionalization on the Dmaxof PSD is rather expectable, in the sense that Dmax tends to lower values (see Table 1). There is an exception with the point at n=0.52 which shows abnormally high Dmax values. This point appears also out of trend in other cases for reasons, which are not clear. The decrease of Dmaxreflects the overall narrowing of pores. But at the same time the effect of functionalization on the variance (2c) of the PSD is rather peculiar. In other words 2c~ initially decreases with n and subsequently increases (see Table 1). That means, at a fist glance, that the PSD is initially getting narrower with the progress of
305 functionalization and then tends to be wider again. Actually this phenomenon is more complex and is due to the initial lowering of the height of PSD with the increase of functionalization. This lowering of the whole PSD, results also in lowering its full width at half maximum, FWHM -~ 2c. As a result of simultaneous decrease of Dmax and of 2g, their ratio Dmax/2a is approximately constant with the progress of functionalization.. The gradual functionalization results in lowering the mean connectivity of the porous network. Indeed as shown in figure 6(c), the connectivity c drops almost linearly from c=12.4 to c=3.1 as the STPI addition increases from n=0 to n=0.6. Then further functionalization (n=0.8) does not seem to affect c which remains steady at c=3.0. Similar results showing a decrease of connectivity from c-~l 1 to c--6 by the process of functionalization of SiO2 with noctadecyl-dimethyl-monochlorosilane, have been reported by Liapis [9]. We draw attention to the fact that the algorithm used by Liapis for the estimation of c is different to the one proposed by Seaton and employed in the present study, so the results in this paper can not be quantitatively compared with the ones in [9]. Another interesting point is also the appreciable drop of tortuosity z occurring at the beginning of surface functionalization (n=0.23) while this decrease becomes almost negligible at higher (n>0.23) values (see Table 1). The initial decrease of 1; reflects the overall phenomenon of pore structure alteration due to functionalization. Higher ~ values are normally associated with wider hysteresis loops provided that there in no substantial interference of capillary condensation metastability effects that can give rise to enhanced hysteresis phenomena and wider hysteresis loop. But clearly this is not the case h e r e - the adsorption/desorption loops are getting rather smaller (see figure 1) which means that the STPI functionalization does not result in the development of additional hysteresis. Finally we should add a comment for the difference observed between the specific surface areas Sp and ScPsM (Table 1). Evidently, a clear drop of ScPsM with respect to the degree of functionalization (n) was observed and this drop moves parallel to the drop of Sp. The ScPsM values are higher by 26 to 75 m2g'lwhich is 8-23% higher than the corresponding Sp ones but in satisfactory agreement with those calculated from the Langmuir equivalent model (Table 1) over the linearity region P/P0=0.02-0.12. The difference between BET and CPSM surface areas may be attributed to the idealizations involved in both methods. For instance, the BET theory assumes the presence of a flat pore surface area whereas the CPSM model the presence of cylindrical pores. The latter method evaluates cumulative surface areas from the respective pore surface area distributions, it does not involve the evaluation of a monolayer capacity and unlike the BET method it does not depend on the selection of a BET-plot linearity region. However, CPSM surface areas do depend on the quality of fitting of the gas sorption hysteresis data. 5.
CONCLUSIONS The gradual functionalization of an commercial SiO2 with silano-(trimethoxy)-propylimidazole group to an extend equal to n = 0.00, 0.23, 0.30, 0.40, 0.52, 0.60 and 0.85 of its surface acid sites results in a gradual drop of specific surface area Sp (m2g1) from-~280 m2gl to 210 m2g1 and the specific pore volume Vp (cm3g"1) of the original material from 1.14 cm3g 1to 0.77 cm3g~. The Dmax of the PSD somehow moves to lower values while the variance 2~ of distribution remains almost constant. The mean connectivity c of the porous network, calculated according to the method of Seaton, drops from c --12.4 at n=0 to c -~3.0 at n=0.85. The Tortuosity ~ of the seven samples, calculated according to the Corrugated Pore Structure M o d e l - CPSM, also appears to drop, presumably because of blocking of various pore channels. The same CPSM model, provide pore volume values VcPsM which are practically identical the one calculated using the classical method (Vp). The specific surface areas SsPcM
306 found via the CPSM simulation are by 8-23% higher that the Sp values found using the BET method applied to the experimental data. This is due to some overestimation of N2-adsorption volume in the range P/Po=0.05 -0.25 by the CPSM algorithm. It seems that the CPSM model can be used as an additional methodology for enriching the data we can extract from N2adsorption/desorption measurements with the values of the mean Tortuosity (~) of solids with random porous network. A definite trend of interrelation between connectivity c and tortuosity r is not clear for the moment, but there are indications that they move in the same direction- at least for the ranges examined in this work. However, the latter issue needs further investigation. REFERENCES
(1) Gregg S.J.; Sing, K.S.W., "Adsorption, Surface Area and Porosity", 2"a ed., Academic Press, London, 1982. (2) Rouquerol, F.; Rouquerol, J.; Sing, K.S.W.; "Adsorption by Powders and Porous Solids", Acad. Press, 1999. (3) Brinker, C.J.; Scherer, G.W.; "Sol- Gel Science: The Physics and Chemistry of Sol-Gel Processing", Acedemic Press, New York, 1990. (4) Trimm, D.L.; Stanislaus, A.; Applied Catal. 1986, 21, 215. (5) (a) Oliver, S.; Kuperman, A.; Ozin, G.A.; Angew. Chem. Ind. Ed., 1998, 37, 46, (b) Oliver, S.; Kuperman, A.; Coomds, N.; Lough A.; Ozin, G.A.; Nature, 1995, 378, 47. (6) (a) Mitchel I.V.; (Ed.), "Pillared Layered Structures: Current Tends and Applications" Elsevier Applied Science, Amsterdam, 1990; (b) Barrer, M.R., "Zeolites and Clay Minerals as Sorbents and molecular Sieves", Academic Press, New York, 1978. (7) Lowell, S.; "Introduction to Powder Surface Area", John Wiley and Sons, New York, 1979. (8) (a) Seaton, N.A.; Chem. Eng. Sci., 1991, 46, 1895. (b) Liu, H.; Zhang, L.; Seaton, N.A.; Chem. Eng. Sci., 1992, 47, 4393. (c) Liu, H.; Zhang, L., Seaton, N.A.; J. Colloid Interface Sci., 1993, 156, 285. (d). Liu, H.; Zhang, L.; Seaton, N.A.; Langmuir, 1993, 9, 2576. (e) Liu, H.; Seaton, N.A.; Chem. Eng. Sci., 1994, 49, 1869. (9) (a) Meyers, J.J.; Nahar, S.; Ludlow, D.K.; Liapis, A.I.; J. Chromatogr. A, 2001, 907, 57, (b) Meyers J.J.; Liapis, A.I.; J. Chromatogr. A, 1998, 827, 197. (10) Pomonis, P.J.; Kolonia, K.M.; Armatas, G.S.; Langmuir, 2001, 17(26), 8397. (11) Aris, R.; "The Mathematical Theory of Diffusion and Reaction in Permeable Catalysis ", Vol. 1, Clarendon Press, Oxford 1975, p25. (12) Satterfield, C.N.; "Mass Transfer in Heterogeneous Catalysis", M.I.T. Press, London, 1970. (13) Salmas, C.E.; Stathopoulos, V.N.; Pomonis, P.J.; Androutsopoulos, G.P.; Langmuir, 2002, 18, 423. (14) Carniglia, S.C., J. Catalysis, 1986, 102, 401. (15) Salmas C.E., Androutsopoulos, G. P., Ind Eng. Chem. Res., 2001, 40, 721. (16)(a) Androutsopoulos, G. P.; Salmas, C. E.; Ind. Eng. Chem. Res. 2000, 39, 3747, (b) Androutsopoulos, G. P.; Salmas, C. E.; Ind Eng. Chem. Res. 2000, 39, 3764.. (17)(a) Sklari S., Rahiala H., Stathopoulos V., Rosenform J. and P.J. Pomonis, Microporous and Mesoporous Materials, 2001, 49, 1; (b) Stathopoulos V.N., Ladavos A.K., Kolonia K.M., Skaribas S.D., Petrakis D.E. and P.J. Pomonis, Microporous and Mesoporous Materials, 1999, 31, 111; (c) Ladavos A.K., Trikalitis P.N. and P.J. Pomonis, J. Mol. Catalysis A: Chemical, 1996, 106, 241. (18)Kirkpatrick, S.; III-Condensed Matter, Eds. Ballian, R.; Mayward, R.; Toulouse, G.; North Holland, Amsterdam, 1979.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
307
Preparation of Porous Silica by Acid Activation of Metakaolins C. Belver, M.A. Bafiares and M.A. Vicente Departamento de Quimica Inorgfinica. Universidad de Salamanca. Plaza de la Merced, s/n. 37008 Salamanca. Spain. Different metakaolins were prepared from a Spanish natural kaolin, by calcination at various temperatures. These solids were submitted to acid activation under different conditions. The behaviour of these samples was studied by different techniques paying special attention to the study of their porosity. This treatment produces the modification of the starting materials and the result was the synthesis of porous silica. Optimal conditions of activation were found. 1. INTRODUCTION Kaolin is an abundant mineral in soils and sediments with a large variety of applications, such as paper coating, ceramic ingredient, rubber filler, cement [1] and adsorption of dyes [2]. This clay mineral is a 1:1 layer aluminosilicate in which an alumina octahedral sheet is fused to a silica tetrahedral sheet forming a layer that is held to another by hydrogen bonding. Isomorphic substitution and cationic vacants are close to zero, so this clay has no exchangeable cations. This structure is the cause of the chemical inertia of this material, it being quite difficult to improve its properties by chemical methods [3,4]. However, different studies have revealed that the amorphization of kaolin produces solids more reactive under chemical treatments. This amorphization can be performed by calcination, obtaining metakaolin by heating from 550~ to 900~ The metakaolin is an amorphous phase formed by the dehydroxylation of the octahedral layers and the deformation of the tetrahedral sheets [5-7]. Also the grinding produces the amorphization of kaolin [8], by means of delaminating [9,10]. Acid activation has long been known as a preparation method for porous materials. In general, the acid solution dissolves the octahedral sheets of the clay and produces a significant modification over the tetrahedral sheets [ 11 ]. The solids obtained have higher surface area and better adsorptive and catalytic properties than the original clays, depending on the starting material and the conditions of the treatment [12-15]. The activation of metakaolin has been reported only in a few articles, and the application of the solids obtained to different catalytic reactions has been described [3,7,16-18]. This study focuses on a systematic analysis of the chemical activation of kaolin with HC1 solutions under different conditions of time and temperature, with a complete discussion of the phenomena governing this process and a wide characterization of the solids obtained. Such a systematic study lacks in the literature and the interest in looking for new applications of natural clays justifies it. By this reason, we have carried out the acid activation of the metakaolins obtained by calcination of a kaolin between 600-900~ and a complete study of the solids prepared, where the porous properties of the prepared materials receive special attention, due to the high importance that these properties have over the industrial applications of the final solids.
308 2. E X P E R I M E N T A L 2.1 Synthesis The parent kaolin used proceeds from Navalacruz (Zamora, Spain). X-ray diffraction of the raw sample shows the presence of quartz, smectite and mica as impurities. The amounts of these species were reduced by aqueous decantation of the < 2gm fraction. The metakaolins were prepared by calcination of the < 2gm fraction at 600, 700, 800 and 900~ with a heating rate of 10~ rain 1. Calcination was maintained for 10 h, under air atmosphere. The solids obtained were named MK-T, where T indicates the calcination temperature. The metakaolins were treated with 6M HC1 solutions using the following conditions: 6 hours at room temperature and 6, 12, 18 and 24 hours at 90~ under reflux conditions. 6.0 g of the metakaolin were suspended in 180 mL of the acid solution and stirred during the times indicated. The leached samples were separated by centrifugation, washed until free of chloride anions and dried at 50~ The nomenclature employed for these samples is as follows: MK-T-HC1-C-T'-t, C being the acid concentration, T' the treatment temperature and t the time of the treatment. 2.2 Characterization methods The chemical compositions were determined by Inductively Coupled Plasma Spectroscopy (ICPS) and Atomic Absorption Spectroscopy (AAS), which were carried out by Activation Laboratories Ltd., Ancaster, Ontario, Canada. The phase transformations were studied by X-ray powder diffraction using a Siemens D-500 diffractometer with Ni filtered Cu K~ radiation. FT-IR spectra were measured by means of the KBr disk technique (1 mg sample/300 mg KBr) employing a Perkin-Elmer 1730 Infrared Fourier Transform Spectrometer. Nitrogen gas adsorption-desorption isotherms at 77K were performed on a static volumetric apparatus (Micromeritics ASAP 2010). Prior to each measurement the samples were outgassed at 110~ up to 501am Hg. The isotherms were obtained following a 40 points P/P0 table with values from 0.12 to 0.99. For low relative pressure P/P0<0.1, the injection of 4 cm 3 N2 g-1 doses was performed, equilibrating for 1.5 h. The surface area of the solids submitted to soft leaching conditions was studied on a Micromeritics Gemini apparatus, estimating their surface area with only four adsorption points. The SEM micrographs were carried out on a Digital Scanning Microscope Zeiss DSM-940. The samples were pre-treated with gold on a Bio-Rad ES 100 SEN Coating System.
3. RESULTS AND DISCUSSION The chemical composition of some acid leached samples is listed in Table 1. Acid leaching performed at room temperature does not significantly modify the composition of the original metakaolins, whereas treatment under reflux produces the diminution of the content in A1203. This decrease is more effective as the time of the treatment increases, about 90% at 6h and 95% at 24h. In these conditions, the acid solution is able to dissolve most of the A13+ cations from the octahedral sheets of the metakaolins; this situation caused the formation of solids with a high percentage of silica, about 85%. The behaviour of the different metakaolins was quite similar, except MK-900 whose A1203 content only decreased in 25%, probably because of a partial sintering of the metakaolin when calcined at such high temperature.
309 Table 1. Chemical composition, weight %, of the parent kaolin and leached samples from MK-600 series.
Sample Kaolin MK-600-HC1-6-RT-6 MK-600-HC1-6-90-6 MK-600-HC1-6-90-12 MK-600-HC1-6-90-18 MK-600-HC1-6-90-24
SiO2
A1203
Fe203
MgO
CaO
TiO2
K20
46.97 53.36 75.98 84.89 84.05 79.16
33.57 37.30 6.70 3.01 2.48 2.67
2.08 2.07 0.30 0.20 0.14 0.10
0.44 0.40 0.29 0.20 0.14 0.10
0.20 0.16 0.06 0.15 0.09 0.02
0.31 0.53 0.74 0.78 0.73 0.69
0.64 1.39 1.17 0.80 0.70 0.71
H20 15.27 4.66 14.25 9.85 11.57 16.48
Acid activation also reduces the percentage of the impurities, Fe203 and K20, which came from the mineralogical impurities of the parent kaolin. The dissolution of the smectite occurred in all samples [ 15,19], whereas the decrease in mica needed stronger conditions. The XRD patterns of kaolin and metakaolins (Fig. l a) clearly show the amorphization of kaolin produced during its thermal activation. The characteristic peaks from kaolin [20] disappear and instead a halo at 20 from 15 to 30 ~ due to amorphous silica appears. These XRD patterns also reveal the thermal stability of the mineralogical impurities. Mrvaca Qquartz Kkaolinite
a)
Mmica Q quartz
MK-900-HC1-6-90-6
b) MK-600-HCI-6-90-24
=..
] ~
I
~
I'
10
'
I
20
IM 500 cps
'
I
'
I
30 40 20/degrees
MK-600
'
I
50
'
I
60
10
20
30 40 20/degrees
50
60
70
Figure 1. X Ray diffraction patterns of: a) kaolin, metakaolins and the solids treated at 90~ for 6 h; b) MK-600 and the leached solids obtained from this metakaolin. Acid leaching of the metakaolins produces the increase of the intensity of the halo from amorphous silica when acid activation is carried out at 90~ The XRD patterns of the samples (Fig 1b) show how the formation of amorphous silica is more effective as the time of the treatment increases. The environment of the Si atoms changes during the acid activation of metakaolins, the A1 sheet is dissolved leading to the condensation of the SiO4 layers and the formation of amorphous silica [3,7]. When the conditions of the treatment are stronger, the content of A13+ decreases leading to a higher content of silica.
310
The formation of silica strongly depends on the parent metakaolin. The XRD patterns (Fig l a) show how the MK-900 sample is the least activated. The high temperature of the calcination employed causes the sinterization of the particles and complicates the attack of the acid. The behaviour of the other metakaolins is quite similar for all of them. Figure 2 shows the FT-IR spectra of kaolin and MK-600 metakaolin and the samples treated at 90~ (the treatment at room temperature did not modify the metakaolin). The spectrum of the metakaolin shows the structural changes produced during the calcination. The four A1-OH stretching bands, from 3700 to 3600 cm l [21-24], disappear due to the dehydroxylation, the water bands being observed at 3470 and 1630 cm -~. Calcination also removes the A1-OH bending bands [25]. The Si-O vibrations are also modified, the three stretching bands (1115-1000 cm -1) merge into one broad band at 1090 cm ~ and in the OSiO bending region only bands at 800 and 476 cm -I appear [8,26]. These changes evidence, as by XRD, the amorphization of the crystalline structure during the calcination of the kaolin.
.i MK-600
4000
3 6 0 0 3200
2800
2400
2000
1 6 0 0 1 2 0 0 800
400
Wavenumber/cm ~
Figure 2. Infrared spectra of kaolin, MK600 metakaolin and some leached solids.
The spectra of the acid leached samples show new bands at 1200 and 950 cm -~. The band at 1200 cm 1 is attributed to Si-O vibration, in addition to that at 1090 cm -1 [7]. The band at 950 cm -1 is assigned to the stretching vibration of silanol groups, species that are formed during the acid treatment [7,9]. This process leads to the reorganization of the SiO4 sheets after the dissolution of A13+ cations, giving rise to silanol and siloxane groups. In Fig. 2 it is possible to observe how the acid-treated solids show quite similar IR spectra, there are not many differences depending on the time of the treatment and on the calcination temperature, only the solids derived from MK-900 display less modified spectra, by the effect above mentioned. The SEM micrographs of the kaolin and metakaolins show plate shape particles for both types of solids (Figure 3a). Calcination of the kaolin produces its dehydroxylation but does not destroy its platy shape. For samples treated under reflux conditions the plates are dissolved and the micrographs (Fig 3b,c) show amorphous particles that coexist with the metakaolin plates. There are not many differences between the samples activated at different times, although the chemical composition indicated that the amount of A13§ removed by activation for 24 h was higher than during activation for 6 h.
311
Figure 3. SEM micrographs of MK-600 (left), MK-600-HC1-6-90-6 (centre) and MK-600HC1-6-90-24 (fight). The nitrogen gas adsorption-desorption isotherms of the metakaolins are classified as type II (BDDT classification [27]). They are almost reversible with a closed hysteresis cycle, indicating the absence of micropores. The samples obtained at room temperature show N2 isotherms similar to those from metakaolins. This treatment did not significantly modify the structure of the metakaolin and the porous properties of these samples are very close to those of the parent metakaolin. The samples obtained under reflux conditions for 6h show nitrogen gas adsorption-desorption isotherms different from those of the parent metakaolins. They show an increase of adsorption at low relative pressures and reach a plateau at intermediate values of P/P0. This kind of isotherm is classified as type I (BDDT classification, [27]) and it is characteristic of microporous materials. For treatment times higher than 6 h, the isotherms are analogous to those of metakaolins, classified as type II [27], which indicates the loss of the microporosity formed at lower times. The BET specific surface area values are listed in Table 2. Acid treatment at room temperature did not significantly modify the surface area of the metakaolin, thus confirming the low effectiveness of this treatment. Activation at 90~ during 6 h produces solids with relatively high surface area, ~ 200 m 2 g-l, when the kaolin is calcined from 600 to 800~ These high values are due to the formation of an amorphous silica phase during this acid treatment. However, the solid obtained from MK-900 has a surface area of only 59 m 2 g-l, due to the above mentioned sinterization of the kaolin at this temperature. For treatment times higher than 6 h, the surface area of the samples decreases until 22 m 2 g~ for the four series of samples. For the acid activation of other clays, such as montmorillonite [19] and sepiolite [13], this effect has been reported and explained in different ways. In our case, the characterization of the acid-treated samples showed that they did not have great structural differences with increasing leaching times, and the chemical composition only differed in about 5% (SiO2, A1203) when the time was higher than 6h. Thus, the lower surface area of the samples activated under strong conditions may be due to a change in the morphology and the shape of the silica particles formed, which could be caused by an amorphization of this silica.
312 Table 2. Porous properties of the kaolin, metakaolins and the acid leached samples: BET surface area (from BET method), external surface area, micropore surface area and micropore volume (from t-method). Sample Kaolin MK-600 MK-900 MK-600-HC1-6-RT-6 MK-900-HC1-6-RT-6 MK-600-HC1-6-90-6 MK-700-HC1-6-90-6 MK-800-HC1-6-90-6 MK-900-HC1-6-90-6 MK-600-HC1-6-90-12 MK-600-HC1-6-90-18 MK-600-H C 1-6-90-24 MK-700-HC1-6-90-24 MK-800-HC1-6-90-24 MK-900-HC1-6-90-24
SBET/m2g-1
SEXT/m2g -I
SMP/m2g -I
VMP/cm3g -1
18.2 10.6 8.0 13.4 6.9 219 172 209 59 25 24 22 22 23 23
16.6 9.8 7.8 10.4 -85 126 104 59 23 21 20 18.4 20 21
1.6 0.8 0.1 3.0 -135 46 105 0.4 2.5 2.7 1.6 3.5 2.9 1.8
0.0007 0.0003 0.0015 0.0648 0.0223 0.0511 0.0001 0.0011 0.0013 0.0007 0.0017 0.0014 0.0008
The t-plot analysis [28] was employed to enlarge the micropore study. The data and the t-curves for the leached samples are shown in Table 2 and Fig. 4, except for the less activated samples, whose SBETwere analyzed with only four adsorption points. The high surface area of the samples activated at 90~ for 6 h is due to the evolution of both the external and the internal surface, except for MK-900-HC1-6-90-6, only slightly modified during the acid treatment. The values varied between 84-126 m 2 g-i for the SEXT and between 46-135 m 2 g-1 for the SMp. The contribution of these effects depends on the parent metakaolin, for MK-600 and MK-800 it is quite similar whereas for MK-700 the higher contribution is from the external surface, which indicates the importance of the calcination temperature of the kaolin over the properties of the acid leached solids. As the time of the acid treatment increases, the external surface of the samples decreases considerably, to ~ 20 m 2 g-l, and the internal surface is reduced to values analogous to the parent kaolin, 2 m 2 g-l, thus indicating the loss of microporosity when the treatments became more aggressive. At the same time, the t-plots showed the presence of micropores in the samples leached for 6 h, while the samples treated for higher times did not show microporosity. The pore size distribution is displayed in Fig. 5 for the samples activated under reflux conditions during 6 h, calculated by the Dubinin-Astakhov method [28]. These samples have a similar pore diameter, with a value between 17 and 19 A but the intensity of the curves is different. This difference shows the influence of the starting metakaolin over the formation of porosity, where MK-900 shows again the worse properties. The samples activated at longer time did not show internal surface.
313 70 --I~ ~o~ ~tx~ ~---
0,012 o
9 9 9 9 60"
0,010 -
~
MK-600-HCI-6-90-6 MK-700-HC1-6-90-6 MK-800-HC1-6-90-6 MK-900-HCI-6-90-6
0,008-
9 IMK--600
9
i
30-
o
9
MK-600-HCI-6-90-6 MK-600-HCI-6-90-12 MK-600-HCI-6-90-24
_=
0,006-
0,004-
20
0,002 -
|1
9
9
|
9
3,5
9 9
|
4,0
9 |
9 |
9
9 |
4,5
Statistical Thickness/J~
Figure 4. t-plot curves of MK-600 and the acid leached solids from MK-600 series,
0,000.,
,
|
2'o Equivalent
, ~ 3o
,
40
Pore Diameter/~
Figure 5. Pore size distribution of the samples activated at 90~ for 6 hours.
The acid leaching of the metakaolins produces the dissolution of the A13+ layers leading to the distortion of Si-layers and the formation of silanol and siloxane species, which hold the platy morphology from the metakaolin. These rearrangements allow the generation of micropores in the layer structure, as was proposed by Okada et al. [7], during the first stages of the activation. But after longer leaching, the micropores present in the leached samples can collapse due to condensation reactions between the species that formed the silica particles, resulting in a decrease in both the external and the intemal surface, and in micropore volume. Recently, Temuujin et al. [9,10] have studied the acid leaching of mechanically amorphized kaolin, and attributed the decrease of micropores at longer leaching to the condensation between the Si-OH groups in the forming of a three-dimensional framework. 4. CONCLUSIONS Acid leaching of metakaolin removed aluminium from its octahedral sheets and produced the distortion of its structure, resulting in the formation, under optimal conditions, of porous silica with high surface area. The calcination of the kaolin must be performed between 600 and 800~ because higher temperatures produce the sinterization of the particles, preventing acid activation. The treatment carried out under reflux conditions during 6 h developed both the internal and the external surface of the solids and produced microporous silica. For leaching times higher than 6 h, a negative effect on the porous properties was observed, due to the amorphization of the silica formed. ACKNOWLEDGEMENTS The authors are grateful for financial support from the Spanish Comisi6n Interministerial de Ciencia y Tecnologla, CICYT (ref. MAT99-0956) and Junta de Castilla y Le6n (ref. SA26/00B). C.B. acknowledges the Predoctoral Grant (FPI) from the Spanish Ministerio de Ciencia y Tecnologia (MCyT) (ref. PN-05424782).
314 REFERENCES
1. H.H. Murray, Appl. Clay Sci., 17 (2000) 207. 2. R.G. Harris, J.D. Wells and B.B. Johnson, Colloids Surf., 180 (2001) 131. 3. R.J. Lussier, J. Catal., 129 (1991) 225. 4. C. Belver, M.A. Bafiares and M.A. Vicente, Chem. Mater., 14 (2002) 2033. 5. J.F. Lambert, W.S. Millman and J.J. Fripiat, J. Am. Chem. Soc., 111 (1989) 3517. 6. Q. Liu, D.A. Spears and Q. Liu, Appl. Clay Sci., 19 (2001) 89. 7. K. Okada, A. Shimai, T. Takei, S. Hayashi, A. Yasumori and K.J.D. MacKenzie, Microporous Mesoporous Mater., 21 (1998) 289. 8. R.L. Frost, E. Mak6, J. Krist6f, E. Horv/tth and J.T. Kloprogge, J. Colloid Interface Sci., 239 (2001) 456. 9. J. Temuujin, G. Burman, J. Amgalan, K. Okada, Ts. Jadambaa and K.J.D. Mackenzie, J. Porous Mater., 8 (2001) 233. 10. J. Temuujin, K. Okada, K.J.D. Mackenzie and Ts. Jadambaa, Powder Technol., 121 (2001) 259. 11. M.A. Vicente, M. Su~irez, M.A. Bafiares and J.D. L6pez Gonz~.lez, Spectrochim. Acta A, 52 (1996) 1685. 12. M.A.Vicente, M. Su~.rez, J.D. L6pez Gonz~.lez and M.A. Bafiares, Clays Clays Miner., 42 (1994) 724. 13. M.A. Vicente, J.D. L6pez Gonz~dez and M.A. Bafiares, Clay Miner., 29 (1994) 361. 14. M.A.Vicente, J.D. L6pez Gonz~.lez and M.A. Bafiares, Microporous Mater., 4 (1995) 251. 15. S. Mendioroz, J.A. Pajares, I. Benito, C. Pesquera, F. Gonzhlez and C. Blanco, Langmuir, 3 (1987) 676. 16. M. Perissinotto, M. Lenarda, L. Storaro and R. Ganzerla, J. Mol. Catal. A: Chem., 121 (1997) 103. 17. K.R. Sabu, R. Sukumar, R. Rekha and M. Lalithambika, Catal. Today, 49 (1999) 321. 18. C. Breen, S. Taylor, E. Burguin and M. Centeno, J. Colloid Interface Sci., 247 (2002) 246. 19. C. Pesquera, F. Gonz~ilez, I. Benito, C. Blanco, S. Mendioroz and J.A. Pajares, J. Mater. Chem., 2 (1992) 907. 20. G.W. Brindley and G. Brown, Crystal structures of clay minerals and their X-ray identification, Mineralogical Society, London (1980). 21. V.C. Farmer, Spectrochim. Acta A, 56 (2000) 927. 22. V.C. Farmer, Clay Miner., 33 (1998) 601. 23. R.L. Frost, J. Krist6f, E. Horv~tthy and J.T. Kloprogge, Spectrochim. Acta A, 56 (2000) 1191. 24. S. Shoval, S. Yariv, K.H. Michaelian, I. Lapides, M. Boudevilley and G. Panczer, J. Colloid Interface Sci., 212 (1999) 523. 25. R.L. Frost, Clays Clay Miner., 46 (1998) 280. 26. N.B. Colthup, L.H. Daly and S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, London (1990). 27. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by powders and porous solids. Principles, methodology and applications, Academic Press, London (1999). 28. B.C. Lippens and J.H. De Boer, J. Catal., 4 (1965) 319. 29. M.M. Dubinin and A. Astakhov, Adv. Chem. Series, 102 (1971) 69.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
315
The effect of particle shape on the filtration rate and shear strength of quartz and dolomite mineral filter cakes B. Benli GOniil, O. Ozcan Istanbul Technical University, Chemical Engineering Department, 80626, Maslak- Istanbul/TURKEY E- mail: [email protected] In this study, the effect of particle shape on the filtration rate and shear strength of the mineral filter cakes was investigated. Shear strength of the mineral filter cakes is dependent on the particle shape, size and surface tension as well as the applied pressure and saturation degree. One of the most effecting parameter of the shear strength of the mineral filter cakes is the particle size and shape. The x-ray diffractometer results showed that dolomite is rhombohedral and quartz is hexagonal. Also, the surface analysis of the two minerals by multipoint BET analysis showed that the surface area and pore volume of quartz are about two times higher than that of dolomite. The laboratory experiments showed that the filtration rate of quartz is greater about two times than that of dolomite. Although the two minerals have the same particle size, i.e. 15 pm, the shear strength of quartz is higher about 1.5- 2 times than that of dolomite at the same conditions again. These results clearly show that the shape factor of mineral (the mineral type) has a determining parameter on the filtration rate and the shear strength of the mineral filter cakes. 1. INTRODUCTION Factors influencing properties of the cake during the cake filtration are particle shape, size, packing and dimensions of the cake in addition to the properties of the fluids, interfacial properties and the other factors such as temperature, pressure gradient and the rate of displacement. Particle shape may be an important factor in determining the drainage characteristics (1). The theoretical description of the cake formation in the constant pressure filtration is based on Darcy' s Permeation Law, which describes the single-phase laminar flow of an incompressible fluid through a porous incompressible system due to an applied pressure difference. It can be written as follows (2, 3); dV L dt
=
-A-
I"/
AP R
(1)
where, dVL/dt, volumetric flow rate of liquid through the filter cake and filter medium, A, Filter surface, AP, Pressure difference, rl~ Dynamic viscosity of the liquid. The permeation resistance R is the sum of the cake and cloth resistance and can be calculated as follows:
316 (2)
R = r~ * hk + R~
Here, rc is the filter cake resistance, hk is the cake height and Rm is the resistance of the filter cloth, hk is calculated from the following equation: From the equations (1) to (4), the filtration equation can then be rearranged for the constant pressure filtration as follows: d t / d V = [(rlt. rc. K) / (2A 2 . A p ) ] . V + [(tit. Rm) / (A . A p ) J
(3)
After separating the variables and integrating within the limits; The filtrate quantity V=0 and V=Va for the cake formation on time t=0 and t--h. The following linear equation can be derived with a constant applied pressure difference. t VL - KC . VL + B
(4)
m
The determination of the specific cake and medium resistance can be extracted from the slope and axis intercept of the graph for the cake formation using Equation 6. It can be seen that, the inverse slope of Equation 6, 1/Kc, can be taken as the indication of the filtration rate, dVL/dt. When 1/Kc increases the filtration rate increases (4). The filter cakes obtained from mineral and chemistry industrials are mostly wastes and sent to the transportation and storage without any thermal operations. The stability properties of the filter cakes such as tensile, shear and compression strength are very important for the deposit of slimes (5). The shear strength of the mineral filter cakes is influenced by the particle size, shape and surface tension as well as the applied pressure and the saturation degree (6, 7). It was mentioned in the recent studies that particle shape has also a very substantial effect on the shear strength of the cake. The shear strength of the mineral filter cake is defined as follows; F r = (5) Dc
.
hc
D~ and h~ are the diameter and the height of the cake, where F is the force applied by an apparatus. The change of the tensile strength of calcite agglomerate as a function of a liquid saturation (so called Schubert Diagram) is given in Figure 1. Several saturation regions can be identified on this curve. S<0,3 represents a pendular saturation state or bridging range. For S>0,8 no liquid bridges exist in the capillary state and ends when the first liquid bridges form between the particles. The relation between maximum tensile strength and the saturation of the cake with the special case of the capillary entry pressure can be written as follows (9, 10): .
.
.
.
f':
7 cos6
(6)
where 7 is the surface tension of the mineral suspension, ~i is the contact angle, f is the shape factor of the mineral particles and x is the particle size as Sauter diameter. Between these states there is a transition region~ Both bridges and liquid filled regions contribute to the strength in this region. The aim of this study is to show the effect of particle shape and size on the filtration rate and the shear strength of the mineral filter cakes.
317
~6
" '
\
N/~ 2
1
i\
~h
!
[
Icalcite ,~.o,~Is[
- !
~=71pm ~ -
1,2
oz
! 1 oo
0,2
o,4 o,6 Saturation, s
o,a
Figure 1. Tensile strength of calcite agglomerate as a function of saturation degree, Schubert Diagram(8). 2. EXPERIMENTAL 2.1. Materials and Methods
Fine grained dolomite, quartz and calcite have been used in the experiments. The average particle sizes and sources of these minerals are given in Table 1. Tal 91e 1. Average particle sizes of used minerals Mineral
Particle Size, xso,
Dolomite" Quartz*
Source
16 15
Kale Maden, ~anakkaie, Turkey Kale Maden, (~anakkale, Turkey
Calcite
15
Kale Maden, (~anakkale, Turkey
Calcite Calcite
5,8 10
Blau Siegel, Germany Mahlung KH, Germany
Calcite
37
Juraperle MHS, Germany
SIKRON, SF800, Quartz
6
Quartzwerke GmBH, Germany
SIK ON, SF600, Quartz
10
Quartzwerke GmBH, Germany
..... SIKRON SH300, Quartz
34
Quartzwerke GmBH, Germany
,
The filtration of the prepared suspensions was performed using a standard pressure filtration unit (bought from BOKELA-Germany) at 1, 2, 4 and 6 bar pressure and laboratory vacuum apparatus at 340 mmHg.
318 The cakes obtained from the apparatus in the disc form have 10 mm height and 50 mm diameter. The preparation of the filter cakes from the suspensions having approximately %20 by volume concentration consists of two steps: The first step is the application of the filtration and the second step is the compression of the cake for about 20 minutes in the cell. The cakes obtained at these conditions were fully saturated with water. In order to obtain partially saturated cakes, these cakes were dried for a defined time in the open atmosphere in the laboratory. The measurement of the shear strength of these cakes were then made using the apparatus BOS which was designed in the laboratory by ourselves (7). 5~t polyester filter cloths were used as a filter medium. The filtering surface area had the constant value of approximately 19,63cm2. 3. RESULTS AND DISCUSSION: The experiments consisted of three parts. In the first part, the characterization of minerals was explained by using x-ray diffractometer and electron microscope studies. Also, it was performed electrokinetic's studies of suspension. The effect of particle shape and size on the vacuum and pressure filtration of minerals has been investigated in the second part of the study. At the last part, the comparison of the particle shape and size effect on shear strength of the mineral filter cakes was performed. 1. The Characterization of Minerals The purifications of the dolomite and quartz minerals were investigated by x-ray diffractometer. The results of the x-ray spectra of minerals are given in Fig 2. According to the JCPDS- International Center for Diffraction Data in the part of Inorganic Phases, it was identified that the example of quartz mineral is synthetic quartz with PDF number 33-1161. The example of dolomite is synthetic Dolomite with PDF number 36-426 and a little amount of synthetic Calcite with PDF number 5-586. The surface analysis results of the two minerals by Quantachrome Autosorb 1-C system using with multipoint BET analysis are given in Table 2.
Table 2. Multipoint BET results of the minerals Mineral Dolomite Rhombohedral Mineral Shape Group Surface Area, m2/g 0,938 0,0057 Pore Volume, cc/g Ave. Pore Diameter, ,& 245
Quartz Hexagonal 1,797 0,0165 367
Multipoint BET analysis showed that the surface area and average pore diameter of quartz are about two times higher than that of dolomite. These results are important especially in the random packing on the filtration and the compaction behavior of the cakes in shear strength measurements.
319 Isamo~. ~ 4
i d = n t i f ,i c a t i o n :
0 0
~ t
D
[counts] 2
~
0
0
(" ~
9
~
l
I)
~
""" ' ' " '" " ' _ ~ d "
I)C, I)
"''" ~ "~
I)
C
"
(' I)
'~r
'
~
'
D I) ( ' C I)
'
. . . .
~ d
(" 1111
'
"
"
~" . . . .
I) D 11111) 11
~o .....
' ....
I)
-)~""
~ - T
(a) I~amole~~c--,atg.X.O-t:~Q~
1OoOO
K ~
K K
2~oo
K
K K
K
K K
KK K
K K K K K
60
(b)
Figure 2. X-ray spectra of minerals a. Dolomite, b. Quartz (Quartz is labeled with K). 2. Filtration Behavior The effects of the pressure and the mineral shape groups on vacuum and pressure filtration are shown in Fig 3. The changes of the 1/K~ values as an indication of the filtration rate at different pressures for dolomite and quartz can be given as two results; 2,50 2,00 1,50 1,00
%f
0,50
9 Calcite
& Quartz
0,00 10
20
30
Particle Size, micron
Figure 3. The effect of mineral shape groups on vacuum and pressure
320 First, 1/Kc values increases with the increasing pressure for both of the two minerals. Second, the filtration rate of dolomite is about two times of quartz' s rate at approximately all pressures. In order to see the effect of the particle size on the pressure filtration at 6 bar for calcite and quartz, the data graphically given in Figure 4. 250
I
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
mQuartz
200
150
-
IE!
,
Dolomite ,
I
-
100
50
Vacuum
1bar
Filtration
2bar
4bar
Pressure,
bar
6bar
Figure 4. The effect of particle size on the pressure filtration at 6bar The two distinct particle size regions can be identified in the Figure 3. The first part of the curve, for the particle size approximately smaller than 15~, represents a sharp increase. For particle size greater than 15g, there is no sharp change on the filtration rate of both minerals.
3. The shear strength behavior of filter cakes To determine the changes in the shear strength of the filter cakes of dolomite and quartz experiments have been done in the seven different saturation degree under three different pressures. The results are given in Figure 5 and 6 for dolomite and quartz, respectively. The relation between the shear strength and the saturation is similar to the Schubert' s relation between tensile strength and saturation. According to the Figs. 5, 6 and Schubert's conclusion, the relation can be separated into three regions; 1. region: 0 ___S < 0,3 ; The bridging region in which the shear strength increases with the saturation. 2. region: 0,3 < S < 0,8 9The transition region in which the shear strength has a minimum value around S- 0,5- 0,6. 3. region: 0,8 < S < 1
9 Capillary region in which the shear strength has a maximum value.
321
Figs. 5 and 6 clearly show that the shear strength of the cakes increases with the increase of the compression pressure in all of the saturation values.
0000
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5ooo oooo
40000 T- .........................................................................
Dolomite
350001 .
i
Quartz
30000 &
/ - ~ , , ,
o
:,~176176 20000
y
-
\
15000
~ 5ooo ,~ r./3
0
0,0
I0000 i
1
0,1
0,2
,.
i
0,3
0,4
, ....
0,5
,. . . . . .
i
0,6
0,7
i
0,8
,,
0,9
1,0
Saturation, %
50O0
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
~.ur~ion,'/,
I " 2 bar [] 4 bar i 6 bar.----Polinom (4 bar) l
92 bar o 4 bar
Figure 5. The effect of the compression pressure on the shear strength of dolomite
96 bar ~Polinom (4 bar )
Figure 6. The effect of the compression pressure on the shear strength of quartz
Table 3 shows the effect of the particle size on the shear strength. For the effect of the particle size on the shear strength in the zero saturation degree (completely dried filter cakes) the following equation has been found. 1 r = a xb (7) For 0 _<S < 0,2; Bridging Region ~ a = (0,5 - 1) 106 and b = 1,4- 1,6 As it can be seen from the Table 3 that for the saturation region, S=0%, the shear strength is independent of the shape factor, namely independent of the type of the mineral. In contrast to that, as the saturation degree increases, the shear strength is absolutely dependent on the shape factor of the mineral particles. The reason is probably the change in the adhesion forces and liquid bridges between the particles with different shape factors in the more saturated regions. Table 3. The effect of particle size on the shear strength of filter cakes Shear Strength, N/m z Particle Size, g 6 lo
36212 . . . . 11192
37 45
4800
5000
.
Quartz
Calcite
s, 70- 80%
S, 0% 44501 35706 4737 3330
S, 70-80% ,
,
35980 34452 ' 22004 21882
S, 0%
42369 39791 4989 1690
322
4. CONCLUSIONS The results of the present study can be concluded as follows: 9 Multipoint BET analysis showed that the surface area and pore volume of quartz are about two times higher than that of dolomite. 9 The filtration rate of the minerals is dependent on the particle shape and size as well as the applied pressure. 9 Shear strength of the mineral filter cakes is mostly dependent on the particle size. Shear strength increases with the decreasing particle size. 9 Particle shape is also an important parameter for the shear strength depending on the saturation of the filter cakes. For the fully dried filter cakes, shear strength is independent of the shape factor. The relation between the shear strength and the particle size and the other related parameters for the saturation region S=0% can be written as: 1 r
=
a
b
(8)
X
For 0 _<S < 0,3; Bridging Region ~ a = (0,5 - 1) 106 and b = 1,4- 1,6
5. REFERENCES [ 1] R. J., Wakeman, Filtration Post- Treatment Processes, Amsterdam, 1975. [2] H.,Anlauf, Filtrieren und Separieren, No.8 (1994) 63. [3] H., Darcy,. Victor Dalman (ed.), Les Fontaines Publiques de la Ville de Dijon Libraire des Corps Imp6riaux des Ponts et Chauss6es et des Mines, Paris, 1856. [4] J.M., Coulson, J.F., Richarson, Chemical Engineering, Vol.2, Third Edition, UK. 1987. [5] O., Ozcan, B., Benli G0ntil, N., Bulutqu, 8th World Filtration Congress, UK, 2000. [6] O., Ozcan, M., Ruhland, W., Stahl, Int. Journal of Mineral Processing, No.59 (2000) 185. [7] B., Benli, The investigation of the parameters effecting the shear strength of the filter cakes, PhD Thesis, Istanbul Technical University, 2001. [8] G., Barthelmes, Einfluf3 der Entw~isserungsbedingungen auf das Festigkeitsverhalten von feink0migen Filterkuchen, Diplomarbeit, Universit~t Karlsruhe, 1995. [9] H., Schubert, Untersuchungen zur Ermittlung yon Kapillardruck und Zugfestigkeit yon feuchten Haufwerken aus k0rnigen Stolen, Dissertation, Institut for Mechanische Verfahrenstechnik und Mechanik der Universitat Kadsruhe (TH), 1972. [10] 0., Ozcan, B. Benli GOn01, A.N., Bulut~}u, H., Manav, Colloids and Surfaces ,4: Physicochem. Eng. Aspects, No. 187 (2001) 405
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
323
Characterisation of silica low-density xerogels in presence of additives by image analysis and nitrogen adsorption-desorption S. Blachera, C. Ali6a, C. Gommes a, P. Lodewyckxb, R. Pirard a and J.-P. Pirard a aLaboratoire de G6nie Chimique, B6a, Universit6 de Li/~ge, B-4000 Li/~ge, Belgium bBelgian Army Service of Technological Applications, NBC Division, Martelarenstraat 181, B- 1800 Vilvoorde (Peutie), Belgium Micro- and mesopore structure modifications in silica low-density xerogels induced by additives in TEOS-based alcogels precursors are discussed on the basis of image analysis of transmission electron microscopy (TEM) micrographs and nitrogen adsorption-desorption experiments. To perform image analysis, novel image processing has been developed on the basis of classical signal treatment and mathematical morphology theory. The obtained results have been correlated with classical and fractal interpretation of nitrogen adsorptiondesorption measurements.
1. INTRODUCTION Silica aerogels are highly porous materials obtained via supercritical drying of cluster assembled gels. The absence of capillary forces during the supercritical drying process allows most of the porosity to be preserved. Aerogels are characterised by a hierarchic continuous porosity, a high surface area and a microstructure that depends on the synthesis conditions. Because of their complex texture, aerogels are very attractive material for catalysis. Nevertheless, cost and risk of the supercritical drying technique limit large-scale aerogel production. Recently, some of us have developed a new process for preparing silica xerogels with similar textural properties as silica aerogels, i.e. a structure formed by a complex hierarchy of micro- meso- and macropores, instead of supercritical drying, drying under vacuum at 150~ has been applied [1, 2]. The xerogels were prepared by adding, before gelation, 3-(2aminoethylamino)propyltrimethoxysilane (EDAS) to a tetraethylortho silicate (TEOS) sol prior to gelation taking place in a single base-catalysed (NH 3) step. It has been shown that varying the EDAS/TEOS ratio, textural properties can be modified. Transmission electron microscopy (TEM) observations, nitrogen adsorption-desorption and mercury porosimetry measurements indicated that increasing EDAS/TEOS ratio results in: (a) a decrease of the building block particle size, (b) an increase of the specific surface area (SBEr), (C) an increase of mesopore volume determined at saturation pressure of N 2 (Vp) and a decrease of the total pore volume (V v) (d) a general shift of the pore size distribution towards smaller pores, (e) an increase of the pressure of transition (Pt), above which mercury can intrude the sample without destroying the pore structure [ 1-3]. To explain this behaviour
324 it has been argued that EDAS acts as a nucleation agent for the formation of the silica particles. The decrease of the particle size with the increase of the EDAS/TEOS ratio would be responsible for the texture changes and for the singular sample behaviour under mercury porosimetry measurements. In this work we discuss in detail the texture modifications produced by this additive in a limited pore size range (pore width < 50 nm). To achieve this goal, the influence of the EDAS/TEOS ratio on the aggregate size of silica particles is determined by image analysis on TEM micrographs. Then, classical and fractal models are used to analyse adsorption-desorption nitrogen measurements.
2. EXPERIMENTAL The xerogels were synthesised via a sol-gel process, i.e. hydrolysis and condensation of tetraethylorthosilicate (TEOS) and a certain proportion of additive, 3-(2aminoethylamino)propyltrimethoxysilane (EDAS), in an alcoholic solution [1]. TEOS and EDAS were hydrolysed by an aqueous 0.18 M NH4OH solution in ethanol at room temperature and under magnetic stirring. The dilution parameter R = ethanol/(TEOS + EDAS) was taken equal to 10. The hydrolysis ratio is the molar ratio H = H20/(main reagent + 3/4 additive) (factor 3/4 being due to the fact that these additives contain only three hydrolysable groups). H was taken equal to 4 for all samples. The gelation, ageing, drying and calcination conditions were identical to those employed in [1]. The EDAS/TEOS ratio in the seven samples considered in this study are: 0.025 (EDAS/E-1), 0.04 (EDAS/E-2), 0.06 (EDAS/E-3), 0.1 (EDAS/E-4), 0.15 (EDAS/E-5), 0.2 (EDAS/E-6). Nitrogen adsorption isotherms measurements were performed at 77 K on a Micrometrics ASAP 2010M and on a Fisons Sorptomatic 1990. Each isotherm point is acquired when the pressure variation, within a fixed time, is lower than a fixed pressure deviation. Xerogel samples were impregnated with an epoxy resin and 60 nm slices were cut. TEM micrographs with magnifications between 34000 times and 64000 times were digitised on a matrix of 750x575 pixels with 256 grey levels. Five images of different areas of the same sample were analysed. Grey level image filtering and image binary processing were performed with the software 'Visilog5.0' from Noesis (France).
3. RESULTS
3.1. Image analysis Figures la and b show typical TEM micrographs of xerogels cuts. It can be seen that xerogels are composed of a complex structure of aggregates where the building block particles size depends on the EDAS/TEOS ratio [1, 2]. The quantitative analysis of this type of morphology requires knowing the density, length per surface unity (Figure 1) and the mean size of the aggregates. To achieve this goal, preliminary grey level image transformations and binary image processing have been performed according to the foUowing steps: (a) enhancement of local discontinuities or edges that delimit the darkest regions by gradient techniques [4], (b) thresholding of the resulting grey level image with the purpose to extract only the darkest regions. Threshold value is chosen automatically using Otsu's method [4], which minimizes the intraclass variance of the thresholded black and white pixels, (c) erosion transformation to eliminate small artifacts, (d)
325 image reconstruction using the eroded image as marker, (e) smoothing of the aggregate boundaries by a closing transformation. Figures lc and ld show the resulting binary images that were used for the following measurements: (a) aggregate density (~5), defined as ~5 = total area of aggregates/area of the image, (b) length density (~), defined as ~SL=total length of the skeleton aggregates/surface of the image, (c) mean diameter of aggregates ( D ), defined as D = total surface of the skeleton aggregates/total length of aggregates, (d) morphological diameter ( D morph)defined as D morph= 2 x mean value of the local maximum of the distance function. The distance function is represented in Figures le and If. In these images, every pixel belonging to the binary image of aggregates is represented by a grey level value (colour) equal to the shortest Euclidean distance from the pixel to the boundary. Then, as each local maximum corresponds to a local radius, local maxima mean value takes into account the various thickness of the aggregate structure. D = D mo~ for cylindrical structures.
Figure 1. Transmission electron micrograph of EDAS/E-1 (a) and EDAS/E-3 (b) samples.The line corresponds to a 50 nm length. Corresponding binarized images (c) and (d). Illustration of the distance function. Images (e) and (f) show the successive erosions of the binarized images [4]. m
Mean values and standard deviations for 8, ~SL, D and D mowhare reported in Table 1. TEM observation of EDAS/E-6 samples requires a large magnification because of the small particle and aggregate sizes. Unfortunately, at this magnification the obtained images are not contrasted enough for reliable two-dimensional image analysis to be performed. The density of aggregates, ~5, increases with the EDAS/TEOS ratio, reaches a maximum at a ratio = 0.06 and then decreases for larger values. On the contrary, ~ increases with increasing EDAS/TEOS ratio. D and D mowhfollow the same trend, i.e. they decrease with increasing the EDAS/TEOS ratio. The gap between D and D mowh indicates that aggregate branches present a high
326 thickness variation. Maximum gap appears for a ratio = 0.06. It must be noticed that image analysis measurements are scale-dependent. Indeed, magnification of TEM micrographs has been chosen in the mesopore range, which allows to well distinguish the aggregates. Then, taking into account the complex structure of samples, the measured densities represent the mass concentration at this scale. This is the reason why there is not a simple relation between this local measurement and bulk density.
Table 1. Textural properties of samples obtained by image analysis
EDAS/TEOS Ratio (Sample)
8
8L
D
(nm-1)
(nm)
D ~o~ph
(nm)
0.025 (EDAS/E- 1)
0.32+_0.03
0.040__.0.003 25.0+-1.7
29.0+_2.7
0.040 (EDAS/E-2)
0.46+_0.04
0.051+_0.002
19.8__.0.9
22.3+-2.1
0.06 (EDAS/E-3)
0.53_+0.05
0.051+-0.003
19.8+_1.1
23.9__.1.7
0.10 (EDAS/E-4)
0.47__.0.01
0.065+_0.003
15.4_+0.7
18.7___1.3
0.15 (EDAS/E-5)
0.28+-0.01
0.070+_0.003
14.2+-0.5
16.3+_1.1
0.20 (EDAS/E-6)
a .
a .
a .
a
.
aTEM observations of EDAS/E-6 samples are not contrasted enough for reliable twodimensional image analysis to be performed.
3.2. Nitrogen adsorption-desorption isotherms analysis Nitrogen adsorption-desorption isotherms measured for the studied samples were presented previously [ 1, 2]. In all cases isotherms are a combination of type I for low pressure and type II for high pressure according to BDDT classification [5] and t-plots show a downward deviation, which is characteristic of micropores [5]. Table 2 shows that the specific surface SB~ and the Dubinin micropore volume VD increase with the EDAS/TEOS ratio. Figure 2a presents the evolution of the cumulative surface distribution over the micro- and the mesopore range obtained by applying the Brunauer and the Broekhoff-de-Boer methods in their respective domains. For low EDAS content, 0.025 < EDAS/TEOS _<0.06, the structure is mainly microporous, whereas for high EDAS content, 0.1 _< EDAS/TEOS _< 0.2, mesopores are also present. Indeed, a large mesopores distribution
327
developed for EDAS/E-4. From EDAS/E-4 to EDAS/E-6 (0.1 < EDAS/TEOS < 0.2) mesopore distributions become more and more narrow and shift towards the smallest mesopores. The evolution of the specific surface of mesopores SBdBwith the EDAS/TEOS ratio is shown in Table 2. In order to complete the study of the structural differences in xerogels with various additive concentrations, fractal analysis has been performed on nitrogen adsorptiondesorption data. The generalisation of the classical Frenkel-Halsey-Hill (FHH) equation [6, 7] to fractal surfaces has been used. In this approach a film thickness of more than 1.5 layers, is used to probe the geometry of the surface. It describes the multilayer coverage of a fractal structure by the equation [7]: V/Vm ~ (In(p/p))(-l/m)
(1)
m = 3/(3-D s) for m = 1/(3- Dp) for
(la) (lb)
with m>3 m<3
where V is the adsorbed volume at the relative pressure P/P0, P0 is the saturation vapour pressure of the adsorbate and Vm is the adsorbed volume in a monolayer calculated by the BET theory, Ds;P is the surface/pore fractal dimension of the solid. Eq (la) characterises the polymolecular adsorption or FHH regime, in which the dominant force is the substrate potential. Eq (lb) is valid for the capillary condensation (CC) regime, i.e. when the liquid-gas surface tension forces control the interface.
!
I
.... EDAS/E-1
600 (a)
t'q
o k
400
l~NN,,~ 300
.~. ~ 200
(b)
--EDAS/E-2 --EDAS/E-3 --EDAS/E-4 .........E D A S / E - 5 EDAS/E-6
500
i
0
5
-3
N 100
-4
0
-5
0
10
20
30
D (nm)
40
50
-4
.... E D A S / E - 1 -- EDAS/E- 2 --EDAS/E-3
--EDAS/E-4 : EDAS/E-5 ---EDAS/E-6 I
I
I
-3
-2
-1
,
I
I
0
1
'
2
3
log(In(p/p))
Figure 2. (a) Cumulative surface area as function of pore diameter obtained by applying Brunauer and Broekhoff-de-Boer methods, (b) FHH plots (Equation 1). The exponents m and the linear range A(p/po) used for their extraction are presented in Table 2. Figure 2b shows the log-log plot of the V/Vm versus ln(p0/p) and Table 1 presents the values of m obtained with the best fit in the linear region and the pressure length scale in which self-
328
similarity extends. In the range 0.025 <-EDASfrEOS <-0.06, m>3 that signifies that FHH adsorption regime is dominant. On the contrary, m<3 in the range 0.1 <-EDAS/TEOS <_0.2 indicates that adsorption is led by capillary condensation. From equations l a and l b, increasing the EDAS content results in a change of the xerogel structure from a surface to a pore fractal. Table 2. Textural properties of samples by obtained by nitrogen adsorption-desorption measurements
EDASfrEOS Ratio (Sample)
SBET VD (mE/g) (cma/g) +5 _+0.01
SBdB (mE/g) _+5
Bulk densit), (g/cm =) _-*0.02
m, A(p/po)a
0.025 (EDAS/E-1)
345
0.14
15
0.28
7.14, (0.14-0.95)
0.04 (EDAS/E-2)
370
0.15
18
0.30
3.45, (0.054-0.94)
0.06 (EDAS/E-3)
415
0.17
25
0.33
3.45, (0.035-0.97)
0.1 (EDAS/E-4)
425
0.17
148
0.37
2.86, (0.034-0.97)
0.15 (EDAS/E-5)
520
0.21
268
0.41
2.56, (0.14-0.84)
0.2 (EDAS/E-6)
615
0.23
347
0.56
2.22, (0.025-0.92)
~A(p/po) is the fit range used for the extraction of exponents m. Let us note that the target density for all samples is 0.07 g/cm3, the skeleton density is around 2.10 g/cm 3 and the final bulk density obtained without any precaution during drying is around 0.7-0.8 g/cm 3. The actual bulk density is presented in Table 2. 4. DISCUSSION
It is well known that aerogel and low-density xerogel porous structure is highly dependent on synthesis conditions, especially the pH of the precursor solution. From considerations based on the dynamic of the hydrolysis-condensation equilibrium during the sol-gel process, silica gels may occur mainly by two types of growth mechanisms: the condensation of monomers with clusters or the condensation reactions of cluster with monomers or clusters [8]. The first class, which results from a sol-gel process under neutral or basic conditions, gives rise to a compact branched cluster structure. The second class, synthesised under acidic conditions, results in lightly cross-linked chains of smaller particles. Both kinds of structures,
329 named "colloidal" and "polymeric" respectively, present a self-similar structure as has been proven using independent techniques, such as small angle X-ray and neutron scattering (SAXS and SANS), gas adsorption methods and mercury porosimetry [8]. Colloidal aerogels have been identified as surface fractal objects whereas polymeric aerogels are pore fractal ones. In previous papers [1, 2] some of us have demonstrated that the use of EDAS as additive to TEOS-based alcogels allows obtaining low-density silica xerogels with textural properties similar to those of silica aerogels. It has been shown that the role of this additive was double: on one hand, it acts as a nucleation agent, and on the other hand, it slightly increases the basicity of the precursor solution. This last behaviour results in a decrease of the building block silica particles that form xerogel aggregates[I] and in an increase of the overall shrinkage of the gels upon drying (see in Table 2, final apparent density of the samples). At low EDAS content, a micro- and macropore texture was revealed by nitrogen adsorptiondesorption and mercury porosimetry experiments, respectively. At higher EDAS content, when the size of silica particles decreases and the basicity slightly increases, pore size distribution shifts towards smaller pores and macroporous structure disappears. To better understand the influence of the additive on the xerogel structure we studied in this work the evolution of the micro- and mesopore texture when the EDAS/TEOS ratio increases. Image analysis has been performed on transmission electron micrographs; local density, length density and average diameter of aggregates have been determined. These measurements indicate that aggregates are longer and thinner for higher EDAS content. The gap between mean and morphological diameter is an indication of the tortuosity. The local density, ~5, increases for EDAS/TEOS<0.06 and then decreases for higher value of EDAS/TEOS ratio, which indicates that mesopores appear. However, the bulk density increases from 0.28 to 0.56 g/cm3 in the studied EDAS/TEOS range mainly due to the disappearance of macropores [2]. Both classical and fractal analysis of adsorption-desorption isotherms of the studied xerogels show a sharp frontier between two kinds of structures. This texture evolution is summarised in the cumulative surface curve (Figure 2a) in which the contribution to the specific surface is mainly due to micropores for low additive content whereas for high additive content an equivalent mesopore specific surface is developed (see Table 2). In fractal analysis, the behaviour described above is expressed by a self-similarity that extends over a large pressure interval. In this pressure range (mainly mesopore range), a cominuous decrease of the parameter m (Equations la and lb, Table 2) as the ratio EDAS/TEOS increases indicates that silica particles arrangement evolves from a compact structure with a very strong surface roughness to a sparsely cross-linked network with a hierarchy of pores spanning between the interconnected aggregates of smaller particles. In terms of fractal geometry, the first kind of structure is characterised by a surface fractal dimension D s whereas the second, by a pore fractal dimension Dp. This picture, which agrees with image analysis results described above, indicates the paradoxical influence of the additive, which on one hand causes a global sample densification and on the other hand, induces at certain scales more open structures. We think that this behaviour is related with the double role played by the additive, which simultaneously acts as a nucleation agent and increases the basicity of the solution. To complete this study, SAXS measurements will be performed in the future.
330 5. CONCLUSIONS Micro- and mesopore structure modifications produced in silica low-density xerogels by additives (EDAS) in TEOS-based alcogels precursors were discussed on the base of data derived from image analysis of transmission electron microscopy (TEM) micrographs and nitrogen adsorption-desorption experiments. Image analysis allows determining local density, 6, length density and average diameter of aggregates. These measurements indicate that aggregates are longer and thinner for higher EDAS content. Local density increases for low EDAS content and then decreases for higher values of EDAS/TEOS ratio, which indicates that mesopores appear despite the fact that the overall bulk density increases mainly due to the disappearance of macropores. Both classical and fractal analysis of adsorption-desorption isotherms of the studied xerogels show a sharp frontier between two kinds of structures. Indeed, at the studied scale (micro and mesopore range), around molar EDAS/TEOS ratio equal to 0.06, xerogel structure changes from a 'colloidal-like' arrangement to a 'polymeric-like' one.
Acknowledgements The authors thank the Belgian Fonds National de la Recherche Scientifique, the Region Wallonne -Direction G6n6rale des Technologies, de la Recherche et de l'Energie-, the Minist6re de la Communaut6 franqaise -Direction De la Recherche scientifique- (action concert6e 00/05-265) and the Fonds de Bay for their financial support.
References 1. C. Ali6, R. Pirard, A.J. Lecloux, J.-P. Pirard, J. Non-Cryst. Solids 246 (1999)216 2. C. Ali6, F. Ferauche, R. Pirard, A.J. Lecloux, J.P. Pirard, J. Non-Cryst. Solids 289 (2001)88 3. C. Ali6, R. Pirard, J-P. Pirard, J. Non-Cryst. Solids 292 (2001)138 4. M. Coster, J.-L.Chermant. Pr6cis d'analyse d'image, CNRS, Paris, 1985 5. A.J. Lecloux,: in Catalysis, Science and Technology, (eds.). J.R. Anderson and Boudadrd, Vol. II (Springer, Berlin, 1981. 6. D. Avnir, M. Jaroniec, Langmuir 5 (1989) 692 7. O. Pfeifer, M.W. Cole, J. Chem. 14 (1990) 221 8. C. Brinker, G. W. Scherer, Sol-Gel Science, Academic Press, New York, 1990
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
331
Texture characterization of ultramacroporous materials using non-destructive methods S. Blachera'c, V. Maquet b'~, A. L6onard a, G. Chapelle b, M. Crine a, R. J6rSmeb'c, J.-P. Pirard a aDepartment of Chemical Engineering bCenter for Education and Research on Macromolecules Clnterfacultary Center for Biomaterials, University of Liege, Sart-Tilman, B6, B-4000 Liege, Belgium
Image analysis, impedance spectroscopy and X-ray microtomography have been used as non-destructive methods in order to characterize ultramacroporous materials (pore width > 1 lam). These methods are used to investigate the texture of different kinds of materials, i.e., highly oriented polylactide (PLA) foams prepared by freeze-drying, isotropic polyolefm foams prepared by thermal degradation of a chemical foaming agent and convective heat dried wastewater sludges. The image analysis of SEM micrographs recorded at different magnifications gave information on the pore size distribution and pore morphology. Impedance spectroscopy was used to investigate the transport properties of the threedimensional porous matrices by measurement of the water ionic conduction. Finally, 3D image reconstruction from 2D cross-sections obtained by X-ray microtomography allowed visualizing the pore structure.
1. INTRODUCTION The analysis of the porosity and texture of highly porous materials (pore width >1 tam) is a major issue in several domains, particularly in polymer foam science. Intrusion techniques, such as mercury porosimetry, are traditionally used to characterize foam textures. Nevertheless, various authors reported on the possible alteration of the porous structure under mercury pressure [1, 2]. Moreover, it has been shown that poly(L-lactide-co-e-caprolactone) foams shrunk under the high pressure required for Hg intrusion [3]. As a consequence, the classical Washburn theory cannot be applied to analyze the experimental data. Thus, mercury porosimetry must be used with great care and the data need to be confirmed by nondestructive methods. We recently demonstrated that image analysis, impedance spectroscopy and X-ray microtomography techniques can be used to characterize highly porous materials of various origins [4-7]. In the present study, these three techniques are presented as altemative non-destructive methods for the quantification of ultramacroporous materials in which fragility or large pore size are not compatible with the use of intrusion method. Image analysis and impedance spectroscopy were used to characterize three kinds of materials exhibiting
332
different morphologies, i.e., anisotropic PLA foams prepared by freeze-drying, isotropic polyolefm foams prepared by degradation of a chemical foaming agent at high temperature and convective heat dried wastewater sludges. The PLA foams are composed of an oriented network of ultramacropores of ca. 100 ~tm diameter dispersed in a ten times less macroporous matrix (i.e. median pore size of 10 lam). The orientation of the ultramacropores is parallel to the cooling direction and depends on both polymer concentration and crystaUinity. These highly oriented porous foams have a great potential as temporary scaffolds for supporting axonal regeneration in transected rat sciatic nerves [8] and spinal cords [9]. The second type of porous material, i.e., PE foams, is constituted by an isotropic network of ultramacropores where pore size distribution and pore connectivity depend on the composition of the foam formulation. Such kind of foams are used for industrial insulation and packaging applications [ 10 a, b]. For these two types of polymer foams, SEM micrographs of two-dimensional crosssections were recorded at different magnifications and the resulting images were analysed using traditional tools of mathematical morphology and signal processing [ 11]. The transport properties of the three-dimensional foam structures were assessed by the ionic conduction measured by the application of an alternative current to water-saturated polymer matrices. Finally, X-ray microtomography was used to visualize the 3-dimensional structure of convective heat dried wastewater sludges in which internal cracks result from liquid evaporation during convective drying.
2. EXPERIMENTAL Poly D,L-lactide (DL-PLA), or Resomer | R206 (Mn: 50,000) and poly L-lactide (L,PLA), or Resomer | L206 (Mn: 60,000, 70% of crystallinity), were supplied by Boehringer Ingelheim (Ingelheim, Germany). Macroporous foams were prepared by freeze-drying of polymer solutions in dioxane, as previously reported [12, 13]. The metallocene polyethylene (PE, Engage 8400, Dupont Dow Elastomers) foams were chemically foamed and chemically cross-linked ones, prepared by conventional foaming in oven [ 14]. As soft and pasty material we used wastewater treatment sludge collected in a domestic wastewater treatment plant of 9000 equivalent inhabitant, after secondary settling and thickening. Mechanical dewatering was proceeded at the laboratory to ensure the most reproducible material. Sludges were flocculated in a jar-test by adding 0.006 kg/kg dry solid of a cationic polyelectrolyte (Zetag 7587 from Ciba). Dewatering took place in a normalized filtration-expression cell (norm) under 0.3 M of pressure. Small cylindrical samples were extruded from the cake through a circular die of 12 mm like in some industrial belt dryers. Samples were cut at a height of 15 mm, yielding extrudates with volume and mass of approximately 1.7 cm 3 and 2 g, respectively [7].
2.1. Image analysis The polymer foams were cut in two directions, i.e. parallel and perpendicular to the surface and, the resulting transverse and longitudinal sections were sputtered with platinum for 120 s under argon atmosphere. Samples were investigated with a Jeol JSM-840 A at an accelerating voltage of 20 kV. SEM micrographs were analyzed by image analysis. Image treatment and statistic analysis were performed with a WorkStation Sun SPARC30, and the software 'Visilog5.0' from Noesis. Images were digitized on a matrix of 1024 x 1024 pixels with 256 gray levels. Five images of different areas of the same sample were analyzed.
333
2.2. Impedance spectroscopy 1 cm 3 specimen of each between two stainless steel electrical conductivity was supplied with the Chelsea frequency range.
foam was degassed in distilled water and placed in an UV ceil electrodes. The ceil was fiUed with distilled water and ionic AC measured by the Schlumberger Frequency Analyzer SI 1255, Dielectric Interface (Model CDI 4t) probing the 10-3-106 Hz
2.3. X-ray microtomography The X-ray microtomograph used was a "Skyscan-1074 X-ray scanner" (Skyscan, Belgium). The X-ray source operates at 40 kV and 1 mA. The detector is a 2D, 768 x 576 pixels, 8-bit X-ray camera with a spatial resolution of 41 tam. The sample, whose maximum size was a few cm 3, can be either rotated in a horizontal plane or moved vertically in order to get 2D scans at different vertical positions. The minimum vertical distance between two scans is 41 tam. The reconstruction of two-dimensional slices from the object was achieved by a classical back projection method.
3. RESULTS AND DISCUSSION 3.1. Image analysis characterization Figures la-d shows typical cross-sections of isotropic PE foams and longitudinal and transverse sections of PLA foams. In the first case, image analysis characterization was used to determine the macropore size distribution. In the second case, a complete morphological description requires the determination of the density, size distribution, orientation and dispersion of the ultramacropores as well as the orientation and the size distribution of the macropores. The preliminary steps of the image quantification, i.e., gray level image transformations and binary image processing are detailed elsewhere [ 15].
Figure 1. Scanning electron micrograph of (a) isotropic PE foam at a x20 magnification; (b,c) transverse sections of PLA foam at a xl0 (b) and xl00 (c) magnification; (d) longitudinal section of PLA foam at a xl00 magnification. (e-h) Extraction of ultramacropores (e, f), macropores (g) and pore orientations (h) of PE (e) and PLA (f-h) foams.
334 Figure le-h represents the final binarized images, which have been evaluated. For PE and PLA foams, transverse SEM sections (Figures l e and f, respectively), the distribution of the equivalent circular diameter of the ultramacropores, D m =~/4A/rc, where A = Number of pixels for each pore multiplied by a calibration constant, has been taken. Taking into account the complex morphology of the ultramacroporous in PLA foams, additional parameters for these pores have also been measured: (1) the ultramacropore density 5M, which is defined as 5M =Number of pixels characteristic of the ultramacropores / Number of pixels of the whole image, (2) the average distance between ultramacropores ()~), which is defined as the average nearest neighbor distance. The shape factor for the PLA macropores (Figure l g) has been determined. The factor shape (C) of an object, defined as C=perimeter2/4re x Area, describes the deviation of an object from a true circle. Finally, the area distribution (A) of macropores has been determined instead of the equivalent circular diameter which would be irrelevant in the case of macropores with irregular shape. Figure 2a-b shows the ultramacropore size distribution of both PE and PLA foams-while the macropore area distribution of PLA foams is illustrated by Figure 2c. Mean values, standard deviations and statistical parameters as the skewness (sk) and the kurtosis (k), of these parameters are reported in Figure 2. For PE foams the pore size distribution follows a decreasing exponential low, large pores are probably due to inhomogeneous porous growth during the foam formation. For PLA foams, the ultramacropore size distribution follows an almost gaussian law whereas the macropore area size obeys a log-normal distribution (Figure 2c). The mean factor shape value is indicative of a very elongated macropore morphology. i
|
,
|
1
,
,
,
i
|
|
|
,
|
|
,
0.6 0.4
~ 0.2
oo,,- Ihl]
sk=0.35,
II I I!1
~=
0.2
0.4 0.6 D(mm)
0.8
1.0'
0.1
Illl [
Jill /
llli +lt 1 ~-1
00
,
-I
IIIh
III!1 lllll_
T
,
|
g =(0.19+
~0.8
~
|
0.2 0.3 D(mm)
0.4
t
,
,
,_.,'
i'
,
,
,"'",'
,
,'T'
c /
A =(0.161 ___ J 0.124)10"2ram2t sk=2"76'k= 12"52 t C=1.88 _
0.59
t
|
0.5
0.5 21.0 2 A(10 n ~ )
1.5
Figure 2. Size distribution of the ultramacropores in PE (a) and PLA (b) foams. (c) Area size distribution of macropores in PLA foams. Spectral analysis, which is a standard technique for detection and quantification of orientation and/or periodicity of images, has been used for the characterization of pore orientation on gray level images [ 16]. This technique has been mainly applied to characterize the surface structure studied by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) [16]. This method is based on the calculation of the Fourier power spectra,
defined
in
the
frequency
or
k
space,
as
where
F(-k) ~ f(r)exp(-2~, r)dr, with r = (x, y), are the Fourier coefficients of the image. The
I
-
335
frequency space representation of a pure sinusoidal structure of orientation 0, consists of two points, located on each side of the 2D power spectra center, at a distance that corresponds to the wavelength of the original image and having the orientation 0+rd2. Usually, (S~)o
, where 13=0 corresponds to the equal power at all frequencies (white
noise), 15=1 to a random structure in which the altitude variations Af(x,y) are completely uncorrelated (pink noise), and 13=2 to a smooth surface. For anisotropic surfaces, the slope of a log S(k) v s . log frequency plot in function of the direction, i.e. a rose plot, allows evaluating the degree and direction of anisotropy [ 16]. Figures 3a and b show the 2D power spectra of porous structures represented in Figure la and c respectively, drawn in a perspective view, with power intensities represented in logarithmic scale. The power spectrum illustrated in Figure 3b presents patterns oriented around the line 0=0, which signifies that most of the pore edges in the original images are nearly vertically oriented. A fiat structure is surrounded by a halo that hides the frequencies related to the characteristic lengths of the original images. On the contrary, the power spectrum shown in Figure 3a is rather isotropic, with all the excited frequencies distributed in a central circle and surrounded by a halo with decreasing density. The corresponding rose plots (Figures 3c and d) exhibit the degree of anisotropy of the images shown in Figures la and c in the various directions.
Figure 3. (a, b) 2D power spectra of the SEM images illustrated in Figures la and lb respectively, drawn in a perspective view. (c, d) Normalized rose plots obtained from power spectra.
336 For the isotropic structure (Figure la) the rose plot is almost a circle (Figure 3c) whereas for the anisotropic image, a gradual variation of the slope with direction gives rise to noncircular patterns (Figure 3d). For the image in Figure la, the rose plot indicates that the majority of edges are oriented in the direction 0---~/2 but the rose pattern also includes a main orientation nearly 0--x/3. A direct observation of Figure l c suggests that the orientation 0--x/2 account for most of the largest channels that correspond to the ultramacropore structure, whereas the other directions characterize the edges inside those channels (ladder-like macrostructure). 3.2. Dielectric
properties
The conducting properties of a liquid in a porous medium can provide information on the pore geometry and the pore surface area [ 17]. Indeed, both the motion of free carriers and the polarization of the pore interfaces contribute to the total conductivity. Polymer foams are three-dimensional solids with an ultramacropore network, through which ionic species can migrate depending on the network structure. Based on previous works on water-saturated rocks and glasses, we have extracted information about the three-dimensional structure of the freeze-dried foams from the dielectric response. Let be e' and o" the dielectric constant and the conductivity, respectively. Dielectric properties are usually expressed by the frequencydependent real and imaginary components of the complex dielectric permittivity: e*(co) = e ' ( w ) - i e " ( w ) = e ' ( W ) e o
-i~
, where i = ~L-~, eo is the dielectric permittivity
OJ
of the vacuum, f is the frequency in Hz and co = 2 n f . Experimental data are usually shown as a log-log plot of e' and e" against frequency. Figures 4a and b compare the dielectric responses for PE and PLA foams, the electrodes being placed in the longitudinal and transverse directions. 1 0 -3
,..,~..-....-, ....~ ....~ ..... , , . - . ....~ ....~ ....,,..-, ..... cr ' ( F ) (a)
1 0 -4
cr ' ( F )
o
**ss~~o~o~;~ss* s"
+.§247247247 00000 00000000000 **********************
1 0 "s
.t
~,*
10 ~
10. 7
~Tm 1
1 0 -7
co1/2:1--0
10 -s
/o~Zo~-~"
~
1 0 "s r
PE
1 0 .9 lff5
(Longitudinal)
§ PE *
(Transverse)
Water co
....
_,
....
_1
....
1 0 "3
.m . . . .
-,
....
1 0 "1
--,
....
d
......a
lO 1
....
-,
....
lO 3
-,
....
-'
(Hz~ ...
lO s
.1..t-§
§
9
4- t -
10 "6
8 /
(b)
1 0 -4
~t
1 0 "s
1 0 3 L . . . ~ . , . - , , . . . - .... ~ ....~ ..... .... . ..... ..... , .... ~ .... . .....
r ....
lO 7
1 0 .9 1 0 "s
.
.
.
1 0 .3
! o
PLA(Longitudinal)
§
PLA(Transverse)
*
.
Water
. 10"1
.
.
.
101
.
...O.( H z ) : 10 3
l0 s
10 7
Figure 4. Dielectric response of water in PE (a) and PLA (b) foams, the electrodes being placed in the longitudinal and transverse directions. The dielectric response of pure water is added as a reference in order to compare free ionic conduction and ionic conduction within polymer foams. For pure water, ~'o~ C01for co <10lHz
337
which corresponds to the electrode polarization, o' being constant for higher frequency. The conductivity plateau width increases and its height decreases with increasing distance between electrodes. This behavior is characteristic of ionic conduction. For the PE foams, the dielectric response is independent of the sample orientation, which is not the case for the PLA foams. In these latter, the dielectric response of free water and of water within longitudinally oriented foams are almost superimposed whereas in the transverse direction, the size of the conductivity plateau is narrower and higher as compared with water response. This indicates that longitudinally oriented PLA foams exhibit a higher degree of connectivity than transversally orientated ones, in which the motion of charge carriers is confined to restricted areas.
3.3. X-ray microtomograph Figure 5 shows the 3D reconstruction of a PE loam (Figure 5a), and the bulk (Figure 5b) and porous structure (Figure 5c) of a wastewater sludge after drying completion. These images have been obtained from 16 2D cross-sections separated by 41 lam in order to visualize the porous structure and connectivity. Currently, image analysis is performed on each cross section to determine the 2D pore density and the total pore volume, which is obtained by multiplying the average sample height by the average pore area [7]. New developments concerning direct 3D image treatment will soon be addressed in our laboratory.
Figure 5. (a) 3D reconstruction of a PE foam, (b) 3D reconstruction of the bulk and (c) the pore structures of a pasty soft material after drying completion.
4. CONCLUSIONS Pore size, structure and morphology are key features for the characterization of highly porous materials. Mercury porosimetry is currently used to measure pore volume and pore size distribution. However, this technique is upper limited to pore sizes of 75 lam, and compression of samples due to mercury penetration under high pressure sometimes leads to incorrect pore size distributions [3]. In this study, three methods were presented to investigate the texture of highly porous materials: image analysis, impedance spectroscopy and X-ray microtomography. These techniques have the advantage to be non-destructive for the pore structure and to allow extracting information at different scales. The lower limit of image analysis is fixed by the quality of the SEM pictures. Analysis of SEM images or crosssections obtained by microtomography allows the porous texture to be determined with the
338
assumption of a homogeneous material. The success of this technique mainly relies on the use of a suitable image processing algorithm for obtaining accurate binary images. In this work, it has been shown that image analysis of SEM micrographs can be used to determine the pore density, pore size distribution and pore shapes of isotropic or anisotropic polymer foams. On the other hand, image analysis can be performed on microtomograph cross-sections of a dried soft material in order to characterize the crack morphology. Pore orientation is another key parameter that can be studied by spectral analysis on gray level images. Finally, impedance spectroscopy showed that the dielectric response is influenced by the proper structure of porous materials. Therefore, ionic conduction can be used to determine the degree of connectivity of the porous structure. As a conclusion, image analysis, impedance spectroscopy and microtomography are nondestructive methods able to provide valuable information on the texture of porous materials. These three techniques are complementary to mercury porosimetry, which does not allow the analysis of morphological details.
Acknowledgements The authors thank the Belgian Fonds National de la Recherche Scientifique, the Region Wallonne -Direction G6n6rale des Technologies, de la Recherche et de l'Energie-, the Minist~re de la Communaut6 fran~aise -Direction de la Recherche scientifique- (Action de Recherche Concert6e 00/05-265) and the Fonds de Bay for their financial support.
REFERENCES 1. J. Tija, P. Moghe, J. Biomed. Mater. Res. (Appl. Biomater.), 43 (1998) 291. 2. Y. Nam, J. Yoon, T. Park, J. Biomed. Mater. Res.( Appl. Biomater.), 5 (2000) 1. 3. V. Maquet, S. Blacher, R. Pirard, J.-P. Pirard, M. N. Vyakarnam, R. J6rSme, J. Biomed. Mater. Res., in press. 4. V. Maquet, S. Blacher, R. Pirard, J.-P. Pirard, R. J6rSme, Langmuir, 16 (2000) 10463. 5. S. Blacher, V. Maquet, R. Pirard, J.-P. Pirard, R. J6rSme, Colloids Surfaces A, 187-188 (2001) 375. 6. S. Blacher, V. Maquet, F. Schils, D. Martin, G. Moonen, J. Schoenen, R. J6rSme, J.-P. Pirard, Biomaterials, submitted. 7. A. L6onard, S. Blacher, P. Marchot, M. Crine, Drying Technology, in press. 8. V. Maquet, D. Martin, B. Malgrange, R. Franzen, J. Schoenen, G. Moonen, R. J6rSme, J. Biomed. Mater. Res., 52 (2000) 639. 9. V. Maquet, D. Martin, F. Scholtes, R. Franzen, J. Schoenen, G. Moonen, R. J6rSme, Biomaterials, 22 (2001) 1137. 10. (a) S. C. Smith, MetaUocenes '95, Int. Congr. MetaUocene Polym. (1995) 159. (b) C. P. Park, Foams '99, Int. Conf. Thermoplast. Foam (1999) 61. 11. M. Coster, J.L. Chermant, Pr6cis d'analyse d'images, (CNRS, Paris), 1985 12. C. Schugens, V. Maquet, C. Grandffls, R. J6r6me, P Teyssi6., J. Biomed. Mater. Res., 30, (1996) 449. 13. C. Schugens, V. Maquet, C. Grandflls, R. J6rSme, P. Teyssi6, Polymer, 37 (1996) 1027. 14. D. Klempner, K.C. Frisch. Polymeric Foams. (Hanser: Munich) 1991. 15. S. Blacher, V. Maquet, R. J6r6me, J-P. Pirard, Image Analysis Stereology, 21 (1) (2002) 43. 16. J. C. Russ, Fractal Surfaces, Plenum Press, New York, 1994. 17. R. Hilfer, Phys. Rev. B, 44 (1991) 60.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
339
Formation of hierarchically ordered silicas prepared by spray drying of nanosized spheres C. du Fresne von Hohenesche a, V. Stathopoulos a, K.K. Unger a, A. Lind b, M. Linden b Institut Rir Anorganische Chemie und Analytische Chemie, Johannes Gutenberg Universitiit, Duesbergweg 10-14, D-55128 Mainz, Germany Department of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5, FIN20500 Turku, Finland. We report on our strategy to synthesise defined arrangements of macro- and mesopores in one single material with spherical morphology using the spray drying technique. As starting materials, nano-sized silica spheres and colloidal suspensions were chosen. The intra-particle as well as inter-particle pore size was independently controlled which allowed to obtain materials with varied bimodal pore networks. The spherical agglomerates were characterised regarding their pore structural parameters and are potential adsorbents in liquid chromatographic separations. 1. INTRODUCTION Even today, adsorbents which are employed in separation techniques do not fulfil the following criteria: a high capacity combined with fast mass transfer kinetics. The solution to this problem is provided by nature itself. There, all adsorption processes are performed in a very effective manner using the principle of hierarchy. It is the ultimate goal to mimic this property and to manufacture a man-made adsorbent featuring a high degree of hierarchically ordered pore structure. For a given process such as the chromatographic separation technology certain aspects have to be met in order to design novel adsorbents: 9 Possibility to control the pore structural parameters either by in-situ synthesis or by posttreatment, 9 Ability to tune the surface chemistry by chemical reaction, e.g. silanisation or coating processes, respectively, 9 Control of particle morphology, particle size and particle size distribution, 9 Reproducible and rugged manufacturing process with up-scaling possibilities. The first step in the generation of hierarchical pore structured materials is the implementation of two different pore systems which build up a highly interconnected pore network in one single bead. Other morphologies, e.g. monoliths with a bimodal pore size distribution, have already been shown to have superior chromatographic performance by Nakanishi et aL [1,2,3].
340 The focus of this paper is on the manufacture and characterisation of spherical beads in the micron-size range which are build up by spherical nano-particles using the spray agglomeration procedure. 2. EXPERIMENTAL SECTION 2.1
Synthesis of Materials
The synthesis of the MCM-41 analogue spherical beads was carried out as following: 5.1 g of n-hexadecylamine (tech. 90 %) was dissolved in a mixture of 500 ml isopropanol (p.a.) and 450 ml of deionised water. After this, 6 ml of aqueous ammonia (32 wt%) were added. Then 29 ml tetraethoxysilane (TES 28) were added while stirring and the temperature was adjusted from 15 to 65 ~ The reaction time was 24 h and the product was isolated by filtration or directly spray dried or subjected to hydrothermal after-treatment. For hydrothermal treatment, l l of the synthesis mixture was diluted with l l deionised water, capped and sealed in pressure stable glass bottles and stored in an oven at 110 ~ for 24 h. Removal of the template was performed either by solvent extraction in isopropanol or by calcination at 823 K for 5 h after filtration. Agglomeration was performed using a Labplant SD 05 spray drier. 2.2
Characterisation
Adsorption and desorption isotherms were measured at 77.4 K on a Quantachrome Autosorb 6B (Quantachrome Corporation, Boynton Beach, FL, USA) using nitrogen. The samples were out-gassed at 423 K and 1 mPa for 14 h before adsorption measurements. Scanning electron micrographs were obtained using a Zeiss DSM 962 scanning electron microscope (Zeiss, Jena, Germany). Samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. 3. RESULTS AND DISCUSSION 3.1
Concept
The concept of this work is visualised in Fig. 1. Firstly, the primary particles have to be synthesised in the range between 10 and 1000 nm. These beads can either be porous or non-porous. Important features of the beads are their porosity, dimension and morphology. If non-porous primary particles are employed, the agglomeration step leads to products with pores generated by the inter-particle volume. In the case of porous primary particles agglomerates with a bimodal pore size distribution are obtained. The inter- and intra-particle pores are highly inter-connected. The target parameters of the agglomerates are therefore a controlled pore connectivity, modality and structure as well as a defined morphology and agglomerate size distribution. These features are decisive for application.
341
Synthesisof Mesoporous primaryparticles Non-porous Materials ~ Materials ~parameters) Agglomeration
Applications Figure 1: Concept for the synthesis of highly ordered pore structures. 3.2
Synthesis of primary particles
The synthesis route of the primary particles possess two distinct points where one can influence the target features (Fig. 2).
Figure 2: Synthesis steps for the preparation of MCMoidal sub-micron beads. In this work the primary pore structure of the beads was varied according to a hydrothermal treatment procedure (step 2). In order to control the secondary pore structure of the agglomerates, the primary particle size has to be adjusted. This can be carried out varying synthesis parameters (step 1) like the concentration of ammonia and water [4], the chain length of the alcohol [5] and the synthesis temperature [6].
342
The latter possibility to tune the particle size was applied in our approach. The pore texture of the spherical MCMoidal beads has been studied using high resolution scanning electron microscopy (HREM). It can be seen from Fig. 3 that the primary particles are aggregates build up by spheres of approximately 10 nm diameter which form the pore structure. XRD measurements show only minor long range order of the pores. Thus, the n-alkylamine merely acts as a porogen during the synthesis than as a structure directing template like found out for ionic surfactants [7,8].
Figure 3" HREM picture of spherical MCMoidal beads. 3.3
Pore size enlargement
Material
aOH before rehydroxylation / pmol m -2
aOH after rehydroxylation / lamol m -2
as (BET)
Vp (G)
~d (BJH)
/ m 2 g-1
/ ml g-1
/ nm
5 2-8 > 1.0 > 1000 (loss of structure) 5 2-4.5 > 1.0 > 1000 MCM-48 (loss of structure) 5-6 2-10 0.2-1.0 300-1000 MCMoidal 5-6 (structure remains) 8-9 50-1000 0.2-4.0 1-50 Xerogel 8-9 Table 1" Comparison of chemical and pore structural data of highly ordered and amorphous silica adsorbents, aOH: silanol group concentration; rehydroxylation: treatment with hydrochloric acid under reflux for 12 h. MCM-41
2-4
As previously published [9], the MCMoidal material is classified as bridging the gap between highly ordered (MCM-41, MCM-48) and totally amorphous (xerogel) silica adsorbents in respect to their crystallinity, pore structure and behaviour towards water.
343
In other words, the loss in long range order and specific surface area is accompanied with an increase in stability towards water or water vapour as shown in Tab. 1. This advantage can be exploited by applying a hydrothermal treatment in order to enlarge the pore size. The hydrothermal treatment is conducted under reduced basic conditions as the mother solution is diluted with water. However, a pH adjustment aider dilution to a value of 10 did not show any changes in the resulting pore size enlargement. The concentration of water was found out to have a significant effect on the hydrothermal process (Tab. 2).
Ratio R a, (BET) vp (G) pa (BJI~ / ml g-1 / nm mother solution / water / m z g-1 1 no treatment 908.6 0.64 2.7 2 1.2 413.5 0.69 5.1 3 1.7 367.3 1.04 9.1 Table 2 Effect of the ratio between volume of mother solution to volume of added water prior to hydrothermal treatment on the resulting pore structural parameters. Material
.
.
.
.
.
.
.
When no water was added (R-value of oo) and hydrothermal conditions were applied, the material totally lost its pore structure. With increasing amount of water (at constant total volume) and decreasing ratio R, we could create graded porosity starting from a parent silica (material 1 in Tab. 2). The pore size distributions of these MCMoidal beads are displayed in Fig. 4.
9v
-E ~,
A
8
il
ii ,' I!
i
II il,, I i
II | I I |
Iliil, l
.;
t
t
.I
1
i
i
as synthesised, 2.7 nm --. - -- hydrothermally treated, 5.1 nm -- -- hydrothermally treated, 9.1 nm
t
i
i
i'l"
lO
i
i
I
i
1
i
i
lOO
I
average pore diameter Pd (BJH) / nm
Figure 4: Pore size distributions ofhydrothermally treated samples in comparison to the untreated parent silica. In order to understand the mechanism of the pore size enlargement by hydrothermal treatment, transmission electron microscopy (TEM) was applied. Both treated materials in Fig. 5 consist of primary particles of the same dimension. The difference in contrast indicate that the pore size enlargement is accompanied by partial dissolution and rearrangement of those spheres.
344
Figure 6: Comparison of bimodal macro- and mesopore arrangements of agglomerates in dependence of the synthesis temperature of primary MCMoidal particles. The spray drying process is a common technique for particle size enlargement [10] and was applied to create agglomerates in the micron size range l~om suspensions of nano-particles with different dimensions. The synthesis of the primary particles was carried out like mentioned above with temperature control between 15 and 50 ~
345 The resulting suspensions were spray dried directly and the products were characterised by SEM and nitrogen sorption at 77.4 K. The bimodal pore size distributions of the agglomerates are shown in Fig. 6. The syntheses lead to primary pore diameters of approximately 2.2 nm regardless from the temperature applied. A slightly broader primary pore size distribution is observed at high reaction temperatures indicating a modified behaviour of the n-alkylamine. The secondary pore size distribution depends on the primary particle size which decreases with increasing synthesis temperature and the inter-particle voids create the second pore system. Thus, the packing of the spheres in the agglomerates is a crucial parameter. It was found out for spherical particles, however, that the average interstitial void diameter corresponds to about 40 % of the particle diameter. The mean particle diameters were calculated according to this correlation and show good consistency with the observed values obtained by SEM pictures (Tab. 3). synthesis temperature
secondary pore mean particle size mean particle size diameter (SEM) (2.s xt,,O T/~ pa2 (BJH) / nm dpso / nm dpso / nm 15 200.1 500 400- 750 25 166.4 416 300-500 30 103.3 258 200-450 35 43.2 108 -100 40 26.1 65 -* 45 27.9 70 -* 50 26.0 65 -* Table 3" Comparison of primary particle sizes in agglomerates. *: too small to measure.
Figure 7: SEM pictures of primary particles (left) and resulting agglomerates (fight). The bimodal agglomerates possess spherical morphology with sizes between 4 and 20 gm depending on the parameters of the spray drier. Agglomeration of smaller primary particles results in smoother agglomerate surface and better morphology.
346 4.
SUMMARY AND OUTLOOK
Bimodal spherical agglomerates where prepared using a two step procedure. The porosity, shape and size of the primary particles were adjusted using a templated St0ber synthesis in combination with temperature control and hydrothermal alter-treatment of the products. In a second step, the primary particles were subjected to agglomeration and the secondary pore size and morphology of the materials was tailored by the applied primary particles and the spray drying parameters. Thus, the hierarchy exists on two levels. On the first level spherical particles of 10 nm build up the primary pores and on the second level the packing of primary particles build up the secondary pores in the agglomerates. The possibility of tuning both pore systems independently makes these hierarchical materials valuable for designed applications. The secondary pore system enables a fast access (high mass transfer kinetics) to the primary pore system which are responsible for the obtained high specific surface areas (high loadability). 5. L I T E R A T U R E
10
K. Nakanishi, Journal of Porous Materials, 1997, 4, 67-112. K. Nakanishi, H. Minakuchi, N. Soga and N. Tanaka, Journal of Sol-Gel Science and Technology, 1997, 8, 547-552. K. Cabrera, G. Wieland, D. Lubda, K. Nakanishi, N. Soga, H. Minakuchi and K.K. Unger, Trends in Analytical Chemistry, 1998, 17, 50-53. W. Strber, A. Fink, E. Bohn, J. of Colloid and Interface Sci., 1968, 26, 62-69. Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, C.J. Brinker, G.W. Scherer, Academic Press, London, 1990. C.G. Tan, B.D. Bowen, N. Epstein, J. Colloid Interface Sci., 1987, 118, 290. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature, 1992, 359, 710. D. Kumar, K. Schumacher, C. du Fresne von Hohenesche, M. Gr'tin and K.K. Unger, Colloid and Surfaces, A: Physiochem. and Eng. Asp., 2001, 187, 109. H.R. Wuthrich, Ullmann "sEncyclopedia oflndustrial Chemistry, Volume B2, 5 th Edition, John Wiley & Sons, Chapter 7, 1988.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
U l t r a t h i n p o r o u s glass m e m b r a n e s
347
with controlled texture properties
D. Enke a, F. Friedel a, F. Janowski a, T. Hahn a, W. Gille b, R. Miiller c, H. Kaden c Institute of Technical Chemistry and Macromolecular Chemistry, University of Halle, SchloBberg 2, D-06108 Halle/Saale, Germany b Department of Physics, SAS-Laboratory, University of Halle, Hoher Weg 8, D-06120 Halle/Saale, Germany c Kurt-Schwabe-Institute of Measuring and Sensor Technique Meinsberg, Fabrikstrasse 69, D-04720 Ziegra-Knobelsdorf, Germany a
Porous glass membranes were prepared in shape of ultrathin plates from a sodium borosilicate initial glass by phase-separation and combined acid and alkaline leaching. The thickness of the membranes varies between 100 and 300 gm. The textural characteristics of these materials were determined using nitrogen physisorption, mercury intrusion, scanning electron microscopy and small angle X-ray scattering. Gas permeation measurements were used to investigate the transport parameters of the porous glass membranes. Furthermore, the mechanical stability and the optical transparency (by UV-VIS spectroscopy) were characterized. The pore structure of the membranes can be tailored in the range between 1 and 120 nm. 1.
INTRODUCTION
In the last years increasing research activities in the fields of membrane science [1, 2], chemical sensors [3], confined matter [4] and micro-reaction engineering [5] have evoked a new interest on porous glass membranes. Furthermore, such membranes are ideal model systems for the investigation of transport processes in porous structures. This broad spectrum of applications demands variable texture properties. Porous glass membranes are currently available only on the basis of Porous VYCOR Glass (PVG) in shape of tubes or discs. That means, the pore size is fixed between 4 and 10 nm [6]. However, pore sizes of 1 nm or lower are required for an adequate selectivity in gas separations. That's why several procedures were developed to reduce the pore size in the VYCOR substrate down to the molecular size level (below 1 nm). Levy et al. [1] modified porous VYCOR tubes using low pressure chemical vapor deposition of SiO2. Beltsios et al. [2] modified the surface of VYCOR membranes by a commercial silicone which is subsequently subjected to oxygen plasma and converted to SiO2. In another approach, these authors used the multilayer Langmuir-Blodgett (LB) deposition of fatty acid salts, sesquioxane polymers and pitch products, followed by a plasma treatment to prepare gasseparating asymmetric membranes on the basis of PVG [5]. Dong and Wong [7] prepared composites on the basis of a microporous MFI-type zeolite on the surface of a flat mesoporous glass membrane by substrate self-transformation. On the other hand, pore sizes above the range covered by PVG are demanded for a good performance of a porous glass membrane in optical sensor devices (fast sensor response) and for the systematical investigation of confinement effects. The various applications require the determination of the texture and transport characteristics of such porous glass membranes. This can be done by combination of equilibrium
348 and dynamic methods, as nitrogen adsorption, mercury porosimetry, molecular probes and permeability measurements, with small angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) [8]. This contribution describes the preparation of mechanically stable flat porous glass membranes with variable texture properties on the basis of phase-separated sodium borosilicate initial glasses and their characterization with a series of techniques. 2.
EXPERIMENTAL
Two sodium borosilicate initial glasses were used in this study. The initial glass (1) in shape of 1 0 - 15 mm thick plates was prepared from a glass melt of the composition 70 wt.-% SIO2, 23 wt.-% B203 and 7 wt.-% Na20 by roller quenching, followed by an optical fine cooling at 300~ for 1 h and a subsequent cooling in air. The initial glass (2) was prepared by moulding the same glass melt in a thicker plate in air, followed by an optical fine cooling. The latter is characterized by a more progressive phase-separation. The initial glasses were cut with a diamond circular saw (SAW 15, Logitech) in smaller glass blocks (i.e. 25x25xl 0 mm), which were phase-separated in the temperature range between 530 and 720~ The initial glass (1) was used up to 610~ The initial glass (2) was employed above this temperature. In the next step, the phase-separated glass blocks were cut in ultrathin flat plates (i.e. 25x25x0.1 mm) using an annular precision (Annular 55, Logitech) and a diamond band saw (SAW 15, Logitech). After that, the ultrathin plates were leached in special baskets with 1 N hydrochloric acid at 90~ for 1 h. The colloidal silica deposits remaining in the pore system of the glass membranes after acid leaching of initial glass plates phase-separated above 550~ were removed by treatment with 0.5 N sodium hydroxide solution at room temperature for lh. Nitrogen sorption measurements were performed by use of a Sorptomatic 1900 Turbo apparatus by Carlo Erba Instruments. All samples were degassed at 393 K before measurement for at least 24 hours at 10-5 mbar. The mercury porosimetry measurements were carried out on a Porosimeter 2000 apparatus by Carlo Erba Instruments. A contact angle of 141.3 ~ for Hg was used. The samples were degassed at 393 K before measurement for 24 h. SEM of the porous glass membranes was carried out on a Phillips ESEM XL 30 FEG microscope. SAXS measurements were performed using nickel-filtered CuK~ radiation. The membranes were studied directly. About 120 points of the smeared relative scattering intensities in the region of the scattering vector h, 0.05 < h < 3 per nm, were recorded point by point using a Kratky camera. After background subtraction about 2500 impulses from the scattering of the sample were obtained at the position hmax= 3 per nm. The experimental intensities have a dynamic of 5 decimal powers after the application of the special collimation correction procedure [9]. The SAXS curves measured were analyzed applying the theory of chord length distribution (cld) [ 10, 11 ]. The ultrathin porous glass membranes were examined for the permeability of air at 25~ The measurements were performed as follows: The membrane was sticked on a brass plate containing a bore with 2 and 5 mm diameter, respectively. After that, the plate was fixed with a special dome on a turbo molecular pump (TMU 261, Pfeiffer). Now, the dome was evacuated. The gas flow was determined by measuring the pressure below the membrane. Finally, the integral permeability was estimated using the pumping speed, the measured pressures and the membrane thickness. The UV-VIS spectroscopic studies on the ultrathin porous glass membranes were performed in the wavelength range 2 0 0 < ~ , < 8 0 0 n m using a 8452 Diode Array Spectrophotometer by Hewlett Packard. A standard cuvette possessing a special membrane
349
holder was used. The self absorption of the cuvette was estimated prior to each measurement and deducted from the spectrum of the investigated membrane. The mechanical stability of the membranes was characterized using the material specific fragility which was standardized on the membrane thiclmess. The measurements were performed with an universal material testing machine Zwicki 1120 by Zwick. The membranes were placed on a mounting plate possessing a bore with 4.4 mm in diameter and breaked through by a punch with 3.2 mm in diameter. The force required was recorded in combination with the distance covered by a 200 N force detector. The test velocity of the punch was 0.01 mm/min. The measurement was finished after a force drop of 85 %. 3.
RESULTS AND DISCUSSION
The texture properties of the ultrathin porous glass membranes prepared in our laboratory were initially characterized by the equilibrium based methods nitrogen gas adsorption and mercury porosimetry. The nitrogen sorption isotherms of two membranes are shown in Fig. 1. The fully reversible isotherm of the membrane in Fig. 1 (A) can be classified as a type I isotherm according to the IUPAC nomenclature which is characteristic for microporous materials. The membrane in Fig. 1 (B) shows a typical type IV isotherm shape with hysteresis of type H1 (IUPAC classification). This indicates the presence of fairly uniform mesopores. The texture characteristics of selected porous glass membranes are summarized in Tab. 1. The variable texture demanded the application of various characterization techniques and methods of evaluation. It is obvious that the porosity of the membranes can be varied by the conditions of heat treatment of the initial glass blocks. The time and temperature of the heat treatment for phaseseparation act in combination with the leaching conditions as structure directing parameters in the preparation process of porous glasses [6]. The great flexibility of the practicable pore sizes allows a tailoring of the membrane properties to many different applications. However, there are no general relationsships between the specific surface areas and pore volumes and the average pore diameters of the membranes. This may originate from the peculiarities of the characterization techniques, residues of colloidal silica in the pore system of the meso- or macroporous glass membranes or from differences in the microstructure of the initial glasses. Systematic investigations are in progress. The mercury porosimetry intrusion plot of a mesoporous glass membrane is given in Fig. 2. That curve and the isotherm shapes in Fig. 1 are indicators for a relatively narrow pore size distribution of the ultrathin porous glass membranes. 200 180160140~'g 120"~ 10080-
60i
B
t
300] . B ~ ii . . L ~ ' L T J ' l l ~
~---
-
~
200
l.l"
,? ,
40202
0
0,0
t-t d
0;2
0;4
0',6
P/P0
0;8
1,0
O/
0,0
T
.
.
0,2
.
0]4
.
0]6
P/P0
Fig. 1 Nitrogen sorption isotherms at 77 K of two ultrathin porous glass membranes (A microporous, B mesoporous)
.n"
0',8
1,0
350 450 ,---, 400 e~
& o o
350 300
250 200 150 100 50 0
.....
O,1
9..-:7"-
1
~~
. . 7 o -
o~
~~
10 1 O0 pressure [bar]
~"7--7-. .~] .... , 1000
Fig. 2 Mercury porosimetry intrusion curve of a mesoporous glass membrane Table 1 Textural properties of some ultrathin porous glass membranes prepared from a sodium borosilicate initial glass by varying of the conditions of heat treatment Temperature Time Spec. surface area Spec. pore volume Avg. pore dia[~ [h] [m2/g] [cm3/g] meter [nm] _ _ 4121, 2 0.1461 540 24 2861'2 0.1681 570 24 1811'3 0.7191 111'4 570 24 1795 0.5645 125 610 24 1955 0.7515 185 630 24 665 0.4705 475 680 24 525 0.4825 725 700 24 365 0.4465 965 720 24 415 0.4635 1125 1N2 sorption at 77 K, 2DUBININ-RADUSHKEVICH, 3BET, 4BARRETT-JOYNER-HALENDA, desorption branch, 5Mercuryporosimetry
Fig. 3 Scanning electron microscope images of a mesoporous (A) and a macroporous glass membrane (B)
351
0,14 0,12 lO
_
0,10
-
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'|
-
,~, "z3
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-
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I 1
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d/nm
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I 4
' 5
Fig. 4 Chord-length distribution 7"(d), 0.5 n m < d < 5 nm, inset: the corresponding SAXS curve obtained for the microporous glass membrane Scanning electron micrographs given in Fig. 3 clearly show a homogeneous texture of the corresponding meso- or macroporous glass membranes. Fig. 4 shows the chord-length distribution 7"(d), obtained from a SAXS experiment, of the microporous glass membrane, see Fig. 1 (A). 7"(d) allows, in combination with the results of other characterization techniques (nitrogen sorption, mercury porosimetry, scanning electron microscopy), a detailed characterization of the microstructure of this sample. Here, the lower resolution limit of the SAXS experiment, dmin = 0.5 n m , leads to difficulties in the assignment of the peaks and shoulders obtained. However, it is possible to explain all peaks using a linear simulation model, recently published by Gille [12]. Furthermore, the porosity of the membrane determined by nitrogen sorption at 77 K can be inserted as a check. In Fig. 4 the main maximum of the chord-length is observed below d = 1 nm. This maximum can be interpreted in agreement with the results of the nitrogen sorption experiment. It reflects the mean chord-length of the micropores. The resolution limit of the SAXS experiment slightly influences this peak position. It is shifted to larger chord-lengths. Additionally, a shoulder is found at about 1.5 nm. Based on the simulation model, Fig. 5, the peak and the shoulder can be explained in terms of <1> = 0.5 nm (mean chord-length of a pore) and <m> 1.55 nm (mean chord-length of the wall). This is supported by the porosity c of the investigated membrane. On one hand, the porosity is given by c = /(+<m>)= 24 % [11 ]. On the other hand, c follows from the network density of the membrane, 9 = 2.2 g/cm 3, and the specific pore volume, Vp= 0.146 cm3/g, determined by nitrogen sorption. Therefore c = 1 / ( I + I / ( 9 " V P ) ) = 24%. Based on the curvature behaviour of the SAS correlation function 7(0, the distribution laws of random distances between the interfaces were analyzed [12]. This is examplified for a sequence of 4 pores and 3 walls in Fig. 5. Furthermore, the appearance of all minima and maxima in the chord-length distribution, see Fig. 4, at d = 2.1 nm, 2.5 nm, 2.9 nm .... is traced back to the mean chord lengths <1> and <m>. The permeabilities of air at 25~ were determined to get first informations about the transport properties. This was done by comparison of the values of the individual ultrathin porous glass membranes. The results obtained (Tab. 2) were used to check the quality (homo-
352
Fig. 5 Linear simulation model geneity, absence of cracks or defects, reproducibility) and to characterize the correlations between the texture and transport properties. All membranes are completely porous. The reproducibility of the permeabilities of membranes of the same pore size indicates the absence of cracks, defects or pinholes. This is confirmed by scanning electron microscope images (Fig. 3). The transport characteristics of the ultrathin porous glass membranes are strongly related to their textural characteristics. The permeabilities of the investigated membranes vary in dependence on the average pore diameter in a range between 3.10 -5 and 3" 10-2 cm2/s. The low air permeability of the membranes prepared by leaching of the roller-quenched initial glass of 3.5"10 -5 cmZ/s, is remarkable. These membranes possess in combination with their microporous texture (Fig. 1 and 4) potential for molecular sieve applications. However, only a "rough" estimation of the transport properties of the ultrathin porous glass membranes was performed in this study. Currently, the permeabilities of various gases, the relations of the textural and structural characteristics to the transport properties, the flow regime and molecular sieve effects are investigated. Table 2 Permeability of air for various membranes Temperature of Time of phaseAvg. pore phase-separation separation [h] diameter [~ [nm] 540 570 610 630
24 24 24 24
< 11) 1.42) 123) 183) 473)
Permeability [cm3(SyP).cm .minl .cm2. atm -1]
Permeability at 25~ [cm2. s -1]
1.95.10-3 9.6" 103 0.37 1.56 1.72
3.5.104 1.7" 10-4 6.6.10 .3 0.028 0.031
~SAXS, 2Nz sorption 2"Vp/Os,3Mercuryporosimetry The optical transparency of the porous glass membranes is interesting for sensor applications. It depends on the structural features of the membranes [ 13]. The UV-VIS spectra of micro- and mesoporous glass membranes are compared in Fig. 6. The membranes with an
353
.
200
.
.
.
300
.
.
.
400
.
.
500
18nm,
600
.
700
800
Z[nm] Fig. 6 UV-VIS spectra of porous glass membranes with different pore sizes average pore diameter of < 1, 12 and 18 nm show a high transmittance within in spectral range )~ = 200 - 800 nm. In contrast to that, the membrane with an average pore size of 47 nm is characterized by a fourfold higher extinction at k = 300 nm. This is also manifested in the strong opalescence of these membranes. Regarding the optical properties, the membranes with < 1, 12 and 18 nm average pore size are better suited for optical sensor applications. The fast response is another important parameter characterizing the quality of an optical chemosensor. Here, membranes with larger pore sizes are to be preferred. This means, these contrary parameters must be optimized for a given sensor application. The great variability of the membrane textures represents an important advantage compared to other porous materials used for the immobilization of chromophors. The characterization of the mechanical stability of the ultrathin porous glass membranes is an essential precondition to the application in micro-reaction engineering or for gas separations in the high pressure range. The material specific fragility was used in this study. The force and the distance covered by a punch were measured up to the fracture of the membrane. This is shown in Fig. 7 for porous glass membranes with < 1, 12, 18 and 47 nm pore size. In all cases, the increase of the force is linear up to the fracture, followed by a vertical decrease. This is characteristic for brittle materials. However, there are important differences between the investigated membranes. The fracture of the microporous membrane occurs at a force which is approximately twice as high as for the various mesoporous membranes. Consequently, the microporous membrane possesses the highest mechanical stability. The transformation of the network SiO2 to colloidal SiO2 and its alkaline removal in the preparation process leads to a decrease of the wall thickness of the resulting mesoporous membranes. This results in a decrease of the mechanical stability. The fine three-dimensional network of the microporous membrane reduces formation and propagation of fractures. The longer distance of the punch and the reduced specific fragility are indicators for a slightly higher "flexibility" of the membrane with an average pore size of 47 nm. 4. C O N C L U S I O N S Ultrathin porous glass membranes with variable texture properties were prepared from a SiO2-rich sodium borosilicate initial glass by careful fine tuning of the conditions of heat treatment for phase-separation. Pore sizes between < 1 and 120 nm can be realized. The membranes are characterized by a narrow pore size distribution. The transport, optical and mechanical properties vary with the pore size. The tailorable texture and transport characteris-
354
3,5 3,0 i
! 2,5
/
/
<1 nm
2,0 E
1,5
/ 18nrr
1,0 - ~
~'~'~
0,5 0,0 0,00
o,b2
~ 47
' o,N o,b6 distance [ram]
0,08
Fig. 7 Force-distance diagrams of various ultrathin porous glass membranes tics in combination with the optical and mechanical properties open the ultrathin porous glass membranes a wide field of possible applications, i.e. as component of optical chemosensors, in heterogeneous catalysis and micro-reaction engineering or for gas separations.
REFERENCES 1. R.A. Levy, E.S. Ramos, L.N. Krasnoperov, A. Datta and J.M. Grow, J. Mater. Res., 11 (1996) 3164. 2. K. Beltsios, G. Charalambopoulou, G. Romanos, N. Kanellopoulos, Joumal of Porous Materials, 6 (1999) 25. 3. Y.Y. Maruo, T. Tanaka, T. Ohyama, T. Hayashi, Sensors and Actuators B, 57 (1999) 135. 4. E. Hempel, A. Huwe, K. Otto, F. Janowski, K. Schr6ter, E. Donth, Thermochimica acta, 337 (1999) 163. 5. K. Beltsios, E. Soterakou, N. Kanellopoulos, G. Tsangaris, Materials Science and Engineering C, 15 (2001) 257. 6. F. Janowski, D. Enke in F. Schtith, K. Sing, J. Weitkamp (Eds.), Handbook of Porous Solids, Wiley-VCH, Weinheim, in press. 7. W.Y. Dong, Y.C. Long, Chem. Commun., (2000) 1067. 8. A.Ch. Mitropoulos, K. Beltsios, Th. Steriotis, F.K. Katsaros, P. Makri, N.K. Kanellopoulos, Journal of the European Ceramic Society, 18 (1998) 1545. 9. W. Gille, PhD Thesis, University of Halle/Saale, 1983. 10. D. Enke, F. Janowski, W. Gille, W. Schwieger, Colloids and Surfaces A: Physicochemical and Engineering Aspects 187 - 188 (2001) 131. 11. W. Gille, D. Enke, F. Janowski, Journal of Porous Materials, 8 (2001) 179. 12. W. Gille, Waves in Random Media, 12 (2002) 85. 13. I.S. Smirnova, T.V. Antropova, M.P. Sidorova, L.E. Ermakova, G.P. Roskova, Glass Physics and Chemistry, 22 (1996) 388.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
355
Reconstruction of the mesoporous silica glass Gelsil| N. Eschricht, E. Hoinkis, F. M/idler, P. Schubert-Bischoff Hahn-Meitner-Institut Berlin, Glienickerstr. 100, D- 14109 Berlin, Germany A stochastic optimization technique is applied to the construction of three-dimensional models of Debye random media and random ensembles of mono-disperse penetrable spheres. The theoretical values of the geometric parameters and the values determined from the models agree within narrow limits. We measured small angle neutron scattering data for a monolithic sample of the mesoporous silica glass Gelsil| and calculated the correlation function. The latter is used to generate a spatial model of Gelsil| The reconstruct reproduces the essential features of the microstructure as observed by transmission electron microscopy. Mean pore size and surface area determined from the spatial model agree closely with the values calculated by use of Porod' s law. 1. INTRODUCTION The pore size distribution (PSD) and other geometrical properties effect the performance of many economically important mesoporous solids. The shape of pores varies in most adsorbents and catalysts, and the pores form an interconnected network. However, commonly used methods for the determination of the PSD presuppose that pores of different size have the same regular shape and behave independently [1,2]. By Monte Carlo simulations of adsorption in digitized three-dimensional models of real materials the Barrett-Joyner-Halenda method [3] can be tested against the PSD obtained directly from the pore structure. For this purpose Gelb and Gubbins [4-6] generated a series of mesoporous controlled pore glasses by quench molecular dynamics which imitates the chemical production processes. Probably only a few random porous solids of interest can be modelled by such an ab initio simulation. X-ray computed microtomography is very useful for media with micron-sized pores but this technique cannot be applied to mesoporous solids. A "reconstruct" of a real medium can also be generated by experimentally obtained statistical functions that describe the geometrical properties of the medium. Such "descriptors" or "target functions" are the volume fraction ~0 of pores, the surface S, the correlation function 7o(r), see Fig. 2 (left), the two-point probability S2(r), see Fig. 3 (left), the lineal path function L(r), the distribution p(r) of chord-lengths in pores, and the nearest-neighbor distribution P(6). The most important descriptor is [7] S+(r) = ~o(1-~o)~o(r) + ~o2.
(1)
"Constructs" are generated from theoretical descriptors representing a certain medium. The correlation function for a Debye random medium [8] is
7o(r) = exp(-r/a),
(2)
356
where a is a correlation length. Such a medium does not show any short-range order, see Fig. 2 (left), and the exact realization of S2(r) in a reconstruction is believed to be possible [9]. Analytical expressions of the target functions are known for a random ensemble of monodisperse penetrable solid spheres, abbreviated as RES [10-13]. The latter ensemble and a random Debye medium are well suited for testing construction procedures. We used the twopoint probability S2(r) and combinations of S2(r) and one of the other descriptors as target functions for the realization of three-dimensional constructs of both media. The microstructural parameters calculated from the construct were compared to the theoretical data. Gelsil| is a commercially produced transparent rigid mesoporous silica glass available in small discs [ 14]. The monolithic Gelsil | samples are used as substrates for photoactive dyes, separation devices, catalyst supports etc. The one peak (see Fig. 2, right) observed in the small angle neutron scattering (SANS) curve indicates short-range order without long-range order. A similar peak was observed with Vycor 7930 [15,16]. In situ SANS experiments [17-21] showed that the adsorption of nitrogen in Gelsil| changes the character of the fluid/solid system continuously towards a Debye random medium, that is the peak disappears gradually with increasing nitrogen adsorption, and the scattering data satisfy the Debye-Bueche equation within a certain range of the adsorption branch of the isotherm [22]. We reconstructed Gelsil| by use of the two-point probability S2(r) obtained from SANS data. The interface area and the mean values of the chord-lengths in pores and within the solid agree with the data obtained from SANS and from the nitrogen adsorption isotherm. 2. PREVIOUS WORK
The Gaussian random field approach (GF) was used [23,24] to reconstruct Vycor 7930. However, GF has two shortcomings: only tp and S2(r) can be used as descriptors, and the properties of the reconstruct depend on the underlying Gaussian statistics [7,25,26]. Torquato applied a stochastic optimization technique [27] which is independent of a particular statistical model and allows for the use of more than one descriptor [28-30,32]. By this technique a number of two-dimensional systems were reconstructed [7,29-33]. The target medium for the three-dimensional reconstructions [30-37] was an X-ray computed microtomographic image of Fontainebleau sandstone which has a mean pore size of 6.5 lam. Previous studies showed that the target descriptors and the descriptors calculated from the reconstructs matched closely. Porosity, mean pore size and surface area of target and reconstruct agreed, too. However, the visual appearence inspection often revealed differences. Microstructural information is limited by principle, and thus perfect reconstructions are impossible. According to Torquato [30], the reconstruction can be considered successful if the structural properties of the reconstruct and the target medium agree closely. 3. THEORY The intensity l(q) scattered by a three-dimensional isotropic two-phase heterogeneous medium without longe-range order is given by Eq.(3), with scattering vector q = 4rd2sinO, wavelength ~, scattering angle 20, and distance r [22,38]: oo
I(q) = 4 z q~(1- tp) (Apb) 2 f r 2 yo(r) sin(qr)/qr dr,
(3)
0 oo
~o(r) = 0--I f q2 I(q) sin(qr)/qr dq, 0
(4)
357
(d?'o(r)/dr)r--.o = - S/(4 V tp(1-qg)) = - 1/rove.
(5)
Apb is the scattering length density difference, Q is Porod's invariant, and '%e the mean chord length. For the calculation of yo(r) we approximated I(q) by a cubic spline. The equations used for the calculation of m~epore and mvgsolid a r e to be found in [8,30,39-41,47]. Analytical expressions for the descriptors of RES were published in [10,11,13,42,43]. In its most simple variant, the stochastic optimization procedure evolves the two-point probability s2b(r) of a binary representation of the sample towards S2(r) by randomly exchanging binary cells of different phases, starting from a random configuration which meets the preset volume fractions. Atter each exchange the objective function
E(r) : 2 (S2(r) - s2b(r)) 2
(6)
is compared to its former value. The improvements in E finally result in almost indentical descriptors $2 and $2 b. This is also true if more than one descriptor is used in hybrid reconstructions.The procedure is well known as "Metropolis algorithm" or "simulated annealing" [27]. In order not to get trapped in local minima, the method must accept deteriorations in the values of E(r) with a certain probability. However, in all our reconstructions a pure evolution strategy - which only accepts improvements in E(r) - was sufficient to minimize E(r). We introduced an "interface heuristics" which suppresses the generation of isolated single cells (of either phase). Together with a complexity reduction from 0(]7 2) to O(N) in the calculation of the changes of s2b(r), this heuristics considerably improved the conformity in s2b(r) and S2(r) and reduced the computing time from days to hours (on a PC with 1GHz). 1. M A T E R I A L S A N D M E T H O D
We studied a Gelsil| disc with a thickness of 2 nma, a diameter of 6 rmn, and a skeletal density of 2.12 g crn-3 (product code A050-060-020-K, lot 72401). The sample was obtained from Geltech Inc., 3267 Progress Dr., Orlando, F1. 32837, USA. It was shipped evacuated, and was used as received. The details of the preparation of monolithic Gelsil| samples are proprietary. According to [14] the preparation of one sample takes about 300 hours and involves the following steps: hydrolysis of tetramethyl-orthosilicate with a drying control chemical additive, casting, gelation, aging, drying, dehydration, and densification at a temperature of about 1173 K. The nitrogen sorption isotherm was measured [44] by means of an ASAP 2010, made by Micromeritics Corp., Norcross, USA. The BET surface area, pore volume and mean pore size are SBEr = 548m2g -1, Vpore = 0.674 cm3g-1, and 4Vpore/SBET = 4.9 nm. The porosity is ~0- 0.631. The SANS data were measured at the V4 instrument of the Berlin Neutron Scattering Center (BENSC) at HMI. The neutron wavelength was 2=0.602 rma, the diameter of the incident beam was 5 mm. Sample/detector distances were 1.4, 4 and 16 m, which correspond to a range of 0.08 < q (nm-1) < 3. The raw data were treated by standard procedures [45]. The n-transmission of the sample was 0.92. The sample for transmission electron microscopy (TEM) was prepared by polishing and ion milling. TEM images were obtained by a Philips CM12 instrument.
358
Fig. 1. The computer-generated RES A (left) and its stochastic reconstruction B (right). 5. R E S U L T S Constructs of a Debye random medium with r
0.631 and a - 3.3 nm were calculated with
Se(r) as a descriptor and voxel sizes of 1, 1/2 and 1/3 nm, because in a digitized medium the slope of S2(r) at r -- 0 depends on the voxel size. For the construct we determined a values of 3.82, 3.55 and 3.46 nm, respectively. Thus, the a value nearly matches that of the target at the highest feasible resolution, where S/V of the construct (322 m2cm -3) almost agrees with the theoretical value (317 m2cm-3). RES A, shown in Fig. 1, was generated by randomly positioning thousand spheres with radius R - 5 nm in a cube of side length 100 nm. The voxel size is 1/2 nm. The surface area S of the construct was calculated by an algorithm of Lorenzen and Cline [46]. The size distributions of gpore and gsond, and the value of S/V result from the random positioning process and the preset porosity of r 0.59. The mean values rngpore ,me~olid, m g, and S/V were calculated from the theoretical 70(0, see the first row of Table 1. The test of our computer programs then consisted in the following three steps: i) We calculated Se(r), L(r), and p(r) for RES A by sampling in thirteen directions. The calculated functions agreed well with the theoretical ones. ii) W e computed the distributions gpo~e and gsond fi'om RES A and calculated their first moments. Finally S/V was calculated. The values are listed in the second row of Table 1. The values for RES A are less than 8% higher than the theoretical ones. This deviation m a y be due to the limited size of the model. Table 1 Mean chord length mg, mean pore size mgp.... mean wall thickness mg~olid, and the specific surface area S for random ensembles of mono-disperse penetrable spheres (RES). Data calculated from: Correlation function RES A Reconstruct B with Sz(r) +p(r)
m ~, ( a m )
5.2 5.6 5.6
m ~'pore ( a m )
12.7 13.6 13.7
ms
(rim) 8.8 9.5 9.5
S/V (m2/cm3) 186 185 189
359 1.0
4.0 i:
9
. ~~
30r
~--- Gelsi150 Debye medium
" . 0.5
o.oL, 4)001
~ i_~ . ' - ~
0.0"
5
0
0
,0
10
......... i
,
i
- ,
x experiment
-~l~,
,
--theory
.
3 k I(q) (a.u.)
4
2.0-
-
1.0
-
~
,0
15
r (nm)
,
20
0
0 "0
1 qt )-'nm-" 2
......... ' ~ -
3
Fig. 2. Lejk Correlation function 70(0 calculated from the experimental scattering data of Gelsil| shown in the right panel (crosses). Right: Experimental scattering data compared to the theoretical curve I(q) which was recalculated from the correlation function To(r) by use of Eq. (3).
.._
~ "
~ " ~
--
theory reconstruction
0.08
0.5-
0.06 0.04
0.4
w
f .... , ........................... ~--~"~7.~. 1
10
r(nm)
*.~p(r)
theory 9 reconstruction
(nm -1)
~ _
]
i~ 7,~:.
100
r (nm)
Fig. 3. Theoretical two-point probability (left) and chord-lengths distribution (right) for a random ensemble A of mono-disperse penetrable solid spheres compared to the same descriptors recalculated from A's reconstruct B.
Fig. 4. TEM image of Gelsil|
(left), and slice of the three-dimensional reconstruct (righO. Size - (150 nm)2.
360 iii) We reconstructed an RES by using the Yeong-Torquato method. Attempts to construct an RES by using only the descriptor S2(r) were not successful. The visual inspection of the constructs showed large discrepancies with RES A. Therefore we performed hybrid constructions with the two-point probability S2(r) and one of the other descriptors weighted equally. We found the best result with S2(r) and the chord-lengths distribution p(r). The size of the model amounted to 1003 voxels, and the resolution was 1 nm/voxel. The descriptors were calculated in thirteen directions. 7x106 iterations were needed, which required 6 hours of computing time. Fig. 3 shows the theoretical descriptors S2(r) and p(r) and their recalculations from construct B depicted in Fig. 1. RES A shows high angularity, and the deviation of the computed and the theoretical p(r) at low r reflects the difficulty of constructing the shape of contacting and slightly penetrating spheres precisely. However, the principal character of the morphology is recovered. A complete agreement between target and spatial reconstruction was observed at no time [29,30,34,35]. The construction was fairly successful with respect to the geometrical parameters, as the comparison of the data in the third row in Table 1 with the theoretical data in the first row shows. Let us now turn to the reconstruction of Gelsil| Fig. 2 (left) shows the respective yo(r) calculated from SANS intensity data, I(q). The insert gives the data of yo(r) in the range -0.01 to +0.03. For comparison we also plotted yo(r) for a Debye random medium, yo(r) for Gelsil| shows a minimum which corresponds to the maximum observed in the scattering curve. At r ~ 15 nm yo(r) decays to zero. Fig. 2 (right) shows the experimental and the theoretical I(q) data recalculated by use of ~(r). We performed the reconstruction by using S2(r), ~0= 0.63, a volume of 1503 voxels and a voxel size of 1 nm. The geometrical parameters calculated from yo'(r) at r = 0 and the data determined from the reconstruct agree reasonably well, as Table 2 shows. The data in the first row were calculated from the invariant and the Porod constant and agree reasonably well with the data in the second row. For mean pore size and surface area, a complete agreement of the values calculated from SANS and from the nitrogen adsorption isotherm (4.9 nm and 548 m2g-1) cannot be expected, but the values are reasonable. A TEM image is shown in Fig. 4 (left), together with a thin slice of the spatial reconstruct (right). The white spots in the reconstruct represent the straight look through most pores, which are irregular and exhibit a size larger than the white spots suggest. The comparison of the images in Fig. 4 indicates that the reconstruct reproduces the essential features of the microstructure: there is a continuous solid phase, a high porosity and pores with sizes in the range --3-12 nm. A maximal pore size of --11 nm was derived from in situ SANS data measured near the upper closure point of the hysteresis loop of the nitrogen adsorption isotherm. Table 2 Mean chord length me, mean pore size m~epore,mean wall thickness m~e~oad,and the specific surface area S for Gelsil| Data calculated from: Porod's law and invariant Q 3,0'(r) Reconstruct with S2(r)
m,g(nm)
m,gpore(rim)
m,gsolia(rim)
1.9 2.0 2.0
5.2 5.5 5.5
3.0 3.2 3.2
S (m2/g) 518 518 504
361 6. SUMMARY
Based on the Yeong-Torquato method for the stochastic optimization of three-dimensional two-phase random media with one or more descriptors algorithms were developed. We have tested our tools by constructing Debye random media and random ensembles of monodisperse penetrable solid spheres. Mean pore size and surface area of target and construct agree reasonably well (Table 1), and the morphology of target and construct is quite similar. The mesoporous silica glass Gelsil| was reconstructed by use of the descriptor S2(r) calculated from SANS data. The geometric parameters determined from the reconstruct and calculated from the Porod law agree within narrow limits (Table 2). A comparison with a TEM image shows that the reconstruct exhibits the essential features of the sample. ACKNOWLEDGEMENTS
The authors would like to thank F. Mezei for his continuous support of this work, U. B loeck for her help with electron microscopy, and K. Diederichsen for refining our English. REFERENCES
[1] [2] [3] [4] [5] [6]
K. Sing, Colloids and Surfaces A 187-188 (2001) 3. P.I. Ravikovitch, A.V. Neimark, Colloids and Surfaces A 187-188 (2001) 11. E.P. Barrett, L. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. L.D. Gelb, K.E. Gubbins, Langmuir 14 (1998) 2097. L.D. Gelb, K.E. Gubbins, Langmuir 15 (1999) 305. L.D. Gelb, K.E. Gubbins, in Proc. Conf. Fundamentals of Adsorption, Nagasaki, Japan, 2001, p.333.Intern. Adsorption Soc., K. Kaneko, H. Kanoh, Y. Hanzawa (eds.), [7] D. Cule, S. Torquato, J. Appl. Phys. 86 (1999) 3428. [8] P. Debye, H.R. Anderson, H. Brumberger, J. Appl. Phys. 28 (1957) 679. [9] S. Torquato, Random Heterogeneous Materials. Springer,New York, Berlin, 2000, p.299. [10] H.L.Weissberg, J. Appl. Phys. 34 (1963) 2636. [ 11 ] B. Lu, S. Torquato, Phys. Rev. A 45 (1992) 922. [12] S. Torquato, B. Lu, Phys. Rev. E 47 (1993) 2950. [ 13] D.A. Coker, S. Torquato, J. Appl. Phys. 77 (1995) 6087. [14] L.L. Nogues, W.V. Moreshaed, J. Non-cryst. Sol. 121 (1990) 136. [15] P. Levitz, Adv. Colloid Interface Sci. 76-77 (1998) 71. [ 16] S.K. Sinha, in Methods in the Physics of Porous Media, Academic Press, San Diego, 1999, Vol.35, p.223, Po-Zen Wong (ed.). [ 17] E. Hoinkis, Langmuir 12 (1996) 4299. [18] E. Hoinkis, BENSC Experimental Report, HMI-B559 (1998) p.212. [19] E. Hoinkis, BENSC Experimental Report, HMI-B565 (1999) p.295. [20] E. Hoinkis, B. R6hl-Kuhn, in Proc. Conf. Fundamentals of Adsorption FOA7, Nagasaki, Japan, 2001, p.601. Intern. Adsorption Soc.,K. Kaneko, H. Kanoh, Y. Hanzawa (eds.). [21] B.Smarsly,C.G61tner,M.Antonietty,W.Ruland,E. Hoinkis, J.Phys.Chem.B 105 (2001)831. [22] P. Debye, M. Bueche, J. Appl. Phys. 20 (1949) 518. [23] R.J.-M. Pellenq, S. Rodts, V. Pasquier, A. Delville, P. Levitz,Adsorption 6 (2000) 241. [24] E.S. Kikkinides, M.E. Kainourgiakis, et al. J. Chem. Phys. 112 (2000) 9881. [25] P. Levitz, Adv. Colloid Interface Sci. 76-77 (1998) 71. [26] S. Torquato, Random Heterogeneous Materials, Springer, New York, Berlin,2000, p.294. [27] A. van Laarhoven, Simulated Annealing. Reidel Publ. Comp., Dordrecht, 1987.
