摘要:

Contents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes. A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M.Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. PlonkaContents 3663 3669 3675 3683 3693 370 1 3709 3717 3725 3737 Normal and Abnormal Electron Spin Resonance Spectra of Low-spin Cobalt(r1) IN,]-Macrocyclic Complexes.A Means of Breaking the Co-C Bond in B12 Co-enzyme M. Green, J. Daniels and L. M. Engelhardt The Interaction between Superoxide Dismutase and Doxorubicin. An Electron Spin Resonance Approach V. Malatesta, F. Morazzoni, L. Pellicciari-Bollini and R. Scotti Biomolecular Dynamics and Electron Spin Resonance Spectra of Copper Complexes of Antitumour Agents in Solution. Part 2.-Rifamycins R. Basosi, R. Pogni, E. Tiezzi, W. E. Antholine and L. C. Moscinsky An Electron Spin Resonance Investigation of the Nature of the Complexes formed between Copper(I1) and Glycylhistidine D. B. McPhail and B. A. Goodman A Vibronic Coupling Approach for the Interpretation of the g-Value Temperature Dependence in Type-I Copper Proteins M. Bacci and S. Cannistr aro The Electron Spin Resonance Spectrum of Al[C,H,] in Hydrocarbon Matrices J. A. Howard, B. Mile, J. S. Tse and H. Morris N; and (CN); Spin-Lattice Relaxation in KCN Crystals H. J. Kalinowski and L. C. Scavarda do Carmo Single-crystal Proton ENDOR of the SO, Centre in y-Irradiated Sulphamic Acid N. M. Atherton, C. Oliva, E. J. Oliver and D. M. Wylie Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice 1,. Part 1.-The 0- Radicals Single-crystal Electron Spin Resonance Studies on Radiation-produced Species in Ice I,. Part 2.-The HO, Radicals J. Bednarek and A. Plonka J. Bednarek and A. Plonka

ISSN:0300-9599    

DOI:10.1039/F198783FX037

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P. N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J.F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S.P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P.N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the Institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 12 January 1988 Further information from Mrs Y.A. Fish, Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the Institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex l w 1 1 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R.Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 40E Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London W1V OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 302 (xiii)

ISSN:0300-9599    

DOI:10.1039/F198783BX039

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 83(10) 3092-3228 (1 987) JOURNAL OF THE CHEMICAL SOCIETY 3093 3107 31 15 3129 3139 3149 3161 3167 3177 3189 3199 3207 3223 Faraday Transactions I Physical Chemistry in Condensed phases CONTENTS The Crystal Structure of Sodium Diheptylsulphosuccinate Dihydrate and Comparison with Phospholipids Doping Effect of Sodium on y-Irradiated Magnesium Oxide T. Matsuda, K. Yamada, Y. Shibata, H. Miura and K. Sugiyama Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part &-The Investigation of Reduced H,(SiW,,O,,) xH,O and Ag,(SiW,,O,,). xH,O and.. the Effects of Oxygen Adsorption R. Fricke, H-G. Jerschkewitz and G. Ohlmann The Ethane- 1,2-dio1-2-Methoxyethanol Solvent System. The Dependence of the Dissociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Kinetics and Mechanism of Oxidative Dehydrogenation of Ethane and Small Alkanes with Nitrous Oxide over Cobalt-doped Magnesium Oxide K.Aika, M. Isobe, K. Kido, T. Moriyama and T. Onishi Temperature-programmed Desorption Study of the Interactions of H,, CO and CO, with LaMnO, L. G. Tejuca, A.T. Bell, J. L. G.Fierro and J. M.D. Tasc6n An X-Ray Photoelectron Spectroscopy Study of the Influence of Hydrogen on the Oxygen-Silver Interaction L. Lefferts, J. G. van Ommen and J. R. H. Ross Binary Systems of 1,2-Dichloroethane with Benzene, Toluene, p-Xylene, Quinoline and Cyclohexane. Part 3 .-Dielectric Properties and Refractive Indices at 308.15 K Radical Spectra and Product Distribution following Electrophilic Attack by the OH' Radical on 4-Hydroxybenzoic Acid and Subsequent Oxidation R. F.Anderson, K. B. Pate1 and M. R. L. Stratford Control of Ni Metal Particle Size in Ni/SiO, Catalysts by Calcination and Reduction Temperatures H. Tamagawa, K. Oyama, T. Yamaguchi, H. Tanaka, H. Tsuiki and A. Ueno Neutron Spectroscopic Study of Polycrystalline Benzene and of Benzene adsorbed in Na-Y Zeolite H. Jobic, A. Renouprez, A. N. Fitch and H. J. Lauter Hydrogen- 1 Nuclear Magnetic Resonance, Differential Thermal Analysis, X- Ray Powder Diffraction and Electrical Conductivity Studies on the Motion of Cations, including Self-diffusion in Crystals of Propylammonium Chloride and Bromide as well as their N-Deuterated Analogues S. Fukada, H. Yamamoto, R. Ikeda and D. Nakamura" Reviews of Books K.C. Waugh; R. Rudham; C. Price; P. Day; M. W. Roberts; J. F. Griffiths; C. F. Cullis; N. M. Atherton J. Lucassen and M. G. B. Drew G. C. Franchini, L. Tassi and G. Tosi J. Nath" and G. Singh 102 FAR IISSN 0300-9599 JCFTAR 83(10) 3092-3228 (1 987) JOURNAL OF THE CHEMICAL SOCIETY 3093 3107 31 15 3129 3139 3149 3161 3167 3177 3189 3199 3207 3223 Faraday Transactions I Physical Chemistry in Condensed phases CONTENTS The Crystal Structure of Sodium Diheptylsulphosuccinate Dihydrate and Comparison with Phospholipids Doping Effect of Sodium on y-Irradiated Magnesium Oxide T. Matsuda, K. Yamada, Y. Shibata, H. Miura and K. Sugiyama Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids. Part &-The Investigation of Reduced H,(SiW,,O,,) xH,O and Ag,(SiW,,O,,).xH,O and.. the Effects of Oxygen Adsorption R. Fricke, H-G. Jerschkewitz and G. Ohlmann The Ethane- 1,2-dio1-2-Methoxyethanol Solvent System. The Dependence of the Dissociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Kinetics and Mechanism of Oxidative Dehydrogenation of Ethane and Small Alkanes with Nitrous Oxide over Cobalt-doped Magnesium Oxide K. Aika, M. Isobe, K. Kido, T. Moriyama and T. Onishi Temperature-programmed Desorption Study of the Interactions of H,, CO and CO, with LaMnO, L. G. Tejuca, A.T. Bell, J. L. G.Fierro and J. M.D. Tasc6n An X-Ray Photoelectron Spectroscopy Study of the Influence of Hydrogen on the Oxygen-Silver Interaction L. Lefferts, J. G. van Ommen and J. R.H. Ross Binary Systems of 1,2-Dichloroethane with Benzene, Toluene, p-Xylene, Quinoline and Cyclohexane. Part 3 .-Dielectric Properties and Refractive Indices at 308.15 K Radical Spectra and Product Distribution following Electrophilic Attack by the OH' Radical on 4-Hydroxybenzoic Acid and Subsequent Oxidation R. F. Anderson, K. B. Pate1 and M. R. L. Stratford Control of Ni Metal Particle Size in Ni/SiO, Catalysts by Calcination and Reduction Temperatures H. Tamagawa, K. Oyama, T. Yamaguchi, H. Tanaka, H. Tsuiki and A. Ueno Neutron Spectroscopic Study of Polycrystalline Benzene and of Benzene adsorbed in Na-Y Zeolite H. Jobic, A. Renouprez, A. N. Fitch and H. J. Lauter Hydrogen- 1 Nuclear Magnetic Resonance, Differential Thermal Analysis, X- Ray Powder Diffraction and Electrical Conductivity Studies on the Motion of Cations, including Self-diffusion in Crystals of Propylammonium Chloride and Bromide as well as their N-Deuterated Analogues S. Fukada, H. Yamamoto, R. Ikeda and D. Nakamura" Reviews of Books K. C. Waugh; R. Rudham; C. Price; P. Day; M. W. Roberts; J. F. Griffiths; C. F. Cullis; N. M. Atherton J. Lucassen and M. G. B. Drew G. C. Franchini, L. Tassi and G. Tosi J. Nath" and G. Singh 102 FAR I

