Studies in Surface Science and Catalysis 114 ADVANCES IN CHEMICAL CONVERSIONS FOR MITIGATING CARBON DIOXIDE
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Studies in Surface Science and Catalysis Advisory
Editors:
B. Delmon and J.T. Yates
Vol, 114
ADVANCES IN CHEMICAL CONVERSIONS FOR MITIGATING CARBON DIOXIDE Proceedings of the Fourth International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997
Editors T, Inui
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
M.Anpo
Department of Applied Chemistry, Faculty of Engineering, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 593, Japan
K. Izui
Division of App/ied Biosciences, Graduate School of Agricu/ture, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
S. Yanagida
Department of Materia/ and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565, Japan
T, Yamaguchi
Research Institute of lnnovation Technology forthe Earth (RITE), Kizu-cho, Soraku-gun, Kyoto 619-02, Japan
1998 ELSEVIER Amsterdam
m Lausanne--
New York--
Oxford m Shannon m Singapore m Tokyo
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Library of Congress Cataloging in Publication Data. A catalog record from the Library of Congress has been applied for.
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CONTENTS Preface
xv
Organization
xviii
Sponsoring
xix
Special Lecture International Energy Agency action on climate change issues M.A. Preville and H.J. Koch (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Plenary Lecture Japan's basic strategy concerning countermeasures to mitigate climate change T. Namiki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
Research and development on new synthetic routes for basic chemicals by catalytic hydrogenation of CO 2 H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 New approaches in CO 2 reduction A. Fujishima, D.A. Tryk and T.N. Rao (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
Development of electrocatalysts for carbon dioxide reduction using polydentate ligands to probe structure-activity relationships D.L. DuBois (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Carbon dioxide and microalgae N. Kurano, T. Sasaki and S. Miyachi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
55
Perspectives of carbon dioxide utilization in the synthesis of chemicals, coupling chemistry with biotechnology. M. Aresta (Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
Keynote Lecture Scope of'studies on CO 2 mitigation K. Yamada (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
Hydrogenation of CO 2 toward methanol Influence of the catalysts composition and preparation on the catalytic behavior R. Kieffer and L. Udron (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Photochemical carbon dioxide reduction with metal complexes: Differences between cobalt and nickel macrocycles E. Fujita, B.S. Brunschwig, D. Cabelli, M.W. Renner, L.R. Furenlid, T. Ogata, Y. Wada and S. Yanagida (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
vi
Electrochemical reduction of CO 2 at metallic electrodes J. Augustynski, P. Kedzierzawski and B. Jermann (Switzerland) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
107
Super-RuBisCO: Improvement of photosynthetic performances of plants A. Y okota (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
117
Organometallic reactions with CO 2 - Catalyst design and mechanisms E. Dinjus (Germany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
127
Oral Presentation Catalytic fixation of CO2: CO 2 purity and H 2 supply J.N. Armor (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
141
Reduction of carbon dioxide to graphite carbon via methane by catalytic fixation with membrane reactor H. Nishiguchi, A. Fukunaga, Y. Miyashita, T. Ishihara and Y. Takita (Japan) . . . . . . . . . . . . . 147 Catalytic reaction of CO 2 with C2H 4 on supported Pt-Sn bimetallic catalysts J. Llorca, P. Ramfrez de la Piscina, J. Sales and N. Homs (Spain) . . . . . . . . . . . . . . . . . . . . . . . . . . .
153
Initial transient rates and selectivities of Fischer-Tropsch synthesis with CO 2 as carbon source H. Schulz, G. Schaub, M. Claeys, T. Riedel and S. Walter (Germany) . . . . . . . . . . . . . . . . . . . . . 159 Palladium-catalyzed carboxylation of allyl stannanes and carboxylative coupling of allyl stannanes and allyl halides M. Shi, R. Franks and K.N. Nicholas (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Interaction between CO 2 and propylene on Ru-Co/A12Oa-catalysts of cluster type G.D. Zakumbaeva, L.B. Shapovalova and I.A. Shlygina (Kazakhstan) . . . . . . . . . . . . . . . . . . . . . . . 171 Photocatalytic reduction of CO 2 with H 20 on titanium oxides anchored within zeolites M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, Y. Ichihashi (Japan), S.G. Zhang, D.R. Park (Korea), S. Ehara (Japan), S.-E. Park, J.-S. Chang and J.W. Yoo (Korea) . . . . . . . . . . . . . . . . . 177 Photocatalytic reduction and fixation of CO 2 on cadmium sulfide nanocrystallites S. Yanagida, Y. Wada, K. Murakoshi, H. Fujiwara, T. Sakata and H. Moil (Japan) ......
183
Abiotic photosynthesis of amino acids, nucleic acid bases and organic acids from CO 2 dissolved in aqueous solution S. Kihara, K. Maeda, T. Hori and T. Fujinaga (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Aspects of CO 2 utilization toward the goal of emission reduction in Romania L. Dragos, N. Scarlat, M. Neacsu and C. Flueraru (Romania) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
195
CO 2 capture and utilization for enhanced oil recovery (EOR) and underground storage A case study in JiLin Oil Field, China G. Yun, D. Liu, T. Wu, J. Wu, X. Ji and Z. Li (China) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
201
Electrocatalytic reduction of CO 2 to worthier compounds on a functional dual-film electrode with a solar cell as the energy source K. Ogura, M. Yamada, M. Nakayama and N. Endo (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
vii
Incorporation of CO 2 into organic perfluoroalkyl derivatives by electrochemical methods E. Chiozza, M. Desigaud, J. Greiner and E. Du~ach (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
213
Molecular tailoring of organometallic polymers for efficient catalytic CO 2 reduction: mode of formation of the active species R. Ziessel (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Electroreduction of CO 2 using Cu/Zn oxides loaded gas diffusion electrodes S. Ikeda, S. Shiozaki, J. Susuki, K. Ito and H. Noda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
225
Recent slow rate of CO 2 increase and vegetation activity K. Kawahira and Y. Maeda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
231
Production of PHA (poly hydroxyalkanoate) by genetically engineered marine cyanobacterium H. Miyasaka, H. Nakano, H. Akiyama, S. Kanai and M. Hirano (Japan) ................... 237 Cellulose as a biological sink of CO 2 T. Hayashi, Y. Ihara, T. Nakai, T. Takeda and R. Tominaga (Japan) . . . . . . . . . . . . . . . . . . . . . . . . .
243
Possibility of molecular protection of photosynthesis under salinity stress F. Sato, Y. Arata, K. Matsuguma, M. Shiga, Y. Kanda, K. Ifuku, K. Ishikawa and T. Yoshida (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Organometallic CO 2 complexes in supercritical CO2: a time-resolved infrared study M.W. George, D.C. Grills, X-Z. Sun and M. Poliakoff (UK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
255
Methanation of carbon dioxide on catalysts derived from amorphous Ni-Zr-rare earth element alloys H. Habazaki, T. Yoshida, M. Yamasaki, M. Komori, K. S himamura, E. Akiyama, A. Kawashima and K. Hashimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Development of high performance Raney copper-based catalysts for methanol synthesis from CO 2 and H 2 J. Toyir, M. Saito, I. Yamauchi, S. Luo, J. Wu, I. Takahara and M. Takeuchi (Japan).. 267 Global carbon-recycling energy delivery system for CO 2 mitigation (I) Carbon one-time recycle system towards carbon multi-recycle system H. Sano, Y. Tamaura, H. Amano and M. Tsuji (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Oil extraction by highly pressurized CO 2 produced in zero emission power plants Ph. Mathieu, E. Iantovski and V. Kushnirov (Belgium) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
Global carbon-recycling energy delivery system for CO 2 mitigation (III) Fossil/solar energy hybridization system for utilization of carbon as solar energy carrier Y. Tamaura, M. Tsuji, H. Amano and H. Sano (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Review of measures to mitigate carbon dioxide emissions in the Slovak Republic and modes of utilization A. Moncmanov~i (Slovak Republic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Proposal of a new high-efficient gas turbine power generation system utilizing waste heat from factories P.S. Pak, H. Ueda and Y. Suzuki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
viii
Acetogenesis and the primary structure of the NADP-dependent formate dehydrogenase of Clostridium thermoaceticum, a tungsten-selenium-iron protein D.J. Gollin, X.-L. Li, S.-M. Liu and L.G. Ljungdahl (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Biochemical CO 2 fixation by mimicking zinc(II) complex for active site of carbonic anhydrase K. Ichikawa, K. Nakata, M. Ibrahim and S. Kawabata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 The biological CO 2 fixation using Chlorella sp. with high capability in fixing CO 2 M. Murakami, F. Yamada, T. Nishide, T. Muranaka, N. Yamaguchi and Y. Takimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
315
Photobiological production of hydrogen gas Y. Asada (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
321
Hydrocarbon synthesis from CO 2 over composite catalysts Y. Souma, M. Fujiwara, R. Kieffer, H. Ando and Q. Xu (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . .
327
CO 2 for petrochemicals feedstock. Conversion to synthesis gas on metal supported catalysts. P. Gronchi, P. Centola and R. Del Rosso (Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Iron catalyzed CO 2 hydrogenation to liquid hydrocarbons R.A. Fiato, E. Iglesia, G.W. Rice and S.L. Soled (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
339
Support effects of the promoted and unpromoted iron catalysts in CO 2 hydrogenation K.-W. Jun, S.-J. Lee, H. Kim, M.-J. Choi and K.-W. Lee (Korea) . . . . . . . . . . . . . . . . . . . . . . . . .
345
Methanol synthesis from COJH 2 over Pd promoted Cu/ZnO/AI203 catalysts M. Sahibzada, I.S. Metcalfe and D. Chadwick (UK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
351
A 50 kg/day class test plant for methanol synthesis from CO 2 and H 2 K. Ushikoshi, K. Moil, T. Watanabe, M. Takeuchi and M. Saito (Japan) ..................
357
Poster Presentation
Comparison of CO 2 sources for the synthesis of renewable methanol M. Specht, A. Bandi, M. Elser and F. Staiss (Germany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
363
Characteristics and economics assessment of power generation systems utilizing solar energy in various regions T. Kosugi, P.S. Pak and Y. Suzuki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Solar/chemical energy hybridization via Boudouard reaction H. Ono, M. Kawabe, M. Nezuka, M. Tsuji and Y. Tamaura (Japan) . . . . . . . . . . . . . . . . . . . . . . . .
371
Development of active and stable nickel-magnesia solid solution catalysts for CO 2 reforming of methane K. Tomishige, Y. Chen, X. Li, K. Yokoyama, Y. Sone, O. Yamazaki and K. Fujimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Global carbon-recycling energy delivery system for CO 2 mitigation (II) Two possible ways for introducing solar energy M. Tsuji, H. Amano, Y. Tamaura, H. Sano and S. Maezawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . 379
ix
Efficient thermochemical cycle for CO 2 reduction with coal using a reactive redox system of ferrite T. Kodama, A. Aoki, S. Miura and Y. Kitayama (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Oxidative dehydrogenation of ethylbenzene with carbon dioxide over ZSM-5-supported iron oxide catalysts J.-S. Chang, S.-E. Park, W.Y. Kim (Korea), M. Anpo and H. Yamashita (Japan) ......... 387 Nature of CO 2 adsorbed on MgO surface at low temperatures T. Ito, J. Isawa, H. Kishimoto, H. Kobayashi and K. Toi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . .
391
CO 2 behavior on supported KNiCa catalyst in the carbon dioxide reforming of methane S.-E. Park, J.-S. Chang, H.-S. Roh (Korea), M. Anpo and H. Yamashita (Japan) ......... 395 Utilization of CO 2 in the reforming of natural gas on carbon supported ruthenium catalysts. Influence of MgO addition P. Ferreira-Apaficio, B. Bachiller-Baeza, A. Guerrero-Ruiz and I. Rodrfguez-Ramos (Spain) .......................................................................................................... 99 Catalytic conversion of carbon dioxide to polymer blends via cyclic carbonates D.W. Park, J.Y. Moon, J.G. Yang, S.M. Jung, J.K. Lee and C.S. Ha (Korea) ...........
403
The selective synthesis of lower olefins (C 2 - C 4 ) by the CO 2 hydrogenation over Iron catalysts promoted with Potassium and supported on ion exchanged(H, K)Zeolite-Y H. Kim, D.-H. Choi, S.-S. Nam, M.-J. Choi and K.-W. Lee (Korea) . . . . . . . . . . . . . . . . . . . . . . . 407 Hydrogenation of carbon dioxide over rhodium catalyst supported on silica M. Kishida, K. Onoue, S. Tashiro, H. Nagata and K. Wakabayashi (Japan) ...............
411
Dehydrogenation of ethylbenzene over iron oxide-based catalyst in the presence of carbon dioxide N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma and M. Ando (Japan) ......... 415 Promoting effects of CO 2 on dehydrogenation of propane over a SiO2-supported Cr203 catalyst I. Takahara, W.-C. Chang, N. Mimum and M. Saito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Hydrogenation of carbon dioxide over Fe-Cu-Na/zeolite composite catalysts Q. Xu, D. He, M. Fujiwara, M. Tanaka, Y. Matsumura, Y. Souma, H. Ando and H. Yamanaka (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 Fe promoted Cu-based catalysts for hydrogenation of CO 2 N. Nomura, T. Tagawa and S. Goto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
427
The effect of rhodium precursor on ethanol synthesis by catalytic hydrogenation of carbon dioxide over silica supported rhodium catalysts H. Kusama, K. Okabe, K. Sayama and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 Selective formation of iso-butane from carbon dioxide and hydrogen over composite catalysts Y. Tan, M. Fujiwara, H. Ando, Q. Xu and Y. Souma (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 Vanadium-catalyzed acetic acid synthesis from methane and carbon dioxide Y. Taniguchi, T. Hayashida, T. Kitamura and Y. Fujiwara (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . .
439
Fuels and petrochemicals from CO2 via Fischer-Tropsch synthesis - steady state catalyst activity and selectivity T. Riedel, S. Walter, M. Claeys, H. Schulz and G. Schaub (Germany) ..................... 443 Effective conversion of CO2 to methanol and dimethyl ether over hybrid catalysts K.-W. Jun, M.-H. Jung, K.S. Rama Rao, M.-J. Choi and K.-W. Lee (Korea) ............
447
Characterization of CO2 methanation catalysts prepared from amorphous Ni-Zr and Ni-Zr-rare earth element alloys M. Yamasaki, H. Habazaki, T. Yoshida, M. Komori, K. Shimamura, E. Akiyama, A. Kawashima K. Asami and K. Hashimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Hydrogenation of CO 2 over Rh ion exchanged zeolite catalysts K.K. Bando, K. Soga, K. Kunimori, N. Ichikuni, K. Asakura, K. Okabe, H. Kusama, K. Sayama and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 Interconversion of Ru-CO and Ru-rll-CO2 through reversible oxide transfer reaction K. Tsuge, K. Tanaka and H. Nakajima (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
459
Physiological properties of phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase from Rhodopseudomonas sp. No.7 T. Fujii, M. Sadaie, M. Saijou, T. Nagano, T. Suzuki, M. Ohtani and H. Shinoyama (Japan) .......................................................................................................... 63 Production of alkane and alkene from CO z by a petroleum-degrading bacterium strain HD-1 M. Morikawa, T. Iwasa, K. Nagahisa, S. Yanagida and T. Imanaka (Japan) ............... 467 Cultivation of cyanobacterium in various types of photobioreactors for biological CO 2 fixation I.S. Suh, C.B. Park, J.-K. Han and S.B. Lee (Korea) ........................................ 471 Application of photosynthetic bacteria for porphyrin production H. Yamagata, R. Matoba, T. Fujii and H. Yukawa (Japan) ....................................
475
Utilization of micro-algae for building materials after CO2 fixation T. Otsuki, M. Yamashita, T. Hirotsu, H. Kabeya and R. Kitagawa (Japan) ................
479
Photosynthetic COz fixation performance by a helical tubular photobioreactor incorporating Chlorella sp. under outdoor culture conditions Y. Watanabe, M. Morita and H. Saiki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 Carboxylation reaction with carbon dioxide. Mechanistec studies on the Kolbe-Schmitt reaction. Y. Kosugi and K. Takahashi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 From carbon dioxide to C 2 organic molecules mediated by Aresta's nickel carbon dioxide complex J.K. Gong, C.A. Wright, M. Thorn, K. McCauley, J.W. McGill, A. Sutterer, S.M. Hinze and R.B. Prince (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Methanol homologation using carbon dioxide catalyzed by ruthenium-cobalt bimetallic complex system K. Tominaga, Y. Sasaki, T. Watanabe and M. Saito (Japan) .................................. 495
Atmospheric CO 2 fixation by dinuclear Ni(II) complex, [TRANi(II)(/~-OH)2Ni(II)TPA](C104) 2 (TPA=Tris(pyridymethyl)amine) M. Ito, T. Ishihara and Y. Takita (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 Carbon dioxide fixation with lanthanoid complex S. Inoue, H. Sugimoto, N. Ishida and T. Shima (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
503
A study on methanol synthesis through CO 2 hydrogenation over copper-based catalysts S.-K. Ihm, Y.-K. Park, J.-K. Jeon, K.-C. Park and D.-K. Lee (Korea) ....................
505
Mechanistic studies of methanol synthesis from C O 2 / H 2 o n C u / Z n O / S i O 2 catalyst D.-K. Lee, D.-S. Kim, C.M. Yoo, C.-S. Lee and I.-C. Cho (Korea) . . . . . . . . . . . . . . . . . . . . . . .
509
Highly effective synthesis of ethanol from CO z on Fe, Cu-based novel catalysts T. Yamamoto and T. Inui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
513
A study for the durability of catalysts in ethanol synthesis by hydrogenation of carbon dioxide K. Higuchi, Y. Haneda, K. Tabata, Y. Nakahara and M. Takagawa (Japan) ............... 517 Development of stable catalysts for liquid-phase methanol synthesis from CO 2 and H 2 H. Mabuse, T. Watanabe and M. Saito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
521
Ethanol synthesis from carbon dioxide and hydrogen M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa and H. Arakawa (Japan) ..............
525
New preparation method of Cu/ZnO catalysts for methanol synthesis from carbon dioxide hydrogenation by mechanical alloying H. Fukui, M. Kobayashi, T. Yamaguchi, H. Kusama, K. Sayama, K. Okabe and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529 Promoting effect of calcium addition to P d / S i O 2 catalysts in CO 2 hydrogenation to methanol A.L. Bonivardi, D.L. Chiavassa and M.A. Baltan~ (Argentina) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 Direct synthesis of gasoline from carbon dioxide via methanol as the intermediate H. Hara, T. Takeguchi and T. Inui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
537
Comparison of CO 2 hydrogenation in a catalytic reactor and in a dielecric-barrier discharge A. Bill, B. Eliasson, U. Kogelschatz and L.-M. Zhou (Switzerland) . . . . . . . . . . . . . . . . . . . . . . . . . 541 Methanol synthesis from carbon dioxide on CuO-ZnO-Al203 catalysts M. Hirano, T. Akano, T. Imai and K. Kuroda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
545
Optimization of preparation conditions and improvement of stability of Cu/ZnO-based multicomponent catalysts for methanol synthesis from CO 2 and H 2 S. Luo, J. Wu, J. Toyir, M. Saito, M. Takeuchi and T. Watanabe (Japan) .................. 549 Effect of solvents on photocatalytic reduction of carbon dioxide using semiconductor photocatalysts T. Torimoto, B.-J. Liu and H. Yoneyama (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553
xii
Applications of electrospray mass spectrometry and high performance liquid chromatography in the elucidation of photocatalytic CO2-fixation reactions H. Hori, J. Ishiham, M. Ishizuka, K. Koike, K. Takeuchi, T. Ibusuki and O. Ishitani (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557 Photocatalytic reduction of CO 2 with HjO on Ti/Si binary oxide catalysts prepared by the solgel method H. Yamashita, S. Kawasaki, M. Takeuchi, Y. Fujii, Y. Ichihashi, Y. Suzuki (Japan), S.-E. Park, J.-S. Chang, J.W. Yoo (Korea) and M. Anpo (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561 Photoelectrochemical reduction of CO2 at a metal-particle modified p-Si electrode in nonaqueous solutions Y. Nakamura, R. Hinogami, S. Yae and Y. Nakato (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565 Infrared spectroscopic study of CO, and CO reduction at metal electrodes O. Koga, T. Matsuo, H. Yamazaki and Y. Hori (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
569
Influence of anions on the production efficiency in pulsed electroreduction of CO z on metal and alloy electrodes R. Shiratsuchi, S. Ishimaru and G. Nogami (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Electrochemical reduction of CO 2 by using metal supported gas diffusion electrode under high pressure K. Hara, N. Sonoyama and T. Sakata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Electrochemical reduction of carbon dioxide at a platinum electrode in acetonitrile-water mixtures Y. Tomita and Y. Hori (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581 Electrochemical reduction of CO 2 in micropores T. Yamamoto, D.A. Tryk, K. Hashimoto, A. Fujishima and M. Okawa (Japan) ...........
585
Photoelectrochemical reduction of highly concentrated CO 2 in methanol solution K. Hirota, D.A. Tryk, K. Hashimoto, M. Okawa and A. Fujishima (Japan) . . . . . . . . . . . . . . . .
589
Studies on CO 2 fixation in PNSB : Utilization of waste as the additional source of carbon for CO 2 fixation by PNSB V. Brenner, M. Inui, N. Nonoura, K. Momma and H. Yukawa (Japan) . . . . . . . . . . . . . . . . . . . . 593 Studies on CO 2 fixation in PNSB : Analysis of CO 2 metabolism in purple non-sulfur bacteria M. Inui, J.H. Roh, K. Momma and H. Yukawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Crystallization and preliminary X-ray studies of phosphoenolpyruvate carboxylase from Escherichia Coli H. Matsumura, T. Nagata, T. Inoue, Y. Nagara, T. Yoshinaga, K. Izui and Y. Kai (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 Molecular characterization of recombinant phosphoenolpyruvate carboxylase from an extreme thermophile T. Nakamura and K. Izui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605
xiii
Revertant of no-active RuBisCO tobacco mutant, Sp25, obtained by chloroplast transformation method using microprojectile bombardment K. Tomizawa, T. Shikanai, A. Shimoide, C.H. Foyer and A. Yokota (Japan) .............. 609 Reductive TCA cycle in an aerobic bacterium, Hydrogenobacter thermophilus strain TK-6 M. Ishii, K. Yoon, Y. Ueda, T. Ochiai, N. Yun, S. Takishita, T. Kodama and Y. Igarashi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613 Carbon dioxide fixation and biomass production with blue-green algae Spirulina platensis S. Hirata and M. Hayashitani (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 Chitosan-calcium carbonate composites: Biomimetic mineralization of aqueous carbonate ions into chitosan-calcium alginate hydrogels S. Hirano, K. Yamamoto, H. Inui, K.I. Draget, K.M. Varum and O. Smidsrod (Japan).. 621 Tolerance of a green alga, Scenedesmus komarekii, to environmental extremes N. Hanagata, R. Matsukawa, M. Chihara and I. Karube (Japan) ............................. Over-expressed
effect of
carbonic
anhydrase
on
CO2 fixation
in
625
cyanobacterium,
Synechococcus sp. PCC7942 M. Murakami, N. Yamaguchi, T. Nishide, T. Muranaka and Y. Takimoto (Japan) ........
629
CO2 removal by a bioreactor with photosynthetic algae using solar-collecting and light-diffusing optical devices M. Nanba and M. Kawata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 Isolation and characterization of a green alga Neochloris sp. for CO 2 fixation M. Kawata, M. Nanba, R. Matsukawa, M. Chihara and I. Karube (Japan) .................
637
Antioxidant activity of CO2 fixing microalgae R. Matsukawa, Y. Wada, N. Tan, N. Sakai, M. Chihara and I. Karube (Japan) ...........
641
Screening of polysaccharide-producing microalgae Y. Shishido, M. Kawata, R. Matsukawa, M. Chihara and I. Karube (Japan) ...............
645
A marine microalga utilization for a paper: Semi-batch cultivation of Tetraselrnis sp. Tt- 1 by a tubular bioreactor and the partial substitution of whole kenaf pulp for a paper Y. Samejima, A. Hirano, K. Hon-Nami, S. Kunito, K. Masuda, M. Hasuike, Y. Tsuyu and Y. Ogushi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 Conversion of CO2 into cellulose by gene manipulation of microalgae: Cloning of cellulose synthase genes from Acetobacter xylinum Y. Umeda, A. Hirano, K. Hon-Nami, S. Kunito, H. Akiyama, T. Onizuka, M. Ikeuchi and Y. Inoue (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 Ethanol production from carbon dioxide by fermentative microalgae S. Hirayama, R. Ueda, Y. Ogushi, A. Hirano, Y. Samejima, K. Hon-Nami and S. Kunito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Production of polyethylene glycol and polyoxyalkylenealkylphenyl ether microspheres using supercritical carbon dioxide K. Mishima, K. Matsuyama, Y. Taruta, M. Ezawa, Y. Ito and M. Nagatani (Japan) ...... 661
xiv Carbon dioxide separation from nitrogen using Y-type zeolite membranes S. Morooka, T. Kuroda and K. Kusakabe (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
665
Comparative study of various amines for the reversible absorption capacity of carbon dioxide Y. Nagao, A. Hayakawa, H. Suzuki, S. Mitsuoka, T. Iwaki, T. Mimura and T. Suda (Japan) .......................................................................................................... 669 Kinetic analysis of COz recovery from flue gas by an ecotechnological system M. Tabata, T. Chohji and E. Hirai (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 Formation of peroxocarbonates from I_,3Rh(Oj)Cl and ~Ni(COj): a unique reaction mechanism with carbon dioxide insertion into the O-O bond M. Aresta, E. Quaranta, I. Tommasi, J. Mascetti, M. Tranquille and M. Borowiak (Italy). 677 Keyword Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
681
Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
687
XV
Preface The global environmental problems, especially the global warming caused by the accelerative accumulation of carbon dioxide in the atmosphere, are the most crucial for human beings. The population of the world is now approaching 6 billion, and is still increasing. Developments in communication systems and transportation tools have made circulation of information, technologies and materials easy, and are bringing the rapid economic growth, particularly in the area of east and southeast Asian countries.
The natural wish of human
being to live their comfortable lives directly connects the increase in consumption of fossil fuels, and consequently, carbon dioxide emission and environmentally-burden materials increase, resulting in the acceleration of evident climate change on a global scale. According to the most recent news, the increase in carbon dioxide emission in last year was the largest among the past seven years, and the total amount of carbon dioxide emission from all over the world attained to 6.5 billion tons. Furthermore, we cannot overlook the report which appeared recently in a distinguished scientific journal, "Nature" that the floor-area of the iceberg in the South Pole has already decreased by 25% in the past five decades. The anxiety has resulted in the operation of COP (United Nation's Framework Convention of Climate Change), and the third conference on COP will be held in the beginning of December 1997 at Kyoto International Conference Hall.
Naturally, such
scenarios planned on governmental or political basis would be focused on strategic topics such as the restriction on the amount of consumption of fossil fuels through the tax for the use of carbon resources and encouraging the temperate habits in the life style of people. On the other hand, proposal and discussion on progressive technological methods to mitigate and utilize carbon dioxide, which scientists and engineers must owe, are seems to be not so clear in the topics of COP. Therefore, the scientists and engineers, who had a strong interest and wished to contribute to solve the carbon dioxide problem
from technological aspects, gathered and
discussed the countermeasures in the conference. First conference on Fixation of CO2 by Chemical Ways was held in 1991 at Nagoya Japan by late Professor Kaname Ito as the chairman, who was the president of Research Association for Chemical Fixation of CO2, which had been established in The Chemical Society of Japan. Second conference was held at Bali, Italy in 1993, which was organized by Professor Michele Aresta as the chairman.
The name of the conference was changed to the present
xvi
Intemational Conference of Carbon Dioxide Utilization (ICCDU).
On that occasion,
Professor Aresta proposed the logo of the I CCDU, and he entered upon the duties of the permanent secretary. Third conference was held at Oklahoma University of USA in 1995, which was organized by Professor Kenneth Nicholas, and the venue of the 4th Conference was decided there. The logo of the 4th Conference was designed by myself with the concept to maintain the beautiful and comfortable globe. The Organizing Committee consists of Chairpersons, Emeritus chairpersons, Advisory Board, and Executive Board.
Five members among them participated in the program
committee, who contributed to publish the proceedings from Elsevier. I have to express my grateful thanks to the invaluable contribution of sponsors listed in the following page to the conference, who made the smooth operation and progress of the conference possible.
It is noteworthy that the Commemorative Association for the Japan
World Exposition (1970) donated the significant fund to the conference, which could realize the financial assistance, even partly for the participants who come from overseas countries. A wide variety of academic society also gave the strong support and sympathy to our activities. Scientific program consists of seven sessions which cover most of possibilities of chemical conversion of carbon dioxide.
As for carbon dioxide exhausts from large
generation sources, it should be concentrated and separated, followed by the rapid conversion into valuable compounds.
Once carbon dioxide diffuses into the atmosphere, it is preferable
to be incorporated into the plant through the biochemical methods.
For six sessions, one
plenary lecture and one key note lecture were done. One special lecture was delivered by Dr. Mary Preville of IEA. The unique and important schedule was the panel discussion in this session, and summary of each session was conducted by distinguished persons; moreover, from more than ten countries, the representative speakers presented the status and perspectives of chemical approaches of CO2 reduction in each country. Technical tour to visit Research Institute of Innovation Technology for the Earth (RITE) was planned on one day moming during the period of the conference, and made a strong impression on the visitors. There are the first world demonstration plant for methanol synthesis from carbon dioxide and other related technologies such as membrane devices for separation of CO2 from the flue gas.
The visitors could see also biochemical studies
conceming CO2 fixation, and studies on total systems for CO2 mitigation.
On the aftemoon
of the same day, they visited to Todai-ji temple located in the world famous ancient capital of
xvii Japan, Nara, where the largest bronze statue of Budda among the world is situated with its accommodation of the largest wooden building.
Nobody could avoid a great and strong
impression and admiration for the high grade technologies achieved by the people in 1300 years ago and their most reverential mind. This conference had been watched with keen interest, and was open to the informal~ion media with considering the public character of this conference, and unexpected number of information media came to get good hope in the future of humankind. Over 260 active participants from 21 countries joined to this conference.
On behalf
of the Organizing Committee, I thank the authors for their high quality presentations and for contributing to this volume.
I also thank the referees for their conscientiousness which
ensured the high scientific quality of this volume. Thanks are also extended to the members of the Program Committee whose invaluable effort make the publication of the proceedings possible. Finally, I believe that the conference afforded a great success and gave a brilliant hope to the people, and the conference became an effective core for developing the strong and everlasting roles to keep our dearest globe in the pleasant place. September, 1997
Tomoyuki Inui Chairman of the ICCDU IV Editor in Chief
xviii
ORGANIZATION Organizing Committee Chairman: Tomoyuki Inui (Kyoto University,)
Vice-Chairman: Tsutomu Yamaguchi (RITE)
Emeritus Chairperson: Kaname Ito (Late) Jiro Kondo (RITE)
Advisory Board:
Executive Board:
T. Hattori (Tokyo Gas Co. Ltd.) Y. Hori (Chiba Univ.) K. Izui (Kyoto Univ.) K. Kanai (RITE) T. Kodama (The Univ. of Tokyo) R. Kurane (National Res. Inst. Biosci., Human Tech.) E. Nakanishi (Osaka Gas Co. Ltd.) H. Nakano (The Kansai Electric Power Co. Inc.) M. Misono (The Univ. of Tokyo) T. Sakata (Tokyo Inst. of Tech.) K. Tanaka (Inst. for Molecular Sci.) K. Urano (Yokohama National Univ.) H. Yoneyama (Osaka Univ.)
M. Anpo (Univ. Osaka Pref.) A. Fujishima (The Univ. of Tokyo) S. Ikeda (Nagoya Inst. of Tech.) M. Ikenouchi (RITE) M. Hattori (RITE) M. Saito (RITE) Y. Souma (Osaka National Res. Inst.) Y. Takita (Oita Univ.) Y.Tamaura (Tokyo Inst. of Tech.) S. Yanagida (Osaka Univ.) M. Y anai (RITE) H. Yukawa (RITE)
International Advisory Board (Scientific Committee) Michele Aresta (Italy), Permanent Secretary of I CCDU Donald Darensbourg (USA) Martin M. Halmann (Israel) Shohei Inoue (Japan) Tomoyuki Inui(Japan) Aaron Kaplan (Israel) Roger Kieffer (France) Lars G. Ljungdahl (USA) Kenneth M. Nicholas (USA) Giuseppe Silvestri (Italy) Ralph Thauer (Germany) Yasuyuki Yamada (Japan)
xix
SPONSORING
The Organizing Committee gratefully acknowledges financial support from:
Research Association of CO2 Chemical Fixation of the Chemical Society of Japan Research Institute of Innovation Technology for the Earth (RITE) The Commemorative Association for the Japan World Exposition (1970) New Energy and Industrial Technology Development Organization (NEDO) The Federation of Electric Power Companies Japan Automobile Manufacturers Association, Inc. The Japan Iron and Steel Federation The Japan Gas Association Petroleum Association of Japan The Japan Society of Industrial Machinery Manufacturers The Japan Chemical Industry Association Engineering Advancement Association of Japan The Cement Association of Japan
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
International Energy A g e n c y action on climate change issues Mary A. Preville and Hans Jorgen Koch International Energy Agency 75739 Paris Cedex 15, France 1. INTRODUCTION
The fourth International Conference on Carbon Dioxide Utilization (ICCDU IV) is very timely. Governments from around the world will meet in Kyoto in December 1997 to agree on a new Protocol or Another Legal Instrument to the United Nations Framework Convention on Climate Change (UNFCCC). The conference provides a very valuable opportunity to take stock of activities in a potentially important area of climate change responses - CO2 utilization. As in any large societal development, technology will be a key ingredient in addressing the effects of climate change. Negotiators and policy makers in the climate change arena are increasingly recognizing that technologies, including technologies for utilizing CO2, will be a key response strategy for addressing climate change. But technology does not develop in a vacuum. Technology is developed and deployed when the people, the knowledge, the financing - and most importantly- the determination to succeed,exist. The organisers of the conference should be congratulated for bringing together representatives from both developed and developing countries to work together and exchange ideas and experiences. All economies need to be involved in finding solutions to mitigate climate change. Analysis of global emission trends of greenhouse gases shows that the stabilisation of concentrations in the atmosphere cannot be achieved by the industrialised countries alone. Wider participation and efforts to limit emissions and enhance sinks will be required. Co-operation among countries to share experiences will, within the limits imposed by differing national circumstances, bring benefits ranging from replication of the actions or technologies that have proved successful in one country, to actions undertaken within a coordinated policy that might otherwise not be possible at the national level. This paper highlights some of the key activities that the International Energy Agency (lEA) undertakes in the climate change area, and highlights some of the factors which the lEA believes will set the framework for future action in the energy sector to respond to the climate change challenge. The major focus of the paper, however, is on the technology dimension to climate change. Although rising in profile, this is an area where much greater attention is needed. Not only do we need to better understand the
role of technology in our economies but we also need to develop strategies for strengthening innovation and understanding its impacts. 2. THE INTERNATIONAL ENERGY AGENCY
Society's increasing sensitivity to environmental problems is reflected in the evolving nature of the International Energy Agency's mandate. Initially, the lEA was established, in 1974, to facilitate co-operation among industrialised countries in their shared quest for energy security. Over the years, the lEA has expanded its scope to be a forum in which 24 OECD Member countries come together to cooperate on the whole range of energy policy and technology issues. The IEA's objectives have taken on three dimensions. We often refer to them as the 'Three-Es': Energy Security, Economic Growth, and Environmental Sustainability. All three objectives simultaneously guide the IEA's activities and programmes. In the area of environmental sustainability, and in particular in the area of climate change, the lEA undertakes analysis and provides advice on a wide array of issues related to the energy dimension of climate change. This includes activities ranging from modelling energy and carbon dioxide projections to policy and economic analyses which inform the decision-making process for sound energy policies and programmes, to research and development for climate friendly technologies. Energy efficiency indicators, transportation trends and competitiveness issues related to the electricity sector are a few examples of analyses undertaken by the lEA. The development of methodologies in support of the concepts of Activities Implemented Jointly (AIJ), tradeable emission permits and voluntary agreements are also undertaken. The IEA's action on climate change issues is closely linked with the implementation of the UNFCCC. The lEA is one of only a handful of international organizations, and the only non-United Nations body, to be granted the status of Partner Organisation in the United Nations Climate Convention process. The lEA has been asked to play that role in order to bring expertise on energy matters to the Convention. This is done in close co-operation with other organisations. 3. THE ENERGY DIMENSION OF CLIMATE CHANGE
The sustainable development challenge of today will need to be addressed by a multitude of actions, and instruments to mitigate climate change and promote sound development have to be flexible. Lifestyles have changed, and standards of living have increased, with the increased use of energy. However, its use entails inherent environmental effects. The challenge is to limit those effects, in a flexible way, to levels and to forms that present and future generations can live with. To meet the challenge, a deeper grasp of the complexities of the energy sector must be developed, and this understanding must be passed on to climate change policy-makers and negotiators.
In this regard, a key analytical study entitled the lEA Statement on the Energy Dimension of Climate Change (EDCC) has recently been submitted as an official document to the UNFCCC. The lEA Statement describes the principal driving forces, key underlying factors and prevailing boundary conditions relevant to the energy aspects of the global greenhouse gas problem. In particular, the lEA Statement points out that: national circumstances and energy systems vary widely among both lEA and non-lEA countries; different dynamics of the main energy services (mobility, electricity and heat) involve different infrastructures, different technologies and capital stocks, and different behaviour among end-users, and different trends in relation to GDP growth over time; the extent and age of existing energy infrastructures constrain the rate at which cost-effective reductions in energy-related emissions can be achieved in the near-term; enhanced use of the best available technologies could help reduce energy requirements and associated emissions within the possibilities of current infrastructure, but barriers to the adoption of currently available and costeffective technologies will have to be overcome; every time energy-using capital stock and infrastructure is installed anywhere in the world, there is a unique opportunity to adopt climate-friendly technologies; and early involvement of all actors concerned will help foster innovation and change in long-term trends and infrastructure and achieve emission reductions at minimum cost. The promotion of sustainable economic development of course requires providing and expanding energy services while simultaneously reducing their energy and CO2 content. Market dynamics, the rigidity of infrastructures and attitudes, and the rate of capital stock turnover define the basic parameters of viable response options, pointing both to limitations and significant opportunities. Before examining new energy technological opportunities, it is useful to briefly review the nature of the energy sector and its contributions to greenhouse gas emissions. 4. THE ENERGY AND EMISSIONS OUTLOOK
The IEA's World Energy Outlook is an annual publication offering its perspective on Global Energy Demand and Energy Supply issues. The projections in the 1996 edition illustrate how energy use will grow in the absence of effective policies to alter established patterns of energy production and use. The bulk of energy demand and
CO2 emission growth will occur in the developing world. Emissions of the countries belonging to the Organisation of Economic Co-operation and Development (OECD) are projected to grow by 26 percent between 1990 and 2010, while those in the rest of the world (excluding the FSU) are projected to grow by 126 percent in the same time period, unless effective policy measures are introduced. The most dramatic increase in energy demand is projected to occur in the developing Asian countries, which are expected to account for between 44 and 55 percent of the increase in total world energy demand. Similarly, developing Asian countries are expected to account for more than 40 percent of the incremental demand for oil, and between 36 and 52 percent of incremental electricity demand. The message of the IEA's World Energy Outlook for energy-related CO2 emissions is clearly not a happy one. In fact, whatever assumptions were made about economic growth, energy prices and energy efficiency, emissions projections in the lEA Outlook rises substantially. In the simplest terms, this message confirms that the world's economy is highly geared to the use of fossil fuels. Nevertheless, the work underlying the lEA model does suggest there is room for policies that could result in faster than expected efficiency improvements, which would reduce the rate of growth in emissions. But, even theoretical "no regrets" policies, when viewed against the projections of the IEA's World Energy Outlook, will not be adequate to stabilise, much less reduce, energy-related CO2 emissions in the OECD by the year 2010. If more is to be done to reduce greenhouse gas emissions, costs will clearly have to be incurred. At the same time, there must be realistic expectations which do not disregard the inherent rigidities in the energy system. Realistic opportunities which are properly defined will vary widely from country to country, even within the OECD. 5. TECHNOLOGY OPTIONS
In the short to medium-term this means faster deployment of existing energy technologies which emit fewer greenhouse gases, and of those which use energy more efficiently. Given current energy prices, additional policy measures will, however, be needed to enhance the market opportunities for many of these options. But, there are limits to potential energy efficiency improvements and many fuel switching options; more innovative solutions will be needed in the longer term if the current goals are to be met. The attractiveness of technology options will vary among countries. As already stated, existing infrastructures constrain the rate at which cost-effective reductions in energy-related emissions can be achieved in the near term. This infrastructure extends beyond strictly energy-related infrastructures, such as power plants.
Infrastructures such as buildings, road networks, and energy grids embody the technology choices and the patterns of energy use of an immense web of economic agents. For the most part, past technology choices did not systematically take into account their energy-use implications, still less their environmental implications. But, these past choices will shape lifestyles and patterns of energy use for decades. They will also influence the selection of longer-term technologies. Long lead times for introducing new technologies and lengthy lifetimes of plant and equipment will influence those choices. For these reasons, CO2 capture and disposal remains an attractive option for the medium to longer term if the current pattern of energy supplies continues, based on the existing fossil fuel infrastructure and reliability of associated technologies. Longer-term technological options will need considerable work to bring them into the market place as commercial options. This applies not only to CO2 capture and disposal but also to options involving, for example, hydrogen fuel systems, renewable energy technologies and biotechnologies. In the case of CO2 capture and disposal, the challenges are significant. Most of the technology options for capture and disposal involve very considerable cost increases in energy supplies. As energy producers and consumers seek to reduce greenhouse gas emissions from their activities, market focus will favour the most cost-competitive solutions. While capture and disposal has the potential to play a significant role, costs will need to come down. The environmental impacts will also need to be fully understood. This will involve assessment of Iocalised impacts as well as the regional and global impacts, particularly in the case of the innovative, ocean sequestration options. The results of these studies must also be communicated widely, so that properly informed decisions can be taken. Also, technology reliability and maintainability will need to be established - as with other emerging technologies. This means the full range of technology risks must be clearly understood as early as possible. The more significant risks must be reduced and the information communicated to potential customers for those technologies. Considerable motivation will be required to mobilise and sustain the substantial efforts needed in these areas. Experiences to date with CO2 capture, utilisation and disposal are a good illustration of the efforts required to link R&D efforts and bring them to fruition. At the national level, various groups have been studying aspects of CO2 capture for some time. Nevertheless, it took considerable effort to create the networks, stimulate interest and bring together the different players working on the various chemical, physical and biological approaches to CO2 capture. The same can be said about other groups working on disposal and utilisation options. The ICCDU conference series is fine example of these efforts to link R&D efforts. Two other examples of international collaboration in the field of CO2 capture, disposal
and utilisation include the lEA Greenhouse Gas R&D Programme and the Climate Technology Initiative (CTI). The lEA Greenhouse Gas R&D Programme has the objective to evaluate (on a full fuel cycle basis) the technical and economic feasibility and environmental impacts of technologies for the abatement, control, utilisation and disposal of carbon dioxide and other greenhouse gases derived from fossil fuel use. One of the goals of the programme is to identify targets for R&D in this field and to facilitate practical activities. The programme is also encouraging more broadly based use of a systems approach to assessment of greenhouse gas mitigation options. The lEA Greenhouse Gas R&D Programme works within a structure the lEA calls
ImplementingAgreementswhich enable countries to work together co-operatively on
projects. Implementing Agreements provides the legal contractual mechanism for establishing the objectives of the projects and the rights and commitments of its participants. There are presently forty active lEA Implementing Agreements, each with between three and twenty participating countries. The lEA Greenhouse Gas R&D Programme has fourteen lEA Member countries participating, plus the European Union, Poland and Venezuela. Industry is also actively represented in the programme. lEA Implementing Agreements are not restricted to lEA Member countries. The co-ordinated research expenditures of lEA Implementing Agreement exceeds $100 million (US dollars) per annum. Thus, the Implementing Agreement mechanism provides very substantial leveraging of domestic expenditures. Another technology collaboration vehicle which has been established by lEA Member countries is the Climate Technology Initiative (CTI). The CTI is a voluntary initiative to foster and strengthen the development and deployment of climate-friendly technologies. Its aim is to share the experience and benefits of national and international measures, practices and processes in all parts of the energy chain. It builds upon existing efforts. The CTI was launched in 1995 at the first Conference of the Parties to the UNFCCC. The CTI responds to the spirit of several UNFCCC provisions, but avoids negotiation. Its key activities are undertaken by cross-sectoral government representatives, in consultation and collaboration with developing countries, the private sector, and others, as appropriate. The CTI has undertaken to report back to the UNFCCC Conference of the Parties on the progress and development of its activities, which it will do in Kyoto in December. The CTI has both a short and a long-term focus. The shorter-term focus is on enhancing markets for currently available technologies. The longer-term focus is on stimulating the research, development and diffusion of new and improved technologies that can contribute to meeting the UNFCCC goal of stabilising concentrations of greenhouse gases in the atmosphere.
More specifically, the CTI aims to: promote awareness of technology-related activities already underway to assist with responses to climate change concerns; identify and share expertise and experiences between countries already working on particular relevant topics, sharing also with countries having limited expertise in particular areas; identify gaps in national and multilateral technology programmes which could be addressed in order to strengthen climate response strategies; strengthen and undertake practical collaboration activities between countries to make technology responses to climate change concerns more effective. One of the five CTI Task Forces set up to implement the objectives of the CTI focuses on greenhouse gas capture and disposal. This includes the role of capture and disposal options as part of a hydrogen fuel chain based on fossil fuels. This task includes an assessment of the feasibility of developing longer-term technologies in these fields and ways to strengthen relevant basic and applied research. While much of the core work on CO= capture and disposal is undertaken at the national level, the international collaboration vehicles stated above play an important role in enhancing technology progress and in improving awareness of the CO2 capture, disposal and utilisation options. Continuing international collaboration will be needed to bring these technologies successfully to the market. The benefits of international R&D collaboration are numerous, but will not be elaborated in this paper. The lEA, through its programme of collaborative research, can facilitate international cooperation as it offers a flexible mechanism for interested players to pool their scarce R&D resources for mutual benefit, and for the long-term benefit of industry and consumers alike. 6. CONCLUSION
Although the climate change issue has been actively discussed in government and industry circles now for almost a decade, discussion is only now turning actively to market and sectoral realities. Society is likely to remain highly dependent on fossil fuels for power, heat and transport; existing infrastructures will shape climate change responses for a considerable time. Innovative solutions will be needed. Technology will play an important role in achieving longer term greenhouse gas emission reductions and enhancement of sinks. Technologies to capture and dispose of greenhouse gases offer huge potential for reducing greenhouse gas emissions. While this option should not be viewed as the sole solution to greenhouse gas concerns, it does provide an option to help meet these climate change challenges.
Expanded and intensified efforts will be needed to speed up the otherwise lengthy technology development and deployment process and so realise those potential technology contributions. Enhanced international co-operation involving all of the players can speed up that process by: 9 sharing the costs of research, development and demonstration; and 9 sharing the lessons learned and so avoiding costly replication of unproductive R&D. The process also has the ability to increase awareness by decision-makers in government and industry about the potential of CO2 capture and disposal technology, and its reliability. Technology has always been a key driver for societal development, and it will be a key driver for providing options for reducing global greenhouse gas emissions. Reducing the stress on our common environment will not happen by accident. It will only happen if the necessary intellectual and financial resources are devoted to developing and deploying new and improved technologies, and this within an appropriate policy framework. Moreover, it will only happen if there is the determination and motivation to succeed. Leadership will need to come from government and industry as well as from the research community. International organisations, such as the lEA, can assist in the process.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
Japan's Basic Strategy C o n c e r n i n g C o u n t e r m e a s u r e s to Mitigate C l i m a t e Change Tom Namiki Director-General for Environmental Protection and Industrial Location Bureau Ministry of International Trade and Industry
1. INTRODUCTION I would like to start by congratulating Professor Inui, Chairman of the Organising Committee, and all those concerned on opening the Fourth International Conference on CO2 Utilization, and to express my deep admiration for all the hard work everyone has done to prepare for the conference. I would also like to express my welcome to all the participants who have traveled to Japan from around the world to be here today. The theme of my presentation is "Japan's Basic Strategy Concerning Countermeasures to Mitigate Climate Change."
2. CURRENT STATUS OF CLIMATE CHANGE (GLOBAL WARMING) ISSUES Although a good deal of research has been carried out on global warming since the late 1980's, there is an increased sense of urgency among meteorologists and environmental researchers around the world, since there is undeniable evidence that the level of carbon dioxide in the atmosphere is continuing to rise. The Intergovernmental Panel on Climate Change (IPCC) was established to handle scientific information related to climate change as a joint proposal by the kIN Environment Program (UNEP) and the World Meteorological Organization (WMO) in 1988. The IPCC has the function of gathering and assessing scientific information and publishing reports on the results of their efforts. IPCC's first assessment report was issued in 1990 in which they announced that, although there was some tmcertainty about the problem of global warming, there was increasing international awareness that a precautional approach was necessary because it would be too late to act when real damage became apparent. As a result, the UN Framework Convention on Climate Change was adopted at the UNCED Earth Summit held at Rio de Janeiro in 1992.
l0 IPCC's second assessment report was issued in December 1995, and I will briefly summarize them. Their contents can be divided into the next five items; (1) According to the most realistic forecasts, by the end of the next century, global temperatures will increase by 2 ~ and the ocean level will rise 50 cm. (2) Climatic effects in the future depend on the volumes of greenhouse gases released from now on. Although there is a great deal of uncertainty regarding predictions, the cost of countermeasures and for damage repair may increase if countermeasures are delayed. (3) To stabilize the emission of greenhouse gases, it is necessary to limit to a fairly low level the human-induced emissions of such gases. Technologies (espeoially their promotion and transfer) have great potentiality to contribute to the decrease of such emissions. They are still practical from the financial viewpoint. (4) The more countries that carry out such measures, the greater the overall effect. (5) It is important to overcome difficulties in implementing effective policies on climate change issues, to promote the taking of measures in each country suitable to each situation, to further broaden scientific knowledge, and to develop, introduce, and transfer technologies. 3. THE BASIC POLICIES BEHIND THE JAPANESE COUNTERMEASURES FOR THE CLIMATE CHANGE
In 1990, Japan decided on an action program to arrest global warming. At the same time, by working within the UN Framework Convention on Climate Change adopted in 1992 and other treaties, we have been taking positive steps to address this issue. Considering the emergency directives for measures based on the new .scientific information given in the IPCC's second series of assessment reports and the need to conclude studies concerning specific measures for 2000 and beyond by COP3, MITI organized the Global Environmental Committee of the Industrial Structure Council from April 1996 to March 1997. The working group submitted a report in March 1997 which laid out a strategy concerning the direction in which to address the climate change issue and specific policy deployments. I would like to present my opinion on this matter, which draw on this IPCC' s report.
11 3. 1 The f i r s t
issue relates
to the future
directions
of measures
t o be t a k e n (1) Comprehensive measures encompassing environmental protection, economic growth, and energy demand/supply stabilization.
Since the major generator of CO2, which causes climate change, is the use of energy, this problem is closely related to economic growth and energy demand/supply stabilization. To continue with sustainable development worldwide, it is necessary to plan comprehensive tripartite measures for three tasks: suppression of CO2 emissions, maintenance of economic growth, and energy demand/supply stabilization, thereby realizing a socioeconomic world in harmony with the environment. In addition, while taking these measures, it is necessary to take those issues deeply related to climate change, such as population problems in developing nations, into account. (2) Consideration of overall environmental concerns, in addition to climate change.
There is a possibility that the promotion of countermeasures for climate change will affect or be influenced by other environmental problems. For example, the promotion of the use of diesel for automobiles will reduce CO2, but will increase SOx and NOx. Recycling of resources consumes energy, and may increase CO2 emissions. Accordingly, when promoting measures thoroughly, it is necessary to identify the position of the climate change problem in all environmental problems, and fully analyze the impact of such measures on the environment as a whole. (3) Importance of voluntary measures
Considering our experience in realizing significant energy conservation after the oil shock, in addition to the uncertainty of scientific information related to climate change, in my view it will be necessary to implement a range of approaches in good balance without simply immediately enforcing compulsory means to tackle climate change problems. In particular, it is important to promote voluntary actions by each economic sector. (4) Key importance of comprehensive measures by all sectors: industry, society, and transportation
The emission of C02, which is the main cause of climate change, is broadly related to human activity, and so increased emissions not only from industry but also society and the transportation sector is a serious problem. For this reason, in making a tripartite plan in the future, although it is of course essential to continue to promote policies in the industry sector, it is crucial to implement comprehensive measures that include society and the transportation sector.
12 (5) Raising the consciousness of all people. Although most people are aware of the seriousness of the climate change problem, there is as yet no consensus as to the urgency of the measures required to tackle it. Accordingly, it is important to conduct activities designed to improve people's awareness, especially by extending provision of education and information to the general public, and to promote lifestyle changes in ordinary consumers.
(6) Importance of breakthrough by development, promotion and transfer of innovative technology. To stabilize CO2 concentrations, it is necessary to see a drastic, worldwide reduction in the volume of CO2 emissions, and to realize comprehensive tripartite measures. In my view it is of key importance to achieve a breakthrough via the development, promotion and transfer of innovative technology. (7) The long--term vision In responding to the climate change problem, working toward comprehensive tripartite measures, considering the need for a breakthrough achieved by innovative technology, there must be a long-term vision.
3 . 2 Based on t h e s e f u t u r e address
specific
directions,
I would
like
to
now
policies.
(1) The direction of measures toward achieving the goals for 2000 in Japan Based on the Action Program to Arrest Global Warming, Japan has made a worldwide declaration of intent to stabilize per capita emissions of CO2 in 2000 at the 1990s level. At the same time, however, viewing the present situation, along with the continued trend in increasing emissions of CO2 centered on society and the transportation sector, there has also been a worsening trend in energy consumption ratio in the industry sector in recent years. If the present trend continues, we will face very severe difficulties in achieving the goals for the year 2000. It is of key importance to implement additional measures as soon as possible to get back on schedule as we head toward 2000. Specifically, measures concerning energy supply and demand, energy conversion, promotion of energy conservation in the industry, society and transportation sectors, and acceleration of the introduction and promotion of new types of energy that have a reduced impact on the environment, such as photovoltaic power generation, are essential. In this connection, Japan in April this year adopted "Comprehensive Energy Conservation Measures Towards the Year 2000", which strengthens the enforcement of current law concerning the rational use of energy to ensure the thorough implementation of energy conservation. Following this in June, Law on Special Measures to promote the utilization of New
13 Energy was newly stipulated to promote the introduction of new types of energy. Furthermore, from the viewpoint of promoting voluntary action by industry, the industrial environment vision was revised centering on measures against the climate change problem. (2) Domestic and international measures viewed from 2000 and after
The key aim of these measures is the promotion of technological development. To achieve the ultimate goal of countermeasures against global warming - - the stabilization of the concentration of greenhouse gases in the atmosphere m it is necessary to sharply reduce CO2 emissions over the mid- and long-term. If concentrations are to be stabilized at double the level seen before the industrial revolution, it will be necessary ultimately to reduce emissions of CO2 to less than half the current level. At the present juncture, however, the means of achieving this have not yet been found, and so in the future it will be of key importance to achieve a breakthrough in technology. Japan has made an international proposal for the New Earth 21 Program from this point of view, and the concrete deployment of this program has been to promote the development of innovative energy and environmental technologies based on the New Sunshine Program. As we promote the development of these technologies, it will become necessary to clarify the development time frame and share a long-range vision among all nations, including developing countries, to promote international cooperation. I believe it is necessary, to examine specific actions, taking into account scientific advancement, in line with the New Earth 21 Program which has recently been restructured based on statistical models. Specifically, in terms of the possibility of restrictions on the supply and demand of major energy resources in the latter half of the 21st century, it will be necessary to concentrate on R&D for technologies with less limitations in supply that can be expected to be broadly introduced and promoted via technological breakthroughs. For example, we should establish basic technologies for photovoltaic energy, hydrogen energy, superconductivity, and fuel cells by 2020-2030. By systematizing these basic technologies, we will be able to implement them on a global scale by the latter half of the 21 st century. To significantly reduce CO2 emissions over the mid-and long-term, it is also necessary to come to grips with the development of innovative environmental technologies, namely applied CO2 fixation technology, environment-friendly production technologies, and CO2 treatment technologies. It is expected that these will be firmly established by around 2020. There will be presentations here today on R&D results related to innovative environmental technologies. MITI will provide about 9.7 billion yen this year for such R&D, and we expect to further increase the budget next year. The development of innovative energy and environmental technologies is not something to be handled by Japan alone, and it is important to seek out international
14 cooperation. This makes the active utilization of the approach of the Climate Technology Initiative (CTI) essential. The objective of the CTI is to promote international cooperation in the following two areas: (i) The improvement and promotion of existing energy conservation and alternative energy technologies, and (ii) the development of innovative technologies to capture, treat and utilize greenhouse gases. CTI was jointly proposed and approved by the 24 countries in IEA/OECD at COP 1 in May 1995. Japan was appointed as task force leader for the development of innovative environmental technologies at the CTI assembly in February 1996, and in February this year we were selected as the country to chair CTI. I hope we will positively realize the initiative toward international cooperation. The second point is the importance of policies that include developing countries. Considering the dramatic increase in C O 2 emissions in developing countries, especially in Asia in the future, it is necessary to substantially promote restrictions on CO2 emissions in such countries. In particular, based on our experience in conserving energy along with economic growth, we expect to positively cooperate in working on this problem, focusing on technology transfer and promotion, taking into account the industrial structure and technological levels of developing countries. Concerning the transfer of environmental and energy technologies to developing nations, we are promoting a green aid plan whose pillars are individual projects involving discussion on policy measures and model projects. We believe it is necessary to continue to promote this plan. In addition, I consider joint implementation to be an especially effective method in further promoting specific measures. Joint implementation is a method for controlling the greenhouse gases specified in the UN Framework Convention on Climate Change. It makes it possible to collectively allot results (the volume of greenhouse gas reduce) among member countries implementing controls on greenhouse gas emissions. Concerning joint implementation, at COP 1 in 1995, agreement was reached on the start of a pilot phase for joint implementation activities, with participation understood to be on a voluntary, basis. In November 1995, Japan held two joint meetings, the Executive Committee Meeting of the Cabinet to Promote Comprehensive Energy Policies and the Council of Ministers for Global Environment Conservation. Agreements were made for a basic framework for the "Joint Implementation Activities m Japan Program." In January 1996, at the Joint Ministry Conference for Activities Implemented jointly chaired by MITI and the Environment Agency, assessment guidelines were approved. Then, Ministries related to the Joint Implementation Activities m Japan Program, centering on MITI and the
15 Environment Agency, approved the 11 plans for the first project related to Japan Program. We will proceed with negotiations with member countries and continue to work on examining conditions which are easy for developing nations to agree on, with the aim of reaching an early agreement among the nations participating in this scheme. Also to promote further positive participation at each industrial level, we are this year making a public announcement to contract out a feasibili .ty study survey on the Japan Program.
4. APPROCHES TO BUILDING FUTURE INTERNATIONAL FRAMEWORKS As you all know, COP3 will be held here in Kyoto in three months' time. Japan, as the chair of COP3, is expected to make a positive contribution toward building a new international framework for tackling the climate change problem. In COP3, I hope to create a framework which will encourage each participating country to work dedicatedly and effectively on the climate change problem, without its being merely noted as a diplomatic event. It is instead important to aim at a framework tbr concrete action. At the G7 summit meeting held in Denver this June, agreement was reached on a number of fundamental principles, which I will list, concerning COP3. (i) To agree on a meaningful, practical and equitable goal. (ii) To identify responsibilities and assure transparency in the COP3 agreement, and at the same time accept flexibility among participating nations regarding measures to achieve the goals. (iii) To recognize the need for action by developing countries to tackle climate change problems and the need for cooperation with developed countries, to include technology transfer and environmental education. At this summit meeting, Japanese Prime Minister Ryuichi Hashimoto proposed a "Global Remedy for the Environment and Energy Use (Green Initiative)," which promotes implementation of an international cooperative action program for the development and transfer of technology. This initiative was again proposed by Prime Minister Hashimoto at a special UN general meeting held in New York. I believe it is necessary for Japan to consider the following basic policies towards COP3. (1) Establishment of goals which d i f f e r e n t i a t e among developed nations Establishment of quantitative goals is eftbctive at promoting positive measures for suppressing emission of carbon dioxide by developed nations. In securing a new
16 international framework that is "equitable" considering the similarities and differences among the nations, it is essential to promote positive action by each nation. For example, the current pledge and review method obliges all countries to achieve uniform reductions based on 1990 levels. In my view, it is important to develop and propose a new index for setting different quantitative goals in line with past efforts, taking into account the fact that the levels of effort devoted by developed countries to energy conservation are far from uniform. (2) The need f o r
l o n g - t e r m a c t i o n s based on R&D
Since the problem of global warming involves continuing reductions of CO2 over the very long term, emphasis must be placed on long-term actions using technological developments. MITI proposed the New Earth 21 Project from this perspective in 1990. To promote international cooperation in future innovative technological development efforts, it is necessary for each country, including developing nations, to share a long-term vision. Based on future scientific advancement, it will be necessary to restructure the Project to realize these intentions. (3) F l e x i b i l i t y
of time frame
Looking at those countries where it is possible to make dramatic short-term reductions in CO2, the cases of the reunion of East and West Germany and the conversion from coal to North Sea natural gas by Britain show that each country has its own individual circumstances. Since these differ among countries, and technological development takes a long time, I think it will be necessary to set a more flexible time frame, allowing a longer period for some of the goals depending on their content without just imposing a uniform short time frame, such as by 2005, on each nation. (4) Importance of p o l i c i e s energy u t i I i z a t i o n
and measures t o
improve the e f f i c i e n c y
of
Reflecting on the experience of the significant conservation of energy that Japan has implemented since the 1970s, to realize a tripartite approach, policies and measures promoting the efficient utilization of energy should become more effective and efficient. In doing this, and in order for flexibility to be granted for working on policies by each country so that they are able to promote independent responses by industry and so on, and for objective evaluations of the results of work on policies encouraging the effective utilization of energy by each country, one strategy would be to establish measures aimed at increasing the efficiency of energy utilization, and a scheme established for secure reviews of the handling of such measures by each participating country. (5) Promot i on of measures i n deve Iop i ng countr i es
17 Considering the anticipated significant future increases in carbon dioxide emissions in developing countries, we should not simply debate policies for developed countries. We must study measures, including the strengthening of environmental policy dialogues, technology transtbrs from developed nations, and participation in the Annex 1 group in the semi-advanced nation UN Framework Convention on Climate Change to substantially promote as many measures as possible for developing countries within the possible negotiation range. (6) Promotion of the green i n i t i a t i v e Furthermore, I believe it will be important to make the Green Initiative that Prime Minister Hashimoto presented at the Denver Summit this past June more concrete as a comprehensive action program in relation to the promotion of introduction of energy conservation, non-fossil fuel energy sources, worldwide deployment of forest planting, and promotion of innovative technological development and technological transfer. 5. OONOLUSION It bears repeating that the climate change problem is one that is expandable in terms of both time and space, and it is one that requires independent action along a spectrum of independent bodies. In working towards a solution to the problem, viewed as a global scale issue, the key challenge worldwide is to implement environmental energy measures from a local level. Finally, to tackle global environmental problems, it is important to continue to increase scientific knowledge and vigorously promote technological developments concerning CO2. I am sure that, through the active discussions that will soon take place, this conference will contribute to the further advancement of R&D in environmen{al energy research institutes worldwide, including RITE. In closing, let me emphasize that your efforts will play a significant role in solving climate change problems. Thank you very much for your kind attention.
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
19
R e s e a r c h a n d d e v e l o p m e n t o n n e w s y n t h e t i c r o u t e s for b a s i c c h e m i c a l s by c a t a l y t i c h y d r o g e n a t i o n of CO2 Hironori Arakawa National Institute of Materials and Chemical Research (NIMC), Higashi 1-1, Tsukuba, Ibaraki 305, Japan This paper gives a review of recent research work in catalytic hydrogenation of CO2 to various kinds of valuable chemicals and fuels. Recently, CO2 hydrogenation has been positively studied in relation to a possible countermeasure against global warming caused by CO2 emission. Advances in the effective syntheses of methanol, dimethylether, ethanol, lower paraffin, lower olefins, gasoline fraction, formic acid, acetic acid and others are introduced. Additionally the progress of solar hydrogen production for this CO2 fixation is described and its importance is pointed out. 1. INTRODUCTION Global warming caused by a remarkable increase of CO2 emission into the atmosphere is an important and urgent problem to be solved. Effective countermeasures should be carried out to reduce such a problem for sustainable prosperity of the human race. Catalytic hydrogenation of CO2 has been recently attracting considerable attention as one of chemical fixation and recycling technologies for emitted CO2.[1] Therefore, extensive studies on this technology have been conducted now. Catalytic hydrogenation of CO2 has some advantages compared with other countermeasures such as CO2 deposit and disposal. It is noted that catalytic hydrogenation can fix or convert CO2 very quickly. In addition to the reduction of CO2 emission into the atmosphere, it can produce valuable chemicals from emitted and useless CO2 and it can save carbon resources, such as petroleum and natural gas, for industrial synthesis of valuable of chemicals. Industrial process for syngas (H2/CO) conversion such as methanol synthesis gives another advantage to CO2 hydrogenation technology. Because the industrial production facilities for syngas conversion process is supposed to be easily applicable to H2/CO2 conversion process without any significant replacement of facilities. Further, CO2 hydrogenation reaction is regarded as one of artificial photosynthesis technologies. It is well known that photosynthesis by plants consists of two main reactions, that is, the light reaction and the dark reaction. In the light reaction, water is decomposed to H2 and 02 under the solar visible light irradiation. Produced 02 is emitted into the atmosphere. On the other hand, produced H2 is carried by NADP § as NADPH. In the dark reaction, adsorbed CO2 is hydrogenated by H2 from NADPH and CO2 is fixed finally as carbohydrates under the non-
20 irradiation conditions. Catalytic hydrogenation of CO2, in a sense, corresponds to the dark reaction of photosynthesis. Remarkable advances in catalytic hydrogenation of CO2 have been established by extensive research and development for last 10 years. As shown in Fig.l, various kinds of chemicals have proved to be produced effectively by CO2 hydrogenation process. This paper reviews such advances in catalytic hydrogenation of CO2.
I Lowerhydrocarbons (Etia
Ethane,.. etc.)]
] Methane ( C H 4 ) ~ r b o n [Carbon Monoxide (CO) ~
[ Higher hydrocarbons (Oil, Wax, .etc.) ]
(~x) 1~ J
(Benzene, Toluene, etc.) [ 1"Aromatics _~,,~,~,,4P'. . . . . . . . . . . .
Carbon Dioxide (CO ]1' 'i " " (CO2) Q . . . . .
[ Formic Acid ( H C ~ H ) ~ / - I " ' " " ~
-[Methanol (CH3OH) i
~
,
~ "~~ ~ m e t h y l e t h e r
'
..
( CH3OCH3) 1
! Formic Ester [ ]-Carboxyiic Acid (CH3COOH, etc.) ] [ i-ii#er A!coh.o.!s(C2HsOH, etc.)] Figure 1. Chemicals synthesized by catalytic hydrogenation of CO2 2. ALCOHOL AND ETHER SYNTHESIS 2.1. Methanol synthesis Methanol synthesis from H2/CO2 has been studied so far in relation to that from H2/CO.[2,3] One of the reasons studied before is due to an interesting phenomenon that an addition of small amount CO2 into H2/CO feed improves methanol yield significantly in the industrial process.[2] The role of added CO2 was noted. Rozovskii showed by tracer analysis study that carbon species of produced methanol originated from CO2 in the H2/CO feed , suggesting methanol was produced v/a CO2.[4] Recent researches have been aiming at the development of highly efficient catalysts for methanol synthesis for the industrial process.
Thermodynamic equilibrium for methanol synthesis Figure 2 shows thermodynamic equilibrium yields of methanol from both H2/CO2 and H2/CO at 5 Mpa. Equilibrium yield of methanol from H2/CO2 is lower than that from H2/CO at all reaction temperatures. However, experimental results show a higher yield of methanol from H2/CO2 at around 260 ~ An example over Cu-ZnO/SiO2 catalyst is shown in Fig. 3.[5]
Catalysts for methanol synthesis from H2/C02 Various kinds of metal catalysts are reported to be active for methanol synthesis from H2/CO2. Activity of metal catalyst for methanol yield increased with following order, Cu~Co=Pd=Re~Ni~Fe~Ru=Pt~Os~Ir=Ag=Rh~Au.[8] Needless to say, catalytic activity is much dependent on metal dispersion, additives and type of support. It is apparent, however, Copper is the most active metal species for methanol production. Effect of metal oxide support to 5wt%Cu catalyst was studied.[9]
21 I00
.
.
.
(a)
.
.
.
.
.
: C O + 21-I2 - CI-I301-I
2H2/CO2--*CH3OH g
3H2 = CH3OH
80
-~
~
0 /
O~~/~O
~ 9 60-
~ 9
9
0
o /
40
/
r 20 5wt%Cu-ZnO(l" 1)/Si02 150
200 250 300 Temperature (~ Figure 2. CH3OH yield at equilibrium
350
220 240 260 2so .... 9
.l
l
l
Temperature (~ Figure 3. Practical yield of CH3OH
Though methanol yield increased with following order of Cu/A1203~Cu/ZnO~ Cufl~O2~Cu/SiO2, the order of turnover frequency(TOF) was as follows, C u / Z n O ~ CufFiO2=Cu/ZrO2~Cu/Cr203=Cu/CeO2~Cu/A1203~Cu/SiO2. TOF indicates an intrinsic activity of reaction sites over Copper. Alumina and Silica improved Cu dispersion of catalyst because of high surface area of support itself and they increased methanol yield, but they did not improve intrinsic activities of reaction sites of catalysts.[9] Obtained results suggest ZnO. TiO2 and ZrO2 improve reaction sites remarkably. Doubly promoted Cu catalysts were investigated using 5wt%CuZnO(l:l) catalysts. Methanol yield in each catalyst increased with following order, 5wt%Cu-ZnO/Al203=5wt%Cu-ZnO/SiO2=5wt%Cu-ZnO/TiO2 ~ 5wt%Cu-ZnO/ZrO2 =5wt%Cu-ZnO/Cr203>5wt%Cu-ZnO/CeO2.[10] From these results, it is apparent that ZnO, TiO2, ZrO2, Cr203, CeO2, Al203 and SiO2 act as promising promoters to Cu catalyst for methanol synthesis. Concerning co-precipitated catalyst, Cu-ZnO based multi-component catalysts such as Cu-ZnO-Al203, Cu-ZnO-ZrO2 and Cu-ZnOCr203 are extensively investigated. Recently Ga203 and La2Zr207 were found to act as promising promoters for Cu catalyst by Fujitani et al. [11] and Kieffer et a1.[12], respectively. Inui et al.[13] and Lee et al.[14] have shown, respectively, that the addition of small amount of noble metals such as Pd and Rh to Cu-ZnO based catalysts improve methanol formation remarkably. As non-Copper catalysts for methanol synthesis, Ag/ZrO2115], Au/ZrO2,TiO2115,16], Pd/CeO2117], Mo2C[18] and PtW/SiO2119] have been investigated. Reaction mechanism and the structure of active site over Cu-ZnO based catalyst Many experimental results suggest methanol is produced not from CO, but from CO2 directly. As mentioned above, methanol yield from H2/CO2 is higher than that from H2/CO at a wide range of reaction conditions.[5,6,7,20] A study of influence of contact time on product distribution suggested methanol was a primary product[21]. Fujita et al.[7] and Koeppel et a1.[22] showed methanol and CO were produced
22 through parallel pathways. In situ FT-IR observations of surface species over CuZnO based catalysts showed the presence of Copper formate as well as Zinc formate.[10,21,23,24] A formate located at the Cu-Zn interface was also observed by TPD study.[25] Because of low reactivity of Zinc formate with H2 stream over catalyst, Copper formate was suggested as a main reaction intermediate.[21] Millar et al.[23]and Vanden Bussche et a1.[29] have also concluded Copper formate was the pivotal intermediate for methanol. Fujita et a1.[7,24] suggested, however, that transformation of Copper formate to Zinc methoxide by hydrogenation proceeded and Zinc methoxide, which was a observed species by in situ FT-IR, was easily converted to methanol by hydration at atmospheric reaction condition. Joo et a1.[26,27] have concluded from TPD and TPD studies that formate species migrated from Cu to ZnO and Zinc formate was converted to methanol by hydrogenation at atmospheric conditions. Various proposed reaction mechanisms were summarized in a unified reaction mechanism shown in Fig. 4. f H
oI I c /
o -
o,
CO--.CuxO
~
d /. - 9.k
o-,
;9"--9-
Ha
CH3
---->
o,
l
o
i orT
coz
.
I (Cu)m-(ZnO)n ] i Cu
Ha k
0I
/
Cu
\
0i -
Cu
IH~ z-.x
" 0 : - :0 I: :1 Cu Cu
I
>
,
0
0
Cu
Cu I
---[col
Figure 4. A unified reaction mechanism of CH3OH formation over Cu based catalyst
It is apparent that main active site of Cu-ZnO based catalyst exists on the surface of Copper particles. For example, over ZnO catalyst, methanol formation is low about two orders of magnitude compared with that over Cu catalyst.[5] Zinc oxide is just a promoter to Copper. Therefore, interfacial sites between Copper and ZnO is very important for effective methanol synthesis. Recently Fujitani et al. have found a good correlation between oxygen coverage of Copper surface of catalyst and specific activity for methanol production over various Cu based catalysts.[ll] The maximum specific activity was obtained at oxygen coverage=0.17. Oxygen coverage was determined by in situ N20 titration of catalyst. This result shows that about 20% of metallic Copper surface was oxidized and, in other words, coexistence of metallic Copper and partially oxidized Copper is essential for effective production of methanol. They have also concluded that ZnO stabilizes Cu § state of Copper particle.[28]
23
Effective methanol synthesis using Cu-ZnO based catalysts Several kinds of excellent Cu-ZnO based catalysts, such as Cu-ZnO-A1203-Cr203 [21], Cu-ZnO-TiO2 [30], Cu-ZnO-Ga203 [31] and Cu-ZnO-ZrO2-A1203-Ga203 [31], were developed so far. Table 1 shows reaction behavior of these catalysts. A fairly large amount of methanol (STY) is produced over these catalysts. Addition of small amount of CO to the H2/CO2 feed increases methanol formation significantly.[32,33] This is a favorable phenomenon for a practical methanol production from H2/CO2. Because unreacted H2, CO2 and produced CO have to be recycled in the industrial production. Methanol formation from the feed gas of H2:CO2:CO=75:22:3 is also shown in Table 1. Extremely high yields of methanol were obtained by Pd modified CuO-ZnO-Cr203-A1203-Ga203 [32] and Cu-ZnO-ZrO2A1203-Ga203 [31]. Methanol production yields from H2/CO/CO2 feed in commercial process are shown in Table 1, too.[34.35] It is apparent that methanol production from H2/CO2 feed is already competitive to industrial methanol production from syngas,H2/CO feed. Bench scale test for 50kg/day methanol production are now conducted at RITE, Japan. Table 1 Effective methanol synthesis from H2/CO2 compared with that from H2/CO Ref. Catalyst Press. Temp. GHSV CH3OHSTY (Mpa) ( ~ ) (l/h) (g/l-cat.h) H2/C02=3/1 240 20000 411 [21] CuO-ZnO-A1203-Cr203(43-20-34-3) 3 240 20000 504 [30] CuO-ZnO-TiO2(30-35-35) 5 250 4700 502 [29] La/CuO-ZnO-A1203-Cr203(25-42-32-1) 8 250 18000 738* [31] Cu-ZnO-Ga203(50-25-25) 5 H2/C02/C0=75/22/3 250 18000 785* [31] Cu-ZnO-ZrO2-A1203-Ga203 5 270 18800 1300 [32] Pd/Cu-ZnO-A1203-Ga203-Cr203 8 (lwt%/38-29-13-18-2) H2/CO/CO2/CH4=70/20/7/3 226 12000 700 ICI [34] CuO-ZnO-A1203(24-38-38) 5 CuO-ZnO-A1203(64-32-4)** 5 250 10000 300 Academic [34] CuO-ZnO-Cr203 10 253 12000 1225 Lurgi [35] *:STY:g/kg-cat.h, **:H2/CO(no CO2 in feed gas)
2.2. Dimethylether synthesis Dimethylether (DME) can be synthesized effectively from H2/CO2 feed by one-pot synthesis using hybrid catalyst. Hybrid catalyst is composed of mixture of methanol synthesis catalyst and solid acid catalyst. DME is an important and valuable chemical used as solvent, propellant and raw material to liquid fuels. As shown in Fig.l, methanol from H2/CO2 has a severe limitation of thermodynamic equilibrium compared with that from H2/CO. To overcome such a limitation, in situ transformation of methanol to DME is reasonable way to improve total methanol yield (CH3OH + DME). Table 2 shows reaction behavior to DME synthesis over hybrid
24 catalysts.[36] It is apparent that Y-zeolite and Mordenite show good performance for in situ dehydration. Total methanol yield increased 1.7 times compared with that over CuO-ZnO-A1203 catalyst alone and 55% selectivity of DME was obtained at 3 Mpa and 240~ Inui et a1.[37] obtained 70% selectivity of DME and 736g/1-cat.h of total methanol STY using the mixture of CuO-ZnO-A1203-Cr203 and SAPO-34 (1:2 in volume ratio) at the condition of 8 Mpa, 300 ~ GHSV=9400/h and H2/C02/C0=75/22/3 feed gas. Practical application of this process will be expected. Table 2 Dimethylether (DME) synthesis from H2/CO2 using hybrid catalyst Catalyst CO2 cony. Selectivity(%) Total CH3OH ~eld(%) (g/g) (%) CO DME CH3OH (DME + CH3OH) A/SIO2(1/0.56) 20.5 50.3 0.0 49.6 10.2 A/7-A1203(1/1) 21.6 46.1 16.9 37.7 11.6 A/SiO2-A1203{98-2}(1/1.6) 23.6 34.4 47.1 18.4 15.5 A/Y-Zeolite(I/0.8) 24.4 32.4 54.8 12.8 16.5 A/Mordenite(1/1.1) 25.0 31.7 55.1 13.0 17.0 1.2 ml of A catalyst was mixed with 1.7 ml of solid acid catalyst. A:CuO-ZnO-A1203(32-66-2); *:Reference 2.3. E t h a n o l synthesis Synthetic ethanol is now produced by hydration of ethylene. Ethanol is one of important chemicals. Though ethanol synthesis from H2/CO was extensively studied so far, there are a few reports about ethanol synthesis from H2/CO2. Recently, some efficient catalysts for ethanol formation from H2/CO2 were developed by Arakawa and his co-workers. Ethanol synthesis by K / Cu-Zn-Fe mixed oxide catalyst It is known that potassium promoted Fe catalyst can produce C2+-alcohols by H2/CO .[38] Therefore, the combination of reverse water gas shift reaction (1) and syngas reaction over K/Fe catalyst (2) might produce ethanol. This idea is a general concept for K/Cu-Zn-Fe catalyst development. H2 + CO2 -(shift catalyst) ..+ CO + H20(inverse shift reaction) (1) 2CO + 4H2-(K/Fe catalyst) ..+ C2H5OH + H20(syngas reaction) (2) As a result of screening test, the combination of K/CuO-ZnO and K/Fe oxide has proved to be a efficient catalyst.[39.40] The results are shown in Table 3. At reaction temperature below 250~ main product was CO. Alcohol formation, however, increased with increasing reaction temperature. Catalysts were prepared by impregnation of K2CO3 onto Cu-Zn-Fe mixed oxide. Over K/Cu-Zn-Fe(0.08/I-I-3) catalyst, ethanol was produced with 20% selectivity at 44% conversion of CO2 under the condition of 7 Mpa, 300~ GHSV=5400/h and H2/CO2=3/1. In case of GHSV=20000 using this catalyst, 270g/l-cat.h of ethanol STY was established. Similar catalyst such as Fe-Cu-AI-K/Cu-Zn-AI-K was also applied to ethanol formation. [41]
25
Table 3 Effective synthesis of ethanol over K/Cu-Zn-Fe mixed oxide catalyst Catalyst CO2 cony. SelectiviW in carbon efficiency(%) ( % ) CO MeOH EtOH C3+OH CH4 K/Fe (0. 4/2) 32.7 11.9 0.2 6.5 4.3 13.4 K/Cu-Zn-Fe(0.4/1-2-1) 39.0 8.7 1.9 13.2 6.2 12.4 K/Cu-Zn-Fe(0.08/1-1-3) 44.4 5.9 2.0 19.5 5.5 13.6 Conditions: 7 Mpa, 300~ GHSV=5400/h, H2/C02=3/1.
C2+ H.C. 63.6 57.6 53.5
Ethanol synthesis by promoted Rh/Si02 catalyst It is known that ethanol can be synthesized efficiently from H2/CO by promoted Rh/SiO2 catalyst.[42.43] Based on this result, promoted Rh/SiO2 catalysts were tested for CO2 hydrogenation to ethanol.[44,45] As a result, it has proved that Sr, Li and Fe additives are effective for ethanol formation.[46,47] Typical results are shown in Table 4.[48] It is speculated reaction proceeds v/a equation(l) and (2). Acetyl species, formed by CO insertion to methyl species on Rh surface, is supposed to be a possible reaction intermediate. Table 4 Selective synthesis of ethanol over promoted Rh/SiO2 catalyst Catalyst CO2 cony. SelectiviW in carbon efficiency(%) (%) CO MeOH EtOH CH4 5wt% Rh-Sr( 1-1)/SiO2 1.9 52.0 20.5 8.2 18.6 24.7 12.7 37.6 25.0 5wt%Rh-Li(1-1)/SIO2 4.4 5wt%Rh-Fe(l- 1)/SIO2 4.4 23.8 36.7 8.8 30.4 33.4 22.8 34.0 9.8 5wt%Rh-Fe-Li(l- 1-1)/SIO2 14.1 Conditions: 5 Mpa, 260~ GHSV=6000/h, H2/CO2=3/1
Homogeneous catalyst system Ethanol is synthesized using homogeneous catalyst in the batch reactor, too. Tominaga et al. [49] reported that the Ru3(CO)12-Co2(CO)s-KI catalyst system in NMP solvent could produce ethanol as well as methanol at the condition of 12 Mpa, 200~ H2/CO2=5/1 and 15 hrs. Reaction proceeds by homologetion of methanol to ethanol. Best result was obtained by Isaka et al.[50] Ethanol was produced with 36.2% selectivity at CO2 conversion of 42% using the Ru3(CO)12-Co2(CO)s-LiBr in Bu3PO solvent at the condition of 20 Mpa, 200~ H2/CO2=5/1 and 18hrs.
2.4. Higher alcohol synthesis Few study on higher alcohol synthesis such as propanol and butanol was conducted so far. Kieffer et al. studied higher alcohols synthesis using Co modified CuLa2Zr207 catalyst.[51] The yield of C3+-alcohols was very low compared with that from H2/CO. This is partly because of low chain propagation probability in case of H2/CO2 reaction. However, higher alcohols syntheses from H2/CO using modified Cu-ZnOA1203152], K-Mo catalyst[53] and etc. are positively studied so far. Therefore, this reaction is still of interest.
26 3.
HYDROCARBON SYNTHESIS
Catalytic hydrogenation of CO2 to hydrocarbons is classified into two categories. The one is direct hydrogenation from H2/CO2 to hydrocarbons. The other is indirect process which includes methanol synthesis from H2/CO2, followed by in situ methanol conversion to hydrocarbons using solid acid catalyst in H2/CO2 feed. Study on indirect hydrocarbon synthesis is now popular.
3.1. Lower paraffin synthesis Perfect hydrogenation of CO2 to methan is not difficult. Various kinds of supported metal catalysts are available for this reaction. Ni-La203-Ru on ceramic fiber support catalyst is known as an efficient rapid conversion catalyst.[54] Though selective and effective synthesis of C2-C5 paraffin by direct hydrogenation is difficult, indirect process is promising for selective synthesis. The concept of hybrid catalyst system for lower paraffin synthesis was firstly demonstrated by Fujimoto et al. [55] Fujiwara et al. showed the composite catalyst of Cu-ZnO-Cr203 and H-Y-zeolite could produce a high C2-C5 hydrocarbon selectivity such as 95% in total hydrocarbons produced, though CO is a major product as shown in Table 5.[56] Joon et al. reported the selective production of propane and iso-butane using hybrid catalyst composed of Cu-ZnO-ZrO2 and SAPO-44 or SAPO-5, respectively.[57] As direct hydrogenation to lower paraffin, Fe/TiO2 and Fe/ZrO2 were studied.[58,59] Table 5 Lower paraffin synthesis from H2/CO2 using hybrid catalyst Catalyst Press.Temp. GHSV CO2 cony. Conv. to(%) H.C. selectivity(%) Ref. (Mpa) (~ (l/h) (%) H.C. C O C1 C2 C3 C4 C5 C 6 + A/H-Y 5 400 3000 39.9 12.0 27.3 4.2 25.8 40.8 21.7 6.7 0.8 [56] B/SAPO-44 2.8 340 20* 25.8 8.0 17.5 5.9 23.5 43.3 23.9 2.6 0.S [57] B/SAPO-5 2.8 340 20* 25.0 9.5 15.4 3.9 5.9 18.5 54.4 12.9 4.5 [57] A:Cu-ZnO-Cr203 (3-3-1), B:Cu-ZnO-ZrO2(60-30-10), *:W/F(g-cat.h/mol) _
_
3.2. Lower olefin synthesis Selective synthesis of lower olefin by direct hydrogenation CO2 is relatively difficult in a similar manner as selective lower paraffin synthesis. However, Choi et al. reported lower olefin synthesis using Fe-K/Alumina catalyst.[60] According to their result, selectivity of lower olefin from C2 to C4 was about 44% in produced total hydrocarbons and total hydrocarbon selectivity was 95% at CO2 conversion of 68% over Fe-K(1-1)/Alumina catalyst under the condition of 2 Mpa, 400 ~ and GHSV=1900/h. In direct synthesis, Inui et al. [41] showed a selective synthesis of lower olefin using two-stage series reactor packed an effective methanol synthesis catalyst such as Pd modified Cu-ZnO-il203-Cr203 in the first stage and SAPO-34 in the following stage, as shown in Fig.5. More than 90% selectivity for C2 - C4 olefins was established. Different from H-ZSM-5, which is prefer to paraffin hydrocarbon synthesis, SAPO-34 was suitable for lower olefin synthesis because of its weak acidic and narrow pore structure.
27
3.3. Gasoline fraction synthesis U s i n g two-stage series reactor system, gasoline fraction of hydrocarbons w a s also synthesized.[41] As a m e t h a n o l conversion catalyst, MFI-type metallosilicate such as H-Fe-silicate a n d H-Ga-silicate were o p t i m u m for gasolin fraction synthesis. As shown in Fig.5, gasoline fraction w a s produced with 65% selectivity in case of HGa-silicate. H y d r o g e n inverse spillover feature of Ga p a r t in silicate suppress hydrogenation ability to paraffin formation a n d promote oligomerization of lower olefins to gasoline component. CiC2 C3
CO2-rich Syngas 22% CO,. ~ 68%H2
C~
C~
C~
A
H-Fe-silicate ff/~-r-r~_ t ' -1--:::-":.::.::--~;:.-~ I Si/Fe=400 -------~t~[~~'/,I/]",/',/< .[.::":.:..~.. ~ Pd-modified ] / 3 ~ Se146%o, STY 208 g/1.h MSCg I / MeOH cony. i00% + 33.9%CUO I ] n C2C3 C4 C5 C6 A 26.2%ZNO /1 _ IH-Ga-silicate/ ~2q-I I f:::: :::: : : ~:~ 37.8%Ah03 / ~k 300oc 15atm 0.7% Pd J ~M ' eOH CO•V. 100% . o ~ 8o a, Is A Po-34 1- - - - " SV 18,800 h Cony. to MeOH 19.4% 4500C' 1 atm MeOHSTV MeOHconv. 100% paraffin 1,028 g/l.h C2-C4 ~ olefin ,
Cl C2-4
II
Se165%, STY 294Jl.h = = +
C:2 _
_ C32 ~_C,i_C_5
Se194%, STY 425 g/1.h [ , I , l , I , 1 , I 0 20 40 60 80 100 Hydrocarbondistribution (C-wt%)
Figure 5. Lower olefins a n d gasoline synthesis u s i n g two-stage reactor s y s t e m [41] 4.
CARBOXYLIC ACID SYNTHESIS
4.1. Formic acid synthesis It is k n o w n t h a t formic acid is synthesized from H2/CO2 as ester in alcohol solvent using m e t a l complex catalysts such as HM(CO)5- (M:W, Cr,Ru) in batch reactor system.J61] However, specific activity (TOF) of these system are relatively low. Recently, Noyori et al. found a significant increase of formic acid in a supercritical mixture of H2/CO2 with N(C2H5)3 using RuH2{P(CH3)3}4 complex at the condition of 20.5 Mpa, 50~ a n d H2/CO2=1/1.4.[62] T O F increased one order of m a g n i t u d e over t h a t of conventional process because of high miscibility of H2 w i t h supercritical CO2. It is also noted t h a t m e t h y l formate produced from H2/CO2 is easily converted to acetic acid by isomerization reaction.
4.2. Acetic acid synthesis In heterogeneous system, H a t t o r i et a1.[63] observed a direct formation of acetic acid from H2/CO2 over Ag-Rh(0.2-1)/SiO2 catalyst at the condition of 2 Mpa, 200~ GHSV=12000 a n d H2/CO2=1/2. Carbon monoxide w a s a m a i n product w i t h 96% selectivity, however, acetic acid w a s produced w i t h 2.4% selectivity. They speculated t h a t direct insertion of CO2 to surface m e t h y l species on Rh led acetate formation, followed by hydrogenation to acetic acid. Acetic acid formation w a s ascribed to a
28 remarkable suppression of CO2 dissociation and desorption over highly dispersed Rh catalyst at lower reaction temperature. In homogeneous system, Fukuoka et a1.[64] found acetic acid formation from H2/CO2 and CH3I using bimetallic catalysts system such as Ru3(CO)12 + Co2(CO)8 and Ni(cod)2+Co2(eO)s in DMF solvent at the condition of 4 Mpa, 150~ H2/C02=1/1 and 24h. Acetic acid was produced with 50% selectivity and by product was mainly CO. Their proposed reaction mechanism was composed of CO2 insertion to Ru-CH3 species, followed by hydrogenation of its intermediate to acetic acid by HCo(CO)4.
5. OTHERS 5.1. Graphitic carbon synthesis Takita et al. proposed a unique conversion of CO2 to graphitic carbon v/a both direct and indirect hydrogenation processes. They found CO2 was converted to graphitic carbon with 40% selectivity at more than 60% conversion of CO2 over WO3 or Y203 catalyst under the condition of 0.1 Mpa, 700 ~ W/F=10g-cat.h/mol and H2/CO2/N2=2:l:5.[65]. Indirect process is composed of two series reactions, that is, methanation of CO2 and its decomposition to carbon and hydrogen using Ni/SiO2 catalysts. Decomposition proceeded remarkably at 500~ And this reaction was much improved using Pd membrane reactor for overcoming with the thermodynamic limitation of methane decomposition to carbon and hydrogen.[66]
5.2. Methyl amines synthesis Baiker et al. demonstrated an interesting reaction process of direct methylamines synthesis from H2/CO2/NH3.[67] Methylamines are produced now commercially from ammonia and methanol. Mono- and dimethylamine were produced effectively with by-product, CO, over 51wt%Cu/A1203 at the condition of 0.6 Mpa, 277~ GHSV=3000/h and H2/CO2/NH3=3/1/1. A new technology for amine derivatives synthesis might be developed by catalytic hydrogenation of CO2. 6.
SOLAR HYDROGEN PRODUCTION BY PHOTOCATALYST
To realize CO2 hydrogenation process as a countermeasure against global warming, solar hydrogen providing system from water should be established. Extensive studies on photocatalytic and photoelectrochemical production of H2 from water have been conducted in the world. For example, recently Sayama and Arakawa have succeeded to produce H2 from water using Na2CO3 + 5wt%NiO/TiO2 photocatalyst system under the solar light irradiation.[68] This is the first demonstration that water is decomposed to H2 and 02 stoichiometrically and steadily by a cheap powder photocatalyst under solar light. About 400ml/m 2 of H2 and 200 ml/m 2 of 02 were obtained under solar hght irradiation for 6.5 hrs in one summer day in Japan. Further, a new interesting approach using tandem system for efficient visible light water cleavage is proposed by Graetzel et a1.[69] A remarkable progress has been observed in this field, too.
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi(Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
31
N e w a p p r o a c h e s in C O 2 r e d u c t i o n A. Fujishima, D. A. Tryk, and Tata N. Rao Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ABSTRACT
In order to make a significant impact on increasing atmospheric CO 2 levels, electrochemical and photoelectrochemical processes with both high current density and high current efficiency need to be developed. Two general approaches have been followed in our recent work. The first involves the use of gas diffusion electrodes in aqueous electrolyte in order to enhance mass transport and electrode kinetics. Recently developed high area Ni catalysts on activated carbon fibers were found to have encouragingly high current efficiencies for CO 2 reduction to CO, mimicking high pressure conditions. In the second approach, high pressure CO 2 in a methanol electrolyte has been used to enhance mass transport, which results from the high CO 2 concentration. Our work with metallic electrodes has recently been extended and now involves p-type semiconductor electrodes such as p-InP, with which we have achieved both high current densities and high current efficiencies. 1. INTRODUCTION Carbon dioxide fixation has been of global interest from both fundamental and practical viewpoints [1-3]. The conversion of CO 2 to useful chemicals has been studied intensively during the past 10 years, and the electrochemical and photoelectrochemical reduction of CO 2 have been of particular interest. The advantage of electrochemical reduction is that the reaction occurs using water as a proton donor under ambient conditions. In an aqueous solution, carbon dioxide is known to be reduced to HCOOH, CO, C H 4 o r alcohols. It is known that the conversion and the distribution of these reduction products depend strongly on the nature and catalytic activity of the electrode material [4]. For example, we have demonstrated the efficient reduction of CO 2 at hydrogen-storing materials such as unmodified and modified Pd electrodes [5, 6]. The use of the hydrogen-storing material has suppressed the evolution of hydrogen and improved the production of CH 4, HCOOH and CHgOH. In addition to control of the product distribution, the attainment of much higher reaction rates is essential for practical use. Recently, the electrochemical reduction of CO 2 with high current density has been studied by many researchers, e.g., using high pressure aqueous systems [7] and gas diffusion electrodes [8]. We
32 are also studying the high rate reduction of CO 2 in CO 2 - methanol solution [9]. The CO 2 concentrations are high, reaching a mole fraction of ~0.3 at 40 arm. In this system, the total current density of CO 2 reduction reaches values comparable to those used in typical industrial electrolyses. The CO 2 concentration is sufficiently high that the reduction rate is not limited by mass transport. The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid threephase boundary. CO 2 reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which "nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. Another promising way to reduce CO 2 is by photoelectrochemical means, as first reported by Halmann [10]. Although a number of groups have examined photoelectrpchemical CO 2 reduction, high photocurrent densities are difficult to achieve due to mass transport limitations. One way to overcome this limitation is the use of high concentrations of CO 2 in the electrolyte. In the present work we have used p-type InP and GaAs semiconductor electrodes in the high pressure CO2-methanol medium in order to achieve high photocurrent densities. 2. E X P E R I M E N T A L 2.1. Gas d i f f u s i o n electrodes
Metal oxide supported Cu catalysts were prepared by the alkaline precipitation method. The metal oxide powders were added to an aqueous Cu (NO3) 2 solution and were stirred at 80~ and then 0.1 M KOH was used to precipitate the Cu hydroxide. The precipitates were washed and reduced under hydrogen atmosphere at 400~ after drying. For electrode layers, a mixture of carbon black and PTFE was ultrasonically dispersed in water. The gas diffusion layer and reaction layers contained 20 wt% and 10 wt% PTFE, respectively. The prepared electrocatalysts were mixed (50 wt%) with a carbon black and PTFE mixture and then pressed, together with a stainless steel mesh to form a disk-type electrode (13 mm dia.). The exposed area of the electrode was 0.49 cm 2. The electrode was then heat-treated at 350~ under hydrogen. A 0.5 M KOH aqueous solution was used as the electrolyte. The electrodes containing ACF-supported metal catalysts were prepared in similar way, expect that the gas diffusion layer contained carbon black and PTFE in a 3:1 weight ratio, and the active layer contained carbon black, PTFE and ACF in a 9:3:1 weight ratio. Prior to the
33 preparation of the electrodes, the ACF fibers were soaked in the metal nitrate solutions overnight and washed with water. The adsorbed metal ions were reduced under hydrogen atmosphere at 350~ The electrolyte was 0.5 M KHCO 3. 2.2. High pressure COz-methanol systems The electrochemical reduction of CO 2 in the CO2-methanol solution was carried out under high CO 2 pressure. The high pressure apparatus was assembled from a SUS-316 stainless steel tube. A glass inner tube was used to avoid contact of the electrolyte with the metal apparatus. Various metal electrodes were used in this study. The details have been described previously [9]. A Pt counter electrode and a silver quasi-reference electrode were used. Reagent grade methanol was used as the solvent. Tetrabutyl or tetraethyl ammonium salts were used as supporting electrolytes. 2.3. Photoelectrochemistry For the photoelectrochemical experiments, p-type InP and GaAs wafers were cut into 0.4 cm x 0.5 cm electrodes. Ohmic contacts were made with successive vapor deposition of Zn (30 nm) and Au (100 nm), which were annealed afterward at 425~ in Ar. A stainless steel pressure vessel was equipped with a 2 cm thick quartz window for illumination. The electrolyte solution (3 c m 3, 0.3 M tetrabutylammonium perchlorate (TBAP) in C H g O H ) w a s placed in a glass cell liner in the stainless steel vessel. The gas pressure in the cell was set at the desired pressure (1 to 40 arm). A Xe lamp was used as the light source. Photoelectrolysis was carried out at 1 to 40 atm of N 2 and CO 2. A total charge of 2.2 to 10 C was passed galvanostatically at 5 to 100 mA c m -2 using a potentiostatgalvanostat. The products were analyzed using gas chromatography. 3. RESULTS A N D DISCUSSION 3.1. CO z reduction at gas diffusion electrodes Several metal oxides (ZnO, ZrO2, T i O 2, A1203, Nb205) were used as supports for the Cu metal catalyst. Table 1 shows the reduction products obtained with various electrocatalysts. The metal oxide catalysts without Cu exhibited very little activity for CO 2 reduction, and H 2 evolution was the main reaction. Similarly, low contents of Cu (5 wt%) in the reaction layer showed very little activity for CO 2 reduction. However, high contents of Cu (50 wt%) in the reaction layer produced 44% HCOOH and 4.4% CO. High current densities obtained by this method have indicated the advantage of gas diffusion electrodes. The products detected in the gas phase were carbon monoxide, methane, and ethylene, and that in the liquid phase was formic acid. Figures 1 ( a ) a n d 1 (b)show the current distribution and total current density at various electrode potentials for C u / Z r O 2 and Cu/ZnO, respectively. It is interesting to note that hydrocarbons were produced at high selectivity with the C u / Z r O 2 catalyst, i.e., ethylene was obtained with a maximum current efficiency of 20%, reaching 70 mA c m -2 partial current density at -2.2 V vs SCE. The XRD analysis of samples before and after the electrolysis has confirmed that neither degradation nor dissolution of the electrocatalyst
34 occurred. In the absence of Cu or metal oxides, the activity for C O 2 reduction was very low (mainly hydrogen evolution). These results indicate that the metal oxide supports enhance the selectivity and catalytic activity of metal catalysts. On the various supports, differences in the reaction mechanisms may give rise to the observed differences in the product distributions and catalytic activities. Table 1 Electrochemical reduction of CO 2 on various electrocatalysts a Current Efficiency (%) Catalyst Potential
Current Density c TotaF
mA cm -2
101
-199
102
-149
98
-299
80
-238
96
-299
88
-179
105
-517
2.2
95
-199
....
98
-199
V vs. SCE
H2
CO
CH 4
HCOOH
ZrO2
-1.75
101.2
0.09
0.17
....
TiO2
-1.80
99.45
0.97
0.14
1.37
Cu 5 wt%
-1.66
95.68
0.07
0.12
1.95
Cu 50 wt %
-1.60
31.41
4.41
0.07
44.20
0.2
Cu/ZnO
-1.70
50.69
45.60
0.13
Cu / ZrO2
-1.80
40.5
8.81
3.26
32.50
2.9
Cu/TiO2
-1.60
96.89
0.14
0.07
8.02
Cu / AI203
-1.64
60.38
6.68
0.7
25.40
Cu / Nb205
-1.70
72.83
0.63
0.04
24.60
C2H 4
0.07
" Reaction temperature: room temperature, electrolyte: 0.5 M KOH, charge passed: 30 C; bTotal current efficiency; CTotal current density. 100
!
v
|
|
i
|
u
!
u
u
i
i
i
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-----~---G0
80
|
u ~
w
'''--N---H2''' '''l''''l''' ---O--- CO --. CH4 = current
.,
i - - - .-' o - - HCH4 COOH / ~" , C2H4 Itl ~ . l 60 -- ~- -current , ~ " ~ 40 r,j
.A
/
,I,,B, i~
600 500 400
~
~
m
300 ,..,~. 200
2O
100 T r,
-1.4
,
i
.~6'
'
i.'8
,
,
,
-2
,
,
,
2.2
,
,
I , , , , n ,
?.14-1.4
-1.6
T
T
, , , l , , , , u , , ,
-1.8
- 2
T I'. ,I,,,,'l
-2.2
1",3 0
- 2.4
Potential, V vs SCE
Figure 1. Potential dependency of reduction products and current density using (A) C u / Z r O 2 and (B) C u / Z n O electrocatalysts.
35 As an extension of the above work, we have used various metal catalysts supported on activated carbon fibers (ACF). The ACF fibers contain slit shape pores with widths on the order of nanometers. In the present work we have used Ni and Fe metal catalysts supported on ACF. We have also used non-activated carbon fiber supports for comparison. Table 2 shows the product distribution for various catalysts at a potential of-1.8 V vs. SCE. When ACF alone was used, hydrogen evolution was found to be very high. Even when the metal was supported on the non-activated carbon fiber (CF/Ni, CF/Fe), there was little activity showed low activity for CO 2 reduction. However, significant activity for CO 2 reduction was observed for ACF/Ni and ACF/Fe electrodes, the partial current density for CO 2 reduction at-1.8 V vs. SCE ranging from 5 to 15 mA cm-2. The effect of porosity was further demonstrated by two types of Ni catalysts supported on ACF. The ACF/Ni-2 catalyst, with a larger surface area, exhibited a high partial current density for CO 2 reduction. It also exhibited high selectivity for CO production. The highest current efficiency for CO 2 reduction to CO reached values of 70% at-1.6 V vs SCE; negligible amounts of CO are produced on conventional nickel catalysts at ambient pressures.
Table 2 Reduction products for various catalysts at-1.8 V vs. SCE Specific Current Efficiency (%) Catalyst Surface Area a H2 CO CH 4 HCOOH Total ACF only
1500
84.69
2.30
0.22
0.00
87.21
Current Density" / mA cm "2 Total C O 2 tot. b 125
2.88
CF / Fe
69.40
0.11
0.05
0.00
69.56
94
0.15
ACF / Fe
64.10
0.16
0.28
9.05
73.58
78
7.40
CF/Ni ACF / Ni-1
700
80.48 69.88
3.38 2.86
0.08 0.15
0.00 12.17
83.94 85.06
109 31
3.68 4.77
ACF/Ni-2
1300
53.14
30.31
0.14
0.00
83.66
47
14.25
a BET surface area, m 2 g - 1 ; b C O 2 reduction partial current density, mA c m -2.
We have also tested the effect of the matrix material on the activity of CO 2 reduction. Figure 2 shows the catalytic activities of A C F / N i comparing two carbon black matrix materials Vulcan XC-72 (Cabot Corp.) and Denka acetylene black (AB). When XC-72 was used, the partial current density for CO 2 reduction saturated at-2.0 V vs SCE. However, in the case of AB, the partial current density continued to increase with increasing potential. The difference may be due to the fact that AB, being more hydrophobic than XC-72, allows more CO 2 molecules to reach the catalyst pores.
36 04
350 3OO < E 25O
.,
O
09 c-
(p E)
,
!
9 Total'
~.
9 CO2 Red.
200 150
/.../".
,
9 Tota 2 Re 9 CO
,
60 d.
.
.."
c"
(D O m
o !--
/
......-'~ f
100 J 50
7
A
.
t/~
70
9"9
/"]
150
# 9, ~ ~ r 40 30
1
z/
B
10
33
~ o
~..,
~
~__. 3 > O
I
-1.6
1.8
~
,
I
-2.2 -2.4 -1.6 -1.8 Potential, V vs SCE
-2
,
,
-2
-2.2
Figure 2. Potential dependence of current density using acetylene black as the electrode matrix material.
-2.4
0
3
rb
(A) XC-72 and (B)
3.2. Electrochemical r e d u c t i o n of CO s in h i g h pressure C O s - m e t h a n o l s o l u t i o n
First, the electrochemical reduction of CO 2 on the Cu electrode was studied in a CO2-methanol medium at various pressures of CO 2. Under both atmospheric and high pressure, CO, CH 4, C 2 H 4, and H 2 w e r e detected as products in the gas phase. In the liquid phase, methyl formate, HCOOCH 3, and dimethoxymethane, C H 3 O C H 2 O C H 3 , w e r e detected. Cyclic voltammograms at various CO 2 pressures are presented in Figure 3 . The cathodic current was observed with an onset potential of-1.0 V under 1 atm of N 2. A shoulder was observed around -1.2 V during the scan in the negative direction. When N 2 w a s replaced with 1 atm of
// 4,--' C" (D
/
/
O
(a)
I
(b)(c)
-1.5
I 2 mA cm-2 I
-1.0
I
-0.5
i
0
Potential vs. Ag QRE / V Figure 3. Cyclic and linear sweep voltammograms (IR compensated) of methanol and CO2-methanol mixture. Scan rate was 50 mV s -1 Dashed line: under 1 atm of N2; solid line: under (a) 1 atm, (b) 20 atm and (c) 40 atm of CO 2.
37 CO2, a larger shoulder was observed with the same onset potential. The cathodic current under CO 2 in the more negative potential region approached the curve under N 2. This result indicates that CO 2 reduction is favorable only in the narrow potential range around -1.3 V. The magnitude of the shoulder increased with increasing CO 2 pressure, indicating an increase in reduction with increasing gas pressure. Figure 4 shows the efficiencies of product formation at various electrode potentials under 40 atm of CO 2. Under this pressure, a marked increase in the efficiency of CO 2 reduction occurred at around -1.0 V (Fig. 4A), as in the CV. CO and HCOOCH 3 were the main products. Formation of CO increased with increasingly negative potential, while that of HCOOCH 3 reached a maximum at -1.8 V. It is interesting to note that the current efficiency of CO 2 reduction did not decrease with increasing potential in the negative direction, indicating a sufficient supply of CO 2 to the electrode surface even under high current densities at high applied potentials. These results are in contrast to the observations under 1 atm or in aqueous systems, where the current efficiency decreases with increasing potential after reaching a maximum value.
100
'
'
~ ?'
'
'
'
I
'
'
'
'_
8o ~9
60
E
40
80
60
40
20 o
~ , -2.5
T -2
i
I
,
,
I
-1.5
,
,,
,-"
-2.
-
- .
-1
Potential, V vs Ag QRE
Figure 4. Effect of the electrode potential on the current efficiencies of products under 40 atm (25~ A) current efficiencies for (a) CO 2 reduction and (b) H 2 evolution; B) current efficiencies for various CO 2 reduction products; (c) CO, (d) HCOOCH 3 (e) CH 4, and (f) C2H 4. The Tafel plots obtained for the formation of different products under 40 atm are shown in Figure 5. Hydrogen evolution and HCOOCH 3 formation become diffusion-controlled at-1.4 and-1.8 V, respectively. However, the current density for CO production keeps increasing even a t - 2 . 3 V. This result demonstrates that the production of CO is not mass transport-limited in this potential range. A high current density of 500 mA c m -2 w a s recorded at -2.3 V. At this potential, the partial current densities for the production of CO and HCOOCH 3 were 134 and 173 mA cm -2, respectively. The total current density for
38 CO 2 reduction was found to be 436 mA c m -2. It is emphasized that these current density values are comparable to those used in industrial electrolytic processes. 1000
E o <E ~_ s
''
100
10
'
''
I
'
'
''
I
'
''
(b "(c ) ' ~ -
'"
-,
(a)~"--..,[3 ~,
!~"
"
..
-
. ,.,
c(!.)
0
1
,"
~,
0
m
.
e3 m
Q_
0.1 -2.5
-2
-1.5
-1
Potential, V
Figure 5. Tafel plots for (a) H 2 evolution, formation under 40 atm (25~
(b) CO formation,
and (c) H C O O C H
3
Hydrocarbon formation is more interesting in the electrochemical reduction of CO 2, since multielectron transfer is required in this process. In the electrochemical reduction of concentrated CO 2 in the CO2-methanol medium, the major products are still the two-electron transfer products, CO and methyl formate, at the Cu electrode, when tetrabutylammonium salts are used (Table 3). However, when tetraethylammonium salt was used as the supporting electrolyte, efficient formation of methane and ethylene was observed with good reproducibility. We defined the hydrocarbon selectivity (THC) as the ratio of the total current efficiency for hydrocarbon production to the total current efficiency for all CO 2reduction products. The 7HC values for were 29% at a CO 2 mole fraction (X) of 0.007 and 6.9% at X = 0.33 with the TBAT electrolyte. These results show that CO 2 reduction to hydrocarbons is less efficient with a high CO 2 concentration. Such behavior was reported using high pressure aqueous systems [11]. When TEAP was used as the supporting electrolyte, the 7HC increased to 49%. The increase in 7HC can be attributed to the effect of the cation of the supporting electrolyte. Earlier, we reported that the electroreduction of CO 2 in a CO 2methanol medium with a TBA salt yields predominantly CO, while that with a lithum salt yields mainly HCOOCH 3. We concluded that the hydrophobic atmosphere of the TBA ion and the hydrophilic atmosphere of Li ion are conducive to CO and HCOOCH 3 formation, respectively [12]. These results also indicate that the use of TEAP instead of TBAP does not affect HCOOCH 3 and H 2 formation markedly but significantly affects the hydrocarbon formation process (Table 3).
39 Table 3 Current efficiencies (q) for various products and hydrocarbon selectivity (THC) in the reduction of CO 2 in CO2-methanol at a Cu electrode a
q (%) Salt
jc
Xb
Y,c
m A . c m -2
H2
CO
HCOOCH 3 CH 4
C2H 4
%
TBATd
0.007 e
500
63.6
13.0
9.7
8.5
0.8
29.0
TBAT
0.33 f
500
4.0
46.4
34.6
3.4
2.6
6.9
TBAP
0.33 f,g
200
9.7
48.1
25.4
7.6
5.2
14.8
TEAP
0.33 f
200
6.5
24.1
27.7
40.7
9.0
48.9
TEAP
0.33 f
1000
9.3
40.9
22.0
15.8
6.9
25.3
aElectrolyses were performed at 25~ bMole fraction of C O 2 at 25~ from ref. 10; CCurrent density at a Cu cathode; dtetrabutyl a m m o n i u m tetrafluoroborate; el atm.; f40 atm; gelectrolysis was conducted at 20~ and thus the mole fraction was slightly larger than 0.33.
HCOOCH 3
t
CO
02H6
I
I
02H4
I
I
OH 4
t
I
H2
I
!
I
Sn
Pb Ag -
Zn ~
Pd
~
~
/
:
,.,, ,,
I I I I I
_
Cu
/
W Ti Pt Ni
-
0
t I I
i:~NNN ,
,
,
I
20
,
,
,
I
40
Current
,
,
,
I
60
,
,
,
I
80
Efficiency,
,
,
,
l
100
,
I
,
120
%
Figure 6. Electrochemical reduction of C O 2 at various metal electrodes. Electrolyses were performed galvanostatically at 200 mA c m -2 and 41 atm.
40 It is well known that the product distribution also depends on the electrode material used [13, 14]. We have examined the effect of various electrode materials on the product distribution in the CO2-methanol system. The current efficiencies of the reduction products are shown in Figure 6. The production of formate was fairly favorable at all electrodes, in comparison with that in aqueous systems. For example, the production of formate was more than 20% on Pt and Ni electrodes, which is much higher than that observed in aqueous systems [14]. At the metals Sn and Sb, formate production was favoured, as in the aqueous systems, but CO formation was also somewhat favored. This is due to the effect of supporting electrolyte. The electrodes Ag, Zn and Pd showed similar activities for CO production, as in aqueous systems. The efficiency of hydrocarbon formation at the Cu electrode was found to be lower, whereas that at the Ni electrode was found to be higher than that in aqueous systems. The balance of hydrogen and carbon atom concentrations on the electrode surface may explain this difference. 3.3. Photoelectrochemical reduction of CO 2 in high pressure CO2-methanol solution Photoelectrochemical experiments using p-InP and GaAs photocathodes were carried out in CO2-methanol medium under high pressure. The currentpotential curves obtained on these electrodes are shown in Figure 7. Negligibly small dark currents were observed at both electrodes. Under argon atmosphere, InP and GaAs showed photocurrent onset potentials of-1.4 V and -1.6 V vs. Ag QRE, respectively. However, under 1 atm pressure of CO 2, both electrodes showed a shift in the photocurrent onset potential in the positive direction, the shift being higher for the InP electrode. This indicates that CO 2 reduction is more favored on the InP electrode.
Ar ,"~//~CO 2 40atm I
//
c'(1)
o
Ar , / / ~ ~ O
E3
I5 mAcm2
mAcro-2
CO2 latm
COe latm I
-2.0
i
i
l
I
p-lnP I
-1.0
l
i
l
I
I
0
2 40atm
-2.o. Potential vs. Ag-QRE, V
p-GaAs -1.o
o
Figure 7. Current-potential curves for methanol under Ar and CO 2 atmospheres using p-InP and p-GaAs photocathodes.
41 The product distribution is summarized in Table 4. The potentials given in the table are IR-free potentials. Under a nitrogen atmosphere, hydrogen was found to be the only product. This is due to the decomposition of methanol. On both electrodes, the main product was CO. Methyl formate was also formed. Insignificant amounts of hydrocarbons were produced. Comparing the two electrodes, InP has higher activity for CO 2 reduction. Table 4 Photoelectrolysis product distributions Photocathode
Gas Pressure Photocurrent (mA cm -2) (atm)
Potential a
(V)
H2
Current efficiency (%) CO H C O O C H 3 Total
p-InP
N2
1
5.0
-1.5
92
0
0
92
p-Inp
CO2
40
100
-1.4
3
93
11
107
p-GaAs
CO2
1
50
-2.0
76
11
19
106
p-GaAs
CO2
40
50
-1.5
15
78
16
109
p-GaAs
CO2
40
100
-1.8
31
48
21
100
aPotential vs. Ag QRE, corrected according to the ohmic resistance measured by current interruption. 4. CONCLUSIONS The use of gas diffusion electrodes appears to be a promising approach to achieve high CO 2 reduction current densities, even at ambient pressures. Another way of achieving high current densities is to use high pressure solvent systems, e.g., CO2-methanol. In addition to high current densities, it is also necessary to consider the source of the input energy needed to drive the reduction reactions. Solar energy is the primary free energy source, if one can utilize it effectively. There are two ways to use solar energy to drive CO 2 reduction. One way is to use a highly efficient solar cell such as a state-of-the-art photovoltaic cell to generate electricity and then to use the generated electricity to drive the reduction reaction in an electrochemical cell at a metal catalyst-based electrode. Another way is to use semiconductor electrodes such as p-InP to convert solar energy directly into chemical energy in a photoelectrochemical cell. In this respect, the use of InP and GaAs in high pressure systems have provided encouraging results. Another promising electrode material for CO 2 reduction is boron-doped diamond, which can be used in both electrochemical and photoelectrochemical reduction. Highly doped diamond is a good material for electrochemical reduction, as hydrogen evolution is extremely inhibited. Lightly doped diamond is semiconducting and the photogenerated electrons in this material have strong reducing power, as the conduction band is close to the vacuum level. Use of this electrode for the photoelectrochemical reduction of CO 2 is expected to yield interesting results.
42 REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 277 (1979) 637. P.G. Jessop, T. Ikariya and R. Noyori, Nature, 368 (1994) 231. K.R. Thampi, J. Kiwi and M. Gr/itzel, Nature, 327 (1987) 506. M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Soc., 137 (1990) 1772. K. Ohkawa, K. Hashimoto, A. Fujishima, Y. Noguchi and S. Nakayama, J. Electroanal. Chem., 345 (1993) 445. K. Ohkawa, Y. Noguchi, S. Nakayama, K. Hashimoto and A. Fujishima, J. Electroanal. Chem., 369 (1994) 247. A. Kudo, S. Nakagawa, A. Tsuneto and T. Sakata, J. Electrochem. Soc.,140 (1993) 1541. R.L. Cook, R. C. MacDuff and A. F. Sammels, J. Electrochem. Soc., 137 (1990) 607. T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Electroanal. Chem., 404 (1996) 299. M. Halmann, Nature, 275 (1978) 115. K. Hara, A. Tsuneto, A. Kudo and T. Sakata, J. Electrochem. Soc., 141 (1994) 2097. T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Electroanal. Chem., 390 (1995) 77. H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda and K. Ito, Bull. Chem. Soc. Jpn., 63 (1990) 2459. Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochim. Acta, 39 (1994) 1833.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
D e v e l o p m e n t o f Electrocatalysts for C a r b o n Dioxide R e d u c t i o n Polydentate Ligands to P r o b e Structure-Activity Relationships
43
Using
Daniel L. DuBois National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado, 80401, USA"
This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO 2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity.
I. INTRODUCTION The major possible sources of nonfossil energy are photosynthesis, photovoltaics, wind, geothermal, hydroelectric, and nuclear fusion and fission. With the exception of photosynthesis, which produces biomass that can be converted directly to fuels, all the remaining nonfossil sources of energy produce electricity as their primary product. Today fossil fuels are burned to produce electricity, but as we rely increasingly on nonfossil energy sources, electricity will need to be converted to fuels to meet the needs for agriculture and transportation. These sectors of our economy will require fuels with high energy densities. Although biomass can produce some of the fuels that will be needed in the future, increasing food demands will likely limit the land mass available for fuels production from biomass. The efficient electrochemical reduction of CO 2 to a liquid fuel such as methanol, ethanol, or methane would provide a route to renewable fuels with high energy densities that will be important in ensuring a balanced distribution of energy sources in the future. For the electrochemical reduction of CO 2 to a fuel to be feasible, the following requirements will have to be met. 1 First, the overpotential for reduction of CO 2 should be less than 150 mV in order to achieve energy conversion efficiencies of 70% or greater. Second, the catalytic rates must be sufficiently high to permit reasonable current densities---on the order of 0.1 A/cm2. For a monolayer of a homogeneous catalyst immobilized on an electrode surface, this corresponds to a second-order rate constant for the reaction of CO 2with the catalyst of approximately "The financial support of the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division is gratefullyacknowledged. I would also like to thank my coworkers, Calvin J. Curtis, Alex Miedaner, Sheryl A. Wander, AndrewM. Herring, BryanD. Steffyand Paul R. Bernatis for their many contributions to this work.
44 104 M ' I s " 1 . Third, current efficiencies for a single product, such as methanol or methane, must be 90% or greater. Producing a variety of products will lead to lower energy efficiencies and lower energy densities. Fourth, the turnover numbers for the catalysts will have to be high-on the order of 107. This requirement is based on the estimated costs of catalysts and/or the availability of raw materials. Finally, in order to achieve high-energy-density products such as methanol or methane, multi-electron reduction of CO 2 must occur. This can be carried out in a single process or in sequential processes, all operating close to the thermodynamic potential. During the past 20 years, a number of electrocatalysts for CO 2 reduction have been 2 9 9 developed. These may be grouped into four major classes: (1) Group 9 and 10 metal complexes wath macrocychc hgands; (2) Group 7-10 metal complexes with blpyndme 9 4 9 5 9 hgands; (3) Group 10 metal complexes with phosphorus hgands; and (4) various metal electrodes. However, none of the currently known catalytic systems for CO 2 reduction meets all the rigorous requirements described above. Continued development of known and new catalysts will be required before practical systems for direct electrochemical reduction of CO 2 to high-energy-density fuels are possible. 9
9
9
3
.
6
,
.
.
.
.
2. CATALYTIC MECHANISM Our research has focused on developing transition metal phosphine complexes as catalysts for the electrochemical reduction of CO 2. A systematic study of Fe, Co, and Ni complexes containing polyphosphine ligands and weakly coordinated acetonitrile ligands 7 revealed that Ni complexes, although not catalytic, exhibited two, closely spaced, one-electron reductions that occurred at potentials of interest for possible CO 2 reduction catalysts. This led to the evaluation of palladium complexes as well, because they should have similar redox potentials. As a result of screening several palladium complexes by cyclic voltammetry, we found that [Pd(triphosphine)(PR3)](BF4) 2 complexes, 1, would catalyze the reduction of CO 2 to CO in acidic acetonitrile solutions, but not in more weakly coordinating solvents such as dimethylformamide (DMF) or acetone, s 3~p NMR studies clearly indicated that, in acidic acetonitrile, reaction 1 lies well to the right. In DMF and other poorly-coordinating solvents, however, the equilibrium lies to the left. These results indicate that complexes containing ga triphosphine ligand and a weakly coordinating solvent molecule are the true catalytic species. 2+ 2+
+
1
PR3
(1)
2
Preparation of [Pd(triphosphine)(CH3CN)](BF4) 2 complexes resulted in catalysts that were active in more weakly coordinating solvents such as DMF and acetone as well as acetonitrile. For example, CO 2 is reduced to CO with current efficiencies in excess of 95% in acidic DMF solutions using [Pd(etpC)(CH3CN)](BF4) 2 as the catalyst (where etpC is bis(2-dicyclohexylphosphinoethyl)phenylphosphine). The closely related complexes, [Pd(dppe)2](BF4) 2 and
45 [Pd(dppp)(CHaCN)2](BF4)2 (where dppe is bis(diphenyl-phosphino)ethane and dppp is bis(diphenylphosphino)propane), are not active catalysts. These results support complex 2 as the active form of the catalyst and demonstrate the utility of using polydentate ligands with different hapticities to probe the optimum number of strongly and weakly coordinating ligands. Electrochemical studies of [Pd(triphosphine)(CI-13CN)](BF4)2 complexes were carried out under both catalytic and noncatalytic conditions to probe mechanistic aspects of CO 2 reduction. The mechanism that we have proposed for this catalytic reaction is outlined in Scheme 1. In this scheme, L represents a triphosphine ligand and solv represents a solvent molecule such as DMF or acetonitrile. The data supporting the various steps shown will be summarized next. [LPd(solv)]2+ (soN)] +
1
[LPd(CO)(H20)] 2+ /
CO2 ~ 2
(
H+_~ [LPd(s~
+
#3
H
[LPd'c~OH] 2+ ~
..~.
le" N~//[LPd`s~
6
2+
4
solv + [LPd(COOH)]+ ~ Scheme 1 Comparison of the cyclic voltammograms of [Pd(triphosphine)(CHaCN)](BF4) 2 complexes under N 2 and CO 2 atmospheres indicates that the first reduction wave is decreased in height and shifted in a positive direction as shown in the left-hand graph of Figure 1. This behavior has been interpreted as evidence for the reduction of the Pd(II) complex to a Pd(I) complex, followed by rapid reaction with CO2, as shown in steps 1 and 2 of Scheme 1. Support for a one-electron reduction of [Pd(triphosphine)(CH3CN)](BF4) 2 in the presence of CO 2 comes from a comparison of the magnitude of the current for the first reduction wave of this complex to that of [Pd(triphosphine)(PRa)~](BF4)2 complexes under N2 (which have been shown to undergo two-electron reductions).- For both cyclic voltammetry and chronoamperometric experiments, the relative magnitudes of the currents for these complexes are consistent with the acetonitrile complexes undergoing one-electron reductions. Controlled potential electrolysis experiments of [Pd(tfiphosphine)(CHaCN)](BF4) 2 under CO 2 atmospheres are also consistent with one-electron reductions. Finally, recent electrochemical studies of similar palladium complexes containing triarsine ligands are consistent with two
46
0 . 5
~
I
'
I
'
I
'
2.0
i
I-
r E o I,,.. r
32 :;
0.0
(3.
............,,
-0.5
E
-2.0
0 L_ 00 .-
-4.0
-1.0 -1.5r
0.0
'
I
'
'
I
=
I
-6.0 ,
-1.4
,
,
,
-1.2
,
-1.0
,
,
-0.8
Volts vs Ferrocene
-8.0
I
,
I
,
I
,
I
,
l
-1.6 -1.4 -1.2 -1.0 -0.8 V o l t s vs F e r r o c e n e
Figure 1. Cyclic voltammograms of a 1.0 x 10"3 M solution of [Pd(IPNetpE)(CH3CN)](BF4)2 in DMF under N2 (left-hand graph, open circles) and in the presence of 0.18 CO2 (felt-hand graph, solid line). The fight-hand graph shows cyclic voltammograms of the same solution after adding acid (0.04 M HBF4) under N2 (open circles) and CO2 (solid line). sequential one-electron reductions, in which the electron transfer for the Pd(I/0) couple is significantly slower than that of the Pd(II/I) couple. ~~ All these results support a one-electron reduction of [Pd(triphosphine)(CH3CN)](BF4)2, followed by reaction with CO 2. The Pd(I) species reacting with CO 2 is probably four-coordinate. Reduction of [Pd(triphosphine)(solvent)](BF4) 2 complexes in the presence of CO 2 results in a shift of the peak potential of the Pd(II/I) couple to more positive potentials, as shown in the left-hand graph of Figure 1. This is consistent with the solvent molecule remaining bound to palladium or exchanging reversibly with bulk solvent. If an irreversible loss of solvent occurred following reduction but prior to CO2 binding, CO 2 should have no effect on the peak potential. The rate of reaction of [Pd(triphosphine)(solvent)]+ with CO 2 can be measured under noncatalytic conditions (i.e., in the absence of acid) by using the shift in peak potential. The rate can also be measured under catalytic conditions by measuring the magnitude of the catalytic current. The fight-hand graph in Figure 1 shows the current observed for [Pd(IPNetpE)(CH~CN)](BF4) 2 (where IPNetpE is bis(diethylphosphinoethyl)(diisopropylamino)phosphine) in the presence of 0.04 M HBF a under N2 and CO 2. It is clear that a catalytic wave is observed when both CO 2 and acid are present. Equation 2 shows the relationship between the peak or plateau current for a catalytic wave and the concentrations of substrate and catalyst for a simple catalytic reaction following a reversible electron-transfer. ~ ic = n F A
[cat](Dk [S] y) 1/2
(2)
In eq. 2, ic is the catalytic current, n is the number of electrons transferred, F is the Faraday constant, A is the area of the electrode, [cat] is the concentration of the catalyst, D is the diffusion coefficient of the catalyst, k is the apparent rate constant, [S] is the concentration of the substrate under investigation, and y is the order of the rate-determining reaction in substrate S. From eq. 2 it can be seen that the square root dependence of the catalytic current on CO 2 concentration (Figure 2 curve A) and its linear dependence on catalyst concentration (Figure 2 curve B) are consistent with a rate-determining step that is first order in catalyst and first order in CO 2. Within the margins of experimental error, the rate determined under
47
25.0
|
I
|
!
|
A
ca. 20.0 E o 32 15.0
E : o 32 t._
,X,
10.0 0.0
I
0.1
-
I
I
0.2
I
0.3
=
I
0.4
[002] 112 i 1/2
I
0.5
!
100.0
.
50.0
i
0.0 0.0
!
|
!
I
!
|
!
|
B
0.5 [cat]
1.0 ma
Figure 2. The let~-hand graph is a plot of the catalytic current vs. the square root of the CO2 concentration for a 2.0 x 10-4 M solution of [Pd(etpC)(CH3CN)]BF02 in acidic DMF (0.04 M HBF4). The fight-hand graph is a plot of the catalytic current vs catalyst concentration for an acidic DMF solution (0.04 M HBF0 saturated with CO2 at 620 mm Hg (0.18 M). catalytic conditions by measuring the peak current of the catal~ic wave is the same as that determined in experiments based on the shiR of peak potentials. These results indicate that the reaction of the Pd(I) intermediate with CO 2 is the rate-determining catalytic step (step 2 of Scheme 1) at high acid concentrations. In subsequent studies, we have investigated the effect of varying the substituents on the triphosphine ligand on the rate of reaction of [Pd(triphosphine)(solvent)]+ with CO2, step 2 of Scheme 1. ~2 A linear free-energy relationship exists between the redox potential of the catalyst and the log of the rate constant for the reaction of the catalyst with CO 2 (Figure 3, open circles). This is reasonable if this reaction is viewed as a nucleophilic attack of the metal on CO 2 with a transfer of charge from the metal to CO 2. A second observation is that bulky substituents on the central phosphorus atom retard the catalytic rates by approximately a factor of 2 (open squares), whereas bulky substituents on the terminal phosphorus atoms have little effect. This result can be understood from the structure of the [Pd(MesetpE)(CH3CN)] 2+ cation (where MesetpE is bis(2-diethylphosphinoethyl)mesitylphosphine) shown in the inset of Figure 3. In this structure, a methyl group of the mesityl substituent on the central phosphorus atom effectively blocks one face of the catalyst. Consequently, only one side of the catalyst is available for CO 2 binding, and the rate decreases by a factor of two. The presence of the methyl group above the palladium atom also suggests that six-coordinate intermediates can be excluded, because such complexes are not sterically feasible. This result is significant, because coordination of a sixth ligand has been suggested to play an important role in CO z binding for some Ni(I) catalysts containing macrocyclic amine ligands. ~3 The observation that bulky substituents on the central phosphorus atom of the triphosphine ligand have a greater effect on the catalytic rate than bulky substituents on the terminal phosphorus atoms indicates that CO 2 approaches the catalyst along the axis perpendicular to the plane of the catalyst. The next two steps in the catalytic mechanism, protonation (3) and a second electron transfer (4), can be inferred by comparing cyclic voltammograms of catalyst solutions saturated with CO 2 in the presence and absence of acid. In the presence of CO 2 but no acid, the peak current of the first reduction wave is consistent with a simple one-electron reduction (Figure 1, left-hand graph). When acid is added to this solution, the current increases
48 3.0 2.5 2.0
A
O
I
I
HOP, Et - ~" Me,E
'
4
A
1.5
I
J
I
I
M e s P+ , Et
Me,Cy
A
Me2N, Et
I
Bu 3P ,
N, N, Et Et Mes, Et [-7-"
,Cy h, Et
t-Bu, Et F]
Ph, Ph
Mes, Cy [3
1.0
Mes, Ph
0.5 -1.6
-1.4
-1.2
-I.0
Ell2(volts vs Ferrocene)
Figure 3. Plot of the log of the rate constant for the reaction of the Pd(I) intermediate with CO2 as a function of E~/2 for various [Pd(triphosphine)(CHaCN)](BFa)2 catalysts in DMF. Each ordered pair of substituents lists the substituent on the central phosphorous atom of the tridentate liquid first, and the terminal substituents second. BuaP + represents a tri(nbutyl)ethylphosphonium substituent, HOP is hydroxypropyl, and other substituents are defined in text or have their usual meaning. (Figure 1, right-hand graph). This implies that protonation of the CO 2 complex formed in step 2 must occur before the second electron transfer can take place at the potential of the first wave. Loss of the solvent ligand must also occur during the catalytic cycle. As discussed above, if the coordinated solvent such as acetonitrile or DMF is replaced with strongly coordinating ligands such as a monodentate phosphine ligand, catalytic activity is suppressed. In addition, dimethylsulfoxide (a strongly coordinating solvent) also suppresses catalytic activity, although reaction of the catalyst with CO 2 does not appear to be significantly affected as judged by peak shift measurements of the catalyst in the presence of CO 2 but in the absence of acid. These observations suggest that solvent loss occurs during the catalytic cycle and that it is important for cleavage of a C-O bond as shown in step 7. A biphasic dependence of the catalytic current on acid concentration is observed for these catalysts, as shown in Figure 4 for [Pd(IPNetpE)(CH3CN)](BF4) 2. For this complex, the current exhibits a first-order dependence on acid at low acid concentrations, but no acid dependence is seen at high acid concentrations. The linear dependence of the catalytic current on acid concentration is consistent with two protons being involved in the transition state (steps 3 and 6 of Scheme 1). Although the second protonation step could be rate-determining, we have interpreted this result to imply that, at low acid concentrations, the rate-determining step is the cleavage of the C-O bond to form coordinated CO and water, as shown in step 7 of Scheme 1. This bond cleavage reaction is sensitive to the number of carbon atoms in the 9
9
49 30
i0.0 tn
~' 20
1"13 O
u
-,.-.I
lO iJO0
9
9
9
9
9
.3 O ~4 tO "rq
0.i
0.15
[HBF 4 ] M
'
8.0
'
I
'
'
I
i
i
'
I
'
J
I
I
O
6.0 4.0
2.0~ V 0.05
'
l
0.000
I
i
0.020
0.040
[HBF 4 ]
Figure 4. Left-hand graph shows the dependence of the peak current on acid concentration for a 2.2 x 10.3 M solution of [Pd(IPNetpE)(CH3CN)](BF4)2 in DMF purged with N2 (solid circles) and CO2 (open circles). The fight-hand graph shows the peak current versus acid concentration for 2.3 x 10.3 M solutions of [Pd(ttpE)(CH3CN)](BF4)2 in DMF saturated with N2 (solid circles) and CO2 (open circles). backbone of the triphosphine ligand. As just discussed for [Pd(IPNetpE)(CH3CN)](BF4)2, the catalytic currents for all [Pd(triphosphine)(CH3CN)](BF4) 2 complexes with two-carbon linkages between phosphorus atoms show a linear dependence on acid concentration, consistent with the loss of water from the activated complex. For complexes with a threecarbon linkage in the triphosphine ligand, such as [Pd(ttpE)(CH3CN)](BF4)2, the catalytic current shows a square root dependence on the acid concentration consistent with a ratedetermining step that is first order in acid (Figure 4, fight-hand graph). This implies that a hydroxide ion is lost from the activated complex, and that the C-O bond is more easily cleaved than it is in the two-carbon chain analog. A structural study of [Pd(ttpE)(CH3CN)](BF4) 2 shows that a significant steric interaction exists between two of the terminal ethyl groups of the tridentate ligand and the acetonitrile ligand. ~4 We believe that a similar steric interaction between the substituents on the terminal phosphorus atoms and the coordinated hydroxycarbonyl ligand promotes the cleavage of the C-O bond in the catalytic cycle of [Pd(ttpE)(CH 3CN)]03F,)=. As discussed in the preceding paragraph, the [Pd(triphosphine)(CHaCN)](BF4) 2 catalysts exhibit a biphasic dependence on acid concentration, which is consistent with steps 2 and 7 of Scheme 1 being the rate-determining steps at high and low acid concentrations, respectively. If a steady state approximation is applied to Scheme 1 with steps 2 and 7 as the rate-determining reactions, and the resulting rate constant is incorporated into eq. 2, eq. 3 results. In this eq., id is the peak current for a reversible reduction of the catalyst under noncatalytic conditions, tr is a constant that depends on whether the second electron transfer occurs in solution (1.414) or at the electrode surface (2), ~5 n is the number of electrons involved in catalyst reduction, v is the scan rate, k~ is the second-order rate constant for the reaction of CO 2 with the Pd(I) intermediate (step 2), and k2 is a third-order rate constant, which is undoubtedly not a simple rate constant.
i d - 0.4471 n F V ~ k l [ C 0 2 ] +
k2[H+] 2
(3)
50 The solid line shown in the lett-hand graph of Figure 4 is a best-fit line to (3), and can be seen to account well for the acid dependence observed. For the complexes with a three-carbon linkage between the central phosphorus atom and the terminal phosphorus atoms of the triphosphine ligand, a similar fit can be obtained by assuming a first-order dependence on acid concentration in eq. 3 (see fight-hand graph of Figure 4). The dashed line in the fight-hand graph is the best-fit line assuming a second-order acid dependence for the second ratedetermining step. The catalytic cycle, Scheme 1, is completed by a rapid loss of CO from the Pd(II) species and solvent is coordinated (step 8). Consistent with CO loss is the observation that no CO adducts were detected by 3~p NMR spectroscopy when [Pd(triphosphine)(CH3CN)](BF4) 2 complexes were dissolved in non-coordinating solvents such as dichloromethane in the presence of CO gas (60 psi). Based on the data described above, we believe that Scheme 1 represents a good working model for the mechanism of CO 2 reduction by [Pd(triphosphine)(CH3CN)](BF4) 2 complexes. 3. SIDE REACTIONS Side reactions are of two types: one leads to products other than CO and the other to catalyst degradation. Our studies have shown that the nature of the tridentate ligand can play an important role in both types of side reactions. The major reduction product, other than CO, for all the catalysts we have studied is hydrogen. We have found that the production of hydrogen does not appreciably affect the kinetics of the complexes studied. For example, electrochemical reduction of [Pd(PCP)(CH3CN)](BF4) , 3, in the presence of CO 2 and acid produces only hydrogen. ~6 However, the catalytic current at high acid concentrations is consistent with a first-order reaction of a Pd(I) species with CO 2 and is independent of acid concentration similar to the [Pd(triphosphine)(solvent)](BF4) 2 complexes discussed above. For complex 3, CO 2 acts as a cofactor for hydrogen production, but CO 2 is not reduced. This
..'H~ H 0 P /
'-- IFPh2 P d - - NCCH3
~<,.~__________/"~Pd
/'.. p/
%
o
4
suggests that the hydrogen that forms during C O 2 reduction by the [Pd(triphosphine)(solvent)](BF4) 2 catalysts involves the same CO 2 intermediate that produces CO. The branching that leads to the different pathways resulting in hydrogen or CO production occurs after formation of the CO 2 complex. The preference for forming hydrogen or CO appears to depend on the basicity or redox potential of the complexes. A more negative redox potential favors protonation at Pd to form a hydride (as shown for structure 4) with subsequent FL2 production. However, less negative redox potentials or a less basic Pd atom favor protonation at an oxygen atom of coordinated CO 2 (step 6 of Scheme 1), which results in CO formation.
51 The size of the chelate bite of the triphosphine ligand significantly affects the catalyst decomposition products as well as the catalytic mechanism. The main decomposition products of complexes with two ethylene bridges between the central and terminal phosphorus atoms of the tridentate ligand are Pd(I) dimers with bridging triphosphine ligands, e.g., 5. Two of these dnners have been characterized by X-ray dsffractmn studies. When the triphosphine ligand contains two trimethylene linkages, the catalysts decompose by forming [Pd(triphosphine)H](BF4) complexes, 6.14 One of the hydride complexes has been characterized by an X-ray diffraction study as well. Complexes 5 and 6 are also formed when the reductions of the corresponding Pd(II) complexes are carried out under CO 2 in the absence 9
9
Ph\f--x
/ ~ P /PR2 R2P--PdPd--PR2 / /J R2P, P\Ph
9
,
,
9
2+
PhP--~d-- H
PR2
5
of acid. The formation of both 5 and 6 is faster in the presence of CO 2 than under nitrogen atmospheres. This result suggests that the Pd(I)CO 2 intermediates discussed above play an important role in both catalytic reactions and in catalyst decomposition. At first sight, it would appear that hydride 6 should not produce catalyst deactivation, because this complex could protonate to form a dihydride or dihydrogen complex, eliminate hydrogen, and regenerate the catalyst. However, hydride 6 is relatively stable to acid, but its two-carbon chain analog is not. The greater stability observed for 6 is attributed to the much lower hydridic character for the hydride ligand in 6, which results from the larger chelate bite. 4. SUMMARY A series of palladium complexes containing polyphosphine ligands and weakly coordinated acetonitrile molecules were synthesized and screened for their ability to catalyze the electrochemical reduction of CO 2. As a result of this screening process, we found that [Pd(triphosphine)(CH3CN)](BF4) 2 complexes catalyze the electrochemical reduction of CO 2 to CO. Kinetic studies were carried out to determine the mechanism of this reaction. The first step is the reduction of the Pd(II) cation to Pd(I), followed by a rate-limiting reaction with CO 2. In the presence of acid, this intermediate enters into a reaction channel that will lead to the production of either CO or hydrogen. If no acid is present, the Pd(I)CO 2 complex decomposes to form either a Pd(I) dimer such as 5 for catalysts with two ethylene bridges between the three phosphorus atoms, or a palladium hydride such as 6 for catalysts with two trimethylene bridges. In the presence of acid, the Pd(I)CO 2 complex is protonated and undergoes a second reduction. Again, the pathway taken depends on the bite size of the triphosphine ligand. For a triphosphine with trimethylene bridges, cleavage of the C-O bond occurs to produce CO and hydroxide. In this case, hydroxide loss is assisted by an unfavorable steric interaction between the triphosphine ligand and the coordinated hydroxycarbonyl. For a triphosphine with ethylene linkages, a second protonation occurs at an
52 oxygen atom of the hydroxycarbonyl ligand to produce an activated complex that undergoes a C-O bond cleavage reaction to produce CO and water. If the Pd(II/I) reduction potential is sufficiently negative, the second protonation reaction can occur at the metal to form a metal hydride that leads to hydrogen production. REFERENCES
1. D. L. DuBois, in the Proceedings: The 1995 Symposium on Greenhouse Gas Emissions and Mitigation Research, U.S. Environmental Protection Agency, June 27-29, 1995 Washington D. C., p. 3-76. 2. For reviews of electrochemical reduction of CO 2 see: (a) B. P. Sullivan, K. Krist, H. E. Guard, Electrochemical and Electrocatalytic Reactions of Carbon Dioxide; (eds.), Elsevier, New York, 1993. (b) J.P. Collin and J. P. Sauvage, Coord. Chem. Rev., 93 (1989) 245. 3. (a) S. Meshitsuka, M. Ichikawa and K. Tamaru,. J. Chem. Soc., Chem. Cornm. (1974) 158. (b) B. Fisher and R. Eisenberg, J. Am. Chem. Soc., 102 (1980) 7363. (c) M. Beley, J.-P. Collin, R. Ruppert and J.-P. Sauvage, J. Am. Chem. Soc., 108 (1986) 7461. (d) C.A. Kelly, Q. G. Mulazzani, M. Venturi, E.L. Blinn and M.A.J. Rodgers, J. Am. Chem. Soc., 117 (1995) 4911. 4. (a) J.-M. Lehn, and R. Ziessel, Proc. Nat. Acad. Sci. USA, 79 (1982) 701. (b) F.P.A. Johnson, M.W. George, F. Hartl and J. Turner, J. OrganometaUics, 15 (1996) 3374. (c) M.N. Collomb-Dunand-Sauthier, A. Deronzier and R. Ziessel, Inorg. Chem. (1994) 2961. (d) K. Toyohara, K. Tsuge and K. Tanaka, Organometallics, 14 (1995) 5099. (e) J.A.R. Sende, C.R. Arana, L. Hernhndez, K.T. Potts, K.M. Deshevarz and H.D. Abrufia, Inorg. Chem., 34 (1995) 3339. 5. (a) S. Slater, J.H. Wagenknecht, J. Am. Chem. Soc., 106 (1984) 5367. (b) A. Szymaszek and F.P. Pruchnik, J. Organomet. Chem., 376 (1989) 133. (c) R.J. Haines, R.E. Wittrig and C.P. Kubiak, Inorg. Chem., 33 (1994) 4723. 6. (a) Y. Hori, A. Murata, T. Tsukamoto, H. Wakebe, O. Koga and H. Yamazaki, Electrochimica Acta, 39 (1994) 2495. (b) Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett. (1985) 1695 7. D.L. DuBois and A. Miedaner, Inorg. Chem., 25 (1986) 4642. 8. D.L. DuBois and A. Miedaner, J. Am. Chem. Soc., 109 (1987) 113. 9. D.L. DuBois, A. Miedaner and R.C. Haltiwanger, J. Am. Chem. Soc., 113 (1991) 8753. 10. A.J. Downard, A. M. Bond, A. J. Clayton, L.R. Hanton and D. A. McMorrau, Inorg. Chem., 35 (1996) 7684.
53 11. J.-M. Sav6ant and E. Vianello, Electrochem. Acta, 10 (1965) 905. 12. P.R. Bernatis, A. Miedaner, R.C. Haltiwanger and D.L. DuBois, Organometallics, 13 (1994) 4835. 13. (a) S. Sakaki, J. Am. Chem. Soc., 114 (1992) 2055. (b) G.B. Balazs and F.C. Anson, J. Electroanal. Chem., 322 (1992) 325. 14. S.A. Wander, A. Miedaner, B.C. Noll and D.L DuBois, Organometallics 15 (1996) 3360. 15. J.-M. Sav6ant and K.B. Su, J. Electroanal. Chem., 196 (1985) 1. 17. B.D. Steffey, A. Miedaner, M.L. Maciejewski-Farmer, P.R. Bematis, A.M. Herring, V. Allured, V. Carperos and D.L. DuBois, Organometallics, 13 (1994) 4844.
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
55
Carbon dioxide and microalgae Norihide Kurano a, Takayuki Sasaki a and Shigetoh Miyachi b aMarine Biotechnology Institute, 3-75-1 Heita, Kamaishi, Iwate 026, Japan, bMarine Biotechnology Institute, 1-28-10 Hongo, Bunkyo-ku, Tokyo 113, Japan Abstract- Photosynthesis in ambient air is much more efficient in aquatic microalgae than in terrestrial C 3- and C4-plants. Microalgae are, therefore, important for the prevention of increase in atmospheric CO 2 concentration. To perform efficient photosynthesis under very low level of CO 2 in the air (0.036%), microalgae developed so-called inorganic carbon concentrating mechanism which is operated by carbonic anhydrase and active transport system for inorganic carbon. On the other hand, it has been generally assumed that plants and algae are intolerant to high CO 2 concentrations. Recently a new species of marine green alga, Chlorococcum littorale, was found to grow rapidly at extremely high concentrations of CO 2 (10-20%) which are equivalent to those in industrially discharged waste gases. Several physiological, morphological and genetic features underlying the resistance to extremely high CO 2 concentrations were elucidated. A novel photobioreactor was developed to estimate the potential of growth and CO 2 fixation capacity of C. littorale. T h e maximum value of CO 2 uptake rate, 0.72 g CO 2 1-1 h -1 w a s achieved. Based on the expected capacity of CO 2 fixation by microalgae and solar energy, required land areas forthe treatment of CO 2 emitted as a result of various human activities were estimated. 1. INTRODUCTION Photosynthesis of terrestrial and aquatic plants, from tall trees to microscopic phytoplanktons, has a significant influence on the global carbon cycle. The better utilization of this biological process is clearly one of the solutions to cope with the rise in atmospheric CO 2 concentration and the possible global warming. Aquatic microalgae are characterized by their efficient photosynthesis and their fast proliferation compared with terrestrial C 3- and C4-plants. Hence, microalgae can be considered as a candidate for biological catalyst in C O 2 fixation. Since dissolved inorganic carbon, such as free CO 2 and bicarbonate, acts as a sole carbon source in photoautotrophic growth of microalgae, the partial pressure of CO 2 in the atmosphere or in the supplied gas mixture is an important factor for their growth. Studies on the mechanisms underlying the efficient photosynthesis of microalgae in air level CO 2 concentration (L-CO2, 0.036%) will be described.
This work is partly supported by New Energy and Industrial Technology Development Organization (NEDO).
56 Industrially discharged gases usually contain 10-20% CO 2. Generally plants and algae are assumed to be susceptible to such extremely high CO 2 concentral~ons (H-CO2). Recently it was found that a new species of marine green alga, Chlorococcum littorale can grow rapidly under H-CO2, indicating the possibility that this alga can be utilized for bioremediation of CO 2 in flu e gas. When these algal cells which had been grown in air condition were transferred to H - C O 2 conditions, transient changes were observed in photosystem I and II activities, gene expressions, and number and size of vacuoles, but intracellular pH value remained constant. In contrast, no such change was observed in Stichococcus bacillaris cells which are intolerant to H - C O 2. By using a newly developed thin-plate type photobioreactor, the capacity for CO 2 uptake rate in C. littorale was evaluated. Taking the energy of average solar radiation and the photosynthetic efficiency of microalgae into account, the required land area for reducing the CO 2 emission from various intensive sources by microalgal cultivation was roughly calculated. 2. LOW CO~ AND MICROALGAE While ancient composition of air had been dominated by CO2, oxygenic photosynthesis performed by primitive blue-green algae (cyanobacteria) gradually reduced atmospheric level of CO 2 during geological length of time. The partial pressure of CO 2 in the present air, 0.036%, is a limiting factor of photosynthesis, as is obvious from the fact that the photoautotrophic growth of microalgal strains can be enhanced with the supplementation of 2-3% CO 2 to air. Some kinds of higher plants developed a clever system, so-called C 4 photosynthesis, to attain high productivity under L-CO2 condition. However, photosynthesis is much more efficient in microalgae than in C4 plants [1]. The high efficiency of photosynthesis in microalgae is due to the existence of CO 2 concentrating mechanism (ccm) which is carried out by the active transport system of inorganic carbon and by an enzyme, carbonic anhydrase (CA, carbonate hydrolyase, EC 4.2.1.1). At neutral pH, the most abundant molecular species of dissolved inorganic carbon is bicarbonate, while free CO 2 is a minor component. On the other hand, free CO 2 can easily cross the cell membrane with the driving force of density gradient, but bicarbonate ion can not. Some microalgae have CA outside the cell membrane (in the periplasmic space). Such extracellular CA acts to enhance inorganic carbon uptake of microalgal cells by catalyzing the conversion of bicarbonate to free CO 2 on the cell surface [2]. A green alga, Chlamydomonas reinhardtii, has two kinds of periplasmic CA [3]. The molecular characteristics and gene sequences of these enzymes have been extensively studied (Table 1). Some microalgae have also intracellular, or chloroplastic CA. The role of chloroplastic CA is supposed to be supplying free CO 2 to ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) under L - C O 2 conditions. A red alga, Porphyridium purpureum, contains only chloroplastic CA [4]. This enzyme was purified to nearly a single band in SDS-PAGE, and its nucleotide and deduced amino acid sequences were elucidated [5]. Primary structure of this enzyme is characterized by two almost identical domains (Fig. 1), each of domains has a homology with CAs found in eukaryote. It is supposed that the Porphyridium CA has evolved through duplication
57 of an ancestral and hypothetical CA gene followed by the fusion of the duplicated gene. Recent observation by Mitsuhashi et al. [6] indicated that CA of P. purpureum is exclusively localized in the pyrenoid. Table 1 Characteristics of two periplasmic CAs in Chlamydomonas reinhardtii Enzyme
CA1
CA2
Specific activity (Unit / rag)
2,000
3,300
Holoenzyme
76-79.8
84.5-87.9
Large subunit
35-36.5
38
Small subunit
4.14
4.24
Subunit structure
LaS2
LRS2
Disulfide bonds
+
+
Glyco-residues
+
+
Acetazolamide
1 X 109
3 X 10.9
Ethoxyzoleamide
2 X 10-9
2 X 10.9
Localization
periplasmic
periplasmic
Corresponding gene
CAH1
CAH2
mRNA induction
L-CO2, fight
H-CO 2 (5% CO2)
Molecular mass (kD)
Affinity to inhibitors (Is0, M)
ATO
TAA N-terminal half
C-terminal half
Signal peptide
I
I
100 bp Figure 1 Schematic primary structure of Porphyridium CA.
58 Compared to green micro algae, CA activity is relatively low in blue green microalgae. Active transport system of inorganic carbon (mostly bicarbonate) is well developed in these blue green algae. ATP produced by cyclic phosphorylation is used as an energy source for the transport system [7]. 3. HIGH CO 2 AND MICROALGAE There are two possible targets for biological C O 2 fixation: atmospheric CO 2 characterized by very low partial pressure, and CO 2 in discharged gases from heavy industries characterized by high concentrations (10-20%). As described above, microalgae are working for the bioremediation of atmospheric level of CO 2 . However, it has been regarded that they are not suitable for treating high partial pressure of CO 2 in waste gases, because studies on Chlorella sp. revealed that CO 2 above 5% was not advantageous for growth [8]. Although this growth inhibition was described as 'narcotic', the mechanism of inhibition remains unclear. A new species of a marine green alga, Chlorococcum littorale, was isolated from the coastal region of Kamaishi, Iwate, Japan [9]. This alga survived the selection pressure of vigorous aeration with air supplemented with 20% CO2, and is characterized by its outstanding ability of growth and C O 2 fixation under 207o or higher concentrations of CO 2 [10]. The growth capability of C. littorale and two other reference strains, Chlorella vulgaris Beijerinck IAM C-133 and Chlorella regularis IAM C-533, both are well known due to their excellent growth rate, was compared under various concentrations of CO 2 [10]. In the linear growth phase, the growth rate of C. littorale, C. vulgaris and C. regularis at 40 % CO 2 was 0.20, 0.00, 0.06 mg dry weight 1-1 d -1, respectively. Although the optimum CO 2 concentration for the growth of C. littorale is 5-10%, this alga can grow rapidly at very high CO 2 concentration such as 40%. With step-wise increase of CO2, the cell grew up to 60%. Thus, the finding of C. littorale arose a hope for direct biofixation of CO 2 in industrial flue gases. Many studies have been done on L-CO2 adaptation. However, there has been practically no investigation o n H - C O 2 adaptation. C. littorale serves as a very good model for studying the acclimation to extremely high CO 2 concentrations. As a first step to approach this problem, changes in photosynthetic characteristics during the course of adaptation of air-grown cells to 40% CO 2 was investigated [11]. When air grown cells were inoculated into fresh culture medium and aerated with ordinary air supplemented with 40% CO2, they did not grow for the first 3-4 days. Then, logarithmic growth started. During 3-4 days of adaptation period, specific growth rate was nearly zero and inhibition of photosynthesis measured by oxygen evolution and 14CO 2 uptake was observed. Temporal decrease and recovery of photosystem II (PS II) activity, and opposite change of PS I activity were also remarkable. These results were obtained in batch cultures,. Moreover, the same phenomena were found in continuous cultures and confirmed with fluorescent measurement [12]. When air-grown cells of a green alga, Stichococcus bacillaris, which has a little tolerance to H-CO2, was aerated with air enriched with 40% C O 2 in continuous culture, the specific growth rate of S. bacillaris gradually reduced and the cells were washed out in 8-10 days. No recovery in photosynthesis and PS II activity was observed [12].
59 In C. littorale, the inhibition of photosynthetic oxygen evolution and carbon uptake, and the growth of air-grown cells subjected to H - C O 2 conditions can be explained by the activity change in PS II. The increase in PS I activity found in the adaptation period suggests that ATP produced by cyclic electron flow around PS I should be used to cope with H - C O 2 stress and for the recovery of PS II activity. The supply of extremely high concentrations of CO 2 into the cultures of microalgae can cause acidification in both extra- and intracellular environment due to the formation of acidic substances in the course of CO 2 hydration. Many enzymatic reactions in photosynthesis and metabolism may be inhibited under low intracellular pH. Therefore, the ability to maintain intracellular pH stable seems to be one of the important factors for the cells to grow rapidly at the exposure to H-CO 2. The 31p-NMR spectroscopy was applied for measuring pH values in different cellular compartments, which were confirmed with the DMO method. The drop in cytoplasmic pH was observed when S. bacillaris cells were exposed to 40% CO 2, while pH in C. littorale was stable up to 60% CO 2. This indicates that the growth inhibition by high CO 2 was associated with cytoplasmic acidification [13]. By staining cells with neutral red or chloroquine, an increase in number and size of vacuoles in C. littorale was found under H - C O 2 , while no such change was observed in S. bacillaris cells (Table 2). Table 2 Vacuoles in L- and H - C O 2 grown cells of C. littorale and S. bacillaris
Chlorococcum littorale
Air
Average number of vacuole per
40%
CO 2
S tichococcus bacillaris
Air
20% CO 2
5.2
9.3**
2.0
2.1
1.0
4.0**
1.0
0.96
1.0
2.7**
1.0
0.99
cell Average cross-sectional area of vacuoles per cell relative to LCO 2 cells Average cross-sedional area of cells relative to L-CO2 cells ,i
**: Statistically different from the value of L - C O 2 cells at the 1% significant level. Number of counted cells was 20- 30.
60 Two cDNA clones were isolated from H - C O 2 adapting cells of C. littorale, by using the differential screening method (Sasaki et al., in preparation). Out of 40,000 plaques of cDNA library, 78 clones were found to be exclusively expressed in 2 day culture at 20% CO 2. These clones were finally classified into two groups, HCR ( H - C O 2 response) 1 and 2, cDNA sizes of which were 1373 and 3095 bp, respectively (Table 3). The deduced amino acid sequence of HCR2 had partial homology with yeast ferric reductase. Ferric reductase activity of C. littorale was also induced in H - C O 2 , and the period of HCR2 induction coincided with the induction of enzyme activity. Expression analysis of mRNA revealed that induction of two genes were regulated by both CO 2 conditions and iron concentration in the medium, indicating that H-CO2 and iron deficiency are necessary for the induction. We consider that gene product of HCR2 should be ferric reductase, though no direct evidence is obtained.
Table 3 Characteristics of HCR cDNA clones and deduced amino acid sequences
Clone
HCR 1
HCR2
Nucleotide size (bp)
1373
3095
No. of amino acid residues
336
815
Predicted molecular mass (kD)
36.5
90.8
There are many other phenomena specifically observed in H - C O 2 conditions. For example, H - C O 2 cells of C. littorale exhibit higher Rubisco activity than L - C O 2 cells, due to the increase in the amount of this enzyme detected by antibody [11]. Rubisco activase is also higher, but phosphoenolpyruvate carboxylase activity is lower in HCO 2 cells than in L-CO2 cells. 4. CAPACITY CONDITIONS
OF
MICROALGAL
CO 2 FIXATION
UNDER
NATURAL
A new type of fiat plate photobioreactor has been developed in the Marine Biotechnology Institute to elucidate the potential of growth and CO 2 fixation capacity of C. littorale cells. The reactor is mainly characterized by its short light path, curved buffles (Figure 2), and intensive aeration. The short light path, less than 2 cm, gives sufficient irradiation to each microalgal cell and minimize the negative effect of mutual shading. Turbulent flow caused by curved buffles and strong bubbling also reduces the dark period of each cell when cell concentration has reached very high. Thereby very high cell growth rate in the linear growth phase (0.38 g cell dry weight 11 h-l), CO 2 uptake rate (0.72 g C O 2 1-1 h-l), and the maximum cell concentration (84 g
61 dry cell weight 1-1) were achieved in this reactor under the continuous irradiation by white cool fluorescent lamps with the light intensity of 2,000/~mol m -2 s -1 ( H u et al., in preparation). The highest rate of CO 2 uptake reported thus far was 0.19 g CO 2 1-1 h -1 with a marine cyanobacteria, Synechococcus sp. by using a sophisticated optical fiber photobioreactor [14].
air outlet
air outlet
curved buffies
perforated air tubing
compressed
air
Figure 2 Side view of flat plate photobioreactor. An outdoor mass culture facility could also achieve a very high growth rate under natural solar irradiation with the use of the modular plate photobioreactor [15]. According to Pultz, 1994 [15], a daily harvest of 175 g cell dry weight m -2 w a s obtained in summer 1991 on the basis of installed ground area, indicating that it may be possible to design microalgal culture system which can fully utilize the solar energy. Based on several estimations and assumptions, such as average solar irradiation in Japan, energy conversion efficiency of photosynthesis, and the rate of respiration in light and in the dark, the expected maximum CO 2 fixation rate by microalgal photosynthesis is 140 g CO 2 m -2 d -1. In Japan, industrial CO 2 emission is huge, and required land area to absorb this emission by microalgae can be calculated from the above CO 2 fixation rate (Table 4). It seems to be hard to fix such CO 2 by microalgal photosynthesis. However, there is no need to fix all the emission. The total increase in emission between 1990 and 1994 was 7.2 % [17], corresponding with average yearly increase of 2.25 X 10~3g CO 2 y-l, and this portion is a target for fixation. Land area necessary for the fixation of this amount of CO 2 is 440 km 2 (Table 5).
62 Table 4 Required land area for treating industrial CO 2 emission (1988) [16] by microalgal mass culture Industry
Annual CO 2 emission ( g C O 2 y-l)
Land area required to absorb total C O 2 emission (km 2)
Electric power generation
3.30 X 10 TM
6,460
Steel
1.83 X 1014
3,590
Ceramic
7.33 X 1013
1,440
Chemical
3.67 X 1013
718
Table 5 Annual
CO 2
emission and required land area for treating average annual increase of
CO 2 emission between 1990 and 1994 in Japan Annual CO 2 emission in Japan (g CO 2 y-l) 1990
1.17 X 10is
1994
1.26 X 10 is
Average annual increase of CO 2emission (g CO a y-l) 2.25 X 1013 Land area required to absorb average annual increase of CO 2 emission (km 2) 440
We should also consider the other kinds of and smaller size of CO 2 emission site, for example, disposal and incineration sites of waste, and activated sludge
63 incinerators. The combustion facility for excess amount of activated sludge is often attached to wastewater treatment plants. In Japan, annual CO 2 emission from this type of incinerators was 1.5 X 1012 g in 1986 [18]. The required land area for the complete treatment of this amount of CO 2 was 29 km 2. The wastewater treatment plants always have a large area for plantation, and total area of this green part was 17 km 2. This means ca. 60% of CO 2 emission can be converged into microalgal biomass by making use of the green area. About 40% higher CO 2 fixation rate is expected in low latitude area because of higher solar radiatiorL Further study is necessary to develop a low cost, easy operation and less maintenance culture facility which is suitable for low latitude countries. REFERENCES
1. K. Aizawa and S. Miyachi, FEMS Microbiol. Rev., 39 (1986) 215. 2. S. Miyachi, M. Tsuzuki and Y. Yagawa, Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms, W. J. Lucas and J. A. Berry (eds.), 1985, 145. 3. T. Kamo, K. Shimogawara, H. Fukuzawa, S. Muto and S. Miyachi, Eur. J. Biochem., 192 (1990) 557. 4. Y. Yagawa, S. Muto and S. Miyachi, Plant Cell Physiol., 28 (1987) 1253. 5. S. Mitsuhashi and S. Miyachi, J. Biol. Chem., 271 (1996) 28703. 6. S. Mitsuhashi, N. Kurano, S. Harayama and S. Miyachi, Third International Symposium on Inorganic Carbon Utilization by Aquatic Photosynthetic Organisms, Vancouver (1997). 7. M. Tsuzuki and S. Miyachi, Bot. Mag. Tokyo, Special Issue 2 (1990) 43. 8. E. S. Nielsen, Physiol. Plant., 8 (1955) 317. 9. M. Chihara, T. Nakayama, I. Inouye and M. Kodama, Arch. Protistenkd., 144 (1994) 227. 10. M. Kodama, H. Ikemoto and S, Miyachi, J. Mar. Biotechnol., 1 (1993) 21 11. I. Pesheva, M. Kodama, M. L. Dionisio-Sese and S. Miyachi, Plant Cell Physiol., 35 (1994) 379. 12. I. Iwasaki, N. Kurano and S. Miyachi, J. Photochem. Photobiol. B: Biol. 36 (1996) 327. 13. N. A. Pronina, M. Kodama and S. Miyachi, XV Int. Bot. Cong., Yokohama, Japan, 1993, p. 419. 14. H. Takano, H. Haruyama, N. Nakamura, K. Sode, J. G. Burges, E. Manabe, M. Hirano and T. Matsunaga, App1. Biochem. Biotech., 34/35 (1992) 449. 15. O. Pultz, Algal Biotechnology in the Asia-Pacific Region, Phang et al. (eds), 1994, p. 113. 16. Y. Uchiyama and H. Yamamoto, Technical Report, No. Y91005, 1992, Central Research Institute of Electric Power Industry, in Japanese. 17. The Japanese government's White Paper on the Environment in 1997, p. 62 in Japanese. 18. Y. Oshima and S. Ochi, Global Warming Handbook H. Komiyama (ed), 1990, p. 337, in Japanese.
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
65
Perspectives of carbon dioxide utilisation in the synthesis of chemicals. coupling chemistry with biotechnology. Michele Aresta Dipartimento di Chimica and Centro CNR-MISO, Campus Universitario, Universita di Bari, 70126 Bari, Italy.
Carbon dioxide or its reduced forms have been used for the synthesis of chemicals using enzymes and biomimetic metal systems as catalysts. In several cases quite interesting results have been obtained that make this way v e r y promising for a number of applications. 1. INTRODUCTION Carbon dioxide as a raw material [1] for the Chemical a n d / o r Energy Industry is receiving a growing attention because: i) if the recovery of CO2 from flue gases will be implemented as a technology for controlling its accumulation in the atmosphere, large amounts of CO2 will be available; ii) environmental issues urge to develop new processes/products, avoiding toxic materials. Several uses of CO2 appear to be responding to (ii), as categorized below [2]: * Use as a solvent (supercritical or liquid) in chemical processes - supplanting organic solvents, such as chlorinated and benzene. * Building block for organic carbamates-isocyanates-carbonates, supplanting phosgene, and for carboxylates, avoiding multistep procedures. * Carbon-source in the synthesis of fuels, supplanting CO or coal/hydrocarbons. From the energetic point of view, the carbon dioxide-utilizing reactions can be divided into two main categories. Class 1: Reactions
in which the entire carbon dioxide molecule is used
(fixation of CO2). The amount of extra energy, if required, is very low. These reactions can be divided into two sub-sets: la. Carboxylation reactions with the formation of C-C bonds. lb. Carboxylation reactions with the formation of C-heterotaom bond (C-E; E=O, N, P, other elements). Class 2" Reduction reactions.
In this case a C1 or Cn species is formed with the use of extra-energy in the form of electrons or hydrogen, or both.
66 Both classes of reactions are common to various biological systems: plants and bacteria. Enzymes involved in carbon dioxide fixation or utilisation may have a metal as the active site [3]. In this paper aspects of the carbon dioxide chemistry we have developed will be reviewed and discussed with reference to relevant natural processes. A few examples of utilisation of either enzymes or biomimetic systems will be presented, covering both the fixation and reduction reactions. 2. CARBON DIOXIDE CHEMISTRY. The utilisation of carbon dioxide in synthetic chemistry appears very attractive since this molecule is very versatile. Figure 1 gives an example of the reactions that can be found in the literature.
RNHC//?OR, [HCOOH] R'RNCONRR' CO & 9 ~I~ RNH2~ ~
\
..J
~
-"
%r -
\
~\~/
-o
//
(3H.-~)H
r,coo:-
C02
r- ~ ~ u
I~2NHJ1\'~
~1
CnH2n+lOH CnH2n+INH2
~ ~,.,.COONaHC//O orHC//O "OR ~NP~ COOH Figure 1. Utilisation of carbon dioxide in synthetic chemistry. The left part of the figure shows the reactions in which the entire CO2 molecule is used, the right part shows those processes that imply the reduction of carbon dioxide to C1 or Cn species. The species formed from carbon dioxide, according to their use, can be divided into two groups: i) Intermediates or fine chemicals for the Chemical Industry. i.e. molecules containing the following functionalities: -C(O)Oacids, esters, lactones.
67 O-C(O)O- N-C(O)OR - NCO
organic carbonates. carbamic esters. isocyanates, that can be obtained from primary amine methyl- or phenyl-carbamates by thermal elimination of methanol or phenol, respectively. ureas.
-
- N-C(O)-N ii) Products for the Energy Industry. i.e. products rich in energy, like CO. -
- CH3OH.
- Cn hydrocarbons and their derivatives. 3. UTILISATION OF THE ENTIRE CO2 MOLECULE. In this paragraph some of the results of our studies on the incorporation of the entire carbon dioxide molecule into organic products are presented. In each case the relevant biological process is briefly illustrated. 3.1 Carboxylation of Organic Substrates. Selective Synthesis of 4-OH-Benzoate from Phenol and Carbon Dioxide Catalyzed by a "Supported Enzyme". Carboxylation reactions based on carbon dioxide have a great interest as the direct introduction of the carboxylic functionality represents, with respect to the conventional synthetic methodologies, a way for both saving energy and reducing the production of wastes. Despite such aspects, that have a positive environmental impact, the only "direct carboxylation" process exploited at the industrial level is the more-than-one-hundred-years old Kolbe-Schmitt reaction. This process converts phenol (in the form of a Group 1 element salt) and carbon dioxide into a mixture of o- and p-OH-benzoic acid. The major product depends on the reaction conditions and the metal used (Na or K, respectively). This reaction has been recently reviewed [4] by several research groups and extended to other substrates. Our interest in the enzymatic functionalization of phenol derived from our discovery of a Mn- and K-dependent Carboxylase enzyme isolated from some anaerobic bacteria growing on phenol. This enzyme converts, under mesophilic conditions, very specifically phenol into 4-OH-benzoate that is thereafter dehydroxylated to benzoic acid and metabolized (Scheme 1) [5].
)
CO2 + H20
P032"
I
--()
HP042"
COOH
OH
II
O--(~-SCoA
O=C-SCoA
- - ~ C%+ H20 ATP
AMP + PPi
Scheme 1. Metabolism of phenol under anaerobic conditions.
68 We decided to explore the possibility of using the purified enzyme (or the enzymatic pool) for catalyzing the net carboxylation of phenol. However, we have recently developed a quite interesting synthetic procedure that converts phenol (I) into 4-OH-benzoate (II) at room temperature and sub-atmospheric pressure of carbon dioxide. Either the enzymatic pool or the partially purified enzyme can be used, supported on low melting agar (that has been shown to be the best support) [6]. As shown in Scheme 1, in order to be carboxylated, phenol must be in the O-phosphorylated form. We have developed an unexpensive and rapid phosphorylation process of phenol that occurs at room temperature [6b]. By passing the solution of the phosphorylated phenol on the supported enzyme the quantitative conversion of phenol into 4-OH-benzoate is observed. This reaction appears to have a great potential as the carboxylated product has a large utilisation in the synthesis of new materials. 3.2
Oxidative Carboxylafion of Olefins Catalyzed by Rh-Complexes. A topic we have investigated for some time is the reaction of olefins towards a 02/CO2 mixture. We have shown that carbon dioxide acts as regulator of the oxidative properties of dioxygen [7]. In fact, transition metal catalysts (Rh, Cu) behave as "mono-oxygenases" when a 02/CO2 mixture is used (III in Scheme 3 is formed only in traces), and as "dioxygenases" in the presence of 02 only (III is the major product). We have postulated the intermediacy in such reaction of peroxocarbonates, species known for a long time.
/ *0 M
Ia
/ *0(+) or
NO.
c%
M
NO
jO~0 jO~
0
Ib
.(-)
~
~r
~r
(+)jo--*o(-)
/O~
2b
O*
~0~C% O
Scheme 2. Mechanism of formation of peroxo-carbonates from a dioxygen complex and CO2. Despite the cospicuous synthetic work available in the literature, very little was reported about the mechanism of formation of peroxocarbonates from dioxygen complexes and CO2. In principle, two ways (routes 1-2, Scheme 2) are
69 possible, that imply formal CO2 insertion into the O-O (route 1, Scheme 2) or MO (route 2, Scheme 2) bond. We have undertaken a detailed study in order to ascertain which mechanism is operating, making an extensive use of labeled compounds (160 2, 180 2, C1602, C1802), combined with a theorethical study [8]. The results of these studies support the insertion into the O-O bond. This reaction pathway finds a rationale in the value of the bond energy of Rh-O (60 kcal/mol)[9] and O-O bond (28-48 kcal/mol for most inorganic and organic peroxides) [10]. Such mechanism and situation are reminiscent of those of organic dioxiranes. The reaction we have observed of Rh-dioxygen complexes with labeled CO 2 brings to the synthesis of an asymmetrically labeled *O-O peroxo bond", either a Rh -160-180 or a Rh -180-160. Such asymmetric peroxo-groups are not very common and constitute an opportunity for studying the reaction mechanism of oxygen transfer to oxophiles. Peroxocarbonates are prone to transfer one single oxygen atom to an olefin (or any other oxophile), while 02 may cause the splitting of the olefinic double bond, using both oxygen atoms. This tendency is explained by the fact that peroxocarbonates convert into carbonates according to Eq. 1. "Rh(CO4)" + XY ...... > "Rh(CO3)" + XYO
(1)
We have studied the oxygen transfer reaction using phosphine as acceptor, that makes simple the reaction as only one product (phosphine oxide) is found. When C1Rh(-180-160-C(O)O)(PEt2Ph)3 was dissolved in CH2C12 at room temperature the O-transfer reaction to phosphine took place according to Eq. 2. CIRh(-180-160-C(O)180)(P) 3 .......... > RhCI(CO3)(P)2+ p - 1 8 0
(P=PEt2Ph) (2)
The GC-MS analysis of the solution has allowed to determine that phosphine oxide formed contained more than 85-90% of 180. This suggests that the oxygen atom of the peroxo group transferred is that linked to Rh, more than that linked to carbon. This result is confirmed also by the reaction of C1Rh(-160 180-C(180)160)(PEt2Ph) 3 that affords RhCI(CO3)(P) 2 and p=160 when dissolved in CH2C12 at room temperature with a similar selectivity (85-90 % 160 on phosphine oxide). More detailed studies are in progress in order to elucidate if this specific step takes place v/a an O-O bond splitting followed by the reaction with the entering oxophile or via an intramolecular reaction. Another product of the reaction of an olefin with the 0 2 / C O 2 mixture in the presence of Rh is the cyclic carbonate (VII) [3]. Only few reports can be found in the literature on the direct synthesis of carbonates from olefins, dioxygen and carbon dioxide, despite the usefulness of this reaction that avoids, with respect to the reaction of epoxides with carbon dioxide, the preliminary synthesis of epoxides. We have found that the product distribution in the oxidative carboxylation of styrene depends on the 0 2 / C O 2 ratio and on the temperature (Scheme 3). As the epoxide is one of the oxidation products of styrene, it could be the origin of styrene carbonate. We have evidence that the formation of the
70
PhHC=CHz
Rh(1) = RhCIP3 RhClLz
02 / C02
,.=__
Rh-, Cu-catalyst
(P = PEtzPh; PPhzEt) (Lz = diphos; 2,2'-dipyridyl)
Cu or Cu20 supported on pumice
PhCHO
( III )
PhCH2CHO
(IV)
PhC(O)CH3
(V)
PhHC - CH2
(VI)
0 PhHC--~H21 O\ /0
(vii)
C 0II
Scheme 3 Products formed in the reaction of styrene with 02/C02. carbonate from styrene, 02 and CO2 takes place at higher rate than from styrene oxide and CO2 in the presence of the same catalysts. This may suggest that the ring opening of the epoxide may be the rate determining step. However, we have demonstrated the formation of a metallacycle [(dipy)(C1)Rh(O-CH2-CHPh) or (dipy)(C1)Rh-(O-CHPh-CH2)] from styrene and dioxygen. These intermediates could give rise to both styrene oxide and the carbonate. The higher reaction rate when starting from styrene and dioxygen with respect to the epoxide can be, thus, justified. High temperature (> 353 K) often cause decomposition of the catalyst. Two mutually free cis positions are necessary for the formation of the metallacycle, that interacts with carbon dioxide and yields the carbonate: so, in the presence of Rh(diphos)2C1 and Rh(dipy)2C1, no conversion at all into the carbonate has been observed, either starting from styrene or from styrene oxide. In the latter case, only a minor isomerization into acetophenone and phenylacetaldehyde has been observed. Synthetic procedures affording organic carbonates without using phosgene are of great industrial interest. Several processes are now available also for the synthesis of open-chain carbonates [11]. This makes useful the study of these compounds with amines as a methodology for the synthesis of carbamates that, in turn, can originate isocyanates. In the following section we report on our studies on the synthesis of carbamates from amines and carbon dioxide, and on the reaction of amines with carbonates. Both aliphatic and aromatic amines have been investigated and we have developed several methodologies that allow the synthesis of carbamates that are precursors of isocyanates used by the chemical industry for the synthesis of large market polyamides. 3.3 Reactions Leading to the Synthesis of Carbamates and Isocyanates. The interest in the synthesis of carbamate esters [12] remains very high owing to their wide utilization [13]. The synthesis of these compounds generally uses phosgene [14] or isocyanates [15] as starting material. These are toxic, harmful compounds and, therefore, it is of interest to discover new synthetic routes to carbamate esters involving the use of less noxious starting materials. Carbon dioxide is a good candidate as a substitute: its fixation by amines and other suitable organic substrates is an attractive way to synthesize carbamate
71 esters. The direct interaction of amines with carbon dioxide leads to ionic carbamates RNH3+-O2CNHR (VIII), (Eq. 3) [16]. (R = Alkyl)
2 RNH2 + CO2 --> RNH3+'O2CNHR
(3)
In the presence of metals [17], metal salts, [18], metal amides, [19] or metal complexes [20], metal carbamates (M[O2CNR2)mLn] (IX) can be obtained, p-Block amides E(NR2)n (E = B, [21e] Si, [21a, b, c, i] Ge, [21f, i] Sn, [21d, g] As, [21c] Sb, [21b] P, [21c] ) also react with CO2 through a formal insertion of the heterocumulene in the E-N bond to afford p-block carbamates E(NR2)n-x(O2CNR2)x (X, n= 3 or 4). Compounds VIII-X can play an important role as potential carriers or source of the carbamic group O2CNR2 that can be easily transferred to alkylating or arylating agents affording organic carbamates [22]. We have carried an extensive
RNC(O)OCH3
13CO2
RNH2 ~
RNH2"13CO2
RNH13C(O)OC(O)OGI-13
CH#DH -~
RNH2
DMC
[RNH3]-[O213CNHR]
RNH2
Scheme 4. The catalytic role of carbon dioxide in the reaction of the carbamate anion with carbonates. study on the synthesis of organic carbamates [11] from amines, carbon dioxide and alkylating-, arylating-agents. We have found that carbon dioxide itself can play an important role as catalyst in the reaction of carbonic esters with amines to afford carbamates in mild conditions and with high selectivity [23]. More recently we have investigated the role of non metal catalysts in the synthesis of carbamates and discovered a biomimetic phosphorous acid catalyst that promotes the selective conversion of aromatic amines (mono- and di-amines) into carbamates A few carboxylating enzymes [24], such as carbamoyl phosphate synthetase (CPS), biotin dependent carboxylases, phosphoenolpyruvate carboxylase, use hydrogencarbonate, a poor electrophile that is activated via the formation of carboxyphosphate, -OC(O)OP(O)O22-, resulting from the interaction of HCO3with ATP. Interestingly, in natural systems CPS also promotes the reaction of
72 - O C ( O ) O P ( O ) O 2 2- with ammonia to produce the carbamate anion, that is phosphorylated in a subsequent step.
(4) (5)
ATP + HCO3- -..... > ADP +-OC(O)OP(O)O22-OC(O)OP(O)O22- + NH 3 ...... > H2NCO2- + HOP(O)O22-
We have used organo-phosphorus acids [25] as promoters of the reaction of aromatic amines with dimethylcarbonate (DMC) or diphenylcarbonate (DPC) in the presence of carbon dioxide to generate N-alkyl- or aryl-carbamates. We have applied this methodology to the carbamation of aniline, naphtylamine, toluendiamine, 4,4'-diaminophenyl-methane, among others. P-acid H 2 N A r N H 2 + ROC(O)OR ........ > H2NArNHC(O)OR + ROH (R = Me, Ph) (6) H2NArNHC(O)OR + ROC(O)OR ...... > RO(O)CNHArNHC(O)OR + ROH (7) The carbonate itself can be used as solvent and the reaction is very selective as no methylation or arylation products of the amine are found. The catalyst can be easily and quantitatively recovered as arylammonium salt at the end of the reaction and recycled. Thereof, the carbamate is not contaminated with P. These features make {Ph2P(O)OH} very attractive from a practical point of view. This represents the first example of non-metal catalysis for the synthesis of carbamates from carbonates. Kinetic experiments support the "nucleophilic catalysis" depicted below. X2P(O)OH + PhNH2
X2P(O)O- +H3NPh
(R- Ph, Me; X: Ph,PhO)
X2P(O)O- +H3NPh + (RO)2C=O---> X2P(O)OC(O)OR + ROH + PhNH2 PhNH 2 + X2P(O)OC(O)OR ..... > X2P(O)OH + PhNHC(O)OR
(8) (9) (10)
This mechanism requires the intermediate formation of the carbonic diphenylphosphinic(phosphoric) mixed anhydride X2P(O)OC(O)OR. Upon reaction with the free aromatic amine, the anhydride converts to the carbamic ester, and regenerates the acid catalyst, X2P(O)OH. The mechanism is analogous to the mechanisms suggested for several enzymatic reactions that use H C O 3 - a s the carboxylating agent. The mixed anhydride X2P(O)OC(O)OR (XI) is structurally O
0
O
O
[I
II
II
II
x """"
C oR X
-0 """"
0j c OH
XI
reminiscent of carboxyphosphate (HO)OP(O)OCO22- (XII).
XII
73 In biological systems XII is the key intermediate in the carboxylation reactions. This similarity is not fortuitous as X2P(O)OC(O)OR and (HO)OP(O)OCO2 2represent the activated form of (RO)2C=O and HCO3-, respectively. The recent application of this synthetic methodology to the carbamation of diamines [25b] is of great interest as di-carbamates are the source of di-isocyanates used in the synthesis of polyamides. Interestingly, the catalyst can be used for the selective synthesis of both mono- and di-carbamates. 4. REDUCTION OF CO2 TO CO AND FIXATION OF THE REDUCED FORM. We have studied for long time the co-ordination chemistry of carbon dioxide and the reactivity of the co-ordinated molecule [26]. Also this aspect of the chemistry of carbon dioxide is relevant to biological systems. Carbon dioxide is the electron accepter in metabolic processes characteristic of several anaerobic microorganisms, methanogens and acetogens among others. Their enzymatic reaction mechanism has surprising relevance to chemical facts. An example of such similarities is given in Scheme 5. Indusld~ Processes O3) Water gas shift reactions.
Enzymatic Processes (a) CODH mediated synthesis of CO from CO2
[M] + CO2 + 2 [H+ + e'] --->
M-CO + H20
CODH mediated synthesis of the acetyl-moiety Corrin-Cl-13 + Ni--Fe(CO) ..... > Corrin + ( C ~ O 0 ) (CHo)Ni--Fe(CO) ..... > Ni--Fe-C(O)CH 3. or
(CH3)Ni--Fe(CO) ..... > Ni--Fe(CO)(GH0) ..... > N-FeC(O)Ot3 NI-Fe-C(O)CH3 ..... > CH3(O)C-Ni--Fe
CH3-G(O)-Ni--Fe
+
CoAS" .... > Ni--Fe + CoAS~O)CH3
co2+ CO+l~
H20 ~'--'~
%
CO + H2 CO2+H2.
Homologation of methanol to the acetyl-moiety. CH~ + HI -->CH31 + I..nM(CO) + 01"131 ~
co.
LnlM(CO)(CH3) ~
LnlM(CO)(CHs) LnI(CO)M-C(O)CH3
Ln represents ancillary ligands, neutral or charged. M is Co, Fe, Rh, Ru.
Scheme 5. Comparison of enzymatic and chemical reduction of CO2 to CO and fixation of the reduced form. The enzymatic mechanism presents, to date, some major points that deserve investigation: i) the existence of different sites where the reduction of carbon dioxide and the formation of the C-C bond take place, respectively, ii) The role of the Ni and Fe centers, and of-SH groups in the above mentioned processes. Under the correct conditions, model systems can contribute to shed light on enzymatic reactions. We have started an extended investigation on Fe and Ni complexes as CO2-fixation catalysts. We report here on some Ni-model systems
74 that fully mimic the enzymatic activity converting CO2 into an organic thioester, v/a reduction to CO and reaction with thiols and an olefin. For a long time, Ni has been considered to be, in the enzymatic system, the metal center binding CO and, probably, catalyzing the CO2 reduction [28]. The involved couple of oxidation states of the metal (NiLNi II, NiII-Ni III Ni0-Ni I) has been matter of discussion and investigation. If Ni or Fe is the center where CO2 is reduced to CO is still unclear, but it has been proposed that CO is bound to Fe prior the acetyl moiety is generated. Chemical models say that both Ni and Fe complexes are able to bind carbon dioxide. Concerning the CO2 reduction to CO, at the present time evidences that the Ni complex (PCy3)2Ni(CO2) (XIV) (Cy = cyclohexyl) catalyzes the CO2 reduction reaction in the presence of proton and electron donors can be found in the literature [26]. The fluxional behaviour of the carbon dioxide moiety coordinated to the metal has been unambiguously established in the case of XIV and the rotational free energy barrier (AG # = 39.6 kcal mo1-1) determined. XIV in toluene, under dinitrogen atmosphere, reacts with several Broensted acids, under electron transfer conditions, and shows a different behaviour according to the species and the reaction conditions. PhSH is a quite interesting reagent: in fact when it is added to a solution of XIV in toluene the i.r. spectrum of the solution shows the immediate disappearance of the band at 1745 cm -1 (due to coordinated CO2) while two new bands at 1980 and 1912 cm-1 indicate the formation of the bound-carbonyl species (PCy3)2Ni(CO)2 (XV). XV and (PCy3)2Ni(SPh)2 (XVI) were isolated from the reaction mixture as thermodynamic products and characterized by elemental analysis, i.r. and 13C, 31p n.m.r, spectroscopy. Water is also an end product of the reaction, while PhSSPh, that is a kinetic product with "(PCy3)2Ni(CO)" (XVII), was detected in the reaction medium but not isolated. In fact the disulphide reacts with the Ni(0) existing in solution: (11)
"(PCy3)2Ni" + PhS-SPh ...... > (PCy3)2Ni(SPh)2 The overall stoicheiometry of the reaction is:
3(PCy3)2Ni(CO2) + 4PhSH --> (PCy3)2Ni(CO)2 + 2(PCy3)2Ni(SPh)2 + CO2 + 2H20 (12) We have extended our studies on Ni complexes [17] in order to ascertain if Ni itself is able to catalyze the formation of a thioester, reproducing it alone the full enzymatic cycle that uses CO2 for the formation of the thioester group.
SPh 0
PhS~c~O
~ SPh XVIII
XIX
By reacting (PCy3)2Ni(CO2)with PhSH in the presence of 1-heptene, under dinitrogen atmosphere, the formation of thioesters was observed.
75 XVIII and XIX demonstrate that the formation of a thioester from CO2 and thiols can be p r o m o t e d by a single Ni center. The coordinatively unsaturated species "(PCy3)2Ni(CO)" (XVII), that is the kinetic product in the N i - p r o m o t e d CO2 reduction, must play a key role. In fact, when tri- or di-carbonyl species, n a m e l y (PCy3)Ni(CO)3 (XX) or (PCy3)2Ni(CO)2 (XXI) were used in the same reaction conditions, the formation of thioesters was not observed. This finding helps to explain w h y the reaction affording the thioester may give quite variable yields with the reaction conditions. XVII can either generate the dicarbonyl (Eq. 13) (that is not reactive towards olefins and the thiol), or interact with the olefin to afford 2 "(PCy3)2Ni(CO)"
..... > (PCy3)2Ni(CO)2 + "(PCy3)2Ni"
(13)
XXII (Eq. 14) that generates the thioester. "(PCy3)2Ni(CO)" + CH2=CHR . . . . >
(PCy3)2Ni(CO)(CH2=CHR) (XXII)
(14)
The reaction conditions play an important role addressing the reaction towards one or the other of the pathways. The direct utilisation of the C O D H enzymatic complex in synthetic chemistry is now one of our objectives. 5. CONCLUSIONS The utilisation of carbon dioxide in synthetic chemistry is a promising way for developing benign synthetic methodologies, avoiding toxic species and saving energy and carbon. New catalysts, that should be at the same time active and selective, must be synthesized. Metal systems are excellent cadidates for such reactions, although non-metal systems are also of interest. Nature provides very interesting examples of catalytic fixation of both the entire carbon dioxide molecule and its reduced forms. The utilisation of either biosystems or mimetic complexes is very challenging for chemists. We have found that in some cases this approach can give interesting results that might be exploited at the industrial level. REFERENCES .
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4.
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6.
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26. 27. 28.
B.B. Wayland, Polyhedron, 7 (1988) 1545. R. Curd, J. O. Edwuards "Activation of hydrogen peroxide by organic compounds" in Catalytic oxidations with H202 as oxidant, G. Strukul Ed.; Catalysis by metal complexes Series; Reidel-Kluwer. Dordrecht, Netherlands, (1992), Chapter 3. M. Aresta, E. Quaranta, ChemTech. 27, (1997)32. P. Adams, F. A. Baron, Chem. Rev., 65, (1965) 567. a) P. Piccardi, Chem. Ind. (Milan), 68 (1986) 108. b) T. Teh Wu, J. Huang, N.D. Arrington G.M. Dill, J. Agric. Food Chem., 35, (1987) 817; c) F. Rivetti, U. Romano, M. Sasselli, US Pat., 4 514 339, (1985) (to ECS). H. Babad, A.G. Zeiler, Chem. Rev., 73, (1973) 75. a) W. Lorenz, I. Hamman, Get. Pat., 2258805, 1972 (to Bayer AG) (Chem. Abstr., 1974, 81, 77701); b) B.A. Teicher, A.C. Sartorelli, J. Med. Chem., 23, (1980) 955; c) F. Maurer, I. Hamman, B. Homeyer, W. Behrenz, Eur. Pat., 23326, 1979 (to Bayer AG) (Chem. Abstr., 95, 43096q, 1985) a) T. Hayashi, Bull. Inst. Phys. Chem. Res. (Tokyo), 11, (1932) 133; b) H.B. Wright, M.B. Moore, J. Am. Chem. Soc., 70, (1948) 3865: c) J.K Wolle, K.L. Temple, J. Am. Chem. Soc., 70, (1948) 1414; d) K.R. Zahradnich, Chem Tech., 11, (1959) 546; e) J. B. Lallau, J. Masson, H. Guerin, M.F. Roger, Bull. Soc. Chim. Fr., 1311 (1972); f) S. Theodoropulos, Eur. Pat., 62161, 1981 (to Union Carbide Corp.) (Chem. Abstr., 98, 88842h, 1983). T.W. Martinek, US Pat. 3 061 637; 1958 (to The Pure Oil Co.) (Chem. Abstr., 58, 6700 g, 1963). F. Calderazzo, S. Ianelli, G. Pampaloni, G. Pelizzi, M. Sperile, J. Chem. Soc., Dalton Trans., (1991) 693. H. Noth, D. Schlosser, Chem. Ber., 121, (1988) 1715. R. L. Cowan, W. C. Trogler, Organometallics, 6, (1987) 2451. a) R.H. Cragg, M. F. Lappert, J. Chem. Soc., (1962) 82. b) H. Breederweld, Red. Trav. Chim. Pays-Bas, 81, (1962) 276. c) G. Oertel, H. Malz, H. Holtschmidt, Chem. Ber., 97, (1963) 891; d) T. A. Georges, K. Jones, M. F. Lappert, J. Chem. Soc., (1965) 2157; e) M. F. Lappert, B. Prokay, Adv. Organomet. Chem., (1969) 1356; f) M. R. Bandet, J. Sotge, Bull. Soc. Chim. Fr., (1969) 1356; g) R. F. Dalton, and K. Jones, J. Chem. Soc. A., (1970) 590; h) J. Koretsu, Y. Ishii, J. Chem. Soc. C., (1971) 511; i) L. K. Peterson, K. I. Th6, Can. J. Chem., 50, (1972) 562. a) M; Aresta, E. Quaranta, Tetrahedron, 48, (1992) 1515; b) M. Aresta, E. Quaranta, Ital. Patent 1 237 208, (1993). a) M. Aresta, E. Quaranta, Ital. Patent, 1 237 207, (1993); b) M. Aresta, E. Quaranta, Tetrahedron, 47, (1991)9489. M. Aresta, A. Ciccarese, P. Giannoccaro, E. Quaranta, I. Tommasi, Gazz. Chim. Ital., 125 (1995) 509. a) M. Aresta, C. Berloco, E. Quaranta, Tetrahedron, 51 (1995) 8073. b) M. Aresta, E. Quaranta, A. Bosetti, Ital Patent Appl. MI 96 A002202 (1996). For more recent works see: a) M. Aresta, E. Quaranta, I. Tommasi, R. Gobetto, Inorg. Chem., 31, (1992), 4286. b) C. Jegat, M. Fouassier, M. TranquiUe, J. Mascetti, I. Tommasi, M. Aresta, F. Ingold, A. Dedieu, Inorg. Chem., 32, (1993), 1279. a) S.W. Ragsdale, J. E. Clark, L. G. Ljungdahl, L. Lundie, H. L. Drake, J. Biol. Chem., 258 (1983),2364. b) S.W. Ragsdale, Coord. Chem. Rew., 96, (1996) 2515. M. Aresta, E. Quaranta, I. Tommasi, C. Fragale, Inorg. Chim. Acta, 1997, in press.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
77
Scope of studies on C O 2 m i t i g a t i o n
K. Yamada Dept. of Chemical System Engineering, School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 1. I N T R O D U C T I O N The atmospheric concentration of CO 2, which is a main greenhouse gas, has grown significantly since pre-industrial times and caused an increase in global mean surface temperature. Therefore, CO 2 mitigation measures should intensively be implemented. There are two methods to mitigate the increase in the atmospheric CO2 concentration. One is to reduce CO 2 emissions by improving energy-efficiency (relating to energy intensity) or developing fuel substitutes (relating to carbon intensity), and the other is to collect and then immobilize atmospheric or other CO 2 in the ocean or on land. An economic revenue over the short term is most important in realizing such measures. However, results of evaluations of energy consumption and its environmental impact as well as the potentM for CO2 reduction should also be taken into account for a long term plan of such systems. In this paper, studies on CO2 mitigation options such as CO 2 capture, sequestration, biological fixation, energy conservation and renewable energy technologies are discussed. Regarding renewable energy technologies, our estimated results are shown herein. 2.
CO 2
CAPTURE TECHNOLOGY
Energy requirements to capture and sequester
CO 2 are
very high, even flue gases from fossil
fuel power plants which are large stationary sources of CO2 are treated. Moreover, capture and sequestration technologies are said to be effective for CO 2 mitigation and emergency measures. It is very important to reduce the energy penalty due to CO 2 capture for the realization of such technologies. A chemical absorption method with a monoethanolamin (MEA) solution is used in commercial CO 2 capture plants and considered to be the lowest energy requirement process. When it is applied to a conventional coal power plant, a typical energy penalty is in a range of
78 20-3O%. Mimura et al [1] found an effective amine solvent (KS-2) with a long lifetime and reported that the KS-2 solvent system could reduce the energy penalty by 20% compared with a MEA system. The power plant losses based on generator output power were calculated to be 5.4-5.8% for a natural gas-fired plant and 9.0% for a coal-fired plant at CO 2 recovery of 90%. The CO 2 capture from the flue gas of power plants always causes energy penalties, on theother hand, if CO2 is separated from the recycling gas of fuel cells, the energy merit may be obtained. Sakaki et al [2] reported that the energy efficiency of a solid oxide fuel cell (SOFC) could be increased by CO 2 separation from its recycling gas at energy requirements of less than two times the theoretical separation energy. It means the possibility of CO 2 capture in fuel cell systems without the use of additional energy. 3. CO z C H E M I C A L U T I L I Z A T I O N AND S T O R A G E T E C H N O L O G Y I E S
3.1 Chemical utilization As Aresta [3] has pointed out, the major issues for the CO 2 utilization are the determination of the actual amount of fixed CO 2 and the life of the product. When a standard of 10Mt-C/y as the fixed CO 2 and 100 years as the life is taken into consideration, it is a big challenge to find effective methods for chemical utilization. Intensive research on catalysts to convert CO 2 to methanol have been undertaken mainly in Japan. Saito et al [4] have found effective catalysts (Cu / ZnO / ZrO2 / AI203 / Ga203) with which very pure methanol (99.96%) could be produced from CO 2 and H 2. However, only a limited effectiveness for the methanol from CO 2 could be found from the viewpoint of energy efficiency.
3.2 Ocean storage technology The ocean is the largest potential sink for
C O 2.
Research on the direct injection of
CO 2
into
the ocean has been done mostly in Japan, USA and Norway. Herzog et al [5] compared and summarized five ocean storage options as shown in Table 1. Fujioka et al [11] reported cost evaluation results on ocean disposal (capture and storage) options as shown in Table 2.
79
Table 1 Comparison of ocean storage options Development
Option
Cost
Required
Environmental
Leakage to
Impact
Atmosphere
a. Dry Ice [6]
Lowest
High
Low
Low-Medium
b. Towed Pipe [7]
Medium
Low-Medium
Lowest
Medium
c. Droplet Plume [8]
Low
Low
Low-Medium
Medium
d. Dense Plume [9]
Medium
Lowest
Highest
Medium
e. CO 2 Lake [10]
Highest
High?
Low
Lowest
Table 2 Electricity and CO 2 disposal costs Option Base Relative electricity cost C O 2 disposal
cost [$/tonC-avoided]
a
b
d
e
100
350
250
230
250
0
660
370
330
370
Power plant capacity = 2 • 554MW In any case, the cost of electricity increased more than twofold by applying the
CO 2
disposal
system and this CO 2 disposal cost is calculated to be higher than $330/ton-avoidedC. Improvement in a CO 2 separation step seems an effective method to decrease the CO 2 disposal cost. In a storage step, a new method to dispose CO 2 as hydrate crystal formed at a depth of 500m in the ocean to lower the disposal cost has been proposed by Yamasaki [12].
3.3 Geological storage technology The high pointial for underground-storage and lower cost than that of ocean storage has led to geological storage as a major option for CO 2 disposal. Table 3 shows the comparison of geological storage options reported by Herzog et al [5]
80 Table 3 Comparison of geological storage options Storage Option Active oil wells (EOR)
Relative Capacity
Relative Cost
Storage
Technical
Integrity
Feasibility
Small
Very Low
Good
High
Coal beds
Unknown
Low
Unknown
Unknown
Depleted oil/gas wells
Moderate
Low
Good
High
Deep aquifers
Large
Unknown
Unknown
Unknown
Mined caverns/salt domes
Large
Very High
Good
High
4. B I O L O G I C A L F I X A T I O N Atmospheric CO 2 can be addressed from an organic viewpoint by considering biological processes using solar energy. Various kinds of biological CO 2 fixation methods such as afforestation, marine fertilization and microalgae production have been proposed. The Chem. Eng. Soc. Japan has surveyed and summarized these processes [13]. In terrestrial ecosystems, afforestation and reforestation in tropical and temperate zones are effective for CO 2 fixation from the viewpoint of energy and cost efficiency. A serious difficulty in afforestation arises in ensuring the preservation of the land. The development of the land is strongly restricted by the policies and traditions of the nation and the community. Involved in many developing countries, food shortage has become a serious problem due to the increasing population. This means that much land must be utilized for agricultural production rather than for afforestation. This is why arid land must be considered for afforestation. The most essential and serious problem in afforestation of arid land is the difficulty in maintaining a water supply. Afforestation in desert areas requires additional water supplies other than precipitation. At present, an afforestation system combined with reverse-osmosis desalination or other processes is not a CO 2 sink but a source. New sustainable systems such as precipitation enhancement [14], efficient water management or proper selection of plants should be developed for afforestation of deserts. In marine ecosystems, fertilization of the main nutrients (N. P) or a micronutrient (Fe) seems a promising process for CO 2 fixation. However, it still remains uncertain at what ratio the biomass formed by such fertilization can reach the deep ocean.
81 A study on the resulting carbon cycle in the ocean to clarify the effectiveness of fertilization is required. 5.
ENERGY-EFFICIENCY
IMPROVEMENT
TECHNOLOGY
(STATUS
OF
ELECTRIC VEHICLE) Energy-efficiency improvements and measures in energy conversion,
manufacturing,
transportation and commercial/residential sectors should be very effective for CO 2 mitigation. Energy use in transportation is projected to grow to 90-140 EJ in 2025 without new restrictions to address the 61-65 EJ in 1990. Electric vehicles (EVs) are expected to be a highly energy-efficient means of transportation compared to internal combustion engine vehicles (ICEVs) and their unit energy requirements can be half of those of ICEVs or less. However, there are significant problems in introducing EVs into commercial markets on a large scale, mainly the low energy density of the batteries inplying a shorter driving range, a long charging time and high costs. Many attempts are being made to solve these problems. Key technologies are in batteries and fuel cells. The development status of EVs is explained herein. Stula et al [15] have reported the advanced battery technologies in the USA. Nickel-metal hydride, lithium ion, lithium polymer batteries and ultracapacitors have attracted an attention and been under development. Lithium battery systems will be a strong contender as the main system for the future. Their current performance data on specific energy, specific power and cycle life and also current costs should be improved by a factor of more than two. In order to overcome the problem of the short distance range, researches on proton membrane fuel cell (PEM) technology for EV application are underway in many places. PEM has the disadvantage of requiring high purity hydrogen as fuel. Solid oxide fuel cells can use a variety of fuels and be used in the future [16], however, they are as yet in the intitial stage of development. 6. R E N E W A B L E E N E R G Y T E C H N O L O G Y
Renewable sources of energy contribute about 20% of the world's primary energy consumption at present. Most of them are fuelwood and hydropower. However, there are other renewable sources such as solar, wind, wave and geothermal energies. Their realization in commercial markets can be useful for CO 2 mitigation.
82 This commercialization requires the intensive cost reduction of energy from renewable sources. Herein, photovoltaic (PV), biomass and geothermal power generation systems are evaluated based on cost and CO2 emissions. 6.1 Photovoitaic energy systems PV systems using polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) cells have been evaluated [17]. The PV systems considered here are large-scale, centralized systems directly connected to the utility grid and include the balance of system (BOS) with supporting structure, inverter, and DC control device and installed in Tokyo. Assumption for energy conversion efficiencies and PV cell production rates are shown in Table 4. Costs and energy pay-back times of present PV systems at an annual production rate of 10MW were calculated to be u (u
(u
and 5.7 years for poly-Si and u
and 6.4 years for a-Si PV systems, respectively. These cases correspond to the
present level of PV technology. Table 4 Assumption of production rate and energy conversion efficiency Annual production rate (GW)
0.01
Energy conversion efficiency (%) Poly-Si a-Si
1
100
15
17
20
8
13
16
Application of the PV system in Tokyo (dispersed system). PV systems with a-Si cells used on roof tops in Tokyo were evaluated using improved values for the weight of supporting racks and inverter efficiencies. The results are shown in Table 5. EPT, cost, and CO2 emissions could be reduced by the use of light-weight BOS components and high-efficiency inverters. The CO 2 emissions of-5g-C/kWh for PV systems were found to be very low. The electricity cost of u desirable to realize a decrease of u
is "almost competitive to current rates, however, it may be or more for commercialization.
83
Table 5 EPT, cost and CO 2 emissions fi)r PV systems using a-Si solar cells. House roof
Apartment roof
42.8
22.2
Area (km 2) Cell production rate
1GW/y
100GW/y
1GW/y
100GW/y
Maximum output (GW)
5.12
6.30
2.66
3.27
Electricity output (GWh/y)
5.12"103
6.31"103
2.66"103
3.27"103
EPT (y)
0.64
0.49
0.75
0.56
Cost (Y/W)
173
122
194
140
Cost (Y/kWh)
26
18
29
21
032 emissions (g-C/kWh)
6
5
7
5
Application of PV system in Australian desert (centralization system). The Gibson desert in Australia where the insolation energy is 2,100kwh /m2, 1.8 times higher than in Tokyo, was selected for the power plant site. The electric power generated is transmitted 1,000 km to Perth. The total power plant area of 860km 2 can provide electricity of 1.1 • 105GWh/y. Evaluation results on centralized PV systems using poly-Si cells (Production rate 100GW/y) are presented in Table 6. Table 6 EPT, cost and
CO 2
emissions for PV system using poly-Si cells in Australian desert EPT
Cost
C O 2 emissions
Without battery systems
0.9y
18Y/kWh
9g-C/kWh
With lead battery systems
1.7
101
15
With NaS battery systems
1.1
32
12
Installation of batteries which store PV electricity generated for ~,o days increases EPTs, CO 2 emissions and especially costs. A high cost with lead batteries can be reduced by use of a new type of battery such as NaS. Without battery system, the electricity cost is near a present level.