362 [28] M.D. Rintoul, S. Torquato, J. Coll. Interf. Sci. 186 (1997) 467. [29] C.L.Y. Yeong, S. Torquato, Phys. Rev. E 57 (1998) 495. [30] C.L.Y. Yeong, S. Torquato, Phys. Rev. E 58 (1998) 224. [31] M.G. Rozman, M. Utz, Phys. Rev. E 63 (2001) article 066701 [32] N. Sheehan, S. Torquato, J. Appl. Phys. 89 (2001) 53. [33] C. Manwart, R. Hilfer, Phys. Rev. E 59 (1999) 5596. [34] B. Biswal, C. Manwart, R. Hilfer, S. Bakke, P.E. Oren, Physica A (1999) 452. [35] C. Manwart, S. Torquato, R. Hilfer, Phys. Rev, E 62 (2000) 893. [36] R. Hilfer, C. Manwart, Phys. Rev. E 64 (2001) article 021304 [37] J.-F. Thovert, F.Yousefian, P. Spanne et al. Phys. Rev. E 63 (2001) article 061307 [38] A. Guinier,G. Fournet, Small Angle Scattering, J.Wiley & Sons,New York, 1955. [39] G. Porod, Kolloid Z. 124 (1951) 83. [40] K. MUller, in O. Glatter and O. Krattky (eds.), Small Angle Scattering, Academic Press, London, 1982, p. 232 [41 ] G. Porod in O. Glatter and O. Krattky, Academic Press, London, 1982, p. 17 [42] S. Torquato, B. Lu and J. Rubinstein. Phys. Rev. A 41 (1990) 2059. [43] S. Torquato, M. Avellaneda, J. Chem. Phys. 95 (1991) 6477. [44] B. R6hl-Kuhn, BAM Berlin, Priv.Comm. (2000). [45] U. Keiderling, Physica B 234 (1997) 1111. [46] W.E. Lorensen, H.E. Cline, Comp. Graph. 21 (1987) 163. [47] E. Hoinkis in Chemistry and Physics of Carbon, Vol. 25, P.A. Thrower (ed.), Marcel Dekker, New York, 1997, p. 118-131.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
363
Pore structural characteristics of m e s o s t r u c t u r e d materials p r e p a r e d u n d e r different conditions A. Est~vio Candeias ~, M.M.L. Ribeiro Carrott ~, P.J.M. Carrott ~, K. Schumacher b'~, M.Griin b'~ and K.K. Unger b aCentro de Quimica de l~vora, University ofl~vora, l~vora, Portugal blnstitut ftir Anorganische Chemic und Analytische Chemie, Johannes Gutenberg Universit~it, Mainz, Germany Cpresent address" Degussa AG, Postfach 1345, 63403 Hanau This paper presents an overview of the pore structural properties of different mesostructured materials prepared at room temperature, namely MCM-41, MCM-48 and MMS. The stability towards water vapour of some samples is inferred by XRD and nitrogen adsorption after and prior to water adsorption. It is shown that MMS materials are more stable than MCM-41. A new method for the stabilisation of MCM-41 using tetraethoxysilane (TEOS) in hexane as silicification agent is proposed and it is shown to be effective in stabilising the pore structure and to increase significantly the surface hydrophobicity. I. INTRODUCTION Since the disclosure of mesostructured materials in 1992 [1] a variety of synthesis procedures has been proposed, leading to the preparation of materials with different structures and/or pore regularity, morphologies and surface chemistry. The room temperature synthesis[2-5] has been a versatile alternative to the conventional hydrothermal synthesis for the preparation of high quality materials, allowing the independent tailoring of both morphological and pore structural properties. For instance, by varying the molar ratio TEOS/alkyltrimethylammonium it is possible to prepare highly regular mesostrutured materials with cubic or hexagonal structures, by using a co-solvent (like ethanol) one can control the morphology and produce regular spherical mesostructured particles, and finally the use of non ionic primary amines as templates allows the preparation of MMS materials with less regular pore structures but with thicker walls in relation to MCM-41 materials. In recent years, several studies [6-8] have pointed out that prolonged exposure to water vapour can provoke structural changes in MCM-41 and MCM-48 materials, the most significant effects being a loss of pore shape uniformity and a large decrease in surface area, pore size and volume. Different methods aimed at stabilising MCM-41 materials have been devised and applied, like trimethylsilylation [6,8] and carbon deposition [9]. As to the MMS materials it has been claimed [10] that these materials possess higher thermal stability than MCM-41 but, to our knowledge, there are no studies concerning their stability towards water vapour. In this work we report a comparison of the structural properties of different mesostructured materials prepared at room temperature and present a study of the stability of MMS materials. We also describe a new and alternative route to stabilisation of MCM-41 materials, using TEOS (in hexane) as a post-synthesis silicification agent.
364 2. E X P E R I M E N T A L
2.1. Synthesis MCM-41 samples (designated HX/) were prepared by room temperature synthesis based on a previously reported method [2], using ammonia as catalyst, TEOS as silica source, and cationic suffactants, namely alkyltrimethylammonium halides (CiTMAX, X=CI,Br) and hexadecilpyridinium chloride (C16PyCI), as templates. The sample HXl6S1 is composed of spherical particles and was prepared by another synthesis procedure [3] using ethanol as morphology agent. Another sample (HXl6S2) was prepared in the same manner as the previous one with the exception that, during the addition of TEOS, non porous silica spheres (Merck monospher 1000) were added to the mother liquor. The molar ratio of TEOS to total silica (non porous silica+TEOS) was 0.72. An MCM-48 sample (designated CB 16S) and an MMS sample (designated MS16S) were prepared following procedures [4,5] which are modifications of the previous method for preparation of homogeneous spherical particles [3], using hexadeciltrimethylammonium bromide and hexadecylamine as templates, respectively. The molar ratios of reagents used are presented in Table 1. In all cases the synthesis products were recovered by filtration and washed with deionized water, dried at 363 K and finally calcined in air at 823K (heating rate 5K/min) for 5 h. Table 1. Synthesis parameters of the MCM-41, MCM-48 and MMS samples Sample
TEOS /mol
NH3 / mol
H20 / mol
HX12 HX14 HX16-1 HX16-2 HX16-3 HX18 HX16Py HX16S1 HX16S2 CB16S MS16S
1 1 1 1 1 1 1 1 1 1 1
2.96 2.96 2.96 2.96 2.1 2.96 2.96 10.4 10.4 10.4 2.96
155 155 155 155 52 155 155 116 116 310 155
Template Type / mol C12TMABr 0.145 C14TMABr 0.145 C16TMABr 0.145 C16TMAC10.145 C16TMABr 0.124 ClsTMABr 0.145 C16PyCI 0.145 C16TMABr 0 . 2 9 0 C16TMABr 0 . 2 9 0 C~6TMABr 0 . 3 7 0 C16H33NH2 0 . 1 1 5
Co-solvent Type / mol 54 Ethanol 54 Ethanol 47.9 Ethanol Isopropanol 22.4
2.2. Silieification with TEOS A suspension was prepared by adding 0.2g of calcined MCM-41 sample HX16-2 to 10cm 3 of dehydrated hexane. 8 cm 3 of 0.76% (w/w) TEOS solution in hexane were added to the previous suspension under stirring and kept for 1 hour. The sample was isolated by filtration and the excess TEOS was washed with hexane. After drying at 353K for several hours the sample was calcined in air at 823K during 5 hours. The final product is designated HXl 6-2T. Elemental analysis prior to calcination showed a carbon content of 10%. 2.3.Characterisation Nitrogen adsorption isotherms at 77K were determined on a CE Instruments Sorptomatic 1990, while the water adsorption isotherms at 298K were determined gravimetrically using a CI Electronics vacuum microbalance. Each water vapour isotherm took about 2 weeks to determine due to the long equilibration times. X-ray diffraction measurements were carried out on a Seifert TT3000 powder diffractometer, in reflection mode, using Bragg-Brentano geometry and Cu Ktx radiation.
365 3. RESULTS AND DISCUSSION
3.1. Structural characterisation by N2 adsorption and XRD The XRD patterns are shown in Fig. 1 and are typical of each class of mesostructured material. For the MCM-41 samples, 3 well defined peaks are observed and it indicates that long range order is present in the mesostructure. The MCM-48 sample (CB16S) shows 1 sharp peak with a shoulder followed by 4-5 peaks, which is characteristic of a cubic mesophase. As expected, the XRD of the MMS material (MS 16S) has only one peak which is an indication of a less ordered structure in relation to the MCM-41 samples. It is also shown that the silicification treatment with TEOS does not alter the structural order of the original sample. An interesting result is that the XRD pattern of sample HX16S1 is similar to that of sample HX16S2, which was prepared under the same conditions but with the addition of nonporous silica beads. The values of ao obtained from the XRD patterns are included in Table 2. The nitrogen adsorption-desorption isotherms determined are shown in Fig.2. It can be seen that each isotherm exhibits a pore filling step within a fairly narrow range of p/p o, characteristic of ordered meosporous solids. The step is somewhat less defined in the case of sample HX12 due to the small pore size and in the case of the MMS and MCM-48 samples possibly due to a wider pore size distribution and/or a different pore geometry. After the mesopore filling, the isotherms are almost horizontal and, with the exception of sample MS 16S, are reversible suggesting the absence of significant inter-particle agglomeration. The total and external surface areas and the pore volume were estimated from the corresponding Cts plots constructed using standard data for adsorption of nitrogen on non-porous partially hydroxylated silica and usin~ a value of 0.808 gcm -3 for the density of liquid nitrogen at 77 K and the value of 0.137 nm for the corrected cross-sectional area of nitrogen adsorbed in MCM-41 [ 11 ]. The mean pore radius was estimated using the hydraulic radius concept. These results are given in Table 2.
_.__.____...MS.16S HX16-2T HX16-2
~ ~ ~
_2 J / ~ ~
HX16-1
_....// ~ . ~ J 0
~
;
~
20P
~
lb
0
HX14
"--..----",-~. I-IX.12 ~
~
~
20/~
~
lb
Fig. 1. X-ray powder diffraction patterns of siliceous mesostructured materials
366 30
30
HXI8.~ 16-
HX16-2
20
"-"
20
o
E E
,--
c-
10
0.0
'
I
0.2
'
I
0.4
'
I
0.6
'
I
0.8
'
10
oi I
1.0
'
0.0
I
'
0.2
I
'
0.4
p/pO
I
'
0.6
I
'
0.8
1.0
p/pO
30
30
HX16-3
"
2O
9-
m o
E E "o t~ ,--
20 o
E E 10
c:
'
0.0
I
0.2
'
I
'
I
0.4
'
0.6
I
10
'
0.8
'
1.0
0.0
I
0.2
p/pO
'
I
'
0.4
I
0.6
'
I
0.8
'
1.0
p/pO
Fig.2. Nitrogen adsorption-desorption isotherms determined at 77K on a series of samples of siliceous mesostructured materials (empty symbols: adsorption, filled symbols: desorption). Table 2. Results of the structural characterisation by nitrogen adsorption at 77K and XRD Nitrogen adsorption at 77K Sample
Vp/cm3g-1
HX12 0.59 HX14 0.70 HX16-1 0.81 HX16-2 0.83 HX16-3 0.75 HX18 0.88 HX16Py 0.72 HX16S1 0.74 HX16S2 0.50 CB16S 0.55 MS16S 0.52 HX16-2T 0.59 (1) using GN2= 0.137 n I n 2
Aext~
(2)
1
As~
15 27 48 15 20 70 47 9 9 26 32 28 for MCM-41 and
-1
XRD rp(H) / n m
ao/nm
861 1.39 878 1.64 878 1.95 894 1.88 787 1.96 891 2.14 821 1.86 879 1.70 602 1.69 626 1.82 606 1.81 684 1.80 MMS: t=ao-2rp(H); for MCM-48:
t/am (2)
3.45 0.67 3.91 0.63 4.61 0.71 4.36 0.60 4.68 0.76 4.89 0.61 4.83 1.11 4.13 0.73 4.08 0.70 8.21 0.84 4.73 1.11 4.30 0.70 t=ao/~o-rp(H), ~o= 3.0919
367 It can be seen that different synthesis conditions lead to different pore volumes and radii. In the case of samples HX12, HX14, HX16-1 and HX18 it follows that increasing the template chain length results in a relatively large increase in mean pore radius and pore volume without any increase in surface area. In the case of the MCM-41 samples composed of spherical particles, HX16S1 and HX16S2, the pore radius is reduced in comparison with samples HX16-1 and HX16-2 which were prepared using the same surfactant. This reduction suggests that the ethanol used in the synthesis acts not only as a morphology agent but also as a co-surfactant having a shrinking effect in the micelar aggregate. It is interesting to note that the ratio of pore volumes of samples HX16S2 and HX16S1 is 0.68 which is very similar to the molar ratio TEOS/total silica used in the synthesis of the former. With regard to the MCM-48 (CB16S) and MMS (MS16S) samples the results show that these materials have lower pore volumes than the analogous MCM-41 materials. One way to check the structural quality ofMCM-41 samples is to verify if the equation: V p~" = 2--~ O,,z
trp)
(1)
- 9s,z
obtained on the basis of the geometrical model ie, assuming a regular hexagonal array of non intersecting tubular pores, is applicable. According to this2geometrical model proposed for the MCM-41 materials, a plot of 1/Vp as a function of (a0/rp) for a series of samples of different pore size should be linear. It should be noticed that ao and rp refer to the mesostructured phase and don't depend on the amount of non-mesostructured material present. On the other hand this will affect Vp as it is expressed per unit mass of sample. In Fig.3 the plot of eq. 1, assuming p~ii-2.2gcm 3 for the density of silica, is indicated by the dotted line. Also included on the figure are the experimental points calculated from the values ofVp, ao and rp(H) given in Table 2. It should be noted that for the sample HX16S2, 2 values for the pore volume were used, namely the real pore volume per unit mass of total material and the pore volume per unit mass of only the mesostructured phase, that is corrected by 0.72, according to the ratio TEOSAotal silica. It is immiediately apparent from the figure that the experimental results are in good agreement with the theoretical prediction thus showing that, despite the differences in the exact pore structure parameters, most of the samples conform with the hexagonal model proposed and have a mean pore wall density close to 2.2gcm-3 and insignificant amounts of non-mesostructured material. Deviations from the ideal model 5.0
4.0
/~
HX12
V
HX14
[]
HX16-1
[]
HX16-2
[] E o
C)
HX16S1 HX16S2 (uncor.)
~)
HX16S2(cor. 0 72)
HX16-3
3.0
HX16Py HX18
9
~ 2.0
Ps~2.2.gcm --3
1.0
0.0
'
5.0
I
5.5
'
I
6.0
'
I
6.5
'
7.0
(ao/rp)2 Fig. 3. Comparison of experimental data with hexagonal model predictions. Dotted line- theoretical line Symbols- experimental points
368
may be due to a series of factors such as uncertainties in the determination of ao, and in the exact values of density of the silica walls, density and cross-sectional area of nitrogen adsorbed, as well as to the presence of amorphous material. This last factor is well illustrated with the results for sample HX16S2. When the corrected value for the pore volume is used, the experimental point is almost coincident with the theoretical line, however, when the real pore volume is used it shitts upwards due to the presence of amorphous material. It is interesting to note that for samples HX16-1, HX16-2, HX16S1 and HX16S2 (cor. 0.72) the agreement is very good even though these materials have different structural parameters. The result obtained for sample HX16Py may be due to the fact that this sample has higher pore wall thickness and also high pore volume and surface area. These features combined suggest that the pore walls of this material may have a lower density than the normal value assumed for silica of 2.2 gcm 3. In fact, the experimental value falls on a theoretical line estimated using psil=l.9gcm3. The results shown here clearly indicate that this type of representation is a useful and simple tool for the structural evaluation of MCM-41 materials. 3.2. Structural stability towards water vapour The samples involved in this study are the MMS (MS 16S), and the MCM-41 obtained after a post-synthesis silicification treatment (HX16-2T) In order to evaluate the stability of these materials, two complete water isotherms, with outgassing of the samples between runs at 423K, were determined At the end of this prolonged contact with water vapour, the samples were again characterized by means of XRD and nitrogen adsorption. The water vapour isotherms for sample MS 16S are shown in Fig.4 and it clearly indicates that some hydroxylation of the surface occurred during the course of the experiments. These results are similar to those obtained for MCM-41 samples [8]. However, the limiting uptake of the second isotherm is only slightly lower than the uptake of the first isotherm. This behaviour indicates that the structural changes in the MMS materials are less profound than in the MCM-41 materials. This evidence is corroborated by the nitrogen adsorption isotherms and the XRD patterns shown in Fig.5. The results show that atter the prolonged exposure to pure water vapour the sample still retains a high degree of 35
20
-
a)
30-
o
1625-
s" _m
o)
"6 2 0 E E "~m 1 5 "0 tll t,-
o17[]
~
~
[] O ~J
_
E before)
I-
105-
DE] O O
12
4E
2
Y
o 0.0
4
6
e
lo
7e/* '
0.2
I
0.4
0.6
'
I
0.8
'
1.0
p/pO
Fig. 4. Water vapour adsorption-desorption isotherms determined at 298K on MMS sample (MS 16S). O / O : l st isotherm, Dill: 2"d isotherm.
0.0
0.2
0.4
0.6
0.8
1.0
plpO
Fig. 5. a) N2 adsorption-desorption isotherms determined at 77K on MMS sample (MS16S) before (O) and after(D) water vapour adsorption. b) corresponding XRD patterns.
369 Table 3. Results of analysis of water vapour isotherms and of characterisation, before and after water vapour adsorption, by nitrogen adsorption at 77K and XRD. V0.95(1), V0.95(n)- volume adsorbed from uptake of water vapour at 0.95p ~ on first and last runs. Nitrogen adsorption
Water adsorption Sample
V0.95(1)
MS16S After water HX16-2T After water
K ! cm~g 1
V0.95(n)
/ cm3(liq)g-~ / cm~(!iq)g1 0.517 0.466 0.070
0.148
A~ ~ ll
/
m2g"1
DRX
A~~ / m2,g-1
rp(H) /
am
ao i
/
0.520
32
606
1.81
0.456
27
610
1.56
4.63
0.592 0.586
28 26
684 687
1.80 1.77
4.30 4.24
III
4.73
(1) using (YN2-- 0.13 7 nnl 2
structural ordering. This is shown not only by the continued presence of the XRD peak and the pore filling step on the redetermined isotherm, but also by the fact that the initial parts of the isotherms are coincident. Furthermore, from Table 3 it can be seen that the total pore volume decreases by only 12% while the surface area remains constant. This is in marked contrast to the results obtained on as-synthesised MCM-41 materials where the pore volume and the total surface area were reduced by about 61%-43% and 35%-23% [8], respectively, after the determination of the water vapour isotherms. This enhancement in stability may be due to the higher pore wall thickness and/or higher degree of crosslinking inside the walls of these materials. The water vapour isotherms for the treated MCM-41 sample, HX16-2T, are shown in Fig.6 together with the nitrogen isotherm obtained before water exposure. For a better comparison the data is expressed as equivalent liquid volumes by assuming the normal liquid densities. It is evident that this silicification treatment was very effective in making the surface extremely hydrophobic, as the uptake of water vapour is very low. Only when a 3ra consecutive water isotherm was done some water molecules were able to penetrate the pore system, and even then, only at quite high values of relative pressure. Furthermore, the nitrogen isotherms shown
25~[ a)
0.8
0.6 ._O" m v O3 [::: o "-= "0
2o
f
"7
"7
o E E
0.4
"o m c 0.2
9
9
A
&
o.o ~~162 ~' a AI ~ *~~ ' I 0.0
0.2
0.4
I 0.6
p/pO
A
=~ 9' I ~ 0.8
0
~
0
2
%e/S.
s
10
' 1.0
Fig. 6. Water vapour adsorptiondesorption isotherms determined at 298K on modified MCM-41 sample (HX16-2T) : O / e : l ~t isotherm, D / l : 2 "d isotherm, A/A:3 rd isotherm; : N2 isotherm at 77K.
0.0
0.2
0.4
0.6
0.8
1.0
p/p0
Fig. 7. a) N2 adsorption-desorption isotherms determined at 77K on modified MCM41 sample (HX16-2T) before (O) and after (D) water vapour adsorption. b) corresponding XRD patterns.
370
in Fig. 7a), determined before and after water exposure are identical, indicating that there were no structural changes during the course of the water contact. Also, it can be seen from Fig. 7b) that the exposure to water vapour had virtually no effect on the XRD patterns. These results are identical to the results obtained on samples subjected to silylation treatment [8]. However, for the presently proposed silicification treatment, the stabilization is achieved with only a slight decrease in pore width and pore volume, in relation to those of the unmodified material, while for the silylation it is achieved at the expense of a considerable reduction in pore width and pore volume. A possible explanation for the improvement in stability and surface hydrophobicity is that, during this treatment, the silanol groups are stripped off from the surface by the TEOS molecules and after calcination, a thin silica film is formed on the surface, thus protecting the inner walls and increasing the degree of crosslinking. 4. CONCLUSIONS It was shown that room temperature synthesis allows the preparation of highly regular materials with different types of mesophases and morphologies. It was revealed by this study that a useful way to combine the nitrogen adsorption and XRD data in order to check the structural quality of the samples is by means of plotting 1/Vp against (ao/rp(H))2. This work showed also that MMS materials are more stable towards prolonged exposure to pure water vapour than MCM-41 materials. Finally, the proposed silicification post-synthesis procedure seems to be more promissing than the previously studied methods, including the widely used silylation method, for increasing the hydrophobicity of MCM-41 samples and stabilising the structure in the presence of water vapour. REFERENCES
1.
J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmidt C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins & J.C. Schlenker J. Am. Chem. Soc. 114 (1992) 10834. 2. M.C~n, K.K. Unger, A. Matsumoto & K. Tsutsumi in Characterisation of Porous Solids IV; B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing & K.K. Unger, Eds.; Royal Society of Chemistry: Cambridge, 1997, p. 81. 3. M.GrOn, K.K. Unger, A. Matsumoto & K. Tsutsumi Micropor. Mesopor. Mater. 27 (1999) 207. 4. K. Schumacher, M.Gr~n & K.K. Unger Micropor. Mesopor. Mater 27 (1999) 201. 5. M. GrOn, G. B0chel, D. Kumar, K. Schumacher, B. Bidlingmaier & K.K. Unger Stud. Surf. Sci. Catal. 128 (2000) 155. 6. K.A. Koyano, T. Tatsumi, Y. Tanaka & S. Nakata J. Phys. Chem. B 101 (1997) 9436. 7. M.M.L. Ribeiro Carrott, P.J.M. Carrott, A.J.E. Candeias, K.K. Unger & K.S.W. Sing in Fundamentals of Adsorption 6; F. Meunier, Ed.; Elsevier: Amsterdam, 1998, p. 69. 8. M.M.L. Ribeiro Carrott, P.J.M. Carrott, A.J.E. Candeias, K.K. Unger & K.S.W. Sing Langmuir 15 (1999) 8895. 9. M.M.L. Ribeiro Carrott, A.J.E. Candeias, P.J.M. Carrott, K.S.W. Sing & K.K. Unger Langmuir 16 (2000) 9103. 10. P.T. Tanev & T.J. Pinnavaia Chem. Mater. 8 (1996) 2068. 11. M.M.L.Ribeiro Carrott, A.J.E. Candeias, P.J.M. Carrott, P.I. Ravikovitch, A.V. Neimark & A.D. Sequeira Micropor. Mesopor. Mater 47 (2001) 323.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
371
A grand canonical Monte Carlo simulation study of water adsorption in a Vycor-like disordered mesoporous material at 300 K. J. Puibasset and R. J.-M. PeUenq Centre de Recherche sur la Mati~re Divisre, CNRS et Universit6 d'Odrans, lb, rue de la Frrollerie, 45071 Oflrans, cedex 02, France.
Confinement in porous materials is known to modify the thermodynamical properties of fluids. Capillary condensation is an example where a dense phase appears before saturating pressure is reached. Such phenomenon is not yet well understood in disordered mesoporous materials presenting highly interconnected pores. Grand Canonical Monte Carlo simulation is used to investigate the properties of water (fluid of most importance) confined in mesoporous Controled Porous Glass (Vycor-like) numerically obtained by the off-lattice method developed by P. Levitz [Adv. Coll. Int. Sci. 76-77 (1998), 71]. We first outline the interaction model and give the adsorption isotherm obtained at 300 K. Good agreement is found with available experimental results.
1. INTRODUCTION Considerable effort has been dedicated to understand the changes on thermodynamical properties of gas and liquids due to their confinement in porous media [ 1-3]. The special case of water in porous silica (silica gels, zeolites, vycor...) [4-7] has grown interest due to large field of application, from geophysics (water and silica are the most represented on earth) to industry and environment (phase separation, catalysis...). In between experiment and theory, simulation of large hydrated systems is a powerful tool to understand how structural, dynamical and thermodynamical properties are modified in the vicinity of a surface. Previous simulation work focused on dynamical and structural properties of water confined in geometrically simple pores (slit [8,9], or cylinders cut in an initial cubic silica [10,11 ]), supposed to mimic real mesoporous materials. The aim of this work is to investigate, by way of Grand Canonical Monte Carlo simulations, the adsorption properties of water confined in a more realistic disordered mesoporous material previously obtained by off lattice reconstruction [12]. It has been shown [13] that the sample reproduces quite well the geometrical complexity of real high specific surface vycor, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra [ 14]. The PN-TrAZ potential [ 15] is used to describe interactions between water and substrate, and the SPC model for water-water interaction. Adsorption isotherm, and isosteric heat of
372
Figure 1 : 3D representation of the numerical sample of vycor used in this study. Porosity (in gray): 28%, density 1.56 g/cm3, specific surface area 210 mZ/g, mean pore size 3.6 nm, and OH surface density 7 0 H / n m 2. The electrostatic potential along the white line through the little pore is drawn in figure 2.
adsorption have been calculated and compared to experimental data found in literature. The good agreement shows that the model is able to give a satisfactory description of hydrophilic properties of vycor glass.
2. COMPUTATIONAL DETAILS To study adsorption properties in a Controlled Porous Glass (Vycor 7930 like) by molecular simulation, an off-lattice reconstruction algorithm has been used to generate an atomistic vycor-like mesoporous structure. Portions of an initial cubic crystal of cristobalite 106.95 ]k of edge are cut out by applying the off-lattice functional representing the gaussian field associated to the volume autocorrelation function of the studied porous structure [ 12,14]. Periodic boundary conditions are applied to the gaussian field, so as to generate the periodic structure shown in figure 1. To have a realistic description of the surface chemistry, all silicon
Table 1 : PN-TrAZ parameters used for the simulation. water atoms q (e) A (hartree) b (bohr radius -1) Polarizabilities (bohr rad. -3)
Neff
O -0.82 247.7 2.075 7.56 4.476
H 0.41 1.338 2.11 2.655 0.414
Si +2 6163.4 2.395 2.36 1.52
vycor O -1 1543.5 2.19 8.03 4.656
H +0.5 1.338 2.11 2.25 0.324
373
.m_ ,0..i r.
q) O. (,1 !_. 4,,i (,1
_o q)
_'2
-50
-45
-40
-35
-30
-25
-20
X (A)
Figure 2" Electric potential across the small pore (white line in Fig.l). 1-Solid thick line" all atoms. 2-Dash-doted line" contribution from atoms inside a sphere 6.5 ]k in radius. 3-segrnents with a circle on the left" interpolated contribution from atoms out of the sphere 6.5 .~ in radius (grid). 5-Solid line" sum of the contributions 2 and 3, indistinguishable from 1
atoms in an incomplete tetrahedral environment are removed, and all oxygen dangling bonds are saturated with hydrogen atoms placed at 1 A from oxygen perpendicular to the surface. It has been previously shown that this numerical sample reproduces quite well micro and meso textural properties of real vycor [ 12-14]. The water adsorbed on the surface is described by the SPC model [16]. This fast computable model is well suited for very large systems, as it reproduces quite well the thermodynamical properties around ambient temperature, like vapor pressure (0.044 bar against 0.035 bar experimentaly) and enthalpy of vaporization [17]. The extended SPC/E model [ 18] is not adapted to study adsorption properties since the polarization correction that it introduces cannot be well defined in the highly inhomogeneous environment of a molecule adsorbed on a surface. Furthermore, the predicted vapor pressure is only half the experimental value [ 19]. The interaction between water molecules and silica substrate is described in the framework of the PN-TrAZ model [15] which has proven to model successfully the adsorption of simple adsorbates on various zeolites [20]. In this model, the pair potential decomposes in two parts: a repulsion term A e -br due to electronic clouds and the attractive dispersion terms. The repulsive parameters (A ,b) for silica atoms (Si, O, H) are those obtained from studies of adsorption of simple gazes on various zeolites [20] and mesoporous glass [21 ]. Those for water oxygen are chosen to fit the repulsive part of Lennard-Jones from SPC model in the range around equilibrium distance, and those for water hydrogen are taken equal to the parameters for surface hydrogen of vycor. The cross repulsive parameters A and b are obtained by Bohm and Ahlrichs [22] combination rules. The dispersion terms are calculated from polarizabilities and effective number of electron Neff according to the PNTrAZ model up to order r 1~ Values are listed in table 1. The electrostatic contributions are calculated by assigning to atoms the partial charges
374 1,0
A e~ 1,0 o
~-
0,8
0,8
0,6
0,6
./'
qO 0,4
9
9
m 0,2
0,4 0,2
9 O J
0
e~
0,0
80
J
--
70
60 Qst
50 b~u40
(kd/mol)
Ik
8
0
.
0,2
.
.
0,4
.
0,6
0,8
...
1,0
0,0
PIPO
Figure 3 9Circles" GCMC adsorption isotherm (fight) and isosteric heat of adsorption (left) obtained in the numerical mesoporous pseudo-vycor shown in Fig. 1. Experimental curves: solid line :T. Takei et al. [25] (gravimetry in vycor; pore size :4.72 nm). dashed line : N. Markova et al. [26] (micro-calorimetry in vycor; pore size : 4 to 7 nm). The vertical arrow on the left corresponds to bulk water (44 kJ/mol).
(given in table 1) extracted from SPC model for water molecule, and taken equal to those used to calculate the dispersion terms in the framework of the PN-TrAZ model for the silica's atoms. Minimal image convention is adopted to calculate all interactions (cut off equal to half the simulation box). No mean-field correction is added to dispersion contributions since the density is not uniform. Actually, the size of the box is large enough so that large distance contributions are negligible. The electrostatic contribution is evaluated by summing on neutral subgroups of atoms. Implementation of Ewald summation procedure [23] has proven to be of little improvement, probably due to the large box size (107 A), absence of isolated charges, and zero total dipolar moment of the substrate. Calculations show that most of the substrate contribution to electrostatics comes from the surface dipoles (hydroxyls), the tetrahedral SiO4 giving rise to weak quadrupolar term only. To save computer time, the interactions between atoms of water molecules and substrate are calculated in advance and saved on a cubic grid, which is possible since the substrate is kept rigid in this preliminary study. A distance of 1 A has been chosen between two grid points, due to memory constraint. For small and dipolar molecules like water, this is too large to get smooth values close to the silica surface, and prevents direct interpolation. This is why the contribution of the substrate has been divided in two parts, inside and outside a sphere of 6.5 ,~ in radius, centered on the grid cell to be evaluated. Only the contribution from outside is saved on the grid, the rest being calculated directly during simulation. The choice of the radius is a compromise between CPU time saving and smoothness of the grid. Three different grids are necessary: two dispersion-repulsion grids for both O and H atoms, plus one for electrostatic potential. The figure 2 shows the electrostatic potential for a positive
375
Figure 4 : Molecular configurations at three different relative pressures (T=300K).
test charge following the white line acrossthe small pore (see fig. 1). The segments with a circle on the left corresponds to the linear interpolation of the electrostatic grid, the circles beinog one of the grids. The dash-dotted curve is the contribution from atoms inside the sphere 6.5 A in radius, calculated directly during simulation. The solid line is the sum of the interpolated grid plus the direct contribution, undistinguishable from the thick solid curve obtained by a direct complete calculation on all atoms of the substrate with minimal image convention: the difference in total energy never exceeds 4%. This procedure is then well suited to study surface properties of large systems, since it is fast and keeps the advantage of high local precision in the description of the interaction between water molecules and the silica wall, as for a direct calculation. The contribution to energy from water-water interaction is calculated directly with minimal image convention (cutoff equal to half size of the box) to keep high precision, and added to the interaction with the substrate.
3. RESULTS AND DISCUSSION The amount of water adsorbed at 300 K has been determined by way of Grand Canonical Monte Carlo Simulation, which mimics a real adsorption experiment where the temperature, the volume and chemical potential (related to the pressure of the gas) are kept constant. An equal number of translation, rotation, creation or destruction of molecules has been chosen, around 2.104 per water molecule at full saturation, up to 105 at low coverage. At full saturation, corresponding to a vapor pressure of 0.044 bar at 300 K (determined by Gibbs Ensemble Monte Carlo [24] for SPC model), the total number of water molecules in the pores is around 10200, which corresponds to a density around 0.90 g/cm3, close to the density of SPC saturating water at 300 K (0.97 g/cm3, by GEMC method). The discrepancy between both values is probably due to the slow convergence caused by a very low acceptance level of insertion of water molecules in liquid phase. The amount of water adsorbed, normalized to full saturation, is given in figure 3 (fight part) as a function of pressure normalized to the saturating pressure for SPC model (0.044 bar). The isotherm is of type IV in IUPAC classification, showing a rapid increase at low
376 pressure characteristic of high adsorbent-substrate interaction, as expected for hydrophilic surfaces. The three molecular configurations presented in figure 4 show the formation of a monolayer film covering the whole surface prior to further condensation. The steep rise in adsorption at a pressure arround 70% of the saturating pressure is related to capillary condensation in the mesoporosity of the sample. For comparison, two experimental adsorption isotherms are shown. The first one is a gravimetric measurement by T. Takei, et al. [25] in a 7930vycor sample characterized by nitrogen adsorption (pore size around 4.72 nm in diameter). The other one has been obtained by N. Markova, et al. [26] with a double twin microcalorimeter, in a 7930 vycor glass (pore size between 4 and 7 nm). Our simulation results are in good agreement with experimental data, specially at low pressure where water/substrate interactions are predominant. This shows the ability of the PN-TrAZ potential to describe water silica interaction with parameters identical to those deduced from previous studies of adsorption in zeolites. To get more insight in the description of interaction properties of water with silica surfaces, the isosteric heat of adsorption Qst has been calculated from cross-fluctuations in energy U and adsorbed quantity N through equation 1 [27] :
Qst =- - + RT -2
(1)
The values are presented in figure 3 (left part) as a function of the amount adsorbed. The heat of adsorption at very low coverage (75 kJ/mol) is well above the heat of liquefaction of bulk water (44 kJ/mol) which means that the surface is hydrophilic, as previously deduced from the shape of adsorption isotherm at low pressure. We also report the result obtained by Markova, et. al. [27] on vycor. The agreement is quite good, taking into account the fact that our sample has pores slightly smaller than real vycor which enhances the strength of the interaction. 4. CONCLUSION A Grand Canonical Monte Carlo simulation study of water adsorption on vycor has been presented. The large size mesoporous numerical sample previously obtained by offlattice reconstruction method, has proven to mimic quite well the geometric complexity of real vycor, like its specific surface area, pore size, chord distribution and correlation peak in small angle neutron scattering spectra. Its surface chemistry is obtained in a realistic way since all dangling oxygen bonds are saturated with hydrogen which gives 70H/nm 2 in agreement with experiment. The water is described by the SPC model, able to reproduce the thermodynamical properties of water close to ambient temperature with minimal computer cost. The interaction between water and silica is entirely derived in the framework of the PNTrAZ model with the intrinsic parameters for silica previously deduced from simulation study of adsorption of rare gases in vycor. This model describes the dispersion and repulsion contribution, and the electrostatic part is evaluated by using the same partial charges as those used in the PN-TrAZ and SPC model. The Ewald summation correction for electrostatics has proven to be of little improvement, due to the large size of the box (107 A.), absence of isolated charges, and zero substrate total dipolar moment. To save computer time, the
377 interaction of water molecules with the 80 000 atoms of the vycor is divided in two parts defined by a sphere 6.5 A in radius and an electroneutrality condition ; the inner contribution is calculated directly during simulation, the outer one is pre-calculated and saved on a grid. The result for energy is within 4% of the value obtained from a completely direct calculation with all the vycor atomic species. The adsorption isotherm calculated is of type IV in IUPAC classification, showing a rapid increase at low pressure as expected for hydrophilic surfaces. The steep rise in adsorption arround P/P~ is due to capillary condensation in the mesoporous solid. The result is comparable to two available experimental adsorption isotherm of water measured by very different techniques (gravimetry and calorimetry). This result, and the good agreement of the simulated isosteric heat of adsorption at very low coverage (75 kJ/mol) with experimental data, show that the model presented is able to describe quantitatively the hydrophilicity of the vycor surface with no adjustable parameters.
ACKNOWLEDGEMENTS
:
A. DelviUe and P. E. Levitz are gratefully acknowledged for fruitful discussions on long range interactions in large systems and water silica interactions. We also thank the Institut du Drveloppement et des Ressources en Informatique Scientifique (CNRS, Orsay, France) for the IBM computing grant 991153.
REFERENCES
:
[ 1] S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. [2] K. S. Page and P. A. Monson, Phys. Rev. E., 54 (1996) 6557. [3] L. D. Gelb, K. E. Gubbins, R. Radhakrishnan and M. Sliwinska-Bartkowiak, Rep. Prog. Phys., 62 (1999) 1573. [4] J. Dore, Chemical Physics, 258 (2000) 327. [5] M.-C. Bellissent-Funel, S. Longeville, J. M. Zanotti and S. H. Chen, Phys. Rev. Lea., 85 (2000) 3644. [6] E. W. Hansen, R. Schmidt, M. Strcker and D. Akporiaye, J. Phys. Chem., 99 (1995) 4148. [7] S. Takahara, M. Nakano, S. Kittaka, Y. Kuroda, T. Moil, H. Hamano and T. Yamaguchi, J. Phys. Chem., B 103 (28) (1999) 5814. [8] S. H. Lee and P. J. Rossky, J. Chem. Phys., 100 (4) (1994) 3334. [9] E. Spohr, A. Trokhymchuk and D. Henderson, J. of Electroanalytical Chemistry, 450 (1998) 281. [10] M. Rovere, M. A. Ricci, D. Vellati and F. Bruni, J. Chem. Phys., 108 (23) (1998) 9859. [ 11] P. Gallo, M. A. Ricci, M. Rovere, J. Chem. Phys., 116 (1) (2002) 342. [12] P. E. Levitz, Adv. Coll. Int. Sci., 76-77 (1998) 71. [13] R. J.-M. Pellenq, A. Delville, H. Van Damme, and P.E. Levitz, Studies in Surface Science and Catalysis, Vol. 128 (2000) pp.l-8, Elsevier, Amsterdam. [14] P. Levitz, G. Ehret, S. K. Sinha and J. M. Drake, J. Chem. Phys., 95 (8) (1991) 6151. [15] R. J.-M. Pellenq, and D. Nicholson, Mol. Phys. 95 (3) (1998) 549. [16] H. J. C. Berendsen, J. P. M. Postma, W.F. van Gunsteren, and J. Hermans in
378
Intermolecular Forces, ed. B. Pullman, Reidel, Dordrecht, 1981, p.331. [17] W. L. Jorgensen, J. Chandrasekhar, J.D. Madura, R.W. Impey and M.L. Klein, J. Chem. Phys., 79 (2) (1983) 926. [ 18] H.J.C. Berendsen,J.R. Grigera, and T.P. Straatsma J. Phys. Chem., 91 (24) (1987) 6269. [19] J. R. Errington, and A. Z. Panagiotopoulos, J. Phys Chem., B 102 (38) (1998) 7470. [20] D. Nicholson and R. J.-M. Pellenq, Adv. Coll. Int. Sci., 76-77 (1998) 179. [21] R. J.-M. Pellenq, B. Rousseau, and P. E. Levitz, Phys. Chem. Chem. Phys. 3 (2001) 1207. [22] H. J. Bohm and R. Ahlrichs, J. chem. Phys., 77 (1982) 2028. [23] P. Ewald, Ann. Phys., 64 (1921) 253. D. M. Heyes, Phys. Rev. B : Condens. Matter, 49 (2) (1994) 755. [24] A. Z. Panagiotopoulos, Mol. Phys., 61 (4) (1987) 813. [25] T. Takei, A. Yamazaki, T. Watanabe, and M. Chikazawa, J. Coll. Int. Sci. 188 (1997) 409. [26] N. Markova, E. Sparr, and L. Wads6, Thermochimica Acta, 374 (2001) 93. [27] D. Nicholson and N. G. Parsonage, Computer Simulation and the Statistical Mechanics of Adsorption, Academic Press, London, 1982, p 97.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
379
A review of the application of the BET equation to experimental data: the C
parameter F. Salvador; C. Sfinchez-Jim6nez; M. J. S~nchez-Montero; A. Salvador Dpto. de Quimica-Fisica. Universidad de Salamanca. 37008 Salamanca. Spain.
Here we offer a review of the BET plot proposing other linear transformations of the original equation, with a view to checking the model. Comparative study of the results obtained with the different equations leads to the conclusion that the C constant of the BET equation cannot be constant; instead it varies with the amount of molecules adsorbed. We describe how to determine the values of C as a function of surface coverage. These values are compared with the molar adsorption enthalpies obtained using the isosteric method. The similarity of the results allows us to confirm the energetic meaning attributed to C. The variation in C with surface coverage also serves to explain the different types of isotherm, including Types III and V, and why the BET plot is only linear for a limited stretch of the isotherm. 1. I N T R O D U C T I O N Interpretation of the isotherms of the physical adsorption of gases has been the motive of considerable debate among surface chemists. Many models, theories and equations have been proposed to explain these isotherms in qualitative and quantitative terms, although -currently- there is no general treatment available that is able to fit the whole adsorption isotherm and explain the different types of isotherm that arise. One of the most relevant theories is undoubtedly that of multilayer adsorption, proposed by Brunauer, Emmett and Teller (BET) [ 1]. From its inception, this theory received much criticism [2,3], both from the point of view of how the equation was deduced and, indeed, the very assumptions contemplated in the model. The assumption that the energy of adsorption in the first layer remains constant, and hence independent of the amount of molecules adsorbed, is unrealistic. The energetic heterogeneity, as a result of different adsorbent-adsorbate and adsorbateadsorbate interactions, is a general characteristic of all adsorption systems. Despite the lack of consistency of the theory, however, the model has persisted and the BET equation continues to be one of those most commonly used. Moreover, the BET method for determining the surface area of solids is internationally accepted and is routinely used in the determination of that parameter. This is because the method is able to provide fairly acceptable results concerning the surface area of many solids, despite the defects inherent to the theory. This and other paradoxes of the BET theory remain to be elucidated and it is because of this that we were prompted to make a review of the BET plot, proposing other ways to check the model that serve to clarify some aspects of the theory. 2. T H E B E T P L O T In their original work Brunauer, Emmett and Teller obtained the following equation:
380 n --
nm
Cx =
(1)
(1 - x)(1 - x + Cx)
where n represents the moles of gas adsorbed at the equilibrium pressure p; nm are the moles of gas adsorbed when the surface of the adsorbent is covered by a complete monomolecular layer; x is the relative pressure (p/pO), pO is the vapour pressure of the liquid adsorbate; and C is a constant defined as: C = a~v2/a2v 1exp[(E~ - E t ) / R T ]
(2)
where al and ae are the condensation coefficients for the first and second layers; Vl and ve are the frequencies of oscillation of the molecule in a direction normal to the surface; E1 is the heat of adsorption of the first layer, and EL is the liquefaction heat. It is generally assumed that al v2/a2 vl = 1, such that the constant C would be: C~exp((E1-EL)/RT); El-EL is called the "net heat o f adsorption" Currently, however, it is more acceptable to use the term "net molar energy o f adsorption" However, if the BET equation is deduced using a statistical mechanical treatment [4], the C parameter takes another meaning: C = J~/Jt exp[(c~ - e L ) / k T ]
(3)
where jlexp(c1/kT) is the partition function of a molecule in the first layer and jtexp(ei/kT) is the partition function in a higher layer. The C constant therefore now involves energetic and entropic terms [2]. To check the validity of their model, Brunauer, Emmett and Teller proposed the transformation of the original equation (1) into the linear form: x 1 C-1 = ~+ x (4) n(1-x) nmC nmC According to this, the plot of x/[n(1-x)] against x should be a straight line, from whose slope and intercept it should be possible to deduce the values of the constants nm and C. In practice, the authors of the model observed that the linearity of the BET representation was reduced to a small range (p/pO= 0,05 - 0,3). Later studies have shown that for many systems that range is much more reduced, although there is still no criterion to select the range. Moreover, for Type III and V isotherms, the BET plot is never linear. One also has the paradox that in general linearity occurs at low coverage values, where the energetic heterogeneity, not taken into account in the model, is higher. The BET plot is still used today as originally proposed by the authors. However, there are other linear transformations [5] of the original equation (1) that can be used to check the BET model" x 1 1 x ~
n ( 1=- x -) 2n(1-x) 2
nm l - x
(5)
= nmC - Cn(1 - x)
(6)
~
nmC
X
x
1
n(1-x)
nm
1 1-x nmC
x
(7)
Some of them have certain advantages over the BET plot (equation (4)). Thus, equation (5) allows one to determine the values of nm directly from the slope without having
381
to use the intercept, such that the determination is simpler and precise because it is not affected by the error of the intercept. Likewise, and for the same reasons, the constant C could be obtained with better precision from the slope of equation (6). Equation (7) has been proposed and used by other authors [6,7]. Here we conducted a comparative study with these three linear transformations (equations (5), (6) and (7)) and transformation (4), proposed by the authors of the model, using the p/pO range in all of them, in which the standard BET plot (equation (4)) is linear. This study was performed with a large number of adsorbents, although here we only report four examples. Figure 1 shows the isotherms of the adsorption-desorption of N2 at 77.3 K for the four samples studied. These isotherms were obtained at high resolution (p/pO= 10-7_1) after a prolonged outgassing of the samples at high temperature, using an ASAP 2010 device from Micromeretics. 450
105
~0
"7,
70
N2/activated c a r b o ~
300
e~
~
35
150
0
0
,
~
0,5
1
0,5
1
X 600
N 2/graphite
"7, e~
]~ 400
"~
200
10
5
T
0
,
0,5
,
X
1
i
0
0,5
,
X
1
Figure 1. Isotherms of the adsorption/desorption of N2 at 77.3 K of different adsorbents. The figures 2 and 3 show the treatment of the four isotherms with each of the linear transformations. It is clear that when equations (4) and (5) are used the regressions are always linear. However, when equations (6) and (7) are employed they are not. This type of behaviour is general in all the isotherms assayed. This apparent mathematical contradiction can be explained as follows: 9 The non-linearity observed with equations (6) and (7) clearly indicates that the slope and/or intercept of these plots does not remain constant; that is, that C is not constant but instead varies along the isotherm. 9 Equations (4) and (5) provide linear fits of the isotherm, even though C is not constant, as long as the values taken by that parameter are high. Under these conditions (C-1)/C 1 and I/C 0, such that the slope and intercept of equations (4) and (5) hardly change at all. Only when C takes small values does the fit cease to be linear. Equations (4) and (5) are able to mask, or neutralise, the changing effect of the C parameter when this takes high values. Therefore, both equations can be used to obtain the true value of the constant nm. Figure 2 shows the values of that constant for the different isotherms obtained from the two equations; the values are almost identical.
382 2,1E-02
2,7E-02
N 2~clay nm-16'60 cm3/g
1,4E-02
j
N 2~clay
nm--6,6cm//"/ 'g
,',, 1,8E-02 (a)
(b)
"~9,0E-03
"~ 7,0E-03
0,0E+00
i
1
0,08
0,16 X
0,0E+00
|
0,24
0,24
0,36
0,48
x/(~-x)
9,0E-04
9,0E-04
N 2/activated carbon n m5248'O2cm3/g 7
6,0E-04
0,12
0,32
N2/activated carbon
~..:.,6,0E-04
n m?247'99c3/g f
(a)
) "~ 3,0E-04
"~ 3,0E-04
0,0E+00
0,0E+00 0
0,05
0,1 x
0,15
0
0,2
0,05
0,1
0,15
0,2
x/(1-x) 2,1E-03
2,1E-03
N 2/silica
N 2/silica 1,4E-03
" 1,4E-03
nm--91'67cm3/g J
n m=91,79 cm3/gf
(a)
(b)
"~ 7,0E-04
"~7,0E-04
0,0E+00 o
0,0E+00
I
i
!
0,04
0,08 x
o,12
o,16
i
o
9,0E-02
9,0E-02
x/(1-x)
i
0,12
0,16
n m-l,67 cm3/g
~.....~6,0E-02
f
|
0,08
N 2/graphite
N 2/graphite
6,0E-02 n m~1,68 cm3/g
0,04
)
(a) "~3,0E-02
"~ 3,0E-02 0,0E+00 0
i
i
i
0,03
0,06 x
0,09
0,0E+00 0,12
0
0,03
0,06
x/(1-x)
Figure 2. Fittings of the BET equation using: (a) equation (4); (b) equation (5).
0,09
0,12
383 4,7E+03
1,3E-01
N 2~clay ~,s 1,1E-01
3,1E+03 e,,i
(c) I
I
f
I
J
(d)
~ 8,0E-02
1,6E+03
0,0E+00
,
i
i
1
11
14
17
20
5,5E-02 0
125
n(1-x)
250
375
500
(1-x)/x
8,7E+04
5,9E-03
N2/activated carbon ~[ 5,1E-03
5,8E+04
f
(c)
(d)
~" 4,3E-03
2,9E+04
0,0E+00
. . . .
180
,. . . . . . . .
198
3,5E-03
~,
216
234
252
i
i
!
125
250
375
n(1-x)
500
(1-x)/x
6,0E+03
1,6E-02
N 2/silica 4,0E+03
.~1,4E-02
(d)
(c) 2,0E+03
~1,2E-02
0,0E+00 60
69
78 n(1-x)
87
96
1,0E-02 . . . . . . . . . . ,. . 0 25
, ..... 50
(1-x)/x
, 75
I00
0,95
150
N 2/graphite I
~oo
~
I
I
0,8
(c)
(d)
50
~'0,65
i
t
i
1,2
1,4
1,6
n(1-x)
0,5 1,8
0
50
100
(1-x)/x
Figure 3. Fittings of the BET equation using: (c) equation (6); (d) equation (7).
150
200
384
3. T H E C P A R A M E T E R The results obtained endow the C parameter with a hitherto unknown importance. It can now be understood why the part of the isotherm that shows a good BET fit is the one in which C takes high values. It is therefore the values of the C parameter that govem the range in which the BET representation is linear. The need for C to take high values explains why the Type III and V isotherms do not "fit" the BET model, because these types of isotherms are attributed very low C values. Also, the variation in the C parameter along the isotherm serve to account for the different shapes of the isotherms. From a mathematical point of view, the C constant of the BET equation is intimately related to the shape of the isotherm. A detailed analysis of this can be found in the book by Grengg and Sing [2], according to which when the C constant is lower than 2 the BET equation affords a convex curve, with the shape of the Type III isotherm. However, when the C constant is above 2 the curve acquires the shape of the Type V isotherm. What is absolutely clear is that the most important consequence of C changing with the surface coverage is that this circumvents one of the most important criticisms that have been made about the BET model. Now, the adsorption heat in the first layer changes with the amount adsorbed, as happens in real systems. 72000
100000 1
I N2/activated carbon
ooot
54000
[
75000 1 ~
18000
I~
/tI 9~176176
i 100001 I
~ 36000
12000-[I[. - . . . . .
s~176176176 ]II 6~176176 ~, 3000 25000
0
~ Surface coverage n/n m 36000 ] ........
I N2/silicagel
o~~
[
500
0 {
0
/t
~ It 3~176176 t _
0
,
I
6000 ] |
~6ooootI 250 ~
,,
12~176176 I N2/graphite
I]
18000
9000
1,3
9ooootl 9~176176
1I
i
1
Surface coverage n/n m I
750 ..........
7000
0,7
- - - 7 ......... - " - " : : : : , - ' -
0,5 1 1,5 2 Surface coverage n/n ,,
I
30000 ] I
Lo
0l
oi, ; 1~,, '~ ""-""~
* : :',
2,5
0
0,5 1 1,5 2 2,5 Surface coverage n/n m
F i g u r e 4. Variation in the C p a r a m e t e r o f the B E T against surface coverage.
385
The values of the C parameter for each surface coverage can be derived from equation (1), according to the following expression: C= n(1-x)2 x[nm-n(1-x)] (8) The constant nm should be determined previously by applying one of the equations (4) or (5) to the part of the isotherm in which the fit is linear. Figure 4 shows the values of C, obtained using this procedure, against surface coverage. It may be seen that for low coverage values the values of C are very high, thereafter decreasing rapidly when the monolayer is completed. In practice, the values of C are closely related to nm, such that if nm is not sufficiently precise, C could take values that are not permitted for that parameter. Thus, in the case of equation (8) C could reach an infinite value in the case where n(1-x) =nm. It should be taken into account that although nm is a constant its value is slightly affected by the range of p/pO selected to perform the linear regression. Table 1 shows that behaviour in the case of a clay isotherm. Table 1.- Calculation of nmfrom BETplot for different p/p Orange. ......... ~/~~ ......r a.n. g e................. .n...u.. .m... .b.er.,..o. f a. .a.. t a. . ..........n...:. ..(c......m..~/. .~ ...... c.. o.. rr.e., l..a....a.o..n. . . ..c.o. e...f.f.t. .c.. ie n. ..t .....
1,0 1,0 2,3 1,2 1,2 5,0
10-7 - 0,28 10-7 - 0,33 10-3 - 0,28 10-2 - 0,28 10-2- 0,12 10-2- 0,33
61 64 28 21 15 15
16,50 16,37 16,56 16,53 17,11 15,95
0,9997 0,9996 0,9997 0,9996 0,9999 0,9990
According to equation (2), lnC is directly related to the heat of adsorption in the first layer. Accordingly, the values of the C parameter were compared with the differential enthalpies of adsorption. These were obtained using the isosteric method from the adsorption data of N2 at two different but close temperatures: 77.3 and 87.3 K. Figure 5 shows that lnC and the differential enthalpies of adsorption vary with the surface coverage in the same way, confirming the energetic nature of the C parameter.
N 2~clay
3300
lpy
2750
12
3500 ~3000
10
~ 2500
~
2200
8
~
1650
6~
"~
1100
4
~ 1000
~
550
2
.~ 500
~N2/silicagel
~2000
6~
1500
0
0
0,35
0,7 ~'n m
1,05
1,4
1
,
0
0,5
,
1 n/n m
,
1,5
2
Figure 5. Comparison of the C parameters of the BET equation with the differential enthalpies obtained using the isosteric method.
386 CONCLUSIONS This review of the BET plot allows us to suggest that there are two different types of linear transformation of the original equation that can be used to check the model. One of them, to which equations (6) and (7) belong, is characterised because those transformations unveil the energetic heterogeneity contained in the adsorption data. With these transformations, the regression is not linear, clearly indicating that the C constant cannot remain constant but, instead, changes with the surface coverage. The other type of linear transformation, to which equations (4) and (5) belong, is characterised because those transformations neutralise or mask the energetic heterogeneity of the adsorption data when C takes high values. Under these conditions, the regression is linear and can be used to obtain the value of the nm constant. Of both equations, (4) and (5), the second one permits a simpler and more precise determination of the nm constant since it is calculated directly from the slope. The energetic heterogeneity of adsorption is reflected in the value of the C parameter. Its values can be obtained directly from the BET equation once the value of nm is known. The fact that the C parameter changes with surface coverage implies an important improvement in the model since it approaches it to real systems. It also explains many of the paradoxes and contradictions hitherto present in the BET model such as: 9 That is it only possible to fit a reduced part of the isotherm 9 That there is no criterion for selecting the linear part. 9 The impossibility of confirming the BET model in Type III and V isotherms. The energetic meaning of the C parameter is also confirmed when one observes that it is closely related to the differential enthalpies of adsorption. Finally, one could say that if Brunauer, Emmett and Teller had used the linear transformations (6) or (7) to check their model, the BET theory would not have been published. Indeed, we would have been left without one of the most interesting theories and one of the simplest models for the determination of the surface area of many solids.
Ackowlegements The authors thank DGESIC for financial support. Project PPQ2000-1249 References [ 1] Brunauer S., Emmett P.H. and Teller E. J. Am. Chem. Soc., 60, (1938), 309. [2] Gregg S.J. and Sing K.S.W. Adsorption, Surface Area and Porosity, Academic Press, London. (1982). [3] Rouquerol F., Rouquerol J. And Sing K. Adsorption by Powders and Porous Solids, Academic Press, London. (1999). [4] Hill T.L.J. Am. Chem. Soc. 68, (1946), 535. [5] Salvador F., Merch~in M.D., S~nchez C., Salvador A. Proceedings of XXIII Reuniao IbOrica de Adsor(ao. Pg. 49, 1998. [6] Keii T., Takagi, T. And Kanataka, S. Anal Chem. 33, (1961), 1965. [7] Parra J.B., de Sousa J.C., Bansal R.C., Pis J.J. and Pajares J.A. Adsorption Sci. Tech. 11, (1994), 51.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
387
T h e r m o g r a v i m e t r i c and Sorption M e a s u r e m e n t Techniques/Instruments Jfirgen U. Keller l, Erich Robens 2, Cedric du Fresne von Hohenesche 2 lInstitut mr Fluid- und Thermodynamik, Universitfit Siegen GH, D - 57068 Siegen, Germany, [email protected] 2Institut mr Anorganische Chemie und Analytische Chemie der Johannes GutenbergUniversitfit, Duesbergweg 10-14, D - 55099 Mainz, Germany, [email protected], [email protected] The survey covers commercially offered instruments for the determination of the surface structure of porous solids based on isothermal measurement of physisorption of inert gases and of thermogravimetric instruments. Basic assumptions, features and experimental uncertainties are compared.
1. GENERAL PROBLEMS OF SORPTION MEASUREMENT The geometric surface structure in the sub-micrometer range of porous materials is usually characterised by specific surface area, specific pore volume, pore size distribution and density [ 1,2]. A variety of methods is available for the determination of those parameters, however all of them provide no absolute values. On account of the fractal nature of the surface structure, the results depend on the size of the yardstick or unit-measure used [3,4]. In the range of nanostructures we find no reliable edges at which the yardstick could be placed. In this case, it is advisable to apply a measuring method which corresponds to the application. Adsorption measurements using chemical inert gases can give the required information of the surface structure. Here, the yardstick is the area required by one adsorbed test molecule or its volume, respectively. However, measurements using model gases like He, Ar, N2 need to be supplemented by field-experienced investigations. To obtain the adsorbed amount as a function of pressure and temperature different variables can be measured: adsorbed mass by means of a balance, consumption of the sorptive gas by means of calibrated volumes, concentration changes in a carrier gas by means of gas chromatography, changes of the dielectric constant, heat of adsorption, nuclear magnetic resonance of special adsorptives, radiation dose of radioactive labelled gas. All these methods, however, give not the pure adsorbed amount because adsorption is always accompanied by other physical phenomena, e.g. buoyancy in gravimetric or dead space in volumetric measurements. To give an example we would like to emphasise that the result of both, volumetric and gravimetric measurements is the quantity ~-m_ofvv. (1) where m is the adsorbed m a s s , pf the density of the adsorptive, and V v the volume which is not accessible for the sorptive gas. Assuming that this volume is identical with the solid volume of the sample VV= V s, (2a)
388
implies that the sorptive gas phase comprises also the adsorbed phase. Then m in eq. (1) is the Gibbs excess mass: m = mGEX. (3a) It can be calculated if we have at least an approximate value for V s, e.g. by determining the sample volume by means of Helium pycnometry and assuming that Helium is not adsorbed: V s - Vr~e (4a) Using eq. (2a-4a) in eq. (1) we get: s (5a) mOEX= f 2 + p f V~e. In this way today all adsorption measurements are evaluated. Alternatively we can define V v as the solid volume of the sample plus the volume of the adsorbate va: V v - - V s q" V a
(2b)
Now, m in eq. (1) is the absolute value of the adsorbed mass of gas: m = m a. (3b) Determining again the volume of the solid sample V s by means of Helium pycnometry a nd using eq. (4a) together with v a = m__a_~_
(4b)
p0 we get m a from eqs. (1, 2b - 4b): ma m a= ~2+pf(v~e + - ~ ) or m ~ =
f s + p Vile mGEX = pf pf "
1-~
p;
1
(5b)
p;
Because the density of the adsorbate p; cannot be measured directly, the liquid density at the triple point may be used as an approximation. It should be born in mind that according to eqs. (5b) mGFX and m a are practically identical for low densities or at low pressures of the sorptive. At high densities, e.g. near saturation pressure remarkable differences may exist. Consequently, the same effects should be taken into account when calculating the inner surface of porous solids from mGEX or m a, respectively. Recently, Keller published a method allowing for the absolute determination of the adsorbed mass m a of porous materials [5]. It consists of a combination of calorimetric and impedance spectroscopic measurements. Unfortunately, that method is experimentally difficult and needs improvements for practical applications [6]. We are faced with special problems if the volume of the sample, e.g. of a polymer, is varied by swelling. Sorption on such materials may be investigated by means of a horizontally arranged rotary pendulum [7,8] in combination with gravimetric density determinations [9]. Sophisticated pendulum experiments may be replaced by impedance measurements for the determination of the (frequency-dependent) dielectric constant [ 10]. Hardly investigated is concurrent adsorption from gas mixtures. For binary mixtures measurements using two independent methods are necessary, e.g. gravimetric and volumetric measurements [11,12,13] or head space analysis using a gas chromatograph or a mass spectrometer. For more than two components at any rate gas analysis is required.
389 2. SORPTION MEASURING METHODS AND INSTRUMENTS Four types of sorption measuring instruments are on the market: volumetric/manometric, gravimetric, carrier gas and calorimetric. The carrier gas method is a variant of gas chromatography and an available gas chromatograph may be modified for that purpose. Carrier gas apparatus are simple, cheap and easy in handling (Table 1). Vacuum is not required. The measurement of the adsorbed amount is indirect and the method does not claim for high accuracy. The method is usually applied for fast single point determinations of the specific surface area. Multipoint measurements of isotherms are complicated. Micropore filling can be distinguished from surface covering with a pulsed gas stream [14]. In gravimetry the adsorbed amount is measured generally by means of an electronic beam microbalances with relative sensitivity up to 10.7 (Table 3). For investigations in corrosive gases the magnetic suspension balance is favourable but expensive. A cheap alternative is the helical quartz spring balance which, however, is not commercially manufactured. For the measurement of adsorption isotherms the pressure is changed stepwise and then kept constant, measured by an electronic manometer and controlled by means of solenoid valves. Sample degassing is performed at the balance and thus optimum degassing conditions (temperature, pressure and time) can be found and the mass of the dry sample determined. The gravimetric method has in its favour the fact that all variables: adsorbed mass, gas pressure and temperature are measured independently. Apparatus are more complicated and more expensive on account of the additional vacuum microbalance. Balance operating requires some skills. The sample is not in direct contact with the thermostat and must be shielded against false heat radiation. Measures to avoid the influence of eddy gas flow may be required. At low pressure the Knudsen pressure difference causes thermal gas flow which seriously interferes in measurements in the Henry region. A volumetric/manometric sorption measuring apparatus consists of a calibrated and thermostated sample bulb, a burette for controlled admission of the sorptive gas and some manometers. The sample is dried mostly in a separate degassing station in vacuum at elevated temperature. Preferably optimum degassing conditions are found out thermogravimetrically. Because samples of any size can be investigated using suitable sample bulbs high sensitivity can be achieved but unfortunately not high accuracy. In practice, the relative sensitivity of both methods is often equivalent. However, using a modem magnetic suspension balance gravimetric measurements can be more sensitive and accurate by one magnitude [6]. This is due to the simple fact, that gravimetric measurements allow to detect the approach to adsorption equilibrium in gas-solid sorbent systems, whereas volumetric/manometric measurements due to the limited possibilities of today's manometer do not. The most serious error in volumetry is the error in calibration of the dead space; in gravimetry it is the error in the determination of buoyancy. Indeed with gravimetry not the adsorbed mass but the difference between mass and buoyancy is measured and in volumetry the difference between the volume of sorptive gas consumed and the gas remaining in the dead space, cp. Eq. (1). Both erroneous influences are equal because both are based on the sample volume. Therefore, this influence cannot be cancelled out by simultaneous measurements using both methods. In volumetry the two variables, pressure and adsorbed amount, are determined by only one instrument and this results in a larger measuring error which is added up at each step of the adsorption isotherm. At low pressures a pressure difference due to the Knudsen effect should be taken into account. Stepwise measurements using any of both methods can be remarkably shortened by extrapolation of the equilibrium values. Evaluation of the kinetic curves allow conclusions on the mechanism of adsorption [ 15]. Results of buoyancy or dead space calibration may be used to calculate the sample density.
390 Table 1. Manufacturer of volumetric / carrier gas sorption instruments, gas pycnometers Manufacturer
A PCD
Beckman Coulter UK Ltd., Oakley Court, Kingsmead Business Park, London Rd. High Wycombe Bucks. HP 11 1JU, U.K. (including Omicron), Tel.: +44149444-1181, Fax: -7558 [email protected], www.beckmancoulter.com
9 .
.
9
Thermo Finnigan, Thermoquest Italia, Strada Rivoltana, s.n., 1-20090 Rodano, Milano, Italy, Tel.: +39-02950592-66, Fax: -56, [email protected], www.thermo.com, www.porotec.de, [email protected]
9 ~ ~ ~
Jouan Robotics - Gira, Rue des Bruy~res, Z.I. Berlanne, F - 64160 Morlaas, France, Tel: +33-559-308-38-3, Fax: -4, [email protected], www.gira.fr
~
Micromeritics, 1 Micromeritics Drive, Norcoss, GA 30093-1877, USA, Tel." +1770-662-36-60, Fax: -96 [email protected], www.micromertics.com
~ ~ ~
Porous Materials, Inc., Comell University Research and Technology Park, 83 Brown Road, Bldg. 4, Ithaca, NY 14850, USA, Tel.: +1-607-257-554, Fax: 5639, www.pmiapp.com, [email protected]
.
Quantachrome Corp., 1900 Corporate Drive, Boynton Beach, Florida 33426 USA, Tel.: + 1-561-731-4999 Fax: -732-9888, www.quantachrome.com, [email protected]
~ ~ ~ ~
Zeton Altamira, 149 Delta drive, Suite 200 Pittsburgh, PA 15238, USA Tel: + 1-412-9636-385, Fax: -485, www.zetonaltamira.com, [email protected]
9
9 9
9
9 ~ ~
Table 2. Manufacturer of gravimetric gas sorption measuring instruments Manufacturer Thermo Cahn, 5225 Verona Road, Madison, WI 53711 USA Tel.: + 1-608-276-633, Fax -273-6827, www.cahn.com, www.thermocahn.com, www.theromhaake.com
Balance Cahn
CI Electronics Ltd., Brunel Rd. Churchfields, Salisbury, Wiltshire SP2 7PX, U.K., Tel.: +44-1722-424100, Fax: 323222, [email protected] www.cielec.com
CI
Hiden Isochema Ltd., 420 Europa Bd., Gemini Business Park, Warrington WA5 7UN, England, Tel: +44-1925-445225, Fax: -416518, [email protected], www.hidenanalytical.com
CI
Rubotherm, Universit~itsstr. 142, D-44799 Bochum, Germany Tel.: +234-70996-0, Fax:-22, www.rubotherm.com, [email protected]
Rubotherm+ Mettler or Sartorius
Surface Measurement Systems, 3 Warple Mews, Warple Way, London W3 0RF, U.K. Tel.: +44-20 8749 4900, Fax: -6749 www.smsuk.co.uk, [email protected] VTI Corp. 7650 W. 26 th Ave., Hialeah, FL 33016-5704, USA Tel.: +1-305-828-4700, Fax-0299, www.vticorp.com, vti@vticorp
Cahn
Cahn or CI
APCWD 9 9 9 9
.
.
9 9 9 9
9 9 9
9 9 9 9
,,
9 ,,
9
A: spec.surface area, P: pore size distribution, C: chemisorption, D: density, W: water vapour
391
Figure 2. Scheme of a thermogravimetrical apparatus equipped with magnetic suspension balance. Typical thermogram of the decomposition of Ca-Oxalate with reaction gas analysis by mass spectromety.
392 In Table 1 manufacturers of carrier gas and volumetric adsorption measuring instruments are compiled. Most of these companies offer also mercury porosimeter and gas pycnometer. Mercury porosimetry extends the measuring range of the sorption method towards larger pores. Table 2 gives a survey on the commercial offer of gravimetric apparatus. Calorimetric measuring techniques give additional information on thermodynamic data. The measuring technique is sophisticated and specialised instruments are not yet on the market. In general Tian-Calvet calorimeters are used by which the heat of adsorption is measured (Fig. 1). From this the adsorption enthalpy and the adsorbed mass in principle can be calculated. With thermogravimetric techniques (Table 4) the desorption is measured of volatile species set free from the solid sample at increasing temperature at low partial pressure of the vaporising component. An example of an sophticated apparatus is shown in Fig. 2. The mass curve as a function of time or temperature gives a characteristic "fingerprint" of the material. Measuring very slowly, at quasi-isothermal conditions, from the thermogram the specific surface area and the pore size distribution can be assessed [5]. Table 3. Manufacturer of vacuum balances Manufacturer Beckman Instruments Inc., Fullerton, CA 82834, USA / 90 Boroline Rd., Allendale, NJ 07401-1613, USA (including Stanton-Redcroft), Tel.: + 1-201818-8900, Fax: -9740, www.beckmancoulter.com Thermo Cahn, 5225 Verona Road, Madison, WI 53711 USA, Tel.: +1-608-276633, Fax -273-6827, www.cahn.com, www.thermocahn.com CI Electronics Ltd, Brunel Rd Churchfields, Salisbury, Wiltshire SP2 7PX, UK, Tel.: +44-1722-424100, Fax: -323222, [email protected], www.cielec.com Linseis Mel3ger~ite GmbH, Viellitzer Str. 43, D-95100 Selb, Germany, Tel.: +49-9287-880-0, Fax:-70867, [email protected], www.linseis.com Mettler-Toledo AG, Im Langacher, P.O. Box, CH-8606 Greifensee, Switzerland Tel.: +41-1-944-45-45, Fax: -10, [email protected], www.mt.com Perkin-Elmer Instruments, 710 Bridgeport Ave, Shelton CT 06484-4794, USA + 1-203-925-4600, Fax: -4654, www.perkinelmer.com, [email protected] Rheometric Scientific, Inc. One Possumtown Road, Piscataway NJ 08854, USA, Tel.: +1-732-560-8550, Fax: 7451, www.rheosci.com, [email protected] (formerly Omnitherm & PL) Rubotherm, Universit~itsstr. 142, D-44799 Bochum, Germany, Tel.: +49-23470996-0, Fax: -22, www.rubotherm.com, [email protected] SETARAM, 7 rue de l'Oratoire, F-69300 Caluire, France, Tel.: +33-472102525, Fax: -8286355, [email protected], www.setaram.com Sartorius AG, 37070 G6ttingen, Tel.: +551-308-0, Fax: -289 www.sartorius.com, webmaster, [email protected]
VTS 9
9 9 9 9
9 9 9 9
9 9
TA Instruments, 109 Lukens Driv e, New Castle, DE 19720-2795, USA, + 1-302-427-400-0, Fax: -1, www.tainst.com, [email protected] (former DuPont) V - vacuum balance, T - thermobalance, S = suspension balance, M = microbalance
9
M
393 Table 4. Manufacturers of thermogravimetric instruments Manufacturer Balance B~.hr-Thermoanalyse GmbH, Altendorfstr. 12, D-32603 Hfillhorst, Germany, B~ihr Tel.: 05744-9302-0, Fax: -90, [email protected], www.baehr-thermo.de Beckman Instruments Inc., Fullerton, CA 82834, USA / 90 Boroline Rd., Beckman Allendale, NJ 07401-1613, USA (including Stanton-Redcrofl), Tel.: + 1-201-8188900, Fax: -9740, www.beckmancoulter.com Thermo Cahn, 5225 Verona Road, Madison, WI 53711 USA Cahn Tel.: + 1-608-276-633, Fax -273-6827, www.cahn.com www.thermocahn:com Instrument Specialists Inc., 2402 Spring Ridge Drive, Suite B, Spring Grove, IL 60081, USA, Tel.: +1-815-675-1550, Fax: -1552, [email protected] www.instrument-specialists.com Linseis Mel3ger~ite GmbH, Viellitzer Str. 43, D-95100 Selb, Germany, Tel.: Linseis 09287-880-0, Fax: -70867, [email protected], www.linseis.de M C 2 Thermal Systems, 667 Pinewood Ave., Troy, NY 12180, USA Mettler-Toledo AG, Im Langacher, P.O. Box, CH-8606 Greifensee, Switzerland, Mettler Tel.: 0041-1-944-45-45, Fax:-10, [email protected], www.mt.com Sartorius Netzsch Ger~itebau GmbH, Wittelsbacher Str. 42, D-95100 Selb, Germany Tel.: 09287-881-0, Fax: -44, [email protected], www.ngb.netzsch.com Perkin-Elmer Instruments, 710 Bridgeport Ave, Shelton CT 06484-4794, USA, Perkin01-203-925-4600, Fax: -4654, www.perkinelmer.com, [email protected] Elmer Rheometric Scientific, Inc. One Possumtown Road, Piscataway NJ 08854, USA, Rheometric 01-732-560-8550, Fax: 7451, www.rheometricscientific.com, www.rheosci.com, Scientific (formerly Omnitherm & PL) Rigaku, Segawa Bldg., 2-8 Kandasurukadai, Chiyoda-ku, Tokyo / Rigaku Rigaku International Corporation 3-9-12, Matsubara-cho, Akishima-shi Tokyo 196000, Japan, Tel.: ++81 425 45 8184 Fax ++81 425 45 7984 [email protected] Rubotherm, Universit~itsstr. 142, D-44799 Bochum, Germany, Rubotherm Tel.: +49-234-70996-0, Fax: -22, www.rubotherm.com, [email protected] + Mettler or Satorius Seiko Seiko Instruments Inc., Scientific Instruments Div., 1-8 Nakase, Mihama-ku, Chiba-shi, Chiba 261-8507, Japan, Tel.: +81-43-211-7974, Fax: -8067 [email protected] SETARAM, 7 rue de l'Oratoire, F-69300 Caluire, France Setaram Tel. : +33 (0)4 72 10 25 25, Fax: +33 (0)4 78 28 63 55, www.setaram.com Shimadzu Corporation, 3 Kanda-Nishikicho 1-chome, Chiyoda-ku, Tokyo 101, Japan, www.shimadzu.com TA TA Instruments, 109 Lukens Drive, New Castle, DE 19720-2795, USA, Tel. :01302-427-400-0, Fax : -1, www.tainstruments.com, [email protected] Instruments
REFERENCES [1]A. D@rowski: Adsorption- from theory to practice. Advances in Colloid and /nterface No~ 93 (2001) 135-224.
394
[2] A. D~browski, M. Billow, Z. Hubicki, E.Robens: Novel environmental sorbents and methods for their characterization. Intemational Conference "Adsorption Against Pollution", Miszkolc, Hungary, 2002, in print. [3] A.V. Neimark, E. Robens, K.K. Unger: Berechnung der Fraktaldimension einiger por6ser Feststoffe aus der Stickstoff-Adsorptionsisotherme. Z Phys. Chem. 187 (1994) 265-280. [4] P. Staszczuk, D. Stemik, G. W. Chadzyfiski: Application of quasi-isothermal thermogravimetry and sorptometry for estimation of heterostructure and fractal dimension of high temperature superconductors. J. Therm. Anal. Cal. (2003) in print. [5] J.U. Keller, W. Zimmermann, A. Schein, R. Staudt: Determination of absolute gas adsorption isotherms by combined calorimetric and dielectric measurements. A#sorpn'on (2002) in print. [6] J.U. Keller, W. Zimmermann, A. Schein, R. Staudt: Determination of absolute gas adsorption isotherms by combined calorimetric and dielectric measurements. This volume. [7] J.U. Keller: Theory of measurement of gas-adsorption equilibria by rotational oscillations. Adsorption 1 (1995) 283-290. [8] J.U. Keller, H. Rave, R. Staudt: Measurement of gas absorption in a swelling polymeric material by a combined gravimetric-dynamic method. Macromo/ Chem. Phfs. 200 (1999) 2269-2275. [9] H. Rave, R. Staudt, J.U. Keller: Measurement of sorption and swelling behaviour of polymer/CO2 systems by a combined oscillometric-gravimetric method. Proeedings. o f PBAC2, Brisbane, May 14-18, 2000, World Scientific, Singapore 2000. [10] R. Staudt, H. Rave, J.U. Keller: Impedance spectroscopic measurements of pure adsorption equilibria on zeolithes. Adsorption 5 (1999) 199-214. [11] J. U. Keller, F. Dreisbach, H. Rave, R. Staudt, M. Tomalla: Measurement of gas mixture adsorption equilibria of natural gas compounds on microporous sorbents. Adsorption 5 ( 1999) 199-214. [12] M. Tomalla, R. Staudt, J.U. Keller: Determination of gas adsorption equilibria by volume-gravimetric measurements. In: J.U. Keller, E. Robens (eds.): 3/Bcrobalance Techniques. Multi-Science Publishing, Brentwood 1994, S. 193-204, ISBN 0 906 522 102. [13] R. Staudt, G. Saller, F. Dreisbach, M. Yomalla, J.U. Keller: Determination of gas adsorption equilibria by volume-gravimetric measurements. In: J.U. Keller, E. Robens (eds.): Microba/ance Techm'ques. Multi-Science Publishing, Brentwood 1994, S. 205210. [14] E. Baumgarten, F. Thielmann: Schnelle Porenanalyse. Chemie Techm'/c27 (1998) 1, 38-40. [15] J.A. Poulis, C.H. Massen, E. Robens, G. Reichenauer: The application of J~intti's method for the fast calculation of equilibrium in case of multilayer adsorption. This volume.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
395
Influence of method of washcoat preparation on hydrothermal stability of alumina support M. Kutazyfiski, J. Trawczyfiski, J. Walendziewski Institute of Chemistry and Technology of Petroleum and Coal, Wroctaw University of Technology, ul. Gdafiska 7/9, 50-344 Wrodaw, Poland E-mail: [email protected]. pl The aim of the research was determination of the influence of thermal treatment of the alumina (catalyst, washcoat or membrane precursor) in the range of temperature 550 - 850 ~ and impregnated with TEOS solution, silica sol and cerium nitrate solution on stability of the obtained alumina precursors under hydrothermal conditions (temperature 150-225 ~ time 24 72 hours, water steam pressure up to ca 1.5 MPa). Influence of preparation parameters on hydrothermal stability was estimated on the base of porosimetry measurements. It was found that the best hydrothermal stabilization of alumina can be acquired by using of silica compounds, e.g. TEOS solution or silica sol. Double impregnation of alumina with TEOS solution and silica sol does not change noticeably good hydrothermal stability of alumina obtained in the case of single impregnation. In comparison to the TEOS hydrothermal stabilization efficiency of stabilisation of alumina by cerium oxide was very low. 1. INTRODUCTION It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 1 5 0 - 225 ~ and time up to 72 hours. 2. EXPERIMENTAL
2.1 Supports preparation Alumina samples for investigation were prepared from aluminum hydroxide (Condea, type: DISPERAL). A batch of aluminum hydroxide was kneaded either with water dematerialized solution of HNO3 (1%) or with water solution of cellulose derived product (commercial name: CULMINAL), extruded with using of piston extruder, dried for 24 h at room temperature, then for 24 at 110 ~ and calcined for 4 h at 550, 650, 750, 800 or 850 ~ Obtained samples of alumina extrudates are described as DN (aluminum hydroxide "Disperal"
396 kneaded with nitric acid solution) or DC (aluminum hydroxide "Disperal" kneaded with "Culminal" solution) and value of the calcinations temperature. For example sample DN-550 is a sample prepared from Disperal type aluminum hydroxide kneaded with 1% water solution of nitric acid and calcined 4 h at 550 ~ Selected extrudates were then coated with TEOS (Tetraethoxysilane, 98 % purity, Fluka) (dipping for 5 rain in the TEOS), dried 24 h at room temperature, then at 110 ~ and calcined 4 h at 400 ~ in air (samples described with additional letter T: DN-550T is a sample prepared from Disperal type aluminum hydroxide kneaded with 1% water solution of nitric acid and calcined 4 h at 550 ~ and coated with TEOS). Extrudates of alumina covered with silica layer (by impregnation of alumina extrudates with TEOS) were additionally coated with silica sol. A silica sol was prepared by controlled hydrolysis and condensation of TEOS in acidic ethanol. The molar composition of the reaction mixture: T E O S : ethanol : w a t e r : HNO3 = 1 : 3.8 : 6.4 : 0.085 In order to obtain silica sol, nitric acid solution in water was added drop wise to the vigorously mixed mixture of TEOS in ethanol and aider finishing of the dosing of hydrolyzing agent temperature of the obtained reaction mixture was increased to the boiling point and was hold for two hours. Samples covered with silica sol were then dried 24 h in ambient temperature, for 24 h at 110 ~ and calcined for 3 h at 400 ~ in air. Additional letter S describes these samples: DN-550TS = a sample prepared from Disperal type aluminum hydroxide kneaded with 1% water solution of nitric acid, calcined 4 h at 550 ~ coated with TEOS and coated with silica sol. We have selected concentrations of TEOS, silica sol and cerium nitrate at the level allowing obtaining ca 1.8 wt % of the individual stabilizers in final alumina material. 2.2. f l y d r o t h e r m a i t r e a t m e n t
In the next step of investigation, samples prepared according to the above-described procedures were treated under hydrothermal conditions. Experiments were carried out in autoclave, 500 ml volume, heated in electric furnace (Fig. 1). The prepared supports were placed in autoclaves in special wire baskets, hanged above distilled water placed at the bottom of the autoclave (filled with deionised water). Experimental conditions of hydrothermal treatment: T = 150, 200, 225 ~ (water steam pressure 0.9 1.3 and 1.5 MPa). I Manometer _____.~.__.]~~
L_ ampl___2ei autolave ~Furn~
Steam
IIL Water
t
i.i!iI
1 Fig. 1. Schematic presentation of autoclave for hydrothermal treatment of support samples.
397 Time of conditioning = 24 or 72 hours. After hydrotreatment samples were overnight dried in dryer at 110 ~ Results of these measurements are shown in Tables 1-3. 2.3. D e t e r m i n a t i o n of d i s t r i b u t i o n of surface area and pore v o l u m e in relation to pore radius
Pore structure of the obtained materials was determined by benzene adsorption methods using of mercury porosimetry. o Benzene adsorption method - determination in the range of pore radius 1.5 - 50 nm. The method applies quartz springs ( M c B a i n - Backr's weight) of mass sensibility about 400 mm/g. vacuum 10-3 Pa, samples of adsorbate ca 0.35 g in quartz sample holders, The isotherm points were determined by applying of the specified, increasing benzene partial pressure (p/po) from 0.175 up to 0.96. o Apparatus Pascal 440 (Carlo Erba, maximum pressure 400 MPa, pore radius in the range 1 . 8 7 - 7500 nm, results are analysed with MIELS W software). 3. R E S U L T S AND DISCUSSION In the first series of experiments alumina support was formed with nitric acid as peptising agent. The support samples were calcined at three different temperatures (550, 750, and 800 ~ DN-650, DN-750 and DN-800). These alumina samples were submitted to the hydrothermal treatment (in water steam media) also at three different temperatures (150, 200 and 225 ~ Table 1 and Figs. 2 and 3). The results presented in Fig 2. clearly indicate that the alumina support calcined at 550 ~ is not resistant against hydrothermal treatment in the applied temperature range. One can see that increase in hydrothermal treatment temperature results in appreciable decrease in specific surface area (from 188 to 16,5 m2/g) and an increase in average pore radius from 4.5 to 42.8 nm). These changes are accompanied by a decrease in pore volume from 0.389 to 0.305 cm3/g This is of course result of very strong transformation of support under hydrothermal treatment conditions (water steam pressure ca 1.5 MPa). In the next step alumina support DN-550 was impregnated with TEOS and the obtained support sample was submitted to hydrothermal treatment. The obtained results are presented in Fig. 3. We can see that supporting of alumina with TEOS (compare samples DN and DN + T) gave increase in 2OO r
d
50
o....
160
40
cD
. ,....
120 = .,-~
80 40
are
I
None
l0
~ !
I
150
"~
2O
,, Averagepore I ladius O
0
30
Specific surface
I
4
I
200
I
.~ <
0
225
Hydrothermal treatment temperature,~ Fig. 2. Influence of temperature of hydrothermal treatment of DN-550 support on its specific surface area and average pore radius
398 200
................................................................................................................................................... 10
180 ~ o
6
160 ,
~ 140
o
~ 4
o
~
~ 120 m 100 DN
DN+T
DN+T+ HT- DN+T+ HT- DN+T+ HT150 200 225 Sample symbol
Fig. 3. Influence of preparation method and temperature of hydrothermal treatment of DN-550 support covered with TEOS on its specific surface area and average pore radius specific surface area and not large decrease in average pore radius. Hydrothermal treatment of DN-550 +T samples in the applied temperature range, showed that specific surface area and average pore radius was almost unchanged. It can be stated that increase in temperature of hydrothermal treatment from 150 to 225 ~ caused only decrease of pore volume from 0.3 54 to 0.335 cm3/g. It means that impregnation of alumina support with TEOS sol can be very good hydrothermal stabilizing method. Similar dependencies were found for alumina supports calcined at 750 and 800 ~ (Table 1). For instance hydrothermal treatment of DN 750 sample (calcined at 750 ~ at 150 and 200 ~ gave supports with surface area 142.5 and 41.4 m2/g, respectively. However impregnation of DN 750 support with TEOS prevents from transformation of support structure and specific surface area and average pore radius after hydrothermal treatment was almost unchanged. Exactly the same results were obtained for supports calcined at 800 ~ However the rate of decreasing in specific surface area and pore volume in the case of supports calcined at higher temperatures was found to be little lower than in the case of supports calcined at 550 ~ The further results given in Table 1 also indicate that some influence on supports structure transformation under hydrothermal conditions exerts type of used peptising agent. It was found that supports formed with using of nitric acid solution are more hydrothermally stable than alumina material formed with using of Culminal (DN-800 and DC-800 supports). The average pore radius in the case of DC-800 sample after hydrothermal treatment was two times larger than for DN-800 sample after the same treatment. The combined influence of calcination temperature (650-850 ~ and long time hydrothermal treatment (200 ~ 72h) of alumina support impregnated with TEOS is presented in the end part of Table 1 Irrespective of calcination temperature and hydrothermal treatment, surface area of the al.umina supports was higher than 100 m2/g and average pore radius was in the range 6,29 to 6,66 nm. The other methods of thermal stabilization were also tested in our laboratory. For
399 instance we have found that simultaneously using of TEOS and silica-sol give similar results as in the case of using TEOS stabilisation method. The exact determination of porous structure of alumina samples, in the range of pore radius 0.4 50 nm) with using of benzene adsorption method generally confirms results obtained by porosimetry method. -
Table 1 The influence of support preparation parameters and parameters of hydrothermal processing on .S.UPport.prgPerti es . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time (h) and temperature of ....Porevoiumel ......Totalspecific .......Average pore radius, surface area Samples hydrothermal treatment m2/g nm .... ....... (~ . . . . . . . . 5.7 14215 0.384 DN-750 24, 150 22.7 41.4 0.332 DN-750 24, 200 5.35 126.9 0.334 DN-750T 24, 200 4,2 162,5 0,342 DN-800 none 6.7 132.7 0 365 DN-800 24, 150 22.8 33.85 0316 DN-800 24, 200 34.6 24.9 0315 DN-800 24, 225 5.36 146 1 0 372 DN-800T 24, 200 6.5 158.2 0 380 DC-800 24, 150 44.7 24 7 0 338 DC-800 24, 200 65.4 21.6 0312 DC-800 24, 225 4.67 141.0 0313 DN-650TS None 5.70 132.3 0 341 DN-650TS 24, 200 5.48 138.2 0.321 DN-750TS None 5.48 122.8 0.303 DN-750TS 24, 200 6.29 131.30 0.363 DN-650T None 6.66 114.05 0.319 DN-650T 72, 200 6.42 105.94 0.317 DN-750T None 6.50 126.46 0.351 DN-750T 72, 200 6 66 113.18 0.350 DN-850T None 6 50 106.27 0.327 DN-850T 72,200 .
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Both calcination and hydrothermal treatment results in decrease in lowering of specific surface area and pore volume. Similarly, in the both cases some part of small pores disappears as a result of connecting. Therefore one can observe decrease in specific surface area and pore volume of micro- and mesopores (range 2-10 nm) and increase in specific surface area and pore volume of mesopores (10-50 nm) and macropores. The next series of experiments was carried out using additionally cerium nitrate as stabilising agent (Table 3, samples 1-16). In the first group of experiments (1-4) alumina precursors shaped with nitric acid and calcined, D1 at 650 ~ and D3 at 850 ~ were impregnated using cerium nitrate solution. The results of porous structure determination of hydrothermally treated samples indicated that there is no visible influence of the presence of cerium oxide on thermal stability. However, in the case of sample calcined at 850 ~ the tenfold decrease in specific surface area was stated while in the case of sample calcined at 650 ~ only fivefold. This observation supports our assumption that the highest temperature of the initial calcination has to be not
400 higher that 750 ~ Table 2 The influence of support preparation parameters and parameters of hydrothermal processing on specific surface area of supports Surface area of pores m2/8; ...... Micropores Mesopores of effective diameters Surface area Samples C6H6 of micro0.4-2nm 2-3nm 3-5nm 5-10nm 10-50nm 2-50nm and mesopores DN-650 94 37.0 34.6 56.2 101.4 229.2 323 without HT DN-750, 81 32.5 28.7 46.6 105.9 213.7 295 without HT DN-650T, 82 27.0 29.9 63.7 66.9 187.5 270 HT-200~ 72 h, DN-750T, 72 24.2 24.3 41.1 75.2 164.8 237 HT-200~ 24 h, Table 3 The influence of support preparation parameters and parameters of hydrothermal processing on support properties Time (h) and Total Total specific Average No Samples temperature of cumulative surface area pore radius hydrothermal volume treatment (~ (cm3/g) (m2/g) (nm) 1. D1C/650 ~ 0.390 118.78 649 2. D 1C-HT/650 ~ 72, 200 0.316 25.38 26.72 3. D3C/850 ~ 0.395 101.36 7.61 4. D3C-HT/850 ~ 72, 200 0.292 11.20 75 69 5 D1CT/650 ~ 0.284 106.03 649 6. D 1CT-HT/650 ~ 72, 200 0.278 96.99 570 7. D3CT/850 ~ 0.329 104.96 6 98 8. D3CT-HT/850 ~ 72, 200 0.301 90.22 6.66 9. D1CTS/650 ~ 0.222 79.30 5.47 10. D 1CTS-HT/650 ~ 72, 200 0.231 80.18 6 66 11. D3CTS/850 ~ 0.237 70.57 6 28 12. D3CTS-HT/850 ~ 72, 200 0.277 81.84 6 50 13. D1TSC/650 ~ 0.301 109.68 5 7O 14. D1TSC-HT/650 ~ 72, 200 0.266 97.67 5.48 15 D3TSC/850 ~ 0.306 101.44 6.98 16 D3TSC-HT/850 ~ 72, 200 0.276 91.31 6.50 Additional impregnation of cerium oxide containing alumina samples with TEOS (samples D1CT and D3CT, D1CT-HT and D3CT-HT, 5 - 8) gave similarly as it was earlier elucidated appreciably hydrothermal stabilization of the structure. Not large decrease in pore volume and surface area gave small decrease in average pore radius in comparison to the multiple increases in the case of previous series of alumina precursor.
401 Similar effect was observed in the next series (experiments 9-12, samples D1CTS and D3CTS). Precursors of this series were obtained by triple impregnation of alumina with cerium nitrate and TEOS solutions and silica sol. In the result of the sequential impregnation pore volume of precursor was diminished almost twice and surface area by 30 %. However in this series of experiments hydrothermal treatment gave results quite reverse result. In this case small increase in pore volume and specific surface area was observed while increase in average pore radius size was negligible. In the last series of the experiments 13-16, samples D1TSC and D3TSC, the influence of impregnation with cerium salt of membrane precursor coated earlier with TEOS and silica sol was studied. In this method of stabilization the contemporaneous decrease in pore volume, surface area and average pore radius was found. It is very interesting that although chemical composition of samples D 1CTS - D3CTS and D 1TSC - D3TSC before hydrothermal treatment was the same their pore volume and surface area were quite different. On the other hand pore structure of the final samples after hydrothermal treatment i.e. D1CTS - HT and D1TSC-HT and D3CTS-HT and D3TSC-HT was quite similar. If one compare pore structure of samples with TEOS and silica and 9-16 samples impregnated additionally with cerium salt it is visible that impregnation with cerium nitrate reduces appreciably pore volume and surface area of the aluminas. However in the case of cerium containing samples hydrothermal treatment do not cause so drastic decreasing pore volume and surface area as in the case of cerium free samples. 4. CONCLUSIONS 9 Hydrothermal treatment causes much more intensive transformation of alumina structure in comparison to thermal treatment without water steam. It is especially important in the case of alumina used in hydrothermal conditions, e.g. in membrane separation technology and washcoats in car catalytic converters. 9 Impregnation of alumina with silica compounds, e.g. TEOS solution, very strong stabilises its porous structure and therefore there is small influence of water steam on physiochemical properties of alumina covered with silica layer. 9 The additional impregnation with silica sol of alumina earlier stabilised by TEOS does not change noticeably good hydrothermal stability of alumina. 9 In comparison to TEOS hydrothermal stabilization efficiency of alumina by cerium oxide is low. REFERENCES [1] K.L. Yeung, J. M. Sebastain, A. Varma, J. Membr. Sci., 13 (1997) 9. [2] S.F. Tikhov, G.V. Chernykh, V.A. Sadykov, A. N. Salanov, G.M. Alikina, S.V. Tsybulya, V.F. Lysov, Catal. Today, 53 (1999) 639. [3] J.B. Miller, E. I. Ko, Catal. Today, 43 (1998) 51. [4] H.P. Hsieh, R.R. Bhave, H. L. Fleming, J. Membr. Sci., 39 (1988) 221.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
403
Limestone Influence on P A H Emissions from Coal A F B C . Catalytic or/and Adsorption Effect? A M Mastral, T Garcia, M S Call6n, J M L6pez, R Murillo and M V Navarro Instituto de Carboquimica, CSIC, M Luesma Cast,in 4, 50015-Zaragoza, Spain Phone: 34 976 733977, Fax: 34 976 733318, e-mail: [email protected]
Limestone incorporation to coal feed during coal atmospheric fluidized bed combustion (AFBC) to control legislated emissions promotes the polyaromatic hydrocarbon (PAH) emissions. This is a fact that has been corroborated working in a laboratory pilot plant at the same conditions used at the coal FBC power stations. PAH emissions increase one order of magnitude but, is it due to a Ca catalytic effect or is it due to limestone is a porous material? The aim of this work was to check the role played by limestone and it is concluded that, at the working conditions, the PAH emissions promotion is due to the limestone porosity. No Ca catalytic activity was found at the coal AFBC standard conditions.
Keywords: Limestone, porous adsorption, Ca, PAH, Coal FBC 1. INTRODUCTION Coal FBC technology was promoted by the search of cheaper and cleaner energy generation together with a more and more pressing legislation on environmental aspects [1]. At the beginning, the aim of all of these technological changes was addressed to decrease combustion temperature, to avoid thermal NOx and to improve limestone performance concerning to SOx adsorption, without losing efficiency during the coal combustion process. However, the effect of control technologies used for the abatement of inorganic pollutants, like SOx, NOx and COx. has not be contrasted with the variations on the corresponding organic emissions. Organic emissions are emitted in lower amount than inorganic emissions but their hazardous character is much higher, which in consequence show a greater negative environmental impact. Mechanisms involved in organic emissions at coal combustion are very complex and not well known. It is not necessary for the emitted organic compounds to be part of the volatile coal structure, neither the particulate matter emission needs to be unburned material [2]. The devolatilization and the pyrolytic processes joined to any fuel combustion imply radicals' release, which is the cause of the new organic and particulate matter emissions by pyrosynthesis retrogressive reactions [2].Therefore, the radicals interactions and the
404 conditions at which they interact (temperature, excess oxygen, flow, pressure, etc) are determinant concerning volatile organic emissions. In this context, limestone added for inorganic emissions abatement might have some influence on the interactions between the released organic radicals that cause the organic emissions. That is why the topic search in this paper is organic emissions, polyaromatic hydrocarbons compounds emissions, from coal AFBC when limestone is added to the feeding blend, in comparison to the ones taking place in a coal AFBC when coal is burnt in an inert bed like sand. The aim is to get information on the limestone effect on volatile organic compounds (VOC), specifically on polycyclic aromatic hydrocarbons. 2. EXPERIMENTAL
The limestone used in the AFBC was deeply characterized. The limestone SEM-EDX analysis shows Ca as the unique identified element and its XRD analysis shows calcite as the only crystalline chemical species. After heat treatment at 100~ calcite showed an apparent surface area of 19 m2/g, 48.5% porosity with unimodal small pores of 5.5nm. The coal burned in the AFBC plant was a low-rank coal with 0.5-1 mm particle size. The combustion experiences were performed keeping constant the total air flow (800 1/h air flow high enough to ensure a proper fluidization avoiding slugging regime), the percentage of excess oxygen (10 %), the combustion temperature (850 ~ and modifying the feed and the nature bed (coal/sand (Rf: CS), limestone-coal blend/limestone (Rf: LCL), and coal/limestone (Rf: CL)). The limestone-coal blends were prepared man ually by mixing coal and limestone taking into account that the Ca/S ratio should be equal to 3. The prepared blend was stored in 5 kg containers and introduced in the combustion pilot plant feeding system when experiments were carried out. The variables were fixed according to a previous work [3] in order to reach high efficiency values (see Table 1). Because of the usually low reproducibility of the combustion processes, each combustion experience, regardless of the bed were sand or limestone, was repeated from two to five times in order to ensure the results validity. Table 1
Coal AFBC efficiencies as a function of the bed (850~ 10% excess oxygen).
800 l/h,
N ~ Run
BED Sand Limestone
Feed Coal Coal-limestoneCa/S=3 Coal
Ref.
1
2
3
4
5
SC LCL LC
99.2 99.0 99.4
99.2 98.6 99.4
99.1 98.5 -
99.3 98.5 -
99.2 98.5 -
The sampling system was formed by two cyclonees, a condenser, a Teflon filter (1 lam) and an adsorbent, XAD-2 resin. The samples were extracted [4] by sonication three times with 25 mL of dimethylformamide (DMF) for 15 min. Concerning to the solution trapped at the condenser, it was extracted after water evaporation. The extracts were filtered and concentrated in a rotary vacuum until a final volume of 10 mL. Percentage recoveries of PAH spiked at appropriate concentrations onto clean particulate matter, filter and XAD-2 resin, reaching between 88 to 102%.
405 The samples were analysed by fluorescence spectroscopy at the conditions for each specific PAH [5] previously determined with the model compounds. The PAH studied are those listed by the US Environmental Protection Agency as priority pollutants [6]: Fluorene, Benzo(a)Pyrene, Pyrene, Chrysene, Anthracene, Acenaphthene, Bezo(a)Anthracene, Dibenzo(a,h)Anthracene, Coronene, Perylene and Benzo(k)fluoranthene. In addition, Coronene emissions were also reported due to their important role on PAH stabilization at extreme conditions [7]. These 16 PAH were analysed from all runs in each of the four samples. 3. RESULTS AND DISCUSSION The coal combustion was optimized at the laboratory plant in order to obtain high efficiencies and to minimize emissions during the combustion process. As Table 1 shows, the efficiencies reached were very high in all runs, regardless of the feeding and the bed nature. For all combustion processes, the total PAH emitted, as a function of the feed and the bed nature, are shown in Table 2. This Table 2 shows that the total PAH emissions depend on the feed. The PAH total quantity detected in the combustion runs carried out with limestone-coal blend feed are one order of magnitude higher than the detected ones for the corresponding runs carried out just with coal feed.
Table 2
Total PAH amount emitted (~tg/kg bumt coal) from each run as a function of the bed (850~ 800 I/h, 10 % excess oxygen).
BED
FEED
Ref.
Inert (Sand)
Coal Coal-limestone Ca/S=3 Coal
Limestone
1
N ~ Run 4
2
3
SC 0.7 LCL 14.3
1.2 31.5
1.9 19.9
LC
2.0
2.9
2.5 11.0
5 1.7 17.1
The distribution of the emitted PAH between the four traps, the cyclones, the condenser, the Teflon filter and the XAD-2 resin, with sand and limestone beds, is shown in Table 3. In spite that the 16 PAH listed by USEPA were comprised in this study, only Fluorene, Pyrene, Acenaphthene, Benzo(a)Anthracene and Coronene could be quantified. Three other PAH, Benzo(a)Pyrene, Chrysene and Anthracene, were detected but were emitted below quantification limits. The remaining studied PAH were not detected. At FBC, elutriation and attrition of the solid material in the bed can facilitate that unburned fuel can be emitted, mostly at high airflow [7]. The formation of volatiles and particulate matter, PM, will be a function of the conditions at which the process is carried out. While the unburned material emissions can be minimized by combustion optimisation and will be mainly composed of the organic and inorganic components of the fuel, other PM emission,
406 from pyrosynthesis processes, is more difficult to control and will be mainly composed of organic matter - soots [8]. It could be thought that the small efficiency differences were relevant on the PAH emissions. However, by comparing the data obtained from LCL runs No 3, 4 and 5, at which the same efficiency value is reached but the emitted PAH amount was different, and those obtained from LCL runs No 2 and No 4, with very close efficiencies but at which the emitted PAH amount were the most extreme values detected, it can be deduced that this influence is negligible and the small differences among the high efficiencies reached in all the runs carried out do not seem to be the only determinant effect on PAH emissions. These results allow inferring the importance of the pyrosynthesis step on the PAH emissions, when a limestone-coal blend is added to the reactor.
Table 3
Total PAH emitted (~tg/Kg burned coal), average value, and total PAH collected on each trap at coal AFBC as a function of the feed and bed (850~ 800L/h, 10% excess oxygen).
AFBC Feed and Bed Feed: coal Bed: sand SC Feed: coal-limestone Bed: limestone LCL Feed: coal Bed: limestone LC
XAD-2 Teflon
Traps Condenser Cyclones
Total
1.57
none
none
none
1.57
None
13.13
0.49
7.99
21.61
1.57
0.98
none
none
2.55
The radicals released by coal devolatilization, due to its high reactivity and its short average lifetime, undergo different reactions that compete among themselves as a function of the combustion conditions. The reactions experimented by the radicals could include two opposite reactions: condensation reactions, which imply the association between radicals, generating compounds of higher molecular weight (PAH), and oxidation reactions, which imply radicals elimination, leading to COx and H20 formation. Once formed, the PAH can be emitted as gas or as generating by themselves the smallest particulate matter. These competitive reactions, among the present compounds in the freeboard, will be the ones which determinate the total PAH emissions and will depend on the feed and the bed nature. By comparing the experiments carried out at the SC and LCL runs, there are significant differences among the total PAH emissions (Table 2), and among the PAH distribution by traps (Table 3). A possible explanation of these results could be due to species in the reactor and their relationship. In the SC runs, the species involved in the combustion
407 mechanisms will be the released radicals from coal devolatilization and thermolysis, the remaining solid nucleus from coal thermolysis, H20 and COx from radicals elimination by oxidation reaction, SOx, and the remaining oxygen. In LCL runs, in addition to all the species involved in SC runs and previously mentioned, a much higher COz concentration as well as CaO, from limestone decomposition, will be also involved in the inside reactor mechanisms. Taking into account that the CO2 concentration at LCL runs is higher than the one at SC runs, on the one side the interactions between the coal released radicals and the available oxygen will be impeded, and so the radicals elimination by oxidation. However, the radical interactions among themselves will be also more difficult, and so the pyrosynthesis process driving to a higher PAH formation and emissions will be hindered. That is to say, a higher CO2 concentration would not imply, in principle, any relevant influence on the total PAH emissions. The CaO from CaCO3 decomposition, the other new specie involved in the combustion mechanisms at LCL runs, is a solid porous material, which behaves like a fluid at the fluidised bed. That means that its interactions with radicals from coal are not limited and the radicals could be adsorbed [9] into its porous structure, hindering their total oxidation and, in consequence, promoting their interaction. This fact is corroborated by the Coronene formation in LCL experiments. Coronene (Co) is the most stable of the PAH studied and the radicals trend in their stabilization will be towards Co formation. Besides, as result of the fluidisation movement and the high temperature, CaO can be fragmented into smaller particles, elutriation and attrition phenomena, the smaller particles formed undergoing entrainment by the airflow. Those facts will justify that, with limestone, not only higher PAH emissions are detected, but the PAH emitted are also trapped on the cyclonee, the condenser and the filter, that is to say, on the solid phase emissions. Moreover PAH emissions were not detected in the XAD2 resin, gas phase, at LCL runs. However, the PAH solid/gas partitioning results, see Table 3, show that when limestone is not included in the feed, all the PAH emitted are adsorbed in the XAD-2 resin, that is to say, are emitted in the gas phase. Therefore, and because of the Ca presence, a possible Ca catalytic role could be guessed. The chelating mechanisms reported [10,11] at polymeric pyrolysis can not be applied to coal combustion because the inert-reducing atmosphere at pyrolysis process has not relation to the oxidation conditions at the combustion atmospheres. However, other additional effect in the LCL runs could influence the PAH emission and distribution. This effect would be a possible catalytic role performed by Ca. To assess a possible Ca catalytic effect, two additional combustion runs (LC runs) were performed, feeding only coal into the reactor but using the bed obtained in the LCL runs. This way, the Ca presence was assured without feeding any additional porous material.
408
*Fig. 1.- PAH (~tg/Kg) Distribution between Solid an Gas Phases as a Function of the Feeding and Bed Nature: SC, sand/coal; LCL: Limestone/Limestone-coal blend; and LC: Limestone/coal
The results obtained (see Tables 2 and 3 and Fig. 1) show that they are closer to the ones obtained with coal as feed than to the ones obtained with coal-limestone feed. Moreover, not only the total PAH emissions are similar to the ones in absence of Ca, but also the PAH solid/gas phase distribution coincide because none PAH was detected in the cyclone and condenser traps, solid emissions. Therefore, all the PAH were emitted on gas phase, as it happens at SC runs. These results allow inferring that the Ca catalytic role promoting PAH formation and emissions at coal combustion can be discarded, and that the higher PAH emissions seem to be due to the physical characteristic (porosity) of the limestone. These results also allow deducing that despite the limestone increases the PAH emissions, its total performance is positive. Limestone not only abates SOx emissions, but also PAH emissions on gas phase are avoided by adsorption, which means that gaseous PAH could be easily controlled by an effective particulate matter trapping system. 4. CONCLUSIONS From the data obtained, it could be concluded that, at coal fluidised bed combustion, not only higher PAH amounts are emitted but these PAH undergo different distribution between solid and gas phase. While, with coal-limestone feed, the PAH emitted are supported on the particulate matter, in limestone absence when only coal is fed, the PAH are emitted on the phase that goes through a filter with a 1 micrometer pore size. It could be also concluded that the porous character of the limestone facilitates the radical interactions, pyrosynthesis reactions, by hindering the oxidation process. That is corroborated by the Coronene emissions detected when limestone-coal blend is fed into the fluidised bed to control SOx emissions. Coronene formation, the most stable PAH studied
409 in this work, would be the preferential trend in the released radicals stabilization leading to PAH formation. Coronene is only detected with limestone-coal blend. In addition, and according to the results obtained, a possible Ca catalytic role promoting PAH formation can be discarded at the coal AFBC burning conditions. REFERENCES
[1].- M. Takeshita, Environmental Performance of Coal-fired FBC, IEA Coal Research, IEACR/75, London, 1994. [2].-A.M. Mastral, M.S. Call6n, T. Garcia., Environ. Sci. Technol. 33 (1999) 3177. [3].- A.M. Mastral, M.S. Call6n, T. Garcia, Energy and Fuels, 14 (2000) 275. [4]..- M.S. Call6n, Ph. D., PAH Formation and Emission in Power Generation. AFBC, Zaragoza, Spain, December 1998. [5].- A.M. Mastral, C. Pardos, B. Rubio, and J. Galban, J. Anal. Lett. 28 (1995) 1883. [6].- US-EPA, Code of Federal Regulation, Title 40, Part 60, Washington, DC, USA, Environmental Protection Agency, 44, 1997. [7].-A.M. Mastral, M.S. Call6n., Environ. Sci. Technol. 34 (2000) 3051. [8].-A.M: Mastral, M.S. Call6n, R. Murillo, In CARBON'97 Conference Proceedings, University Park, 1997, 140. [9].-A.M. Mastral, T. Garcia, M.S. Call6n, M.V. Navarro and J. Galban, Energy and Fuels 15 (2001) 1. [ 10].-Y.L. Wei and J.H. Lee, Sci. Total Environ., 212 (1998) 173. [ 11 ].- Y.L. Wei and J.H. Lee, Sci. Total Environ., 228 (1999) 59.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
411
Freezing point elevation in nanospace detected directly by atomic force microscopy M. Miyahara, M. Sakamoto, H. Kanda and K. Higashitani Department of Chemical Engineering, Kyoto University, Kyoto 606-8501, Japan Following the earlier molecular simulation work by the authors, in which freezing point elevation in nanospace caused by the attractive potential energy from pore wall had been predicted, an experimental trial for finding the elevation phenomena was conducted, employing the so-called colloidal-probe Atomic Force Microscopy. A carbon microparticle was attached to the top of the cantilever tip, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned. For two kinds of liquids, freezing behavior has been detected above the bulk freezing point. Further, the degree of elevation was in good accord with the prediction by our model based on the attractive potential energy of the wall. Though the extent of the elevation itself was rather small, the finding of the definite existence of the elevation, not only in the micropores but also in a nanospace with the size of a few nanometers, would be of much importance in the research field of the phase behavior in nanopores. 1. INTRODUCTION An understanding of phase behavior of fluid confined in narrow pores is of importance not only in adsorption but in many other relevant fields, e.g., the fabrication of nanomaterials, nanotribology and in the determination of pore sizes. As for vapor-liquid coexistence (capillary condensation), many studies have pointed out the incorrectness of the Kelvin model in the scale of nanometers. Understanding of freezing phenomena (solid-liquid coexistence) in nanopores has been more difficult. Many experimental studies of freezing in porous materials have reported that freezing points are usually lowered [1]. The porous bodies often used include Vycor glass, controlled-pore glass, silica gels, and so on. Such glasses do not have variety in the base material, and the shape of the pores is thought to be roughly cylindrical. In contrast to the above results, a study with SFA (Surface Force Apparatus) reported an increase in the freezing temperature for liquids confined between mica surfaces [2]. The general trends of freezing in confined space have not been made clear, nor have been the mechanisms. Freezing phenomena in confined space must be affected, we suppose, by at least the following three factors: i) strength of pore wall potential energy (compressing effect), ii) geometrical shape of pore (geometrical hindrance), and iii) equilibrium vapor-phase pressure (tensile effect). Molecular simulation techniques are appropriate for clarifying each contribution, while experimental measurements may suffer from complication by
412 simultaneous affection of the above factors or others. We have been exploring each of the effects employing molecular simulation techniques, and have been successful in understanding and modeling the quantitative nature of the effects [3-5]. Our study on the first point [3] clarified the following. Depending on the strength of the attractive potential energy from pore walls, fluid in a slit pore in equilibrium with saturated vapor showed freezing point elevation as well as depression, and the critical strength to divide these two cases was the potential energy exerted by the fluid's solid state. The "excess" attraction relative to the critical one was considered to bring the confined liquid to a higher-density state that resembles a compressed state, which would result in the elevated freezing point. The above result is in accord with other recent studies. Dominguez et al. [6] examined freezing of LJ fluid in slit pores of purely repulsive and weakly attractive walls, employing thermodynamic integral technique to find out true equilibrium points. The freezing points that were determined rigorously by free energy calculation showed significant downward shift, relative to the bulk, in purely repulsive walls, while the downward shift was much smaller in magnitude for weakly attractive walls. Further, Radhakrishnan and Gubbins [7] used a different approach of determination of freezing point in slit pores employing the Landau free energy calculation, and simple fluid in strongly attractive slit pore was shown to exhibit elevated freezing points. Now the research effort goes toward experimental verification of the elevation phenomena in the simplest geometry, a slit. Our main interest is in the range of a few to several nanomenters. Some experimental studies have already reported freezing point elevation in slit pores [8-10], but the materials used were activated carbon fibers (ACFs), which have only micropores less than 2nm. In such small pores the first layer adjacent to the attractive pore wall, which is known to form a frozen phase at a temperature well above the bulk freezing point, will occupy most of the pore spaces, and the freezing behavior in the interior of the pore space is difficult to be detected. Further, there may still remain some controversy if a liquid confined in a larger nanopore would exhibit elevation unless an experimental verification is made over such sizes. The employed technique for this purpose was the so-called colloidal-probe AFM (Atomic Force Microscopy). A carbon microparticle with high degree of carbonization was attached to the top of the cantilever tip, forming the colloidal probe, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned here. The results clearly demonstrate that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes, e.g., when the distance comes down to 4 nm, even at 8.4~ which is above the bulk freezing point of 6.4~ Further, the degree of elevation was in good accord with the prediction by our model [3] based on the attractive potential energy of the wall. Though the extent of the elevation itself might look rather small, we believe that the finding of the definite existence of the elevation, and its accord with the prediction of our model would be of much importance in the research field of the phase behavior in nanopores. 2. E X P E R I M E N T A L
An AFM apparatus, PicoSPM manufactured by Molecular Imaging (MI), which is schematically shown in Fig.l, was used. The benefit of this apparatus is the top-down
413
Fig.l Schematic drawing of AFM apparatus and image of the nanospace tbrmed between surthces, scanner, which controls the location of the cantilever tip from upper side of the liquid cell, so that the temperature of the liquid cell can easily be controlled from the bottom side of" the cell. We used the sample stage with Peltier cooling device attached on the bottom side of the sample plate manuf~tctured by MI. They guarantee temperature control with 0~ ~ accuracy A slit-like nanospace can be made up by applying so-called the ;'colloidal probe .AFM" technique, which was in its origin developed to measure the force between a solid surfiace and a particle of micrometer size. The particle was glued on the cantilever tip, and this colloidal probe is to be used instead of the usual cantilever. The vertical scanning of the cantilever controls the distance between the particle surfi~ce and a solid surface, and the usual manner of detecting the bending of cantilever gives the f0rce-distance relation. If we look at the system with the separation distance of single nanometers, the separated space between the two surfiaces would be almost slit, geometry because the radius of the particle (order of 10 l ~tln) is fiar larger than the separation distance, V-shaped silicon-nitrlde cantilever tips (Nanoprobes lnc.) were used: whose spring constant was either 0.32 N/m or 0.58 N/rrl. A graphite plate (HOPG) and carbon particles, which were made from phenolic: resin by pyrolyzlng above 2000~ were used to form a nanospace with carbonaceous surfaces,. The particles were evacuated at I I0~ for 24 h before use. The graphite plate was freshly cleaved before measurement. Three kinds of liqmds of analytical grade (Wako Pure Chemicals) were used. .All liqmds were desiccated with zeolite 4A molecular sieve fbr one week before experimental use, Cyclohexane, and octamethylcyclotetrasiloxane (OMCTS), whose bulk freezing point were 6.4~ and 18~ respectively, were used for the examination of freezing behavior. The reasons of the choice were: i)affinity to carbonaceous surface, ii)freezing point near ambient temperature, and iii)almost spherical molecular shape. Methanol, with bulk freezing point of-98~ was also used as a reference. All the force measurement were done after confimalng the stability of the temperature under control fbr at least 10 minutes, The temperature was set at various values above the bulk freezing point. 3,. R E S U L T S A N D D I S C U S S I O N
414
'
I
~'
'
o
0.3 0.2
I
18.6~
9 10.4~
A
o 17.0~ 9 11.O~ zx 10.O~ 9 9.0~ n 8.0~
E
9.4~
9 8"4~
O.1
a
7.4~
r/3 .,..~
0,
-
~--
~0
-0.1
AV 9 o
o -0.2 ~ -0.3
Cyclohexane t ,
i , 0 10 20 Distance [nm] Fig.2 Force curves between carbon-graphite surfaces immersed in cyclohexane measured at various temperatures above the freezing point.
-1 -
Methanol I
,
I
,
0
10 20 30 Distance [nm] Fig.3 Force curves between carbon-graphite surfaces immersed in methanol measured at various temperatures.
3.1. Ultrahigh sensitivity of vdW force on temperature and its trick Figure 2 illustrates the force curves between the carbon particle and graphite plate immersed in cyclohexane at various temperatures above the bulk freezing point. As is expected the force at higher temperature, e.g., at 18.6~ exhibits typical van der Waals (vdW) force that acts mainly in the single nanometer range. On the other hand what would NOT be expected is its ultrahigh sensitivity on the temperature. Only about 10 degree C of temperature change brought multi-fold variation in the force. From theoretical point of view the effect of temperature on the vdW force would be only from the variation in density of solids and liquid, which at most would affect only a few percent in this small range of the temperature examined. As a matter of fact, the same measurement in methanol gives the results shown in Fig. 3: Quite naturally the force curves do not show any detectable difference against the temperature variation. We should, then, not take the abnormal temperature dependence as is apparently shown, but should interpret it on the basis of the insensitivity of vdW force on temperature. The observed abnormality can be reasonably interpreted if we look at the trick in the determination of the separation distance in the AFM measurement, which follows below. Unlike the surface force apparatus the AFM measurement does not have a direct method to detect the distance between the surfaces. Instead, one will take the linear signal of cantilever deflection against sample displacement as the origin of the surface distance: The linear signal results because the two surfaces are in contact and move together. This manner of "wall detection" usually works well. However, what would result if the "freezing point elevation" is the case? Suppose that the liquid between the two surfaces may freeze when the two surfaces come close to a certain distance, at which the superposition of the potential energy from each wall exceeds a critical strength that would be needed for the liquid to freeze. The two surfaces, then, can never go any closer because of the steric repulsion of the frozen phase (Fig. 4). The system replies with a linear signal as if it reached the situation of real contact. The observed force curve would then represent a part of the real vdW force cut out at this distance, taking this point as the origin of the surface distance. A "weak" force appears.
3.2. Force curves against real separation distance: freezing point vs. distance relation
415
Fig.4 Schematic representation of the "trick".
Fig.5 Shifted force curves between carbon-graphite surfaces immersed in cyclohexane.
Now we try constructing the real force curves plotted against real separation distance. The key assumption for this trial would be the insensitivity of the vdW force on the temperature. The attractive force before the freezing or the apparent contact should stay almost unchanged. Thus each force curve was slid laterally to exhibit a best fit in the region of attractive force with the long separation distance. The results are shown in Fig. 5. Each force curve converges into a single curve in longer distance region, which is in line with the insensitivity of the vdW force on temperature and stands for the validity of the above assumed interpretation of the apparent phenomena. Then the "walls" standing at certain distances for lower temperatures would be a direct reflection of the freezing at a temperature above the bulk freezing point. Namely, the graphitic/carbonaceous surfaces in their nature exert strongly attractive potential energy to cyclohexane confined within, and the superposition of the two potential energies grows with decreasing distance between them. At the position where the "wall" stands in Fig. 5, the potential energy in the nanospace exceeds the critical value for the confined liquid to be frozen. Form the results of Fig. 5, a quantitative discussion can be made on the model of freezing proposed earlier [3] because it gives the relation between freezing point and the "pore" size expressed as the location of the "wall", which is described in the next section. Before going into the model examination the results employing the other liquid, OMCTS, are described. Similarly to Fig. 2, an apparent ultrahigh sensitivity of the force on temperature was detected as shown in Fig. 6. Of course this would not be the case in reality, and the same procedure of shifting the curve was attempted. Figure 7 illustrates the result. The agreement in the longer distance region is even more pronounced than that for cyclohexane, and three "walls" can clearly be obtained for this liquid, which would again suggest the freezing of confined liquid above the bulk freezing point by the superimposed potential energy in nanospace. The superposition of the potential energy is typically the case for the micropores. Note that, however, superposition of potential energy in some larger pores of a few nanometers would still be sufficient to cause detectable elevation in the freezing point because the potential energy of carbon surfaces itself shows quite a large value in the unit of temperature
416
[] 26.0~
~1.0
9
o
o
20.8oC
~
~-,
[
!
/
l
r [] 26.io(3 9 22.0~ <> 20.8~ 9 19.5~
~0.0
o:'O
....
1
o.o
I
OMCTS
-1.0
20 lO Distance [nm] Fig.6 Force curves between carbon-graphite surfaces immersed in OMCTS measured at various temperatures above the freezing point.
-1~
lO 20 Distance [nm] Fig.7 Shifted force curves between carbon-graphite surfaces immersed in OMCTS.
when converted with Boltzmann's constant: it will be more than hundreds of Kelvins in the vicinity of the surface and can easily be tens of Kelvins even with a distance of a few nanometers. The relation between the size of the nanospace and the shift in freezing point is summarized in Table 1. Though the extent of the elevation itself might look small, we believe that the finding of the existence of the elevation would be of much importance in the research field of the phase behavior in nanopores. It would be worth noting here that the spacing between the surfaces at which the frozen phase is thought to emerge is likely to be exhibiting some multiple of the molecular diameter (ca. 0.5nm for cyclohexane and ca. 0.9nm for OMCTS). This is more deducible from the results on OMCTS. As already examined by our molecular simulation work [4], freezing is a little bit enhanced if the pore spacing matches a multiple of the molecular diameter, resulting in a small oscillation of freezing point against pore size over a general monotonous trend. The confined liquid is thought to find this small benefit when the two surfaces gradually approach. 3.3. E x a m i n a t i o n
of the model
Through a simple thermodynamic treatment the following equation was obtained to express the extent of freezing point elevation b7' [3].
67" -A~ = ~ Tj Ahm
(1)
where Tf is bulk freezing point, and dhm - (SrSs)Tf is the latent heat absorbed on melting (>0). Table 1. Summary of freezing point elevation Approx. number of 6T[K] Distance [nm] molecular layers Cyclohexane 1.0 2.0 OMCTS
7.0 3.5
1.5
4.6
5
2.8 4.0
2.8 1.7
3 2
14 7
417 10
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L.)
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,
10 Observed pore width Wobs [nm] Fig.8 Comparison in terms of the width of nanospace for cyclohexane. The "excess" potential A~b is defined as the difference between the pore potential ~ore and that for a fictitious pore whose wall was made up of the liquid's solid state ~iquid. The underlying assumption includes: The perturbation in the structure of the solid phase is negligible in the slit pore; thus the temperature dependence of the chemical potential of solids in slit pore can be expressed with the entropy of the bulk solid Ss. Further details on the model are available in Ref. 3. For a strongly attractive wall A~b is negative, thus resulting in elevated freezing point. For larger pores [A~ becomes small and the freezing point approaches to the bulk value. The model was tested quantitatively with the simulation results, and showed good agreement. The potential function that is suitable to express carbon/graphite surfaces would be the so-called 10-4-3 potential [11]. If we look at, however, the potential energy at a distance in the range of a few nanometers, it can be approximated by the so-called 9-3 potential (qfl-3(z)=(2/3)rcpsCo3[(2/15)(cr/z)9-(cr/z)3]), and only the attractive part will be prevailing. Thus a single wall contributes to the A~b by the solid's ~b9-3 minus the liquid's one. Taking the potential energy at the center of the pore of width w as the simplest representative value, the A~b in question can be expressed as follows. A~b "-
w
2[Op9or3e J.
-- Oliquid( z )] =
0
W3
6
6
; where C = -~Jr(p,e,SCrsS - pse•Cr•)
(2)
With the above treatment, all that has to be determined for estimation by the model is now the parameter C. The definition of the parameter naturally reminds us of its tight relevance with the Frenkel theory for adsorption on a planar surface. Utilizing the theory, a method to determine the above parameter from a standard adsorption isotherm was developed by the authors based on the molecular simulation as an ideal experimental system [12], and successfully applied to a real experimental system [13]. Here to be needed for the determination is an adsorption isotherm of cyclohexane and OMCTS on a nonporous carbon or graphite. We found the data for cyclohexane [14], but not for OMCTS: The comparison can only be made here for the former liquid, whose parameter was determined as C=2.3x10 -22 [Jnm3]. Equations (1) and (2), together with the parameters, connect the freezing point elevation and the width of the nanospace. For a given elevation the comparison was made in terms of the width as shown in Fig. 8. Though the number of data points is quite limited, the predicted widths are roughly in good accord with the observed results. Note that the "parameter" needed for the prediction is NOT a fitting parameter but is the one determined from adsorption data, independently, based on its physical meaning. Considering the above, the agreement would be thought as a satisfactory one to demonstrate the validity of the model.
418 An examination of the results of OMCTS shall be made in the near future to confirm the reliability. 4. CONCLUSION Following the earlier molecular simulation work by the authors, in which freezing point elevation in nanospace had been predicted, an experimental trial for finding the elevation phenomena in the simplest geometry, a slit, was conducted. The employed technique for this purpose was colloidal-probe Atomic Force Microscopy. A carbon microparticle with high degree of carbonization was attached to the top of the cantilever tip, forming the colloidal probe, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned here. The results demonstrated that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes when the distance comes down to 4 nm, even at 8.4~ which is above the bulk freezing point of 6.4~ Measurements with octamethylcyclotetrasiloxane, bulk freezing point being 18~ also detected the freezing behavior, e.g., at the distance of 1.7 nm at 22~ Further, the degree of elevation was in good accord with the prediction by our model based on the attractive potential energy of the wall. Though the extent of the elevation itself might look rather small, we believe that the finding of the definite existence of the elevation, and its accord with the prediction of our model would be of much importance in the research field of the phase behavior in nanopores. REFERENCES
[ 1] Over more than half a century many studies were reported: e.g., W.A. Patric and W.A. Kemper, J. Chem. Phys., 42 (1938) 369; J.A. Duffy, N.J. Wilkinson, H.M. Fretwell, M.A. Alam and R. Evans, J. Phys. Cond. Matter, 7 (1995) L713. [2] J. Klein and E. Kumacheva, Science, 269 (1995) 816. [3] M. Miyahara and K.E. Gubbins, J. Chem. Phys., 106 (1997) 2865. [4] H. Kanda, M. Miyahara and K. Higashitani, Langmuir, 16 (2000) 8529. [5] M. Miyahara, H. Kanda, M. Shibao and K. Higashitani, J. Chem. Phys, 112 (2000) 9909. [6] H. Dominguez, M. P. Allen, and R. Evans, Mol. Phys., 96 (1999) 209. [7] R. Radhakrishnan and K. E. Gubbins, Mol. Phys., 96 (1999) 1249. [8] K. Kaneko, A. Watanabe, T. Iiyama, R. Radhakrishnan and K.E. Gubbins, J. Phys. Chem. B, 103 (1999) 7061. [9] A. Watanabe and K. Kaneko, Chem. Phys. Lett., 305 (1999) 71. [ 10] M. Sliwinska-Bartkowiak, R. Radhakrishnan and K.E. Gubbins, Molecular Simulation, 27 (2001) 323. [ 11 ] W. Steele, The interaction of gasses with Solid Surfaces, Pergamon, Oxford (1974). [ 12] M. Miyahara, T. Yoshioka, J. Nakamura and M. Okazaki, J. Chem. Eng. Jpn., 33 (2000) 103. [13] H. Kanda, M. Miyahara, Y. Yoshioka and M. Okazaki, Langmuir, 16 (2000) 6622. [ 14] A.L. Myers, C. Minka and D.Y. Ou, AIChE J., 28 (1982) 1.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
419
Is it possible to obtain a coherent image o f the texture o f a porous material? Francis Noville a, C6dric Gommes a, Catherine Doneux b, Alain Brasseur a, Ren6 Pirard a, Jean-Paul Pirard a a Universit6 de Li6ge, Laboratoire de G6nie chimique, Institut de Chimie, B6a, B-4000 Li6ge b Solvay Research & Technology, Analytical Technologies Department, rue de Ransbeek, 310, B- 1120 Brussels
This study consists in verifying the coherence of a few commonly used analysis methods of nitrogen adsorption-desorption isotherms. These methods were tested on model samples obtained by mechanically mixing two micro- and mesoporous solids respectively with known mass proportions. Although the individual analysis methods may lead to discrepancies in the interpretation of the isotherms, their systematic comparison allows drawing a coherent picture of the porous texture. 1. INTRODUCTION The analysis of nitrogen adsorption-desorption isotherms is one of the most commonly used methods to assess the texture of porous materials. It has given rise to numerous theoretical studies and many mathematical models have been developed to analyze the results. These models establish a relationship between pressure and pore size on the basis of the real physicochemical adsorption mechanisms. However, the user is often bewildered by the diversity of the models, the disparity of basic hypothesis, the difficulty checking them and the apparent incoherence of the results. The aim of this work is to test and to compare the performances of various nitrogen adsorption-desorption isotherms analysis methods. These models were applied to model samples obtained by mechanically mixing two micro- and mesoporous solids respectively in perfectly known proportions. The relevant morphological characteristics of the porous texture of the mixtures, such as the specific surface and volume, are physically additive. A criterion that allows determining the reliability of the analysis methods tested is thus to check the linearity of the relation between a given parameter and the weight percentage of the pure solids.
420
2. E X P E R I M E N T A L Two materials have been used: a mesoporous alumina (Pural SCC30 - Condea) and a microporous amorphous Si-Ti co-gel. Five mixtures were prepared by mixing these materials in various mass proportions. The nomenclature of samples is given as X(y), y is the weight percentage of microporous material. The nitrogen adsorption-desorption isotherms were determined at nitrogen boiling temperature by the classical volumetric method with a CE Instruments SORPTOMATIC 1990 series of THERMOQUEST. The apparatus is equipped with an additional 10 torr pressure gauge and a turbomolecular pump. Nitrogen of high purity (99.999%) was used. The samples were first outgased at 10.4 Pa during 16 h, then heated from 25 to 200~ at a rate of 1~ and kept at 200~ during 12 h. The weight loss of each sample during this treatment was determined and removed from the sample weight.
Table 1" Samples' textural properties. Sample
CBET
SBET m2/g
St m2/g
Sext
SBdB SBJH
m2/g
m2/g
m2/g
m2/g
SDH
X(0)
98
173
169
(a)
157
282
280
X(t 3)
166
228
233
194
126
229
228
X(29)
205
317
335
203
104
191
190
X(50)
264
408
442
152
74
135
134
X(76)
393
545
536
92
35
64
64
X(85)
333
603
582
92
28
44
43
X(100)
380
698
691
_(a)
_(.)
_(a)
_(a)
Sample
VBaB
VBjH
VDH
•m
cm3/g
cm3/g
cm3/g
cm3/g
VD
VHK
VB
cm3/g
cm3/g
cm3/g
Vp cm3/g
X(0)
0.527
0.747
0 744
0.060
_(a)
_(a)
_(a)
0.563
X(13)
0.435
0.607
0 605
0.078
0.082
0.107
0.015
0.507
X(29)
0.366
0.487
0486
0.110
0.116
0.145
0.048
0.487
X(50)
0.256
0.326
0 324
0.141
0.160
0.181
.0114
0.452
X(76)
0.129
0.164
0 164
0.189
0.218
0.237
0.186
0.374
X(85)
0.099
0.139
0 139 (a)
0.208
0.234
0.259
0.283
0.354
0.241
0.252
0.307
0.347
0.327
X(100) _(a) (a) not applicable
_(a)
421 400
@
Adsorbed Volume (cm31g)
400
Adsorbed Volume (cm 3/0 )
4,
~,~
-
,~,"
t
<; 200
200
--r
==--- ---"--. . . . . . . . . . . . . . . . .
b 0
0
0.5
Relative pressure
p/po
1
................