ISSN:0300-9599    

DOI:10.1039/F198783FP129

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II, Issue1 0,1987 Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions [I, Issue 10, is reproduced below. This issue contains the proceedings of Faraday Symposium 22 on Interaction-induced Spectra in Dense Fluids and Disordered Solids, held in Cambridge in December 1986. 1743 General Introduction A. D. Buckingham 175 1 Depolarized Interaction-induced Light Scattering in Dense Phases U. Bafile, L. Ulivi, M. Zoppi and F. Barocchi 1759 Structural Slowing Down and Depolarized Light Spectra in Dense Noble-gas Fluids 1 765 Dipole-induced Static and Dynamic Liquid Structures A. Gerschel 1777 Density Effects on Collision-induced Spectra in Fluids J. Jonas 179 1 Molecular Dynamics Study of Intercollisional Interference in Collision- induced Absorption in Compressed Fluids R.D. Mountain and G. Birnbaum 1801 Far-infrared Interaction-induced Spectra of the Halogens J. Yarwood and B. Catlow 18 15 Density Dependence of Interaction-induced Scattering Contributions to the Depolarized Rayleigh Band of Ethane H. Versmold and U. Zimmermann 1825 Non-linear Optical Spectroscopy, Ultrafast Dynamics and Quantum Transport in Disordered Media G. A. Kenney-Wallace, T. Dickson and M. Golombok 1843 A Simulation Study of the Induced Infrared Absorption in Liquid CO, W. A. Steele and H. Posch 1 859 Collision-induced Light Scattering from Growing Clusters. Depolarization by Fractals 1867 Infrared and Raman Spectra of Hexagonal Ice in the Lattice-mode Region M.Marchi, J. S. Tse and M. L. Klein 1875 Interaction-induced Spectra of Molten Alkali-metal and Alkaline-earth Halides R. L. McGreevy 1891 Light Scattering by Liquid and Solid Sodium Chloride. A Simulation Study P. A. Madden and J. A. Board 1909 The Interaction-induced Q-Branch A. I. Burshtein I92 I General Discussion 1939 Index of Names 1940 List of Posters I. M. de Schepper and A. A. van Well T. Keyes, G. Seeley, P. Weakliem and T. Ohtsuki612028 612100 71040 71 139 71240 71341 71369 71388 71436 71486 71544 71557 7/65 1 71653 717 16 71717 71726 71747 The following papers were accepted for publication in Faraday Transactions I during July 1987 : Interfacial Tensions and Microemulsion Formation in Heptane-Aqueous NaCl Systems containing AOT and SDS R.Aveyard, B. P. Binks and J. Mead Multicomponent Ion Exchange in Zeolites. Part 3.-Equilibrium Properties of the Sodium/Potassium/Cadmium-Zeolite-X System K. R. Franklin and R. P. Townsend I.R. Dielectric Response of UO, P. A. Thiry, J. Pireaux and R. Caudano Thermal Decomposition of Gamma-irradiated Silver Malonate A. K. Galwey, P. J. Herley and M. Abel-Aziz Mohamed Monolayer Adsorption of Non-spherical Molecules on Solid Surfaces. Part 2.-Adsorption of Nitrogen on the Structureless Surface of Graphite J. Penar and S. Sokolowski Dehydrogenation of Alcohol on Hydride-forming Rare Intermetallic Com- pounds (RFe, and R,Co,) H. Imamura, S. Kasahara, T. Katada and S. Tsuchiya Raman Spectra of Adsorbed Aniline on an Ag Electrode in Acidic Solutions H.Shindo and C. Nishihara Interpretation of Free Energies of Transfer and Solute Partial Molar Volumes to Mixed Solvents using Kirkwood-Buff Theory Electrochemical Regeneration of NAD: A New Evaluation of its Actual Yield J. Bonnefoy, J. Moiroux, J-M. Lava1 and C. Bourdillon Domain Complexions in Capillary Condensation. Part 1 .-The Ascending Boundary Curve Solvation Thermodynamics of Ethidium in Mixed Solvents G. Varani, G. Chirico and G. Baldini Modification of y-Alumina by Barium and Lanthanum and the Consequential Effect of the Reducibility and Dispersibility of Nickel S. Narayanan and K. Uma Water Dynamics and Aggregate Structure in Reversed Micelles at Sub-zero Temperatures. A Deuteron Spin Relaxation Study Molar Gibbs Energies of Transfer for Cu2+, Zn2+, Cd2+ and Pb2+ G.Gritzner Stability Criteria for Charged Interfaces and their Role in Double-layer Theory D. G. Hall A Thermodynamic Analysis of Common Intersection Points in Potentio- metric Titration Studies of Solid Surfaces D. G. Hall The Influence of Water on the Oxygen-Silver Interaction and on the Oxidative Dehydrogenation of Methanol L. Lefferts, J. G. van Ommen and J. R. H. Ross Charge Trgnsfer Band of N-Alkylpyridinium Iodides in Mixed Aqueous Solvents K. E. Newman V. Mayagoitia, F. Rojas and I. Kornhauser P. Quist and B. Halle K. Medda, M. Pal and S. Bagchi (ii)71769 71777 71797 71817 7/82 1 71822 71867 71946 71970 71999 71 1042 71 1044 Preferential Solvation of Europium(rI1) Ion in Water-Non-aqueous Solvent Mixtures. A Luminescence Lifetime Study F.Tanaka, Y. Kawasaki and S. Yamashita Selective Conversion of Methanol into Aromatic Hydrocarbons over Zinc- exchanged ZSM-5 Zeolites Y. Ono, H. Adachi and Y. Sendoda Activation and Chain Carrying CH, Species for Terminal Alkene Metathesis on Molybdena-Titania Catalysts Practical Limitations of Polyacetylene used as a High Power Density Cathode J. B. Schlenoff and J. C. W. Chien Electron Energy-loss Spectroscopy and the Crystal Chemistry of Rhodizite. Part 1 .--Instrumentation and Chemical Analysis W. Engel, H. Sauer, R. Brydson, B. G. Williams, E. Zeitler and J. M. Thomas Electron Energy-loss Spectroscopy and the Crystal Chemistry of Rhodizite. Part 2.-Near-edge Structure W. Engel, R. Brydson, B. G. Williams, E. Zeitler, H. Sauer, Th. Lindner, R. Schlogl, M.Muhler and J. M. Thomas Differential Thermo-osmotic Permeability in Water-Cellophane Systems C. Fernandez-Pineda and I. Vazquez-Gonzalez Domain Complexions in Capillary Condensation. Part 2.--Descending Boundary Curve and Scanning V. Mayagoitia, B. Gilot, F. Rojas and I. Kornhauser Rotational Isomerism in 1,2-Dinitro- 1,2-diphenylethane and 2,3-Dinitro-2,3- diphenylbutane L. H. L. Chia, B. G. Tan and H. H. Huang Electron Spin Resonance Studies of the *CS; Radical Anion and its Conjugate Acid J. S. Lea and M. C. R. Symons Dynamic Studies of the Interaction between Diols and Water by Ultrasonic Methods. Part 4.-3-Methylbutane- 1,3-diol and 2,2-Dimethylpropane- 1,3- diol Solutions Difference in the Thermal and Mechanochemical Polymorphism. Effects of Impurity on the System Aragonite-Calcite K.Tanaka and K. Tanaka S. Nishikawa, N. Nakayama and N. Nakao T. Isobe and M. Senna (iii)Cumulative Author Index 1987 Abraham, M. H., 2867 Agnel, J-P. L., 225 Aika, K., 3139 Akalay, I., 1137 Akasheh, T., 2525 Akitt, J. W., 1725 Albano, K., 2113 Alberti, A., 91 Albery, W. J., 2407 Alexandrova, I., 2841 Ali, A.-K. M., 2391 Allen, G. C., 925, 1355 Amorebieta, V. T., 3055 Andersen, A., 2140 Anderson, A. B., 463 Anderson, J. B. F., 913 Anderson, R. F., 3177 Antholine, W. E., I51 Ardizzone, S., 1159 Arias, S., 2619 Arriaga, P., 2705 Atay, N. Z., 2407 Atherton, N. M., 37, 941, 3227 Aun Tan, S., 2035 Aveyard, R., 2347 Avnir, D., 1685 Axelsen, V., 107 Bahneman, D. W., 2559 Baker, B. G., 2136 Baldini, G., 1609 Ball, R. C., 2515 Balon, M., 1029 Barford, W., 2515 Barratt, M.D., 135 Barrer, R. M., 779 Bartok, M., 2359 Basosi, R., 151 Bastein, A. G. T. M., 2103, 2129 Bastl, Z., 51 1 Bateman, J. B., 841 Battesti, C. M., 225 Baussart, H., 1711 Becker, K. A., 535 Beezer, A. E., 2705 Bell, A. T., 2061, 2086, 2087, Bennett, J.E., 1805 Bennett, J. E., 2421, 2433 Berclaz, T., 401 Berleur, F., 177 Bernal, S., 2279 Berroa de Ponce, H., 1569 Berry, F. J., 615, 2573 Bertagnolli, H., 687 Berthelot, J., 231 Au, C-T., 2047 2088, 3149 Beyer, H. K., 511 Bianchi, H., 3027 Bianconi, A., 289 Binks, B. P., 2347 Bird, R., 3069 Bjorklund, R. B., 1507 Blandamer, M. J., 559, 865, 1783, 3039 Blyth, G., 751 Boerio-Goates, J., 1553 Bogge, H., 2157 Bond, G. C., 1963, 2071, 2088, 2129, 2130, 2133, 2138, 2140 Borbely, G., 511 Botana, F.J., 2279 Boucher, E. A., 1269 Brandreth, B. J., 1835 Brandt, B. A,, 2857 Braquet, P., 177 Brazdil, J. F., 463 Breault, R., 2119 Brede, O., 2365 Brillas, E., 2619, 2813 Briscoe, B. J., 938 Bruce, J. M., 85 Brunton, G., 2421, 2433 Brustolon, M., 69 Brycki, B., 2541 Budil, D. E., 13 Bugyi, L., 2015 Bulow, M., 1843 Burch, R., 913, 2087, 2130, 2134, 2135, 2141, 2250 Burgess, J., 559, 865, 1783, 3039 Burggraaf, A. J., 1485 Burke, L. D., 299 Busca, G., 853, 1591, 2213 Buscall, R., 873 Butt, J. B., 2757 Cairns, J. A,, 913 Care, C. M., 2905 Carley, A. F., 351 Caro, J., 1843, 2301 Carthy, G., 2585 Cassidy, J. F., 231 Celalyan-Berthier, A., 401 Chadwick, D., 2227 Chalker, P. R., 351 Chandra, H., 759 Chengyu, W., 2573 Chieux, P., 687 Chinchen, G. C., 2193 Chittofrati, A., 1159 Choudhery, R.A., 2407 Christensen, P. A., 3001 Christmann, K., 1975 Chu, D-Y., 635 Chu, G., 2533 Chudek, J. A., 2641 Clark, B., 865 Clausen, B. S., 2157 Clifford, A. A., 751 Coates, J. H., 2697, 2751 Colin, A. C., 819 Coller, B. A. W., 645, 657 Coluccia, S., 477 Colussi, A. J., 3055 Compostizo, A., 819 Compton, R. G., 1261 Conway, B. E., 1063 Copperthwaite, R. G., 2963 Corti, H. R., 3027 Corvaja, C., 57 Costa, J. M., 2619 Cottrell, M. R., 3039 Couillard, C., 125 Courbon, H., 697 Craven, J. B., 779 Crossland, W. A., 37 Cullis, C. F, 3227 Cummins, P. G., 2773 Cunningham, J., 2973 Czarnetzki, L., 3015 D’Alba, F., 267 Daniel de Namor, A. F., 2663 Danil de Namor, A. F., 1569 Darvell, B. W., 2953 Dash, A. C., 1307, 2505 Dash, N., 2505 Daverio, D., 705 Davies, M.J., 1347 Davoli, I., 289 Dawber, J. G., 771 Day, P., 3225 de Beer, V. H. J., 2145 De Doncker, J., 125 De Laet, M., 125 De Ranter, C. J., 257 Declerck, P. J., 257 Dega-Szafran, Z., 2541 Delafosse, D., 1137 Delahanty, J. N., 135 Delobel, R., 1711 Despeyroux, B. M., 2081, 2139, 2171, 2243, 2255 Di Lorenzo, S., 267 Diaz Peiia, M., 819 Dimitrijevid, N. M., 1193 Dimov, A. D., 2841 Dodd, N. J. F., 85 Doddridge, B. G., 2697 Domen, K., 2765AUTHOR INDEX Dongbai, L., 2573 Drew, M. G. B., 3093 Du, J., 2671 Duarte, M. A., 2133 Duce, P. P., 2867 Ducret, F., 141 Dudikova, L., 51 1 Dusaucy, A-C., 125 Dutkiewicz, E., 2847 Eicke, H.-F., 1621 Elbing, E., 645, 657 Elders, J. M., 1725 Empis, J. M. A,, 43 Endoh, A., 411 Engberts, J. B. F. N., 865 Evans, J. C., 43, 135 Fahim, R. B., 1601 Fan, G., 323 Fatome, M., 177 Feakins, D., 2585 Fejes, P., 1109 Fernandez-Prini, R., 3027 Fierro, J.L. G., 3149 Fischer, C-H., 2559 Fitch, A.N., 3199 Fletcher, P. D. I., 985, 1493 Flint, N. J., 167 Formaro, L., 1159 Formosinho, S. J., 431 Forrester, A. R., 211 Forste, C., 2301 Forster, H., 1109 Foster, R., 2641 Fouassier, J. P., 2935 Fox, G. G., 2705 Fraissard, J., 451 Franchini, G. C., 3129 Freude, D., 1843 Freund, E., 1417 Fricke, R., 1041, 3115 Fujii, K., 675 Fujitsu, H., 1427 Fukada, S., 3207 Funabiki, T., 2883, 2895 Galli, P., 853 Gampp, H., 1719 Gao, Y., 2671 Garbowski, E., 1469 Garcia, R., 2279 Garrido, J., 1081 Garrido, J. A., 2813 Garrone, E.. 1237 Gellings, P. J., 1485 Geoffroy, M., 401 Germanus, A., 2301 Gervasini, A,, 705, 2271 Giddings, S., 2317, 2331 Gilbert, B.C., 77 Gilbert, R. G., 1449 Goates, J. R., 1553 Goates, S. R., 1553 Goffredi, M., 1437 Golding, P. D., 1203, 2709, Goodman, D. W., 1963, 1967, 2719 2071, 2072, 2073, 2075, 2082, 2086, 2251 Goralski, P., 3083 Gottschalk, F., 571 Gozzi, D., 289 Grampp, G., 161 Grant, R. B., 2035 Gratzel, M., 1101 Grauer, G. L., 1685 Grauer, Z., 1685 Gray, P., 751 Greci, L., 69 Greenwood, P., 2663 Grieser, F., 591 Griffiths, J. F, 3226 Grigorian, K. R., 1189 Grimblot, J., 2170 Grossi, L., 77 Groves, G. S., I1 19, 1281 Grzybkowski, W., 281, 1253, Gu, T., 2671 Guardado, P., 559 Guilleux, M.-F., 1137 Gunasekara, M. U., 2553 Hada, H., 1559 Hagele, G., 1055 Hakin, A. W., 559, 865, 1783, Halawani, K. H., 1281 Hall, D. G., 967 Hall, D. I., 2693 Hall, M. V. M., 571 Haller, G.L., 1965, 2072, 2080, 2089, 2091, 2129, 2131, 2132, 2133, 2135, 2136, 2137, 2138, 2243 226 1 3039 Halpern, A., 219 Hamada, K., 527 Hanke, V.-R., 2847 Harbach, C. A. J., 2035 Harendt, C., 1975 Harland, R. G., I261 Harrer, W., 161 Harriman, A., 3001 Harris, R. K., 1055 Hartland, G. V., 591 Hasegawa, A., 759, 2803 Hatayama, F., 675 Haul, R., 2083 Hayashi, K., 1795 Hayashi, O., 3061 Hayter, J. B., 2773 Healy, C. P., 2973 Heatley, F., 517, 2593 Hemminga, M. A., 203 Henglein, A., 2559 Henriksson, U., I515 Hermann, R., 2365 Herold, B. J., 43 Hertz, H. G., 687 Hidalgo, J., 1029 Higgins, J. S., 939 Hikmat, N. A., 2391 Hilfiker, R., 1621 (v) Hill, W., 2381 Hinderman, J. P., 21 19, 2142, Hindermann, J. P., 21 19 Holden, J. G., 615 Holloway, S., 1935 Hongzhang, D., 2573 Hounslow, A.M., 2459, 2697 Howarth, J. N., 2787, 2795 Howe, A. M., 985, 1007 Howe, R. F., 813 Hudson, A., 91 Hunger, M., 1843 Hunter, R., 571 Hunter, W. H., 2705 Hussein, F. H., 1631 Hutchings, G. J., 571, 2963 Ichikawa, K., 2925 Ikeda, R., 3207 Ikeyama, N., 1427 Imamura, H., 743 Imanaka, T., 665 Inoue, Y., 3061 Ishikawa, T., 2605 Ismail, H. M., 1601, 2835 Isobe, M., 3139 Isozumi,Y., , 2895 Ito, T., 451 Iwaki, T., 943, 957 Iwamoto, E., 1641 Jackson, S. D., 905, 1835 Jaenicke, W., 161, 2727 Janata, E., 2559 Janes, R., 383 JenE, F., 2857 Jerschkewitz, H-G., 31 15 Jobic, H., 3199 Jones, P., 2735 Joyal, C. L. M., 2757 Joyner, R. W., 1945, 1965, 2074, 2085, 2138, 2249 Juszczyk, W., 1293 Kakuta, N., 1227, 2635 Kameda, Y., 2925 Kaneko, M., 1539 Kanno, T., 721 Kapturkiewicz, A., 2727 Karger, J., 1843, 2301 Kariv-Miller, E., 1169 Karpinski, Z., 1293 Katime, I., 2289 Kato, C., 1851 Kawaguchi, T., 1579 Kazansky, V.B., 2381 Kazusaka, A., 1227, 2635 Kemball, C., 3069 Kerr, C W., 85 Kido, K., 3139 Kiennemann, A., 21 19 King, D. A., 1966, 2001, 2079, 2080, 2081 Kinoshita, N., 2765 Kira, A., 1539 Kiricsi, I., 1109 Kitagawa, H., 2913 2143AUTHOR INDEX Kitaguchi, K., 1395 Kiwi, J., 1101 Klein, J., 1703 Klinszporn, L., 226 1 Klofutar, C., 2311 Knoche, W., 2847 Knozinger, H., 2088, 2171 Kobayashi, J., 1395 Kobayashi, M., 721 Koda, S., 527 Kondo, Y., 1089 Konishi, Y., 721 Koopmans, H. J. A., 1485 Kordulis, C., 627 Korf, S . J., 1485 Korth, H-G., 95 Koutsoukos, P. G., 1477 Kowalak, S., 535 Kristyan, S., 2825 Kubelkova, L., 511 Kubokawa, Y., 675, 1761 Kumamaru, T., 1641 Kuroda, K., 1851 Kusabayashi, S ., 1089 Kuzuya, M., 1579 La Ginestra, A., 853 Lackey, D., 2001 tajtar, L., 1405 Lambelet, P., 141 Lambert,'R. M., 1963, 1964, 2035, 2082, 2083, 2084 Lamotte, J., 1417 Lang, N. D., 1935 Laschi, F., 1731 Laurin, M., 2119 Lauter, H. J., 3199 Lavagnino, S., 477 Lavalley, J-C., 1417 Lawin, P. B., 1169 Lawrence, C., 2331 Lawrence, S . , 1347 Le Bras, M., 1711 Leach, H. F., 3069 Leaist, D. G., 829 Lecomte, C., 177 Lee, E. F. T., 1531 Lefferts, L., 3161 Lengeler, B., 2157 Lercher, J. A., 2080, 2255 Leroy, J-M., 1711 Letellier, P., 1725 Levin, M. E., 2061 Lima, M. C. P., 2705 Lin, C. P., 13 Lin, Y-J., 2091 Linares-Solano, A., 1081 Lincoln, S . F., 2459, 2697, 2751 Lindgren, M., 893, 1815 Lippens, B.C., 1485 Liu, R-L., 635 Liu, T., 1063 Liu, Y., 2993 Liwu, L., 2573 Loliger, J., 141 Lorenzelli, V., 853, 1591 Loretto, M. H., 615 Lougnot, D. J., 2935 Lucassen, J., 3093 Luckham, P. F., 1703 Lund, A., 893, 1815, 1869 Luo, H., 2103 Lycourghiotis, A., 627, 1179 Lynch, J., 1417 Lyons, C. J., 645 Lyons, M. E. G., 299 Machin, W. D., 1203, 2709, Makkowiak, M., 2541 MacLaren, J. M., 1945, 1965 Maestre, A., 1029 Maezawa, A., 665 Makela, R., 51 Manfredi, M., 1609 Maniero, A. L., 57, 69 Manzatti, W., 2213 Marchese, L., 477 Marcus, Y., 339, 2985 Mari, C. M., 705 Markarian, S . A., 1189 Martin Luengo, M. A., 1347, Martin-Martinez, J. M., 1081 Mashkovsky, A. A., 1879 Masiakowski, J. T., 893, 1869 Masliyah, J. H., 547 Matralis, H., 1179 Matsuda, T., 3107 Matsuura, H., 789 Maxwell, I.A., 1449 McAleer, J. F., 1323 McCarthy, S. J., 657 McDonald, J. A., 1007 McLauchlan, K. A,, 29 Mead, J., 2347 Mehandru, S . P., 463 Mehnert, R., 2365 Mehta, G., 2467 Mkriadeau, P., 2140 Miriaudeau, P., 2 1 13 Merwin, L. H., 1055 Micic, 0. I., 1127 Mijin, A., 2605 Mills, A., 2317, 2331, 2647 Mintchev, L., 2213 Miura, H., 3107 Miyahara, K., 1227 Miyata, H., 675, 1761, 1851 Mochida, I., 1427 Molina-Sabio, M., 1081 Monk, C . B., 425 Montagne, X., 1417 Morazzoni, F., 705, 2271 Morimoto, T., 943, 957 Moriyama, T., 3139 Morris, J. J., 2867 Moseley, P. T., 1323 Mosseri, S . , 3001 Moyes, R. B., 905 Mozzanega, M-N., 697 Miiller, A., 2157 2719 1651 (vi) Muiioz, M. A., 1029 Nabiullin, A. A., 1879 Naccache, C., 2113 Nagao, M., 1739 Nagaoka, T., 1823 Nair, V., 487 Naito, S ., 