84
6.2 Power-generation systems by biomass At present, biomass power plants have low thermal efficiencies of about 20-25 %. Herein, biomass power plants with advanced gasification and liquefaction technologies [18], [19] are designed and have been evaluated [20]. The capacity of a power plant is assumed to be 100MW. Fuel for power plants is produced by the liquefaction and gasification of eucalyptus. Its heat of combustion is assumed to be 16.7MJ/kg-dry basis. The annual growth rate of biomass in a plantation area is assumed to be
1.25kg-dry wood/m 2. The plantation - harvesting cycle is 10 years. The area of the plantation site required for the 100MW plant is calculated to be about 600km 2. The evaluation results are shown in Tab. 7. As indicated in the results, thermal efficiencies are in the range of 27---31% which are 20-30% higher than for those of current biomass power plants. CO z emissions and electricity costs by biomass power plants are low compared to those by PV systems as shown in the previous section (6.1). Technology barriers to realize a new biomass power plant seem not so high, however, a high barrier is to find a plantation site. Table 7 Evaluation results (material quantities, CO 2 emissions, EPT and cost). ...................
Process
LCC
LST
GCC
Combined cycle Steam turbine Combined cycle (gas-steam
Process
(boiler)
turbines)
Thermal efficienies (%)
GST Steam turbine
(gas-steam
(boiler)
turbines)
29.0
26.7
30.5
26.7
2
3
2
3
EPT (d)
78
155
53
130
Electricity cost (u
7.1
6.5
6.9
5.2
Electricity cost (Y/kWh)**
10.1
10.0
9.9
9.9
Unit CO 2 emissions (g-C/kWh)
* Labor cost of Y1500d-lman-lat plan site
** Labor cost of u
1
85
6.3 Geothermal power generation system A geothermal power plant with a hot dr3' rock system constructed in Japan is evaluated herein. The capacity of the power plant is assumed to be 280MW and hot dry rocks (300A6) at a depth of 4km is used to generate steam. The plant area is calculated to be 1.4kin 2. The evaluation results are presented in Tab. 8. Table 8. Evaluation results on geothermal power generation system CO 2 emissions
EPT
(g-C/kWh)
(days)
3
80
Electricity cost (u 5
The evaluation results suggest the geothermal system can be a promising candidate from the viewpoint of CO 2 emissions and the electricity cost. 7. C O N C L U S I O N An overiview of verious studies
on CO 2
mitigation were presented in this paper. The research
summarized have been conducted with significant progress. However, there are no single solutions. We should investigate the near-, mid- and long-term projects considering their priority. CO 2 reduction costs and barriers for the realization of the different options discussed in this paper are shown below.
CO 2
(u CO 2 capture and sequestration
reduction cost
Barrier
avoided) 36,000
9High energy consumption 9Environmental uncertainty
Afforestation
at present
6,000
9Land-use competition
(tropical / temperate zone) PV system
at present future roof-top future centralized
800,000 0 100,000
9Conversion efficiency 9Energy storage
86 Biomass power generation system
0
9Land-use competition
Geothermal power generation system
0
9Water path structure
EV
at present
200,000
9Battery 9Short distance range
REFERENCES
1) T. Mimura et al, Energy Conrers. Mgmt Vol. 38 (1997) $57-$62 2) K. Sakaki et al, Kagakukougaku-Ronbunsyu Vol. 23, No. 2 (1997) 292-295 3) M. Aresta and I. Tommasi, Energy Confers. Mgmt Vol. 38 (1997) $373-$378 4) M. Saito et al, ibid $403-$408 5) H. Herzog, E. Drake and E. Adams, "CO 2 capture, reuse and storage technologies for mitigating climate change", DOE order No. DE-AF22-96PCO1257 (Dec. 1996) 6) N. Nakanishi et al, "Sequestering of CO2 in a Deep Ocean -- Fall Velocity and Dissolution Rate of Solid CO2 in the Ocean", CRIEPI Report (EU 91003), Japan (1991) 7) M. Ozaki et al, Energy Convers. Mgmt. 36(6-9), (1995) 476-478 8) C. Liro et al, ibid 33(5-8), (1992) 667-674 9) PM. Haugan and H. Drange, Nature 357 (1992) 318-320 10) T. Ohsumi, Marine Technology Society Journal, 29(3), (1995) 58-66 11) Y. Fujioka et al, Energy Conrers. Mgmt Vol. 38 (1997) $273-$277 12) A. Yamasaki, Reported at 1997 annual meeting of Japan Chem. Eng. Soc 13) K. Yamada, T. Kojima et al., NEDO-GET-9638 (1997) 14) D. Li and H. Komiyama, "Numerical Simulation of Limited-area Weather Modification in Australia" 1997 Australia Meteorology Soc. 15) R.A. Stula et al, Symposium Proceedings of The 13 International Electric Vehicle Vol. 1, (1996) 303-310 16) K. Sakaki, F. Nishikawa and K. Yamada, ibid, (1996) 686-692 17) K. Yamada, K. Kato and H. Komiyama, Energy Convers. Mgmt. 36, (1995) 819 18) S. Yokoyama et al., Kagakugogaku Ronbunshu, 17, (1991) 326-333 19) S. Fujinami, "Fluidized-bed gasification of cellulostic wastes", Ebara Jiho 151 (1991) 20)K. Yamada et al., "Evaluation of Electric Power Generation System by Biomass", Developments in Thermochmical Biomass Conversion (Banff- Canada, 20-24 May, 1996), 1582-1589 (1996)
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
87
Hydrogenation of CO2 toward Methanol Influence of the catalysts composition and preparation on the catalytic behavior R. Kieffer, L. Udron. L E R C S I [UMR 7515 du CNRS] E C P M Universit6 Louis Pasteur 1, rue Blaise Pascal - 67008 Strasbourg France The influence of the preparation method of methanol catalysts composed of copper associated with rare earth oxides (eg Cu-La2Zr207 and ZnO promoted Cu-La2Zr207 systems) on the catalytic behaviour is discussed. Good activities and improved aging properties are always associated with a high copper surface area and a reasonnable crystallinity of the La2Zr207 pyrochlore. For Cu-La2Zr207, as well as for Cu-ZnO catalysts, an almost linear correlation can be observed between the methanol yield, the copper surface area and the amount of formates located on the catalyst's surface. A similar correlation cannot be evidenced on ZnO promoted Cu-La2Zr207 catalysts. The results are discussed and a mechanism for the hydrogenation of CO2 to methanol is proposed 1. I N T R O D U C T I O N CO2 hydrogenation to methanol is one of the most favorable valorization of the carbon dioxide considering that the methyl alcohol can easily be converted into hydrocarbons on zeolite type catalysts [ 1,2] even in a "one step" process using composite material. Many catalytic formulation are proposed for the hydroconversion of CO2, most of them are based on promoted copper-zinc oxides given by the long industrial experience on methanol synthesis from syngas (CO+CO2+H2) [3-6]. Specific methanol catalysts working for CO2 are proposed including promoted Cu-Zn catalysts [3,6], zirconia supported systems [7] as well as copper associated with stabilized rare earth oxides [8,9]. In the last case Cu-LaZr and CuZnLaZr catalysts were proposed and showed interesting catalytic properties in the methanol formation. The objective of the present work was to demonstrate the importance of the catalyst's preparation technique on the catalytic behavior and hence to establish the possible links between solid state chemistry and the catalytic process.
2. EXPERIMENTAL 2.1. Catalyst preparation The copper-zinc catalysts were obtained from aqueous solutions of copper and zinc nitrates using NaaCO3 (Cu-Zn [ex carbonate]) or oxalic acid (Cu-Zn [ex oxalate]) as precipitant. The filtered precipitate was washed 3 times with water, dried 14 h. at 100~ and calcined (in air) at 350~ The copper-pyrochlore catalysts were prepared from aqueous solutions of the corresponding nitrates using the precipitation with Na2CO3 (Cu-LaZr [ex carbonate]) or with oxalic acid (Cu-LaZr [ex oxalate]). The solid was washed, dried and calcined (in air) at 350~ 550~ and/or 710~ The zinc promoted catalysts were mainly obtained in the same way, i. e. by addition of zinc nitrate in the metal salts solution before precipitation. For the CuZn +LaZr
88 systems the carbonate precipitation was operate in presence of a slurry of the promoters (La2Zr207). For all the given preparations the calcination processes were monitored using TGADTA data obtained with a Setaram 92-12 TGA device.
2.2 Characterization of the catalysts BET and porosity were measured by N2 chemisorption (volumetric technique) on a Coulter SA 3100 equipment. The accessible copper surface area (SCu) is determined by the conventional N 2 0 adsorption technique on reduced catalysts (H2, 270~ 15 h). XRD measurements were performed on a Siemens D 5000 equipment using the CuKc~ radiation.
2.3. Catalytic Activities The catalytic tests were carried out in a stainless steel continuous flow reactor (6 mm inner diameter) containing 0.5 g of catalyst as described elsewhere [8]. Standard reaction conditions are : pressure = 6 MPa, CO/H2 ratio = 1/2 (CO2/H2 = 1/4), flow rate = 4 1.h-lg cat.-1. The carbon balance was always higher than 95%. 3. R E S U L T S AND D I S C U S S I O N
3.1. Copper-zinc oxide catalysts The copper-zinc [ex carbonate] catalysts, prepared by the conventional precipitation technique using Na2CO3 often described in literature [6,12], lead to efficient catalysts containing, despite washing of the precipitate, small amounts of sodium (0.05-0.1 wgt%) able to change the properties of the catalytic system [ 13]. To make a fair comparison with a Na free Cu-La-Zr catalysts Cu-Zn [ex oxalate] catalysts samples were prepared using the precipitation with oxalic acid described in the experimental part.
3.1.1. Characterization of the Cu-Zn catalyst. The characteristics of the catalytic systems depend not only on the preparation technique but also on the annealing temperature. Both catalysts have a poor thermal stability and a calcination above 350~ led to very low BET and copper surface areas e.g. 3m2/g (Cu-Zn [ex carbonate]) and lm2/g (Cu-Zn [ex oxalate]) at 550~ Since the copper surface area determines for the catalytic activity only the samples calcined at T = 350~ have been used for the catalytic tests. Table 1 Characteristics of Cu-Zn catalysts Catalyst T~C Preparation Annealing Cu-Zn [ex carbonate] 350 Cu-Zn [ex oxalate] 350
SBET m2/g 64 48
SCu m2/g 37 21
3.1.2. Catalytic activity of Cu-ZnO catalysts The catalytic activity of the catalysts in presence of a CO2 + H2 mixture between 250~ and 320~ (figure 1) can be more or less related to the copper surface areas as observed in table 1. Thus the catalysts prepared using carbonate precipitation are the most active in methanol formation. This can be attributed to the higher selectivity easily related with the high copper coverage of the catalyst. It can be noted that both catalysts do not produce any heavier products and in our reaction conditions, the presence of sodium in the catalyst originated from carbonates is not able to induce any chain growth despite some literature indications [ 13].
89
MeOH Yield % MeOH Sel. % I~ Yield [carb] l~ Yield [ox] " l - Sel [carb](x 0.3) Sel [ox](x 0.3)
I... m !
-
10 i 9 i
A !
i
!
250
270
300
?~
320
Figure 1. Influence of the preparation technique on the activity of Cu-ZnO catalysts 3.1.3. Aging of the Cu-Zn catalysts. During a 70 h. run at 300~ both catalysts, originating from carbonate and oxalates precursors, show (figure 2) a decrease of the methanol yield which represents respectively 9 and 19% of the initial activity of the systems .The absence of stabilising promotors (A1203, Cr304, ZrO2 ..) can explain these deactivation in agreement with literature data [ 14]. 3.2. Copper-pyrochlore catalysts As discussed in previous papers, the conventional firing-milling technique developped by inorganic chemists for La2Zr207 preparation [ 15] cannot be used for the preparation of catalytic materials since the used calcination conditions e.g. 1000-1200~ 6-24 h. lead to samples with a BET surface area lower than 1 m2/g without any catalytic activity. To obtain well distributed copper oxide on a pyrochlore matrix two "soft chemistry" techniques were compared.
1 - precipitation of mixed oxalates from ethanolic solutions of the metal salts followed by a calcination at relatively low temperatures (e.g. 550-710~ 2 - precipitation of the precursor from aqueous solutions by Na2CO3 followed by a calcination in the conditions given previously. 3.2.1. C h a r a c t e r i z a t i o n of C u - L a 2 Z r 2 0 7 catalysts Table 2 show that the characteristics of the catalysts depend strongly on the preparation technique and on the calcination temperature. According to these results it appears that, after annealing at 550~ the Cu-La2Zr207 [ex carbonate] catalysts is poorly crystallised whereas on Cu-LaZr [ex oxalate] a crystalline pyrochlore structure was not identified, but an ordered arrangement of La and Zr in an amorphous phase cannot be excluded. The well crystallized product obtained at 900~ has an extremely low BET and copper surface areas for an use as catalyst. The increase of the copper surface area observed on CuLaZr [ex oxalate] catalysts after calcination at 710~ can be explained by a phase rejection of CuO described previously [9,16] according to the following reaction pathways : CuO + L a 2 0 3 ...... > C u L a 2 0 4 C u L a 2 0 4 + 2ZrO2 ..... > CuO + L a 2 Z r 2 0 7 .
90 These solid-solid reactions may also be possible in the carbonate catalysts but do not appear so distinctly in this experiment. The high stability of the reaction intermediate La202CO3, can hinder, in this case, the formation of CuLa204 and so the rejection process and can therefore explain these results. ATG-ADG experiments under H2 - He flow on both catalytic systems calcined at 550~ show a lower reduction temperature of Cu-LaZr [ex carbonate] (165~ than Cu-LaZr [ex oxalate] (190~ which can be related with a stronger LaCu interaction in the latter case caused by the possible presence of non crystallized CuLa204. A lower reduction temperature of the catalysts annealed at 710~ (170~ compared to that obtained at 550~ (190~ may explain the disappearence of the La-Cu interaction in accordance with the rejection of CuO at 710~ ATG experiments on the precursors of both catalytic systems show after the weight loss corresponding to the escape of CO2, an exothermic peak (very small in the case of the carbonate catalyst), in the 700-710~ temperature range which can be attributed to the formation of the crystalline La2Zr207 compound. Table 2 Characteristics of copper-pyrochlore catal~csts Catalyst Annealing SBET T~ m2/Ig Cu-LaZr 550# 60 [ex carbonate] 710 25
Cu-LaZr [ex oxalate]
SCu m2/g 12 7
900
3
<1
550
39
9
710#
20
12
900
5
<1
XRD (crystallinity) CuO* La2OZr207 ~ CuO* La2Zr207* CuO** La2Zr207** CuO* CuO* La2Zr2OT* CuO** La2Zr207**
~ poor crystalline compound, CuO* crystalline compound, ** v. good crystallization
3.2.2. Catalytic properties of Cu-La2Zr207 catalysts Both catalytic systems even calcined at 550 or 710~ are active in methanol synthesis in presence of CO2 + H2. Comparing the catalysts annealed at 550~ the Cu-LaZr [ex carbonate] sample is the most active due to the higher copper surface area (e.g. 12 m2 compared to 9 m2) as shown in figure 3. Considering the catalysts annealed at 710~ it can be observed that the system originating from the carbonates has a lower activity that the sample calcined at 550~ whereas on the Cu-LaZr [ex oxalate] system the methanol yield is increased if the annealing temperature goes from 550 to 710~ All these results can more or less be explained by the change of the copper surface area with the preparation and the annealing temperature as described in table 2. Finally each preparation technique needs the use of the best selected annealing temperature labelled # in table 2. It can also be observed that despite the same copper surface areas of 12 m2/g the CuLaZr ([ex carbonate] 550) system has a noticiable different catalytic behavior that Cu-LaZr ([ex oxalate] 710). This proves that, appart the copper surface areas, variation of morphology, presence of impurities as well as other factors related with the preparation can be responsible for difference of catalytic properties related to the preparation technique.
91 As extensively described elswhere [9, 16] the copper content of the catalyst and the La/Zr ratio play also a determining role on the catalytic behavior on Cu-LaZr catalysts.
MeOH Yield % MeOH Sel. %
[==! Y i e l d [ c a r b ] 550 l~l Yield[carb] 710 IrA Yield[ox] 550 Yield [ox] 710 -~" Sel [carb] 550 (x 0.1) 0 - Sel [ox] 710 (x 0.1)
m
250
270
,
300
r~
320
Figure 2. Influence of the preparation technique on the activity of Cu-La2ZrO7 catalysts
3.2.3. Aging of the Cu-La2Zr207 catalyst. At 300~ both methanol yield and methanol selectivities are decreasing during a 70 h. run. Figure 4 shows that no essential difference appears during this aging process if the Cupyrochlore (Zr/La ratio = 1) prepared via the oxalate or the carbonate intermediates are compared. Thus the catalysts obtained from the carbonate precipitation show a 39% loss of the MeOH yield and a 45% loss of selectivity. In the same conditions the decreases are respectively of 25% and 34% with the system originating from oxalates. The better crystallinity of this catalyst, in relation with a lower content of non bonded La203 responsible for total oxidation can explain these results. Only a sur-stoichiometry of Zr (Zr/La ratio of 1,2 or 1,6), which avoids also the presence of non bonded La203 particles, allows to obtain good aging properties (only 9% yield loss) as shown for Cu-LaZr [ex oxalate] (Zr/La= 1,6) catalysts.
Me( )H Yield %
I~ CuLaZr[carb] 1 17YJ CuLaZr[ox] 1 ml CuLaZr[ox] 1,6 -O- CuZnLaZr[ox] CuZnLaZr [carb]
m l
-O O
4t
~ l l
2
.
12
~ .
24
~/,
i ~ ! .
48
- l - Cu-'-'Zn[carb]+LaZr , me
(hours)
70
Figure 3. Influence of the preparation technique on the aging of Cu-La2ZrO7 and CuZn-La2ZrO7 catalysts.
92
3.3.Copper-zinc-pyrochiore
catalysts.
The interesting promoting effect of ZnO on Cu-La2ZrO7 catalyst has been proven in previous studies [9,16] as well as the favorable effect of doping copper-zinc catalysts with rare earth oxides [6]. To illustrate the effect of the preparation technique on the characteristics and on the catalytic activity of these systems, three samples of catalytic material with the weight composition Cu (33%), ZNO(33%), La2Zr207 (34%) are compared.
1 - precipitation of the precursors from aqueous solutions of the metal salts by the use of Na2CO3. 2 - precipitation of the precursors from ethanolic solutions of the salts using oxalic acid 3 - precipitation of the precursor using Na2CO3 as described in position 1 but in the presence of a La2Zr207 slurry. The physical characteristics of the different samples, summarized in table 3 show that optimal annealing conditions have to be chosen for each catalytic system to have both a good copper surface area and a reasonnable crystallization of the La2Zr207 pyrochlore.
3.3.1. Characterization of CuZnO-La2Zr207 catalysts From table 3 it appears that the highest copper surface area associated with the presence of La2Zr207 pyrochlore structure is observed after annealing the CuZn+LaZr catalyst at 350~ the CuZn-LaZr [ex carbonate] system at 550~ and finally the CuZn-LaZr [ex oxalate] sample at 710~ The comparison of the catalytic activities can only be achieved by the use of these optimized systems labelled (#) in table 3. Table 3 Caracteristics of CuZnO-La2Zr207 catalysts Catalysts T~ SBET (annealing) (m2/g) CuZn-LaZr 550# 50 [ex carbonate] 710 22 CuZn-LaZr [ex oxalate] CuZn+LaZr [ex carbonate]
SCu (m2/g) 20 5
550
27
19
710#
16
15
350#
52
31
550
12
2
XRD CuO* ZnO* La2ZrO7~ CuO* ZnO* La2Zr207* CuO* ZnO* CuO* ZnO* La2Zr207* La2Zr207*
CuO* ZnO* La2Zr2OT* 710 2 <1 CuO* ZnO* La2Zr207* o poor crystalline compound, CuO* crystalline compound, ** v. good crystallization
3.3.2. Catalytic activity of CuZnO-La2Zr207 systems. In the presence of CO2+H2 the observed conversion and methanol yields can be roughly related to the copper surface areas measured for the different catalyst samples. Thus the CuZn+LaZr [ex carbonate] catalyst with 31 m2/g copper surface area shows the best catalytic activity. For the same catalyst, the methanol selectivities illustrated in figure 4 are generally lower than those of the other samples. This phenomenon can be related to the presence of La2ZrO7 which is not in interaction with copper and which induces mainly the reverse water gas shift reaction.
93 The CuZn-LaZr [ex oxalate] system shows a lower activity but a higher selectivity. On the other hand this catalyst seems to have a better thermal stability since its activity is maintained at high temperature (320 ~ Compared with the unpromoted catalysts the positive effect of the addition of ZnO on Cu-LaZr catalysts can be revealed by comparing the data of paragraph HI to that of paragraph II The promoting effect of ZnO at 300~ for the different catalysts preparations can be evidenced by comparing figures 2 and 4. Thus a MeOH yield increase of 60 % for (Cu-LaZr [ex oxalate] and of 100 % for (CuZn-LaZr [ex carbonate] catalysts can be observed. M e O H Yield % M e O H Sel. % [~ Yield [ox] l=:l Yield [carb] ~i Yield [carb] + LaZr
"O- Sel [ox] (x 0.3)
10
Sel [carb] (x 0.3)
Sel [carb] + LaZr (x0.3)
0
1 '~ 250
270
300
320
Figure 4. Influence of the preparation on the activity of CuZn-La2ZrO7 catalysts 3.3.3. Aging of the C u Z n - L a Z r
catalysts.
Figure 3 shows that in a 70 h. run at 300~ both the methanol yield and the methanol selectivity are decreased for all the three samples The CuZn-LaZr [ex oxalate] catalyst seems to be the most stable (yield loss 7%) and the less active one. The pyrochlore promoted copperzinc catalyst (CuZn + LaZr) shows also a good stability (yield loss 3.5%) better in any case than the tested unpromoted Cu-Zn catalyst (9%) . Finally the CuZn-LaZr [ex carbonate] samples given its low pyrochlore crystallization undergo to the highest deactivation (yield loss 14%). 3.4. R e l a t i o n b e t w e e n catalytic mechanistic informations. H ~-- ~ O.~-CH3OH
CO+H2 --~C -
I
~'. C O2 §
~:
....,,
~
~....
-
"''-"', I)e(
_L _L Formate
..J_ Carbonate Figure 5
characteristics
/ II
O
H
--,
C /f"~\
catalysts H
\
.,,.~
~ O
H
H
".~,~rm-,,'~"?IkI
_ _
"'#"
activity,
Formaldehyde
H
\c/
/
O _L
H
H
\1/ H
,/
\
O ..J_
C
CH3OH
I
o
_.L
Methoxy
Dioxymethylene
Mechanisms of CO2 hydrogenation to methanol
and
94 The previous results will be discussed on hand of mechanistic proposals described in the literature [ 17-20] and which can be summarized as on the scheme represented in figure 5. An important step in this mechanism is the formation of the formate species after chemisorption of CO2 on copper sites. Thus the number of metallic copper sites and of formate species as well as the stability of these intermediates are likely to be in relation with the catalystic activity as shown in table 5. Table 5 Correlation between catalytic activit,r and catalysts characteristics Catalysts T~ SCu Formates ** (m2/g) (A.U.)
MeOH* (yield %)
Cu-Zn [carbonates]
350
37
17
8.8
Cu-Zn [oxalate] Cu-LaZr [carbonates]
350 550 710 550 710
21 12 7 9 12
14 17 13 15 17
6.3 4.1 3.0 3.8 4.0
Cu-LaZr [oxalates] Cu-LaZr [oxalates] (10% Cu)
710
6
11
2.1
Cu-LaZr [oxalates](33% Cu)
710
12
17
4.0
Cu-LaZr [oxalates](50% Cu)
710
17
18
4.3
Cu-LaZr [oxalates]
710
12
17
4.0
Cu-LaZr [carbonates]
710
12
17
4.1
CuZn-LaZr [oxalates]
710
15
16
6.4
CuZn-LaZr [carbonates]
550
20
6
8.5
CuZn + LaZr [carbonate] 350 * T= 300~ ** calcination temperature
31
18
8.9
The use of temperature programmed desorption makes possible a qualitative and a quantitative determination of formate species as well as their localization on the copper or on the support [21]. Figure 6 shows that after chemisorption of CO2 (or MeOH given the reversibility of the reaction) a CO2 desorption attributed to copper formates can be observed in the temperature range 160-200~ whereas the desorption above 300~ corresponds to formates or carbonates located on the support [21 ]. The relation between the copper surface areas, the amount of formates and the catalytic activity is given in table 5. For copper-zinc catalysts if the same preparation technique [carbonates] or [oxalates] is used a nearly linear correlation can be established between the copper surface area, the amount of formates and the methanol yield. But if the preparation technique is changed the formates and methanol formation, related to a unit of copper surface area, is not maintained and the observed correlation is restricted to catalysts prepared by the same technique as shown in figure 7. For copper-pyrochlore catalysts an almost linear correlation can be also established between the copper surface area, formate and methanol formation can be observed. The change of the of copper loading leads in figure 8 to a good correlation between the same values (copper surface area, amount of formates, methanol yield).
95
co2
(A.U.)---q "-"-O---
CuLaZr (+ CO2) M e O H Yield % CuLaZr (+ MeOH)formate s (A.U.)
20
20
oo
10
10 0
-0
~ '
100
'
T~ '
200
,
'
300
,
,
0~
0 D
I
Yield [ox] 270 formate [ox] Yield [carb] 270
D
formate [carb]
O
0
D
D
D
'
I
m
2
/
g
)
I
400
500
0
Figure 6 Determination of formates using TPD of CO2
10
20
30
40
50
60
Figure 7 Correlation between MeOH yield, Formates and S. Cu on Cu-Zn catalysts
The promoting effect of a ZnO addition to Cu-LaZr catalysts cannot be correlated with an increase of copper surface area or copper formate formation. On figure 9 three CuZn-LaZr catalysts with the same composition, but prepared by different ways, are compared and not any correlation can be observed between the catalytic activity and the characteristics of the catalytic materials. According this result the role of zinc oxide can be attributed rather to the enhancement of the hydrogenating properties (by formation of H...ZnO species) that to the formation of formates [22]. Thus a small amount of ZnO (1 wgt. %), well located on the surface of the catalyst using the excipient wetness impregnation technique, allows a comparable activity (methanol yield 7.9% at 300~ ) that a catalyst promoted by 33% ZnO using the coprecipitation technique [23]. Me S. for
D Yield 300~ [D S. Cu (x 0.3) O formate (x 0.3)
[
=
z
Cu z
1
sts
0 10
33
50
55
Figure 8 Correlation between MeOH yield, Formates and S.Cu on Cu-LaZr catalysts
1
2
3
4
5
Figure 9 Correlation between MeOH yield, Formates, S.Cu on CuZn-LaZr catalysts 1= Cu-LaZr [ox]; 2= CuZn-LaZr [ox]; 3= CuLaZr [carb]; 4= CuZn-LaZr [carb]; 5= CuZn + LaZr [carb].
96 4. C O N C L U S I O N The preparation technique of the methanol catalysts plays an important role on their catalytic behavior. For each catalytic composition a specific preparation technique is able to optimize the catalytic properties. The precipitation by Na2CO3 seems to be the most suitable way for the preparation of copper zinc catalysts and also of promoted systems. But the use of precipitation of mixed oxalates allows the best methanol yield on copper-pyrochlore catalysts. In most of the catalysts composed of copper zinc and copper-pyrochlore a correlation between the copper surface area, the amount of formates located on the copper sites after CO2 or methanol chemisorption, and the catalytic activity can be found, but in most cases the relation is not strictly linear. The promoting effect of ZnO on Cu-LaZr catalysts cannot be ascribed to an enhancement of copper coverage or formate formation. Zinc plays therefore rather a positive role in the formate hydrogenation than its formation. REFERENCES
[1 ] I. Inui, Catal. Today, 29 (1996) 329 [2] M. Fujiwara, R. Kieffer, L. Udron, H. Ando, Y. Souma Catal. Today 29 (1996) 343-348 [3] H. Arakawa, J.L. Dubois, K. Sayama, Proceedings 1st Int. Conf. on Carbon Dioxide Removal Amsterdam March 1992 [4] Skrypek, J. Sloczynski, S. Ledakowicz, "Methanol Synthesis" Pol. Scientific Pulishers, Warszawa 1994 [5] J. Bart, R.P..A. Sneeden, Cat. Today, 2 (1989) 1 [6] E. Ramaroson, R. Kieffer, A. Kiennemann, Appl. Catal. 4 (1982) 281 [7] Y. Amenomiya, A. Emesh, K.W. Oliver, G. Pleizier, Proceedings 9th I.C.C. Calgary 2, 634 (1988); Appl. Catal. 18,285 (1985) [8] R. Kieffer, M. Fujiwara, L. Udron, Y. Souma Catal. Today 36 (1997) 15-24 [9] R. Kieffer, Proceedings 6th International Conference on Ferites Tokyo, 29-09 - 2-12/1992, (paper ApS-3) p. 254 [ 10] P. Chaumette, A. Kiennemann, R. Kieffer, P. Poix, J.L. Rehspringer, Eur. Patent, 92902115-2 (1992) [11] D. Andriamasinoro, R. Kieffer, A. Kiennemann, J.L. Rehspringer, P. Poix, A. Vallet, J.C. Lavalley, J. Mat. Science, 24 (1989) 1757-1766 [12] R.G. Hermann, K. Klier, G.V. Simmons, B.P. Finn, J.B. Bulko, T.P. Kobylinsky, J. Catal., 56 (1979) 56 [13] J. G. Nunan, R.G. Herman, K. Klier, J. Catal., 116 (1989), 222 [ 14] B. Denise, O. Cherifi, M. Bettahar, R.P.A. Sneeden, Appl. Catal. 48 (1989) 365 [15] R. Collongue, F. Queyroux, M. Perez, Y. Jorba, J.C. Gilles, Bull. Soc. Chim. Fr. (1965) 1141 [16] R. Kieffer, P. Poix, J. Harison, L. Lechleiter International Conference on CO2 Utilisation Bari 26-30/09/1993 [17] A. Kiennemann, J.P. Hindermann, Stud. Surf. Sci. Catal., 35 (1988) 181-255 [18] G.C. Chinchen, K.C. Waugh, J. Catal. 97 (1986) 280-283 [19] A. Ya. Rozovski, Y.B. Kagan, G.I. Lin, E.V. Slivinskii, A.N. Bashkirov, Kinet. Katal. 16 (1975) 809 [20] T. Kakumoto, Energ. Conv. Management 36 (1995) 661 [21 ] A. Kiennemann, H. Idriss, R. Kieffer, P. Chaumette, D. Durand, I. and E.C. Research 29, 1130 (1991) [22] R. Burch, R.J. Chapell, M.S. Spencer, J. Chem. Soc. Faraday Trans. 86 (1990) Catal. Lett. 5, (1990) 55 [23] L. Udron, Ph.D. Thesis, Universit6 Louis Pasteur Strasbourg 1997
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
97
Photochemical carbon dioxide reduction w i t h m e t a l complexes: Differences between cobalt and nickel macrocycles Etsuko Fujita, a Bruce S. Brunschwig, a Diane Cabelli, a Mark W. Renner b Lars R. Furenlid, c Tomoyuki Ogata, a,d Yuji Wada, d and Shozo Yanagida d a Chemistry Department, b Department of Applied Science, and c National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973-5000, USA, d Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565, Japan
1. INTRODUCTION Problems related to the increase of greenhouse gases in the atmosphere and the depletion of fossil fuels have made the conversion of CO2 into useful chemicals and fuels an important area of research. However, CO2 reduction poses many scientific challenges. Despite intense interest in photochemical and electrochemical CO2 reduction, the kinetics and mechanism of the reduction remain unclear in many systems. A number of 14-membered tetraazamacrocyclic complexes serve as catalysts for photochemical and electrochemical CO2 reduction. [CoHMD(H20)](C104)2 (HMD = 5,7,7, 12,14,14-hexamethyl- 1,4,8,11-tetraaza-cyclotetradeca-4,11-diene) [ 1,2 ] and Ni(cyclam)Cl2 (cyclam = 1,4,8,11-tetraazacyclotetradecane) [3] have been used as electrocatalysts for the reduction of CO2 in H20 or aqueous CH3CN. The ratio for CO/H2 production is -1 for [CoHMD(H20)](C104)2 and > 100 for Ni(cyclam)Cl2. Metal(I) complexes, metal(III) hydride complexes, and metallocarboxylates such as [NiIII(cyclam)(COz2-)]+ are postulated as intermediates in the electro- and photo-chemical CO2 reduction [4]. Our research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO2 reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis, X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here we summarize our research on photochemical carbon dioxide reduction with metal macrocycles.
2. NATURE OF C o - C O 2 ADDUCTS We and others have characterized the interaction of low-spin d 8 CoIH]V[D + with CO2 in CH3CN [5-9] and in H20 [10,11]. Schmidt et al. [12] have characterized the binding thermodynamics as a function of organic solvents. The chiral N-H centers of the macrocycle give rise to two diastereomers, N-rac and N-meso. The CoL complexes are shown below.
98 The equilibration between the N-rac- and N-meso cobalt(II) isomers is slow in acidic aqueous and organic media, but equilibration of the two cobalt(l) isomers is relatively rapid (>2 x 10-3 s -1) in CH3CN.
N
N
~N
1~
E rac
HMD
3
meso H M D
The CO2 binding constants of the corresponding [CoHMD] + isomers are quite different: Nrac-[CoHMD] +, (1.2 + 0.5) x 104 M-I; N-meso-[CoHMD] +, 165 +_ 15 M -1 [5,6]. While hydrogen bonding interactions between the bound CO2 and amine protons of the macrocycle will tend to stabilize both adducts, the N-meso adduct is destabilized by the steric repulsion by the macrocycle methyl group. Although the N-rac-[CoHMD(CO2)] + adduct decomposes to N-rac-[CoHMD] 2+ and CO in wet CH3CN [5], it is stable enough to handle in dry CH3CN under a CO2 atmosphere. The complex is thermochromic [6,7] being purple at room temperature and yellow at low temperature (-100 ~ as shown in Figure 1. The equilibrium between five-coordinate [CoHMD(CO2)] + (purple) and six-coordinate [CoHMD(COz)(CH3CN)] + (yellow) has been studied by UV-vis, 1H NMR, FT-IR, XANES and EXAFS in CH3CN [6,7,9] [CoHMD] + + CO2 ~
[CoHMD(CO2)] +
[CoHMD(CO2)] + + CH3CN ~
[CoHMD(CO2)(CH3CN)] +
Ks = [CoHMD(CO2)(CH3CN) +] / [CoHMD(CO2) +]
(1) (2) (3)
The singular value decomposition (SVD) [13-15] spectral analysis of the temperaturedependent UV-vis data between 26 and -40 ~ is consistent with the presence of two species in CH3CN. The fit gives AH ~ -7.0 kcal mo1-1 and AS ~ -27 cal K -1 mo1-1 for eq. 2 [7]. The equilibration is rapid on the NMR time scale. The pressure dependence of the equilibrium constant shows that increasing pressure shifts the equilibrium toward the six-coordinate species with an overall reaction volume of AV ~ -17.7 + 1.0 mL mo1-1 at 15 C ~ in CH3CN [16]. The FT-IR spectra measured over the range 25 to -75 ~ in CD3CN and in a CD3CN/THF mixture indicates [7] the existence of four CO2 adducts with and without intramolecular hydrogen bonds between the bound CO2 and the amine hydrogens of the ligand: a five-coordinate, non-hydrogen-bonded form (vc=o = 1710 cm "1, VNH = 3208 cm'l), a five-coordinate hydrogen-bonded form (vC=O = 1626 cm'l), a six-coordinate non-hydrogen bonded form (vc=o = 1609 cm "1, VNH = 3224 cm-1), and a six-coordinate hydrogen-bonded form (v C=O = 1544 c m " 1, VNH - 3145 cm" 1).
99
2(){)(),() E
I
a
. L
'
:A j
"~ lOOO.o
b
51111.{1
0.11
401)
5(X)
6(X)
700
800
Wavelength, nm
Figure 1. UV-vis spectra of [CoIHMD] + (a), [CoHMD(CO2)] + at room temperature (b), and [ColIIHMD(COz2-)(CH3CN)] + at-100 C ~ (c)in CH3CN. X-ray absorption spectroscopy is an attractive tool for the characterization of metal complexes in solution. The metal coordination number, geometry, and electronic properties can be studied using XANES and the metal-ligand bond distances are obtained through analysis of EXAFS. Previous work [17-19] has also shown that the edge energy correlates with the oxidation state of the metal. The XANES spectra (Figure 2) for a series of CoHMD complexes [9] indicate that the edge positions (E()) are sensitive to the oxidation state of the metal.
1.2
-A
'
I
'
I
'
I
'
I
'
1.2 .
=""
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I
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I
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7725
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,
77O5
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Energy (eV)
Figure 2. XANES spectra for a series of CoHMD complexes in various oxidation and ligation states. (A) [ColIHMD](CIO4)2 in acetonitrile at 150 K ( ), five-coordinate [CoHMD(CO2)]CIO4 in acetonitrile at room temperature ( . . . . ) 03) [ColHMD(CO)]C104 in acetonitrile at room temperature ( _ _ - - ) , [ColIHMD](C104)2 in acetonitrile at 150 K ( ), six-coordinate [CH3CN-CoHMD(CO2)]C104 in acetonitrile at 150 K ( _ _ __.) and [ColIIHMD(CO32")]C104 in H20 at room temperature ( . . . . ).
100 The edge energy, relative to [CoIIHMD] 2+, decreases 1 eV upon reduction and increases 2 eV upon oxidation. As seen from Figure 2A, the E0 for five-coordinate [CoHM (CO2)] + at room temperature is similar to that of [CoIIHMD] 2+. This is consistent with theoretical predictions [20,21] that the bound CO2 receives 0.71 electrons mainly from the Co dz 2 orbital. The sixcoordinate [CoHMD(CO2)(CH3CN)] + species shows a 1.2 eV shift towards Co(III) and is interpreted as a Co(III)-CO22- carboxylate complex. Although the Co(III) carboxylates have been postulated as intermediates in CO2 reduction and water-gas shift reactions, the XANES results provide the first unambiguous evidence that active metal catalysts, such as [CoIHMD] +, can promote two-electron transfer to the bound CO2 and thereby facilitate its reduction.
3. P H O T O C H E M I C A L CO2 REDUCTION W I T H COBALT M A C R O C Y C L E S : MECHANISTIC AND KINETIC STUDIES Our previous studies indicated that cobalt macrocycles mediate the photoreduction of CO2 to CO with p-terphenyl (TP) as a photosensitizer and a tertiary amine as a sacrificial electron donor in a 5:1 acetonitrile/methanol mixture [22]. The system enhances the activity of the TP by suppressing the formation of dihydroterphenyl derivatives and produces CO and formate efficiently with only small amounts of H2. The total quantum yield of CO and formate is 25% at 313 nm in the presence of triethanolamine (TEOA) and Co(cyclam) 3+. Transient absorption measurements provide evidence for the sequential formation of the p-terphenyl radical anion (TP'-), the CoHMD + complex, the [CoHMD-CO2] + complex and the [S-CoHMD(CO2)] + complex (S = solvent) in the catalytic system containig triethylamine (TEA) [23 ]. The electron-transfer rate constant (k6) for the reaction of TP'- with C o I I H M D 2+ is 1.1 x 1010 M -l s -1 and is probably diffusion controlled because of the large driving force (--- 1.1 V). Flash photolysis studies yield a rate constant (k7) of 1.7 x 108 M -1 s-1 and an equilibrium constant of 1.1 x 104 M -1 for the binding of CO2 to CoHMD +. These values are consistent with those previously obtained by conventional methods in CH3CN [5]. TP
*
+ TEA
k
4 :~TP'-+ TEA +
(4)
~
T P ' - + CO 2 TP'-
k5 }TP
kc'' } TP + CoIHMD §
+ C o II H M D 2+
CoIHMD + + CO 2
+ CO 2
~
(5)
(6)
CoIHMD(CO2) * k-v
k8 CoIHMD(CO2) + + S ~ k_8
(7) [SCoIIIHMD(CO22-)] +
(8)
101
Kco~
[CoHMD - CO2 ] _ AA = [CoIHMD+][CO2] - A~[CO2----------~
[S-CoIIIHMD-(CO22")] + + HA ~
(9) [S-ColIIHMD-(COOH)'] 2+ + A"
(10)
HA = TEA, MeOH, TEAH + [S-CoIIIHMD-(COOH)-]2+ + e- ~
ColIHMD 2+ + CO + OH-
(11)
e" = CoIL +, Et2NC'HCH3, TP'OH" + CO2 ~
HCO3"
(12)
In eq (9) the ratio of [CoHMD-(CO2)] + / [CoIHMD] + is given by AA/Aoo where AA = Aoo- Ao, and Aoo and Ao are the absorbances at 670 nm at long times and at t = 0, respectively. The dependence of the decay rate of TP'- on [CO2] in the absence of the cobalt macrocycle (eq 5) is not linear. We estimate a rate constant k5 < 106 M -1 s-1 for electron transfer between TP'- and [CO2]. This rate constant is consistent with the large reorganization energy of the CO2/CO2"couple (associated the geometry change from a linear to a bent molecule) [24,25] and small driving force for the reaction (0.3 V). Under our photocatalytic conditions the cobalt reacts with the TP'" > 20 times faster than does the CO2. Thus the direct reduction of CO2 by TP'- plays a negligible role here and all of the photochemically generated reducing equivalents are captured by the cobalt macrocycle. The production of CO from CoL(CO2) + requires a second reducing equivalent. The source of this equivalent is of interest. Under flash photolysis conditions the TP'- has completely reacted before the CoL(CO2) + is formed. On the other hand, under continuous photolysis T P ' - can react with the ColIL 2+ or the CoL(CO2) + complexes. In the flash photolysis, where only Et2NC'HCH3 and/or CoIL + may act as the electron donor, the decomposition of CoLCO2 + is slow owing to the low concentrations of these two species. In fact, since CoLCO2 + decomposes faster with low [CO2] (i.e. higher [COL+]), CoL + is the likely electron donor under flash photolysis conditions. We suggest that reactions 10-12 are responsible for the production of CO in the photolysis. The slow step is likely to be the C-O bond breakage of the bound carboxylic acid with either Et2NC'HCH3, or CoIL + acting as electron donor. Unfortunately the UV-vis transient spectrum of [S-ColIIHMD(CO22-)] + is too weak to study the proton dependence of its disappearance.
4. P H O T O C H E M I C A L CO2 REDUCTION WITH NICKEL M A C R O C Y C L E S
4.1 Photochemical CO2 reduction In contrast to the cobalt-based system, small amounts of H2 and no CO are produced when nickel cyclam or other saturated 14-membered tetraazamacrocycles (L) in Figure 3 are used to replace the cobalt complex in the above system [22]. Flash photolysis studies indicate that the electron-transfer rate constant (k13) for the reaction of the p-terphenyl radical anion with NilI(cyclam)2+ is 4.3 • 109 M -1 s-1. However, when CO2 is added to the solution, the decay of the TP anion becomes slower! Flash photolysis studies of the acetonitrile solutions
102 suggest the existence of a minor pathway for MIL + formation that does not involve TP. When TEA (or TEOA) is used with UV excitation (<320 nm), a minor pathway is observed that can be suppressed by the addition of methanol in the case of CoHMD 2+, but not Ni(cyclam) 2+. TP'- + NiXlcyclam2+
kl_~ - >TP + Ni xcyclam +
k14
Nilcyclam+ + CO 2 ~ ~
(13)
Nilcyclam(CO2) +
k-14
(14)
Both NiL + and NiL(CO2) + species are formed under COa atmosphere by irradiation at 313 nm in acetonitrile solutions containing TEA and NiL 2+. In order to understand the interesting behavior of these nickel-based systems we have studied the nature of the ground-state complexes, electrochemical CO2 reduction, and the differences in CO2 binding between cobalt and nickel macrocycles.
eq.... IIu~
ax
~"~
eq
NH HN/Itlt' ..... ~ ~
ax
RSSR-HTIM
N~~j~ MCC
RSSR-HTIM
.eq
~
e
NH HN- ~
eq ~
RRSS-HTIM
eq
~ NH HN
NH HN
NH NH
~ , , , , ....
NH NH
DMC
cyclam
NH~NH
NHLv,,jNH
NHL~NH
MTC
TM
OMC
RRSS-HTIM
cyclam,Tralt~ III
Figure 3. Structures and geometries of metal macrocycles
cyclam,Trans I
103 4.2 Electrochemical CO2 reduction with nickel macrocycles The electrocatalytic activity of various nickel macrocycles in aqueous solution was studied. Cyclic voltammograms indicate that RRSS-NiHTIM 2+, NiMTC 2+ and NiDMC 2+ are better catalysts than Ni(cyclam) 2+ in terms of more positive potentials and/or their larger catalytic currents [26]. Bulk electrolyses with 0.5 mM Ni complexes confirm that these complexes are excellent catalysts for the selective and efficient CO2 reduction to CO. The macrocycles with equatorial substituents showed increased catalytic activity over those with axial substituents. These structural factors may be important in determining their electrode adsorption and CO2 binding properties.
4.3 Properties of NiIIL 2+ complexes Ni(cyclam) 2+ is oxidized at 0.98 V and reduced at -1.45 V vs SCE in CH3CN under argon. Under a CO2 atmosphere the reduction wave shifts about 10-20 mV more positive, indicating a very small binding constant in CH3CN. When TEA is added to the solution under argon, the reduction remains at -1.45 V as shown in Figure 4. The oxidation potential is not observed due to the oxidation of TEA. The CV under a CO2 atmosphere shows a reversible oxidation at 0.31 V. The reduction becomes irreversible and occurs at a very negative potential, -1.8 V in TEA-containing CH3CN (Figure 4). This indicates that the [NiI(TEA)(CO2)] + adduct is unstable. This also explains the slower decay of TP'- under CO2 atmosphere, since the driving force for electron transfer from the TP'- to [NilI(cyclam)] 2+ becomes smaller upon addition of CO2.
I
I
I
I
I
1 10.4
=,
10 .4
-2
10 .4
I n
I
1000
9
nu
I 9
| i
m" 9 i
0 I0~
r~ -1
um 1
m 9m
I
0
i
I
- 1000 Potential, mV
I
I
-2000
Figure 4. CV for 1 mM Ni(cyclam) 2+ with TEA under Ar (solid) and CO2 (dot) in CH3CN.
104 The reaction of NiII(cyclam)2+ with TEA/CO2 was monitored by UV-vis and FT-IR. The d-d absorption intensity of NiII(cyclam)2+ decreases with TEA binding in CH3CN and shifts to lower energy with CO2 binding in a TEA-containing CH3CN solution as shown in Figure 5. Both TEA and CO2 binding are reversible. The IR spectrum of the [Ni(cyclam)(TEA)(CO2)] 2+ adduct indicates two kinds of CO stretching bands at 1615 and 1653 cm -1 due to two isomers (lrans I and III in Figure 3). With RRSS-NiIIHITM 2+, a single isomer, we observed only one CO stretching band at 1630 cm -1. We have determined CO2 binding constants for both Ni(I) and Ni(II) in CH3CN. The CO2 binding constant to Ni(II) cyclam is 1000 M -l, much larger than that of Ni(I). (See below.)
0.16
i
|
i
i
!
i
-B
I
I
I
I
I
I
I
I
I
I
0.12
0.12 -~ <
I
().()8
0.()8
<
0.04
0.04
()
()
i
-
450
550 Wavelength,nm
35(1
650
45O 55{) Wavelength,nm
35{)
T--4 I
650
Figure 5. A: Spectral change of NiII(cyclam)2+ by the addition of two equivalent of TEA in CH3CN. B Spectral change of [NiII(cyclam)(TEA)2] 2+ by the addition of CO2 in a TEA containing CH3CN.
Table 1 Differences in CO2 binding and pKa of ML(H') 2+ rac-CoHMD + Ni(cyclam) + RRSS-NiHTIM + NiTM + KCO2 in CH3CN (M "l)
1.2 • 104
KCO2 in H20 (M "l)
4.5 • 108a
kco2 in CH3CN (M'ls -l)
4
<1
11, 16b
6.0
<1
1.7 • 108
___10 7
_< 10 7
___
kco2 in H20 (M'ls -l)
1.7 • 108a
3.3 x 107b
3 • 10 7
---
pKa of hydride in H20
11.4 a
a ref. 11, b ref. 28
4
1.8 b
1.9
< 0.5
105 This behavior was not observed when H20 was used instead of CH3CN. [NiII(cyclam)] 2+ reacts with both TEA (or OH-) and CO2 in H20 to form a carbonate-bridged dimer, [(Nicyclam)2(CO3)] 2+ (UV-vis: 352, 548 and 900 nm; vco2: 1517, 1460, 1374 cm-1). The structure was confirmed by an X-ray diffraction study [26]. CO2 binding constants of Co(I) and Ni(I), and the pKa of ML(H') 2+ are shown in Table 1. As can be seen, the CO2 binding constant for CoHMD + is much larger than those for the Ni macrocycles. In H20, the binding constants are larger than the corresponding values in CH3CN. CO2 binding constants for Ni macrocycles are very small, however we see some effect due to ligands. Complexes with axial methyl groups, such as NiTM, show almost no binding of CO2. The trend of the binding constants does not parallel the electrocatalytic activities which is RRSS-NiHTIM 2+ > Ni(cyclam) 2+ > NiTM 2+ [27]. The rate constants for CO2 binding by CoHMD 2+ are also about 10 times larger than those by Ni macrocycles. The pKa of cobalt hydride is 11.4, but the corresponding pKa values for the nickel macrocycles are less than 2. 5. CONCLUDING REMARKS CoHMD 2+ and Co(cyclam) 2+ are good catalysts for photochemical CO2 reduction because of the small CoIIL2+/CoIL + reorganization energy, the fast CO2 binding to CoIL+ (1.7 x 108 M -1 s -1) and the large KCO2. Our XANES results clearly indicate that active metal catalysts, such as [CoIHMD] +, can promote two-electron transfer to the bound CO2 (reduce CO2 to CO22-) and thereby facilitate reduction of CO2. However since CoIL + reacts with H + in CO2 saturated water (pH -4) the selectivity of CO2 reduction in water is not high. NiL 2+ (L = cyclam and its derivatives without axial groups) are excellent electrocatalysts for CO2 reduction. It is known that adsorbed NilL + is the active species, however the CO2 binding constants are not known. We find high selectivity for CO2 reduction due to the low pKa of the hydride. Ni(cyclam) 2+ may not be a good photocatalyst because of the large NiIIL2+/NiIL+ reorganization energy, small CO2 binding constant to NiL + and instability of the trivalent state. We note that TEA is not an innocent electron donor. It can bind to the nickel center and make the energetics unfavorable for CO2 reduction. The Ni(I) species is formed by irradiation of the solution containing [Ni(cyclam)(TEA)2] 2+ species at 313 nm probably due to the intramolecular electron transfer from TEA to Ni. ACKNOWLEDGMENT We thank Drs. Norman Sutin and Carol Creutz for their contribution to earlier work and for their helpful comments. Prof. Rudi van Eldik, Prof. Horst Elias, Prof. Kazuya Kobiro, Ms. Mei Chou, and Dr. David J. Szalda are acknowledged for high pressure work, preparation of NiHTIM 2+, preparation of NiTM +, single crystals of [(Ni(cyclam))2(CO3)] 2+, and its structural determination, respectively. We gratefully acknowledge financial support for travel from the Monbusho International Scientific Program: Joint Research (No.07044148). This research was carried out at Brookhaven National Laboratory under contract DE-AC0276CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences.
106 REFERENCES .
2.
.
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26 27. 28
B. Fisher and R. Eisenberg, J. Am. Chem. Soc. 102 (1980) 7361. A.T.A. Tinnemans, T.P.M. Koster, D.HM.W. Thewissen, and A. Mackor, Recl. Trav. Chim. Pays. -Bas 103 (1984) 288. M. Beley, J.-P. Collin, R. Ruppert, and J.-P. Sauvage, J. Am. Chem. Soc. 108 (1986) 7461 N. Sutin, C. Creutz,and E. Fujita, Comments Inorg. Chem. 19 (1997) 67. E. Fujita, D.J. Szalda, C. Creutz, and N. Sutin, J. Am. Chem. Soc. 110 (1988) 4870. E. Fujita, C. Creutz, N. Sutin, and D.J. Szalda, J. Am. Chem. Soc. 113 (1991) 343. E. Fujita, C. Creutz, N. Sutin, and B.S. Brunschwig, Inorg. Chem. 32 (1993) 2657. J.S. Summers, Ph.D. Thesis, Georgia Institute of Technology (1989). E. Fujita, L.R. Furenlid,and M.W. Renner, J. Am. Chem. Soc. 119 (1997) 4549. C. Creutz, H.A. Schwarz, J.F. Wishart, E. Fujita, and N. Sutin, J. Am. Chem. Soc. 111 (1989)1153. C. Creutz, H.A. Schwarz, J.F. Wishart, E. Fujita, and N. Sutin, J. Am. Chem. Soc. 113 (1991) 3361. M.H. Schmidt, G.M. Miskelly, and N.S. Lewis, J. Am. Chem. Soc. 112 (1990) 3420. G.H. Golub and W. Kahan, J. SIAM Numer. Anal., Ser. B 2 (1965) 205. J. Hofrichter, E.R. Henry, J.H. Sommer, R. Deutsch, M. Ikeda-Saito, T. Yonetani, and W.A. Eaton, Biochemistry 24 (1985) 2667. R.I. Shrager and R.W. Hendler, Anal. Chem. 54 (1982) 1147. E. Fujita and R. van Eldik, Inorg. Chem. submitted. A. Manthiram, P.R. Sarode, W.H. Madhusudan, J. Gopalakrishnan, and C.N.R. Rao, J. Phys. Chem. 84 (1980) 2200. M.D. Wirt, M. Kumar, S.W. Ragsdale, and M.R. Chance, J. Am. Chem. Soc. 115 (1993) 2146. M.D. Wirt, I. Sagi, E. Chen, S.M. Frisbie, R. Lee, and M.R. Chance, J. Am. Chem. Soc. 113 (1991) 5299. S. Sakaki and A. Dedieu, J. Organomet. Chem. 314 (1986) C63. S. Sakaki and A. Dedieu, Inorg. Chem. 26 (1987) 3278. S. Matsuoka, K. Yamamoto, T. Ogata, M. Kusaba, N. Nakashima, E. Fujita, and S. Yanagida, J. Am. Chem. Soc. 115 (1993) 601. T. Ogata, S. Yanagida, B.S. Brunschwig, and E. Fujita, J. Am. Chem. Soc. 117 (1995) 6708. D.W. Ovenall and D.H. Whiffen, Mol. Phys. 4 (1961) 135. O.P. Chawla and R.W. Fessenden, J. Phys. Chem. 79 (1975) 2693. E. Fujita, M. Chou, and D.J. Szalda, to be published. E. Fujita, J. Haft, R. Sanzenbacher, and H. Elias, Inorg. Chem. 33 (1994) 4627. C.A. Kelly, Q.G. Mulazzani, M. Venturi, E.L. Blinn, and M.A.J. Rodgers, J. Am. Chem. Soc. 117 (1995) 4911
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
107
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Electrochemical reduction of C02 at metallic electrodes J. Augustynski, P. Kedzierzawski and B. Jermann D6partement de chimie min6rale, analytique et appliqu6e, Universit6 de Gen6ve, Sciences II, 30, quai E. Ansermet, CH-1211 Gen6ve 4
1.
INTRODUCTION
The quantities of carbon stored in the form of atmospheric carbon dioxide, C O 2 in the hydrosphere and carbonates in the terrestrial environment, substantially exceed those of fossil fuels. In spite of this, the industrial use of carbon dioxide as a source of chemical carbon is presently limited to preparation of urea and certain carboxylic acids as well as organic carbonates and polycarbonates. However, the situation is expected to change in the future, if effective catalytic systems allowing to activate carbon dioxide will become available. In this connection, the electrochemical reduction of CO2, requiring only an additional input of water and electrical energy, appears as an attractive possibility. For more than 100 years formic acid and formates of alkali metals were considered as the only significant products of the electroreduction of carbon dioxide in aqueous solutions. The highest current efficiencies, exceeding 90 %, were obtained with mercury and with amalgam electrodes [1-5] as well as at gallium, zinc, indium, tin, lead and cadmium [6-8] i.e., at the metals exhibiting high hydrogen overvoltages. It is, however, to be mentioned that under particular conditions (in the solutions containing quatemary ammonium salts) the same metal cathodes produced C2, C3 and C4 carboxylic acids [8]. The only comprehensive study regarding kinetics of CO2 reduction in aqueous solution has been performed by Eyring et al. [9,10] using a mercury cathode. The proposed mechanism includes two charge transfer steps, eq. 1 and 3, separated by a rapid chemical reaction, eq. 2. CO 2 + e + CO2"'(ad)
(1)
CO2"(ad) + H2O---~ HCO2(ad ) + OH
(2)
HCO2(ad ) + e --~ H C O 2"
(3)
The involvement of the radical anion CO2as the reaction intermediate at the mercury electrode has been confirmed by the photoemission measurements [11]. Small amounts of CO2"radical anions have also been identified at a lead cathode using modulated reflectance spectroscopy [12]. It is important to recall, in this connection, that both these cathodes exibit high overvoltages for the CO2 reduction (for the mercury electrode, for example, it exceeds 1V at a current density of l mA/cm2). This is consistent with a strongly negative value of the
108 half-wave potential of the reaction (1) close to -2V versus standard hydrogen electrode (SHE). Other "soft" metal cathodes, particularly indium and tin allow still to obtain formates with high current efficiencies but at lower overvoltages [6,7]. It was the observation by Hori et al. [13-15] that medium hydrogen overvoltage cathodes, gold, silver and copper are able to promote formation of gaseous products of CO2 electroreduction which has led to a marked regain of interest in this process. These authors have in fact demonstrated that the electrolysis of slightly alkaline solutions containing alkali metal hydrogen carbonates and CO2, when conducted at gold and silver, leads to the formation of carbon monoxide with faradaic yields attaining 100%. On the other hand, in the case of a copper cathode, the CO2 reduction continues further to form hydrocarbons-methane and ethylene as well as ethanol. These findings were rather unexpected as, for the long time, carbon monoxide together with oxalic acid had been considered as typical products of CO2 reduction in non-aqueous solutions [8]. Despite a large number of studies devoted, since more than 10 years, to the reduction of CO2 at the Au, Ag and Cu cathodes, several important features of this reaction remain still unclear. This concerns, in particular, (i) the nature of the first charge transfer step leading to the subsequent formation of CO and (ii) the reasons of deactivation of the above cathodes occurring during continuous electrolysis runs. The next paragraphs are devoted to the discussion of the most important aspects of the CO2 reduction at the copper cathode.
2.
EXPERIMENTAL
Electrolysis experiments conducted under atmospheric pressure were carried out in a two-compartment, tight Teflon| cell. The cathodic compartment contained ca. 30 cm 3 of electrolyte and was separated from the anodic one by a Nation| membrane. The cell was equipped with a cyclic gas flow system. Before each electrolysis run, CO2 supplied from a gas cylinder was passing through catholyte and gas circuit during 2 h to saturate the solution and to fill the system with CO2. The total volume of the gas enclosed in the system was 185 cm 3 and its circulation rate 12 cm 3 min l. High purity metals rods (7 mm in diameter, 99,999%) served as cathodes. All electrodes were mechanically polished with 1200 grit polishing paper and 0.3 ~tm alumina. The copper cathode was in addition subjected to an etching in 10 % HC1 for 15 s or, in some cases, to an anodic cleaning in concentrated H3PO 4. The oxide-modified Cu electrodes were prepared by attaching Cr203 and ZrO2 (Merck) powders to the mechanically polished copper surface by means of poly(vinylidenefluoride), PVDF, and annealing at 250~ in air. A suspension of Cr203 (ZrO2) in DMF containing dissolved PVDF (typical composition: 1 g oxide and 0.2 g PVDF per 5 cm 3 DMF) was deposited on a crosssection (0.28 cm 2) of Cu rods and dried in ambient air for 20 min before the final annealing. Solutions were prepared from reagent grade chemicals and twice distilled water. If not otherwise indicated, all potentials are given with respect to normal calomel electrode (NCE). In order to eliminate heavy metal contaminants, a constant current pre-electrolysis (25 ~tA/cm2) was usually conducted for at least 48 h, under nitrogen atmosphere, between two platinum electrodes separated by a Nation| membrane. 99.99% CO2 was further purified by passing through an activated charcoal filter before being introduced to the cathodic compartment of the cell and to the gas flow system. Electrolysis experiments under increased
109 pressure of C O 2 w e r e performed in a stainless steel autoclave including a two-compartment electrolysis cell. Analyses of the gaseous as well as solution products of the reaction were carried out on a Hewlett Packard 5890 Model gas chromatograph. The gas was sampled periodically with a gas syringe during the electrolysis, but products in the electrolyte were analysed once electrolysis run was completed. The chromatographic column Porapak HayeSepQ was employed to determine hydrocarbons and the reaction products in the solution, whereas analyses of CO were performed with the Carbosieve S-II column. 3.
RESULTS AND DISCUSSION
3.1 CO2 reduction at copper under atmospheric pressure The progressive poisoning affecting Au, Ag and Cu cathodes during electroreduction of CO2 [ 16-20] renders their behaviour in some ways similar to that of the catalysts employed in the gas phase hydrogenation of CO and CO2.
40
A) 15
................. 2 ............
40
,0
10 .~176176 ..... .---~ *"'~ .....
" ' ~ 1 7 6 1 7 6 1~7C6~1 7 6
ta.l
,i
o
......; .... i..... ;-'-0
.50
0 I00
"nME(mirO
,0if
50
----(:I-t4
io
15 lOg
-o- ~I--I4 ~
~.,~
5
' 0 0
50
100
150
100
150
TtME(ni~)
C3
rO~
0
150
4o
, IOE
~0~
I
.-~
.............................
-----13t4
5
i
0 0
50
100
150
Yn~(nm)
Fig. 1. Temporal evolution of faradaic efficiencies for methane and ethylene and of the cathodic current for CO2 reduction in 0.5 M KHCO3 at 22~ ; electrode potential, E =-2 V. 99.9 % and 99.999 % copper was used as electrode material, Figs A and C, respectively B and D. Solution was pre-electrolyzed in the case of C and D and without pre-electrolysis for A and B.
110 This poisoning is less severe for gold, where it results in a decay of the current (at constant controlled potential) without affecting faradaic efficiency of CO formation [18] than in the case of silver for which 11 (CO) tends to decrease as a function of time [ 17,21 ]. The heaviest deactivation is observed for the copper electrode, where CO2 reduction is virtually stopped after 20-40 min of continuous electrolysis, being replaced by hydrogen evolution [ 19,20]. This phenemenon is illustrated by a series of curves in Fig. 1. Importantly, the copper electrode behaved in a similar way whether the electrolyte was pre-electrolyzed or not. Moreover, XPS (X-ray induced photo-electron spectroscopic) analysis of the copper samples polarized for 2h at -2V in both kinds of 0.SM KHCO3/CO 2 solutions (ie, with and without pre-electrolysis) revealed only the presence of Cu, O and C signals. In particular, none of the metal elements present in trace amounts in KHCO3 such as, for example, Fe, Zn, Cr, Pb or Cd was detected. The time associated with the deactivation of the cathode was no more influenced by the degree of purity of the copper metal (99.999% vs. 99.9%). It is to be pointed out that the observed poisoning of the Cu electrode affects selectively the CO2 reduction while, at the same time, the rate of hydrogen evolution tends to increase (cf. variation of the electrolysis current vs. time in Fig. 1. This kind of behaviour can be expected in the case of formation of the elemental carbon [20] or of a layer of organic products [21,22] at the Cu surface. An in situ electrochemical activation method has been demonstrated to act very efficiently against the inhibition of the copper cathode versus the CO2 reduction. Such a treatment involves a periodic anodic stripping of the nascent poisoning species from the electrode surface by means of a series of 2-3 rapid voltammetric sweeps, repeated every 510 min. over the entire electrolysis run [16,19]. 50
--
40 --
. . . . . . after 28 h after 72 h
30 . . . . .
after 120 h
-)
i
~
201_
---------afler168h
10 0 ]
. ,~
.,....-~.~. 0
-10 --12 -~(~t~'>~-l:5"~
0.5
-30 -40 -50 POTENTIAL (V)
Fig. 2. Cyclic voltammograms (scan rate 5 V/s) representing sequences of the activation treatment of the Cu electrode (0.28 cm 2) performed periodically at different stages of a longterm electrolysis of CO2 in 0.5 M KHCO3.
111 The electrode activation requires less than 1 percent of the total electrolysis time (i.e., 23 s every 5 or 10 min.) and consumes a negligible extra amount of electrical charge. A typical series of cyclic voltammograms corresponding to the activation sequence is shown in Fig. 2. No features directly associated with the oxidation of the poisoning species are perceptible on any of these voltammograms. An initial increase of the current on the anodic side, connected with the oxidation of the copper surface is followed, during the cathodic sweep, by a large peak due to the reverse process. The increasing area under voltammograms is indicative of a continuous increase in the true surface area of the electrode. Application of such a potential program allows high faradaic efficiencies of CH4, C2H4 and C2HsOH to be maintained over long electrolysis runs. Thus, the amount of methane formed at an activated Cu electrode increases considerably along a 24h electrolysis run (cf. Fig. 3).
200, E 0
=- lO0-
ff
I
-2.2
-2.1
I
-2
I
-1.9
-1.8
I
-1.7
-1.6
POTENTIAL (V) Fig. 3. Methane formation rates recorded at the beginning and after 24h of continuous electrolysis runs performed at increasingly cathodic potentials. Activated Cu electrode, 0.5 MKHCO3/CO2 solution, 22~ As shown in Fig. 4, no electrode deactivation was observed during a 8-days long continuous electrolysis experiment. While the total rate of hydrocarbon formation remained remarkably constant as electrolyses progressed, an increase of the amount of ethylene accompanied by a decrease of the amount of methane were in general observed. These variations in the product distribution are probably associated with the structural changes occurring at the electrode surface and, in particular, with the increasing presence of Cu+(s) species [ 19,23]. Interestingly, experiments performed using Cu electrodes modified with attached oxide (Cr203,ZrO2) particles showed a dramatic change in the distribution of the CO2 reduction products [23]. In fact, under such conditions, ethylene and ethanol are formed in larger amounts than methane (cf. Figs 5 and 6). This is in contrast with the results of electrolyses conducted using a bare Cu cathode, under otherwise identical conditions, where methane remained the major product.
112
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T I M E (days) CH4
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Total
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Fig. 4. Faradaic efficiencies of C H 4 and C2H 4 vs. time for C O 2 reduction at an activated Cu electrode (0.28 cm 2) in 0.5 M KHCO3 at 22~ ; E = -2V. Dotted line shows evolution of the apparent current density during 8 days-long electrolysis run. 40
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0 500
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- ~ - " C2H4
. . . . . . CURRENT
Fig. 5. Faradaic yields for C2H 4 and CH 4 recorded together with cathodic current during electrolysis of CO2 at a ZrOE-modified, periodically activated, Cu electrode (0.28 cm 2) in 0.5 M K2SO 4 at 5~ ; E = -1.8 V.
113
4O
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-
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Fig. 6. Behaviour of a Cr203-modified Cu electrode ; all other features as in Fig. 5.
3.2 CO2 electrolysis
experiments
at a c o p p e r
cathode
conducted
under
increased
pressure
Because of a relatively small CO2 solubility in water under atmospheric pressure (0.033 mol/dm 3 at 15~ the reduction process meets limitations due to the slow transport of the reactant to the electrode. Such limitations appear clearly for an activated Cu electrode where a continuous increase in the cathodic current during prolonged electrolysis runs (cf. Fig. 4) leads finally to a decrease of the faradaic yield for hydrocarbons. To overcome this problem, several authors conducted electrolysis experiments at elevated CO 2 pressures. Ito et al. [24] examined the effect of CO 2 pressure up to 20 atm on its reduction at metals displaying high overvoltage for hydrogen evolution: Zn, In, Pb and Sn. They found that the yield of formic acid (the main reaction product at these electrodes) was increased at higher pressures. Nakagawa et al. [25] investigated the CO2 reduction at pressures up to 60 atm on group VIII metal electrodes: Fe, Co, Ni, Pd and Pt. At ambient pressure these metals yield almost exclusively hydrogen. At the CO2 pressures of J0 and 60 atm, CO was produced at all the above electrodes with faradaic efficiencies of 62 % for Pd, 37 % for Ni and 34 % for Pt. The same group of researchers investigated the effects of CO2 pressure, stirring and current density on the reduction product distribution at a Cu electrode [26,27]. They found that in order to obtain high faradaic efficiencies of hydrocarbons, a balance between CO2 supply and current density should be maintained. If the flux of CO2 was too high with respect to the current density, CO and HCOOH were the major products. If, on the contrary, CO2 was in a short supply (too low pressure for a given current or no stirring) water reduction to hydrogen prevailed. It should be stressed that all these results concern short term experiments.
114
60
I
50
-
o
40
-
u_
30
Z W m 0m LL. Ill
I
I
I
I
0
2.
_
20 <
la.
10 0
0
I
I
I
I
I
1.0
2.0
3.0
4.0
5.0
6.0
TIME/h
Fig. 7 Faradaic yields of hydrocarbons (CH 4 and C2H4) obtained during electrolyses of CO2 (20 atm) in 0.5 M KHCO3 at an activated, respectively, non-activated copper electrode ; 22~ E =-2.85 V.
Our work [28] has extended to significantly longer electrolysis periods in order to check whether the Cu cathode undergoes poisoning also under increased pressure conditions. As shown in Fig. 7, the deactivation of copper is, in fact, much slower than under ordinary pressure, even if the amount of produced hydrocarbons remains definitely smaller than at an activated Cu electrode. Typically, the highest faradaic yields of CH 4 and CzH 4 are reached at more negative potentials than under 1 atm CO2 pressure (of. Fig. 8), in spite of a careful compensation for the ohmic drop. However, the cathodic current densities recorded under a 20 atm CO2 pressure, exceeding 600 mA/cm 2, are almost 6 times larger than those attained during an electrolysis conducted under ordinary pressure. 3.3
N a t u r e o f the i n h i b i t i n g p r o c e s s e s at the C u e l e c t r o d e .
Recent surface enhanced Raman scattering (SERS) measurements, carried out during the reduction of CO2 on the Cu surface [29], provide some insight into a series of events associated with deactivation of the electrode. These include: (i) the time-dependent decay of the SERS bands corresponding to adsorbed CO paralleled by (ii) the increase of a new band attributed to the formation of a "patina" like species (including copper oxide, hydroxide and carbonate). At this stage, the latter surface compound appears as the most likely poisoning species.
115
70
60
-
50
-
z w 0
40
-
0
30
-
<
20
-
10
-
>0
LL LL W
O - CH 4 V
[]
- C2H 4
El - C H 4 + C2H 4
8 0
[] []
[] 0
u_
B v
0 -1.6
v
v I -2.0
v I
v I
-2.4
POTENTIAL N
-2.8
-3.2
NCE
Fig. 8. Effect of the Cu electrode potential upon faradaic yields of hydrocarbons, formed during electrolyses of CO2 under increased, 20 atm pressure in 0.5 M KHCO3 at 22~
On the other hand, the fact that graphitic carbon, visible on the Cu surface at less cathodic potentials (-0.64 V), is readily hydrogenated to CHx already at -1.54 V rules it out as a possible poison (the formation of a layer of graphitic carbon was evoked by De Wulf et al. [20] as the reason of deactivation of the Cu electrode observed in the course of CO2 reduction.
Acknowledgements - This research was supported by the Swiss Federal Office of Energy and, in part, by the Swiss National Science Foundation.
116 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] [25] [26] [27] [28] [29]
M.E. Royer, C.R. Acad. Sci. Paris 70/(1870) 73. A. Cohen, S. Jahn, Ber. Deutsch. Chem. Ges. 37 (1904) 2836. R. Ehrenfeld, Ber. Deutsch. Chem. Ges. 38 (1905) 4138. F. Fischer, O. Prziza, Ber. Deutsch. Chem. Ges. 47 (1914) 256. M. Rabinovich, A. Mashovets, Z. Elektrochem. 36 (1930) 846. K. Ito, T. Murata, S. Ikeda, Bull. Nagoya Inst. Technol. 27 (1975) 369. Yu.B. Vassiliev, V.S. Bagotzld, N.V. Osetrova, O.A. Khazova, N.A. Mayorova, J. Electroanal. Chem. 189 (1985) 271. G. Silvestri, S. Gambino and G. Filardo, in Carbon Dioxide Fixation and Reduction in Biological and Model Systems (Proceedings of the Royal Swedish Academy of Sciences Nobel Symposium 1991), 185-209, Oxford University Press, Oxford, 1994. W. Paik, T.N. Andersen, H. Eyring, Electrochim. Acta 14 (1969) 1217. J. Ryu, T.N. Andersen, H. Eyring, J. Phys. Chem. 76 (1972) 3278. D.J. Schiffrin, Faraday Discuss. Chem. Soc. 56 (1973) 75. A.-W.B. Aylmer-Kelly, A. Bewick, P.R. Cantril, and A.M. Tuxford, Faraday Discuss. Chem. Soc. 56 (1974) 96. Y. Hori, K. Kikuchi, and S. Suzuki, Chem. Lett. (1985) 1695. Y. Hori, A. Murata, K. Kikuchi, and S. Suzuki, Jr. Chem. Soc. Chem. Commun, (1987) 728. Y. Hori, K. Kikuchi, A. Murata, and S. Suzuki, Chem. Lett. (1986) 897. J. Augustynski, A. Carroy, A.S. Feiner, B. Jermann, J. Link, P. Kedzierzawski, and R. Kostecki, Proceedings of the International Conference on Carbon Dioxide Utilisation, pp. 331-338, Bari, 26-30 September 1993. R. Kostecki and J. Augustynski, J. Appl. Electrochem. 23 (1993) 567. P.Kedzierzawski and J. Augustynski, J. Electrochem. Soc. 141 (1994) L58. B. Jermann and J. Augustynski, Electrochim. Acta 39 (1994) 1891. D.W. De Wulf, T. Jin, and A.J. Bard, J.. Electrochem. Soc. 136 (1989) 1686. R. Kostecki and J. Augustynski, Ber. Bunsenges. Phys. Chem. 98 (1994) 1510. S. Kapusta and N. Hackerman, J. Electroanal. Chem. 134 (1982) 197. B. Jermann, PhD Thesis, University of Geneva 1996. K. Ito, S. Ikeda and M. Okabe, Denki Kagaku, 48 (1980) 247. S. Nakagawa, A. Kudo, M. Azuma and T. Sakata, J. Electroanal. Chem., 308 (1991) 339. A. Kudo, S. Nakagawa A. Tsuneto and T. Sakata, J. Electroanal. Chem., 140 (1993) 1541. K. Hara, A.Tsuneto, A. Kudo and T. Sakata, J. Electrochem. Soc., 141 (1994) 2097. P.Kedzierzawski and J. Augustynski, to be published. B.D Smith, D.E Irish, P.Kedzierzawski and J. Augustynski, J. Electrochem. Soc., 144 (1997) 4288.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Super-RuBisCO: plants
Improvement
117
of p h o t o s y n t h e t i c
performances
of
A. Yokota Plant Molecular Physiology Laboratory, Research Institute of Innovative Technology for the Earth (RITE), Kizu, Kyoto 619-02, Japan Graduate School of Biological Sciences, Nara Institute of Science and Technology (NAIST), Takayama, Ikoma, Nara 630-01, Japan
1. INTRODUCTION The atmospheric CO2 concentration is increasing at the rate of 2 to 3 ppm per year. The rate is fostered by deforestration and overgrazing in many countries. The concentration is deduced to reach 700 ppm in the end of the next century. The high concentration of CO2 will cause the global temperature to increase 3 ~ from the present temperature. Our lands for sequestration of the atmospheric CO2 into plants and crop cultivation will be dried and lose resouces in the soil including water and nutrients under these conditions. Creation of plants 3-phosphoglycerate
carboxylase reaction
CO2 CH20PO3 2 ~ . ~ 0 CHOH
CH20PO3 2 .~.~2 x
I COO-
, CH20PO3 2 -
02
RuBP
oxygenase reaction
|CH20PO3 2 -
~,--H20PO32 -
~,I-IOH I GOO"
COO-
+
2-phosphoglycolate Figure 1. Reactions of RuBisCO
'This study was partly supported by the Petroleum Energy Center (PEC) subsidized from the Ministry of International Trade and Industry of Japan.
118
requiring only a limited amount of water and with a high productivity would be essential to stop the present deforestration and overgrazing. Plants requires 250 to 500 molecules of water to fix 1 molecule of CO2 [1]. This is mainly caused by the inefficiency of the enzyme to fix CO2 in photosynthesis. Ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes this step. RuBisCO, even the enzyme of higher land plants, has several disadvantages [2]. 1"he reaction turnover rate is only approximately 3/sec/reaction site; i.e., 1/100 to 1/1000 of the rate of most other enzymes. The affinity of the enzyme for CO2 is 10 to 15 mM; only a quarter of the enzyme in chloroplasts may participate in photosynthesis [3]. Of much greater consequence is the unavoidable oxygenase reaction. Plant RuBisCO enzymes which are well adapted to the present oxygenic atmosphere still fix one 02 for every 3 to 4 CO2 substrates [4]. Only in part are the oxygenation products again withdrawn into the carbon reduction cycle, but the majority is oxidized to CO2. Overall, the oxygenase reaction can reduce the productivity of crop plants by up to 60% [5] and, equally, renders plants very drought-sensitive [6]. The ratio of the specificity in the carboxylase reaction to that of the oxygenase reaction is called "relative specificity (Sr)" [7]. This is a constant determined by the enzymatic properties of RuBisCO. RuBisCO from a photosynthetic bacterium, Rhodospirillum rubrum, has the Sr value of 10; the oxygenase reaction proceeds two-times faster than the carboxylase reaction in the ordinary atmosphere. The upper most in the Sr value has been thought to be possessed by RuBisCO from higher plants. The value reaches 90 to 95 [8]; the carboxylase reaction is three to four times faster than the oxygenase reaction. The Sr value is a determinant of the CO2 compensation point, where CO2 fixation by RuBisCO and CO2 release from photorespiration are balanced and no net CO2 fixation is observed.
.@
t
L_ C 0 . n
Kc decrease Vc increase
. n
E
present Sr increase C02
Figure 2. Expected effects of improvements of RuBisCO properties on the photosynthetic CO2 assimilation rate. Kc, Km for CO2; Vc, muximum turnover rate.
119 Improving the enzymatic efficiency of RuBisCO is a meaningful direction for improvement of crop productiviy and water use efficiency in photosynthesis. Figure 2 shows a prediction of photosynthetic performances of a plant, RuBisCO of which has been improved as indicated. The present C3 plants utilize 60 to 70% of light energy fixed on thylakoid membranes for CO2 fixation under well watered condition [9]. If these plants are transferred to a semi-dried or dried land, the stomata on the leaves are soon closed to prevent the cell components from high water potential, which inhibits cell activities. The plant with RuBisCO, Km for CO2 or the turnover rate of which are improved, can fix CO2 at much higher rates under well-watered conditions than the present plants, but the sensitivity to drought cannot be improved at all. The plant with RuBisCO with a much higher Sr value can fix CO2 even if the stomata close and the CO2 concentration in the leaved is decreased. All in all, RuBisCO with a lower Km for CO2, a higher turnover rate and a higher Sr value would be able to render the plant droughtinsensitive and efficient in productivity.
2. STRUCTURE-FUNCTION RELATIONSHIP OF RUBISCO REACTIONS
We have begun to work on improvement of the enzyme with a strategy that is based on the molecular and biochemical mechanisms of enzyme catalysis with a view on the comparative evolution and adaptation of the enzyme to the present atmosphere. It is known that RuBisCO enzymes of extant photosynthetic organisms reflect the atmospheric conditions of the times when the enzymes began to function in their organisms. For example, RuBisCO of higher land plants, which appeared after the atmosphere of the earth became oxygenic, has a higher affinity for CO2 and possesses a stronger specificity for the carboxylase reaction than for the oxygenase reaction compared to cyanobacterial enzymes [10]. Useful approaches that allow for improving the enzyme can be gained from the information that is provided by comparing the structure-function relationships and the changes in these parameters during RuBisCO evolution. 2.1. Hysteresis in reaction RuBisCO reveals its activity after the previous activation of the enzyme by CO2 and Mg 2+. The activated enzyme from plant sources shows an apparently biphasic reaction course with time in the presence of 1 mM or less ribulose 1,5bisphosphate (RuBP) which is comprised of the initial burst which slows down gradually in several minutes and a subsequent course where RuBisCO loses its activity much more slowly compared to the initial burst [11] (Fig. 3). The biphasic reaction course has been called "fallover". It has been proposed that the initial rapid decrease in activity is due to reaction hysteresis accompanying a change in the protein conformation of RuBisCO [11]. The latter is the slow inhibition phase where suicide inhibitors including xylulose 1,5-bisphosphate is formed from RuBP on the catalytic sites of the enzyme [12-14]. Hysteresis is always accompanied by the subsequent slow decrease in activity by the suicide reaction. Fallover has been seen in RuBisCOs from C3 and C4 plants. Lys-21 and Lys-305
120 from the large subunit of the spinach enzyme have been identified as two of the residues involved in a change of the protein conformation in hysteresis [15] (Fig. 4). The activity after the fallover increases about 50% in the presence of more than 1 mM RuBP. It has been proposed that the increase is due to the alleviation of the decrease of the activity in fallover through the binding of RuBP to the noncatalytic RuBP-binding sites [15-17]. The non-catalytic RuBP-binding sites have been designated as the regulatory sites affecting the enzymatic activity. In contrast to the enzyme from higher plant sources, RuBisCO from green
0.6 0.5
1~
[
...-,,,
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I I
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"
0.0 0
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40
60
"
, 80
' 100
0
Time (min)
Figure 3. Long-term measurement of the reaction course of spinach RuBisCO. The measured reaction course with 0.5 mM RuBP is shown with dots. After the reaction for 80 min, a further 0.5 mM RuBP was added. The solid line with and without closed circles are the courses calculated by our hysteresis models (see [15] for details). The broken line shows the reaction rate at that reaction time. The inset is the semilogarithmic plots of the ratios of the activities after various reaction time (v) to that of the initial activity (vo). The rapid decrease seen in the initial several minutes is due to reaction hysteresis. Suicide inhibitors required a longer time for inihition of RuBisCO.
121
24 19 D Y K LTY 9 9 E
spinach tobacco pea
9
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p e t u n i a
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I
9
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.
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9
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o
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9
barley rice wheat
9
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S
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122 microalgae including Euglena gracilis and cyanobacteria, and from photosynthetic bacteria do not show fallover [11]. Residues 21 and 305 which are lysine in the spinach and other higher plant enzyme, are other amino acid residues in RuBisCO of green microalgae and photosynthetic bacteria [15] (Fig. 4). RuBP of more than 1 mM does not increase the activity of the Euglena enzyme. Figure 4 also shows the relationship between the occurrence of the two lysine residues and the Sr value of RuBisCO. One soon notices that the Sr value increases along with evolution and the occurrence of the lysine residues. Our preliminary results showed that the organisms included in Conjugatae had RuBisCO that showed a very small decrease in activity during reaction compared to the plant enzyme. Lysine is partly incorporated at the fallover-inducible site (Fig. 4). Conjugatae is believed to be the first organism in the evolution from green alage to land plants. It is interesting to speculate that fallover was attained by RuBisCO in this step to increase its Sr value; a higher Sr value of RuBisCO is essential for photosynthetic organisms to live in the oxygenic atmosphere. If this is true, introduction of the two lysine residues into RuBisCO of a photosynthetic bacterium, Chromatium vinosum, will change the bacterial enzyme to the plant enzyme in the Sr value. The results are shown in Table 1. In this experiment, the entire mutated RuBisCO genes (rbcA-rbcB) where Arg-21 and Pro-305 were changed into lysine was expressed in Escherichia coli and the enzyme synthesized was purified to homogeneity. The maximum turnover rate increased from 8.8 to 15.8 turns/sec/site by introducing the two lysine residues without any significant decrease in the Sr value. The rate was 5 times higher than that of the plant enzyme. Together with the increase in activity, hysteresis was also introduced into the bacterial enzyme by the double mutation.
Table 1 Comparison of characteristics of purified double mutant and wild-type RuBisCO of Chromatium vinosum with those of the enzyme of other organisms RuBisCO
Specific activity kcat (~tmol CO2 fixed / min / mg) (turns / sec / site)
wild double mutant spinach cyanobacteria
7.94
Sr
8.8
43.8 + 0.8
13.5
15.8
41.8 + 0.5
2.5
3.3
93.9 + 1.7
11 - 13
41.0
Figure 4 (left page). Alignment of amino acid sequences around Lys-21 and Lys305 and the Sr values of RuBisCO. Numbering of the residues is for the spinach enzyme.
123 2.2. Relative specificity of R u B i s C O from n o n - g r e e n algae
Figure 4 indicates another interesting group, which includes RuBisCO having lysine at the hysteresis-inducible sites. The phylogenetic tree of the gene for the large subunits of RuBisCO gives three branches for evolution [16]. One is the branch which includes green algae and land plants and all RuBisCO consists of eight large Table 2 Relative specificties of RuBisCO from some non-green algae RuBisCO source
Sr value
spinach
94
Cylindrotheca (diatom)
105 -111
Porphyridium (red alga)
138
Porphyra (red alga) . . . . . . .
144
400
6.0 w
r./9
A
~
B
o
o
300 -
--
5.5
200 -
~
5.0
(/3
.,..~
o . , ~
o t~
-
loo
9
o~
-
~
~
9 ~ J
0
0
10
.~_,
-
~
J
J
20
30
Temperature (~
o
4.5
9 _
t
40
J
50 .5
....
4.0
1.6
1.7
1.8
1/RT (• 103 mol.kca1-1)
Figure 5. A. Effects of the reaction temperature on the Sr value of RuBisCO from Galdieria [18]. Open circles, Galdieria RuBisCO; open squares, spinach RuBisCO [18]; closed circles, spinach RuBisCO determined by Jordan and Ogren [19]. B. ln(Sr) of RuBisCO are plotted as: ln(Sr) = AGo_ c*/RT where AGo_ c* is the difference in the activation energies at the transition states of the carboxylase and oxygenase reactions, R is the gas constant and T is the absolute temperature. (Courtesy of the publisher).
124
and eight small subunits. The second group includes some photosynthetic bacteria, RuBisCO of which is composed of only the large subunits and has very low Sr values. The third group is the branch for some chemical bacteria and nongreen algae. Our preliminary studies [8] and the results from Tabita's laboratory [17] indicated that some organisms included in this branch have RuBisCO with 10 to 50% higher Sr values than the spinach enzyme (Table 2). These studies induced us to analyze RuBisCO of other red algae. Particularly, it was very interesting to recall that in the phylogenetic tree of rbc-L, there were two separate branches for the red algae; the branch for Porphyridium and Phorphyra and the other for Galdieria partita and Cyanidium caldarium. While the former organisms grow at moderate temperatures, the latter organisms prefer higher temperatures around 45 ~ for growth. It is known that the concentration of CO2 in water decreases with increasing temperatures much more steeply than that of 02. The active growth of Galdieria and Cyanidium at these high temperatures suggests the occurrence of RuBisCO with a high relative specificity for the carboxylase reaction, since these organism cannot grow under these conditions if they have the plant-type enzyme.
oxygenase reaction carboxylasc ,lase reaction
MD r
II
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-4+
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%
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lunit: kcal/mol]
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RuBisCO spinach RuBisCO
Galdieria
Figure 6. Comparison of activation energies in carboxylase and oxygenase reactions of RuBisCO of spinach and Galdieria
125 The Sr value of the Galdieria enzyme was 238; 2.5-fold that of the spinach enzyme [18]. The value increased to 360 at 15 ~ Cyanidium also had the similar enzyme. The Km for CO2 of RuBisCO from these red algae was 6.6 to 6.7 ~tM; the affinity of this enzyme was 2 times higher than that of the spinach enzyme. This is the surprising results because it has been believed that the plant enzyme should be most evolved in the nature. The reason for the higher Sr value of the Galdieria enzyme was further examined in Fig. 5B. The slopes of these lines give the difference in the activation energies in the both reactions; 10.4 kcal/mol for the Galdieria enzyme and 5.2 kcal/mol for the spinach enzyme. From these calculations and other results, the activation energies in the carboxylase and oxygenase reactions of Galdieria RuBisCO can be calculated as depicted in Figure 6. From comparison of the reaction velocities of the carboxylase reaction between RuBisCOs of spinach and Galdieria, the Galdieria enzyme requires 2.7 kcal/mol more than the spinach enzyme. Since the oxygenase reaction of the Galdieria enzyme needed 10.4 kcal/mol more than the carboxylase reaction (Fig. 5B), the calculated activation energy in the oxygenase reaction of the enzyme was 28.6 kcal/mol. This indicates that the oxygenase reaction of Galdieria RuBisCO is a very energy-consuming reaction and, thereby, is very slow at moderate temperatures (Fig. 5A).
3. PROSPECTIVES
In our studies on the structure-function relationships of RuBisCO, we could creat an artificial RuBisCO that can react at the highest rate among RuBisCO examined so far. Galdieria RuBisCO is another extreme in that it can strongly discriminate the oxygenase reaction. These new findings will again induce us to start the study for improvement of the characteristics of this important enzyme. In another project in our laboratories, transformation of the tabacco plastid genome has been achieved [20]. Introduction of the gene for improved RuBisCO into the plant will be our next interest. If a C3 plant has Galdieria RuBisCO for its photosynthetic CO2 fixation, the CO2 compensation point of the transformant will decrease from 50 p p m to 20 ppm. The transformant can fix CO2 at a considerable rate photosynthetically at 50 p p m where the ordinary C3 plants cannot adsorb CO2. Thereby, the water-use efficiency of the transformant will be substantially improved.
REFERENCES 1. E.D. Schluze and A.E. Hall, in Encyclopedia of Plant Physiology, Vol. 12B., O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Ziegler (Eds.), pp. 181, Springer-Verlag, Berlin, 1982. 2. I.E. Woodrow and J.A. Berry, Annu. Rev. Plant Physiol. Plant Mol. Biol., 39
126 (1988) 533. 3. S.D. McCurry, J. Pierce, N.E. Tolbert and W.H. Orme-Johnson, J. Biol. Chem., 256 (1981) 6623. 4. W.A. Laing, W.L. Ogren and R.H. Hageman, Plant Physiol., 54 (1974) 678. 5. I. Zelitch, Science, 188 (1975) 626. 6. M.K. Morell, K. Paul., H.J. Kane and T.J. Andrews, Aust. J. Bot., 40 (1992) 431. 7. D.B. Jordan and W.L. Ogren, Nature, 291 (1981) 513. 8. K. Uemura, Y. Suzuki, T. Shikanai, A. Wadano, R.G. Jensen, W. Chmara and A. Yokota, Plant Cell Physiol., 37 (1996) 325. 9. G.D. Farquhar, S. von Caemmerer and J.A. Berry, Planta, 149 (1980) 78. 10. T.J. Andrews and G.H. Lorimer, in The Biochemistry of Plants, Vol. 10, M.D. Hatch and N.K. Boardman (eds.), pp. 131, Academic Press, New Yorks, 1987. 11. A. Yokota, J. Biochem., 110 (1991) 246. 12. S.P. Robinson and A.R. Portis, Jr., Plant Physiol., 90 (1989) 968. 13. D.L. Edmondosn, M.R. Badger and T.J. Andrews, Plant Physiol., 93 (1990) 1390. 14. G. Zhu and R. G. Jensen, Plant Physiol., 97 (1991) 1348. 15. A. Yokota and H. Tokai, J. Biochem., 114 (1993) 746. 16. K. Ueda and H. Shibuya, Endocytobiology V (5th International Colloquim on Endocytobiology and Symbiosis), S. Sato, M. Ishida and H. Ishikawa (eds.) pp. 369, T~ibingen University Press, T~ibingen. 17. B.A. Read and F.R. Tabita, Arch. Biochem. Biophys., 312 (1994) 210. 18. K. Uemura, Anwaruzzaman, S. Miyachi and A. Yokota, Biochem. Biophys. Res. Commun., 233 (1997) 568. 19. D.B. Jordan and W.L. Ogren, Planta, 161 (1984) 308. 20. K. Tomizawa, T. Shikanai, A. Shimoide, C. Foyer and A. Yokota, Proceedings of ICCDU IV, 1998.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
127
O r g a n o m e t a l l i c reactions with CO2 - Catalyst design and m e c h a n i s m s E. Dinjus Forschungszentrum Karlsruhe GmbH, Institut far Technische Chemie, Bereich ChemischPhysikalische Verfahren, Postfach 3640, 76021 Karlsruhe, Germany
The use of carbon dioxide (CO2) as a raw material in chemical synthesis is a research area of extraordinary scientifical and of great economical and ecological interest. 1'2 Owing to the generally high activation barriers for reactions involving the highly oxidized and thermodynamically stable molecule CO2, catalysts are required in most of these reactions. 3 Apart from hydrogenation of CO2 C-C-coupling reactions are hitherto the domain in homogeneous catalytic reactions which is shown exemplary by the catalyst development for synthesis of lactones and pyrones (Figure 1). Examples for both above mentioned approaches to CO2 activation will be given in this article.
,atura,e0 u
hydrocarbons
/ acids
cooligomedsation]
@ l act,te0C--0on0 I carboxylation C-H-insertion
H2
]
Kolbe-Schmidt-reaction esters
lactones pyrones
\ --C--COOH + =C--COOH / I
, hydrogenation I
/ / I
CH4
CHaOH
CO
HCOOH
heterogeneous heterogeneous homogeneous or catalysis catalysis enzymaticalcatalysis
Fig. 1" Industrial useful reactions o f carbon dioxide with energy rich cosubstrates Homogeneous organometallic catalysts posses an adjustable molecular structure and offer high selectivity concerning the formation of a wide range of products. Industrial applications of catalytic processes so far use heterogeneous catalysts by reason of getting higher reaction rates and a quite easy separation from the reaction product. The catalytic efficiency is often determined by the nature of the coordinating ligands. The electronic and steric nature of the ligand has a remarkable influence on the activity of the catalytic systems and many attempts have been made to obtain a comprehensive concept of ligand classification, that allows correlation of catalytic activity and ligand structure. In this article it will be shown how homogeneous catalysts can be developed and optimised on basis of these concepts by combination of experimental and theoretical work.
128 1.
C O - O L I G O M E R E S F R O M BUTADIENE AND CARBON DIOXIDE
1,3-Butadiene as raw material is available in high amounts from the C 4 fraction of raffination processes. The telomerization of butadiene itself catalyzed by different metal catalysts is a well documented reaction 4. Depending on the catalyst and on the conditions different telomeres may be synthesized. Carrying out the reaction under carbon dioxide instead of argon atmosphere, 1,3,7-octatriene becomes the main product 5. Pioneering work of Inoue and co-workers in 19766 showed that the same reaction under carbon dioxide atmosphere lead to co-oligomeres 2, 5 and 6 when palladium complexes are used as catalysts (Scheme 1). Scheme 1. n
n=2
~
+
n=3
CO 2
catalyst
>
n=4
O
1
2
<
~
COOH 4
3
l
COOH
COOH 5
Since then, much effort has been made to increase selectivities and conversion rates 7'8. The developed catalytic systems (refer to 8) may be devided into two groups: preformed catalysts and "in situ" systems. Some of the "in situ" catalysts are prepared by up to five additions, but there has not been evidence for the real active species yet. A wide spectrum of accessable compounds 1 to 11 (Scheme 1) has been characterized. Depending on the reactivity of the olefinic side chains, some products are formed by double bond isomerization. Three trends determine the co-oligomerization: Palladium seems to be the most active metal centre, forming 2:1 and 4:1 co-oligomers, whereas rhodium catalysts also yield 3:1 products. Some nickel and ruthenium complexes were found to show little activity. The second trend is that the activity of the palladium catalysts is often increased by the addition of certain donor ligands such as aliphatic or aromatic nitrogen compounds. Similarly, product formation is strongly influenced by the used solvent. At last, steric and electronic factors at the active centre lead to quite different results. For example, palladium catalysts with aromatic substituted phosphines tend to the formation of 4:1 co-oligomers 8 to 11. Similar catalysts with bulky aliphatic substituents at the phosphorous yield 8-1actone 1 with high selectivities.
129 From a mechanistic point of view the first steps of the catalytic cycle should be similar to the telomerization of butadiene itself (Scheme 2). The catalytic precursor generates the Pd(0) species A that reacts to the bis-(rl3-allyl) complex C. The C,C bond formation between two C4 units is followed by insertion of carbon dioxide into a Pd,C bond affording the carboxylate intermediate D. Different pathways have been discussed to describe the multiple product formation (refer to 8). Interestingly, a bis-(carboxylato) complex may be prepared directly from the reaction of lactone 1, palladium acetate and P(i-Pr)3. This complex was structurally characterized by Behr and co-workers and shows good activity as catalyst. 9 Reviewing the literature, there are some remarkable facts and open questions of theoretical and technical interest: 1. In spite of some patents which have been filed covering this field, there is still no industrial application of the catalytic co-oligomerization. 2. The formation of a variety of products, the large number of possible catalysts and at last, a significant solvent effect complicate mechanistic interpretation and selective developement of further optimized catalysts. Scheme 2. ,f""'~
Pd~ B
PdL~ A C
00~~ ~ + PdL~
C02
D
- Pdl~ 2,4
~
L
I
~
J
9
8,9, 10, 11
+2
Our investigation of certain ligand effects led to some interesting results~~ Earlier studies revealed that acetonitrile strongly favours the formation of 8-1actone 1. In order to understand this solvent effect, we determined the influence of phosphinoalkyl nitriles on catalysis. Using palladium catalysts with this ligand type, the reaction becomes much more independent from the solvent (Table 1). Even in the absence of solvent, we find high butadiene conversion and good lactone selectivities. Table 2 shows a comparision of catalysts with ligands differing in length of the alkyl chain. Preferably, catalysts are prepared by ligands which are able to support the chelating hemilabile coordination mode as outlined in Scheme 3. Variation of the stoichiometry Pd/L
130
has only little influence on the results. The dependence from the chain length indicates that bior multinuclear palladium species do not play a significant role as intermediates during the transformation. Consequently, the solvent effect is rather a ligand effect because without nitrile solvent the nitrile function within the ligand structure is essential. Intermediates easily change their coordination sites on addition or removal of the weakly bound nitrile group. We have further focussed our interest on lactone 1 as potential product of value. It is known, that 8-1actones are substructures of some natural compounds. They are useful monomers for the synthesis of polyester resins and can also be used in the synthesis of fungicides and pesticides. Ring opening reactions, the chirality at C 5 and reactive olefinic side chains should also offer a wide spectrum of interesting synthetic applications. Table 1. Catalytic co-oligomerization in different solvents. "P " P (i-PO 3; ''P ~--,CN": (i-Pr) 2P-(CH2) 6-CN; Pd precursor." (rl5-C5H 5) Pd(rl3-C 3H5). Selectivity to 1 (%, by IH-NMR)
Conversion of butadiene
(%) Solvent
"P . . . .
"P . . . .
P~-~CN"
P~--~CN"
Isolated yield of 1 (%) "P . . . .
P~--~CN"
39
61
32
58
>44
2
78
_a
30
>24
<1
62
-"
14
>5
>49
2
70
_a
32
>12
100
3
74
- ~
67 a
acetonitrile
85
>95
benzene
>3
thf
>4
thf / pyridine (10/1) pyridine hexane
>2
>3
>1
20
a
sc carbon dioxide
>4
>3
<1
a
a
>3
>90
45
_a
39
b
a
The amount of 1 is not sufficient for further distillation.
b
No additional solvent is used, resulting in a liquid mixture of butadiene and CO2 as reaction medium.
T a b l e 2.
Catalytic cooligomerisation.
Pd precursor." (rl5-C5H5)Pd(rl3-C3H5). Selectivity to 1 (%, by lH-NMR)
Conversion of butadiene
(%)
a
Isolated yield of 1 (%) Pd/ligand (2:1)
Pd/ligand (1"1)
Ligand
Pd/ligand (2:1)
Pd/ligand (1:1)
Pd/ligand (2:1)
Pd/ligand (1"1)
(i-Pr)2P-(CHz)3-CN
>22
>17
22
19
a
a
(i-Pr)aP-(CH2)s-CN
>76
>66
40
36
29
26
40
39
31
(i-Pr)2P-(CH2)6-CN
>90
>83
45
(i-Pr)aP-(CH2)v-CN
>87
>80
44
39
37
33
(i-Pr)2P-(CHa),o-CN
>74
>63
32
30
23
20
The amount of 1 is not sufficient for further distillation.
131 Scheme 3.
_~,
i_npd 2 R2P\
LnPd
slow
FC=NI
/
F
L(n_l~Pd--N~ C - ~ ' ~ P R 2 R2P\ F C~ N ~ PdL(n_l / )
+2L
c~NI
-L -~ +L
~n-l~Pd-N=C--~ R2P~
/~
Most recently, we found that thiols can be co-polymerized with 1 in an easy manner 12 (Scheme 4), unless homopolymerization via radical or ionic initiation was not successful. Attempts to polymerize 1 by metallocene catalysis have not afforded any polymers yet. SHEne reaction under mild conditions does not affect the lactone structure. Scheme 4.
I•0
+
HsJR~sH
radical initiation
=
siRes n
Linear polymers with average molecular weights of more than 7000 g.mo1-1 are synthesized starting from dithioles, whereas tetrathiols offer access to insoluble networks carrying intact lactone substructures. These polymers are of potential interest in composites for optical applications. Currently, the mechanic properties of such materials are beeing measured. A principal disadvantage of homogeneous catalysis is the need to seperate the catalyst from the product mixture. Firstly, metallic impurities in the product do not permit following reactions. Secondly, recovery and regeneration of the catalyst would be much more difficult. On the other hand, heterogeneous catalysis often does not give high selectivities. Because a heterogeneous system for the co-oligomerization has not been found so far, we tested some immobilized catalysts. The principles of synthesizing phosphines anchored to a polymer are well known. We found that useful catalysts may be prepared by ,,in situ" reaction of polystyrene linked phosphines with (qs-CsHs)Pd(r13-C3Hs)13. Similar to homogeneous catalysts, phenyl substituents at the phosphorous mainly result in open chained C17 esters 8 to 11, whereas bulky alkyl substituents yield lactone 1 highly selectively. The same catalyst may be used in multiple replications, with decrease of butadiene conversion and slight increase of lactone selectivity (Table 3). The catalyst may be also preformed separately on a large scale and is stable over a long period. Analysis of the product mixture indicates only small amounts of byproducts such as butadiene dimers. On a technical scale, unreacted butadiene is easy recovered and the process can be optimized with respect to TON and selectivity. Further studies of several reaction
132 parameters and to characterize the polymer bound catalysts by various spectroscopic and analytical techniques are in progress. Table 3.
Co-oligomerization with catalyst from (rl5-C5H5)Pd(q3-C3H5) and polystyrene linked phosphine. Conversion of butadiene (%)"
a
Selectivity to 1 ( % )
number of replications with same catalyst
,,in situ"catalyst
preformed catalyst
,,in situ"catalyst
preformed catalyst
0
> 12.9
> 24.7
59
56
1
> 7.3
> 10.3
65
68
2
> 5.6
> 7.2
66
72
3
> 5.8
> 6.4
67
71
4
> 5.0
> 5.0
64
66
The amount of butadiene dimers was not quantified.
0
NICKEL CATALYSED COTRIMERISATION OF ALKYNES AND CO2 TO 2-PYRONES
The formation of 2-pyrones 12 from CO2 and alkynes was first described by Inoue and co-workers using in situ catalysts consisting of Ni(cod)2 and different chelating phosphanes of the type Ph2P(CH2).PPh2 ~4. Yields were very low, however, even under drastic reaction conditions. Later we showed that the catalytic system Ni(cod)2 / PR3 in acetonitrile / THF gave higher turnover numbers and a very high selectivity under mild conditions (Scheme 5) ~5. The catalytic conversion of alkynes with CO 2 represents up to now the sole example for a homogeneously catalytic reaction which yields to C-C-bond formation with CO2 and selective formation of cyclooligomeres using a cheap 3d metal complex catalyst. The variation of alkyne substituents allows synthesis of a wide range of 2-pyrones ~6. Scheme 5:
Formation of 2-pyrones 12from C02 and alkynes.
Ni(cod) 2, PR3
CO 2
+
2 R
CH3CN/THF, CO2 , 60~ 10 bar
up to 200 turnovers
~'~
R 3.o 12
96% selectivity
By systematic variation of the phosphane ligands it was found that catalysts formed from basic phosphanes with small cone angles gave the highest activity and selectivity. The optimum ligand to metal ratio lies between 1:1 and 2:1. Excess phosphane decreases the catalytic activity drastically, possibly due to the formation of stable coordinative saturated nickel complexes. The catalytic system works in a temperature range between 20 and 120~ most effectively at 60~ with a low but constant reaction rate over a long time. Furthermore,
133 Tsuda and coworkers 17synthesised novel polymeric materials based on CO2 as C1 building block and long chained Gt,o~-alkynes as coupling partners using the same catalytic system. Dunach et al. synthesized instead of pyrones unsaturated carboxylates by using electrochemically generated Ni(0) centres from alkynes and CO2. The formation of unsaturated acids is a catalytical process relating to Ni(0) but the nessesary presence of Mg 2+ ions is realized by a sacrificial Mg-anode TM. In 1993 Reetz et. al 19 reported on a nickelcatalysed 2-pyrone synthesis in sc CO2 (Scheme 6) by means of the same catalyst ([Ni(cod)2]/PhaP(CHR)nPPh 2 (dppb)) presented by Inoue ~4'2~in 1977. That is one of the first examples in which sc CO2 did not only function as solvent but also as reaction partner, i.e. substrate, in homogeneously catalysed reactions ~ga. Scheme 6: 2-pyrone-synthesis from 3-hexyne and C02 under supercritical conditions.
/
/
Ni( cod )2 / Ph2P(CH2)4PPh2 ] +
Et
Et
CO2
50 bar, 20 h, 120~
Et"
0
0
Reactions in supercritical CO2 show- apart from the well-known and already used fields extraction 2~'22and chromatography 23possibilities of also achieving advantages in chemical reactions with the application of SCF by using the special properties of SCF such as variable density between gas and liquid, high fluidity, miscibility with other gases 24. The main problem to investigate catalytic processes in sc solvents is reflected by the question: WHEN happens WHAT process in WHICH phase?. In order to elucidate this problem we developed a method to add the catalyst to reaction mixtures in the sc state. Working in this way the following details of the catalyst formation could be stated: - Using dppb as phosphane, no catalytically active species is formed in the sc mixture, dppb
must be added before the beginning of the reaction. The active catalyst is only formed in the liquid solution. - With PMe 3/ Ni(COD)2 = 2:1 as catalyst precursors added to the mixture in the sc state, an active catalyst is formed and the reaction starts (Figure 2). This can be explained by the fact that PMe 3 - in contrast to dppb - forms a homogeneous solution with sc COR and therefore, reacts more quickly with Ni(COD)2 to the catalytically active spezies. The selectivity of the 2-pyrone formation in sc CO2 with the catalyst PMe 3 reaches values far beyond 90% and thus, it has a clear advantage compared to the reaction in conventional solvents such as THF/acetonitrile. The reaction rate in THF/acetonitrile can also be reached in sc CO2 although at much more drastic reaction conditions. The example is to show that, if sc CO2 is used as solvent and reaction partner in catalytic reactions, not only the more complicated phase transitions and phase equilibria than in conventional solvents, but also the adjustment of the catalyst system must be taken into consideration. On principle, it can be expected that further applications of sc CO2 as an environmentally safe solvent and as C~-building block will be found when catalysts were correspondingly adapted.
134 Figure 2" Kinetic plot of the formation of 2-pyrone from 3-hexyne and C02 with
Ni[cod]2 / phosphane under various conditions. 12-
PMe3
11-
(2mmol per I mmol Ni), 95~ 175bar
10.a
98-
r
7-
i...
6-
> O =-
4-
~.
E
t...
~
dppb (1 mmol per I mmol Ni), 95~ 175bar
catalystis addedto the cold liquid reaction mixture at the beginning
5-
PMe3,40~ 80bar
no pyroneformationwiththe catalystwith dppb added to the sc. mixture
1 0
I
I
22
0
33
I
I
44
55
time [h]
1
TRANSITION METAL CATALYZED FORMATION OF FORMIC ACID AND ITS DERIVATIVES FROM COs AND H2
The catalytic addition of hydrogen on CO2 presents also an important starting point for the utilization of CO 2 as a couple of technical important basic chemicals can be produced on this way (Scheme 7). The formation of formic acid from carbon dioxide and dihydrogen is an exothermic but strongly endergonic process under standard conditions. Scheme 7" Theoretical possibilities for the reduction of C02. H2CO
H3COH
_2~ 0 + H2
CO
~
- H20
+ 3/_H2OH2/
CO 2
- 2 H20
~-2
HCO2H
H20
CH4
CnH2n+2 Fischer-Tropsch
135 The equilibrium in Equation 1 lies therefore far to the left. This unfavourable situation is ruled by the large difference in entropy between two gaseous reactants and a liquid product that forms very strong intermolecular hydrogen bonds. High pressure and relatively low temperatures will obviously help to shift equilibrium to the right. Equation 1- Thermodynamicparameters for the hydrogenation of CO2 to formic acid. catalyst CO2 (g) +
H2 ( g )
-
HCOOH
(I)
O
A H = - 31.6 kJ/mol A G~
+ 32.9 k J/tool
Even more important is the choice of the right solvent, as solvatation will not only lower the entropy of the reactants by enclosing them in a solvent cage, but may also break up the strong hydrogen bonds between HCO2H molecules. The small negative value of the Gibb's free energy in aqueous solutions strongly supports these considerations. Base addition will work in the same direction, especially if amines are used, which are known to form stable adducts with carbon dioxide. Another possibility of shifting the equilibrium to the right is trapping formic acid in form of derivatives like esters or N,N-dimethylformamide (DMF). The Rh-catalysed hydrogenation of CO2 to formic acid is a fully reversible reaction as experimently demonstrated in Fig. 3. This fact leads straightforward to the possibility of using this system as a reversible hydrogen carrier. Using dihydrogen as energy source is a very promising way to reduce pollution. The main problem is storing molecular hydrogen in a simple way. Figure 3:. Reversible formation offormic acid
2.0
formation
-
pt~
_~
1.5-
0
"0
1.00
o 0
/
0.5-
/
/ / / / /
/ /
/
/
~ decomposition
= 40atm
/
/ P
/
,
formation
;mbient pressure
0 Ptot
=
\
40arm ..__-
r
/ ~ -- -- --o-- -~
/ \
/ \
/ \
/ \
\
/ ! \
\
\
\
/ q
\
(i)
/ !
(
\ (H)
/
2
\
(iH)
g~ 0.0
~ 0
, 15
, 30
I
45
I
60
75
90
t [h]
conditions" [(g-C1)Rh(cod)] 2/dppb, c(Rh) = 2 . 4 8 m m o l / 1 , T = 23 ~
acetone/NEt 3, c(NEt3) = 1.20 mol/1
The first homogeneously catalysed example was demonstrated by Inoue et. al. in 1976 TM. They used Rhodium(I) phosphane complexes such as Wilkinson's catalyst
136 [Rh(PPh3)3C1 ] for the catalytic hydrogenation of CO2 in benzene solution in presence of tertiary amines. Inoue's catalyst showed a better performance when small amounts of water were added but the TON did not arrive more than 150 even under drastic reaction conditions. Other investigations showed the possibility getting higher yields when an isopropanol/amine mixture containing a water content up to 20% was used TM. Aqueous solutions often have higher rates and yields than the systems in organic solvents. The accelerating effect of small amounts of water in organic solvents 25f'26 allows several mechanistic explanations. It is possible that a donative interaction between water and the CO2 carbon atom increases the nucleophilicity of the CO2 oxygen atoms and that its capacity to bind to a metal centre is intensified in this way. Calculations by ab initio SCF methods confirm that a CO2 - water interaction in the described way is more stable than each of the two species 27. Rhodium formate complexes 14 (Scheme 8) have been inferred as possible key intermediates during the catalytic cycle of CO2 hydrogenation in DMSO/NEt3 mixtures 28. Recently the complexes [{R2P-(X)-PR2}Rh(hfacac)] 13 has been introduced as stable model compounds for 14 29. Complexes 13a-g were synthesised in order to further improve the catalytic activity by variation of the ligand structure (Table 4). Scheme 8: [{R2P-(X)-PR2}Rh(hfacac)] 13 as stable model compoundsfor rhodium formate
complexes 14. CF 3
p/
p/
o-=4(
Rh
\-
H
o
GF a 13
14
Table 4: Selected analytical data and catalytic activities of complexes
[{R2P-(X)-PR2}Rh(hfacac)] 133~ -(X)-
R
13a
(CH2)2
Ph
8 (103Rh) [ppm]
tof
438
170
vrel
[h-1 ]
P-Rh-P
[~ 8.5
84.34(3)
13b
(CH2)2
Cy
368
77
3.9
84.97(2)
13c
(CH2)2
ipr
323
95
4.8
86.01(7)
13d
(CH2) 2
Me
370
20
1
85.08(5)
13e
(CH2) 3
Ph
567
300
15.0
90.77(6)
13f
(CH2)4
Ph
646
565
28.3
93.08(3)
13g
(CH2)4
Cy
845
1335
66.8
98.93(6)
Very fast formation of HCO2H is observed when a solution of [{Ph2P(CH2)nPPh2}Rh(hfacac)] 13f (2.5 x 10 -3 mol dm -3) in DMSO/NEt3 (5:1) is stirred under
137 HJCO 2 (1:1, 40 atm) in a stainless steel autoclave at 25~ The equilibrium concentration of 2.0 mol dm -3 HCO2H is reached within approximately 5 hours. Kinetic measurements reveal that there is no induction period and that catalysis starts immediately with a maximum turnover frequency (toJ) of 565 h -~ (Figure 4) 3~
Figure 4: Increase offormic acid concentration during catalytic hydrogenation of using
catalysts 13b, 13f and 13g30.
25 I
%
2.o ] 2.0
'
'
I
i
=
i
i
I
~ i=
,, n
i
i
t
t
i
13g
,
,
13f
=
9
1.5
o
13b 1.0
*
-I-
"6
0.5
o.0
-1-
I
I
I
I
o
100
200
300
400
'
500
t [min] Complexes 13 are ideally suited for a systematic study of structural changes in rhodium phosphane chelates upon small changes in the ligand structure as there is no steric interaction between the phosphane ligand and the hfacac moiety 29. As expected, the ligand structure has a marked influence on the catalytic acitivity of complexes 13. The influence of the ligand on the coordination sphere of rhodium complexes 13a-g in the solid state is prevalent also in solution as seen from the linear correlation between the P-Rh-P angles and the ~~ chemical shifts as determined from 2D-(3~P, ~~176 For the series of ligands R2P(CH2)nPR2 of complexes 13a-g an increase of the relative catalytic activity in CO2 hydrogenation with increasing 8-values is observed. The fact that larger ligands coordinated to the rhodium centre accelerate the catalytic activity is reflected by the results of CAMD calculations 32. In search for a model giving a concise description of all these observed sterical effects, the concept of the accessible molecular surface (AMS) of the rhodium center within the flexible [(P2)Rh] fragment has been developed for the catalyst system of type 13. It has been demonstrated how the shape and the size of the open cavity is determined not only by the PRh-P angle and the steric bulk of the groups R at phosphorus, but also by the flexibility of R and by possible internal movements within the chelate ring. All relevant parameters for the description of the intrinsic steric properties of the [(P2)Rh] fragment is reflected by the cavity. The AMS analysis readily quantifies the obvious difference in the accessibility of the rhodium center within the cavity. The ligand in complex 13g yields the smallest AMS for the rhodium center and ligand in 13d provides the most accessible central metal atom in this series of ligands. The AMS of the rhodium center in the [(P2)Rh] fragment has been plotted
138 versus the TOF values observed for the corresponding complexes 13 in carbon dioxide hydrogenation. The catalytic activity increases strongly with decreasing accessibility of the metal center. The AMS model has been introduced as a unique approach for the description of steric ligand effects in homogeneous catalysis. Its use was demonstrated for the first time in the rhodium-catalysed hydrogenation of carbon dioxide to formic acid using complexes 13 as catalyst precursors. In processes mainly governed by steric interactions this concept may serve as a general approach to understand ligand effects on activity and selectivity. The elimination of the product (formic acid) seems to be the cinetical determining step 33. Up to 2200 mole of HCO2H per mole of rhodium with turnover frequencies as high as 374 h -1 can be achieved with the in situ catalyst [Rh(eOD)H]4/dppb 34. As CO2 removal from process waste gases is predominantly carried out in water the hydrogenation of CO2 in aqueous solution is a very attractive starting point for the utilization of the raw material CO 2. Only a few attempts have been made in the last decades to carry out catalytical hydrogenation of CO2 in water as solvent 35'36'37. Transition metal complexes incorporating phosphane ligands which have been proved as catalysts in organic solvents are not suitable for the use in aqueous solution for reasons of non-solubility under these conditions. Only when complexes of rhodium containing the water soluble phosphane P(C6H4-m-SO3Na)3 (TPPTS) 38 were used homogeneous catalytic systems could be obtained, which show higher activities and better yields as catalysts in organic solvents 39. For the hydrogenation of CO2 in aqueous solution catalysts formed in situ from suitable precursors and TPPTS are used, but the most effective system up to now is found with the water soluble analogon of Wilkinson's catalyst [C1Rh(TPPTS)3]. Equation 2 presents the reaction conditions leading to TON of 3440 and TOF of 1365 h ~ 39. It is noteworthy that the amine concentration is never passed over by formic acid concentration in aqueous systems and formic acid formation is absolutely suppressed without addition of any amine 39a. [CIRh(TPPTS) 3 ] CO 2
4-
H2
40 bar rt, 12 h
~"
HCOOH 3440 mol per mol Ru
H 2O, Me 2 NH
Equation 2: Rhodium catalysed hydrogenation of C02 in aqueous solution. Carbon dioxide in its supercritical state is a reaction medium of great interest. Noyori et. al. 4~ recently detected that Ruthenium(II)-phosphine-complexes of typ [(X)2Ru(PMe3)4] 15 (X = H) and 16 (X = C1) can act as highly active catalysts for an effective transition metal catalysed hydrogenation of CO2 to formic acid in a supercritical mixture of CO2, H2 and NEt3 without use of any further solvent. 4.
ACKNOWLEDGEMENTS
I thank my coworkers Dr. S. Pitter, Dr. F. Ga6ner and Dr. R. Fomika, my colleagues and technical stuff at Karlsruhe and Jena for their contribution to our work described in this chapter. Additional thanks are due to Dr. R. Fornika for editing the manuscripts and preparing
139
the final paper. Financial support from the M a x - P l a n c k - G e s e l l s c h a f t greatfully a c k n o w l e d g e d .
and the B M B F
is
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22K. Zosel, Angew. Chem. 1978, 90, 748. 23 E. Klesper, Angew. Chem. 1978, 90, 785. 24 a) J. A. Hyatt, J. Org. Chem. 1984, 49, 5097 and references cited therein; b) E. Dinjus, R. Fornika, M. Scholz in Chemistry under extreme or non-classical conditions (Eds.: R. v. Eldik, C. D. Hubbard), Spektrum Akademischer Verlag, Heidelberg, Germany, 1996. 25 a) Mitsubishi Co. (Y. Hashimoto, Y. Inoue) JP 138614 (1976) [Chem. Abstr. 1977, 87, 67853v]; b) Tjin Ltd. (T. Yamaji) JP 166146 (1981) [Chem. Abstr. 1982, 96, 122211x]; c) Tjin Ltd. (Y. Yamaji) JP 140948 (1981) [Chem. Abstr. 1982, 96, 68352d]; d) BP Ltd. (D. J. Drury, J. E. Hamlin) EP 95321 (1983) [Chem. Abstr. 1984, 100, 174262k]; e) BP Ltd. (A. G. Kent) EP 151510 (1985) [Chem. Abstr. 1986, 104, 109029h]; f) Y. Inoue, H. Izumida, Y. Sasaki, H. Hashimoto, Chem. Lett. 1976, 863; g) C. P. Lau, Y. Z. Chen, J. Mol. Catal. 1995, 101, 33. 26 J.-C. Tsai, K. M. Nicholas, J. Am. Chem. Soc. 1992, 114, 5117. 27 M. Y. Ngyen, T.-K. Ha, J. Am. Chem. Soc., 1984, 106, 599. 28 a) T. Burgemeister, F. Kastner, W. Leitner, Angew. Chem. 1993, 105, 781" Angew. Chem. Int. Ed. Engl. 1993, 32, 739; b) W. Leitner, E. Dinjus, F. Gal3ner, J. Organomet. Chem. 1994, 475, 257 c) E. Graf, W. Leitner, J. Chem. Soc., Chem. Commun. 1992, 623. 29 a ) P. J. Fennis, P. H. M. Budzelaar, J. H. G. Frijns, A. G. Orpen, J. Organomet. Chem. 1990, 393, 287; b) W. Leitner, E. Dinjus, R. Fornika, H. G0rls, to be submitted; c) R. Fornika, PhD Thesis, Universit~it Jena, 1994. 3oR. Fornika, H. G0rls, R. Seemann, W. Leitner, J. Chem. Soc. Chem. Commun. 1995, 1479. 3~ a) R. Benn, H. Brenneke, R.-D. Reinhardt, Z Naturforsch. 1985, 40b, 1763; b) R. Benn, H. Brenneke, A. Rufinska, J. Organomet.Chem. 1987, 320, 115. 32 W. Baumann, E. Dinjus, R. Fornika, H. G0rls, M. Kessler, C. KrUger, W. Leitner, F. Lutz, Chem. Eur. J. 1997, 3,755. 33F. Hutschka, A. Dedieu, M. Eichberger, R. Fornika, W. Leitner, J. Am. Chem. Soc. 1997, 119, 4432. 34 W. Leitner, E. Dinjus, F. GaBner, J. Organomet. Chem. 1994, 475, 257. 35K. Kudo, N. Sugita, Y. Takeszaki, Nippon Kagaku Kaishi 1977, 302. 36 C. J. Stadler, S. Chao, D.P. Summers, M. S. Wrighton, J. Am. Chem. Soc. 1983, 105, 6318. 37 a) M. M. Taqui Khan, S. B. Halligudi, S. Shukla, J. Mol. Catal. 1989, 53, 305; b) M. M. Taqui Khan, S. B. Halligudi, S. Shukla, J. Mol. Catal. 1989, 5 7, 47. 38 a) Ruhrchemie AG (R. G~irtner, B. Cornils, H. Springer, P. Lappe) DE 3235030 (1982) [Chem. Abstr. 1984, 101, 55331t]; b) Ruhrchemie AG (L. Bexten, B. Cornils, D. Kupies) DE 3431643 (1984) [Chem. Abstr. 1986, 105, 117009n]; c) W. A. Herrmann, C. W. Kohlpaintner, Angew. Chem. 1993, 105, 1588; Angew. Chem. Int. Ed. Engl. 1993, 32, 1524. 39 a ) F. GaBner, W. Leitner, J. Chem. Soc., Chem. Commun. 1993, 1465; b) F. Gal3ner, PhD Thesis, Universit~it Jena, 1994. 4o a) P. G. Jessop, T. Ikariya, R. Noyori, Science 1995, 269, 1065; b) P. G. Jessop, T. Ikariya, R. Noyori, Nature 1994, 368, 231; c) T. Ikariya, P. G. Jessop, R. Noyori,.Japanese Patent Application 274721 (1993).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
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C a t a l y t i c f i x a t i o n o f C O 2 : C O 2 p u r i t y and H2 s u p p l y J. N. Armor Air Products and Chemicals, Inc., 7201 Hamilton Blvd, Allentown, PA 18195 (USA) An analysis of factors affecting proposed solutions to the CO2 problem is provided. To consider CO2 as a feedstock, one has to consider the purity, reactivity, operational conditions, customer preferences, transportation costs, and availability of the CO2. Since CO2 is a global problem, local efforts to reduce CO2 emissions will have limited impact, except to convert waste CO2 to a more valued chemical product. Some have suggested that H2 offers a good approach to remove CO2, but use of H2 as the reducing agent must address the source and cost of the H2. Most H2 is produced by steam reforming of hydrocarbons which is also a source for CO2; thus use of conventional sources of H2 is not a practicable solution to destroying CO2. New non-fossil fuel routes to H2 production might enhance the use of H2 as the reductant, but initial production of chemicals, even if cost competitive, from CO2 is expected to have limited impact on worldwide CO2 emissions. Issues that impact Hz supply and cost will be discussed, since these may be a part of any CO2 solution. 1. INTRODUCTION An excellent summary [ 1] of the volume and sources of major atmospheric pollutants was published by the US Dept of Energy in 1994. Highlights of their report can be broken down into the types, volumes and sources of a variety of pollutants. In particular, for CO2, it is estimated that 160,000 million metric tons (mmt) are generated naturally, worldwide: 8,000 mmt from human derived sources, globally;165,000 mmt are absorbed by earth, with the balance being a global increase of-~3,400 mmt. There is some disagreement about the accuracy of the latter number since it is based on the difference of two large nmnbers. Further, a small group of scientists contend that global warming is not related to CO2, but to other factors such as water vapor. This manuscript will describe the solutions being considered for CO2 removal, the chemical and political limitations on use and reduction of CO2 levels, and the role of Hz in affecting a solution. Since Hz is such an important part of the potential solution, some introduction into the current supply, availability, and cost for Hz will be provided as well as alternative approaches to making more Hz. 2. PROPOSED SOLUTIONS TO CO2 BUILDUP
As described in an earlier publication [2], a number of solutions [refer to Figure 1] have been proposed to reduce this imbalance, including the establishment of a "carbon tax", minimizing CO2 emissions (already underway), demanding zero emissions of CO2 (solar, hydroelectric, wind, nuclear, or geothermal), burying CO2 by storing in deep in ocean pools or
142
use it for enhanced oil recovery, absorbing it (already done with monoethanolamines), using it to produce clean C02 for carbonated beverages, etc, and finally considering it as a feedstock for valuable chemicals. - Carbon Tax Minimize Emissions Zero Emissions - Bury it [Oil recovery, oceans, MEA] - CO 2 as Feedstock -
-
3,400 MMt
Figure 1. Potential solutions to offset the perceived imbalance in CO~ in our upper atmosphere 2.1. T a x e s
Some progress [3] has been made using taxes to enforce environmental regulations. For example, in the 1980s in France & Germany provided incentives to invest in waste water treatment. In the 1990s in USA, laws and taxes were enacted to reduce the impact of ozone depleting chemicals. In the 1990s revenue producing taxes were imposed on CO2 emissions in Sweden and Norway and on sulfur and NOx emissions in Sweden as well as on dumping and incineration in Denmark. Any tax on CO2 would have to be implemented worldwide to avoid upsetting the competitiveness already existing between nations of the world. 2.2. C 0 2 as a f e e d s t o c k
To consider CO2 as a feedstock, one has to consider the purity, reactivity, operational conditions, customer preferences, transportation costs, and availability of the CO2. There is a tendency by some to think that CO2 from a powerplant stack can simply be used to supply the same CO2 for chemicals production, such as the production of H2 by reaction of CO2 with CH4. In fact this cannot be done without a lot of purification. In the example to produce H2, CO2 reforming of CH4 will doubtless need very pure CO2, that is one will have to remove SO2 or NOx from the process stream. This will add cost to the CO2 for the necessary purification steps. Another way to look at the issue of purifying CO2 is to examine how CO~ is produced today. The Caloric catalog [4] provides a diagram of the many unit operations. Thus, one can see that CO: produced from a powerplant is recovered as high purity liquid CO2 which can be converted to cylinder grade gas for use in carbonated beverages. Typically one has to absorb the CO2 in an aqueous amine such as monoethanolamine. If the presence of water is unacceptable that has to be stripped out of the process stream. Steam is used to recover the CO2 from the amine solution. There are filters and scrubbers in the process before liquid CO: is produced. In addition there may be the need for compression of the CO2, which is a very energy intensive process operation thus adding more cost to the CO~. [One must remember that because CO2 is so stable, it will not be sufficient to react it at one atmosphere; it will probably have to be pressurized to enhance its reactivity.]
143 In addition one must also remember that where the CO2 is produced may not be the place where it is needed. It probably will need to be transported [via vehicles or pipeline] to the process operation. Transportation also adds more cost to any feedstock. In another process related issue, the CO2 will be needed on a continuous basis for the production of chemicals. This is because most commodity scale chemicals are produced around the clock at the same level of productivity. Chemical plants run efficiently when running full out. This continual need for CO2 is not consistent with the major anticipated source of CO2 which is from power plants. Power plants do not run at a constant output; power is produced during peak user periods and the plants reduce power production at night. This means that CO: production is reduced at night. Thus one gets a non-uniform production of CO2 which is unacceptable for commodity chemicals production. J. Rostrup-Nielsen pointed out [5] in 1994 that it is "questionable whether C1 chemistry can contribute significantly to solve the greenhouse problem created by CO2." For example the present world production of acetic acid is about 5 billion pounds per year. If one were to use CO2 + CH4 to produce acetic acid this would amount to the CO2 emission from only ONE 500 MWatt coal fired power plant- "a small drop in a big bucket." 3. OTHER NON TECHNICAL ISSUES RELATED TO CO~ CONVERSION The perceived CO2 problem is a global one, not a local one. This means that unlike NOx removal, localized removal of CO2 will not provide significant reduction in worldwide CO2 levels in the upper atmosphere, unless all countries are equally participating in rigid CO: emissions control. The undesirable cost of any added CO: emission control will have to be passed onto the consumer. It is anticipated that emerging nations will resist controls to growth; naturally, they will be more interested in doing whatever they can to enjoy the comforts of more prosperous societies without additional cost. Their rate of growth will be high and new laws will probably impact new construction greater than existing production facilities; thus they may be expected to bare a greater proportion of the CO2 reduction. The passage of international laws will require a good deal of compromise and negotiation. Naturally there will be local issues and pressures applied to politicians to minimize the cost burden to any one country. At the same time, environmentalists will be demanding strict reduction of CO:, thus we can expect governments around the world to be swayed by the "politically correct" lobbies. In addition, considerable uncertainties about the origin of any greenhouse effect will delay implementation of any globally binding agreements. Politics and business influence these efforts to legislation and changes. Since the commercial energy section impacts wide regions of any economy, therefore, its hard to control a single business. It's not simply a matter of focusing on the oil companies who supply the fuel, the automobile companies who produce the cars, the power companies (who are just trying to meet the demands of the consumer for increasing levels of power generation), or the water companies trying to quench the thirst of populations living in arid regions of the world- it's a collection of vested business and consumer interests that vary around the globe. Trying to tackle this uniformly will not be easy, if at all possible on a global scale. Indeed, one general approach that at least make a dent in the pollution of our planet is energy conservation. I personally believe this offers greatest impact and has a realistic chance
144 of making some impact. Currently, economic pressures (cheap fossil fuel), not legislation control. Since CO2 is a global problem, local efforts to reduce CO2 emissions will have limited impact, except to convert waste CO2 to a more valued chemical product.
4. WHERE DO WE GET THE HYDROGEN WE NEED? I sense another false impression is that a solution to the removal of CO2 is to just use H2 to reduce it back to CO or CH4. This just is not an acceptable solution, except in some micro economies around the world. The strong pressure for cleaner fuels has forced refineries to become net consumers of H2, whereas, 20 years ago, they were producers of surplus H2. As the article by P. Courty and A. Chauvel [6] indicates, H2 demands will continue to escalate into the next century which is expected to result in a substantial demand for H2 which cannot be matched by existing supply. The strong demand is driven by need of refineries to meet existing legislation for removing S and N from fuel. This is also aggrevated by the lower net H2 production [due to the reduced demand for adding aromatics to enhance octane number of fuels]. Independent of all this pressure from the refineries, H2 also offers some distinct advantages as a future fuel which may put much more demand pressures on H2, since it is a clean fuel when combusted and no CO2 is produced when H2 is derived from non-fossil fuels. Fortunately, the earth possess a huge H2 reserve [our oceans], if only we could figure out how to convert water to H2 in a cost effective manner. All these market and technology forces will keep the price of H2 relatively high, and it is probably unreasonable to use it for destroying vast amounts of CO2.
4.1. Cost of H2 to produce gasoline One estimate for the cost for pure H2 is -~$1.50 for 1000 std cu fl, which coverts to $ 0.00127/mole H2. If we assume that for CO2 + H2 to gasoline, we can represent gasoline as [(CH2)x with x=7]. This means that one will produce 2 moles water/mole CO2. This means that one must consume 2x moles H2 for every x moles CO2 to make (CHz)x, plus x moles of H2 to make water. Thus there are 3 moles of H2 for every mole of CO2. This means that (CH2)7 requires 21 moles of H2. This stoichiometry means $0.027/mole gasoline. If gasoline sells for $20/barrel, and we assume that for the density of gasoline we can use the density of methylcyclohexane, this converts to $0.015/mole gasoline as methylcyclohexcane. This means that gasoline from CO2/H2 is more than two times the current market price. This unacceptable price difference between gasoline and the cost of H2 is driven by 2 issues: gasoline is terribly cheap and the cost or credit one assigns to CO2. One might expect some relief in this cost if one could use a cheaper source of He, such as from an off gas process stream. Alternatively, if there were a credit [or tax] on CO2 emissions, that would help to reduce the large difference in costs. As pointed out earlier CO2,will not be free, since it will cost something to purify and pressurize it for suitable reactivity. However, many nations are proposing a tax on CO2 emissions or providing CO2 with a credit price which will add some cost incentive to CO2 conversion to chemicals. Getting money for disposing of CO2 to chemicals will depend on the world wide acceptance of such a philosophy, the chemical to be produced, market pressures, and commercially acceptable catalytic processes.
145
5. A ROLE F O R H2 TO R E M O V E COs?
Some have suggested that H2 offers a good approach to remove CO2, but use of H2 as the reducing agent must address the source of the H2. Most H2 is produced by steam reforming of hydrocarbons which is also a source for CO2; thus use of conventional sources of H2 is not a practicable solution to destroying CO2. New non-fossil fuel routes to H2 production might enhance the use of H2 as the reductant, but initial production of chemicals, even if cost competitive, from CO2 is not expected to have significant impact on worldwide CO2 emissions [5]. 5.1. Production of H2 also Produces COs There are two primary sources of commercial production of H2 [other than by-product H2 from dehydrogenation, etc]. They are SR [Steam Reforming] and the partial oxidation of heavier hydrocarbons. SR uses a variety of hydrocarbon sources. Both approaches convert the carbon components to CO2, but a large portion of H2 is derived from added steam. The amount of CO2 generated depends [7] upon the hydrocarbon feedstock. Most of the current chemical approaches to H2 production also produce CO2 as a by-product; however, SMR coproduces much less CO2 than partial oxidation. Therefore, it does not make sense to use H2 to remove CO2 when more CO2 is produced whenever one makes H2. There is a very small need for making CO/H20 or CH4 from COdHz, and we already have ample catalysts for these reactions.
6. H O W CAN W E G E T M O R E H2? With the building demand for Hz - and preferably relatively cheap H2- how are we going to produce H2 to meet the future generation's needs? Steam methane reforming is one of the preferred approaches with natural gas accounting for about 50% of the feed for H2 production. The lower levels of CO2 produced via the use of natural gas feedstocks will continue to make this an attractive feedstock for H2 production. There are some areas of opportunity to consider in modifying or displacing in future decades the current approaches to SMR. These include: the fact that steam Reforming (SR) is energy intensive, endoergic process and large quantities of CO2 are co-produced. Currently considerable H2 purification is necessary to meet the customer's demands, and H2 is needed at >10 atm pressure by most customers. In particular, refineries need H2 a t 500-2000 psi. A number of alternative approaches are being pursued worldwide to generate H2. Some of the more attractive processes include, 9 Oxidative dehydrogenation 9 C02 reforming without carbon formation: C02 4- CH4 - ~ 2 CO + 2 H2 9 Use of methane 9 Solar energy for water electrolysis 9 Selective Oxidation of CH4 9 Thermochemical water splitting combined with solar or nuclear sources of energy 9 Fuel cells [8] 9 Photoassisted water splitting 9 Biomass conversion
146 7. OTHER ISSUES IMPACTING H2 PRODUCTION
Just as with CO2, there are some issues that will restrict our technical approaches, and these need to be appreciated in considering alternative routes to H2 production [9]. The type of feedstock available [NG, heavy oil, etc] will have an impact on the preferred process approach; these feedstocks are controlled by regional issues and supplies out of the direct control of R&D. For a partial oxidation plant one will need a supply of 02 from a nearby air separation plant. Once again the needs of customer [pressure, purity, volume, etc] will have a strong influence on new plant construction. In some regions of the world, the cost of power is strongly influenced by governmental tax credits and subsidies which can make some fuels, technologies and feedstocks much more acceptable. Certainly, the availability of large amounts of capital can severely limit not only the decision to build a plant, but also the type of process chosen. One often forgets about what one does with all the Hz produced. Investments must be made in H2 storage, separation, and purification. Finally environmental regulations [the degree and breadth] will impact the process approach and costs. 8. CONCLUSION I believe that any large scale removal of CO2 will be impacted by the continuing huge, broad, and expanding production of CO2 vs. what one can do with it. The potential production of chemicals from CO2 is small and can only have a limited and localized impact on a global problem. With regard to the use of CO2 as a feedstock, process issues will prevail regarding purity and pressure limitations on CO2 value. In order to use H2 as a reducing agent for such a huge quantity of CO2 one will need CHEAP H2, which simply is not possible in a world market where Hz is in high demand and the price of Hz is set to match the demand. There is no simple solution to destroying all the excess CO2 now produced. I believe the only meaningful approach that we can take immediately to attack the issue of CO2 emissions with technology within the reach of today's knowledge is the insistence on greater energy efficiency in chemical processes, automobile production, power generation, etc. With regard to the production of H2, we need to continue to try to avoid or minimize the coproduction of CO2 in fossil fuel based plants. REFERENCES
1. "Emissions of Greenhouse Gases in the United States: 1987-1992," DOE/EIA Report # 0573, October, 1994, US Government Printing Office, Washington, DC. 2. J.N. Armor, Catalysis Today, to be published 3. M. Burke, Env. Sci. & Techn., 31 (1997) 84A. 4. Catalog "Make your own CO2", Caloric GMBH, Lohenstrasse 12, D-8032 Graefelfing, Germany. 5. J. Rostrup-Nielsen, Natural Gas Conversion II, H. E. Curry-Hyde and R. F. Howe, Eds., Elsevier Science Publishers BV, Amsterdam, The Netherlands, 1994, 25-41. 6. P. Courty and A. Chauvel, Catalysis Today 29 (1996) 3. 7. W. Scholz, Gas Separation & Purification, 7 (1993) 131 8. Chemical Engineering, August 1996, 46 9. J. Abrardo & V. Khurana, Hydrocarbon Processing, Feb. 1995, 43
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
R e d u c t i o n of c a r b o n dioxide to g r a p h i t e catalytic fixation with membrane reactor
147
c a r b o n via
methane
by
Hiroyasu Nishiguchi, Akira Fukunaga, Yumi Miyashita, Tatsumi Ishihara, and Yusaku Takita Department of Applied Chemistry, Faculty of Engineering, Oita University Dannoharu 700, Oita 870-11, JAPAN Catalytic fixation of CO2 to graphite carbon in the temperature range above 500 ~ was investigated with membrane reactor. The process investigated in this study was consisted of two stage reaction, i. e., decomposition of CH4, into C and H2 and CO2 methanation with formed H2. Thus formed CH4 from CO2 was fed to first bed reactor to decompose to graphite carbon. It became evident that nickel supported on SiO2 was active for both reactions. Since the decomposition of CH4 was drastically increased with the application of membrane reactor system which considered of Pd alloy tube to CH4 decomposition. As a result, about 70% of CO2 can be reduced to graphite carbon at 500~ in this system. Furthermore, the high activity of catalyst was stably sustained over long period.
1. I N T R O D U C T I O N
On a global scale, concentration of carbon dioxide in atmosphere have increased by nearly 25 % since the mdustrial revolution. Carbon dioxide is one of the so-called "greenhouse effect" and makes a significant contribution to the global warmmg. Therefore, CO2 fixation should be demanded urgently. Since graphite carbon is harmless and not required the large space to keep, fixation of CO2 into graphite C is one of the promising process for CO2 fixation. In our previous study, reduction of CO2 to C via CO was investigated and it was found that WO~ catalyst is active for this reaction 1). However, excessively high reaction temperature such as 800 ~ was required for this process. In this study, catalytic reduction of carbon dioxide to graphite carbon with methane expressed by the following reaction was investigated for the CO2 fixation from the large CO2 emission sources such as electric power plants.
148 CH 4(g) + CO 2(g) ~
2C (s) + 2H20 (1)
(1)
The AG298of this reaction is -12.1 k J mol 1 and this reaction is exothermic reaction. Therefore, this reaction (1) can decrease the amount of CO2 totally. Furthermore, the amount of CH4 reserves is sufficient as a reductant for CO~ fixation. However, CO2 reforming of CH4 into CO and H9 only proceeds when gaseous mixture of CO~ and CH4 is fed simultaneously.
2CH 4 CO2 + 4H2
~ " ~
2C
+
CH4 +
4H2 2H20
AG298 -10 1.5 k J tool "1
(2)
AG298- - 113.6 kJ mol 1 (3)
/
total
CH4(g ) + CO2(g )
2C (s) + 2H20 (I)
AG298--12.1 k J m o l 1 (1)
In this study, CO2 reduction with methane was investigated on the second step reaction [reaction (2) and (3)]. Comparing with the reduction of CO2 to carbon via CO, high yield of carbon was expected in the low temperature region by the methanation of CO2 followed by decomposition of it. In this process, decomposition of CH4 [reaction (2)] is a rate determining step due to a chemical equilibrium. If the formed H2 is removed immediately from the reaction system 2), the chemical equilibrium in CH4 decomposition can be shifted to the product side. The present study also investigate the catalytic reduction of CO2 into carbon with CH4 applying for membrane reactor system. 2. EXPERIMENTAL Supported metal catalyst were prepared by the incipient wetness techniques. SiO 2 supported Ni and other metal catalysts (10wt%) were used for both CO 2 methanation and CH 4 decomposition. Catalytic activity to CO2 methanation and the decomposition of methane were performed with a pair of conventional fixed bed micro flow reactors at an atmospheric pressure connected in series, of which temperature controlled separately. In the case of membrane reactor system, double tubular type reactor where Pd-Ag tube was used as inner tube of hydrogen permeable film, was used for the reactor of CH 4 decomposition. Ar gas was fed to the inside of Pd-Ag tube at an atmospheric pressure for sweeping the permeated hydrogen. Gaseous mixture of C02, H2, and N2 (CO2" H2" N2 - 1:4" 3) was fed to the catalyst bed at W/F - 50gcat-h/mol, where W and F stand for catalyst weight and flow rate, respectively.
149
The reaction products were analyzed by the gas chromatograph, and the amount of carbon yield was estimated based on the carbon balance.
3. RESULTS AND DISCUSSION 3.1. Methanation of CO2 on supported catalysts
100
silica
0 ............. 0 ............. 0 "
75
The methanation of C02 was proceeded with high conversion over group VIII metals supported on SiO2, in particular, it was found that Ni supported on SiO2 was highly active to the CO2 methanation. Conversion of CO2 attained on this Ni/SiO2 catalyst to the equilibrium conversion, 95%, at 300 ~ On the other hand, decomposition of CH4 only proceeded upon Co or Ni catalyst. In particular, high conversion of CH4 decomposition and almost the theoretical amount of H2 were obtained on Ni/SiO23). Consequently, it became evident that Ni supported on SiO~. is active for both CO2 methanation and decomposition of the formed methane.
cO
~
50
> cO
Ao
o
o
o
25
. . . . . . . A--'"
A "- "- "-,'-'-'-"-I
4-1~]0
....
450
," " "[~
500
~
...D
" " ~ " ..... J
550
,
I
600
Temperature / ~
Fig. 1 Catalytic fixation of CO into carbon via methane with conventional fixed bed reactor. temperature at CO2 methanation was fixed at 300~ (2):of CO ~ i n t o carbon @into CH4 D:into CO
3.2. CO2 fixation with conventional fixed bed reactor Figure 1 shows the dependencies of the activity and the selectivity on the temperature of CH4 decomposition, when the temperature of CO2 methanation was fixed at 300~ Carbon dioxide was completely converted into CH4, CO and C. Although the CH4 was the main product in a low temperature range below 400~ conversion into CH4 and carbon extremely decreased and mcreased, respectively, as the temperature of CH4 decomposition increased. Comparing with CO2 reduction via CO over WO3 catalyst in the previous study, 1) conversion into C was greatly increased by the methanation of CO2 and decompose it, and conversion into C attained to 60% at 700~ However, the conversion into C at 400~ was lower than 10%. This is because the decomposition of methane is restricted by the chemical equilibrium. It is expected that the activity of C H 4 conversion can be exceeded the equilibrium conversion by removing the formed H2
150
from the reaction system. We investigated on the application of m e m b r a n e reactor consisted of H2 permeable film for CH4 decomposition in order to increase the conversion into C in the low t e m p e r a t u r e range. 100
3.3. CO2 fixation with m e m b r a n e reactor
0---- :---v ~ ...... ~. --
Figure 2 shows the comparison of the conversion of CO2 into C with conventional reactor with t h a t of m e m b r a n e reactor. It is clearly shown t h a t the conversion into C drastically increased with the application of memb r a n e reactor to CH4 decomposition. Although the conversion into C was as
75
to Carbon
o~ C 0 .m (].)
-. ............. 0 .of 002 ".,, ",
50
C 0
A
into CH4
A
..'
25
low as 10% at 500~ in the conventional fixed bed reactor, it a t t a i n e d 72% on the m e m b r a n e reactor. The conversion 4"6"o 4so soo sso 600 into C was further increased with Temperature / ~ increasing the flow rate of sweep Ar, Fig. 2 Comparison of the conversion of since the permeation rate of H2 was C O 2 into carbon with conventional reactor increased. Figure 3 shows the effects of with that of membrane reactor. Temperature flow rate of sweep Ar of m e m b r a n e at CO2 methanation was fixed at 300~ reactor system on the catalytic re- closedsymbol;membranereactor duction of CO2. Although the con- open symbol; conventional fixed bed reactor version of CO2 was independent on the flow rate of sweep Ar and attained a I I I I I I
100~_<~
'0'
'
0value as high as 32% at 400~ which is Oof C02 available t e m p e r a t u r e for CO2 fixation in a practical application. On this process, carbon is deposited into CH4 on the catalyst surface and causes the \ deactivation. However, the high g 5o~ activity of CO2 reduction was sustained over 60 h examined. The weight of into Carbon O r,.) Ni/SiO2 catalyst increased from 0.4 to 2.04 g after CO2 reduction for 60 h. The molar a m o u n t of carbon deposited on catalysts was 200 times larger t h a n i , i I 560 i i i i i 1000 t h a t of supported Ni. SEM observation Flow rate of Sweep Ar ff ml 9min-1 and XRD analysis suggest t h a t the formed carbon on the catalyst surface Fig. 3. Effects of flow rate of sweep Ar of memwas a filament shaped graphite. Figure brane reactor on the conversion of CO2 into C. 4 shows the TEM photograph of the Temperature: 400~
151
*~'~:: ~
,~
......
.,,.L...
....
...
,;
..
'
....
.
~ : : .~ ,.
,, : .
,,,r.
,.
..
40nm
Fig. 4. TEM photograph of carbon filaments formed during the carbon via CH4.
CO 2
reduction into
formed carbon filament. The average diameter of formed carbon filament was about 20 nm and it is clearly shown in Fig. 4 that the graphite filament has a tube like shape. The formations of tube-shaped carbon filament were also reported in the CO disproportionation with Ni catalysts 4). Furthermore, the EDX spectra at the points shown in the TEM photograph (Figure 4.) indicated that a small Ni particle was observed at the tip of filament. Therefore, it is expected that that the activity of Ni/Si02 catalysts was maintained even after large amount of carbon was deposited. 3.4. Total process of CO2 fixation Based on the above results, the new CO2 fixation process by using natural gas is mainly consisted of CH4, the first step in this process is the decomposition of
1st step reactor
2nd step reactor
CO 2
~ ~ CH 4 ~- C+2H2 ) ~-
I
Carbon
~
T__
CH 4 + 2H20
Heat Condensor
CH 4
(Natural gas) Fig. 5.
Proposed
C O 2 fixation
process
)
152 C H 4 into C and H2 with membrane reactor shown in Fig. 5. Second step is the methanation of CO2 separated from effluent gases and thus formed CH4 is fed to the first reactor after removing of H20. Although one molecule of C H 4 is consumed for the fixation of one molecule of CO2 in this process, further C02 molecules did not form by the fixation of CO2 and the amount of carbon dioxide can decrease totally by this proposed process. Since the amount of natural gases reserves is
sufficiently large and heat energy for keeping reaction temperature around 400~ is also available from waste heat in the large power plants, the proposed fixation process for C02 has a possibility as practically used.
4. CONCLUSIONS Ni is the most effective catalyst for C H 4 decomposition into solid carbon and H2. SiO2 is the most suitable support for Ni catalyst. Graphite filament was formed, deposition of graphite at least 40 times of moles of the catalyst does not deactivate the catalyst. Application for membrane reactor, there is a marked increase (8-10 times ) in the conversion into solid carbon. REFERENCES i. T. Ishihara, T. Fujita, Y. Mizuhara, and Y. Takita, Chem. Lett., (1991) 2237. 2. J. Shu, B.P.A.Grandjean, A. van Neste, and S. Kaliaguine, Can. J. Chem. Eng., 69, (1991) 1036. 3. T. Ishihara, Y. Miyashita, H. Iseda, and Y.Takita, Chem. Lett., (1993) 93. 4. M. Audier and M. Coulon, Carbon, 23, (1985) 317.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
153
Catalytic reaction of CO 2 with C2H 4 on supported Pt-Sn bimetallic catalysts Jordi Llorca, Pilar Ramirez de la Piscina*, Joaquim Sales and Narcis Homs*. Departament de Quimica Inorghnica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona. Spain.
Abstract
The catalytic activation of CO2 and its reaction with C2H4 and H20 was studied over several silica-supported platinum-tin catalysts under different reaction conditions. The lactic acid production is related to the content of the PtSn alloy in the catalyst. 1. INTRODUCTION Catalytic CO2 activation and its transformation to chemicals is a growing area of research interest, and CO2 has been used as a building block in organic synthesis [ 1-3]. In this context, we are addressing studies on the catalytic reaction of CO2 with low molecular-weight alkenes, which are available in large amounts and at a rather low price, from the petrochemical industry. Although the reaction of carbon dioxide and unsaturated hydrocarbons mediated in homogeneous phase by nickel and palladium systems had been studied extensively [4-6], no processes had been reported by means of heterogeneous catalysis. We have reported that the CO2 and C2H4 molecules coordinate to the silica-supported [PtCI(SnCI3)(PPh3)2] complex, and the coupling of both molecules after reaction over the anchored complex at 42 bar and 393 K. More precisely the methyl 3-hydroxypropanoate has been obtained from the reaction mixture after extraction with a water-methanol solution [7]. On the other hand, we have achieved, for the first time, the catalytic coupling of CO2 and C2H4 in the presence of water or hydrogen to yield selectively lactic acid over a heterogeneous phase catalyst with the silicasupported PtSn (1:1) alloy [8]. Recently, we have acomplished the study of the evolution of the silica-supported [PtCI(SnC13)(PPh3)2] complex after a thermal treatment under H 2 to the formation of only the silica-supported PtSn alloy [9]. We have also tailored the preparation of different silica-supported PtaSnb phases, changing the ratio of [PtCI2(PPh3)2] and SnC12 used as precursors [I0]. In this paper, we present the utilization of several silica-supported
*To whom all correspondence should be addressed. Thanks are due to CICYT (MAT96-0859-C02) for financial support.
154 platinum-tin catalysts of different compositions, in the catalytic reaction of CO 2 with C2H 4 and water. The goal of this paper is to increase &knowledge of the use of PtSn systems in this reaction and to relate the structural characteristics of these PtSn systems to their catalytic properties. 2. EXPERIMENTAL
2.1. Catalyst Preparation Two series of silica-supported Pt-Sn samples were prepared and labelled PtSnT and aPtbSn respectively. In both eases the support was a partially dehydrated Degussa Aerosil-type silica (BET surface-area 200 m2.g'l), and samples were prepared to have ca. 2.5 % (wt/wt) platinum loading. PtSnT samples were prepared by anchoring the cis-[PtCl(SnC13)(PPh3)2] bimetallic complex on silica, following a H 2 treatment at temperatures from 473 K to 873 K as reported elsewhere [9]. The samples obtained were PtSn473, PtSn573, PtSn673 and PtSn873 depending on the reduction temperature. aPtbSn samples were prepared as reported elsewhere [10], by two impregnation steps from cis-[PtCl2(PPh3)2] and SnCI2 solutions, where a/b was the Pt/Sn atomic nominal ratio of the catalysts. These samples were treated under H 2 flow at 673 K. The samples obtained were 5PtlSn, 2PtlSn, 1Pt2Sn and 1Pt5Sn.
2.2. Catalytic activity The reaction between CO 2 and C2H4 was carried out in two different reaction systems, one was a batch reactor and the other one a microreactor operating under continuous flow and differential conditions. The batch reactor was inconel made with a volume of 80 cm3. Other reaction conditions used were: sample charge 350-500 mg, total pressure 40 bar, CO2:C2H4=3:2, reaction temperature 393 K and reaction time 16 h. In this ease, in order to extract reaction products, the solid was treated with 5 cm3 of a 1"1 water-methanol mixture, and after filtering, solution was acidified with HC1 and analyzed by gas chromatography. When reaction was carried out in a continuous flow, reactant gases, CO2, C2H 4 and H 2, were supplied from a pressurized manifold via individual mass flow controllers. H20, when necessary, was introduced into the reactant mixture by a mechanical pump. The reaction pressure was maintained by means of a back-pressure regulator, in the range 25 to 35 bar. Reaction products were analyzed by on-line gas chromatography. In all cases products were characterized by mass spectrometry.
155 3. RESULTS AND DISCUSSION
Over the PtSnT samples the reaction of CO 2 with ethylene was studied in the batch reactor. The results of the extraction of reaction products with an acidified 1:! water-methanol solution are shown in Table 1. In these solutions, 3- and 2-hydroxypropanoic methyl esters were present. The products obtained depended on the reduction temperature T of PtSnT sample. For lower reduction temperatures (473 K and 573 K) both, 3- and 2hydroxypropanoic methyl esters were obtained. The lower reduction temperature produced the higher quantity of HOCH2CH2COOMe and the lower of CH3CH(OH)COOMe. We have reported for the [PtCI(SnC13)(PPh3)2] complex anchored on silica the production of only the 3-methylhydroxypropanoate [7]. When the [PtCI(SnCI3)(PPh3)2] complex has been treated under H 2 at increasing temperatures, its evolution to the welldefined silica-supported PtSn alloy has been shown. This has been the only phase identified after a treatment at 673 K [9] and it is active in the catalytic obtention of lactic acid from CO 2, C2H4 and H20 [8, 9]. After all these considerations, over PtSnT samples, the production of 3hydroxypropanoic methyl ester can be related to the silica-supported [PtCI(SnCI3)(PPh3)2] complex, and the catalytic production of 2-hydroxypropanoic methyl ester with the silicasupported PtSn alloy. Table I. Products extracted after reaction of CO2 and C2H4 over ~SnT samples.
Catalyst
PtSn473 PtSn573 PtSn673 PtSn873
HOCH2CH2COOMe (l.tmol.g" 1 cat)
CH3CH(OH)COOMe (lamol.g" 1 cat)
1.0 0.1 ---
23 63 1800 380
Total Pressure: 40 bar, T=393 K, reaction time 16 h, CO2/C2H4-3/2. The results obtained under the same experimental conditions for aPtbSn catalysts are shown in Table 2. In this case the only product extracted was the methyl 2-hydroxypropanoate. After reaction the presence of lactic acid in the gas phase was confirmed, indicating its formation before the extraction process. Thus, the participation of water, which could be provided by the support, was necessary.
156 Catalysts aPtbSn, which were treated under H 2 at 673 K before the reaction, have no more Sn or Pt molecular species on surface, and several mono- or bimetallic phases have been identified; Pt and PtSn for 5PtlSn and 2PtlSn catalysts, PtSn 2 and PtSn for 1Pt2Sn catalyst and PtSn, PtSn 2 and Sn for 5PtlSn catalyst [10]. All samples presented the silica-supported PtSn alloy, but only in IPtlSn sample, which showed the highest value of CH3CH(OH)COOMe production, did this phase not coexist with other metallic phases. Table 2. Lactic acid methyl ester extracted after reaction of CO 2 and C2H4 over aPtbSn samples.
Catalyst
5PtlSn 2PtlSn 1PtlSn 1Pt2Sn 1Pt5Sn
CH3CH(OH)COOMe (~tmol.g"1 cat) 340 910 2160 1370 304
Total Pressure=40 bar, T=393 K, reaction time 16 h, CO2/C2H4=3/2. Since we have shown that silica-supported PtSn alloy produces catalytically lactic acid from CO2/C2H4/H2 or from CO2/C2H4/H20 mixtures, a study of these processes was carded out over the aPtbSn (a+b) .samples. These studies were carried out under continuous flow of reactants and the lactic acid produced was analyzed avoiding any extraction or esterification step. In Table 3 the catalytic activity of aPtbSn (a~b) samples for the reaction between CO2, C2H4 and H20 is shown. Under the experimental conditions used in this work, lactic acid, was the only reaction product. In fact, only aPtbSn samples with a>b showed lactic acid production. The 2Pt 1Sn catalyst, which has a higher content of PtSn phase, showed a higher production than the 5PtlSn sample, aPtbSn catalysts with a>b have shown by XPS much higher Pt/Sn and Sn0/Sn surface ratios than aPtbSn catalysts with a
157 reaction are shown, Again, it was only possible to detect lactic acid with aPtbSn samples having a>b, these samples also showed the higher C2H4 hydrogenation. Table 3. Catalytic activity in the production of lactic acid from CO2, C2H4 and H20 over aPtbSn samples (a4:b).
Catalyst
F/W (ml.min- 1.g- 1 cat.)
Bmol lactic acid.g- 1 cat.min'l
5Pt 1Sn 2PtlSn 1Pt2Sn 1Pt5Sn
13
167
33 ---
157 140 133
Total Pressure: 35 bar, T=423 K, CO2/C2H4/H20=l/1/1.
Table 4. Catalytic activity in the production of lactic acid from CO 2, C2H 4 and H 2 over aPtbSn samples (a4:b).
Catalyst
lamol lactic acid.g-1 cat.min "1
C2H6/C2H 4 (%)
5PtlSn 2PtlSn 1Pt2Sn IPt5Sn
5 14 ---
11 9 2.5 0.5
F/W (ml.min-l.g "1 cat.) 18 17 17 14
Total Pressure: 35 bar, T=423 K, CO2/C2H4/H2=5/5/1.
The production of lactic acid in this case is interpreted through the reverse water gas shift reaction, which will produce the water necessary for the lactic acid synthesis. In order to compare the catalytic activity of samples in this reaction, a study was carded out at 35 bar of total pressure and 423 K. In Table 5 appears the results obtained with these catalysts in the reverse water-gas shift reaction. CO was obtained in all cases, and although in the experimental conditions used it
158 was not possible to analyze quantitatively the water produced, it was detected for 5PtlSn, 2PttSn and 1Pt2Sn catalysts. Again, aPtbSn samples with a>b showed higher activities. Table 5. Catalytic activity in the reverse water-gas shift reaction of aPtbSn samples (a#b).
Catalyst
F/W (ml.min'l.g -1 cat.)
5PtlSn 2Pt 1Sn 1Pt2Sn 1Pt5 Sn
~tmol CO g-1 cat.min-1
28 27 27 23
37 35 6 2
H20 detected detected detected non-detected
Total Pressure: 35 bar, T=423 K, CO2/H2=1/1.
4. CONCLUSIONS The catalytic activation of CO 2 and its reaction with C2H 4 and H20 occurred over silica-supported platinum-tin catalysts which contained the PtSn phase. A direct relation between lactic acid production and the content of the PtSn alloy in the catalyst is established. A negative effect of tin on the catalyst surface is observed for this process.
REFERENCES
.
5. 6. 7.
10.
A. Behr, Angew. Chem. Int. Ed. Engl., 27 (1988) 661. A. T. Ashcrott, A. K. Cheetham, M. L. H. Green and P. D. F. Vernon, Nature, 352 (1991)225. D. Walther, G. Braunlich, U. Ritter, R. Fischer and B. Sch6nocker, in Organic Synthesis via Organometallics, K. H. DOtz (ed.), Vieweg, Braunschweig, 1991, p.77. G. Burkhart and H. Hoberg, Angew. Chem. Int. Ed. Engl., 21 (1982) 76. P. Braunstein, D. Matt and D. Nobel, Chem. Rev., 88 (1988) 747. T. Tsuda, K. Maruta and Y. Kitaike, J. Amer. Chem. Soc., 114 (1992) 1498. J. Llorca, P. Ramirez de la Piscina, J. Sales and N. Homs, J. Chem. Soc. Chem. Commun., (1994) 2555. J. Llorca, P. Ramirez de la Piscina, J. L. G. Fierro, J. Sales and N. Homs, J. Catal., 156 (1995) 139. J. Llorca, P. Rarnirez de la Piscina, J. L. G. Fierro, j. Sales and N. Homs, J. Mol. Catal., A, 118 (1997) 101. J. Llorca, N. Horns, J. L. G. Fierro, J. Sales and P. Ramirez de la Piscina, J. Catal., 166 (1997) 44.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
159
Initial transient rates and selectivities of Fischer-Tropsch synthesis with C O 2 as carbon source Hans Schulz, Georg Schaub, Michael Claeys, Thomas Riedel, Stefanie Walter Engler-Bunte Institut, Universit~it Karlsruhe, Kaiserstral3e 12, 76128 Karlsruhe, Germany
Initial transient changes of conversion and selectivity during Fischer-Tropsch synthesis with a potassium-promoted iron catalyst with H2/CO 2 and H2/CO syngases were determined and several episodes of catalyst transformation distinguished. Selectivity changes are related to changes in elemental reaction steps probabilities using the model of "non trivial surface polymerisation". With the H2/CO 2 syngas the catalyst transformation episodes are extended to about one week. Transient episodes are discussed as for compositional and structural catalyst changes and related Fischer-Tropsch surface chemistry. 1. INTRODUCTION Knowledge about transient episodes in catalytic conversions may contribute to the understanding of their stationary state. In Fischer-Tropsch CO-hydrogenation, the mechanism is undoubtedly complex, and the reaction steps occur among chemisorbed species; their rates are not directly measurable, however, they can be calculated with the help of a kinetic model. Particular methods of performing the reaction and the sampling and analysis of the products are required to obtain the time resolution of reaction rates and selectivities. Early FT-literature reveals (particularly for iron) an initial catalyst transformation time [ 1]. FT-iron catalysts generally exhibit WGS-activity, which is advantageous in case of CO-rich syngas (from coal) and should be essential for FT-synthesis with a H2/CO 2 syngas. In this investigation a strongly potassium-promoted iron precipitation catalyst is used. Through momentaneous product sampling and internal calibrations, time resolved mass balances are attained [2, 3] and with the help of special GC techniques a detailed analysis of product composition is achieved, which is converted into kinetic data (reaction probabilities) for individual repeatedly at each carbon number occurring steps of surface reactions on the basis of the kinetic model "non trivial surface polymerisation" [4], the term being introduced for characterizing the FT-reaction to some extent as a heterogeneous polymerisation, however, combined with a number of further catalytic reactions in particular for synthesizing the chemisorbed monomer from CO and H 2. 2. EXPERIMENTAL
The FT-conversion has been conducted in a fixed bed reactor with the finely powdered catalyst (dp < 0.1 mm) covering larger fused silica particles (dp = 0.25-0.4 mm) as an adhering layer, the weight ratio of catalyst to fused silica particles being 1:10. By this means a very isothermal catalyst bed was provided with highly uniform flow of the gas phase, minor pressure drop and no noticeable intraparticle resistance influence. The catalyst was prepared through quick precipitation from a hot aqueous solution of the nitrates with an N H 3 solution. The precipitate was washed, dried and the potassium added (as
160 potassium carbonate solution) to yield the catalyst composition 100 Fe/13 A1203/10 Cu/25 K (weight ratios). In the reactor the catalyst (1 g of iron) was dried/calcined at 673 K for 16 hours in an Ar stream (0.1 MPa, flow = 40 ml/min (NTP)) and subjected to a reductive treatment (heated with 2 K/min to 673 K and held for 10 hours at this temperature at 0.1 MPa in a H2/Ar stream (molar ratio = 1/3) with a flow rate of 40 ml/min (NTP)). For the synthesis the reactor was cooled in Ar to the reaction temperature of 523 K, the synthesis gas flow adjusted to 30 ml/min (NTP) in the bypass line, and the reference stream of cyclopropane in nitrogen added to the reactor effluent for determination of conversions and yields from the chromatogram. A liquid product fraction was recovered in a hot trap at about 523 K before the pressure reducing needle valve. At zero time the synthesis gas flow was switched to the reactor and ampoule samples were taken from the gaseous product stream at 523 K for later GC analysis. GC analysis has been developed for high resolution of a wide product carbon number range C 1C20 and precolumn hydrogenation for olefin saturation. 3. R E S U L T S A N D D I S C U S S I O N In this section, comparatively, results obtained with the H2/CO 2- and the H2/CO-syngases are presented, regarding in particular the changes of activity and selectivity occurring with increasing duration of the experiment and discriminating distinct episodes of transformation of the catalyst, respectively, distinct episodes of transient kinetic regimes, until the stationary state of synthesis is established. The first figure shows the conversion of CO 2 (for H2/CO 2 syngas) and of CO (for H2/CO syngas) and the yields of CO respectively of CO 2, of organic c o m p o u n d s and of carbon >.-
3O
._1 uJ
I
II
m
10
i
0
100
. l01
60
c.b
40
20
i,j:
.i~i!"'~Yorg.cpds.
y
~/co :..--~.,~,<~
i
~a,c~.,n..a..i.~.:i~3.':..~:::s~ii:[..;,............!..... 102 103 104
I
n
::Ill
80
.~ 0~
..~"
;:~ol
~t~ .... ~::-~i~ ,--.....::.~r ~, .~.~p,. . . . .
v,,~:~x---~::~?::
0 0
iii.:.~ ~':.
i i
~;i~ '~ ,
::~:i'~--- C2+-HCs. ~" ! , Oxygenates
:~:. . . . . . igtz::~.::~iz i i.: : :.:. : ..o.:::.:~:~::-~::..~...:::..9:.[..L'S--_
. . . . . . .
i
tI
>_m-
, . . . . . . . .
!
(C1+)
ell4
. . . .
0101 102 103 DURATION OF EXPERIMENT.
! "
~. . . .
I
104 texp. min
I II
100
80
5<'o~
60
0 0
t~i
0
V
.r
~"" ....... .~. i i~i~. . . .
~::
:i:i!i.. :
:.t~
i'~ ~ ....
i
', .....
: : :::-. ! i i. ~ : "::: : ~ - - - Y c o z i
hJLL~';O~"' .... ' : 101 102
I...t
r
80 [-
11i 111 i
i
VI
i":.~- X~o
:
L i .~t~t~'~/"..~:"
10011'
>.-
IV
[" !~.~..~..~:.~"
rr' uJ
Z
HI
:. H'2/C0-23
_.1 I.U >.
IV :: V
_ :~-~......... ,~
]
:~i ~'- Xco2
i :
(1I)
V
I
H2/C02=3
2O
70 d
IV
HI
....
i 103
:g
: ........
u.I ._1 u.I u~
40_
20 L ~
H2/CO=2"3 , !
I
..:#" ~! '~::~:%
L .....~esi~::'%-,~_j_. 02 ~.H C s 60 r ~ ' : i ' "
L
,
104
11/ , , r ~ ........i
I-
>_.o~
.rg.cpas.
i
. ' !~Oxygenates'Cl+l
i
i ......
101 102 103 DURATION OF E X P E R I M E N T
I
104 texp, rain.
Fig. 1; above: Conversion of CO 2 respectively CO and yield of organic compounds, carbon deposited on the catalyst and CO respectively CO 2 as function of duration of the experiment with the synthesis gases H2/CO2=3 (left) and H2/CO=2.3 (right). Transient episodes of catalyst transformation are indicated at the upper rim of the diagrams; below: Group selectivities of organic compounds in dependence of duration of experiment. (100 Fe/13 A1203/10 Cu/25 K; 523 K; 1MPa; Feed gas flow: 30 ml (N'I?) per minute and g-Fe)
161 deposited on the catalyst. This carbon yield Y zxc is determined as the difference in the carbon balance between conversion and yield of products in the reactor effluent. The yield of organic compounds comprises also the waxy products accumulated in the hot trap, which have been calculated by extrapolating the product chain length distribution. YaC reflects the carbon formation through CO-decomposition ( CO ~ C + O ). It is to be seen in Fig. ! that with both synthesis gases initially carbon deposition on the catalyst is the almost exclusive reaction. This carbiding is an intrinsic process of iron Fischer-Tropsch catalyst transformation. With the CO 2 syngas the yield of catalyst carbon is relatively high for about 2 hours (102 minutes) and then declines to about zero within one day (103 minutes). With the CO syngas carbon formation initially is higher and continues all over the experiment (as to be explained by the high carbon formation tendency of the strongly alkalized catalyst) leading to catalyst deactivation; the catalyst bed being finally plugged by the produced black carbon after 3 days. With the CO 2 syngas the CO, which is used for carbon formation, is produced through the reversible water-gas-shift- reaction ( CO 2 + 1-12 ~ CO + 1-120 ) and Fig. 1 (left) shows that in the first 100 minutes "no" CO is left, all being decomposed to carbon and oxygen. During the episode III with the CO 2 syngas, the CO yield increases and carbon deposition declines to very low values, this indicating the WGS-activity of the catalyst to increase and the catalyst carbiding process coming to an end. Then only in episode IV after about 103 minutes (about 1 day) the Fischer-Tropsch activity develops ( CO 2 + 3 l-I2 ~ (CH2) + 2 H20 ). In this episode also the CO yield increases, indicating the WGS-activity of the catalyst also to increase. It is assumed, that the now higher CO partial pressure makes the gas phase more reductive, causing increased surface reduction. It needs about 6000 minutes (4 days) until the stationary state (episode V) is attained. The decline of CO yield from about 3000 minutes onwards is not caused by a decreasing WGS-activity. Calculations show the WGS-reaction to be in equilibrium here and the decreasing CO yield being due to the decrease of its equilibrium concentration as a consequence of increasing b-T-conversion (more H20, less CO 2, less H2). It is concluded that at the stationary state the WGS-activity is very high and the reaction rate constant much higher than that of the FT reaction. It is also evident, that the Fischer-Tropsch reaction needs the CO, which is formed via the WGS-reaction, and cannot proceed through direct CO 2 hydrogenation. Then the slow establishing of the stationary FT-kinetic-regime with the CO 2 syngas results from the only slow carbiding of the iron catalyst. With the common H2/ CO=2.3 syngas (Fig. 1, right) the catalyst transformation episodes are much shorter, however, similar trends can be noticed. Additionally the episode VI of deactivation occurs after an about 1 to 2 days lasting pseudo stationary state episode. The selectivity of organic compounds, as grouped for hydrocarbons C2+, methane and oxygen compounds in dependence of duration of the experiment, is presented in Fi~. 1, below. Comparing the results with the CO 2 and the CO syngas reveals surprisingly the stationary selectivity to be almost the same for both syngases and this being true for all of the further selectivity data in Figs. 2, 3 and 4. It is concluded that the catalyst transformations as (1) Deposition of carbon from CO, (2) formation of bulk and surface carbides and elemental carbon, (3) reducing and oxidizing reactions of the iron with H 2, CO, CO 2 and H20 and (4) redistribution of the alkali (spreading on the surface) approach the same kind of FT active surface with its unique set of reaction capabilities: (1) Dissociative chemisorption of H 2, (2) dissociative chemisorption of CO, (3) transfer of activated hydrogen to chemisorbed species, (4) formation of C-C bonds and (5) inhibition of desorption reactions, particularly of associative desorption of paraff'ms (a prerequisite of the surface polymerisation mechanism).
162
100 o D~
~
80
!1~\
60
//~
uiE 40 I--
~" x 0
~"
g
20 0
!
~
2
80
<+
60'
a=o .J < 40 -1I.U 0
..J "~
\\
_
~
4
6
10
4710 men,(V) . ~ ~ _ _ ~ . 9 6 0 min, (V)
-
,
w
I
50 rain, (111)
00 min, (IV) 80 min, (IV) 20 rain, (V)
2
4
6
8
100 - H2/CO=2.3 ~
-o. i
0
80
<+
60
_
10
(hi / iv)
c:r"
~200 mqn, (IV) v
I
"vt
4o
,~r "1"
H2/002=3 0
~
0
o
A -!:
(IV)
60
20
m,n,(V)
8
H2/CO=2.3
z-~ ~ E 40
4710 min,.(V)
lin, (111)
20 0
480~in, (111) ~
80
"-1o~
(11)
~ 9 9 6 0
~300min,
100
O, uj--'r: ~
,
0
~ 1 2 min, (I)
~ m i n
100
au) 13.
H2/CO2=3
t
2 4 6 8 CARBON NUMBER, Nc
10
'"
o_1 <:
20
0
~
0-
2
i
n
4
'
(V)/
6
8
10
CARBON NUMBER, Nc
Fig. 2, above: Content of oxygen compounds in carbon number product fractions at different episodes of catalyst transformation with the synthesis gases H2/CO2=3 and H2/CO=2.3; below: Aldehydes among aldehydes plus alcohols; (For further exp. data see Fig. 1) With the CO 2 synthesis gas, with its slow establishing of the FT-kinetic-regime and its initially much oxidizing (CO 2, H20 ) and less reducing (CO, H 2) atmosphere in the episodes I, II and III when the FT-activity of the catalyst is still very low, the produced organic compounds are mainly oxygenates, specifically mainly aldehydes (Fi~. 2). It is assumed that in these episodes the availability of reactive hydrogen is low and that of OH-species is high, this favouring the reaction of hydrocarbon species with OH-species, which can desorb as aldehydes via [3-H-abstraction from the OH-group: H3C \
J.L
H3C OH \/ >
H3C- CH
=O
+
H
Methane selectivity never becomes excessive as to be explained by the alkali promoter action inhibiting associative paraffin desorption. Time dependence of chain growth probability (Fi~. 3) exhibits the same trends with both synthesis gases, however more pronouncedly with the CO 2 syngas. The growth probability generally increases with time until the (almost) same curve of growth probability as a function of carbon number is attained at the stationary state in both experiments. This increase of growth probability reflects the establishing of the FTH r e g i m e . In an ideal p o l y m e r i s a t i o n scheme the growth R - C H =CH 2 + ,.j..,.. probability is carbon number independent, as represented by a Jb, horizontal line. The deviations hereof to be seen in Fig. 3, above, specifically in the episodes III and IV with the CO 2 syngas, are R- CH 2 - ~ H 2 H),H attributed to olefin readsorption on FT sites (see kinetic scheme).
163
o) CL
>i-.,J m
H2/CO2=3
0.8
'
~
9960 rain, (V) ~
~J~"~-o'~--1800min, (iV)
0.6
~
i
~
i
n
,
I
I
I
(V)
~ 7 8 0 rain, (111)
t:m C1.
>.. I-
0.8
m < m 0rr I1.
0.6
"I-
0.4
I
I
I
I
I
I
I
2
4
6
8
10
12
14
(
1
/-1120 min, (V) J 1 / IV)
1
~~I~-.i~..~-50
min, (111)
o--o"---12 min, (I)
0.4
"
0n,"
H2/CO=2. 3
,, ~
,-I
I
I
2
o
,
4
I
6
I
8
10
!
I
12
14
.
=....
.Q
_ H2/CO2=3
>: 0.06
,_1
m
0
H2/CO=2.3
I-.- 0.06
..J
~ 9 9 6 0
< 0.04 Ill O n-" ". 0.02 (:3 .Io
min, (V) min.,V,
1800 mm, Uv~ (O) I
0
2
I
I
I
I
I
I
z
4 6 8 10 12 14 CARBON NUMBER, Nc
0
< in
~~.~-1120
min, (V)
. ~ ~ ~ 0 o
min, (Ill/IV)
50min,=(lll) = ~ I
0
2
= !
4 6 8 10 12 CARBON NUMBER, Nc
14
Fig. 3; above: Chain growth probability in dependence of carbon number at different transient episodes of catalyst transformation with the synthesis gases H2/CO 2 = 3 and H2/CO = 2.3; below: Branching probability; (For further exp. data see Fig. 1) 100
/~
II ~
~..r.
J z 40 O z
--~100 ~ 0 80
~ 9 9 6 0 m i n , ( v )
_z~. 60 .J
J
H2/CO2=3.
5 80 u~E
~ ~0'~,,,,x~__'4710 rain, IV) Z "~''"~-~ -'~>'1800min, (IV) u~f~"60
o~o
(Ill)
rain,
u "r"
1120rain,
200 min, (V) (iv)
,
,
,
,L 101 min'' (111/ IV)I
2
4
6
8
~ 20
20 0
40 "
~
I
H2/CO=2.3 . . . .
I
0
2'
100
' 4'
6
8' ' 0 '12
1
!
O0 -
14
10
12
14
100
e-"::e:-".-~.-~-v....~.::,:,.~. '
,........
0
mE
<>
.z ~ 80
H2/C02-3
99960rain, (V) D 4710min, (V) 0 1800rain, (IV)
UJ
oI 0
'-z
60
--I
~E z
0
H2/CO---2.3
8o
91120min, (V)
n 200 min, (IV)
0 100 rain, (111/IV) o 50min, (111)
,0
"z
60
.J I
z
o
2
I
I
I
I
I
I
4 6 8 10 12 14 CARBON NUMBER, No
z
I
o
2
I
I
I
I
I
I4
4 6 8 10 12 1 CARBON NUMBER, Nc
Fig. 4; above: Olefin content in carbon number product fractions at different transient episodes of catalyst formation with the synthesis gases H2/CO2=3 and H2/CO=2.3; below: 1-olefin content among n-olefins; (For further exp. data see Fig. 1)
164
Time dependence of branching probability (Fig. 3, below) is low and hardly significant in both cases. Typically shaped curves of branching probability were obtained showing a strong decrease with carbon number. As branching reactions are assumed as space demanding, the respective spacial constraints would arise from surface carbiding and surface alkali coverage. Olefinicity of the product is pictured in the diagrams of olefin content in carbon number hydrocarbon fractions and 1-olefin content in n-olefin carbon number fractions (Fi~. 4). Horizontal lines (no influence of carbon number) represent primary olefin selectivity. Then these diagrams show that with both synthesis gases, from the beginning of FT-product formation, the olefins are almost exclusively of primary nature (almost solely t~-olefins). This primary olefinicity increases with time, e.g. in Fig. 4, left, from about 35 to about 80 per cent. Selectivity of oxygenates as a function of time, shown in Fi z. 2, features a high initial aldehyde formation tendency as discussed already above. Among the oxygenates, acetaldehyde is the dominating compound. Whereas in the early episodes the selectivity of alcohols is much lower than that of the aldehydes, the alkanol selectivity at the stationary state is much higher than of the aldehydes, the overall selectivity of oxygenates being generally low in the stationary state. 4. CONCLUSIONS With a H2/CO 2 syngas and a potassium-promoted iron catalyst relatively long lasting catalyst transformations occur. 5 transient episodes have been distinguished: I The first about 20 minutes include the dead time of the apparatus (ca. 10 minutes) and the initial adsorption of reactants. II About 20 to 180 minutes of experiment duration: The catalyst is being carbided through CO-decomposition. The CO is formed by the WGS-reaction. The formation rate of organic compounds is very low and preferentially oxygen compounds are obtained indicating a preferred reaction of hydrocarbon species with OH-species. III About 180th to 1000th minute of the experiment: Declining rate of carbon deposition, increasing WGS-activity, change of selectivity from oxygenates to hydrocarbons. The surface properties are assumed as shifting to a more reduced and carbided state. IVAbout 1000th to 6000th minute of the experiment: Only now the FT-activity begins to develop. This is explained by a now sufficient availability of activated hydrogen for being added to the surface carbon. V After ca. 6000 minutes the stationary state is reached. On the same catalyst using H2/CO syngas the carbon deposition continues after attaining maximum FT-activity and leads to deactivation. The CO decomposition rate then is too high as related to hydrogen transfer to carbon and chemisorbed species. These findings about transient episodes of catalyst transformation are useful for the improved understanding of the unique kinetic regime of Fischer-Tropsch synthesis. REFERENCES
1. 2. 3. 4.
H. Pichler, Advances in Catalysis IV, 271-342 (1952) Ed. P.H. Emmett H. Schulz, E. Erich, H. Gorre and E. van Steen, Cat. Letters 7 (1990) 157 H. Schulz, E. Bamberger and H. Gorre, Proc. 8th Int. Congr. on Catal., Berlin 1984, Vol. H, p. 123, Ed. Dechema, Verlag Chemie, Weinheim 1984 H. Schulz, K. Beck and E. Erich, Proc. 9th Int. Congr. on Catal., Calgary 1988, Vol II, p. 829, Eds. M.J. Phillips and M. Ternan, The Chem. Inst. of Canada, Ottawa 1988
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
165
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Palladium-catalyzed carboxylation of allyl stannanes and carboxylative coupling of allyl stannanes and allyl halides Min Shi, Russell Franks and Kenneth M. Nicholas* Department of Chemistry and Biochemistry, University of Oklahoma, Norman, OK, 73019 U.S.A. Abstract. The reaction of allyl stannanes with CO 2 to form stannyl carboxylates (esters) is catalyzed by Pd(PR3) 4 complexes.
Thus, R 3 S n C H 2 C H = C H 2 [R-Me and Ph] are converted to
R3SnO2CCH2CH=CH 2 and R3SnO2CCH=CHCH 3 under 33 atm of CO 2 (70 ~
THF) in moderate
to excellent yield in the presence of 8 mol% Pd(PPh3)4; polycarboxylation of diallyldibutyltin and tetraallyltin
also
is effected,
producing
the respective
di-
and
tetracarboxylates,
Bu2Sn(O2CCH2CH=CH2) 2 and Sn(O2CCH2CH=CH2) 4, along with the corresponding isomeric crotyl derivatives. In the presence of allyl halides, allyl stannanes and CO 2 undergo carboxylative coupling to produce allyl esters. Under these conditions other Sn-C (Sn-alkyl, -awl, -vinyl) and Si-C (Si-allyl, -alkyl, -aryl, -vinyl) bonds are inert to carboxylation and carboxylative coupling with allyl halides. A tentative mechanism is proposed to account for this catalytic carboxylation of a main group metal-carbon bond.
Introduction The world's dwindling petroleum reserves and increasing atmospheric concentrations of carbon dioxide have stimulated considerable interest in the capture and chemical conversion of carbon dioxide. Among several approaches being examined towards this objective is the activation of carbon dioxide by transition metal complexes. 1 Reactions of CO2 which result in carbon-carbon bond formation are especially synthetically attractive, e.g. the classic carboxylation of Grignard reagents. Indeed, insertions of CO2 into the metal-carbon bonds of electropositive main group metals 1b,2 and many transition metals 1 are common but corresponding reactions with less polar organometaUics, though still thermodynamically favorable, 3 are rarer. Promoting such transformations would be highly desirable since the resulting metal carboxylates should be more amenable to subsequent transformation of the weaker M-O bond, enhancing the prospects for catalytic conversions to organic products. We report here the first examples of transition metal catalyzed insertion of CO2 into otherwise unreactive tin-carbon bonds and a catalytic coupling reaction between allylstannanes and allyl halides.
166 Results
and D i s c u s s i o n
Although allyltributyltin does not react with CO2 in THF even at 70 ~ and 33 atm (24 hr), under the same conditions in the presence of 8 mol % Pd(PPh3) 4 or Pd(PPh3)2C12/PPh 3 this stannane is quantitatively converted to the isomeric tin esters (eq. 1).4,5 Under the same carboxylation conditions a 30% conversion of allyltriphenyltin to isomeric insertion products (7:3) was found. Surprisingly, crotyltributyltin failed to undergo carboxylation entirely.
R,Sn.CH2.CH=CHR,
9,
CO 2 (30 a t m ) .__ pd(PPh3)4 o r ~
RsSn-O-C-CH2-CH=CHR'
+
I R3Sn-O-C-CH=CH-CH2R'
(R=Bu, R'=H)
Pd(PPhs)2CI2/PPh 3
(90 %)
(R=Bu, R'=H)
(10 %)
(R=Ph, R'=H)
THF, 70 ~
(70 %)
(R=Ph, R'=H)
(30 %)
24 h
(R=Bu, R'=Me)
no reaction
(R=Bu, R'=Me)
Multiple carboxylation of poly(allyl)stannanes also can be effected with Pd(0) catalysis. Thus, diallyldibutyltin reacted completely with CO 2 under the standard conditions to afford dicarboxylates (overall yield >90%). In the Pd-catalyzed reaction of tetraallyltin with CO 2 a complex mixture of
Bu2Sn(CH2-CH=CH2)2
co,
>
Pd(PPh3)4, THF, 70 oC, 24 h
~
Bu2Sn(O-C-CH2-CH=CH2)2
C02
-~
f'
Bu2Sn(O-C-CH=CH-CH3) 2
( 2.2 : 1 )
?
[ Bu2(CH2=CHCH2)SnOCCH2CH=CH,]
O-C-CH2-CH=CH2 I 0
Sn(CH2-CH=CH2)4
+
(i) Sn(O-C-CH2-CH=CH2) 4
Oi Sn(O-C-CH=CH-CH3) 4
Pd(PPh3)4, THF, 70 ~ 72 h
Sn(O-C-CH2-CH=CH2)4.~O.C-CH=CH-CH3)n
(n=l, 2, 3, 4)
carboxylates (allyl)4_nSn(O2C-allyl) n (n=l-4) was produced after 24 hr. However, if the reaction time was extended to 72 h, the major products (ca. 80 % yield) were the isomeric tetracarboxylates, i.e. all four Sn-allyl bonds had inserted CO2. Under these conditions double bond isomerization products dominated ca. 4:1. Of the various stannanes tested thus far only those possessing allyl-tin bonds were found to undergo carboxylation. Thus, tetrabutyltin, tetraphenyltin, vinyltributyltin, and benzyltributyltin each
167 were recovered unchanged after contact with CO2/Pd(PPh3) 4 under the previous conditions. The failure of the crotyl- and benzyl-tin derivatives to react with CO 2 and the diminished reactivity of allyltriphenyltin suggests that a specific Pd-allyl interaction may play an important role in catalysis. The allyl silanes aUyltrimethylsilane, allyltriphenylsilane, and allyltrimethoxysilane also did not react. These observed structure/reactivity features in combination with the established reactivity of Pdallyl complexes with CO26 lead us to propose Scheme 1 as a tentative catalytic mechanism: Scheme 1
stannaneactivation/f----'~ ~
~-~
PPh3/Pd~SnR3
• ' •~'snR3
Pd(PPh3)2
"~
(PPh3)2Pd-- SnR3
R3Sn
17~
Pd(PPh3)n
~ , ~ . . ~
coJ
(PPh3)2Pd"1~: 0"~ Sn
(PPh3)2Pd-- SnR3 Pd(PPh3)n +
C02
C02 activation
_
",--- (PPha)2Pd :'? "C~0
~_] R3Sn
i) oxidative addition of allylstannane to Pd(0)L n gives rl 3- and 1"11-allyl palladium complexes; ii) insertion of CO2 into the 111-allyl palladium bond (or the Pd-Sn bond); and iii) reductive elimination of the carboxylate and organotin groups to give the product and regenerate Pd(0). The sensitivity of the reaction to substitution on or adjacent to the Sn-allyl unit suggests that an initial rl2-complex may precede the oxidative addition step but no indication of complexation of the allyl stannane by Pd(PPh3) 4 was provided by 1H NMR analysis. Although transmetalation from Sn to Pd(II) 7 and the
168 activation of allyl electrophiles (e.g. halides, carbonates) by Pd(0) complexes 8 are both well documented, Pd(0) activation of aUyl tin reagents has little precedent. 9 However, the viability of an (rl3-allyl)Pd(stannyl) intermediate is supported by formation of a Pt analog in the reaction of Pt(ethylene)2(PPh3) with (allyl)SnMe3 .10 The possibility of CO 2 insertion into the Pd-C bond of the latter is bolstered by prior examples of CO 2 insertion reactions of ri 1_ and rl3-allyl-Pd complexes. 6 Since neither starting material nor product isomerization occurs under the reaction conditions (op cit), isomerization of the intermediate CO 2 inserted species apparently occurs prior to product release. 11 An alternative mechanistic pathway involving reaction of an intermediate Pd(rl2-CO2) complex 12 with the allyl stannane cannot be excluded at this time. However, IR spectra of Pd(PPh3)4/CO 2 solutions (1 atm, THF) showed no absorptions in the 1625-1700 cm- 1 region expected for such a species. We have also begun to investigate the possibilites for modifying the stannane carboxylation to produce wholly organic products. Bond energy considerations suggested that metathesis of the tin carboxylate with an alkyl halide might be possible. Although no reaction was observed betweeen the tin carboxylate and 2-methallyl chloride (65 ~ THF), in the presence of Pd(PPh3) 4 under the same conditions, clean conversion to the corresponding allyl ester (below) was observed. Moreover, combined reaction of allylSnBu 3, CO 2 and methallyl chloride in the presence of Pd(0) or Pd(II) complexes produced a mixture of organic esters (below) in moderate (unoptimized) yield. A major component was that expected from allyl-Sn carboxylation followed by metathesis with the allylic halide; a second major product, however, appears to be derived from initial carboxylation of the methallyl chloride. c..3 0
Bu3Sn~,~
+
Pd(PPha)4 THF, 65 ~ 12 h
+
~
Cl
co=
.tm)
Pd(PPh3)4 or Pd(PPha)2CI2/PPh3 THF, 70 ~ 24 h
+
BuaSnCI
Bu3SnCl
U ( 50 % isolated yield )
As in the direct stannane carboxylation the carboxylative coupling is thus far limited to allyl stannanes; RSnBu 3 with R = PhCH 2, Ph, vinyl, PhC-C- all fail to react. The structure of the
169 isomeric ester (propenyl 3-methyl-3-butenoate) and existing precedents for the formation of allyl-Pd species 8 and their insertion reactions with CO2 6 suggest the involvement of a second pathway. involving initial carboxylation of the aUyl halide (Scheme 2). Scheme 2
I C'l
cl~Pd~ppha
co2
R3Sn"~'~
~,~pph P~snR3 ~
Pd(PPh3)n
o3-.A I
(PPh3)2Pd--Cl RzSn,/",,~
]
I
O ) CO2
I
(PPh3)2Pd~SnR3
~,~
C.H3
-vc,
In conclusion we have reported on two new reactions relevant to the goal of carbon dioxide utilization. In the first, Pd complexes catalyze the net insertion of CO 2 into otherwise unreactive Snallyl bonds. The product organotin carboxylates, especially diorganotin esters, have a wide range of commercial applications. 13 In the second reaction the same Pd complexes catalyze the coupling of allyl stannane, allyl halide and CO 2 to produce allyl esters. Efforts are underway to further elucidate the mechanistic details of these reactions and to extend their scope to include a range of organometals and organic halides. Acknowledgments. We are grateful for financial support provided by the U.S. Department of Energy, Office of Basic Energy Sciences.
170 References.
1)
2) 3)
a) Behr, A. "Carbon Dioxide Activation by Metal Complexes", VCH publ., Weinheim, Germany, 1988; b) Ito, T.; Yamamoto, A. "Organometallic Reactions of Carbon Dioxide", chap. 3 of Organic and Bio-organic Chemistry of Carbon Dioxide, Inoue, S.; Yamazaki, N., ed. Kodansha Ltd., Tokyo, Japan, 1982; c) Gibson, D.H. Chem. Rev. 1996, 96, 2063. Sanderson, R.T.J. Am. Chem. Soc. 1955, 77, 4531. Bond enthalpy considerations for M-CR 3 + CO 2 ---> M-OC(=O)-CR 3 indicate an exothermic
4)
C bond and a C-C c-bond is formed at the expense of a comparably strong C-O re-bond. The following procedure is representative. Allyltributyltin (0.33 g, 1.0 mmol), Pd(Ph3P) 4
process for virtually all metals since a strong M-O bond is formed at the expense of a weaker M-
(0.093 g, 0.08 mmol), 20 mL of dry THF, and a magnetic stir bar were placed in the 50 mL glass liner of a stainless steel autoclave under a nitrogen purge. After the autoclave was purged several times with CO 2, it was pressurized with CO 2 (33 atm), sealed, and heated at 70 ~ for
5) 6)
7) 8) 9)
24 h with stirring. After cooling and release of the pressure, the solvent was removed under reduced pressure and the residue was passed through a short column of flash silica (1:1 ethyl acetateJhexane) to afford a spectroscopically pure mixture of the tin carboxylates. A preliminary report of this study has appeared recently: Shi, M.; Nicholas, K.M.J. Am. Chem. Soc. 1997, 119, 5057. a) ref. la, p. 54-60; b) Ito, T.; Kindaichi, Y.; Takami, Y. Chem. Ind (London), 1980, 19, 83; c) Hung, T.; Jolly, P.W.; Wilke, G. J. Organomet. Chem. 1980, 190, C5; d) Santi, R.; Marchi, M. J. Organomet. Chem. 1979, 182, 117; e) ref. la, p. 56; f) Behr, A.; Ilsemann, G. v.; Keim, W.; Kruger, C.; Tsay, Y.-H. Organometallics, 1986, 5, 514; g) Behr, A.; Ilsemann, G. j. Organomet. Chem. 1984, 276, C77. Stille, J.K. Angew. Chem. Int. Ed. Eng. 1986, 25, 508. Trost, B.M. Acc. Chem. Res. 1980, 13, 385; Tsuji, J. Pure Appl. Chem. 1982, 54, 197. Catalysis of allyl-Sn additions to aldehydes by Pd(II)L2C12 complexes recently has been
reported; one, less efficient example using Pd(PPh3) 4 was included. Nakamura, H.; Iwama, H.; Yamamoto, Y. J. Am. Chem. Soc. 1996, 118, 6641. 10) Christofides, A.; Ciriano, M.; Spencer, J.L.; Stone, F.G.A.J. Organomet. Chem. 1979, 178, 273. 11) Extensive double bond isomerization has been found in the reactions of (rl3-allyl)Pd complexes with CO2; ref. 6 c-f. 12) Sakamoto, M.; Shimizu, I.; Yamamoto, A. Organometallics, 1994, 13, 407. 13) Kirk-Othmer Concise Encyclopedia of Chemical Technology, Grayson, M. and Eckroth, D. eds., 1985, Wiley, NY, pp. 1179-1181; Sijpesteijn, A. K.; Luijten, J. G. A.; van der kerk, G. J. M. in Fungicides, An Advanced Treatise, Torgeson, D. C., ed., Academic Press, New York, 1969, Vol. 2, p. 331; Evans, C. J. Tin and Its Uses, 1976, 110, 6; van der kerk, G. J. M.; Luijten, J. G. A. J. Appl. Chem. 1954, 4, 31.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
171
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
I N T E R A C T I O N B E T W E E N CO2 C A T A L Y S T S OF C L U S T E R T Y P E
AND
PROPYLENE
ON
Ru-Co/A1203
G.D.Zakumbaeva, L.B.Shapovalova, I.A.Shlygina
Institute of Organic Catalysis & Electrochemistry, 142 Kunaev St., 480100 Almaty, Republic of Kazakhstan, Grant TA-MOU-CA 13 041 US Agency for International Development.
I. Introduction The availability of CO2 makes it highly attractive for synthesis of oxygen containing compounds. CO2 molecule is inert and its catalytic activation is necessary. Application of heterogeneous catalytic systems opens wide possibilities for carrying out of organic reactions with CO2. Recently the interaction between CO2 and hexene-1 in solvent on Ru-Co/AI203 catalysts has been studied [ 1].
2.Experimental In this work the interaction between CO2 and C 3 H 6 o n Ru-Co (1:1)/A1203 - catalysts of clusters type has been studied. The process was carried out in flow type reactor in the range of 423 - 623 K and pressure variation from 0,1 to 2,0 MPa. The catalyst was prepared by impregnation of 7 - A1203 with mixture of RuOHCI3 and Co(NO3)2.6H20 solutions, and reduction with H2 during 3 hours at 773K.. Then it was washed from C1 " and N O 3 " ions and dried in the air at 302-323K. The catalyst was then reduced directly in the reactor during 1 hour at 473-673K before the reaction of CO2 and propylene. The preliminary experiments have shown independence of catalyst's activity on the reduction temperature in the indicated range. The Tred =573K was chosen as a standard in further experiments. The mixture C O 2 : C 3 H 6 = 4:1 with small concentration of propane impurity (<1,5%) was used. The space velocity of reagents was 150-200h -1. To elucidate the mechanism of CO2 + propylene reaction the physicochemical and quantum methods have been used. IR-spectra of CO2 were registered by UR-75-Spectrometer at room temperature in the 1200 - 3500 cm -l range. Quantum-chemical calculations have been made on the basis of EH method modified by the core-core repulsion in Anderson's ASED MO approximation [2]. Minimum number
172 of metal atoms has been examined because the chemisorption was-considered as highly localized phenomenon. The total optimization of geometry of CO2 molecule on a few rigid metal atoms has been made to estimate CO2 and its fragments' state on the catalyst surface. The structure and the state of Ru-Co/A1203 were studied by electron microscopy, electron diffraction method and ESXA. Quantum-chemical calculations were carried out by EHT method [3].
3. Results and discussion. The interaction of CO2 with propylene was studied under the mild conditions (Tex=423K, Pex=0,2 MPa). The propylene conversion is 10,5%. An increase of pressure from 0,2 to 1,0 MPa leads to raising the propylene conversion to 25,1%. The conversion is not changed with further increase of pressure up to 1,4 MPa and then at higher pressure (P=2,0 MPa) it decreases. At 1,1 MPa pressure the propylene conversion grows from 25,1 to 49,4% with the increase of temperature from 423 to 523K. At higher temperature (Tex=623K) the conversion decreases more than 2 times (20,5%). The optimal conditions for formation of oxygen containing compounds were Tex=523K and Pex=l 1 atm. The conversion of propylene is 49,4% under these conditions and total yield of oxygen containing products is 70%. The reaction products were analyzed by chromatography and chromatomassspectrometry. The complex mixture of oxygen containing organic compounds and C~ - C6 hydrocarbons was formed. Oxygen containing products consist of Cl - Ca aldehydes (formaldehyde, propionic aldehyde, butiraldehyde), C2 - C4 acids (acetic, propionic, butyric), acetone, ethanol and the traces of C3 - C4 alcohols of normal structure. The formation of C l - C6 hydrocarbons shows that the main reaction is accompanied with side processes of propylene destruction and dimerization with participation of CHx species and hydrogen is formed at propylene decomposition. The carbon oxide were detected in some experiments due to the dissociation of CO2 => Oads + COads on the specific centers of catalyst surface at high temperatures. Complex composition of reaction products shows that interaction of CO2+propylene is not selective process and occurs in several directions simultaneously. The IR-spectroscopy study of CO2 chemisorption was carried out to determine the mechanism of CO2 +propylene reaction. In a case of CO2 chemisorbtion on Ru-Co/AI203 reduced in H2 at 773K and stirred in the air there was not registered adsorption bands typical for adsorbed CO and CO2 structures in IR-spectrum. Intensive adsorption bands of OH-group (3400cm-1), at the range of deformation oscillation of OH-group (1620cm l ) and adsorption bands at 1570-1360cm -1 were recorded. The intensity of this bands was not changed after its treatment in H2 at 373-673K. Appearance of the adsorption bands at 2350cm 1 (CO2 ads), 1950-2020cm-l(COads) and 29302920cm -l (CHads) in IR-spectrum were observed for catalyst freshly reduced at 373K. The intensity of these adsorption bands increases at catalyst Treo rise to 673K. The influence of temperature on CO2 adsorption has been studied by IR spectroscopy. It was found that in the range of 373-573K (Tred=573K) the position of adsorption bands CO2 ads is not changed significantly. But the intensity of adsorption bands at 2350-2310cm -1,
173 1950-2020cm q and 2930-2920cm l grows due to the increase of CO2 amount and at the same time the dissociative processes are strengthened, giving rise to COads and CHx ads. The opportunity of deeper dissociation of CO2 according to scheme: C02 ads => COads => Cads +Oads
and interaction of Cads with adsorbed hydrogen together with CHx- structure formation (29302920cm l ) takes place. Ruthenium in Ru-Co/Al203 is in Ru 0 and RuO2 forms ( R u 3 p 3 / 2 = 460,8 - 461,8 eV) and cobalt is in oxidized state with C o 2 p 3 / 2 - 777,8 - 780,0 eV. However separate spots of aCo 0 and 13-Co 0 have been found by electron microscopy. Also the fine dispersed particles have been observed, its amount was increased with the rise of Co concentration and got maximum when content of Co was 70%. These particles were X-Ray amorphous. Oxidized states of Co bonded with tiny particles of Ru were presented in these formations. The clusters formation is available .Particles of Ru 0 bonded with A1203 have been also found. Quantum chemical results of full optimization of mono and bimetallic cluster geometry containing 2-4 atoms of metal of different multiplicity has been published in the Ref.[3]. It has been shown that bond energy decreases in the series: Co-Co > Co-Ru > RuRu. In mixed Co-Ru clusters the Co-Co bond is more stable and Ru-Ru bond is weakened in comparison with monometallic particles. So in process of cluster growth the formation of CoCo bond is preferable and it should be proposed that content of Ru atoms on bimetallic cluster surface is higher than in its volume. These results are in good correlation with experimentally found particle size of catalyst: Ru - clusters of d>40A, bimetallic particles forming in plenty of Ru form 80 to 160A, and in plenty of Co - 320A. Therefore we suppose, that the X-Ray amorphous structures are Co-Ru clusters. This suggestion is in accordance with quantum chemical calculations. The quantum chemical calculations showed, that depending on cluster structure and initial orientation of CO2 molecule either associative or dissociative (a) (b) CO2 adsorption can take place (Fig.l) . The transfer of CO2 molecule parallel to Ru7 and Co7 monometallic clusters plane is accompanied with complete OC )O destruction of CO2 molecule according to equation Figl. Optimized structure of adsorbed CO2 on Ru7 (a) and Co7 (b). Horizontal orientation CO2 in start C02 ads => Cads -i- 20ads 9 point of optimization and symmetrical structure of fixed monometallic 7-atomic clusters. At vertical approach of ,,
'
,-...~
,
,,
,_..~,
ec.
adsorbed molecule to monometallic Ru- and Co- clusters' plane or clusters containing one atom of second metal (Co6Rul and Ru6Co 1) the associative vertical adsorption occurs.
174 It must be noted, that axis deflection of CO2 molecules from vertical position (up to value of angle between normal to surface and axis of molecule CO2 ads close to 90 ~) accompanies with increase of total energy of the system to -1 eV. The complete optimization of the CO2 geometry from initial position at <45 ~ to surface brings to vertical reorientation of molecule. As follows from these results the approaching of COzadsorbed molecule to plane part of the surface must cause its vertical (linear) orientation, moreover in this case the adsorption proceeds via oxygen atom (Fig.2a,2b). The complete optimization of geometry and position of molecule C02 ads over bimetallic clusters from location corresponding to minimum of potential energy on adsorption curve showed: if content of second metal was > 30% and there was not local symmetry in the sample the dissociative adsorption was observed according to the scheme:
(b)
(a)
~.
(c)
_
~Ru
~Co
OC
eO
Fig.2a Optimized structure of adsorbed C02 on Ru7 (a), Co7 (b), Co7(H3). Vertical orientation C02 in the start point of the optimization and symmetrical structure of fixed monometallic 7-atomic clusters.
(b)
(a)
9
eCo
4
OC
eO
Fig.2b. Optimized structure of adsorbed C02 on Ru6Co(H3) (a), Co6Ru (b). Vertical orientation C02 in the start point of optimization and symmetrical structure of fixed bimetallic 7-atomic clusters.
O
(a) ,0
(b)
C02 ads => COads %-Oads (Fig.3)
The presence of defects on surface and small sizes of Ru-Co clusters lead to dissociative adsorption. As follows from IR-spectroscopy data and investigations of interactions process between CO2 and propylene the dissociative adsorption of CO2 is caused by high temperatures.
~Ru
~Co
OC
eO
Fig3. Optimized structure of adsorbed C02 on Co4Ru3 (a), Co5Ru2(H3) (b). Vertical orientation CO2 in the start point of optimization and nonsymmetrical structure of fixed bimetallic 7-atomic clusters.
175 Quantum-chemical calculations showed that Co and Ru in cluster have different affinity to CO2, CO and 02 molecules. Ru is characterized by lower affinity to CO than Co, but more high affinity to oxygen. So it may be suggested that during the CO2 dissociation on the Co-Ru-clusters the preferable formation of new bonds Co-COads and Ru-Oaos takes place. In complex Ru-Oads metal has positive charge and could activate the olefin molecule - typical donor of electrons. To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantumchemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms ~-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to dorbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. Thus, analysis of experimental data and quantum-chemical calculations shows that the direction of interaction of propylene and CO2 or fragments of dissociation is determined by the mechanism of CO2 and propylene adsorption on clusters of different composition.