0
0.5
1
Statistical thickness
1.5
t (nm)
Figure 1. Adsorption-desorption isotherms (a) and corresponding t-plots (b) for samples X(0) ~, X(50) .,and X(100) II, 3. RESULTS AND DISCUSSION
3.1. Qualitative analysis of nitrogen adsorption-desorption isotherms Figure 1a shows the nitrogen adsorption-desorption isotherms of the two pure materials as well as mixture X(50). According to BDDT classification, the adsorption-desorption isotherm obtained on the pure alumina sample, X(0), is of type IV with a type E hysteresis loop. This kind of isotherm is typical of a mesoporous material with cylindrical pores closed at one end. In contrast, the adsorption-desorption isotherm obtained on the pure Si-Ti co-gel, X(100), is of type I without hysteresis loop, which corresponds to an exclusively microporous material
[1].
The five mixtures' isotherms are linear combinations of the pure materials' isotherms. At any pressure, the adsorbed volume is proportional to the weight percentages of each pure compound. This result is statistically confirmed by an Z 2 test with a threshold of 1%. The closing of the hysteresis loop always occurs at the same pressure and its height is proportional to the weight percentage of alumina in the mixture.
3.2. Specific surface area The specific surface areas, SBET, as estimated from the BET theory for the pure materials and for each mixture are given in Table 1. This method consists in determining the specific surface area from the monomolecular layer volume of the adsorbed nitrogen. It is clearly seen on Figure 2a that the specific surface area, SBET, is a linear function of the mixing proportions. As discussed in the introduction this observation suggests that the BET surface is a reliable parameter.
422 ,.... 800 0')
a
~"
r
g
b
300 ,
:
9
~
O :3
---...
4oo
=
150
o e~ 0
50
100
Microporous material content (wt. %)
O e~ O :3 ,n o
0 .................. o
~
50
,
0
lO0
Microporous material content (wt. %)
Figure 2. Specific surface area, SBET (a), and estimated mesoporous surface area (b), Sext 9 SBJH &, and SBdB *, as a function of the weight fraction of microporous material y.
However, Dubinin and co-workers do not accept the concept of monolayer formation in micropores and propose determining the microporous volume, VD, on the basis of the thermodynamic theory of Polanyi adsorption. However, one can observe that the monolayer volume, Vm, when expressed in liquid nitrogen volume per unit mass, is very close to the Dubinin volume, VD. The proportionality of the BET monolayer volume, Vm, and the so-called micropore volume, VD, (VD ~ 1.1 Vm) has been observed for many materials, as shown in different studies [2, 3]. This means that both variables are correlated, so determining one is equivalent to the determining the other. The discussion on the physicochemical meaning of these parameters may be interesting from a theoretical point of view but as far as practical characterization of porous materials is concerned, both methods can often be considered as equivalent. 3.3. t-plot
The t-plot compares the adsorbed volume on a porous material with the amount adsorbed at the same pressure on a given non-porous material. In the case of a non-porous material, the thickness of the adsorbed layer, t, if plotted versus the relative pressure p/po can be represented by a single curve called "standard isotherm". According to Lecloux [1, 4], the selection of a standard isotherm can be based on the magnitude of the adsorbent-adsorbate interaction as assessed by the CBET constant value. As long as the multilayer is formed unhindered on the solid surface, the t-plot is a straight line passing through the origin. Its slope is proportional to the total surface area, St, of the adsorbent. The surface areas SBET and St are found to be very similar for the two pure samples as well as for the five prepared mixtures (Table 1). At higher relative pressure (higher t value), the t-plot of the pure Si-Ti co-gel sample exhibits a downward deviation from the linearity, which is typical of micropores. On the contrary, an upward deviation is observed for the pure alumina sample, which corresponds to capillary condensation in the mesopores (Figure 1b). For the five mixtures, a downward deviation from linearity is first observed and then an upward deviation corresponding to mesopores. It is sometimes possible to estimate the mesoporous surface area, Sext, from the slope of the second linear portion of the t-plot,
423
observed after the filling of micropores and before capillary condensation. In order not to take capillary condensation into account we only considered the part of the t-plot corresponding to relative pressures smaller than the closing of the hysteresis loop. However, as the micropore filling coexists with the capillary condensation in mesopores in a given pressure range, the external surface area, Sext, overestimates the real surface area of mesopores (Table 1). Indeed, the mesoporous surface area versus y should be a straight line decreasing from 170 m2/g, in the case of the pure mesoporous material, X(0), to 0 m2/g for X(100). Furthermore, it is clearly seen from Figure 2b that Sext versus y is not a straight line and should thus be considered as a poor estimator of mesoporous surface area. Nevertheless, the t-plot is a useful tool in textural studies since it gives valuable qualitative information on the type of pores present in the samples. Even if micropores are present in very low proportions, such as in the X(13) sample, a downward deviation is observed. In the same way, a very low mesoporous volume can also be detected from an upward deviation from linearity.
3.4. Cumulative specific surface area and pore volume
Mesopore size analysis is usually based on the application of Kelvin's relation between vapor pressure of a capillary condensed phase and pore size. It is conventionally admitted that mesopores are pores whose width lies between 2 and 50 nm. For narrower pores, the capillary condensation phenomenon does not take place and Kelvin's relation is irrelevant. These methods are thus strictly limited to pores in which the capillary condensation phenomenon occurs, as can be visualized by the hysteresis loop. BJH and Dollimore-Heal methods are based on the assumption that the statistical thickness of the adsorbed layer is independent of the surface curvature and assume that the meniscus between vapor and condensed phase is hemispherical. This kind of meniscus is met in the case of a cylindrical pore during desorption. The hypothesis of constant thickness, independently of surface curvature, is justified for large mesopores, but generally leads to underestimation of pore size [7].
Figure 3. Mesopore volume distribution as a function of pore radius, obtained by the BdB (a) and BJH (b) methods. X(100) m, X(85) at, X(76) i,, X(50) o, X(29) c~, X(13) A, X(0) 0.
424 In order to correct these discrepancies, Broekhoff and de Boer [5] generalized Kelvin's equation by taking pore shape into account, as well as the influence of surface curvature on the thickness of the adsorbed layer. The BdB method can be applied to both adsorption and desorption isotherms using four different pore models defined by a shape factor. Unfortunately, owing to computational difficulties, this last method, although more general, has been far less applied than the first two. For the pure alumina sample the chosen model for computing cumulative surface area SBdB, cumulative volume VBdBand surface area and volume distribution as a function of pore size is a cylinder closed at one end. This choice is motivated by the type IV shape of the isotherm and by the E type hysteresis. In the case of the cylinder model closed at one end, the relevant branch of the isotherm is the adsorption. The calculation is carried out from the saturation pressure down to the pressure corresponding to the hysteresis loop's closing. The cumulative specific surface area, SBdB, is close to both SBET and St values for the pure mesoporous material, X(0) (Table 1). For the five mixtures, the cumulative mesoporous volume, VBdB, and mesoporous surface area, SBdB, and are both linear decreasing functions of the micropore content y (Figure 2b). The cumulative specific surface area SBdB is definitely a better estimator of the mesoporous surface than the specific surface Sext computed from the t-plot. The IUPAC classification states that mesopores are pores whose width is larger that 2 nm. In the case of the cylindrical pore model retained for the pore size distribution, this is equivalent to radii larger than 1 nm. It should however be stressed that the calculation of the cumulative surface and volume of the mesopores must not be continued at lower pressures than the closing of the hysteresis loop (gray zones of Figures 3a and 3b). If a black box analysis tool is used and if the calculation is systematically continued down to 1 nm, severe overestimation of the mesopores surface and volume may occur. The BJH and Dollimore-Heal methods, owing to the inadequacy of the basic hypothesis, lead to an underestimation of pore size and hence to an overestimation of the total mesoporous surface area. Mean mesopore radius, as estimated from the BdB method is found to be close to 7 nm (Figure 3a), while BJH method predicts a mean size of 5 nm (Figure 3b). In the case of the pure mesoporous sample, X(0), SBJH, SDH are found to be greater than SBET and St, which have been shown to be very reliable estimators. SBdB is in better agreement with both SBET and St (Table 1).
2]-E
b E
&
"o 0
O--i. . . . .
0.2
1 Pore width (nm) . . . . .
1.8
0.2
1 Pore width (nm)
1.8
Figure 4. Micropore volume distribution as a function of pore width, w, obtained by the Brunauer (a) and HK (b) method, X(100) m, X(85) A, X ( 7 6 ) . , X(50) e, X(29) cz, X(13) A,
x(o) o.
425 0.4
Microporous volume (cmSlg)
9
Figure 5. Cumulative microporous volume as estimated from Dubinin, 0.2 VD 0 Brunauer VB 9 , and Horvath-Kawazoe, VHK A, theories. 50
Microporous material content (wt. %)
100
3.5. Micropores Three different methods were tested, namely the Horvath-Kawazoe (HK) [6], the Dubinin and Brunauer methods [ 1]. The HK method is based on the energetic interaction of the adsorbent and adsorbate in micropores, and therefore requires that the values of four physical constants be known. The appropriate values are seldom available, and the method is strictly limited to a few adsorbates. However, we applied the method using values corresponding to the adsorption of nitrogen on carbon black. The obtained cumulative microporous volume is slightly higher than that obtained from the Dubinin method. Using both methods, the estimated microporous volumes are linear function of y, but the lines do not pass through the origin. This observation suggests that these methods are not only sensitive to micropores, but also to the mesoporous surface area. The Brunauer method assumes that the slope of the t-plot, even in non-linear regions, is proportional to the surface of the not yet filled micropores. The analysis is thus limited to t values lower than the second linear portion of the t-plot. The cumulative surface of micropores can be evaluated as St minus Sext, i.e. as the fraction of the total surface that cannot be explained by the mesopores. Sext being overestimated, it should not be surprising that VB underestimates the true microporous volume.
4. CONCLUSIONS Various commonly used nitrogen adsorption-desorption isotherms analysis methods have been applied on model samples containing micro and mesopores in known proportions. The very observation of the isotherms, as well as of the corresponding t-plots, allows a qualitative conclusion to be drawn about pore type and shape. It has been checked that even a very low amount of micropores leads to a visible downward deviation in the t-plot, such as in the X(13) sample. This conclusion is also true for mesopores that are already visible in the t-plot of sample X(85). These two observations confirm the usefulness and reliability of the t-plot as a tool for qualitative texture analysis. It has also been verified that the BET surface area, SBET, as well as the total surface area estimated from the t-plot, St, are both very reliable estimators of the specific surface of a
426 porous solid. These estimators, when plotted against the weight fraction of microporous solid present in the mixture, y, lead to an almost perfect straight line. BJH, Dollimore-Heal and BdB methods were compared for assessment of mesoporosity. Owing to the pore shape of the material used, the first two methods lead to an underestimation of pore size, and hence to an overestimation of total mesoporous surface. The last method is more general, and when the appropriate shape factor is used, a reliable estimation of the mesoporous texture is obtained. The SBdB estimator is almost a perfectly linear function of y, and vanishes in the X(100) sample. The external surface area, Sext, computed from the t-plot leads to an overestimation of the true mesoporous surface. Concerning microporosity, this study confirms the fact that Dubinin method leads to a porous volume very close to the monolayer volume estimated from the BET theory. This observation suggests that this methods is sensitive to the mesoporous volume as well and is thus unsuitable for estimating the micropore contribution to the total porous volume. The HK method leads to porous volumes close to the ones estimated with the Dubinin method. Furthermore, this method requires knowing physical constants characterizing the adsorbent as well as the adsorbat. Since the appropriate values are generally not available, this method should strictly be applied to some well known adsorbate-adsorbent systems only. Brunauer method, although leading to non linear plots of estimated volume versus y, is the less influenced by the mesoporous volume. Although independent analysis methods may lead to contradictory conclusions, a systematic comparison of theses methods leads to a coherent picture of porosity, as already advocated by Lecloux [ 1] in the 1980s, insofar as the user is aware of the applicability of the methods.
REFERENCES
1
A.J. Lecloux, in Catalysis Science and Technology, edited by J.R. Anderson and M. Boudart (Springer-Berlin, 198 !), Vol. 2, p. 171.
2
G.O. Wood, The European Carbon Conference "Carbon 96", Newcastle, U.K. (1996) p. 606.
3
R. Pirard, J.P. Pirard in R6cents Progr6s en G6nie des Proc6d6s - Science et Technologie des Poudres edited by H. Muhr (Lavoisier Technique et Documentation Paris, 2001), Vol. 15(77), pp. 149-160.
4
A. Lecloux, J.P. Pirard, J. Colloid Interface Sci. 70, 165 (1979).
5
J.C.P. Broekhoff, J.H. de Boer, J. Catal. 9, 8 and 15 (1967), 10, 153, 368, 377 and 391 (1968).
6
D.D. Do, Adsorption Analysis: Equilibria and Kinetics, Series in Chemical Engineering, Volume 2, Imperial College Press, London (1998).
7
P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics Instrument Corporation (1997).
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
427
Preparation and characterisation of Cr-Co spinels for methane oxidation in presence of sulphur compounds J.R. Paredes, S. Ord6fiez, F.V. Diez, H Sastre Department of Chemical and Environmental Engineering, University of Oviedo, C) Julian Claveria s/n, 33071-Oviedo, Spain. Phone: 34-985 103 508, FAX: 34-985 103 434. e-mail: [email protected]
Three different Cr-Co spinels were prepared and tested as catalysts for the oxidation of methane in the presence of SO2, a typical catalyst poison. The spinels were prepared from nitrate precursors using a co-precipitation method, followed by calcining at three different temperatures, (400, 600 and 800 ~ for 5 hours. Characterisation results indicate that the catalyst calcined at 800~ presents a structure of pure spinel, whereas the presence of single oxides is observed in the catalyst calcined at 600~ and the catalysts calcined at 400~ presents a very complex structure (probably a mixture of several single and binary oxides). Experiments show an important influence of calcining temperature on the catalyst performance. In absence of SO2, catalysts calcined at 400~ and 600~ performs similarly, whereas the activity of the catalysts calcined at 800~ is worse. When sulphur compounds were added to the feed, catalyst calcined at 600~ deactivated faster than the other two catalysts.
1. INTRODUCTION Catalytic incineration has shown to be a technically, economically and environmentally interesting alternative for the abatement of organic pollutants in gaseous emissions. In the last years the interest for decreasing methane emissions from industrial sources has increased, due to the large amount of methane emitted and the high global warming potential of this compound. In most cases the decrease in emissions can only be achieved using end-of-pipe technologies. One of the most promising technologies, widely used for the abatement of VOCs, is catalytic combustion [ 1]. However, the catalytic combustion of methane is more difficult than the combustion of other VOCs, since this compound is very difficult to activate. This aspect leads to higher operation temperature, causing fast deactivation of conventional supported precious metal catalysts due to crystallites coalescence. In addition, in most methane-containing emissions (emissions from coke ovens, composting plants, etc.), sulphur compounds (SO2, H2S or organosulphur compounds) are present. The presence of these compounds hinders most the treatments. So, in the case of
428 catalytic incineration, sulphur compounds have been demonstrated to be a strong poison for the most active catalysts, such as supported palladium. However complex metal oxides such as perovskites, hexaaluminates or spinels are considered as more resistant to poisoning. Although single oxides are considered as active for this reaction their thermal stability is quite lower [2]. In this work the performance of Cr-Co spinels as catalysts for total combustion of methane is studied. The spinels were prepared from nitrate precursors. The effect of temperature and time of calcining was studied using x-ray diffraction (XRD) in order to check the crystalline structure and the absence of other phases, N2 physisorption (BET) in order to study the porous structure of these solids and Temperature Programmed Reduction (TPR) in order to determine the presence of different metallic species. The deep oxidation of methane was chosen as model reaction because this reaction is considered as one of the most difficult catalytic combustions, and the simultaneous presence of methane and sulphur compounds is common in many off-gases. Sulphur dioxide was chosen as the model sulphur compound because in previous works it was observed that other sulphur compounds were oxidised to SO2 at the operation temperatures.
2. EXPERIMENTAL
2.1. Catalyst preparation Spinel-type oxides of Cr and Co were prepared by coprecipitation of a solution of Cr(NO3)3"9H20 and Co(NO3)2"6H20 in a mass ratio of 2.74 at pH=8. The precipitation was accomplished by adding dropwise a diluted solution of ammonium hydroxide to a cosolution of precursor salts under continuous stirring until a pH=8 was reached. The precipitate was then filtered, washed with distilled water and dried overnight at 100 ~ Afterwards, three samples were taken and calcined each one at different temperature, 400, 600 and 800 ~ for 5 hours. All chemicals used were of analytical grade, supplied by Panreac.
2.2. Catalyst Characterisation Surface area measurements of fresh and used catalysts were carried out using a Micromeritics ASAP 2000 apparatus with nitrogen as the adsorbing gas, considering a value of 0.164 nm 2 for the cross-section of the nitrogen molecule. All samples were outgassed at 200~ before analysis. TPR analyses were carried out using a Micromeritics TPD/TPR 2900 Analyzer, operating at room conditions. The reactor was a quartz glass vertical tube and the catalyst sample (20 mg) was kept in position by plugs of quartz wool. This reactor was placed inside an electric furnace, temperature being controlled by a PID controller (Eurotherm). The gas flow rate was measured by rotameters and the gas leaving the reactor was analysed by a TCD detector. The flow rate of the carrier gas (5% hydrogen in nitrogen) was 50 mL/min. The temperature was increased at 10~ from room temperature to 900~ The hydrogen consumed was measured by the TCD. Powder X-ray diffraction patterns were recorded in a D-5000 Siemens diffractometer, using nickel-filtered CuK~ as monochromatic X-ray radiation. The patterns were recorded
429 over a range of 20 angles from 20 to 70 ~ and crystalline phases were identified using JCPDS files. The external surface morphology of the catalysts (around 1 pm depth of a gold-coated sample) was determined by the X-ray microanalyser of a JSM 6100 scanning electron microscope (SEM).
2.3. Reaction studies Combustion experiments were performed in a continuos packed bed reactor. The reactor consisted of a stainless steel cylinder of 400 mm length and 9 mm internal diameter. Two grams of catalyst, crushed and sieved to 250-355 ~tm, were placed in the middle part of the reactor, whilst the upper and lower parts were filled with glass balls (lmm). The reactor was placed inside an electric furnace, temperature being controlled by a PID controller (Honeywell) connected to a thermocouple placed inside the reactor, which monitored the reaction temperature. The system was provided with 5 additional thermocouples that measured the reactor wall temperature at different positions. The reactor was fed with 1.6 N1/min of 1000, 2000 and 4000 ppm of methane in air. The mixtures were obtained by mixing N-50 synthetic air and 2.5 % (vol.) CH4 in N-50 synthetic air (Air Products). 40 ppm of SO2 (from a cylinder of 370 ppmV SO2 in N-50 synthetic air, Air Products) were added when the effect of sulphur on the catalysts activity was studied. Flow rates were controlled by calibrated mass flow controllers (Brooks 5850 TR). Exhaust gas was analysed by gas chromatography (Hewlett Packard HP 5890 Series II). Methane in the inlet and outlet streams was analysed using a 30 m fused silica capillary column with apolar stationary phase SE-30, and a FID detector. CO and CO2 were analysed using a HayeSep N 80/100 and a molecular sieve 45/60 columns connected in series, and a TCD detector. Neither CO, nor partial oxidation were detected in any experiment, the carbon mass balance fitting in all the cases within 2%. Methane conversions were calculated both from outlet methane and CO2 concentrations, being both values very close in all the cases. Methane (2000 ppmV) and SO2 (40 ppmV) concentrations have been selected because they are representative of industrial emissions, such as coke oven emissions.
3. RESULTS AND DISCUSSION
3.1. Catalyst characterisation Most works found in the literature studying the catalytic performance of spinel-like materials, used a fixed calcining temperature (in the range 500-700~ As far as we know, the influence of calcining temperature in the performance of spinels as combustion catalysts has not been studied, although this parameter largely affects the properties of the solids [3]. TG analysis of the dried precursor shows that all the weight losses take place at temperatures below 300~ So, in all the samples the metals are present as oxides. So, the only process occurring in the catalyst is the solid phase transition to form the spinel structures. However, this technique, as well as DSC analysis, is not suitable for studying the formation of spinels, since they are formed in a non-instantaneous solid phase reaction.
430
XRD profiles (Fig. 1) show important differences between the calcined samples. So, in the case of solid calcined at 800~ a well-crystallised spinel structure (JCPDS file number: 22-1084) is observed. The sharpness and intensity of the peaks indicate that the spinel
A ,,,,,
em
C
400~
.Jk
.................. 7 -
Sk A Sk .......
30
s~k
.k~_-_. . . . . . . . . . . . . . 60
90
Angle 20 Fig. 1. XRD pattems for the catalysts calcined at three different temperatures, The unlabelled peaks correspond to spinel structure, Sk to skolaite
50
250
450
650
850
Temperature (~ Fig. 2. TPR profiles for the catalysts calcined at different temperatures
presents a very crystalline structure. Peaks corresponding to other crystallographic structures were not detected. By contrast, in the case of the catalyst calcined at 400~ the solid is less crystalline, the intensity of the peaks being markedly lower. In addition, 325 peaks of other crystallographic phases, such as Cr203 (skolaite) were detected. In the case of the spinel calcined at Co304 600~ no additional peaks were detected but the structure is clearly less m co'1 crystalline than the structure of the solid Cr203 ,m calcined at 800~ It is important to remark that cobalt oxide (Co304), w a s not detected in any case. Although this C0304 q- C r 2 0 3 solid has also spinel structure (JCPDS i , i number 42-1467), the characteristic 100 250 400 550 700 850 angles and intensities are different from Temperatu re (~ the ones of Co-Cr spinels, allowing the differentiation using XRD. Fig. 3. TPR profiles for pure Co304, Cr203 TPR profiles of all the catalysts and a mechanical mixture of both with used in this work are shown in Fig. 2, similar chemical composition to spinels whereas the TPR profiles for the same
___A J
431 amount of Cr203, Co304 and a mechanical mixture of these oxides in proportions similar to the composition of spinel is depicted in Fig. 3. It is important to remark the presence of a peak at temperatures around 230~ in the spinels calcined at lower temperatures. On the other hand, pure Co304 present a big peak with the maximum at 325~ This aspect leads to think that this peak corresponds to cobalt oxide, which does not react to form the spinel (it appears at lower temperature, probably due to its smaller particle or a highly defective structure). So, in the case of the solid calcined at 800~ this peak is not observed. Concerning to the reduction peak that appears at higher temperatures, it could be caused by the reduction of the spinel structure, since it does not appear in the case of single oxides. This peak is shifted to higher temperatures in the case of the solid calcined at 800~ This behaviour could be caused by the higher crystallinity and smaller porosity of this material. In the solid calcined at 400~ several peaks, which do not appear in the case of pure Cr203 or Co304, are observed. The complexity of the profile corresponding to this material, indicates that the solid-phase reaction 0.035 leading to spinel structure is not A completed, being present several o.030 - 400~ 800oc intermediate compounds. ']:: 0.025 Nitrogen physisorption analysis o.02o reveals important morphological 1:: differences between the solids. So, BET = o o.o15 surface area decreases as the calcining >~ 0.OLO temperature increases, being the surface area 101.7, 29.4 and 16.93 m2/g for the a. o.oo5 solids calcined at 400~ 600~ and o.ooo , 800~ respectively. The porous structure lo 100 lO00 (Fig. 4) is also clearly different in these materials, especially for the solid log(D) calcined at 400~ for which pore diameters are markedly lower than for the Fig. 4. Pore distribution for the spinel other two materials. Comparing the solids calcined at three different temperatures (D calcined at 600 and 800~ it is observed in Angstroms) that their porous structure is similar, although the peaks are sharper in the case of the solid calcined at 800~ This aspect leads to think in a better defined crystallographic structure and is in a good agreement with the results obtained in the XRD analysis. SEM micrographs of the catalysts (Fig. 5), also show the differences between the three samples. So it is observed that the sample calcined at the lowest temperature presents a more porous texture, this aspect decreasing for the solid calcined at 600~ The morphological aspect of the catalyst calcined at 800~ is quite different, fused particles being observed in the surface. This is in a good agreement with the measured lower surface area of this material.
432
Fig. 5. SEM micrographs of the catalysts calcined at different temperatures (x20000). 3.2. Reaction studies
Stable performance of the 100 three catalysts for the oxidation of methane, at the conditions 80mentioned in the experimental section, in absence of sulphur compounds was observed. So, c 0 60deactivation phenomena were not detected for a period of 100 hours on ! stream at 425~ for the solids 4Ocalcined at 400, 600 and 800~ O The activity of these materials 0 20for methane oxidation was tested by recording light-off curves at different methane concentrations (1000, 2000 0 and 4000 ppm). The light-off curves for the combustion of 1000 ppm of 150 350 550 methane over the different catalysts Temperature (~ are shown in Fig. 6. In this figure, it is observed that the solids calcined at Fig. 6. Light-off curves for the combustion of 400 and 600~ present very similar 1000 ppm of methane over the catalysts calcined behaviour, whereas the solid at 400~ (A), 600~ ( , ) and 800~ (,,) calcined at 800~ is less active. The differences between the behaviour of these catalysts is not only caused by the differences in the surface area, since the variation of the relative activity (i.e. mol of CH4 converted per min and m 2 of catalyst surface area) does not follow any clear trend with the calcining temperature. So, at 400~ the relative activities are 0.225 gmol/min.m 2 for the catalyst calcined at 400~ 0.717 gmol/min'm 2 for the catalyst calcined at 600~ and 0.471 ~tmol/min.m 2 for the catalyst calcined at 800~ These results suggest that different active mill
433
phases are involved in the catalytic oxidation of methane. So, in the literature is reported that cobalt oxides and, in minor extension, chromium oxides are active for deep oxidation of hydrocarbons [4, 5]. On the other hand the interaction between the different oxides, which takes place in the catalyst calcined at 400~ could increase the activity of the catalysts. Concerning to the behaviour in presence of sulfur, an ageing experiment carried out at 425~ in presence of 40 ppm of SO2 (Fig. 7), showed that the behaviour of the three catalysts is very different. So, the catalysts calcined at 400 and 600~ present a progressive deactivation during the time interval studied, this deactivation being more pronounced in the case of the catalyst calcined at 600~ By contrast, the solid calcined at 800~ has lower initial activity (which is in a good agreement with the findings of the light-off curves) but in two hours it reaches a period of constant activity. The explanation of this behaviour is very complex, since different active phases are involved in the catalytic reaction, such as spinel, single oxides and non-spinel complex oxides. Results suggest that these phases are poisoned by the SO2 in a different way. So, depending the predominant phase in each catalyst the deactivation phenomena occur in different extent. In the case of catalyst calcined at 800~ which seems to be formed basically by spinel, it is observed that although is less active than 8O other catalysts, it presents good resistance to poisoning. The catalysts 60 calcined at 600~ is more r L~ ~ 9 active, but it deactivates 0 9 4, faster. This behaviour could ~ 40 be tentatively explained ! A considering an important > C /k L~ /k /k A z role of cobalt oxide in the o 020 n n n n n n n catalytic activity. At this point, some authors state [5] that cobalt oxides are very active as deep 0 2 4 6 8 10 oxidation catalyst, but not resistant to sulfur Time (h) poisoning. The explanation Fig, 7. Effect of 45 ppm of 802 on the combustion of of the behaviour of the 1000 ppm of CH4 at 400~ over catalyst calcined at catalyst calcined at 400~ is not so clear, since it 400~ ( , ) , 600~ (A) and 800~ (==) presents a resistance to deactivation even better than the catalysts calcined at 800~ Although further work must be carried out on the identification of the active phases of this catalyst, it seems that these unidentified phases could present high resistance to poisoning. Further experiments were carried out using pure Co304 as catalyst at these conditions. It was observed that, although the initial activity is higher than the observed for Co-Cr spinels (near 90 %,) the catalyst is fatally deactivated after ten hours on stream. A
=m=
s
]
i
i
434
4. C O N C L U S I O N S As general conclusion, it can be observed that spinel-like materials are interesting as thioresistant deep oxidation catalysts, in addition to operate at conditions close to the used with the more active supported palladium catalysts. On the other hand, this work shows that the calcining temperature of the precursor plays an important role in the catalytic properties of the catalyst, being quite different the structure and catalytic behaviour of the solids calcined at different temperatures. ACKNOWLEDGEMENTS This work was financed by the Spanish Minister of Science and Technology (PPQ20013675). Lara S. Escand6n is acknowledged for her cooperation in this work.
REFERENCES 1. E.C. Moretti, N. Mukhopadhyay, Chem. Eng. Prog. 7 (1993) 20. 2. M.F.M. Zwinkels, S.G. J~iras, P.G. Menon, Catal. Rev.- Sci. Eng. 35 (1993) 319 3. D.Ch. Kim, S.K. Ihm, Environ. Sci. Technol. 35 (2001) 222 4. C.J. Hayes, J.G. Irwin, J.G., H.A. Johnson, R.L. Moss, J. Chem. Tech. Biotech., 32 (1982) 1025 5. M. Paulis, L.M. Gandia, A. Gil, J. Sambeth, J.A. Odriozola, M. Montes, Appl. Catal. B 26 (2000) 37. 6. C.J. Hayes, H.A. Johnson, R.L. Moss, J. Chem. Tech. Biotech., 32 (1982) 1034
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
435
Characterising the porous structure of Egyptian mortars using thermoporometry, mercury intrusion porometry and gas adsorption manometry Jehane Raga'i a, Thomas Poyet b, Isabelle Beurroies b, Franqoise Rouquerol b and Philip Llewellyn b Chemistry Department, American University in Cairo, 113 Kasr E1 Aini St., PO-Box 2511 Cairo 11511, Egypt b MADIREL, (UMR6121) CNRS/Universit6 de Provence, 26 rue du 141 ~ RIA, 13331 Marseille cedex 3, France a
A comparison of different methods to characterise the porosity of Egyptian mortars presented. The results obtained using adsorption manometry, mercury porosimetry and thermoporometry are complementary and overlap in certain regions of pore size. 1. INTRODUCTION The Ancient Egypt of the Pharaohs still holds numerous secrets. One stares at awe at the magnificent monuments that were erected over 5000 years ago. For this reason it is interesting to take a closer look at the building materials used to construct such wonders. It is possible to consider that one of the strongest direct technical interventions in this building process is the mortar. Whilst mortars were well employed at this time, several questions persist. Is it possible to relate their porous structure to factors such as the various 6poques, different regions of manufacturing and different conditions of ageing ? It is well known that gypsum was the most abundant material used as mortar in Ancient Egypt [ 1]. From the Greco-Roman period, the use of lime was gradually introduced followed by the appearance of Portland cement in the 18 th century. It is suggested that although lime was available in Ancient Egypt, it was not used for building due to the heat treatment required and the ensuing high cost of fuel. Two qualities of gypsum mortar have been distinguished in Ancient Egypt. The first is a relatively poor quality gypsum mixed with small quantities of lime and sand and the second type of mortar used consists of a purer gypsum phase. In the present study we have had access to various mortars obtained from the Sphinx and the Kephren Valley Temple. The characterisation of the porous structure of these mortars was carried out using thermoporometry, mercury intrusion porometry and gas adsorption. The aim of this communication is to discuss the relevance of these techniques to the study of Egyptian mortars and to compare the superposition of the results thus obtained.
436
2. EXPERIMENTAL Several samples were obtained from the Sphinx were compared to a sample extracted from the Kephren Valley Temple. The mortars consist predominantly of gypsum with small quantities of lime and quartz. Table 1 gives a description of the sample origin. The nitrogen adsorption - desorption isotherms were obtained with a commercial manometric device (ASAP 2010, Micromeritics) using a point by point procedure of adsorptive introduction. The experiments were obtained at 77 K in a liquid nitrogen cryostat. The adsorptive was obtained from Air Liquide (Alphagaz, purity greater than 99.999 %). Around 4 g of sample were weighed into the adsorption cell, which was pretreated under vacuum of 10-2 mbar for several hours. No heat treatment was used to avoid any degradation of the sample. Table 1 : Sample designation and description. Sample ml0 mE1 mN2 mVT4
Description Sphinx, mortar bonding large rectangular limestone block and body slabs Sphinx, masonry attached to chest, north side Sphinx, mortar between small stonework against core body Kephren valley temple, mortar limestone roofing block of air shaft
The mercury porosimetry experiments were carried out using an Autopore 9220 apparatus (Micromeritics). Around 4 g of sample were placed into the penetrometer. The initial results were obtained on a low pressure port between 0.004 and 0.1 MPa. The high pressure curve was obtained up to 400 MPa before extrusion and reintrusion. A contact angle of 130 ~ was used to interpret the results using the Washburn equation. Thermoporometry [2] measurements were made using a DSC from Setaram (DSC92). Around 30 mg of sample were weighed into the crucible and a slight excess of water was added to the sample before being cooled to -60~ (213 K). The temperature program used consisted of heating from -60~ (213 K) to -0.5~ (272 K) and back down to -60~ (213 K) at a rate of 1 K.min ~. The cooling curve was used for subsequent calculations.
3. RESULTS AND DISCUSSION
3.1 Nitrogen adsorption The nitrogen isotherms obtained with the various samples are shown in Fig. 1. The corresponding pore size distributions calculated using the BJH method are shown in Fig. 2. The isotherms all indicate the presence of porosity although it is not evident to distinguish whether it is rigid or not. Whilst in the case of ml 0 and mVT4 an inflexion in the adsorption branch is visible, this is not the case for mE1 and mN2. In the case of these latter samples, the form of the hysteresis loop which closes at p/p~ = 0.42 would seem to confirm the nonrigidity of the porosity. The source of such non-rigidity could simply be the sheet-like nature of natural gypsum. One can question the use of adsorption manometry to study such samples. Firstly, one can question the pretreatment with respect to sample degradation. Whilst Hammond et al. [3] have shown that large gypsum aggregates retain their structure even after dehydration, Badens et
437 al. [4] show that, even at room temperature, it is possible to dehydroxylate the gypsum to form the hemihydrate under vacuum. Finally, Raga'f et al. [5] have shown that thermal
pretreatment leads to the formation of smaller pores. In the present case, pretreatment of the sample under vacuum for a short time is sufficient to dehydrate the sample of excess physisorbed water. However, heating the samples above 40~ (313 K) under the current vacuum conditions leads to dehydroxylation of structural water.
Figure 1 : Nitrogen isotherms at 77 K on samples m l 0, mE1, mN2 & mVT4.
Figure 2 : BJH treatment of the nitrogen isotherms shown in Fig. 1.
A second criticism that can be levelled at the use of nitrogen adsorption manometry is the measurement of porosity in the upper mesopore range. Indeed, accurate pressure measurements in the region close to the saturation pressure are difficult and can be influenced by small variations in temperature (e.g. due to a decrease in level of liquid nitrogen in the cryostat). In such cases, the use of a constant level cryostat can be useful. Errors made in this pressure region greatly affect the calculation of pore size as the BJH calculation introduces a log pressure term. Whilst the use of the Kelvin equation can be questioned in the case of smaller mesopores, this is not the case in the present case where, on the contrary, the pores are situated in the upper mesopore range. However, use of the BJH method implies the use of a t-curve. On commercial adsorption equipment, the software proposes the use of several equations to fit the t-curve. In the present case, the Harkins and Jura equation or Halsey equation is proposed. Unfortunately neither of these fit the original t-curve data of de Boer very well. Nevertheless, bearing in mind the above-mentioned reservations, it is possible to interpret the pore size distributions obtained using the BJH method with the Harkins and Jura t-curve (Fig. 2). Firstly, it would seem obvious that the peaks centred on 4 nm relate to the closing of the hysteresis at around p/pO = 0.42. One should ignore these, as they are artefacts due to the non-stability of the nitrogen meniscus under these conditions. However, the peaks observed in the case of m l 0 and mVT4, centered on 40 nm and 100 nm respectively, are significant. It is these peaks that can be confidently used for further comparison.
438
3.2 Mercury porosimetry The mercury porosimetry intrusion-extrusion data is shown in Fig. 3. Two types ofbehaviour can be observed. In the case of mN2 and mE1 the pores are filled at relatively low pressure and the extrusion curve is horizontal with negligible hysteresis, mVT4 and m l 0 show intrusion at higher pressures and hysteresis on extrusion - reintrusion corresponding to this high pressure region.
Figure 3 : Mercury porosimetry results on samples ml0, mE1, mN2 and mVT4.
Figure 4 : Treatment of the mercury intrusion data (1 st intrusion) from Fig. 3.
This data can be interpreted with respect to a previous study on Egyptian mortars [5]. An uptake in the 0.1 to 5 MPa region would seem to correspond to region 'I' mentioned in [5]. This study suggests that a filling of intragranular pores occurs in this region. These pores were described as large three-dimensional cavities, situated between gypsum granules, connected through narrow throats or channels. The uptake in the 5 to 500 MPa region corresponds to region 'II' described by Raga'f et al. [5]. This region would seem to correspond to the filling of intergranular pores. Raga'f et aL [5] suggest that these pores can be formed during thermal pretreatment. The present study suggests that either a certain quantity of these pores can be present in the untreated sample or that the vacuum treatment used during the mercury porosimetry measurements can be enough to form these pores. Mercury porosimetry experiments themselves can be criticised from two points of view. Firstly, the sample is pretreated under a primary vacuum of around 10-2 mbar. As mentioned above, care has to be taken to ensure that this is not already sufficient to degrade the sample, especially in the case of hydrates. Secondly, the pressures that are attained (400 MPa) can be sufficient, in some cases, to crush the samples. This explains the interest to perform intrusionextrusion-reintrusion curves. This procedure was used in the present study. The extrusion occurs until 0.1 MPa which was enough just to investigate the smaller pores. However, the second reintrusion curve follows the first intrusion curve in both cases (ml0, mVT4) indicating that destruction did not occur for this class of pore.
439
3.3 Thermoporometry
The pore size distributions obtained using thermoporometry are shown in Fig. 5. It can be seen that each of the samples studied show significant peaks. One should note however, that in the case of mVT4, the solidification of water inside the pores occurs at the upper limit of detection. In this region, very close to 0~ care should be taken in the experiment not to increase the temperature above this bulk freezing point temperature. Nevertheless, in the case of samples, mE1 and ml0, the peaks at 40 nm and 12 nm respectively are significant. A lower limit for thermoporometry can be estimated in the region as low as 3 nm.
E m o
0.8
>
._G] 0.6 (.-
i ~ 0.4
.__ "~ 0.2 w'
1
10
100
1000
Pore diameter/nm
Figure 5 9Thermoporometry results on samples mE1, ml 0and mVT4. Thermoporometry has the advantage of not requiring any sample pretreatment (either vacuum or heat treatment), thus one can postulate that no sample degradation occurs. However, the freeze-thaw cycle that the sample undergoes during the experiment itself can lead to a small amount of sample destruction. The other problem of using water as the probe molecule is chemical interaction with the sample. A rearrangement of the sample may occur in the present case where hydration of gypsum can lead to a reorganisation of the non-rigid structure. Although, this is unlikely during the experiment, one can imagine some variation if the sample is prepared well in advance. In the present study, the experiment was performed as soon as the sample was prepared. 3.4 Comparison of the three measurements on the individual mortar samples
Figures 6 to 9 show the comparison of the three measurements on the individual mortar samples. In the case of sample ml 0, a single peak is observed for the three techniques. This peak is centered on approximately the same value. However, one can note that thermoporometry gives a slightly higher value than the other techniques. Nevertheless, from the BJH analysis, one can be confident that this porosity is rigid. However, neither the adsorption nor the thermoporometry measurements permit the observation of the larger pore sizes, above around 300 nm.
440
The case of mE1 is somewhat more surprising, whilst mercury porosimetry and thermoporometry detect a peak just above 10 nm, the gas adsorption results show no indication of any porosity in this range. This may be due to a non-rigid structure that can not be detected using the former techniques. 1
1
E 0 >
0.8
i.. 0 EL 0.6
0.8
o
rt 0.6, ._~ t-
c-
0.4
.
BJH desorption i l--e- Hg porosimetry t ' - " Thermoporometry j
[-O-
E
_=
. 0.4 o_ 121
._ a
~--BJHdesorption + Hg porosimetry ---,Thermopormetnj
".~, 0.2 ~ n,"
o
i
1
> :~
9 10
9
0
100
1000
Pore Diameter / nm
~
10
w ,._ . . . . . . . . lOO
Pore Diameter / nm
Figure 6 9Comparison of the pore size distributions obtained with sample ml0.
E
0.2
,.1 lOOO
Figure 7 9Comparison of the pore size distributions obtained with sample mE 1.
BJH desorption I
BJH desorption -.i- Hg porosimetry
E
ll~
~\
"
~5
>=
~3
>=
0.2 0.
~ 0.2
1
lO
10o
Pore Diameter / nm
Figure 8 9Comparison of the pore size distributions obtained with sample mN2.
lOOO
0, 10
100
Pore Diameter / nm
1000
Figure 9 9Comparison of the pore size distributions obtained with sample mVT4.
The case of mN2 is once again interesting. It would seem that an overlap is obtained for the smaller pore size whereas adsorption manometry is not able to probe the larger pores. However, the smaller pores are at the limit at which the BJH method is valid. That is to say that from the adsorption data alone, one would discard the peak obtained at around 4 nm as an artefact. From the mercury intrusion data though, a peak is observed which would seem to confirm the presence of a meaningful porosity. Finally, the case of mVT4 again shows a good coincidence between the pore size distributions obtained between the three methods.
441 4. CONCLUSIONS Each of the three characterisation techniques explored in the present study can be criticised in terms of experimental procedure or mathematical interpretation of the raw data. Nevertheless, in the present study, a remarkable agreement is obtained between the results obtained with the three techniques. It is interesting to note that this is the case with such large pore samples of heterogeneous pore size distribution. These results are in good agreement with comparison studies carried out on other more ordered pore systems [6]. We are now left with our initial set of queries, in terms of different 6poques, different regions of manufacturing and different ageing conditions. We hope to address these questions in some following work to the present research. REFERENCES 1. A. Lucas, J. R. Harris, "Ancient Egyptian Materials and Industries", Edward Arnold Ltd., 1962. 2. M.L. Brun, A. Lallemand, J.-F. Quinson and E. Eyraud, Thermochimica Acta 21 (1977) 59. 3. W.A. Hammond, J. R. Withrow, Ind. Eng. Chem., 25 (1933) 1112. 4. E. Badens, P. Llewellyn, J. M. Fulconis, C. Jourdan, S. Veesler, R. Boistelle, F. Rouquerol, J. Sol. State Chem., 139 (1998) 37. 5. J. Raga'f, K. S. W. Sing, M. Yates, in "Characterization of Porous Solids II" (F. Rodriguez-Reinoso, J. Rouquerol, K. Unger and K. S. W. Sing Eds.), Elsevier, Amsterdam, 1991, p.693. 6. Non published results presented at SILICA 2000 and in the present conference by I. Beurroies and R. Denoyel.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
443
M o n i t o r i n g fast pressure changes in gas transport and sorption analysis G. Reichenauer a'b, H.- J. Fella a and J. Fricke a aphysikalisches Institut, Universit~it Wtirzburg, Am Hubland, 97074 Wtirzburg, Germany bcurrent affiliation : Bavarian Center for Applied Energy Research, Am Hubland, 97074 Wtirzburg, Germany, [email protected]
Due to the limited response time of suitable sensors fast sorption or gas transport processes on a time scale below a second are hard to monitor. To significantly improve the resolution in time an interferometric pressure sensor can be applied. The central part of the interferometric pressure sensor presented is a Michelson-interferometer; this set-up is sensitive to changes in gas pressure as the index of refraction, and thus the optical path length for a laser beam within the interferometer, is a function of the gas density. We have built a prototype of such a pressure sensor with a resolution in differential pressure of about 0.3 %. The resolution in time achieved was about 50 ms, limited by features of the manifold rather than the interferometric device itself. 1. I N T R O D U C T I O N Gas pressures in vacuum applications are usually either recorded via membrane transducers, systems that monitor the gas density via partial ionisation of the gas or sensors that make use of the fact that the thermal conductivity or diffusivity of a gas is pressure dependent. The first type of transducer is sensitive to the total gas pressure while the other methods yield gas dependent signals. In terms of application properties such as the response time of the sensor, the sensitivity and the pressure range that the sensor covers are important technical specifications. The response of a membrane pressure sensor to a step-like pressure change is essentially an exponential function characterized by a relaxation time r ; for a MKS transducer, type Baratron 220 [1], z- was determined to be 0.227 s (see Fig. 1), the actual pressure and the value as recorded by the transducer therefore do not match within the error bars given for the sensor until more than a second passed. Gas pressures can also be monitored via the analysis of the refractive index of the gas under investigation. This non-invasive method is a fast, gas sensitive technique that provides a high resolution, both, in time and pressure, combined with the option of its operation in vacuum, at ambient pressure or above. 2. T H E O R E T I C A L B A C K G R O U N D High resolution gas pressure measurements based on the evaluation of the refractive index can be performed with a Michelson-interferometer: The interferometer compares the phases of two beams that result from splitting the light of a coherent source. Using a beam splitter the two coherent beams travel different paths before
444 105 ,'c-
104
s loa t._
(n 102
Q" 101 100
_ _1
o
o'.5 . . . . .
....
1.5
2
t(s) Fig. 1. Response of membrane pressure sensor (Baratron 220, MKS, full pressure range : 1000 mbar, resolution : 0.15 % of full range) after a step-like increase in gas pressure at t=0. The full line represents a fit to the experimental curve (o) with an exponential relaxation function. The dashed lines indicate the error bars at equilibrium pressure. they are reflected into the same direction and finally interfere. As shown in Fig. 2 the interference of the beam that is crossing Lreo/2 twice and the reference beam in the interferometer yields an interference pattern consisting of concentrical rings. The intensity I at a fixed point (e.g. the point where an optical diode is positioned) of the pattern as a function of a change in gas pressure Ap is given by
E /
1~ 1 + cos I ( p ) = --~
-2~a~e~
+
r
.
(1)
Io is the intensity of the incident beam, ALopt is the change in the optical path length of the beam through the gas under investigating, /],Laser is the wavelength of the incident beam and ~o is a phase shift that can be adjusted e.g. through the position of mirror #2 in Fig.2. ALop t is related to the geometric path Lceo via
(2)
ALopt (Ap) = LGeoAn(Ap) ,
where An is the change in refractive index upon a change in gas pressure Ap. For ideal gases the refractive index for a given wavelength is related to the pressure via 1
p
(n - 1) oc -- = ~ , V v.R.T
therefore
An = Ap. TR (n R _ 1). T PR
(3)
Here 1/V is the density of the atoms or molecules in the gas, v is the number of atoms/molecules per mol, R is the gas constant and T the temperature, nR denotes the reference refractive index at temperature TR and gas pressure pR. For pressures and temperatures for which the ideal gas equation represents no longer a good approximation, Eq.
445
Fig. 2. Schematic representation of the Michelson-interferometer for gas pressure monitoring attached to a manifold (e.g. a volumetric sorption unit). (3) has to be modified by including higher terms of the virial representation. With Eqs. (2) and (3) relation (1) transforms into
'~
I(Ap) = -~- 1 + cos
.......... /~Laser " T ' p
il
+ q9 R
(4)
.
Examples of the gas specific refractive indexes nR are given for four different gases in Table 1. Since besides the polarizability of the molecules the gas density is the second factor affecting the index of refraction, the value for He is about an order of magnitude lower than for the other gases listed. 3. E X P E R I M E N T A L SET-UP
3.1. Design We have built a prototype of an interferometric pressure transducer that was designed to be attached to a non-commercial room temperature sorption instrument [3] equipped with a differential membrane sensor (MKS, Baratron 220CD, full range 10 mbar, pressure resolution: 0,15 % of full range value). An additional absolute pressure sensor (MKS, Baratron 220 CA) integrated into the set-up provides the average pressure in the manifold and Table 1 Relative refractive indizes for different gases at pR = 1013 mbar, TR= 273 K, Gas 103 (nR- 1)
He 0.036
N2 0.297
Ar
C02
0.281
0.451
2= 589 nm [2]
446 therewith the reference value for the differential transducers. According to Eq. (4) the highest resolution in pressure can be achieved in the linear part of the cosine (see Fig. 3). Restricting ourselves to + re/6 around the center of the linear range, the full range in differential pressure mpmax is given by the condition rc
2"--"-
2~r . L G e o 9Apmax 9T R (n R 2Laser " T . p R
6
with
C=
2~. TR(n R - 1)
1)
~
1
::~ Apmax = - - ~
(5)
3 Laeo " C
.
(6)
/]'Laser " T " P l~
To make sure that upon the pressure change to be recorded the intensity of the interference pattern on the diode stays within the linear range, the interference pattern can be shifted making use of the arbitrary phase in Eq. (4). In practice the phase shift is provided by changing the position of mirror #2 (Fig. 2) via a piezoelectric transducer. The only free parameter in Eq. (5) is LGeo , the geometrical length of the path that the light travels through the cell containing the gas under investigation. To cover a range in pressure change of about 1 mbar for gases like Ar and N2 (Table 1), Laeo has to be about 0.5 m. On the other hand, the related volume should be as compact as possible to minimize the dead space introduced by the sensor. Therefore the long distance needed was provided by multiple reflections of the beam off the reflectively coated walls of the volume connected to the manifold. That way the additional dead space, incl. the connecting pipes, was only 10cm 3, corresponding to an increase in volume of 10% of the sample cell. To suppress effects of changes in the environmental pressure on the optical paths that the two beams travel separately before they interfere they are fed through evacuated tubings. Changes in the interference pattern can also be induced by fluctuations in temperature resulting in variations of the laser wavelength, the geometrical distances (e.g. between mirrors) or a change of the temperature of the confined gas. The interferometric device is
Fig. 3. Intensity at the diode according to Eq. (4) vs. pressure change at room temperature for nitrogen and LGeo= 0,5 m.
447 therefore shielded from these effects by a thermostated chamber that also contains the sorption unit. A combination of active and passive elements were used to set the temperature for a given experiment to a value between 5 ~ to 40~ on one hand and to effectively damp all external thermal fluctuations on the other hand. With the design of the interferometric sensor we are restricting ourselves to a small part of one period in the interference pattern for the sake of a high resolution in pressure; at the same time we want to achieve a high resolution in time. Fast vibrations of the device resulting in fast and small, but possibly crucial changes of distances within the interferometer therefore have to be avoided. Building a set-up rigid is possible, but not an option for a prototype were the position of the mirrors have to be adjustable on site. The instrument was therefore placed onto a table resting on air cushions (by Spindler & Hoyer GmbH, Germany) that efficiently damp vibrations above 2 Hz. 3.2. Performance of the interferometic pressure sensor
The interferometric set-up was first calibrated at constant pressure by comparing the signal to the reading of the membrane differential pressure sensor. Applying Eq. (4) a geometrical distance LGeo = (0.575 + 0.003) m (Fig. 2) has been determined for N2, Ar and CO2. The value derived for He is with (0.715 + 0.003) m significantly off. Gas impurities in the order of 1 % or less would be sufficient to explain this effect, since the relative refractive index of He is at least an order of magnitude smaller than for most other gases and vapors. An analysis of the gas will clarify the situation in the near future. Analysis of the stability of the signal at constant pressure versus time reveals that the relative error in pressure for analysis periods t < 2 s is dominated by the inaccuracy of the calibration parameter LGeo (caused by thermal drifts during the long calibration runs) rather than vibrational contributions or thermal effects upon analysis itself. Fig. 4 shows the comparison of the signals detected via the Baratron differential transducer and the
Fig. 4. (a) Pressure change as a function of time as detected after a step-like change in gas pressure of about 5 mbar via the interferometric (o) and the membrane (x) pressure sensor (average gas pressure : 500 mbar, N2 ). (b) Interferometric versus membrane transducer (data taken from Fig. 4 (a)). The gray shaded regime (t > 1.5 s) indicates the range where the two signals coincide. The dashed lines are guides to the eye.
448 interferometric sensor aider a step like change in pressure of about 5 mbar. The pronounced relaxation in pressure (Fig. 4 (a)) is caused by the expansion of flexible connections in the manifold (to extract equilibrium data for sorption analysis this effect is taken into account via a volume calibration routine). Fig. 4 shows that the agreement between the two datasets is excellent for t > 1.5s while for t < 1.5s the response of the membrane sensor shows the expected damping behavior. The resolution in time of the interferometric device is demonstrated in Fig. 5 that shows the response of the optical sensor to a pressure step. The pressure step was induced by opening a dosing valve for 50 ms. This period is reflected in the interferometric signal by a sharp change with vibrations superimposed that are induced by the opening and closing of the valve, respectively. Note that the signal in Fig. 5 presents the raw data, i.e. the voltage at the diode. 4. A P P L I C A T I O N OF THE I N T E R F E R O M E T R I C P R E S S U R E S E N S O R
Every sorption set-up also allows for the extraction of information about sorption dynamics and gas transport provided that the resolution in time of the sensor applied is adjusted to the process under investigation. In an earlier publication we studied the transport of gases into the mesopores of monolithic materials by analyzing the pressure relaxation after the dosing of a defined amount of gas onto the sample [3]. An improvement of the resolution in time allows to investigate also small monoliths or samples with larger pores. Fig. 6 demonstrates the advantage of a fast pressure monitoring for a diffusion experiment on a silica aerogel with a porosity of 96 %, a mesopore size of about 60 nm and a volume of 2x3x4 cm 3. The fit of the analytical solution for the related diffusion problem [3] to the experimental data taken with the interferometric transducer yields a diffusion coefficient for nitrogen of (19.5 +1.5). 10-6 m2/s. |
1.5
i
|
i
i
i
w
~
!
,
,
,
I
!
m
i
I
,
~
~
i
1.0
0.5
6/V
-0.5
-1.0
I
0
,
,
,
!
,
0.1
0.2
0.3
t/s Fig. 5. Change of voltage measured at the diode as a function of time after changing the gas pressure by opening a dosing valve for about 50 ms. The oscillations within the first 30 ms and between about 60 and 120 ms are due to vibrations induced by opening and shutting the valve.
449
7.5 7"6LI "u"
~-........... -r~---,--~, . . . .
, ....
i
7.4
E 7.a
7.2I
0
/ membrane transducer 1
2
3
i 4
5
t(s) Fig. 6. Dynamic expansion experiment : Relaxation of the gas pressure, after a step like change of the pressure in the sample cell, due to molecular diffusion of the gas into the open porous monolithic material. 5. SUMMARY AND CONCLUSIONS A prototype of an interferometric pressure transducer has been built and tested with a room temperature sorption unit. The sensor proved to provide a resolution in time clearly below 50 ms; in practice the resolution is limited by features of the manifold (e.g. characteristic times of valves, gas transport through tubing thermal equilibration) to which the senor is connected rather than its intrinsic properties. The resolution in pressure of currently 0.01 mbar for He (at full range 6 mbar) and 0.002 mbar for Ar, N2 and CO2 (at a full range of 0.3 to 0.7 mbar) can be further increased by - improving the stability of the laser : highly stabilized laser diodes (AX/2) < 0.0003 are state of the art (e.g. by LASER2000) - increasing the geometrical distance Lgeo that the light is traveling through the gas to be analysed; special optical set-ups, e.g. with spherical mirrors, are available that provide up to several hundreds of meters compared to 0.5 m used in our prototype. The pressure range covered can drastically be extended by using a diode array instead of a single diode, so that the restriction to only part of the full cosine can be avoided. Alternatives to the presented interferometric configuration are Mach-Zehnder-type interferometers that are also available as integrated optical devices; the performance of such optical sensors will be tested for the above application in the near future.
REFERENCES
[ 1] MKS Instruments Inc., Andover, MA, USA. [2] CRC Handbook of Chemistry and Physics, CRC Press, 63th Edition. [3] C. Stumpf,K. von G~issler, G. Reichenauer, J. Fricke, Journal of Non-Cryst. Solids 145 (1992) 180.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
451
Effect of coke deposition on catalyst texture during catalytic cracking reaction. J.P. Reymond, C. Delattre and M. Forissier. LGPC ; ESCPE ; 43 Bd 11 Novembre, 69616 Villeurbanne cedex, France The active phase of industrial catalytic cracking catalysts is a Y-type zeolite. Deactivation occurs during reaction by deposition of large aromatic molecules (coke). Changes in porous texture of an industrial catalyst have been investigated for different coke quantities, using physisorption isotherms of argon (77K and 87K) and nitrogen (77K). Apparent microporous texture of the catalyst and its change with increasing coke content depend on both the adsorbate type and the sorption temperature.
1. Introduction 9 Fluidized catalytic cracking (FCC) is still the major refining operation to convert heavy petroleum compounds to lighter products The industrial catalyst, shaped as small spheres (70 pm), circulates in the process trough a fluidized bed reactor. It is composed of an active phase, Y-zeolite, embedded in a matrix [1]. Two ways of catalyst deactivation occur during the cracking reaction. First, the deposition of polyaromatic carbonaceous compounds (coke) produced during by-side reactions leads to a fast but reversible deactivation [2,3]. Burning coke can regenerate the initial activity. Then, an irreversible deactivation occurs due to the partial destruction of the zeolite cristallinity, induced by the drastic hydrothermal conditions existing during the regeneration, and to the harmful presence of metallic ions in the feed (Ni, V...). Study of the irreversible deactivation is beyond the scope of this paper. Coke can induce texture modifications, such as active surface covering, pore diameter reduction or pore closing. Typically, the analysis [4,5] of solid texture consists in measuring gas physisorption isotherms. Texture representative parameters (specific surface area, volume, diameter and diameter distribution of each pore c l a s s e s - micro, meso, macropores) can be extracted using models (B.E.T., B.J.H., t-plot, Horvath-Kawasoe, Dubinin .... [4]). This paper aims at locating coke inside the porous catalyst particles, using the texture modifications induced by coke deposition on an industrial catalyst. Isotherms using Nz and Ar are used in order to extract texture data for samples containing an increasing quantity of coke. Due to the presence of Y type zeolite, a large fraction of FCC catalyst volume is mainly microporous. Literature studies [6] emphasize that Nz is not suitable to determine microporous parameters characteristic and instead strongly recommend using Ar at its boiling temperature. However, N2 is a more commonly used and less expensive adsorbat than Ar. As this paper is devoted to studying texture evolution and not measuring exact surface area and porous volume, does argon have to be used ? In order to answer to this question, different adsorbates have been used at different temperatures and the results obtained from these experiments are compared.
2. Experimental Adsorption isotherms were recorded with a Micromeritics ASAP 2010 instrument. Experimental porous data (surface area, volume, pore diameter distribution) have been
452 obtained with the models and the related parameters provided by Micromeritics. N2 was used at 77.35 K (adsorption and desorption branches), while Ar was used at 77.35 K (adsorption branch only, up to P/P0 = 0.7) and 87.29 K (adsorption and desorption branches). Prior to any measurement, each sample was outgased under vacuum (~0.1Pa) during 24 hours, with a heating time of 2 hours at 623 K. Coke deposition was performed via cracking reactions of a real feedstock (gas-oil) operated in a fixed bed reactor which allows a wide range of experimental conditions [7] : catalyst mass from 0.5 to 10 g ; reaction temperature from 723 K to 873 K ; pressure from 1 to 4 bar ; injected feed mass between 0.4 and 4 g ; feed injection time from 10 to 300 s. This reactor induces a coke formation very similar in quantity and nature to that observed on industrial plant catalysts [7]. Coke combustion was performed at 1773 K under oxygen flow in a Leco CR12 carbon analyzer. The global carbon content was extracted from the total volume of carbon dioxide produced during combustion
3. Materials Two different catalyst samples were obtained from TotalFinaElf : a fresh carbon-free catalyst, (reference catalyst) and an equilibrium catalyst, which was working in an industrial FCC unit. So, the equilibrium catalyst is a mixture of regenerated catalyst particles, plus some spent catalyst particles and steamed fresh catalyst particles (the fresh catalyst is introduced in the regenerator of the industrial FCC unit). It contains a residual carbon rate : 0.276 wt%. Catalyst samples with various carbon contents (from 1.7 wt% to 4.2 wt%) were also prepared with the MAT reactor. Moreover, two reference microporous solids have been also tested : NaY and USY zeolites (from Zeocat).
4. Results and discussion 4.1 Physisorption isotherms The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/P0 values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. Table 1 gives the values of SBET (specific surface area), Smicro and Vmicro (surface area and volume of micropores, calculated from the t-plot model), VT, the total pore volume is measured at P/P0 = 0.97 with N2-77K and Ar-87K, and at P/P0 = 0.7 with Ar-77K (corresponding values are in italic) as well as the ratio Smi~ro/SazT and VmiCroNT ,for the two zeolites and the fresh catalyst with all sorption conditions. Table 1 : total and micropore areas and volumes of NaY and USY zeolites. Adsorbate SBET Smicro Smicro/SBE T VT Vmicro mZ/g m2/g cm3/g cm3/g Vmicro/Vtotal Temperature 644 580 0.901 0,27074 0,32578 0,831 Nz-77K 0,894 541 491 0.908 0,25060 0,2802 Ar-77K NaY 450 371 0.824 0,18813 0,26329 0,715 Ar-87K 0,577 564 430 0.762 0,23353 0,40443 Nz-77K 0,634 449 321 0.715 0,16178 0,25523 Ar-77K USY 0,416 520 336 0.670 0,16756 0,40262 Ar-87K 0,654 222 176 0,8 0,14960 0,09788 Na-77K 0,549 215 163 0,76 0 , 1 4 9 6 0 0,0822 Ar-77K Fresh catalyst 0,569 213 169 0,8 0,1412 0,08034 Ar-87K
453
Both the values in this table and the position of the inflexion point on high-resolution isotherms strongly depend of probe molecule and adsorption temperature. Values calculated from N2 isotherm are greater than values calculated from Ar isotherms. On one hand, the differences could be attributed to interactions between Nz molecule and the adsorbent (strong electrostatic fields exist in zeolites), leading to a preferential orientation of the N2 molecule, more perpendicular to adsorbent surface than expected, leading to higher calculated surface areas [10]. On another hand, the cross-section of Nz molecule (0.3 nm) is smaller than that of Ar atom (0.38 nm). Nz could penetrate in pores remaining non accessible to Ar, leading to larger adsorbed volumes for N2 and consequently higher surface areas. Due to the presence of the binder, the surface areas (SBET, Smicro) of the fresh catalyst are smaller than that of zeolites. It is possible to provide some estimate of the amount of zeolite in the catalyst, assuming that : the fresh catalyst contains USY zeolite, micropores are exclusively present in the zeolite (Szeol = Smicro and SBEX= Szeol + Sbinder), binder and zeolite have same density. From Ar isotherms the fresh catalyst would contains --50 wt% zeolite and --40 wt% from Nz isotherms., From similar calculations, binder surface area is near 90 mZ/g. Figure 1 shows the isotherms obtained for the equilibrium catalyst : adsorption-desorption branches and the hysteresis loops are similar for all isotherms (Ar and Nz ). These isotherms are close to type I (microporous solid) or type II (adsorbed volume increases at high P/P0.) 100
30
80
25
"~
60
E o ">"
40
20
Ar-87K Ar-77K
~ N2-77K
20
10
_
5 0
0.2
0.4
0.6
0.8
P/Po
Figure 1 9Ar (87K and 77K) and N2 (77K) isotherms of equilibrium catalyst.
1
0 1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
P/Po
Figure 2 9low pressure region of high resolution isotherms of equilibrium catalyst.
High-resolution isotherms of the equilibrium catalyst are represented in Figure 2. The observed sigmoid curves demonstrate the microporous character of the catalyst, but the relative pressure corresponding to the inflexion point depends on both the adsorbate and the adsorption temperature (Ar at 77 K or 87 K). Thus, mesopores and micropores are qualitatively evidenced from each physisorption isotherm but quantitative differences exist. The presence of hysteresis loop (H3 type, corresponding to slit pores) reveals the presence of mesopores. Typically, in a FCC catalyst, micropores are attributed to the zeolite fraction while mesopores are mainly related to the amorphous matrix. Upper part of Table 2 gives specific surface area (SBE~), surface areas of micropores (Smicro) and mesopores (Smeso), total pore volume (VT) and micropore volume (Vmicro) of fresh, equilibrium, 2.17 %C and 4.2 %C catalyst samples. Lower part of table 2 gives the same characteristics of equilibrium, calcined equilibrium, calcined coked 2.17 %C catalysts (calcination is operated at 873 K, during 24 hours, in air flow) as determined from N2-77 K or Ar-87 K isotherms. All values depend on the probe molecule and sorption temperature, which confirm the observations deduced from the isotherm shape. Comparison of fresh and equilibrium catalysts (lines 3 and 4) shows that irreversible deactivation strongly changes the porous texture of the catalyst, mainly the microporous
454 fraction (see Smicro and Vmicro values). Coke deposition induces a decrease of all measured surfaces and volumes. Calcination of coked samples leads to a reproducible state (lines 7 and 8), demonstrating the reversible effect of carbon deposition on the catalyst texture. Table 2 9texture characteristics as determined from N2-77 K and Ar-87 K isotherms. Nitrogen 77.35 K Catalyst Fresh Equilibrium (0.276% C) Coked cata. (2.17 % C) Coked cata (4.2 % C)
m 2 / g cm3/g cm3/g mZ/g 46 0.1496 0.0978 212.5
Smicro mZ/g 169
Smeso Vtotal Vmicro mZ/g cma/g cm3/g 43.4 0.14120 0.0803
2 5 . 2 0.1093 0.0350
78.2
48.5
29.7
0.0970 0.0241
57
20.6
0.1011 0.0264
64.5
37.5
26.9
0.0959 0.0186
62.7
41.3
21.4
0.0849 0.0191
54.8
30.5
24.3
0.0712 0.0151
89.6
64.4
2 5 . 2 0.1093 0.0350
78.2
48.5
29.7
0.0970 0.0241
94.2
68
24.3
0.1116 0.0369
88.3
65.2
23.1
0.1353 0.0298
94.4
67.3
27
0.1115 0.0365
90
59.7
3 0 . 4 0.1134 0.0296
Smicro
mZ/g 221
mZ/g 176
89.6
64.4
77
Equilibrium (0.276% C) Calcined eq. cata (o % c) Calcined coked cata (0 % C)
Argon 87.29 K SBET
SBET
Smeso
Vtotal
Vmicro
Figure 3 presents the variations of specific surface area (SBET) and micropore area (Smicro) as a function of carbon content for all the tested catalysts. Similarly, figure 4 presents the corresponding variations of total pore volume (VT) and of micropore volume (Vmicro). As evidenced by curves on figures 4 and 5, changes in texture characteristics are quasilinearly related to increasing carbon content. It also appears that calculated surface areas strongly depend on the physisorption conditions, while measured adsorbate volumes are less affected. This observation is in favor of erroneous assumptions on nitrogen molecule area for the calculations of specific surface area [10]. But, as mentioned above, nitrogen molecule could penetrate in smaller pores, increasing measured micropore volume and surface areas. 100 9N2,77K 9A r, 87K o A r, 77K
o~
~
80
O ..........
E
v
60
VT
0.08
E
m O >
(D O t~
't: :3 co
0.12
-9
O n
40
20 0
|
|
|
|
1
2
3
4
Carbon content (%)
Figure 3" SBET and Smicro of coked catalysts.
0.04
9N2 77K OAr 77K l i a r 87K
Vmicro ,~--__ . . . .
1
2
3
4
Carbon content (%)
Figure 4" Micropore and total pore volume (liquid) of coked catalysts.
On one hand, extrapolation of each line to 0% carbon content gives values very close to that of calcined catalysts. On the other hand, extrapolation to a null micropore or BET surface area gives an estimation of the maximal carbon content of the catalyst. From Ar-87 K experiments
455 Smicro and SBZTdrop to zero for a carbon content of 1 1 % of the catalyst weight. In the case of Nz experiments, these areas become null for respectively 19 % and 27 %. Once again, it has to be emphasized that the description of changes in catalyst texture due to coke deposition strongly depends on physisorption conditions. 4.2 M i c r o p o r e c h a r a c t e r i z a t i o n f r o m H o r v a t h - K a w a z o e plots : The zeolite fraction, the active phase of FCC catalysts, looses its activity through deactivation by coke deposition. Change in the zeolite texture or the microporous texture will give some insight on the deactivation phenomena. Changes in micropore diameter and volume have been studied from the curves calculated from the Horvath-Kawazoe model (denoted H-K) applied to slit pore geometry (hysteresis loop type H3). This quantitative model, developed for adsorption on carbons [11], is only a qualitative measure of pore diameter for others solids but it can be used to compare changes in micropore diameter for a given solid. Pore size distributions (PSD) of NaY and USY zeolites are presented on figure 5 and 6 respectively. 8 E ,= O
9 E
3 0.45
_ . _ N2,77K + Ar,77K ~Ar,87K
6
0.65
4
o
E
O
>
N2, 77K = Ar,77K :. At, 87K
~" 2.5 ,= 2
~ 0.68 ~1
IT
1.5
O >
2
O
......
9
|
|
,
0.5 0.6 0.7 micropore diameter (nm)
0.4
,.
0.4
0.3
-
0.5
=
'
0.6
t'
i
0.7
0.8
0.3 0.25 02
E 0.2 _=
E 0.15 _=
O >
O >
~
0
,-, 0.1 o E
|
-
Figure 6" Pore size distributions (H-K) of U S Y zeolite.
E =
'~
"--
-
0.4
micropore diameter (nm)
Figure 5" pore size distributions (H-K) of NaY zeolite.
E ~
0
0.8
t
0
J
)
~ ,
0.4
J
A
r ,
,
0.6 0.8 1.0 micropore diameter (nm)
Figure 7 9Cumulative pore volume plots for NaY zeolite
0.1
0
__,_At, 771< , 87K
.~_o 0.05 , 1.2
E
0
'
04
I
06
018
....i:o
,12
mieropore diameter (nm) Figure 8" cumulative pore volume plots (H-K) for USY zeolite.
Experiments using Ar at 77K and 87 K lead to similar PSD and maxima of pore mean diameter, but resuts obtained with N2 are very different, with smaller mean pore diameters (particularly for NaY) and a bimodal PSD for USY zeolite centered on 0.46 nm and 0.58 nm. Aperture of Y zeolite micropore is 0.7-0.8 nm (cristallographic data [1]) and diameter of internal cage is --1.2 nm. N2 leads to underestimate Y zeolite micropore diameter, while Ar
456
leads to a correct value.Bimodal PSD encountered with N2 for USY zeolite has been previously described attributed to the dealumination process [9]. H-K cumulative pore volume plots as a function of pore diameter for NaY and USY zeolites are presented on Figure 7 and 8 respectively : micropore volumes calculated from nitrogen isotherm are overestimated compared to values calculated from argon isotherms. Figures 9 to 14 present the PSD and cumulative pore volume plots (H-K) relative to several coked samples (from 1.76 to 4.2 wt%C) and the equilibrium catalyst as calculated with nitrogen (fig. 9 and 10), argon at 77 K (fig. 11 and 12) and argon at 87 K (fig. 13 and 14). As shown by figure 9 (Nz-77K), the PSD for both the equilibrium catalyst and the coked samples have two micropore populations, centered on 0.59 nm (DI(N2)) and 0.45 nm (Dz(N2)). With increasing coke content, the mean diameter of micropores corresponding to DI(N2) is not affected but the number of pores tends to decrease. The mean diameter of pores corresponding to Dz(Nz) seems to increase (from 0.45 nm to 0.51 nm) as carbon content increases.Curves of figure 10 show that increasing of carbon content leads to a decreasing of the sample micropore volume. Their relative position to each other suggest that the volume of larger micropores (diameter > 0.58 nm) is more affected than the volume of smaller ones (curves are merged for low micropore diameters).
I N , 77K J
0.4 E
r
i
(0.51)
0.3
X
IN , 77K ~
0.04
D. (0,59)
A
_
~tE 0.03 o v
E 0.2
D2
\
...... Eq. Cata
O >
E := o 0.02 ,- 0.01 ._o E
0
E X~
0.5 OJ6 017 micropore diameter (nm)
0.4
Figure 9 9pore size distribution (H-K method ) of coked catalysts with N2 at 77K.
E 0.5 d~ t-
_.,,_2.8%0 --0--3.5% C - x - 4.2 %C ..... Eq. Cata
E 0.4 .9.o E -, 0.3
O
i
~
/,~
/I 1
"5 2 .9 E
~ 2 (0 50) D
0.4
i
1.2
IAr, 77K I
0.03
0.02
i
I
|
,
0.5
0.6
0.7
0.8
micropore diameter (nm)
Figure 11 9pore size distribution (H-K method ) of coked catalysts with Ar at 77K.
"
/.~, /~9
0
i
1.0
a) E
0.2
0
i
0.8
Figure 10" cumulative pore volume plot (H-K method) of coked catalysts with N2 at 77K.
2 0.1
!
0.6
0.04
i /1/ !1/
O >
_ x _ 4.2 % C _7 Eq. Cata
micropore diameter (nm)
......... 1 Ar, 7rK i I D, (0.67)
0.6 ,.-.,
._ E
0.4
018
; 3.s%c
~'~ ,~x~~, J ;5
0
: 1.8 %C __ 2.6 %C
l#
O
o. 0.1
O
~'#,,'t,--
O
0
0.01
,~x
_~:_ 4.2 %C ..... Eq. Cata
XX"
0 0.4
06
1.8 % C 218 %C
08
micropore diameter (nm)
Figure 12" cumulative pore volume plot (H-K method) of coked catalysts with Ar at 77K.
Argon adsorption experiments at 77K (figure 11) shows that PSD for the equilibrium catalyst is monomodal, with a mean pore diameter centered on 0.67 n m (DI(Ar)). This value is higher
457 than that reported for nitrogen (0.59 nm). PSD for coked samples is slightly bimodal with appearance of a population of smaller micropores centered on --0.5 nm (DZ(Ar)). As the carbon content increases, both mean diameters (DI(Ar) and DZ(Ar)) remain constant and the micropore volume decreases (fig. 12) with a larger drop for pores larger than 0.67 nm compare to smaller ones. Same observations can be made from the results of argon adsorption experiments at 87 K (fig. 13 and 14), with for DI(Ar) = 0.67 nm and D2(Ar) = 0.47 nm. The calculated values of D1 (fig. 9, 11 and 13) match the corresponding values of micropore diameter for the USY zeolite (fig. 6). Ar sorption results (77 and 87 K) show that the micropores of diameter D2 do not exist neither on the U SY zeolite nor on the equilibrium catalyst but appear only for coked samples. In this case the presence of these small pores would be a consequence of the coke coating which would decrease the diameter of larger pores. On the contrary, nitrogen sorption indicates their presence on both the zeolite (fig. 6) and the equilibrium catalyst. So, existence of these small pores would be an intrinsic property of the solids, not linked to the coke coating. Whether the adsorption experiment is run with nitrogen or argon, micropore population centered on Dz and its changes are different. A general trend in the data shows that when carbon content increases the mean diameter of micropores remains constant, while the volume of micropores decreases. --,--E ~E
0.3
I Ar, 87KI
_._
' 1.8 %c
~2.8 %C . - o - - 3.5 %C - x - 4.2 %C ....... Eq. C a t a
0.2
E
I
IAr, 87K I
;~D1 (0,67) /1/
.-.
/ I~ / li //\ / ~l ~
0,03
E ._. o ~
0,02
o o ..~-
0,01
~l,~.~s,........
>
~t. o
F=
~' x-x/" xXJ /
E
0
0.4
, 0.5
016
, 0.7
_1i
,~~
I
'
018
micropore diameter (nm)
Figure 13 : pore size distribution (H-K method ) of coked catalysts with Ar-87K.
0,00
,
0,4
~. _x_ __
J~, ,~J',t
,
1,76% C 2,83% C 3,5%C 4,2% C Eq. C a t a .
0,6 0,8 10 micr0p0re diameter (nm)
|
1
Figure 14 : cumulative pore volume plot (H-K method) of coked catalysts with Ar-87K.
The modification of pore size distribution and the decrease of pore volume could be explained by two simultaneous phenomena described previously [12] : Coke deposition on the walls of larger micropores (diameter > D1), appearing predominant at the beginning of cracking reaction, e.g. for the lowest coke content. As a result, smaller micropores centered on Dz are created (argon experiments). - Coke deposition progressively closing the entrance of micropores centered on D], in such a way that D1, representing the diameter of open pores, remains constant. The decrease of pore volume is then induced by the diminution of the number of pores accessible to adsorbate. As carbon content increases, the volume of large micropores decreases because coke coating of largest pores can occur, leading to the formation of the 0.51 nm pores appearing on figure 10. Nitrogen molecule, due to its lower cross-section, could be able to penetrate in these small pores, detecting changes of pore diameter while argon cannot do it. Thus, nitrogen could give a better description of changes in the population of the small micropores than argon do [13]. However, it has been seen that N2 underestimate the zeolite micropore diameter (fig. 6 and 7) and overestimate micropore surface area and volume. Only qualitative changes in micropore characteristics can be detected when Nz is used as the probe molecule. -
458
5. Conclusions The cracking of complex hydrocarbon feedstocks in presence of an industrial catalyst leads to the deposition of carbonaceous compounds (coke) on the catalyst, inducing a loss in activity of catalytic sites. As a general observation, increase of coke content leads to a decrease in micropore area, BET area, micropore and mesopore volumes, and the micropore diameter distribution. The mean diameter of zeolitic micropores (0.67 nm) is slightly modified. Mesoporosity of the solids studied in this work is satisfactorily described from Nz or Ar (87 K) isotherms. Description of the microporosity and its changes is more complicated. We have studied the micropore texture modifications due to the coke deposition using the HorvathKawazoe model (slit pore geometry). From a qualitative point of view, N2 and Ar isotherms describe similarly a great part of changes observed during catalyst coking. However, all micropore surface and volume values depend strongly on the adsorbate nature. N2 physisorption leads to overestimate micropore surface and volume and underestimate micropore diameters. These overestimations can be explained by the differences of molecular cross-section between argon and nitrogen , or by interactions between Nz and the solid, inducing a more perpendicular orientation of the Nz molecule towards the surface. Due to a smaller cross-section N2 molecule could penetrate in smaller pores than Ar, leading to a better description of the micropore populations. But H-K diameters of the zeolitic micropores calculated with N2 as probe molecule are very different of the real diameters of Y zeolite micropores. As H-K diameters calculated with Ar are very similar to real zeolitic diameters, and as Ar-adsorbent interactions are very weak, Ar (at 87 K) must be preferred to Nz in order to evaluate micropore texture. Understanding of coke deposition process can be improved from Ar experiments (77 K and 87 K) : at low carbon content coke deposition occurs predominantly on micropore walls leading to the formation of smaller micropores. Then, coke progressively plugs the micropore entrances in such a way that the number of pore decreases while the diameter of open micropores remains constant.
Acknowledgements: The authors want to thank TotalFinaElf for providing financial support.
References: [1] P.B. Venuto and E.T. Habib Jr, Catal. Rev. Sc. Eng., 40 (1975) 1. [2] E.E. Wolf and A. Alfani, Catal. Rev. Sc. Eng., 24 (1982) 329. [3] T.C. Ho, Ind. Eng. Chem. Res., 31 (1991) 2281. [4] S.J. Gregg, K.S.W. Sing in "Adsorption, Surface Area and Porosity" (Ac. Press, London, 1982). [5] A.J. Lecloux in "Catalysis, Science and Technology" (J.R. Anderson, M. Boudart eds, 1981), 171. [6] S. Storck, H. Bretinguer and W.F. Maier, Applied Catal. A: General 174 (1998) 137. [7] C. Delattre, M. Forissier, I. Pitault, D. Schweich, J.R. Bernard, Chem. Eng. Sci. 56 (2001) 1337. [8] G de la Puente and U.A. Sedran, Microporous Materials 12 (1997) 251. [9] J. Lynch, F. Raatz and C. Delalande, in "Characterization of Porous Solids" (K.K. Unger and al. Eds, Elsevier, Amsterdam, 1988) 547. [10] P.L. Llewellyn, C; Sauerland, C. Martin, Y. Grillet, J.P. Coulomb, F. Rouquerol, J. Rouquerol, in "Characterization of Porous Solids IV" (B. Mc Enaney, and al. Eds., R.S.C, Cambridge, 1997) 111. [11] G. Horvath and K. Kawazoe, J. Chem. Eng. Japan, 16 (1983), 470. [12] M. Guisnet and P. Magnoux, Applied Catal. 54 (1989), 1-27. [13] S.W. Weber, W.C. Conner, in "Characterization of Porous Solids II" (F. Rodriguez-Reinoso and al., Elsevier, Amsterdam, 1991) 31.
Studies in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. MeEnaney, J. Rouquerol and K.K. Unger (Editors) ' 9 2002 Elsevier Smence B.V. All nghts reserved
459
Precision of porosity measurements on cementitious mortars K. Rtibner a , Th. Fritz a and F. Jacobs b aFederal Institute for Materials Research and Testing (BAM), D-1200 Berlin, Germany bTechnical research and consulting on cement and concrete (TFB), Lindenstrage 10, CH-5103 Wildegg, Switzerland
1. INTRODUCTION Mercury porosimetry has been a well-established method to study the porosity and the pore size distribution of cement pastes, mortars and concretes for many years [1-6]. It provides information about a wide range of pores that are intimately associated with engineering properties of building materials such as strength, permeability and durability. However, the measurements are strongly influenced by test parameters, like sampling, preparation techniques and drying methods, because of the heterogeneity of the cementitious material and the delicate colloidal structure of their cement paste matrix. A correct analysis requires a knowledge of the effect of test parameters and the accuracy of the measuring method. A key issue is whether differences in the results of measurements on various samples are caused by the difference between the samples or by the inaccuracy of the measuring method. The heterogeneity of the material, the discrepancy between test conditions and/or the dissimilarity between laboratories produce further problems. To find an answer, the uncertainty of a measuring method has to be determined by repeat measurements on identical test items. The use of a uniform test material that is produced artificially under controlled conditions is necessary to investigate the precision. Furthermore, the test material has to represent its material class with reference to the precision of the method. The precision of the measuring method mercury porosimetry on the material class cementitious mortar was investigated by an interlaboratory study, which was organized by BAM and TFB from 1997 to 1999 [6]. The interlaboratory tests were based on ISO 5725 [7]. Six different samples of a Portland cement mortar were analysed. The repeat and the reproduce standard deviations of the numerical characteristics total pore volume at 200 MPa, median pore radius and bulk density were determined. Furthermore, the complete pressure/volume curve became the focus of analysis by calculating laboratory average curves and total average curves with uncertainty intervals. The results reported here are discussed with regard to the influence of drying technique and sample geometry.
460 2. E X P E R I M E N T A L
2.1 Laboratories 30 institutes and companies from Germany, Switzerland, Austria, Italy and Slovakia took part in the interlaboratory test, involving 32 testing laboratories that used 13 measuring instruments. 2.2 Samples The test material was a cementitious mortar made of a Portland cement CEM 142.5 and sand with maximum grain size of 4 mm. The blend ratio was 1 : 3 cement to sand by mass. The water/cement ratio was 0.44. Five 140 x 140 x 560 mortar prisms were produced. After demolding they were stored under water for 90 days. The mortar is characterized by a compressive strength of 61.9 N mm 2, a bending strength of 8.83 N mm -2 and a bulk density of 2.37 g cm -3 (determined according to DIN-EN 196-1 [8] at 28 days). Six different samples were prepared from the 90 day old mortar. Two groups of samples were produced: ~ 1 0 mm x 25 mm cylinders by drilling and 3-8 mm granules by crushing and sieving. Each sample group was pre-treated by three methods: 105 ~ drying, vacuum drying at 5 kPa, 20 ~ and wet storing until drying by the participating laboratories. The characteristics of the samples are given in Table 1. Table 1. Samples of the interlaboratory test Sample B1 G1 B2 G2 B3 G3
Bulk density
Density
(g cm-3)
(gcm -3)
2.202 similarto B 1 2.330 similarto B2 2.352 similar to B3
2.539 2.554 2.421 2.541 ---
Material cylinders, 105 ~ drying granules, 105 ~ drying cylinders, vacuum drying granules, vacuum drying cylinders, wet material granules, wet material
Cement content Residual moisture content (M.-%) (M.-%) 20 18 20 18 20 18
0.89 1.10 6.00 3.60 ---
2.3 Test design Each testing laboratory performed four measurements of each sample B 1, G1, B2, G2, B3 and G3. The mercury intrusion measurements were carried out according to DIN 66133 [9]. Additionally, the bulk density was determined pycnometrically by mercury immediately after the low-pressure measurement. The experiments were performed according to routine procedures of the participating laboratories and instructions of the test coordinator. 2.4 Statistical evaluation The data delivered by the laboratories were sample mass, bulk density and pressure/volume values (p/V curve) within minimal range from 0.01 to 200 MPa. The evaluation was performed in the following steps: - critical examination of the data in order to identify anomalies and irregularities;
461 -
determination of the total pore volume at p = 200 MPa by extra- or interpolation of the pressure/volume data for every single measurement; - calculation of pore size distribution (pore radius/volume curve) according to DIN 66133 [9] (8 = 140 ~ cr = 0,48 N m -1) and the median pore radius (as the radius, at which the pore volume reaches 50 % of the total volume); discarding of invalid values due to erroneous computation or obviously irregular application of the test method; - statistical evaluation according to ISO 5725 [7] for the numerical characteristics total pore volume, median pore radius and bulk density; statistical evaluation of the pressure/volume curves. According to ISO 5725 [7] the total mean X, the repeat standard deviation Sr, the between laboratory standard deviation So and the reproduce standard deviation SR were determined. The relation is given by -
-
The repeat standard deviation describes the scattering of the measuring results under repeat conditions (same laboratory, same equipment, same staff). Whereas, the between laboratory standard deviation expresses the differences between the laboratories. The reproduce standard deviation contains the two above mentioned scatter components. It is the deviation under reproduce conditions (different laboratories, different equipment, different staff). To get a unique repeat standard deviation it must be assumed that it does not vary (significantly) with the laboratory. For this reason the standard recommends a statistical outlier test (Cochran test) for the individual standard deviations of the laboratories. Furthermore, the individual laboratory means are a subject to an outlier test (Grubbs test). ISO 5725 [7] relies on a statistical analysis of variance model with two variance components "laboratory" and "repetition". Hence, a homogeneous material is assumed. Only under this homogeneity condition the calculated precision values are true method characteristics. For a heterogeneous material, like the tested cementitious mortar, the precision values are contaminated by the variance component of the material. Therefore, the precision values represent both material and method characteristics. To analyse the complete pressure/volume curve simple statistical methods were applied: - The p/V curves of every single measurement are linear interpolated and then evaluated on a fixed lattice of points at the logarithmic pressure scale. - For each laboratory and for each sample B1, G1, B2, G2, B3, G3 the average pressure/volume curve is calculated. Then the total average curve (non-weighted) of these laboratory averages and their standard deviation (uncertainty interval) are calculated for each sample. -
3. RESULTS AND DISCUSSION The results of the statistical analyses according to ISO 5725 [7] are presented in Tables 2, 3 and 4. Fig. 1, 2 and 3 show the total average curves with uncertainty interval of some particular samples. Full results are reported and discussed in [6].
462 Table 2. Results of statistical evaluation for the total pore volume
Sample
Number Number oflaboratories i
of values
Mean
Repeat standard deviation
X
Sr
mm 3 g-1 mm 3 g-1
Between laboratory standard deviation
Reproduce standard deviation
Sb
%
mm 3 g-I
SR %
mm 3 g-i
%
B1
24
102
59.34
4.131
6.96
1.659
2.80
4.451
7.50
B2
26
108
35.19
4.025
11.44
7.324
20.82
8.357
23.75
B3
27
111
53.74
3.909
7.28
4.205
7.83
5.741
10.68
G1
22
92
48.14
2.622
5.45
3.035
6.31
4.011
8.33
G2
25
101
37.79
2.468
6.53
3.735
9.88
4.477
11.85
G3
26
106
45.10
2.771
6.15
5.044
11.18
5.755
12.76
Table 3. Results of statistical evaluation for the median pore radius Mean Number
Sample
oflaboratories i
B1
Number of values
Repeat standard deviation
X
Between laboratory standard deviation
Sr
Reproduce standard deviation
Sb
SR
nm
nm
%
nm
%
nm
%
41.8
3.13
7.5
1.46
3.5
3.45
8.3
25
104
B2
24
100
17.1
2.56
15.0
2.21
12.9
3.38
19.8
B3
26
107
31.2
1.82
5.9
8.37
26.8
8.57
27.5
G1
22
92
37.3
1.60
4.3
2.30
6.2
2.80
7.5
G2
24
98
24.7
1.43
5.8
3.10
12.6
3.41
13.8
G3
22
89
31.2
0.80
2.6
6.70
21.5
6.74
21.6
Table 4. Results of statistical evaluation for the bulk density Mean Number
Sample
of laboratories i
Number of values
Repeat standard deviation
X
Between laboratory standard deviation
Sr
Reproduce standard deviation
Sb
SR
g c m -3
g c m -3
%
g c m "3
%
g c m "3
%
1.31 1.10
0.014 0.016
0.62 0.70
0.032 0.030
1.45 1.31
B1 B2
24 23
103 99
2.19 2.32
0.029 0.026
B3
24
103
2.20
0.027
1.21
0.011
0.49
0.029
1.31
G1
24
103
2.27
0.020
0.88
0.024
1.08
0.031
1.39
G2
24
101
2.31
0.019
0.84
0.038
1.64
0.042
1.84
G3
26
108
2.26
0.025
1.10
0.028
1.25
0.038
1.67
The mercury intrusion measurements were performed with a good precision within each individual laboratory (repeat standard deviation) as well as in different laboratories (reproduce standard deviation) for all mortar samples B1, G1, B2, G2, B3, G3. The reproduce standard deviations are 1.5 to 2 times higher than the repeat standard deviations. In particular cases,
463
like the total pore volume of B 1 e.g., they have almost the same value due to a marginal between laboratory standard deviation. This factor differs a little from findings for other measuring methods [ 10, 11 ]. Obviously, the participating laboratories are able to apply the measuring method correctly. But the precision values are affected by the inhomogeneity of the test material. It is remarkable that the pressure/volume curves of all samples have similar shapes regardless of sample preparation methods. They can be called a fingerprint of the porous material tested here, i.e. of the cementitious mortar with this composition. The techniques of sample preparation and sample drying influence the pore size distribution, the total pore volume, the median and the bulk density as well as the uncertainty of the determined characteristics.
3.1 Sample drying The preparation methods of 105 ~ drying and vacuum pre-treatment yield samples with different residual moisture contents (see Table 1). The pressure/volume curves of 'dry' (B1; G1) and 'wet' (B2; G2) samples are compared in Fig. 1 and 2. Compared with B2; G2 the 'dry' samples B1; G1 show p/V curves with a greater slope. Their pore size distribution is shifted to smaller pressures (i.e. greater pore radii). They have the highest values for the total pore volume and the median pore radius. First of all, the residual water in the pores of B2; G2 is responsible for these effects. The adsorbed water reduces the pore size and their whole volume as well. Additionally, irreversible changes of the microstructure of B1; G1 due to the heat treatment, such as the breakdown of pore walls, the collapse of small pores or the degradation of hydrate phases, have to be associated with the findings [5, 6]. The influence of the contact angle should also be considered, as it is not constant as assumed in the calculations [ 1, 12]. The repeat and the reproduce standard deviations of the total pore volume and the median pore radius are only slightly affected by different moisture contents of the samples (Tab100 000 . . . . .
70
B1
r Inm] 1 000
10 000 i. . . . . . . .
i . . . . . . . .
100
I. . . . . . . .
10
I. . . . . . . .
.....
1
i . . . . . . . .
I-"
P = 200 MPa
B2
4O: V [m~/gl 3O
10 0
0.001
.
.
.
0.01
.
.
.
.
.
0.1
.
~ ,~
1 p [aPa]
.
.
.
.
.
.
.
.
10
.
.
.
.
.
.
.
.
.
.
.
100
.
.
.
.
1 000
Fig. 1. Total average curves with uncertainty interval of cylindrical samples dried by various methods (B 1 - 105 ~ drying, B2 - vacuum pre-treatment)
464 100 000 ':"~
"
9
"
i . . . . . .
70
G1 .....
60
G2 ~
10 000 '
"
9
'1""
. . . . . .
rInml 1 000 I . . . . . . . .
100 I .....
10 " .
.
.
.
1
.i . . . . . .
"
P = 200 MPa
i "4
I
i
50
o..-
l
40 V [mrn3/g]
30 20 10 (]
-
0.001
__
. . . . . .
0.01
"0.1
1 p [MPa]
10
100
1 000
Fig 2. Total average curves with uncertainty interval of granular samples dried by various methods (G1 - 105 ~ drying, G2 - vacuum pre-treatment) les 2 and 3). B2 is an exception with a very high reproduce standard deviation of 8 mm 3 g-1. Obviously, the high moisture content of 6 % and the relatively large cylindrical bodies together cause specific measuring problems. So the testing laboratories reported difficulties with outgassing of B2. The results of B3 and G3 dried by the laboratories by a method of their own choice happen to fall in between that of the other two samples. They were averaged from samples, which were dried by 13 different procedures. As expected, the reproduce standard deviations are relatively high. But it is remarkable that they lie below the value of the 'wet' sample B2. Every laboratory treated B3 and G3 by their own method very well so that proper dried samples with low moisture contents were produced. Thus measuring samples of almost similar state could be measured relatively precisely.
3.2 Sample geometry The preparation techniques drilling and crushing yield samples with different geometry. Fig. 3 shows the p/V curves of the cylindrical sample B1 and the granular sample G1. The volume increase of the B1 cylinders is very small up to a pressure of about 1 MPa (corresponding pore radius 735 nm). Then, at this threshold pressure, the pore volume goes steadily up and at 10 MPa (74 nm) the slope becomes steeper. In contrast, the pore volume of the G1 granules increases steadily direct from the beginning of the measurement. A sharp bend at 10 MPa leads to the second part of the p/V curve. The increase of the pore volume slows down. And in the end the granules have a smaller total pore volume than the cylinders. These findings can be explained by the different geometrical forms of the sample bodies themselves as well as by differences of the drilling and crushing techniques. Thus, the granules consist of fine-grained particles with a rough surface, which can simulate additional large pores and interparticle spaces. Pore walls, blockades and narrow pore entrances can be broken due to the crushing. This leads to a greater amount of large pores. Possibly, they lie outside of
465 rInml I00 000 .....
70
!. . . . . . . .
B1
.....
G1
m
I0 000 i........
1 000 i.......
I00 '.'
10
'l ........
i ........
p = 200
1 t-.
MPa
50
40
V [mm3/g] 30
20
10
0
0.001
0.01
0.1
1 p [MPa]
10
100
.... i'b00
Fig. 3. Total average curves with uncertainty interval of cylindrical sample B 1 prepared by drilling and granular sample G1 yielded by crushing and sieving (105 ~ drying) the measuring range too. Additionaly, the granular material is richer in unporous aggregates than the cylinders (see section 2.2). However, the cylindrical samples can contain pore entrances that are blocked or partially reduced by fine abrasion particles of the drilling. The large cylindrical bodies can act as a barrier against the intrusion of mercury at low pressures. The same, but poorer effects are found for the samples B3 and G3. The 'wet' samples B2 and G2 do not show significant differences because the influences of sample geometry are interfered by other irregularities of the measurement. The repeat standard deviations of total pore volume show the tendency to be better (lower) for granules than for cylinders (Table 2). Therefore the inhomogeneity of the material supplies an explanation. So a measuring sample of B-group consists of single cylinders, i.e. a defined part of the sample volume. But a measuring sample of the granules G is produced by dividing of whole sample volume, which partly equalize the material inhomogeneities. The reproduce standard deviations of all samples, except B2 (see section 3.1), have almost the same value independently of sample geometry. The median pore radius does not show clear correlations to sample geometry (Table 3). The repeat standard deviations of the granules are a little smaller than that for the cylinders. 4. CONCLUSIONS The precision of the mercury porosimetry method on the material class cementitious mortar was investigated by an interlaboratory study of six different samples. The results are summarized in the following points. 9 Standard deviations and uncertainty intervals indicate a good precision of mercury intrusion measurements on cementitious mortars. The reproduce standard deviations are 1.5 to 2 times higher than the repeat standard deviations because the precision values are contaminated by the variance component of the heterogeneous material.
466 9 All results are significantly influenced by the techniques of preparation (sample geometry) and drying (moisture content). 9 The mercury intrusion measurements show the smallest deviations for samples with low residual moisture content (< 1%), granules and measuring samples with masses above 5 g. 9 For comparisons between several laboratories it is recommended to use samples of same geometry, i.e. same preparation technique, that are well dried or pre-treated by the same method. 9 The precision values are both method and material characteristics. They are valid for the tested material class, i.e. a cementitious mortar with this composition. 9 Based upon the determined precision values, laboratories can estimate the uncertainty of their single measurements on mortars of a comparable composition. According to [ 13] the uncertainty of a result S can be calculated from 1 2 S 2 -- S 2 q- ~ S r
(2)
m
if a mean X of k single measurements is determined. Sb is the between laboratory standard deviation and Sr is the repeat standard deviation of the measuring method. REFERENCES
[1] D.N. Winslow and S. Diamond, J. of Materials, JMLSA, 5 (1970) 564 [2] K. Hinrichsmeyer, S. Abdul-Maula, U. Diederichs and F.S. Rostfisy, Quecksilberporosimetrie- Ringversuche an erh~irtetem Zementstein, DFG-Bericht des Instituts for Baustoffe, Massivbau und Brandschutz der Technischen Universit~it Braunschweig, 1988 [3] L. Zhang and F.P. Glasser, Adv. in Cement Research 12 (2000) 79 [4] S. Diamond, Cem. Con. Res. 30 (2000) 1517 [5] C. Gallr, Cem. Con. Res. 31 (2001) 1467 [6] K. Rfibner, Th. Fritz and F. Jacobs, Ringversuch zur Quecksilberporosimetrie an Zementm6rtel, Forschungsbericht 250 der Bundesanstalt f'tir Materialforschung und -prtifung (BAM) Berlin, Wirtschaftsverlag NW, Verlag f'fir neue Wissenschaft GmbH, Bremerhaven, 2001 [7] ISO 5725, Accuracy (trueness and precision) of measurement methods and results, December 1994 [8] DIN EN 196-1, Prtifverfahren f'tir Zement- Teil 1: Bestimmung der Festigkeit, Mai 1995 [9] DIN 66133, Bestimmung der Porenvolumenverteilung und der spezifischen Oberfl~iche von Feststoffen durch Quecksilberintrusion, Juni 1993 [10] B. R6hl-Kuhn, K. Meyer, P. Klobes and T. Fritz, Fresenius J Anal Chem 360 (1998) 393 [ 11 ] W. Unger, Th. Gross, O. B6se, Th. Fritz and U. Gelius, Nachrichten aus der Chemie 48 (2000) 1108 [12] J. Adolphs and M.J. Setzer, GDCh-Monographie Bauchemie yon der Forschung bis zur Praxis, Bd. 11, Gesellschaft Deutscher Chemiker, Frankfurt, 1998, pp. 178-180 [ 13] ISO/IEC Guide 43-1, Proficiency testing by interlaboratory comparisons, part 1, 1999
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
467
Freezing in Mesopores" Aniline in Silica Glasses and M C M - 4 1 M.Sliwinska-Bartkowiak ~, G. Dudziak a, R.Radhakrishnan b, and K.E.Gubbins e a Institute of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland bMassachusetts Institute of Technology, Chemical Engineering Dept., Cambridge, MA 02139-4307 c North Carolina State University, 113 Riddick Labs, Raleigh, NC 27695, USA
1. ABSTRACT We report a study of the freezing of aniline in silica porous materials, using dielectric relaxation spectroscopy and light transmission measurements. The porous materials include controlled pore glasses with pore sizes H in the range 7.5 to 50 nm, Vycor glass (H=4.1 nm) and MCM-41 (H=2.8 nm). The freezing temperature is lowered due to the confinement, and in the larger pores crystallization occurs. In the MCM-41 material no crystallization is observed; instead a glassy phase is formed at low temperatures. In Vycor the experiments indicate a mixture of microscopic domains of crystal and glass at low temperature. The results are consistent with recent molecular simulation results. 2. INTRODUCTION Both experimental and molecular simulation studies of freezing and melting in porous materials suffer from several difficulties [1]. Difficulties in the experiments include the lack of well-characterized porous materials, difficulty in identifying the nature of the adsorbed phases, and long-lived metastable states. In the simulations, large systems and finite size scaling analysis may be needed to feel confidence in the results, and models of the porous materials may be over-simplified. The somewhat complementary nature of the difficulties in experiment and simulation make it profitable to use both approaches in a combined study. We have therefore adopted such a strategy in our recent work in this area [2-7]. For simple adsorbates composed of spherical molecules in pores of simple geometry, it is possible to map out freezing phase diagrams based on the simulation studies [3,6]. A corresponding states analysis [3] of the partition function for such a system shows that the freezing temperature in the pore, Tfpore, relative to the value for the bulk material, Tfbulk, is a function of three variables: H/try, a, and af~/trff, where H is pore width, tr is the molecular diameter, subscripts f and w refer to fluid and wall, respectively,
468 and a is a dimensionless ratio of the fluid-wall to the fluid-fluid attractive interaction when the molecular centers are at the minimum of the potential well. In practice it is found that the freezing and melting behavior is insensitive to the diameter ratio, trf,c"trff, for small molecules except when the pore width approaches the molecular diameter (molecular sieving regime). The qualitative behavior (whether the freezing temperature goes up or down, types of new phases occurring, etc.) is found to be controlled to a large extent by a, while H/aft determines the magnitude of shills in transition temperatures. The global freezing behavior predicted in the simulations is in qualitative agreement with the experimental measurements that have been reported [6]. In this paper we report new measurements of freezing and melting behavior for aniline in controlled pore silica glasses (CPG), Vycor and MCM-41 having a range of pore widths from 2.8 to 50 nm. The results are discussed in terms of the global freezing behavior predicted in molecular simulations. 3. METHODS
Freezing of a dipolar liquid is accompanied by a rapid decrease in its electric permittivity [8-10]. Following solidification, dipole rotation ceases and the electric permittivity is almost equal to n2, where n is refractive index, as it arises from deformation polarisation only. Investigation of the dynamics of a confined liquid is possible from the frequency dependences of dielectric properties, which allows both the determination of the phase transition temperature of the adsorbed substance and characteristic relaxation frequencies related to molecular motion in particular phases. The temperature of the phase transition to the solid state has also been determined recently by the light transmission method, recording changes in intensity of the light beam passing through the medium studied. At the melting/freezing transition, the number of absorption centers (nuclei of the new phase) changes discontinuously, leading to a sudden change in intensity of the light beam. 3.1. Dielectric relaxation spectroscopy The complex electric permittivity, • = ~:' + i~:", where K' = C/C 0 is the real, and K" - tan(6) / •'is the complex part of the permittivity, was measured in the frequency interval 300 Hz - 1 MHz at different temperatures by a Solartron 1200 impedance gain analyser, using a parallel plate capacitor made of stainless steel. From the capacitance, C, and the tangent loss, tan(6), the values of K' and ~" were calculated [2]. The temperature was controlled within 0.1K using a platinum resistor Pt(100) as a sensor and a K30 Modinegen external cryostat coupled with a N-180 ultra-cryostat. The aniline sample was twice distilled under reduced pressure and dried over A1203. The conductivity of purified aniline was on the order of 10.9 ~1m1. The porous silica samples used were the commercially available Controlled Pore Glass (CPG) from CPG Inc., with a pore size distribution of about 5% around the mean pore diameter. Different CPG samples having average pore diameters ranging from 50 nm to 7.5 nm were used. We also studied confinement effects in Vycor glass from Coming Inc., having a mean pore size of 4,1 nm, and a silica - based MCM-41 material with mean pore diameter of 2.8 nm. The pore samples were heated to about 600 K, and kept under vacuum (-~10.3 Tr) for 6 days prior to the introduction of the fluid. The MCM-41 samples
469
were synthesized at A. Mickiewicz University, and were characterized using x-ray diffraction and nitrogen adsorption measurements [11]. The characterization results for MCM-41 showed that these crystalline materials consisted of uniform pores in a hexagonal arrangement with a narrow pore size distribution (dispersion less then 5%). For an isolated dipole rotating under an oscillating field in a viscous medium, the Debye dispersion relation is derived in terms of classical mechanics, .
tr =tr
,
+
K ' s + K'~
, (1) 1 + (i6o r) where co is the frequency of the applied potential, and x is the orientational relaxation time of a dipolar molecule. The subscript "s" refers to static permittivity (low frequency limit when the dipoles have enough time to be in phase with the applied field). The subscript oo refers to the high frequency limit, and is a measure of the induced component of the permittivity. The dielectric relaxation time was calculated by fitting the dispersion spectrum of the complex permittivity near resonance to the Debye model of orientational relaxation of the induced component of the permittivity.
3.2. Light transmission method The light source was a He-Ne laser of 6 mW power, whose light beam was split into two beams by a set of prisms. One beam was collected directly by a photodiode, while the other was passed through the measuring cell and the focusing lens and was directed to the second photodiode. The signals from both photodiodes were analysed by the differential amplifier. The measuring cell was thermostatted by a cryostat N-180, and temperature was measured by a copper-constantan thermocouple, with an accuracy of _+ 0.1 K. The relative change in the light intensity after passing through the sample studied is proportional to the voltage change at the photodiodes and characterises the absorption of a given medium. A strong increase in the light absorption is observed in the vicinity of the phase transition as a consequence of the appearance of many nuclei of the new phase, which act as absorption centres. The systems studied are transparent in the liquid state, and the change in light absorption at the solid - liquid phase transition is easily seen. 4. EXPERIMENTAL RESULTS The dielectric relaxation method was applied to study the process of freezing and melting for samples of confined liquid aniline in CPG, having mean pore sizes of H = 50, 25, and 7.5 nm, and in Vycor with H = 4.1 nm. The sample was introduced between the capacitor plates as a suspension of CPG or Vycor in pure aniline. Therefore, the capacitance measurement yields an effective relative permittivity of the suspension of porous silica glass in pure aniline. Results of the measurements of C for aniline in these silica glasses as a function of T at a frequency of 0.6 MHz are shown in Fig.1 (a-d). There is a sharp increase in C at T=267 K corresponding to the melting point of pure aniline, due to the contribution to the orientational polarisation in the liquid state from the permanent dipoles [2,7]. The second sharp increase at 262K (Fig. l a), 260K (Fig. l b), 246K (Fig.lc) and about 227K (Fig.1 d ) is attributed to melting in the pores. The signal corresponding to melting becomes increasingly rounded as the pore size becomes smaller (Fig.ld), suggesting a heterogeneous melting process in the pores.
470
Similar results (not shown) were obtained from the light transmission measurements, which were carried out for aniline in CPG, Vycor and MCM-41 as a function of temperature. Melting temperatures were indicated by sharp increases in the photodiode voltage, which is proportional to the intensity of the light beam passing through the sample. Melting temperatures were in good agreement with those determined from dielectric relaxation spectroscopy, except for Vycor where a somewhat lower melting temperature of 229.8 K was observed. For MCM-41 a melting transition was observed at 223.1 K. '
|
'
Aniline
i
-
!
,
|
'
+ C P G 50 nm.
!
' i
,/*',-k.~
400
r
,
240
9 -
!
:
!d
,
i d
.
.
.
.~,...~.
i i: '
--...,....~,= |
;ZOO.
1; ill,
300,
.
+ CPG 25nm,
Aniline
220.
~.
180-
(,3
160.
I40 200,
120. f,~
193
233
....
ti
!
300
|
"
"
Xl~]
"
'
!
i '
+ CPG 7.5 nm . . . . .
Aniline
!~
273
2Y3
r~l '
!
"
!
Aniline
,
+ Vycor
i
"i
'1
9
|
9
i .....
,
4,1 ~Im.
i
"
..,,..m
,._,..,_~ : -..=.-
,1
73 rO
m.=- . - r 72
.... ....
,
t"/
,.--,~-'~"
.
,
,
t
:~
,
i.~
i~
|
.
,k . . . .
r~
r~s . . . .
z~'
'
"
i~ ~-'
Fig. 1 (a-d) Capacitance C vs. temperature T for Aniline in CPG and Vycor for different pore widths at frequency c0=0.6 MHz. 50
II
.
.
.
.
I
Melting
,
,
!
of Aniline
,
!
in C P G
,
20
10 9
0
o.oo
,
/
o o5
,'
i
o~o
,
i
0,,5
,
0,20
H'l[nm
Light transmission
,~ D i e l e c t r i c M e t h o d 9 i . i ,
i
0,25
"11
0,30
Method
!
o.~
,
o 40
'
471
Fig. 2. Shift in the melting temperature ATm as a function of 1/H for Aniline in CPG, Vycor and MCM-41. The straight line is a fit to the data for H values of 25 nm and above, and is consistent with the Gibbs-Thomson equation. The size dependence of the melting temperature of the confined aniline is shown in Figure 2. The linear relationship between the shift in the pore melting temperature and the inverse pore diameter for larger pore widths is consistent with the Gibbs-Thomson equation [1], but strong departures from this linear behaviour are observed for smaller widths, particularly below pore widths of about 10 nm. Such departures arise from the assumptions made in the derivation of the Gibbs-Thomson equation, and have been observed in other systems [e.g. 1,2,4]. The dielectric and optical measurements are in generally good agreement. Aniline in C P G 7.5nm | Bulk ~d
!
f
!
'
I
'
Maxwell - Wagner
Pore phase r
om ~ ~ - ir-II~-. I II
\,t;" "'m..
"" v
o) c3
-3
.
m
"-n i
m
"A m
-4
Contact layer phase - _
-
2)3
'
2~
'
3)3
'
373
'
423
T [K] Fig. 3. z vs. T for aniline in CPG with average pore size of 7,5 nm. The spectrum of the complex permittivity (~:' and •" vs ~o ) was fit to the dispersion relation, Eq.(1), to determine the dielectric relaxation time z. The frequency range in this study encompasses the resonant frequencies corresponding to the dielectric relaxation in solid and glass phases. According to Eq.(1), K' shows a point of inflection and 1<" goes through a maximum at the resonant frequency. Therefore, from a spectrum plot of ~:' and ~:" vs lOgl0(O~), the relaxation time can be calculated as the reciprocal of the frequency corresponding to a saddle point of the ~:' function or a maximum of the K" function. An alternative graphical representation of the Debye dispersion equation is the Cole-Cole diagram in the complex ~:* plane [2]. The behavior of the relaxation times as a function of temperature for aniline in CPG of 7,5 nm pore size are depicted in Fig. 3. For temperatures greater than 246 K (melting point inside the pores), there are two different relaxations. The longer component of the relaxation that is of the order of 10x 10 -3 s is divided into three regions. The response in the region T > 267K is due to Maxwell-Wagner polarization.
472
This relaxation mechanism occurs for heterogeneous system due to interfacial polarization, when a slightly conducting liquid is enclosed in an insulating material, and has a relaxation time of the order 10-3 s; the porous glass particles, suspended in bulk aniline were electrically neutral. The response in the region 246 < T < 267 K corresponds to the relaxation of the bulk crystal; the Maxwell-Wagner effect disappears because the porous glasses particles are arrested in the crystalline matrix of bulk aniline, thereby preventing interfacial dispersion. The shorter relaxation component, of the order 10-5 s, is too slow to represent the liquid phase relaxation in the pore. However, for dipolar liquids confined in nanopores, the molecules in the contact layer show a slower dynamics, with a relaxation time of the order of 10-5 s [12,13]. Thus, the shorter branch of the relaxation time that occurs above 246 K can correspond to the response of the contact layer. The response of the liquid phase in the bulk and in the inner layers of the pore are not accessible in our experiments, as the sub-nanosecond relaxation times characteristic of liquids is not affected by the frequency range we use in our experiment. The disappearance of the 10-5 s branch at 246 K is due to the freezing of the liquid in the pores. Below this temperature, the millisecond relaxation time corresponds to the crystal phase relaxation in the bulk [7] and in the pore. Aniline + Vycor -1.0 ~" -2.o
Bulk and Pore phase crystals
!
Maxwell-Wagner
9
~ -3.0 ~ .E_ -4.0' ._o ~9 -5.o ~
rr" -6.0 t~
" gz . '
Amorphous region in Ihe inner layers _
-7.0 I
173
"
I
223
|
"
~ I
~
273 T[K]
'
'
i
323
"
1 '
373
-
Fig. 4. x vs. T for aniline in Vycor with average pore size of 4.1 nm. For mean pore diameters larger than 7.5 ran, the behavior of the relaxation time with temperature is similar to that shown in Fig. 3, and suggests that at lower temperatures the phase in the pore is a continuous (presumably defective) crystal; i.e. molecules have a single relaxation component throughout the confined crystalline phase. For smaller pores, e.g. Vycor with H = 4.1 nm, the behavior is quite different. In Fig.4 the corresponding relaxation times for aniline melting in Vycor glass are shown. The 10-5 s branch of the relaxation time shows a sharp increase at T = 230 K, which is taken to be the melting temperature inside the pores. The millisecond branch is again divided into three regions: T>267K (Maxwell-Wagner effect), 230
473 However, there is a new branch of relaxation times of the order of a few hundred nanoseconds which occurs below the melting temperature of aniline in the pores (T < 230 K). This strongly suggests that the confined crystalline phase is not homogeneous, but that there are regions that are glasslike (amorphous), having a relaxation component of the order one hundred ns. Thus, for pore diameters as small as 4.1 nm, the confinement poses a serious constraint on the formation of a homogeneous crystalline phase in the pore. The exact structure of the confined crystalline phase cannot be determined from dielectric relaxation spectroscopy experiments alone. One would need to resort to more direct methods, such as x-ray diffraction and molecular simulation, in order to obtain the fluid structure of the inhomogeneous crystalline phase. The light transmission measurements for aniline in MCM-41 (H=2.8 nm) showed only one signal, corresponding to a phase transition at 223.1 K. Dielectric relaxation measurements showed that the distribution of the relaxation times vs temperature was similar to that for nitrobenzene in MCM-41 [4], and qualitatively different from the behavior in Vycor. The melting transition at low temperature is absent for the 10-5 s branch; this branch of the relaxation time does not show any discontinuity. Once again there is a branch of relaxation times of the order of a few hundred ns, suggesting the presence of an amorphous region, which appears at a temperature of about 223 K, corresponding to the temperature of the phase transition observed in the light transmission experiments. We conclude that the pore size of 2.8 nm is too small for even partial crystallization to occur.
Hlnm Bulk 50 (CPG) 25 (CPG) 7.5 (CPG) 4.1 (Vycor) 2.8 (MCM-41 , Si)
H/on
TIK
Ordered phase
...
266.8
Crystal
100 15 9 5.6 5.6
262 260.2 246.1 229.8 223.1
Crystal Crystal Crystal + glass Glass Glass
Table 1. Melting temperatures: Experimental measurements. The melting behavior of aniline is summarized in Table 1 for a range of pore sizes in various silica-based pores. Aniline freezes to a crystalline structure when confined in the larger pores H > 7.5 nm (H >15oer). For a pore size of 4,1 nm, only part ofthe fluid freezes, with the rest forming an amorphous phase. Morishige and Kawano [14] reached similar conclusions in their x-ray diffraction study of fi'eezing of N2 and CO in MCM-41 materials of varying pore diameter. For these latter fluids, the x-ray studies indicated a homogeneous crystalline phase for pore diameters greater than 6 nm ( H _> 16 o~r ). For smaller pore sizes, the x-ray diffraction patterns were consistent with a partially crystalline confined phase coexisting with an amorphous phase. For pore size of 2.8 nm the confined fluid does not undergo a freezing transition; however a glass transition is observed.
474 5. DISCUSSION AND RESULTS
The proposition that the quasi-two-dimensional contact layer may exist as a hexatic phase [6] implies that at higher temperatures, the "hexatic" contact layer can undergo a transition to an orientationally disordered, liquid-like contact layer. In Fig.4 the disappearance of the ~ts branch at 363 K suggests such a transition from a hexatic (with ~s relaxation time) to a liquid-like contact layer (with nanosecond relaxation time). Such a transition is a Kosterlitz-Thouless (KT) transition [15,16], and involves a large entropy change. In two dimensions, the specific heat shows a nonuniversal peak at the KT transition. Previous DSC measurements [4] for the case of nitrobenzene adsorbed in Vycor and MCM-41 showed an additional peak at a temperature of about 373 K, which is consistent with the KT behavior (hexatic to liquid transition). These results are in qualitative agreement with recent molecular simulation results [6].
Acknowledgements. This work was supported by the KBN (grant no. 2 PO3B 175 08) and the National Science Foundation (grant no. CTS-9908535). REFERENCES
[1 ] L.D. Gelb, K.E. Gubbins, R. Radhakrishnan and M. Sliwinska-Bartkowiak, Reports on Progress in Physics, 62 (1999) 1573. [2] M. Sliwinska-Bartkowiak, J. Gras, R. Sikorski, R. Radhakrishnan, L.D. Gelb and K.E. Gubbins, Langmuir, 15 (1999) 6060 [3] R. Radhakrishnan, K.E. Gubbins and M. Sfiwinska-Bartkowiak, Journal of Chemical Physics, 112 (2000) 11048. [4] M. Sliwinska-Bartkowiak, G. Dudziak, R. Sikorski, R. Gras, R. Radhakrishnan and K.E. Gubbins, Journal of Chemical Physics 114 (2001) 950. [5] M. Sliwinska-Bartkowiak, G. Dudziak, R. Sikorski, R. Gras, K.E. Gubbins and R. Radhakrishnan, Phys. Chem. Chem. Phys. 3 (2001) 1179. [6] R .Radhakrishnan, K.E. Gubbins and M. Sliwinska-Bartkowiak, Journal of Chemical Physics 116 (2002) 1147. [7] M. Sliwinska-Bartkowiak, R. Radhakrishnan and K.E. Gubbins, Molecular Simulation 27 (2001) 323. [8] N.HilI, W.E. Vaughan, A.H.Price, M.Davies, Dielectric Properties and Molecular Behaviour, ed. T.M.Sugden, Van Nostrand Reinhold Co., 1970. [9] A.U. Ahadov, Dielektricieskoje svoistva tchistih ztfidkosti, Izdaitelstwo Standardow, Moskva, 1975. [ 10] B.Szurkowski, T.Hilczer, M.Sliwinska-Bartkowiak, Berichte Bunsenges. Phys. Chem. 97 (1993) 731. [11 ] I.Nowak and M.Ziolek, Third Polish-German Zeolite Conference, edited by M.Rozwadowski (N. Copernicus University Press, Torun 1998), p. 161. [12] P. Pissis, A. Kyritsis, D. Daoukaki, J.Phys..Condens. Matter 10 (1998) 6025. [ 13] M. Urbakh and J. Klaffer, J.Phys. Chem. 97 (1993) 3344. [ 14} K. Morishige and K. Kawano, J. Chem. Phys. 112 (2000) 11023. [ 15] J.Kosterlitz and D. Thouless, J. Phys. C 5 (1972) L 124. [ 16] J.Kosterlitz and D. Thouless, J. Phys. C 6 (1973) 1181.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
475
Transport characteristics of porous solids derived from chromatographic measurements O. ~;olcovfi, P. Schneider Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojov/t 135, 165 02 Praha 6, Czech Republic The chromatographic method, which employs Single-Pellet-String Column (SPSC) and takes, via convolution, into account the extra-column effects of the measuring system, can provide consistent transport characteristics of porous solids. These characteristics are constants of the porous medium and are independent of the kind of gases used, as well as of the temperature and pressure. A set of four commercial catalysts was used for testing of the method. Peclet numbers (which characterize interparticle mixing) were determined for a set of inerts (hydrogen, helium, nitrogen, argon) and tracer/carrier gas pairs flowing through a column packed with nonporous pellets. It was confirmed that Peclet numbers determined in a column packed with nonporous particles are applicable for porous packing; this has a positive effect in the transport parameters confidence. The extra-column effects (due to sampling loop, connecting elements, detector, etc.) were determined experimentally and taken into account by the convolution integral during the time domain fitting of experimental column responses. The obtained transport parameters for the Mean-Transport-Pore-Model (~, ~) were compared with transport parameters obtained from multicomponent counter-current diffusion in Graham's diffusion cell and from mercury porosimetry pore-size distributions. 1. INTRODUCTION In the modern chemical and biochemical research porous materials play an irreplaceable role. The mass transport resistance in the pore structure of the porous solids significantly affects rates of transport processes, which take place inside the porous material (Keil [1], Haugaard and Livbjerg [2], t2apek and Seidel-Morgernstern [3]). Inclusion of transport processes into the description of the whole process is essential when reliable simulations/predictions have to be made. The transport properties of porous solids can be obtained by evaluation of parameters of a suitable simple model applied to experimental results of a simple gas-transport process that takes place in the porous solid in question. The model parameters are material constants of the porous solids and are, thus, independent of temperature, pressure, kind and concentration of gases. At present two models are available for description of pore-transport of multicomponent gas mixtures: the Mean Transport-Pore Model (MTPM)[4,5] and the Dusty Gas Model (DGM)[6,7]. Both models permit combination of multicomponent transport steps with other rate processes, which proceed simultaneously (catalytic reaction, gas-solid reaction, adsorption, etc). These models are based on the modified Maxwell-Stefan constitutive equation for multicomponent diffusion in pores. One of the experimentally performed transport processes, which can be used for evaluation of transport parameters, is diffusion of simple gases through porous particles packed in a chromatographic column.
476 The chromatographic method for obtaining effective diffusivities in porous materials is well established [8,9] and has been frequently used [10,11]. Effective diffusivities are evaluated from response (chromatographic) curves of columns packed with the tested porous particles. The carrier-gas (C) flows at constant rate and a pulse of another gas (tracer - T) is injected into the carrier stream. Tracer concentration is monitored at the column outlet by a suitable detector and the recorded outlet response peak is then analysed. Several processes occur during the passage of the tracer band through the column: besides convection and axial dispersion, transport of tracer through the laminar film around the packing particles takes place, followed by diffusion in the pore structure and possibly adsorption (for adsorbable tracers) on the internal surface of the porous packing. Porous particles can be packed in the column in two ways: a wide bed packed with particles; the column to particle diameter ratio should be larger than about 20 to satisfy conditions for axially dispersed plug-flow, or, particles are packed one by one into a column with diameter that is only 10-20% larger than the particle diameter. This arrangement is known as Single Pellet String Column (SPSC) [12] and it is usually used for spherical or cylindrical porous particles. In SPSC high linear carrier-gas velocities - which suppress the laminar film around particles and tracer peak broadening due to axial dispersion - are easily attained. This is even more significant when inert (nonadsorbable) gases are used as tracers, which move through the column with the high carrier-gas velocity. The use of inert tracers prevents their adsorption and the possible surface diffusion, which obscures the effective diffusion coefficients [13] and transport characteristics. Another advantage of the SPSC arrangement is the averaging of obtained transport characteristics over many pellets. In addition: SPSC guarantees low carrier gas consumption. The analysis of outlet peaks is based on the model of processes in the column. Today the Kubin - Ku~era model [ 14,15], which accounts for all the above-mentioned processes, as long as they can be described by linear (differential) equations, is used nearly exclusively. Several possibilities exist for obtaining rate parameters of intracolumn processes (axial dispersion coefficient, external mass transfer coefficient, effective diffusion coefficient, adsorption/desorption rate or equilibrium constants) from the column response peaks. The moment approach in which moments of the outlet peaks are matched to theoretical expressions developed for the system of model (partial) differential equations is widespread because of its simplicity [16]. The today's availability of computers makes matching of column response peaks to model equations the preferred analysis method. Such matching can be performed in the Laplace- [17] or Fourier-domain [18], or, preferably in the time-domain [19,20]. 2. E X P E R I M E N T A L 2.1. Porous and nonporous catalysts Cylindrical pellets of four industrial and laboratory prepared catalysts with mono- and bidisperse pore structure were tested. Selected pellets have different pore-size distribution with most frequent pore radii (rma• in the range 8 - 2500 nm. Their textural properties were determined by mercury porosimetry and helium pycnometry (AutoPore III, AccuPyc 1330, Micromeritics, USA). Description, textural properties of catalysts pellets, diameters of (equivalent) spheres, 2R, (with the same volume to geometric surface ratio) and column void fractions, ~, (calculated from the column volume and volume of packed pellets) are summarized in Table 1. Cylindrical brass pellets with the same height and diameter as porous catalysts were used as nonporous packing.
477 Table 1 Textural properties of catalysts Pellets
Manufacturer
Description
Pellet heigh x diameter
Porosity, 13
Pore radii from PSD
(mm) unreduced watergas conversion and methanol synthesiscatalyst 45% ZnO, 20% CuO, A]2O3 Pt, Ni an alumina carrierfor removal of 02 and H2 0.1% pt, 3% Ni, A1203
ICI5 2/1
ICI Ltd, Great Britain
G43 A
St~d-Chemie AG, Germany
G-4
a)
580/1200 b)
G- 1
a)
(nm)
Equivalent sphere diameter (mm)
Colum n void fraction c~
8
4.8
0.491
3.8 x 5.5
0.623
4.4 x 4.4
0.600
3.5
254
4.4
0.601
c~-alumina;
4.35 x 4.1
0.52
46
630
4.2
0.651
c~-alumina; 190/1200 b)
4.17 X 4.1
0.579
46
2330
4.1
0.633
a)Valug J., Schneider P., Appl. Catal. 16, 329 (1985). b)compression pressure (atm)/calcinations temperature (~
2.2. Gases Hydrogen, helium, nitrogen and argon were used as carrier gases (C). The same gases plus methane (99.9 % purity) were used as tracers (T).
2.3. Set-up The chromatographic set-up with a detail of column packing in the SPSC arrangement is shown in Figure 1. The system consists of calibrated mass flow-meter/controllers (Brooks
Fig. 1. Set-up 1 carrier gas source; 2 tracer sampling valve; 3 column with pellets; 4 tracer detector + A/Dconversion + data logger; 5 detail of the packed column
478 Instrument, Netherlands) for regulation of carrier and tracer gases, a six-way sampling valve for tracer input with sampling loop (volume 0.273 cm3), SPSC and thermal conductivity detector (Micro-TCD 10-955; Gow-Mac Instruments Co., Gillingham, England; cell volume 20 btl). Metal capillaries (id. 1 mm) with minimized length were employed for connecting the system components. Two stainless steel tubes (i.d. 6.8 mm, length 50 and 100 cm) were used as SPSC column. All measurements were made at laboratory temperature and pressure. 3. R E S U L T S AND DISCUSSION 3.1. Data analyses
The Kubin - Ku6era model describes correctly the intracolumn processes but excludes the effects of peak distortion due to sampling valve, connection between sampling valve, column, detector and detector itself (Extra Column Effects - ECE). In Figure 2 ECE peak is compared with a peak for 50 cm column packed with porous particles. It is clearly seen that ECE peak is not negligible and its omitting can significantly influence the obtained effective diffusion coefficients and, thus, transport parameters. We suggested [21] that ECE peaks could be included in the time-domain matching through the application of the convolution theorem. This requires, besides the knowledge of the experimental system response, also the knowledge of the ECE response. This can be replaced by experimental system responses for two columns with different length. Because the ECE responses are very rapid, which decreased their accuracy, we have applied the second approach for removing ECE effects. The convolution theorem states that the column response, c(t), is given by the convolution integral t c(t)
I g ( t - u) h(u) du
---
(1)
0
h(t) is the column impulse response and g(t) describes the shape of the signal entering the column instead of the Dirac impulse. In linear systems it is immaterial if the ECE are distributed in different places of the system or if they are concentrated in one place and in what order they are arranged. Therefore, it is possible to use the experimental responses for the shorter column as g(t). Both experimental peaks for different l~ngth of columns and the resulting impulse response peak are shown in Figure 3 for N2(tracer)-+He(carrier) and G43A '
'
'
i
'
'
'
1.0
.A.
2
C/Cmax
C,/Cmax 9
9
9
9
~
9
.5
(
0.0
1/ , \ , 0
20
10
t(s)
20
30
Fig 2. ECE and column responses 1 ECE peak (experimental), 2 column with porous pamcles lines experimental, points calculated
30
t(s)
Fig 3 N2(tracer) - > He(carrier) peaks for SPSC with G43A pellets Experimental column responses: 1 column 100 cm, 2 column 50 cm. 3 calculated impulse response for column length: 100 cm - 50 cm = 50 cm Peaks are normalized to the same height
479 101
101
,_..,] t'q
10 0
10 0
10 ~
101
10 2
101
10 2
ReSc ReSc Fig. 4. Peclet numbers for G4 and G43A catalysts from nonporous packing; points: experimental, line: calculated from Equation (2) pellets. Because the difference between lengths of columns is the same as the length of the shorter column, the difference between the position of peak 3 (impulse response) and peak 2 (shorter column) corresponds to ECE effects. There are four unknown parameters in the theoretical impulse response for porous particles, h(t); the pellet diffusion time, tdif (which contains the effective diffusion coefficient of the pair T-C, Dvc, taft = R2[~/DTc, R is the radius of the pellet equivalent sphere), the mean residence time of the carrier-gas in the interparticle space, tc (tc = v/L with the carrier gas linear interstitial velocity, v, and column length, L), Peclet number, Pe (Pe = L.v/E, with E the effective axial dispersion coefficient) and the adsorption parameter, 80 (see below). Because matching with four unknown parameters would give highly correlated parameters, it is better to determine some parameters independently. 3.2. Axial dispersion It was confirmed that Peclet numbers for the same carrier and tracer gases and carrier flow-rate are the same for column packed with nonporous and porous particles. This permits determination of Peclet numbers from measurements in SPSC with nonporous packing. In this case only two parameters appear in the theoretical impulse response: Pe and tc; their determination by time-domain matching is very rapid and parameters are not correlated. Using the obtained Peclet number decreases the number of parameter for matching of porous packing response from four (tdif, Pe, 80, tc) to three (tdif, 8o, tc). In the Figure 4 the obtained optimum Peclet numbers for G4 and G43A pellets are plotted versus the product of Reynolds and Schmidt numbers, ReSc (ReSc = v*2R/D~c ). It is seen that Peclet numbers are obtained with quite a good accuracy in the whole range of carrier-gas velocities used (from 30 to 250 cm3/min). The obtained data were correlated by a semi empirical expression for axial dispersion 2 [22]. Table 2. Obtained parameters 7, X and [3 for tested catalysts Pellets ICI G43A G1 G4
7 1.28 1.27 1.15 1.18
X 0.69 0.61 0.64 0.31
[3 16.4 21.5 16.7 12.8
480 1 L
7
Pe2R
ReSc
kReSc _
j
r
(2)
_
13+ReSc
The obtained parameters y, X and [3 for all tested catalysts are summarized in Table 2.
3.3. Tracer adsorption The tracer adsorption parameter, ;5o, includes tracer adsorption equilibrium constant KT; ~5o = ~/(1 + KT), and 3' = 13(1-c0M. 13 is the pellet porosity and ct is the column void fraction (interstitial void volume/column volume). Thus, 7, is the pore volume per unit interstitial volume. For an inert tracer KT= 0 and ;5o = 3'. Parameters ;50 were determined from the difference of first peak moments for two column lengths. Thus, only tdif and tc were searched by the time-domain matching; these parameters are not correlated. From our experience the simplex algorithm [23] for minimization of the sum of squared differences between the calculated and experimental SPSC responses proved to be sufficiently rapid and stable.
3.4. Pore diffusion and transport parameters determination The effective diffusion coefficients, Dwc, obtained from the parameter tdif, include contributions from the Knudsen diffusion mechanism and from the molecular diffusion mechanism. Because of the very low tracer concentrations the Bosanquet formula (3) is applicable. 1 1 1 + ~
:
DTC
< r > ~l/KT
(3)
in
~l/D~c
Here D~c is the molecular diffusion coefficient of the pair C-T and K T - (2/3)~/(8RgT/~MT) is the Knudsen constant for the tracer T, Rg is the gas constant, T temperature, and MT the tracer molecular weight. ~ and ~ are parameters characterizing the porous medium (transport parameters). stands for the integral mean radius of pores through which the
cmB/g
9
I
o=84nm ,=73nml
c=302nm ] !~2nm]
2 7nm
/J o v
oo t/
-o
j ~
--'
o
'
r
1
.
.
.
.