2475 Nakai, S . , 1579 Nakajima, T., 1315 Nakamura, D., 3207 Nakata, M., 2449 Napper, D. H., 1449 Narayanan, S., 733 Narducci, D., 705 Nath, J., 3167 Nayak, R. C., 1307 Nazer, A. F. M., 1119 Nebuka, K., 2605 Nedeljkovic, J. M., 1127 Nenadovic, M. T., 1127 Neta, P., 3001 Niccolai, N., 1731 Niemann, W., 2157 Nishida, S., 1795 Nogaj, B., 2541 Nomura, H., 527 Nomura, M., 1227, 1779, 2635 Norris, J. 0. W., 1323 Norris, J. R., 13 Nerrskov, J. K., 1935 Notheisz, F., 2359 Nukui, K., 743 Nuttall, S . , 559 O'Brien, A. B., 371 Ochoa, J. R., 2289 Odinokov, S. E., 1879 Ogura, K., 1823 Ohlmann, G., 31 15 Ohno, M., 1559 Ohno, T., 675 Ohshima, K., 789 Okabayashi, H., 789 Okamoto, Y., 665 Okubo, T., 2487, 2497 Okuda, T., 1579 Okuhara, T., 1213 O'Malley, P. J. R., 2227 Onishi, T, 2765, 3139 Ono, T., 675, 1761 Ono, Y., 2913 Otsuka, K., 1315 Ott, J.B., 1553 Oyama, K., 3189 Page, F., 2641 Paljk, S . , 231 1 Pallas, N. R., 585 Parry, D. J., 77 Patel, I., 2317, 2331 Patel, K. B., 3177 Patil, K., 2467 Patrono, P., 853 Peden, C. H. F., 1967 Pedersen, E., 2157 Pedersen, J. A., 107 Pedulli, G. F., 91AUTHOR INDEX Penar, J., 1405 Pendry, J. B., 1945 Penfold, J., 2773 Pkrez-Tejeda, P., 1029 Pethica, B. A., 585 Pethrick, R. A., 938 Pfeifer, H., 2301 Pichat, P., 697 Pielaszek, J., 1293 Pilarczyk, M., 281, 2261 Pilz, W., 2301 Pizzini, S., 705 Pletcher, D., 2787, 2795 Poels, E. K., 2140 Pogni, R., 151 Pomonis, P., 627 Pomonis, P. J., 1363 Ponec, V., 1964, 1965, 2071, 2072, 2074, 2083, 2103, 2136, 2138, 2139, 2244, 2251 Price, C., 3224 Primet, M., 1469 Prins, R., 2087, 2136, 2137, 2145, 2169, 2170, 2172 Priolisi O., , 57 Pritchard, J., 1963, 2085, 2249 Prugnola, A,, 1731 Puchalska, D., 1253 Purushotham, V., 2 1 1 Radulovic, S., 559 Rafi, J.J., 225 Rajaram, R. R., 2130 Ramaraj, R., 1539 Ramirez, F., 2279 Ramis, G., 1591 Rees, L. V. C., 1531, 1843 Renouprez, A., 3 199 Renyuan, T., 2573 Resasco, D. E., 2091 Reyes, P. N., 1347 Richards, D. G., 2138 Richoux, M-C., 3001 Richter-Mendau, J., 1843 Riley, B. W., 2140, 2253 Ritschl, F., 1041 Riva, A., 2213 Riviere, J. C., 351 Roberts, M. W., 351, 2047, Robinson, B. H., 985, 1007, Rodriguez, R. M., 2813 Rodriguez-Izquierdo, J. M., Rodriguez-Reinoso, F., 108 1 Rollins, K., 1347 Roman, V., 177 Romiio, M. J . , 43 Rooney, J. J., 2077, 2080, 2086, Ross, J. R.H., 3161 Rosseinsky, D. R., 231, 245 Rossi, C., 1731 Rowlands, C. C., 43, 135 Rubio, R. G., 819 2084, 2085, 2086, 2248, 3225 2407 2279 2089 Rudham, R., 1631, 3223 Sabbadini, M. G. C., 2271 Sakai, K., 2895 Sakai, T., 743, 1823 Sakakini, B., 1975 Sakata, Y., 2765 Sakurai, M., 2449 Salazar, F. F., 2663 Saleh, J. M., 2391 Salmeron, M., 2061 Salmon, T. M. F., 2421, 2433 Sanchez, M., 1029 Sanfilippo, D., 2213 Sangster, D. F., 657 Saraby-Reintjes, A., 271 Sato, K., 3061 Sato, T., 1559 Saucy, F., 141 Savoy, M-C., 141 Sayed, M. B., 1149, 1751, 1771 Scholten, J. J. F., 1966, 2073, Schuller, B., 2103 Seebode, J., 1109 Segal, M. G., 371 Segre, U., 69 Self, V. A,, 2693 Sendoda, Y., 2913 Sermon, P. A., 1347, 1369, 1651, 1667, 2175, 2243, 2256, 2693 Seyedmonir, S., 813 Shelimov, B. N., 2381 Sheppard, N., 1966, 2075 Shibata, Y., 3107 Sidahmed, I.M., 439 Simonian, L. K., 1189 Singh, G., 3167 Smith, B. V.. 2705 Smith, D. H., 1381 Smith, G. V., 2359 Smith, J. R. L., 2421, 2433 Soderman, O., 151 5 Sokolowski, S., 1405 Solymosi, F., 2015, 2074, 2078, 2081, 2082, 2086, 2137, 2142, 2247 2246, 2255, 2257 Schulz-Ekld, G . , 30 1 5 Somorjai, G. A.. 2061 Spencer, M. S., 2193, 2245, Staples, E., 2773 Steenken, S., 113 Stevens, D. G., 29 Stevenson, S., 2175 Stone, F. S., 1237, 2080, 2084, Stratford, M. R. L., 3177 Strumulo, D., 2271 Stuckey, M., 2525 Su, Z., 2573 Suda, Y., 1739 Suematsu, H., 2605 Sugahara, Y., 1851 Sugiyama, K., 3107 Suppan, P., 495 2246, 2247, 2248, 2249, 2250 2254 (vii) Sustmann, R., 95 Suzuki, T., 1213 SvetliEiC, V., 1169 Swartz, H.M., 191 Swift, A. J., 1975 Symons, M. C. R., 1, 383, 759, Szafran, M., 2541 Szostak, R., 487 Tabner, B. J., 167 Taga, K., 789 Takahashi, N., 2605 Takaishi, T., 41 1, 2681 Tamagawa, H., 3 189 Tan, W. K., 645 Tanaka, H., 1395, 3189 Tanaka, K., 1213, 1779, 1859 Tanaka, K-i., 1859 Tanimoto, M., 2475 Tannakone, K., 2553 Tascon, J. M. D., 3149 Tassi, L., 3129 Taylor, P. J., 2867 Tejuca, L. G., 3149 Tempere, J.-F., 1137 Tempest, P. A., 925 Theocharis, C. R., 1601, 2835 Thiery, C. L., 225 Thomas, T. L., 487 Thomson, S. J., 1893, 1964, 1965, 2083 Thurai, M., 841 Tiddy, G. J. T., 2735 Tilquin, B., 125 Timmons, R. B., 2825 Tkaczyk, M., 3083 Tomellini, M., 289 Tonge, J. S., 23 1, 245 Toprakcioglu, C., 1703 Topsne, H., 2157, 2169, 2171 Topsne. N-Y., 2 157 Torregrosa, R., 108 I Tosi, G., 3129 Toyoshima, I., 1213 Trabalzini, L., 151 Trifiro, F., 2213, 2246, 2251, Tsuchiya, S., 743 Tsuiki, H., 1395, 3189 Tsukamoto, K., 789 Turner, J.C. R., 937 Tyler, J. W., 925, 1355 Ueno, A., 1395, 3189 Ukisu, Y., 1227, 2635 Uma, K., 733 Unwin, P. R., 1261 Vaccari, A,, 221 3 Vachon, A., 177 van de Ven, T. G. M., 547 van den Boogert, J., 2103 van der Lee, G., 2103 van der Riet, M., 2963 van Ommen, J. G., 3161 van Santen, R. A., 1915, 1963, 2803 2254 1964, 2077, 2140, 2250AUTHOR INDEX Varani, G., 1609 Vattis, D., 1179 Vickerman, J. C., 1975, 2075 Villani, R. P., 2751 Vincent, P. B., 225 Vink, H., 801, 941 Vissers, J. P. R., 2145 Vong, M. S. W., 1369, 1667 Vordonis, L., 627 Vuolle, M., 51 Vvedensky, D. D., 1945 Waddicor, J. I., 751 Waddington, D.J., 2421, 2433 Waghorne, W. E., 2585 Waller, A. M., 1261 Wang, E., 2993 Waters, D. N., 1601 Waugh, K. C., 2193, 3223 Wells, C. F., 439, 939, 1119, 1281 Wells, P. B., 905 Whalley, E., 2901 Whan, D. A., 2193 White, A., 2459 White, L. R., 591, 873 Whyman, R., 905 Wickramanayake, S., 2553 Williams, D. E., 1323 Williams, G., 2647 Williams, J. O., 323 Williams, R. J. P., 1885 Williams, W. J., 371 Wilson, H. R., 1885 Wilson, I. R., 645, 657 Winstanley, D., 1835 Wojcik, D., 1253 Wurie, A. T., 1651 Wyatt, J. L., 2803 Wyn-Jones, E., 2525, 2735 Xyla, A. G., 1477 Yamada, K., 743, 3107 Yamaguchi, T., 3189 Yamamoto, H., 3207 Yamamoto, Y., 1641, 1795 Yamasaki, S., 1641 Yamashita, H., 2883, 2895 Yanagihara, Y ., 1579 Yanai, Y., 1641 Yangbo, F., 2533 Yariv, S., 1685 Yonezawa, Y., 1559 Yoshida, S., 2883, 2895 Yoshikawa, M., 2883 Yoshino, T., 1823 Yun, D.L., 2251 Zaki, M. I., 1601, 2835 Zhang, Q., 635 Zikanova, A., 2301 Zsigmond, A. G., 2359 Zukal, A., 3015 (viii)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbo!s' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the 'Systhme International d'Unit6s' (SI). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Ph ysicochemical Quantities and Units (Pergamon, Oxford, 1 979).Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, 8, C, 0, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now publis- hed by Pergamon).Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. Soc., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 23 Molecular Vibrations University of Reading, 15-16 December 1987 Organ ising Com m ittee : Professor I. M. Mills (Chairman) Dr J. E. Baggott Professor A. D. Buckingham Dr M. S. Child Dr N. C. Handy Dr B.J. Howard The Symposium will focus on recent advances in our understanding of the vibrations of polyatomic molecules. The topics to be discussed will include force field determinations by both ab initio and experimental methods, anharmonic effects in overtone spectroscopy, local modes and anharmonic resonances, intramolecular vibrational relaxation, and the frontier with molecular dynamics and reaction kinetics. The preliminary programme may be obtained from Mrs. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London WIV OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 8 5 Solvation University of Durham, 28-30 March 1988 Organising Committee: Professor M. C. R. Symons (Chairman) Professor J. S. Rowlinson Professor A.K. Covington Dr I . R. McDonald The purpose of the Discussion is to compare solvation of ionic and non-ionic species in the gas phase and in matrices with corresponding solvation in the bulk liquid phase. The aim will be to confront theory with experiment and to consider the application of these concepts to relaxation and solvolytic processes. Topics to be covered are: clusters, (c) Gas phase ionic clusters, (d) Liquid phase ionic solutions, ( e ) Dynamic processes including so Ivo I ysi s. Speakers include: H. L. Friedman, B. J. Howard, M. J. Henchman, S. Tomoda, 0. Kajimoto, M. H. Abraham, Yu Ya Efimov, J. L. Finney, P. Suppan, J. P. Devlin, D. W. James, G. W. Neilson, T. Clark, M. L. Klein, J. T. Hynes, G. A. Kenney-Wallace, G. R. Fleming, M. J. Blandamer and D.Chandler. The preliminary programme may be obtained from: Mr. Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. Dr J. Yarwood Dr A. D. Pethybridge Professor W. A. P. Luck Dr D. A. Young (a) Gas phase non-ionic clusters, (b) Liquid phase non-ionicTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION N o . 86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. 6. Davies Dr 6. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested in similar problems but viewed from the perspective of different environments.Contributions for consideration by the Organising Committee are invited. Titles should be submitted as soon as possible and abstracts of about 300 words by30 September 1987 to: Professor A. C. Legon, Department of Chemistry, University of Exeter, Exeter EX4 4QD. Full papers for publication i n the Discussion volume will be required by May 1988. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE I T A L I A N A DI CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OF THE SOCIETE FRANCAISE DE CHlMlE A N D DEUTSCHE J O I N T MEETING BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee.M. Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and charact (i) Metals (both in single crystal and dispersed form) rization by physical meth ds: (ii) Insulators and semkonductors (oxides, sulphides, halides, both in single crystal and dispersed forms) (iii) Mixed systems (with special emphasis on metal-support interaction) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods. Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Fisica, Corso Massimo D’Azeglio 48, 10125 Torino, Italy.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Collisions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S.Stoke Professor R. N. Dixon Professor J. P. Simons Dr K. Burnett Professor H. Loesch Professor R. A. Levine The Symposium will focus on the study of vector properties in reaction dynamics and photodissociation rather than the more traditional scalar quantities such as energy disposal, integral cross-sections and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interactions.The Symposium will provide an impetus to the development of 3-D theories of reaction dynamics and assess the quality and scope of the experiments that are providing this impetus. Contributions for consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ‘collisions’ in van der Waals complexes (D) Polarisation of the products of full and half-collisional processes Abstracts of about 300 words should be sent by 31 October 1987 to: Professor J. P. Simons, Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Full papers for publication in the Symposium volume will be required by 15 August 1988. (x ii)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Electrochemistry Group with the Society of Chemical industry Batteries To be held at the Society of Chemical Industry, London on 13 October 1987 Further information from Dr S. P.Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GLI 3 9PB Polymer Physics Group with the Institute of Marine Engineers Polymers in a Marine Environment To be held in London on 14-16 October 1987 Further information from Professor G. J. Lake, MRPRA, Brickendonbury, Herts SG13 8NL Division London Symposium: Kinetics and Spectroscopy of Alkali and Ionic Species in the Laboratory in Flames and in the Upper Atmosphere To be held at the Scientific Societies Lecture Theatre, London on 3 November 1987 Further information from Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN Division with the Institute of Ph ysics and the British Society for Electro Heat Seminar in Drying and Curing To be held a t Boddington Hall, University of Leeds on 17 Movember 1987 Further information from the Meetings Officer, Institute of Physics, 47 Belgrave Square, London SW1X 8QX Electrochemistry Group with the Electroanalytical Group Chemical Sensors To be held in London on 9-10 December 1987 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Electrochemistry Group with the Society of Chemical Industry Electrosynthesis To be held a t the University of York on 15-17 December 1987 Further information from Dr S.P. Tyfield. Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Colloid and interface Science Group with the Society of Chemical Industry The Preparation of Colloid Particulates and the Annual General Meeting To be held at the Society of Chemical Industry, Belgrave Square, London on 16 December 1987 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Laboratory, PO Box 11, The Heath, Runcorn, Cheshire WA7 4QE ~ High Resolution Spectroscopy Group with CCP6 Spectroscopy and Dynamics of highly Excited States of Molecules To be held at the University of Reading on 16-18 December 1987 Further information from Professor I.M. Mills, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD Neutron Scattering Group Scattering from Disordered Systems To be held at the University of Bristol on 16-18 December 1987 Further information from Dr G. W. Neilson. H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL Electrochemistry Group Electrochemistry/Surface Science: Must the Relationship remain Platonic? To be held at the University of Southampton on 6-7 January 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB (siii)Electrochemistry Group Workshop on Electrochemical Techniques and Instruments To be held at the University of Warwick on 6-7 January 1988 Further information from Dr P.N. Bartlett, Department of Chemistry, University of Warwick, Coventry CV4 7AL Surface Reactivity and Catalysis Group with the Process Technology Group and the Institute of Chemical Engineers Opportunities for Innovation in the Application of Catalysis To be held at Queen Mary College, London on 6-7 January 1988 Further information from Professor J. Pritchard, Queen Mary College, London Division with the institute of Mathematics and its Applications Mathematical Modelling of Semiconductor Devices and Processes To be held at the University of Loughborough on 7-8 January 1988 Further information from the Institute of Mathematics, Maitland House, Warrior Square, Southend-on-Sea SS1 2JY Division London Symposium: Modern Electrochemical Systems To be held at Imperial College, London on 21 January 1988 Further information from Mrs Y. A.Fish, Royal Society of Chemistry, Burlington House, London WIV OBN ~~ ~ Polymer Physics Group with the 3Ps Group Plastics, Packaging and Printing To be held at the institute of Physics, 47 Belgrave Square, London on 18 February 1988 Further information from Dr M. Richardson, National Physical Laboratory, Teddington, Middlesex Tw11 OLW Theoretical Chemistry Group Postgraduate Students’ Meeting To be held at University College, London on 2 March 1988 Further information from Dr G. Doggett, Department of Chemistry, University of York, York Colloid and Interface Science Group with The Society of Chemical Industry and British Radio frequency Spectroscopy Group Spectroscopy in Colloid Science To be held at the University of Bristol on 5-7 April 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE ~ ~ ~ Annual Congress: Division with Electrochemistry Group Solid State Materials in Electrochemistry To be held at the University of Kent, Canterbury on 12-15 April 1988 Further information from Dr J. F. Gibson, Royal Society of Chemistry, Burlington House, London WIV OBN Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AY Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9BP Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G. Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ (xiv)