References. 1. G.D.Zakumbaeva, L.B.Shapovalova, Japan-FSU. Catalysis seminar'94. October 31 November 2, 1994, Japan, Tsukuba(1994)28-34. 2. A.B.Anderson,S.J.Hong,J.L.Smialek, J.Phys.Chem.No 16,V.91 (1987)4245-4250. 3. G.D.Zakumbaeva, L.B.Shapovalova, I.G.Efremenko, A.V.Gabdrakipov, J.Neftechimia, No5 (1996)427-438. The research was carried out at support of grant TA-MOU-CA 13 041 US Agency for Intemational development.
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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
177
P h o t o c a t a l y t i c r e d u c t i o n of C O 2 w i t h H 2 0 on t i t a n i u m o x i d e s anchored w i t h i n zeolites Masakazu Anpo a*, Hiromi Yamashita a, Keita Ikeue a, Yo Fujii a, Yuichi Ichihashi a, Shu Gou Zhang a, Dal Ryung Park a, Shaw Ehara b, Sang-Eon Park c, Jong-San Chang c, Jung Whan Yooc a'Department of Applied Chemistry, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 5939, Japan. bIon Engineering Research Institute Corporation, Hirakata, Osaka 577, Japan CKorea Research Institute of Chemical Technology, Daeduck Science Town Daejeon, 305-606, Korea
Titanium oxide species anchored within the zeolite cavities and framework by an ion-exchange method and a hydrothermal synthesis exhibit high and unique photocatalytic reactivity for the reduction of CO2 with H20 at 328 K with a high selectivity for the formation of CH3OH in the gas phase. In situ photoluminescence, ESR, diffuse reflectance absorption and XAFS investigations indicate that the titanium oxide species are highly dispersed within the zeolite cavities and f r a m e w o r k and exist in t e t r a h e d r a l coordination. The charge transfer excited state of the highly dispersed titanium oxide species plays a significant role in the reduction of CO2 with H 2 0 exhibiting a high selectivity for the formation of CH3OH, while the catalysts involving the aggregated octahedrally coordinated titanium oxide species show a high selectivity to produce CH4. 1. I N T R O D U C T I O N The efficient photocatalytic reduction of CO2 with H 2 0 is one of the most desirable and challenging goals in the research of environmentallyfriendly catalysts [1-8]. Although pioneering works on the photoreduction of CO2 on semiconductors in aqueous suspension systems were summarized by Halmann [1] and recent work in solid-gas systems were reviewed by Anpo and Yamashita [2], the efficiency of CO2 reduction was low when water was used as the reductant. Recently, several researchers have reported that the photocatalytic reduction of CO2 with gaseous H20 proceeded on powdered TiO2 at room temperature to form small amounts of CH4 [2-8]. It was also found that the highly dispersed titanium oxide catalyst anchored on Vycor glass exhibits a high and characteristic photocatalytic reactivity compared to bulk TiO2 powder [3]. The titanium oxide species prepared within the zeolite cavities or the zeolite framework have recently revealed a unique local structure as well as a high photocatalytic reactivity for the decomposition of NOx into N2 and 02 [9]. However, the true chemical nature of these titanium
178 oxide species as a p h o t o c a t a l y s t are yet little known, especially the photocatalytic reactivity for the reduction of CO2 with H20. In the present study, highly dispersed titanium oxides included within the zeolite cavities (Ti-oxide/Y-zeolite) and framework (Ti-MCM-41,-48) were prepared using an ion-exchange method and h y d r o t h e r m a l synthesis to be used as photocatalysts for the reduction of CO2 with H 2 0 at 328 K. The characterization of these catalysts by means of in situ photoluminescence, diffuse reflectance absorption, XAFS (XANES and FT-EXAFS), and ESR m e a s u r e m e n t s have been carried out and special attention has been focused on the relationship between the structure of the titanium oxide species and the reaction selectivity in the photocatalytic reduction of CO2 with H 2 0 to form CH3OH. 2. E X P E R I M E N T A L
The Ti-oxide/Y-zeolite (1.1 wt% as TiO2) was prepared by ion-exchange w i t h an aqueous t i t a n i u m a m m o n i u m oxalate solution using Y-zeolite samples (SIO2/A1203 = 5.5) (ex-Ti-oxide/Y-zeolite). The Pt-loaded ex-Ti-oxide/Yzeolite (1.0 wt% as Pt) was p r e p a r e d by i m p r e g n a t i n g with an aqueous solution of H2PtC16. Ti-oxide/Y-zeolites having different Ti contents (1.0 wt% and 10 wt% as TiO2) were prepared by impregnating the Y-zeolite with an aqueous solution of" titanium ammonium oxalate (imp-Ti-oxide/Y-zeolite). TiMCM-41 (Si/Ti=100) and Ti-MCM-48 ( S i / T i = 8 0 ) w e r e h y d r o t h e r m a l l y synthesized according to procedures reported previously [7]. TiO2 powdered catalysts (JRC-TIO-4: a n a t a s e 92 %, rutile 8 %) were supplied by the Catalysis Society of Japan. The photocatalytic reduction of CO2 with H20 was carried out with the catalysts in a quartz cell with a flat bottom connected to a conventional vacuum system (10 -6 Torr range). Prior to photoreactions and spectroscopic m e a s u r e m e n t s , the catalysts were heated in 02 at 725 K for 2 h and then evacuated at 475 K for 1 h. In the case of Pt-loaded catalysts, the pretreated catalyst was heated in H2 at 475 K for 2 h and evacuated at 475 K for 1 h. UV irradiation of the catalysts in the presence of CO2 (24 pmol) and gaseous H 2 0 (120 umol) was carried out using a high-pressure Hg lamp (~ > 280 nm) at 328 K. The reaction products collected in the gas phase were analyzed by gas chromatography. The photoluminescence spectra were m e a s u r e d at 77 K using a Shimadzu RF-5000 spectrophotofluorometer. The diffuse reflectance absorption spectra were recorded with a Shimadzu UV-2200A spectrometer at 295 K. The ESR spectra were recorded at 77 K using a J E O L JES-RE2X spectrometer in the X-band mode. The XAFS spectra were m e a s u r e d at the BL-7C facility of the Photon Factory in Tsukuba. 3. R E S U L T S A N D D I S C U S S I O N
UV irradiation of powdered TiO2 and Ti-oxide/Y-zeolite catalysts in the presence of a mixture of CO2 and H20 led to the evolution of CH4 and CH3OH in the gas phase at 328 K, as well as trace amounts of CO, C2H4 and C2H6.
179 The evolution of small amounts of 0 2 w a s also observed. The rates of these photoformed products increased linearly against the UV irradiation time and the r e a c t i o n i m m e d i a t e l y ceased w h e n i r r a d i a t i o n was d i s c o n t i n u e d , indicating the photocatalytic reduction of CO2 with H 2 0 on the catalysts. The specific photocatalytic reactivities for the formation of CH4 and CH3OH are shown in Fig. 1. It is clear t h a t the photocatalytic reaction rate and selectivity for the formation of CH3OH strongly depend on the type of catalyst. It can be seen t h a t the specific photocatalytic reactivities of the Ti-oxide/Yzeolite catalysts which have been normalized by unit gram of Ti in the catalysts are much higher t h a n bulk TiO2. The ex-Ti-oxide/Y-zeolite exhibits a high reactivity and a high selectivity for the formation of CH3OH while the formation of CH4 was found to be the major reaction on bulk TiO2 as well as on the imp-Ti-oxide/Y-zeolite. Although the addition of Pt to the ex-Ti-oxide/Yzeolite is effective in i n c r e a s i n g the photocatalytic reactivity, only the formation of CH4 is promoted, accompanied by a decrease in the C H 3 O H yields. The absorption spectra of the Ti-oxide/Y-zeolite and bulk TiO2 catalysts were measured by the UV diffuse reflectance method. A significant shift to shorter w a v e l e n g t h s in the absorption band was observed with the ex-Tioxide/Y-zeolite, clearly suggesting t h a t the dispersion of the Ti-oxide species on this catalyst was higher t h a n on catalysts prepared by i m p r e g n a t i o n methods. The Pt-loaded catalyst exhibited the same spectra, indicating t h a t the local structure of the Ti-oxide species was not changed by Pt-loading. Figure 2 shows the XANES spectra of the Ti-oxide/Y-zeolite catalysts. The ex-Ti-oxide/Y-zeolite exhibits an intense single preedge peak, indicating t h a t the Ti-oxide species in this catalyst has a tetrahedral coordination [9]. 15
y:.-
I~ CH4 "i C|t3OHJ
0
& o 2, <
k: a..
5 cd ---
(b~
~~"" Ti-O
5
0 0
z (a)
(b)
(c)
(d)
/~j
(e)
R = 1.78 N=3.5
Catalysts
Fig. 1.
(B)= 1.78
The yields of CH4 and C H 3 O H in the photocatalytic reduction of CO2 with H20 on the TiO2 powder (a), the imp-Ti-oxide/Yzeolite (10.0 wt% as TiO2) (b), the imp-Ti-oxide/Y-zeolite (1.0 wt% ) (c), the ex-Ti-oxide/Y-zeolite (d), and the Pt-loaded ex-Ti-oxide/Y-zeolite (e) catalysts.
4920
4960
5 0 0 0 5040
Energy / eV
2
4
6
Distance / A
Fig. 2. The XANES (a-c) and FT-EXAFS (A-C) spectra of the imp-Ti-oxide/Yzeolite (1.0 wt% as TiO2) (a, A), ex-Tioxide/Y-zeolite (b, B), and Pt-loaded ex-Tioxide/Y-zeolite (c, C) catalysts. N: coordination numbers, R: atomic distances.
180 The Pt-loaded catalyst (Fig. 2-c) also exhibits the same preedge peak, indicating that Pt-loading does not lead to any change in the local structure of the Ti-oxide species. On the other hand, the imp-Ti-oxide/Y-zeolite exhibits weak preedge peak attributed to formation of crystalline TiO2. Figure 2 also shows the FT-EXAFS spectra of the catalysts and all data are given without corrections for phase shifts. The ex-Ti-oxide/Y-zeolite (Fig. 2-B) and Pt-loaded catalysts (Fig. 2-C) exhibit peaks only at around 1.6 /k assigned to the neighboring oxygen atoms (Ti-O) indicating the presence of isolated Ti-oxide species in these catalysts. From the curve-fitting analysis of the EXAFS spectra, it was found that the ex-Ti-oxide/Y-zeolite catalyst consists of 4coordinate titanium ions with a coordination number (N) of 3.7 and an atomic distance (R) of 1.78/k. On the other hand, the imp-Ti-oxide/Y-zeolite catalysts exhibit an intense peak at around 2.7 ~ assigned to the neighboring titanium atoms behind the oxygen (Ti-O-Ti), indicating the aggregation of the Ti-oxide species in these catalysts. Figure 3 shows t h a t the ex-Ti-oxide/Y-zeolite catalyst exhibits a photoluminescence spectrum at around 490 nm by excitation at around 290 nm at 77 K. The observed photoluminescence and absorption bands are in good agreement with those previously observed with the highly dispersed tetrahedrally coordinated Ti-oxides prepared in silica matrices [3,9]. We can therefore conclude t h a t the observed photoluminescence spectrum is attributed to the radiative decay process from the charge transfer excited state to the ground state of the highly dispersed Ti-oxide species in tetrahedral coordination as shown in the hv (Ti4+mO2-) :-. : :~ (Ti3+--O-) * hv' following scheme. On the other hand, the imp-Ti-oxide/Y-zeolite catalysts did not exhibit any p h o t o l u m i n e s c e n c e spectrum. Thus, these results clearly g E indicate t h a t the ex-Ti-oxide/Y-zeolite catalyst consists of highly dispersed isolated t e t r a h e d r a l Ti-oxide species, ..... while the imp-Ti-oxide/Y-zeolite catalysts involve the aggregated octahedral Ti350 450 550 650 oxide species which do not exhibit any Wavelength / n m photoluminescence spectrum. As shown in Fig. 3, the addition of Fig. 3. Photoluminescence spectrum of the ex-Ti-oxide/YH 2 0 or CO2 molecules onto the ex-Tizeolite catalyst (a), and the oxide/Y-zeolite catalyst leads to an effects of the addition of CO2 and efficient quenching of the H20 (b, c) and the loading of Pt photoluminescence. The lifetime of the (d) on the photoluminescence charge transfer excited state was also spectrum. Measured at 77 K, found to be shortened by the addition of excitation at 290 nm, amounts of added CO2: b) 8.5, and H20; c) CO2 or H20, its extent depending on the 2.9 gmol/g. a m o u n t of added gasses. Such an
181
efficient quenching of the photoluminescence with CO2 or H 2 0 suggests not only t h a t t e t r a h e d r a l l y coordinated Ti-oxide species locate at positions accessible to the added CO2 or H20 but also that added CO2 or H20 interacts and/or reacts with the anchored Ti-oxide species in both its ground and excited states. Furthermore, as shown in Fig. 3, Pt-loading onto the ex-Tio x i d e / Y - z e o l i t e c a t a l y s t l e a d s to an efficient q u e n c h i n g of t h e photoluminescence, accompanied by the shortening of its lifetime. Because the results obtained by EXAFS and absorption m e a s u r e m e n t s indicate t h a t the local structure of the Ti-oxide species in the ex-Ti-oxide/Y-zeolite was not altered by the Pt loading, the effective quenching of the photoluminescence can be a t t r i b u t e d to the electron t r a n s f e r from the photo-excited Ti-oxide species to Pt metals. As a result, on the Pt-loaded ex-Ti-oxide/Y-zeolite catalyst, photocatalytic reactions which proceed in the same m a n n e r as on bulk TiO2 become p r e d o m i n a n t , m e a n i n g t h a t the reduction reaction by electrons and the oxidation reaction by holes occurring separately from each other on different sites becomes p r e d o m i n a n t , leading to the selective formation of CH4. UV irradiation of the anchored titanium oxide catalyst in the presence of CO2 and H 2 0 at 77 K led to the appearance of ESR signals due to the Ti 3+ ions, H atoms, and carbon radicals [5,6]. From these results the reaction mechanism in the photocatalytic reduction of CO2 with H 2 0 on the highly dispersed titanium oxide catalyst can be proposed in the following way. CO2 and H20 molecules interact with the excited state of the photoinduced (Ti 3+m O-)* species and the reduction of CO2 and the decomposition of H 2 0 proceed competitively. Furthermore, H atoms and OH. radicals are formed from H20 and these radicals react with the carbon species formed from CO2 to produce CH4 and CH3OH. UV irradiation of the Ti-mesoporous zeolites and the TS-1 zeolite in the presence of CO2 and H20 also led to the formation of CH3OH and CH4 as the main products. The yields of CH4 and CH3OH per unit weight of the Ti-based ~ 10 catalysts are shown in Fig. 4. It can be :~ O seen t h a t Ti-MCM-48 exhibits m u c h 9 higher reactivity t h a n either TS 1 or Ticc7.~ ~, MCM-41 Besides the higher dispersion -~ ~k= 5 s t a t e of the Ti-oxide species, o t h e r 8 distinguishing features of these zeolite ~r =~. catalysts are: TS-1 has a smaller pore ~ " size (ca. 5.7 /~) and a three-dimensional channel s t r u c t u r e ; Ti-MCM-41 has a 0 l a r g e pore size (>20 /~) b u t one(a) (b) (c) (d) Catalysts dimensional channel structure; and TiMCM-48 has both a large pore size (>20 Fig. 4. The yields of CH4 and /~) and t h r e e - d i m e n s i o n a l c h a n n e l s . C H 3 O H in the photocatalytic Thus, the higher reactivity and higher reduction of CO2 with H20 on TiO2 selectivity for the formation of C H 3 O H powder (a), TS-1 (b), Ti-MCM-41(c), observed with the Ti-MCM-48 zeolite and Ti-MCM-48(d) catalysts.
cC.":o.]
-
9
>~
/
182 than with any other catalyst used here may be a combined contribution of the high dispersion state of the Ti-oxide species and large pore size having a three-dimensional channel structure. 4. CONCLUSIONS A high photocatalytic efficiency and selectivity for the formation of CH3OH in the photocatalytic reduction of CO2 with H20 was achieved with the ex-Ti-oxide/Y-zeolite catalyst having highly dispersed isolated tetrahedral Ti-oxide species, while the formation of CH4 in the photocatalytic reduction of CO2 with H20 was found to proceed on the bulk TiO2 catalysts and on the imp-Ti-oxide/Y-zeolite c a t a l y s t s involving aggregated octahedrally coordinated titanium oxide species. On the isolated tetrahedral Ti-oxide species, the charge transfer excited complexes of the oxides, (Ti3+mO-) *, formed under UV irradiation plays a significant role in the formation of CH3OH. On the other hand, with the aggregated or bulk TiO2 and Pt-loaded catalysts, the photo-formed holes and electrons rapidly separate from each other with large spaces between the holes and electrons, thus preventing the reaction between the carbon radicals and OH. radicals on the same active sites, resulting in the formation of CH4 due to the reaction between the H atoms and carbon radicals formed at the electron trapped center. The present study clearly demonstrates that zeolite catalysts involving Ti-oxide species highly dispersed in their cavities and framework are promising candidates as new and efficient photocatalysts for the photoreduction of CO2 with H20 and the control of the charge separation is important in developing highly efficient and selective photocatalysts. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9.
M. Halmann, in "Energy Resources through Photochemistry and Catalysis", Grfitzel M. (ed.), Academic Press, New York, 1983, 507. M. Anpo and H. Yamashita, in "Heterogeneous Photocatalysis", M. Schiavello. (ed.), John Wiley & Sons, London, 1997 (in press). M. Anpo and K. Chiba, J. Mol. Catal., 74 (1992) 207. H. Yamashita, N. Kamada, M. Anpo, S. Ehara, L. Palmisano, M. Schiavello, and M. A. Fox, Res. Chem. Intermed., 20 (1994) 815. M. Anpo, H. Yamashita Y. Ichihashi, and S. Ehara, J. Electroanal. Chem., 396 (1995) 21. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B, 101 (1997) 2632. S.C. Zhang, Y. Fujii, H. Yamashita, K. Koyano, T. Tatsumi, and M. Anpo, Chem. Lett., (1997) 659. F. Saladin, L. Forss, and I. Kamber, J. Chem. Soc., Chem. Commun., (1995) 533. H. Yamashita, Y. Ichihashi, M. Anpo, C. Louis, and M. Che, J. Phys. Chem., 100 (1996) 16041.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Photocatalytic reduction nanocrystallites
and fixation
of CO2
183
on c a d m i u m
sulfide
Shozo Yanagida a Yuji Wada a, Kei Murakoshi a Hiroaki Fujiwaraa, Takao Sakatab, and Hirotaro Mori b aMaterial and Life Science, Graduate School of Engineering, bResearch Center for Ultra-high Voltage Electron Microscopy, Osaka University, Suita, Osaka 565, Japan The detailed studies of the surface of CdS nanocrystallites prepared in N,Ndimethylformamide (CdS-DMF) by means of emission measurements, in-situ Cd K--edge EXAFS analysis, and theoretical MO calculations reveal the correlation of the photocatalysis of CdS-DMF and the formation of sulfur vacancies on its surface. It has been experimentally proved that CO2 interacts with the sulfur vacancies and is converted into its radical anion under irradiation as an intermediate in the photocatalysis. The knowledge on the photocatalysis obtained above has led to the achievement of the photofixation of CO2 into benzophenone, acetophenone and benzyl halides under visible light irradiation in the presence of TEA as an electron donor. 1.
INTRODUCTION
Semiconductor photocatalysis is one of the most active research fields in view of the chemical utilization of CO2 at ordinary temperature and pressure. Photocatalysis of semiconductor nanocrystallites such as ZnO,[1] CdS,[1] GAP,[1] SIC,[1] TiO2,[1] RuO2/TiO2,[2] CrTiO2,[2] NbTiO2,[2] and Q-ZnS/SiO2,[3] was reported for CO2 photoreduction. However, most of the systems required UV light and gave just low conversions. Recently we have reported on the photocatalytic reduction of CO2 under visible-light irradiation using CdS-DMF at a high quantum yield of 0.098 at ~--405 nm.[4,5] The induction period observed in the formation of CO suggests the importance of the surface conditions for the photocatalysis. This paper deals with the mechanistic investigations of the photoreduction of CO2 using CdS nanocrystallites as photocatalysts in view of the changes in surface structures of CdS-DMF induced by the addition of excess Cd 2+ and the interaction of adsorbed CO2 molecules with the surface. The work is extended to the achievement of photocatalytic CO2 fixation into organic substrates.
2.
EXPERIMENTAL
2.1. Preparation of CdS-DMF as photocatalysts and the procedure for photoreduction of CO215,61 CdS-DMF was prepared by introducing H2S into a DMF solution of Cd(C104)2.6H20 under stirring on an ice bath. The effect of the excess Cd 2+ was investigated after adding a DMF solution of Cd(C104)2 into the CdS-DMF solution. The excess amount of Cd 2+ was
184 indicated by a ratio of excess Cd 2+ concentration to the diatomic concentration (2.5 mM) of CdS nanocrystallites. CO2 was introduced into a stirred DMF solution (2 ml) containing CdS-DMF and triethylamine. The resulting CO2-saturated DMF solution was irradiated under magnetical stirring with a 300 W tungsten halogen lamp through a saturated aqueous sodium nitrite solution filter (Z, > 400 nm) in a water bath. 2.3. Emission measurements and EXAFS measurements[6] Steady-state emission spectra were recorded with a fluorescence spectrophotometer (Model 850, Hitachi Ltd.). Emission lifetime measurements were carried out using laser excitation pulses and a time--correlated single-photon counting system as described elsewhere. Cd K-edge (26710 eV) EXAFS measurements were performed on the BL-14A at the Photon Factory of the National Laboratory for High Energy Physics. The details of these measurements are described elsewhere. 3.
R E S U L T S AND D I S C U S S I O N
3.1. Photoreduction of CO2 and effects of addition of excess Cd 2+ CdS-DMF showed excellent photocatalysis for CO2 reduction in DMF under k>400 nm irradiation in the presence of TEA as an electron donor.[4,5] Carbon monoxide (CO) was efficiently and selectively formed along with the evolution of a negligible amount of hydrogen. Diethylamine (DEA) and acetaldehyde were detected while maintaining the electron-balance with the CO formation during the initial stage of the reaction. The quantum yield for the formation of CO, measured after the induction period, was determined to be ~ 1 / 2 C O = 0.098 at Z, = 405 nm. An induction period of 30 min was observed at the initial stage of the CO formation in the photocatalysis for CO2 reduction on CdS-DMF, suggesting that the surface structures change photochemically during this period, giving catalytically active sites.
30
~20
10
0,1 0
i,
i,,
t
,
i
,
0.2 0.4 0.6 0.8 l Molar Ratio of Cd 2+ to CdS Fig. 1. Effect of the addition of excess Cd 2+ on efficiency of the CdS-DMF-catalyzed CO formation.
185 In order to observe the effect of the structural change of the surface on the photocatalysis, effect of excess Cd 2+ on the photoproduction of CO was examined (Fig.l). The CO formation was increased by a factor of two when 0.2 equivalent amount of excess Cd 2+ to the concentration of CdS-DMF was added into the system. Induction period of the CO formation became shorter by the addition of excess Cd 2+. The addition of more than 0.2 equivalent of excess Cd 2+, however, led to a decrease in the photocatalytic activity. During the reaction with the system containing more than 0.2 equivalent of excess Cd 2+, photo-Markening of the CdS-DMF solution was observed after 30 min irradiation. 3.2. Emission properties[6] Fig. 2 shows the changes in the emission spectra of the CdS-DMF solution by excess addition of Cd 2+ at 400 nm excitation. The spectra consist of two broad bands at ~,=650 and 480 nm, attributed to radiative recombination at deep trap sites originating from lattice imperfection at the surface, i.e., the surface states, and the direct recombination of electron and hole pairs at the band gap, respectively. The emission intensity at Z,=650 nm increased as excess Cd 2+ increased up to 0.2 equivalent, and then decreased until 1.0 equivalent. The emission-lifetime measurements in ns-time region were also carried out for the two emission bands. Multi-exponential decay behavior was observed for both the emission bands. Fast decay component at ~,=480 nm less than the order of ns was attributed to the recombination of electrons and holes. Slow decay component at Z,=480 nm in the order of a few ns was attributed to thermal detrapping of the electron from the surface states to the conduction band since such thermal activation could enhance the lifetime at the band-edge emission. The emission lifetime at 2~--480 nm increased as excess Cd 2+ increased up to 0.2 equivalent, and then decreased. This elongation of the lifetime can be explained by increment of the shallow trap sites. 15
O.
t
500
600 Wavelength /nm
,
I
700
Fig. 2 The effect of excess Cd 2+ addition on emission spectra excited at ~,=400 nm. Excess amount of Cd 2+ was induced with 0.0 ( ~ ) , 0.1 ( - - - - ) , 0.2 ( . . . . . ), 0.5 ( ....... ) and 1.0 ( 9 ) equivalent of excess Cd 2+ to CdS-DMF nanocrystallites.
186 We assumed that the added excess Cd 2+ form not only the deep trap sites but also the shallow trap sites. Accordingly, the trap sites should be originated from lattice imperfection which should be formed on the surface of CdS-DMF as a result of the adsorption of excess Cd 2+. The effects on the emission intensity and lifetime proved that the maximum number of trap sites was achieved when 0.2 equivalent of excess Cd 2+ was introduced into the system. 3.3. EXAFS analysis[6] We already reported on the analysis of CdS-DMF by means of EXAFS that not only sulfur atoms in CdS-DMF but also oxygen atoms of DMF molecules solvate to surface Cd atoms of CdS-DMF nanocrystallites. Analysis of Fourier-filtered k3;~(k) of the CdS-DMF solution gave an excellent fitting (R=6.43 %) with a two-shell fit of Cd-O and Cd-S, but not with a one-shell fit of Cd-S. When excess Cd 2+ was added to a CdS-DMF solution, the coordination numbers of Cd-O and Cd-S determined by the EXAFS analysis were changed as displayed in Fig.3. As excess Cd 2+ increased, CNcd-S decreased and CNcd-o increased. The changes in CNcd-S and CNcd--O of CdS-DMF became more apparent as the added excess Cd 2+ was increased. Based on these analyses of EXAFS data, we propose that the structure of CdS-DMF nanocrystallites changes as shown in Scheme 1. When 0.2 equivalent of excess Cd 2+ was added into the system, the adsorption of Cd 2+ solvated by DMF occurred to the CdS-DMF surface and resulted in the formation of the sulfur surface vacancy on the surface of CdS-DMF. The change in the coordination number and the square of the Debye-Waller factor of the Cd-S and Cd-O shell support such changes in the surface structure on CdS-DMF nanocrystallites.
2.5
.5
4
I
"--
2
=
= 1.5 o
~ I,,,~
1
o o
0.5 r" I
0
J2.5 ,
I
.
0.2 0.4 Molar ratio of Excess Cd 2§ Fig. 3 The effect of excess Cd 2+ addition on the coordination number of Cd-O (Q) and Cd-S (O) shell obtained by EXAFS measurement.
187
~F ~ ~.
DMF..~~
/ , ~ . d f ~ S ~ j ' ~ .~.S ~
i-,,, ,2 DMF~. , ~ . ~ ~ . l - DMP.~.IWrMF DMF,~'~-I~. " ~ uI v~..~ ~ /,,,~17'- D M F . . = ~ ~ s ]~ DMF::-- IDMF
DMF
/~,Y~ , G - . S ~ ,r S..~'~
Scheme I 3.4. Molecular orbital calculations[6] The molecular orbital calculations for the catalyst surface models for a vacancy-free surface (ideal surface) and a surface with a sulfur vacancy (S-defect surface), respectively, indicated that the bidentate-type adsorption of the CO2 molecule on CdS surface with a sulfur vacancy should be more stable than the other types of adsorption, the O-end-on models and Cadsorbed models. 3.5. Observation of CO2-" as an intermediate[7] In order to prove the formation of CO2-', direct EPR analysis of the system was attempted. Although the direct observation of CO2 -~by EPR was unsuccessful, when the CdSDMF solution containing CO2 and DMPO was irradiated by visible light for 30 min, EPR signals reasonably assigned to the adduct of DMPO formed by trapping CO2 -~could be observed (aN= 14.2 G and all= 17.3 G).
3.6. Mechanism of Photoreduction of CO2 molecules The Cd atoms in the vicinity of sulfur vacancies should act as adsorptive sites for CO2 molecules. Here we postulate a mechanism for the photoreduction of CO2 to CO on the CdSDMF nanocrystallites by focusing on the importance of sulfur vacancies (Scheme 2). Upon visible light irradiation of the CdS-DMF system, the photo-formed electron on the conduction band is injected to the adsorbed CO2, forming adsorbed CO2-" on Cd atoms of CdSDMF nanocrystallites, where CO2 interacts with the Cd atom through the C and O atoms. On the surface of CdS-DMF with sulfur vacancies, CO2-" accepts successive electrons after forming a Cd2+OCOCO2 complex through reaction with another CO2. This complex is reduced, leading to CO elimination, as has been reported in electrochemical CO2 reduction. c92 iOi
,
--- CdN...~ -
-CO32, CO
e"~'~-n
"C
~
,J•,%
~
--Cd-'tb-I-Scheme 2
CO~
2
188
3.7. Photocatalytic fixation of CO2 into organic substrates[7] Considering the formation of CO2-" on CdS-DMF, we successfully applied the C d S DMF photocatalysis to the fixation of CO2 into benzophenone (BP), acetophenone (AP), and benzyl halides (BnC1 and BnBr) (Table 1). Four substrates gave benzilic acid, atrolactic acid, and phenylacetic acid as respective fixation products, with dimerized and hydrogenated products. Considering the mechanism proposed above, CO2-" formed through the same route as described therein should participate in the fixation reaction. Table 1 Photofixation of CO2 into aromatic ketones and benzyl halides with TEAa Product yield (%)b Reaction, Conversion t/h (%)
Ac
Bd
Ce
Substrate
Eredl/2 V vs.SCE
BP AP
-1.83 -2.14
0.5 5
100 100
1.3 33
43 25
8.7 3.8
BnBr BnCI
- 1.68 -2.18
1 8
92 97
16 34
23 trace
12 55
a) Each substrate (20 mmol dm-3) was admitted to the photoreduction system of CO2. b) Yields were calculated on the basis of the substrate converted, c) fixation products, d) dimerized products, e) hydrogenated products. 4.
CONCLUSIONS Visible-light induced photoreduction of CO2 proceeds on CdS nanocrystallites when they are stabilized by specific solvation of organic solvent molecules like DMF. Formation of sulfur vacancies as CO2 adsorptive sites and sustained quantization effect of CdS nanocrystallites are requisites for their effective photocatalysis. Adsorptive activation of both CO2 molecules and appropriate organic molecules leads to photofixation of CO2 to the organic molecules on CdS nanocrystallites (CdS-DMF).
REFERENCES 1. T. Inoue, A. Fujishima, S. Kohishi, and K. Honda, Nature, 277, 637 (1979). 2. M. Halmann, V. Katzir, E. Borgarello, and J. Kiwi, Solor Energy Materials, 10, 85 (1984). 3. A. Henglein, M. Gitierrez, and C. Fischer, Ber. Bunsenges. Phys. Chem., 88, 175 (1984). 4. M. Kanemoto, tt. Ankyu, Y. Wada, and S. Yanagida, Chem. Lett., 2113(1992). 5. S. Yanagida, M. Kanemoto, K. Ishihara, Y. Wada, T. Sakata, and H. Moil, Bull. Chem. Soc. Jpn., 70, 2063 (1997). 6. H. Fujiwara, H. Hosokawa, K. Murakoshi, Y. Wada, S. Yanagida, T. Okada, and H. Kobayashi, J. Phys. Chem. B, 101, 8270 (1997). 7. H. Fujiwara, M. Kanemoto, H. Ankyuu, K. Murakoshi, Y. Wada, and S. Yanagida, J. Chem. Soc., Perkin Trans. 2, 317 (1997).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
189
Abiotic photosyntheses of amino acids, nucleic acid bases and organic acids from CO2 dissolved in an aqueous solution Sorin Kihara a, Kohji Maeda a, Toshitaka Hori b and Taitiro Fujinaga c Department of Chemistry, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606, Japan.
a
b Graduate School of Human and Environmental Studies, Kyoto University, Yoshida, Sakyo, Kyoto 606-1, Japan.
c The Research Institute of Oceanochemistry, c/o Faculty of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-1, Japan.
Amino acids, nudeic acid bases and organic acids were synthesized by the UV irradiation of aqueous solutions in which CO2 and NH3 gases or ammonium carbonate had been dissolved. For the photosyntheses of amino acids and nucleic acid bases, the irradiation of UV light of wavelengths shorter than 280 nm at temperatures higher than 80 ~ and the coexistence of Mg 2+ in the solution were found to be effective. Oxalic acid and/or oxamic acid were assumed to be possible intermediates for the syntheses. 1. I N T R O D U C T I O N
Since the synthesis of amino acids from CH4, NH3, H2 and H20 by Miller in 1953 [1], a wide variety of molecules of importance for the origin of life have been produced from mixtures of nonbiological substances with the aid of various energy sources. The carbon sources adopted in most previous works have been those which provide a reducing atmosphere such as CH4 [2]. However, judging from the geological and cosmochemical evidence such as the concentrations of rare gases and the isotopic ratio of 4~ in the present air [3], compositions of volcanic gases [4] and the atmosphere on Venus or Mars [5], and geochemical estimations based on the buffer action of carbonates [6,7], it is reasonable to consider that the air on the primitive earth had been rather oxidizing and contained CO2 instead of CH4. Here, if the abiotic synthesis of substances relative to the origin of life is confirmed under the oxidizing atmosphere containing CO2, it might provide not only support for the oxidizing nature of the atmosphere of primitive earth but also many useful suggestions for the research on the fnxation of CO2. The photosyntheses of amino acids, nucleic acid bases and organic acids from an aqueous solution containing CO2 or carbonates are discussed along with synthetic processes in the present paper.
190
2. EXPERIMENTAL A high pressure mercury arc lamp (H400-P, product of Toshiba Co. Ltd.) whose spectrum is distributed between 220 and 600 nm was used as a light source. The quantity of light irradiated (dose-rate) was determined by a uranyl oxalate actinometer to be 2.4• photons/s. Water used was distilled over permanganate followed by two-time distillations. Although reagents of reagent grade were used without further purification, blank tests for amino acids, nucleic acids and organic acids were carried out thoroughly for each reagent. In a typical experiment, 35 ml of aqueous solution into which both CO2 and NH3 or ammonium carbonate had been dissolved (pH = 8.9) was sealed in a quartz ampule and irradiated at 90 ~ for 100 hr. The resulting solution was heated at 90 ~ to dryness under the reduced pressure in order to remove the remaining NH3 or ammonium carbonate and the residue (white) was hydrolyzed at 105 ~ for 48 hr in 6 M HC1. After evaporating the HC1 solution at 90 ~ to dryness, the residue was dissolved with 10 ml of water. Amino acids in the solution were separated by HPLC (SCL-6A, product of Shimadzu Co.) equipped with a column of cation exchange resin (Shim-pack ISC-07/S1504Li, a product of Shimadzu Co.) and detected based on the ninhydrin reaction [13]. Nucleic acid bases and organic acids such as oxalic and oxamic acids were separated by HPLC with an ODS reversed phase column (Shimpack CLC-ODS) and detected spectrophotometrically. The concentrations given hereafter are those in 35 ml of the irradiated solutions which were converted from the results obtained by the analyses. 3. R E S U L T S AND D I S C U S S I O N
3.1 Concentrations of amino acids, nucleic acid bases and organic acids formed under various conditions An aqueous solution was prepared by bubbling CO2 gas into a 1 M ammonium solution [designated as a (CO2 + NH3) solution], which had been prepared from NH3 gas and distilled water, until the pH of the solution attained 8.9, and the solution was irradiated for 100 hrs at 90 ~ When the irradiated solution was analyzed by HPLC without hydrolysis, small peaks appeared in the chromatograms at positions of glycine and alanine. The concentrations were estimated to be less than 5 2 1 0 -7 M. Upon hydrolysis of the same sample, however, 1 . 5 2 1 0 -5 M glycine, 2.1X10 6 M alanine, 8X10 7 M aspartic acid, 3.7X10 "6 M cytosine, 1.2X10 6 M uracil were detected, which suggested that the irradiation product contained polymer(s) composed of several kinds of amino acids and nucleic acid bases. The results given in the following are those determined after the hydrolysis. Yields of amino acids and nucleic acid bases were nearly proportional to the irradiation time and increased with the initial concentration of CO2 or NH3 in the solution (more than proportional), e.g., concentrations of glycine after irradiation (for 100 hrs at 90 ~ of solutions prepared by bubbling CO2 into 0.5, 1 and 2 M NH3 solutions were 7 X 10 6, 1.5 X 10 -5 and 5.3 X 10 -5 M, respectively. The effect of the temperature of the solution during the irradiation was remarkable, i.e., the irradiation
191
Table 1 Amino acids and nucleic acid bases formed by the UV irradiation of aqueous ammonium carbonate (AC) solutions (pH = 8.9) at 90 ~ Conc. of Irradiation Additional AC (%) time (hr) Reagents
Conc. of Amino acids and nucleic acid bases ( X 10-7 M') Glycine Alanine Serine Aspartic Cytosine Uracil acid 170 28 6 14 40 18 650 175 22 51 164 25 83 19 3 8 17 7 , ,
10 20 5
100 100 100
none none none
10 10
440 20
none none
575 22
78 3
21 N.D.
74 2
10a
100
none
21
N.D.
N.D.
N.D.
10b
1000
none
7
2
N.D.
N.D.
10c
100
none
18
6
N.D.
N.D.
10 10 10 10 10
100 100 100 100 100
120 131 107 86 1230
21 24 29 23 83
10
100
0.1 M MgC12 270 0.1 M MgSO4 293 0.05 M oxalic acid 364 0.05 M oxamic acid 431 0.1 M MgCI2 + 1200 0.05 M oxalic acid 0.1 M MgCI2 + 1430 0.05 M oxamic acid
1170
72
a Irradiated with light of wavelengths longer than 280 mn. instead of 90 ~ N.D.: not detected.
b
127 13
33 4
29 32 32 28 186
68 73 170 265 -
26 24 81 178 -
158
-
-
Not irradiated, c Irradiated at 25 ~
at 25 or 50 ~ yielded negligible a m o u n t s of amino acids a n d nucleic acid b a s e s (less t h a n 10 % of the yields obtained b y the i r r a d i a t i o n at 90 ~ Practically no amino acids were formed b y h e a t i n g of the solution for 1000 hrs at 90 ~ in the dark. Selecting the r a n g e of w a v e l e n g t h of the light with the aid of color filter glasses, UV light shorter t h a n 280 nm was found to be effective for the production of amino acids and nucleic acid bases. In a n o t h e r e x p e r i m e n t , a n aqueous solution containing 10 % a m m o n i u m carbonate (pH = 8.9) was i r r a d i a t e d for 100 hrs at 90 ~ C o n c e n t r a t i o n s of amino acids and nucleic acid bases produced were similar to those observed with the (CO2 + NH3) solution as follows; 1.7 • 10 -5 M glycine, 2.8 X 10 -6 M alanine, 6.0 X 10 -6 M serine, 1.4 X 10 -6 M aspartic acid, 4 . 0 X 1 0 -6 M cytosine a n d 1 . 8 X 1 0 -6 M uracil. Effects of various conditions during irradiation were also similar to those with (CO2 + NH3) solutions, as s u m m a r i z e d in Table 1. Therefore, s u b s e q u e n t e x p e r i m e n t s were carried out b y using a m m o n i u m c a r b o n a t e solutions i n s t e a d of (CO2 + NH3) solutions. W h e n 10 % a m m o n i u m c a r b o n a t e solutions of which pH h a d been adjusted to be 9.9
192 or 7.0 by adding LiOH or HC1, respectively, were irradiated for 100 hrs at 90 ~ yields of amino acids were smaller t h a n those at pH = 8.9, e.g., concentrations of glycine produced at pH = 9.9, 8.9 and 7.0 were 6 • 10 6, 1.5 • 10 -5 and 2 • 10 -6 M, respectively. The coexistence of MgC12 or MgSO4 in the ammonium carbonate solution resulted in the increase of the yields of amino acids and nucleic acid bases, especially alanine, as can be seen in Table 1, although the presence of NaC1 up to 2 M, alkaline metal ions up to 0.1 M, and Mn 2+ up to 0.1 M produced no effect. By HPLC analysis, 1.4 X 10 -4 M oxalic acid and 1.3 X 10 -4 M oxamic acid were found in the solution containing 10 % ammonium carbonate and 0.1 M MgC12 after irradiation for 100 hrs at 90 ~ The voltammetric determination of oxalic and oxamic acids based on oxidation waves at a stationary platinum electrode supported the the HPLC result. Table 2 Amino acids formed by UV irradiation (100 hrs) of aqueous ammonium hydrogen carbonate, ammonium carbamate, ammonium oxalate or oxamic acid solutions (pH=8.9) at 90 ~ Aqueous solution
Reagent coexisted
Conc. of amino acids ( • 10-7 M) Glycine Alanine Serine Aspartic acid
0.5 M Ammonium oxalate
none 0.1 M MgC12
803 43600
129 12500
58 2320
52 1600
0.5 M Oxamic acid
none 0.1 M MgC12
11500 41900
3080 12900
398 3120
325 996
1.0 M Ammonium hydrogen carbonate
none 0.1 M MgC12
193 351
26 128
7 32
12 22
1.0 M Ammonium carbamate
none 0.1 M MgC12
40 105
8 66
1 13
N.D. 8
N.D.: not detected
3.2 Intermediates in photosyntheses of amino acids and nucleic acid bases Since oxalic acid and/or oxamic acid were assumed to be the possible intermediates to form amino acids and nucleic acid bases from ammonium carbonate, 0.05 M oxalic acid or oxamic acid were added into the solutions and irradiation was carried out. These results are included in Table 1. The presence of intermediates facilitated the formation of amino acids and nucleic acid bases significantly. When Mg 2+ was present in addition to the intermediates, the yields of amino acids and nucleic acid bases increased remarkably, although the reproducibility for nucleic acid bases was fairly poor. Table 2 summarizes the concentrations of amino acids formed when 0.5 M ammonium oxalate or 0.5 M oxamic acid were irradiated for 100 hrs at 90 ~ in the absence of ammonium carbonate. Tremendous amounts of amino acids were formed
193 from ammonium oxalate in the presence of 0.1 M MgC12 and from oxamic acid both in the absence and in the presence of 0.1 M MgC12. The results aider irradiation of solutions containing ammonium hydrogen carbonate or ammonium carbamate are added in Table 2. Here, ammonium carbonate is considered to be a mixture of ammonium hydrogen carbonate and ammonium carbamate. The results suggest t h a t ammonium hydrogen carbonate is more available to form amino acids between two components of ammonium carbonate. 3,3 Consideration on synthetic pathways of amino acids and nucleic acid
bases Reaction processes involved in the syntheses of amino acids and nucleic acid bases from aqueous ammonium carbonate solution, which is equivalent to the (CO2 + NI-I3) solution, were estimated to be the sequence of reactions as follows: (i) The photoproduction of oxalic acid, which exists as ammonium oxalato in the alkaline ammonium solution. This process has not been fully understood yet. However, if we assume the production of CO from CO2 by the UV irradiation, even though the yield is very small, CO produces formic acid in an alkaline solution, and formic acid can be converted to oxalic acid at high temperatures such as 90 ~ In this regard, the formation of CO from gaseous CO2 by the irradiation of UV of wavelengths between 120 and 200 nm through a forbidden transition [14] and that from CO2 adsorbed on the surfaces of solid alkali halides by the irradiation of UV of wavelengths between 230 and 250 nm [15] have been reported. Hence, we consider that it might be possible to form CO from CO2 dissolved in a solution containing NH3, NH4 + and Mg2+ since rather strong interactions m a y operate between CO2 and NH3, NH4 + or/and Mg 2+ in the solution. (ii) Thermal conversion of ammonium oxalate into oxamic acid. This process is supported by the evidence t h a t the yields of amino acids are significant at high temperatures such as 90 ~ The presence of Mg 2+ enhances this process through the stabilization of oxamic acid caused by the complex formation between oxamic acid and Mg 2+, and hence the yields of amino acids from ammonium oxalate increase remarkably in the presence of 0.1 M MgC12. Here, the thermal conversion of solid ammonium oxalate to oxamide via oxamic acid is well known. (iii) The formation of amino acids from oxamic acid. The formation is attributable to a photochemical process, since the photoabsorption spectrum of aqueous oxamic acid shows a peak at ca. 235 nm and a shoulder in the range between 250 and 290 am, and the yields of amino acids from aqueous oxamic acid by the heating at 90 ~ in the dark were negligible. Although further investigation is necessary, we assume nucleic acid bases might be formed by UV irradiation of oxamic acid or decomposition products such as oxamide since yields of nucleic acid bases in the presence of oxamic acid were somewhat higher than those in the presence of ammonium oxalate (cf., Table 1). 4. CONCLUSIONS The formation of amino acids, nucleic acid bases and organic acids such as oxalic
194 and oxamic acids in aqueous solutions containing CO2 or carbonate ion by UV irradiation at high temperature has been reported in the present work, and Mg 2+ has been demonstrated to be an effective catalyst for the formation of amino acids and nucleic acid bases. These results suggest that primary life emerged in the ocean, i.e., in an aqueous salt solution, under an oxidizing atmosphere that contained high concentrations of CO2. In this regard, there are several reports in which the atmosphere of 3.8 billion years ago was considered to contain CO2 at concentrations much higher, and O2 (and hence Os) at concentrations much lower, than those in the present atmosphere [19]. Accepting these views, it can be imagined that the temperature of the air which covered the earth was quite high because of the greenhouse effect and UV light penetrated to fairly deep level in the sea at the time of the origin of life. Finally, further investigation is necessary to separate the synthesized products before their decomposition in order to improve the reaction yields, since photosyntheses and photodecompositions of amino acids, nucleic acid bases and organic acids may occur simultaneously under UV irradiation.
REFERENCES 1. S.L. Miller, Science, 117 (1953) 528. 2. S.L. Miller and L.E. Orgel, The Origin of Life on the Earth, Prentice-Hall, New Jersey, 1974. 3. Data of Geochemistry, Volcanic Emanations, Geochemical Survey Professional Paper, U.S. Government Printing Office, Washington, D.C., 1963. 4. W.D. Metz, Science, 194 (1976) 924. 5. H.D. Holland, Petrologic Studies - A Volume to Honor A. F. Buddington, Geological Society of America, Colorado, p. 447, 1962. 6. S. Matsuo, Origin of Life, Japan Sci. Soc. Press, Tokyo, p. 27, 1978. 7. R. Paschke, R.W.H. Chang and D. Young, Science, 125 (1976) 881. 8. P.H. Abelson, Science, 124 (1956) 935. 9. J. Oro', A. Kimball, R. Fritz and F. Master, Arch. Biochem. Biophys., 85 (1959) 115. 10. F. Egami, J. Mol. Evol., 4 (1974) 113. 11. H. Hatanaka and F. Egami, Bull. Chem. Soc. Jpn., 50 (1977) 1147. 12. H. Yanagawa, Y. Makino, K. Sato, M. Nishizawa and F. Egami, Origins of Life, 14 (1984) 163 and references cited therein. 13. T. Hori and S. Kihara, Fresenius Z. Anal. Chem., 330 (1988) 627. 14. Y. Matsumi, N. Shafer, K. Tonokura, M. Kawasaki, Yu-Lin Huang and R. J. Gordon, J. Chem. Phys., 95 (1991) 7311. 15. V. K. Ryabchuk, L. L. Basov and Yu P. Solonitzyn, React. Kinet. Catal. Lett., 31 (1988) !19. 16. A. Bar-run and H. Hartman, Origins of Life, 9 (1978) 93. 17. W. Groth and H. Suess, Naturwiss., 26 (1938) 77. 18. S.L. Miller, J. Am. Chem. Soc., 77 (1955) 2351. 19. L.V. Berkner and L.C. Marshall, J. Atoms. Sci., 22 (1965) 225.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
195
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Aspects of CO 2 utilization toward the goal of emission reduction in Romania Liviu Dragos, Nicolae Scarlat, Mircea Neacsu and Cat~lin Flueraru Energy and Environment Department, ICPET CERCETARE 236 Vitan Str., 74369 Bucharest 3, Romania
The climate change regarding precisely the emissions of carbon dioxide from industrial sources related to energy production from the combustion of fossil fuels represents nowadays an important environmental issue. The CO2 emissions receivable from human activities, energy production, transportation, and industry are relatively low, around 16 Gt/year, in contrast with an estimate of atmospheric CO2 content of around 750 Gt. But, there is the dynamic increase in its presence in atmosphere due to human activities that concerns society, while CO2 addition produces perceptible changes in the natural carbon cycle, affecting the global equilibrium. Solutions to solve related aspects to this global concern are identified and investigated to determine various appropriate CO2 utilization options.
1. OVERVIEW OF CO2 EMISSIONS AT THE LOCAL SCENE
Romania, one of the East European countries moving away from the command economy, made progress toward privatizing its economy and establishing the legal framework for a market economy. Economy indicators presented in Table 1 show that Romania's economy registered a continuous increase over the past 5 years [ 1]. Table 1. Basic Energy and Industrial Products Indicators Electric power Thermal power Mining Net coal Crude petroleum Natural gas yield Crude processing Fuel oil Cement Steel
Units GWh Teal. thousand tons thousand tons mil. c.m. thousand tons thousand tons thousand tons thousand tons
1992 54,195 109,675 38,371 6,615 22,138 13,299 3,693 6,946 9,241
1993 55,476 110,806 39,751 6,676 21,317 13,191 3,731 6,864 9,538
1994 55,136 80,414 40,547 6,693 19,590 14,744 4,688 6,676 10,310
1995 59,134 83,067 41,128 6,712 21,128 15,250 4,697 7,562 11,411
1996 61,425 84,374 41,320 6,822 21,682 16,685 4,781 7,856 11,936
The total Romanian inventory of the CO2 emissions in the atmosphere amounting to a total estimate of around 178,000 Mt/y can be attributed to different sources, especially to the
196 industry sectors entirely responsible for the main participation of 34% of the total, followed closely by fossil fuels firing, for electricity and heating demands, registered with a significant part of 24 %. Substantial sources of CO2 emissions are represented by the household sector with 16% and transportation with 12% of the total. (Fig. 1). E x t r a e t i v e ~
Energy production
_=,
n~nn
Agrieultltrl
C3aemiF~~
Population
M
Metalurgy
4"1
} i~ I
.....
-_,
Transport
~
Industry processes
[]
Industry 0
5
1o
15
20
25
30
35
40
[~1
Figure 1. CO2 emission sources in Romania
,
0
......
,-
5
[%]
10
15
20
Figure 2. C02 emissions sources from industry
A more in depth structural analysis of the industrial sectors, responsible for actual CO2 emissions, reveals the participation of primary producing sectors, as shown in Figure 2. The energy producing sector provides an installed power capacity of 22,300 MW, but uses only one third of it (6-7,000 MW), the other two thirds being in low-efficiency facilities. The structure of the resources used for energy generation, displayed on a country scale, approach a fair estimate of CO2 emissions attribuable to the sector, as shown in Figure 3. The participation of the main available resources occurs in a balanced partaking, for the natural gas, fuel oil, and the use of coal for energy production. It is important to consider the hydraulic energy, and the lately commissioned nuclear plant, each supporting the sector by almost 7%. A significant feature of the National energy system, with respect to the CO2 emitted, is the ample development of an efficient district heating strategy for all the main cities. ./t [] SOLID FUELS 1 50 [] LIQUID FUELS ( 40 .................................................... ] [] GAS FUELS [
iti!!ii!
lO
I
i
-r
~
120,000-~
o.o|
(Teal) (OWh) (thousand tom) Thermal power
,/ 2o, o o o u
/
i
Electric power
Steel
Cement (thousand tons)
Year
1992 1993 1994 1995 1996 Year
Figure 3. CO2 emission from the energy sector Figure 4. C02 emission sources from industry Regarding the development of the environmental sector, there is a high demand for environmental equipment. The domestic environmental protection equipment production is still in its beginning, although there are local companies that produce the metal bulk components of some environmental equipment or could become co-production venture partners, important end users of developed know-how, components, parts, and process controls.
197 2. RESEARCH RESULTS IN CO 2 EMISSION CAPTURE
Investigations through deep research of all experimental aspects has been directed toward the determination and CO2 chemical absorption efficiency and optimum operating conditions in aqueous solutions of ethanolamine. Furthermore, the program includes in depth experiments, at a pilot scale facility, designed and commissioned by the specialized team, as shown in Figure 5. The CO2 production capacity is designed for 35 kg/h, dealing with a flue gas flow rate of 200 STP m3/h, with a CO2 content in flue gas of 10 to12 %. The flue gas is fed in the absorption tower where the CO2 contained is absorbed. The rich solution is then pumped throughout the heat exchanger where it is heated by the flue gas, then to the desorber where the CO2 is released [2]. G ~ (;02 flue gas The lean solution is then passed back from x. ~ ~ rich sokltiort ! j 4 ~ ~ ~ pool S o t u t i o n the desorber to the absorber after cooling, passing through the lean/rich solution heat ' 5 exchanger and the second heat exchanger as ~ --,,,. the water is cooled. The trials revealed a high efficiency of C02 recovery from flue gas, the high carrying capacity of absorbent solution, and satisfactory regeneration process with reduced energy consumption. The experiments demonstrated the capacity of the preferred absorption process using monoethanolamine. The results have shown a t~ tfigh CO2 recovery efficiency of more than 1. Forced draft fan 6. Desorber 80% at low liquid per gas ratios, Figure 6. 2. Bumer 7. Reboiler The registered CO2 removal efficiency, as it 3. Absorber 8. Absorber pump was determined for different desorption 4. Heat exchanger I 9. Reboiler pump temperatures conditions, and distinct MEA 5. Heat exchanger II 10. MEA storage tank flow rates of up to 2 m3/h is presented in Figure 5. Scheme of the CO2 removal facility Figure 6. An important grade of dependency of the CO2 removal efficiency by the solution flow rate, has been noticed, revealing distinct increase of efficiency up to the maximum level, with different desorption temperatures. High removal efficiency was determined for high desorption temperatures at a low MEA flow rate. The trials on this pilot facility go further to I%] study more aspects regarding MEA loss 90 .................................................................................................. a0~ .... I rates, and the optimum quality and quantity 80 70 of the inhibitors and stabilizers needed to ,0oi 60 improve the amine tolerance at high oxygen 50 40 levels to avoid the formation of corrosive 30 compounds. The research aims to complete 20 10 the process development and to identify 0 potential applications for utilization of the 0,5 1 1.5 2 recovered CO2 and to establish appropriate Absorbant solution flow [m3/h] Figure 6. C02 removal efficiency obtained
198 technologies in order to implement projects for the cycle of C02 recovery and utilization.
3. ASPECTS OF CO 2 UTILIZATION IN ROMANIA With respect to C02 utilization possibilities, the potential of using the recuperated C02 to enhance oil recovery, and two distinct case studies for implementing demonstrative projects for the C02 utilization have been identified and analyzed. The research started for C02 utilization in the chemical industry and the process of hydrocarbon production has been delayed on the basis of the significant resources needed for the achievement of the experimental facility.
3.1. CO2 Utilization for Enhancing Oil Recovery The local potential of CO2 utilization, to raise productivity of crude exploitation fields is worth careful analysis through further research, while the technology of injecting pressurized CO2 into oil reservoirs is proving to be effective for enhancing crude production. Taking into account that in many of the applications reported, the CO2 used to recover oil that cannot be produced because of pressure depletion or of the unsuitable use of secondary recovery by waterflooding enhances the oil recoveries in a significant way, there is an evident interest and need for further research efforts and foreign co-operation to be developed in this sector to address and develop distinct methods that suits specific cases of local oil reservoirs. Estimates over the Central and East European countries potential in oil production [3], presented in Figure 7, show good and effective CO2 utilization possibilities, taking into account that for Romania the figures indicate a considerable quantity of 91 Mt CO2 needed to further exploit the estimated oil resources. The main source for such a substantial quantity would be the flue gas from thermal power plants that are located near the oil fields, providing low transportation costs. The utilization of the CO2 emissions recovered from flue gas in such a considerable amount to fulfill the estimate potential for the injection into the local oil fields enhancing the bulk of oil recovery, would provide a significant positive environmental impact. Foreign co-operation and investment is willing to be developed in scientific research and technology development, Serbia . . . . . . . . . . -and in the exploration of Romania ...... natural resources. C02 utilization as a Poland reliable method to raise Hungary productivity of oil fields Czech Reo. ~ ___ marks this application as Croatia an effective method that Bulgaria i needs more in depth Albania analysis in co-operation 0 50.000 100.000 150.000 200.000 250.000 300.000 with advanced research New Oil Production by CO2 Injection (1000 bbl) CO2 Needed (1000 tons) centers and technology developers toward C02 Figure 7. Potential increase in oil production in Eastern Europe emission reduction. (CIS not included) by injection of CO2
199
3.2. Case study no. 1
On the Black Sea the main Romanian town is Constanta, which is an important port having the energy demand provided by the TPP Palas, the town utility with an output of 220 MW electricity provided by four fuel oil fired boilers. Considering the specific local climate, this would be the most suitable location for a biological CO2 fixation system. From the flue gases ducts, a part is to be directed to a chemical absorption plant, 5000 kg/h CO2 will be produced, providing the necessary amount of CO2 for the culture ponds with salt sea water. The utilization of chemical separation for CO2 would eliminate contaminants from the flue gases, enabling the use of biomass not only for energy as renewable source to be used in addition to the base fuel of TPP Palas, but also for foods, nutrients and fertilizers. The maximum daily rate of CO2 fixation is about 50 g per square meter, assuming 4% of total solar radiation (5636,391 MJ/m2/year) can be converted by algae. Productivity Assumptions: Annual productivity ............................................. 182.5 t/ha/year CO2 fixation .......................................................... 86.7 t C/ha/year Annual CO2 quantity used ............................... 35000 t CO2/year Land area required ................................................ 72 ha Total capital cost ............................................. 51800 S/ha Profit: Biomass fuel ................................................. + 15038 S/ha/year Power, nutrient, maintenance, labor ................. - 8610 S/ha/year Annualized capital cost .................................... - 5324 S/ha/year Net profit ....................................................... + 1104 S/ha/year C O 2 mitigation profit ............................................... 8.3 $/t C As it can be observed, a small profit is registered from algae production, on the basis that Romanian production costs are much lower than in developed countries. This demonstration project will lead to the development of specific research directed to measure the yield of algae grown at high CO2 concentration at different temperatures during the whole year. The results would be further scaled-up and incorporated in a large program of CO2 utilization through direct biofixation. 3.3. Case study no. 2
The project addresses the implementation of a CO2 recovery plant based on the chemical absorption in MEA aqueous solution from flue 5,000,000 gas produced by a greenhouse heating utility 4,500,000 4,000,000 and natural gas fired. The utilization of CO2 is 3,500,000 - ~ - $ a v i n g s J-............... i.................... ensured, within the greenhouse for enhancing 3,000,000 D the greenhouse vegetable and flower crop r 2,500,000 D while production fairly covers the investment 2,000,000 1,500,000 and operation costs in comparison with buying 1,000,000 the necessary C O 2 at the market price. 500,000 The CO2 recovery plant proposed would 0 have a CO2 production capacity of 2,000 kg/h, 0 1 2 Year 3 4 5 treating a flow 10,000 STP m3/h flue gas, with Figure 8. Economicparameters 10- 12 % CO2.
200 The economic assessment completed took into consideration 7,200 operation hours per year (300 days) at an expected production rate of 2,000 kg/h for a minimum plant operation life of 15 years. Related costs such as electricity, steam, MEA, potassium permanganate, wages, CO2 plant investment and installation cost, and the CO2 market price, have been considered. The results shown in the Figure 8 present attractive economic effectiveness with a short pay back period and good profit rates. On the basis of the clear economic parameters, this application becomes visible as a good and practical solution for CO2 utilization potential.
4. CONCLUSIONS The Romanian inventory of the CO2 emissions in the atmosphere, amounting to a total estimate of around 178,000 Mt/y are chargeable to different sources, especially to the industry sectors entirely responsible for the main participation of 34% of the total, followed closely by fossil fuel firing, for electricity and heating demands significantly registered with 24 %. Romanian demand for environmental equipment is important, while domestic environmental protection equipment production is still in its beginning, although there are local companies that produce components of environmental equipment or could become joint venture partners. The research locally developed on a CO2 removal pilot plant currently in use for research aims, revealed the high efficiency of CO2 recovery from flue gas, high carrying capacity of the monoethanolamine solution and satisfactory regeneration with low energy consumption. CO2 utilization possibilities identified and analyzed, address the potential of using the recuperated CO2 to enhance oil recovery, given that Romania would need a substantial quantity of 91 Mt of CO2 expected to be captured from flue gas from thermal power plants. Developing a biological CO2 fixation system for culture ponds in salty water based on the treatment of flue gas from town's utility located on the Black Sea coast would provide the opportunity of using the algae crop as additional fuel to the power station. Advantages are offered by the significant solar radiation that provides a notable daily rate of CO2 fixation. The potential of implementing a COz recovery plant based on chemical processes to suit the demands of greenhouses to enhance crop productivity, is the subject of the second ease study. The COz would be captured from the flue gas produced by the greenhouse heating utility. The appreciable investment effectiveness given the good profitability fairly covers investment and operation costs, avoiding the purchasing of the CO2 from an external supplier. Considering that Romania reveals good potential for CO2 utilization, there is also notable interest in foreign co-operation and investment to develop scientific research and technology development and to implement COz utilization technologies toward CO2 emission reduction.
REFERENCES
1. National Commission for Statistics, 1994, 1995, 1996, Romanian Statistical Yearbook 2. Dragos L., Scarlat N., Flueraru C "Trends in evolution of C02 emissions in Romania and perspectives for diminishing their environmental impact" Proceedings of 3rd International Conference on Carbon Dioxide Removal, Cambridge, U.S.A. (1996), pp. 679 - 684. 3. Oil & Gas Journal, Worldwide Production Report, (1993)
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
201
CO2 capture and utilization for enhanced oil recovery (EOR) and underground storage A case study in JiLin Oil Field, China Yun Guichun a, Liu Deshun b, Wu Tianbao, Wu Jiaquan, Ji Xiaoyuan a and Zhuang Lir a. Institute of Nuclear Energy Technology Tsinghua University, Beijing, 100084, China b: Institute for Techno-Economics and Energy System Analysis, Tsinghua University, Beijing, 100084, China c: JiLin Oil Field AdministrativeBureau, 76 Guorlos Road Song Yuan City 131100, JiLin Province, China 1. INTRODUCTION China is a large developing country with the most population but comparatively inadequate natural resources per capita in the world. So, China has adopted a sustainable development strategy, and the capture and reuse of CO2 is one of the technical measures for the mitigation of carbon dioxide emissions. China's oil fields are widely located in the country's 14 provinces. The total number of oil fields put into service by the end of 1990 is 238, of which 84% of reserves is recovered by the injection of water and the enhanced recovery in average is accounted for at about 32.5% of the oil originally in place, then, 60 to 70% of the oil in the reservoir can not be recovered. Oil and water do not mix; interfaces are formed which must be forced by the available energy. Therefore the injection of miscible agents to recover additional oil is needed. Development of a least-cost option as an environmental sound way for enhanced oil recovery is an issue the petroleum industry sector must address. In an effort to recover the remaining oil, the use of CO2 for large injection volume and more effective displacement process in West Texas U.S.A. is a well proven technology[I]. Recovery of CO2 from large point sources and reuse as resources for EOR can certainly be adopted as an preferential technology option for CO2 emission reduction, therefore China National Petrol Natural Gas Corp. (CNPNG) has set up a program to conduct CO2 flooding process tests in allocated sites. JiLin Oil Field was chosen as one of the testing oil fields.
202
2. P R O J E C T DESCRIPTION JiLin Oil Field is located at Songyuan City in SongLiao Basin of the North Eastern China. It owns 11 oil production areas with 3.55 MT of annual oil output and 135 Mm 3 of annual natural gas output. The main parameters of oil reservoir are: depth of oil reservoir 1200-1500m, permeability in average 6.5• 2, porosity in average 14.4% and viscosity of crude oil in the range of 5.0-10.3mPa.s. In the most oil production area, oil reservoir is characterized by the low permeability and the production wells are water saturated with water content above 80% since put into operation in 1984. It has drilled 988 water dependent wells and reached 6.4 MT of sum total crude oil with the production rate being 16.3% when compared to the expected final exploitable 30%. It was revealed that the efficiency of the water drive is rather poor. The remaining resources in the oil reservoir constitutes the target for enhanced recovery process. The physical simulation tests in the laboratory indicated that the parameter of the oil reservoirs are satisfied with the requirements for the CO2 flooding process. A test on CO2 flooding on a small scale in place was conducted that showed 7% of recovery ratio could be enhanced by injection of CO2 reaching 0.2 PV. Similarly, about 3.15 MT of recoverable reserves could be recovered by injecting 14 MT-CO2. Based upon these tested results, the XinLi 228 Zone of production with 2.25 klTl 2 w a s selected for the CO2 flooding process tests. Process design consists of 4 CO2 injecting wells and surrounded by 21 production wells. The geological reserve in place is estimated as 2.12 Mt. By using water driving process the expected recovery per cent would eventually be 27%, corresponding to 0.57 MT of the recoverable reserve. It is evaluated that when CO2 miscible flooding process is applied to reach 0.2 PV within 5 years, the recovery per cent could be enhanced by 7%, that means 0.148 MT of the recoverable reserve. Within 10 years of the life time of the project, the accumulated CO2 amount injected would be 0.346 MT-CO2, and approximate 0.173 MT-CO2 will be stored underground stably. The CO2 source will be taken from the Chang Shan Synthetic Amonia Factory 5 km away, which will emit 4.0 t-COJh in average with concentration of 98%. The process flow chart is shown in Fig. 1 3. POTENTIAL CO2 MITIGATION BY USING CO2 I N J E C T I O N FOR EOR
203
CO2 Sources ]
Anti-corrosive Tank ,,,
Dosage Pump 3.0 MPa
~
~
3.0 MPa
CO2
Separator]
I
Ammonia Cooling
.~] CO2Storage ~ "-I Tank
CO2 Injecting I Pump [
~[ Well ,,,
Fig.1 Process flowsheet of CO2 injection for EOR China's oil fields are widely distributed in the 14 provinces. Up to 1990, 84% of oil fields produced crude oil by using water injection teclmology with 32.5% of the recovery ratio only. By the simulation test and calculation conducted by the Ministry of Petroleum Industry during 1985-1995, it was evaluated that the geological oil reservoirs suitable for miscible flooding would be about 6.39x109 t and mainly located in Zhongyuan, Jianghan, Kelamayi, Liaohe, JiLin, Jiangsu, Daqing and Yumen Oil fields etc. with bright prospect in using CO2 flooding process for EOR and disposal of CO2 underground. If CO2 flooding process is adopted for EOR, additional recovery ratio of 17.8% could be enhanced equivalent to 86 MT of the recoverable reserve and 236.2x108 Nm 3 CO2 gas consumed. By the experiences learned in U.S.A[2] that showed 4.5 t-CO2 is required for recovery of an additional tons of oil. In other words, a half of these amounts of CO2 will be stably stored in the oil reservoir without any leakage. Furthermore, CO2 emission can be reduced additionally by 182.5 Mt-C based on a replacement of coal by the additional oil as fuel. 4. THE BASELINE DETERMINATION To evaluate the C02 emission abatement and its cost-benefit, the baseline for the project design should be deterlnined. In light of the real conditions of the XinLi Oil area, some of oil-beating reservoirs are facing the end of conventional recovery operations. The recovery factor is extremely small and the cost of production becomes economically unfeasible. It is essential to adopt the recovery through injection of miscible agents among which the available miscible agents are currently polymer and gas. In the case of JiLin Oil Field, the baseline options are: water drive and polymer drive.
204
4.1 The baseline option 1: water drive For some areas, especially for those new areas, water drive will continue in operation for a long period. If sludge injection (CO2 and water altemative injection) is adopted, not only can oil output be increased, but also CO2 mitigation benefits can be achieved additionally. In this case the baseline option is water drive, by which the CO2 emission is mainly caused by the electricity consumption for water pumping and oil production, and the cost levels are lower. 4.2 The baseline option 2: polymer drive The other available recovery process in JiLin Oil Field could be polymer drive plus water drive. It can be taken as another baseline option, by which more electricity energy are consumed for the process and production of polymer, therefore the cost level is higher than the water drive case. The production curve of the CO2 flooding versus the two baseline-H20 and POM is illustrated in Fig.2. 18000 16000 14000 " , ~ 12000 10000 8000 i 6000 4000 2000 0 !
mmmmmm w m mmmmmmm
mmmm.~~mmmmmmmm
)-
mmm~~mmmMmmmmm m
m
m
1993
m
m
m
m
m
m
m
m
m
1999 ~H20
~POM
Imm! -.
~
2005 ~CO2
[
Year
Fig.2 JiLin Oil Field XL 228 Zone Q-T Curve 5. THE NET EMISSION MITIGATION Considering the leakage of the C02 injection, i.e. assuming about 50% of C02 injected would be eventually stored underground, while another 50% would escape to the atmosphere with the oil recovery during the C02 flow cycling, the net C02 emission could be calculated by the following formula: net C02 emission = C02 leakage + C02 emission (caused by energy consumption in the process) C02 injection, in which C02 leakage = leaking rate (50%) C02 injection.
205
6. THE I N C R E M E N T A L COST OF CO2 M I T I G A T I O N The incremental cost is defined as the levelized life cycle incremental cost for per unit of CO2 emission reduction[3]. Given the discount rate of 12% and exchange rate of 1US$:8.31 RMB Yuan, and life time of 10 years, the incremental costs are evaluated as: 9 For baseline option as water drive: 70.52US$/t-CO2 or 258.58US$/t-C 9 For baseline option as polymer drive: 27.86US$/t-CO2 or 102.15US$/t-C. It should be noted that some uncertainty in the capital cost and the leaking rate of GEF option will influence the resulted incremental costs of GHG mitigation. For instance, if the capital cost reduce by 27% and the leaking rate would be 30% instead of 50%, then the incremental cost will be: 9 For baseline option as water drive: 36.7US$/t-CO2 or 134.76US$/t-C 9 For baseline option as polymer drive: 10.79US$/t-CO2 or 39.54US$/t-C. 7. THE ANNUAL CO2 EMISSION M I T I G A T I O N 9 For baseline option as water drive: By the scale of the pilot project in JiLin Oil Field, it is estimated that annual CO2 emission mitigation will reach 3.56 kt-C or 13.07 kt-CO2, in which the net CO2 emission mitigation per unit oil production is 0.64 t-CO2/t-oil. 9 For baseline option as polymer drive: By the scale of the pilot project in JiLin Oil Field, it is estimated that annual CO2 emission reduction will reach 4.22 kt-C or 15.46 kt-CO2, in which the net CO2 emission mitigation per unit production is 0.758 t-CO2/t-oil. 9 Increased energy production: By the scale of pilot project, for polymer drive option the accumulative enhanced recovered oil production is estimated to be about 148.4 kt, and for CO2 injection option the accumulative enhanced recovered oil is about 38.5 kt. The Cost-Benefit assessment of the C02 injection for EOR and storage underground on the two baseline scenarios are listed in Table 1. REFERENCES 1. T. Holt and E. Lindeberg, Thermal power-without greenhouse gases and with improved oil recovery, Proceedings of the First Intemational Conference on Carbon Dioxide Removal, Amsterdam 4-6 March (1992) 595-602
206
2. P.L Bondor, applications of carbon dioxide in enhanced oil recovery, Proceedings of the First International Conference on Carbon Dioxide Removal, Amsterdam 4-6 March (1992) 579-586 3. Liu Deshun, Ouyang Lihui and Zhang Yanlin, Feasibility of AIJ Pilot Options: CFBC&CHP in China, Case Study: Cogeneration power plant by using CFBC boiler, International Conference on Technology for AIJ, May 26-29 (1997), Vanacouver, Canada. 4. Yun Guichun, Jin Guangyu, Preliminary Analysis of Environmental And Economic Effects of EOR With Recovered CO2, Abstract of ICCDU, Norman, Oklahoma, April 30-May 4 (1995). Table 1 The Cost-Benefit assessment of the CO2 injection for E O R Item Unit Baseline1 Baseline2 GEF project (water inj.) (po!ymer inj.) (C02 inj.) 450 2481.6 80 1Ok Yuan Capital cost 56.25 310.2 10k Yuan 10 Annual fix cost 384.23: 290.22 10k Yuan 42.88 Annual variable cost 12,700! 12,700 T 12,700 Annual oil production by water inj. 6,452.2 7,697.4 T Annual enhanced oil production by 19,152.2 20,397.4 T 12,700 Total annual oil production 239.05 2,380.95 4,047.62 MWh/year Electricity consumed 0.266 0.452 0.027 10kT-CO2 CO2 emission by Electricity 1.732 10kT-CO2 CO2 net injection 0.266 -1.28 10kT-CO2 0.027 Net CO2 emission -0.628 0.139 0.021 Net CO2 emission per unit oil prod T-CO2/T-Oil T-CO2/T-Oil 0.766 0.649 Net CO/mitigation per unit oil prod. 520.59 84.29 15.86 1Ok Yuan yearly levelized LCC of fixed capital cost 384.23 290.22 42.88 1Ok Yuan Annual variable cost 427.92 250.50 47.83 Yuan/T-Oil Unit production cost 231.50 586.04 Yuan/ Incremental cost per unit of CO2 T-CO2 mitigation US$/T-CO2 70.5 27.86 Yuan/T-C 2,148.80 848.85 US$/T-C 258.58 102.15 10kT-CO/ 1.323 1.563 i GEF annual net CO2 10kT-C 0.361 0.426 emission mitigation 10k Yuan 775.30 361.89 GEF annual incremental 43.55 93.30 10k US$ cost for CO2 mitigation .
. . . . . . . . .
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
207
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Electrocatalytic
r e d u c t i o n o f CO2 to w o r t h i e r
compounds
on a functional
d u a l - f i l m e l e c t r o d e w i t h a s o l a r cell as the e n e r g y s o u r c e
K. Ogura, M. Yamada, M. Nakayama, and N. E n d o
Department of Applied Chemistry, Yamaguchi University, Ube 755, Japan
The electrochemical reduction of CO 2 in aqueous solution on a functional dual-film electrode consisting of Prussian blue and polyaniline doped with a metal complex using a solar cell as the energy source led to the formation of lactic acid, formic acid, methanol, etc., and the maximum current efficiency for the CO 2 reduction was more than 20 % at -0.8 V vs Ag I AgCI.
1. I N T R O D U C T I O N The chemical conversion of CO 2 to more valuable substances is a very important subject in connection with the mitigation of the concentration of green-house gas in the atmosphere. Many conversion processes are proposed, including the hydrogenation at high temperature and the electrochemical reduction [ 1]. However, a matter of importance in the chemical conversion of CO 2 is to achieve it under an input energy as low as possible to avoid a secondary generation of
CO2~ In
this regard, the electrochemical method is promising since the electroreduction of
CO 2 in aqueous solution can provide a process similar to natural photosynthesis if the required energy is given by a solar cell.
In this process, however, it is indispensable that the
overpotential for the CO 2 reduction is low enough because, otherwise, hydrogen evolution woul become predominant resulting in negligible reduction of CO 2. Furthermore, reduction products are expected to be worthy as high as possible to be a match for natural photosynthesis.
For this purpose, we have developed the modified electrode with two
laminated films consisting of an inorganic conductor and a conducting polymer [2, 3]. The
208 mediation effect was much improvedby immobilizing a metal complex to the conducting polymer. In the present study, a solar cell was used as the energy source for the reduction of CO 2 on the functional dual-film electrode, and the electrolysis was carried out under the sunlight.
2. E X P E R I M E N T A L A commercial solar cell (model BL-432, Showa Shell Petroleum Co.) was used as the energy source for electrolysis. The maximum output of power of this solar cell was 8 W, and the open-circuit voltage 20 V. A given voltage for electrolysis was obtained through a variable resistance. The functional dual-film electrode was prepared by the following procedure.
Prussian
blue (PB) and polyaniline (PAn)were used as an inorganic conductor and a conducting polymer, respectively. The PB film was first deposited from an aqueous ferric ferricyanate solution, and the PAn film was then deposited on the PB film electrode by repeated potential cycling in hydrochloric acid solution (pH 1) containing 0.1 M aniline. The prepared PAn I PB I Pt electrode was immersed in the phosphate buffer solution (pH 7) for 30 min to release chloride ions incorporated into the PAn film during the electrodeposition. After the dual-film electrode was rinsed thoroughly with distilled water, it was put in a solution containing bis(1,8 -dihydroxynaphthalene-3,6-disulphonato) iron(II) complex (Fe-L(C)) where the
iiiiiiiiiiiiiiiiiiiiiHiiiiiiiiiiiiiiiiiiiil Solar cell
immobilization of this complex onto the PAn
film
was
achieved
by
the
electrochemical method. The electroreduction of CO 2 was performed with the prepared electrode (WE) in a gas-tight H-type electrolysis cell as shown in Fig. 1. The electrolyte was 0.5 M KCI solution of pH 3.0. The pH of the solution was regulated at 3.0 throughout the electrolysis with a pH Fig. 1 Schematic representation of the electrolysis system using a solar cell as the energy source.
controller (NPH-660 ND-type).
The
volume of
the
the
catholyte
after
electrolysis was about 125 cm 3, which
209
was precisely measured.
CO 2 (Iwatani Co., 99.7 % ) was purified by the repeated
freeze-pump-thaw cycles.
The reference electrode used was a saturated calomel electrode
(SCE), and the counter electrode (CE) a platinum plate. The products were analyzed with a Shimadzu organic acid analyzer (LC-10AD type) and an Okura SSC-1 steam chromatograph with a flame ionization detector and a Porapak R column.
The products were found both in the solution and within the coated film.
samples for these analyses were the distillate that was prepared by evaporating 20
cm 3
The of the
catholyte until 2 crn3 under reduced pressure. The products adsorbed on the coated film were released into 25 cm 3 of distilled water under ultrasonic irradiation for 5 min. The identification of lactic acid (product) was performed by liquid chromatograph / electrospray mass spectrometry (LC/MS) examining negative ions. The apparatus used was a Hitachi M-1200 LC/MS. The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FI'IR) reflection absorption spectroscopy 13]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ F'I'IR measurements were carried out in a spectroelectrochemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Agl AgCI I saturated KCI electrode. The IR spectra are represented as A R/R in the normalized form, where A R=R_R(Eb) ' and R and R(E b) are the reflected intensity measured at a desired potential and a base potential, respectively.
3. R E S U L T S A N D D I S C U S S I O N In situ FTIR reflection spectra of a PAn I PB I Pt electrode are shown in Fig. 2 where the
potential was stepped from 0 V vs Ag I AgCI to the cathodic side. The downward and upward bands correspond to the increase and decrease in concentration of adsorbed species on the electrode, respectively.
The band observed at the wavenumber of 3600 to 3200 cm ~ is
attributable to the vibrational absorption of H20. The negative-going peak at 2100 cm I, which increases with stepping the potential to less noble side, means the electroreduction of PB to its reduced form (ES). A broad band appearing in the wavenumber region higher than 1600 cm -~ is ascribed to the electronic absorption of PAn that is caused by free carders (unpaired
210
electrons,
positive
charges)
[4].
Upward direction of this band indicates
0.3
\
F
the
-0.4 /
/-~x
H
transformation
nonconducting form.
0.2
of
PAn
to
The absorption
peaks at 1471 and 1561 cm 4 are both assigned to semiquinoid ring vibrations
/
n.-. rr' 0.1
1
<1
'rA .z
[5], and the peak at 1514 cm 4 to a benzenoid ring vibration [6].
o-.,
q CJ
4000
The
enhancement of the former two peaks
I
2000
u3
TI
1500
1000
directed upward and the latter peak directed downward upon the negative polarization
suggests
semiquinoid
structure
that of
the
PAn
is
transferred to the benzenoid by the
w a v e n u m b e r / cm -1
electrochemical reduction. The prolonged electrolysis of CO 2
F i g . 2 In situ FFIR reflection spectra of a PAn I PB I Pt electrode in N2-sarurated 0. I M KCI solution. The electrode potential was stepped from 0 V (base) to various cathodic potentials as indicated (Ag I AgCI).
was carried out on the functional dual-film electrode with a solar cell as the energy
source.
The
results
obtained are shown in Table 1 where
Table 1 Yields and current efficiency of the products obtained in the electrochemical reduction of CO 2 with a solar cell as the energy source a~ Run
1 2 3 4 5 6 7 8
Time h
MeOH
0.2 0.5 1 6 18 30 48 72
0 0 0.2 0.8 2.2 1.1 1.9 4.5
Products / F mol dm 3 EtOH Acetone I_aaicAdd FormicAcid 0.5 0.5 0.2 1.3 1.7 0.5 0.4 4.7
0.4 0.7 2.8 0.4 1.3 2.6 0.9 4.4
0 0 1.4 8.6 10.9 14.5 19.7 21.1
3.5 5.8 2.8 2.2 2.1 1.3 10.5 10.6
Qbl C
r/c~ %
0.9 1.7 3.4 12.3 33.4 55.3 137.3 167.4
26.4 20.0 24.8 13.6 7.0 5.3 2.6 3.3
a) 0.5M KCI solution of pH 3; electrolysis potential, -0.83----0.75 V vs SCE. b) Electric charge passed during the electrolysis. c) Current efficiency for the CO_, reduction.
211 the electrolysis experiments were
,
8O
conducted with the solar cell under
-~ 30 ~, 60 -
o
!40
~'E "o
the irradiation of the
sunlight
(daytime) and a fluorescent lamp
-
O
20 E
(nightime). The initial electrolysis potential was adjusted t o - 0 . 8 V
o o
20-~
- 10-~ _J
(SCE), but it varied in the potential range between-0.75 and-0.83 V. As the reduction products, lactic acid, formic acid, acetone, ethanol,
0 0
i 40
! 80
I 120
0
and methanol were obtained, and the
Q/C Fig. 3 Plots of 5; c (total carbon content)(O) and the amount of lactic acid ( O ) versus the electric charge passed during the electrolysis with the solar cell. E c = [CH3OH]+2[C2HsOH]+ 3 [ (CH3) 2CO !+3 ICH3CH(OH)COOH 1. 100 %
reduction reached more than 20 % in a relatively short electrolysis time (< l h). The overpotential required for the
reduction
of
CO2
was
considerably lower compared to that referred for the electroreduction o f
a
s9
current efficiency for the CO2
CO2 with a metal electrode in aqueous solution.
50~
The worthiness
of the products was considerably
i
1 I
25
50
.ll,lll.
1o0
h.,
,!1
150
m/z
200
improved
since
products
usually
the
reduction
obtained
in
aqueous solution at metal electrodes are formic acid or methane.
m/z=89(M-1)
The amount of lactic acid, that was the most abundant product, and the total carbon content ( E c) are plotted as a function of the electric charge passed during the electrolysis
0
!
1
2
I
t / min
3
I
4
5
Fig. 4 Negative-ion electrospray mass spectra of the catholyte (a) and the mass chromatogram of the peak at m/z value of 89 (b).
in Fig. 3. Both quantities increase in proportion to the electric charge although the increment became sluggish beyond 15 coulombs. Itis therefore indicated that the products
212
including lactic acid are generated through electrochemical reactions. Negative-ion electrospray mass spectra of the catholyte are shown in Fig. 4a where the electrolysis of CO2 was conducted for 6 h with the solar cell in 0.5 M KCI solution. The peaks at m/z values of 89, 45 and 31 correspond to CH3CH(OH)COO-, HCOO-and C H 3 0 - , respectively, confirming the formation of lactic acid, formic acid and methanol. In Fig. 4b, the mass chromatogram of the peak at m/z value of 89 is shown, and the retention time at 2.19 min was precisely in agreement with that of the standard lactic acid. From these results, it is considered that the electrophilic carbon atom of CO 2 is bound to the amino group of PAn and the basic oxygen atom coordinates to the central metal of the complex. Such bifunctional activation of the substrate may lead to the reduction at the electrode potential close to the thermodynamic one.
Lactic acid is probably generated through the
formation of formaldehyde, the insertion of CO 2 to the C-H bonds to form C-C-C bond and the reduction by Had s.
4. C O N C L U S I O N S It was demonstrated that CO2 can be reduced to C~
"- C 3
compounds involving lactic acid
on the functional dual-film electrode with a solar cell in aqueous solution. This process is accompanied by the simultaneous evolution of oxygen at the counter electrode, which bears a close resemblance to natural photosynthesis.
REFERENCES 1.
B.P. Sullivan, K. Krist, and H. E. Guard (eds.). Electrochemical and Electrocatalytic Reactions of Carbon Dioxide, Elservier Publishing, Amsterdam, 1993.
2.
K. Ogura, N. Endo, M. Nakayama, and Y. Otsuka, J. Electrochem. Soc., 142 (1995) 4026.
3.
K. Ogura, M. Nakayama, and C. Kusumoto, J. Electrochem. Soc., 143 (1996) 3606.
4.
H. Kuzmany, N. S. Sariciftci, H. Naugebauer, and A. Neckel, Phys. Rev. Lett., 60 (1988) 212.
5.
Y. Furukawa, T. Hara, Y. Hyodo, and I. Harada, Synth. Met., 16 (1986) 189.
6.
J. Tang, X. Jing, B. Wang, and F. Wang, Synth. Met., 24 (1988) 231.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
I n c o r p o r a t i o n of CO2 i n t o o r g a n i c p e r f l u o r o a l k y l electrochemical methods
213
derivatives
by
E. Chiozza a, M. Desigaud a, J. Greiner b, E. Dufiach a aLaboratoire de Chimie Mo!~culaire, Associ~ au CNRS, URA 426, Universit~ de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France bLaboratoire de Chimie Bioorganique, Associ6 au CNRS, ESA 6001, Universit6 de Nice-Sophia Antipolis, 06108 Nice Cedex 2, France
The CO2 fixation into (perfluoroalkyl)iodoalkanes, (perfluoroalkyl)iodoalkenes and (perfluoroalkyl)alkenes, catalyzed by e!ectrogenerated nickel complexes, afforded perfluoroa!kyl carboxylic acid derivatives. The electrocarboxylation of perfluoroalkyl olefins proceeded with double bond migTation and loss of an ally!ic fluorine atom. 1. I N T R O D U C T I O N
We have been interested in the carbon dioxide fixation into organic substrates for the s3~nthesis of speciality products of the carboxylic acid family. We focussed our attention on the formation of new carbon-carbon bonds between CO2 and perfluoroa!ky! derivatives, particularly perfluoroalkyl olefins. The perfluorinated chain being highly hydrophobic, the expected perfluoroa!kyl carboxy!ic acids are precursors of amphiphi!ic molecules, of interesting applicability in the field of surfactants and medicinal chemistry [1]. The fixation of carbon dioxide into functionalized organic substrates to selectively afford carboxylic acids in catalytic reactions is still a challenge in carbon dioxide chemistry [2]. In the field of o!efin carboxy!ation, stoichiometric reactions have been described to occur between non-activated alkenes, CO2 and an electron-rich transition-metal complexes, such as Ni(0) [3], Ti(!!) [4] or Fe(0) [5]. A Pdcatalyzed CO2 fixation occurs into methylenecyclopropane derivatives affording !actones [6]. The reaction of carbon dioxide with ethylene is difficult and its carboxylation to propionic acid, catalyzed by Rh derivatives [7], needs drastic experimental conditions. The direct carboxylation ofperfluoroalkyl iodides has been reported to afford RFCO2H in the presence of a Zn-Cu couple [8] or that of zinc and ultrasounds [9]. To our knowledge, no reports on the carbon dioxide fixation into perfluoroalkyl olefins have been yet described.
214 Our synthetic method for CO2 fixation was based on the use of transitionmetal catalysis combined with electrochemical techniques [10]. Within this methodology, the electrochemical C02 fixation into some a!kenes has been reported to afford carboxylic acids in a reductive hydrocarboxylation-type reaction c a t a l y z e d by nickel complexes, u n d e r mild conditions [1!]. The electrocarboxylation of organic halides to the corresponding carboxylic acids has also been reported [12], )4e!ds and efficiency of the reaction being strongly dependent on the reaction conditions. 2. ELECTROCARBOXYLATIONS The catalytic incorporation of carbon dioxide into alkene derivatives remains an interesting goal in CO2 chemistry, and in the present study, we wish to CnF2n+l) on the examine the influence of the perfluoroalkyl (RF) chain (RF electrocarboxylation of a double bond, as well as on that of other perfiuoroalkyl iodo derivatives. Substrates 1-4, containing RF substituents (Cf. eqs. 1-3) have been prepared according to literature procedures [13]. Their electrochemical behaviour and in particular, the CO2 fixation under mild conditions have been examined in the presence of several nickel(II) complexes used as catalyst precursors. Ni(!I) derivatives associated to polydentate nitrogen ligands have been reported as efficient catalysts in the electrochemical carboxylation of alk)~es [14] and di:~es [15]. Thus, Ni(II) complexes such as Ni(bipy)32+, 2BF4- (bipy = 2,2'-bipyridine) [16], Ni(cyclam)Br2 (cyclam = !,4,8,11-tetraazacyclotetradecane) [17], or NiBr2(dme) with N,N,N',N",N"-pentamethyldiethylene triamine (PMDTA) (dme = dimethoxyethane) [18] have been prepared and used in catalytic amounts in e!ectrocarboxylation. The general electrochemical procedure for the carbon dioxide incorporation was based on the use of one-compartment cells fitted with consumable anodes of magnesium or zinc [12]. E!ectrocarboxylations were carried out in DMF at constant current density, using tetrabutylammonium tetrafluoroborate (10 -2 M) as supporting electrolyte. The catalyst was introduced in a 10% molar ratio with respect to the substrate and carbon dioxide was bubbled through the solution at atmospheric pressure. Electrolyses were generally run at room temperature and reactions were stopped when starting material was consumed or when the farada'~c yield attained 30%. =
2.1. Electrocarboxylation reactions of (perfluoroalkyl)iodoalkanes and (perfluoroalkyl)iodoalkenes The electrocarboxylation of 1 took place under mild conditions (C02 pressure of I atm, T = 20~ in the presence of Ni(bipy)32+, 2BF4- as the catalyst precursor and a Mg/stainless steel couple of electrodes, to afford the corresponding perfluoroalkyl carboxylic acid 5 in 30% yield, together with the olefin 3 [20%, E / Z 90:10)] (eq. 1).
215 CO2H I ,~H I 1) Ni-bipy,cJ C8F17~ CsFI7CH 2 CHC4H 9 + --"C C8FITCH 2 CH C4H 9 + CO 2 DMF,Mg anode H ~C i~C4H 9 1 2) hydrolysis • 3
(eq. 1) The electrocarboxylation of the same substrate in the absence of the nickel catalyst did not afford any carboxylic acids derived from 1. The electrocarboxylation of(perfluoroalkyl)iodoalkene 2 (E/Z 90:10) under 1 atm of CO2 (Mg/Ni electrodes, Ni-bipy catalytic system) allowed the formation of unsaturated carboxylic acid 6 in 10% yield (E/Z 85:15) (eq. 2). Olefin 3, issued from the reduction of the halide function by protodehalogenation, was obtained in 30% together with 25% of unreacted starting material.
C8F17 ..C-'-" C~r + CO I) Ni-bipy,e- ~C8Fi7 "H/C=C~'r CO2H c+8 F 1H7 ~ C : C H C4H9 2 DMF,Mg anode ~ C4H 9 2 2) hydrolysis 6 3
~,jH I~C4H9 (eq. 2)
2.2. Electrocarboxylation of (perfluoroalkyl)alkenes The Ni-catalyzed electrocarboxy!ation of differently activated olefins has been reported to afford selective CO2 incorporation via hydrocarboxylation [11]. However, no CO2 incorporation occurred with non-activated alkenes such as 1- or 4-octene. Carboxylation of olefins 3 and 4 should give some indication on the influence of the RF substituent on the double bond. The electrolyses of olefins 3 and 4 (eq. 3) in the presence of CO2 led to the synthesis of carboxylic acids, and have been carried out using bipy, cyclam or PMDTA as the ligands on nickel (Table 1). Table 1 Influence of the ligands of the Ni(II) catalyst on the electrocarboxylation of substrates 3 and 4 (room temperature; CO2 1 atm; electrodes: M~stainless steel)
Starting compounds CsF17-CH=CH-C4H9 3 C6F13-CH=CH-CsH17 4
Nature of the !igand bipy cyclam bipy cyclam PMDTA
Carboxylic acids 20% 65% 30% 92% 64%
216 The results indicate that the Ni-cyclam catalytic system offers the best yields of carboxylation, and up to 92% of CO2 incorporation into 4 could be achieved. However, the carboxylic acids were not issued from the expected hydrocarboxylation of the double bond. We could respectively identify the two (E) and (Z) isomers of the ~,y-unsaturated acids 7 and 8 (isolated as their methyl esters, eq. 3), containing a vinyl fluorine substituent. The relative yields of products 7 and 8 were 45% ( E / Z = 30/70) and 56% ( E / Z = 37/63), respectively. R~\C= / H C\CH_R u F/ i
Rp-CF2 H;C:C 3
~H +
RH
COeMe
]) e-, Ni(II)L CO 2
Z
+
2) K2CO3, MeI
or 4
Rt=~ 3, 7 RF= C7F15, RH= C4H9 4,8 RF=CsFll, RH=C8H17
7 or 8
CO2Me / CH- RH
F/C=C\H
7 or 8 E (eq. 3)
Thus, both perfluoroalkyl olefins 3 and 4 incorporated CO2 on one of the initial vinyl carbons, regioselectively on the site of the hydrocarbon chain. The carboxylation process involves a shift of the double bond, with the loss of one fluorine atom from the allylic positi'on. The influence of the temperature on the electrocarboxylation was examined, and showed that the Ni-bipy catalytic system was more efficient at 60~ than at 20~ Thus, 58% and 86% yields of carboxylic acids were obtained at 60~ from 3 and 4, respectively. 3. M E C H A N I S T I C
STUDIES
In order to explain the results involving RF-olefin electrocarboxylation reaction with allylic double bond migration, cyclic voltammetric studies were carried out and electrolyses were conducted under stoichiometric conditions in the particular case of the Ni-bipy catalytic system with perfluoroalkyl olefin 3. Several experiments with electrogenerated Ni(0)(bipy)2 complex at -1.0 V vs Ag/AgC1 indicated that no activation of the allylic C-F bond occurred at this potential. Controlled potential experiments at-1.7 V under CO2 led to the formation of the corresponding RF-carboxylic acid, 7. According to these results, the catalytic cycle shown in Figure i is proposed. Reduced nickel species, such as [Ni(bipy)2]', could be responsible for the catalytic activity in the carboxylation process.
217
[Ni(bipy!,,3,1 F,,x '
~ + 2e" (-1,2 V)
R/ ~ ~ RH Mg ~ F'~ Anode:
RFCF2~""--~RH
L2Ni(0)
e" (-1,7v)
FNiO L
CO2
Figure 1. Proposed m e c h a n i s m for the electrocatalytic carboxylation of (periluoroalkyl)alkenes 4. CONCLUSIONS The electrochemical incorporation of CO2 into perfluoroalkyl derivatives has b e e n e x p l o r e d in t h e case of ( p e r f l u o r o a l k y l ) a l k y l iodides and (perfluoroalkyl)alkenes, with an electrochemical system based on the use of consumable anodes combined with organometallic catalysis by nickel complexes. Iodide derivatives have been functionalized to the corresponding carboxylic acids by reductive carboxylation. Interesting and new results have been obtained from the fixation of CO2 into perfluoroalkyl olefins. Good yields of carboxylic acids could be reached by a carefull control of the reaction conditions and of the nature of the catalytic system. The main carboxylic acids are derived from the incorporation of carbon dioxide with a double bond migration and loss of one fluorine atom from the CF2 in ~ position of the double bond. The formation of carboxylic acids from perfluoroalkyl olefins reveals an important influence of the perfluoroalkyl chain on the carboxylation process. Thus, no carboxylation occurred in the case of related non-activated alkyl olefins under the same reaction conditions. These results constitute the first example in which an allylic reactivity involving a double bond migration is observed in electrochemical carboxylations.
218 REFERENCES
1.
a) E. Kissa, Fluorinated Surfactants. Synthesis, Properties, Applications, Surfactant Science Series, Vol. 50, M. Dekker, New York, 1994. b) J. G. Riess, J. Greiner and P. Vierling, In Organofluorine Compounds in Medicinal Chemistry and Biomedical Applications, R. Filler, Y. Kobayashi and L. 1Y[ Yagupolskii (eds), Elsevier, Amsterdam, London, New York, Tokyo, pp 339380, 1993. 2. M. Aresta and G. Forti (eds.), Carbon Dioxide as a Source of Carbon, Nato ASI Series, Series C, Vol. 206, Dordrecht, 1987. 3. a) H. Hoberg, Y. Peres, C. Krtiger and Y. H. Tsay, Angew. Chem., Int. Ed. Eng., 26 (1987) 771. b) D. Walther, E. Dinjus, J. Sieler and L..4~dersen, J. Organomet. Chem., 276 (1984) 99. 4. S.A. Cohen and J. E. Bercaw, Organometallics, 4 (1985) 1006. 5. H. Hoberg, K. Jenni, K. Angermund and C. Kruger, Angew. Chem., Int. Ed. Eng., 99 (1987) 141. 6. P. Binger and H. J. Weintz, Chem. Ber., 117 (1984) 654. 7. A. L. Lapidus, S. D. Pirozhkov and A. A. Koryakin, Bull. Acad. Sci. USSR., Div. Chem. Sci. (Engl. Trans), (1978) 2513. 8. H. Blancou, P. Moreau and A. Commes~as, J. Chem. Soc., Chem. Commun., (1976) 885. 9. N. Ishikawa, M. Takahashi, T. Sato and T. Kitazume, J. Fluorine Chem., 22 (1983) 585. 10. H. Lund and M. Baizer in Organic Electrochemistry, M. Dekker, New York, 3 rd Ed., 1990. 11. S. Derien, J. C. Clinet, E. Dufiach and J. P~richon, Tetrahedron, 48 (1992) 5235. 12. a) J. Chaussard, J. C. Folest, J. Y. Ndd~lec, J. P~richon, S. Sibille and M. Troupel, Synthesis, (1990) 369. b) G. Silvestri, S. Gambino, G. Filardo and A. Gulota, Angew. Chem., Int. Ed. Engl., 23 (1984) 979. 13. a) N. O. Brace, J. Org. Chem., 27 (1962) 3033. b) N. O. Brace, J. Fluorine. Chem., 20 (1982) 313. c) A. Manfredi, S. Abouhilale, J. Greiner and J. G. Riess, Bull. Soc. Chim. Fr., (1990) 872. d) D. J. Burton and L. J. Kehoe, Tetrahedron Lett., (1966) 5163. 14. S. Derien, E. Dufiach and J. P~richon, J. Am. Chem. Soc., 113 (1981) 8447. 15. S. Derien, J. C. Clinet, E. Dufiach and J. Pdrichon, J. Org. Chem., 58 (1993) 2578. 16. E. Dufiach and J. P6richon, J. Organomet. Chem., 352 (1988) 239. 17. B. Bosnich, C. I~ Poon and M. L. Tobes, Inorg. Chem., 4 (1965) 1102. 18. E. Dufiach and J. Pdrichon, Synlett, (1990) 143.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
219
M o l e c u l a r tailoring of organometallic polymers for efficient catalytic CO2 reduction: m o d e of formation of the active species Raymond Ziessel Laboratoire de Chimie, d'Electronique et de Photonique Molrculaires, Ecole Europrenne Chimie, Polymrres, Matrriaux, 1 rue Blaise Pascal, 67008 Strasbourg Cedex, France We here report the first example of an electrochemical polymerization process which leads to formation of a modified electrode having the generic formula [Ru0(bpy)(CO)2C1]n, and which displays outstanding electrochemical activity towards reduction of carbon dioxide to either carbon monoxide or formate. A crucial stereochemical effect of the leaving groups on the feasibility of polymerization is demonstrated. Formation of the polymer occurs stepwise, through the formation of a dimeric or a tetrameric intermediate. 1. INTRODUCTION Carbon dioxide fixation is the basic process by which natural photosynthesis produces organic matter on earth. Appreciable effort has been devoted to the design of artifical systems capable of carbon dioxide fixation with the viewpoints of converting solar energy and/or electricity into chemical energy and mimicking biological carbon assimilation. In light of the twin problems of global warming and depletion of fossil fuels, much attention has been paid to electrochemical reduction of CO2 as a potential C 1 or C2 source for chemicals and fuels. These processes are difficult to catalyse as they typically involve not only multiple electron transfers but are often coupled to chemical steps such as protonation. In general, there can be multiple competing reaction pathways leading to a variety of reactions products. These products differ in the number of redox equivalent (2 to 8e-) and the kinetics for their formation can vary broadly and depend on factors such as proton avalaibility. Although thermodynamically these processes should take place at moderately cathodic potentials, the direct reduction of CO2 at bare metal cathodes, which act as simple outer-sphere electron donors, typically requires very large overpotentials because of the formation of high energy intermediates such as CO2-" [1]. This uncatalyzed electrochemical reduction of CO2 requires very high overpotentials in the range of 1 to 2 volts (e.g. the reduction of CO2 in DMF takes place at about - 2.0 V vs SCE [2]). Transition metal complexes used in conjunction with metal cathodes decrease the activation energy barrier and circumvent the formation of high energy intermediates. These complexes can efficiently mediate the electron transfer from the cathode to CO2. In addition to lowering the overpotential, a good catalyst could in principle, increase the selectivity of the product being produced and yield high current efficiencies for a single product. A number of transition metal complexes have been shown to be effective in the electrocatalytic reduction of CO2
[3,4].
In many of these examples, the catalyst is capable of undergoing more than one reduction and thus storing multiple redox equivalents. During the reduction process the catalyst make available at least one open coordination site where CO2 can bind and ultimately be reduced
220
(case of CO formation), or the open coordination site can protonate to form a hydride in which CO2 can be inserted (case of HCOO-formation). The use of electrodes modified with surface-immobilized transition metal complexes exhibiting electrocatalytic activity towards CO2 is especially attractive and this subject has recently been reviewed [5,6]. These surface-modified electrodes could be classified in three main categories depending on the mode of preparation: (i) by chemical derivatization of the surface via covalent bonding; (ii) by coating the electrodes by sputtering or paining, e.g. cobalt phthalocyanine immobilized in this manner has been shown to reduce CO2 to CO at 0.6 V vs SCE [7]; (iii) by electropolymerization of complexes containing polymerizable substituted ligands (e.g. vinyle, pyrrole, acetylenic groups) giving rise to the formation of redox active polymeric films which can reduce CO2 to CO electrocatalytically. These modified electrodes can mediate the electron transfer from the cathode to the substrate and it has been ascertained that the potential at which CO2 is reduced strongly depend on the nature of the metal center [8]. We discovered a new way to prepare coated electrodes by using an electro-precipitation process which allows the deposition of films onto an electrode surface. The protocol of the overall process is illustrated in Fig. 1, where the use of soluble complexes beating trans-axial leaving groups afforded during the reduction process metal-metal bonds [9]. -
f J X-M-M-M-M-X
....:--:-!i'iiii
;-i-i:-::-~i-ii" X-M-M-X :~~i! ~i0.~}'.iI
[
X-M-X
X- M- M- M-X
I
{ X-M-M-Xx_M_X
--{--M---}-
X-M-X
for
% ,. fo
X--M--Xo
M
for
/ \
Figure 1. Schematic representation of the synthetic protocol used for formation of o p e n - c h a i n clusters. The complex is stabilized by various ligands such as polypyridyl or n-acceptor cart)owl or phosphine groups.
The increasing number of n motive leads to the formation of insoluble polymers on the electrode surface. A relatively uniform coating could easily be obtained and the film thickness could be electrochemically controlled. The use of this technique is advantageous from a number of standpoints. The effective concentration of electroactive material can reach levels that are not accessible in homogeneous solution. The distance between adjacent metal centers are close enough that cooperativity effects are enhanced or effective. Finally, the process of film formation could be studied step-by-step starting either from chemically or electrochemically prepared dimers, tetramers... The unique stereochemical positionning of the leaving groups could be studied in details. This alternative and powerful novel method socalled "electrogeneratedopen-chain clusters" for immobilizing of CO2 redox active catalysts onto electode surfaces is the purpose of the present account.
221
2. RESULTS AND DISCUSSION Electrolysis of monomeric mono-bipyridine bis-carbonyl ruthenium(H) complexes bearing two trans leaving groups (e.g. chloride anions or solvent molecules) generate at the working electrode a strongly adherent deep-blue film (Fig. 2A). This modified electrode demonstrate outstanding catalytic activity for the reduction of CO2 to CO (Fig. 2B) and was introduced in an effort to overcome the above limitations [10]. The overpotential was decreased to about 0.8V, and selective and quantitative formation of CO was obtained in aqueous electrolyte.
(A)
I~ i A
(B) ~
i/ILtA 100
I10 p.A t
-2
JF~"I~/--S,-~-,I ~
0 E/V I
-2
E/V
-100-
//
a
-200 Fi2ure 2. (A) Cvclic voltammo~rams recorded for 3 (cf Fie 4~ in CI-hCN enntninino T R A P (3 1 M n f a t carbon electrode.( ...... ) First scan between - 0.45 and- 2.00 V and (. . . . . ) 2nd to 28 th successive scans between - 0.85 and - 2.00 V. (B) Cyclic voltammogramsrecorded in H=O solution containing LiCIO4 (0.1M) for the C/[Ru(bpy)(CO)2], modified eleclrode prepared by electrolysis of 3 at -1.65V a) argon-purged solution, b) CO2-purged solution. .
.
.
.
.
.
~
- "
,.3
.
.
.
.
.
The bulk material has been characterized and identified as an organometallic polymer consisting of [Ru~ repeating units (Fig. 3A). The polymer which morphology is shown in Fig. 3B comprises an extended Ru~ ~ backbone having a staggered arrangement. Eq. 1 summarizes the overall process (bpy for 2,2'-bipyridine). Polymerization results from the overall addition of two electrons per mole of trans-(C1)-[RuII(bpy)(CO)2C12] and is associated with the loss of both coordinated chloride ligands.
n trans-(C1)-[RuII(bpy)(CO)2C12] + 2 ne"
[Ru0(bpy)(CO)2]n + 2 nCl"
(1)
Oxidation of the resulting polymer at - 0.6V vs SCE induces breakage of the Ru~ ~ bonds and causes desorption of the film and ultimate quasi-quantitative formation of the soluble [Ru II(bpy)(CO)z(CH3CN)2 ] 2+ complex (eq. 2).
222 [Ru~
- 2 ne" ~
[RuII(bpy)(CO)2(S)2] 2+
(2)
In light of this results it could be concluded that within the polymer the basic structure of the ruthenium "Ru(bpy)(CO)2" core is maintained in the molecular film. ""
,..-'-.tit
WN
.
Ru o -
Ru o _
"
.
. ~
-,~
~
:"~
.~"
" "t"
-
"~ " ~ -
.:~d,=
"
-
q
I/~
RuO.--- Ru o
oc' l/ql o/\
il/),1% C~ L,%
co
~0 .
"..~
:
"
""
.
" - ~--
(A)
"/
e,
" ~"
=~
"d"
.
.
g
.t~
,=
." t = =
(B)
(A) Schematic representation of the [Ru(bpy)(CO)2].polymer; (B) Scanning electron micrograph of the polymeric film formed electrochemicallyon an ITO electrode; the length of the bar corresponds to 10 l~m.
Figure
3:
An important endeavour in the understanding of polymer formation arise from the discovery that the chemically prepared trans-(C1)-[Ru(bpy)(CO)2C1]2dimer 4 (Fig. 4) allows the formation of the same polymeric material by continous cycling of the potential between - 0.9 and - 2.0V (Fig. 5A). It is worth noting that if the potential range is limited to - 1.5V, growth of the film is highly inefficient. Exhaustive electrolysis at Ep = - 1.40V after consumption of one-electron per mole of 4, produces a red-brown solution due to the formation of the trans-(Cl)-[{Ru(bpy)(CO)2}4C12]tetramer 5 (Fig. 4). This complex exhibits one irreversible reduction peak at Epc = - 1.60V and growth of the [Ru0(bpy)(CO)2]n film is ensured by continuous cycling of the potential between - 0.9 V and - 2.0 V (Fig. 5B).
CI F~ = -1.46 v E~ = -1.60 V CI
CI
Figure 4: Schematic representation of the different steps effective during formation of the suitable polymer Moreover, exhaustive electrolysis at- 1.60 V of a solution of 4 or 5 leads to deposition of the deep-blue, strongly adherent [Ru0(bpy)(CO)2]n film on the working electrode, after exchange of two electrons per mole of complex. The reaction results in quantitative conversion into a polymeric film upon exhaustive electrolyses.
223
14HA
11
)
(B)
(c)
Figure 5 9Cyclic voltammograms in DMSO solution containing 0.1 M TBAP under an argon atmosphere at a Pt electrode showing in each case (..... ) initial sweep and ( ) 2nd to 30th successive scans between - 0.9 4 ; c) from compound 5. and - 2.0 V of a) solution of 3 ; b) from
trans-(C1)-[RuI(bpy)(CO)2Cl]2
These results clearly show that polymerization occurs directly upon reduction of 3 by an electrochemical propagation process (eqs. 5-7 and Figure 4). This is a consequence of the easier or similar reducibility of dimer 4 and parent oligomer 5. In terms of mechanism it means that the polymerization proceeds via the initial formation of a Ru I species (eqs. 5-6), which dimerizes into compound 5 after the release of one chloride ion, rather than through a direct two electron reduction of 3 into a Ru ~ species followed by an aggregation process [ 11 ]. [RuII(bpy)(CO)2C12] + e" [RuII(bpy-')(CO)2C12] [RuI(bpy)(CO)2C1]
=___ [RuII(bpy")(CO)2C12]" =
~ 4
[RuI(bpy)(CO)2C1] + el= 5
=-- polymer
(5) (6) (7)
These modified electrode having the generic formula [Ru0(bpyRR)(CO)2C1]n, display outstanding electrochemical activity towards the reduction of carbon dioxide to either: (i) carbon monoxide, 100 % faradic yield in water at -1.2 V vs SCE, bpy = 2,2'-bipyridine, R = H; (ii)or formate, 95 % faradic yield in aqueous electrolyte at -1.2 V vs Ag/Ag +, R = isopropylesters groups. 3. C O N C L U S I O N The use of these surface-immobilized electrocatalysts allows for the easy removal of the catalysts from the reaction vessel, and the use of much lower quantities of catalyst which is here highly concentrated in the reaction layer. In many cases the immobilization of the catalyst on the electron source provides its stabilization and allows an marked increase of the turnover frequency compared to the numbers found in related homogeneous systems. Taking
224 into account the low yield of formiate which is produced we suggest that the major pathway for CO2 reduction involves interaction of a [Ru(bpy")(CO)]" fragment with CO2 to form a metallo-carboxylate intermediate which after protonolysis and reduction regenerates the starting [Ru(bpy)(CO)2] fragment which then perpetuates the catalytic cycle (Fig. 6). The [Ru(bpy")(CO)]" fragment is formed in dimer 2 or in the polymer by the reduction of the bpy ligand (electron reservoir) followed by the release of a carbonyl ligand. o...,,,''c~ ]~
~CO
I-L J" /
Ri "~CO
N,~ .CO l~l R0 ..... I "~*CO CO
Ru
CO]" CO2,H+
Figure 6: Schematicrepresentationof the proposedcatalyticcycle. One fragmentof the polymer is shown. The application of such systems to the multi-electron reduction of carbon dioxide to formate, methanol or methane is now under active investigation in our laboratory. REFERENCES 1. C. Amatore and J.-M. Sav6ant, J. Am. Chem. Soc., 103 (1981) 5021. 2. R. Kostecki and J. Augustynski, Ber. Bunsen-Ges Phys. Chem., 98 (1994) 1510. 3. J.-P. Collin and J.-P. Sauvage, Coord. Chem. Rev., 93 (1989) 245. 4. R. Ziessel, In "Photosensitization and Photocatalysis using Inorganic and Organometallic Compounds" K. Kalyanasundaram and M. Gratzel (eds) Kluwer Academic Publishers, 1993, pp 217-240; B. P. Sullivan, K. Krist and H. E. Guard (eds) in "Electrochemical and Electrocatalytic Reactions of Carbon Dioxide", Elsevier, Amsterdam, 1993. 5. H. D. Abruna, Coord. Chem. Rev., 86 (1988) 135. 6. A. Deronzier and J.-C. Moutet, Coord. Chem. Rev., 147 (1996) 339. 7. P. A. Christensen, A. Hammett and A. V. G. Muir, J. Electroanal. Chem., 241 (1988) 361. 8. J. A. Ramos Sende, C. R. Arana, L. Hernandez, K. T. Potts, M. Keshevarz-K and H. D. Abruna, Inorg. Chem., 34 (1995) 3339. 9. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, J. Chem. Soc., Chem. Comm., 1994, 189. 10. M.-N. Collomb-Dunand-Sauthier, A. Deronzier et R. Ziessel, Inorg. Chem., 1994, 33, 2961. 11. S. Chardon-Noblat, A. Deronzier, D. Zlodos, R. Ziessel, M. Haukka, T. Pakkanen and T. Ven~il~iinen, J. Chem. Soc., Dalton Trans., (1996) 2581.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
225
E l e c t r o r e d u c t i o n o f CO2 u s i n g C u / Z n o x i d e s l o a d e d gas d i f f u s i o n e l e c t r o d e s Shoichiro Ikeda a, Satoshi ShiozakP, Junichi Susuki a, Kaname Ito a, and Hidetomo Noda b a Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, 466 Japan, b Chubu Electric Power Co. Inc., Kika-sekiyama, Midori-ku, Nagoya, 459 Japan
Gas diffusion electrodes (GDEs) consist of a gas layer (mixture of hydrophobic carbon black (CBphob) and PTFE dispersion) and a reaction layer (mixture of catalyst powder, CBphob, hydrophilic CB (CBphi~), and PTFE) laminated on a Cu mesh as a current collector. As the catalyst, CuO/ZnO (3:7 by mole ratio) mixed oxides and a mixture of a Cu powder (4N, -325 mesh) and a ZnO powder were examined. Electroreduction was performed potentiostatically passing in general 200 C using a m-shaped Pyrex cell having one gas and two liquid chambers, with two lines of gas circulating systems. When using a GDE of (CuO / ZnO = 3 / 7 : CB = 6 : 5 [by weight], the reduction products were mainly C2HsOH with slight amounts of CO and HCOO-, and a comparable amount of H2. Faradaic efficiency maximum of 16.7% for C2HsOH formation with maximum selectivity of 88% was observed at -1.32 V vs. Ag-AgC1, at a partial current density of 4.23 mA/cm 2, which is about 50 times greater than that obtained on a sintered oxide electrode. In the case of the GDE of (Cu / ZnO = 3 : 7) : CB = 3 : 1 reduced by H2, the selectivity of the reduction products became poorer, like in the case of a Cu foil electrode, with lower current density, although the total faradaic efficiencies for CO2 reduction was 40.5 % with additional formation of n-C3H7OH and C2H4 at -1.30 V.
1. I N T R O D U C T I O N Gas diffusion electrodes (GDEs) having large areas of three phase boundaries are promising for use in electrochemical reactions of gaseous reactants to enhance their rates, i.e. current densities. Sammells et al. reported formation of alcohols on a perovskite loaded GDE in KOH solution [1]. We have reported massive electroreduction of CO2 using Cu-loaded GDEs [2-4], and also reported selective formation of ethanol from CO2 on a CuO/ZnO sintered electrode in KHzPO4 aqueous solution [5], however, its formation rate was extremely low as 85/zA/crff. In this paper, we will show the results on the electroreduction of CO2 using CuO/ZnO- and Cu/ZnOloaded GDEs in 0.5 M KHzPO4 aqueous solution.
226 2. EXPERIMENTAL
The active material, Le. catalyst, of CuO/ZnO (3 : 7 by mole ratio) mixed oxides was prepared from the aqueous solution (aq. soln.) of reagent grade copper nitrate and zinc nitrate by adding diluted ammonium water. The precipitate was washed and calcined at 360~ in air for 1 day. On the other hand, Cu/ZnO powder was prepared by mechanical mixing of Cu powder (Rare Metallic Co., Ltd.; 4N, -325 mesh) and ZnO (Nacalai Tesque Inc.; >99.0%) with a mole ratio of 3:7. The GDEs were prepared by almost the same manner as described in the case of Cu-GDEs [3]. GDEs consist of a gas layer (mixture of hydrophobic carbon black (CBphob)(Denki Kagaku Kogyo; AB-7) and polytetrafluoroethylene (PTFE) dispersion (Daikin Kogyo; D-l) and a reaction layer (mixture of catalyst powder, CBphob, hydrophilic CB (CBphil) (Denki Kagaku Kogyo; AB-11), and PTFE) laminated on a Cu gauze of 30 mm~ as a current collector. The electrodes were hot-pressed under 1.2 MPa in N; atmosphere. In the case of H2 reduced GDEs, they were reduced in an electric furnace at 300 ~ for 60 min under H2 flow at 50 cm3/min. The apparent working area of GDE was about 4.5 cm2. Electroreduction was performed at 25 ~ in 0.5 M KH2PO4 aq. soln. (pre-electrolyzed under N2 flow) potentiostatically, passing in general 200 C using a potentio-galvanostat (Hokuto Denko; HA-303), an electronic coulometer (Hokuto Denko; HF-201), and a LU-shaped Pyrex cell having one gas and two liquid chambers for anolyte and catholyte, separated by a cation exchange membrane, Nation NX90209. The cell had two lines of gas circulating systems for gas and catholyte. The counter and reference electrodes were Pt-Pt 60 i I ~ I I i 30 and Ag-AgC1 saturated with V: q (Total) ,,,/ 9 : /d KC1, respectively. The reduction 50 25 products were analyzed by gas chromatographs and a high % 40 20 performance liquid chromatograph as described in the previous paper [6]. g 30 15 3. RESULTS DISCUSSION
AND
3.1. CuO/ZnO loaded GDEs When using a GDE of (CuO / ZnO = 3 / 7 [by mole ratio]) : CB = 6 : 5 [by weight]; the standard composition, the reduction products were mainly C2HsOH (EtOH) with slight amounts of CO and HCOO-, and
o : .ooo
o 20
X
-,:co
.
9"" I
I
-1.7
I
-1.6
I
I
I
-1.5 -1.4 -1.3 V vs. Ag-AgCI
lo
I
-1.2
Figure 1. Faradaic efficiency for reduction products of CO2 on a CuO/ZnO-GDE, (CuO / ZnO = 3 / 7 ) : CB - 6 : 5, in 0.5 M KH2PO4 aq. soln. at 25 ~
227
a comparable amount of H2 a s a byI I I I I product as shown in Figure 1. 50 I A./d Faradaic efficiency (1]) maximum of 16.7% for EtOH formation with _ - 20 .IV ........ maximum selectivity of 88% was 40 V " ---V- ............................. -V observed a t - 1 . 3 2 V vs. Ag-AgC1, v . rl (Total) E where total current density and the ~ 30 o >, < faradaic efficiency for CO2 0 E 1-) H2 9 9 (CO2) reduction, 1"1 (CO2), showed maxi.0_ .mo ma. The partial current density for ~o 20 10 ~O9 r EtOH formation was 4.23 mA/cm 2, .o_ o...--~-~ 9 J O "o which is about 50 times greater than ~ r 9 "C2HsOH ~ that obtained on a sintered (CuO / u_ 10 ZnO = 3 / 7) electrode [5]. rl(H2) o m:CO increased with the potential 0 _ ~-~-~- :-~ .................... 8 0 becoming more negative. The 9 I I I I I I selectivity for EtOH formation 0 100 200 300 400 500 600 during the CO2 reduction was more Q/C than 75% in the potential range o f Figure 2. Dependence of the faradaic efficiency for CO2 1.2 to -1.7 V as shown in Figure 1. reduction products and current density on the quantity rl(COz)M,x was, however, close of electricity passed with the standard CuO/ZnO-GDE to 19% at a maximum. The in 0.5 M KH2PO4 aq. soln. at -1.30 V. distribution of reduction products, their q, and current density at -1.3 V were maintained almost constant up to 500 C as shown in Figure 2. This fact indicates that the activity of catalyst is not changed during continuing the electrolysis up to 500 C. To improve the electrode performance, the amount of catalyst of the GDE was increased to 2.5 times as much as the standard composition, i.e. (CuO / ZnO) : CB = 3 : 1. Contrary to the Table 1 Reduction products of C02 on a GDE with (CuO/ZnO):CB = 3"1 for electrolysis 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1
Current density / mA c m 2
-1.20
Faradaic efficiency / % C2H4
CO
H2
T](CO2) rl(total)
EtOH
HCOO-
4.1
4.2
0.9
0.3
1.3
20.5
6.7
27.2
-1.25
4.4
7.9
1.1
0.8
2.4
22.7
11.5
34.9
-1.30
5.2
12.8
1.4
1.5
3.3
25.6
19.0
44.6
-1.35
4.8
9.6
1.2
1.3
3.0
29.9
15.1
45.0
-1.40
3.9
7.2
1.1
0.6
2.8
34.5
11.9
46.4
228 expectations, the current density became only about 1/5 of the previous one, although CzH4 was additionally produced as listed in Table 1. This fact may indicate the decrease of gas diffusion rate in GDEs. After reduction of the GDE of (CuO / ZnO) : CB = 3 : 1 by H2 at 300~ for 60 min, the current density became 10.1 mA/cm 2 at -1.30 V and rl (CO2)Max became 34.5%, and nC3HvOH (n-PrOH) was additionally produced with rl = 9.2%. However, the selectivity for EtOH formation in the CO2 reduction became poorer than that in the case of the GDE with the standard composition as shown in Table 2. Consequently, pre-reduction of the GDE increased the 1"1(total) and rl (CO2), and did not change I"I(EtOH), but decreased the selectivity because of the produced metallic Cu, which usually leads to a variety of CO2 reduction products [6]. Table 2 Reduction products of CO2 on a H;-reduced GDE with (CuO/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq soln. at 25~ Potential / V vs. Ag-AgC1
Current density / m A c m -2 EtOH
Faradaic efficiency / % n-PrOH
HCOO-
C2H4
CO
H2
1"1(CO2) 1"1(total)
-1.20
8.2
10.1
2.9
1.0
2.1
2.3
20.0
18.4
38.4
-1.25
9.3
16.6
5.0
0.7
4.2
3.1
22.8
29.6
52.4
-1.30
10.1
11.9
9.2
1.3
7.6
4.5
25.3
34.5
59.8
-1.35
9.7
8.4
3.9
1.0
7.1
4.2
29.5
24.6
54.1
-1.40
7.8
6.8
1.3
1.1
3.8
4.0
34.8
17.0
51.8
3.2. C u / Z n O loaded GDEs To confirm the effects of Cu formed by reduction with H2 and/or in the course of the CO2 reduction in the CuO/ZnO-GDE, GDEs containing a mixture of Cu powder and ZnO have been prepared and examined. Results obtained by the as-prepared and H2-reduced GDEs are summarized in Tables 3 and 4, respectively. Table 3 Reduction products of CO; on an as-prepared GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KH2PO4 aq. soln. at 25~ Faradaic efficiency / %
Potential / V vs. Ag-AgC1
Current density / m A c m -2
EtOH
n-PrOH
HCO0-
C2H4
CO
H2
1"1(CO2)
1"1(total)
-1.20
3.9
7.2
2.4
0.8
1.5
1.8
18.2
13.7
31.9
-1.25
4.3
14.8
4.3
1.1
3.3
2.4
20.1
25.9
46.0
-1.30
5.1
11.7
7.8
0.8
5.8
3.9
24.7
30.0
54.7
-1.35
4.6
6.4
4.8
0.9
5.6
3.7
28.3
21.4
49.7
-1.40
4.2
4.3
2.9
1.1
2.7
3.1
35.1
14.1
49.2
229
n-PrOH was also produced with both GDEs. The selectivity for EtOH formation became poorer and the current density lesser than that for CuO/ZnO-GDEs of the standard composition. Faradaic efficiencies of the reduced GDE are larger than those of the as-prepared GDE. rl(EtOH)Max of 16.2% at -1.25 V, rl(COz)Max of 40.5% at -1.30 V, and rl(total)Max of 66.4% at -1.30 V were obtained with the H2-reduced GDE. These tendencies are similar to those observed for the Cu foil electrode, even though in the different electrolyte, i.e. 0.1 M KHCO3 aq. soln. [6]. Table 4 Reduction products of CO2 on a H2-reduced GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1
Current density / mA cm 2
-1.20 -1.25
Faradaic efficiency / % H2
I"I(COz)
rl (total)
EtOH
n-PrOH
HCOO-
C2H4
7.3
9.8
3.6
1.1
3.1
3.2
21.3
20.8
42.1
8.6
16.2
6.1
1.5
6.2
4.8
23.4
34.8
58.2
-1.30
7.9
12.8
10.2
1.3
9.1
7.1
25.9
40.5
66.4
-1.35
6.8
8.3
6.5
1.1
8.6
6.9
30.2
31.4
61.6
-1.40
6.5
5.6
3.8
1.3
5.5
6.6
36.7
22.8
59.5
CO
Table 5 Summary of C02 electroreduction results for 200 C passed at Cu/Zn oxides loaded gas diffusion electrodes in 0.5 M KHzPO4 aq. soln. at 25~ CuO/ZnO-GDE Weight ratio of catalyst : CB
6:5
IdMax/mA cm -2
25.3
Pot. of rl (total)Ma,,, / V
-1.70
-1.40
1"1(total)Max / %
48.9
Pot. of rl (CO2)Max / V
3 :1 5.2
Cu/ZnO-GDE
3 : 1(red.) 10.1
3 :1
3 : 1(red.)
5.1
8.6
-1.30
-1.30
-1.30
46.4
59.8
54.7
66.4
-1.32
-1.30
-1.30
-1.30
-1.30
rl (CO2)Max / %
19.0
19.0
34.5
30.0
40.5
Pot. of rl (EtOH)Ma,,, / V
-1.32
-1.30
-1.25
-1.25
-1.25
rl (EtOH)Ma~, / %
16.7
12.8
16.6
14.8
16.2
Selectivity of EtOH / %
87.9
68.7
56.1
57.1
47.1
Pot. of rl (n-PrOH)Max / V
-
-
-1.30
-1.30
-1.30
rl (n-PrOH)Max /
-
-
9.2
7.8
%
10.2
(red.): Hz-reduced, CB: Total amount of carbon black, IdMax"Maximum of current density, Pot.: Potential vs. Ag-AgC1, rl (X)Max: Maximum faradaic efficiency for formation of X.
230 From these results, it is found that there are zinc oxide and copper oxide, lather than the metallic copper, which control the selective formation of ethanol from CO 2.
4. CONCLUSION Using ZnO/CuO-loaded gas diffusion electrodes, ethanol has been selectively produced with ca. 17% of faradaic efficiency by electroreduction of CO2 the same as with the sintered ZnO/CuO electrode in aqueous KH2PO4 solution but with about 50 times higher current density than the latter. H2-reduced GDEs or Cu/ZnO-loaded GDEs produced in addition ethylene and n-propanol, but with lower current density and selectivity. Optimum conditions for electroreduction of CO 2 on each GDE are summarized in Table 5.
REFERENCES
1. R.L. Cook, R.C. McDuff, and F. Sammells, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 39. 2. T. Ito, S. Ikeda, M. Maeda, H. Yoshida, and K. Ito, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 313. 3. S. Ikeda, T. Ito, K. Azuma, K. Ito, and H. Noda, Denki Kagaku, 63 (1996) 303. 4. S. Ikeda, T. Ito, K. Azuma, N. Nishi, K. Ito, and H. Noda, Denki Kagaku, 64 (1996) 69. 5. S. Ikeda, Y. Tomita, A. Hattori, K. Ito, H. Noda, and M. Sakai, Denki Kagaku, 61 (1993) 807. 6. H. Noda, S. Ikeda, Y. Oda, and K. Ito, Chem. Lett., 289 (1989).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
231
Recent Slow Rate of CO2 Increase and Vegetation Activity K. Kawahira* and Y. Maeda** * Department of Bioscience, Fukui Prefectural University, 4-1-1 Kenjyojima, Matsuoka Town, Fukui 910-11, Japan ** Faculty of Electrical Engineering, Toyama University, 12 Gofuku, Toyama City 939, Japan Recent slower rate of the atmospheric carbon dioxide, which abruptly happened in 1989 through 1993, has been studied in relation to the vegetation activity in the Northern Hemisphere. It is found that the gradual increase of the active vegetation area was noticed after 1986. Most active vegetation was seen in July near the high latitudes around 60 ~ N. Since the growth in the vegetation was in accord with the continuous rise of the surface temperature in all seasons after 1987, the slow rate owes to growth in the active vegetation which uptakes the atmospheric carbon dioxide through photosynthesis. 1. I N T R O D U C T I O N Rapid and continuous increases in the greenhouse gases have been predicted to cause global warming at the surface by 1-4 K in the next century compared to pre-industrial time [1]. Among these gases, the atmospheric carbon dioxide increases have main role in the warming due to the anthropogenic emission by combustion of fossil fuel and cement production [1]. Contrary to the atmospheric carbon dioxide increase 'prediction', the anomaly of the increase rate, discovered by Keeling et al. [2], happened mainly in the Northern Hemisphere during the years of the 198993. The dramatic slow down in the increase rate was estimated to be large enough to uptake half as large as the total combustion of fossil fuels [3]. We hypothesize as a cause of the anomaly that the global warming induce the active period (area) of the vegetation to become longer (wider), especially over the lands in the Northern Hemisphere [4]. In the present study we focus on how the active vegetation area changed. 2. ANOMALY IN THE RECENT CO2 INCREASE In order to make the recent CO2 anomaly clear, we used the following definition? NMVj
- ((C02)j/(CO2)j. 1 -
where j mean the year.
1) x 100 ( % / year)
(1)
232 The definition is named as the normalized variation (NMV), which shows the year-to-year changes relative to the mean amount. The application of this definition to the annual changes of the observed carbon dioxide concentration at Mauna Loa [5] was made and shown in Fig. 1. 380 370
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::: .. i i i i i i i i i i i i i i i i i i i i i i i i i i i i i PifiaLubd
i i
1.2
360 >
350
0.8
~, r o
EQ. 340
0.6
Q.
O 330 320
0.4
z
310 0.2
300
l
l
I
l
:
l
:
:
l
i
:
l
l
:
l
l
:
i
l
l
l
:
:
:
:
i
:
l
l
I
:
:
:
l
l
:
i ! i ! i i i i i i ! i i ! ! i ! ! ! i i i i i i ! i ! i i i ! ? ! ! i
290
l
60
l
l
62
t
t
-
64
l
J
l
66
l
l
i
68
J
l
'
70
|
L
l
72
~
l
'
74
l
l
l
76
L
I
l
~
L
'
78 80 YEAR
"
l
J
82
l
d
84
|
'
'.... |
86
'
l
88
'
l
"
90
l
0.0
'
92
94
96
Figure 1. Trend of Annual CO2 and NMV at Mauna Loa (20 N) Although the annual mean concentration increases monotonically, the NMV show variable. Sudden drop of the NMV is seen from 1988 to 89, about 0.8 to 0.4 (%/year). About 0.2 of the NMV are shown in 1992-93. In the following we determine the period of the recent CO2 anomaly as the 1989-93. Similar changes of the NMV at Mauna Loa were seen in the Point Barrow Observatory (71 N), being more remarkable compared to the Mauna Loa Observatory. 3. VARIATIONS OF THE VEGETATION AND SURFACE TEMPERATURE The surface temperature trend is an important effect on the vegetation activity. The trend is analyzed from the radiosonde observations in the world by Angell [6]. The temperature data are in a form of the anomaly, which is the deviation from the 1959-77 mean. Fig. 2 shows the long-term trend for annual mean, winter, and summer in the Northern Hemisphere. The annual mean trend shows continuous temperature rise after 1986. F u r t h e r it is interesting that the winter temperature rise is remarkable compared to the summer trend. The warming trend can bring about that the winter season in the high latitudes became a shorter period t h a n in the no warming period. The negative temperature anomaly in the 1992-93 has been explained as that the massive aerosol loading in the atmosphere due to the eruption of the Pinatubo Mountain in July 1991 causes the worldwide cooling at the surface by strong reflectance of solar radiation. This effect could induce the ocean surface temperature drop, which in turn help uptake of the CO2 [1]. However,
233 since the CO2 anomaly began in 1989, the anomaly in the 1989-91 has no relation to the eruption and~or temperature fall.
N"u
o >,
o
<
0.0
"~
, ,
~
, ,~,
, ,
',
, , , ,
, , , ,
-0.5 -1.0 58
60
62
64
66
68
70
72
74
76
78
80
82
84
86
88
90
92
94
96
YEAR
Figure 3. Surface t e m p e r a t u r e trend in the Northern Hemisphere for annual mean, winter (December, January, and February) and summer (June, July, and August). Analysis of the vegetation is based on the NDVI (normalized difference vegetation index), which is between -1 ~- +1. The area over 0.05 presents the vegetation area. The NDVI data set is the NOAA/NASA Pathfinder AVHRR LAND (PAL) for the period J a n u a r y 1983 to August 1994 [7]. The used set is l x l degree (latitude/longitude) and monthly data set. From the NDVI the GVI (global vegetation index) data set are calculated as follows. GVI = NDVI x 100 + 100
(2)
Therefore the GVI is between 0 - 200, and over 105 corresponds to over 0.05 in the NDVI which shows the vegetation area. The trend of the vegetation area is counted for the every month in the Northern Hemisphere land. Most active month is July, and most weak month is February. First result is the global distribution for July 1990 shown in Fig. 4. Most active regions are shown in yellow and red and concentrate in high latitude belt centered 60N. There are active belts from Siberia to Europe, and over Canada in the North America, which consist of the forests named as the taiga. Deserts like as S a h a r a presented by blue are clearly shown; the vegetation in Fig. 4 can describe realistic large scale pattern of plants or forests. The trend is simply shown in Fig. 5, which takes a global difference of the GVI in July between 1990 and 86. This period is continuous t e m p e r a t u r e rise as shown in Fig. 3. Then the differences are counted over the value of 110 GVI, t h a t is, excluded desert or soil regions. Increased areas are
234 p r e s e n t e d by r e d regions, a n d d e c r e a s e d one by blue regions. It is a p p a r e n t t h a t t h e d e s e r t s are excluded, a n d t h e t a i g a in t h e h i g h l a t i t u d e s is i n c l u d e d GVI Jul. 1990 in r e d area; t h e forests in high z latitudes had so ~ .;~ .:~ .. . . grown. It is ~- 6o also n o t e d t h a t ~ 40 ~- ~ the changes in v 2[} the Southern ~ 0 Hemisphere ~ -20 ~, (SH) show a ~ -40 growing 5 -60 :" tendency of -so ....... v e g e t a t i o n like ~ -'~ ~' -' " - ' . . . . . 5'0 ' ' as NH, -150 -I00 -50 0 I00 150
-i
"
although the J u l y is w i n t e r season. Global warming may help grow up the
~
LONGITUDE
(deg .)
E
i00 I i 0 1 2 0 1 3 0 1 4 0 1 5 0 1 6 0 1 7 0 1 8 0
vegetation
activity even in GvI the SH winter season. Figure 4. Global distribution of GVIin Ju]y 1990. The difference of the Global GVI distribution is furthermore shown in more detail in Fig. 6, where only for the area over ii0 of GVl, plant or grass area, the difference between 1990 and 86 (90 minus 86) is taken. Active areas (yellow or red) are noticed over high latitude belt in Russia and in spot-like distributions over North America high latitudes. In accord with the distributions in Fig. 4, high latitude forests as the taiga can induce higher activity of the vegetation in NDVI DIFF. 90-86 N H due to c o n t i n u o u s rise 75of t h e surface temperature 5o a n d i n c r e a s e in z precipitation ~@ 25[4]. It is n o t e d ~ othat the ~ i i n c r e a s e d a r e a s ~ -25 of t h e active ~ 5o vegetation 75 ~ c o r r e s p o n d to i - '" ! ! i .... t t' h i g h active -150 -i00 -50 0 50 I00 150 regions: h i g h W Long. (deg) E active a r e a Fig. 5 GVI chTferencebetween 1990 and 86 (90 minus as t h e t a i g a 86). Positive (negative) areas are shown in red (blue). has grown ~
-
235
to become more GVI Jul. (90 - 86) active area compared to other vegetation ~" c. 60 ~ - areas. In SH 40 U there are 20 . .;~ growing 0 vegetation areas like as ~ -40 east Australia, ~ -60 middle Africa, -80 , " ', ..... , . . . . . . . . ! i and South o so - 1so - loo-so I00 I~0 America. Furthermore W LONGITUDE (deg,) E the decaying regions are noticed in north ! I I I I 1 i China, near -30-20-I0 0 I0 20 30 Mexico, and India. The GVI decay may Fig. 6 Like as Fig.5 but the chTferences are connect to shown in G V I values. deforestation. In order to estimate how global w a r m i n g (1986-90) influences the vegetation activity in NH winter but SH s u m m e r season, the difference in F e b r u a r y global distributions GvI Feb. (90 86) are shown in = Fig. 7. Like as 8o "-" 60 Fig.6, the ~ 40 analysis is ~ 2o done only to ~ 0 the regions ~ - 2 0 over 105 ~ - 4 0 (GVI), ~ -- 6800 plants, grass, and forests. In NH, apparent increase in the GVI is noticed over Europe; the expanding forests may
-I'50
-I'00 %7
-50
0
LONGITUDE
-30 -20 -I0
50
I00
(deg.)
0
I0
20
150
E
30
GVI
Fig. 7 Like as Fig. 6, but for February. Difference is obtained only over 105 G VI area.
236 cause this growth due to warming and precipitation increase [4]. Decaying regions are also seen in southeast part of China, which is also confirmed by recent study [1]. In SH where is summer season, apparent decaying area is found over Amazon forests, which is also possibly due to deforestation [1]. 4. DISCUSSIONS AND CONCLUSIONS The global distributions of the GVI in July 1990 clearly indicate that the active vegetation area lies in the high latitudes around 60N. Highly active region noticed in the present study is consistent with the study [4] where the growth in the forests in east Europe was clear as that the 1970s-80s reached to the increase between 1971 and 1990 by 25 and 30%, respectively. The difference of the GVI in July by taking the 1990 minus 86 in Fig.6 indicates that the high active area around 60N is the most growing area where are consist of the forests as the taiga. This shows that more growth occurred in more active vegetation area. This growth has continued for the 1984-91, and affects the recent slow rate of the atmospheric carbon dioxide increase [8-9]. Global warming mainly due to the atmospheric carbon dioxide increase brought about this condition, which is considered to one of the feedback effect of the global warming. Recent slow rate of the atmospheric carbon dioxide is studied by analysis of the observed CO2, vegetation index, and temperatures. The following results were stressed. (1) The anomaly occurred in 1989 through 1993. (2) In the 1989-91, the anomaly is an intimate relation to the continuous surface temperature rise not only in summer, but also in winter seasons. (3) The vegetation activity has grown in the Northern Hemisphere in accord with the surface warming in the 1986-91. (4) The most active region lies and limited to the high latitude belts of the Northern Hemisphere where lie the forests as the taiga. (5) The slower rate, thus, owes to wide and favorable condition of the vegetation, which in turn was brought about by the carbon dioxide increase.
REFERENCES 1. J. T. Houghton et s]. (eds.) Climate Change 1995, Cambridge Univ. Press, 1995. 2. J. L. Sarmiento, Nature, 365 (1993) 697. 3. T. J. Conway et a/., J. Geophys. Res., 99 (1994) 22831. 4. P. E. Kauppi et al., Science, 256 (1992) 70. 5. C.D. Keeling & T. P. Whorf, Trends, CDIAC, 1996. 6. J. K. Angell, Trends, CDIAC, 1997. 7. NASA, NOAA/NASA Pathfinder AVHRR Land (PAL) Program, 1996. 8. C. D. Keeling et al., Nature, 375 (1995) 666. ,9. R. B.Mynemi et al., Nature, 386 (1997) 698.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
237
Production of PHA (poly hydroxyalkanoate) by genetically engineered marine cyanobacterium H. Miyasaka ", H. Nakano ~, H. Akiyama ~, S. Kanai ~and M. Hirano b "Kansai Electric Power Co., Inc., Technical Research Center, 11-20 Nakoji 3Chome, Amagasaki, Hyogo 661, Japan ~Bioiogical Sciences Department, Toray Research Center, Inc., 1111 Tebiro, Kamakura, Kanagawa 248, Japan To develop a basic system for the biological conversion of 002 into useful industrial materials, the vector-promoter system for the expression of foreign genes in the marine cyanobacterium was established. Using this system, the production of a biodegradable plastic, PHA (poly hydroxyalkanoate), by the genetically engineered cyanobacterial cells was examined. The transformant cyanobacterial cells carrying the poly hydroxybutyrate (PHB)-synthesizing genes of hydrogen bacterium (Alcaligenes eutrophus) produced up to 17 % of the cell dry weight of PHA. 1. INTRODUCTION Although the fixing of 002 by photosynthetic microorganisms can be an efficient system for the removal of CO2 in flue gases from thermal power plants and other industrial sources, one of the major problems of this system is the effective utilization of the fixed biomass. The biomass produced by photosynthetic microorganisms must be utilized as a resource, or it will be easily degraded by microorganisms into CO2 again. There have been, however, only a few reports on the possible utilization methods of fixed biomass, such as the utilization for animal feeds [1] and fuels [2]. Thus the introduction of foreign genes into photosynthetic microorganisms for the production of useful materials is an important technological approach. Cyanobacteria are procaryotic photosynthetic microorganisms and can provide a simple genetic transformation system. In this study, we established an efficient vector-promoter system for the introduction and expression of foreign
238
genes in the marine cyanobacterium Synechococcus sp. PCC7002, and examined the production of biodegradable plastic, PHA, by genetically engineered cyanobacteria. PHA has already been commercially produced by bacterial cultures using organic compounds as substrates. The production of biodegradable plastics by photoautotrophic organisms has several advantages on the protection of the global environment as follows: (i) CO2 in flue gases from industrial sources can be converted into useful resources; (ii) the use of plastics made from CO2 can reduce the consumption of fossil fuel resources by substituting the chemical plastics made from petrochemicals; and (iii) the use of biodegradable plastics can reduce the environmental pollution caused by the chemical plastics. 2. EXPERIMENTAL
The unicellular marine cyanobacterium Synechococcus sp. PCC7002 was grown under continuous illumination at 32 ~ C in A2 medium [3]. The nucleotide sequences were determined using an ABI 373S DNA sequencer (Perkin-Elmer). The CAT (chloramphenicol acetyltransferase) activities in bacterial and cyanobacterial cells were determined by the spectrophotometric assay method [4]. The PHB gene [5] was cloned from the originally constructed genomic library of Alcaligenes eutrophus, using the PCR amplified DNA fragment as the probe. For the extraction of PHA from cyanobacterial cells, the cells were disrupted by a sonication and extracted with chloroform. The PHA was then precipitated by adding methanol to the chloroform solution, dried, and weighed. For GC-MS analyses, the PHA was alkaline hydrolyzed and silylated by N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA). The tert-butyldimethylsilylated derivatives of I~-hydroxybutyric acid and lactic acid were identified by comparing the retention time of GC and the mass fragmentation patterns of MS with commercial standard samples. The molecular weight (M.W.) of PHA was determined by gel permeation chromatography (GPC). 3. RESULTS
AND
DISCUSSION
3.1. Construction of shuttle-vectors For the construction of a shuttle-vector between E. coil and the marine cyanobacterium Synechococcus sp. PCC7002, we isolated and characterized the smallest endogenous plasmid pAQ1 (DDBJ Accession No. D13972) of this cyanobacterium. The DNA sequence analysis revealed that plasmid pAQ1 was 4809 bp long and had four ORFs, ORF943, ORF64, ORF71, and ORF93 (numbers show the putative amino acid numbers). The construction of the shuttle-vector was
239 done by digesting pAQ1 plasmid and pUC19 plasmid of E. coli with restriction enzymes, which cleave each plasmid at a unique site, and by ligating the linearized plasmids. The plasmid pUC19 and the plasmid pAQ1 were linearized by Sma I and Stu I digestions, respectively, and were ligated to generate the shuttle-vector pAQJ6 (Fig. 1; both Sma land Stul are blunt-end forming restriction enzymes). EcoRI Sacl
Stul
a,
Stu,
~ ' ORF64i4~O:;:P43)~'--Sac'
SaClHind~l EcoRI Sacl
Amp
pAQJ6
EcoRI Sacl (partial digestion)
Ligation
Sacl
~
ac'
v
Hindlll
\
Hindlll / ~ ~ : , a c i Sacl~
sa
Sail
Figure 1. Construction of shuttle-vector betweenE, coli and Synechococcus sp. PCC7002. The effect of four ORFs on the transformation efficiency of the shuttle-vector was examined by introducing various deletions into these ORFs. Figure 2 shows the effects of the deletions in ORF943 on the transformation efficiency of the shuttlevector. When the deletions were introduced into ORF943 from 5' side, the transformation efficiency decreased stepwise, indicating that this ORF plays an important role in the maintenance of shuttle-vectors in cyanobacterial cells. The other ORFs, ORF64, ORF71, and ORF93 showed no significant effect on the transformation efficiency of this shuttle-vector (data not shown). From these results the simplified shuttle-vector pAQJ4 with full ORF943 was constructed from the pAQJ6 vector (Fig. 1). The transformation efficiency of the shuttle-vector pAQJ4 was about 3.6 x 105 (cfu / l~g DNA), when we transformed 4 x 107 of cyanobacterial cells with 0.3 l~g (0.1 pmol) of pAQJ4 vector in 1 ml solution. This transformation efficiency was 10 -~ 100 times higher than the shuttle-vectors for this cyanobacterium previously reported [6,7].
240
Vector (1-3201) pAQJ4-D 1 (1142-3201 )
Sac~/Sac/ S~cl
Hind/l/
pUC19
I
Sicl/Ec~
Hind/I/
Transformation efficiency (cfu / l~g DNA) 3.6 x 10 5
~ ORF943 .... ................................................. ! ~
pAQJ4-D2 (1978-3201)
I
Amp
5.2 x 10 4
.................................. ~
pAQJ4-D3 (2264-3201)
3.6 X 10 4
3.7 X 10 3
. . . . . . . . . . . . . . . . . . . . . . . . . . .
~
Figure 2. Effects of the deletion in ORF943 on the transformation efficiency of shuttle-vectors
3.2. Development of effective promoter Next, we developed the effective promoter for the expression of foreign genes on the shuttle-vector, pAQJ4. The promoter of the RuBisco (rbc) gene of this cyanobacterium was chosen for the source of strong promoter, and the rbc gene was isolated by screening the genomic library of this cyanobacterium. Our genomic clone of the rbc gene (DDBJ Accession No. D13971) was 4234 bp long and had 962 bp in the 5' upstream region of the rbc large subunit (Fig. 3a). We introduced various deletions into this 5' upstream region and determined the precise promoter region by both bacterial and cyanobacterial CAT assays [8]. The promoter activity existed in the region close to the coding region of the rbc large subunit (Fig. 3b, c). Sau3AI/BamHI
(a) Structure of rbc gene
/
EcoRI
Sau3AI/BamHI
(b) 5' Upstream region (962 bp)
(c) Promoter region (possible -35 and -10 sequences are underlined)
300bp
vtl
...... , .......
rbcL
BamHI
,,,,,,,J
rbcS
I
i
i
........................
__GCTAATCAGCCCAAAAAACAAAAGCAATCTTTTTTTGTTGCTAAAAGATAAAA
-i0
ATAAGTCGAGGCTGTGGTAACATATCCCACAGATTAAAGAAA
Figure 3. Structure of rbc gene of
Synechococcussp. PCC7002.
241
We also examined the effects of the 5' upstream region of the rbc gene on the promoter activity, by dividing the 962 bp of upstream region into three fragments, as shown in Fig. 3b, and connecting these fragments in various combinations to pAQJ4-CAT vector [8] (Fig. 4). We found that the AT-rich region of -303 to -654 upstream of the rbc gene had some repressive effects on the promoter activity (by the comparison of pAQ-EX6 and pAQ-EX8), and that the -655 to -962 region had some enhancing effects on the promoter activity (by the comparison of pAQ-EX1 and pAQ-EX6). When the -655 to -962 region was connected to the upstream of the bacterial tac promoter, the activity of the tac promoter was also enhanced both in E. coli and cyanobacterium (pAQ-rbc+trc of Fig. 4), indicating that the enhancing effect of this region might work universally in procaryotic cells. From these results, we designed the new strong promoter by removing the -303 to -654 region from the 5' upstream region of the rbc gene (pAQEX6 promoter of Fig. 4). The pAQJ4 vector with the pAQEX6 promoter, however, was found to be unstable in the cyanobacterial cells, and the modifications to increase its stability are in progress. Thus, for the production of PHA, we used the pAQJ4 vector with the pAQEXl promoter. Sau 3AI/Bam HI
Promoter
-962
Bam HI
Eco R I
-655
-304
k\~-~t,~,~,~,~~~,~,~,~:~,!
pAQ-EXl
i
pAQ-EX3
iii~i~i!i~~i~iii~i~l
pAQ-EX6
L~"-.I
I
-1
- I
E.coli
=- I
P0C7002~
=1 ~ I
pAQ-EX8 No promoter (pAQJ4/cat)
tac promoter V ~ A
pAQ-trc pAQ-rbc+trc
~
tac promoter ~'/~/,,! 0
0
1
i
20
I
30
40
CAT activity (~mol/mg/min) Figure 4. Effects of 5' upstream region of rbc gene on the promoter activity.
242
3.3.
Production of PHA in the genetically engineered cyanobacterial cells For the production of PHA in the cyanobacterial cells, the PHB genes from A/ca/igenes eutrophus were introduced into the pAQJ4 vector under the control of the pAQEXl promoter. The growth rate of the cyanobacterial cells with PHB genes, and with only the pAQJ4 vector (control), did not show any difference. The production of PHA by the transformant cells was examined after more than 5 passages of the culture. The transformant cells showed different PHA contents depending on the culture conditions, and the maximum productivity was about 17 % of the cell dry weight. This productivity was several times higher than that of the fresh water cyanobacterial transformant cells, carrying the PHB genes, previously reported [9]. The PHA produced by the transformed cyanobacterial cells was identified by GC-MS analysis. The constituents of PHA of the cyanobacterial cells were 15hydroxybutyric acid, lactic acid, and other unknown hydroxyalkanoic acids, and the major constituent was 13-hydroxybutyric acid. The average molecular weight (M.W.) of PHA produced by the cyanobacterial cells was about 1,000,000, similar to the average M.W. of PHA from A/ca/igenes eutrophus. REFERENCES 1. Y. Watanabe and D.O. Hall, Energy Convers. Mgmt., 36 (1995) 721. 2. J.R. Benemann, Energy Convers. Mgmt., 34 (1993) 999. 3. E.R. Tabita, S.E. Stevens and R. Quijano, Biochem. Biophys. Res. Commun., 61 (1974) 45. 4. W.V. Shaw, Methods Enzymol, 156 (1975) 737. 5. O.P. Peoples and A.J. Sinskey, J. Biol. Chem., 264 (1989) 15293. 6. J.S. Buzby, R.D. Porter and S.E. Stevens, J. Bacteriol., 154 (1983) 1446. 7. R.L. Lorimier, G. Guglielmi, D.A. Bryant and S.E. Stevens, J. Bacteriol., 169 (1987) 1830. 8. H. Akiyama, S. Kanai, M. Hirano and H. Miyasaka, submitted for publication in Gene . 9. T. Suzuki, M. Miyake, Y. Tokiwa, H. Saegusa, T. Saito and Y. Asada, Biotech. Lett., 18 (1996) 1047.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
243
Cellulose as a biological sink of C O 2 T. Hayashi, Y. Ihara, T. Nakai, T. Takeda, and R. Tominaga Wood Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611, Japan
One strategy to enhance CO2 fixation is to increase the biological deposition of cellulose in woody plants, because cellulose which is the most abundant organic compound on the earth is made from CO2 through photosynthetic pathways in the walls of plant cells. Cellulose has a strong tendency to selfassociate into fibrils which are not easily hydrolyzed, either chemically or biologically, and accumulate in the walls. Certainly, cellulose is a good biological sink for CO2 on the earth, but the mechanism of cellulose biosynthesis is still unknown (the cellulose synthase activity in vitro in higher plants has not been completely identified or defined by anyone yet, and its gene is unknown [1]). In addition, the cellulose biosynthesis has not onlybeen identified and defined as chain polymerization but is also involved in a dynamism of cortical microtubule association during the developmental growth of woody plants. We report here our lab update on cellulose biosynthesis in higher plants to improve woody plants by genetic engineering through studies on the biosynthetic mechanism. 1. CELLWALL LOOSENING The plasma membranes of growing plant cells select to incorporate sugars, amino acids, ions and other low molecular weight compounds from the apoplastic space, and then, the cells have a certain level of osmotic pressure. The difference between their osmotic pressure and their wall pressure (=turgor pressure) is due to motive power (suction force) to suck water from the apoplastic space (Figure 1). The plant hormone auxin, which decreases the wall pressure in a growing plant cell, therefore induces cell elongation or expansion.
Wall pressure (Turgor pressure)
Osmotic pressure Suction force = OP--- WP Figure 1. The relationship between osmotic pressure (OP), wall pressure (WP), and suction force.
244 The p h e n o m e n o n is called cell wall loosening [2], in which the wall controls plant cell growth. The cell enlargement (elongation and/or expansion) substantially associates cellulose deposition during development. Xyloglucans probably contribute to the cross-linking of each cellulose microfibril network in the walls of growing plant cells [3]. The binding of adjacent microfibrils probably gives cell wall its rigidity. The cross-linking between perpendicular fibrils may function as a bracket, and that between parallel fibrils as a beam. The primary growth-promoting action of auxin is identified and defined by the cell wall modification inv olv ed in the xyloglucan solubilization in the walls of growing plant cells [3, 4], because the solubilization may cause the weakening of the wall which allows it to stretch, and subsequent turgor-driven wall expansion [5]. A u x i n - a n d / o r acid-induced growth has been proven to be accompanied by xyloglucan solubilization in the apoplastic space of Pisum sativ u m [6] and in poplar and soybean cells in suspension culture [7, 8], and with changes in xyloglucan molecular weights in higher plants [9]. Although there are several enzyme candidates responsible for xyloglucan turnover, i.e., xyloglucanase [10], xyloglucan endotransglycosylase (XET) [11, 12], expansin [13] and cellulase [14], the mechanism of xyloglucan turnover has not yet been clarified. The overexpression of the sense or antisense m R N A might help reveal the function of each enzyme in plant tissues. 2. DEPOSITION OF CELLULOSE
2.1.
Cellulose synthase Random sequencing of 1,000 clones from the cDNA library of the fiber cells revealed 750 clones of DNA sequences [15], which were computor-simulatively translated according to their nudeotide sequences and their potential 4,500 polypeptides in deduced amino acid sequences were subjected to a homology search with Acetobacter xylinum cellulose 4-~-glucosyltransferase [16]. The full length cDNAs of pcs A1 and pcs A2 have been obtained by using 5'-RACE m e t h o d [17] and sequenced. The cotton pcs A1 which appears to be a full length clone of 3,228 bp contains an open reading frame of 2,934 bp that encodes a polypeptide of 978 amino acids with a calculated molecular mass of about 110 kDa as cel A 1 shown by Pear et al. [18]. The cotton pcs A2 which appears to be a full length clone of 3,311 bp contains an open reading frame of 3,120 bp that encodes a polypeptide of 1,039 amino acids with a calculated molecular mass of about 125 kDa. Each deduced amino acid sequence contains one consensus sequence for UDP-glucose binding motif (Table 1). The cellulose 4-~-glucosyltransferase of Acetobacter x y l i n u m exhibits 42.8 % identity at the DNA level and 26.2 % identity at the whole deduced amino acid level to the pcs A2 polypeptide. The cotton cel A1 polypeptide exhibits 53.9 % identity at the DNA level and 68.7 % identity at the amino acid level to the pcs A2 polypeptide. The hydropathy profiles suggest at least two transmembrane helices, e.g., one is located in the N-terminal region and one is in C-terminal region. The central regions of the polypeptides are rather hydrophilic and are
245 probably catalytic sites in the cytoplasm. The hydrophilic regions have the conserved UDP-glucose binding motif which has been believed to bind to the substrate and to catalyze the transfer of glucose into pre-formed 1,4-~glucan. Table 1 Characterization of pcs A2 cDNA and its deduced amino acid sequence Length
Identity with bcs A (%)
UDP-glucose-
(bp)
Nucleotide Amino acid
binding motif
Gene
pcs A2 pcs A 1 bcs A
2.2.
Source
3,311 3,228 2,262
42.8 42.4 100
26.6 25.4 100
YPVEKVCCYVSDDG Cotton YPVDKVSCYISDDG Cotton WPPDKVNVYII.DDG Acetobacter
Formation of UDP-glucose Higher plants have two systems for the formation of UDP-glucose with UDP-glucose pyrophosphorylase (EC 2.7.7.9) and sucrose synthase (EC 2.4.1.13), although bacteria contain only one system (Figure 2). The sucrose synthase catalyzes the reaction: UDP-glucose + fructose = sucrose + UDP, a freely reversible reaction. The a m o u n t of the enzyme is much higher in nonphotosynthetic tissues, where sucrose is the source of carbon that is Figure 2. Formation of UDP-glucose translocated and cleaved by the in higher plants. enzyme to produce UDP-glucose for synthesis of cellulose as a major sink in plants. Therefore, the enzyme may function to produce UDP-glucose rather than to synthesize sucrose in plant tissues. UDP formed from UDP-glucose by 4-~-glucosyltransferase reactions can be recycled in a short time to produce UDPglucose by sucrose synthase. The production of UDP-glucose by the enzyme is a method of conserving energy ATP [18. 19], which only occurs in higher plants. In developing cotton fibers, the sucrose synthase, localized in arrays that parallel the helical pattern of cellulose deposition, may participate in the biosynthesis of cellulose [20]. The m u n g bean (Vigna radiata, Wilczek) sucrose synthase is a tetramer corn posed of identical subunits of 95 kDa, and its cDNA contains an open reading frame of 2,415 bp that encodes a polypeptide of 805 amino acids with a calculated molecular mass of 92,087 daltons. The recombinant sucrose synthase expressed
246 in Escherichia coli harboring an expression plasmid containing m u n g bean sucrose synthase cDNA conserves the activity of sucrose synthase [21]. 3.
ORIENTATION OF MICROFIBRILS BY CORTICAL MICROTUBULES
In higher plants, the microtubules have two functions, one is to determine the plane of cell division by the formation of the mitotic spindle, and the other is to orient the deposition of cellulose microfibrils by the assembly of microtubules in growing cells [22]. In fact, in growing plant cells, cellulose microfibrils are mostly transversely oriented against an elongating or expanding direction as a result of microtubule reorganization (Figure 3). However, this has been shown onlyby com paring the assembly of cortical microtubules with the orientation of microfibrils in the freeze fracture micrographs. To examine the interaction between cortical microtubules and microfibrils more directly, we prepared an isolated plasma membrane sheet Figure 3. The orientation of cellulose with cortical microtubules from microfibrils in elongating plant cells. tobbaco cells and demonstrated that ~glucan synthases penetrating through the membrane move in the fluid m e m b r a n e along cortical microtubules, forming microfibrils. In the presence of UDP-glucose, ~-glucan microfibrils were formed abundantly in the interface between the prepared membrane sheet and a polylysine-coated coverslip. The microfibrils appeared to be formed as short fibers at m a n y loci in the presence of taxol within a few minutes after the start of incubation, and longer fibers were formed after incubation for 30 min. The microfibrils formed during incubation were arranged closely in parallel to the microtubules. The rate of ~glucan elongation directly determined on the exoplasmic surface was 1.288 ~tm per min. W h e n the ordered structure of microtubules was disrupted by the treatment with propyzamide during the preparation of protoplasts, ~-glucans were deposited in masses on the prepared membrane sheet not in arrays. This suggests that the arrayed cortical microtubules are required for the formation of arranged microfibrils on the prepared membrane sheet.
247 4.
C A N WE IMPROVE TREES BY OVEREXPRESSION OF THE GENES?
One strategy to the enhance CO2 fixation in woody plants is to enhance the expression of genes required for cellulose deposition, which should enhance plant growth either by cell wall or just cellulose deposition. W e have already isolated the full length of cDNAs for three kinds of cellulases, XET and expansin as plant cell growth regulators, and for two kinds of cellulose synthases and sucrose synthase as a system of cellulose synthesis, as summarized in Table 2. If cellulose deposition is increased by the increased activity Of each enzyme in the transformants of Arabidopsis, the gene can be introduced to woody plants to determine the cellulose deposition. Each cDNA fragment was redoned into the binary plasmid pBE2113 containing a chimeric promoter E12~ [23]. Arabidopsis plants were transformed by the vacuum infiltration method [24]. Transform ant seeds were selected in the presence of 100 , g / m l kanamycin. Table 2 Potential cDNAs for enzymes responsible for cellulose deposition Molecular size
Length (bp)
Enzyme
Plant origin (kDa)
cDNA
ORF
Growth regulation Cellulase Cellulase Cellulase (EGL1) XET Expansin
55 56 54 34 28
1,580 1,550 1,473 1,341 779
1,482 1,518 1,458 879 774
Poplar Pea Pea Pea Pea
Cellulose synthesis Pcs I Pcs 2 Sucrose synthase
110 125 92
3,228 3,311 2,652
2,934 3,120 2,445
Cotton Cotton Mung bean
The carbohydrate analysis of the transformants would be expected to reveal the changes in xyloglucan and cellulose because carbohydrates are genetically regulated to form in the cell and to be secreted into the wall. In fact, there is evidence that novel cell walls are produced by hybridization of plants [25]. The polysaccharides may be changed not only in amount but also in molecular weight, and therefore, the cell walls of the transform ants will differ from those
248 of wild plants. Such modification of the cell wall is the kind of cell-wall engineering that should make useful contributions to plant biotechnology.
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. 25.
D.P. Delmer and Y. Amor, Plant Cell, 7 (1995) 987. Y. Masuda, Bot. Mag. Tokyo Special Issue, 1 (1978) 103. T. Hayashi, Annu. Rev. Plant Physiol. Plant Mol. Biol., 40 (1989) 139. J.M. Labavitch, Ann. Rev. Plant Physiol., 32 (1981) 385. C.W. Lloyd (ed.), The Cytoskeletal Basis of Plant Growth and Form, Academic, London, 1991. M.E. Terry, R.L. Jones and B.A. Bonner, Plant Physiol., 68 (1981)531. T. Hayashi and T. Takeda, Biosci. Biotech. Biochem., 58 (1994) 1707. T. Hayashi, Y. Kato and K. Matsuda, Plant Cell Physiol., 21 (1980) 1405. E.P. Lorences and I. Zarra, J. Exp. Bot., 38 (1987) 960. T. Matsumoto, F. Sakai and T. Hayashi, Plant Physiol., 114 (1997) 661. S.C. Fry, R.C. Smith, K.F. Renwick, D.J. Martin, S.K. Hodge, K.J. Matthews, Biochem. J., 282 (1992) 821. K. Okazawa, Y. Sato, T. Nakagawa, K. Asada, I. Kato, E. Tomita and K. Nishitani, J. Biol. Chem., 268 (1993) 25364. D.J. Cosgrove, Plant Cell, 9 (1997) 1031. Y. Ohmiya, T. Takeda, S. Nakamura, F. Sakai and T. Hayashi, Plant Cell Physiol., 36 (1995) 607. N. Shiraishi (ed.), Kyoto Conference on Cellulose, The Cellulose Society of Japan, Kyoto, 1994. H.C. Wong, et al., Proc. Nail. Sci. USA, 87 (1990) 8130. M.A. Frohman, M.K. Dush and G.R. Martin, Proc. Natl. Sci. USA, 85 (1988) 8998. J.R. Pear, Y. Kawagoe, W.E. Schreckengost, D.P. Delmer and D.M. Stalker, Proc. Natl. Sci. USA, 93 (1996) 12637. P.S. Chourey and O.E. Nelson, Biochem. Gen., 14 (1976) 1041. Y. Amor, C.H. Haigler, S. Johnson, M. Wainscott and D.P. Delmer, Proc. Natl. Acad. Sci. USA, 92 (1995) 9353. T. Nakai, N. Tonouchi, T. Tsuchida, H. Mori, F. Sakai and T. Hayashi, Biosci. Biotech. Biochem., (1997)in press. R.J. Cyr, Annu. Rev. Cell Biol., 10 (1994) 153. I. Mitsuhara, et al., Plant Cell Physiol., 37 (1996) 49. I. Potrykus and G. Spangenberg (eds.), Gene Transfer to Plants, Springer, Berlin, 1995. C. Ohsumi and T. Hayashi, Biosci. Biotech. Biochem., 58 (1994) 959.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
249
Possibility of m o l e c u l a r protection of p h o t o s y n t h e s i s u n d e r salinity stress Fumihiko Sato*, Yuto Arata, Kazuyo Matsuguma, Minae Shiga, Yutaka Kanda, Kentaro Ifuku, Kaoru Ishikawa and Takahiro Yoshida Division of Applied Life Sciences, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-01, Japan Water stress dramatically limits plant growth and crop productivity. Plants have evolved several adaptive strategies to counter these types of abiotic stresses. Most common type of osmotic adaptation is the accumulation of compatible solutes in the cytoplasm. Genetic engineering of compatible solute production is very useful technique to create new stresstolerant plant species, but it is time-consuming and their performance is still limited. Here, we report the effects of exogenously applied chemicals on the protection of plant metabolism under stress and discuss the possibility to use exogenous chemicals with transgenic approaches. 1. I N T R O D U C T I O N Continuous increase of fossil fuel consumption builds up the CO2 in atmosphere (about 3 GtC/year; from 280 ppm in 1800 to 355 ppm in 1995). Especially, half of that increase has been concentrated in the past three decades. Now global warming due to greenhouse gases is more serious concerns for human beings. While reduction of fossil fuel consumption is needed, our most social activities depend on the high energy input technology. Modern agriculture is not exceptional for high energy input technology; although plants can fix solar energy directly, high energy inputs like irrigation, fertilizer, pesticide, herbicides and so on are applied to optimize the plant growth and to maximize the crop yield. High yield crop varieties are also developed on the basis of high energy input. Now we have to develop new system to reduce the CO2 emission and to establish sustainable systems for the production. Biological fixation of CO2 by plants has several advantages, while it is not as efficient as chemical fixation; total amounts of biologically fixed carbon by plants per year is about 100 GtC (IPCC 1995) and few percentage of improvement of biological fixation is sufficient to compensate the carbon emission from fossil fuels. And biologically fixed carbon can be easily re-used as food, fiber, pulp, organic oil, industrial raw materials and so on. However, because the available land for plantation is very limited and civilization continuously reduce the rich farm land, the improvement of plant productivity based on low energy input is urgently needed. As described above, high salinity, drought, and low temperature are environmental factors that dramatically limit plant growth and crop productivity (Boyer, 1982, Bohnert et al., 1995). Dehydration and loss of turgor induced by external solutes induce osmotic stress to reduce the plant growth immediately. Dehydration also often leads to irreparably destructive events in proteins and cellular membranes (Crowe et al., 1988). An excess of K + caused by cell shrinkage and the uptake of external NaCI result in toxicity to many intracellular enzymes (Greenway and Munns, 1980). In addition, excess Na + may disturb mineral nutrition by *Correspondenceshould be addressed;
[email protected] We appreciate Dr. J. Mano of Kyoto University for his kind help in EPR analysis. We also appreciate for kind gift of ectoine from Mr. Y. Toyoda and Mr. K. Yamatoya of Dainippon Pharmaceutical Co. This work was supported in part by a Grant-in Aid for Scientific Research, JSPS-RFTF9616001, and NEDO's International Joint Research Program (to F.S.).
250 inhibiting the uptake of essential cations such as K+ and Ca2 + (Greenway and Munns, 1980 ; Serrano and Gaxiola, 1994). Plants as well as other organisms have evolved several adaptive strategies to counter these types of abiotic stresses (Csonka, 1989; Bohnert et al., 1995). At the cellular level, the most common type of osmotic adaptation is the accumulation of compatible solutes in the cytoplasm and the sequestration of NaCI into the vacuole (Rhodes and Hanson 1993, Bohnert et al., 1995). Compatible solutes are small molecules that can act as nontoxic cytoplasmic osmolytes to raise osmotic pressure, and stabilize enzymes and membranes against damage by high salt levels (Wyn Jones, 1984). To examine the physiological role of compatible solutes, transgenic plants which accumulate glycinebetaine (Hayashi et al., 1997), mannitol (Tarczynsky et al., 1993), proline (Kishor et al., 1995), and trehalose (Holmstrom et al., 1996) have been established and their drought/salt tolerance has been shown. As shown, genetic engineering is very useful technique to create new stress-tolerant plant species, but the establishment of new transgenic plants is timeconsuming, and their performance is still limited. We think that the combination of chemical regulation and genetic engineering is more practical way to establish the system for the biological CO2 fixation (biomass production) with low energy input. Thus, we examined the effects of exogenously applied chemicals on the protection of plant metabolism under stress condition. In the present work, we chose 11 compounds shown in Figure 1 and determined their effects on the growth of tobacco seedlings and photosynthetic activities of isolated thylakoid membranes. The possibility to use these chemicals in the field and the combination with transgenic plants like compatible solute overproducer is discussed.
9P o l y h y d r i c
Sorbitol CH2OH H_l_OH H H
alcohols-polyols CH2OH HO.__t__H
OH OH
OH
OH
CH2OH
Sucrose
CH2OH
N
OH
.+o. CH=OH
H
H
H3N+- CH=- CH=- SO3" OH
OH
\ \//I---O--I l o l~l HOH 2CI//IOH I OH
H~~~I~O
-
Choline chloride
OH
Trehalose
Cl-b
Taurine
Ectoine
CHzOH
HOO H ~ ~ ~ - - - O - - ~ O ~
CH2OH
I I-bC -- N+ "CI-I=-COOI -
O
CH2OH
I OH Glycerol
Glycine betaine
Proline
H--l--OH H--~OH
CH~DH
Inositol
9A m i n o a c i d s and a m i n o acid d e r i v a t i v e s
Mannitol
CH~
I H3C-- N+-cH2-CH2-OH " CI-
'
O
Figure 1. Chemicals used 2. M A T E R I A L S AND METHODS 2.1. P l a n t m a t e r i a l s Tobacco seedlings (Nicotiana tabacum cv. Samsun NN) after 7-10 days of germination were grown on half strength of Murashige-Skoog basic agar media with different NaC1 concentrations (0 - 0.3 M). Seedlings were cultured for 2 weeks under different light conditions
251 (nearly 0, 20-30 and 90-170 mE/m2/s) at 24 *C and their fresh weight increase and chlorophyll content were determined. The effect of compatible solutes (1 mM) on the growth of tobacco seedlings under 0.2 M NaC1 stress were determined under 90-170 mE/m2/s after 10 days of culture at 24 *C. These experiments were carded out under sterile condition in petri-dishes 2.2. I s o l a t i o n of t h y l a k o i d m e m b r a n e s and m e a s u r e m e n t s of photosynthetic activities. To examine the effects of chemicals on thylakoid membranes, chloroplasts were prepared from commercially purchased spinach leaves as described by Shigematsu et al (1989). Concentrations of chlorophyll were calculated from the equations of Mackinney (1941). Isolated thylakoid membranes were stored at -80 ~ until use. Photosynthetic activity was determined using PAM2000 (Heinz Walz GmbH, Effeltrich, Germany) or oxygen electrode (Rank Brothers, Cambridge, UK). The isolated thylakoid membranes (10 mg Chl) were suspended in buffer (50 mM Tricine-KOH, pH 7.8, 25 mM NaCI, 2.5 mM NH4CI and 2.5 mM potassium ferricyanide as the electron acceptor ) with/without NaCI (0.2 M) or compatible solutes (0- 0.5M). Photosynthetic activities were measured under saturated light intensity (around 700 mE/m2/s) at 25 ~ 120
120 - - - m - ~ 90-170 ~tE I
m2s
o~ loo
-
ca. 0
1001
~tElm2s
A
r N,,-
r.0 u
r. - 90-170 t t E l m 2 s -_
20-30 ~tE I
=
ca.0
m2s
~tElm2s
80
0
r..
-
20-30 ~tE/m2s
o
O1 6O
6O
40
20
2O
0
0
!
0
(a)
100
200
3OO
400
NaCI conc. (mM)
0
I
I
100
200
(b)
I
300
400
NaCI conc. (mM)
Figure 2 High light intensity enhances the salt toxicity in tobacco seedlings 3. R E S U L T S AND D I S C U S S I O N 3.1. C h a r a c t e r i z a t i o n of the salt stress m e c h a n i s m Several factors affect the salt sensitivity of plants. We first evaluated the effect of light intensity on the salt-stress. Figure 2 clearly showed that tobacco seedlings were more severely affected by salt stress under high light intensity both on the basis of chlorophyll content and fresh weight increase. This result indicated that light, probably photosynthetic process, would be involved in the salt-stress. To characterize the salt-stress mechanism, we examine the effect of salt on the photosynthetic activities of isolated thylakoid membranes. Previously, we reported that the presence of salt in assay inhibited the photosystem II activity of tobacco thylakoid membranes but not photosystem I activities (Murota et al., 1994). Then, we further examined the effect of salt on the irreversible photodamage of thylakoid membrane activity.
252 PAM analysis indicated that salt treatment in the light irreversibly damaged the photosynthetic activity of thylakoid membranes (data not shown). While actual mechanism of photoinhibition is not clear yet, our electron paramagnetic resonance (EPR) analysis indicated that the addition of salt increased the generation of superoxide radicals (data not shown). Chloroplasts generally produce active oxygen species and several scavenging enzymes scavenge them (Asada, 1994). Superoxide radicals produced by salt stress would overflow the normal scavenging system. Therefore, it is reasonable that transgenic plants producing compatible solutes acting as active oxygen scavenger (Shen et al., 1997) or scavenging enzyme itself (SOD; McKersie et al., 1996; catalase, Shigeoka et al, personal communication) can show some salt/drought tolerance. These results suggested that at least active oxygen generation would be involved in salt-stress. 3.2. Effect of chemicals on the growth of tobacco seedlings and photosynthetic activity of thylakoid membranes under salt stress As mentioned above, endogenous accumulation of compatible solutes showed some protective effects on the growth of plant under salt/drought stress. Then, the effects of exogenously applied chemicals on the protection of tobacco seedlings under 0.2 M NaCI stress were determined. Most compatible solutes examined showed clear protective effects on the growth and greening of tobacco seedlings under the stress (Figure 3); Ectoine, a novel compatible solute in halophilic bacteriaEctothiorhodospira (Galinski et al., 19985), and glycerol, a compatible solute in yeast, were also effective as other known compatible solutes in higher plants (glycinebetaine, mannitol, sorbitol, trehalose and inositol). Moderate protection was observed with sucrose and choline, a precursor of glycinebetaine. But, proline was not so effective. Growth stimulation by these compounds were only observed under stressed condition and no effect was found under normal non-stressed condition. To examine the mechanism of these compounds, we measured their effect on the oxygen evolving activities of isolated thylakoid membranes under salt stress. As shown in Figure 4, addition of many compatible solutes (glycinebetaine, mannitol, sorbitol, inositol and sucrose) linearly increased the oxygen evolving activity under salt stress. These compounds also showed some increase of activity under normal condition, but their effect was marginal. Trehalose was effective at low concentration both under salt stressed and normal conditions, but higher concentration than 0.3 M was rather toxic. Proline did not show evident effect under salt-stressed condition. Taurine showed some increase under salt stressed condition, but the
(a) fresh weight ectoine glycerol trehalose sucrose mannitol sorbitol inositol choline chloride taurine proline glycine betaine control 200
n +NaCI .....................~......~.........
i ~...-.-.-.-,-.-.-.-....,...........
~...,
..,.......................
i
.............................-.
1O0
'
0 1O0 200 relative growth (%)
(b) Chl. content ectoine glycerol trehalose sucrose rnannitol sorbitol inositol choline chloride taurine proline glycine betaine control m 200
9-NaCl
[] +NaCl . . . . . . . . . . . . . . . .
'
. . . . . . . .
~ w ~ . ~ . . . . . . . . . . . Y ~ .
i
/ i
~
~..........~...........
100 0 100 200 relative content of Chl. (%)
Figure 3. Effect of compatible solutes (1 mM) on the growth and chlorophyll content of tobacco seedlings on the medium with/without NaCI (0.2 M) grown at 90-170 mE/m2/s
253 stimulation was rather small. On the other hand, glycerol and ectoine did not show any effect in this assay (data not shown). The comparison of the results of seedling assay (in vivo assay) and thylakoid membrane assay (in vitro assay) clearly indicted that many compounds (glycinebetaine, mannitol, sorbitol, inositol and so on)showed good correlation between the in vivo protection activity and those found in vitro experiments, while some compounds like glycerol and ectoine which showed low protective activity in in vitro assay also showed clear in vivo protective activity under stress. These results suggest that chemicals which can protect photosynthetic activity under stress conditions would be useful candidates for the in vivo protection, while target site(s) of salt-stress are multiple and the combination of several chemicals would be more effective. Because genetic engineering of metabolism is not easy task, and there is limitation of substrate pool for metabolite production, the combination of exogenous application of chemicals with transgenic approach to modify the plant metabolism would be more practical strategy to maximize the tolerant potentials of plants. Very preliminary experiment showed that exogenous application of glycerol was effective to protect the plant growth in greenhouse, while effective plant species was limited to cotton. While further characterization was needed, chemical regulation of stress-tolerance would have high potential.
200
initial
[
[] 9,NaCI [/
I 0 500 mannitol conc. [mM] 9m a n n i t o l 9g l y c i n e betaine 9s o r b i t o l 9i n o s i t o l 9s u c r o s e
2oo r
i
I
initial
[~ [] control ,NaCIT
~'-'-'% 100~
~
~~'
Fl control
[] +NaCl
~-~100
01 proline conc.
5o0
[mM]
~ 0 o soo ~ taurineconc. [mM]
~
9p r o l i n e
effective
200
initia...__[
9t a u r i n e
Initial
[] control [] +NaCI
E 1
.
[~,
0 trehalose conc.
5OO
[mM]
9t r e h a l o s e
ineffective
Figure 4. Effect of compatible solutes on oxygen evolving activity of spinach thylakoid membranes. Experimental conditions are shown in "Materials and methods". 3.3. Further improvement
of salt-tolerance
Above experiments indicated that protection of photosynthesis under salt/drought stress would be suitable target for the improvement of plant productivity. Previously, we selected high NaCl-adapted photoautotrophic tobacco cells and characterized their characteristics of salt-tolerance (Sato et al., 1992; Murota et al., 1994). Our characterization indicated that photosynthetic activity of photosystem II (PSII),
254 especially extrinsic 23 kDa (OEC23) of oxygen-evolving complex of PSII would be one of the target sites of salt-stress. Molecular characterization of salt-sensitive cucumber indicated that cucumber OEC23 was very sensitive to salt and had many amino acid changes in conserved residues (Sato et al., in preparation). Now, molecular breeding of cucumber OEC23 is conducted to determine the function of amino acid change in cucumber OEC23 and to make salt-tolerant cucumber. On the other hand, we should remind that dehydration and loss of turgor reduce the plant growth immediately and plants respond to this stress by the closure of stomata to prevent the water loss. However, this stomata closure concomitantly inhibit the diffusion of CO2 into leaf mesophyll cells and decrease the photosynthetic carbon fixation. Thus, too fast response to dehydration seems to be harmful for the plant growth. To maximize the plant productivity, some delayed closure of stomata might be beneficial because plant can fix more CO2 to produce more metabolites for the adaptation and growth and also to reduce the risk of overenegized states under high light and low CO2 condition. Transgenic plants which can produce high level of compatible solutes would have more opportunity to grow when they have delayed response to dehydration. We believe that this optimistic adaptation hypothesis would be worthy to be evaluated. We have selected some ABA-insensitive mutants of tobacco. An ABA-0 line showed slow response to dehydration and about quarter of self-fertilized M2 seeds of ABA-0 showed much better growth in medium with 2% NaCI under high humidity condition. Now, transformation of ABA-0 M2 plant with compatible solute overproducing gene is conducted. REFERENCES
K. Asada, In Causes of photooxidative stress and amelioration of defense systems in plants. C.H. Foyer and P.M. Mullineaux (ed), CRC Press, Florida, pp. 77 (1994) H.J. Bohnert, D.E. Nelson, R.G. Jensen, Plant Cell, 7:1099 (1995). J.S. Boyer, Science, 218:443 (1982). J.H. Crowe, L.M. Crowe, J.F. Carpenter, A.S. Rudolph, C.A. Wistorm, B.J. Spargo, T.J. Anchordoguy, Biochim. Biophs. Acta., 947:367 (1988). L.N. Csonka, Microbial. Rev., 53:121 (1989). E.A. Galinski, H-P. Pfeiffer, H.G. Trueper, Eur. J. Biochem., 149" 135 (1985) H. Greenway, R. Munns, Annu. Rev. Plant Physiol., 31 : 149 (1980). H. Hayashi, Alia, L. Mustardy, D. Patchraporn, M. Ida, N. Murata, Plant J., in press IPCC: Climate change 1995 - The science of climate change (1995) K-O. Holmstrom, E. Mantyla, B. Welin, A. Mandal, E.T. Palva, Nature, 379:683 (1996). P.B.K. Kishor, Z. Hong,G.-H. Miao, C-A.A. Hu, D.P.S. Verma, Plant Physiol., 108 9 1387 (1995). G. Mackinney, J. Biol. Chem., 140:315 (1941). B.D. McKersie, S.R.Bowley, E. Harjanto, O. Leprince, Plant Physiol., 111" 1177 (1996). K. Murota, F. Sato, Y. Oshita, A. Watanabe, S. Aso, Y. Yamada, Plant & Cell Physiol., 35: 107 (1994). F. Sato, K. Murota, S. Aso, Y. Yamada, In Research in Photosynthesis, Vol. IV, N. Murata (ed.), Kluwer Acad. Pub., Netherlands, pp. 259 (1992). R. Serrano, R. Gaxiola, Critical Reviews in Plant Sciences, 13 9121 (1994). B. Shen, R.G. Jensen, H.J. Bohnert, Plant Physiol., 113:1177 (1997) Y. Shigematsu, F. Sato, F. Y. Yamada, Plant Physiol., 89 : 986 (1989). M.C. Tarczynski, R.G. Jensen, H.J. Bohnert, Science, 259:508 (1993). R.G. Wyn Jones, RecentAdv. Phytochem., 18 : 55 (1984).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
255
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Organometallic CO 2complexes in
supercritical C02: a time-resolved
infrared study M. W. George*, D. C. Grills, X-Z. Sun and M. Poliakoff Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom. e-mail:
[email protected] 1. I N T R O D U C T I O N There has recently been considerable interest in the interaction of carbon dioxide with metal centres. When a molecule of CO2 reacts with a metal centre there are three possible adducts which depend on the nature of the metal, the steric and electronic factors, as shown below [1-4]:
M +
, or
O---C=O
or M ~ - O - - - C = O
0
(I)
~0
(II)
(III)
Examples of Type I adducts (q '-C), and Type II adducts (q2-CO) have been isolated and the mode of co-ordination has been confirmed by crystallography [ 1-4]. By contrast, Type III (q 1-O) has only been predicted theoretically and deduced from spectra of low temperature matrices [ 1-4]. Time-resolved Infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR spectroscopy (ns), has been outstandingly successful in identifying reactive intermediates [5] and excited states [6] of metal carbonyl complexes in solution at room temperature. We have used infrared spectroscopy to probe the mechanism of photo[7] and electrochemical [8] catalytic reduction of CO 2. We have used TRIR to study organometallic reactions in supercritical fluids on a nanosecond time-scale [9-10]. This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have reactivity comparable to organometallic alkane complexes. In addition, we have studied the co-ordination of CO 2 to metal centres in supercritical CO 2 (scCO2) and shown that v(C-O) bands provide a very sensitive probe for the oxidation state of the metal centre. We found evidence, albeit circumstantial, for the formation and reactivity of 1"1~-Obound metal CO 2 complexes in solution at or above room temperature and found these highly reactive CO 2complexes have similar reactivity to the analogous Xe complexes [11-12]. We have also used TRIR to examine the reactivity of CpMo(CO) 3 radicals in scCO 2 and found evidence for an interaction, possibly Lewis Acid/Base, between CpMo(CO)3 and scCO 2 [13].
256 2. E X P E R I M E N T A L The Nottingham TRIR apparatus has been described in detail elsewhere [5]. Briefly, a pulsed Nd:YAG laser (Quanta-Ray GCR-11; 266 nm or 355 nm), is used to initiate the photochemical reaction and a cw infrared source, (Mtitek IR diode laser) monitors the changes in infrared transmission following the UV/visible pulse. IR spectra are built up on a "point-by-point" basis by repeating this measurement at different infrared frequencies. The stainless steel high pressure cells for supercritical TRIR have been described previously [ 14]. All solutions were characterised by conventional FTIR prior to use. 3. R E S U L T S AND D I S C U S S I O N 3.1 18-electron
Complexes:
Perutz and Downs characterised [Cr(CO)5(CO2)] following photolysis of Cr(CO)6 in a low temperature Ar matrix doped with CO 2 and suggested that the mode of co-ordination was 111-O [ 15]. Recent calculations have supported this assignment [161. We have recently reported the characterisation of [M(C0)5(C02) ] (M = Cr, Mo and W) in scCO 2 using TRIR and thus provided the first tentative observation of organometallic rll-O CO2 complexes, in solution at or above room temperature. The downwards shift in v(CO) of [M(CO)5(CO2)] relative to complexes such as [M(CO)5(Xe)] indicates rl 1-O CO 2co-ordination, Figure 1.
W ( C O ) 5 ( C O 2) ee 9
(b) 0 r"
,
.Q
o
9
w(co)5(xe
)
Oo
<~
I
I
I
I
I
I
I
1980 1940 1900 Wavenumber / cm -1
Figure 1" TRIR spectra obtained 1 gs after irradiation of W(CO)6 in (a) scXe and (b) scCO 2. The negative peaks show depletion of W(CO) 6 and the positive peaks are assigned to the production of (a) W(CO)5(Xe) and (b) W(CO)5(CO2).
257 M(CO)5(CO2) complexes are highly reactive and exist in scCO 2 at 35 ~ for 1-10 Its. The comparison of the reactivity of M(CO)5(Xe) and M(CO)5(CO2)is quite intriguing. For a given metal (Cr, Mo or W), the reactivity of M(CO)5(CO2) towards CO in scCO 2 is very similar to the reaction of the analogous M(CO)5(Xe ) with CO in supercritical xenon (scXe), see Table 1. We have investigated both the steric and electronic factors which govern the co-ordination of CO z to the metal centre, using the v(CO) bands as an indication of the oxidation state of the metal. We found that CpMn(CO)2(CO2) and CpRe(CO)2(CO2) have different co-ordination modes of CO 2, Figure 2.
(b)
I
I 2000
I 1950
I 1900
W a v e n u m b e r (cm -1 )
I
I
I 2000
I 1960
I 1920
Wavenumber (cm -1 )
Figure 2 TRIR spectra obtained 1 Its after irradiation of (a) CpMn(CO)3 and (b) CpRe(CO) 3 in scCO 2. The negative peaks show depletion of CpM(CO) 3 (M = Mn or Re) and the positive peaks are assigned to the production of (a) CpMn(CO)2(CO2) and (b) CpRe(CO)2(CO2). The downward shift in the v(CO) bands of CpMn(CO)2(CO2) is suggestive of a 1"!~-O CO2 complex. However, co-ordination of CO 2 to the CpRe(CO) 2 moiety resulted in oxidation of the Re centre, probably via rl ~-C co-ordination of the CO 2. All these assignments were based on the position of v(C-O) bands because our TRIR data were restricted to this region of the spectrum. We are extending these studies using step-scan FTIR to cover IR regions where co-ordinated CO 2 absorbs. Comparing the reactivity of CpM(CO)2(CO2) with CO in scCO 2 and CpM(CO)2(Xe ) with CO in scXe we find that CpMn(CO)2(CO 2) has very similar reactivity to CpMn(CO)2(Xe). This is analogous to the reactivity of M(CO)5(COz)/M(CO)5(Xe) and provides further evidence that we have xl~-O co-ordination. However, CpRe(CO)2(CO2) is significantly less reactive (see Table 1) than CpRe(CO)z(Xe ) and this is consistent with CO 2 having different modes of co-ordination to Mn and Re.
258
Table 1 2nd Order Rate constants for the reaction of weakly bound Xe and CO 2 complexes LmM(CO)x.z(L ) +CO --->LmM(CO) x + L ( L m = (CO)3 and Cp (qS-CsHs); L = Xe and CO2) in either scXe or scCO 2 at 35 ~ Complex
Solvent
Cr(CO)s(Xe ) Cr(CO)s(CO2) Mo(CO)s(Xe ) M0(CO)5(CO2) W(CO)s(Xe ) W(CO)5(CO2) CpMn(CO)2(Xe ) CpMn(CO)2(CO2) CpRe(CO)2(Xe ) CpRe(CO)2(CO2)
scXe scCO 2 scXe scCO 2 scXe scCO 2 scXe scCO 2 scXe scCO 2
k 7 (M-~s1) 9x 1x 8x 8x 3x 4x 3x 4x 7x 2x
106 10v 106 106 106 106 10 6 10 6 10 3 10 3
3.2 R a d i c a l C h e m i s t r y We have studied the reactivity of CpMo(CO) 3 in s c C O 2. The reactivity of CpMo(CO)3 in n-heptane has already been elucidated using TRIR [ 17]. Visible photolysis of trans-[CpMo(CO)3] 2 generates CpMo(CO) 3 radicals which recombine at a diffusion controlled rate to form the stable trans- and unstable gauche-[CpMo(CO)3] 2. gauche-[CpMo(CO)3]2 slowly isomerises to trans-[CpMo(CO)3] 2, Scheme 1.
hv,vi,)j
[CpMo(CO)3] 2 - ~ (gauche)
~
slow
fast
2CpMo(CO)3
fast
[C pMo(CO)3]2 (trans)
Scheme 1: Summarising the photochemistry of trans-[CpMo(CO)3] 2 in nheptane following visible photolysis
259
CpMo(CO) 3 has 2 v(CO) bands in n-heptane solution, consistent with C3v symmetry, Figure 1(a). In scCO 2 CpMo(CO) 3 has 3 v(CO) bands, Figure 3(b), which indicates the symmetry has been lowered to C s. We suggest that this is a result of CpMo(CO)3 interacting with CO 2 [ 16]. This assignment is supported by observing 3 v(CO) bands for CpMo(CO) 3 in n-heptane doped with CO 2, Figure 3(c).
s o
<~
I
I
I
2000 1950 1900 W a v e n u m b e r / cm -1
Figure 3 IR spectra of CpMo(CO)3 in (a) n-heptane, (b) scCO 2 and (c) n-heptane doped with CO 2 (100 psi). Note the highly significant arrowed shoulder which is observed in the spectra recorded in the presence of CO 2.
260 Although we have found evidence for interaction of CO 2 with the CpMo(CO)3 radical it appears that this has little or no effect on the reactivity of the radical. CpMo(CO) 3 decays at an identical 2nd order rate (3 xl09 M-~s~) in both CO 2 doped and undoped n-heptane. The 2nd order rate of decay of CpMo(CO) 3 in scCO 2 (k 2 = 1 x 10 l~M ~ s 1) is also close to diffusion control. CONCLUSIONS We have shown that TRIR has been highly successful for probing organometallic reactions in conventional solvents. Here we have shown it is equally successful for probing such reactions in supercritical fluids, particularly scCO 2. The combination of TRIR and scCO 2 provides a unique approach to study the interaction of CO 2 to metal centres and hence the co-ordination chemistry of CO 2. With improved sensitivity of TRIR instrumentation the technique is no longer restricted to organometallic compounds and we have successfully shown it can be applied to short-lived organic species in conventional solvents [ 18]. Work is currently underway at Nottingham to develop TRIR for probing organic reactions in scCO 2.
REFERENCES D. H. Gibson, Chem. Rev. 96 (1996) 2063. [1] K. K. Pandey, Coord. Chem. Rev. 140 (1995) 37. [2] W. Leitner Coord. Chem. Rev. 153 (1996) 257. [3] J. Mascetti and M. Tranquille J. Phys. Chem. 92 (1988) 2177. [4] M. W. George, M. Poliakoff and J. J. Turner, The Analyst 119 (1994) 551. [5] J. J. Turner, M. W. George, F. P. A. Johnson and J. R. Westwell, Coord. [6] Chem. Rev. 125 (1993) 101. M. W. George, F. P. A.; Johnson, J. R. Westwell, P. M. Hodges and J. J. [7] Turner, J. Chem. Soc. Dalton Trans. (1993) 2977 F. P. A.; Johnson, M. W. George, F. Hartl and J. J. Turner, Organometallics [8] 15 (1996) 3374. S. M. Howdle, M. Jobling, M. W. George and M. Poliakoff, Proc. 2nd Intl [91 Symp. on Supercritical Fluids (Boston), M. A. McHugh, (ed.), Johns Hopkins University, ( 1991) 189. X-Z. Sun, M. W. George, S. G. Kazarian, S. M. Nikiforov and M. Poliakoff, [10] J. Am. Chem. Soc. 118 (1996) 10525. X-Z. Sun, D. C. Grills, M. W. George, S. M. Nikiforov and M. Poliakoff, [111 J. Am. Chem. Soc. in press X-Z. Sun and M. W. George to be published [12] X-Z. Sun, D. C. Grills, M. Poliakoff and M. W. George to be published. M. Poliakoff, S. M. Howdle and S.G. Kazarian, Angew. Chem. Int. Ed. [14] Eng. 34 (1995) 1275. M. J. Almond, A. J. Downs and R. N. Perutz, lnorg. Chem. 24 (1985) 275. U. Pidum and G. Frenking Organometallics 14 (1995) 5325. [17] J. Peters, M. W. George, and J. J. Turner, Organometallics 14 (1995) 1503. [18] G. W. Sluggett, C. Turro, M. W. George, I. V. Koptyug, and N. J. Turro, J. Am. Chem. Soc. 117 (1995) 5148.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
261
M e t h a n a t i o n of c a r b o n dioxide on c a t a l y s t s d e r i v e d from a m o r p h o u s N i - Z r - r a r e e a r t h e l e m e n t alloys H. Habazaki a, T. Yoshida a, M. Yamasaki ~, M. Komorib, K. Shimamura b, E. Akiyama ", A. Kawashima . and K. Hashimoto ~ aInstitute for Materials Research, Tohoku University, Sendal 980-77, Japan bMitsui Engineering and Shipbuilding Co. Ltd., Ichihara, Chiba 290, Japan The nano-grained Ni/ZrO2 catalysts containing rare earth element oxides were prepared by oxidation-reduction pretreatment of amorphous Ni-(40-x) at% Zr-x at% rare earth element (Y, Ce and Sm; x=l - 10) alloy precursors. The conversion of carbon dioxide on the catalysts containing 1 at% rare earth elements was almost the same as that on the rare earth element-free catalyst, but the addition of 5 at% or more rare earth elements increased remarkably the conversion at 473 K. In contrast to the formation of monoclinic and tetragonal ZrO2 during pretreatment of amorphous Ni-Zr alloys containing 1 at% rare earth elements, tetragonal ZrO2, which is generally stable only at high temperatures, was predominantly formed during the pretreatment of the catalysts containing 5 at% or more rare earth elements. The surface area of the catalysts increased with the content of rare earth element. Thus, the increase in the surface area and stabilization of tetragonal ZrO~ seem to be responsible for the improvement of catalytic activity of the Ni-Zr alloy-derived catalysts by the addition of rare earth elements. 1. INTRODUCTION For the conservation of global atmosphere and abundant energy supply, we are proposing global CO2 recycling [1, 2], which consists of electricity generation at deserts, H2 and CH4 production at coasts close to the deserts and energy consumers. At deserts, electricity is generated by solar energy. At coasts close to the deserts, the electricity is used for hydrogen production by electrolysis of seawater, hydrogen is used for methane formation by the reaction with carbon dioxide, and methane is sent to energy consumers. The energy consumers, after using methane as a fuel, recover carbon dioxide and send it to the coasts close to the deserts. Production of methane by catalytic hydrogenation of carbon dioxide is selected in the proposed system due to its extremely higher reaction rate, higher conversion and higher selectivity on the well designed catalysts, compared with the production of other valuable organic substances. The possibility of this global CO2 recycling, proposed by us, has been substantiated by successful development of the CO2 recycling plant built on the roof of the Institute for Materials Research, Tohoku University. Among a range of solid catalysts amorphous alloys have attracted attention as
262 catalysts or catalyst precursors due to their high catalytic activity for some catalytic reactions and their unique catalytic properties. It has been reported that amorphous alloys show higher activity for hydrogenation of carbon monoxide than the corresponding crystalline alloy catalysts[3]. It has also been reported that some amorphous alloys containing zirconium are converted to highly active zirconiasupported metal catalysts during hydrogenation reactions or oxidation-reduction pretreatment[4]. Recently the present authors have been studying methanation of carbon dioxide on amorphous alloy-derived catalysts and have found that Ni/ZrO2 catalysts prepared by oxidation-reduction treatment of amorphous Ni-Zr alloys show high activity for methanation of carbon dioxide at ambient pressure[5, 6]. The catalysts thus prepared contain both tetragonal and monoclinic ZrO2, and the relative amount of tetragonal ZrO2 increases with nickel content in the catalysts. Interestingly, the turnover number increases with increasing nickel content in the catalysts, possibly relating to the relative amounts of tetragonal ZrO217]. However, the surface area of the catalysts or the number of surface nickel atoms decreases with an increase in the nickel content. Thus, the highest conversion has been obtained on the catalysts containing medium amounts of nickel. Rare earth elements, such as yttrium, cerium and samarium, are well known to stabilize tetragonal ZrO2 at lower temperatures, although the tetragonal ZrO2 is generally stable at and above 1373 K. In the present study the effect of rare earth element addition on the catalytic properties of the amorphous Ni-Zr alloy-derived catalysts has been examined in order to improve the catalytic activity of the catalysts. 2. EXPERIMENTAL Amorphous Ni-(40-x) at% Zr-x at% RE (x = 0, 1, 5 and 10; RE = Y, Ce and Sm) alloy ribbons of about 1 mm width and about 20 tmi thickness were prepared by a single-roller melt spinning method. The structure of the alloys prepared was co~ed by X-ray diffraction with Cu K~ radiation. The amorphous alloy ribbons were oxidized at 773 K in air for 5 hours and then reduced at 573 K under flowing hydrogen for 5 hours. During this treatment the amorphous alloys transformed to nickel catalysts supported on zirconia or zirconia-rare earth element oxides. The catalytic reaction was performed in a fixed bed flow glass reactor. A gas mixture of CO2 and H2 (1:4, volume ratio) was passed continuously on the catalyst with F/W = 5400 ml g-i h-i, unless otherwise mentioned. After the reaction the gas mixture was analyzed using a Chrompac MicroGC CP2002 gas chromatograph equipped with a thermal conductivity detector. Nitrogen physisorption and hydrogen chemisorption measurements were carried out at 77 K and 293 K, respectively, with a Belsorp 28SA automatic gas adsorption apparatus. Prior to the measurements, the catalysts were reduced in flowing hydrogen at 573 K for 2 h and then evacuated at the same temperature for 3 h. The distribution of pore sizes in the catalysts were measured from nitrogen physisorption at 77 K using the D-H method [8]. The structure of catalysts was determined by Xray diffraction using Cu K~ radiation.
263 3. R E S U L T S AND D I S C U S S I O N
3.1. C a t a l y t i c a c t i v i t y Figure I shows the conversion of carbon dioxide on the Ni-Zr catalysts containing yttrium, cerium and samarium at 473 K as a function of the content of rare earth element. Methane was exclusively produced on all the catalysts at all temperatures examined, and the formation of only a trace amount of ethane was detected as a byproduct. From this figure it is clear that the conversion of carbon dioxide on the catalysts containing 1 at% rare earth elements is almost the same as that on the Ni40Zr catalyst. However, the conversion on the Ni-40Zr catalyst is significantly enhanced by the addition of 5 at% or more rare earth elements. In particular, the cat~ysts containing 5 and 10 at% samarium and 10 at% yttrium show more than 80% conversion at this temperature. At higher temperatures all the catalysts showed about 90% conversion. The effect of reactant gas flow rate on the conversion of carbon dioxide was examined. The conversion of carbon dioxide on the most active Ni-30Zr-10Sm catalyst is shown in Figure 2. Interestingly, even at high flow rates of more than 1001 g-~ h ~ the conversion is higher than 60% at and above 523 K. Similar trend was observed for the catalyst containing 5 at% samarium. In contrast, the conversion on the catalyst containing only 1 at% samarium decreased drastically even at 573 K with increasing gas flow rate. Since the conversion on the Ni-40Zr catalyst without rare earth elements showed the gas flow rate dependence similar to Ni-30Zr-10Sm catalyst, the addition of a small amount of samarium, such as 1 at%, is rather detrimental to the conversion of carbon dioxide at high flow rates. 3.2. C h a r a c t e r i z a t i o n o f c a t a l y s t s For a better u n d e r s t a n d i n g of the role of rare earth elements in the catalytic properties of the amorphous aUoy-derived catalysts, characterization of the catalysts was carried out. Figure 3shows the change in the n u m b e r of surface nickel atoms, determined by hydrogen chemisorption, as a functionof the content of rare earth 100 o
I~i-(4:0-xl atds Z}-x at% ~RE' F/W=5400 ml g-1 h - ~ ~
r
L) 80 -473K o
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Content of rare earth e l e m e n t / a t %
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Flow rate, FAN / ml g-1 h-1 Figure 1. Change in the conversion of carbon dioxide on the Ni-Zr-rare earth Figure 2. Change in the conversion of element catalysts at 473 K with the carbon dioxide on the Ni-30Zr-10Sm content of rare earth element, catalyst with reactant gas flow rate.
264 element. Clearly, t h e n u m b e r of "-2, ~ 25 surface nickel atoms increases with i , i , i , i , i , i 0 N i-(4 O - x ) a t % Zr-x at o ~ ~ increasing rare earth element content. ~: o20 3 m Among three rare earth elements ~6 ~ 0 15 examined yttrium is the most effective element in increasing the number of ~ 10 surface nickel atoms. In particular, the 3 0 z -~ 5 _ number of surface nickel atoms in the Z Ni-30Zr-10Y catalyst is more than 3 0 0 2 4 6 8 10 times as high as that in the Ni-40Zr catalyst. Similar trends were observed Content of rare earth element / at% for BET surface areas of the catalysts. The BET surface areas of yttrium- Figure 3. Change in the number of containing catalysts were found to be surface nickel atoms in the catalysts higher in comparison with the with the content of rare earth element. corresponding other rare earth elementcontaining catalysts. . . . . . . . . i The pore size distribution of the I10 mm3 nm-1g-1 catalysts has also been examined, and o typical examples for the Ni-30Zr-10Sm and Ni-40Zr catalysts are shown in Figure 4. The Ni-30Zr-10Sm catalyst > has nanoscale pores of about 1.7 nm OZr-lOSm radius. This is quite similar to that of the Ni-40Zr catalyst, although total ~ _ _ _ Ni-40~r . . . . pore volume for the former catalyst is 1 10 larger than that for the latter catalyst. Pore radius, R / nm P Almost similar distribution of pore sizes was also observed for all the catalysts Figure 4. Pore size distribution in the containing other rare earth elements. In this manner, the addition of Ni-40Zr and Ni-30Zr-10Sm catalysts. Rp sufficient amounts of rare earth and Vp denote pore radius and pore elements is effective in increasing the volume, respectively. surface area of the Ni-Zr catalysts. A similar effect has been reported for the carbonsupported nickel catalysts containing lanthanum and cerium; the dispersion of nickel in the catalysts is increased by the addition of rare earth elements[9]. From Figs. 1 and 4, however, the increase in the conversion is not linearly correlated with the increase in the number of surface nickel atoms in the catalysts. This implies that other factors, such as rare earth oxides themselves and structural change in zirconia support, are expected to affect the catalytic activity of the catalysts. Figure 5 shows X-ray diffraction patterns of the Ni-Zr-Sm catalysts after the oxidation-reduction treatment. Reflections corresponding to metallic fcc nickel are clearly seen in the patterns of all the catalysts. Although no reflections corresponding to samarium oxide are detected, the structure of ZrO2 changes with the composition of the catalysts; in the Ni-40Zr and Ni-39Zr- 1Sin catalysts, monoclinic and tetragonal I
,
I
,
I
,
I
~
I
,
I
265
ZrO~ are present, while the catalysts containing 5 at% or more samarium contain only tetragonal ZrO~. Similar structural change was observed for the catalysts containing yttrium and cerium, although the catalyst containing 10 at% Ce contained a small amount of monoclinic ZrO2 in addition to tetragonal ZrO2. Tetragonal ZrO~ is well known to be present at and above 1373 K and is reversibly transformed to monoclinic phase at lower temperatures. However, the tetragonal phase can be stabilized at low temperatures by Zr 4§ substitution with Ca% Mg ~-§and rare earth element ions[10]. Further, metastable tetragonal phase of pure zirconia can be present even at ambient temperature by reducing the grain size to less than 30 nm[11]. In the Ni-40Zr catalyst the tetragonal ZrO2 is present, probably due to the presence of nickel as well as the formation of nano-scale grained ZrOg[7]. In the present study the tetragonal phase is further stabilized by the addition of rare earth elements and hence, this phase is predominantly formed in the catalysts containing 5 at% or more rare earth elements, although the addition of only 1 at% rare earth elements is not effective in stabilizing the tetragonal phase. Since the turnover numbers of the catalysts prepared from amorphous Ni-Zr alloys increase with an increase in the relative amount of the tetragonal phase[7], the predominant formation of the tetragonal ZrO2 also appears to be responsible for the enhancement of the catalytic activity of the catalysts containing 5 at% or more rare earth elements. 3.3. Effect o f w a t e r r e m o v a l Finally, an attempt was made to increase the conversion of carbon dioxide using two reactors connected in series. In this reaction system water produced in the first reactor was removed by bubbling the reaction gas into water at room temperature. The results are shown in Fig. 6. Clearly, the conversion on the Ni-30Zr-10Sm catalyst after passing two reactors is significantly higher than that using single reactor,
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0!