2
KT/DmTc 10-5 (cm 4)
Fig. 5. Diffusion times for G4 pellets from parameter matching in co-ordinate: tdifKT versus KT/DmTC
0.0
}L
II
c=330nm d=670 nm ~,-
I)/I l1 I
,
I
nm nm T ~233~ , ,
d=763
, ,
./
0
40n
f!
dV/dlog(r) cm3/g 5
b
100
10 ~
/
102
r(nm)
103
104100
10 ~
102
103
104
r(nm)
Fig.6. Pore-size distributions from mercury porosimetry and mean transport-pore radii c from SPSC and d from multicomponent counter-current diffusion
481 Table 3. Transport parameters from SPSC and counter-current diffusion SPSC u/
~
Diffusion cell ~
ICI 2.27 0.027 84 2.7 0.037 G43A 1.95 0.013 302 7.8 0.031 G1 8.03 0.071 112 152 0.199 G4 12.90 0.039 330 61 0.091 majority of the mass transport takes place (transport-pores) and ~ is the ratio porosity and tortuosity. Eq. (3) can be rearranged into the form (4),
73 252 763 670 of transport-pore
td,rKw =
e2[~ F R213 Kv (4) ~ ~ D~c which permits easy graphical data determination of ~ and ~ from effective diffusion coefficients, Dsc, for all T--+C pairs. For each catalyst the diffusion times, td,f, were determined by parameter matching for at least six T ~ C pairs and several carrier gas velocities, v. In Figure 5 the t u for G4 pellets are plotted in co-ordinates tdifKvversus K s/Drc. The lowest deviation of diffusion times were found for tracer-carrier pairs and carrier gas velocities that correspond to the maximum of the Peclet n u m b e r - ReSc dependence (figure 4), i.e. to the lowest axial dispersion coefficient. Transport parameters ~ and V were determined from the slope and intercept of the (least squares) straight-line tdirKTversus KT/D~c (equation 4). The obtained transport parameters together with calculated mean transport pore radii ( = ~@) are summarized in Table 3 and compared with transport parameters obtained by another method, viz. multicomponent counter-current diffusion in Graham's diffusion cell [24]. The transport parameters have the same trend and are mostly in agreement. In Figure 6 calculated mean transport pore radii, , from both methods are compared with pore-size distribution from mercury porosimetry. It is seen that the obtained mean transport pore radii either agree with porosimetric peak, or, for bidispersed pellets, with the porosimetric peak of wider pores, or, are positioned between porosimetric peaks closer to the peak for wider pores. It was confirmed that the gas diffusion transport takes place predominantly through wider pores and the role of narrower pores depends on their size and amount. The resistance, ~K, of the Knudsen diffusion mechanism relative to the sum of Knudsen and bulk diffusion resistances was calculated for two pairs tracer-carrier and all porous pellets. Calculated results are summarized in Table 4. The obtained ~K for both pairs of gases for all catalysts are in a good agreement, the largest deviation is less than 12%. It can be seen that the significance of Knudsen diffusion mechanism decreases from ICI pellets (where it has nearly the same significance as bulk diffusion) to G4 pellets (where only about 15-20% of the total diffusion resistance is of the Knudsen type). Figure 7 illustrates the 95% Table 4 Relative Knudsen diffusion resistance
ICI G43A G1 G4
~K (%)
~K (%)
N2 --+ He 46 33 38 18
Ar --+ N2 43 30 36 16
482 confidence region for pellets G43A. The shape of this region, protracted along the ~ axis, implies higher accuracy of the parameter ~ that characterizes the more significant molecular diffusion mechanism. In agreement with the ratio of diffusion resistances (Table 4) Knudsen diffusion is less important, hence its characteristic parameter, ~, varies in wider limits. 4. C O N C L U S I O N S On a set of four porous solids it was confirmed that consistent transport characteristics, 2 3 pertinent to gas transport, can be obtained by the ~ (nm) chromatographic method with SPSC columns. Fig. 7 95% confidence region of transport parameters The developed method takes into account extra ~F and qJ for G43A catalyst, optimum parameter pair ~s shown as point column effects of the measuring system. It was also confirmed that Peclet numbers for equally sized nonporous and porous cylindrical pellets of the same shape are identical; this permits to decrease the number of parameters to be searched during the time-domain matching of experimental peaks. The decrease of the number of parameters searched for porous packing significantly increases the transport-parameters confidence. 1
~
i
_1
i
i
i
~
i
L
i
1
Acknowledgement The financial support of the Grant Agency of the Czech Academy of Sciences (# A 4072915) and of the Grant Agency of the Czech Republic (104/01/0546) is gratefully acknowledged.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
F. Keil, Diffusion und Chemische Reaktionen in der Gas/Feststoff- Katalyse, Springer Press, Berlin, 1999. J. Haugaard, H. Livbjerg, Chem. Eng. Sci 53 (16) (1998) 2941-2948. P. Capek, A. Seidel-Morgenstern, Appl. Catal. A General 211 (2001) 227-237. E.A. Mason, A.P. Malinauskas, Gas Transport in Porous Media, The Dusty Gas Model., Elsevier, 1983. R. Jackson, Transport in Porous Catalysts, Elsevier, Amsterdam, 1977. P. Schneider, D. Gelbin, Chem. Eng. Sci., 40 (1985) 1093. P. Fott., G. Petrini, P. Schneider, Coll. Czech. Chem. Commun., 48 (1983) 215. H.W. Haynes, Jr. Catal. Rev.-Sci. Eng. 30 (1988) 563. M. Suzuki and J. M. Smith, Adv. Chromatogr. 13 (1975) 213. H.S. Nan, M. M. Dias, V. G. Mata and A. E. Rodrigues, Chem. Eng. Commun 146 (1996) 201-229. P.J.A.M. Kerkhof, M. A. M. Geboers and K. J. Ptasinski, Chem. Eng. J 83 (1) (2001) 107-121. D.S. Scott, W. Lee, J. Papa, Chem. Eng. Sci. 29 (1974) 2155. P. Schneider, J. M. Smith, AICHE J. 14 (1968) 886. M. Kubin, Collect. Czech. Chem. Commun. 30 (1965) 1104. E. Ku~era, J. Chromatogr. 19, (1965) 237. P. Schneider, J.M. Smith, AICHEJ. 14,(1968)237. K. Kamiyanagi, S. Furusaki, Intemat. Chem. Eng. 25,(1985) 301. K. Zygourakis, K. Moudgalya, Chem. Eng. Commun. 61, (1987) 107. M.A. Fahim, N.Wakao, Chem. Eng. J. 25, (1982) 1. N. Wakao, S. Kaguei, J. M. Smith, J. Chem. Eng. Japan 12, (1979) 481. O. Solcovfi, P. Schneider, Collect. Czech. Chem. Commun., 61 (1996) 844. M.F. Edwards, J. F. Richardson, Chem. Eng. Sci., 23 (1968) 109 D.M. Himmelblau, K. B. Bischoff, Process Analysis and Simulation, Wiley 1968, New York. O. ~;olcowi, H. Sajdaufovfi and P. Schneider, Chem. Eng. Sci. 56 (2001) 5231-5237
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
483
Effects of porous solid structures on the electrical behaviour : prediction key of transport properties in sedimentary reservoir rock A. Cerepi, R. Burlot, S. Galaup, J-P. Barde, C. Loisy and L. Humbert Institut EGID-Bordeaux 3, 1, all6e Daguin, 33 607, Pessac, cedex, France. Tel. : 33 (05) 56 84 80 72 ;Fax : 33 (05) 56 84 80 73 ; E-mail : [email protected] This paper reports an investigation of the effects of porous solid structures on their electrical behaviour at different frequencies (from 100 Hz to 100 kHz). For that, we study different parameters such as : formation resistivity factor, cementation factor, chargeability, resistivity index and saturation exponent. Different porous solid structures are quantified from the petrographic image analysis and Hg-injection technique. Then, by using different models we obtain the permeability prediction from the electrical behaviour and structure parameters. 1. INTRODUCTION The electrical conductivity of a porous solid depends on different parameters such as the spatial distribution of the constituent minerals and pore space, saturation distribution, mineralogical content, wettability and temperature, with the pore structure playing the dominant role in clean-water saturated sandstone [1]. The effect of pore geometry is even more crucial when an non-wetting, non-conducting phase occupies part of the pore space, because the configuration of the conducting phase is a strong function of geometry. The transport properties such as the electrical conductivity and permeability depend not only on porosity but are also strongly sensitive to the micro-structure of the porous system, the connectivity of the pore space and their micro-geometry [2, 3]. During the last decade the study of the electrical properties of porous solids (sandstone and carbonate) has played a important role in many scientific fields such as surface chemistry, geology, geophysics and reservoir engineering including oil industry logging and geothermal systems. The electrical behaviour of porous solids can be explained by different parameters such as: formation resistivity factor, cementation factor, resistivity index and saturation exponent. These laboratory electrical properties are important to know for [4-6]: i) the improvement of the determination of water-oil saturation in reservoir from well logs, ii) the calibration of resistivity logs. Different recent studies have shown that the complex electrical behaviour of porous solid submitted to an alternative electrical field can be used to estimate rock petrophysical properties such as the specific surface area and permeability [7]. One of the most important conclusions that has emerged from the recent studies is that the permeability may be estimated by combining the electrical behaviour properties as the formation factor with the micro-geometric length scale such as the pore size, throat size or the grain size [ 1-3]. 2. THEORETICAL BACKGROUND The complex electrical behaviour of porous solids can be characterized by the dependance of the complex resistivity and conductivity on frequency as well as the water content, the pore
484 microgeometric, the pressure and state parameters temperature [3, 8]. The complex electrical impedance vector (Z) can be explained by two parts: the real part (resistance r) and the imaginary part (reactance x) ; following the relationship: Z = r+jx where j = ~/-1 is the complex operator. The phase angle (0) by which current and voltage are shifted is given as 0 = t a n - l ( x / r ) . From the impedance vector it is possible to obtain the complex resistivity R -- Z-A/L = R' + jR" where A is the cross-sectional area of the sample ; L is its length; R' and R" are the real and imaginary parts of the complex resistivity. In the same way, the complex conductivity o can be expressed by the following relationship : o = YL / A = o'+jo" where 0' and 0" are the real and imaginary conductivies ; Y is admittance. Seigel (1959) [7] introduced the "chargeability" concept linked to the variation of the observed voltage in time-domain measurements. This method has evolved from direct current measurements where the polarization effect is observed in the variation of the apparent resistivity with the frequency. The empirical parameter of chargeability factor (M) explained the frequency dispersion andis defined as follows [7, 8] : M = RL / (RL + RH) where RL and RH stand respectively for the low and high resistivity. This relationship is very strongly influenced by the rock texture and water saturation [7, 8]. So, when the brine is displaced by non-wetting and non-conducting fluid, the "chargeability" increases. In isotropic homogeneous saturated porous media (Sw-100 %), the electrical conductivity and the porosity are usually related by the empirical Archie's law [5] : 1 = 0__0_ 0 = R w = acI)m ( 1) F ow Ro where F is the formation factor; Ow is the electrical conductivity of the brine; Oo is the electrical conductivity of the whole saturated core, Ro is the saturated core resistivity; Rw is the brine resistivity ; 9 is the porosity of porous medium ; a is the tortuosity. The exponent m is usually called the cementation factor. The factor a ranges from 0.6 to 1.3. It represents a combination of several second-order textural factors that affect the geometry of the pore structure, and a is sometimes considered as a type of tortuosity factor. For clean sand with intergranular porosity and for brine saturation Sw<100% the resistivity index versus brine saturation is obtained from Archie's equation [5]: Rt IR- Swn (2) Ro where Sw is the water saturation; n is called the Archie saturation exponent; Rtis the resistivity of the rock partly saturated with brine, and partly with oil or gas. Both, the formation resistivity factor and the saturation exponent depend strongly on microtextures, pore-types, rough surface and frequency [4, 9, 10, 12]. 3. SAMPLE DATABASE, TEXTURES AND PORE-TYPES USED The 15 studied samples coming from Danien dolomite and Oligocene carbonate rocks in the Aquitain basin, are chosen for this study because of their high textural heterogeneity due to variable depositional environments and different diagenetic evolutions. Detailed petrographic analyses of the porous microstructures were made from thin-sections in conventional optical microscopy and Scanning Electron Microscopy (SEM), resulting in the four main defined rock-types e.g., Table 1, Fig. 1. Texture I(mudstone-wackestone-packstone with mudsupported texture and with grain-supported texture) is characterised by primary pore-types: intramatrix (within carbonate mud) and intragranular of small size (<1-2 ~tm) (within carbonate grains). Their pore networks are monomodal, with low porosity-low permeability trends.
.~
0
485
o=
o
b~
o
9.
o,,,
~0 , z , =
r
,-.M
o
~: ~ - ~
o
~
~
o
9,-,~
~
o
o
~
o
~:.~
.,-,~
.~~
486 Table 1 Samples used in this study, their formation, petrographic, petrophysical and mineralogical characteristics. C: crystal carbonate; M-W: mudstone, wackestone; P-G: packstone-grainstone; Vac: vugs; iX: intercrystalline pores; iM; intramatrix pores; iG: intragranular pores; IG: intergranular pores; K: karsts ; Fr: fractures; F: formation factor; m: cementation factor; n: saturation exponent. Petr%raphy Samples number 1-LS3 2-LS3 3-LS3 4-LS3 5-LS8 6-LS8 7-LS8
8-LS8 9-kS8 1-sgrc5 24-12B 7-SC3 23-12B 10-SC2 9-SC2
Formation
Mineralogy
Aquitam Danien dolomite dolomite " calctte-dolomite " dolomite " dolomite " dolomite " dolomite " dolomite " dolomite-calcite " dolomite Aqmtam Stampien calcite hmestone " calcite " calcite " calcite " calcite " calcite
Electricalparameters Texture
Pore-types
F
m
C
Vac, IX
35.56
2 05
Chargeability M=RIJ(RL+Rz0 0 388
M-W C C C C C M-W C P-G
IM, iX,Vac JX iX iX K, Fr, iX K, Fr, iX IM, IX,Vac Vac, ~X IG, IG, iM
8 70 20.60 9 54 23 28 19.67 13 77 36 97 28.20 8.62
1 68 2.48 2.14 1 98 2.07 2.00 2.31 2.27 2.39
0.491 0.366 0.478 0.411 0.447 0.423 0.420 0.483 0.483
P-G G P-G M-W M-W
IG, IG, iM IG, IG, Vac IG, iG, iM IM, Vac IM, Vac
5.47 13.17 5 18 6 91 15 52
1.98 2 47 1.94 1.62 1.98
0.494 0.446 0 502 0.488 0.492
n
2.4 34
Water Permeability porosity (mD) 0 17 27 4 027 0.31 0 35 0 20 0.24 0 27 0.21 0 23 0 41
93 203 2 90.34 142.9 185 5 303 4 07 ! 70 189 4
0 42 0 35 0 43 0 30 025
263 4 41 3 491 1 2047 2 50
Texture H (packstone-grainstone with grain-supported texture) shows three primary poretypes : large and well-connected intergranular pores ; intercrystalline (in carbonate mud) and intragranular pores of small size. Its pore network presents high porosity-middle permeability trends, a three-modes structure. Texture III (grainstone) is characterized by large and wellconnected primary pore-types (inter-intragranular). Its pore network has a two-modes structure, high porosity-high permeability trends. Texture IV (crystal carbonate) is a dolomitic rock-types characterized by the dominance of well-developed dolomite crystals. Pore-types were mainly intercrystalline with some vugs and karsts. The pore structures varied from monomodal to three-modes with an increase of porosity and permeability according to developments of vugs and karts.
4. EXPERIMENTAL SETTING 4.1. Electrical measurements
A special new configuration of loan Mk2 "LLC" Modular Universal Stackable Pr system is developed to obtain different electrical measurements and capillary pressure curves e.g., Fig. 2. The apparatus contains multi-potential electrodes along the sample length which allows us to identify the electrical behaviour heteorgeneity of each sample. The sample plugs are 3.8 mm in diameter varying in length from 3 to 8 cm. The elecrical cell is connected with an impedencemeter HP which allows us to make different frequencies for resistivity measurements from 100 Hz to 100 kHz for a voltage of 1000 mV. The two end metal plates serve as current electrodes and two additional electrodes around the samples measure the voltage e.g., Fig. 1. The volume of the sample whose resistivity is measured, is situated between the two additional voltage electrodes. In order to study the frequency dependence of partially saturated rocks, the desaturation technique using semi-permeable capillary diaphragms (ceramic membranes) as used. The brine used for all the IR experiments was 5 g/l NaC1. The main advantages of this method are the reduction of capillary end effects and the uniform saturation distribution along the core length. Core plugs are drilled in parallel with the orientation of the stratification. Then, the samples are oven dried for 48 hours at a temperature of 50 ~ The dried samples were saturated with brine under 0.001 mbar vacuum
487 for 24 hours. We gradually increased the capillary pressure up to the atmospheric pressure. Then, the effective water porosity was obtained by weighing. Once the sample is saturated at 100%, we assemble it with the jacket and the electrode pins into the pressure tube (confining pressure 29,5 bars max). Resistivities were measured at different saturations and different frequencies. 4.2. Water permeability measurements The water permeability kw is obtained from pressure drop LIP versus Darcy velocity F~ curves for different limestone samples. The volume flow rate Qy is set by the syringe pump, and the Darcy velocity Vf is found from the following relationship: Vf = Q f / A where A is the crosssectionnal area. The pressure drop AP is measured and then converted into the pressure gradient AP by being divided by the sample length l. The permeability is given by the relationship kw = r / / m , where m is the slope of the line fitted to the data and r/is the water viscosity.
Fig. 2 "Multi - electrode core flow cell configuration.
4.3. Pore structure measurements by Hg-injection The theoretical basis of the Hg-injection method is defined by Laplace law. By using a capillary model where the porous medium is assimilated to a bundle of cylindric capillary tubes the capillary pressure is" Pc = Y(1/Rcl+1/Rc2) = 2y]cos0I/Rc (3) where Pc is the capillary pressure; Rcl and Re: are mutually perpendicular radii of a surface segment ; Re is the average pore-throat size (~m); 0 is the angle
between mercury menisc and pore wall (for mercury 0=140 ~ ; y is the interfacial tension (for mercury y = 0.480 N/m). Hg-injection curves give us the pore structures measurements such as mercury porosity, distribution of Hg-saturation versus pore-throat size; specific surface area deduced from vpr V(Pcmax)" A p ~ v ~-~os~ (in m2/gr). Hg-injection allows us to obtain also the
characteristic length lc defined as the smallest pores size in the connected path of the largest pores following the relationship 9lc = 4yIcos01/P [2]. 5. PERMEABILITY PREDICTION Two models were tested to predict the permeability from the electrical behaviour and microstructure parameters of porous solids. The first one is the Katz and Thompson's model (1986) which interprets transports within pore solids in terms of these percolation ideas [2]. From that, the authors introduced a fractal percolation model to predict the permeability of a disordered porous media. In invasion percolation, a non-wetting fluid can have access to the first connection from one face of the sample to the other only when the driving pressure is sufficient to penetrate the smallest porethroat of radius re in the most efficient conducting pathway. So, the permeability of rocks saturated with a single liquid phase is given from the following relationship : kKT = clc2 cy~ = clc2 ~
:clc ro a~
(4)
488 where the permeability k is defined by the Darcy relation ; c is a constant of 1/226. Here lc is the characteristic length of the pore space which sets the scale for the permeability. It is a unique transport length scale which dominates the magnitude of the permeability. The second is the model proposed by Johnson, Schwarty and co-workers who introduced a dynamic length scale Ae which is a function of the electrical field intensity E in the medium and is related to the formation factor and so to Archie's exponent m by the relationship [ 12]: _22 = dln__~F. S O = m((I)) S~ (5) A e d l n ~ (I3 q3 where So is the specific surface area. This means, physically, that Ae is a modified hydraulic radius. From the Eq. (5) the permeability can be derived from the following equation : A2 ~2 k j s c = 8F
=
2F- m 2 . S 2
Fig. 5 : Chargeability factor M can be predicted by a multi-linear model composed by different parameters: formation factor F, water porosity ~, Hg-specific surface Asp and water permeability k for different textures.
(6)
Fig. 6 : Permeability prediction from electrical behaviour and structures parameters of porous solids, kKx: Katz and Thompson' model; kjsc: Johnson, Schwartz and co-workers model.
6. R E S U L T S AND DISCUSSION The electrical behaviour versus frequency for different porous solids textures is given in Figure 1. The complex impedance Z and phase angle 0 decrease with a frequency ranging from 100 Hz to 100 kHz for any porous solids texture at full water saturation. However, the
489 gap between maximum and minimum values of complex impedence versus frequency is higher in ~primary textures ~ than in ~ secondary textures ~. This frequency dispersion is explained by the chargeability factor M. Different results can be summarized as follows : - Chargeability factor M depends strongly on the rock-texture e.g., Table 1, Fig. 1. The higher value of M is observed in calcite P-G textures which ranged from 0.483 to 0.502. The lower value of M is reached in the dolomitic crystal carbonate texture with a intercrystalline pore-type where it ranges from 0.366 to 0.483. - Chargeability factor M depends on the brine~gas saturation o f porous solids. Figure 3 gives the relationship between the chargeability and brine saturation for two samples. We noted that the M decreases hardly with the decrease of the brine saturation. The presence of vugs and karsts pore types (sample 9-LS8) seems to speed up the decrease of the M. - Chargeability factor M can be explained by a multi-linear model composed of different structures parameters such as the formation resistivity factor, water porosity, Hg-specific surface area and water permeability, e.g., Fig. 5. In the F - ~ cross-plot, different experimental data sets satisfied the relationship F>(39 )/2~ e.g., Fig. 7, Fig. 8. The cementation factor (m) and the structural parameters (a) were interpreted following two methods : i) The Archie method assumes a = 1, and m is determined for each sample from a logF-log~ curve using Eq. (1), e.g., Table 1, Fig. 8 ; ii) The best-fit method is applied for each textural class in order to obtain a and m e.g., Fig. 7. The values of m obtained by using the first method, range from 1.62 to 2.48.
Fig. 7 : Relationship between F and 9 for different pore textures, the best-fit of F-~ relationship for each texture.
Fig. 8 : Relationship between F and 9 for different pore textures using the Archie method. The comparison with some predicted models.
The dolomitic cristal carbonate texture shows the highest value of m (1.98 to 2.48) and F (9.54 to 36.97) while M-W and P-G textures have low values of m (1.62 to 1.98) and F (5.47 to 15.52). This high Archie cementation factor m of crystal carbonate is due to its high cementation degree. On the other hand, the best-fit electrical parameters were: a factor (ranged from 0.02 to 2.26) and m (ranged from 1.57 to 4.74) e.g., Fig. 7. Figure 4 gives the relationship 1OgIR - logSw and saturation exponent n whose values range between 1.2 to 3.4. These values are different that those proposed by the Archie'law and are usually used in petrophysics (n = 2). A non-linear behaviour of IR-Sw curve is observed. This is due probably to the high microporosity development and its roughness [4,10]. The saturation exponent n depends strongly on the frequency. So, in sample 9-LS8 when the frequency increases from 100 Hz to 100 kHz the saturation exponent n decreases strongly from 3.56 to 2.81. The comparison between the measured and predicted permeability from electrical parameters using two models is given in Figure 6. The best result is obtained with the Katz and Thompson's model. In this model, the best-fit is obtained for a constant c of about 1/171. So,
490 the characteristic length lc of pore space is the best adapted structural parameter of porous solids. 7. CONCLUSION The electrical behaviour of different carbonate textures is investigated as a function of the frequency, from 100 Hz to 100 kHz. This frequency dispersion is explained by the chargeability factor M which depends strongly on both the rock-texture and the brine/gas saturation of porous solids. The higher value of M is observed in the calcite P-G texture. The M decreases hardly with the deacrease of the brine saturation. The presence of vugs and karsts pore types seems to speed up the decrease of the M. The saturation exponent n depends strongly on the frequency. The best predicted permeability using electrical measurements and the micro-geometry is obtained by Katz and Thompson's model. REFERENCES
[ 1] Bryant, S. & Pallat, M., 1996. Predicting formation factor and resistivity index in simple sandstones, J. Pet. Sci. Eng, 15, pp. 169-179. [2] Katz, A.J. & Thompson, A.H., 1986. Quantitative prediction of permeability in porous rock, Physical review B, vol. 34, N ~ 11, pp. 8179 - 8181. [3] Worthington, P.F., 1997, Petrophysical estimation of permeability as a function of scale, from Lovell, M.A. & Hervey, P.K. (eds), in Developments in Petrophysics, Geological Society Special Publication, N ~ 122, pp. 159-168. [4] Worthington, P.F., Pallat, N., and Toussaint-Jackson, J.E., 1989. Influence of microporosity on the evaluation of Hydrocarbon saturation, SPEFE (June 1989), 203; Trans, AIME, 287. [5] Archie, G.E., 1942. The electrical resistivity log as an aid in determiny some reservoir characteristics. AIME Petroleum Technology, pp. 1-8. [6] Saner, S.Abdulaziz A1-Hasthi, Maung Than Htay, 1996, Use of tortuosity for discriminating electro-facies to interpret the electrical parameters of carbonate reservoir rocks, Journal of Petro. Science & Engineering, 16, pp. 237-249. [7] Denical, P.S. & Jing, X.D. 1998. Effects of water salinity, saturation and clay content on the complex resistivity of sandstone samples, In: Harvey, P.K. & Lovell, M.A., (eds) Core-log Integration, Geological Society, London, Special Publications, 136, pp. 147157. [8] Da Rocha, B. R. P. & Habashy, T.M., 1997. Fractal geometry, porosity and complex resistivity: from hand specimens to field data, from Lovell, M.A. & Harvey, P.K., in Developments in Petrophysics, Geology Society Special Publication, N ~ 122, pp. 287297. [9] Swanson, R.G., 1981. Visualing pores and nonwetting phase in porous rock. Journal of Petroleum Technology, vol. 31, pp. 10-18. [10] Diederix, K.M., 1982, Anomalous relationship between resistivity index and water saturations in the Rotliegend sandstone (The Netherlands), SPWLA Annual Logging Symposium, Corpus Christi, Texas, 6-9 July. [l l] Stalheim, S.O., Eidesmo, T., Rueslatten, H., 1999. Influence of wettability on water saturation modelling, J. Pet. Sci. Eng, 24, N~ pp. 243-253. [12]Clennell, M.B., 1997. Tortuosity: a guide through the maze, from Lovell, M.A. & Harvey, P.K., in Developments in Petrophysics, Geology Society Special Publication, N ~ 122, pp. 299-344.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
491
Pore and surface characteristics of porous melamine- and phenolicformaldehyde polymers by sorption and XPS measurements A. Deryto-Marczewska and J. Goworek aFaculty of Chemistry, M. Curie-Sktodowska University, 20-031 Lublin, Poland E-mail: [email protected] Mesoporous melamine-formaldehyde and phenolic-formaldehyde resins were synthesized in the process of polymerization in the presence of fumed silica as an inorganic template. The surface and structural characteristics of the obtained sorbents were investigated using XPS technique and sorption from gas phase. The parameters characterizing porous structure of the synthesized resins in a dry state were determined from nitrogen adsorption/desorption isotherms. The sorption processes of benzene and water vapor accompanied by simultaneous swelling of both polymers were also studied. 1. INTRODUCTION Porous organic polymers have attracted a considerable attention in recent years because of their important industrial applications. They are widely used in separation techniques, e.g. gas and liquid chromatography, and in other processes relevant to polymer-solvent mixtures [ 1,2]. In contact with gaseous or liquid media porous polymers exhibit adsorption and absorption properties [3-5]. Both sorption phenomena start almost simultaneously with resin swelling in vapors or liquids. Specific interactions of adsorbate molecules with polymer surface and degree of swelling in contact with various media determine the combined mechanism of sorption. Hence, cross-linked polymers are capable to increase their volume by a factor 2 or even 3. Saturation of polymers with organic solvents and water results in substantial change of pore diameters and pore size distributions. As a result the porosity of organic sorbents is different in dry and wet state [6]. Textural characterization of polymeric sorbents is difficult to obtain in comparison to rigid inorganic adsorbents. Several methods can be used to investigate the characteristics of pores, including adsorption of inert gases, mercury intrusion porosimetry, small angle scattering of x-rays or neutrons, thermogravimetric porosimetry, and more recently NMR spectroscopy and positronium annihilation spectroscopy [7-15]. In wet state, when the solute size is known, size exclusion chromatography can be used to characterize the pore structure of the column packing material [ 16]. The pore dimensions differ usually when evaluated in swollen state in comparison to dry state as in low-temperature nitrogen adsorption.
492 In nitrogen adsorption at 77 K polymers demonstrate some tendency to crush and deform, however, due to their glassy state at this temperature and weak interactions with nitrogen molecules the swelling effect may be neglected. The porosity parameters derived from lowtemperature adsorption data are usually different than those obtained by other methods [6]. Thermal desorption of liquids indicates that the pore radii of melamine-formaldehyde resins wetted with water are higher than those estimated from nitrogen isotherm. Textural parameters depend strongly on the particle dimensions and chemical character of wetting liquid [17-20]. The rate of swelling depends mainly on interactions of adsorbate with the polymer network and the interactions of the molecules with themselves. Thus, the same porous polymers, being in contact with various gas or liquid media, especially when crosslinking process is not completed, may exhibit various structural characteristics. Internuclear chain elasticity of porous polymer influences the pore properties and swelling ability. The aim of the present paper was to investigate the structural and surface characteristics of two types of porous polymers. The sorption isotherms of nitrogen at 77 K, and benzene and water vapor at room temperature were measured by the static method. The specific surface areas, pore volumes and pore dimensions were derived for the investigated polymers from different experimental data. The structural characteristics of the investigated porous polymers differ considerably for various types of adsorbates and temperatures. The surface characterization of both resins was made by XPS method. 2. EXPERIMENTAL
2.1. Resin synthesis The synthesis of hyper-cross-linked phenolic-formaldehyde resin was performed by contacting a reacting mixture of bisphenol, paraformaldehyde with inorganic template in the form of fumed silica at controlled pH [6,20]. Fumed silica is available as individual particles ranging from 7-40 nm. The dispersion of fumed silica particles to the size of primary particles is very difficult due to formation of aggregates whose dimensions depend on interactions of surface hydroxyls of the silica and surrounding species. In our case it is reasonable to assume that strongly polar molecules like formaldehyde (or melamine) and water are adsorbed on silica surface. Hence, the dispersion of fumed silica particles in reacting mixture is very high, however, is not complete to the scale of individual particles. After synthesis, the reacting mixture was aged during 3 days and heated. Next the template particles were removed from the cross-linked copolymer by dissolution in alkaline solution (NaOH) at pH of about 12, without destroying the polymerizate. The obtained phenolic-formaldehyde resin was marked as PHD. Prior to further experiments the polymer was washed with hot water and acetone. The chemical character of the phenolic-formaldehyde resin surface was determined mainly by the presence of polar hydroxyl groups and hydrophobic benzene rings. The synthesis of hyper-cross-linked melamine-formaldehyde resins was conducted by contacting a reacting mixture of melamine and paraformaldehyde with fumed silica as inorganic template at controlled pH [21]. Further procedure was the same as in the case of synthesis of phenolic-formaldehyde polymers. The obtained resin was marked as MEA. The chemical character of the melamine-formaldehyde resin surface is determined mainly by the presence of polar amine groups, hydrophobic hydrocarbon segments and nitrogen atoms in triasine rings.
493 2.2. Experimental methods 2.2.1. XPS measurements XPS spectra were obtained by means of ESCALAB-210 electron spectrometer (VG Scientific Ltd., Sussex, U.K.), employing mg Kc~ X-rays. The measurements were performed at base pressure lower than 8"109 mBa. The X-ray gun was operated at 15kV and 20mA. Survey scans were collected from 0 to 1200eV with pass energy of 50eV for each sample followed by the regions C ls, N ls, and O 1s. Each spectral region was scanned between 10 and 20 times depending on the signal intensity. Data processing was performed according to ECLIPSE program, applying Schirley-type background subtraction. Curve fitting was performed using the nonlinear list-squares algorithm and assuming a mixed Gaussian/Lorentzian peak shape of variable proportion. This peak-fitting was repeated until an acceptable fit was obtained. XPS data were also used to examine the atomic composition and surface species of initial and modified polymer. Atomic percentage values and elemental ratios were calculated from the peak-area ratios after correction with the experimentally determined sensitivity factors, being reliable within _ 10%. 2.2.1. Measurements of vapor sorption isotherms The sorption isotherms of benzene and water vapor were measured at 298K by a volumetric method. Before the experiment the samples were degassed (10 .3 mmHg) at 393 K. 3. RESULTS AND DISCUSSION 3.1. Surface characteristics from XPS spectra The XPS spectra for the polymer PHD are presented in Figure 1. The optimum fitting was achieved by resolving each Cls spectrum into four peaks. Peak I (BE=284.7eV) - aromatic carbon, peak II (BE=285.7eV)- carbon present in phenolic, alcohol or ether groups, peak III (BE=287.4eV) -carbonyl groups, and peak IV (BE=291.1 e V ) - shake-up satellite peak due to n - n* transitions in aromatic systems [22-25]. Ols spectra reveal of the presence of three peaks: peak I (BE=531.3eV) corresponding to ketone C=O groups, peak II (BE=532.85eV) hydroxyls or ethers, and peak III (BE=534.0eV) - the oxygen atoms in carboxyl groups. In Table 1 the content of carbon and oxygen atoms for the resin PHD is presented. For comparison, the content of carbon, oxygen and nitrogen atoms for the polymer MEA is also given. According to these data it may be presumed that the PHD polymer surface contains mainly carbon atoms. A part of them bear oxygen functional groups C=O, C-OH, C-O-C and COOH. These groups are strongly polar adsorption sites interacting specifically with polar adsorbates; however, hydroxyl groups predominate. The atomic composition of MEA surface is quite different. The amount of carbon and oxygen is lower in comparison to the resin PHD; however, three types of nitrogen species were found [26]. Table 1 Surface composition of the PHD and MEA [26] resins as determined by XPS Atom % Resin Cls
Nls
Ols
PHD
85.15
-
15.00
MEA
50.48
45.46
4.06
(CH/CC- 1.97)
494
A
4O
v
1
294
290
286
282 Binding Energy
[eV]
B
1,4
t0
i
.
.
.
.
.
.
I
,
.1
,
I,
.
i.,
,
z
_
Binding Energy [eV]
Figure 1. XPS Cls (A) and Ols (B) spectra for the phenolic-formaldehyde resin PHD. 3.2. Adsorption from gas phase Data of low-temperature nitrogen adsorption were used to evaluate the parameters characterizing the pore structure of the obtained polymeric materials in dry state. The BET specific surface area, SBET, and the total pore volume, Vt, were estimated by applying the standard methods (SBETfrom the linear BET plots and Vt from adsorption at relative pressure p/po=0.975) [7]. The mesopore structure was characterized by the distribution function of mesopore volume calculated by the Barret-Joyner-Halenda (BJH) method [27]. In Table 2 the values of these parameters are given for both synthesized polymers. The melamine2 3 formaldehyde resin MEA has a more developed pore structure (SBEr=220m/g, Vt=0.45cm/g) and narrower mesopores (D=7.3nm) in comparison to the phenolic-formaldehyde polymer PHD. Table 2 Values of parameters characterizing the porous structure of polymeric sorbents calculated from nitrogen adsorptio~desorption !s0therms [20,25 ] . . . . . .
SBEr
Vt
D
[mZ/g]
[cm3/g]
[nm]
PHD
125
0.28
11.7
MEA
220
0.45
7.3
Resin
495
In order to study the adsorption/absorption processes on both synthesized resins, sorption isotherms of benzene and water vapor were measured. In Figure 2 the benzene isotherms on PHD and MEA resins are shown. For all studied experimental systems one can observe hysteresis loops typical for mesoporous materials. All measured isotherms were nonreversible, which accounts for a mixed adsorption/absorption character of the sorption process and swelling of the resins in contact with both sorbates. Both benzene and water vapor were more strongly sorbed by the resin MEA, which reflects its greater porosity and shows a high share of physical adsorption in the total sorption phenomena for both polymers. The thermogravimetric experiments conducted earlier indicated swelling of cross-linked melamine-formaldehyde polymers in water [21,28]. The mass of liquid taking part in swelling process may be estimated on the basis of thermal desorption curves measured for a nonporous sample of the same polymeric material [6]. Thus, the measurements of thermodesorption under quasi-isothermal conditions made it possible to estimate the degree of swelling. We estimated that water absorption for studied polymers did not exceed 20-25 w/w%. In order to compare the sorption processes on the synthesized polymers, the experimental sorption values were expressed as the volumes of liquid sorbate per gram of solid. These isotherms are presented in Figure 3 in the co-ordinates: a [cm3/g] vs. P/Po and in a logarithmic scale loga vs. log(p/po) exposing the sorption behaviour over the range of low relative pressures. In comparison to nitrogen isotherms showing the resin pore structure in dry state, the sorption of benzene is stronger as a result of physical adsorption in mesopores and absorption in the polymer network. However, for very low relative pressures nitrogen adsorption is higher than that of benzene or water. Considering the structure of the investigated porous polymeric resins one can find three possible types of micropores: those formed between polymer domain boundaries and on the domain surface, and those originating from the imprint of colloidal silica template. Nitrogen molecules can easily penetrate these small pores, thus, for small pressures its adsorption is relatively high.
~ PHD/benzene -A- MEA/benzene
=o
§
,Y
4
~ ,8
/'
§
/ '~"
"Z~'"Z~"" 2
.~" ,.I'<>-
1
6
-dr MEA/watervapour
, . /
Z~ ...Z~"
4
.0/~'//
.0"--
~ ~
2
0
0
0.2
0.4
0.6
0.8
9
1
0
0 0.2 0.4 0.6 0.8 P/Po P/Po Figure 2. Benzene and water vapor adsorption/desorption isotherms on PHD and MEA resins.
496
The mechanism of water vapor adsorption on solids like the investigated polymers is very complex. However, it is generally agreed that the main factor is the presence or absence of surface hydrophilic groups; one of the proposed mechanisms of adsorption on solid surfaces of mixed, hydrophobic - hydrophilic character is formation of clusters of chemisorbed water on specific active sites. Both investigated polymers are differentiated with respect to hydrophobicity of the internal network. In the case of PHD polymer polar hydroxyl groups are present mainly on the external surface. However, for MEA sample, due to the presence of nitrogen heteroatoms inside the polymer network, the hydrophobic character is depressed in the whole bulk material. As a result adsorption/absorption of water at lower pressure is very low for phenolic resin. It is worth to note that within relative pressures p/po
0.5 - -O- - PHD/nitrogen
E u s_,
'~
PHD/benzene
A
~'~--- 0.4
MEA/nitrogen
?~1
&
MEA/benzene
I f " "~
---•-- MEA/wat
-9- I - - PHD/water vapour
0.3
r
0.3 0.2 0.2 ..O. j , gill
0.1
0.1"
0~
~
~
0 0
0.2
0.4
0.6
0.8
0
0.2
0.4
0.6
0.8
plpo
plpo
- -l- - PHD/nitrogen
E u
&
- -o- - MEA/nitrogen
PHD/benzene
&
---I-- PHD/wat
ICl
0.1
MEA/benzene
--El.9 MEA/water vapour
0.1
0 -.0 ..
. . ~ . . i t . . . i . j /
El'"
...l
m'" ...o'"
'B-
"
0.01
0.01 0.01
0.1
0.01
plpo
0.1
1
plpo
Figure 3. Comparison of nitrogen, benzene and water vapor adsorption isotherms on PHD and MEA resins.
497 4. CONCLUSIONS The synthesis of phenolic-formaldehyde and melamine-formaldehyde resins in the presence of fumed silica allows obtaining porous organic materials with a differentiated porous structure and surface properties. The pore characteristics of the studied resins in dry state were determined from nitrogen adsorption isotherms. The differences in surface character of the synthesized polymers were estimated satisfactorily by XPS spectra showing the presence of various functional groups. The adsorption/desorption mechanism of water and benzene on the investigated porous polymers was different due to differentiated hydrophobicity of the bulk material.
Acknowledgement The authors would like to thank the State Committee for Scientific Research (KBN, Warsaw), (Project No. 3 T09B 007 17) for financial support.
REFERENCES 1. .
.
4.
10. 11.
12. 13. 14. 15. 16. 17.
L.D. Belyakova, A.V. Kiselev, N.P. Platonova and T.I. Shevchenko, Adv. Colloid Interface Sci., 21 (1984) 55. M.P. Tsyurupa, A.S. Shabaeva, L.A. Pavlova, T.A. Mrachkovskaya and V.A. Davankov, in "Characterisation of Porous Solids IV", Eds., B. McEneney, T.J. Mays, J. Rouqu6rol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger, The Royal Society of Chemistry, London 1997, p. 398. H. Omidian, S.A. Hashemi, P.G. Sammes and I. Meldrum, Polymer, 39 (1998) 6697. W.-Y. Chuang, T.-H. Young, D.-M. Wang, R.-L. Luo and Y.-M. Sun, Polymer, 41 (2000) 8339. Ch. Soles and A.F. Yee, J. Polym. Sci., Part B: Polym. Phys., 38 (2000) 792. J. Goworek, W. Stefaniak and W. Zgrajka, Mat. Chem. Phys., 59 (1999) 149. S.J. Gregg and K.S.W. Sing, "Adsorption, Surface Area and Porosity", Academic Press, 1982. C.A. Fyfe, G.C. Gobbi and G.J. Kennedy, J. Phys. Chem., 89 (1985) 277. H. Brumberger (Ed.), "Modern Aspects of Small-angle Scattering", Kluwer Academic Publisher, Dortrecht 1995. J. Goworek and W. Stefaniak, Colloids Surfaces, 80 (1993) 251. W.L. Wiliams, K. Yong-Wals and M. Douglas, in "Characterisation of Porous Solids III", Eds., J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger, Elsevier, 1994, p. 301. T. Goworek, K. Ciesielski., B. Jasifiska and J. Wawryszczuk, Chem. Phys. Lett., 272 (1997) 91. P.W. Schmidt, D. Avnir, D. Levy, A. H6hr, M. Steiner and A. Roll, J. Chem. Phys., 94 (1991) 1471. T.W. Perkins, T.W. Root and E.N. Lighfoot, Anal. Chem., 69 (1997) 3293. M. Brun, A. Lallemand, F. Jean and Ch. Eyrand, Thermochimica Acta, 21 (1997) 59. K. Jerabek, A. Revillon and E. Puccili, Chromatographia, 36 (1993) 259. M. Ousalem, X.X. Zhu and J. Hradil, J. Chromatogr,. A 903 (2000) 13.
498 18. B.J. Brune, J.A. Koehler, P.J. Smith and G.F. Payne, Langmuir, 15 (1999) 3987. 19. S. Kiatkamjornwong, S. Traisaranapong and P. Prassassarakich, J. Porous Mat., 6 (1999) 205. 20. A. Deryto-Marczewska, J. Goworek and W. Zgrajka, in "Fundamentals of Adsorption", in press. 21. A. Deryto-Marczewska, J. Goworek and W. Zgrajka, Langmuir, 17 (2001) 6518. 22. E.T. Kang, K.G. Neoh, and K.L. Tan, in "Handbook of Organic Conducting Molecules and Polymers"; Ed., H.S. Nalwa, John Wiley, Chichester, 1997; vol. 3, chapter 3. 23. G. Beamson, and D. Briggs, "High Resolution XPS of Organic Polymers. The Scienta ESCA 300 Data Base"; John Wiley, Chichester, 1992. 24. S.L. Lim, K.L. Tan and E.T. Kang, Langmuir, 14 (1998) 5305. 25. K. L~iszlo, E. Tomb~icz and K. Josepovits, Carbon, 39 (2001) 1217. 26. A. Deryto-Marczewska, J. Goworek, S. Pikus, E. Kobylas and W. Zgrajka, Langmuir, submitted for publication. 27. E. P. Barrett, L. G. Joyner and P. P. Halenda, Am. Chem. Soc., 73 (1951) 373. 28. A. Deryto-Marczewska, J. Goworek, R. Kusak, W. Zgrajka, Appl. Surf. Sci., submitted for publication.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
499
Porous structure of multifunctional mineral - carbon and zeolite-carbon sorbents E. B. Dr~g, M. Kula~yfiski, J. Kaczmarczyk
Institute of Chemistry and Technology of Petroleum and Coal Wroclaw University of Technology, e-mail: [email protected] ul. Gdar~ka 7/9, 50-344 Wroctaw, Poland Presented are the examinations of the multifunctional mineral-carbon and zeolitecarbon sorbents prepared from kaolinite with an admixture of carbonaceous materials: industrial waste deposits, municipal sewage sludge and cellulose. The mixture of raw materials was thermally and hydrothermally pretreated in order to facilitate their specific structure. The parameters of capillary structure (micro and mesopores) were determined. For examinations of porous structure the mercury porosimetry method was used. In order to evaluate the solid phase transformation during the each step of sorbent preparation the SEM observation with quantitative X-ray microanalysis were made. 1. INTRODUCTION Clay minerals, natural and synthetic zeolites, silica and aluminum oxide forms generally are a mineral phase in mineral-carbon adsorbents. Natural aluminosilicates, particularly zeolites, due to the existence in their structure of ultramicropores and micropores (with pore diameter below 2 nm) with hydrophilic properties, exhibit high sorption capacity for particles of water vapor as well as sieve properties. They also demonstrate very good ion exchange properties. For instance, the ion exchange capacity of zeolite NaA is about 700 mval/100 g. The regular microporous structure of zeolites, can additionally be modified with ion exchange. The high temperature resistance, even to 873 K, with simultaneous maintaining the sorption activity appeared to be an extremely useful property of zeolites. For these reasons zeolites are frequemly used in drying operations, for separation of undesirable components of gases and liquids, including volatile hydrocarbons, and radioactive isotopes from solutions. Particularly attractive method for preparation of synthetic zeolite is recrystallization of natural aluminosilicates, such as kaolinite (halloysite), previously formed for elimination of plastic flow of highly thixotropic, pulverized zeolite. Some additional components of initial mixtures, such as texture modifiers (hard coal, lignite, cellulose, silica, aluminum oxide) are also introduced. They enrich the structure of zeolite adsorbent in transport pores and prevent an excessive compression of the clay material during the formation process. This results in an increase in product efficiency during the crystallization of zeolite phase. Active carbons, cokes and other substances exhibiting in their structure the presence of pores with wide range of diameters, can be the components of carbonaceous phase [1,2]. The following pores exist in the structure: ultramicropores with diameters below 0.4 nm, micropores- accessible for benzene particles- with diameters within 0 . 4 - 2 nm, transport mesopores ( 2 - 50 nm) and macropores (above 50 nm).
500 The pores carbon phase indicate hydrophobic properties, responsible for the affinity for particles of organic compounds. Due to low specific surface of macropores (0.5 - 2 ma/g), their adsorption capacity is considered negligible. However they play the role of the transportation routes, enabling the diffusion of adsorbed substances to narrower pores. The similar function can be attributed to intermediate pores - mesopores with the specific surface in the range of 10 - 400 m2/g. The adsorbent consisting of both phases: mineral and carbon phase combine the properties of both inorganic material and active carbon. However, the properties of mineralcarbon adsorbents are not additive. Therefore, the synergetic effect can be observed in this type of adsorbent and the activity of resultant adsorbent is higher than the activities of individual components. The zeolite-carbon adsorbents from mineral-carbon adsorbents group are novel and exhibit not quite well recognized properties with their unique, modified porous structure. The characteristic structures for zeolite, active carbon and intermediate structure exist in these materials. Such a structure results from the modification of a surface of a mineral matrix by depositing carbon material. The effectivity of enrichment of the structure of zeolite-carbon adsorbents (in relation to crystalline zeolite structure) in hydrophobic micropores (0.4 - 2 nm) and rnacropores (above 50 nm) is proportional to the fraction of carbon phase. Such combination of hydrophilic properties of mineral phase and hydrophobic properties of organic phase results in various sorptive properties of the material and the range of their application can be consequently extended. Additionally, the chemical resistance of these adsorbents for their exploitation in aggressive conditions takes place. An alternative method of preparation of zeolite-carbon adsorbents is the treatment of mixtures: clay mineral with hard coal and waste carbon deposits. The treatment consists of several physicochemical processes i.e.: formation, carbonization, activation and crystallization, presented in this paper. The adsorbents prepared with this procedure are not a simple mixture of two components but strongly dispersed material resulting from thermochemical transformation, thus facilitating the surface structure. 2. EXPERIMENTAL
The adsorbents have been prepared from the halloysite (H) - mineral from kaolinite group with an admixture of carbonaceous materials: refinery waste deposits (RS1), sediment communal sewage (CSew) and cellulose (Ce), and the fraction of these admixtures were within 30 - 70 wt.%. The mixture of raw material was thermally (carbonaceous materials carbonization, 973 K) and hydrothermally (crystallization of the amorphous metahalloysite in alkaline solution to zeolitic structure of NaA type, 373 K) pretreated in order to facilitate their specific structure [1,2]. The parameters of pore structure (micro- and mesopores) were determined on the basis of specific sorption examinations. Benzene and carbon dioxide were applied as sorption agents taking into account their particle shape and dimensions. The experiments were performed in high vacuum gravimetric apparatus with Mc Bain-Backr balance at 298 K. The distribution of mesopores (with a diameter within 2-50 nm) volume and surface was calculated from Kelvin equation according to benzene desorption isotherm [3]. The volume of micropores (with a diameter within 0.4-2 nm) was determined from benzene sorption isotherm. The volume of pores determined from Gurvitsch law and mesopores volume were used for calculations. The volume of submicropores with diameter below 0.4 nm (unattainable for benzene particles) was calculated as a difference between volume of micropores accessible only for CO2 (found from Dubinin- Raduszkiewicz equation) and volume of micropores accessible for benzene [4].
501 For examinations of porous structure of manufactured sorbents, the mercury porosimetery method, enabling determination of pore radius in the range 1.87 - 7500 nm, was used. In order to evaluate the solid phase transformation during each step of sorbent preparation the SEM examinations with qualitative microanalysis were conducted. The results of investigations for manufactured adsorbents were compared with those obtained using a similar method with hard coal (gas - coking coal), industrial active carbon and zeolite type A. 3. RESULTS AND DISCUSSION
The distribution of pores volume in carbonized coal deposits (Car), mineral-carbon and zeolite-carbon adsorbents (A) under examination, as well as those included in commercial standards is given in Table 1 and illustrated by adsorption isotherms for carbon dioxide and benzene in Figures 1 - 4. The observed results of carbon dioxide adsorption were presented in the form of CO2 isotherms in Fig. 1 and 2. Active carbon is a typical example for hydrophobic adsorbents whereas zeolite 4A is an example for hydrophilic adsorbents. For the former, specific pore volume accessible for benzene is 0.227 cm3/g, whereas for the later, these pore volume was an order of magnitude lower, i.e. 0.021 cm3/g. The volume of other pores, inaccessible for benzene is 0.017 cm3/g for carbon and 0.172 cm3/g for zeolite. It can be noticed, that active zeolite-carbon adsorbents have to be characterized by maximally developed structure of both kinds of pores. According to the data presented in the Table 1, the fraction of pores inaccessible and accessible for benzene molecules was essentially affected not only by fraction of introduced carbonaceous deposit to the adsorbent material, but also by its quality and circumstances of carbonization process. 2,5
3,5 3,0
2,0 -
~o
=1,5
~'
~ 2,0 1,5
]
1,o
C~ 0,5
0,5 0,0
0,0 0
100 200 300 400 500 600 700 CO2pressure, mm Hg
0 100 200 300 400 500 600 700 CO2 pressure, mm Hg
Fig. 1. Isotherms of CO2 adsorption for adsorbents Fig. 2. Isotherms of CO2 adsorption for standards with sediment of communal sewage, and zeolite-carbon adsorbent with kaolin and coal.
502 Table 1 The effect of carbonization, activation and calcination of mixtures of clay minerals and carbonbearing materials on the structure of pores with diameters below 50 nm for chars, mineralcarbon and zeolite-carbon adsorbents. Pore volume, cm3/g Samples
micropores micropores mesopores <0.4nm
micro + mesopores <50nm
Pore Surface volume area WoC02 SOC02 cma/g m2/g
0.4-2nm 2 - 5 0 n m Refinery sludge Car 100RS1 0.002 0.005 0.048 0.055 0.007 18 Car 50H/50RSI 0.008 0.009 0.122 0.139 0.017 45 A Car 70H/30RS1 0.115 0.011 0.060 0.186 0.126 331 A Car 50H/50RS1 0.109 0.017 0.083 0.209 0.126 331 A Car 30H/70RS1 0.086 0.021 0.116 0.223 0.107 281 Sediment communal sewage Car 100 Csew 0.028 0.017 0.015 0.060 0.045 118 Car 50H/50 CSew 0.010 0.014 0.066 0.090 0.024 63 A Car 50H/50 Csew 0.081 0.021 0.058 0.160 0.102 268 Cellulose Car 100 Ce 0.006 0.063 0.014 0.083 0.069 181 Car 50H/50 Ce 0.000 0.019 0.040 0.059 0.019 50 A Car 50H/50 Ce 0.089 0.030 0.050 0.169 0.119 313 0.119 0.007 0.064 0.190 0.126 331 A50H/50Ce (calc.823K)* Hard coal Car 100 Hc 0.000 0.140 0.066 0.206 0.128 336 Car 50K/50 Hc 0.014 0.022 0.018 0.054 0.036 95 Act50K/50 Hc 0.008 0.067 0.024 0.099 0.075 197 A Car 50K/50 Hc 0.095 0,034 0.040 0.169 0.129 339 A Act 50K/50 Hc 0.154 0.076 0.049 0.279 0.230 604 Commercial standards Zeolite NaA 0.172 0.003 0.015 0.190 0.175 461 Active carbon 0.017 0.143 0.084 0.244 0.160 461 *Zeolite adsorbent A50H/50Ce was prepared by the calcination of carbon phase from the zeolite-carbon sample ACar50H/50Ce at 823 K The comparison of pore volume distribution for chars only indicates that they are very different. The char of oiled refinery sludge was enriched in mesopores, the char of communal sewage sedimem was enriched mainly in ultramicropores, and the char of scrapped cellulose was enriched in micropores, whereas hard coal char was found to be enriched in micro- and mesopores. Generally, the assumption was apparently confirmed that chars of carbon deposits create the pores accessible for benzene molecules with dimension above 0.4 nm. The admixture of clay mineral (halloysite) to carbonaceous deposit (as for waste materials) remarkably enriches the texture of mineral-carbon adsorbems in mesopores and leads to the decrease of magnitude of micropores volume with dimension of 0 . 4 - 2 nm. In the case of hard coal and kaolinite mixture this char contains the maximum sub- and micropores at the lowest content of mesopores. The hydrothermal crystallization process of mineral-carbon adsorbents results in appearing of
503
0,9 |/ ~o 0,8
--O-- Car50H/50CSew -O--CarCSew
]]
eat) o
E
0,7
2,5
--0-- Active carbon _ 1 - D - Zeolite NaA
] , ~
2
~s 0,6 "~ 1,5
"~0,5 ~ 0,4
1
~ 0,3
=o
m 0,5
0,2
0,1
0
0
0 0
0,2
0,4
0,6 p/po
0,8
Fig. 3. Isotherms of benzene adsorption for adsorbents with sediment of communal sewage.
0,2
0,4
0,6
0,8
1
1
p/po Fig. 4. Isotherms of benzene adsorption for standards and zeolite-carbon adsorbent from kaolin and gas-coking coal.
ultramicropores in porous structure. This is provides an evidence that metahalloysite and metakaolinite crystallized to the zeolite structure. The samples of zeolite-carbon adsorbents were also richer in micropores than mineral-carbon adsorbents. Therefore, recrystallization of aluminosilicate in alkaline medium to zeolite structure enriches the zeolite-carbon adsorbents in hydrophific pores with dimension <0.4 and 0.4-2.2 nm. The fraction of carbon deposit introduced to the raw material mixture apparently influences the character of zeolite-carbon adsorbents. It mainly increases the specific volume of mesopores and, to a lesser extent, the pores with dimension of 0.4 - 2 nm. On the other hand, the increasing fraction of mineral phase resulted in increase of ultramicropores specific volume. It is a result of growing yield of forming zeolite phase in the adsorbents. The presence of transport pores leads to the higher reaction extent of amorphous aluminosilicate with sodium hydroxide and facilitates crystallization of zeolite. The most of developed pore structure was detected for A Act50 K/50 Hc adsorbent, prepared from the gas - coking coal and kaolinite mixture, subjected to the thermochemical activation alter carbonization process. The thermochemical activation step resulted both in opening of carbon micropores with diameter of 0.4 - 2 nm and in slight increase of mesopores number. It was also observed that zeolite-carbon-adsorbent evidently enriched in ultramicropores alter hydrothermal reaction. The zeolite-carbon adsorbents with developed structure of micropores exhibit high magnitudes of structure coefficients: Woco2 and Soco2. These magnitudes can be specified in an ascending order: Car50K/50He < Aet50K/50He < ACarSOK/50He < AAetSOK/50He for the successive steps of preparation. The structure coefficients for AAct50K/50Hc sample are: pore volume - 0.230 cm3/g and specific surface - 604 m2/g. These magnitudes exceed the data for commercial samples. For zeolite NaA and active carbon Woco2 increased for 32 % and 12.5 % while for Soco2 increased for 24 % and 34.5 %, respectively. It should be emphasized, that such exceptional results were achieved for mineral-carbon adsorbents subjected to the
504
thermochemical activation. The zeolite-carbon adsorbents prepared from chars subsequently produced with the admixture of waste carbon deposits and hard coal, exhibited similar structure parameters: W0co2 - 0.102 - 0.129 cm3/g and S0co2= 268 - 339 m2/g The porosimetric investigations performed for characterization of porous structure of meso- and macropores range (1.87 - 7500 nm) indicated that higher total specific volumes of mineral-carbon adsorbents were achieved than for the analogical zeolite carbon adsorbents. These magnitudes observed for samples Car50H/50RSI and ACar50H/50RSI were 0.689 and 0.641 cm3/g, respectively. For activates prepared with admixtures of hard coal and subjected to the thermal-chemical treatment, Act50K/50Hc and AAct50K/50Hc, the values were 0.882 and 0.750 cm3/g, respectively. It was assumed that the presence of zeolite crystals deposited inside the transport pores reduces the pore volume in comparison with char's samples without zeolite phase. According to the presented microscopic and structural investigations the individual samples of zeolite-carbon adsorbents are differentiated with regard to quality of raw material used. The zeolite crystals .and carbon phase were found in all examined samples. In adsorbents prepared with the presence of hard coal the semicoke phase was the matrix for zeolite crystals deposition. These adsorbents exhibited remarkably higher mechanical strength. Carbonaceous materials do not form the adsorbent's matrix, they are deposited on the aluminosilicate frame. The following impurities were introduced into the structure of adsorbents with raw materials: ilmenite, hematite (with clay minerals) and other impurities (Fe, Ti, Ni, Ca, V, S) with carbon deposits (Fig. 5-10).
Fig. 6. SEM micrographs and analysis of zeolite-carbon adsorbent ACar50H/50RS1. Magnific. 3000x.
505
Fig. 9. SEM micro graphs and analysis of char Car 50H/50 Ce. Magnific. 3000x.
506
Fig. 10. SEM micrographs and analysis of zeolite-carbon adsorbent ACar50H/50Ce. Magnific.3000x. 4. CONCLUSIONS 1. The usability of waste carbon deposits was demonstrated for preparation of active adsorbents with both zeolite and active carbon properties. 2. It was indicated during the performed investigations that porous structure formed during the carbonization, activation and crystallization processes. 3. The pores in zeolite-carbon adsorbents exhibited dual hydrophobic-hydrophilic character. 9 The ultramicropores structure can be attributed to the existence of zeolite phase and exhibits hydrophilic superficial properties. 9 Within the micropores both hydrophilic and hydrophobic pores, of mineral and carbon origin, could be detected. The rich carbonaceous materials (gas - coking coal, cellulose) enriched the adsorbents structure in the hydrophobic micropores. 9 Mesopores exhibited dual properties of mineral and carbon origin. 9 The specific macropores volume for mineral-carbon adsorbents decreases in the crystallization process. This can be attributed to the deposition of zeolite crystals inside the solid structure. ACKNOWLEDGEMENT
Polish State Committee for Scientific Research has supported this work. Grant No.6 P04G 055 18 REFERENCES
[ 1] K. Bratek, W. Bratek and E. B. Drag, 1st Intem. Conf. On Carbon ,,Eurocarbon 2000", Berlin, Deutsche Keramische Gesellschafl, K~ln (2000) 565. [2] E. B. Drag, 1st Intern. Conf. On Carbon ,,Eurocarbon 2000", Berlin, Deutsche Keramische Gesellschait, KSln (2000) 567. [3] A. Jankowska, T. Siemieniewska, K. Tomk6w, M. Jasieb&o-Hatat, J. Kaczmarczyk, A. Albiniak, J. J. Freeman and M. Yates, Carbon, 31, 6 (1993) 871. [4] T. Siemieniewska, K. Tomk6w, J. Kaczmarczyk, A. Albiniak, Y. Grillet, M. Francois, Energy & Fuels, 4 (1990) 61.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
507
Waste air cleaning using activated carbon fibre cloths regenerable by direct electric heating E. Schippert, C. M0hner, B. Pannwitt, Horst Chmiel Institute for Environmentally Compatible Process Technology, Im Stadtwald 47, D-66123 Saarbrficken A new process was developed for VOC adsorption and desorption on activated carbon fibre cloths (ACC) using electric current to directly heat the cloth for desorption purposes. Besides the industrial application of this process for waste air cleaning, a new measurement method for isotherm fields for VOCs on ACC was developed, enabling isotherm fields in broad temperature (20~176 and VOC concentration ranges (5-200 000 mg/m 3) to be measured within a few days. It is possible to describe the isotherm fields measured with sufficient accuracy by using a modified Langmuir isotherm model considering the dependence of adsorption enthalpy and adsorption entropy on ACC loading. 1
INTRODUCTION
In numerous industrial processes, VOCs and/or air streams containing odorous substances, sometimes in very large quantities, are generated by production. These substances must be concentrated if they are to be disposed of economically. Conventional concentration processes are usually limited to a factor of 10 or a maximum of 20. Due to considerably faster adsorption kinetics, compared to commercial granular activated carbon as well as more favourable isotherm trends for low concentrations, commercial activated carbon cloths (ACC) are particularly suitable for VOC removal at low concentrations ranging from several lag/m3
Fig. 1: Principle of the process- lab-scale adsorber
508 plants: a laboratory adsorption unit for a maximum of 30 m3/h; an equilibrium apparatus for measuring adsorption isotherms; a pilot plant for approx. 1000 m3/h. 2
MEASUREMENT RESULTS AND DISCUSSION
A test programme in the laboratory adsorption unit was implemented to obtain basic data on the use of activated carbon fibre cloths in adsorption and desorption processes. The test series was performed using the substances, acetone, isopropanol, toluene, glycole ether acetate, and decane, thus providing information on the adsorption behaviour of the three relevant groups of substances, namely low, medium and high boilers. The results and experience gained were subsequently used for upscaling to pilot level. The determination of the isotherm fields in concentrations ranging from 10-100 000 mg/m 3 and at temperatures from 60~ to 240~ which was required to describe the entire process, was carried out in the equilibrium apparatus.
Adsorption A raw gas concentration of between 10-2000 mg/m 3 was selected so as the tests in the laboratory adsorption unit and pilot plant could be conducted in an acceptable time span. Flow velocity during adsorption was 0.1-0.5 m/s while relative humidity was set at between 20% and 70%. Each of the measurements was performed at room temperature (20~ to 25~ The number of ACC layers used in the tests varied between 2 and 10 layers, depending on the parameter to be measured. Table 1 contains specific data on the plant as well as the test conditions. Table 1: Typical specific plant data and test conditions. Dimensions of adsorber module (height*width*length): Structural parameters of the plant 1172 mm * 770 mm * 1422 mm (outside) No. of filters: 3 Filter dimensions: 900 mm * 300 mm * 5 layers ACC Total mass of the ACF cloth : 1012.5 g No. of distributing pipes: 3 Volume flow: 900 m3/h; Gas velocity: 0.3 m/s Specific test conditions for Temperature: 22-23~ Relative humidity: 40-45% adsorption Raw gas concentration: 213 mg/m 3 tolouene Clean gas concentration: up to max. 20 mg/m 3 Average clean gas concentration: 2.33 mg/m 3 Pressure loss: 300 Pa Adsorption time before breakthrough: 59.75 min Volume flow: 30 m3/h; Gas velocity: 0.01 m/s Specific test conditions for Desorption time: 15 min; No pre-heating desorption Rated value for concentration: 10.000 mg/m 3 solvent Final temperature: 200~ Max. power supply: 9.25 KW/m 2 Average power supply - regeneration: 3.5 KW/m 2 Average energy requirements for 1.000 m3/h based on Specific energy requirements 3/ adsorption and desorption: 0.66 KW for 1000 m h
509
Fig. 3: Typical breakthrough curve for toluene Fig. 4: Specific depletion factor (per layer) as a function of gas velocity Since each fibre has a very small diameter of roughly only 10 ~tm, the volume-related rate of mass transfer for the cloth is approximately 100 times higher than that of granular activated carbon. Therefore only a few layers of ACC are needed to achieve high rates of efficiency. A degree of efficiency of more than 99% is possible, using a joint thickness of only 5 mm of ACC layers (Fig. 3). This is particularly significant for industrial applications since pressure loss for this thickness is very low (compared to bed height of granular activated carbon needed to achieve similar efficiency levels). The VOC depletion factor for each layer is roughly 3 at v = 0.3 m/s. Fig. 4 shows the expected trend when mass transfer resistance is predominantly in the gaseous phase. Depletion factor (raw gas concentration/clean gas concentration), working capacity (massrelated loading as a percentage for each test taking into account the residual loading present after the previous test) and the adsorption time before breakthrough (set at 10% of the raw gas concentration value) were considered as important parameters for evaluating the measurement results. Adsorption isotherms Accurate information on the isotherm trends of various VOC groups (in relation to the individual boiling points of low, medium and high boilers) was essential in order to describe the entire process and to elaborate a suitable design for the plant. The isotherms of various VOCs were measured on a commercial microporous ACC using the equilibrium apparatus previously described. The specific surface of this activated carbon fibre cloth was 1000 mVg, pore diameter was between 1 and 2 nm and the specific pore volume for solvents was 0.45 cmVg. Figure 5 shows an example of the way in which the individual isotherm points were determinedl Ten minutes after the solvent has been injected into the loop, concentration is determined in the gaseous phase by a flame ionisation detector (FID) or process chromatograph. The gas phase concentration was measured for a maximum of 2 minutes so that equilibrium was disturbed as little as possible. Each individual loading process can be determined after a re-balancing, which takes into account the operating conditions, is conducted. Thus, each measurement point is expressed as one point on the adsorption isotherm. The number of measurement points and the distance between them can be influenced by the quantity of the solvent injected. Adsorption isotherms were measured on ACC using the laboratory adsorption unit for concentrations ranging from 10 to 100000 mg/m 3 and in a temperature range of 60~ to 240~ as well as for toluene at 35~ and 45~ (Fig. 6). The new measurement method allows measurements to be performed for a
510
ioo
1 ~
Measurement
"-,.-.....~..
" ' ~
\\ ,w, o
u
1
Tlml"4[h]e
t
Nl,i0*C
2.s
~ 86mg
Dosage:
172rag
"eo~
~%\120"C
'
' ; o.1
....
t
1o
1DO
Gas phase concentration of toluene [g I m =]
264mg
Fig. 6: Adsorption isotherms for toluene on Fig. 5: Determination of the adsorption ACC ~ isotherms for toluene on ACC at 150~ gaseous phase concentration of approx. 5 mg/m 3 upwards. The measurement values can be described for all solvents in the entire temperature and concentration range by using a modified Langmuir isotherm. The adsorption isotherms for toluene on ACC, additionally measured at 35~ and 45~ by the conventional method (flow-through apparatus) coincide well with the results obtained with the new measurement method. This can be considered as verification of the new method. The evaluation of the measurement values showed that a Henry range begins only at loadings below 0.5%. The individual isotherms were determined several times to receive information on the reproducibility of the measurement values. It was shown that the trends of the individual isotherms were easily reproducible but that fluctuations in quality occurred as a result of the production process of the ACC. The isotherm equation and its limiting cases used to fit the measurement values will be presented briefly in the following. The requirements for the isotherm model were defined as follows: it should suitable to describe the measurement data in a temperature range of 20~ to 200~ and in a gas concentration range of 0.1 mg/m 3 to 200 000 mg/m3; it should be possible to correlate the parameters of the isotherm equation with chemical~physical properties of the adsorbate; it must be possible to extrapolate loadings at room temperatures from measurements taken at higher temperatures. The following approach using a modified Langmuir isotherm was elaborated:
C(X,T)=/Xmax(T-~~)-I i X
-1
"e
AH(X) (1 1 / R " 298.K .e-K(X)
AN(X) : (AHA - ARE)" exp(maH" X)+ AR E K(X) = const + mE-X + (AG o - const) .exp
lim X --> 0
C(X,T)
X
= KHenry (T) =
/ x/m'-- con., o0 9
1
Xmax (T)
lim C(X, T) - oo x --> Xrnax(T) with C in [g/m3], X in [wt.-%], AH in [kJ/mol] und K in [(g/m3)wt.-%]
511
8 AHA
1
1
4 ~
298 K
-,...~ "8 0
5'
iO
1'5 x [wt.-%!
iO
25
398 K 30
Fig. 7: Fitting parameters In Figure 7, the respective parameters are illustrated in a graph. The isotherm model selected is explicit in C and therefore is well suited for computer simulation of the entire process. Physically meaningful values result for the limiting cases. The average relative deviations between the measured and the calculated (for X = funct. (C)) loading values were in between 2 and 6% for the different solvents. They were in the same order of magnitude as expected for variations caused by the adsorbent properties due to variations in production quality of the ACC. This was documented by measurement series on quality. Adsorption enthalpy and its dependence on loading can be calculated from the isosteres measured. As for polar and low-boiling solvents, it was determined that AH depends only slightly on ACC loading whereas, in the case of nonpolar and high-boiling VOCs, AH shows a greater dependence on loading, resulting in a strong decrease in AH between 0% and 30% pore filling. Apart from acetone, AH is between 50 and 60 kJ/mol for all other solvents at higher loading. The errors in the adsorption enthalpies calculated from the isosteres were in the range of +/-10 kJ/mol at a maximum. Therefore, it is not possible to determine a potential temperature dependence of AHads from the measurement data. Only for VOCs with poor adsorbability (e.g. acetone) and for VOCs with very high boiling points does AHads at high degrees of pore filling approach the value of evaporation enthalpy AHeva. As for the other VOCs investigated, AHads was significantly higher than AHeva, indicating that a considerable interaction between adsorbent-adsorbate was present even at degrees of pore filling of 80-90%. By plotting the left side of equation 1 in Fig. 9 along X, all measurement points for each solvent can be situated along one curve. This fact justifies the approach of the isotherm equation. Xmax(T) was measured directly in a pressure apparatus. The isotherm fields measured could be described with sufficient accuracy by using a modified Langmuir isotherm model, considering the dependence of adsorption enthalpy and adsorption entropy on ACC loading [4]. Basically, it can be assumed that, considering the VOC adsorption and desorption processes currently applied for this concentration range, it is desorption which places limitations on the entire process. Adsorption wheels are generally used for enrichment purposes. The temperatures, possible during the desorption process (max. 140~ are the restricting factor [5]. Depending on the VOC used and its boiling point, VOCs with high boiling points can be concentrated on the adsorbent. The enrichment factors of the desorption gas are also limited (max. factor 10 - 20). Therefore large quantities of air are needed for the desorption process. As will be shown in the following, these constraints do not occur in the newly developed process involving the direct electrical heating of activated carbon fibre cloths.
512 18
"
'%
9
"
'~'o.
d c , x .,-, (x,,,,,,('r) -'I .... L,~-"JI
:[,
51
~(x) +-~ -
~,(x, (,
~', ,, ~ LY - i - ~ - ~ ) =
,
I]
14 ill~,. 6
"2 "4 6
32
ILoading [wt.-%] I
2
4
6
8
10
12
14
16
18
:20
'12
24
:26
28
30
Loading [wt.-%]
Fig. 9: Negative temperature-normalised Fig. 8: Enthalpy of adsorption of VOC on natural logarithm of the KLangmuir ACC Figure 10 shows the pilot plant set up at the Institute for Environmentally Compatible Process Technology (upt). The plant comprises a unit for raw gas conditioning in which humidity, temperature and solvent concentration can be set, as well as the adsorber unit. Total regeneration time is between 5 and 20 minutes, depending, to a great extent, on VOC load and concentration as well as the quantity of co-adsorbed water. It can be assumed that VOC equilibrium is established between gaseous and solid phase at a regeneration temperature between 120~ and 220~ and a desorption gas velocity of roughly 0.01 m/s. Enrichment factors (maximum regeneration gas concentration / raw gas concentration) of up to 1000 were achieved. During the desorption process, temperature on the ACC surface is measured and controlled without contact by a radiation pyrometer, allowing desorption temperature to be monitored and controlled without delay. The strong temperature dependence of ACC electrical resistance, in principle, can also be used for control purposes. As Figure 11 shows, the maximum desorption concentration increases as temperature rises. Increasing the temperature from 125~ to 155~ and from 155~ to 190~ results in the maximum concentration doubling. The regeneration time needed to reach the required desorption quality decreases as desorption temperature increases. Depletion factor, working capacity and adsorption time before breakthrough deteriorate at desorption temperatures below 155~ due to higher levels of residual loading.
Fig. 10: Pilot plant at the Institute for Environmentally Compatible Process Technology (upt)
513
Desorption with regular of concentration alter loading with relative humidity, T 922~ 5 layers ACC
I
350 m g l m s acetone, 42%
['\ ii- 235"cl
_E 100,00 q
'cc.::n,-
E 80,00 ._=
i
60,00
~, 40,00
! l / ~
P'! = 3.4 - 10.2 k W l m 2
..
e~
-i 200
[temperature last layer ACCI
100
2,500
20,00
50
1,000 0
5
10
Time in min
15
20
0
0
0 200
400
600
800
Time in s
Fig. 11" Desorption of toluene as a function of Fig. 12: Desorption desorption temperature concentration
with
controlled
By measuring the VOC concentration by a flame ionisation detector and using a control algorithm which takes into account temperature dependence C(T)x=constant, known from isotherm measurements, it is possible to control regeneration temperature so that a nearly constant regeneration gas concentration is maintained throughout the regeneration time. Thus the explosion limit can be adhered to even when air is used as regeneration gas. As Figure 12 shows, a set concentration can be specified and maintained [3]. 3
CONCLUSIONS
A new process for cleaning waste air streams containing VOCs and/or odorous substances was developed. The first plants are already under construction. Applications areas in the following industrial sectors can be earmarked for process implementation: 9 Solvent removal in the chemical or pharmaceutical industries, painting plants, printing works and the electronics industry, 9 Odour mitigation in the food industry, sewage treatment plants, animal carcass rendering plants, composting plants, waste disposal plants and refuse pits. The next objective is to develop the process further to accommodate very low VOC concentrations, particularly aimed at cleaning the incoming air to buildings and indoor areas. We would like to express our gratitude to Philip Morris Incorporated for partially funding this research work. REFERENCES [1]
[2] [3] [4] [5] [6]
M. Cal, ,,Characterization of Gas Phase Adsorption Capacity of Untreated and Chemically treated Activated Carbon Cloths", Thesis University of Illinios at UrbanaChampaign, (1995) HIDEN ANALYTICAL, ,,Gravimetric Analysers for the Characterisation of the Sorption Properties of Materials", Handbook, (1996) C. M6hner, Paper presented at GVC/DECHEMA Congress, Bingen 11./12.04, (2002) E. Schippert, Paper presented at GCV/DECHEMA Congress Bingen 11./12.04, (2002) G. Konrad, G. Eigenberger, ,,Rotoradsorber zur Abluftreinigung und L6semittelRiackgewinnung", Chem.-Ing.-Tech., Nr.3 (1994) VDI Guidelines, ,,Waste gas cleaning by adsorption- process and waste gas cleaning", VDI 3674, May (1998)
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Adsorption of inflammatory cytokines polymers and activated carbons
515
and endotoxin
by mesoporous
M.C. Murphy a, S. Patel a, G.J. Phillips a, J.G. Davies a, A.W. Lloyd a, V.M. Gun'ko b and S.V. Mikhalovsky a* aSchool of Pharmacy and Biomolecular Sciences, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2 4GJ, United Kingdom blnstitute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine Adsorption of E. coli lipopolysaccharide (LPS) and an inflammatory cytokine TNF-~ from model solutions on uncoated 'hyper-crosslinked' polystyrene polymers MN200 and MN500 and activated carbons Carboxen 1003 and Carboxen 1010 has been studied. It has been shown that TNFot can be efficiently removed by both non-functionalised MN200 and MNS00 functionalised with cation exchange functional groups. On the contrary, surface chemistry of the resins is important in LPS adsorption, MN500 being significantly more efficient than MN200. Presence of mesopores in Carboxen 1003 correlates with its higher adsorption capacity towards LPS in comparison with purely microporous Carboxen 1010. It has been suggested that hydrophobic interactions play an important role in TNF(z adsorption, whereas in LPS adsorption electrostatic interactions dominate the process.
1. INTRODUCTION Adsorbents are used in medicine mainly for the treatment of acute poisoning, whereas other extracorporeal techniques based on physico-chemical principles, such as dialysis and ultrafiltration, currently have much wider clinical applications [1]. Nevertheless, there are medical conditions, such as acute inflammation, hepatic and multi-organ failure and sepsis, for which mortality rates have not improved in the last forty years. These conditions are usually associated with the presence of endotoxin - lipopolysaccharide (LPS) or inflammatory cytokines - molecules of peptide/protein nature [2]. Advantages of adsorption over other extracorporeal techniques include ability to adsorb high molecular mass (HMM) metabolites and toxins. Conventional adsorbents, however, have poor biocompatibility. They are used coated with a semipermeable membrane of a more biocompatible material to allow for a direct contact with blood. Respectively, ability of coated adsorbents to remove HMM solutes is dramatically reduced. In this paper, preliminary results on adsorption of LPS and one of the most common inflammatory cytokines, TNF-ot, on uncoated porous polymers and activated carbons, are presented. The aim of this work is to estimate the potential of extracorporeal adsorption technique to remove these substances and to relate it to the porous structure of adsorbents.
To whom correspoiadence should be addressed.
516
2. MATERIALS AND METHODS 2.1. Adsorbents Commercial carbons of Carboxen series (Supelco, Inc.) are prepared from synthetic polymers, which are then converted to ion exchange resins and subsequently pyrolysed to form spherical, amorphous carbon sieves. In this work Carboxen 1003 and Carboxen 1010 were studied. Commercial resins MN200 and MN500 (Purolite Int., Ltd) belong to the series of Hypersol-Macronet T M 'hyper-crosslinked' polystyrene polymers. MN-200 is nonfunctionalised and MN-500 has strong acidic (sulphonic) functional groups. Prior to their use, adsorbents were demineralised with a 10% solution of concentrated HC1 and then washed with distilled water until the pH of the solution was between 6 and 7. 2.2. Adsorption of LPS LPS from E. coli serotype 0.111:B4 was obtained from Sigma-Aldrich. The RAW 264.7 cell line, derived from murine bone-marrow macrophages was obtained from the European Collection of Animal Cell Cultures (ECACC), Salisbury, UK. Cells were cultured in Dulbecco Modified Eagle Medium (DMEM) containing 10% v/v foetal calf serum (FCS) and 1% v/v penicillin/streptomycin. In the batch experiments, the response of macrophages to LPS in the presence of Tyrode buffer was measured. It was quantified using nitrite and protein assays [3, 4]. It was found experimentally that the seeding cell density of 7.5 x 105 cells gave the best response in the assays. Initial concentration of LPS was 10 pg mL -1. In experiments with MN resins, the ratio of adsorbent amount/volume of solution was 3 g in 15 mL of Tyrode buffer/DMEM. In experiments with Carboxens, the ratio was 1 g in 3 mL of Tyrode buffer/DMEM. 2.3. Adsorption of Inflammatory Cytokines Test solutions of TNF-ot (IDS Ltd.) were prepared in a Tyrode buffer containing 0.1% of bovine serum albumin, BSA, with final cytokine concentrations ranging from 0 to 1000 pg mL -l. Solutions (15 mL) were incubated at 25~ and shaken, (100 rpm) with 0.3 g of adsorbent for 6 hours. Aliquots of solutions taken at various times were analysed using ELISA kits (IDS Ltd.). 2.4. Characterisation of adsorbents Low-temperature nitrogen adsorption isotherms were recorded at 77.4 K using a Micromeritics ASAP 2010 adsorption analyzer. Specific surface area (Table 1, S,Er) was calculated according to the standard BET method [5, 6]. Table 1 Structural parameters of the adsorbents Parameter* MN200 MN500 Carboxen 1003 Carboxen 1010 S~Ev, m 2 g-1 1131 530 1168 827 SDs, m 2 g-i 768 353 1129 900 Vp, cm 3 g-1 0.773 0.442 0.850 0.46 VDs, cm 3 g-1 0.483 0.258 0.533 0.407 xDs, nm 0.63 0.47 0.471 0.436 * SsET is the specific surface area calculated using standard BET method; Vp is the total pore volume computed from adsorption at the maximal p/po value; parameters with the DS subscripts were computed using the modified Dubinin-Stoeckli equation [7].
517 The total pore volume Vp was evaluated by converting the volume of nitrogen adsorbed at p/po 0.98 (p and po denote the equilibrium pressure and the saturation pressure of nitrogen at 77.4 K, respectively) to the volume of liquid nitrogen. The mean pore half-width Xmea, (Table 1) was calculated as Vp/SaEr assuming a slit-like shape of pores. The pore size distributions (PSD) f(x) (Fig. 1) were calculated using the overall isotherm equation based on the combination of the modified Kelvin equation and the statistical adsorbed film thickness [8, 9]. The desorption data (as the overall isotherms) were used to compute the f(x) distributions with a modified regularization/singular value decomposition (CONTIN) [ 10] procedure under nonnegativity constraints for f(x) OC(x)>_0 at any x) with a fixed regularization parameter a = 0.01-0.001 re-normalized in comparison with PSD [9]"
xf(x)G Aew(X) = ~xf (x)clx Additionally, the PSDs were calculated using Micromeritics software with the Density Functional Theory (DFT) for the slit-like pore model. Based on the Dubinin's theory of volume filling of micropores, a modified DubininStoeckli (DS) equation was used to estimate micropore contribution with correction related to adsorption in mesopores [7]. The SDs, Vos and other parameters with the DS subscript (Table 1) were calculated over the pore range at the half-width xos = 0.2-1.0 nm. 3. R E S U L T S AND DISCUSSION
3.1. Polymeric adsorbents The PSD of MN200 and MN500 show the presence of micropores at the pore half-width x < 1 nm, narrow mesopores at x < 5 nm and larger pores at x up to 100 nm (Fig. 1). Relative contribution of micropores Fos/Vp is 0.62 (MN200) and 0.58 (MN500) as well as the shape of their PDSs reveal relatively greater contribution of large pores in MN500 than that for MN200. (a)
(b)
,-, 5001 0
400 E 300,
-
.oQ. 2001~ ~
500
/
' Carboxen ~ lOO3
1
-o <~176176
-0-- 1010
I I
I 0.0 0.2 0.4 0.6 0.8 1.0
OJ .
.
.
.
.
.
.
P/Po
.
.
.
"
e-
.o 200. c~
MN500 j
oo 100. "O < o
0.0 0.2 0.4 0.6 0.8 1.0 P/Po
Fig. 1. Nitrogen adsorption isotherms at 77 K. (a) for Carboxens; (b) for MN resins.
518
The PSDs calculated by the CONTIN procedure and the Micromeritics DFT are similar; i.e., represented estimations of fix) are quite reliable, which are in agreement with the structural parameters calculated by the DS method. Notice that the availability of broad mesopores x < 50 nm and transport pores at x > 50 nm (Fig. 1, a and b) is important with respect to adsorption of such biopolymer molecules as endotoxins or inflammatory cytokines of peptide/protein nature. 3.2. Carboxens From the analysis of the standard nitrogen adsorption isotherms (Fig. 1), and structural parameters (Table 1) related to the total porosity (Vp, SBEr) and microporosity (VDs, SDs), it follows that Carboxen 1010 is almost a purely microporous carbon, with VDs close to Vp, and SDs close to SsEr. Carboxen 1003 is a bi-porous carbon with a marked contribution of mesopores, as there is a substantial difference between VDs and Vp, as well as between SDs and SBeT (Table 1). Textural features of these carbons are also presented in Fig. 2, showing the pore size distributions. For Carboxen 1010 high intensity off(x) (Fig. 2c) is observed only at x < 1 nm. Significantly lower intensity is seen at x > 1 nm both for narrow and broad mesopores, as this adsorbent is almost purely microporous. The PSD for Carboxen 1003 (Fig. 2c) is akin to that of MN200 (Fig. 2a) with a marked contribution of mesopores at x > 10 nm that can be of importance for adsorption of larger biopolymer molecules. Although MN200 has parameters of pore structure superior to MN500 (Table 1), the latter is characterised by relatively larger contribution of mesopores (Fig. 2). 3.3. Adsorption of TNF-(z and LPS Results of adsorption experiments are presented in Table 2. In solution, TNF-cz, a polypeptide with MW 17 kD, exists as a trimer with MW 51 kD. The monomeric units are kept together due to the hydrophobic interaction. From the experimental data it is clear that large mesopores play a relatively minor role in adsorption of TNF-cz, as both MN resins adsorb it rather efficiently. Isoelectric point of TNFcz is similar to that of serum albumin (pI between 4.8 and 5.6) [11]. It has been shown that TNF(z can be removed from biological media using non-polar cross-linked polystyrene adsorbents of the Amberlite | type (XAD-2, XAD-4 and XAD-6) [12]. This observation confirms that hydrophobic interactions play an important role in the adsorption of TNFcz at neutral pH. Our data support this conclusion. a
0.050.04.
j~ II
MN200 1--0-- DFT
I
0.03-
2
CONTIN
~" 0"010t
0.02
llll
1 - - 0 - - DFT
c 0.04.
~
--O--Carboxen 1003 Carboxen 1010
2 0.02"
0.005]
0.01.
0.00
0.2
1
10
Pore Half-width
(nm)
100
0.000;
0.2
.
.
.
.
1
.
.
. .. . .. . .. ... . . 10
Pore Half-width
.
.
.
(nm)
100
0.00~ 0.2
.....................
1
10 (nm)
60
Pore Half-width
Figure 2. Pore size distribution in (a) MN200, (b) MN500, and (c) Carboxen 1003 and Carboxen 1010, calculated using a model of slit-like pores and the regularisation procedure CONTIN (a, b, c); and Micromeritics DFT (a, b).
519 Table 2 Removal of TNF-tx and LPS from solution, % of initial concentration Adsorbent TNF-ot* LPS** LPS*** MN200 82 negligible negligible MN500 96 35 81 Carboxen 1003 not measured 89 not measured Carboxen 1010 not measured 18 not measured * From a solution with initial concentration of 500pg mL l after 360 minutes, at 25~ Similar results were obtained at other initial concentrations. ** After 360 minutes, at 25~ *** After 24 hours, at 25~ Molecular mass of LPS varies, and its subunits of 10-20 kD normally aggregate into complexes with a molecular mass between 100 kD and 1,000 kD [ 13, 14]. Both hydrophobic interactions and electrostatic forces are responsible for the aggregation, which is also promoted by bivalent cations. In case of LPS, MN200 demonstrates much lower adsorption capacity than MN500. Additionally, MN500 has a very different surface chemistry (strong acidic groups) from MN200 (nonionic neutral surface), which perhaps is responsible for such a striking difference in their adsorption affinity towards LPS. Despite significant efforts put into finding an efficient adsorbent for LPS removal from biological media, factors affecting its adsorption are yet not well understood and the data published so far, are controversial. It is generally accepted that both hydrophobic and electrostatic interactions contribute to the retention of LPS by surfaces [15]. There is no agreement, however, about their relative contribution to LPS adsorption. The isoelectric point of endotoxins is between 1.3 and 2.0; therefore, endotoxins are negatively charged at neutral pH [15, 16]. For this reason, anion-exchangers and affinity adsorbents with basic ligands, such as poly-L-lysine, polyethylenimine, histidine or polymyxine B have been used to purify protein solutions and other biological media from endotoxins. On the other hand, it is well known that LPS is strongly adsorbed by hydrophobic polystyrene and bound to serum albumin, the latter also being negatively charged at pH above 5.0, thus indicating an important role of non-polar interactions [ 17]. At this stage it is difficult to explain the role of the surface chemistry in adsorption of such a very complex molecule as LPS. Strong acidic groups on the surface of MN500 under the experimental conditions (neutral pH, serum albumin and Tyrode buffer containing Ca 2+) most likely exist in (-SO3)2 Ca 2+ form. It has been shown that metal ions, particularly divalent cations, strongly interact with the LPS molecule via phosphate and carboxylic groups of its core region [ 18]. Hence, LPS may be attached to the surface via Ca 2+ bridge. The real picture may be even more complicated taking into account possible adsorption of serum albumin and other proteins that could bind LPS in the adsorbed state. Presence of mesopores in Carboxen 1003 (Fig. 2c) correlates with its higher adsorption capacity towards LPS in comparison with purely microporous Carboxen 1010. Although this study raises more questions than provides answers, the results are very encouraging from the practical point of view, as they suggest that manipulating pore structure and surface chemistry of adsorbents it would be possible to design adsorbents with high capacity towards removal of LPS and inflammatory cytokines - a task, which other methods failed to fulfil.
520 ACKNOWLEDGMENTS This work was supported by the Wellcome Trust grant 051017 (M.C.M) and EPSRC grant GR/R05154. V.M.G. thanks the Royal Society for financial support of his visit to the University of Brighton, UK. Authors thank Dr. W.R. Betz (Supelco, Inc.) and Dr. J.A. Dale (Purolite Int., Ltd.) for providing samples of carbons and resins, respectively. REFERENCES
[ 1] S.V. Mikhalovsky, Microparticles for Hemoperfusion and Extracorporeal Therapy, in: Microspheres, Microcapsules & Liposomes, R. Arshady (ed.), Vol. 2, Cirrhus, London, (1999) 133-169. [2] R.L. Patterson and N.R. Webster, J. R. Coll. Surg. Edin., 45 (2000) 178. [3] L.C. Green, D.A. Wagner, J. Glogowski, P.L. Skipper, J.S. Wishnok and S.R. Tannenbaum, Anal. Biochem., 126 (1982) 131. [4] M.M. Bradford, Anal. Biochem., 72 (1976) 248. [5] A.W. Adamson and A.P. Gast, Physical Chemistry of Surface, 6th ed., Wiley, New York, 1997. [6] S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, 2 nd ed., Academic Press, London, 1982. [7] M.M. Dubinin and F. Stoeckli, J. Colloid Interf. Sci., 75 (1980) 34. [8] C. Nguyen and D.D. Do, Langmuir, 16 (2000) 7218. [9] V.M. Gun'ko and D.D. Do, Colloids Surf. A, 193 (2001) 71. [10] S.W. Provencher, Comp. Phys. Comm., 27 (1982) 213; 229. [ 11 ] G.S. Rees, C.K. Gee, H.L. Ward, C. Ball, G.M. Tarrant, S. Poole and A.F. Bristow, Eur. Cytokine Netw., 10 (1999) 383. [12] M. Nagaki, R.D. Hughes, J. Lau and R. Williams, Int. J. Artif. Organs, 14 (1991) 43. [ 13] L.P. Li and R.G. Luo, Separ. Sci. Technol., 34 (1999) 1729. [ 14] G.S. Worthen, N. Avdi, S. Vukajlovich and P.S. Tobias, J. Clin. Invest., 90 (1992) 2526. [ 15] F.B. Anspach, J. Biochem. Bioph. Meth., 49 (2001) 665. [ 16] Y. Shi, H. Hogen-Esch, F.E. Regnier and S.L. Hem, Vaccine, 19 (2001) 1747. [ 17] H. Yokota, H. Kiyonaga, H. Kaniwa, T. Shibanuma, J. Pharm. Biomed. Anal., 25 (2001) 1001. [ 18] R.D. Lins and T.P. Straatsma, Biophys. J., 81 (2001) 1037.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
521
S e p a r a t i o n of a d s o r p t i o n i s o t h e r m s of N2 in i n t e r n a l i n t e r s t i t i a l n a n o p o r e s of s i n g l e - w a l l e d c a r b o n n a n o h o r n c o m p a r a t i v e s t u d y with e x p e r i m e n t and s i m u l a t i o n
and - A
T. Ohbaa, H. Kanoha, K. Muratab, M. Yudasakab, S. Iijimab, and K. Kaneko a aGraduate School of Natural Science and Technology, Chiba University 1-33 Yayoi, Inage, Chiba 263-8522, Japan bjapan Science and Technology Corporation, NEC Corporation, 34 Miyukigaoka, Tsukuba 305-8501, Japan
N2 adsorption isotherms in internal and ineterstitial nanopores of single wall carbon
nanohorn
(SWNH)
were calculated
compared with experimental one.
by GCMC
simulation
and is
Fitting of the GCMC-simulated isotherm
to experimental one in internal nanopores gave the average pore width w - 2.9 nm.
The N2 adsorption isotherm in the interstitial nanopores of the bundled
SWNH particles well coincided with the observed one.
I. Introduction Single wall carbon nanohorn (SWNH) known as a new nanocarbon material is expected as a hopeful applicant for methane and hydrogen storage. [1-3] A single particle of SWNH has a horn shaped structure and the tube part is similar to single wall carbon nanotube (SWNT). "Dahlia-flower" mesopores.
like
aggregate,
giving
SWNH particles form a
external
micropores
and
internal
Thus the SWNH assembly has both of micropores and mesopores.
Then the term of nanopores covering micropores and small mesopores (pore width < - 5
nm) is used in this article.
SWNH is highly pure, because the
SWNH is prepared by CO2 laser ablation of graphite without metal catalysts. Therefore, SWNH has an advantage for a better comparison with simulation study. [4-6]
Authors divided experimentally adsorption isotherms of N2 in
internal pores and external pores of SWNHs. [7]
Elucidation of adsorption in
522 internal and external nanopores
should be helpful to design an o p t i m u m
adsorbent for methane and hydrogen. In this study, N2 adsorption in the internal pore of single SWNH particle and on external
pores
of bundled
SWNH
canonical Monte Carlo (GCMC)
particles
is simulated
with
grand
method and the simulated isotherms are
compared with the experimental results.
2. Simulation and Experiment The preceding high resolution TEM examination suggests the shape o f a single SWNH particle. [1 ]
One end of a single SWNH particle has a corn like
structure whose tip has the average angle of 7t / 9 rad from the TEM image and closed pore structure in Fig. assemblies
must be studied
properties.
In
this
1.
The average pore structures of SWNH
in order to understand
GCMC
simulation,
a
their
single
gas adsorption
SWNH
particle
is
a p p r o x i m a t e d by the smoothed wall structure consisting of the corn and tube parts. The intermolecular interaction between N2 molecules was a p p r o x i m a t e d by one center Lennard-Jones (L J) potential function in eq (1).
(1)
Here, ooff and off are the potential well depth between N2 molecules and the effective diameter,
respectively.
The
used
LJ
p a r a m e t e r s for an N2 molecule are e f f / kB = 104.2
K
and
off
=
0.3632
nm.
The
m o l e c u l e - p o r e interaction was a p p r o x i m a t e d by the smoothed graphitic wall function for the
structure-less
tube.
[4,5]
Even the
m o l e c u l e - p o r e interaction for the corn part was a p p r o x i m a t e d by the spinning fishing rod model. [6]
Here, the 6cc / kB = 30.14 K
and o'c~ = 0.3416 nm were used for a carbon
Fig. 1 TEM image of SWNH (a) and the Model (b).
523
atom.
The
cross
parameters
L o r e n t z - B e r t h e l o t rule.
N2 and
of
carbon
were
given
by
the
The r a n d o m m o v e m e n t , creation, and r e m o v e m e n t o f
a molecule
give
a new
configuration
whose
total
potential
calculated.
As the c o n f i g u r a t i o n is a c c e p t e d in the c o n d i t i o n o f M e t r o p o l i s ' s
s a m p l i n g s c h e m e , the s y s t e m r e a c h e s an e q u i l i b r i u m state.
energy
was
The r e l a t i o n s h i p
of the tube d i a m e t e r D at the c a r b o n atom p o s i t i o n and the e f f e c t i v e pore w i d t h w w h i c h can be d e t e r m i n e d by the gas a d s o r p t i o n , is d e s c r i b e d by the f o l l o w i n g equation. w = D-0.3
/nm
(2)
In this study, s i m u l a t e d a d s o r p t i o n i s o t h e r m s o f N2 in the internal n a n o p o r e s were c a l c u l a t e d over the r e l a t i v e p r e s s u r e range o f 10 .6 to 1 for different tube d i a m e t e r s f r o m D = 2.0 nm to 3.6 nm by every 0.1 nm.
The a d s o r p t i o n
i s o t h e r m was c a l c u l a t e d on the e x t e r n a l n a n o p o r e s o f b u n d l e d S W N H p a r t i c l e s , whose D is 3.2 nm.
Here 0.4 nm was a d o p t e d as the i n t e r p a r t i c l e spacing for
the oriented SWNH assembly using the XRD data. [8] The S W N H used in this study is p r e p a r e d by CO2 laser a b l a t i o n o f graphite under Ar a t m o s p h e r e
at 101 kPa.
A s - g r o w n S W N H s were only used for
a d s o r p t i o n in the e x t e r n a l n a n o p o r e s .
The a s - g r o w n S W N H s were p a r t i a l l y
oxidized at 693 K; o x i d i z e d S W N H have n a n o - s c a l e w i n d o w s on their walls and t h e r e b y a d s o r p t i o n occurs in the e x t e r n a l and internal pores.
The w e i g h t
d e c r e a s e o f as g r o w n S W N H on the o x i d a t i o n t r e a t m e n t is less than 3 % and thereby
it
does
not
affect
the
s e p a r a t i o n o f a d s o r p t i o n in the internal and
external
pores.
adsorption
The
was
N2
measured
v o l u m e t r i c a l l y after the p r e h e a t i n g at 423
K and
1 mPa.
in
obtained
f r o m the s u b t r a c t i o n of the
adsorption
the
The a d s o r p t i o n
isotherm
internal
isotherm
pores
of
was
500 _
~.7~ ~7'
~4oo
-
~'~
-
E
300-<2oo~
~~
J
O
]
10o
as-grown
S W N H s from that on o x i d i z e d S W N H s . were
_ . . . . 0.4 ~ _ 0.~6 0.18 00 _ _ _ L0.2 1] P/P0 Fig. 2 Adsorption isotherms of N2 in the
separated, which will be used for the
internal and external pores at 77 K.
comparison
Internal pores: O , external pores: ZX.
Thus, internal
adsorption and
isotherms
external with
the
in
pores
the
simulated
524 isotherms.
1.2 1
3. R e s u l t s a n d D i s c u s s i o n
-
-
0.8
"~ 0.6
Adsorption isotherms of N2 in internal and external pores at 77 K are shown in Figure 2. Interaction potential for adsorbed N2 in the external nanopores is stronger than that in internal nanopores because of
0.2 0 -6
I
J I I -4 -2 log(P/P )
I 0
0
the bundled structure. The adsorption processes in the internal pores of the carbon tube structure are quite different from those in the slit pore due to an intensive packing restriction. ~[9] Therefore, 2] nm
P/Po = 3 x 10 .6
Fig. 3 Simulated adsorption isotherms of w = 2.1 (O), 2.5 (Z~), 2.9 (!"!), and 3.3 ( x ) nm and experimental adsorption isotherm (O) in internal nanospaces of SWNH
P/Po = 6 x 10-4
(A] 2;9 nm
tube
L4rm
I
I
~ /
Q
!
inec] /'7(C~
.I l
,
I
,..
@ ,
I I[
,
@ V//
!
I I
I
I I
0 @ ,
I i
i
I l
.
@
i
t. 9
P/Po = 6 x 10-2
@ I i
I I
..
I
i
,
l
t
l I
,
@ 9
I
I I
.
@
o tiPnrn
Fig. 4 Snapshots from axial direction at different relative pressure P/Po. The snapshots at the tube (A), neck (B), and tip (C) are showrr
525 subtracting pore effect (SPE) method using high resolution C~s-method cannot be applied to the internal pores of the cylindrical tubes. bundled
tube
system
having
nano-scale
windows
Adsorption on the
includes
adsorption
in
internal and external pores, as mentioned above.
Then, it is difficult to
determine the average pore width experimentally.
Thus, the fitting o f the
experimental
isotherm with GCMC
simulation in internal nanopores was
carried out for the d e t e r m i n a t i o n of the average pore width.
Figure 3 shows
simulated a d s o r p t i o n isotherms in the internal nanopores o f different D values together with the experimental one. near P/P0 = 106-10 -4 and 10 ~. initial adsorption uptake.
All simulated isotherms have two steps
The smaller the pore width, the greater the
Here the simulated adsorbed amount is expressed
by the fractional filling under the assumption that adsorption in the internal pores completes at P/P0 = 0.4.
The simulated adsorption isotherm in w = 2.9
nm (D - 3.2 nm) gives the best fitting.
Thus, the average pore width w o f the
internal nanopores of SWNHs is determined to be 2.9 nm. Figure 4 shows snapshots of the GCMC simulation for adsorption in the internal pores from the axial direction corresponding at different P/P0 values. The snapshots are illustrated on three regions of tip, neck, and tube spaces, which have the different molecule-pore interaction.
At first, molecules are
adsorbed in the tip space due to the strongest interaction and then adsorption develops in the neck space with the increase of P/P0.
Finally, the internal
tube space is filled with molecules Simulated a d s o r p t i o n isotherms on the bundled SWNH assembly of the hexagonal s y m m e t r y for D = 3.2 nm (w = 2.9 nm) is shown in Figure 5.
Here
it is assumed that the internal pores are available
for
adsorption
and
interparticle spacing is 0.4 nm. simulated
isotherm
in
the
the
~ .,.,
1.5
= o
1
The
external
pores well expresses the experimental isotherm, the
indicating the presence
bundled
shows
N2
structure. adsorption
Figure behaviors
of 6 in
0.5
0 -5
i
i
I
I,
-4
-3
-2
-1
log ( P / P 0 )
external pores at different P/P0 values using the snapshot.
Molecules
are
Fig.5 Simulated adsorption isotherms in
adsorbed at the tube and neck sites
extemal pores of bundled SWNH ( 0 )
from an extremely, low pressure region,
and experiment (A).
526 6 x l0 "4
P/P0 = 3 x 10-6
6 x 10-2 1
~ ~ _ _ ~ ( A ) ~
v ~ ~ ~!~ tube
o
V
W
& ..............
i111
!1
~
. . . .
(B> neck~ .....
~ ...........
o
(c) tip
:;o
&
&
......
&,
- , ' ~ ~
~ .,~
'~oO~Oo ~ o
AA ~.~
~~ ~ ~ ~
~
~ o
0%~ '~
~o ~ ~ ~o~~~% ~
%~
~
o
o,.s ~.a ,
~o
o~
~
<~
#,~', ~-..... ;-,~,._
~ i_~.,~
~
.-
+'d ~_~r,-, ~, -
,<_~ .... -".2", ....-,-!/"
Fig. 6 Snapshots from axial direction at different relative pressure P/Po. The snapshots in interstitial nanopores at the tube (A), neck (B), and tip (C) are shown. which is different from adsorption in the internal pores. Adsorption in the tip part begins at a high pressure region. Thus, the comparative study using GCMC simulation and experiment can provide a good description of N2 adsorption on the SWNH assembly at 77 K.
References [1] S. Iijima, M. Yudasaka, R. Yamada, S. Bandow, K. Suenaga, F. Kokai, and K. Takahashi, Chem. Phys. Lett. 309 (1999) 165. [2] S. Bandow, F. Kokai, K. Takahashi, M. Yudasaka, L. C. Qin, and S. Iijima, Chem. Phys. Lett. 321 (2000) 514. [3] K. Murata, K. Kaneko, W. A. Steele, F. Kokai, K. Takahashi, D. Kasuya, K. Hirahara, J. A. Nisha, M. Yudasaka, and S. Iijima, J. Phys. Chem. B 105 (2001) 10210. [4] W.A. Steele, and M. J. Bojan, Adv. Colloid Interface Sci., 76-77 (1998) 153.
527 [5] [6]
G. Stan, and M. W. Cole, Surf Sci., 395 (1998) 280. T. Ohba, K. Murata, K. Kaneko, W. A. Steele, F. Kokai, K. Takahashi, D.
Kasuya, M. Yudasaka, and S. Iijima, Nanolett. 1 (2001) 371. [7]
K. Murata, K. Kaneko, F. Kokai, K. Takahashi, D. Kasuya, M. Yudasaka,
and S. Iijima, Chem. Phys. Lett. 14 (2000) 331. [8]
S. Bandow, F. Kokai, K. Takahashi, M. Yudasaka, L. C. Qin, and S.
Iijima, Chem. Phys. Lett. 321 (2000) 514. [9]
T. Ohba and K. Kaneko, J. Phys. Chem. in press.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
529
Visualizing the porous structure of different carbon materials: a scanning tunneling microscopy study J.I. Paredes, A. Martinez-Alonso, J.M.D. Tasc6n Instituto Nacional del Carb6n, CSIC, Apartado 73, 33080 Oviedo, Spain
Scanning tunneling microscopy (STM) has been employed to visualize the porous structure of selected carbon materials. Model micropores artificially produced on an atomically fiat graphite surface were imaged at the atomic scale. Several types of perturbations in electronic density near the Fermi level around the micropore region were observed, constituting an indication of its enhanced adsorptivity. STM imaging of activated carbon fibers revealed mesopores up to 16 nm in size within highly developed spongy morphologies. Also, large numbers of interconnected micropores were present on the fibers surface. By contrast, a nonporous carbon black did not show evidence of any of these features, which agrees with its poor sorptive capabilities. The ultramicroporosity of nonactivated carbon fibers was also tentatively probed, but its detection was inconclusive owing to limitations of the STM technique. 1. INTRODUCTION Among the prominent features of sp2-bonded carbon materials, their ability to form porous structures lending very high specific surface areas (between 1000 and 3000 m 2 g-l) is particularly significant [ 1]. The development of a network of pores in such materials, which is typically attained by controlled reaction of the host carbon with 02, H20 and/or CO2 at 1000-1400 K, becomes crucial for their application in a variety of fields, including heterogeneous catalysis and the adsorption and separation technology of gases and liquids [2]. An example can be found in the recent synthesis of ordered arrays of nanoporous carbon capable of supporting extremely high and uniform dispersions of diameter-controllable platinum nanoparticles [3]. Another example is that of activated carbon fibers, which are being currently explored as potential efficient adsorbents for use in environmental applications, such as the removal of contaminants (aromatic compounds, microorganisms, etc.) from water [4-6] and the elimination of SOx/NOx from air [7,8]. A considerable number of different techniques has been employed in the past to characterize the porosity and surface chemistry of porous carbon materials. These include gas adsorption (mostly N2 and CO2) [9-14], immersion calorimetry [9], small-angle X-ray [ 11,15] and neutron [ 14] scattering, inverse gas chromatography [ 12,13], differential thermal analysis [ 12], Fourier transform infrared [ 12], Raman [16] and X-ray photoelectron [17] spectroscopies and electron spin resonance [ 16]. It is worth mentioning that the information about the porous structure of the material provided by this array of techniques is only indirect
530 and, in order to directly visualize the pores, very high resolution microscopy techniques should be employed. Such a task can be partly fulfilled with the use of transmission electron microscopy, which is capable of resolving the micropores in, e.g., activated carbon fibers [18]. By contrast, scanning electron microscopy is generally limited to the detection of macropores or large mesopores [19]. More recently, the advent of scanning probe microscopies, and in particular of scanning tunneling microscopy [20,21], has provided an excellent opportunity to visualize the micro- and mesoporous structure of carbonaceous materials [22-24]. The objective of the present work was to study and compare by scanning tunneling microscopy (STM) the microporosity and mesoporosity of several different carbon materials with various types and amounts of pores: highly oriented pyrolytic graphite with artificiallygenerated model pores, activated carbon fibers, nonporous thermally treated carbon black and nonactivated carbon fibers with an ultramicroporous texture. 2. EXPERIMENTAL Highly oriented pyrolytic graphite (HOPG) was obtained from Advanced Ceramics Corporation. To generate pores on its surface, the samples were exposed to an oxygen plasma in a Technics Plasma 200-G apparatus. The plasma was generated by means of microwave (MW) radiation (2.45 GHz). Two different etching conditions were implemented to produce different types of pores: a MW power of 40 W for 6 s and a MW power of 100 W for 9 min. The activated carbon fibers (ACFs) used in the present work were produced by physical activation (CO2, 1023 K, 73% burnoff) of pyrolyzed Kevlar pulp as detailed in Ref. [25]. These ACFs display a BET specific surface area of 874 m 2 g-l, as measured from N2 adsorption at 77 K. The thermally treated (3000 K) carbon black (Sterling FT N880, from Cabot) presented a BET (N2, 77 K) surface area of 15.3 m 2 g-1. The nonactivated carbon fibers (NACFs) were prepared from the pyrolysis of Nomex aramid fibers in a quartz reactor under Ar, heating at 10 K min l to 1173 K, followed by cooling to room temperature under Ar. Their BET surface area was only 0.3 m 2 g-i (N2, 77 K) but their equivalent microporous surface area determined by CO2 adsorption at 273 K and calculated by the DRK method, was as high as 750 m 2 g-~, which means that, as opposed to the carbon black, these nonactivated fibers possess an ultramicroporous texture (pore size-~0.4 nm). STM imaging was performed with a Nanoscope Multimode IIIa, from Digital Instruments, in air at room temperature. The measurements were carried out in the constant current mode (variable height) using mechanically prepared Pt/It (80/20) tips. The tunneling parameters typically employed were between 30 mV and 1 V and between 0.35 and 1 nA for the bias voltage and tunneling current, respectively. Such parameters could be varied substantially without considerable changes in the corresponding images. Special care was required to mount the fiber and carbon black samples onto the metallic STM sample holders. Only very small quantities were used, paying attention to avoid the presence of aggregates protruding a great length from the sample surface which could interfere with the tip and prevent the attainment of a stable current signal. To confirm the reproducibility of the images, several different tips were employed to study the samples. Also, to improve their visual presentation, the images were flattened and then smoothed by a low-pass filter, taking care to ensure that no noticeable artifacts or distortions were introduced by this procedure.
531 3. RESULTS AND DISCUSSION Since pristine HOPG displays a highly ordered structure and a surface with large atomically flat terraces, it has been traditionally considered as a model carbon material where different types of fundamental studies can be carried out. Furthermore, it has been previously shown [26] that pits of different sizes can be produced on its surface by oxygen plasma treatments. Consequently, HOPG is a p r i o r i an ideal substrate to study micropores on carbon surfaces by STM at the atomic scale. Fig. 1 shows several images of the defects produced in HOPG following plasma exposures at a MW power of 40 W for 6 s (a) and at a MW power of 100 W for 9 min (b-d). Fig. 1a reveals the structural changes brought about by the treatment: a 1 nm wide and 0.1-0.2 nm high protrusion can be observed at the bottom half of the image on an otherwise unaltered graphitic structure (displaying the well-known STM triangular pattern). The density of protrusions on the HOPG basal plane was about several thousands per square micrometer. Such protrusions were never observed on the untreated material and their origin can be attributed to the effect of atomic oxygen (the main active species from the plasma), which is able to react with carbon atoms from the graphite basal plane [26], creating isolated atomic vacancies in the initial stages of attack that may subsequently evolve into nanometer-sized pits or trenches following extended exposures. Moreover, atomic vacancies on the graphite surface are known to induce an enhancement in electronic density near the Fermi level in the area immediately surrounding the vacancy [27-29]. Note that what the STM detects are changes in this partial electronic density [30], along with topography variations. Therefore, the observed STM protrusions (Fig. l a) are ascribed to the atomic vacancies created on the HOPG surface by the plasma and reflect the enhanced Fermi level electronic density in their surroundings. In turn, such atomic vacancies can be considered as model micropores in a carbon material and the electronic density increase near the Fermi level in the micropore region implies its enhanced adsorptivity/chemical reactivity, particularly for polar or polarizable molecules [31 ]. Upon longer exposures to the plasma, the HOPG surface becomes in general atomically rough and mesopores develop, presumably from the micropores/atomic vacancies created at earlier stages [26]. This can be appreciated in Fig. lb, where rounded or elongated pores with diameters/widths roughly between 2 and 8 nm have been created after a 9 min plasma exposure. It should be noted that such pores present depths normally between 1 and 3 nm, which is about one order of magnitude greater than the typical height variations arising from electronic effects (e.g., protrusion of Fig. l a). This means that in the case of Fig. l b, the possible electronic effects associated with the pores will be largely overshadowed by purely topographic effects and the pores are visualized essentially as physical voids on the surface. This result is also generally applicable to the disorganized carbon materials considered subsequently in the present work, whose topographies tend to be rather rough and where, in consequence, electronic effects are obscured by topographical ones. Having said this, however, it should be mentioned that in the 9 min sample it was possible to find certain limited and relatively unaltered areas where some electronic effects arising from the pores could still be ascertained. Fig. 1c shows one of such areas. A number of pores 1.5-2.0 nm in size can be seen in the bottom part of the image. It is noticed that the atomic-scale structure in a rim about 2-3 nm wide around the pores differs from that of perfect, unperturbed graphite (observed, for instance, at the bottom right comer of the image). Such type of
532 structure is presented in more detail in Fig. l d. It corresponds to a (x/3x 4r3)R30 ~ superstructure, which is believed to reflect not a surface atomic reconstruction but again a perturbation in the electronic density near the Fermi level due to defects [32], which are pores or pits in the present case. Line profiles taken from Fig. l d indicate that two out of three spots in the superstructure possess the same height as that of the spots of unchanged graphite (restored at the bottom right comer of the image). By contrast, the third spot lies approximately 0.15 nm above the other two. This indicates that there is a net increase in electronic density near the Fermi level in the 2-3 nm wide superstructure region around the pores. Accordingly, such nanometer-sized region will exhibit enhanced adsorption/reaction capabilities when compared to those of the unperturbed HOPG surface.
Figure 1. STM images of the HOPG surface following oxygen plasma exposure to develop porosity. Treatment conditions: (a) MW power of 40 W, 6 s. (b-d) MW power of 100 W, 9 min. The following step in the investigation consisted in visualizing the porous structure of ACFs. First, it must be noted that the N2 (77 K) isotherms of the studied Kevlar pulp-derived ACF are essentially of type I, corresponding to a microporous solid, but with a capillary condensation hysteresis loop at high relative pressures, indicative of the presence of mesoporosity. The BET surface area (N2, 77 K) of this sample is 874 m E g-l, which is about three orders of magnitude larger than its corresponding geometric area (0.5 m 2 g-l). Fig. 2 shows evidence of mesopore development in the sample. The right and bottom areas of Fig. 2a display a large number of deep mesopores, lending such areas a highly spongy texture.
533 These mesopores have diameters ranging between 4 and 16 nm, with an average value of 8.3 nm. Although not so evident from the image, mesopores are also present on other parts of Fig. 2a, but in this case they do not display the same type of spongy texture as in the former case. This can be appreciated in more detail in Fig. 2b, where pores are typically between 3 and 10 nm, with a mean value of 6.5 nm. The different development of mesoporosity in nearby areas of the ACFs studied here can be possibly attributed to inhomogeneities in the crystallinity of the starting material (Kevlar pulp): it has been shown that regions of different crystallinity are present within the same Kevlar fiber [33,34]. Due to their higher reactivity in gasification, the less crystalline areas of Kevlar should be more liable to develop porosity than those displaying a higher degree of order. Such behavior is very likely to be behind the general structural heterogeneity that was observed in the STM images of this fiber.
Figure 2. STM images of the mesoporous structure of ACFs.
As a rule, to resolve micropores very high resolution STM imaging needs to be accomplished and this is largely dependent on the tOpography of the sample. Highly corrugated morphologies are much more difficult to track accurately than fiat or smooth surfaces and will usually imply a reduction in resolving power of the instrument. Indeed, B6ta et al. [ 11 ] have previously reported the difficulty in attaining very high resolution STM images of rough active carbons. For this reason, in the present work STM imaging of the micropore structure was preferably pursued in relatively fiat areas as opposed to rough ones. Some images of the microporous structure of the ACFs are shown in Figs. 3a and b. A great number of micropores along with some small mesopores (2-4 nm) can be seen in both images. The micropores have sizes from about 0.7 to about 1.8 nm with an average value around 1 nm, which is reasonably consistent with the mean pore width obtained from CO2 adsorption at 273 K (-1.1 nm [25]), considering that only a tiny fraction of the total fiber surface is probed by STM. A great number of micropores are slit-shaped, particularly those of Fig. 3a, where they make up a large interconnected network. Slit-shaped or elongated porosity has been previously shown to be present in activated carbon materials and particularly in ACFs [22,23,35]. In addition, approximately rounded and more isolated pores are also found on the ACF surface (Fig. 3b).
534
Figure 3. Microporous structure of ACFs as imaged by STM.
Fig. 4 shows two STM images of the surface structure of a carbon black. The sample exhibits a specific surface area, determined by N2 adsorption at 77 K, of 15.3 m 2 g~, which is almost coincident with its geometric area (16.9 m 2 g-l). Therefore, this is a nonporous carbon and its STM images should be expected to differ from those of the ACFs. As a matter of fact, this is what can be observed in Fig. 4. First, it is noted that the carbon black does not display any mesoporosity (Fig. 4a) such as that of the AFCs (Fig. 2). Second, at the micropore scale the carbon black porosity is also very poorly developed (Fig. 4b) in comparison with the pore development of ACFs (e.g., Fig. 3a). In the former case (Fig. 4b), altough some trenches are also present, they are very shallow and, consequently, are simple topographic variations of a smooth surface and cannot be considered as pores penetrating deeply into the material as in Fig. 3a. Also, pores of the type shown in Fig. 3b for the ACFs were not normally seen on the carbon black surface. Hence, all these observations agree with the lack of adsorption capabilities of this material.
Figure 4. STM images of thermally treated carbon black evidencing a lack of meso- and microporosity. The nonactivated carbon fibers (NACFs) were a difficult case to study by STM. As mentioned previously, this sample possesses a BET surface a r e a (N2, 77 K) of 0.3 m 2 g-l, coincident with its geometric area. By contrast, it displays a high adsorptivity toward CO2 at
535 273 K (equivalent surface area of 750 m 2 g-l), implying an ultramicroporous structure (typical of carbonized materials [36]) with pore widths of-~0.4 nm. Fig. 5 shows an image of the NACFs. The general appearance is similar to that of the carbon black (i.e., shallow trenches). However, exploring the image of Fig. 5 in detail, there seems to be a number of minute voids of about 0.4 nm, absent from the carbon black images, and which would be the ultramicropores of the NACF. Nevertheless, this observation must be treated with caution, since it is possible that such tiny features might not be detected with sufficient accuracy by STM in a disordered and rough material, such as the present NACFs, so as to be totally confident that they are true features and not just artifacts. As a final remark, a brief comment should be made on the capabilities of other scanning probe microscopy techniques, particularly atomic force microscopy (AFM), for the visualization of pores in carbon materials. Fig. 6 shows an image of the studied ACFs obtained by tapping mode AFM. It is evident from Fig. 6 that the pore structure of the ACFs is not resolved and the surface appears extremely smooth. Such deficient performance of the AFM is mainly attributed to the relatively large curvature radius of the tips used (5-10 nm, among the sharpest available). This limits the width of the trenches that can be probed with reasonable accuracy and, in this case, prevents the detection of the micro- and mesopores of the ACF sample.
Figure 5. STM image of nonactivated carbon fiber (NACF). It displays a fine structure which could be indicative of the ultramicroporous texture of this fiber.
Figure 6. Tapping mode AFM image of the studied ACFs. The technique lacks enough resolution to detect the micro- and mesoporosity typical of this sample.
4. CONCLUSIONS STM has proved to be a powerful technique to visualize the porous structure of carbon materials. Model micropores artificially produced on graphite showed evidence of atomicscale electronic perturbations in the micropore region which imply an enhanced reactivity/adsorptivity of the micropores. Mesopores lending spongy textures and slit-shaped micropores forming interconnected networks were observed in activated carbon fibers. On the other hand, and as expected, carbon black showed no sign of porosity. Finally, the observation of the ultramicroporous texture of nonactivated carbon fibers was not conclusive, due to the difficulty in resolving such small features in a rough and disordered surface.
536 ACKNOWLEDGEMENT
Financial support from DGICYT (project PB98-0492) and CICYT (project 1FD1997-1915) is gratefully acknowledged. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
M.S. Dresselhaus, Annu. Rev. Mater. Sci., 27 (1997) 1. R.C. Bansal et al., Active Carbon, Marcel Dekker, New York, 1988. S.H. Joo et al., Nature, 412 (2001) 169. C. Brasquet and P. Le Cloirec, Langmuir, 15 (1999) 5906. C.L. Mangun et al., Chem. Mater., 13 (2001) 2356. A.V. Pimenov et al., Sep. Sci. Technol., 36 (2001) 3385. I. Mochida et al., Carbon, 38 (2000) 227. K. Li et al., Carbon, 39 (2001) 1803. F. Rodriguez-Reinoso and M. Molina-Sabio, Adv. Colloid Interface Sci., 76-77 (1998) 271. 10. N.J. Foley et al., Langmuir, 13 (1997) 2083. 11. A. B6ta et al., Langmuir, 13 (1997) 6502. 12. F. Adib et al., Langmuir, 16 (2000) 1980. 13. I. Salame and T.J. Bandosz, Langmuir, 17 (2001) 4967. 14. K.C. Littrell et al., Chem. Mater., 14 (2002) 327. 15. D. Cazorla-Amor6s et al., Carbon, 36 (1998) 309. 16. A. Manivannan et al., Carbon, 37 (1999) 1741. 17. Y. Kaneko et al., Langmuir, 11 (1995) 708. 18. K. Oshida et al., J. Mater. Res., 10 (1995) 2507. 19. C. Brasquet et al., Carbon, 38 (2000) 407. 20. G. Binnig et al., Phys. Rev. Lett., 49 (1982) 57. 21. S.N. Magonov and M.-H. Whangbo, Surface Analysis with STM and AFM, VCH, Weinheim, 1996. 22. M.A. Daley et al., Carbon, 34 (1996) 1191. 23. V. Vignal et al., J. Mater. Res., 14 (1999) 1102. 24. P. Pfeifer et al., Phys. Rev. Lett., 88 (2002) 115502. 25. A. Martinez-Alonso et al., Microporous Mater., 11 (1997) 303. 26. J.I. Paredes et al., J. Mater. Chem., 10 (2000) 1585. 27. J.R. Hahn et al., Phys. Rev. B, 53 (1996) R1725. 28. M. Hjort and S. Stafstr6m, Phys. Rev. B, 61 (2000) 14089. 29. K.H. Lee et al., THEOCHEM- J. Mol. Struct., 506 (2000) 297. 30. J. Tersoff and D.R. Hamann, Phys. Rev. B, 31 (1985) 805. 31. F. Rodriguez-Reinoso and A. Linares-Solano, Chemistry and Physics of Carbon, P.A. Thrower (Ed.), vol. 21, Marcel Dekker, New York, 1989, p. 1. 32. N. Takeuchi et al., Surf. Sci., 380 (1997) 190. 33. S. Rebouillat et al., Polymer, 40 (1999) 7341. 34. J.F. Graham et al., Polymer, 41 (2000) 4761. 35. R. Radhakrishnan et al., J. Chem. Phys., 111 (1999) 9058. 36. J. Garrido et al., Langmuir, 3 (1987) 76.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Textural characterisation of activated carbons obtained poly(ethylene terephthalate) by carbon dioxide activation
537
from
J.B. Parra, C.O. Ania, A. Arenillas and J.J. Pis Instituto Nacional del Carb6n, C.S.I.C. Apartado 73, 33080, Oviedo (Spain) ABSTRACT Polymer waste nowadays accounts for a large part of municipal solid waste and so poses a problem of considerable concern for the environment. In this study, it is reported that poly(ethylene terephthalate) pyrolysis is a valuable process with no residual solids, as all the products obtained are of potential use, i.e., energy from the gases produced (58%), terephthalic acid (approximately 20%), and a solid black residue (22%) which, when activated, shows interesting properties, making it useful as an adsorbent. Pyrolysis of raw PET was performed under nitrogen atmosphere, and ulterior activation was achieved at 925 ~ with a flow rate of 10 mL min -1 of CO2, up to different burn-off degrees. Textural characterisation of the samples was carried out by measuring apparent density (mercury at 0.1 MPa), mercury porosimetry, and N2 and CO2 adsorption isotherms, at-196 and 0 ~ respectively. The active carbons obtained were mainly microporous, with BET apparent surface areas of up to 2468 m2gl.
Keywords: PET, activated carbon, texture 1. INTRODUCTION Poly(ethylene terephthalate), PET, has become one of the major post-consumer plastic wastes. PET products do not present a direct threat to the environment, but they do pose a problem of considerable concern due to the huge amount of solid waste produced [1 ]. The disposal of this waste together with its low bio-degradability and photo-degradability, now represents a serious challenge for industrial countries worldwide. The most common ways to eliminate PET wastes are incineration with energy recovery or, less often, recycling. The increasing interest in PET recycling is associated with the use of this polymer for packaging, mainly in the form of bottles. Like other plastic items, PET bottles have been the target of recently introduced restrictive laws. As a result, the new challenge to PET companies is to find the best way to collect, separate, reprocess and market their low-cost products [2]. Chemical recycling consists in the decomposition of polymer macromolecules in molecules with lower molecular weight by means of reactions like glycolysis, hydrolysis and alcoholysis, among others
538 [3]. Both incineration and chemical recycling techniques produce: i) gaseous products that could represent potential health hazards and, therefore, should be treated carefully, and ii) residual solid products, which need further disposal [ 1]. In this work, special attention was paid to minimising the potential residues from PET recycling. Some works have been published on active carbons from granulated non-used PET [4], but none concerning the use of plastic wastes. The advantage of the pyrolysis of post-consumer PET is that all the products obtained present interesting applications (i.e., chemical products, gases with calorific value and a solid that can be used as an active carbon), as a result of which residual wastes can be minimised considerably. The aim of this work was to obtain valuable active carbons from the solid residue from PET pyrolysis. 2. E X P E R I M E N T A L
PET from post-consumer soft-drink bottles was cut into small pieces and batches of this raw material (~ 40 g) were pyrolysed in a first step at 400 ~ under nitrogen atmosphere, in a 35 mm internal diameter vertical quartz reactor. Then, a second heat treatment was performed, at 725 ~ for 2 hours. The distribution of the final products of the pyrolysis process was approximately 58% of gaseous compounds (CO, CO2 and a complex mixture of hydrocarbons), 20% of a yellow crystal solid that condensed in the upper part of the reactor, and 22% of a black solid residue (i.e., char, denoted as P) with a glassy sheen, retrieved from the bottom of the reactor. The characterisation of the yellow crystal solid revealed that terephthalic acid was the principal component, which can be recycled for PET synthesis. The char was ground, sieved and a particle size fraction of 0.5-1.0 mm was selected for physical activation with carbon dioxide. Previously, char reactivity was evaluated in a CO2 atmosphere, in order to optimise the operating temperature for the activation process. A thermogravimetric analyser (TGA) was used for this purpose. Approximately 25 mg of sample was heated at 15 ~ min -1 from room temperature up to 1200 ~ under 50 mL min -~ of CO2. The reactivity test revealed that the char is a very stable carbon material, that does not begin to react until 900 ~ Thus, an activation temperature of 925 ~ was selected, at which point mass loss started to be significant (5% of the initial mass). Table 1 Analysis of the samples studied SAMPLE P PC PC12 PC35 PC58 PC76
1.2 0.9 0.6 0.6 0.6 0.5
Ultimate Analysis (wt%) C 97.0 98.6 99.1 99.1 99.2 99.4
H 1.8 0.5 0.3 0.3 0.2 0.1
539
The activation process involved the previous treatment of the P char at 925 ~ under nitrogen atmosphere for one hour (sample named PC). Then, around 6 g of the carbon material was activated in a 10 mL min 1 flow rate of carbon dioxide, at 925 ~ Samples with different activation degrees (12, 35, 58 and 76% burn-off) were obtained and denoted as PC12, PC35, PC58 and PC76, respectively. The ultimate analysis of all the samples studied is given in Table 1. Textural characterisation of the samples was carried out by measuring apparent density (mercury at 0.1 MPa), mercury porosimetry and N2 and CO2 adsorption isotherms, a t - 1 9 6 and 0 ~ respectively. The apparent surface areas of the samples were obtained by using the BET equation [5]. The micropore size analysis was performed by means of the t-plot and the Dubinin-Astakhov methods [6]. 3. RESULTS AND DISCUSSION
The ultimate analysis of the samples shows significant low oxygen and hydrogen contents, especially in the activated samples (cf., Table 1). No other heteroatoms were detected (i.e., nitrogen or sulphur). Thus it can be stated that the samples are materials that are composed mainly of carbon. The activated samples exhibit different textural properties. The apparent densities decrease significantly in accordance with the degree of activation, from 1.3 to 0.6 g cm -3 for PC12 and PC76, respectively. This means that the activation process creates and/or enlarges pores, thereby producing carbon materials that are more porous. The adsorption isotherms of nitrogen at-196 ~ on the PC activated samples are shown in Fig. 1. 1000 9OO 8OO .~
700 ~
600 "~
~
PC7
-*-- P C 5
500
+
400
--~ PC1
PC3
-*- P C 30O 200 100 0
. . . .
0.0
I
0.2
. . . .
I
. . . .
0.4
t
0.6
. . . .
I
0.8
. . . .
1.0
p/p*
Fig. 1. Nitrogen adsorption isotherms at-196 ~ for the samples studied.
540 Table 2 Textural parameters derived from N2 adsorption isotherms BET Dubinin-Astakhov method SAMPLE Area W0 E0 L Smic (m 2 g-l) C Range oflinearity (cm 3 g-l) (kJ mol "l) (nm) (m 2 g-l) PC PC12 PC35 PC58 PC76
340 668 1405 1920 2468
431 1976 532 203 100
0.027 _
. 0.24 0.47 0.63 0.79
.
. 26 22 19 17
. 0.8 1.0 1.4 1.8
629 909 930 870
All the adsorption isotherms are type I on the BDDT classification [7], indicating that they are mainly microporous materials. The desorption isotherms are reversible, except for sample PC76, which presents a small hysteresis loop. This is indicative of a slight development of mesoporosity. On the other hand, only the original sample (i.e., PC) presents gas retention at low pressures. Fig. 1 shows that as the activation degree increases, there is a clear enhancement of the amount of nitrogen adsorbed. This indicates that the activation process has induced important modifications in the texture of the materials. Although the BET theory is based on an unrealistic model, there is a generally accepted convention [8] of applying the BET-nitrogen method in order to obtain the values of the apparent surface area. The results are shown in Table 2, which also includes the range of linearity of the BET equation. It can be observed in this table that the apparent surface areas increase significantly with the degree of burn-off. Thus, the apparent surface area of the most activated sample (PC76) is nearly four times higher than that of sample PC12 (2468 versus 668 mZg-1, respectively). In addition, the CBETvalue decreases from 1976 for PC12 to 100 for PC76, indicating that the samples present bigger micropores as the bum-off degree increases. The Dubinin-Astakhov (D-A) [6] equation was applied to the N2 adsorption isotherms. The accessible pore width, L, was calculated from the expression proposed by Stoeckli and Ballerini [9]. The corresponding textural parameters obtained from the D-A equation, such as micropore volume -W0-, surface of micropores -Stoic- (Smic = 2 Wo/L), characteristic energy -E0-, and pore width-L-, are presented in Table 2. It can be observed that, as the burn-off degree increases, pore width also increases. This is corroborated by the Eo values, which are generally related with the adsorbent microporosity: it was found that the higher the Eo values, the narrower the pores [9-10]. The enlargement of the pores with the burn-off degree led to a higher micropore volume as the activation degree increased (see Table 2).