ISSN:0300-9599    

DOI:10.1039/F198783BP131

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chern. Soc., Faraday Trans. I, 1987, 83 (lo), 3093-3106 The Crystal Structure of Sodium Diheptylsulphosuccinate Dihydrate and Comparison with Phospholipids Jacob Lucassen*? Unilever Research, Port Sunlight Laboratory, Wirral, Merseyside L63 3JW Michael G. B. Drew Department of Chemistry, The University, Whiteknights, P. 0. Box 224, Reading RG6 2AD Sodium diheptylsulphosuccinate can be crystallised in two polymorphic modifications, a monohydrate and a dihydrate. The dihydrate gave crystals of sufficient quality for a single-crystal structure analysis and its full structure has been elucidated. Th? crystal is monooclinic, spacegrpp P2,/a, with cell dimensions a = 7.79 (1) A, b = 9.80 (1) A, c = 32.63 (3) A, p = 92.5 (1)". There are 4 surfactant and 8 water molecules per unit cell.The crystal is racemic, i.e. there are two molecules of each optical enantiomer of the sulphosuccinate per cell. The water molecules form hydrogen-bond links between sulphonate oxygens of adjacent molecules. The discrimination in the crystal between the two optical enantiomers is through these hydrogen bonds. One of the water molecules causes enantiomer pairs to form, straddling two monolayers. The other water molecule is involved in the formation of infinitely long ribbons of alternating enantiomers in one monolayer in the direction of the a axis of the crystal. In the dihydrate, the aliphatic chains have a virtually zero angle of tilt with respect to the normal on the monolayer plane. From X-ray long spacings it is deduced that the monohydrate crystal has a tilt angle of ca.38". The molecular conformation, the packing in the crystal, the array of hydrogen bonds and the tendency to crystal polymorphism show a great similarity with recently elucidated crystal structures of phospholipids. It is suggested that the polymorphic transition between crystal types finds its parallel in monolayer-covered surfaces of sulphosuccinate solutions. Here, slow surface-tension changes could be ascribed to cooperative changes in tilt angle for rows of molecules, as found for phospholipid monolayers. Subsequent rapid formation of intermolecular hydrogen bonds would then consolidate this new con- formation. Sulphosuccinates, mainly in the form of Aerosol OT, the diethylhexyl derivative, find a wide application as wetting and emulsifying agents. AOT itself cannot be obtained in crystalline form; this can be attributed to the branched alkyl chains and to the presence of three asymmetric carbon atoms, which means that the compound is a mixture of diastereoisomers.While there is a vast amount of literature on AOT, sulphosuccinates with straight alkyl chains have received far less attention. Williams et a2.l showed that di-n-alkyl sulpho- succinates could be adequately recrystallised from methanol-water mixtures. Samples obtained by crystallisation had a high degree of surface chemical purity, as indicated by the absence of a surface-tension minimum near the critical micellar concentration. t Present address: Unilever Research Laboratorium, Postbus 1 14,3130 AC Vlaardingen, The Netherlands.3093 102-23094 Sodium Diheptylsulphosuccinate Dihydrate Table 1. Atomic coordinates ( x lo4) and equivalent isotropic thermal parameters (A2 x lo3) with estimated standard deviations in parentheses atom X Y Z Ua 3330 (3) 5098 (9) 2586 (9) 3041 (10) 354 (15) 3074 (14) 2274 (16) -30 (5) 2202 (12) -652 (10) -67 (9) - 1857 (17) - 2249 (21) - 1956 (22) -2481 (22) -2031 (29) -2615 (34) -2072 (46) 1307 (14) 2820 (1 2) 2170 (17) 3047 (23) 2287 (22) 3065 (31) 2312 (32) 3134 (57) 2446 (61) - 2736 (1 0) -121 (13) 357 (4) 2277 (5) 716 (9) 977 (8) 664 (1 2) 1095 (12) 732 (1 2) 1432 (14) 1059 (9) - 1087 (8) -280 (9) -711 (15) - 1526 (19) -890 (19) - 1552 (24) -969 (21) - 1634 (31) - 1142 (34) 2371 (11) 916 (9) 1551 (18) 929 (19) 1557 (22) 846 (22) 1403 (28) 1353 (62) 916 (9) 2191 (13) 735 (45) 857 (1) 307 (1) 926 (2) 484 (2) 879 (2) 1231 (3) 1270 (3) 1675 (3) 2028 (4) 975 (2) 1506 (2) 1486 (4) 1851 (5) 2248 (5) 2625 (5) 3035 (6) 3406 (7) 3797 (8) 2004 (3) 2382 (2) 2738 (4) 3110 (4) 3506 (4) 3882 (5) 4282 (5) 4658 (7) 5024 (1 0) 210 (2) -399 (3) 51 (3) 68 (5) 76 (9) 58 (8) 64 (8) 46 (1 1) 48 (10) 52 (12) 55 (14) 68 (9) 60 (8) 89 (16) 85 (19) 100 (20) 106 (24) 135 (26) 160 (33) 204 (43) 94 (12) 78 (10) 97 (18) 108 (20) 103 (20) 139 (28) 164 (34) 281 (58) 358 (90) 64 (10) 91 (18) In recrystallising a sample of sodium-di-n-heptyl sulphosuccinate from aqueous sol- utions we found evidence for crystal polymorphism. Thin leaflets, formed initially, were slowly replaced by fairly large, well shaped, elongated hexagons.X-Ray powder analysis showed a significant difference in long spoacing between the two types of crystals.The initial precipitate had a spacing of 25.6 A, while for the large crystals 32.6 A was found. Such differences in long spacings for amphoteric molecules are usually ascribed to different angles of tilt of the aliphatic chains with respect to the normal to the plane of the monolayers. Both types of crystal were hydrates. From n.m.r. measurements it appeared that the initial precipitate had just one molecule of hydration water per surfactant molecule, while the final precipitate was a dihydrate. In view of the lack of information on crystal structures of surfactants in general, especially on those crystallised from aqueous solution, we decided to attempt a single- crystal structure analysis.Experimental Only the relatively large, slow-forming crystals were found suitable for a single-crystal analysis. The crystal used was selected out of two precipitate samples which had been leftJ. Lucassen and M. G. B. Drew 3095 Table 2. Anion dimensions atoms length/A atoms angle/" s-C(2) s-O(2) s-O( 1) S-0(3) W - C ( 1) C(2)-C(3) C( 1)-O(22) C( 1)-O(2 1) 0(22)-C(21) C(21)-C(22) C(22)-C( 2 3) C(2 3)-C( 24) C(24)-C( 25) C(2 5)-C(26) C(26)-C(27) C(3)-C(4) 0(32)-C(4) 0(32)-C(3 1) O(3 1)-C(4) C(3 1)-C(32) C(32)-C(33) C( 3 3)-C( 34) c (3 4)-C( 3 5) C( 3 5)-C(36) C( 3 6)-C(3 7) 1.792 (10) 1.458 (8) 1.430 (8) 1.434 (8) 1.500 (15) 1.501 (14) 1.339 (1 3) 1.186 (12) 1.455 (14) 1.477 (1 9) 1.448 (21) 1.464 (21) 1.481 (23) 1.466 (24) 1.41 1 (28) 1.499 (1 7) 1.315 (13) 1.429 (15) 1.189 (14) 1.497 (19) 1.568 (20) 1.5 13 (23) 1.554 (26) 1.51 (3) 1.46 (4) C(2)-S-O( 2) C(2)-S-O( 1) O(2)-s-O( 1) C(2)-S-O( 3) 0(2)-S-O( 3) O( 1)-S-0(3) S-C(2)-C( 1) S-C(2)-C( 3) C( 1)-C(2)-C(3) C(2)-C( 1)-O(22) C(2)-C( 1)-O(2 1) O(22)-C( 1 )-O(2 1 ) C( 1)-0(22)-C(2 1) 0(22)<(21)-C(22) C(22)-C(23)-C(24) C(23)-C(24)-C(25) C(2)-C( 3)-C(4) C(2 l)-C(22)-C(23) C(24)-C(25)-C(26) C(25)-C(26)-C(27) C(4)-0(32)-C(3 1) C( 3)-C (4)- O( 3 2) C(3)-C(4)-0(3 1) 0(32)-C(3 1)-C(32) C(3 1)-C(32)-C(33) C(32)-C(33)-C(34) C(33)-C(34)-C(35) C(34)-C(3 5)-C(36) C(35)-C(36)-C(37) 105.7 (4) 106.4 (4) 112.0 (4) 106.0 (5) 113.3 (4) 112.8 (5) 108.8 (7) 110.4 (7) 113.8 (9) 113.5 (9) 124.8 (10) 121.6 (10) 115.5 (8) 110.5 (10) 117.4 (14) 121.5 (15) 121.6 (18) 120.4 (21) 120.3 (26) 112.4 (9) 115.8 (10) 111.8 (11) 125.8 (11) 108.5 (11) 109.5 (13) 109.5 (16) 11 1.5 (19) 11 1.6 (25) 109.0 (30) to crystallise without induction at room temperature for ca.6 months. The samples had an initial sulphosuccinate concentration of 5 x lop3 mol dmP3 and added NaCl concentrations of 2 x and mol dmP3, respectively. Crystal Data NaSO,C,,H,,, .A4 = 452.3, 2 = 4. The crystal wa? monoclinic, spacegroup P2,La. d, = 1.19 g ~ r n - ~ , d($ = 1.21 g cm-3, F(OO0) = 976, II (Mo K,) = 0.7107 A, ,u = 1.92 ern-'. A crystal of approximate dimensions 1.0 x 0.4 x 0.1 mm was set to rotate about the a axis on a Stoe Stadi 2 diffractometer. Data were taken via w scan with a variable width of (1.5 + sin ,u/tan 8) and a speed of 0.0 166" s-l. A total of 3549 independent reflections were measured with 28 < 50°, of which 1570 with I > 30 ( I ) were used in subsequent refinement.The structure was solved by direct methods. Atoms other than hydrogen were refined using anisotropic thermal parameters. Hydrogen atoms bonded to carbon were positioned tetrahedrally and refined ; those bonded to the same carbon atom were given a common thermal parameter. After refinement, a difference Fourier map was calculated in order to locate the hydrogen atoms bonded to the oxygens of the two hydrating water molecules, W(2) and W(1) (see below). Two peaks were located around W(2) but there were three peaks around W(1), u = 7.79 ( I ) A, b = 9.80 (1) A, c = 32.63 (3) A, p = 92.5 I/= 2488.7A3,3096 Sodium Diheptylsulphosuccinate Dihydrate Table 3.The sodium ion environment atoms bond length/A N a a ( 2 ) 2.450 (8) Na-W( 1) 2.501 (9) Na-W(2) 2.303 (10) Na-0(21) 2.548 (9) Na-0(2”) 2.606 (9) Na-O( 1 ji) 2.817 (10) Na-W( 1 i, 2.544 (9) Table 4. Intermolecular hydrogen bonding atoms length/A atoms angle/’ W( 1) - * * O( Pi) 2.95 W(1)-H(W1 1) * . *O( liii) 154 W(1)-0(2”) 2.93 W( 1)-H(W 12) * - O(2”) 93 W(1). .0(21) 2.92 W( 1)-H(W 13) * * eO(21) 148 W(2) * O(3’) 2.85 W(2)-H(W22) * . * O(3”) 166 W(2)..*0(3”) 2.9 1 W(2)-H(W21)..*0(3”) 152 Symmetry elements : i 0.5+x, 0.5-y, z ii -0.5+x, 0.5-y, z 111 -1 +x, y, z iv -x,-y,-z vi - l + x , y, z ... v 0 . 5 - ~ , 0.5+y, - 2 viii 0.5-x, -OS+y, -z indicating some positional disorder. These three positions were all included in the refinement with occupancies of 0.66.The parameters of all these five hydrogen positions were allowed to refine independently and converged successfully. The structure was refined by full-matrix least-squares to R = 0.075 (R, = 0.079). The weighting scheme was w = [a2F+ 0.003F2]-’, a(F) being taken from counting statistics. Calculations were carried out using SHELX 762 and some home-made programs on the Amdahl V7 computer at the University of Reading. Final atomic coordinates are given in table 1 and anion dimensions in table 2. Details of close contacts between oxygen and the sodium ion are given in table 3 and the hydrogen bonds involving the two water molecules in table 4. Torsion angles, thermal parameters, structure factors and hydrogen atomic coord- inates are available as Supplementary Publication No.56672. t Results and Discussion Molecular Conformation in the Crystal The unit cell contains four surfactant molecules (four sodium ions and four sulpho- succinate ions) and eight water molecules. The crystal is racemic, i.e. there are two molecules of each optical enantiomer of the sulphosuccinate per cell. Fig. 1 shows the conformation of the sulphosuccinate ion in the crystal together with -f See Notice to Authors, J. Chem. SOC., Faraday Trans. I, 1987, 83, Part 1.J. Lucassen and M. G. B. Drew y chain 3097 Fig. 1. Conformation of the diheptylsulphosuccinate anion in the crystal together with the atomic numbering scheme. The numbering scheme is based on the convention for phospholipids (see text and fig. 2). y chain O W ) p chain Fig.2. Conformation of dilauroylphosphatidylethanolamine together with the atomic numbering scheme. Coordinates are taken from those of ref. ( 5 ) with the addition of some hydrogen atoms in calculated tetrahedral positions. the atomic numbering scheme. Because of similarities with other dialkyl amphiphiles, notably phospholipids, we used the system suggested by S~ndaralingam~. * for this class of compounds. Thus in fig. 1 the more and the less protruding aliphatic chains are indicated by y (or 3) and /3 (or 2), respectively. In fig. 2 the conformation for dilauroylphosphatidylethanolamine5 is shown for comparison. As is to be expected, the two hydrophobic chains are parallel to each other and approximately parallel to the c direction, but the heptyl group closest to the sulphonate only extends to the fourth carbon of the other chain.This leads to a staggered arrangement of the chain ends in the crystal. Both chains have an all-trans configuration, and the planes through their carbon atoms intersect at an angle of 74.3". By comparison, the same angle for the phospholipid (fig. 2) is only 7.0". As is apparent from fig. 3 (the c projection), this is caused by a rotation in the ab plane of the chains relative to each other. Both chains are aligned approximately parallel to the c direction, as can be seen in fig. 4. This can only be realised when at least one of the two carboxy groups makes an angle with the adjoining carbon chain. In the event this is the C(2)0(22)0(21)C(l) group, which has an angle of intersection with the /3 chain of 52.9' (see fig.1). The other carboxy group is virtually coplanar with the y chain (angle of 4.3").3098 Sodium Diheptylsulphosuccinate Dihydrate Fig. 3. The c projection of the unit cell. The intermolecular hydrogen bonds are shown as dotted lines. For clarity these are not superimposed on the diagram but shown separately. In the top half of the figure the continuous - ' O(3). . - W(2) - - O(3) ribbons are shown, while in the bottom half of the bonding around a centre of symmetry involving W(1), 0(2), 0(1) and O(21) is shown. The lower right-hand part qf the same figure shows the environment of the sodium ion. There are seven close contacts < 3.00 A (listed in table 3). Six are shown as arrows. The seventh interaction is with W(2), which is immediately below the sodium cation.0 d Crystal Packing and Intermolecular Interactions As is expected for this sort of compound, the crystal consists of a 'stack' of monolayers, arranged back-to-back and face-to-face, i.e. with close mutual contact between hydro- philic and hydrophobic groups, respectively, in adjacent layers. The b-projection (fig. 4) gives the best impression of the molecular packing. In this figure it can be seen that the hydrating water molecules, indicated by W( 1) and W(2), participate in creating an intermolecular network in the hydrophilic layers. They form hydrogen-bond links be- tween neighbouring molecules involving all three oxygens of the sulphonate groups. This is in addition to the interactions between the sodium ion and surrounding sulphonate and water oxygens.As illustrated by table 3, there are seven sodium-oxyg%n distances c 3.0 A, of which six fall yithin the range 2.30-2.61 A. The seventh is 2.82 A, but the next largest distance is 3.75 A. The sodium is bonded to three water molecules, W( l), W(2) and W( li), one monodentate sulphonate oxygen, 0(2), one bidentate sulphonate group O( li) and 0(2i)'f and one carboxyl oxygen, O(21). The geometry around the sodium ions is irregular. The geometry around the water molecules is given in table 2. As stated in the t Roman numerals as superscripts refer to atoms in symmetry-related positions. A full list is given at the bottom of table 4.J. Lucassen and M. G. B. Drew 3099 0 1 B3100 Sodium Diheptylsulphosuccinate Dihydrate bL c sinp Fig.5. Enantiomeric pairs, connected by water W( 1) : (a) in the b projection, (b) in the a projection. Experimental section, three hydrogen atoms were refined for W(1), but only two for W(2). The arrangement around W(2) is straightforward, with hydrogen atoms pointing towacds O(3") and O(3") to form hydrogen bonds with bond lengths of 2.91 and 2.85 A, respectively. This water molecule connects opposite enantiomers in the a direction, giving rise to infinitely long ribbons. The c projection (fig. 3) clearly illustrates the zig-zagging nature of the hydrogen-bonded chains, whereby it connects adjoining rows of molecules in the b direction (see fig. 4). The arrangement around the other water qolecule, W( l), isomore complicated. It js close to three oxygen atoms, O(2'"l at 2.93 A, O(21) ,at 2.92 A and 0(liii) at 2.95 A, and to two sodium ions, Na at 2.50 A and Na" at 2.54 A.A study of the bond angles for W( 1) shows that the hydrogen bonds with the latter two oxygens are complicated by the close proximity of the sodium ions.? Fig. 5 illustrates the hydrogen bonds in which W(1) participates. As can be seen from this figure, it connects pairs of opposite enantiomers located in adjoining crystalline monolayers through the sulphonate oxygen bonds. Finally, the arrows in fig. 3 illustrate the environment of the sodium ion in the c projection. Comparison with known Crystal Structures Remarkably few surfactant crystal structures have been elucidated. In some instances, e.g. for ethylene oxide adducts, dodecylbenzenesulphonate or AOT, impediments pre- t This steric hindrance between sodium ions and hydrogen atoms could give rise to acid hydrolysis6 once the constraint of strict stoichiometry has been removed, as it is in a monolayer at the air-water surface.If for such a monolayer intermolecular distances were comparable to those in the crystal, sodium ions could be ejected from the surface while the combination of water molecule and sodium ion would be replaced by a hydrated proton.'J. Lucassen and M. G. B. Drew 3101 venting the formation of suitable crystals are obvious. A wide distribution of molecular weights, chain branching, ortho-para isomerism and diastereoisomerism are all detri- mental to controlled growth of single crystals. Many of the remaining surfactants, even if they are pure, crystallise in unmanageably thin leaflets or long needles, unsuitable for single-crystal X-ray crystallography.This seems to be the case especially for crystals grown from aqueous solutions and, as far as we could establish, all structure deter- minations have been carried out on crystals obtained from organic solvents. Thus the sulphosuccinate crystal grown from aqueous solution seems to be exceptional.? The reason for this may be related to the need to form an intricate network of hydrogen bonds in the crystal. For thin leaflets, crystal growth in the direction perpendicular to the monolayers is relatively slow, while growth in the plane of the monolayers, i.e. equivalent to our a and b axes, is fast. We suggest that the need for forming hydrogen bonds in these directions, and to manoeuvre the right enantiomer in the appropriate position, will slow down the growth rate relative to that in the c direction, thus resulting in crystals with a more ‘ balanced ’ size.It has been found for a number of crystal structures that water molecules play an essential role in the formation of a three-dimensional network of hydrogen bonds. Early examples include mesotartaric acid monohydrate, a oxalic acidg and acetylenedicar- boxylic acid dih~drate.