Two reactors (0.2 g + 0.2 ~)
,t-t2o
'
J
500
'
Ni-30Zr-10Sm F/W = 13500 ml g-1 h4 I
525
t
I
550
,
I
575
Temperature / K 30
40
50
60
70
80
90
Figure 6. Comparison of the conversion of carbon dioxide on the Ni-30Zr-10Sm catalyst using single reactor and two Figure 5. X-ray diffraction patterns of reactors connected in series. In the the Ni-Zr-Sm catalysts after the oxida- latter case, water was removed after tion-reduction pretreatment, passing the first reactor.
2e / Degree (Cu Ko~)
266 which contains the same amount of catalyst as the total amounts of the catalyst in the two reactors. Under the conditions the conversion is independent of reaction temperature between 473 and 573 K, and amounts to about 98% under the selected relatively high flow rate. Consequently, it can be said that the removal of water is effective in increasing the conversion of carbon dioxide. 4. CONCLUSIONS 1. The nano-grained nickel catalysts supported on zirconia or zirconia-rare earth element oxides are prepared by the oxidation-reduction pretreatment of amorphous Ni-Zr-rare earth element alloys. The conversion of carbon dioxide to methane on the catalyst prepared from amorphous Ni-40Zr alloy is improved by the addition of 5 at% or more rare earth elements (Y, Ce and Sm). 2. The addition of 5 at% or more rare earth elements leads to an increase in the surface area of the catalysts and to the predominant formation of tetragonal ZrO2. In contrast, the catalysts with 1 at% rare earth elements contain both monoclinic and tetragonal ZrOe, and have similar BET surface area to the Ni-40Zr catalyst. The increase in the surface area and preferential formation of nickel catalysts supported on tetragonal ZrOe appear to be responsible for the higher activity of the catalysts containing 5 at% more rare earth elements. 3. Removal of water between two reactors connected in series is effective in increasing the conversion of carbon dioxide. Using the two reactors about 98% conversion of carbon dioxide is attained on the Ni-30Zr-10Sm catalyst even at 473 K. REFERENCES
1. K. Hashimoto, Mater. Sci. Engng., A179/A180 (1994) 27. 2. K. Hashimoto, E. Akiyama, H. Habazaki, A. Kawashima, M. Komori, K. Shimamura and N. Kumagai, Sci. Rep. RITU, A43 (1997) 153. 3. H. Komiyama, A. Yokoyama, H. Inoue, T. Masumoto and H. Kimura, Sci. Rep. RITU, A28 (1980) 217. 4. T. Takahashi and T. Kai, J. Chem. Engng. Japan, 21 (1995) 961. 5. H. Habazaki, T. Tada, K. Wakuda, A. Kawashima, K. Asami and K. Hashimoto, Symp. on Corrosion, Electrochemistry, and Catalysis of Metastable Metals and Intermetallics, eds. C. R. Clayton and K. Hashimoto, p.393. The Electrochemical Society, Pennington (1993). 6. K. Shimamura, M. Komori, H. Habazaki, T. Yoshida, M. Yamasaki, E. Aldyama, A. Kawashima, K. Asami and K. Hashimoto, Mater. Sci. Engng., A226-228 (1997) 905. 7. M. Yamasaki, H. Habazaki, T. Yoshida, E. Akiyama, A. Kawashima, K. Asami and K. Hashimoto, Appl. Catal. A. General, (1997) in press. 8. D. Dollimore and G. R. Heal, J. Appl. Chem., 14 (1964) 109. 9. J. Barrault andA. Chafik, Appl. Catal., 67 (1991) 257. 10. T. K. Gupta, J. H. Bechtold, R. C. Kuznicki and L. H. Cadoff, J. Mater. Sci., 12 (1977) 2421. 11. R. C. Gravie, J. Phys. Chem., 69 (1965) 1238.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
267
Development of high performance Raney copper-based catalysts for methanol synthesis from CO2 and H2 J. Toyira, M. Saito b, I. YamauchiC, S. Luoa , J. Wu a, I. Takahara b and M. Takeuchid a Research Institute for Innovative Technology for the Earth (RITE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan* b National Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan c Department of Material Science and Processing, Osaka University, 2-1 Yamadagaoka, Suita-shi, Osaka 565, Japan d RITE, 9-2 Kizukawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan Catalytic hydrogenation of 002 into methanol has been investigated over Raney Cu-based catalysts. The Raney catalysts leached in NaOH/ZnO solutions showed high activities and selectivities for methanol synthesis. The deposition of Zn on the surface of Cu particles increased the surface area and the specific activity of Raney Cu-M. Raney Cu-Zr developed was significantly more active than a commercial catalyst. 1. I N T R O D U C T I O N
Catalytic hydrogenation of carbon dioxide is one of the crucial issues and processes to present a serious option for the global warming control. In particular, methanol synthesis has been considered to play an important role in the transportation of hydrogen energy derived from natural energy such as solar energy, hydropower and so on. In previous studies the authors have reported that metals oxides such as Ga203, AI203, ZrO2 and Cr203 contained in Cu/ZnO-based catalysts have an important role to improve simultaneously the activity and the selectivity[ 1, 2]. Unlike Cu/ZnO-based catalysts, Raney copper catalysts have not been widely reported in the literature as practical catalysts for methanol synthesis. However, 20 years ago Wainwright and co-workers have been the first to report the potentiel use of Raney Cu and Raney Cu-Zn as catalysts to produce methanol from syngas to use as synthetic liquid fuel [3]. Recent works of Wainwright et al. on methanol synthesis New Energyand IndustrialTechnologyDevelopment(NEDO)researchfellowship
268
and WGS reactions from syngas showed that the alkaline leaching of copper/zinc/aluminium alloy to produce zinc-promoted Raney Cu leads to an excellent mixing of the components and consequently allows a high activity for methanol synthesis by improving the surface area and the porosity of the catalyst [4]. This work also demonstrated that the production of MeOH was promoted by copper and the major role of zinc oxide and that the carbon dioxide is the major reactant forming methanol under the industrial conditions. On the basis of these studies, it is completly legitimate to envisage an interesting route for the methanol synthesis directly from CO2 and H2 over an adequate Raney Cu catalyst. In this conference, we report an unprecedented high catalytic performance for methanol synthesis from CO2 and H2over a new metal-promoted Raney Cu catalysts. 2. EXPERIMENTAL Raney Cu-M catalysts (M=Metal added to Cu/AI alloy) were prepared by leaching of the metal alloys in a moderately stirring NaOH aqueous solution and/or a sodium zincate (Na2Zn(OH)4)/NaOH solution which was prepared by adding ZnO to a NaOH aqueous solution. The concentrations of NaOH were respectively 250 (g-NaOH in 1 kg-H20) for NaOH leaching solution and 300 (g-NaOH in 1 kg-H20) for NaOH-Zincate leaching solution. Leaching in zincate gives zinc deposition on the catalyst. The Raney-Cu catalysts prepared after leaching were washed with distilled water until completely removing Na from the catalyst. The catalysts were stored in distilled water to avoid the re-oxidation of the metallic particles. The total specific surface area of the catalyst after reaction was determined by flow nitrogen adsorption at 77K. The total copper surface area of each catalyst after reaction was determined by the technique of N20 reactive frontal chromatography (RFC) after rereducing the post-reaction catalyst with hydrogen at 523 K [5]. The catalyst fixed in a flow reactor was reduced in H2 at 523 K and 573 K during 2 hours under 5 MPa before flowing CO2 and H2 feed gas mixture (CO2/H2 = 1/3). The reaction products were analysed by adequate gas chromatographs connected to the reactor. The main products of the reaction were methanol, water, and carbon monoxide. Byproducts were dimethyl ether, methane and methyl formate but their selectivities were less than 0.1%. 3. R E S U L T S AND D I S C U S S I O N 3.1. Influence of the metal added to Cu/AI alloy on the activity and the specific activity of Raney Cu catalysts Figure 1 shows methanol synthesis activities and specific activities of Raney Cu-M (1 atomic%) catalysts leached in aqueous solutions of NaOH or NaOH/Zincate under the same reaction conditions. Raney Cu-Zr has exhibited the best activity among the catalysts tested. At the present stage of this study, the reason why Zr has a strong effect for enhancing the activity and the copper surface of Raney Cu is not determined. However, Zr might be very important to enhance the interdispersion of Zn into copper and then allows a higher Cu surface area. On the other hand, for every catalyst tested, the presence of zincate in the leaching solution led to the deposition of Zn on the surface of Cu particles and had a strong effect to improve the activity and the Cu surface area (respectively by 95% and
269
50% for Raney Cu-Zr). The highest activity due to the presence of zincate seems to be partially related to the increase in Cu surface area. Figure 1 shows also another important finding which indicates that zincate doping leads to better specific activity. As it was reported in others 10110 works [6], the increase in copper surface area could be related to ' (a) the formation of smaller copper particles on the surface of Raney copper due to the slower rate of leaching when the zincate is - + present. Therefore, both the nature of precursor alloy and the nature of leaching solution were m 400 found to be key factors in the preparation of high performance Raney Cu for methanol synthesis from CO2 and H2. _
3.2. Role of ZnO contained in the leaching solution for the preparation of Raney Cu catalysts Table 1 shows the atomic composition of Raney-Cu and Raney Cu-Zr 1% leached in NaOH and NaOH-Zincate solutions and after reaction. For both catalysts the presence of zincate leads to a significant deposition of metallic zinc. The amounts of metallic zinc deposited on the surface were 1.1% for Raney-Cu and 1.9% for Raney Cu-Zr (1 atomic%). The effect of ZnO concentration in the leaching solution on the composition of Raney Cu-Zr (1 atomic%) and its activity has been examined by varying the concentration of ZnO from 0 g to 91 g in l kg-H20 (which corresponds to maximum concentration of ZnO). It is clearly shown in Table 2 that the content of zinc metal in the catalyst after reaction increased with ZnO concentration. On the other hand, the Cu surface area and the activity of Raney Cu-Zr
J
~
0 . . . . . . . none Ti
V
Cr
Zn Ga Zr Nb Mo Pd La
Ce
Metal added to a Cu/Al alloy 120
100
=~
(b)
8O
-
-
~6o
,.++
0 II 0
.
.
.
noneTi
.
I
.
V
Cr
Zn Ga Zr Nb Mo Pd La Ce
Metal added to a Cu/AI alloy
Figure 1. Effect of metal added to a Cu/AI alloy on: (a) the activity and (b) the specificactivity of Raney Cu catalysts leached with aqueous solutions of NaOH(20%, II) and zincate (NaOH + ZnO, O). Conditions for leaching alloys : 313 K in N2 Reaction conditions : 523 K, 5 MPa, SV=18,000 h"~,
H2/CO~--3
270
Tabel 1 Compositions and activities of Raney Cu catalysts leached with aqueous solutions of NaOH and zincate(~) Catalyst (alloy)
Leaching solution
Composition of catalyst (atomic %) CulAI 98.6/1.4 Cu/AI/Zn 98.4/0.5/1.1 Cu/AI/Zr 89.5/7.6/2.9 Cu/AI/Zr/Zn 89.9/5.0/2.9/1,9
NaOH Cu/AI (1/2) Zincate NaOH Cu/Al+Zr(1 at"/,,) Zincate
SCu (m21ml)
Activity (g-CH3OH/I-cat.h)
Specific activity (mg-CH3OH/m2-Cu.h)
6.0
239
39.8
7.4
540
73.0
12.7
415
32.7
18.2
814
44.7
(1) Leaching and reaction conditions are the same as those of Figure 1.
Table 2 Effect of ZnO concentration on the composition and the activity of Raney Cu/Zr(1) leached with aqueous solution of NaOH and zincate [ZnO] in a leaching solution (2) (g-ZnO/kg-H20)
Composition (3) (Cu/AI/Zr/Zn) (atomic %)
Sarea (4) (m2hnl-cat)
Scu (4) (m2/ml-cat)
91(a) 84(b) 67(c)
94.6/1.5/1.8/2.1 92.5/3.2/2.3/2.0 93.4/2.5/2.2/1.9
26.1 29.2 28.1
18.8 19.1 19.5
Methanol activity (5) Specific activity (s) (g-CH3OH/I-cat.h) (mg-CHaOH/m2-Cu.h) 931(a) 936 939
842(b) 838 836
44.7 43.7 43.6 38.7
51(d)
93.3/2.7/2.3/1.5
31.4
20.9
923
809
34(e)
-
23.5
16.5
881
798
48.4
27(f)
94.3/2.6/2.1/1.2
12.6
9.4
721
738
78.6
(1) Zr content in CuAI2 alloy was 1 atomic % (2) Conditions of prel~ration : Leaching in an aqueous solution of NaOH/ZnO in the stream of N2 at 323K for lhr before fixing the temperature at 333 K. The concentrations of NaOH were (a) 341, (b) 333, (c) 316, (d) 300, (e) 283, (f) 276 g in lkg-H20. (3) By elemental analysis of the post-reaction catalyst. (3) Surface area and Cu surface area were measured after the reaction at 523 K following the reduction at 573 K. (4) Conditions : reduction : (a) 523 K, (b) 573 K, 2 hrs in pure H2 reaction : 523 K, 5 MPa, SV=18,000 h-1, H2/CO2=3 (5) Specific activity was calculated by dividing the activity (4-b) by Cu surface area (3).
are significantly improved by increasing the ZnO concentration. Therefore, the chemical composition and the texture of Raney copper tested changed by the addition of zincate and increasing the content of metallic zinc deposited on the surface of Cu particles had an important role to improve both the activity and the Cu surface area of Raney Cu catalyst.
271
3.3. Temperature of leaching the alloy Table 3 reveals that leaching temperature influences also the surface area and the activity of Raney Cu catalysts. An optimum range of temperature for leaching the alloy was between 293 K and 333 K. The activity of Raney Cu-Zr (1 atomic %) reached a maximum at 333 K. The highest Cu surface was obatined for the catalyst leached at 323 K. When the temperature was increased between 293 and 333K, the activity and specific activity increased. This is another experimental result showing that there is a strong interdependence between the activity of the catalyst and its Cu surface area. On the other hand, as shown in Table 3, increasing the temperature between 293 K and 333 K leads to the increase in the amount of zinc metal deposited on the surface of the catalyst. However, if the temperature of leaching the alloy is over 333 K, the activity of the catalyst and its Cu-surface area significantly decreased with the increase in the temperature. At high temperature a strong intraction between copper and zinc could affect the dispersion of Cu particles. Consequently, keeping the temperature in a moderate range leads to leach completely the alloy without affecting the physical structure of the catalyst. Table 3 Effect of leaching temperature on the composition and the activity of Raney Cu/Zr(~) leached with aqueous solution of NaOH and zincate Leaching(2) temperature(K)
Composition(3) (Cu/AI/Zr/Zn) (atomic %)
S area (3) (mJ/ml-cat)
293
91.2/4.7/2.311.3
32.0
SCu (3) Methanol activity(3) Specific activity (3) (m2/ml-cat) (g-CH3OH/I-cat ,h) (mg-CH3OH/mZ-Cu, h) 19.0
881(a)
742(b)
38.8
313
93.0/3.3/2.2/1.5
34.7
19.0
900
782
41.0
323
93.4/2.7/2.3/1.7
33.4
21.7
906
801
36.9
323-333 (a)
93.5/2.7/2.3/1.5
31.4
20.9
923
809
38.7
333
93.8/2.4/2.0/1.8
24.5
19.8
927
816
41.2
343
94.3/1.9/2.0/1.8
24.0
18.4
911
817
44.0
353 95.1/1.3/1.9/1.7 23.2 17.3 877 790 45.7 (1) Zr content in CuAI2 alloy was 1 atomic % (2) Conditions of preparation Leaching 9 in an aqueous solution of NaOH(300 g)/ZnO (51 g) in 1 kg-H20 in the stream of N2. (a) 1 hr leaching at 323 K and then the temperature was fixed at 333 K. (3) Same conditions as mentioned in table 2. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
3.4. Catalytic performances of the optimum Raney Cu catalysts and those of a commercial catalyst for methaonl synthesis from CO2 and H2 Catalytic performances of the best Raney Cu obtained (Raney Cu-Zr 1.5 atomic %) were compared with those of the optimum multicomponent catalyst developed in our laboratory and the commercial catalyst tested in the same conditions for methanol synthesis from CO2 and H2 (Table 4). The initial activity (with the catalyst reduced at 523K) of Raney Cu was 30% higher than commercial catalyst and 10% higher than the multicomponent catalyst. In addition, the Raney Cu-Zr (1.5 atomic%) exhibited a higher specific activity which was 57% higher than the commercial catalyst and 25% higher than that of the multicomponent catalyst. These findings clearly indicated that the synergetic alloying between copper and
272
Table 4 Catalytic performances of a typical Raney Cu based catalyst compared with those of an optimal Cu/ZnO-based multicomponent catalyst and a commercial catalyst Catalyst
S area (3) (m2/ml-cat)
Scu (3) (m2/ml-cat)
Methanol activity (3) (mg-MeOWml-cat.h)
Specific activity (3) (mg-MeOH/m2-cat.h)
Raney Cu/Zr (1)
31.2
20.6
941(a)
853(b)
41.4
Cu/ZnOFZrO21AI203
90.7
26.4
845
839
31.8
Commercial catalyst (2)
72.5
34.5
626
591
17.1
(1) Conditions of preparation : leaching of an alloy CuAI2/Zr containing 1.5 atomic % of Zr in an aqueous solution of NaOH(341 g)/ZnO (91 g) in lkg-H20 in the stream of N2 at 333K. (2) Cu/ZnOIAI2Oa (3) Same conditions as described in Table 2.
the metallic components is more exerted in the case of Raney Cu catalyst than in the case of mixture oxides catalysts. 4. C O N C L U S I O N
In summary, this study clearly showed that Raney Cu catalyst prepared in the optimal conditions determined was highly active for methanol synthesis from CO2 and H2. For every Raney Cu-M catalyst, the presence of zincate with NaOH in the leaching aqueous solution led to the deposition of Zn on the surface of Cu particles. The degree of zinc deposition was improved by increasing ZnO concentration in the leaching solution. The presence of Zn metal along with Cu on the surface of Raney Cu catalyst had a strong double effect: to increase the surface area and to enhance the specific activity. Raney Cu-Zr (1.5 atomic %) was the most active catalyst among Raney Cu-M catalysts tested. The initial activity of the present catalyst was significantly higher than that of a commercial catalyst. REFERENCES
1. M. Saito, T. Fujitani, M. Takeuchi, T. Watanabe, Appl.Catal. A: General, 138 (1996) 311. 2. M. Saito, T. Fujitani, I. Takahara, T. Watanabe, M. Takeuchi, Y. Kanai, K. Moriya, T. Kakumoto, Energy Convers. Mgmt, Vo1.36, 1995, p. 577. 3. M.S. Wainwright, Proc. Alcohol Fuels, Sydney, Aug. ,1978, p. 8. 4. M. S. Wainwright, D. L. Trirnm., Catalysis Today, 23 (1995) 29. 5. G. C. Chinchen, K. C. Waugh and D. A. Whan, Appl. Catal., 251 (1986) 101. 6. H.E. Curry, M.S. Wainwright and D.J. Young, in D. Bibby and C.C. Chang (eds) Methane Conversion, (Studies in Surface Science and Catalysis, Vol. 36), Elesevier, Amsterdam, 1988, p. 239.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
273
Global carbon-recycling energy delivery system for CO2 mitigation (I) Carbon one-time recycle system towards carbon multi-recycle system Hiroshi Sano a, Yutaka Tamaura b, Hiroki Amano b and Masamichi Tsuji u "Laboratory Office of Global Energy System, Makioti, Minoo 562 bTokyo Institute of Technology, Research Center for Carbon Recycling and Utilization Ookayama, Meguro-ku, Tokyo 152
I. INTRODUCTION
An energy-shift from fossil fuel to renewable energy in the next century will be a very big issue as well as CO2 control and its management. Considering a serious need for liquid fuels in the coming century, authors proposed a Global Carbon-recycling Energy Delivery System (GCRED) which would be able to supply enough liquid synthetic fuels, even in the condition of a shortage of fossil fuel. The target of the new energy source is solar energy, which exists enormous quantity comparing with a fossil fuel supply nowadays. The carbon source for the synthetic fuels is CO2, or sometime partly fossil fuel. There are two ways for the synthesis of the new fuel: (1) CO2 + solar H2 (from water electrolysis) --, liquid fuel, such as methanol (CH3OH) (2) Coal + solar energy (+ H20) ~ (CO, H2, etc.) ~ liquid fuel where the electric power for the electrolysis is derived from renewable energy1) such as photovoltaic (PV) or solar thermal (ST), and the coal conversion is a solar thermo-chemical reaction such as high-temperature steam gasification. The first case is comparatively simple system, and is often called as "CO2 recycling system" 2) Herein we mainly discuss this system for the estimation of its feasibility. In order to avoid a social shock when the global energy system should transfer from fossil system to the CO2 recycling system using renewable energy, a hybridization between fossil and solar energy should be also discussed. That is, there are various kind of energy sources (solar, fossil fuels and their complex sources), and of carbon sources (CO2 from flue gas of combustion, CO2 from exhaust gas of fossil fuel chemical conversion, and carbon of fossil fuel itself). Fossil and solar hybridization system would be expected as a suitable candidate of sofUanding path from fossil system onto sustainable energy system in future.
274 2. CURRENT FOSSIL FUEL SYSTEM
For comparative estimation of those systems, the first typical fossil fuel system will be discussed, concerned with the including problems. Principally fossil fuel has a serious problem in the resource. Petroleum oil will reach the final stage at the middle of next century3 a), only coal resource is so rich that it can survive ~b) at least in 21st century. I Ic
q
(transportation)
>t
coal /
(combustion) I > ! ~]Energyusell
t__.._~ C02 emission Figure 1 Current fossil fuel system (global) However the coal system inevitably has a high CO2 emission which would contribute global warming. The problem "CO2 emission" in Fig. 1 will be technologically able to be removed by C02 recovery & disposal. But the total consumption of fuel resource would be rather accelerated (Fig. 2, -AE 1 ) that we cannot find any sustainable prospect over there. [! COAL I
(transportation)
> I coal /
(combustion) ~ Energy use [[ : ........ <--" AE1
V
(ocean) < ~ L-CO2 L disposal F"
(C02 recovery)----> (underground) disposal
Figure 2 Fossil fuel system in addition of C02 recovery & disposal -AE 1' process energy for CO2 recovery. At simple coal use (the coal composition - - C H o . 8 O o . l ~ C H o . 6 0 0 . 0 9 ) , the C02 emission index is about 0.84 +0.02(g-CO2/kJ). The index is shown in the equation: CO2 emission index -
ZCO2
ZE
where s is total CO2 emission, and ZE is the energy utilized in the total system. If one want to decrease the CO2 emission index of coal in total system, one way is decreasing ZCO2 by such as CO2 recovery and disposal, and another way would be increasing of ZE in total system. Then we propose to introduce a renewable energy such as solar power which is much abundant although in very remote place from consumer's land, by the energy-conversion into a portable fuel (methanol,etc.). These conceptual designs, that was using CO2 as an energy carrier, had been proposed by RITE 1~ 2 Estimation of sunlight energy resource.
The sunlight energy on the Earth is stepwise decreased after the arrival at the surface of the Earth 3). (Table 1)
275 Table 1 Sunlight radiation power (10 ~~W) place
radiation power 173,000TW 81,000TW 3,000TW 300TW ( if PV efficiency =10%) 10TW
at the outer sphere of atmosphere on the surface of the Earth on the surface of desert area possible PV power on desert cf. energy consumption nowadays
Therefore, the solar energy in a partial use of desert is sufficiently enough to cover the global energy demand theoretically.
3. R E N E W A B L E E N E R G Y C O M B I N E D S Y S T E M W I T H F O S S I L F U E L S Y S T E M (RITE type)
Coal system (Fig. 2) can join with a solar hydrogen system which is very difficult in the global transportation, at the processes of CO2 recovery and fuel synthesis (Fig. 3).
II ~
I
(transportation)
" I coal /
\
[
,
PV
F u e l ] - - ~ CH3OH/
(c~176
.-
i ) ~ Energy use II AE1
(C02 recovery) ~ .... z
(combustion).] > [ "1 Energy usel[II ~C02
emission
Figure 3 Renewable energy combined system with fossil fuel system (RITE type) -AE 1 the same as Fig. 2. [Syn-fuel]: synthetic fuel. (elec.): water electrolysis. The combination of two systems makes the solar energy more easily transportable for overseas, by the formation of liquid fuel with the following conventional synthetic reaction: CO2 + 3H2 -- CH3OH (liq) + H20 In this system of Fig.3, all fossil carbon will be finally emitted. Therefore, it looks like no decrease of CO2 emission, we must check the change of CO2 emission index. The ZCO2 is constant, but the ZE becomes twice for the sake of carbon-double use in the total system. ]~CO2/ZE ~ 0.42g-COz/kJ Strictly speaking, the synthesized energy on 1 mol-C/methanol is not same to the energy on 1 mol-C/coal. The former is larger (over 30%) than the latter, by these equations'
276 CH3OH(methanol)+ 1.502 -- C02 + 2H20 + 725kj CH0.800.1(average coal) § 1.1502 -- C02 q- 0.4H20 + 510kJ Then theoretical ZE in Fig.3 is about (725+510)/510-2.4. Therefore the C02 emission index of total system will decrease to smaller than half of the value of standard coal system in Fig. 1. In practice, there are considerable energy deficits in every process in this system. For example: (1) CO2 recovery process (about 20-25% of coal energy, in recent technology" a) ). (2) CO2 transportation (2-3% of synthesized methanol energy, 1000km by tanker 4a' 6) ) (3) Synthetic process of methanol (about 20% of the supplied hydrogen energy 4 a) ). These process energy deficits cause a ZE decrease, which makes the CO2 emission index larger. Considering both the merit of high calorie of the synthesized methanol and the demerit of the process energy deficits, we can expect about half value of CO2 emission index in Fig.3 comparing with that of Fig. 1. In the middle of the next century, additional severe reduction of the C02 emission index will be necessary. Such reduction wilt demand us a partial recovery of CO2 from the synthetic fuel use.
4.
RENEWABLE
ENERGY COMBINED
SYSTEM WITH DECREASING
FOSSIL
FUEL SUPPLY There are some problems to recover the C02 from synthetic fuel use, although it is technically possible. The synthetic fuel such as methanol or synthetic gasoline are not only clean fuel but high cost fuel. Mainly economical reason, these synthetic fuel will not apply to coal-type electric generation, but mostly to automobile engine fuel. The application of CO2 recovery for such small apparatus is quite unfavorable in nowadays technology.
II coALI
I coal / X L-C02_I Fuel]-q CHOH/ >
I~
PV
(combustion) ~Energy use II ' AE1 ( C 0 2 r e c o v e r y ) <. . . . q A !AE2
(comb>)
q~ Energy use II . . . . . . >~irC02 e m i s s i o n
Figure 4 Renewable energy combined system with decreasing fossil fuel system However, multi-time use of the carbon as the energy carrier is very much effective on the reduction of CO2 emission index. In Fig. 4, if half CO2 of the synthetic fuel can be
277 recycled, the CO2 emission index of the total system becomes 1/4 (about 0.21g-CO2/kJ), owing to the increase of the average carbon recycle-times in the system. Then we can control the CO2 emission index by cutting the fossil fuel consumption or by increasing the gross energy supply, either. In this point of view, a CO2 recovery from synthetic fuel use is quite desirable. How to increase a percentage of CO2 recovery from synthetic fuel use?
It is very difficult to stop the increasing of number of cars. The small size CO2 recovery apparatus for flue gas will be needed but very much difficult. Perhaps only hope will be found in fuel cell generation. In principle, fuel cell is able to avoid air-N2 dilution at the oxidation of fuel on the surface of electrode. However, at the present fuel cells, for example PAFC (Phosforic acid fuel cell) or MCFC (Molten carbonate fuel cell), the residual fuel is finally burned by the already N2-diluted exhausted gas for the heat supply in order to convert the fuel to hydrogen and CO. On the 4) other hand, SOFC (Solid oxide fuel cell) could more easily separate the CO2 recycling gas Much more research and development should be necessary for the recovery of CO2 in the fuel cell system.
5. ULTIMATE CARBON RECYCLE SYSTEM (PURE RENEWABLE ENERGY
SYSTEM) If we succeed to increase a percentage of C02 recovery from synthetic fuel use, finally, we may approach to perfect Carbon-recycle system, without an abrupt change. The ultimate scheme is shown in Fig. 5.
(CO2
[
PV
F u e l ] - - ~ CH3OH/
(comb~
recovery)<
]AE2
~ ~ Energy usell
Fig. 5 Ultimate carbon recycle system (=pure renewable energy system) This system would be feasible, even if all fossil fuels had disappeared. Of course there are several problems. (1) PV high cost, and long energy pay back time (EPT). (2) too remote distance to transport CO2. The average distance from desert to consumer's land is about 10,000km. (3) CO2 recovery cannot become 100% (90-95%, in conventional method). The problem (1) will be gradually solved in the future. Especially, the EPT became already shortened to 2-3 years 5) The problem (2) is partly solved by that L-CO2 tanker may use the return cargo 6) of the
278 methanol tanker. Sometimes as a lucky case in a CO2-rich gas field near desert 7) , we can find CO2 source without transportation. The problem (3). A chemical process in general, 100% yield of recovery will never achieved. In the case of CO2 recovery from flue gas, it becomes very difficult when higher than 95%, considering the process energy consumption. Even in future, about 5% of carbon should be helped from outside of the system. However, this percentage of carbon sources 8~ will be able to supply from the municipal wastes and industrial refuse in future society.
Synopsis CO2 global recycling system using desert solar energy will be hopeful to realise an energy-shift from fossil fuel to renewable energy, and also to CO2 control. The serious needs for liquid fuels in future, will also invite this global carbon recycling delivery system. The new energy source should be a solar energy in desert, which is only rich resource for the fossil fuel alternative in the consideration of the suppliable quantity. The carbon source for the synthetic fuels is primarily the CO2 from flue gas of fossil combustion, but finally, perfect recycling CO2 will be supplied without any fossil fuel. The revolution of global energy system often bring us social shock, but the transfer from fossil system to the CO2 recycling system is sufficiently gradual system-transformation using hybrid systems. All the system can provide a liquid fuel transportable globally. The CO2 emission index (52CO2/ZE) will be gradually decreased from 0.84g-CO2/kJ (simple coal system) to 0.42g-CO2/kJ (one-time C-recycle), 0.21g-CO2/kJ (twice C-recycle), and finally 0g-CO2/kJ (c~-time C-recycle).
REFERENCES 1) H.Sano: Introduction for Energy from Foreign Countries, Sunshine Journal, No.2, NEDO, (1991) 15-22. 2) RITE: Feasibility Study on CO2 global recycling system by using natural energy, NEDO-ITE-9002-5 (1991)173; ibid., NEDO-ITE-9110-2 (1992) 170; ibid., NEDO-GET-9210-2 (1993) 226. 3a) RITE: Handbook of Mitigation Technology for Global Environmental Problems, (1997) 97; 3b) ibid.,110; 3c) ibid.,46. 4) K.Sakai et al.: Kagakukougaku-Ronbunsyu Vol.23, No.2 (1997) 292-295. 5a) RITE and NEDO:The Report of Action Plan for New Earth in the Next Century, NEDO-GET-9527 (1996) 109,189; 5b) ibid., NEDO-GET-9623 (1997). 6) H.Sano, P.Pak and T.Honjou: CO2 Global Recycling System by using Solar Energy, New Energy System Conversions (HESS, JAPAN) Yokohama, Univ. Academy Press. Inc. The Proc. (1993) 491-494. 7) H.Sano: CO2 global recycling system: via CO2-methanol or via CO2-methane?, Energy Convers. Mgrnt. vol.36, No.6-9, Elsevier Sci. Ltd. (1995) 895-898. 8) NEDO: NEDO-GET-9639 (1997) 181.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
279
Oil extraction by hiehlv pressurized CO~_produced in zero emission power plants Mathieu Ph., Iantovski E., Kushnirov V. University of Lirge - Institute of Mechanics Dpt of Nuclear Engineering and Power Plants 21, rue E. Solvay, B-4000 Lirge, Belgium The concept of a zero emission coal-fired power plant as a large source of very high pressure flow of CO2 aimed at oil extraction is presented. The results from a simple model are discussed when pure carbon is considered as the fuel. The issues raised by the use of C02 turbines for high and low pressure expansions are addressed. The key point of the oil extraction process is in the delay between the start of the injection of CO2 and the oil lifting in order to let the supercritical CO2 dissolve all the oil in place. A rough calculation shows that the CO2 produced by a set of new CO2-based power plants with an installed power of 14 GW can be used for the extraction of 300 Mton of oil during 20 years. It is a recommended option in Europe where a lot of oil fields can still be exploited. 1. INTRODUCTION In this paper, we propose a joint solution of the two problems, especially interesting for Europe, namely the elimination of the fuel-fired plants atmospheric emissions on the one hand, and the recovering of the oil remaining in European oil fields on the other hand. This solution involves the fruitful matching of a zero-emission coal fired power plant and of a high-pressure sequestering of the produced CO2 into oil wells. A recent report (Combe et al [13]) indicates a tertiary oil potential for carbon dioxide flooding in Western European reservoirs ranging from 80 to 160 106 m 3. This report also states that the potential application is restricted by the lack of a carbon dioxide source. CO2 methods for oil recovery combined with the reduction of CO2 emissions of power plants are widely discussed in the literature [1-4,6]. The practice of the use of CO2 for that purpose is quite significant, in the USA for example around 125000t of CO2 are injected daily and 16000 t/day of tertiary oil has been produced in 1990 [2]. This CO2 was extracted with a low efficiency from natural subterranean sources. However, techniques of the capture of CO2 from power plants flue gas by means of scrubbers or membrane devices, but with only a partial CO2 removal from the flue gas, are currently available or under development. Instead of reducing emissions of power plants by some 90%, the proposed technology here has the objective of a total elimination of all the emissions, that is a true zero-emission power plant. Instead of increasing the extraction of Original-Oil-In-Place (OOIP) from 30 to 50%, as it is possible by the known tertiary methods, our target is the oil extraction of 90%, as it is achieved with the extraction of natural gas. This is usually called "total oil extraction" in the relevant literature. The basements of our concept come from two sources. First, the well known, published papers, mainly of Amsterdam, Oxford, Kyoto and London Conferences, involving our own papers. The second are less known analysis of the phase diagrams of the oil-gas two-phase systems under high pressure [7,8,10,11]. Because our concept of quasicombined cycle with zero-emission [9] inherently contains the production of a very high pressure CO2 flow, namely 240 bar and more, it is possible to combine the power generation and the CO2 injection without any need of additional pumping. In order to give the background of CO2 use, we quote here a text by Bondor [1]: "If the pressure of the system is sufficiently high (at or above the "minimum miscibility pressure") then the exchange process will proceed until the enriched carbon dioxide mixture is completely miscible with the crude oil. The minimum miscibility pressure varies with the type of crude oil and system temperature, but in general is above 100-150 bar.
280 This developed miscibility process results in a miscible fluid, that is capable of displacing all the oil which it contacts in the reservoir... The efficiency of this displacement is controlled by the mobility (ratio of relative permeability to viscosity) of each fluid. If the displacing fluid (i.e. carbon dioxide) is more mobile than that being displaced (i.e. crude oil) then the displacement will be relatively inefficient. Some of the residual oil saturation will never come into contact with carbon dioxide. Both laboratory and field tests have indicated, that even under favourable condition, injection of 0.15-0.6 103 m 3 of carbon dioxide is required for recovery of an additional barrel (0.16 m 3) of oil". Here our goal is to obtain a mass ratio of CO2 to incremental oil of 1 to 4, on the basis of the Bondor's data. In this paper, we address the two following issues: -the supply of large amounts of high pressure CO2 in Europe. -the technology of CO2 injection and oil extraction, which gives the highest contact of oil and CO2. 2. ZERO EMISSIONS POWER PLANT AS THE CO2 SOURCE Merely half of the European power plants are coal-fired. The associated CO2 emissions are rather small, when compares to the total emissions in the world. There are a lot of schemes of power plants with CO2 removal from the flue gas flow, for example the monoethanolamine scrubbing. The dimensions of this type of scrubber are prohibitively large, due to the high mass flow rates and small concentration of CO2. In addition, the heat consumption of the reboiler in the stripper section produces an efficiency penalty of around 10% pts in a typical coal-fired plant. We do not see any possibility to retrofit economically existing fuel-fired power plants with CO2 scrubbers. The only way we see is to set up the design and the construction of the new zero-emission quasicombined power plants as described in [9,12]. Fig. 1 shows an improved version of the power plant flowsheet as compared to the previous one given in [9, 12]. Here we consider that the fuel of the quasicombined cycle can be identified to pure carbon. The latter represents coal more or less correctly. As hydrogen is not present in the considered fuel, the calculations can be carried out using the best known tables of CO2 thermodynamic properties [5], which include the region of CO2 condensation. Here the oxygen is separated from air and is injected in liquid state into the CO2 flow before the recuperators. It is hence preheated along with CO2 and works in the high pressure turbine expansion as well in the combustion chamber, the complete combustion of pure carbon in the mixture COJO2 produces some additional percents of CO2 in the flow. The compressor and turbines are schematically represented with only one stage. Actually, the CO2 compressor has 6 stages with intercooling and partial recuperation of the compression heat. The expander has two stages with an intermediate combustion chamber, not shown in the picture. All these features have been taken into account in the calculations. 3. MASS AND ENERGY BALANCE Assuming complete combustion, one has: C + 02 ::~ C02 + 393137 kJ/kmol
The heating value of the fuel C is 393137/12 = 32.76 MJ/kgC. For a typical coal this quantity is 29.3 MJ/kg of fuel so that the assumption to identify coal with pure carbon is acceptable. The two most important temperatures, tf = t d (atter the recuperators) and t h (at the expander inlet) are taken as: tf = t d = 600 C and t h = 1300 C. These temperatures are currently achieved in the boiler and gas turbine practice. The lower cycle temperature in the cooling tower t m is 25 C. The pressure limits are: Pb = 280 bar (the upper limit in the table) and atter the gas turbine expansion, Pi is 1 bar. The intermediate pressures are 60 and 8 bar, at the inlet of the first and second expanders. Now, the total cycle on the t-S diagram is calculated (fig.2). The line b-d actually corresponds to the mixture of recirculated CO2 and oxygen without fuel mass. However, due to the smaller molecular mass of oxygen, the specific heat capacity of the mixture is only a little bit less than of the total fluid was CO2. The mixture CO2/O2 is assumed to be totally equivalent to a CO2 fluid as a first approximation. The isentropic efficiency of the first, high pressure expander is 0.80, that of CO2 compressor is 0.80, and that of both lower pressure expanders is 0.85. From energy balance, one has :
281
M [Oah - he)- (hh - hf)] = m c H c
where M is the mixture mass flow rate and m e that of carbon, H e is the low heating value. Hence the carbon mass fraction mc/M is 0.0404 and the oxygen mass fraction m 0 is 0.0404.32/12 = 0.1077. The fluid entering the first combustion chamber consists of 10% of oxygen and 90% of carbon dioxide. The mass flow rate of the CO: produced in the combustion process is 0.1481, that is about 15% of the total flowrate. This flow has to be deflected and injected in the oil wells. The cycle key points are presented in the table: point a b c d e' f h' e" h" i k 1 m
t,C 25 62 75 600 435 600 1300 930 1300 965 790 100 25
P, bar 60 280 280 280 60 60 60 8 8 1 1 1 1
S, kJ/kgK 3.36 3.36 3.50 4.86 4.92 5.91 5.95 6.30 6.39 5.051 4.85
h, kJ/kg 576 616 650 1398 1204 1412 2252 1819 2303 1852 1638 874 806
The compression work of the CO2 compressor equals the sum of 6 enthalpy rises in each stage, starting from t m = 25 C. The maximal temperature increase in the intermediate stages is 82 C, which is enough for a partial recuperation. The total compression work amounts to 265 kJ/kg. The CO2 pumping work (from 60 to 280 bar) is : Wp = (1/rip) AP / p = 40 kJ/kg. The compression and pumping works are 265 + 40 = 3135 kJ/kg. If, instead of CO2, we used an ideal gas with the same gas constant R, the isothermal compression would be 9 Wid = (1/rip) RT ln(Pb/Pm) = 440 kJ/kg. Due to the actual non-ideality of CO2 and especialy near the saturation line, a reduction of the compression work of 135 kJ/kg is obtained. In addition, CO2 is pumped in liquid state. The oxygen production in the ASU (0.2 kWh/kg O2) needs 77.5 kJ/kg CO2. Pumping 02 from 1 to 280 bar with an effectiveness of 0.6 is consuming 4.5 kJ/kg. The total oxygen power consumption is 82 kJ/kg CO2. It is much less, than the mentioned benefit in CO2 compression (135 kJ/kg). By definition of the cycle efficiency, one has: r 1 = 194 + 433 + 451 - (265 + 40 + 82) = 0.522 840 + 484 The cycle is not optimized yet, especially with respect to intermediate pressures. Also, the expanders effectivenesses might be higher than those used in the calculation (0.85). That means, that some potential still exists for efficiency increase which could offset some small losses, not taken into account. The energy balance in the recuperators is fulfilled when the recuperator effectiveness erec is taken equal to 0.98. Indeed, the available heat is : (h i - hi) 6rec = (1852 - 874) 0.98 = 958 kJ/kg
282
whilst the needed heat for the preheat of the CO2/O2 mixture is: (hf- he) + (lad - he) = 956 kJ/kg 4. THE CO 2 PRODUCTION BY A ONE GW POWER PLANT The net work being 691 kJ/kg, the total CO2 flowrate for a one GW power plant is 106 / 691 = 1447 kg/s. The CO2 fraction to be injected is 0.1481.1447 = 214 kg/s. If the power plant is operating 5500 hours in a year, the annual CO2 production amounts 4.3 Mton at a pressure of 280 bar. 5. THE HIGH PRESSURE CO2 SEQUESTERING IN A WELL OF OIL FIELD The aim of the present concept is twofold, namely zero-emission power generation and oil extraction. These two actions are separated by a significant time span, which depends upon the amount of oil to be extracted and of the local pressure. First, when an appropriate oil field is found, all the wells should be used as sites for CO2 injection and sequestering. The CO2 flows at a pressure of 280 bar in a special pipe at the ground surface. At the bottom of the well, CO2 will experience higher pressures of 350-550 bar due to gravity at the depth of 1000-3000 m. Therefore if CO2 and oil are in full contact, we can expect the total dissolution of oil into the CO2, as show experimental proofs of the fate of the gas-oil mixtures under high pressures [8]. Experiments have been carried out with hydrocarbon gases, however the applicability to carbon dioxide looks easy, because CO2 dissolves oil better. The behavior of gas-oil mixture in the highpressure bomb Alsthom-Atlantique, with a volume of 4140 m 3 and a maximum pressure of 1200 bar, at a constant temperature of 100~ is represented in figure 3. The phase diagram is shown in the coordinate system P-F (P = pressure, F = mass ratio of gas to oil). The lines with constant percentage represent the percentage of oil dissolved in the gas. The upper line shows the boundary, above which, on the right of the critical point, the dissolution of oil in the gas is complete. We see that this process needs more than 450 bar, however when the gas is CO2, this pressure might be lower. The delay to the beginning of oil production is significant, a few years. It is not detrimental for the power plants. In due time, when the oil dissolution is observed to be total by appropriate means of observation of the wells, the extraction is started. At the surface CO2 is separated from oil, recompressed and injected back into the empty reservoir and this time forever. The amount of recoverable oil in Western Europe could consequently be two times greater than the figures given by Combe et al. to our mode of operation with CO2 injection during a long time without extraction. The round figure of 300 Mton of oil is realistic. If the CO2/oil ratio equals 4, which we consider as a minimum, the capacity of the oil fields in Western Europe for CO2 storage is 1.2 Gton. It is enough to store the CO2 emissions of all the German power plants (255 Mton/y, [14]) during 5 years. 6. CONCLUSION The concept, presented in the paper, forecasts a big profit when using new coal-fired power plants without any atmospheric emissions and injection of high pressure carbon dioxide, produced in the plant, into old oil fields for almost total oil extraction. With a capacity of 14 GW in Europe it is possible to extract 300 Mt of oil. The concept is undoubtedly worth being funded for further studies for the design of a demonstration plant. 7. NOMENCLATURE M = total flowrate of working substance, (kg/s) m c = carbon flowrate m 0 = oxygen flowrate h = specific enthalpy, (kJ/kg) S = specific entropy, (kJ/kg.K) p = density, (kg/m 3) 11 = efficiency
283
8. REFERENCES 1. P. Bondor, Applications of carbon dioxide in enhanced oil recovery energy conversion mgmt v. 33 No 5-8, pp 579-586, (1992) 2. R. Bayley, McDonald M., Capture and use COs for EOR in Western Canada. Energy convers. mgmt v.34, No 9-11, pp 1145-1150, (1993) 3. M. Todd, Grand G. Enhanced oil recovery using carbon dioxide. Energy convers, mgmt v.34, No 9-11, pp 1157-1164, (1993) 4. P. Tontiwachwuthikul, New feasability study of carbon dioxide production. Energy conversion mgmt V.37 No 6-8, pp 1129-1134, (1996) 5 V.V. Altunin, Thermodynamic properties of carbon dioxide. Moscow; Standard, (1975) 6. G. Moritis, EOR dips in US but remains a significant factor OGJ Special, Sept. 26, pp 51-73, (1994) 7. V. Kushnirov, Retrograd liquid-gaseous entrails systems. Tashkent Fan, (in Russian), (1987) 8. V. Kushnirov, Printsev A. new experimental data on the aggregate state of gaseous and liquid hydrocarbons under high pressure. Doclady Acad. Sci. of the Uzbekistan Republic No l, pp 41-42 (in Russian), (1993) 9. E. Iantovski, Stack Downward. Zero-emmission fuel-fired power plant concept. Energy convers, mgmt V.37, No 6-8, pp 867-877, (1996) 10. V.N. Martos, et al., The features of the phase state of liquid-gaseous systems. Oil and gas geology No 10 (in Russian), pp 27-28, (1995) 11. T.P. Juze, The role of compressed gases as solvents. Moscow, Nedra (in Russian), p. 165, (1981) 12. E. Iantovski, Energy and Exergy currents. NOVA Sci Publ. NY p.180, (1994) 13. Combe et al., EOR in Western Europe. 5th European Symp. on Improved Oil Recovery, Budapest, Apr. 25-27, (1989) 14. Umweltpolitik. Klimschutz in Deutschland. Eine Information des Bundesumweltministeriums, s.143
l
e'
_1
0
t
d
c
h
P
1
z Nz
1
,
12
,
1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)
Air separation unit Combust. chamber Recuperator Coolingtower CO2turbine HP CO2turbine LP (*) CO2 compressor Generator CO2 condenser CO2 treatment unit Pumps Well Cyclone separator
(*) It might be doubled with reheat as in the cycle diagram on fig. 2
L|qCO= fig. 1 Zero emission quasicombined cycle power plant outline
284
1400
1200
h'
-
C02 (pure)
h"
/~
g~/~
1000
/
^
,
,
/
800
600
400
200
3
35
4
4.5
5
55
6
6.5
S, kJ/(kg.K)
Fig.2 Quasicombined cycle t-S diagram on pure CO2, bcde'opa=topping Rankine-hke part, oe'fh'e"h"ilmo=bottoming brayton-like part
P,bars 40C
300
20
I
,/
30
10( 1Yo ,
I
,
0.3% I
,
,
I
2 2 6 8 ....10 1"2 14 46 fig.3 Phase diagram of gas oil mixture by 100~ (F=ratio gas/oil by mass in the bomb).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
G l o b a l c a r b o n - r e c y c l i n g e n e r g y delivery s y s t e m for CO~ m i t i g a t i o n
Fossil/solar energy hybridization system for utilization of carbon as solar energy carrier
285
(III)
Y. Tamaura a, M. Tsuji a, H. Amano a, and H. Sano b aTokyo Institute of Technology, Research Center for Carbon Recycling and Utilization Ookayama, Meguro-ku, Tokyo 152, Japan bLaboratory Office of Global Energy System, Makioti, Minoo,Oosaka 562, Japan
Scenario to start-up global carbon-recycling energy delivery system (GCREDsystem) has been studied from a point of fossil/solar energy hybridization using solar thermochemical process. Construction of the solar farm of SCOT (Solar Concentration Off-Tower; central receiver beam-down configuration) is the key step to launch the project. The solar-assisted coal gasification system with the SCOT-solar farm can produce solar methanol, whose cost is competitive to the present gasoline cost. For further reduction of the CO2 emission, the coal for production of solar methanol should be gradually replaced by the CO2 recovered at fossil-fired power plant (shift to GCRED-system). In this stage, the solar-assisted coal gasification reactor with the SCOT-solar farm is gradually used for natural gas decomposition reaction and the 2-step water splitting reaction using metal oxides to produce the solar H 2. 1. I N T R O D U C T I O N Transportation of the solar energy from the remote sunbelt prefers liquid or gaseous hydrocarbon fuel t o H 2 and electricity[i,2]. In the ultimate stage of the global carbon-recycling energy delivery system (GCRED-system), carbon is recycled as a solar energy carrier between energy consuming sites and sunbelt (CO2 is converted to solar fuel), where solar H 2 is indispensable to convert CO2 to solar fuels (solar methanol or methane)Ill. The solar H 2 by PV electrolysis would be a candidate, but transformation of the solar electricity (PV) t o H 2 is exergy downgrading process. It is preferable to produce the solar H 2 directly from solar energy by solar thermochemical process (STC). Also, we have to consider the economic point that solar fuels from sunbelt should be commercially competitive to fossil energy. As a starting technology for GCRED-system, production of solar fuel by direct hybridization of fossil and solar energy using a solar thermochemical process (STC)[2-6] is conceivable.
286 In this paper, we have studied the solar methanol production by a solar-assisted coal gasification system, which will be able to start-up GCRED-system, from a point of fossil/solar energy hybridization using solar thermochemical process (STC).
2. SOLAR-ASSISTED COAL GASIFICATION FOR SOLAR METHANOL PRODUCTION AND COST ANALYSIS TO START-UP GCRED-SYSTEM The primary target material for exergy-upgrading hybridization process will be coal which is an abundant and cost-effective fossil energy. The solar-assisted coal gasification is an exergy-upgrading process. Coal gasification is highly endothermic and energy-intensive process; solar methanol produced from the syngas carries a 26% portion of its chemical energy as a solar energy[7-8]. In this solar-assisted coal gasification process, solar energy conversion efficiency is expected to be around 2040% (theoretical conversion efficiency is 70-80% at 1000sun by solar thermochemicl process)[ 8-9]. Figure 1 shows the system for solar-assisted coal gasification, which has been studied in the present paper. In the system of Fig.l, coal is gasified using the heat generated by direct irradiation of concentrated solar energy, and the produced gas (syngas) is fed to steam turbine to recover the heat and to convert electricity. Then, the syngas is converted to methanol(Fig.I). A fossil fired combustor is operated to compensate for insolation fluctuations (mainly operated using concentrated solar energy).
(SCOT Solar farm) Concentrated solar energy Coal]direct H20 [radiation
~
<'x. (Solar reactor)
x,~
j
.~
~
/ / I (Combustion')
heat ]~chamber J /
/
J
heat Iheat X,~as-~gas "'~ .~,x
~" "ethanol"l-" [,lvl ~"
/
compensation for insolation fluctuation Fossil I fuel /
CO, ~ . f H2
Steam "N~ turbine~
N . . . ~ f~lectri city') ~J
Fig. 1 Schematic diagram for solar-assisted coal gasification
287
Tower
cPc
ilf
.
-.
: .'.:'.~ t-C 2 ~ , ~ ....:;'_.!-iic.~.?,,:'-;::::..... :; -::."!:i:~D(;!;:i'7)i;:~i$';!
Fig. 2
SCOT-solar farm (Solar Concentration Off-Tower) [10-11]
W e h a v e made cost analysis for the solar m e t h a n o l production for the system of Fig. 1. In this analysis, SCOT-solar farm (Solar Concentration Off-Tower; central receiver b e a m - d o w n configuration) is used for solar concentrating system(Fig.2). This solar concentrating system has an economical advantage, since the heavy chemical plant can be installed on the ground. Since the high temperature of 1000-1200~ is obtained by the SCOT-solar farm, chemical plant (or reactor) for solar-assisted coal gasification can be operated. Table 1 shows estimated i n v e s t m e n t cost Table 1. I n v e s t m e n t c o s t for S C O T s o l a r farm c o n s t r u c t i o n 500m x 600m farm (Reflection Tower = 1) Subsystem
Reflection Tower
Subsystem
Cost (MS)
SCOT solar farm
32
Solar gasification
25
22
boiler, turbine
35
4
Methanol plant
30
Cost
(MS)
reactor (16t/h)
Heliostat
(2.5milion yen/m2)
Table 2. I n v e s t m e n t c o s t for s o l a r a s s i s t e d coal g a s i f i c a t i o n s y s t e m
CPC
6
Total
32
Total
122
only for construction of SCOT-solar farm (500mx600m) (heliostat, reflective tower), based on the work by Kribus et al. [10-11] and Spiewak[12]. Table 2 lists the i n v e s t m e n t cost estimated for the system of Fig. 1 (solar-assisted coal gasification). In the cost analysis of Table 2, solar reactor for the gasifier is assumed to be operated by a salt m o l t e n process. This type of reactor has an advantage of an easy
288 startup for the daytime operation (Fig.3). To simplify the calculation, we assume that the SCOT-solar farm is operated for 5 h at 1 k W / m 2 insolation. If the solar energy is converted to chemical energy by the solar-assisted coal gasification with an efficiency 40% (theoretically 75-80%), 85t/day of coal can be gasified and 410kl/day of methanol is produced. The estimated cost was 0.6-0.8S/gallon methanol. This figure shows that the solar m e t h a n o l produced by the solar-assisted gasification is equal to that by the conventional process. Also, this is competitive to the gasoline (1.0-1.2S/gallon). These results show that the solar methanol production by the solar-assisted coal gasification with SCOT-solar farm can be operated in the commercial base. The cost of the solar methanol produced from H 2 by PV is estimated to be 8-10 times higher than that by the conventional process[13].
'-
SECONDARY
// TRANSPARENT
PI-L~,SE
Fig. 3 Solar-assisted coal gasification reactor with a salt molten process [14] The system of Fig. 4 is the alternative of the system of Fig.1. In this system, coal is pre-treated by liquefaction process, and the coke is used for the gasification of the system of Fig. 1. Also, two kinds of liquid fuel of oil and methanol are produced. The H 2 required for liquefaction can be supplied from the gasification of coke. This system will take an important role for production of syn-oil, w h e n oil is shortage. In this system with SCOT-solar farm (500mx600m), we assumed that 180t/day coal is treated, and that 90t/day oil (conversion efficiency =50%) by the liquefaction process and 410kl/day of methanol by the solar-assisted coke gasification are
289 produced. Since this capacity is close to the NEDOL process, the oil cost would be 20-305/bbl, which is estimated for the NEDOL process(15). This cost is slightly higher than the present oil cost (17-20/bbl). The solar methanol was estimated to be 0.7-0.9S/gallon, assuming that the investment cost for liquefaction plant is 20MS/one SCOT solar farm (one liquefaction plant of 6000t/day is assumed to be constructed against about 100 farms (one farm=500mx600m) of solar-assisted coke gasification plants). Several other chemical reactions involving natural gas and CO 2, besides coal, can be also adopted to this system. By selecting those chemical reactions depending on cost-effectiveness, oil shortage, carbon-tax introduction, and the international implementation on CO2 mitigation, we could operate the systems which meet minimizing technological and economical barriers to shift it to the ultimate stage of GCRED-system (soft landing scenario from fossil energy to solar energy).
H2 by Solar/coal gasification Coal
pipe 1 ~ / / ( Liquefaction~ plant (L)
Solarhybridsyn-oU (fuelfor power station)
Coke ~ fr
/i
Solar Energy
Fig. 4.
one~_ portion--
\
J
(Production of solar methonol"1 ~ (TransportationSector ~nergy Fuel)
Solar farm syngas (H2 + CO) S~ (solar furnace) 1 Coke Gasification
~~.
~
(G)
(L/G)plant number ratio = 1 91000
Alternative of the system of Fig.l, where coal is pre-treated by liquefaction process, and the coke is used for the gasification of the system of Fig. 3.
3. SOLAR H Y D R O G E N P R O D U C T I O N BY STC FOR GCRED-SYSTEM During the shift to the ultimate stage of GCRED-system, the coal will be gradually substituted with the CO2, which could be recovered at the solar fuel consuming site. In this sense, we may say that the solar and fossil-energy hybridization system with STC will take an important role in starting-up the GCRED-system For further reduction of the CO 2 emission, the coal for production of solar methanol by the systems of Figs. 1 and 4 should be gradually replaced by the CO2 recovered at fossil-fired power plant (shift to GCRED-system).
290 At this stage, CO2 is partially recycled and GCRED-system will be smoothly started. But, to replace the coal with the recovered CO2, solar H 2 should be produced by another STC process. Natural gas decomposition reaction and the 2-step water splitting reaction using metal oxides[3,10-14] are candidates. The solar-assisted coal gasification reactor with the SCOT-solar farm is gradually used for natural gas decomposition reaction and the 2-step water splitting reaction using metal oxides to produce the solar H 2. In the solar H 2 production farm, the produced amount of the solar methanol will be reduced to 1/4, since the solar share in the solar methanol is increased from 26% (coal) to 100% (recovered CO2). This will result in the increase in the solar methanol cost by 4 times, 0.6x4= 2.45/gallon methanol (This cost dose not include the cost for CO2 recovery). Thus, the solar methanol produced by 100% of solar energy (solar H2) and the recovered CO2 can not be competitive to the present cost, even though it is produced by solar thermochemical process using the SCOTsolar farm. This suggests that there is an optimum where the solar methanol cost is competitive and the coal is replace by the solar H 2 with a m a x i m u m solar share.
REFERENCES ,
2. 3. ,
5. 6. 7. o
9. 10. 11. 12.
13. 14. 15. 16. 17. 18.
T.Asami, S.Maezawa, S.Niwa, Y.Yanagisawa, IEW/JSER'96, Osaka (1996) 53. Y.Tamaura, M.Tsuji, IEW/JSER'96, Osaka (1996) 59. Y.Tamaura, Int.Workshop on High Temp Solar Chemistry, Switzland, (1995) 43. A. Steinfeld, R. Palumbo, Sun at Work in Europe (1997), 12. Y.Tamaura, M.Tsuji, K.Ehrensberger, A.Steinfeld, Energy, 22 (1997) 337. M.Tsuji, Y.Tamaura, IEEW/JSER'96 (1996) 65. Y.Tamaura, M.Tsuji, Proc. of Symp. on CO2 Fixation, (The Chemical Society of Japan), Tokyo (1996),pp6-7, "Report on Global Carbon Recycling System", (RITE) (Aug. 1997), pp.20-32. P.Kesselring, "Highflux Disch-Solar Reactor" by Deutsche Forschungsanstalt fur Luft und Raumfahrt, (1994), pp4-14, June. A.Kribus, A.Segal, R.Zaible, D.Carey, S. Kusek, ECE/WEC/ UNESCO/ MOST Workhsop on the Use of Solar Energy, Bet Berl, Israel (1995). Solar Chemistry News, 1,(2) winter(1996). I. Spiewak, "Economic Analysis of the Co-Production of Zinc and Synthesis Gas Using Solar Process Heat" (June, 1997), Report by W e i z m a n n Institute of Science. "Report on R&D for CO2 Fixation and Utilization Using Catalytic Hydrogenation" (NEDO) (1993), p.799. M. Epstein, Int.Workshop on High Temp Solar Chemistry, Switzland, (1995),86. H. Saruhashi, Journal of Denki Kyokai, (1997), 17. Y.Tamaura, A.Steinfeld, P.Kuhn, K. Ehrensberger, Energy 20, (1995), 325. A.Steinfeld, P.Khun, and Y.Tamaura, Energy Convers. Mgmt., 37, (1996),1327. A. Steinfeld, P. Kuhn, A.Reller, P.Palumbo, J.Murray, Y.Tamaura, Hydrogen Energy Progess XI, (1996), 601.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
291
Review of measures to mitigate carbon dioxide emissions in the Slovak Republic and modes of utilisation Ane~ka Moncmanova Department of Environmental Sciences, Faculty of Chemical Technology, Slovak University of Technology, Bratislava, Radlinsk6ho 9, 812 37 Bratislava, Slovak Republic 1. INTRODUCTION The Slovak Republic (SR), as one of the countries in the process of the EU accession is harmonising its policy with the other EU member countries. In the field of the environment protection the SR has accepted requirements for energy saving and for enforcement of measures to reduce the greenhouse gases emissions. Paralelly, in compliance with the Framework Convention on the Climate Change the SR is committed to reduce its CO2 emissions by 20% during 1988-2005 [1], which represents approximately 12 Tg of CO2. 1.1. National Conditions The Slovak Republic is located in the heart of the Europe, with area of 49036 km2, 50% of which is agriculturally utilised area, 41% is covered with forests, 2% is free area and 3% is urban area. Slovak Republic has currently 5.39 million inhabitants. The Slovak Republic is an industrial country. Industry and construction contribution to GDP reached approximately 52% in 1991. The:economy of Slovakia has high demand for raw material and energy, but it is not a country with large raw material resources. 1.2. Contributions to the anthropogenous emissions of C02 The Slovak Republic share on the global anthropogenous emissions of greenhouse gases is approximately 0.2%, CO2 emissions represents the largest portion thereof. With the annual CO2 emissions per capita (11 tons in 1990) the SR belongs among the first 20 countries with the largest emissions per capita.[ 1] The CO2 emissions significantly decreased during 1990-1995 reaching 42.9 Tg in 1995. This amount is lower by one third than in 1990. In this period the Slovak economy was facing problems related to the transfer from centrally planned to free market economy. The transformation process caused significant decrease of industrial production, which was before the transformation characterised by high portion of heavy industry using energy consuming technologies. This explains the radical CO2 emissions reduction. The scenaria estimate the CO2 emission absolute volume for the year 2005 to be within the range 45-51 Tg. Table 1 summarises the anthropogenous CO2 emission projection till the year 2010 for the scenario, which assumes significant innovation and restructuring of industrial technologies, resulting in decreased energy demand.
292 Table 1" Projection of anthropogenic CO2 emissions in Tg CO2 Year ........ 1990 Energy consumption . . . . . Energy and transformation 12.673 Energy consumption- Industry 25.242 Energy consumption- TransPort 5.108 Energy cgnsumption - Other sectors .... 13.92.3 Total energy consumption 56.946 Industry- non-energy CO, emissions . 3.167 60.113 Total COz emissions .
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L.
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1995 . 17.365 13.229 ~4.72.2 4.816 40.132 2.769 42.901 .
.
.
.
2000
2005
2010
17.728 19.016 13.847 14.172 ~ 5.760 7.345 4.303 4.549 41.883 44.837 3.439 2.769 44.652 48.276 .
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.
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,
,
,
. . . . .
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21.079 14.036 8.263 4.194 47.572 3.930 51.502 .
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2. COz MITIGATION P R O G ~ S High energy demand of the Slovak economy resulted from its specific development during the last 50 years. Industry had consumed more than half of the produced energy and 63% of generated electricity. The largest consumption was in metallurgy (production of aluminum, iron and steel), chemical industry including production of construction materials. In preparation of optimal CO2 mitigation alternatives attention is therefore paid to the energy saving and effective utilisation of energy in these sectors. The CO2 reduction programmes include relevant legislative, economic and technical measures. One of the most important already enforced legislative tool to reduce the CO2 emissions is the Act on the Air Protection Against Pollutants (the Clean Air Act), introduced in the year 1992. Though the requirement to reduce the CO2 emissions is not explicitly stipulated in this Act, it contains a general provision which requires for the new and reconstructed facilities the best utilisation of available technology, maintaining acceptable investment and operational costs. Regulations to this Act set mandatory emision limits, which are harmonised with the relevant EU standards for the fuel combustion. The impact is reflected in the change in consumption of fuels as the primary energy sources towards utilisation of fuels producing less of CO2. Currently there is the Act on Energy Management prepared, aimed to stimulate development of programmes supporting energy management and increase of energy efficiency. Economic tools already applied include charges, paid by each source operator for generating emissions. There are no emission limits set directly for the CO2, though its generation is influenced indirectly through charges for other pollutants. From other applied economic tools the following should be mentioned: the consumer tax on carbohydrate fuels and oils and the road tax. Prepared economic tools expect to establish the State Fund of Energy Saving and utilisation of alternative energy sources. This include also the Energy Saving Fund to provide financially attractive loans to support small and medium size investment energy saving projects. Tax relief to support the energy saving projects are expected to be introduced as well. The carbon tax shall be introduced in future to be already harmonised with the EU measures. Planned and already implemented technical measures include: * reconstruction or replacement of energy sources with higher energy efficiency sources
293 9 fluidised bed combustion which increase energy efficiency of electricity and heat sources 9 change of coal and heavy heating oils to the natural gas 9 implementation of steam-gas cycles, leading to increase effectiveness in combined electricity and heat generation 9 wider utilisation of renewable energy sources, mainly hydro potential and completion of new nuclear power plant (4x400 MWe) [2,4]. In the transport sector the attention is paid to checking of vehicles, limiting ineffective transport within the city agglomerates, introduction and development of combined transport, preference is given to the electrical power to diesel in the railway transport and utilisation of alternative fuels. 3. CARBON DIOXIDE REDUCTION AND UTILISATION IN INDUSTRY Direct CO2 emissions in the industrial sector include the CO2 generated in the fuel burning directly in production (energy-related CO2), and non-energy emissions produced in the technology itself. Indirect CO2 emissions are related to the electricity consumption in production. These emissions are not included to the industrial emissions, but are accounted for in the energy sector contributions. Direct emissions from industrial processes decreased during 1990-1995. In 1995 these reached approximately 16 Tg. This decrease was influenced mainly by the energy related CO2 reduction, resulting from industrial restructuring and recession were the most important ones. Slight increase of the energy-related CO2 is expected to be reached in future development till the year 2010, but their relative portion on the total CO2 emissions shall be decreasing. Taking into account a real assumption, that these emissions are proportional to the energy consumption in production, the main effort to reduce these shall be focusing to decrease energy demand of productions and to increase the energy efficiency. Aluminum production can be given as a good example, where the current efficiency (CE) increased from 80% to 92%. Good results has been reached also 'in the cement manufacturing, where the fuel efficiency in kiln systems increased as well. The US reports give significant progress in energy savings by the cement industry (nearly 40% since 1972).[5] Non-energy CO2 emission development trend differs to the energy. Though these emissions don't contribute significantly, their relative portion on the total CO2 emissions shall be gradually increasing to 7.0-7.5% till the year 2010 comparing with the year 1990, when this reached approximately 5%. In absolute amounts this is higher approximately by 1/3. Mainly the cement and lime manufacturing productions, the steel iron and aluminum productions and production of some chemicals are generating non-energy CO2 emissions. Cement production is one of the largest industrial productions as far as the production volume is concerned, aluminum produ~ion belongs to the most energy demanding. These technologies are good example to present some rational measures resulting in the CO2 emissions decrease.
3.1. Potential Reduction of CO2 Emission in Cement Manufacturing. 3.1.1. The Portland Cement Manufacturing Contributions to the CO2 Emissions The portland cement is the most common part of the construction materials. The Slovak Republic annual production of the Portland cement is 3.5 million tons (data for the year 1996). Amount of the CO2 emissions generated during its production are the same. Cement
294 production includes several operations in which direct and indirect CO2 emissions are generated. It consists of operations characterised by the electricity consumption (quarrying, crushing, raw grinding, raw mix preparation, clinker grinding, cooling, storing), consumption of carbon fuels or carbon waste used in the pyroprocessing operation. Non-energy CO2 emissions are generated in the decomposition of carbonates contained in the raw mix. It is estimated, that the CO2 generated from the fuel combustion and raw mix is nearly 1 ton per ton of product. Thereof almost 0.6 tons of CO2 per ton of clinker released by calcination of the calcium carbonate in raw materials. 3.1.2. Reduction of Carbon Dioxide Generation in Manufacture of the Portland Cement
There are several possibilities to reduce the CO2 emissions in the cement production. Research and development in the Slovak Republic in this field mainly concentrate on: * improvement of the energy efficiency in the pyroprocessing * reduction of emissions in the clinker production, possibly to be gained by: a) decreasing the carbonate content in the raw material b) decreasing the clinker content in the cement * utilisation of waste in the pyroprocessing (this will not reduce the direct CO2 emissions, but global emissions). Improvement of the energy efficiency in the pyroprocessing is reached by increasing the fuel efficiency of the kiln for example by replacing the wet kilns with preheater and precalciner systems, resulting in significant reduction of the CO2 emissions. Utilization of different blended cements leads to the highest reduction of the CO2 emissions during the cement production. This is realised by replacement of a portion of clinker in cement by granulated materials. Blast-furnace slag, fly ash with accelerators were used as additive materials. Practical decrease of the CO2 emissions from both fuel and raw material can reach a reduction by 50% or even more [5]. Utilization of industrial by-products, replacing the high portion of limestone in the raw mix with certain types of slags or other materials reach on calcium may reduce the CO2 emissions up to 50% in addition to reducing fuel consumption by 10% to 20%. Several materials are being subject to research currently, such as the steel making slags, electric furnace slag, slag from the industrial waste incinerators, brown mud, red mud from at!uminia plant [5]. The CO2 emission reduction from both the fuel combustion and calcination in intergriding clinker with extenders is directly equal to the amount of the extender. It is proved, that mixing moderate amount of limestone with clinker up to 10% has not detrimental effect, but some characteristics of cement are improved. Therefore there are several countries, allowing mixing up to 5% of limestone to portland cement. Applying 5 % of limestone, which replaces the clinker in cement shall reduce CO2 emissions by 0.05 ton CO2 per ton of cement [6]. Changing the raw rnixfor the clinker production, e.g. producing clinker of adequate quality with reduced amount of lime. Though these changes impact the COz production only slightly. For example reducing the tricalcium silicate content of clinker from 60% to 20% a proportional increase in dicalcium silicate shall be the CO2 of the raw mix reduced from 35.3% to only 34.1%. Emissions from calcination are thus reduced by approximately 3.5% and from the fuel burning by additional 1-2%. [6]. Waste fuels are m the pyroprocess used practically in each Slovak cement works. Good results were gained during burning the car tires and organic waste.
295 Other possibilities to reduce the non-energy C O 2 emissions in the cement manufacturing are researched, for example by preparing a mix based on monocalcium aluminate with significantly lower CaO content, and respectively less carbonate in the raw mix. These cements, known as the calcium aluminate cements, may be used only for specific purposes. Group of materials, so called geopolymers may be a new trend in the mineral binders. These materials develop strength through creating zeolites and their further polycondensation. I have to emphasise, that each treatment of the raw mix or the final products has to be an energy efficient process, based on available and cheap raw materials. 3.2. Potential Reduction of Carbon Dioxide Emission in the Aluminum Production
In the electrolytical production of aluminum carbon dioxide and other greenhouse gases: CF4 and C2F6 are produced. 0.333 tons of carbon is needed to produce 1 ton of aluminum. Theoretically, from 1 ton of produced aluminum 1.222 ton of CO2 is generated. The carbon consumption is though higher by nearly 50%, reaching 0.5 tons per ton of aluminum. Higher consumption is caused by subsidiary reactions in the melted electrolyte. Part of carbon is used during mechanical breaks on the anode. These losses are reduced to one half in new technology, which was applied also in the Slovak Republic. The aluminum production has a high energy demand. 13 000-16 000 kWh are used to produce 1 ton of aluminum, thereof only one half is used for the electrochemical reaction, the rest is turned to heat. Energy consumption is influenced also by secondary electrolyte reactions, during the reversed aluminum oxidation. This decreases the electrolysis current efficiency (CE). However, energy saving requires to reach the highest CE as possible. This requirement is met by new technologies with 12% higher increase CE compared with classical technologies. 3.2.1. Slovak Aluminum Production Contributions to COz Emissions
During modernisation in the electrolysis aluminum production in 1992-1995 when the aluminum electrolysis S6derberg cells were exchanged with electrolysis prebaked cells, there was a temporary decrease in production resulting in significant decrease of CO2 emissions. Emissions amounted 47.9 thousand tons of COz in 1995, which was less nearly by 60% than in 1988. The full production with modem technology, which started again in 1996 allows better energy utilisation and reduced CO2 emissions volume by 17%. Usage of electrolysis prebaked cells limited significantly the anode effect and generation of other greenhouse gases. 3.3. Carbon Dioxide Reduction and Utilisation in Other Productions
Manufacturing of lime, production of calcium acetylide (carbide) and amonia belong to other industrial productions generating higher volume of non-energy CO2. Annual production of calcium aceWlide and lime is stable. Emissions from the lime production reached approximately 0.8 million tons CO2. Some increase may be expected due to projected increased capacity. In difference to these productions, transformation processes undergoing in the Slovak economy in 1990-1995 significantly affected production of amonia and the whole sector of industrial production of fertilisers. When this production declined rapidly. CO2 emissions from the amonia production are generated during the synthesis gas preparation. In past more than 50% of generated CO2 had been used to prepare the urea. The urea was then used in production of industrial fertilisers and production of urea formaldehyde polycondensates. Research of its application in the Nox reduction from the flue gases and
296 other waste gases started. However, the industrial restructuring reduced production of amonia and urea. Since the turn in 1995 the amonia production significantly increased again, which again opened problems with CO2 utilisation. Further development of urea production and utilisation is expected in chemical industry, in particular for purposes of the Nox abatement from the flue gases. It has to be emphasised that application of flue gas denitrification is currently limited in the Slovak Republic and urea allows a relatively simple way of their reduction. 4. SUMMARY
CO2 emissions decreased by nearly 1/3 during 1990-1995 in the Slovak Republic. This decrease resulted from several factors, the most important of which was decreased industrial production and its restructuring. Adopted legislative measures proved positive impacts leading to the energy savings and increase of energy efficiency. Situation turned in 1996, since when the emissions started again gradually increase. The CO2 emission projections till 2005 or 2010 are uncertain, due to the economic transformation and establishment of the new state. Further development projection was based on the assumption, that CO2 emissions in energy shall be decreased, along with decrease of energy demand and energy saving within the whole economy. These goals may be reached through increased energy efficiency, limiting utilisation of fossil fuels and their replacement by renewable energy sources in primary energy sources accompanied with the liberalisation of fuels and energy. There are several options also to decrease non-energy CO2 emissions from industrial production. Two important directions are arising here: reduction of emissions in clinker production in the cement industry and CO2 utilisation for production of urea, which will be used as a semi-product for chemical industry, as well as a denitrification agent.
References
1. Second National Communication on Climate Change. Slovak Ministry of Environment, Bratislava, Slovak Republic, 1997 2. Balajka, J.: Country Study of Slovak Republic, Report- Element 3, Profing, Ltd., Bratislava, Slovak Repblic, 1996 3. Moncmanov~, A. Liquid and Solid Waste Energy Recovery in Co-generate Steam-gas Cycles. Intern. conf. APLICHEM '94, Bratislava, Slovak Republic, 22 - 23 June 1994 4. Moncmanov~, A." Mitigation of Carbon Dioxide Emissions. Intern. confer. EKOTECH "94, Bratislava, Slovak Republic, 20 - 21 June 1994 5. Mishulovich, A.' Potential Reduction of CO2 Emissions in the Manufacture of Portland Cement, PCA R&D Serial N~ PCA, USA. 1996 6. Nisbet, M.A." The Reduction of Resource Input and Emissions Achieved by Addition of Limestone to Portland Cement, PCA R&D Serial N~ PCA, USA. 1996
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
297
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Proposal of a new high-efficient gas turbine power generation system utilizing waste heat from factories Pyong Sik Pak, Hiroshi Ueda and Yutaka Suzuki Faculty of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565, Japan Abstract It has been required to reduce the emission of CO2 for mitigating the global warming. As one of means for reducing CO2 emission, constructing high-efficient power generation s3;stems is important, since the quantity caused by power generation is enormous and high-efficient system has a possibility to be widely installed from the economic point of view. In this paper, a H20 turbine power generation system utilizing waste heat from factories is proposed. In the proposed system, the steam produced by utilizing waste heat from factories is adopted as working fluid of a gas turbine. Thus, the system can be high efficient because the most part of a turbine output can be used for driving electric power generator: this is different with a conventional gas turbine power generation system where approximately two third of turbine output is consumed to compress the working fluid gas (air). Simulation programs have been developed by using object-oriented-language C + + to evaluate various characteristics of the proposed system. The total exergetic efficiency of the system has been estimated to be 57.5% and the fuel-base exergetic efl:iciency to be 6:1.6%, when a refuse incineration plant has been taken as an example of waste-heatemitting factories. The comparison with the characteristics of a gas turbine system using a dual fluid cycle has also been performed. It has been shown that the proposed systeln has higher exergetic eflqciency. The potentia.l of CO2 reduction by installing the proposed system was also investigated. 1. I N T R O D U C T I O N It has been required to reduce the emission of CO2 for mitigating the global warming. To reduce the amount of CO2 emission, constructing high-efficient power generation systems is indispensable, since the CO2 quantity caused 1)37power generation is enormous and high efficient system has a possibility to be widely installed from the economic point of view. In this paper, a new high-efficient gas turbine power generation system utilizing waste heat from factories is proposed, and its thermodynamic characteristics are estimated. Its energy saving potential and potential of reducing CO2 emission are also evaluated. 2.
STRUCTURE
AND
CHARACTERISTICS
OF T H E P R O P O S E D
SYS-
TEM Figure 1 shows the fundamental structure of the proposed system. The system can be constructed based on gas turbine power generation technologies, but is different in the following point; that is, the working fluid is not the air but the steam produced by utilizing waste heat energy[I]. Thus, the system has no compressor (air compressor) of
298 working gas. The compressing work can be efficiently performed during the state of liquid by using a feed water pump. In the system only the air quantity required for combusting the fuel is compressed by using the air compressor. Hence, the system can be remarkably high efficient since the most part of a turbine output is used for driving a generator. This is different from a conventional gas turbine power generation system where approximately two third of turbine output is consumed to compress the air[2]. Waste heat recovery boiler
.I Superheated steam
Steam
produced by
using waste heat
,4_
~i~
{l
Pump
~,,
ue,
~] | l
Waste heat
recocery ,, condenser
Oeo- ;iJ _ Filter
silencer Return water
!......J " Com-
pressor
L~ ~ Gas
I ~
turbine
IPump I I k
Cooling water
J 1___,.(~._~
Condensate
Gas (N2 ,CO2 )
Figure 1. Structure of the proposed system.
The following is a brief explanation how to generate electric power. The steam temperature used as the working fluid in the system are usually low, because the steam (H20) is produced by utilizing the waste heat from a factory. The I120 is introduced to a combustor and its temperature is raised higher than 1400 I( by combusting tile fuel with tile air for combustion. The reason tile temperature of the H20 can be made higher than 1400 K, being different from the case of steam turbine systems, is that the pressure is significantly low compared with that in stealn turbine systems. The high temperature combustion gas, whose main component is H20, N2, and CO,_) formed 1U combustion of the fuel, is introduced into the turbine and is used to drive a generator. The turbine exhaust gas, which has still a considerable heat energy, is utilized to produce superheated steam at a waste heat recovery boiler (\VHB) to increase power output as well as to improve power generation efficiency[3]. The low-telnperature outlet gas fi'om the WHB is lastly led into a condenser and is cooled with the cooling water. Most of H20 included in the exhaust gas is condensed into water at the condenser outlet, and the condensate, after being heated, is returned to the waste-heat-emitting factory as the feed water to produce the steam. The condenser outlet gaseous component, which is constituted of N2, CO2, and saturated steam not. condensed in the condenser, is emitted into the atmosphere. 3. C H A R A C T E R I S T I C S
ESTIMATION
3.1 E s t i m a t e d c h a r a c t e r i s t i c s of t h e p r o p o s e d s y s t e m It has been assumed in estimating thermodynamic characteristics that the amount of 60.3 t / h of saturated steam with temperature 548 K (275 ~ whose pressure is 5.95 MPa
(60.7 kg/cm 2) is used in the proposed system. This assumption was made by supposing case of utilizing the waste heat from a refuse treatment factory; specifically, a municip refuse incineration plant, which treats the amount of 400 t/d refuse whose lower caloriJ value is 8250 MJ/kg (1970 kcal/kg), was taken as an example of a waste-heat-emittil factory[4]. Simulation programs have been developed by using object-oriented-language C + + evaluate the proposed system. Table 1 shows major exogenous variables and paramete of simulation models, and the values of the exogenous variables and parameters used f thermodynamic characteristics simulation. As shown in Table 1, turbine inlet temperatu is assumed to be 1473 K, condenser outlet pressure is 39.2 kPa (0.40 kg/cm2), and t] fuel used is the natural gas composed of only methane. The exergetic efficiency of the system has been estimated to be 57.5% and the fuel-ba efficiency to be 63.6%, as shown in Table 2. The net fuel-base efficiency is the efficien, that is calculated by the following equation: net fuel-base efficiency (%) = (net generated electric energy net electric energy that can be gen&ated by using the waste heat) / (fuel consumption) x 100 (1) It has been assumed that the steam produced by utilizing the waste heat can genera electric energy with efficiency of 16.0%[4]. 3.2 C o m p a r i s o n w i t h c h a r a c t e r i s t i c s of a conventional s y s t e m The comparison with the characteristics of a conventional power generation syste using a gas turbine system has also been performed. Figure 2 shows the structure of system, taken as an example of a conventional system, that consists of a gas turbine ar a steam turbine power generation system to utilize the waste heat energy. In Figure the temperature of the steam produced by using the waste heat is raised by utilizing tl gas turbine exhaust gas to improve the steam turbine power generation efficiency; and t] system is called a repowering system. Table 3 shows simulation results of the repoweril system. It can be seen from Tables 2 and 3 that the proposed system has remarkab higher exergetic and net fuel-base efficiency than the repowering systenl. Filtersilencer Generator Waste heat
recoveryboiler usingwasteheat
Returnwater
Fuel
l
Superheat~ steam Steal~mGenerator Goad.... t e ~ _ turbine [ P'L; / ' ~ - Condenser Coolingwater
Figure 2. Structure of a compared system (repowering system).
300 Table 1. Major exogenous variables and parameters of simulation models. (a) Exogenous variables
Reference value Definition
Proposed system 548 5.95 60.3 471.7 1473
Repowering system Steam temperature produced by waste heat (K) 548 Steam pressure produced by waste heat (MPa) 5.95 Steam quantity proposed by waste heat .1 (t/h) 47.5 Return water temperature (K) 349.7 Turbine inlet temperature (K) 1473 Composition of fuel gas CH4 CH4 Condenser outlet pressure (kPa) 39.2 39.2 91. The reason the steam quantity produced by waste heat for the proposed system is greater than that for the repowering system is that the return water is heated by utilizing waste heat in the proposed system. (b) Exogenous parameters Definition Adiabatic efficiency of turbine (%) Adiabatic efficiency of air compressor (%) Flow loss rate at equipments .2 (%) Combustion efficiency of combustor (%) Excess air ratio in combustor (%) Pressure loss rate at combustor (%) Pressure loss rate at fuel gas nozzle (%) Pressure loss rate a,t air injection nozzle (%) Pressure loss rate at steam injection nozzle (%) Adiabatic efficiency of pump (~ Generator efficiency (%) \VItB terminal temperature difference (K) Pressure loss of gas at \VHB (MPa) Pressure loss rate of steam at \VHB (%) Enthalpy exchange efficiency of \VHB (%) ._9. Colnbustor, compressor, turbine, and \VHB.
Reference vahm Proposed Repowering system system 85 85 85 85 0.5 0.5 98 98 5 5 2 2 20 20 20 20 75 75 95 95 40 40 0.049 0.049 10 10 95 95
Table 2. Estimated thermodynamic characteristics of the proposed system. Generated power (M\u Inhouse power (MWh/h) Net generated power (MW) Fuel consumption (MWh/h) Exergetic efficiency (%)
35.10 -1.05 34.05 43.90 57.54
Net fuel-base efficiency (%)
63.56
301 Table 3. Estimated thermodynamic characteristics of the repowering system. " Power generated by steam turbine Generated power (MW) 9.85 Inhouse power (MWh/h) -0.49 Net generated power (MW) 9.36 Power generated by gas turbine Generated power (MW) 3.55 Inhouse power (MWh/h) -0.11 Net generated power (MW) 3.44 Total generated power (M\V) 12.80 Fuel consumption (MWh/h) 11.82 Exergetic efficiency (%) 50.43 Net fuel-base efficiency (%) 56.64 4. E V A L U A T I O N OF P O T E N T I A L S EMISSION REDUCTION
OF E N E R G Y S A V I N G A N D CO2
The potentials of, energy saving and CO2 emission reduction by installing the proposed system or the repowering system have been investigated, by taking a case where the waste heat energy generated fl'om all the refuse incineration plants in Japan in 1991 (98,821 t/d) were assumed to be used. In evaluating potentials, the steam that could be produced by utilizing the waste heat has been assumed to have the potential of generating electric power with efficiency of 16.0%. That is, the steam energy itself was estimated to have the potential of generating electric power of 1.51 GW and electric energy of 36.7 GWh/d[4]. It has also been assumed in estimating energy saving and CO2 reduction potentials that the amount of natural gas (NG) is saved which is required for generating the same amount of electric power energy in NG-firing thermal plants whose efficiency is 44.5%. Table 4 shows evaluation results. We can see from Table 4 that introducing the proposed systems has the potential of generating 8.67 GW and of saving 12.0% of NG (29.178 million ton) consumed in NG-firing thermal plants in Japan in the year 1993 and that it has the potential of reducing CO2 emission of 9.62 x 106 t/y. Table 4. Estimated potentials for saving the amount of consumed natural gas and for reducing CO2 emission. Proposed system Repowering system Net generated power 8.67 (G\V) 3.i6 (GW) Net generated electric energy 208.1 (GWh/d) 75.9 (GWh/d) Natural gas (NG) consumption 18732.3 (t/d) 5046.1 (t/d) Amount of N G to be saved 33657.8 (t/d) 12278.7 (t/d) Net amount of NG to be saved 9605.8 (t/d) 1912.9 (t/d) 3506.1 (103t/y) 698.2 (103t/y) Ratio* 12.0 (%) 2.39 (%) Potential of CO2 reduction 9618.7 (10at/y) 1 9 1 5 . 5 (10at/y) Ratio*: Ratio of the net amount of NG to be save to the amount of NG (29.178 million ton) consumed in NG-firing power plants in Japan in 1993.
302 5. C O N C L U T I O N Thermodynamic characteristics of the proposed system, which generates power by utilizing the waste heat from a factory, were estimated. It has been shown that the system has exergetic efficiency of 57.5% and fuel-base efficiency of 63.6%. Potential of CO2 emission reduction was also evaluated assuming the case where the waste heat emitted from all the refuse incineration plants in Japan in 1991 were utilized in the system. It has been shown that introducing the system has the potential of reducing CO2 emission of 9.62 x 106 t/y. The system has a remarkably high efficiency and is considered to be relatively easily realized based on conventional gas turbine technologies, and therefore the wide installation of the proposed system could be expected for reducing C02 emission in the near future. REFERENCES
1. P. S. Pak: Proceedings of IEA International Conference on Technology Responses to Global Environmental Challenges, 1 (1991) 191. 2. P. S. Pak and Y. Suzuki: Electrical Engineering in Japan, 114 (1994) 86. 3. P. S. Pak, K. Nakanmra and Y. Suzuki: Proceedings of IFAC/IFORS/IAEE International Symposium on Energy System Mana.gement and Economics (1989) 249. 4. P. S. Pak and Y. Suzuki: Joint Meeting of the International Energy Workshop and the Energy Modelin~ Forum, Laxenburg, Austria (1997).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversionsfor Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
303
A c e t o g e n e s i s and the p r i m a r y structure o f the N A D P - d e p e n d e n t formate d e h y d r o g e n a s e o f C l o s t r i d i u m t h e r m o a c e t i c u m , a t u n g s t e n - s e l e n i u m - i r o n protein. D. Gollin a, X.-L. Li a, S.-M. Liu b, E. T. Davies a and L. G. Ljungdahl a aCenter for Biological Resource Recovery and Department of Biochemistry & Molecular Biology, The University of Georgia, Athens, GA 30602-7229, USA bInstitute of Marine Biology, College of Fisheries Science, National Taiwan Ocean University, 2 Pei-Ning Road, 20224 Keelung,Taiwan, R.O.C. SUMMARY Acetogenic bacteria have the autotrophic acetyl-CoA pathway, which may be responsible for recycling more than 10% of the earth's carbon. The fixation of CO2 is catalyzed by two metalcontaining enzymes; formate dehydrogenase and carbon monoxide dehydrogenase/acetyl-CoA synthase. Huber and W~ichtersh~iuser [Science, 276 (1997) 245] have considered the acetyl-CoA pathway as a model in studies of primordial reactions for the development of chemoautotrophic origin of life. The acetogenic bacteria could be used for fixation of large amount of CO2 provided a cheap s o u r c e ofH 2 or another reducing substrate is available. Some properties of the formate dehydrogenase are presented. The primary structure of this enzyme was determined recently. 1. I N T R O D U C T I O N OF A C E T O G E N I C B A C T E R I A Drake [ 1] recently defined acetogens as being obligatory anaerobic bacteria, which can use the acetyl-CoA pathway as their predominant mechanism for (i) the reductive synthesis of acetyl-CoA from CO2, (ii) terminal electron-accepting, energy-conserving process, and (iii) mechanism for the synthesis of cell carbon from CO2. This implies that acetogens can grow autotrophically with CO2 as the carbon source and with a suitable electron donor. This concept is rather new. Most of the work leading to the elucidation of the acetyl-CoA pathway was done using Clostridium thermoaceticum (Moorella thermoacetica, [2]), which was described in 1942 by Fontaine et al.[3] as a heterotrophic bacterium fermenting glucose, xylose and fructose with acetate as the only product (reactions 1-3). C6H1206 + 2H20--> 2CH3COOH + 2 C 0 2 +8H + + 8e(1) 2 C O 2 +8H + +
8e-
--> CH3COOH
Sum = C6H1206 --> 3CH3COOH
+
2H20
(2) (3)
304 The fermentation of glucose (reaction 1) proceeds by the Embden-Meyerhof glycolytic pathway. The electrons generated reduce CO2 formed in the fermentation with the formation of acetate (reaction 2), which occurs by the acetyl-CoA pathway described below. The result is the formation of three mol of acetate out of one mol of glucose (reaction 3). The ability of C. thermoaceticum to grow autotrophically was realized when it was discovered it has hydrogenase [4], and subsequently was found to grow on a mixture of H2 and CO2 or CO alone as sole source of carbon and energy [5]. Until 1967 C. thermoaceticum was the only acetogenic bacterium available. At this time Clostridiumformicoaceticum was discovered. It was followed by Acetobacterium woodii (1977), Acetogenium kivuii and Clostridium thermoautotrophicum (1981). At present over 60 different acetogenic bacteria have been described. Most of them are presented in the review by Drake [1 ], who also considered their very diverse metabolic capabilities including the use of different carbon sources as well as electron donors and acceptors. The assessment is that acetogenic bacteria are probably the most versatile anaerobes encountered [6 and chapters by B. Schink, H.L. Drake et al., J.A. Breznak, R.I. Mackie and M.P. Bryant, M.J. Wolin and T.L. Miller, Zavarzin et al. and A.C.Frazer in 1]. The impact by acetogens on the environment and in ecological settings is still being evaluated, but it appears to be enormous. Annual worldwide fixation of CO2 by photosynthesis has been estimated to be about 150 x 1 0 9 tons of dry plant mass. About 70% of this material consists of cellulose and hemicellulose, and as much as 10% of it may be converted in the anaerobic environment to methane and CO2 by consortia of anaerobic bacteria [7]. It appears as if acetogenic bacteria play a substantial role in this consortia. At the global level approximately 1013 kg of acetate is metabolized annually in the anaerobic environment and about 10% of this may be derived by CO2 fixation via the acetyl-CoA pathway. Breznak [Chapter 11 in 1] discusses the role of acetogenesis in the guts of termites from which he and his associates have isolated three acetogenic bacteria. They have estimated that bacterial formation of acetate in these guts amounts to about 1012kg annually [8]. Similarly, several different acetogenic bacteria are present in the human gut and they may form as much as 1.25 x 101~kg of acetate per year in the human population by fixation of CO2 using the acetyl-CoA pathway [Chapter 13 in 1]. Other anaerobic environments in which acetate is formed by fixation of CO2 include forest soils [9], rumen of cows [10] and intestines of other animals. Acetogenic bacteria metabolize also a number of methoxylated aromatic acids including 3,4,5-trimethoxybenzoic, syringic, ferulic, and vanillic acids. These compounds, which are lignin degradation products, undergo O-demethylation by the acetogens in the presence of CO2 or CO forming acetate with the methyl group derived from the methyl of the methoxylated aromatic acids [Chapter 17 in 1, and 11 ]. 2.THE ACETYL-CoA PATHWAY Reaction 2 above shows the reductive formation of acetate from C O 2. This occurs via the acetylCoA pathway so called because the acetyl group of acetyl-CoA is the first 2-carbon moiety in which both carbons originate from CO2. The acetyl-CoA pathway is now recognized as an autotrophic pathway of CO2 fixation. The pathway is outlined in Fig. 1. Carbon dioxide enters the pathway via two reductive reactions. One, catalyzed by an NADP-dependent formate hydrogenase (FDH), leads to the formation of formate that, in a series of reactions involving tetrahydrofolate (THF)intermediates is reduced to methyl-THF, the methyl group of which is
305 NADPH
2H++2e-
NADP ~
CO2
HCOOH ATPADP~ ~ ~
[ CODH/ACSl
THF
CiO C~I3
HCO -THF
o e-S
CH = THF
I
CoA
Celllaterial
~._
CH3COOH
THF " * - " t
NADP ~~ ~ CH 2 = THF
f~
~ CH3-THF
2H + + 2e-
Figure 1 Simplified acetyl CoA pathway of C. thermoaceticum. transferred to the cobalt atom of a corrinoid/Fe-S protein to be the precursor of the methyl group of acetyl-CoA. The properties of FDH will be discussed below. The second reaction is catalyzed by carbon monoxide dehydrogenase/ acetyl-CoA synthase (CODH/ACS). Since it is a COz fixing enzyme, some of its properties are summarized here. Ragsdale and Kumar [ 12] have recently published an extensive review of CODH/ACS. As the name of the enzyme indicates, CODH/ACS catalyzes the reversible oxidation of CO to CO2, and the final step in the synthesis of acetyl-CoA from the methyl group, CO, and CoA. CODH activity in C. thermoaceticum was first discovered by Diekert and Thauer [13]. The final conclusive evidence for the ACS activity was presented by Ragsdale and Wood [ 14]. CODH/ACS has been studied extensively by the groups ofRagsdale [ 12] and Lindahl [ 15]. The enzyme, first thought to be a hexamer, has now been shown to be a tetramer consisting of two subunits with the composition a2132. The gene acsA encodes the 13subunit consisting of 674 amino acids with a molecular mass 72,928 Da. It is followed by acsB, that encodes the c~ subunit having 729 amino acids with a molecular mass of 81,730 Da [16]. The enzyme contains two nickel, 12 iron, one zinc and 14 acid labile inorganic sulfide per al3 dimer. The metals are arranged in three clusters A, B, and C. Clusters A and C are similar with a Ni bridged to a Fe4_S4 cluster, whereas the B cluster is a regular [Fe4-S4] cluster. Clusters B and C reside in the 13subunit. Results show that the CODH
306 activity is catalyzed by the [3 subunit and involves cluster C. The ACS activity may reside in the subunit containing cluster A. The B cluster may be involved with internal electron transfer. See the review by Ragsdale and Kumar [12] for a detailed description of the clusters and other properties of CODH/ACS enzymes. 3. N A D P - D E P E N D E N T F O R M A T E D E H Y D R O G E N A S E C. thermoaceticum contains a NADP-dependent formate dehydrogenase that catalyzes the reversible reduction of CO2 with NADPH (Reaction 4). It was found in 1966 [17], and subsequently its formation was shown to be dependent on several metals present in the growth medium [18]. It was purified by Yamamoto et al. [19]. C O 2 -~- NADPH
(4)
--> HCOO- + NADP +
The enzyme has a mass of 340 kDa and consists of two hetereodimers with subunit masses of 96 kDa and 76 kDa as determined with SDS-PAGE; thus the composition is %1~2-The purified tetrameric enzyme contains per mol two tungsten, two selenium, 36 iron, and about 50 inorganic sulfide. The Se is in the form of seleno-cysteine situated in the larger Gt subunit. Tungsten exists as a pterin cofactor similar to molybdopterins or tungsto-pterins of many molybdo- and tungsto-enzymes [20, 21]. The structure of the pterin cofactor has not been established. Progress with the C. thermoaceticum 8 2 120 0.5 FDH has been slow. This is partly due to the enzyme being extremely oxygen sensitive. An apparent >, ~ 0.3 for 02 is 7.6 lam. The oxygen seems to ~6 ~ 1 60 ~ have also a secondary ~ 0.2~ slower effect involving 40 a nonreversible 5 - = ~ 0.1 inactivation of the 20 ~ enzyme. Thus all work with FDH must be 4 0 't0 0.0 0 0 20 30 40 50 60 performed in an Time (hours) anaerobic chamber. An additional problem with FDH is its Figure 2 C. thermoaceticum cultures were grown at 60~ with apparent regulation glucose (56 mM). CO2 was bubbled through the medium to maintain during the growth an anaerobic environment and as an external electron acceptor. cycle. This is shown in Optical density (O), pH (ll), glucose (A), acetate (V), and FDH Fig. 2. Cells at the specific activity ( , ) were assayed as in [19].
il~17604
307 beginning of the logarithmic growth phase have low FDH activity. During the log phase over a period of four hours a fast increase of the FDH activity occurs. It then decreases rapidly to almost nil when the cells enter the stationary phase. The regulation of FDH activity in the cells involves the metals, which are constituents of the enzyme, and also molybdenum, but additional factors which may be responsible include pH, CO2/bicarbonate ratio, and acetate concentration. The sharp regulation of the FDH activity in the cells has the practical consequence that it is difficult to harvest cells when FDH is at its highest, which is needed to obtain very active preparation of the enzyme. By using molecular biological methods involving cloning and sequencing we have recently obtained the nucleotide sequence of thefdh operon in C.thermoaceticum. The sequence has been deposited in the Genbank under the accession number U73807. The gene coding the 13 subunit,fdhB, precedes that of the a subunit,fdhA, andfdhA overlapsfdhB by eight nucleotides. Both genes are preceded by putative ribosomal binding sites andfdhB is preceded by a putative promotor sequence. Only one copy of the genes was detected in the C. thermoaceticum genome. The predicted translation product offdhA, the a subunit, has 893 amino acids with a calculated mass of 98,145 Da, and that offdhB, the [3 subunit, consists of 708 amino acids with a mass of 76,445 Da. Both values are consistent with determinations of the sizes of the subunits of the purified enzyme using SDS-PAGE. The sequence data are now being compared with those of other molybdopterin and tungstopterin enzymes and especially with those of known structures e.g. formate dehydrogenase of Escherichia coli [22], aldehyde ferredoxin oxido-reductase from Pyrococcusfuriosus [23], aldehyde oxido-reductase from Desulfovibriogigas [24], and DMSO reductase from Rhodobacter sphaeroides [25]. What has emerged is that the a subunit contains potential binding sites for four [4Fe-4S] and two [2Fe-2S] clusters whereas the 13subunit may have one [4Fe-4S] and one [2Fe2S] cluster. Thus the enzyme has the potential of binding 48 Fe, which agrees quite well with chemical analyses. It has been pointed out that the reduction of CO2 with NADPH is energetically unfavorable [19]. It is possible that the reduction is mediated through an internal electron transport chain involving the many iron-sulfur clusters. In addition to the iron centers, the 13 subunit has a binding motif for NADP(H). The tx subunit contains the selenocysteine (residue 358), which is encoded by an in frame UGA codon. It has also a molybdopterin guanine dinucleotide binding motif which presumably is the binding site for the tungstopterin cofactor. It is still not possible to predict the structure of the cofactor. ACKNOWLEDGMENTS Support for work on acetogenic bacteria from National Institute of Health grant 5 RO 1 DK 27323 and U.S Department of Energy grant DE-FG05-93ER20127 is gratefully acknowledged as is the support of a Georgia Power Distinguished Professorship from Georgia Power Company. REFERENCES 1.
H.L. Drake (ed.), Acetogenesis, Chapman & Hall, New York, (1994).
308
,
,
,
5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
M.D. Collins, P.A. Lawson, A. Willems, J.J. Cordoba, J. Femandez-Garayzabal, P. Garcia, J. Cai, H. Hippe and J.A.E. Farrow, Int. J. Syst. Bacteriol., 44 (1994) 812. F.E. Fontaine, W.H. Peterson, E. McCoy, M.J. Johnson and G.J. Ritter, J. Bacteriol., 43 (1942) 701. H.L. Drake, J. Bacteriol., 150 (1982) 702. R. Kerby and J.G. Zeikus, Curr. Microbiol., 8 (1983) 27. G. Diekert and G. Wohlfarth, Antonie van Leeuweenhoek 66(1994) 209. L.G. Ljungdahl and K.-E. Eriksson, Adv. Microbial Ecology, 8 (1985) 237. J.A. Breznak and M.D. Kane, FEMS Microbiol. Rev., 87 (1990) 309. K. Ktisel and H.L. Drake, Appl. Environ. Microbiol., 60 (1994) 1370. F. Rieu-Lesme, B. Morvan, M.D. Collins, G. Fonty and A. Willems, FEMS Microbiol. Lett., 140 (1996) 281. S.L. Daniel, E.S. Keith, H. Yang, Y.-S. Lin and H.L. Drake, Biochem. Biophys. Res. Commun. 180 (1991) 416. S.W. Ragsdale and M. Kumar, Chem. Rev., 96 (1996) 2515. G. Diekert and R.K. Thauer, J. Bacteriol., 136 (1978) 597. S.W. Ragsdale and H.G. Wood, J. Biol. Chem., 260 (1985) 3970. J.Q. Xia, J.F. Sinclair, T.O. Baldwin and P.A. Lindahl, Biochemistry, 35 (1996) 1965. T.A. Morton, J.A. Runquist, S.W. Ragsdale, T. Shanmugasundaram, H.G. Wood and L.G. Ljungdahl, J. Biol. Chem., 266 (1991) 23824. L.-F. Li, L.G. Ljungdahl and H.G. Wood, J. Bacteriol., 92 (1966) 405. J.R. Andreesen and L.G. Ljungdahl, J. Bacteriol., 116 (1973) 867. I. Yamamoto, T. Saiki, S.-M. Liu and L.G. Ljungdahl, J. Biol. Chem., 258 (1983) 1826. R. Hille, Chem. Rev., 96 (1996) 2757. M.K. Johnson, D.C. Rees and M.W.W. Adams, Chem. Rev., 96 (1996) 2817. J.C. Boyington, V.N. Gladyshev, S.V. Khangulow, T.C. Stadtman and P.D. Sun, Science, 275 (1997) 1305. M.K. Chan, S. Mukund, A. Kletzin, M.W.W. Adams and D.C. Rees, Science, 267 (1995) 1463. M.J. Romeo, M. Archer, I. Moura, J.J.G. Moura, J. LeGall, R. Eng, M. Schneider, P. Hof and R.Huber, Science, 270 (1995) 1170. H. Schindelin, C. Kisker, J. Hilton, K.V. Rajagopalan and D.C. Rees, Science, 272 (1996) 1615.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
309
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Biochemical CO 2 fixation by mimicking zinc(II) complex for active site of carbonic anhydrase Kazuhiko Ichikawa *, Kou Nakata, Mohamed M. Ibrahim, and Satoshi Kawabata Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan
The complex coordinated by three benzimidazolyl moieties and a single water molecule was syntesized as a model complex of the active site in carbonic anhydrase. A catalytic reaction of CO2 fixation was simulated by using the model complex.
1. INTRODUCTION The increase of CO2 in the atmosphere is the H serious problem on the global environment. In biological system CO2 is hydrated by catalytic function of carbonic anhydrase, which is an Hydrophilic ] H H "'~176 Half / i o'enzyme containing zinc(II) in its active site. / O-H~ \H f Since the native enzyme is not so suitable to use H,, H Hydrophobic as a catalyst from the view of its stability and handling, it is important to design a catalyst which fixes CO2 and the artificial model --~9 N" ~, N / complex can be used for many times as a /--\ 'N. catalyst. The crystal structure of human HN~ ~~-~~NNH"~ ~ l( His119) carbonic anhydrase II was revealed by X-ray (His 96) / (His 94) crystallography[ 1,2]. In the active site of carbonic anhydrase, zinc ion has tetrahedral Chart The active site of carbonic geometry and coordinated by three imidazoles anhydrase. of histidines and H20 (or OH-) (chart). The catalytic mechanism of carbonic anhydrase is discussed from the data of pH dependence of the catalytic activity[3-5] and X-ray crystallography[I,2]. A number of studies on the syntheses and structures of the model complexes as the active site of carbonic anhydrase have been reported[6-11], most of these complexes consist of pyrazolyl ligands[6,7] or macrocyclic amine ligands[8-10]. The kinetic studies of hydration reaction using model complexes of carbonic anhydrase active site revealed the rate constants of hydration [ 12-14]. This paper reports in vitro simulation of CO2 hydration with the aid of [LZn(OH2)] 2+ which is a model complex of the active site in carbonic anhydrase.
/.o
* the author to whom correspondence should be addressed.
310 2. E X P E R I M E N T A L SECTION 2.1. Synthesis of tris(2-benzimidazolylmethyl)amine L and [LZn(OHz)](PF6) 2 1-(PF6) z The ligand of L was p r e p a r e d from o H p h e n y l e n e d i a m i n e and nitrilotriacetic acid[15]. 1.(PF6) 2 and its D 2 0 d e r i v a t i v e I'.(PF6) 2 were synthesized according to the literature[ 16]. L
2.2. Reaction of 1' with CO2
2+ X D e p r o t o n a t i o n of 1' in D M F by base (i.e., I triethylamine, N-methylmorpholine, imidazole, or 2,6lutidine) was characterized by 2H NMR. The chemical shifts were recorded vs. acetone-d6, the shift of which was assumed to be 2.0 ppm. The dried reagent of DMF (water content is less than 0.005% ) was used as a 1 X = H20 solvent. 1' X = D20 Reaction of 1' with CO2 was monitored with the product of zinc-bound HCO 3- by 13C NMR and i.r techniques. To compare the results of the presence and absence of Et3N, the two solutions which contain I'.(PF6) 2 + Et3 N and only I'.(PF6) 2 were prepared and CO2 gas was introduced to each solution at the same time using Y-shape tube. For NMR measurements samples were made as follows, l"(PF6)2 (ca. 100 mM) with or without equimolar Et3N was dissolved into DMSO-d6. CO2 gas was bubbled for 3-5h to each solution. The new signal resulted from the hydration was clearly observed after bubbling of CO2 for 3-5h. The chemical shifts of 13C NMR in DMSO-d6 were indicated vs. TMS. For i.r. measurements samples were made as follows. After the equimolar base was dissolved into a CH3CN solution of I'.(PF6) 2 (ca. 50 mM), the solution became suspended as a result of the product of [LZn(OH)]+. The slow evaporation of the suspended solution by bubbling CO2 produced a solid compound for i.r. measurements.
3. RESULTS AND DISCUSSION We designed the model complex for the active site of carbonic anhydrase providing (i) imidazole ligand which corresponds to histidine imidazole, (ii) coordinated water molecule and (iii) hydrophobic pocket, as mentioned in Experimental Section. Since the ligand, tris(2benzimidazolylmethyl)amine L used in this work plays a role of steric hindrance, it will be able to reproduce the tetrahedral geometry which is identical with the active site of carbonic anhydrase. Furthermore, the benzene rings of benzimidazolyl groups can fix a hydrophobic pocket, in which there exists the active site of the native enzymes.
3.1. In vitro simulation of enzymatic CO2 hydration The in vitro simulation consists of the three processes of (1) deprotonation of coordinated water to give the active zinc hydroxide derivative, (2) nucleophilic attack of zinc-bound hydroxide to CO2 substrate, and (3) displacement of the bicarbonate anion by H20[4,6], as
311
shown in Scheme. 3.1.1. Deprotonation of coordinated water molecule The deprotonation of coordinated water is presented as follows 9 [LZn(OD2)] 2+ +
B ..
~ [LZn(OD)] +
+
(1)
BD +
and the equilibrium constant K of eq.(1) can be given by pK
=
pKa,D20-
(2)
pKa,BD+
Ka,D20 and Ka,BD + stand for dissociation
B
-
BH+
c o n s t a n t s o f c o o r d i n a t e d D 2 0 and (1) %,,,~j,,,4 (His)3Zn-OH (His)3Z-OH2 c o n j u g a t e acid BD+ of weak base B, respectively. The degree of deprotonation (~2) C02 HCO3of z i n c - b o u n d w a t e r m o l e c u l e is proportional to pK. The smaller pKa,D20 H~O~ ' ~ (His)3Zn-OCO2 H as well as the larger pKa,BD+ bring about Scheme The catalytic mechanism of the l a r g e r c o n c e n t r a t i o n of the zinc carbonic anhydrase. hydroxide derivative according to eq.(2). For the in vitro simulation the reagents of weak base B [ LZn(OD2) ]2+ are triethylamine, N-methyLut D+ D+ Et3N D+ ~,,,~r,fD2 O morpholine, imidazole and 2,6-1utidine. In h u m a n c a r b o n i c a n h y d r a s e II a i ! [ | i ! ! ' I ' ' couple of water m o l e c u l e s 15 lo De0 + kut, Ira, 0 connected to His 64 and Thr 199 through hydrogen bond may d e p r o t o n a t e a zincFigure 1. The evidence of deprotonation of the zincb o u n d w a t e r m o l e c u l e [ 17, bound water molecule by 2H NMR chemical shifts in 18]. F o r the c o o r d i n a t e d DMF. The concentrations of 1', DaO, and base are 50 D 2 0 of 1', the free D 2 0 , mM. Et3 N : triethylamine, MeM : N-methylmorpholine, /'+base, and D20 + base, the Lut : 2,6-1utidine, Im : imidazole. (a) 1' + Et3N, (b) 1' observed chemical shifts of + MeM, (c) 1'+ Lut, (d) 1'+ Im. 2H NMR in DMF are shown in Figure 1 : the other data of chemical shifts are also added for the salts of these bases, Et3NDC1, MeMDC1, ImDC1 or LutDBr. Figure 1 shows that (1) the larger pKa,BD+ gives rise to the smaller difference of chemical shifts between 1' +base and the salt of base, (2) the base of Et3 N provided the largest production of [LZn(OD)]+, (3) [LZn(OD2)] 2+ showed almost no d e p r o t o n a t i o n without the aid of B, and (4) free D 2 0 showed almost no deprotonation even with B.
I In~D+MeT
""orMeM (tl Et3N
3.1.2. Hydration of C O
2
The hydration of CO2 is given by
312 [LZn(OD)]+
+
CO2(g)
~
~
(3)
[LZn(OCO2D)] +
The formation of the hydrogencarbonate complex [LZn(OCO2D)] + as the reaction product of [LZn(OD)]+ with CO2 gas has been revealed by 13C NMR and i.r. studies, as shown in Figures 2 and 3. The 13C NMR spectrum (Figure 2) i n d i c a t e d the evidence of CO2 hydration. The new signal at 167 ppm observed after bubbling CO2 for 5 hours into DMSO solution of I'.(PF6)2+Et3N has been attributed to the production of z i n c - b o u n d HCO 3- and shifted to downfield from the signal of HCO3( 158 ppm ) in D M S O solution of ((Ph3P)2N)HCO 3. The yield of [LZn(OCO2D)]+ was estimated to 50% based on I'. The difference of i.r. spectra between the product from the CH3CN solutions of I'.(PF6)2+Et3N and I'-(PF6) 2 with bubbling of CO2 gas shows the two bands at 1440 cm-1 and 1675 cm-] (marked by * in Figure 3), which were assigned to the symmetric and asymmetric CO stretching bands. The separation of these two bands, Av=235 cm-1 demonstrates unidentate for HCO 3ligand[19]. The i.r. and 13C NMR results obtained from the I'-(PF6) 2 solution without Et3N did not show any evidence of CO2 hydration. In the previous studies, the ligand OHof the model complex was reacted with CO2 to produce CO32- complexes, such as [LZn(bt-CO3)ZnL], where L=HB(3,5i -Pr2pz)3[8]. Since the dimerization does not take place in vivo, these c o m p l e x e s are not preferred as an enzyme model. 1H NMR evidence showed that the [LZn(OH)], where L-HB(3-But-5-Mepz) 3 , is reacted with CO2 in benzene to produce bicarbonate complex [LZn(OCOzH)], reversibly[9].
(a)
C02
Benzimidazolyl
I
tA,,-I
,bt .t&,.~ ~ , 1 ~ ~ , J .,==~,.,=,,1., It,,w, J=J I~,l,,,l,~,~l W,,p- ~1 m,,p,,m,l~.,nm,~
(b)
'
''
'1'
I
'''
I'
''
J__L
,I
1 'i'
'''
I''
160
''
I ' ' '
140
'i
120
''
' ' 1 '
,5 (ppm)
Figure 2. 1 3 C NMR spectra after bubbling CO2 for 5h. (a) I'.(PF6)2+Et3 N solution and (b) I'.(PF6) 2 solution. The peak with * was assigned to zinc-bound HCO3-.
100 rv/'-", r e - ' - , O,i
F~AVAI~ [ //IJV ~1 F,,f'kll ~ t"~, ,',
....,.
,, ,~ A A
v
.-X-
50
2000
.3(-
1000
v / c m -1
Figure 3. I.r. spectra of the product after bubbling CO2 into I'.(PF6)2+Et3N solution ( - - ) and l"(PF6)2 solution (--). The bands with * were assigned to the symmetric and asymmetric stretching modes of HCO3- ligand.
313 3.2. Structures of 1 and characterization of coordinated water molecule The structure of 1 d e t e r m i n e d by X-ray crystallography[20] is shown in Figure 4. The O1 zinc ion in 1 was coordinated by three nitrogen atoms of benzimidazolyl groups and one oxygen atom of water molecule. The coordination N1 geometry around zinc ion is tetrahedral. The Zn-N, Zn-OH2 distances and the bond angles of N-Zn-N, N-Zn-OH 2 of 1 were consistent with the value for zinc c o m p l e x e s with four coordination[21]. The geometry around zinc Figure 4. O R T E P d r a w i n g of 1. ion in carbonic anhydrase is tetrahedral and Ellipsoids are depicted at the 50% c o o r d i n a t e d by three nitrogen atoms of probability level. Bond distances(nm) : imidazoles and H20. The observed zinc ion Znl-N1 0.2052(4), Znl-O1 0.201(3). geometry of 1 is quite similar to that of the active site in carbonic anhydrase[ 1]. The pKa value of coordinated water molecule for 1' was obtained as follows. From eq.(1) the ratio of [LZn(OD2)]2+ : [LZn(OD)]+ was indicated by [ [LZn(OD)]+ ] [ [LZn(OD2)]2+ ]
(4)
[ BD+ ] [B]
The following equation was led from eqs.(2) and (4). log(
[BD+ ]) [B]
1 = T
(5)
(pKa'BD +-pKa,D20 )
The eq.(5) m e a n s that the intercept of the linear plot of log ([ BD+] / [B] ) vs. pKa,BD+ gives the pKa of z i n c - b o u n d D 2 0 . F i g u r e 5 shows the r e l a t i o n b e t w e e n log ([ B D+] / [B] ) calculated from the chemical shifts of 13C N M R spectra in D M S O - d 6 vs. pKa,BO+. The weak base B are triethylamine, benzylamine, morpholine, 2methylimidazole, N-methymorpholine, and 2,6-1utidine. The pKa value of zinc-bound water molecule for 1' was ca. 8.6,
J9
koJ"~
o
.y,<,-
-1.
-2.0i 6.0
o
H
o,~,N Me
Me.
710
8'.0
9:0
lO.O
1 .o
PKa, g D+ Figure 5. The plot of log ( [BD+] / [B] ) vs. pKa, B D+ with the structures of the bases.
314 which is lower than that of free H20 (pKa = 15.6) and somewhat higher than the active site of carbonic anhydrase (pKa = c a . 7) [3,22].
4. CONCLUSION In conclusion, we have carried out the syntheses the complex coordinated by a single water molecule, [LZn(OH2)] 2+ as a model complex of the active site in carbonic anhydrase. The geometry around zinc ion is similar to that of human carbonic anhydrase II. The deprotonation of coordinated water molecule was characterized by 2H, 13C NMR. It was demonstrated that the pKa of zinc-bound D20 is 8.6. The formation of hydrogen-carbonate complex, [LZn(OCO2D)]+ as a product of reaction of [LZn(OD)]+ with CO2 gas has been revealed by 13C NMR and i.r. studies. REFERENCES
1. 2. 3. 4. 5. 6.
A.E. Eriksson, T. A. Jones and A. Liljas, Proteins : Struct., Funct., Genet., 4(1988) 274. S.K. Nair and D. W. Christianson, J. Am. Chem. Soc., 113(1991) 9455. D.N. Silverman and S. Lindskog, Acc. Chem. Res., 21(1988) 30. Y. Pocker and D. R. Bjorkquist, Biochemistry, 16(1977) 5698. J.E.Coleman, J. Biol. Chem., 242(1967) 5212. E. Kimura, T. Shiota, T. Koike, M. Shiro and M. Kodama, J. Am.Chem.Soc., 112 (1990) 5805. 7. E. Kimura, Y. Kurogi, M. Shionoy and M. Shiro, Inorg. Chem., 30(1991) 4524. 8. N. Kitajima, S. Hikichi, M. Tanaka and Y. Moro-oka, J. Am.Chem.Soc., 115 (1993) 5496. 9. A. Looney, R. Han, K. McNeill and G.Parkin, J. Am.Chem.Soc., 115(1993) 4690. 10. S. Kawabata, K. Nakata and K. Ichikawa, Acta Crystallogr., C51(1995)1554. 11. R. Gregorzik, V. Hartmann and H. Vahrenkamp, Chem. Bar., 127(1994) 2117. 12. R. S. Brown, N. J. Curtis and J. Huguet, J. Am.Chem.Soc., 103(1981) 6953. 13. X. Zang and R. Eldik, Inorg. Chem., 34(1995)5606. 14. X. Zang, R. Eldik, T. Koike and E. Kimura, Inorg. Chem., 32(1993)5749. 15. L. K. Thompson, B. S. Ramaswamy and E.A.Seymour, Can. J. Chem., 55(1977) 878. 16. K. Nakata, M. K. Uddin, K. Ogawa and K. Ichikawa, Chem. Lett., (1997) 991. 17. Y. Pocker and N. Janji'c, J. Am.Chem.Soc., 111(1989),731. 18. L. L. Kiefer, S. A. Paterno and C. A. Fierke, J. Am.Chem.Soc., 117(1995),6831. 19. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, John Wiley and Sons, New York, 1986. 20. K. Nakata and K. Ichikawa, unpublished result. 21. A. G. Orpen, L. Brammer, F. H. Allen, O. Kennard, D. G. Watson and R. J. Taylor, Dalton Trans., (1989) S 1. 22. Y. Pocker and J. E. Meany, Biochemistry, 6(1967) 668.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
315
The biological CO2 fixation using Chlorella sp. with high capability in fixing CO2 ,b 9 F.Yamada,b T.Nlshlde, 9 9 b T.Muranaka~b, N.Yamaguchi "b and M.Murakaml', Y.Takimoto ~b Research Institute of Innovative Technology for the Earth (RITE), Minato-ku, Tokyo 105, JAPAN b Biotechnology Laboratory, Sumitomo Chemical Co., Ltd., Takarazuka, Hyogo 665, JAPAN The algal strain with highly efficient in fixing C O 2 , Chlorella sp. UK001, was isolated. The cells showed tolerance to high temperature and high CO2 concentration, making the strain advantageous in fixing CO2 in flue gas from power plants. In the experimental small scale cultures, the CO2 fixation rates of the alga exceeded the target value of 1gCO2/1/day at 10hr illumination so as to simulate the actual daylight period. The biomass produced by the cells can be used as feed for poultry or swine. 1. INTRODUCTION In order to mitigate the potential problems associated with increasing atmospheric CO2 levels, Research Institute of Innovative Technology for the Earth (RITE) has started the biological CO2 fixation and utilization project in 1990 using microalgae under a commission from the New Energy and Industrial Technology Development Organization (NEDO), which is subsidized by the Ministry of International Trade and Industry (MITI) [1 - 3]. Microalgae were selected because of their following distinctive features; 1) capability to assimilate CO2 into proteins, carbohydrates, lipids and other useful substances by using solar energy, 2) higher rates of CO2 fixation rates than land plants, and 3) better suitability for incorporating the CO2 removing system into industrial processes than other photosynthetic systems using higher plants. In addition, many microalgal strains are able to grow well in saline water and are able to tolerate widely fluctuating temperatures, high CO2 concentrations and high light intensities. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).
316
Although feasibility of CO2 mitigation using microalgae was described [4] and the cost effectiveness was calculated [5], large areas of land are required for biological CO2 fixation. Therefore, it is essential to obtain microalgae with high photosynthetic performances. In order to increase the productivity and to reduce the land area required, we have screened a green alga, Chlorella sp. UK001, which showed high capability in fixing CO2 and tolerance to high temperature and high CO2 concentration. The characteristics of this alga will be described. 2. MErHODS
2.1. Sampling and isolation of microalgae The algal samples were collected from hot springs, limestone caves and lakes in Japan in order to obtain microalgae with tolerance to high temperature and/or high concentration of CO2, because the high temperature flue gas from power plants containing high concentration of CO2 will be introduced into algal culture. The samples were then placed in culture tubes with several different culture media to allow their growth under experimental conditions (25 ~ standing culture, fluorescent light). Cells grown in test tubes were isolated by picking up single colony on agar plates, diluting the cells in 96 well culture plates or by using micromanipulater with pipettes. Isolated algal cells were then characterized for their CO2 fixation rates by means of evaluating photosynthetic rate and growth rate. The algal samples were also examined their tolerance to high temperature and high CO2 concentration. 2.2. Characterization of microaigae MC medium or HD medium were used for culturing Chlorella sp. UK001. Cells were placed in a flat culture vessel with 2.5 cm thickness and with 100 ml medium. The vessels were placed in temperature controlled water bath and illuminated by projector lamps. CO2 enriched air was aerated into culture at the flow rate of 100 ml/min (1 vvm). 2.3. High density culture of Chlorella sp. UK001 The algal cells were analyzed for the elemental composition (C, H, N, O, P, Fe, Mg, S, Mn) and the spent medium during growth of the cells were also analyzed for inorganic nutrients (NO3, PO4, SO4, Fe , Mg and . The required amount of nutritional composition for high density culture was estimated from the elemental composition and inorganic nutrients in the spent medium.
2.4. Nutritional composition of Chlorella sp. UK001 The cells grown in high density culture medium at 30~ were analyzed for protein, carbohydrate, lipid and vitamins. Amino acid, monosaccharide and fatty acid compositions were also determined.
317
3. RESULTS AND DISCUSSION Among more than 700 algal samples collected from nature, Chlorella sp. UK001 was selected as one of the microalgae showing the high capability in CO2 fixation and the tolerance to high temperatm'e and high CO2 concentration. This alga is classified as Chlorella sp. from its morphological features with a close resemblance to Chlorella vulgaris or Chlorella sorokiniana. Specific growth rate of Chlorella sp. UK001 showed quite higher specific growth rate than Chlorella vulgaris or Chlorella sorokiniana under the culture temperature between 25 ~ through 40 ~ Therefore, fin'ther characterization of Chlorella sp. UK001 was conducted. 3.1. Characterization of Chlorella sp. UK001 The results of light saturating specific growth rate change and Tamiya plot analysis [6] are shown in Fig. 1. The maximum specific growth rate t/z max), light imensity at 1/2/z max and light compensation point were 0.32 h , 112 /z E/m'/s and 8.15 /z E/m2/s, respectively.
1/hr
hr
14-
~ 0.4"~ 0.3
0
0
~
~ 0.20
,...,,~ o,
~6
~0 0.1 1~ 0.0 0 r.~
.
.
400
.
.
8~)0
1:~00
1600
I (light intensity, /z E/m2/s)
0
'
I
0.01
1/I
'
0.~02
1/(/z E/m~/s)
Figure 1. Light saturating specific growth rate curve and Tamiya plot Effects of temperature (Fig.2), CO2 concentration(Fig.2), NaC1 concentration and pH on the growth rate were examined. The optimum growth temperatures are between 25 ~ - 40 ~ but the cells do not grow at 45 "C. The specific growth rates under the CO2 concentrations of ordinary air level to 40 % in air were almost constant with the highest value at 5% - 10% CO2 in air. The optimum pH levels are between 5 to 9. The cells grow in the medium containing NaC1 at up to 2% with relatively slower rate, but do not grow at 3%. The cells disperse easily in a culture medium and do not adhere to culture vessel indicating the advantage in mass culture. Furthermore, scum formation is rarely observed during the culture.
318
1/hr
1/hr
0.3" ::t
=" 0.2" 0
~ 0
0.1
f
0.10" 0
o.os-
o
0
r.~ 0.0
9
20
I
30
'
!~
40
"
r,/3 0.00
5'0
o
1'o
2'0
~o
~o
C02 concentration (%)
Culture temperature (~
Figure 2. Effects of temperature and CO2 concentration on the growth 3.2. High density culture of Chlorella sp. UK001 According to the elemental composition of the cells and the depletion rate of inorganic salts in the spent medium, we designed a medium for high density culture (HD medium). Ingredients of the medium are presented in Table 1. The results of high density culture using HD medium under continuous illumination or under light and dark cycle (10 hr/14 hr) are shown in Fig. 3. Under continuous illumination, the maximum concentration of dry cell weight reached about 10 g/1 and CO2 fixation rate was elevated to 3.0 gCO2/1/day. CO2 fixation rate under light and dark cycle was 1.2 gCO2/1/day. (g) 10
Table 1 Ingredients of medium Ingredients of
medium
KNO3 KH2PO4 MgSO4~ FeSO4.7H20
As-solution
H3BO3 ZnSO4o7H20 MnSO4~ CuSO4.7H20 Na2MoO4
HD
MC
(mg/I)
(mg/I)
3750 750 500 20
1250 1250 1250 2
2.860 2.500 0.222 0.079 0.021
2.860 2.500 0.222 0.079 0.021
3.0*
9 HD (L/D=24/0) O MC (L/D=24/0) ,~ HD (L/D=10/14)
r
._o~ 1.2 0.74
9
48
96
,
144
hr
* gCOz/1/day (38-158hr) Figure 3. High density culture of Chlorella sp. UK001
319
3.3. Nutritional composition of Chlorella sp. UK001 Elemental and nutritional compositions, fatty acid and amino acid compositions, and monosaccharide and vitamin contents of the cells are shown in Fig. 4 - 6, respectively. Protein is the major nutrient followed by the content of carbohydrate. All the essential amino acids are contained in the cells and the high content of linoleic acid which is the essential fatty acid was found. Nutritional composition of Chlorella sp. UK001 suggested usefulness of the cells as feed for poultry or swine.
[--~
--Carbohydrate 32% Lipid 12% Ashes 5%
H 7%
~
Mn 0.01%
Figure 4. Elemental and nutritional compositions
C14:0 C15:0 C16:0 C16:1A 7 C16:1A9 C16:2A7,10 C16:2A 9,12 C16:3 A 7,10,13 C17:0 C17:1A9 C18:0 C18:1A9 C18:1All C18:2A9,12 C18:3A 9,12,15
- - - - - - ] Palmitic acid Palmitoleic acid ---1 Hexadecadienic acid I
Oleic acid
0
1
2
I
Linoleic
3
4
%
5
o
Figure 5. Fatty acid and amino acid compositions
1
I
I
I
I I
I
6
%
320 Glucose 1 Galactose~ Rhamnose- -] Arabinose- -1 Mannose- ] Xylose-] Ribose-I GIucuronic acid-I 0
VitaminCt Carotene-
O~Tocopherol-'~
V~minD-~] * ,
5
,
10
,
15
'
20
%
~
0
T
*lU/100g ~
50
T
~
100
150
rng/lOOg
Figure 6. Monosaccharide and vitamin contents
4. CONCLUSION AND PERSPECTIVES The algal strain with highly efficient in fixing CO2 was isolated. The cells showed tolerance to high temperature and high CO2 concentration, making the strain advantageous in fixing CO2 in flue gas from power plants. The biomass produced by the cells can be used as feed for poultry or swine. In the experimental small scale cultures, the CO2 fixation rates of the alga exceeded the initial target value of 1gCO2/1/day at 24hr illumination. The initial target value was then raised to l gCO2/1/day at 10hr illumination so as to simulate the actual daylight period. After optimizing the culture conditions, the second target value was also attained. In order to bring out the ability of cells in mass culture, it is necessary to develop highly-efficient photobioreactor, evaluating the energy and CO2 balances. Development of gene transfer methods into algal cells and cloning of genes for producing useful materials will extend the possibilities of microalgal CO2 mitigation. REFERENCES 1. M. Murakami and M. Ikenouchi, Energy Convers. Mgmt., 38, (1997) 493. 2. N. Usui and M. Ikenouchi, Energy Convers. Mgmt., 38, (1997) 487. 3. H. Michiki, Energy Convers. Mgmt., 36, (1995) 701. 4. J.R. Benemann, Energy Convers. Mgmt., 38, (1997) 475. 5. K.L. Kadam, Energy Convers. Mgmt., 38, (1997) 505. 6. H. Tamiya et al, B.B.A, 12, (1953) 23.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Photobiological
production
of h y d r o g e n
321
gas
Yasuo A s a d a National I n s t i t u t e of Bioscience and H u m a n - T e c h n o l o g y , Higashi 1-1, T s u k u b a , I b a r a k i , 305 J a p a n
AIST/MITI,
H y d r o g e n p r o d u c t i o n by c y a n o b a c t e r i a and p h o t o s y n t h e t i c b a c t e r i a has b e e n s t u d i e d f r o m a p p l i c a t i o n a l points of view. In this p a p e r , r e c e n t p r o g r e s s m a i n l y from our w o r k s i n c l u d i n g c o o p e r a t i v e r e s e a r c h w i t h the Project of E n v i r o n m e n t a l l y - F r i e n d l y T e c h n o l o g y for the P r o d u c t i o n of H y d r o g e n by RITE ( R e s e a r c h I n s t i t u t e of I n n o v a t i v e T e c h n o l o g i e s for the Earth) will be r e v i e w e d . 1. I N T R O D U C T I O N H y d r o g e n gas could be a n e w e n e r g y c a r r i e r t h a t can c o n t r i b u t e to m i t i g a t i o n of w o r l d - w i d e e n v i r o n m e n t a l p r o b l e m s c a u s e d by c a r b o n d i o x i d e since h y d r o g e n gas does not e v o l v e c a r b o n d i o x i d e in c o m b u s t i o n . A l t h o u g h t h e r e h a v e b e e n v a r i o u s m e t h o d s for h y d r o g e n production, photo-biological method is attractive due to its e n v i r o n m e n t a l l y - a c c e p t a b l e c h a r a c t e r i s t i c s ; e n e r g y s o u r c e of sunlight, r e q u i r e m e n t of r e l a t i v e l y little f a c t o r y - m a d e e q u i p m e n t , a v a i l a b i l i t y of c a r b o n d i o x i d e (for the case of c y a n o b a c t e r i a ) , c o m b i n a t i o n w i t h w a s t e w a t e r t r e a t m e n t (for the case of p h o t o s y n t h e t i c b a c t e r i a ) , etc. A l t h o u g h the m a x i m a l c o n v e r s i o n efficiency f r o m light e n e r g y to h y d r o g e n gas p r o d u c e d was as high as a b o u t 6 to 8% in s o m e i n d o o r e x p e r i m e n t s (see the text, s e c t i o n 4, i.e.), f u r t h e r d e v e l o p m e n t s to i m p r o v e c o n v e r s i o n e f f i c i e n c y are r e q u i r e d for practical h y d r o g e n p r o d u c t i o n . I will i n t r o d u c e m a i n l y our w o r k s i n c l u d i n g c o o p e r a t i v e r e s e a r c h with the RITE project, " E n v i r o n m e n t a l l y - F r i e n d l y T e c h n o l o g y for Biological H y d r o g e n P r o d u c t i o n " and give an o v e r v i e w of w o r l d - w i d e r e s e a r c h activities on p h o t o b i o l o g i c a l p r o d u c t i o n of h y d r o g e n in special r e l a t i o n s h i p w i t h the p r o g r a m of IEA ( I n t e r n a t i o n a l E n e r g y A g e n c y , an OECD a g e n c y ) H y d r o g e n Committee, A n n e x 10. 2. M E T H O D O L O G Y T h e r e h a v e b e e n two m a j o r factors in p h o t o b i o l o g i c a l h y d r o g e n production as a technology; productivity of hydrogen by m i c r o o r g a n i s m s , a n d c h a r a c t e r i s t i c s of p h o t o b i o r e a c t o r s . M i c r o b i o l o g y , m i c r o b i a l p h y s i o l o g y , b i o c h e m i s t r y , g e n e t i c e n g i n e e r i n g could c o n t r i b u t e to s u r v e y and b r e e d i n g of p o t e n t h y d r o g e n - p r o d u c i n g m i c r o o r g a n i s m s .
322 For efficient and e c o n o m i c a l l y feasible p h o t o b i o r e a c t o r s , biochemical e n g i n e e r s h a v e b e e n s t u d y i n g m u c h on p h o t o s y n t h e t i c m i c r o o r g a n i s m s . Sun light s h o u l d be p r o v i d e d h o m o g e n e o u s l y at a p p r o p r i a t e i n t e n s i t y inside of p h o t o b i o r e a c t o r s , which s e e m s a key factor for i m p r o v e m e n t of efficiency. 3. HYDROGEN P R O D U C T I O N BY C Y A N O B A C T E R I A
3.1. N i t r o g e n a s e - m e d i a t e d cyanobacteria
h y d r o g e n p r o d u c t i o n by
N i t r o g e n a s e is r e s p o n s i b l e not only for n i t r o g e n fixation but also h y d r o g e n p r o d u c t i o n [1]. Most of n i t r o g e n - f i x i n g c y a n o b a c t e r i a h a v e u p t a k e h y d r o g e n a s e s that are re-utilizing h y d r o g e n p r o d u c e d by n i t r o g e n a s e . The u p t a k e h y d r o g e n a s e s r e d u c e net h y d r o g e n p r o d u c t i o n [2,3]. We r e p o r t e d a e r o b i c h y d r o g e n p r o d u c t i o n by a n i t r o g e n - f i x i n g A n a b a e n a sp. that was s u p p o s e d to be u p t a k e h y d r o g e n a s e - d e f i c i e n t s t r a i n [4]. A f t e r 12 days culture s t a r t e d with a small i n o c u l u m , the s t r a i n a c c u m u l a t e d a b o u t 10% h y d r o g e n and 70% o x y g e n gas in the gas p h a s e of the v e s s e l by the side reaction of n i t r o g e n a s e . A l t h o u g h the c o n v e r s i o n efficiency ( c o m b u s t i o n e n e r g y of h y d r o g e n / i n c i d e n t light e n e r g y ) was s m a l l e r than 0.1%, this strain could be a s t a b l e h y d r o g e n p r o d u c e r w i t h o u t use of inert gas such as argon.
3.2. H y d r o g e n a s e - m e d i a t e d cyanobacteria
h y d r o g e n p r o d u c t i o n by
T h e r e was some c o n t r o v e r s y in the existence and p h y s i o l o g i c a l role of r e v e r s i b l e h y d r o g e n a s e in c y a n o b a c t e r i a [2, 3]. We h a v e r e p o r t e d that c y a n o b a c t e r i a can p r o d u c e h y d r o g e n u n d e r d a r k and a n a e r o b i c c o n d i t i o n s [5-8] w h e r e r e v e r s i b l e h y d r o g e n a s e s s e e m e d active. Spirulina s p e c i e s w e r e d e m o n s t r a t e d to have the highest a c t i v i t y a m o n g the tested[7]. R e s p o n s e by h y d r o g e n - p r o d u c i n g c y a n o b a c t e r i u m , M i c r o c y s t i s aeruginosa to light r e s u l t e d not in s t i m u l a t i o n of h y d r o g e n p r o d u c t i o n but h y d r o g e n c o n s u m p t i o n [5, 6]. N e v e r t h e l e s s of r e c e n t p r o g r e s s in gene s t r u c t u r e s t u d i e s [3], electron t r a n s p o r t concerning native h y d r o g e n a s e , e s p e c i a l l y its r e l a t i o n s h i p with p h o t o s y n t h e t i c s y s t e m is still u n c l e a r [9-11]. T h e r e f o r e , w e have not chosen n a t i v e h y d r o g e n a s e s in c y a n o b a c t e r i a but a n o t h e r h y d r o g e n a s e as the t a r g e t of g e n e t i c improvement.
3.3. E x p r e s s i o n of h e t e r o l o g o u s h y d r o g e n a s e into cyanobacteria Our s t r a t e g y is coupling of clostridial h y d r o g e n a s e w i t h p h o t o s y n t h e t i c s y s t e m . To check the r e l e v a n c y of our strategy, in vivo c o u p l i n g was t e s t e d . The c y a n o b a c t e r i a l cells into which clostridial h y d r o g e n a s e p r o t e i n w a s e l e c t r o - i n d u c e d p r o d u c e d h y d r o g e n by light i r r a d i a t i o n [121. Then, we h a v e b e e n t r y i n g to o v e r e x p r e s s the h y d r o g e n a s e in a
323
Synechococcus cyanobacterium, e n g i n e e r i n g s y s t e m [13, 14]. The for the h e t e r o l o g o u s e x p r e s s i o n h y d r o g e n a s e a c t i v i t y , yet.
PCC7942 by developing genetic S h i n e - D a l g a r n o s e q u e n c e was critical [15]. We h a v e not d e t e c t e d the
4. HYDROGEN P R O D U C T I O N BY P H O T O S Y N T H E T I C B A C T E R I A P h o t o s y n t h e t i c b a c t e r i a c a r r y out a n o x y g e n i c p h o t o s y n t h e s i s w i t h o r g a n i c c o m p o u n d s or r e d u c e d s u l f u r c o m p o u n d s as e l e c t r o n d o n o r s . Some n o n - s u l f u r p h o t o s y n t h e t i c b a c t e r i a are p o t e n t h y d r o g e n p r o d u c e r s utilizing o r g a n i c acids such as lactic, succinic and b u t y r i c acids [16]. From p r a c t i c a l p o i n t of view, p h o t o s y n t h e t i c b a c t e r i a are i m p o r t a n t since t h e y can be u s e d for dual aims of w a s t e - w a t e r t r e a t m e n t and h y d r o g e n p r o d u c t i o n as d e s c r i b e d in the next section. M i y a k e and K a w a m u r a [17] d e m o n s t r a t e d t h a t 6 to 8% of i n c i d e n t light e n e r g y can be c o n v e r t e d to h y d r o g e n gas ( c o m b u s t i o n e n e r g y ) in l a b o r a t o r y e x p e r i m e n t s by using our isolate, R h o d o b a c t e r sp. 8 7 0 3 [18] w i t h lactic acid as an e l e c t r o n donor. 4 . 1 . H y d r o g e n p r o d u c t i o n by c o m b i n a t i o n of p h o t o s y n t h e t i c bacteria with anaerobic bacteria A n a e r o b i c b a c t e r i a d e g r a d e s u g a r to h y d r o g e n gas and o r g a n i c acids [19, i.e.], but can not b r e a k d o w n the organic acids any m o r e . M i y a k e et aI. [20] p r o p o s e d a c o m b i n a t i o n use of p h o t o s y n t h c t i c b a c t e r i a w i t h a n a e r o b i c b a c t e r i a for c o m p l e t e c o n v c r s i o n of s u g a r to h y d r o g e n . T h e o r e t i c a l l y , one m o l e glucose can be c o n v e r t e d to 12 m o l e s of h y d r o g e n , and the e x p e r i m e n t a l r e s u l t was 7 m o l e s p e r c o n s u m e d glucose. From p r a c t i c a l point o f v i c w , organic w a s t e s f r e q u e n t l y c o n t a i n s u g a r or s u g a r p o l y m c r s . The c o m b i n a t i o n usc of thc two k i n d s of b a c t e r i a w o u l d e n l a r g e p r o b a b i l i t y of a p p l i c a t i o n in p h o t o b i o l o g i c a l h y d r o g e n p r o d u c t i o n [21]. F u r t h e r m o r e , p h o t o s y n t h e t i c b a c t e r i a , by t h e m s e l v e s , can a p p l i c a b l e to v a r i o u s k i n d s of o r g a n i c w a s t e w a t e r [22, 23].
4.2. Genetic engineering to c o n t r o l p h o t o s y n t h e t i c proteins of p h o t o s y n t h e t i c bacteria For e f f i c i e n t h y d r o g e n p r o d u c t i o n , the b a l a n c e of p h o t o s y s t e m and h y d r o g e n p r o d u c i n g e n z y m e is r e q u i r e d . C o m p a r i n g to n i t r o g e n a s e a c t i v i t y in p h o t o s y n t h e t i c b a c t e r i a also, too m u c h light e n e r g y is c o n v e r t e d to b i o c h e m i c a l e n e r g y by p h o t o s y s t e m t h a t will be lost by other biochemical processes. We h a v e b e e n t r y i n g to control p h o t o s y s t e m to t h e a p p r o p r i a t e level for n i t r o g e n a s e a c t i v i t y . For this operon of Rhodobacter sphaeroides RV e n c o d i n g purpose, puf p h o t o r e a c t i o n c e n t e r and light h a r v e s t i n g p r o t e i n s was i s o l a t e d at first and c h a r a c t e r i z e d [24]. F u r t h e r m o r e , we h a v e o b t a i n e d a m u t a n t by UV-treatment that alters in photosynthetic protein complexes. C h a r a c t e r i z a t i o n of the m u t a t i o n and its h y d r o g e n p r o d u c t i o n u n d e r m o n o c h r o m a t i c light is going on.
324 5. B A S I C
STUDIES
FOR R & D OF P H O T O B I O R E A C T O R S
There has b e e n a difficult p r o b l e m of ' s e l f - s h a d o w i n g ' in utilization of solar e n e r g y w i t h c u l t u r e s of p h o t o s y n t h e t i c m i c r o o r g a n i s m s . Quite a small portion of p h o t o b i o r e a c t o r s is active due to inability of d e e p light p e n e t r a t i o n w h e n microbial cell c o n c e n tr a tion is high. T h e r e f o r e , basic analysis on d i s t r i b u t i o n of light intensity and shift in c o m p o s i t i o n of w a v e l e n g t h in r e l a t i o n s h i p to h y d r o g e n production [25, 26] is i m p o r t a n t . P h o t o b i o r e a c t o r for h y d r o g e n production by c y a n o b a c t e r i a was s t u d i e d by M a r k o v e t al. [27]. Various types of p h o t o b i o r e a c t o r s for photosynthetic bacteria in RITE Hydrogen Project have been d e m o n s t r a t e d [28-31]. Moreover, immobilization of bacteria could contribute to prolongation, stabilization and ease in exchange of c ulture broth. We have d e v e l o p e d m e t h o d s with glass plate [32], glass beads [33] and agar gel [34]. 6. RESEARCH AND DEVELOPMENT PRODUCTION IN THE WORLD
FOR B I O L O G I C A L HYDROGEN
A l t h o u g h some r e s e a r c h groups have been active in basic or a p p l i e d fields r e l a t e d to biological h y d r o g e n production, r e c e n t w o r l d - w i d e e n v i r o n m e n t a l p r o b l e m s u rg ed formation of national projects for biological h y d r o g e n production. Federal Ministry for Research and Technology, G e r m a n y c o n d u c t e d and funded Biological h y d r o g e n p r o d u c t i o n and h y d r o g e n a s e s ( 1 9 8 9 - 1 9 9 4 ) , w h e r e the p a r t i c i p a n t s w e r e from u n i v e r s i t i e s and m a d e basic research. As m e n t i o n e d above, we are c o o p e r a t i v e l y i n v o l v e d in the project of E n v i r o n m e n t a l l y - F r i e n d l y Technology for the Production of Hydrogen ( 1 9 9 1 - 1 9 9 8 ) by RITE, w i t h financial o p e r a t i o n by New Energy and I n d u s t r i a l Development Organization (NEDO). Ministry of I n t e r n a t i o n a l Trade and I n d u s t r y , Japan is the final s p o n s o r and conductor. The project includes total technologies for biological h y d r o g e n production; s c r e e n i n g and b r e e d i n g of m i c r o o r g a n i s m s and basic research and developments for p h o t o b i o r e a c t o r and a n a e r o b i c bioreactor. H y d r o g e n c o m m i t t e e of I n t e r n a t i o n a l Energy Agency (IEA, u n d e r control of OECD) has r e a r r a n g e d annex c o m m i t t e e s for h y d r o g e n technologies. The t a r g e t of Annex 10 is p h o t o p r o d u c t i o n of h y d r o g e n . It is c o m p o s e d of t h r e e subtasks: A. P h o t o e l e c t r o c h e m i c a l h y d r o g e n production, B. Photobiological h y d r o g e n production, C. S t a n d a r d i z a t i o n . T h r e e - y e a r plan ( 1 9 9 5 - 1 9 9 8 ) is aiming w o r l d - w i d e and i n t i m a t e r e s e a r c h n e t w o r k to p r o m o t e h y d r o g e n production technologies. 7. CONCLUSIONS AND FUTURE PROSPECTS Photobiological h y d r o g e n p r o d u c t i o n is the most c ha lle nging item of b i o t e c h n o l o g y for e n v i r o n m e n t a l problems. We are not c o n f i d e n t t h a t photobiological h y d r o g e n p r o d u c t i o n is economically feasible at this
325 momont. Practical a p p l i c a t i o n of this t e c h n o l o g y s e e m s p r i m a r i l y d e p e n d e n t on i m p r o v e m e n t s of the c o n v e r s i o n efficiency a l t h o u g h R & D for p h o t o b i o r e a c t o r s are also i m p o r t a n t . I s u p p o s e that our e f f o r t s m a y h a v e d e m e o n s t r a t e d n e w w a y s for i m p r o v e m e n t s of the c o n v e r s i o n e f f i c i e n c y of light e n e r g y to h y d r o g e n gas by t h e s e p h o t o s y n t h e t i c m i c r o o r g a n i s m s a l t h o u g h we h a v e not r e a c h e d the final goal. The f u t u r e a n d t e c h n o l o g i c a l p r o b a b i l i t y of p h o t o b i o l o g i c a l h y d r o g e n p r o d u c t i o n d e p e n d s on not only a d v a n c e of r e s e a r c h but also e c o n o m i c a l c o n d i t i o n s (price of fossil fuels) or social a c c e p t a n c e and d e v e l o p m e n t of h y d r o g e n e n e r g y s y s t e m s . ACKNOWLEDGMENTS Our w o r k s w e r e p e r f o r m e d p a r t i a l l y u n d e r m a n a g e m e n t s of RITE as a part of E n v i r o n m e n t a l l y - F r i e n d l y T e c h n o l o g y for the P r o d u c t i o n of Hydrogen supported by New E n e r g y and I n d u s t r i a l Technology D e v e l o p m e n t O r g a n i z a t i o n (NEDO). REFERENCES
1. J. Miyake, Y. A s a d a a n d S. K a w a m u r a , In Biomass h a n d b o o k , Kitani. O. and Hall. C . W . ( e d s ) , Gordon and Breach Science P u b l i s h e r s , New York. ( 1 9 8 9 ) 362. 2. J.P. Houchins a n d R.H. Burris, J. Bacteriol., 146 ( 1 9 8 1 ) 125. 3. R. Schulz, J. M a r i n e Biotech., 4 ( 1 9 9 6 ) 16. 4. Y. A s a d a and S. K a w a m u r a , Appl. Environ. Microbiol., 5 1 ( 1 9 8 6 ) 1063. 5. Y. A s a d a and S. K a w a m u r a , Agric. Biol. Chem., 4 8 ( 1 9 8 4 ) 2 5 9 5 . 6. Y. A s a d a and S. K a w a m u r a , Rept. F e r m e n t . Res. Inst., 63 ( 1 9 8 5 ) 39. 7. Y. A s a d a and S. K a w a m u r a , J. F e r m e n t . Technol., 64 ( 1 9 8 6 ) 553. 8. IC A o y a m a , I. U e m u r a , J. M i y a k e and Y. Asada. J. F e r m e n t . Bioeng., 83 ( 1 9 9 7 ) 53. 9. Y. Asada, S. K a w a m u r a and K-K. Ho. P h y t o c h e m i s t r y , 26 ( 1 9 8 7 ) 637. 10. Y. Asada, M. M i y a k e , Y. Koike, I. U e m u r a and J. M i y a k e , In Proc. of B i o H y d r o g e n 97 (Int. W o r k s h o p on Biological H y d r o g e n P r o d u c t i o n , 2 3 - 2 6 June, Kona, Hawaii), Zaborsky, O. (ed) in s u b m i s s i o n ( 1 9 9 7 ) . 11. J. Appel, S. P h u n p r u c h , S. Stangier, R. W u e n s c h i e r s and R. Schulz, I n A b s t r a c t s of H y d r o g e n a s e s 97, 5th Int. Conf. on the m o l e c u l a r biology o f h y d r o g e n a s e s , A l b e r t v i l l e , France, 1 2 - 1 7 July, ( 1 9 9 7 ) 103. 12. M. M i y a k e a n d Y. A s a d a , Biotech. T e c h n i q u e , in press, ( 1 9 9 7 ) . 13. M. Miyake, J. Y a m a d a , K. A o y a m a , I. U e m u r a , T. Hoshino, J. M i y a k e and Y. Asada, J. M a r i n e Biotech. 4 ( 1 9 9 6 ) 61. 14. K. A o y a m a , M. M i y a k e , J. Y a m a d a , J. Miyake, I. U e m u r a , T. H o s h i n o and Y. Asada, J. M a r i n e Biotech. 4 ( 1 9 9 6 ) 64. 15. Y. Koike, K. A o y a m a , I. U e m u r a , J. M i y a k e and Y. Asada, I n A b s t r a c t s of H y d r o g e n a s e s 97, 5th Int. Conf. on the m o l e c u l a r biology of h y d r o g e n a s e s , A l b e r t v i l l e , France, 1 2 - 1 7 July, ( 1 9 9 7 ) 119.
326 16. J. Miyake, In ECB6: Proceeding of the 6th European Congress on Biotechnology, Alberghina, L., Frontali, L. and Sensi, P. (eds), Elsevier, (1993) 1019. 17. J. Miyake and S. Kawamura, Int. J. Hydrogen Energy, 39 ( 1 9 8 7 ) 1 4 7 . 18. X.-u Mao, J. Miyake and S. Kawamura, J. Ferment. Technol., 64 (1986) 245. 19. Y. Ueno, T. Kawai, S. Sato, S. Ohtsuka and M. Morimoto, J. Ferment. Bioeng. 79 ( 1 9 9 5 ) 395. 20. J. Miyake, X.-Y. Mao and S. Kawamura, J. Ferment. Technol. 62 (1984) 531. 21. H.-G. Zhu, T. Suzuki, A . A . Tsygankov, E. Nakada, Y. Asada and J. Miyake, J. W a t e r T r e a t m e n t , 10 (1995) 61. 22. H.-G. Zhu, J.-L. Shi and Y.-T. Xu, J. Env. Sci. Shanghai, 4 (1994) 10. 23. H.-G. Zhu, Y.-L. Zao, and J.-L. Shi, J. Appl. Ecology, 8 (1997) 194. 24. Y. Nagamine, T. Kawasugi, M. Miyake, Y. Asada and J. Miyake, J. Marine Biotech., 4 (1996) 34. 25. E. Nakada, Y. Asada, T. Arai and J. Miyake, J. Ferment. Bioeng., 80 (1995) 53. 26. E. Nakada, S. Nishikata, Y. Asada and J. Miyake, In Photosynthesis: from light to biosphere, Proc. 10th Int. Photosynthesis Congress, Montpellier, France, 20-25, Aug., Mathis, P.(ed), Kluwer Academic Publishers,, Dordrecht, 5 (1995) 837. 27. S.A. Markov, M.J. Bazin and D.O. Hall, Advances in Biochem. Eng. Biotech., 52 (1995) 59. 28. M. Minami, T. Uechi, M. Kimura, T. Nishishiro, H. Kuriaki, K. Sano and T. Kawasugi, ln H y d r o g e n Energy Progress XI, Proc. l l t h World Hydrogen Energy Conf., Stuttgart, Germany, 23-28 June, Veziroglu, T.N., Winter, C.-J., Baselt, J.P. and Kreysa, G. (eds), p u b l i s h e d by DECHEMA, F r a n k f u r t ) 3 (1996) 2583. 29. M. Morimoto, S. Kawasaki, R.M.A. E1-Shishtawy, Y. Ueno and N. Kunishi, ln H y d r o g e n Energy Progress XI, Proc. 11th World 9 Hydrogen Energy Conf., Stuttgart, Germany, 23-28 June, Veziroglu, T.N., Winter, C.-J., Baselt, J.P. and Kreysa, G. (eds), p u b l i s h e d by DECHEMA, Frankfurt, 3 (1996) 2761. 30. E.D'Addario, E. Fascetti and M. Valdiserri, ln Hydrogen Energy Progress XI (Proc. l l t h World Hydrogen Energy Conf., Stuttgart, Germany, 2 3 - 2 8 June, Veziroglu, T.N., Winter, C.-J., Baselt, J.P. and Kreysa, G. (eds), p u b l i s h e d by DECHEMA)vol.3(1996) 2577. 31. S. Uchiyama, T. Ohtsuki and S. Fukunaga, In Hydrogen Energy Progress XI, Proc. 11th World Hydrogen Energy Conf., Stuttgart, Germany, 2 3 - 2 8 June, Veziroglu, T.N.,Winter, C.-J.,Baselt, J.P. and Kreysa, G. (eds) p u b l i s h e d by DECHEMA, Frankfurt, 3 (1996) 2607. 32. A.A. Tsygankov, Y. Hirata, Y. Asada and J. Miyake, Biotech. Technique,7 ( 1 9 9 3 ) 283. 33. A.A. Tsygankov, Y. Hirata, M. Miyake, Y. Asada and J. Miyake, J. Ferment. Bioeng., 7 (1994) 575. 34. E. Nakada, S. Nishikata, Y. Asada and J. Miyake, J. Marine Biotechnol. 4 (1996) 38.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
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Hydrocarbon synthesis from C O / o v e r composite catalysts Yoshie Souma, Masahiro Fujiwara, Roger Kieffel~, Hisanori Ando and Qiang Xu Osaka National Research Institute, AIST, MITI, 1-8-31 Midorigaoka,Ikeda, Osaka563,Japan a ECPM Universite de Strasbourg, 67008, Strasbourg, France
Hydrogenation of C O 2 w a s carded out using various composite catalysts containing a methanol synthesis catalyst and a zeolite. Hydrocarbons were obtained via methanol by MTG(Methanol to Gasoline) reaction. Cu-Zn-Cr mixed oxide prepared from CuO, ZnO and CrO3 was an effective component of the composite catalyst for the synthesis of light paraffins. The composite catalyst containing Cu-Zn chromate or Fe-ZnO gave higher hydrocarbons via olefin intermediate formed from methanol. Fe-ZnO had two active sites; iron species formed from Fe304 and Z n F e 2 0 4. When Fe-ZnO was used alone, F-T reaction over iron species was predominant. The addition of HY zeolite to Fe-ZnO deactivated the iron species for F-T reaction, and thus MTG reaction over ZnFe204 became predominant.
1. INTRODUCTION Carbon dioxide is an important carbon source to produce chemicals and fuels. Hydrogenation of CO2 using composite catalyst, containing a methanol synthesis catalyst and a zeolite, is a good method to give C/+ hydrocarbons, in which hydrocarbon distribution is not restricted by Schulz-Anderson-Flory law[l]. We studied hydrocarbon synthesis from CO: using Cu-Zn-Cr (3:3:l)/HY and Fe-ZnO/HY composite catalysts[2"-6]. In this case, first CO2 converted to methanol, and then methanol converted to hydrocarbon over zeolite. The main products formed among hydrocarbons was ethane, propane and butane, and hydrocarbon distribution was different from Schulz-AndersonFlory rule. In MTG reaction, methanol converted to dimethyl ether, then to ethene, and ethene converted to higher hydrocarbons or ethane. In order to obtain higher hydrocarbons, it is important to f'md active catalysts which have no hydrogenation activity of olefm. In this paper, we wish to report the higher hydrocarbon synthesis via olefin over composite catalyst and the influence of zeolite on Fe-ZnO catalyst.
328 2. EXPERIMENTAL
2.1 Catalyst Cu-Zn-Cr oxides were prepared as follows. CuO was added to an aqueous solution of CrO3, and ZnO was added after 1 h aging. The obtained paste was dried without heating. Fe-based catalysts were prepared by the coprecipitation of the corresponding nitrates using sodium hydroxide. The precipitate was washed five times, dried at 120~ for 6 h and calcined at 350~ for 3 h. The composite catalysts were obtained by the physical mixing of the equal amounts of a methanol synthesis catalyst and a zeolite. HY [JRC-Z-HY4.8(2)] and NaY(JRCZ-Y4.8) were provided from the Catalysis Society of Japan as the Reference Catalyst. 2.2 Reaction procedure Hydrogenation of carbon dioxide was carded out using a pressurized flow type fixed bed reactor. In a typical experiment, 1 g of catalyst was packed in the stainless steel tubular reactor with an inner diameter of 10 mm. After in situ reduction of the catalyst at 250 "-~ 400~ reaction gas (H2/CO2=3 or 4) was introduced into the reactor under 0.1"~5MPa. Then the reaction conditions were set (200~400~ and hydrogenation was started. An effluent gas was analyzed by on-line gas chromatographs using Porapak Q for carbon dioxide, MS-13X for methane and carbon monoxide, PEG for methanol and VZ-10 for hydrocarbons. The tubing from catalyst bed to gas chromatograph was heated at 100 "~ 150~ to avoid the condensation of all products.
3. RESULTS AND DISCUSSION Hydrocarbons were produced from 002+H2 methanol synthesis cat
zeolite v
-..--
CO 2
and
H2
by the following path 9
CH30 H zeolite
methanol synthesis cat CnH2n ,, zeolite(hydrogen transfer)
The results of hydrogenation of
CO 2 over
zeolite
~ CH3OCH 3 .......
.~ CH2=CH 2
CnH2n+2
various catalysts are shown in Table 1.
(1)
329 Table 1 Hydrogenation of carbon dioxide over composite catalysts?) Run Catalyst Conv.of Conv.to(%) HC CO2(% ) HE Oxyb) CO C1 1 Cu-Zn-Cr(3:3:1)/HY 37.3 9.2 0.8 27.3 8 2 Cu-Zn-Cr(3:3:1)/HY d> 32.4 2.1 1.1 29.2 14 3 Cu-Zn-Cr(3:3:1)/HY ~> 40.9 14.4 0.5 26.0 5 4 Cu-Zn-chromate~/HY 35.5 5.2 0.1 30.2 3 5 Fe-Zn(4:1)/HY g) 13.3 4.9 0.2 8.2 8
Distribution(C-mol%) C2__SelP C2 C3 C4 C5+ (%) 18 37 23 14 0 38 33 10 5 0 24 35 24 12 0 9 24 39 25 58 15 14 26 37 80
a) 4000C, 50 atm, SV=3000ml/g-cat-h, H2/CO2=3, b) MeOH+Me20. c) C2jC2_PC2=" d) Prepared by coprecipitation. ~) Mixing using granules, o Cu/(Cu+Zn)=0.01. g)350~ When methanol synthesis catalyst, prepared from CuO, ZnO and CrO3, was mixed with HY zeolite, C2+ hydrocarbons were obtained in a good selectivity(Run 1)[2]. The same catalyst made into granule gave better results (Run 3). The selectivities to ethane, propane and butane were higher and that to methane was lower. No olefin was observed. The hydrocarbons distribution of typical Cu-Zn-Cr/HY is shown in Figure 1. This is a typical distribution of MTG reaction, and quite different from Schultz-Anderson-Flory law.
[-7 Paraffin ~" 3 O
E
0
1
2
I
3
4
5
I
6
C number Figure 1. Hydrocarbons distribution over Cu-Zn-Cr/HY
The yield of hydrocarbons (14.4%) were higher than that of equilibrium conversion of C O 2 to methanol (ca 7% at 400~ 50 atm). It means that methanol formation was accelerated by MTG reaction. When methanol synthesis catalyst was prepared by coprecipitation, the yield of hydrocarbons decreased (Run 2). This seems to be due to the deactivation of zeolite by the sodium remaining after 5 times wash. Similar tendency was observed on the hydrocarbon synthesis between two Cu-Zn/HY composite catalyst, in which one Cu-Zn catalyst was precipitated by Na2CO3, and another Cu-Zn catalyst was precipitated by oxalic acid[3]. When methanol synthesis catalyst was prepared by sodium compound, remaining sodium deactivate an active site of zeolite on MTG reaction.
330
Secondly, Zn-chromate catalyst, which is useful for the reduction of ester and does not hydrogenate coexisting double bond, was applied for composite catalyst[5]. A Cu-Znchromate catalyst containing 1% of Cu was most effective among various composite catalysts (Run 4). The formation of ethene was observed, and the selectivity of higher hydrocarbon increased comparing to Run 1. It was shown that higher hydrocarbons were obtained via olefin. Iron catalyst is well-known as Fisher-Tropsh catalyst to give hydrocarbons with Schulz Flory law. However, hydrocarbons distribution changed drastically with the addition of zeolite(Run 5)[6]. The selectivity to ethene was high, and the selectivity to higher hydrocarbons increased comparing to those of other composite catalysts. The main hydrocarbons produced were branched. Detail studies were carried out on Fe-ZnO and Fe-ZnO/zeolite catalyst further more. Hydrocarbon synthesis from CO2 over various Fe-ZnO/zeolite catalyst is shown in Table 2. By the addition of HY zeolite, the formation of olefins and the selectivity of C2§ hydrocarbon increased and that of methane decreased. The hydrocarbon distribution of Fe-ZnO and FeZ n O / H is shown in Figure 2. Hydrocarbon distribution followed Schulz-Flory rule over FeZnO catalyst, indicating F-T reaction took place. However, it changed to the formation of higher hydrocarbons over Fe-ZnO/HY, indicating MTG reaction took place. Table 2. The effect of zeolite on the formation of hydrocarbons Catalyst CO2 Conv. Yield(%) Sel. of olefin (Fe:Zn=4:1) (%) CI-I4 C2. Oxya CO Cz= C3= Fe-ZnO 17.2 7.6 3.1 0.5 6.0 1 4 Fe-ZnO/HY 13.3 0.4 4.5 0.2 8.2 80 30 Fe-ZnO/HM 11.5 0.2 3.0 2.6 5.7 90 73 FeZnO/H-ZSM-5 14.4 0.0 2.8 0.0 11.6 40 11 Condition: 350~ 50 atm, SV=3000ml/g-cat. h, H2/CO2=3, 6h. a MeOH + Me20.
2
(A) Fe-ZnO(4:1)/HY
(B) Fe-ZnO(4:1)
r-1 Paraffin I I Olefin
o~" 1.5 !
0
!
1'
.,.,.,
.~_
>" 0.5 ,
1 Figure 2.
I
I.
2 3 4 5 6 Carbon Number
I
7
-
1
2 3 4 5 6 Carbon Number
7
Hydrocarbons distribution over Fe-ZnO(4:I)/HY (A) and Fe-ZnO(4:I)(B).
331
The effect of the zinc content on the catalytic behaviors of Fe-ZnO is shown in Figure 3(A). When the zinc content is higher than 33%, methanol was obtained in up to 3% yield and the yields of hydrocarbons including methane was very low. These Fe-ZnO catalysts acted as methanol synthesis catalyst rather than F-T catalyst. On the other hand, Fe-ZnO catalysts with a zinc content lower than 33% produced hydrocarbons in good yields. The distribution of hydrocarbons approximately followed the Schulz-Anderson-Flory model, indicating that those catalysts work as F-T catalysts. The dependence of the catalytic behaviors of Fe-ZnO/HY on the zinc content is also shown in Figure 3(B). Fe/HY (without zinc) and Zn/HY (without iron) gave hydrocarbons in very poor yields. However, Fe-ZnO/HY with various zinc contents produced hydrocarbons in up to 5% yields, and the selectivities of C2+ hydrocarbons in all hydrocarbons were high. Although the zinc content was not a crucial factor in the hydrocarbon synthesis, the best yield of hydrocarbons was observed with Fe-ZnO (4:I)/HY. XRD Patterns for Fe-ZnO catalysts containing less than 33% ZnO before the reaction showed two phases : a-Fe~O3 and ZnFe204. After the reaction, the diffraction patterns of a-Fe203 disappeared completely while those of ZnFe204remained. This suggests that a Fe~O3 was transformed to other iron species effective for F-T reaction, and ZnFe204 was not active for F-T reaction. On the other hand, the diffraction patterns of ZnO and ZnFe204 were detected in Fe-ZnO with a zinc content higher than 33%. After the reaction, all peaks still remained but became sharper. The catalytic activity for methanol synthesis of Fe-ZnO(l:2) was higher than that of ZnO as shown in Figure 3(A), indicating that not only ZnO but also ZnFe204 is responsible for methanol synthesis. 30 (A)
c,
(B)
20
10
5
4~
15 ......
20
.....
002
I- / ~
~-
<
:>
cony.
I
2~
Figure. 3.
20
40
60
Zn Fe+Zn (%)
80
~
o , ~
"']~176176 ~. . . . . . . . .
2 ~] ,.,....
0
.=.........e...
,
MeOH
0 100
o_.
O 5 0 0
20
40
60
80
=~0 100
Zn Fe+Zn (%)
Effect of zinc content on the activity of Fe-ZnO(A) and Fe-ZnO/HY(B). Reaction conditions: 350~ 5MPa, SV=3000ml/g-cat. h, H2/CO2=3 [6]
The Fe-ZnO catalyst shows two kinds of catalytic sites, that is, iron species effective for F-T reaction formed from a-Fe203(Fe304) and Fe promoted ZnFe204 effective for methanol synthesis. In the absence of zeolite, the F-T reaction sites are very active to produce hydrocarbons with the Schulz-Anderson-Flory distribution. On the other hand, the sites for F-T reaction are deactivated and the sites for methanol synthesis ZnFe204 exhibit the catalytic activity in the case of the composite catalyst. Therefore, hydrocarbons were obtained by MTG reaction with a non-Schulz-Anderson-Florv distribution over F e - Z n O / H (Figure 4).
332
,.,,,.,
CO ,...~...t,
tjk.)2~ t ~ "1-
H2
i
..........
~ . . . . . . .: . . . . . . . .. . . . . . "
F-T Reaction - ~ . . . .' -
~ c a t a l y s t
~
[._~_~_~-~..
MeOH
z' z - .".'.....~. ., . . . . . . . . . .' .: .
,
..,: :.1
:.....: :~ ~ " + ~
,,Ee7(fr0m Fe3o4) '
I Hydrocarbons] Schulz-AndersonFiery Distribution
(High Methane Selectivity) Non-
Schulz-AndersonFiery
Distribution
(Low MethaneSelectivity) Figure 4.
Reaction scheme of hydrocarbon synthesis over Fe-ZnO/HY [6]
4. CONCLUSION Hydrocarbons were obtained from CO2 by MTG reaction using various composite catalysts. Cu-Zn-Cr oxide prepared from Cue, ZnO and CrO3 was an effective component of the composite catalyst for the synthesis of light paraffins. Methanol synthesis was accelerated on composite catalyst, and much hydrocarbons were obtained above equilibrium conversion to methanol. Cu-Zn-chromate/HY or Fe-ZnO/HY, which gave ethene or propene, were good catalysts to give higher hydrocarbons by oligomerization of olefin. The main hydrocarbons produced were branched. The XRD analysis showed that Fe-ZnO has two active sites: iron species formed from Fe304 for F-T reaction and ZnFe204 for methanol synthesis. When Fe-ZnO was used alone, F-T reaction over iron species was predominant. The addition of HY zeolite to Fe-ZnO deactivated the iron species for F-T reaction, and thus MTG reaction over ZnFe204 became predominant.
REFERENCE
1. K. Fujimoto, T. Shikada, Apple. Catal., 31, (1987)13. ; T. Inui, T. Takeguchi, Catal Today, 10, (1991) 95. 2. M. Fujiwara, Y. Souma, J. Chem. Soc., Chem. Commun., (1992)767. 3. M. Fujiwara, R. Kieffer, L. Udron, H. Ando, Y. Souma, Catal. Today, 29, (1996)343. 4. M. Fujiwara, R. Kieffer, H. Ando, Y. Souma, Appl. Catal. A, 121, (1995) 113. 5. M. Fujiwara, H. Ando, M. Tanaka, Y. Souma, Appl. Catal. A, 130, (1995)105. 6. M. Fujiwara, R. Kieffer, H. Ando, Q. Xu and Y. Souma, Appl. Catal. A, 154, (1997)87.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
333
Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
C02 for petrochemicals feedstock. Conversion to synthesis gas on metal supported catalysts. P. Gronchi, P. Centola, and R. Del Rosso Industrial Chemistry and Chemical Engineering Department, CIIC, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133 Milano, Italy.Tel/Fax+39-2-23993274. E-mail Paolo.
[email protected]
1. INTRODUCTION The direct conversion of methane to synfuels or petrochemicals is a very arduous reaction and few research papers appear in the literature [ 1] the passage through the synthesis gas remains the obligatory industrial route towards Cn (n> 1) organic compounds (Fig. 1). -- -
CH3OH
~[ .... ALCOHOLS C
4
/
,,
--
Cn
•
L
-..
f-F i - ,
/'~,, New ,~"'\ "H20
]
+ nil2
ETHYLENE/PROPYLEN~,~
- -
GAsoL.,,~E 4
-
~J
Fig.1. Synfuels and Cn (n> 1) organic compoundsproduced trough synthesis gas from CI-L.
/, C 0 2 ' i WAX
.J- 02 /.- .,
Synthesis gas
-
-
--II~[
INTERMEDIATE
,
Due to the large natural gas reserve (same order of magnitude than crude [2]) methane appears the most cheap and available carbon source. The catalytic steam reforming is actually the most frequently industrial route to synthesis gas. However the carbon and thermal efficiency deeply depend on the CO2. The dioxide is formed during the reaction from the hydrocarbon partial oxidation or from CO, or it may be present in the natural gas (some reserves have till 40% of CO2). As a consequence the reactivity of pure CO2 in the reforming conditions must be studied at first to optimise the reforming catalyst and at second to design industrial reforming processes with CO2 and H20 mixture as reactants from which synthesis gas with customised CO/H2 ratio can be obtained for specific organic hydrocarbon synthesis [3]. Carbon dioxide weakly adsorbs at high temperature and in presence of CH4 and H20 on the traditionally used metal catalyst (Ni supported on A%03). A relevant role can be however
334 assumed by the support when the basic character prevails [4]. In fact by using a basic support the adsorbed CO2 could become more reactive and more available to the near metal active site [5]. In this paper we investigate the behaviour of Ni metal catalyst on pure SiO2 and on La203 (very recently investigated also by other authors [6]) pure or as additive of SiO2, with the aim of more deeply understanding the surface reaction.
2. EXPERIMENTAL
Catalysts. SiO2 (70-230 mesh, Merck 7734, 60~ 430 m2/g BET method), and La203 (Fluka; La 99,98%; 9-12 m2/g BET method) were used as support following two different procedures of the Ni deposition. Using the first one, Ni was deposited by wet impregnation starting from Ni(NO3)2.6H20 alkaline solution of NH3. Alternatively a solution of Ni(NO3)2.6H20(1 ml/g of support) was slowly feeded to a SiO2 fluidized bed maintaining silica at incipient wetness (dry method). Both the obtained catalysts were dried under vacuum at 333K, and successively in an oven at 383K (24h) and finally calcined at 873K (2h). FTIR analysis does not reveals nitrate adsorption. The La203-SiO2 supports also were obtained by wet impregnation of La(NO3)3 (Prolabo) alkaline solutions on SiO2 or dry impregnation as described for Ni impregnation. All the catalysts were used fresh prepared and reduced with H 2 before the catalytic tests. A Philips P-W1130 diffractometer was used for the X-ray powder analysis. Reaction Apparatus. A quartz tube flow reactor (10 mm in diameter, 400 mm overall length) filled with 200-500 mg of catalyst between two layers of carborundum, placed in a ventilated oven, with a thermocouple located inside the catalytic bed, was used. The flow rates of the gases (>_100 ml/min, CH4 + CO2, diluted with N2) were controlled by mass flow meters (Brooks 5850).The concentration of reagents and products was determined using a GC (Dani 3800) equipped with a TCD and a Carbosieve S II packed column. Blank tests showed negligible conversions of the reagents. The discrepancy from 100% mass balance was ascribed to coke and tar formation. A direct measurement of the amount of deposited coke was made by an extensive hydrogenation of the catalyst to CH4 after each experiment in the 823-923K temperature range. Thermal analysis. A Mettler TA2000 system was used for TGA. The analyses, on supports and catalysts, were performed under CO2+ CH4 (1:1) at atmospheric pressure in isothermal conditions at 823, 873,923 K with the H2 initial reduction and H2 final treatment (60 min He purge after each gas change). XPS analysis. A M-Probe XPS instrument [7] was used for the analysis of catalyst surface that was treated under the reaction conditions at 873K.
3. RESULTS 3.1. Preparation methods. Two different routes were used for the preparation of Ni/SiO2 catalysts. The starting material was always Ni(NO3)2 but following the wet impregnation the metal hydroxide is the precursor.
335 The nitrate salt itself is the precursor following the dry impregnation procedure. The CO production rate vs. the reaction time is reported in Fig. 2; the data indicate that the catalyst obtained using the hydroxide precursor has a very stable activity for more than 2 hours and that, on the contrary, the nitrate precursor causes a quick loss of activity. We tested also the effect of the time to reach the calcination temperature from ambient temperature, by the comparison between the calcination performed using a tubular reactor with a 5~ of increasing programmed temperature and that using a large oven, which has a time to final temperature about 8 times longer. No effect was ascertained on the wet prepared catalysts. The time of calcination (873K) is a determining parameter only on the activity of the catalysts prepared by the dry impregnation method as the CO production rate of the other materials (not reported in the figure) is not modified by 2 or 3 times longer calcination time..
20 18 ;
[CO production rate ]
16 l
/
.mmo]_E~
h.gNi
1412I 10,,.7-..
......
-
•
=
-
-
,~ W E T
IMPREGNATION
t
6 ;] ~,%~.'\,'kDRY IMPREGNATION 4 ~- -,,~.!~,. ,,~ 2 ~ : : : ~ . : : . . ~ : ......."~:
DRY IMPREGNATION + LONG CALCINATION
0 20 40 60 80 100120140 Time (rain) Fig. 2. CO production rate dependence on the impregnation and calcination method of Ni/SiO2 catalysts.
The physical characteristics of the prepared catalysts are reported in Table. 1. The pore distribution is monomodal and the medium pore size does not change from that of the support when nitrate is used as precursor and while it increases using Ni(OH)2. Tab. 1. Comparison of the physical characteristics of catalystsa. Catalysts
Precursor
Ni (%w/w) // 3.2 3.2 4.0 4.0
Surfaceareab (m2/g) 550 491 438 317 252
Porosity (mediumvalue; A) 31 27 26 48 47
SiO2 // Ni/SiO2c Nitrate Ni/SiO2 Nitrate Ni/SiO2 Hydroxide Ni/La203Hydroxide Si02d a)Calcination at 873k (3h); b) BET area; c) Calcinated for 8h at 873K d) La203 (2% w/w) was obtained by dry impregnation (from La(NO3)3) Useful indications can be also acquired from the XRD analysis. Using the wet deposition the powder does not show NiO reflections at low Ni per cent (5% max.); a 30% minimum amount of Ni is necessary in order for the reflections appear. On the contrary following the dry method just 4% of Ni give NiO reflection.
336 3.2. XPS analysis Cls XPS spectra recorded on catalysts at incipient carbon deposition [8] after 7 and 16 hours of reaction are presented in Fig. 3. Three carbon species can be observed: the first at low binding energy range, (284-285 eV) attributable to -CHx- species or adventitious carbon, the second at slightly high binding energy ( at about 286-287 eV) probably due to carbon linked to -O- or -OH groups, and at last that in the 288-289 eV range characteristic of-CO3= groups[9]. 5000
4o00-
Incipient C deposition
Fig. 3. Cls XPS binding energies at increasing reaction time (T=823K; P= 101KPa; CH4/CO2/N2=40/40/20 ml/min): Aincipient carbon deposition, B- after 7 hours, C-after 16 hours.
Ni/La203 Ni/SiO 2
c 0
0
3000
-
2000
-
1000
--
// / / / / / //
/ / U
I
I
I
I
eV
284 285 286 287 288 289 290
10000
50000
420 min
8000u) ~9 6000 c
o r
4000
30000
-
20000
20000
I
284
960 min
40000
285
|R.
10000
I
I
I
286
287
288
289
eV
284
I
I
I
I
285
286
287
288
289
3.3. Effect o f the catalyst r e d u c t i o n
The analyses have been performed with the same apparatus of the catalytic tests using the H2reduced and the unreduced catalysts (Fig.4); at first the reactants were feeded together ( A serie) and then separately (B serie), The data refer to the GC analysis at 5 min. of reaction time. The catalysts used were Ni(4%)/SiO:, Ni(3.2%)/SiO2-La:O3(2%) and Ni(4.4%)/La203 all prepared by the wet method. The degree of reduction of the catalyst surface influences the conversion of CO2 and CH4. The conversion of CO2 appears strongly different between the two serie of analyses (A and B). If CO2 is feeded together with CH4 the conversion is always greater than in the separate reaction, while with the same type of feeding, comparing the reduced and the unreduced materials, the conversion is greater on the unreduced La203-SiO2 support that on the same reduced one. The figure shows the same conversion pattern as CO2 for the CH4 but all the values are less than those of CO2. On the contrary when CO2 and CH4 are feeded separately CH4 conversion is greater than CO2 and it is significantly higher on the unreduced La203-SiO2 and La203 supports when CO2 is absent that on the reduced one. With the pure lanthana
337 support the CH4 conversion ratio is much higher than with the reduced one when CO2 is absent. Conversion 513
Reduced [77"~ Unreduced 25
CH 4
00 2
A
N iS iLa T-~ NiLa ~i'.~i
:;
Conversion (%)
F--"-!
(%)
-
20
C02
-
l ~ !OH 4
B
l
I
Reduoed Unreduced
NiSiLa
ii
I
rt
zl 15
-
10
-
NiSiLa NiSi
NiSi
]
r 6
~;
""
-
s.,
Fig. 4. C02 and CH4 conversions at 5 min. and 873 K, on reduced (white bar) and unreduced catalysts when the reactants are co-feeded (A) and separately feeded (B). 4. DISCUSSION By using as precursor the Ni(OH)2 obtained following the wet impregnation method, we observed a more stable activity than with the Ni nitrate salt (dry impregnation). The behaviour could be related to the type of the surface metal: the hydroxide seems to close the small pores while the nitrates penetrate them. The lower decomposition temperature of hydroxides than nitrates determines a better dispersion together with a more abundance in the macropores which have a greater accessibility. The easy CO desorption preserves the metal site from an excess of carbon formation from Boudouard reaction and the dispersion promotes the reaction of the adventitious (or "status nascendi") carbon with the surface oxygen before its ageing or transformation into less reactive form [8]. In fact it is well known that the CHx (where x=0-3) fragments, from which originate the carbon polymers, take place from both the CI-I4 dissociation and the CO disproportionation deactivating the catalyst [10]. We very recently reported a mechanistic hypothesis where the Ni-surface oxygen takes a relevant role [11]; others reported the importance of the metal-oxide perimeter on the activity [12]. Moreover the data presented here confirms the ox-red mechanism of the CH4 reforming with CO2 as the conversion of both the reactants is greater when they are feeded at the same time than alone. CO2 acts as an oxidant and becomes converted on the reduced catalyst but more extensively on that treated with CH4 (A serie) than H2 (B serie). The conversion occurs both on the CH4 reduced metal (see the Ni/SiO2 catalyst) and on the CH4 reduced support (Ni/La203) as it appears that low amount of easy reducible La203 increases the CH4 conversion when it is feeded alone, probably due to the reduction of the oxide surface. Similarly on CeO2 Japanese researchers observed an CO2/CH4 ox-red reaction [13] suggesting that low ox-red potential of the support may play an important role.
338 CH4 reacts both on the oxidised surface of the support giving CO and on the reduced metal producing CHx specie. The conversion is lower than CO2 when co-feeded and, on the contrary, higher than CO2 when it is feeded alone. Really other reactions have to be considered; La203 easily forms carbonate specie or, at reduction conditions, cover the metal surface thus, due to the stability of carbonate groups or the excessive metal decoration, inhibiting the CH4 reaction. At this respect as appears from XPS analysis, the oxygenated C ls population on La203 seems more numerous than on the SiO2 support, at the initial reaction time being carbonate and successively C-O-X or C-O-H. The carbon is also formed more easily on the lanthana containing support, probably due to the oxide promotion of the decomposition of carbonate species into CO which disproportionate to C specie on the metal.
5. C O N C L U S I O N We investigated two methods of preparation observing that the wet impregnation produces a more stable catalyst with an increased dispersion of the metal due to the Ni(OH)2 precursor. About the support, La203 used as pure support or as a promoter on SiO2 puts in evidence the support effect as it modifies the conversions and the carbon deposition. Indeed investigating more deeply about the differences of CO2 reactivity between silica and lanthana supported Ni catalysts, particularly from the tests on reduced and unreduced catalyst, it appears that the oxidation degree of the catalytic surface plays a relevant role on the reactivity. A concerted ox-red scheme of reaction between CH4 and CO2 is confirmed involving the surface oxygen of the support. At low temperature, lanthana promotes carbon formation more than the silica support.
REFERENCES 1. 2. 3. 4. 5. 6.
E.E. Wolf, Natural Gas Conversion IV, Kruger National Park (South Africa), Dec. 1995, Closing remarks D.Sanfilippo, ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.IV, pag.2219-2220 J.Rostrup-Nielsen,ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.I, pag.327-330 E. Ruckenstein and Y.Hang Hu, J.ofCatal., 162, 230-232 (1996) P.Gronchi, R.Del Rosso, and P.C6ntola, Applied Catalysis A, 152 (1997) 83-92 Z. Zhang and X. Verykios, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M.de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M.S.Scurrell (eds.), Vol. 107, pag.511-516, Elsevier (1997). 7. C.L.Bianchi, M.G.Cattania, V.Ragaini, Mat.Chem. & Phys, 29 (1991) 297-302 8. P.Gronchi, D.Fumagalli, R.Del Rosso, and P.C6ntola, J. of Thermal Anal., 47, 227-235 (1996). 9. J.Chastain, (ed.), Handbook of XPS. Eden Praire: Perkin-Elmer (1992) 10. A.Erdtshely. K.Fodor and F.Solymosi, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M.de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M.S.Scurrell (eds.), Vol. 107, pag.525-530, Elsevier (1997) 11. P.Gronchi, P.C6ntola, and R.Del Rosso, ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.I, pag.375-378 12. J.H. Bitter, PhD Thesis, University ofTwente (NL), (1997) 13. K.Otsuka, E.Sunada, T.Ushiyma and I.Yamanacha, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M. de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M. S. Scurrell (eds.), Vol. 107, pag.531-536, Elsevier (1997)
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
339
Iron Catalyzed CO2 Hydrogenation to Liquid Hydrocarbons Rocco A. Fiato* E. Iglesia, G. W. Rice and S. L. Soled Exxon Research and Engineering Company, Florham Park, New Jersey 07932 USA
Many of the catalysts which are useful in Fischer-Tropsch synthesis are also capable of catalyzing the hydrogenation of CO2 to hydrocarbons. Our structure-function studies have shown that it is possible to control the selectivity of CO2 hydrogenation by specific iron-based catalysts to generate yields of C5+ hydrocarbons that are comparable to those produced with conventional CO based feedstocks.
1. INTRODUCTION Catalyst structure-function studies have shown that alpha-olefins are the primary products of CO2 hydrogenation, with normal paraffms being formed by secondary hydrogenation. Catalysts that contain mainly iron oxide, or iron carbide with substantial concentrations of matrix carbon, favor secondary reactions and limited chain growth. The relative concentrations of these phases depend on the ease of reduction of the starting catalyst precursor, which can be changed to some degree by substitution of various cations into the initial formulation. In cases where excess matrix carbon is introduced into the reduced/carbided catalyst system, we are able to demonstrate the importance of diffusion constraints on overall process selectivity. We have developed proprietary iron catalysts that are essentially free of metal oxide and/or matrix carbon phases, with unprecedented selectivity toward C5+ olefinic products.
2. BACKGROUND ON CO AND CO2 HYDROGENATION Studies on CO2 hydrogenation can be traced back to the early 1900's [1] with emphasis on cobalt, nickel and iron-based catalysts. While this work clearly demonstrated the overall reactivity of CO2 under a wide range of reaction conditions, the primary product(s) consisted of methane, CO and other light hydrocarbons. Emmett and others have evaluated the large body of experimental work that was performed through the early 1960's and concluded that CO2 hydrogenation to higher molecular weight hydrocarbons remained a significant challenge. Selectivity control continues to be a critical issue in Fischer-Tropsch chemistry, a catalytic process that dates back more than seventy years [ 1]. Operating conditions can be adjusted to control selectivities but overall effects are limited [2-4]. During Fischer-Tropsch synthesis with conventional bulk iron catalysts, various phases, including metal, metal carbides and metal oxides are present at steady-state catalytic conditions [5-7]. * Corresponding author
340 Previous studies indicate that alkali increases the heat of CO chemisorption while decreasing the heat of H2 chemisorption [8,9]. As a result of alkali promotion on iron catalysts, the average chain length and the olefin content of the products increase, whereas the activity and methane selectivity decrease [ 10,11 ]. Recent studies indicate that ot-olefins, the major primary products formed during FischerTropsch synthesis, participate in secondary reactions [12]. Chains can terminate either by 13-hydrogen abstraction to form an ct-olefin or by H-addition to form a paraffin [13,14]. Olefins can undergo secondary reactions by subsequent readsorption leading to isomerization or hydrogenation. We observe selectivity relationships that are consistent with Egiebor's proposal that significant secondary hydrogenation reactions can occur on iron catalysts [ 12].
3. EXPERIMENTAL ON CO AND CO2HYDROGENATION The details of the preparation of the iron oxide catalyst precursors are described elsewhere [1517]. For those carbides made by exsitu carburization, the oxide was loaded into a 1" diameter quartz tube and heated in a 1/1 H2/CO mixture at a space velocity of 10,000 v/v/hr at 350~ for 24 hours. Iron carbide catalysts were also prepared by laser pyrolysis of iron carbonyl and ethylene using a 150 watt continuous wave CO2 laser to provide both a rapid high temperature reaction (-.1 sec with T-1000~ and quench [18]. The procedures used to produce the wet chemical and laser generated iron carbides of this paper have been disclosed in detail [22]. Catalyst tests were performed in a 300 cc Parr stirred tank reactor with octacosane as the slurry medium. Wide-angle powder x-ray diffraction identified catalyst phases present. Thermogravimetric reductions were recorded on a Mettler TA-2000~ using 100-200 mg of sample and a heating rate of 8 deg/min. Gravimetric titrations were performed using a specially designed heated entry port enabled injection of the pyridine titrant [19]. Mossbauer spectra were collected with an Austin Sciences instrument with a radiation source consisting of Co 57 diffused into Rh (New England Nuclear).
4. RESULTS AND DISCUSSION Unpromoted iron oxide catalysts (surface area 30-50m2/gm) and iron oxide converted ex-situ to iron carbide were compared under standard reaction conditions. Figure 1 illustrates the sequential conversion of an iron surface in 1:1 H2:CO at 1 atm under programmed heating conditions. In region 1 CO adsorbs onto the metal surface. At about 300~ Fe begins to convert to (Haag) carbide and at slightly higher temperature amorphous carbon begins to grow. Gravimetric acidity titrations with 3,5 dimethylpyridene at 250~ showed an acid site concentration of 99 ~t-moles/grn catalyst for the oxide and only 27 for the carbided catalyst. These numbers compare with --250 ~t-moles/grn for a solid acid like ~/-A1203. Figure 2 summarizes the different product selectivities measured at CO conversions of-50%. In the carbided system we find ethylene present as well as a higher olefmic content in Ca, apparently as a result of inhibited secondary hydrogenation. Consequently, a carbided iron surface produces a more olefmic, heavier product, consistent with the observations of Egiebor and Cooper who suggest that acid sites on a precipitated FeOx/SiO2 catalyst lead to secondary reactions of tz-olefins [ 12].
341
Alkali titrates acid sites on the iron surface and also increases the strength of the CO-surface interaction. Figure 3 compares the effect of potassium promotion on the iron carbide catalyst where we had substantial amounts of matrix carbon. A cementite Fe3C phase was synthesized by laser pyrolysis with a carbon content corresponding closely to the Fe3C stoichiometry, i.e., without any excess matrix carbon. Some of it was heated in H2/CO mixtures (1/1) at 350~ in order to introduce 40-50% by weight matrix carbon. Figure 4 shows that the excess carbon reduced olefin selectivity while favoring CH4 and lighter products. The pure carbide, as prepared, does not respond to alkali treatment, Figure 5, suggesting that potassium is only needed when removal of the olefins is not fast enough to prevent secondary hydrogenation. The oxide catalyst reduces at lower temperature with substitution of Co into Fe304. Figure 6 shows the onset of reduction is initiated 20~ lower with the cobalt substitution. This may explain the enhanced olefin selectivity attributed to iron-cobalt catalysts in the literature [20,21]. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts is also sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, we find that iron has partitioned into carbide in the front of the bed but shows increasing amounts of oxide near the exit, whereas the same catalyst in the stirred tank reactor remains all iron carbide, Figure 7. Comparative tests were conducted on CO2 hydrogenation over a conventional coprecipitated Fe/Cu/K/Si catalyst versus a wet chemical derived sample of FesC2, see Figure 8. At 7/1 H2/CO2 feed ratio, the carbide exhibits significantly higher selectivity to C2+ products and higher olefin yields than the conventional catalyst. The higher molecular weight fractions produced from the 7/1 H2/CO2 feed consisted of a mixture of alpha- and beta-olefins, n-paraffins and n-alcohols together with large fraction of methylbranched and internal olefin isomers, see Figure 9, while a 2/1 mixture of H2/CO operated at >80% CO conversion (i.e., effective H2/CO ratios >10/1) over this catalyst would produce nearly 65% alphaolefin, 15% n-paraffm, 1-3% n-alcohol and 15% methyl-branched and internal olefin isomers. Laser generated carbides that contain virtually no matrix carbon overlayer have been tested and show much higher selectivity to desired products than wet chemical analogs. The Zn and Mn containing systems required potassium promoters for optimum selectivity, see Figure 10. A series of 13-CO2 labeling studies were conducted to determine the extent of CO2 conversion to hydrocarbons in the presence of varying quantities of CO, see Figure 11. These studies showed that no CO2 was converted in mixtures where >5% carbon atom CO was present, and that CO2 conversion to hydrocarbons did not occur until virtually all of the CO was consumed. Preliminary kinetic studies showed that product formation rates in the presence of low levels of unlabeled CO were consistent with a mechanism involving dissociative CO2 chemisorption followed by "CO" and surface "C" formation with hydrogenation to hydrocarbons, Figure 12.
342 REFERENCES
M. E. Dry, "The Fischer-Tropsch Synthesis" in Catalysis, Science and Technology. Vol. 1, p. 159, ed. J. R. Anderson and M. Boudart, NY (1981); "The Fischer-Tropsch and Related Synthesis," H. Storch, N. Golumbic and R. Anderson, Wiley NY 1951. C. D. Frohning and B. Cornils, Hydrocarbon Processing, p. 143, Nov. 1974. B. Comils, B. Buessmeier and C. D. Frohning, Inf. Ser..-Alberta Res. Council, 85, 126 (1978). B. Buessemeier, C. D. Frohning and B. Comils, Hydrocarbon Processing, p. 105, Nov. 1976. F. Fischer and H. Tropsch, Bennstoff-Chem. 7, 97 (1926). V. V. Niemanstuerdiert, A. M. vanderKraan, W. L. Van Dyk and H. S. vanderBaan, J. Phys Chem. 84, 3363 (1980). F. Blanchard, J. P. Reymond, B. Pommier and S. J. Teichner, 3rd International Symp on Scient Basis for Prep. ofHet. Catal. (Belg), Sept. 1983 and J. Mol. Catal. 176, 171 (1982). .
9.
M. E. Dry, T. Shingles, L. J. Boshoff and G. J. Ostuizien, J. Cat. 15, 190 (1969). J. Benziger and R. J. Madix, Surf. Sci. 94, 119 (1980).
10.
D. L. King and J. B. Peri, J. Cat. 79, 164 (1983).
11.
H. Storch, N. Golumbic, and R. B. Anderson, "Fischer-Tropsch and Related Synthesis", Chap. 6, Wiley N. Y., 1951.
12.
N. O. Egiebor and W. C. Cooper, Appl. Catal. 17, 47 (1985).
13.
P. Biloen, J. N. Helle and W. Sachtler, J. Cat. 58, 95 (1979).
14.
G. Henrici-Olive and S. Olive, Angew. Chem. Int. Ed. 15, 136 (1976).
15.
R. A. Fiato and S. L. Soled, U.S. Pat. 4,618,597 (1986).
16.
S.L. Soled and R. A. Fiato, U.S. Pat. 4,544,671 (1985).
17.
S. L. Soled and R. A. Fiato, U.S. Pat. 4, 584, 323 (1986).
18
G. Rice, R. A. Fiato and S. L. Soled, U.S. Pat. 4,788,222 (1988).
19.
S. L. Soled, G. McVicker and B. DeRites, Proc. 1lth North American Thermal Analysis Conference, 1981.
20.
M. Nakamura, B. B. Wood, P. Y. Hou and H. Wise, Proc. 7th Int Conf of Catal., part 7a, June '80.
21.
R. M. Stanfield and W. N. Delgass, J. Cat. 72, 37 (1981).
22.
R. A. Fiato, S. L. Soled, G. W. Rice and S. Miseo, U.S. Pat. 4,687,753 (1987) and U.S. Pat. 5,140,049 (1992).
ACKNOWLEDGMENTS The authors would like to thank Professor Carl Lund for the Mossbauer measurements and Professor Charles Mims for the 13-CO2 labeling experiments.
343
Figure 1 T H E R M A L C O N V E R S I O N O F I R O N TO IRON C A R B I D E ,
,
l
.
F i g u r e 2. OXIDE SURFACE SHOWS M O R E SECONDARY PRODUCTS THAN CARBIOED SURFACE % seioctlvlty
70
,
Fe in HdO0 at 1 atm
i "~I
oo" 50-
~
~
I
40301~0
2~I0 3~10 4~
5~i0
t (~ 1--.Low Temp. CO Adsorption 2---OxlckDConverts to Carbide 3---Sur/ace Carbon Grows
2o 10--
/
0
l
C2 olefln/C2 total
l
C4 olefln/C4 total
l
C5 plus
CH4
F i g u r e 4. E X C E S S MATRIX C A R B O N D E C R E A S E S S E L E C T I V I T Y ON L A S E R G E N E R A T E D Fe3C
% selectivity
/i--I.
i
Condition*: 270~ 75 l~i, 2/1 H2/CO 1500 vlg~r, conv 9SO%,CSTR
F i g u r e 3. A L K A U P R O M O T I O N I N H I B I T S SECONDARY HYDROGENATION IO0
i t
/
I
J
I
100
% selectivity
li- +-.c+!
'~ ! "~''"~
J
I
-J
I
60
40
C2 olefln/ C2 total
C4 olefln/ C4 total
C10 olefln/ C10 total
C:2 olefln/ C2 totaJ
CS plus
F i g u r e 5. POTASSIUM P R O M O T I O N H A S M I N O R EFFECT ON L A S E R - G E N E R A T E D IRON C A R B I D E S % sek~-tJv~
~"
:
C4 oleftn/ C4 total
//'-++"L m
C10 olefln/ C10 total
Conditions: 270~ 75 p~, 2/1 H2/CO
8000 vlg/hr, CSTR
C10 C10
CS plus
total
CX4
Figure 6 COBALT SUBSTITUTION FACIUTATES R E D U C T I O N OF F e 3 0 4 I
:1//
total
Conditions: 270"(:, 7513411,2/1 H2/CO 1000-0000 v/Whr, CSTR L a i r G4menm~ Cata~W
Conditions: 270~ 75 psi, 2/1 H2/CO lSOOvlg/hr, cow > 50%, CSTR
100]
C10 olefin/ C10
CS plus
I m ~ 3 c
Thm111~ Grlvlmetry: Fe304 R e d ' ~
i
CH4
9 ....
i ....
i ....
i ....
i ....
:
. . . .
~ Loss Spinel
/
I
....
! .... 260
+X
! .... 270
1 .... 280
1 .... 290
I .... 300
310
T (~
Onset 9 of Reduction Lowertd by 20~ with Co Substitution
344
Figure 7
Figure 8
MOSSBAUER SPECTROSCOPY REVEALS DIFFERENT PHASES PRESENT IN SLURRY VS. FIXED BED REACTORS Re~eve zmn~y
CO2 HYDROGENATION PERFORMANCE OF CARBURIZED F5C2/2% K
~red~ S ~ ~mW ~
r, _
~!.~',
_
--Top
IS0A
,
I
,
i
9
7.0
7.0
3.0
1.7
21
37
23
13
CH4
64
16.5
6.2
4.2
C.~
36
83.5
93.8
95.8
28
80
95
99
Figure 9
Figure 10
CO2 HYOROGENATION PROOUCES WlOE RANGE OF C10+ PROOUCTS
LASER GENERATED IRON CARBIDES FOR SELECTIVE CO2 HYDROGENATION
FeC
FeZn-C
FeMn-C
Colllplrstlve Fe/Cu/K/Si
Conversion
22
25
31
21
Seie~lviW (based on Ct+1 CH4 C2+ % Oleftn in C2-C4
5.5 94.5 94.0
5.8 942 93.0
S.1 94.9 96.0
64 36 28
Catalyst 9
FesC2/2%K with > 50% wt. Matrix Carbon
% CO2 Conversion
37
S e k ~ i v ~ (based on c1+) CH4 C2+ % Oiefin in C2-C4
16.5 83.5 80
C|Q Product Dlstrlbutlon (% wt) r,-olefln I~-olefin n-paraffin n-alcohol CH3-Branched4ntsmal Isomers
36.7 0~ 13.3 14.2 28.4
Figure 11 REACTION MECHANISM STUDIES VIA 13CO2 L A B E U N G
Reaction
Fes C212% K With >50% wL Matrix Carbon
Conditions: 260-270~ 3800 cm31gFe/hr,75 psig, 10% vol catalyst in octacosane, Parr CSTR, 30-50% vol N2 In Feed gas
,
~0 0.0 s.0 10.0 vek~(nv~)
Catalyst:
H2/CO2 Feed Ratio % CO2 Conversion
% Oiefln in C2-C4
~ t'~,~~ '~ ~,~.~ i
Comparative Fe/Cu/IOSI
Selectivity (based on C1+)
~!!? l
Catalyst
CO/'CO2 + H2 --~ --CH2-- + --CH2*-- + H20
9 Feeds with >5% (Carbon Atom) CO Feed Show No Label In Product 9 Mbmd Feeds Show Labeled Product After All CO is Consumed 9 CO2/tt 2 Alone Yields Mixture of Hydrocarbon Plus CO as s By-Product 9 Preliminary Kinetics Suggest Two Routes for Hydrocarbon Formation - Reverse Shift Followed by "CO" Hydrogenation - Oissocistive CO2 Chemisorptlon Followed by "C" Hydrogenation
~, r
Conditions: 270~ 7/1 H2/CO2 3800 cm31gFe/hl, 75 psig, 26% N2 vol In CSTR Feed 9Laser generated FeZn and FeMn carbides contain <2% g atom K snd <10% wt matrix carbon; laser generated Fe-C Contains No Promoters.
Figure 12 SUMMARY 9 Proprietary Iron Carbide Based Catalysts Generated for Selective Conversion of CO2 to Uquid Hydrocarbons - Laser Generated Carbides that are Free of Other Metals/Metal Oxides Provide Highest Selectivity to Oleflnic Liquid Hydmcart~ns 9 CO2 Hydrogenation Over Iron Carbon Catalysts Provide an Efficient Means of Utilizing H2/CO2 Containing Streams, e.g., Process Waste Gas, Fe-Catslyzed Fischer-Tropsch Of/gas 9 Mechanistic issues Remain on the Extent to Which Reverse Shift Followed by "CO" Hydrogenation Versus Oiasoctative CO2 Adsorption Followed by "C" Hydrogenation Contribute to Overall Product Formation
T. Inui, M. Anpo, K. Izui, S. Yanagida,T. Yamaguchi(Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 ElsevierScienceB.V. All rights reserved.