541 120 110
--u.-PC76
100
.-o- PC58
90
a.
70 Z=
E m
+
Pc35
80
-*- PC
60 50
40
40 30 20 10
0.000
0.002
0.004
0 0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
plpO
Fig. 2. C 0 2 adsorption isotherms at 0 ~ for the samples studied. The adsorption isotherms of C02 at 0 ~ on the samples studied are presented in Fig. 2. At low relative pressures, the adsorption of CO2 decreases with the increase in burn-off degree, confirming the enlargement of pore size with the increase in the activation degree of the samples. However, at higher relative pressures, the adsorption isotherms intersect each other. This behaviour can be attributed to the presence of different pore size distributions, as will be discussed below. The Dubinin-Astakhov [6] equation was also applied to the CO2 adsorption isotherms, using 0.36 as [3 parameter [11]. The values of the textural parameters are given in Table 3. It can be seen that the micropore volume W0, increases substantially from 0.19 cm 3 g-1 for PC12 to 1.11 cm 3 g-1 for PC76. The increase in micropore volume is due to an enlargement of the pore size (L values, cf. Table 3), as evidenced by the decrease in Eo values. The decrease in the values of the n exponent with the increase in burn-off degree is indicative of a stretching in the porosity and thus a wider range of pore size distribution [12]. It has to be mentioned that the micropore surface area values, calculated by Stoeckli-Ballerini equation [9], present a maximum value with the burn-off degree of the samples, either from the N2 or CO2 adsorption isotherms data. This is due to the increase in the pore size (L values), and the assumptions made in the equations. Thus, as the pore size -L- increases, the micropore surface area for a determined micropore volume -W0- is smaller.
542 Table 3 Textural parameters derived from the Dubinin-Astakhov equation applied to CO2 adsorption isotherms n Eo (kJ mol "1) L (rim) Smic(m2 g-l) SAMPLE Wo (cm3g-1) PC PC12 PC35 PC58 PC76
0.19 0.29 0.57 0.88 1.11
2.4 2.0 1.6 1.4 1.3
33 28 21 16 14
0.5 0.7 1.2 2.3 -
746 882 969 748 -
The pore size distribution (see Fig. 3) can be obtained from the mercury porosimetry data and the t-plot from N2 adsorption isotherms, using an active carbon with a very low surface area as a reference [13]. It was observed that the volumes of mercury intruded were very small. As a consequence, the volumes of meso (the largest ones) and macropores are low. Thus, the samples studied are mainly microporous, as already mentioned in the N2 and CO2 adsorption isotherm results.
Fig. 3. Pore size distribution for the activated samples. Regarding microporosity, the t-plot is able to distinguish between medium and small micropores [12-14]. In the series of activated samples, the small and medium-sized micropores display different trends as can be seen in Fig. 3. There is a decrease in the volume of small micropores with the increase in degree of bum-off, while the volume of the medium-sized micropores increases gradually as burn-off increases.
543 4. CONCLUSIONS The activation of PET waste produce carbon materials suitable for using as active carbons, with very high BET apparent surface areas -up to 2468 m2g-1-. These active carbons are mainly microporous, with low meso and macroporosity. It was observed that the increase in the activation degree (i.e., burn-off) produces a gradual increase in the volume of total micropores, probably by enlarging the small micropores besides creating new porosity. Further studies on this topic are ongoing. REFERENCES [ 1] P.S. Bhandare, B.K, Lee, K. Krishnan, J. Thermal Anal., 49 (1997) 361. [2] C.C. Lin, Macromol. Symp., 135 (1998) 129. [3] M. Marcec, B. Tryba, R.J. Kaleficzuk, A.W. Morawski, Polym. Adv. Technol., 10 (1999) 588. [4] K. Laszlo, A. Szucs, Carbon, 39 (2001) 1945. [5] J.B. Parra, J.C. de Sousa, R.C. Bansal, J.J. Pis, J.A. Pajares, Adsorption Sci. Technol., 12 (1995) 51. [6] M.M. Dubinin, V.A. Astakhov, Adv. Chem. Series, 102 (1970) 69. [7] S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Am. Chem. Soc., 62 (1940) 1723. [8] P.L. Llewellyn, F. Rouquerol, J. Rouquerol, K.S.W. Sing, (Eds. K.K. Unger, G. Kreysa, P. Baselt), Studies in Surface Science and Catalysis, Elsevier Science, 128 (2000) 421. [9] F. Stoeckli, L. Ballerini, Fuel, 70 (1991) 557. [10] M.C. Blanco-L6pez, A. Martinez-Alonso, J.M.D. Tasc6n, Microporous and Mesoporous Materials, 34 (2000) 171. [11] A. Guillot, F. Stoeckli, Carbon, 39 (2001) 2059. [12] G. Finger, M. Biilow, Carbon, 17 (1989) 87. [13] M.J. Sell6s-P6rez, J.M. Martin-Martinez, J. Chem-Soc. Faraday Trans., 87 (1991) 1237. [14] J.B. Parra, J.J. Pis, J.C. de Sousa, J.A. Pajares, R.C. Bansal, Carbon, 34 (1996) 783.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
545
A M o n t e Carlo study on the structure of carbon dioxide adsorbed in microporous carbons. Th. A. Steriotis a*, G.K. Papadopoulos a, A. K. Stubos b and N. Kanellopoulos a Institute of Physical Chemistry, NCSR Demokritos, 15310 Ag. Paraskevi Attikis, Greece b Institute of Nuclear Technology & Radiation Protection, NCSR Demokritos, 15310 Ag. Paraskevi Attikis, Greece
a
C O 2 permeability data through a microporous activated carbon (AC) membrane at 308 K reveal maximum permeance at approximately 35 bar, in close analogy with mesoporous membranes. The shape of the permeance versus pressure curve implies a transition within the micropores, which does not occur at higher temperatures (333 K). Recent neutron diffraction experiments in conjunction with in-situ CO2 adsorption provide clear evidence of an orientational transition of adsorbed molecules in the vicinity of the maximum. In the present study the Grand Canonical Monte Carlo (GCMC) method is employed in order to reveal structural configurations of CO2 molecules adsorbed in microporous carbons. Specifically, the CO2 density inside single, slit shaped, graphitic pores of given width is found for a predefmed temperature and different pressures. Local density profiles as well as angular distributions of molecular axes within the pores are examined and discussed. The results provide useful insight concerning the densification process and the molecular packing in the micropores. Furthermore, an orientational transition in pores of sizes around 1.15 nm at 308 K and at ca. 35 bar is observed. This transition is absent at higher temperatures (333 K) and can be invoked to explain the above-mentioned experimental results.
I.
INTRODUCTION
Gas flow processes through microporous materials are important to many industrial applications involving membrane gas separations. Permeability measurements through mesoporous media have been published exhibiting a maximum at some relative pressure, a fact that has been attributed to the occurrence of capillary condensation and the menisci formed at the gas-liquid interface [ 1,2]. Although, similar results, implying a transition in the adsorbed phase, have been reported for microporous media [3] and several theoretical studies [4-6] have been carried out, a comprehensive explanation of the static and dynamic behavior of fluids in micropores is yet to be given, especially when supercritical conditions are considered. Supercritical fluids attract, nowadays, both industrial and scientific interest, due to their unique thermodynamic properties at the vicinity of the critical point. For example supercritical CO2 is widely used in industry as an extraction solvent as well as for chemical *Author to whomcorrespondenceshouldbe addressed(e-mail: [email protected])
546 and biochemical reactions [7], while structural investigations of CO2 have been extensively performed by neutron or x-ray diffraction techniques in a wide range of thermodynamic conditions [8, 9]. Very recently neutron diffraction data concerning CO2 adsorbed on microporous AC at 308 K have been reported [10] and reveal different orientational configurations of molecules as the equilibrium pressure changes. As conventional macroscopic descriptions are generally invalid for the study of sorption in micropores, improved approaches have been developed. In particular, density functional theory (DFT) has been used to provide accurate descriptions of simple (in practice atomic) fluids in confined spaces [11, 12]. This approach has been also employed to develop practical methods for the evaluation of the pore structure and the estimation of pore size distribution (PSD) over a wide range of pore sizes [13-20]. Molecular simulation based on the Monte Carlo (MC) technique has been established lately as an efficient alternative approach for the study of adsorption of gases in microporous solids [21-29]. Structural investigations for the packing of molecules in micropores have appeared accounting for effects of pore shape and size, temperature and molecule length [30, 31 ]. Recently, parallel attempts have been reported to use the technique to determine PSD and pore network connectivity from adsorption data of different molecules [32]. Gusev et al. [33] obtained the PSD of a microporous carbon by using MC simulations to analyze CH4 adsorption at 308 K. Lopez-Ramon et al. [34] have simulated the adsorption of CH4, CF4 and SF6 to produce estimates of PSD's in carbon, employing additionally elements of percolation theory to estimate the network connectivity. The present authors have used the GCMC method in order to calculate PSDs from experimental isotherms [35] and to study structural details of the packing of molecules adsorbed within micropores [36]. In this paper, the work is focused on relatively high temperatures (308 K, i.e. slightly above the CO2 critical temperature, and 333 K) and attempts to relate simulation findings on CO2 molecular packing in micropores with recent experimental permeance and diffraction data. Specifically, by means of GCMC simulations the mean CO2 density inside single slit shaped graphitic pores of given width is found for predefined temperatures and different pressures. Furthermore, local density profiles as well as angular distributions of molecular axes within the pores are produced. Thus, a clear picture of the packing of the adsorbed molecules is gained, while an orientational transition is observed only at 308 K, in accordance with both permeability [3] and neutron diffraction data [10]. 2. MODELING OF THE MOLECULAR INTERACTIONS
CO2 is modeled as a three charged centers molecule, according to [37] with the parameters eoo/kB=75.2 K, aoo=0.3026 nm, ecc/kB=26.3 K, trcc=0.2824 nm. The O-O and C-O distances of the model are 0.2324 nm and 0.1162 nm respectively. The intermolecular potential u/j is assumed to be a sum of the interatomic potentials between atoms a and fl of molecules i and j respectively (taken of Lennard-Jones 12-6 form), plus the electrostatic interactions due to CO2 quadrupole moment with the point partial charges qo = -0.332e and qc = +0.664e, i.e. u/j (r) = Z
~eap[(~ /raft~2 _((:rap/raft )6]+ qaq fl /(4~reorafl )}
(I)
where e0 is the permittivity of vacuum. Pore walls are treated as stacked layers of carbon atoms separated by A=0.335 nm, and having a number density pw=114 atoms/nm 3 per layer. The adsorbate-wall interaction at distance rz was calculated following Steele [38] with tss/kB=28.0 K and ass=0.340 nm:
547
Uw (rz ) = 2~rPwea~ a a~ A
7 a~
- (tTafl /rz )4-174afl
rz ) 3
(2)
The energetic inhomogeneity of the surface along the x and y directions is not taken into account, but this is not expected to affect the results significantly at 308 and 333 K [39]. The cross interaction potential parameters between different sites were calculated according to the Lorentz-Berthelot rules: a ~ = (aaa + a~B)/2 and e,r = (eaa @p)l/2. The potential energy Uw due to the walls inside the slit pore model for each atom of the CO2 molecule is given by the expression Uw = uw(r~) + u~(H-rz) where H is the distance between the carbon centers across the slit pore model. For the determination of PSDs the corrected width H' should be used since this is the one involved in the experimentally obtained isotherms, namely H ' - H - 2zo + ag, where ag is the root of the adsorbate-adsorbent Lennard-Jones function, and zo the root of its first derivative. In the present CO2-graphite system H ' = H - 0.24 nm [40]. 3. SIMULATION EXPERIMENTS The GCMC method was employed to probe the statistically important regions of the configuration space in the (IX, V, T) ensemble according to [41 ]. For the CO2 molecules three types of move are attempted with equal probability: (a) a random displacement and reorientation, with the maximum allowed displacement being adjusted so that the acceptance ratio of the move is about 20% in order to sample phase space more efficiently; (b) a random insertion of the center of mass of a molecule in a random orientation, by generating a unit vector distributed uniformly on the surface of a sphere centered at the origin of the Cartesian system of coordinates of the simulation box (Marsaglia's algorithm [41]); and (c) a random deletion of a molecule. Periodic boundary conditions have been applied in the directions other than the width of the slit. For a given simulation, the size of the box (i.e. the two dimensions other than H) was varied in order to ensure that sufficient particles (ca. 500) remained in the simulation box at each pressure. Statistics were not collected over the first 3x106 configurations to assure adequate convergence of the simulation. The tmcertainty on the computed equilibrium properties such as ensemble averages of the number of adsorbate molecules in the box and the total potential energy is estimated to be less than 4%.
CO2, T=308K
E
8
t,t) "5
6
v
4
to
^
- . ~ H=0.65
~v
H=1.75
~
- - ~ H=0.85
--~H=0.95 -e-.. H=1.05
0
0
0.2
0.2
0.4
0.4
0.6
PIPc
0.8
0.6
1 .-o--H=1.25
0.8
Fig. 1. Computedadsorption isotherms of C O 2 at 308 K, for different slit sizes.
1
548
4.
RESULTS AND DISCUSSION
The detailed CO2 density profiles across the slit pore have been computed (at 308 K and 333 K) for the whole range of micropore widths (0.6-2.0 nm, in steps of 0.05 nm). From this information, the average density in the micropores can be calculated and used to construct the corresponding isotherms, shown in Fig. 1 for 308 K only, since the general behavior found is quite similar to the 333 K case, both qualitatively and quantitatively. Data are plotted versus the ratio P/Pc (Pc=73.8 bar, being the CO2 critical pressure). It is observed in Fig. 1 that at low pressures the adsorbate density is highest in the smaller pores while at high pressures, the larger pores exhibit higher adsorptive capability. This reversal can be explained with reference to the adsorbate-adsorbate (a-a) and adsorbate-pore (a-p) interaction energies [30]. At low loadings, molecules occupy the energetically most favorable positions in the pore and the a-a interaction is much smaller than the a-p interaction. The attractive potentials due to each wall overlap most in the smallest pore, resulting in deep energy wells. In the wider pores at high loadings, molecules can occupy the central region, as well as the wall regions of the pore. This increased packing efficiency leads to higher densities in the pore [30-32].
0.85 nm
50 "~ 4O m
50
4o
'r 4O
3o ~
30
o
I~ 20
20
lO
10
0 -0.4
-0.2
0 z (nm)
0.2
1.15 nm
50
1.05 nm
5o
0.4
~
50 40
30
30 A
20
20
lO
0
0 -0.4
I 50
,
,
,
-0.2
0 z (nm)
0.2
50 t
1.35 nm
r0 0.4
1 50
,o
,o
v
10 0 -0:4
-0.2
0
Z (rim)
0.2
0.4
v
10
10
0
0 -0.4
10
-0.2
0
0.2
0.4
0
z (nm)
Fig. 2. Examples of singlet densities (solid line) and angular distribution of molecular axes (open symbols) computed for CO2 adsorption at 308 K and P/Pc=0.474 (35 bar).
549
Fig. 3. "Artistic"representation of adsorbed CO2 molecules (308 K, P/Pc=0.474)based on angular distributions of Fig. 2.
On the other hand the consideration of the singlet density of the position of the carbon atom under various conditions can elucidate the packing of CO2 molecules within the pores. The singlet density is the local number density at a distance z from the pore center. Computationally it is obtained by dividing the pore width into several parts (presently 100) and keeping record of the number of times a molecule enters each part during the simulation (Fig. 2, with z=0 at the pore center). The corresponding local orientation of the molecules is also indicated in these plots as the ensemble average of 0, the angle formed between the molecular axis (Coo) and the pore wall. It can be seen (Fig. 2 and the "artistic" representation in Fig. 3) for example that in the H=0.85 nm pore, molecules span the pore with the oxygen atoms lying in the potential well of each wall and the carbon atom near the pore center, while the angle formed between the molecular axes and the pore wall is 45-50 ~. In larger pores (H=l.05 nm) CO2 molecules tend to lie fiat against the wall and this configuration (all three atoms of the molecule in the potential minimum) leads to high average densities in the pore even at relatively low loadings. As the pore size increases further (H=l.15 nm) the same configuration holds but a distinct sub-layer is also forming. This particular molecular configuration is of great importance in our considerations and will be further discussed. In even larger pores (H=1.35 nm) the sub-layer disappears while molecules are also found in the central region of the pore, a configuration that is more pronounced in pores with H>l.35 nm. Following experimental CO2 permeability (through a microporous AC membrane) results [3], exhibiting a maximum at 308 K and a mean pressure of around 35 bar (P/Pc=0.474), which is absent at 333 K (see Fig. 4), Papadopoulos has recently presented in [42] a possible explanation of the CO2 behavior based on GCMC studies. The simulation results predicted an orientational ordering transition of the Coo axis of supercritical CO2, inside individual graphite micropores, only at 308 K and not at 333K. By studying the diffusion equation in terms of the Darken factor over a real porous medium, he concluded that this transition has a significant influence on the adsorbate permeability. On the other hand very recently, neutron diffraction experiments were performed [ 10], in conjunction with in-situ adsorption of CO2 at 308 K on the same carbon membrane. Based on these experiments the intermolecular part of the total radial distribution function, G(r), for adsorbed CO2 can be calculated (Fig. 5).
550
201 :~'~, ,.--,
/
4O
,,
, PIPc=0.813 ~
~
P/Pc=0.474
0
30
g
20
1.2
X
% ,-
0.8 10
o
0
I
I
I
0.2
0.4
0.6
'
P/Pc
Fig. 4. C O 2 permeancethroughan AC membrane versus P/Pc [3]
I
0.8
0.4
-------
0.3
0.5
0.7
0.9
r (nm)
Fig. 5. Radial distribution function of adsorbed CO2 molecules in microporous AC [ 10]
The G(r) functions exhibit a first neighbor peak (0.4 nm), followed by a damped oscillation and a second neighbor peak (0.8 nm). At the low-pressure state (10 bar) and especially at the high-pressure one (60 bar) the main peak reveals the presence of two structures centered at 0.37 and 0.415 nm respectively. The first is attributed to the C-O and OO interacting distances between orientationally correlated molecules whereas the latter arises mainly from the C-C distance between orientationally uncorrelated molecules. Such effects extend to the second neighbor molecules, since a split of the second neighbor peak appears in both curves (10 and 60 bar). In the case of the medium pressure (35 bar) no splitting is observed, while a shoulder appears at 0.5 nm, implying a different molecular arrangement. Considering the above experimental evidence of orientational transitions of adsorbed CO2 molecules, the detailed structure of the layer near the wall is of special interest. As implied by the density distribution and the pertinent 0 values of our simulations in H-l.15 nm pores (Fig. 2), this layer consists of two distinct sub-layers, one closer to the wall (sub-layer a) with molecules lying almost parallel to it and a second (sub-layer b) with molecules oriented at an average 45 ~ with respect to the pore wall. Similar sub-layers have also been observed in the case of ethane adsorption by Vishnyakov et al. [31 ]. The presence of these two entities has been attributed to the quadrupole-quadrupole interactions, which contribute essentially to the stability of such a herringbone like configurations [8]. This is further supported by GCMC simulations, since the absence of the coulombic term in Eq. (1) leads to the degeneration of the sub-layers into one uniform layer [36]. In Fig. 6 we present detailed plots of CO2 singlet density distributions in 1.15 nm pores at 308 and 333 K, "focused" on the layer close to the pore wall. The two maxima located at approximately 0.18 and 0.24 nm from the pore center indicate the position of CO2 molecular centers that constitute the two sub-layers. By tracing the density distribution changes with pressure at 308 K (Fig. 6a) it is observed that when the pressure increases the molecular arrangement changes in a way that sub-layer b is favored, since the fraction of molecules that lie flat on the pore wall is decreasing while the fraction of molecules that are oriented in a 45 ~ angle with the pore wall, is increasing. These simulation results imply that in the pressure range under consideration an orientational transition takes place, since molecules are re-oriented by literally turning away from the wall in order to achieve the most energetically "convenient" configuration.
551
40
o'2x j~ ~ ','6 t
1.15 nm, 308 K . . . . . P/Pc=0.542
40 ] i
35 1/) _.e
30
1.15 nm, 333 K
~-*-- P/Pc=O.542 I'-"-- P/Pc=0.474 35 PIPc=0.407
~
~ 3o
,::_~,-...~
25
25
20
j
0.
0.15
0.2 Z (rim)
(a)
0.25
0.3
20
0.1
0.15
0.2
0.25
0.3
Z (nm)
(b)
Fig. 6. Details of the CO2 density distribution within a 1.15 nm slit carbon pore at (a) 308 and (b) 333K, revealing an ordering transition at 308 K.
On the contrary, this not the case at 333 K, where the pressure increase leads to a proportional increase in the density of both sub-layers, implying that more and more molecules are simply added over the already adsorbed ones without any significant configurational changes. We may also note that this orientational transition was only observed for the 1.15 nm pore, at 308 K within the 30-40 bar pressure range, since the results for other pore sizes and pressure ranges follow the general trend shown in Fig. 6b (i.e. proportional increase of density all over the pore with increasing pressure). These results indicate the possible nature of the orientational transition proposed for the explanation of the observed permeability and neutron diffraction experimental data. More work is underway to confirm the present findings. In such a case, GCMC simulations will once more prove their potential as a tool to follow closely and probe adsorption phenomena in the molecular level. 5.
CONCLUSIONS
A series of experimental results imply that an orientational transition of CO2 molecules adsorbed in graphitic micropores takes place at the supercritical temperature of 308 K (but not at 333K) and at an equilibrium pressure of approximately 35 bar. In this work we employed the GCMC method to reveal structural details of adsorbed CO2 molecular configurations within microporous carbons. After a detailed examination of the local density profiles and angular distributions of molecular axes at different pressures within pores of different size at 308 and 333K, we observed an orientational transition of molecules confined in 1.15 nm slit graphitic pores at 308 K and at c.a. 35 bar. Specifically in this pore size two distinct sublayers are forming one being closer to the wall with molecules lying almost parallel to it and a second one with molecules oriented at a 45 ~ angle with the pore wall. At 308 K as the pressure increases from 30 to 35 and finally to 40 bar, the molecules in order to account for the increased total density, tend to re-orient themselves by turning their molecular axes away from the pore wall in a way that the second sub-layer is enriched in the expense of the first. This transition is absent at different pore sizes and outside the specific pressure range or even within the same pore and pressure range but at higher temperature (333 K). Our results could serve as a plausible explanation of the orientational transition implied by experimental data.
552
REFERENCES
[1] K-H. Lee and S-T. Hwang, J. Colloid Interface Sci., 110 (1986) 544. [2] R.J.R. Uhlhom, K. Keizer and A.J. Burggraaf, J. Membr. Sci., 66 (1992) 259. [3l F.K. Katsaros., Th. A. Steriotis, A. K. Stubos, A. Ch. Mitropoulos, N. K. Kanellopoulos, and S. Tennison, Microporous Mater., 8 (1997) 171 R. Evans, U.M.B. Marconi and P. Tarazona, J. Chem. Soc. Faraday Trans., 82 (1986) 1763. Bitsanis, S.A. Somers, H.T. Davis and M. Tirrell, J. Chem. Phys., 93 (1990) 3427. D. Nicholson, and R. Cracknell, Langmuir, 12 (1996) 4050. A. Akgerman in "Supercritical fluids: Extraction and Pollution Prevention", ACSSS No 670, M. A. Abraham and A. K. Sunol (eds.), American Chemical Society, Washington, 1997, pp. 208-231. [8] P. Cipriani, M. Nardone, F.P. Ricci and M.A. Ricci, Mol. Phys., 99 (2001) 301. [9] T. Morita, K. Nishikawa, M. Takematsu, H. Iida and S. Furutaka, J.Phys. Chem. B, 101 (1997) 7158. [lO] Th.A. Steriotis, K.L. Stefanopoulos, A.Ch. Mitropoulos, N.K. Kanellopoulos, A. Hoser and M. Hofmann, Appl. Phys. A, (2002) in press [11] Z. Tan and K.E. Gubbins, J. Phys. Chem., 96 (1992) 845. [12] E. Kierlik and M.L. Rosinberg, Phys. Rev. A, 44, (1991) 5025. [13] C. Lastoskie, K.E. Gubbins and N. Quirke, J. Phys. Chem., 97 (1993) 4786. [14] V. Neimark and P. I. Ravikovitch, Microporous Mesoporous Mater., 44-45 (2001) 697. [15] P.N. Aukett, N. Quirke, S. Riddiford and S.R. Tennison, Carbon, 30 (1992) 913. [16] J.P. Olivier, W.B. Conklin and M. Szombathely, in "Characterization of Porous Solids (COPS-lID, Proceedings of the IUPAC Symposium", F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing, K.K. Unger (eds.), Elsevier, Amsterdam, 1994, p.81. [17] A.V. Neimark, P.I. Ravikovitch, M. Grun, F. Schuth and K.K. Unger, Langmuir 13 (1995) 4765. [18] K.A. Sosin and D.F. Quinn, J. Porous Mater., 1 (1995) 111. [19] S. Scaife, P. Kluson and N. Quirke, J. Phys. Chem. B, 104 (2000) 313. [2o1 P I Ravikovitch, A. Vishnyakov and A V Neimark, Phys. Rev. E, 64 (2001) 11602. [211 G.M. Davies and N.A. Seaton, Carbon, 36 (1998) 1473. [22] G. M. Davies and N. A. Seaton, Langmuir, 15 (1999) 6263. [23] G.M. Davies, N.A. Seaton and V.S. Vassiliadis, Langrnuir, 15 (1999) 8235. [24] P.I. Ravikovitch, A. Vishnyakov, R. Russo and A.V. Neimark, Langmuir, 16 (2000) 2311. [25] Vishnyakov, P.I. Ravikovitch, and A.V. Neimark, Langmuir, 15 (1999) 8736. [26] R.F. Cracknell, D. Nicholson and N. Quirke, Mol. Phys., 80 (1993) 885. [27] R.F. Cracknell, D. Nicholson and N. Quirke, Mol. Sim., 13 (1994) 161. [28] R.F. Cracknell and D. Nicholson, J. Chem. Soc., Faraday Trans., 90 (1994) 1487. [29] M.B. Sweatman and N. Quirke, J. Phys. Chem. B, 105 (2001) 1403. [301 D. Keffer, H.T. Davis and A.V. McCormick, Adsorption, 2 (1996) 9. [31] Vishnyakov, M.E. Piotrovskaya and N.E. Brodskaya, Langmuir, 12 (1996) 3643. [32] S. Samios, A.K. Stubos, N.K. Kanellopoulos, R.F. Cracknell, G.K. Papadopoulos and D. Nicholson, Langmuir, 13 (1997) 2795. [331 V.I. Gusev, J.A. O'Brien and N.A. Seaton, Langmuir, 13 (1997) 2815. [34] M.V. Lopez-Ramon, J. Jagiello, T.J. Bandosz and N.A. Seaton, Langmuir, 13 (1997). 4435. [351 S. Samios, G.K. Papadopoulos, Th.A. Steriotis and A.K. Stubos, Mol. Sim., 27 (2001) 441. [36] S. Samios, A. Stubos, G.K. Papadopoulos, N.K. Kanellopoulos and F. Rigas, J. Colloid Interface Sci., 224 (2000) 272. [37] C.S. Murthy, S.F. O'Shea, and I.R. McDonald, Mol. Phys., 50 (1983) 531. [38] W.A. Steele, The Interaction of Gases with Solid Surfaces, Pergamon, Oxford, 1974. [39] D. Nicholson, J. Chem. Soc., Faraday Trans., 90 (1994) 181. [40] K. Kaneko, R.F. Cracknell, and D. Nicholson, Langmuir, 10 (1994) 4606. [41] M. Allen, and D.J. Tildesley, Computer Simulation of Liquids, Clarendon, Oxford, 1987. [42] G. K.Papadopoulos, J. Chem. Phys., 114 (2001) 8139.
[4] [5] [6] [7]
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
553
Preloading of GAC by natural organic matter: effect of surface chemistry on TCE uptake James E. Kilduff ~, Reena Srivastava a, and Tanju Karanfil b aRensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA bClemson University, 342 Computer Court, Anderson, SC 29625, USA Trichloroethylene (TCE) adsorption by activated carbon previously loaded ("preloaded") with natural organic matter (NOM) is reduced significantly in comparison to asreceived carbon. Granular activated carbon (GAC) surface chemistry was modified as a means to preferentially reduce the uptake of natural organic matter foulants and thus increase the uptake of TCE. Heat treatment (up to 1000 ~ under N2) and oxidation (conc. NO3) were successful in changing the surface chemistry while not extensively changing the pore structure of two GAC samples. The uptake of organic foulants decreased significantly with increasing surface acidity (as measured by NaOH neutralization). While the subsequent effects of preloading on TCE uptake were mitigated, TCE uptake by hydrophilic carbons was not satisfactory. In contrast, heat treatment of acid-activated wood-based carbons mitigated the effects of preloading while maintaining high TCE uptake. The observed effects could not be explained by a reduction in NOM uptake, suggesting a shift in the sorption mechanism, possibly resulting from the mesoporous structure of these carbons. 1. INTRODUCTION Granular activated carbon (GAC) is widely used for removing synthetic organic contaminants from potable water supplies, contaminated groundwaters, and industrial wastewaters. All natural waters and many industrial wastewaters contain natural macromolecular dissolved organic matter (NOM) that can compete with TCE and other target compounds, and thus significantly reduce the efficiency of activated carbon adsorbers [1-8]. As shown in Fig. 1 (left panel), preloading occurs when the mass transfer zone of NOM moves through a fixed bed at a faster rate than the target compound, and loads the carbon ahead of it. As shown in Fig.1 (middle panel), such preloading by NOM can result in direct competition for adsorption sites, pore blockage or both [9-11 ]. This results in a reduction in equilibrium uptake of TCE (Fig. 1, right panel). In addition, a reduction in the rate of synthetic organic compound adsorption is often observed, which may include an increased diffusion resistance at the particle surface [12], an intraparticle diffusion resistance [6,8], or both. Several factors could potentially influence the impact of preloading on adsorber performance. One such factor is the composition of the background NOM; for example, in many systems the effect of preloaded organic matter on SOC uptake is related to the amount of the smaller size fraction adsorbed [13,14]. Characteristics of the adsorbent, especially the pore size distribution [15] and surface functional group composition, are also important. Surface acidity has been implicated as a factor controlling the uptake of organic compounds [ 16-19]. Surface acidity may reduce the catalytic activity of the surface, increase
554 selectivity for water, modify the charge distribution on graphitic basal planes, change the adsorbate configuration, or produce some combination of these effects. It has been postulated that increasing the number of polar oxygen molecules within the carbon matrix or oxygencontaining surface functional groups increases the polarity of carbon surfaces and their selectivity for water. Previous work in our laboratories has shown that increasing carbon surface acidity and negative surface charge caused a significant reduction in the uptake of natural organic foulants [20]. Given that preloading effects are related to the loading of foulant [14], and that the loading of foulant can be controlled by modification of carbon surface chemistry, we hypothesized that such surface chemistry modification may be a viable route to producing carbons that resist fouling by natural organic matter. The objectives of this work were to (i) investigate carbon surface chemistry modification as a means to preferentially reduce the uptake of natural organic matter foulants and thus increase the uptake of TCE by GAC preloaded with these foulants; (ii) investigate the effects of surface chemistry and pore structure using a series of carbons prepared from different starting materials, and having a wide range of surface acidity; and, (iii) evaluate the effects of natural organic matter foulant physicochemical properties (e.g. molecular weight, acidity, hydrophobicity) by using several different natural and model humic substances (humic and fulvic acids) and natural waters.
2. EXPERIMENTAL METHODS All carbons used in this research were pretreated to remove fines by sonication (Bransonic 220, Branson Cleaning Equipment) for 30 seconds followed by a rinse with Type I reagent-grade water (Millipore Corp). Coal-based carbons were then extracted with 2-N HC1 in a soxhlet extractor for 42 hours to remove ash components and alkaline impurities. After extraction, the sample was boiled for four hours, then exhaustively rinsed until the pH of the suspension was in the range 5 to 6. All carbon samples were dried at 100 ~ under vacuum, and stored in a vacuum desiccator. The surfaces of a thermally-activated coal-based carbon (Calgon F400) and an acidactivated wood based carbon (Westvaco WVB) were modified using liquid-phase oxidation (HNO3) and heat treatment in an inert atmosphere (N2). Adsorbents were heat treated in a tube furnace (Model LSTF341C, Lindberg). An oxygen-free atmosphere was maintained by flowing purified nitrogen gas at a rate of 30 ml/min with a back pressure of 4 to 5 psi.
Figure 1.
Preloading in fixed bed adsorbers (left); preloading mechanisms (middle); effects of preloading on trichloroethylene uptake by as-received WVB GAC (right).
555 Adsorbents were de-gassed at 100~ for 24 hours prior to heat treatment. Target temperatures were attained at a ramping rate of 10 ~ per minute and maintained for a "soak" time of 24 hours, after which the furnace was allowed to cool to ambient conditions. Heat treated samples are coded as either HT1000 (1000 ~ or HT650 (650 ~ Activated carbons were oxidized in 70% HNO3 using a solution-to-solid ratio of about 17:1 w/w. Solutions were kept stirred for the duration of the contact, and temperature was maintained constant within 3~ After oxidation, the carbon was rinsed exhaustively until the pH of the suspension was in the range 5 to 6. Oxidized samples are coded by the symbol "OX" followed by the duration of the treatment in hours and the temperature in degrees Celsius (e.g., OX 2/50). Surface area and pore size distribution were measured with a Gas Adsorption Analyzer (Quantachrome Autosorb 1). Aliquots of carbon weighing approximately 50 mg were transferred to a glass sample cell and outgassed for 48 hours at 120 ~ The weight of the sample cell containing GAC did not change after outgassing, confirming negligible loss of material during this step. Nitrogen isotherms were measured at 77 K, and surface area was determined by fitting the Brunauer, Emmett and Teller (BET) adsorption isotherm equation to at least five low-pressure isotherm points (P/Po < 0.2) using a non-linear regression algorithm. Precision of replicate samples averaged 5%. Pore size distributions were calculated using the Barret, Halenda and Joyner method. Carbon surfaces were wetted either by boiling in water for 30 min followed by centrifugation to remove interstitial water, or by equilibrating washed samples with water in a thermostatted bath at 25 ~ for 48 hours, followed by centrifugation. The two techniques gave similar water uptake values, and both were reproducible within 2%. Water uptake values measured using the second technique are reported in Table 3. The acid/base characteristics of adsorbents were characterized using the Boehm technique [21 ] and are reported in Table 3. A 0.4-gram aliquot of carbon was equilibrated for at least 48 hours with 20 g of a solution 0.05N in HC1, NaOH, Na2CO3 or NaHCO3. After equilibration, the solution was filtered and titrated with either standard base (NaOH) or acid (HC1); endpoints were determined from the first derivative of the titration curve. NaOH titrant was standardized against 0.05-N potassium hydrogen phthalate. All solutions were prepared using reagent-grade chemicals, in CO2-free water. Reproducibility of the technique was within a few percent. It is often assumed that NaHCO3 uptake corresponds to strong carboxyl acidity, while NazCO3 further reacts with weak carboxyl and f-lactonic functionality, and NaOH further reacts with phenolic acidity [22]. Model NOM included polymaleic acid (PMA), a fulvic acid surrogate; natural humic (LaHA) and fulvic acids (LaFA) extracted from Laurentian soil; and Aldrich humic acid (AHA) purified to remove ash components. Physicochemical characteristics of these compounds have been reported in one of our previous publications [23]. Surface water samples were collected from the Edisto River, Charleston, S.C; the Intercoastal Waterway, Myrtle Beach, S.C.; and the Tomhannock Reservoir, Troy, NY. Physicochemical characteristics of these compounds have been reported previously [20]. Preloading was done in 250-ml well-mixed batch reactors (amber bottles sealed with teflon septa). First, 10 to 15 mg of adsorbent was contacted with an organic matter solution (6 to 20 mg/L as organic carbon) for 14 to 45 days. After preloading, reactors were sampled with glass syringes, and the solution was filtered through a 0.45-micron polysulfone filter. TCE isotherms were also measured in 250-ml well-mixed batch reactors (amber bottles sealed with teflon septa). Aliquots of TCE stock solution (4,000 mg/L in methanol) were added to reactors containing 10 to 15 mg of either as-received or surface treated carbons to yield initial TCE concentrations ranging from 0.030 to 8 mg/L. The solution phase consisted
556 of either buffer solution or NOM solution. TCE was equilibrated for 21 days, a time that was sufficient to reach an operational equilibrium. Hexane extracts of TCE were analyzed using gas chromatography. All isotherms were conducted in the presence of a 0.01 M phosphate buffer, at pH 7 and room temperature of 21 + 3 ~ 3. R E S U L T S A N D D I S C U S S I O N Examination of the data shown in Table 1 reveals that surface treatments were successful in changing the surface chemistry while not extensively changing the pore structure. For the coal-based carbon (i.e., F400), heat treatment and surface oxidation did not change the total surface area or the pore volume significantly. As-received coal-based carbons were more microporous than the acid-activated wood-based carbons, and a maximum shift of 14% in the surface area distribution from micropores to mesopores was observed with increasing extent of surface treatment. For the wood-based carbon, the initial heat treatment decreased the total surface area and pore volume by up to 35%. However, subsequent chemical oxidation had little impact on the pore structure, and a shift of only 4 to 7% in the surface area distribution from mesopores to micropores was observed. In contrast to these minor changes in pore structure, a wide range of surface acidity and functional group density was created on both types of carbon surfaces. In general, heat treatment decreased the surface acidity selectively (depending on the temperature) whereas oxidation in HNO3 solution increased the acidity, presumably by creating new carboxylic, lactonic and phenolic type functionality, as reported previously by others [21]. As expected, the acid-activated woodbased carbons had a more acidic character as exemplified by a negative surface charge (pHpzc values ranging from 3.45 to 5.35), whereas coal-based carbons had basic surfaces (pHpzc values ranging about 9.0-9.5). As a result, heat treatment to remove oxygen functionality had a large effect on the wood based carbons, decreasing their initially high surface acidity, but had little effect on the as-received coal-based carbons, which had low initial surface acidity. On the other hand, oxidation of the coal-based carbons had a dramatic effect on their surface chemistry, making them more hydrophilic and negatively charged. Table 1 Carbon characterization Physical Analysis Carbon Type
Surface Treatment
Surface Area, m2/g
Water Uptake gig
Chemical Analysis HCI Uptake l~eq/m2
NaOH Uptake ~eq/m2
Na2CO3 Uptake peq/m 2
NaHCO3 Uptake peq/m2 0.00
F400
AW
1044
0.63
0.34
0.14
0.00
F400
AW HT1000
1000
0.64
0.37
0.00
0.00
0.00
F400
OX 2/70
1041
0.67
0.22
0.94
0.40
0.22
F400
OX 9/70
1031
0.68
0.11
1.71
0.96
0.61
F400
OX 2/70 HT650 1029
0.67
0.29
0.26
0.00
0.00
F400
OX 9/70 HT650 1066
0.72
0.24
0.55
0.22
0.05
WVB
AW
1745
1.55
0.18
0.57
0.24
0.12
WVB
AW, HT1000
1169
1.13
0.23
0.38
0.08
0.00
WVB
OX 2/50
1272
1.14
0.17
0.76
0.40
0.19
WVB
OX 2/70
1295
1.13
0.13
1.14
0.62
0.35
557 The ability to control NOM adsorption through modification of surface chemistry was demonstrated previously [20]; the uptake of several model humic substances and natural organic matter isolated from surface waters decreased significantly with increasing surface acidity (as measured by NaOH neutralization). The uptake was partially restored by subsequent heat treatment of the oxidized surfaces (i.e. OX 9/70 HT650). For the wood-based carbons, the impact of surface treatment on adsorption of organic matter was surprisingly small or absent. Overall, the reactivity of carbon surfaces to DOM uptake depended on the raw material type, activation conditions and surface treatment. Previous work in our laboratories has shown that surface acidity also plays an important role in the uptake of TCE [19]. Because TCE is adsorbed by a hydrophobic adsorption mechanism, and because increasing surface acidity reduces the hydrophobicity of the surface, it was found that in general, increasing surface acidity reduced uptake. Examples of such behavior are shown in Fig. 2. For the F400, having low oxygen content initially, uptake decreased significantly upon exposure to nitric acid, which increased acidity. For the WVB carbon, having high acidity initially, solute TCE uptake increased significantly after heat treatment to reduce surface acidity and polarity. Heat treatment temperature had a significant effect on uptake; however, the effects of heat treatment were the same for reaction times ranging from 2 to 16 hours (data not shown). It was our goal to optimize the effects of surface treatment- to decrease the uptake of organic matter, thereby minimizing the effects of preloading, but without making the surface so hydrophilic as to reduce the uptake of TCE to an unacceptable degree. In assessing the success of various surface treatments, two comparisons are of interest. These are depicted schematically in Figure 3. For a given surface treatment, a comparison of preloaded and non-preloaded carbons gives the effect of preloading. This is shown as comparison 1 in Figure 3. The effect of preloading can then be assessed for different surface treatments, and the role of surface treatment in mitigating the impact of preloading can be determined. However, only those treatments for which uptake by preloaded carbon is higher than preloaded as-received carbon are of economic interest. Therefore, a comparison of preloaded as-received carbon with preloaded surface-treated carbon gives the overall effectiveness and economic viability of surface treatment for preloading control. This is shown as comparison 2 in Figure 3. 100
100 WVB
F400 it II
d m O I---
1
o"
O o 9
o-
~"
~2"
el 10-
9 qtgl, i
."
~
. . . . . . . .
,
10
. . . . . . . .
9 9149
,
100
. . . . . . . .
t
1000
TCE Concentration, gg/L Figure 2.
1
9As-recieved 9HT1000 9OX 2/70 9OX 9/70 o OX 9/70 HT650
A 0.1
Ii
.
.
.
.
.
9As-recieved 0 400 ~ A 600 ~ o 800 ~ [] 1000 ~
9 9
I~ .
.
.
.
10000
.
.
.
.
0
.
.
1
.
i
10
. . . . . . . .
f
100
. . . . . . . .
i
1000
. . . . . . . .
10000
TCE Concentration,gg/L
The effect of carbon surface treatment on the uptake of TCE by GAC. Plate A: the effects of surface oxidation and subsequent heat treatment on uptake by F400 carbon. Plate B: the effects of heat treatment temperature (6 h reaction time) on uptake by WVB carbon.
558 I O0
Surfacetreated, preloaded --7 ~--- Surfacetreated
~
..~.... ~ ".............. " " " "i"
\
. . : : .......... ~ s
~
1
""I
1
I
...o,.
.....,*"
_~........ " I - / - - - A s - r e c i e v e d
..........
0.1
~..
10
As-received, preloaded
100
1000
10000
TCE Concentration, lag/L
Figure 3. Assessing effects of preloading and surface treatment.
In this figure, the surface treatment mitigates the impact of preloading, and the uptake after preloading is higher than the uptake by preloaded as-received carbon. Therefore, in this example, surface treatment would be of economic interest. The impacts of surface acidity on the effects of preloading were found to depend on the surface treatment, carbon type (wood or coal-based) and the organic matter source (i.e. aquatic, soil, or synthetic) in a complicated way. In several cases, particularly for the F400 carbon as shown in Figure 4, the effects of preloading are mitigated with increasing surface acidity, a trend consistent with the effects of surface acidity on humic loading. Not all experiments showed such trends however. Indeed, some data (not shown) suggests that specific interactions may occur between preloaded humic materials and surface functional groups, especially those remaining after heat treatment of oxidized surfaces at 650 ~ When specific interactions occur, the high affinity between the carbon surface and organic matter can cause large reductions in uptake resulting from preloading. In general, the effects of preloading were smallest for the most hydrophilic carbons, i.e., the as-received WVB carbon and the most oxidized F400 carbon (OX 9/70). However, even though the effects of preloading were low (or in some cases actually increased the uptake of TCE), the uptake was also low. 100
PMA
o,J
E
LU tO I-
9
9~
t:~ =k 10
9 <>
dv .i.,., e-s
100
9 <>
*o
.o.O~
9
E
o
LFA
<>
9~
10
9 o
9
1
9
o
Ip
9
Io 9
As-received
o As-received, preloaded
9 OX 9/70 o o x 9/70, preloaded
A 0.1
9
Q.
10
100
1000
TCE Concentration,l~g/L
Figure 4.
10000
LU O I--
9
<>
9 0
o o 9
9
o
1
goo
o
o
9 9
%
~ m Q
9
R 0.1 10
9As-received <> A s - r e c e i v e d , p r e l u d e d 9OX 9/70 o OX 9/70, p r e l o a d e d 100
1000
10000
TCEConcentration,gg/L
The effect of carbon surface treatment on the uptake of TCE by preloaded GAC. Plate A: carbon preloaded by poly(maleic acid); Plate B: carbon preloaded by Laurentian fulvic acid.
559 Therefore, even though the effects of preloading were mitigated, the use of highly oxidized carbons to control preloading effects is not economically attractive. The wood-based carbons exhibited different trends than the coal-based carbons in one important way. Figure 5 presents data for TCE uptake by heat-treated WVB carbon. A comparison of preloaded and non-preloaded carbons (comparison 1 in Figure 3) suggests that the impact of preloading on TCE uptake by heat treated carbon is less than the impact on uptake by as-received carbon- i.e., the percentage reduction in single solute uptake by heat treated carbon as a result of preloading is smaller than the reduction observed for as-received carbon. More importantly, a comparison of preloaded, as-received carbon with preloaded, surface-treated carbon (comparison 2 in Figure 3) reveals that uptake by preloaded, heat treated carbon (open symbols) is significantly higher than both that of single solute uptake by as-received carbon (black circles) and preloaded, as-received carbon (filled diamonds). The observed effects cannot be explained by a reduction in the uptake of NOM (data not shown). Consistent with previous work [20], heat treatment of wood based carbons does not significantly affect the uptake of natural organic matter (or other organic macromolecules). The data shown in Figure 5 confirm the efficacy of heat-treated WVB carbon for mitigating preloading effects of both natural organic matter from a surface water, and several different model organic macromolecules. Therefore, the results are valid for organic macromolecules having a wide range of physicochemical properties including molecular weight, acidity, and polarity [20]. A comparison of the data in Fig. 2 (Plate A, filled circles) and Fig. 5 (Plate B, open symbols) reveals that the performance of the heat-treated wood-based carbon, even under some preloading conditions, is similar to single solute TCE uptake by coal-based activated carbons in the absence of preloading [9]. The observed effect may result from some combination of optimum surface acidity, optimal type of surface functional group, and/or pore structure effects. The WVB carbon has a mesoporous pore structure, which has been observed to minimize the impacts of preloading in preliminary comparative experiments designed to isolate this effect (data not shown). Future work will employ carbon surface characterization techniques that will allow identification of functional groups and more accurate correlation with surface reactivity. 100
100
Heat Treated W V B TMK Preloaded
10
r
o~
E
E ~
Heat Treated WVB Model Compound Preloaded
Q~
o
o~
o
o
o e 9
q'
Q. U.l
1
....
9Preloaded, 4 wks o Heat treated, single sol Preloaded, 2 wks DOPreloaded, 4 wks Preloaded 6 wks
I--
A 0.1 0.1
1
10
TCE
Figure 5
100
1000
Concentration,gg/L
10000
9 g 0.1 1
9 ~ 9
9 t 9
9As-received single solute 9LFA preloaded O Heat treated - single solute <> LFA preloaded [] AHA preloaded PMA preloaded
lO lOO lOOO TCE Concentration,l~g/L
10000
A comparison of TCE uptake by as-received (filled symbols) and heat treated carbons (open symbols), in single solute (circles) and preloaded systems; carbon preloaded with natural organic matter (Plate A) or model compounds (Plate B)
560 ACKNOWLEDGEMENTS
James Kilduff and Reena Srivastava gratefully acknowledge partial financial support for this research from the Eastman Kodak Company. Support from the U.S. EPA (R 82815701-0, Kilduff, Karanfil), the U.S. NSF (BES 0093600, Karanfil; BES 9871241, Kilduff) and the Eastman Kodak Co. (Kilduff) is also gratefully acknowledged. This research has not been subject to funding agency peer or policy review, it does not necessarily reflect their views, and no official endorsement should be inferred. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
M. Pirbazari and W.J. Weber, Jr., J. Environ. Eng., 110 (1984) 656. E.H. Smith, S. Tseng, and W.J. Weber, Jr., Environ. Prog., 6 (1987) 18. T.F. Speth and R.J. Miltner, J. AWWA, 81 (1989) 141. R.S. Summers, B. Haist, J. Koehler, J. Ritz, G. Zimmer, and H. Sontheimer, J. AWWA, 81 (1989) 66. T.F. Speth, J. Environ. Eng., 117 (1991) 66. M.C. Carter and W.J. Weber, Jr., Environ. Sci. Technol., 28 (1994) 614. E. Orlandini, T.G. Gebereselassie, J.C. Kruithof, and J.C. Schippers, Wat. Sci. Technol., 35 (1996) 295. D.R.U. Knappe, V.L. Snoeyink, P. Roche, M. Jose-Prados, and M-M. Bourbigot, J. AWWA, 91 (1999) 97. M.C. Carter, W.J. Weber, Jr., and K.P. Olmstead, J. AWWA, 84 (1992) 81-91. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J. Colloid Interface Sci., 205 (1998a) 271. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J. Colloid Interface Sci., 205 (1998b) 280. K. Wilmanski and A.N. Van Breeman, Wat. Res., 24 (1990) 773. G. Newcombe, M. Drikas, and R. Hayes, Wat. Res., 31 (1997) 1065. J.E. Kilduff, T. Karanfil, and W.J. Weber, Jr., J AW WA, 90 (May 1998a) 76. C. Pelekani and V.L. Snoeyink, Wat. Res., 33 (1999) 1209. V.L. Snoeyink and W.J. Weber, Jr., Environ. Sci. Technol., 1 (1967) 228. L.R. Radovic, I.F. Silva, J.I. Ume, J.A. Men6ndez, C.A. Leon y Leon, and A.W. Scaroni, Carbon, 35 (1997) 1339. P. Pendleton, S.H. Wong, R. Shumann, G. Levay, R. Denoyel, and J. Rouquerol, Carbon, 35 (1997) 1141. T. Karanfil and J.E. Kilduff, Envir. Sci. Tech., 33 (1999) 3217. T. Karanfil, J.E. Kilduff, M. Kitis, and A. Wigton, Environ. Sci. Technol., 33 (1999) 3225. T.J. Bandosz, J. Jagiello, and J.A. Schwarz, Anal. Chem., 64 (1992) 891. J.J. Noh, and J.A. Schwarz, Carbon, 28 (1990) 675. T. Karanfil, J.E. Kilduff, M.A. Schlautman, and W.J. Weber, Jr., Environ. Sci. Technol., 30(1996)2187.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
561
Structural studies of saccharose- and anthracene-based carbons by high energy X-ray scattering. A.Szczygielska a, A.Burian a, S.Duber b, J.C.Dore c and V.Honkimaki d. aInstitute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland bFaculty of Earth Sciences, Interdepartmental Laboratory of Structural Researche, University of Silesia, Bedzinska 60, 41-2000 Sosnowiec, Poland CSchool of Physical Sciences, University of Kent, Canterbury CT2 7NR, UK dEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble cedex 9, France A series of porous carbon materials, produced by pyrolysis of saccharose and anthracene and heat-treated at 1000~ 1900~ and 2300~ have been studied by wideangle X-ray scattering. The X-ray data were collected at European Synchrotron Radiation Facility (ESRF) in Grenoble on the ID 15 beam line (high-energy X-ray diffraction) using the wavelength 3~=0,1067A (E=I 16,2 keV). The data were recorded in the scattering-vector range from 0.5 to 24A-1 which allowed to conversion to real space via the Fourier transform yielding the radial distribution function of a good quality. Analysis of the experimental radial distribution function shows that carbons produced from anthracene transforms into graphite at 1900~ and this process is almost complete at 2300~ The saccharose-based carbons remain disordered even at high temperature. 1. I N T R O D U C T I O N Although pyrolytic carbons have been produced for about fifty years and studied by Xray, electron and neutron diffraction, which are the most direct probes of their structure, interpretation of experimental data and explanationof different behaviour of these materials upon annealing is rather complicated, due to interplay of order and disorder in interatomic correlations. It has been known that some carbon materials are more susceptible to graphitization than others. The first detailed study of graphitizing and non-graphitizing carbons was performed by Rosalind Franklin [1 ]. The graphitizing carbons are soft and less porous and can be easily transformed into graphite under thermal treatment. The latter are much harder, have a complicated porous structure and cannot, or can only with difficulty, be transformed into graphite, even at temperatures of 3000~ or above. The internal surface of graphitizing carbons can be enhanced by mild oxidation with carbon dioxide or steam, which makes them useful for a wide range of commercial applications. The traditional view, about the structure of such carbonaceous materials, is that their microstructure consists of twisted networks of carbon layers cross-linked by bridging groups, but nature of such cross-links is not explained precisely [2]. Earlier suggestions that sp3 bonding may be present in this kind of carbons are now questioned [3]. The discoveries of the
562 fullerenes [4] and the carbon nanotubes [5] has given a new perspective on sp2-bonded carbons structures. They have prompted several authors to develop models for nongraphitizing carbons containing non-six-membered rings (e.g. pentagons or heptagons) besides the graphitic six-membered rings [6-8]. Generally, the structure of non-crystalline forms of carbon can be described in terms of disordered graphite-like layers with the very weak interlayer correlations or random stacking [1,9]. The purpose of the present work is to trace changes in the structure of two disordered carbons obtained from anthracene (graphitizing) and saccharose (non-graphitizing) depending on temperature of preparation and annealing processes. The effect of heat treatment at temperatures in the range of 1000-2300~ on these carbons, is studied by detailed analysis of the reduced radial distribution function (RRDF) computed from the X-ray scattering data_ The model based on the limited in size graphite-like structure with very weak interlayer correlations was taken to simulate of the RRDF's. 2. E X P E R I M E N T A L
TECHNIQUES
The carbons studied in the present work were prepared by the carbonization of anthracene (Cl4H10) and saccharose (C12H22Oll) in flowing argon. The sample preparation has been described in detail in our previous paper [10]. The high-energy X-ray scattering experiment were performed on the ID15A beam-line at the European Synchrotron Radiation Facility (ESRF, Grenoble). The photon energy E=ll6.2keV 0~=0.1067 A) was used for the measuremems. The powdered samples were placed in capillary tubes of 3mm diameter and mounted on the diffractometer axis. The intensities were recorded using a Ge solid-state detector. The measured intensities cover a Krange up to 24 A -1. 3. T H E O R E T I C A L
OUTLINE
Investigation of the structure of disordered systems can be performed directly by diffraction methods. The reduced radial distribution function d(r) is related to the structure factor S(K) as follows:
d(r)
.
4rCpo[g(r . . ).
1]
2 ~ -SoK[S(K)7c
where po is the number density and
(1)
1]W(K) sin(Kr)dK
W(K) is the window function:
W(K)-sin( xK )/( ~ ) 1(max
(2)
/~max
and g(r) is called the pair correlation function and can be written as: g(r) - 1+ ~
1
eK
~~ K [ S ( K ) -
2rc2rPo ,
llW(K) sin(Kr)dK.
The structure factor is defined as S(K)-I(K)/fl, where intensity andfis the atomic scattering factor.
I(K) is
(3) the corrected and normalised
563
S(K)
I(K) -
-[
1 sin(Kr~ ) -N,:,,:, ( K r ~ )
]
e
cr~
k.;
-
+1
(4)
2
The structure factor S(K) is independent on the scattering power of individual atoms and depends only on the structure of the investigated sample. The experimental RRDF provides information about the probability of finding an atom in a spherical shell at a distance r from an arbitrary atom. Successive peaks correspond to nearest-, second- and next-neighbour atomic distribution. Assuming three-dimensional Gaussian distribution of the interatomic distances with standard deviation a .The d(r) function can be finally expressed as follows [11,12]:
4zcr2g(r)P~ =
r~,%
1
N, e x p
; Jo~-~cr, r,
2cr7
-exp
2cr2,
-
-
-
+
where
P ( r ) = --1 x.~ !rv (IC) c o s ( K r ) d K
(6)
The sum in Eq. (5) was taken over all coordination spheres. A model of the atomic distributions may be constructed, based on Eq. (5), in terms of a series of peaks convoluted with the peak-shape function P(r) to include the effects of data termination at Km~. In order to compensate for the edge effects in a finite-sized model the correction term e(r), which corresponds to probability of finding an atom at a distance r from another atom lying inside the volume of the considered model, is introduced as multiplier of the RRDF computed for the infinite model. According to [ 13], for the model in the form of a rectangular prism with the edged a, b and c, e(r) is given by:
c(r)
=
1
-
lit3
abe
4-x
3re
!- ~
(7)
2
for r
1- exp(-d /
4nr2p~
= ~ as for one layer
+ 4ztrna
e x - ~ / - L - ) -L1) . . . . . . .
tr/d
cl -,- .
L [l_exp(r/Lc) .
.
]~
,
(8)
.
turbostratlc term
where no is the number of atoms per unit area on plane Po = na / d , dc the intra-layer spacing, Lr is the mean crystallite size along the direction perpendicular to planes, [r/dc] denotes the
564
greatest integer in r/dc [ 14] and g(r) is the pair correlation function. In order to simulate the paracrystalline structure in layer stacking the turbostratic term has been averaged assuming that the inter-layer spacing d has normal distribution. 4. R E S U L T S A N D D I S C U S S I O N 4.1. A n a l y s i s in reciprocal space The scattered intensities for the three investigated samples were corrected for scattering coming from the empty capillary, Compton scattering and then normalised using the high angle method. The structure factors S(K) calculated according to Eq. (4) for the saccharose- and anthracene-based carbons are shown in Fig. 1 together with the structure factor of graphite. The RRDF's were computed according to Eq. (1) in the K-range from 0 to 24 A "1. A comparison of the d(r) functions for the saccharose- and anthracene-based carbons annealed at 1000~ 1900~ and 2300~ with graphite is shown in the r-range of 0-20 A in Fig. 2, on the top and bottom, respectively. The structure factors of both carbons annealed at 1000~ are very similar and typical for highly disordered materials called turbostratic carbons [9]. In the turbostratic structure the graphitic layers are stacked without spatial correlations. As the temperature of annealing increases, the two investigated carbons exhibit quite different behaviour. The structure factors of the saccharose carbons heat-treated at 1900~ and 2300~ show sharper and higher peaks but the diffraction patterns differ substantially from that of graphite and remain turbostratic in nature.
20
9
18
'
|
,
i
,
|'
,
!
16
, '1
,
i
,
i
,
|
,
|
saccharose-based
.
,
i
,
i
,
30
....
i ....
i ....
i ....
i ....
i ....
i ....
i ....
saccharose-based
carbons
i ....
i ....
carbon
.
14
20
12 10
g~ ~ 0
15 ~
8
cO
4
lOOO ~
2 0
.~
~
o
C 0
-,,,,i
0
~
20
'
I
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
I
'
,
a n t h r a c e n e - b a s e d carbonsS
'~
601_ ....
....
i ....
2
4
, ....
I ....
I ....
6 ] ....
_~'~-5o ~9
I ....
8 I ....
i ....
10
i ....
12
,''"',
i ....
14 ....
, ....
i ....
16 I ....
la,iI
18
20
, ....
a n t h r a c e n e - b a s e d carbon _
16 14
~
12
"10
40
10 8
"~
20
-
6 10
4 2
f,
,
1 I
0 0
2
4
6
8
,
I
10
,
I
12
,
I
14
,
I
16
K=4nsinO/Z, [A 1]
,
I
18
,
I
20
o ,
I
22
,
24
o
lo o= ....
0
=
I ....
4
I ....
6
I ....
8
I ....
10
I ....
12
I ....
14
I ....
16
I ....
is
zo
radial distance [AI
Fig. 1. The structure factors S(K) for the Fig. 2. The reduced radial distribution saccharose- and the anthracene-based carbons functions d(r) for the saccharose- and the annealed at 1000~ 1900~ and 2300~ anthracene-based carbons annealed at 1000~ 1900~ and 2300~ Information about disorder comes also from the position of the first diffraction peak, which defines the inter-planar spacing. The K values of the first diffraction peak are: 1.675 A"
565 1, 1.825 A -1 and 1.838 A -1 for the saccharose carbons annealed at 1000~ 1900~ and 2300~ respectively. Using the Bragg equation, the values of the corresponding inter-layer distances are: 3.75 A, 3.44 A and 3.42 A. These values are substantially greater than that of graphite (3.35 A) supporting the conclusion about the turbostratic nature of the structure of these carbons, in which such increase is expected [9,10, 14-15]. The structure factor of the anthracene carbon heated at 1000~ is very similar to that of the saccharose one and the value of the inter-planar spacing is 3.47 A (the first diffraction peak appears at 1.813 AI), which is an indication of the turbostratic structure. At higher temperatures the inter-planar distances are appreciably reduced to the value of 3.37 A, which suggests that the structure of the anthracene-based carbons is much more ordered and similar to the graphite structure. This conclusion can be reinforced by comparison of the structure factors of the anthracene samples at 1900~ and 2300~ with S(K) for graphite in the whole K-range. All peaks for the samples annealed at 1900~ and 2300~ are in a good agreement with graphite, considering both positions and amplitudes. Such observation suggests that the anthracene-based carbon transforms into graphite at 1900~ and this process is almost completed at 2300~
4.2 Analysis in real space In order to study the graphitization process in more details the RRDF's were simulated for the models in which the graphitic structure within a single layer and a random stacking sequence of layers was assumed. The experimental RRDF's are shown in Fig. 2 together with the data of graphite. The RRDF's were simulated according to Eq. (5) and Eq. (8), taking the values of the interatomic distances and the coordination numbers characteristic of a single graphitic layer. The model size was taken into account multiplying d(r) computed for the infinite model by e(r). The values of a and b were estimated for all investigated carbons on the base of the range of oscillations of the reduced radial distribution function in a high rrange. The c-parameter, was obtained according to dependence: c-dc(Nrl), where dc is interlayer spacing and N1 is the number of layers. The number density/90=0.112 atoms/A 3 was taken for calculations, which corresponds to the macroscopic density about 2-3% less than that of graphite. This value has only a weak effect on the turbostratic term in Eq.(8) and influence of this lower value on the resulting simulations can be practically neglected. The standard deviation of the interatomic distances tri Eq. (5), increasing proportionally to the root-square of the interatomic distance, according to the paracrystalline distortion of the two dimensional network within a single layer tri = tro rm was used for computations. The paracrystalline theory proposed by Hosseman and his group, implies the so-called a* relation [ 11 ]:
a*
=
~
Adc
de
~0.15
(9)
where Nt indicates the average number of the netplanes in the paracrystal. This relation has the physical meaning that the real paracrystals have limited size controlled by a degree of disorder specified by Adc/dc. The validity of the models was checked by comparison of the experimental and modelled functions (Figs. 3 and 4). The model parameters are listed in Table 1.
566 Table 1 Modelling parameters a, b, c, ao obtained for the anthracene- and the saccharose-based carbons. The anthracene-based carbons The saccharose-based carbons a : b [A] c [A] or0 [A] a : b [A] c [A] or0 [A] 1000~ 20 13.8 0.06 18 7.2 0.058 1900~ ......... 48 24.8 0.037 2300~ 55 37.6 0.037
Because d(r) functions of the anthracene-based carbons heated at 1900~ and 2300~ were only compared with the reduced radial distribution functions calculated for the threedimensional graphite, the tabledoes not include the simulation parameters. The experimental RRDF's are compared with the simulated functions and the experimental data for graphite in Fig. 3. From an inspection of Fig. 3 and 4, it can be seen that the paracrystalline model with turbostratic layer stacking accounts very well for the experimental data at 1000~ for both carbons. The sizes and the distortions of the ordered regions within a single layer are practically the same. However, the model size in the direction perpendicular to the layers is for the anthracene carbon about twice of that of the saccharose carbon. This result is consistent with the appreciably higher amplitude of the first diffraction peak for the former, as can be seenin Fig. 1.The amplitude of the first diffraction peak is related to the number of correlated layers in graphitic materials. As the temperature increases, the model sizes are significantly greater and the network distortions clearly lower for the saccharose carbons. The experimental and model RRDF's at higher temperatures show greater discrepancies suggesting that, the structure is less distorted and deviations from the paracrystalline turbostratic model are possible.The paracrystalline distortions may relax when temperature increases making the model size larger, according to the relation expressed by Eq. (9). Such network relaxation is not enough to allow the transformation of the saccharose-based carbons into graphite. The RRDF's for the samples annealed at 1900~ and 2300~ are clearly different when compared with the graphite data. From this comparison we can conclude that these carbons remain disordered even at relatively high temperatures. A quite different behaviour is observed in the case of the anthracene carbons. These carbons are strongly disordered at the early stage of their preparation and their structure can be satisfactorily described by the paracrystalline turbostratic model at 1000~ At 1900~ the RRDF is very similar to that of graphite and at 2300~ the material is almost completely transformed into graphite, as can be seen in Figs. 2 and 4
567 t
4~
-
l-
.
"
anthracene-based carbon 10001 ex~.me.=, function- solid hne -~ ino=,,unc.on - ~ o ~ li..
~
t
!1
4 j-
j~
.
|l
~
sacharose-based e• ....
carbon 1000
1
function -solid Ilt~
.
r .Q 1~ (" :~
0 12!, ~9
.--
"0
I-I-
t-
"0 12[
0
~ L
o(1) ~1
--
6
1 ~
2 ~ ,;
i ~
I
1
2
L j
,
4 5 '= '
,
~
,
I
3 i
,
I
4
,
, ~
"~
6 7 8 9 10 11 12 ' ~'-~-~' =' ~' ~ ' ~ "--1 anthracene-based carbon 1900]
=
, ,
J__L_,
5 t
,
6 i
I ,
,
7 i
,
I
8
~
, ,
I
,"
9
t
,
10
~
0
,
i--r--]~-,=-y--.,-~
2
I
0
1
S I-
3
,
I
4 radial
,
I
,
I
,
I
=
I
I
l
iv
I
10
i
I
,
I
2
,
I
3
,
4
I
, _L~_
I
5
6
7
,
I
8
j
I
,..~
9
.
--"~"
o
"0
8 i---,---F-~ ] L ".
model ~ n c t l o n
dotted hne
,
11
12
1900
_~
/
.~A
"
"
-
I
10
sacharose-basedcarbon
:
4
_]
o
1 .__~..__d
11
12
Fig. 3. Comparison of the experimental and model reduced radial distribution functions for the anthracene-based carbons.
1
;;}eL-
"~
~
0
5 6 7 8 9 distance r [ A ]
~
"..~
12
~ anthrac,,r,~-ba=,d car~,on 2300
~ e x p e d m e n t a l f u n c t i o n - solid line _ ~ | | expedmental function for gral)h~e - dotted Itne
--
I
1
.41_~
A
I -bJ
11
. vw "
t-"
.0
~ expenmental function - sohd line -1 | | experimental function for graphite - dotted line - I
.'.
1 i
3 ~'
-
-4
3
,
~
R
ti
ll
I
1
4
'
!
'
s
6
, 1 , 2 3
~'
8
9
if .......
-oo~ ..... --
11
12
hdkne
....
-
,
lo
i ' J ' I--'--F--'-~ ' I ' ~ ' sacharose-based carbon 2300 .......
g
"
'
0
2
, i , , , 4 5 6 7 8 9 radial distance r [ A ]
-
,
, 10
, 11
12
Fig. 4. Comparison of the experimental and model reduced radial distribution functions for the saccharose-based carbons.
5. C O N C L U S I O N S The high energy X-ray scattering experiments, performed using synchrotron radiation, yielded a good quality RRDF's, which can be numerically analysed. This analysis demonstrates that the structure of the saccharose-based carbons at 1000~ 1900~ and 2300~ and the anthracene-based carbon at 1000~ can be described in terms of a graphitelike model with very weak interlayer correlations. Moreover, the structure of investigated samples can be described on the base of the model consisting of the small ordered regions, which are randomly distributed in space. The carbons produced from saccharose are clearly non-graphitizing, whereas the anthracene-based carbon can be easily converted into graphite under the high temperature heat-treatment and their structure can be described on the base of graphite structure. It can be also concluded that, together with the temperature, the size of model increases both within a single layer and in the c-axis direction. Consequently, the ordered domains increase together with the temperature. This behaviour confirms Oberlin's suggestions based on the electron microscopy investigations [8,16]. The results of the present work are also in agreement with our earlier electron microscopy and Raman scattering studies [8,17-19].
568
Acknowledgment The work was supported by the State Committee for scientific Research (KBN) grant no 5 PO3B 113 21.
REFERENCES 1. 2. 3. 4.
R.E. Franklin, Proc. R. Soc. (London) A, A209 (1951) 196-218. B.McEnaney, Carbon, 26 (1988) 267. S.Ergun and L.E. Alexander, Nature, 195 (1962) 765. H.W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl and R. E. Smalley, Nature, 318 (1985)162. 5. S. Iijima, Nature, 354 (1991) 56. 6. P.J.F Harris, Int. Mater. Rev., 42 (1997) 206. 7. P.J.F Harris, S.C. Tsang, Phil. Mag. A, 76 (1997) 667. 8. P.J.F.Harris, A.Burian and S.Duber, Philos. Mag. Lett., 80 (6) (2000) 381-386. 9. B.E.Warren, Phys. Rev., 9 (1941) 693. 10. A. Szczygielska, A. Burian, S.Duber, J.C. Dore and V.Honkimaki, Journal of Alloys and Compounds, 328 (2001) 231-236. 11. R. Hosemann and S.N. Bagchi, Direct Analysis of Diffraction by Matter., NorthHolland Amsterdam (1962) 69. 12. S.N. Bagchi, Adv. Phys., 19 (1970) 119. 13. J. Bell, Nature, 218 (1968) 985. 14. D.F.R Mildner, J.M. Carpenter, Proc. 5th International Conference on Amorphus and Liquid Semiconductors, Garnish-Partenkirchen, eds: J.Stuke and W.Brening, 1 (1973) p.463. 15. S.Ergun and R.R. Schehl, Carbon, 11 (1973) 127. 16. A. Szczygielska, A. Burian, S.Duber, J.C. Dore and V.Honkimaki, Journal of Alloys and Compounds, 328 (2001) 231-236. 17. A. Oberlin, Carbon, 22 (1984) 521-541. 18. A.Burian, J.C. Dore, Acta Physica Polonica A, 98 (2000) 457. 19. A. Burian, Ph. Daniel, S. Duber, J.C. Dore, Phil. Mag. B, 81 (2001) 525.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
569
The dynamic adsorption behaviour of volatile organic compounds on activated carbon honeycomb monoliths M. Yates a, J. Blanco a and M.A. Martin-Luengo b aInstituto de Catfilisis y Petroleoquimica (ICP), Cantoblanco 28049 Madrid, Spain. bInstituto de Ciencia de Materiales Madrid (ICMM), Cantoblanco 28049 Madrid, Spain. Adsorption offers an efficient technology for removing volatile organic compounds (VOCs) from air pollution sources. Often activated carbons (ACs) are employed owing to their large specific surface areas, high micropore volumes, rapid adsorption capabilities and selectivity towards organic molecules compared to water vapour or air. However, when large volumes of gas have to be treated pressure drop limitations may arise from the use of conventional adsorption beds. For these applications conformation of the adsorption bed as honeycomb monoliths take advantage of the negligible pressure drop of these structures. Commercially available ACs were conformed as honeycomb monoliths with a magnesium silicate clay as binder. The textural and mechanical properties of the raw materials and the monolith composites were determined. These results were analysed together with the dynamic adsorption capacities towards o-dichlorobenzene (o-DCB) a chlorinated probe molecule used to simulate a dioxin. With this data, criteria by which the dynamic adsorption capacity could be related to the textural properties of the adsorption units were established. Key words: Activated carbon, Gas purification, Honeycomb monolith, VOCs. 1. INTRODUCTION In relation to price/performance, physical adsorption is one of the most important techniques to control air pollution. It is well known that the efficiency of ACs as adsorbents is due to their high micropore volumes and large specific surface areas [1,2,3]. The adsorption capability of these materials is further enhanced due to their selectivity towards organic vapours compared to water vapour or air [4,5,6]. In systems that require the treatment of large volumes of gas conventional adsorption beds can suffer from limitations due to pressure drop. For these applications conformation of the adsorption bed as an array of honeycomb monoliths take advantage of the almost null pressure drop to which these open channel structures give rise [7]. However, the integrity of an AC composite can be a problem if thermal regeneration of the adsorption bed is a requirement. Results obtained previously by the authors [8] have shown that the static adsorption capacities of several commercially available ACs, conformed as monolith composites, towards o-DCB were directly related to their micro and narrow mesopore volumes. Of the original twelve ACs studied in these static adsorption measurements the four most promising materials were selected for further study in dynamic adsorption experiments. These samples
570 were chosen in order to maintain a wide selection of AC sources: coal, wood, coconut shell and peat among the composites that had high adsorption capacities and good mechanical strengths. Two further samples were also characterised, a monolith composite produced with a mesoporous alumina [9] and a monolith in which only the magnesium silicate clay binder was employed. All of the monoliths produced were dried at 150~ and fractions subsequently heattreated at either 500~ or 850~ in a nitrogen atmosphere. The static and dynamic adsorption capacities of these monolithic composites at 30~ were determined using o-DCB as a probe molecule. This molecule was chosen since it may be considered as approximately corresponding to half a molecule of tetrachloro-dibenzene dioxin (TCDD), the most toxic isomer of the dioxin family. 2. EXPERIMENTAL
2.1 Monolith preparation All the monolith composites were prepared at a 1:1 ratio between the magnesium silicate clay binder and the AC or alumina. After premixing of the dry powders by careful addition of water a dough was formed. This dough was extruded as honeycomb monolithic structures with parallel channels of square section at a cell density of 8 cells cm 2 and a wall thickness of 0.9 mm using a Bonnot single screw extruder. The honeycomb monoliths were dried at room temperature and then heated in air to 150~ for 4h. Samples of each composite were also heat-treated at 500~ or 850~ in nitrogen. Heat-treatment in an inert atmosphere was necessary to avoid oxidation of the AC. These materials were subsequently used in all of the characterisation techniques and adsorption performance tests. 2.2 Characterisation techniques and adsorption tests Nitrogen adsorption/desorption isotherms at -196~ were determined using a Carlo Erba 1800 Sorptomatic. Raw materials and composites previously treated at 150~ were outgassed overnight at this temperature, other composites were outgassed at 250~ to a vacuum of < 10~ Pa, ensuring a dry clean surface free from any loosely held adsorbed species. The specific surface areas (SBET) were calculated by application of the BET equation [10] taking the area of the nitrogen molecule as 0.162 nm 2 [ 11 ]. For the alumina composite and the monolith produced solely from the silicate binders the linear range of the BET equation was located between 0.05-0.35 p/p~ Owing to the microporous nature of the ACs this linear range was much narrower and displaced to lower relative pressures: p/p~ = 0.02-0.15. The micropore volume and external surface area, i.e. the area not associated with the micropores, were calculated using a t-plot [12] analysis, taking the thickness of an adsorbed layer of nitrogen as 0.354 nm assuming that the arrangement of nitrogen molecules in the film is hexagonal close packed [ 13]. In this study mercury intrusion porosimetry (MIP) analyses were employed to determine the pore size distribution and pore volume over the range of approximately 100 ~tm down to 7.5 nm diameter, utilising CE Instruments Pascal 140/240 apparatus, on samples previously dried overnight at 150~ The pressure/volume data were analysed by use of the Washburn Equation [14] assuming a cylindrical nonintersecting pore model and taking the mercury contact angle as 141 ~ and surface tension as 484 mN ml[10]. For the monolith
571 samples a single piece two channels wide and approximately 1 cm in length was employed for the measurement. The dynamic adsorption measurements were carried out in a glass reactor of 2.54 cm i.d. at 94 kPa and 30~ The monolithic adsorbent of four channels 2 x 2 and 25 cm length was first dried overnight at 150~ and then weighed. The monolith was then placed in the reactor and the space between the external wall of the monolith and the internal wall of the reactor filled with silicon carbide with an average particle diameter of 0.84 ram. This ensured that the gas flowed through the monolith channels. The samples were then preconditioned at 150~ in a dry air stream ovemight. The reactor was subsequently cooled to 30~ and the air stream switched to one containing 100 ppm of o-DCB at a linear velocity of 0.321 ms l. The amount of o-DCB in the gas stream was controlled by bubbling air through the organic maintained in a thermostatic bath and then diluting this spiked air stream with further air. The o-DCB concentrations in the inlet and outlet were measured with a sensitivity of +0.1 ppm by means of a Beckman flame ionisation detector (model 400A). The breakthrough point was taken when 0.3 ppm of o-DCB (99.7% efficiency) was detected in the exit gas, indicating that the system was beginning to fail. 3
RESULTS AND DISCUSSION
3.1 Textural properties of the raw materials Four commercial microporous ACs and a mesoporous alumina were formed by the addition of a magnesium silicate binder into ceramic composites. In the nomenclature adopted for the ACs the first two letters designate the manufacturer, the third the carbon and the last its source: coal (c), wood (w), coconut shell (n), or peat (p). Thus, the ACs produced as honeycomb composites were Fluesorb B (ChFc) from Chemviron, AC35 (EAAw) and NC35 (EANn) from Elf Atochem and GL50 (NoGp) from Norit. The alumina, Pural SB from Condea and the magnesium silicate binder were designated as Pural and MSB, respectively. The final heat-treatment at 150~ 500~ or 850~ is designated by the letter a, b or c respectively. In Table 1 the most relevant textural properties of the raw materials used in this study are collated.
Table 1 Textural properties of the raw materials Designation
ChFc EAAw** EANn** NoGp Pural/a Pural/b Pu ral/c MSB/a MSB/b MSB/c
Particle Size 90%<* (IJm) 12.3 3.7 1.9 8.5 41.0 41.0 41.0 0.3 0.3 0.3
Area SBET
(m2g-1) 1093 1271 1210 775 264 233 154 182 133 55
External Area (m2g-1) 115 54 23 160 264 233 154 129 111 55
* Calculated from MIP ** Supplied as pellets and milled by the authors
Micropore Mesopore 0-2 nm 2-50 nm (cm3g-1) (cm3g-1) 0.418 0.567 0.487 0.267 0.000 0.000 0.000 0.025 0.011 0.000
0.222 0.090 0.037 0.293 0.394 0.531 0.493 0.424 0.366 0.252
Pore Volume 0-50 nm (cm3g1) 0.640 0.657 0.524 0.560 0.394 0.531 0.493 0.449 0.377 0.252
572 For the ACs the data are representative of the samples after heat-treatment at all three temperatures since during their fabrication these materials have already been treated at temperatures in excess of 850~ However, for the alumina and clay samples the surface areas and pore volumes are shown after treatment at each temperature as these materials undergo various phase transitions that lead to sintering of the samples and shifts in their relative pore size distributions with heat-treatment. The particle size was determined from the corresponding MIP curve for the powder raw material. The SBETin the case of microporous ACs should be considered as an apparent surface area due to the micropore filling mechanism associated with these materials [15]. The external area and micropore volumes were calculated from the slope and intercept of the t-plots of the corresponding isotherms. The total pore volume was taken as the amount of gas adsorbed at a relative pressure of 0.96 on the desorption isotherm, equivalent to a pore diameter of 50 nm. The mesopore volume was calculated from the difference in the total pore volume and the micropore volume. In Table 2 the textural properties of all the composites heat-treated at 150 ~ 500~ and 850~ are presented. The sample designation is the same as that used for the raw materials with the addition of the letter m to indicate that the results refer to monolith composites. The total pore volume is the sum of the micro- and mesopore volumes (0-2 nm and 2-50 nm) calculated from the corresponding nitrogen adsorption/desorption isotherms, and the macroporosity (50 n m - 100 gm) determined from MIP, respectively. The threshold diameter was that at which in the MIP analysis there was a sudden upswing in the cumulative volume curve where a large part of the porous network became filled. This pore size can be considered as that which controls any transport phenomena through the solid sample. Table 2 Textural properties of monolithic samples Monolithic Sample ChFc-m/a ChFc-m/b ChFc-m/c EAAw-m/a EAAw-m/b EAAw-m/c EANn-m/a EANn-m/b EANn-m/c NoGp-m/a NoGp-m/b NoGp-m/c Pural-,m/a Pural-m/b Pural-m/c MSB-m/a MSB-m/b MSB-m/c
Area External Total Pore SBET Area Volume (m2g-1) (m2g~) 0-101Jm (cm3g~) 593 125 0.819 603 122 0.906 571 92 0.824 710 121 0.816 732 122 0.877 685 62 0.856 662 86 0.740 670 93 0.785 631 54 0.725 444 157 0.879 449 147 0.889 400 107 0.839 216 214 0.573 196 196 0.693 107 107 0.594 182 129 0.552 130 120 0.532 53 53 0.422
Pore Volume (cm3g1) Threshold Micropore Mesopore Macropore Diameter 0-2 nm 2-50 nm 50 nm-10 IJm (IJm) 0.198 0.326 0.295 1.78 0.207 0.316 0.383 1.78 0.205 0.247 0.372 1.78 0.262 0.258 0.296 0.89 0.273 0.282 0.322 0.89 0.284 0.158 0.414 0.89 0.238 0.232 0.270 0.28 0.240 0.249 0.296 0.28 0.238 0.166 0.321 0.28 0.122 0.378 0.379 2.24 0.130 0.339 0.420 2.24 0.122 0.253 0.464 2.24 0.001 0.415 0.157 2.82 0.000 0.461 0.232 2.82 0.000 0.388 0.206 2.82 0.025 0.449 0.078 0.22 0.005 0.390 0.137 0.22 0.000 0.303 0.119 0.22
The surface areas of the monolith composites were very close to those expected for purely physical mixtures of the raw materials, calculated from the data given in Table 1. This
573
indicated that there was no significant chemical interaction between the two components during fabrication and thermal treatment of the monoliths over this temperature range. The slight rise in the measured surface areas of the AC composites heat-treated at 500~ compared to those treated at 150~ was due to the removal of the more strongly bound species adsorbed in the narrow micropores with the higher outgassing temperature used before surface area determination. Thus, a subsequent slight rise in the measured micropore volume was also observed. However, the almost negligible changes in the measured micropore volumes for all of the AC composites with heat-treatment were indicative that heating up to 850~ in an inert atmosphere had little effect on the AC component of these materials. The major textural changes observed with heat-treatment were the loss in surface area at 850~ especially the external area and the rise and subsequent fall in the total pore volume. These effects could be attributed to changes in the magnesium silicate binder. In the case of the Pural composites the changes in porosity were due to sintering of both the alumina and clay binder. The shift to wider mesopores with increasing heat-treatment led to a rise and then fall in the total pore volume. Typical pore size distributions for MSB-m/b, an AC monolith (EANn-m/b) and Puralm/b are shown in Fig. 1. 12-
10 r
-~ o >
9 , ,, ',
8
II I| II
s~
6
, '
.
b.r
9
,
I
',
I ,
._.~,,
....j
~
~/
/
i
I
I I |
I
~
:
'".
,
-
2 IF-.
1
.
9
~
~
10
100
9 -
9
.
1000
10000
Pore Diameter (nm) Fig 1. Pore size distributions for MSB-m/b (m), EANn-m/b (---) and Pural-m/b ('"). The data for MSB-m/b in Fig. 1 has been halved so that its contribution towards the PSD of the two composites may be more easily appreciated. The PSD of MSB-m/b was bimodal due to contributions of both intraparticulate and interparticulate porosities that may be easily distinguished by the minima at 23 nm. The contribution to the overall PSDs, obtained with the composite materials, due to the magnesium silicate binder were clearly distinguished, indicating that the different raw materials act almost independently in the final overall PSD. The result obtained with EANn/b is shown as being representative of the results obtained with all the AC composites. With this material apart from the porosity due to the inorganic binder there is a significant contribution at c. 200 nm due to the interparticulate porosity formed among the carbon particles. The position of this contribution is dependent on the particle size distribution of the AC used. Thus, for the other AC's with larger primary
574
particle sizes this peak is displaced towards wider pores. With the Pural-m/b composite the PSDs displayed contributions of interparticulate and intraparticulate porosities of both the alumina and the binder. Two contributions may be attributed to the alumina: at c. 2000 nm, due to its particle size, and at c. 8 nm due to its mesopores.
3.2 Dynamic adsorption capacities Through careful selection of the samples to be tested in dynamic adsorption, the importance of the different textural characteristics of these materials could be judged. Typical breakthrough curves of o-DCB in the exit gas against time in operation for ChFc-m treated at 150~ 500~ and 850~ and MSB-m/b are shown in Fig. 2.
&2 ~0 +.a
X 9 ,...i
!
0 0
24
48
72
96
120
144
Time in operation (h) Fig. 2. o-DCB breakthrough curves for ChFc-m/a (O), ChFc-m/b (A), ChFc-m/c (0) and MSB-m/b if'l). From Fig. 2 it may be appreciated that with heat-treatment up to 500~ the dynamic adsorption capacity of this AC composite was practically unchanged, but after treatment at 850~ the adsorption capacity was severely reduced. For MSB-m/b breakthrough came after only 2.6 h and reached almost 100% o-DCB in the exit gases by 24 h in operation. The breakthrough time was taken when 0.3 ppm of organic was detected in the exit gases (99.7% efficient). This was then used to calculate the amount adsorbed in terms of the liquid volume of o-DCB taking the liquid density as 1.3048 [ 16]. This amount adsorbed was then compared with the measured static adsorption capacity [8] and the micropore volume. Considering that the organic first forms a monolayer, the amount adsorbed was converted into an equivalent area per gram covered by o-DCB. The cross sectional area occupied by a molecule of o-DCB was calculated to be 0.357 nm 2 from substitution of the various values in the following equation [ 15]: = f(M/pL) 2/3 (1) where: cy is the cross sectional area, f the packing factor ( = 1.091 for hexagonal closepacked), M the molar mass of adsorptive, P the liquid density and L Avogadro's constant.
575 This equivalent area was then compared with the measured external area and SBET. The full results of the dynamic adsorption experiments are given in Table 3. Table 3 Dynamic adsorption results Sample
ChFc-m/a ChFc-m/b Ch Fc-m/c EAAw-m/b EANn-m/b EANn-m/c NoGp-m/b NoGp-m/c Pural-m/b MSB-m/b
Time to Amount Static Micropre Equivalent External Area Breakthrough Adsorbed Capacity Volume Area Area SBET (h) (cmag-1) (%) (%) (m2g1) (%) (%) 86.1 89.9 50.0 50.2 45.6 1.05 39.8 39.8 3.5 2.6
0.198 0.207 0.153 0.143 0.125 0.003 0.125 0.125 0.009 0.007
41 45 41 30 30 1 30 33 2 2
100 100 75 52 52 1 102 102 146 131
317 331 244 229 200 5 200 199 14 10
254 271 265 188 215 9 136 186 8 9
53 55 43 31 30 1 44 50 7 8
From the results shown in Table 3 and the textural characteristics of the adsorbents presented in Table 2 several conclusions may be drawn. Under these dynamic adsorption conditions with a contact time of <1 second the most favourable conditions lead to the amount adsorbed before breakthrough being equivalent to the micropore volume of the adsorbent. Thus, it would appear that the organic is stored in the micropores. However, this condition may only be met if other textural properties of the monolithic adsorbent are also present. For samples ChFc-m/a and ChFc-m/b, with external areas of c. 120 m2g1 the dynamic adsorption capacity was equal to the micropore volume. But for sample ChFc-m/c with an external area of 90 mZgl the dynamic adsorption capacity was only 75% of the micropore volume. Thus, a high external area was necessary. This was confirmed by the results obtained with NoGp-m/b and NoGp-m/c where the dynamic capacity was equal to the micropore volume even when the external area was reduced to 110 mZgl . For sample EAAw-m/b although the external area was 120 m2g-1 the dynamic capacity was only c. 50% of the micropore volume. However, this sample had a narrow threshold diameter (0.9 gm) compared to the previous two series. Thus, it would appear that the presence of wide pores that aid internal diffusion into the monolith walls was also important. This was confirmed by the results with samples EAN-m/b and EAN-m/c that only gave dynamic adsorption capacities of 50% and 1% of their micropore volumes, respectively. These samples had low external areas 90 mZgl and 50 mZgl and narrow threshold diameters (0.3 gm). Samples that had no micropore volume still cause some hold up before breakthrough. This must be due to adsorption on their available surface area. Samples Pural-m/b and MSBm/b both adsorbed under dynamic conditions until about 9% of their external areas were covered with adsorbate. Both of these samples subsequently showed rapid increases in the concentration of the organic not adsorbed, reaching almost 100% in 24 hours. This result may be contrasted with that of EANn-m/c where breakthrough also occurred when 9% of the external area was covered but then the rise in the o-DCB in the exit gas was extremely slow. This slow rise in the amount of organic not adsorbed indicated that the rate of diffusion into
576 the micropores was approximately the same as the rate of organic reaching the adsorption unit. 3 CONCLUSIONS The dynamic adsorption capacity of activated carbon containing monoliths has been shown to be equivalent to the micropore volume. However, this condition can only be met when the external area is above c. 100 m2gl and the threshold diameter wide. In systems with no micropore volume or poor internal diffusion due to a low external surface area and narrow threshold diameter the breakthrough point is reached when c. 9% of the external area is covered. Future work will concentrate on using higher linear velocities and adsorption temperatures and different monolith geometries (wall thickness and channel width) in order to study the internal diffusion limitations of these types of adsorption units.
Acknowledgements The authors appreciate the financial assistance received from the Comunidad Autonoma de Madrid through its project Ref. 07M/0106/2000. REFERENCES 1. M. Iley, H. Marsh and F. Rodriguez-Reinoso, Carbon, 11 (1973) 633. 2. M. Domingo-Garcia, I. Fernandez-Morales, F.J. L6pez-Garz6n and C. Moreno-Castilla, Langmuir, 7 (1991 ) 339. 3. F.J. L6pez-Garz6n, I. Fern~indez-Morales, C. Moreno-Castilla and M. Domingo-Garcia, in: A. Dabrowski (Ed.), Adsorption and its Applications in Industry and Environmental Protection, Studies in Surface Science and Catalysis, Vol. 120, Part B, Elsevier, Amsterdam, 1998, p. 397. 4. S.J. Gregg and K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London (1982). 5. S.S. Barton, M.J.B. Evans, J. Holland and J.E. Koresh, Carbon, 22, 3 (1984) 265. 6. E.N.Ruddy and L.A.Carroll, Chemical Engineering Progress, July (1993) 28. 7. R.K. Shah and A.L. London, Dept. Mech. Eng. Stanford University CA, Tech. Rep. 75 (1971). 8. M. Yates, J. Blanco, P. Avila and M.P. Martin, Microporous and Mesoporous Materials, 37 (2000) 201. 9. J. Blanco, M. Yates, P. Avila and A. Bahamonde, J. Mat. Sci., 29 (1994) 5927. 10. S. Brunauer, P.H. Emmett and E. Teller, J. Amer. Chem. Soc., 60 (1938) 309. 11. J. Rouquerol, D Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pericone, J.D.F. Ramsay, K.S.W. Sing and K.K. Unger, Pure and Appl. Chem., 66, 8 (1994) 1739. 12. B.C. Lippens and J.H. de Boer, J. Cat., 4 (1965) 319. 13. B.C. Lippens, B.G. Linsen and J.H. de Boer, J. Cat., 3 (1964) 32. 14. E.W. Washburn, Proc. Nat. Acad. Sci. U.S.A., 7 (1921) 115. 15. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by powders and porous solids, Academic Press, London (1999). 16. CRC Handbook of chemistry and physics, 59th Edition, 1978.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
577
Structural properties of Cu-MCM-41 and Cu-AI-MCM-41 (Si/AI=30) catalysts Giovanni Ferraris
a,
Giuliano
M o r e t t i a,b,
Giuseppe Fierro a and Mariano Lo Jacono b
alstituto IMIP del CNR, "Sezione Materiali Inorganici e Catalisi Eterogenea" bDipartimento di Chimica, Universith "La Sapienza", p.le Aldo Moro 5, 00185 ROMA Italy; (e-mail: [email protected]) Cu-MCM-41 and Cu-A1-MCM-41 samples have been obtained by ion exchange of the MCM-41 and A1-MCM-41 matrices prepared by hexadecyltrimethylammonium cloride, tetraethyl orthosilicate, aluminum isopropoxide and an ammonia solution. The aluminum concentration affects the MCM-41 textural properties and large amount of extra-framework aluminum species are supposed to be present in A1-MCM-41 with Si/A1 = 30. Cu-MCM-41 and Cu-A1-MCM-41 catalysts have been tested for NO selective catalytic reduction by propane in the presence of oxygen, in comparison with microporous Cu-S-1 and Cu-ZSM-5 catalysts with similar copper loading and Si/A1 atomic ratio. Cu-A1-MCM-41 catalysts are less active and selective with respect to the Cu-ZSM-5 catalysts indicating that they are not suitable for NO abatement reactions. 1. INTRODUCTION Copper ions exchanged microporous molecular sieves, in particular Cu-ZSM-5, are active catalysts for the selective catalytic reduction of NO and N20 with hydrocarbons in the presence of 02 (HC-SCR). It has been reported that the catalytic activity may be controlled by intra-crystalline diffusivity and by geometry-limited diffusion depending on the hydrocarbon molecular size and the zeolite pore size [1]. Therefore, it is of interest to prepare Cu-A1-MCM-41 mesoporous molecular sieves and to compare their activity with that of Cu-ZSM-5. The synthesis of A1-MCM-41 has been reported by several authors and the distribution between framework and extra-framework aluminum species has been found to depend strongly on the silica and aluminum sources, the nature of templating surfactant and synthesis conditions [Ref. 2 and references therein]. In this contribution we investigate a simple preparation method based on hexadecyltrimethylammonium cloride, tetraethyl orthosilicate, aluminum isopropoxide and an ammonia solution. The Cu-A1-MCM-41 catalysts where prepared by standard ion exchange procedure and tested for the selective catalytic reduction (SCR) of NO with propane in the presence of O2, under the same experimental conditions employed for Cu-ZSM-5 catalysts with similar copper loading and Si/A1 atomic ratio.
578 2. EXPERIMENTAL
2.1. Catalysts preparation MCM-41 (all silica) was synthesized using hexadecyltrimethylammonium cloride (CTA-C1) (Fluka), tetraethyl orthosilicate (TEOS) (Fluka-purum) and an ammonia solution (33 wt%) (Allied Signal). CTA-C1 was dissolved in an ammonia solution at room temperature. After addition of TEOS the mixture was stirred for 90 rain at room temperature before being heated in a 350 mL stainless autoclave in an oven at 343 K for 24 h, without stirring. The molar ratios of the components were the following: SiO2 : CTA-C1 : NH3 : H20 = 180 : 95 : 3397 : 6515. The solid product was washed with water, dried at ambient temperature overnight and treated in air at 393 K for 1 h and finally at 823 K for 5 h. This procedure, first described by Chiola et al. in a patent filed in 1969, was reported by Di Renzo et al. [Ref. 3 and references therein]. A1-MCM-41 samples were prepared following the experimental procedure reported above, by adding to the reaction mixture the appropriate amount of aluminum isopropoxide (Fluka-puriss.) to give the nominal Si/A1 atomic ratios of 30 and 150. The H-ZSM-5 (Si/Al=25, PQ Corp.) and Silicalite (S-l) zeolites were used to prepare the microporous copper-containing catalysts, Cu-ZSM-5 and Cu-S-1. ( We recall that S-1 has the same framework topology of ZSM-5, but without A13§ ions in the framework, and therefore S-1 is an all silica materials as MCM-41.) S-1 was prepared using TEOS and a 20% aqueous solution of tetrapropylammonium hydroxide (TPA-OH) (Fluka-purum). TEOS was poured in the TPA-OH solution. The resulting mixture was kept at 333 K for 3 h and then heated under autogenous pressure in a 350 mL stainless autoclave in an oven at 448 K for 24 h, without stirring. The solid was washed with water, dried 2 h at 383 K and finally treated in air at 823 K for 5 h. Further details were reported in Ref. 4. The copper containing catalysts, Cu-ZSM-5 (Si/Al=25), Cu-S-1, Cu-A1-MCM-41 (Si/AI=30 and 150) and Cu-MCM-41, were prepared by ion exchange using copper acetate solutions. In the preparations, 1 g of support (H-ZSM-5, S-l, A1-MCM-41 and MCM-41) was treated with 125 mL of 0.1 M copper acetate solution for 2h under stirring at room temperature. After filtration, samples were washed several times with distilled water, dried at 383 K for 2 h, and finally treated in air at 823 K for 5 h. Samples were stored over a saturated solution of NH4C1 at room temperature (relative humidity = 79%). The Cu contents of the catalysts, determined by atomic absorption (Varian SpectrAA-30), were: 2.90 wt% for Cu-MCM-41; 2.94 wt% for Cu-A1-MCM-41 (Si/A1 = 30); 3.92 for Cu-A1MCM-41 (Si/A1 = 150); 1.38 wt% for Cu-S-1 and 1.79 wt% for Cu-ZSM-5 (Si/A1 = 25).
2.2. Catalysts characterization The XRD patterns were obtained with a Philips automated PW 1729 diffractometer. Scans were taken at 20 step of 0.02 ~ (2.5 s per step) in the range 1.5-10 ~ using Cu-I~ radiation (Ni-filtered).
579 Textural characterization was performed by N2 adsorption-desorption at 77 K using a Micromeritics ASAP 2010 analyzer. The samples were preheated under vacuum in three steps of lh at 423 K, lh at 513 K, and finally 4 h at 623 K. BET specific surface area, SBET, was calculated using adsorption data in the relative pressure range, P/Po, from 0.05 to 0.2. Total pore volume, Vp t~ was estimated by Gurvitsch rule on the basis of the amount adsorbed at P/Po of about 0.95. The primary mesopore diameter, Dp, was evaluated using the BJH method from the desorption data of the isotherm. The primary mesopore volume, Vp, and the external surface area, Sext, were determined using the t-plot method with the statistical film thickness curve of a macroporous silica gel [5].
2.3 Catalytic tests The catalytic tests were carried out in flow mode with the effluent gas analyzed by an on-line gas chromatograph which was provided with an Alltech CTR-1 column and a hot wire detector. The catalyst (ca. 120 mg) underwent the standard pretreatment in flowing 10% 02 in He at 773 K overnight. The reactant blend (0.46% NO + 0.46% C3H8 + 5% 02 + He balance) was passed through the catalyst at the total flow of 100 mL (NTP)/min and total pressure of 1 atm in the range of temperature 423-873 K with a 0.5 h isothermal step at any selected temperature. GHSV = 25.000 h-~. 3. RESULTS AND DISCUSSION
3.1. Preparation of the catalysts MCM-41 and S-1 present Al-independent exchange behavior, previously investigated by many authors [Ref. 4 and references therein]. Internal silanols are believed to be responsible for the phenomenon of excess ion exchange. We recall that S-1 may contain a highly concentration of defective sites caused by the lack of [SiO4] building units, which are substituted by up to four OH groups. High concentration of silanols, up to ca. 2 OH nm 2, is also present at the surface of MCM-41 [6]. The more acidic silanols could represent useful grafting sites for the copper species in solution; in our Cu-MCM-41 sample we can calculate a Cu/SiOH ratio of about 0.25, considering the stoichiometry Cu 2++ 2 -SiOH = (-SiO)2Cu 2+ + 2 H +. As regards the Cu-A1-MCM-41 (Si/AI=30 and 150) and Cu-ZSM-5 (Si/Al=25) catalysts, the copper ion exchange is related to the presence of both Bronsted Si-OH-A1 acid sites and internal silanols. In the case of Cu-ZSM-5 (Si/Al=25) it appears that only the Bronsted acid sites are responsible for the ion exchange (Cu/A1 = 0.5). The fact that the copper loading of Cu-A1-MCM-41 (Si/AI=30) is similar to that of Cu-MCM-41 and less than that of Cu-A1-MCM-41 (Si/AI=150) may be an indication of the presence of extraframework octahedral A1 species with no ion exchange capacity (vide infra). 3.2. Textural characterization All the samples exhibit the nitrogen physisorption isotherms with the typical sharp inflection characteristic of capillary condensation within the narrow mesopores of MCM-41 structure (Fig.l). The incorporation of aluminum into the silicate walls causes a
580 less ordered arrangement. In particular, for a low A1 content (Si/A1 = 150) the inflection shifts to higher P/Po values and adsorptions are no more perfectly reversible. By increasing the content of A1 in the synthesis (Si/A1 = 30), an additional hysteresis appears in the relative pressure range of secondary mesopores ( P/Po > 0.4). The introduction of copper by ion exchange does not modify the shape of the isotherms, but lowers BET areas and total pore volumes (see Table 1).
1200 7~o)~ 9 1000 I
>
~,,~,
~ *o"o '~ o~ o~O o'o ~
600 !
~ & AI-MCM-41 SilAI= 150 A~ .~=[]ID=u=D-D-~,,D=O'[]n=~ 400 a , , ~ ~ 200 ~ . " c ~ ~"~ MCM-41 0
0
,
,
,
i
0.2
,
,
,
I
0.4
L
,
a
I
0.6
,
,
,
i
0.8
PIPo
,
,
,
1
Fig. 1. Nitrogen adsorption (open symbols) and desorption (closed symbols) isotherms at 77 K for the calcined MCM-41 and A1-MCM-41 samples. Amounts adsorbed for A1-MCM-41 (Si/AI=150) and A1-MCM-41 (Si/AI=30) are incremented by 200 and 500 cm3(STP)g -l, respectively. The t-test analysis (Fig. 2) excludes the presence of micropores and completes the determination of textural data reported in Table 1. By comparing the samples with or without copper, it appears that Cu ion exchange decreases the total surface area and the primary mesopore volume, but without a dramatic reduction of internal accessibility. Table 1. Textural data of the calcined samples Sample
MCM-41 Cu-MCM-41 A1-MCM-41 (Si/AI=30) Cu-A1-MCM-41 (Si/AI=30) A1-MCM-41 (Si/A1= 150) Cu-A1-MCM-41 (Si/AI=150)
SBET (mEg-l) 972 845 948 819 872 781
VP t~
Vp
Sext
Dp
(cm 3 g-l)
(cm 3 g-l)
(mEg-l)
(A)
34 32 158 152 56 43
26 26 26 26 31 30
0.65 0.55 0.82 0.61 0.80 0.70
0.58 0.50 0.57 0.38 0.71 0.63
581
The BJH pore size distributions (Fig. 3) show that at low content of A1 (Si/AI=150) the average diameter of the channels is shifted to value larger than pure siliceous MCM-41, whereas a bimodal distribution is present in the sample with higher A1 content.
,;., ol I1. I,O3
E u
600
~
400
D [] D
000
"o f
2OO
/
/
MCM-41 [] AI-MCM-41 (SilAl=30) ~ AI-MCM-41 o
~
2
4
............ 6
8
10
12
14
tlA
Fig.2. t-test for MCM-41 and A1-MCM-41 samples.
A '9,
0.1 AI-MCM-41
__~ 0.08
si/~u = 1so
~ 0.06 "o "o
0.04 o.o2~
10
/I
20
/
\ \
~
30
~-MCM4~
40
50
60
DplA
Fig. 3. BJH pore size distribution for MCM-41 and A1-MCM-41 samples.
582
3.3. X-ray diffraction The XRD pattems of MCM-41 and A1-MCM-41 are shown in Fig. 4. The crystallinity of the MCM-41 samples decreases in the Al-containing sample. The patterns can be indexed in a hexagonal unit cell. From the interplanar distance, d~00, the hexagonal unit cell parameter (a = 2 dl00 / ~/3) are calculated and reported in Table 2. From the pore size diameter, Dp, reported in Table 1, the thickness of the silicate walls (s) are obtained by using the equation s = a - Dp (see Table 2).
d
00)
c -"--t~ ~ ( ~
10)
AI-MCM-41 (Si / AI = 30)
_
MCM-41 , I
2
,
~
3
,
4
,
l
5
i
lilir i I~I -, -n--I-,
- -
6 7 8 9 10 2 0 I degrees
Fig.4. XRD patterns of MCM-41 and A1-MCM-41 samples. The textural, XRD and ion exchange data suggest that in the A1-MCM-41 (Si/AI=30) and Cu-A1-MCM-41 (Si/AI=30) samples, most of the aluminium species are extraframework. At low A1 loading (Si/AI=150) the framework A1 species lead to sample with increased pore size diameter, increased thickness of silicate walls and reduced total surface area. Our results for the A1-MCM-41 (Si/AI=30) sample are in agreement with the finding of Cesteros and Hailer [7] which reported a careful characterization by 27A1 MAS NMR of A1-MCM-41 samples with Si/A1 ratio ca. 30 prepared following several procedures and using different A1 sources. Table 2. Hexagonal Unit Cell Parameter, a, and thickness of silicate walls, s, for the calcined matrices Sample
a/A
MCM-41 A1-MCM-41 (Si/A1 = 30) A1-MCM-41 (Si/A1 =150)
41 46 51
s/A 15 20 25
583
3.4. Catalytic tests The plots in Figs. 5 and 6 show the HC-SCR activity and selectivity of Cu-containing mesoporous samples together with data obtained for Cu-containing microporous Cu-S-1 and Cu-ZSM-5 samples of similar Cu content and Si/A1 ratio. 9 Cu-S-1 (Cu=1.38 wt%)
[]
Cu-ZSM-5 (Cu=1.79 wt%, SilAl=25) 9 Cu-MCM-41 (Cu=2.90 wt%)
22
9 Cu-AI-MCM41 (Cu=2.94 wt%, SilAl=30)
O Im z
o~ o 21
201.3 . . . . . . . . .
115. . . . . . . . . 1'.7. . . . 1000 / T
Fig.5. HC-SCR Arrhenius plots (rNO expressed as NO molecules sl Cu mole -1) of microporous and mesoporous Cu-containing catalysts. C) = Cu-S-1 (Cu=1.38 wt%) - ~ - - ~ - C u - Z S M - 5 (Cu=1.79 wt%. Si/AI=25) [] "- Cu-MCM41 (Cu=2.90 wt%) Cu-AI-MCM~,I (Cu=2.94 wt%, SilAI=30) 100
O
80
or) 60
4 0
. . . .
500
J
. . . .
i
. . . .
600
i
. . . .
i
. . . .
700
i
T/K
. . . .
i
. . . .
800
i
. . . .
900
Fig.6. HC-SCR Selectivities to N2 (open symbols) and to CO2 (closed symbols) of microporous and mesoporous Cu-containing catalysts.
584 The activity sequence is: Cu-ZSM-5 > Cu-A1-MCM-41 = Cu-S-1 > Cu-MCM-41, i.e. microporous catalysts are more efficient than the corresponding mesoporous ones. The higher activity of Cu-A1-MCM-41 with respect to Cu-MCM-41 indicates that part of copper ions are linked to framework A1, i.e. part of Aluminum is structural. A further drawback of Cu-A1-MCM-41, in comparison with Cu-ZSM-5, is the low selectivity, in particular that to CO2. At our knowledge, this is the first time that the catalytic behavior of Cu-A1-MCM-41 for the HC-SCR of NO is reported. HC-SCR of NO was previously reported on the Pt-MCM-41, Rh-MCM-41 and Co-MCM-41 catalysts [8]. Pt-MCM-41 resulted the most active catalyst, but no comparison was made with the activity of Pt-ZSM-5 catalysts measured under the same experimental conditions.
4. CONCLUSIONS In agreement with literature data, we found that aluminum insertion into the MCM-41 framework is difficult also with our preparation method based on the use of hexadecyltrimethylammonium cloride, tetraethyl orthosilicate, aluminum isopropoxide and an ammonia solution. The A1 insertion causes loss of crystallinity and less ordered mesopore structure. By comparing the copper ion exchange capacity of Cu-MCM-41 and Cu-A1-MCM-41 it turns out that for low A1 insertion (Si/AI=150) most of A1 is framework, whereas in the sample with Si/AI=30 a large part of the aluminum species are extra-framework with no ion exchange capacity and different catalytic behavior. For the SCR of NO with propane, that is in the absence of diffusion limiting structural effects, Cu-A1-MCM-41 (Si/AI=30) catalyst results substantially less active and selective than Cu-ZSM-5 catalyst with similar Cu content and Si/A1 atomic ratio. Moreover, the presence of extra-framework aluminium lowers the selectivity to CO2. These results indicate that Cu-A1-MCM-41 catalysts are not suitable for NO SCR reactions with hydrocarbons. REFERENCES 1. A. Shichi, A. Satsuma and T. Hattori, Appl. Catal. A: General, 207 (2001) 315. 2. G. A. Eimer, L. B. Pierella, G. A. Monti and O. A. Anunziata, Catal. Lea., 78 (2002) 65. 3. F. Di Renzo, H. Cambon and R. Dutartre, Microporous Mater., 10 (1997) 283. 4. G. Moretti, C. Dossi, A. Fusi, S. Recchia and R. Psaro, Appl. Catal. B: Environmental, 20 (1999) 67. 5. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 6. N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Soc. Chem. Commun., (1994) 967. 7. Y. Cesteros and G.L. Hailer, Micropor. Mesopor. Mater., 43 (2001) 171. 8. W. SchieBer, H. Vinek and A. Jentys, Catal. Lett., 56 (1998) 189.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
585
Comparative study of the textural properties of alumina-pillared saponites synthesised from the intercalation with various aluminium oligomers L.M. Gandia a, M.A. Vicente b and A. Gil a aDepartamento de Quimica Aplicada. Edificio Los Acebos, Universidad P6blica de Navarra. Campus de Arrosadia, s/n. E-31006 Pamplona. Spain. bDepartamento de Quimica InorgS.nica. Facultad de Ciencias Quimicas, Universidad de Salamanca. Plaza de la Merced, s/n. E-37008 Salamanca. Spain. A comparative study of the textural properties developed by the intercalation and pillaring of a saponite with various aluminium oligomers is reported. Three aluminium polycations, [A113(~t3-OH)6(~t2-OH)12(heidi)6(H20)6] 3+ and [A11304(~t2-OH)24(H20) 12] 7+, [AIt3(OH)z4(H20)24] tS+, have been considered for this purpose. Significant differences have been found among the properties of the solids with respect to both the aluminium oligomer and the temperature of calcination used. Intercalated and pillared solids with basal spacings (d(001)) in the range of 18.4-23.1 and 15.0-17.6 A, respectively, and specific surface areas (St.ang) in the range of 28-391 and 119-282 m2/g, respectively, have been obtained. The Dubinin-Astakhov (DA) method, the Horvath-Kawazoe (HK) one and the Jaroniec-GadkareeChoma (JGC), have been applied in the analysis of the nitrogen adsorption data in order to characterise the microporous structure of the solids. 1. INTRODUCTION The development of inorganic pillared interlayered clays (shortly termed as PILCs), an important family of high surface area microporous materials, in the late 1970s created remarkable new opportunities in the field of the synthesis and applications of clays [1-3]. These materials are prepared by exchanging the charge-compensating cations present in the interlamellar space of the parent clays with hydroxy-metal polycations, formed by the partial hydrolysis of some metal salts. On calcining, they yield thermally-stable oxide pillars that prop apart the layers, giving rise to an interesting two-dimensional porous structure in the interlayer space with diameter and acidity comparable to that of zeolites [4,5]. Moreover, the porous structure and physicochemical properties developed by the pillared clays can be
Financial support by the Ministerio de Ciencia y Tecnoiogia(MAT2000-0985) is gratefully acknowledged.
586 controlled to a great extent by adjusting the several parameters involved in the synthesis process [6]. The preparation and characterisation of PILCs have been the object over the last two decades of a large number of studies, and various reviews are available in the literature [7]. Alumina-pillared clays are by thr the most documented PILCs. The use of the [Al1304(gzOH)24(H20)I2] 7+ polycation synthesised from the hydrolysis of aluminium salts or from commercially available products (aluminium chlorhydrate (ACH) solutions) are the most commonly intercalation solutions used. The intercalation with other metallic solutions aiming to improve the thermal stability and the textural properties as well as the creation of new active sites has received less attention. Only in very few cases the structure of the intercalated species has been precisely established. There are some results on elements such as A1, Cr, Fe, Si, Ti or Zr showing that well-defined polycations with original structures may be obtained, especially if structuring ligands are used. This strategy may be of interest for intercalation purposes and it has been considered in this work for preparing aluminium polycations. Although the possible catalytic applications of PILCs is the subject more extensively studied, large eftbrts have also been devoted to the characterisation of the microporous structure of these materials. To this end, the application of both classical and novel models based on gas adsorption has been reported in the literature. The adsorption data depend on the internal physical and chemical structure of the solid and on the nature of the adsorbate molecule. Thus, the adsorption results contain information about structural and energetic properties of the materials surface [8]. In the following, the methods considered in this work to investigate the microporous properties of several alumina-pillared saponites are briefly presented. 2. THEORETICAL APPROACH Several methods have been proposed for the characterisation of the Micropore Size Distribution (MPSD) that take into account the energetic heterogeneity of solid surfaces [9,10]. The Dubinin-Radushkevich (DR) and Dubinin-Astakhov (DA) equations have been used to describe the adsorption process on structurally heterogeneous solids [11,12]. From these equations, the adsorption isotherm can be expressed as ~bllows: Ill
0 Efexp[-(A/E,)"] =
(1)
i--I
The energetic heterogeneity associated with micropores of microporous materials can be characterised by means of the adsorption potential distribution, and the distributions (X(A)), related to the Dubinin equations:
587
111 Ii X( A) = - dO(A__.__~): nA,,_, i~.f f __exp[_( A / Ei ),, ] dA .= E,'
(2)
A simple method for evaluating MPSD is based on the Horvath-Kawazoe (HK) procedure [13] and its modifications. In this method, the pressure scale of the experimental adsorption results is converted to a pore size scale assuming micropore-filling and using a theoretical relation between pressure and pore size, which is equated to the free-energy of the adsorbing molecules [14], the resulting function being finally differentiated to give a MPSD. The HK method has been widely applied to characterise various microporous solids as activated carbons and pillared interlayered clays. The HK equation for a slitlike geometry considering that the adsorption process follows Henry's law can be written as: N a A a + NAA A
-A = NA,.
O.4( x _
2d0)
0.4
0.10
0.4
0.1o ]
(3)
3(x_do)3-9(x-do) 9 3d3 + ~-~ ]
Jaroniec et al. [15] also proposed a simple thermodynamic approach to characterize microporous solids (JGC model). The MPSD, J(x), is related to the adsorption potential distribution through the following equation:
J(x) = -X( A)( dA'~ dxJ
(4)
dA/dx depends on the pore size range and pore geometry. In the case of slitlike micropores, the derivative of the adsorption potential, A, with respect to the pore width, x, can be written as"
dA
C~
dx- (x-r
[ 3C2 )--------~- 9G ),o
(X+do ( +do
(x-r
( +do
(X+do
(5)
In this work, a comparative study of the textural properties developed by the intercalation and pillaring of a saponite with various aluminium oligomers is presented. The adsorption models above summarized have been applied to the nitrogen adsorption at 77 K results in order to evaluate the effect of the oligomer nature and the calcination temperature on the MPSD of the prepared solids.
588 3. EXPERIMENTAL
3.1. Preparation of the intercalated and pillared samples A saponite from Ballarat (California, USA) was used as raw material. The as-received material was purified by careful aqueous dispersion and decantation and the fractions with particle size lesser than 2 ~m separated and subjected to intercalation experiments with aluminium oligomers. The parent clay was intercalated with [Al1304(~t2-OH)24(H20)12]7§ polycation (A1, in short) following a standard procedure [ 16]. The A1 polycation solution was prepared by slow titration of a solution of AICI3"6H20 (Panreac, PA) with a solution of NaOH under vigorous stirring, using a OH/A13+ mole ratio equal to 2.2 (pH = 4.1) [17,18]. The hydrolysed solution was allowed to age tbr 24 h at room temperature under constant agitation. The interlayered saponite was obtained by addition of 8 g of the clay to an aqueous solution of hydroxyaluminium, using an A13§ ratio of 5 mmol/g. The slurry was stirred for 24 h at room temperature and then centrifugated and washed by dialysis with distilled water until no chloride was present in the filter wash water. The preparation of the [Al13(~t3-OH)6(~t2-OH)12(heidi)6(H20)6] 3§ polycation (Al~e,d,, in short, where heidi refers to N-(2-hydroxyethyl)iminodiacetate) was reported elsewhere [19]. Two polycation solutions with A1/heidi mole ratios of 2.0 and 3.0, respectively, were prepared from a solution of A1C13"6H20 (Panreac, PA) and the heidi ligand precursor, [N-(2hydroxyethyl)iminodiacetic acid] (Acros Organics, 98 %), used as received, the pH being adjusted to 5.0 with NaOH. After ageing overnight at room temperature, the intercalated clays were obtained by addition of these solutions to previously prepared suspensions of 6 g of the saponite in 500 cm 3 of water, using an A13+/clay ratio of 5 mmol/g. The slurries were stirred for 24 h at room temperature and then centrifugated and washed by dialysis with distilled water until no chloride was present in the filter wash water. The synthesis of the [AI13(OH)24(H20)24]Is+ polycation (All3, in short) was described by Breuil in 1965 [20] and a similar method was used in this work. 2.0 g of alumina (Institut Fran~ais du P6trole, ref. E8648, 150-400 ~tm in size), 9.1 g of AIC13"6H20 (Prolabo) and 7.3 cm 3 of water were heated under reflux for 24 h and then centrifugated to separate the alumina that had not reacted. The suspension thus obtained was used directly for intercalating the clay. 2.0 cm 3 of the suspension and 1.0 g of the saponite were put in a dialysis bag, and washed by dialysis until no chloride was present in the filter wash water. The intercalated solid was finally separated by centrifugation. The resulting intercalated clays were dried in air at 323 K for 16 h and then calcined at 773 K for 4 h in order to obtain the alumina-pillared clays. The solids are designated hereafter as (BAsap-A1)T, where BAsap refers to the starting clay used. The following letter indicates the aluminium polycation used (A1, Alh~,d, or All3) and T the calcination temperature. In the case of the Alh~,d;polycation, the Al/heidi mole ratio used (2 or 3) is also included ((BAsap-Alhe,d,2) and (BAsap-Alhe,d/-3)773, respectively).
589
3.2. Characterization techniques X-ray powder diffraction (XRD) patterns were obtained by using a Siemens D-5000 difffactometer employing nickel filtered Cu Kc~ radiation and operating at 40 kV and 30 mA. Textural analyses were carried out from the corresponding nitrogen (Air Liquide, 99.999 %) adsorption at 77 K, obtained from a static volumetric apparatus (Micromeritics ASAP 2010 adsorption analyzer). The nitrogen adsorption data were collected in the relative pressure range of 10-5 < p/p0 _<0.99. To obtain an adequate characterization of the microporous region, sufficient data points at low pressures are needed, and this requires the addition of constant small nitrogen volumes. The nitrogen adsorption data were obtained using about 0.2 g of sample and successive nitrogen doses of 4 cm3/g until p/p0 = 0.01 was reached. Each point of the adsorption isotherm in this range was equilibrated for at least 2 h in order to characterize correctly the smallest micropores. Further nitrogen was added and the volumes required to achieve a fixed set of p/p0 were measured. Previous to analysis, samples were degassed at 473 K for 24 h (p < 10.3 mmHg).
4. RESULTS AND DISCUSSION The basal spacings (d(001)) of the intercalated and pillared samples determined from the (001) reflection in the XRD powder patterns ranged from 18.4 to 23.1 and from 15.0 to 17.6 A, respectively. These values fit well into the ranges generally reported in the literature for alumina-pillared clays and indicate that the intercalating and pillaring processes have been succesfully accomplished in every case. Relative pressure (p/pO) 0
0.2 ,
150
-
I
~
0.4 ,
I
0.6 ,
I
0.8 ~
I
p/p~
1 ,
]
150
[
r~
~
100
~
50
~00
~.
50 ~
0
0 0 -5
10 -4
10 -3
10 -2
10 -I
10 ~
Relative pressure (p/pO)
Fig. 1. Nitrogen adsorption at 77 K starting from very low pressures of the samples, (O,o) (BAsap)773, (1:1,11) (BAsap-Al)773, ( , V ) (BAsap-Alheld,-2)773, ( , II~) (BAsap-Alhetd,-3)773, (A,A) (BAsap-Al13)773.
590 The nitrogen adsorption at 77 K of the samples treated at 773 K starting from very low pressures are shown in Figure 1. The data corresponding to the natural clay and the intercalated samples are not presented in this work. The adsorption isotherms of (BAsap)773 and (BAsap-Al13)773 are of type II in the Brunauer, Deming, Deming and Teller (BDDT) classification [21]. The adsorption isotherms at low relative pressures of (BAsap-A1)773, (BAsap-Alheiai-2)773 and (BAsap-Alheiai-3)773 are of type I in this classification. The nitrogen adsorption experiments at low pressures (p/p0 < 0.1) show the greatest differences between the isotherms of the samples. The textural properties of the samples are more explicitly given in Table 1. From these results, remarkable differences among the solids are found depending both on the aluminium oligomer and the calcination temperature. Intercalated (BAsap-Alheld,) samples exhibited a noticeable increase of specific surface area and micropore volume on calcining, which could be related to the elimination of the organic part of the polycations. On the contrary, intercalated (BAsap-Al) and (BAsap-All3) showed specific surface area and micropore volume losses that could be related to the pillar density and the thermal decomposition of the aluminium oligomers [6]. Table 1 Textural properties of the samples indicated. DA method a SLang
Sample (BAsap) (BAsap)773 (BAsap-Al) (BAsap-A1)773 (BAsap-Alheidi-2) (BAsap-Alheidi-2)773 (BAsap-Alheidi-3) (BAsap-Alheidi-3)773 (BAsap-Ali3) (BAsap-Al~3)773 a
(m2/g) 37 27 391 256 28 226 28 282 366 119
(cg=196) (C=196) (C-518) (C=308) (C=246) (C=452) (C=215) (C=521) (C=441) (C=267)
VP b (cm3/g) 0.107 0.106 0.232 0.183 0.072 0.164 0.120 0.250 0.242 0.163
c Sext
V~pttK
(m2/g) (cm3/g) 27 23 30 33 11 20 22 28 61 63
0.017 0.012 0.145 0.096 0.012 0.085 0.012 0.105 0.140 0.049
d
d
VptpDA
Ee f
(cm3/g) (kJ/mol) 0.016 0.011 0.141 0.088 0.012 0.083 0.012 0.101 0.137 0.046
13.9 14.3 19.4 17.0 16.3 20.0 14.7 20.5 18.8 17.3
n 1.6 1.7 2.7 2.3 1.5 2.3 1.6 3.0 2.1 1.9
Specific surface areas from the Langmuir equation (0.01 < p/p0 < 0.05, interval of relative pressure). Specific total pore volumes at p/p0 = 0.99. ,~Specific external surface areas obtained from the t-method. Specific micropore volumes derived from the Horvath-Kawazoe (ttK) and Dubinin-Astakhov (DA) methods. e Characteristic energies from the Dubinin-Astakhov equation. [ Exponents of the Dubinin-Astakhov equation. g Langmuir C-values, characteristic of the intensity of the adsorbate-adsorbent interactions. h
591
The DA equation was applied in the nitrogen relative pressure range between 10 .6 and 0.2 to estimate both E and V~tpDA. These parameters are necessary to calculate the adsorption potential distribution (see Eq. (2)). V~tpHK values were evaluated from the MPSDs as the adsorbed volume corresponding to a pore size of 20 A [22]. These results have been included in Table 1. (BAsap-A1)773 and (BAsap-A11,e,d,-2)773 can be considered as representative samples of the solids synthesised in this work, and their MPSDs are compared in Figures 2, 3 and 4. It has been pointed out that in some cases the adsorption potential distributions (Fig. 2) can be related with the MPSD of the samples [11]. Differences between the maxima of the distributions and their width can be observed, suggesting that (BAsap-Alhe,d,-2)773 shows smaller micropores than that of (BAsap-A1)773. The MPSDs derived from the HK slitlike model (Fig. 3) present two maxima in both cases, 5.1 and 7.4 A for (BAsap-A1)773 and 5.3 0.075 * ( B Asap-AI )773 - - - o - - (BAsap-AI -2)773 h,.lch
0.050" E
~ o.o25-
0.000
,
0
i 5
,
J 10
,
i 15
,
i 20
,
i 25
,
i 30
,
35
A ( kJ/mol )
Fig. 2. Adsorption potential distributions. 0.15
0.003
~"
.~
"- (BAsap-AI)773 - - ~ - - - ( BAsap-AIh,.,,/,-2)773
o< 0.10
[,r~
~E
e~ "~
""
0.05
6
8
!0 Pore width
|2 (A~)
14
Fig. 3. MPSDs calculated from the Horvath-Kawazoe method.
16
(BAsap-AI)773
I
'
0.002
0.001
0.000
0.00
*
l
6
'
I
'
8
!0 Pore
width
'
112
'
114
(~)
Fig. 4. MPSDs calculated from the Jaroniec-Gadkaree-Choma method.
592 and 6.4 A for (BAsap-Alhe~a,-2)773, respectively. These results also indicate that (BAsapAlhe,a,-2)773 shows smaller micropores than that of (BAsap-A1)773. Pore diameters with maxima at 5.5 A are obtained when the JGC method is applied (Fig. 4), being the distribution of (BAsap-Alhe,d,-2)773 sharper than the one of (BAsap-Al)773. The microporous structure of the PILCs is characterised by the distance between the clay layers and the distance between the intercalated units (pillars), the interlayer spacing and the interpillar spacing respectively [6]. Several phenomena can influence these distances such as the elimination of organic moieties from the polycations and the dehydration or sintering of the pillars. Regarding the MPSDs presented in Figures 2-4, more information about the distribution of the aluminium fixed and the evolution of the textural properties with the calcination temperature is required in order to achieve a better knowledge of the microporous structure of these solids. REFERENCES
1. G.W. Brindley and R.E. Sampels, Clays Clay Miner., 12 (1977) 229. 2. S. Yamanaka and G.W. Brindley, Clays Clay Miner., 26 (1978) 21. 3. H. Lahav, V. Shani and J. Shabtai, Clays Clay Miner., 26 (1978) 107. 4. A. Gil, A. Massinon and P. Grange, Micropor. Mater., 4 (1995) 369. 5. A. Gil, H.L. Del Castillo, J. Masson, J. Court and P. Grange, J. Mol. Catal. A, 107 (1996) 185. 6. A. Gil, M.A. Vicente and L.M. Gandia, Micropor. Mesopor. Mater., 34 (2000) 115. 7. A. Gil, L.M. Gandia and M.A. Vicente, Catal. Rev.-Sci. Eng., 42 (2000) 145. 8. M. Jaroniec in T.J. Pinnavaia and M.F. Thorpe (Eds.), Access in Nanoporous Materials, Plenum Press, New York, 1995, p. 259. 9. M. Jaroniec, R. Madey, J. Choma, B. McEnaney and T.J. Mays, Carbon, 27 (1989) 77. 10. J. Jagiello and J.A. Schwarz, Langmuir, 9 (1993) 2513. 11. A. Gil and P. Grange, Colloids and Surfaces A. Physicochemical and Engineering Aspects, 113 (1996) 39. 12. A. Gil, Adsorption, 4 (1998) 206. 13. G. Horvath and K. Kawazoe, J. Chem. Eng. Jpn., 16 (1983) 470. 14. D.H. Everett and J.C. Powl, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 619. 15. M. Jaroniec, K.P. Gadkaree and J. Choma, Colloids and Surface A: Physicochemical and Engineering Aspects, 118 (1996) 203. 16. N. Lahav, U. Shani and J. Shabtai, Clays Clay Miner., 26 (1978) 107. 17. J.Y. Bottero, J.M. Cases, F. Fiessinger and J.E. Poirier, J. Phys. Chem., 84 (1980) 2933. 18. S.M. Bradley, R.A. Kydd and R. Y amdagni, J. Chem. Soc., Dalton Trans., (1990) 2653. 19. M.A. Vicente and J.-F. Lambert, Phys. Chem. Chem. Phys., 1 (1999) 1633. 20. H. Breuil, Ann. Chim. 136me s6rie, 9-10 (1965) 467. 21. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1991. 22. A. Gil and P. Grange, Langmuir, 13 (1997) 4483.
Studiesin SurfaceScienceandCatalysis 144 F. Rodriguez-Reinoso,B. McEnaney,J. RouquerolandK.Unger(Editors) 92002ElsevierScienceB.V. Allrightsreserved.
593
Chord length distribution and pore size distribution of porous VYCOR glass W.Gille a, D. Enke b and F. Janowski b aMartin-Luther-University Halle-Wittenberg, SAS Laboratory, Hoher Weg 8, D-06120 bDepartment of Technical Chemistry, Schlossberg 2, D-06108 Halle
Nitrogen adsorption (NA) experiments yield diameter distribution densities V(d) of the random pore diameter d. Based on the assumed particle shape (circular cylinder, random diameter d), the function V(d) can be transformed into a chord length distribution density (CLD) Aft). CLDs are connected with the porosity p of the material. The direct transformation step, V(d) ~ Aft), has been performed via Mathematica procedures. Small-angle scattering (SAS) experiments include information about the texture. Here, geometric information about pores and walls is intermixed into the correlation function y(r). After separation of both effects, the determination of a pore size distribution from a CLD, ),(r)-,V(d), can be handled.
1. INTRODUCTION Even anisotropic materials can be described by scattering methods [ 1], isotropic materials all the more. Scattering methods are related to the field of CLDs. The simplest case is a single convex geometric figure. The isotropic uniform random chord length distribution density A(/) reflects size and shape of each single geometric object (particle). Scattering experiments of isotropic samples yield isotropic CLDs. Here, the particle is detected in space via an indicator density function i(r). The length r stretches along a test line. Inside a particle i(r) = 1 and outside i(r)=0. According to the Babinet theorem, pore or particle produce the same scattering intensity I(h) (h=amount of the scattering vector). The existence of pores and walls implies the existence of two different, mostly independent, CLDs in one and the same sample: There is the CLD of the pores A(/) (first moment 7) and the CLD of the walls (first moment m). According to the linear integration principle of Rosiwal (1898), these moments define the porosity p via p=l/(l+m). Unfortunately, the moments are intermixed in the so-called SAS correlation function ),(r), which can be obtained from scattering data I(h) by solving the inverse problem, [2], I (h) - ;o 4 ~r2 - u (r) fo
9sin (h. r) / ( h - r ) ~ r 2 9y (r) d r
dr
(1)
594 On the other hand, based on geometric pore models, the method NA yields the pore size distribution density of the random pore diameter d, V(d). In this field, CLDs are not of urgent interest. But, assuming a certain pore shape, there is a connection between V(d) and A(/). This is useful to compare CLDs directly obtained by SAS with those resulting from a transformation V(d) ~ A(l). Such a procedure has to include a separation of pore-portion and wall-portion of the scattering intensity function I(h), operating with all the resulting structure functions of SAS, especially with y"(r) [3-6]. Distinction and separation of pores and walls is not possible without the useful information of NA and electron microscopy. The first step of data evaluation is
I(h)~y"(r). The second step uses NA for interpreting y" and the CLD of the walls results. Finally, based on the assumption of cylindrical pores, V(d) is obtained. 1.1. Theory of chord length distributions of cylinders Analytical expressions for A(/) of circular cylinders are known. The formulas simplify for infinitely long cylinders [4]. Indeed, the first moment 7 coincides with the diameter d, which is important for the following. The transition from a limited right circular cylinder to an unlimited one is studied in Fig. 1. Here, d=l is a surprisingly exact approximation, if the cylinder is longer than 2d.
Figure 1" The function A(l,d,H) of circular cylinders has a spike at I=H and a pole at l=d. In the case H~oo, A(0)=0. Already in the case H=4.d, A(l,d,H) is very close to the case of an infinitely long cylinder Ao(l, d) [4,10].