~ Another group of related molecules for which various aspects of conformation and packing are being thoroughly studied are the phospholipids. As late as 1972, Sundar- alingam3 stated : ‘The phospholipids form a major fraction of biological membranes but the X-ray structure of none has yet been determined’.During recent years, however, considerable progress has been made in elucidating their crystal structures, and they show features which are remarkably similar to those of the sulphosuccinate crystal reported here. The similarities are in the general conformation of the molecules (see fig. 1 and 2) as well as in the characteristic hydrogen-bonded organisation of their polar groups. In lecithin dihydrate,’O which (in spite of having been crystallised from an ethanol-ether-water mixture) contained two hydration molecules per lipid molecule, the analogy is nearly perfect. One of the water molecules is located between phosphate oxygens of adjoining lecithin molecules in a monolayer, thus forming an infinite hydrogen-bonded phosphate-water-phosphate ribbon and providing stability in the a direction.The other water molecule establishes a link across the bilayer interface, in this case between one phosphate oxygen and the corresponding symmetry-related water molecule in the adjacent monolayer. In D,L-dilauroylphosphatidylethanolamine, 5* l1 the conformation of the aliphatic chains and the arrangement of optical enantiomers in the crystal is virtually the same as for the sulphosuccinate. For this compound, which was crystallised from glacial acetic acid, the arrangement of the headgroups is compared with that of sulphosuccinate in fig. 6. Hydrogen-bonded ribbons are formed linking up amine and phosphate groups in adjacent molecules. The chemical bond between phosphate and amine in the same molecule will provide a ‘ cross-link ’ between the abovementioned ribbons and will cause additional rigidity in the crystal.Hydrogen-bonded links between the monolayers are formed via an acetic acid molecule of crystallisation. Both in the phospholipid and in the sulphosuccinate, the p and y chains, i.e. the less and the more protruding chains, respectively, form a triangular lattice. The inserts in fig. 6 show that the lattices are slightly more complex than those based on simple translation, suggested by Albrecht et aZ.12 It can be seen that while in the phospholipid the a and /? chains are arranged in single rows, in the sulphosuccinate they form double rows. f It is noteworthy that the mean thermal parameters of the atoms in the anion (table 1) increase markedly towards the ends of the chains.This reflects considerable freedom of movement in this region and indicates the difficulty in obtaining crystalline order for long-chain surfactants.3102 Sodium Diheptylsulphosuccinate Dihydrate P Fig. 6. The disposition of head groups for (a) a phospholipid and (b) a sulphosuccinate. (a) Dilauroylphosphatidylethanolamine [coordinates from ref. (5)] is here plotted out down the long axis. Hydrogen bonds [of the type N-H.--O(PO,)] that hold the molecules together are illustrated as dotted lines. (b) The diheptylsulphosuccinate anion plotted down the long axis. The - - - W(2) - - - O(3). . - W(2) continuous hydrogen bonding is shown. The inserts in this figure illustrate the triangular lattice formed by the protruding aliphatic chains.The shorter (B) chains are shown as open circles and the longer (7) chains as closed circles.J . Lucassen and M. G. B. Drew 3103 Fig. 7. Suggested packing (b projection) for crystal with tilted chains. Angle of tilt is 38". Ref. (4) describes some other phospholipid crystal structures, all showing similar conformation, packing and hydrogen-bond arrangements. Alternative Crystal Structure of Sodium Diheptylsulphosuccinate We mentioned before that sodium diheptylsulphosuccinate crystallises in two modifi- cations, distinguishable by their X-ray long spacings. The alternative crystal form, which precipitates first, was shown by n.m.r. to contain only one molecule of hydration water per surfactant molecule. Obviously, the hydrogen-bond network will in that case be less intricate and more readily formed, resulting i? strongly asymmetric crystals.The reduction of the long spacing from 32.6 to 25.6 A must increase the intermolecular distances in the ab plane; hydrogen-bond links in this plane are the most likely to have disappeared. Fig. 7 shows a possible structure for this metastable crystal in which W(2) has disappeared and in which an angle of tilt of arccos(25.6/32.6) = 38" between the chains and the normal to the monolayer planes has been introduced. This type of crystal could then grow by simple juxtaposition of doublets of enantiomers connected by W( 1). Molecular arrangements with similar tilt angles (41 ") have been found for crystals of lysophosphatidylcholine l3 and of cerebroside. l4 We can compare the postulated tilted structure in fig. 7 with the equivalent projection of the dihydrate in fig.4. The correct arrangement of enantiomers in the dihydrate crystal depends upon the ability to form the proper intermolecular hydrogen bonds with W(l) and W(2). In the tilted structure the hydrogen bonds in the a direction, which primarily involve W(2), are absent and the packing conditions in that direction may well be relaxed. In the dihydrate W(l) also participates in hydrogen bonding with 0(21), but it is impossible to tell if such a bond would persist in the tilted structure. However, it does seem clear that there is less likelihood of a strict ordering of optical enantiomers in the monohydrate than in the dihydrate.3 104 Sodium Diheptylsulphosuccinate Dihydrate Table 5.X-Ray long spacings for various sodium dialkylsulphosuccinates long spacingjA alkyl group untilted tilted - hexyl 23.5 heptyl 32.6 25.6 27.5 hexadecyl 55.6 - octadecyl 60.4 octyl - nonyl 38.4 - - We attempted to prepare single crystals of the monohydrate to test these tentative conclusions. This proved impossible, but for the sodium dihexyl sulphosuccinate homologue it was found that a tilted structure (c = 23.50 A) could be grown from concentrated aqueous solution in fairly large, diamond-shaped crystals. However, these crystals were not suitable for single-crystal analysis as the diffraction pattern was weak and the spots were very broad. This is not inconsistent with our speculation that in the tilted structure the constraining lateral hydrogen bonds and the strict enantiomer ordering are absent. Table 5 shows X-ray powder diagram long spacings for a number of pure dialkyl- sulphosuccinates.Both for untilted and tilted structures there is a linear relationship between long spacing and chain length. Least-sqtares fits for both structures gave increments per methylene group of 10.255 and 1.00 A, respectively. This compares well with the increments of 1.25 and 0.98 A expected from carbonxarbon distances in an all- trans chain. The extrapolated long spacing for zero aliphatic chain length could be thought to represent the effective thickness of the hydrophilic layers in the crystal. The fact that it changes by roughly the same factor as the abovementioned increment in going from untilted to tilted (0.748 compared to 0.797) provides another argument in favour of the crystal structure suggested in fig.7. Once again, there is a striking analogy between sulphosuccinates and phospholipids, this time with regard to crystal polymorphism, reorganisation of hydrocarbon chains and changes in hydration. For n-palmitoylgalactosylsphingosine15 an untilted anhydrous crystal transforms into a tilted hydrated modification. Dilauroylphosphatidyl ethan- olamine," on the other hand, dehydrates upon conversion into a more stable form. Thus, conversion into more stable forms can involve either hydration or dehydration. De- pending on molecular geometry and available hydrogen- bond donors and acceptors, inclusion of an extra water molecule can apparently either increase or decrease the internal energy of a crystal.While for the sulphosuccinates, transformation from metastable to stable appears to proceed through a nucleation-and-growth mechanism, for similar transitions in the phospholipids rearrangements occur in existing crystals through cooperative processes. Such processes, involving changes in state of hydration and in hydrophobic chain orientation, are of potential importance in determining the biological function of lipid membranes. Relation between Ordering Processes in Crystals, Membranes and Surfaces It had been observed'' that submicellar solutions of sulphosuccinates showed a slow surface-tension equilibration which could not be explained on the basis of diffusion toJ. Lucassen and M. G. B. Drew 3105 the surface only. Our very pure samples of diheptylsulphosuccinate showed a similar slow surface-tension change and a simultaneous increase in the surface dilational mod- ulus. It has been argued'' that slow changes in surface tension and increased modulus values are likely to be caused by cooperative ordering in the surface rather than by adsorption barriers between surface and solution.The question arises whether polymorphic transitions in crystals and restructuring of monolayers in single surfaces have in fact a common cause. Certainly, those forces which are operative in determining crystalline order in three dimensions will make their influence felt in one or two directions only, before crystallisation sets in. One possibility is that after sulphosuccinate ions arrive at the surface in an on-average tilted position, they move into a more upright position, allowing them to form intermolecular hydrogen bonds, similar to those observed in the a direction of the crystal [involving W(2)].Such an ordering in one direction would result in polymer-like hydrogen-bonded rows of molecules. In the liquid-air surface such rows will, at least originally, consist of random mixtures of both optical enantiomers. If there were an additional ordering in alternate optical enantiomers, such polymeric units in the surface could serve as nuclei for crystallisation. Evidence for any enantiomer ordering in surfaces is hard to obtain.lg We did observe, however, that above the solubility limit rather well shaped crystals do form in the air-water surface in such a position that their ab planes are parallel to the surface.Thus a change in angle of tilt of adsorbed molecules, giving then a closer packing and a more perpendicular arrangement of their chains, could be the cause of measured slow changes of surface tension with time. The slowness of the rearrangement should be ascribed to its cooperative character, to the necessity of moving molecules into another conformation and possibly to the need for a limited enantiomer ordering. In itself, formation of hydrogen bonds, once the intermolecular distance is as required, should be a very fast process. Thus the knowledge of the detailed crystal structure of a surfactant may give a better idea of its behaviour in surfaces. It should also give useful information on the ordering in thin films and membranes.Especially interesting is the tendency to form intermolecular hydrogen bonds, either with or without direct involvement of water. If such bonds are formed in the lateral direction, i.e. in the monolayer plane, they can be expected to lead to an increased rigidity, or elastic modulus for surfaces and thin films. The measured slow increase' of surface elastic modulus during equilibration does, therefore, fit in with the general picture. For lipid membranes, with hydrophilic groups, on the outside of the membrane, these lateral bonds will provide the required stability and rigidity. All lipid crystal structures solved so far show lateral hydrogen bonding. For phospholipid monolayers there is considerable evidence2O. 21 for polymorphism which is related to changes in tilt angle.Thus for phospholipids there is a clear relation between rearrangement in monolayers and the change of crystal structure, and the case for a similar relation for the sodium diheptylsulphosuccinate appears to be quite convincing. There is possibly a structural similarity between isolated bimolecular layers in crystals and free Newton black films. Such films consist of bimolecular layers as well, with only a few more water molecules associated per surfactant molecule than found in the sulphosuccinate dihydrate. For sodium dodecylsulphate films, for example, de Feyter and Vrij22 estimated the number of associated waters as 5-8. They also found evidence that the dodecyl sulphate and sodium ions in Newton black films form a highly ordered two-dimensional lattice.Thus, just as in the crystal, water-mediated hydrogen bonds could well have a struc- turing function for Newton black films. Indirect evidence supporting the presence of such bonds is present in ref. (23). Both urea and sucrose had definite but opposite effects on the free energy of film formation.3 106 Sodium Diheptylsulphosuccinate Dihydrate Conclusion The work reported here represents the first full crystal-structure determination of a surfactant crystallised from aqueous solution. Some hitherto little-known aspects of intermolecular interaction forces between the ionic headgroups in crystals have been revealed. Hydrogen bonding through hydrating water molecules plays an important role just as it does for phospholipids, and this could be a more general feature in surfactant crystal structures than has been realised so far.The authors thank Dr C . D. Adam for carrying out the X-ray powder analysis. References 1 E. F. Williams, N. T. Woodberry and J. K. Dixon, J. Colloid Sci., 1957, 12, 452, 2 G. M. Sheldrick, SHELX 76, Package for Structure Determination (University of Cambridge, 1976). 3 M. Sundaralingam, Ann. N. Y. Acad. Sci., 1972, la5, 324. 4 H. Hauser, I. Pascher, R. H. Pearson and S. Sundell, Biochim. Biophys. Ada, 1981, 650, 21. 5 M. Elder, P. Hitchcock, R. Mason and G. G. Shipley, Proc. R. SOC. London, Ser. A, 1977, 354, 157. 6 D. J. Salley, A. J. Weith, A. A. Argyle and J. K. Dixon, Proc. R. SOC. London, Ser. A , 1950, 203, 42. 7 J. Lucassen, to be published. 8 G. A. Bootsma and J. C. Schoone, Acta Crystallogr., 1967, 17, 462; 1967, 22, 522. 9 F. R. Ahmed and D. W. J. Cruickshank, Acta Crystallogr., 1953, 6, 385. 10 R. H. Pearson and I. Pascher, Nature (London), 1979, 281, 499. 11 P. B. Hitchcock, R. Mason, K. M. Thomas and G. G. Shipley, Proc. Nut1 Acad. Sci. USA, 1974, 71, 12 0. Albrecht, H. Gruler and E. Sackmann, J. Phys., 1978, 39, 301. 13 H. Hauser, I. Pascher and S. Sundell, J. Mol. Biol., 1980, 137, 249. 14 I. Pascher and S. Sundell, Chem. Phys. Lipids, 1977, 20, 175. 15 M. J. Ruocco, D. Atkinson, D. M. Small, R. P. Skarjune, E. Oldfield and G. G. Shipley, Biochemistry, 16 J. M. Seddon, K. Harlos and D. Marsh, J. Biol. Chem., 1983, 258, 3850. 17 I. I. Germasheva and S. A. Panaeva, Kolloidn. Zh., 1982, 44, 661. 18 M. van den Tempe1 and E. H. Lucassen-Reijnders, Adv. Colloid Interface Sci., 1983, 18, 281. 19 M. V. Stewart and E. M. Arnett, Top. Stereochem., 1982, 13, 195. 20 C. Gebhardt, H. Gruler and E. Sackmann, 2. Naturforsch., 1977, 32, 581. 21 A. Fisher and E. Sackmann, J. Phys. (Paris), 1984,45, 517. 22 J. A. de Feyter and A. Vrij, J. Colloid Interface Sci., 1979, 70, 456. 23 J. A. de Feyter, Ph.D. Thesis (Utrecht, 1973). 3036. 1981, 20, 5957. Paper 61 1228 ; Received 17th June, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303093