345
Support effects of the promoted and unpromoted iron catalysts in CO2 hydrogenation Ki-Won Jun, Soo-Jae Lee, Ho Kim, Myoung-Jae Choi and Kyu-Wan Lee* Chemical Technology Lab. I, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, S. Korea The effects of support on the catalytic behavior of Fe and Fe-K have been investigated with the hydrogenation of CO2 into hydrocarbons. The catalysts were prepared by impregnation using silica gel, y-alumina and titania (anatase) as supports. The chemisorption and XPS results suggest that only y-alumina support provides favorable dispersion of the active phase, Fe or Fe-K. From the reaction results, it was found that y-alumina support increases the activity and the selectivity for long chain hydrocarbons. The combination of Fe, K and yalumina makes the catalyst highly active and selective for CO2 hydrogenation to long chain hydrocarbons and light olefins. It seems that strong metal-support interaction exists in Fe-K/y-A1203. 1. INTRODUCTION Carbon dioxide has received much attention as a useful carbon source for the valuable chemicals in recent years [1-7]. One of the challenging aims in carbon dioxide utilization is the synthesis of relatively valuable hydrocarbons such as olefins or liquid hydrocarbons by the hydrogenation. There have been two kinds of methods employed in the hydrocarbon synthesis from CO2 hydrogenation: one is the combination of methanol synthesis and the methanol conversion to hydrocarbons (MTG process) using the composite catalysts and the other is direct synthesis using Fischer-Tropsch (F-T) type catalysts. Although many efforts have been made with the composite catalysts, this method usually gives light alkanes as major hydrocarbon products because of further hydrogenation of alkene intermediate by methanol synthesis catalysts [8-11]. On the other hand, Lee et al. [12,13] reported that CO2 hydrogenation over the Fe-K catalyst, which is an F-T type catalyst, gives relatively high selectivity for the formation of light olefins. In our previous study [14], it was found that high activity and selectivity toward light olefins and C5+ hydrocarbons can be achieved at the high concentration of K in the Fe-IUalumina catalysts which were prepared by *Corresponding author. E-mail:
[email protected], Fax. (+82-42)860-7590
346 impregnation. In the present study the effects of different supports on the catalytic properties of K-promoted and unpromoted iron catalysts were investigated comparatively. 2. EXPERIMENTAL Unsupported catalysts Fe and Fe-K were prepared by decomposition of Fe(NO3)3.9H20 and the homogeneous mixture of Fe(NO3)3-9H20 and K2CO3 respectively at 773 K. Supported catalysts (Fe/support = 0.25 wt ratio) were prepared by impregnating the supports like ?-alumina (surface area = 157 m2/g, pore diameter = 104 A), silica gel (462 m2/g, 63 A) and titania (anatase, 10 m2/g, 118/k) with an aqueous solutions of Fe(NO3)3.9H20. To prepare iron-potassium catalysts (K/Fe molar ratio = 0.5), an adequate amount of K2CO3 was added into the aqueous solution of Fe(NO3)3.9H20 before impregnation. After the impregnation, the catalyst samples were dried at 383 K for 24 hr and calcined in air at 773 K overnight. The X-ray photoelectron spectroscopy (XPS) measurements were carried out on a VG Escalab MK II photoelectron spectrometer, which was equipped with a magnesium anode operated at 15 kV and 20 mA. The chemisorption measurements of carbon dioxide and hydrogen were obtained at 298 K using the chemisorption apparatus (Micromeritics ASAP 2000). The samples (1.0-1.2 g) used in the chemisorption studies were reduced for 12 hr at 723 K in a flow of hydrogen, evacuated at the reduction temperature and then cooled to the adsorption temperature. The difference between the adsorption isotherms obtained by the repeated use of gas dosing and degassing system gives the amount of chemisorbed species on the catalysts. The catalytic hydrogenation of carbon dioxide was performed in a continuous fixed bed reactor. The catalyst was reduced in a flow of hydrogen at 723 K for 20 24 hr. After the reduction, the catalyst was brought to the following conditions: 573 K, 10 atm, space velocity of 1900 h -1 and H~JCO2 = 3. The activity data was taken after 24h of reaction. The products were analyzed by a gas chromatograph (Chrompack CP 9001) equipped with thermal conductivity and flame ionization detectors. Carbon monoxide, carbon dioxide and water were analyzed on a Porapak Q column and the hydrocarbons on a GS Q capillary column. -
3. RESULTS AND DISCUSSION Table 1 lists the surface area of prepared catalysts and the chemisorption results of CO2 and H2 on the catalysts. In case of supported Fe catalysts, the surface area is support-dependent. In contrast, Fe-K/SiO2 shows much smaller surface area compared with silica or Fe/SiO2. This indicates that with increased mass of active phase, iron and potassium blocks the micro-pore of silica support. This seems to be due to the smaller pore diameter of silica (see experimental). In addition, if Fe-K exists mainly as poorly dispersed masses, it would block easily the micro-pores of the support. From the chemisorption results, it is apparent that only T-alumina-supported catalysts, Fe/AleO3 and Fe-K/A12Oa give greatly
347 improved CO2 uptake and H2 uptake. The other supports, silica and titania do not increase CO2 uptake and H2 uptake, suggesting that the dispersion of active phases is very poor on silica or titania. The blank test with 7-alumina indicated that either of the chemisorption of CO2 or H2 on the support itself is negligible. The observations indicate clearly that only alumina support provides effectively increased surface area of active phases due to the high dispersion of Fe and K. Moreover, with Fe-K/alumina, especially much higher CO2 uptake ability is obtained, most probably due to increased basicity by the addition of potassium. Table 2 represents the dispersity index estimated from XPS data, from which the information about relative dispersity of catalysts is given. The results support the view that using 7-alumina leads higher dispersion of Fe phase compared to the other supports.
Table 1 BET surface areas of catalyst samples and chemisorbed amounts of H2 and CO2 on catalysts Catalyst Fe
Surface area (m2/g) C02 uptake (~mol]g)
H2 uptake (~mol/g)
9
16.8
2.1
Fe/SiOe
320
7.2
2.0
Fe/A1203
128
Fe/TiO2
13
5.5
2.7
Fe-K
2
12.5
-
Fe-K/SiO2
7
0.9
-
Fe-K/A1203
92
Fe-K/TiO2
16
167
514 18.3
31.0
20.1 1.7
Table 2 Comparison of the dispersity estimated from XPS data Catalyst
Fe/SM ratio a
Imaginary no. of Fe monolayer b
Dispersity index c
Fe/SiO2
0.05
0.44
0.11
Fe/A12Os
0.27
1.09
0.24
Fe/TiO2
0.75
10.76
0.07
a The surface ratio (Fe : support metal) observed from XPS. b Calculated number of Fe monolayer based on Fe area = 6.49 • 10 .20 m2/atm on the assumption of uniform dispersion of iron on the support. c (Fe/SM) ratio divided by imaginary number of Fe monolayer.
348 Fig. 1 shows the catalytic activity of unsupported and supported Fe and FeK in the CO2 hydrogenation. Relatively enhanced catalytic activity is obtained with alumina-supported catalysts: Fe-K/A1203 catalyst gives the highest CO2 conversion and Fe/A1203 catalyst gives the highest hydrocarbon yield. It is likely that the high activity of alumina supported catalysts is attributed to the high dispersion of active phase as evidenced by chemisorption results. Hydrocarbon distributions from CO2 hydrogenation on various catalysts are displayed in Fig. 2 for comparison. The results apparently show that alumina support and potassium promotion favor the formation of long chain hydrocarbons. With Fe-K/A1203, the best improvement in hydrocarbon distribution is achieved: i.e., low methane formation and better ability of chain growth. Fig. 3 shows the olefin selectivity in C2-a hydrocarbons of CO2 hydrogenation on various catalysts. The results show that K-promotion enhances olefin selectivity and this is achieved significantly with Fe-K/A1203. While unpromoted Fe catalysts show mainly the formation of light alkanes in hydrocarbons, the addition of potassium to iron results in increased selectivities towards light olefins and to liquid hydrocarbons. This is in agreement with the fact that potassium has long been known as an effective promotor of iron-based catalysts for the production of olefins and long chain hydrocarbons in F-T reaction [15].
35
mConversion mHC yield
30 25 20 15 10
,L, Fe
Fe/ Si 02
, Fe/ A1203
,L,L, Fe/ Fe-K Ti 02 Catalyst
Fe-K/ Fe-K/ Fe-K/ Si 02 A1203 Ti02
Fig. 1. CO2 hydrogenation activity of various Fe and Fe-K catalysts (reaction conditions: 573 K, 10 atm, 1900 h "1, H~JCOe- 3).
0.8 0.6 0.4 0.2 .
Fe
Fe/Si02
.
Fe/ A1203
i
Fe/Ti02 Fe-K
F e - K / Fe-K/ Si02 A1203
m
Fe-K/ Ti02
Catalyst
Fig. 2. Hydrocarbon distribution of the CO2 hydrogenation on various catalys (reaction conditions: 573 K, 10 atm, 1900 h -1, H2/CO2- 3).
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 Fe
Fe/Si02
Fe/ A1203
Fe/Ti02 F e - K
F e - K / Fe-K/ Si02 A1203
Fe-K/ Ti02
Catalyst Fig. 3. Olefin selectivity in C2.4 hydrocarbons of the CO2 hydrogenation on various catalysts (reaction conditions: 573 K, 10 atm, 1900 h -1, H2/CO2 = 3).
350 In addition, the present results clearly show that ?-alumina improves the catalytic activity and selectivity to long chain hydrocarbons. Particularly, when the Fe-K is supported on ~,-alumina, the selectivity to olefins is improved as well as the activity and chain growth ability. It is likely that the marked improvements achieved by using both K and alumina is due to well dispersed active phase Fe-K. The better dispersion of Fe and K should provide the efficient interaction of Fe and K as well more active sites for the CO2 hydrogenation. The improved dispersion seems to be the result of strong metal-support interaction. ACKNOWLEDGMENT This research being one of the National G7 projects is supported by the Ministry of Science and Technology and the Ministry of the Environment in Korea. REFERENCES
1. B. Delmon, Appl. Catal. B, 1 (1992) 139. 2. J.H. Edwards, Catal. Today, 23 (1995) 59. 3. G.C. Chinchen, P.J. Denny, D.G. Parker, M.D. Spencer and D.A. Whan, Appl. Catal., 30 (1987) 333. 4. Y. Amenomiya, Appl. Catal., 30 (1987) 57. 5. K.G. Chanchlani, R.R. Hudgins and P.L. Silveston, J. Catal., 136 (1992) 59. 6. R.A. Koeppel, A. Baiker and A. Wokaun, Appl. Catal. A, 84 (1992) 77. 7. S.G. Neophytides, A.J. Marchi and G.F. Froment, Appl. Catal. A, 86 (1992) 45. 8. K. Fujimoto and T. Shikada, Appl. Catal., 31 (1987) 13. 9. T. Inui, K. Kitagawa, T. Takeguchi, T. Hagiwara and Y. Makino, Appl. Catal. A, 94 (1993) 31. 10. M. Fujiwara, R. Kieffer, H. Ando and Y. Souma, Appl. Catal. A, 121 (1995) 113. 11. M. Fujiwara, H. Ando, M. Matsumoto, Y. Matsumura, M. Tanaka and Y. Souma, Chem. Lett., 839 (1995). 12. M.-D. Lee, J.-F. Lee and C.-S. Chang, Bull. Chem. Soc. Jpn., 62 (1989) 2756. 13. J.-F. Lee, W.-S. Chern, M.-D. Lee and T.-Y. Dong, Can. J. Chem. Eng., 70 (1992) 511. 14. P.-H. Choi, K.-W. Jun, S.-J. Lee, M.-J. Choi and K.-W. Lee, Catal. Lett., 40, 115 (1996). 15. J.A. Amelse, L.H. Schwartz and J.T. Butt, J. Catal., 72 (1981) 95.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
M e t h a n o l s y n t h e s i s f r o m CO2/H 2 over Pd p r o m o t e d Cu/ZnO/A1203 catalysts
Mortaza Sahibzada, Ian S. Metcalfe and David Chadwick Department of Chemical Engineering & Chemical Technology, Imperial College of Science, Technology & Medicine, London SW7 2BY, UK
The kinetics of methanol synthesis from CO2/U 2 o v e r Pd-promoted Cu/ZnO/A1203 catalysts have been investigated at finite conversions using an internal recycle reactor and at differential conditions in a microflow reactor. Methanol production at finite conversions was promoted by addition of Pd in physical mixtures or to a greater extent by impregnation. There was no evidence of Pd promotion at strict differential conditions, although the rates of CO2 hydrogenation to methanol were much greater than methanol production rates at finite conversions. Water addition confirmed that Pd promotion was active only in the presence of the product water from CO2 hydrogenation. 1. INTRODUCTION The recognition that the hydrogenation of carbon dioxide to methanol is a promising approach to the recycling of emitted CO2 has given rise to considerable interest in the development of selective catalysts. The addition of precious metal promoters to conventional low pressure methanol synthesis catalysts in an attempt to provide spillover hydrogen has proved an attractive approach. Inui and co-workers [1-3] have shown that addition of Pd/A1203 and Ag/A1203 to Cu/ZnO based catalysts in the form of physical mixtures significantly promoted methanol synthesis from CO2. Subsequently, Fujimoto and Yu [4], investigating the effect of Pd and Pt on Cu/ZnO-based catalysts, arrived at a similar conclusion and in addition demonstrated the stability of the Pd promoted catalysts against poisoning of methanol synthesis activity by water. Lanthanide oxides [5] and TiO2 [6] have also been shown to promote CO2 conversion to methanol over Cu/ZnO-based catalysts. We have previously reported [7] preliminary results concerning the effect on methanol production from CO2/H2 of Pd addition by impregnation of an industrial-type Cu/ZnO/A1203 catalyst. Pd impregnation achieved significant increases in methanol production over physical mixtures. These studies were carried out using an internal recycle reactor over a range of conversions which approached those typical of industrial reactors. In such a reactor the catalyst is exposed to the product concentration, leaving open questions concerning the influence of the products on the hydrogenation of CO2. In the present study, we have sought to clarify the influence of the products and their interaction with the promoting effect of Pd. This has been achieved by a study of the kinetics of methanol synthesis from CO2/H 2 in an internal recycle reactor at normal yields, and in a microflow reactor at differential conditions using high space velocities. In both cases a comparison has been made between incorporation of Pd by impregnation and addition of Pd via physical mixtures ofPd/A1203 + Cu/ZnO/A1203. 2. E X P E R I M E N T A L CuO/ZnO/A1203 (Cu/Zn/A1) catalysts were prepared by two-stage precipitation according to a proprietary recipe [8]. PdO/CuO/ZnO/A1203 (Pd/Cu/Zn/A1) catalysts were formed by impregnation of the precipitate before calcination. The dried precipitate was reslurried in
351
352 dilute Na2CO 3 before neutralising with Pd(NO3) 2 solution. The quantities of Na,2CO 3 and Pd(NO3) 2 w e r e pre-calculated to achieve the desired Pd loadings. For the physical mixtures, PdO/A1203 (Pd/A1) was prepared by precipitation of Pd aluminate, followed by calcination. The calcined Pd/A1 and Cu/Zn/A1 powders were thoroughly mixed before pelletisation. Metal compositions of the calcined catalysts are given in Table 1. Pellets of 2 g/cm 3 density were formed with the addition of 2% (w/w) graphite, then crushed and sieved to required particle size. Prior to reaction, catalysts were activated in situ by reduction under flowing 5% H 2 in N2 at 215~ approached at 1~ Table 1. Compositions and surface areas of the calcined catalysts Catalyst Cu/Zn/A1-1 Cu/Zn/A1-2 0.04 Pd/Cu/Zn/A1 0.09 Pd/Cu/Zn/A1 0.07 Pd/AI+Cu/Zn/A1 0.21 Pd/AI+Cu/Zn/A1
Pd/Cu (at ratio)
PdO (wt%)
CuO (wt%)
ZnO (wt%)
(wt%)
BET surface area (m2/g)
0 0 0.04 0.09 0.07 0.21
0 0 3.7 7.7 4.0 12.0
60.3 60.1 58.2 55.6 36.2 36.2
28.6 28.3 27.2 25.7 17.2 17.2
10.8 11.3 10.7 10.6 42.5 34.5
115 112 112 110 -
A1203
Kinetics were measured in an internal recycle reactor ( 3 0 0 c m 3 volume, 100 c m 3 catalyst basket) [7] at 5 MPa and 250~ using an impeller speed of 1500 r.p.m and a synthesis gas with a 4 H2 : 1 CO2 mix. The flow rate (defined as mol (of synthesis gas) h 1 gcu1) was varied to achieve a range of conversions. 10% He was added to the synthesis gas as an internal analytical standard. Experiments at differential conversion were carried out in a microflow reactor at the same temperature and syngas pressure and with the same H2/CO2 ratio. The 6 mm internal diameter reactor was charged with -5 mg catalyst giving a bed length of approximately lmm. Packing of SiC above (175 mm) and below (100 mm) the catalyst with the same particle size ensured the flow was well distributed. A flow rate > 240 ml/min (s.t.p.) was sufficient to prevent extra-particle mass transfer limitation. Catalyst particle size ranges of 250-500 lam for the internal recycle reactor and 106-250 gm for the microflow reactor were used and were small enough to prevent any limitation from intra-particle diffusion. Product analysis was performed by on-line G.C. using a Poropak Q Column and T.C.D. 3. R E S U L T S 3.1 Finite conversions in the internal recycle reactor The reaction of CO2 and H2 produced only methanol, CO and water with trace quantities of methane. The sum carbon selectivity of CO and methanol was always greater than 99%. No trend in the trace methane production was identified, either with Pd promotion or with deactivation. Methanol and CO production were measured every hour over a period of approximately 50 h. All the catalysts deactivated with approximately 10% loss of activity in 40 hours. From the clear trend it was possible to define initial activities which are reported in Figure 1 and throughout this paper. The two Cu/Zn/A1 catalysts studied gave similar methanol production rates (+5%) across a range of conversions, demonstrating the reproducibility of the activity.
353 The addition of Pd in 40 the form of physical mixtures enhanced CO z ~ 35 0.04 Pd/Cu/Zn/A1 conversion to methanol, == 30 which increased with the .2 O 0.09 Pd/Cu/Zn/A1 higher Pd content. The z 25 activity was even greater 20 in the case of the --O 0.21 PdAI+Cu/Zn/A1 impregnated catalysts, N 15 although the two Pd "~ loadings studied gave very ~ 10 0.07 Pd/AI+Cu/Zn/A1 similar results. Figure 1 "'= 5 shows the observed promotion in methanol ~ 0 production relative to 0 2 4 6 8 10 12 14 Cu/Zn/AI-1. A promotion flow rate (mol h -1 gcu"1) of approximately 35% was found for the two Figure 1. Pd promotion of initial activity for methanol Pd/Cu/Zn/A1 catalysts at synthesis from COJH 2 in the internal recycle the highest flow rate reactor relative to Cu/Zn/A1-1 (lowest conversion). Here, the intermediate promotion of the physical mixtures is clearly seen. At the highest flow rate, the selectivity to methanol ranged from approximately 58% in the case of Cu/Zn/A1 to 65% in the case of the Pd/Cu/Zn/A1 catalysts. For all catalysts the extent of promotion observed in the net methanol production rate decreased as the flow rate decreased (CO2 conversion to methanol increased). This is probably associated with the methanol reaction approaching closer to equilibrium (which lies at-~25% conversion [1,2]) and the higher CO partial pressure. Pd had no observable effect on CO production by the reverse water-gas shift reaction across the range of conversions studied [7]. 3.2 Methanol production at differential conversion A study of the kinetics of methanol synthesis at differential conversion was performed in order to eliminate the influence of the products. The only products observed were methanol, CO and water. Data was obtained as initial activity measurements to eliminate the interference from catalyst deactivation in comparisons of the activities of different catalysts. The initial rate of deactivation in the microflow reactor at differential conditions was < 1%/hour for all catalysts. Each data point reported in Figure 2 and Table 2 is the initial activity from a unique experiment using a fresh catalyst charge. In order to establish the experimental parameters necessary for differential conditions, we studied the effect of increasing flow rate of COJH 2 through the microflow reactor on methanol yields and production rate over Cu/Zn/AI-1. At flow rates above 650 mol h ~ gcu1, where the methanol yields were < 0.33%, the rate of methanol production became independent of flow rate, signalling that true differential reaction conditions were achieved (Figure 2). This constant methanol production rate, 0.45 mol h 1 gcu "1, w a s the intrinsic forward rate of CO2 hydrogenation to methanol at the given conditions for these catalysts. The high space velocity necessary to achieve differential conversion under COJH2 is
354 indicative of the inhibition of CO2 6 tE hydrogenation associated - 0.4 with the presence of the reaction products (in particular with the -9 4 -- 0.3 presence of product water - see below). 3 -- 0.2 Methanol production rates over the Cu/Zn/A1, E 2 = 0.04 Pd/Cu/Zn/A1, 0.09 ~ 1 ~ --0.1 Pd/Cu/Zn/A1, and 0.21 E Pd/AI+Cu/Zn/A1 catalysts 0 in the differential kinetic regime are given in Table 0 200 400 600 800 1000 2. Each point was the flow rate (mol h l gcu "1) average of several experiments which were Figure 2. Methanol production from C O 2 / I - I 2 o v e r Cu/Zn/A1-1 quite reproducible. All in the microflow reactor (initial activities) four catalysts gave the same rate of methanol production within experimental error. Thus, in contrast to the results obtained at finite conversions with the internal recycle reactor, the promotion of methanol production by Pd was no longer apparent at differential conversions (that is, above a flow rate of 650 tool h -~ gcu-~). At these conditions the methanol production rate is the forward rate of CO2 hydrogenation, and we may conclude that this is unaffected by the presence of Pd in the absence of the reaction products (the product water - see below). This result applies whether Pd was incorporated by impregnation or by use of a physical mixture. 7 ~
0.5
Table 2. Methanol production from CO2/H 2 at differential conversion* Catalyst
methanol production rate (mol h -~ gcu~)
Pd promotion (%)
0.44 0.45 0.45 0.46
2 1 3
Cu/Zn/AI-1 0.04 Pd/Cu/Zn/A1 0.09 Pd/Cu/Zn/A1 0.21 Pd/AI+ Cu/Zn/A1 * CO2/H 2 flow rate 698 mol h -1 gcu~
CO production by the reverse water-gas shift reaction reached differential conversion at relatively low flow rates compared to methanol production. Above a flow rate of 150 mol h -~ gcu-1, the CO production rate became approximately constant at 0.10 mol h -1 gcu-1, corresponding to CO yields < 0.33%. In other words, the intrinsic rate of CO2 hydrogenation to methanol was much faster than the reverse water-gas shift reaction. Similar results were obtained over the three catalysts providing no evidence for Pd promotion of the reverse watergas shift reaction.
355
3.3 M e t h a n o l p r o d u c t i o n
f r o m C O J H 2 + H 2 0 at d i f f e r e n t i a l c o n v e r s i o n
The addition of water to the inlet gas of the microflow reactor operating at differential conditions severely inhibited the production of methanol from CO2/H2 over the Cu/Zn/AI-1 and Pd-impregnated Cu/Zn/A1 catalysts. The inhibition was slowly reversed when the water was switched off. The addition of water, which amounted to 2.6% (mol/mol) of the syngas resulted in an order of magnitude loss of activity over both Cu/Zn/AI-1 and the Pd impregnated catalyst, and there continued to be a similar slow transient loss of activity during exposure to water. Importantly, however, in the presence of water there was 33% greater methanol production over the Pd-promoted catalyst compared to the Cu/ZrdAI-1 catalyst (see Table 3). These results demonstrate that the presence of water causes severely inhibition of CO2 hydrogenation to methanol, but also that Pd promotion is only active in the presence of water. Table 3. Methanol production from CO2/H 2 (differential conversion), CO2/H 2 + H20 (differential conversion) and CO2/H2 (finite conversion, internal recycle reactor)
(I) CO/H 2 (differential conversion)
Cu/Zn/A1-1
0.09 Pd/Cu/Zn/A1
0
0
0.44
0.45
-
1
Cu/Zn/A1-1
0.09 Pd/Cu/Zn/A1
water concentration (mol%) methanol production rate (mol h -1 gcu-l) Pd promotion (%)
2.6 0.043 -
2.6 0.057 32
(III) COzO~: (internal recycle reactor)
Cu/Zn/Al-1
0.09 Pd/Cu/Zn/A1
water concentration (mol%) methanol production rate (mol h "l gcu-1) Pd promotion (%)
2.8 0.046
3.1 0.057 24
water concentration (mol%) methanol production rate (mol h -1 gcu-1) Pd promotion (%)
(II) COz/H2+H20 (differential conversion)
-
(I) CO2/H 2 (698 mol h-' gcu-'); (II) CO2/H2 (698 mol h-' gcu-') + H20 (18.5 mol h -1 gcu-'); (III) CO2/H 2 (3.47 mol h -1 gcu"), H20 production rate (0.096 / 0.109 mol h-' gcu-')
The results at differential conversions with water addition can be compared with methanol production at the finite conversion in the internal recycle reactor where the water concentration as a result of water production was similar (Table 3). The two types of experiment are analogous in that at differential conditions in the microflow reactor the catalyst was uniformly exposed to the feed concentration, whereas at finite conversions in the internal recycle reactor the catalyst was uniformly exposed to the product concentration. The methanol production rate at finite conversion was similar to the methanol production rate from CO2/H2/H20 at differential conditions for both the Cu/Zn/AI-1 and Pd impregnated catalyst. Therefore, the kinetics at the particular finite conversions, well away from equilibrium, can also be described by methanol production by CO2 hydrogenation, and the inhibition of this reaction associated with the presence of the product water. Furthermore, the Pd promotion was similar under the two reaction regimes (Table 3), reinforcing the conclusion that Pd promotion of CO2 hydrogenation is active only in the presence of water.
356 4. D I S C U S S I O N
Inui et al [1-3] have proposed that hydrogen spillover from Pd, maintaining a more reductive state of Cu, is the mechanism of Pd promotion of methanol production from CO2/I--I 2. Since Pd/AI203 catalysts were found to be inactive, the promotion observed over the physical mixtures in the present work supports this view. T.P.R. results have shown that Pd acts as a reduction promoter for CuO in the impregnated catalysts and in the physical mixtures [3,7] and may provide a reasonable guide to the efficiency of hydrogen spillover from Pd to Cu in this catalyst system [7]. Fujimoto and Yu [4] suggested that the reductive effect of spillover hydrogen from Pd opposed the oxidising effect [9] of the product water from methanol synthesis. While it has often been supposed that a CO2/CO redox process controls the oxygen surface coverage on Cu [ 10], leading to a lower net rate at high CO2 fractions, the results presented here suggest the HzO/H 2 ratio is at least as important. The results at true differential conversions make it clear that Pd promotion is only active in the presence of the product water, and that a high CO2 concentration does not in itself lead to a low rate of CO2 hydrogenation. Indeed, the intrinsic rate of CO2 hydrogenation is observed to be very high over conventional methanol synthesis catalysts in the absence of the products. An implication might be that the Cu surface is oxygen free in the absence of water [ 11 ]. However, rather than a high surface oxygen coverage reducing the rate, it may be that the combination of high CO2 and water concentrations leads to a Cu surface blocked by carbonate and formate [10], which would also be offset by spillover hydrogen from Pd. For the impregnated catalysts we are uncertain over the precise form of Pd in the reduced catalysts, so the explanation of the promoting effect in terms of hydrogen spillover is partly speculative. 5. C O N C L U S I O N S Results at true differential conversions have demonstrated that the intrinsic rate of CO2 hydrogenation is unaffected by the presence of Pd, whether it is incorporated by impregnation or added in a physical mixture, in the absence of water. For catalytic conversion of CO2 to methanol, a limiting factor at industrially useful yields is the high concentration of product water the effect of which can be ameliorated to some extent by Pd probably due to hydrogen spillover. ACKNOWLEDGEMENTS M.S. thanks EPSRC (UK) and ICI Katalco for a CASE Award. We are grateful to G. Chinchen (ICI Katalco) for help with catalyst preparation. REFERENCES [ 1] T. Inui and T. Takeguchi, Catal. Today, 10 (1991) 95. [2] T. Inui, T. Takeguchi, A. Kohama and K. Kitagawa, 10th. Int. Congress Catal., (1992) 1453. [3] T. Inui, Catal. Today, 29 (1996) 329. [4] K. Fujimoto and Y. Yu, 2nd. Int. Conf. Spillover, (1993) 393. [5] T. Inui, T. Takeguchi, A. Kohama and T. Tanida, Energy Convers. Mgmt., 33 (1992) 513. [6] H. Arakawa, K. Sayama, K. Okabe and A. Murakami, 2nd. Int. Conf. Spillover, (1993) 389. [7] M.Sahibzada, D.Chadwick and I.S.Metcalfe, Catal. Today, 29 (1996) 367. [8] D. Comthwaite (I.C.I.), U. K. Patent 1 296 212 (1966). [9] E. Colboum, R. A. Hadden, H. D. Vandervell, K. C. Waugh and G. Webb, J. Catal., 130 (1991) 514. [10] K. C. Waugh, Catal. Today, 15 (1992) 51. [ 11] M. Muhler, E. Tomqvist, L. P. Nielson, B. S. Clausen and H. Topsoe, Catal. Lett., 25 (1994) 1.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
357
A 50 k g / d a y class test plant for m e t h a n o l synthesis from CO2 and H2
Kenji Ushikoshi
a*,
Kouzou Mori a, Taiki Watanabe a, Masami Takeuchi a
and Masahiro Saito b a Research Institute of Innovation Technology for the Earth (RITE) 9-2 Kizukawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan bNational Institute for Resources and Environment (NIRE) 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan
A small scale test plant with a methanol production capacity of 50 kg/day has been designed and constructed in order to examine the performance of a Cu/ZnO-based multicomponent catalyst under practical reaction conditions. The production rate of methanol over the multicomponent catalyst was around 600 g/1-cat.h under the reaction conditions of 523 K, 5 MPa, and SV-10,000h-1 The purity of methanol produced was 99.9%.
1. INTRODUCTION The greenhouse effect of carbon dioxide has been recognized as one of the most serious problems in the world, and a number of countermeasures have been proposed. Catalytic hydrogenation of carbon dioxide to produce various kinds of chemicals and fuels has received much attention as one of the most promising mitigation options. In particular, methanol synthesis by CO2 hydrogenation (CO2+3H2=CH3OH+H20) has been considered to play an important role in the conversion and transportation of Ha energy derived from natural energy such as solar energy, hydro power and so on. RITE and three national institutes (NIMC, NIRE and ONRI) of MITI have been jointly carrying out a R/D project "CO2 Fixation and Utilization Using Catalytic Hydrogenation Reaction". In this project, methanol synthesis from CO2 and Ha is one of the key technologies being developed. A practical methanol synthesis process greatly requires a high performance catalyst, which must be highly active and selective for methanol synthesis and also stable for a long period in a continuous operation. RITE and NIRE have been jointly implicated in the development of high performance methanol synthesis catalysts since 1990. We developed the Cu/ZnO-based multicomponent catalyst on the basis of the role of metal oxides contained in a Cu/ZnO-based catalyst, which were found to be highly active and selective for methanol synthesis, and also stable for a long period in continuous methanol synthesis. * To whom correspondence should be addressed.
358 In the present kg/day has been developed under designing a pilot
study, a small scale test plant with a methanol production capacity of 50 designed and constructed in order to examine the performance of catalysts practical reaction conditions, and to collect experimental data useful for plant in the future.
2. EXPERIMENTAL
2.1. Catalyst A multicomponent catalyst (Cu/ZnO/ZrO2/A1203/Ga203) prepared by a conventional coprecipitation method [ 1] was used for the present study. The catalyst was pelletized to a cylindrical shape, the size of 3 mmD x 3 mmH. The multicomponent catalyst was developed on the basis of the role of metal oxides contained in a Cu/ZnO-based catalyst [2]. Figure 1 shows the activity of the multicomponent catalyst compared with the activities of Cu/ZnO, Cu/ZnO/A1203 and a commercial catalyst for methanol synthesis from syngas. Target Value
Cu/ZnO Cu/ZnO/AI203 C u/Z n0/ZrO 2/AI203 T
Cu/Z n O/ZrO 2/AI203 / Ga203 Commercial Catalyst (Cu/ZnO/AI203)
i iii::i:iiii::i.i!i:iiiiil..:
200
300
1
r
i .:" .i: :!: :: .'ii :i ::!'!:
T ~ ::! :.i ..: .. :i."
400 500 600 700 STY, g - M e O H / I - c a t / h
800
Figure 1. Activities of the catalysts developed in the projects. The reaction condition was as follows: Catalyst loading = 1 ml, temperature = 523 K, pressure = 5 MPa, SV = 18,000 h - 1 H2/CO2 ratio =3.
2.2. Test plant Practical methanol synthesis must be performed by using a reactor with recycling equipment for unreacted gases, because the conversion of CO2 to methanol at reaction equilibrium is low under ordinary reaction conditions such as 17% at 523 K, 5 MPa and H2/CO2 ratio is 3. Therefore, a test plant with a methanol production capacity of 50 kg/day was designed and constructed. The flow diagram of this plant is shown in Figure 2. The mixture of CO2 and H2 supplied from gas cylinders was compressed along with recycled gases, and then fed into the reaction tube through a pre-heater. Reaction products were cooled down, and then the mixture of methanol and water was separated at the gas-liquid separator from unreacted gases. Unreacted gases and gaseous products such as CO, methane and so on were recycled back to the reactor. The mixture of methanol and
359 water was taken out of the plant and stored in a container.
Recycle Gas Catalyst
DT~-P.Vent
\Z
_~ ~Gas-Li qu i d iSeparator
H2 Feed Gas Compressor
002
!
Recycle Gas
Reactor~
Compressor
0il
Heater I
Oil
Cooler
Crude CH30H Tank
OH3OH/H20
Figure 2. Flow diagram of a 50 kg/day test plant for methanol synthesis from CO2 and Ha The reaction conditions were as follows: reaction tube of 38.4 mmID x 4 mL, catalyst loading = 0.5-3 1, temperature = 503-543 K, pressure = 3-11 MPa, SV in the r e a c t o r - 5,000-20,000 h-1
2.3. Reaction method and analysis of product The catalyst fixed at the reactor was reduced in a gas mixture of 5% of Ha and 95% of N2 at a space velocity 1000 h-1 before the reaction. The reduction of the catalysts was performed in three steps/ (1) temperature was maintained at 413 K for 5 h, (2) temperature was raised to 543 K during 2 h, (3) temperature was maintained at 543 K for 2 h. The reaction was carried out under the following range of conditions; temperature = 503-543 K, total pressure = 3-7 MPa, space velocity = 5,000-20,000 h-1, feed gas composition - CO2 (25%)/H2 (75%). Gas chromatography was employed for analysis of the reaction products; Ha, CO and CO2 were analyzed by thermal-conductivity detector (TCD); methanol, dimethyl ether, methyl formate and hydrocarbons were analyzed by the flame ionization detector (FID). 3. R E S U L T AND DISCUSSION
3.1. The performance of the catalyst under practical reaction conditions The present test plant for methanol synthesis is beeing easily operated. Figure 3 shows the rate of production of methanol, i.e., space time yield of methanol (STY), as a function of time on stream over the multicomponent catalyst under the reaction conditions of 523 K, 5 MPa and SV = 5,000 h-1, 10,000 h-1. The production rate of methanol at SV = 5,000 h-1 was almost the same as that at reaction equilibrium. In the case of SV = 10,000 h - 1, the methanol production rate was 600 g/1-cat'h, which is 20% lower than that at reaction equilibrium. No significant difference was observed between data obtained from the present
360 test and from the previous laboratory scale test [ 1].
t-
1200
800
-~' 700
L 1000
0 I ._I
0
"
G~
t"
600
"I"
o 500
[3~.
"G
-~.
800
o
~ 400 '-I-
600 >-" 400
300 I-..
oo 200 0
i
i
i
20
40
60
I-09
200 490
80
TIME ON STREAM, h
20
.i.a -
800
--r-
c.) o10 l--
o (.)
550
1000
o ~15
z o (.)
530
Figure 4. Production rate of methanol under various reaction conditions. Reaction conditions: catalyst (Cu/ZnO/ZrO2/ A1203/Ga203) loading = 3 1, pressure, A=3 MPa; 11=5 MPa; 0 = 7 MPa; SV in the reactor=l 0,000h -' Broken lines are equilibrium values.
Figure 3. Activity of a Cu/ZnO/ZrO2/ A1203/Ga203 catalyst for methanol synthesis, Reaction conditions: catalyst loading = 3 1, temperature = 523 K, pressure = 5 MPa, SV O=5,000 h -' 9 h -' Broken lines are equilibrium values.
z
510
REACTION TEMPRATURE, K
600
400 5 0
200 ,
0
5
~
,
10 15 SV, IOOOh-I
L
20
0
o
I ._J
"I-
o
9
I
>: m
25
Figure 5. CO2 conversion to methanol ( A ) and space time yield ( O ) space velocity (SV). Reaction conditions: catalyst (Cu/ZnO/ZrO2/A1203/GazO3) loading = 3 1, temperature = 523 K, pressure = 5 MPa.
as a function of
Figure 4 shows the production rate of methanol at SV = 10,000 h - ' under different pressures as a function of reaction temperature ranging from 503 K to 543 K. The production rate of methanol increased with increasing reaction pressure. The production rate of methanol at a given pressure approached the value at equilibrium with increasing temperature. The methanol synthesis reaction over the multicomponent catalyst was found
361
to attain equilibrium at 543 K. Figure 5 shows CO2 conversion to methanol and STY at 523 K, 5 MPa as a function of space velocity ranging from 5,000 h-1 to 20,000 h-1 The CO2 conversion to methanol decreased with space velocity. This suggests that the methanol synthesis should be suppressed by the water produced along with methanol, as described by Saito et al. [2].
3.2. Purity of the methanol produced Table 1 shows gas compositions of the gases at the inlet and the outlet of reactor during the methanol synthesis operation under the condition of 523 K, 5 MPa, SV = 10,000 h -1, a purge rate = 1% of the inlet gas. The CO concentration in the gas at the inlet of the reactor was around 3% and almost unchanged during the operation. This level is much lower than that in the methanol synthesis from syngas. It is well known that CO is much more reactive with Ha to produce higher alcohols and higher hydrocarbons than CO2. Therefore, this finding suggests that a lower CO concentration in the feed gas to the reactor should result in a lower yield of by-products and thus a higher methanol purity. Table 2 shows the composition of the liquid products obtained from the operation under the conditions of pressure = 5 MPa, SV = 10,000 h-1, purge rate = 1% of the Inlet gas and different temperatures 503 K, 523 K and 543 K. The main compositions of the liquid products were methanol and water, but a very low concentration of methyl formate, ethanol, higher alcohol and so on were observed as the byproducts. The purity of the methanol produced at any reaction temperature tested was higher than 99.9 wt%. The purity of the methanol produced at 523 K was the highest, mainly because there was more production of methyl formate at 503 K and more higher alcohols were formed at 543 K.
Table 1 Gas compositions at the inlet and the outlet of the reactor Compound Hydrogen Carbon dioxide Carbon monoxide Methanol Water Dimethyl ether Methane Methyl formate Ethane Ethanol
Make up gas Ha CO2 CO CH3OH HaO
74.5 % 25.5% -
(CH3)20
-
CH4 HCOOCH3 C2H6 CzHsOH
-
Inlet 73.2% 23.4% 3.0% 0.2% 0.1% 0.13% 0.02% 24 ppm 5 ppm ND
Outlet 66.9% 21.1% 3.3% 4.5% 4.0% 0.16% 0.02% 51 ppm 6 ppm ND
Reaction conditions: catalyst = Cu/ZnO/ZrO2/A1203/Ga203, temperature = 523 K, pressure = 5 MPa, SV=10,000h- 1, H2/CO2 ratio in the make-up gas - 3, purge r a t e - 1% of the inlet gas.
362 Table 2 Compositions of liquid products at various reaction temperatures Composition at the reaction temperature of
Compound Methanol Water Methyl formate Ethanol 1-Propanol 2-Propanol 2-Butanol
CH3OH H20 HCOOCH3 C2H5OH 1-C3H7OH 2-C3HTOH 2-C4H9OH
CH3OH/Products except t-I20
503 K
523 K
543 K
63.3 wt% 36.7 wt% 450 ppm 12 ppm ND 7 ppm 9 ppm
63.0 wt% 36.9 wt% 290 ppm 18 ppm 6 ppm 9 ppm 12 ppm
64.4 wt% 35.6 wt% 270 ppm 46 ppm 8 ppm 17 ppm 21 ppm
99.92 %
99.95 %
99.94 %
Reaction conditions: catalyst = Cu/ZnO/ZrO2/A1203/Ga203, pressure = 5 MPa, SV=10,000h -1, H2/CO2 ratio in the make-up gas = 3, purge rate = 1% of the inlet gas. 4. CONCLUSION (1) The methanol production rate was 600 g/1-cat.h under the conditions of 523 K, 5 MPa and SV=10,000h-1, which is 20% lower than that at reaction equilibrium. No significant difference was observed between data obtained from the present test and from the previous laboratory scale test.. (2) The production rate of methanol increased with increasing reaction pressure. The production rate of methanol at a given pressure approached the value at reaction equilibrium with increasing temperature. The methanol synthesis reaction over the multicomponent catalyst was found to attain equilibrium at 543 K. (3) The selectivity of the multicomponent catalyst for methanol synthesis was extremely high. Accordingly, the purity of methanol produced was 99.9%. ACKNOWLEDGEMENT The present work was supported by New Energy and Industrial Technology Development Organization (NEDO). REFERENCES
1. M. Takeuchi, M. Saito, T. Fujitani, T. Watanabe and YI Knai, Abstracts of ICCDU-III, 1995. 2. M. Saito, T. Fujitani, M. Takeuchi and T. Watanabe, Appl. Catal. A:General, 138(1996),311. 3. M. Saito, M. Takeuchi, T. Watanabe, J. Toyir, S. Lou and J. Wu, Proc. ICCDR-III, 1996, 403.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
363
C o m p a r i s o n of CO2 sources for the synthesis of r e n e w a b l e m e t h a n o l M. Specht, A. Bandi, M. Elser, F. Staiss Center for Solar Energy and Hydrogen Research (ZSW), Hessbruehlstr. 21 C, D-70565 Stuttgart, Germany 1. INTRODUCTION The reduction of fossil fuel consumption, as well as the recycling of CO2 contributes to the reduction of climate-relevant emissions. ZSW develops concepts for CO2 recycling aimed to produce a liquid secondary energy carder. For both methanol vectors it was assumed that renewable energy is available in remote areas. The generated fuel will be transported from the fuel production site to the consumption site to be used in road transportation. Various carbon sources for the fuel production process can be used, e.g. flue gases from fossil powered plants and the atmosphere [ 1-3]. Other carbon sources for the methanol production are biomass and waste, but they are not considered for this paper. ZSW gives special attention to CO2 recovery from the atmosphere which enables a closed CO2/fuel cycle. This technology was demonstrated in a bench scale test plant [ 1]. For the CO2 recovery from fossil based power stations an additional primary energy input is required, producing supplementary CO2 emissions. The overall energy efficiency of the methanol vectors and the CO2 balance of the fossil based recovery process will be compared to the CO2 recovery from the atmosphere and to the conventional crude oil-gasoline systen~ 2. METItANOL AS TRANSPORTATION FUEL In this paper interest is focused on methanol as a transportation fuel. Compared with other alternative fuels, methanol is still considered to be an attractive option for the future from the ecological, technical, and strategic point of view. Methanol can be produced from a large variety of raw materials and it can be used in different ways for: the reformulation of conventional fuels, methanol spark ignition engines, methanol high-compression direct injection (DI) engines, engines for methanol/gasoline mixtures, the production of oil methyl esters from plant oil, and fuel cells as hydrogen storage medium 3. METEIANOL PRODUCTION; EXPERIMENTAL RESULTS The assumptions for this study regarding CO2 recovery from the atmosphere and methanol synthesis from CO2 are based on experimental results. The generation of methanol from electricity, water and air with an overall process efficiency of 38.1% represents a technology available today. At ZSW a new process for methanol generation from atmospheric CO2 has been developed [5]. New, highly efficient technologies with a low specific energy consumption are used whereby CO2 is absorbed from the air by a NaOH scrubbing solution to form a Na2CO3 solution. CO2 is recovered from the carbonate solution by acidif3dng with sulphuric acid. The caustic scrubbing liquid and the acidic solution are regenerated by an electrodialytic process with bipolar membranes. Methanol is then produced by catalytic conversion of CO2 and electrolytically generated hydrogen over a Cu/ZnO-based catalyst. The overall energy demand for the absorption/stripping/electrodialysis (ED) unit was estimated to 430 kJ/mol CO2 at ED current densities of 100 mA/cm2 including 70 kJ/mol CO2 for the fan blower. The value used in our calculation is higher due to higher current densities assumed for a technical process. The methanol generation from CO2 and 1-12was carded out in a bench scale synthesis plant with a reactor volume of about 400 ml. A modified Cu/CuO based catalyst (C79-05-GL of
364 Siidchemie, Germany) was used for the synthesis. An intermediate value of 0.66 kgMcOHlcat-1 h "1 for the space time yield was obtained at the following experimental conditions: once-throughreactor, cat.vol. 385 ml cat., 260~ 80 bar, feed gas ratio H2/CO2 = 3 and a space velocity of 8120 l~nga~Lat-1 h -1. This corresponds to a CO2-conversion rate to methanol of 0.23. 4. M E T I ~ O L VECTOR D E S C R I F r l O N In order to obtain reliable results the process design for the renewable methanol production is based on technologies available today. For the vector calculation the following assumptions were used: Two methanol energy vectors have been investigated for two different CO2 sources, capture from the flue stack of a coal fired 500 MWo thermal power station, or absorbed from the atmosphere. For the CO2 recovery from power stations different methods are feasible. Our calculation is based on a pulverised coal fired power station with MEA scrubbing. The energy for the CO2 capture is provided by extra fossil fuel, so that the electric output remains at nominal 500 MWo. The efficiency declines from 40 to 29 % at a CO2 recovery rate of 90% [4]. The additional primary energy input in the form of coal and the related CO2 emissions are considered for the CO2 balance of this methanol vector. If air serves as carbon source in the case of the second methanol energy vector, renewable energy is used for the CO2 recovery process. The reference vehicle is a gasoline passenger car with a fuel consumption of 8.2 1 gasoline/100 km driving distance corresponding to 73,1 kWh/100 km and a CO2 emission of 21.7 kg/100 km considering the entire energy vector. The cost of gasoline is 1.50 Deutschmark (DM/1) which includes 0.98 DM mineral oil tax and 0.20 DM value added tax according to applied regulations in Germany. Due to the higher efficiency of the methanol powered car, the energy demand for driving 100 km with a M100 (100% methanol) car is 15.2 1MoOH/100km corresponding to 65,1 kWh/100 knl CO2 emission factors for coal (323 g CO2/kWh coal), Diesel (267 g CO2/kWh Diesel), heavy fuel oil (284 g CO2/kWh heavy oil) and hydroelectric power (CO2 emission free) are used for the calculation in Tab. 1. All costs are given in Deutschmark (1 DM = 0.56 US-S; Aug. 1997). The costs are mainly determined by the energy input and the capital costs of the production plants. For both vectors it was assumed that hydroelectricity is available at 0.05 DM/kWh~ and 8,300 h/a. Capital costs are calculated on a real interest rate of 8% and depreciation periods (usually 15 years; power station 20 years) correspond to about 50% of the expected technical lifetime of the plants. The plant capacity was calculated to produce about 200 tons of methanol per day. It is assumed that no additional costs for a M100 car are necessary in a series production compared to a conventional gasoline car. Further assumptions were mainly taken from Ref. [ 1]. 5. RESULTS AND DISCUSSION Since renewable methanol requires - besides carbon dioxide - hydrogen derived from renewable energy, production sites would be in general located in remote areas, where these energy sources are abundant but far away from the methanol demand site. A process for the recovery of CO2 from the atmosphere can be located at the methanol production site, whereas the CO2 from concentrated emissions has to be transported from the recovery to the fuel production site. The "concentrated CO: path" is more energy efficient than the "atmospheric CO2 path", but the latter offers the advantages of avoiding long-distance CO2 transportation and additional CO2 production caused by the recovery process itself. If atmospheric CO2 and renewable energy are used for the fuel production, this energy system is almost climate neutral. A direct comparison of the energy demand for the CO2 recovery, production, transport, and distribution related to one litre methanol is given in Fig. 1. It is obvious that the production efficiency of the methanol vectors is lower than gasoline production efficiency, due to the conversion of the primary energy (hydro)electric power to a completely different energy carder. As shown in Fig.1 the primary energy demand for the production of methanol is 9.5 (conc.-CO2) and 11.4 kWlffl methanol (air-CO2). This is corresponding to an efficiency for the fuel generation of 45.8 and 38.1%, respectively
365 (efficiencies are related to the lower heating value of methanol). The main energy demand of the methanol synthesis column (Fig.l) requires the electrolytic hydrogen generation (4.2 kWhJNm 3 1-12). The overall efficiencies of the vectors including the car efficiency of 23.5% are 10.8 and 9.0%, respectively. For comparison, the gasoline production efficiency is 89.6 and the overall efficiency 19.2% [1]. A further improvement for the conc.-CO2-methanol path is the CO2-recovery upstream the combustion after CO shift in an IGCC-plant with the selexol absorption process. This technology has a better CO2 balance than the pulverised coal fired plant with MEA-process technology due to the less decrease in IGCC efficiency from 42% (reference efficiency) to 36% only (after CO2 capture) [4]; but it should be mentioned, that this technology is not state-ofthe-art today. The efficiency of the air-CO~,methanol vector was calculated to be 38.1%. With more advanced technologies (e.g. high temperature processes), the efficiency can be improved to more than 44% [6]. IW~/I
o.o 6.o 4.0
j m Power8/tion: I).6kWh ] Ak: 11~ kWh
3~;1
J []
3.0
i
2.0
1.0 0.0
___.
~
/
,e~o~~ oO~
9~
0.2
0~1
. ~ . ~
.~!
or
o~.
oO
Fig. 1 Primary energy comparison for the production of one litre methanol from conc.-CCh and from air-CCh 0.6 DM/I
0.67~
o.s
0.39
0.4
Jm Power Station: 0.82DMII J n Air: t.lt DM/I
0.3
0.18
0.18
0.2
0.t
0.08
~6
0
~o-~
oo ~,
,,,,,-,,
~,.o ~
Fig.2 Cost comparison for the production of one litre methanol from conc.-C02 and from air-C02 The costs are mainly determined by the energy input and the capital cost of the production plants (Fig.2). The cost of producing and providing methanol to the consumer can be calculated as 0.82 (conc.-CO2) and 1.11 DM/1 methanol (air-CO2) untaxed. Comparing the untaxed methanol fuel costs with taxed gasoline, the cost difference is rather small. If the untaxed gasoline costs are used as reference, methanol costs are 4 - 6 times higher. The results concerning efficiency, CO2 emissions and fuel costs are summarised in Tab. 1. The comparison of the two vectors shows a CO2 reduction of about half of that of the
366 conventional gasoline passenger car for the conc.-CO2 vector and an almost CO2 neutral vector if air serves as carbon source. In a bench scale plant the synthesis of renewable methanol from hydrogen and atmospheric CO2 has been investigated at ZSW. The results show that this method is a realistic option besides the recycling of CO2 from concentrated emissions. In the case of tax-free alternative fuels, the conc.-CO2-methanol option is competitive to the gasoline system today. From the point of view of fuel costs, the air-CO2 path is more expensive, but it saves fossil resources and reduces substantially the CO2 emissions, which are convincing advantages. Therefore the conc.-CO2 path has to be considered as the first step and the air-CO2 path as a future option for the generation of renewable fuels to obtain a sustainable mobility. Table 1 Energy efficienc~r CO2 emission and fuel costs of the ~asoline and the methanol vectors Fuel Generation Efficiency [%] Overall Efficiency [%] C02 Emission [kg/lO0 km] Fuel Costs for 100 km [DM/IO0 km] Taxed: Untaxed:
Gasoline
Methanol (Power Station)
Methanol (Atmosphere)
89.6
45.8
38.1
19.2
10.8
9.0
21.7
9.2
1.1
12.30 2.71
12.46
16.87
6. OUTLOOK Besides CO2 from concentrated emissions, other carbon resources have to be considered for the generation of liquid fuels. In this paper special attention is paid to methanol synthesis from atmospheric CO2 (artificial photosynthesis). Another approach is to use concentrated carbon of biomass (waste), which was recovered from the atmosphere through the natural photosynthesis, as feedstock. The hydrogen deficit of the generated synthesis gas from biomass gasification containing CO2, CO and 1-12 could be compensated by electrolytic hydrogen to achieve a high carbon conversion. If carbon taxes, mineral oil taxes, environmental damage costs and biomass waste revenues are included in the fuel costs, climate friendly fuels might be competitive to fossil based fuels in the near future. Synfuels from renewable hydrogen and carbonaceous resources (CO2, biomass, waste, etc.) are the link to conventional fuels, which allows the continuous transition from fossil to renewable energy carriers. Increasing environmental awareness and the growing significance of renewable energies may lead methanol to achieve considerable acceptance as an alternative fuel. The main advantage of the described concepts is related to the greenhouse gas emission reduction potential. Renewable methanol can contribute to a substantial reduction of CO2 emissions in the transport sector. REFERENCES
[1] M. Specht, F. Staiss, A. Bandi, T. Weimer in Hydrogen Energy Progress XI, T.N. Veziroglu, C.-J. Winter, J.P. Baselt, G. Kreysa (Eds.), DECHEMA, Frankfim (1996) 227 [2] M. Steinberg, Fuel 57 (1978) 460 [3] S. Stucki, A. Schuler, M. Constantinescu, Int. J. Hydrogen Energy 20 (1995) 653 [4] IEA Greenhouse Gas R&D Programme: Carbon Dioxide Capture from Power Stations (1993) [5] M. Specht, A. Bandi, K. Schaber, T. Weimer in CO2 Fixation & Efficient Utilization of Energy, Y. Tamaura, K. Okazaki, M. Tsuji, S. Hirai (Eds.), Tokyo Institute of Technology, Research Center for Carbon Recycling & Utilization, Tokyo, Japan (1993) 165 [6] T. Weimer, K. Schaber, M. Specht, A. Bandi, Energy Convers. Mgmt. 37 (1996) 1351
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
367
Characteristics and economics assessment of power generation systems utilizing solar energy in various regions Takanobu Kosugi a, Pyong Sik Pak b and Yutaka Suzuki b aCooperative Research Center for Advanced Science and Technology, Osaka University 2-1 Yamadaoka, Suita, Osaka 565, Japan bFaculty of Engineering, Osaka University 2-1 Yamadaoka, Suita, Osaka 565, Japan 1. I N T R O D U C T I O N Under the restriction on CO2 emissions, power generation systems utilizing solar energy (referred to as solar options) are expected to be introduced worldwide. Solar thermal system and solar photovoltaic system are well known as the solar option. However, these systems have difficulty to be introduced especially in the regions with lower solar radiation conditions from the economic point of view. As a kind of solar options, a CQ-capturing hybrid power generation system utilizing solar thermal energy (hybrid system) has been proposed by the authors [1,2]. The hybrid system emits no CO2 in principle by using oxygen combustion method and has high net fuel-to-electricity efficiency of higher than 60 % on lower heat value basis. The system is thus expected to be applied for mitigating CO2 emissions and for saving fossil-fuel instead of introducing high cost solar options. In this paper, the characteristics and economics of these three solar options constructed in five different locations in the world are evaluated and compared. 2. C H A R A C T E R I S T I C S
EVALUATION
Table I shows the annual mean values of daily total horizontal, direct-normal and diffuse solar radiation energy in five locations assumed. The hourly mean values of direct and diffuse components of solar radiation have been estimated from the given monthly mean values of daily total horizontal solar radiation [3]. Table 1 Annual mean values of daily total horizontal, direct-normal and diffuse solar radiation energy. (kWh/m 2- day) Total DirectLocation horizontal normal Diffuse Osaka (Japan) 3.63 2.50 1.99 New York (U.S.) 3.84 3.54 1.63 Ottawa (Canada) 3.91 4.42 1.38 Miami (U.S.) 5.31 5.60 1.55 Port Hedland (Australia) 6.53 7.68 1.22
368 Table 2 Assumed conditions for characteristics evaluation. (a) Solar thermal system Total aperture area of collectors Type of collector Tracking scheme of collector Optical efficiency of collector Concentration ratio of collector Effective emittance of collector Temperature difference at high and low temperature sides of heat exchanger Enthalpy loss rate in thermal transportation and heat exchanger Capacity of steam turbine power generation system Steam turbine inlet temperature Steam turbine inlet pressure Adiabatic efficiency of steam turbine Generator efficiency Condenser outlet pressure (b) Solar photovoltaic system Total area of photovoltaic arrays Tracking scheme of photovoltaic array Conversion efficiency of photovoltaic array Energy loss rate at inverter (c) Hybrid system Characteristics and capacity of collector and heat exchanger Steam accumulator inlet temperature Internal volume of steam accumulator Capacity of CO2-capturing H20 turbine power generation system Inlet steam pressure of regenerator Condenser outlet pressure Turbine inlet temperature Return water temperature
10 ha (100000 m 2) parabolic trough single north-south axis 70 % 3O 0.4 20 ~ ; 2O ~ 10% to be optimized 346 ~ 14 kg/cm2a 80 % 95 % 0.1 kg/cm2a
10 ha (100000 m 2) single north-south axis 15% 10% same as those of solar thermal system 220 ~ 2000 m 3 to be optimized 10 kg/cm2a 0.1 kg/cm2a 1200 ~ 100 ~
The assumed conditions used in evaluating characteristics of the systems are summarized in Table 2. The total areas of the solar collectors and photovoltaic arrays are assumed to be 10 ha. In the solar thermal and hybrid systems, the type of collector is parabolic trough, the condenser outlet pressure 0.1 kg/cm2a, and the capacity (maximum net power output) has been determined so as to minimize the unit cost of generated power energy. In the solar photovaltaic system, the conversion efficiency at the array is assumed to be 15 %. The tracking scheme of the solar collector and photovoltaic array is single north-south axis. Table 3 shows the estimated capacity and annual generated power energy. The capacity
369 Table 3 Estimated capacity and annual net generated power energy, NGPE.
Location Osaka New York Ottawa Miami Port Hedland
Solar thermal system Capacity NGPE (MW) (GWh/y) 3.80 8.05 5.49 12.2 6.34 15.3 7.21 22.1 10.1 32.1
Solar photovoltaic system Capacity NGPE (MW) (GWh/y) 13.5 19.3 13.5 22.0 13.5 24.0 13.5 31.8 13.5 40.6
Table 4 Assumed cost data for economics evaluation. Solar collector* 1
Steam turbine power generation system .1 Solar photovoltaic system*l
H20 turbine power generation system .1 02 production and compression equipments .1 CO2 liquefaction equipments*l Steam accumulator*l Fuel cost Disposal cost of captured CO2 Annual cost rate 9i.
construction
Hybrid system Capacity NGPE (MW) (GWh/y) 3.96 26.1 4.68 30.5 5.20 34.0 7.02 48.4 9.52 58.7
14 • 103 yen/m 2 (low) 18 • 103 yen/m 2 (medium) 22 • 103 yen/m 2 (high) 150 • 103 yen/kW 250 • 103 yen/kW (low) 350 • 103 yen/kW (medium) 450 • 103 yen/kW (high) 200 • 103 yen/kW 440 • 106 yen/(tC/h) 310 z 106 yen/(tC/h) 47 z 103 yen/m 3 1.0 yen/MJ 8000 yen/tC 0.1433 *2, 0.1733 *3
cost
2: 9 solar thermal system 3: 9 solar photovoltaic system and hybrid system
of the solar thermal system and hybrid system has been estimated to become larger in favorable solar condition. The estimated annual generated power energy are greater in larger solar radiation districts in all the systems. In all locations assumed, the hybrid system generates the largest power energy and the solar thermal system does the smallest. 3. ECONOMICS
COMPARISON
Table 4 shows the assumed cost data in near future around 2020. Three levels of the construction costs of the solar collector and the solar photovoltaic system are assumed. For evaluating the economics of the systems, the unit cost of generated power energy, denoted by Cp, has been estimated. Figure 1 shows the estimated values of Cp in two cases where both low and high construction costs are assumed. We can see that the unit cost Cp of the solar thermal system changes most widely corresponding to the change in solar radiation and that of the hybrid system does the least.
370
Osaka New York Ottawa Miami Port Hedland
/
c-
/
/
/
Osaka New York Ottawa Miami Port Hedland
/,
/
v,
c- 60
::', Solar thermal system
..
$
/
sYstem
ii i iT,dii/
40
::
~ 3O
"4>.
", x ! /
!i i
~ 40
0
Solar photovoltaic system ', ' ~ ' ~ ',,
'~.\
Hybrid system
"-~.~:,, -
~ 30
"
!
g
.
'
o 20
o 20
D
Hybr,d system . i i
10
i
..
2
Figure 1. Estimated unit cost where low and high construction costs are assumed.
/
~,
g o
,
60
50
"\.~
/,
>.,
50
cQ)
/, / ,
._ E
i / '
CF = I.5 yen/MJ
<,x %,
!
!
i
i
Solar thermal s y s t e m
4
6
8
Figure 2. Estimated unit cost taking the rise in fuel cost CF into account.
In the case of the high construction cost, the solar thermal system is the most economical in higher solar radiation and the hybrid system is the most economical in lower solar radiation. In the case of the low construction cost, the solar photovoltaic system is estimated to be more economical than the other systems in higher solar radiation; the hybrid system is still the most economical in lower solar radiation. Figure 2 shows the estimated value of Cp when the medium construction cost is assumed. For taking the rise in fuel cost into consideration, the fuel cost, CF, of 1.5 yen/MJ is assumed in addition to that of 1.0 yen/MJ. The value Cp of the hybrid system becomes higher by 2.82 yen/kWh caused by the rise in CF by 0.5 yen/MJ. However, the hybrid system keeps its economical advantage in lower solar radiation districts even when the value of (JR is raised to 1.5 yen/MJ. 4. C O N C L U S I O N Since the characteristic dependencies on solar radiation are different among solar options, the preferable option changes with the change in location from characteristic and economic points of view. We hope the results of this paper are helpful for selecting a highly recommendable solar option in various locations. REFERENCES 1. P. S. Pak, T. Hatikawa and Y. Suzuki, Electrical Engineering in Japan, Vol. 116, No. 5, pp. 50-60 (1996). 2. P. S. Pak, Y. Suzuki and T. Kosugi, Energy- The International Journal, Vol. 22, No. 2/3, pp. 295-299 (1997). 3. T. Kosugi, P. S. Pak and Y. Suzuki, Proc. of the 13rd Conference on Energy System and Economics of Japan Society of Energy and Resources, pp. 135-140 (1997) (in Japanese).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
371
S o l a r / c h e m i c a l e n e r g y h y b r i d i z a t i o n via B o u d o u a r d r e a c t i o n H. Ono, M. Kawabe, M. Nezuka, M. Tsuji, and Y. Tamaura Tokyo Institute of Technology, Research Center for Carbon Recycling and Utilization, Ookayama, Meguro-ku, Tokyo 152, Japan.
A solar thermochemical energy hybridization system for stating-up the global carbon recycling energy delivery system (GCRED-system) was studied. It involves a solar thermochemical methane decomposition and Boudouard reaction. In this hybridization system, natural gas is decomposed into carbon and H2, and 3/5 of the carbon is converted solar-thermochemically into CO using the Boudouard reaction. Half of the CO2 recovered from energy consuming site will be recycled. CO2 emissions can be reduced to 60% on the same calorific heat basis at energy consumption site.
1. INTRODUCTION Solar energy can be delivered in the form of the syn-fuel, e.g. solar methanol, in a global carbon recycling energy delivery system (GCRED-system) proposed[i,2]. Other candidate may be a solar H2 global energy delivery system (H2-GED system), where the solar H2 is transported by H2 pipe line and H2 tanker to an energy consuming site and used in H2 gas. However, a corresponding energy infrastructure is not available commercially for this system, and the former will be a better option for the near and medium term chemical system. In the GCRED-system, carbon is used as a solar energy carrier from a solar energy site to a syn-fuels consuming site, where existing oil tankers and fossil-fuel power plants can be used. Since additional energy for CO2 recovery from flue gas and CO2 shipping can be supplied by solar fuel, it is considered more feasible to start the GCRED-system by combining solar energy with fossil fuel and converting it to solar fuel such as syn-methanol. This fluid solar syn-fuel is transportable using the conventional energy infrastructure; here carbon of the fossil fuels is used as a solar energy carrier. Chemical systems for starting-up the GCRED-system have been proposed in the present study, where solar energy is hybridized with fossil energy through a solar thermochemical (STC) natural gas decomposition (CH4 ~ C + 2H2) and Boudouard reaction (C + C02 ~ 2CO, AH~ +171.54kJ at 900K). The STC process can give the theoretical maximum efficiency of 70-80% at 1000sun (1000 time concentrated solar energy)[3]. Kinetics of the solar thermochemical process of the Boudouard reaction using a concentrated solar radiation
372
constitutes a vital portion for the GCRED system and will be also described.
2. HIGH CONVERSION EFFICIENCY OF SOLAR E N E R G Y INTO CHEMICAL ENERGY IN SOLAR/FOSSIL ENERGY HYBRIDIZATION Solar furnace reaction system of Fe304 and coal can absorb the heat generated by the irradiation of a concentrated solar flux. The overall reaction can be represented by CHx + Fe304 = CO + 3FeO + 1/2xH2. Solar energy can be hybridized with fossil energy into the gaseous products of H2 and CO, and eventually into methanol (solar methanol)[4]. Solar-driven experiments were conducted using a high-flux solar furnace. The carbon content decreased rapidly after only 1 second exposure with efficient heat transfer and high chemical conversion by direct absorption of concentrated solar energy (47%) [5]. The proposed solar thermochemical process offers the possibility of performing simultaneously gasification of coal and reduction of iron oxide and also producing a fuel with an upgraded calorific value. This hybridization system will take an important role after the natural-gas shortage. 1 CH4
3 -5--C for preservation
-~1 CH4
t 6 CH4 --~-
_ 65 C + ~ . - ~ - H 2 ~
Solar AH=211
olar
nHrg0~
Combustion
(Q1) 6 ~f~12
~I~
--uCO + -~-H2 ----
l
6 CH30H 5
Recycling
3
1 C02
~:
Q1=890
~E CO2~ 53 missions
/
)
(Q2)
CO2
-~-5CO2 -"
C02
I Combustion I
F Q2=890 ( 237 ) =744xl.2 Combustion heat of preserved C AH in kJ.
Figure 1. GCRED-system with the STC process for fossil and solar energy hybridization
Figure 1 shows solar thermochemical energy hybridization system for stating-up the GCRED-system by the STC process. In this system, natural gas is decomposed into carbon and H2 by STC, and 3/5 of the carbon is converted solar thermochemically into CO by the Boudouard reaction. The CO2 emitted at the energy consuming site can be reduced to 60% and half of the CO2
373 recovered at the energy consuming site will be recycled. Thus, this system will find an important role as a near-term technology, which can be applicable before n a t u r a l gas shortage. The Boudouard reaction using CO2 as an oxidant has been reported to constitute an efficient solar thermochemical process[6]. The solar efficiency attained 40%.
3. S I M U L A T I O N OF SOLAR T H E R M O C H E M I C A L P R O C E S S F O R BOUDOUARD REACTION Basic knowledges on kinetics of the carbothermal redox process using a concentrated solar radiation are required for process and receiver/reactor designs of the solar thermochemical transformation. The present paper deals with kinetic modeling of the Boudouard reaction. The following solar/chemical transformation model was applied to the specimen and simulated by numerical calculation obtained by the energy and material balance equations. The energy balance equation is given by Ei = CphTt + Hchem + Hrad + Hcond
(1)
where the left h a n d side denotes incident light energy per sec, the first term on the right h a n d side energy used for increase in the temperature of the reaction cell and specimen, the second energy converted chemically, the third heat loss by black body radiation, the fourth heat loss by thermal conduction. Eq. (1) gives the temperature profile. Material balance gives the following equations. Assuming that carbon used in the experiment consists of spherical particles, CO evolution rate at time t, nt, is proportional to the product of carbon surface area at time t, St, and the rate constant at time t, kt. nt = - dnc/dt = kt St where nc denotes the quantity of carbon in mole at time t. given by the Arrhenius-type equation. kt = A exp(-Ea/RTt)
(2) kt is assumed to be
(3)
where Ea denotes the activation energy and R the gas constant. Moreover, if a carbon particle is consumed by the infinitesimal decrease dr, the material balance gives Vm dnc -- St dr
(4)
where Vm is the molar volume of carbon. Eqs. (2) and (4) are combined to give the decreasing rate of the particle surface area. Based on the above concept, kinetic data of the thermochemical process of Boudouard reaction was analyzed. The data was obtained as follows; the
374
sunlight concentrated with a Fresnel lens was irradiated to active carbon (0.30g or 0.025moles) while flowing C02 at a rate of 7 cm3/min. The specimen was placed in a quartz tube of 8mm i.d. supported by an alumina cement furnace. In the beginning of irradiation, the CO evolution rate rapidly increased with increase in the sample temperature and the maximum CO evolution rate attained 3.8 mmol sec -1 at 640 sec-irradiation where the specimen temperature reached 940K (Fig.2). Figure 3 shows the result of the simulated profile. As can be seen here, the above concept allowed satisfactory simulation results to the CO evolution profile of the Boudouard reaction in the solar thermochemical process.
r
1200
1200
-0- 4 " 900
G3 4~
~2 0
~9
1
~0 N?
0 0
'
0
600
,
1200
~
- 900
~ 3
6oo "~ ~
~~ 2
600
3O0 ~
.~ 1
300
o
~0
1800
Irradiation time [see]
Figure 2. CO evolution rate with irradiation time.
~ 0
o
G)
0 0
600
1200
1800
Irradiation time [sec]
Figure 3. Simulation results of temperature and CO evolution.
REFERENCES 1. Y. Tamaura and M. Tsuji, Proc. Joint IEW/JSER Int. Conf. Energy, Economy, Environment, June 25, (1996) 59. 2. Y. T a m a u r a and M. Tsuji, Abstr. Symposium on CO2 Fixation, December 9, The Chemical Society of Japan, Tokyo, (1996)6. 3. P. Kesselring, Proceedings of International Workshop on High-temperature Solar Chemistry, PSI, Switzerland, August 17, (1995) 21. 4. Y. Tamaura, Y. Wada, T. Yoshida, and M. Tsuji, International Symposium on CO2 F i x a t i o n a n d Efficient U t i l i z a t i o n of E n e r g y , Tokyo I n s t i t u t e of Technology, J a p a n , October 23, (1995) 337 5. Y. Tamaura, Y. Wada, T. Yoshida, M. Tsuji, K. Ehrensberger and A. Steinfeld, Energy-The International Journal, 22 (1997) 337. 6. R. W. Taylor, R. Berjoan and J. P. Coutures, Solar Energy, 30 (1983) 513.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
375
Development of active and stable nickel-magnesia solid solution catalysts for CO 2 reforming of methane K. Tomishige, Y. Chen, X. Li, K. Yokoyama, Y. Sone, O. Yamazaki, and K. Fujimoto Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
Nickel-magnesia solid solution catalyst with low Ni content had excellent stability for methane reforming with carbon dioxide at 1123 K without the formation of carbon deposition. Nickel-magnesia solid solution and magnesia supported nickel catalysts were characterized by means of FFIR, TEM, and the amount of H 2 and 0 2 adsorption. Ni metal particles on Ni0.o3Mg0.970have considerably high dispersion and the interaction with support surface. It is suggested that high resistance to carbon deposition is caused by supplying CO 2 or oxygen species through the interface between nickel metal and support surface.
1. INTRODUCTION CO 2 reforming of methane (equation 1) has been proposed as one of the most promising technologies for utilization of these two greenhouse gases, and this synthesis gas is suitable for Fischer-Tropsch synthesis and oxygenated chemicals. A serious problem is carbon deposition via Boudouard reaction (equation 2) and/or methane decomposition (equation 3). CH4+CO2--->2CO+2H2 2CO--->C+CO2 CHa--->C+2H2
AH=+247 kJ/mol AH=-173 kJ/mol AH=+75 kJ/mol
(1) (2) (3)
Carbon deposition has been reported to cause the catalyst deactivation, plugging the reactor. This has also been observed in steam reforming, but much more serious in CO 2 reforming of methane as expected by thermodynamic calculations [1]. Noble metals are found to be less sensitive to coking than nickel [2-4]. However, considering high cost and limited availability, it is more desirable to develop nickel-based catalysts which are resistant to carbon deposition. Recently we have reported that nickel-magnesia solid solution Ni0.03Mg0.970 has high and stable activity [5-9]. Our purpose is to elucidate catalytic active site and inhibition mechanism of carbon deposition by means of temperature programmed hydrogenation (TPH) of the sample after catalytic reaction and catalyst characterization.
2. EXPERIMENTAL Nickel-magnesia solid solution catalysts were prepared by coprecipitating nickel acetate
376 and magnesium nitrate aqueous solution with potassium carbonate. After being filtered and washed with hot water, the precipitate was dried overnight at 393 K, and then calcined in air at 1223 K for 10 h. Supported nickel catalysts (Ni/MgO and Ni/A1203)were prepared by impregnating MgO or A1203 with Ni(C5I-I702)2 acetone solution. MgO was prepared by the same method as solid solution catalyst. The loading was denoted as the molar ratio Ni/(Ni+Mg or A1). CH4-CO~ reaction was performed in a fixed bed flow reaction system equipped with gas chromatograph. Catalysts were reduced with H2at 1123 K before the reaction. Reaction condition was CH4/CO2=1/1, 773 K-1123 K, W/F=0.1-1.2 gh/mol. Carbon deposition was characterized by TPH method, in which pure hydrogen was introduced, and then temperature was raised from room temperature to 1123 K at a heating rate of 20 K/min. The sample after CH4-CO 2 reaction was quickly cooled down to room temperature under Ar flow, followed by replacing Ar with H 2. The signal corresponding to C H 4 formation was recorded continuously by FID detector without separating column. FYIR spectra were obtained in a transmission mode using in-situ IR cell connected to closed circulating system. The sample was reduced with hydrogen at 1123 K for 0.5 h. The samples for TEM observation were stored under vacuum until the measurements. Sample powder was dispersed in tetrachloromethane by supersonic waves and put on Cu grids under atmosphere. 100
3. RESULTS AND DISCUSSION
, %
80
o
. 9
Nio.=Mg0.9,O O
O
O
O
C~ - O
O O
O
{3
Figure 1 shows the catalytic activity on Ni0.03Mgo.97O' Ni/MgO, Ni/A12O3 .~ catalysts at 1123 K. Ni0.03Mgo.970was ~> 60 Ni(3 moi%)/MgO found to have very high and stable activity o for a long period (100 days). 3mo1% ~ 40 Ni/MgO did not so high activity, but rather -=_ stable. 3 mol% Ni/ml20 3 catalyst ~deactivated very rapidly and finally the 20 Ni(3 mol%)/AI203 reactor was plugged with the deposited carbon. We measured the amount of total 0 0 20 40 60 80 100 carbon species on the catalyst surface after Time on stream / day the reaction for 120 h. The results were 0.1 wt% on Ni 0.03Mg0.97O, 1.6 wt% Ni/MgO Figure 1. Reaction time dependence of and 10 wt% on Ni/A1203 catalysts. From methane reforming with carbon dioxide. Reaction condition: 1123 K, W/F=1.2 these results, it was found that Ni0.03Mgo.970 gh/mol, CHJCO2=I/1, 0.1 MPa, 0.1 g. catalyst has high resistance to carbon deposition in CO 2reforming of methane. Figure 2 shows the TPH results on Nio.03Mgo.970 and 3.0 mol% Ni/MgO after the reaction at 773 K, and the activity was listed in Table 1. Under this reaction condition, methane conversion is far from the thermodynamic equilibrium level. Two peaks were observed in the TPH profiles. One appeared at 550 K-700 K (a-carbon), and the other above 873 K (fl-carbon). It is found that the peak intensity of a-carbon was almost constant, while that of fl-carbon increased linearly with the time on stream. From the behavior and reactivity, fl-carbon is ascribed to deposited carbon, fl-carbon formation rate and selectivity were also in Table 1. Selectivity to carbon is much related to the dispersion of Ni metal particles. This suggested that carbon formation tended to proceed on the larger Ni particles. And carbon was formed on solid solution catalysts with higher Ni content. O
i
i
i
i
377 Table 1. Catalyst properties of nickel-magnesia solid solution and supported catalysts. Catalyst BET O2 a) HEb) Ni'~ c) D , Rco~ fl-carbon rate ~ selP /mE/g /btmol/g-cat /% /% /btmol g~s -~ /bt C-mol g~s ~ /% 0.00 Nio.03Mg0.970 22 10.5 3.1 2.9 29.5 31 0.00 0.019 Nioa0Mgo.900 33 130.4 14.9 11.4 11.4 155 0.03 Ni/MgO h) 25 21.8 2.4 58.6 11.0 87 0.02 0.023 Ni/MgO i) 25 226.5 3.9 62.4 1.7 138 0.15 0.109 a: adsorption temperature 873 K, b: adsorption temperature 298 K, c: reduction degree of Ni was estimated by 2x(amount of 0 2 adsorption )/(total amount of Ni), d: dispersion of Ni was estimated by (amount of H 2 adsorption)/(amount of 0 2 adsorption), e: CO formation rate in the reforming of c n 4 with CO 2 under 773 K, 0.1 MPa, CHJCO2=I/1, W/F=0.1 gh/mol, catalyst weight: 0.05 g, f: r-carbon formation rate under the same reaction condition, g: r-carbon selectivity is estimated by ( r-carbon rate)/( r-carbon rate + CO formation rate), h, i: Ni loading of supported catalyst was 0.3 and 3.0 mol%, respecitively.
3 mol% N i / M g ~
Nio.03Mgo.970
9
_./x.__
3
9
s I
270
1
I
470 670 870 Temperature / K
I
1070
I
I
l
1
270 470 670 870 1070 Temperature / K
Figure 2. TPH profiles on the samples after being exposed to cn4-1-CO 2 for (1) 2, (2) 30, and (3) 60 min. Reaction conditions: 773 K, W/F=0.1 gh/mol, CH4/CO2=1/1, 0.1 MPa, 0.05 g. Figure 3 shows FUR spectra of CO adsorption on nickel magnesia catalysts. On Ni0a0Mg0.900 and Ni/MgO catalysts, linear (2100-2000 cma), bridge (2000-1850 cm -~) and physisorbed Ni(CO)4 (2057 cm -~) were mainly observed. In contrast, on Nio.03Mgo.970 nickel monomer and dimer carbonyl species which are interacted with MgO were mainly observed as previously reported[10]. These species were increased with the CO pressure, therefore they are found to be formed via CO induced structural change. On Nio.o3Mgo.970 solid solution, Ni metal particles seem to be highly dispersive. Figure 4 shows TEM image of reduced Ni 0.03Mg0.970. Large cube and small sphere on it were observed. Large cube is nickel-magnesia solid solution, small sphere is a nickel metal particle. The average size of small particle is about 4 nm. This is larger than that estimated by the dispersion as listed in Table 1. In addition, the number of metal particles are much smaller than that of solid solution cube, and that expected by reduction degree as listed in Table 1. This means that a lot of solid solution cubes with no metal particles were observed, and most nickel atoms can not be observed in TEM image. This is because the metal particle size is beyond the detection limit of TEM, or very small metal particle is oxidized during TEM sample preparation. It is strongly suggested that Ni o.o3Mgo.97O has more highly dispersive nickel metal particles than other three catalysts.
378
3.0 mol% Ni/MgO Figure 3. FTIR spectra of CO adsorption on the samples. Pco=13.3 kPa, 298 K.
:!;~
0.3 m o l % N i / M g O
,~'.~
,, ~ _
~: ..,;.
Ni 0.10Mgo.900
Nio.03 Mgo.97 O
100 nm Figure 4. TEM image of reduced Ni 0.03Mg0.97 O . c G ~,4
r
~ ~ ---.
c G --.
Wavenumber/cm-t
4. CONCLUSION
Ni0.03Mgo.970 solid solution catalyst has high resistance to carbon deposition in C O 2 reforming of methane. From the characterization results, this catalyst was found to have highly dispersive nickel metal particles with the interaction with support surface. The inhibition mechanism is suggested to be the activation of adsorbed CO 2 at the interface between metal and support surface and rapid supply of oxygen species to nickel surface. REFERENCES
1. J.R. Rostrup-Nielsen, Catalysis Science and Technology, J. R. Anderson, M. Boudart (eds), Germany, Berlin, Springer, 5 (1984) 3. 2. A.T. Ascchcroft, A. K. Vermon, M. L. H. Green and P. D. E Vermon, Science 352 (1991) 225. 3. J.R. Rostrup-Nielsen and J. H. B. Hansen, J. Catal., 144 (1993) 38. 4. J.T. Richardson and S. A. Paripatyadar, Appl. Catal., 61 (1990) 293. 5. O. Yamazaki, K. Omata, T. Nozaki and K. Fujimoto, Chem. Lett. (1992) 1953. 6. O. Yamazaki, K. Tomishige and K. Fujimoto, Appl. Catal. A:General 136 (1996) 49. 7. K. Tomishige, Y. Chen, K. Yokoyama, Y. Sone and K. Fujimoto, Shokubai, 39 (1997)70. 8. Y. Chen, K. Tomishige and K. Fujimoto, Appl. Catal. A:General, in press. 9. Y. Chen, O. Yamazaki, I~ Tomishige and K. Fujimoto, Catal. Lett., 39 (1996) 91. 10. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1891.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
379
Global c a r b o n - r e c y c l i n g e n e r g y delivery s y s t e m for C 0 2 m i t i g a t i o n (II) Two p o s s i b l e w a y s for i n t r o d u c i n g solar e n e r g y M. Tsuji a, H. Amano ~, Y. Tamaura a, H. Sano b and S. Maezawa c aTokyo Institute of Technology, Research Center for Carbon Recycling and Utilization, Ookayama, Meguro-ku, Tokyo 152, Japan bLaboratory Office of Global Energy System, Makioti, Minoo 562, Japan cResearch Institute of Innovative Technology for the Earth, Toyokaiji-Bldg., Nishi-shinbashi, Minato-ku, Tokyo 105, Japan
Two possible ways of H2 production technologies were studied for starting up a global carbon-recycling energy delivery system (GCRED-system). They are based on conventional electrolysis using electric power produced by solar energy. Solar/electricity conversion methods studied include solar thermal power generation (STP) and PV technology. The installation cost, levelized electricity cost, solar/electricity conversion efficiency and specific CO2 emissions by these solar power stations were compared for feasibility of their practice. These considerations showed that the STP technology is more cost-effective and has approximately the same specific CO2 emissions level as the PV technology.
1. INTRODUCTION In the global carbon-recycling energy delivery system (GCRED-system), solar energy is carried in form of CH3OH [1-4]. Cost-effective H2 production from renewable energies has an essential role for the feasibility of the GCRED-system. In the GCRED-system, the H2 produced by electrolysis of solar electricity will be used for chemical conversion of CO2 into solar methanol transportable for end use from world's sunbelt. It is needed to close the economic gap between the low prices of fossil fuel and the higher prices of the solar methanol. The solar energy is the only major renewable energy that can meet the primary energy requirement on a global scale. There are two possible ways to produce solar electricity by PV and STP. The commercial STP (Solar Electricity Generation System, SEGS plant in California, Table 1, STP a) uses trough-shaped
380 mirror collectors and is currently operated using concentrated solar energy as its primary heat source and fossil back-up fuel in the boiler (solar share 75%). LEC for THESEUS project of SEGS plant in Crete (Greece) (1998-2000 construction term) will be 0.085ECU/kWh [7] (Table 1, STpc). The present paper compares key parameters such as solar/electricity conversion efficiency, solar share, levelized electricity cost and installation cost of these two possible ways.
2. COMPARISON EMISSIONS
OF
ELECTRICITY
COST
AND
SPECIFIC
CO2
Solar/electricity conversion efficiency, capital cost and levelized electricity cost (LEC) are briefly compared in Table 1. The solar/electricity conversion efficiency is an important factor determining land required for both systems, though nonconcentrated sunlight is used in PV power plant, while concentrated sunlight is employed in solar thermal power plant. The conversion efficiency ranges between 14 and 21% for the STP technology, and between 8 and 12 for the PV technology, depending on the plant capacity in MW. The capital cost of STPs is one quarter to a half of that of PV power plant (Table 1). The LEC is defined by the total cost, [(capital cost)x(fixed charge rate) + O&M + fuel cost], per annum net electricity production in kWh. It varies depending on the plant scale and solar share. The LEC ranges between $0.05kWh and $0.12/kWh for the STP, and attains $0.4/kWh-$1/kWh for PV electricity. Thus, the solar H2 production cost by PV and STP electricity is not comparable in terms of capital cost and LEC. However, the cost of STP electricity is still not competitive with that of a conventional coal-fired power plant. To close the price gap, the solar thermal parabolic trough power plant industry has introduced integrated solar combined cycle systems (ISCCS) which are able to offer a competitive base-load electricity cost of $0.05-0.07/kWh at solar shares of 15-25% [7] (Fig.l). Direct solar steam generation (DISS) is being developed as more cost-effective STP system[8]. It does not use oil, but steam as heat transfer medium. The expected benefit is a 30% reduction in the electricity generation cost. It may be able to close the economic gap coming from fossil fuel price and be competitive with a conventional power plant (Table 1 STP e, Fig. 1). Other technology is Solar Concentration Off-Tower optical configuration with Combined Cycle (SCOT/CC) [9] (Table 1 STP d, Fig.l). The SCOT design offers several advantages having better collection optics, stable flux distribution and ground-level plant. As to PV technology, though silicon-cell systems are currently best developed, several other technologies are in various stages of development and it is unclear which of these will prove least expensive under what conditions in the future. Among them, polycrystalline thin films are expected to reduce PV module costs to $0.50-1.00 per peak Watt. In this case, flat-plate thin-film systems may be capable of producing
381 Table 1 Costs of construction and electricity by STP and PV electric power generation System
Capacity (MW)
Solar/elec. convers, eft.
(%)
Solar share
Capital cost* ($/kW)
LEC (S/kWh)
(%)
STP a STC b
80 100
14.0
55
3,011 4,683
0.07-0.12 0.068
STP c STP d STP e
55 34 200
15.9 21.3
55.2 80-90 70-85
2,462 2,460
0.085 0.06-0.11 0.05-0.08
PV f Coal-fired b
0.01 100
8- 12
100 0
10,000 2,440
0.4- 1 0.032
Coal-fired g
700
0
2,467
*Cost is given in US$, unless described. 1US$=u 1ECU=IUS$. Sources: a: Solar Electric Generating System (SEGS VIII, 1989). b: Solar Thermochemical Power Plant with Solar/Gas Turbine Combined Cycle[6]. c: THESEUS plant in Frangokastello, Sfakia, Crete, Greece [7]. d: Solar Concentration OffTower/Combined Cycle (SCOT/CC) [8]. e: Direct Solar Steam Generation in the Solar field (DISS) [9]. f: [5]. g: a conventional coal-fired power plant (Japan, 1988).
electricity for $0.04-0.08/kWh [5] (where detailed evaluation is not given, therefore this is not included in Fig. 1). Thus, in comparison of the present costs (* in Fig.l), the STP system is superior to the PV system. CO2 emissions level is comparable between PV and STP (Fig.2). These findings recommend the solar H2 production using STP electricity for the GCRED-system. Further study on the solar/electricity conversion using STP and STC-systems should be also conducted while being compared with the solar H2 production by PV electricity. The other possible way for producing the solar H2 will be a direct- or two-stepwater splitting by the solar thermochemical process (STC-process). In the STCprocess, the maximum solar/chemical (H2) conversion efficiency of the solar radiation is 70% at 1000sun. This high conversion efficiency in STC-system suggests that the solar H2 production by the STC-process will be one of the promising options for the GCRED-system. Thus, the solar energy conversion s y s t e m s u s i n g c o n c e n t r a t e d solar beam give a h i g h e r efficiency in solar H2 production, which will enable to lower the economic gap from fossil fuel price.
382
* at present # (at the year
PV*---- ""
5.2
given in Figure) o
0.4
~ !,,,,,i r~
3.8
4
o
~ t
SCOT/CC#(2010) Trough _ \ (thermal m e d i a = o i l ) ~ ........ ~
o
rr O1
9
.
9 1,,,,,i
r N
Coal-fired i
,
0
roug DISS (Steam) I
t
[
~
i
t
I
50 Solar Share (%)
100
1
1.1
PV
STP
Oil
Coal
Figure 2. Comparison of specific CO2 emissions by various power sources.
F i g u r e 1. Change in levelized electricity cost (LEC) with increase in solar share
REFERENCES 1. Y. Tamaura and M. Tsuji, Proc. Int. Conf. Energy, Economy and Environment, Osaka, (1996) 59. 2. M. Tsuji and Y. Tamaura, Proc. Int. Conf. Energy, Economy and Environment, Osaka (1996) 65. 3. Y. Tamaura, Kinou-zairyou, 16 (1996) 31. 4. Y. Tamaura and M. Tsuji, Proc. Symp. CO2 Fixation, Chemical Society of Japan, (1996) 6. 5. N. Nakicenovic(ed.), Energy-The International Journal, 18 (1993) 437. 6. J. H. Edwards, K. T. Do and A. M. Maitra, Energy Convers. Mgmt., 37 (1996) 1339. 7. Pilkinton Solar Int'l: Status Report on Solar Thermal Power Plant, Pilkington Solar Int'l, January 1996. 8. A. Kribus, A. Segal, R. Zaibel, D. Carey and S. Kusek, Proc. ECE/WEC/UNESCO/MOST Workshop on the Use of Solar Energy, Bet Berl, Israel, Aug. 1995. 9. E. Zarza, J. I. Ajona, K. Hennecke, Proc. 8th Int. Symp. Solar Thermal Concentrating Technologies, K(iln, Germany, October 6-11 (1996) 397.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
383
Efficient thermochemical cycle for C O 2 reduction with coal using a reactive redox system of ferrite Tatsuya Kodama, Akira Aoki, Satoshi Miura and Yoshie Kitayama Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-21, Japan A two-step thermochemical process using a redox system of metal oxides for converting CO 2 and coal to CO was studied on a number of iron-based oxides (ferrite) for the purpose of utilizing solar high-temperature heat. Reactions are done in a two-step redox cycle in which ferrite is reacted with coal powder at high temperatures to produce COx, H2 (the lst-step reaction) and metals which are reoxidized with CO2 to generate CO at lower temperatures in a separate step (the second-step reaction). In the lst-step reaction, pure magnetite (Fe304) was readily reduced to FeO but the reduction from FeO to o~-Fe scarcely proceeded by a short time reaction (30 min) with coal below 900~ It was found that Ni(II)- or In(III)-substitution for Fe(II) or Fe(III) in magnetite strongly enhances the phase transition from FeO to metallic phase to improve the efficiency for coal conversion. The efficiency of the reaction by the two-step cycle using a reactive ferrite is superior to that by a conventional single-step CO2 gasification of coal. Our findings suggest that highly endothermic CO2 reduction with coal will be conducted using concentrated solar radiation as the energy source for high-temperature process below 900~ 1. I N T R O D U C T I O N Efficient conversion of solar high-temperature heat below 1000~ to fuels or chemicals is a current subject for research[ 1]. The goal is an industrially important endothermic reaction which can be driven by high-temperature heat. On the other hand, the mitigation of CO2 emission has been a major environmental concern. Recently, a two-step thermochemical process using a redox system of metal oxide is proposed for steam gasification of coal utilizing concentrated solar radiation[2]. Reactions are done in a two-step redox cycle in which metal oxide is reacted with coal powder at high temperatures to produce CO, H 2 and reduced metal oxide (or metal) which is reoxidized with H20 to generate H2 at lower temperatures in a separate step. The net reaction is represent by: CHn (coal) + H 20 ~ CO + (n/2+ 1)H2. This two-step process will be applied for a thermochemical reduction of CO2 to CO with coal as follows: metal oxide + CHn ~ metal + xCO + (1-x)CO2 + n/2H2 metal + (2-x)CO2 ~
metal oxide + (2-x)CO
(1) (2)
384 The net reaction is so-called CO2 gasification of coal, CHn + CO2 ~ 2CO + n/2H2. The coal gasification is conventionally performed directly in a single step using steam or CO2 as an oxidant for coal[3]. It is highly endotherlnic and, in the conventional single-step gasification, the process heat is supplied by direct combustion of fossil fuels which release large amounts of
CO2. In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO + CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/c~-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200~ It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe z+ or Fe 3+ in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. In the present work, a number of redox systems of ferrites were studied to find the most reactive working materials. Efficient two-step process using the most reactive In(III)-doped ferrite was demonstrated below 900~ 2. E X P E R I M E N T A L STUDIES
Ultrafine particles of magnetite and metal-bearing ferrites (>50nm) were synthesized by coprecipitation method. The molar ratio M]Fetotal in the starting solution was 0.50 (except for In 3+) to synthesize stoichiometric MFe204. The synthesis of In(III)-bearing ferrite was carried out at an In/Fetotal molar ratio of 0.15 because high level In 3+-substitution for Fe 3+ in magnetite is difficult in this method. Ferrite thus prepared was mixed with Australian bituminous coal pulverized smaller than 3001am[5]. The reactivities for the coal-gasification step [equation (1)] and the CO2-reduction step [equation (2)] were examined for the ferrite. Detailed procedure have been reported on the previous paper[5]. 3. R E S U L T S AND D I S C U S S I O N
To study the variations of reactivities and selectivities of metal oxides with temperature in the coal-gasification step, the mixture of coal and metal oxide was slowly heated from 300 to 900~ at 3.3~ min -1 under N2 gas flow. Figure 1 shows a typical temperature variations of the partial pressures of evolved CO and CO2 for magnetite. Evolution of CH4 was also observed but the partial pressure was negligibly small. The values of Pco
o~" 30
o:g 09
=
)
CO
09
15
oe~
:5 m~
0
~27
300
k..J
500 700 Temperature(~
900
Figure 1. Evolution profiles of CO and CO 2 during the coal-gasfication step using the magnetite
385
and Pco2 increased with increasing temperature and reached a maximum at 900~ For the other ferrites, the maximum peaks for CO and CO2 evolutions were also observed at 900~ except for Ni(II)-ferrite. With Ni(II)-ferrite, the maximum peaks for both CO and CO2 evolution appeared at 800~ The formation rates of COx (CO + CO2) at the maximum level of Pcox are shown in Table 1. For Mg(II)-, Mn(II)- and Zn(II)-ferrites, the formation rate of COx at 900~ were rather lower than that for magnetite. However, this rate was greatly increased with In(III)-ferrite and was 1.6 times as fast as that with magnetite. For Ni(II)-ferrite, nearly the same value of COx formation rate as that for magnetite at 900~ was obtained at a lower temperature of 800~ These results indicate that the Ni(II)-, and In(III)-ferrites are promising materials for the two-step process. The mixture of coal and magnetite, Ni(II)-, or In(III)-ferrite was rapidly heated to 900~ (heating rate = 30~ min -1) and the coal-gasification step was carried out for 30 min at a constant temperature of 900~ Ferrite was mixed with coal (0.2g) at a molar ratio of oxygen in the ferrite to carbon in coal - 1.2. Table 2 shows the conversions and selectivities for products. Only coal reacted directly with CO2 under similar reaction Table 1 Reactivities of metal oxides in the coal-gasification step. Material Magnetite Ni(II)-ferrite M g(II)- ferrite
(~
Products b (mol %) CO CO2
Max. formation rate of COx c (ktmol min -1)
900 800 900
23.7 16.1 13.9
11.5 15.1 4.42
270 225 136
Th a
Mn(II)-ferrite
900
13.3
4.43
125
Zn(II)-ferrite
900
14.1
5.99
148
In(III)-ferrite
900
39.0
8.12
437
CO2
900
19.9
-
-
aTh is the temperature at which the highest level of the formation of CO and CO2 w e r e observed. bThe partial pressures of CO and CO2 when the reactor reached to the temperature of Th. CCOx-production rate when the reactor reached to the temperature of Th. Table 2 Coal conversions to CO,CO2 and CH4 and the selectivities in the coal-gasification step for 30 min at 900~ Material a In(III)-ferrite Ni(II)-ferrite Magnetite CO2
selectivity ( % )
coal conversion to (%)
CO 48.7 16.8
CO 2 18.6 26.7
CH4
9.5
9.3 -
13.1
CO 70.8 36.0
CO2 27.0 56.7
CH4
3.4
total 68.8 46.9
2.6
21.4
44.4
43.3
12.3
0.4
13.5
-
-
1.5
2.2 7.5 -
aMetal oxide was mixed with coal (0.2g) at a molar ratio of oxygen in metal oxide to carbon in coal = 1.2.
386 conditions to carry out the direct single-step C O 2 gasification of coal (coal-CO2 reaction) and the total coal conversion was only 14% (Table 2). A higher conversion of 21% was obtained with the coalmagnetite reaction. With Ni(II)-ferrite, it was improved to 47%. However, the In(III)-ferrite reaction showed a much higher conversion of 69% which was 5 times larger than that by the single-step coal-CO2 reaction. Figure 2 shows the XRD patterns of the solid phases of magnetite, Ni(II)- and In(III)ferrites following the coal-gasification step at 900~ The magnetite showed strong peaks due to wustite (FeO) along with very small peaks of ~-Fe (Fig. 2a). The spinel peaks disappeared completely. This indicates that magnetite was readily reduced to FeO
a) Magnetite tO
~
0 FeO
/."~-Fe o
b)Ni(II)-ferrit~ 9
I
9 Ni-Fe alloy 0 FeO
ooll
i'
o
~.~_'.a::~......._L~I~,,,,~ _~. . . . . .
~.......... _,1
cI2'II'-e'te 1 1__
30
I
40
I
I
50 60 20CuKo t (deg.)
I
70
Figure 2. XRD patterns of magnetite but the reduction from FeO to ~-Fe scarcely and Ni(II),In(III)-ferrite after use for proceeded by a short time reaction (30 rain) with coal the coal-gasification step for 30min. below 900~ On the other hand, in the XRD pattern The O/C molar ratio in the mixture = of the Ni(II)- or In(III)-ferrite, the spinel peaks also 1.2. disappeared but strong peaks of metallic phases appeared. The peaks of FeO were small or little here. Strong peaks of Ni-Fe alloy appeared with small peaks of FeO in the XRD pattern of Ni(II)-ferrite (Figure 2b). For In(III)-ferrite, the peaks of FeO were scarcely observed and the strong peaks of a-Fe appeared with peaks due to metallic In (Figure 2c). The presence of Ni(II) or In(III) in the ferrite phase enhances the phase transition from FeO to the metallic phase. From these results, we concluded that the most reactive iron-based oxide for the proposed two-step process was the In(III)-doped ferrite. After performing the coal-gasification step, the reduced In(III)-ferrite reacted with CO2 at 800~ to carry out the CO2-reduction step. Significant amount of CO was evolved and the evolution was completed within 30 rain. In the XRD pattern of the ferrite after the CO2reduction step, the peaks of the metallic o~-Fe and In disappeared completely, and strong peaks of a spinel appeared with small peaks of In203. The metallic phases formed in the coalgasification step were almost reoxidized to the oxidized phases such as the ferrite. This twostep process was twice repeated in the temperature range of 800-900~ and more than 80% of coal used could be converted to CO in total. REFERENCES 1 .E.A. Fletcher, J. Minnesota Academy of Science, 49 (1983) 30. 2. Y. Tamaura, Y. Wada, T. Yoshida and M. Tsuji, EnergymThe International Journal, 22 (1997) 337. 3.J. Kellar, Fuel Process. Technol., 24 (1990) 247. 4.T. Kodama, Y. Wada, T. Yamamoto, M. Tsuji and Y. Tamaura, J. Mater. Chem., 5 (1995) 1413. 5. T. Kodama, S. Miura, T. Shimizu and Y. Kitayama, Energy--The International Journal, in press.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
387
O x i d a t i v e d e h y d r o g e n a t i o n o f e t h y l b e n z e n e with carbon dioxide over Z S M - 5 supported iron oxide catalysts Jong-San Chang a, Sang-Eon Park a, Woo Young Kim a, Masakazu Anpo b and Hiromi Yamashita b alndustrial Catalysis Research Laboratory, Korea Research Institute of Chemical Technology (KRICT), Taejon 305-606, Korea bDepartment of Applied Chemistry, Osaka Prefecture University, Osaka 593, Japan Carbon dioxide was proposed as an oxidant in dehydrogenation of ethylbenzene over zeolite-supported iron oxide catalyst, which was highly dispersed in zeolite matrix. The dehydrogenation was mainly proceeded under the oxidative pathway in the presence of carbon dioxide. The presence of carbon dioxide contributed to remarkable enhancement not only in dehydrogenation activity of catalyst but also of its coke resistance. 1. INTRODUCTION Great efforts and considerable attention have recently being given to the transformation of carbon dioxide into valuable products through catalytic reactions [ 1,2]. These transformations are mainly based on the reduction of carbon dioxide as a carbon source. However, in other way, carbon dioxide has been also utilized as an oxygen source or oxidant since this could be considered a nontraditional or mild oxidant and oxygen transfer agent [3-5]. As an example of this view, it was reported that oxidizability in the gasification of coke is as follows: O2(105) > H20(3) > CO2(1) > H2(0.003) where the numbers in parenthesis indicated the relative ratios of the gasification with oxidants [6]. This oxidizability suggests that carbon dioxide can play as an oxidant for the oxidative transformation of hydrocarbons. Large amounts of styrene are commercially produced by dehydrogenation of ethylbenzene (EB) in the presence of steam using iron oxide-based catalysts. Carbon dioxide, small amounts of which are formed as a by-product in the ethylbenzene dehydrogenation, was known to depress the catalytic activity of commercial catalyst [7,8]. However, it has been recently reported that several examples show the positive effect of carbon dioxide in this catalytic reaction [5,9,10]. In this study, we investigated the effect of carbon dioxide in dehydrogenation of ethylbenzene over ZSM-5 zeolite-supported iron oxide catalyst.
2. E X P E R I M E N T A L
Zeolite-supported iron oxide catalysts were prepared by precipitation of aqueous suspension of Fe(II) hydroxide in slightly alkaline solution at 333 K under N2 atmosphere.
388 The support used was high siliceous NaZSM-5 zeolite (Uetikon PZ-2/980, Si/A1 = 1900). These newly prepared catalysts were dried in vacuo at 353 K followed by calcination under N2 flow at 673 K for 3 h. Loading of iron oxide, Fe304 on the catalysts was in the range of 1.5 - 20 wt.%, which were designated as FeNaZ catalysts hereinafter. For example, 5FeNaZ denotes 5 wt.% loading of iron oxide supported on NaZSM-5 zeolite. Bulk Fe304 oxide prepared by the above method without zeolite support was compared the catalytic activity with FeNaZ catalyst. Dehydrogenation of ethylbenzene into styrene was carried out in a conventional flow-type reactor made of a quartz tube (10 mm i.d., 300 mm length) at 873 K and atmospheric pressure. Ethylbenzene during the reaction was fed to the reactor as passing carbon dioxide or nitrogen through ethylbenzene saturator kept at 298 K. Several methods such as powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and BET isotherms from N2 adsorption-desorption were selected to characterize iron oxide in zeolite-supported catalysts. 3. RESULTS AND DISCUSSION The XRD patterns of zeolite-supported catalysts containing iron oxide below 10 wt.% loading did not show crystalline phases of iron oxides but the crystalline pentasil-type structure. It was found that XRD peaks related to Fe304 oxide in zeolite-supported catalysts was appeared when loading ratio is more than 20 wt.%. No change in surface area of catalyst exhibited until up to 10 wt.% loading of iron oxide. Characterizing zeolite support by BET full isotherms from adsorption-desorption cycle of N2, it showed hysteresis loop at p/po = 0.2 - 0.4, indicating mesopores in the range of 30 - 40 A. As presented in Figure 1, the Fe 2p spectra of zeolite-supported iron oxide 3 catalysts showed the intrinsic pattem of Fe304 oxide without the significant satellite structure at 719.1 eV which is present in the a ,~. , ~ (a) oxidized surface of bulk Fe304. In addition, the O ls binding energy was observed at 2 530.3 eV to support the presence of iron oxide, while that of zeolite lattice was shown at 532.4 eV. These results explain that iron ~ oxide phase in zeolite-supported catalysts r~ 1 would be Fe3Oa-like and highly dispersed on mesopore and external surface of zeolite support below the level of critical dispersion 719.1 capacity which denoted the border line to start the appearance of crystalline oxide 0 phase. 730 720 710 700 740 Dehydrogenation activity on ethylbenzene Binding Energy (eV) was measured by using catalysts treated with nitrogen at 873 K for 1 h before the reaction. Figure 1. XP spectra of Fe 2p core levels: (a) Among zeolite-supported iron oxide catalysts the highest catalytic activity was obtained in 1.5FeNaZ, (b) 3FeNaZ, (c) 5FeNaZ, and (d) the case of 5.0 wt.% loading of iron oxide Fe304 (oxidized). -
I
,
l
a
I
,
I
389 onto zeolite. As shown in Figure 2, the presence of carbon dioxide over ZSM-5-supported iron oxide (5FeNaZ) catalyst induced the enhancement of the activity as compared with the condition of nitrogen stream. Dehydrogenation of ethylbenzene with carbon dioxide over FeNaZ catalysts produced styrene, water and carbon monoxide along with small amount of hydrogen and stoichiometrically only less than 25 % of carbon monoxide produced. Moreover, even small amount of oxygenated by-product such as benzaldehyde was formed together with benzene and toluene. Formation of water as well as carbon monoxide in this reaction implied that CO2 molecule could be dissociated into CO and surface oxygen which could abstract hydrogen from ethylbenzene forming water. On the other hand, water would be also produced partially via reverse water-gas shift reaction. It was known that the alleviation of the equilibrium limitation in the dehydrogenation of ethylbenzene was achieved by effective removal of produced hydrogen through coupling reaction by the reverse water-gas shift reaction [10]. When the reaction was carried out under an inert nitrogen stream, unsupported Fe304 oxide showed similar dehydrogenation activity to 5FeNaZ catalyst. However, this Fe304 oxide exhibited considerable decrease of catalytic activity in the presence of carbon dioxide [ 11 ]. In the presence of CO2, styrene yield on FeNaZ catalyst was more than 2.5 times comparing with that on bulk Fe304. This result showed that dehydrogenation over zeolite-supported iron oxide catalyst was predominantly enhanced by oxidative pathway and carbon dioxide played a role on the dehydrogenation of ethylbenzene. The activity of 5FeNaZ catalyst under a N2 stream decreased monotonically with reaction time and the reaction was abruptly stopped by plugging the reactor after 5 h due to coke deposition but the catalytic activity under a CO2 stream maintained without significant decay of the activity [ 11 ]. To examine the effect of iron oxidation state on the dehydrogenation the activity of the iron oxide catalyst was compared as in various pretreatment. As presented in Figure 3, the catalyst pretreated with nitrogen showed rather high activity compared to those pretreated with carbon ~.
60
[ 9 EB Conv. 50
|
CO2
.~_
EB Conv. ~ Styrene Yield C02
40
40
~
30
o.,
t...
I-,
~1
~.
~
20
2O
r,.) 1.o
0 i
3
s
Reaction Time (h) Figure 2. Catalytic activity in dehydrogenation of ethylbenzene depending on carrier gas at 873 K. W/F=298 g.h/mol; CO2 (or N2)/EB=80 (mol. ratio).
N2
CO 2
H2
Air
Pretreatment G a s e s Figure 3. Activity of 5FeNaZ catalyst depending on pretreatment condition at 873 K. Reaction conditions: see Figure 2. Pretreatment: heating at 873 K for 10 min (H2, air) or 60 min (N2, CO2), respectively.
390 dioxide, air or 5% H2 in N2 stream. This result indicates that partial reduction during heat treatment under nitrogen flow reaches a certain optimum oxidation state of iron oxide which has oxidation state of +2 and +3 although the optimum population between two oxidation states is not obvious now. Since the reaction occurs in the presence of hydrogen, ethylbenzene and carbon dioxide it is possible for the iron oxide to undergo both partial oxidation by carbon dioxide and partial reduction by hydrogen and ethylbenzene. Thus this population would be controlled properly in gaseous mixture of ethylbenzene, hydrogen and excess carbon dioxide during the reaction. Oxygen deficiency of iron oxide in FeNaZ catalysts also had influenced in the catalytic activity [11]. The highly dispersed iron oxides in zeolite matrix had much more isolated oxygen-deficient sites comparing with that on unsupported Fe304. It is speculated that oxygen deficient sites of zeolite-supported iron oxide stimulate the CO2 dissociation into CO. The dissociated oxygen on the catalyst surface would abstract hydrogen of ethylbenzene or convert hydrogen molecule produced into water. Dispersion of the oxygen deficient sites or oxygen defects seemed to be more efficient for the CO2 dissociation and thereby the oxidative dehydrogenation of ethylbenzene. Therefore, it can be concluded that carbon dioxide was utilized as an oxidant on dehydrogenation of ethylbenzene to improve its activity and that an active phase of iron oxide in the catalyst is considered to be rather reduced iron oxide, like Fe304, dispersed in zeolite matrix. ACKNOWLEDGMENTS We acknowledge the financial support by Korea Electric Power Research Institute (KEPRI), Ministry of Environment (MOE), and Ministry of Science and Technology (MOST) in Korea. REFERENCES
1. M.M. Halmann (ed.), Chemical Fixation of Carbon Dioxide, CRC, FL, Boca Raton, 1993. 2. M. Aresta, C. Fragale, E. Quaranta and I. Tommasi, J. Chem. Soc., Chem. Commun., 315 (1992). 3. O.V. Krylov and A.Kh. Mamedov, Ind. Eng. Chem. Res., 34 (1995) 474. 4. J.S. Yoo, P.S. Lin and S.D. Elfline, Appl. Catal., 106 (1993) 259 5. S.-E. Park, J.-S. Chang and M.S. Park, Prepr. Am. Chem. Soc., Div. Fuel Chem., 41(4) (1996) 1387. 6. C.H. Bartholomew, Chem. Eng., Nov. 12, (1984) 96. 7. T. Hirano, Appl. Catal., 26 (1986) 65. 8. J. Matsui, T. Sodesawa and F. Nozaki, Appl. Catal., 67 (1991) 179. 9. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and M. Suzuki, Appl. Catal., 121 (1995) 125. 10. S. Sato, M. Ohara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 11. J.-S. Chang, S.-E. Park and M.S. Park, Chem. Lett., accepted (1997).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
391
Nature of COz adsorbed on MgO surface at low temperatures T. Ito, a J. Isawa, a H. Kishimoto, a H. Kobayashi, b and K. Toi c aDepartment of Social Information Studies, Otsuma Women's University, Karakida, Tama, Tokyo 206, Japan bDepartment of Chemical Technology, Kurashiki University of Science and the Arts, Tsurajima, Kurashiki, Okayama 712, Japan CDepartment of Chemistry, Tokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-03, Japan
Among five types of species formed by CO2 adsorption on MgO, dynamic behavior of unidentate-type carbonate (species-l) upon desorption was investigated. Surface diffusion, O-exchange, and molecular substitution appreciably take place prior its desorption even below the room temperature. Since these dynamic processes necessitate bond rupture in species, MgO may be useful in CO2 conversion at low temperatures.
1. INTRODUCTION Magnesium oxide may be expected to function as an active catalyst for CO2 utilization since CO2 is easily chemisorbed on MgO [1-3]. Most works so far reported on this adsorption system were carried out above the room temperature, and recent works have demonstrated the occurrence of oxygen isotope exchanges between adsorbed species and surface 02. [4-6]. However, dynamic behaviors of adsorbed species, especially below the room temperature, are still open to question.
2. EXPERIMENTAL
An MgO sample used was a smoke-type with a specific surface area of 10-15 m 2 g-1 and was pretreated in vacuo at 1123 K before each run. CO2 gas below 133 Pa was introduced onto MgO surface at and below the room temperature. The isotopic concentrations of labeled gases are 88.3-180 and 99-13C atom % for C1802 and 13C02, respectively. Other details have been described elsewhere [6].
392 3. RESULTS AND DISCUSSION
3.1. Types of adsorbed species TPD curves measured after the adsorption of a small amount of CO2 at a low temperature indicate the presence of five types of chemisorbed species, species-0 to -4. Their desorption temperature and structure which has been determined from observed IR bands, theoretical models [6], and isotopic studies are summarized in Table 1. In species-0, CO2 is adsorbed as a base molecule but all the other species are basically carbonate-type, CO32. Species-1 has a unidentate structure, while species-3 and -4 have a bidentate structure which are known as major species in CO2 adsorption and stable above the room temperature. The difference between species-3 (type b) and -4 (type a) seems to come from the coordination number of the adsorption sites. Their adsorption models are illustrated in Figure 1. Dynamic behaviors of species-1 which is stable only below room temperature are mainly described hereafter. 3.2. TPD observations for species-1 Figure 2(A) shows TPD curves observed after the C1802adsorption at 110 K, where species-1 is desorbed with the nearly same isotopic composition as the source gas and the difference in desorption temperature among isotopic species is within 10 K as shown in Table 2. However, species-1 formed by the C1802 adsorption on the surface carrying species-3 and -4, which was previously formed by the C~602 adsorption at the room temperature, is desorbed in a complex manner [Two-step adsorption, Figure 2(B)]. For example, the isotopic composition of species-1 is considerably diluted by 160, and C1802 species which Table 1 Adsorbed species of CO z on MgO Species Structure Species-0 Linear CO 2 on Mgcc 2+ Species-1 Unidentate CO32Species-2 (Unknown) Species-3 Bidentate CO32-(type b) Species-4 Bidentate CO~2- (type a)
0
\ /
0
oN
C
---OLc
Desorption temperature / K 190 230 300 390 ca. 800
2+
MgLc
Unidentate C032 (Species-I) Figure 1. Adsorbed models.
/
C ~0
---0~
I 2+
MgLc
Bidentate C032 (Species-3,4)
IR band / cm -1 2355 1274, 1711 951, 1274, 1626 1022, 1326, 1660
393
!
0.8
!
|
(B)
(A)
3
_
1
~-6 t--
_C1802,,,,~~
0.6
3 L._
~4 ~t - 9 0.4
Or) fO
O c N
--=2
0.2
ii .
o 100
200
300 Temperature / K
400
500
lOO
200
300
400
500
Temperature / K
Figure 2. TPD curves observed after adsorption of 1.3-Pa C1802 at 110 K on MgO carrying (A) no preadsorbed species and (B) preadsorbed species-3 and -4 (not shown in this figure) consisting of C1602 (Two-step adsorption). Table 2 Desorption temperatures of species-1 observed for TPD in two-step adsorption a Adsorption pressure in the Desorption temperature / K first step b /Pa C16Oz C160180 C18Oz 0 (No first step) 220 225 230 1.3 223 237 250 6.6 227 246 257 27 228 246 266 aIn the second step 1.3-Pa C1802gas was admitted at 110 K. bc1602 gas was admitted under indicated pressure at the room temperature. was directly formed in the second step is desorbed at a temperature close to that observed in the single step adsorption of the C1802 gas while the desorption of C160180 and C1602 species is shifted to much higher temperatures in this order. The degrees of this dilution by 160 and the shift in desorption temperature are enhanced with increase in the amount of previously adsorbed species-3 and -4 (cf. Table 2). These facts indicate that a part of species-1 can diffuse on the surface, through which O isotope is exchanged with surface O 2, and then tends to be desorbed at a higher temperature in TPD process. The longer the diffusion distance is, the larger both the content of 160 included in species-1 and the shift in the desorption temperature become. A similar two-step adsorption using the 13CO2 gas in the second step also gives complex TPD curves. Especially the appearance of species-1 consisting of 12CO2, which is also desorbed at a higher temperature than ]3CO2, strongly suggests that molecular substitution between adsorbed species occurs in the TPD process since no species-1 can be present in the first step.
394
3.3. IR observations for species-1 IR spectra observed after two-step adsorption using C1802in the second step at 180 K show that isotopic composition of species-1 formed is nearly same as that of the original gas. This fact corroborates that, during the adsorption at low temperatures, first, no CO2 in the gas phase undergoes O-isotope exchange with surface 02. or previously adsorbed speices-3 and -4, and second, no molecular substitution occurs between gaseous CO2 and preadsorbed species. The IR bands due to species-1 formed in the single step adsorption at 180 K completely diminishes with increasing temperatures up to 270 K while the bands due to species-3 and -4 are much increased in intensity with this temperature increase. This indicates that a part of species-1 is readsorbed as these more stable species through surface diffusion upon heating. 3.4. Dynamic behaviors of species-1 The TPD and IR observations mentioned above indicate that many dynamic processes for species-1 can occur even below the room temperature. The fate of this species upon heating can be classified into the following four cases. First, species-1 is directly desorbed from the adsorbed site, which ideally leads to no isotope exchange and no temperature shift in desorption. The other three cases are all accompanied by surface diffusion of species-1 during which an O-exchange takes place between the diffusing molecule and surface 02. . Second, the diffusing molecule is desorbed as species-1 without changing the adsorption state after diffusion, which causes a temperature shift in desorption. Third, the diffusing molecule undergoes molecular substitution with a more stable species (e.g., species-3), and consequently the diffusing molecule (species-l) remains as a more stable species and, instead, the substituted molecule is desorbed as species-1 with a temperature shift. Fourth, the diffusing molecule is readsorbed as a more stable species on a vacant active site, resulting in no desorption of species- 1. Each molecule adsorbed as species-1 is diminished through any of the four cases mentioned above during heating. These dynamic processes occur through the bond rupture of species, and hence MgO catalysts seem to be able to convert CO2 into other useful substances at low temperatures when a second component is admitted. REFERENCES 1. J. V. Evans and T. L. Whateley, Trans. Faraday Soc., 63 (1967) 2769. 2. Y. Fukuda and K. Tanabe, Bull. Chem. Soc. Jpn., 46 (1973) 1616. 3. R. Philipp and K. Fujimoto, J. Phys. Chem., 96 (1992) 903 5. 4. Y. Yanagisawa, Y. Shimotama, and A. Ito, J. Chem. Soc., Chem. Commun., 1993 (1993)610. 5. H. Tsuji, T. Shishido, A. Okamura, Y. Gao, H. Hattori, and H. Kita, J. Chem. Soc., Faraday Trans., 90 (1994) 803. 6. Y. Yanagisawa, K. Takaoka, S. Yamabe, and T. Ito, J. Phys. Chem., 99 (1995) 3704.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
395
C02 b e h a v i o r on s u p p o r t e d K N i C a catalyst in the carbon dioxide r e f o r m i n g of methane Sang-Eon Park a, Jong-San Chang a, Hyun-Seog Roh a, Masakazu Anpo b and Hiromi Yamashita b aIndustrial Catalysis Research Laboratory, Korea Research Institute of Chemical Technology (KRICT), Yaejon 305-606, Korea bDeparment of Applied Chemistry, Osaka Prefecture University, Osaka 593, Japan
CO2 behavior in the carbon dioxide reforming of methane over supported KNiCa catalyst was investigated by using 'in-situ' FT-IR, CO2-TPD and pulse reaction method. The role of alkali metals on the catalyst was to improve catalyst stability through a high surface coverage of adsorbed CO2 and formation of surface carbonates. It was demonstrated that the oxidation step of surface carbon with adsorbed CO2 or surface oxygen on the catalyst contributes to high catalyst stability by the effective removal of surface carbon species. 1. INTRODUCTION There has been an increasing interest in the catalytic transformation of carbon dioxide, a greenhouse gas, into more valuable compounds [1]. The carbon dioxide reforming of methane (so-called "dry reforming") to produce CO-rich synthesis gas (H2 and CO) is considered to be one of the promising ways of CO2 utilization. Previously, we suggested that zeolite-supported KNiCa catalyst (KNiCa/ZSI) exhibited high activity and coke resistance for the CO2 reforming of methane into synthesis gas [2]. To understand detailed chemistry in the dry reforming, it is essential to elucidate the behavior of CO2 in the reaction and on the catalyst surface. The aim of the present study is to examine the adsorption behavior of CO2 on our designed catalyst and the role of CO2 in the reaction. 2. E X P E R I M E N T A L
Zeolite-supported Ni and KNiCa catalysts were prepared by molten-salt method [2], which were designated as Ni/ZSI and KNiCa/ZSI, respectively, hereinafter. The support was a high siliceous ZSM-5 zeolite (UOP S-115) mixed with an alumina. Experiments for pulse reaction on reduced KNiCa/ZSI catalyst were conducted with a gas sampling valve in a pulse microreactor, which was incorporated between the sample inlet and the column of gas chromatograph. FT-IR spectroscopic studies were performed in a quartz vacuum cell using self-supported wafers which were treated under vacuum or underwent the
396 same treatment as catalyst testing. The chemisorbed amounts of C02 on reduced catalysts were measured by a static vacuum volumetric system (Micromeritics' model ASAP 2000C) with stainless steel tubing greaseless valves. Temperature-programmed desorption (TPD) of CO2 on reduced catalysts was performed by IGA (Intelligent gravimetric analyzer, Hiden IGA-002). 3. RESULTS AND DISCUSSION In the previous study, KNiCa/ZSI catalyst exhibited high activity and high resistance to coke in the CO2 reforming of methane at 700 ~ [2]. Its high activity showed near equilibrium conversions of CO2 and CH4 as well as near equilibrium yields on CO and H2, which were unchanged during over 140 h. On the other hand, Ni/ZSI catalyst was also highly active, but this showed severe coke deposition within several hours. Upon the in-situ FT-IR analysis of KNiCa/ZSI catalyst, no C-H bands were detected in the region of 2700 - 3100 cm l corresponding to CHx adsorbed species after the reaction and CH4 introduction at 700~ into in-situ cell. The adsorbed species formed upon CH4 adsorption was postulated mainly as a type of Ni-C. Figure 1 shows FT-IR spectra of reduced Ni/ZSI and KNiCa/ZSI catalysts before and after CO2 adsorption, and under the CO2-reforming conditions. The behavior of adsorbed CO2 on KNiCa/ZSI was different from that on Ni/ZSI. FT-IR spectrum of KNiCa/ZSI even after reduction at 700 ~ showed two evident bands at 1480 and 1410 cm 1 which did not appear that on Ni/ZSI catalyst. These bands could be assigned to mainly asymmetric (Vas(OCO)) and symmetric stretching vibration (vs(OCO)) modes of monodentate carbonate species coordinated to Ca oxide or K oxide of the catalyst, respectively [3]. The intensities of these bands on KNiCa/ZSI increased by the C02 .12 adsorption at 25 ~ or upon the CO2-reforming (d) reaction, but on Ni/ZSI no changes were detected by above treatment. This formation of carbonate species formed on the basic catalyst -,2~ surface by the CO2 adsorption in air or under vacuum would lead to the high catalyst stability of KNiCa/ZSI due to reducing the formation of coke on surface. In other words, the enrichment -.52of surface oxygen upon CO2 adsorption as carbonate species could play a role on the elimination of coke by converting it into CO, which gave high CO/H2 ratio as near 1. -.84 ........... i .....1 The metallic Ni and CaO sites adjacent to 1800 1600 1400 1200 reduced nickel metal sites on KNiCa/ZSI W a v e n u m b e r (cm-1) catalyst were considered as major sites of CO2 Figure 1. FT-IR spectra of (a) Ni/ZSI after chemisorption. The chemisorbed amounts of reaction at 600~ KNiCa/ZSI after (b) CO2 on KNiCa/ZSI catalyst were increased to a reduction at 700~ (c) 20 Torr CO2 adsorption value 40% higher than that of Ni/ZSI catalyst. at RT followed by evacuation at 400~ and (d) As shown in the CO2 desorption profiles in the reformingreactionat600~ Figure 2, the integrated amounts of CO2
i
397 desorption on KNiCaJZSI catalyst were higher than those on Ni/ZSI catalyst which corresponded to the result of volumetric 5 CO2 adsorption. In addition, significant difference in the CO2-TPD profiles 4 o between Ni/ZSI and KNiCa/ZSI catalysts ~ ~ 6 ~~ was observed in the high temperature region. CO2-TPD profile of KNiCa/ZSI 2 exhibited a strong desorption peak above 600~ which was not observed on Ni/ZSI. The desorption peak at higher temperature indicated the formation of surface 0 , i , i , i , i , i , i , i carbonate, mainly on Ca promoter due to 0 100 200 300 400 500 600 700 its great amounts of composition, and the Desorption Temp. (~ presence of this peak seemed to be directly related with excellent stability of Figure 2. CO2-TPD profiles of reduced (a) Ni/ZSI KNiCa/ZSI catalyst compared to Ni/ZSI. and (b) KNiCa/ZSI catalysts (13 = 10 ~ Results of pulse reactions showed that the dissociation of reactant molecules and the formation of CO on KNiCa/ZSI catalyst might greatly affect the state of catalyst surface. Figure 3 displays the amounts of CO produced as a function of ordinal number of CO2 and CH4 pulses dosed at 600~ over reduced KNiCa/ZSI catalyst. At the first several CO2 pulses the formation of CO was great but decreased fastly and then reached to steady state with an increase of CO2 pulse number, showing steady production of CO after 7th pulse. This catalyst exposed to several CO2 pulses was subjected to CH4 pulse. Then, CO and H20 were generated together with H2. The amounts of CO formed were 4.3 pmol at the 1st CH4 pulse although they were getting decreased later. This means that surface oxygen species on the catalyst were formed and remained on catalyst surface during CO2 pulses. These species played a role as oxidant for methane oxidation. The formation of H2 from CH4 pulse after the reaction with several CO2 pulses was lower than that on the freshly reduced catalyst. This is ascribed to the loss of hydrogen by its oxidation into water with the formed 16 oxygen species from CO). When o the catalyst was exposed again E 4 o CO2 pulses 12 E 2 pulses:H 4 puL, es ~ O ::I. to the CO2 pulse after sequential 0 3 9 CO2 pulses and CH4 pulses, the 8 amounts of CO increased with 4 ~= 2 = times higher than those of the freshly reduced catalyst at initial 4 <~ 1 stage. The amounts of CO formed after 4th pulse reached 0 0 4 8 12 16 20 24 to steady-state which were similar to those of the freshly No. of Pulses reduced catalyst, suggesting that Figure 3. Pulse test on the dissociation of CO2 and the surface oxygen present on the oxidation of CH4 over reduced KNiCa/ZSI catalyst at 600~ catalyst could be consumed by 12
,
,
,
,
,
V - l r -
r-lr-s
.
,
,
398 methane oxidation. From the results of both pulse reaction and adsorption experiments, it could be confirmed that Ni has a strong affinity with methane, while alkali promoters with carbon dioxide. The retardation of coke deposition on KNiCa/ZSI catalyst must be ascribed to the abundantly adsorbed CO2 species. This explanation is similar to the suggestion of Horiuchi et al. [5], showing that the surface of the Ni catalyst with basic metal oxides was labile to CO2 adsorption, while the surface without them was labile to CH4 adsorption. Since coke deposition was mainly caused by methane decomposition, the catalyst surface covered with adsorbed CO2 or reactive oxygen species from the dissociation of CO2 would suppress coke deposition. The addition of alkaline promoters also seemed to greatly suppress the activity of supported Ni catalyst for the direct decomposition of methane. In this study it was found that dissociation of CO2 and CH4 is an elementary step in the CO2 reforming of methane and that an active site for the dissociation of CO2 and CH4 (eqns. (1) and (2)) is metallic Ni on the KNiCa catalyst. Ni surface of KNiCa/ZSI catalyst was mostly occupied by adsorbed C and O species as intermediates during the reaction. Surface reaction of these species produced carbon monoxide and simultaneously rejuvenated nickel species (eqn. (5)), which was considered to be rate-determining step under the following reaction scheme. CO2(g) + 2Ni(s) CH4(g) + 5Ni(s) Ni-CO(s) 2Ni-H(s) Ni-C(s) + Ni-O(s)
-~ -~ -~ -~ -~
Ni-CO(s) + Ni-O(s) Ni-C (s) + 4Ni-H(s) Ni(s) + CO(g) 2Ni(s) + Hz(g) 2Ni(s) + CO(g)
(1) (2) (3) (4) (5)
Therefore, it can be concluded that carbonates species formed on mainly Ca promoter of KNiCa/ZSI catalyst contributed to high catalyst stability due to enriching surface oxygen from CO2 by providing great ability to the CO2 adsorption. It was also confirmed that the oxidation step of surface carbon with gaseous CO2 or surface carbonates over KNiCa/ZSI catalyst was essential to effectively eliminate surface carbon species. ACKNOWLEDGMENTS We acknowledge the financial support by Ministry of Environment (MOE), and Ministry of Science and Technology (MOST) in Korea. REFERENCES
1. M.H. Halman (ed.), Chemical Fixation of Carbon Dioxide, CRC, CL, Boca Raton, 1993. 2. J.-S. Chang, S.-E. Park and H. Chon, Appl. Catal., 144 (1996) 111. 3. T. Yamazaki, M. Katoh, S. Ozawa and Y. Ogino, Mol. Phys., 80 (1993) 313. 4. J.C. Lavalley, Catal. Today, 27 (1996) 377. 5. T. Horiuchi, K. Sakuma, T. Fukui, Y. Kubo, T. Osaki and T. Mori, Appl. Catal., 144 (1996) 111.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
399
Utilization of CO2 in the reforming of natural gas on carbon supported nathenium catalysts, hlfluence of M g O addition. P. Ferreira-Aparicio a, B. Bachiller-Baeza a, A. Guerrero-Ruiz b and I. Rodriguez-Ramos a. Instituto de Cat~,lisis y Petroleoquimica. CSIC. Campus de Cantoblanco. 28049 Madrid. Spain.
a
b Dpto. de Quimica Inorganica y T4cnica. Facultad de Ciencias. UNED. 28040 Madrid. Spain.
The addition of MgO to activated carbon-supported ruthenium catalysts in an optimal Ru:Mg ratio results in efficient catalyts for the CO2 reforming of methane, with stable selectivities towards CO and H2 production.
1. INTRODUCTION The use of carbon dioxide in catalytic reactions with other compounds is a very attractive alternative not only for the reduction of its emissions to the atmosphere but also to produce valuable chemical products. Along this line, the reaction of CO2 with methane (major component of the natural gas) to obtain syn-gas (CO+H2) has been largely studied [1-3]. Methane reforming reactions, either dry reforming with CO2 or steam reforming, are strongly endothermic. The endothermicity of the CO2 reaction with methane may be interesting for energetic aspects concerning applications such as solar energy storage [4]. In addition to those of conventional reforming catalysts, catalytically active absorber materials have to fulfill functions as solar absorber and as heat transferer. The use of carbonaceous materials as support of this type of catalysts may be a good option. On the other hand, it is known that catalyst support exerts a great influence on the catalytic properties of the metallic particles deposited on it during the carbon dioxide reforming of methane. So for a given metal, catalytic activities can be changed [5], product selectivities modified [6] and carbon deposition resistivity altered [7]. Also the addition of certain promoters can improve the catalytic behavior of a given supported metal catalyst. In particular we have shown the benefit of the MgO addition to cobalt catalyst [ 8, 9]. In this work we have studied ruthenium based catalysts. It is known that ruthenium is among the most active and stable catalysts for the reforming of CI-I4 with CO2 [2]. As catalystsupport we have used a high surface area activated carbon (AC). In order to improve the catalytic properties of the Ru/AC system, different loadings of magnesium oxide have
400 been added in the formulation of the catalysts. Apart from the catalyst testing in the CH4+CO2 reaction, complementary characterization of samples was accomplished by chemical analysis, selective hydrogen chemisorption and temperature programmed reduction experiments.
2. EXPERIMENTAL The catalysts were prepared by consecutive impregnation with aqueous solutions of Ru(NO)(NO3)3 and Mg(NO3)2. The support was an activated carbon (commercial one provided by ICASA, Spain, SBET= 960.7 m2-g1) purified by treatment with HC1 solution, to remove inorganic compounds. For comparative purposes, a ruthenium catalyst supported on a "y-A1203 (Puralox condea, S B E T -- 191.9 m2-g-1) was also prepared by similar procedure. The impregnants were dried at 383 K and subsequently reduced. Before reaction and chemisorption measurements, samples were "in situ" reduced at 673 K for 2 h. Activity, selectivity and stability under reaction conditions were measured at atmospheric pressure in a fixed-bed quartz reactor kept at 823 K by cofeeding CH4, CO2 and He as diluent. An equimolecular mixture of CH4 and CO2 (10% CH4, 10% CO2 and balance He) was adjusted by mass flow controllers (Brooks) and passed through the catalyst at a flow rate of 100 cm3-min-I (space velocity = 1.2.104 h-l). The effluents of the reactor were analysed by an on-line gas chromatograph with a thermal conductivity detector. The prepared catalysts and the chemical compositions measured by atomic absorption, are listed in Table 1. Complementary characterization experiments such as hydrogen chemisorption in a pulse apparatus and temperature-programmed reduction (TPR) were performed using experimental systems and methods described in detail elsewhere [ 10].
3. RESULTS AND DISCUSSION The data in Table 1 show that ruthenium dispersion is higher for Ru/A1203 catalyst than for the Ru/AC one. The addition of small amounts of MgO to carbon supported catalysts improves the dispersion of the metal, very probably because the MgO is avoiding sinterization processes. However, addition of higher amounts of MgO causes a diminution of the hydrogen uptakes. This fact indicates that a part of the metallic surface could be covered by MgO hindering the hydrogen chemisorption. Some interesting conclusions can be drawn from the TPR experiments (Figure 1). First, the reduction feature of Ru/A1203 catalyst differs from that observed for Ru/AC. Ruthenium precursors supported on carbon are reduced at lower temperatures. This fact is indicative of different metal-support interactions. Furthermore, in all the AC supported catalysts a second HE consumption peak appears at temperatures close to 673 K. This peak is accompanied by the production of CH4, which can be originated by the partial gasification of the carbon species of the support near the metal particle [ 10]. Also, this peak near 673 K could indicate the presence of some Ru + species stabilized by interaction with the carbonaceous support, which would become reduced at this temperature. Moreover, the addition of MgO to the Ru/C catalyst shifts the reduction of ruthenium to higher temperatures. Thus, we can deduce that in the Ru-Mg/AC catalysts the ruthenium particles are in close interaction with the MgO.
401 Tablel Some characteristics of the catalysts Catalyst* Chemical Hydrogen Adsorption Analysis Atomic Ratio Ru:Mg Dispersion Uptake (%) (ktmol'g1) Ru/A1203 1:0 12 14.1
Catalytic activity (lamol'g-l's-1) Consumption Consumption of CH4 of C O 2 178.7 215.1
Ru/AC
1:0
6
6.0
37.8
65.5
Ru-Mg2/AC
1:2
11
11.1
54.8
85.0
Ru-Mgl0/AC
1:10
9
8.7
30.5
66.4
Ru-Mg20/AC
1:20
7
7.1
34.4
66.5
* Ruthenium loading in all samples is close to 2 wt%. Table 1 gives the values of catalytic activities for conversion of CH4 and C O 2 after 10 h in the CH4+CO 2 reaction at 823 K. Considering the values of metallic surface determined by hydrogen chemisorption (Table 1), it appears that the catalytic activities per ruthenium surface atom are at least twice for Ru/A1203 compared with AC supported catalysts. This fact indicates that the support can modify the catalytic behavior of the metallic particles supported on it. The effects of support (A1203 vs AC) and promoter (MgO) can also affect the catalytic selectivities. Figure 2 displays, for the different ruthenium supported catalysts, the selectivities towards CO and H2 production as a function of reaction time. While Ru/A1203 exhibits quite stable and high selectivities towards CO and H2, over Ru/AC catalyst a decrease in the catalytic selectivities during reaction can be observed, the decline of the H 2 production selectivity being very pronounced. These differences in catalytic behavior can be explained by the participation of secondary reactions, such as the reverse of water gas shift (CO2+H2r H20+CO), whose occurrence depends on the nature of the catalytic support and on the species and intermediates adsorbed on the support surface. These surface species can be modified by addition of promoters such as MgO [8, 9]. In the case of the Ru/AC catalysts, an optimal M g ~ u ratio of 1:10 produces a significant improvement of the CO and H 2 selectivities. In conclusion, the adequate molecular design of the Ru/AC samples by addition of a convenient promoter such as MgO results in efficient catalysts for the CO2 reforming of methane, with stable selectivities towards CO and H2. These catalysts could be a good choice to be employed as solar energy absorbers.
REFERENCES
1. I.R. Rostrup-Nielsen and J.H. Bak Hansen, J. Catal., 144 (1993) 38. 2. S.C. Tsang, J.B. Claridge and M.L.H. Green, Catal.Today, 23 (1995) 3.
402 3. J.R.H. Ross, A.N.J. van Keulen, M.E.S. Hegarty and K. Seshan, Catal. Today, 30 (1996) 193. 4. A. W6rner and R. Tamme, in 5th European Workshop on Methane Activation, University of Limerik, Ireland, June 1997. 5. A. Erd6helyi, J. Cser6nyi and F. Solymosi, J. Catal., 141 (1993) 287 6. P. Ferreira-Aparicio, I. Rodriguez-Ramos, C. Marquez-Alvarez, C. Mirodatos and A. Guerrero-Ruiz, submitted for publication. 7. Z.L. Zhang, V.A. Tsipouriari, A.M. Efstathiou and X.E. Verykios, J.Catal., 158 (1996) 51. 8. A. Guerrero-Ruiz, I. Rodriguez-Ramos and A. Sepulveda-Escribano, J.C.S. Chem. Commun (1993) 487. 9. A. Guerrero-Ruiz, B. Bachiller-Baeza, P. Ferreira-Aparicio and I. Rodriguez-Ramos, J. Catal, in press. 10. A. Guerrero-Ruiz, A. Sepulveda-Escribano and I. Rodriguez-Ramos, Appl. Catal A:Gen., 120 (1994) 71.
Ru/AC
Ru/A1203
4
.
Ru-Mgl0/AC I
4-
,..., r~
"-62 E
::3..
,
|
--
!
473 673 T(K)
273
,
_ _
873
273
673 473 T(K)
0 873 273
473 673 T(K)
873
Figure 1. Profiles of temperature programmed reduction for the different catalysts. ( ~ ) consumption of H2, ( + ) production of CH4. 100
~100
75
75
or,.) o
~ ~
r~
50 6
. 460 860 Time (min)
1200
50
6
460 860 Time (min)
1200
Figure 2. Evolution of selectivities to CO and H 2 as function of the time in C O + C H 4 reaction at 823 K. D Ru/A1203, O Ru/AC, A Ru - Mg2/AC, V Ru - Mg 10/AC, 0 Ru - Mg20/AC.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
403
Catalytic conversion of carbon dioxide to p o l y m e r blends via cyclic carbonates D. W. Park a, J. Y. Moon a, J. G. Yang a, S. M. Jung a, J. K. Lee b and C. S. Ha b aDept, of Chemical Engineering, College of Engineering, Pusan National University, Kumjung-Ku, Pusan, 609-735, Korea bDept, of Polymer Science and Engineering, College of Engineering, Pusan National University, Kumjung-Ku, Pusan, 609-735, Korea
In the present study, synthesis of (2-oxo-1,3-dioxolan-4-yl)methyl vinyl ether(OVE) from carbon dioxide and glycidyl vinyl ether(GVE) was investigated using quaternary ammonium salts as catalyst. Among the salts tested, the one having larger alkyl group and more nucleophilic counter anion exhibited a better catalytic activity. Polymers bearing pendant 5membered cyclic carbonate groups were prepared by radical copolymerization of OVE with acrylonitrile. The monomer reactivity ratio could be estimated as r1(OVE)=0.36 and rz(AN)=I.21 by Finemann-Ross method. The miscibility of blends containing poly(OVE-coAN) and poly(styrene-co-acrylonitrile)(SAN) has been studied by the optical clarity and DSC. All of the OVE-AN/SAN blends studied were miscible.
1. INTRODUCTION Chemical fixation of carbon dioxide is a very attractive subject in view of resource utilization and environmental problems. The application of carbon dioxide as a monomer for the synthesis of various polymers has received much attention in recent years. The reactions of carbon dioxide with oxiranes leading to 5-membered cyclic carbonates are well-known among many examples. The synthesis of cyclic carbonates from the reaction of CO2 with oxirane has been performed using Lewis acids, transition-metal complexes, organometallic compounds as catalysts under high pressure. The successful synthesis of a new functional monomer (2-oxo-l,3-dioxolan-4-yl)methyl vinyl ether(OVE), containing both reactive vinyl ether and cyclic carbonate groups, have been described by the reaction of glycidyl vinyl ether(GVE) with carbon dioxide in recent year[ 1]. In the present study, we have investigated the synthesis of OVE from CO2 and GVE in view of the characteristics of phase transfer catalysts and reaction mechanism as well as its application with acylonitrile(AN) to the synthesis of corresponding copolymer. We have tested monomer reactivity ratio and thermal properties of the copolymer. The blends of poly(OVE-co-AN) and SAN were prepared, and the degree of miscibility was examined by *This work was supported by the Korea Research Foundation through the Research Institute of Industrial Technology of Pusan National University.
404 visual observation and differential scanning calorimetry. 2. EXPERIMENTAL 2.1. Materials Glycidyl vinyl ether(GVE), acylonitrile and reaction solvents were used after distillation on Call2. Quaternary ammonium salts were all in reagent grade and used as supplied without purification. 2.2. Experimental The synthesis of OVE was carried out under a slow stream of carbon dioxide. GVE(30mmol) and 1 mmol of catalyst were dissloved in 50 mL of solvent, and the solution was heated to a desired temperature. Reaction was allowed to start by stirring the solution. Periodically, a small portion of reaction mixture was taken out and analyzed by gas chromatography(HP5850A). Radical copolymerization of OVE(5 mmol) with acrylonitrile (AN, 5 mmol) was performed using AIBN(5mg) as an initiator at 60 ~ for 10 hr under N2, and then the solution was poured into diethyl ether to give a precipitate. The obtained polymer was reprecipitated twice, and dried in v a c u o at 60 ~ for 12 hr. To prepare polymer blends, weighed amounts of poly(OVE-co-AN)(27 wt.% AN) and SAN(40 wt.% AN) for a mixture(about 0.4g) were cast from 10wt% solution in DMF. The films were dried slowly in a Petri dish at room temperature and then kept under vacuum to constant weight. 3. RESULTS AND DISCUSSION The synthesis of 5-membered cyclic carbonates is carried out by the reaction of oxirane with carbon dioxide in the presence of various quarternary ammonium salts catalyst as shown in Scheme 1 and their results are listed in Table 1.
H2c-CH I O i
H2C---~ GVE O
H2c-CH I
CO 2
O
~
Catalysts
i
H2C
/
\
O.,~O OVE H O
Scheme 1 Table 1 Effects .0f.catalyst structure .on.the pseudo-first orde r rate.constant(k ') ................................C_...ata!.y st .................................................
...................................................... Ca!.a!yst
..............................
.......................................... k:_x..!
....................
TBAB
4.2
TPAC
20.6
TBAI
2.9
TBAC
24.0
NaI
4.3
TOAC
26.6
405 The catalytic activity of quarternary ammonium salt usually depends on the corresponding catalyst cation and counter anion[2]. For a series of tetraalkylammonium chlorides, the activity increased in the order of TPAC < TBAC < TOAC. Bulky quaternary salts, having longer distance between cation and anion, are generally known to exhibit higher activity in activating anions[3]. This explains why they are more effective in nucleophilic attack of the anion to oxirane ring of GVE. Table 1 also shows that the rate constant with different halide anions of the quaternary ammonium salts decreases in the order of C1- > Br > I-. This is consistent with the nucleophilicity of the halide anions. Radical copolymerization of OVE with AN was carried out using AIBN in acetonitrile at 60~ for 10h. Poly(OVE-co-AN) was identified by an FT-IR spectrometer. The FT-IR spectrum of poly(OVE-co-AN) exhibited characteristic peaks of cyclic carbonate C=O band at 1790 cm -1, ether C-O band at 1720 cm l and CN band at 2250 cm l. In order to estimate the monomer reactivity ratio for the copolymer, the copolymer composition was calculated by Table 2 Mole fraction of OVE and molar ratio of OVE to AN for the poly(OVE-co-AN) copolymers (a)M l (b)m1 ....... (c)F (d)f f/F 2 (f- 1)/F 12.2
9.5
0.14
0.11
5.59
-6.42
30.0
24.2
0.43
0.32
1.74
-1.59
50.0
38.3
1.00
0.62
0.62
-0.38
71.6
57.3
2.52
1.34
0.21
0.14
67.7
7.33
2.38
0.04
0.19
88.0 ...........
(a) mole fraction of OVE in feed (b) mole fraction of OVE in copolymer measured by 13C-NMR (c) F = M1/M2, (d) f = ml/m2
1 I
m
~t
-1 -2
O/lOO
_
40160
- ~
60140 80120
-
100/0
-3 v
-4 -5
m
-6
-7
0
I
1
i
2
I
3 Elf2
i
4
ill
I-',
5
6
OVE-AN/SAN I
I
I
I
0
50
100
150
Temperaure(~
Figure 1. Finemann-Rossplot of poly(OVE-co-AN). Figure2. DSC thermodiagramof poly(OVE-co-AN)/SAN.
406 Table 3 Optical clarity ofpoly(OVE-co-AN)(SAN blends Poly(OVE-co-AN)/SAN 20/80 40/60 Composition Optical clarity ............................................................
-...................
--~-:-: ................
-----~ .................
Clear --~
............
-=--~=-:
.................
----~ ...............
Clear -~--:
.............
-
.~ . . . . . . . . . . . . . . . . . . . . .
60/40
80/20
Clear
Clear
-: ...........................................................................................................................................................................................
a high resolution nuclear magnetic resonance spectroscopy. The monomer conversion was adjusted to be less than 10% and the monomer ratio([OVE]/[AN]) in the feed was varied from 0.14 to 7.33. By measuring areas of C=O peak(OVE) in 165 ppm and CN peak(AN) in 120 ppm, the mole fraction of OVE and AN in the copolymer can be determined. The mole fractions of OVE and AN both in the feed and copolymer are shown in Table 2. The Finneman-Ross plot of the copolymer of OVE(M1) and AN(M2) is shown in Figure 1. From the slope and intercept, the monomer reactivity ratio can be estimated as r~=0.36 and r2= 1.21. In order to examine the degree of miscibility of the poly(OVE-co-AN)/SAN blends, optical clarity was first investigated(Table 3). All the poly(OVE-co-AN) formed clear films when blended with SAN, which means that the blends are miscible over the whole concentration range. Differential scanning calorimetry measurements of poly(OVE-co-AN)/SAN blends shown in Figure 2 reveal the shift of glass transition temperature from the lowest Tg, which corresponds to the poly(OVE-co-AN) to the highest Tg of SAN. One Tg value of the blend confirmed the miscibility of the poly(OVE-co-AN)/SAN blends. 4. CONCLUSION Carbon dioxide can effectively be added to the epoxide ring of GVE to produce the corresponding cyclic carbonate, OVE. Quaternary ammonium salt catalysts showed good catalytic activity even at atmospheric pressure of carbon dioxide. Since the blends of poly(OVE-co-AN) and SAN showed good miscibility, catalytic fixation of carbon dioxide to polymer blends via cyclic carbonate could be one of choice for the reduction and utilization of the greenhouse gas. REFERENCES
1.T. Nishikubo, A. Kameyama and M. Sasano, J. Polym. Sci., A, Polym. Chem., 32 (1994) 308. 2.J.Y. Moon, D. W. Park, J. G. Yang, S. M. Jung and J. K. Lee, React. Kinet. Catal. Lett., 61 (1997)315. 3.D.W. Park, S. W. Park, C. F. Kaseger, J. Y. Moon and J. B. Moon, React. Kinet. Catal. Lett., in press. 4.C.M. Starks, C. L. Liotta and M. Halpern, "Phase Transfer Catalysis", Chapman & Hall, New York, 1994.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
407
The selective s y n t h e s i s o f l o w e r olefins(C2 - C4) by the C O 2 h y d r o g e n a t i o n over Iron c a t a l y s t s p r o m o t e d w i t h P o t a s s i u m and s u p p o r t e d on ion e x c h a n g e d ( H , K) Z e o l i t e - Y
Ho Kim, Dae-Ho Choi, Sang-Sung Nam*, Myung-Jae Choi and Kyu-Wan Lee Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, Korea Iron catalysts promoted with potassium and supported on ion exchanged (H, K) zeolite-Y were examined for catalytic activity and product selectivity in the CO2 hydrogenation. The catalysts were prepared by impregnating the support with iron nitrate followed by calcination and in-situ reduction in hydrogen and were characterized by XRD, AAS, BET, CO2 & H2 Chemisorption, and Temperature Programmed Reduction Technique. The catalytic tests on CO2 conversion were performed at 10 atm and at 573K in a continuous flow system using a fixed bed reactor. It was found that iron oxide was highly dispersed on the zeolites and potassium exchanged zeolite-Y increased the basicity of the catalyst surface. This significantly improved the hydrocarbon yield and the C2 - C4 olefins selectivity. On the other hand, hydrogen exchanged zeolte-Y depressed surface basicity and hence resulted in an increased amount of methane production. Therefore, an attempt has been made to correlate the CO2 hydrogenation activity with the surface properties of the catalysts. 1. INTRODUCTION For the last few years, our group has been extensively engaged in the studies of the hydogenation of CO2 to form hydrocarbon and oxygenates products ~). In this paper, zeolite-Y was ion exchanged with hydrogen and potassium which did not chemisorb CO2, and used as a support to find a correlation between metallic iron and the basicity of the catalyst surface in CO2 hydrogenation. Potassium was added to iron supported on the ion exchanged zeolite-Y to obtain higher yields of hydrocarbons with high selectivity towards lighter olefins in the CO2 hydrogenation. 2. E X P E R I M E N T A L The ion exchanged(H, K) zeolite-Y were prepared according to the method described elsewhere 2) and the structure of the synthesized zeolites were verified by XRD. To prepare the catalysts, the zeolites were impregnated with an aqueous solution of Fe(NO3)3 and Fe(NO3)3 + K2CO3 ( the atomic ratio of Fe : K was 2 : 1 ), respectively by the incipient wetness procedure, dried at 393K and calcined in air at 753K for 12hrs.
* To whom correspondence should be addressed
408 The catalysts were characterized by BET surface area measurement, XRD, in-situ CO2 & H2 chemisorption measurements, and Temperature Programmed Reduction (TPR). CO2 hydrogenation was carried out in a fixed bed flow reactor made of stainless steel. Prior to the activity studies, the catalysts were reduced in 99.99 % H2 flow at 723K for 12hrs. After this, the reaction gas (H2/CO2 = 3) was introduced into the reactor at 573K at 10 atm. The gas phase effluents were analyzed by on-line GC. 3.
RESULTS AND DISCUSSION
Catalyst preparation The physical and chemical characteristics of prepared HY and KY zeolite were analyzed by AAS and BET surface area measurements. These results are summarized in Table 1 and show that they are highly crystalline and free from impurities. Table 1. The physical and chemical characteristics of HY and KY. MY zeolite Surface area (mZ/g) Si/A1 % Exchanged* HY zeolite 903 2.3 88 KY zeolite 865 2.3 95 *It was calculated by assumption that N a Y zeolite(parent material) was 100% ion-exchanged
The XRD patterns of HY and KY exhibit similarity. At 5 wt.% loading of iron on HY and KY, the structural peaks of zeolite-Y was detected. The absence of these peaks in 17 wt.% loading of iron on HY and KY may be due to the screening of the zeolite surface by iron oxide particles with very small particle size. This indicated that the metal oxide was highly dispersed on the zeolie-Y. At 30 wt.% loading of iron on HY and KY, large particle size (> 100A) hematite species was formed by the aggregation of iron oxides on the supports.
CO2 and H2 chemisorption _
CO2 and H2 chemisorption studies were performed on supporting materials and iron supported on HY & KY zeolite catalysts to determine the relative basicity. The results were listed in Table 2. No chemisorption of CO2 was observed on the HY and KY zeolite. However, the chemisorbed amount of CO2 increased with increasing the iron content on the supports. By the way, iron supported on potassium ions in zeolite-Y catalyst showed a much higher chemisorption capacity of CO/. From these results, it is concluded that CO2 appears to chemisorb on the free iron surface and on the iron surface on the potassium present in the zeolite matrix. The addition of potassium into Fe/HY and Fe/KY catalysts slightly increased the chemisorption amount of CO2 due to the electron donating ability of potassium to neighboring surface iron atoms. On the other hand, the chemisorbed amount of H2 did not show considerable difference in all samples.
409 Table 2. The chemisored amounts of CO2 & H2 o n prepared catalysts samples
CO2 uptake(/.t mol/g)
HY KY - - ~'e/H-Y-Fe/KY Fe/HY Fe/KY Fe-K/HY Fe-K/KY -
H2 uptake(/.t mol/g)
6 .-5-. . . . . . . . . 25.0 13.8 54.2 17.4 58.5
(F~ 5-wt.%3. . . . . . . . (Fe" 5wt.%) (Fe: 17wt.% fie: 17wt.%) (Fe: 17wt.% fie: 17wt.%)
2.1 1.7 1.8 1.3 1.7 1.1 1.5 1.2
not observed
Temperature Programmed Reduction(TPR) Temperature programmed reduction experiments of prepared catalysts were done to understand the strength of metal-support interaction. The results were shown in Fig. 1. There are two peaks on each profile. As shown in Fig. l-b, both peaks in Fe/KY catalysts are shifted toward higher temperature region compared with Fe/HY. This implied that iron component interacted more strongly with potassium ion exchanged zeolite-Y than hydrogen exchanged zeolite-Y. TPR profile of potassium added in Fe/KY catalyst (Fig. l-c) was very similar to that of Fe/KY catalyst. This indicated that potassium addition to the catalysts did not strongly affect the interaction between iron and KY zeolite. 440
460
I
~
I
I
I
250
3 0
450
550
600
isothermal
Temperature / *C Fig. 1. TPR profiles of Fe & Fe-K catalysts a supported on HY and KY.
410
Activities and selectivities of various catalysts in CO2 hydrogenation The results of catalytic activity and selectivity were presented in Table 3. Iron supported on HY could not yield olefin and was mainly responsible in producing methane due to the strong acidity of hydrogen exchanged zeolite-Y. This was well coincident with the low level of CO2 chemisorption uptake and reduction degree from the TPR profile. When the potassium was added to the Fe/HY catalyst, it also could not change selectivity to lower olefins. For the Fe/KY catalyst, potassium ions in zeolite-Y bifunctionally acted as a support to increase catalyst surface basicity as well as a chemical promoter to increase C2+ hydrocarbon distribution. It can be explained by the fact that it can enrich the local electron density towards active iron metals 3. The addition of potassium into Fe/KY catalysts slightly increased the catalytic activity (21% conversion) and C2-C4 olefin selectivity (80 %). Therefore, it is suggested that not only the electron donating ability of the chemical promoter but also the basicity of the support are very important factors to synthesis C2+ hydrocarbons and olefins from CO2 hydrogenation. Table 3. C02 hydrogenation over Fe and Fe-K (2:1 atomic ratio) catalysts a supported on ion exchanged Y zeolites. (*SFe 5wt. % loading, *17Fe 17wt. % loading) CO2 Selectivity Catalysts conv. (c mol %)
Hydrocarbon Distribution (C mol %)
(%) CO HC C1 C2= C2 C3= C3 C4: C4 C5> Fe/HY .5 3.15 79.06 20.94 75.70 0.00 17.29 0.32 4.90 0.00 1.80 0.00 Fe/KY .5 14.33 62.16 37.84 36.77 0.96 14.79 3.79 10.45 4.21 5.45 23.40 -Fs qT- 10.14 Fe/KY *17 17.95 Fe-K/HY *17 11.90 Fe-K/Ky*1721.28
39.14 31.35 33.87 26.53
60.86 66.49 66.13 69.35
72.56 12.54 52.70 11.17
0.02 8.94 0.14 9.12
15.23 3.18 16.91 2.08
0.07 13.02 0.79 13.62
7.64 2.98 1.49 3.24 10.40 3.83 44.50 12.72 0.69 7.81 8.24 2.33 10.76 2.75 47.64
O1.(%)b /(Ol.+Pa,) C2-C4 1.30 22.48 0.36 75.93
-47i .
.
.
.
.
.
82.38
aco: hydrogenation at 1900 ml/g/h, 573 K, and 10 atm, bSelectivity to olefins (C mol %)
CONCLUSIONS Prepared catalysts showed iron was highly dispersed on the supporting materials and they are free from impurities. Chemisorption studies suggest that CO2 appears to chemisorb on the free iron surface and on the iron surface on the potassium present in the zeolite matrix. TPR profiles implied that the iron component interacted more strongly with potassium ion exchanged zeolite-Y than with hydrogen exchanged zeoliteY. Potassium ions in zeolite-Y bifunctionally acted as not only a support to increase the catalyst surface basicity but also as a chemical promoter to shift C2+ hydrocarbon distribution and olefin selectivity. From these results, we proposed that the basicity of catalyst surface is a very important factor in CO2 hydrogenation. 4.
REFERENCES 1. Pyung-Ho Choi, Ki-Won Jun, Soo-Jae Lee, Myung-Jae Choi, Kyu-Wan Lee, Catalysis Letter, 40, 115-118 (1996) 2. U.S. Patent, 3-130-009 (1970) 3. H. P. Bonzel, H. J. Krebs, Surf. Sci., 117, 639 (I982)
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998.Elsevier Science B.V. All rights reserved.
411
H y d r o g e n a t i o n of c a r b o n dioxide over r h o d i u m catalyst s u p p o r t e d on silica Masahiro Kishida, Kyotaro Onoue, Shizuka Tashiro, Hideo Nagata, and Katsuhiko Wakabayashi. Chemical Engineering Group, Department of Materials Process Engineering, Graduate School of Engineering, Kyushu University, Higashiku, Fukuoka, 812-81, Japan. In this work, it was found that the surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600m2/g by using the preparation method which we have developed using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. By using these catalysts, it was also found that turnover frequencies for CO2 hydrogenation increased linearly with the catalyst surface area. 1. INTRODUCTION Although it is important, it is difficult to study the effect of catalyst surface area on the catalytic behavior because it has been impossible to control only catalyst surface area by using a conventional method for catalyst preparation. For example, the metal particle size of the catalyst probably becomes small if the metal salts were impregnated on the support with a large surface area. On the other hand, we have developed a novel method for catalyst preparation using water-in-oil (w/o) microemulsion. By using our method, the metal particle size of the catalyst has been controlled regardless of metal content [ 1,2]. The objectives of this work are to control the catalyst surface area of silica-supported rhodium catalysts and to investigate the effects of the catalyst surface area on the catalytic behavior in the hydrogenation of carbon dioxide. 2. EXPERIMENTAL
Rh/SiO2 catalysts were prepared using cetyltrimethylammonium chloride (CTAC) / 1hexanol / RhC13 aq. w/o microemulsion in the same manner as reported previously [ 1-3]. The concentration of CTAC in 1-hexanol and the aqueous concentration of RhC13 were 0.5 and 0.19 mol/dm 3, respectively. The water-to-surfactant molar ratio in the starting microemulsion was 12. The Rh-N2H4-complex nanoparticles were formed in the microemulsion by adding hydrazine directly. After tetraethylorthosilicate (TEOS) as the silica source and a diluted ammonium solution were added to the microemulsion containing the nanoparticles, silica gel containing the nanoparticles were precipitated by the hydrolysis of TEOS. The precipitates were filtered, thoroughly washed by ethanol, dried at 80 ~ overnight, and calcined under air flow at 500 ~ for 2 h. The catalyst thus obtained was pelleted, crushed, sized to ca. 16-24 mesh, and reduced at 450 ~ for 2 h. This preparation method will be denoted by ME method. The catalyst surface areas were determined using the BET equation from the nitrogen isotherms at 77 K. The rhodium particle was characterized by X-ray diffraction (XRD, Rigaku,
412 RINT2500), transmission electron micrography (TEM, Nihon Denshi, JEM-2000FX). Rhodium particle size was determined by the broadening technique. The CO2 hydrogenation was carried out at 220~ under a pressure of 5.0 MPa using a reactant gas mixture composed of H2/CO2/Ar=60/30/10. The flow rate of the gas mixture was adjusted to obtain the CO2 conversion in the range between 4 and 6%. The reactants and products were analyzed with on-line gas chromatographs (Shimadzu GC-4BPT, GC-7BPT, GC-8A). 3. RESULTS AND DISCUSSION 3.1 Effects of hydrolysis conditions on catalyst surface area In the ME method, silica as a support was formed by the hydrolysis of TEOS. Thus, in order to control a catalyst surface area, it is necessary to prepare catalysts changing the hydrolysis conditions of TEOS and to investigate the change in surface area of the resulting catalysts. Table 1 shows the relationship between hydrolysis conditions and the catalytic properties. The amount of TEOS charged considerably affected the BET surface area of the catalysts prepared by the ME method. It was found that the BET surface area increased with decreasing the amount of TEOS. The silica yield, which is defined as {weight of silica obtained }/{ weight of silica when all the TEOS charged were converted into silica }, was independent of the amount
Table 1 Catalyst preparation condition and the catalytic properties. Composition before hydrolysis
Catalyst
Solution a) [cm 3]
TEOS [g]
NH3 aq. [Vol % ]
[NH3]aq b) [mol/dm 3]
Silica yield c) [%]
Rh cont. [wt%]
ABETd) [m2/g]
191 191 191 191
10 20 30 50
30 30 30 30
13.5 13.5 13.5 13.5
94 94 98 93
6.3 3.1 2.1 1.2
420 240 140 31
191 191 191 191 191 191 191
50 50 50 50 50 50 50
30 30 30 30 30 30 30
2.9 4.7 6.8 8.8 10.3 11.7 13.5
55 69 78 80 92 86 93
2.0 1.6 1.4 1.4 1.2 1.3 1.2
700 605 413 327 141 68 31
a) Volume of the solution before adding TEOS. b) The aqueous concentration of NH 3 in water-pool. c) The ratio of the weight of silica obtained to the weight of silica when all the TEOS charged was converted into silica. d) The N2-BET surface area of the catalyst.
413
of TEOS. This result indicated that the amount of silica obtained was approximately proportional to the amount of TEOS charged. Here, the amount of rhodium contained in the starting solution was kept constant at 4.56x10 -3 mol. Consequently, the rhodium content of the resulting catalyst increased with decreasing the amount of TEOS. The aqueous concentration of NH3 before adding TEOS also greatly affected the BET surface area of the resulting catalysts. It was noteworthy that the BET surface area extremely increased with decreasing the concentration of NH3 and was 700 m2/g at the NH3 concentration of 2.9 mol/dm 3. Here, the amount of TEOS charged was always 50 g, but the rhodium content was changed because silica yield was changed with the NH3 concentration. As it was generally known that both increasing the amount of TEOS and the concentration of NH3 promote the hydrolysis rate, these results show that the silica prepared at a faster hydrolysis rate has a smaller BET surface area. The surface area of the catalyst was changed in this way, however the rhodium content was also changed at the same time. Thus, we made an attempt to control only the surface area at a constant rhodium content. From Table 1, the catalyst surface area and the rhodium content were found to be a function of both the amount of TEOS and the NH3 concentration as follows; (1) (2)
(Surface area) = - 2 . 2 x (amount of TEOS) + 62 x (NH3 conc.) + 1071 (Rh cont.) =-0.027 x (amount of T E O S ) - 0.024 x (NH3 conc.) + 2.91
The amount of TEOS and the NH3 concentration were determined by the eqs. (1) and (2) for the purpose of controlling the catalyst surface area with a constant Rh content. Consequently, as shown in Table 2, the catalyst surface area could be controlled in the range between 68 and 605 m2/g. The rhodium particle sizes and the rhodium contents of all the catalysts were approximately 5 nm and 1.6 wt%, respectively. 3.2 Effect of catalytic surface area on catalytic behavior Next, the effects of the catalyst surface area on the catalytic behavior in CO2 hydrogenation were investigated using the catalysts with a controlled surface area. The time dependence on turnover frequencies (TOF's) for CO2 hydrogenation were shown in Fig. 1. The product was methane only. The deactivation was observed in all the catalysts, Table 2 Control of catalyst surface area. Target
Condition
Result
Rh cont. [wt%]
BET S.A. [m2/g]
TEOS [Vol%]
[NH3]aq [mol/dm 3]
Rh cont. [wt%]
BET S.A. [m2/g]
Rh size [nm]
1.6 1.6 1.6 1.6 1.6
50 150 300 400 600
32.4 33.9 36.1 37.6 40.6
15.4 13.7 11.1 9.6 6.0
1.5 1.6 1.6 1.7 1.6
68 118 299 371 605
4.9 5.0 4.4 4.8 5.0
414
0.15
'
I
'
O
'7
O
o t'-
(D
0.1
O
ABET [m2/g] 605 371 299 118 68 O
_
O
O
O
A
A
0.08
'
0.06 o to
0.04 >
a~ > 0.05 o t-
A
x_
D
9
9
9
9
F-I
0
200
,
I
'
I
o 0.02
o00
9
\7 i
'
././
"7,
x.,.
x.,.
I
540 min on stream
9
I
9
S
0 ,
400
600
Time on stream [min]
Fig. 1 Time dependence on turnover frequency for C O 2 hydrogenation.
,
0
I
200
,
I
400
,
I
,
600
800
BET surface area [m2/g]
Fig.2 Relatiionship between the surface area and the turnover frequency for the catalyst.
however the change in activity became smaller at 540 min on stream. Since carbon was detected in all the catalysts subjected to the reaction, the deactivation was suggested to be due to carbon deposition on the catalyst surface. It was noteworthy that the TOF increased with an increase in the catalyst surface area. The TOF at 540 min on stream was found to increase linearly with the surface area, as shown in Fig. 2. This result suggests that the species adsorbed on silica surface play an important role in the CO2 hydrogenation over the silica-supported Rh catalyst. 4. C O N C L U S I O N 1. The catalytic surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600 m2/g by the novel preparation method using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. 2. The effects of the surface area on the catalytic behavior of silica-supported rhodium catalysts in CO2 hydrogenation were examined. It was found that the turnover frequencies for CO2 hydrogenation increased lineally with the catalytic surface area.
REFERENCES 1. M. Kishida, K. Umakoshi, W.Y. Kim, T. Hanaoka, H. Nagata, K. Wakabayashi, Kagakukogaku Ronbunshu, 21, 990 (1995). 2. M. Kishida, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Catal Today, 29, 355 (1996). 3. M. Kishida, T. Fujita, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Chem. Soc., Chem. Commun., 1995, 763.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
415
D e h y d r o g e n a t i o n of e t h y l b e n z e n e o v e r i r o n o x i d e - b a s e d c a t a l y s t in t h e p r e s e n c e of c a r b o n dioxide N. Mimura a, I. Takahara ~, M. Saito ~, T. Hattori b, K. Ohkuma c, M. Ando d aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi,Ibaraki 305, JAPAN bDepartment of Applied Chemistry, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-01, JAPAN cSystems Research & Development Institute of Japan, 16-5 Tomihisacho, Shinjuku-ku, Tokyo 162, JAPAN dJapan Fine Ceramics Association, Halifax Onarimon Bldg. 6F, 3-24-10, Nishishinbashi, Minato-ku, Tokyo 105, JAPAN
The energy required of a new process using CO2 for the dehydrogenation of ethylbenzene to produce styrene was estimated to be much lower than that of the present process using steam. A Fe/Ca/A1 oxides catalyst was found to exhibit high performance in the dehydrogenation of ethylbenzene in the presence of CO2.
1. INTRODUCTION Styrene is one of the most important substances as a raw material of polymers. In Japan, 1.5 million tons of styrene is produced every year. It is commercially produced by the dehydrogenation of ethylbenzene (equation 1), which is made from benzene and ethylene (equation 2). C~H6 + C2H4 ---> C6Hs-C2H5 C6Hs-CzH5 --> C6H~-C2H3 + H2
(i) (2)
A large quantity of high temperature steam (steam/ethylbenzene=7-12 mol/mol) is used in the commercial plant. The important roles of the steam in the dehydrogenation of ethylbenzene are considered as ibllows. (1) The medium for supplying heat to the endothermic dehydrogenation
416 (2) Dilution of ethylbenzene to increase equilibrium conversion (3) Avoiding coke deposition on the catalyst. It has been pointed out that latent heat of condensation of steam is lost at a separator in a commercial process. Recently, dehydrogenation of ethylbenzene in the presence of carbon dioxide instead of steam has been studied 1)z). It is considered that carbon dioxide can play above-mentioned three roles. In this paper, we will also report a result of calculation of energy required to produce styrene in comparison between a present process using excess steam and a new process using carbon dioxide, and some experimental results of the dehydrogenation of ethylbenzene in the presence of CO2.
H20
CO2
One step pathway
Step Step(H)
Catalyst ~iv
H 2"[-CO 2 ~ H20 + CO Two step pathway
2. E X P E R I M E N T A L
The iron oxide-based catalysts were prepared by Figure 1. Pathways of dehya coprecipitation method. In a typical experiment, drogenation of ethylben1.4 g of catalyst (0.18-0. 30mm) was set in a quartz zene in the presence of C02 tube reactor. Ethylbenzene was fed through a vaporizer, and was mixed with CO2. The flow rate was 130 ml/min. The dehydrogenation was conducted at 550~ under atmospheric pressure. The product was analyzed by GC. 3. R E S U L T and D I S C U S S I O N
1.0
One step pathkvay COz/EB=9 \ - / ' ~ - f o " 3.1. T h e r m o d y n a m i c consideration of 0.8 the d e h y d r o g e n a t i o n of e t h y l b e n z e n e Two step ,/~//,"(" 0.6 - p a t h w a y ~ / ,'" . \ There might be two possible pathways COz/EB=9 7 , / ,~ Simple. for the dehydrogenation in the presence 0.4 ~ / / dehydrogenation of CO2, as shown in Figure 1. Figure 2, "/~'/s" "~ H20/EB=9 which shows the effect of temperature on 0.2 the equilibrium yield of styrene, clearly indicates that the yield of styrene in the 0.0 presence of CO2 is much higher than that 300 400 500 600 700 Temperature fC of the process using steam. On the other hand, at given temperature, the two step Figure 2. Equilibrium yield of styrene pathway appears to be more favorable for in the dehydrogenation of ethylbenthe yield of styrene, zene o~,,~
417 3.2. E s t i m a t i o n of e n e r g y r e q u i r e d for d e h y d r o g e n a t i o n processes Figure 3 shows model flow sheets for a typical present commercial process and new process using CO2. Table 1 gives some basic parameters corresponding to the production of styrene ~ a steam process and CO2 process. In the presence of CO2 the temperature of the dehydrogenation was assumed to be 50K lower than that for commercial process on the basis of equilibrium data shown in figure 2. Table2 summarize the estimated energies required to produce styrene by dehydrogenation of ethylbenzene in the presence of CO2 as well as in the presence of steam. The quantity of energy required for the new process using CO2 is much lower than that for the present process, mainly because a large quantity of latent heat of water condensation cannot be recovered in the commercial process. Consequently the dehydrogenation in the presence of CO2 should be a energy saving process. Table1 Basic parameters of the model process Present process New process 630 580 Atmospheric pressure b) H20/EB=9, 25~ CO2fEB=9, 25~ Simple One step dehydrogenation pathway Yield of styrene (Selectivity= 100%) R1:35%, R2:35%, Total:70% a) Temperature at the top of R1, which is indicated by * in Fig.4 and 5 b) The pressure in commercial plant is about 0.5-0.8 atm Reaction temperature(~ Pressure Feed gas and temperature Reaction mechanism
'aporator EB 25~
Heat excnanger EB 25~
Water Separator
,!
/~, Off Gas Product 40~
"
Separator
R1
R2
,~Off Gas Product 40~
~oiler
J~BoilLer
Water~___._~ :~~~_ C02 r25~ Present process
_
25~
New process
Figure 3. Model flow sheets of a present commercial process and a new process
418 Table 2 Energy required for producing styrene Commercial process New process 10 8 cal / t-styrene 10 8 cal / t-styrene Input Boiler 17.8 12.2 (1) Evaporator 2.2 Output Combustion of off gas 5.0 5.9 (2) Surplus energy 4.4 Energy required (1)-(2) 15.0 1.9(6.3 a)) a) Without the surplus energy recovered by heat exchanger (el. Fig.3) 3.3. D e v e l o p m e n t of c a t a l y s t s 6o I 9 :Fe/Ca/Ai(14/10/76wt%) Figure 4 shows that activities of several I: Fe/AI(14/86wt%)a) kinds of iron oxide based catalysts. A 5o I'[ -~. &: Ca/Al(10/86wt%) Fe/Ca/A1 oxides catalyst exhibited the best ~ 4o performance among the catalysts tested. Fe/Ca/A1 and Fe/A1 oxides catalysts were "~ 30 highly active, whereas Fe/Ca and Ca/A1 ox20 ides catalyst were extremely low in activity, lO The selectivities of Fe/A1 oxides and Fe/Ca/A1 I o oxides catalysts were almost the same (97% at o 1 2 3 4 5 5.25 h), and the main by-products were benTime / h zene and toluene. Therefore the addition of an optimum amount of CaO to Fe/A1 based cata- Figure 4. Activity of Fe oxidelyst could suppress the deactivation of the based catalysts catalyst during long term reaction. Further Feed gas" CO2/EB=ll experiment are under achievement to eluci- a)catalyst weight=l.0g date precisely the role of CaO.
3. C O N C L U S I O N The energies required for the process using steam and for the new process using CO2 were estimated to be 1.5xl0%al/t-styrene and 6.3xl0Scal/t-styrene, respectively. Therefore, the new process using CO2 should be a "energy-saving process." A Fe/Ca/A1 oxides catalyst was found highly active and promising catalyst for the dehydrogenation of ethylbenzene in the presence of CO2. REFERENCES 1. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. ikenaga, and T. Suzuki, Appl.Catal.A, 121(1995)125 2. S. Sato, M. Ohhara, T. Sodesawa, and F. Nozaki, Appl.Catal.,37 (1988) 207
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
419
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Promoting effects of C O 2 supported Cr203 catalyst
on
dehydrogenation of propane over a SiO2-
I. Takahara a, W.-C. Chang b, N. Mimura a and M. Saito a aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan bjKF (Japan-Korea Industrial Technology Onogawa, Tsukuba-shi, Ibaraki 305, Japan
Co-operation Foundation)
researcher,
16-3
The effects of carbon dioxide on the dehydrogenation of C3H8 to produce C3H6 were investigated over several Cr203 catalysts supported on A1203, active carbon and SIO2. Carbon dioxide exerted promoting effects only on SiO2-supported Cr203 catalyst. The promoting effects of carbon dioxide over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation.
I. I N T R O D U C T I O N The utilization of carbon dioxide has recently received much attention since the global warming mainly due to carbon dioxide was recognized as one of the most serious problems in the world. The catalytic hydrogenation of CO2 to produce methanol, hydrocarbons, etc., and the CO2 reforming of methane to syngas have been extensively studied. Furthermore, it has been reported that CO2 has several promoting effects on the conversion of hydrocarbons, for example, oxidative coupling of methane [1], aromatization of propane [2] and dehydrogenation of ethylbenzene [3, 4]. The authors have investigated the effect of CO2 on the dehydrogenation of propane over several supported Cr203 catalysts, and found that CO2 has promoting effects on a silica-supported Cr203 catalysts. 14 2. EXPERIMENTAL
12
B
C
~lO Several supported Cr203 catalysts were prepared by an impregnation method using an aqueous solution of chromium nitrate. The supports used were y- A1203, active carbon (AC) and SiO2. The catalysts prepared were calcined at 823 K in air for 2 h. The catalysts were characterized by X-ray diffraction (XRD). The XRD pattems of CrzO3/A1203 and CrzO3/AC showed the diffraction lines ascribed only to the phases of respective
=~ 8 o 6 -~ ~ 4 2 06
~ x
2
4 6 0 2 4 0 ' t/h t/h 2/h Figure 1. Catalytic activities of several supported Cr203 catalysts as a function of time on stream. supports. In the case of Cr203/SiO2, a Cr203 phase A=Cr203(5wt%)/Ai203, B=Cr203(5wt%)/AC' and an amorphous SiO2 phase were observed. The C-Cr203(Swt%)/SiO2' 823 K, W/F=2 g-cat.h/mol, dehydrogenation of C3H8 was conducted under Feed gas:C3H8/CO2=l/l(O), C3H8/Ar=-I/I(O)
420 100
atomospheric pressure of C3H8+CO2 (At) at 823 K by using a fixed bed flow reactor. TPR/D studies were carried out for elucidating the behavior of Cr203 in the catalyst during treatment with H2 and CO,. Pulse reaction technique was also employed for examining the initial activity of the catalyst.
A
88~
B
~
-''-
.
_.
~:~ 60 Q ~ .,,,~ ..,.4
3. R E S U L T S A N D D I S C U S S I O N
"~ 20 r~
3 . 1 . Effects of CO2 on d e h y d r o g e n a t i o n o f C3Ha
%
The main products of the conversion of C3H8 in the presence of Ar were C3H6 and H2, while those in the presence of CO, were C3H6, H2, and CO. Since the selectivities for C3H6 were more than 90%, the dehydrogenation of C3H8 to C3H6 should be the main reaction both in the presence of CO2 and in the absence of CO2. The yield of H2+CO was found higher than C3H6 yield on all catalysts used in the present study. There might be three possible routes for CO formation; the first one via the successive reactions (1) and (2), the second o n e via the reaction (3) and the third one via CO2 reforming of C3H8 (reaction 4) as shown below.
' 2 ' 4 6 L 8 ' 10 Yield of C3H6 / % Yield of C3H 6 / %
Figure 2. Selectivities of H2 and CO in the
reactionof C3H8+CO2as a function of C3H6 yield. A=Cr203(5wt%)/Al203, B=Cr203(5wt%)/Si02, Selectivity" H2(ll), CO(O),823 K, W/F=2g-cat-h/tool, Feed gas:C3H8/CO2=l/1
7 6 ~
/
~
~5 ~:~4
~
rS 3
~
f a'~,,,,o
/
o .,-,2
C3H8- C3H6 + H2 CO2 + H2 = CO + H20 C3H8 + CO2 = C3H6 + CO + H20 C3H8 + 3CO2 = 6CO + 4H2 Figure
1 shows
the activities
(1) (2) (3) (4)
of
>' 1 06
:~
3, t/h ~i
~
10
Figure 3. Change in the C3H6 yield with alternate several feeds of C3Hs/Ar and C3Hs/CO2/Ar over a Cr203/SiO 2.
supported Cr203 catalyst for the dehydrogenation C3H8/CO2/Ar=I/2/7(O) ' C3Hs/Ar=I/9(O) ' of C3H8 in the presence of CO2 as well as in the 823 K, W/F----0.62g-catoh/mol ~. . 1.3.~ absence of CO2. The activity of the Cr203]AI203 catalyst was much lower in the presence of CO2 l ~ ,~ than that in the absence of CO2. The activity of the 8 Cr203/AC was independent of the presence of CO2. ~ - t.2~ ~:1~6~.~' ~,~ L) On the other hand, the activity of Cr203/SiO2 catalyst in the presence of CO2 was surprisingly ~2/~\ "~ found to be 40% higher than that in the absence of .~ 4 t ~ "d CO2. ~ ~.~r~ Figure "~ C shows the O selectivities _ for HE and ~2t x~~__ ~ ~ as a function of C3H6 yield. In the case of 0/ i t ~ ~ t 1 o Cr203/AI203 catalyst, the, selectivity for H2 0 20 Cr20340/wt% 60 "= decreased with an increase in C3H6 yield, whereas Content of the selectivity for CO increased with an increase in Figure 4. Yield of C3H6 in the dehydrogenation of C3H8in the presence of CO2(l) and in the C3H6 yield, as shown in Figure 2A. On the other absenceof CO2(O) and their ratio(O) as hand, the selectivities of Cr203/SiO2 catalyst for a function of Cr203 contenton a Cr203/SiO2. CO and H2 did not change irrespective of C3H 6 C3Hs/CO2(Ar)=I/1,823K, W/F=2g-catoh/mol
421
yield, as shown in Figure 2B. These findings suggest that CO might be formed via successive reactions (1) and (2) over Cr203/A1203 catslyst, whereas both the reaction (1) and the reaction (3) could take place simultaneously over Cr203/SiO2 catalyst. In order to study the contribution of CO2 in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst, catalytic tests with alternate feeds of CaH8/Ar and C3H8/CO2/Ar were carried out. The results shown in Figure 3 clearly indicate that the presence of CO,,_ markedly improved the yield of C3H6. This catalytic performance is a proof that CO,, plays a promoting role in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst. Furthermore, the ratio of C3H6 yield in the reaction in the presence of CO2 to that in the absence of CO 2 decreased with increasing Cr203 content in the Cr203/SiO2 catalyst, as shown Figure 4. This suggests that the boundaries between Cr203 and SiO2 particles might have an important role in the promoting effect of CO2. The effect of CO2 addition on the deactivation of a Cr203/SiO2 catalyst was also examined (Figure 5). The reaction conditions for the both cases with and without CO2 were the same except the catalyst weight, which was 50% larger in the case of the reaction without CO2, in order to obtain the same initial yield of C3H6. The decrease in the yield of C3H6 was found much less in the presence of CO~ than in the absence of CO2. This finding suggests that the addition of CO2 could suppress the deactivation of the catalyst.
lo
8 :~ 6 e~
~ g7. O
2 I
I
I
2 4 6 0 t/h Figure 5. Effect of C O 2 o n the deactivation of Cr203(5wt%)/SiO2. Reaction conditions: 823K 9 : C3H8/CO2/Ar=1/2/7, W/F=0.62g-catoh/mol O: C3Hs/Ar=-1/9, W/F=0.93g-cat.h/mol
(A) in He -I
~B) in H2(10)/Ar
~ .~
3.2. TPR/D result Figure 6 shows TPR/D profiles during various treatments of Cr203/SiO2 catalyst. The Cr203 was (C) in He after CO2 treatmentat 823 K found to be reduced in a stream of H2. And smaller peaks were observed when the catalyst, which was reduced with H2 and then treated with CO2, was re(D) in H2(10)/Arafter (C) reduced with H2. This suggests that CO2 could 323 423 523 623 723 823 oxidize some part of the surface of Cr203 in the catalyst. Accordingly, these findings suggest that Temperature / K --823K= the surface of Cr203 on a Cr203/SiO2 catalyst could Figure 6. TPR/D profile for treatment be reduced during the dehydrogenation of C3H8 in A---*B----C----Dover a Cr203/SiO 2 catalyst. the absence of CO,_, whereas the surface of Cr203 Operation conditions 9H2(10)/Ar(90), 10 K/min might be maintained partially oxidized during the reaction in the presence of CO2. ,.._.___
-
- -
_.
_
422
3.3.
Pulse
reaction
A pulse reaction technique was employed tbr | 50 examining the initial activity of the catalyst. Figure 25 H2 treatment CO2 treatment / 7 shows the conversion of C3H8 and the yield of I hydrocarbon products as a function of the number 20 ~-._~:~ :i 40~. of C3H8 pulses on Cr203/SiO2 catalyst. The yield of C3H6 reached a maximum of about 21% at the - 30~ second pulse. After the second pulse, the yield of ~15 0 C3H6 slightly decreased with increasing pulse ~ "~ number. The yield of C3H6 decreased to 17% by ~,10 20-~ the treatment of the catalyst with H2 after filth pulse, o and remained constant from sixth pulse to ninth 5 10~ pulse. The treatment of the catalyst with CO2 after ninth pulse raised the yield of C3H6 to 19%, but the 0 yield of C3H6 gradually decreased with increasing 0 0 2 4 Pul6enumber810 12 14 pulses number from 10th pulses. These findings 7. Conversionof C3H8 and yield of confirm that partially oxidized Cr203/SiO2 catalyst Figure hydrocarbon products as function of could be more active for the dehydrogenation of the number of C3H8 pulse on Cr203/SiO2. C3H8 than a reduced catalyst. The results obtained Reaction conditions: 823K, He carrier from pulse reaction studies and from TPR/D studies Yield: CH4(i"I), C2H4(A), C2H6(O), C3H6(. ) might explain slower deactivation of the catalyst Conversion: C3H8(1) during the dehydrogenation in the presence of CO2. Furthemore, pulse reaction studies were performed for comparing between the yield of C3H6 during the reaction in stream of CO2 and in a stream of He. The C3H 6 yield at the first pulse and the second pulse in the presence of CO2 was found to be higher than those in the absence of CO2. This strongly suggests that CO2 could enhance the rate of the dehydrogenation of C3H8 over a Cr203/SiO2 catalyst. This promoting effects of CO2 might occur at the boundaries between Cr203 and SiO2 particles, as described in 3.1. 4. C O N C L U S I O N The effects of CO2 on the dehydrogenation of C3H8 to produce C3H6 exhibits only on SiO2-supported Cr203 catalyst. The promoting effects of CO, over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation. The partially oxidized Cr203 supported on SiO2 catalyst is active for the dehydrogenation of C3H8. In the presence of CO2, the surface of Cr203/SiO2 might be maintained partially oxidized during the reaction.
REFERENCES
1. T. Nishiyama and K. Aika, J. Catal., 122 (1990) 346. 2. S. Yamauchi, A. Satsuma, T. Hattori and Y. Murakami, Sekiyu Gakkaishi ( J. of Jpn. Petrol. Inst.), 37 (1994) 278. 3. S. Sato, M. Ohhara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 4. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and T. Suzuki, Appl. Catal. A:General, 121 (1995) 125.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
423
H y d r o g e n a t i o n of c a r b o n dioxide over F e - C u - N a / z e o l i t e c o m p o s i t e catalysts Qiang Xu a, Dehua He b, Masahiro Fujiwara a, Mutsuo Tanaka a, Yasuyuki Matsumura a, Yoshie Souma a, Hisanori Ando a and Hiroshi Yamanaka a aOsaka National Research Institute, AIST, MITI, Midorigaoka, Ikeda, Osaka 563, Japan bDepartment of Chemistry, Tsinghua University, Beijing, P. R. China.
Physical mixing Na-rich Fe-based catalysts with zeolites gave rise to a great improvement of the activity for the hydrogenation of carbon dioxide at 250~ Branched and higher hydrocarbons were obtained over such modified composite catalysts. Sodium migration from the surface of the iron-based catalyst to the zeolite via the solid-solid reaction accounts for the change of catalytic activity. Evidence for Na migration was obtained by XRD measurements of the catalysts before and after the reaction. 1. I N T R O D U C T I O N Hydrogenation of carbon dioxide is an important reaction for the utilization of CO2 as a carbon resource [1]. The hydrogenation of carbon dioxide over Fe catalysts, the typical Fischer-Tropsch (F-T) catalysts, produces mainly linear hydrocarbons in the Anderson-SchulzFlory distribution [2]. Composite catalysts with zeolites have been employed to improve the distribution [3-5]. We have reported in a preliminary communication that physical mixing of zeolites with Na-rich Fe-Cu catalysts improved the activity of the F-T catalysts and enhanced the generation of branched and higher hydrocarbons [6]. The idea of Na-migration from the Fe-based catalyst to the zeolite has been proposed to explain the improvement of catalytic activity. Evidence for Na migration has been obtain by examining the reductive behavior of the catalysts by TG measurements [7]. In the present paper, we report evidence for Na migration obtained by XRD studies and describe the influence of the solid-solid interaction between the iron-based catalysts and the zeolites on the catalytic activity and on the distribution of products in the hydrogenation of carbon dioxide.
2. E X P E R I M E N T A L Fe-Cu-Na catalysts were prepared by coprecipitation from the iron and copper nitrates with sodium hydroxide. Sodium contents were controlled by washing the precipitates. The composite catalysts were obtained by physical mixing of equal amounts of Fe-Cu-Na oxides and zeolites. After reducing the samples in a flow of 10% H2/N2 for 6 h at 350~ the catalysts were kept at 250~ in a flow of reaction gas under 20 atm. The reactants and products were analyzed with an online gas chromatograph system. The XRD powder patterns of the catalysts before and after the reaction were obtained with CuKa radiation at 40 kV and 30 mA on a RIGAKU X-ray diffractometer.
424
3. RESULTS AND DISCUSSION Table 1 summarizes the results of the hydrogenation of carbon dioxide at 250~ catalyzed by Fe-based catalysts and their composite catalysts. Over the Fe-based catalysts without zeolites physically mixed (C1-C4), the distribution of hydrocarbons in the products follows the Anderson-Schulz-Flory law [2]. Na addition resulted in (1) an increase in the average molecular weight of hydrocarbon products, (2) an increase in olefin selectivity, (3) an increase of CO selectivity, and (4) an increase in hydrocarbon yield at low alkali concentrations (C2), followed by a decrease at higher levels of Na addition (C3 and C4). These observations are consistent with the previous reports on the effects of alkali promotion on iron catalysts for the hydrogenation of monoxide (F-T synthesis) [8,9]. The results can be interpreted in terms of electronic effects from alkali, the base (electron donor), which weakens the bonds between the metals and hydrogen (electron-donor), lowers the degree of reduction and therefore decreases the CO2 conversion. The electronic effects from alkali also lead to low activity for hydrogenation of olefins and therefore high selectivities of olefins. Table 1 Hydrogenation of carbon dioxide over Fe-Cu-Na/zeolite composite catalysts Cat
Fe-Cu-Na a zeoliteb, c
No.
Conv. of
Convert to C-mol%
C=
Cbrch
CO2/%
CO
C1
C2+
C1
99:1:0.06
-
13.8
20.8
26.5
47.5
Oxy d Ratio e Ratiof 5.2
2.9
7.2
C2
99:1:0.17
-
14.4
19.5
27.6
48.4
4.5
3.2
6.2
C3
99:1:1.45
-
6.8
69.0
4.7
23.4
2.9
70.5
2.9
C4
99:1:27.0
-
6.7
71.9
4.7
21.2
2.1
64.5
3.3
C5
99:1:0.06
HY(4.8)
C6
99:1:0.06
US-Y(10.7)
C7
99:1:1.45
HY(4.8)
C8
99:1:1.45
C9
99:1:1.45
CI0 Cll
8.4
25.5
33.5
41.1
0
0.6
61.5
12.5
14.8
35.1
50.1
0
0.5
76.4
8.2
33.4
21.3
45.3
0
1.1
69.6
HMOR(9.8)
11.8
23.7
17.6
58.4
0.3
15.7
44.0
US-Y(10.7)
11.5
22.7
24.0
53.3
0
0.3
77.3
99:1:1.45
HZSM-5 (25)
12.3
19.6
22.9
57.5
0
3.1
64.6
99:1:1.45
HZSM-5 (1000)
7.9
54.6
8.4
31.0
6.0
41.5
3.1
C 1 2 99:1:1.45 NAY(4.8) 12.3 18.5 20.4 54.4 6.7 6.0 4.2 250~ 20 atm, SV=3000 ml/g-cat./h, H2/CO2=3. Results after 5 h. aMolar ratio, bRatio, SIO2/A1203, in the parentheses. CReference catalysts of the Catalysis Society of Japan. dMeOH+EtOH+PrOH, eOlefin/(olefin+paraffin)/% of C2, C3 and C4. fBranched/(branched+linear)/% of C4, C5, and C6. Mixing HY zeolite with the Na-poor catalyst (C1) resulted in deactivation of the F-T activity of Fe-Cu catalysts (C5). The H-type zeolite with a higher Si/A1 ratio, such as US-Y zeolite (C6), has less acidic sites, which deactivated the F-T activity to a slight degree, and the Natype one, which has no acidic sites, did not deactivate it. These facts indicate the deactivating process is closely related to the acidic sites. Physical mixing of zeolites with Na-rich catalysts of Fe-Cu-Na(99:l:l.45) (C3) drastically improved the activity (C8 -C10). It has been proposed that sodium migration from the surface of the metal catalyst to the zeolite via the solid-state reaction accounts for the change of catalytic activity [6,7]. Na migration resulted in a decrease of the inhibitory effect of Na on reduction of the iron oxide, giving rise to an increase in F-T activity. Light olefins were formed on the surface with a moderate Na level,
425 and branched and higher hydrocarbons were then formed via the acidic carbon-homologation of the light olefins on the partially Na + ion-exchanged H-type zeolites. The slight change in the activity observed in the reaction with HY (C7) arose from the deactivation of the F-T sites by the large number of acidic sites on the zeolite which competed with the enhancement of reduction arising from Na migration. A large increase of CO2 conversion was observed in the reaction with US-Y zeolite, which has a relatively small amount of acidic sites to deactivate the F-T sites. The zeolite having higher Si/A1 ratio allowed only a small amount of Na to migrate in and therefore no apparent change in activity appeared with HZSM-5(1000) ( C l l ) . However, a remarkable enhancement of activity was observed in the reaction of the Na-type zeolites (C12).
4-
+
A 4-I, ,4-+/ 4-l+
A
A
A
A
+
b 4-
4-4-
+§ 0
I+
20
a
Oo 40
60
80
20 / degree Figure 1. XRD patterns of (a) the catalyst of Fe-CuNa(99:1:2.9)/HY before the reaction; (b) the catalyst of Fe-Cu-Na(99: l:2.9)/HY after the reaction; and (c) the catalyst of Fe-Cu-Na(99:l:27)/HY after the reaction. o, Fe203; Lx,Fe304; +, HY zeolite. In the present study, we obtained evidence for Na migration by measuring the X-ray diffraction of the catalysts before and after the reaction. Figure 1 shows the powder patterns of X-ray diffraction for (a) the catalyst of Fe-Cu-Na(99: l:2.9)/HY before the reaction; (b) the catalyst of Fe-Cu-Na(99:l:2.9)/HY after the reaction; and (c) the catalyst of Fe-CuNa(99:1:27)/HY after the reaction. The reflections of Fe203 and Fe304 were observed before and after the reaction, respectively. The absence of Fe after the reaction may be due to the reoxidation of the samples, which were exposed to the air after the reaction and during the XRD measurements. Although no distinct differences appeared in the XRD patterns of the HY zeolite for the composite catalyst containing a small amount of Na before and after reaction (Figs. 1a and lb), the crystallinity of the HY zeolite decreased during the reaction over Fe-Cu-
426 Na(99:1:27)/HY, the composite catalyst containing a large amount of Na (Fig. l c). This observation indicates that a large amount of Na migrated from the metal oxides with a high Na content to the zeolite, resulting in a lower hydrothermal stability and destruction of the crystal structure of zeolite underthe reaction conditions [ 10]. For the composite catalysts with a low Na content, it is reasonable to consider that Na migration occurred between the metal oxide catalysts and the zeolites, although the small amount of Na did not cause the destruction of the crystal structure of the zeolites. Similar solid-solid reactions have been reported for alkaline, and alkaline-earth metal chlorides/zeolite systems [11-14]. We have recently revealed evidence for Na migration from the surface of the metal-oxide to the zeolite by examining the reductive behavior of the catalysts [7]. It was found that there was almost no effect on the reduction rate as a result of physically mixing the HY zeolite with the Na-poor catalyst. On the other hand, physical mixing increased the rate of reduction of the Na-rich catalyst as a result of the increased efficiency of Na migration. The effect was more significant for zeolites of either of H- and Na-type having lower SIO2/A1203 ratio, indicating that the Na affinity is affected by the polarization effect of [A104]-; it does not depend on the structure of the pore in the zeolite or whether the zeolite is of the H-type or not. These observations supported the results of the hydrogenation of carbon dioxide over the composite catalysts as described above.
4. C O N C L U S I O N We presented a facile route for the modification of zeolites and for the preparation of bifunctional catalysts possessing both acidic and hydrogenation functions via solid-solid reaction. Branched and higher hydrocarbons were obtained over such modified composite catalysts. Sodium migration from the surface of the iron-based catalyst to the zeolite during the solid-solid reaction accounts for the change of catalytic activity. XRD measurements exhibited evidence for Na migration.
REFERENCES 1. W. M. Ayers, Catalytic Activation of Carbon Dioxide, American Chemical Society, New York, 1988. 2. R. B. Anderson, The Fischer-Tropsch Syntheses, Academic Press, New York, 1984. 3. M. Fujiwara, H. Ando, M. Tanaka and Y. Souma, Appl. Catal. A, 130 (1995) 105. 4. K. Fujimoto and K. Yokota, Chem. Lett., (1991) 559. 5. P. D. Caesar, J. A. Brennan, W. E. Garwood and J. Ciric, J. Catal., 56 (1979) 274. 6. Q. Xu, D. He, M. Fujiwara and Y. Souma, J. Mol. Catal., 120 (1997) L23. 7. Q. Xu, D. He, M. Fujiwara, M. Tanaka, Y. Souma and H. Yamanaka, J. Mol. Catal., submitted. 8. R. A. Dictor and A. T. Bell, J. Catal., 97 (1986) 121. 9. S. L. Soled, E. Iglesia, S. Miseo, B. A. DeRites and R. A. Fiato, Topics in Catalysis, 2 (1995) 193. 10. T. Masuda, M. Ogata, T. Ida, K. Takakura and Y. Nishimura, J. Japan Petrol. Inst., 26 (1983) 344. 11. H. G. Karge, Stud. Surf. Sci. Catal., 83 (1994) 135. 12. H. G. Karge, H. K. Beyer and G. Borbely, Catalysis Today, 3 (1988) 41. 13. H. K. Beyer, H. G. Karge and G. Borbely, Zeolites, 8 (1988) 79. 14. M. S. Tsou, H. J. Jsiang and W. M. H. Sachtler, Appl. Catalysis, 20 (1986) 231.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
427
Fe p r o m o t e d C u - b a s e d c a t a l y s t s for h y d r o g e n a t i o n of CO2 Naofumi Nomura, Tomohiko Tagawa and Shigeo Goto Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya, 464-01, J a p a n
The effects of compositions and reaction conditions on product distribution were investigated over various metal promoted Cu-based catalysts to improve the performance for synthesis of hydrocarbons. The formation of carbon monoxide was suppressed and the formation of hydrocarbons increased with the increase in the amount of Fe. The synergetic effect between copper and iron was required for hydrocarbon synthesis.
1. INTRODUCTION Hydrogenation of CO2 to produce valuable chemicals has received much attention in recent years. Not only synthesis of methanol as the energy carrier but also syntheses of hydrocarbons and higher alcohols as the carbon resources will be possible. CO is usually formed as a sideproduct at the same time. Therefore, the control of the product distribution is important. We have carried out hydrogenation of CO2 over copper-based catalysts [1-4]. These results showed t h a t the formate species formed on copper was a common reaction intermediate to both methanol and CO and that CuO-ZnO/TiO2 was the most effective for methanol from the viewpoint of activity and selectivity [1,2,4]. Furthermore, we have found that Fe added catalyst reduced CO with increasing hydrocarbon products [3]. In this study, the effects of compositions and reaction conditions on product distribution were investigated over various Fe promoted Cu-based catalysts.
2. EXPERIMENTAL Table 1 s u m m a r i z e s the tested catalysts. CuO-ZnO/TiO2 catalyst (cat. A) was prepared by precipitating CuO-ZnO onto TiO2 powder. Metal promoted catalysts (cat. B) were prepared by impregnating solution of a metal compound at d i f f e r e n t m o l a r r a t i o s to CuO-ZnO/TiO2. Then, t h e y were d r i e d a n d c a l c i n e d in air. Fe/TiO2 c a t a l y s t (Cu free) w a s p r e p a r e d in t h e s a m e procedure as cat. B (cat. C). In addition, the c a t a l y s t (cat. D) which was
428 Table 1 Tested catalysts Cat.
Composition (weight fraction)
A B
CuO-ZnO/TiO2 (30:30:40) M-CuO-ZnO/TiO2
C D
Fe/TiO2 (0.3:99.7) CuO-ZnO/TiO2 + Fe/TiO2 (mixing of A and C) (50:50)
Molar ratio (Cu:M)
1:0.005 (M=Fe) 1:0.01 (M= Fe, Co, Ni, Ru, Rh, Pd) 1:0.05 (M=Fe)
p r e p a r e d by physical mixing of cat. A and cat. C was also tested. After p r e t r e a t m e n t by hydrogen at 623K for 1 hour, the reactions were started. Reactions were carried out with a conventional continuous flow reactor at a pressure of 1.0MPa. The reaction conditions were as follows: mole ratio of HJC02=4/1, W/Fco2,o=570kg-cat's/mol and T=553K. The reaction mixture was analyzed by a TCD gas c h r o m a t o g r a p h connected to the reactor t h r o u g h a gas sampling valve.
3. RESULTS AND D I S C U S S I O N 3.1. Effect of addition of metals Several metals (cat. B) with the hydrogenation activity were added to CuOZnO/TiO2. The r e s u l t s are s u m m a r i z e d in Tables 2 and 3. H y d r o c a r b o n s
Table 2 Effect of addition of noble metals Additive a)
Conversion
None Ru Ru b) Rh Rh b) Pd Pd b) a) Cu:M=I:0.01
Selectivity (%)
(%)
CO
CH30H
CH 4
C2H6
23.0 12.4 7.7 13.3 8.4 0.9 0.8
97.6 79.8 63.9 92.0 84.1 20.0 18.5
2.4 10.0 20.4 3.8 7.8 trace trace
0.0 10.2 15.7 4.2 8.1 80.0 81.5
0.0 trace trace 0.0 0.0 0.0 0.0
b) p r e t r e a t m e n t by hydrogen at 673K.
429 Table 3 Effect of addition of t r a n s i t i o n metals Additive a)
Conversion
None Fe Fe only b) Co Ni a) Cu:M=I:0.01
Selectivity (%)
(%)
CO
23.0 23.4 2.6 24.9 23.1
97.6 60.5 61.3 87.0 97.7
CH3OH CH4 2.4 5.2 9.3 7.0 2.3
0.0 17.3 29.4 4.6 trace
C2H~ C,~Hs C4H,o 0.0 6.6 0.0 1.1 0.0
0.0 5.8 0.0 0.3 0.0
0.0 4.6 0.0 0.0 0.0
b) cat. C (Fe/TiO2, Cu free).
were not formed without additives. Addition of noble metals (Table 2) decreased conversion markedly, but formed m e t h a n e and suppressed CO formation. C2+ hydrocarbons were not produced. Since these catalysts were prepared by using the chlorides as raw materials, they were also calcined at higher t e m p e r a t u r e (673K) to remove residual chlorine. However, they were deactivated due to the sintering. On the other hand, addition of t r a n s i t i o n metals (Table 3) did not decrease conversion. Ni did not show the promotion effect. Co mainly promoted the formation of methanol and methane. Fe produced C2+ hydrocarbons. Furthermore, Co and Fe suppressed CO formation. However, Fe/TiO2 catalyst (cat. C) showed poor activity. This indicates t h a t Cu is also necessary for hydrocarbon synthesis. It was shown from these results t h a t addition of Fe to CuO-ZnO/TiO2 was the most effective for the formation of C2+ hydrocarbons. Thus, the effects of Fe addition were further investigated in the following study. 3.2. Effect of a m o u n t of Fe
Figure 1 shows the effect of a m o u n t of Fe. The a m o u n t of Fe was varied from 0 to 5%. Hydrocarbon selectivity increased and CO selectivity decreased with the increase in the a m o u n t of Fe. On the other hand, conversion and methanol selectivity were almost independent on the a m o u n t of Fe. Therefore, the addition of Fe allowed the shift from CO formation to synthesis of hydrocarbons. 3.3. Effect of method of Fe addition
Figure 2 shows the effect of method of Fe addition on product distributions. CuO-ZnO/TiO2 (cat. A) w a s active for m e t h a n o l s y n t h e s i s , b u t it w a s not effective for the s y n t h e s i s of h y d r o c a r b o n s . This i n d i c a t e s t h a t Cu species alone is not e n o u g h to produce h y d r o c a r b o n s . On t h e c o n t r a r y , F e - b a s e d c a t a l y s t s are k n o w n as h y d r o c a r b o n s y n t h e s i s c a t a l y s t s from CO, t h a t is, Fischer-Tropsch reaction. However, Fe/TiO2 c a t a l y s t (cat. C) showed poor
430 activity in this study of CO2 hydrogenation, since the pressure in this study (1.0MPa) I ~ 9 8O was quite low compared to the t ~f Fischer-Tropsch reaction o~20 & conditions (5 10MPa). This c60 O indicates that Cu is also > o3 necessary for hydrocarbon (D o > 40 csynthesis. In the case of /x o10 CO physically mixed catalyst (cat. o D), the product distribution was 20 almost the same as that of CuOI ,[~E? ZnO/TiO2 (cat. A). Therefore, ,~ ] synthesis of hydrocarbons was Amount of Fe [%] not a simple consecutive route that CO formed on Cu-Zn Figure 1. Effect of amount of Fe: O - c o n v . ; catalyst was converted into A - s e l . of CO; C ] - s e l . of CHaOH; hydrocarbons on Fe catalyst. ~ - sel. of CnHm. Only when copper and iron were supported simultaneously (cat. B), hydrocarbons 2522.4 22.7 were produced and CO 20formation was suppressed. This strongly 14.2 o'~ 15 _ suggests that the synergetic "o effect between copper and ~J lO8.0 iron is required for synthesis of hydrocarbons. In F-T synthesis, the synergetic 0 8 1"6 effect between copper and iron is explained as Cu being Cu-Zn Cu-Zn-Fe Fe Cu-Zn + Fe a reduction promoter for Fe. (A) (B) (C) (D) 3q~
.
.
.
.
I
100
'
~
,
_
Me0H ~CnHm ~ C 0 Figure 2. Effect of method of Fe addition.
REFERENCES 1. 2. 3. 4.
Y. T. T. T. 21
Amenomiya Tagawa, M. Tagawa, N. Tagawa, N. (1995) 193.
and T. Tagawa, Proc. 8th Intern. Congr. Catal., 2 (1984) 557. Shimakage and S. Goto, ISCF-CO2 (1991) 409. Nomura and S. Goto, Proc. ICCDU (1993) 369. Nomura, M. Shimakage and S. Goto, Res. Chem. Intermed.,
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
431
The effect of rhodium precursor on ethanol synthesis by catalytic hydrogenation of carbon dioxide over silica supported rhodium catalysts Hitoshi Kusama, Kiyomi Okabe, Kazuhiro Sayama and Hironori Arakawa National Institute of Materials and Chemical Research (NIMC), Tsukuba, Ibaraki 305, Japan
Ethanol synthesis by CO2 hydrogenation was carried out over Rh/SiO2 catalyst. Several catalysts were prepared using acetate hydrate, nitrate and chloride precursors of rhodium, and the effects of the precursor on reaction behavior were investigated. The remarkable effect of Rh precursor on the ethanol selectivity as well as activity of reaction was observed. The highest ethanol selectivity was obtained over the 5 wt% Rh/SiO~ catalyst prepared from rhodium acetate hydrate precursor. It has been suggested that the difference in Rh precursor changed the mean particle size of Rh/SiO2 catalysts, resulting in the change in the ethanol selectivity.