595 2. DIAMETER DISTRIBUTION and CLD for INFINITELY LONG CYLINDERS 2.1. Procedure from V(d) to A(/) For many cylinders with random diameter d, A(/) is defined in terms of
V(d) and the
CLD of a single cylinder A0(r, d) by the normalized parametric integral [7], see Fig. 2.
A (r, V (x))
:A
(r) :
"
.,~x.
Ao
(r, X) x.
v
v
(x)
(x)
d x
(2)
dx
From V(d) to A(I) ,
- . . . . . . . . .
-]
0.08 .~ 0.06 ~" 0.04
0.02 i
"~-'-'~
0 0
20
40 60 80 Pore diameter d, Chord length I
100
Figure 2: By use of Eq (2), from an assumed pore diameter distribution density V(d)(full line) follows the corresponding CLD A(/) (dashed line). Basic behaviour and maximumpositions of V(d) and A(/) nearly agree. 2.2. Procedure from A(/) to V(d) The following three steps are required to transform applied and
(h)
V(d) follows
V(d). Altogether, A0(l, d) is
explicitly [7,8]. At first, a working function
f ) A ( r) [ 2 - 2 -
A(r) to
9c o s
(h . r) - h
~o I / 12
A (r)
9r o
9s i n
It(h)
follows from A(r)
(h . r) ] / h 4 d r
r 4 ~r
.
(3)
I~,(h) is normalized in the origin, Iv(0) = 1, see [2], and defines another function I~(h) via I~(h)-..I~,(h) / h. From I~(h) follows the normalized CF/3(r) of the mean circular area (twodimensional cross-section of the cylinders) using a Hankel transformation, Eq. (4),
596
/3 (r) - f ~
(h) . Jo (h. r) cYh
foIr (h) d h
(4)
Here, Jo(x) is the Bessel function of the first kind with index zero, and/3(0) = 1 holds. Now, V(d) follows operating with the third derivative fl'"(r) at r = ~]x2 + d 2 , see Eq. (9) in [8], V (d) : - k
9
/3'''
%Ix 2 + d 2
dx
,
V (x) d x
= i.
(5)
The coefficient k normalizes V(d). Eqs. (2,3) and finally Eq. (5) presume the existence of an isotropic two-phase system, in which cylindrical pores are arranged. Interferences between two cylinders are not allowed. The latter is not fulfilled in the scattering experiment. A step of separation performed in real space, operating with the CLD of the pores and the CLD of the walls, will separate the interference between pore and wall.
3. EXPERIMENTAL BACKGROUND and POROSITY p PVG disks with thickness 0.4 mm, Corning Co., Code-7930, have been used. PVG has been prepared by phase separation of an alkali borosilicate glass and subsequent acid leaching. The NA measurement has been performed at 77K on a Sorptomatic 1900 (Carlo Erba Instruments). The sample has been degassed at 393K for 8h. Adsorption and desorption isotherms were measured over a range of relative pressures P/P0 from 0 to 1.0. The total pore volume Vpt has been estimated from the amount of gas, Vads,adsorbed at a relative pressure P/P0=0.99. Assuming a straight cylindrical pore model, V(d) has been determined according to the BJH (BARRET, JOYNER and HALENDA) method. The porosity is defined by the silica network density 2.2 g / cm 3 and the specific pore volume 0.227 cm 3 / g. It follows c~30%. The relative isotropic scattering intensity I(h), (h=4rc/~.sin(O), 20=-scattering angle, ,~=0.154 nm) was recorded by use of a Kratky plant. From the tightly packed arrangement of pores and walls the Porod plot of the scattering curve, see Fig. 3, results. Furthermore, for the isotropic twophase material, for a fixed range order (here L=30 nm), y(r) follows via Eq. (1).
4. RESULTS The NA measurement yields a mean chord length of the pores ]:10.8 nm, via Eqs. (2). By use of this value, it follows a normalized Porod plot, defined by PI (h) =
7r. i
h 4 9I (h)
4
fo h2 I ( h ) d h
9
(6)
597 The Porod asymptote is normalized to unity in Fig. 3. The insert shows the CF in the analyzed r-interval, 2 nm
=
1
(7)
2d2m" I Y' (0) I
The length parameter dim is the average mean chord length, dim = (l + m)/2 = 15 nm, see 4.1. Eq. (7) can be used in many cases, but exclusively based on Eq. (7), an a priori assignment of the numbers p and 1-p to a certain phase (pore or wall) is impossible (Babinet theorem).
-2_
~
.
.
.
.
.
.
.
0.8
Z
0.6
0.8
.
,.., 0 . 6
i
0.4
0.4 0.2
-
.
.
.
.
.
-
.
.
.
.
.
. 10
...................... 0
0.25
12
14
r/,nm, 0.5
0.75
1
1.25
1.5
1.75
h/rim -1
Figure 3: Normalized Porod plot Pl(h) of the scattering intensity I(h). The resolution limit in real space is marked in the subfigure. 4.1. Comparison between CLDs obtained from SAS and NA Based on Eq. (1), Fig. 4 operates with experimental CLDs, I(h)~y"(r)~A(r). The experimental function y"(r), which reflects pores and walls, is represented by the filled plot in insert (a). The grey area allows to control y'(0)=-1/(7.5 nm). The dashed line in (a) is the CLD, obtained after application of the transformation Eqs. (2) with experimental NA-data VNA(d). Obviously, y"(r) can be split into two parts: A pore-part (b) and a wall-part (c), which are approximated by smooth functions. In agreement with the NA-experiment (dashed curve in (a)), the resulting pore-part is unsymmetrical. The wall-part is a nearly symmetrical function reflecting a mean chord length m=20 nm. Thus, dim=(10 nm+20 nm)/2 = 15 nm.
598 In subfigure (d) the step from A(/) of the pores (SAS experiment) to V(d) is illustrated. The difference between the curves A(/) and V(d) is small,
A(r)~V(r). These
functions have been
calculated without using any presupposition about the type of the distributions. No function system has been used. Definitely, the only assumption made is L=70 nm, the integration limit in Eq. (1), which controls all integral transformations.
Figure 4: From the CLDs of pores and walls to V(d) of the pores. The inverse transformation A(/)~ V(d), see Section 2.2., has been applied. This demands a separation procedure for the CLD of the pores. Operating with y"(r) and Eqs. (3-5), from the CLD of the pores A(/) ((d), dashed line) the diameter distribution density V(d) ((d) full line) results.
599 4.2. A linear simulation model This type of model, see Fig. 5, is a one-dimensional approximation of i(r) along a fixed test line through the material [9]. In Fig. 5, the sequence pore/wall/pore ... is simulated for/=]=const, and m=~=const. This simulation predicts sharp peaks in the curvature to the left hand side at r=l 0 nm and r-20 nm. Here, y"(r) should have a negative peak at r=30 nm. Further, a sequence of more or less "weaker" positive (+) and negative (-) peaks is expected at r-40(+), 50(+), 60(-) nm. The clearest stereological information about the pores is limited to the interval 0
Figure 5: Presupposing fixed, well-definedchord lengths, it is possible to construct the theoretical behaviour of
y(r) and the function of occupancy Z(r) as well. Both functions are connected,Z(r)=(y(r)-c)/(1-c) [9,13].
4. SUMMARY NA is a routine method for texture analysis and yields information about the pores. Contrary to this, scattering experiments include stereological information about the walls too. Two overlapping positive main peaks in y"(r) result. Considering the CLD of the pores, derived from NA, it
600 follows a clear allocation of the y'(r) main peaks: The left one (clear maximum position at r = 7 nm), indicates the pores, 7= 10 nm results. The right one indicates the walls m=20 nm. Consequently, p=33% as to be expected for the VYCOR glass considered, see Section 3. Finally, describing the shape of the "pore peak" analytically by a smooth function, a separation of both peaks makes it possible to determine the size distribution of the pore diameter from the scattering experiment. An explicit solution (based on the sequence of three integral transformations) represents V(d) in terms of A(/), see Eqs. (2). In the end, it is possible to compare the pore diameter distributions obtained from NA and SAS experiments. It is useful in each case to combine several methods of investigation in order to steady the results [ 12-14]. The geometric shape of the walls is characterized by a selected CLD too, see Fig. 4. Here, the shape is more complex and a description exclusively based on one random variable only (like wall thickness), the size distribution of which could be estimated then, is not simple.
REFERENCES [1]
[2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14]
J. S. Rigden, J. C. Dore, A. N. North, In: J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing, K.K. Unger (Eds.), Characterization of Porous Solids III, Studies in Surface Science and Catalysis, Vol. 87 (1994) 263-271. L. A. Feigin, D. I. Svergun, Structure Analysis by Small-Angle X-Ray and Neutron Scattering, Plenum, New York, 1987. F. Janowski, D. Enke, in: F. Schtith, K.S.W. Sing & J. Weitkamp (Eds.), Handbook of Porous Solids, Chapter "Porous Glasses", WILEY-VCH, Weinheim, 2002, in press. W. Gille, D. Enke, F. Janowski, J. Porous Mat. 8 (2001) 179-191. P. Levitz, S. K. Sinha, J. M. Drake, J. Chem. Phys. 95 (1991) 6151-6161. D. Enke, K. Otto, F. Janowski, W. Heyer, W. Schwieger, W. Gille, J. of Mat. Sci. 36 (2001) 2349-2357. Wolfram Research, Inc., Mathematica, version 4, Champaign, IL, 2000. I. S. Fedorova, J. of Coll. and Interface sci., 59 (1977) 98-104. W. Gille, Waves Random Media 12 (2002) 85-97. W. Gille, Comp. Mat. Sci. 22 (2001) 318-332. Wu Hsin-I., P. W. Schmidt, J. Appl. Cryst. 6 (1973) 66-75. P. W. Schmidt, private communication during NATO Advanced Study Institute: Como, Italy, May 1993. H. Herrmann, Stochastic models of heterogeneous materials, Materials Science Forum, Vol. 78, Ztirich, Trans. Tech. 1991. K. L. Stefanopoulos, K. Beltsios, P. K. Makri, Th. A. Steriotis, A. Ch. Mitropoulos, N. K. Kanellopoulos, Physica B 276-278 (2000) 477-4787.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
601
A Structural Study of Dehydration/Rehydration of Tobermorite, a Model Cement Compound A. Gmiraa&, R. J.-M. Pellenq a, I. Rannou a, L.Duclaux a, C. Clinard a, T. Cacciaguerra a, N. Lequeux b and H. Van Damme b aCentre de Recherche sur la Mati&e Divis6e, CNRS et Universit6 d'Orl6ans, lb rue de la F&ollerie, 45071 Orl6ans, France bEcole Sup6rieure de Physique et de Chimie Industrielle, 10 rue du Vauquelin 75231 Paris, France & to whom correspondence should be sent We have investigated the structure and the dehydration/rehydration process of Tobermorite-like Calcium Silicates Hydrates (CSH) by using High Resolution Transmission Electronic Microscopy (HRTEM) and a new in-sire temperature and pressure controlled XRay Diffraction set up. CSH samples were synthesized with a Calcium/Silicium ratio 0.9 following a procedure which allows to produce model compounds of cement: a calcio-silicate layered structure with an interlamellar spacing of 14 .~. Atomic scale resolved TEM data confirm the layered structure of Tobermoritic CSH: in particular we were able to show directly for the first time the silicate chain alignment and measure the so-called dreierketten pattern within the silicate chains. A newly developed X-Ray diffraction cell which allows to control temperature and pressure is used to study the dehydration/rehydration of Tobermorite and Ca-MontmoriUonite. In the particular ease of Tobermorite, the variations of the interlayer distance, as measured with the new in-situ X-Ray cell at atmospheric pressure and upon increasing temperature, are very close to those obtained with a standard ex-situ X-Ray apparatus coupled to a Controlled Rate Thermal Analysis heating technique (which allows for quasi-equilibrium conditions). This validates the (T,P) in-situ X-Ray cell: in agreement with literature, we have clearly identified two structural transitions upon heating under atmospheric pressure" (14 .~ -11 ,~) and (11 .~-9.5 ,~) at T=330 K and 510 K respectively. Furthermore, we demonstrate that the first structural transition of Tobermorite upon dehydration is irreversible by contrast to that observed for Ca-Montmorillonite clay.
1. I N T R O D U C T I O N Calcium Silicates Hydrates (CSH) is the major hydration product of Portland Cement. It has been intensively studied for several decades. CSH can be obtained by hydration of C3S and [3-C2S or by precipitation from aqueous solutions containing a Ca salt and silicate ions. CSH has a wide range of chemical composition. Many studies [1-5] indicate that its molar ratio Ca/Si can vary from 0.7 to 2 (or more) [6] and is in most eases near to 1.7 [7]. There has been indirect evidence that CSH (obtained by reaction between CaO and Silica) has a layered structure similar to that of natural Tobermorite and/or Jennite [8]. Recent solid state NMR [910], infrared spectroscopy [11], EXAFS [12] and thermogravimetry works [14] have
602 confirmed the similarities between the CSH and the 14A-synthetic Tobermorite. The structure of natural 11A-Tobermorite, Ca225[Si307s(OH~.5].H20,has been solved by Hamid [13] and Armbruster [ 14]. The structure of 14A-Tobermorite (equivalent to the CSH cement phase) is usually inferred from that of l lA-Tobermorite and its ideal chemical formula is The primary structural unit of Tobermorite contains CaO polyhedral sheets sandwiched between single silicate chains. The silicate chains are based on a three units repeating pattern called dreierketten arrangement of 7.8A in length in which two of the tetrahedra point toward the CaO polyhedral sheet while the third points toward the interlayer void.
Ca2.25[Si307.5(OH~.5].8H20.
2. E X P E R I M E N T S
2.1. Samples preparation The Tobermorite-like CSH samples were synthesized in hydrothermal conditions: an aqueous reaction between CaO and silicic acid with different Ca/Si ratios. CaO was calcinated from CaCO3 at 1100~ while silicic acid was calcinated at 150~ for 2 hours in order to remove adsorbed vapours. The water/solid ratio was fixed at 20. Plastic bottles containing the samples were placed in autoclave at 60~ with a permanent rotation system for 100 days. The samples were then rinsed with pure water and stored at 30% of relative humidity. Here, we report results for synthetic Tobermorite material with Ca/Si=0.9.
2.2. Transmission Electronic Microscopy (TEM) Morphological and high-resolution transmission observations were performed with a TEM Philips CM20 instrument, equipped with a low-dose camera and which can work at 200kV. The sample were prepared by inclusion. First, a drop of the hydrate dispersion (previously homogenised with ultrasounds) was deposited on a Millar film (polyethylene terephtalate). This technique allows to obtain an orientated deposit. Aiter evaporation, the deposit was embedded in a Spur resin in order to maintain the material. The resin was then thin-sectioned by microtomy. The final step was to depose the thin sections on a carbon membrane supported by a fine metallic grid. A TEM observation of hydrates raises many problems because these solids are easily disturbed by local heating due to the electron beam impact under vacuum [ 18]. The electron beam can provoke the fusion of the metallic particles deposed on the membrane and can also cause contamination of the preparations. It is possible to reduce the damages by using a cryogenic trap and decreasing the electron beam intensity. Usually, the electronic diffraction spectrum measurement and TEM photography acquisition are not possible at the same location on the sample because of beam damages.
2.3. In-situ X-Ray Diffraction We have used a new cell developed by INEL : FUREQUI__H. This cell is attached on a standard INEL 120 diffractometer with a Cu wavelength (A = 1.54056 A). The furnace allows to heat polycrystalline samples (bulk or powder) from room temperature ~p to 220~ Because of its sealed chamber, it is possible to work under vacuum (around 10 bars) or in controlled vapour flowing conditions. The inner part of the furnace is constituted by : 9 a brass for the main chamber, and stainless steal for the platelet support
603
9 a sample holder is an aluminium plate 9 a kapton or aluminium foil for the window In this new cell, a X-Ray diffractogram is recorded during 15 minutes at constant temperature after a heating period 5 minutes; the temperature step between two recordings being +5 K and the pressure is set to the atmospheric pressure value. Note that these parameters are disposable. The advantage of this new method is to investigate the timeresolved evolution of the structure during dehydration and rehydration (if it occurs). Note that the heating procedure as implemented in our in-situ cell does not guarantee equilibrium conditions.
2.4. Ex-situ X-Ray Diffraction and Controlled Rate Thermal Analysis (CRTA) The X-Ray analysis were performed on a INEL 120 diffractometer with a Mo wavelength (A = 0.70926 A). The dehydration process is investigated here by a heating using a Controlled Rate Thermal Analysis (CRTA).
3. R E S U L T S A N D D I S C U S S I O N "Figure 1" shows a micrograph obtained by Transmission Electronic Microscopy of a Tobermorite sample. To our knowledge, it is the first time that the layered structure of Tobermorite is so clearly demonstrated. The sheet length is over 3000A in agreement with the domain size obtained from XRD data. The interlayer distance is around 10A.
Fig. 1. Tobermorite TEM Photography
Fig. 2. Zoom
The difference between the initial interlayer distance (14,~) and the TEM observed distance is due to the water resin exchange process during the sample preparation. The zoom shown in "Figure 2" is obtained after 2D-image analysis coupled to a noise reduction technique. It shows the chain alignment of silicate tetrahedra. One can consider this photography as being the first direct visualisation of the silicates chain structure in Tobermorite. The silicate tetrahedra in the chains have the expected molecule dreierketten arrangement as first inferred from XRD data [13]. The measured length of the dreierketten pattern is 7.3A in relative good agreement with the reported XRD value (7.8A) [13 ].
604 "Figure 3" shows the evolution of Tobermorite interlayer distance with temperature during dehydration: it compares results obtained with the two different methods namely the ex-situ XRD coupled to quasi-equilibrium CRTA outgassing and the new pressure and temperature controlled X-Ray cell with the heating and recording conditions above described. Results for Tobermorite dehydration obtained with the new cell are close to those obtained in quasi-equilibrium conditions. The as-synthesized material is well crystallised and present fine Bragg peaks indicating large coherence domains. The interlayer distance of the starting Tobermoritic structure is 14A. Figure (3) allows to locate two structural transitions hence three type of Tobermorite materials: the so-called 14.~, l lA and 9.5A,-Tobermorite in agreement with literature results [7]. These (14,~ -11,~,) and (11A -9.5A) transitions occur at T=330 K and 510 K respectively. The agreement between the two outgassing techniques validates the in-situ X R D cell; the (ex-situ X R D + CRTA) approach being our reference method since it operates in quasi-equilibrium conditions. Tobermorite dehydration 0
,~
16 15
h3 =
~
Ex-situ
:
14 ~, 13 ~ 12
"'In-situ
11
o9
.---
0
50 100 150 200 250 300 350 400 450 500 Outgassing temperature (~
Fig. 3. Tobermorite dehydration process. "Figure 4" represent the XRD results for Tobermorite dehydration using the in-situ XRD set up. It clearly shows the time-resolved evolution of the Tobermorite structure during dehydration obtained with the above mentioned heating procedure under atmospheri.'c pressure. Many authors [7,13] describe the first step of Tobermorite dehydration [14 A-11 A] as being a reversible process.
Tobermorite dehydration
30~ : 14.12A 60~ : 14.06A 80~ : 11.7A 120~ 11.5A 220~ 11.36A 240~ 10.97A
1500 1000: Intensity 500 0 Or
T(oC) t~
120 "' eq m (30 " - ' - ' ~ ~ ~ oeq Fig. 4. Tobermorite dehydration process 2Theta
605 "Figure 5" describes the evolution of the Tobermorite interlayer distance upon rehydration at room temperature and water vapour pressure equal to the saturating pressure value (i.e. relative humidity RH=100%, P/P0=I). No swelling phenomenon was observed after 48 hours hydration: the interlamellar distance remains unchanged at 11.5 ~,. The dehydration process (14 A-11 A) is thus irreversible with these rehydration conditions. Tobermorite dehydration 12 o11,8 ;~11,6
~ 11,4 ~ 11,2
P/Po=I
11
..........
0
i ................
6
I. . . . . . .
12
f
!
I
18
24
30
' "
I................
36
I
42
.........
48
Time (hours) Fig. 5. Tobermorite dehydration process This contrasts sharply with that obtained for Ca-Montmorillonite Wyoming clay, another negatively charged lamellar calcio-silicate mineral for which a reversible dehydration(shrinkage)/rehydration(swelling) process is observed and well documented in the literature [16]. In the case of calcium clay minerals, rehydration is often described as a limited swelling (also called crystalline swelling) with an interlayer variations smaller than a nanometer (few angstroms in most cases). This is at variance with sodium clay minerals for which hydration is unrestricted and usually called osmotic swelling (layers can get separated). In both cases, reversible dehydration/re-hydration cycles are observed. Sodium clay/water behaviour is well described within the frame of the well-known DLVO theory for colloidal suspensions which predicts repulsive electrostatic interaction between charge-like layers immersed in a dielectric continuum containing a number of counter-ions insuring global electro neutrality. The (electrostatic) cohesive (attractive) behaviour of calcium clay in water (restricted swelling) results from ionic correlation interactions [ 17-20]. In order to further validate our in-situ approach, we have realised a dehydration/rehydration cycle for Ca-Montmorillonite in the same conditions as for Tobermorite. As presented in "Figure 6", the clay dehydration occurs at constant atmospheric pressure with increasing temperature: the interlayer distance shifts from 16 .~, to 11 .~. The inverse process, rehydration, is easily obtained at 300 K boy increasing the water vapour pressure: the interlamellar distance first shifts from 11.5 A to 14 .~ for P/P0<0.2; at larger pressure the swelling is more progressive and easily detected as shown in "Figure 6". The origin of the cohesive property of Tobermorite, a model of cement compound remains to be clarified. In a first approach mainly based on Monte-Carlo simulations, the cohesive behaviour of Tobermorite was attributed to correlation forces in a similar fashion as
606 for calcium clay minerals on the basis of the so-called anisotropic primitive model [21-23] due a possible very large electrostatic coupling.
30
Clay dehydration/rehydration -,"'~,, i . . . . J . . . . I . . . . I . . . . i . . . .
40
-
50
-
~
-
80
-
~-~
I
--"
90
Rehydration
00 ,~
-
60
i
80 60
_
- 4 ~ Dehydration ~ 40 I
. . . . .
17
"-"
/
100 110
20
. . . .
16
I
. . . .
I . . . .
a . . . .
15 14 13 Interlamllar distance (A)
i , , ,
12
'q 20 11
Fig. 6. Ca-Montmorillonite clay dehydration/rehydration process The irreversible dehydration process indicates that the underlying dielectric continuum approach used in the anisotropic primitive model does not hold. Further numerical simulations are presently undertaken at the atomic scale in the frame of the polarized ion model (so-called "shell model") in order to give a better description of Tobermorite dehydration/rehydration and cohesive property of cement [24].
4. CONCLUSION Dehydration/rehydration process of Tobermorite-like Calcium Silicates Hydrates (CSH) was investigated by means of High Resolution Transmission Electronic Microscopy (HRTEM) and X-Ray diffraction using a new in-situ temperature and pressure controlled experimental cell. The following points summarise the results obtained in our investigations: 1. CSH samples were synthesized with a Calciurn/Silicium ratio 0.9 following a sott Chemistry hydrothermal procedure which allows to produce model compounds of cement: a calcio-silicate layered structure with a pristine interlamellar spacing of 14 ,~. 2. Atomic scale resolved TEM data confirm the layered structure of Tobermoritic CSH: in particular we were able to show directly for the first time the silicate chain alignment and measure the so-called dreierketten pattern within the silicate chains. 3. A newly developed X-Ray diffraction cell which allows to control temperature and pressure is used to study the dehydration/rehydration of Tobermorite and Ca-Montmorillonite. In the particular case of Tobermorite, the variations of the interlayer distance, as measured
607 with the new in-situ X-Ray cell at atmospheric pressure and upon increasing temperature, are very close to those obtained with a standard ex-situ X-Ray apparatus coupled to a Controlled Rate Thermal Analysis heating technique (which allows for quasi-equilibrium conditions). This validates the (T,P) in-situ X-Ray cell: in agreement with literature, we have clearly identified two structural transitions upon heating under atmospheric pressure 9(14 A -11 ,~) and (11 ,~ -9.5 A) at T=330 K and 510 K respectively. Furthermore, we demonstrate that the first structural transition of Tobermorite upon dehydration is irreversible 9there is no 11 A to 14 ,~ transition upon rehydration at constant room temperature and increasing water vapour pressure. This last result contrasts sharply with that observed for Ca-Montmorillonite day, another lamellar calcio-silicate.
ACKNOWLEDGMENTS The authors would like to thanks the ATILH (Association Technique de l'Industrie des Liants Hydrauliques) for the financial support of A. Gmira and INEL.Ine for technical support in XRay diffraction experiment.
REFERENCES [ 1] H. F.W. Taylor, J. Chem. Soc., (1950) 3682. [2] D. L. Kantro, S Brunauer, C. H Weise, Adv. Chem. Ser., 33 (1961) 199. [3] M. W. Grutzeck, D. M. Roy, Nature., 223 (1969) 492. [4] G. W. Groves, P. J. L. Sueur, W.J. Sinclair, Am. Ceram. Sot., 69 (1986) 353. [5] H. F.W. Taylor, A. B. Turner, Cem. Concr. Res., 17 (1987) 613. [6] S. Mindess, J. F. Young, (eds.), Concrete, Englewood Cliffs, NY, 1981. [7] H. F.W. Taylor, (eds.), Cement Chemistry, Academic press, London, 1990. [8] H. F.W. Taylor, J. Am. Ceram. Soc., 69 (1986) 464. [9] X. Cong, R. J. Kirkpatrick, Adv. Cem. Bas. Mat., 3 (1996) 144. [ 10] X. Cong, R. J. Kirkpatrick, J. Am. Ceram. Soc., 79 (1996) 1585. [11 ] P. Yu, R. J. Kirkpatrick, B. Poe, P. F. Me Millan, X. Cong, J. Am. Ceram. Sot., 82 (1999) 742. [12] N. Lequeux., A. Moreau, S. Philippot, J. Am. Ceram. Soc.,8 (1999) 1299. [ 13] S. A. Hamid, Zeitsch. Fur Kristall., 154 (1981) 189. [14] C. Hoffmann and T. Armbruster, Zeitschritt fur Krist., 212 (1997) 864. [ 15] M. Rautureau, Steinberg, J. Micros. Electro., 10 (1985). [ 16] K. Norrish, Discuss. Faraday Soc., 18 (1954) 120. [17] A. Delville, R. J.-M. Pellenq, J.-M. Caillol, J. Chem. Phys., 106 (1997) 7275. [18] R. J.-M. Pellenq, J.-M. Caillol, A. Delville, J. Phys. Chem. B., 101 (1997) 8584. [19] A. Delville, R. J.-M. Pellenq, J.-M. Caillol, (eds.), Proceedings of the Xlth International Clay Conference, H. Kodama, 1998. [20] A. Delville, R. J.-M. Pellenq, Mol. Sim., 24 (2000) 1. [21] H. Van Damme, R. J.-M. Pellenq, A. DelviUe, Rev. Fran. G6n. Civil., 2 (1998) 767. [22] R. J.-M. Pellenq, A. Delville, H. Van Damme, (eds.), Characterization of Porous Solids IV, Roy. Soc. Chem, Elsevier, U. K, Elsevier, 1997, 596.
608 [23] R. J.-M. Pellenq, M. Crespin, N. Lequeux, C. Menager, L. Costalin, A. Delville, J.-M. Caillol, H. Van Damme, (eds.), Proceedings of a RILEM International Workshop, E & FN Spon, 2001, 535. [24] A. Gmira, R. J.-M. Pellenq, to be published.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
609
Support mesoporosity: a tool for better control of catalytic behavior of cobalt supported Fischer Tropsch catalysts A. Griboval-Constanta, A.Y. Khodakov ~, R. Bechara ~ and V. L. Zholobenko b "Laboratoire de Catalyse, Cit6 Scientifique, B~t. C3, 59655 Villeneuve d'Ascq, France bSchool of Chemistry and Physics, Keele University, Staffordshire ST5 5BG, UK Influence of support porous structure on Fischer-Tropsch reaction rates and selectivities was studied over cobalt supported catalysts with a wide range of cobalt contents (5-50 wt %). Periodic (SBA-15 and MCM-41) and commercial mesoporous silicas were used as catalytic supports. A combination of characterization techniques (adsorption, XRD, and TGA) showed that the sizes of supported cobalt particles and cobalt reducibility strongly depended on pore diameters of mesoporous silicas: small pore diameters led to smaller sizes of supported Co clusters and their lower reducibility in hydrogen. The catalytic measurenaents in a tixed bed reactor at total pressures of 1-20 bar showed that the catalytic behavior of silica supported Co catalysts over the wide range of cobalt contents was primarily governed by catalyst porous structure. The reaction rate and C5+ selectivity increased with increasing catalyst pore diameters. The results suggest that the support mesoporosity provides an efficient tool for the design of Fischer-Tropsch catalysts with desired catalytic properties. 1. INTRODUCTION The Fischer-Tropsch (FT) synthesis leads to a broad range of products, i.e. hydrocarbons, alcohols, acids, esters, etc. The increasingly stringent regulations on the sulfur and aromatics content in fuels are the reasons for renewed interest in this reaction [I]. More efficient catalysts are required to improve FT activity and selectivity to the desired products. Cobalt catalysts have been found to be most suitable for the production of higher hydrocarbons [2]. Optimization of the metal function (Co, Fe, Ru, Mo) in supported FT catalysts has been studied in a large number of papers [3-6]. Little information is however available about the effect of catalyst mesoporosity on FF catalytic behavior. Broad distribution of mesopore sizes in amorphous oxides makes it difficult to quantify this effect. On the contrary, SBA-15 and MCM-41 periodic mesoporous silicas [7-8] with uniform pore size distributions could provide new insights into the effects of catalyst pores on the structure and catalytic behavior of FF catalysts. This paper focuses on the effects of support mesoporosity on the structure and catalytic behavior of cobalt supported FT catalysts. In addition, we aim to demonstrate that the support mesoporosity could provide an efficient tool for the control of catalytic performance of supported FT catalysts.
610 2.
EXPERIMENTAL
Catalyst preparation. Two periodic (S1 and $2) and two commercial ($3, $4) mesoprous silicas were used as catalytic supports. MCM-41 type S1 periodic mesoporous silica was synthesized as described in reference [9]. 1 g of 28% NH4OH solution was added to 21 g of 25% solution of cetyltrimethyl ammonium chloride under stirring. This solution was combined with 5.3 g of tetramethylammonium hydroxide pentahydrate (Aldrich, 97%), followed by addition of 5.6 g of fumed silica (Cab-o-sil M-5, Cabot) and 11.4 ml of water under stirring. $2 periodic mesoporous silica (SBA-15 structural type) was obtained using block copolymer (P123) poly (ethylene glycol)- block- poly (propylene glycol)- block- poly (ethylene glycol), average M, ca. 5,800 (Aldrich, 43,546-5) as a template [ 10]. 90 g of dimethylformamide, 180 g of water and 180 g of 4M HC1 were mixed in a glass flask under stirring. Then 12 g of P123 was dispersed in the resulting solution under stirring. Alter complete dissolving of P123, 24 g of TEOS were added. The mixture was kept at 313 K under stirring for 24 h. Then, for the two silicas, the reacting mixture was transferred into a hermetically closed polypropylene flask and heated in an oven at 373 K for 24h. The resulting gel was washed with distilled water, then dried at room temperature for 24 h and calcined in air at 773 K for 6 h. The rate of temperature ramping was I K/min. $3 and $4 commercial fumed silicas were respectively Aerosil 380 (Degussa) and Cab-o-sil M5 (Cabot). These materials were agglomerated by wetting and dried in an oven at 393 K overnight. Cobalt was introduced to silicas by repeated incipient wetness impregnations using aqueous solutions of cobalt nitrate. Between the impregnations the samples were dried in an oven at 373 K overnight, and calcined in the flow of dry air at 773 K for 5 h. The catalysts were pressed and sieved to obtain 200-500 ~tm size fractions. They were labeled as xCoSy, where x designates the weight percentage of cobalt and y is a number in reference to the silica supports (S1, $2, $3 and $4). Co content in the samples was measured by atomic absorption at the "Service Central d'Analyses du C.N.R.S." (Vernaison, France). Cha.racterization. The catalysts were characterized by nitrogen adsorption, XRD and TGA techniques. BET surface area and total pore volume are calculated from nitrogen isotherms measured at 77 K using a Micrometrics ASAP 2010 system. Average pore diameters are calculated from desorption branches of the isotherms using BJH method [11]. Prior to the adsorption measurements the samples were outgassed at 473 K for 5 h. The catalysts were characterized by XRD technique using a Siemens D5000 diffractometer. Average Co304 particle size was calculated from the Sherrer equation [ 12] using a (440) Co304 XRD peak at 20 = 65.344. The extent of reduction was calculated from TGA weight loss in the atmosphere of hydrogen at 753 K assuming stoichiometric reduction of Co304 to metallic cobalt. The experiments were carried out using a Sartorius 4102 electronic ultramicrobalance equipped with a controlled atmosphere cell. TGA technique was successfully used in our previous reports [13, 14] to evaluate the extent of cobalt reduction in silica and alumina supported catalysts. Catalytic measurements'. The catalytic tests were performed in fixed bed reactors operating at 463-498 K and total pressure of 1-20 bar. The Hz/CO ratio was 2 in all experhnents. Prior to the reaction, the catalysts were reduced in the flow of hydrogen at 753-773 K for 5 h. FT catalytic rates and selectivities were measured at the stationary reghne after 24 h thne-on-stream. FT reaction rates were normalized by the number of cobalt atoms ha the reactor. The reaction products were analyzed by gas chromatography.
611 0.05 0.04
a) b) c)
0.03 E
c~
"~ 0.02 > 0.01
I00
1000
Pore Diameter, A
Figure 1. Pore size distribution curves of: a) S 1, b) $2, c) $3, d) $4. Hydrocarbon selectivities were calculated on carbon basis. The chain growth probabilities c~ were calculated from the slope of the curve ln(Sa/n), where n is the carbon number and S,, the selectivity to a corresponding Ca hydrocarbon. 3. RESULTS AND DISCUSSION 3.1. Catalyst structure The B.E.T. surface area, total pore volume and average pore size of the mesoporous silicas are reported in Table 1. The pore size distributions (Figure 1) are much narrower in periodic mesoporous than in the commercial silicas. After impregnation with cobalt, specific area and total pore volumes slightly decrease. The shape of the pore size distribution curves remains however unchanged after Co impregnation, even if the average pore size diameters were a little smaller. XRD, XPS and XANES techniques show [14, 15] that Co304 is the only phase present in the oxidized catalysts. In the catalysts with low cobalt loadings (5wt. %), the sizes of Co304 crystallites have been found to increase with increasing silica pore sizes. The extent of cobalt reduction also increases as a function of silica pore sizes [15]. The effect of cobalt content on the structure of cobalt species was studied using cobalt catalysts supported by $2 and $4 silicas. The characterization results are presented in Table 2. Specific area and total pore volumes decrease with increasing Co content in the catalysts. The specific area remains however, relatively high (> 90 mZ/g) even at higher cobalt loadings. Table 2 also shows that the average pore size diameter of mesoporous silicas is not affected by varying cobalt content in the catalysts. The XRD patterns (not
Table 1 Adsorption properties Silica SBET (m2/g) S1 742 $2 887 $3 311 $4 213
of lnesoporous silicas TPV Pore diameter (cm3/g)
(A)
0.59 1.91 1.38 0.84
--20 91 280 330
612 Table 2 Characterization of Co catalysts Co Co Cobalt SBET TPV catalyst content surface (mZ/g) (cm3/g) (%) density, Nco/(100 ~5CoS2 1()COS2 2()COS2 30COS2 50COS2 5COS4 10COS4 20COS4 30COS4 50COS4
5.39 10 19 27.3 31.9 4.75 10.5 17.9 26 43.4
0.82 1.61 3.93 6.88 13.07 2.32 5.52 12.01 19.79 48.13
674 634 493 405 249 206 194 152 134 92
1.11 1.03 0.83 0.63 0.38 0.76 0.75 0.63 0.52 0.35
Pore diameter (/~)
Co304 crystallite diameter from XRD
Extent of reduction measured by TGA (%)
(A) 75 75 75 75 75 -230 --330 -330 -330 -330
121 112 106 125 137 230 231 315 286 250
94.9
96.3 94.4
100.0
represented here) are shnilar for all catalysts. The size of cobalt oxide particles estimated from the width of the XRD patterns does not increase with increasing concentration of cobalt atoms on the support surface. It appears that Co304 crystallite diameter (Table 2) depends only on catalyst pore sizes. These results are consistent with the suggestion [15] that the most of the cobalt particles are located inside the pores of mesoporous silicas. The extent of reduction was close to 100% in both $2 and $4 supported catalysts with the cobalt content varying from 5 to 50 wt %. High extent of reduction of $2 and $4 cobalt supported catalysts is likely to be attributed to the presence of relative large cobalt particles (100-2()0 A). Earlier reports showed that large cobalt oxide particles supported by silica could be easily reduced to metal phases [ 16].
3.2. Catalytic behavior 3.2.1. FT synthesis at atmospheric pressure The FT catalytic results obtained at atmospheric pressure after 24 hours on-stream are presented ha Table 3 and in Figure 2. Water and hydrocarbons have been observed as reaction products. Carbon dioxide has not been detected at the reaction conditions. FT reaction rate decreases slowly with the time of stream. At atmospheric pressure the steady state conditions have been usually attained after 7 h of the reaction. Figure 2 shows that catalytic behavior of Co catalyst supported by silicas is strongly affected by catalyst porous structure. Indeed, with the same cobalt content, the reaction rate increases 1()-20 times as the average pore diameter of the support increases from 20 to 330 A~. The lowest reaction rate has been observed with the narrow pore MCM-41 type support (S 1, pore diameter _--_20 A). One of the reasons for the lower catalytic activity of 5CoS 1 catalysts seems to be related to the lower reducibility of smaller supported cobalt particles [14]. Possible reoxidation of small cobalt particles by water at FI' reaction conditions as was suggested earlier by Iglesia et al. [17] could be another reason for the lower concentration of active sites in 5COS1 catalyst. C5+ selectivity observed with Co/SiO2 catalysts is between 50 and 70% (Figure 2), the chain growth probability varies from 0.7 to
613 80
~
I ,cos. ~-------...~...._~..
9
Cs+
5CoSi3 o s--.
40 > 0
0
Or)
o q)
.~ CH4
-4-O
0.1 1oo
200
300
400
P o r e d i a m e t e r , .&.
Figure 2. Effect of support pore size on FT reaction rate and hydrocarbon selectivities. 0.8. Note that slightly lower C5+ selectivity and chain growth probability c~ are observed (Figure 2) with commercial silicas ($3, $4). Table 3 shows that for the catalysts prepared from periodic mesoporous silica ($2) FT reaction rate normalized by the number of cobalt atoms increases with increasing Co content. Higher catalytic activity at high cobalt loadings could be attributed to higher metal dispersion and higher concentrations of active sites in Co catalysts supported by periodic silicas than in those supported by commercial silicas (Table 2, 3). It appears that porous structure of periodic mesoporous silicas prevents small cobalt particles from sintering. Thus, high cobalt dispersion can be maintained at higher cobalt surface density (>10 NcJ(100 A2). On the contrary, we observe a decrease in FT reaction rate at higher cobalt content tbr the catalysts prepared from commercial silica support ($4) with wide pore size distribution (Figure 3). No significant changes however are observed for all catalysts in CH4 and C5+ selectivity. The chain growth probability remains almost the same at different Co loadmgs (Table 3). Table 3 Catalytic results (H2/CO=2, P=l bar, T=463 K, 24 h on-stream, conversion <5%) Co catalyst FT reaction CH4 C5 + rate selectivity selectivity 5COS2 10COS2 20COS2 30COS2 50COS2 5COS4 10COS4 20COS4 30COS4 50Co $4
(10 -4 S-1)
(%)
(C-%)
1.38 1.73 2.45 2.54 2.47 2.68 1.75 1.60 1.88 1.00
15.3 15.2 12.8 15.2 13.3 16.9 16.0 15.8 16.5 18.2
68.4 67.1 70.5 68.0 71.8 60.7 63.0 63.4 62.4 62.3
0.77 0.77 0.78 0.77 0.79 0.70 0.74 0.72 0.70 0.72
614 3 [] 2.5
0.5
0 0
10
20 "30 Cobalt surface density, Nco/(lO0 A 2)
40
50
Figure 3. FT reaction rates on $2 and $4 silicas with different cobalt surface densities.
3.2.2. FT synthesis at higher reactant pressure In agreement with the literature data [ 19-20], we found that catalytic activity of cobalt supported catalysts increases with increasing total pressure from 1 to 20 bar (Table 4). An increase ha the reaction temperature at 20 bar from 463 to 498 K leads to higher FT reaction rates and to a decrease both in C5+ selectivity and in chain growth probability (Table 5). This observation is also consistent with the previous reports [2, 18]. Wide pore catalysts have been found to be more active in FT synthesis than narrow pore ones both at 1 and 20 bar (Table 4). Figure 4 shows that the effect of pore sizes on FT reaction rate was also significant at higher reactant pressure. The concentrations of hydrocarbons adsorbed in catalyst pores, especially of the heavier products, depend on their partial pressure in the catalyst bed. The partial pressure of the products at the same conversion and selectivity levels depends on the total pressure ill the reactor. Therefore, Fischer-Tropsch synthesis at total pressure of 20 bar (H2/CO=2) would result in a higher partial pressure of hydrocarbons. The higher partial pressure would lead to the condensation of reaction products which are normally in gaseous phase Table 4. Catalytic results: effect of total pressure (H2/CO=2, T=463 K, 24 h on-stream) C1+C2 selectivity C5+ selectivity Co catalyst Reaction rate (10 4 s l ) (%) (C-%) lbar 20 bar lbar 20 bar lbar 20 bar lbar 5CoS 1 0.1 4.11 24.5 23.0 57.0 69.9 0.78 5COS2 1.38 4.51 16.7 13.3 68.4 79.2 0.77 5COS3 1.47 21.1 19.4 16.5 59.5 72.2 0.73
20 bar 0.80 0.804 0.70
615
20
""
a)
9
15
5
0
0
9
9
100
200
Pore diameter,
A
Figure 4. Effect of pore diameter on FT reaction rate at a) P=20 bar, b) P= 1 bar. at atmospheric pressure. Kelvin's equation suggests that capillary condensation would occur at much lower partial pressures in narrow pores of the catalyst than in wider ones. Therefore, narrow pores are more likely to be filled by liquid reaction products than wider ones. Capillary condensation in narrow mesopores would lead to diffusion limitations due to a more difficult access for the reacting molecules to the catalyst active sites through the liquid phase. Thus, two phenomena seem to be responsible for the effect of catalyst pore size on FT reaction rate at a higher reactant pressure. First, higher activity of wide pore catalysts could be attributed to the higher extent of reduction of larger cobalt particles located in wider pores [14, 15]. Secondly, wider mesopores seem to be affected to much lesser extent by capillary condensation of FT reaction products at high reaction pressure than narrow ones. Capillary condensation of hydrocarbons in the catalyst pores at FT reactant pressure above atmospheric has been reported previously in a large number of publications [ 17,18]. For all the catalysts, higher reactant pressure results in lower selectivity to light hydrocarbons (C1 and C2) and in higher C5§ selectivity. The chain growth probabilities ((x) seem not to be affected by the increase in total pressure. These results agree with previous studies showing a marked increase in the product molecular weight [18] that occurs over cobalt catalysts at higher reactant pressures. Table 5 Catalytic performance of Co catalysts (H2/CO=2, P=20 bar, T=498 K, 24 h on-stream, conversion <7%) Co catalyst Reaction rate C5+ selectivity c~ (10 -4 S-1) (C-%) 5COS1 12.1 43.8 0.77 5COS2 9.91 45.1 0.74 5COS3 25.0 32.1 0.55 4. C O N C L U S I O N S The results show that catalyst porous structure has a strong hnpact both on the structure of Co species and on the catalytic behavior of the catalysts. The size of supported cobalt particles is related to the size of silica pores: supports with small pores stabilize smaller
616 cobalt oxide particles with lower reducibility. The size of supported cobalt particles is determined by the support pore diameter even at relatively higher cobalt contents (50 wt % of Co). Higher FT reaction rates have been observed with the supports with large pore diameters both at atmospheric pressure and at 20 bar. Cobalt catalysts prepared from periodic mesoporous silicas have been found to be more active at high cobalt contents than those supported by commercial silicas. Narrow pore size distribution in catalysts prepared from periodic mesoporous silicas seems to stabilize cobalt particle size and metal dispersion at higher cobalt loadings and thus to provide higher concentrations of active sites tbr FT synthesis compared to silicas with wide pore size distributions. The mesoporous structure of the support appears to be an efficient tool controlling the catalytic performance of metal supported catalysts. REFERENCES
1. 2. 3. 4.
A.M. Thayer, Chem. & Eng. News, March 13 (2000) 20. P. Chaumette, Revue IFP 51 (1996) 711. H.F.J. van't Blik, D.C. Koningsberger and R.J. Prins, J. Catal. 97 (1986) 210. D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff and A. Holmen, J. Catal. 156 (1995), 85. 5. S.A. Eliason, C.H. Bartholomew, Appl. Catal. A: General 186 (1999) 229. 6. F. Tihay, G. Pourroy, M. Richard-Plouet, A.C. Roger and A. Kienneman, Appl. Catal. A: General 206 (2001) 29. 7. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J.Am. Chem. Soc. 114 (1992) 10834. 8. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. 9. C.-Y. Chen, H.-X. Li and M.E. Davis, Micropororous Mater. 2 (1993) 17. 10. D. Zhao, J. Sun, Q. Li, and G.D. Stucky, Chem. Mater. 12 (2000) 275. 11. E.P. Barrett, L.G. Joyner, and P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. 12. B.D. Cullity, "Elements of X-ray diffraction", Addision-Wesley, London 1978. 13. R. Bechara, D. Balloy, J.-Y. Dauphin, J. Grhnblot, Chem. Mater. 11 (1999) 1703. 14. A.Y. Khodakov, A. Griboval-Constant, R. Bechara, and V. L. Zholobenko, J. Catal. 206 (2002) 230. 15. A.Y. Khodakov, A. Griboval-Constant, R. Bechara, and F.Villain, J.Phys Chem. B, 105(2001), 9805. 16. A. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P. Chaumette, J. Catal. 168 (1997), 16. 17. E. Iglesia, S.L Soled, and R.A.Fiato, J.Catal. 137 (1992), 212. 18. E. Iglesia, S.C. Reyes, R.J. Madon, and S.L.Soled, Adv. Catal. 39(1993), 221. 19. H. Schulz, " Uhnans Encyklop/Sdie des technischen chemie" Ed. Verlag Chemie GmbH Wemhefin, RFA, 46me edition, 14 (1977), 329. 20. C. Hubert, PhD thesis, Universit6 Libre de Bruxelles (1997).
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
Textural characterization of aluminum/lanthanum polyoxycations
montmorillonite
617
pillared
with
J.A. Morante, C. Pesquera, C. Blanco, F. Gonz~lez. Inorganic Group. Department of Engineering Chemistry and Inorganic Chemistry. University of Cantabria. Avda de los Castros, s/n 39005-Santander. SPAIN This study compares the porosity created in montmorillonites pillared with aluminum/lanthanum pillars with that of a sample pillared only with aluminum. Studies of the pore size distribution indicate that the A1/La-pillared samples have a new porous system, with a pore size at the limit between microporosity and mesoporosity. The presence of larger pores in the A1/La-pillared samples is due to the intercalation of inorganic polyoxycations between the clay layers. These polyoxycations are larger than the Keggin ion intercalated in the sample pillared only with aluminum. After successive thermal treatments the micropore volume is still high in the A1/La-pillared samples, whereas the thermal stability of the micropore volume, developed after the pillaring process, is lower in the Al-pillared sample, being reduced practically to zero after thermal treatment at the same temperature. Moreover, this work distinguishes and analyzes the different thermal evolution of both types of pores and compares them with the thermal evolution of the one single type of pores generated in the A1pillared sample.
1. INTRODUCTION Pillared montmorillonites are clay minerals that have been modified by exchanging the interlayered cations of the mineral for large inorganic species, followed by calcination, in order to prepare microporous materials. The separation between layers can be kept stable and depends on the polyoxycation used, and heating these materials results in the formation of clusters that keep the layers open permanently, generating a microporous structure with a high specific surface area [1-3]. Polymeric compounds of A1, Zr, Cr and Fe are among the principal pillaring species quoted in the literature [ 1, 4-7]. To prevent the clay layers from sintering at the high temperature of the catalytic reactions, the thermal resistance of the pillars must be increased. One well-tested way of achieving this is to use mixed pillars in the materials [8-9], and the most widely used methods have been the mixing of Al-pillaring solutions with lanthanide cations. Sterte [ 10] and McCauley [ 11] found that the incorporation of lanthanide elements in the preparation of the pillaring agent resulted in material whose basal spacing was greater than that of conventional materials. In this study, we have prepared and characterized montmorillonite pillared with A1 and La in different proportions. The structural and textural parameters of the materials were compared with those of montmorillonite pillared only with A1. We have applied classical and new models to low pressure nitrogen adsorption data to obtain a quantitative evaluation of the microporosity of the synthesized materials and their evolution under thermal treatments.
618 2. EXPERIMENTAL SECTION
2.1. Starting material The starting material used in this work was a montmorillonite from Wyoming, supplied by Missouri University and denominated here as Wy. The fraction <21.tm of this sample had a surface area of 33 m2/g and a volume adsorbed of 0.042 cm3/g at p/p0 = 0.98. Its exchange capacity was 106 mequiv/100 g of clay. 2.2.Preparation of the pillaring agent The solutions of the pillaring agent, which had different A1/La ratios (25/1; 50/1; 75/1 and 100/1), were prepared by adding suitable amounts of LaC13.7H20 to 2.5M A1 solutions obtained from Locron [AlffOH)sC1.2-3H20]. These solutions were heated in a reactor autoclave for 72 hours at 130~ The reactor reaches pressures of 3 bar. After returning to room temperature and atmospheric pressure, the reaction mixture was diluted with the quantity of water necessary to yield an A1 concentration of 0.1M. The pillaring agent with aluminum, the Keggin ion [A1OaAl12(OH)24(H20)12]7+, was obtained from a 0.1M A1 solution from Locron at room temperature. The different synthesis conditions for each pillaring agent were optimum after several experiments in the laboratory. 2.3. Pillaring Process The solutions of pillaring agent were added with vigorous stirring to a clay slurry of 2.5 g/100 mL. The final proportion in all cases was 20 mequiv of A1/g of clay, with a solid/liquid ratio of 0.5%. The reaction mixture was stirred continuously for 24 h at room temperature. It was then washed by means of dialysis with distilled water, using 1 L of water/g of clay. Dialysis was continued with water being renewed every 24 h until the CI ion concentrations decreased to the point where the conductivity of the water was < 30 ~tS. Finally, the samples were freeze-dried. Samples were then calcined for 2 hours at 400~ to obtain the pillars. The materials obtained are denominated A1La-Wy-25, A1La-Wy-50, A1La-Wy-75 and A1La-Wy100 according to the proportion of A1/La used in the pillaring agent. The sample denominated A1-Wy was obtained under the same conditions from a pillaring agent prepared with the solution containing only aluminum. In order to study the thermal stability of the samples, the same sample was treated in successive sets of thermal treatment at 500~ 600~ and 700~ for two hours at each temperature. Equipment and Methods. The following equipment and techniques were used for the physicochemical characterization of the materials: X-ray diffraction. The X-ray diffraction diagrams were obtained on powder with the particles oriented so as to increase the intensity of the (001) reflection. The apparatus used was a Philips PW- 1710 diffractometer with CuKot radiation. Textural Parameters. Adsorption-desorption isotherms of N2 at 77K were determined in a Micromeritics ASAP 2010 with a micropore system. Prior to measurement, the samples were outgassed at 140 ~ for at least 16 h. The specific surface area was determined by the BET method, assuming that the area of a nitrogen molecule is 0.162 nm 2 [12]. Micropore volume was calculated by the t-plot method using the Harkins and Jura [ 13] thickness. We used model isotherms calculated from density functional theory (DFT) to determine the pore size distributions and cumulative pore volume of the pillared samples by taking the adsorption branch of the experimental nitrogen isotherm, assuming slit-like pores [ 14].
619 3. RESULTS AND DISCUSSION Table 1 shows the values of the basal spacings for all the synthesized materials and the original sample. The A1-Wy sample displays a d(001) peak at 18.8 A from XR diffactograms, while samples A1La-Wy-25, A1La-Wy-50, A1La-Wy-75 and A1La-Wy-100 present a peak between 20-24A, which corresponds to inorganic polyoxycations with sizes different from that of the Keggin ion [ 15-17]. The textural parameters of the samples were studied by means of the adsorption-desorption isotherms of N2 at 77K. Since all the A1La-Wy pillared samples behave in a similar way, the A1La-Wy-50 sample has been chosen as representative in the figures.
n 140
140
co 120
120
~'100
100
-~- AI-Wy - * - AI-Wy-500 --*- AI-Wy-600
80
80
- - - AI-Wy-700
60
60
< 40 E = 20
40
I--
0 "O
9 L_ 0
~
20
0
>
0
0 0
0.2
0.4
0.6 P/Po
0.8
1
1E-05 0.0001 0.001
0.01
0.1
1
P/Po
Figure 1. N2 adsorption isotherms at 77K of the A1-Wy sample after thermal treatments, between 400 and 700~ Relative pressure axis" left, linear and right, logarithmic.
Figures 1 and 2 show the nitrogen isotherms at 77 K for A1-Wy and A1La-Wy-50 respectively, after successive thermal treatments at temperatures between 400 and 700~ with a linear relative pressure axis (left) and with a logarithmic relative pressure axis (right). The adsorption isotherms are type I in the Brunauer, Deming, Deming and Teller (BDDT) classification [12] at low relative pressures, which indicate the presence of micropores, generated in the pillaring process to intercalate inorganic polyoxycations between the clay layers. All samples show a hysteresis loop type H4 in the IUPAC classification [ 18]. The most important difference between the pillared samples' isotherms is shown in the adsorption branch at low relative pressures. In the A1La-Wy-50 sample (Figure 2), two levels of nitrogen adsorption are observed, a new increase in the volume of adsorbed nitrogen taking place at approximately p/p0 =0.1. This new step in the level of nitrogen adsorption is seen more clearly when the isotherms of nitrogen adsorption are represented in semilogarithmic scale, right of figure 2. The presence of these two different steps at the low relative pressure of nitrogen adsorption could be due to the different sizes of polyoxycations inserted between the clay sheets. Moreover, this is reflected in the greater basal spacing, thus generating a greater
620
140
140
120
120-
~100
100-
Or) ETJ
0
-
0
-
0
-
--.- ~La-Wy-50 ~ - AILa-Wy-50-500 --~ AILa-Wy-50-6(X) --,- ~La-Wy-50-700
"t3
-a60 t,_
0
"o
m40
E
V
20
0
>
0
i
0
i
i
i
0.2
0.4
0.6
i
1
i
i
0.8 P/Po 1 0.00001 0.0001 0.001
i
0.01
i
I
0.1 P/Po 1
Figure 2. N2 adsorption isotherms at 77 K of the A1La-Wy-50 sample after thermal treatments, between 400 and 700~ Relative pressure axis: left, linear and right, logarithmic. space interlayer and, consequently, pores of larger diameter. This adsorption took place in micropores at the limit between microporous and mesoporous size. However, in the A1-Wy sample we observed that there is a single level of adsorption at low pressures, in figure 1 (right) the step at approximately p/p0 =0.1 does not appear, which indicates a unique micropore size. Figure 2 shows that the step at p/p0 =0.1, present in the samples pillared with A1/La, remained after treatments up to 700~ Although as temperature is increased, a decrease in intensity of the step is observed with inferior nitrogen adsorptions when the samples are treated at higher temperatures. This relationship between the level of nitrogen adsorption and the increase in thermal treatment, is also noted in the A1-Wy sample (figure 1). The adsorption branches of the isotherms (figures 1 and 2), at high relative pressures P/P~ remain parallel to each other after the consecutive thermal treatments, which indicates that the mesoporosity of the materials is not modified, affecting the decrease in the adsorption of nitrogen at low relative pressure values and therefore in the microporous zone. In addition, the adsorption branches of the Al-pillared material (A1-Wy), are seen to display lower values which decrease more quickly with the thermal treatment. Table 1 shows the values for specific surface area, SBET and volume of N2 adsorbed at P/P~ Vad, of the pillared samples and the raw material. All the pillared samples show a more developed porosity than the raw material (montmorillonite). The specific surface area and pore volume increase in all the pillared samples, but the increase is greater in the A1/Lapillared samples than in the Al-pillared sample. The increase in specific surface area was consistent with the expansion of the structure observed by XRD. The A1-Wy sample had a SBETof 195 m2/g while the A1La-Wy-100 had a value of 349 m2/g.
621 Table 1 Structural and textural parameters of the samples. Samples
d (/~)
A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy- 100 A1-Wy Wy
SBET(m2/g)
19.9 21.2 23 24 18.8 13.9
Vad (cm3/g)
299 321 317 349 195 33
Vmp (cm3/g)
0.181 0.190 0.192 0.210 0.133 0.042
0.109 0.115 0.120 0.131 0.061 0
Figure 3 shows the t-plot for two of the pillared samples. From this plot, the micropore volume, Vmp, was calculated (Table 1). The micropore volume is seen to increase in the pillared samples, and again the increase is greater in the A1/La-pillared samples. This increase in micropore volume, which is greater than that in specific surface area shows a two-fold increase in the A1La-Wy-100 sample over the A1-Wy sample (0.131 and 0.061 cm3/g respectively). The former sample presents a second step in the adsorbed volume at greater thickness, t, than in the A1-Wy sample, which presents only one step. This second step is related to relative pressure of adsorption around p/p0 = 0.1, which corresponds to the abovementioned increase in the adsorption isotherm.
80
120
Al-Wy ---4-- Al-Wy-500
70
100
a. 60 1-O9 o)
80
50
60
o 40 uf -o 30 <
40
>o 20
--e-- AILa-Wy-50 --e-- AILa-Wy-50-500
20
10
--~,-.- AILa-Wy-50-600 --.-I- AILa-Wy-50-700
0
0
5
10
Thickness-Harkins-Jura (A)
15
--~'
0
i
5
10
15
Thickness-Harkins&Jura (A)
Figure 3. t-plot of pillared samples: A1-Wy and A1La-Wy-50 after thermal treatments between 400 and 700~
Table 2 shows the values of the specific surface area, SBETof the samples after thermal treatments. The A1/La pillared samples can be seen to show much higher percentages of conservation of the surface area than the A1-Wy sample. Thus the A1La-Wy-100 sample
622 Table 2. Thermal evolution of the specific surface area (mVg) of the pillared samples. Sample A1-Wy A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy-100
400 (~ 195 299 321 317 349
500 (~ 185 248 230 194 296
600 (~ 82 142 228 153 237
700 (~ 41 110 138 103 181
modifies its specific surface area of 349 m2/g at 400~ maintaining 181 m:/g at 700~ whereas sample A1-Wy, in the same range of temperatures, decreases from 195 m2/g to 4 lm2/g, indicating a total collapse of the pillars, with microporosity falling sharply to values similar to those of the initial sample. This can be attributed to the presence of lanthanum in the pillars which gives an increase in the basal spacing of these samples and seems to display greater thermal stability than in the sample that only incorporated A1 in this process.
Table 3. Thermal evolution of the micropore volume Sample A1-Wy A1La-Wy-25 A1La-Wy-50 A1La-Wy-75 A1La-Wy-100
400 (~ 0.061 0.109 0.115 0.120 0.131
500 (~ 0.056 0.088 0.080 0.071 0.109
(cm3/g)of the pillared samples. 600 (~ 0.024 0.050 0.079 0.050 0.080
700 (~ 0.005 0.034 0.046 0.031 0.057
Table 3 presents the micropore volume of the samples, calculated by the t-method, after thermal treatment at 400, 500, 600 and 700~ A high micropore volume is generated with the pillared process in all the pillared samples, because the raw material has no microporosity. Again, this increase is greater in the A1/La- pillared samples than in the A1-Wy sample. Table 3 shows that the volume of micropores remains high in the A1/La-pillared samples after the successive thermal treatments. On the other hand, the thermal stability of the micropore volume developed after the pillaring process is lower in the A1-Wy sample, being reduced practically to zero after thermal treatment at 700~ whereas in the A1/La-pillared samples it is 0.031-0.057 cm3/g at 700~ In order to study the evolution of the microporosity generated in these materials with thermal treatments, the micropore size distribution and cumulative pore volume of the pillared samples have been analyzed. The DFT method has been applied accordingly assuming the slit-like pores model, taking into account the laminar structure of the samples. Figures 4 and 5 show the micropore distribution (fight) and cumulative pore volume (left) for the A1-Wy and A1La-Wy-50 samples after thermal treatments. The A1/La-pillared sample presented micropores of two sizes whereas, the A1-Wy sample presented micropores of only one size, which corresponds to the smallest diameter pore of the A1/La-Wy-50 sample. Moreover, these figures also reveal that up to 700~ the cumulative pore volume in the A1La-Wy-50 sample for the smaller diameter type of pore is not greatly altered, whereas in the A1-Wy sample the decrease is easily observable in this type of pores.
623 0.04
0.14 "0
--~l-VVy
ro
"~ 0.0a5
--~/~i-Wy-500 --*-~WtO:t) /~t-Wy-700
0.12 0.1
0.03 0
> 0.025
E
t,...
_q 0.08
g_ 0.02
O
> e 0.06
o.o15
r
O
a. 0.04
0.01
t,,..
> . - -
0.005
~ 0.02
=_
E O
0 1(10
10
10
100
Fbre ~dth ( ~ o r m )
F ~ Wdth ( , m : ~ m s )
Figure 4. Distribution (right) and cumulative pore volume (left) by the DFT method of the A1-Wy sample after thermal treatments between 400 and 700~
0.0:35 0.14
--~ ~La-W~~" --~ta-V~ --~La-V~
O}
"b 0.12 tO
9
0.1
~>
0.08
~O 13.
0.06
E
0.03 0.025
v
O
> 0.02
--.-/~iLa-V~
0
o_ 0.015 t,-
E
e> 0.04 ~
0.01
L
oe- 0.005
-5 0.02 E
O
AA AAAA--~"
0 1
10 PoreV~dth( ~ )
100
1
10
Pore Wdth ( ~ )
Figure 5. Distribution (right) and cumulative pore volume (left) by the DFT method of the pillared sample A1La-Wy-50 after thermal treatments between 400 and 700~
100
624 4. CONCLUSIONS The montmorillonite pillared with aluminum and lanthanum has inorganic polyoxycations incorporated between the clay sheets, which modifies the textural characteristics of the raw material. An increase is observed both in the specific surface area and also in the porous structure, particularly the micropore volume, with generation of pores at the limit between microporosity and mesoporosity. The A1/La-pillared samples have larger pores than those generated in the A1-Wy material, whose pores clearly belong to the micropore region. In addition to the fact that the textural parameters show higher values, they are thermally more stable, maintaining high values of specific surface area and micropore volume up to 700~ This greater thermal stability in the A1/La-pillared samples, which show two types of pores, could be due to the fact that the larger sized pores prevent the collapse of the smaller-sized pores, or it may be that these types of pores generate smaller-sized pores. ACKNOWLEDGMENT: Our acknowledgment to CICYT for financial support of this work under Project MAT 99/1093-CO2-O2. REFERENCES
[1] F. Figueras, Catal. Rev.- Sci. Eng. 30 (1988) 457. [2] J. T. Kloprogge, J. Porous Mater. 5 (1998) 5. [3] A. Gil, L.M. Gandia, M.A. Vicente, Catal. Rev.- Sci. Eng. 42 (2000) 145. [4] D. Tichit, F. Fajula, F. Figueras, B. Ducouraut, G. Mascherpa, D. Gueguen, J. Bousquet, Clays Clay Miner. 36 (1988) 369. [5] M.A. Martin-Luengo, H. Martins-Carvalho, J. Ladriere, P. Grange, Clay Miner. 24 (1989) 495. [6] F. Figueras, A. Mastrod-Bashi, G. Fetter, A. Therier, J.V. Zanchettr, J. Catal. 34 (1986) 658. [7] B.M. Choudary, V.I.K. Valli, J. Chem. Soc., Chem. Commun. (1990) 1115. [8] X. Tang, W. Q. Shu, Y.F. Shen, S.L. Suib, Chem. Mater. 7 (1995) 102. [9] M.J. Hernando, C. Pesquera, C. Blanco, I. Benito, F. Gonzfilez, Chem. Mater. 8 (1995) 76. [10] J. Sterte, Clays Clay Miner. 39 (1991) 167. [11] J.R. McCauley, U.S. Patent No. 4,818,737 (1988). [12] S.J.Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1982. [ 13] W. D. Harkins, G.J. Jura, J. Chem. Phys. 11 (1943) 431. [ 14] J. P. Olivier, J. Porous Mater. 2 (1995) 9. [ 15] S. M. Bradley, R.A. Kydd, J. Yamdagni, J. Chem. Soc., Dalton Trans. 7 (1990) 102. [ 16] C. Pesquera, F. Gonz~lez, I. Benito, S. Mendioroz, J.A. Pajares, Appl. Catal. 8 (1991) 587. [17] F. Gonz~lez, C. Pesquera, C. Blanco, I. Benito, S. Mendioroz, Inorg. Chem. 31 (1992) 727. [18] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure Appl. Chem. 57 (1985) 603.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
625
Influence of pH in mesoporous silica aluminas (MSA) synthesis C. Rizzo*, A. Carati, C. Barabino, C. Perego and G. Bellussi EniTecnologie, Via Maritano 26, 1-20097 San Donato Milanese, Italy Fax 0039-02-520-56364: [email protected]
The effect of the pH ranging between 11.9-14.0 in the sol-gel synthesis of silicaaluminas MSA and ERS-8 has been studied. A progressive variation from white opalescent to transparent gels was observed. After drying and calcination, the gel textural properties were compared by N2 adsorption/desorption. The pH increase gives rise to evolution from irreversible Type IV to reversible Type I isotherms and a progressive decrease of medium pore size is observed.
1. INTRODUCTION The overall performance of a catalyst is known to depend not only on the inherent catalytic activity of the active phase but also on the textural properties of the solid. The ability to control the specific surface area and the pore size distribution during the synthesis of amorphous silica-aluminas has been described for both surfactant micelle templated syntheses (M41-S (1), FSM-16 (2), HMS (3), SBA (4), MSU (5), KIT-1 (6)) and cluster templated sol-gel syntheses (MSA (7), ERS-8 (8)). All these families of silica-aluminas are characterized by high surface area (larger than 500 m2/g) and narrow pore size distribution in the micro and/or meso region. The synthesis route affects not only the formation mechanism and the mean pore size but also the long-range order of pore system, that can be related to the presence of reflection at low angles in the X-ray diffraction pattern (XRD) from powders. For each family of silica-aluminas several synthesis parameters can be identified and applied to control the textural properties of final products. For example the role of the type and the amount of gelling agent (8, 9), the solvent role (10), the silica/alumina molar ratio (9) have been discussed for MSA and ERS-8 formation. Another important parameter that has to be deeply considered is the pH. It is well known the role of pH on silica chemistry: it affects dissolution and polymerization rate, gel or precipitate formation and the textural properties of the final silica (11). Also for surfactant micelle or cluster templated syntheses of silica-aluminas, the effect of pH on porosity remains relevant and it is strongly influenced by the kind of material and by the synthesis route selected for its preparation. For example, in order to synthesize a MCM-41-type material at room temperature a low pH (about 8.5) is essential, while a micropore material is obtained at pH=ll (12). By contrast when the MCM-41 synthesis is performed by hydrothermal treatment, the pH
626 adjustments at value about 11 permit to increase the structural order and the textural uniformity (13). Besides, in syntheses of MCM-41 by the system cetyltrimethylammoniumtetraethylortosilicate, the pH affects the particle size and the wall thickness (14). SBA syntheses performed at different pH give rise to only poorly-ordered mesoporous solids out of the pH range 10-12, while the best ordered samples are obtained at pH between 11 and 12 (15). In this work different pH values ranging from 11.9 and 14.0 has been studied in the solgel synthesis of MSA and ERS-8. The pH values have been modified using different amount of tetrapropyl ammonium hydroxide (TPAOH). In the synthesis of MSA and ERS-8, TPAOH plays several roles: it is the catalyst in the sol-gel reactions via alkoxides (e.g. hydrolysis, alcoholysis and polymerization/ depolymerization) and the gelling agent too. Besides, it can act as a templating agent, organizing SiO2 and A102"units in polymeric structure and addressing the pore formation through the TPA+ clusters. In particular in this work the contribution of TPAOH to the alkalinity of the reagent mixture during the preparation of silica-aluminas is discussed. Several molar ratios TPAOH/SiO2 (0.050, 0.075, 0.100, 0.125, 0.150, 0.175 and 0.200) have been considered and their effect on porosity and surface area has been examined.
2. EXPERIMENTAL Samples were prepared via sol-gel in alkali-free medium using Si(OC2H5)4 (Dynasil-A, Nobel), Al(iso-OC3H7)3 (Fluka), tetrapropylammonium hydroxide (TPAOH, Sachem), and ethanol (Fluka). All syntheses were performed at the same molar ratio: SIO2/A1203=300, H20/SIO2=8, CzHsOH]SiO2=8. The TPAOH/SiO2 molar ratio was changed between 0.05 and 2 and consequently the pH between 11.9-14. A typical synthesis preparation is following described. AI(i-OC3HT)3 was dissolved at 60 ~ in TPAOH (12 wt % in aqueous solution). The solution was cooled to room temperature, then Si(OC2H5)4 in CzHsOH was added. As the monophasic clear sol was obtained, the pH value was measured with a Ross Sure-Flow Combination, Orion model 8172, after pH meter calibration with two buffers (pH 7 and
lO). Then the sol was transformed in a homogeneous gel (from opalescent white to transparent) without separation of phases. After 15 hour aging at room temperature, the gels were dried at 100 ~ and calcined 8 hour in air at 550 ~ All the syntheses performed are reported in Table 1. The textural properties of all calcined samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area (SBET) was evaluated by full 3-parameters BET equation and by 2-parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume (VT) was evaluated by Gurvitsch rule. BJH method was applied on the desorption isotherm branch only for mesoporous materials, in order to evaluate the mesopore width. Mean pore size (dDFT) was calculated using DFT method (Micromeritics' DFT Plus | software) for all materials with the cylindrical pores in oxide surface model.
627
Table 1 Synthesized materials and their textural properties. SAMPLE pH Gel Molar ratio Isotherm SBET(2p) SBET(3p) TPAOH/SiO2 Type (m2/g) (m2/g) ET- 01 11.9 W 0.050 IV 790 1076 ET- 02 12.4 W 0.075 IV 770 823 ET - 03 12.6 O 0.100 IV + (I) 690 800 ET- 04 12.8 O 0.125 IV + (I) 710 852 ET- 05 13.0 T 0.150 I + (IV) 710 1046 ET- 06 13.6 T 0.175 I 690 1041 ET- 07 14.0 T 0.200 I 740 1108 Synthesis molar ratio: SIO2/A1203=300, H20/SIO2=8, CEHsOH/SiO2=8 W = white opalescent; T = transparent; 0 = opalescent
.
.
dajn (.A) 55 37 36 34 .
.
dDFT VT (A) 51.6 36.3 28.8 22.2 15.6 11.6 13.8 .
.
(ml/~) 0.901 0.684 0.505 0.442 0.374 0.310 0.353 .
3. RESULTS The gelation time is related to the TPAOH/SiO2 molar ratio: at low values a fast gelation is observed and opalescent white gels are obtained, by contrast a slow gelation is observed for the samples synthesized at high values and transparent gels are obtained. Intermediate behaviors are observed between the conditions above described. In Tab. 1 the textural properties of the calcined samples are reported. In Fig. 1 and Fig. 2 isotherms and cumulative pore volume for all samples are shown. The samples derived from white gels show an irreversible Type IV isotherms characteristic of MSA-type materials, while, those derived from transparent gels are characterized by a reversible Type I isotherms typical of microporous ERS-8-type materials. Irreversible Type I + IV isotherms are observed for the samples with intermediate values of molar ratios. Particularly as the TPAOH/SiO2 molar ratio increases a progressive shift of the hysteresis loop toward low relative pressure, till the total disappearance, is observed (Fig.l). All materials are characterized by a very high specific surface area. As general trend the 2-parameters BET specific surface area results underestimated with respect to that evaluated by the full 3-parameters BET. With the exception of the sample ET-01, the observed differences increase with the contribution of micropores. For mesoporous (Type IV isotherms) or essentially mesoporous (Type IV +(I) isotherms) samples, the pore diameter has been evaluated with both BJH and DFT methods obtaining a consistent values. The DFT method gives rise to lower values with respect to BJH, the difference observed for each sample gives a qualitative information about the contribute of micropore in the sample and is in agreement with the isotherm above described. Only DFT method was applied for samples with a large contribution of micropores. Indeed, DFT, based on a molecular statistical approach, is applied over the all range of the relative pressure.
628
TPAOH/SiO2 = 0.050
T P A O H / S i O / = 0.075
S TPAOH/SiO2 = 0.100
m
O
TPAOH/SiO2 = 0.125
TPAOH/SiO2 = 0.150
r~ TPAOH/SiO2 = 0.175
TPAOH/SiO2 = 0.200
r
0
0,25
~5
~75
Relative Pressure (p/pO)
Fig. 1" N2 adsorption/desorption isotherms.
629
T P A O H / S i O 2 = 0.050
T P A O H / S i O 2 = 0.075
E E
f
m
j
o
T P A O H / S i O 2 = 0.1 O0 .... ,,,,,,,., . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I,i
T P A O H / S i O 2 = 0.125
o
m
E j
"-
1
_.,=~.
T P A O H / S i O 2 = 0.175
I
I
10
100
Pore Width
"
I
1000
10000
(Angstroms)
Fig. 2" DFT cumulative pore volume
The mesopore diameter (ET-01 - ET-04), evaluated with BJH method, increases when the molar ratio TPAOH/SiO2 decreases. The sample prepared at TPAOH/SiO2 =0.050 shows the largest mesopore diameter.
630 In the range of conditions studied (ET-01 - ET-07), the mean pore size, evaluated by DFT, decreases when TPAOH content increases. Accordingly both the increase of micropore volume and the shift of hysteresis loop are observed. The total pore volume decreases as the amount of TPAOH and the pH increase.
4. DISCUSSION The variation of TPAOH amount during the synthesis of silica-aluminas has two macroscopic effects: the change of the pH and of the relative amount of TPAOH to silicoaluminate oligomers. The textural properties of the final silica-aluminas may result from the different sequence of hydrolysis and condensation reactions (and the reverse reactions: esterification and alcoholic or hydrolytic depolymerization) of the Si and A1 alkoxides. Indeed an increase in TPAOH content and therefore of the pH corresponds to an increase of O H availability that can favor the hydrolysis and depolymerization reactions, giving rise to different gelation rate and to different network formation. As well-known drift fast hydrolysis and slow condensation favor formation of linear oligomers; on the other hand, slow hydrolysis and fast condensation result in larger, bulkier and more ramified oligomers (16). Also the appearance of gels is in agreement with the type of network produced: the presence of highly cross-linked gel networks yields white opalescent gels (17, 18), as those obtained at pH < 12.5 by a fast gelation. Linear chain of polysilicates yields transparent gels (17, 18), as those obtained at pH > 13 by a slow gelation. The kind of network affects the textural properties of final products: highly branched network favor the formation of open structures (i.e. mesoporous materials), while weakly branched networks tend to collapse forming more densely packed structures (i.e. microporous materials). Besides, according to the mechanism of formation of MSA-type and ERS-8-type silicaaluminas (Fig. 3) proposed in (8), the relative amount of TPAOH with respect to silicoaluminates is very important. Indeed mesoporous MSA is obtained when the silicoaluminate oligomers are enough to completely surround the TPA + solvated clusters. By contrast microporous ERS-8 is obtained when the silico-aluminate oligomers grow as sheets because they can not surround the solvated clusters. According with this mechanism ERS-8 is obtained for molar ratio TPAOH/SiO2 > 0.150. At high TPAOH content both the high availability of O H and the formation mechanism do not allow formation of large, ramified silico-aluminate oligomers favoring the growth in sheets, i.e. ERS-8 type silica-aluminas. By contrast at low TPAOH content the lower availability of OH- favors large and ramified oligomers, surrounding the TPA + solvated clusters. The dimension of TPA + clusters seems independent on the TPAOH amount, indeed the pore diameter decreases when the amount of TPAOH increases.
631
Fig. 3: Formation mechanism of ERS-8 and MSA as proposed in (8).
5. CONCLUSIONS In the sol-gel synthesis of silica-aluminas the TPAOH amount affects the textural properties of the final material modifying the pH and the relative amount of TPAOH with respect to silico-aluminate oligomers. At low TPAOH content (pH = 11.9 - 12.4) a slow gelation with white opalescent gel formation is observed. This behavior is typical of highly branched networks that after drying and calcination give rise to mesoporous materials. At high TPAOH content (pH = 13.6-14.0) a fast gelation with transparent gel formation is observed. This behavior is typical of linear networks that after drying and calcination give rise to microporous materials.
References
1. J. S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonoswicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc., Chem. Commun. (1993) 680. 3. A. Tuel, S. Gontier; Chem. Mater., 8 (1996) 114. 4. Q. Huo, D.I. Margolese, G.D. Stucky; Chem. Mater. 8 (1996) 1147. 5. S.A. Bagshaw, E. Prouzet, T.J. Pinnavaia; Science 269 (1995) 1242. 6. R. Ryoo, J.M. Kim, C.H. Shin, J.Y. Lee; Studies in Surface Science and Catalysis, 105 (1997) 45. 7. G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara, G. Perego; Studies in Surface Science and Catalysis, 84 (1994) 85. 8. G. Perego, R. Millini, C. Perego, A. Carati, G. Pazzuconi, G. Bellussi; Studies in Surface Science and Catalysis, 105 (1997) 205 9. C. Rizzo, A. Carati, C. Barabino, C. Perego, G. Bellussi, Studies in Surface Science and Catalysis, 140 (2001) 401.
632 10. A. Carati, C. Rizzo, M. Tagliabue, C. Perego, Studies in Surface Science and Catalysis, 130 B (2000) 1085. 11. R.K. Iler, The chemistry of silica, John Wiley & Son Eds., (1979), chapter 3. 12. A.C. Voegtlin, A. Matijasic. J. Patarin, C. Sauerland, Y. Grillet, L. Huve, Microporous Materials 10 (1997) 137. 13. R. Ryoo, J.M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 14. F.Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galaeneau, D. Plee, F. Fajula, Microporous and Mesoporous Materials, 28 (1999) 437. 15. H.M.A. Hunter, P.A. Wright, Microporous and Mesoporous Materials, 43 (2001) 361. 16. L.L. Hench, J.K. West, Chem. Rev., 90 (1990) 33. 17. S.D. Jones, T.N. Pritchard, D.F. Lander, Microporous Materials, 3 (1995) 419. 18. B. Handy, K.L. Walther, A.Wokaun, A.Baiker, Studies in Surface Science and Catalysis, 63 (1991) 239.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
633
The adsorption of 1-hexene and 3,3-dimethyl-l-butene on R u - M C M 41 Udayshanker Singh a, Ruth T. Williams a, lan D. Salter b a Department of Chemistry, The Open University, UK. b School of Chemistry, University of Exeter, UK.
The effect of heat-treatment under reduced pressure (> 10-5 torr) at 623 K on a sample of MCM 41 prepared using a ruthenium surfactant as a template (Ru-MCM 41) has been studied by sorption of nitrogen, 1-hexene, and 3,3-dimethyl-1-butene. Nitrogen sorption (77 K) shows a slight decrease in the BET surface area (330 to 282 m2gl ) and pore volume (0.19 to 0.16 cm3gl ) as a result of the heat-treatment. However, adsorption of 1-hexene at 303 K shows a doubling of the monolayer uptake of 1-hexene on heat-treatment, which corresponds to an increase in both (1-hexene derived) BET surface area (144 to 295 meg-1) and pore volume (0.12 to 0.16 cm3g-~). For the non heat-treated sample, less than-~50 % of the available surface was accessible to 1-hexene, suggesting the 1-hexene is sterically hindered compared with nitrogen. However heat-treatment of this sample resulted in improved accessibility of the surface and the pore system to the 1-hexene molecules. This increase in 1-hexene uptake without a corresponding change in the nitrogen BET surface area may explain the observed increase in catalytic activity for 1-hexene hydrogenation following heat-treatment of the RuMCM 41. The adsorption at 303 K of 3,3-dimethyl-l-butene, which is more bulky isomer of 1-hexene, yielded a (3,3-dimethyl-l-butene derived) BET surface area of 161 m2g-1 on the heat-treated sample, i.e. about half that of the 1-hexene derived value (295 m2g-1). The pore volume determined from this sorptive was also lower (0.13 cm3g-~) than that for 1-hexene (0.16 cm3g-~) on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-1butene, which approximates to a spherical shape, is more sterically hindered compared with that of the long chain 1-hexene.
1 INTRODUCTION The mesoporous solids developed by Mobil group 1 in 1991 were found to be catalytically inactive and have attracted a considerable interest from researchers throughout the world to introduce catalytically active sites within these materials. For example, doping of aluminium into the silica2 generates Bronsted acid sites and the resulting materials can be used as solid acids in acid-catalysed reactions. It is also possible to deposit metal particles within the pores and to use these materials as redox catalysts in many chemical reactions. 3 Another avenue for catalytic functionalisation is to tether metal complexes within pores in order to prepare heterogeneous catalysts. 4 It has been observed in the process of functionalisation that MCM materials can lose mesoporosity, surface area and pore volume as shown by nitrogen
634 sorption5. Most of the functionalised materials are produced using a two-step procedure, which involves preparing the mesoporous solids, and then introducing catalytic sites. However Bruce et al. 6 developed a one-step method for production of a mesoporous solid containing deposited metal particles. This method uses a metal (ruthenium) surfactant, exploiting the dual functionality of the ruthenium complex as both structure directing and catalytic site generation agent in the synthesis of Ru-MCM 41. The catalytic properties of RuMCM 41 for the hydrogenation of 1-hexene to n-hexane were found to improve upon heattreatment. 6 Therefore, we wished to investigate the effect of heat-treatment on the surface characteristics ofRu-MCM 41. In this paper, we report the adsorption of 1-hexene at 303 K by Ru-MCM 41 before and after heat-treatment at 623 K. We also report the adsorption at 303 K of 3,3-dimethyl-1butene, which is an isomer of 1-hexene, after heat-treatment ofRu-MCM 41. 2 EXPERIMENTAL
The sample of Ru-MCM 41 was supplied by Bruce and co-workers. 6 Powder x-ray diffraction data were obtained with a Philips PW1050/25 diffractometer operating in Bragg-Brentano geometry with CrKa radiation, (~ = 2.29 A). Data were collected in the 20range 1~ 20 ~ with a step size of 0.05 ~ and dwell time of 6 s per point. Nitrogen sorption at 77 K was performed using an automated Micromeritics Tristar 3000 Surface Area Analyser. The sample was outgassed by purging using the Micromeritics Flowprep 060 at 423 K under nitrogen flow for 6 h prior to analysis. Adsorption studies of 1-hexene and 3,3-dimethyl-l-butene at (303 K) were performed using a McBain-Bakr gravimetric balance: The samples were outgassed at 423 K under reduced pressure (> 10-5torr) for 1.5 h to remove any physisorbed vapours prior to adsorption. The heat-treatment of the sample was performed at 623 K for 16 h under reduced pressure (> 10.5 torr). Isotherms are presented as plots of amount adsorbed (cm3 g-l) against relative pressure, p/pO. The sorptives were exposed to three '~eeze-pump-thaw' cycles to remove dissolved gases before sorption studies. For each data point the sample was exposed to the sorptive and allowed to reach equilibrium, this being defined as the point at which no further decrease in pressure and no further increase in mass of the sample were observed. Sorptive vapour pressure readings were recorded using a pressure transducer (Digitron Instruments, Model P400). 3 RESULTS AND DISCUSSION Powder x-ray diffraction data of Ru-MCM 41 show a dl00 peak at 3.7 ~ on 20 scale, which confirms the hexagonal phase is formed. The powder x-ray diffraction analysis performed on the sample after heat-treatment continued that the material retains the hexagonal symmetry of the mesophase. Nitrogen, 1-hexene and 3,3-dimethyl-1-butene sorption isotherms for both heat-treated and non-treated samples are shown, e.g., Figs. l, 2 and 3, respectively. For nitrogen sorption both the samples display Type IV isotherms, indicating that these materials are mesoporous. The isotherm for the non-treated sample displays a lack of hysterisis on desorption, which is characteristic of nitrogen sorption on MCM 41 materials. 7 The hysterisis observed in the nitrogen sorption isotherm for heat-treated sample suggests that heating of the sample (623 K
635
0.2 ~-
~ "~
o9 O o e
f
0.15 [ e ~ ~ n
~ ' 0 O oO AA A
,
g~
9
9 A"
c9
dj
dDe~e:e~~
JA A AA AA AA ~A ~
'
o
o 9 o 9 o~
AA~6~,A~/,
A
A
A
0.1
A
adsorption (heat-treated) adsorption (non-treated)
o 0.05
00
,
!
=
I
0.2
i
,
i
i
0.4
,
,
f
0.16
i
,
relative pressure, p/pO
0.I8
L
i
~
Fig. 1 Sorption isotherms of N2 at 77 K on Ru-MCM 41, where filled symbols denote desorption.
0.2 ~
'
I
A A
~ o.~5i
A
A
A
A
0 A
o
0
O
0
A
0
0
0
0
0.1
~
F 0.05
Q A
adsorption on non-treated sample adsorption on heat-treated sample !
0
0
o12
,
i
,
,
,
I
,
0.4 0.6 relaffve pressure, p / p o
,
,0.18,
,
,
i
1
Fig. 2 Adsorption isotherm of 1-hexene at 303 K, before and after heat-treatment of Ru-MCM 41.
636
~ F 9
>o,5 I 0
0.1
i
9
0
(>
0
A
A
0
A
O
A
0
0
0
0
2
E
0
1-hexene adsorption on non-treated sample
3, 3-di-methyl- 1-butene adsorption on
0.05
heat-treated sample <>
o~ . . . . .
!
o. 0.2
,
1-hexene adsorption on heat-treated sample
i
i
I
,
J
~
I
,
,
0.4 0.6 relative pressure, p / p o
1_
t
9
,
0.8
Fig. 3 Comparison of the adsorption of 1-hexene and 3,3-dimethyl-1-butene at 303 K on Ru-MCM 41.
under the reduced pressure of> 105 torr) has brought about a change in the nature of the surface characteristics. The sorption data are summarised in Table 1. Table 1 Sorption data for nitrogen, 1-hexene and 3,3 dimethyl 1-butene on Ru-MCM 41. Pore volume / cm 3 g-l
Molecular
BET derived surface
area of the
area / m 2 g-1
sorptive /
Non-
Heat-
A2
treated
treated*
nitrogen
16.2
330
282
0.19
0.16
1-hexene
38.2 ~
144
295
0.12
0.16
3,3-dimethyl-
39.0 ~
Sorptives
161
Non-treated
Heattreated*
0.13
1- butene * Sample heated at 623 K under vacuum (> 10-5 torr) for 16 h. t Molecular area (a,,) calculated from liquid density (pL) using the equation am -- ( M / p z Z ) 2Is • 1016, where M is the molar mass (g mol 1) and L is the Avagadro number.
637 A slight decrease in the BET surface area (330 to 282 mZg-l) and pore volume (0.19 to 0.16 cm3g1) as a result of the heat-treatment was observed for nitrogen sorption.
1-Hexene adsorption carried out on the non-treated and the heat-treated sample yielded a Type I isotherm in each case (Fig. 2). A BET specific surface area of 144 m/g l for non-treated sample was obtained, which was found to be almost doubled (295 m2gl) for the heat-treated sample. The heat-treated material showed a high initial uptake of 1-hexene which levels off around a relative pressure of p/pO = 0.25. The final plateau of the 1-hexene isotherms gives a Gurvitsch pore volume of 0.12 cm3g-1 and 0.16 cm3g"l for non-treated and heat-treated samples, respectively. Heat-treatment was shown to result in improved accessibility of surface and pore system to the 1-hexene molecules. This observed increase in 1-hexene uptake without a corresponding change in the nitrogen BET surface area (Table 1) may partly explain the observed increase in catalytic activity for hydrogenation of 1-hexene following heat-treatment of the Ru-MCM 41. 6 The large size of the 1-hexene molecule with respect to the nitrogen molecule may explain the lower uptake of the 1-hexene (Table 1) compared with nitrogen for the sample prior to heat-treatment. The adsorption of 3,3-dimethyl-l-butene at 303 K on the heat-treated sample is shown in Fig. 3. The 3,3-dimethyl-l-butene adsorption yielded a Type I isotherm. The BET surface area value of 161 m2g-1 obtained, from 3,3-dimethyl-l-butene is lower than those derived from nitrogen and 1-hexene (Table 1). The pore volume determined from this sorptive was also lower (0.13 cm3g1) than that for 1-hexene on the heat-treated sample. This suggests that adsorption of 3,3-dimethyl-l-butene, which approximates to a spherical shape, is sterically hindered 8, whereas in 1-hexene, which is a straight chain molecule, is less sterically hindered to interact with the surface. 4 CONCLUSIONS Heat-treatment (under reduced pressure of > 10-5 torr, at 623 K) of Ru-MCM 41 has been shown to alter the surface characteristics of the material with respect to sorption of nitrogen and hexene isomers, although the change was more marked with 1-hexene. Nitrogen sorption yields a Type IV isotherm, before and after heat-treatment, with hysterisis observed in the isotherm of heat-treated sample. A slight decrease in the BET surface area (330 to 282 m2g-l) and pore volume (0.19 to 0.16 cm3g-~) as a result of the heat-treatment was observed for nitrogen sorption. However, adsorption of 1-hexene yields a Type I isotherm and a doubling of the monolayer uptake of 1-hexene on heat-treatment, which corresponds to an increase in both BET surface area (144 to 295 mZg-l) and pore volume (0.12 to 0.16 mZg-l). For this sample, the heat-treatment resulted in improved accessibility of the 1-hexene molecules to the surface and the pores. This increase in the 1-hexene uptake without a corresponding change in nitrogen BET surface area partly explains the observed increase in catalytic activity for 1hexene hydrogenation following heat-treatment of Ru-MCM 41 sample. The adsorption of 1hexene isomer, 3,3-dimethyl-l-butene (303 K), after heat-treatment showed a Type I isotherm and yielded a BET surface area of 161 m2g-l, i.e. about half of the 1-hexene (295 m2gl) derived value. The derived pore volume (0.13 cm3g-1) was also lower that that of 1-hexene (0.16 m2g1) on heat-treated sample. The lower uptake of 'spherical' 3,3-dimethyl- 1-butene, compared with the 'straight chain' 1-hexene suggests that the former isomer is more sterically hindered on the adsorption sites of Ru-MCM 41 sample.
638 5 ACKNOWLEDGEMENTS
We thank M. Danks and Prof. D. W. Bruce (University of Exeter) for the mesoporous ruthenium sample. U. Singh thanks the Open University for financial support, and University of Exeter, School of Chemistry for the use of its facilities. REFERENCES
1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J.C. Vartuli and J.S. Beck, Nature, 1992, 359, 710. 2. B. Lindlar, A. Kogelbauer and R. Prins, Microporous and Mesoporous Materials, 2000, 38, 167. 3. W. S. Ahn, D. H. Lee, T. J. Kim, J. H. Kim, G. Seo, R. Ryoo, Applied catalysis A: General, 1999, 181, 39. 4. R. J. Clarke and I. J. Shannon, J. Chem. Sot., Chem. Commun., 2001, 1936. 5. C. M. Bambrough, R. C. T. Slade and R. T. Williams, J. Mater. Chem., 1998, 8, 569. 6. H. B. Jervis, M. E. Raimondi, R. Raja, T. Maschmeyer, J.M. Seddon and D. W. Bruce, J. Chem. Sot., Chem. Commun., 1999, 2031. 7. P. J. Branton, P.G. Hall, and K. S. W. Sing, Adsorption, 1994, 1, 77. 8. C. M. Bambrough, R. C. T. Slade and R. T. Williams, Phys. Chem. Chem. Phys., 2000, 2, 3499.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
639
XPS studies of M C M 41 postmodified by a Schiff base copper complex Udayshanker Singh a, Ruth T. Williams l, Ian D. Salterb, Keith R.Hallam r Geoffrey C. AHenC a Department of Chemistry, The Open University, UK. bSchool of Chemistry, University of Exeter, UK. r Interface Analysis Centre, University of Bristol, UK.
Preliminary studies of the distribution of the copper complex in MCM 41 postmodified by a Schiff base (salen) copper complex have been carried out using x-ray photoelectron spectroscopy (XPS) and atomic absorption spectroscopy (AAS). AAS showed 1.3 at% (atomic %) (0.65 mmolg~) of copper loading, which is in close agreement with 1.5 at% obtained from XPS analysis. Nitrogen sorption (77 K) shows a change from a Type IV to a Type I isotherm on postmodification, indicating a change in porosity of the material from mesoporous to microporous. This change in porosity corresponds to a reduction in surface surface area (from 813 m2g-l to 332 m2g-l) and pore volume (from 0.39 cm3g-l to 0.02 cm3gl). The XPS argon ion etching results suggest that about a third of the copper complex is going inside the pores, thereby narrowing some pores and blocking others. The remaining copper complex goes on to the external surface of the material. This hypothesis is consistent with the observed changes detected by nitrogen sorption studies on postmodification of MCM 41.
1 INTRODUCTION The synthesis of mesoporous materials with metal-organofunctional groups has attracted a great deal of interest from many researchers due to their potential applicability as catalysts. MCM 41 materials, 1 comprising hexagonally-packed arrays of one-dimensional cylindrical pores, can provide large surface areas for functionalisation with metals and/or organic species, either by postmodification or direct synthesis. 2' 3 Copper-containing molecular sieve materials are very important catalysts in many liquid-phase oxidation reactions. 4' 5 The analysis of metal content is usually obtained using atomic absorption spectroscopy (AAS) 6, but this provides no information on the distribution of the metal within the material. In this paper, we report on the characterisation of a siliceous MCM 41 material postmodified with a Schiff base copper complex7' 8 by x-ray photoelectron spectroscopy (XPS), AAS and other standard techniques. Quantitative estimations of the copper concentrations and chemical states and its distribution within the material have been made using XPS. The effect of modification by the Schiff base copper complex on the surface characteristics of the MCM 41 was investigated by nitrogen sorption at 77 K.
640 2 EXPERIMENTAL Synthesis Preparation of the parent MCM 41 sample The parent siliceous MCM 41 material was synthesised by hydrolysis of tetraethyl orthosilicate (TEOS) under mild alkali conditions, based on a procedure described by Schmidt and co-workers. 9 Postmodification of the parent MCM 41 with a Schiff base copper complex The chemical postmodification of the parent MCM 41 material was carried out in three steps, as shown in scheme 1. (Et O)3--Si'CH 2-CH 2-CH 2-NH 2 4- HO
-
~H
I ~ reflux in MeOH for3 h
(EtO)3---Si'CH 2"CH 2-OH 2-N
-reflux in toluene
for 6h l----O
C
M ----0\
41
/
1
2
C2H50..
t-
Cu %
\
i - ~ O -Si 43 H2"C H2"C H2-N ---~)/
3
MCM 41
C2H,O
/
~ .J
# jOj c QO C2H50," ~ -~-"~, C2HsO~~'Si - C H2"C H2-C H2"~ ___...~ I.I C 2H 5 o -
Scheme 1 Postmodification ofMCM 41 by a Schiffbase copper complex. In step 1, the Schiff base ligand was prepared by reacting aminopropyl triethoxysilane with salicylaldehyde. 6 Then, in step 2, the Schiff base copper complex was prepared by refluxing for 3 h the Schiffbase ligand (0.75 g, 2.30 mmol) with copper acetate (0.23 g, 1.26 mmol) in an approximately 2:1 molar ratio, using toluene (35 cm3) as the solvent. Finally, in step 3, template-free MCM 41 (1.00 g ) (preheated at 100 ~ for 2 h) was added and the refluxing continued for a further 16 h. The resulting yellowish green solid was filtered and washed, first with ethanol (2 x 25 cm 3) and then with acetone (10 cm3), and then dried, initially under reduced pressure and finally in an oven at 50 ~ for 12 h to give the final product (1.35 g). Characterisation The postmodified MCM material synthesised was characterised by XRPD (powder x-ray diffraction), nitrogen sorption, AAS, CHN micro analysis, XPS, and IR. 9, l0 Powder x-ray diffraction data were obtained with a Philips PW1050/25 diffractometer operating in Bragg-Brentano geometry with CrKt~ radiation, 0~ = 2.29 A). Data were collected in the 20 range 1~ l0 o with a step size of 0.05 ~ and dwell time of 5 s per point. Nitrogen sorption at 77 K was performed using an automated Micromeritics Tristar 3000 Surface Area Analyser. The samples were degassed using the Micromeritics Flowprep 060 under nitrogen flow prior to analysis. The parent material was degassed at 150 ~ for 6 hrs and the postmodified material was degassed at 100 ~ for 6 hrs (the parent MCM 41 samples, which were degassed in temp range 1 0 0 - 200 ~ did not show any significant differences in the sorption results). The lower temperature was used to avoid any possible decomposition of the postmodified material.
641 The quantitative estimation of copper in the postmodified material was carried out by AAS following complete dissolution using an HF/HNO3 mixture. II The amount of copper was determined by a calibration curve method using a copper lamp of wavelength 324.8 nm and air/acetylene fuel (ratio of 3" 1). X-ray photoelectron spectroscopy is a surface analytical (1 - 5 nm depth) technique. For the analyses presented here, it should be remembered that on the scale of the XPS analysis area (4 mm x 3 mm), the individual sample grains would be randomly oriented. The analyses will be averages of measurements from all possible directions of the grains, including those presenting the outer faces of the hexagonal structure to the analyser and others which are end-on, thus contributing information on elements and species within the open ends of the pore structure. Samples were mounted on to a stainless steel holder using double-sided adhesive carbon tabs. Wide-scan survey spectra were acquired to identify the elements present, followed by higher energy resolution scans for the elements identified. The shallow take-off angle analysis was achieved by increasing the angle between the sample and the analyser input to enhance the surface selectivity of the XPS analysis. The argon ion etching for a total of 270 s (at a rate of-~l nm min-I, equivalent to - 4.5 nm depth) was performed followed by scanning the spectra to determine the distribution of complex within the pores. The adventitious hydrocarbon peak, expected at 284.8 eV binding energy 12, was used to correct for sample charging. Quantification and peak-fitting of the data were performed using linear backgrounds beneath the peaks of interest and relative sensitivity factors. 3 RESULTS AND DISCUSSION The hexagonal structures of the parent MCM 41 and the postmodified material were confirmed by XRPD. Both materials showed sharp and intense dloo peaks at lower 20 angles and smaller ones at higher 20 angles. The IR band at 1624 cm-1 for the imine group confirms the immobilisation of the Schiffbase copper complex on the MCM 41 material. 6 Nitrogen sorption isotherms at 77 K for the siliceous MCM 41 and the postmodified material are shown in Fig. 1. The parent MCM 41 material showed a Type IV isotherm, typical of mesoporous material as reported in previous studies. 9' 13 The step-wise rise of the adsorbed amount of nitrogen, caused by capillary condensation of nitrogen in the mesopores, was clearly observed for the parent MCM 41 material in the range 0.3 < p/pO < 0.4. The postmodified material, in contrast, showed a Type I isotherm with significant decreases in pore volume and surface area, as summarised in Table 1. These changes in pore parameter were expected because of the replacement of surface silanol groups by the bulky Schiff base copper complex. 4, 5, 7, ~3 The change in the isotherm from Type IV for the parent MCM 41 to Type I for the postmodified sample suggests that the copper complex is filling and/or blocking the pores, resulting in the decrease in surlhce area and negligible pore volume for the postmodified material. 14 On postmodification the pore width (DBm) was found to decrease from 2.4 nm to 1.6 nm. Assuming a cylindrical pore shape the predicted decrease in pore volume would be expected to correspond to a decrease in pore volume from 0.39 cm3g-1 to 0.17 cm3g-~ on postmodification. However, the measured pore volume for postmodified sample is only 0.02 cm3g-~, supporting the hypothesis that considerable pore-blocking has occurred upon postmodification. The postmodified material analysed by AAS shows 0.65 mmol g-1 of copper loading. The analysis of the filtrate from the preparation of the postmodified MCM 41 material and the analysis of the washings showed a combined total of 0.09 mmol of copper. This suggests
642 ,-..300 ~ ~ I'-"
'
'
'
~
'
I
ct)
' ~
"
'
'
9 ~n~::~ ~
~
o
'
~
'
'
'
o o o ~ o o o
~
'
'
'-]
co
"7
"" 250 i ~ - / ~ 1 7 6 1 7 6 ~
r
Parent MCM41
9
200
<~
"1o
Postmodified MCM
-~ 41 q
.Q 150 i,.. 0
t~ r
100
o
o
e~eoeoe
~
~
~
~
~,
~
~
~
#
~
e
~
0
E 50 < 0
0.2
0.4
0.6
relative pressure,
0.8
1
Wp~
Fig. 1 Nitrogen sorption isotherms (77 K) of the parent MCM 41 and the postmodified MCM 41 materials (filled symbols denote desorption points).
Table 1 Nitrogen sorption results for the parent MCM 41 and the postmodified MCM 41 materials. Sample
Type o f I s o t h e r m
SBET / m2g_ 1
Vpore/ cm3g_ 1
DBJH nm
Parent MCM 41
IV
813
0.39
2.4
Postmodified MCM 41
I
332
0.02
1.6
Table 2 CHN microanalytical results obtained for the postmodified MCM 41 material Carbon /% Observed Expected
13.7 12.5
C:N Hydrogen Nitrogen N" Cu (AAS) /% /% Molar Ratio Molar Ratio 8.1:1 1.7 1.9:1 1.9 8.3:1 1.5 2.0:1 1.6
that the most of the copper goes on to the MCM 41 material and approximately 10 % remains in the solution or is removed by washing. The CHN microanalytical data for the postmodified material are shown in Table 2. The expected percentages of C, H, and N were calculated by using the observed amount of Cu from AAS and the stoichiometry of the postmodified MCM 41 material. When comparing the observed carbon and hydrogen mass percentages with the expected values, it can be seen that the observed values are slightly higher in each case. These slight excesses of carbon and hydrogen might possibly be attributed to some residual ethoxy groups from the copper Schiff base complex, which have not been liberated during the postmodification process. The C/N ratio observed (8.1) is close to the theoretical value (8.3). The observed N/Cu (copper from AAS measurement) ratio (1.9) is also very close to the expected value (2.0). This confirms the complexation of copper with the Schiffbase ligand in a 1:2 ratio and gives a clear idea about the complexation of copper on MCM 41 material via the Schiffbase
643 Ix. ,el
5800 ....." . . . . . . . . . Cu 2p Co5400 u 5000
Cu4
'-"~ . . . . . I Cul
"....
Cu3
n 4600 t s 4200
3800 c148
944
940
936
932
928
Bindi_ng Energy / eV Fig. 2 Peak-fitted Cu
2p region of the XPS spectrum of the postmodified MC]V[ 41 material.
ligand. The CHN microanalysis results support the hypothesis from IR spectroscopy that the Schiffbase copper complex is immobilised on the parent MCM 41 material. Table 3 presents the measured binding energy peak positions and postulated chemical environments for the various elements as determined using XPS on samples of both the parent and the postmodified MCM 41 materials. For the parent MCM 41 material, the O l s peak position of 533.2 eV is as expected for alpha-quartz, SiO2. Likewise, 103.8 eV for the Si 2p peak is in agreement with SiO2-type material. These peak positions did not change significantly upon tilting the sample to reduce the analysis depth. The 399.4 eV binding energy measured for N Is in the spectrum from the postmodified material is as expected for amines, cyanides and organic matrices. The postmodified sample's Si 2p peak position (102.8 eV binding energy) was observed at slightly lower energy compared to the parent MCM 41, which suggests that former is more akin to silicate than silica. The Cu 2p peak position was observed at binding energy of 934.7 eV, as shown in Fig. 2. The two peaks for copper indicated that it was present in both Cu(I) and Cu(II) forms, although Cu(II) was the expected oxidation state. This may be explained by considering the exposure to x-rays under ultra-high vacuum conditions, which is known to reduce Cu(II) to Cu(I). To prove this hypothesis, a series of experiments with increasingly short exposure times were performed on freshly prepared samples of the postmodified material and it was noted that reduction was observed even for very short exposure times. Fig. 3 presents an example of the effect of x-ray exposure on the relative proportion of Cu(I). In addition to a shift in the peaks to lower binding energy, there is also a reduction in the intensity of the satellite peaks, which are both indicative of a loss of Cu(II). Therefore, it was concluded that the presence of two apparent oxidation states in the postmodified MCM 41 was a function of the analysis conditions rather than a fundamental property of the material. Table 4 presents the quantified surface atomic concentrations as determined using XPS on samples of both the parent and the postmodified material. Tilting of the parent sample, to enhance the surface selectivity of the technique, highlighted the slight carbonaceous contamination of the material. There was an apparent doubling of the surface carbon concentration, at the expense of the oxygen signal, showing the carbon to be predominantly present on the very outer surface of the sample. Clear differences are apparent between the spectra of the parent and the postmodified MCM 41. The presence of nitrogen and copper are observed in the spectrum of the postmodified material, but not in the spectrum of the parent material. The carbon
644 concentration was greater than expected from the postmodified sample. In addition, the spectrmn of the postmodified material showed reduced silicon and oxygen levels, which is indicative of the presence of the Schiffbase masking the siliceous substrate. Table 3 Binding energies determined by XPS and postulated chemical environment for the parent MCM 41 and the postmodified materials. (HC = adventitious hydrocarbon and -CH2- groups) Binding Energy / eV
Sample
Parent Parent MCM MCM 41 41 shallow take-off angle 284.8 284.8 HC HC Carbon
Parent MCM 41 after Ar§ etching
Postmodified MCM 41
Postmodified MCM 41 shallow take-off angle
Postmodified MCM 41 after Ar+ etching
284.8 HC 287.2 alcohol/ ether 289.0 carboxyl
284.8 HC 286.2 carbon with nitrogen
284.8 HC 286.4 carbon with nitrogen
284.8 HC 286.7 alcohol/ether
399.4 amines/ cyanides/ organic
399.3 amines/ cyanides/ organic
531.3 hydroxyl 532.8 SiO2-type
532.3 silicate/ SiO2-type
397.8 nitride 399.6 amines/ cyanides/ organic 532.5 silicate/SiO2type
102.8 silicate
102.7 silicate
102.1 silicate
932.9 Cu(I) 934.5 Cu(II)
933.1 Cu(I) 934.7 Cu(II)
933.1
Nitrogen
Oxygen
Silicon
Copper
533.2 SiO2type
103.8 SiO2type
533.3 Si02type 104.0 Si02type
532.9 SiO2type 102.0 silicate 103.7 SiO2type
Cu(I)
645
Cu2p,, b
ttCu
~,te t
't
a
970
965
960
955 950 945 940 Binding Energy / eV
935
930
925
Fig. 3 An example of x-ray induced reduction effect on copper for the postmodified material. (a) Initial XPS spectrum, (b) XPS spectrum following exposure for 2.5 hours. Table 4 Quantified surface atomic concentrations determined by XPS Surface Concentration/Atomic % (at %) Sample
Carbon Nitrogen Oxygen Silicon Copper
Parent MCM 41
3.3
68.5
28.3
6.4
65.5
28.0
Parent angle
MCM 41
shallow take-off
Parent MCM 41 after Ar+ etching Postmodified MCM 41
32.i 71.0
5.8
45.8 16.2
22.1 5.6
1.5
Postmodified MCM 41 shallow take-off angle
71.8
5.3
15.5
6.2
1.2
after Ar ~ 85.7
3.0
6.6
4.1
0.5
,,
Postmodified etching
MCM
41
AAS measurements gave absorption levels of 41.4 mg g-~ of copper on the postmodified MCM 41 material. This is equivalent to a concentration of 1.3 at% and the result compares favourably with the surface concentration of 1.5 at % for the postmodified material, which was determined using XPS. Argon ion etching for a total of 270 s (at a rate o f ~ l nm min-1, equivalent to - 4.5 nm depth) brought about an increase in the carbon concentration for both the parent and the postmodified materials, due to the ion beam redistributing carboncontaining material from the adhesive tab. The oxygen concentration decreased upon etching for both samples. For the postmodified material, the Si:Cu ratio increased, suggesting that the copper was predominantly on the surface and preferentially removed by the ion etching. The copper concentration decreased to one third after etching, suggesting that the CuSchiff base complex was blocking the pores and that a significant proportion (approximately two thirds) was immobilised on the external surface. These results are supported by those from nitrogen sorption, where the change in isotherm from Type IV to Type I indicates narrowing and blocking of pores and loss in both pore volume and accessible internal surface area.
646 4 CONCLUSIONS MCM 41 was successfully functionalised using a Schiff base copper complex. The IR band at 1624 crn-~ for the imine group confirms the immobilisation of the complex on the MCM 41 material. The change in nitrogen isotherm from Type IV for the parent MCM 41 material to Type I for postmodified MCM 41 corresponds to a decrease in the surface area and pore volume from 813 mZg-l to 332 mZg-l and 0.39 cm3gl to 0.02 cm3gl, respectively. AAS shows 1.3 at% of copper loading, which is in close agreement with 1.5 at% obtained from XPS analysis. We have shown XPS to be an important tool in determining the distribution of a Schiff base copper complex within the MCM 41 material. XPS argon etching shows that, for this particular postmodified material, about a third of copper complex goes into the pores of the MCM 41 material and the rest on to the external surface. These results may be explained by the bulky nature of the complex, which gives rise to pore-blocking as well as narrowing of the pores. 5 ACKNOWLEDGEMENTS
We thank Robin Holman (Exeter University) for CHN microanalysis. U. Singh thanks the Open University for financial support, and the University of Exeter for the use of its facilities. REFERENCES
1. C.T. Kresge, H.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 1992, 359, 710. 2. X.S. Zhao, G.Q. Lu and X. Hu, Microporous and Mesoporous Materials, 2000, 41, 37. 3. S.T. Wong, J. Lee, S. Cheng and C. Mou, Applied Catalysis A: General, 2000, 198, 115. 4. J.H. Clark and D.J. Macquarrie, Chem. Soc. Rev., 1996, 303. 5. P.M. Price, J.H. Clark and D.J. Macquarrie, J. Chem. Soc., Dalton Trans. 2000, 101. 6. I.C. Chisem, J. Rafelt, M.T. Shieh, J. Chisem, J.H. Clark, R. Jachhuck, D.J. Macuqarrie, C. Ramshaw and K. Schott, J. Chem. Soc., Chem. Commun., 1998, 1949. 7. Shin-Guang Shyu, Sheau-Wen Cheng and Der-Lii Tzou, J. Chem. Soc., Chem. Commun., 1999, 2337. 8. F. de Juan and E. Ruiz-Hitzky, Advanced Materials, 2000, 12, 430. 9. J.S. Beck, J.C.Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kregese, K.D. Schmidt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. MeCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 1992, 114, 10834. 10. Zhidong Zhu, Zhixiang Chang and L. Kevan, J. Phys. Chem. B, 1999, 103, 2680. 11. E. Armengol, A. Corma, V. Forn6s, H. Garcia and J. Primo, Applied Catalysis A: General, 1999, 181, 305. 12. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, "Handbook of X-Ray
Photoelectron Spectra - A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data", ed. J. Chastain, Perkin-Elmer Corporation, Eden Prairie, Minnesota, United States of America, 1992. 13. G. Oye, J. Sjoblom and M. Stocker, Advances in Colloid and Interface Science, 2001, 80-90, 439. 14. C.M. Bambrough, R.C.T. Slade and R.T. Williams, J. Mater. Chem., 1998, 8, 569.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
647
Influence of synthesis conditions on surface heterogeneity of M41 type materials studied with lattice Monte Carlo Flor R. Siperstein and Keith E. Gubbins Department of Chemical Engineering, North Carolina State University Raleigh, NC 27695-7905, U.S.A. 1. ABSTRACT The synthesis of mesoporous silica materials using surfactants as structure-directing agents is studied using lattice Monte Carlo simulations. The surfactant HmTn is modeled as a sequence of m hydrophilic segments or heads (H) followed by n hydrophobic segments or tails (T). Favorable interactions between the surfactant heads and the silica result in the formation of a surfactant-rich silica-rich phase in equilibrium with a dilute phase. Liquid crystal behavior is observed in the phase containing high-surfactant and high-silica concentration, with different structures depending on the overall system composition that are similar to the M41 family. The formation of a silica hexagonal phase is observed at low surfactant/silica ratios and lamellar or perforated lamellar phases are formed at high surfactant/silica ratios. The structure of the silica materials depends on the surfactant chemistry, the surfactant/silica ratios and the temperature. Heats of adsorption of simple gases on model MCM-41 type materials are calculated using Grand Canonical Monte Carlo simulations. Adsorption properties on model materials that were generated through a mimetic synthesis using lattice Monte Carlo simulations are compared with those on smooth cylindrical pores. Energetic heterogeneity in the materials studied is due to surface roughness and structural defects and not to the presence of areas with different chemical composition. This is in agreement with experimentally measured heats of adsorption of simple gases. At low coverage, heats of adsorption of argon and krypton on MCM-41 decrease with coverage, indicating that MCM-41 is not a homogeneous adsorbent even for non-polar spherical gas molecules, where the presence of polar groups in the adsorbent surface should have little effect. 2. INTRODUCTION Following the introduction of MCM-41 type materials [1], the synthesis of surfactant templated nanostructured materials has attracted the attention of the scientific community because it provides the possibility of tailoring pore size, geometry and surface chemistry through control of the synthesis conditions. Potential applications of these materials range from separations and catalysis [2] to the production ofbiomimetic materials [3] and devices for optical and electronic applications [4]. Several synthesis protocols have been developed in the last ten years and are the focus of many recent reviews [5]. Despite the enormous experimental effort to develop methods to control the structure and composition of templated nanoporous materials, modeling the different processes has remained elusive, mainly due to the overlapping kinetic and thermodynamic effects. The characterization of
648 these materials is also challenging do to the presence of regularly spaced pores with amorphous walls. Therefore, they cannot be considered completely random materials or idealized regular geometries. An attractive alternative to generate model structures of ordered mesoporous materials of the MCM-41 family is to follow some of the most important steps of the synthesis using different molecular simulation techniques. This mimetic approach should provide us with a model material that will have similar properties to real MCM-41 type materials. Detailed modeling of these syntheses is not currently possible because the required system size is larger than what is possible with today's computing capabilities. Simulations containing all atom descriptions of the surfactant, solvent and other components are reported in the literature [6]; the authors indicate that the system may reach an energy plateau but is not necessarily equilibrated. Some coarse-grained simulations are reported in the literature where the formation of organic-inorganic ordered structures is studied [7-9]. Molecular simulations of two-dimensional ionic surfactants in the presence of neutral particles show that in order to have an ordered structure of the neutral particles around the micelles, the dimensionless density needs to be similar to a liquid phase density [7]. Other studies involve lattice Monte Carlo simulation where the ordering of the surfactant liquid crystal phase is driven by evaporation of the solvent [8] similar to what is observed in dip coating techniques [9]. Computational studies of self-assembly of inorganic particles in fluids that involve the ordering of hard particles during spindoal decomposition have also been reported [10]. We have described previously the bulk synthesis of surfactant templated silica materials [11] based on experimental evidence that surfactant-silica liquid crystal phases can be obtained under no silica polymerization conditions where true lyotropic liquid crystal phases (hexagonal or lamellar) are in equilibrium with a solvent rich phase that may contain a small amount of free surfactant and silica species [12]. These mesophases are prepared under highly alkaline conditions from separate isotopic aqueous inorganic and surfactant precursors containing multiply charged anionic silicate oligomers and cationic micelles respectively. Even when the existence of silica-surfactant lyotropic liquid crystal phases has been observed for cationic surfactants (cetyltrimethyl-ammonium bromide, CTAB) under strongly basic conditions, we model neutral surfactants under the assumption that this behavior is not particular to ionic surfactants but is the result of the absence of appreciable inorganic polymerization. On the other hand, synthesis methodologies using nonionic surfactants are especially interesting because the surfactant is only weakly bound to the silica framework and can be removed by solvent extraction without the need of calcination. A wide variety of nonionic surfactants has been used for the synthesis of templated materials [ 13 ], including alkyl-polyethylene-oxide oligomeric surfactants (CnEOm) [ 14], and triblock copolymers of the pluronic family [15]. In general, for a given alkyl chain length, lamellar structures were observed for short ethyleneoxide (EO) chains while the formation of cubic and hexagonal phases were favored with long EO chains, although surfactants with long EO chains (C12EO23 or C16EO20) did not promote the silica precipitation. For a given surfactant, such as C16EO10, cubic structures (SBA-11) were formed from solutions containing low surfactant/silica ratios and lamellar structures were formed at high surfactant/silica ratios. Given the complexity of the synthesis of surfactant templated silica materials and the many variables that influence the structure and properties of the final product, it would be impossible to include all of them in this study. Thus we do not attempt to investigate all the variables involved in the synthesis, but rather select the governing factors that can be used to describe the physics of these syntheses. The detailed simulation of micelle formation
649
alone is a challenging problem for today's computer power. Simulations using detailed atomistic potentials have, for the most part, focused on the evolution of prearranged structures, but are usually unable to span real times that are long enough to observe formation and destruction of micelles [ 16,17]. For the problem we are interested in here, it is necessary to study a large enough system such that the mesoscopic periodicity can be observed while retaining a simplified description of the surfactant. Coarse-grained approaches, either on- or off-lattice, are normally used to study the formation of micelles. Although off-lattice simulations are typically more versatile and realistic, in this work we selected lattice simulations because they allow larger systems to be studied. We have used lattice Monte Carlo simulations to study the equilibrium properties of ordered mesoporous silica materials using linear nonionic diblock surfactants as structure directing agents. The key to predicting structures of templated mesoporous materials is to understand how a dilute solution with spherical micelles becomes a liquid crystal phase when silica is added to the system. We have show that synthesis of templated materials in bulk solution can be interpreted with an equilibrium triangular diagram for surfactant, solvent and silica, where different liquid crystal phases can be located. The equilibrium diagram is calculated using a lattice Monte Carlo approach, specifying the appropriate interaction parameters to represent each component. In this work we concentrate on the characterization of model materials obtained from the mimetic synthesis of templated mesoporous materials. Pore size distributions as well as adsorption isotherms and heats of adsorption are used to characterize the model materials. 3. SIMULATION PROCEDURE The simulation procedure to generate the phase diagrams for surfactant-silica liquid crystal systems in equilibrium with a solvent rich phase is described in detail in a previous publication [11]. We used three dimensional lattice Monte Carlo simulations of different model surfactants of type HINT,,, which contain m hydrophilic segments H, and n hydrophobic segments T. Solvent (S) and inorganic-oxide (/) occupy a single site on the lattice while the surfactant occupies several connected sites. Each molecular unit (H, T, S and/) is characterized by an interaction energy eab (a, b = H, T, S,/). The net energy change associated with any configuration rearrangement depends on a set of interchange energies (0aa:
1
CO~b=Cab --~(eaa + 6bb) with a ~: b z
(1)
and not on the individual interaction energies Sab. The surfactant-solvent interaction parameters are the same as those used by other researchers: c o , r / k B T = cosr / kBT =0.153846 andcoHs = 0. A strong inorganic-head attraction to mimic the strong affinity between silica and surfactant heads was specified as COZH/kBT=-0.307692 and COzr =0.153846. Two extreme cases were studied, one where the inorganic component and the solvent are completely miscible (cois = o) and another where they are immiscible (co/s / kBT =0.153846 ). The dimensionless temperature is defined using the head-tail interchange energy by T* = ksT / co/4TLiquid-liquid equilibrium was calculated using a direct interfacial approach. One dimension of the simulation box was increased with respect to the other two by a factor of 8
650 to favor the formation of two pseudo-fiat interfaces. The box size used in the simulation was 40x40x320 sites. Bulk coexisting densities were estimated from ensemble averages of the system densities away from the interfaces after the system reach equilibrium. Single-phase simulations in relatively small boxes (203-303 sites) were performed after the determination of the surfactant/silica liquid crystal composition to generate model materials for adsorption simulation. The advantage of generating such materials is that they have truly periodic boundary conditions in the three directions. In contrast, extracted model materials obtained directly from the interfacial simulation will have periodic boundary conditions in two directions, but in the third direction (z-direction) there are two liquidcrystal/dilute phase interfaces. After the model materials were generated, the geometrical pore size distribution (PSD) and simple fluid adsorption isotherms and heats of adsorption were calculated to obtain a good description of the model materials. The geometrical pore size distribution was obtained from the first derivative of the void space volume, Vpore(F), coverable by spheres of radius r or smaller. Vpore(r) is a monotonically decreasing function of r and is easily compared with the cumulative pore volume curves. The derivative-dVpore(r)/dr is a direct definition of pore size distribution. Vpore(r) was calculated by Monte Carlo integration [ 18] and the derivative was calculated numerically. To calculate the PSD it is necessary to specify the size of the solid particles in the silica framework. As a first approximation, it is possible to calculate the PSD assuming that the solid particles that constitute the silica framework have a diameter equivalent to the lattice spacing. This assumption would imply that the particles do not overlap and they only touch each other tangentially. It is common practice in modeling silica-gel type materials to assume that the amorphous silica spheres that constitute the material overlap slightly. PSD were calculated for different solid particle sizes. Adsorption isotherms and heats of adsorption were calculated using Grand Canonical Monte Carlo simulations. Details of the simulation are described in a previous publication [ 11]. Surface excess amount adsorbed was calculated by subtracting the bulk density from the pore density at the conditions of the simulation. The pore volume is necessary to calculate the pore density. In this work, the helium pore volume using CYHe= 0.264nm was determined by MC integration, similar to the procedure used for the PSD. Table 1. Interaction parameters for simple gases on siliceous materials
Ar-Ar Ar-O Kr-Kr
e/kb (K) 119.8 93.0 171.0
cr(nm) 3.405 3.335 3.600
The fluid-fluid and solid-fluid interaction parameters used for the simulation were taken from the literature [ 19] and are summarized in Table 1. The solid particles that constitute the silica framework were assume to have the amorphous silica density (2.2 g/cm3). 4. RESULTS AND DISCUSSION 4.1. Solid frameworks
The model materials were obtained under the assumption that silica polymerization is negligible before the ordering of the liquid crystal structure. Such systems have been observed experimentally for silica at very basic conditions or for transition metal oxides at acid conditions.
651 Solid frameworks that have almost cylindrical pores in an hexagonal arrangement can be obtained from different surfactants and at different conditions. In our models, the definition of the cylinders (surface roughness) depends mainly on the simulation temperature (Figure 1). Frameworks obtained from systems that contain immiscible silica/solvent binaries have higher wall density than those where silica/solvent binaries are miscible. Three surfactants were studied at a surfactant/silica ratio of 0.5 and T*=6.5. For a surfactants of equal length but different composition (HaT4 and H2T6), it was observed that the surfactant concentration in the high-surfactant phase increases with increasing number of tail segments. Therefore, for a given overall system composition, surfactants with low head/tail ratio will form lamellar phases and with high head/tail ratio will form hexagonal phases, in agreement with experimental observations. 4.2 Pore Size Distribution PSD curves calculated for a model material obtained from HaT4/silica/water at 0.55/0.35/0.10 volume fractions are shown in Figure 2. The pore sizes and solid particle radius are given in lattice units. Aider removal of the surfactant it is possible to observe the presence of micropores in the regions where surfactant heads were removed, because the surfactant heads interpenetrate into the silica framework. This is not observed in MCM-41 type materials, but is typical of SBA-15 type materials, or other materials that u s e CmEOn as structure directing agents. In order to model materials of the SBA-15 family that contain a combination of micro and mesopores, it is only necessary to scale the average mesopore size to the corresponding size of the real material. The size of the solid particles in the framework can be tuned to obtained the desired micro/mesopore ratio. In general, large particle sizes will have small micropores. It is possible to cancel completely the presence of micropores by selecting a large enough solid particle size. It was observed that particles with a diameter of two lattice spacings cancel completely the presence of micropores. This suggests that there are no agglomerations of surfactant heads that penetrate in the material framework and that will create pores thicker than one lattice spacing. Micropores larger than one lattice site appear only when the solid particle size is smaller than 0.5 lattice sites. This would generate an unconnected solid network and would yield an unrealistic material. A schematic representation of a model material showing different solid particle sizes is shown in Figure 3.
Figure 1. Solid frameworks obtained using H4T4as structure directing agent at (A) T*=6.5 and (B) T*=10.0
Figure 2. Pore size distribution of model material obtained from H4T4 at T*=6.5. Four different solid particle sizes were used.
652
Figure 3. Solid frameworks with different solid particle size. (a) R = 1.05 and (b) R=0.5. The presence of micropores is evident in (b).
4.3 Low coverage adsorption Adsorption isotherms and heats of adsorption of simple gases on model MCM-41 type materials were calculated using Grand Canonical Monte Carlo simulations. The simulations were performed emphazising the region of pressures below the capillary condensation where adsorbent heterogeneity is normally related to a decrease in isosteric heats of adosrption. Energetic heterogeneity in the materials studied is due to surface roughness and structural defects and not to the presence of areas with different chemical composition. This is in agreement with experimentally measured heats of adsorption of simple gases. At low coverage, heats of adsorption of argon and krypton on MCM-41 decrease with coverage, indicating that MCM-41 is not a homogeneous adsorbent even for non-polar spherical gas molecules, where the presence of polar groups in the adsorbent surface should have little effect (Figure 4). The magnitude of the energetic heterogeneity seems to be smaller for Kr than for Ar, which may be due to the following factors: (1) Kr is less sensitive to surface roughness than Ar due to its larger size, or (2) the fluid-fluid interactions for Kr are stronger than for Ar, therefore, the overall decrease in heats of adsorption, which is the sum of the energetic heterogeneity and the fluid-fluid interactions (with opposite signs), is smaller for Kr than for Ar. Figure 5 shows the heat of adsorption of Ar on a smooth cylindrical pore of 3.2 nm diameter at 77K. It is clear that at low coverage, the heats of adsorption increase with loading up to the formation of a monolayer. This behavior is contrary to what is observed experimentally on adsorption of simple fluids on MCM-41 materials [20], but is typical for adsorption on smooth surfaces without any energetic heterogeneity [21]. The increase in heats of adsorption in energetic homogeneous porous materials below the monolayer formation is due to the adsorbate-adsorbate interactions. 14 E
"6
Ar
0
0.05 Adsorbed
[]
0.1 phase
0.15 p*
Figure 4. Isosteric heats of adsorptionof argon (squares) and krypton (circles) on model materials.
653
-- 18 16 ,~ 14 .,..., o.. ~ 12
o
6
--
2
0
0.2
04
06
08
1
Adsorbed phase density, p*
Figure 5 Isosteric heats of adsorption of argon on smooth cylindricalpores with diameter of 3.2 run.
5. CONCLUSIONS Model materials similar to the M41 family can be modeled using lattice MC simulation under the assumption of non silica polymerization conditions. The structures observed are in agreement with experimental evidence with respect to the surfactant/silica ratio, temperature and surfactant architecture. Even when the resulting structures have a strong influence of the lattice constraints, adsorption properties on such materials are in good agreement with experimental observations. Adsorption properties of modeled materials are similar to experimental observations on MCM-41 type materials and show heterogeneity at low pressures. Such behavior is not observed in smooth cylinders. The structures obtained with the surfactants modeled have a significant amount of micropores as observed from the PSD. The micro/mesopore distribution can be tuned by changing the solid sphere size. 6. ACNOWLEDGMENTS We thank the Department of Energy for support of this research under grant no. DEFG02-98ER14847 and the National Partnership for Advanced Computational Infrastructure (grant npa205) for support of this research. REFERENCES
[1] J.S Beck,. J.C. Vartuli, W.J Roth,. M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 6469; T. Asefa, M.J. MacLachan, N. Coombs and G.A. Ozin, Nature 402 (1999) 867. [3] S.A. Davis, S.L. Burkett, N.H. Mendelson and Mann, Nature 385 (1997) 420. [4] B.J. Scott, G. Wirnsberger and G.D. Stucky, Chem. Mater. 13 (2001) 3140. [5] A. Sayari and S. Hamoudi, Chem. Mat. 13 (2001) 3151; F. SchtRh, Chem. Mat. 13 (2001) 3184; U. Ciesla and F. Sch~th, Microporous and Mesoporous Mat. 27 (1999) 131. [6] S. Mohanty, H.T. Davis and A.V. McCormick, Langmuir 17 (2001) 7160.