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I , 1987, 83 ( I 0), 3 107-3 1 14 Doping Effect of Sodium on y-Irradiated Magnesium Oxide Tsuneo Matsuda," Koji Y amada, Y asumasa Shibata, Hiroshi Miura and Kazuo Sugiyama Faculty of Engineering, Saitarna University, 255 Shirno-ohkubo, Urawa 338, Japan The concentration of one-electron donor centres increases markedly when y-irradiated MgO is treated with sodium amide (NaNH,), followed by calcination at 500 "C for 2 h. The drastic increase in the concentration of one-electron donor centres is attributed to the electron-donating effect of Na produced by the decomposition of NaNH, at the anionic vacancy sites which are formed during irradiation. In contrast, the concentration of one- electron donor sites decreases if NaNH, is previously introduced into MgO before y-irradiation.A similar result is obtained with MgO doped with NaNO,. Thus in both cases when NaNH, was doped into the irradiated MgO and when MgO-NaNH, was irradiated, the concentration of one- electron donor centres changed considerably. This is explained by transfer of electrons trapped at anion vacancies to lattice oxygen atoms in MgO. The electrical conductivity was measured at temperatures between 50 and 300 "C. As the sodium content increases the electrical conductivity also increases. The thermal activation energy, described by the conductivity, differs on going from the low- to the high-temperature region, indicating that there exist two kinds of electron levels. The correlation between electroconductivity, one-electron donor and basicity properties is dis- cussed.Defects are produced in an MgO crystal lattice by irradiation with y- or u.v.- irradiati~n.'-~ As a result of irradiation, MgO turns blue, the change of colour being attributed to the formation of F-centres. The F-centre, an electron trapped at an anionic vacancy in the MgO lattice, behaves as a one-electron donor centre.6 According to Tench et al.,' the lattice oxygen ions on the magnesium oxide surface may act as electron donors. Cordishi et a1.8 have also mentioned that electron donor sites consist of 02- ions in a coordinatively unsaturated site and OH- on the magnesium oxide surface. Thus there is still no definite explanation of one-electron donor centres. In either case they occur at sites which are able to transfer electrons to molecules having a high electron affinity, such as nitrobenzene, tetracyanoethylene (TCNE) etc.Lunsford et aL4. investigated the effect of ultraviolet and neutron irradiation on MgO using electron spin resonance (e.s.r.), concluding that irradiation has a more pronounced effect on insulators such as MgO than on metals or semiconductors. Consequently it would be useful to study the effect of irradiation of MgO, especially in connection with the electron-donating effect of sodium. Furthermore, it is expected that one-electron donor centres will increase in concentration on irradiation. However, the experimental results we obtained were contrary to this expectation, i.e. the number of one-electron donor centres decreased remarkably on irradiation. The cause may be that electrons trapped at anion vacancy sites on the surface transfer to oxygen molecules which are produced by the irradiation.' If sodium is doped into pre-irradiated MgO, which is presumed to have a larger concentration of anion vacancies, it can be assumed that the number of one-electron donor centres will increase through the electron- 31073108 Na Doping of y-Irradiated MgO T a- u Fig.1. Apparatus to produce the sample for e.s.r. measurements a, sample; b, quartz tube for e.s.r. measurements; c, quartz-Pyrex glass join; d, greaseless joint with O-ring; e, trap to seal the grease vapour which is dipped in liquid N, during the preparation; f, to vacuum line; g, connection with the air-tight benzene solution of TCNE; 0, greaseless stop valve. donating properties of sodium. This study was carried out in order to verify the above consideration.The basic site in magnesium oxide lattice is O2-.l0 Consequently it is of interest to examine whether one-electron donor centres and the basic site will be related by electric conductivity. On the basis of this consideration the electric conductivity of MgO-NaNO, was measured at various temperatures and at different concentrations of NaNO,. Experimental The magnesium oxide mainly used in this study (purity > 99.9%; surface area 146 m2 g-') was kindly donated by the Kohnoshima Chemical Co. Ltd. Another source of magnesium oxide from Wako Pure Chemicals was also used (surface area 40 m2 g-'). Ca. 7 g MgO was placed in a glass tube of 2 cm outer diameter, outgassed at 500 "C for 3 h and then sealed under vacuum.The MgO in the glass tube was irradiated with y-rays from a 6oCo source at room temperature with either 5 x lo-, or 8 x 10-1 Mrad irradiation dosages. The MgO samples doped with sodium compounds such as NaNH, and NaNO, were also irradiated using the same irradiation dosages as in the case of MgO. Before irradiation the mixture of MgO and the sodium compound was calcined at 500 "C for 3 h under evacuation. Doping the irradiated MgO with NaNH, was carried out as follows; after the irradiation of MgO the seal of the glass tube was broken in a dry box filled with pure N,. NaNH, (which decomposes to give metallic sodium at ca. 400 "C) was mixed with the MgO and then a portion of the mixture (ca. 0.1 g) was placed in glass tube ('A') of fig.1. The glass tube was sealed with a rubber stopper before removal from the box.T. Matsuda, K. Yamada, Y. Shibata, H. Miura and K. Sugiyama 3109 Fig. 2. Apparatus for electrical conductivity measurements; a, thermocouple; b, sample; c, electric furnace; d; metal stop valve. While dry, pure N, was allowed to flow from the narrow glass tube (B), near (d) in fig. 1, the stopper of tube (A) was rapidly removed and the tube connected with (B) through joint (d); thus MgO-NaNH, mixture did not come into contact with air. The MgO- NaNO, sample was prepared by impregnating MgO in an NaNO, aqueous solution, followed by drying at 110 "C and calcining at 700 "C for 5 h in flowing N,. Each sample of MgO, MgO-NaNH, and -NaNO, was evacuated at 500 "C for 2 h in tube (A).The samples doped with sodium as described above were further treated with a benzene solution of TCNE, which flowed down from the top tube (B) into the bottom of tube (A). After the TCNE solution was adequately adsorbed onto the sample, by leaving it for about half a day at room temperature, the benzene was removed by evaporation at 70 "C. A small portion of the prepared sample was transferred into the e.s.r. sample probe [(quartz tube, 0.d. 4 mm, see (b) in fig. 13 and then sealed off. The e.s.r. spectra of the samples were recorded at room temperature in a dual-cavity spectrometer. In the cavity two quartz tubes were prepared; one contained the standard probe, DPPH diluted with sodium chloride, and the other the sample. By using the dual cavity a mode check could be easily operated at the same time for the two samples, and the e.s.r.spectra could be measured under the same conditions of modulation amplitude and microwave frequency. The e.s.r. spectrometer, operating at X-band frequency, was a Varian model E-9 instrument equipped with a 100 kHz modulation unit. The relative spin concentration was determined by comparison of each integrated peak area of the samples and using DPPH. Contamination from grease vapour was prevented by using a liquid-nitrogen cold trap and greaseless stop valves in both the sample preparation and the following electrical conductivity measurements. With the apparatus shown in fig. 2 the electrical conductivity31 10 Na Doping of y-Irradiated MgO I I Na/mmol g i i o Fig.3. Relative spin concentration of the irradiated MgO-NaNH, and MgO-NaNO, us. the sodium content. (a) Sample prepared by the doping of NaNH, in the preirradiated MgO, 0, 0, A, 5 x lo-, Mrad h-l; 0 , 8 x lo-' Mrad h-l. (ti) MgO-NaNO, irradiated with prays. ( c ) A, MgO from Wako Pure Chemicals (left-hand ordinate). was measured by raising the temperature from 50 to 300 "C at the rate of 100 "C h-l. For electrical-conductivity measurements of MgO-NaNO,, the powder was pressed into a thin disc 2 mm thick by 2 cm diameter and then broken into 5 x 5 mm squares; then on both sides gold contacts ( > 99.9% purity) were attached by an evaporation method. Before measurement, the square disc was heated at 700 "C for 1 h in the apparatus. The basicity of the samples, dissolved in a benzoic acid-benzene solution (0.1 mmol), was measured by titration using indicators with various pK, values.Results and Discussion The e.s.r. spectra exhibited a single differential absorption of a shape similar to those cited in the literature.6 However, the g values of the TCNE radical were 2.0035 for MgO and between 2.0027 and 2.0030 for MgO doped with sodium. There are small differences in these g values between our results and those in the literature. The dependence of the relative spin concentration Gel of one-electron donor centres in MgO-NaNH, and MgO-NaNO, as a function of the sodium content is shown in fig. 3. Cel of the sample in which NaNH, was added in the irradiated MgO, followed by calcination at 500 "C for 2 h [denoted as MgO (I)] is remarkably large and is 40 times greater than the value of Gel of irradiated MgO-NaNO, calcined at 700 "C for 5 h [cf.fig. 3(a) and (b)]. Gel of an irradiated sample [MgO (11)] of MgO-NaNH, already calcined at 500 "C also showed the same behaviour as that of the irradiated MgO-NaNO, sample, as shown in fig. 3 (b). A notable decrease in Gel due to irradiation was found with MgO. Thus theT. Matsuda, K. Yamada, Y. Shibata, H. Miura and K . Sugiyama 3111 A I I I I 1 2 4 6 8 I ( Na/mmol g i i o Fig. 4. Electrical conductivity of MgO doped with NaNO, at various temperatures: 0, 100; a, 200; A, 300 "C. addition of sodium compounds followed by calcination greatly affects to the value of Gel of the one-electron donor centres. The cause of the increase of the Gel of MgO (I) may be as follows: the donation of Na electrons into the anion vacancies formed by y-irradiation will produce one-electron donor centres.This result also indicates that these centres are stable after evacuation at 500 "C. On the other hand, in the case of MgO (11) the electrons already trapped in one- electron donor centres will be ejected by y-irradiation, resulting in a decrease in Gel. The ejected electrons may be transfered to the oxygen atoms in the MgO lattice to form the basic site, 02-. One might expect an increase in the basicity, but no increase could be detected in this study. The number of basic sites is ca. 1019 g&,,ll and that of one- electron donor centres is lo1' or 10l6 spin g-l from the results of this study and others.12 Even if the electrons in a one-electron donor centre were transferred to the production of the basic site, the contribution to the basicity would give rise to an increase of only 1 or 0.1 O/O, which would be taken up by the error range of the basicity measurement. Another cause may be considered as follows.According to Wy~ocki,~ oxygen is released during y-irradiation in the initial stage of the process and then readsorbed on the MgO surface, mainly as 0;. The adsorbed site of 0, is neither a one- electron donor centre nor a basic site. If the 0, is produced by y-irradiation with the31 12 Na Doping of y-Irradiated MgO 2.0 2.5 3.0 103 K I T Fig. 5. Plot of electric conductivity vs. the reciprocal of temperature in an MgO-NaNO, sample. Sodium content per g MgO (mmol) as follows: (1) 3.2, (2) 1.6, (3) 1.0, (4) 0.35, ( 5 ) 0.consumption of the electron trapped at the anion vacancy, a remarkable decrease in the concentration of centres would be expected, as shown in fig. 3 (b) in comparison with fig. 3(a). However, this speculation seems to be doubtful, because such spectra as 0, and 0; indicated by Wysocki' could not be detected in this study, perhaps owing to the contamination of Na ions in MgO (I) and (11) samples. Normally, no 0; ions can be directly produced when oxygen is contacted with MgO, but are formed through the agency of presorbed species such as pyridine,l3l1* h ydrogen, ethylene15 etc.16 Consequently, it seems to be difficult to form 0, ions without an oxygen source. The fact that Gel increases with increasing sodium content is a consequence of the high electron- donating ability of Na.This will also contribute to the formation of one-electron donor centres, as in the case of MgO (I). In this case the sodium electrons will be trapped in anion vacancies formed by y-irradiation to produce one-electron donor centres. With increasing sodium content Gel attains a maximum [fig. 3(a)] and then gradually decreases irrespective of the irradiation dosage. The decrease in the Cel signal may be ascribed to sodium covering the MgO surface, especially near anion vacancy sites, thus diminishing the adsorption of TCNE. The surface area of MgO and the corresponding samples doped with sodium compounds did not change before and after y-irradiation. Consequently, one may conclude that irradiation is responsible for the effect occurring only at one-electron donor centres.The surface of zinc oxide irradiated with y-rays sinter~,~' but no sintering was observed in this study of MgO. The most likely cause is probably the higher melting point of MgO compared with that of ZnO.T. Matsuda, K. Yamada, Y. Shibata, H . Miura and K. Sugiyama 3113 0 2 4 6 Na/mmol g,',, Fig. 6. Relationship between the activation energy for electric conduction and sodium content in MgO-NaNO, in two temperature regions : 0, high-temperature region (130-300 "C); A, low- temperature region (50-130 "C). The basic site of MgO is 02- on the electron-rich MgO surface." The basicity was increased by the doping of sodium compounds in MgO, followed by calcination." The electrons on one-electron donor centres or on the basic sites can be considered to be involved with electrical conductivity.The electrical conductivity of MgO doped with NaNO, at various temperatures was examined, and the results are shown in fig. 4. With progressively increasing doping in MgO and on raising the measurement temperature the electrical conductivity increases, which is similar to some extent to semiconductors. The temperature dependence of the average electrical conductivity of the samples is shown in fig. 5, from which the thermal activation energy of the electrical conductivity is obtained. As shown in fig. 5, a plot of temperature dependence against electrical conductivity exhibits a discontinuity. The activation energy in the high-temperature region, (i.e.above 130 "C) attained a constant value of 0.9 eV following the increase in sodium content (see fig. 6). In the low temperature region (below 130 "C) the activation energy is constant (0.4 eV) and independent of the sodium content. It is clear from fig. 5 and 6 that there are two different conduction-electron energy levels. Electrons in the higher energy level can easily jump into the conduction band in the low-temperature region with a low activation energy. On the other hand, the jump from the lower energy level in the high-temperature region requires a higher activation energy. The precise mechanism for electric conduction in Na-doped MgO is not yet clear, and other electrical conductivity measurements need to be made. By comparing the results in fig. 3(a) and 4 the electrical conductivity appears to correlate with the Gel of one-electron donor centres.On the other hand, the relationship between basicity and sodium content goes through a maximum at 0.87 mmol sodiumll g&, which is unlike the result of fig. 3(b). This shows that the electrons in one-electron donor centres will contribute to the electrical conduction. The electrical conductivity of the sample doped with NaNH, in pre-irradiated MgO was not measured in this study since it was difficult to prepare the sample without exposing pre-irradiated MgO to air. This sample will later be examined to study the effect of moisture in the air on the conductivity.31 14 Na Doping of y-Irradiated MgO We thank the Japanese Ministry of Education, Science and Culture for financial support in the form of a Grant in Aid for Special Research, and the Kohnoshima Chemical Co.Ltd for donating the magnesia samples. References 1 R. L. Nelson, A. J. Tench and B. J. Harmsworth, Trans. Faraday SOC., 1967, 63, 1427. 2 C. Naccache and M. Che, Proc. Fifth Int. Congr. Catal. (Amsterdam, 1972), 105-1. 3 J. H. Lunsford, J. Colloid Interface Sci., 1962, 26, 355. 4 J. H. Lunsford and T. W. Leland, J. Phys. Chem., 1968, 66, 2591. 5 J. H. Lunsford, J. Phys. Chem., 1964, 68, 2312. 6 J. Kijenski and S. Malinowski, Bull. Acad. Polon Sci., Ser., Sci. Chim., 1977, 25, 329; 501. 7 A. J. Tench and R. L. Nelson, Trans. Faraday SOC., 1967, 63, 2254. 8 D. Cordishini, V. Indovina and A. Cimino, J. Chem. Soc., Faraday Trans. I, 1974, 70, 2189. 9 S. Wysoki, J. Chem. SOC., Faraday Trans. 1 , 1986, 82, 715. 10 K. Tanabe, Shokubai no Hataraki (Kagakudohjin Pub. Co., Japan, 1974), p. 63. 11 T. Matsuda, Z. Minami, Y. Shibata, S. Nagano, H. Miura and K. Sugiyama, J. Chem. Soc., Faraday 12 S. Coluccia, A. Barton and A. J. Tench, J. Chem. Soc., Faraday Trans, I , 1981, 77, 2203. 13 T. Iizuka and K. Tanabe, Bull. Chem. SOC. Jpn, 1975, 48, 2527. 14 T. Iizuka, Chem. Lett., 1973, 891. 15 D. Cordischi, V. Indovina and M. Occhinzzi, J. Chem. Soc., Faraday Trans. 1, 1978, 74, 456. 16 E. Garrone, A. Zecchina and F. S. Stone, J. Catal., 1980, 62, 396. 17 W. Wysocki and H. Sugier, Radiochem. Radioanal. Lett., 1975, 20, 191. Trans. I, 1986, 82, 1357. Paper 611751; Received 29th August, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303107