1. I N T R O D U C T I O N We have been studying the feasibility of CO2 hydrogenation to oxygenates over silica supported rhodium catalysts based on the results obtained through CO hydrogenation. In previous papers, we reported that the effect of additives to Rh/SiO2 catalysts. As a result, we found that only four additives ( Li, Fe, Sr, and Ag ) showed ethanol formation [1-3]. It has been reported that the precursors of catalyst components over promoted Cu catalysts influence reaction behavior of CO2 hydrogenation. Nitta et al. has reported that the precursor components of the precipitated Cu-ZrO2 catalysts have a great influence on the methanol selectivity as well as CO2 conversion [4]. Based on these findings through CO2 hydrogenation, it is considered that appropriate choice of the precursors in the preparation of supported metal catalyst might improve ethanol formation in CO2 hydrogenation. In this study, the catalysts using different Rh precursors were prepared and the effect of the Rh precursors of Rh/SiO~ on reaction behavior was investigated. Prepared catalysts were characterized by various kinds of methods, in order to
432 elucidate the relation b e t w e e n the p r o p e r t i e s of the c a t a l y s t s a n d reaction behavior.
2. E X P E R I M E N T A L Silica gel ( Fuji-Davison, g r a d e / / 5 7 ), sieved into 16-32 mesh size range and e v a c u a t e d at 473 K for 2 h, was i m p r e g n a t e d with an a q u e o u s solution of Rh(CH~COO)3.5/2H20 ( acetate h y d r a t e , Soekawa Chemicals ) or Rh(NO~)3 ( nitrate, Soekawa Chemicals ) or RhCh~ ( chloride, Wako P u r e Chemical I n d u s t r i e s ) by the incipient wetness method to form Rh/SiO2 catalysts. After drying at 473 K in vacuo, the catalyst was reduced at 623 K for 1 h in an H2 flow of 100 cmS/min. X-ray photoelectron spectra of catalysts were m e a s u r e d using a Shimadzu ESCA-850 after p r e t r e a t m e n t at 623 K for 0.5 h in an H2 flow of 200 cm3/min within the p r e c h a m b e r of the a p p a r a t u s . The binding energies of XPS were referred to the evaporated Au on the surface as the internal s t a n d a r d with the Au 45/2 level at 83.8 eV. H2 adsorption was determined at 308 K using a Micromeritics ASAP 2000 in order to measure dispersion and mean particle size of Rh. Hydrogenation of COs was conducted using a pressurized fixed-bed, flow-type micro-reactor. One gram of pre-reduced catalyst was packed in the reactor tube and was pretreated in-situ at 623 K for 0.5 h in an H2 flow of 200 cm3/min. After cooling to room temperature, the gas was switched to H2 - CO2 premixed gas ( H2 / CO2 = 3 ) containing 1% of Ar as an internal standard for GC analysis, and the reaction was carried out u n d e r a p p r o p r i a t e conditions. The effluent gas was analyzed by on-line gas chromatography with a PEG-1500 column ( 3 m, FID ) and a 2 wt% squalene/active carbon column ( 3 m, TCD ). The tubings from the catalyst bed to the GC were kept hot to avoid condensation of all products.
3. R E S U L T S A N D D I S C U S S I O N 3.1. R e a c t i o n b e h a v i o r Three 5 wt% Rh/Si02 catalysts with different Rh precursors were prepared. Table 1 shows the effects of Rh precursors on reaction behavior. The turnover frequency of C02 conversion increased in the order: nitrate precursor < acetate hydrate precursor < chloride precursor. The remarkable influence of Rh precursor of catalyst on product selectivity was observed. While the main product was CO over the catalysts prepared from acetate hydrate precursor and nitrate precursor, t h a t was m e t h a n e over the catalyst p r e p a r e d from chloride. E t h a n o l was not detected over the catalyst prepared from chloride precursor. The highest ethanol
433 and methanol selectivity was obtained over the catalyst p r e p a r e d from acetate hydrate precursor.
Table 1 Effect of Rh precursor on C02 hydrogenation over 5 wt% Rh/Si02 catalysts a Rh precursor
Rh mean Selectivity in carbon efficiency Turnover % particle size c frequency b h -1
MeOH
EtOH
CO
CH4
nm
Rh(CH3COO)3.5/2H20
44.9
6.0
1.2
89.3
3.3
2.82
Rh(N03)~
12.4
5.4
0.5
68.6
25.5
3.27
144.5
0.1
0
0.1
99.8
5.50
RhCh~
a Reaction temperature = 533 K, pressure = 5 MPa, flow rate = 100 cm3/min, H2/ C02 ratio = 3. b, c Determined by H2 adsorption.
3.2. Characterization of catalysts In order to clarify the reason why Rh precursor influenced the product selectivity, the catalysts were characterized. The residue such as chlorine coming from Rh precursor were not detected on the surface of catalysts by XPS analysis. Table 1 also shows the Rh mean particle size determined by H2 adsorption. The Rh mean particle size of catalysts increased in the order: acetate h y d r a t e precursor < nitrate precursor < chloride precursor. CO selectivity decreased and m e t h a n e selectivity increased d r a m a t i c a l l y in the same order. F r o m these results, it has been suggested t h a t the difference in Rh precursor changed the particle size of Rh, resulting in the change in the product selectivity consequently. 3.3. Correlation b e t w e e n Rh particle size and ethanol selectivity It was shown t h a t Rh p r e c u r s o r influenced ethanol selectivity of C02 hydrogenation over Rh/SiO2 catalysts. Therefore, the catalysts with different Rh dispersion were prepared by changing both Rh precursor and the a m o u n t of Rh loaded on silica supported catalysts. The a m o u n t of Rh loading r a n g e d 0.1 - 10 wt% to Si02. The turnover frequency of C02 conversion decreased with increasing Rh particle size ( correspond to acetate hydrate precursor in Table 1 ), attained a minimum ( correspond to nitrate ), and then increased ( correspond to chloride ). Figure 1 shows the effect of Rh particle size determined by H2 adsorption on ethanol selectivity. The highest ethanol selectivity was obtained when the mean size of Rh particle was about 2.5 nm. The reason why ethanol selectivity reaches
434
to maximum over Rh/Si02 having about 2.5 nm of Rh mean particle size is now under investigation in detail.
2.5
2.0
I
--
I
I
I
I
9
Acetate hydrate precursor
9
Nitrate precursor
l
9 .
Chloride precursor .
.
.
,m
ID
1.5
u
O
1.0
-
0.5
0.0
9
_
I
v
1
9
2
3
v
4
I
5
m
mm
I
6
l
mm
7
Rh m e a n particle size d e t e r m i n e d by H2 a d s o r p t i o n / n m Figure 1. Effect of Rhodium mean particle size on ethanol selectivity. Reaction temperature = 533 K, pressure = 5 MPa, flow rate = 100 cm3/min, H2/CO2 ratio = 3.
REFERENCES 1. H. Kusama, K. Sayama, K. Okabe, and H. Arakawa, Nippon Kagaku Kaishi, 1995 (1995) 875. 2. H. Kusama, K. Okabe, K. Sayama, and H. Arakawa, Catal. Today, 28 (1996) 261. 3. H. Kusama, K. Okabe, K. Sayama, and H. Arakawa, Energy, 22 (1997) 343. 4. Y. Nitta, T. Fujimatsu, Y. Okamoto, and T. Imanaka, Catal. Lett., 17 (1993.) 157.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
435
Selective formation of iso-butane from carbon dioxide and hydrogen over composite catalysts Yisheng Tan, Masahiro Fujiwara, Hisanori Ando, Qiang Xu and Yoshie Souma
Osaka National Research Institute, AIST, MIT1, 1-8-31 Midorigaoka, Ikeda, Osaka563, Japan
The hydrogenation of carbon dioxide was studied using composite catalysts comprised of Fe-Zn-M (M = Cr, AI, Ga, Zr) catalysts and the HY zeolite, where the methanol synthesis and the methanol-to-gasoline (MTG) reaction are combined. The results show that light olefins are important intermediates for iso-butane formation. In all of the cases, the selectivity of isobutane, which can be used as a reactant in subsequent methyl-tert-butyl ether (MTBE) synthesis, was the highest in hydrocarbons. 1. INTRODUCTION The increase of carbon dioxide in atmosphere is believed to be the cause of the serious global warming problem. Therefore, the development of new technologies which suppress the emission of carbon dioxide or convert it into useful chemicals has been intensively studied in recent years [ 1,2]. Especially, the hydrogenation of carbon dioxide is an important method for the utilization of carbon dioxide. We have already reported that the liquefied petroleum gas [3], ethylene and propylene [4,5] could be synthesized from carbon dioxide and hydrogen over various composite catalysts. Recently, methyl-tert-butyl ether (MTBE) has received much attention as an octane-enhancing and exhaust gas-cleaning additive, of which the increased demand has caused a lack of iso-butylene as a key reactant for MTBE synthesis [6]. Conversion of carbon dioxide to iso-butane and subsequent dehydrogenation must be a potential effective process for obtaining iso-butylene. 2. EXPERIMENTAL
The Fe-Zn-M catalysts were prepared by coprecipitation from the corresponding solutions of nitrates with NaOH solution. The two solutions were concurrently added to a 1000 mL beaker and continuously stirred at 65 ~ the pH value of which was kept at 7. The precipitates were aged for 2 h, then filtered and washed with distilled water 10 times. The gels were dried at 120 ~ overnight and calcined at 400 ~ for 4 h. The sodium contents in all catalysts were determined to be less than 0.01% (mass) by atomic absorption spectroscopy (Hitachi Z-8200). The composite catalysts were obtained by physical mixing with the HY zeolite (JRC-Z-HY4.8, SIO2/A1203=4.8) of which the weight ratio of Fe-Zn-M and the HY zeolite was 2:1.
436 The catalytic test was performed in a fixed-bed flow reactor[5]. The reactor was made of a stainless-steel tube with an inner diameter of 9 mm. 1 gram of the catalyst was packed in the reactor, and reduced in a stream of 5% H2 (95%N2) at 340 ~ for 14 h. A reaction gas (H2/CO2=3) was then introduced into the reactor under 5 MPa. All effluent gases were analyzed by an on-line gas chromatograph system [5]. All results in this paper were obtained after reaction at 360 ~ for 6 h. Powder X-ray diffraction was carried out using CuKa radiation at 40 kV and 30 mA on a Rigaku X-ray diffraction meter. 3. RESULTS AND D I S C U S S I O N
Table 1 Hydrogenation of carbon dioxide over Fe-Zn-M/HY composite catalysts a Catalysts Conv. of Selectivity Distribution Ratio of Yield of C02 (%) (%) of hydrocarbons(%) olefins (%) i-C4_ HC Oxy b CO C1 i-C4 Other c C2=d C3=e (C-mol%) Fe-Zn-Zrf 13.9 3.0 27.0 70.0 100 0 0 0 0 0 Fe-Zn-Zr/HY 15.5 40.8 0.7 58.5 2 34 64 72 7 2.5 Fe-Zn-Zrf/HY 17.2 46.8 0 53.2 3 38 59 79 6 3.0 Fe-Zn-Zr/HYg 15.2 42.5 2.8 54.7 6 32 62 74 24 2.1 Fe-Zn-Cr/HY 18.9 31.1 0 68.9 3 39 58 74 6 2.3 Fe-Zn-A1/HY 17.5 35.3 0 64.7 3 39 58 67 3 2.4 Fe-Zn-Ga~Y 15.0 43.3 2.0 54.7 7 33 60 75 24 2.2 Cu-Zn-A1/HY 30.5 6.3 9.3 84.4 10 trace 90 0 0 0 Fe-Zn/HY h 13.3 36.8 1.5 61.7 8 23 69 80 30 1.1 a 360 ~ 5 MPa, S V = 3000 ml/g-cat./h, H2/CO2=3, Fe-Zn-M =1:2:1 (in atomic ratio), Fe-Zn-M/HY=2: l(in weight ratio), b MeOH+MeOMe. c C2H4/(C2H4+C2H6). d C3H6/(C3H6+C3H8). e Other (C2-C7, except for iso-butane), fFe-Zn-Zr =1"11 (in atomic ratio), gFe-Zn-Zr(I'I'I)/HY=I'I, hRef. [5], Fe-Zn(4:I)/HY=II, 350~ Table 1 shows the results of the hydrogenation of carbon dioxide over various composite catalysts. All the composite catalysts except for the Cu-Zn-A1/HY gave considerable amounts of olefins and exhibited high selectivity of iso-butane (32-39%) with low content of methane (2-7%). For the Fe-based composite catalysts, the selectivities of hydrocarbons (31.1-46.8%) depended on the third metal added to the Fe-Zn catalyst, while the conversions of CO2 were relatively constant at 15-18%. In the series of Fe-Zn-M(1:2:1)/HY composite catalysts, the highest yield of iso-butane was observed in the Zr-containing composite catalyst (2.5 C-mol%). The selectivity of hydrocarbons (46.8%) and the yield of iso-butane (3.0 C-mol%)for Fe-Zn-Zr(I:I:I)/HY, as far as we know, are the best for the selective production of iso-butane from carbon dioxide and hydrogen. We compared our novel catalysts with a commercial methanol synthesis catalyst Cu-ZnA1 (Cu-Zn-Al=42:45:13, in atomic ratio). As shown in Fig.la, although the conversion of CO2 for the Cu-Zn-A1/HY composite catalyst was the highest (30.5%), the selectivity of hydrocarbons was the lowest (6.3%) in our study. The decomposition of methanol to CO at high temperatures accounts for the remarkable decrease in the selectivity of hydrocarbons. Moreover, because olefins are easily hydrogenated into paraffins over Cu-based catalysts [7], no olefins and only a trace of iso-butane appeared in the products. The Fe-Zn-Zr (1:1:1)
437 catalyst produced methanol and methane exclusively, although Fe-Zn gave C2+ hydrocarbons as well [5]. For the Fe-Zn/HY composite catalyst, the maximum selectivity of hydrocarbons was 33.8% and the content ofiso-butane only 23% in hydrocarbons [5]. On the other hand, in the case of Fe-Zn-Zr/HY, both of them increased to 46.8% and 38%, respectively. The distribution of hydrocarbons was completely different from the Schulz-Anderson law, indicating that the hydrocarbons are not formed by F-T reaction. We have reported that the Fe-Zn catalyst acts as a methanol synthesis catalyst in the case of the composite catalyst, although the Fe-Zn catalyst is a typical F-T catalyst [5]. However, the Fe-Zn-Zr catalyst without the zeolite behaved as a methanol synthesis catalyst and hydrocarbons were formed via the M T G reaction over the HY zeolite. The XRD powder patterns indicate that the aFe203 and ZnFe204 spinel exist in the Fe-Zn/HY composite catalyst. ZnFe204, ZrO2 and a-Fe203 exist in the fresh Fe-Zn-Zr/HY composite catalyst. However, after the reaction, ZnO appeared and a-Fe203 disappeared. It is known that ZnFe204, ZnO and ZrO2 [8] are favorable for methanol synthesis. It seems that the improvement of iso-butane selectivity by adding the third metal is due to the different behavior of Fe-Zn and Fe-Zn-Zr catalysts, the former as a F-T catalyst and the latter as a methanol synthesis catalyst. 1.0
4.0
I-i
Paraffin
3.5
2
HC Sel. 6.3% Oxy Sel. 9.3% ield (1.92 C-tool%)
0.8
~0.6
CO Conv. 30.5%
~3.0
HC Yield 8.05 (C-mol%) CO Conv. 17.2% m
2
HC Sel. 46.8% _
I
I
I
i
r]
Paraffin
1 ~
Olefin Iso-butane
,E 2.5
B
c..)
-~2.o
(a)
(D
(b)
~0.4 1.0
0.2
0.5
0.0
1
2
3 4 5 6 Carbon Number 360 ~
7
0.0 1
2
3
4
5
6
7
Carbon N u m b e r
5 MPa, SV = 3000 ml/g-cat./h, H2/CO2=3
Figure 1. Hydrocarbon distribution over the composite catalysts of (a) Cu-Zn-A1/HY and (b) Fe-Zn-Zr(l 1 1)/HY The importance of olefins to the distribution of hydrocarbons and the synthesis of isobutane is obvious. In the case of the Cu-Zn-A1/HY composite catalyst, the immediate hydrogenation of the olefins such as ethylene and propylene prevents the oligomerization to higher hydrocarbons. Therefore, C4+ hydrocarbons were scarcely obtained. On the contrary, in the case of the Fe-Zn-Zr/HY composite catalyst (Fig.l, b), the selectivity of methane was
438 low and that ofiso-butane was the highest in all hydrocarbons. Ethylene and propylene were also observed. These olefins seem to be important intermediates to form iso-butane via carbon homologation. Therefore, as predicted in our previous paper [4], to use a metal oxide catalyst which has high activity for methanol synthesis but low activity for the hydrogenation of olefins, is essential for obtaining a high selectivity of iso-butane on the composite catalysts. Our composite catalyst system is also advantageous to produce branched hydrocarbons, especially iso-butane, because of the acidic M T G mechanism. This was confirmed by the predominant formation of iso-butane to n-butane. On the other hand, linear hydrocarbons are formed exclusively by the carbene polymerization mechanism in the F-T reaction. We now wish to propose a plausible reaction path from carbon dioxide to isobutane over the composite catalysts (eq-1). At first, carbon dioxide is hydrogenated to methanol over the Fe-Zn-Zr catalyst, followed by the conversion of methanol to hydrocarbons in the acidic sites of the HY zeolite according to the mechanisms of chain growth [9]. A main route to iso-butane formation is the reaction between methanol and propylene [ 10]. -H20 CO 2 + H 2
~ CHsOH
Fe-Zn-Zr
~CHsOCH s
HY CHsOH C2H 4
~.. C3H6
~ iSO-C4Hlo
(eq-1)
4. CONCLUSION The hydrogenation of carbon dioxide produced iso-butane over Fe-Zn-M/HY (M = A1, Cr, Ga, Zr) composite catalysts with high selectivities. The mechanism of iso-butane formation combines the methanol synthesis reaction and the MTG reaction. The olefins were formed to be important intermediates for iso-butane formation. In order to obtain high selectivity of iso-butane, we found it essential to prepare a composite catalyst which has high activity for methanol synthesis but low activity for the hydrogenation of olefins.
REFERENCES
[1] T. Inui, K. Kitagawa, T. Hagiwara and Y. Makino, Appl. Catal. A., 94 (1993) 3. [2] T. Fujitani, M.Saito, Y. Kanai, T. Kakumoto, T. Watanabe, T. Nakamura and T.Uchijima, Catal. Lett., 25 (1994) 271. [3] M. Fujiwara, R. Kieffer, H. Ando andY. Souma, Appl. Catal. A, 121 (1995) 113. [4] M. Fujiwara, H. Ando, M. Tanaka and Y. Souma, Appl. Catal. A, 130 (1995) 105. [5] M. Fujiwara, R. Kieffer, H. Ando, Q. Xu and Y. Souma, Appl. Catal. A, 154 (1997) 87. [6] A. Sofianos, Catal. Today, 15 (1992) 149. [7] B. Denise and R. P. A. Sneeden, J. Mol. Catal., 37 (1986) 369. [8] B. Denise, R. P. A. Sneeden, B. Beguin and O. Cherifi, Appl. Catal., 30 (1987) 353. [9] W. W. Kaeding and S. A. Butter, J. Catal., 61 (1980) 155. [10] C.D. Chang, Catal. Rev.-Sci. Eng., 25 (1983) 1.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
439
V a n a d i u m - c a t a l y z e d acetic acid synthesis from m e t h a n e and carbon dioxide Yuki Taniguchi, Taizo Hayashida, Tsugio Kitamura, and Yuzo Fujiwara Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-81, Japan I. INTRODUCTION Carbon dioxide is one of the natural Cl-resources which is being watched with keenest interest as a substitute of toxic CO in the C~-chemistry. The chemical fixation of CO2, a process which is appealing for both scientific and environmental reasons since methane and CO2 are well known as the greenhouse gases, is an important task for the human being though it is difficult because of low reactivity of CO2. Alkane activation/functionalization by transition metals under mild conditions is also one of the most challenging problems in modern chemistry since small alkanes including methane, ethane, and propane are the most abundant natural resources of hydrocarbons. In continuing studies on C-H bond activations [1], we have found that methane, ethane, and propane give rise to the corresponding acetic, propionic, and butyric acids, respectively in good yields when allow to react with CO using the Pd(OAc)2/Cu(OAc)2/K2S2Os/CF3COOHcatalyst system [2]. We have also reported that oxygen can be used as an oxidant in lieu of K2S2Os in the Pd/Cu system, and that more interestingly CO2 can also react with methane to give acetic acid [3]. On the acetic acid synthesis from methane and CO2, there is only one example [4] in addition to ours [3]. We have found that carbon dioxide can also react with methane to give acetic acid in the presence of vanadium catalysts.
CH 4
+
CO 2
V cat., K2S20 8
CF3COOH
~
CH3COOH
2. EXPERIMENTAL In a 25-mL stainless steel autoclave fitted with a magnetic stirring bar, VO(acac)2 as catalyst, K2S2Os and CF3COOH were added. The autoclave was closed and then pressurized to 5 atm with CH4 and 5 atm with CO2. The mixture was heated with stirring at 80 ~ for 20 h. After cooling the autoclave was opened and the mixture was analyzed by GLC.
440
3. RESULTS AND DISCUSSION At first, we examined the reaction of methane (5 atm) with CO 2 (5 atm) in the presence of K2820 8 (5.0 mmol) in trifluoroacetic acid (TFA) (10 mL) using various transition metal compounds at 80 ~ for 20 h, and the results are summarized in Table 1. As is apparent from the table, VO(acac)2 (acac: acetylacetonate) gives the highest turnover number (TON) and the highest yield of acetic acid (entry 2). Vanadium(III) oxide is also effective in this reaction (entry 5). In the absence ofK2S208, the TONs of the catalyst are extremely low (entries 2-4), which suggests that K28208 acts as an oxidizing agent. Some vanadiumcontaining heteropolyacids also act as catalyst in this reaction (entries 6-9). This reaction requires strong acid as a solvent. The solvent effect for the synthesis of acetic acid is in the order: TFA (TON = 8.91) >> 2N TFA (1.60)- 2N HCI (1.36)-~ 2N HzSO 4 (1.07) < 2N NaOH (0.36) << H20 (0.06). Use of TFA (entry 1) gives the best result. Table 1 Effect of Catalysts a Entry
Catalyst
1 2 3 4 5 6 7 8 9
none VO(acac)2 NaVO 3 V205 V203 HsPV2Mo loO4oo30H20 HsPMo 12040~ H4PVW1104o~ H20 HsSiVW11040~
TONb 0 8.91 (0.09)d 3.28 (0.29)d 5.88 (0.15)d 8.25 2.67 2.76 5.86 5.30
Yield/%c 0.4 15.7 (0.2) d 6.0 (0.9) d 10.2 (0.4) d 14.7 4.3 4.6 9.6 8.5
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), vanadium catalyst (0.05 mmol), K2S208 (5.0 mmol), CF3COOH (10 mL), 80 ~ 20 h. bTurnover number. CGLC yield based on CH 4. dNo K2S208 added. Table 2 shows the control experiments for the VO(acac)2]K2S208 catalyst system. In the absence of VO(acac)2 and/or K2S208, the carboxylation of CH4 did not occur (entries 1-3). Both of VO(acac)2 and K28208 are indispensable in this reaction. In order to improve the yield of acetic acid, we tested the effect of the amount of TFA (entries 1-3). As the amount of TFA increases, the amount of methane introduced into the autoclave decreases. As a result, the yield based on methane reaches up to 89% by use of 20 mL of TFA (entry 3). However the highest TON obtained in the case of the use of 15 mL of TFA (entry 2). To convert methane perfectly, an excess of CO2 gas was introduced in the autoclave (entries 4 and 5). When the CO2 pressure is increased to 20 atm, the yield of acetic acid based on CH4 reaches up to 97%.
441 Table 2 Control Experiments a Entry
K2S208 (mmol)
1
0
2 3 4 5
5 0 5 10
VO( acac)2 (mmol) 0
0 0.05 0.05 0.05
TONb
Yield/%c
0
0
0 0.1 9.8 13.4
0.4 0.2 15.7 21.6
a25-mL Stainless steel autoclave, CH4 (5 atm), CO 2 (5 atm), CF3COOH (10 mL), 80 ~ bTurnover number. CGLCyield based on CH4.
20 h.
Table 3 Quantitative Acetic Acid Synthesis from Methane and CO2 a Entry
1 2 3 4 5
CH4 (atm) (mmol) 5 5 5 5 5
3.05 2.00 0.95 0.95 0.95
CO2 (atm) (mmol) 5 5 5 10 20
3.05 2.00 0.95 1.89 3.78
TFA (mL)
TON b
10 15 20 20 20
13.4 24.0 16.9 17.5 18.4
a25-mL Stainless steel autoclave, VO(acac) 2 (0.05 mmol), K2S208 (10.0 mmol), 80 ~ bTurnover number. CGLCyield based on CH4.
Yield/%c
22 60 89 92 97 20 h.
Figure 1 shows the effect ofthe amount 0fK2S208. The yield of acetic acid increases in proportion to the amount of K2S208. The kinetic order of the reaction with respect to [K2S208] is clearly first order. The yield of acetic acid depends upon the concentration of VO(acac)2 catalyst (Figure 2). The reaction proceeds rapidly by the addition of very small amounts ofthe catalyst. And then the yield increases slowly. The effect of pressure of CH4 and CO2 is shown in Figure 3. The yield of acetic acid increases sharply in proportion to the pressure of CH4 until 5 atm, and then increases slowly until 30 atm of CH4 pressure and finally it became constant (Figure 3, a). The reaction proceeds with first order with respect to [CH4] at the initial period. This trend can be explained in the following way that the solubility of CH4 in TFA proportionally increases at low CH4 pressure, but at the higher pressure of CH4 it became saturated in TFA. After the saturation point, no CH4 dissolve further with increasing pressure. Moreover, the yield of acetic acid depends on the amount of CH4 dissolved in TFA. Interestingly, Figure 3 (b) indicates that the formation of acetic acid is independent to CO2 pressure, and that even in the absence of CO2 gas, the carboxylation of CH4 occurs. After the reaction, we detected small amounts of CO2 and CHF3 derived from the decomposition of TFA [2] in
442
1.0
1.4
0.8
1.2
i
y
"6 1.0 ~ 0.8
--
~0.6 9 0.4
0.6
<
0.4 0.2-
0.2 ~
...............
0
1 .................
2
~. . . . . . . . . . . . . . . .
7.................
T. . . . . . . . . . . . . . . . .
4 6 8 K2S208/mmol
0
10
..........
0
r ............
r ...........
r ............
r ..........
~ .............
r ...........
l
0.2 0.4 0.6 0.8 1.0 1.2 1.4 VO(acac) 2/mmol
Figure 1. Effect of the Amount of K2S208
Figure 2. Effect of the Amount of Catalyst
Reaction conditions: VO(acac)2 (0.05 mmol), CH4 and CO 2 (5.0 atm each), CF3COOH (20 mL), 80 ~ 20 h.
Reaction conditions: CH4 and CO2 (10 atm each), K2S208 (5.0 mmol), CF3COOH (20 mL), 80 ~ 20 h.
the residual gas by GLC analysis. Although the mechanism is not yet clear at this stage, VO(acac)2 would be converted to active VO(OCOCF3)3 by K2S208 and TFA. Then VO(OCOCF3)3 would abstract Ho from CH4 to form CH3V(OH)(OCOCF3)3 which reacts with CO2 to give (CH3COO)V (OH)(OCOCF3)3. Decomposition of this species gives rise to CH3COOH and VO(OCOCF3)3. The detailed results on acetic acid synthesis from methane and CO2 by vanadium catalysts will be presented and the mechanistic implication discussed.
1.0 a) CH 4 0.8 "6 ~ 0.6 9
b) CO 2
O 0.4 < 0.2 0 0
10
20
30
40 50
60
70 80
Pressure/atm Figure 3. Effect of Pressure of CH 4 and CO 2 Reaction conditions: VO(acac) 2 (0.05 mmol), CF3COOH (20 mL), 80 ~ 20 h. a) CO 2 (5 atm). b) CH 4 (5 atm).
REFERENCES 1. 2. 3. 4.
Y. Fujiwara, T. Jintoku, and K. Takaki, Chemtech, (1990) 636. Y. Fujiwara, K. Takaki, and Y. Taniguchi, Synlett, (1996) 591. M. Kurioka, K. Nakata, T. Jintoku, Y. Taniguchi, K. Takaki, and Y. Fujiwara, Chem. Lett., (1995) 244. H.-J. Freund, J. Wambach, O. Seiferth, B. Dillmann, WO 96/05163.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
443
Fuels and p e t r o c h e m i c a l s f r o m C O 2 via F i s c h e r - T r o p s c h s y n t h e s i s state catalyst activity and selectivity
-
steady
Thomas Riedel, Stefanie Walter, Michael Claeys, Hans Schulz, Georg Schaub Engler-Bunte Institut, Universit~t Karlsruhe, Kaiserstra6e 12, 76128 Karlsruhe, Germany
The behaviour of CO 2 in Fischer-Tropsch synthesis was investigated using a promoted iron and a promoted cobalt catalyst. The decrease in yield of hydrocarbons is more pronounced on cobalt than on iron. The product distribution on iron remains nearly constant with increasing CO 2 concentration, however on cobalt the selectivity to methane increases dramatically.
1. INTRODUCTION A potential way to use CO 2 as the carbon source for the synthesis of organic compounds is the hydrogenation of CO 2 via Fischer-Tropsch (FT) synthesis using a CO 2 rich synthesis gas. Some indication can be found in the literature that the hydrogenation of CO 2 to hydrocarbons proceeds via CO as an intermediate [ 1], which means that the catalyst must have a high activity for the reverse CO shift reaction (1) together with good properties for the FT reaction (2). CO 2 +
H2
CO
2H 2
+
<
-" >
CO
+
H20
(1)
(CH2) +
H20
(2)
Based on an experimental study the present investigation addresses for two different types of catalysts the effect of CO 2 concentration in the reaction gas on carbon conversion rates, yields of organic products and selectivity in the carbon number range C 1 to C20. Two catalysts on Fe- and Co-basis with significantly different CO shift reaction activity were characterized by parameters according to the previously developed model of "non trivial surface polymerisation", based on extended Anderson-Schulz-Flory kinetics [2]. 2. E X P E R I M E N T A L
The reactions were carried out in a fixed-bed micro reactor using Fe- and Co-based catalysts at reaction temperatures of 523 K and 463 K, respectively, a total pressure of 1 MPa, and a reaction gas flow of 30 ml/min (NTP). The H2/C ratio of the reaction gases was kept constant, 7/3 for the Fe-based and 6/3 for the Co-based catalyst, decreasing only the CO/CO 2 ratio. Product samples were taken after steady state activity and selectivity were maintained for at least 24 hours. The catalysts were prepared by precipitation with an aqueous NH 3 solution from the solutions of the metal nitrates, in which Aerosil (Aerosil 200, Degussa) was
444 suspended. The washed and dried catalysts were impregnated with K2CO 3 and H2PtC16 solutions, respectively. Two grams of the catalyst (dp < 0.1 mm), diluted with quartz (dp = 0.25 - 0.4 mm), were placed in the reactor, calcined in Ar (40 ml/min (NTP)) at 673 K and then reduced with an H2/ Ar stream (40 ml/min (NTP), molar ratio = 1/3) at 673 K until no more water was found in the exiting gases. The compositions of the catalysts were 100 Fe / 13 A120 3 / 10 C u / 1 0 K and 100 Co / 60 MnO / 147 SiO 2 / 0.15 Pt. Product analysis was done by off-line gas chromatography with a sampling technique using evacuated ampoules [3]. 3. RESULTS AND DISCUSSION After an initial transient period characterized by activation and deactivation processes of the catalysts [4], a steady state yield of hydrocarbons of 18% was observed with the H2/CO reaction gas on the Fe-catalyst and of 70% on the Co-catalyst (Fig. 1).
15Pt
80 i
6 >d..J
w
>-
r
IO0 ,~
80
09
60
0
40
60
"10
40
m
0
/
,100Fe/13~
9
.J
20 I 0
_ 100Co/60M nO/147Si02/0.15 7
LL!
I
I
I
25
50
75
no02 / ( nco + no02 ), mol-%
7
II 100
m
20
"r ~
0n
IJl--
"
0
I
I
I
25
50
75
1 O0
nco2 / ( nco + nco2 ), mol-%
Fig. 1: Yield of organic compounds (Y, left) and methane selectivity (ScH4, right) as function of the molar CO 2 content of the feed gas 100Fe/13A1203/10Cu/10K : 523 K, 1 MPa, H2/C = 7/3, flow = 30 ml/min (NTP) 100Co/60MnO/147SiO2/0.15Pt : 463 K, 1 MPa, H2/C = 6/3, flow = 30 ml/min (NTP) With increasing CO 2 content, only a slight decrease of the yield of hydrocarbons was found for Fe, whereas with Co a strong decrease was observed. Due to its high activity for the reverse CO shift reaction, a negative CO 2 conversion was found with Fe for low CO 2 concentrations in the reaction gas, in contrast to Co (Fig. 2). With increasing CO 2 concentration in the reaction gas, the selectivity to methane remained nearly unchanged in the case of Fe. For Co, methane became more and more the main product and for the hydrogenation of pure CO 2 an almost exclusive conversion to methane was obtained. When starting the experiment on Fe with H2/CO 2, yield data were slightly different due to irreversible changes of the catalyst [4]. Product distributions can be evaluated for reaction probabilities of elemental surface reaction steps with the model of "non trivial surface polymerisation" [2]. Specific inhibition of desorption of a chemisorbed organic species has been postulated to be the intrinsic principle of the FT-synthesis [5]. A chemisorbed species can react further by linear chain prolongation or chain branching or it can desorb as a paraffin, olefin or an organic oxygen compound. Growth probabilities pg, that contain a similar information as the Anderson-Schulz-Flory parameter or,
445 indicate for Fe no significant effect of the CO 2 concentration, whereas for Co a strong decrease of the C1 value with increasing CO 2 content takes place (Fig. 3).
100 X
75
z
0 03 rr" m > z 0o
100
100 Fe/13AI203/10Cu/1 OK >T
Q Xco O Xco2
z
9
25
A
0 0
75 1
25
50
75
25
100
"v
_....----
-
1.-
100Co/60M nO/147SIO2/0.15Pt
50
o 03 rr LLI > z 0 o
50
f
9 Xco - O Xco2
0
I
I
I
25
50
75
100
nco2 / ( nco + nco2 ), tool-%
nco2 / ( n c o + nco2 ), mol-%
Fig. 2: Conversion (X) of CO and CO 2 as a function of the molar CO 2 content of the feed gas; 100Fe/13A1203/10Cu/1 OK (left), 100Co/60MnO/147 SIO2/0.15Pt (right); (For further information see Fig. 1)
~,
1
100 Fe/13AI203/10Cu/1 OK
-'i 0.75 m 131 < m 0.5
H2/CO/CO2 9 7/3/0 O 7/O/3
-r" 0.25 I--
0
1
100Co/60M nO/147Si02/0.15Pt
0.75
0 iI
0
o,
I
I
I
I
I
2
4
6
8
10
CARBON N U M B E R ,
0.5 0.25
~/~%.,.cro - o ]/
9 6/3/0
v---
V' 6 / 2 / 1 <) 6 / 1 / 2 O 6/0/3
I
12 NC
2
4
6
8
10
CARBON NUMBER,
12 NC
Fig. 3: Chain growth probability (pg) as a function of the carbon number for different molar H2/CO/CO 2 ratios of the feed gas; 100Fe/13A1203/10Cu/10K (left), 100Co/60MnO/147SIO2/0.15Pt (fight); (For further information see Fig. 1)
Linear o~-olefins together with linear paraffins are the main primary products. On Fe the olefin content in the fraction of linear hydrocarbons for small carbon numbers was found to be about 80% (Fig. 4), which is very close to their primary selectivity [6]. This can be due to the high potassium loading, which suppresses the secondary reactions of the olefins. With increasing CO 2 content a slight increase of the olefin content is observed. This can be due to the increasing amount of water formed from the reaction with CO 2 instead of CO. The effect of added water on the olefin selectivity for a potassium promoted fused iron catalyst has been reported earlier by Satterfield [7]. With increasing CO 2 concentration in the reaction gas on Co no more olefins were present in the products.
446
lOO
ee~
80 -1-
60
_z
40
,--I
z -O9 Z
,-rtlJ ._1
o
20 o
100
1oo Fe/13AI203/10Cu/1 OK
8
-
H2/CO/CO 2
-
9 7/3/0 /k 7 / 1 . 5 / 1 . 5 O 7/O/3
I
I
I
I
2
4
6
8
I
I
I
10 12 14 16
CARBON NUMBER,
Nc
8o
o
6o
_z ._1
4o
z
2o -
~ I
100Co/60M nO/147SIO2/0.15Pt
~
z
,7" u.I .-J
o
H2/CO/CO 2
-
00
9 6/3/0 V 6/2/1 O 6/0/3
0
OO
0
I
2
4
6
OOOOooa..." I
8
!
/
--WIJ'
10 12 14 16
CARBON NUMBER,
Nc
Fig. 4: Molar olefin content in linear hydrocarbons as a function of the carbon number for different molar H2/CO/CO 2 ratios of the feed gas; 100Fe/13A1203/10Cu/10K (left), 100Co/60MnO/147SIO2/0.15Pt (fight); (For further information see Fig. 1) 4. CONCLUSIONS Product distribution is not affected by the C O 2 concentration in the reaction gas as long as the catalysts exhibit a high reverse CO shift activity. This can be seen as evidence that the formation of hydrocarbons from CO 2 proceeds via the same way as their formation from CO. Therefore a high reverse CO shift activity is necessary for the conversion of CO 2 to hydrocarbons. A technical process with CO 2 as the carbon source appears feasible with a catalyst like the potassium-promoted iron catalyst used in this investigation. In this case reaction rate is lowered but to a tolerable extent and product selectivity is not affected. Increasing olefin selectivity with increasing CO 2 content is advantageous with respect to the production of chemicals. The manganese-promoted cobalt catalyst did not exhibit a sufficiently high activity for the reverse CO shift reaction, CO 2 being converted almost exclusively to methane, and therefore this catalyst is not suitable for the hydrogenation of CO 2 to higher molecular weight fuels or petrochemicals. There still remains the challenge to combine the advantageous characteristics of catalysts (high FT-activity and selectivity values and high reverse CO shift activity).
REFERENCES
[1] [21 [3] [41 [5] [6]
[7]
P.H. Choi, K.W. Jun, S.J. Lee, M.J. Choi, K.W. Lee, Catalysis Letters 40 (1996) 115 H. Schulz, K. Beck, E. Erich, Stud. Surf. Sci. Catal. 36 (1988) 457 H. Schulz, S. Nehren, Erda51und Kohle- Erdgas- Petrochemie 39 (1986) 93 H. Schulz, G. Schaub, M. Claeys, T. Riedel, S. Walter, "Initial transient rates and selectivities of Fischer-Tropsch synthesis with CO 2 as carbon source", 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, 1997 H. Schulz, E. van Steen, M. Claeys, Topics in Catalysis 2 (1995) 223 H. Schulz, H. Gtikcebay, Proc. 9th Conf. on Catal. of Organic Reactions, Charleston S.C. (1982); in "Catalysis of Organic Reactions" Ed. J.R. Kosak, M. Dekker, New York 1984 C.N. Satterfield, R.T. Hanlon, S.E. Shao, Z. Zou, G.C. Papaefthymlou, Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 407
T. Inui, M. Anpo, K. Izui, S. Yanagida,T. Yamaguchi(Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
447
Effective conversion of CO2 to methanol and dimethyl ether over hybrid catalysts Ki-Won Jun, Mi-Hee Jung, K.S. Rama Rao +, Myoung-Jae Choi and Kyu-Wan Lee* Chemical Technology Lab. I, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-600, S. Korea Catalytic hydrogenation of carbon dioxide was studied for the simultaneous synthesis of methanol and dimethyl ether (oxygenates). Various combinations of methanol synthesis catalysts and methanol dehydration catalysts have been examined for the hydrogenation. The hybrid catalyst of Cu/ZnO/Cr203 and CuNaY zeolite was found to be very efficient for the production of oxygenates. 1. INTRODUCTION Methanol formation from hydrogenation of CO2 is much more thermodynamically unfavorable than that of CO under industrial operation conditions. The simultaneous production of hydrocarbons or dimethyl ether (DME) with methanol by using hybrid catalysts creates a strong driving force for CO2 conversion - thus overcoming the equilibrium restriction. The previous study has indicated that the oxygenates (CH3OH + DME) yield is significantly improved by adding a solid acid to a methanol synthesis (MS) catalyst and a synergistic effect was observed [1]. The present investigation was concerned with the development of an efficient hybrid catalyst system for the co-production of methanol and DME. In this work, the CO2 hydrogenation was conducted on the mixture of various Cu/ZnO-based MS catalysts and T-alumina and then the behavior of various solid acids were investigated as methanol dehydration (MD) catalysts in the reaction and correlated with their acidity measured by NH3 temperature programmed desorption (TPD). 2. EXPERIMENTAL Cu/ZnO-based catalysts with and without promoters for methanol synthesis were prepared by the conventional coprecipitation using Na2CO3 followed by drying overnight at 393 K and calcining at 623 K for 5 h. HY zeolite has been prepared by calcining NH4Y zeolite at 673 K for 24 h. Various cation-containing zeolites were prepared from metal nitrates and NaY zeolite or NH4Y zeolite by § Permanent address: Catalysis Div., Indian Institute of Chemical Technology, Hyderabad 500 007, India. *Corresponding author, E-marl:
[email protected], Fax. (+82-42)860-7590.
448 ion exchange method at 353 K for 24 h, followed by drying at 393 K for 24 h and calcining at 773 K for 12 h. In temperature programmed reduction (TPR) studies on MS catalysts, the activated catalyst at 673 K under He flow was cooled to 323 K, and then 5 % H2/Ar was introduced followed by increasing the temperature at a rate of 10 K/rain up to 873 K. For the NH3 TPD the activated MD catalyst under He flow (30 mYmin) for 1 h at 673 K was cooled to 373 K, where NH3 was introduced and maintained for 3 h followed by replacing NH3 by He flow (30 ml/min). In order to remove physisorbed NH3 He flow was continued for 1 h at 373 K followed by increasing the temperature at rate of 7.6 K/rain up to 873 K where He flow was maintained for 1 h. Both H2 consumption on MS catalysts and the desorbed NH3 from MD catalysts were monitored by a thermal conductivity detector. Prior to the activity test, the hybrid catalyst (mixture of an MS catalyst and an MD catalyst in a 1:1 weight ratio), placed in a fixed bed micro-reactor was reduced at 523 K in a 10 % H2/N2 mixture. The hydrogenation of CO2 was carried out at 513 - 543 K, 30 kg/cm 2, and a space velocity of 1800 ml/g-cat.h by feeding H2 and C02 (I-I2/C02 -~ 3). 3. R E S U L T S AND D I S C U S S I O N Many kinds of Cu/ZnO-based catalysts were prepared according to the composition of which have been reported to be efficient in MS reaction [2-5] and examined in the oxygenates synthesis as they were mixed with T-alumina. Representative data are shown in Table 1. The catalysts Cu/ZnO/Cr203 (No. 3) and Cu]ZnO/Ga203 (No. 4) were proven to be efficient MS catalysts in this reaction. Table 1 Comparison of different types of MS catalysts in the oxygenates synthesis a) No. MS catalyst Conversion (C-mole%) to DME selectivity in (Composition / wt%) CO Oxygenates oxygenates (%) 1 CuO/ZnO 11.07 1.04 15.4 (55.6/44.4) 2 CuO/ZnO/A1203 10.70 8.92 42.2 (57.7/39.7/2.7) 3 CuO/ZnO/Cr203 10.21 9.37 36.7 (55.6/40.0/4.4) 4 CuO/ZnO/Ga203 9.42 9.19 39.9 (55.6/22.2/22.2) 5 CuO/ZnO/ZrO2 13.65 2.08 4.3 (55.6/35.5/8.9) 6 CuO/ZnO/Cr2Oa/A1203 11.51 6.76 41.6 (50.3/23.5/3.6/22.7) a) reaction condition: temp. = 523 K, pressure = 30 kg/cm 2, H2/CO2 ratio = 3, flow rate = 30 mYmin, catalyst = 0.5 g MS catalyst + 0.5 g T-alumina.
449 The results of TPR showed that the addition of promoters like A1203, Cr2Oa and Ga203 to Cu/ZnO system helps the catalyst particularly the CuO phase to be reduced easily, i.e., at relatively low temperatures. However, no promotional effect due to ZrO2 addition is observed from the TPR pattern of this catalyst. The easy reduction of the CuO phase in these catalysts seems to contribute the high yields on Cr203-, Ga203- and A1203- doped Cu/ZnO catalysts. As an efficient MS catalyst, Cu]ZnO/Cr203 was chosen and used in the subsequent investigations. The NH3 TPD patterns of various MD catalysts are shown in Fig. 1. The number of acid sites on these catalysts are in the following order: CuNaY > HY > NaHY > 8i02-A1203 > NaY > A1203. The NH3 TPD results reveal that HY-zeolite bears the broad distribution of acid strength while NaY bears only weak acid sites. The partly ion exchanged NaHY and CuNaY show that they have moderate acid site strength compared to HY. These were supported by the results of the measurement with Hammett indicator: NaY, NaHY and HY gave the values correspondent to pKa > +4.0, pKa > -5.6 and pKa >_-8.2, respectively.
c
. m
_(3 v -I--"
c
o
E 7Z
i 373
I 473
i 573
i 673
I 773
Temperature
I 873-isothermal(K)
Fig. 1. NH3 TPD patterns on MD catalysts. (a) 7-alumina (b) silica-alumina (c) NaY (d) NaHY (e) HY (f) CuNaY.
450 The reaction results on hybrid catalysts (mixtures of Cu/ZnO/Cr203 and MD catalysts) are displayed in Table 2. The acidity of T-A12O3 seems to be insufficient for MeOH dehydration under the conditions of this work. Silica-alumina shows much higher activity for the DME formation than T-A12O3indicating its relatively strong acidity: this agrees with the NH3 TPD. In the test of Y-zeolites, NaY gives very smaU amount of DME because it has only weak acid sites. The lowest oxygenates yield on NaY when compared to A1203 is due to the presence of weaker acid sites than those present on A1203 even though their number is more on NaY. When a portion of Na + is replaced with a weak basic cation (Cu++), the zeolite gives very high conversion to DME. In the case of HY that shows very high activity, there was the formation of light saturated hydrocarbons (i.e. ethane and propane). The presence of strong acid sites on HY zeolite are responsible in yielding little hydrocarbons. However, the catalytic behavior could be modified suitably by the ion exchange with Na as shown in the reaction result of NaHY. From the results, it can be concluded that the combination of Cu/ZnO/Cr203 and CuNaY provides an efficient catalytic system for the oxygenates synthesis. Table 2 Comparison of different types of MD catalysts in the oxygenates synthesis ~) MD catalyst
Conversion (C-mole%) to
DME selectivity
CO
Oxygenates
Hydrocarbons
in oxygenates (%)
none
10.37
6.76
-
-
7-A1203
10.21
9.37
-
36.7
Si02- A1203
9.55
13.43
-
77.3
NaY zeolite
10.91
6.59
-
8.0
Cu72Na2sY zeolite
7.75
15.18
-
79.8
HY zeolite
8.63
15.03
0.52
86.6
Na44Ha6Y zeolite 8.12 14.57 77.8 a) reaction condition: temp. = 523 K, pressure = 30 kg/cm 2, H2/C02 ratio = 3, flow rate = 30 ml/min, catalyst = 0.5 g CuO/ZnO/Cr203 (55.6:40.0:4.4) + 0.5 g MDC.
REFERENCES 1. J.L.Dubois, K.Sayama, and H.Arakawa, Chem.Lett., (1992) 1115. 2. M. Saito, T. Fujitani, I. Takahara, T. Watanabe, M. Takeuchi, Y. Kanai, K. Moriya and T. Kakumoto, Energy Convers. Mgmt, 36 (1995) 577. 3. T. Inui and T. Takeguchi, Catal. Taday, 10 (1991), 95. 4. H. Arakawa, J.-L. Dubois and K. Sayama, Energy Convers. Mgmt, 33 (1992) 521. 5. M. Fujiwara, R. Kieffer, H. Ando, Y. Souma, Appl. Catal. A, 121 (1995) 113.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
451
C h a r a c t e r i z a t i o n of C O 2 m e t h a n a t i o n catalysts p r e p a r e d f r o m a m o r p h o u s N i - Z r and Ni-Zr-rare earth e l e m e n t alloys M. Yamasaki a *~, H. Habazaki a, T. Yoshida a ,2, M. Komori b, K. Shimamurab, E. Akiyamaa, A. Kawashima a, K. Asami a and K. Hashimoto a alnstitute for Materials Research, Tohoku University, Sendai 980-77, Japan bMitsui Engineering & Shipbuilding Co., Ltd., Ichihara, Chiba 290, Japan
Nano-grained Ni/ZrO 2 and Ni/ZrO2-Srn203 catalysts were prepared from amorphous Ni-Zr and Ni-Zr-Sm alloys by oxidation-reduction treatment. Their catalytic activity for methanation of carbon dioxide was examined as a function of precursor alloy composition and temperature. The addition of samarium is effective in enhancing the activity of the nickel-rich catalysts, but not effective for the zirconium-rich catalysts. The surface area and hydrogen uptake of the nickel-rich catalysts are increased by the samarium addition. In addition, tetragonal zirconia, the formation of which is beneficial to the catalytic activity, is stabilized and formed predominantly by the addition of samarium to the nickel-rich catalysts, although monoclinic zirconia is also formed in the zirconium-rich catalysts. As a consequence, the higher conversion of carbon dioxide is obtained on the Ni-Zr-Sm catalysts with relatively high nickel contents.
1. INTRODUCTION In order to solve global warming induced by enormous amounts of carbon dioxide emission, various types of methods for recovering and chemical fixation of carbon dioxide have been proposed. We are proposing "global CO 2 recycling"[ 1], which consists of electricity generation using solar energy at deserts, H 2 and C H 4 production at coasts close to the deserts, and C H 4 combustion and CO 2 recovery at energy consuming district. Methane is produced by the reaction of recovered CO 2 with H 2 formed by seawater electrolysis using electricity generated at deserts. In order to achieve this global CO 2 recycling, tailoring of highly active catalysts without platinum group elements is needed. It has been found in our recent studies[2-4] that the Ni/ZrOz catalysts prepared from amorphous Ni-Zr alloys by oxidation-reduction pretreatment show high catalytic activity for CO 2 methanation. The turnover number of the catalysts increases with increasing nickel content, while the number of active sites decreases with *~ Graduate Student, Tohoku University ,2 Present address: LSI Logic Japan Semiconductor, Inc., 10 Kitahara Tsukuba, lbaraki 300-32, Japan.
452 increasing nickel content. As a consequence, the highest conversion was obtained on the catalysts prepared from the Ni-Zr alloys containing middle amounts of nickel. The Ni/ZrO 2 catalysts thus prepared contain both monoclinic and tetragonal zirconia, and the relative amount of tetragonal zirconia, with respect to the total amounts of zirconia, increases with increasing nickel content[4]. Therefore, the higher turnover number of nickel-rich catalyst seems to be related to the formation of tetragonal zirconia. Furthermore, the addition of 5 at% or more samarium, which stabilizes tetragonal zirconia, to the Ni-Zr catalysts enhances the catalytic activity. In the present study, the changes in structure and metal dispersion of the catalysts with samarium content have been examined, in order to clarify the role of samarium in the catalytic activity.
2. EXPERIMENTAL Amorphous alloy ribbons of Ni-(40-x)Zr-xSm (x=0, 1, 3, 5, 7 and 10 at.%: nominal composition) and Ni-(25, 35, 45, 55 and 65 at.%)Zr-5Sm were prepared by a melt-spinning method. The amorphous structure of these ribbons was confirmed by X-ray diffraction using Cu K~ radiation. Prior to catalytic reaction, the alloy specimens were oxidized in air at 773 K for at least 5 hours and subsequently reduced in flowing hydrogen at 573 K for at least 5 hours. The catalytic reaction was performed in a fixed bed flow glass reactor of 15 mm inner diameter. The amount of catalyst used was 1.0 g. A gas mixture of CO 2 and H 2 (1:4, volume ratio) was passed continuously on the catalyst with F/W = 1.5ml g-~ s~. The catalytic reaction was preformed at temperatures ranging from 423 to 573 K. After the reaction the gas mixture was analyzed using a Chromopac MicroGC CP2002 gas chromatograph equipped with thermal conductivity detector. Nitrogen and krypton physisorption measurements at 77 K and hydrogen chemisorption measurements at 293 K were carried out with a Belsorp 28SA automatic adsorption apparatus in order to obtain BET surface area and the number of surface nickel atoms, respectively. The structure of catalysts was determined by X-ray diffraction using Cu K~ radiation.
3. R E S U L T S AND DISCUSSION
3.1. Catalytic activity Figure 1 shows the effect of samarium addition on the conversion of CO 2. The conversion of samarium-containing catalysts is almost the same as that of the Ni-40Zr catalysts at and above 523 K. At 473 K the catalysts containing 3 at.% or more samarium show higher conversion than the Ni-40Zr catalysts and the conversion of the former catalysts is independent of samarium content. Figure 2 shows the CO 2 conversion on the Ni-Zr-5Sm catalysts as a function of nickel content. An increase in nickel content leads to a gradual increase in the catalytic activity, in contrast to the samarium-free Ni-Zr catalysts, which show the highest conversion on the catalyst containing middle nickel content. The Ni-25Zr-5Sm catalyst reveals the highest activity in all the catalysts examined. In this experiment, methane selectivity on all the catalysts was practically 100%.
453
100
100
, ;
.
80
-* 8~
OLO~ 60 40
~
:
.--n
f i/ i: i ! i
"~
g
.
:
........... : ~ : ~ 7 3 ~ ; ....... i
..... ~ /
tOO 60 5 2 3
g "~
:
. . . . . . . . . .';". . . . . . . . . . . . . . . . .
40 . . . . . . .
~ 20
~
20[
0 420 440 460 480 500 520 540 560 580
~
0 20
30
Temperature / K
.~ ~9
I
I
_
u
too'
. . . . . 9. . . . . ,
40
50
60
: 70
80
[Ni] / ([Ni] + [Zr] + [Sm]) / at%
Fig.1 Temperature dependence of the conversion of carbon dioxide on the catalysts prepared from amorphous Ni-40Zr and Ni-(40-x)Zr-xSm alloy precursors. F/W = 1.5ml g - l s - I , C O J I - I 2 = 114
..................
423.K . . _
It
Fig.2 The conversion of carbon dioxide on the catalysts prepared from amorphous NiZr (D) and Ni-(25, 35, 45, 55 and 65)Zr-5Sm ( 0 A It O) alloy precursors as a function of precursor alloy composition. F/W = 1.5ml g-ls-l , C O 2 ] I - I 2 --- 114
1
IIIN i [ Q ZrO 2 (tetragonal) ] 9 ZrO2 (monoclinic)
.
.
.
.....
.
.
.
i ................................i...........
z !
.~
"N
i~
N
20
30
40
50
60
70
80
90
2theta / degree
Fig.3 X-ray diffraction patterns of the catalysts prepared from amorphous Ni-50Zr and Ni-45Zr-5Sm alloys.
#o=
0.4
~
0.2
30
40
50
60
70
80
[Ni] / ([Ni]+[Zr]+[Sm]) / at%
Fig.4 Change in the relative peak height of 111 reflection for the tetragonal zirconia in the catalysts prepared from the amorphous alloy precursors as a function of nickel content in the catalysts.
3.2. Characterization of catalysts To clarify the role of samarium, Ni-50Zr and Ni-45Zr-5Sm catalysts were analyzed after the reaction using XRD. The results are shown in Figure 3. In the Ni-50Zr catalyst, there are two types of zirconia, that is, major tetragonal and minor monoclinic zirconia. On the other hand, in Ni-45Zr-5Sm catalyst, tetragonal zirconia was formed predominantly. Figure 4 shows the relative peak height of 111 reflection for tetragonal zirconia at the diffraction angle
454 of about 30 ~ with respect to the total peak heights of 111 reflection for tetragonal zirconia and 1 11 reflection for monoclinic zirconia at the diffraction angle of about 28 ~ as a function of nickel content. This indicates clearly that the samarium addition results in the preferential formation of tetragonal zirconia. Since it has been found that nickel supported on the tetragonal zirconia has higher activity for methanation of CO 2 than that supported on the monoclinic zirconia[4], the stabilization of the tetragonal zirconia by the addition of samarium should be one of the reasons for high activity of the Ni-Zr-Sm catalysts. Table 1 shows H 2 uptake, the number of surface nickel atoms and turnover number. The addition of samarium increases the turnover number as well as the number of surface nickel atoms, i.e., the number of active sites. The increase in the turnover number seems to be mostly due to the predominant formation of tetragonal zirconia. Table 1 Summary of hydrogen chemisorption on the catalysts Specimen
H2 uptake ml gl
Surface Ni atoms 1020 atms g~
Turnover Number / s-1 423K 473K
Ni-30Zr
0.63
0.34
0.0013
0.054
Ni-25 Zr-5 Sm Ni-40Zr
1.96 1.19
1.05 0.64
0.0063 0.0054
0.068 0.048
Ni-35Zr-5Sm
1.94
1.04
0.0051
0.065
4. CONCLUSIONS The conversion of carbon dioxide on the catalysts prepared from nickel-rich amorphous Ni-Zr alloys is improved by the addition of samarium. On the other hand, the activity of the zirconium-rich catalysts is not influenced by the addition of samarium. The predominant formation of tetragonal zirconia in the nickel-rich Ni-Zr-Sm catalysts, in contrast to the formation of two types (monoclinic and tetragonal) of zirconia in the zirconium-rich Ni-Zr-Sm catalysts, appears to be responsible for the higher catalytic activity of the nickel-rich catalysts in addition to their higher surface area than the corresponding samarium-free Ni-Zr catalysts.
REFERENCES [1] K. Hashimoto, Mater. Sci. Engng., A179/A180, 27 (1994) [2] H. Habazaki, T. Tada, K. Wakuda, A. Kawashima, K. Asami, K. Hashimoto, Corrosion, Electrochemistry and Catalysis of Metastable Metals and Intermetallics, The Electrochemical Society, p.393 (1993). [3] K. Shimamura, M. Komori, H. Habazaki, T. Yoshida, M. Yamasaki, E. Akiyama, A. Kawashima, K. Asami and K.Hashimoto, Supple. to Mater. Sci. Eng. A226-228, 376 (1997) [4] M. Yamasaki, H. Habazaki, T. Yoshida, E. Akiyama, A. Kawashima, K. Asami, K. Hashimoto, M. Komori and K. Shimamura, Appl. Catal. A, General, in press
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
455
H y d r o g e n a t i o n of CO2 o v e r Rh Ion E x c h a n g e d Zeolite Catalysts Kyoko K. Bando a, Kensaku Soga b, Kimio Kunimori b, Nobuyuki Ichikuni c, Kiyotaka Asakura d, Kiyomi Okabe e, Hitoshi Kusama e, Kazuhiro Sayama e, Hironori Arakawa e. a Japan Science and Technology Corporation, Hon-cho, Saitama 332, Japan, b University of Tsukuba, Ibaraki 305, Japan, c Chiba University, Chiba 263, Japan, d The University of Tokyo, Tokyo 113, Japan, e National Institute of Materials and Chemical Research, Higashi, Ibaraki 305, Japan. Hydrogenation of CO2 over Rh ion exchanged zeolite catalysts (RhY) was studied. The RhY catalyst showed high activity for hydrogenation of CO2. The main product transformed from methane to CO and formation of ethanol was promoted in accordance with Li addition. Characterization of surface sites and adsorbed species was performed. 1.INTRODUCTION CO2 hydrogenation over supported Rh catalysts has been studied for the purpose of selective production of C2 oxygenates from CO2 and HE. It is reported that Li or Fe promoted impregnated Rh/SiO2 catalysts showed high efficiency for ethanol formation from CO2 and HE [ 1]. So in this work we studied on zeolite supported Rh catalysts (RhY) and Li-promoted RhY (Li/RhY), and elucidated their catalytic activity and the effect of Li addition on oxygenates production. Zeolites have regular pore structure and the size of metal particles produced by ion exchange method can be controlled by the pore diameter and becomes relatively uniform, so it is possible to determine the structure of metal particles in the working state. We will report the study on the catalytic activity of RhY and Li/RhY in connection with the surface structure. 2.EXPERIMENTAL
Ion exchanged catalysts were prepared according to the literature [2,3]. RhCI3"3H20 ( Wako, 99.5%) was used as a metal precursor. NaY zeolite (Nishio, SK-40) was suspended in RhC13 solution and stirred at 363 K over night. Ion exchanged catalyst was filtered, washed, dried and calcined at 673 K for 6 h. The amount of Rh loading was determined from the concentration of remaining Rh in the filtered solution, and it was 5-6 wt%. Li addition (Li/Rh
456
=0-15) was carried out by impregnation. The calcined Rh ion exchanged zeolite catalyst was immersed in LiNO3 solution and calcined at 673 K for 6h. CO2 hydrogenation was carried out by a fixed bed flow reactor for high pressure conditions. Prior to the reaction, the catalyst was
1o -(a) 8 .sr~ 6 4 O 2 r (3 0 I
/ + 6w o (~ ---O- 6wt%RhY
,
0
reduced in H2 at 723 K for 0.5 h. The reaction
I
,
I
,
I
400 800 time / min
1200
condition was as follows; 423 - 523 K, H2/CO2 =3, 3 MPa, 100ml/min. The products were analyzed by on-line gas chromatographs. XRD, XPS,
b= 60 -(b) 9
4O
EXAFS, in-situ FI'-IR, and chemisorption (H2 and CO) were performed for characterization of catalysts.
"g 20 9
r162
3.RESULTS
AND
over
I
0
DISCUSSION
3.1.CO2 H y d r o g e n a t i o n
0
RhY
Fig.1 and table 1 show the result of CO2 hydrogenation over 6wt%RhY, compared with a
,
I
,
I
400 800 time/min
,
I
1200
Fig.1 CO2 Hydrogenation over 6wt%RhY and 6wt%Rh/SiO2. Reaction condition; catalyst lg, H2/CO2 = 3, 3 MPa, 423 K, 100ml/min. Intervalsindicate the I-t2reduction treatment.
conventional impregnated 6wt%Rh/SiO2 catalyst. The RhY catalyst showed CO2 conversion as high as 6 % at 423 K. The main product was methane. Production of methanol was observed at such low temperature. The amount of produced methanol was relatively large compared with the impregnated 6wt% Rh/SiO2 catalyst. However, formation of methane was so promoted that the selectivity for methanol was only 2%. A serious deactivation occurred about 1 h after the reaction started. However, activity was fully recovered by reduction in H2 at 523 K. Therefore, it is concluded that the Rh metal particle size in the working state was identical with that obtained after reaction. We carried out volumetric H2 adsorption and observed TEM for both before reaction and after reaction. The Rh particle size determined both method were shown in table 1. This result suggests that before reaction Rh partiTable 1 The results of CO2 hydrogenation and the particle size. Sample CO2 Hydrogenation Particle size / nm c02 Conversion Selectivity/% Before Reaction After Reaction /% CH4 CH3OH Ha adsorptioo TEM H2 adsorption TEM 6wt% RhY 5.9* 99.8 0.1 1.3 1.3 2.7 3.3 2.8** 97 1.9 6wt% Rh/SiO2 0.2 100 0 3.8 Reaction condns." catalyst 1 g, 423 K, H 2 / C O 2 - 3, 3 MPa, 100ml/min. *When CH4 yield showed the maximun value. ** When CH3OH yield showed the maximum value.
457
cles were produced inside zeolite cages, but during the reaction aggregation occurred and Rh metal particles about 3.3 nm in diameter formed outside the cage and worked as reaction centers. Deactivation was attributed to accumulation of produced H20 in the zeolite cages in accompany with methane and/or methanol production, because catalytic activity was easily recovered by
25r- ~
reduction. The high activity for CO2 hydrogenation
20
Q~,
!~
CH4
~
Q m
CH30H_ C2HsOH -
--
100
was due to the zeolite cage which condensed CO2 in it and supplied CO2 to active Rh sites. Such promotion effect was suppressed by water accumulation inside
~0
40~
the cages. 0
3.2. CO2 Hydrogenation over Li/RhY
....
1 ~
0
]Li/R~rati ]o
lO
I
Fig.2 CO2 hydrogenation over Li/RhY (Li/Rh = 1, 3, 10). Reaction condition; catalyst lg, H2/CO2 = 3,523 K, 3 MPa, I00 ml/min.
3.2.1. Catalytic Activity We prepared Li-impregnated RhY catalysts (Li/ RhY, Li/RhY= 1-15, Rh concentration - 5wt%) and
D CH4 N CH3OH .1 I C2H5OH] 80
%
carried out CO2 hydrogenation at 523 K. The result is shown in figure 2. The main product transformed from methane to CO in accordance with Li addition.
~
r', co
10
/
9 .,..~
Finally CO became dominant (90% selectivity) on Li/
4 0 .< .
RhY (Li/Rh - 10). Simultaneously, promotion of not
= 3 5
only methanol but also ethanol was observed. It is a
9
peculiar phenomenon that production of ethanol was enhanced without methane formation though ethanol should be generated from surface methyl groups. We expected that gas phase CO which was produced during the reaction had some contribution to ethanol
2O ~ " ~ m m
m
CO2+H2
CO+CO2+H2
Fig 3. CO2 and CO+CO2 hydrogenation over Li/RhY (Li/Rh = 10). Reaction condition; H2/CO2 = 3, 3 MPa, 100 ml/min, 523 K,CO = 1.8 %.
formation. So we mixed 1.8% of CO with H2+CO2 (H2/CO2- 3), and carried out the reaction at 523 K under 3 MPa. The result was depicted in figure 3. Over Li/RhY (Li/Rh - 10) ethanol production was promoted and selectivity for ethanol became as high as 16%, although no notable change was observed for RhY, over which methane was the dominant product. Enhancement of methane formation by CO Table 3 The results of curve-fitting analysis of EXASF spectra and H2, CO adsorption observed on RhY and Li/RhY (Li/Rh= 10, Rh - 5wt%). Sample EXAFS analysis Number of H and CO adsorbed Condn. bonding CN distance/nm H/Rh CO/Rh RhY RhY H2 reduction Rh-Rh 7.6 0.266 0.95 CO adsorption Rh-Rh 4.2 0.269 1.54 Li/RhY H2 reduction Rh-Rh 7.0 0.267 0.75 CO adsorption Rh-Rh 6.0 0.268 0.58
458
addition was also observed on Li/RhY, To elucidate the
0.5
2016
mechanism of ethanol promotion observed on Li/RhY, we performed characterization of Li/RhY comparing with
4
RhY.
o.o ..Q 9
3.2.2. C h a r a c t e r i z a t i o n At first we observed XRD to confirm whether any
-o.5 /
able growth of metal particles was observed upon Liobserve modification of charge of surface Rh species by Li addition, which might verify reactivity. However, Rh
1615
<
aggregation was caused by Li addition or not. No detectdoping. We also carried out XPS measurements to
I
I i ,,, 2ooo 15oo Wavenumber / cm-1
Fig.4 IR spectra of CO2 adsorption at 303 K on reduced (a) RhY and (b) Li/RhY. Rh = 5wt%, Li/Rh = 10.
species were found to be in metallic state and the observed binding energies were the same within experimental errors, regardless of the amount of Li loading. Since we could not find out any obvious modification of Rh attributed to Li either by XRD or XPS, we performed EXAFS measurements to investigate the influence of Li addition to coordinative environment around Rh. The result was shown in table 3. It was found that on RhY coordination number (CN) of Rh - Rh was drastically decreased by CO addition because of carbonyl cluster formation, meanwhile, only sight decrease was observed for CN on LiRhY. This suggests either that doped Li prohibited CO to be adsorbed on Rh or that Li bound Rh so strongly that dispersion of Rh by CO adsorption was suppressed. We carried out volumetric adsorption of H2 and CO. The results are shown in table 3. The amount of both adsorbed H and CO per unit Rh atom decreased with Li loading, which suggests physical blocking of Rh by Li. Moreover, CO/H ratio adsorbed on unit Rh atom was also decreased, which implies that Li addition may generated different type of adsorbed CO species. To confirm these expectation, we observed in-situ FT-IR of adsorbed CO species which was generated by CO2 introduction onto reduced catalysts. As depicted in figure 4 (a), when CO2 was adsorbed on reduced RhY, bands assigned to terminal CO species were observed at 2016 cm -]. Meanwhile, completely different spectrum was observed when CO2 was introduced on reduced Li/RhY as shown in figure 4 (b), that is, new peak appeared at 1615 cm -1. According to the literature, this band was assigned to either side-on CO or q2-coordinated CO2 on Rh. We speculated that adsorbed CO (or CO2 ) species appeared on Li/RhY were rather stable and hard to be subjected to dissociative hydrogenation. As the result, these CO species promoted formation of acyl species and ethanol formation. REFERENCES 1 H.Kusama, K Okabe, K Sayama, H. Arakawa, Catal. Today, 28 (1996), 261. 2 R. D. Shannon, J. C. Verdine, C. Naccache, and F. Lerebevre, J. Catal., 88, 431, (1984). 3 A. Fukuoka, L.-F. Rao, and M. Ichikawa, Nikkashi, 561, (1989).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
459
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Interconversion of Ru-CO and Ru-rll-CO2 through reversible oxide transfer reaction K. Tsuge, K. Tanaka and H. Nakajima Institute for Molecular Science, Myodaiji, Okazaki 444, Japan
1. I N T R O D U C T I O N Much attention has been devoted to the activation of CO2 with metal complexes. A variety of metal complexes have been used for the reduction of CO2, in which metal-rl 1-CO2 complexes function as precursors for CO2/CO conversion. In protic media, CO2/CO conversion on metal complexes generally takes place through acid-base equilibria among [MrI1-CO2] n+, [M-rI1-C(O)OH](n+I) +, and [M-CO](n+2) + (eq 1). 1 In the absence of proton donors, [M_TII(C)_CO2]n+ --.~
H+
"-
H+
" - [ M _ C O ] (n+2)+ (1) OH OH CO2/CO conversion is achieved by oxide transfer reactions from metal-CO2 to various oxide acceptors such as CO2, 2 another metal-CO2, 3 adjacent oxophilic metals 4 and PR3 ligands. 5 Among these reactions, the oxide transfer from metal-CO2 to CO2 (eq 2) serves for CO generation by reductive disproportionation of CO2 (eq 3).6 [M(CO2)] n+ + CO 2 _.~ ~ [M-CO] (n+2)+ + CO32 (2) 2CO 2 + 2e-
[M_COOH](n+l)+ - ~
~
CO + CO32-
(3)
Highly reduced rl 1-CO2 complexes, [W(CO)5(rl 1-CO2)] 2-, [CpFe(CO)2(rl 1-CO2)]- and [Ru(bpy)2(qu)(rll-CO2)] react with CO2 to produce W(CO)6, [CpFe(CO)3] + and [Ru(bpy)2(qu)(CO)] 2+, respectively (eq 2). 2 To promote smooth CO2/CO conversion in aprotic media, characters of M-CO2 bond should be revealed. The knowledge on the bond character, however, is quite limited due to the lack of well characterized metal complexes with rl 1-CO2 ligands. This paper describes the interconversion of Ru-CO and Ru-CO 2 complexes through a reversible oxide transfer. 2. E X P E R I M E N T AND R E S U L T S 2 . 1 . Oxide transfer from CO3 2- to [ R u ( b p y ) 2 ( C O ) 2 ] 2+.
We conducted the reaction between [Crown~ (Crown = 18-crown-6 ) and [Ru(bpy)2(CO)2] 2§ in dry CH3CN. A colorless CH3CN solution of [Ru(bpy)2(CO)2](PF6)2 rapidly turned to yellow by an addition of equimolar amount of [Crown~ and the yellow solution subsequently
changed to greenish red. The change of color almost ceased in five minutes. The 1H_NMR spectra of the greenish red CD3CN solution showed the existence of two nonequivalent bpy
460 ligands suggesting the formation of a
cis-Ru(bpy)2 moiety with two other different ligands.
The solution also exhibited two non-aromatic signals at ~5= 202.8 and 201.8 ppm in the 13CNMR spectra, and the carbonyl signal of the starting material, [Ru(bpy)2(CO)2] 2+ (~5 = 190.4 ppm) completely disappeared. The IR spectra of the greenish red solution displayed two strong new bands at 1968 and 1620 cm"t and a very sharp band at 2342cm "~.
23
II V
1968cm'l
1620 cm" ! ......
|
I
2000
Figure
t
1500
L
1000
/cm-t
IR spectrum of the greenish red solution
This sharp band shows the generation of free CO2. It has been reported that M-rl ~-CO2 ligand shows a Vasym(COO) band near 1600cmt. The 1968 and 1620cmt bands of greenish red complex can be assigned to v(C-O) and Vasym(COO) bands, respectively. The carbonyl complex reacts with carbonate to afford CO2 and the greenish red complex where the carbonyl ligands should be transformed. Therefore, the oxide transfer reaction from CO32" to [Ru(bpy)2(CO)2] 2+ takes place and carbon dioxide complex [Ru(bpy)2(CO)(CO2) ] and CO 2 are generated (eq. 4) [Ru(bpy)2(CO)2] 2+ + CO32 ~ [Ru(bpy)2(CO)(CO2)] + CO 2 (4) 2.2 Oxide transfer reaction from [Ru(bpy)2(CO)(CO2) ] to CO 2. A greenish red CH3CN solution of [Ru(bpy)2(CO)(CO2)] prepared by a 1"1 mixture of [Ru(bpy)2(CO)2] 2+ and [Crown'K]2CO3 (eq 4) did not show any appreciable change by bubbling CO2 into the solution. Further addition of 5 molar excess of LiCF3SO3 to the solution resulted in a gradual disappearance of the greenish red color accompanied with the generation of white precipitation. The rate of the reaction was slower than that of the above reaction, and it took almost one day until the greenish red color disappeared. The IR spectrum of resulting colorless solution showed that the CO2 complex disappeared and that [Ru(bpy)2(CO)2] 2+ was reformed in the solution. The white precipitation was characterized as Li2CO3 by IR spectrum. As a result, the carbon dioxide complex reacts with CO2 in the presence of Li§ ion, to afford carbonyl complex and CO32. (eq. 5) [Ru(bpy)2(CO)(CO2)] + 2Li § +CO 2 ~ [Ru(bpy)2(CO)2] 2§ + Li2CO3 (5) Since the carbonyl complex, [Ru(bpy)2(CO)2] 2+ was not regenerated by an addition of either Li+ or CO2 to the CH3CN solution containing [Ru(bpy)2(CO)(CO2)] and [Crown-K] +, Li+ is an essential component for the oxide transfer from [Ru(bpy)2(CO)(CO2)] to CO 2 (eq 5). 3. D I S C U S S I O N 3.1 I n t e r c o n v e r s i o n of Ru-CO and Ru-rll-CO~ complexes. We have reported the oxide transfer reaction from carbonate to [Ru(bpy)2(CO)2] 2+ in DMSO solution as the first
461 example of oxide transfer reaction from carbonate to carbonyl complex. 7 In CH3CN solution, the same reaction occurs also, and the reverse reaction, i.e., the oxide transfer reaction from MCO 2 complex to CO 2 takes place. Though the interconversion of M-CO 2 and M-CO complexes plays a key role in the reductive disproportionation of CO 2, there have been only two examples of this reaction. It has been reported that the unstable iron and tungsten C O 2 complexes decompose in the presence of CO2, to afford the corresponding carbonyl complexes and CO32. The reaction between these ruthenium complexes is the first example of the reversible interconversion of M-CO 2 and M-CO complexes, accompanied by the interconversion of CO 2 and CO32. Since the yellow intermediate was observed in the course of the oxide transfer reaction from carbonate to M-CO complex, the reaction should proceed via two steps (eq. 6). [Ru-CO] 2§ + CO32 ~ [carbonate adduct] ~ [Ru-CO] + CO 2 (6) The yellow intermediate was also observed on the reaction in DMSO solution. We studied kinetics of this reaction in DMSO solution and proposed the head-to-tail as the intermediate.
7Ru CO ~~] C~-O Scheme
"7 2+
+
~176 I 0
CO ~ U~c'O(a)
_1o
g c.O I 0
7o
~ CO ~'v ~~..,]u ' c ' O
"o
+ O=C=O
When the bond cleavage takes place at (a) in the scheme, the M - C O 2 complex and C O 2 a r e generated and when it takes place at (b), the M-CO complex and CO32 are generated from this intermediate. The C204 moiety has been referred as the intermediate of the intra-molecular O2 scrambling reaction, and the C204 ligand has been found in the iridium complex [IrC1(C 204 )(PMe 3)3] .7 Since the reverse reaction proceeds by the removal of CO32 from solution as LiE C O 3 precipitation, the interconversion of the ruthenium M-CO 2 and M-CO could be regarded as very shifted equilibrium. We have already reported that these ruthenium carbon dioxide and carbonyl complexes are in an acid-base equilibria (eq. 7). OH OH [Ru(bpy)2(CO)2] 2§ [Ru(bpy)E(CO)(COOH)] § --" "-- [Ru(bpy)2(CO)(CO2)] (7) H§ H§ These ruthenium M-CO and M-CO 2 complexes are the first complexes which interconverse each other both in protic and aprotic solvent. The comparison of eqs. 6 and 7 indicates that the role of CO32 and CO2 in acetonitril solution corresponds to that of OH and H § in aqueous solution, respectively. The proposed head-to-tail adduct corresponds to hydroxycarbonyl complex in which the position of the cleft bond determines the product; when the C(O)-OH bond cleaves, the M-CO complex is generated, and when the C(O)O-H bond cleaves, the MCO 2 complex is generated (eq. 7). The interconversion of Ru-CO 2 and Ru-CO in protic
462 media is the key step of water-gas-shift reaction, and it is the key step of the reductive disproportionation of CO2 in aprotic media. The interconversion of both Ru-CO 2 and Ru-CO in protic media can be explained by the oxide transfer reaction from M-CO 2 to Lewis acid; H § leads the water-gas-shift reaction and CO 2 leads the disproportionation of CO 2. 3.2 Direction of oxide transfer. The other two examples of conversion of M-CO 2 and M-CO only proceed from M-CO 2 to M-CO. In those cases, the highly reduced M-CO 2 moiety easily releases the oxide ion to give M-CO complex and CO32. For the iron and tungsten complexes, the combination of M-CO and CO3v is more stable than that of M-CO 2 and CO 2. However, for the ruthenium complex, the combination of M-CO 2 and CO 2 is more stable. The reaction of Ru-CO and CO32 proceeds spontaneously to give Ru-CO 2 and CO 2, while the reverse reaction is accomplished only by the addition of lithium salt. In the case of iron and tungsten complexes, the reaction proceeds due to the high basicity of rl ~-CO2 moiety. The basicity of Ru-CO 2 complex, however, seems to be slightly smaller than that of CO3v, since the pKa value of the conjugate acid, HCO32 (10.3) is larger than that of [Ru(bpy)2(CO)(COOH)] § (9.5). The relatively basic CO32 passes its oxide ion to the Ru-CO complex, and CO2 and less basic Ru-CO 2 complex are generated as the result. The direction of the interconversion of M-CO2 and M-CO complex should be mainly determined by the difference of the basicity of M-CO~ and CO3v, or M-CO and CO2. When the M-CO~ is more basic than CO32, M-CO2 gives its oxide ion to CO 2 to become M-CO with the generation of CO3v (in case of the iron and tungsten complexes). When CO32 is more basic than M-CO 2, CO32 gives its oxide ion to M-CO complex to become CO2 with the generation of M-CO 2 complex (in case of the ruthenium complex). REFERENCES
1. (a) K. Tanaka, M. Morimoto and T. Tanaka, Chem. Lett., (1983) 901. (b) H. Ishida, K. Tanaka, M. Morimoto and T. Tanaka, Organometallics, 5 (1986) 724. (c) S. Matsuoka, K. Yamamoto, T. Ogata, M. Kusaba, N. N akashima, E. Fujita and S. Yanagida, J. Am. Chem. Soc., 115 (1993) 601. 2. (a) G. R. Lee and N. J. Cooper, Organometallics, 4 (1985) 794. (b) J. M. Maher and N. J. Cooper, J. Am. Chem. Soc., 102 (1980) 7606. (c) H. Nakajima, Y. Kushi, H. Nagao and K. Tanaka, Organometallics, 14 (1995) 5093. 3. R. Alvarez, J. L. Atwood, E. Carmona, P. J. Perez, M. L. Poveda and R. D. Rogers, Inorg. Chem. 30 (1991) 1493. 4. (a) J. C. Bryan, S. J. Geib, A. L. Rheingold and J. M. Mayer, J. Am. Chem. Soc., 109 (1987) 2826. (b) C. Floriani, Pure & Applied Chem., 54 (1982) 59. 5. M. Aresta and C. F. Nobile, Inorg. Chim. Acta., 24 (1977) LA9. (b) T. Ito and A. Yamamoto, J. Chem. Soc., Dalton Trans., (1975) 1398. 6. C. Amatore and J.-M. Saveant, J. Am. Chem. Soc., 10 3 (1981) 5021. 7. H. Nakajima, K. Tsuge and K. Tanaka, Chem. Lett., (1997) 485. 8. T. Herskovitz and L. J. Guggenberger, J. Am. Chem. Soc., 9 8 (1976) 1615.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
463
P h y s i o l o g i c a l p r o p e r t i e s of p h o s p h o e n o l p y r u v a t e carboxylase and p h o s p h o e n o l p y r u v a t e c a r b o x y k i n a s e f r o m Rhodopseudomonas sp. No. 7 Takaaki Fujii, Megumi Sadaie, M a s a a k i Saijou, Takanari Nagano, Tomoaki Suzuki, Masahiro Ohtani and Hirofumi Shinoyama Department of Bioproduction Science, Faculty of Horticulture, Chiba University, 648 Matsudo, Matsudo-shi, Chiba 271, J a p a n 1.
INTRODUCTION
Purple nonsulfur bacteria absolutely require a CO2 source such as bicarbonate for growth when they are incubated in a medium containing ethanol under photoanaerobic conditions. On the contrary, their growth on acetate is independent of bicarbonate. These facts suggest that the Calvin Benson cycle operates in cells growing on ethanol, but not in cells growing on acetate [1-3~ . Therefore, it is expected that the carbon metabolism in the ethanol culture of the purple nonsulfur bacteria is more intricate than that of the acetate culture although both ethanol and acetate are metabolized via acetyl CoA. However, information on the regulation between the Embden Meyerhof Parnas pathway-Calvin Benson cycle side and the TCA cycle-glyoxylic acid cycle side in purple nonsulfur bacteria is limited [4-8~ . Activity-levels of phosphoenolpyruvate carboxylase (EC 4.1.1.31, PEPC) and phosphoenolpyruvate carboxykinase (EC 4.1.1.49, PEPK) were examined with Rhodopseudomonas sp. No.7 grown photoanaerobically in an ethanol-bicarbonate and in an acetate medium. PEPC and PEPK were purified from cells grown under these conditions, and several characteristics of the enzymes were discussed in connection with photoheterotrophy of purple nonsulfur bacteria. 2.
METHODS AND MATERIALS
2.1. O r g a n i s m , m e d i u m , a n d g r o w t h No.7 and its maintenance were described containing both 0.1% ethanol and 0.2% acetate was used for the main culture. screw-capped 1.5 liter Roux bottles filled conditions.
conditions. Rhodopseudomonassp. previously [2~ . A basal salt medium sodium bicarbonate or 0.27% sodium The organism was grown at 30~ in with the medium under photoanaerobic
2.2. E n z y m e a s s a y s . The activity of PEPC was assayed by a coupled system in which the oxaloacetate formed was reduced to malate by NADH [8~ . The activity of PEPK was assayed principally according to the method for the activity of PEPC except for the addition of ADP and aspartate.
464
2.3. P u r i f i c a t i o n of P E P C a n d P E P K . PEPC was purified by the method described previously [8~ . PEPK was purified principally according to the method for the purification of PEPC. The enzymes were purified as chromatographically, electrophoretically and isoelectrophoretically homogeneous proteins. 3.
RESULTS
3.1. P E P C a n d P E P K a c t i v i t i e s i n R h o d o p s e u d o m o n a s sp. No.7 grown on e t h a n o l or a c e t a t e . The maximum growth of Rhodopseudomonas sp.No.7 a t t a i n e d in the 0.1% (w/v) ethanol medium, when sodium bicarbonate was added to the medium to give the concentration of 0.2 %. On the other hand, the growth in the acetate medium was independent of bicarbonate. When the organism was incubated in a medium containing 0.27 % sodium acetate which was almost equivalent to the carbon m a s s of the 0.1% ethanol-0.2% bicarbonate medium, the maximum growth attained in the acetate medium was similar to t h a t of the ethanol medium. The activity of PEPC in the ethanol-grown cells showed about 3 times higher level t h a n t h a t of the acetate-grown cells (Table 1). There was a significant difference between the activity ratio of PEPC to PEPK in the ethanolgrown cells (11.6) and t h a t in the acetate-grown cells (2.76). Table 1 Effect of c a r b o n sources on activity levels of P E P C a n d P E P K in sp. No.7
Rhodopseudomonas
Carbon source for growth
Specific activity (nmol/min/mg)
Rate of activity (PEPC/PEPK)
PEPC
PEPK
Ethanol + bicarbonate
42.4
3.66
11.60
Acetate
14.1
5.11
2.76
Reprinted from: M. Sadaie,T. Nagano,T. Suzuki,H. Shinoyama,and T. Fujii, Biosci. Biotech. Biochem., 61,625(1997). 3.2. P r o p e r t i e s of P E P C . The molecular weights of the purified enzyme and its subunit were e s t i m a t e d to be about 400,000 and about 102,000, respectively. The optimum pH and t e m p e r a t u r e of the activity were about 9.0 and 50~ respectively. The p / o f the enzyme was about 5.8. The enzyme required Mg 2§ or Mn 2§ for appearance of the activity. The enzyme activity in the presence of 2 mM Mn 2§ was about 25 times higher t h a n t h a t of 2 mM Mg 2§ But an allosteric effector, acetyl CoA, acted on the enzyme more effectively in the presence of Mg 2§ t h a n Mn 2§ (Figure 1). When the enzyme was incubated with 20 mM Mg 2§ the Kms against bicarbonate and PEP decreased to about 1/2 (1.7 mM) and 1/20 (0.23 mM) in the presence of 40 tt M
465 acetyl CoA, respectively, and the Vmax increased to about 30 times (90 tt mol/mirgmg protein). The enzyme was inhibited by aspartate (/~, 0.208 mM). Besides, the enzyme of strain No.7 was strongly inhibited by ATP and GTP (Table 2). The enzyme was also inhibited by ADP. This inhibition was completely reversible. The enzyme activity was hardly affected by various concentrations (0-10 mM) of fructose- 1,6-bisphosphate (FBP).
Table 2 Effect of various nucleotides on PEPC and PEPK activity Nucleotides (4mM) None ATP ADP GTP GDP
Relative activity (%) PEPC 100 4.8 35 12 94
PEPK 0
100 0 0 0
3.3. P r o p e r t i e s of P E P K . The molecular weights of the purified enzyme and its subunit were estimated to be about 120,000 and about 60,000, respectively. The optimum pH and temperature of the activity were about 6.0 and 50 to 60~ respectively. The p/of the enzyme was about 5.7. PEPK absolutely required Mn 2§ for the appearance of its activity. Although the activity of PEPC was reversibly inhibited by ATP(ADP), the activity of PEPK appears only in the presence of ATP(ADP) (Table 2). The activity was not affected by acetyl CoA and aspartate. The decarboxylation activity of PEPK was about three times higher than the carboxylation activity (Figure 2). The Kms against oxaloacetate and ATP were calculated to be 0.31 mM and 0.38 mM in the decarboxylation reaction, respectively. The Vmaxwas 82 tt mol/min/mg protein.
100/ ~o
.
60,]:
[]
___--~
f
0
~100
v .~
60 9
20
40 20
oI
I
10
I
I
I
20 30 40 Acetyl CoA (,aM)
50
Figure 1 Effect of Mg 2+ or Mn 2+ on activation of PEPC by acetyl CoA ( 9 2raM Mg 2+
(m), 20raM Mg 2+
(0), 0.5raM Mn 2+ (o), 2raM Mn 2+
o
1 2 Enzyme conc. (tz g/ml)
3
Figure 2 Decarboxylation and carboxylation activity of PEPK (O), Carboxylation activity ( 0 ) , Decarboxylation activity
466
4.
DISCUSSION
PEPC and PEPK, which connect the EMP pathway-Calvin Benson cycle side and the TCA cycle-glyoxylic acid cycle side, were detected in the cell-free extracts of Rhodopseudomonas No.7 grown on C2-compounds. There was a significant difference between the ethanol-grown and acetate-grown cells in respect to levels of PEPC and PEPK. The ethanol-grown cells had a higher level of PEPC than that of the acetate-grown cells. The difference of the carbon assimilation observed between the ethanol culture and the acetate culture of strain No.7 has not been reported in nonphoto-heterotrophic microorganisms. PEPCs from enterobacteria such as Salmonella typhimurium and Escherichia coli are activated by acetyl CoA and inhibited by aspartate. Some properties of the enzyme from Rhodopseudomonas sp. No.7 were similar to those of the enterobacteria. However, the strain No.7 enzyme was different from enzymes of the enterobacteria in important points of metabolic regulation. It have been reported that the enzyme activities of the enterobacteria were increased by FBP and GTP. In contrast with this, the activity of the No.7 enzyme was not affected by FBP and reversibly inhibited by GTP and ATP (ADP). This differences may signify roles of the enzyme of strain No.7 related to the ethanol-assimilation and the enzymes of the enterobacteria to the glucose-assimilation in their own way. It is interesting in connection with the assimilation of ethanol and acetate by purple nonsulfur bacteria that the activity of PEPC from Rhodopseudomonas sp. No.7 was reversibly inhibited by ATP(ADP), but the activity of PEPK from strain No.7 appears only in the presence of ATP(ADP). Besides, it was found that the decarboxylation activity of PEPK was higher than the carboxylation activity. Although further investigation is necessary to clarify whether regulatory properties of PEPC and PEPK from strain No.7 are closely related to photoheterotrophic physiology or not, these enzyme seems to play significant roles in the photoheterotrophic growth of strain No.7 on ethanol and acetate. REFERENCES 1. G.A.Sojka, in "The Photosynthetic Bacteria," ed. by R.K.Clayton and W.R.Sistrom, Plenum Press, New York, 1978, pp. 707-718. 2. T.Fujii, A.Nakazawa, N.Sumi, H.Tani, A.Ando and M.Yabuki, Agric. Biol. Chem., 47, 2747 (1983). 3. K.Isida, H.Shinoyama and T.Fujii, Seibutsukougaku, 71, 397 (1993). 4. J.Payne and J.G.Morris, J. Gen. Microbiol.,59, 97 (1969). 5. H.Albers and G.Gottschalk, Arch. Microbiol., 111, 45 (1976). 6. R.C.Fuller, in "The Photosynthetic Bacteria," ed. by R.K.Clayton and W. R. Sistrom, Plenum Press, New York, 1978, pp. 691. 7. J.C.Willison, J. Gen. Microbiol., 134, 2429 (1988). 8. M.Sadaie, T.Nagano, T.Suzuki, H.Shinoyama and T.Fujii, Biosci. Biotech. Biochem., 61, 625 (1997).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
467
Production of alkane and alkene from CO2 by a petroleum-degrading bacterium strain HD-1 M. Morikawa', T. Iwasa', K. Nagahisa 2, S. Yanagida' and T. Imanaka3 'Dept. Material & Life Science, 2Dept. Biotechnology, Osaka University, Suita, Osaka 565, and 3Dept. Synthetic Chemistry & Biological Chemistry, Kyoto University, Kyoto 606-01, Japan. 1. I N T R O D U C T I O N Alkane/alkene are little but widely distributed among all types of organisms including bacteria, fungi, algae, higher plants, and animals. Based on the labeling experiments in higher plant tissues, it was proposed that alkane/alkene are biologically synthesized by elongation of a fatty acid followed by the loss of the carboxyl carbon [1]. On the other hand, hydrocarbon distribution has been also reported for several bacteria. However, the mechanism of bacterial alkane/alkene production has not been well studied. Here we demonstrate that a new type of petroleum-degrading bacterium, strain HD-1, produces medium chain alkane/alkene from CO2, and that fatty acid and aldehyde are utilized as precursor. 2. E X P E R I M E N T A L S 2-1. Growth conditions
The strain HD-1 was grown on basal medium (BM); 0.5% (NH4)2SO4, 0.1% KH2PO4, 0.05% MgCI2-6H20, pH 7.0, with continuous supply of anaerobic gas mixture (CO2/HZ/N2 = 5:5:90) in the fermentation system. 2 - 2 . Preparation and a n a l y s e s o f h y d r o p h o b i c fraction o f the cell
Cells grown under autotrophic growth condition (in BM) were disrupted by sonication for 1 min five times, and the hydrophobic fraction was extracted with pure redistilled chloroform. Acetone insoluble fraction of chloroform extracts was analyzed by TOF-MS (KOMPACT MALDI II, Shimadzu, Kyoto, Japan) and its methanolysate was analyzed by GC/MS(EI) (JMS-DX303, JEOL, Tokyo, Japan). Acetone soluble fraction was directly analyzed by GC/MS(EI). HD-1 cell extract was prepared as follows; autotrophically grown cells were centrifuged, washed, freeze-dried and weighed. 2 - 3 . A l k a n e / a l k e n e p r o d u c t i o n from aldehyde or fatty acid by H D - 1 cell lysates
Cells were suspended in 0.1M sodium phosphate buffer (pH 7.0) and sonicated for 5 min in an ice bath to prepare the cell lysates. The reaction mixture contained 160 laM 14C(U)hexadec,anal or 14C(U)-palmitic acid (850mCi/mmol), 0.1 M sodium phosphate (pH 7.0), 0.1% Triton X-100, and 159 mg (dry wt.) cell extracts in a total volume of 25 ml. '4C(U)hexadecanal was chemically synthesized from '4C(U)-palmiticacid. Reaction mixture from which HD- 1 cell lysates were omitted was used as control samples. The reaction mixture was bubbled with Ar gas for several minutes and then the vessel was closed tightly.
This work has been supportedby Program for Promotionof Basic ResearchActivities for InnovativeBiosciences.
468 After anaerobic incubation of reaction mixture at 37*(2 for 24h, the alkane containing fraction was prepared by chloroform extraction. Chloroform extracts were separated on silica gel 60 thin layer chromatography (TLC) plate with hexane as a developing solvent. Radioactivity was measured by liquid scintillation counter (LS6500, Beckman). 3. R E S U L T S A N D D I S C U S S I O N 3-1.
Autotrophic
growth of the strain HD-I
The strain HI)- 1 had been isolated as a facultative anaerobic petroleum-degrading bacterium from an oil field [2]. The bacterium could degrade tetradecane to 1-dodecene anaerobically, which suggested a new alkane oxidation pathway without oxygen [3]. The strain HD-1 was found to grow on the BM with continuous supply of anaerobic gas mixture (CO2/H2/N2=5:5:90) in the fermentation system (Fig. la). Both CO2 and H2 were necessary for cell growth. The reason was not clear but NaHCO3 was a poorer carbon source than CO2 gas. Since the strain normally grew under the dark condition, it seemed not to utilize light energy. These findings demonstrated that strain HD-1 fixed CO2 by utilizing H2 as an energy source. Nitrogen gas was examined for the nitrogen source of the strain HI)-1 (Fig. l b). Even when ammonium sulfate was replaced by sodium sulfate, cell growth was observed. This fact shows that the strain HD-1 is able to fix nitrogen gas, too.
0.1 5
0.15 BM
(N2/C 0
8
0.10
8
0.05
Of
0
0
......
~
~
2
4
u
6 Days
8
0.1 0
0.0 5
BR
10
0
4
6
Days
(a)
(b) Fig. 1
3-2. Hydrophobic
2
Growth curve of the strain HD-1
fraction o f the strain HD-1
Even after cells were successively grown in BM, 10 to 30% of HD-1 dry cell weight was chloroform/methanol extractable lipophilic fraction with no addition of organic carbons to the media. Electron microscopic observation of the cells showed that the cell surface was rather irregular, and many granules were accumulated inside the cell (Fig. 2).
469
' ~,:
,~
Fig. 2
Taking into consideration that the bacterium was isolated from an oil field, the strain was expected to produce petroleum from CO2 under anaerobic conditions. The major part of the hydrophobic fraction prepared by chloroform extraction was biological polyester which was insoluble in methanol and soluble in acetone. The molecular mass of the polymer was over 400kDa. According to the GC/MS(EI) analysis data (figure not shown), it was found that this bacterium accumulated poly-13-hydroxy alkanoic acid (PHA) as reported for several bacteria. When the cells were grown on BM agar plate on which crude oil membrane was formed beforehand, a clear oil displaced zone was appeared around the colony. This phenomenon indicated that the strain HD1 produced surface active reagent (biosurfactant) as is often the case with oil-assimilating bacteria [4].
,,~ I
Electron micrographs of strain HD-1 cells.
3 - 3 . A l k a n e / a l k e n e in the strain H D - 1 cell
I
(
I IT
o 7 ci8
9
I
] l
c12
I
Itll Ill,, ..... I
l
[
[ C16c17
~2g
C28
C30
C30
10 |
20 Retention time
Fig. 3
(a)
3'0 (rain)
4'0
50
0
10
20
I
I
Retention
30 I
time
I
40
--
(min)
(b)
Gas chromatograph of HD-I cell (a) and crude oil (b) Cn; Carbon number of alkanes are shown.
When the hydrophobic fraction was analyzed by silica 60 TLC, quite apolar substances were found at the front of the developing solvent (chloroform/methanol/water=-65:25:4). Therefore, we examined whether or not the strain HD-1 produced hydrocarbons from CO2. HD-1 cells were collected from 10 liter BM culture. Non-polar substances contained in the chloroform extracts of the disrupted cells were directly analyzed by GC/MS(EI) (Fig. 3a). The characteristic fragmentation pattems and each mass number of the molecular ion peak of these
470 fractions demonstrated that they were straight alkanes of the carbon number from 14 to 30. These identified substances were eluted at exactly the same retention times on GC with corresponding authentic samples. Interestingly, the distribution pattern of cellular alkanes resemble that of crude oil from which the strain HD-1 was isolated (Fig. 3b). This may suggest that the HD-1 contributes for petroleum production under the ground. Several alkenes (e.g. eicosadiene) were also identified in another batch of culture (data not shown). 3-4.
Alkane/alkene
production
by the strain HD-1 cells
It is reported that long chain alkenes (waxes) are biologically synthesized from fatty aldehyde rather than fatty acid by the experimental results with insects and mammals [5]. Botryococcus braunii, a green alga, is known to produce large quantity of long chain alkenes, and aldehyde decarbonylase has been shown to transform fatty aldehydes to alkene with a loss of carbon monoxide [1]. Then, 14C-fatty aldehyde and 14C-fatty acid were examined as potential precursors of alkane/alkene synthesis in the HD-1 cell. It was previously confirmed that alkane/alkene migrated at Rf 0.9 and Rf values of other polar substances were smaller than 0.8 on the plate (Rf values of both palmitic acid and h e x a d ~ a l are ca. 0.5). Radioactivity of substances at Rf 0.9 was measured by liquid scintillation counter. (Table 1, [6]). Radio activities at Rf 0.9 (alkane/alkene) on the TLC plate were detected only when the cell extract was added to the reaction mixture. Since hexadecanal was a more effective precursor than palmitic acid, fatty aldehyde might be the immediate precursor in the alkane/alkene synthetic pathway. The HD- 1 cell probably maintained such an aldehyde decarbonylase-like activity as reported for B. braunii cell. However, it should be noted that strain HD-1 produces both even and odd carbon numbered hydrocarbons while B. braunii (A-race) produces mainly odd carbon numbered hydrocarbons. Similarities and differences between their hydrocarbon production mechanisms need to be clarified in the future. Table 1 Production of alkane/alkene by strain HD-1 alkane/alkene (nM) Substrate cell extract + 32.3 160/~M ~"C-palmitic acid 0 160/~M '"C-palmitic acid + 56.5 160/~M ~"C-hexadecanal 0 160/~M~"C-hexadecanal 4. C O N C L U S I O N A petroleum-degrading strain HD- 1 was able to grow autotrophically by using CO2 as sole carbon source. Inside the cell there accumulated alkane/alkene besides fatty acids or poly-13hydroxyalkanoic acid. Fatty acids and fatty aldehydes were shown to be reduced to alkane/alkene by anaerobic incubation with the HD-1 cell lysate. REFERENCES
[ 1] T. M. Cheesbrough and P. E. Kolattukudy, J. Biol. Chem., 263 (1988) 2738. [2] M. Morikawa and T. Imanaka, J. Ferment. Bioeng., 76 (1993) 280. [3] M. Morikawa, M. Kanemoto, and T. Imanaka, J. Ferment. Bioeng., 82 (1996) 309. [4] M. Morikawa, H. Daido, T. Takao, S. Murata, Y. Shimonishi, and T. Imanaka, J. Bacteriol., 175 (1993) 6459. [5] M. Dennis and P. E. Kolattukudy, Proc. Natl. Acad. Sci. USA, 89 (1992) 5306. [6] M. Morikawa, T. Iwasa, S. Yanagida, and T. Imanaka, J.Ferment. Bioeng., in press.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
471
Cultivation of cyanobacterium in various types of photobioreactors for biological COe fixation In Soo Suh a, Chan Beum Park a, Jung-Kuk Han a, Sun Bok Lee
a,b,c
aDepartment of Chemical Engineering and bSchool of Environmental Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea cResearch Institute of Industrial Science & Technology, Pohang, 790-784, Korea
Synechococcus sp. PCC6301 was cultivated in various types of enclosed photobioreactors for biological CO2 fixation. In order to maximize the photosynthetic efficiency of the cell, major culture conditions such as inlet CO2 concentration, incident light intensity, aeration rate, temperature, and agitation speed were optimized. To investigate the effects of bioreactor configuration on cell growth and CO2 fixation rate, we constructed stirred-tank photobioreactor (ST-PBR), florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OFPBR) and florescent-lamp/optical-fiber photobioreactor (FLOF-PBR). FL-PBR was found to be most energy efficient, whereas the highest CO2 fixation rate was obtained in the FLOF-PBR system. It is expected that the latter type of photobioreactor is useful for high-density cultivation of photosynethetic microorganisms and biological COe fixation in a large scale.
1. I N T R O D U C T I O N Carbon dioxide (COe), the natural product of fossil-fuel combustion, has been recognized as the main factor for the global warming. Among the various approaches to utilize the CO2 gas, there has been a considerable interest in biological CO2 fixation as a clean and energy-efficient technique [1]. In this research, we studied the cultivation of a cyanobacterium in various types of photobioreactors for biological fixation and utilization of COe. As a model photoautotrophic microorganism, Synechococcus sp. PCC6301 was chosen due to its higher growth rate, simple nutrient requirement, and a great deal of biochemical and genetic information. In order to maximize the photosynthetic efficiency of this unicellular cyanobacterium, major culture conditions were optimized. To examine the effects of bioreactor configuration on cell growth and CO2 fixation, five different types of photobioreactors were constructed and their efficiencies were compared in terms of the cell growth rate, CO2 fixation rate, and
472 energy efficiencies. The results presented in this work indicate t h a t FLOF-PBR, a combined form of externally illuminating photobioreactor (FL-PBR) and internally illuminating photobioreactor (OF-PBR), is suitable for efficient biological CO2 fixation in a large scale.
2. MATERIALS AND METHODS 2.1. S t r a i n and c u l t u r e c o n d i t i o n s Synechococcus sp. PCC6301 (ATCC27144, Anacystis nidulans) was cultivated in B G l l [2]. The cells grown on the agar were inoculated into 300 mL of the sterilized medium in a 500 mL bottle, and these cells were incubated by bubbling with the prehumidified air (0.03% CO2) at 30~ and 40 ~mol/m2/sec. The 7-dayold stock cultures were used as the inocula (10%) for all experiments. In the case of photobioreactor operation, cells were grown at 30~ using 0.03% CO2. 2.2. A n a l y t i c a l m e t h o d s In order to determine the cell concentration, optical density was measured at 600nm using a spectrophotometer (Milton Roy). The dry cell weight was then deduced from the calibration curve between the optical density and dry cell weight. Average light intensities at the surface of photobioreactors or optical fibers were determined with a quantum meter (LI-COR, LI-190SA). The CO2 concentration was measured with a gas chromatograph (Young-In, 680D), equipped with a packed column (Porapak N) and TCD detector. The flow rates of air and CO2 were precisely controlled by using mass flow controllers (Brooks Model 5850E). The elemental composition of the cells was determined by an elemental analyzer (LECO, CHNS-932). 2.3. P h o t o b i o r e a c t o r s y s t e m Based on the agitation and illumination methods, five types of enclosed photobioreactors were constructed. Working volumes, surface-to-volume ratios (S/V), and light intensities at the surface of photobioreactor or optical fibers are shown in Table 1. For mixing of culture broth, internal mechanical agitator was used in a stirred-tank photobioreactor (ST-PBR), while in bubble-column type photobioreactors gas spargers were used. Based on the light illuminating methods, photobioreactors could be divided into florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OF-PBR), and florescent-lamp/opticalfiber photobioreactor (FLOF-PBR). FL-PBR was externally illuminated with eight 20 W florescent lamps. OF-PBR-A was internally illuminated with 200 light-diffusing optical fibers connected to a 150W metal-halide lamp. In the case of OF-PBR-B, 486 light-diffusing optical fibers and a 400W lamp was used to enhance the light intensity. FLOF-PBR, a combined configuration of FL-PBR and OF-PBR-B, was illuminated with florescent lamps and optical fibers simultaneously. For efficient light emission, optical fibers were pretreated by scrubbing with sand papers.
473 T a b l e 1. Comparison of photobioreactor configuration and performance
Reactor type
Volume S/V Light intensity Growth rate CO2 fixation rate c Energy input d (L) (m1) (}~mol/m2/sec) (g/L/day) (g/L/day) (W/g/L/day)
ST-PBR FL-PBR OF-PBR-A OF-PBR-B FLOF-PBR
4.0 2.5 2.5 2.5 2.5
24 75 273 557 632
100 a 100 a 5b 20 b 100 a + 205
0.25 0.43 0.07 0.38 0.53
0.43 0.75 0.12 0.66 0.92
279 213 1250 606 609
a Incident light intensity measured at the surface of photobioreactor. b Incident light intensity measured at the surface of optical fibers. c CO2 fixation rate calculated from the growth rate and carbon content of the cell. dEnergy required for illumination at the CO2 fixation rate of 1.0 gCOJL/day.
3. R E S U L T S A N D D I S C U S S I O N
Among the various photobioreactor systems, we excluded the outdoor open system such as pond-type and raceway-type cultivator. Although these types of photobioreactors are relatively less expensive and easy to operate, there are several disadvantages such as low growth rate, large space requirement, limited light availability, and difficulties in control and optimization [3]. In this research, we designed five types of enclosed photobioreactors and compared their energy efficiencies (Table 1). It appears that energy-efficient photobioreactor system is necessary for large scale CO2 fixation since high energy consumption for illumination and agitation could lead to additional CO2 gas release from a fossilfuel power plant. From the preliminary experiments, it was found that most cell growth (>70%) occurred in the linear growth phase during batch cultivation. Optimal inlet CO2 concentration for the cell used in this work was found to be in the range of 0.54.0% although the lag periods were increased with increasing CO2 concentration. The linear growth rates were increased with increasing the light intensity, followed by light saturation at 180 ~mol/m2/sec. The optimal agitation speed and aeration rate were found to be 200 rpm and 2.5 L/min at 0.03% CO2 and 30~ for ST-PBR and FL-PBR, respectively. In the case of ST-PBR, high electric power might be required for the agitation of culture broth. Since bubble-column type photobioreactors provide energyefficient mixing of culture broth and simple bioreactor configuration ease to scale-up, we designed several types of bubble-column type photobioreactors. Whereas the CO2 fixation rate in the ST-PBR was 0.43 g COJL/day, the CO2 fixation rate in the FL-PBR under the same incident light intensity (100 ~mol/m2/sec) was improved to 0.75 g COJL/day (see Table 1). Under optimal culture conditions in FL-PBR, the CO2 fixation rates reached a
474 maximum value of 4.33 g COJL/day in an exponential growth phase, which is equivalent to 16.5% conversion of input CO2 to biomass, and then decreased steadily with time in the linear growth phase. The incident light intensities were exponentially decreased inside the reactor with the specific absorption coefficient of 2.16 L/g/cm in the Beer-Lambert's law. Thus FL-PBR has a problem of light transfer limitation in large-scale photoautotrophic cultivation. The average CO2 fixation rate was calculated to be 0.87 gCOJL/day and the carbon content of the cell was 47.4%. If the above data are employed for estimating the reactor volume required for the biological t r e a t m e n t of CO2 gas from a typical 150 MW t h e r m a l power plant (assuming the CO2 emission rate of 130 tonCOJh), the required culture volume is calculated to be as much as 6• s liters. The OF-PBR system, which employs light-diffusing optical fibers for light distribution, has been known to provide a higher illuminating surface area per culture volume [4]. Since the light intensity from light-diffusing optical fibers can be changed depending on the light source and the number of optical fibers, we compared the effect of light intensities on reactor performance. As shown in Table 1, the CO2 fixation rate was enhanced from 0.12 to 0.66 gCOJL/day with increasing the light intensities. From the viewpoint of energy efficiency, however, OF-PBR system was not an ideal photobioreactor: OF-PBR requires at least 2.8 times more light energy t h a n FL-PBR when compared at the same CO2 fixation rate. It was also observed that the optical fibers interfered the mixing of culture broth and that cells attached to the surface of optical fibers reduced the light irradiance as the cells grew up. To compensate the limitation of FL-PBR and OF-PBR, we designed the FLOFPBR system, a combined form of externally illuminating photobioreactor (FLPBR) and internally illuminating photobioreactor (OF-PBR), and examined the efficiency of CO2 fixation rate and light energy utilization. As can be seen from Table 1, the highest CO2 fixation rate (0.92 gCOJL/day) was obtained in FLOFPBR among the photobioreactor systems employed in this work. This again indicates that a higher CO2 fixation rate can be achieved if more light is provided to the cell inside the reactor. In order to establish a more energy-efficient photobioreactor system, therefore, more efficient light transmission and light distribution methods need be developed. The results presented in this work implies that the use of FLOF-PBR may be an alternative way for high-density cultivation of photosynethetic microorganisms and biological CO2 fixation in a large scale.
REFERENCES 1. 2. 3. 4.
S.B. Lee, C.B. Park, and I.S. Suh, Chem. Ind. Technol. (Korea), 13, (1995), 347. M.M. Allen, J. Phycol. 4 (1968) 1. M.R. Tredici and R. Materassi, J. Appl. Phycol. 4 (1992) 221. H. Takano, H. Takeyama, N. Nakamura, K. Sode, J. G. Burgess, E. Manabe, M. Hirano, and T. Matsunaga, Appl. Biochem. Biotech. 34/35 (1992) 449.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi(Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
475
A p p l i c a t i o n of p h o t o s y n t h e t i c b a c t e r i a for p o r p h y r i n p r o d u c t i o n H. Yamagata% R. Matoba a, T. Fujii b, and H. Yukawa a a Molecular Microbiology and Genetics Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-Soraku, Kyoto 619-02, JAPAN Department of Bioproduction Science, Chiba University, 648 Matsudo, Matsudo, Chiba 271, JAPAN
b
1. INTRODUCTION It is widely accepted that effective measures must be taken to reduce atmospheric CO2 and its effect on global warming. Our research has concentrated on use of biological activities for this purpose through design of bioreactors using microorganisms for efficient CO2 fixation. The Molecular Microbiology and Genetics Laboratory at RITE has been studying the CO2 fixing ability of photosynthetic microorganisms since 1993. It is essential in this process not only to fix CO2 merely in the form of biomass but, in addition, to convert it to useful materials, facilitating recycling of carbon. Porphyrin, a macrocyclic tetrapyrrole, and its metal-containing derivatives play important roles in many biological systems, including heme (Fe) for electron transport, (bacterio)chlorophyll (Mg, Zn) for photosynthesis, cobalamin (Co) as a cofactor for various enzymes, and F430 (Ni) for methanogenesis [1]. Based on a vast amount of research directed at understanding their mechanistic roles in various reactions, recent attention has focused on utilizing the unique properties of porphyrins [1]. This includes applications as catalysts for chemical reactions, electronic and photonic devices, and sensitizers for photodynamic therapy and diagnosis of tumors. Unmodified forms of porphyrins are more versatile for designing specific properties and better processability, because the physicochemical properties of porphyrins depend on the macrocyclic skeleton, metals chelated inside, and modifications of side chains by functional groups. Development of a process which can produce porphyrins at a low cost will accelerate the application for commercial products, making this material available as a possible CO2 reservoir. Among microorganisms, photosynthetic bacteria are known to contain the greatest diversity of porphyrins (Figure 1). Taking advantage of their great biosynthetic ability and capacity to convert CO2 to porphyrins, we are studying the possibility of using this microorganism for production. In this paper, excretion of porphyrins by purple nonsulfur photosynthetic bacteria, cultural conditions affecting production, and the mechanism of biosynthesis are discussed.
476
co~
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I Bacteri~176176 II I ProtoporphyrinIX Copropo!phyrinogenIII
in~~B~jroporph!rinogenIII
Succiny~-CoA am I ~ /_P..BGdeaminase 5-Aminolevulinate------~Porphobilinogen f. ALA synthase PBG s,,nthase Glycine y ' Figure 1. Biosynthesis of porphyrins by purple nonsulfur bacteria. 2. MATERIALS AND METHODS For the production of porphyrins, photosynthetic bacteria were grown phototrophically at 30~ harvested, suspended in the medium, and incubated anaerobically in the light at 40~ Growth was measured by OD660. Porphyrins in the culture supernatant were quantitated by measuring A404 in water or A401 in 1M HC1. Analysis of porphyrins was done, after extraction and esterification, by silica gel TLC [2,3]. Aminolevulinate (ALA) synthase, porphobilinogen (PBG) synthase, and PBG deaminase activities were determined by measuring (dis)appearance of substrate/product colorimetrically with the Ehrlich reagent [4]. 3. RESULTS AND DISCUSSION 3.1 Excretion of porl~hyrins by Rhodopseudomonas palustris strain No.7 Extracellular accumulation of porphyrins was reported for some species of purple nonsulfur photosynthetic bacteria when some biosynthetic steps (Figure 1) were impailed by mutation or by deprivation of certain biosynthetic precursors [5]. We have found that a natural isolate R.palustris strain No.7 [6] excretes a large amount of porphyrins into the culture supernatant when this strain is incubated in anaerobic light conditions. This phenomevon was noticeable when the incubation was done at 40~ in a m e d i u m containing a mixture of a primary alcohol (ethanol, 1-propanol, or 1-butanol), an organic acid of the TCA cycle (2oxoglutarate, succinate, fumarate, or malate), and an amino acid (glutamate or aspartate) as carbon and nitrogen sources. The alcohol can be replaced by its corresponding carboxylic acid, and the alcohol or acid alone is also effective when its concentration is increased. Thus, the amount rather than the con' n t is important for the excess carbon to be converted to excreted porphyrins. Excretion
477 was associated with H2 evolution due to the derepression of nitrogenase expression, and was strongly inhibited by NH4 or an amide form of the above amino acids (glutamine and asparagine). Under optimum conditions for strain No.7, little porphyrin excretion was observed for other purple nonsulfur bacteria including the type strains of Rhodobacter(Rb.) sphaeroides, Rb.capsulatus, R.palustris, and Rhodospirillum rubrum.
3.2 Analysis of intra- and extra-cellular porphyrin species The species of intra- and extra-cellular porphyrins by R.palustris No.7 were analyzed under excretion (ethanol/malate/glutamate, 40~ and non-excretion (ethanol/malate/glutamate, 30~ and ethanol/malate/glutamate+NH4, 40~ conditions. Porphyrin in the culture supernatant was determined almost exclusively to be coproporphyrin, an oxidized product of coproporphyrinogen III (Figure 1). While protoporphyrin was observed inside cells, the ratio of proto/copro-porphyrin was much higher in non-excretion condition. Free porphyrins were not detected either inside or outside cells when NH4 was added as a nitrogen source. The repression of accumulation by NH4 or amide form amino acids strongly suggests that the regulation of porphyrin synthesis in this microorganism is affected by nitrogen metabolism, though the mechanism remains unclear.
3.3 Activities of porphyrin biosynthetic enzymes The growth, porphyrin excretion, and the activity of the first three enzymes involved in the synthesis of tetrapyrrole skeleton from glycine and succinyl-CoA, ALA synthase, PBG synthase, and PBG deminase, were measured under excretion (ethanol/malate/glutamate, 40~ and non-excretion (ethanol/NaHCO3/NH4C1, 40~ conditions (Figures 2 and 3).
1!
0D66o
20 A4ol 15 10
C 0
50
100
150
Incubation time (hr)
200
Figure 2. Growth and porphyrin excretion by strain No.7. II, excretion; O, non-excretion condition.
478 Growth ( O D 6 6 0 ) w a s almost the same, but there was a great difference in A401 for the two conditions. ALA synthase activity tends to decrease in the time course, but is considerably higher in excretion condition than in non-excretion condition. PBG synthase activity, also higher in excretion condition, shows five fold increase compared to the start of incubation. PBG deaminase activity is maintained at constant level, but higher in excretion conditions. Relative activity of ALA synthase is approximately one tenth of those of PBG synthase and deaminase. This elevated level of biosynthetic enzymes seems to be one reason for enhanced porphyrin excretion. ~.~ 1.5 . . . . .
, ~ L A synthase
2O
PBG
synthase
15-
O 1.o. lO-
~
0.5.
'it
PBGdeaminase
N t-"
U.l
o
50
100
150
200
0
,
50
,
100
,
150
200
0
50
100
150
200
Incubation time (hr) Figure 3. Activity of porphyrin synthesis enzymes, m, excretion; GI, non-excretion condition. Activity is expressed as n m o l / m g protein/min at 40~ 4. CONCLUDING REMARKS The photosynthetic bacterium R.palustris No. 7 presently produces porphyrins at an unsatisfactory level. Understanding the mechanism of excretion from a biochemical and genetic stand point will provide useful information to improve the productivity of porphyrins by this microorganism. A part of this work is supported by a grant from MITI through the Japan Alcohol Association.
REFERENCES
1. L.R. Milgrom, The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds, Oxford University Press, Oxford, 1997. 2. R. Cox and H.P. Charles, J. Bacteriol., 113 (1973) 122. 3. M. Doss, J. Chromatogr., 30 (1967) 265. 4. G. Urata and S. Granick, J. Biol. Chem., 238 (1963) 811. 5. A.J. Biel, in R.E. Blankenship, M.T. Madigan, and C.E. Bauer (eds.), Anoxygenic Photosynthetic Bacteria, p.1125, Kluwer Academic Publishers, Dordrecht, 1995. 6. T. Fujii, A. Nakazawa, N. Sumi, H. Tani, A. Ando, and M. Yabuki, Agric. Biol. Chem., 47 (1983) 2747.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
479
Utilization of micro-algae for building materials after C02 fixation Toshi Otsuki a, Masatada Yamashita a, Takahiro Hirotsub, Hiroshi Kabeyab, and Ryoichi Kitagawab a RITE IHI Laboratory in Research Institute, Ishikawajima-Harima Heavy Industries Co., Ltd. 1, Shin-Nakahara-cho, Isogo-ku, Yokohama 235, Japan bShikoku National Industrial Research Institute. 2217-14, Hayashi-cho, Takamatsu 761-03, Japan
1. INTRODUCTION Increasing of CO2 in atmosphere is so serious problem for mankind that many R&D are implemented to reduce CO2 discharge. One is a biological CO2 fixation using the photosynthetic function of micro-algae. The purpose of this study is to make ecofriendly building materials using the micro-algae as Chlorella sp.(hereafter chlorella) and synthetic polymer as polyvinyl chloride(PVC). After CO2 fixation was completed by the micro-algae, carbon from fixed CO2 should keep in their bodies for a long period. As long as the micro-algae are in the building materials such as floor tiles, laminated plastic boards, CO2 would not come out to the air again. Nowadays some phthalate ester are used as plasticizer when PVC molding materials are made on commercial basis. We started on some feasibility studies in which large amounts of chlorella discharged from the photo-bioreactor after CO2 fixation became substitute to the conventional plasticizer.
2. EXPERIMENTAL 2.1. Materials Dried chlorella powder and PVC (n--1100) were used. As the stabilizer some chemical additives were used. These chemicals were dibasic lead stearate (DBL), tribasic lead sulfate (TC),cadmium/barium/lead fatty acids complex (L-500), barium/zinc fatty acids complex (PSE) and paraffin. Usually we used the mixture of 20% chloreUa and PVC with TC 1.5%, DBL 0.5%, L-500 0.5% of PVC respectively.
This work was performed under the management of Research Institute of Innovative Technology for the Earth (RITE) as a part of the "Biological CO2 Fixation and Utilization Project", supported by New Energy and Industrial Technology Development Organization (NEDO).
480 2.2. Molding methods The mixture (about 8g) was set in the aluminum mold flame (press area is 20 • 120 mm). The mixture was then pressed and heated simultaneously for a certain period to make rigid form using the molding machine (max. 20MPa). After being cooled down in water, the sample was removed from the mold frame, and the physical properties (thickness, weight, tensile strength) were evaluated after more than 48 hr keeping in the constant temperature/humidity (20~ room. 2.3. Measurement The tensile strength, thickness of the samples were measured according to the Japanese Industrial Standard, JIS K-6740. The amounts of burr pushed out the machine were measured as weight ratio to the total.
3. RESULTS AND DISCUSSION 3.1. Effect of chlorella/PVC ratio on tensile strength and thickness Figure 1 shows that the tensile strength decreased as increase of chlorella in the mixture. Suitable content of chlorella was 10 "~ 30 % and the tensile strength showed 17 ~ 20 MPa, which was as same as commercial PVC plastics described in the Japanese standard "Plasticized polyvinyl chloride compounds (JIS K-6723). The thickness of the mold increased and the amounts of burr decreased as content of chlorella increased (Figure 2). It shows that chlorella behaved as fluidity regulator of the melting plastics in the mold frame. 5.0 50 40
~ 4.0
-
~2.0
10 F ~
-
~ u ~
''3.0 ~
30 ,-, 20 , ,,,,,q
_
1.0 I
I
i
I
t
I
i
I
i
I
i
I
I
I
t
u
0.0 0
10
20
30
40
50
60
70
80
Chlorella content in PVC plastic (wt.%) Figure 1. Effect of chlorella content on tensile strength of PVC plastic
I
I
t
I
I
I
I
I
'
I
t
I
l
80 70 t 60 50~ 40 30 m 20 10 L 0
0 10 20 30 40 50 60 70 Chlorella content in PVC plastic (wt.%) Figure 2. Effect of chlorella content on thickness and burr of PVC plastic
3.2. Effect of molding temperature and time on tensile strength Figure 3 shows the results that the tensile strength increased as molding time increased at only 180~ 22MPa after 5 rain. It was difficult to keep the suitable condition over 190~ because PVC proceeded to decomposition over 190~ Melting of PVC was incomplete under very short melting times. Some points to fix the optimum condition were as follows : (i) a little bit over
481 25 Molding
20
Temp.
~
=9e~o 15
-~
180
3.3. Effect of molding pressure on thickness Figure 4 shows the thickness became thinner as the pressure increased. As the metal flame was not airtight, melted plastics happened to leak from the frame. According to Figure 5 the tensile strength and the density had not changed over 1.5MPa of molding pressure. Over this pressure just makes the plastics to be thinner, not to effect the tensile strength and density.
- - l - - 190
10
,
"~
melting temperature of PVC was preferable (180~200~ (ii) it depended on the thermal conductivity of material in the molding frame.
200
~210
5
- I - - 220
0 0
1
2
3
4
5
6
7
8
Molding time (min) Figure 3. Effect of molding time on tensile strength of PVC plastic
~
3.4. Effect of various chemical additives It is well known that thermoplastics as PVC will deteriorate easily by heating and oxidization. We tried to improve the physical properties of plastics by adding some commercial chemicals as stabilizer. Figure 6 shows that addition of PSE 3% and paraffin 0.2% to PVC prevented to decrease the tensile strength. The browning of plastics did not occur and it showed original dark green of chlorella by addition of paraffin. 1.2 Because it is preferable not 1 to use toxic metal substances (DBL, TC and L-500), PSE is 0.8 recommended. 0.6
1.5
r~ r
0.5 I
0
I
0
I
1
I
I
2
I
I
I
3
I
4
I
I
I
5
,
6
Molding pressure (MPa) Figure 4. Effect of molding pressure on thickness of PVC plastic 2O _,___---
ITensile
[
strength
_
15
VOensi'
10 9 "~
0.4
cD
5
0
o,-~
0.2 t
1.5
I
i
2.2
I
t
3.7
I
i
I
5.2
,
I
0
6.6
Molding pressure (MPa) Figure 5. Effect of molding pressure on tensile strength and density of PVC Plastic
3.5. Estimation of chlorella substitute to conventional plasticizer We estimated roughly the chlorella substitute effects after 15% CO2 fixation from
482
30
PSE3%+Paraffin3%
25 ---tl-- TC1.5%+DBL0.5%+L-500 0.5%
x: 20 =
15 DBL3%+L-500 0.5%
r~
~ 10
.,-,i
~
PSE3%+Paraffin0.2%
5
0
2
4 6 Molding time (min)
8
Figure 6. Effect of molding time on tensile strength of PVC containing various chemical additives 1 million kW LNG power station as follows : The yields of chlorella will be 740,000 t/y with 80% water. Assuming that CO2 fixation rate of chlorella in the photo-bioreactor is l kg-CO2 /m3-culture 9d and carbon content of dried chlorella is 48% on the basis of bench scale experiments, the chlorella product could be 211,000 t/y of plasticizer with 30% water. According to these assumption we estimated the effect of CO2 reduction is 600,000 t/y and the effect of energy resources is 190,000 t-oil/y.
4.
CONCLUSION
The optimum molding condition of the mixture of chlorella and PVC was as follows : The suitable molding temperature wasl80~ because the tensile strength increased as the molding time increased and showed 22 MPa at 180~ 5min., but it decreased as the time increased over 180~ The tensile strength did not change and showed about 18 MPa at 1.5 ~" 6.6 MPa of molding pressure and decreased as the chlorella/PVC ratio increased under 2.2 MP of molding pressure. 10~30 wt % of chlorella in the mixture showed about 20 MPa of tensile strength. This could be applicable to "Plasticized polyvinyl chloride compounds (JIS K-6723)". Furthermore, the no-preferable browning of chloreUa on molding process was prevented by paraffin addition and chemical stabilizers. Now we have succeeded to make chlorella/PVC plate of which the size was 150 X 150 • 5mm. In Japan a large amount of PVC floor tiles and their plasticizer (ex. phthalate esters) were produced and estimated to be 96,433 ton, 28,930 ton respectively in 1995. We have great expectation that the phthalate esters could be replaced with the new chlorella plasticizer.
REFERENCES
1. JIS K-6723 (1995) 2. JIS K-6740 (1976) 3. T. Yamaguchi and K. Fukuda, J. Polymer Process, 44 (1995) 394
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
483
Photosynthetic
CO 2 fixation p e r f o r m a n c e by a helical t u b u l a r p h o t o b i o r e a c t o r incorporating Chlorella sp. u n d e r o u t d o o r culture conditions Yoshitomo Watanabe*, Masahiko Morita and Hiroshi Saiki
Bio-Science Department, Abiko Research Laboratory,Central Research Institute of Electric Power Industry(CRIEPI), Abiko 1646,Abiko city,Chiba pref., Japan 270-11.
1.INTRODUCTION Since the photosynthetic capability of microalgae, such as ChloreUa, is higher than any other terrestrial plant, CO2 fixation technology utilizing microalgae would provide an efficient route of CO 2 conversion from a thermal power plant into useful biomass. The utilization of the microalgal biomass could contribute to the mitigation of global warming. It is very important to develop the novel photobioreactor to achieve the higher photosynthetic CO 2 fixation performance. We have been developing a helical tubular photobioreactor(HTP) at a laboratory scale[5,6]. In the next stage, it is necessary to construct an HTP system for outdoor culture which could use the solar radiation directly. In this study, we investigated the photosynthetic CO2 fixation performance of a cone-shaped HTP incorporating Chlorella sp. under outdoor culture condition.
2.EXPERIMENTAL 2.1 Photobioreactor A schematic diagram of the cone-shaped HTP system for outdoor culture experiment is shown in Figure 1. It was constructed with a 1.1 m 2 installation area (top diameter;120 cm) and photosynthetic production of Chlorella sp. was observed from May to November 1996. The inside surface of the cone-shaped photostage was illuminated by the solar radiation. Environmental data(Temperature of ambient, water bath and culture medium, photosynthetic photon flux density(PPFD), etc.) was automatically monitored.
2.2 Organism, Culture Medium,Culture operation and Biomass Assay Chlorella sp. strain HA-112] was used for culture experiments. The culture medium(M4N medium) had the following composition: (amounts in mg/litre): KNO3,5000; MgSO47H20,2500; KH2PO4,1250; FeSO47H20, 3; H3BO3,2.86;MnSO47H20,2.5; ZnSOa7H20,0.222; CuSOa5H20,0.079; Na2MoO4,0.021. The initial pH value was 6.0. A batch culture or a semi-batch culture was investigated to determine the photosynthetic performance under outdoor culture conditions. An air/CO2(10% CO2) mixture(CO2 enriched air) was injected into the two pathway of the HTP at a total flow rate of 1.2 litre / min. The optical density of the culture medium at 750 nm was measured and the biomass concentration was calculated using a standard curve between biomass concentration(dry weight) and optical density. The carbon content of the biomass was analyzed by a C/N analyzer in order to calculate the caloric value.
*To whom correspondence should be addressed.
484
Gas out ||l|
~
...
,,,,,,,
i.
"..
120 cm ., . . . . . .
,,
..
.
.
.
9-._
(~) .tp._~llk\\\Nt Degasser (Diameter.29cm)
Heat exchanger
'8~n~le
Medium recovery . L
, "~In order to allow air-lift i operation, the photostage and~ heat exchanger tube were ! devided2n?~? par.ts((~)&?) t
-"
CO2 enriched air
Water bath
Temp.Controller
Figure 1. A schematic diagram of the cone-shaped helical tubular photobioreactor (HTP) system with 1.1 m 2 installation area for outdoor culture experiment. The photobioreactor is comprised of; a cone-shaped helical photostage made of transparent polyvinyl chloride(PVC) tubing ( 110 m in total length and 1.6 cm internal diameter with 0.2 cm wall thickness. PVC tubing was divided into two parts at the middle so as to allow an air-lift operation.) ; a photostage support made of steel ; a helical heat exchanger set in the water bath(Total length of transparent PVC tubing was 30 m. It was also divided into two parts.); a water bath temperature controller ; a degasser ; CO2 enriched air supply system( air pump, COa gas cylinder and gas flow meter). The outer surface of the photostage was covered with aluminum foil and white cloth to prevent the absorption of outside light in order to determine the accurate photosynthetic efficiency. The inside surface of the cone-shaped photostage was illuminated by solar radiation. The photosynthetic photon flux density(PPFD) at the horizontal level surface was measured using the quantum meter automatically at 5 minute intervals and the light energy input of photosynthetically active radiation(PAR) into the photobioreactor was calculated by using the conversion factor[ 1]. In summer, a mist water spray system was used to cool down the photostage.
3.RESULTS 3.1.
AND DISCUSSION
Photosynthetic performance under outdoor conditions Results of the outdoor culture experiments using the H T P is s h o w n in Table 1. The H T P system was applicable for outdoor culture. Photosynthetic productivity corresponded to the amount of solar radiation. M a x i m u m daily growth rate was 17.9 g dry weight biomass / H T P installation area (m2)/ day, and its photosynthetic efficiency was 4.12 %(PAR). This experimental data was s h o w n in Figure 2. Chlorella had g r o w n well with sufficient solar radiation. The microalgal productivities in outdoor ponds were reported as follows;productivity of Spirulina platensis S P - G was up to 21 g / m / d a y in Israel [3] ; daily productivity of the marine chlorophyte Tetraselmis suecica was 15 to 20 g dry w e i g h t / m / d a y under full sunlight conditions in Hawaii [2].Although photosynthetic productivity in this study is similar to the conventional open pond type system, 20 % less than the results of our previous indoor (laboratory) experiment[7]. It is considered to be due to the slow flow speed of the culture
485
Table 1. Maximum daily growth yield and photosynthetic efficiency of Chlorella sp. strain HA-1 in the cone-shaped helical tubular photobioreactor under field culture operation. Experimental Day of Max. Inputof solar energy Maximumdaily growth. Maximumdaily Photosynthetic term. yield achieved(kJ (PAR)/HTP installation(g dry biomass/HTP energyrecovery efficiency( % ) area(m2)/day) installation area(m2)/day) (kJ / HTP installation (PAR) area(m2)/ day) (~) (~) (~) (~) (~) (~) (Z)
96.5.23-28. 5.26. 96.6.4- 9. 6. 7. 96.6.21-27. 6.24. 96.7.2-10. 7. 6. 96.7.16-20. 7.19. 96.8.30-9.7.8.31. 96.10.23-11.1.10.30.
8819 9701 3993 10107 9560 3962 4635
14.3 9.37 8.19 10.1 17.9 9.65 4.70
325 206 181 222 393 212 103
(A)
~2 y.
Z
/
~o E ~ ~
~'~
1 18
o= 16 "~ 14 12 10~~
8~
~-~ l .s ~
3.69 2.12 4.51 2.20 4.12 5.35 2.22
I
I
10
(B)
4 2 0
~
.,,-
4
0
~ =
g 2 0
!
96"/7/17
!
7/18
!
7/19
7/20
Figure 2. Changes in dry weight biomass concentration and growth rate of Chlorella sp.(A),or daily input of solar energy (PAR)(B) in the cone-shaped HTP outdoor culture experiment. Data. 1996.7.17-7.20.The vertical arrow indicates the time when half of the culture medium was removed for biomass recovery and a new culture medium was added. Mist water was supplied onto the photostage surface to maintain the optimum temperature of the culture medium.
486 medium in the tubing of the HTP. The photostage size of the outdoor culture system was 4 times bigger than the previous laboratory scale to increase the CO2 removal rate. In order to improve the culture medium flow and increase productivity, it is important to consider the size of the photostage.On the other hand, the results also showed that the biomass concentration reached approximately 2g dry weight per litre medium. It is more advantageous compared with the conventional open-pond type culture system, since the biomass concentration was normally less than 1 g dry weight per litre medium. 3.2.1nfluence of environmental temperature of culture m e d i u m .
conditions
on
the
maintenance
of
suitable
Since the illuminated area to culture volume ratio of the HTP was high, temperature change response of the culture medium against a solar radiation or ambient temperature was quick. In spring or autumn, the temperature of the culture medium increased with solar radiation and stayed at a suitable level (data not shown). In summer, the temperature rises above the suitable level, therefore it is necessary to lower the temperature by spraying water onto the photostage, in addition to using an equipped heat exchanger.Higher productivity was maintained at suitable temperature conditions (Figure 3). _,._,1600 1400 '~ 1200 = s 1000 "~ < 800
mmmm
m
9
400 200
m m mmmm~"
0
(A)
9
9
m m
. . . .
m
. . . .
m
'
m_ ' m m
9
m m m m
(B)
40 mmmmmmmm
35
m| u I ~ ~ go go ~ 1 7m 6 19 7o61 4991 4 9
mm
30
owlll o . ' ' ' - . . . . . . . . . . . .
zs 20
[.-,
Figure 3. Changes in light intensity(A) and temperatures of ambient,culture broth and heat exchanger water in the water bath(B) in the cone-shaped helical tubular photobioreactor outdoor culture experiment (Data of 1996.7.19.) Mist water spray for cooling was operated.
~ w ~ O
15 i 99 Ambient Culture medium 10 * Water bath 5 ~ - "l~Mist water spray 0
0
. . . .
'I
6
'
''
'
'r
12
'
',
'
'i
18
',
'
''
I
24
Time (hr.) It is important to develop a cheap and easy control system of culture medium temperature, in order to keep the higher productivity. Also, utilization of a thermophilic microalgae is considered to be an effective choice in summer. REFERENCES 1.D.O.Hall, J.M.O. Scurlock, H.R.Bolhar-Nordenkampf, R.C.Leegood and S.P.Long(eds.),Photosynthesis and production in a changing environment, Chapman & Hall,London, 1993. 2.E.A.Laws and J.L. Berning, Biotechnol. Bioeng.,37,(1991)936 3.A.Vonshak and R. Guy, Plant,Cell and Environ .,15,(1992)613 4.Y.Watanabe,N.Ohmura and H.Saiki,Energy Convers.Mgmt., 33(1992)545 5.Y.Watanabe,J. de la Notie and D.O.Hall,Biotechnol. Bioeng., 47(1995)261 6.Y.Watanabe and D.O.Hall, Appl.Microbiol.Biotechnol.,44,(1996)693 7.Y.Watanabe and H.Saiki, Energy Convers.Mgmt.,38,Suppl. (1997)$499
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
487
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Carboxylation reaction with carbon dioxide. Mechanistic studies on the Kolbe-Schmitt reaction. Yoshio Kosugi* and Kazufumi T a k a h a s h i Advanced Organic Material Chemistry, D e p a r t m e n t of Material Science, Faculty of Science and Engineering, S h i m a n e University ~690
Nishikawatsu, Matsue city, Shimane, J a p a n
The n m r chemical shifts of alkali metal ( K, Na etc.) phenoxide moved to down field upon formation of the complex, [ phenoxide-M 9CO2] with carbon dioxide in the Kolbe-Schmitt reaction. The complex formed under pressure of carbon dioxide showed more increments in weights and chemical shifts, as much as 0.72 ppm of (~ para o f potassium phenoxide at 5 MPa. The kinetic studies on the carboxylation
of resorcinol in aqueous hydrogencarbonates with carbon dioxide revealed an equilibrium between resorcinol and the product or resorcylic acid. 1. I N T R O D U C T I O N The Kolbe-Schmitt reaction[i] has long history related with
'aspirin'
and has
been a n a m e reaction used for the longest period in an industrial process. While the d e m a n d for the m a n u f a c t u r i n g aromatic hydroxycarboxylic acids is still successively coming out today with a n u m b e r of patents, the m e c h a n i s m of the reaction has r e m a i n e d unsolved. The present n m r spectroscopic studies have proved a [substrate 9CO2] complex or an i n t e r m e d i a t e prior to the formation of carboxylic acids.
Another puzzling question about the unstable complex even to
moisture, is why the carboxylation of polyhych'oxybenzenes, such as resorcinol, should proceed in aqueous solutions. Herein also reported are kinetic studies on the carboxylation of resorcinol in aqueous solutions of alkali hydrogencarbonates. * Financial support, by Research & Development Laboratory (Chemical Division) of Ueno Fine Chemicals Industry, Ltd. at, Osaka, and (]rant-in-Aid for Scientific Research by the Educational Ministry of Japan are greatly acknowledged.
488 2. E X P E R I M E N T A L The powdered alkali metal (K, Na etc.) phenoxide (ca.0.1g) was placed in a vial filled with carbon dioxide, and covered with parafilm. Carbon dioxide of purity more than 99.95% was supplied through a capillary tubing at room temperature. After the absorption of the gas, the weight increment was measured, and a portion of the sample was transferred in an nmr tube of with acetone-dG, or in DMSO-dG
5 r
and dissolved in DMF
with addition of TMS. The nmr spectra on a
JEOL-JNM GX 270 spectrometer were taken without delay. For the kinetic studies,
a 150 mL aqueous solution containing resorcinol and potassium
hydrogencarbonate was kept at 80 ~ with or without bubbling carbon dioxide, then an aliquot was taken at given time for nmr and/or HPLC analyses. 3. R E S U L T S AND D I S C U S S I O N
3.1 Structures of [alkali metal phenoxide'CO2] complex In the Kolbe-Schmitt reaction, phenyl carbonate (I) was originally proposed as the intermediate[2], but infrared absorption (i.r.) spectra of the intermediate showed a band at 1684 cm -1 which disagrees with I, because an absorption band at 1754 cm 1 of methyl phenyl carbonate is not much different from 1748 cm -1 for dimethyl carbonate.[3] Therefore, the carbonyl of the complex might be in the structures such as (II) - (IV).
oco2K
f' ]
Cl,:)
Ciu]
C~3
On the other hand, the amount of the complex was estimated by titration method.J4]
The investigation on a variety of phenols indicated the substitution
effect on the amount of the complex formed was in favor of the structure II or III. However, any affirmative supports have not been given to these structures. In the present studies, nmr spectroscopy was used and the existence of the complex has been clearly proved. Carbon dioxide is exothermically absorbed by dried potassium phenoxide, of which weights increase. Upon dissolving in nmr solvents, such as DMF and DMSO-d6, a part of the complex decomposes with liberation of carbon dioxide gas. In spite of the loss of portions of the complex, nmr
489 spectra of the sample showed significant large down field shifts of the signals of ortho, meta and para protons of the benzene ring. Upon the longer introduction of the gas flow onto the phenoxide, the more increments of the weight of the sample and the more down field shifts of all protons were observed (Table 1). The spectrum did not show any peaks of salicyhc acid or p-hydroxy benzoic acid, both might result from the complex . These observations indicate the 7r-electron of potassium phenoxide flows toward to carbon dioxide, and, at the same time, its oxygen atom forms weak bonding with potassium atom (structure IV).
This
structure originally appeared in Dewar's book[5], but any experimental data had not been available until today.
The adulteration with inorganic carbonates in
the sample is more or less inevitable for the preparation of phenoxides, and the i.r. absorption bands of ~ c:o of the carbonates are much stronger than that of the complex. In fact, our reexamination of i.r. spectra of the complex confirms neither sharp nor strong band of the complex as described in hteratures[3]. Moreover, the contamination of carboxylic acids which show strong absorption bands of carbonyl would make the evidence of the complex obscure. On the other hand, insoluble inorganic compounds are removed from nmr solvent and nmr spectra can easily detect the contamination of carboxylic acids, if any. Table 1 Weight increments of [Potassium phenoxide" CO2~ complex, and proton nmr spectral data ( 6 ppm) with the extents of down field shifts (A 6 ppm). CO2 flow
Complex~)
6 o
6 m
6 p
A6 o
A6 m
A6 p
Time (min.) 0
0.000
6.40
6.80
6.02
0.00
0.00
0.00
1
0.244
6.67
6.92
6.32
0.27
0.12
0.30
3
0.335
6.72
6.97
6.40
0.32
0.17
0.38
5
0.351
6.74
6.99
6.44
0.34
0.19
0.42
10
0.430
6.76
7.00
6.76
0.36
0.20
0.44
(1.000)
6.81
7.14
6.74
0.41
0.34
0.72
600 b)
a) Weight increment is assumed to be due to the formation of the complex, which is given in tool fraction based on the amount of potassium phenoxide initially used (ca. 5 mg or 3.75 • 10 -'~ mol). b) Carried out in an autoclave (5 MPa at 45 ~
490 3.2. Carboxylation of resorcinol. The Kolbe-Schmitt reactions of polyhydroxy phenols such as resorcinol, pyrogallol and phloroglucinol are exceptionally carried out in aqueous solution.[6] Any explanation has not yet been given to the reason of the exception. Kinetic studies on the carboxylation of resorcinol (ROH) with hydrogencarbonate (KHC03) were carried out in aqueous solutions. The rate of the formation of ~-resorcylic acid (RA) was a first order with respect both to ROH and to KHCO3. The overall reaction rate with bubbling carbon dioxide gas into the aqueous solution of potassium hych'ogencarbonates i s d[resorcylic acid]/dt = kl[resorcinol][KHCO3] + k2[resorcinol][pCO2]
( 1)
where the rate constant k2 is obtained by subtraction of the value of ki from the overall reaction rate constant (koch.= ki + k2 ). The resulting facts of ki ~> k,~ revealed, for the first time, that the carboxylation proceeds smoothly without bubbling carbon dioxide. The k2 is comparable with kl only in the case that the amount of potassium ion is insufficient to the concentration of hydroxyl groups of resorcinol. At higher conversion, bubbling CO2 may be utilized to recover hydrogencarbonate according to the reaction: KOH + CO2 ~ KHCO:~ (2) Apparently these results are different from the Kolbe-Schmitt reaction, and further examination has disclosed that the reverse reaction or decarboxylation undergoes easily. The measurement of the formation rate of ROH from RA gave k.~ to be 5.87 x 10% 1, thus Ko~ was calculated to be 2.3 at 80 ~ The kl of the carboxylation of resorcinol with ammonium hydrogencarbonate
was about one tenth of that with potassium hydrogencarbonate. These results are in agreement with the reported lower yield (53.9%) [7] of the product even at applying higher pressure (5 MPa) and higher temperatures (110-125 ~ REFERENCES 1. A.S.Lindsey and H.Jeskey, Chem.Rev. 1957, 57, 583. 2. R.Schmitt, J.prakt.Chem. 1885, [2] 31,397 (cited on p. 594 in ref. 1) 3. J.L.Hales, J.I.Jones and A.S.Lindsey, J.Chem.Soc. 1954, 3145. 4. I.Hirao and T.Kito, Bull.Chem.Soc.Jpn. 1973, 46, 3470. 5. M.S.J.Dewar,"The Electronic Theory of Organic Chemistry" ,pp.168 and 227, Oxford Univ.Press, London, 1949. 6. M.Nierenstein and D.A.Clibbens, "Organic Syntheses" coll.vol.2,
p.557, John
Wiley and Sons, New York, 1943. 7. D.K.Hale, A.R.Hawdon, J.I.Jones and D.I.Packham, J.Chem.Soc. 1952, 3503.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
491
From carbon dioxide to C2 organic molecules mediated by Aresta's nickel carbon dioxide complex Jin I~ Gong,* Chris A. Wright, Matthew Thorn, Kevin McCauley, James W. McGill, Angela Sutterer, Shannon M. Hinze, and Ryan B. Prince, Department of Chemistry, Southeast Missouri State University, One University Plaza, Cape Girardeau, Missouri 63701, USA The key to the success of the conversion of carbon dioxide into C2 or higher organic molecules is carbon-carbon coupling. This paper reports the modified synthesis of Aresta's nickel carbon dioxide complex, (Cy3P)2NiCO2, (Cy = cyclohexyl) and a "Wittig Reaction" of the complex with trimethyl phosphorous ylide. The formed nickel ketene complex, (Cy3P)2Ni[~2-(C,O)-CH2=C=O], has an unusual rl2-C,O bonding mode instead of the ~2_ C,C normally found for the later transition metals. The pathway of this '%Vittig Reaction" is an unprecedented example for a transition metal carbon dioxide complex.
1. I N T R O D U C T I O N Carbon dioxide chemistry has been of great interest due to its fundamental importance and practical applications. To develop efficient catalytic processes in which carbon dioxide can be used as a carbon source is one of the main objectives of this research area. m Carboncarbon coupling is the key to achieve C2 or higher organic molecules. We are reporting a carbon-carbon coupling reaction between the carbon of the carbon dioxide in Aresta's complex and a organophosphorus substrate. Many transition metal complexes have played an important role in activating very stable molecules as well as in stabilizing very reactive molecules. Carbon dioxide is a very stable molecules. The carbon oxygen double bonds has a bond energy of 799 kj/mol. Coordination of carbon dioxide to a transition metal is one of the initial steps in the catalytic conversion of carbon dioxide into useful organic molecules. The electronic structure of a coordinated carbon dioxide is perturbed by bonding to a transition metal center. Different types and degrees of altered reactivities have been observed for different coordination modes of carbon dioxide. The two well known coordination modes of carbon dioxide to transition metals are 112 side-on coordination and 111-C coordination for mononuclear metal carbon dioxide complexes [23]. Aresta's complex, (PCya)ENi[rl2-(C,O)-CO2], has been well studied for its physical and chemical properties. The rl:-(C,O)coordination of carbon dioxide to the nickel complex, (PCya)ENi[rl2-(C,O)-CO2], is favored as a starting complex with the ylide due to the electrophilic carbon of the coordinated CO2. The complex was first reported in 1974 [la'4'51. The reaction between the Aresta's carbon dioxide complex and an ylide provides the first example of a ' ~ i t t i g Reaction" on a coordinated carbon dioxide complex.
492
2. E X P E R I M E N T A L SECTION 2.1 Aresta's complex, (PCyshNt[rl 9 2-(C,O)-CO2], Based on their literature procedures, the preparation was modified to a one step procedure:
+ 2Cy3P
CO2 Cy3 P /
\~)
The preparation is carried out under nitrogen atmosphere. Ni(COD)2 (COD = 1,5cyclooctadiene) was mixed with 2 equivalents of Cy3P in the glove box. minimum amount of anhydrous toluene was added to dissolve the mixture. The reaction flask was immediately brought out of the box and placed into a -20 ~ cooling bath. The reaction solution was bubbled with carbon dioxide gas through a syringe needle for 40 min. The product precipitated as a yellow crystalline solid. The reaction mixture was stirred for another 20 min., and then filtered under vacuunl The yield is 89%. The spectral data agree with the _[3] literature values . The C=O in coordinated CO2 is characterized by a strong IR band at 1740 cm1. 2.2 TIL(C, O) nickel ketene complex,
(CysP)2Ni[TI2-(C, O)-CH2-CO],
Aresta's complex, (Cy3P)2NiCO2 was reacted with freshly made trimethyl phosphorus ylides, R3P=-CH2 in toluene at -20 ~ the reaction solution color was changed from yellow to orange yellow. Keep the solution stirring for two hours then vacuum dry the solution until enough solid precipitated out. Filtrate the solid under vacuum to obtain 42% yield of product. The 31p{1H} N]V[R for (Cy3P)2Ni[yl2-(C,O)-CH2=CO] shows a singlet at 8 27.15 ppm The coupled 31p N]V[R spectrum of the byproduct, Me3P=O, shOWS a multiplet of ten peaks centered at 8 37.98 ppm due to the coupling ofphosphine to the nine protons in Me3. The FTIR shows a strong 1611(s) cm1 peak for C=C stretch. The 1H NMR for the CH2 group of the coordinated ketene is at 8 2.45 (br) ppm The variable temperature (VT) 31p{1H} N]V[R spectra for our nickel ketene complex, (Cy3P)2Ni[~i2-(C,O)-CH2=CO], in toluene-d8, show a singlet at 8 27.15 ppm at 298 K and at 193 K. The VT studies were performed on a Bruker ARX-500MHz NMR spectrometer at the University of Missouri-St. Louis. 3. RESULTS AND DISCUSSIONS Trialkyl phosphorus ylides, R3P=-CH2, are strong nucleophiles and react with organic ketones and aldehydes to form alkenes via a [2+2] cycloaddition mechanism (Wittig Reaction). We have discovered recently that such ylides can also react with free carbon dioxide at one atmosphere and room temperature. When Me3P=-CH2 in THF was bubbled with carbon dioxide gas for 30 minutes, free ketene, CH2=C=O, was produced. The formed
493 ketene immediately dimerize to form methylene-13-propiolactone due to the reactive nature of ketene t61. In order to use a transition metal complex to stabilize the formed ketene, Free diphenyl ketene was reacted with Ni(COD)2 and phosphine ligands such as Cy3P to form nickel ketene complexes. Another way to stabilize the ketenes is to react Aresta's complex, (Cy3P)2NiCO2 with trimethyl phosphorus ylide. We believe that the coordinated CO2 in the nickel complex is expected to be more reactive than free CO2. It is also possible that the Aresta's complex could undergo the dissociation of CO2 first and the ylide could react with CO2. The formed nickel ketene complex in the above reaction showed characteristic IR stretches ofa rl2-(C, O) coordinated nickel ketene complextSl. The band at 1570 cm1 may be assigned to the C=O stretch of the ketene coordinated to the nickel with rl2-(C, O). The IR stretch at 1611 cm1 could be assigned to the C=C double bond in the ketene ligand with a rl2-(C, O) bonding mode. The resulting rl2-(C, O) ketene bonding to late transition metal could be caused by the unique '%Vittig like" reaction mechanism or the electronic effect of phosphorus ligands. Further studies of this bonding effect and the MO calculation is under investigation. The only structurally characterized rl2-(C,O) nickel ketene complex, (dtbpm)Ni[rl2-(C,O)-Ph2C20], (dtbpm, = Bis(di-tert-butylphosphino)methane), was reported by Hofmann's group in 1992 t81. The VT NMR results may suggest either the nickel atom has a geometry close to tetrahedral or the low temperature at 193 K may not have been able to freeze out rotation of the ligandtTl. Free organic ketenes are very reactive molecules. They can be stabilized by transition metal complexes. Transition metal ketene complexes are important intermediates in catalysis, such as in the Fischer-Tropsch process tTl. Ketenes can be bonded to transition metal complexes in a wide variety of waystTal. The two common bonding modes for mononuclear ketene complexes are rl2-(C, O) and rl2-(C, C). The rl2-(C, O) mode is considered to be favored by early transition metal complexes due to the electrophih'c properties of the metals, while the rl2-(C, C) mode is preferred by late transition metals. Indeed, several nickel ketene complexes have been characterized spectroscopically to have the rl2-(C, C) bonding mode, and these ketene complexes have demonstrated much interesting chemistl@ 7a]. The coordinated ketene can be readily converted to various organic molecules such as ketenes, alcohols, aldehydes, acetones and acidstTl. Our preliminary investigation has shown that the rl2-(C, O) nickel ketene complex we isolated has its own characteristic reactivity which differs from the rl2-(C, C) species, (Ph3P)2Ni[rl2-(C, C)CH2=CO], reported in the literature tgl. For example, instead of reacting with nucleophiles, the 112-(C, O) nickel ketene complexes we isolated can readily undergo electrophilic addition of IT to the ketene a carbon to form acetaldehyde. Detailed studies of these ketene complexes with electrophiles, such as HX and RX, and the ligand effects on the bonding modes ofrl2-(C, C) and rl2-(C, O) to nickel complexes are currently under investigation. 4. ACKNOWLEDGMENTS We acknowledge ACS-PRF, #32375-B3, and the GRFC of Southeast Missouri State University for the financial support. C. A. Wright, Kevin McCauley are grateful to the NASA JOVE scholarship. We are grateful to Janet Wilking at UM-St. Louis for the VT NMR experiment.
494 REFERENCES
1. (a) Ayers, W. M. Catalytic Activation of Carbon Dioxide; ACS Symposium Series 363, New York, April 13-18, 1986. (b) Palmer, D. A.; Eldik, R. V. Chem. Rev. 1983, 83; 651. (c) Darensbourg, D. J.; Kudaroski, I~ A. Adv. Organomet. Chem. 1983, 22, pp 129-168. (d) Bratmstein, P.; Matt, D.; Nobel, D. J. Am. Chem. Soc.1988, 110, 32073212. (e) Caballol, R,; Marcos, E. S.; Barthelat, J-C. J. Phys. Chem. 1987, 91, 13281333. (f) Gambarotta, S.; Arena, F.; Floriani, C.; Zanazzi, P. F. J. Am. Chem. Soc.1982, 104, 5082-5092. (g) Jessop, P. G.; Ikarlya, T.; Noyorl, R, Nature, 1994, 368, 231-233. 2. (a) Sneeden, R. P. A.; Villeurbanne, C. N. R. S. Comprehensive Organomet. Chem.; Wilkinson, G: Perga Press: New York, 1982; 8, pp 225-283. (b) Eisenberg, R,; Hendriksen, D. E. Adv. Catal. 1979, 28, 79. (c) Braunstein, P.; Matt, D.; Nobel, D. Chem. Rev. 1988, 88, pp 747-764. 3. (a) Aresta, M.; Gobetto, 1~; Quaranta, E.; Tommasi, I., lnorg. Chem., 1992, 31, 4286. (b) Jegat, C.; Fouassier, M.; TranquiUe, M.; Mascetti, J.; Tornmasi, I.; Aresta, M.; Ingold F.; Dedieu, A., lnorg. Chem., 1993, 32, 1279. (c) Aresta, M.; Nobile, C. F.; Sacco, A. lnorganica Chimica Acta, 1975, 12, 167. 4. Aresta, M.; Nobile, C. F.; Albano, V. G.; Forni, E.; Manassero, M. J. Chem. Soc. Chem. Comm. 1975, 636. 5. (a) Aresta, M.; Nobile, C. J. Chem. Soc., Dalton Trans., 1977, 708. (b) Arce, A. J.; Deeming, A. J. Chem. Soc., Chem. Commun., 1982, 364. 6. (a) TidweU, T. T. Ketene, John Wiley & Sons, Inc., 1995. (b) Wilsmore, N. T. M. J. Chem. Soc. 1907, 91, 1938-1941. (c) Chick, F.; ) Wilsmore, N. T. M. J. Chem. Soc. 1908, 93, 946-950. (d) Clemens, R. J. Chem. Rev. 1986, 86, 241-318. 7. (a) Geoffroy, G. L.; Bassner, S. L., Adv. Organomet. Chem., 1988, 28, 1. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, 1~ G. Principles and Applications of Organotrasition Metal Chemistry, Univ. Sci. Books, 1987, p657-659. (c) Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121. (d) Masters, C. Adv. Organomet. Chem. 1979, 17, 61. (e) Blyholder, G.; Emmet, P. H. J. Phys. Chem. 1960, 64, 470. (f) Ichikawa, M.; Sekizawa, K.; Shikakura, K,; Kawai, M. J. Mol. Catal. 1981, 11, 167. (g) Takeuchi, A.; Kratzer, J. 1~ J. Phys. Chem. 1982, 86, 2438. (h) Muetterties, E. L. J. Chem. Rev. 1979, 79, 479. (i) Bell, A. T. Catal. Rev. 1981, 23, 203. (j) Herrmann, W. A. Angew. Chem., lnt. Ed. Engl. 1982, 21, 117. 8. Hofmann, P.; Perez-Moya, L. A.; Steiglmann, O.; Riede, J. Organomettallics, 1992, 11, 1167. 9. (a) Miyashita, A.; Shitara, H.; Nohira, H. J. Chem. Soc., Chem. Commun. 1985, 850. (b) Miyashita, A.; Grubbs, R. H. Tetrahedron Lett. 1981, 22, 1255.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
495
M e t h a n o l h o m o l o g a t i o n u s i n g c a r b o n d i o x i d e c a t a l y z e d by r u t h e n i u m - c o b a l t bimetallic c o m p l e x s y s t e m Ken-ichi Tominaga a, Yoshiyuki Sasaki a,*, Taiki Watanabe b, and Masahiro Saito a aNational Institute for Resources and Environment (NIRE) 16-30nogawa, Tsukuba, Ibaraki 305, Japan bResearch Institute of Innovative Technology for the Earth (RITE) 9-2 Kizugawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan
The ruthenium-cobalt bimetallic complex system catalyzes the homologation of methanol with carbon dioxide and hydrogen in the presence of iodide salts. A synergistic effect is found between these two metals. The yield of ethanol is also affected by the Lewis acidity of the iodide salt, lithium iodide being most effective. The reaction profile shows that methanol is homologated with CO formed by the hydrogenation of CO2.
1. I N T R O D U C T I O N In the transformation of methanol to ethanol, which is called methanol homologation, synthesis-gas (CO/H2) is usually used as the carbon source (eq. 1). MeOH + CO + 2H2 ----)EtOH + H20, AG ~ = -96.7 kJ/mol.
(1)
This reaction is of interest as an alternative route to the formation of ethylene, since the subsequent dehydration step is well-established. Cobalt complexes are usually used for this reaction with various kinds of promoters such as iodides, phosphines, and other transition metal complexes [1 ]. The possibility of the corresponding reaction using CO2 in place of CO is demonstrated by the thermodynamical calculation (eq. 2). MeOH + CO 2 + 3H2 -+ EtOH + 2H20, AG ~ = -68.3 kJ/mol.
(2)
However, only alkyl formates are formed in the conventional reactions of alcohols, CO 2 and H2 using transition metal complexes, because intermediary hydride complexes generally react with CO2 to give formate complexes. On the other hand, we have found that ruthenium cluster anions effectively catalyze the hydrogenation of CO2 to CO, methanol, and methane without forming formate derivatives [2-4]. Ethanol was also directly formed from CO2 and H2 with ruthenium-cobalt bimetallic catalyst [5]. In this paper, we report that this bimetallic catalytic
496 system can be used for the methanol homologation with carbon dioxide.
2. R E S U L T S
AND DISCUSSIONS
Results of some typical experiments are summarized in Table 1. Although Ru3(CO)I2 is known as a catalyst precursor for the methyl formate formation from CO2, H2 and methanol [6], this catalyst gives a small amount of ethanol (0.7 mmol) instead of methyl formate in DMI solution in the presence of LiI (entry 1). Also formed are CO and methane, which are considered to originate from the hydrogenation of CO2 and methanol respectively. Interestingly, when Ru3(CO)I2 and Co2(CO),~ were used in combination at a metal atomic ratio of 1:2 in which the total amount of metals was held constant (entry 3), the yield of ethanol increased to 3.2 mmol (32 % based on methanol). When all the ruthenium is replaced with cobalt, no methanol homologation or CO2 hydrogenation is observed (entry 2). Therefore, for the methanol homologation with CO2, a synergistic effect of the ruthenium-cobalt bimetallic system is apparent. Methanol homologation is significantly influenced by the nature of the added salts. Among the halide salts, iodide was most effective (entries 4,8,9), just as the case of methanol homologation with synthetic gas [1 ]. As for the effect of cations, the ethanol yield increases in the order of Cs + < Rb + < K + < Na + < Li + (entries 3-7); this order corresponds to the order of Lewis acidity of the cations.
Table 1. Ruthenium-cobalt bimetallic complex catalyzed methanol homologation with CO2 a Yield / mmol Entry
Co/(Ru+Co)
Salts CO
EtOH
CH4
1
0
LiI
7.5
0.7
1.9
2
1
LiI
0.0
0.0
2.9
3
0.67
LiI
8.7
3.2
3.6
4
0.67
NaI
8.3
2.9
3.5
5
0.67
KI
8.2
2.8
3.5
6
0.67
RbI
8.8
2.6
3.6
7
0.67
CsI
9.8
2.0
3.8
8
0.67
NaBr
7.1
1.5
3.4
9
0.67
NaCI
1.6
0.0
2.0
aConditions: Ru3(CO)12, C02(CO)8 (total amount of metals was 0.6 mg.atom), Salt (5.0 mmol), MeOH (10.0 mmol), DMI (10.0 mL), CO2 (20 atm), H2 (100atm), 180 ~ 15 h.
497
It is known that iodide salts react with methanol to give methyl iodide which can subsequently react with transition metal complexes [1]. The substitution of hydroxy group with iodide would be promoted by the Lewis acidic cations coordinating to the oxygen atom as illustrated in Scheme 1.
MI
~-~ 'M+ I- H3C"TO,' "~'H
H30--- OH
~
CH3I + MOH
Scheme 1 There are two possible pathways to homologate methanol with carbon dioxide: the CO2 insertion path and CO insertion path (Scheme 2). As for the former, Fukuoka et al. reported that the cobalt-ruthenium or nickel bimetallic complex catalyzed acetic acid formation from methyl iodide, carbon dioxide and hydrogen, in which carbon dioxide inserted into the carbonmetal bond to form acetate complex [7]. However, the contribution of this path is rather small because no acetic acid or its derivatives are detected in this reaction. Besides, the time course
10
8
\ O
6
E ~
4-
20
'
0
'
'
I
4
'
'
'
I
8
'
'
'
I
'
'
12
'
I
16
'
'
'
I
20
'
'
'
24
Time / h Figure 1. Time course of methanol homologation by CO2 and H2. Conditions: Ru3(CO)12 (0.067 mmol), Co2(CO)8 (0.200 mmol), LiI (5.0 mmol), MeOH (10.0 mmol), DMI (10.0 mL), CO2 (20 atm), H2 (100 atm), 180 *C. A - C O , 9 = EtOH, II = CH4, 9 = MeOH.
498
O CO2
H3C- ~ OmM
H3C--M
H2 CH3CH2OH
H3C M CO 2
~- CO H2
~
~-O Scheme 2
of this reaction (Fig. 1) shows that CO is initially formed and then gradually decreases with an increase in ethanol yield, suggesting that mcthanol is homologated with CO derived from CO2.
3. EXPERIMENTAL SECTION All chemicals were of reagent grade. Catalytic experiments were carried out in a 50 mL hastelloy autoclave. Typically, a 1,3-Dimethyl-2-imidazolidinone (DMI) solution (10 mL) of Ru3(CO)12 (0.067 mmol), Co2(CO)8 (0.200 mmol) and LiI (5.0 mmol) was placed in the autoclave, into which CO2 (20 atm) and H2 (100 atm) were introduced with stirring at room temperature. The reactor was then heated at 180 ~ for 15 h. After the reaction, the reactor was cooled to room temperature and depressurized. The products were quantitatively analyzed by gas chromatography. The present work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO).
REFERENCES
1. W. Keim (ed.), Catalysis in C l Chemistry, D. Reidel Publishing Company, Holland, Dordrecht, 1983. 2. K. Tominaga, Y. Sasaki, M. Kawai, T. Watanabe, and M. Saito, J. Chem. Soc. Chem. Commun., (1993) 629. 3. K. Tominaga, Y. Sasaki, T. Watanabc, and M. Saito, Chem. Lett., (1994) 1391. 4. K. Tominaga, Y. Sasaki, T. Watanabc, and M. Saito, Bull. Chem. Soc. Jpn, 68 (1995) 2837. 5. K. Tominaga, Y. Sasaki, M. Saito, K. Hagihara, and T. Watanabe, J. Mol. Catal., 89 (1994) 51. 6. D. J. Darensbourg, C. Ovalles, and M. Pala, J. Am. Chem. Soc., 105 (1983) 5937. 7. A. Fukuoka, N. Gotoh, N. Kobayashi, M. Hirano, and S. Komiya, Chem. Lett., (1995) 567.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
499
Atmospheric CO2 fixation by dinuclear Ni(II) complex, [TPANi(II)(pOH)2Ni(II)TPA](C104)2 (TPA = Tris(pyridylmethyl)amine) Masami Ito *a, Tatsumi Ishihara b and Yu-saku Takitab
Research and Development Centera Department of Applied Chemistryb, Oita University, Dannoharu 700, Oita 870-11
The dinuclear Ni(II) complex [TPANi(II)(I.t-OH)2Ni(II)TPA](CIO4) 2 (TPA = tris(pyridylmethyl)amine) (2) can react with atmospheric CO2 readily, and forms corresponding ~t-carbonate complex, [TPANi(II)(~t-CO3)Ni(II)TPA](CIO4) 2 (3). 1. INTRODUCTION The atmospheric CO 2 fixation is of special interest from the point of view of environmental chemistry [ 1]. The increase of atmospheric CO 2 will cause serious environmental problem in the near future. Thus, the development of effective chemical method to eliminate CO 2 is an urgent matter. Many CO2 insertion reactions t o the metal-anion ligand bonds are known [2]. However most of the complexes having such reactivity are dioxygen reactive, therefore they can not be the appropriate means for atmospheric CO 2 fixation. Here we will describe the atmospheric CO 2 fixation by using Ni(II) complex. Some reports have presented the reaction of the metal-hydroxy complex with CO 2 affording metal carbonato or bicarbonato complex as shown in scheme 1 [3].
M-L
+ C--O--C
O II
- M--O--C--L
(scheme 1)
Our research goal is to develop effective system for atmospheric CO 2 fixation by using such metal hydroxy complex. To design the suitable ligand for the metal hydroxy complex, it is necessary to investigate the reactivity of the complex toward CO2 systematically by using various ligands. 2. EXPERIMENTAL The dinuclear bis(~t-C1) complex, [TPANi(II)(~-C1)2Ni(II)TPA](CIO4) 2 (1) was synthesized by the reaction of TPA(3HCIO4) 3 and NiCI2 in the presence of triethylamine. [TPANi(II)(~t-OH)2Ni(II)TPA ] (CIO4) 2 (2) [4] was obtained immediately as a blue solid by
500 the treatment of aqueous solution of I with 0.1 N NaOH aqueous solution, (yield ca. 88%). Single crystals suitable for the X-ray measurement could be obtained by the slow diethyl ether diffusion into the methanolic solution of 2. The X-ray structure is . ~ " shown in Figure 1 [5]. The dinuclear(lx-carbonato) complex, C [TPANi(II)(~t-CO 3)Ni(II)TPA] (CIO4) 2 3 was made by the reaction of complex 2 with CO 2 gas [6]. The recrystallization of 3 in methanol afforded the single crystal suitable for the crystallography. The X-ray structure of 3 is shown in Figure 2. 3. RESULTS AND DISSCUSSION
.~
'h
Figure 1 The ORTEP view of complex (2) The counter ions (perchlorate) are omitted for clarity Selected distances (A) and angles(deg): Nl(1)O(1) 2.034(4); NI(1) O(1) 1.990(4); NI(1) N(1) 2.118(6); NI(1) N(2) 2.090(5); NI(1) N(3) 2.113(5); NI(1) N(4) 2.114(5);
The dinuclear Ni(II)(B-OH) complex is newly prepared and the reactivity toward the carbon dioxide of 2 was investigated. The X-ray difraction studies show that 2 crystallizes in the monoclinic space group P21/a with the inversion center. The geometry around the each nickel shows slightly distorted octahedral with N402 donor set. The distances between the bridging hydroxy group and nickel are 1.990 A and 2.034 A. These are typical values found in high spin nickel(II) complex [3c]. The l.W-visible spectra in methanol shows the bands at 585, 770(sh) and 890 nm. These bands can be assigned as typical d-d transition bands in high spin Ni(II) octahedral system (3A2g___~ 3T lg (585
N1
nm), 3A2~3T2g(770 and 890 nm)), respectively. The complex 2 can react with CO 2 gas at r o o m temperature instantaneously, and color change from blue to purple was noted. Purple single crystals could be obtained by the diffusion of diethyl ether into the
NI(2) N(6) 2.037(7); NI(2) N(7) 2.022(7); NI(2) N(8) 2.041(8); O(1)NI(1)O(2)64.5(2); O(1) NI(1)N(1) 163.3(3); O(I)NI(1)N(2) 98.2(3); O(1)NI(1)N(3) 112.5(3): O(1)NI(1)N(4) 95.6(3); O(2)NI(1)N(I) 98.9(3); O(2)NI(1)N(2) 87.1(3); O(2)NI(1)N(3) 176.8(3); O(2)NI(1)N(4) 89.9(3);
0(1)NI(I) O(1)82.1(2); O(1)NI(1)N(1)94.5(2); 0(]) Nl(1)N(2) 178.2(2); O(1)NI(1)N(3)90.4(2);
O(1) NI(1) N(4) 96.5(2); 0(1) NI(1) N(2) 99.4(2);
O(1) NI(1) N(1) 99.2(2); O(1) NI(1) N(3) 102.6(2);
0(1)NI(1)N(4) 178.1(2);
~,,~, ~ ,4
o2
c
03 ,6
i~
NrX ~ )
)
"~ c-,/ (
Figure 2 The ORTEP view of complex (3) The counter ions (perchlorate) are omitted for clarity Selecteddistances(A) and angles(deg): NI(1) O(1) 2.084(6); NI(1) 0(2) 2:093(6); NI(1) N(1) 2.068(7); NI(~) N(2) 2.077(8); NI(1) N(3) 2.059(7); NI(1) N(4) 2.056(8);
NI(2)O(1)2.174(6);NI(2)0(3) 2.023(6);NI(2)N(5)2.095(7);
O(1)NI(2)O(3)63.9(2);O(1)NI(2)N(5)108.4(3);O(1)NI(2)N(6)91.9(3);
O(1 )NI(2)N(7) 167.6(3); O(1)NI(2)N(8) 88.0(3); O(3)NI(2)N(5) 172.3(3); O(3)NI(2)N(6) 98.3(3); O(3)NI(2)N(7) 103.7(3); O(3)NI(2)N(8) 98.0(3);
501 purple methanol solution. The crystallography indicated that the 0.5 [TPANi(II)( IX- OH)2Ni(n)TPA] 2. + CO2 purple species was the dinuclear ~-[TPANi(II)( ix- CO3 )Ni(n)TPA]~'*+ H20 Ni(II) (i.t-carbonato) complex, [TPANi(II)(I.t-CO 3)Ni(II)TPA] e._~ (CIO4) 2 (3) [6,7]. The ORTEP view .~ is shown in Figure 2. The coordination geometry of each nickel .~ atom can be described as a distorted octahedral formed by four nitrogens of TPA and two oxygens from the 0.25 carbonate ion, and crystallography suggests that the two nickel T T coordination structures are asymmetric. The Nil-O1 bond is trans to the alkyl amine nitrogen, N1. In contrast, the Ni2-O1 bond is trans to the pyridine nitrogen, N7. Furthermore, the differences of the bond lengths around each nickel 0350- 450 650 850 suggested the asymmetric structure of Wavelength/nm 3, while these bond lengths are not exceptional v a l u e s as high spin Ni(II) Figure 3 The UV-vis absorption spectra showing the formation of complex 3 upon exposure of 2 to an air, Complex: 2 1.5mM, 1 scan per complex [8]. For instance, Ni(1)-O(1) 10min, solvent: MeOH. is 2.084(6) A, in contrast, Ni(2)-O(1) is 2.174(6) A. The IR band at 1570 cm-1 (C=O) also supports the existence of the carbonato anion in the complex. The UV-Vis absorption spectra showing the formation of the complex 3 upon exposure of 2 to an air are shown in Figure 3. The atmospheric CO 2 fixation reaction was completed within several hours. By comparing the UV-vis spectrum of complex 3, the final spectrum shown in Figure 3 indicated that the reaction proceeded quantitatively. The Z, max value at 585 nm in complex 2 is changed to 542 nm with three isosbestic points, 460, 570 and 700 nm. The ligated carbonate ion is easily released by the treatment with aqueous base. Thus, the reaction of complex 3 (5mM) in acetonitrile with 0.1 N NaOH aqueous solution afforded the complex 2, bis(hydroxy) Ni(II) complex quantitatively and instantaneously [9]. Half of the complex 3 was converted to complex 2 within a minute by the addition of 0.01 N NaOH aqueous solution to the 5mM complex 3 in acetonitrile [9]. As the complex 2 can reacts with CO 2 and formed carbonato complex 3 can release the carbonate ion in the presence of aqueous base, it is expected that the catalytic CO 2 hydration system can be developed by use of complex 2. In summary, the new bis(hydroxo) dinuclear Ni(II) complex was synthesized. This complex showed high reactivity toward atmospheric CO2, and the formed carbonato complex could release the carbonate ion easily by the treatment of aqueous base. We are grateful for the measurement of X-ray crystallography performed by Pros Y. Moro-oka and Prof. M. Akita (Tokyo Institute of Technology).
502 REFERENCES
1
2 3
4
5
6
7
8 9
a) J. B. Martson, M. Oppenhaimer, R. M. Fujita, and S. R. Gaffin, Nature(London), 349, (1991)573. b) D. S. Jenkinson, D. E. Adams and A. Wild, Nature(London), 351, (1991) 304. D.A. Palmer and R. Van Eldik, J. Am. Chem. Soc., 83, (1983)651. a) I. Murase, G. Vuckovic, M. Kodera, H. Harada, N. Matsumoto, and S. Kida, Inorg. Chem., 30, (1991)728. b) T. Tanase, S. Nitta, S. Yoshikawa, K. Kobayashi, T. Sakurai, and S. Yano, Inorg. Chem., 31, (1992)1058. c) N. Kitajima, S. Hikichi, M. Tanaka, and Y. Moro-oka, J. Am. Chem. Soc., 115, (1993)5496. d) M. R. Churchill, G. Davies, M. A. E1Sayed, M. F. E1-Shazly, J. P. Hutchinson, and M. W. Rupich, Inorg. Chem., 18, (1979)2296. e) R. Menif, J. Reibenspices, and A. E. Martell, Inorg. Chem., 30, (1991)3446. f) R. Alsfasser, S. Trofimenko, A. Looney, G. Parkin, and H. Vahrenkamp, Inorg. Chem., 30, (1991)4098. Anal. Found C, 46.44; H, 4.03;N, 11.81%. Calcd for2 Ni2C36N8H38010C12" C. 46.44; H. 4.11; N. 11.81%. IR(KBr, cm" 1), v(C=C), 1601, v(C104) , 1150. UV-vis (nm; e/M" 1 cm -1) 585(70), 770(31), 890(43). The structure was solved by direct methods (MITHRIL) and refined by the full matrix least squares techniques with TEXSAN. All non-hydrogen atoms were refined anisotropically, and they were refined isotropically. Hydrogen atoms were calculated and fixed in final refinement cycles. X-ray data for 2, monoclinic space system with P21/a, a = 16.0166(25) A, b = 13.2909(15) A, c = 9.5625(39) A, (x = 90.00 ~ 13= 98.92(24) ~ y = 90.00 ~ V = 2011.0(1.5) A 3. Z = 2, The R(Rw) value is 7.8(6,8)% for 2833 reflections ( 3 ~ < 0 < 50 ~ Fo>6o(Fo)). Analytically pure solid of 3 was obtained by the recrystallization in MeOH (yield ca. 75%). Anal. Found C, 46.50" H, 4.06; N, 11.34%. Calcd for 3 Ni2C37N8H36011C12 C. 46.47; H. 3.77; N. 11.72%. IR(KBr, cm'l), v(C-C), 1603, v(C--O) 1573, v(C104) , 1130. UV-vis(nm; e/M-lcm'l)542(80), 772(45), 890(95). The structure was solved by direct methods (MITHRIL) and refined by the full matrix least squares techniques with TEXSAN. All non-hydrogen atoms were refined anisotropically, and they were refined isotropically. Hydrogen atoms were calculated and fixed in final refinement cycles. X-ray data for 3, monoclinic space system with P21/a, a = 15.0345(37) ~ b = 14.9787(72)/~ c = 18.9072(73)/~ c~ = 90.00 ~ 13 = 98.40(2) ~ 7 = 90.00 ~ V = 4212(2) A 3. Z = 4, The R(Rw) value is 6.6(5.4)% for 4386 reflections( 5~ < 0 <50 ~ Fo> 4o~o)). M. Mikuriya, I. Murase, E. Asato, and S. Kida, Chem. Lett., 1989, 497. These observations are based on UV-vis spectra.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
503
C a r b o n dioxide fixation w i t h l a n t h a n o i d complex Shohei Inoue, Hiroshi Sugimoto, Noriyuki Ishida and Takahiko Shima Department of Industrial Chemistry, Science University of Tokyo Kagurazaka, Shinjuku-ku, Tokyo 162, Japan 1. Introduction
In the course of our studies on organic and macromolecular syntheses with lanthanoid alkoxide, we have found that a lanthanoid complex, formed by the addition of a lanthanoid isopropoxide (Ln(OiPr)8) to a heterocumulene bearing nitrogen atom (R-N=C=X; isocyanates and carbodiimides), served as a novel CO,. carrier for the rapid carboxylation of active methylene compounds (Y-CI-~-Z) under mild conditions such as room temperature and atmospheric pressure (Scheme 1).
Ln(O4Pr)3 R-N:C=X
.•n,N•OiPr
Scheme I
I
R
C02
Y
>-<
y••'lZ
X
H,-l~OiPr
2. Results and Discussion
Carboxylation reaction of active methylene compounds with lanthanoid system was first attained by using a lanthanoid complex obtained from the reaction between lanthanum isopropoxide (La(OiPr)3) and 2 equiv of phenyl isocyanate (Ph-N=C=O). Under COs at atmospheric pressure, phenylacetonitrile, an active methylene compound (equimolar with respect to La((YPr)3), was effectively
504 carboxylated in DMF at room temperature, where the reaction proceeded very rapidly to give the carboxylated product, a-cyanophenylacetic acid, in 57% yield in only 20 sec. The longer reaction time than 1 min (maximum yield; 69%) under otherwise identical conditions resulted in lowering the yield of a-cyanophenylacetic acid. The La(O~Pr)3-Ph-N=C=O-CO~ system brought about the carboxylation of phenylacetonitrile in a wide temperature range (-40-75~ Of much interest to note is that the reaction even at very low temperature such as -40~ gave a-cyanophenylacetic acid in good yield. The systems from other lanthanoid isopropoxides such as Sm((YPr)3 and Yb(OiPr)~ with 2 equiv of Ph-N=C=O exhibited high activities to give the carboxylated product of phenylacetonitrile in 59 and 50% yield at room temperature in 3 min, respectively. Complex prepared from La(OiPr)~, L-alanine ethyl ester isocyanates, and COs gave a-cyanophenylacetic acid in 59% yield. Similarly, when complex prepared from L-valine methyl ester isocyanate or L-leucine methyl ester isocyanate were used, carboxylation of phenylacetonitrile also took place and gave a-cyanophenylacetic acid in 57% and 51% yield, respectively. When using a complex prepared by the reaction of La(OiPr)3 with 2 equiv of diphenylcarbodiimide (Ph-N=C=N-Ph), the carboxylation of fluorene took place in DMF at room temperature to give 9-fluorenecarboxylic acid in good yield (57% in 1 h). In addition to phenylacetonitrile and fluorene, various active methylene compounds such as indene, propiophenone, phenyl propionate, benzyl p h e n y l a c e t a t e afforded the corresponding carboxylated products by the carboxylation reaction with La(O~Pr)8-Ph-N=C=O-CO~ system. Of fundamental and practical importance is t h a t S-benzyl thiopropionate was effectively carboxylated into a thioester of 2-methylmalonate in a good yield, since this reaction is related to the biological carboxylation of propionyl coenzyme A with a biotin enzyme. Other thioesters were also carboxylated similarly, where successful examples were thioesters of phenylacetic, acetic, and isovaleric acids carrying active methylene and methyne groups, respectively. 3. Conclusion Lanthanoid complexes, formed by the addition of a lanthanoid alkoxide to isocyanate and carbodiimide of appropriate structures, serve as a novel carbon
dioxide carrier for the rapid carboxylation of active methylene compounds under mild conditions.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
505
A study on methanol synthesis through CO 2 hydrogenation over copper-based catalysts Son-Ki Ihm, Young-Kwon Park, Jong-Ki Jeon, Kwang-Cheon Park and Dong-Keun Lee* Dept. of Chem. Eng., Korea Advanced Institute of Science and Technology, 373-1 Kusungdong, Yusong-gu, Taejon 305-701, Korea
In CO2 hydrogenation over Cu/ZrO2 based catalysts, the methanol formation activity could be correlated with copper dispersion. The reaction intermediates of methanol synthesis were carbonate, formate, formaldehyde and/or methoxy, and the rate determining step for methanol synthesis seems to be the conversion of formate into formaldehyde or methoxy.
1. INTRODUCTION The synthesis of methanol over copper-based catalysts is an important industrial process and one of the most investigated catalytic reactions. As methanol can be very easily converted to other valuable materials, its add-value can compensate large energy costs needed. Although methanol synthesis from CO2 hydrogenation over supported copper catalysts has been widely investigated, there are still controversies concerning the methanol synthesis mechanism and the effect of copper on the catalytic activity[ 1-6]. In this work, the influence of the copper dispersion in Cu/ZrO2 catalyst on the catalytic activity in CO2 hydrogenation was investigated. In order to understand the reaction mechanism, FT-IR spectroscopy under reaction conditions and TPD of adsorbed methanol were performed.
2. EXPERIMENTAL Binary copper-based catalysts were prepared by coprecipitation method and some components were added as promoters into the binary catalysts. The methanol synthesis reaction was carried out in a continuous flow microreactor operated at 22 atm and at various temperatures. Reaction pathway of the methanol synthesis was investigated through FT-IR spectroscopy. For the catalyst with a copper content over 15wt%, the diffuse reflectance method (DRIFT) was applied, but for the catalyst with a copper content of 7wt%, the transmission technique was used. For more information about intermediates, TPD of adsorbed methanol was carried out and the products were analyzed using mass spectrometer.
*Present Address : Dept. of Chem. Eng., Res. Inst. Environ. Prot., Gyeongsang Nat. Univ., 900 Kajwa-dong, Chinju 660-701, Kyongnam, Korea Acknowldgement : This work was partially supported by Clean Energy Program by R&D Management Center for Energy and Resources of MTI (Korea).
506 3. R E S U L T S AND D I S C U S S I O N
Among copper based binary catalyst systems, CuO/ZrO2 was proved to be the most reactive toward methanol synthesis. The methanol synthesis activity of the CuO/ZrO2 catalyst was greatly affected by the copper dispersion (or copper crystallite size) ; the smaller the crystallite size, the higher the rate of methanol synthesis (Table 1). When some components of Ce, Cr, Pd, K, V and Zn were added as promoters into CuO/ZrO2, the crystallite size of copper particles changed significantly. CeO2 increased the copper crystallite size significantly, while ZnO made the copper crystallite size much smaller than those of the Cu/ZrO2 samples. Table 1. Ph~csical properties and methanol s)mthesis rate of Cu based catal~csts Catalyst SBET(m2/g) d(nm)* Methanol formation rate (104mol/g-cat 9min) Cu/ZrO2a(pH=7) 55 37 5.48 (pH=9) 78 41 4.99 (pH=l 1) 88 43 4.83 Cu/ZrOzb(pH=7) 60 53 4.75 Cu/ZrO2/CeO2 98 69 3.95 Cu/ZrO2/fr203 102 32 5.72 Cu/ZrO//PdO 69 40 5.39 Cu/ZrO2/K20 42 33 5.56 Cu/ZrO2/V205 90 70 3.38 Cu/ZrO~/ZnO 87 21 8.94 Cu/ZrO2:60/40 (wt%), Cu/ZrO2/MexOy:60/30/10 (wt%), precipitating agent ; aNaOH, bNa2CO3 * : Cu crystallite size by XRD, Temperature : 250~ Pressure : 22atm Fig. 1. shows the DRIFT spectra with time on stream in C O 2 hydrogenation. At 5min, bands at 1060, 1280, 1380, 1520 and 1580cm1 were observed. As reaction time increased, the band at 1580cm-' grew apparently and reached a steady state after 30 min. The bands at 1580, 1380cm-1, the bands at 1520, 1280cm", and the bands at 1060-1080cm 1 could be assigned to bidentate formate, bidentate carbonate and methoxy, respectively[I-4]. It was found that the bands of bidentate carbonate, bidentate formate and methoxy continued to grow in transient state and that formate and methoxy reached steady state. A similar spectra were observed for Cu/ZrO2/ZnO. For ZnO catalyst, however, we found that the formate bands were observed but the methoxy band was hardly observed. Fig. 2 shows the DRIFT spectra with increasing temperature in CO2 hydrogenation over Cu/ZrO2. The Cu formate band at 1590cm1 decreased with temperature. The formaldehyde band at 1120-~1150cm1 increased with temperature upto 190~ but beyond that temperature the band decreased again. From the above results, copper formate, formaldehyde and methoxy were believed to be the intermediates of methanol synthesis on Cu/ZrO 2 catalysts. Temperature-programmed methanol decomposition was observed with DRIFT (Fig. 3). At 50~ monodentate formate, formaldehyde and methoxy were observed at 1600, 1130 and 1080cm~, respectively. As temperature increased, the bands due to monodentate formate and methoxy decreased slowly. The bands at 1580 and 1370cm" due to bidentate formate increased with temperature, but disappered over 270~ Gas phase products from the decomposition of adsorbed methanol were analyzed by mass spectrometer(Fig. 4). Methanol began to appear at around 70~ and disappeared about 200~ which supported the IR results
507
a•
1590
c
1140
i
1520 2000
/
1280
I
I
I
I
1800
1600
1400
1200
I
I
I
I
I
I
I
17001600150014001300120011001000
1000
Wave Number (cm"1)
Wave number(cm "1)
Fig. 1. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst at 250~ and 22atm for (a)lmin (b)5min (c)lOmin (d)15min (e)aOmin (f)6Omin (g) 120min (h) 180min
~6oo
Fig. 2. FT-IR spectra during CO 2 hydrogenation over Cu/ZrO 2 catalyst under 22atm (a)100~ (b) 150~ (c)190~ (d)230~ (e)250~
lo8~
5 v
e-
2000
I
I
I
I
1800
1600
1400
1200
1000
Wave number (cm"1) Fig. 3. FT-IR spectra during methanol TPD over Cu/ZrO2 catalyst at (a) 50~ (b) 70~ (c) 110~ (d)lS0~ (e)190~ (f) 230~ (g)270~ (h)300~
50
100
150
200
250
300
Temperature (~ Fig. 4. Methanol TPD curves by GC-Mass over Cu/ZrO 2 catalyst
508 (Fig. 3) that methoxy peak disappeared above 200~ For C u / Z r O 2 / Z n O , similar results were also obtained. 2000 In addition to above results, a 1~o I transmission FT-IR spectra were obtained to confirm reaction intermediates over Cu/ZrO2 (7/93 in wt%). Fig. 5 shows the FT-IR spectra during CO2 hydrogenation. The band at 2130cm 1 was due to dissociation of CO2 into CO and O. Among reaction intermediates, carbonate and formate were 1410 confirmed through transmission IR spectra. The bands at 1410 and 1540cm -~ were due to carbonate species and the bands at 2930, ax26 I 1 I i I 2860, 2770, 1610 and 1360cm ~ were due to 2200 1800 1400 3000 2900 2800 2700 formate species. The intensities of small bands at about 1600cm-1 due to the water Wave number(cm "l) and the CO band at 2130cm 1 were higher than that of formate bands (2930, 2860cm-1). Fig. 5. IR spectra taken during CO2 hydrogenation This result showed that reverse water gas over Cu/ZrO2(7:93) catalyst under 22 atm at shit~ reaction was favored at lower (a)30~ (b)70~ (e)260~ temperature. This was supported by the reaction product analysis in which CO was the only product below 100~ The intensity of formate band increased with reaction temperature in good correlation with the results that methanol synthesis increased with temperature(not shown). However, formate bands were clearly shown at 70~ but methanol was not detected below 100~ It is believed that formate is difficult to be converted into formaldehyde or methoxy, and that the rate determining step is the conversion of formate into formaldehyde or methoxy.
REFERENCES
1. 2. 3. 4. 5. 6.
G.J. Milar, D.H. Rochester and K.C. Waugh, Catal. Lett., 14 (1992) 289 S. Fujita, M. Usui, E. Ohara and N. Takezuwa, Catal. Lett., 13 (1992) 349 S.G. Neophytides, A.J. Marchi and G.F. Froment, Appl. Catal. A:General, 86 (1992) 45 J.F. Edwards and G.L. Schrader, J. Catal., 94 (1985) 175 Y. Sun and P.A. Sermon, Catal. Lett., 29 (1994) 361 Y. Nitta, O. Suwata, Y. Ikeda, Y. Okamoto and T. Imanaka, Catal. Lett., 26 (1994) 345
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
509
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Mechanistic studies of methanol synthesis from CO2/H2 on C u / Z n O / S i O 2 catalyst Dong-Keun Lee, Dul-Sun Kim, Chang Moo Yoo, Chun-Sik Lee and In-Cheol Cho Department of Chemical Engineering/Environmental Protection, Research Institute of Environmental Protection, Gyeongsang National University, 900 Kajwa-dong, Chinju 660-701. Korea
TPD and infrared spectroscopy were used to study CO2 hydrogenation over Cu/ZnO/SiO2 catalyst. Adsorbed CO2 was believed to form copper formate via the hydrogenation and the formate subsequently. The role of zinc oxide seemed to store more hydrogen during reaction, which promoted the hydrogenation of copper formate to methanol.
1. INTRODUCTION The mechanism and kinetics of methanol synthesis over Cu have been the subjects of extensive investigations [1-3]. Despite considerable research, there still remain controversies as to the exact mechanism by which methanol is synthesized over Cu-based catalysts and very little is agreed upon concerning the nature of active site and the role of ZnO phase. The present work was undertaken to obtain a more detailed mechanism of methanol synthesis from CO2/H2 over Cu/ZnO/SiO2. To do this in situ FTIR was to observe the structure and surface concentration of adsorbed species during the reaction. Complementary TPD studies were also conduced to analyze the surface species.
2. EXPERIMENTAL Cu/ZnO/SiO2 sample was prepared by coimpregnating Cu and Zn nitrate on the surface of SiO2 (Cab-O-Sil M5). The weight loading of Cu and Zn were 5.9 and 2.2%, respectively. Cu/SiO2 having 5.9wt% Cu was also prepared for comparison. The prepared sample was dried and calcined in a furnace at 723K for 16hrs. The calcined catalyst was then reduced with H2 at 523K for 24hrs. The copper surface area of the catalyst was determined by the nitrous oxide titration following the procedure in reference [4]. The Cu dispersions of Cu/ZnO/SiO2 and Cu]SiO2 were 11.7 and 11.2%, respectively. Infrared spectra were recorded with a Bruker IFS66 FTIR spectrometer with a resolution 2-4cm~. About 50mg of the catalyst was pressed into a wafer and the wafer was placed inside the IR cell designed by the method of Hicks et at. [5].
* This research University
was
supported
by
Research
Foundation,
Gyeongsang
National
510 3. R E S U L T AND D I S C U S S I O N
The turnover frequency of methanol synthesis and reverse water gas shift(RWGS) reaction at 533K is presented in Table l. The turnover frequency of methanol synthesis was based on the intensity of the mass spectrometer signal at m/e=31. The turnover frequency for RWGS had been tried to be measured from the rate of CO formation, but the measurement could not be achieved because of interference in the m/e=28 signal due to cracking of CO2 to CO in the mass spectrometer. Therefore the ttmover frequency for RWGS reaction was based on the difference between the measured rate of water(m/e=18) formation and the rate of methanol formation. The turnover frequency of methanol formation and RWGS reaction on Cu/ZnO/SiO2 catalyst is much higher than that on Cu/SiO2, which indicates that the presence of ZnO increases the rate of methanol formation and RWGS reaction. What role, if any, does ZnO phase play on the CO2 hydrogenation reaction? There have been many controversies on the origin of the synergistic effects between Cu and ZnO. Frost [6] suggested that the active site of the catalyst was the oxygen defect in ZnO, electronically modified by copper. Burch and his coworkers [7-9] proposed that hydrogen was spilled over and stored on the oxide during the reaction, which promoted the hydrogenation process. On the other hand the synergistic effect between copper and ZnO was proposed to be due to the formation of formates at the interface of copper and ZnO [10-12]. Table 1. Tunrover frequencies of methanol synthesis and reverse water gas shift reaction on Cu/SiO2 and Cu/ZnO/SiO: at 533K and 900KPa. (H2/CO2 - 3/1) Turnover frequency(sec ~) Catalyst Methanol synthesis RWGS Cu/SiO2 Cu/ZnO/SiO2
6.6 x 105 12.7 x 10-5
1.1 x 10-3 1.8 x 10-3
Complementary TPD studies were conducted on the Cu/SiO2 and Cu/ZnO/SiO2 catalysts which had been exposed to a H2/CO2 (3/1) mixture for 30min at 900KPa and 533K (Fig.l). TPD experiments were carried out in flows of helium with programmed heating up to 650K at a rate of 10K/min. On the Cu/SiO2, the main decomposition temperature was 450K which was very similar to that of maximum decomposition temperature of copper formate [13,14]. The hydrogen peak at 355K is believed to be due to the desorption of the adsorbed hydrogen on copper. On the Cu/ZnO/SiO2, however, the decomposition pattern is somewhat different. Besides the copper formate decomposition temperature at 450K, another decomposition occurs at about 520K. Bowker et al. [14] reported that the decomposition of zinc formate was observed at around 530K. Accordingly the peaks at 520K seem to come from the decomposition of zinc formate. Another difference to be emphasized is the appearance of a broad H2 desorption band at around 395K. More hydrogen is believed to adsorb on the Cu/ZnO/SiO2 catalyst during CO2 hydrogenation reaction. From the TPD results in Fig.l, the roles of ZnO phase on the enhancement of the rate of CO2 hydrogenation seem to be related with the formation of zinc formate and/or the activity of the adsorbed hydrogen on the surface of zinc oxide and/or copper. Since the activity of ZnO/SiO2 has been so low when compared with those of Cu/SiO2 and Cu/ZnO/SiO2, the formation of zinc formate alone is not believed to accelerate the rate of CO2 hydrogenation. More plausible role of ZnO phase may be the suggestion by Burch
511 and his coworkers that hydrogen was spilled over and stored on the ZnO during reaction, which promoted the hydrogenation reaction [7-9].
___[_380
HzO(xl)
.180
HzO(xl)
~
co2(x0.1)
I
~\
~o~(x0.1~ 3 ~
/
~
0
Hz(xl) __
H2(xl) MeOH(xl)
MeOH(xl)
3o0 3~o 4~o 4~o 560 51o 60~ 650 TEMPE~TUr~(K)
300 ~
4~0 4~0 560 510 6o3 650 TEMPERATURE(K)
Fig. 1 TPD spectra taken after the exposure of the Cu/SiO2 (A) and Cu/ZnO/SiO2 (B) to a H2/CO2 (3/1) mixture for 30min at 900KPa and 533K. (He flow rate = 600cc/min, 0.5g catalyst, heating rate = 10K/min) Figure 2 shows IR spectra taken after the exposure of the Cu/SiO2 and Cu/ZnO/SiO2 to a HJCO2 (3/1) mixture for 2hrs at 900KPa and 305K and 533K. On Cu/SiO2 methanol is detected as a shoulder at 2961cm"1 at 533K. The shoulder at 2916cm1 can be ascribed to methoxy groups [15,16]. Bidentate copper formate groups are observed by C-H stretching at 2852cm-1 and 2928-2934crn1, and by O-C-O bending at 1350-1353cm-~. Adsorbed CO is observed by bands at 2137crn1 [17]. The band at 2112cm1 seems to be due to gaseous CO. At 305K a carbonate species can be detected at 1410cmx which were ascribed to a carbonate species symmetrically coordinate to the Cu surface through the oxygen atoms [15].
t
(A)
/| 11 ~137
,
1634 1410[ 113521 [
,92928 f 1~27228 3(6[3) I
19_40
t 2900 I I
U
r
3000 2800 2500 2200 1900 1600 1300
I
3000 2800 2500 2200 1900 1600 1300
WAVENUMBER(cm-1) WAVENUMBER(cm-1) Fig. 2 . FTIR spectra taken after the exposure of the Cu/SiO2 (A) and Cu/ZnO/SiO2 (B) to a HJCO2 (3/1) mixture for two hrs at 900KPa and 305K and 533K.
512 The IR spectra of Cu/ZnO/SiO2 are somewhat different. Methoxy groups of Cu are observed at 2916cm 1, 2900cm 1, 2882cm 1 and 2836cm 1. As the reaction temperature increases, these methoxy groups disappear. Formation of methanol becoms more clear, which provides a further evidence for higher reaction rate of methanol synthesis. In spite of the presence of ZnO and the appearance of zinc formate decomposition peaks in Fig.2, no detectable zinc formate peaks can be observed. So the zinc formate is believed to be a very unstable intermediate during reaction. From the IR spectra taken at different temperatures(not shown) and the literatures for methanol synthesis [18-22], adsorbed CO2 is believed to form copper formate via the hydrogenation and the formate subsequently forms methoxy group on copper surface which will be finally converted to methanol. A number of literatures have proposed that the hydrogenation of copper formate is the rate-determining step in methanol synthesis. The role of zinc oxide seems to store more hydrogen during reaction, which will promote the hydrogenation of copper formate to methanol as suggested by Burch and his coworkers [7-9]. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
H.Kung, Catal. Rev. Sci. Eng., 22 (1980) 235. K.Klier, Adv. Catal., 31 (1982) 2434. K.C.Waugh, Catal. Today, 15 (1992) 51. J.W.Evans, M.S.Wainwright, A.J.Bridgewater and D.J.Young, Appl. Catal., 7 (1983) 75. R.F.Hicks, C.S.Kellner, B.J.Savatsky, W.C.Hecker and A.T.Bell, J.Catal., 71 (1981) 216. J.C.Frost, Nature(London), 334 (1988) 557. R.Burch, R.J.Chappell and S.E.Golunski, J.Chem.Soc., Faraday Trans., 85 (1989) 3569. G.J.J.Bartley and R.Burch, Appl.Catal., 43 (1988) 41. R.Burch, S.E.Golunski and M.S.Spencer, Catal.Lett., 5 (1990) 55. G.J.Miller and C.H.Rochester, J.Chem.Soc. Faraday Trans., 89 (1992) 1109. A.Kinnermann, H.Idriss, J.Hindermann, J.Lavalley, A.Vallet, P.Chaumette and P.Courty, Appl. Catal. A, 59 (1990) 165. K.M.Vanden Bussche and G.F.Froment, Appl.Catal. A, 112 (1994) 37. O.S.Joo, K.D.Jung, S.H.Han, S.J.Uhm, D.K.Lee and S.K.Ihm, Appl.Catal. A, 135 (1996) 273. M.Bowker, R.A.Hadden, H.Houghton, J.N.K.Hyland and K.C.Waugh, J.Catal., 109(1988)263. D.B.Clarke, D.K.Lee, M.J.Sandoval and A.T.Bell, J. Catal., 150 (1994) 81. D.B.Clarke and A.T.Bell, J.Catal., 154 (1995) 314. M.A.Kohler, N.W.Cant, M.S.Wainwright and D.L.Trimm, J.Catal., 117 (1989) 188. G.J.Millar, C.H.Rochester and K.C.Waugh, J.Chem.Soc.Faraday Trans., 88 (1992) 188. G.J.Millar, C.H.Rochester and K.C.Waugh, Catal.Lett., 14 (1992) 289. M.J.Sandoval and A.T.Bell, J.Catal., 144 (1993) 227.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
513
Highly effective synthesis of ethanol from CO2 on Fe, Cu-based novel catalysts Tetsuo Yamamoto and Tomoyuki Inui Department of Energy and Hydrocarbon Chemistry, Graduate Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan
School of
To synthesize ethanol more effectively from COs, the Cu-Zn-A1-K mixed oxide catalyst was combined with the Fe-based catalyst. An F-T type Fe-Cu-A1-K mixed oxide catalyst, which has been developed already in our laboratory[l], converted CO2 to both ethanol and hydrocarbons, while the Cu-based catalyst converted CO2 to CO and methanol with high selectivity. Through the combination of these two catalysts, the three functions were harmonized; C-C bond growth, partial reduction of CO~ to CO, and OH insertion to the products. Furthermore, combination catalyst of Fe- and Cu-based ones was modified with both Pd and Ga to maintain the desirable reduced state of the metal oxides during the reaction. As the result, the space-time yield of ethanol was enhanced to 476 g/1.h at SV=20,000 h 1. 1. I N T R O D U C T I O N
In our previous studies, CO2 was converted to methane[2] or methanol[3] at extraordinarily rapid conversion rates. However, rapid ethanol synthesis from CO2 has been much more difficult owing to both equilibrium limitation and retardation caused by HsO, which inevitably forms in the COs hydrogenation. In this study, to synthesize ethanol from COs with higher yield, a catalyst (Cu-ZnA1-K mixed oxides) having a function of partial reduction of COs was combined with a Fe based F-T type catalyst which we had already developed[l]. Then Pd and Ga, which have promotion effect for Cu-Zn based methanol synthesis catalyst[3], were added to modify the catalyst, and the performance was examined. 2. E X P E R I M E N T A L
The Fe-Cu-A1-K ethanol synthesis catalyst(Cat. 1) and Cu-Zn-A1-K catalyst(Cat. 2) were prepared by applying the uniform-gelatin method[4]. A
514 m i x e d solution consisted of each m e t a l n i t r a t e w a s t r a n s f o r m e d into a gel m i x t u r e by b e i n g t r e a t e d w i t h NH3-H20 v a p o r at 60~ T h e gel w a s d r i e d a n d calcined at 350~ in air w i t h a flow r a t e of 100 ml/min. Pd w a s s u p p o r t e d by a n i n c i p i e n t i m p r e g n a t i o n m e t h o d on a 1060~ a l u m i n a . It w a s calcined at 350~ r e d u c e d in 10% H2 a n d m i x e d w i t h Fe, Cu b a s e d catalyst. Ga w a s a d d e d to F e - b a s e d or C u - b a s e d c a t a l y s t before t h e gelation. F e - b a s e d catalyst, C u - b a s e d catalyst, a n d Pd/A1203 w e r e m i x e d ( C a t . 4,5). The catalysts w e r e t a b l e t e d , c r u s h e d a n d t h e n sieved to 1 0 - 2 4 m e s h . Before t h e reaction, t h e c a t a l y s t s w e r e r e d u c e d in situ in a s t r e a m of 10% H2 diluted w i t h N2 at a flow r a t e of 12,000 h -1 at 450~ T h e r e a c t i o n w a s o p e r a t e d u n d e r t h e following r a n g e of conditions; t e m p e r a t u r e : 270-370~ p r e s s u r e : 80 arm, space velocity 20,000~70,000 h -1, a n d r e a c t i o n gas: 25% CO2 - 75% H2.