654 [7] A. Bhattacharya and S.D. Mahanti, J. Phys. Condens. Matter 13 (2001) 1413. [8] Rankin, S.E., Malanoski, A.P. and Van Swol, F., Materials Research Society Symposium Proceedings 636 (Nonlithographic and Lithographic Methods of Nanofabrication: From Ultralarge-Scale Integration to Photonics to Molecular Electronics) D 1.2/1-D 1.2/6, Boston, 2001. [9] Y.F. Lu, R. Ganguli, C.A. Drewien, M.T. Anderson, C.J. Brinker, W.L. Gong, Y.X. Guo, H. Soyez, B. Dunn, M.H. Huang and J.I. Zink, Nature 389 (1997) 364. [10] A.C. Balazs, V.V. Ginzburg, F. Qiu, G.W. Peng and D.J. Jasnow, Phys. Chem. B 104 (2000) 3411.; R.B. Thompson, V.V. Ginzburg, M.W. Matsen and A.C. Balazs, Science 292 (2001) 2469. [11] F.R. Siperstein and K.E. Gubbins Mol. Sim. 27 (2001) 339. [12] A. Firouzi, F. Atef, A.G. Oertli, G.D. Stucky and B.F. Chmelka J. Am. Chem. Soc. 119, (1997) 3596. [13] P.T. Tanev and T.J. Pinnavaia, Science 267 (1995) 865. [14]D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [15] D. Zhao, J.L. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science 279 (1998) 548. [16] J.C. Shelley and M.Y. Shelley, Curr. Opin. Colloid Interface Sci. 5 (2000) 101. [17] R. Rajagopalan, Curr. Opin. Colloid Interface Sci. 6, (2001) 357. [ 18] L.D. Gelb and K.E. Gubbins, Langmuir 15 (1999) 305. [19] Talu, O. and Myers, A.L., Fundamentals of Adsorption 6, editor F. Meunier, Elsevier 1998. pp 861. [20] A.V. Neimark, P.I. Ravikovitch, M. GrOn, F. Schtith and K.K. Unger J. Colloid Interface Sci. 207 (1998) 159; J.P. Olivier Proceedings of the Second Pacific Basin Conference on Adsorption Science and Technology, editor D.D. Do, World Scientific, Singapore, 2000, pp. 472. [21] D. Nicholson and N.G. Parsonage, Computer Simulation and the Statistical Mechanics of Adsorption, Academic Press, London 1982, p. 255.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
655
Porosity of chemically modified silica gels by nitrogen adsorption, positron annihilation and small angle X-ray scattering J. Goworek a, A. Bor6wka a, S. Pikus b and J. Wawryszczuk c a Department of Adsorption, Faculty of Chemistry, b Department of Crystallography, Faculty of Chemistry, c Department of Nuclear Physics, Institute of Physics, Maria Curie-Sktodowska University, Maria Curie-Sk|odowska Sq. 3, 20-031 Lublin, Poland Tel. +4881 5375563, fax. +4881 533348, e-mail: j [email protected]
The progress in the determination of porosity of various types of materials has arisen over the past ten years from advances in application of new spectroscopy techniques. In the present paper the application of small angle X-ray scattering (SAXS), positronium annihilation lifetime spectroscopy (PALS) and low temperature nitrogen adsorption methods to the characterization of mesoporosity is reviewed using different types of silica gels with chemically modified surface. The results from the three methods are compared and discussed. 1. INTRODUCTION The most frequently used classical methods for textural characterization of solids are based on adsorption isotherms, mercury intrusion, freezing point depression or size exclusion in chromatography. To interpret the experimental data and obtain PSD, one must adopt a model for the pore structure and the theory that estimates the process in pores of a particular size. In the case of adsorption-desorption under isothermal or isobaric conditions, an abrupt phase transition of adsorbate filling pores appears at a characteristic pressure or temperature commonly estimated by the Kelvin equation describing capillary condensation. The liquid nitrogen adsorption method measures the available pore volume for the adsorbate through the adsorption isotherm. Hence there is a great interest to compare the pore dimensions derived from adsorption data with those estimated independently when pore filling is not necessary. Recently, a considerable attention has been given to the spectroscopic methods. The progress in understanding of the adsorption mechanism and textural properties has arisen from advances in application of small angle scattering SAXS/SANS (see e.g. 1, 2). In recent years the small angle scattering method (SAXS) has been used more and more frequently in investigations of the profile of electron density changes on the phase boundary. The deposition of a surface film undoubtedly changes the profile of electron density on the phase boundary, which consequently causes changes in the small angle scattering values. SAS methods are useful for the study of gas-solid and liquid-solid interfaces and make possible measurements in situ. Thus they may be applied for investigations of pore structure evaluation during processing. Recent improvements in theory and instrumentation for positron annihilation lifetime spectroscopy (PALS) have greatly enhanced the value of this method
656 [3-5]. Good consistency of the parameters derived from various experimental data is observed for rigid materials with regular pore geometry and sharp boundary between solid surface and pore space. The contrast between empty pores and silica is large both for X-rays and positrons. However, in the case of chemically modified silica this interface boundary is characterized by the presence of transition layer for which the structure and density is not satisfactory established. Thus, pore dimensions determined by using different techniques exhibit some discrepancy. In the present paper the pore textures of several porous silicas have been analyzed on the basis of liquid nitrogen data and compared with the results obtained by SAXS and PALS. Silica gels with chemically bonded hydrocarbon phases C-2, C-8 and C-18 were used in experiment. 2. E X P E R I M E N T A L 2.1. Materials
In this paper, the silica gels selected for investigation were commercially available LiChrosorbs RP-2, RP-8 and RP-18 (Merck, Germany). The numbers in the names of these materials specify the number of carbon atoms in the chains of bounded hydrocarbon phase. Thus, they represent a graded series of materials made by a similar process of involving the silanizing agent, where the pore size changes gradually. The second series of silica materials was obtained by burn-off of the hydrocarbon phase at 400~ in oxygen stream. According to the manufacturer LiChrosorb RP-18 and RP-8 were prepared using standard silica gel Si-100, but LiChrosorb RP-2 with Si-60 silica gel. After removal of the bonded phase we did not obtain initial silica gel due to the presence on the surface of additional silica atoms introduced during silanization. The amount of introduced silicon atoms was different for various samples due to different surface coverage by alkyl radicals. This coverage diminished when the length of bounded molecule increased. Contamination, expressed as a surface coverage, of organic phase calculated from the weight loss during temperature treatment was determined to be 0 = 4.8, 3.4 and 3.0 p,mol/m 2 for RP-2, RP-8 and RP-18, respectively. The lowest surface coverage for C18 phase means a relatively small deformation of silica surface by additional Si atoms from silane reagent. For C18 phase, even after endcapping with smaller silane molecules, nearly half of the original silanol groups on silica surface remained unreacted. 2.2. Methods
Adsorption/desorption isotherms of nitrogen at 77 K were measured with an automated apparatus ASAP 2010 (Micromeritics, USA). The specific surface areas, SBer, were calculated from the linear form of the BET equation, taking the cross-sectional area of the nitrogen molecule to be 16.2"10 -20 m 2. Pore size distributions were calculated in the standard manner by using BJH method [6]. The total pore volumes, lip, for the samples under study were determined from a single point adsorption at a relative pressure of 0.98 by converting the value of the adsorbed gas to the volume of the liquid adsorbate. SAXS measurements were performed on a slit-collimated Kratky camera, using CuI~ radiation. The background scattering curve was each time subtracted from the scattering curve for the investigated sample. The absorption coefficient was also measured for each sample. Subsequently, the SAXS scattering curves were recalculated considering the differences in the absorption coefficient. Consequently, the curve (the intensity scattering [I(q)] vs. module of 4z sin 0 scattering vector (q)) for each sample was obtained; q = ~ , 20 - is scattering angle, 2 ~,- is the X-ray wavelength.
657
Positron lifetime spectra were measured using a conventional delayed coincidence gamma-ray spectrometer [3]. The positron source, 180 kBq of 22Na, sealed in Kapton envelope was placed between two layers of sample powder inside the container which was evacuated to the pressure = 0.5 Pa in order to eliminate the possibility of ortho-para conversion to atmospheric paramagnetic oxygen. The sample chamber was viewed by two scintillation counters. The delayed coincidence spectrometer was a classic fast-slow type. The distribution of time spacings between the positron birth and annihilation was registered in 8000 channels of time scale; the width of one channel corresponded to 116 ps. The start signal for time measurement was produced by 1274 keV gamma ray emitted almost simultaneously with beta decay of 22Na nucleus, the stop signal by one of annihilation quanta. 3. RESULTS AND DISCUSSION
The nitrogen adsorption-desorption isotherms at 77 K for the investigated silicas are shown in Fig.1. All nitrogen isotherms exhibit hysteresis. The parameters characterizing the pore structure of these materials derived from nitrogen adsorption data are summarized in Table 1. Comparison of the results for grafted and pure silica of the same type indicates some qualitative parallelism between the total pore volume and average pore diameter. For silica gels, from which the organic layer was removed, the values of these parameters are higher. Pore volumes increased significantly after evacuation of the hydrocarbon phase only in the case of RP-18 silica. It should be noted that after burning off the total pore volume, Vp, for this sample became similar to volume Vp for non-modified silica gel Si-100, i.e. initial silica before treatment with silanizing reagent. However, after modification the pore radii markedly decreased and the specific areas simultaneously increased. A similar effect was observed for RP-2 silica and Si-60 silica gel which was the initial material for this adsorbent. The specific surfaces for RP silicas may be assumed as the surfaces of cores of pores with hydrocarbon phase present on their walls.
800
800 RP-2
700
800
RP-8
700
_- RP-2 W
_.
600
600
600
a. 500 I--
500
500
~ 400 E
400
400
>" 300
300
300
200
200
200
100
100
100
0 0
0.2
0.4
0.6
P/Po
0.8
1
0 0
0.2
RP-18
700
_" RP-18 W
_
0.4
0.6
P/Po
0.8
1
i
i
i
J
i
0
0.2
0.4
0.6
0.8
P/Po
Fig.1. Adsorption/desorption isotherms of nitrogen at 77 K for RP-2, RP-8 and RP-18 before and after burning off.
658 Table 1 Parameters calculated from liquid nitrogen data characterizing the investigated silica gels
Sample
W3pa
R peak BJH
S b
cm/g
nm
m2/g
Si-60
0.72
3.25
443
RP-2
0.60
2.11
569
RP-2 W
0.68
2.21
615
SI-100
1.15
6.40
334
RP-8
1.09
5.61
389
RP-8 W
1.19
5.71
416
RP-18
0.79
4.21
375
RP-18 W
1.13
5.63
401
Pore volumes per 1 g of pure silica gel; b Specific surface areas derived from the relation: S
a
=
2VpfRpeakBJH
It is worth noting that the specific surface area for silica gels with decomposed hydrocarbon layer is much higher than the surface of initial silica samples. It suggests an irreversible reorganization of the silica surface by silicon atoms during chemical modification. Contrary to substantial increase of silica surface and pore volume in the case of silica gels with evacuated hydrocarbon phase the differences of average pore radii are very small. Some correlation of the decrease of pore radii with hydrocarbon chain length occurs only for C-18 phase (AepeakBJH ~ 1.4 nm). The structure of the hydrocarbon layer depends on the temperature and presence of adsorbate molecules. Thus, the thickness of the surface layer, 6, at the temperature of liquid nitrogen in comparison to room temperature is different. In respect to the latter case estimation of 6 can be done by small angle scattering of X-rays. The scattering intensity is proportional to the square of difference of electron density between the scattering heterogeneities and their surrounding. In porous materials the pores may be assumed as heterogeneities for which electron density differs from that of materials constituting the porous skeleton. There are many porous materials for which power law, I(q)=Io'q -~ (~, Io are constants), is fulfilled in a certain q region. The value a = 4 (3 for slit smeared data) corresponds to three-dimensional, non-fractal heterogeneities with a sharp interface border (this relation is known as the Porod law), whereas 4 < a < 6 (3 < a < 5 for slit smeared data) corresponds to three-dimensional objects with a transition layer on the interface border. Traditionally, the power law scattering can be shown as a Porod's plot: I(q).q 4 vs. q for a point collimated system or I(q).q 3 vs. q for slit collimated one. The presence of a coating film on the support surface causes depletion of the SAXS scattering resulting in a negative slope for Porod's plot (negative deviation). For chemically modified porous materials (especially with organic films), due to the thermal density fluctuations, additional scattering occurs at the end of the SAXS curves (positive deviation). Our analysis of the SAXS data for porous materials coated by organic films is based on Ruland's and Vonk's considerations [7-9], where the surface film can be assumed as the transition layer for which electron density decreases continuously from electron density of
659
solid skeleton to electron density of the medium in pores (in our case - air). Thus, after necessary mathematical reductions the following equation for slit-collimated data was obtained [ 10]:
1
6 2)
J ( q ) . q = c q :Z
-6
(1)
'
where: c - is a constant; (5- thickness of transition layer.
1
It means that the plot J ( q ) . q vs. --5- should be a straight line; from this relation (5 values may q be calculated. Recently, Benedetti and Ciccariello [10] proposed a procedure which allows us to determine not only the thickness but also the electron density of the coated film. They assumed that, within the range of q attainable in SAXS measurements, coated material can be considered as isotropic three-phase system in which each phase has a uniform electron density. The three phases correspond to the skeleton (bulk) of porous material (n 1), the voids (pores) characterized by the electron density of the air (n2) (equal = 0), and the phase formed by the molecules bonded on the solid surface (n3). They also assumed that the thickness of a coating layer is independent of the support loading. This means that the procedure can yield realistic results only in investigations of materials with significant coating close to 100% because, as it may be expected, the number of chemically bonded molecules determines their arrangement on the adsorbent surface. Under the physical conditions specified above, the leading asymptotic term of the standard normalized scattered intensity, It,,t(q), is given by [11]: 2~ I,,, (q) = - - T I P q
where: P = [(n, - ) 2n(3n + A=--t(n ,
n3
(2)
A cos(q6)],
2-
+(n 2
n3)2 ],S , n3
-(n,
n
].S
,
while S approximates the area of the silica interface. Table 2 Thickness of the transition layers for the investigated samples Sample
(5, n m
a
6, n m b
6, n m c
RP-2
0.61 • 0.1
0.93 • 0.1
0.55 • 0.1
RP-8
1.40 • 0.1
1.07 • 0.1
1.00 + 0.1
RP- 18
1.87 + 0.1
1.80 + 0.1
1.65 + 0.1
a 6 values for SAXS data presented in this paper obtained by Ruland-Vonk method; b (~ values for SAXS data taken from Ref. 12 calculated by Ruland-Vonk method; c (5 values for SAXS data taken from Ref. 12 calculated by Benedetti-Ciccariello method
660
---a---- R P - 1 8 - o - - RP-8
.~ C
---r
::3
RP-2 Si-200
.m L
O" v B
q, A "1
Fig.2. Porod plot for RP-2, RP-8, RP-18 and pure silica gel Si-200. From Eq. (2) the thickness of the transition layer, electron density and area per molecule of coating material can be estimated using the best-fit procedure. Fig. 2 shows typical dependences of I(q).q 3 vs. q for the investigated silica gels and, for comparison, the scattering curve for pure silica sample Si-200. From the short linear regions in the curves it is possible to define the exponents of the power low scattering. These exponents were c~ = 3.6, 3.8 and 4.5 for RP-2, RP-8 and RP-18, respectively. The c~ =3 was obtained for two-phase system of silica gel Si-200. The thickness of the transition layers for the same adsorbents is given in Table 2 and compared with the values derived from experimental data of Schmidt for the same type of material. It should be noted that for modified silica samples 6 values decrease as the length of the hydrocarbon chain decreases. There exists good consistency of these values derived from various experiments. In the case of RP-2 silica the 6 value seems to be overestimated, if we take into account that silanizing reagent is dimethyl organic phase. Pore dimensions can be determined also by positron annihilation lifetime spectroscopy (PALS). Positron in a solid can create a bound structure with an electron, called positronium (Ps). Its triplet state (ortho-Ps) has an intrinsic lifetime in vacuum 142 ns, but when trapped in a free volume, like a pore, it lives shorter. The o-Ps lifetime is r = (~
+ XT)-~
(3)
where ~ is the o-Ps decay rate inside the bulk, usually assumed 2 ns l , XTis the intrinsic decay rate, P - the probability to find o-Ps outside the empty volume. For cylindrical geometry oo
P=
g(r
rdr
(4)
R
where R is the cylinder radius; the Ps wavefunction ~b(r) outside the pore is exp(-rr)/r; 1 / t c = [ 4 m ( U - E ) / h 2 ] -1/2 is the mean penetration range of o-Ps into the surrounding bulk ( U - the depth of the potential well represented by the pore, E - the o-Ps energy in the well, h is Planck's constant). For a normal bulk material Uis (1 - 3) eV, while E is of the order of
661
thermal energy kT. The lifetime depends on the pore radius, or generally free space in solid, and respective r vs. R dependence can be found in the literature [4, 5]. That relation is well confirmed by the experiments with silica gels, glasses, e.g. the lifetime of o-Ps in Si-100, which is the basis for RP-18 and RP-8 production fits well to the model expectations. Porous media with bonded organic phase represent an interesting case from the viewpoint of PALS method. The average radii determined by nitrogen method are given in Table 1. Average o-Ps lifetimes should be then 108 ns, 114 ns and 99 ns for RP-18, RP-8 and RP-2, respectively. The experimental values are however much longer: 127 ns, 125 ns, 108 ns. The general tendency of larger radius - longer lifetime is preserved (in the limits of error), however, the lifetime values are shifted upwards; e.g. the lifetime 125 ns corresponds to the radius 13.5 nm (in Gidley's version [5] of the annihilation m o d e l - 11.5 nm)? Removal of the brush (by burning out the alkane layer) in RP-18 increases the radius of empty volume to 6.5 nm, while the lifetime decreases to 119 ns, i.e. changes in the direction opposite to the expected. In the case of RP-8 no difference in lifetime was observed before and after burning off, in RP-2 a small increase of the lifetime, by 1 ns, was seen. This effect (not seen up to now) can be explained as follows: The brush represents a much lower (5 times) electron density than the silica body; nevertheless the potential step vacuum-brush is sufficiently high (more than kT) to be a trap for positronium, and o-Ps is confined in the empty part. The radius of this part is smaller, but Ps wavefunction penetrates a thick brush layer only, and N, depending on electron density (see Eq.(3)), is much smaller than in pure silica, so the lifetime increases. If the potential step at vacuum-brush boundary is small the penetration range 1/K increases, but the alkane layer is sufficiently thick to prevent Ps reaching silica. Removal of the hydrocarbon phase increases the radius, but increases N,, too, giving as a result lifetime shortening.
140
120 m
'- 100
f
/
80 60
. 2
. 4
.
.
. 6
.
.
. 8
,
10
i
12
1
14
R, nm
Fig.3. The theoretical dependence of positronium lifetime on pore radius for: #- RP-2; 9 -RP 8; 9 - RP-18. Full points - LiChrosorbs RP, open points - LiChrosorbs RP after burning off. Triangle - lifetime and pore radius for amorphous silica gel Si-100. The size of symbols is longer than experimental error.
662 4. CONCLUSIONS The presented results indicate a complexity of mesopore assessment in the case of chemically modified silica materials. The structural parameters calculated from liquid nitrogen adsorption data demonstrate that significant changes in silica gel properties occur during chemical treatment with a silanizing reagent. Changes of the silica structure are evident for a long chain modifier (C18). In the case of C2 and C8 phase the differences of the geometric parameters for modified and non-modified sample are very small. The AR value correlates with the surface film thickness obtained from SAXS experiment only for C18 phase. Positronium annihilation methods (in particular PALS) have several advantages: they allow us to study closed pores, have no limit for small radii, and are nondestructive. PALS measurements can be performed at an arbitrary temperature. Hence, this method is successfully applied for determining the pore structure of silica gels, porous glasses, ultra thin porous films and porous polymers. In this paper PALS was used for the first time to chemically modified silica with a transition layer on its surface. The PALS method works well in one-component media; however, in the case of the presence of the transition layer, differing in electron density from the bulk, the theoretical model needs a general modification. Thus, recognizing the drawbacks of the classical methods it is important to note that neither the new approaches satisfy all answers. REFERENCES
[ 1] J.D.F. Ramsay, Studies in Surfaces Sciences and Catalysis, Characterization of Porous Solids (T. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger, Eds.), Elsevier, vol. 87, 1994, p. 235. [2] J.S. Rigden, J.C. Dore and A.N. North, Studies in Surfaces Sciences and Catalysis, Characterization of Porous Solids (T. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger, Eds.), Elsevier, vol. 87, 1994, p. 263. [3] ,,Positron and Positronium Chemistry" Eds. D.M. Schrader and Y.C. Jean, North Holland, Amsterdam, 1986. [4] T. Goworek, K. Ciesielski, B. Jasifiska, J. Wawryszczuk, Chem. Phys. 230 (1998) 305 [5] D.W. Gidley, W.E. Frieze, T.L. Dull, A.F. Yee, E.T. Ryan, H.M. Ho, Phys. Rev. B60 (1999) R5157 [6] E.P. Barrett, L.G. Joyner, P.H. Halenda, J. Am. Chem. Soc. 73 (1951) 373. [7] C.G. Vonk, J. Appl. Cryst., 6 (1973) 81. [8] W. Rulad, J. Appl. Cryst., 4 (1971) 70. [9] S. Pikus, A. L. Dawidowicz, E. Kobylas, D. Wianowska, Apel. Surf. Sci., 137 (2000) 189. [10] A. Benedetti, S. Ciccariello, J. Appl. Cryst., 27 (1994) 27. [ 11 ] G. Porod, "Small Angle X-ray Scattering", Ed.: O. Kratky, O. Glatter, London, Academic Press, 1982. [12] P. W. Schmidt, D. Avnir, D. Levy, A. Hrhr, M. Steiner, A. Rrll, J. Chem. Phys., 94 (1991) 1471.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
663
Preparation and characterization of porous sorbents for hot gas desuffurization Torn,~ Alonso a, F.,* Orenes Femfindez, I. a, Palacios Latasa, J.M. b a Dpto.
Ingenieria Quimica. Universidad de Murcia. Spain.
b Instituto de Catfilisis y Petroleoquimica. CSIC. Madrid. Spain The preparation and performance of some oxides and mixed oxides supported sorbents by different ways for high temperature desulfurization of fuel gases were studied. The results obtained showed that the preparation method has a strong influence on the spatial distribution of the chemical species inside the pellet. The reactivity attained in a fixed bed reactor was high, but simultaneously the efficiency was low, due to formation of inactive phases obtained by masking of ions in the framework of supports. The study also showed that the addition of some binders (NaBO2) in the preparation stage of zinc titanites allows attaining a three-folded strength than the pure sorbent.
I. Introduction Traditionally, the use of fossil fuels and coal in big power generation plants has been recognised as a major source of pollutants to the environment [ 1]. This scenario has changed over the last decade, with the introduction of new, more efficient, cleaner commercial technologies, such as the Integrated Gasification Combined Cycle (IGCC). In these units, H2S, present in the exit gas leaving the gasifier (about 500 to 800~ has to be removed down to a few ppmv. The desulfufization stage takes place by sorption with solid sorbents at temperatures as high as possible (as those existing in the gasifier), in order to recover the maximum energy carried out by the gas. Regenerative mixed metal-oxides are the most common sorbents used: zinc ferrite, [2,3] and zinc titanite [4]. However, these sorbentes certainly exhibit high efficiency and easy regeneration, their use presents well-known drawbacks. In the case of zinc ferrite, the operation temperature is limited to 650~ because of the reduction of the Fe203 phase; furthermore, some metallic Zn may volatilize under reducing atmosphere [5]. The ZnO phase may be stabilized by addition of TiO2, resulting in formation of stable titanite species, mainly a spinel-structure Zn2TiO4 [5,6]. By the formation of this last compound the applicability of zinc titanium mixed oxides could be extended up to 670~ However, as TiO2 does not react with H2S, this sorbent has considerably lower sorbing efficiency due to dilution [6]. It is remarkable here that porosity of the sorbent in the sulphided state changes, because inert TiO2 assures some porosity at the interparticle voids. As sulfidation process is diffusion-restricted, reactivity greatly varies using zinc titanites as sorbents. Kinetic experiments in the thermo-balance carded out with ZnO as sorbent showed that the overall reaction rate at 600~ depended on the particle size in the range 50-350 pm [7] and on the pore structure of the sorbent [8]. In addition, a study of sorbent performance [9] * To whom correspondenceshould be addressed
664 in a fixed-bed reactor through the break-through curves showed an important size effect for particles of diameter higher than 100 ~tm due to generation of inaccessible pores to H2S as sulfufization progresses. Identical conclusions have been derived from the study of zinctitanium oxide sorbents [5,10,11] The performance of zinc titanium oxide sorbents has been investigated in multi-cycle tests in thermobalance, [12] fixed bed bench-scale reactor, [13] and fluidized-bed reactor [4], and the results showed that the long-term performance was dependent on the sorbent capacity to withstand many cycles with minor deterioration of reactivity and mechanical strength. In fact, ageing of the active phase due to the accumulation of sorption- regeneration cycles causes, not only a progressive decay in sulfurization reactivity and diminishing efficiency, but also a large mechanical degradation of the particle itself. Apparently, sulphate formation and thermal sintering play a major role in this process [ 14]. The performance of the sorber/t could be substantially improved if the active phase in the extrudate is formed in a proper degree of dispersion or a suitable macropores network. Provided that macropores network could be generated, during sorbent calcination at high temperature and many sulfidation-regeneration cycles, by using convenient pore-forming additives [ 15]. The aim of this paper is to show the influence of the preparation method, the kind of support, and the presence of binders, on the performance of sorbents in sulfidation reactions. For this purpose, a number of supports are considered, selecting those that give the best porosity as well mechanical properties. Some of the most common supports-alumina, bentonite- would not be adequate for this application. Therefore, after a previous study for selecting the possible supports for the catalysts, it can be concluded that they must be inorganic, in order to resist the high temperatures necessary to obtain the active phase without breakdown. Also, the supports must be inert to the sulfurization and regeneration reactions. Thus, three basic types were chosen in this work [16,17]:a) natural silicates (sepiolite, bentonite) with mesoporosity and low Si/A1 ratio; b) zeolites (USY, BETA, ZSM-5) with microporosity and high Si/Al ratio, and c) inorganic oxides (ZrO2, Ca3(P04)2), with macro and low porosity. On the other hand, the role of both, binders for stabilizing structure and the graphite for increasing porosity was analysed. A literature search [18] showed that the most common binders for these reactions were the bentonite and natural silicates (2-4% w/w) with 1,5-3 Kg/mm in order to provide some cohesion and increase the mechanical strength to the zinc titanite and zinc ferrite particles. In the second step, several supported sorbents (10% of active phase) have been prepared and characterised.
2. Experimental section 2.1. Supports and binders selection. The selection of the more suitable nature for a porous support was based in the criteria of thermal and thermo dynamical stability (AG>>0, chemically inert support) in the sulfurization step. The melting temperatures of 1000~ or higher, are required because zinc ferrite phase is prepared about 950~ One representative material from each of the aforementioned categories has been selected for the preparation of every supported sorbent: Commercial sepiolite (Tolsa, S.A., Spain), USY zeolite (Zeolyst Inc., U.K.), and ZrO2, m.p. 2500~
665 (Merck, A.R.). On the other hand, a wide range of possible binders has been selected in order to research the action on mechanical strength of zinc titanite: Silica gel, AI(PO4), bentonite, kaolin, borax, and Inorganic Oxides (Na~CO3 and NaBO2). 2.2. Preparation of sorbents. No supported sorbents were prepared by mixing the pure oxides and the selected binders (up to 10 or 20% depending on sample) with water, extrusion with a syringe, and cut to the adequate size (2x3 mm). Then they were dried at 110~ 2 hours and calcined 6 hours at 750, 950 and 1100~ Graphite was used as a way to increase porosity. Supported sorbents were prepared by two methods, in order to modify the impregnation profile of the active species. In every case, when preparing the pelleted supports, water is mixed with the powdered sorbent, resulting in a wet paste, that is extruded with a syringe and dried for 10 min at 120~ Then it is cut into 2x3 mm size extrudates. ZrO2 and zeolite required the addition of 20% of bentonite as a binder. Dried extrudates were calcined according to the preparation method selected: Method 1: Classical incipient wetness method [19] of the calcined support with stoichiometric amount of ZnO and Fe203 (1:1) ratio at 10% w/w of the support. Method 2: Graphite co-impregnation, by mixing all powders together (including graphite) and then calcinating. Thus, the pore volume of the support before the impregnation step is maximum. In brief, in the first method, at the expense of reducing the pore volume available to the ferl'ite, all supports should have the maximum resistance due to its previous calcination at 950~ In the second one, the support keeps a higher pore volume. Additional porosity is attained by using graphite, at the risk of having worse contacts between Zn and the Fe oxides. A smamamy of the prepared materials is shown in Table 1. Table 1. Summary of the ZF(1:1) prepared supported sorbents Non impregnated Support
S o r b e n t Composition/ calcination conditions
Ferrite (as oxides) % added
SEP-5%G
SEP-M1
Sepiolite, 5% graphite, 950~
10
USY-20%B
USY-M1
80%USY, 950~
ZR-20%B-5%G
ZR-M1
80% ZrO2, 20% bentonite 5% graphite, 950~ Sepiolite, 5% graphite, 1000~
SEP-M2
20%
Bentonite, 10
USY-M2 80% USY, 20% bentonite 5% graphite, 1000~ 80% ZrO2, 20% bentonite ZR-M2 5% graphite, 1000~
Method
Incipient Wetness of support
10 10 20 l0
Graphite Co-lmpreg
2.3. Techniques.
The mechanical strength of extrudates was measured with an standardized apparatus according to ASTM D-4179 and the crush strength was obtained as the average of 20 measurements. A scanning electron microscope (SEM) ISI DS-130 coupled to a Kevex Si/Li detector and a Sun SparcStation 5 for energy dispersive X-ray (EDX) analysis was used to
666 study both the chemical composition and the distribution of the elements in the sorbents. Xray diffraction (XRD) patterns were recorded by a Seifert 3000 diffi'actometer using Nifiltered CuKtx radiation in order to characterize the structure of powder samples. The stability of supports was obtained in a thermoanalyzer TGA-50H, TA Instrtmaents, with a 25~ 1500~ temperature range, a 0-99~ heating ramp, and a 20 mg top weight. The surface area was calculated by means of the simple impregnation method, and the pore volume by titration with water. The performance of sorbents in sulfidation tests was verified in a quartz upflow fixed-bed reactor with mass flow controllers and downstream traps for collecting vapours and elemental sulphur. Analysis of the outlet and inlet gases from reactor was carried out by gas chromatography (GC) using a thermal conductivity detector (TCD) and a flame photometric detector (FPD) with a sensitivity higher than 10 ppmv for sulphur compounds.
3. Results and Discussion
First of all, the mechanical strength of selected binders was compared to the one obtained for pure zinc titanite. Considering that 20% binder was used, the obtained results for classical binders, as bentonite, were very low (0.9-1.6 kg/mm) compared to the found in the literature (2-4%). This conclusion focussed our research to the inorganic oxides having high melting point (Na2CO3, NaBO2, kaolin, cryolite), and some three-dimensional gel structure oxides (AIPO4 x H20, silica gel, borax), whose surface area is expected to be very high. Table 2 shows that pure zinc titanite presents a mechanical strength of 3.1 Kg/nma, whereas by adding 20% NaBO2 it attains 10.6 Kg/mm. The addition of 5% graphite allows reaching only 6 Kg/mm. A progressive decay of mechanical strength is produced by addition of graphite to zinc titanite. This decay is abrupter in the initial stages of the porogenic reaction (low graphite percentages) because macropores break the structure. Afterwards, increasing amounts of graphite affect scarcely to strength, because CO2 can be easily removed across the macropores network. Table 2. Effect of binders on mechanical strength of zinc titanite, ZT (0.8:1) MATERIAL CALCINATION % w/w BINDER TEMPERATURE, ~ Pure zinc titanite 950 0 Zinc titanite, 5% graphite 950 0 Bentonite 1100 5-20 AIPO4, Kaolin, Cryolite Silica 1100 20 Gel Borax pentahydrate 1100 20 Sodium Carbonate 1100 20 NaBO2 950 20 NaBO2 (5% graphite) 1100 20 NaBO2 (10% graphite) 1100 20 NaBO2 (20% graphite) 1100 20
STRENGTH, Kg/mm 3.1 2.2 0.9-1.6 <1 7.73 1.9 10.57 6 4.34 2.69
Figure 1 shows that, adding NaBO2, an extraordinarily porous material (20% graphite) having the same mechanical strength of the pure titanite, can be obtained. Considering the good performance of this binder, thermogravimetric and pore volume studies were also made.
667
!
~0
.........
2
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . .
. . . . . . . . . .
:
. . . . .
0
i~,
.
,
r
0
,
4
U ,
,
~
,
,
,
8
i
,
,,-,,
12
Graphite added,
~
,
.
16
,,
i
i
. . . . . . . . . . . .
.
,
.
.
,
.
.
.
.
.
,
20
i 24
%
Fig.1. Effect of graphite addition on the mechanical strength of ZT (0.8:1) with NaBO2 (20%) as binder Table 3 shows that the addition o f graphite increases the pore volume of zinc ferrite twice without binder (0.10) than with binder (0.05). On the other hand, metaborate introduces a certain porosity, which allows reaching larger values than pure ferrite, in both fresh and calcined materials. Table 3. Pore Volumes o f zinc ferrites with graphite and binder. Pore Volumes, cm3/g Sorbent ZF(1:1 ) ZF(1:1)-3%G ZF(1:1)-3%G-6%MB ZF(1 :l)-3%G-12%MB Fresh Bentonite G= Graphite; MB= Metaborate
Fresh
Calcined at 950~
0.32 0.33 0.45 0.43 0.43
0.34 0.43 0.50 0.49 0.27
6h
A +0.02 +0.10 +0.05 +0.06 -0.16
The programme of thermal analysis consists of two stages: The first is a drying stage at 500~ in a nitrogen atmosphere in which adsorbed water is eliminated. The second stage is calcination in the presence o f air at 6~ ending at 1200~ (graphite combustion and metaborate decomposition). Results are in Table 4. Table 4. Thermogravimetric study of zinc ferrite calcination MATERIAL
GRAPHITE COMBUSTION PEAK (DTG), ~
ZF(I:I) ZF(I:I)-G ZF(1 :I)-GMB(6%) ZF(I:I)-GMB(12%)
--752 682
WEIGHT LOSS IN GRAPHITE COMBUSTION, % Theoretic value Measured value (prepn. stage) (TG) . . . . . . 3.00 2.35 2.76 2.59
WEIGHT LOSS AT HIGH TF.M-P % T, ~ T, ~ 1000-1100 1000-1200 0.07 0.39 0.10 0.34 0.26 1.75
837
5.0
0.01
6.15
668
We can conclude that pure zinc ferrite remains stable up to 1150~ Besides, graphite combustion is complete, which is indicated by a sharp peak in the 650-825~ range and by a "plateau" in TG and DTG curves over 825~ The graphite combustion takes place at a lower temperature in the presence of metaborate, irrespective of its content. The temperature of the highest oxidation peak depends on the kind of sample: 750~ (zinc ferrites), 680~ (zinc ferrites with metaborate) and 837~ (pure bentonite). These changes can be explained by means of a certain catalytic activity of alkaline metal ions (NaBO2) on the graphite gasification and the different porosity of materials, which affects to diffusional restrictions (mesoporose bentonite). Finally, metaborate is a very stable binder up to 1000~ decomposing to B203 around 1200~ For the characterization of supports, pore volume was measured by water titration, as shown in Figure 2. Results show that higher volume -which is essential to produce a high concentration of active oxides alter impregnation- is found for zeolites (mainly USY). Calcination tends to decrease the pore volume. Silicates and inorganics show relatively low pore volume. For these cases, a way to increase porosity could be the use of graphite [22], as it will be shown later on. As expected, the strength of the extrudates depends on the temperature in an opposing way than pore volume does. Good values are found for inorganic oxides (5-6 Kg/mm) and silicates (4-6 Kg/mm). Zeolites should reach similar values but only alter calcination at temperatures higher than 1000~ In all cases, a substantial improvement with regard to the non-supported zinc ferrite (0.1 - 0.5 Kg/mm) has been observed. A thermobalance was used to study the graphite interactions with supports in air stream (5~ Figure 2 shows the temperature peaks measured for the combustion of graphite. Lower temperature does not produce any change in porosity. The maximum increase in porosity is attained at temperatures close to the peaks, (except in zeolites, where graphite seems to break the structure at any temperature). A further increase of temperature would cause a porosity collapse. For instance, addition of 5% graphite to ZrO2 and calcination at 950~ increases pore volume from 0.16 to 0.21 cm3/g. Nevertheless, no increase in pore volume is found after calcination of this material at 1100~ irrespective of the presence or the absence of graphite. Figure 3 shows that the reactivity of graphite depends on the material. Although, compared to pure zinc ferrite, it will always be less reactive. This indicates that support hinders the accessibility and diffusivity of oxygen molecules through the pore structure. In a similar way, other molecules can be affected by this restriction, as is the case of H2S. This fact can be of great importance in the sulfidation reaction.
Fig.2. Pore volume
Fig. 3, Effect of support
669 The sorbents were obtained according to the formulations shown in Table 1. All of them were prepared by calcinating the precursors at temperatures over 900~ XRD was used for identification of the crystalline phases formed. Results are shown in Table 5. Table 5. Summary of XRD zinc ferrite lines (20) identified in the catalysts. Suppo~: USY Position. Intensity 30.003 13 35.322 38 42.865 13 56.637 18 62.257 18
Support ZrO2 Position Intensity Not found 35.265 53 Not found Not found Not found
Support: Sepiolite Position Intensity 29.850 20 35.689 29 43.003 13 56.939 18 62.405 17
These results show that zinc ferrite has been formed, but in sepiolite and USY zeolite the enstatite and hercynite networks substitute some of their ions for Fe 2+ and Zn 2+, therefore masking the franklinite network. In the ZrO2 case such substitution does not occur, appearing the pure oxide together with the franklinite (6-8%), as theoretically expected. SEM-EDX profiles analysis monitors the metal oxide distribution inside the catalyst particles. Distribution of Z n F e 2 0 4 particles is quite homogenous throughout the support. However, profiles seem to be smoother for ZrO2 and more irregular for USY, what indicates differences in the size of the oxide aggregates at the micrometer scale (about 0.1 ~tm). In addition, scanning photographs of the catalysts show clear textural differences between materials. USY zeolite (also for sepiolite, although not shown) seems to be more compacted. New pores created by graphite combustion in ZrO2 are easily seen as big black structures, about a micron size. Moreover, a strong reduction in the microporosity (BET areas) is observed for all materials, as shown for some selected samples: Fresh sepiolite (280 m2/g), fresh zeolite (600 m2/g), ZF(I: 1)SEP-5G (15 m2/g), ZF(I:I)USY-B (2.9 m2/g), and ZF(I:I) ZrO2-B-5G (2.5 m2/g). Therefore, it can be expected that USY and sepiolite present lower diffusivity for reactants (in agreement with TG data, Figure 3) than ZrO2. This can also have strong implications in the reactivity of the sorbents developed for the sulfidation reaction. Figure 4 shows the sulfidation and the regeneration curves obtained for the selected sorbents. As expected, the activity is low, as the active phase concentration is, and the impregnation profiles are not optimized. Diffusional problems can exist, and neither nonconverted ZnO nor some ferrite phases are accessible and active for the sulfidation reaction.
12000 __ .~. /'-.i_i :-i : i_i~-i-iiI 10000
. . . . . . . . . . . . . . .
I
3,5
I
3.0~ 2,fi
t
8000
oooo
++oo -"
.................................
i
,I
l
--'+~
2000
0,00
.-.i-..SEP-~ _ "- ~Y-IV~2
0,20
t/to
0,40
0,60
o. 2,0 o. ,..,+ 1,5 o 1,0 0,5 0,0 (] O0
0,20
Fig.4. Activity in sulfidation/regeneration (1st cycle)
t/to
0,40
0,60
670 The efficiency of the sorbents for the sulfidation process in a fixed bed reactor agrees notably with the obtained structural data. Although the active phase appears in all samples, the best performance is conferred to the ZrO2 (with 5%G), due to the presence of the ZnFe204 in the expected ratio and to its macroporous structure. 4. Conclusions
The use of binders different from classical bentonite allows setting up a sequence of new, suitable materials for increasing the strength of the zinc ferrites and zinc titanites. So, the highest strength of the final sorbent prepared with zinc titanite is attributed to pure NaBO2 (10.57 Kg/nma, threefold the pure mixed oxide). Strength decreases as the percentage of graphite increases, until 2.69 kg/mm for 20% graphite. Extruded supports present simultaneously a pore volume reduction and a higher strength as the calcination temperature increases. A calcination temperature in the range of 900-1000~ gives mechanical strength values of 3-4 Kg/nun, which are much higher than the values of 0.5-1.0 Kg/mm for the pure ferrite. In both preparation methods for zinc ferrites, the active phase is homogeneously distributed on the support, but co-impregnation is not suitable because it produces low oxide dispersion. All materials present a reduction of porosity, which means diffusional restrictions that causes a decrease in the activity. The best performance in the sulfidation/regeneration tests was obtained for the ZR-M2 sample, what confirms the good prospects for the inorganic oxides as supports in the sulfidation reactions. References
1. Armor, J.N. "Power Plant Emission: An Overview". Environmental Catalysis. ACS Symposium Series 552. Washington, D.C. 1994 2. Focht, G.D., Ranade, P.V., Harrison, D.P. Chem. Eng. Sci.1989, 44, 2919. 3. Gupta, R.; Gangwall, S.K. Energy Fuels. 1992, 6, 21 4. Mojtahedi, W., Salo, K., Abbasian, J. Fuel Process Technol. 1994, 37, 53. 5. Woods, M.C., Gangwal, S.K., Jothimurugesan, K, Harrison, D.P.,. Ind. Eng. Chem. Res. 1990, 29, 1160. 6. Gangwal, S.K., Harkins, S.M., Woods, M.C., Jain, S.C., Bossart, S.J. Environmental Prog..1989,8,265. 7. Efihimiadis, E., Sotirchos, S.V. Chem. Eng. Sci. 1993, 48, 829. 8. Eiihimiadis, E., Sotirchos, S.V. Chem. Eng. Sci. 1993, 48, 1971. 9. Pineda, M., Palacios, J.M., Garcia, E., Cilleruelo, C., Ibarra, J.V. Fuel 1997, 1l, 813. 10. Jothimurugesan, K., Harrison, D.P. Ind. Eng. Chem. Res. 1990, 29, 1167 11. Lew, S., Sarofim, A:F., Flytzani-Stephanopoulos, M. Ind. Eng. Chem. Res. 1992, 31, 1890. 12. Gangwal, S.K., Stogner, J.M., Harkins, S.M. Environ. Prog. 1989, 8, 26. 13. Gangwal, S.K., Harkins, S.M., Woods, M.C., Jain, S.C., Bossart, S.J. Environ. Prog. 1989, 8, 265. 14. Pineda, M., Palacios, J.M., Alonso, L., Garcia, E., Moliner, R. Fuel.2000, 79, 885. 15. Pineda, M., Palacios, J.M., Tomfis, F., Cilleruelo, C. Garcia, E., Ibarra, J.V. Energy and Fuels. 1998, 12,409. 16. Tomfis, F., Orenes, I., Palacios, J.M., Alonso, L. SECAT'99. 1999, 293. 17. Tomfis, F., Orenes, I., Palacios, J.M., Alonso, L., Garcia, E:, Moliner, R. Recent Reports of the 12th International Congress on Catalysis, Granada. 2000, R070. 18. Jha, M.C., Berggren, M.H. Report. DOE Contract no. DE-AC21-86MC23172. 1989. 19. Komiyama, M., "Design and Preparation of Impregnated Catalysts". Catal. Rev. Sci.1985, 362.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
671
Synthesis and characterization of M W W zeolites L.M.Vtjurina and S.S.Khvoshchev Coming Scientific Center in St.Petersburg, Russia WESTPOST PO BOX 109, Lappeenranta, Fin-53101, Finland Mechanical activation of the reaction mixtures by ball-milling before crystallization has been applied to the preparation of high-silica MWW crystals. Adsorption of water and toluene was used to characterize of zeolite products. 1.1NTRODUCTION The effect of mechanical activation by ball milling is mostly used as a technique to combine the components for creation of metals or metal-non-metal composites [1]. For catalysts this technology is known since more than 40 years [2]. Treatment by ball milling may modify the catalytic properties of zeolites due to decrystallization processes [3,4]. The collapse of the zeolite crystal structure leads to the enhancement of the selectivity for base catalyzed reactions [5]. There is not enough information about use of ball milling in the preparation of zeolite synthesis mixtures [6]. Recently effect of hydrogel milling prior to hydrothermal crystallization of NaZSM-5 on the crystal size distribution of the product has been investigated [7]. In this work we used a milling pretreatment of crystallization mixtures for MWW syntheses without stirring.
2. E X P E R I M E N T A L Hydroxide of aluminum (Merck) or sodium aluminate (Merck), sodium fluoride or sodium hydroxide (Merck), hexamethyleneimine (Aldrich) and silica fumed S-5130 (Sigma) were used for the syntheses. Compositions of the initial mixtures, alumina sources and results of the syntheses of MWW zeolite without and with ball milling pretreatment (XRD-1 and XRD-2) are represented in the Table 1. Table 1. Conditions of mixtures preparation and synthesis results. Na20/SiO2 NaF/SiO2 SIO2/A1203 Alumina source XRD-1 XRD'2 0,09 0,09 0,09 0,09 -
1,33 1,33 1,33 1,33
100 200 100 200 100 200 100 200
NaA102 NaA102 NaA102 NaA102 AI(OH)3 AI(OH)3 AI(OH)3 AI(OH)3
MWW+ken kenyaite kenyaite MTW+ken MWW+ken kenyaite kenyaite MTW+ken
MWW+ken kenyaite MWW MTW+ken MWW+ken kenyaite MWW MTW+ken
672 Contents of the hexamethyleneimine and HzO were 0,5 and 45 moles per 1 mole of SiO2 in the all syntheses. Synthesis mixtures were prepared by two ways: 1. Hydroxide of aluminum or sodium aluminate, sodium fluoride or sodium hydroxide, water, HMI and silica fumed were mixed. Resulted mixtures were stirred up to homogenization. Crystallization was performed at 145~ for 7-14 days under static conditions. 2. Hydroxide of aluminum or sodium aluminate and sodium fluoride or sodium hydroxide were milled with silica fumed with addition of water (10%) on the Planet mill during 1 hour (600 rpm). Resulted mixtures were stirred with HMI and remaining water up to homogenization. Crystallization was performed at 145~ during 7-14 days under static conditions. The crystalline products were filtered, washed with water until pH = 9 and dried at 100~ Calcination was carried out in air at 600~ for 2 hours. X-ray powder diffraction and electron microscopy were used to characterize the samples. The adsorption isotherms of water and toluene at 293 K were obtained gravimetrically. 3. RESULTS AND DISCUSSION Fluoride ions are widely used as mineralizing agents in zeolite synthesis [8]. They act as solubilizing agents and participate in polycondensation processes and crystallization. In general, synthesis in fluoride containing media are carried out using NH4F or HF as sources of fluoride ions. These routes with subsequent calcination allow synthesizing zeolites directly in
(a) .
_
_
_
~ |
m .
.
_ .
.
(b) .=
i
5
I
I
I
1~)
115
2D
'
I ~i u
215
I
3b
20
'
Fig. 1. XRD patterns of the calcined samples synthesized from the mixtures: (a) unstirred; (b) stirred by magnetic bar; (c) milled on Planet Mill; (d) - XRD pattern of calcined MWW [ 10].
673 H-form without additional ion-exchange. NaF and KF ai'e the most effective fluoride sources for the synthesis of MWW crystals with SIO2/A1203=30-60 under stirring conditions [9]. For the syntheses we used the crystallization mixtures with the same initial compositions as in [9, 10]. MWW crystals with 100% crystallinity were obtained under dynamic conditions in [9] only from the initial mixtures with SIO2/A1203 = 30. Crystallization results do not depend on the nature of alumina sources (Table 1). Pure MWW crystals with SIO2/A1203 = 80 were obtained only from the mixtures on the base of NaF with SIO2/A1203-- 100 if the ball milling pretreatment was used. Crystallization of the mixtures with the same compositions without preliminary milling resulted in kenyaite. Zeolite MTW with impurity of kenyaite was formed from the mixtures with SIO2/A1203 = 200 in the both cases (with or without ball milling). Fig. 1 and Fig.2 show the XRD and SEM images of the products which were obtained from the mixtures with SIO2/A1203 = 100 on the base of AI(OH)3 and NaF without any pretreatment (a), with stirring by magnetic bar (b) and ball-milling (c) before crystallization. As seen from these figures the pure zeolite MWW crystallized only from the mixture milled on Planet Mill. Otherwise kenyaite and MTW were the main phases. No pure zeolite MWW was obtained from the mixtures on the base of NaOH (Fig.3). MWW with kenyaite impurity was formed after crystallization of mixtures with SIO2/A1203 = 100 for 7-10 days. Pure kenyaite was the main phase in the case of crystallization of the mixtures with SIO2/A1203 = 200. The increase of the synthesis time up to 14 days leads to formation or-quartz [10]. Only slightly difference was observed between the samples crystallized with or without ball milling and without pretreatment.
Fig.2. SEM images of samples synthesized from the mixtures: unstirred (a); stirred by magnetic bar (b); milled on Planet Mill (e).
674
1 iO2/A1203=200
,SIO2/A1203=1 O0
Fig.3. XRD pattems of the calcined samples synthesized from the mixtures on the base of NaOH with different SIO2/A1203 and with (2;4) or without (1;3) preliminary ball milling. Pore volume for pure MWW should be about 0.2 cm3/g [11 ]. This value is observed (Fig.4) for water adsorption on the sample synthesized on the base of NaF with application of ball milling pretreatment. Isotherm of toluene adsorption exhibits the same pore volume. In contrast with this finding almost a half of pore volume is not available for toluene in the case of crystallization product of the mixture without NaF. The difference in water adsorption values between these two zeolites is rather small.
a, cm3/g 0,24-1 (a)
,....
a, cma/g 0,243 |
o,2o-I
/'-"
0,20.
0,16-1
,A-i /j~
0,161
0,12-1
J
j"
0,08
0,04~
, 0 0,0
0,04
0
(b)
0,12,
0,08-1 ...g"' ~
0
~ - -
~ 0,5
0,00 pips
17 - NaF, ball milling pretreatment; 0 - NaOH, ball milling pretreatment; A - Na0H, without pretreatment
0,0
0,5
P/Ps
Fig.4. Isotherms of H20 (a) and toluene (b) adsorption on the MWW crystals with SIO2/A1203 = 80 obtained in different conditions.
675 CONCLUSION Pretreatment of the initial mixture by ball-milling can use for the preparation of highsilica MWW crystals without stirring during the crystallization. REFERENCES
1. L.Schulz, J.Less Common Met.., 145 (1988) 233. 2. E.W.Pitzer, US Pat. 2,866,790 (1958). 3. P.A.Zielinski, A.Van Neste, D.B.Akolekar and S.Kaliaguane, Microporous Materials 5 (1995) 123-133. 4. A.Auroux, M.Huang and S.Kaliaguane, Langmuir (1996) 12, 4803-4807. 5. Jinhai Xie and Serge Kaliaguane, Applied Catalysis A: General 148 (1997) 415-423. 6. D.P.Klevtsov et al. React.Kinet.Catal.Lett., 36,2,319, 1988. 7. C.Falamaki, M.Edrissi and M.Sohrabi, Studies in Surface Science and Catalysis, v.135, Zeolites and Mesoporous Materials at the Dawn of the 2 lth Century, Proceedings of the 13th Intern.Zeolite Conf., Montpellier, France, 8-13 July 2001, CD ROM 02-P-09. 8. E.M.Flanigen and R.L.Patton, US Pat.4073865, 1978. 9. R.Aiello et al. Microporous and Mesoporous Materials 35-36 (2000) 585-595. 10. L.M.Vtjurina, S.S.Khvoshchev and I.V.Karetina, Studies in Surface Science and Catalysis, v.135, Zeolites and Mesoporous Materials at the Dawn of the 21 th Century, Proceedings of the 13th Intern.Zeolite Conf., Montpellier, France, 8-13 July 2001, CD ROM 02-P-42. 11. A.L.S.Marques, J.L.F. Monteiro and H.O.Pastore, Microporous and Mesoporous Materials 32 (1999) 131-145.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
677
Structural Analysis of Oxyanion - Cation Complexes Anchored by Organic Group in Mesoporous Silicas Hideaki Yoshitake ~, Toshiyuki u
~'b and Takashi Tatsumi b
aGraduate School of Environment and Information Sciences, Yokohama National University, Yokohama 240-8501, Japan bGraduate School of Engineering, Yokohama National University, Yokohama 240-8501, Japan We have investigated the structure of oxyanion-Fe3+-amino group complexes in the pore of MCM-41 in order to develop heterogeneous chemistry of oxyanion- cation interactions on the oxide surface with organic groups. XAFS spectroscopies were applied to the structural analyses. Two kinds of structure were shown in arsenate- and molybdate-Fe complexes. One is edge-sharing and the other is corner-sharing coordinations. In contrast, a uniform coordination structure was implied in chromate and selenate adsorptions. The stoichiometry at the adsorption saturation and the coordination numbers suggest that a part of chromate, selenate and molybdate are reduced into monovalent anions. 1. INTRODUCTION The exposure to toxic oxyanions in groundwater is one of rising environmental problems everywhere in the world. The EPA maximum contaminant level for As in drinking water is 10.5 g.L 1, while the naturally excessive arsenic has been widely detected in Chikugo plain, Japan [1]. The chronical toxicity has been actually reported as blackfoot disease in the southwestern coast of Taiwan [2, 3] and various cancers in Bangladesh [4]. Chromium is used in electroplating and tannery and untreated effluents cause a pollution of groundwater [5, 6]. The occurrence of selenium in groundwaters is also oxyanions, selenate and selenite, and the element can either be toxic and essential to animals and plants. The adsorption of these anions on the inorganic oxides has been studied to establish the geochemical models [7]. Searching effective adsorption materials is also important for developing technology of oxyanion removal because the adsorption is one of the most simple and powerful methods for purifying water. The molecular sieves with uniform mesopores are attractive substrates for the researches on ion adsorptions. The mesoporous silicas with various pore-connecting topologies have been prepared by templating with surfactant micelles. Grafting technique of silanes introduces functional groups into the pores of such silicas. Divalent cations, Cu, Zn,
678 Cr and Ni, in wastewater are adsorbed more on amino-functionalized SBA-15 than on thiol-functionalized one, while more Hg 2+ is adsorbed on the latter [8]. The mercury adsorption on the thiol-functionalized mesoporous silicas has been intensively studied probably because of the environmental impact [9-12]. One of the advantages of this research strategy is the molecular nature of the adsorption site. A plenty of works were published on other metal-coordinated functional groups on mesoporous silicas [ 13-20] where the structure of the sites adsorbing cations (or anions) is discussed. These functional groups anchor ions strongly and the adsorptions are usually irreversible, suggesting that the certain groups in humic materials, such as-NH2 and-SH, fix the ions in the surface water. The clarification of the interactions between cations, anions and those functional groups will be helpful for understanding the behaviour of these toxic anions in nature and developing effective adsorbents for the area suffering from the pollutions. In this study we analysed the structure of cation-oxyanion complexes anchored by amino groups in the pores of mesoporous silica. Fe 3+ was used as a cation. The mononuclear oxyanions such as AsO43, CrO4 2, SeO42- and MoO4 2- have structural similarities to SO42-; four oxygen atoms surround the penta- and hexavalent cations tetrahedrally. The aim of this paper is to clarify the differences in the ligand structures. 2. MATERIALS AND METHODS
Commercially available tetraethyl orthosilicate (TEOS), dodecyltrimethyl- ammonium chloride (DTMAC1) and trimethylammonium hydroxide (TMAOH) were mixed in water and the solution was stirred for 4 h at room temperature. The composition of the gel mixture was Si: DTMAC1 : TMAOH : H20 = 1:0.6 : 0.3 : 60. White precipitates were filtered and dried at 393 K. This as-synthesized powder was calcined at 903 K for 4 h to obtain MCM-41. The silica was stirred vigorously in toluene containing a monolayer equivalent amount (1.0 per 1 nrlq2) of [ 1-(2-aminoethyl)-3-aminopropyl]trimethoxysilane (HzNCHzCHzNHCHzCHzCH2 Si(OCH3)3) and heated to 383 K in dry nitrogen for 6 h. The powder was collected by filtration, washed with 2-propanol for 2 h and dried at 373 K. We denote this diamino-functionalized silica as en-MCM-41. The amino functionalized silica powders were stirred in 0.1 M 2-propanol solution of FeC13 at room temperature for 2 h, washed two times with 2-propanol and dried in an oven. The Fe3+-anchored en-MCM-41 is denoted as Fe/en-MCM-41. The standard procedure of adsorption experiments was as follows. 50 mg of Fe/en-MCM-41 was stirred for 10 h in 100 ml of aqueous solution of KHzAsO4, KzCrO4, K2SeO4 or K2MoOa. After removing the solid, the concentrations of the oxyanion were determined by ICP-AES. The detection limit was 1 x 10.6 g/L ( = 1 ppb). Typical pH value of the solution at the beginning was 2, 7, 3 and 5 for arsenate, chromate, selenate and molybdate, respectively. In these pH conditions, the dominant species in the aqueous solution are HzAsO4-, CrO42-, SeO4 2- and MoO4 2-. X-ray absorption experiments were carried out in a transmission mode at BL-9A and BL-10B of Photon Factory, High Energy Accelerator Research Organization, Tsukuba, Japan
679 (Proposal #2001G133), with a ring energy of 2.5 GeV and a stored current at 250 - 400 mA. The data were processed by EXAFS analysis program REX 2000 (Rigaku Co.) which is a program assembly of the background subtraction by cubic spline functions, Fourier transformation and the curve-fitting analysis. After normalised with using the McMaster tables, k 3-weighted EXAFS oscillation, k 3x(k), in 30 - 135 nm -1 region was Fourier transformed into a radial distribution function. The amplitude and phase-shift functions were calculated by using the parameter set by Mckale et. al. [21 ]. 3, RESULTS AND DISCUSSION X-ray diffraction patterns showed that the p6mm mesoporous structure was retained in Fe/en-MCM-41 though the intensities of diffractions became smaller than those of the MCM-41. The elemental analysis of Fe/en-MCM-41 demonstrated that the molar ratio of N/Fe was 3.8, suggesting Fe(en)2 structure. The adsorption experiments were carried out over a wide range of the anion concentration. When the initial concentration of oxyanions was smaller than 0.60 mmol.L -1, no oxyanions were detected after the adsorption. The adsorption isotherms were different between As, Cr, Se and Mo. Nevertheless, the complete removal of oxyanions was observed at the surface coverage below 0.1, which is the amount of oxyanion adsorbed divided by that at the adsorption saturation. Table 1 summarizes the adsorption capacities (= the adsorption at the saturation) and the stoichiometries of oxyanion / Fe. The adsorptions per unit weight of adsorbent shown in Table 1 are the largest adsorption capacities ever reported in the literature. The oxyanion / Fe varies from 1.5 to 2.8. The stoichiometry of cation-anion complexes determined by Coulomb force is calculated by the charge balance and probably gives the upper limit of the adsorption. It is 3 for H2AsO4" and 2 for CrO42-, SeO42 and MoO42-, suggesting As, Cr and Mo fully occupy the Fe site. Fe K-edge XANES spectra are shown in Figure 1. The post edge feature of Fe captured in en-MCM-41 is completely different from that of FeCI3, suggesting a ligand exchange with the amino groups. Some of the pre-edge absorptions of Fe centres, which are usually attributed to the degree of p-d mixing, are composed of two peaks. The peak for Fe/en-MCM-41 is sharp, suggesting that the coordination environment of Fe is relatively uniform. After the adsorption of the anions, with consuming this peak, a new absorption appears at the position 1 eV higher than the original peak. Two kinds of peaks are clearly distinguished in As-, Se-, and Mo-Fe/en-MCM-41 while a single peak is observed in Cr-Fe/en-MCM-41. The blue shift is only 0.4 eV in the last complex. These differences imply that the coordination environment in As-, Se- and Mo-Fe/en-MCM-41 are more heterogeneous than those in Fe/en-MCM-41 and Cr-Fe/en-MCM-41. Table 1 Adsorption Capacity of Fe Captured by Amine-Modified MCM-41. ........................................................................................................................................................
specific adsorption / mg.g -1. oxyanion / Fe (molar ratio)
6s.(y).
...................................
225 2.8
*specific adsorption by 1 g of Fe/en-MCM-41.
.......................... S e ( V ! )
115 1.8
116 1.5
........................
...............
208 2.3
680 i
(a)
i
A/ s e Fe,en
i <
Mo-Fe J / l / Se-Fe ..___.,.-~1 t tit Cr-Fe ______~][
k
~
_ FeCI3
FelM4 FeCI3 I
S
_ i I
I
I
I
7070 7090 7110 7130 7150 7170 E/eV
7097
I
J
7102 7107 7112 7117 E/eV
Figure 1 (a) Normalized XANES and (b) pre-edge peaks of Fe K-edge in Fe/en-MCM-41 before and after adsorption of oxyanions. Figure 2 shows the radial distribution functions of Fe K-edge EXAFS spectra. The curve-fitting results are summarized in Table2. The adsorption complexes for the spectrum measurements were prepared in the condition where the adsorption isotherms were saturated. On the other hand, the sample for the EXAFS of As, Cr, Se and Mo K-edges contained oxyanions with a half amount of the saturation. The largest peak in Fe/en-MCM-41 is attributed to Fe-C1 shell with a shoulder due to Fe-N bonds. The coordination numbers in these shells are 3.3 + 0.6 and 2.1 + 0.4, respectively, suggesting a formula, Fe(en)2C12. The distances are 0.192 and 0.224 nm, respectively. The positive charge of Fe 3+ is probably neutralized by a chloride ion in the outer shell. (Thus the proposed formula for the Fe centre is [Fe(en)2C12]+C1-.) A large Debye-Waller factor for Fe-N bond suggests that the coordination of en is not rigid. We thus consider the coordination structure of Fe centre as a distorted Oh. After adsorption of As, Se and Mo, the FT changes into unresolved two peaks; one appears below 0.23 nm while the top of the other peak is around 0.28 nm. On the other hand, the peaks for Cr-Fe/en-MCM-41 are much narrower than the other adsorbing Fe/en-MCM-41. Two kinds of Fe-As bonds are distinguished in As-Fe/en-MCM-41; one is 0.327 nm and the other is 0.359 nm. The former is due to direct coordination of As to Fe and the latter agrees with the distance of comer sharing complexes of FeO6-AsO4 [22]. The bond lengths of Fe-N and Fe-C1 are not substantially changed. The coordination numbers for Fe-N, Fe-C1 and Fe-As are 2.9 + 0.4, 0.50 + 0.2 and 1.5 + 0.5 (for Fe-As at 0.327 nm) and 1.4 + 0.5 (for Fe-As at 0.359 nm), respectively. This result agrees with As(V) / Fe = 2.8 (Table 1) and suggests
681
that arsenate is coordinated to Fe center in two ways and a part of chloride anions remain even at the adsorption saturation. C1 shell does not fit to the FT of Cr-Fe/en-MCM-41 and the coordination number for Cr shell agrees with the ratio of Cr(VI) and Fe in Table 1. It is likely that the coordination structure of Cr-Fe/en-MCM-41 is uniform and Cr : Fe - 2 : 1. However, Fe(en)2(CrO4)2 is not possible because the charge is not balanced and a part of chromate is probably reduced into HCrO4. The coordination number of Fe-N bond is significantly reduced. The possibility for releasing an amino group of en ligand should be considered. The structure of Se-Fe/en-MCM-41 is reasonably explained by a similar manner; selenate is partially reduced into HSeO4, the ratio of Se to Fe is two and C1 partially remains. Two kinds of coordinations are found in molybdate adsorption and the bond lengths are 0.315 and 0.354 nm. These correspond to a direct coordination and a liganding via O atom, respectively. The coordination numbers for these bonds are 2.3 + 0.6 and 0.60 + 0.2 whose sum shows accordance with the result in Table 1 within errors. The part of molybdate is, again, likely reduced. Two kind of N is also distinguished, suggesting the disproportionation of en ligand.
I-ii
I/V....
0
0.1
0.2
0.3
0.4
0.5
0.6
r/nm
Figure 2 Fourier Transforms of Fe K-edge EXAFS for Fe/en-MCM-41 and anion coordinated Fe/en-MCM-41.
0
0.1
0.2
_
0.3
0.4
_ Se_l
0.5
0.6
rlnm
Figure 3 Fourier Transform of As, Cr, Se and Mo K-edge EXAFS for anion coordinated Fe/en-MCM-41.
The radial distribution functions of As, Cr, Se and Mo K-edge EXAFS are overlaid in Figure 3. Strong peaks appearing between 0.1 and 0.2 nm is assigned to oxygen shell. Although the significant peaks can not be observed in the region more than 0.2 nm, Fe shells were necessary for obtaining good curve-fitting results (with R < 5%) in k-space. Table 3
682 shows the result of curve-fitting analysis of the K-edge spectra of oxyanions. The distance for Fe shells agrees with the data for Fe K-edges in Table 2. The coordination number supports the stoichiometry where the oxyanions coordinate to one Fe atom. The coordination number and the bond length of As-O, Se-O and Mo-O were not changed by the adsorption on Fe/en-MCM-41 and they are very similar to the oxygen environment of respective monomeric oxyanions. This suggests that the interaction of oxyanions with Fe centre is nearly ionic and the adsorption appears as an exchange reaction of potassium cation with the ferric site. On the other hand, Cr-O bond is longer than that in chromate, suggesting a perturbation of Td structure by a strong coordination to Fe. Table 2
Curve-fitting Results of Fe K-edge EXAFS shell N Fe/en-MCM-41 N 3.3(0.6) CI 2.1(0.4) As-Fe/en-MCM-41 N 2.9(0.4) CI 0.50(0.2) As 1.5(0.5) As 1.4(0.5) Cr-Fe/en-MCM-41 N 2.9(0.4) 1.8(0.3) Cr Se-Fe/en-MCM-41 N 1.9(0.3) CI 0.83(0.3) Se 1.6(0.5) Mo-Fe/en-MCM-41 N 1.6(0.3) N 1.3(0.2) Mo 2.30(0.6) Mo 0.60(0.2) Tile numbers in parenthesis are errors. Table 3
Curve-fitting Results of As, Cr, shell As-Fe/en-MCM-41 O Fe Fe Cr-Fe/ea-MCM-41 O Fe Se-Fe/en-MCM-41 O Fe Mo-Fe/ea-MCM-41 O Fe Fe The numbers in parenthesis are errors.
r/nm 0.192(0.03) 0.224(0.03) 0.194(0.03) 0.228(0.03) 0.327(0.04) 0.359(0.04) 0.188(0.04) 0.312(0.03) 0.195(0.03) 0.220(0.03) 0.305(0.04) 0.193(0.03) 0.218(0.04) 0.315(0.03) 0.354(0.04)
0.112 0.062 0.049 0.061 0.063 0.072 0.071 0.084 0.065 0.086 0.081 0.061 0.089 0.105 0.063
Se and Mo K-edge EXAFS N r / nm 3.9(0.4) 0.168(0.03) 0.93(0.3) 0.327(0.04) 0.40(0.2) 0.351(0.04) 3.7(0.5) 0.200(0.03) 1.0 (0.2) 0.308(0.04) 3.2(0.5) 0.166(0.02) 1.2(0.3) 0.309(0.04) 3.8(0.6) 0.175(0.04) 0.93(0.3) 0.318(0.04) 0.62(0.2) 0.358(0.04)
a2 0.046 0.058 0.065 0.063 0.067 0.056 0.096 0.072 0.066 0.063
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
683 The above analysis demonstrates that the coordination of oxyanions varies by the anion species. And it accords well with the stoichiometry of oxyanion / Fe at the adsorption saturations. The coordination structure of en is still unclear and the data analysis is in progress in order to clarify the origin of the small coordination numbers after the adsorption of anions. The broadening of pre-edge peaks of XANES (Figure 1) can be explained by the remaining C1~ (for As and Se) and two kinds of liganding structure (for As and Mo). A smaller FWHM in the peak of Cr-Fe/en-MCM-41 agrees well with the result in Table 2 where one kind of Fe-Cr bond is found. 4. CONCLUSIONS Fe anchored amino-functionalized MCM-41 effectively adsorbs toxic oxyanions, arsenate, chromate, selenate and molybdate, in the aqueous solution. These adsorbents show ones of the largest adsorption capacities reported in the literature. The molar ratio, oxyanion / Fe, is 2.8, 1.8, 1.5 and 2.3, respectively. They are consistent with the coordination numbers in the EXAFS structural analysis of adsorption complexes. Cr-Fe adsorption centre is uniform, where 2:1 complex is uniformly generated. On the contrary, chloride ion remains in As- and Se-Fe complexes and two kinds of coordination with different bonding distances were found in As- and Mo-Fe complexes. 5. ACKNOWLEDGEMENT A part of this study has been supported by Grant-in-Aid for Scientific Research (No. 13780448) from Japan Society for the Promotion of Science (JSPS). REFERENCES
[ 1] H. Kondo, Mizukankyo gakkaishi, 20 (1997) 6. [2] Y. S. Sen, C. S. Shen, J. Water Pollut. Control Fed. 36 (1964) 281. [3] S. L. Chen, S. R. Dzeng, M. H. Yang, K.H. Chiu, G. M. Shieh, C. M. Wai, Environ. Sci. Technol. 28 (1994) 877. [4] R. Nickson, J. McArthur, W. Burgess, K. M. Ahmed, E Ravenscrofl, M. Rahman, Nature, 395 (1998) 338. [5] M.M. Lawrence, Environ. Sci. Technol., 15 (1981) 1482. [6] E. L. Tavani, C. Volzone, J. Soc. Leather Technol. Chem. 81 (1997) 143. [7] W. R. Cullem, K. J. Reimer, Chem. Rev., 89 (1989) 713. [8] A. M. Liu, K. Hidajat, S. Kawi, D. Y. Zhao, Chem. Commun. (2000) 1145. [9] X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu, K. M. Kemner, Science, 276 (1997) 923. [ 10] L. Mercier, T. J. Pinnavaia, Environ. Sci. Technol. 32 (1998) 2749. [ 11] J. Brown, R. Richer, L. Mercier, Microporous Mesoporous Mater. 37 (2000) 41.
684 [12] R. I. Nooney, M. Kalyanaraman, G. Kennedy, E. J. Maginn, Langmuir, 17 (2001) 528. [13] K. Moller, T. Bein, Chem. Mater. 10 (1998) 2950. [ 14] B. T. Holland, C. Walkup, A. Stein, J. Phys. Chem. B, 102 (1998) 4301. [ 15] P. Sutra, D. Brunel, Chem. Commun. (1996) 2485. [16] S. H. Lau, V. Caps, K. W. Yeung, K. Y. Wong, S. C. Tsang, Microporous Mesoporous Mater. 32 (1999) 279. [17] J. Evans, A. B. Zaki, M. Y. El-Sheikh, S. A. E1-Safty, J. Phys. Chem. B 104 (2000) 10271. [ 18] J. E Diaz, K. J. Balkus, Jr., Chem. Mater. 9 (1997) 61. [19] D. H. Park, S. S Park,. S. J Choe,. Bull. Korean Chem. Soc. 20 (1999) 291. [20] G. E. Fryxell, J. Liu, T. A. Hauser, Z. Nie, K. F. Ferris, S. Mattigod, M. Gong, R. T. Hallen, Chem. Mater., 11 (1999) 2148. [21 ] A. D. Mckale, B. W. Veal, A. E Pailikas, C. K. Chan, G. S. Knapp, J. Am. Chem. Soc. 110 (1988) 3764. [22] G. A. Waychunas, B. A. Rea, C. C. Fuller and J. A. Davis, Geochim. Cosmochim. Acta, 57 (1993) 2251.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
685
Adsorption equilibrium of polar/non-polar mixtures on M CM-41: experiments and Monte Carlo simulation J.-H.Yun.*, Y. He*, M. Otero*, T. Diiren w N.A. Seaton** *Instituto de Recursos Naturales, Avda. Portugal n~ 24071 Le6n, Spain w 4, Chemische Reaktionstechnik, Technische Universitiit HamburgHarburg, Eissendorfer Str. 38, D-21073 Hamburg, Germany *School of Chemical Engineering, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh EH9 3JL, United Kingdom We have studied the adsorption of mixtures of polar and non-polar gases in MCM-41. MCM-41 is a suitable model on which to test our understanding of adsorption at the molecular level, and to evaluate methods for the prediction of multicomponent adsorption equilibrium. The adsorption of mixtures of methane, ethane and carbon dioxide (and the corresponding pure gases) was studied using experiment, grand canonical Monte Carlo (GCMC) simulation, and ideal adsorbed solution theory (lAST). Both GCMC and lAST work very well for the adsorption of the methane/ethane mixture in MCM-41. IAST gives good predictions for ethane/carbon dioxide mixture adsorption equilibrium at low and moderate pressures, and exhibits some deviations at relatively high pressures. INTRODUCTION The main aim of this study is to investigate the fundamental bases of classical and statistical mechanical methods for the prediction of adsorption equilibrium in porous solids, using MCM-41 [1-2] as a model adsorbent. MCM-41 is well suited as a model adsorbent because it has a regular structure consisting of an array of long, unconnected, cylindrical pores. The adsorptive gases studied are methane, ethane and carbon dioxide, and mixtures of methane with ethane, and ethane with carbon dioxide. Ideal Adsorbed Solution Theory (lAST), an engineering thermodynamic model, and grand canonical Monte Carlo (GCMC) simulation, a statistical mechanical method, are the two methods used to predict the adsorption equilibrium in MCM-41. EXPERIMENTAL SECTION Pure silica MCM-41 materials were supplied by Chonnam National University, Korea. The preparation of the MCM-41 samples and adsorption experimental operation procedures are described in detail in Ref. 3. An outline is as follows: MCM-41 samples were compacted into 10-12 mesh pellets with a hand-operated press with a relatively small external pressure of 2 MPa for 3 s before it could be used in adsorption apparatus, and then were calcined at 500 ~ for 12 h, 200 ~ for 4 h and 120 ~ for 2 h, in sequence, prior to use. This was done to minimise the pressure drop in the sample cell. Nitrogen adsorption and XRD measurements were carried out to ensure that the MCM-41 structure was maintained. The adsorption apparatus, shown in Figure 1, uses a static volumetric method to measure pure gas adsorption, and an open-flow adsorption/desorption method for mixture
686
adsorption. The apparatus operates at pressures up to 33 bar and at temperatures from 263 K to 373 K. MFCs PRESS. CONTROLLER ~VENT
GC/TCD
Om m~
-Q
Fig. 1. A schematic diagram of the adsorption apparatus used in this study. IDEAL A D S O R P T I O N S O L U T I O N T H E O R Y
Ideal adsorbed solution theory (IAST) [4] is a widely used engineering thermodynamic method, the equivalent to the Raoult's law for adsorption, which uses the pure-gas isotherms as inputs to predict the mixture adsorption equilibrium at the temperature of interest. Before the IAST calculation, the pure gas isotherm must be fitted with a suitable isotherm equation to an acceptable degree of accuracy; we used a piece-wise fit [3]. Here is the summary of the most important equations used in the IAST calculation:
.fi = x i f i~ (~) 7rA :o R-T =~0' n i d l n f i
1 =~L, xi nt
i
(1) (2) (3)
ni
S21 = x2 / Xl
(4)
y21yl Here fi is the fugacity of component i in the bulk gas phase; f0 is the standard-state fugacity, i.e. the fugacity of pure component i at the equilibrium spreading pressure of the mixture, ~ ; x~ and y~ are the mole fraction of component i in the adsorbed and gas phase, respectively; A is the surface area of the adsorbent; n~ is number of moles of pure component i adsorbed (i.e. the pure-component isotherm) and n o is number of moles of pure component i adsorbed at the standard-state pressure.
687 Equation (1) is the central equation of IAST, specifying the equality of chemical potential in the bulk gas and the adsorbed phase (which is assumed to be ideal in the sense of Raoult's law). Equation (2) calculates the spreading pressure from the pure-component isotherm. The total amount adsorbed and the selectivity are given by equations (3) and (4), respectively. M O L E C U L A R SIMULATION Grand canonical Monte Carlo (GCMC) simulation is a statistical-mechanical method, in which a rigorous molecular-level model of adsorption is solved exactly. GCMC simulation is widely used in the simulation of adsorption equilibrium, because it gives the number of moles of molecules in the pore directly. More detailed descriptions of the GCMC method in general are given in Refs. 5 and 6, and details of our implementation of the method in the simulation of adsorption in MCM-41 are given in Ref. 3. In this study, the MCM-41 model pore consists of a regular array of oxygen atoms arranged on a rectangular grid on three concentric cylinders. Three wall layers give a wall thickness of about 1.0 nm, which is a value typical for MCM-41. The silicon atoms in the adsorbent are ignored, as their effect on adsorption is expected to be slight (and is, in effect, subsumed into the fitted oxygen-fluid interaction parameters- see below). The density of oxygen atoms in the model pore corresponds to the skeletal density of amorphous silica (2.2 g/cm3). The Lennard-Jones (L-J) potential was used for the fluid-fluid and the fluid-oxygen interactions. The fluid-fluid L-J parameters were taken from Ref. 7. The fluid-oxygen parameters were related to the Lennard-Jones parameters for oxygen using the LorentzBerthelot mixing rules. The L-J atomic size parameter, c, for oxygen was taken from Ref. 8. The pore diameter, the porosity, and the L-J energy parameter, ~, for oxygen were fitted to a single pure-ethane isotherm at 264.75 K. After fitting these parameters, no further experimental data were used as input to the model. GCMC is thus much less dependent on experimental data than IAST, which requires all the pure-component isotherms. Table 1 is a summary of the parameters used in this study. Table 1. Lennard-Jones Parameters Used in GCMC simulations Molecule e/kB, K a, nm Bond length, nm CH4
149.92
0.37327
C2H6
139.8
0.3512
O in MCM-41
185.0
0.2708
0.2353
The GCMC simulations were carried out in a simulation cell containing a section of the model pore, and periodic boundary conditions were applied along the axis of the pore. The simulation cell had a length of 4.0 nm, which was sufficiently large to make the effect of finite system size negligible. The system was equilibrated for 500 000 GCMC steps, and data were collected for another 500 000 GCMC steps to get the average amount adsorbed. Snapshots of adsorption of ethane in MCM-41 at 264.75 K are shown in Figure 2.
688
Fig. 2. Snapshots of adsorption of ethane in MCM-41 at 264.75 K.
RESULTS AND DISCUSSION 1. Methane and ethane
Figure 3 shows experimental and predicted binary isotherms at 264.75 K for a puresilica MCM-41 material with a pore width of 4.1 nm. Both lAST and GCMC gave accurate predictions for the amount adsorbed over the whole pressure range.
...................... ' i " ........... lAST-methane ....... lAST-ethane . lAST-total O Exp.total Exp. methane 0 Exp.etahne --X-- GCMC,total ~ = GCMC,methane 6 + G C M C, ethane
I
'
.............. | . . . . . . . . . . .
"
I
9
"ll . . . . . . . . . . .
i ......
!
10
"~ O
E E z-
f ~
..~~,,X'~
~
~ /. . . ~ - ~ O ~
./ O." /
.......
.....
4
0
500
1000
1500
2000
2500
3000
Pressure, kPa
Fig. 3. Adsorption isotherms for the binary mixture of methane/ethane in MCM-41 (pore diameter = 4.1 nm) at 264.75 K and a composition of 59.1% methane.
689 13 , 12
,
I
'
.
I
'
'1
....
'
"
I
'
I
lAST
~
11
o
10
--m--
EXP. G C M C
o
o
9 -
0
==8-
o
~7
~6 "~ 5 or) 4 3 2 1 0
.
0
,
500
.
,
1000
.
,
1500
.
.
.
2 0 0 5 0 02
.
.
0
,
3000
Pressure, kPa Fig. 4. Adsorption selectivity of ethane relative to methane in MCM-41 (pore diameter = 4.1 nm) at 264.75 K and a composition of 59.1% methane. Figure 4 shows the selectivity of ethane over methane in the same MCM-41 material at 264.75 K. As the selectivity is a ratio of mole fractions (see Equation (4)), it is sensitive to small absolute errors in the predicted amount adsorbed, and is thus a very demanding test of the quality of the predictions. Both IAST and GCMC give good results at moderate pressures, though the predictions become progressively poorer at high pressure. In addition, GCMC (but not IAST) gives poor predictions at low pressure. The MCM-41 model used in the GCMC simulations is a regular array of oxygen atoms, which omits the surface heterogeneity of MCM-41. As this kind of heterogeneity is important for gas adsorption, especially at low pressure, this is likely to be the reason for the poor prediction of selectivity in the GCMC simulation at low pressure. We are currently developing models of MCM-41 with a more disordered (and hence more realistic) surface. 2.
Ethane/carbon dioxide mixture
Mixture adsorption experiments of ethane and carbon dioxide on MCM-41 (pore diameter = 4.1 nm and 3.6 nm) have been carried out and IAST has been applied to these systems. The results are presented in Figures 5-8. IAST predicts the amount adsorbed quite well at low and moderate pressures (Figures 5 and 7), but slightly overestimates both the ethane and carbon dioxide adsorption in the relatively high-pressure range. This may be because of the chemical dissimilarity of the adsorbates, which cause the adsorption behaviour to be nonideal. On the other hand IAST gave quite good results for the selectivity of carbon dioxide over ethane for the whole pressure range (Figures 6 and 8). Thus, the deviation from IAST for both components is similar, suggesting that they have comparable activity coefficients. This nonideal behaviour of ethane/carbon dioxide mixture will guide future GCMC simulations.
690 12 i
]]
,
i
.
I
,
I
,
I
,
I
,
i
.
I
,
I
,
I
.......... IAST, carbon dioxide i
[]
.............. L A S T , e t h a n e lo.
~
IAST,
9 .
A
Exp. carbon
8
o
Exp. ethane
7
[]
Exp. Total
~6-
dioxide j
Yco = 0 - 4 7 0 6
/
5
~f
~ u
Total
a
n
n
.............:~::g~" ll~"
~
4
2.
.......
0
.
0
.......~ i 2 .-~'-
........
.
'
400 '
200
'
600 '
800 '
'
1000 '
'
1200 '
'
14 ' 0 0 '
16 ' 0 0 '
1 ~'00
Pressure, kPa
Fig. 5. Adsorption isotherms for the binary mixture of ethane/carbon dioxide in MCM-41 (pore diameter = 3.6 nm) at 264.55 K and a composition of 47.06% carbon dioxide.
1.0 O
0.8
O 0.6
0.4 r~
o
0.2
0.0
o
9
I
~oo
'
I
,oo
'
I
~oo
"
I
~oo
.
.
.
.
IAST Exp.
.
lO'OO 12'oo ,4'oo 16'oo 18'oo
Pressure, kPa
Fig. 6. Adsorption selectivity of carbon dioxide relative to ethane in MCM-41 (pore diameter = 3.6 nm) at 264.55 K and a composition of 47.06% carbon dioxide.
691 '
16
I
'
I
'
I
'
I
'
I
'
.......... lAST, carbon dioxide ............... IAST, ethane lAST, Total Lx Exp., carbon dioxide o Exp., ethane
14 12 10
[]
I
'
I
'
I
/
...... ...
/
.......................
I.--'~ .~--~'-"-'"
::::: ....
'
[]
J
6
I
/
Exp., Total
Yr =0-6128
'
~
. ....................... -'"
. ....... ..-"'" .....
0
..........................o
0 ............ 0
'
0
I
200
' 400 ' " ' 600 ' ' ' 800 '
1000
' ~1o o '
' 1400
' 1 goo'
1 goo' 2 0 0 0
Pressure, kPa Fig. 7. Adsorption isotherms for the binary mixture of ethane/carbon dioxide in MCM-41 (pore diameter = 4.1 nm) at 264.75 K and a composition of 61.28% carbon dioxide.
1.0
_
"
I
"
I
"
I
'
I
0.8
o o
0.6
0.4 [/3
IAST o
0.2
0.0 0
9
I
200
9
=
400
9
,
600
'
=
800
'
, 1000
9 , 1200
9 , 1400
9
/
EXP.
9
160800 1
.
0
9
2000
Pressure, kPa Fig. 8. Adsorption selectivity of carbon dioxide relative to ethane in MCM-41 (pore diameter = 4.1 nm) at 264.75 K and a composition of 61.28% carbon dioxide.
692 CONCLUSIONS We have reported experimental data for the adsorption of ethane/methane and ethane/carbon dioxide mixtures in MCM-41 samples at 264.55 K. For the adsorption of the methane/ethane mixture in MCM-41, we have demonstrated that lAST gives generally accurate predictions of multicomponent adsorption equilibrium. For these components, GCMC simulation gives accurate predictions for both pure-gas (see Ref. 3) and binary adsorption, supporting the usefulness of our simple model for MCM-41, which ignores surface heterogeneity. The predictive capability of GCMC simulation supports its use as a design tool for adsorbents in general, particularly given (i) the slight experimental data requirements of GCMC relative to IAST, and (ii) the ability of GCMC to investigate adsorption in hypothetical adsorbents for which experimental data are not available. For the ethane/carbon dioxide mixture in MCM-41, lAST gives good predictions for the mixture adsorption at low and moderate pressures, but exhibits some deviations at high pressures. This non-ideal behaviour might be due to the chemical dissimilarity of the adsorptive species, which will be taken into account in subsequent GCMC simulations. Nevertheless, lAST gives very accurate selectivity results for this mixture over the whole pressure range.
ACKNOWLEDGEMENTS:
We are grateful to Prof. G. Seo (Chonnam National University, Korea) for providing the MCM-41 samples. LITERATURE CITED: 1. Kresge, C.T.; Leonowicz, M. E.; Roth, W.J., Vartuli, J.C.; Beck, J.S., Nature, 359 (1992), 710. 2. J.S. Beck; J.C. Vartuli; W.J. Roth; Leonowica, M.E.; Kresge, C.T.; Schmitt, K.D.;Chu, C. T-W.; Olsen, D.H.; Sheppard, E.W.; McCullen, S.B.; Hinggins, J.B.; Cshlenkerm J.L.J. Am. Chem. Soc., 114 (1992), 10834. 3. Yun, J.H.; Diiren, T.; Keil, F.; Seaton, N. A. Langmuir, 18 (2002), 2693. 4. Myers, A.L.; Prausnitz, J.L. AIChE J., 11 (1965), 121. 5. Allen, M. P.; Tildesley, D. J. Computer simulations of liquids; Clarendon Press: Oxford, 1987. 6. Frenkel, D.; Smit, B. Understanding molecular simulations; Academic Press: San Diego, 1996 7. Fischer, J.; Heinbuch, U., Wendland, M. Molec. Phys. 61 (1987), 953. 8. Koh, C.A.; Montanari, R.I.; Nooney, S.F.; Tahir, S.F.; Westacott, R.E. Langmuir 15 ( 1999 ), 6043.
Studxes in Surface Science and Catalysis 144 F. Rodriquez-Reinoso, B. McEnaney, J. Rouquerol and K.K. Unger (Editors) ~ 2002 Elsevier Science B.V All fights reserved
693
Nondestructive Characterization of Porous Ceramics by X-Ray Refraction K.-W. Harbich, F. E. Fensch-Kleemann, B. R6hl-Kuhn and P. Klobes Federal Institute for Materials Research and Testing, BAM Berlin, Germany X-ray refraction reveals the inner surface concentrations of porous materials within the range of gm- to nm-dimensions. This technique is compared to mercury pressure porosimetry which is a standard method for pore size determination in porous solids. The mean pore size of several ceramics are investigated over a range of porosity between 30% to 60%. Complementary to high pressure mercury intrusion the refraction technique detects the spatially resolved inner surfaces of both, open and closed pores. Furthermore, X-ray refraction scanning topography visualizes integral interface properties up to 50 gm spatial resolution by two-dimensional topographic images. Characteristic micro structure parameters, i. e. inner surface density, porosity and mean pore sizes of some A1203 and SiC ceramics are presented. The specific surface data of A1203 ceramics obtained from X-ray refraction agree well with the results given by mercury porosimetry. In contrast, the related mean pore sizes differs systematically by about 30%.
Introduction X-ray refraction techniques combine analytical capabilities of sub-micrometer structure detection with the requirements of full volume characterization. The method therefore might give a better understanding of agglomeration of pores and pore size fluctuations in porous materials with respect to different states of porosities. The effect of X-ray refraction provides unconventional small angle X-ray scattering techniques which have been applied to meet actual demands for improved nondestructive characterization of advanced high performance ceramics and other heterogeneous materials. The method is based on refraction of X-rays caused by interfaces of microstructures (pores and micro-cracks) in heterogeneous solids. Among the micro structural requirements for porous materials the size of pores and their distribution are two important parameters. X-ray refraction topography in conjunction with Xray scanning radiometry is a nondestructive method to examine the interfacial properties of pores in ceramic materials with open and closed porosity. The analysis of the computer assisted scanning topographies shows that the 2-dim topographic images of porosity and the related inner surface density frequently do not correspond to each other. From our experimental results we suggest the fact of strong pore size fluctuations at regions of nearly constant porosity. The information solely given by the distribution of the mean porosity ot~en is not sufficient to characterize the clusters of different pore sizes as occurring in some ceramic samples. The topographic analysis of the inner surface density in combination with simultaneous measured scanning radiometry permits to control the spatial distribution of the local mean pore size in correlation to inhomogeneities of ceramic porosities.
694 The X-ray refraction effect The physics of X-ray refraction is analogous to the well known refraction of visible light by optical lenses and prisms which generally is governed by Snell's law. The major difference from optics is given by the refractive index n of X-rays in matter. As n is nearly one, deflection is caused at very small angles up to a few minutes of arc. The dependence on the refractive index involves a refraction intensity IR which increases with the square of the electron density difference between interfaces, for example at the interface of a pore and the surrounding solid material. This is in fundamental contrast to porosity measurements which are based on the physics of X-ray absorption : depending on the electron density of the solid material the absorption level IA decreases exponentially. The experimental equipment requires a standard fine structure X-ray generator operated usually with monochromatic Ka-radiation. The measurements of the refraction effect are taken by using a commercial small angle X-ray camera of the Kratky type in combination with two scintillation detectors for simultaneous detection of X-ray refraction intensity IR and sample absorption IA. A standard DOS-computer handles the scattering intensity data acquisition and the micromanipulator scanning-system. Figure 1 shows the experimental setup.
Figure 1: Instrumental setup for simultaneous 2-dim detection porosity and inner surface density. The determination of an average refraction value C of a porous material for analytical purposes is referred to as refractometry [1,2]. It expresses the quantitative X-ray refraction response by a selected domain. This designation corresponds to the well known term Radiometry in nondestructive testing, which characterizes the X-ray absorption behavior. In practice we get the relevant data by scanning the sample over a sufficiently long distance. The essential refraction factor C represents a relative measure for the inner surface density of a ceramic sample. It is determined by the relation of the refraction intensity level IR and the intensity of the attenuated signal IA. Generally the refraction value is proportional to the ratio of pore surface S to pore volume V. To determine the mean pore sizes with respect to absolute values, the refraction value C has to be calibrated by a reference measurement of standard powder with known internal surface density and packing density. Due to the short wavelength of X-rays this method can be applied universally to determine the specific internal surfaces and interfaces down to nanometer dimensions.
695
Quantitative X-ray refraction The angular intensity distribution of X-rays refracted by fibers or spherical pores has been approximated by Hentschel et al. [3]. Quantitative X-ray refraction analysis is implemented at a constant scattering angle of about two minutes of arc and at a fixed wavelength. The refraction value C is proportional to the absolute internal surface density S. With dp denoting the sample thickness the relationship can be given by: C = (1/dp)[(IR/I,O - 11
w i t h C = k ,F,
(1)
The apparatus constant k is obtained by calibration measurement. With the measured attenuation IA = I0 exp(-~tdp) the porosity p is simply determined. I0 represents the intensity of the primary beam. To calculate the porosity p one gets the condition : (2)
p = 1 - (It/luo)
The ratio ~t/~t0expresses the density of the porous ceramic in relation to the solid body. The value of ~t0 can be calculated from listed tables. In consideration of the porosity p and the model of uniform spheres, the surface to volume relation S/V finally yields an expression for the radius R of a pore : (3)
R = 3 p/,~
With this equation the pore size R as a characteristic length is estimated from the measured inner surface density Y~. In our definition the unit of the inner surface density ~; is given by m2/cm3. Inserting the density of the solid phase p of the sample permits to calculate the specific surface Ssp of the ceramics [4]: Ssp - 2 7 / p ( l - p ) , w h e r e p (1 - p )
= pbutk = m a s s / s a m p l e v o l u m e
(4)
Figure 2 illustrates the refraction effect by a model of a solid material with different pore concentrations per unit volume.
Fig 2: Schematic refraction effect of X-rays passing a porous sample. In correlation to the specific surface per unit volume the refraction intensity IR depends on the pore sizes and their concentrations. IA is the attenuated intensity of the primary beam. Due to the interface concentrations the intensity of the deflected beam in a porous ceramic corresponds with the inner surface density of the material.
696 The fundamental difference between X-ray radiometry and refraction technique is demonstrated by analyzing a sample of uniform spherical particles and a constant porosity. Figure 3 demonstrates the effects by a related model structure.
Figure 3 : With exception of small density fluctuations the 2-dim projection image of the mean porosity (middle) does not contain informations on the significant different inner surfaces (topograph below) due to the corresponding particle diameters (top). The diameters d of the uniform spherical glass particles are 0,25 ~tm and 1,20 ~trn, respectively. The arrangement of the small particles has the same package density as the big ones. The image of the porosity represents the attenuation of the beam intensity in a real measurement. It shows the same mean porosity in both sample areas with exception of small density fluctuations. The precise separation of the inner surface becomes obvious in the refraction topograph. It visualizes the selective contrast of different surface concentrations due to the different number and size of particles per unit volume. The comparison of both topographs make clear that the absorption image of the porosity does not contain any information on the significant different internal surfaces. Independently from the kind of model - either glass particles in air or pores in ceramics- in each case the refraction index at the interface determines the relevant refraction signal. The variations are exposed by the colours from blue to red. The range is marked by the colour index bar.
Experimental results The following measurement of the monochromatic scanning topography of a SiC-sample with the mass density p = 3,22 g/cm3 represents the spatially resolved properties of the mean porosity. The local porosity p variation is in the range of about +/- 3 % . It decreases from p= 62 % on the right side (which represents the centre of the sample) to p -- 58 % at the left perimeter.
697 Figure 4 : Spatially resolved scanning topographies of a SiC-sample with minor fluctuations of porosity p (x,y), (lett) and more significant pore size variations resulting exclusively from the measured inner surface density E (x,y), (right). The pore size d (x,y) was calculated using equation (3) with a constant shape factor 3, which denotes the spherical pore model. From the refraction measurements the related variation of the pore size is estimated in the order of about +/- 30 %. It increases from R = 6,0 ~tm at the left to R = 12,0 ~tm in the middle of the sample. With respect to equation (3) for obvious reasons these extent fluctuations in pore size are determined exclusively by the measured fluctuations of the internal surface density E. The constant shape factor 3 denotes the spherical pore model. Since the porosity variations are negligible compared with Z (x,y) the pore size image is given by the inverse of the internal surface density. Both, the local mean porosity and the local pore size are determined by the averages within the scattering volume of 0,05 mm x 0,3 mm times the sample thickness. The inspection of the marked area of Fig. 4 shows that the local porosity and local pore size do not correlate to each other. The relevant differences are demonstrated by analyzing the cross-sections. The inner surface density X obviously leads to significant different informations about the micro structure configuration than the local mean porosity. These results suggest strong fluctuations of inner surface densities within regions of nearly constant porosity. Compared with conventional radiography the method of monochromatic scanning radiometry permits a higher contrast of about a factor of ten with respect to the detection of structure inhomogeneities.
Figure 5: Spatially resolved 2D-topographs (8 mm x 12 mm) of porosity (lett) and poresize (right) of AlzO3-ceramic reference material with mean porosity p = 30,1%. The sample thickness is dp = 1 mm. The 2D plots of porosity and pore size show uncorrelated fluctuations. The topographs in figure 5 show the spatially resolved porosity distribution and the corresponding mean pore size distribution of an A1203 ceramic sample (with a mass density of 9 = 3,97 g/cm3 ). The mean pore size d is varying in the range of 2,3 ~m < d < 2,9 l~m. As in the examples shown above there is no correlation with respect to the porosity image. The scaling of the pore size image results from calibration by a powder of known inner surface density. With equation (3) and the assumption of a spherical pore model from E(x,y).the mean value of the pore size d(x,y) is calculated to d - 2,5 l~m. This value differs from d = 2,0 lam as measured by high pressure mercury intrusion. The results of both methods become more
698 reliable when comparing the determination of the corresponding specific surface. The inner surface density Z measured by X-ray refraction is pure of any model assumption. Using equation (4) we found Ssp = 0,26 m2/g which do agree much better with S = 0,28 m2/g as measured by mercury intrusion. The results of three A1203-samples with different mean porosities are listed in figure 6.
Fig. 6 : Comparison between X-ray refraction and high pressure mercury intrusion: results of mean pore sizes and specific surface corresponding to A1203-samples of varying mean porosities. The differences of the calculated mean pore sizes may be explainable by considering the different pore models. The specific surface Z measured by X-ray refraction solely is caused by geometric characteristics of the sample and agree well with the results of mercury porosimetry using a cylindrical capillary model. It is obvious that the results of the specific surface are in better agreement than those of the related pore sizes which differs systematically from each other by about 30%. This is in correspondence with earlier investigations [5]. Because the estimation of the specific surface by X-ray refraction only is caused by geometric characteristics, the comparison between the listed results may give some aspects to discuss the reliability and the usability of different pore shape models. The good agreement of the specific surface obtained from refraction on the one side and mercury porosimetry on the other side in this case suggests the validity of the cylindrical capillary model. Regarding equation (3) make clear that the shape factor of the spherical pore model should be reduced from 3 to about 2,2 to fred the same mean pore size as measured by mercury intrusion. Conclusions
X-ray refraction techniques combine analytical capabilities of sub-micrometer structure detection with the requirements of nondestructive spatially resolved full volume characterization. The method therefore might give a better understanding of agglomeration of pores and pore size fluctuations with respect to different states of porosity, especially when
699 other methods are less specific. Two-dimensional refraction topography will be useful to analyze ceramic densification processes and to image inner surface, interface and crack densities. The image analysis of the inner surface density in combination with simultaneous scanning radiometry permits to evaluate the spatial distribution of the local mean pore size in correlation to porosity properties. In addition to integral intrusion techniques the X-ray refraction method with selective contrast of inner surface concentrations detects both, open and closed porosity. Because the determination of geometric characteristics of porous materials are not limited to the porosity alone but supplemented by the local distribution of the specific internal surface the refraction method may also be helpful to support microstructure analysis obtained from reconstruction models. Future attempts are necessary to visualize geometric quantities with higher spatial resolution, especially to optimize the determination of the characteristic length of pore size properties. References
[ 1] K.-W. Harbich, M.P. Hentschel and A. Lange: "New X-ray refractometry for nondestructive characterization of high performance composites and ceramics." In: Nondestructive Testing, Rotterdam/Balkema, (1995) 207. [2] U. Mi~cke, K.-W. Harbich and T. Rabe: "Determination of pore sizes on sintered ceramic materials using image analysis and X-ray refraction." Ceramic Forum International/Ber. d. DKG 74 (1997) 95. [3] M.P.Hentschel, R. Hosemann, A. Lange, B. Uther and R. Brt~ckner: "R~ntgenkleinwinkelbrechung an Metalldr~ihten, Glasf~iden und hartelastischem Polypropylen". Acta Cryst. A34 (!987) 506. [4] K.-W. Harbich, M.P. Hentschel and J. Schors: "X-ray refraction characterization of nonmetallic materials." NDT & E International 34 (2001) 297. [5] K.-W. Harbich, K. Meyer, A. Lange, M.P. Hentschel: "Poren in Keramik bestimmen." Materialprfifung 36/6 (1994) 234.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B, McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
701
Ion-exchange properties of a novel layered titanium(IV) phosphate O.A. Khainakova a, A. Espina a, C. Trobajo a, S.A. Khainakov a, J.R. Garcia a and A.I. Bortun b aDepartamento de Quimica Org~nica e Inorgdnica, Universidad de Oviedo, 33006 Oviedo (Spain) bMagnesium Elektron, 500 Point Breeze Rd, Flemington, NJ 08822-9111 (USA)
A novel layered titanium(IV) phosphate, Ti202H(PO4)[(NH4)2PO412 (1), was synthesized in the system TiC14-H3PO4-(NHE)zCO-H20 under mild hydrothermal conditions. The ionexchange behavior of 1 towards alkali, alkaline earth and some divalent metal cations in individual and complex solutions was studied. It was found that, unlike other known titanium phosphates, 1 exhibits an unusually high affinity for the mercury ion, which makes it promising for some specific separations. The porous structure of the compound does not undergo substantial changes as a result of removing the lattice of either ammonia and water. I. INTRODUCTION Due to properties such as thermal, radiation and chemical stability, resistance to oxidation and developed porosity, insoluble acid salts of Group IV elements (phosphates, silicates, arsenates) have received a great deal of attention as selective ion exchangers [1 ], promising catalysts [2], ionic conductors [3], materials for non-linear optics [4], radioactive waste hosts [5] and low thermal expansion materials [6]. Among these salts, titanium-based compounds, and especially titanium phosphates, hold a prominent place. Although only two basic types of titanium phosphates are known, o~-Ti(HPO4)2"H20 and 7-Ti(PO4)(HEPO4).2H20 [7], a large number of their derivatives, including partial and completely substituted ion-exchanged forms and phases with different extents of hydration, has been reported [8]. Recently, renewed attention has been given to so-called "soft chemistry" methods of synthesis of new metastable materials [9]. The synthesis of new microporous materials containing transition metals in the framework is of growing interest due to the expected catalytic redox properties [ 10]. The microporous titanium(IV) silicates [ 11] discovered have already proven the concept by showing very good catalytic activities and are widely used nowadays [12]. Similarly, hydrothermally synthesized titanium phosphates with openframework or layered structures are attracting attention as potential materials with similar properties [ 13]. The investigation of the formation of metastable phosphate phases under hydrothermal conditions has shown the different factors that control synthesis of a specific compound [ 14]. Among these factors are the molar ratio P:Ti in the reaction mixture, the concentration of phosphoric acid and pH. This paper reports the synthesis of a novel layered compound, Ti202H(PO4)[(NH4)2PO412, under mild hydrothermal conditions in the system Ti(IV)-HaPO4(NHE)2CO-H20and data on its characterization.
702 2. EXPERIMENTAL All chemicals used were of reagent grade and were used without further purification. In the synthesis of the novel phosphate, a freshly prepared 2 M aqueous solution of TiCI4 was used as an initial source of titanium and 85% H3PO4 was used as the source of phosphorus. The experimental procedure first included the preparation of a H3PO4-(NH2)2CO solution with an (NH2)2CO:P molar ratio of 2:1 followed by mixing of titanium and phosphorus-containing solutions in a molar ratio P:Ti = 20:1 in a 100 mL stainless steel Teflon-lined vessel. The total volume of a reaction mixture was 60 mL. The reaction mixture was then sealed and heated at 190~ for 5 days. The obtained product was filtered, washed with water and dried in air at 60~ Elemental analysis (% wt.): Ti - 19.7, P - 19.0, N - 10.7 (calculated for TizO2H(POn)[(NHn)2PO412: Ti - 19.72, P - 19.14, N - 11.11). The phosphorus, titanium and alkali metals content in the solids was determined using a SpectraSpec Spectrometer DCP-AEC after dissolving a weighed amount of the sample in HF(aq). Microanalytical data (C, H, and N) was obtained using a Perkin-Elmer model 2400B elemental analyzer. Thermal analysis was performed on a Mettler TA4000-TG50 (nitrogen atmosphere, heating rate 10~ min-~). The infrared spectra were recorded with a PerkinElmer 1000 FT-IR spectrophotometer. N2 adsorption-desorption isotherms were recorded at 77 K using an automatic volumetric system equipped with a turbomolecular pump (~10 -5 Torr) Micromeritics ASAP 2000 instrument. SEM micrographs were recorded on a JEOL JSM-6100 electron microscope operating at 20 kV. Ion exchange of alkali and alkaline earth metal cations was studied using 10.3 M M(NO3)n (M = Li, Na, K, Cs, Mg, Ca, Sr, Ba, Cd, Co, Cu, Hg, Ni, Mn, Pb, Zn; n=l,2) solutions at V:m = 200:1 (mL:g) and 0.1 M M(NO3)n solutions at V:m = 100:1, 25~ 5 days contact time. Uptake of some divalent ions from groundwater simulant and Ringer solution was studied by the batch method at V:m ratio (200-5000):1 (mL/g), ambient temperature and 6 days contact. The groundwater simulant contained (in mg/L): Ca 2+ - 100, Mg 2+ - 10, Na + 15 and M 2§ - 6.0-9.0. The Ringer solution [ 15] contained (in g/L): NaC1 - 8, NaHCO3 - 0.2, KC1 - 0.2, CaCI2 - 0.2, glucose - 1.0 and M 2+ - 4.0-6.0 ppm. The pH of the solutions after equilibration with the adsorbent was measured using a WTW-538 pH meter. The initial and final concentrations of all metal ions in solutions were measured using a Varian SpectrAA300 atomic absorption spectrometer. The affinity of the exchanger for ions of interest was expressed through the distribution coefficient (Kd, mL/g) values, which were obtained according to the formula Ka = [(Co-Ce)/Ce]'V/m, where Co and Ce are the ion concentrations in the initial solution and in the solution after equilibration with adsorbent, respectively, and V/m is the volume to mass ratio. 3. RESULTS AND DISCUSSION On studying the hydrothermal reactions in the system TiC14-H3PO4-~H2)2CO-H20, the formation of several different titanium(IV) phosphate products was found to depend both on the molar ratio of titanium- and phosphorus-containing reagents in the reaction mixture as well as on the acidity of the solution. Regulation of the acidity of the reaction system was achieved by adding different amounts of urea, which decomposes gradually with the release of NH3. Fig. 1 shows the ranges of concentration used for the preparation of individual crystalline phases.
703
2.0 i
.
._o 1.5L_
I I,,,,..
E
1.0-A
~j 0 . 5 -
B 1
1
D I
2
F
E I
4
I
6
.1 .....
10 P/Ti motar ratio
I
15
G
20
Fig. 1. Phase diagram for the formation of titanium(IV) phosphates by reaction of TiC14(aq) and H3PO4(aq) in the presence of urea: TiO2 (anatase) + Ti(OH)PO4 (A), Ti(OH)PO4 (B), NH4TiOPO4 (C), TiOz(PO4)z'2H20 (D), (NH4)z[Ti3Oz(HPO4)(PO4)2] (E), y-TiNH4H(PO4)2 (F), y-Ti(NH4)o.sH1.5(PO4)2"0.5H20 (G) and Ti2OaH(PO4)[(NH4)2PO412 (H). The crystallinity of Ti202H(PO4)[(NH4)2PO412 (1) was sufficiently developed to solve the structure of this compound from its synchrotron X-ray powder diffraction data [16]. The unit cell is orthorhombic, space group Pinna, with a = 6.3943(8), b = 10.106(1), c = 10.850(2) A, and Z - 2. Two different crystallographic forms of phosphorus exist corresponding to orthophosphate and dihydrogenphosphate groups. There are also two crystallographically different titanium atoms joined alternatively by oxo bridges forming one-dimensional chains advancing in zig-zag along the c-axis. These chains are then tied together into layers by the orthophosphate group. The layers contain cavities in which half the ammonium ions reside. The remaining ammonium ions are located in the interlayer space. The position of the hydrogen atom is not evident from the diffraction experimental data. The electron micrograph (Fig. 2) shows how the compound is constituted by platelets of a pseudohexagonal form. Many of the platelets are corroded to an almost circular shape, with a plate diameter ranging from 1 to 5 lam.
Fig. 2. Scanning electron microscopy image of 1 (x3000 magnification).
704
4000
3500
3000
2500
2000
1500
1000
500
Wavenumt~r (cm-1)
Fig. 3. Infrared spectrum of 1. The IR spectrum of the ammonium titanium phosphate is presented in Fig. 3. As can be seen, the adsorption bands are at 532, 557, 639, 791, 1023, 1079, 1405, 1636 and 3210 c m "1. The intense vibrational band at 1023 c m "1 indicates the presence of highly symmetric PO4 [ 17]. The medium intense bands in the OH stretching region might arise from the presence of physically bound water or/and free hydroxy groups. The presence of ammonium manifests itself by bands at 1405 c m -1 and 2800-3250 cm- regions. The total weight loss for Ti202H(PO4)[(NH4)2PO412 (Fig. 4) is 23.3% (calculated: 23.26%). The weight release occurs in three overlapped steps and the final product is TiP207 mixed with (TiO)2P207 [18]. Thermal decomposition does not provoke microporosity in the interlayer space. The 77 K N2 adsorption-desorption isotherm shape of samples is independent of the thermal treatment temperature. In all cases, this corresponds to type II of the BDDT classification [ 19], which indicates that the adsorption process takes place in a mono-multilayer with negligible porosity (Fig. 5). Application of the BET method to the adsorption branches gives rise to values o f SBET (in m2g "l) of 7.3 (120~ 8.0 (250~ and 8.1 (450~
0 DIG 5
10
TG
100
200
300
400
500
600
Temperature(~
Fig. 4. Thermal analysis data: TG and DTG curves for 1.
700
800
900
705
16
III
adsorp .~on
14
m
4 2 fl
O0
0,2
04
0.6
10
08
plpO
Fig. 5. N2/77K adsorption-desorption isotherm of 1 outgassed at 120~ The presence of the ammonium-substituted H2PO4 functional groups in 1 suggests that the new compound could exhibit ion-exchange properties. The theoretical ion-exchange capacity (IEC) is 8.23 meq g-], assuming that all ammonium ions are exchangeable. The ion exchange affinity towards alkali, alkaline earth and some other divalent metals are presented in Table 1. Low values of metal uptake (less than 0.5 meq g~, with the exception of Cu 2+ and Hg 2+ ions) indicate that 1 has an extremely high affinity for the ammonium ions, which cannot be easily replaced by other cations. The high affinity for NH4 + is also confirmed by the fact that among alkali metal cations, the compound preferably uptakes potassium and not cesium or sodium ions, which affords some evidence that its ionexchange sites fit ammonium and potassium ions best (due to having close ionic radii). Table 1 Distribution coefficient values (Kd, mL/g) determined in 10.3 M metal nitrate solutions and ion exchange capacities (IEC, meq/g) determined in 0.1 M solutions. Ion
Ka
IEC
Ion
Kd
IEC
Li +
30 0.10
Cd 2+
36000
0.28
Na +
120 0.15
Co 2+
22000
0.55
K+
960
0.25
Cu 2+
>100000
1.34
70 0.05
Hg 2+
43400
1.09"
Mg 2+
200 0.05
Ni 2+
13500
0.10
Ca 2+
860 0.05
Mn 2+
41100
0.35
Sr2+
1500 0.20
Pb 2+
26000
0.25
Ba 2+
2800
Zn 2+
22000
0.50
Cs +
0.20
*Exch~mge from 0.01 MHg(NO3)2 solution, V : m - 200"1 (mL:g).
706
A preferential uptake of the Ba 2+ ion (due to having an ionic radius close to that of the ammonium ion) among the alkaline earth metals also confirms this hypothesis. At the same time, the affinity for other divalent elements is at least one order of magnitude higher than that for alkali or alkaline earth metal cations. The Kd values for most transition metals and lead are in the range from 20000 to more than 100000. However, the absolute values of their uptake in 0.1 M metal nitrate solutions are relatively low. The IEC values are higher than 1.0 meq g-1 for only two ions, Cu 2+ and Hg 2+. A high affinity of polyvalent metal phosphates for divalent cations is not an unusual fact [ 1]. There are dozens of known exchangers with much higher capacities than that found for the 1. Nevertheless, this layered compound is an extremely interesting ion-exchanger. He shows a uniquely high affinity for the mercury(II) ion, while all other known titanium phosphate or titanium arsenate sorbents do not exhibit any preference for the Hg 2+ cation. For example, 1 retains a high affinity for mercury even in the presence of electrolytes. This is illustrated by the Kd values of Hg 2+ and Cd 2+ ions uptake by the TiP as a function of sodium nitrate concentration in solution (Fig. 6). The KdHg values are very high (> 10000 mL g-l) in the presence of a 100-fold excess of the sodium ion. Even higher Kd values were found for cadmium sorption. A gradual decrease in the Kd values as the concentration of NaNO3 increases suggests that Hg 2+ and Cd 2+ ions are taken up by the ion-exchange mechanism. The high affinity of 1 for some transition elements suggests the possibility of using it for selective removal of toxic heavy metals from contaminated environmental systems and biological liquors. This point is illustrated by the uptake of Pb 2+, Cu 2+, Cd 2+ and Hg 2+ ions in groundwater simulant (Fig. 7a) and Cu 2+ and Cd 2+ ions uptake in the Ringer solution (Fig. 7b).
10 5
10 4
10 3
10 2
---13-- Cd z+ --0-- Hg 2*
,
10-2
. . . . . . . .
I
.
.
.
.
.
.
.
.
10 "1
.
.
.
.
.
10 ~
.
.
.
101
NAN03, M
Fig 6. Ion-exchange behaviour of 1" Kd values (mL/g) of Cd 2+ and Hg 2+ uptake in the presence of NaNO3 electrolyte.
707
100
100
80
80 .,'--
.E 60
> O
> 0
E
E
40
20
60 - o - c u 2" ]
4O
---0- C~"
20
~o-----____~
, 0
.
.
.
.
0
i
2000
,
,
,
,
I
4000
i
,
.
,
,
,
I
|
|
i
.
i
,
6000
V:m Ratio, mug
1000
2000
V'rn Ratio, mug
Fig. 7. Ion-exchange behaviour of 1: a) Pb 2+, Cu 2+, Cd 2+ and Hg 2+ removal from the groundwater simulant, and b) Cu 2§ and Cd 2+ ion removal from the Ringer solution. It was observed that 1 is able to remove almost all the lead from 3000 volumes and copper ions from 1000 volumes of groundwater simulant, while its efficiency for Cd 2+ and Hg 2+ ions removal is much lower (200-500 volumes). The presence of organic compounds in the Ringer solution affects somewhat the affinity of the ammonium-titanium phosphate for divalent metals. Nevertheless, it removes the Cu 2+ ion efficiently from 1000 volumes of this biological liquor and the Cd 2+ ion from 200 volumes. All together, these results indicate that the novel ammonium-titanium phosphate, despite its relatively low exchange capacity, exhibits a high affinity for some toxic heavy elements, which makes it promising for some specific separations.
Acknowledgement. This work was funded by the Ministerio de Ciencia y Tecnologia (Spain), Research Project No. MAT2000-1654.
REFERENCES [1]
[21 [31 [4] [5] [6] [7]
A. Clearfield (ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982; G. Alberti, in Recent Developments in Ion Exchange, P.A. Williams and M.J. Hudson (Eds.), Elsevier, London, 1987; R.G. Anthony, C.V. Philip and R.G. Dosch, Waste Management, 13 (1993) 503; A. Clearfield, Progress Inorg. Chem. 47 (1998) 373. S. Cheng and A. Clearfield, J. Catal., 94 (1985) 455. J-M. Winand, M. Rulmont and P. Tarte, J. Solid State Chem., 93 (1991) 341. G.D. Stucky, M.L.F. Phillips and T.E. Gier, Chem. Mater., 1 (1989) 489; M.E. Hagerman and K.R. Poeppelmeir, Chem. Mater., 7 (1995) 602. R. Roy, E.R. Vance and J. Alamo, Mater. Res. Bull., 17 (1982) 585. J. Alamo and R. Roy, J. Am. Ceram. Soc., 63 (1984) 78; R. Roy, D.K. Agrawal, J. Alamo, and R.A. Roy, Mater. Res. Bull., 19 (1984) 471. A.N. Christensen, E.K. Andersen, I.G.K. Andersen, G. Alberti, M. Nielsen and M.S. Lehmann, Acta Chem. Scand., 44 (1990) 865; M.A. Salvad6, S. Garcia-Granda and J. Rodriguez, Mater. Sci. Forum, 166-169 (1994) 619; S. Bruque, A.A.G. Aranda, E.R.
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[8]
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Losilla, P. Olivera-Pastor and P. Maireles-Torres, Inorg. Chem., 34 (1995) 893; M.A. Salvad6, P. Pertierra, S. Gareia-Granda, J.R. Garcia, J. Rodriguez and M.T. Fem6.ndez-Diaz, Aeta Crystallogr. B 52 (1996) 896. A. Men6ndez, M. Bfireena, E. Jaimez, J.R. Gareia and J. Rodriguez, Chem. Mater., 5 (1993) 1078; A. Espina, E. Jaimez, S.A. Khainakov, C. Trobajo, J.R. Garcia and J. Rodriguez, Chem. Mater., 10 (1998) 2490; A. Espina, J.R. Garcia, J.M. Guil, E. Jaimez, J.B. Parra and J. Rodriguez, J. Phys. Chem. B, 102 (1998) 1713. K. Byrappa and M. Yoshimura, Handbook of Hydrothermal Technology, Noyes, New Jersey, 2001. I.W.C.E. Arends, R.A. Sheldon, M. Wallau and U. Schuchardt, Angew. Chem., Int. Ed. Engl., 36 (1997) 1145; M. Hartmann and L. Kevan, Chem. Rev., 99 (1999) 635. D.M. Chapman and A.L. Roe, Zeolites, 10 (1990) 730; M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347; W.T.A. Harrison, T.E. Gier and G.D. Stucky, Zeolites, 15 (1995) 408; G. Sankar, R.G. Bell, J.M. Thomas, M.W. Anderson, P.A. Wright and J. Rocha, J. Phys. Chem., 100 (1996) 449. T.K. Das, A.J. Chandwadkar and S. Sivasanker, J. Mol. Catal. A, 107 (1996) 199; C.L. Bianchi and V. Ragaini, J. Catal, 168 (1997) 70; T.K. Das, A.J. Chandwadkar and S. Sivasanker, Catal. Lett., 44 (1997) 113. Y.J. Li and M.S. Whittingham, Solid State Ionics, 63-65 (1993) 391; A.I. Bortun, L.N. Bortun, A. Clearfield, M.A. Villa-Garcia, J.R. Garcia and J. Rodriguez, J. Mater. Res., 11 (1996) 2490; A.I. Bortun, S.A. Khainakov, L.N. Bortun, D.M. Poojary, A. Clearfield, J. Rodriguez and J.R. Garcia, Chem. Mater., 9 (1997) 1805; A.M.K. Andersen and P. Norby, Inorg. Chem., 37 (1998) 4313; C. Serre and G. Ferey, J. Mater. Chem., 9 (1999) 579; S. Ekambaram, C. Serre, G. Ferey and S.C. Sevov, Chem. Mater., 12 (2000) 444; T. Loiseau, C. Mellot-Draznieks, C. Sassoye, S. Girard, N. Guillou, C. Huguenard, F. Taulelle and G. Ferey, J. Am. Chem. Soc., 123 (2001) 9642. M.A. Salvad6, P. Pertierra, S. Garcia-Granda, A. Espina, C. Trobajo, J.R. Garcia, Inorg. Chem., 38 (1999) 5944; C. Trobajo, A. Espina, E. Jaimez, S.A. Khainakov, J.R. Garcia, J. Chem. Soc., Dalton Trans. (2000) 787. S. Frankel and S. Reitman (Eds.), Clinical Laboratory Methods and Diagnosis, Vol. 1, Mosby, Saint Louis, 1963. S. Garcia-Granda, M.A. Salvad6, P. Pertierra, J.R. Garcia, A.I. Bortun, A. Clearfield, Mater. Sci. Forum, 378-381 (2001) 665. V.C. Farmer (Ed.), The Infrared Spectra of Minerals, Mineralogical Society, London, 1974. Powder Diffraction Files, International Center for Diffraction Data, Swarthmore, PA, 1996, no. 39-207 and 38-1468. K.S.W. Sing, D.H. Everett, R.A.W. Havl, L. Moscov, R.A. Pierotti, J. Rouquerol and T. Simieniewska, Pure Appl. Chem. 57 (1985) 603.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
709
The Anomalous Sorptive Behaviour Of Zsm-5 And Silicalite-I: Observation Of Low-Pressure Hysteresis In Nitrogen Adsorption Georgia Kyriakou, and Charis R. Theocharis*, Porous Solids Group, Department of Chemistry, University of Cyprus, P.O. Box 20537, 1678 Nicosia, Cyprus A first stage of a systematic study of the nature of the low-pressure hysteresis loop observed in the nitrogen adsorption isotherms of some MFI zeolites is presemed. It was shown that the pressure at which occurrence of this hysteresis loop takes place was linked to the presence of defect sites. The role of the exchangeable cation was also investigated, and it was suggested that in addition to the numbers of defect sites present, the strength of adsorbate-adsorbem interaction, and probably pore shape was also involved.
1. INTRODUCTION Zeolites have been of considerable practical and academic interest in the past several years because of their diverse applications as catalysts, ion-exchangers, sorbents, and latterly because of their environmental applications. It has also been proposed, that zeolites may be used as model sorbent materials that could be used for calibration and comparison purposes. A EU-funded project proposed the use of zeolites of the socalled MFI structural type, such as ZSM-5 and its silica analogue Silicalite I, for this purpose. The reason for this lay in the fact that synthetic routes existed that could consistently produce well-characterised samples with crystallite size in the region of 0.1 to 0.2mm [1]. Such samples would enable the measurement of adsorption isotherms that would not exhibit secondary capillary adsorption characteristics, or effects of activated adsorption. However, Carrott and Sing [2], Unger et al as well as Rouquerol et al observed that the nitrogen adsorption isotherms of such Silicalite or H-ZSM samples, exhibited low-pressure hysteresis loops at a p/pO value of 0.15 for the former, and close to 0.10 for the latter [1, 3, 4]. Clearly such a hysteresis loop could not be attributed to capillary condensation. Other ZSM-5 samples did not exhibit such behaviour towards nitrogen adsorption but did so towards other gases [5]. Silicalite I exhibited similar behaviour with adsorbates other than nitrogen. A theoretical simulation of the adsorption of argon in Silicalite failed to reproduce the above-mentioned step in the isotherm, but has nevertheless indicated that adsorption
710 in the micropores would proceed in at least two steps, the first one being localised adsorption on the strongest sites, and the second corresponding to molecules clustering around other adsorbed molecules [6,7]. The appearance of low-pressure hysteresis was attributed to a solid-liquid transition or a phase transition of the adsorbed phase in the porous system of the MFI lattice, and neutron diffraction data appear to corroborate this. Some work with ZSM-5 samples has indicated a correlation between the pressure at which the low-pressure hysteresis loop appeared as well as its height with the presence of aluminium [4]. Sing has discussed in detail the occurrence of low-pressure in hysteresis in silicalite in several articles [8,9]. A discussion of low pressure hysteresis was presented in [ 10]. However, no systematic work has been carried out on the nature of this unusual behaviour, especially of the factors influencing its position or shape. Furthermore, all data available thus far, were on large crystalline samples, while no data are available on commercially available ones. This paper presents work, as part of an ongoing investigation in this laboratory [11,12] on commercially available ZSM-5 samples, which attempts to clarify the nature of this anomalous sorptive behaviour. It presents a first attempt to study of the effect of changing exchangeable cations on the lowpressure sorptive behaviour of these solids. 2. EXPERIMENTAL
Nitrogen adsorption isotherms were carried out using a Micromeretics ASAP 2000 automated apparatus at 77K, from a starting pressure of 0.13Pa. Prior to adsorption isotherm determination, samples were outgassed at 0.13Pa overnight 573K for all samples, except for NH4§ ZSM-5 which was outgassed at 283K, a temperature at which the ammonium ion does not decompose. ZSM-5 and Silicalite samples used in this work were donated by Degussa and were free of binder. We chose to use ZSM-5 samples with Si:Al=120. Some data were also obtained on samples with Si:Al=80. The pristine form of ZSM-5 had Na + as counterions. Other counter-ions, specifically NH+4, Ca2+, Ce 4+ and Cu2+ were introduced by repeated overnight suspensions of the samples in appropriate solutions at room temperature. Thermogravimetric analysis was carded out in the region 300-1073K in flowing air using a Shimadzu TGA-50 apparatus. FTIR spectra were obtained using an 8500 Shimadzu Spectrometer, and DRIFTS spectra using a Spectratech attachment.
711
3. RESULTS Table 1 shows the nitrogen adsorption results for the zeolite samples with Si:AI =120 under investigation, whereas Figures 1 and 2, show the corresponding curves for samples Ca 2+ ZSM-5 and NH4+ ZSM-5.
Thermogravimetry was used for sample
NH4+ ZSM-5, to determine the optimum outgassing temperature for this sample. Figures 3-5 contain the DRIFTS spectra for the ZSM-5 samples with Si:AI=120 under investigation. The corresponding FTIR spectra have also been measured, but not included. SBET was estimated from the linear form of the well-known BET equation, and the pore volume from the total amount of nitrogen sorbed at p/p~
converted
to liquid.
Table 1
Sample
SBET (mZ/g)
Pore Volume (cm3/g)
Silicalite I
340
0.16
Low Pressure Hysteresis Yes
C a 2+ ZSM-5
327
0.15
NO
Cu 2~-ZSM-5
308
0.16
No
Na + ZSM-5
405
0.20
No
-NH4+ ZSM-5
327
0.17
Yes
C e 4+ ZSM-5
381
0.19
No
Outgassing temperatures: 573K, except for NH4+ ZSM-5 which was outgassed at 383K 4. DISCUSSION
A comparison of the differences in BET surface areas for the ZSM-5 samples with Si:Al=120 and different counter-ions in Table 1 above, cannot be completely rationalized in terms of the ionic radii, since NH4+ ZSM-5 with an ionic radius of 143 pm has a similar SBEX value with Ca 2§ ZSM-5 for which the ionic radius is 97 pm. However, ignoring Na §
C e 4+
ZSM-5 has the lowest ionic radius for the counter-ion
(92 pm) and the highest BET surface area value. Ions Na +, Ca 2§ and Cu 2§ have similar ionic radii but differing BET area values. Examination of the DRIFTS spectra in Figures 3-5 shows significant differences between the various samples in the region
712
around 3500 cmq where the OH stretching frequencies occur. The rest of the spectra were, as expected, almost identical. The differences between the OH stretching bands indicates that both the nature and concentration of structural OH groups or those associated with the exchangeable cations is different from sample to sample. Comparison of the intensity of the band at 1640 crn~, which is the H-O-H bend and thus characteristic of molecular water, in the various samples, suggests that the water content is the same for all of them, thus any differences in the OH stretching bands cannot be attributed to water.
Figure 1 shows the nitrogen adsorption isotherm for Ca2+ ZSM-5 with Si:AI=120, a typical Type I isotherm in the low-pressure range, but with a type H4 hysteresis loop at higher p/pO range. This is a typical isotherm for the rest of the Si:AI=120 samples, except for sample NH4+ ZSM-5 for which the nitrogen adsorption isotherm is shown
Ca 1 1 0
L
'
'
'
i
'
2+ '
ZSM-5 120 "
i
'
'
'
i
'
"
'
i
'
'
"
......... !
100 95 g0 85 80 0
0.2
0.4
0
0.6
0.8
1
pip Figure 1 Nitrogen Adsorption isotherm at 77K for Ca 2+ ZSM-5 outgassed at 573K in Figure 2. This sample by contrast shows a low pressure hysteresis loop centered at p/p~
This is the only sample examined which exhibited such a hysteresis loop.
The shape of the low-pressure hysteresis loop in Figure 2 is different to that reported by previous workers, or that found by this group for Silicalite I. In those cases, the loop was found to be squarer, indicating that the transition occurring within the sorbed species in the micropores occurred over a smaller pressure range [8]. A loop shape similar to that shown in Figure 2 was previously observed for samples with higher
713 aluminium content. In the present work, samples with higher aluminium content (Si:Al=80) did not exhibit a low-pressure hysteresis loop for any exchangeable cation.
NH + ZSM 5-120 4
110 105 100 95 90 85 80 75 0
0.2
0.4
p/p
0 0.6
0.8
1
Figure 2 Nitrogen Adsorption isotherm at 77K for NH4+ ZSM-5 outgassed at 383K The main systematic difference between the samples used in the present work and those used previously, was crystallite size. The samples used in the previous work consisted of large single crystals, whereas the present study was carried out using polycrystalline samples. The main difference between the nitrogen adsorption isotherms for the two sets of samples was in the pressure range in which the hysteresis loop occurred: for the single crystalline samples, this was at p/p~
and moved at
lower values with increasing aluminium content, whereas for the polycrystalline material, the hysteresis loop occurred at p/p~
and appeared to be unaffected by
aluminium content. These results, in view of the observations made from the DRIFTS spectra suggest that the position of the low-pressure hysteresis loop is influenced by the presence of defects in the crystals, which are present in greater numbers in a polycrystalline sample than a single crystal. These defects represent areas of different structure than the rest, and represent the so-called areas of strong adsorption. These are also usually associated with OH groups. Theoretical work by Nicholson et al [7] has suggested that adsorption in a tubular channel system such as that of an MFI zeolite proceeds at two steps. The first of these steps corresponds to localised adsorption at preferential high-energy sites, and the second, clustering around other
714
adsorbed molecules. Clearly, defect sites present preferred sites of adsorption, so since in the samples used here such sites were present in greater concentrations, a low-pressure hysteresis loop would be more prevalent than hitherto. In addition, the greater number of strong adsorption sites, would also have the effect of requiring a higher concentration of adsorbed molecules for clustering away from the walls. Such clustering would also occur more slowly in the presence of a higher concentration of defect sites, this making the clustering of the adsorbed molecules to occur a slower rate.
Figures 3 to 5 show the DRIFTS spectra for ZSM-5 with Si:AI=120 with different
30 25 C a Z S M 5 120 20 o~ 15
10
2950
2100 1/cm
1250
400
Figure 3 DRIFTS spectrum for Sample Ca2+-ZSM-5 counterions. The structural OH groups, ie those not associated with molecular water, are associated with Broensted surface acid sites (Si---O(H)---A1 groups), as well as defects. Given that no appreciable difference in water content has been observed for the samples discussed here, the differences in the OH groups present, are due, presumably, to the counter-ions occupying different positions in the channel system, associating themselves with different OH sites on the surface. This would alter the strength of the adsorbate-adsorbent interaction, but could also alter the shape of the
715
Cu ZSM 5 120
I-.
i
I
i
2950
3800
I
i
I
2100
,
,
I
1250
llcm
400
Figure 4 DRIFTS spectrum for Sample Cu2+-ZSM-5
30
NH4 Z S M 5 120
25,
I--
10
,,
3800
i
I
i
2950
I
2100 l/cm
f
I
1250
i
I
400
Figure 5 DRIFTS Spectrum ofNH4 + ZSM-5 Pores, thus explaining why for some counter-ions a low-pressure hysteresis loop is observed, and not for others. This change of shape may have the effect of disrupting the reorganisation of the adsorbed layer.
716 A question that does remain is why should this unusual hysteresis loop occur in certain MFI zeolites, and not in other zeolitic materials, or other microporous systems. This suggests that it cannot be a case of pore diameter, or the strength of adsorbateadsorbent interaction alone, but has to be a combination of these factors. It is apparent, that the effect of the Si:A1 ratio on the presence of the hysteresis loop is because of the influence this has on the strength of the interaction. A systematic study of microporous materials should continue with the aim of locating other systems that exhibit a similar behaviour.
Acknowledgements We are grateful to Degussa for donating the Zeolite samples, and to Maria Christophidou and Maria Tingiridou for experimental assistance. The financial support of the University of Cyprus is appreciated. References 1. U. Muller, H. Reichert, E. Robens, K.K. Unger, Y. Grillet, F. Rouquerol, J. Rouquerol, D. Pan and A. Mersmann, Fresenius Z. Anal. Chem., 333,433 (1989) 2. P.J.M. Carrott and K.S.W. Sing, Chem. & Ind., 786, (1986) 3. H. Reichert, U. Muller, K.K. Unger, Y. Grillet, F. Rouquerol, J. Rouquerol, J.P. Coulomb, Studies in Surface Science and Catalysis, (Eds F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger), Elsevier, Amsterdam, 62, 535, (1991) 4. U. Muller and K.K. Unger, Studies in Surface Science and Catalysis, (Eds K.K. Unger, J, Rouquerol, K.S.W. Sing and H. Kral), Elsevier, Amsterdam, 39, 101, (1988) 5. P.L. Lllewellyn, PhD Thesis, Brunel University, (1992) 6. R J-M Pellenq and D. Nicholson, Studies in Surface Science and Catalysis, (Eds J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Elsevier, Amsterdam, 87, 21, (1994) 7. D. Nicholson, R.W. Adams, R.F. Cracknel and G.K. Papadopoulos, Characterisation of Porous Solids IV, (Eds B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Royal Society of Chemistry, London, 57, (1997) 8. K.S.W. Sing, Colloids and Surfaces, 38, 113, (1989) 9. K.S.W. Sing, 3rd Fundamentals of Adsorption (Ed. A.B. Mersmann and S.E. Scholl), Engineering Foundation, New York, p78, (1991) 10. F. Rouquerol, J. Rouquerol and K.S.W. Sing, in "Adsorption by Powders and Porous Solids", Academic Press, New York, pp 389-366 (1999) 11. M.-E. Elettheriou, PhD Thesis, University of Cyprus (1995) 12. M.-E. Elettheriou and C.R. Theocharis, Characterisation of Porous Solids IV, (Eds B. McEnaney, T.J. Mays, J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger), Royal Society of Chemistry, London, 475, (1997)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
717
Characterisation of the textural properties of chemically dealuminated Y zeolites R. L6pez-Fonseca, B. de Rivas, J.I. Guti6rrez-Ortiz, and J.R. Gonzfilez-Velasco* Departamento de Ingenieria Quimica, Facultad de Ciencias, Universidad del Pals Vasco/EHU, P.O. Box 644, E-48080 Bilbao, Spain. Phone: +34-94-6012681; Fax: +34-94-4648500; E-mail address: iqp~ovei~l~.ehu.es
The main objective of this work is to characterise the textural properties of a series of Y zeolites dealuminated by ammonium hexafluorosilicate treatment. It was observed that the fluorosilicate treatment produced a highly crystalline product with a contracted unit cell. Both textural and XRD analysis confirmed the samples to be at least 95% crystalline for dealumination degrees <50%. According to N2 adsorption only a minimal contribution of mesopores was observed. However, a notable loss of micropores accompanied by the formation of mesopores was noted for severe dealumination levels, along with a considerable structural degradation. 1. INTRODUCTION The Y-zeolite consists of a three-dimensional cavity system with an opening diameter of 0.74 nm. This zeolite has the most open pore system among the large pore zeolites having a 12-member ring aperture. The dealuminated forms of the Y zeolite plays a major role in the refining industry, since it is one of the main components of FCC and hydrocracking catalysts. The dealumination of Y zeolite can be obtained by different treatments, which can be classified in two groups depending upon whether or not an external source of Si is provided [1,2]. The dealumination with external addition of Si corresponds ideally to a perfect isomorphous substitution of A1 by Si. In the present study, a commercial H-Y zeolite was dealuminated via the procedure described by Skeels and Breck [3,4] using ammonium hexafluorosilicate (AHFS) as the dealuminating agent under closely controlled conditions. The fluorosilicate method is attractive because it allows to produce silicon-enriched zeolites which are in principle perfectly microporous and exempt from framework defects and non-framework A1 species. Typically, the AHFS treatment differs from many of the dealumination methods in that it is carried out in aqueous media under relatively mild conditions [5]. The main objective of this work is to analyse the effect of chemical dealumination by ammonium hexafluorosilicate treatment on the textural properties of Y zeolite.