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1987, 83 (lo), 3115-3128 Electron Spin Resonance Studies of Free and Supported 12-Heteropoly Acids Part 6.-The Investigation of Reduced H,(SiW,,O,,) a x H,O and Ag,(SiW,,O,,) * x H 2 0 and the Effects of Oxygen Adsorption Rolf Fricke, Hans-Georg Jerschkewitz and Gerhard Ohlmann* Central Institute of Physical Chemistry, Academy of Sciences of the GDR, DDR- 1199 Berlin- Adlershof, German Democratic Republic A number of W5+ e.s.r. signals have been observed when reducing 12- tungstosilicic acid and its silver salt with hydrogen or methanol up to 873 K. By means of various adsorption measurements and by comparison with results of molybdenum-containing heteropoly acids, a more detailed conclusion concerning the state of the samples could be drawn.The adsorption of oxygen, leading to the formation of 0; radicals, was shown to proceed on decomposed Keggin anions only. A strong interaction between the acid or silver salt and alumina was found to promote de- composition. Samples used as catalysts in the conversion of methanol showed only a narrow signal due to paramagnetic coke residues, possibly because the catalyst, partly reduced in the course of the reaction, is immediately reoxidized in air. Compared to methanol, the use of hydrogen facilitates reduction of the acid and the silver salt. During the last decade an increasing number of papers has been published, showing that heteropoly acids (abbreviated as HPA) lend themselves as catalysts for various catalytic reactions. Among these HPAs those containing molybdenum and/or tungsten and some of their salts have shown the most promising catalytic properties.'-' For this reason investigations have been extended to the study of properties which are related to the catalytic activity, such as for example, dehydration, thermal stability, acidity etc., applying a great variety of physical methods, including i.r.'-ll and PAS-F.t.i.r.12 spectroscopies, d.t.a./t.g.,?l3 t.p.d.and t.p.r.14-16 and, to a limited extent, e.s.r. spectroscopy. 7-19 In addition, it has been shown that tungsten-containing HPAs and their silver or ammonium salts are capable of transforming methanol to hydrocarbons3* '* 20-22. As far as e.s.r. spectroscopy is concerned, tungsten is an unfavourable element. Compared to molybdenum the six-valent state, which is diamagnetic and therefore not detectable by e.s.r., is very stable,,? 7 9 l9 i.e.its reducibility in hydrogen or methanol, at temperatures where catalytic reactions proceed (below 773 K), is relatively low. In addition, it has been that the spin-lattice relaxation time for 5d and 4d ions (W5+ is 5dl) is relatively short compared to 3d ions, so that observation of their e.s.r. signals is possibly difficult except at low temperatures. This may well be the reason why only few observations of W5+ e.s.r. signals have been reported in the literature : thus for 1Ztungstophosphoric or 12-silicic acids, the work of Prados and Pope17 and of Saidkhanov et aL6 represent the only known examples, both reporting W5+ signals from reduced HPAs in solution. Of course the reduction of HPAs is not just merely a demanding test of the e.s.r.method, as reduced HPAs can favourably increase the activity and stability of catalysts 31 1531 16 E.S.R. of 12-Heteropoly Acids in the conversion of methan01.~~~~ l1 The present contribution continues a series of e.s.r. studies on H,+,(PV,Mo,,.,O,,) (n = 0.. .3) heteropoly acids,lg' ,** 25 with tungsten- containing samples. Experimental Preparation Dodecatungstosilicic acid has been prepared from the appropriate solution of sodium silicate, sodium tungstate and hydrochloric acid according to the method well known in the literature.26 The product was cleaned by twofold extraction with ether followed by crystallization of the acid from aqueous solution. The water content was determined analytically to be 20-22 mol/mol.The silver salt [Ag,(SiW,,O,,)] was prepared by dissolving the stoichiometric amount of silver carbonate in a weak aqueous solution of the acid. After 24 h storage of the solution in a refrigerator the fine crystalline precipitate was separated by filtration. The supports used for the present investigations were Degussa silica, Aerosil-50 (Ox 50), specific surface area S = 50 m2 8-l: and Degussa Alumina-C, S = 100 m2 8-l. The preparation of the supported catalysts has been carried out in the same way as described for the PMo12/support sample^,'^ except that both supports were dehydrated for 3 h at 873 K before impregnation with the HPA. Only the alumina-supported silver salt has been prepared under different conditions. To obtain these samples, silver carbonate was dissolved at 333-343 K in an aqueous solution of the acid.The maximum amount of the solution was chosen to be that which could still be re-adsorbed by the alumina. The limited solubility of the silver salt required prompt working utilizing super- saturation conditions. Catalysts containing up to 13.5 wt. % W could be obtained in this way in one step. Higher concentrations required sample drying and repetition of the procedures described above. The samples are designated as SiW12-HPA and AggSiW12 for the unsupported HPA and its silver salt, respectively. The supported catalysts are additionally annotated by the support used (e.g. SiW12/Si02) and the concentration of the active component is given when necessary. E.S.R. Measurements The reduction of catalysts was carried out under static conditions in the e.s.r.sample tube. In general, 30 Torr (1 Torr = 101 325/760 Pa) of H, or 100 Torr of CH,OH were used for reduction. Owing to the observed instability of signals under vacuum the catalysts were not evacuated after reduction. The time of reduction was 1 h at each temperature, at 50 or 100 K intervals between 293 and 773 K (and 873 K in a few cases). Exposure of the reduced catalysts to the atmosphere was avoided. Oxygen adsorption (30 Torr) at room temperature was performed on catalysts pre- reduced with hydrogen at the temperatures given in the text and the excess oxygen was subsequently pumped off. E.s.r. measurements were performed at 77 K and room temperature with a ZWG ERS 220 spectrometer operating at X-band.An n.m.r. marker and a DPPH sample (g = 2.0036) were used for magnetic field calibration (the field values are given as 1 G = lo-' T) and for calculation of the signal intensity. The main signals obtained (signals 3-5) were computer-simulated using the program cOMPAR. 27R. Fricke, H-G. Jerschkewitz and G. Ohlmann I DPPH 1 31 17 g, 9 2 % m ;-,..'-t, 1 a , a . I . ; , . , ' .. I , , I I , ' I * I ( . I , b , ' 0 Fig. 1. E.s.r. Results Sample Treatment in Air After preparation, neither the HPA nor the Ag salt showed any e.s.r. signal. The samples were colourless or pale yellow,' sometimes also tending to greyness (Ag salt). They retained this colour when treated in air up to 773 K and no sample showed an e.s.r. signal, whether unsupported or supported on SiO, or A1,0,.This behaviour is different from that of PV,Mo,,., - HPAs, which showed a Mo5+ ( n = 0) or V4+ ( n = 1-3) signal immediately after preparation, the intensity of which did not decrease to zero even after treatment at 773 K in air or ~ x y g e n . ' ~ ~ ~ ~ Clearly, the well known stability of the six-valent state of tungsten in these conditions means that, in contrast to the PV,Mo,,., samples, the dehydration process for the current tungsten sample series could not be studied using e.s.r. spectroscopy. Sample Reduction with Hydrogen or Methanol The justification for studying the reduced state of the SiW,,-HPA and its Ag-salt is at least twofold: (a) it is known from X-ray and i.r. investigations that the Keggin anion structure is, at the most, only slightly influenced by reduction;2v28 (b) catalytic investigations have shown that pre-reduction of the silver salt influences the activity as well as the stability of the catalyst.'*'' The reduction which was performed, usually up to 773 K and in very few cases also at 873 K, changed the sample colour from white-pale yellow to blue-greyish, dark blue and black at higher temperatures.A number of e.s.r. signals could be observed, whose shape and parameters depended upon the conditions of reduction and the sample composition. The first signal, which appeared after a brief reduction at 323 K of the SiW,,-HPA, is signal 1 (fig. 1) with the following parameters: g , = 1.944, g,, = 1.901, A , = 41.8 G, A , , = 91.2 G. The hyperfine structure was usually not well resolved but could, however, be evaluated in a few cases which are not shown here in detail.In all of the cases investigated, signal 1 was observed31 18 E.S.R. of 12-Heteropoly Acids Fig. 2. E.s.r. spectra of SiW,,-HPA, reduced in H, at 573 K as a function of the time of reduction q.s.r. = 77 K. (a) 5 min, (b) 1 h, (c) 4 h. to be temperature-independent and was usually the first e.s.r. signal to appear when increasing the reduction temperature. A second signal, designated as signal 2 and also shown in fig. 1, was always of low intensity and was absent at room temperature. It was probably present for all samples, but was not resolved for the supported samples. The g values for signal 2 were calculated as: g, = 1.830, g, = 1.802, and g, = 1.786, being very similar to a low-temperature W5+ signal of reduced polytungstates described by Prados and Pope.” Fig.2 shows the sequence of e.s.r. spectra taken after reduction (H,) at 573 K as a function of time. Signal 1 is still present, but a new signal (signal 5 ) now appears, of comparable intensity. It is characterized by a g, value of 1.771. The g,, value could not be fixed owing to variation of the resonance position, depending upon time and conditions (see amplified dotted lines of the signals in the high-field part of the spectra). Details of this complication are shown for the spectrum recorded after 5 min reduction. Although the parallel component is clearly resolved, showing two different positions at g,, = 1.557 (5*) and g,, = 1.608 (9, the perpendicular component presents itself still as a single line.In a few cases, however, a shoulder was observed, or even an indication of splitting of the perpendicular component. Therefore, a reasonable explanation is that signal 5 is a superposition of signals, with g, values very close to each other (with these peculiarities in mind the signals will still be designated by signal 5, further on in the text). After 4 h reduction several new lines could be observed, one of which was identified asR. Fricke, H-G. Jerschkewitz and G. Ohlmann 31 19 Fig. 3. E m . spectra of SiW,,-HPA, reduced in H, for 1 h at: (a) 673 K, (b) 773 K, ( c ) 873 K. c.s.r, = 77 K (dotted lines show the room-temperature spectra). a new signal 3, which is better shown in fig. 3 and 4. Signal 5, which is the most characteristic signal for samples not supported on A1,0,, is sometimes visible also at room temperature; its intensity is variable, however, as is the intensity ratio 1(77 K):Z(295 K).Increase of the reduction temperature above 573 K, up to 873 K, produced the set of characteristic spectra shown in fig. 3. After reduction at 673 K, when the sample colour is already black, a better-resolved signal 3 is observed, together with signal 5 (or 5*). The resonance at g = 1.504 is not assumed to be a component of signal 5, as it occurs independently of signal 5, although always with low intensity. A sharp and narrow signal at g = 2.004, signal 9, could also be observed increasing in intensity as the temperature increased. After 873 K reduction, both signals 3 and 5 / ( 5 * ) disappeared almost completely and a signal at g,, = 1.907 and g , = 1.678 (signal 7) characterises the high- temperature reduced state.Signal 7 is undetectable, however, at room temperature [fig. 3(c)]. Two variations of the results described above are shown in fig. 5, which presents the spectra obtained after 573 K reduction with CH,OH. Fig. 5(a) and (b) show a superposition of the signals 1, 3 and 5, already identified from fig. 2. However, on increasing the time of reduction up to 6 h two new additional signals appeared : a narrow line at ca. g = 2.00 (signal 9*) and a similarly narrow, axial signal with g1 = 1.744 and g,, = 1.607 (signal 6). As shown by the broken line in fig. 5(c), signal 6 is strongly temperature-dependent and is not visible at room temperature.3120 E.S.R.of 12-Heteropoly Acids Fig. 4. E.s.r. spectra of an Ag,SiW,,/Al,O, catalyst (20 YO W) reduced with H,, 1 h at: (a) 373 K, (b) 473 K, (c) 573 K, ( d ) 673 K, (e) 773 K. q.s.r. = 77 K (dotted spectra were taken at room temperature). The signals described so far were observed more or less unchanged for all samples, provided that the active component was not supported on Al,O,. Fig, 4 gives the general view for the Ag,SiW,,/Al,O, catalyst taken as an example. At low temperature the picture is similar to the free acid or Ag salt (including the silica-supported samples), showing signals 1 and 3 superimposed. After 773 K reduction, however, a superposition of signal 3 with a new signal is obtained, designated as signal 4, with g , = 1.783 and g,, = 1.676.In all experiments, signal 4 was only observed for alumina-supported samples, regardless of whether the acid or the silver salt had been reduced. In contrast to signals 2 and 3 signal 4 could be observed at both low temperature and at room temperature (see the dotted line in fig. 4). For reasons of completeness, it should additionally be noted that another signal, not separately shown in one of the present figures, appeared at ca. g = 4.2 (signal 8) when studying the alumina-supported samples. This signal is well known from the literature and is usually assigned to Fe3+ ions in tetrahedral coordination, caused by impurities in the support. Adsorption of Oxygen These experiments were undertaken because recent results obtained from supported SiMo,,-HPAlg and PV,MO,,-,-HPAS~~ have shown that, if taken with necessaryR.Fricke, H-G. Jerschkewitz and G. Ohlmann 3121 Fig. 5. E.s.r. spectra of a SiW,,/SiO, catalyst (20% W) reduced with CH,OH at 573 K as a function of time of reduction, q+s-r. = 77 K (dotted spectrum is taken at room temperature). (a) 5 min, (b) 1 h, (c) 6 h. DPPH g1 I I g3 Fig. 6. 