3. R E S U L T S A N D D I S C U S S I O N 3-1. P e r f o r m a n c e s o f F e - b a s e d c a t a l y s t a n d C u - b a s e d c a t a l y s t . P e r f o r m a n c e s of each c a t a l y s t is s h o w n in F i g u r e 1. T h e e t h a n o l s y n t h e s i s c a t a l y s t (Fe-based catalyst, C a t . 1) h a v e both functions of F-T s y n t h e s i s a n d alcohol s y n t h e s i s . The m a i n p r o d u c t s w e r e h y d r o c a r b o n s , e t h a n o l a n d m e t h a n o l . W i t h t h e i n c r e a s e of CO in r e a c t i o n gas, t h e yield of e t h a n o l i n c r e a s e d [ I ] . The C u - b a s e d c a t a l y s t ( C a t . 2) c o n v e r t e d CO2 to CO w i t h selectivity m o r e t h a n 70% at a t e m p e r a t u r e r a n g e from 270 to 370~ and other products were methanol a n d a slight a m o u n t of m e t h a n e . E t h a n o l a n d C2+ h y d r o c a r b o n s w e r e not produced. In order to h a r m o n i z e t h e t h r e e functions, C-C b o n d g r o w t h , p a r t i a l r e d u c t i o n of CO2 to CO, a n d O H i n s e r t i o n to products, t h e m i x e d r a t i o of Feb a s e d c a t a l y s t to C u - b a s e d catalyst w a s coordinated at t h e r a n g e from C u / F e = EtOH Cat. 1
MeOH
CO2 Conv. Selec.(C-mol%) (%) EtOH MeOH H.C. CO 41.4 11.4 5.07 69.5 12.4
Cat. 2
30.8
0.00
Cat. 3
39.5
15.8 6.22 60.6 14.9
Cat. 4
47.0
17.4 5.84 62.8 12.3
Cat. 5
54.5
17.0 5.18 64.5 9.72
0
22.5 0.31 77.2
2OO 4O0 600 Space-time yield (g/1. h) Fig. 1 The peformance of catalysts for CO2 hydrogenation Cat. 1 Fe-basedcatalyst : Fe:Cu:AI:K= 1:0.03:2:0.7 Cat. 4 Pd-modified(Fe-based + Ga-modifiedCu-based) Cat. 2 Cu-basedcatalyst : Cu:Zn:AI:K= 1:1:1:0.1 Cat. 5 Pd-modified(Ga-modifiedFe-based + Cu-based) Cat. 3 Fe-based+ Cu-based Cu/Fe = 0.53 CO2/H2= 1/3, SV=20,000h-1,80 atm, 330~
515
0.03 to 1.03. With the decrease of C u ~ e ratio selectivity to hydrocarbons increased, and with the increase of Cu/Fe ratio, selectivity to methanol increased. The selectivity to ethanol was the highest at Cu/Fe = 0.5 (Cat. 3) and CO2 conversion was also the highest at this ratio. 3-2. E f f e c t o f t h e a d d i t i o n o f P d a n d Ga Pd and Ga was added to Cat. 3(Cat. 4,5). As shown in Fig. 2, with an increase of Pd content, yield of ethanol increased, and ethanol STY attained a maximum at around Pd/Fe = 0.02, and above at that content the ethanol yield decreased while the crystallite size of Pd increased monotonously. Addition of Ga was more effective when Fe-based catalyst was modified with Ga before the gelation. As shown in Figure. 3, when the content of Ga was 0.16 (Ga/Fe, atomic ratio) the ethanol yield attained a maximum (Cat. 5). The results of TPR m e a s u r e m e n t indicates that Ga suppresses the reduction in H2. As the result of coordination of Pd and Ga modification, the space-time yield of ethanol amounted to 476 g/1.h at a SV of 20,000h -1 (Fig. 1). The reason for the high yield is ascribed to the catalyst beeing maintained in the desirable reduced state of the metal oxides for exhibiting the optimum catalytic performance which could be controlled by the combined catalyst components Pd and Ga through their functions of hydrogen spillover and inverse spillover, respectively.
450
300 ~
300[
21~,
400! [-., 35O
~9 9
o
300
2o~. ._=
200~ ~
19
[-~
11~176"~ 2401 ~
,""
25O
~ _
O
18~
._~ ~9
2000 "
0.01
'
0:02
'
"o
Pd/Fe Fig. 2 Effect of Pd modification on space-time yield of ethanol Fe:Cu:Zn:Al:K:Ga=l:0.53:0.5:2.75:0.7:0.16 C021H2=1/3, SV=20,000 hl , 80atm, 330~ *calculated from XRD
2ooi
17 ~l: , - -7"--, 0.10 0.20 0.30 0 ., 4 0 0.50 Ga/Fe Fig. 3 Effect of Ga modification on space-time yield of ethanol Fe: Cu:Zn:A1 :K:Pd= 1:0.5 3:0.5:2.75:0.7:0.02 CO2/H2=1/3, SV=20,000 h1, 80atm, 330~
0.00
3-3. E f f e c t of t h e s p a c e v e l o c i t y o n t h e Pd- a n d G a - m o d i f i e d c a t a l y s t The effect of space-velocity (SV) on the Pd- and Ga-modified catalyst(Cat. 4) is shown in Figs. 4 and 5. With an increase of SV, the conversion to hydrocarbons
516 decreased markedly, and on the contrary the conversion to CO increased. At a SV of 70,000 h 1, the yield of ethanol on Pd-modified catalyst decreased; however, compared to that, the yield was maintained on the Ga-modified catalyst. That means, the excessive effect of Pd for the hydrogen spillover reduced the surface of catalyst, and the activity of the catalyst decreased. At higher SV, a large quantity of hydrogen remained because of low CO2 conversion, and the effect of hydrogen spillover was promoted, and was too excessive even on the catalyst of low Pd loading. Furthermore, the results indicate that at the short contact time or high SV with catalyst surface and reaction gas, the main product was CO and at the long contact time or low SV, CO decreased and hydrocarbons, ethanol, and methanol increased. This means that CO is an intermediate of the formation of ethanol and hydrocarbons.
o
30
o
9
"~O
8 ~ d 7
o~- 20
H.C.
Pd
r..)~ 6 5
~r..)
.~ l0
~ >
4
O
3
(.9
~9
> = o
Ga modification
0 10000
30000 50000 70000 Space velocity (h -~)
Fig. 4 Effect of space velocity on the conversion to each product Fe:Cu:Zn:AI:K:Pd:Ga = 1:0.53:0.5:2.75:0.7:0.02:0.16 C02/H2- 1/3, 80 atm, 330~
2 ' 20000 3;000 40000 5;000 6;000 70000 Space velocity(h1) Fig. 5 Effect of Pd- and Ga-modification on conversion CO2 to ethanol Fe:Cu:Zn:AI:K:Pd:Ga -1:0.53:0.5:2.75:0.7:0.02:0.16 C02/i--I2 = 1/3, 80 atm, 330~
REFERENCES [1] T. Inui, [2] T. Inui, [3] T. Inui, [4] T. Inui, (1982).
M. Inoue, T. Takeguchi, J. Lee, Catal. Lett., to be submitted. M. Funabiki, M. Suehiro, T. Sezume, JCS Faraday I, 75, 787(1979). H. Hara, T. Takeguchi, J. Kim, Catal. Today, 36 (1997) 25-32. M. Suehiro, Y. Saita, T. Miyake and Y. Takegami, Appl. Catal., 2,389
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
517
A study f o r the durability of catalysts in ethanol synthesis by h y d r o g e n a t i o n of c a r b o n dioxide* Katsumi Higuchi a, Yoko Haneda b, Kenji Tabatab, Yoshiko Nakahara b, and Makoto Takagawa a aCorporate Research Laboratory of Mitsubishi Gas Chemical Co. Inc. (MGC) 22, Wadai, Tsukuba, Ibaraki, 300-42 Japan bResearch Institute of Innovative Technology for the Earth (RITE) 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto, 619-02 Japan The durability of catalysts in ethanol synthesis by the hydrogenation of C O 2 w a s investigated by means of XRD, TEM, and EDS. The K/Cu-Zn-Fe oxides catalyst was deactivated by the segregation of catalyst components to FeCO 3, ZnO, and Cu during the reaction. The segregation was prevented by the addition of Cr component to the catalyst. Consequently, the K/Cu-Zn-Fe-Cr oxides catalyst indicates long catalytic life. 1. I N T R O D U C T I O N It was reported that the K/Cu-Zn-Fe oxides catalyst efficiently converted a mixture of C O 2 and H 2 into ethanol by Mitsubishi Gas Chemical and National Institute of Material and Chemical Research [1]. However, the catalyst was deactivated quickly during the reaction. To improve the catalytic life, an addition of various kinds of components was tried. It was found that the addition of Cr component to the catalyst prevented the deactivation of catalyst [2]. In this paper, we describe the effect of the addition of Cr component to the catalyst from the results of XRD analysis, transmission electron microscope observation (TEM), and energydispersive X-ray microanalysis (EDS) of the catalysts before and after the reaction. 2. E X P E R I M E N T A L
The catalysts were prepared by co-precipitation method from aqueous solution of metal nitrates of Cu, Zn, Fe, and Cr and NaOH aqueous solution. Potassium was impregnated to the precipitate with K2CO 3 aqueous solution. The composition of catalysts were as follows; CAT A: K/Cu-Zn-Fe=0.077/1-1-3, CAT B: K/Cu-Zn-Fe-Cr=0.077/1-1-3-0.1. The hydrogenation of CO 2 was performed with a conventional flow reactor for about 150 hours at 300~ and 7.0MPa. The structures of catalysts were identified by means of Rigaku RINT 2000 X-ray diffractometer. The observation of catalyst particles and the micro analysis of their compositions were carded out by means of Hitachi HF-2000 field emission transmission electron microscope and Kevex DELTA plus 1 energy-dispersive X-ray spectrometer. ,
We wish to thank MITI and the Japan Alcohol Association for their support and approval to the presentation of this paper.
518 3. R E S U L T S A N D D I S C U S S I O N
The results of hydrogenation of C O 2 w e r e shown in Figure 1. In the reaction using K/CuZn-Fe oxides catalyst (CAT A), the CO 2 conversion at the start was 45%, and the ethanol selectivity was 19%. However, the CO 2 conversion fell down quickly. After 114 hours, the CO 2 conversion was 30 %, and the ethanol selectivity was 16%. The study concerning catalytic life revealed that the addition of Cr component to K/Cu-Zn-Fe oxides catalyst prevented the deactivation of catalyst. In the reaction using K/Cu-Zn-Fe-Cr oxides catalyst (CAT B), the CO 2 conversion at the start was 37%, and the ethanol selectivity was 19%. The catalytic activity of CAT B at the start was lower than that of CAT A. However, after 20 hours, the catalytic activity of CAT B reached a steady state, 35% CO 2 conversion and 16% ethanol selectivity, and kept these conversion and selectivity even after 170 hours run. 50
50 --0--
CO2 Conv. of C A T A
40
~
.,..~
~o
30
I*
c7 ~
ztoIIS~ ~ATB I
20
lo
rj
-0
50
1 O0
150
2t
Time / h Figure 1. Hydrogenation of CO 2 on CAT A and CAT B.
I
'l
"
....
I ' ' ' ' I ' ' ' ' I
\
I , , , , I , ' , , I ' ' ' ' I ' '
9
I,a)
I
9
9
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'.
9 Fe304
"
0 FeCO3
-"
] ~zu~
-
. I
. '''I'' .
.
. I ' ' ' ' I.
.
.' ' ' I
.
. ''I'
I
'
'.
O)Fe304 FeCO 3
(a)
Ocu
A ZnO
::
O
9
~
.
....
9
..o
9
(b)
(b 9
9
9
9
I0
20
30
~O~I)
40
50
60
70
20 / deg. Figure 2. XRD profiles of CAT A. (a) Before reaction. (b) After reaction for 114 h.
80
90
I0
20
30
40
50
60
70
80
20 / deg. Figure 3. XRD profiles of CAT B. (a) Before reaction. (b) After reaction for 190 h.
90
519
%~.~,
~,
,"
.~..-
.
,, ~....
.i' ~ ~" ~.
m m
:.~
9
.
,
. , . j.:!
Figure 4. Transmission electron micrographs of CAT A. (a) Before reaction. (b) After reaction for 114 h.
m
10 n m
(a)
.~.,,
9
i:,'
,.
. ~:,.':~: i..~!
,-'"L?~ .........
.._,
Figure 5. Transmission electron micrographs of CAT B. (a) Before reaction. (b) After reaction for 190 h.
m
10 n m
520 To clarify the reason of slow deactivation rate in the reaction using CAT B, we characterized the catalysts before and after the reaction by means of XRD analysis, transmission electron microscope observation, and energy-dispersive X-ray microanalysis. The XRD profile and the TEM of CAT A were shown in Figure 2 and Figure 4, respectively. The structure of CAT A before reaction was the same as that of Fe304 which has a spinel type structure. The small peaks assigned to CuO and ZnO were observed. The existences of metallic Cu and Zn could not be ascertained by XRD analysis. On the TEM observation, the CAT A before reaction consisted of the particles of uniform size, 10-20 nm diameters. The compositions of particles were uniform also. These results suggest that Cu and Zn components are dissolved into the spinel type structure. The CAT A after the reaction for 114 hours was identified as the mixture of Fe304, FeCO3, ZnO, and Cu by XRD analysis. On the TEM observation, the various sizes and shapes of particles were observed. On the microanalysis of composition, the segregation of Cu, Zn, and Fe components was observed. Based on these results, it is cleared that the K/Cu-Zn-Fe oxides catalyst is segregated to FeCO3, ZnO, and Cu during the reaction. This fact suggests that the deactivation of catalyst is caused by the segregation of catalyst components. The XRD profile and the TEM of CAT B were shown in Figure 3 and Figure 5, respectively. The structure of CAT B containing Cr component before reaction was as same as that of CAT A before reaction. Regardless of the long reaction time for 190 hours, the CAT B after reaction retained the spinel type structure. The existences of FeCO 3 and Cu were observed a little. On the TEM observation, the CAT B before reaction consisted of the particles of uniform size, 10 nm diameters. The compositions of particles were uniform. After the reaction, the particles of similar size, 10-20nm diameters, were observed. Most of the panicles had the same composition as that of panicles before reaction. The segregation of catalyst components was observed on only a few panicles. Based on these results, it is concluded that the addition of Cr component to the K/Cu-Zn-Fe oxides catalyst prevents the segregation of catalyst components during the reaction. This fact can explain slow deactivation rate in the reaction using CAT B. It is considered that the Cr component stabilizes thermally the structure of catalyst. 4. C O N C L U S I O N S We make clear the following facts in this study. The K/Cu-Zn-Fe oxides catalyst has a spinel type structure. The K/Cu-Zn-Fe oxides catalyst is deactivated by the segregation of catalyst components to FeCO3, ZnO, and Cu during the reaction. The segregation is prevented by the addition of Cr component to the catalyst. The long life of K/Cu-Zn-Fe-Cr oxides catalyst can be explained by its slow segregation rate. It is considered that the Cr component stabilizes thermally the spinel type structure of the catalyst.
REFERENCES 1. H. Arakawa and A.Okamoto, Chem. Chemical Ind., No.47, (1994) 1314. 2. M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa, and H. Arakawa, Proceedings of ICCDU IV, P-057, Kyoto, Japan, 1997, Elsevier.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
521
D e v e l o p m e n t o f stable catalysts for liquid-phase methanol synthesis from CO2 and H2 H. Mabu sea,T. Wat anab ea,M. Sait o b "Research Institute of Innovative Technology for the Earth(RITE) 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto,619-02, Japan bNational Institute for Resources and Environment(NIRE) 16-3, Onogawa, Tsukuba-shi, Ibaraki, 305, Japan
This work focuses on the investigation of the stability of catalytic activity in the liquid phase methanol synthesis process. Novel catalysts with a long-term stability have been developed by the addition of hydrophobic materials. The addition of hydrophobic materials were effective for slowing down the crystallite size growth and inhibition of deactivation of catalyst as compared with the original catalyst without modification.
1. INTRODUCTION Methanol synthesis from CO2 and H2 has received much attention as one of the most promising processes to convert CO2 into chemicals. Gas-phase methanol synthesis process should recycle a large quantity of unconverted gas and furthermore the single pass conversion is limited by the large heat release in the reaction. Liquid-phase methanol synthesis in solvent has received considerable attention, since temperature control is much easier in the liquid phase than in the gas phase. Several types of reactors have been proposed such as the liquid entrained reactor(I) and the Trickle bed reactor(2). The authors have been studying a liquid-phase methanol synthesis process in order to develop a new technology as an alternative for a gas-phase process, and reported that a new process employing liquid-liquid separation of the products from the solvent has several advantages in practical methanol synthesis(3). Lee et a1.(4,5,6) have studied the phenomenon of crystallite size growth and the post-treatment using carbon dioxide in Cu/ZnO-based methanol catalysts. They have concluded that the produced water is one of the most strongly suspected species promoting the crystallite size growth and the existence of ZnCO3 slows down the rate of crystallite size growth in the liquid phase methanol synthesis. In the present study, novel catalysts with a long-term stability for the liquid-phase methanol synthesis process have been developed by the addition of hydrophobic materials.
522 2. E X P E R I M E N T A L 2.1. Catalyst A Cu/ZnO-based multicomponent catalyst(Cu/ZnO/ZrO2/Al203) prepared by a conventional coprecipitation method was used in the present study. A mixture of aqueous solution of metal nitrates and an aqueous solution of Na2CO3 were added dropwise to distilled water. Subsequently, the precipitate was filtered out, washed with distilled water, dried in air at 393K overnight, calcined in air at 623K for 2 hr. Two kinds of hydrophobic silica and the special silicone oil replaced a part of methyl group in dimethyl silicone oil by hydrogen were employed for the hydrophobic treatment of the catalyst. Two kinds of hydrophilic silica were used for comparison. The hydrophobic silica and hydrophilic silica were mixed physically with the powder of calcined catalyst. The special silicone oil diluted with isopropyl alcohol to the prescribed concentration was impregnated with the powder of calcined catalyst, removed isopropyl alcohol in air at room temperature and polymerized in air at 523K. The catalysts were pelletized to ca. 5 x 20mm cylindrical pellets under a pressure of 20MPa and crushed to the size of 1-2ram.
2.2. Apparatus and procedures The activity of the catalyst was examined using a liquid-phase continuous reactor described elsewhere (3). The reaction conditions were as follows:temperature=523K,total pressure=15 Mpa, H2/CO2=3/1, recycle rate of solvent=100 l-solvent/l-cat./hr. XRD measurements were performed to analyze the structure of the catalyst. The contact angle between the catalyst and water was measured in order to evaluate the hydrophobicity of the catalyst. The thermal treatment in presence of mixture of water, methanol and solvent(n-dodecane) was performed in order to evaluate the modified catalyst in a brief period.
3. RESULTS AND DISCUSSION 3.1. Crystallite size of Cu The results in Table 1 show the contact angles before the thermal treatment and the crystallite sizes of Cu after the thermal treatment. The contact angles of the catalyst modified with the hydrophobic materials were larger than that of the catalyst without modification. The Cu crystallite sizes of the catalysts modified with the hydrophobic materials were smaller than
Table 1 Contact angles and Crystallite sizes of various modified catalysts Additive Hydrophobic silica A Hydrophobic silica B Special silicon oil Special silicon oil Hydrophilic silica A Hydrophilic silica B Original catalyst
Component (wt%) 10 10 5 10 10 2 m
Contact angle (degree)
Crystallite size of Cu (nm)
30 60 85 150 m ---
22.4 20.0 12.6 9.2 28.1 22.0 26.1
523 that of the catalyst without modification. The special silicone oil was most effective among another hydrophobic materials. It seems that the hydrophobicity of materials are connected with the sintering of Cu with the exception of the hydrophilic silica B. Another experiment showed that water caused a great growth of Cu crystallite size of the catalyst,while methanol had a small effect. These findings suggest that the addition of the hydrophobic materials to the catalyst could suppress the sintering of Cu particles in the catalyst. 3.2. A c t i v i t y
A long-term methanol synthesis test was performed using the catalyst modified with the special silicon oil(5wt%), the catalyst mixed physically with the original catalyst without modification and the catalyst modified with the special silicon oi1(10wt%)(50/50), and the catalyst modified with the hydrophilic silica B(l%). It was performed using the original catalyst without modification for comparison. Figure 1 shows the change in the activities of these catalysts. The activity of the original catalyst without modification decreased gradually with time. The deactivation of the catalyst modified with the hydrophilic silica B was lower than that of the original catalyst. On the other hand, the activities of the catalysts modified with the special silicon oil were relatively stable at higher level.
8O0 [ 7OO
/muI-i._....
ur
~
o+,-4b
/ 600
<
/++,~/ 5OO /,y
L400
,
,
,-
..+..::!./
-
9 special silicone oil 9 original/silicone oil 9 hy'drophilicsilica B 0 original catalyst
+.~_+~+_0~. . . .
+,"
-'S 300 2()0 0
100
200
300
400
500
600
Time on stream(hr) Figure 1. Change in the activities of the catalysts 3.3. X - r a y d i f f r a c t i o n p a t t e r n
Figure 2 shows X-ray diffraction patterns of the catalysts after the liquid-phase methanol synthesis. The crystallite size of the catalyst modified with the special silicone oil was lower than that of the original catalyst without modification. The crystallite size of the catalyst modified with the hydrophilic silica B was a little lower than that of the original catalyst. And also the formation of Zn2SiO4 phase was detected in the catalysts modified with the special silicone oil and the hydrophilic silica B after the liquid-phase methanol synthesis. The catalyst modified with the hydrophilic silica B was more stable than the original catalyst without
524 modification. Therefore, it is also possible that the formation of Zn2SiO4 in the catalyst improve the stability of the catalyst.
-
".
OZn2SiO4 phase catalvst modified with ] a special silicone oil catalyst modified with hydrophilic silica B
original catalyst 20
30
40
l
I
I
50
60
70
80
20( ~ ) Figure 2. X-ray diffraction patterns of catalysts after the liquid-phase methanol synthesis 4. CONCLUSION The catalysts with a long-term stability for the liquid-phase methanol synthesis process have been developed. The addition of hydrophobic materials to the catalyst could suppress the sintering of Cu particles in the catalyst and then result in a long-term stability of the catalyst. The modification of Cu/ZnO-based catalyst by the hydrophobic treatment is very useful for improving a long-term stability of the catalyst for the liquid-phase methanol synthesis from CO2 and H2.
ACKNOWLEDGMENT This work is partly supported by New Energy and Industrial Technology Development Organization.
REFERENCES 1. S.Lee,V.R.Parameswaran,I.Wender, and C.J.Kulik, Fuel Sci. and Tech.Int'l,7(1989)899-918 2. A. Akgerman, S. Tjandran and R.G.Anthony, Ind.Eng. Chem.Res.,32( 1993)2602-2607 3. K.Hagihara, H.Mabuse, T.Watanabe,M.Kawai and M.Saito Energy Convers. Mgmt., 36(1995)581-584 4. A.Sawant, S.Lee and A.Foos, Fuel Sci. and Tech. Int'1,6(1988)569-589 5. V.R.Parameswaran, S.Lee and I. Wender, Fuel Sci. and Tech. Int'l,7(1989)899-918 6. S.Lee,B.G.Lee and C.J.Kulik, Fuel Sci. and Tech. Int'1,9(1991)977-998
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
525
Ethanol synthesis from carbon dioxide and h y d r o g e n Makoto Takagawa a, Atsushi Okamoto", Hiromitsu Fujimura a, Yuriko Izawa a, and Hironori Arakawa b Corporate Research Laboratory, Mitsubishi Gas Chemical Co. Inc., 22 Wadai, Tsukuba City, Ibaraki 300-42, Japan b National Institute of Material and Chemical Research, 1-1 Higashi, Tsukuba City, Ibaraki 305, Japan
Abstract K/Cu-Zn-Fe oxides catalyst was found effective for ethanol synthesis by the catalytic hydrogenation of C O 2. The catalyst gave an ethanol selectivity of 20C-% with a CO2 conversion of 44% at 7.0MPa, 300~ GHSV 5,000, and H2/CO 2 in the feed 3(mol). The ethanol STY of 290 g/L-cat-h was achieved at the GHSV of 20,000. The addition of Cr reduced the deactivation of the catalyst remarkably. The K/Cu-Zn-Fe-Cr oxides catalyst was revealed useful for practical purposes. 1.INTRODUCTION Re-utilization of C O 2 a s a chemical feedstock becomes very important from the standpoint of preventing the global warming. Recently, studies concerning methanol synthesis by the hydrogenation of CO2 have been increased [1,2]. However, studies to produce ethanol, more valuable than methanol, have been quite meager. We have investigated to develop a method for ethanol synthesis by the catalytic hydrogenation of CO2, and found a catalyst having a good availability.
2.EXPERIMENTAL The hydrogenation of C O 2 w a s carried out with a conventional flow reactor equipped with a fixed catalyst bed. The inner diameter of the reactor was 8mm and lg of catalyst granules ( 0 . 5 ~ lmm) was loaded. Standard reaction conditions were as follows; 7.0 MPa, 300~ GHSV 5,000 ml/ml-cat.h, H2/CO2 in the feed 3(mol). The reactor-effluent gas was analyzed with on-line connected gas chromatographs. Catalysts were prepared principally by co-precipitation method and were reduced in the reactor with H2 stream prior to the reaction. 3.RESULTS AND DISCUSSION
3.1.Catalyst Investigation Catalyst search was carried out in order to find the effective ones for ethanol synthesis from CO2/H 2 and the results were summarized in Table 1.
526 Table 1
Summary of reaction results in catalyst search .
Catalyst Temperature Conversion (atomic ratio) (~ (%)
CO
K/Fe (1/5) Cu-Zn (1-2) K/Cu-Fe (1/5-5) K/Cu-Zn-Fe (0.077/1-1-3)
73.8 18.8 33.0 57.4 75.9 14.6 23.6 5.9
L:aci'ionL;n:diii;'n:si
250 300 250 300 250 300 250 300
3.27 25.9 23.1 27.0 10.9 34.0 22.3 44.2 '::Mi;al
........SH
i0:0Oml
Selectivity (C-%) MeOH EtOH Oxy.* H.C.** 0.19 0.15 67.0 42.1 0.60 1.01 1.7 2.0 mi:L;ii
7
1.34 5.46 0 0 1.76 10.8 10.5 19.5 .....
0.25 5.79 0 0 2.19 6.97 3.9 7.4
20.9 47.9 0 0 19.6 33.4 28.6 46.1 .......
* C3H7OH + C4H9OH + CH3CHO + CH3COOCH 3 + CH3COOC2H 5 ** C 1 ~ C 5 hydrocarbons In order to produce ethanol by CO2 hydrogenation, the catalyst should have two functions; C-C bond formation and C-O bond partial preservation. In the case of the CO/H2 feed gas system, the former is industrially performed in Fischer-Tropsch synthesis, while the latter in methanol synthesis. K/Fe oxides catalyst, being effective in Fischer-Tropsch synthesis, was found to produce C-C bond in CO2 hydrogenation. It converted CO2 into CO, alcohols, and hydrocarbons. Cu-Zn oxides catalyst, practically used in methanol synthesis from CO/CO2/H2 mixture, was found unable to produce C-C bond; it converted CO: to CO and methanol without any other detected compounds. The addition of Cu to K/Fe oxides catalyst enhanced its ability of ethanol production. K/Cu-Fe oxides catalysts, prepared by kneading K 2 C O 3 with Cu-Fe co-precipitate, gave the ethanol selectivity of more than 10C-% at 3000(;. The combination of K/Fe and Cu-Zn gave remarkable results on ethanol production. In the reaction over K/Cu-Zn-Fe oxides catalyst, CO2 conversion of 44% and ethanol selectivity of 20C-% were obtained under the standard reaction conditions. Besides ethanol, hydrocarbons were produced with a selectivity of almost 45C-%. The formation of hydrocarbons seems inevitable as long as Fe-based catalysts are employed. The reaction products obtained over K/Cu-Zn-Fe oxides catalyst showed SchulzFlory distribution. This suggests that C-C bonds were formed under the mechanism similar to that in Fischer-Tropsch reaction.
3.2. Activities of K/Cu-Zn-Fe Catalyst Figure 1 shows the influence of temperature upon the reaction over K/Cu-Zn-Fe oxides catalyst. In the temperature range lower than 250~ CO was the main product among oxygenated compounds. As the reaction temperature rose, however,
527 CO formation was suppressed and the production of ethanol became dominant. At 300~ ethanol selectivity reached a maximum value of about 20C-%. Figure 2 shows the influence of GHSV, which was varied between 2,000 and 20,000. The ethanol selectivity remained almost constant in the whole range, and the CO2 conversion decreased slightly as the GHSV value increased. As a result, with an increase in the GHSV value, ethanol STY increased monotonously up to 290 g/L-cat.h at the GHSV of 20,000. Figure 2 shows that methanol selectivity remained quite low in the whole range studied. It suggests that ethanol was not formed from methanol. In order to elucidate the role of CO in CO2 hydrogenation, activity tests were carried out with the feed gas, in which CO2 was partly replaced with CO. The results are illustrated in Figure 3. It shows that the ratios of the yields of ethanol, methanol, and hydrocarbons (C1--~C5) scarcely changed with the replacement of CO2 by CO. The total yield of the products increased with an increase in the CO/CO2 ratio. The total yield dependence on the CO/CO2 ratio is attributable to the difference of reactivity between CO and CO2. In Figure 2 CO selectivity remained low in the whole range. These results shown in Figure 2 and 3 suggest that ethanol was produced directly from CO2 as well as CO. In methanol synthesis from CO/CO2/H2 mixture, it was reported that methanol is produced directly both from CO and CO2 [3]. 50 f
50 CO2 c o n v . ~
i~40
/ ~30
~
350
"~40
"-~
300 250
CO selec. selec
~30 IEtOHselec.
~
I STY
200 O
g 2O lOO
~10 H selec.
O
~ o
o 150
200 250 300 350 400 Temperature, ~ Figure 1. Influence of temperature 7 MPa, GHSV = 5000, Hz/COz = 3
0
5
10 15 GHSV Figure 2. Influence of GHSV 7 MPa, 300~ H2/CO2 = 3
o 20~ xl03)
3.3. Catalyst Life K / C u - Z n - F e oxides catalyst has good activity and s e l e c t i v i t y in ethanol synthesis from CO2/H2 as described above. However, its activity was found to decline quickly during the reaction. In order to prevent the deactivation of the catalyst, the additions of 5 th component to the catalyst were studied extensively and Cr was found to have a remarkable effect. The effects of Cr addition are shown in Figure 4. In the reaction over K/Cu-Zn-Fe-Cr oxides catalyst CO2 conversion as well as ethanol selectivity attained steady values after 40 hours. At the steady state,
528
50
CO/CO2 [IIEtOH EIMeOH EOxy. FAHC v l O t h e r j
9
40
0/25
O
S 3o
5/20
Q
00
o
O K/CuZnFe 9K/CuZnFe-Cr ~ OOoO
9
o
25
10/15
I
25/0 0
20
40 60 Yield, %
80
r~ 20 ~
15
~
lO
ID,
0
oo
10
O
~
100
go
i
200
oOoo
i
300
i
400
500
Time on stream, h
Figure 3. Influence of CO/CO2
Figure 4. Effect of Cr on durability
7 MPa, 300 ~
7 MPa, 300~
GHSV = 5000, H2 / (CO+CO2) = 3
9
GHSV = 5000, H2 / CO2 = 3
the ethanol STY was 150 g/L-cat.h at the GHSV of 10,000. This STY value exceeds that generally needed for industrial processes (at least 100 g/L-cat.h). Therefore, it is said that K/Cu-Zn-Fe-Cr oxides catalyst has a good availability for the ethanol synthesis by the hydrogenation of CO2 in industrial scale. The role of Cr for preventing the deactivation of K/Cu-Zn-Fe oxides catalyst will be described elsewhere [4]. 4.CONCLUSION K/Cu-Zn-Fe oxides catalyst was found effective in ethanol synthesis by CO~_ hydrogenation. It gave a CO2 conversion of 44% and an ethanol selectivity of 20C-% under the standard reaction conditions; 7.0MPa, 300~ GHSV 5,000, H2/CO2 in the feed 3(mol), but had a drawback that its activity declined rapidly during the reaction. However, the addition of Cr to the catalyst showed a remarkable effect on preventing its deactivation. K/Cu-Zn-Fe-Cr oxides catalyst obtained in this study is revealed available for the ethanol synthesis by the hydrogenation of CO2 in practice. 5.ACKNOWLEDGEMENT We wish to thank MITI and the Japan Alcohol Association for their support and approval to the presentation of this paper. REFERENCES 1. T.Fujitani, I.Nakamura, T.Watanabe, T.Uchijima and J.Nakamura, Catal. Lett., 35 (1995) 297. 2. D.Andriamasinoro, R.Kieffer and A.Kiennemann, Appl. Catal. A, 106 (1993) 201. 3. M.Takagawa and M.Ohsugi, J.Catal., 107 (1987) 161. 4. K.Higuchi, Y.Haneda, K.Tabata, Y.Nakahara and M.Takagawa, ICCDU IV P055, Kyoto, Japan (1997).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
529
New preparation method of Cu/ZnO catalysts for methanol synthesis from carbon dioxide hydrogenation by mechanical alloying H. Fukui a, M. Kobayashi a, T. Yamaguchi a, H. Kusama b, K. Sayama b, K. Okabe b and H. Arakawa b aSendai Institute of Material Science and Technology, YKK Corporation, 38, Shimosakuranoki Akaishi Tomiya, Kurokawa, Miyagi, 981-33, Japan bNational Institute of Materials and Chemical Research (NIMC), Tukuba, Ibaraki, 305, Japan
Hydrogenation of carbon dioxide to methanol was investigated over Cu/ZnO catalysts prepared by mechanical alloying(MA) method, which is suitable for excellent mixing of different materials to make alloys or composites. The catalytic activity increases with mechanical milling time, and methanol yield over the catalyst milled for 120 hour is about 1.5 times higher than that of conventional coprecipitated Cu/ZnO catalyst. The reason for increasing catalytic activity by MA method can be attributed to the preparation of well mixed structure of Cu and ZnO nanocrystals. 1.
INTRODUCTION
Global warming caused by CO2 emissions is at present a very crucial problem. Methanol synthesis by CO2 hydrogenation over catalyst has been recognized as one of very effective methods for CO2 fixation, and Cu/ZnO based catalysts are well known for their high performance in this reaction. They are usually prepared by wet processing method using metal salt solutions, t) But little attention has been given to apply for preparation of catalyst by dry processing method. We applied the preparation of catalyst by dry processing methods. Especially, MA is one of promising methods, which is suitable for well mixing of different materials to make alloys or composites, for the preparation of the Cu/ZnO catalysts. Consequently, we have succeeded to prepare highly effective catalysts for methanol synthesis. 2.
2.1
EXPERIMENTAL
Catalyst preparation
Cu/ZnO catalyst preparation was conducted by MA method of a mixed powder of Cu and ZnO (Cu/ZnO =50/50wt%) to MA treatment in a ball mill of alumina, with the milling time varied from 2 to 120 hour.
530
2.2
Reaction procedure
Each catalyst thus obtained was tested for CO2 hydrogenation to methanol using a pressurized flow type fixed bed reactor. Reaction was performed under the concentration of H2/CO2(75/25vol%) =3/1, space velocity (SV)=26000hr-1, temperature=250~ and pressure= 5MPa. An effluent gas was analyzed by on-line gas chromatograph.
2.3
Catalyst characterization
X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis were carried out to observe any structural changes in the catalyst powders during milling. The specific surface area and Cu metal surface area of these catalysts were determined by BET measurement and N20 titration. 2) 3. 3.1
RESULTS AND DISCUSSION
Effect of MA method on catalytic activity
Figure 1 shows the effect of milling time of the Cu and ZnO mixing powders on catalytic activity comparing with the results 400 of the conventional coprecipitated Cu/ZnO catalyst of the same composition prepared by the ~. 300 method are additionally. Before milling, this mixed powder had not 200 catalytic activity. But, by CH3OH yield of coprecipitated mechanical milling, catalytic Cu/ZnO catalyst activity increased, and methanol ~ 100 yield over the catalyst milled for 120 hour is about 1.5 times higher than that over conventional 0 20 40 60 80 100 120 140 coprecipitated Cu/ZnO catalyst. It is Milling time (h) significantly interesting that active catalyst was prepared by this easy Figure1 Methanol synthesis activity (STY) method, so we studied the cause of overCu/ZnO(50/50wt%) catalysts as a function increase of the catalytic activity due of millingtime. Reaction conditions: 250~ to MA method. 50MPa, SV=26000h", H2/CO2=3 !
3.2
Structure and surface area
Figure 2 illustrates the variation of XRD patterns of Cu/ZnO catalysts with milling time. this figure indicates that the Cu/ZnO phase compositions are invariant to milling time. However, as suggested by changes in the width of the XRD-peaks, crystal grains of the
531
Cu/ZnO powder become small with milling time. Thus the diameter of Cu and ZnO particles were estimated from XRD peaks corresponding to Cu(lll), ZnO(101) by Sherrer's equation. The results are shown in Figure 3. Both Cu and ZnO powder grains decreased to nano meter size rapidly within 24 hour, but over 24 hour milling, the grain size did not change markedly. A milling time of 120 hour reduced grain size to 20nm, resulting in superior mixing of the Cu and ZnO particles. As a result of these phenomenon, one of the reason of the increase catalytic activity is supposed to be due to the decrease of grain size of Cu and ZnO powders. Figure 4 shows a TEM photograph of the Cu/ZnO catalyst powder with 120 hour milling. In this result, both Cu and ZnO powders crystal grains size are about 20nm, which corresponds with the particle size estimated by using Sherrer's equation from XRD peak. In addition, Cu and ZnO powders are well mixed. From this result, it is found that the catalytic activity is affected by mixing of Cu and ZnO powder. This result is agreement with hypothesis, that active site of catalyst is the interface of Cu and ZnO particle. 3)
Figure 5 shows the change of specific surface area of Cu/ZnO
|
.
.
.
.
.
.
I oCu 9 ZnO 120h agO
o
9
o
.+
.L '1.~ 9
o
i
20
30
i
40
i
i
i
50 60 70 2 0 (degree)
i
80
i
90
100
Figure 2 XRD pattern of the Cu/ZnO(50/50 wt %) catalysts after 2 ~ 120 hour milling
2o0 150
~ 9 100
~
511
0 0
20
40 60 80 100 Milling time (h)
120
140
Figure 3 Grain size of Cu and ZnO in Cu/ZnO(50/50 wt %) catalysts as a function of milling time
532
3 ~
Total(Cu+ZnO)
t~
N 2 ~
1 O 0
Figure 4 TEM image and electron diffraction of the Cu/ZnO(50/50 wt %) catalyst after 120h milling
~ 20
40
60 80 100 120 140 Milling time (h) Figure 5 Specific surface area and Cu surface area of Cu/ZnO(50/50 wt %) catalysts as a function of milling time
catalyst and Cu surface area with the milling time. The specific surface area of Cu/ZnO catalyst powders were invariant to milling time, but Cu surface area was increased with mechanical milling. This result suggests that these catalyst powders particle is not broken by mechanical milling. Because of these powder is relatively soft, and only extend and junction is repeated. In addition, Cu is softer than ZnO, so Cu is exposed to the surface of catalytic powder. Judging from the above, the reason for the increase of catalytic activity is due to the well mixed structure of Cu and ZnO nanocrystals and the increase of surface Cu by MA method. We conclude that MA method is an effective method for obtaining high performance Cu/ZnO catalysts compared with conventional metal oxide precipitation method by wet processing method.
REFERENCES 1. H. Arakawa, J-L. Dubois and K,Sayama, Energy Convers. Mgmt, 33, 1992, 521 2. G.C. Chinchen, K.C. Waugh, D. A. Whan, Appl. Catal., 25, 1986,101 3. J.Nakamura, I. Nakamura, T.Uchijima, Y.Kanai, T. Watanabe, M. Saito, T. Fujitani, Catal. Lett., 31, 1995,325
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
533
Promoting effect of calcium addition to Pd/SiO2 catalysts in CO2 hydrogenation to methanol A. L. Bonivardi, D. L. Chiavassa and M. A. Baltan~ Instituto de Desarrollo Tecnol6gico para la Industria Quirnica (INTEC) Gtiemes 3450, 3000 Santa Fe, Argentina - E-mail:
[email protected]
Earlier work by this group has focused on the selective production of methanol by syn-gas activation on Pd/SiO2. New experimental data suggest that these Ca-promoted catalysts are also promising materials for CO2 recycling via its selective hydrogenation to methanol.
1. INTRODUCTION A reduction of the present levels of CO2 in the atmosphere has become a subject of increasing concern as regard the environmental pollution problem. Beside other choices currently under consideration its catalytic transformation at point sources, by hydrogenation into more valuable products (e.g.: methanol), has been found particularly attractive. To this end, both commercial and novel (poison-resistant) types, among which supported catalysts based on Pd and other noble metals are gaining acceptance [1 ] and are now under scrutiny. Yet, the activity and selectivity of supported Pd catalysts in the activation of carbon oxides strongly depends on the nature of all their active components. In a previous presentation of some of us it was reported that on well-purified silica (DAVISON G-59, mesoporous, Sg = 270 m2/g) and in the absence of anionic or cationic promoters, Pd/SiO2 catalysts obtained via ion exchange (I.E.) produced methanol with very high selectivity from syn-gas. However, the catalytic activity was modest, almost that of Pd black [2]. More recently we have investigated the impact of Ca promotion on these I.E. catalysts by adding them calcium acetate, a decomposable salt that would not lead to anionic promotion (viz., residual chloride ions). Our strategy was to either minimize or maximize the Ca-Pd interaction. So, while the catalysts obtained by incorporating Ca on air-precalcined Pd/SiO2 materials (423K) showed TORcr~3OH that could reach up to 40-fold that of the unpromoted ones (ScH3OH-->98%), the activity of those materials where calcium was added on pre-reduced Pd did not differ from that of the unpromoted Pd/SiO2 The present piece of work, in an effort to contribute in establishing a correlation between catalytic performance, catalyst structure and surface composition, presents combined results of reactivity in activation of syn-gas (CO/H2) and CO:M: mixtures. Hydrogen chemisorption (HChS), TEM, and XPS and Raman spectroscopic techniques have also been used.
Thanks are given to Universidad Nacional del Litoral and to CONICET. The continuous support of the Japanese International Cooperation Agency (JICA) is gratefully acknowledged.
534 2. EXPERIMENTAL
Palladium acetate in aqueous ammonia (pH = 11) was used to ion exchange Pd (2 % w/w) onto the pre-purified, calcined support. After air drying in stove, the Pd tetraammine complex was decomposed to the diaammine one at 423K. Part of the stock was used to prepare a first set (S Series) of Ca promoted catalysts where maximum Ca-Pd interaction was expected; a second part was H2 reduced instead (723K @ 2 K/min) to minimize the said interaction (R Series). Different amounts of Ca(AcO)2 were added by incipient wetness, in vacuo, to aliquots of both stocks (Ca/Pd = 0.1, 0.2, 0.5, 1.0, and 2.0 at/at) and then water was sublimated. Both series were calcined at 673K (@ 2 K/min) in dry, CO2-fi'ee air and then reduced in H2 at 723K. Catalytic activity was evaluated at 3.0MPa, 493-523K, using the following binary mixtures: CO/HE: 1/2.5 [B1], CO2/H2" 25/75 [B2] in a copper-plated differential microreactor using always the same W/F ratio. The reaction products were analyzed by GC (FID/TCD). Surface composition and oxidation states were determined with an ESCA PHI 5300 unit (Mg anode; pass energy of 8 eV; BE values referred to the Si 2p peak). TEM studies were performed in a JEOL-100 CX microscope. Raman spectra were obtained using a Jasco Laser Raman System (Model TRS-600SZ-P) equipped with a CCD 9000 detector. The 514.5-nm line of a 200 mW powered argon laser was used to excite the sample.
3. RESULTS AND DISCUSSION
The values of total surface area by BET and mean pore diameters indicate that the structure of the different materials is independent of the CaJPd ratio and/or the preparation method (Table 1). On the other hand, the size of the palladium crystallites is not modified by the presence of calcium only on the R Series catalysts. On the S Series materials, though, the larger the Ca contents is the larger the Pd crystallite diameter (TEM) becomes. The agreement between TEM and hydrogen chemisorption particle size data is very good. Table 1 Catalysts structure vs. Ca content and catalysts preparation method. Code Ca/Pd Stot(BET) Dpore dp(TEM) (*) (at/at) m2/$ (nm) (nm) SiO2 G-59 271 16.6 R0 0.0 266 16.8 1.4 R2 0.2 266 16.5 1.4 R3 0.5 259 1.4 R Series R4 1.0 259 1.5 R5 2.0 255 16.5 1.4 SO 0.0 274 16.7 1.5 $2 0.2 264 16.9 1.4 S Series $3 0.5 259 1.4 $4 1.0 256 1.5-2.0 (:l:) $5 2.0 232 17.2 1.4-3.5($)
FE (t) 0.70 0.69 0.71 0.68 0.65 0.75 0.75 0.71 0.61 0.41
dp(FE)
(nm) 1.6 1.6 1.6 1.6 1.7 1.5 1.5 1.6 1.8 2.7
(*) 2.0 % Pd w/w. (~') Fraction of exposed Pd (hydrogen chemisorption). (~) Bimodal distribution. The Pd 3d5/2 BE of the catalysts in the R Series was always 335.5 eV, and constant, which corresponds to small Pd metal particles [3]. Instead, the BE values decreased steadily in the S
535 Series, from 335.5 eV for Ca/Pd = 0 to 335.1 eV when Ca/Pd = 2. The latter result is consistent with TEM findings, as well as HChS data, which indicate that the Pd crystallites are larger the higher the Ca to Pd ratio is (Table 1). The Ca 2p3a signal was always typical of Ca 2+ (347.5 eV), with consistently shifted values (+ 0.7 eV) from those of bulk CaCO3 or CaSiO3, which can be attributed to final or initial state effects quite similar to those observable whenever an oxide is dispersed onto an inert support [4]. The presence of a C ls signal at 289.3 eV only in Ca-containing samples, and in both series, indicates the possible association of the carbonate anion with C a 2+ cations. The Raman spectra of high Ca loading Pd/SiO2 catalysts are shown in Fig. 3 together with the ones of the silica and reference materials.The band at 964 cm~ for the catalysts of both Series can be assigned to the support. The band at 1088 cm1 for sample R5 strongly resembles the features observed in CaCO3 spectra due to the A~g symmetric stretching for calcite [5]. However, the spectrum of $5 catalyst showed a broad band appearing at 1058 cm~ and a weak shoulder at 634 crn~ which are difficult to assign. While it is expected that the calcination of Ca(AcO)2 will proceed to calcium carbonate, the formation of a calcium carbonate silicate can not be ruled out. Therefore, these results together with XPS results confirm the presence of calcium carbonate on R Series and suggest the association of the carbonate and silicate anions with calcium cations on S Series. 9
_
I
9
I
9
i
~
A
,
.
S5
6
..aldlr
~ I
9
CalPd=2
CalPd=2
CalPd=d
v
,m
w c 0
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m
. C a C
1400
0
s---;
'
. . . . . . . . . . . . .
1
2100
'
-~:-
.........
I
_. . . . . . . . . . . . . . . . . . . . .
,
1000 V
(cm
I
800
,
_. . . .
I
600
,
400
"1 )
Figure 1. Raman spectra of R5 and $5 catalysts (Ca/Pd=2.0) and the support (G-59) referred to calcite (CaCO3) and wollastonite (CaSiO3). TEM results on the series S showed small (1.5-2.0 nm) Pd particles on the unpromoted Pd/SiO2 and larger ones (3-5 nm) for Ca/Pd = 2; the former were structurally unresolved. Nevertheless, the XPS spectra gave identical C/Si surface signals in both series. Moreover, the HChS measurements of exposed metal fraction indicate that the average size of the Pd crystallites coincides with that evaluated from TEM data, and allows us to conclude that the noble metal is not significantly covered by either C nor Ca. As said above, we have reported recently that the catalytic activity toward methanol synthesis from a [B 1] mixture is strongly affected by both the method of addition of the Ca promoter to the Pd-loaded silica and the Ca/Pd ratio. Catalysts of the S Series can be 40-fold
536 more active (Ca/Pd=2) than unpromoted Pd/SiO2 or any from the R Series. Using the [B2] mixture, however, the increase in the TORcH3OHowing to the Ca addition was independent of the Ca/Pd ratio or the preparation method (Figure 2.a). a
40-
b
60-
rlCO~ i COZfl~
II
30-
~1~ 45 20
F
]
Dcom2
.1, 4
0 R0 R2 R4
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S2
S4
R0 R2 R4
S0
S2
S4
Figure 2. Activity to methanol as a function of the catalysts preparation method and the reaction mixture (T=523 K, P= 3 MPa, SV = 104 hl). a) Turnover rates; b) Specific rates. Yet, as the fraction of exposed Pd at pseudo steady-state conditions was severely reduced upon exposure to the [B1] mixture (FE _<0.12), while it did not decrease as much using [B2] (0.3 < FE < 0.6), the specific rate for methanol formation was almost equal in the S Series when CO2 was hydrogenated (Figure 2.b). With both reaction mixtures the average Eaet(CH3On) was about equal: 65 + 9 kJ/mol. This similitude fails to support the idea that methanol is produced in a direct reaction of CO2 and not through formation of CO and its consecutive hydrogenation. For both catalyst series the selectivity to methanol was always better than 85% while using the [B 1] mixture. The selectivity could be "boosted" to 95+% in the Ca-containing samples. However, the simultaneous production of CO when carbon dioxide is used as feedstock (reverse WGSR) significantly lowered the selectivity to CH3OH, which could imply that large recycle ratios would have to be employed in real-life processes. Carbon monoxide is known to restructure Pd crystallites but its partial pressure in the reacting system is 20-fold lower when carbon dioxide is hydrogenated, which may explain why the dissimilar degree of Ca-Pd interaction achieved during the pretreatments that led to the S and R Series can be responsible for the marked differences of their catalytic performance for activating the components of syn-gas (but not CO2/H2) under reaction conditions. REFERENCES 1. L. Fan and K. Fujimoto, J. Catal., 150 (1994) 217. 2. A.L.Bonivardi, D.L. Chiavassa and M.A. Baltan~, "New Frontiers in Catalysis - Proc. lOth. Int. Congr. on Catalysis" (L. Guzci et al, Eds.), Vol. B., p. 1801 (1993). 3. Yu. A. Ryndin, L.V. Nosova, A.I. Boronin and A.L. Chuvilin, Appl. Catal., 42 (1988) 131. 4. A. Fern~dez, A. Cavallero and A.R. Gonz~ilez-Elipe, Surf Interface Anal., 18 (1992) 392. 5. K. Nakamoto, in "Infrared and Raman Spectra of Inorganic and Coordination Compounds", 4th Edition. Wiley, New York, 1986.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
537
Direct synthesis of gasoline from carbon dioxide via methanol as the intermediate Hideki Hara, T a t s u y a Takeguchi, and Tomoyuki Inui D e p a r t m e n t of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, J a p a n In order to synthesize gasoline effectively from carbon dioxide through one-pass reaction system, methanol synthesis catalyst was improved. Pd and Ga were added to Cu-Zn based catalyst to optimize the s t a t e of Cu during the reaction. As the result, the space-time yield (STY) of methanol from CO 2 was 1,410 g/l.h at 270~C, 80 a t m and SV=18,800/h. In second stage reactor in which H-Ga or Al-silicate was packed, methanol was converted to gasoline. Maximum selectivity to gasoline fraction was 54.4 % and STY was 312 g/l.h at 320~C and 15 atm. 1. I N T R O D U C T I O N
Catalytic hydrogenation of CO 2 is one of the most effective methods to convert CO 2. In our laboratory, CO 2 was converted to methanol or gasoline via methanol form the viewpoint of effective syntheses of high-value compound from carbon oxides. Hydrocarbon distribution in FT synthesis on classical Co- or Fe-based was wide, and gasoline selectivity was low. Moreover, both conversion rate and octane value were low. In Mobil MTG process, high octane-number gasoline containing a large fraction of aromatics was produced with space-time yield of ca. 0.5 kg/1.h. Considerable amounts of light paraffms were incrutably produced on H-ZSM-5 catalyst as the by products owing to the strong hydrogen shift property of H-ZSM-5 catalyst. Aromatics in gasoline fraction was recently regarded as the cause of harmful compound in the exhaust gas. This process could not be accepted by the modern society. In our previous study, it was found out t h a t the yield of methanol from CO 2 could be much increased by modification of the methanol-synthesis c a t a l y s t with Pd, because it played a roll of the porthole of H 2 spillover [1]. On the other hand, Ga functioned as the porthole of inverse hydrogen spillover [2]. Pd and Ga were then added to the methanol-synthesis catalyst to combine these two functions [3], and a second-stage reactor was connected to the first-stage reactor to synthesize gasoline
538 from methanol [1]. In this study, the catalytic property of H-Ga-silicate packed in the second reactor was compared with that of H-Al-silicate (H-ZSM-5).
2. EXPERIMENTAL 2.1. Methanol synthesis catalyst A methanol synthesis catalyst was prepared by the uniform-gelation method [4]. A concentrated mixed solution of Cu, Zn, Cr, A1, and Ga nitrates was treated with an NH 3 vapor at 60~C for 15 min to transform into a gel state. This was dried and calcined at 600~C. An A1203-supported Pd catalyst was prepared by impregnation method. It was calcined at 350~C for 10 min, followed by hydrogen t r e a t m e n t at 400~C for 30 min. Pd/A1203was physically mixed with the methanol synthesis catalyst. The combined catalyst was tabletted, and crashed to 10 - 24 m e s h to provide the reaction. It was reduced in situ with 1% H~N 2 s t r e a m at 250~C before the reaction. 2.2. Metallosilicate catalyst MFI-type A1- and Ga-silicate catalysts were synthesized by the rapid crystallization method [5], and they were dried at 120~C over night and calcined at 540~C for 3.5 h. The calcined samples were ion-exchanged by 1N NH4NO solution twice, and they were calcined to form H-type silicate. 2.3. R e a c t i o n A pressurized reactor was used for methanol synthesis, and the second reactor was used for methanol-to-hydrocarbon conversion. In the series reactor, a mixture of H 2 and CO 2 was allowed to flow in the first reactor, and all products were introduced into the second reactor. The reaction gas was analyzed by gaschromatographs. 3. R E S U L T AND D I S C U S S I O N
3.1. Effect of modification of Ga and Pd to methanol synthesis catalyst Effect of Ga203 content to the activity for CO 2 hydrogenation to methanol are shown in Figure 1. The conversion of CO 2 to methanol increased, and selectivity to CO decreased with increasing Ga contents. Temperature programed reduction (TPR) was measured under 5% H 2 in N2, at a heating rate 10~C/min and 150-350~C. The weight change of the methanol synthesis catalyst without Ga was ca. 10 wt%. On the other hand, Ga-containing catalyst was ca. 8 wt%. These m e a n t h a t Ga species control the reduction of the catalyst surface. The effect of the addition of Pa and Ga on the catalytic performance of methanol are shown in Table 1. Catalytic activity was increased by the addition of Pa and Ga especially at lower-
539 t e m p e r a t u r e r a n g e a r o u n d 250~C. T h e T P R p a t t e r n s for G a - c o n t a i n i n g a n d Pdmodified G a - c o n t a i n i n g c a t a l y s t s a r e s h o w n in Figure 2. The peak temperature d e c r e a s e d 30~ b y Pd-modification. T h e Pd-modified G a - c o n t a i n i n g c a t a l y s t could c o n v e r t CO 2 to m e t h a n o l with a n extraordinarily high yield of 1,410 g/l. h due to t h e
..~" 30 I o G~ Ga,
o
!
!
o
200
Ga,~.117.8 wt% I 30 ~10.3 wt%
20 ~ O rj
20
or,.)
Ga-containing
~ J
O oo
Pd-modified
""~ 100
Ga-containing f " " , t 9
o o
~
[--
10
o
o
O ~
O 0
250
310 270 290 Temperature (~ C)
0 150
0
i
|
200
i
250
i
300
350
Reduction Temperature (~C)
Figure 1 Effect of Ga203 addition to four components catalyst
Figure 2 TPR profiles of each catalysts 5% H2 in N2, 10 ~
c o m b i n a t i o n of t h e f u n c t i o n s of h y d r o g e n spillover of P d a n d i n v e r s e spillover of Ga. This m e a n s t h a t s t a t e of C u w a s o p t i m i z e d b y p r o m o t i o n a n d c o n t r o l effects, a n d b e c a m e s t a b l e a g a i n s t s e v e r r e a c t i o n condition. Table 1 Effect of p d o G a - modification on methanol synthesis from CO2 CO2 Selectivity (C - mol %) *MeOH STY Catalysts Temperature conversion (~ (%) MeOH CO H.C. (g/1 9h) Cu-Zn-Cr-A1
250 270 250
17.4 21.5 22.3
82.8 17.2 0.0 78.1 21.9 0.0 86.1 13.5 0.4
850 990 1230
270
26.1
8 4 . 3 14.9 0.8
1410
....................................................................................................................................................
Cu-Zn-Cr-AI-Ga-Pd
22% CO2 - 3% CO - 75% H2, SV 18800 h-~ 80 atm Cu-Zn-Cr-A1 -- 38.1-29.4-1.6-30.9 wt% (as oxides) Cu-Zn-Cr-A1-Ga = 38.1-29.4-1.6-13.1-17.8 wt% (as oxides) * on the basis of methanol synthesis catalyst volume
3.2. G a s o l i n e s y n t h e s i s f o r C O 2 v i a m e t h a n o l W h e n H-ZSM-5 w a s m i x e d w i t h a b o v e m e n t i o n e d c a t a l y s t , t h e s p a c e - t i m e yield
of gasoline w a s as low as 20 g/1.h in a single r e a c t o r s y s t e m , a s s h o w n in Figure 3.
540
Since the pressure of H 2 w a s so high that fight olefms were hydrogenated to fight paraffms. On the other hand, when the second reactor was connected to the first reactor and was operated at 15 atm at 300~ the gasoline fraction was obtained on H-ZSM-5 with a higher selectivity of 36.8% and a space-time yield of 222 g/l.h, but fight paraffms were also obtained owing to the strong hydrogen-shift function of the A1 in H-ZSM-5. When H-Ga-silicate was used in the second reactor, the selectivity and the space-time yield of gasoline increased to 54.4% and 312 g/l-h, respectively. The reason for the increase is ascribed to that hydrogen is removed from the reaction system, because the Ga parts in H-Ga-silicate play a role of the porthole of the inverse hydrogen spillover. Feed gas
lst-stage reactor C5C~ A.
H-ZSM-5
CO2-rich Syngas 22% C02 3% CO 75% H2
Feed gas
CO2-rich Syngas 22% CO2 3% CO 75% H2
mixed with Pd-modified composite catalyst 38.1% Cu 29.4% ZnO 1.6% Cr203 13.1% A1203 17.8% Ga203
CO2 cony. 30.3 %
I r ,- ,~ ,~,-t ,, ,, ,,, = CO V MeOH"A"DME,'I 80 atm, 320 ~ [ It ~ ~ / J , " ," t ,4 SV 18800/h (on the basis of methanol synthesis catalyst volume) I ~ I ~ I ~ I
1st-stage reactor Pd-modified composite catalyst 38.1% Cu 29.4% ZnO ~ . 1.6% Cr203 13.1% A1203 17.8% Ga203 ~ l 270~ 80 atm SV 18,800/h Conv. to MeOH 22.0% MeOH STY 1,410 g/1.h
C2-C4 ~
paraffin olefin
0
20
40
u Sel 3.6% STY 20 g/l. h I I [
60
80
100
Products distribution (C-wt%)
2nd-stage reactor C IC2C3
i
(Si/A1 = 100) I
~
C4
~ Se138.6% Gasoline C IC2C3
C4
C5
C2-C4
STY 222 g/1.h C; A.
~ ~~JC~fJA:iii!!ii::i:!!i!~ii::ii~ii!ii~!ii~ - t
Gasoline Sel 54.4%
15 atm, 320~ MeOH conv. 100% paraffin olefin
C~ A.
ILg,,,,,,,,,,,,J
15 atm, 300~ ? e O H cony. 100%
H-Ga-silicate(si/Ga = 400) 11
C5
STY 312 g/l" h [ 0
,
I 20
,
I 40
,
I 60
,
I ~ I 80 100 Hydrocarbon distribution (C-wt%)
Figure 3 Results of gasoline synthesis from carbon oxides via methanol as the intermediate product using the reactors connected in series REFERENCES 1. T. Inui, T. Takeguchi, A. Kohama, K. Kitagawa, Stud. Surf. Sci. Catal., 75, 1453 (1993). 2. T. Inui, Y. Makino, F. Okazumi, S. Nagano, A. Miyamoto, I&EC., Res., 26, 647 (1987). 3. T. Inui, H. Hara, T. Takeguchi, J.-B. Kim, Catal. Today, 36, 25 (1997). 4. T. Inui, T. Takeguchi, A. Kohama, K. Tanida, Energy Convers. Mgmt., 33, 513 (1992). 5. T. Inui, ACS Symp. Series, 3 9 8 , 4 7 9 (1989).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
541
C o m p a r i s o n o f CO2 h y d r o g e n a t i o n in a catalytic r e a c t o r a n d in a d i e l e c t r i c - b a r r i e r d i s c h a r g e Alain BilP, Baldur Eliasson a, Ulrich Kogelschatz a and Li-Ming Zhou b a
ABB Corporate Research Ltd, 5405 Baden, Switzerland
b On leave from Xi' an Jiaotong University, Xi'an, P.R. China
Conversion of the greenhouse gas CO 2 with hydrogen to methanol is studied in a conventional tubular packed-bed reactor filled with a copper-based catalyst. The performance of different catalysts was tested. An alternative approach is investigated using a dielectric-barrier discharge (DBD) reactor with and without the aid of a catalyst inside the discharge space. Comparison of the results shows that the simultaneous presence of discharge and catalyst led to a substantial lowering of the optimum temperature range from 220 ~ in the packed-bed reactor, to about 100 ~ inside the discharge. Regarding the DBD reactor, the remarkable finding is a substantial increase of the methanol yield (factor of ten) when the catalyst is introduced into the discharge space. 1. I N T R O D U C T I O N One technological option that may contribute to reducing carbon dioxide emissions is CO 2 control and CO 2 chemistry [1 ]. This approach concerns recovering CO 2 from sources that contain it in much higher concentrations than the atmosphere. The technology (flue gas scrubbing) is available, for example, in a 300 MW coal-fired power plant at Shady Point in Oklahoma where 200 t of CO 2 are removed daily from the flue gases [ 1]. The question then arises: what can one do with the recovered CO2? One option is CO 2 disposal (disposal in ocean, injection in aquifers, confination in depleted gas or oil wells, injection for enhanced oil recovery). This is now done in Norway (Sleipner gas field) where 2'740 t of CO 2 per day are removed from natural gas. This CO 2 is then compressed and injected into a water-filled sandstone reservoir (deep aquifer) [2]. Another option is to use it directly (without conversion) as a solvent, additive for beverage, propellant in place of CFC . . . . . In Shady Point, all the recovered carbon dioxide is liquefied and sold to the food industry [1 ]. And the last option, which attracts our attention, is to use it as a source in the synthesis of chemicals (synthesis of fuels, intermediates and/or fine chemicals) like the catalytic CO 2 hydrogenation to methanol. Carbon dioxide can be converted to methanol by combining it with hydrogen and using the right kind of catalyst. Of course, the hydrogen must be available on a large scale and its production must be achieved without emission of CO 2 to the atmosphere. This implies that the
542
hydrogen must be produced by using renewable energies like solar, hydro or biomass fuel. Methanol, in addition to being a fuel on its own, can be looked at as a storage and transport medium for hydrogen. Methanol has also the advantage that it is liquid under normal conditions. It can be stored and transported as easily as gasoline, and can be used in conventional combustion engines without requiring any major adjustments. Methanol has three times the energy density (per volume) of liquid hydrogen, and half the energy density of gasoline. Methanol synthesis can thus be looked upon as a way of converting hydrogen into an energy carrier that can be more conveniently stored and transported. We present results on the catalytic hydrogenation of carbon dioxide to methanol in a conventional tubular packed-bed reactor filled with a copper based catalyst. In addition, results of an alternative approach using a dielectric-barrier discharge (DBD) reactor with and without the aid of a catalyst are presented.
2. E X P E R I M E N T A L The experimental configurations of the packed-bed reactor [3], as well as of the DBD reactor [4], are described elsewhere.
3. RESULTS AND DISCUSSION 3.1. P a c k e d - b e d
reactor
The synthesis of methanol is primarily accomplished by passing a H2/CO 2 mixture (3" 1) over CuO/ZnO/A1203 catalysts. The influence of temperature on the experimental CO 2 conversion is given in Figure 1 (thermodynamic equilibrium values are also plotted). 50.0
.~ 40.0. 9 O
"i
30.0 .
O
J
20.0 O r,.)
q
CH,OH equilibrium yield
"
lo.o /"
r,.) I~- ~ ~z
.---"
o.o
lOO
9
140
,
~ "~'
:
.i
'
CH,OH exp. yield I
,
I
180 220 Catalyst temperature [~
,
I
260
.
300
Figure 1. Yield and conversion in a packed-bed reactor (in comparison to equilibrium values) (most active catalyst, P = 20 bar, H2/CO 2 = 3:1, Space velocity = 4500 h -1) The graph shows that temperature has a considerable influence on the conversion of CO2, as well as on product selectivity. With the best catalyst investigated, a methanol yield of 7.1% per
543
single pass is measured at 220 ~ 20 bar and at a space velocity of 4500 h-l.The methanol selectivity is 43.8 %. Major other products are carbon monoxide and water (not plotted) due to the reverse water-gas shift reaction [5].
3.2. Dielectric-barrier discharge reactor Secondly a H2/CO 2 mixture (3:1) is passed through the DBD reactor without catalyst at a pressure of 1 bar and at a total flow rate of 1 Nl.min -1. The electrical power in the reactor is set to 400 W. Figure 2 shows the influence of temperature on the CO 2 conversion. CH,OH 10.0
~Z r..)
c5 r..) z" Q
~z L)
CO 5.0
~D
CH4 0.0
,
50
I
I
100
150
"-... ,
200
Reactor wall temperature [~ Figure 2. Yield (CH3OH, CO, CH4) in a dielectric-barrier discharge reactor (Elec. Power = 400 W, P = 1 bar, H2/CO 2 = 3, Qtot = 1 N l . m i n -1) Temperature does not have a very strong influence on the conversion of C O 2 in the presence of H 2. Methanol formation is observed and its concentration decreases at higher temperatures as expected for an exothermic reaction. A maximum CH3OH yield (0.2 %) is obtained at the lowest investigated temperature (T = 50 ~ and at 1 bar. Conversely, CO and CH 4 yields increase with rising temperature. Maximal measured values at 200 ~ are 11.0 % and 0.5 % for CO and CH 4 respectively. Similar temperature dependency were observed for pure CO 2 [6].
3.3. DBD packed with a catalyst It is easy to imagine the simultaneous use of discharges and catalysts in a technical process. It could be demonstrated that the combination of a DBD and an appropriate catalyst substantially improves the selectivity of the discharge and the performance of the DBD reactor. A summary of the experimental conditions found to maximise methanol formation is given in Table 1.
544 Table 1. Best results for the methanol synthesis from a H2/CO 2 (3:1) mixture Comparison of DBD and packed-bed reactors
Parameter
Experimental configurations DBD reactor DBD reactor Packed-bed reactor with catalyst without catalyst with catalyst E
50
100
100 i
220
Power [W]
400
500
500
500
Pressure [bar]
1
8
8
8
Total flow rate [Nl.min-1]
0.25
0.50
0.50
0.50
Methanol yield [%]
0.2
<0.1
Temperature [~
0 . 8 - 1.0 E
0.22
100
220
0.50
0.50
0 ~
2.2
The presence of a catalyst has a positive effect (factor 10) on the methanol formation at low temperatures. Compared to the performance of the same catalyst in the packed-bed reactor, the simultaneous presence of discharge and catalyst led to a substantial lowering of the optimum temperature range from 220 to about 100 ~
4. C O N C L U S I O N S The experiments with a thermal packed-bed reactor show that copper based catalysts can be used to produce methanol from carbon dioxide and hydrogen. Optimum performance is reached at a catalyst temperature of 220 ~ and a pressure of 20 bar. The experiments with the dielectricbarrier discharge show that small amounts of methanol already form at atmospheric pressure and room temperature. The introduction of a catalyst in the discharge space drastically increases methanol formation in the temperature range around 100 ~ This demonstrates that the presence of activated species generated in this non-equilibrium discharge can lower the apparent activation energy of the catalyst for this process. Its optimum temperature range is lowered by more than 100 ~ Thus the combination of catalysts and discharges can be regarded as an important step in tackling the problem of CO 2 recycling and the development of a new technology to convert greenhouse gases to liquid fuels.
REFERENCES [1]
[2] [3] [4] [5] [6]
B. Eliasson, CO 2 Chemistry: An Option for CO 2 Emission Control?, C.-M. Pradier, J.P. Pradier (Ed.), Carbon Dioxide Chemistry: Environmental Issues, Stockholm (The Royal Society of Chemistry, 1994). A. Smith, in Greenhouse Issues, Number 27, (November 1996), pp. 1. A. Bill, B. Eliasson, E. Killer, Proc. 11th World Hydrogen Energy Conf. (HYDROGEN '96), Stuttgart, Germany. Proc. Vol. II, pp 1989-1995. A. Bill, B. Eliasson, E. Killer, U. Kogelschatz, Proc. 11th World Hydrogen Energy Conf. (HYDROGEN '96), Stuttgart, Germany. Proc. Vol. III, pp 2449-2459. A. Bill, ETH-Lausanne PhD Thesis, to be published. A. Bill, B. Eliasson, E. Killer, U. Kogelschatz, Wokaun A., Energy Conversion and Management, 38, (1997), pp $415-$422.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
545
Methanol synthesis from carbon dioxide on C u O - Z n O - A l ~ O ~ catalysts Masaki Hirano " , Toru Akano a, Tetsuya Imai b, and Kennosuke Kuroda b a Technical Research Center, The Kansai Electric Power Company,Inc., 11-20 Nakoji 3-chome, Amagasaki, Hyogo 661, Japan b Chemical Plant Engineering & Construction Center, Mitsubishi Heavy Industries, Ltd., 3-3-1 Minatomirai, Nishi-ku, Yokohama 220-84, Japan
The activities and durabilities of catalysts having two different A1 z O s content were investigated using a microreactor. The microreactor tests showed that the catalyst containing 5 wt% A1 2 0 a exhibited better durability than the catalyst containing 8 wt% A1 2 0 a . Additional durability test was performed on the catalyst selected on the basis of the microreactor test results by recycling unreacted gases using a bench-scale test facility. The recycling test, conducted for 3000 hours, demonstrated that about 95% of supplied carbon dioxide was converted into methanol under the following conditioins: temperature, 513 ~ 521K; pressure, 9 MPa; make-up gas composition, H z/CO z =75/25 mol%; GHSV, 5000 h - 1 ; recycling ratio, 4 m 3 N/m 3 N.
I.INTRODUCTION Methanol synthesis by catalytic hydrogenation of carbon dioxide is evaluated as one of the most promissing processes for conversion of carbon dioxide into valuable chemicals[ 1]. While various catalysts for methanol synthesis from carbon dioxide and hydrogen have been investigated, their durability - an important factor for evaluating their practicability-have not been reported[2,3,4,5]. The authors prepared CuO-ZnO-A1 2 O s catalysts for methanol synthesis from carbon dioxide and examined their activities and durabilities. 2.EXPERIMENTAL Two kinds of CuO-ZnO-A1 2 0 s catalyst, M-1 having a CuO/ZnO mol ratio 67/33 and 8 wt% A1 2 0 a, and M-2 having a CuO/ZnO mol ratio 67/33 and 5 wt % A1 2 0 a were prepared by the coprecipitation method. Precipitation was carried out by mixing of Cu, Zn and A1 nitrate aqueous solutions with a sodium nitrate solution. The precipitate was filtrated, washed, dried and calcined at 573K.
546 The activities and durabilities of the M-1 and M-2 catalysts were investigated in once-through reaction using a microreactor. Additionally,two kinds of commercial CuOZnO-A1 ~ O 3 catalyst for methanol synthesis (P-l) and for CO shift(P-2) were joined in the microreactor experiment to compare with M-1 and M-2 catalysts. In each microreactor experiment, 1.5 ml of catalyst ( 0.6-1.0 mm in particle size ) was filled in the reactor. The microreactor tests were conducted under the following conditions : temperature, 483 "~ 523K; pressure, 4 MPa; GHSV, 6000h-1 ; feed gas composition,H ~/CO ~./CO=77/17/6 tool%. Additional durability test was performed on the catalyst selected from the microreactor test results by recycling unreacted gases using a bench-scale test facility. In the bench-scaletest, 100 ml of the catalyst (3 mm~X 3raml) was f i l l e d in the r e a c t o r . The schematic diagram of the bench-scale test facility is shown in Figure 1. The composition of the gas at the outlet of the reactor was analyzed by gas chromatography. 3
5
_
6
3
'
N
9
'
N r
9
1 :
9Carbon
3 :
Pressure regulator
4 : Mass 5 :
dioxide cylinder
flow controller
Mixer
6 : Vessel 7 - Make-up 8 9 : 10" 11
L____'t!
Hydrogen cylinder
2
12 : 13
gas compressor
9Recycle gas
compressor
Heater Reactor 9Thermocouple
Condenser 9T r a p
Fig. 1. Schematic diagram of bench-scale test facility 3. RESULTS and DISCUSSIONS 3.1. Activity and durability test with mieroreactor Figure 2 shows the initial methanol synthesis activities of M-l, M-2, P-1 and P-2 catalysts at 483 "~ 523K in once-through reaction using the microreactor. The methanol yield is def'med by the following equation to evaluate the methanol synthesis activity of each catalyst; methanol flow late at the outlet of the reactor (mol/h) Methanol yield(%)= • 100 (CO 2 +CO)flow late at the inlet of the reactor(mol/h) Test results show that M-1 and M-2 catalysts have better methanol synthesis activity than the commercial P-1 and P-2 catalysts. The methanol yields of M-1 and M-2 catalysts were 24 and 22 % at 523 K respectively, and those values were roughly equal to the equilibrium methanol yield (25%). The difference of A1 ~ O 3 content between M-1 and M-2 catalysts did not cause a remarkable difference of the methanol yield.
547
%% % %
X~Equilibriurn yield
3O ,-..,
>, 20
~~
oK: j..ig 4.o
o~
%%%%
o7
0 460
,~o
S~o 5~o Temperature
%\%%%% 9
5~o
5~o
580
[K]
Pressure : 4 MPa, 6HSV : 6000 h-1 Feed gas composit i on :
Figure 3 shows the result of durability test on M-1 and M-2 catalysts using the microreactor. Both the catalysts showed neary the same initial activity, but the methanol yield of M-1 catalyst declined to 10% after about 500 hours, while that of M-2 catalyst declined to 10% after about 3000 hours. This result also showes M-2 catalyst containing 5 wt% A1 2 0 3 have better durability than M-1 catalyst containing 8 wt% A1 ~ O 3. It is concluded from the result that minor difference of A1 2 0 3 content in catalyst composition does not bring about any significant difference in initial activity, but considerable difference in durability .
H=/C0=/C0=77/17/6 mol% Catalyst No. ZX: II1-1, O : B-2, V : P-l, r-I: P-2
Fig.2. Comparison of methanol synthesis over various CuO-ZnO-A1 2 0 3 catalysts 4O
Pressure 94 MPa, Temperature 9503 K, 6HSV " 6000 h-1 Feed gas composition 9H~/C0~/C0=77/17/6 mol% Catalyst No. A " ~ 1 , O" ~ 2
~" 30
20
A lO
O0
Fig.3.
1000 '
Time on stream
Result of durability test on M-1
2000 ' [h]
3 oo 0
and M-2 catalysts using microreactor
3.2. Recycling test using bench-scale test facility Generally, the methanol yield in once-through reaction is low due to the limitation of chemical equilibrium. Therefore, in a practical process, an unreacted gas recycling system should be used to enhance the reaction efficiency. The durability of M-2 catalyst was investigated by recycling unreacted gases for 3000 hours using the bench-scale test facility. Figure 4 showes the change of reaction temperature
548 with time to keep the methanol yield of M-2 catalyst at about 95%. The methanol yield in the recycling test is defined as follows; Produced methanol flow rate (mol/h) Methanol yield(%)= • 100 CO z flow rate in make-up gas (mol/h) It was found that about 95% methanol yield could be kept for 3000 hours by controlling the temperature at 513K after from 400 to 1000 hours, at 516K after from 1000 to 2000 hours and 521K after 3000 hours at 9MPa under the recycled urtreacted gases. 1oo
.
.
A A .
.
.
.
" " .
.
. ~ _ .~ A _.A
. -
zgy-A
. . . . .
or ~ L_. A:: - I ~
80
m "m
>:
Recycl i ng rat io " 4 maN/maN, GHSV " 5000 h-1 Make-up gas composition " H2/C02=75/25 mol%
6o
A D'
Pressure [MPa]
9
lo ,,
~ ' s2o
--A--Z~
6_ 51o E Q~
I"1
I--. 500
i
4O0
I
I 800
I
I 1200
I
I 1600
Time
Fig.4.
I
I 2000
I
I 2400
I
I 2800
I
3200
[h]
Result of durability test on M-2 catalyst underrecycledunreacted gases
4. CONCLUSIONS Two kinds of CuO-ZnO-A1 2 0 3 catalyst with different A1 z O 3 content named M-1 and M-2 for methanol synthesis from carbon dioxide were prepared. Both the catalysts exhibited higher activity than the commercial methanol synthesis catalyst and the commercial CO shift catalyst. The initial activity of M-1 and M-2 catalysts were neary equal to the equiliblium methanol yield (25%) at 523K. M-2 catalyst containing 5 wt% A1 2 0 a showed better durability than M-1 catalyst containing 8 wt%A1 2 0 3 . Those results suggest that minor difference of A1 2 0 3 contents in catalyst composition cause considerable difference in durability despite a little difference in initial activity. Additionally, M-2 catalyst showed about 95% methanol yield for 3000 hours by gradually raising the temperature from 513K to 521K at 9MPa with recycling of the unreacted gases. REFERENCES
1. H.Kumazawa, Kemikaru Enjiniaringu, 2 (1993) 22-26 2. Y.Arakawa, Shokubai, 35 (1993) 96-97 3. T.Fujitani, M.Saito, Y.Kanai, M.Takeuchi, K.Moriya, T.Watanabe, M.Kawai and T.Kakumoto, i b i d , 35 (1993) 92-95 4. M.Saito, i b i d , 35 (1993) 485-491 5. T.Inui, Kemikaru Enjiniaringu, 2 (1993) 61-65
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
549
Optimization of preparation conditions and improvement of stability of Cu/ZnObased multicomponent catalysts for methanol Synthesis from CO2 and H2 S. Luo a, J. Wu a, J. Toyir a, M. Saitob, M. Takeuchi c and T. Watanabe e aResearch Institute for Innovative Technology for the Earth (RITE), 16-30nogawa, Tsukubashi, Ibaraki 305, Japan* bNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan eRITE, 9-2 Kizukawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan
The preparation conditions of Cu/ZnO-based multicomponent catalysts were optimized. The temperature during copreciptation should be less than 313 K, and the removal of Na from the catalyst by washing the precipitates is most important. The addition of a small amount of silica to the catalyst greatly improved the stability of the catalyst.
1. INTRODUCTION Catalytic hydrogenation of CO2 to produce various kinds of chemicals and fuels has received much attention as one of the most promising options for mitigating CO2 emission. In particular, methanol synthesis by CO2 hydrogenation has been considered to play an important role in the transportation of hydrogen energy produced from natural energy such as solar energy, hydropower and so on. At ICCDU-III held at Oklahoma in 1995, the authors reported that highly active Cu/ZnO-based multicomponent catalysts had been developed after elucidatng the role of metal oxides contained in a Cu/ZnO-based catalysts [1 ]. In the present conference, the effects of the conditions for preparing Cu/ZnO-based multicomponent catalysts on their methanol synthesis activities and the effect of silica on the stability of the catalyst in a long-term methanol synthesis operation will be reported.
2. EXPERIMENTAL Cu/ZnO-based multicomponent catalysts (Cu/ZnO/ZrO2/A1203/Ga203 and Cu/ZnO/ZrO2/A1203) were prepared by a coprecipitation method using Na2CO3 as a precipitant, as described in detail elsewhere [2]. The operation conditions for preparing the catalyst such as the temperature during coprecipitation, the time of aging precipitate, the extent of washing the precipitate and so on were varied. A small amount of silica was added New Energy and Industrial TechnologyOrganization (NEDO)research fellow
550 to the catalysts by using a colloidal silica supplied by Nissan Chemical Co. A fixed bed flow reactor was used both for short-term methanol synthesis tests and for long-term tests. The surface areas of the catalysts were measured by N2 adsorption with a flow method. The Cu surface areas of the catalysts were measured by the same way as described elsewhere [2]. Xray diffraction measurements were performed for analyzing the structure of the catalysts.
3. RESULTS AND DISCUSSION
3.1. Optimization of preparation conditions of the catalyst Among the various operation conditions for preparing the precipitate, only the temperature during the coprecipitation had a significant effect on the activity of the multicomponent catalyst. This finding is favorable for preparing a practical catalyst. No significant difference has been observed among the activities of the catalysts prepared at temperatures between 273 K and 313 K, whereas the activity of the catalyst prepared at 333 K was slightly (only 7%) lower, as shown in Figure 1. XRD measurements showed that the crystallite size of the precipitate prepared at 333 K was slightly larger than those of the precipitates prepared at temperatures ranging from 273 K to 313 K. Sodium (Na) remaining in the catalyst after washing the precipitate with distilled water greatly decreased the activity, as shown in Figure 2. The precipitates after being washed different times and then dried overnight at 383 K gave almost the same XRD patterns, whereas the XRD patterns for the catalysts calcined at 623 K became sharper with the increase in the amount of Na in the catalyst. These findings clearly indicate that the Na remaining in the catalyst causes the crystallization of the components in the catalyst, and thus decreases the surface area, the Cu surface area and the activity of the catalyst.
30
800
~" 800 -
O
o 600
2o~
30
600 q |
"~ 400
*6 400
t~
8 o
~200
200
0
0
273 293 313 333 Temperature during coprecipitation (K)
Figure 1. Effect of temperature in coprecipitation on the activity (D) and Cu surface area (e) of a Cu/ZnO/ZrO2/A1203/Ga203 catalyst, Reaction conditions : 523 K, 5 MPa, SV=I 8,000, feed gas composition=CO2(25)/H2(75 )
0
5 1 2 3 4 Amount of Na in the catalyst (wt%)
Figure 2. The activity (o) and the Cu surface area (e) of the multicomponent catalyst as a function of the amount of Na in the catalyst. Reaction conditions were the same as shown in Figure 1.
551
3.2. Improvement of long-term stability of the catalyst The addition of a small amount of silica to the catalyst was found to improve a long-term stability of the catalyst. Calcining the catalyst at high temperatures of around 873 K is also important for stabilizing the catalyst activity. The activity of a Cu/ZnO/ZrO2/A1203 catalyst containing silica and calcined at 873 K decreased by 10% during initial 40 h in methanol synthesis, but after that no significant decrease in the activity was observed until 500 h, as shown in Figure 3. On the other hand, the activity of the catalyst without silica decreased monotonously and was not stabilized until 500 h. Table 1 shows both the surface area and Cu surface area of the catalysts without and with silica as a function of time on stream in methanol synthesis. Both the surface area and the Cu surface area changed in the same manner during the methanol synthesis as the catalytic activity.
80o 700~A~
o
%00
.~,~~o,~o
600 rk,,
A~ Z ~ A
1w1-~
i io0
r
i i i 200 300 400 Time on stream (h)
i 500
600
Figure 3. The effect of a small amount of silica added to a Cu/ZnO/ZrO2/A120 3 catalyst on its long-term stability in methanol synthesis. A: without SiO2, calcined at 873 K, A: without SiO2, calcined at 623 K, o: with 0.6 wt% SiO2, calcined at 873 K, e: with 0.6 wt% SiO2, calcined at 623 K. Reaction conditions: 523 K, 5 MPa, SV = 10.000, CO2/CO/I-I2=22/3/75
Table 1 Changes in the surface area and Cu surface area of Cu/ZnO/ZrO2/A120 3 catalysts without and with silica during methanol synthesis from CO 2 and H 2 Catalysta
Time on stream Surface areab (h) (m2/ml) 1
Cu/ZnO/ZrO2/A120 3 without SiO 2
Cu/ZnO/ZrO2/A120 3 with 0.6wt% SiO 2
Cu/ZnO/ZrO2/A12 03 with 2.2wt% SiO 2
Cu surface areab Activity c (m2/ml) (g-CH 30H/ml.h)
82.9
24.9
750
141
75.2
23.6
679
500
62.7
19.3
583
1
85.3
27.9
721
20
78.8
23.2
632
90
76.9
22.1
607
500
76.8
22.6
609
1 163
73.3 73.5
20.0 13.8
636 395
aCalcined at 873 K. bDetermined after the methanol synthesis reaction. eReaction conditions : 523 K, 5 MPa, SV=10,000, CO2/CO/H2=22/3/75.
552 Figures 4 and 5 show the XRD patterns for the catalysts without and with silica as a function of time on stream in methanol synthesis. The XRD peaks corresponding to ZnO in the catalyst without silica greatly increased in intensity with time on stream, whereas those in the catalyst with silica hardly increased. Furthermore, the effects of silica in the case of Cu/ZnO/A1203 w e r e similar to that in the case of Cu/ZnO/ZrO2/A1203, although the actvity of Cu/ZnO/A1203 was around 20% lower than that of Cu/ZnO/ZrO2/A1203. These findings suggest that the catalyst deactivation should be caused by the crystallization of the metal oxides in the catalyst, especially ZnO, which leads to the reduction of the surface area and the Cu surface area. Therefore, the silica added could suppress the crystallization of the metal oxides in the catalyst and thus lead to the improvement of the stability of the catalyst. On the other hand, when a larger amount of silica (2.2 wt%) was added to the catalyst, the initial loss of the activity was larger than that of the catalyst with 0.6 wt% of SiO2. As shown in Table 1, the surface area of the catalyst with 2.2 wt% of SiO2 did not change during the methanol synthesis for 160 h, whereas the Cu surface area and the activity of the catalyst decreased by 31% and by 38%, respectively. This finding suggests that some of silica might move to the active sites and block them. This could explain the decrease in the activity of the catalyst with 0.6 wt% of silica during 40 h from the beginning. The present work was supported in part by New Energy and Industrial Technology Development Organization (NEDO).
C A: ZrO2 B: ZnO C: Cu
C
A: ZrO2 B: ZnO
500h
C: Cu
B
500h
B AB
C
141h
10
20
30
40 50 20(~)
60
70
80
Figure 4. XRD patterns for Cu/ZnO/ZrO2/ without silica as a function of time on stream in methanol synthesis.
A1203
10
20
30
40
50
60
70
80
2o( ~ )
Figure 5. XRD pattems for Cu/ZnO/ZrO2/A1203 containing 0.6 wt% of silica as a function of time on stream in methanol synthesis.
REFERENCES
1. M. Takeuchi, M. Saito, T. Fujitani, T. Watanabe, Y. Kanai, Abstracts of ICCDU-III, 1995. 2. M. Saito, T. Fujitani, M. Takeuchi, T. Watanabe, Appl. Catal. A 9General, 138(1996), 311.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
553
Effect of solvents on photocatalytic r e d u c t i o n of c a r b o n dioxide using s e m i c o n d u c t o r photocatalysts
Tsukasa Torimoto, Bi-Jin Liu, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University Yamada-oka 2-1, Suita, Osaka 565, Japan
Photocatalytic reduction of carbon dioxide on TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and bulk CdS particles with and without surface modification by several thiol compounds was investigated in various kinds of solvents. Formate and carbon monoxide were obtained as the major reduction products and the ratio of the former to the latter was increased with an increase in the dielectric constant of the solvents used for the use of Q-TiO2/SiO 2 and bare CdS particles as photocatalysts. The surface modification of CdS particles with thiol compounds was effective in enhancing the ratio of formate to carbon monoxide. The observed selectivity of CO 2 reduction products was explained well in terms of the stabilization of reaction intermediates on the photocatalyst surface.
INTRODUCTION Artificial photosynthesis has been intensively studied in view of the chemical storage of light energy. One of the promising approaches is to utilize photoinduced reductions of carbon dioxide using semiconductor photocatalysts [1-7], and so far, several compounds such as formate, carbon monoxide, methanol and methane have been reported. What kind of reduction products are obtained may depend on preparation conditions of the photocatalysts and the environments under which the photocatalytic experiments are carried out. The importance of the latter factor has been suggested from the results that changes in the charged conditions of the CdS photocatalyst surface [3], adsorption of the In3+ metal ion on CdS photocatalyst surface [4], metal deposition on TiO 2 photocatalysts [5], and changes in the dispersion of photocatalysts in zeolite matrices [6] changed selectivities of CO 2 reduction. In the case of photocatalytic reaction using solution systems, environments under which the CO 2 reduction occurs are changed depending on the kinds of solvents used, so that the reduction products seem to be varied. However, little is known about the role of solvents in the reduction behavior of CO 2. In this paper, photocatalytic reduction of CO 2 in various kinds of solvents with use of TiO 2 and CdS as photocatalysts and the role of the solvents in CO 2 reduction behavior are reported.
554
EXPERIMENTAL
TiO 2 nanocrystals embedded in SiO 2 matrices (Q-TiO2/SiO2) and surface-modified bulk CdS particles were used as the photocatalysts. Q-TiO2/SiO 2 was prepared by hydrolysis of Ti(OEt)4 in the presence of Si(OEt)4 [7]. 25 cm 3 ethanol solution containing 80 mM HC1 and 4.4 M H20 was added to an ethanol solution of the same volume which contained 0.10 M Ti(OEt)~ and 1.0 M Si(OEt)4, resulting in a sol containing TiO 2 and SiO 2. After stirring for 2 h, 0.14 cm ~ of sol were cast on a quartz plate (2.0 cm 2) to give a transparent gel film of Q-TiO2/SiO 2 containing 7 ~tmol of TiO 2. In order to modify the surface of CdS particles, five kinds of thiol compounds were dissolved in appropriate solvents, that is, 0.1 M 2-aminoethanethiol in 2-propanol, 0.1 and 0.5 M 1-dodecanethiol in 2-propanol, 0.1 M sodium 2-mercaptoethane sulfonate in water, and 0.1 M sodium sulfide in water. CdS particles (0.72g) were suspended in these solutions and the suspensions were agitated overnight. By repeating centrifugation and washing with water, followed by drying in vacuum, surface-modified CdS particles were prepared. The photoreduction experiments were carried out using CO2-saturated solutions containing the photocatalyst and 1 M 2-propanol as a hole scavenger. 5 cm 3 of the solution was put in a quartz cell whose top was sealed with a rubber septum and irradiated with light of wavelengths longer than 300 nm, which was obtained by passing light from a 500 W high pressure Hg lamp through a colored filter. R E S U L T S AND D I S C U S S I O N
The photocatalytic reduction of CO 2 on the bare CdS particles suspended in acetonitrile in the presence of 2-propanol resulted in the formation of formate and carbon monoxide as the reduction products with a simultaneous production of H 2 (Figure 1). As an oxidation product of 2-propanol, acetone was produced, and no other oxidation products were obtained. As shown in 100
<>
o
E r o ...I -o o EL
0 0
o ro r
1
L
LL
<>
75 50
e
25
"o" "o. ~
f
"'-
h m
0
2
4
6
Time / h
Figure 1 Time course of the production of formate ( II ), carbon monoxide ( 9 ) hydrogen ( V ) and acetone (<~) in acetonitrile solution containing 1.0 M 2propanol. The photocatalyst used was naked CdS.
20
40
60
80
Dielectric constant
Figure 2 The fraction of carbon monoxide as a function of the dielectric constant of the solvent used. The results were obtained by irradiation of the naked CdS (11) and Q-TiO2/SiO 2 photocatalyst ( I--I). Solvents used were (a) carbon tetrachloride, (b)dichloromethane, (c) 2-propanol, (d) propionitrile, (e) ethylene glycol monoethyl ether, (f) acetonitrile, (g) sulfolane, (h) propylene carbonate, and (i) water.
555 Figure 1, the amount of the products increased linearly with time, indicating that the activity of photocatalyst did not deteriorate during the photoreduction experiments. By comparing the sum of the amount of reduction products with that of the oxidation product, it was found that the chemical stoichiometry of the reduction and oxidation products were maintained. In order to investigate the effect of solvents on the reduction behaviors of CO 2 in more details, photoreduction experiments were carried out using the naked CdS and Q-TiO2/SiO 2 in various solutions. It was found that in all cases, formate and/or carbon monoxide were produced and no other CO 2 reduction products were detected. Figure 2 shows the fraction of carbon monoxide in CO 2 reduction products as a function of the dielectric constant of the solvent used. In both cases, the fraction of carbon monoxide decreased with increase of the dielectric constant, though the selectivity of the reduction reactions was a little different, that is, CdS has a tendency to produce more CO than Q-TiO2/SiO 2. These results are explained well in terms of stabilization of reaction intermediate. The CO 2o- anion radicals formed by reduction of CO 2 with photogenerated electrons may be adsorbed on the photocatalyst surface, the degree being dependent on the solvation of CO2~ In high polar solvent, CO 2~ is greatly stabilized with solvent molecules, resulting in the weak interaction between CO2~ photocatalyst surface, as shown in Scheme 1. Then carbon atom of CO2otends to react with a proton to give formate. On the other hand, CO2~ in low polar solvent is strongly adsorbed on the photocatalyst surface, and then an attack of oxygen atom of CO2owith a proton to yield CO becomes feasible. If this view is valid, the results shown in Figure 2 suggest that the CdS surface has a higher affinity for CO2o- than TiO 2.
O'~'.;;O H+ O'~c/OH/ --x-M-x-> \--x-I~1-x-- ~ CO+OH"
-~ 0
1 O0
"10 0 i,..
80
',0 tO
60
9-
20
a
Solvation
H
--~ HCO0
b) high - polar solvents
r
1 O0
0 L
80
'*-0
60
b
c
d
e
(B)
g 40
.~
~U_
20
a
Scheme 1 Illustration of hypothetical reduction pathway of CO 2 on photocatalyst surface. M: metal ion site on the surface.
(A)
40
a) l o w - polar solvents
I
HCOOH
fl co
b
c
d
e
Photocatalyst
Figure 3 The fraction of C O 2 reduction products obtained by irradiation of various surface-modified CdS photocatalysts in acetonitrile (A), and dichloromethane (B). Photocatalysts of (a) to (e) were the same as those given in Table 1.
556 Table 1 The molar ratio of adsorbed thiol to CdS determined by elemental analyses.
Catalyst
Surface-modification condition
a
none
b
Ratio of adsorbed thiol to CdS (mol%)
surface coverage (%)
0
0
0.1M NH2(CH2)2SH
0.60
29
c
0.1M n-C 12H25SH
0.65
30
d
0.1M NaSO3(CH2)2SH
0.90
45
e
0.5M n-C lzH25SH
1.30
65
In order to confirm the above hypothesis, photoreduction experiments of C O 2 w e r e performed with use of the surface-modified CdS particles. Figure 3 shows the fraction of formate and carbon monoxide in CO 2 reduction products obtained for use of various kinds of CdS photocatalysts in acetonitnle and dichloromethane. It is clearly seen that the fraction of formate was varied greatly depending on the kind of CdS photocatalyst used, and in both solutions, it increased in the order of photocatalyst (a) to (e). Though the same modifier of 1-dodecanethiol was used in the photocatalysts (c) and (e), the fraction of CO 2 reduction products was greatly different. Table 1 shows the molar ratio of the adsorbed thiol compounds to CdS determined by elemental analyses, and also listed is the surface coverage with thiol compounds which were obtained by using the molar ratio of thiols to CdS and the average diameter of 50 nm of CdS particles and by assuming that the surface density of Cd 2+ sites was the same as that of (0001) facet of hexagonal CdS. Comparing the results in Figure 3 with the surface coverage by thiol compounds on the CdS surface shown in Table 1, the fraction of formate production seems to be related to the degree of the surface coverage, and the presence of the functional groups o f - N H 2 a n d - S O 3- in thiol compounds did not greatly alter this tendency. It seems likely that CO2o-anion radicals in low polar solvents tend to predominantly adsorb on the positively charged Cd 2+ sites, resulting in carbon monoxide formation. Since the thiol compounds should be bound to the surface Cd 2+ sites, the amount of the adsorbed CO,~oon Cd 2+ sites must decrease with increase of the degree of surface modification, and then formate production becomes predominant with increasing the surface coverage. REFERENCES
1. 2. 3. 4. 5. 6
T. Inoue, A. Fujishima, S. Konishi, and K. Honda, Nature 277 (1979) 637. A. Henglein, M. Guttieretz, and C. H. Fischer, Ber. Bunsenges. Phys. Chem. 88 (1984) 170. H. Inoue, R. Nakamura, and H. Yoneyama, Chem. Lett. (1994) 1227. M. Kanemoto, M. Nomura, Y. Wada, T. Akano, and S. Yanagida, Chem. Lett. (1993)1687. Z. Goren, I. Willner, A. J. Nelson, and A. J. Frank, J. Phys. Chem. 94 (1990) 3784. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B. 101 (1997) 2632. 7. H. Inoue, T. Matsuyama, B. -J. Liu, T. Sakata, H. Mori, and H. Yoneyama, Chem. Lett. 653 (1994).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
557
Applications of electrospray mass spectrometry and high performance liquid chromatography in the elucidation of photocatalytic CO2-fixation reactions H. Hori a, J. Ishihara a, M. Ishizuka a, K. Koike a K. Takeuchi a T. Ibusuki a, and O. Ishitani b aNational Institute for Resources and Environment, 16-3, Onogawa, Tsukuba 305, Japan bGmduate School of Science and Engineering, Saitama University, Urawa 338, Japan Photocatalytic CO2-fixation using [Re(bpy)(CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), PEt3 (2), PPh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), P(OEt)3 (6)] was examined by electrospray mass spectrometry and high performance liquid chromatography. The complexes 1, 2, 4, $ and 6 remained at close to initial levels during the catalytic CO formation and a formate complex Re(bpy)(CO)3OC(O)H (7) was detected after prolonged irradiation. On the other hand, 3 changed completely to solvent (DMF, triethanolamine) complexes with a quantum yield of >>1 before catalytic CO formation, followed by the generation of 7. The ligand exchanges with solvent molecules were explained in terms of a chain reaction mechanism.
1. INTRODUCTION Rhenium bipyridine complexes have received a great deal of attention because they act as photoreduction catalysts from CO2 to CO in the presence of amine with high selectivity and high efficiency [ 1-3]. The first step of the reaction is an electron transfer from the amine to the excited complex. However, the subsequent processes are not clear. Difficulties in the elucidation of the reaction mechanisms are ascribed to the instability of the intermediates. Furthermore, the presence of a highly non-volatile amine, e.g., triethanolamine (TEOA), makes it more difficult to isolate the intermediates. FJectrospray mass spectrometry (ESMS) is a powerful tool for identifying unstable complexes and high performance liquid chromatography ( ~ ) is also useful for the quantification of relatively stable species such as the initial and f'mal complexes during the reaction. A great advantage of these techniques is that there is no requirement for pretreatment, in other words, the reaction solutions can be directly subjected to the measurements. In this work, we examined CO2-photoreduction by six rhenium complexes [Re(bpy)(CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), PEt3 (2), VPh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), P(OEt)3 (6)] using ESMS, HPLC, and in situ UV/vis spectral measurements. 2. EXPERIMENTAL The complexes [Re(bpyXCO)3L] + were prepared according to a previous paper [4]. A high-pressure mercury lamp with suitable glass filters was used to obtain fight at 365 or >330 nm. As a typical photochemical run, a TEOA-DMF (1 : 5, v/v) solution of the complex was placed in a glass cell. The solution was purged with CO2 and then irradiated. After irradiation, the gas was analyzed by gas chromatography while the liquid was analyzed by HPLC and
558 ESMS [3]. In situ UV/vis spectra were measured by a spectrometer connected to the photochemical cell through optical fibers. e0 3. R E S U L T S AND D I S C U S S I O N 60 3.1. Photocatalytic reduction of CO2 All of the complexes caused catalytic reduction of ~ 40 CO2 to CO. Fig. 1 shows the time conversion plots of CO formation by 365 nm irradiation. After J so irradiation started, CO was generated linearly with irradiation for about 4 hours, and then the rate o m reduced. The quantum yields of CO formation (Oco) 10 of 1-6 in the steady CO formation period were 0.02, 0.01, 0.05, 0.20, 0.20, and 0.17, respectively (fight o - 0 4 8 12 16 intensity = 1.27 • 10 .8 einstein s- 1). The O c o values Irradiation time I h of the phosphine complexes (1-3) were much lower Fig. 1. than those of the phosphite complexes (4-6). It is Rots of CO fommtion against irradiation time. A TEOA-DMF solution of each known that the electron-acceptor strength of phosphite complex (2.6 mM) was irradiated (365 nm) is higher than that of corresponding phosphine. The under CO2. tendency observed here indicates that the elecmm2.0 acceptor strength of L significantly affects the catalysis. 1.5
I
3.2. In situ UV/vis spectra Fig. 2 shows the in situ UV/vis absorption spectra of $. When irradiation started, new bands around 400 and 500 nm appeared. These bands are attributable to the one-electron reduced species $ ' , comparable with the spectrum of $" produced by an electrochemical technique [2]. The spectral changes of 1, 2, 4, and 6 were similar to that of $. The reduced species were generated with yields of 20 100 % during irradiation, indicating that they act as precursors for CO formation. On the other hand, the in situ spectra of 3 were much different (Fig. 3). After accumulation of 3", another absorption maximum at -380 nm was found and no further changes occurred. This final species was extracted with dichloromethane- water and then purified by silica gel chromatography using methanol - ethyl acetate as an eluenL It was identified as the formate complex Re(bpy)(CO)3OC(O)H (7) on the basis of its NMR, IR, and UV/vis spectra.
1.0 O.5 0.0 8O0
.
4OO
SO0
eO0
Wavelength / nm
Fig. 2.
In silu UWvis spectra of TEOA-DMF
solution containing $ under CO2 during irradiation (365 nm). Timeinterval is 20 s 2.O
1.5
,~1.0
:
-.-~,
0.5
o,o
300
Fig. 3.
400
~::-~---500
600
Wavelength I nm
In sire UV/vis spectra of TEOA-DMF
solution containing 3 under CO2 during irradiation (365 nm) for (a) 0, (b) 70 and (c) 230 s.
559 3.3. H P L C Fig. 4 shows a HPLC chromatogram of $ after prolonged 365 nm irradiation. The starting complex $ remained at close to initial levels during the steady CO production. After prolonged irradiation, a peak corresponding to 7 appeared. The yields of 7 after 13 h were 3-11% for the complexes except 3. It is known that 7 has low CO producing ability [ 1,3]. Hence, a reason for the decrease in CO production rate after prolonged irradiation is the formate complex formation. Comparatively, the complex 3 showed drastic changes in H ~ C with irradiation (Fig. 5). When irradiation started, 3 showed an 81.6 % loss even after 5 s of >330 nm irradiation accompanied by the a ~ of new peaks, X and Y. Further irradiation caused decreases in X and Y, which resulted in the formation of 7. When the yield of 7 attained a maximum (52.2 % based on 3 used) and all of the other species disappeared, the turnover number (TN) of CO formation was 1.03. On the other hand, the overall CO formation process occurred with a TN reaching 12. These findings indicate that 7 acts as a real photocatalyst of CO2 reduction using 3. As for the species corresponding to peaks X and Y, they could not be isolated due to their instability. However, when excess CI- was added and then warmed after the solution had been irradiated until X and Y attained maxima, Re(bpy)(CO)3Cl formed with a 78.3 % yield. Therefore, these two species have an easily removable ligand, such as a solvent molecule. Consistently, [Re(bpy)(CO)3(DM~] + (8) showed the same retention time as that of X. To obtain more information on these intermediates, ES-mass spectra were measured. Solvent peaks Solvent peaks 5
[
,__
Fig. 4. HPLC chromatogram of $ after 13 h irradiation (365 nm). Retention time of $ is 32.5 min.
i, -l i
,~'l[t
'%"
b
I m
..
7
3.4.
ESMS Fig. 6 shows changes in the ES-mass spectra of 3 upon irradiation. Before irradiation, a sole peak of 3 (m/z = 689) was detected. After irradiation, another peak (m/z = 576) for a single positively-charged species was detected. This is clearly attributable to [Re(bpyXCO)3(TEOA)] + (9). Although the presence of 8 was expected from HPLC, no corresponding peak was observed. Presumably, 8 should be labile enough to undergo substitution of the DMF Ugand with an anion such as OMe- or OH- present in the mobile phase. This ligand substitution produces neutral complexes, which are hard to detect by this method. In fact, the neutral species 7 gave no peaks
C I
_ I
I
_I'-,~ jl ,
~. 0
5
/,_
~0
,'-7--'-~., /
~s
Retention
'~,s
,
,"
50
_
I.
ss
time / min
Fig. 5. Changes in HPLC chromatograms due to irradiation for (a) 0, (b) 5, (c) 600 and (d) 1200 s. A TEOA-DMF solution containing 3 was irradiated (>330 nm) under CO 2.
560 in the ES-mass spectra. Therefore, Y was identified as the TEOA complex 9 whereas X is supposed to be derived from the DMF complex 8. :i The photochemical fommtion of 8 and 9 from 3 should be a chain reaction because the quantum yield of the decrease in 3 was 16.9 upon irradiation at 365 i nm with an intensity of 8.30 • 10-10 einstein s-1. The mechanism can be e x p ~ as follows. 300 3 + hv -* 3* ( 3 ~ ) 3* + TEOA ~ 3" + TF~A .+ 3" + DMF or TEOA ~ 8" or 9" + PPh3 3 + 8 " org" --- 3" + 8 o r 9
(1) (2) (3) (4)
689
,.
,
400
500
,
,
nVz
600
b
700
800
689
d 576
The excited species 3" generated by irradiation (eq. 1) undergoes an electron transfer with TEOA to give 300 the reduced species 3" (eq. 2), which then undergoes the substitution of the PPh3 ligand with TEOA or c DMF to give S" and 9" (eq. 3). Subsequent electron :i exchange of 8" and 9" with another 3 leads to the d formation of g and 9 accompanied by the regeneration of 3" (eq. 4). I
.......
t.,al&,_l.=.
400 ,
9 ._
500
s.
m/z
L~..I
600
,
I._.
. . . . .
700
,
800
,
576
J
4. C O N C L U S I O N 700 800 The photocatalytic CO2-fixation using [Re(bpy)- 300 400 500 m/z 600 (CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), Fig. 6. PEt3 (2), PVh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), Changes in ES-mass spectra of 3 due to P(OEt)3 (6)] was examined by ES-mass specUv- irradiation for (a) O, (b) 5 and (c) 600 s. metry and HPLC. In the case of 1, 2, 4, $, and 6, the initial complexes remained largely undimimshed during the catalytic CO formation, and a formate complex 7 produced after prolonged irradiation which caused a decrease in the catalytic activity. In contrast, 3 was rapidly substituted with solvent to form [Re(bpy)(CO)3(DMF)] + (8) and [Re(bpy)(CO)3(TF.DA)] + (9) prior to catalytic CO formation by a chain reaction mechanism, followed by the formation of 7. These differences indicate that the nature of the phosphorus ligands (steric bulk, electron-accepter strength, etc.) is very important for the design of CO2-reduction photocatalysts. REFERENCES 1. J. Hawecker, J.- M. Lehn and R. Ziessel, Helv. Chim. Acta, 69 (1986) 1990. 2. H. Hod, F. P. A. Johnson, K. Koike, O. Ishitani and T. lbusuki, J. Photochem. Photobiol. A: Chem., 96 (1996) 171. 3. H. Hod, F. P. A. Johnson, K. Koike, K. Takeuchi, T. Ibusuki and O. Ishitani, J. Chem. Soc., Dalton Trans., (1997) 1019. 4. H. Hod, K. Koike, M. Ishizuka, K. Takeuchi, T. lbusuki and O. Ishitani, J. Organomet. Chem., 530 (1997) 169.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
561
Photocatalytic reduction of CO2 with H20 on Ti/Si binary oxide catalysts prepared by the sol-gel method Hiromi Yamashita a, Shinchi Kawasaki a, Masato Takeuchi a, Yo Fujii a, Yuichi Ichihashi a, Yasuo Suzukib, Sang-Eon Park c, Jong-San Chang c, Jung Whan Yoo c, Masakazu Anpo a* a'Department of Applied Chemistry, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599, Japan bIon Engineering Research Institute Corporation, Hirakata, Osaka 577, Japan CKorea Research Institute of Chemical Technology, Daeduck Science Town, Daejeon, 305-606, Korea
Titanium-silicon (Ti/Si) binary oxides prepared by the sol-gel method with different Ti contents exhibit high photocatalytic reactivity for the reduction of CO2 with H20 to form CH4 and CH3OH. A dramatic enhancement in the photocatalytic reactivity was found in regions of lower Ti content. In situ spectroscopic investigations of these Ti/Si binary oxides indicate that the titanium oxide species are highly dispersed in the SiO2 matrices and exist in tetrahedral coordination. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the oxides as a function of the Ti content clearly indicates that the high photocatalytic reactivity of the oxides having a low Ti content is associated with the high reactivity of the charge transfer excited state of the titanium oxide species in tetrahedral coordination. 1. I N T R O D U C T I O N
Thc utilization of solar energy through chemical storage can be achieved by the photocatalytic activation of light-sensitive catalytic surfaces. The photocatalytic reduction of CO2 with H20 is of special interest and one of the most desirable goals in this field [1]. Although, recently, the photocatalytic reduction of CO2 with gaseous H20 to form CH4 and CH3OH has been found to proceed on the titanium oxide catalysts highly dispersed on silica and zeolites at room temperature [2-4], the efficiency of the photocatalytic reaction is still low. It has been shown that the "sol-gel method" is an effective and fascinating way to design highly active photocatalysts [4-6]. The present study deals with the preparation of Ti/Si binary oxide photocatalysts by the sol-gel method and the relationship between the local structure of the titanium oxide species and its photocatalytic properties in the photoreduction of CO2 with H20 at 328 K.
562 2. E X P E R I M E N T A L
Ti/Si binary oxides having different Ti contents were prepared by the sol-gel method from mixtures of tetraethylorthosilicate and titaniumisopropoxide. Ti/Si gels were obtained by keeping the mixture at room t e m p e r a t u r e for several days, washed with sufficient amounts of boiled w at er and then calcined in dry air at 725 K for 5 h. The Ti/Si binary oxides were crushed and sieved to 0.25-mm-size particles. Prior to spectroscopic measurements and photocatalytic reactions, the catalysts were treated with 02 at 725 K for 2 h and then evacuated for 2 h at 475 K. The photocatalytic reduction of CO2 with H 2 0 was carried out with the catalysts (150 mg) in a quartz cell connected to a conventional vacuum system. UV irradiation of the catalysts in the presence of CO2 (24 pmol) and gaseous H20 (120 pmol) was carried out using a high-pressure Hg lamp (~ > 280 nm) at 328 K. The reaction products collected in the gas phase were analyzed by gas chromatography. 3. R E S U L T S AND D I S C U S S I O N XRD patterns of the binary oxides exhibited only diffraction lines which were attributed to the crystalline anatase phase of TiO2. When the Ti content was decreased, these X-ray diffraction lines decreased in intensity and finally disappeared, indicating t h a t the t i t a n i u m oxide species were present in amorphous structure within the SiO2 matrices. FT-IR spectra of the oxides showed a peak due to the stretching of the Si-O- of the Si-O-Ti bond at around 950 cm -1. The binding energy of the Ti(2p3/2, 5/2) XPS bands shifted to higher values when the Ti content was decreased, especially the less than 20 wt% as TiO2. This shift could be attributed to the smaller relaxation energy for the highly dispersed titanium oxide species as compared to powdered bulk TiO2. UV-Vis absorption spectra of the Ti/Si binary oxides with low Ti content exhibited a large shift towards shorter wavelength regions. This large shift was attributed to the size quantization effect arising from the presence of extremely small t i t a n i u m oxide particles and/or the presence of highly dispersed titanium oxide species having a low coordination number. These results obtained by XRD, IR, XPS and UV-Vis absorption m e a s u r e m e n t s clearly show that a decrease in the Ti content causes the crystalline structure of the titanium oxides in SiO2 matrices to change from aggregates in an a n a t a s e phase to ultrafine t i t a n i u m oxide species with an amorphous structure and eventually to isolated titanium oxide species having a local coordinate geometry different from those of the crystalline anatase TiO2. As shown in Fig. 1, the Ti K-edge XANES and FT-EXAFS spectra of the binary oxides with 80 wt% TiO2 content are similar to those of pure anatase TiO2. On the other hand, the oxides with a low TiO2 content (< 20 wt% TiO2) exhibit a single and intense preedge peak in the XANES spectra, indicating t h at the titanium oxide species are present in tetrahedral coordination. The corresponding FT-EXAFS spectra indicate the presence of isolated titanium oxide species. ESR signals due to the Ti 3+ ions g e n e r a t e d by the photoreduction of the oxides with H2 at 77 K also indicated the presence of
563 f, tetrahedrally coordinated 81 Ti 0 FT-EXAFS t i t a n i u m oxide species in the XAN ES ~ 1 wt,,y~ oxides with low Ti content, c O 5 As shown in Fig. 2, the Ti/Si ~ 4 L i wt% b i n a r y oxides h a v i n g low Ti o content of less t h a n 20 wt% as < TiO2 exhibit a c h a r a c t e r i s t i c N p h o t o l u m i n e s c e n c e s p e c t r a at E around 490 nm upon excitation TiO2 at around 280 nm at 77 K. The z observed photoluminescence --if| J_ spectra are in good a g r e e m e n t 0 2 4 6 4960 5000 5040 with those of highly dispersed Distance / Energy/eV tetrahedrally coordinated t i t a n i u m oxides anchored onto Fig. 1. The Ti K-edge XANES and FTVycor glass where the EXAFS spectra of the Ti/Si binary oxides. a b s o r p t i o n of UV l i g h t at around 280 nm brought about 250 an electron t r a n s f e r from the lattice (A) (a) H20 oxygen (O/2-) to the t i t a n i u m ion (Ti/4+) 5 to form a charge t r a n s f e r excited state, (Ti3+mO-) * [1,2]. These findings clearly .~ 125 show the presence of highly dispersed .m t i t a n i u m oxide species in the oxides c" h a v i n g a t e t r a h e d r a l coordination. On cthe o t h e r h a n d , Ti/Si b i n a r y oxides o c 0 having a large Ti concentration did not 0 exhibit any photoluminescence. (B) C02 (a) . .c_" As shown in Fig. 2, the addition of H20 E or CO2 molecules onto the Ti/Si binary 0 oxides leads to an efficient quenching of ~r " 100 the photoluminescence and shortening of I% its lifetime, their extent depending on the a m o u n t of a d d e d gasses. Such an efficient quenching of the 0 I p h o t o l u m i n e s c e n c e w i t h CO2 or H 2 0 300 475 650 i n d i c a t e s t h a t a d d e d CO2 or H 2 0 Wavelength / nm interacts and/or reacts with the titanium Fig. 2. Photoluminescence oxide species in the excited state. For the spectrum (a) of the Ti/Si binary quenching of the photoluminescence in oxide (5 wt% as TiO2) upon its i n t e n s i t y and lifetime, CO2 is less excitation at 280 nm at 77 K effective t h a n H20, indicating t h a t the and the effects of addition of i n t e r a c t i o n of CO2 w i t h the c h a r g e gasses. The a m o u n t of added t r a n s f e r excited s t a t e of the t i t a n i u m gasses: (A) H20; b, 0.1, c, 0.5, d, oxide species is weaker t h a n H20. 1.0, e, 5.0, f, 10, (B) CO2; b, 0.5, c, UV i r r a d i a t i o n of the Ti/Si b i n a r y 1.0, d, 5.0, e, 10 Torr. oxides in the p r e s e n c e of a gaseous
oF A ' I P
564 mixture of CO2 and H 2 0 was 150 found to lead to the reduction of : 6 Yields of C1 products I 5 CO2 to produce CH4 and CH3OH APotoluminesc,ence Intensity ] ~ as the m a i n products. The .~ .zphotocatalytic reaction rate and 100 ~f::: E 4 selectivity for the formation of =L f:: u C H 3 O H strongly depended on I {9 the Ti content of the oxides. The t-(D {9 oxides having a low Ti content ~" 2 50 ~ exhibits high r e a c t i v i t y and r E selectivity for the formation of CH3OH (22 mol% on the binary ~ oxide at 1 wt% as TiO2) while the ~_ 0 formation of CH4 was found to be o, 0 20 40 60 80 100 the major reaction on bulk TiO2 Ti Content / wt% asTiO2 (selectivity of CH3OH formation: F i g . 3. The effects of the Ti/Si 1 mol%) as well as on the oxides composition of Ti/Si binary oxides on having a high Ti content. As their photoluminescence yields and shown in Fig. 3, the parallel specific photocatalytic reactivities for relationship between the specific the photocatalytic reduction of CO2 photocatalytic reactivity of the with H20 to produce CH4 and CH3OH titanium oxide species and the at 328 K. photoluminescence yields of the oxides clearly indicates that high photocatalytic reactivity and selectivity for the formation of CH3OH is closely associated with the high reactivity of the charge transfer excited complex of the highly dispersed tetrahedrally coordinated titanium oxide species. XAFS and photoluminescence investigations of the oxides indicated that these catalysts can sustain tetrahedral coordination of the titanium oxide species until the Ti content reaches up to approximately 20 wt% as TiO2. Thus, it can be seen that the Ti/Si binary oxides having a Ti content of less t h a n 20 wt% can be successfully utilized as active photocatalysts for the efficient reduction of CO2 with H20 at 328 K. (9
O
e-
9
0 r-
REFERENCES
1. 2. 3. 4. 5. 6.
M. Anpo and H. Y a m a s h i t a , in Heterogeneous Photocatalysis, M. Schiavello. (ed.), John Wiley & Sons, London, 1997 in press. M. Anpo and K. Chiba, J. Mol. Catal., 74 (1992) 207. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B, 101 (1997) 2632. M. Anpo, H. Yamashita Y. Ichihashi, and S. Ehara, J. Electroanal. Chem., 396 (1995) 21. S.C. Moon, M. Fujino, H. Yamashita, and M. Anpo, J. Phys. Chem. B., 101, (1997) 369. H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Harada, M. Anpo, M. A. Fox, J. M. White, C. Louis, and M. Che, Chem. Mater, 1997 (in press.).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
565
Photoelectrochemical reduction of CO2 at a metal-particle modified p-Si electrode in non-aqueous solutions Yasushi Nakamura, Reiko Hinogami, Shinji Yae, and Yoshihiro Nakato* Department of Chemistry, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560, Japan A p-type silicon (p-Si) electrode modified with copper particles (particulate-Cu/p-Si) was applied to photoelectrochemical (PEC) reduction of carbon dioxide (CO:) in acetonitrile electrolyte solutions with and without 3.0 M H20. The particulate-Cu/p-Si electrode generated high photovoltages of 0.50 to 0.75 V, and produced methane, ethylene, etc., under addition of 3.0 M H:O, similar to a Cu metal electrode, indicating that the particulate-Cu/pSi electrode acted as an efficient electrode for the PEC reduction of CO2 in non-aqueous solutions. 1. INTRODUCTION Photoelectrochemical (PEC) reduction of CO: with a p-type semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from a view-point of the global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made [ 1], but the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. We reported [2-4] that a p-Si electrode modified with small metal (Cu, Au and Ag) particles worked as an ideal-type electrode for the PEC reduction of CO2 in aqueous solutions. In the present paper we will report that the electrode of this type is also effective for the PEC reduction of CO: in non-aqueous solutions which have high CO2 solubility. ///sio2 A schematic cross-section of D - ...... h v the present-type electrode is shown ...... in Figure 1. Metal is deposited on p....." Si sparsely in the form of small D -metal particle "#~,," ~ " ,,:~ % , particles. The ideal size of the Cu e t c Si/metal contact areas and their separation are estimated I theoretically to be about 5 and 20 CH4 etc. nm, respectively [5]. Photogenerated holes in p-Si enter into its interior, D C O 2 + H§ whereas photogenerated electrons non-a q u e o u s come out to the surface due to the electrolyte presence of band bending near the pSi surface. The electrons then Figure 1 A schematic cross section of a transfer into the metal particles and metal-particle modified p-Si electrode.
~
566 reduce CO2 at the surface which has high catalytic activity. This-type electrode has prominent features in that it not only generates a high photovoltage but also has high catalytic activity for the electrode reactions. 2. E X P E R I M E N T A L
Single crystal p-Si(100) wafers [CZ, 1.37 - 1.7 s Shin-Etsu Handotai Co., Ltd.] were used. Ohmic contact was made with In-Ga-Zn alloy. Metal (Cu) particles were deposited photoelectrochemically in a 0.01 M CuSO4 acidic solution. Details of the electrode preparation were described in our previous papers [2-4]. Copper metal electrodes were prepared using a Cu sheet (99.994 %). They were electrochemically polished at 2.0 V vs. Cu plate counter-electrode in 85.0 % phosphoric acid for ca. 1 min. The PEC reduction of CO2 was performed using an H-shaped Pyrex cell, with an Ag/AgC1 electrode as the reference electrode and a Pt plate as the counter electrode. The cathode and anode compartments were separated with a cation exchange membrane (Nation 117) in order to avoid mixing of products at the cathode and the anode. The electrolyte was acetonitrile (infinity pure, Wako Chemical) solutions containing 0.1 M tetrabutylammonium perchlorate (TBAP; 99 %, Acros Organics) with and without 3.0 M H20. Current-potential curves were measured with a potentiostat (Nikko-Keisoku NPOT-2501) and a function generator (Hokutodenko HB-III). Because the electrolyte has a relatively high resistivity, the iR drop was corrected by measuring the solution resistance by an iR compensation instrument (Hokutodenko HI-203). Photoelectrolyses for product analyses were performed under a sealed condition; namely highly pure CO2 (99.99 %) was bubbled into the stirred electrolyte until the air in the cell was completely substituted for CO2, then the cell (cathode compartment) was sealed and the photoelectrolysis was carried out potentiostatically under illumination with a tungsten-halogen lamp (light intensity: 100 mW/cm2). The temperature of the catholyte was kept at 20.0 + 0.1 ~ Reduction products in the gas phase of the cathode compartment were analyzed by gas chromatography, and those in the catholyte were analyzed by high performance liquid chromatography and gas chromatography. Special-grade chemicals and water purified with a Milli-Q water purification system (Nihon Millipore Kogyo) were used for all experiments. 3. RESULTS
Figure 2 shows the photocurrent (j) - potential (U) curves in a CO2 bubbled acetonitrile (AN) solution containing 0.1 M TBAP and 3.0 M H20. The j-U curves for Cuparticle modified p-Si electrodes (denoted as Cu/p-Si in Fig.2) varied from electrode to electrode, probably due to a variety of the form of deposited copper particles or the thickness of silicon oxide on p-Si, but the onset potentials of photocurrents were almost unchanged (0.5 V vs. Ag/AgC1). In Fig. 2, two typical curves (ordinary and best) are shown. The potentials at j = -5 mAcm -2 for the particulate-Cu/p-Si lay 0.50 - 0.75 V more positive than that for a Cu electrode, showing that high photovoltages were generated in the particulateCu/p-Si electrodes. The j-U curve for a naked p-Si electrode was almost the same as that for the particulate-Cu/p-Si(ordinary) in Fig. 2, but the saturated photocurrent of the particulateCu/p-Si(ordinary) was decreased due to a decrease in incident light intensity by the deposited Cu particles. Table 1 shows current efficiencies of products for CO2 reduction on various electrodes. Note that the results in Table 1 (and also Fig. 2) are obtained at high current densities compared with those in aqueous solutions previously reported by us, by taking account of high CO2 concentration in non-aqueous solutions. In an AN electrolyte solution with no addition of H20 (first row), a Cu metal electrode produced mainly CO as reported [6]. Small amounts of H2 and HCOOH were also produced due to a trace amount of H20
567
(0.016 %) contained in AN used in the present Potential vs. Ag/AgC1 / V work. With the addition -2.0 -1.5 -1.0 -0.5 0.0 -2.5 of 3.0 M H:O, a Cu electrode produced CH4 and C2H4together with CO, HCOOH and H2. C1 - 5 This result indicates (d~ ~) that H20 acted as a -10 proton donor in an AN Cu/p-Si (best) ~. s o l u t i o n for h y d r o -15~ carbon production. A naked p-Si -20 naked p-Si electrode / gave mainly H2 together with small amounts of -25 ~. CO, HCOOH and CH4, Figure 2 Photocurrent-potential curves for CO 2 reduction indicating that the Si surface has low in a CO 2 bubbled 0.1 M TBAP + 3.0 M H20/AN solution. catalytic activity for CO2 r e d u c t i o n . The parti c ul ate- Cu/p- S i (ordinary) generated CH4 and C2H4, similar to the Cu metal electrode, showing that the copper particles on p-Si acted as a good catalyst. The production of a large amount of H2 for this electrode, however, suggests that the reaction proceeds also on the naked Si surface. The total current efficiency for all the reduction products, especially that for the electrodes producing CH4 and C2H4(second and fourth rows), does not reach 100 %. This is partly because the electrolysis experiments were performed in a sealed cell for long periods of time of 30 min and thus parts of the reduction products escaped from the cell, e.g., through the Oring seals and the cation exchange membrane, and partly because certain amounts of CH4
Table 1. Current efficiencies of products of CO2 reduction in CO2 bubbled 0.1 M TBAP + 3.0 M H20/AN solutions a~. Potential
Electrode
Current Efficiency [%]
j b~
HCOOH CH4 C2H4 [mA/cm 2]
[V vs. Ag/AgC1]
H2
CO
Cu c~
-2.08
6.4
75.2
5.1
0.0
0.0
16.55
Cu
-2.21
3.9
24.8
7.4
11.8
9.9
28.71
naked p-Si
-2.68
87.8
1.5
2.1
1.5
0.0
28.37
Cu/p-Si(ordinary) d~
-1.68
40.3
20.8
6.6
2.1
4.7
12.98
a) The electricity which passed during the electrolysis is 10 C. The area of the electrodes is 0.5 cm 2. b) The average current density during the electrolysis. c) A result without any addition of H20. d) This is a particulate-Cu/p-Si electrode. The meaning of "ordinary" is shown in Fig. 2. Most of the tested electrodes showed the "ordinary" j-U curve.
568 and C2H4 were left dissolved in the AN electrolyte solution and not detected. However, the above-mentioned essential features for the reduction products were well reproduced by several-time experiments. 4. DISCUSSION The results described in the preceding section indicate that the particulate-Cu/p-Si electrodes generate high photovoltages and reduce CO2 at 0.50 - 0.75 V less negative potentials, producing methane and ethylene as reduction products due to catalytic effect of the deposited copper particles, though a naked p-Si electrode produces mainly H2. Thus, the particulate-Cu/p-Si electrode works as an effective electrode for the PEC reduction of CO2 in non aqueous solutions. The mechanism of the efficient CO2 photoreduction on particulateCu/p-Si was explained in detail in our previous papers [2-4]. The important point is that a high energy barrier height is formed at the p-Si/Cu/solution contact, irrespective of the barrier height of the p-Si/metal contact. A problem remains in the present result: As already mentioned, the current efficiency of H2 for the particulate-Cu/p-Si(ordinary) electrode is much higher than that on a Cu electrode (Table 1), indicating that the reaction proceeds not only on the Cu particles but also on the naked Si part of the particulate-Cu/p-Si(ordinary) electrode. This effect should be much smaller on the particulate-Cu/p-Si(best) as is understood from Fig. 2, but it is not easy to obtain such an electrode reproducibly. To reduce the H2 evolution may be possible by suppressing the reaction on the naked Si part, e.g., by making a silicon oxide layer on it or by increasing the amount of Cu particles. The photovoltages of 0.50 to 0.75 V obtained in the present work are fairly larger than those obtained in aqueous solutions (-- 0.5 V) [2-4]. This may be due to a decrease in surface carrier recombination centers (such as surface penetrated H atoms) in p-Si by a decrease in the H § concentration in solution, or due to a beneficial effect of TBA+ cations on the CO2 reduction reported in the literature [7-9]. Further studies are now in progress. REFERENCES
1. I. Taniguchi, in: Modern Aspects of Electrochemistry No. 20, Eds. J. O'M. Bockris, Ralph E. White and B. E. Conway, (Plenum press, New York, 1989) Chap. 5, and papers cited there. 2. R. Hinogami, T. Mori, S. Yae, and Y. Nakato, Chem. Lett., (1994) 1727. 3. R. Hinogami, Y. Nakamura, S. Yae, and Y. Nakato, Appl. Surf. Sci. (Proc. of 6th Iketani Intern. Conf. on "Surface Nano-control of Environmental Catalysts and Related Materials", Nov. 25 - 27, 1996, Waseda University, Tokyo), 121/122 (1997) 301. 4. R. Hinogami, Y. Nakamura, S. Yae, and Y. Nakato, J. Phys. Chem., submitted. 5. Y. Nakato, K. Ueda, H. Yano, and H. Tsubomura, J. Phys. Chem., 92 (1988) 2316. 6. S. Ikeda, T. Takagi, and K. Ito, Bull. Chem. Soc. Jpn., 60 (1987) 2517. 7. I. Taniguchi, B. Aurian-Blajeni, and J. O. Bockris, Electroanal. Chem., 161 (1984) 385. 8. J. O'M. Bockris and J. C. Wass, J. Electrochem. Soc., 136 (1989) 2521. 9. T. Saeki, K. Hashimoto, N. Kimura, K. Omata, and A. Fujishima, J. Electroanal. Chem., 77 (1995) 390.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
569
Infrared spectroscopic study of CO2 and CO reduction at metal electrodes O. Koga, T. Matsuo, H. Yamazaki, and Y. Hori Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi-cyo, 1-33, Inage-ku, Chiba, 263, Japan In order to elucidate intermediate species of electrochemical reduction of CO2 and CO to hydrocarbons, surface species at Cu, Ni, and Fe electrodes were investigated by infrared spectroscopy. The results show that adsorbed CO is intermediate species to hydrocarbons. The reduction activities of adsorbed CO on metal electrodes were discussed. 1. I N T R O D U C T I O N
Electrochemical reduction of CO 2 is one of the most important topics in connection with environment, energy and natural resources. A few papers have reported spectroscopic detection of intermediate species. CO2 or adsorbed CO is detected in the electroreduction of CO2 on a Pb or a Pt electrode.[1,2] However, these electrodes scarcely produce hydrocarbons from CO2 or CO. Cu electrodes electrochemically produced CH4, C2H4 and alcohols from CO2 and CO in aqueous media at high current densities, and Ni electrodes at lower current densities.[3-7] Fe, not active in CO2 reduction, can reduce CO to hydrocarbons.[8] It is thus interesting to reveal intermediate species on these electrodes in the electrochemical reduction of CO2 and CO by in-situ spectroscopic method. 2. E X P E R I M E N T A L
Two kinds of phosphate buffer solutions were used as the electrolyte after preelectrolysis purification with a platinum black cathode overnight; the one is 0.10 M KH2PO4 + 0.10 M K2HPO4 (1 M = 1 moi dm 3) saturated with CO and Ar or N2 (pH 6.8), and the other one, initially 0.05 M KH2PO4 + 0.15 M K2HPO4, gives pH 6.8 after equilibration withsaturated CO 2. The electrochemical equipments are a potentiogalvanostat (Tohogiken Co. Ltd., 2001) and a function generator (Tohogiken Co. Ltd., FG-02T). The electrode potentials were measured with respect to an Ag/AgCI reference electrode, and the potential values are given against SHE in this article. The counter electrode is a platinum wire. A copper sheet (99.999 % purity, 8 x 10 mm, thickness 0.2 mm), a nickel sheet (99.99 % purity, 10 x 12 mm, thickness 0.2 mm) and an iron sheet (99.99 % purity, 10 x 12 mm, thickness 0.1 mm) attached with a lead strip of the same metal were used as electrodes in infrared measurements. The electrodes were polished to a
570
mirror finish with alumina compounds down to 0.05 pm, degreased with acetone and electropolished in 85 % phosphoric acid. The infrared-electrochemical cell, originally designed by Bewick and his coworkers, was partly modified to introduce an electrode from the upper part of the cell. The front side of the cell is attached with a CaF 2 optical window, and the backside with a glass syringe which pushes the electrode against the window. The Fourier transform infrared measurements were conducted at 0 ~ for Cu electrodes and at ambient temperature for Ni and Fe electrodes by JIR-6000 (Nihon Densi, Co. Ltd.) externally equipped with an MCT (mercury-cadmium-telluride) detector. Infrared spectra were acquired by the subtraction of two spectra reflected from the electrode at different potentials (SNIFTIRS). The other details were described previously.[9] 3. R E S U L T S 3.1 I n f r a r e d s p e c t r a of a d s o r b e d s p e c i e s on metal electrodes.
CO 2 is not reduced at potentials more positive than -0.4 V at Ni electrodes. After the potentials were scanned negative direction to-1.2 V in the solution saturated with CO 2, the cathodic hydrogen formations were significantly suppressed. The previous studies showed that a reduced species is formed and adsorbed on the surface.F] The adsorbed species prevents cathodic currents. The reduced species in CO2 reduction was investigated by the following procedure. The Ni electrode, initially separated from the CaF 2 window, was polarized at-1.2 V in CO2 saturated solution. Hydrogen evolution is rapidly suppressed by this treatment. The electrode was subsequently pushed against the window and the IR spectroscopic measurements were carried out. In the same way, cathodic currents on Cu electrodes were also significantly prevented when the electrode potentials were kept at-1.0 V in CO 2 atmosphere. The reduced species on the Cu electrode was measured by the similar procedure as indicated above. When CO was introduced into the solution, the cathodic currents were suppressed on Fe electrodes. The adsorbed species on Fe in CO atmosphere was measured by infrared spectroscopy. The resulting infrared spectra of adsorbed species on each metal are given in Fig. 1. There
a)
2081
b)
. 1884
1986'-"
V1851 !
c)
v 1955 I
2200
I
I
I
2000
I
I,
1800
w a v e n u m l ~ r / c m "1
Fig. 1. SNIFTIRS spectra of adsorbed species: a) the reduced CO2 on Cu (-0.4 V vs.-1.0 V); b) the reduced CO 2 on Ni (-0.3V vs. -0.5 V); c) the adsorbed CO on Fe (-0.5 V vs. -0.7 V).
571
are one monopolar peak on Cu, two bipolar peaks on Ni and one bipolar peak on Fe in the wavenumber region 2300~1700 cm ~. The absorption peaks above 2000 cm 1 are assigned to Vco of linearly adsorbed CO and those around 1900 cm 1 are assigned to Vco of bridgedly adsorbed CO. CO adsorbs linearly on Cu and adsorbs both linearly and bridgedly on Ni. The peak at a little lower than 2000 cm 1 on Fe is assigned to Vco but is not clear due to linear or bridged adsorption. Thus adsorbed CO is evidently formed on Cu and Ni electrodes during the CO2 reduction and on Fe electrodes during the CO reduction. The downwards peaks correspond to the absorption of adsorbed species at more negative potentials and upwards peaks vice versa. The bipolar peaks on a Ni and a Fe electrode mean that CO exists on the electrode surface at both potentials and the absorption bands shift lower wavenumbers as more negative potentials are applied. Two types of CO on Ni and one type on Fe are stably adsorbed. The monopolar downwards peak on Cu indicates the adsorbed CO exists on the surface at more negative potentials but does not at less negative potentials. CO may be easily desorbed from the surface of Cu electrodes at less negative potentials.
3.2 Reduction of adsorbed CO at a Ni electrode by negative polarization. The electrochemical and infrared spectroscopic measurements have confirmed that the adsorbed CO is formed during CO 2 reduction. However, it still remains ambiguous whether the adsorbed CO is a real intermediate species to produce hydrocarbons or stable poison species. Thus negative polarization of CO adsorbed on Ni electrode was studied by infrared spectroscopy at ambient temperature. CO adsorbed at-0.4 V with the Ni electrode kept apart from the IR window in CO atmosphere and the dissolved CO was subsequently purged with N2. Then the Ni electrode was polarized at -1.0 V in N2 atmosphere. After the polarization, the electrode was maintained a t - 0 . 4 V and pushed against the window. The S NIFTIRS operation was conducted between -0.3 and -0.5 V. The results are shown in Fig. 2. The heights of two bipolar peaks decrease and disappear after the polarization 15 min. Figure 2b)indicates the linear-CO species decreases rather faster than the bridge-CO species. The adsorbed CO is probably reduced to hydrocarbons at-1.0 V on the electrode. Free CO will not be desorbed at the negative potential, since the products from CO2
a)
2031
2025
b)
A 1905
1898 A Jl
lx10"aa
1867
c)
I
I
I
I
I
2200 2000 1800 wavenumber I cm 1 Fig. 2. SNIFTIRS spectra of adsorbed CO on Ni affected by the cathodic polarization at-l.0 V. Polarization time: a) 0 s; b) 40 s; c) 15 min.
572
reduction do not contain any trace of CO.[7] Thus the CO adsorbed on Ni electrodes is stable at-0.4 V but easily reduced at-1.0 V. 3.3 Nature of the adsorbed CO as an intermediate species in CO reduction. We can compare the results on three metals, Cu, Ni and Fe. The order of wavenumbers of adsorbed CO at-0.9 V is Cu:2080 cm -1 > Ni: 2010 cm 1 (linear) > Fe:1960 cm ~ > Ni:1880 cm ~ (bridged). Blyholder related the C-O stretching band frequency of adsorbed CO with the adsorption strength to the metal.[10] Weak adsorption will lead to high C-O stretching frequency, approaching the value of free CO molecule, i.e. 2140 cm ~. Thus the adsorption strength of CO on the electrodes in the aqueous solutions may be given in the following order; Cu < Ni (linear) < Fe < Ni (bridged) . The CO molecule on Cu electrodes is easily desorbed from the surface, as discussed in 3.1. The infrared bands of adsorbed CO may be related to the activity in the electrochemical reduction. Ni and Fe electrodes reduce CO to methane, ethylene and ethane electrochemically with the current efficiency 3 % at a constant current density 2.5 mA / cm 2 (the electrode potentials are around-1.5 V ) i n 0.1 M KHCO3.[8,11] The Cu electrode effectively reduces CO to hydrocarbons with the current efficiency 50 % at 2.5 mA /cm 2 (the potential is-1.4 V).[4,5,11] Thus the activity order in CO reduction is Cu > Ni ,~ Fe. The linear CO is more easily reduced than bridged one on the Ni electrode. The order of the electrochemical activity of metals in CO reduction roughly agrees with the reverse order of the adsorption strength of CO. We demonstrated that the adsorption strength between CO and electrode metals relates closely with the product selectivity in CO and CO2 electroreduction, and that moderate strength of CO adsorption on Cu electrode is suitable for production of hydrocarbons.[11] The present results of IR spectroscopy evidently confirm the validity of our hypothesis. REFERENCES 1. A. W. B. Aylmer-Kelly, A. Bewick, R. Cantrill, A. M. Tuxford, Faraday Discuss., Chem. Soc., 5 6, 96 (1973). 2. B. Beden, A. Bewick, M. Razaq, J. Weber, J. Electroanal. Chem., 139, 203 (1982). 3. Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett., 1985, 1695. 4. Y. Hori, A. Murata, R. Takahashi, S. Suzuki, J. Am. Chem. Soc., 109, 5022 (1987). 5. Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 1, 85, 2309 (1989). 6. Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta, 3 9, 1833 (1994). 7. Y. Hori, A. Murata, Electrochim. Acta, 3 5, 1777 (1990). 8. A. Murata, Y. Hori, Denki Kagaku, 5 9,499 (1991). 9. Y. Hod, O. Koga, Y. Yamazaki, T. Matsuo, Electrochim. Acta, 40, 2617 (1995). 10. G. Blyholder, A. C. Allen, J. Am. Chem. Soc., 91,3158 (1969). 11. Y. Hori, A. Murata, R. Takahashi, S. Suzuki, Chem. Lett., 1 987, 1665.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yarnaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Influence of anions on the production efficiency in pulsed electroreduction of on metal and alloy electrodes
573
CO 2
R. Shiratsuchi a, S. Ishimaru b and G. Nogami a aDept, of Elect. Engn., Kyushu Inst. ofTech., Tobata-ku, Kitakyushu 804, Japan bDept.of Elect.Engn., Ariake Nat. Coll.of Tech., Higashihagiomachi, Omuta 836, Japan
A pulsed electrolysis of C O 2 o n Au, Ag and Cu and their alloyed electrodes was performed, laying special emphasis on an improvement of the selectivity of reduction products. The effect of anions (CI-, Br-, I-) intentionally added to a KHCO3 blank solution on the selectivity was investigated. The pulsed electrolysis was found to have a remarkable effect in enhancing hydrocarbonization reactions on Ag and Cu, while little on Au. It is concluded that oxide surface formed during an anodic period may play a key role in the selectivity of reduction products of CO2.
1. INTRODUCTION As reported elesewhere [1-3], a galvano/potentiostatic electroreduction of C O 2 o n Cu electrode shows poisoning effect of electrode due to graphitic carbons produced as one of the final products of CO2. We proposed a pulsed electroreduction which has an advantage over the conventional methods in the following two respects [4-7]: i) the high selectivity of the reduction products could be obtained by selecting optimum bias conditions, ii) long term electrolysis could be run probably because the poisoning effect can be avoided. Improvement of the selectivity of electrode reaction is undoubtfully of vital importance in electrochemistry. Uniqueness of the pulsed method may be a temporal interruption of chemical reaction sequences from intermediates to final products by a sudden bias change from cathodic to anodic, with which some of competitive reaction pathways may be interrupted. Therefore, an anodic bias can be a key parameter in the pulsed electrolysis. When bias is changed from cathodic to anodic, charges should be forced to redistribute across the interface. Electrolyte anions may also take part in charge redistribution which can afl'ect the electrode reaction taking place during the next cathodic period. The influence of the electrolyte anion and cation on the reduction products of CO2 under the static condition has been reported by Y. Hori et.al. [8] and G. Kyriacou et.al. [9]. In the present paper, the influence of anions (F, CI-, Br-, I, SO42-, NO3-, and C10;) intentionally added to a CO2-saturated 0.1M KHCO3 solution on the pulsed reduction products of CO2 was investigated for Au, Ag, Cu, and their alloyed electrodes. 2. EXPERIMENTAL Metal wires used as an electrode material were Au (99.95%), Ag (99.99%) and Cu
574 (99.999%) purchased from Nilaco Co. Ltd.. CuAu, AuAg, and AgCu electrodes were prepared by melting these metal wires in vacuum. The weight ratio for the alloy electrodes was Au:Cu=59:41, Au:Ag=57:43, and Cu:Ag=68:32. The experimental procedure, analytical method, and electrolysis system were the same as before [7]. Cathodic period and anodic period in the pulsed electroreduction were both set at 5s. All potentials were referred to Ag/AgC1 electrode. The electrolyte used (blank solution) was a CO2-saturated 0.1M KHCO3 buffer solution (pH 6.8) at 10~ The electrolyte containing a certain anion was prepared in each experiment by adding KF, KC1, KBr, KI, KNO3, K2SO4, or K C 1 0 4 to the blank solution. A conventional H-type gastight glass cell was divided by an ion exchange membrane (Nation 417). The Ag/AgC1 reference electrode was also separated from the working electrode compartment by the membrane so as to avoid a contamination of KC1. A gas chromatograph (Yanaco G-3810) was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Molecular Sieve 5A and Porapak Q were used for CO and H2 analysis in the TCD and CH4 and C2H4 analysis in the FID, respectively. Soluble products such as CH3OH , CH3CHO , and C2H5OH were analyzed by the FID after electrolysis for 5 h. Formate ions and other anions in the solution were analyzed by means of an ion chromatograph(Dionex DX-100) equipped with an anion exchange column (IonPac ICE-AS1), an anion exchange micromembrane suppressor, and a conductivity detector module. 3. RESULTS The electrolyses for the solutions containing lmM of CI-, F, Br-, or I- and the blank solution were performed at Vc=-I.5V and V,=-0.9~ 1.2V. Au: Figure 1 shows the faradaic efficiencies for CO (77 (CO)) and H2( 77(H2)) as a function of the anodic bias with the additives. 80
"~
60
'
':'
o
Bla,~
[]
KBr
Q
[] KF
9
60"
o
,' o~ 0
Blank KC1 KBr
m
40
o
4O
'
CO
CH 4
.o
.o
20
20 -
o
-65 Va vs.
'
0
H2
-| .....
'
Ag/AgC1
9
0:5
/V
Figure 1. Faradaic efficiency 77 (CO) for CO and 77 (H2) for H 2 o n an Au electrode in pulsed reduction at a cathodic bias of Vc= -1.5V vs. Ag/AgC1, as a function of anodic bias with and without the addition of lmM halogen ion.
-0.5 V,
vs.
0 Ag/AgC1
0.5 /V
Figure 2. Faradaic efficiency 77 (CH4) for CH4 and 77 (CzH5OH) for C z H s O H o n a Cu electrode in pulsed reduction at a cathodic bias of Vc =-2.25V vs. Ag/AgC1, as a function of anodic bias with and without the addition of 1 mM KC1 or KBr.
575
Table 1 Typical faradaic efficiencies(%) for pulsed electroreduction products of CO z Electrode Vc Va Anion H2 CO CH4 C2H4 CzHsOH CH3CHO -2.25 -0.6 blank 36.2 tr 20.1 5.8 8.2 11.0 -2.25 -0.6 cr 47.4 tr 12.7 7.8 7.2 5.0 Cu -2.25 -0.25 cr 26.7 tr 30.4 4.0 4.8 3.5 -2.25 0 C113.0 tr 26.0 1 0 . 1 10.3 6.1 -2.25 -0.6 blank 9.1 15.1 17.4 1.8 7.5 3.4 -2.25 -0.6 cr 7.6 49.7 0.2 0.2 2.7 4.2 Ag -2.25 -0.5 cr 3.8 47.8 3.5 2.5 8.6 14.9 -2.25 -0.4 C110.8 1 5 . 1 13.0 1.4 9.3 5.5 CuAu AgAu AgCu
-2.25 -2.25 -2.25 -2.25 -2.25 -2.25
-0.25 -0.4 -0.6 -0.5 -0.25 -0.25
blank C1blank cr blank C1-
45.1 45.7 44.8 60.6 30.4 44.2
7.6 5.7 17.9 17.4 tr tr
6.7 3.8 tr 1.3 23.1 18.9
3.1 2.4 tr 1.1 3.8 2.8
6.2 7.2 1.4 3.9 4.0 2.4
13.8 11.2 tr 10.2 tr 5.3
HCOOH 6.1 3.7 1.4 1.3
Total 87.4 82.8 70.8 66.8
14.5 7.7 7.4 9.1
68.8 72.3 88.5 64.2
3.8 2.4 5.8 2.2 15.8 7.6
86.3 78.4 69.9 96.7 77.1 81.2
77 (CO) was dependent on these anions over the anodic bias range between -0.6V and 0.9V. 77 (CO) at Va=0V was the maximum value for I-(57.6%) and the minimum value for F (34.5%). However, 77 (CO) for the solution with SO42-, NO3-or C104- was almost the same value as that of the blank solution at Va=0V. 77 (CO) for the blank solution was monotonously increased with increasing the anodic bias. 77 (H2) was suppressed by adding these anions to the blank solution. Cu: Figure 2 shows the dependence of the faradaic efficiency, 77 (CH4) and 77(CzHsOH) on anodic bias for the blank solution and the solution containing lmM KC1 or lmM KBr. The addition of KC1 or KBr gave a maximum value of 77 (CH4)=30.4% at Va=-0.25g. 77(CzHsOH) reached about 10% for the pulsed reduction at Va=0V and was almost independent of the addition of anion. The dependence of the yield of CzH 4 and CH3CHO on V, was similar to that observed for77 (CzHsOH). The total faradaic efficiency of hydrocarbonization reactions to CH4, C2H4, CzHsOH and CH3CHO was 53.8%. 77(CH4) in the blank solution was independent of Va between Va=-0.25 and 0.25V probably because of the surface oxide layer produced by the reaction between Cu metal and OH-. Although main reduction product of CO2 on Ag under galvano/potentiostatic electrolysis is CO, CH4 was found to be effectively formed under the pulsed electrolysis at Vc=-2.25V and Va =-0.6~ -0.4V. The present result shows that the peak potential at which 77 (CH4) and 77 (CO) become a maximum shifts positively. Amaximum faradaic efficiency of about 30% for hydrocarbonization reaction on Ag electrode was achieved as shown in Table 1. The yields of hydrocarbons increased while 77 (CO) decreased. In the meanwhile, the addition of C1- has little effect on alloy electrodes. Only exception is that hydrocarbonization reaction may be enhanced on the AgAu electrode when C1- is added. The production of CO for the pulsed reduction on the CuAu electrode occurred, while that on the Cu electrode and the CuAg electrode is trace. The increase in the production of HCOOH on the AgCu electrode reflected the behavior of the Ag electrode.
576 4. DISCUSSION It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduction of CO2. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. Main product of CO2onAu and Ag electrode under galvano/potentiostatic electrolysis is known to be CO. Even under a pulsed electrolysis, this is also the case for Au electrode. On the contrary, the pulsed method has a remarkable effect for Ag electrode. The largest difference between pulsed and conventional methods lies in that, in the former, anodic reactions and/or charge redistribution can take place during the anodic period. As well known, Cu and Ag are more easily oxidized to form an oxide layer than Au. Therefore, the oxide layer formed during the anodic period may play a key role in the selectivity of reduction products. In addition, Au has another disadvantage over the hydrocarbonization reactions because hydrogen adsorption is reported to be quite limited on Au surface[12]. The effect of anions was pronounced both on Au and on Ag. The fact that r/(CO) on Au and Ag increases when halogen ions are added implies that the amount of adsorbed hydrogen may be decreased by a specific adsorption of anions to the metal electrodes. This may be the reason why r/(CO) on Au and Ag depends on anions.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
R.L. Cook, R. C. MacDuff, and A. E Sammells, J. Electrochem. Soc., 135 (1988) 1320. D.W. DeWulf, T. Jin, and A. J. Bard, ibid., 136 (1989) 1686. G. Kyriacou and A. Anagnostopoulos, J. Electroanal. Chem., 322 (1992) 233. R. Shiratsuchi, Y. Aikoh, and G. Nogami, J. Electrochem. Soc., 140 (1993) 3479. G. Nogami, Y. Itagaki, and R. Shiratsuchi, ibid., 141 (1994) 1138. S. Ichikawa, and R. Doi, Catal. Today, 27 (1996) 271. R. Shiratsuchi, and G. Nogami, J. Electrochem. Soc., 143 (1996) 582. Y. Hori, A. Murata and R. Takahashi, J. Chem. Soc., Faraday Trans. 1, 85 (1989) 2309. G. Kyriacou, and A. Anagnostopoulos, J. Appl. Electrochem., 23 (1993) 483. M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Soc., 137 (1990) 1772. Y. Hori, H. Wakebe, T. Tsukamoto, and O. Koga, Electrochim. Acta, 39 (1994) 1833. G.M. Schmid and M. E. Curley-Fiorino, in: Encyclopedia of Electrochemistry of The Elements Vol.IV, Ed. A.J. Bard (Marcel Dekker, New York 1975) p. 131.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
577
E l e c t r o c h e m i c a l r e d u c t i o n of CO2 by using m e t a l s u p p o r t e d gas diffusion electrode under high pressure Kohjiro Hara, Noriyuki Sonoyama and Tadayoshi Sakata
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan The electrochemical reduction of CO2 under high pressure was carried out using various metal supported gas diffusion electrodes (GDEs). By using Pt supported GDE in reverse arrangement, in which the Pt catalyst layer of GDE is directed toward CO 2 gas phase, methane was produced at faradaic efficiencies of 38.8 %, while in the normal arrangement in which the Pt catalyst layer of GDE is directed toward the electrolyte hydrogen was the main product. By using Ag and Pd supported GDEs, CO was produced at faradaic efficiencies of 57.5-86.0 %. The partial current density of CO formation at Ag supported GDE reached 3.0 A cm-2. 1, INTRODUCTION We have investigated the electrochemical reduction of CO2 under high pressure on various metal electrodes in aqueous electrolytes.[1-2] The effect of CO2 pressure on the electrochemical CO2 reduction on metal electrodes was studied. The CO2 reduction at large current density could be achieved on metal electrodes by the high pressure electrolyses due to high concentration of CO2 in the aqueous electrolyte. By using the gas diffusion electrode (GDE) which have been developed and employed for fuel cells, a large current density electrolysis can also be achieved. The electrochemical CO2 reduction on GDEs supported by various metals and metal compounds under 1 atm were conducted by several workers.[3-5] They reported the achievement of larger current density for CO2 reduction, compared to the electrolyses in aqueous electrolyte using metal electrodes. In the present study, we have carried out electrochemical reduction of CO2 under high pressure using various metal supported GDEs to demonstrate the efficient electrochemical conversion of CO2 in high current density. 2. EXPERIMENTAL The electrolyses under high pressure were carried out in a stainless steel autoclave. The apparatus for the electrolyses is shown in Figure 1. A GDE and Pt supported GDE are purchased from Tanaka Noble Metal, Ctd. Other metals were supported on the GDE by the impregnation method. These metal supported GDEs were used as working electrodes. The aqueous electrolyte was 0.5 mol dm-3 KHCO3 and 2.0 mol dm-3 KOH aqueous solution, which was prepared from reagent-grade chemicals and distilled water (Wako Pure Chemical Industries, Ltd.) The electrolyte was purified by the pre-electrolysis with a Pt black cathode at
578 a current density of 0.25 mA cm-2 to eliminate heavy metal impurities. A Pt wire was used as an anode, and a reference electrode was Ag/AgCl/saturated KC1. High purity CO2 (99.99%) was introduced into the autoclave. The electrolyses were carried out galvanostatically at 25~ using a potentiostat/galvanostat (Hokuto, HA-501) connected in series with a coulometer (Hokuto, HF-201). The electrode potential was corrected with an IR compensation instrument (Hokuto, HI-203). Electrolysis products such as hydrocarbons, ethanol, CO, and hydrogen were analyzed quantitatively by a gas chromatograph, and formic acid was determined by an HPLC. Other details were described in previous papers.[6-8]
3. RESULTS AND DISCUSSION 3.1. Electrolyses of CO2 by using Pt supported GDE Table 1 shows the faradaic efficiencies of the reduction of CO2 under 20 atm on GDEs with and without Pt catalyst at a constant current density of 600 mA cm-2 for passed charge of 150 C. Where, "Normal" and "Reverse" are arrangements of GDEs. "Normal" means the arrangement that the Pt catalyst layer of GDE is directed toward the electrolyte while the gas diffusion layer faces CO2 gas phase and "Reverse" means the arrangement that the Pt catalyst layer of GDE is directed toward CO2 gas phase. In the case of the normal arrangement, CO and formic acid were formed at faradaic efficiencies of 0.6 and 4.0 %, respectively, and hydrogen was predominantly formed. In contrast to this, CO2 was efficiently reduced and methane was the predominant product formed at the faradaic efficiency of 38.8 % in the case of the reverse arrangement. Moreover, CO and formic acid were produced at faradaic efficiencies of 3.8 and 6.3 %, respectively. From these results, it is clear that the Pt catalyst and the
Highpressure / ~ \ Pressure 002111' L" )gauge ~' yr~ Sampling
[I
II
I II Ill
80
Autoclave n
6O
o
o
E =L
r 40 0
Ag/A ~ /
~-
E
ii1~-~'~ ~ ~ e l e c t r o d e I ~ - - " - ~ ~ L ~unter)
Gasd/iiffusi0n electrode
).8 E E
\ ~
Electrolyte Magneticstirrertip
Figure 1. Schematic diagram of the electrolysis equipment.
<
o
).4 <E
20 0
0
100
200
300
400
0
500
Passed Charge / C
Figure 2. The dependence of the amounts of the reduction products on the charge passed
579
Table 1 . Effect of catalyst and electrode arrangement on the high-pressure reduction of CO2 using GDEs. Faradaic efficiency/% Catalyst
Arrangement
Ea ] V
CH4
C2H6
C2H4
CO
HCOO-
H2
Total
Normal
-2.12
0.16
0.03
Tb
Nc
2.1
84.4
86.7
Reverse
-1.91
1.5
0.02
0.03
N
5.7
92.4
99.7
Pt
Normal
-1.45
0.03
N
N
0.6
4.0
74.5
79.2
Pt
Reverse
-1.93
38.8
0.15
0.57
3.8
6.3
42.2
94.4
aCorrected with an IR compensation instrument (vs. Ag/AgC1) bTrace cNotdetected. electrode arrangement are important for high efficiency CO2 reduction. Figure 2 shows the dependence of the amounts of products (methane, CO and hydrogen) on the charge passed at the current density of 600 mA cm-2 under CO2 30 atm. As shown in this figure, the amounts of methane and CO monotonously increased until 250 C, and then leveled off. In contrast, the amount of hydrogen increased with increasing of charge passed even more than 250 C. The leveling off of methane and CO indicates that the activity of the electrocatalyst deteriorated after passing a charge of 250 C. The deterioration of the catalyst activity might be due to catalyst poisoning by some intermediates formed by the CO2 reduction.
3.2. Electrolyses of CO2 by using various metal supported GDEs Faradaic efficiencies of the reduction products produced at the electrochemical reduction of CO2 under 20 atm using various metal supported GDEs were summarized in Table 2. The electrodes other than Pd, Cu and Ag showed low or no catalytic activity for the reduction of CO2. Ag and Pd showed very high efficiency and selectivity of CO formation. Figure 3 is the relationship between the current density and the partial current densities of CO and formic acid
Table 2. Electrochemical reduction of CO2 under high pressure on metal supported GDEs at the normal arrangement. Catalyst
Current density /mA cm-2
E /V
Faradaic efficiency/% CH4
C2H6
C2H4
CO
HCOO-
H2
Total
Co
600
-1.56
0.20
0.14
N
N
0.6
92.3
93.2
Rh
300
-1.34
0.09
T
N
9.8
3.6
82.0
95.5
Ni
300
-1.88
0.76
0.21
0.40
2.4
0.9
91.0
96.1
Pd
300
-1.33
0.03
0.02
N
57.5
2.3
26.5
86.7
Cu
900
-1.27
0.5
0.02
0.2
31.0
27.8
45.4
104.9
Ag
300
-1.22
N
N
N
86.0
1.8
2.0
89.8
580 formed in the CO2 reduction using a Ag supported GDE under 30 atm in 2.0 mol dm-3 KOH aqueous solution. Partial current density of CO increased up to 3.0 A cm-2 with increasing the current density. It was for the first time that such a high rate over 1 A cm-2 for CO 2 reduction was achieved on the electrocatalyst. This is due to the efficient supply of CO2 to the electrocatalyst in the electrolysis using the GDE under high pressure. Figure 4 is the relationship between the yield of CO and the passed charge in the electrochemical CO2 reduction on Pd supported GDE under 20 atm in 0.5 mol dm-3 KHCO3 solution. The yield of CO increased in proportion to the passed charge until 900 C. This indicates that deactivation of Pd catalyst, as in the case of Pt supported GDE, did not occur and the electrocatalytic activity of CO hardly changed until 900 C. These results suggest the possibility of the electrochemical conversion of CO2 into CO in high efficiency and high selectivity by using Ag and Pd supported GDEs.
E 3.0 IIO:CO. to <
.
2.5
1.0
2.0
O
1.5
tE E
t.-
3 ~
n 0
.
Io:col
O o t,-
0.1
1.0
o
0
E < 0.5 0.01 0.2
0.4
0.6
1.0
3.0
Current Density / A cm -2
Figure 3. Relationship between the current density and the partial current densities of reduction products formed on a Ag-GDE.
0
200
400
600
800
1000
Passed Charge / C
Figure 4. The dependence of the yield of CO on the passed charge on a Pd-GDE.
REFERENCES 1. K. Hara, A. Tsuneto, A. Kudo and T.Sakata, J.Electrochem.Soc., 141 (1994) 2097. 2.K.Hara, A.Kudo and T.Sakata, J.Electroanal.Chem., 386 (1995) 257 3.M.N.Mathmood, D.Masheder and C.J.Harty, J. Appl.Electrochem., 17 (1987) 1159 4.N.Furuya and K.Matsui, J.Electroanal.Chem., 271 (1989) 181 5.R.L.Cook, R.C.MacDuff and A.F.Sammells, J.Electrochem.Soc., 137 (1990) 607 6.K. Hara, A. Kudo, T. Sakata and M. Watanabe, J.Electrochem.Soc., 142 (1995)L57 7.K.Hara and T.Sakata, J.Electrochem.Soc., 144 (1997) 539 8.K.Hara and T.Sakata, Bull. Chem. Soc. Jpn., 70 (1997) 571
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
581
Electrochemical reduction of carbon dioxide at a platinum electrode in acetonitrile-water mixtures Yugo Tomita and Yoshio Hod Department of Applied Chemistry, Faculty of Engineering, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba, 263, Japan Electrochemical reduction of CO2 yields (COO')2 as a main product in water free electrolyte. The current efficiency of CO and HCOO" initially increases and then decreases with increase of water concentration in the electrolyte. CO, reduced from CO2, is strongly adsorbed on Pt electrode, and prevents further reduction of CO2 in aqueous media. Adsorbed CO is present on Pt in nonaqueous media as well, but reduction of CO2 still proceeds. Since the supply of H20 is severely limited in nonaqueous media, 1-12evolution is suppressed. CO2, diffusing to the electrode, may be readily reduced to CO2"'. CO2"" will react with CO2, eventually forming (COO)2(oxalate). (COO)2 may also be formed by coupling of CO2"'. With increase of H20 concentration, H2 evolution is enhanced. HCOO" and CO may be produced in the reaction of CO2"" and H20. 1. INTRODUCTION Electrochemical reduction of CO2 to useful products is an important subject in modern chemistry, which will contribute to a new energy storage technology in the next generation. Numerous papers have reported the electrochemical reduction of CO2 at metal electrodes in aqueous media[I-12]. H2 evolution and reduction of CO2 occur competitively in aqueous solution. Water molecule is involved with CO2 reduction as a reaction reagent. We employ acetonitrile as an aprotie solvent, in order to control the concentration of water in the electrolyte solution. We attempt to reveal the effect of water concentration in the electrochemical reduction of CO2 by adding water into acetonitrile electrolyte. 2. EXPERIMENTAL The purification of acetonitrile was performed by distillation with Call2 and P205 as drying agents. The concentration of water was analyzed by a gas chromatograph. Synthesized tetraethylammonium perchlorate (TEAP) [13] was used as the electrolyte. Water was purified with a Milli Q Low-TOC system (Millipore) after distillation. A Pt metal plate (1.70 cm2, purity higher than 99.9 %) was employed as a working electrode. The surface of the electrode was etched by hot concentrated sulfuric acid. The electrode potential of the cathode was measured with respect to an Ag wire reference electrode, which was corrected by the redox peak of ferrocene ; +0.267 V vs.SHE[ 14]. Voltammetric and coulometric measurements were conducted with the Pt electrode in the nonaqueous electrolyte saturated with CO2 (purity 99.99 %). The electrolyses were carried out in a closed electrolytic cell with COz saturated solution for 60 minutes. The current density was -5 mA/cmz. The products were analyzed by gas chromatographs and a
582 liquid chromatograph 9 3. RESULTS AND DISCUSSION Figure 1 shows negative sweep voltammograms of the nonaqueous electrolyte (water concentration was 31 ppm). The reducing current in CO2 saturated
solution starts at more positive potential than in Ar atmosphere
~ 100 < 0 ~t -10o -~
~ Ar sat. " - - - - - CO, sat.
Figure 2 shows voltammograms with various water -3oo concentration. The onset of the cathodic current shifts to more positive potential with increase of water concentration in the electorlyte. H2 evolution will be -500 favored in the electrolytes with high water content. Figure 32plots the potential at the current density-100 31o z'o l ~ 0 o~ l i0 ~z AJcm against logarithms of the water concentraE / V vs. S H E tion. The reduction of CO,, not only H~O, takes place at more positive potential with increase of H20 Figure 1. Voltammogram in 0.1 M concentration. TEAP / Acetonitrile 100 <
............ 9 Ar sat. COa sat.
0
::1.
........... Ar s a t ~ C O
........... Ar sat. CO2 s a t
r
-~ -100
-300
-500 3077 ppm
576 ppm -3'.0
~ -3'.o-a.o-1'.o o~ 1.~0
~ z -zo-1.o o, 1.'0
50171 ppm
ao' o.o
E / V vs. S H E
Figure 2. Voltammograms in 0.1 M TEAP/Acetonitrile with various water concentration 0.01
I
i
I
| O Ar atmosphere tu-0"5 E " CO2 a t m ~ -1 0 >
LU
o
.
9
O e
-1.5
-
~-
-2.o
-2.5
1
3 4 2 log [H20 (ppm)]
5
Figure 3. The electrode potential at the current density -100/~A / cm2 plotted against log[H20(ppm)]
Table 1 presents the current efficiencies of the products obtained in the controlled current electrolyses a t - 5 mA/cm "2 with various water concentration. Figure 4 shows the current efficiencies of the products as a function of water concentration. A major product from 31 ppm water electrolyte is (COO')2. With increase of water concentration in the electrolyte, (COO)2 formation steeply decreases, and the current efficiencies of CO and HCOO" initially increase and finally decrease. In 100% aqueous electrolyte, CO2 is not reduced, and only H2 is yielded.
583
Table 1 Various products from the electroreduction of CO2 in nonaqueous electrolyte HzO concentration Potential (V) Faradaic efficiency/% ppm vs.SHE CO H2 (COO')2 HCOO" 31 -2.51 5.30 0.98 52.9 12.4 72 -2.06 55.2 0.32 62.2 8.70 220 -1.96 52.6 1.82 37.6 17.7 576 -1.94 66.0 1.35 25.6 22.3 1083 -1.89 68.0 6.35 4.33 32.8 3077 -1.84 33.5 12.7 1.13 47.9 9508 -1.67 33.5 19.7 0.00 39.2 50171 -1.26 10.6 55.7 0.00 23.4 *1000000 -1.07 0.00 95.7 0.00 0.10 Current density" '5 mA/cm 2 Electrolyte" 0.1 M TEAP/Acetonitrile ; temperature 9room 90.1 M KHCO3 soln. ; temperature 918.5 ---+ 0.5 ~
Total 71.6 126.4 109.7 115.3 111.5 95.2 92.4 89.7 95.8
Figure 5 depicts a reaction scheme of CO2 100 reduction at a CO adsorbed Pt electrode in oco 'aN2 acetonitrile-water system. CO reduced from -O (000")2 A HCOOCO2, is strongly adsorbed on the Pt electrode, o>, 80 preventing the reduction of CO2 in aqueous " .~- 60 media[15-18]. The solubility of CO2 in ._ aqueous solutions is about 30 mM, or the a= w 40 ratio of water molecule to CO2 molecule is E approximately 2000. More water molecules ~. 20 will closely approach the electrode than CO2. Thus the charge transfer proceeds favorably O 0 to H20, resulting in H2 evolution. 1 2 3 4 5 Adsorbed CO will be present on Pt in log [H20 (ppm)] nonaqueous media as well. Despite the Figure 4. Current efficiency of products presence of adsorbed CO, CO2 reduction as a function of water concentration proceeds in nonaqueous electrolyte with low H20 content, probably because the supply of H20 is severely limited in nonaqueous solution. The solubility of CO2 in acetonitrile is about 240 mM[ 19], and the electrode surface is abundant in CO2 molecules. Consequently H2 evolution is suppressed, and the electrons react directly with CO2 in spite of adsorbed CO on the electrode surface. CO2, dirt-using to the electrode, may be readily reduced to CO2", which will further react with CO2 as a Lewis acid to form (COO')2. (COO')2 may also be produced by coupling of CO2"'. CO2"', produced from electronation of CO2, will readily react with H20 as a Lewis acid in higher H20 concentration. Water molecules participate in CO2 reduction as a major reactant. Thus (COO')2 formation will decrease, and the current efficiencies of CO and HCOO" will increase. Over a certain water concentration, the electrode neighborhood is enriched with water molecules. H2 evolution by decomposition of water molecule is gradually enhanced, predominating over CO2 reduction. Amatore and Sav6ant previously published the electrochemical reduction of CO2 at Hg and Pb electrodes in the mixtures of dimethylformamide(DMF) and water in the identical concentration range with ours[20]. They showed that much more (COO)2 was formed with less formation of CO in 0.1 to 0.2 M H20 in DMF in comparison with ours. The differences
584 may be rationalized in terms of CO selectivity of metal electrodes which we previously described[ 12]. The detailed study is now in progress, and will appear in the near future. In nonaqueous solution
In aqueous solution CO, + 2H.~'-*C0.., + H20 e-
0
CO CO CO CO
! Q~~~~)
i ~
.____.,. e ~---CO
-.4-,.c.-e--:-_. ~l--c-e--
i
@
ce a_ ~x.~ ~ _ ~ e r ~ c C g ""
~
i
}
i cc o,rc o , , ~,,,..e~ ~
:.
~---:.c-e--~_.., i ~'Tb-'~c o,., .~ 9t---:c-e ...... ! ' ~ v - ~
-c~
OHP
C02
2H, O + 2e--.Hz + 2 O H H,O : C 0 , = 2 0 0 0
With adding water
:I
+
OHP
e--.CO,--
.C_Q_L~+CO~-+e-
\
...... [C O0-I,
CO,
-
+H20+
e - --*
CO+20H-
\ Hcoo-+oa"
c_o_d_ + 9_9_LL /
Figure 5. Reaction scheme of the electrochemical reduction of CO2 REFERENCES 1. Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., (1985)1695. 2. Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, Chem. Lett., (1986)897. 3. K. Ogura, J. Appl. Electrochem., 16(1986)732. 4. Y. Hori, A. Murata, R. Takahashi and S. Suzuki, J. Chem. Soc. Chem. Commun., (1988)17. 5. J. J. Kim, D. P. Summers and K. W. Frese. Jr, J. Electroanal. Chem., 245(1988)223. 6. R. L. Cook, R. C. MacDuff and A. F. Sammells, J. Electrochem. Soc., 135 (1988)1320. 7. D. W. Dewulfand A. J. Bard, Catal. Lett., 1(1988)73. 8. Y. Hod, A. Murata and R. Takahashi, J. Chem. Sot., Faraday Trans. 1, 85 (1989)2309, and references cited therein. 9. H. Noda, S. Ikeda, Y. Oda and K. Ito, Chem. Lett., (1989)289. 10. M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and Y. Sakata, J. Electroanal. Chem., 260(1989)441. 11. S. Wasmus, E. Cattaneo and W. Vielstieh, Eleetroehim. Acta, 35(1990)711. 12. Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochim. Acta, 39(1994)1833, and references cited therein. 13. H. O. House, E. Feng and N. P. Peet, J. Org. Chem., 36(1971)2371. 14. J. N. Butler, Advances in Electrochemistry and Electrochemical Engineering, Vol. 7, Interscience Pub. New York, 1970. 15. J. Giner, Electrochim. Aeta, 8(1963)857. 16. M. W. Breiter, Electrochim.Acta, 12(1967)1213. 17. M. C. Arevalo, C. Gomis-Bas, F. Hahn, B. Beden, A. Arevalo and A. J. Arvia, Electrochim. Acta, 39(1994)793, and references cited therein. 18. B. Beden. A. Bewick, M. Tazaq and J. Weber, J. Electroanal. Chem., 139(1982)203. 19. T. Mizen and M. A. Wrighton, J. Eleetroehem. Sot., 136(1989)941. 20. C. Amatore and J.-M. Savrant, J. Am. Chem. Soe., 103(1981)5021.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
585
E l e c t r o c h e m i c a l r e d u c t i o n of CO~ in micropores T. Yamamoto a, D. A. Tryk a, tC Hashimoto b, A. Fujishima a, and M. Okawa c aDepartment of Applied Chemistry, Faculty of Engineering, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan bResearch Center for Advanced Science and Technology, the University of Tokyo, 4-6-1 Komaba, Megro-ku, Tokyo 153, Japan CElectric Power Development Co. Ltd., 6-15-1 Ginza, Chuo-ku, Tokyo 104, Japan
1.
INTRODUCTION
The use of high area metallic electrocatalysts supported on nanoporous media is of great interest. Such materials, for example, activated carbons and zeolites, are highly porous with large surface areas. With pores widths on the order of nanometers, microporous materials provide spaces in which unusual effects can occur, termed nanospace effects [1]. N anospace effects include the ability to mimic a high pressure environment, i.e., because the reactants are both concentrated and contained in small spaces, the products of certain types of reactions may be similar to those found at high pressure with conventional catalyst supports. This effect can be quite attractive in cases in which desirable products are favored by high pressure conditions, e.g., CO 2 reduction. In the present work, CO 2 electrochemical reduction was examined on high area metal electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths~on the order of nanometers.
Such
electrocatalysts were used in the form of gas diffusion electrodes (GDE), which are used in the fuel-cell field. The structure of this type of electrode is shown in Figure 1. The reaction takes places at the gas phase / electrolyte (liquid phase) / electrode interface, the so-called three-phase boundary.
586 2.
EXPERIMENTAL
ACF-supported metal catalysts were prepared as follows.
The fibers were
placed in contact with aqueous solutions of the metal nitrates and were stirred at room temperature.
The fibers were then washed with water, and the adsorbed
metal ions were reduced under hydrogen atmosphere at 350~
after drying.
For
the GDE active layers, the mixture of carbon black, for example, acetylene black (AB) (Gunbai, Denki Kagaku Kogyo) and PTFE (Daikin D-l) was ultrasonically dispersed in water.
The gas diffusion layer contained AB and PTFE in a 3:1
weight ratio, while the active layer contained AB, PTFE and ACF in a 9:3:1 weight ratio.
The mixture of carbon black and PTFE was dried using a rotary
evaporator and was then pressed, together with a stainless mesh using a die to form a disk-type electrode, 13 mm in diameter.
The apparent surface area of the
working electrode was 0.49 cm 2. This electrode was then heat-treated at 350~ in a hydrogen atmosphere. electrochemical cell.
Figure 2 shows a schematic illustration of the
CO 2 was fed into the gas compartment and 0.5 mol L 1
KHCO 3 aqueous solution was used as the electrolyte. sparging with argon before the electrolysis.
Oxygen was removed by
A saturated calomel electrode (SCE)
and a platinum wire were used as the reference electrode and counter electrode, respectively.
The electrolysis was carried out using a potentiostat-galvanostat
(Hokuto HA-501) with a coulometer (Hokuto HF-201). The electrode potential was corrected using an I-R compensator (Hokuto HI-203). The reduction products (gas compartment, gas phase in liquid compartment, and electrolyte) were analyzed using a gas chromatograph (Ohkura GC-202, Porapack-Q column, FID; Hitachi 163, MS-13X column, TCD), and a high performance liquid chromatograph (Tosoh UV-8010, Shodex KC811 column, 210 nm UV). e =
I Products
I
WE RE CE (Gas Diffusion (SCE) (Pt wire) Electrode) I i.._1 I /Ar
o7 ]] in- ,
<._ J\ J Gas Diffusion Reaction Layer Layer
Fig. 1. Electrode Structure.
Fig. 2. Electrochemical Cell.
587 3.
R E S U L T S AND D I S C U S S I O N To investigate the effect of micropores, we conducted electrolyses using the
following catalysts, unmodified ACF, iron and nickel catalysts supported on nonactivated carbon fibers (CF/Fe, CF/Ni), iron catalyst supported on activated carbon fibers (ACF/Fe) and two types of nickel catalysts supported on activated carbon fibers (ACF/Ni-1, ACF/Ni-2).
Table 1 shows the reduction product
distributions for the various catalysts at -1.8V vs. SCE.
The ACF catalyst itself
has very little activity for CO 2 reduction, and hydrogen evolution was the principal reaction.
The CF/Fe and CF/Ni catalysts showed very little activity as well.
The ACF/Fe and ACF/Ni catalysts, however showed significant activity for CO 2 reduction.
These results can be understood in the light of the work of H a r a et al.
on electrochemical reduction with high pressure CO 2 [2]. Iron and nickel bulk electrodes show little activity for CO 2 reduction at ambient pressure.
However,
with high pressure CO 2, these metals showed high activity for CO 2 reduction.
In
this case, we achieved electroreduction under high pressure-like conditions assisted by the nanospace effect. We have also examined the influence of the type of support using for the metallic catalysts.
For example, the main difference between two samples,
ACF/Ni-1 and ACF/Ni-2, was the specific surface area.
The ACF/Ni-1 c a t a l y s t
showed a greatly reduced surface area compared to unmodified ACF, while the ACF/Ni-2 catalyst showed a smaller reduction.
The ACF/Ni-2 catalyst also
showed higher CO 2 reduction activity t h a n the ACF/Ni-1 catalyst.
This also
Table 1. Reduction products for various catalysts at -1.8V vs. SCE Specific Current Efficiency (%) Current Density C a t a l y s t Surface / mA cm 2 Hz
CO
CH 4
1500
84.69
2.30
0.22
0.00
87.21
125
2.88
treaa)
ACF only
HCOOH Total
Total COztot. b)
CF/Fe
....
69.40
0.11
0.05
0.00
69.56
94
0.15
ACF/Fe
....
64.10
0.16
0.28
9.05
73.58
78
7.40
CF/Ni
....
80.48
3.38
0.08
0.00
83.94
109
3.68
ACF/Ni-1
700
69.88
2.86
0.15
12.17
85.06
31
4.77
ACF/Ni-2
1300
53.14
30.31
0.14
0.00
83.66
47
14.25
a) BET surface area, b) C O 2 reduction
m 2
g-~
partial current density, m A c m
"2
588 indicates that the nanoporous support may be playing a role in the C O 2 reduction electrocatalysis. Then we measured the reduction product distribution at various potentials for the ACF/Ni-2 catalyst. The highest current efficiency for CO 2 reduction to CO reached values of approximately 70%, while, in contrast, negligible amounts of CO are typically generated on conventional nickel catalysts at ambient pressure. Figure 3 shows the total current density and the CO 2 reduction partial current density using ACF/Ni catalyst. When we used Cabot Vulcan XC-72 as the matrix material for the electrode, the CO 2 reduction partial current density saturated at -2.0V vs. SCE. But when we used acetylene black (AB), in contrast, the partial current density continued to increase with increasing potential. The electrodes fabricated using AB are more hydrophobic than those using XC-72, and thus more CO 2 molecules and few protons are provided to the pores of the ACF. 350 ,-1'E 300 <
250
.~
200
9
~
,
,
(a) XC-72
.............. 70
350
,
~
- 70
~ 300 [ (b) acetylene black
,i0~
! '60
~,
50
<
,,; .'
.o'"
" t
o-"
150 -f
100
40
2
"~
-" 9, O " 9
30
~
20
r~
Io
"~ ~'= 200 r
150
~ ~1oo
0
.
-...... 40 I
.... f
.s
"'" #~
::i ii
o
30
o'" //
20
."
10
~= ~ r
0 -1.6
i
. t
1
-1.8 -2.0 -2.2 Potential vs SCE / V
l
-2.4
0 -1.8
t__
-2.0 -2.2 Potential vs SCE / V
0
-2.4
Fig. 3. Potential dependence of the current density using (a) XC-72 and (b) acetylene black as the electrode matrix material" --o-- Total current density; -'-..... Partial current density due to CO2 reduction
REFERENCES
1. J. Imai, M. Souma, S. Ozeki, T. Suzuki and K Kaneko, J. Phys. Chem., 95, 9955 (1991). 2. K Hara, A. Kudo and T. Sakata, J. Electroanal. Chem., 391,141 (1995).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
589
P h o t o e l e c t r o c h e m i c a l r e d u c t i o n o f h i g h l y c o n c e n t r a t e d C O 2 in m e t h a n o l solution K. Hirotaa, D. A. Tryka, K. Hashimotob, M. 0kawa c and A. Fujishimaa aDepartment of Applied Chemistry, the University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan bResearch Center for Advanced Science and Technology, the University of Tokyo, Komaba, Meguro-ku, Tokyo 153, Japan CElectric Power Development Co. Ltd., Ginza, Chuo-ku, Tokyo 104, Japan
High-rate photoelectrolysis of CO 2 was conducted in a high pressure CO 2 + methanol medium using p-type semiconductor electrodes. Current densities of up to 100 mA cm -2 were achieved, with current efficiencies of up to 93 % for CO production on a p-InP photocathode. The effect of CO2 pressure on the product distributions was examined for p-InP and p-GaAs.
1. INTRODUCTION Halmann was the first to report the photoelectrochemical reduction of CO2 in 1978, using a p-type GaP electrode 1. Since then, a number of studies have been carded out on this subject2, 3, aimed at reducing CO2 to produce useful products with a low input energy. In these reports, however, the photocurrent densities for CO2 reduction have been limited to maximum values of about 10 mA cm -2. Even when the light intensity was increased, the partial current density of CO2 reduction reached saturation4. This was explained in terms of CO2 mass transport limitations and competition with hydrogen generation. As stated by Halmann2, the chemical fixation of CO2 can be effective in impeding the rise of atmospheric CO2 only if the process is highly efficient. In the case of an electrochemical process, for example, a high current density and a high current efficiency are both required. In order to obtain high current densities, high CO2 mass transport rates to the electrode surface are necessary. We have previously reported on high rate CO2 reduction on metallic electrodes using high pressure CO2 in methanol 5-7. The CO2 concentration in this medium becomes quite high (8.0 mol dm -3 at 40 atm CO2 pressure) 8, so that the partial current densities for CO2 reduction were found not to be limited by CO2 mass transportS, 6. Current densities of up to 1 A cm -2 have been achieved on copper electrodes in this medium 7. We are now examining p-type semiconductor electrodes in order to achieve high current densities photoelectrochemically in the high pressure CO2-methanol medium. In this solution, the CO2 concentration is controlled by changing the pressure of CO2. The influence of the variation of the CO2 concentration on the product distribution was also examined.
590 2. EXPERIMENTAL The equipment and the experimental procedures using the CO2-methanol medium have already been described in previous papers5, 6. For the photoelectrochemical experiments, a stainless steel pressure vessel was equipped with a 2-cm thick quartz window for illumination, p-type InP and GaAs wafers were cut into ca. 0.4 cm • 0.5 cm electrodes and were mounted using epoxy resin. Ohmic contact was made with successive vapor deposition of Zn (30 nm) and Au (100 nm), which was annealed afterward at 425 ~ in Ar. A silver wire (0.8 mm dia) was used as a quasi-reference electrode (Ag-QRE, ca. +80 mV vs. SCE). A P t wire (0.8 mm dia) was used as the counter electrode. The photocathode was etched in hot aqua regia for ca. 5 s before each experiment. The electrolyte solution [3 cm 3, 0.3 mol dm -3 tetrabutylammonium perchlorate (TBAP) in CH3OH] was placed in a glass cell liner in the stainless steel vessel. Gases were introduced into the pressure vessel and were left to equilibrate for one hour at the desired pressure (1 to 40 atm). A xenon lamp was used to illuminate the photocathodes. Light with wavelengths shorter than 370 nm was filtered out in order to avoid photodecomposition of the supporting electrolyte. The current-potential measurements were performed without ohmic loss compensation. The potential was corrected according to the measured solution resistance during galvanostatic electrolysis, using an I-R compensator. Photoelectrolyses were conducted at 1 to 40 atm of N2 and CO2. A total charge of 2.2 to 10 C was passed galvanostatically at 5 to 100 mA cm -2 using a potentiostat-galvanostat. After the electrolysis, the gas and liquid-phase products were analyzed using gas chromatography.
3. RESULTS AND DISCUSSION The current-potential curve for the p-InP photocathode under illumination in CO2 (40 atm)-methanol exhibited a relatively large photocurrent (solid line), while the dark current was negligibly small (dotted line, < 1 mA cm -2) at potentials down to -2.0 V vs. Ag-QRE (Fig. 1). The onset photopotential was approximately -0.6 V. When CO2 was replaced with Ar, the onset of the cathodic photocurrent shifted toward the negative direction by 0.4 V (dashed line). This indicates that, in the highly concentrated CO2 solution, CO2 reduction on the Potential vs. Ag-QRE / V p-InP surface occurs in preference to the reaction -2.0 -1.0 0 -4.0 -3.0 occurring under Ar atmosphere, which is 0 ~'"'" Dark ~/--=/,,~r I predominantly hydrogen evolution. The cathodic photocurrent reached 20 mA (approximately 100 E 50 mA cm -2) at a potential of-2.4 V vs. Ag-QRE. E The ohmic loss in this system was found to be a p p r o x i m a t e l y 50 ohms using an I-R = 100 2 compensator when galvanostatic electrolysis was 0 conducted. The open circles in Fig. 1 show the .~ ~ (IR-corrected 150 photocurrent vs. the IR-free potential, assuming / (uncorrected) C ..... that the solution resistance does not change with potential. Thus, for current densities exceeding I-E curve for the p-lnP 100 mA cm -2, the IR correction exceeds 1 V with Fig. 1 photocathode in CO2-methanol medium. the present geometry. .
.
.
.
|
.
.
.
.
|
.
.
.
.
.r"/:
,i.-,
7:
i
.
.
.
.
i
591 Table 1 Photoelectrolysis product distributions Photocathode
Gas
Pressure (atm)
Photocurrent Potentiala (mA cm -2) (V)
Current efficiency (%) H2 CO HCOOCH3 Total
p-InP p-InP
N2 CO2
1 40
5.0 100
-1.5 - 1.4
92 3
0 93
0 11
92 107
p-GaAs p-GaAs p-GaAs
CO2 CO2 CO2
1 40 40
50 50 100
-2.0 - 1.5 - 1.8
76 15 31
11 78 48
19 16 21
106 109 100
apotential vs. Ag-QRE, compensated according to the measured ohmic loss
Galvanostatic electrolysis under illumination was conducted and the results are summarized in Table 1. The photocurrent was stable, and no visible damage of the electrode surface was observed after electrolysis. Under atmospheric pressure of nitrogen, hydrogen was the only product obtained in both gas and liquid phases. This presumably is produced via the decomposition of methanol. The total current efficiency does not reach 100 %, and this may be due to the reoxidation of the evolved hydrogen, because a one-compartment cell was used, containing both working and counter electrodes. In methanol saturated with CO2 (40 atm), a photoelectrolysis was carried out at 19.8 mA (100 mA cm-2). The solution resistance was estimated as to be 50 ohms and the ohmic loss can be calculated as 1.0 V (= 0.0198 A x 50 ~). The measured potential was -2.4 V, and therefore the IR-free potential was -1.4 V vs. Ag-QRE. The main product was CO, with a current efficiency of 93 %. Methyl formate was also formed. Methyl formate in the present medium is assumed to be analogous to formic acid production from CO2 in aqueous media. Insignificant amounts of hydrocarbons were observed. Hydrogen evolution was suppressed, which resulted in a current efficiency of only 3 %. This is in accord with the fact that, in the current-potential curve (Fig. 1), at -1.4 V (IRcorrected), the photocurrent for hydrogen evolution under Ar atmosphere is still very low. This accordance also supports our calculation for I-R correction. Thus, in high pressure CO2methanol, photoelectrochemical CO2 reduction proceeded efficiently at the high current density of 100 mA cm -2 on p-InP. The partial current density for CO2 reduction is approximately tenfold higher than previously reported values. When p-GaAs was used as the photocathode, photoelectrolysis at 50 mA cm-2 resulted in a current efficiency of 78 % for CO production. The importance of high CO2 concentration is apparent, because, with 1 atm CO2, hydrogen was the principal product. At 100 mA cm -2, however, the current efficiency for CO production decreased to 48 %, with the other products being hydrogen and methyl formate. This indicates that, at 50 mA cm -2 on p-GaAs, a sufficient amount of CO2 exists in the vicinity of the electrode surface for efficient CO formation, while, at 100 mA cm-2, the CO2 supply is not enough. The balance between the CO2 mass transport to the electrode surface and the current density seems therefore to be important as one of the factors which determine the product distribution. In methanol, the CO2 concentration can be changed by increasing the CO2 pressure in the system. The dependence of the product distribution on CO2 pressure at a current density of 50 mA cm -2
592
100
CO2 concentration / M 2.5 5.0 7.5
0
(a) p-lnP
80
i,~~r -
~
.
CO2 concentration / M 2.5 5.0 7.5
100 o~
(b) p-G aAs
!,
0
0
0
~ D~D~ ~
40 %
'
co
g ao
= 20
'
.~ 20
H2
~
- ~Cl..fl_ _ n
10
20 30 40 Pressure / atm
50
Fig. 2(a) CO2 pressure dependence of product distribution on p-lnP electrode in CO2-methanol medium.
0
1'o
Q
-. H2
' ' 20 3'o 40 Pressure / atm
50
Fig. 2(b) CO 2 pressure dependence of product distribution on p-GaAs electrode in CO2-methanol medium.
was examined on both p-InP and p-GaAs photoelectrodes (Fig. 2a,b). The current efficiencies for CO2 reduction to CO increased and those for hydrogen evolution decreased when the CO2 pressure was increased. Especially at pressures higher than 10 atm of CO2 (CO2 concentration is approximately 1.6 mol dm-3), H2 formation was suppressed and CO formation became predominant on both p-InP and p-GaAs. Thus, for efficient CO2 reduction at 50 mA cm-2, high CO2 concentrations (> 2 mol dm-3) are necessary. The concentration of protons, which can be involved in H2 generation, decreases as the CO2 pressure is increased. The range of proton concentrations, however, was found to be relatively small and thus the influence on the product distribution was negligible. p-InP and p-GaAs exhibited differing types of product distributions. On p-InP photocathodes, CO production was predominant at every CO2 pressure (Fig. 2a), while on pGaAs hydrogen evolution was the principal reaction at lower CO2 pressures (1 to 10 atm, Fig. 2b). This difference may be due to differing catalytic activities for CO2 reduction of the two different semiconductor surfaces. Further studies are now in progress in our laboratory.
REFERENCES
1. M. Halmann, Nature, 275, 115 (1978). 2. M. Halmann, Chemical Fixation of Carbon Dioxide - Methods for Recycling CO2 into Useful Products, CRC Press, Boca Raton, Florida, 1993. 3. See, e.g., I. Taniguchi et al., Elecrochim. Acta, 29, 923 (1984). 4. H. Noda et al., Chem. Lett., 1757 (1990). 5. T. Saeki et al., J. Electrochem. Soc., 141, L130 (1994). 6. T. Saeki et al., J. Phys. Chem., 99, 8440 (1995). 7. T. Saeki et al., Chem. Lett., 361 (1995). 8. E. Brunner et al., J. Chem. Thermodynnamics, 19, 273 (1987).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 O 1998 Elsevier Science B.V. All rights reserved.
593
Studies on CO2 fixation in PNSB : Utilization of w a s t e as the a d d i t i o n a l source of c a r b o n for CO2 fixation b y PNSB V.Brenner, M.Inui, N.Nunoura, K.Momma, and H.Yukawa Molecular Microbiology and Genetics Lab., Research Institute of Innovative Technology for the Earth (RITE), 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto, 619-02, JAPAN
1. INTRODUCTION Global warming is thought to be caused to a large part by CO2 accumulation in the atmosphere and poses serious environmental threat. A variety of different technologies are being developed to decrease the CO2 emissions. We are convinced that biotechnology could significantly contribute to the solution of this problem. This time we present a new concept for complex utilization of different wastes in biotechnology employing purple non-sulfur photosynthetic bacteria (PNSB) under anaerobic light conditions for chemical production and for SCP production with subsequent fixation of CO2 released during industrial processes. PNSB have great bioindustrial potential due to their metabolic flexibility and relatively rapid growth [1]. They also utilize various organic compounds as electron donors for the fixation of carbon dioxide and molecular nitrogen under anaerobic light conditions [2]. Large volumes of agroindustrial wastes are discharged all over the world and their accumulation causes environmental problems in the agricultural districts. Our experiments presented in this paper were performed to obtain preliminary data for setting parameters to suggest convenient technology for PNSB mediated CO2 fixation during waste treatment. Therefore, waste related carbon source utilization by model PNSB, their optimal light range for the growth, and their efficiency regarding CO2 utilization, were studied. 2.EXPERIMENTAL
Growth of PNSB on a broad range of selected carbon sources was monitored by measuring absorbance at 650nm . The optimal light range for the growth of PNSB was measured by LI- 192 SA pyranometer sensor with the light intensity 100W/m 2 during the growth on ethanol at 35oc. Changes in UV absorption spectra of the PNSB diluted culture filtrates during the growth on aromatic acids (AA) were monitored on spectrophotometer Beckmann DU 640. CO2 fixation rates were calculated by TOC 5000A analyzer.
594
3.RESULTS AND DISCUSSION 3.1.Metabolism of aromatic acids (AA) in PNSB O u r p o i n t in s t u d y i n g of A A p h o t o m e t a b o l i s m in PNSB w a s to find o u t the s p e c t r u m of A A u t i l i z a t i o n (Table 1) a n d to i n v e s t i g a t e m a j o r m e t a b o l i c r o u t e s in the b e s t p e r f o r m i n g strain. Lignin d e r i v a t i v e s k n o w n as c i n n a m i c acids are u b i q u i t o u s in a n y w a s t e c o n t a i n i n g p l a n t m a t e r i a l [3] . C h a n g e s in UV a b s o r p t i o n spectra of PNSB d i l u t e d culture filtrates d u r i n g the g r o w t h on AA w e r e m o n i t o r e d . F r o m t h e s e d a t a a n d f r o m the m e t a b o l i c s t u d i e s t w o m a j o r m e t a b o l i c r o u t e s of A A in the best p e r f o r m i n g AA d e g r a d e r , strain No.7, w e r e suggested: Ph en yl v al era te--p h e n yl p r o p i o n a te-- ci n n a m a te--benzoate .......... m i n e r ali z a ti on p- cou m ara te 4-hyd roxybenzoa t e.......... m i n e r ali z a ti on Table 1 G r o w t h of PNSB on a r o m a t i c acids Su-bstrate- . . . . . . . . . . Benzoate Cinnamate o-Coumarate p-Coumarate Ferulate 4- H y d r o x y b e n z o a t e Mandelate Vannilate Salicylate Phenylpropionate Phenylvalerate
R.pa!u. stris # 7 + ++ + +. + +. -+ + ++
R.capsulatus-R~spheroides # 11166 # 17023 + + + -+ + . . . -+ + . . . -+ + + + +
R.rubrum #11170 + + +-+ -+
3.2.Growth of PNSB on other carbon sources Bacteria w e r e g r o w n a n a e r o b i c a l l y on M M m e d i a in i l l u m i n a t e d c o n d i t i o n s . The e v a l u a t i o n of g r o w t h is c h a r a c t e r i z e d by scale w h e r e u s e d s y m b o l s h a v e this meaning: - no g r o w t h ( O . D . 6 5 0 < 0 . 1 ) ; - + negligible g r o w t h ( 0.2 >O.D.650>0.1) +- m o d e r a t e g r o w t h (0.4 >O.D.650>0.2); + fair g r o w t h (1.5>O.D.650>0.4); ++ excellent g r o w t h (O.D.650> 1.5 ). D a t a are s u m m a r i z e d in Table 2 a n d d i s c u s s e d in the section "Conclusion". 3.3. Effect of light range on the growth of Rhodopseudomonas palustris No.7 In the i l l u m i n a t i o n i n t e n s i t y of 1 0 0 W / m 2 w e h a v e s t u d i e d the effect of light r a n g e o n the g r o w t h of s t r a i n Rhodopseudomonas palustris No.7. F r o m the data obtained, the o p t i m a l light r a n g e 850-950nm, w a s e s t i m a t e d (Fig.2). 3.4 Analysis of CO2 fixation by Rhodopseudomonas palustris No.7 C O 2 fixation rate in the strainR.palustris No.7 w a s m e a s u r e d in p h o t o a n a e r o b i c c o n d i t i o n s . The h i g h e s t fixation rate p e r r e a c t o r (0.43g/L/hr) w a s o b t a i n e d d u r i n g the g r o w t h on e t h a n o l u n d e r i l l u m i n a t i o n 4 1 6 W / m 2. CO2 fixation rates nn
cliffprpnt
earhnn
~nllreoq
arp
(~cwnnaroct
in FiCrllro 1
595
Table 2 Selected waste
related
carbon
Substrate
R.palustris
substrate
utilization
R.capsulatus
#7
# 11166
by
PNSB
R.spheroides
R.rubrum
# 17023
11170
VFAs acetate
+
+
+
+
propionate
+
++
+
+-
butyrate
+
+
+
+
malate
++
+
+
+
succinate
+
+
+
+
glutarate
++
+-
+
+
pyruvate
+
++
++
+
caproate
++
+
+
+
SUGARs sucrose
-
-+
-+
fructose
-
+
++
glucose
-
+
++
xylose
-
-
+ +
mannose
-
-
mannitol
-
-
+
sorbitol
-
-+
+
+-
ALCOHOLs +
ethanol
++
-+
-+
n-propanol
++
-+
-+
+-
n-butanol
+
-
-
-+
glycerol
-
+-
benzylalcohol
+
-
+
1.5
ethanol f,'valerate
5.0"
I::
//
4.0"
acetate
:..:z'propionate
acetate te
r
///btlyratea
3.0"
2.0'
L~
~.~..~
/;:
rJ
5
1.0.
0.0 :/
; ~
0
10
. 20
30
.
. 40
. 50
0 60
Culture time (hr) Figure
eth and
....;'
1. G r o w t h
i
0
i
10
.
i
20
,
i
30
i
i
40
!
l
50
i
i
60
Culture time (hr)
and CO2 consumption
Rhodopseudomonas palustris
70
No.7
on selected substrates
by the strain
,
70
596 1.6 m
..
> 700 nm
A
-
> 800 nm
-
> 900 nm
1.4 1.2 l
500-600 nm
o
0.8
----O---
9
600-700 nm
0.6 700- 800 nm
0.4
800-900 nm
0.2
900-1000 nm 0
24
48
72
96
120 144 168
---~--
Control
Growth time (hr) Figure 2. Influence of selected light range on the growth of the strain
Rhodopseudomonas palustris No.7 4. CONCLUSION The strain Rhodopseudomonas palustris No.7 is good utilizer of lower alcohols , volatile fatty acids (VFA), and aromatic acids and therefore may be useful for the treatment of wastes containing either material from plants (such as bean industry), or containing ethanol (breweries), and VFAs (complex wastes after anaerobic treatment). Sugars are the major components of wastes released by food industry and agroindustry. They were preferentially utilized by the strain Rhodopseudomonas spheroides 17023 which in opposite did not grow solidly on alcohols. The optimal light range for the strain No.7 was found in the interval between 850nm and 950nm. The CO2 fixation rate for the same strain in bioreactor was 0.43g C O 2 / L / h r , measured at 35~ pH 7.5, at light intensity 4 1 6 W / m 2. Two alternative models , the first one considering anaerobic pretreatment of waste with subsequent utilization of prevalently created VFAs by PNSB in a laser operated photobioreactor [4], and the second one with direct application of waste into the photobioreactor, were suggested. REFERENCES
[11 K.Sasaki,
N.Noparatnaraporn, S.Nagai, Bioconversion of Waste Materials to Industrial Products, Martin A.M. eds., Chapter 7, 223 (1991) [2] M. Kobayashi, M.Kurata, Process Biochem., _1_3,27, (1978) [3] P.G.England, D.A.Pelletier, M.Dispensa, J.Gibson, C.S.Harwood, Proc.Natl.Acad. Sci.USA, 94, 6484, (1997) [4] H.Sawada, P.L.Rogers, J.Ferment.Technol. _55, 311, (1977) This work was supported by a grant from the New Energy and Industrial Technology Development Organization.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
597
S t u d i e s on CO2 fixation in PNSB : A n a l y s i s of CO2 m e t a b o l i s m in p u r p l e n o n - s u l f u r bacteria M. Inui, J. H. Roh, K. Momma a, and H. Yukawa Molecular Microbiology and Genetics Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-Soraku, Kyoto 619-02, JAPAN aResearch Institute of Food Science, Kyoto University, Gokasho, Uji, Kyoto 611, JAPAN 1. INTRODUCTION Purple non-sulfur bacteria (PNSB), anoxygenic phototrophic prokaryotes, are a phenotypically and genotypically diverse set of species. They are able to grow aerobically as chemoheterotrophs, anaerobically using respiration or fermentation, and photosynthetically in an autotrophic or heterotrophic mode. Therefore, PNSB exhibit metabolic diversity reflecting the complex modes of growth. CO2 fixation in most PNSB is catalyzed by ribulose 1,5-bisphosphate carboxylase /oxygenase (RubisCO) in the Calvin pentose phosphate cycle. Besides the plant like formI RubisCO, which is composed of both large(L) and small subunits(S) and has an L8S8 structure, formII RubisCO containing only the large subunit exists in PNSB. Both forms of RubisCO are differentially expressed according to the culture conditions. In addition to CO2 fixation by RubisCO, anaplerotic enzymes such as p h o s p h o e n o l p y r u v a t e carboxylase(PEPC), phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate carboxylase(PC) also play important roles in carbon metabolism, supplying intermediates for energy production and cell materials. Understanding the expression and regulation of these enzymes will furnish important information for utilizing PNSB in biotechnological applications. In this study, we have constructed genomic disruptants of formI and formII RubisCO, PEPC, and PEPCK, and analyzed the function of each enzyme. 2. EXPERIMENTAL
The genes encoding the two RubisCOs, PEPC, and PEPCK were cloned and sequenced from Rhodopseudomonas palustris No. 7 [1]. Each cloned gene was inactivated by insertion of a kanamycin(Km) cassette. Inactivated genes were transferred from Escherichia coli to R. palustris No. 7 by conjugation. Homologous replacement took place in vivo between the chromosomal gene and the Km inactivated copy carried on the suicide vector, pGP704 [2]. Exoconiugants
598 were initially selected by Km resistance because the suicide vector can not replicate in R. palustris No. 7, and then tested for double crossover with the loss of vector-mediated gentamycin resistance. The integration of the Km cassette via a double crossover was confirmed by Southern hybridization experiments. Growth and the enzyme activities of the disruptants were investigated under various growth conditions as previously reported [3]. 3. RESULTS A N D DISCUSSION
3. 1. Characterization of R u b i s C O deficient mutant. R. palustris No. 7 possesses two forms of RubisCO, formI and formII, as reported for Rhodobacter(Rba) species. In Rba. sphaeroides, formI RubisCO is mainly e x p r e s s e d u n d e r photoautotrophic and formII u n d e r photoheterotrophic conditions [4]. When ethanol is used as a carbon source of R. palustris No. 7 under anaerobic light condition, addition of NaHCO3 is needed for an electron acceptor to maintain the redox balance. Although mutants deficient for either RubisCO did not show any change in growth rate compared to wild type under this condition, the RubisCO activity in the formII deficient mutant decreased to 45% of the wild type, while a formI deficient mutant did not(Fig. 1). This result indicates that both forms of RubisCO are functional for CO2 fixation in this strain and active enough to support the photoheterotrophic growth of either disruptant. To investigate the regulation of the expression of two RubisCO, studies are in progress utilizing both p h o t o a u t o t r o p h i c conditions and p h o t o h e t e r o t r o p h i c conditions. 10[~ Table 1. RubisCO activity of R. pahistris No. 7, FormI and FormI! deficient mutant E Strains
-
Activity(nmol/mg/rain)
R. pahlstris No. 7
6.9 (:t:0.2)
FormI deficient mutant
7.3 (•
FormlI deficient mutant
3.2 (•
O
n R. palustris No. 7 9 FormI deficient mutant [] FormII deficient mutant 50
100
Time(hr) Figure 1. Growth of R. palustris No. 7 and the RubisCO deficient mutants on 20mM ethanol plus 20mM NaHCO3 medium under anaerobic light conditions.
599
A
o
B
1
R.p
.1
o
.1
ient mutant I
I
I
30 Time(hr)
I
I
I
60
i
I
I
30
I
I
I
60
Time(hr)
Figure 2. Growth of PEPC deficient mutant on pyruvate(A) and pyruvate plus 0.05 % malate(B) as a carbon source under anaerobic light conditions.
3. 2. Characterization of PEPC d e f i c i e n t mutant. PEPC is an anaplerotic CO2 fixing enzyme catalyzing the irreversible conversion of phosphoenolpyruvate to oxaloacetate which is an intermediate of the TCA cycle and a precursor of many cellular compounds. Most PNSB contain at least one anaplerotic enzyme, PC or PEPC, to supply oxaloacetate. A PC deficient mutant of Rba. capstllatus where PC is the sole anaplerotic enzyme in the wild type did not grow on glucose unless the m e d i u m was s u p p l e m e n t e d with TCA cycle intermediates [5], but a PEPC deficient mutant of Corynebacterium glutamicum where the wild type contains both PC and PEPC shows the same growth when compared to wild type on glucose m e d i u m [6]. The PEPC deficient mutant of R. palustris No. 7 showed reduced growth rate under anaerobic light condition on pyruvate or substrates such as lactate and gluconate metabolized via pyruvate(Fig. 2). However, in the presence of a TCA cycle intermediate, growth is almost the same as that of wild type. Therefore, PEPC has an important function in R. palustris No. 7. 3. 3. Characterization of PEPCK deficient mutant. PEPCK of most bacteria appears to function as a gluconeogenetic enzyme rather than as an anaplerotic one. The PEPCK deficient mutant of R. palustris No. 7 showed a similar growth rate to that of wild type on p y r u v a t e and on TCA cycle intermediates necessary for gluconeogenesis(Fig. 3). PEPCK mutants of most bacteria show reduced growth on pyruvate and TCA cycle intermediates, but malic enzyme and PEP synthase, or pyruvate phosphate dikinase and PEP synthase are able partly to support the growth by forming an alternative pathway to synthesize PEP [7]. The existence of NAD+-dependent malic enzyme activity suggests that R. palustris No. 7 utilizes this alternative pathway for gluconeogenesis.