718 2. E X P E R I M E N T A L
2.1. Dealumination procedure The Y zeolite (CBV400) in its H-form (H-Y) was supplied from Zeolyst Corp. and used as received. The series of dcaluminated samples H-Y(d) was prepared following this procedure: prior to dealumination the starting material was obtained by two successive ion exchanges with a 3 M ammonium nitrate solution of the commercial H-Y sample at room temperature for 24 h in order to reduce the sodium content. Then, the NHaH-Y zeolite was preheated in a 0.5 M ammonium acetate solution at 80~ An aqueous solution of ammonium hexafluorosilicate was added dropwise at a rate of 50 cm 3 h 1 under vigorous stirring. The (NHa)2SiF6-to-zeolite ratio was adjusted to remove 15, 30, 50 and 75% of the aluminium in the zeolite, respectively. Afterwards, the temperature was raised to 95~ and the slurry was kept at this temperature for 3 hours to ensure that silicon could be inserted into vacancies created by the extraction of aluminium. Subsequently, the zeolite was recovered by filtration and repeatedly washed with hot deionised water to remove the unreacted (NH4)2SiF6 completely. Finally, the dealuminated samples were dried at l l0~ and stabilised by calcination in air at 550~ for 3 hours. 2.2. Zeolite characterisation The determination of the compositions was carried out using a Philips PW 1480 wavelength dispersive X-ray fluorescence (XRF) spectrometer. All the chemical elements were analysed using a side window Sc-Mo target X-ray tube and under a vacuum path. X-ray diffraction (XRD) studies were carried out on a Philips PW 1710 X-ray diffractometer with CuKc~ radiation (L=l.5406A) and Ni filter. The X-ray tube was operated at 40kV and 20mA. Samples were scanned from 20 equal to 2 ~ up to 60 ~ and the X-ray diffraction line positions were determined with a step size of 0.02 ~ and a counting time of 2.5 s per step. The crystallinity of the zeolites was established according to the method proposed by ASTM D-3906-91 considering the crystallinity of the parent H-Y as 100%. The unit cell size a0 of the zeolites was estimated from the X-ray diffraction patterns using a methodology similar to ASTM procedures D-3942-91 using silicon as internal standard. The number of framework aluminium (FAL) atoms per unit cell, All, was calculated from a0 using the correlation given by Fichtner-Schmittler et al. [6]. The atomic framework Si/A1 ratio was derived from the calculated NAI. The number of extra-framework aluminium (EFAL) atoms per unit cell was calculated by the difference between the total aluminium, as determined by XRF analysis, and the framework aluminium Alf. The BET surface areas of the zeolite samples were determined by N2 adsorptiondesorption at -196~ in a Micromeritics ASAP 2010 equipment. Prior to the determination of the adsorption isotherm, the calcined sample (0.5 g) was outgassed at 400~ under a residual pressure of 1 Pa in order to remove moisture. The adsorption data were treated with the full BET equation. The "t-plot" method using the universal t-curve was applied in order to obtain an estimation of the micropore volume, microporous surface and external surface area [7].
719 3. R E S U L T S AND D I S C U S S I O N
Expectedly, increasing amounts of AHFS added led to increased degrees of dealumination of the samples. For moderate dealumination levels (<50%) in which 15, 30 and 50% of the original aluminium was calculated to be removed, respectively, the reaction was stoichiometric within the experimental error. Hence, the actual dealumination degrees obtained were 16, 32 and 50%, respectively. However, for higher degrees (75%) the reaction was not complete any more since only 64% dealumination was achieved. XRD studies showed that the starting Y zeolite contained no amorphous phase and had a well-crystallised framework. This was manifested as the high intensity of the main peaks and a uniform low background. The crystallinity of the dealuminated samples is listed in Table l. It was observed that H-Y(d16o/o), H-Y(d320/o) and H-Y(ds0o/o) retained high degrees of crystallinity [8,9]. In addition, no changes in the background of the pattems because of the presence of amorphous material were observed. However, a noticeable crystallographic degradation was noted for H-Y(d64%) sample since the crystallinity decreased to 40% as a result of the massive aluminium extraction [10,11 ]. On the other hand, it is seen from Table 1 that the replacement of aluminium in the framework (increasing Si/A1 ratio) by the smaller silicon atoms caused a reduction in the unit cell size [ 12,13]. For the parent material XRF analysis gave aluminium contents that were noticeably larger than the framework aluminium concentration indicating the presence of large amounts of extralattice or non-framework aluminium. A comparison of the results from Table 1 clearly demonstrate that at relatively low dealumination levels, i.e. 16 and 32%, the AHFS treatment preferentially removed EFAL species while both EFAL and FAL were removed when the AHFS concentration was increased [14,15]. Hence, for H-Y(ds0%) sample, almost 100% EFAL and 30% FAL was extracted from the parent material. Indeed, it was found for this sample that the framework Si/A1 ratio obtained from the unit cell size was very close to the value obtained by XRF measurements. Our observations were consistent with the results Table 1. Physicochemical characteristics of the dealuminated samples:
a
b c a e
Zeolite
Crystal., %
a0, A
(Si/A1) a
(Si/A1) b
Alt c
A1d
Alextraf e
H-Y
100
24.52
2.3
4.9
53.3
32.2
21.1
H-Y(d16%)
99
24.51
3.3
5.2
44.6
31.1
13.5
H-Y(d32o/o)
97
24.49
4.3
5.6
36.2
28.9
7.3
H-Y(ds0%)
95
24.46
6.2
6.5
26.7
25.5
1.2
H-Y(d64%) 40 .... 8.9 11.5 19.4 15.4 4.0 (Si/A1): global (Si/A1) molar ratio. (Si/A1)f: framework (Si/A1) molar ratio. The total aluminum content was estimated from XRF data. The framework aluminum (FAL) was estimated from the unit cell size (determined by XRD) and using the Fichtner-Schmittler equation [framework AI=112.4(ao-24.233)]. The extraframework aluminum (EFAL) content was estimated as the difference between the total aluminum content determined by XRF and framework content estimated from the FichtnerSchmittler equation.
720 found in the literature where it has been reported that dealumination with AHFS up to 50% dealumination does not damage the zeolite structure and EFAL is practically free absent from such dealuminated samples [ 16]. The complete nitrogen isotherms of dealuminated Y zeolites are reported in Fig. 1. The curve of the parent H-Y zeolite corresponded to type I in the Brunauer classification, which was typical for the crystalline microporous materials [17]. As expected, the starting material showed no evidence of mesopores. Fig. 1 shows that the AHFS-treated samples with dealumination levels equal or lower than 50% were characterised by a very flat adsorptiondesorption isotherm with nearly no hysteresis loop [ 18]. The adsorption data were treated with the full BET equation, which was more suitable for solids containing micropores [19]. The results obtained are listed in Table 2. The total pore 300
300
280
H-Y
260 I1.
i(n
~ :)20 21111
r
180
2801 260
240
6 E
240 220 200
180o) 160 )
160 140q(
0 > 10 4) J2
140 ) 120
1201 100:
O (n 10
801
<1:
H'Y(dl w/') ~
601
r
100 )) 80 ) ) 60 )) 40 b ) 20 )
Adsorption
Desorption
C
40:
.
0
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
0
0
:
',
;
:
u) u =; E -= O > 10 4) .a
o 10 <
;
;
l
:
;
:
:
:
:
:
:
:
:
:
,
1
Relative pressure
240
70
220
65
200
60
g
On
55
18o
50
160~ b 4b 1404) 4) 1204) 4) 1004) ~) 804)
) 60 ,
40'
:
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative pressure
a.
Adsorption Desorption
--.
0
)
45 40 35 30 25 r C
20
Adsorption Desorption
10
20
5
0
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative pressure
r O
15
1
0
Adsorption Desorption
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Relative pressure
Fig. 1. Nitrogen adsorption-desorption isotherms from Y zeolites.
1
721 volume, Vpore, was calculated by Gurvitsh rule at P/P0=0.95. The 't-plot' method was applied in order to obtain an estimation of the microporous surface area, Smicro, the external surface area, Sext, and the microporous volume, Vmicro. The determination of the t-plot was established from the universal t curve [20]. The mesopore volume, Vmeso, was calculated by subtracting the micropore volume from the total pore volume. The sum Smicro plus Sext represents the total surface area. For all the samples, the total surface areas agreed well with the BET (SBEv) surfaces determined by the full BET equation. According to N2 adsorption, the microporous character of the modified zeolites was largely retained and these samples did not possess any appreciable mesopore volume (Table 2). This finding was in agreement with their unaffected X-ray crystallinity. Interestingly, it must be pointed out that the AHFS dealumination to a low extent (16%) of the parent material resulted in a slight increase in BET surface area and pore volume. This could be attributed to the removal of a great fraction of EFAL species eventually reopening the pore system. By contrast, the values of surface area and pore volume for the sample with a dealumination level of 64% were severely affected, probably due to the collapse of the crystalline structure and to the blockage of the zeolite pore system to a very large extent. Under such conditions, a secondary mesoporous system developed which was also detected by N2 adsorption [21 ]. As mentioned above, the structure collapse was also revealed by XRD analysis. Accordingly, Goyvaerts et al. [22] reported mesoporosity and structural degradation for Y zeolites severely dealuminated by SIC14 treatment. Concerning the values of Vmicro and Sext, it was deduced that mild dealumination induced a slight decrease in the micropore volume for the samples H-Y(d32%) and H-Y(ds0%). This effect could be ascribed to the formation of mesopores during dealumination, which could be accompanied by the destruction or blocking of part of the micropores [23]. For these samples, the contribution of the microporous surface to the total surface area was similar to that of the parent material (around 90-95%). As said before, the extent of this structural degradation appeared to be not very severe for dealuminated samples with a Si/A1 ratio lower than 6.2 as evidenced from the results in Table 2. Table 2. Textural properties of the dealuminated samples.
Zeolite
SBET,m 2 g-1
Smicro,m 2 g-1
Sext, m 2 g-1
Vp, cm 3 g-1
Vmicro,cm 3 g-1
H-Y
900
880
66
0.382
0.312
H-Y(d16%)
955
940
44
0.406
0.334
H-Y(d32%)
865
860
17
0.355
0.305
H-Y(dsoo/o)
820
810
38
0.334
0.288
H-Y(d64o/o)
155
150
15
0.085
0.054
4. CONCLUSIONS The joint use of N2 adsorption and XRD led to a coherent description at least on a semiquantitative basis, of the effect of the dealumination via ammonium hexafluorosilicate treatment on the textural properties of the Y zeolites.
722 Y zeolites with dealumina::ion levels lower than about 50% were, according to N2 adsorption, exempt from mesopores. Moreover, since no trace of amorphous material was apparent in these solids, it could be concluded in agreement with other physico-chemical properties, such as an excellent X-ray crystallinity that the isomorphous Si-A1 substitution took place successfully. For solids with higher dealumination levels (>50%) the isomorphous substitution was no longer ideal resulting in the formation of a secondary pore system. ACKNOWLEDGEMENTS
The authors wish to thank Universidad del Pals Vasco/EHU (9/UPV 0069.31013517/2001) and Ministerio de Ciencia y Tecnologia (PPQ2001-0543) for the financial support. REFERENCES .
2. 3. 4. .
6. .
8.
9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
J. Scherzer, ACS Symp. Ser., 248 (1984) 157. B.Sulikowski, Heterog. Chem. Rev., 3 (1996) 203. D.W. Breck, G.W. Skeels, US Patent No. 4 503 023 (1985). G.W. Skeels, D.W. Breck, in: D. Olson, A. Bisio, (Ed.), Proceedings of the 6th International Zeolite Conference, Butterworths, Guilford, 1984, p. 87. Y. He, C. Li, E. Min, Stud. Surf. Sci. Catal., 49 (1989) 189. H. Fichtner-Schmittler, U. Lohse, G. Engelhardt, V. Patzelova, Cryst. Res. Technol., 19 (1984) K1. J.H. de Boer, B.G. Linsen, TH.J. Osinga, J. Catal., 4 (1965) 643. Q.L. Wang, G. Giannetto, M. Guisnet, Zeolites, 10 (1990) 301. M. Neuber, V. Dondur, H.G. Karge, L. Pacheco, S. Ernst, J. Weitkamp, Stud. Sure Sci. Catal., 37 (1987) 461. A.P. Matharau, L.F. Gladden, S.W. Can, Stud. Surf. Sci. Catal., 94 (1995) 147. A. Corma, V. Forn6s, F. Rey, Appl. Catal., 59 (1990) 267. G. Garral6n, V. Forn6s, A. Corma, Zeolites, 8 (1988) 268. A.V. Abramova, E.V. Slivinskii, E.A. Skryleva, Kinet. Katal., 39 (1998) 411. J.M. Cruz, A. Corma, V. Forn6s, Appl. Catal., 50 (1989) 287. P.V. Shertukde, W.K. Hall, J-M. Dereppe, G. Marcelin, J. Catal., 139 (1993) 468. A. Gola, B. Rebouis, E. Milazzo, J. Lynch, E. Benazzi, S. Lacombe, L. Delevoye, C. Fernandez, Microporous Mesoporous Mater., 40 (2000) 73. G. Leofanti, M. Padovan, G. Tozzola, B. Venturelli, Catal. Today, 41 (1998) 207. J. Lynch, F. Raatz, Ch. Delalande, Stud. Surf. Sci. Catal., 39 (1988) 547. A. Gervasini, Appl. Catal. A, 180 (1999) 71. M.J. Remy, G. Poncelet, J. Phys. Chem., 99 (1995) 773. H. Ajot, J.F. Joly, J. Lynch. F. Raatz, P. Caullet, Stud. Surf. Sci. Catal., 62 (1991) 583. D. Goyvaerts, J.A. Martens, P.J. Grobet, P.A. Jacobs, Stud. Sure Sci. Catal., 101 (1996) 731. J. Lynch, F. Raatz, P. Dufresne, Zeolites, 7 (1987) 333.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
723
Complementarity of microcalorimetry, manometry and gravimetry in the study of gas adsorption by microporous solids up to 50 bar St~phanie Moret a, Thomas Poyet a, David Bigot b, Bernd Polster b, Simon Crispel b, Jean Rouquerol a and Philip Llewellyn a MADIREL, CNRS/Universit6 de Provence, 26 rue du 141 +meRIA, 13331 Marseille cedex 3, France
a
b Air Liquide, Claude-Delorme Research Centre, 1 chemin de la Porte des Loges, Les Logesen-Josas, France This work compares and contrasts the experimental results using three different experimental techniques : manometry, gravimetry and microcalorimetry. The system studies was the adsorption of argon, nitrogen and carbon dioxide on a NaLSX zeolite between 20 and 60~ and up to 50 bars. 1. INTRODUCTION The adsorption of gases at room temperature and up to pressures in the region of 50 bars is of interest from several points of view. From an industrial standpoint, both gas separation (by pressure-swing adsorption (PSA)) and gas storage operate in this pressure range [1 ]. From a fundamental point of view however, the nature of the adsorbate-adsorbent interactions close to and within supercritical regions merit detailed study [2]. Now, these measurements are somewhat difficult to carry out, not only because of the question of the pressure but also because of the problems of accuracy, due to the corrections and errors - due to the void volume (in adsorption manometry) [3-5] or to the buoyancy (in adsorption gravimetry) [6]. For this reason, it may be found advisable to cross-check the experimental results by making use of a variety of different techniques. The aim of this paper is therefore to check the consistency of three basic techniques (adsorption manometry, adsorption gravimetry and adsorption microcalorimetry) in the 0-50 bar pressure range, using a standard NaX zeolite adsorbent and selecting three adsorbable gases, namely: - Argon, with nearly perfect behaviour and known to usually give rise to non-specific interactions during adsorption - Nitrogen, again with nearly perfect behaviour, but now giving rise to specific adsorption interactions, due to its permanent quadrupole moment - Carbon dioxide, which gives rise to more important specific interactions due to the larger quadrupole. Furthermore carbon dioxide does not behave as a perfect gas and needs important corrections to be taken into account After presenting the experimental details and the results, the latter will be compared to each other and we shall then comment on the merits and conditions of reliability of each of these approaches.
724
2. E X P E R I M E N T A L
2.1. Adsorptives and Adsorbents The zeolite sample used in the present study is a sodium containing faujasite NaX of Si/A1 ratio = 1.3. This sample was supplied by CECA S.A. (ref. G5CO2M). The sample was in bead form of diameter 1.6-2.5 mm. The adsorptives were obtained from Air Liquide (quality Alphagaz), of purity greater than 99.99%)
2.2. Adsorption manometry The commercial equipment used for this purpose makes use of a Mensor pressure gauge covering the 0 - 70 bar range, with a sensitivity of 0.01% of the full scale and it can accommodate 4 samples at the same time. The isotherms were carried out at 20, 40 and 60~ and up to a final pressure of 50 bars. A sample mass of around 3 g was used for each experiment. The sample was outgassed in the adsorption cell on a separate apparatus attached to a turbomolecular pump. The sample was heated up to a final temperature of 400~ in 10 hours with several intermediate plateau's. The sample was kept at this final outgassing temperature for 6 hours, with a final pressure of around 10-5 mbar. For the determination of each isotherm, the sample cell was kept to within 0.1~ of the experimental temperature by use of a liquid thermostat. Dead space correction was carried out using a helium calibration. A difference in the measured dead space of up to 0.4 % was observed over the experimental pressure range. When varying the temperature within the 20 60~ range however, this variation in the measured dead space was far less. The gases were introduced using a point by point procedure. The a d s o r b a t e - adsorbent equilibrium for each point was considered attained for a pressure change of less than 8 mbar over 5 minutes. Under such conditions a typical isotherm with 20 points was obtained in around 24 hours.
2.3. Adsorption microealorimetry The adsorption up to 50 bars was carried out by means of a Tian-Calvet type isothermal microcalorimeter built in the former CNRS Centre for Thermodynamics and Microcalorimetry. For these experiments, around 2 g of sample was used which were outgassed by Controlled Rate Thermal Analysis (CRTA) [7]. The experiments were carried out at 30~ (303 K). Approximately 6 hours is required after introduction of the sample cell into the thermopile for the system to be within 1/100 th of a degree Celsius. At this point the baseline recording is taken for 20 minutes. After this thermal equilibrium was attained, a point by point adsorptive dosing procedure was used. Equilibrium was considered attained when the thermal flow measured on adsorption by the calorimeter returned to the base line. For each point the thermal flow and the equilibrium pressure (by means of a 0-70 bar MKS pressure transducer providing a sensitivity of 0.5% of the measured value) were recorded. The area under the peak in the thermal flow, Qme,,s, is measured to determine the pseudo-differential heat of adsorption, Aads/l , v i a :
Aads]l .__ G e a s -- Glank o" nmeas
(1)
725 where
Qblank is the heat output measured from a blank experiment. This value takes into
account gas compression of the non-adsorbed phase, dose.
n:eas is the
amount adsorbed during the
2.4. Adsorption gravimetry and density measurements The equipment used was built in-house around a high-pressure magnetic suspension balance marketed by Rubotherm, Germany. A similar balance has been used to measure high temperature isotherms [8]. Our current set-up has added features such as CRTA control unit for the in-situ outgassing of the sample and an adsorption manometry system. The coupling of manometry and gravimetry can be used for the study of coadsorption [9]. It is equipped with three Mensor pressure gauges (0-10, 0-50 and 0-70 bar, respectively), with a sensitivity of 0.01% of the full scale. The balance, with 10 ~g sensitivity, is able to alternately weigh either the sample proper (sample mass up to 10 g) or a cylindrical titanium sinker of known volume (ca. 4 cm 3) whose apparent weight provides a direct assessment to the buoyancy effect, i.e. to the density of the gas phase. This is valuable information each time when the gas does not behave ideally, such as carbon dioxide at room temperature, in order to make an adequate buoyancy correction. The isothermal conditions are provided by a water jacket fed by a thermostat controlled within 10~ K. Prior to any new series of adsorption isotherms and also after each adsorption isotherm of carbon dioxide, the sample (ca. 5 g) was in-situ outgassed by CRTA up to 450~ The adsorptive was then automatically introduced in successive pressure steps. The "height" of the steps was selected so as to provide ca 20 points on each adsorption isotherm up to 50 bars. Equilibrium was considered to be satisfactory when the sample weight varied of less than 80 ~g in 5 min. At each equilibrium point both the sample weight and the sinker weight were recorded. The total duration of an adsorption experiment proper (without taking into account the outgassing) was ca. 15 hours. 3. RESULTS AND DISCUSSION
We report in Figures 1 and 2, with the same presentation and scale, the experimental results obtained by adsorption manometry and gravimetry, for the three gases and the three temperatures. Incidentally, this is a good example where the formerly used representation in "adsorbed volume" or "adsorbed mass" is clearly less convenient than the more universal representation in "adsorbed amount" (or still more precisely in the present case, in "surface excess amount"). The shapes of the adsorption isotherms, with very different curvatures from one gas to the other, are a clear indication of the increased gas/solid interaction as one passes from argon to nitrogen and then to carbon dioxide, for which the isotherms are practically of type I.
726
Nitrogen /%
-O
E E
/%
2
/%
60~ "T 03 --~ 0
/%
/%
20oc
9
9
9 Anr,.,uo
9 9
"uv%, ~"
2
E E ~ 1.5
/% 15
9
2.5
/%
/%
/%
3
400C
/%
/%
A
/%
Nitrogen
/% 20~
/%
/%
/%
/% /% /%
O 9 t~
o5
1"o
o
2"0
~o
Pressure / bar
;o
~o
go
A /%
21
/%
/%
1.8
/%
/% /%
/%
/%
09,
/%/%
/%
06,
A A A AAA
A
/%
15,
E
/%
/%
/%
/%20~
/% /%
/%
/%
60~
/%
o
9
200C 40~
21 '7
600C
1.8,
O E
1.5. 1.2
~
&
~
A
A
~o
dioxide
A
&
~
0
60
A
;0
0
~
2"0
40~
A
"60~
55
Carbon
9
9 9
Ooee
sl'~
5,
;0
Pressure / bar
7
20~ A
A
,,,
,
10 ~o ;0 Pressure / bar
Carbon
9
9
9
s'o
6"0
dioxide
9 9
4"0
9
9 9
9
20~
9 9
9
9 9
9
9
9
9
~0o~ 9
~O~
4'5 ' ~ O ~ O ~
A
4.
~o
o3p
7
4.5 /
~o
0.9
1"o
~
;o
2.41
I~. D
/%
/%
/%
o, ff
_3? 4.~ ~
m O E 35,
F:
~o
Pressure I bar
271
40oC
/%
03, / ~ # A
03.
2'0
Argon
24
,-
1"o
o
Argon
3 2.7
0 E
r
0
0
E 3.5~p
3,
E
3,
%;25.
2,
2
15~
1.5
1
1
05
0.5
0. 0
;
;
;
6
;5
;8
Pressure / bar
2"1 2'4
Fig 1" A d s o r p t i o n i s o t h e r m s on zeolite determined by manometry
2'~
~;o
NaLSX
0
;
;
;
;2
1"5
;8
Pressure / bar
Fig 2" A d s o r p t i o n isotherms zeolite determined by gravimetry
2"1
2"4
on
2"7
~0
NaLSX
727 Nitrogen
30"C o
,-
.=$e
o
,i,. 9 '~
9+ _ ~ 9
20
40
15
30 -~
E
E o.
J= <1, 10
~
Nitrogen
,60
- 0 microcalorimetry i _~_ manometry
2
zxo
E15
20
oe
9
h a I oe
40oc
-<:,a9 e a 9 ~ o
'~ .<> '~
9 9
9
+'
me
"60~
~ ~
zx*Cx~ ~ ~,~
1,
9149 e~ 20"C
~x gravimetry o manometry
microcalomietry
0.5 _j..- j n f ,~ 0
;
0"5
~:5
gad s I
;
0
2:5
0
mmol.g "~
Argon
27
50
30"C
L_~L:-_m a _ n ~
=o so
q>~
2.1
e-
I~. 1.8 O E
2O
~
*9
++, .+
12 0.9
10
10
0.6 0.3
0
0
3
o
#
7
i
!__~_rnanom__et_ry __ _, "6 50 E
~30 ,,0 -.,,.
* +
e-
w
9~.3o
o 80~ o~
# e ~e 9
,g 9 gravimetry z~ manometry
~
o microcalomietry
1"o
2"0
3"0
Pressure / bar
0 E
3.5
E
3
4"0
C a r b o n dioxide
ss] o+'* ~*`'~'+"*~ ~* 9 9"~,,o," ". ,a
30"
<>+
.~9 40
1
@o
g~ 9
n,.as ! m m o l . g "1
C a r b o n dioxide
o %0~
9
~5
E
9 20
94xe
~'~ ooO~ ~ t ~'~ 9 ~o~
"7 i.. r 30 .O
~o
9 A 20~
~ o o~
2.4 40
s'o
Argon
3,
,=
40
2"0 ~o ;o Pressure / bar
1"o
0
4
~E 2.5
s'o
6"0
20~
9 %~o.c
9
9 A~OoC
9 gravimetry /~ man 9 o microcalomietry
2 1.5 1 0.5 0 0
;
~
;
n .T--;--.f
4
n.d, / mmol.g "+
0
7
Fig 3 9Pseudo-differential enthalpy (diamonds) and isotherm (triangles) on NaX determined by microcalorimetry/manometry.
0
;
;2 l"s ;8 2"1 Pressure / bar
2"4 2"7 3"0
Fig 4 9Comparaison of adsorption isotherms on NaX zeolite obtained by manometry, gravimetry and microcalorimetry
In Figure 3, we report the microcalorimetric results obtained with the three gases at 30~ Two curves are provided for each experiment. One, with distinct points, gives the pseudo-
728 differential enthalpies of adsorption (corresponding to the ordinate on the right) which can be assessed by a point-by-point procedure. Strictly speaking, differential enthalpies of adsorption are indeed only assessed through a procedure of slow and continuous introduction of the adsorptive which ensures, at any time, quasi-equilibrium conditions and allows a continuous recording of the differential enthalpy [2]. Unfortunately, such a procedure is not easily amenable to high-pressure adsorption experiments. The second, continuous, curve is simply the adsorption isotherm as determined by the manometric equipment connected with the microcalorimeter (the unusual orientation of this adsorption isotherm is due to the presentation of the calorimetric results in the same figure). Now, these calorimetric results allow, in principle, to recalculate adsorption isotherms at other temperatures than 30~ by simple use of the "isosteric method" derived from the ClausiusClapeyron equation, for instance in the following form:
'ash
-T2 -fl
(2)
In this way the 9 adsorption isotherms, for the three gases at the three temperatures were recalculated. They are compared, in Fig 4, with those obtained with the two other techniques. As it can be seen, the consistency between the three techniques is excellent in the case of nitrogen adsorption, still good in the case of argon adsorption and slightly less satisfactory in the case of carbon dioxide. We can notice that the isotherms obtained by gravimetry tend to be very slightly above those determined by manometry: this can be explained either by a slight residual error in the correction used for non-ideality of carbon dioxide in adsorption manometry or by the higher outgassing temperature used in the gravimetric experiments (450~ instead of 400~ 4. CONCLUSION
At this stage, it can be said that the three techniques used are satisfactory for this type of highpressure gas adsorption experiments. Adsorption manometry allows an easy outgassing in a separate unit and lends itself to multiple sample operation, ie to numerous experiments, but requests a satisfactory knowledge about the non-ideality of the gas used. Adsorption gravimetry lends itself to in-situ outgassing (up to 500~ with our equipment) but can only accommodate one sample at a time. A major advantage only obtained with the special assembly making use of a sinker is the permanent measurement of the density of the gas phase: it allows not only to make at any time a correct buoyancy correction but it also allows to provide the users of adsorption manometry equipment with the data they need to make a safe void volume correction. Adsorption microcalorimetry, finally, allows to check the differential enthalpies of adsorption derived from various adsorption isotherms through the isosteric method. What is more, it is much safer and meaningful in the first, raising, part of the isotherm, specially when it is close to the ordinate, such as for instance for carbon dioxide adsorption. This part is probably the most interesting from the viewpoint of specific interactions and gas separation or storage.
729 5. R E F E R E N C E S
[1] S. Sircar, Ind. Eng. Chem. Res., 41 (2002) 1389. [2] F. Rouquerol, J. Rouquerol, K. Sing, "Adsorption by powders & porous solids", Academic Press, London (1999). [3] S. Sircar, I. & Ec. Res. 38 (1999) 3670. [4] S. Sircar, in "Proceedings of 7th Fundamentals of Adsorption", (K. Kaneko, Ed.), Elsevier, Amsterdam, 2001. [5] P. Malbrunot, D. Vidal, J. Vermesse, R. Chanine, T. K. Bose, Langmuir, 13 (1997) 539. [6] H. B. Gimler, R. Kobayashi, A.I.Ch.E.J, 10 (1964) 77. [7] J. Rouquerol, Thermochim. Acta, 144 (1989) 209. [8] G. De Weireld, M. Fr6re, R. Jadot, Meas. Sr Techn., 10 (1999) 117. [9] J. U. Keller, R. Staud, M. Tomalla, Ber. Bunsenges. Phys. Chem., 96 (1992) 28.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
731
The applicability of the Dubinin-Radushkevich equation to the very low pressure region of isotherms of various microporous solids. P. L o d e w y c k x a and L. V e r h o e v e n b
aRoyal Military Academy- Department of Chemistry, Renaissancelaan 30, B-1000 Brussel, Belgium. E-mail: [email protected] bBelgian A r m y - Service of Technological Applications, Martelarenstraat 181, B-1800 Vilvoorde (Peutie), Belgium.
Over the years there has been a lot of debate concerning the applicability of the DubininRadushkevich equation on the very low pressure region of isotherms of microporous solids. The experimental downward deviation of the DR-plot for very low pressures is generally attributed to kinetic barriers, especially in the case of nitrogen adsorption at 77K. This low pressure region of isotherms of various adsorbents can be fitted with the Langmuir equation. Hence it is shown that the downward deviation is not due to experimental factors but reflects a different adsorption mechanism.
1. INTRODUCTION Over the years there has been a lot of debate concerning the applicability of the DubininRadushkevich equation (Eq 1) on the very low pressure region of isotherms of microporous solids. Most DR-plots exhibit a downward deviation for very low pressures (see Fig 1). W = expl-BflT22
Wo
ln2(po/p)l
(1)
Where W = volume of the pores that has been filled at p/po [cm3/g] = total volume of the micropore system [cm3/g] B = structural constant related to the width of the Gaussian pore distribution [K z] T - temperature at which the isotherm has been taken [K] fl = similarity constant, depending solely on the adsorbate (fl=l for benzene) [-] - inverse of the relative pressure of the adsorbate [-]
Wo
po/p
This is generally attributed to kinetic barriers, especially in the case of N2-adsorption at 77K [ 1,2,3]. Hence it is regarded upon as an artifact of the experimental set-up and parameters, rather than being the sign of a different adsorption mechanism. If this would be true, these points should ideally be part of the straight line of the DR-plot if they are given sufficient time to reach equilibrium, i.e. the experimental points of the isotherm should be situated on a
732
unique line from P/Po -- 0 till the start of capillary condensation. This could be attained by increasing the allowed equilibration time for each point of the isotherm (see Fig 2). However, at our knowledge, nobody has yet succeeded in getting all the experimental data points on this straight (DR) line. An additional problem with the DR-equation is its thermodynamical inconsistency: for very low partial pressures (approachingp/po = 0) it does not converge to a zero adsorption uptake. Several authors [4,5] have tried to overcome this by rearranging or adapting the equation. The aim of this study was to investigate these two phenomena in order to be sure that the deviations of the Dubinin-Radushkevich plot are really the result of experimentally induced errors. And to gather additional information about possible reasons for its thermodynamic inconsistency. 2. E X P E R I M E N T A L
Isotherms of various microporous solids (activated carbons, zeolites, ACF's), involving different adsorbates (N2, CO2, Ar), have been measured: Norit R1Extra (N2, CO2), Norit RB 1 (N2), Norit C Granular (N2), Chemviron BPL-HA (N2), Chemviron ASC-TEDA (N2), an experimental activated carbon AC 147 (N2), a pitch-based activated carbon fibre (P3200CO2/900~ manufactured at DSTL-Porton Down (N2), zeolite 13X14H (N2) and zeolite 13X12L (Ar). The last one was copied from the reference isotherms provided with the Micromeritics ASAP 2010 poresizer that was used to measure the different isotherms. All the nitrogen and argon isotherms could be fitted with the Dubinin-Radushkevich equation between, typically, p/po = 10-5 and p/po --- 102. At higher pressures capillary condensation causes the isotherm to diverge. At lower pressures the typical deviation described in Fig 1 was observed. The carbon dioxide isotherm showed only a minor deviation from the DR-plot (one point) and was hence excluded from this study.
ln2(po/p)
ln2(po/p) 0
50 I
100 m
80
150 I
-1
100
120
140
!
!
I
DR
-0,5
-1,5
-1 -2
-1,5
g~ "
9 -2,5
-2,5 -3
-3 -3,5
,
-3,5
AT increasing
,
-4
Fig 1" Example of a typical N2-isotherm of an activated carbon with corresponding Dubinin-Radushkevich plot.
Fig 2: Theoretical isotherms for increasing equilibrium times.
733 At first, equilibrium time was increased in order to try to eliminate the low-pressure deviation. This proved to be impossible. The upward shift of the isotherm predicted in Fig 2 was only observed when the initial equilibrium times were very short. Once a "reasonable" equilibrium time was reached (in the order of several hours per point) the isotherm stayed identical. Even for equilibrium times as high as 12 hours per point the isotherm did not shift towards the Dubinin-Radushkevich line. From this, it was concluded that the deviation for low partial pressures was not an experimental artifact. In fact, this has already been suggested by several authors [6,7], but little attention has been given to these results. Partly because their observations and conclusions were mainly directed towards the fractal characterisation of porous materials. In a next step, the part of the isotherm for pressures smaller than p/po = 10.5 was fitted with various equations. As only a limited number of data points were available (in some cases as few as three points) a reasonable fitting could be achieved with various equations. However, to be accepted, these equations had to obey two supplementary conditions: general applicability and physico-chemical consistency. To answer to the first condition the same equation (possibly with different parameters) had to be applicable to all adsorbents, and preferably to both argon and nitrogen (the carbon dioxide data being discarded). The second condition is rather straightforward: if the first part of the isotherm really depicts a different adsorption mechanism, the equation that describes this part should represent a physico-chemical adsorption phenomenon.
3. RESULTS AND DISCUSSION
Ultimately, the best fit for all isotherms was obtained for a slightly modified Langmuir equation (Eq 2): n,, c ( p / p o - x o )
(2)
n = [1 + c ( p / p o - x o)1 With n the amount adsorbed, nm the monolayer capacity, c a parameter related to the net heat of adsorption, p/po the partial pressure and Xo a parameter differing from one experiment to another. This last parameter was in most cases equal to the lowest measured partial pressure. So it seems to be an artifact of the experimental set-up, i.e. it represents the offset of the adsorption apparatus: in stead of n ---) 0 for p/po --4 O, the amount adsorbed n tends to zero for ( p / p o - Xo) ---) O. So the model considers Xo to be the lowest attainable pressure. Quite surprisingly, the monolayer capacity nm turned out to be equal (or at least proportional) to the Dubinin-Radushkevich micropore volume Wo. This constitutes another proof of the physico-chemical consistency of the equation. The last parameter, c, is related to the net heat of adsorption and, consequently, depends on both adsorbent and adsorptive. As Ar and N2 are quite similar adsorptives, e should only be dependent on the adsorbent. This is reflected in the results: for activated carbons c varied between 25000 and 35000 (with a value of 30000 for the ACF), and for zeolites c was situated between 2500 and 3000. The limited variation of c for a given type of adsorbent shows the general applicability of the equation.
734
The results of the fitting are shown in Figs 3-6. Figure 3 shows a Dubinin-Radushkevich plot for an activated carbon with an additional fitting of the "deviation" by Eq 2. Figure 4 shows the same three series (experimental isotherm, DR-plot and Eq 2) but on a linear scale. Figure 5 shows only the low pressure part of Fig 4. Finally, Fig 6 is similar to Fig 5 but for an argon isotherm on a zeolite. In all examined cases, equation 2 provided a very good fit of the first part of the isotherm. As this equation represents a layer-by-layer adsorption rather than a pore filling mechanism, this strongly suggests a different adsorption behaviour for the very first part of the isotherms. The fact of nm being proportional to Wo shows this type of adsorption to be exclusively related to the micropore system. 0,6 0
In2(p/po) 100
50 n
150
200
I
i
'
0,5
i
0,4
-1
t'r_ 0,3
-2
Nitrogen isotherm --DR --Langm_ui_r 9
0,2
-4
[ * Nitrogen isotherm / ----DR-plot I'_-- Lan_gmuir
-5
*~ *~ ~'
0,1
0
-6
9
0,04
0,05
_@ o "
,"
0,25 -
0,03
p/po
021
,,"
i
0,02
Fig 4: N2-isotherm o f N o r i t R1Extra, fitted with DR-eq and Eq 2
Fig 3" DR-plot and Eq 2 for activated carbon Norit R1 Extra 0,3 -
0,01
0,15 0,2-
9
0,15 -
9Nitrogen isotherm
0,1 -
mD R
0,050
I
:t r-~
- - Eq.2 !
!
o,o, !
|
t
9
0E+00 2E-05 4E-05 6E-05 8E-05 I E-04
p/po Fig 5" N2-isotherm o f N o r i t R1Extra, fitted with DR-eq and Eq 2 (low p/po)
0
0
,,t r
--~,rgon-i-s~h~m---11
.....................
i "" E
.2
0,0005
p/po
..... j
0,001
0,0015
Fig 6: Ar-isotherm of zeolite 13X, fitted with DR-eq and Eq 2 (low P/Po)
735 4. CONCLUSIONS Combining all observations we can deduce the following general scheme for the adsorption of nitrogen and argon (and possibly other adsorbates) on microporous solids such as zeolites and activated carbons: first odd molecules will be adsorbed at the inner surface of the micropores. Then, at a certain pressure, the process of volume filling of the micropores will take over. From this we can conclude that the deviations from the DR-plot in the low pressure part of the isotherms are not an experimental problem but do reflect a different adsorption mechanism. Hence there is no "need" for the DR-equation to be thermodynamically consistent as it starts at a certain pressure and never at p/po = 0. The proposed mechanism is very similar to that of capillary condensation, also starting at a certain partial pressure and only after the formation of a (mono)layer on the pore walls. The only real differences between the two mechanisms being the driving force and the size of the pores (micropores versus mesopores).
REFERENCES
[1] Byrne, J.F. and Marsh, H. in Porosity in Carbons (J.W. Patrick ed.) London: Edward Arnold 1995:35
[2] Zawadski, J in Chemistry and Physics of Carbon Vol. 21 (P. Thrower ed.) New York: Marcel Dekker 1989:231
[3] Rodriguez-Reinoso and Linares-Solano in Chemistry and Physics of Carbon Vol. 21 (P. Thrower ed.) New York: Marcel Dekker 1989:59 [41 Bansal RP, Donnet J-P and Stoeckli F in Active Carbon New York : Marcel Dekker 1988:139-141 [5] Aranovich, G.L. and Donohue, M.D. Vapor adsorption on microporous adsorbents. Carbon 2000; 38:701-708 [6] Ehrburger-Dolle, F. A new way to analyze adsorption isotherms. Langrnuir 1999; 15: 6004-6015 [7] Blacher S, Heinrichs B, Sahouli B, Pirard R and Pirard J-P. Fractal characterisation of wide pore range catalysts: Application to Pd-Ag/SiO2 xerogels. Journal of Colloid and Interface Science 2000; 226:123-130
Acknowledgements: The authors whish to thank Chemviron and Norit for kindly providing the activated carbon samples, as well as the researchers from DSTL-Porton Down for sharing their ACF data.
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Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
737
Controlled porosity and surface area titania gels as novel photocatalytic washcoats M. Yates a and E. Garcia b aInstituto de Cat~lisis y Petroleoquimica, Cantoblanco 28049 Madrid, Spain. bInstituto de Energias Renovables, CIEMAT, Avda Complutense 22, 28040 Madrid, Spain. Since natural sunlight can only penetrate a few microns depth, the use of thin films of titania applied to ceramic or metallic supports as maintenance free decontamination catalysts for the photocatalytic oxidation of volatile organic compounds is of interest for the abatement or control of these emissions. The sol-gel technology can be readily incorporated as a washcoating step of the catalyst supports that may be subsequently heat-treated to fix the titania to the support. The surface area, porosity and crystalline phases present in these gels is important in controlling their catalytic activity. Furthermore, the thermal stability and development of porosity with heat-treatment was important if the sol-gel route is to be used as a washcoating step to produce thin films. Key words: Titania, sol-gel, photocatalysis, VOC 1. INTRODUCTION Volatile organic compounds (VOCs) are considered as major anthropogenic pollutants generated in urban and industrial areas. Due to the tighter and more stringent regulations concerning gaseous emissions [1], there is an increasing interest in VOC abatement technologies. The main difficulty in removing VOCs is that in many industrial sources the gaseous emissions produce low levels of these toxic compounds. Thus, although combustion of hazardous waste is widely practised when the concentrations to be removed are traces this technology cannot be considered. Gas phase photocatalytic oxidation is being investigated as a means to destroy a broad range of contaminants. Attractive advantages of this technology for air treatment and purification are near ambient temperature and pressure operation (mild conditions) and use of atmospheric oxygen as the oxidant. Photocatalytic oxidation has been demonstrated as an efficient technology for the destruction of aromatic compounds [2,3,4], halocarbons [5] and alcohols [6]. Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors.
738 The production of high purity titania via a sol-gel routes has two main advantages: the fine control over the final textural properties, specific surface area and porosity and the production method can be employed as part of a wash-coating step prior to heat-treatment to fix the thin active layer to the metallic or ceramic support [ 10]. In this investigation a series of titania gels were produced at differing pH of precipitation, since it was known that this could be used as a method to control the specific surface area and porosity [11,12,13]. One of the members of this series was subsequently calcined at various temperatures up to 500~ and characterised at each stage by X-ray diffraction and nitrogen adsorption/desorption. These studies indicated that the specific surface area, pore volume and pore size distribution and phase composition of the material were dependent on the initial pH of formation and the final calcination temperature. 2. EXPERIMENTAL
2.1 Sol-gel preparation of titania Titania gels produced in this study were formed by the acid hydrolysis of titanium tetra-isopropoxide [ 14]. Although all the sols were produced in a similar manner, the final pH for the formation was controlled by the addition of ammonium hydroxide. The acid hydrolysis of titanium tetra-isopropoxide may be considered as follows: Ti-(O-C3H7)4 + 4H20 ~ Ti(OH)4 + 4C3H7OH
(1)
In this sol-gel process the molecular precursors are metal alkoxides that are bound by organic ligands to the central metal or metal ion. The addition of water causes a rapid hydrolysis of the metal. In the case of Ti(OH)4 the particles do not have a surface charge at a pH of 6.8. At lower pH the surface of the particles are positively charged leading to the formation of a stable sol. Thus, in the series of sols produced in this study the final pH of the solution was controlled by the careful addition of ammonium hydroxide solution to give a range of pH values at which the gel was formed between 1.0 and 6.7. The experimental method followed for the preparation of the sols was the following: a small quantity of tetra-isopropoxide was added to doubly distilled water with continuous stirring. Nitric acid (65%) was then added and subsequently ammonium hydroxide (3%) to control the final pH. The first sol with no ammonium hydroxide led to a pH of 1.0. Further samples with addition of varying amounts of ammonium hydroxide were produced at pH's of 3.0, 4.5, 5.7 and 6.7, respectively. A typical preparation was as follows: 15 ml Ti(OPr)4 + 180 ml H20 + 1.3 ml HNO3
(2)
All of the sols were subsequently dried at 60~ to remove the organic part and form the gel. The textural properties of these materials were then characterised by nitrogen adsorption/desorption isotherms and X-ray diffraction. The development of the textural properties with heat-treatment was studied for the material formed at a pH of 5.7. This sample was divided into several fractions that were subsequently calcined with a heating rate of 5~ min l in air from ambient up to 200~ 300~ 400~ or 500~ respectively, and maintained at this temperature for 3 h. Higher temperatures were not employed since the loss in surface area and the phase transformation to rutile would severely diminish any photocatalytic activity.
739 2.2 Characterisation techniques The specific surface area (SBET) and porosity: pore volumes and pore size distributions, of the gels were determined from measurement of nitrogen adsorption/desorption isotherms at-196~ on a Carlo Erba 1800 Sorptomatic. The samples were previously outgassed overnight to a vacuum of _< 10.2 Pa, to ensure a dry clean surface free from any loosely held adsorbed species. The SBET was calculated by application of the BET equation [ 15] taking the area of the nitrogen molecule as 0.162 nm 2 [16]. However, it should be noted that the SBET calculated for microporous materials may only be taken as a guideline since the adsorption in narrow micropores is a volume filling effect [ 17]. Owing to the microporous nature of the gels the linear range of the BET equation was usually located between p/p~ 0.02 to 0.15. The micropore volume and external surface area, i.e., the area not associated with the micropores, were calculated using a t-plot analysis [18], taking the thickness of an adsorbed layer of nitrogen as 0.354 nm assuming that the arrangement of nitrogen molecules in the film is hexagonal close packed [19]. The mesopore volume was calculated from the amount adsorbed at p/p~ = 0.96 on the desorption branch of the isotherm, equivalent to 50 nm pore diameter, minus any micropore volume calculated from the corresponding t-plot. The average micropore diameters were calculated by the MP method applied to the corresponding t-plots of the samples [20]. The mesopore size distributions were calculated by the BJH [21 ] method, assuming cylindrical nonintersecting pore geometry. The t-plot, MP method and BJH method used the Harkins-Jura [22] relationship between the relative pressure and the thickness of the adsorbed layer since their data used nitrogen adsorption over a nonporous anatase sample. The phase compositions of the gels were determined from their X-ray diffraction patterns, measured on a Philips "X'Pert-MPD using Cu Ka radiation (~ = 0.1518 nm) between 20-60 ~ 20 in steps of 20 = 0.05 ~ The identification of the anatase, rutile and brookite phases were made by comparison with the corresponding standards. The fractions of these three phases in the gels were calculated as relative amounts from the intensities of the major peaks of each phase. Absolute concentrations could not be given due to the possible presence of amorphous material.
3. RESULTS AND DISCUSSION Using the sol-gel technique five titania gels were prepared at final pH for the formation of the gels ranging from pH 1-6.7. The nomenclature adopted in this study starts with the letters SG followed by the pH of formation and finally a letter to indicate the final heat-treatment temperature: a, b, c, d, or e being equal to 60~ 200~ 300~ 400~ or 500~ respectively. The original five materials were prepared at pH's of 1.0, 3.0, 4.5, 5.7 and 6.7. The results obtained on these five materials dried at 60~ will be first discussed. The sample prepared at a pH of 5.7 was then calcined at various temperatures. The effects on the SBET, porosity and phase composition will be discussed separately. 3.1 Textural characterisation of materials dried at 60~ The results obtained from the textural analysis of the five gels after drying at 60~ in order to eliminate the organic are presented in Table 1. The SBET values are in parentheses since due to the microporous nature of the materials they are only apparent surface areas. It should be noted that under these mild conditions the X-ray data were only relative due to the amount of amorphous phases present in the samples.
740
Table 1 Textural Results for Titania gels prepared at different pH Sample Area External Micropore Mesopore Micropore Relative Composition SBET Area Volume Volume Diameter Anatase Rutile Brookite (m2g-1) (m2g-1) (cm 3g-1) (cm 3g-l) (nm) (%) (%) (%) SG SG SG SG SG
1.0a 3.0a 4.5a 5.7a 6.7a
(108) (198) (389) (323) (241)
2 4 10 3 11
0.045 0.112 0.231 0.203 0.171
0.003 0.009 0.027 0.006 0.020
0.7 1.1 1.3 1.4 1.7
76 77 58 77 88
10 7 0 7 0
14 16 42 16 12
From the results presented in Table 1 the wide variation in the SBET with the pH of formation was most obvious. The SBETwas lowest for the sample prepared at pH 1 but rose to a maximum at pH 4.5 and then fell. All of the materials dried at 60~ were microporous, giving rise to type I isotherms with very little external surface area, or mesopore volume. However, the width of the micropores determined by the MP method was found to progressively increase as the pH at which the sol was formed was raised. This gradual widening of the micropores was apparent from the change in the shape of the nitrogen isotherms shown in Fig. 1. 200 "7 ~
150
a~ or/)
100
<
~
5o
o
>
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/p ~ Fig. 1. Nitrogen isotherms of titania gels: SG 1.0a (o), SG 3.0a(A), SG 4.5a ( , ) , SG 5.7a (11) and SG 6.7a (-~). Thus, the material prepared at a pH of 1 had only narrow micropores and gave rise to a very sharp knee at low relative pressm;e. The other samples displayed progressively wider micropores with a less sharp knee in the isotherm and the characteristic slope up to a relative pressure of c. 0.4. It may also be noted that samples produced at a pH of 3 or more also developed a hysteresis loop between the adsorption and desorption branches, that was indicative of the development of a mesoporous structure. However, the closures of these
741 desorption branches at relative pressures close to 0.42 indicated that the mesoporous structures were not well defined. Closure of the hysteresis loop at this relative pressure is generally considered to be due to percolation effects, where due to the complexity of the pore structure the liquid adsorbate is trapped within the pores until finally the liquid meniscus breaks and the pores suddenly empty. Calculation of the pore size distribution from the adsorption branch of the isotherms gave a maximum in the pore diameter that gradually increased from about 2.5 nm for SG 3.0a to 3.8 nm for SG 6.7a. Although this range is on the lower limit of applicability for the BJH method [21] it was indicative of the shift in the average pore size to supramicropores and narrow mesopores as the pH of formation was raised. The increase in the pore diameter as the pH of formation was raised has been observed with other titania sol-gel systems [11,12,13]. In these systems the development of the gel porosity was linked to the removal of the water ligands from the solvated cations. The increase in pH leads to a greater degree of ligand displacement and thus to a more highly developed pore structure with a corresponding increase in the SBET.However, in this study the pore volume and SBET reached a maximum at a pH of 4.5 and then reduced, although the average micropore diameter always increased with increasing pH. The analysis of the crystalline phases present by XRD was only relative due to the presence of amorphous fractions. However, it was observed that the crystallinity decreased in the order: SG 1.0 > SG 3.0 >_ SG 6.7 > SG 5.7 >> SG 4.5. This order was the inverse of the S~ET: materials with greater crystallinity had lower specific surface areas, while samples that were more amorphous had higher specific surface areas. 3.2 Textural characterisation of heat-treated materials The possible changes in the textural properties with heat-treatment were studied with sample SG 5.7. This sample was divided into various fractions and subsequently heat-treated at 200~ 300~ 400~ and 500~ The results of the textural analysis of these heat-treated materials, together with those for the sample dried at 60~ are given in Table 2.
Table 2 Textural properties of SG 5.7 after heat-treatment Sample Area External Micropore Mesopore S~EV Area Volume (m2g-1) (mZg-1) (cm 3g-l) SG SG SG SG SG
5.7a 5.7b 5.7c 5.7d 5.7e
(323) 219 162 98 48
3 219 162 98 48
0.203 0.000 0.000 0.000 0.000
Pore
Relative Composition
Volume Diameter Anatase Rutile Brookite (cm 3g-l) (nm) (%) (%) (%) 0.006 0.296 0.281 0.211 0.182
1.4 4.7 5.9 7.4 11.4
77 91 90 80 42
7 4 5 12 58
16 5 5 8 0
From the results presented in Table 2 several interesting features in the changes in the texture of this material with heat-treatment may be observed. The sample dried at 60~ gave rise to a type I isotherm due to its microporous nature, with a negligible extemal surface area and mesopore size contribution. However, on heat-treatment at 200~ a mesoporous material was formed with a severe reduction in the surface area, but an increase in the overall pore
742 volume. As the heat-treatment temperature was increased the SBETwas further reduced but the mesoporous nature of the material was maintained although the average pore diameter steadily increased. These changes in the porosity of the samples with heat-treatment may be appreciated from the observed changes in the form of the nitrogen isotherms shown in Fig. 2 and the PSD's presented in Fig. 3. 200 "7 el)
~
r
150
or~ 100
< ~
5o
0
0.0
0.2
0.4
0.6
Relative Pressure Fig. 2. Nitrogen isotherms of SG 5.7 heat-treated at 60~ (11) and 500~ (-~).
0.8
1.0
(p/p ~ (o), 200~
(A), 300~
( , ) , 400~
2.0
1.5
>
1.0
0.5
0.0
"'"
0
5
10
15
20
Pore Diameter (nm) Fig. 3 Pore size distributions for SG 5.7 heat-treated at 200~ (A), 300~ ( , ) , 400~ (11) and 500~ (-*).
743 The average pore size for sample SG 5.7 a in Table 2 was determined by the MP method. The average pore diameters for the heat-treated samples were taken from the maxima in the PSD curves drawn using the BJH method from the desorption branches of the corresponding isotherms presented in Fig. 3. It may be noted that the most significant change in pore diameter was caused by heat-treatment at 500~ This shift along with the severe reduction in the SBETto only c. 50 mZg1 was mainly due to the transformation into rutile. Since for photocatalytic reactions titania should be in its anatase form it is important to note that with heat-treatment up to 300~ the sample was 90% anatase. With higher temperatures both the loss in SBET and the conversion to rutile would reduce the photocatalytic activity of the material. However, it has been shown that for high photocatalytic mineralisation of VOCs the reaction temperature should be maintained below c. 150~ [9]. Thus, with this material heat-treatment at between 200~176 would be sufficient to form the anatase washcoat and maintain a high SBET. The choice of the correct heat-treatment temperature depends not only on forming a high surface area anatase support but also on the width of the pores produced since these may control limitations due to diffusion of the gases to be treated into the active layer. Thus, although in principal a higher surface area would be expected to lead to a more active catalyst, diffusion limitations of the gases to be treated onto the active surface, especially for the microporous materials produced on drying at 60~ could be a concern. 4. CONCLUSIONS These results demonstrate that by using a sol-gel technology high surface area titanias can be produced. Furthermore, by careful control over the pH of formation the pore width of the microporous gel may be controlled. With heat-treatment it was demonstrated that the microporous gel was transformed into a mesoporous material. If the heat-treatment temperature did not exceed 300~ the titania was 90% anatase. At higher temperatures the transformation into the less active rutile phase began to take place. As the heat-treatment temperature was increased there was a steady reduction of the SBET, with a corresponding widening of the mesopores, due to sintering of the material. Thus, by careful selection of the final heat-treatment temperature the pore size distribution and SBETcould be controlled. Since after any wash-coating step it is necessary to calcine the composite material in order to fix the thin active layer to the surface this sol-gel preparation technique could be readily adapted to produce photocatalytic coatings with a tailor made porosity. REFERENCES .
2. 3. 4. .
6. 7.
European Community Official Directive, 94/63, No. L 365/24 (1994). T.T. Ibusuki and K. Takeuchi, Atmos. Environ. 20 (1986) 1711. J. Peral and D.F. Ollis, J. Catal. 136 (1992) 554. J. Blanco, P. Avila, A. Bahamonde, E. Alvarez, B. S~mchez and M. Romero, Catal. Today 29 (1996) 437. L.A. Dibble and G.B. Raupp, Environ. Sci. Technol. 26 (1992) 492. L.A. Dibble and G.B. Raupp, Catal. Lett. 4 (1990) 345. M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann, Chem. Rev. 95 (1995) 69.
744
10. 11. 12. 13. 14. 15. 16.
17. 18. 19. 20. 21. 22.
X. Fu, W.A. Zeltner and M.A. Anderson, in Semiconductor nanoclusters, P.V. Kamat, D. Meisel (Eds.), Elsevier Science, New York, 11 (1996) 445. P. Avila, A. Bahamonde, J. Blanco, B. Sfinchez, A.I. Cardona and M. Romero, Applied Catalysis B: Environmental 17 (1998) 75. R.J. Candal, W.A. Zeltner and M.A. Anderson, J. Adv. Oxid. Technol, 3, 3 (1998) 270. J. Ragai, K.S.W. Sing and R. Mikhail, J. Chem. Tech. Bioltechnol., 30 (1980) 1. J. Ragai and K.S.W. Sing, J. Chem. Tech. Bioltechnol., 32 (1982) 988. J. Ragai and K.S.W. Sing, J. Colloid and Interface Sci. 101, 2 (1984) 369. Q. Xu, and M.A. Anderson, J. Mater Res. 6 (1991) 1073. S. Brunauer, P.H. Emmett and E. Teller, J. Amer. Chem. Soc., 60 (1938) 309. J. Rouquerol, D Avnir, C.W. Fairbridge, D.H. Everett, J.H. Haynes, N. Pericone, J.D.F. Ramsay, K.S.W. Sing and K.K. Unger, Pure and Appl. Chem., 66, 8 (1994) 173. M.M. Dubinin, E.D. Zaverina and D.P. Timofeeva, Zhur. Fiz. Khim., 23 (1949) 1129. B.C. Lippens and J.H. de Boer, J. Cat., 4 (1965) 319. B.C. Lippens, B.G. Linsen and J.H. de Boer, J. Cat., 3 (1964) 32.9. R.S. Mikhail, S. Brunauer and E.E. Bodor, J. Colloid Interface Sci., 26 (1968) 45. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Amer. Chem. Soc., 73 (1951) 373. W.D. Harkins and G. Jura, J. Amer. Chem. Sot., 66 (1944) 1362.
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
745
The effect of geometry and axial orientation of spheroidal particles on the adsorption rate in a granular porous medium
F. A. Coutelieris
National Center for Scientific Research 'Demokritos', 15310 Aghia Paraskevi Attikis, Greece
The mass transport problem from a Newtonian fluid to a swarm of prolate and/or oblate spheroidal adsorbers under creeping flow conditions is considered here. The spheroidal-incell model is used for the analytical description of the flow field within the swarm. The convective diffusion equation along with the appropriate boundary conditions for the description of the adsorption upon the solid surface is solved analytically for high Peclet numbers and numerically in the low Peclet regime. In both cases, analytical expressions for the adsorption rate are obtained. It is found that the oblate geometry offers significant advantage for capturing the diluted mass compared with the prolate one even in strongly convective environments. It is also shown that the assumption of instantaneous adsorption overestimates significantly the adsorption efficiency.
1. INTRODUCTION Modeling mass transport through swarms of particles has attracted significant interest mainly in relation to fluid flow and the associated physicochemical processes. Most of the proposed models derive analytical solutions for the mass transport problem under creeping flow conditions by assuming spherical or cylindrical shape of the particles [1]. Happel and Kuwabara have presented models that solve analytically the creeping flow problem for spherical geometry [2,3]. Both these models are based on the representation of the overall solid mass of the swarm by just one spherical particle, which is embedded in a spherical or cylindrical liquid envelope keeping the porosity equivalent to that of the swarm. However, in almost all practical applications, the particles are of spheroidal shape instead of spherical [4]. An analytical model for the representation of the flowfield within the swarm of spheroidal particles has recently been proposed for both Happel- and Kuwabara-type boundary conditions [5,6]. This analytical solution has already been applied in the study of mass transport processes within swarms of spheroidal particles for both high and low Peclet values [7,8] in a way quite analogous to previous investigations concerning particles of spherical shape [9-11 ].
746 ~0=0
z
~
q=q~
I " O=n Fig. 1. Prolate and oblate "spheroid-in-cell" models The weak point of all these approaches is the postulation of instantaneous adsorption occurring on the liquid-solid interface. This approximation, based on the assumption of the very thin diffusion layer, which is valid only for high Peclet values, produced analytical expressions for the concentration profile in that regime, while, for low Pe a numerical treatment is necessary. Unfortunately, instantaneous adsorption is rather rare corresponding to a very limited range of applications. A more realistic approach is adopted here based on an adsorption - heterogeneous reaction - desorption mechanism, which describes the adsorption of the diluted mass upon the solid surface with high accuracy [12-15]. More precisely, it can be supposed that the component A, which is diluted in the bulk phase, is initially adsorbed by the solid surface where a heterogeneous reaction takes place and its products, which are considered inactive and of very low concentration, are again desorbed in the bulk phase. The adsorption is assumed to occur due to vacant sites that are normally distributed over the solid surface while the whole process can be described by an overall rate according to basic thermodynamics analysis [ 16].
2. THEORY
Consider a solid spheroid having long semi-axis a3 and semi-focal distance a = ~f~ - 1, which is surrounded by another confocal spheroidal liquid envelope, whose thickness is adjusted so that the porosity of the granular medium is equal to that of the model. The spheroidal-in-cell model, predicts the stream function, ~, for creeping flow conditions in the prolate coordinates system (r/,0) as follows [5]: a [A2G2(coshrl)+ A3 [ 5G4(cosh r/~ ) Gl(cOsh~7)+ G4(coshrl)] ~(rl,O)= --~ GI (cosh r/p )
+A4H2(coshrl)] G2(cosO)
(1)
where D, A2, A3 and A4 are r/- and 0-dependent coefficients defined in Dassios et al. [5] and G,,(x) and H,,(x) are the Gegenbauer polynomials of the first and second kind, respectively, of degree -1/2 and of order n. The governing equation for the steady state mass transport in the fluid phase within the porous medium can be written in prolate spheroidal coordinates and in dimensionless form as: OCA 9CA Pe~ "920A U q ~ + U O ~ -90 a4sinh 2 r/+sin2 0 ( - ~ 2 +c~ av
9CA
02CA + - ~ +c~
OCA 90 )
(2)
747 The above equation can be integrated with the following boundary conditions: Ca
(r/=r/~,0)= 1,
(~CA]
0 _<0 _
=0,
(3a)
0_<0< ~r
(3b)
(~CAI --0,
r/~ _
(3c)
OCa I --0, 00 o--0
r/~ _
(3d)
O0 o=,
and
Pe-' IOCA] .._ksCAS' a4sinh 2 r/~ + sin 2 0 k or~ J,~=~
0<0<~r
'
(3e)
The first boundary condition is equivalent to the well-known Levich approach (CA=I for q--+ m ), according to which, it is supposed that the concentration values vary only within a very thin concentration layer while it is supposed to keep its bulk value elsewhere [9]. Eq. (3b) has been proposed by Coutelieris et al. [8] in order to ensure the continuity of the concentration upon the outer boundary of the cell for any Peclet number. Furthermore, eq. (3c) and (3d) express the axial symmetry that has been assumed for the problem. The boundary condition (3e) can be considered as a significant improvement of Levich approach, where instantaneous adsorption on the solid-fluid interface (Ca(~=~a,O)=O) is also assumed for any angular position 0. In particular, eq. (3e) describes a typical adsorption, 1st order reaction and desorption mechanism for the component A upon the solid surface [ 12,16] where ks is the rate of the heterogeneous reaction upon the surface and the concentration of component A upon the solid surface, Cas, is calculated by solving the non linear equation
[k~+kd +kaCA(rl~,O)NICAs -kaCa(rl~,O)~m= 0
(4)
which relates the surface concentration of A, Cas (a quantity that is difficult to determine experimentally), with its concentration in the bulk phase very close to the solid surface, Ca(rl~,O). In the above eq. (4), the terms ka and kad denote the adsorption and desorption rate of component A, respectively, ~m is the concentration of the vacant sites on the solid surface and N is the Avogadro number [12]. In the high Peclet number regime (Pe >> 1), the concentration boundary layer is very thin compared to the local radius of curvature of the particle, and the curvature term coth r/0 CA/0r/ can be neglected. The same holds for the tangential diffusion terms 0 zcA / OOz and cotOOCA/O0 , which were also shown to be insignificant for high Peclet values [8]. In this case, eq. (2) becomes parabolic on 0 and it can be solved analytically in a manner quite similar to that of Coutelieris et al. [7], providing concentration profiles in the fluid phase as follows z 4t3
CA(Z) = C2 ~e -~ dt + c 3
o
(5)
748 where
I
aEPe
z =
(6)
4 D (sinh2/7,~ _1) /7 f (0)
and c2 and c3 are coefficients that can be calculated by solving the non-linear system
3•
(7a)
1.17c2+c3=1
(7b)
E f (O) c2 - R(c 3) = 0 4aSPe2 4sinh 2/7,~ + sin20
where E and flO) are terms defined by Coutelieris et al. [7]. Further mathematical manipulations lead to the following expression for the overall adsorption efficiency, 20,
1.26 33~e 2~r Iczf(O)dO 2o -- O~2sinh2/7p o
(8)
which expresses the ratio of the adsorbed mass to the overall amount of mass moving within the porous material. The corresponding expressions for the case of oblate spheroids-in-cell are quite analogous to those of the prolate case. Differences are observed only in the following equations
~.
E
f (O)
c2_R(c3)=0
4a 5Pe2 4CONh2 /Ta - sin 2 0
(7a')
and
-
20 -
1.26 a~ cosh~ r/p
3 3 ~ e 2,~ ICzf(O)dO o
(8')
The moderate and low Peclet regimes are characterized by significant magnitudes of all the terms of eq. (2), which should therefore be solved numerically. A non-uniform finite difference discretization scheme has been chosen for the system, estimating the overall adsorption efficiency, 20, as follows:
sinh o
2~ = Pe O~2 sinh 2/Tp
Isin0 (CA1
/1"
0
\ 0/7 )q=,7,,
and
sinh o
Isin0
2~ = Pe a 2 cosh 2/7,0 0
~. 0/7 )q=r/~
for the cases of prolate and oblate spheroids, respectively.
749 3. RESULTS & DISCUSSION Fig. 2 shows the concentration profiles for the component A at different angular positions for the cases of prolate (a) and oblate (b) spheroids-in-cell. For high Peclet numbers (Pe=1000), higher concentration gradients are found for prolate spheroids compared to those for oblate ones, as it has also been observed previously for the case of instantaneous adsorption [7]. Principally, the concentration decreases, as the angular position is closer to the stagnation point and approaches its bulk value at distances less than 25% of the envelope thickness, i.e. close enough to the solid surface in all cases. Dashed lines in Fig. 1 denote the concentration profiles for a low Pe value (Pe=15). Note that this value has been selected to be high enough to ensure the satisfaction of the condition C3CA/ = 0 at r/=r/p and 0=0 [8]. A /0r/ large reduction of the concentration gradients towards the solid surface is observed, compared with the case of high Peclet numbers, because the diffusion starts playing a dominant role over convection as Pe decreases. Furthermore, an important weakness of Levich approach can be observed in the very low concentration regime at r/=rh~ within the tail region (0---~r) where the concentration tends to reach the constant bulk value. Levich's approach fails to predict acceptable concentration values in this area because the fundamental assumption of very thin concentration boundary
1.0
~j0
' --0
0.8 .
j 0.6
""
""
....
"
............
~ ~ ""
" ....
-
.
.
.
.
.
.
.
"
-
Pe=1000
. . . . .
8=n'/2 ./
0.4
Pe=l
5
a3=2, E=O.9
0.2
(a)
O=n .
0.0
~ .
~
............
0.0
0.2
9 ......
.
.
.
.
~--
0.4
.
.
.
.
.
.
.
.
,
0.6
,
0.8
1.0
(q'qa)/(q~-qa)
1.0 0.8
~
J .......... ........
0.6
.--~
t9 0 I
0.4
~
e=~/2 , !.,. * . . . .
0.2
......... Pe=1000
......
Pe=15
................. a3=0.5, E=0.9 8=n ,4,.../ . . . . . . . . . . . . .
o.o
....
0.0
T .......
0.2
7 ......
0.4
(b)
,
,
,
0.6
0.8
1.0
(n-no)/(n~-qo) Fig. 2. Concentration profiles for prolate (a) and oblate (b) spheroidal geometry for high and low Peclet at three different angular positions
750 layer is not valid in this region even for high Pe. Finally, it should be stressed that the concentration upon the surface, Ca(rl,~,O), takes its higher value at the impact point and decreases monotonically as 0 tends to zr for the case of prolate spheroids and high Peclet number. The maximum concentration value appears at the equator for the case of oblate spheroids where the thickness of the diffusion film becomes minimum [8]. This behavior is not observed for low Peclet numbers where the surface concentrations are extremely higher than those of the high Pe case and therefore, it can be considered that the accessibility of the solid surface is high enough to produce smooth concentration profiles for the adsorbed mass. The dependence of the overall adsorption efficiency on the Peclet number for both prolate and oblate geometries is presented in Fig. 2. A considerable decrease of the adsorption efficiency is observed as Pe increases. This is expected, as the more convective flows do not allow the component A to be captured by the solid surface. In general, oblate spheroids present higher frontal surface than the prolate ones, and therefore, their ability for adsorption is higher. This advantage of oblate geometry disappears in the case of low Peclet where almost all parts of the adsorbing solid surface become active as the environment becomes more diffusive. The values for the overall adsorption efficiency predicted considering the model of instantaneous adsorption are 10-35% higher, depending on the Pe, the geometry, the porosity and the order of the reaction, than those calculated by suggesting the more realistic model of adsorption, described by eq. (3e) & (4) and low Peclet values, because the concentration on the solid surface attains in the latter case non zero values making the difference Ca(l'lfl,O)-CA(Fla,O) very small for some 0-values. The impact of this effect is higher than that of the decrease of the concentration gradients observed when the realistic adsorption process is adopted instead of the instantaneous adsorption postulation.
0.12
,
~a3=2
0.08 0.04
0.00 10
100
1000
10000
Pe Figure 3: Influence of the Peclet number on the adsorption efficiency for typical prolate and oblate spheroids-in-cell. 4. CONCLUSIONS The problem of mass transfer from a moving Newtonian fluid to a swarm of prolate and/or oblate stationary spheroidal adsorbing particles under creeping flow conditions is solved using a spheroidal-in-cell model. The flow field through the swarm was obtained by using the spheroid-in-cell model proposed by Dassios et al. [5]. An adsorption- 1st order reaction - desorption scheme is used as boundary condition upon the surface of the spheroid in order to describe the interaction between the diluted mass in the bulk phase and the solid surface. The convective diffusion equation is solved analytically for the case of high Peclet numbers where the adsorption rate is also obtained analytically. For the case of low Pe a non-
751 uniform finite difference scheme is used to treat the problem numerically. It is found that the adsorption rate is higher for oblate spheroids-in-cell compared to spheres-in-cell and prolate spheroids-in-cell. Consequently, the oblate geometry offers a significant advantage compared with the prolate one for convective environments. However, this tendency fades away as the environment becomes diffusive. Results presented here are more accurate than previous ones produced under the assumption of instantaneous adsorption, which leads to a 10-35% overestimation of the adsorption efficiency. REFERENCES 1. C. Tien, "Granular filtration of aerosols and hydrosols", Series in Chemical Engineering, p.65, Butterworths, Boston (1989) 2. Happel J.,AIChEJ., 4, 197 (1958) 3. Kuwabara S., J. Phys. Soc. Japan., 14, 527 (1959) 4. F.S. Ham, Quart. Appl. Math., 17, 137 (1959) 5. Dassios G., Hadjinicolaou M. and A.C. Payatakes, Quart. Appl. Math., 52, 157 (1994) 6. Dassios G., Hadjinicolaou M., Coutelieris F.A. and A.C. Payatakes, Int. J. Eng. Sci., 33, 1465 (1995) 7. Coutelieris F.A., Burganos V.N. and A.C. Payatakes, J. Colloid Interf. Sci., 161, 43 (1993) 8. Coutelieris F.A., Burganos V.N. and A.C. Payatakes, AIChE J., 41, 1122 (1995) 9. Levich V.G., "Physicochemical hydrodynamics", p. 80, Pentice-Hall, Englewood Cliffs, N.J. (1962) 10. Pfeffer R. and J. Happel, AIChE J., 10, 605 (1964) 11. Tardos G.I., Gutfinger C. and N. Abuaf, AIChE J., 22, 1147 (1976) 12. P.Atkins and J. DePaula, "Physical chemistry", 7th ed., Freeman, N.Y. (2001) 13. Suzuki M. and J.M. Smith, AIChE J., 18, 326 (1972) 14. Weber T.W. and R.K. Chakravati, AIChE J., 20, 229 (1974) 15. Peters M.H., Jalan R.K. and D. Gupta, Chem. Eng. Sci., 40, 723 (1985) 16. Smith J.M., "Chemical engineering kinetics", McGraw-Hill, Tokyo (1981)
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
753
The effect of Peelet on the Sherwood number in high porosity granular media
F. A. Coutelieris, M.E. Kainourgiakis and A.K. Stubos National Center for Scientific Research 'Demokritos', 15310 Aghia Paraskevi Attikis, Greece
The Sherwood number representing the mass transfer coefficient for transport from a moving fluid to a swarm of spherical particles is calculated in the range of high porosities. The mass transport problem is solved both analytically and numerically under the assumption of instantaneous adsorption upon the liquid-solid interface. The velocity components within the liquid phase are obtained either by using the analytical formulations of the sphere-in-cell model or by solving numerically the creeping flow problem in a stochastically constructed packing of spheres. It is found that the agreement between numerical and analytical data is in general reasonably good for the porosity range considered. Limitations of both the numerical and analytical approaches are identified. Numerical predictions are deteriorating rapidly for extremely high porosities (above 0.95). On the other hand, the accuracy of analytically obtained Sherwood values becomes weaker as the porosity decreases because the fundamental assumptions of the analytical model are not satisfied in this case.
1. INTRODUCTION Mass transport within high porosity granular porous media is a physicochemical process, which attracts considerable interest from both the research and technological point of view. As porous media are generally characterized by highly complex internal geometry, appropriate modeling of such a process is necessary for the derivation of meaningful conclusions. Several analytical and numerical models have been proposed in this respect, dealing in general with the estimation of the overall mass transport characteristic parameters such as the Sherwood number and the adsorption rate for a wide range of different geometries [1-4]. Most of these models derive analytical solutions for the mass transport problem under creeping flow conditions based on the representation of the overall solid mass of the granular medium by just one spherical particle, which is embedded in a spherical liquid envelope [5,6]. The ratio of the solid volume to the total volume represents the solid volume fraction, i.e. the porosity of the medium. On the other hand, considerable effort has been invested in performing numerical simulations for the study of convection, dispersion and interfacial transport in homogeneous porous media. The macroscopic approach developed by Quintard
754 and Whitaker [7] based on the volume averaging method, is a representative example where efficient numerical schemes for the solution of the local closure problems were also proposed. In almost all the above models it is supposed that the solid surfaces instantaneously adsorb the mass, which is diluted within the fluid that flows through the medium. This approach has been initially proposed by Levich [8] who obtained analytical expression for the overall Sherwood number for the case of a single isolated sphere in an unbounded fluid. Significant improvements to this model, including the sphere-in-cell model and an intelligent representation of the mass transport conditions, have been presented later [9-11 ]. The objective of this work is to determine the influence of the Peclet number (expressing the relative importance of convection and diffusion in the medium) on the fluid/solid mass transfer coefficient, expressed by the Sherwood number. The focus in this part of the work is on granular media of relatively high porosities (0.6 to 0.95). To attain this objective, mathematical models for the mass transport problem through a high porosity granular medium under creeping flow conditions are employed. Two approaches are considered: a numerical solution for moderate Peclet values (500_>Pe>70) and relatively low porosities (e>0.6) and an analytical approach based on the sphere-in-cell model for high porosities (e>0.8) and high Peclet values
(Pe>100). The overall Sherwood number is calculated in all cases and
comparisons between analytical and numerical results are performed.
2. M O D E L I N G APPROACH Consider a Newtonian incompressible fluid containing a component A in high dilution (<0.05M) and moving under creeping flow conditions within a relatively high porosity porous medium. The solid surface adsorbs instantaneously the component A. The mass transport regime (convection and/or diffusion) is expressed by the value of the Peclet number, defined as
Pe=
(1)
a uo
D where a is the characteristic length, uo is the characteristic velocity and D is the diffusion coefficient.
2.1. Analytical solution Consider a solid sphere of radius a, which is surrounded by another concentric spherical liquid envelope of radius fl, whose thickness is adjusted so that the porosity of the medium is equal to that of the model. The governing equation for the steady state mass transport in the fluid phase within the porous medium can be written in spherical coordinates as
Ur
OCA l~o OCA +__
8r
r
80
= p e -~
-'-"-"7"-( 82CA 2 8 c a +_
~ Sr ~
. +
r 8r
1 a2CA
FTU
cot 00C A + ~ ~ r z 80
where the velocity components are given as [5,6]
)
(2)
755
~r---V 1--~ r
-~r;
(3a)
cosO
3Iar I - - l(a)31sinO uo=U [ 1--~ 4\r) j
(3b)
The above transport equation can be integrated with the following boundary conditions"
CA(r=fl,O)=l, OCA1 = O,
0 < 0 < zr
(4a)
0 < 0 < Jr
(4b)
-"0,
a < r
(4c)
I~CA1 -" O,
a < r _
(4d)
Or r=p
I~CAI
O0 o:~ O0 0--0 and
CA(r=a,O)=O,
0 SO _<~r
(4e)
The first boundary condition is equivalent to the Levich approach, given elsewhere as r--~ oo [8]. Eq. (4b) has been proposed by Coutelieris et al. [1] to ensure continuity of concentration at the outer boundary of the cell for any Peclet number. Moreover, eq. (4c) and (4d) represent the axial symmetry assumption considered for this problem. The last boundary condition describes a typical instantaneous adsorption mechanism for the component A upon the solid surface. For the case of high Peclet numbers (Pe >>1), the concentration boundary layer is very thin compared to the local radius of curvature of the particle, and, thus, the curvature term as well as the tangential diffusion terms can be neglected as it has already been shown [1,8,11]. In that case, eq. (2) becomes parabolic on 0 and can be solved analytically in a manner quite similar to that of Levich, providing concentration profiles in the fluid phase of the form
Ca=l for
Z
CA(Z)
4t3
= 1.17Ie ~
o
dt
(5)
where
3 Pe sin 0 4 a 3 r 0 - 0.5sin 20
3•
z =
(6)
The overall Sherwood number, Sho, is defined as:
Sho=
a k0 D
(7)
756 where ko is the overall mass transport coefficient. After some mathematical manipulations, the following expression for Sho, can be derived 2~
Sho = 0 . 7 9 3 ~
; f (O)dO
(8)
o
The moderate and low Peclet numbers correspond to cases where all terms of eq. (2) are significant and numerical solution is required. A non-uniform finite-difference discretization schema has been chosen for solving the boundary value problem of eq. (1) with boundary conditions (3a-e), estimating the overall Sherwood number, Sho, as follows:
S ho = 4 rc21 a ~ CA ( -'~ "~) --CA-'(~' O) dO
(9)
Finally, a predictive form for the overall Sherwood number can be used providing satisfactory predictions for any Peclet value [12]:
Sh o = Sho(a, Pe ---~O) +O.997 g(a)Pe '/3
(10)
where numerically derived values of g(a) have been tabulated by Coutelieris et al. [13] and respective values for Sh o (a, Pe -~ 0)by Burganos et al. [12]. 2.2. Numerical solution The pore scale boundary value problem is described by the advection-diffusion equation ~C a
0t
+ V'('VCA ) -- DV2CA
(11)
where CA is the concentration of component A in the flowing phase, t is time and v is the interstitial fluid velocity. At the fluid-solid interface it can be considered (12)
CA "-0
To obtain numerically the mass transfer coefficient, a porous medium is stochastically constructed in the form of a sphere pack. Specifically, the representation of the biphasic domains under consideration is achieved by the random deposition of spheres of radius R in a box of length L. The structure is digitized and the phase function (equal to zero for solid and unity for the pore space) is determined in order to obtain the porosity and to solve numerically the convection- diffusion problem. The next for this purpose is to obtain the detailed flow field in the porous domain through the solution of the Stokes equations:
Vp = ,uV2v
(13a)
V.v =0 v=0
(13b) (13c)
at fluid- solid interface
757 where p and p are the pressure and the viscosity of the fluid, respectively. A staggered marker-and-cell (MAC) cell is used for the solution of the 3D stokes flow problem in a box made of cubic elements. An initial guess for p is determined through the solution of a Laplace equation. Next, v is calculated from the corresponding momentum balances and the continuity term V. v = 0 is determined. The pressure is corrected through an artificial compressibility equation of the form: dP = V . v
(14)
dt
The above steps are repeated until convergence is reached. Having determined the velocity field in the medium, the procedure outlined by Quintard and Whitaker [7] is followed to determine through an upscaling procedure the mass transfer coefficient. This approach proceeds by decomposing the local concentration into its interstitial average and a fluctuation Cw = (CA)W q- CA, . . --S W (C A )w
(15)
where the symbol ( )Wrepresents volume averaging in the fluid phase. The variable Sw is the solution of the equation [7]: 2
DV Sw-V.VSw-C-~k
*
=0
(16)
where s w = 1 at fluid- solid interface, Sw is spatially periodic, (Sw) w = O, and k* is the mass transfer coefficient defined as: = -~
, nws . V s w d A
(17)
By nws is denoted the unit vector normal to the fluid-solid interface pointing from fluid to solid while Aws reperesents the area between the fluid-solid phases. By adopting the decomposition sw =l+k*~
(18)
where g is chosen to be solution of the following boundary value problem: DV2~ - v. V N - ,z-~ = 0 ~=0
atAws is spatially periodic
(19a) (19b) (19c)
the mass transfer coefficient is given by k*=
c
(20)
758 because the average of Sw over the porous medium is nullified. The Sherwood number is then determined as:
Sho= k*l.... V DAw,
(21)
where Ic is a characteristic length. The equations (15a-c) are discretized with upwind finite differences and the resulting linear systems of equations are solved using the Succesive Over Relaxation (SOR) technique. 3. RESULTS & DISCUSSION Fig. 1 presents the overall Sherwood number derived either analytically (continuous line) or numerically (discrete points) as a function of the Peclet number for a relatively high porosity value (e=0.9). The Sho presented here has been unified by simple manipulations to be consistent for both the analytical and numerical approach. An almost monotonic increment of Sho is observed as the Peclet values increase because of the higher concentration gradients close to the adsorbing surfaces for highly convective regimes. The agreement between analytical and numerical solutions is very good with a relative difference less than 10% for any Pe value. This kind of agreement can be attributed to the high porosity value used, taking into account that the analytical approach has been shown to be highly accurate in that range [1,11,12]. This is valid the basic assumption made in all such analytical models for an adequate representation of both the velocity components and the concentration profile is that
~: = 0.9
Y l
50
100
--
f
150
-
200
250
-
I
i
300
350
-
-
i
400
Pe
Fig. 1. Dependence of the overall Sherwood number on the Peclet for a high porosity value
759 6 P e = 70
|
5i
9
4-
34 0.7
~
,
0.8
0.9
Fig. 2. Influence of the porosity on the overall Sherwood number derived both analytically (compact line) and numerically (discrete points) of a very thin boundary layer, which corresponds to an outer sphere of radius that is significantly higher than that of the inner one, i.e. the porosity takes quite high values. The dependence of the overall Sherwood number on the porosity is shown in Fig. 2 for a rather low Peclet value. Evidently, increasing porosities do not favor mass exchange efficiency and the Sho drops. High porosity values correspond to low available surface for mass transfer leading to low adsorption rates for the porous material. Numerical predictions are generally in reasonable agreement with the analytical results despite the most "erratic" behavior of the former. This is due to the fact that the high porosities, where the analytical models are in principle applicable, correspond to a very low number of spheres for the numeri
20
aft15 ~ 1 7 6 1 7 6
C
"0
10
.~
0
,,,~
o.O
0~
~ = 0.96
~
........ e=0.9 f
.,., ,,.,.
~
......
J-
~: = 0 . 8
5
L
-
50
-
'
1
m
100
q
150
~
200
250
T
1
i
300
350
400
Pe
Fig. 3. Relative difference of the analytical to the numerical results for various porosities cal representation of the porous medium. Obtaining solutions on an ensemble of stochastically constructed sphere packs may improve the accuracy of the numerical data. On the other hand, it must be pointed out that at porosities lower than 0.8, the available analytical approaches become gradually less accurate and therefore less dependable.
760 This behavior is further exemplified in Fig. 3, where the relative difference of the analytical to the numerical results is presented as a function of the Peclet number for various porosity values. The general agreement can be considered as good enough for any practical use independent of Pe. 4. CONCLUSIONS The problem of mass transfer from a moving Newtonian fluid to a high porosity granular medium considered in this work is characterized by the presence of instantaneously adsorbing spherical particles under creeping flow conditions. Analytical and numerical solutions are derived and compared for high porosities and various Peclet numbers. The well known sphere-in-cell model has been used for the analytical calculation of the flow field while the convection-diffusion equation is solved analytically in accordance to the Levich approach [8] and the associated improvements [1,12,13]. A stochastically constructed sphere pack is considered for the numerical solution of the problem. It is found that the overall Sherwood number is higher as the porosity and Peclet number increase. The comparison of the analytical and numerical approaches shows that both of them are sufficient in different ranges of parameter values (Pe & e). Indeed, the analytical model is valid for porosities higher than 0.8 while the numerical simulation fails for very high porosity values (e>0.95).
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Coutelieris F.A., Burganos V.N. and A.C. Payatakes, AIChE J., 41, 1122 (1995) Clift R., Grace J.R. and M.E. Weber, "Bubbles, drops and particles", p. 133, Academic Press, N.Y. (1978) C. Tien, "Granular filtration of aerosols and hydrosols", series in Chemical Engineering, p.65, Butterworths, Boston (1989) Kikkinides E.S. and V.N. Burganos, Phys. Rev. E, 59, 7185 (1999) HappelJ.,AIChEJ., 4, 197 (1958) Kuwabara S., J. Phys. Soc. Japan., 14, 527 (1959) M. Quintard and S. Whitaker, Adv. Wat. Resour., 17, 221 (1994) Levich V.G., "Physicochemical hydrodynamics", p. 80, Pentice-Hall, Englewood Cliffs, N.J. (1962) Pfeffer R. and J. Happel, AIChE J., 10, 605 (1964) Pfeffer R., Ind. Eng. Chem. Fundam., 3, 380 (1964) Tardos G.I., Gutfinger C. and N. Abuaf, AIChE J., 22, 1147 (1976) Burganos V.N., Coutelieris F.A. and A.C. Payatakes, AIChE J., 43, 844 (1997) Coutelieris F.A., Burganos V.N. and A.C. Payatakes, J. Colloid Interf Sci., 161, 43 (1993)
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
761
The application of J~intti's method for the fast calculation of equilibrium in case of multilayer adsorption J. A. Poulis a, C. H. Massen a, E. Robens b, G. Reichenauer c aFaculty of Applied Physics, Technische Universiteit Eindhoven, P.O.Box 513, NL - 5600 MB Eindhoven, The Netherlands, [email protected] / [email protected] blnstitut f'tir Anorganische Chemie und Analytische Chemie der Johannes GutenbergUniversitat, Duesbergweg 10-14, D - 55099 Mainz, Germany, [email protected] CPhysikalisches Institut, Julius Maximilians Universit~it, W0rzburg. Current address: Bavarian Center of Applied Energy Research, Am Hubland, D - 97074 WOrzburg, Germany, [email protected]
On the basis of a molecular model for sorption kinetics Jantti introduced a method to calculate equilibria shortly after a change of the pressure of the sorptive gas. In the present paper we apply that method for the description of multilayer adsorption. I. INTRODUCTION On the basis of a molecular model for adsorption kinetics J~intti [ 1,2] introduced a method for the fast calculation of adsorption equilibria. He measured the actual adsorbed amount at three times shortly after a change of the sorptive gas pressure. The method was designed for use in the case of an adsorption satisfying the equation
m(t):miO-e
xt )
(1)
where m (t) is the adsorbed amount observed as a function of time after a stepwise change of the gas pressure, m i is the asymptotic equilibrium value, and x is a characteristic constant for the gas/solid system under consideration. This equation is based for gas/solid systems in which simple adsorption processes occur and for an infinite number of adsorption sites [3]. In former papers we showed that the measurement of only two points may be sufficient [4] and we discussed the possibilities to make use of the method for the calculation of adsorption data in a broader scope [5]. We introduced extra parameters and adaptations to Eq. (1) [6]. We discussed for each case how the J~intti approach could be used [7]. We restricted our calculations to a small coverage and to the case that only a single layer can be adsorbed [8]. Recently we discussed the adsorption for continuously increasing gas pressure [9]. In the present paper we discuss the case that multilayer adsorption occurs without limitations of the coverage.
762 Jantti suggested to use triple points of kinetic adsorption curves. He showed that this could result in a fast determination of the equilibrium value m i. To get such a quick estimate of the values of the parameters from measured values of m, he introduced equation (2):
m(t)Z-m(t-At).m(t+At) J (t, At ) = i m - ~ - m ~ - -~) Z-m-(t +--~) "
(2)
In the case the adsorptions are characterised by Eq. (1) and J (t, At) is independent of the values of both t and At it follows:
J = m i.
(3)
In more complicated situations where the independence of J with regard to t and A t is not certain, we will use the limit function J (t) = lim At = 0 to indicate that the value of At has been chosen small enough to ensure that J (t, At) is independent of At. For this limit function we will use the symbol J* when it is the result of using an m(t) function, which results from a theoretical consideration:
J*=m -(dm/dt)2 . d2m/dt 2
(4)
We used this procedure recently in explaining measured data in the case of adsorption on a porous solid 5. Also in that situation we were able to calculate the asymptotical value of the adsorbed mass, very shortly after the beginning of the measurements. Besides the method resulted in values of the parameters used to explain the mechanism of this combination of adsorption and porosity. In the present paper we shall deal with two intriguing matters which we shall refer to as "the layer effect" and the "the coveting effect" respectively, which, at first sight, could well be cause complications when using Jantti's method. The "layer effect" will play a part when on the solid surface the number of sites available for adsorption is not unlimited so that for unrestricted adsorption more than one layer has to be considered. In the below we investigate whether this assumption intervenes with the results of Jantti's procedure. The "covering effect" is due to the fact that the molecules adsorbed on a clean surface will be bound to the surface molecules in a way which differs from that of gas molecules which are bound to a layer of previously adsorbed gas molecules. When, following J~intti's method, calculations are based on the mass of the first adsorbed molecules, the resulting value for the asymptotical adsorbed mass, may be expected to differ from the real value. As experiments with clean surfaces demand specialised procedures which are rich of pitfalls, we shall base our treatment on a computer simulation. 2. M U L T I L A Y E R A D S O R P T I O N In the following we discuss the adsorption of a finite number of layers and we discuss first the case of two layers where treatment can be simple and results in an easy insight. The fact that
763
we neglect the possible existence of a third layer brings along the restriction of the results to small coverage's. This is not too serious a restriction as J~intti's method is specially designed for use during the beginning of the adsorption measurement. We also will show the applicability of his model in the case of four layers where conclusions about the values of the parameters are not so simple but where we will be nearer to reality. The number of sites available for adsorption on the solid we refer to as n o. The number of molecules in the first and second layer are n~ and n 2 respectively. The total number n of the adsorbed molecules satisfies: n = n I + n2 .
(5)
We suppose that each molecule of the first layer, so adsorbed on one of the no sites available on the solid surface is available for second layer adsorption. This leads to the following equations:
dn, =X(no_n,)_X,(n _n2 )
(6)
dt dn 2 = X
n 1- X 2 n 2
(7)
dt where X, X1 and X2 are constants determining the probability of molecules to leave the gaseous state or the first or second adsorbed layer respectively. We shall restrict the use of these equations to situations where no > nl > n2. 3. A N A L Y S I S O F " M E A S U R E D " A D S O R P T I O N D A T A Let the data resulting from an adsorption curve be presented as an n versus t curve, and let there be at our disposal a J versus t curve, which has been calculated using Eq. (2). The original aim of J~intti's method was to get a quick (at small values of t) estimate of the asymptotical value of n. Also in the more complicated molecular situation we discuss in this paper, we get a very early information of this asymptotical value as is clearly shown by Figs. 1a+b. From Eqs. (6) and (7) we see that this asymptotical value na, satisfies:
rtas
I+ X / X 2 =no
.
l+Xl/X-Xl/X
(8)
2
At t = 0 we can, using Eqs. (6) and (7) derive:
: no.
(9)
t=O
Near t = 0 we get, using Eqs. (6) and (7) into Eq. (4)" X
J,=o = ~ "o
(1 o)
764 ~, dt J,=o
X,
X2
X n o.
(I I)
With both n and J plotted as functions of t, we can derive the values of the parameters no, X, X~ and )(2 with Eqs. (8) - (11). Examples of the use of these tools for the evaluation of the parameters are shown in Fig 1. Instead of measured data, use has been made of computer simulations by numerically solving Eqs. (6) and (7). The values of J are calculated with Eq. (2). In these figures different combinations of the parameter values are used. Care has been taken throughout that the parameter At, for use in Eq. (2), was small enough to guarantee the equality of J and J*. The resulting Fig. 1 shows in the first place that the early evaluation of the asymptotical value nas is very well possible also in the case of the more complicated molecular model used X=0,2 )(1=0,25 X2-43,3 120
.
,~,, ~ /
v
V
v
v
v
v
v
%,t ~ J
~,/ ~J
100 8O
=.6o
4O 20
v
v
v
v
v
v
v
v
v
--e- nl
oooo o ~
0
--13- n 2
_ ....
i,-Ii--1r-'lr"lr"l
I
I
1
6
11
I
16
I
I
I
i
n --X-j i
21
I
26
time step X=0,1 X1=0,25 X2-4),3 0
,~,, , ~
~_V
V
V
V
V
V
V
V
V
V
V
V
V
V,,.,V
V
V
V
~.r
m 25
j 0
i liR i i=../i - -
1
i
I
i
6
i
i
i
i
i
11
i
i
i
i
i
16
i
i
i
i
i
21
i
i
i
nl
o
n2
--~ n
ooo . . . . . . . . . i
e
--x-j i
i
26
tirre step Fig. 1. Computer simulations of the number of molecules (nl, he) adsorbed in the different layers and J as a function of time t by numerically solving Eqs. (6) and (7) in the case of two layers with different values of the binding parameters, n designates the total number of molecules adsorbed, no is taken to be 100. The values of J approach the asymptotical value nas much faster then the values of n, indicating that Jantti's method is useful for the saving of time.
765
in this paper. Besides, the evaluation of the four parameters using Eqs. (8), (11) proofs to be a very simple matter. Fig.1 shows the reaction of the adsorption on two different steps of the gas pressure from vacuum, the parameter X can be taken to be proportional to this gas pressure. To link the parameters with temperature is not so easy as X, X1 and X2 will depend in different ways on temperature. The use of two layers allows us in principle to distinguish between the gas molecules adsorbed on solid material and those adsorbed on other adsorbed molecules. The latter situation may be compared with molecules at the frontier between a gas and a liquid state. This comparison is a rough one as in many experiments there will, before the pressure step is activated, be already adsorbed molecules present at the solid surface. One might however still use different values for the parameters )(1 and X2 because of the different distances of the two layers involved from the solid surface. When the occupation of the second layer becomes too large, the results of the calculations should be considered with reserve as in reality a third layer might appear. To demonstrate that also in the case of four layers the quick evaluation of the asymptotical value n,s of n is possible, Figs. 2 and 3 are added. In Fig. 2 special attention is paid to the "layer effect", so we have restricted our treatment to situations where: X4=X3 =X2 =X~.
(12)
In that case we may use, instead of Eqs. (8) - (11):
for large values oft:
na, = n o
for small values of t:
nt=o=
+-~-S- +-~-5- + - ~ )
,
no X t
x Jt*--o = no ~ , X1
d J*)
X
2
no t=0
(14)
(15)
=
and ~ , - ~ )
(13)
(16) Sl
When using an infinite number of layers we get instead of Eq. (16):
X nas = n o
X 1-X
In Fig. 3 special attention has been paid to the "coverage effect".
(17)
766
Fig. 2. Computer simulations of adsorption in the case of four layers with equal values of the binding parameters, no is taken to be 100. J~_ntti's method proves to be useful in spite of the "layer effect". X=0,2 X 1=)(2=-X 3=X 4--~,4
n 0=100
100
.......... * v
t'-
=
- 9 50
.......
nl n2 n3
e ..... n4
1
11
21 tirne step
31
"
n
^
j
41
Fig. 3. Computer simulation of adsorption in the case of four layers with different values of the binding parameters, no is taken to be 100. J~intti's method proves to be useful in spite of the "covering effect".
4. CONCLUSIONS J~intti's approach was originally developed enabling short adsorption measurements when a simple molecular adsorption model is applicable and the number of sites available for adsorption is infinite. In the present paper we show that with also in the case of a restricted number of sites combined with the possibility of more than one layer of adsorbed molecules J/intti's approach can lead to the saving of time. An estimate of the equilibrium value of the adsorbed mass as well as of other kinetic parameters in an early stage of the measurements is
767 possible with J~intti's method. The procedure is not hindered when the binding parameters of the different layers are not equal. REFERENCES
1. O. J~intti, J. Junttila, E. Yrj ~inheikki: Mikropunnitusaj an Lyhent~imisest~i Ekstrapolaatiomenetelm~ill~i. (On curtailing the microweighing time by an extrapolation method). Suomen Kemistilehti A 43 (1970) 214-218. 2. O. J~intti, J. Junttila, E. Yrj~aheikki: On curtailing the micro-weighing time by an extrapolation method. In: T. Gast, E. Robens (eds.): Progress in Vacuum Microbalance Techniques, Vol. 1. Heyden, London 1972, p. 345-353. ISBN 0 85501 073 8. 3. C.H. Massen, J.A. Poulis, E. Robens: Criticism on J~intti's three point method on curtailing gas adsorption measurements. Adsorption 6 (2000) 229-232. 4. J.A. Poulis, C.H. Massen, E. Robens, K.K. Unger: A fast two-point method for gas adsorption measurements. In K.K. Unger, G. Kreysa, J.P. Baselt (eds.): Characterisation of Porous Solids V. Studies in Surface Science and Catalysis, Elsevier, Amsterdam 2000, p. 151-154. ISBN 0 444 502 599. 5. J.A. Poulis, G. Reichenauer, C.H. Massen, E. Robens: A Jantti approach for quick calculations of sorption equilibria. Z. Physikalische Chemie (2002) in print. 6. E. Robens, J.A. Poulis, C.H. Massen: Fast gas adsorption measurements for complicated adsorption mechanisms. In: V.A. Tertykh, V. Pokrovskiy (eds.): Proceedings of the 2 8 th International Conference on Vacuum Microbalance Techniques, 1999, Kyiv. Journal of Thermal Analysis and Calorimetry 62 (2000) 429-433. 7. E. Robens, C.H. Massen, J.A. Poulis, P. Staszczuk: Fast measurements of adsorption on porous materials using J~intti's method. Adsorption Sci. and Tech. 17 (1999) 10, 801-804. 8. C.H. Massen, J.A. Poulis, E. Robens: The Jantti approach in the case of a limited and Experimental Studies of lnterfacial Phenomena and Their Technological Applications, pp. 186-190, SCSEIO, Odessa 2001, ISBN 966-7635-13-9. 9. J.A. Poulis, C.H. Massen, E. Robens: Measurement of gas adsorption with J~intti's method using continuously increasing pressure. Journal of Thermal Analysis and Calorimetry 68 (2002) 2, 719-725.
This Page Intentionally Left Blank
Studies in Surface Science and Catalysis 144 F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger (Editors) 9 2002 Elsevier Science B.V. All rights reserved.
769
Characterization of controlled pore glasses by small angle x-ray scattering and evaluation of the scattering data by he indirect Fourier transformation method. A. Christoforides a, N. Kanellopoulos b, A. Mitropoulos a, K.L. Stefanopoulos b and K. Tarchanides" Cavala Institute of Technology, Dpt. of Petroleum Technology, 65404 St. Lucas, Cavala, Greece. b NCSR "Demokritos" Institute of Physical Chemistry, 15310,4g. Paraskevi Attikis, Greece. A series of controlled pore glasses (CPG's) having nominal pore diameters from 120 to 350 A and the related Vycor porous glass were examined by Small-Angle X-ray Scattering (SAXS). The spectra of these glasses show a rather broad peak at low scattering angles and, after that, the scattered intensity decays by following a non-integer power law. The location of the peak is related to the mean pore size of each particular glass and the power law is found to be independent from the pore sizes indicating that all glasses have a fractally rough interface. In the case of CPG's the fractal dimension is similar and equal to about DF (cPG's)=2.2, whereas for Vycor the fractal dimension is rather higher, DF (VVCOR)~2.5. Since the SAXS data cover a wide Q range, 0.005
INTRODUCTION
Porous glasses are applied in many industrial processes such as gas separation, catalysis, or membrane technology. Controlled pore glasses (CPG's) are primarily used as inorganic supports in liquid chromatography. Derivatized CPG's are also used as enzyme reactors, immunosorbents, biosensors, as well as for solid phase synthesis. Porous glasses can be prepared with a wide range of porosities and average pore sizes having a high mechanical strength, narrow pore size distribution, and high surface area. Because of these unique properties, porous glasses are considered as model pore systems and often used as a substrate for studying sorption and transport processes [ 1-5].
770 Although Vycor porous glass does not belong to the series of CPG's its properties as well as its preparation procedure are closely related to them. Since Vycor has attracted the interest of many investigators a comparison with CPG's may be useful in studying the structural characteristics of these particular glasses. On Vycorization, a melt composed of 62.7% SIO2, 26.9% B203, 6.6% Na20 and 3.5% A1203 undergoes a phase separation into a SiO2 rich phase and a B203 rich phase. After heat treatment, the B203 rich phase is removed by acid leaching leaving an almost pure SiO2 skeleton; a fully interconnected structure is obtained. CPG's are prepared in a similar way. The molten composed of 50-75% SiO2, 1-10% Na20 and the remainder B203 is phase separated by cooling to between 500 and 750 ~ The resulting average pore size is determined by the time required for this treatment. The structural morphology of porous glasses has been the subject of several studies. Three mechanisms have been proposed to explain the phase separation process, namely: mass fractals [6], spinodal decomposition [7-9], and nucleation and growth [10-11 ]. By using small angle x-ray or neutron scattering, the texture of the pore interface has also been examined. According to Bale and Schmidt [12], the scattered intensity, I(Q), at high Q values (Porod's region), from a fractally rough surface is given by: I(Q) oc QD,-6
(1)
where DF is the surface fractal dimension; Q=4nsin0/k where ~, is the wavelength and 20 is the scattering angle. In most of the cases the results converge that porous glasses possess a fractally rough interface with a fractal dimension raging between 2.2 and 2.5 [ 11, 13-16]. 2. EXPERIMENTAL
A series of CPG's and Vycor 7930 in a powder form were used in the present study. The samples were cleaned by 30% hydrogen peroxide and exposed to an oxygen stream at 350 ~ for 24 h. The x-rays were produced by an EnrofNonius Diffractics 582 x-ray generator fitted with a Philips orange-spot copper-anode x-ray tube. The most intense of the characteristic lines Ka, at 1.54 A, was selected as the experimental wavelength and monochromatization was attained by diffraction from the 110 plane of pyrolitic graphite. The collimation system was consisted of a short collimator (100 mm) attached to the housing of the x-ray generator, over the x-ray shutter, followed by a long collimator tube (2 m) at the end of which a pinhole of-~l mm diameter was defined. The main feature of the apparatus was a two-dimensional xray imaging system based on a gas-filled multi-wire proportional counter coupled to an artificial delay line. The sample to detector distance was equal to 2727 mm resulting to scattering vector values ranging between 0.005 and 0.15 A ~. 3. RESULTS AND DISCUSSION Fig. 1 shows the scattering curves from CPG's having average pore sizes 120, 170 and 350 A. The spectrum of Vycor porous glass is included too. At low scattering angles, the spectra show a rather broad peak. The location of the peak is related to the mean pore size of each particular glass, that is, the larger the average pore size the lower the scattering angle where the peak is located. At the outer part of the spectrum, the scattered intensity decays by following a non-integer power law as described in Eq. (1). For the CPG's the power law is found to be independent from their pore sizes indicating that all glasses have a similar fractally rough interface with a fractal dimension DF~2.2, whereas the fractal dimension of
771
10000000I
~
~oooooo t "~ ooooo i'"
w c ,,,,,,
10000 I
"
1000
~i
100 I 10 1 )
0.1 i 0.01 I ........ ~ ........ ~ ........ I 0.001 0.01 0.1 1 Q [A~] Fig. 1. Small angle x-ray scattering curves for Vycor and controlled pore glasses (from top to bottom): Vycor, CPG-120, CPG-170, and CPG-350. For clarity reasons, intensity is in arbitrary units. A Q-4 curve has been drawn as a guide to the eye.
Vycor is somehow greater, DF---2.5. Following HShr et al. [11 ], this difference is attributed to a different time treatment during the preparation procedure. Since in both cases the phase separation is stopped before completion, porous glasses inherit a fractally rough interface. In the case of Vycor, however, the phase separation process is stopped at an earlier stage than that of CPG's resulting to DF (VYCOR)>DE (CPO's). In the general case, the scattering from a homogeneous and isotropic dispersion of spherical particles (pores) can be written as the product of, I(Q) oc np P(Q) S(Q)
(2)
where np is the number density, P(Q) is the single particle (pore) form factor, and S(Q) is the structure factor arising from interference effects in the scattering from particles (pores) being in close separation. The SAXS data are further evaluated by using a generalized form of the indirect Fourier transformation method for concentrated systems that allows the determination of both the form factor and the structure factor simultaneously [17-18]. This is possible due to the different analytical behavior of these functions, which leads in most cases to the existence of a global minimum in the parameter surface.
772
Fig. 2. Solid/pore sizes for porous glasses (ds = size of the solid entities and dp= size of pores).
The averaged structure factor, containing the interference effects, is based on an analytical solution of the Percus-Yevick approximation, which is a model for hard spheres without any charge. By using SANS, Allen et al. [ 19] studied the relation between grain and pore sizes for ceramic nanophase materials. They concluded that the grain size is greater than the pore one when agglomeration is present and subsequently introduced a modified structure factor taking into account the mean distance, ds, between pore centroids. Since the indirect Fourier transformation technique is solving for a global minimum, the averaged structure factors for CPG's and Vycor should be related to that ds, which subsequently should give a measure of the solid phase, provided that pores are arranged around solid entities (Fig. 2).
1.2 I
0.9
A
0.5
~" 0.6
a
0.3 0 0
1
i
0.03
0.06
Q [A'q
Fig. 3a. Structure factors (from left to right): CPG-350, CPG-170, CPG-120, and Vycor.
0 0
100
200
300
Rp[A]
Fig. 3b. Pore radii distributions (from left to right): Vycor, CPG-120, CPG-170, and CPG-350.
773
Table 1 Fitting parameters for the indirect Fourier transformation method.
Sample
~:(%)
d~s (A)
~ (%)
dp (A)
VYCOR CPG-120 CPG-170 CPG-350
28 55 57 63
270 362 390 840
6 18 19 20
44 143 175 348
E is the volume fraction of the pores (porosity). dHs is the diameter of the hard sphere (dHs=ds). tx is the polydispersity, defined as the ratio between the width of the Gaussian distribution and its maximum value ((ins). do is the pore size (dp=2 x Rp).
Therefore, the three significant parameters for the evaluation of the structure factors of glasses are: a) porosity, b) polydispersity, and c) the size of solid entities. Fig. 3a illustrates the calculated averaged structure factors. Note that as the size of the hard spheres is increased the maxima are shifted to the lower Q region corresponding to the increasing distance between pore centroids. Based on the setting parameters for the structure factors, the form factor is calculated for a polydisperse pore size distribution function, Dn(Rp), that states the number of pores defined by the radius parameter Rp (Fig. 3b). Table 1 summarizes the results. For Vycor, an average pore size 44 A is calculated, which is in an excellent agreement with that claimed by the manufacturer and also reported to several other studies [ 14, 20]. For CPG's the calculated pore sizes are in good agreement with their nominal ones. One may note that the polydispersity in CPG's is slightly overestimated. Further detailed analysis of the results and comparison with gas adsorption and mercury porosimetry data is under progress. 4. CONCLUSIONS Controlled pore glasses are possessing a fractally rough surface of about DF=2.2 which is independent from their pore sizes. This fractal dimension is somehow lower than that of Vycor porous glass (DF-~2.5) indicating a longer time treatment for CPG's. An analysis based on the indirect Fourier transformation technique suggests that the distance of pore centroids, ds, that is the size of solid entities, influences the structure factor. The pore radii distributions corresponding to these structure factors are in a good agreement with the nominal sizes of the porous glasses. Similar results are obtained for Vycor.
774
REFERENCES
[1] J-C. Li, D.K. Ross, L.D. Howe, K.L. Stefanopoulos, J.P.A. Fairclough, R. Heenan and K. lbel, Phys. Rev. B, 49 (1994) 5911.
[2] A.Ch. Mitropoulos, P.K. Makri, N.K. Kanellopoulos, U. Keiderling and A. Wiedenmann, J. Colloid Interface Sci., 193 (1997) 137. [31 L.D. Gelb and K.E. Gubbins, Langmuir, 14 (1998) 2097. [4] L.D. Gelb and K.E. Gubbins, Langmuir, 15 (1999) 305. [5] E.S. Kikkinides, M.E. Kainourgiakis, K.L.Stefanopoulos, A.Ch. Mitropoulos, A.K. Stubos and N.K. Kanellopoulos, J. Chem. Phys., 112 (2000) 9881. [6] U. Even, K. Rademann, J. Jortner, N. Manor and R. Reisfeld, Phys. Rev. Lett., 52 (1984) 2164. [7] J.W. Cahn, J. Chem. Phys., 42 (1964) 93. [S] N.F. Berk, Phys. Rev. Lett., 58 (1987) 2718. [9] P. Wiltzius, F.S. Bates, S.B. Dierker and G.D. WignaU, Phys. Rev. A, 36 (1987) 2991. [10] W.J. Hailer, J. Chem. Phys., 42 (1965) 686. [11] A. H6hr, H.B. Neumann, P.W. Schmidt, P. Pfeifer and D. Avnir, Phys. Rev. B, 38 (1988) 1462. [12] H.D. Bale and P.W. Schmidt, Phys. Rev. Lea., 53 (1984) 596. [13] M.J. Benham, J.C. Cook, J-C. Li, D.K. Ross, P.L. Hall and B. Sarkissian, Phys. Rev. B, 39 (1989) 633. [14] P. Levitz, G. Ehret, S.K. Sinha and J.M. Drake, J. Chem. Phys., 95 (199 l) 6151. [15] A.Ch. Mitropoulos, J.M. Haynes, R.M. Richardson and N.K. Kanellopoulos, Phys. Rev. B, 52 (1995) 10035. [16] E. Hoinkis, Adv. Colloid Interface Sci., 76-77 0998) 39. [17] J. Brurmer-Popela and O. Glatter, J. Appl. Cryst., 30 (1997) 431. [18] B. Weyerich, J. Brtmner-Popela and O. GlaRer, J. Appl. Cryst., 32 (1999) 197. [19] A.J. Allen, S. Krueger, G.G. Long, H.M. Ketch, H. Hahn and G. Skandan, Nanostructured Mater., 7 (1996) 113. [20] T.H. Elmer and S. J. Schneider (eds.), ASM Engineered Materials Handbook, Vol. 4, p. 427, Materials Park, Ohio, USA, 1991.
775 AUTHOR INDEX
A Alcafiiz-Monge, J. Ali6, C. Androutsopoulos, G.P. Ania, C.O. Arenillas, A. Armatas, G.S.
Bagreev, A. Balys, M. Bandosz, T.J. Bafiares, M.A. Barabino, C. Barata-Rodriguez, P.M. Barde, J-P. Bashkova, S. Beehara, R. Bellussi, G. Belver, C. Benfield, R.E. Benli G6ntil, B. Berenguer-Mureia, A. Beurroies, I. Bhatia, S.K. Bigot, D. Bilingmaier, B. Blaeher, S. Blaneo, C. Blaneo, J. Borowka, A. Bortun, A.I. Brasseur, A.
193,201,291
Buezek, B. Burian, A. Burlot, R.
225 561 483
323 27,299 537 209,537 299
217 225 217, 247 307 625 139 483 217 609 625 307 163 315 83
Caceiaguerra, T. Call6n, M.S. Calo, J.M. Candeias, A.E. Carati, A. Carrott, P.J.M. Cazorla-Amor6s, D. Cerepi, A. Chapelle, G. Charalambopoulou, G. Ch. Chmiel, H. Christoforides, A. Christophidou, M. Clinard, C. Coasne, B. Cohaut, N. Coutelieris, F.A. Crine, M. Crispel, S.
601 283,403 59 363 625 363 51, 83 483 331 147 507 769 75 19,601 35 19 745,753 331 723
435 123 723 115 323, 331 617 569 655 701 419
Davies, J.G. de Rivas, B. De Weireld, G. Delattre, C. Denoyel, R. Derylo-Marczewska, A. Diez, F.V. Dombrowski, R.J.
515 717 231 451 171 491 427 99
776 Domingo-Garcia, M. 239 Doneux, C. 419 Dore, J.C. 163, 561 Drag, E.B. 499 du Fresne v. Hohenesche, C. 115, 339, 387 Duber, S. 561 Duclaux, L. 601 Dudziak, G. 467 DiJren, T. 685 Dutour, S 267
E1-Sayed, Y. Enke, D. Eschricht, N. Espina, A.
Fella, H.-J. Fensch-Kleemann, F.E. Ferraris, G. Fierro, G. Fierro, V. Figueiredo, J.L Findenegg, G.H. Forissier, M. Freitas, M.M.A. Fr6re, M. Fricke, J. Friedel, F. Fritz, Th. Fuller, E.L., Jr
247 347, 593 355 701
443 693 577 577 255 261 177 451 261 231,267 443 347 459 275
G Galaup, S. Gandia, L.M. Garcia, E.
Garcia, J.R. Garcia, T. Garcia-Martinez, J. Gil, A. Gille, W. Gmira, A. Gommes, C. Gonz~ilez, F. Gonz~lez-Velasco, J.R. Goworek, J. Grandjean, D. Griboval-Constant, A. Groen, J.C. Grosman, A. Groszek, A.J. Grtin, M. Gubbins, K.E. Guet, J.-M. Gun'ko, V.M. Gutierrez-Ortiz, J.I.
701 283, 403 83 585 347, 593 601 323, 419 617 717 491,655 163 609 91 35 239 363 19, 467, 647 19 515 717
H
Hahn, A. Hahn, T.. Hall, P.J. Hallam, K.R. Harbich, K.-W. Hattori, Y. He, Y. Higashitani, K. Hoinkis, E. Honkimaki, V. Houtrnann, S. Humbert, L.
67
Iijima, S. Ismadji, S.
521
347 59 639 693 11 685 411 355 561 59 483
483 585 737
123
777
Izquierdo, M.T.
Jacobs, F. Janowski, F. J6rome, R. Jorge, M. Jullian, S.
255
459 347, 593 331 131 267
K
Kaczmarczyk, J. Kaden, H. Kainourgiakis, M.E. Kallus, S. Kanda, H. Kaneko, K. Kanellopoulos, N. Kanoh, H. Karanfil, T. Keller, J.U. Khainakov, S.A. Khainakova, O.A. Khodakov, A.Y. Khvoshchev, S.S. Kikkinides, E.S. Kilduff, J.E. Klobes, P. Kroll, M. Kulazynski, M. Kyriakou, G.
Ladavos, A.K. Lastoskie, C.M. Le Bolloc'h, D. L6onard, A. Lequeux, N.
499 347 147, 753
Limborg-Noetinger, S. Linares-Solano, A. Lind, A Lind6n, M. Llewellyn, P. Lloyd, A.W. Lo Jacono, M. Lodewyckx, P. Loisy, C. L6pez, J.M. L6pez-Fonseca, R. L6pez-Garz6n, F.J. Lozano-Castell6, D. Lubda, D.
267 51, 83 339 339 171,435, 723 515 577 323, 731 483 283, 403 717 239 51, 59, 201,291 115
67 411 11,521 545, 769 11,521 553 387 701 701 609 671 147 553 693 163 395, 499 709
27 99 163 331 601
M
M~idler, F. Maquet, V. Martinez-Alonso, A. Martin-Luengo, M.A. Massen, C.H. Mastral, A.M. Mays, T.J. Mikhalovsky, S.V. Mirodatos, C. Mitropoulos, A. Miyahara, M. Moggridge, G.D. M6hner, C. Monson, P.A. Morante, J.A. Moret, S. Moretti, G. Mtiller, M. Mtiller, R. Murata, K. Murillo, R. Murphy, M.C.
355 331 1, 83, 529 569 761 283, 403 139 515 255 769 411 139 5O7 155 617 723 577 51 347 521 283, 403 515
778 N Navarrete, R. Navarro, M.V. Nokerman, J. Noville, F.
171 283, 403
6zcan, 6.
Palacios-Latasa, J.M. Pannwitt, B. Papadopoulos, G.K. Paredes, J.I. Paredes, J.R. Parra, J.B. Patel, S. Peffer, L.A.A. Pellenq, R.J.-M. Perego, C. P6rez-Mendoza, M. P6rez-Ramirez, J. Pesquera, C. Phillips, G.J. Pikunic, J. Pikus, S. Pirard, J.-P. Pirard, R. Pis, J.J. Polster, B. Pomonis, P-J. Poulis, J.A.
435, 723 371
R
267 419
O Ohba, T Ord6fiez, S. Orenes-Femmadez, I. Ortega, C. Otero, M.
Poyet, T. Puibasset, J.
11,521 427 663 35 685 315
663 507 545 1,529 427 209, 537 515 91 19, 35, 371,601 625
Radhakrishnan, R. RagaL J. Ramsay, J.D.F. Rannou, I. Raymundo-Pifiero, E. Reichenauer, G. Reinhardt, S. Reymond, J.P. Reznik, B. Ribeiro-Carrott, M.M.L. Riekel, C. Rigby, S.P. Rizzo, C. Robens, E. Rodriguez-Reinoso, F. R6hl-Kuhn, B. Rouquerol, F. Rouquerol, J. Rouzaud, J.-N. Rubiera, F. Rubio, B. RiJbner, K.
467
Sakamoto, M. Salmas, C.E. Salter, I.D. Salvador, A. Salvador, F. Sfinchez-Jim6nez, C. S~_nchez-Montero, M.J. Sarkisov, L. Sastre, H. Schippert, E.
411
435 67 19, 601 51 443, 761 177 451 217 363 51 185 625 387, 761 107 693 171,435 171,723 19 209 255 459
239 "91 617 515 19 655 323, 331,419 323, 419 209, 537 723 27, 299 761
27, 299 633, 639 379 379 379 379 155 427 507
779 Schneider, P Schreiber, A. Schubert-Bischoff, P. Schumacher, K. Schuurman, Y. Seaton, N.A. Seifert, S. Sepfilveda-Escribano, A. Silvestre-Albero, J. Singh, U. Siperstein, F.R. Sliwinska-Bartkowiak, M. Solcov& O. Srebnik, S. Srivastava, R. Stathopoulos, V.N. Stefanopoulos, K.L. Steriotis, Th.A. Stubos, A.K. Sunaga, M. Szczygielska, A.
475 177
355 363
Unger, K.K.
131,139, 685 59 107 107
633, 639 647 467 475
43 553 27, 339 769
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STUDIES IN SURFACE SClENCEAND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1
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Preparation of Catalysts I.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, RA. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsofthe Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, R Grange, RJacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t~ de Chimie Physique,Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedingsof an International Symposium, Ecully (Lyon), September 9-11,1980 edited by B. Imelik, C. Naccache,Y. BenTaadt, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.E Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis,Tokyo, June 30-July4,1980. PartsA and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov andV.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyfie, September 29-October 3,1980 edited by M. LdzniEka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, R Meriaudeau, R Gallezot, G.A. Martin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties-Applications. Proceedings of aWorkshop, Bremen, September 22-24, 1982 edited by RA. Jacobs, N.I. Jaeger, RJiKJ and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings of theThird International Conference, Asilomar, CA, September 1-4,1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets
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Volume 17 Volume 18 Volume 19 Volume 20 Volume 21 Volume 22 Volume 23 Volume 24 Volume 25 Volume 26 Volume 27 Volume 28 Volume 29 Volume 30 Volume 31
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Preparation of Catalysts III. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsof theThird International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, R Grange and RA. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13,1984 edited by RA. Jacobs, N.I. Jaeger, R Ji E=,V.B.Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3,1984 edited by S. Kaliaguine and A. Mahay Catalysis byAcids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27,1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. BenTaarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29,1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure,Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8,1984 edited by B. Dr~.aj,S. HoEevarand S. Pejovnik Catalytic Polymerization of Olefins. Proceedingsof the International Symposium on Future Aspects of Olefin Polymerization,Tokyo, July 4-6,1985 edited by 1".Keii and K. Soga Vibrations at Surfaces 1985.Proceedingsofthe Fourth International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven~ New Developments in Zeolite Science andTechnology. Proceedings of the 7th International Zeolite Conference,Tokyo, August 17-22,1986 edited by Y. Murakami,A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis andAutomotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq andA. Frennet Preparation of Catalysts IV. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe Fourth International Symposium, Louvain-laNeuve, September 1-4, 1986 edited by B. Delmon, R Grange, RA. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by RWissmann Synthesis of High-silica Aluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.E Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine
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Volume 45 Volume 46
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Volume 51 Volume 52 Volume 53 Volume 54
Methane Conversion. Proceedingsof a Symposium on the Productionof Fuelsand Chemicals from Natural Gas,Auckland, April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.E Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by RJ. Grobet,W.J. Mortier, E.EVansant and G. Schulz-Ekloff Catalysis 1987.Proceedingsof the 10th North American Meeting of the Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a.Ts.,Apri126-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987.Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11,1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P~rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Padl Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings of theWorldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium,WLirzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo andT. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16,1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. PartsA and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting,Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G.Anthony New Solid Acids and Bases.Their Catalytic Properties by K.Tanabe, M. Misono,Y. Ono and H. Hattori RecentAdvances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19,1989 edited by J. Klinowsky and RJ. Barrie Catalyst in Petroleum Refining 1989. Proceedi~ngsof the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L.Trimm, S.Akashah, M.Absi-Halabi andA. Bishara Future Opportunities in Catalytic and SeparationTechnology edited by M. Misono,Y. Moro-oka and S. Kimura
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New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F.Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts,Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of SurfaceAnalysis edited by J.L.G. Fierro Volume 57B SpectroscopicAnalysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Introduction to Zeolite Science and Practice Volume 58 edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals,Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba andT.Tatsumi Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Characterization of Porous Solids II. Proceedings of the IUPACSymposium Volume 62 (COPS II),Alicante, May 6-9,1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of CatalystsV. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990 edited by G. Poncelet, RA. Jacobs, R Grange and B. Delmon NewTrends in COActivation Volume 64 edited by L. Guczi Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT90, Leipzig, Volume 65 August 20-23, 1990 edited by G. (~hlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~ired, September 10-14,1990 edited by L.I. Simdndi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli andA.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13, 1991 edited by P.A.Jacobs, N.I. Jaeger, L. Kubeikov~ and B.Wichterlovd Volume 70 Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova
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Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C.Sanford Angle-Resolved Photoemission.Theory and CurrentApplications edited by S.D. Kevan New Frontiers in Catalysis, PartsA-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F.Solymosi and RTdtdnyi Fluid Catalytic Cracking: Science andTechnoiogy edited by J.S. Magee and M.M. Mitchell, Jr. NewAspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of theThird International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto,T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals Iii. Proceedings of the 3rd International Symposium, Poitiers, April 5- 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P@rotand C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals ofAdsorption, Kyoto, Japan, May 17-22,1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of theThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.E Howe New Developments in Selective Oxidation II. Proceedings of the SecondWorld Congress and Fourth European Workshop Meeting, Benalm~dena, Spain, September 20-24,1993 edited by V. Cort@sCorberdn and S.Vic Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan,August 22-25, 1993 edited byT. Hattori andT.Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by J.Weitkamp, H.G. Karge, H. Pfeifer andW. H61dedch Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPACSymposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F.Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger
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Volume 101 Volume 102 Volume 103 Volume 104 Volume 105
Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Delmon and G.E Froment Catalyst Design forTailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M.Terano Acid-Base Catalysis Ii. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono andY. Ono Preparation of CatalystsVI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A.Jacobs and P.Grange Science andTechnology in Catalysis 1994. Proceedings of the SecondTokyo Conference on Advanced Catalytic Science andTechnology,Tokyo, August 21-26, 1994 edited by Y. Izumi, HoArai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.EVansant, P.Van DerVoort and K.C.Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13,1995 edited by H.K. Beyer,H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution Control III. Proceedings of theThird International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites:A RefinedTool for Designing Catalytic Sites. Proceedings of the International Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dqbrowski andV.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26,1995 edited by M. Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 11th International Congress on Catalysis - 40th Anniversary. Proceedings of the 11th ICC, Baltimore, MD, USA, June 30-July 5,1996 edited by J.W. Hightower,W.N. Delgass, E. Iglesia andA.T. Bell RecentAdvances and New Horizons in Zeolite Science andTechnology edited by H. Chon, S.I.Woo and S.-E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by RV. Kamat and D. Meisel Equilibria and Dynamics of GasAdsorption on Heterogeneous Solid Surfaces edited by W. Rudzifiski,W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings of the 11th International Zeolite Conference, Seoul, Korea, August 12-17,1996 edited by H. Chon, S.-K. Ihm andY.S. Uh
787 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.I-. Froment, B. Delmon and R Grange Volume 107 Natural Gas Conversion IV Proceedings of the 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23,1995 edited by M. de Pontes, R.L. Espinoza, C.P.Nicolaides, J.H. Scholtz and M.S. Scurrell Volume 108 Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12,1996 edited by H.U. Blaser,A. Baiker and R. Prins Volume 109 Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings of the International Symposium, Antwerp, Belgium, September 15-17,1997 edited by G.I-. Froment and K.C.Waugh Volume 110 ThirdWorld Congress on Oxidation Catalysis. Proceedings of theThirdWorld Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T.Oyama,A.M. Gaffney and J.E. Lyons Volume 111 Catalyst Deactivation 1997. Proceedings of the 7th International Symposium, Cancun, Mexico, October 5-8,1997 edited by C.H. Bartholomew and G.A. Fuentes Volume 112 Spillover and Migration of Surface Species on Catalysts. Proceedings of the 4th International Conference on Spillover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin Volume 113 Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings of the 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4, 1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Volume 114 Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings of the 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo, K. Izui, S.Yanagida andT.Yamaguchi Volume 115 Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Volume 116 Catalysis and Automotive Pollution Control IV. Proceedings of the 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11,1997 edited by N. Kruse,A. Frennet and J.-M. Bastin Volume 117 Mesoporous Molecular Sieves 1998 Proceedings of the 1st International Symposium, Baltimore, MD, U.S.A., July 10-12,1998 edited by L.Bonneviot, F.B61and,C. Danumah, S. Giasson and S. Kaliaguine Volume 118 Preparation of Catalysts VII Proceedings of the 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4,1998 edited by B. Delmon, RA. Jacobs, R. Maggi, J.A. Martens, R Grange and G. Poncelet Volume 119 Natural Gas ConversionV Proceedings of the 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25, 1998 edited by A. Parmaliana, D. Sanfilippo, F.Frusteri,A.Vaccari and F.Arena Volume 120A Adsorption and itsApplications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. Dabrowski
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Volume 120B Adsorption and itsApplications in Industry and Environmental Protection. Vol I1:Applications in Environmental Protection edited byA. Dabrowski Volume 121 Science andTechnology in Catalysis 1998 Proceedings of theThirdTokyo Conference in Advanced Catalytic Science and Technology,Tokyo, July 19-24,1998 edited by H. Hattori and K. Otsuka Volume 122 Reaction Kinetics and the Development of Catalytic Processes Proceedings of the International Symposium, Brugge, Belgium, April 19-21,1999 edited by G.F.Froment and K.C.Waugh Volume 123 Catalysis:An IntegratedApproach Second, Revised and Enlarged Edition edited by R.A. van Santen, RW.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Volume 124 Experiments in Catalytic Reaction Engineering by J.M. Berty Volume 125 Porous Materials in Environmentally Friendly Processes Proceedings of the 1st International FEZAConference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. PdI-Borb61y,J.B. Nagy and H.G. Karge Volume 126 Catalyst Deactivation 1999 Proceedings of the 8th International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.F.Froment Volume 127 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/Tth European Workshop, Antwerpen, Belgium, November 14-17, 1999 edited by B. Delmon, G.F.Froment and R Grange Volume 128 Characterisation of Porous SolidsV Proceedings of the 5th International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2, 1999 edited by K.K. Unger, G. Kreysa and J.R Baselt Volume 129 Nanoporous Materials II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari, M. Jaroniec andT.J. Pinnavaia Volume 130 12th Intemational Congress on Catalysis Proceedings ofthe 12th ICC, Granada, Spain, July 9-14, 2000 edited by A. Corma, F.V.Melo, S. Mendioroz and J.L.G. Fierro Volume 131 Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization byV. Dragutan and R. Streck Volume 132 Proceedings of the International Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited byY. Iwasawa, N. Oyama and H. Kunieda Volume 133 Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings of the 3rd International Symposium, Oostende, Belgium, April 22-25, 2001 edited by G.F.Froment and K.C.Waugh Volume 134 Fluid Catalytic CrackingV Materials and Technological Innovations edited by M.L. Occelli and R O'Connor
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Zeolites and Mesoporous Materials at the Dawn of the 21st Century. Proceedings of the 13th International Zeolite Conference, Montpellier, France, 8-13 july 2001 edited by A. Galameau, F. di Renso, F. Fajula ans J. Vedrine Natural Gas Conversion VI Proceedings of the 6th Natural Gas Conversion Symposium, June 17-22, 2001, Alaska, USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2ndcompletely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, P.A. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces edited by A. Guerrero-Ruiz and I. Rodriquez-Ramos Catalyst Deactivation 2001 Proceedings of the 9 th International Symposium, Lexington, KY, USA, October 2001. edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11,2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 34 International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec Impact of Zeolites and Other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2ndInternational FEZA (Federation of the European Zeolite Associations) Conference, Taormina, Italy, September 1-5, 2002 edited by R. Aiello, G. Giordano and F.Testa Scientific Bases for the Preparation of Heterogeneous Catalysts Proceedings of the 8th International Symposium, Louvain-la-Neuve, Leuven, Belgium, September 9-12, 2002 edited by E. Gaigneaux, D.E. De Vos, P. Grange, P.A. Jacobs, J.A. Martens, P. Ruiz and G. Poncelet Characterization of Porous Solids VI Proceedings of the 6 th International Symposium on the Characterization of Porous Solids (COPS-VI), Alicante, Spain, May 8-11,2002 edited by F. Rodriguez-Reinoso, B. McEnaney, J. Rouquerol and K. Unger
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