600
a
1.0
:
R. palustris No. 7
1.0 i
B R. palustris No. 7
~D
9 0.5 9
:~
....~
0.5"
nt
.
PEPCK dificient mutant |
9
40
,
|
,
|
80 Time(hr)
.
i
,
i
'
'
120 Time(hr)
Figure 3. Growth of PEPCK deficient mutant on pyruvate(A) and succinate(B) as a carbon source under anaerobic light condition. 3. 4. PC of R. palustris N o . 7. PC is a biotin containing anaplerotic enzyme catalyzing the conversion of pyruvate to oxaloacetate. PC activity of R. palustris No. 7 was detected by coupling the production of oxaloacetate with the oxidation of N A D H using malate dehydrogenase. NADH oxidation was dependent on acetyl-CoA, ATP, pyruvate, NaHCO3, and MgC12. In addition, specific band with a molecular mass of 130kDa was detected by western blot analysis for biotinylated protein(data not shown). Purification experiment are in progress to clarify the existence of PC in this microorganism. REFERENCES
1. T. Fujii, A. Nakazawa, N. sumi, H. Tani and A. Ando, Agric. Biol. Chem., 47 (1983) 2747. 2. V. L. Miller and J. J. Mekalanos, J. Bacteriol., 170 (1988) 2575. 3. M. Inui, V. Dumay, K. Zahn, H. Yamagata and H. Yukawa, J. Bacteriol., 179 (1997) 4942. 4. D. L. Falcone, R. G. Quivey Jr. and F. R. Tabita, J. Bacteriol., 170 (1988) 5. 5. J. C. Willson, J. Gen. Microbiol., 134 (1988) 2429. 6. P. G. Peter-Wendisch, B. J. Eikmanns, G. Thierbach, B. Bachmann and H. Sahm. FEMS Microbiol. Lett., 112 (1993) 269. 7. M. Osteras, B. T. Driscoll and T. M. Finan, Microbiology, 143 (1997) 1639.
This work was supported by a grant from the New Energy and Industrial Technology Development Organization.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
601
C r y s t a l l i z a t i o n a n d p r e l i m i n a r y X - r a y s t u d i e s of p h o s p h o e n o l p y r u v a t e c a r b o x y l a s e f r o m E s c h e r i c h i a Coli H. Matsumura a, T. Nagata a, T. Inoue a, Y. Nagara a, T. Yoshinaga b, K. Izui c, Y. Kai ~ aDepartment of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan bDepartment of Public Health, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan cDepartment of Agricultural Biology, Faculty of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan We have crystallized the enzyme and determined the X-ray structure of PEPC by a multiple isomorphous replacement method. Our current structural model suggests that PEPC forms a 'dimer of dimers' and provides the mechanism for allosteric regulation. 1. I N T R O D U C T I O N
Phosphoenolpyruvate carboxylase (PEPC) catalyzes the fixation of bicarbonate ion to form oxaloacetate and inorganic phosphate in the presence of Mg 2§ The native enzyme is present in all plants and in a variety of bacteria but is absent in mammals and fungi. PEPC is a tetramer comprising of four identical subunits with molecular weight of about 100 kDa. The enzyme plays a key role in the initial CO 2 fixation of photosynthesis in C4 and CAM plants. The PEPCs from most sources are allosteric, and their activities are regulated by a variety of effectors. The Escherichia coli enzyme is regulated by four different activators: acetyl-CoA, fructose 1,6-bisphosphate, GTP, and long chain fatty acids, as well as by the inhibitors Laspartate and L-malate. In order to study its biological function and allosteric regulatory properties based on the three dimensional structure, we have determined the X-ray structure of PEPC. 2. S T R U C T U R E D E T E R M I N A T I O N
Crystals of PEPC were grown by the hanging drop vapor diffusion method as
602 reported previously [1]. Cell constants and a space group are determined by precession photographs. Crystals are of the space group/222 with the unit cell dimensions of a = 117.6, b = 248.4, and c = 82.7 A. The asymmetric unit contains one PEPC monomer. The statistics of diffraction data and initial phases determined by the multiple isomorphous replacement method are summarized in Table 1. Although the electron density map at 2.8 h. resolution was sufficient enough to recognize secondary structure, it was further improved by a series of density modification methods. The final model includes 6,875 non-hydrogen protein atoms and 120 solvent molecules. The final R-factor and free R-factor [2] for the 25,652 independent reflections between 10.0 and 2.8 A resolution with F>2(~(F) is 0.224 and 0.290, respectively. Table 1 Crystallographic data statistics Diffraction data Resolution
Completeness
Native
2.8 A
91.3%
7.0%
CH3HgC1
2.8 .~
67.2%
11.3%
mersalyl acid
3.5
60.2%
7.7%
EMTS
3.5 A
90.1%
9.8%
No. of heavy-atoms
Rcullis* 0.68 0.77 0.80
Phasing power + 1.38 1.05 0.87
Re*
Heavy-atom phasing statistics
CH3HgC1 mersalyl acid EMTS
6 3 4
Refinement statistics Resolution limits
10- 2.8 s 22.4% / 29.0%
Rfactor / Rfree R.m.s deviaton in bond length
0.009 A
R.m.s deviaton in bond angle
2.0 ~
*Rmerge=21 I-/average I/ Z I
*RCun~=21 IFpH- FpI- Fr~(calc) I/ ~E IFpH- FpI +Phasing power=2 IFn(calc) I/2 I IG.- G I- IF~(calc) I 3. D E S C R I P T I O N OF T H E S T R U C T U R E .
As shown in Figure la, PEPC monomer has overall dimensions of approximately
603 42 x 42 x 60 ,~. The monomer is abundant in a-helices and consists of the 13barrel structure with eight ~-strands. Whereas the 13barrel region including the active site was fitted well on the map, nine residues (K702- G708) near the region are disordered and could not be modeled. The high mobility of these residues along with three disordered residues (M1- E3) at the N-terminus might contribute to the high overall temperature factor for X-ray data that is estimated as 60 ~2 by Wilson scaling [3]. PEPC t e t r a m e r has an overall size of approximately 60 x 110 x 140 A. The four subunits of PEPC are related by the crystallographic 222 symmetry. The entire PEPC molecule forms a 'dimer of dimers' (Figure lb), which corresponds to the results of biological experiments.
a)
~' ,
b)
t
!' )
Figure 1. Three-dimensional structure of phosphoenolpyruvate carboxylase from E. coli. a) Subunit structure of PEPC. b) The entire PEPC molecule. 4. A L L O S T E R I C R E G U I ~ T I O N
PEPC has a highly conserved unique sequence, 578-FHGRGGSIGRGGAP-591, in which a GRGG motif is repeated twice with two intervening residues. Aspartic acid in one of the effector molecules for the allosteric regulation of PEPC. As shown in Fig. 2, aspartic acid binds to PEPC through the hydrogen bonds to R587, K773, R832, and N881. As reported from previous studies [4], some Lys and Arg residues are essential for catalytic activity and allosteric regulation. One of them, Arg587 in a highly conserved sequence is the residue to cause the inactivation of PEPC by site-directed mutagenesis. Based on these results, the highly conserved loop region including R587 may be regulated by the binding of Asp even though the region is far from the active site.
604
R832
K773 N881
K773 N881
Figure 2. Binding structure of asparate in the regulatory site. 5. M E T H O D S 5.1 C r y s t a l l i z a t i o n a n d d i f f r a c t i o n d a t a
PEPC from E. coli was crystallized as described previously [1] with minor modifications. X-ray diffraction data were collected at station BL-6B of the Photon Factory, Japan. Intensity data were obtained using a Weisssenberg camera for macromolecule crystallography and imaging plates as a detector [5]. The data were processed using DENZO and scaled by the program SCALEPACK [6]. The crystal structure was determined by multiple isomorphous replacement method. 5.2 M o d e l b u i l d i n g a n d r e f i n e m e n t Interpretation of the electron density map allowed the chain tracing for most parts of the polypeptide chain. Refinement has been carried out using X-PLOR and REFMAC, and manual model rebuilding resulted in an R-factor of 22.4% for all 2c F data between 10.0 and 2.8 A resolution. Using a 5% reflection test set (1,380 reflections) the free R-factor [2] value is 29.0%. REFERENCES
1. Inoue, M., Hayashi, M., Sugimoto, M., Harada, S., Kai, Y. and Kasai, N. J. Mol. Biol. 208 (1989) 509. 2. Briinger, A. T. Nature 355 (1992) 472. 3. Wilson, A. J. C. Acta Crystallogr. 2 (1949) 318. 4. Terada, K., Murata, T. and Izui, K. J. Biochem. 109 (1992) 49 5. Sakabe, N. Nucl. Instrum. Methods Phys. Res. A303 (1991) 448. 6.Otwinowski, Z. & Minor, W. in Proc. CCP4 Study Weekend, 29. Jan. (1993)
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
605
M o l e c u l a r characterization of r e c o m b i n a n t p h o s p h o e n o l p y r u v a t e c a r b o x y l a s e f r o m an e x t r e m e t h e r m o p h i l e Tsutomu Nakamuraa, * and Katsura Izuib aGraduate School of Science and bGraduate School of Agriculture, Kyoto University, Sakyoku, Kyoto 606-01, Japan Phosphoenolpyruvate carboxylase (PEPC) of Thermus sp., was characterized and its mechanism of stabilization was studied by means of site-directed mutagenesis. A divergent sequence at a Gly-rich region of Thermus PEPC was revealed to contribute to the activity at high temperature but not to the tolerance to the irreversible heat inactivation. 1. INTRODUCTION Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) catalyzes the reaction of phosphoenolpyruvate (PEP) with HCO3- to form oxaloacetate (OAA) and orthophosphate in the presence of Mg2+, and plays an anaplerotic role by replenishing C4-dicarboxylic acids in the citric acid cycle [1]. In C4 plants such as maize and sugarcane, PEPC plays a key role in the photosynthetic CO2-fixation [2]. PEPC consists of four identical subunits with a molecular mass of around 100 kDa, and its activity is, in most cases, regulated by wide variety of allosteric effectors. Since PEPC utilizes HCO3-, which is chemically less reactive than CO2, as a substrate, its reaction mechanism has been attracting much attention. According to the stepwise reaction model of PEPC-reaction, formation of carboxyphosphate (CP) and enolate anion of pyruvate (Pyr), which must be very unstable in water, is supposed to precede the carboxylation [3, 4]. In order to demonstrate CP and to convert CP to useful compounds, use of organic solvent-resistant PEPC seems more advantageous. Enzymes of thermophiles are potentially suited to application because of their high stability. PEPC from an extreme thermophile, Thermus sp., was highly stable to heat and also to high concentrations (up to 4 M) of dioxane. In order to study thermostable PEPC using genetic engineering technique, we have cloned and sequenced the gene for PEPC from Thermus sp. [5]. The polypeptide of Thermus PEPC consisted of 857 amino acid residues and its molecular mass was 95,632 Da. In the present study, Thermus PEPC was expressed in E. coli and enzymological characterization was performed. Furthermore, site-directed mutagenesis was carried out to identify the residues which contributes to the expression of catalytic activity at high temperature.
2.1. Expression and purification of recombinant Thermus PEPC The gene for Thermus PEPC was inserted to an expression vector pTVll9N and expressed in E. coli JM109. Although the expression level in the E. coli cells was low, Thermus PEPC *Present address: National Institute of Bioscience and Human-Technology, 1-1 Higashi, Tsukuba 305, Japan.
606 was purified to homogeneity with ammonium sulfate fractionation, heat treatment at 65~ for 10 min, column chromatography with Butyl-Toyopearl, Mono-Q, and Superose 12 [6]. Thermus PEPC had a strong interaction with hydrophobic-interaction column as reported for PEPCs from the other thermophilic organisms [7, 8].
2.2. Catalytic and regulatory properties of Thermus PEPC The activity of Thermus PEPC was enhanced by acetyl-CoA (CoASAc), as is the case with the E. coli enzyme. Acetyl-CoA affected not only maximum velocity (Vmax) but also halfsaturation concentrations (S0.5) of PEP and Mg2+. Thermus PEPC was inhibited by aspartate and malate. Unexpectedly, it was also inhibited by various phosphorylated compounds such as fructose 1,6-bisphosphate and GTP which are known to be activators of E. coli PEPC. The inhibition by these compounds showed tendencies to be released by increasing CoASAc. These properties were essentially the same with those of native Thermus PEPC.
2.3. Behavior of Thermus PEPC at high temperature The activity of Thermus PEPC was highest at 80~ when monitored with Vmax in the presence of I mM CoASAc (cf. Fig. 3). Above the optimum temperature, Thermus PEPC showed decreased activity. This decrease of activity was reversible and was not due to irreversible heat inactivation. Inactivation of Thermus PEPC at high temperature proceeded slowly even at 95~ Fig. 2) and the inactivation was negligible during measurement of enzyme activity which took about 5 min. Thus, the mechanism for loss of catalytic activity prior to irreversible heat denaturation remains to be elucidated.
2.4. Identification of the region contributing to the expression of activity at high temperature Alignment of Thermus PEPC with the enzymes of mesophilic organisms showed a significant divergence of Thermus PEPC in a Gly-rich conserved region. PEPCs of mesophilic sources have highly conserved and unique sequences with repeated Gly-rich motifs (GRGG) and two intervening residues (Fig. 1, ref [9]). This region is presumed to be exposed to the solvent and to form a flexible loop since it is rich in Gly and polar residues. Previous studies by means of site-directed mutagenesis of His579 and Arg587 in E. coli PEPC showed the involvement of this region in the catalytic activity [10, 11]. In Thermus PEPC, the amino acid sequence in the corresponding region is 562-FHGRGTSTARGGGP575. At three sites (underlined above), two Gly residues are substituted by bulkier amino acids, Thr and Ala, and an aliphatic residue by hydroxylated one, Thr. These substitutions seem to decrease the flexibility of the Gly-rich region of Thermus PEPC. In order to examine the role of the divergent sequence of Thermus PEPC in the behavior at high temperature, we prepared a mutant Thermus PEPC in which Thr567, Thr569, and Ala570 were replaced with the residues of the E. coli enzyme, Gly, lie, and Gly, respectively, and characterized it [12].
(A)
Thermus sp. 562-FHGRGTSTARGGGP-575 E s c h e r i c h i a coli FHGRGGSIGRGGAP Coryn eba ct eri um FHGRGGTVGRGGGP gl u tami cum A n a c y s t i s nidulans FHGRGGSVGRGGGP S o r g h u m vulgare (C4) FHGRGGTVGRGVGP Zea mays (C4) FHGRGGTVGRGGGP
(s)
Mutant Thermus PEPC
562-FHGRGGSIGRGGGP-575
Fig. 1 Gly-rich highly conserved region of PEPCs.(A) Amino acid sequences of the Gly-rich region of Thermus PEPC and other 17 ones aligned by Toh et al. (1994). (B) Amino acid sequence of the Gly-rich region of mutant Thermus PEPC. Mutagenic residues are indicated by boldfaces.
607 As shown in Fig. 2, rate of heat inactivation at high temperature (90 and 950C) was almost the same with the wild-type and the mutant. Thus, the tolerance to heat inactivation of Thermus PEPC was not affected by the mutation. On the other hand, catalytic activity of Thermus PEPC and its dependence on the temperature was affected by the mutation (Fig. 3). When assayed below 65~ Vmax and S0.5 of PEP of the mutant Thermus PEPC were around 70% and 15-fold, respectively, of those of the wild-type. The optimum temperature was lowered, from 80~ to 65~ by the mutation. Therefore, the Gly-rich region of Thermus PEPC does not contribute to heat stability of the enzyme, but does to its activity at high temperature. A |
1~176 t
i
,
I
,
&
,
o
200
100
Incubation time (min)
Fig. 2 Heat inactivation of Thermus PEPCs. Heat treatments were carried out with 0.1 mg/ml of wild-type ( e , A) and mutant (O, A) Thermus PEPCs at 90 (0, O)and 95~ (A, A). The residual activities were assayed at 60~ in the reaction mixture containing 10 mM potassium PEP, 10 mM KHCO 3, 10 mM MgSO4, 0.3 mM NADPH, 1.0 mM CoASAc, 0.1 M Ches-KOH, pH 8.6, 2.0 U malate dehydrogenase from Thermus sp., and the enzyme. Relative activities were plotted against incubation time.
300 0")
E c
20o
X m 100
040
50
60
70
80
Temperature (~
90
oo
Fig. 3 The effect of temperature on the activity of wild-type (4,) and mutant (<>) Thermus PEPCs. Vmax was calculated from the saturation curves for PEP. Enzyme assay was performed in the reaction mixture described in the legend to Fig. 2 except for the concentration of PEP.
2.5. Subsidiary activity of Thermus PEPC As described in "INTRODUCTION," postulated reaction intermediates of PEPC, CP and enolate anion of Pyr, are extremely unstable in water. They are hydrolyzed to produce HCO3and Pi, or protonated to Pyr, respectively. Thus, impairment of the partial reaction of the latter step of PEPC-reaction leads to the formation of Pyr due to bicarbonate-dependent hydrolysis of PEP. For example, the mutant enzymes of E. coli PEPC in which His137 or Arg587 are replaced with Asn or Ser, respectively, catalyze the subsidiary bicarbonatedependent hydrolysis of PEP[3, 11]. Figure 4 shows the authentic OAA-forming and subsidiary Pyr-forming activities of wild-type and mutant Thermus PEPCs. In mutant Thermus PEPC whose Gly-rich region had been converted to that of the E. coli enzyme, higher ratio of Pyr/OAA-formation was observed than the wild-type especially at high temperature. Judged from the sequences, the Gly-rich region seems to be more flexible in the mutant than in the wild-type enzyme. Thus, exceeding flexibility in the Gly-rich region may cause the subsidiary activity of Thermus PEPC by exposing the intermediates to the solvent.
608 3. CONCLUSION In Thermus PEPC, stability of the enzyme can be expressed in two ways, activity at high temperature and tolerance to heat inactivation. Site-directed mutagenesis of Gly-rich region of Thermus PEPC revealed that the divergent sequence at the region contributed to the former but not to the latter. Moreover, the divergence at the region was suggested to be important for avoiding the subsidiary activity of Thermus PEPC at high temperature.
.2oo
. . . . . . . . .
"~ 8o
. . . . . . . . .
il
ut
,oo
4o._ 20
"N
o
0
20
40
60
80
100
Temperature (~ lOO
(B)
60
40
a. 20 0 40
50
60
70
80
Temperature (*C)
0
20
40
60
80
100
Temperature (~
80
0
o
90
100
Fig. 4 0 A A - and Pyr- forming activities of wild-type and mutant Thermus PEPCs. (A) OAA-forming activity (0, A)or Pyrforming activity (O, A)were assayed with wild type (left) or mutant (right) Thermus PEPC in the presence (e, O)or absence (A, A)of 1.0 mM CoASAc. (]3) Relative Pyr-forming activity to OAAforming activity in the presence of 1.0 mM CoASAc is represented in % for wild type (l I) and mutant (r-l) Thermus PEPC.
REFERENCES 1. M.F. Utter and H.M. Kolenbrander, in The Enzymes (Boyer, P.D., ed,) 3rd ed., Vol. 6, 117, Academic Press, New York 1972. 2. R. Chollet, J. Vidal and M.H. O'Leary, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996) 273. 3. K. Terada and K. Izui, Eur. J. Biochem., 202 (1991) 797. 4. J.W. Janc, J.L. Urbauer, M.H. O'Leary and W.W. Cleland, Biochemistry, 31 (1992) 6432. 5. T. Nakamura, I. Yoshioka, M. Takahashi, H. Toh and K. Izui, J. Biochem., 118(1995)319. 6. T. Nakamura, S. Minoguchi and K. Izui, J. Biochem., 120 (1996) 518. 7. Y. Sako, K. Takai, A. Uchida and Y. Ishida, FEBS Lett., 392 (1996) 148. 8. K. Takai, Y. Sako, A. Uchida and Y. Ishida, J. Biochem., 122 (1997) 32. 9. H. Toh, T. Kawamura and K. Izui, Plant Cell Environ., 17 (1994) 31. 10. K. Terada, T. Murata and K. Izui, J. Biochem., 109 (1991) 49. 11. M. Yano, K. Terada, K. Umiji and K. Izui, J. Biochem., 117 (1995) 1196. 12. T. Nakamura and K. Izui, Biotechnol. Lett., 19 (1997) 335.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversionsfor Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
609
Revertant of no-active RuBisCO tobacco mutant, Sp25, obtained by chloroplast t r a n s f o r m a t i o n method using m i c r o p r o j e c t i l e b o m b a r d m e n t Ken-Ichi Tomizawa a, Toshiharu Shikanai b, Ayako Shimoide a, Christine H. Foyerc and Akiho Yokota a,b aplant Molecular Physiology Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Kyoto, 619-02, Japan bGraduate School of Biological Sciences, Nara Institute of Science and Technology (NAIST), Takayama-cho, Ikoma, Nara, 630-01, Japan CLaboratoire de Metabolisme, INRA, F78000 Versailles, France 1. I N T R O D U C T I O N Ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO, EC 4.1.1.39) is the key regulatory enzyme of CO2 fixation in photosynthesis [1]. RuBisCO from higher plants is composed of eight large (L) and eight small (S) subunits; the L subunits contain the active site of the enzyme. RuBisCO activities have been enhanced in some bacteria and lower plants by introducing single amino acid substitutions into the L subunit by techniques of in vitro mutagenesis [2]. This approach has not been used, to date, to engineer higher plant RuBisCO because reliable techniques have not been available to introduce foreign DNA sequences (such as an altered rbcL, the chloroplast gene for the L subunit) into the plastid genome. In addition, appropriate hosts that lack rbcL and/or RuBisCO activity have not been available. In this report we take advantage of recent advances in chloroplast transformation technology to introduce a wild type rbcL gene from Nicotiana tabacum into Sp25, a tobacco mutant that lacks RuBisCO activity [3]. Sp25 was generated by EMS mutagenesis and is maternally-inherited. Compared to the wild type gene, rbcL in the mutant has a single nucleotide change, resulting in an amino acid substitution of Gly-322 to Ser-322 [4]. We show that transformation of the mutant with the wild type rbcL gene can restore RuBisCO activity. This suggests that the amino acid substitution in rbcL is the sole reason for the loss of RuBisCO activity in Sp25. These experiments further represent the first successful attempt to genetically engineer higher plant RuBisCO by altering the sequence of the L subunit. 2. MATERIALS AND METHODS 2 . 1 . Chloroplast transformation A vector was constructed that contained coding and flanking regions (5' and 3' ) of the wild type N. tabacum rbcL gene. The aadA gene, conferring spectinomycin resistance was introduced into the 3'-flanking region of rbcL for purposes of selection. Gold particles were coated with vector sequences and then applied to mature Sp25 leaves (2-4 cm 2) by particle bombardment, according to the methods of Svab and Maliga [5]. After bombardment, transformants were screened on a solidified MS medium containing spectinomycin. Because it
This work was partly supportedby PEC/MITI
610 is photosynthetically incompetent, Sp25 (and putative transformants) were maintained on sucrose-containing medium.
2.2. Analyses of rbcL gene sequences Total cellular DNA was prepared by the method of Taylor and Powell [6]. PCR (polymerase-chain reaction) was performed using two oligonucleotides, 5'AACATATACCACTGTCAAGG-3' and 5'-TCACTCAGAAAAGAATGATC-3', corresponding to ca. 300 bp upstream and ca. 1150 bp downstream, respectively, of the coding region of the tobacco rbcL gene [7]. The PCR products were fractionated by electrophoresis, purified and then subjected to restriction enzyme analysis. A small aliquot of the PCR reaction mixture was used for DNA sequencing using the dye terminator cycle sequencing FS ready reaction kit (Perkin-Elmer Japan Corp., Tokyo, Japan) with a synthetic oligonucleotide, 5'-AACAAAATCATCACGCAGTA-3'. The products were analyzed using an ABI PRISMTM377 DNA Sequencer (Perkin-Elmer Japan Corp., Tokyo, Japan). 2.3. Analyses of RuBisCO content and activity To determine RuBisCO contents, leaf tissues were quick-frozen in liquid nitrogen then homogenized with a mortar and pestle in a buffer containing 50 mM HEPES-KOH (pH 7.6), 1 mM dithiothreitol, 2% polyvinylpolypyrrolidone, 1raM phenylmethylsulfonylfluoride and 10~tM leupeptin. The homogenate was passed through two layers of cheesecloth, centrifuged at 20,000 x g for 10 min, then fractionated on a PD-10 gel filtration prepacked column (Pharmacia Biotech, Uppsela, Sweden). The fractionated sample was analyzed by SDS-PAGE To analyze RuBisCO activities, 200 ~1 of the soluble protein sample (lmg/ml) was added to 0.5ml of a buffer containing 100mM HEPES-KOH (pH 8.0), 20mM MgC12, 0.1mM DTF, lmM EDTA, and 20mM NaHCO3. The mixture was incubated at 25 ~ for 10 min (for RuBisCO activation to occur), then 25 ~tl of 20mM RuBP was added for 10 min (for the carboxylation reaction to occur). The sample was then treated with Dowex and the filtrate (5~tl) was analysed using a Dionex DX-AQ 1110 anion-exchange chromatographic system [8]. PGA was detected at about 6 min.
2.4. Analysis of chlorophyll content and photosynthetic activity Chlorophyll contents were determincd by Arnon's method [9]. In brief, fresh leaf tissue was ground to a fine powder in liquid nitrogen. After adding 5 volumes of 80% acetone, thc suspension was incubated overnight at 4 ~ thcn centrifuged. Pigment contents werc determined spectrophotometrically on the supernatants. Photosynthetic activities were measured using a portablc photosynthcsis system (LI6400, LI-COR, Lincoln, NB, USA) according to thc manufacturcr's instructions. 3. R E S U L T S AND D I S C U S S I O N Wild type rbcL sequences were introduced into tobacco Sp25 leaves by the biolistic method, and the transformed plants were regenerated on sucrose-containing medium supplemented with 500~tg/ml spectinomycin. Out of 120 leaves that were bombarded, 8 independent putative transformants were identified that survived three rounds of screening on the spectinomycin. Non-bombarded Sp25 leaves served as controls. Total cellular DNA was prepared from the control and putatively-transformed plants, and PCR amplification was carried out using the two primers. It would be predicted that a
611 single 2.9 kbp rbcL-containing band would be amplified from Sp25 plants; this band should not contain a HindIII site. On the other hand, it would be predicted that the primers should amplify a 4.2 kbp band from the transformed plants (rbcL + aadA); this fragment should contain two HindIII sites, giving rise to subfragments of 2.0kbp, 1.4kbp and 0.8kbp. One of these HindIII sites arises from the Gly-322 (versus the Ser-322) in the wild type rbcL sequence. Of the 8 transformants, four behaved like Sp25; these are likely spontaneous mutants, as described by Svab and Maliga [5]. Three of the transformants were heteroplasmic and contained the three HindIII restriction fragments, as well as faint amounts of the 2.9 kbp band. The final transformant contained a single HindIII site and likely suffered a deletion of the introduced rbcL gene, but not of the aadA gene. Only the three heteroplasmic transformants will be further discussed in this report. Direct sequence determination of the 4.2kbp product in the three transformants confirmed that it contained a Gly-322 instead of Ser-322, as well as the HindIII site (Fig. 1).
Hindlll
9
!~Kpnl
!i W
Fig. 1. DNA sequence analysis of a portion of the 4.2 kbp rbcL-containing fragment in the transformed plants. Both strands of the amplified sample were sequenced. The Ser-322 to Gly-322 change, as well as the new HindIII site, are underlined.
Soluble proteins from the transformed and Sp25 plants, as well as from wild type N. tabacum plants, were separated by SDS-PAGE. As illustrated in Figure 2, RuBisCO L and S amounts are nearly the same in the wild type and transformed plants. This contrasts with their virtual absence in Sp25. RuBisCO activity assays revealed that PGA is detectable after about 6 min in the products of the transformed plants, but that it is not present in Sp25 (data not shown). Taken together, these data indicate that RuBisCO content and activity are restored to wild type levels in the transformed plants. Consistent with these findings, the in vivo photosynthetic activities of the transformants, as assayed by gas exchange, were normal. Compared to the pale green color of Sp25 leaves, the transformants also had dark green leaves and their chlorophyll contents were 1.4 times higher than those of Sp25. It is clear that the loss of RuBisCO in Sp25 has a pleiotropic effect on the physiology of the plant. The results in this report strongly support the hypothesis that the conversion of Gly-322 to Ser-322 leads to the loss of RuBisCO activity in Sp25. These results also pave the way for the introduction of mutations into the tobacco rbcL gene. This will allow us to investigate the
612 structure-function relationships of higher plant RuBisCO, as is being done for the prokaryotic RuBisCO [10].
:3-,' .,,.
, ~ . ~ ~,~ ~ , ~.
A
B
C
Fig. 2. SDS-PAGE of soluble proteins. Lane A: N. tabacum (wild type). LaneB: Sp25. Lane C: Transformant REFERENCES 1. T.J. Andrews and G.H. Lorimer. M.D. Hatch and N.K. Boardman (eds), The Biochemistry ofPlants, vol. 10 (1987) 131 2. G.Schneider, Y. Lindqvist and C.I. Brand6n, Ann. Rev. Biophys. BiomoL Struct., 21 (1992) 119 3. C.H. Foyer, A. Nurmi, H. Dulieu, M.A.J. Parry, J. Experi. Bot..,44 (1993) 1445 4. T. Shikanai, C.H. Foyer, A. Nurmi, H. Dulieu, M.A.J. Parry, A. Yokota, Plant Mol. BioL, 31 (1996) 399 5. Z. Svab, P. Maliga, Proc. Natl. Acad. Sci. USA, 90 (1993) 913 6. B, Taylor and A. Powell, Focus, 4 (1982) 2 7. K. Shinozaki, M. Ohme, M. Tanaka, T. Wakasugi, N. Hayashida, T. Matsubayashi, N. Zaita, J. Chunwongse, J. Obokata, K. Yamaguchi-Shinozaki, C. Ohto, K. Torazawa, B.Y. Meng, M. Sugita, H. Deno, T. Kamogashira, K. Yamada, J. Kusuda, F. Takaiwa, A.Kato, N. Tohdoh, H. Shimada and M. Sugiura, EMBO J 5 (1986) 2043 8. K. Uemura, Y. Suzuki, T. Shikanai, A. Wadano, R.G. Jensen, C. Wendy and A. Yokota, Plant Cell Physiol., 37 (1996) 325 9. D.I. Amon, Plant Physiol, 24 (1949) 1 10. S. Gutteridge and A.A. Gatenby, The Plant Cell, 7 (1995) 809
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi(Editors) Advances in Chemical Conversionsfor Mitigating CarbonDioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Reductive TCA cycle in an aerobic bacterium, thermophilus strain TK-6
613
Hydrogenobacter
Masaharu Ishii, Ki-Seok Yoon, Y a s ~ Ueda, Toshihiro Ochiai, Nare Yun, Seiichi Takishita, Tohru Kodama*, and Yasuo Igarashi Department of Biotechnology, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Introduction Hydrogenobacter thermophilus strain TK-6 is an aerobic thermophilic hydrogen-oxidizing bacterium isolated from hot spring in Izu, Japan [1]. As for C02 fixation pathway of the strain, 14C02 labeling experiment [2], in vitro enzyme assays [2], and purification of ATP:citrate lyase [3] in our group supported the idea that the reductive TCA cycle is operative in strain TK-6. However, identification of a strong reductant which is needed for the pyruvate synthase and 2-oxoglutarate synthase and clarification of an electron transport system were definitely needed. H. thermophilus was shown to belong to a very early branching order, the Aquificales [4]. Also, strain TK-6 was isolated from hot spring, which implies that the strain may have its original ecological niche underground where little or no oxygen exists. Taking these things into consideration, it is of great interest to examine if the strain has its ability to grow anaerobically. Results and Discussion 1. Purification and characterization of ferredoxin [5] Purification was performed aerobically by adding octyl- B-glucoside to buffers. Columns of Q-Sepharose Fast Flow, DEAE-5PW, and Superdex 75 were used in these order. The ferredoxin had a molecular mass of 13000. The sequence of Nte rminal amino acids was MKDWKIYEKKLGELKDYLEKNYATNPDVEFRLLXPYDXGF. The sequence had a long stretch at the N-terminal region like those from Thermoplasma acidophilum [6], Sulfolobus [7], and Halobacterium [8]. However, identity was less than 20%. 2. Purification and characterization of pyruvate synthase (Pyruvate:ferredoxin oxidoreductase (POR)) [9] POR activity was determined spectrophotometrically by following the *Faculty of Textile Science and Technology, Shinshu University, Ueda-city, Nagano 386, Japan
614 pyruvate-dependent reduction of methyl viologen under nitrogen gas atmosphere at 70 ~ Buffers used throughout the purification procedure contained 0.1% Triton X-100. Purification was performed by using columns of DEAE-Sepharose CL-6B, PA-QA, Hydroxyapatite, and Superdex-200 in these order. The enzyme had a molecular mass of 135 kDa and was composed of four subunits (46, 31.5, 29, and 24.5 kDa). The enzyme did not react with 2-oxoglutarate. The enzyme reacted with ferredoxin isolated from strain TK-6, FMN, and FAD as an electron acceptor. However, NAD, NADP, and ferredoxin from Chlorella spp. or Clostridium pasteurianum were not effective.
3. Purification and characterization of 2-oxoglutarate synthase (2-Oxoglutarate:ferredoxin oxidoreductase (OGOR)) [10] OGOR activity was determined spectrophotometrically by following 2oxoglutarate-dependent reduction of methyl viologen under nitrogen gas atmosphere at 70~ The buffers used throughout the purification procedure contained 0.1% Triton X-100. For the purification of OGOR, the order of columns usage was the same as that for POR. OGOR and POR could be separated by the first column; DEAE-Sepharose CL-6B. OGOR had a molecular mass of 105 kDa, and was composed of two subunits (70 and 35 kDa). The enzyme only slightly reacted with pyruvate (less than 0.4% relative to that with 2-oxoglutarate). The enzyme reacted with ferredoxin isolated from strain TK-6, FMN, and FAD as an electron acceptor. NAD, NADP, and ferredoxin from Chlorella spp. or Clostridium pasteurianum were not effective. 4. Carboxylation reactions of POR and OGOR [11] Demonstration of synthetic reaction of pyruvate or 2-oxoglutarate by using only purified components has not been successful probably because of the instability of acetyl-CoA and succinyl-CoA. In the next step, attempt was made to measure carboxylation reactions (14CO2 incorporation) of POR and OGOR by using ATP:citrate lyase and succinyl-CoA synthetase, respectively, within the cell extract. The results are given in Tables I and II. Also, the CO2 fixation products were analyzed by using HPTLC and the Bio-imaging analyzer system. The major products of 14CO2 fixation by POR were pyruvate and oxalacetate and the major one by OGOR was 2-oxoglutarate. The reason why oxalacetate was detected in the carboxylation reaction of POR is because pyruvate carboxylase which exists within the cell extract carboxylates pyruvate to produce oxalacetate. These results strongly demonstrate that the reductive TCA cycle is operative as the CO2 fixation pathway in strain TK-6. 5. Electron transport system Formerly, we isolated a new quinone from strain TK-6 (Methionaquinone; 2- m ethyl thio-3-VI,VII-t etrahydro multipr enyl 7-1,4naphthoquinone) [12] and demonstrated that the quinone is involved in the
615 m e m b r a n e electron t r a n s p o r t system of strain TK-6 [13]. Purification of membrane-bound hydrogenase was performed as follows; the hydrogenase was solubilized by 0.5% Triton X-100 at pH 10, and the solubilized enzyme was successively put on columns of Butyl-Sepharose, Mono-Q, and Hydroxyapatite. The enzyme was composed of two subunits (63 kDa and 32 kDa). Recently, we d e m o n s t r a t e d t h a t the activities of soluble NAD-reducing hydrogenase and NADH:Fd reductase exist within cell extract. Based on these results, we proposed a mechanism for energy generation in strain TK-6 [14]. Table 1. The carboxylation reaction of POR Treatment
[14C]incorporated (cpm)
Complete
22600
Citrate omitted
103
CoA omitted
101
ATP omitted
1060
Ferredoxin omitted
3000
NaH14C03 omitted
35
Table 2. The carboxylation reaction of OGOR Treatment Complete
[14C]incorporated (cpm) 10500
Succinate omitted
114
CoA omitted
145
ATP omitted
1150
Ferredoxin omitted
1500
NaH14CO3 omitted . . . . . . . . . .
36
,
6. Terminal electron acceptor Elemental sulfur, thiosulfate, or sulfate was tested as a terminal electron acceptor for growth of strain TK-6. Among these, the strain was able to grow on elemental sulfur; the strain produced H2S during its growth and the obtained cells were able to grow aerobically again (H2:02:CO2=75:15:10). These facts show that the strain can grow anaerobically. It is widely believed that there was little or no oxygen on the earth when the
616 first living cell emerged. Because the strain TK-6 is thought to belong to a very early branching order and because the strain can grow anaerobically, the strain might evolve so that it can use and tolerate a little amount of oxygen. In fact, when the cultivation of the strain is performed, much care should be taken so that dissolved oxygen concentration is around 1 ppm.
Conclusions (1) Purification and characterization of components involved in the reductive TCA cycle in H. thermophilus strain TK-6 confirmed the operation of the cycle. (2) Analysis of energy transport system demonstrated the overall energy flow in the strain. (3) The strain was shown to grow anaerobically.
REFERENCES 1. T. Kawasumi, Y. Igarashi, T. Kodama, and Y. Minoda. Int. J. Syst. Bacteriol., 34 (1984) 5. 2. H. Shiba, T. Kawasmni, Y. Igarashi, T. Kodama, and Y. Minoda. Arch. Microbiol., 141 (1985)198-203. 3. M. Ishii, Y. Igarashi, and T. Kodama. J. Bacteriol., 171 (1989) 1788. 4. C. Pitulle, Y. Yang, M. Marchiani, ERB moore, JL Siefert, M. Aragno, P. Jurtshuk Jr, GE Fox. Int. J. Syst. Bacteriol., 44 (1994) 620. 5. M. Ishii, Y.Ueda, K.-S. Yoon, Y. Igarashi, and T. Kodama. Biosci. Biotech. Biochem., 60 (1996) 1513. 6. S. Wakabayashi, N. Fujimoto, K. Wada, H. Matsubara, L. Kerscher, and D. Oesterhelt. FEBS Lett., 162 (1983) 21. 7. T. Iwasaki, T.Fujii. T. Wakagi, and T. Oshima, Biochem. Biophys. Res. Commun., 206 (1995) 563. 8. T. Hase, S. Wakabayashi, H. Matsubara, M. Mevarech, and M.M. Werber, Biochim. Biophys. Acta, 623 (1980) 139. 9. K.-S. Yoon, M. Ishii, T. Kodama, and Y. Igarashi, Arch. Microbiol., 167 (1997) 275. 10. K.-S. Yoon, M. Ishii, Y. Igarashi, and T. Kodama, J. Bacteriol., 178 (1996) 3365. 11. K.-S. Yoon, M. Ishii, T. Kodama, and Y. Igarashi, Biosci. Biotech. Biochem.,61 (1997) 510. 12. M. Ishii, T. Kawasumi, Y. Igarashi, T. Kodama, and Y. Minoda, J. Bacteriol., 169 (1987) 2380. 13. M. Ishii, T. Omori, Y. Igarashi, O.Adachi, M. Ameyama, and T. Kodama, 55 (1991) 3011. 14. K.-S. Yoon, Y. Ueda, M. Ishii. Y. Igarashi, T. Kodama, FEMS Microbiol Lett., 139 (1996) 139.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
617
Carbon dioxide fixation and biomass production with blue-green algae Spirulina platensis S a t o s h i H i r a t a a b and M a s a o H a y a s h i t a n i a b a R e s e a r c h I n s t i t u t e of I n n o v a t i v e Technology for the E a r t h (RITE) 9-2, Kizugawa-dai, Kizu-cho, Kyoto 619-02, J a p a n bAkashi Technical I n s t i t u t e , K a w a s a k i H e a v y I n d u s t r i e s , Ltd., 1-1 Ka was ak i- cho, A k a s h i 673, J a p a n * 1. I N T R O D U C T I O N
In p h o t o a u t o t r o p h i c culture of microalgae, CO2 is a sole c a r b o n source for the growth. CO2 supplied to the s u s p e n s i o n culture of microalgae is first dissolved to the medium and utilized by the cells. In u s u a l culture o p e r a t i o n of microalgae, gas mixture containing CO2 is c o n t i n u o u s l y supplied to the medium. As CO2 supply r a t e is more t h a n CO2 c o n s u m p t i o n r a t e by the cells, m o s t of supplied CO2 e s c a p e s to the atmosphere. In the p r e s e n t study, with the regulation of gas supply r a t e , the efficiency of CO2 utilization by the cells is improved. In microalgal cells, CO2 is fixed as organic c o m p o u n d s such as protein, sugars and lipids. For the p u r p o s e of fixation and utilization of CO2 in the e x h a u s t gas from t h e r m a l power generation, a basic r e s e a r c h of biological CO2 fixation process using microalgae was carried out. 2. E X P E R I M E N T A L
A f ila men to u s blue-green alga, Spirulina platensis N I E S 3 9 was u s e d i n this study. This algal s t r a i n was provided from Global E n v i r o n m e n t a l Forum, J a p a n . For the algal cul t i vat i on, SOT medium [ 1] was used. The pH value of the medium was a d j u s t e d at 9.0 with 1 mol dm -3 NaOH. The medium was a u t o c l a v e d at 121 ~ for 15 min prior to use. P r e c u l t u r e of the cells was p e r f o r m e d at 30 ~ in a 0.3 dm 3 E r l e n m e y e r f l a s k for 7 - 10 days. The flask was s h a k e n at 50 rpm on a r o t a r y s h a k e r u nd er irradiation with white fluorescent lamps. The cells h a r v e s t e d by c e n t r i f u g a t i o n were us ed as an inoculum in s u b s e q u e n t culture experiments. For culture e x p e r i m e n t s of S. platensis, a cylindrical culture a p p a r a t u s The present work was supported by a grant from the New Energy and Industrial Technology Development Organization (NEDO), Japan. *Corresponding address.
618 (12 c m in d i a m e t e r , 16 cm in h e i g h t , 1.8 dm 3 in c u l t u r e v o l u m e ) was used. The s c h e m a t i c d r a w i n g o f t h e c u l t u r e a p p a r a t u s is s h o w n i n F i g u r e 1. A f t e r t h e i n o c u l u m was added to t h e m e d i u m , a b a t c h c u l t u r e s t a r t e d a t 30 ~ The light of w h i t e f l u o r e s c e n t l a m p s was c o n t i n u o u s l y i l l u m i n a t e d to the c u l t u r e a p p a r a t u s from t h e side. The m e d i u m was a g i t a t e d a t 60 r p m u s i n g a s t i r r i n g propeller. Air c o n t a i n i n g 10 or 20 % CO2 was s u p p l i e d to t h e m e d i u m t h r o u g h a g l a s s t u b i n g . In t h e c a s e of i n t e r m i t t e d s u p p l y of t h e g a s , w h e n the p H v a l u e of the m e d i u m was m o r e t h a n 9.1, t h e g a s e o u s m i x t u r e was i n t r o d u c e d to t h e m e d i u m . I n c o n t i n u o u s s u p p l y of t h e gas, t h e g a s e o u s m i x t u r e w a s also i n t r o d u c e d a t a c o n s t a n t flow r a t e of 0.54 dm 3 h-1. Cell c o n c e n t r a t i o n in the m e d i u m was e s t i m a t e d as dry cell weight. The h a r v e s t e d cells were c o l l e c t e d b y c e n t r i f u g a t i o n ( 1 2 , 0 0 0 • g, 4 ~ for 15 min) a n d w a s h e d w i t h distilled w a t e r . The cells dried u n d e r r e d u c e d p r e s s u r e were s u b j e c t e d to c h e m i c a l a n a l y s i s . The calorific v a l u e of t h e cells was m e a s u r e d u s i n g a n a u t o m a t i c b o m b c a l o r i m e t e r (CA-3, S h i m a d z u , J a p a n ) . E l e m e n t a l c o m p o s i t i o n of the cells w a s m e a s u r e d u s i n g an automatic elemental analyzer (EAll08, Fisons Instruments Co., Italy). 3. R E S U L T S
AND DISCUSSION
3.1. E f f e c t of i n t e r m i t t e n t s u p p l y of CO~ for algal g r o w t h I n o r d e r to e x a m i n e the effect of i n t e r m i t t e n t s u p p l y of C 0 2 for algal g r o w t h , b a t c h c u l t u r e s of S. platensis were c a r r i e d out for 39 d a y s w i t h t h e i r r a d i a t i o n of light i n t e n s i t y of 18 W m "2. T i m e c o u r s e s of cell c o n c e n t r a t i o n X were s h o w n in Figure 2. T h e r e was little difference b e t w e e n t h e profiles of cell c o n c e n t r a t i o n c h a n g e in i n t e r m i t t e n t s u p p l y and
Intermittent Continuous supply supply X [kg m"3] o Vt [dm3] 1.5
. . . . . . .
"I~ = 18 w m 2 ~ t ~ 1.o
,1 5OO
400
..."
300
0.5 (~) cylindrical culture apparatus, (~) pH probe, (~) white fluorescent lamp, (~)pH controller, (~) CO2 cylinder, (~) compressor, (~) gas mixer, (~) mass flow controller, (~) flow meter, Figure 1. Schematic drawing of a culture apparatus for S. platensis.
o o
lO 20 3o Culture time [days]
39
Fugure 2. Batch cultures ofS. platensis with different gas supply methods.
619 c o n t i n u o u s s u p p l y of CO2. I n F i g u r e 2, t o t a l v o l u m e s of C O 2 - e n r i c h e d air s u p p l i e d to t h e m e d i u m , Vt, w e r e also i n d i c a t e & I n t h e c a s e of c o n t i n u o u s s u p p l y , C O 2 - e n r i c h e d air of 8.7 t i m e s a s m u c h a s in i n t e r m i t t e n t s u p p l y w a s s p a r g e d to t h e m e d i u m . The e f f i c i e n c y of CO2 u t i l i z a t i o n b y t h e cells w a s e s t i m a t e d as 34.5 % in i n t e r m i t t e d s u p p l y a n d 4.2 % in c o n t i n u o u s s u p p l y , r e s p e c t i v e l y . F r o m t h e s e r e s u l t s , it w a s f o u n d t h a t i n t e r m i t t e n t s u p p l y of CO2 c o r r e s p o n d i n g to t h e p H v a l u e of t h e m e d i u m w a s a v a i l a b l e in a p h o t o a u t o t r o p h i c c u l t u r e of a l g a l cells. I n b a t c h c u l t u r e s s h o w n in F i g u r e 2, light i n t e n s i t y w a s s u p p o s e d to r e s t r i c t t h e cell g r o w t h . T h e r e f o r e , b a t c h c u l t u r e of S. platensis w a s p e r f o r m e d u n d e r i r r a d i a t i o n of light i n t e n s i t y of 87 W m -2 w i t h i n t e r m i t t e n t s u p p l y of CO2 g a s . C o n c e n t r a t i o n of CO2 w a s r e g u l a t e d a t 20 %, w h i c h is a p p r o x i m a t e l y t h e s a m e c o n c e n t r a t i o n a s in t h e e x h a u s t gas f r o m t h e r m a l p o w e r g e n e r a t i o n . T i m e c o u r s e s of cell c o n c e n t r a t i o n a n d t o t a l a m o u n t of CO2 s u p p l i e d to t h e m e d i u m are s h o w n in F i g u r e 3 a n d t h e c a l c u l a t i o n r e s u l t s from t h e c u l t u r e a r e also s h o w n in T a b l e 1. The c o n c e n t r a t i o n of t h e cells i n c r e a s e d w i t h e l a p s e d t i m e a n d r e a c h e d 10.9 k g m "3 a f t e r 70 d a y s . The m e a n g r o w t h r a t e of t h e cells d u r i n g 70 d a y s w a s c a l c u l a t e d as 0.16 k g m -3 d -1. S i n c e t o t a l v o l u m e of CO2 s u p p l i e d to t h e m e d i u m for 70 d a y s w a s 55.8 dm 3, it w a s also c a l c u l a t e d t h a t
lO
~i0=87 w m 2 .
I
o' o o
400 3oo
o
200 i00
,~. ~-~
o
0
20
40
60
80
Culture time [h] Figure 3. Batch culture of S. platensis with intermitted supply of 20% CO2. Table 1. Calculated results from batch culture of S. platensis with intermitted supply of 20% CO2. 0.16 kg-dry cells m "3 d -1 Mean growth rate of the cells 0.28 kg-CO2 m 3 d -1 Mean CO2 fLxation rate by the cells 55.8 dm 3 Total volume of C02 supplied to the medium for 70 days 19.7 g-dry cells Produced cells in the culture for 70 days 49% Carbon contents of the cells 32.3 % Efficiency of CO2 utilization by the cells for 70 days 3.25 x 107 J Furnished light energy for 70 days 1.94 x 107 J kg-l-dry cells Calorific value of the cells 1.2 % Efficiency of conversion from light to chemical energies
620 32.3 % of supplied CO2 was utilized by the cells and fixed as organic compounds. The c a r b o n c o n t e n t of the cells was 49 % and the calorific v a lu e of the cells was 1.94• J kg -1 on dry weight basis. The m e a n CO2 fixation r a t e c a l c u l a t e d from the c a r b o n c o n t e n t was 0.28 kg-CO2 m -3 d "1. The efficiency of conversion from light energy to c h e m i c a l energy was e s t i m a t e d as 1.2 %. From t h e s e r e s u l t s , it was possible t h a t CO2 in the e x h a u s t gas from t h e r m a l power g e n e r a t i o n was efficiently r e m o v e d with p h o t o s y n t h e s i s of microalgae with r e g u r a t i o n of gas supply r a t e to the medium.
3.2. C o m p o s i t i o n of p r o d u c e d c e l l s Table 2 r e p r e s e n t s gross c h e m i c a l compositions of produced cells in b a t c h c u l t u r e s for 39 days. The cells were rich in p r o t e i n s and the p r o t e i n c o n t e n t was e s t i m a t e d as a p p r o x i m a t e l y 60 % of the dry weight. C a r b o h y d r a t e s including sugars and fibers was a b o u t 13 % of the dry weight and lipids was also a b o u t 8 %. These r e s u l t s r e f l e c t e d t h e p o t e n t i a l of the cells as h u m a n food, a n i m a l feed and fertilizers. In this e x p e r i m e n t s , a lot of a s h e s were d e t e c t e d in the cells. The sodium and p o t a s s i u m c o n t e n t s were a b o u t 5.7 % and 2.4 % of the dry cell weight, r e s p e c t i v e l y . It was s u p p o s e d t h a t the n u t r i e n t s from the medium r e m a i n e d i n the sam pl e u sed for the chemical a n a l y s i s . Table 2. Gross chemical composition of produced cells Composition g per 100g of dry cells
Composition g per 100g of dry cells
Protein Lipids Sugars Fibers Ashes
P Fe Ca Na K
60.1 7.6 12.2 1.2 18.9
1.29 0.054 0.077 5.69 2.35
4. C O N C L U S I O N (1) In p h o t o a u t o t r o p h i c c u l t u r e s of microalgae, the ratio of C02 fixed by the cells was i m p r o v e d by m e a n s of regul at i ng gas supply r a t e corresponding to the pH value of the medium. (2) In a b a t c h culture o f S . platensis for 70 days with the r e g u l a t i o n ofgas supply r a t e , cell c o n c e n t r a t i o n b e c a m e 10.9 kg m -3 and the a v e r a g e C02 fixation r a t e c a l c u l a t e d on the b a s i s of the carbon c o n t e n t of the cells was 0.28 kg-CO2 m "3 d -1. (3) The produced cells were rich in p r o t e i n s , c a r b o h y d r a t e and lipids. It was s u p p o s e d t h a t foods, feed and fertilizer were p r o m i s i n g for utilization of microalgal b i o m a s s produced using CO2.
REFERENCES 1. Marquez et al., J. F e r m e n t . Bioeng., 76 (1993) 408.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors)
Advances in Chemical Conversions for Mitigating Carbon Dioxide
621
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Chitosan-calcium Biomimetic into
carbonate
mineralization
chitosan-calcium
Shigehiro Hirano,Koichi Kjell M. V a r u m , b a n d O l a v ~Department University, bDepartment
Norway
composites: of
alginate
Biotechnology,
University
carbonate
ions
hydrogels
Yamamoto, ~ tliroshi Smidsrod b
of A g r i c u l t u r a l Biochemistry Tottori 680, Japan, of
aqueous
and
of
Inui, -
Kurt
I.
Biotechnology,
Trondheim,
Draget,
b
Tottori
N-7034,
Trondheim,
Two hydrogels of c a l c i u m a l g i n a t e and chitosan-calcium alginate composite were prepared f r o m Na a l g i n a t e and its composite with chitosan by soaking in O.1M a q . C a C l z . A biomimic fixation of a q u e o u s C 0 3 2 - i o n s a s CaC03 was performed. E a c h of t h e h y d r o g e l s was p u t i n t o a d i a l y s i s membrane tube, and was s o a k e d in 0.1M a q . Na2C03 (pH 11) a t room t e m p e r a t u r e for 1 day to afford a white precipitate o f CaCOa or c h i t o s a n - C a C 0 3 and a watersoluble sodium alginate. The d r y c h i t o s a n - C a C 0 3 composite (60% c h i t o s a n and 23% C 0 3 2 - ) w as s t a b l e in aqueous alkaline solutions and unstable in aqueous acidic solutions. 1
.
I N T R O D U C T
I ON
Chitin, a (l~4)-linked N-acetyl-i~-D-glucosaminan, and chitosan, the Ndeacetylated derivative of chitin, are widely found in shells of many marine animal such as crabs and shrimps. Alginic acid, a heteropolysaccharide consisting of ( l ~ 4 ) - l i n k e d ~-D-mannuronic acid and ~-L-guluronic acid residues, is found in various seaweeds. Calcium carbonate is the main structural c o m p o n e n t of c r a b s h e l l s , seashells and coral reefs [1]. The c r a b s h e l l i s o n e of n a t u r a l composite materials of CaCOa, chitin and p r o t e i n as the main components [2]. Several artificial composite materials of c h i t i n or c h i t o s a n with inorganic salts have been prepared [3]. In t h e e c d y s i s of c r a b s h e l l s [4-6], CaC03 i n t h e shells is solubilized into the body fluids and into the specific organs, and the solubilized Ca i o n s a r e u t i l i z e d for the mineralization of a q . HC03ions as CaC03 in t h e h y d r o s p h e r e to f o r m new s h e l l s . Crabs assemble chitin fibrils into a cylindrical architecture at the endocuticle layer, using Ca 2§ i o n s in t h e body fluids and aq. C032- ions in the hydrosphere. The calcification of HC03- i o n s i n t h e h y d r o s p h e r e has been reported in coral reefs [7]. Sea water is slightly alkaline ( pH 7 . 5 - 8 . 5 ) due to p r e s e n c e of several metal cations including Na § Mg 2 § a n d K§ [ 8 ] . In our previous works [9-11], f r e e C a C l 2 was u s e d a s a c a l c i u m source for the fixation of HCOaand COa 2 - i o n s i n w a t e r . However, the greater part of CaCl2 was released out of t h e g e l a t i n o u s matrixes of c h i t i n and chitosan, and the maximum 0 . 4 4 m o l e C032§ i o n s p e r GlcN ( o n l y a b o u t 10% o f t o t a l weight) was mineralized as CaC03. Zhang and Gonsalves [12] reported a crystal growth of CaC03 on c h i t o s a n surface in a s u p e r s a t u r a t e d CaC03 solution. Taking these previous reports into consideration, it is essential to develop an efficient and mild method for the mineralization o f a q . COa 2 and HC03-
622
ions in water. We w i s h t o r e p o r t a novel biomimic mineralization o f a q . C0~ 2 - i o n s i n t o a Ca a l g i n a t e hydrogel and into a chitosan-Ca alginate hydrogel in water. 2 . MATER I ALS AND METHODS Crab shell chitosan (Flonac N, K y o w a T e c h n o s , Co., Ltd. Chiba) was treated one t i m e w i t h a q . 40% Na0H a t 100 ~ for 4 h to give a purified sample of chitosan, MW 3 x 1 0 5 ; d . s . 0 . 1 f o r NAc; [ ]D 2~ - 6 ~ (C 0 . 8 % , 2% a q . Ac0H); p ~ x KBr 1 6 2 0 cm -~ (NH2) a n d no a b s o r p t i o n s were detected a t 1 6 5 0 arid 1550 cm-1 (NAc). Sodium alginate (MW 3 6 x 1 0 s , 69% L-guluronic acid) was a product of Protan Co., Ltd., Norway (Protanal L F I 0 1 6 0 SLP 4 6 6 ) , and its source was from a brown algae, Laminaria. hyperborea. FTIR spectra were recorded on a J a s c o F T I R 5 3 0 0 s p e c t r o m e t e r (Jasco Company, Tokyo). 3
.
3.1
EXPER
.
I
MENTAL
C a l c i u m
a l g i n a t e
h y d r o g e l
A solution o f Na a l g i n a t e ( 0 . 5 0 g) i n w a t e r (20 m l ) w a s p u t i n t o a d i a l y s i s membrane tube (diameter 1.5 cm), and soaked in aq. 0.1M CaC12 (500 ml) at room temperature overnight to afford a rigid hydrogel. The hydrogel was dialyzed against running water and against distilled water to give a transparent Ca a l g i n a t e hydrogel in the tube. 3 . 2 .
T r e a t m e n t
N a 2 C O 3
of
the
Ca
a l g i n a t e
h y d r o g e l
in
O.1
M
aq.
s o l u t i o n
The hydrogel obtained above was put into a dialysis membrane tube, and was soaked i n 5 0 0 ml o f a q . 0 . 1 M Na2C03 solution (pHll) with gentle stirring at room temperature for 1 day. Calcium carbonate appeared as a white precipitate ( 0 . 1 0 g ) , a n d Na a l g i n a t e was solubilized. The precipitate was collected by filtration and dried to afford CaC03 in 0 . 1 0 g y i e l d . ~ma• in yacuo, 1450, 8 7 0 a n d 710 c m - 1 ( C 0 3 2 - ) . The filtrate was concentrated and 3 volumes of ethanol was added to afford Na a l g i n a t e as a precipitate. 3 . 3 .
C h i t o s a n - - c a l c
ium
a l g i n a t e
gel
A chitosan ( 0 . 3 0 g) s o l u t i o n in aq. 0.2% acetic acid (6 m l ) was diluted with water (17 m l , pH a b o u t 6.5). To t h e s o l u t i o n , w a s a d d e d a Na a l g i n a t e ( 0 . 4 0 g) s o l u t i o n i n 20 ml d i s t i l l e d water to afford a viscous transparent solution. The solution was put into a dialysis membrane tube (diameter 1.5 cm), and was soaked in 0.1M aq. CaCl, solution (500 ml) overnight to give a rigid hydrogel. The hydrogel was soaked in distilled water overnight to afford a transparent hydrogel of chitosan-Ca alginate composite. The hydrogel was stable in boiling water, but unstable in aq. strong acid and alkaline solutions. A part of the solution was lyophilized. ~ ms• KBr 3 5 0 0 (OH), 1640 (C00-), 1600 ( N H a ) , a n d 1100 ( C - 0 ) c m - 1 ; A n a l . N 3.73% (43% chitosan). 3 . 4 . in
T r e a t m e n t O.1
M
aq.
of N a z C O 3
a
c h i t o s a n - - C a
a l g i n a t e
h y d r o g e l
s o l u t i o n
The hydrogel obtained above was put into a dialysis membrane tube, and was soaked in 500 ml o f 0 . 1 M a q . N a z C 0 3 s o l u t i o n (pH c a . ll) with gentle stirring at room t e m p e r a t u r e for 1 day. A chitosan-CaC03 composite was
623
obtained as a white precipitate, a n d Na a l g i n a t e was solubilized. The composite was collected and dried to afford a white chitosan-CaC03 composite in 0.39 g yield. ~ ms• KBr (KBr) 1 6 0 0 ( N H z ) , 1 4 2 6 , 8 7 6 a n d 7 1 2 c m -1 (C032-). Anal. N 5 . 2 2 % (60% c h i t o s a n ) . The composite contained 23% CO~ 2 as estimated f r o m C/N r a t i o of the elemental analyses. The filtrate was c o n c e n t r a t e d in yacuo, and 3 volumes of ethanol was added to afford Na alginate
as
a precipitate.
Fig. 1. ~ i~hot~graphic scerlc of a chitosan-CaCO~ composite. Left, an a i r - d r i e d slice of the chitosan-CaC03 composite hydrogel; an a i r - d r i e d chitosan of the same size as the composite hydrogel.
4.
R e s u l t s
and
Right,
D i s c u s s i o n
In t h e Ca a l g i n a t e hydrogel, Ca 2§ i o n s a r e b o u n d a s s a l t to the region of contiguous L-guluronic acid residues (G-block) more stronger than those of contiguous D-mannuronic acid (M-block) and alternative L-guluronic and Dmannuronic acid (GM-block) residues [1]. The a l g i n a t e sample used in the present study contains 69% o f t h e G-block. The chitosan-Ca alginate hydrogel was s o a k e d in O.1M a q . Na2CO~ s o l u t i o n to afford a chitosan-CaC03 composite as a precipitate, a n d Na a l g i n a t e is solubilized into water. The c a l c i f i c a t i o n reaction o f a q . COs 2 - i o n s i n t h e Ca a l g i n a t e hydrogel is as followings: Na a l g i n a t e + C a C I 2 ---> Ca a l g i n a t e Ca a l g i n a t e + N a 2 C 0 3 ----> CaCOo The hydrogel
calcification reaction is as followings:
of
+ NaC1 + Na a l g i n a t e
CO~ 2 -
ions
in
the
Chitosan-Na alginate + C a C 1 2 ---> C h i t o s a n - C a alginate Chitosan-Ca alginate + Na2C03 ---> C h i t o s a n - C a C 0 3
chitosan-Ca
alginate
+ 2NaCI + Na a l g i n a t e
One m o l e o f Ca 2§ i o n r e a c t s with one mole of C032- ion, and one mole of CaC03 is produced in the reaction with NazC03 as demonstrated here. However, o n e m o l e o f Ca 2§ i o n r e a c t s w i t h two m o l e s o f H C 0 3 - i o n s , and one m o l e o f C a C 0 3 a n d o n e m o l e o f COe a r e p r o d u c e d in the reaction with NaHC03 as demonstrated previously [13]. Fig. 1 shows a photographic scene of a dry piece of the chitosan-CaC03 composite, which did not shrink after drying, although a chitosan piece shrank significantly. The d r y chitosanCaC03 c o m p o s i t e is white and elastic, and stable in alkaline solutions, and
624
unstable in acidic solutions. The c o m p o s i t e is probably usable as a new material f o r mimic c r a b s h e l l s and a s a f u n c t i o n a l feed additive of a n i m a l s and f i s h e s an d a s a g r i c u l t u r a l materials [14]. The r e c o v e r e d alginate is repeatedly usable for the preparation of c h i t o s a n - C a alginate hydrogel. ACKNOWLEDGMENTS
The work was supported by r e s e a r c h g r a n t from the the Industrial Technology Development Organization (NEDO), Research Institute of I n n o v a t i v e Technology for the Earth
New E n e r g y and Tokyo, and the (RITE), Kyoto.
R E F E R E N C E S
1. 2.
0. S m i d s r o d and A. H a u g , A c t a Chem. S c a n d . 22 ( 1 9 6 8 ) 1 9 8 9 - 1 9 9 7 . T. K. P a r s o n s , M. T a k a h a s h i and B. H a r g r a v e , Biological Oceanographic Processes, 3rd Ed., vol. 1, 5 5 - 5 8 ( 1 9 8 4 ) 3. S. H i r a n o , H. I n u i , K. Yamamoto, E n e r g y . C o n v e r s . Mgmt. 36 ( 1 9 9 5 ) 7 8 3 786. 4. I . Yan o, N i p p o n Suisan Gakkaishi 38 ( 1 9 7 2 ) , 7 3 3 - 7 3 9 . 5. I . Y a n o , N i p p o n S u i s a n G a k k a i s h i 40 ( 1 9 7 4 ) 7 8 3 - 7 8 7 . 6. I . Y a n o , K a g a k u to S e i b u t s u 15 ( 1 9 7 7 ) 3 2 8 - 3 3 6 . 7. J. K o n d o , T. I n u i and K. Wasa ( E d s ) , P r o c e e d i n g s of t h e 2nd International C o n f e r e n c e on C a r b o n Dioxide Removal, Pergamon Press, 1995. 8. J . P. R i l e y and G. S k i r o w , C h e m i c a l O c e a n o g r a p h y , Academic Press, New York, 1975. 9. S. H i r a n o , K. Yamamoto, H. I n u i , E n e r g y . C o n v e r s . Mgmt. 38 ( 1 9 9 7 ) $ 5 1 7 $521. 10. S. H i r a n o , K. Yamamoto, H. I n u i , M. J i , M. Z h a n g , M a c r o m o l . Symp. 105 (1996) 149-154. 11. S. H i r a n o , K. Yamamoto, M. Yamada, H. I n u i , M. J i , A d v a n . C h i t i n Sci. 1 (1996) 137-142. 12. S. Z h a n g , K. E. G o n s a l v e s , J . A p p l . P o l y m . S c i . 56 ( 1 9 9 5 ) 6 8 7 - 6 9 5 . 13. S. H i r a n o , K. Yamamoto, H. I n u i , K. I . D r a g e t , K. M. Varum a nd O. Smidsrod, 7th Int. Conf. Chitin/Chitosan, Lyon, Sept 3-5, 1997. 14. S. H i r a n o , A p p l i c a t i o n s of C h i t i n a n d C h i t o s a n . Ed. M . F . A . G o o s e n , Technomic, Lancaster, 1997, pp. 237-258.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
625
T o l e r a n c e o f a g r e e n alga, S c e n e d e s m u s k o m a r e k i i , to e n v i r o n m e n t a l e x t r e m e s N. Hanagata a'b, R. Matsukawa c'd, M. Chihara c'e and I. Karube c a Research Institute of Innovative Technology for the Earth (RITE), Minato-ku, Tokyo 105, Japan b Chiba Laboratory, Mitsui Engineering & Shipbuilding Co., Ltd., Ichihara, Chiba 290, Japan c RCAST, University of Tokyo, Meguro-ku, Tokyo 153, Tokyo d New Energy and Industrial Technology Development Organization (NEDO), Toshima-ku, Tokyo 170, Japan e
Japanese Red Cross College of Nursing, Shibuya-ku, Tokyo 150, Japan
The capacity of a bark-inhabiting Scenedesmus komarekii to tolerant environmental extremes was examined. The optimum temperature for growth was 40~
The alga had high viability even
after an incubation period at 45~ for 3 days and remained after freezing at-20~ for 2 days. This alga was able to grow after the cells were dried at 25~ and 45~
alga could tolerate
an atmosphere up to 30% CO 2. The capacities of the strain to tolerate a high CO 2 concentration was enhanced by preculturing under slightly low CO 2 concentrations. I. I N T R O D U C T I O N Some freshwater algae can adapt to aerial or subaerial habitats although environmental conditions are generally harder on land than in water. Bark-inhabiting green algae are exposed to high and low temperatures, sometimes freezing, dry conditions, air pollutant gases, and strong sunlight. Furthermore, the environmental changes are more rapid on land. To survive in the terrestrial environment, the algae have to be able to tolerate these environmental conditions. In this study, we examined the capacity of bark-inhabiting Scenedesmus komarekii to tolerate environmental extremes and determined which environmental factor was the most important as regards this species. Footnote: This work was in part supported by a grant from the New Energy and Industrial Technology Development Organization to the Research Institute of Innovative Technology for the Earth, Japan.
626 2. M A T E R I A L S AND M E T H O D S
2.1. Algal strain Scenedesmus komarekii Hegewald was isolated from tree bark (1). The culture conditions have been previously described (2).
2.2. Tolerance of the algae to environmental extremes During high temperature tolerance experiment, ten ml of the sub-sample from the preculture was transferred to a test tube and incubated at 45~ for 3 days. The algal suspension was inoculated into a 100-ml Erlenmeyer flask containing 40 ml medium and cultured at 25~ on a rotary shaker (130 rpm). During freezing tolerance experiment, ten ml of the sub-sample from the preculture was transferred to a test tube and frozen at-20~ for 2 days. The frozen algal culture was defrosted and inoculated into a 100-ml Erlenmeyer flask containing 40 ml medium. The culturing was carried out at 25~ on a rotary shaker (130 rpm). In the dryness tolerance experiment, the precultured alga was harvested by centrifugation at 3,000 rpm for 10 min and washed twice with distilled water. The pellet was resuspended in sterilized water so that the optical density was 1.0 at 660 nm. One ml of the suspension was filtered using a filter paper of 2.5 cm in diameter. Two filter papers were prepared. One was dried at 25~ for 7 days, the other was dried at 45~ for 2 days. Each of the dry filter papers was transferred into 40 ml medium in a 100-ml Erlenmeyer flask and cultured at 25~ on a rotary shaker (130 rpm). Cell growth was determined by the optical density of the culture medium at 660 nm. 3. R E S U L T S
AND D I S C U S S I O N
3.1. Effect of temperature on the cell growth Figure 1 shows the effect of temperature on the growth rates. Increasing the incubation temperature from 25~ to 40~ increased the growth rate. The growth rate drastically decreased at 42~
and no growth occurred at 43~
40~ and 42~
The optimum and growth limiting temperatures were
respectively.
3.2. Tolerance to temperature and to dry conditions The alga showed a high recovery of its viability after incubation at 45~ (Figure 2). This result indicates that this alga can maintain cell viability at 45~ Figure 3 shows the growth after incubation at -20~
although no growth occurs.
The alga grew after a 5-day lag. This alga
had 3-day and 5-day lags before growth after drying 25~ and at 45~
respectively (Figure 4).
627
, -|
0.3
2.0
"o "7, ..J
E to r r (g a
0.2 "o O) 0.1
r
0
L
0 ~
0.0 2O
25
30
35
40
1.0
0.5
t"
r
1.5
0.0 q
v
I
Temperature (~
I
I
3
0
45
. 6
.
.
.
.
9
. . 12
15
18
Time (d)
Figure 1. Effect of incubation temperature
Figure 2. Tolerance of high temperature.
on the growth rate
The alga incubated at 45~ for 3 days was cultured at 25~
The lag time shows the cell damage injured by these treatments. The longer lag time and the lower growth rate indicate that freezing is more harmful than high temperatures for this alga. 0.3
E c:
o r r
CI O
1.5
E
0.2
1.0
o ~D ~D 0.1
m a o
v
0.0
.
0
.
5
.
10 Time
.
.
15
0.5
0.0 c
20
0
3
(d)
6
9
12
Time
(d)
15
18
21
Figure 3. Tolerance to freezing. The alga
Figure 4. Tolerance to dry conditions at
frozen to -20~ for 2 days was cultured
25~ and 45~
at 25~
7 days and at 45~ for 2 days was
The alga dried at 25~ for
cultured at 25~ 3 . 3 . E f f e c t o f CO 2 c o n c e n t r a t i o n o n the c e l l g r o w t h
The effects of the
CO 2
concentration of the aeration gas on the cell growth at the optimum
temperature is shown in Figure 5. The alga was precultured in air. The growth rates of the air-adapted alga were not affected by CO 2 concentrations of 10% and 20% in the bubbling air. A four-day lag before growth was observed in the culture bubbled with 30% CO z in the air. The
628 alga was precultured with air containing 20% grow in
CO 2
CO 2 .
Consequently, the CO2-adapted alga could
concentrations of 40% and 50% (Figure 5). This result shows that the alga is able
to adapt to higher concentrations of
CO 2
after preculture in a high concentration of
CO 2 .
Hanagata et al. (3) and Kodama et al. (4) also reported that the capacities of Chlorella sp., S. acuminatus and Chlorococcum lithorale to tolerate a high CO 2 concentrations were enhanced
after preculturing high concentrations of CO 2 . Although S. komarekii used in this study could also adapt to the high concentration of CO 2, the mechanism is still not clear. Further studies will be needed on the mechanism of adaptation to higher concentrations of CO 2. 0.5 "7, ..J
A I-"
air-adapted alga
0.4
0.5
"o ol
0.3
~" o1
0.3
m (n
0.2
m
0.2
t~
m
E
=
CO2-adapted alga
0.4
E
0.1
0.1
0
0.0 I 0
0.0 2
4
6
8
10
12
, 0
, 1
Time (d)
Figure 5. Effect of
CO 2
,
, 2
9 3
9 4
Time (d)
concentration on the algal growth. The alga preincubated in air (left) and
CO2-riched air (fight) were cultured in air (open circle), 10%
CO 2
(open square), 20% CO 2
(open triangle), 30% CO 2 (closed circle) and 40% CO 2 (closed square) in air. The cultivation temperatures were maintained at 40~
REFERENCES
1. N. Hanagata, I. Karube and M. Chihara, J. Jpn. Bot., 71 (1996) 87. 2. N. Hanagata, T. Takeuchi, Y. Fukuju, D. J. Barnes and I. Karube, Phytochemistry, 31 (1992) 3345. 3. M. Kodama, H. Ikemoto and S. Miyachi, J. Mar. Biotechnol., 1 (1993) 21.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
629
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
Over-expressed effect of carbonic anhydrase cyanobacterium, Synechococcussp. PCC7942
on CO2 fixation
in
M. Murakami .'b, N. Yamaguchi ~b, T. Nishide ~, T. Muranaka ~band Y.Takimoto "b a Research Institute of Innovative Technology for the Earth (RITE), Minato-ku, Tokyo 105, JAPAN b Biotechnology Laboratory, Sumitomo Chemical Co., Ltd., Takaram~a, Hyogo 665, JAPAN Over-expressed effect of carbonic anhydrase (CA) on CO2 fixation in cyanobacterium, Synechococcus sp. PCC7942, was evaluated. CA overexpression in carboxysomes induced elevated levels of growth rate and CO2 fixation rate, suggesting the possibility to improve photosynthetic performance. 1. INTRODUCTION Cy anob acterium is a pro c ary oti c i.~'''"'r"''''''~ organism with the ability of oxygenic Cettwatt photosynthesis. In cyanobacterium, Cell membrano~~ the external dissolved inorganic carbon Carboxysom (DIC), CO2 and/or bicarbonate, enters the cells and accumulates in the ~ c / cytoplasm via energy-dependent Hi Oa" I system(s). Regardless of whether HCO; r~ T [~ ,.-.,.,, 9 ~ HCOa" r~ bicarbonate or CO2 is supplied, bicarbonate is the predominant species Periplasmic CA in the cytoplasm. Bicarbonate emers the carboxysomes, polyhedral subcellular C02 bodies, where localized carbonic anhydrase (CA)catalyzes the formation of CO2 from bicarbonate, increasing the Figure 1. Schematic view of concentration of substrate at the site of inorganic carbon transportation carboxylating enzyme, ribulose 1,5and concentration mechanisms biphosphate carboxylase/oxygenase in cyanobacteria. (RuBisCO) (Fig. 1) [1 ]. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).
630
When the cDNA encoding human CA was introduced into Synechococcus sp. PCC7942, CA was expressed in the cytoplasm and the" transformant showed a high CO2 requiring phenotype, possibly due to a rapid conversion of cytoplasmic bicarbonate into CO2, followed by a CO2 leakage from the cells to the medium[2]. This showed the importance of CA localization in the cells. In order to evaluate the effect of increased CO2 supply to RuBisCO on CO2 fixation, we constructed transformed Synechococcus sp. PCC7942 which overexpressed CA at the site of RuBisCO. In this report, we examined the effect of CA over-expression on CO2 fixation rate. 2. MEIItODS 2.1. Construction of CA over-expression vector and transformation Plasmids containing the cyanobacterial CA gene (icfA)[3], promoter and terminator derived from RuBisCO and the ampicillin resistant gene were constructed (Fig.2). The resulting plasmids were introduced into Synechococcus sp. PCC7942 [4]. 2.2. Western blot analysis of CA protein Total soluble protein of Synechococcus sp. PCC7942 was used in SDS-PAGE and Western blotting analysis using a Multigel 10/20 (Daiichikagaku) and an ECL Western
blotting detection kit (Amersham). Rabbit
pUG118
Sac-~,~.~
Xho l
icfA ~ T ~ P v u
II
Sac I--~ g%UH24-pUC-CA-R] ori ~ X h o~l . ~ , , ~ , M pUH24
Bg/ll
P : Synechococcus sp.PCC7942
RuBisCO promoter
T : Synechococcus sp.PCC7942
RuBisCO terminator
icfA : Synechococcus sp.PCC7942
CA gene
Figure 2. Construction of CA expression
antiserum against cyanobacterial CA protein vector in Synechococcus sp.PCC7942. was used for Western blot analysis. Total protein was measured by Lowry method. 2.3. Tolerance to acetazolamide Cells were grown aerated (2vvm) with ordinary air in BG11 medium with several concentrations of acetazolamide (AZA, CA inhibitor) [5]. Ratio of packed cell volume (PCV) with AZA to PCV without AZA was calculated after culturing the cells for one day. PCV was determined by using a COULTER MULTISIZER (COULTER). 2.4. Growth rate of transformed Synechococcussp. PCC7942 Cells were grown in 15 ml test tubes with 8 ml of culture medium under a photon flux density of approximately 150/zE/m2/s and were aerated with ordinary air at 4 ml/min or 16 ml/min. Cells were also cultured under the same conditions except for CO2 concentration in air (20 ppm). The PCV change during the growth of cells was measured by COULTER MULTISIZER
631
(COULTER). The linear growth rate (mg dry w e i g h t / ~ ) was calculated by using PCV to dry cell weight converting factor. 2.5. CO2 fixation rate of CA over-expressed transformants
Cells were grown in 15 ml test tubes under a photon flux density of approximately 150tzE/m~/s and were aerated (2vvm) with ordinary air containing 20 ppm CO2. Linear growth rate was converted to CO2 fixation rate (gCO2/1/day) multiplying the conversion factor (1.65). The conversion factor is calculated as follows: 44(CO2)/12(carbon)X0.45(carbon content of cells). 3. RESULTS AND DISCUSSION CA over-expression vector was introduced into Synechococcus sp. PCC7942 resulting ampicillin resistant transformants. Southern blot analysis revealed the existence of introduced gene (icfA) in transformants. The antiserum against icfA was raised and was used for Western blot analysis. Western blot analysis revealed increased levels of the 30kDa icfA protein in eleven transformants, but not in five vector-control clones (Fig.3). vector-control
CA over-expression
IB 1 2 3 4 5 1 2 3 4 5 ,",'
,,' ". ,
,~,,,'c'
,
'
'
,
,
, ,
6 7 8 910111B
,' ",, '
, '
,
"'
, , , ", ',
',,"i
,i
,~"'~"~''~'~~~~'*''~ ~ , , , , ~ ~ , ~ " " " ,"91--- 30kDa .
.
.
.
.
Figure 3. Western blot analysis of CA over-expressed in Synechococcus sp.PCC7942. IB, 30kDa icfA protein in inclusion bodies produced by transformed E. coli
Transformants showed increased tolerance to AZA comparing to control cells, suggesting that over-expressed CA exerts its function. By immunological electron microscopy, CA protein was found in carboxysomes of transformants, where most of RuBisCO protein was also found, but CA protein was not found in cytoplasm, showing that the site specific expression of CA in the cells [6]. In order to clarify the effect of over-produced CA in carboxysomes on photosynthetic performances, growth rate and CO2 fixation rate were evaluated. CA over-expressed transformants when cultured under the CO2 concentration in the air, showed elevated levels of growth rate comparing with wild type or vector-control strains. This effect was enhanced as the aeration rate was decreased or under the concentration of 20ppm CO2 in the culture (Fig.4), suggesting that over-expressed CA is especially important under the limited supply of DIC.
632
360ppm C 0 2 (16ml/min) 360ppm C02 (4ml/min) ,, 20ppm C02 w~//~//7//~///////~ ~ ,,'HHHHHHH~ (16ml/min) 0 50 15 10 5 0 Growth rate % increase of growth rate (mg dry weight/I/hr) in CA over-expressed transformants(%) BB CA over-expressed transformants r-! Wild type and vector-control
Figure 4. Growth rate of transformed
Synechococcussp.PCC7942.
The enhanced CO2 fixation rate in the transformants was further confirmed under the concentration of 20ppm CO2 in the culture, using independent eleven clones of CA over-expressed transformants and five vector-control clones. As a result, CA over-expression induced elevated levels of CO2 fixation rate, suggesting the possibility to improve photosynthetic performances (Fig. 5). 200 A
o or E
G)
Q
150
lo0
c o
.x "-O4 o o
I
CA over-expressed transformants , J L . 131 -I-23.6 Vector-control transformants
.----82.4___8.14 O
50
* Mean -I- SD of eleven clones # Mean :1: SD of five clones
Figure 5. CO2 fixation rate of CA over-expressed transformants.
REFERENCES 1. D. A. Bryant (ed.), The Molecular Biology of Cyanobacteria, Kluwer Academic Publishers, The Netherlands, 1994 2. G.D.Price and M.R.Badger, Plant Physiology, 91, (1989) 514. 3. H.Fukuzawa et al, Proc. Natl. Sci. USA, 89, (1992) 4437. 4. Methods in Enzymology, 153, (1987) 199. 5. S.Bedu et al, Plant Physiol., 90, (1989) 470. 6. M. Murakami et al, in preparation.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversionsfor Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
633
CO2 removal by a bioreactor with photosynthetic algae using solarcollecting and light-diffusing optical devices* M a s a r u N a n b a a'b and Miyuki K a w a t a ~'b a Hitachi, Ltd. Hitachi Research L a b o r a t o r y 1-1, Omika-cho 7-chome, Hitachi-shi, Ibaraki-ken, 319-12 J a p a n b Research Institute of Innovative Technology for the E a r t h (RITE) 2-8-1, Nichi-shinbashi, Minato-ku, Tokyo 150, J a p a n
A photo-bioreactor was designed and constructed, in which solar light was supplied by light-diffusing optical rods connected to solar-light collector through optical fiber cables. The performance of biological CO2-fixation with photosynthetic microalgae, Chlorella sp. UK001, under the natural rays of sunlight was estimated. A r a t e of CO2-fixation was obtained of 24 g m2d -1 on a sunny day, in which energy density of direct-sunlight was 780 W m 2. Photosynthetic efficiency of converting light energy to biomass was e s t i m a t e d as 1.6% for total incident energy on the surface of the solar-light collector, and 12% for photosynthetically available visible-region. 1. I N T R O D U C T I O N The atomospheric concentration of so-called greenhouse effect gases such as carbon dioxide (CO 2) is increasing as a result of the rapid increase in the use of energy and other resources [1]. Microalgal photosynthesis has increasingly been paid attention as a m e a n s of producing industrially valuable compound and reducing the emmission of CO 2 into the atomosphere. Mass production of photosynthetic microalgae such as Chlorella or S p i r u l i n a have been realized for commercial purposes. Various types of photobioreactors have been used or proposed, such as open-pond types, raceway types and tublar types [2, 3]. Light supply as an energy source would be the limiting factor of microalgal production, ff the reactor was properly operate& For efficient solar energy utilization, a novel illumination s y s t e m b a s e d on light-diffusing optical fibers or rods have been proposed in order to increase the ilumination area to culture volume ratio [4, 5]. These systems have supposed the solar light collector. We have designed a photo-bioreactor, in which solar light was supplied by light-
* This study was supported by New Enegy and Industrial TechnologyDevelopment organaization.
634 diffusing optical rods connected to solar-light collector through optical fiber cables. In this work, the performance of biological CO~-f~xation with photosynthetic microalgae under the natural rays of sunlight is presented. 2. MATERIALS AND M E T H O D S 2.1 P h o t o - b i o r e a c t o r Figure 1 shows the schematic diagram of the photo-bioreactor
O2-CO2 ITemp 9 I Gas-meter x ~ ~, Icontrollerl I ~ ~.L__/., ~ ~ ~[.~ o ~ ~][ ;::--I.Rec()rder I ; Solar col ~ " | ~- ,, (Reflecting Mirror , , r , , , , . . ,, ,.r~--, .~ ~,::~--m @300mm) I~1 I IIll]' Ir-~J'l (~~~11 Light-difusin'(rn.~~~l ~L 02 il opticalrods ' ~ ~ I!!!!I~~N2 (013x300m o
used in this study. T h e r e a c t o r was constructed in the form of a cylindrical glass vessel 500 mm tall with a n internal diameter o f o Sensors io o o (.HDOTemn) 300 m m (20 L in culture Reactor vesse .. . ' ' 9 volume). Solar light or artificial (O300x500mm, k, . . : - ~ - / compressor culture volume2L) Gas-dispersion / plate light from a metal-halide lamp
'8r~ | e!.oI li
could be transmitted through Figure 1. Schematic diagram of the photooptical fibers into light-diffusing bioreactor and control devices. optical rods (Bridgeston Co. Ltd.), 13 mm in diameter and 300 mm in length, which were arranged in the reactor vessel. The radiation surface area to volume ratio was 18 m -1. A mixed air with CO~ was used for aeration at the bottom of the reactor. Gas flow rate was 0.5L min -1 , not otherwise state& CO 2 concentrations in the supply and exhaust gases were measured by infrared gas analyzer (VIA-510, Horiba, Ltd.) and used to estimate CO~ removal. Solar light collector, developed by Asahi Glass Company, was constructed with reflection mirror (300 mm in diameter) and designed to concentrate solar light into the input end of 18 optical glass fiber cables. Photosyntbetically active radiation with wavelengths between 400 and 700 nm was transmitted into the light-diffusing optical rods. Light transmission efficiency through the solar collector to the optical rods was determined to be 34% for visible region of the sun light. 2.2 Culture strain A green alga, Chlorella sp. UK001, was obtained from M. Murakami et al. (Sumitomo Chemical Co., Lt&) and used in this study. This strain possesses high capability in fixing CO2 at high concentration of CO2 (10-40%) in aeration gas and high temperature (30-40 ~ The Closterium medium was prepared at a pH of 7.5, and maintained at a temperature of 35~ throughout the experiment. CO 2concentration of 10% (v/v) in air was aerated into the medium at a flow rate of 0.51.0 L/min.
635 3. R E S U L T S AND D I S C U S S I O N 12 3.1 CO2 r e m o v a l a n d b i o m a s s y i e l d
1
Figure 2 shows the cell growth curve .i : of Chlorella sp. UK001 strain using a n :', ', ', '11 artificial light source of m e t a l halogen i .... ', ' , , , i lamps. Light illuminations were carried "~ ' "' ' out under periodic llight-dark cycles, 10 light period of 10 h and dark period of ~14 h. Cells were grown a t light periods " o~ " c, 0.5 Cell density decreased s o m e w h a t in =, the dark, especially a t higher cell ~ 9 o density t h a n 0.2 g L ~. The growth r a t e ~ a t the light periods showed a c o n s t a n t ~ value of 0.26 g h ~ by dry weight. Thus, 8 the r a t e of CO9 fmation was t a k e n as 0.48 g h ~ from the carbon content of the cells, which was analyzed as 50% 0 7 0 50 100 150 in weight ratio. Culture Time (h) At the experiment shown in Figure 2, a mixed air with CO2-concentration of 10.5% was a e r a t e d into the culture Figure 2. Changes of cell density and vessel. CO 2 concentration in the outlet CO2-concentration in outlet gas of the Chlorella sp. UK001 culture vessel. gas from the vessel decreased during strain was cultured in the photothe light illumination. CO~ removal was bioreactor, under condition of light (10h)calculated from CO 2 concentration in dark (14h) cycles with artificial halogen CO 2 concentration of supply and exhaust gases and the feed lamps a t 35~ rate. The rate of CO~ removal was aeration gas into the vessel was 10.5%, and gas flow rate was 1 L min -1. Light t a k e n as 0.47 g h ~ , which agreed well intensity at the surface of light-diffusing with the rate of CO9 fixation obtained optical rods was 160 ~E m 2 s 1 above. It was concluded t h a t almost all the CO 2 removed from the feed gas was fixed as algal biomass in Chlorella sp. UK001 strain. 3.2 CO~ r e m o v a l w i t h s u n l i g h t
Figure 3 shows some examples of changes of CO 2 concentration in the outlet gas of the culture vessel, when solar light collector was used as a source of light-supply. The CO 2 concentration in the outlet gas was decreased t h a n t h a t in the inlet gas, while direct-incident rays of sunlight was obtained. Daffy a m o u n t of CO 2 removed in the feed gas was calculated by integration of difference between inlet- and outlet-gases in a whole day (see Fig. 3, shadowed area
636 in upper part). Table 1 summarized the performance of the photobioreactor used in this study. 1.7 g of CO 2 was fixed as algal biomass in a free day. The rate of CO2-fmation per unit area of the sun-light collector was 24 g m2 d-i. Photosynthetic efficiency of converting light energy to chemical energy of algal cells was 1.6% based on the total incident energy. Photosynthetically useful radiation (400-700 nm) is 39% of the solar energy, and 34% of that is transmitted to the photobioreactor. So, the photosynthetic efficiency was estimated as 12% based on the photosynthetically available visible-region of sunlight. The efficiency would be higher than that of conventional type reactors, considering that the energy loss of solar light collector used here was 66%. It may be safe to say that lightcollecting type photo-bioreactor is helpful to produce industrially valuable compound and reducing the emmission of CO 2 into the atomosphere.
v
o m
o
~
~ ~.~-
I
c/.oudy .~-fin~
"
" " ~ " -nine
0.8
................
0.6
=,,.,,,.,,.,,.~,.....,,.,,
& .................
" ~'":"
,~ . . . . . . . . . . . . . . . .
- ..........
,,,.,,,~,,
cloudy
E-*fine ~ ...........
-:
,,,,,,~,,,.
= = , 0.4 .~_.~v o.2 ~ 0 . ~ om
"7.'-
oo
20
40 60 Culture time (h)
80
100
Figure 3. Changes of CO2-concentration in outlet gas of the culture vessel and the light intensity of direct irradiation of sun-light. Chlorella sp. UK001 strain was cultured in the photo-bioreactor under the natural rays of sun-light. Cultural temperature was controlled at 35 ~ CO 2 concentration of aeration gas into the vessel was 9.8%, and gas flow rate was 0.5 L min -~. Table 1. Summary of culture of Chlorella sp. UK001 strain using sunlight. Fixed CO 2 through a day
[g-CO2/d ]
Light collection area Fixed CO 2 per unit area Photosynthetic efficiency of light energy to biomass Light irradiation area to culture volume ratio
[m 2] 0.071 [g-CO2/m2/d] 24.1 [%] [m-l]
Direct sun-light irradiation [MJ/m2/d] Averaged light intensity [W/m 2] Efficiency of solar collector [%]
1.7
1.6 18 21 778 34
REFERENCES
1. S. H. Schneider, Science, 243 (1989) 771. 2. E. A. Laws, US DOE Rep. SERI-SP-231-3071 (1987) 209. 3. A. Vonshak and A. Richmond, Biomass, 15 (1988) 233. 4. K. Mori, H. Ohya, I~ Matsumoto, H. Furuune, K. Isozaki and P. Siekmeier, Adv. Space Res., 9 (1989) 161. 5. H. Takano, H. Takeyama, N. Nakamura, K. Sode, J. G. Burgess, E. Manabe, M. Hirano and T. Matsunaga, Appl. Biochem. Biotech., 34/35 (1992) 449.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
637
Isolation and characterization of a green alga Neochloris sp. for CO 2 fixation* M. Kawata a,b, M. Nanba a,b, R. Matsukawa c,d, M. Chihara c,e and I. Karube c a Hitachi Research Laboratory, Hitachi Ltd, 7-1-10mika-cho Hitachi-shi, Ibaraki 319-12, Japan b Research Institute of Innovative Technology for the Earth, 2-8-1, Nishi-shinbashi, Minato-ku, Tokyo 105, Japan c Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, 153 Tokyo, Japan d New Energy and Industrial Technology Development Organization*, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, Japan e Japanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, Japan A green alga Neochloris sp. TS4F isolated from a pond at Ueno Park, Tokyo, is tolerant of the concentration of 10% CO 2 bubbling at 35~ This microalga can fix CO 2 at the rate of 0.027 g L - l h -1 by photosynthesis at 30 ~ and produces lipid as 24% by dry weight when grown under nitrogen starvation. 1. I N T R O D U C T I O N The increase in the carbon dioxide (CO 2) concentration in the atmosphere is suspected to be causing global warming via the greenhouse effect [1]. Atmospheric CO 2 has been increased by burning of fuels, especially fossil fuels, in association with the increase in world population. The use of photosynthetic microalgae to convert CO 2 into usable materials should be expected to contribute to decrease the CO 2 concentration in the atmosphere. We have studied CO 2 fixation of the exhaust gas from thermal power stations by microalgal photosynthesis. In order to utilize the resulting large quantities of biomass for fuel, compost, feed and other useful chemical substances, we have screened microalgae which produce lipids or hydrocarbons. In this study, we report the isolation and characterization of a green alga tolerant of the concentration of 10% CO 2 at 30~
*This study was supported by New Energy and Industrial Technology Development Organization (NEDO).
638 2. MATERIALS AND METHODS 2.1. Selection of oil-producing microalgae We collected water or soil samples containing microalgae from ponds and swamps in Japan, on May, 1995. They were cultured in modified Fitzgerald medium[2]. Firstly, microalgae were screened under atmosphere of air at 25 or 30 ~ CO 2 concentration in the feed gas was increased in sequence to the concentration of 10% CO 2 at each temperature. Secondly, a cytochemical staining procedure for semiquantitative estimation of hydrophobic materials of algae was employed [3]. Cells were stained with a solution of Nile Red [3], a fluorescent reagent for lipids. The oil droplets or lipid storage centers of the cells appeared as yellowish-orange structures, while the remaining cell structures were stained deep red. Nile Red-dyed cells were isolated on agar plates. Optical density of algal suspension was used as a measure of cell density. Specific growth rate was determined at logarithmic phase of the cell growth by dry weight. 2.2. Mass culture and chemical profiles of selected microalga After the unialgal culture was established, the strain was cultured under various conditions in order to investigate the effects of temperature, pH, light/dark cycle and nutrient deficiency in the medium. Cell suspensions were inoculated into 0.8 L (~ 90 mm ) or 4 L (~ 160 ram) of cylindrical glass vessel, which contained MDM medium [4]. The medium was enriched with 3200-4200 mM potassium nitrate (the original amount of nitrogen) and 200-600 mM magnesium sulfate heptahydrate and 360-460 mM dipotassium hydrogen phosphate. The culture medium was continuously stirred mechanically and by bubbling with 10% carbon dioxide in air. Continuous light (160 or 200 /1 E m-2s-1)was provided by cool-white fluorescent light. To determine the optimum temperature, cultures were grown at 25, 30 and 35~ To promote good lipid accumulation in the cells, the culture was transferred to a nitrogen-absent medium. The rate of depletion of soluble potassium nitrate in culture medium was determined with Hitachi HPLC L6000. And the fluorescent intensity of the Nile Red-dyed microalgal cells was measured by Hitachi fluorescent spectrophotometer UV-2000. Stainability with Nile Red was determined as fluorescent intensity at 575 nm per cell density. After the cultivation, cells were harvested by centrifugation, washed twice with distilled water and lyophilized. Cellular lipid was extracted by the method of Bligh and Dyer [5] and determined gravimetrically. Elemental cell compositions were analyzed with Yanagimoto CHN analyzer MT-3 and heat values of the biomass were analyzed with Shimazu bomb calorie meter CA4PJ or Ogawa OS meter 150.
639
0.04 i
c~
0
~ ,,.,.i
0.03 0.02 0.01 I
I
I
25 30 35 Culture temperature (~ Figure 1. Neochloris sp.TS4F.
Figure 2. Effect of temperature on the specific growth rate of Neochloris sp. TS4F.
3. RESULTS AND DISCUSSION 3.1. Selection of oil-productive microalgae Among 273 samples collected from ponds and swamps, 236 samples grow at the concentration of 10% CO 2 at 30~ Only 28 samples show yellowish-orange structures in the Nile Red-dyed cells. Finally, we obtained eight algal samples, because 20 samples were diatom and died out. Among the eight green algal strains, one microalgal strain with the highest growth rate was selected and isolated. It was temporarily identified as Neochloris sp. TS4F (Chlorococcales Neochloridaceae) with light and electron microscopic observations (Figure 1). Neochloris sp. TS4F isolated from Shinobazu-ike in Ueno Park, Tokyo, has the following characters ; (1) cells are spherical, with 5-18 ju m in diameter; (2) zoospores are without walls and with two flagella; (3) vegetative cells have one or more pyrenoids in the chloroplast; (4) vegetative cells are sometimes multinucleate. Additionally, Neochloris sp. TS4F strain tends to aggregate with hundreds of cells, so its sedimentation rate was higher than that of other green algae such as Chlorella. This characteristic should be advantageous at harvest of cells after cultivation. 3.2. Mass culture and chemical profiles of selected microalga The optimum growth condition of Neochloris sp. TS4F was determined with 10% CO 2 at 30~ and pH 7.5. Specific growth rate at 30 ~ was 0.03 h -1, which meant doubling time of 23 h (Figure 2). Under continuous lighting condition, biomass yield after eight days culture was 1.45 times that of 16:8 h light/dark cycle. This results showed that the biomass yield was proportional to the lighting time. The growth rate of Neochloris sp. TS4F strain was 0.018 g L-lh -1 based on dry cell mass, under light intensity of 160 ,u E m-2s -1 and 30 ~ The rate of CO2-fixation was estimated to be 0.027 g-CO 2 L-lh-1, according to the carbon content of cells.
640 Cultivated cells in a nitrogen-rich medium were harvested by centrifugation, and inoculated into a nitrate-deficient medium. Figure 3. shows the effect of nitrogen starvation on the growth rate, stainability with Nile Red and cellular content of total lipids and nitrogen. The growth of alga slowed gradually and stopped finally. In contrast, the stainability with Nile Red increased gradually. It shows that nutrient deficiency in the medium caused the increase of the fluorescent intensity of algal cells stained with Nile Red. The total lipid content was found to increase with the depletion of nitrogen in the medium. Cellular lipid content increased from 13% to 24% by dry weight. It means that lipid accumulation caused by nitrogen starvation requires continuous cultivation for at least 140 hours under nitrogen starvation. Cellular lipid content by nitrogen starvation became considerably high, a heat of combustion increased from 5.3 kcal g-1 to 6.1 kcal g-l, as cellular lipid content increased. 20
0.02
25
5
| o~..~ .F.-I |
9
el?
9
4 ~
15 10 ~
~0.01
~20
3 N
9
~
~10
2
( o
~
Z 0
~
5
1~
0 0
50 100 150 200 Nitrogen starved time (h)
0
0 50 100 150 200 Nitrogen starved time (h)
Figure 3. Effect of nitrogen starvation on the growth rate and stainability with Nile Red (A), and on the cellular total lipid and the cellular nitrogen content (B) of Neochloris sp. TS4F. In conclusion, the feasibility of increasing the lipid content of Neochloris sp. by nitrate depletion in the medium suggests the possibility of using this strain in a biological CO2-fixation system and as a source of liquid fuels or feed in the future. REFERENCES
1. S. H. Schneider, Science 243 (1989) 771. 2. E. O. Hughes, P. R. Gorham and A. Zehnder, Can. J. Microbiol. 4 (1958) 225. 3. D. Berglund, B. Cooksey, K. E. Cooksey and I. R. Priscu, 1986 Aquatic Species Program Annual Report, No. SERI / sp-231-3071 41-52. Solar Energy Research Institute, 1987. 4. A. Watanabe, J. Gen. Appl. Microbiol. 6 (1960) 283. 5. E.G. Bligh and W. J. Dyer, Can. J. Biochem. Physiol. 37 (1959) 911.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
641
A n t i o x i d a n t activity of CO2 fixing microalgae* R. Matsukawaa, b, Y. WadaC,d, N. TanC,d, N. SakaiC,d, M. Chiharaa, e and I. Karube a aResearch Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153 Tokyo, Japan bNew Energy and Industrial Technology Development Organization, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, Japan CResearch Institute of Innovative Technology for the Earth, 2-8-11, Nishishinbashi, Minato-ku, Tokyo 105, Japan dToyo Engineering Corporation, Research Engineering, Biotechnology, 1818 Fujimi, Togo, Mobara-shi, Chiba 297, Japan ejapanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, Japan Microalgae from hot spring and ponds in Japan were screened for antioxidant activity using a lipoxygenase inhibition test. The ethanolic extracts of some microalgae showed high lipoxygenase inhibition activity. The ethanolic extracts of isolated Chlorella species with tolerance to high temperatures and high concentrations of CO2 possessed high radical scavenging activity. These results suggest that CO2 fixing microalgae may be potential sources of natural antioxidants. 1. INTRODUCTION Carbon dioxide (CO2) is the principle greenhouse effect gas and the increase in the atmospheric CO2 concentration is suspected to be causing global warming via the greenhouse effect [1]. In order to decrease the CO2 concentration in the atmosphere, the development of CO2 fixation and removal systems, in conjunction with a system which converts the fixed CO2 to useable material, is desirable. "This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.
642 It is well known that photosynthetic plants convert CO2 to organic matter using solar energy. Photosynthesis is more efficient in microalgae than in terrestrial higher plants and CO2 from industrial exhaust could be a useful carbon source for growing algae. In order to fix excessive atmospheric CO2, microalgae could be cultured and then be exploited as a useful resource. Photosynthesizing plant cells, including microalgae, are exposed to a combination of intense light and high oxygen levels [2, 3], leading to the formation of free radicals and other strong oxidizing agents. The ability of microalgae to thrive under such extremely oxidizing conditions led us to focus our investigation on identifying potential antioxidant activity in microalgae. In this study, we report the screening of microalgae for antioxidant activity and the radical scavenging activity of microalgae with tolerance to high temperatures and high concentrations of CO2. 2. MATERIALS A N D METHODS 2.1 Preparation of microalgal extract Over 100 species of microalgae were collected from hot springs and ponds in Japan. Some of these were isolated and cultured in suitable media. The microalgae were separated from the medium by centrifugation at 3,000 rpm for 10 min. After washing the solid algal pellets with distilled water, a volume of solvent (water, ethanol or methanol) weighing ten times the sample weight was added to the pellets. The sample was then homogenized in a sonicator for 30 rain in 30 sec intervals. The homogenate was then centrifuged at 3,000 rpm for 10 rain, and the supernatant was used as the algal extract. 2.2 Determination of antioxidant activity As an assay for antioxidant activity, we examined the inhibition of lipoxygenase activity and radical scavenging activity using the o~, cz-diphenyl-~picrylhydrazyl (DPPH)decolorization test. Aqueous, ethanolic and methanolic extracts of microalgae were used in the assays. The assay of lipoxygenase activity was carried out using the method of BenAziz et al. [4]. The enzyme reaction was carried out in the cuvette of a spectrophotometer monitored at 234 nm until the reaction rate reached a steady state. That wavelength is the peak of absorption for the hydroperoxides generated by the action of lipoxygenase on linoleic acid, with the uptake of oxygen. The extent of inhibition was defined as the ratio of the rate of increase in OD234 in the absence of algal extract, to that measured in the presence of the sample. The assay of radical scavenging activity was determined using of a stable free radical, DPPH, according to the method of Blois [5]. The decrease in absorbance due to DPPH was measured at 540 nm using a microplate reader. The activity was defined as the ratio of OD540 in the absence of algal extract, to that measured in the presence of the sample.
643 3. RESULTS AND DISCUSSION
3.1 Screening of microalgae for antioxidants Several microalgae samples from hot springs and ponds showed lipoxygenase inhibition activity in both aqueous and ethanolic extracts. The ethanolic extracts of Chlorella sp. were observed to be more effective than aqueous extracts. The methanolic extract of another Chloretla sp. showed lipoxygenase inhibition. The inhibition of lipoxygenase by the extracts of microalgae suggested the existence of a cellular mechanism protecting them from oxidation in vivo. 3.2 Effects of temperature on cell growth Our previous paper reported the isolation and growth characteristics of Chtoretla strains H-84 and A-2 with tolerance to high temperature and high concentrations of CO2 [6]. Figure 1 shows the effects of temperature on the growth of Chlorella sp. H-84. The strains showed a high growth rate at 40~ but the growth rate was significantly decreased at 30~ and above 45~ The algae was unable to grow at temperatures higher than 50~ Thus, the optimum temperature for this alga seemed to be around 40~ The isolated Chlorella spp. were identified as Chlorella sorokiniana. Previously, the thermophilic Chloretla species known was only Chlorella sorokiniana [7]. 3.3 Effects of CO2 concentration on cell growth Growth of Clorella sp. H-84 was examined under different CO2 concentrations as shown in Figure 2. The alga grew well at concentrations of CO2 from 5 to 40%. It was able to grow in the culture medium with CO2 concentrations up to 60% (data not shown). The optimum concentration of CO2 is about 20%. Similar results were obtained for Clorella sp. A-2. These results suggest that these microalgae may be suitable for the biological fixation of CO2 from
' 30"C 9 40"C A
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,
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,
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Figure 2. Effects of CO2 concentration on cell growth.
644 industrial flue gases.
3.4 Radical scavenging activity The radical scavenging activities of aqueous, ethanolic and methanolic extracts of microalgae with tolerance to high temperatures and high concentrations of CO2 are shown in Table 1. The stable free radical DPPH was oxidized and decolorized in the presence of extracts of the microalgae. The ethanolic extracts of Chlorella sp. A-2 and Chlorella sorokiniana UTEX-1230 showed high radical scavenging activity. Ethanolic extracs of Chlorella sp. H-84 showed a radical scavenging activity of about 53%. These species of microalgae have antioxidant activity and may be used for purposes such as food, animal feed, pharmaceuticals and cosmetics.
Table 1 Antioxidant activity of extracts of Chlorella species Chlorella sp.
Activity (%) aqueous
ethanolic
methanolic
H-84
39.2
52.8
43.9
A-2
32.4
98.5
77.1
52.7
98.3
97.5
sorokiniana
REFERENCES 1. S.H. Scheider, Science, 243 (1989) 771. 2. A. Abeliovich and M. Shilo, J. Bacteriol., 111 (1972) 682. 3. R.R. Bidigrare, M.E. Ondrusek, M.C.II., Kennicutt, R. Iturriaga, H.R. Harvey, R.W. Hoham and S.A. Macko, J. Phycol., 29 (1993) 427. 4. A. Ben-Aziz, S. Grossman, I. Ascarelli, P. Budowski, Anal. Biochem., 34 (1970) 88. 5. M.S. Blois, Nature, 181 (1958) 1199. 6. N. Sakai, Y. Sakamoto, N. Kishimoto, M. Chihara and I. Karube, Energy Convers. Mgmt., 36 (1995) 693. 7. C. Sorokin and J. Myers, Science, 117 (1953) 330.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
645
Screening of polysaccharide-producing microalgae Yoshiko Shishido a b, Miyuki Kawata a c , Ritsuko Matsukawa Mitsuo Chihara d f and Isao Karube d
de
Research Institute of Innovative Technology for the Earth (RITE), 2-8-11, Nishi-Shinbashi, Minato-ku, Tokyo 105, J a p a n b Sumitomo Heavy Industries, Ltd., 1-15, Kuryo-zutsumi, Hiratsuka-shi, Kanagawa 254, J a p a n c Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1, Omika, Hitachi-shi, Ibaraki 319-12, J a p a n a Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, J a p a n New Energy and Industrial Technology Development Organization, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, J a p a n f Japanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, J a p a n
e
1. INTRODUCTION The study has been carried out for the purpose of investigating the fLxation of CO 2 in exhaust gas from thermal power stations by microalgae and the effective utilization of microalgal products. It is important to develop technologies to convert photosynthesized organic products into useful substances such as fuels, feed, manure, materials for plastics and concrete, and fine chemicals which have a high added value. We have screened microalgae which produce polysaccharides. In this study, we report a method for the screening and detection of polysaccharide-producing microalgae, which tolerate atmospheres with high CO 2 concentrations and high temperatures.
2. METHODS AND MATERIALS Samples were collected from ponds, marshes, hot springs and softs at various places in Japan. Collected samples were cultivated on 6 kinds of
* This study was supported by the New Energy and Industrial Technology Development Organization (NEDO).
646 Table 1. Cultivation media medium Fitzgerald 1) 9
pH
I microalgae
usage
7.0
general
BG-11 2~
9.0
blue-green algae
CSi 3)
7.0
diatoms
CC ~)
3.0
red & green algae
MC 4)
6.0
green algae
Pro 4)
6.0
green algae
]
cultivation under 10% CO 2,3O ~
dying with Chinese ink .
,
,
precipitation by ethanol
*Vitamins were added in the medium. Isolation Figure 1. Method of screening and detection media (Table 1).
2.1. Screening of polysaccharide-producing microalgae The method of screening and detecting polysaccharide-producing microalgae is shown in Figure 1. Firstly, microalgae were screened under an atmosphere of 10% CO 2 at 30 ~ Secondly, microalgae growing under these conditions were selected by dying in Chinese ink and microscope observation. Thirdly, we picked out microalgae which secreted a non-toxic polymer from the cell. Lastly, the presence of polysaccharides was determined by precipitation with ethanol. Microalgae which deposited a precipitate were selected as polysaccharide-producing microalgae and isolated.
2.2. Growth of polysaccharide-producing microalgae Microalgae were cultured in a test tube containing 5ml of medium with s h a k i n g at 120 rpm under an atmosphere of 10% CO 2 at 30 ~ The growth of microalgae in the test tube was m e a s u r e d directly by means of the optical density at 690 nm. The growth velocity was defined as a relative rate by calculation of the slope of the growth curve during the linear phase.
3. RESULTS AND DISCUSSION
3.1. Screening of polysaccharide-producing microalgae We screened 273 microalgae in total, as shown in Table 2. Twenty six samples of polysaccharide-producing microalgae were selected and were
647 classified into 17 green algae and 9 blue-green algae by microscopic observation. These results show that the combination of dying using Chinese ink and precipitation of polysaccharides with ethanol is useful for the easy detection of polysaccharide-producing microalgae. 3.2. Growth characteristics of polysaccharide producing microalgae Three typical growth curves of polysaccharide-producing microalgae are shown in Figure 2.
Table 2. Results of screening for polysaccharide-producing microalgae. polysaccharide-producing microalgae
total number medium
total
of sample
Fitzgerald BG-11 Csi CC MC Pro
80 69 67 19 30 8
13 0 6 1 6 0
Total
273
26
green algae
blue-green algae
17
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120 240 CULTURE TIME(hr)
360
F i g u r e 2. G r o w t h c u r v e s of p o l y s a c c h a r i d e producing microalgae
648 0.05 m green algae
0.04 . . . .
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D blue-green algae
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O0
0
MICROALGA STRAINS F i g u r e 3. R e l a t i v e g r o w t h r a t e of p o l y s a c c h a r i d e producing microalgae
The relative growth rates of the 26 strains is shown Figure 3. The results show that blue-green algae grew faster than green algae. The strains OK72F1, ON1F, OT51F and ON2F1, which were classified as blue-green algae by microscopic observation, were rapidly growing microalgae. More examination is needed to establish the absolute growth rate and CO 2 fixation rate. The simple examination of growth characteristics is useful for screening for rapidly growing microalgae. We are continuing the characterization of polysaccharide-producing green algae, and the function of the polysaccharides in order to find a way to utilize these metabolites.
REFERENCES
1. H. Tamiya and A. Watanabe (eds.), Sourui Zikkenhou(in Japanese), Nankoudou Ltd., Tokyo, 68-104, 1965 2. R. Rippka, J. Deruelles, J. B. Waterbury, M. Herdman and R. Y. Stanier, J. Gen. Microbiol., 111 (1979) 1-61. 3. National Institute for Environmental Studies, LIST OF STRAINS, 1994 4. K. Nishizawa & M. Chihara (eds.), Methods in phycological studies, Kyouritsu shuppan Ltd, Tokyo, 281-305, 1979 5. H. Miyashita, H. Ikemoto, N. Kurano, S. Miyachi and M. Chihara, J. Gen. Appl. Microbiol., 39 (1993) 571. 6. M. S. Blois, Nature, 181 (1958) 1199.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
649
A marine microalga utilization for a paper: Semi-batch cultivation of Tetraselmis sp. Tt-1 by a tubular bioreactor and the partial substitution of whole kenaf pulp for a paper Y. Samejimaa, A. Hirano a, K. Hon-Nami a, S. Kunito a, K. Masuda b, M. Hasuike b, Y. Tsuyuki~, and Y. Ogushib aEnergy and Environment R&D Center, Tokyo Electric Power Co., 4-1 Egasaki-cho, Turumi-ku, Yokohama 230, Japan bHiroshima R&D Center, Mitsubishi Heavy Industries, Ltd., 4-6-22 Kan-on-shinmachi, Nishi-ku, Hiroshima 733, Japan ~Chemical Plant Engineering & Construction Center, Mitsubishi Heavy Industries, Ltd., 3-3-1 Minatomirai, Nishi-ku, Yokohama 220, Japan A semi-batch cultivation of a Prasinophyceae, Tetraselmis sp.Tt-1, was examined using a 50L scale tubular bioreactor under lamps with a view to estimating its performance. The mean productivity of more than 20g/m2/day could be obtained by fixing the flow rate of the culture medium at 0.5m/s, which was the maximum in this apparatus, by drawing a partial medium once in two days, and by adjusting an initial cell concentration to 0.5-1.0g/L. A long-term and stable cultivation for more than two months could be carried out by this system. Our previous study, on the other hand, has shown that cell bodies of this microalga are of use as the partial substitute of a wood pulp paper [ 1]. In the present study, whole kenaf pulp paper including algae obtained by the above cultivation was made and its properties were examined. It was indicated that they also could be used as an agent for surface improvement of kenaf pulp paper in addition to a partial substitute for the pulp as in the case of a paper made from wood pulp. 1. INTRODUCTION The increase of CO2 in the atmosphere has been considered to be one of the causes of global warming. The amount of CO2 exhausted from the thermal power stations of electric power companies has reached approximately one fourth of the total amount of CO2 exhausted in Japan. Therefore, as one of the measures to cope with this problem, technologies to fix CO2 using the photosynthetic ability of the microalga have been studied [2]. The establishment of the utilization technology of CO2 which is fixed by microalga is also
650 indispensable [3]. In the present study, it is reported the performance of a tubular bioreactor system having advantages such as a higher alga-productivity and a lower liability for a contamination with various germs compared to an open pond system. In our previous study [ 1], on the other hand, it was shown that Tetraselmis sp. Tt-1 was useful as a partial substitute to wood pulp for a paper. Kenaf, an annual herbaceous plant of the genus Hibiscus, grows relatively quick and absorbs more CO2 compared to other herbaceous plants. Since its fiber is long and strong, the use of this fiber as an alternative of wood pulp for a paper has already been tried in order to contribute to the preservation of forest resources [4]. We also describe some properties of microalga-added paper using kenaf pulp of its whole stem. 2. EXPERIMENTAL PROCEDURES A microalga used in this study was Tetraselmis sp.strain Tt-1 [ 1] which was
isolated from seawater ladled at the Sagami Bay in Japan. The f/2 medium [5, 6] was prepared as described previously [7], and
gas inlet
was used with a slight modification; concentrations of nitrate and phosphate were enriched 4 times and 8 times, respectively. The semi-batch cultivation of this strain was carried out using a tubular Figure 1. The drawing of a tubular bioreactor similar to that described in [7] bioreactor. (Fig. 1). The inner diameter of the tubes made of transparent acryl resin was 70mm. The size of the apparatus was 4,350mm(L), 235mm(W), and 2,270mm(H). A working volume and an irradiation area were 50L and 0.84m 2, respectively. The tubular reactor was put in a room controlled at 25~ with metal haloid lamps (M700-L/BU-SC, Matushita Denki k.k., Osaka,). Illuminance was maintained at about 30klux and the irradiation time was 12 hours a day. The CO2 concentration injected was set at 1.8%. Alga cells harvested were washed with deionized water before use. Microalga-added paper was prepared according to the method stipulated by the Japanese Industrial Standard (JIS) P8209 [8]. Indexes of paper quality, including tensile strength, smoothness, air permeability and receptivity to printing ink were measured by the methods of JIS series P 8113, P 8119, P 8117 and J.TAPPI No.46, respectively [8]. 3. RESULTS AND DISCUSSION 3.1. Indoor cultivation using a tubular bioreactor
With a view to optimizing the cultivating conditions in this apparatus, three parameters were considered; the flow rate of the medium, the drawing frequency of grown cells, and the initial cell concentration, which is designated in this paper as the cell concentration after
651
the partial drawing of grown cells, followed
25
by the addition of a flesh medium. The flow rate tested were 0.3m/s and 0.5m/s. The drawing frequency tested were once a day, once in two days, and once in three days. The initial cell concentration were changed
E
20
~0
~15 ,. 10
among 0.3g/L and 2.0g/L. As shown in Fig.2, the productivity depended on the all these parameters; each optimum value for the flow rate, the drawing frequency, and the initial concentration were 0.5m/s, which
~0
5 0
i
ii
iii
iii
0.5 1.0 1.5 2.0 2.5
Concentration [g/L]
was the maximum in this apparatus, once in
Figure 2. Relationship between the cell
two days, and 0.5-1.0g/L, respectively. The
productivity and the initial cell concentration
mean productivity up to more than 20g/m2/day was obtained when each optimal
at two kinds of the flow rate and at various
conditions were employed simultaneously.
drawing frequencies. All closed symbols show the flow rate of 0.5m/s. Drawing frequencies of symbols O, I1, and A are once a day, once in two days, and once in
It was suggested that a long-term and stable cultivation in this system would be possible from the fact that a stable semi-
three days, respectively. Open circle(O)
batch cultivation for more than two months
shows 0.3m/s and once a day for the flow
was carried out (data not shown).
rate and the drawing frequency, respectively.
3.2. Use of cultured microalgae for a paper
Figure 3 shows the mixture of the microalgae and kenaf pulps, which indicates that the size of this microalga is extremely smaller than that of kenaf pulp fiber. The surface of microalgae-added paper looks to be smoother than that of pure kenaf-pulp paper, as shown in Fig.4.
Figure 3. Light micrograph of Tetraselmis
Figure 4. Electron micrographs of the
sp.Tt- 1 cells and kenaf pulp fibers. • 100
surface of pure kenaf-pulp paper (A) and microalgae-added paper (B). The content of alga, Tetraselmis sp. Tt- 1, was 15%.
652 As shown in Fig.5, some physical properties of microalgae added-kenaf pulp paper clarified that the paper quality could be improved by the addition of these algae to the pulps. With the increase in the content of microalgae, the tensile strength was slightly declined without any problem in its practical use. Both the air permeability and the smoothness were increased, while the receptibility to printing ink was decreased 9It was also demonstrated that one of the advantages of kenaf pulp paper, that is, being stronger than that of hardwood pulp paper, was still maintained after the substitution of pulps even 15% algae (Fig.5). In summary, Tetraselmis sp.Tt-1 could be used as a partial substitute of kenaf pulps and an agent for surface improvement of kenaf pulp paper as in the case of hardwood pulp paper.
o o
o =
o
[-2
5 10 Microalga content (%)
15
100 ~" 80 60 o 40 o 20 0
~ r~
0
~'~
60 40
0
10 5 Microalga content (%)
15
o o
~.,~lOO ;~ 80 ,.Q o
~ 20 o 0 <9-~
20
0
5 10 Microalga content (%)
15
9
0 o
5
lo
15
Microalga content (%)
Figure.5. Physical properties of microalga-added paper. Symbolsll,kenafpulp 9U],hardwood pulp REFERENCES 1. Hon-Nami,K., A.Hirano, S.Kunito, Y.Tsuyuki, T.Kinoshita and Y.Ogushi, Energy Convers. Mgmt, 38,Suppl.(1997) 481. 2
Benemann, J.R. Energy Convers. Mgmt 38, Suppl.(1997) 475.
3
Hon-Nami,K. and S.Kunito, Chin.J.Oceanol.Limnol.(in press).
4
Hara, H. Insatsu Zasshi 77 (1994) 42 (in Japanese).
5 6.
Ong, L.J., A.N.Glazer and J.B.Waterbury, Science 224, (1984) 80. Castenholz, R.W. Methods in Enzymol., 167, (1988) 68.
7.
Hirano,A., Y.Samejima, K.Hon-Nami, S.Kunito, S.Hirayama, K.Ueda and Y.Ogushi, in Proceedings of the 3rd Biomass Conference of the Americas, Montreal, Canada, August (1997) 1069.
8.
JIS Methods, JIS Handobukku Kami-pulp (Japanese Standard Association, ed.) Japanese Standard Association, Tokyo (1997) (in Japanese).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
653
C o n v e r s i o n of C 0 2 into cellulose by gene m a n i p u l a t i o n of m i c r o a l g a e : Cloning of cellulose s y n t h a s e g e n e s from Acetobacter xylinum Yoko Umeda a, Atsushi Hirano a, Koyu Hon-Nami ~, Shunji Kunito ~, Hideo Akiyamab, Takuo Onizuka b, Masahiko Ikeuchi c, Yorinao Inoue a ~Energy and Environment R&D Center, Tokyo Electric Power Company, 4-1 Egasaki-Cho, Tsurumi-Ku, Yokohama 230, Japan bToray Research Center, Inc., 1111 Tebiro, Kamakura 248, Japan cUniv, of Tokyo, 3-8-1 Komaba, Meguro 153, Japan aRIKEN, 2-1 Hirosawa, Wako 351-01, Japan 1.INTRODUCTION Our objective is to devise effective countermeasures against the CO2 increase in the atmosphere through utilization of the solar energy. Photosynthetic CO2 fixation of microalgae is supposed to be a promising candidate for this. However, most of photosynthetic products easily decompose to regenerate atmospheric CO2 via various biological pathways. Cellulose microfibril would be a plausible candidate for this purpose, since it has relatively long life-cycle and can be applied to various industrial use. Mass production of cellulose by biotechnology is expected to support forest protection. The cyanobacteria, oxygenic photosynthetic prokaryotes, would be the best for introduction of cellulose synthesis because of their higher feasibility for genetic engineering, higher rate of photosynthetic production and simpler carbon metabolism compared with eukaryotic plants. For the purpose of heterologous gene expression in cyanobacteria, we attempted to clone bacterial genes involved in cellulose biosynthesis. Acetobacter xylinum is one of the best characterized bacteria in cellulose synthase, of which two types have been reported to date [1-3]. The type I consisting of three or four genes encodes a cellulose synthase to produce majority of bacterial cellulose. Type II consisting of a single gene encodes an enzyme having synthase activity detectable only in vitro. The functional role of the type II enzyme is not known. Here we report cloning of both type I and type II genes from Acetobacterxylinum JCM 7664 and demonstrate that the type II may consist of more genes with a novel function. 2.EXPERIMENTAL We chose Acetobacter xylinum JCM 7664 (equivalent to IFO 13693), which shows high activity of cellulose synthesis. Parts of bcsA and bcsCwere amplified by PCR based on consensus sequences of A. xylinum strains 1306-3 [1] and ATCC 53582 [2] and used as probes for initial screening of cosmid and plasmid libraries.
654 Genomic DNA digested with various restriction enzymes were probed with a bcsA probe (Fig. 1). Clearly, there were multiple DNA fragments which hybridized with the bcsA probe (about 660bp). The type II genes were first cloned from a broad 78kbp band of SalI/BamHI digest. The genomic DNA was screened with a fragment of cloned bcsABII-A (446bp) for cloning of downstream region (Fig. 2). The type I genes were cloned from the cosmid library by using both bcsA and bcsCprobes.
kbp
kbp 9
~
~10 ~
"-5 --4
--
. . . .
,~,,,~
'
--lo
i'
2
~
1.6
o
Figure 1. Southern hybridization of JCM 7664 DNA with ~sA probe
~,~,
Figure 2. Southern hybridization of JCM 7664 DNA with bcsABH-A probe
3. RESULTS 3.1. Structure and homology of type I genes The type I consisted of four genes, which are highly homologous to bcs genes of A. xy]inum strain 1306-3 (Fig. 3). We named those genes in JCM 7664 bcsAZ, bcsBI, bcsCIand bcsDI. Unlike the fusion gene acsABin A. xy]inum ATCC 53582 and strain AY201, bcsA/and bcsBIgenes are separated in the JCM 7664 strain as in the strain 1306-3. Furthermore, deduced amino acid sequences of bcsA~, bcsBI, bcsCIand bcsDIwere 81, 83, 86 and 94% identical to bcsA, bcsB, bcsCand bcsD of the strain 1306-3, respectively, while 67, 60, 64 and 78% identical to N-terminal part of acsAB, C-terminal part of acsAB, acsC and acsD of ATCC 53582.
JCM7664
~ cmc
1306-3
ATcc53582
ccp
bcsAI ~
bcsBI ~
bcsCI
==~ /bc~A ~
bcs.B ~
bcsC ac c
Figure 3. Structure of bcs operons of A. xylinum
3.2. Flanking regions of type I genes In the flanking region of the type I genes, we found three ORFs which were
655
located on the same strand. An ORF in the downstream region was homologous to beta-glucosidase, although its 3' end was not yet determined. In the upstream region, ORF346 (cellulose complementing protein) and an ORF homologous to endoglucanase were found in the same arrangement as in A. xylinum ATCC 23769 [41.
3.3. Structure and homology of type II genes We obtained two clones for the type II gene and named bcsABII-A and bcsABII-B. They were highly homologous to each other (>99% at nucleotide level), although the upstream regions were distinctly different [5]. Clearly, N-terminal part (homologous to bcsAD and C-terminal part (homologous to bcsB]) of the gene product were fused to form a single gene bcsABIIas already reported in acsAIIof ATCC 53582 [3]. In the downstream region of bcsABII-A, two more ORFs and homolog of bcsC (named bcsCII) were found on the same strand [6]. Therefore, we assumed that the two ORFs are co-transcribed with bcsABII-A and bcsCII and tentatively designated bcsXand bcsYas in Fig. 4.
pCELI+pCEL3~ ~ ~ i I
( Melibiose carrier protein. ) 1i
pCEL2
ORF448l
4 U-A
ORF261ORF386
>
~s~(~I') /rsABH-B I
U~b
Figure 4. ORFs of type II genes of A. xylinum JCM 7664
bcsABIIof JCM 7664 was 66% identical at amino acid level to a type II gene, acsAII of A. xylinum AY201 [3], while about 30% identical to type I genes. Similarly, bcsCIIwas about 23% identical to bcsCI. Thus, we predict that bcsCII homolog would be present in downstream of acsAIIin A. xylinum AY201. We are currently searching bcsD homolog in the downstream of bcsCII.
3.4. BcsY possibly encodes transacylase Homology search of the protein databases revealed that bcsYis significantly homologous to several transacylases including oaf_A in Salmonella typhimurlum, which encodes an acetylase for O-antigen oligosaccharide on the surface lipopolysaccharide [ 7 ] . Moreover bcsY carried the consensus residues for transacylase family and showed hydropathy profile very similar to oa[A (Fig. 5), which is supposed to span the cytoplasmic membrane seven to nine times. However, bcsYdid not have a hydrophilic extension of oaf_A, which is not conserved in other transacylases. Provided that bcsY is co-transcribed with bcsABII-A/ bcsCII, it is suggested that bcsY encodes a transacylase, which is located on the cytoplasmic membrane and acts on cellulose in cooperation with bcsABII and bcsCII. On the other hand, bcsXdid not have any clear homolog in the databases.
656
BcsY
I
" H • 1 .... ~ . . . . . . . . . . . . . . . . . . . . .
i : ~....... i'"ii ....... ; ..... P u t a t i v e
Transacylase
IIIllllB ~ ~ Y 0
100
200 Amino
300 400 500 Acid Position
600
........O a f A
Figure 5. Domain Structure and Hydropathy of bcsYand oafA products 3.5. F l a n k i n g regions of type II genes In the upstream region of the bcsABII-A, three ORFs including putative melibiose carrier protein were found on the opposite strand. By contrast, there was a transposase gene in the upstream of bcsABII-B. Interestingly, not only the bcsABII gene but also its 5' untranslated region (13bp) of the two clones were almost identical to each other. This suggests that duplication of bcsABII region and insertion of an IS element occurred very recently as an independent or concomitant event. 4. D I S C U S S I O N We cloned from A. xylinum JCM 7664 three sets of cellulose synthase genes. Most bands of the genomic Southern hybridization (Fig. 1) fitted with the sequence data of the three. However, a 4kbp band of BamH I hybridized with the bcsA probe could not be accounted for and may indicate the presence of more genes involved in cellulose biosynthesis in A. xylinum JCM 7664. There are at least two related types of cellulose synthase enzymes, type I and II. The type I produces most of extracellular cellulose fibrils, while physiological function of the type II enzyme is not known. Our findings of novel genes (bcsX, bcsY, bcsCII) would provide a new insight into the function; production of acylated cellulose, which may be anchored on the cytoplasmic membrane. In this context, we should discuss a set of genes of Escherichia coli homologous to the bcs genes [8]. Four genes, yhjO, yhjN, yhjM and yhjL are significantly homologous to bcsA~, bcsBI, endoglucanase and bcsCI/CII, respectively, and are located in this order possibly as an operon. This arrangement in E. coliis essentially the same as for our type II genes in A. xylinum except that endoglucanase gene is replaced with bcsX and bcsY in the latter. Although we do not have an idea about a precise role of endoglucanase, the set of genes in E. coli might be involved in production of some variant of extracellular cellulose. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.
H. C. Wong, et al. Proc. Natl. Acad. Sci. USA, 87 (1990) 8130. I. M. Saxena, et al. Plant Mol. Biol., 16 (1991) 947. I. M. Saxena, et al. J. Bacteriol., 177 (1995) 5276. R. Standal, et al. J. Bacteriol., 176 (1994) 665. Y. Obata, et al. XV International Botanical Congress, Abstracts (1993) 561. Y. Umeda, et al. Nippon Nougeikagaku Kaishi 71, Suppl., (1997) 197. J. M. Slauch, et al. J. Bacteriol., 178 (1996) 5904. H. J. Sofia, et al. Nuc. Acids Res., 22 (1994) 2576.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
657
Ethanol production from carbon dioxide by fermentative microalgae S. Hirayama a, R. Ueda a, Y. Ogushi b, A. Hirano c , Y. Samejima c, K. Hon-Nami ~ , and S. Kunito ~ ~Adv. Technol. Res. Center, Mitsubishi Heavy Industries Ltd., Kanazawa-ku, Yokohama 236, Japan bHiroshima R&D Center, Mitsubishi Heavy Industries, Ltd., Nishi-ku, Hiroshima 733, Japan CEnergy and Environment R&D Center, Tokyo Electric Power Co., Tsurumi-ku, Yokohama 230, Japan 1. A B S T R A C T Microalgae were screened from seawater for C O 2 fixation and ethanol production by self-fermentation and tested for their growth rate, starch content, and conversion rate from starch to ethanol. More than 200 strains were isolated, and some of them were found to be suitable properties for the purpose. One of the excellent strains, Chlamydomonas sp. YA-SH-1, which was isolated from the Red Sea showed (1) a growth rate of 30 g-dry biomass/m 2.d, (2) a starch content of 30 %(dry base), and (3) a conversion rate from intracellular starch to ethanol of 50 % in the dark and anaerobic condition. Proposed new ethanol production system consists of microalgal cultivation, algal cells' harvest, self-fermentation of algae, and ethanol extraction processes. The system seems more simple and less energy consuming compared with the conventional one. If the microalgal productivity, starch content, and ethanol conversion rate are improved, the system should be an effective means for CO 2 fixation and energy production. 2. I N T R O D U C T I O N Microalgal CO 2 fixation and energy production are expected to be potential measures that will mitigate atmospheric CO: [I]. A new microalgal ethanol production system has been proposed in which intracellular starch is converted to ethanol by self-fermentation [2-3]. This system seems more simple and less energy consuming compared with convensional one. In the course of screening microalgae from seawater, more than 200 strains were isolated, and as one of the excellent strains, Chlamydomonas sp. YA-SH-1 was selected from evaluation for their growth rate, starch content, and conversion rate from starch to ethanol. In this paper, we report on some properties of this strain related to the ethanol production under the dark and anaerobic conditions. The proposed new system with ethanol production will be discussed.
658
3. EXPERIMENTAL Isolation of microalgae from seawater samples were performed by micro-pipette manipulation or colony formation on an agar gel [3]. Growth rate (g-dry weight/m 2~
was obtained at the
linear growth phase by using a flat culture bottle [3]. For the examination of self-fermentation, algal cells in the late linear growth phase were harvested by centrifugation and resuspended in the 0.4M potassium phosphate buffer (pH 7.7) or seawater at a final density of 150 ~ 250 mg dry wt/mL, and put into a light-shielded airtight tube (10 mL). The starch content was measured by a coupled method with perchloric acid and glucose oxidase[4]. Ethanol concentration was measured by gas chromatography. 4. R E S U L T S AND DISCUSSION
4.1. Algal productivity, starch content and ethanol conversion rate Many strains over 200 were isolated from seawater samples and many of them showed adhesive growth to culture flasks and/or flocculated growth. More than 10 strains were tested to examine algal productivity, starch content and ethanol production. Table 1 shows some strains having a productivity of ca. 30 g/m2.d, accumulated a starch more than 30 % (vs dry weight), but had a variety of starch conversion rate to ethanol. One of the excellent strains,
Chlamydomonas sp. YA-SH-1, which was isolated from the Red Sea showed (1) a growth rate of 30 g-dry biomass/m2-d, (2) a starch content of 30 %(dry base), and (3) a conversion rate from starch to ethanol of 50 % in the dark and anaerobic condition. Table 1 Characteristics of the typical microalgae from seawater Strain
Ch. sp.YA-SH- 1
Productivity -, (~rffod)
Starch content (%)
30
30
Starch conversion#1 Reference rate to ethanol (%) 50
This study
9A3
31
32
13
This study
Tit- 1
30
25
30
This study
Sag-1
20
29
5
[3]
Yok-1
30
2
5
[31
#1): Data were calculated according to an assumption described in [5] 4.2. Characteristics of ethanol production with microalgae Chlamydomonas sp. YA-SH- 1
659 4.2.1. Time course of ethanol production Figure 1 shows the time course of the ethanol production in the dark and anaerobic algal slurry. It was evident that the ethanol concentration increased during intial 44 hours and were remained constant thereafter.
1.5 1.0 ~D
o 0.5 r O
U~
I
0
I
20
I
40
60
Time (hr) Figure 1. The time course of the ethanol production by
Chlamydomonas
sp. YA-SH-1 strain under dark and anaerobic conditions. A slurry of
Chlamydomonassp. YA-SH-1
cells of 15 %(w/w) dry
weight was kept at 25 ~ in a light-shielded airtight tube. 4.2.2. Effect of temperature Figure 2 shows that the optimum temperature range for ethanol production was 30-35~ which indicates that further additional energy for cooling would not be necessary in the process of the fermentation if it is performed at room temperature.
100 O .,~
~
80
= 60
0
0
>
40200
0
I
10
I
I
20 30 Temperature (~
I
40
I
50
Figure 2. Effect of temperature on the ethanol fermentation. Ethanol in the slurry of Chlamydomonassp. YA-SH-1 cells was assayed at each 24-hour incubation.
660
4.2.3. Ethanol production system by fermentative microalgae From some properties of the ethanol production by microalgae, we propose new ethanol production system by fermentative microalgae. The proposed ethanol production system shown in fig. 3 consists of microalgal cultivation, algal cells' harvest, slurry preparation, algae selffermentation, and ethanol separation processes. The system seems more simple and less energy consuming compared with the conventional one [6]. The algal cultivation has advantages of being usable of devastated lands and seawater, which shows that this system would be noncompetitive with the agriculture in general. If improvements in the microalgal productivity, the starch content, and the ethanol conversion rate are achieved, the system should be an effective means for CO 2 fixation and energy production. Further investigation needs for the above improvements including the isolation of superior microalgae.
Microalgal cultivation (Seawater)
Slurry preparation
Harvest
Self-fermentation Ethanol separation Ethanol
Figure 3. Ethanol production system by microalgal fermentation.
REFERENCES 1. J.R. Benemann, Energy Convers. Mgmt 38, Suppl. (1997) $475-$479. 2. S. Hirayama, R. Ueda, Y. Ogushi, A. Hirano, K. Hon-Nami and S. Kunito (1996) Proceedings of the Annual Meeting of the Japanese Society for Marine Biotechnology, Sendai, Japan (May). p 64. (in Japanese). 3. A. Hirano, R. Ueda, S. Hirayama and Y. Ogushi, Energy, 22, (1997) 137. 4. S. Ohta, K. Miyamoto and Y. Miura, Plant Physiol., 83, (1987) 1022. 5. A. Hirano, Y. Samejima, K. Hon-Nami, S. Kunito, S. Hirayama, R. Ueda and Y. Ogushi, in Proceedings of the 3rd Biomass Conference of the Americas, Montreal, Canada, August (1997) 1069-1076. 6. S. Hirayama, R. Ueda, Y. Ogushi, A. Hirano, K. Hon-Nami and S. Kunito (1997) Proceedings of the Annual Meeting of the Japanese Society for Marine Biotechnology, Tokyo, Japan (June). p78. (in Japanese).
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.
661
Production of polyethylene glycol and polyoxyalkylenealkylphenyl ether microspheres using supercritical carbon dioxide K. Mishima,* K. Matsuym~ Y. Taruta, M. Ezawa, Y. Ito, and M. Nagatani Department of Chemical Engineering, Faculty of Engineering, Fukuoka University, Nanakuma Jormku, Fukuoka 814-80, Japan To form polymeric microspheres of polyethylene glycol(PEG) and polyoxyalkylenealkylphenylether, we demonsWated a process using the rapid e ~ i o n from ~ t i c a l fluid mLxnm~ containing a polar solvent such as ethanol, which was antisolvent for PEG and polyoxyalkylenealkylphenylether, restx~vely. Microspheres obtained from the rapid expansion of sutxtcfitical solufion(RESS) were very small, on the order of 10 u m in diameter, regular in shape and had a globular form. They were not adhesive to each other. The smqace properties of produced microspheres were investigated by Raman spectroscopy. As the results of spectroscopic analyses, it was found that surface property change of produced microspheres was caused by RESS process. INTRODUCTION Recently, the reduction of the amount of toxic organic solvent, such as toluene and xylene has been atWacting much attention for preserving the biologically mild environment. The development of a new process, in which the toxic organic solvent is not used, has been desired in the coating industry and related industries. To reduce the amount of toxic organic solvent, carbon dioxide(CO2) may be utilized as an environmentally benign solvent substitute. Sutxa~tical state of CO2 has already brought the new extraction and separation processes ofbiomolecules[1 ]. Sutxa~tical fluids including CO2 have been used in a process called by the rapid expansion of ~ t i c a l solution (RESS) to produce a variety of polymeric powders and films[2, 3]. In general, the solubility of polymer is very smaU. So the majority of RESS studies of polymers has used organic solvents, such as C1- C5 alkanes and alkenes, as the cosolvent. The organic solvent used is good solvent for polymer. In routine RESS ~ , there was difficulty to control the quality of polymeric microparticles because remained cosolvent could resolve the produced polymeric particles after the particle formation. In our previous work, we proposed the new method to produce acrylic microspheres to use as the powder paint material by using RESS with nonsolvent[4]. In this work, polymeric microspheres of polyethylene glycol(PEG) and polyoxyalkylenealkylphenyl ether were formed using the rapid expansion from the supercritical fluid mixtures containing a polar solvent 1.
*Corresponding author
662 such as ethanol which was antisolvent for them. These polymers cannot solve in supercdtical CO2 nor in ethanol. BUt mixttwes of ~ t i c a l CO2 and ethanol can solve enough amount of them. The surface properties ofprodtr.ed polymeric microspheres were investigated by Raman ~ s c o p y . 2. EXPERIMENTAL
2.1. Reagent Polyethylene glycol 600(0PEG; Avg. M.W.=7500) and polyoxyalkylenealkylphenyl ether were purchased from Wako Pure Chemical Ind. Ltd. and Dai-ichi Kogyo Seiyaku Co., reSlXX:tively. The structtwe of polyoxyalkylenealkylphenyl ether is shown in Figure 1. Ethanol used as cosolvent was purchased from Wako Pure Chemical Ind. Ltd. and its purity was believed to be more than 99.5 %. Carbon dioxide(CO2) used as the supetcritical fluid was purchased from Fukuoka Sanso Co. and was of a minimum 99 % purity.
2.2. Experimentalmethod The schematic diagram foran e~ental apparatus to make the polymeric microspheres is presented in Figure 2. In this apparatus, the head of the pump used for CO2 was cooled to 268.15 K in order to prevent wanning and vaporization of the liquid CO2. Impurities in CO2 were removed through an in-line dryer and a filter. The liquid was brought to its supercfitieal phase(SC-CO~) by a ptmap capable of delivering up to 59 MPa at a rate of up to 5.2 dm3 'rain"l (liquid CO2 basis). SC-CO2was pumped through a preheater which was kept at 308.15 K and then to an equilibrimn cell. The cell, about 500 dm 3 in volume, is equipped with pressure resistance observation windows. The system pressure was controlled by a back pressure regulator(V-l) having an accuracy of -0.1 MPa, and monitored by a digital presst~ gauge. Its temperature was kept at 308.15 +--0.1 K with a water bath(12). PEG or polyoxyalkylenealkylphenyl ether (about 25g) was placed in the equih'brium cell with ethanol(about 200g). This mixtures were stirred at 200 rpm for about 4 hours and then left to stand for more than one hour. The final solution was confirmed to be
R:Alkyl group n<30 Figure 1. Structure of polyoxyalkylenealkylphenyl ether. 6
V-I
V-~ -5
8
"J
10 9
1 Gas cylinder 2 Dryer 3 Cooling unit 4 Filter 5 Pump 6 Pressure gauge 7 5aSty valve 8 Preheater
19
I
~,~
12
9 10 11 12 13 14 15 16
Stopper Extraction cell Moter Water bath .Pressuregauge Safetyvalve Heating unit Thermometer
17 18 19 20 21 V-1
17,xpazsionnozzle Plate Airbath Samplingspots Samplingspots Back pressure regulator V-2 -,, V-5 Stop valve
Figure 2. Experimental apparatus for the rapid expansion of supercfitical solution.
663 homogeneous by observation through the high pressure resistance windows. The solution was discharged through a capillary nozzle, 0.28 mm in diameter(17), for a short time(less than 3 seconds) by opening a valve(V-5) placed behind the nozzle. The nozzle was maintained at 313.15 K with an electric heater. PEG and polyoxyalkylenealkylphenylether were sprayed on a target plate placed in a chamber (70 cm • 80 cm • 80cm) under almospheric pressure. The distance from the tip ofthe noTyleto the plate was set to 30cm. The size of produced microspheres collected on the plate was detemfined by the laser diffraction particle size analyzer(SALD-2000, Shimadzu Co. Ltd.) and its configtnation was observed with a scanning electron mieroscope(SEM S-2100B, Hitachi Ltd.). The glass tramition temperatures and molecular weight distributions of the polymer before and after the process were meastred by differential scanning calorimelw(DSC120, Seiko lnstnmaents Inc.) and high performance liquid chromatography(HPLC-8000, Tosoh Co., Ltd.). To explore the aaface properties of produced polymeric microspheres, st~ace of them was analyzed by the Raman S l X ~ t m m ~ - 1800, Jasco Co., Ltd.). 3. RESULTS AND DISCUSSION After RESS, polymericmicrospheres were precipitated and their morphology was observed. A SEM photograph of the polymeric microspheres obtained from the p r e - e ~ i o n condition of 20 MPa and 308.15 K is illusWatedin Figure 3. The polymeric microspheres are very small, on the order of 10 ~zm in diameter, and uniform in shape. Becausethe lower alcohol is volatile and is an antisolvent for the PEG and polyoxyalkylenealkylphenylether, the produced polymeric microspheres do not adhere to each other. The physical properties ofthe microspheres were compared to that ofthe source material. Glass transition temperatt~s of the microspheres showed the same behavior at the observationtemperattwe as the source material. Both the HPLC elusion pattem and retention time are identicalto each other. The polymeric microspheres have the same molecular weight distributions as the source materials. The produced polymeric microspheres include no impurities and only a slight amount of solvent remained in them. We concluded that the physical properties such as glass wansitiont e m ~ and molecular weight distribution of the polymer were not altered after the RESS process. The size of polymeric microspheres was measm~ by a laser differential particle size analyzer. Mean diameters of produced PEG and polyoxyalkylenealkylphenylether microspheres are 11 ttm and 9 ~t m, restx~vely. In order to analyze the sta'face properties of produced microspheres, the Raman SlX~ctmwere obtained. At the case ofpolyoxyalkylenealkylphenyl ether, the intensity of benzene group of produced polymeric microspheres(698, 1624cmq) are lower than that of source material as shown in Figure 4. As the results of ~ s c o p i c analysis, it may be considered that surface properties of polyoxyalkylenealkylphenylether was changed by RESS process. The Raman ~ of produced polyoxyalkylenealkylphenylether microspheres was nearly equal to that of these high pressure mixtures in the equilibrium cell. For the reasons given above, morphology and surface property changes in the polymer may occur in super~tical solution. This particular phenomenon of the polymer in the SCF mixture containing the lower alcohol is in agreement with the results ofDeSimone's work[5] using small angle X-ray photoelectron spectroscopy. Q
CONCLUSION PEG and polyoxyalkylenealkylphenyl ether microspheres were produced by using a new polymeric
664
r
'
' ;~; ". ~,." ,!~:~" ~:"~' :.a,-.:" "
":
~-.Feedof polyoxyalkylenealkylphenylether I [-'-Micr~ ~ poly~ 1ether / /~ Micr~ ~ polyethylene glyc~
~",i:":':;i,."' . .
I ' (a)polyethylene glycol ,.
'1624c7n1 . . . . .
'
. . . . 69. ~ - 1
,~ !.~
1800
1000
600
Raman shift[cm-1] (b)polyoxyalkylenealkylphenylether Figure 3. SEM of PEG and polyoxyalkylenealkylphenyl ether microspheres formed by using the rapid expansion ofsupercritical solution at 308.15 and 20MPa.
Figure 4. Raman ~ of PEG, polyoxyalkylenealkylphenyl ether microspheres and feed material of them.
micmspheres producing process ufili~g RESS without toxic organic solvent, such as toluene and xylene, and the new solubilization method of polymers in supe~-'riticalmixture containing a lower alcohol which is an antisolvent for PEG and polyoxyalkylenealkylphenylether. This new process for polymeric microspheres is original and a novel process utilizing a specific solubilization phenomenon, that a large amount of PEG and polyoxyalkylenealkylphenyl ether extensively can be dissolved and d i ~ in mixtures of SC-CO2 and lower alcohol, such as ethanol, which are not solvents for the PEG and polyoxyalkylenealkylphenyl ether, respectively. The produced polymeric microspheres using RESS are very small, on the order of 10 u m, regular in shape and have a globular form. They do not adhere to each other. The surface properties of produced polyoxyalkylenealkylphenyl ether microspheres were investigated by Raman stx~oscopy. As a result, it may be considered that the surface properties of produced polyoxyalkylenealkylphenyl ether microspheres was changed by RESS process. REFERENCES
1. 2. 3. 4. 5.
K.P. Johnston and J. M. L. Penninger, Supercritical Fluid Science and Technology, American Chemical Society, Washington, DC(1989). D.W. Matson, R. C. Petersen and R. D. Smith, J. Material., Sci., No.22(1987)1919. S. Mawson, K.P. Johnston, J.M. Combes and J.M. DeSimone, Macromolecules, No.28(1995)3182. K. Mishima, M. Ueno, J. Sugino, A. Miyake, N. Komorita, M. Nagatani, H. Umemoto and S. Yamaguchi, The 4thAsian Thermophysical Properties Conference, Tokyo, No.3(1995)675. J.B.McClain, D.E.Betts, E.T.Samulski and J.M.DeSimone, AIChE Annual Meeting, Florida(1995~50h.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
665
C a r b o n d i o x i d e s e p a r a t i o n f r o m nitrogen using Y - t y p e zeolite m e m b r a n e s Shigeharu Morooka, Takahiro Kuroda and Katsuki Kusakabe Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-81, Japan
A polycrystalline Y-type zeolite membrane was formed by hydrothermal synthesis on the outer surface of a porous a - a l u m i n a support tube, which was polished with a finely powdered X-type zeolite for use as seeds. When an equimolar mixture of CO 2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the singlecomponent system, and the N 2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. At 30~ the permeance of CO 2 was higher than 107 mol.m-2.s-l.pa-1, and the permselectivity of CO 2 to N 2 was 20-100. 1. I N T R O D U C T I O N Carbon dioxide is the major compound of greenhouse gases the emission of which should be reduced. Membrane technology is one of the most promising methods for this purpose since it may be able to recover CO 2 at elevated temperatures without losing sensible heat [1-4]. In this study, Y-type zeolite membranes were developed, and their CO2-selective permeation was evaluated. 2. E X P E R I M E N T A L A porous a-alumina tube (2.8 mm o.d. and 1.9 mm i.d.) with an average pore size of 150-170 nm was used as the support of a zeolite membrane. Each support tube was cut to a length of 3 and 20 cm, and the outer surface of the tube was rubbed with NaX zeolite particles of 7 0 - 8 0 gm to implant crystal fragments as nucleation sites. Water glass, sodium aluminate and NaOH were dissolved in distilled water (A1203:SiOz:Na20:H20 1"10:14:798 in a molar basis). The support tube was fixed vertically in a 40 ml Tefloncoated autoclave containing the solution, and hydrothermal synthesis was carried out at 90~ for 6-24 h. After synthesis, the tubes were washed thoroughly with distilled water and dried. Each end of the membrane was then connected to a stainless steel tube with epoxy resin, and permeance was measured at 30-130~ Helium was used as the sweep gas on the permeate side, and ambient pressure was maintained on both sides of the membrane. The partial pressure of each permeant on the permeate side was maintained at less than 10 kPa by dilution with the sweep gas. Permeance was determined for singlecomponent and mixed gases, and selectivity was defined by the ratio of permeances. 3. RESULTS AND DISCUSSION 3.1. Formation of membranes Without implanting seeds, no continuous film was formed even after a 24 h syn-
666 thesis. When seed particles were implanted, however, a continuous layer of zeolite was formed on the outer surface of the tube as shown in Figures 1 (a) and (b). There are two zones of zeolite in the fractured section. The top layer (I) is composed of zeolite polycrystals, and the inner layer (II) is the or-alumina support whose macropores are filled with deposits. The layer of the or-alumina support is the white part below layer I in Figure 1 (b). The crystal size and top layer thickness increased with increasing reaction time. Since the inside of the tube was not rubbed with the zeolite crystals, no continuous film was formed. Membranes were characterized by X-ray diffraction. The XRD pattern of a membrane formed after 12 h was similar to that of the purchased NaX-type zeolite particles. Crystals recovered from the bottom of the reactor showed the same XRD pattern.
3.2. Permeation properties Permeation was determined with single-component gases and equimolar mixed gases. After the membrane was air dried, it was fixed in the permeation test unit. The He carrier was introduced in the permeate side, the temperature was then raised to 130~ in 1 h, and permeating gases were introduced to the feed side. Measurement was started after stabilizing the flow system for 6 h and was completed in about 10 h The temperature was then decreased to 80~ in 1 h, and the measurement was repeated. The temperature was further lowered to 30~ and the procedure was again repeated. As indicated in Figure 2,
(a)
~
106
!
I
CO2 (C02N 2 mixture) o_ 10.7 So~ i'M !
E 5 10-8
~
le),
E 0 o c-
0-9 0
N2 (C02-N 2 mixture)
1010
I
I
50 1O0 Temperature [~ Figure 2. Permeances of a Y-type zeolite membrane.
Figure 1. Top (a) and fractured (b) surfaces of a Y-type zeolite membrane.
150
667 the permeance of CO 2 was of the order of 10 -7 mol.m-2.s-l-pa -1, which was equivalent to the permeances of H 2 and Ar through an MFI-type membrane reported by Bai et al. [5]. Permeances to CO 2 and N 2 were weakly depended on the CO 2 partial pressure. The Y-type membrane in the present study was unique in that the N 2 permeance was greatly retarded when the mixture of CO z and N 2 was fed at 30~ The permeance to N 2 for single component system was (1-2)x10-8 mol.m-2.s-l.pa -1, and that for mixed gas system was 5x10-10 mol.m-2.s-l.pa-1. The permeance to CH 4 was also greatly decreased when the CO2-CH 4 mixture was fed. The activation energies for permeances to CO2, CH 4 and N 2 were positive. Li and Hwang [6] reported that the activation energy for CO 2 permeance through macropores was negative. Thus, the permeation mechanism of the Ytype membrane is different from that of macroporous membranes. In order to determine the reproducibility of membrane formation, nine membranes of 3 cm length were formed under the same conditions except for reaction time. One membrane was accidentally fractured during the setup procedure. The others obeyed the same relationship between selectivity and permeance as indicated in Figures 3 (a) and (b). The CO2/N 2 selectivity was decreased when the CO 2 permeance exceeded 10 -6 mol-m -2. s-l.pa-1. Figure 4 shows the effect of membrane length on the CO2/N 2 selectivity at 30~ The membranes of 3 cm length showed higher selectivities than those of 20 cm length, but the effect of the membrane length was not serious. These results suggest that the reproducibility of membrane formation was maintained in the present experiment. It was questioned if permeances were affected by desorption of water and adsorption of impurities during the permeation test. Thus, COg and N 2 permeances were determined as a function of time. An air-dried membrane was placed in the permeation test unit, the temperature was maintained at 30~ for 30 min, and an equimolar CO2-N e mixture was introduced. The CO 2 and N 2 permeances increased by the desorption of water in the initial stage of the measurement, and then gradually decreased. However, the CO2/N2 selectivity did not greatly change with time, ranging from 50 at zero time to 75 after 15 h.
100
,
,
,
,,
, , , 1
,
,
,
,
100
, , , ,
9
9 "
'
' ' " |
I
I
.... I
9
9
9 9 "'"'l
"
'
"
I
......
""""
.m
>
.m
0
o
~ 10 04
04 Z 04
10
z
04
0 0
9 0
I
10-7
.
. ......l
CO 2 permeance
10-6
,
, ......
10-5
[m01-m'2.s-l-pa -1]
I
10-9
I
10-8
........
I
10-7
10-6
N2 permeance [m01.m-2-s-l.pa -1]
Figure 3. Relationship between CO2/N 2 selectivity and permeances to CO 2 and N 2 at 30~
668 The free aperture of the main 100 channels in Y-type zeolite is 0.74 nm [7] and is much larger than the diameter of CO2 and Nz molecules. If the concentrations of CO z and N 2 in the micropores of > the Y-type zeolite membrane are equal to o those in the outside gas phase, these 10 t/) molecules permeate through the membrane r at a low C O z / N 2 selectivity. However, this Z Membrane length o,! was not the case. Carbon dioxide moleO o 3cm cules adsorbed on the outside of the o membrane migrate into micropores by sur9 20cm 1 ........ I ....... face diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micro10-7 10-6 10-5 pores by translation-collision mechanism CO 2 permeance [mol.m-2-s-l-pa -1] from the outside gas phase. The mouth of the micropores may Figure 4. Effect of membrane length be narrowed by adsorbed CO 2 molecules, on separation performance at 30~ which block N 2 molecules from entering the pores. Furthermore, C O 2 diffuses in pores of the zeolite membrane at a faster rate than N z. This selection mechanism is plausible for micropores with a width of up to six molecules [7]. When CO 2 molecules are strongly adsorbed on the pore wall, the CO 2 permeation rate will be low even if CO 2 is concentrated in the pore. If the pore size is close to the size of molecules, CO z molecules cannot pass N 2 molecules. Thus, balances between pore size and molecule size and between adsorptivity and mobility as well as difference in polarity of competitive species are important to attain both high permeance and selectivity. '
'
'
' ' " ' 1
'
'
'
' ' "
gs oo
.
m
4. CONCLUSIONS A Y-type zeolite membrane was formed on a porous a - a l u m i n a support tube. The membranes produced separated CO 2 from N2 at a permeance of the order of 10 -7 mol.m-2-s-l.pa-1 and a selectivity of 20-100 at 30~ This rapid and selective permeation was due to the pore-size controlled adsorption. REFERENCES 1. 2. 3. 4. 5. 6. 7.
J.-i. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe and S. Morooka, Ind. Eng. Chem. Res., 35 (1996) 4176. B.-K. Sca, M. Watanabe, K. Kusakabe, S. Morooka and S.-S. Kim, Gas Sep. Purif., 10 (1996) 187. K. Kusakabe, S. Yoneshige, A. Murata and S. Morooka, J. Memb. Sci., 116 (1996) 39. K. Kusakabe, T. Kuroda, A. Murata and S. Morooka, Ind. Eng. Chem. Res., 36 (1997) 649. C. Bai, M.-D. Jia, J.L. Falconer and R.D. Noble, J. Memb. Sci., 105 (1995) 79. D. Li and S.-T. Hwang, J. Memb. Sci., 66 (1992) 119. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conrersions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
669
Comparative study of various amines for the reversible absorption capacity of carbon dioxide Y. Nagao a, A. Hayakawa a, H. Suzuki a, S. Mitsuoka b, T. IwakP, T. Mimura c and T. Suda c aDepartment of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-01, Japan bHiroshima Research Institute, Mitsubishi Heavy Industries, Ltd. Kan-on-shin-machi, Nishi-ku, Hiroshima 733, Japan CTechnical Research Centre, The Kansai Electric Power Co. Inc. Nakoji, Amagasaki 661, Japan 1. A B S T R A C T
Aqueous solution of a-amino amides has been found to exhibit good reversible absorption capacity of carbon dioxide compared to amino alcohols when carbon dioxide is absorbed under 1 atm. However, the absorption capacity of (x-amino amides is highly dependent on the partial pressure of carbon dioxide, the absorption capacity being considerably decreasing when carbon dioxide is absorbed under 0.1 atm. 2. I N T R O D U C T I O N
Global warming is one of the most serious present-day issues for the human to overcome. The enhanced greenhouse effect was mainly caused by the rapid increase of carbon dioxide in the atmosphere since the Industrial Revolution. To resolve the problems about carbon dioxide emissions, several new methods to recover and utilize carbon dioxide have recently been developed [1]. Alkanolamine solutions are generally used as chemical absorbent for the recovery of acid compounds from flue gases. Although the reaction between carbon dioxide and a variety of amino compounds, represented by monoethanolamines, has been widely studied [2],[3],there is still a great need to develop more effective and chemically more stable absorbents. In the present study, we have prepared several different types of amino compounds and compared their capacity for the reversible absorption of carbon dioxide at different temperatures and under different pressures of carbon dioxide.
670
3. RESULTS AND D I S C U S S I O N 3.1. Comparison of amino alcohols and a-amino amides With the aim to develop more effective and chemically more stable absorbents than conventionally used amino alcohols, we have prepared a-amino amides 1 ~8 and compared their capacity for the reversible absorption of CO 2 with those of commonly used amino alcohols. Absorption capacity was examined by bubbling CO2 under 1 atm. As seen in Table 1, CO 2 loading of a-amino amides at both 40 and 100 ~ was generally smaller than those of amino alcohols. The solution of 8 absorbed CO 2 more slowly than 9 and 10 at 40 ~ as shown in Figure 1, but the same solution desorbed CO 2 considerably faster than that of 9 at 100 ~ as shown in Figure 2. These results mean that compound 8 was better in the desorption ability of CO 2 than compound 9, though the absorption ability of a-amino amides for CO 2 was found to be generally lower than that of amino alcohols. This difference in the reversible absorption capacity between amino alcohols and a-amino amides is noteworthy and may be attributed to the difference of nucleophilicity and coordination ability between two functional groups involved, i.e. hydroxyl and amide groups. Table 1 Comparison of CO2 loading (%) of amino alcohols and amino amides Substrates
Loading (%)
Capacity (%)c
40 ~
100 ~
EtNHCH2CONH 2 (1) ~PrNHCH2CONH 2 (2)
62
13
66
12
54
nBuNHCH2CONH2 (3)
57
14
43
tBuNHCH2CONH2 (4)
74
11
63
49
Me2NCH2CONH 2 (5)
50
7
43
Et2NCH2CONH 2 (6)
67
8
59
Me2NCH2CONMe 2 (7) Me2NCH2CONEt2 (8) N H2CH2CH2OH (9)
80 74
12 13
68 61
74
33
41
EtNHCH2CH2OH (1 0)
93
44
49
Et2NCH2CH2OH (1 1) 97 51 46 aAmount of CO2 absorbed within 1 h under 1 atm of CO2. bAmount of CO2 remaining after 1 h. ~ in loading values at 40 and 100 ~ As shown above, the reversible absorption capacity of a-amino amides was comparable with those of amino alcohols when CO 2 was absorbed under 1 atm. However, aqueous solutions of compounds 1 ~7 slowly underwent hydrolysis when they were kept at 100 ~ for a long time. Chemical stability is one of the
671
indispensable prerequisites for the chemical absorbents. So, at first sight, o~-amino amides may be thought to be unsuitable for the present purpose. But taking advantage of the property that o:-amino amides desorbs CO 2 with less thermal energy, we synthesized a compound 8, which has a hindered amide function alkylated by two ethyl groups and is quite difficult to be hydrolyzed even heated for prolonged time at high temperatures. 100
O O
80 ~ 70 v 60 .c_ 50
o
A
401 30
[]
A
D
O
O
O
A
A
I
II
D
c]3
/
20-1 10 0 0
1.
9
A9
98 o 10
70 ~. 60~50= 40:5 30o ,.,,.,
1'0 20 3'0 4'0 50 6'0
absorption time (min) Figure 1. Absorption behavior of compounds.3, 8~ 10
e8 •
,~
A 9 I I A
A
9
101 0
5 10 15 20 25 30 35 40 desorption time (min) Figure 2. Desorption behavior of compounds 8 and 9.
3.2. C o m p a r i s o n under different pressures of C O 2 As discussed above, the reversible absorption capacity of the solution of amide 8 was better than those of amino alcohols under 1 atm of CO2, though the absorption rate of the former compound was slower. However, when CO2 absorption was carried out by bubbling a gaseous mixture of CO2 and N 2 in a ratio of 1 : 9, the CO2 loading value of the solution of 8 significantly dropped as compared to that of monoethanolamine, as shown in Table 2. The absorption ability of the solution of 8 became poor and its reversible absorption capacity was no better than that of monoethanolamine, either, under these conditions employed. This may be interpreted that the absorption ability of the tertiary amide 8 was highly dependent
Table 2 Comparison of reversible CO 2 loading (%) Substrates a
Me2NCH2CONEt 2 (8)
Loading (%)
Capacity (%)~
40 ~
100 ~
29 (74) e
13
16 (61)'
NH2CH2CH2OH (9) 59 (74) e 33 26 (41)' aUsed as 1M aq. solution, bAmount of CO2absorbed within 2 h under 0.1 atm of CO2.CAmount of CO2 remaining after 1 h; see Table 1. dDifference in loading values at 40 (under0.1 atm of CO2)and 100 ~ "Amount of CO2 absorbed within I h under 1 atm of CO2. 'Difference in loading values at 40 (under 1 atm of CO2)and 100 ~;.
672
on the partial pressure of CO2. Thus, a-amino amides are suitable as the absorbent for CO2 under a normal partial pressure of CO 2, but not so satisfactory under a low partial pressure of CO2.
4. EXPERIMENTAL
SECTION
A 1 M aqueous amine solution was placed in a two-necked flask, through which CO 2 was bubbled with stirring for 1 hour at a flow rate of 50 ml/min at 40 ~ under 1 atm. After the end of CO 2 bubbling, the solution was heated to 100 ~ and kept at this temperature for an additional hour. The amount of CO2 remaining in the amine solutions at 40 ~ and 100 ~ was determined by an apparatus SHIMADZU TOC10B and was shown here as the "loading capacity" in mol COJmol amine (%). Similar determinations of CO 2 Ioadings were made under a CO 2 partial pressure of 0.1 atm, using a 1 : 9 gaseous mixture of CO 2 and N2. Compounds 1--6 were prepared from 2-chloroacetamide and the respective amines, while compound 7 was obtained from chloroacetyl chloride and dimethylamine and compound 8 from chloroacetyl chloride, diethylamine and dimethylamine, as previously reported [4],[s]. 5. C O N C L U S I O N a-Amino tertiary amides were found to be attractive as a potential candidate for the new CO2 absorbent with respect to their high stability toward oxidative degradation as well as thermal economy. The CO 2 loading of c(-amino amide 8 under 1 atm of CO 2 was comparable with that of monoethanolamine, but the former compound exhibited better desorption ability than the latter. REFERENCES 1. D. H. Gibson, Chem. Rev., 96 (1996) 2063. 2. P. V. Danckwerts and M. M. Sharma, Chem. Engng., (1966) CE244. 3. G. F. Versteeg, L. A. J. Van Dijck and W. P. M. Van Swaaij, Chem. Eng. Commun., 144 (1996) 113. 4. M. M. Werber and Y. Shalitin, Bioorganic Chemistry, 2 (1973) 202. 5. V. P. Y. Gadzekpo, J. M. Hungerford, A. M. Kadry, Y. A. Ibrahim, R. Y. Xie and G. D. Christian, Anal. Chem., 58 (1986) 1948.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
673
K i n e t i c a n a l y s i s o f C 0 2 r e c o v e r y f r o m flue g a s b y a n e c o t e c h n o l o g i c a l system M. Tabata, a T. Chohji a and E. Hirai b aDepartment of Ecomaterials Engineering, Toyama National College of Technology, 939 Toyama-si, Hongou-mati 13-banti, Japan bLibrary Center, Hokuriku University, 920-11 Kanazawa-si, Taiyougaoka 1-1, J a p a n Kinetic analysis of Ca 2§ desorption due to the displacement of Ca 2§ from cation exchange resin by H § which is generated by bubbling CO~. to the resin containing water at room temperature has been studied. Ca 2§ was rapidly displaced by H § at room temperature. This indicates that cation exchange resin or soil which acts as cation exchange resin may be good material for CO2 recovery from flue gases. 1. I N T R O D U C T I O N Development of CO2 removal system is the one of the urgent problems to be solved[I]. The CO2 removal system should satisfy the following requirements. (1) Consume low energy. (2) Using no or less harmful materials to environment. (3) Products of CO2 removal system is non toxic and/or commercially valuable materials. Recently, we have proposed a new system to recover CO2 from flue gas in the form of carbonate salts which satisfy the above requirements[2, 3]. The system uses water as absorbent of CO2 and soil as cation exchanger. The principal of the new system consists of the following four steps. (1) Dissolution of CO2 separated from the flue gas into soft(or cation exchange resin)-containing water which is accompanied by a formation of carbonate ions and a decrease in the pH value of the soil-containing water. (2) The decrease in the pH value of the soil-containing water results in the enhancement of the displacement of alkaline earth metal ions (such as Ca 2§ adsorbed on the soil (or resin) with H § by ion exchange reaction. (3) Separation of the soil (or resin) from the soil-containing water and aeration of the supernatant. (4) Carbonate ions react with alkaline earth metal ions to form carbonate salts which are insoluble in water. Although a portion of dissolved CO2 is released after the aeration step, soluble and/or insoluble carbonates are eventually formed in the soil-separated solution. All the materials used in the system are not harmful to the environment and all the reaction undergo at room temperature. We have tested this new system for CO2 recovery using soil samples taken in Hokuriku district, Japan[4, 5], and have
674 found that CO2 was fixed in the form of calcium carbonate. However, kinetic analysis of this reaction system has not been analyzed. In the present paper, reaction rate for Ca ~-+ desorption step during bubbling pure CO2 gas to the resincontaining water is analyzed. 2. E X P E R I M E N T A L Schematic experimental procedure is shown in Figure 1. All the chemicals used were of analytical grade, and ion-exchanged distilled water was used for all the procedure. Amberlite IRC-76 (Organo K.K.) was used for cation exchange reactions. Its cation exchange capacity for 1 dm 3 of wet resin is 200 g of CaCO3. The resin was treated in the diluted HC1 solution to displace Na + by H +, and then treated in saturated CaCO3 solution to displace H + by Ca 2+. After washing with the distilled water, 1 cm 3 of wet Ca2+-resin was dispersed in the 300 cm 3 of the distilled water. Pure CO2 gas was introduced into the resin-dispersed solution at the constant flow rate (10 cm3.minl). Time variation of the pH value and Ca 2+ concentration of the resin-dispersed solution was analyzed by using pH / ion meter (Horiba K.K. model F-23 with pH and calcium ion electrodes).
mixing with diluted HC1 solution -i 9
l Na+.resin
i
-~
H+-resin
I
i
I addition of CaZ+-eontaining solution .....................
C02 bubbling
CO2 f i x a t i o n a n d r e l e a s e of Ca 2+
lI
t
Ca2+.resin
:
'.
Figure 1. Schematic diagram of the present reaction system
3. R E S U L T S A N D D I S C U S S I O N
The pH value of the resin-dispersed solution decreased from 7 to 3.5 when C02 was introduced. Immediately after few seconds of C02 introduction, the pH value increased to 4.5, and the pH value kept constant. The decrease in the pH value just after C02 introduction is due to that ionization of C 0 2 by the following equation.
675 C02
+
C02-H~O
H20
~-
~-
HCOa- ~--
C02"H20
(1)
COa 2-
(3)
+
H+ H§ +
HCO3-
(2)
The increase in the pH value indicates that some of H § formed by dissolving CO2 into the resin-dispersed solution were displaced by cation exchange reaction with Ca 2§ High pH value of C02-bubbled resin-dispersed solution compared to 3.5, which is attained by bubbling CO2 into pure water, is attributed to t h a t most of the H § formed by CO2 bubbling continuously displaced with Ca 2§ released from the resin. Time variation of the Ca 2§ concentration in the resin-dispersed solution in Figure 2 shows that the Ca 2§ concentration rapidly increased after introducing CO2 gas; the Ca 2§ concentration increased to 1 ppm in few seconds and to 1000 ppm in 10 min. The evolution of Ca 2§ is due to that pH value is increased because of CO2 dissolution. Variation of the Ca 2+ increase is not linear to the CO2 bubbling time. This suggests that rate determining step for Ca 2§ evolution is not CO2 dissolution step, since the CO2 bubbling rate was kept constant. From the fact that pH value of the resin-dispersed solution kept constant value, displacement of cation on the surface of cation exchange resin seems to be proceeded at a constant rate. The rate determining step may then be among following reaction steps. Ca2+-resin + 2 H + ~ -- H+-resin-H + Ca 2+ + CO32" ~-- CaCO3
40000
30000 o 20000 o9 r
9 ~9 + r
i0000
~
0
A . ~
A.
A . A .
5
-
_
10
15
20
Time (min) Figure 2. Time variation of Ca 2+ concentration in the resin-dispersed solution
+
Ca 2+
(4) (5)
Because the amount of Ca 2+ evolved by 10 minutes C O 2 bubbling was larger t h a n 1000 ppm, the present system may sufficiently fix CO2 from flue gas. Ca 2+ displacement by H + will proceed fast under acidic condition. This indicates that CO2 fixation rate will be enhanced when the pH value of resin-dispersed solution is low. Flue gas contains not only CO2 but also nitrogen oxide compounds, so that when flue gas is bubbled into the resin-dispersed solution instead of pure CO2 gas, evolution rate of Ca 2§ from the resin is expected to be enhanced. At the same time, if the pH value of resin-dispersed solution is too low, CO2 will not be dissolved in the solution. Further study on the reaction kinetics is necessary.
676 4. COUCLUSIONS Present C02 recovery system using materials harmless to global environment and mild reaction condition (ecotechnological system) was examined for the practical application from the viewpoint of reaction kinetics. Cation exchange reaction of Ca 2§ by H § generated by bubbling pure CO2 proceeded very fast, indicating that cation exchange resin may be applied for the present ecotechnological C02 recovery system. Rate determining step of the cation exchange reaction is either cation exchange step or the formation of insoluble CaC03 step. REFERENCES
1. C. Hendriks, Carbon Dioxide Removal from Coal-Fired Power Plants, Kluwer Academic Publishers, The Netherlands (1994). 2. T. Chohji and T. Korenaga, Proceedings of the International Symposium on C02 Fixation and Efficient Utilization of Energy, Tokyo, (1995) 227. 3. T. Chohji, M. Tabata and E. Hirai, Energy, 22 (1997) 151. 4. C. Nakagawa, T. Chohji and E. Hirai, Mizu Kankyou Gakkai-Si (J. Japan Soc. Water Environ.), 16 (1993) 114. 5. C. Nakagawa, T. Chohji and E. Hirai, Mizu Kankyou Gakkai-Si (J. Japan Soc. Water Environ.), 16 (1993) 175.
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide
677
Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
F o r m a t i o n of p e r o x o c a r b o n a t e s from L3Rh(O2)CI a n d L2Ni(CO2) : a u n i q u e r e a c t i o n m e c h a n i s m w i t h c a r b o n d i o x i d e i n s e r t i o n into t h e OO bond. Michele Aresta, a Eugenio Quaranta, a Immacolata Tommasi, a Jo/~lle Mascetti b, Michel Tranquille, b Marek Borowiak. c a Dipartimento di Chimica and Centro CNR-MISO, Universitb di Bari, Bari, Italy, b Laboratoire de Spectroscopie Mol(~culaire et Cristalline, Talence, France c Industrial Chemistry Research Institute, Warsaw, Poland. 1. INTRODUCTION Transition-metal peroxocarbonates of formula LnM(CO4)X m (L = ancillary ligand; n = 2, 3; X = halogen; m = 0, 1) are known since a long time but, despite the cospicuous synthetic work available in the literature, [1] very little was reported about their mechanism of formation. If the peroxocarbonate complexes are prepared by reaction of dioxygen complexes of transition metals LnM(O2)X m (M = P d , Pt, Rh, Ir) with carbon dioxide, [la] in principle, two ways are possible, that imply the formal insertion of carbon dioxide into the O-O (route 1, Scheme 1) or M-O (route 2, Scheme 1) bond. Scheme 1
route 1 / *0 M
/ *0
or
\
(+) route
2
M
COz
M
\.0 -
.ol*
d"
/ *0(+)
~.0( -)
-) d
~
/0~0.
M
/
~.< ) ~ C ~ O .
.
/~
COz
~
M
([
\ o / ~o
The mechanism of formation of RhCI(CO4)(PEt2Ph) 3 [2] from CO 2 and RhCl(~!2O2)(PEt2Ph) 3 and of Ni(CO4)(PCy3)2 from Ni(CO2)(PCy3)2 and 02 has been ascertained through the comparison of a normal coordinate analysis for the fragment [RhCI(CO4)(PC3)3] and [Ni(CO4)(PC3)2] respectively and the IR spectra of the labeled analogues of peroxocarbonate complexes obtained through the following reactions : RhCI0802)(PEt2Ph)3 + CO2 (reaction 1A) RhCI(O2)(PEt2Ph)3 + 0 8 0 2 (reaction 1B) Ni(CO2)(PC~3)2 + 1802 (reaction 2A) Ni(C1802)fPCy3)2 + 02 (reaction 2B)
678 2. EXPERIMENTAL SECTION General P r o c e d u r e . Unless otherwise stated, all reactions and manipulations were conducted under a dinitrogen or CO 2 atmosphere by using vacuum-line techniques. All solvents were dried as described in the literature [2] and stored under dinitrogen. CO 2 (>99.95%) and 02 (>99.998%) were from SIO SpA, C1802 080 97.7%) was from CEA-ORIS, 1802 080 99%) from SIC. FTIR spectra were recorded using a Bruker 113V Fourier transform interferometer. Frequencies are accurate to • cm-1. Solid samples were studied as Nujol mulls (Nujol was previously dried on sodium wire and bubbled with argon). Structural parameters and initial force field were taken from literature [3] and the program used to calculate the generalized valence force field was similar to Schachtschneider's one. [4]
Synthesis of ClRh(-160-lsO-C(1sO)160)(PEt2Ph)3 and ClRh(-laO-160C060)aaO)(PEt2Ph)3 A solid sample of RhCl(~I2-1802)(PEt2Ph)3 (0.300 g, 0.45 mmol) was dissolved in toluene (30 mL) saturated with C1602 at 243 K. The solution was then added with pentane (50 mL) and stored overnight at 243 K. The yellow solid that separated was filtered under Ar, washed with pentane and dried in vacuo. It was characterized by elemental, IR and NMR analyses. O-Transfer to Phosphine Ligand 0.150 g (0.21 mmol) of C1Rh(-180-160-C(160)lso)(PEt2Ph)3 were dissolved in 5 mL of CH2CI2. The solution was stirred at 313 K for I h and analyzed by GC-MS. The GC-MS analysis showed the formation of 180=P(Et2Ph) 3 and 160=P(Et2Ph) 3 in molar ratio 85:15. The experiment was repeated in the same conditions using CIRh(-1.60-180-C080)160)(PEt2Ph)3 . In this case, 180=P(Et2Ph) 3 and 160=P(Et2Ph) 3 were formed in 15:85 molar ratio.
Synthesis of Ni(-a60-lsO-C(180)160)(PCy3)2 and Ni(-180-160C060)180)(PCy3)3 0.150 g (0.36 mmol) of Ni(CO2)(PCy3)2 (solid state) were put in a Schlenk tube under 1802 atmosphere at 278 K. The system was allowed to react, u n d e r continuous stirring, until the initial yellow color of the powder was changed into pink-salmon. The same procedure was used by reacting Ni(C1802)(PCy3)2 with 160 2. Samples of equilibrium gas were withdrawn at regular time intervals and examined using the FTIR technique. The solid resulting sample was characterized by Infrared spectroscopy and elemental analysis. 3. RESULTS AND DISCUSSION 3. 1. The vibrations due to the peroxocarbonate moiety The analysis of the FTIR spectra of solid RhCI(CO4)(PEt2Ph)3 and its 180 labeled derivatives obtained through reactions 1A and 1B allowed a preliminary assignement of the spectroscopic vibrations due to the RhOOC(0)O moiety. The strong absorption present in the IR spectrum of RhCI(CO4)(PEt2Ph) 3 at 1665 cm -1 is assigned to the carbonyl stretching v(C--O).
679 Others absorption shifted by C1802 labeling are the medium band at 1249 cm q associated to one of the v(C-O) modes and the two sharp bands at 579 and 545 cm q which have isotopic effects of 6 and 10 cm q, respectively (assigned to the two stretching modes v(Rh-O) ). The medium strong sharp absorption at 963 cm q, which is shifted to 943 cm -1 upon 1802 labeling is assigned to the stretching of the O-O bond and the assigment of the bands at 579 and 545 cm q to v(Rh-O) is confirmed by their isotopic effects of 23 and 17 cm-1 by 1802. Bands at 808, 779, 380 and 322 cm -1 also show weak isotopic effects by C1802 a n d / o r 180 2 labeling and are therefore associated to vibrations of the I~hOOC(O)C) moiety. [5] In the case of the Ni(CO4)(PCy3)2 complex the bands sensitive to isotopic labeling were the strong absorption at 1685 cm-1 (assigned to v(C=O)), the medium bands at 1258 and 1250 cm -1 (assigned to v(CO)) and the medium band at 995 cm -1 (assigned to v(O-O)). 3.2. Mechanism of formation of peroxocarbonates from "Rh~2-O2 '' and CO2: a normal mode analysis Chart 1 shows all derivatives obtainable from RhCI(CO4)(PEt2Ph) 3 by isotopic labeling with 180 2 (molecules 2 and 3, Chart 1) and C180 2 (molecules 4 and 5, Chart 1) Chart 1
Rh(~lz-leOz) + C160z P
P. I ,,o
Rh(2 1802) + C160z (Reaction 1A)
P
-
p
P-I,,,"80--. 0 P
2
A
Rh(~lZ-160z) + C180 (Reaction 1B)
1
p
P-.I ,,'-0 "0 P
o
"E, c/
3
p
P'I
P
4
'~--~0
A
2
P
P . I .... 0--9 P
5
A normal mode coordinate analysis on the RhCl(CO4)(PEt2Ph)3complex has shown that from all derivatives obtainable the observed frequencies are in agreement only with the formation of species 2 and 4. As reported in the Experimental Section the Ni(CO4)(PCy3)2 complex is prepared by reacting Ni(CO2)(PCy3)2 with 02 according to reaction 2A or 2B. Also in the case of the Ni-complex 6 there were, in principle, 4 possible labeled analogues of Ni(CO4)(PCy3)2 species that could be observed. (Chart 2) Interestingly, a normal mode coordination analysis of the Ni(CO4)(PC3)2 fragment has shown that also in the case of the Ni-complex only species 7 and 9 are formed which can form only by insertion of the CO2 molecule in one O-O bond. This result and the finding that the FTIR spectrum of the gas evolved during the reaction have shown the presence of the CO2 free molecule, bring to
680
Chart 2
Ni( Z_ClsOz ) + lSOz R,,N i""Oz'--/04 P" 6
Ni('qZ-C160z) + 180z (Reaction 2A) P.
"
.,
0----0
CI'''N~'~'~)'~'/ 9 7
A
%0
P'"N -." O ~ " o
Cl'" ~0---/, 8
%0
Ni('qZ-C180z) + 1602 (Reaction 2B)
A
P.,
CIf N 9
o
,,, 0 ~ 0 CI~Ni'~'" / ~'O---C~ 0
10
the conclusion that the Ni-CO2 complex reacts with 02 by releasing CO2. Dioxygen is co-ordinated with formation of a Ni-O2 complex that undergoes CO2 insertion into the O-O bond affording the peroxocarbonate species. The described Nichel reactions are among tke very few solid-state organometallk reactions reported in the literature. 3. 3. Oxygen Transfer from Peroxocarbonate to Oxophiles When ClRh(-180-lSO-C(O)O)(PEt2Ph) 3 was dissolved in CH2CI 2 at roorr temperature the O-transfer reaction to phosphine took place according tc equation (1). ClRh(-180.160.C(O)180)(P)3 - - - - - >
RhCI(CO3)(P)2 + p=180
(1)
The GC-MS analysis of the solution has allowed to determine that phosphinc oxide formed contained more than 85-90% of 180. This suggests that the oxyger atom of the peroxo group transferred is that linked to Rh, more than that linkec to carbon. This result is confirmed also by the reaction of ClRh(A60-180 C(180)160)(PEt2Ph)3 that affords RhCI(CO3)(P) 2 and P=160 when dissolved ir CH2CI 2 at room temperature with a similar selectivity (85-90 % 160 on phosphin( oxide). [2] REFERENCES 1. a) P. J. Hayward, D.M. Blake, G. Wilkinson, C.J. Nyman, J. Am. Chem. Soc., 92 (1970) 5873; b) L. Malatesta, R. Ugo, J. Chem. Soc. (1963) 2080; c) M. Aresta, C.F. Nobile, J. Chem. Soc., Dalton Trans. (1977) 708; d) M. Aresta, E. Quaranta, A. Ciccarese, C 1 Mol Chem., 1, (1985) 267. 2. D.D. Perrin, W.L. Amarego, D.R. Perrin, Purification of Laboratory Chemicals; Pergamon press: Oxford England, 1986. 3. a) M. Fouassier, M.T. Forel, J. Mol. Struct., 26 (1975) 315; b) A. Nakamura, Y. Tatsuno, M. Yamamoto, S. Otsuka, J. Am. Chem. Soc., 93, (1971) 6052; c) C. Jegat, M. Fouassier, J. Mascetti, Inorg. Chem., 30 (1991) 1521; d) C. Jegat, M. Fouassier, M. Tranquille, J. Mascetti, I Tommasi, M. Aresta, F. Ingold, A. Dedieu, Inorg. Chem., 32 (1993) 1279. 4. J.H. Schachtschneider, R.G. Snyder, Spectrochim. Acta, 19 (1963) 117. 5. M. Aresta, I. Tommasi, E. Quaranta, C. Fragale; J. Mascetti, M. Tranquille, F. Galan, M. Fouassier, Inorg. Chem., 35 (1996) 4254.
681
Keyword Index A abiotic photosynthesis of amino acids acetic acid synthesis Acetobacter xylinum acetogenesis acetonitrile-water mixtures active site of carbonic anhydrase aerobic bacterium alloy 9 electrodes allyl halides allyl stannanes amorphous Ni-Zr alloys amorphous Ni-Zr-rare earth element alloys analysis of CO2 metabolism antioxidant activity Aresta's nickel CO2 complex aspects of CO2 utilization atmospheric CO2 fixation
B biochemical CO2 fixation biological CO 2 fixation biological sink of CO2 biomass production biomimetic mineralization of aqueous carbonate ~ons bioreactor biotechnology blue-green algae Spirulina platensis Boudouard reaction building materials C C 2 organic molecules cadmium sulfide nanocrystallites carbon dioxide behavior carbon dioxide capture carbon dioxide fixation carbon dioxide fixation by PNSB carbon dioxide fixation in PNSB carbon dioxide fixing microalgae carbon dioxide for petrochemicals feedstock. carbon dioxide hydrogenation carbon dioxide insertion into the O-O bond carbon dioxide methanation carbon dioxide mitigation carbon dioxide recovery from flue gas carbon dioxide reduction carbon dioxide reforming of methane carbon dioxide separation from nitrogen carbon dioxide utilization carbon multi-recycle system carbon-recycling energy delivery system carboxylation of allyl stannanes
189 439 653 303 581 309 613 573 165 165 451 261, 451 597 641 491 195 499 309 315,471 243 617 621 633 65 617 371 479 491 183 395 201 503,637 593 593,597 641 333 345, 407, 505, 533, 541 677 451 77, 273 673 31,43 375,395 665 65 273 379 165
682 carboxylation reaction with CO2 catalyst design catalytic behavior catalytic CO2 reduction catalytic conversion of CO2 catalytic fixation of CO2 catalytic hydrogenation of CO2 catalytic reaction of CO2 cellulose cellulose synthase genes chitosan-calcium alginate hydrogels chitosan-calcium carbonate composites Chlorella sp. chloroplast transformation method climate change cluster cobalt macrocycle composite catalysts conversion of CO 2 into cellulose copper-based catalysts countermeasures Cr203 catalyst crystallization Cu/Zn oxides Cu/ZnO catalysts Cu/ZnO/SiO 2 catalyst Cu/ZnO-based multicomponent catalysts cultivation of cyanobacterium CuO-ZnO-A1203 catalysts cyanobacterium cyclic carbonates
487 127 87 219 403 141 19,431 153 243 653 621 621 315, 483 609 1,9 171 97 327,435 653 505 9 419 601 225 529 509 549 471 545 629 403
D
dehydrogenation of ethylbenzene dehydrogenation of propane dielecric-barrier discharge dimethyl ether dinuclear Ni(II) complex durability of catalysts
415 419 541 447 499 517
E
economics assessment of power generation systems ecotechnological system effect of solvents electrocatalysts electrocatalytic reduction of CO2 electrochemical methods electrochemical reduction of CO2 electroreduction of CO2 electrospray mass spectrometry energy delivery system Escherichia Coli ethanol production from CO 2 ethanol synthesis ethanol synthesis from CO2 and H 2
367 673 553 43 207 213 107, 577, 581,585 225 557 285 601 657 431,517 525
683
ethene extreme thermophile
153 605
F
Fe promoted Cu-based catalysts Fe, Cu-based novel catalysts Fe-Cu-Na/zeolite composite catalysts fermentative microalgae Fischer-Tropsch synthesis formate dehydrogenase of Clostridium thermoaceticum formation of peroxocarbonates from L3Rh(O2)C1 and l_,zNi(CO2) fossil/solar energy hybridization system fuels and petrochemicals from CO2 functional dual-film electrode
427 513 423 657 159, 443 303 677 285 443 207
G gas diffusion electrodes gas turbine power generation system gene manipulation of microalgae global carbon-recycling energy delivery system graphite carbon green alga Neochloris sp.
225 297 653 273 147 637
H
helical tubular photobioreactor high performance liquid chromatography highly pressurized CO2 hybrid catalysts hydrocarbon synthesis from CO2 hydrogenation of CO2 Hydrogenobacter thermophilus strain TK-6
483 557 279 447 327 87,411,423,427,455, 517 613
I
IEA action incorporation of CO2 influence of anions influence of MgO addition infrared spectroscopic study initial transient rates lnterconversion of Ru-CO and Ru-h 1-COz ion exchanged(H, K)Zeolite-Y iron catalysts iron catalyzed CO2 hydrogenation iron oxide catalysts iron oxide-based catalyst
1
213 573 399 569 159 459 407 345, 407 339 387 415
K
kinetic analysis KNiCa catalyst Kolbe-Schmitt reaction
673 395 487
L lanthanoid complex light-diffusing optical device
503 633
684
liquid hydrocarbons liquid-phase methanol synthesis
339 521
M
marine cyanobacterium marine microalga utilization for a paper mechanical alloying mechanisms membrane reactor metal complexes metal electrodes metal supported gas diffusion electrode metal-particle modified p-Si electrode metallic electrodes methanation of CO2 methane methanol methanol as the intermediate methanol homologation using CO2 methanol solution methanol synthesis from CO2 and H 2 MgO surface microalgae micropores microprojectile bombardment mitigate CO 2 emissions molecular characterization molecular tailoring
237 649 529 127 147 97 569 577 565 107 261 147, 439 87, 447 537 495 589 267, 351, 3 57, 505, 509, 529, 533,545, 549 391 55 585 609 291 605 219
N
nature of CO 2 new synthetic routes for basic chemicals nickel macrocycle nucleic acid bases
391 19 97 189
O oil extraction oil recovery orgam c acids organic perfluoroalkyl derivatives organometallic CO2 complexes organometallic polymers organometallic reactions over-expressed effect of carbonic anhydrase oxidative dehydrogenation of ethylbenzene
279 201 189 213 255 219 127 629 387
P
Pd promoted Cu/ZnO/A1203 catalysts Pd/SiO 2 catalysts Pd-catalyst petroleum-degrading bacterium strain HD-1 phosphoenolpyruvate carboxykinase phosphoenolpyruvate carboxylase photobiological production of H 2
351 533 165 467 463 463, 601 321
685 photobioreactors photocatalytic CO2-fixation photocatalytic reduction of CO 2 photochemical reduction of CO 2 photoelectrochemical reduction of CO2 photosynthesis under salinity stress photosynthetic algae photosynthetic bacteria photosynthetic COg fixation photosynthetic performances plants platinum electrode polydentate ligands polymer blends polysaccharide-producing microalgae porphyrin production production of alkane production of alkene Production of poly hydroxyalkanoate production of polyethylene glycol microspher production of polyoxyalkylenealkylphenyl ether microsphere promoting effect of calcium addition promoting effects of CO 2 propylene Pt-Sn bimetallic catalysts pulsed electroreduction of CO 2 purity of CO2 purple non-sulfur bacteria
471 557 177, 183,553,561 97 565, 589 249 633 475 483 117 117 581 43 403 645 475 467 467 237 661 661 533 419 171 153 573 141 597
R
Raney copper-based catalysts recombinant phosphoenolpyruvate carboxylase redox system of ferrite reduction of COz reductive TCA cycle reforming of natural gas reversible absorption capacity of CO2 reversible oxide transfer reaction Rh ion exchanged zeolite catalysts rhodium catalyst rhodium catalysts Rhodopseudomonas sp. No.7 Ru-Co/AI2 O3-catalysts RuBisCO ruthenium catalysts ruthenium-cobalt bimetallic complex system
267 605 383 147 613 399 669 459 455 411 431 463 171 609 399 495
S
Scenedesmus komarekii selective formation of iso-butane semi-batch cultivation semiconductor photocatalysts sol-gel method solar cell solar energy
625 435 649 553 561 207 367, 379
686 solar energy carrier solar/chemical energy hybridization solar-collecting device Sp25 stable catalysts stable nickel-magnesia solid solution catalysts strategy structure-activity relationships super-RuBisCO supercritical CO2 supply of H 2 support effects Synechococcus sp. PCC7942 synthesis gas synthesis of chemicals synthesis of ethanol from CO2 and H 2 synthesis of gasoline from CO2 synthesis of lower olefins synthesis of renewable methanol
285 371 633 609 521 375 9 43 117 255, 661 141 345 629 333 65 513 537 407 363
T test plant tetraselmis sp. Tt- 1 thermochemical cycle Ti/Si binary oxide catalysts time-resolved infrared study titanium oxides tobacco mutant tolerance of a green alga tungsten-selenium-iron protein
357 649 383 561 255 177 609 625 303
U underground storage utilization of micro-algae
201 479
V
vanadium catalyst vegetation activity
439 231
W
waste heat from factories
297
X
X-ray studies
601
Y Y-type zeolite membranes
665
Z
zeolites zero emission power plants zinc(II) complex ZSM-5
177 279 309 387
687
Author
Index
A Akano, T. Akiyama, E. Akiyama, H. Amano, H. Ando, H. Ando, M. Anpo, M.
Arata, Y. Aresta, M. Armor, J.N. Asada, Y. Asakura, K. Asami, K. Augustynski, J.
545 261,451 237,653 273,285, 379 327, 423,435 415 177,387,395, 561 383 19,431,455, 525, 529 249 65, 677 141 321 455 451 107
B Bachiller-Baeza, B. Baltan~, M.A. Bandi, A. Bando, K.K. Bill, A. Bonivardi, A.L. Borowiak, M. Brenner, V. Brunschwig, B.S.
399 533 363 455 541 533 677 593 97
Aoki, A. Arakawa, H.
C Cabelli, D. Centola, P. Chadwick, D. Chang, J.-S.
Chiozza, E. Cho, I.-C. Chohji, T. Choi, D.-H. Choi, M.-J. Claeys, M.
97 333 351 177,387,395, 561 419 375 533 625, 637, 641, 645 213 509 673 407 345, 407, 447 159, 443
D Del Rosso, R. Desigaud, M.
333 213
Chang, W.-C. Chen, Y. Chiavassa, D.L. Chihara, M.
Dinjus, E. Draget, K.I. Dragos, L. DuBois, D.L. Dufiach, E.
127 621 195 43 213
E Ehara, S. Eliasson, B. Elser, M. Endo, N. Ezawa, M.
177 541 363 207 661
F Ferreira-Aparicio, P. Fiato, R.A. Flueraru, C. Foyer, C.H. Franks, R. Fujii, T. Fujii, Y. Fujimoto, K. Fujimura, H. Fujinaga, T. Fujishima, A. Fujita, Z. Fujiwara, H. Fujiwara, M. Fujiwara, Y. Fukui, H. Fukunaga, A. Furenlid, L.R.
399 339 195 609 165 463,475 177, 561 375 525 189 31,585, 589 97 183 327, 423,435 439 529 147 97
G George, M.W. Gollin, D.J. Gong, J.K. Goto, S. Greiner, J. Grills, D.C. Gronchi, P. Guerrero-Ruiz, A.
255 303 491 427 213 255 333 399
H Ha, C.S. Habazaki, H. Han, J.-K. Hanagata, N. Haneda, Y. Hara, H. Hara, K. Hashimoto, Kazuhito
403 261,451 471 625 517 537 577 585, 589
688 Hashimoto, Koji Hasuike, M. Hattori, T. Hayakawa, A. Hayashi, T. Hayashida, T. Hayashitani, M. He, D. Higuchi, K. Hinogami, R. Hinze, S.M. Hirai, E. Hirano, A. Hirano, M. Hirano, S. Hirata, S. Hirayama, S. Hirota, K. Hirotsu, T. Horns, N. Hon-Nami, K. Hori, H. Hori, T. Hori, Y.
261,451 649 415 669 243 439 617 423 517 565 491 673 649, 653,657 237, 545 621 617 657 589 479 153 649, 653,657 557 189 569, 581
Ishitani, O. Ishizuka, M. Ito, K. Ito, M. Ito, T. Ito, Y. Iwaki, T. Iwasa, T. Izawa, Y. Izui, K.
557 557 225 499 391 661 669 467 525 601,605
J Jeon, J.-K. Jermann, B. Ji, X. Jun, K.-W. Jung, M.-H. Jung, S.M.
505 107 201 345, 447 447 403
I Iantovski, E. Ibrahim, M. Ibusuki, T. Ichihashi, Y. Ichikawa, K. Ichikuni, N. Ifuku, K. Igarashi, Y. Iglesia, E. Ihara, Y. Ihm, S.-K. Ikeda, S. Ikeuchi, M. Ikeue, K. Imai, T. Imanaka, T. Inoue, S. Inoue, T. Inoue, Y. Inui, H. Inui, M. Inui, T. Isawa, J. I shida, N. Ishihara, J. Ishihara, T. Ishii, M. Ishikawa, K. Ishimaru, S.
279 309 557 177, 561 309 455 249 613 339 243 505 225 653 177 545 467 503 601 653 621 593,597 513,537 391 503 557 147, 499 613 249 573
Kawabata, S. Kawabe, M. Kawahira, K. Kawasaki, S. Kawashima, A. Kawata, M. Kedzierzawski, P. Kieffer, R. Kihara, S. Kim, D.-S. Kim, H. Kim, W.Y. Kishida, M. Kishimoto, H. Kitagawa, R. Kitamura, T. Kitayama, Y. Kobayashi, H. Kobayashi, M. Koch, H.J. Kodama, Tatsuya Kodama, Tohru Koga, O. Kogelschatz, U. Koike, K. Komori, M. Kosugi, T. Kosugi, Y. Kunimori, K.
K Kabeya, H. Kai, Y. Kanai, S. Kanda, Y. Karube, I.
479 601 237 249 625, 637, 641, 645 309 371 231 561 261,451 633, 63 7, 645 107 87, 327 189 509 345, 407 387 411 391 479 439 383 391 529 1 383 613 569 541 557 261, 451 367 487 455
689 Kunito, S. Kurano, N. Kuroda, K. Kuroda, T. Kusakabe, K. Kusama, H. Kushnirov, V.
649, 653,657 55 545 665 665 431,455, 529 279
Moil, H. Moil, K. Morikawa, M. Morita, M. Morooka, S. Murakami, M. Murakoshi, K. Muranaka, T.
183 357 467 483 665 315, 629 183 315, 629
L Lee, C.-S. Lee, D.-K. Lee, J.K. Lee, K.-W. Lee, S.B. Lee, S.-J. Li, X. Li, X.-L. Li, Z. Liu, B.-J. Liu, D. Liu, S.-M. Ljungdahl, L.G. Llorca, J. Luo, S.
5O9 505, 509 403 345, 407,447 471 345 375 303 201 553 201 303 303 153 267, 549
N Nagahisa, K. Nagano, T. Nagao, Y. Nagara, Y. Nagata, H. Nagata, T. Nagatani, M. Nakahara, Y. Nakai, T. Nakajima, H. Nakamura, T. Nakamura, Y. Nakano, H. Nakata, K. Nakato, Y. Nakayama, M. Nam, S.-S. Namiki, T. Nanba, M. Neacsu, M. Nezuka, M. Nicholas, K.M. Nishide, T. Nishiguchi, H. Noda, H. Nogami, G. Nomura, N. Nonoura, N.
467 463 669 601 411 601 661 517 243 459 605 565 237 309 565 207 407 9 633,637 195 371 165 315, 629 147 225 573 427 593
O Ochiai, T. Ogata, T. Ogura, K. Ogushi, Y. Ohkuma, K. Ohtani, M. Okabe, K. Okamoto, A. Okawa, M. Onizuka, T. Ono, H. Onoue, K. Otsuki, T.
613 97 207 649, 657 415 463 431,455, 529 525 585, 589 653 371 411 479
M
Mabuse, H. Maeda, K. Maeda, Y. Maezawa, S. Mascetti, J. Masuda, K. Mathieu, Ph. Matoba, R. Matsuguma, K. Matsukawa, R. Matsumura, H. Matsumura, Y. Matsuo, T. Matsuyama, K. McCauley, K. McGill, J.W. Metcalfe, I .S. Mimura, N. Mimura, T. Mishima, K. Mitsuoka, S. Miura, S. Miyachi, S. Miyasaka, H. Miyashita, Y. Momma, K. Moncmanov& A. Moon, J.Y.
521 189 231 379 677 649 279 475 249 625, 637, 641, 645 601 423 569 661 491 491 351 415, 419 669 661 669 383 55 237 147 593,597 291 403
690 P
Pak, P.S. Park, C.B. Park, D.R. Park, D.W. Park, K.-C. Park, S.-E.
297,367 471 177 403 505 177,387,395, 561 505 255
Park, Y.-K. Poliakoff, M. Preville, M.A. Prince, R.B.
491
Q Quaranta, E.
677
1
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Rama Rao, K.S. Ramfrez de la Pisca, P. Rao, T.N. Renner, M.W. Rice, G.W. Riedel, T. Rodrfuez-Ramos, I. Roh, H.-S. Roh, J.H.
447 153 31 97 339 159, 443 399 395 597
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Sadaie, M. Sahibzada, M. Saijou, M. Saiki, H. Saito, M. Sakai, N. Sakata, Tadayoshi Sakata, Takao Sales, J. Samejima, Y. Sano, H. Sasaki, T. Sasaki, Y. Sato, F. Sayama, K. Scarlat, N. Schaub, G. Schulz, H. Shapovalova, L.B. Shi, M. Shiga, M. Shikanai, T. Shima, T. Shimamura, K. Shimoide, A.
463 351 463 483 267, 357, 419, 495, 549 641 577 183 153 649, 657 273,285, 55 495 249 431,455, 195 159, 443 159, 443 171 165 249 609 503 261,451 609
415, 521,
Shinoyama, H. Shiozaki, S. Shiratsuchi, R. Shishido, Y. Shlygina, I.A. Smidsrod, O. Soga, K. Soled, S.L. Sone, Y. Sonoyama, N. Souma, Y. Specht, M. Staiss, F. Suda, T. Sugimoto, H. Suh, I.S. Sun, X-Z. Susuki, J. Sutterer, A. Suzuki, H. Suzuki, T. Suzuki, Yasuo Suzuki, Yutaka T Tabata, K. Tabata, M. Tagawa, T. Takagawa, M. Takahara, I. Takahashi, K. Takeda, T. Takeguchi, T. Takeuchi, K. Takeuchi, Masami Takeuchi, Masato Takimoto, Y. Takishita, S. Takita, Y. Tamaura, Y.
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87 297 657 613 653 357
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693 S T U D I E S IN SURFACE SCIENCE A N D CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume
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Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.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 Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings ofthe 32nd International Meeting ofthe Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, u Ben Taarit, 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.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.l. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Ldzni~ka 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, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties- Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third 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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Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. 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 P.A. Jacobs, N.I. Jaeger, P.JinX, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit 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 byJ. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~evar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the 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. Cerveny New Developments in Zeolite Science and Technology. 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 and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. 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.F. Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine
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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings ofthe 10th North American Meeting ofthe 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., April 26-29,1987 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. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary ofthe Catalysis Society of Japan edited byT. Inui Transition Metal Oxides. Surface Chemistry and Catalysis byH.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wfirzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T, 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. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings ofthe 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 Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura
696 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals I1. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, 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 and T. Tatsumi Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Characterization of Porous Solids I1. Proceedings of the IUPAC Symposium Volume 62 (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of Catalysts V. 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, P.A. Jacobs, P. Grange and B. Delmon New Trends in CO Activation Volume 64 edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. (Shlmann, 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, Balatonf0red, September 10-14, 1990 edited by L.I. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13, 1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Experiments by M. Kiskinova Volume 55
697 Volume 71 Volume 72
Volume 73 Volume 74 Volume 75 Volume 76 Volume 77
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Volume79 Volume80 Volume81 Volume82
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Volume85 Volume86 Volume 87
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 ofthe 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 Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings ofthe Third 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 ofthe 3rd International Symposium, Poitiers, April 5-8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third 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 Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich 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 II1. Proceedings of the IUPAC Symposium (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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Volume90 Volume91
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Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis I1. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. 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 and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by u Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F. Vansant, P. Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, I-lungary, 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 II1. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool 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. Dajbrowski and V.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 1lth International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 1lth ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J. W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon, S.I. Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzifiski, W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 1lth International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon, S.-K. Ihm and Y.S. Uh
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Volume 113
Volume 114
Hydrotreatment and Hydrocracking of Oil Fractions Proceedings ofthe 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment, B. Delmon and P. Grange 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 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. Balker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium, Antwerp, Belgium, September 15-17, 1997 edited by G.F. Froment and K.C. Waugh Third World Congress on Oxidation Catalysis. Proceedings of the Third World 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 Catalyst Deactivation 1997. Proceedings ofthe 7th International Symposium, Cancun, Mexico, October 5-8, 1997 edited by C.H. Bartholomew and G.A. Fuentes 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 Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 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 Advances in Chemical Conversions for Mitigating Carbon Dioxide Proceedings of the Fourth International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997 edited by 1".Inui, M. Anpo, K. Izui, S. Yanagida and T. Yamaguchi
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