0; e.s.r. spectrum of an SiW,,/SiO, catalyst (20% W) (prereduced at 723 K with H, and 25 Torr 0, adsorbed at room temperature), q.s.r. = 295 K.3122 A 7 / / / E.S.R. of 12-Heteropoly Acids A 12 n c U .r( d 2 8 - g - U n 4 4 - - B !"i I '\ I \ I \ I \ \ I ' I 1 \ I \ I \, ( b ) I I 373 573 773 TIK Fig. 7. A, Amount of coke [I(C'), in arbitrary units] us. the time of methanol conversion derived from: (a) Ag,SiW,,/A1,03 (20% W), e.s.r. signal intensity I; (b) Ag,SiW,,/SiO, (20% W), e.s.r.signal intensity I; (c) sample (a), chemical determination of carbon contents. B, e.s.r. signal intensity I(C') of a paramagnetic coke signal us. the temperature of reoxidation (1 h at each temperature). The catalysts have been used for 1 h in the conversion of methanol at 573 K. (a) Ag,SiW12/A1,0, (20% W), (b) Ag,SiW,,/SiO, (20% W), (c) WO,/SiO, (20% W), (d) WO,+Ag,/SiO, (20 % W), (e) WO,/Al,O, (10 % W), (f) Ag,SiW12, unsupported. caution, the results can give information not only on the properties of oxygen radicals formed under special conditions but also on the state and structure of the Keggin anion. Neither unsupported SiW ,,-HPA nor its silver salt ever generated stable oxygen radicals (0- or 02).This observation is in accordance with previous results on other unsupported H P A S ' ' ~ ~ ~ and supports the conclusions drawn in these papers that the inability of the HPAs to generate stable oxygen radicals is probably a result of fast electron recombination in the bulk of the Keggin anion. In contrast to the unsupported samples, a radical signal was observed after oxygen was adsorbed on supported catalysts, pre-reduced at 723-773 K in hydrogen (fig. 6). Comparison of the parameters of this signal (8, = 2.026, g , = 2.014, g , = 2.005) with the literature data2' allows this signal to be assigned to 0; oxygen radicals stabilized at W6+ sites. Adsorption of H, on a sample showing this signal caused no change, showing that 0- radicals, which react very quickly with H,, were not present on the surface of the catalyst.The Ag,(SiW,,O,] Catalyst after Use in the Methanol Conversion E.s.r. spectra were recorded for both the unsupported and the supported silver salt after being used in the conversion of methanol to alkenes at 573 K.* After reaction, the catalysts were stored in air for some hours. For all of the samples studied, only signalM rn 2 6 1 s cj % 9 8 - I I I 9 8 q 8 I € I I 9 8 I (M % oz) 'O~IV/"M!S'~V (M % OZ) zO!S/"M!S'~V "M!S 'SV (M O h 0 I ) cOzW/zlM!S ( M %OZ> zO!S/zTM?S VdH-"M!S (M % 02) E ~ Z ~ ~ / Z K ~ ~ ~ ' ~ ~ (M % 02-S) zO!S/zlM!S'%' zKM!S'SV (M % 0 I) 'OzIV/"M!S (M YoOZ) ZO!S/ZKM!S VdH-"MIS €11 EL9 u s ElP €L€3124 E.S.R. of 12-Heteropoly Acids 9* could be observed, the signal intensity depending upon the time of reaction [fig.7(a)]. The conditions under which signal 9* appeared, whether after reduction with methanol or after use in methanol conversion, clearly indicate that this signal originates from paramagnetic coke residues on the catalyst. Obviously, this signal does not represent all the coke present on the catalyst because, as shown in fig. 7(b), treatment in air at temperatures up to 773 K led to very pronounced changes in the signal intensity. Other supported tungsten catalysts containing WO, or a mixture of WO, and Ag,O were treated in the same way as the silver salt, and the following conclusions can be derived [fig. 7(b)]: (a) the most intense coke signals were found on silica-supported samples, showing maximum intensity at a reoxidation temperature of ca. 573 K ; (b) the silver component is responsible for the promotion of the coke formation, represented by signal 9*, because W/SiO, and W/Al,O, catalysts studied for comparison showed no signal.The absence of any signal for the unsupported Ag,SiW,, salt can be explained both by the absence of silica and the low surface area, which does not allow deposition of great absolute quantities of coke on its surface. Discussion Owing to the lack of any e.s.r. signal under oxidative conditions, it is impossible to discuss the dehydration process of SiW,,-HPA as has been recently accomplished for the PV,Mo,,-,-HPAs (n = 0-3).19* 24 In addition, the present studies show that the reduction of tungsten proceeds at temperatures above 473 K, i.e.temperatures where large amounts of the water of crystallization have already e s ~ a p e d . ~ , ~ For a better understanding of the state of the HPA under the conditions described in this paper it is necessary to elucidate the origin of the e.s.r. signals summarized in table 1 in terms of the type of sample and the temperature of reduction. Signal 1 A signal similar to signal 1 was found by Saidkhanov et aL6 for a SiW,,-HPA in solution and by other authors for various tungsten-containing catalysts,30’ 31 and is unanimously attributed to W5+ ions. It is, however, proposed from the present results, that this signal originates from Mo5+ impurities rather than from W5+ species, for the following reasons. The g-values for this signal are relatively too high to be attributed to the W5+ species.More conclusively, a detailed analysis of the hyperfine structure, clearly shows 6 h.f.s. lines for each signal component (h.f.s. separation is shown in table 2), whereas there should only be 2 h.f.s. lines per signal component for lE3W, for which I = 1/2 with 14.28 % natural abundance. Indeed, the presence of molybdenum in all samples, owing to Mo impurities in the WO,, has been proved by emission spectral analysis. HCl was adsorbed onto several samples in an attempt to produce ligand exchange which would be reflected in g-value inversion from g , > g,l to g, < g,l.32 No changes were, however, induced in signal 1, showing that the Mo5+ impurities were fully incorporated in the Keggin anion structure, rather than existing at the ‘surface’.Signal 2 In an early paper, Prados and Pope17 obtained a very similar signal, with slightly higher g, and g, values, from various frozen solutions of one-electron, electrochemically reduced pofytungstates, and attributed the signal to electrons trapped on rhombic tungsten centres. Owing to the observed similarity of the g values and the conditions of generation (one-electron redu~tion’~ and low-temperature reduction in our case) this signal is assumed to originate from the same type of tungsten ions. The signal was foundR. Fricke, H-G. Jerschkewitz and G . Ohlmann 3125 Table 2. E.s.r. signals observed after reduction (H, or CH,OH) of SiW,,-HPA and its silver salt (unsupported and supported on SiO, or A1,0,) with the possible assignment to the HPA structure as discussed in the paper signal g , gll origin 1 1.944 1.901 impurity of Mo5+ ions 2 1.830 1.802 1.786 W5+ (electrons trapped on rhomb.W centres") 3b 1.838 1.743 W5+ of Keggin anions (KA) fragments (incompletely 4b 1.783 (1.676) W5+-O-Al ' phase ' 5' 1.771 W5+, KA strongly distorted 5** 1.771 :%I} but undestroyed 6 1.744 1.607 unknown 7 1.678 1.907 W5+ of destroyed KA 8 ca. 4.2 Fe3+ impurity of A1,0, support 9 2.00 lattice defect of KA 9* 2.00 paramagnetic coke residue (41.8)" (91.2)" crystallized regions) " H.f.s. splitting. reduction). Spectra ~imulated.,~ Parallel components derived from fig. 2 (5 min to be of low intensity at 77 K but, owing to the strong temperature-dependence of this signal,17 it can be expected that its intensity would increase drastically at temperatures lower than 77 K.Signal 3 This signal is especially well characterised by its g-values and by the behaviour of the signal on the adsorption of different species. The signal disappeared when oxygen or air was adsorbed, suggesting that the appropriate species were surface ions. The signal which was only observed at low temperature (77 K), disappeared when the sample was evacuated after reduction. Adsorption of H,O, CH,OH, 0, or air regenerated the original signal and for the last two gases, an 0; signal was superimposed. (However, observation of the signal requires removal of the excess oxygen.) To our knowledge this signal has not been described before, perhaps because of its instability. According to the g-values, the signal can be attributed to a dl species, i.e.W5+ ions in the present case. From the temperature-dependence of the signal and the sample behaviour under vacuum it can be deduced that the spin-lattice relaxation time is short, pointing to a rather symmetric coordination sphere which is further enhanced on sample evacuation (and, vice versa, decreased by adsorption). One may therefore conclude that signal 3 originates from low (possibly tetrahedral) coordination W5+ surface ions, which were not incorporated in the Keggin anion structure and which probably belong to incompletely crystallized regions of the HPA. Signal 4 Because this signal was only observed on alumina-supported samples, it may be concluded that it is the special interaction between the HPA (or its silver salt) and A1,0, which defines the signal.It is well known from e.s.r. investigations of PV,MO,,~,/A~,O,~~~~* or ESCA studies of WO,/AI,O, catalyst^,^^,^^ that the inter- 103-23126 E.S.R. of 12-Heteropoly Acids action of the active components with the alumina support is rather strong, giving rise to facile decomposition of the Keggin anion s t r u c t ~ r e . ~ ~ ~ 24 Analogously, this is also suggested here for the reduced SiW,,/Al,O, sample (or the Ag-salt/Al,O,), especially as infrared studies have shown that destruction of the Keggin anion under oxidizing conditions is already complete at 613 K (and at ca. 660 K, respectively). l3 It is reasonable to expect formation of A14(SiW1204,,)3 salts under the present conditions, because both the SiW,,/Al,O, and the aluminium salt showed similar properties in the catalytic conversion of methanol as well as in i.r.and Raman ~pectroscopy.~~ On the other hand, for WO,/Al,O, samples, Salvati et aZ.,, did not find any evidence for the appropriate compound Al(W0,). E.s.r. measurements of the unsupported salt as well as silica-supported A14(SiW1204,,), showed signal 5 rather than 4 after stepwise reduction in hydrogen up to 773 K. Therefore, signal 4 is, with the necessary caution, attributed to a ' W5+-O-Al phase' from the destroyed SiW,,-HPA and not to the formation of the aluminium salt. Signal 5 This is the most characteristic signal of the free SiWl,-HPA and its silver salt, as well as of the silica-supported catalysts. There is sufficient information available concerning the stability of the Keggin anion structure of SiW,,-HPAs and of PW,,-HPAs, to show that the anion structure of these unsupported acids is stable up to ca.773-823 K.,, '9 16$ l3 It is possible to conclude that signal 5 is, in addition to signal 2, a further W5+ signal representing the undestroyed Keggin anion, which appears at conditions of increased reduction (i.e. at higher temperatures than signal 2). The coordination sphere is rather stable because adsorption of H,O, CH,OH or 0, did not influence signal shape or intensity. Prolonged evacuation led to decreased intensity, observed especially at room temperature, which was, however, reversed after the re-adsorption of H,O, CH,OH or 0,. The formation of 0; radicals was not observed for samples showing signal 5.It is suggested that these W5+ species are coordinatively saturated under the conditions specified, with approximately distorted octahedral or square-pyramidal symmetry, because oxygen is unable to enter the coordination sphere to broaden the signal or to form oxygen radicals. The changes observed during evacuation can be attributed to smaller structural variations of the W5+ coordination state which influence the spin-lattice relaxation time to a certain degree. A similar W5+ signal (8, = 1.767, g , , = 1 S89) described by Abdrakhmanov et al.35 has no relationship to the HPA structure because it originates from [WOF5I2- species in frozen solution. It should be mentioned that there exists some similarity between the signal 5 described here and a Mo5+ signal (signal F) observed for reduced PMo,,-HPA, the latter being attributed to highly distorted but undestroyed Keggin ani0ns.l' To explain the appearance of the different g values (signals 5 and 5*), further investigations will have to be made.Signals 6 and 7 The origin of these signals, and especially of signal 6, is not fully clear. However, because signal 7 could only be obtained after reduction at temperatures as high as 773-873 K, it is suggested that it arises from W5+ ions in a destruction product of the Keggin anion. No further information is available at the moment.R. Fricke, H-G. Jerschkewitz and G. Ohlmann 3 127 Signals 9 and 9* Both signals have approximately the same g value of 2.00, their lines are very narrow and were obviously caused by radicals.Signal 9 is only observed after high-temperature reduction with hydrogen and can be attributed to paramagnetic lattice defects of the Keggin anion or to its destruction products. It is well known from the literature36 that reduction of catalysts with hydrocarbons generally produces coke, which is often paramagnetic and shows an e.s.r. signal. This is also the case for the HPAs reduced by methanol or following use in the catalytic conversion of methanol, thereby generating signal 9*. It is possible that signal 9 may be superimposed but cannot be distinguished, while the different origins of the signals are obvious. Conclusions The present e.s.r. studies have shown that, besides the W5+ signal already described by Prados and Pope,17 additional characteristic W5+ signals could be observed and described, representing the state of the SiW,,-HPA (table 2).The conditions of formation of the 0, oxygen radicals allow some further conclusions. As shown in more detail for the supported SiMo,,-HPA,ls the undestroyed Keggin anion is unable to stabilize oxygen radicals. This property of HPAs can be confirmed here also for SiW,, -HPA, supporting the conclusion above, that signals 3 and 4 represent only anion fragments, whereas signal 5 represents the distorted but undestroyed Keggin anion. For reasons of completeness it should also be mentioned that, in addition to the signals summarized in table 1, several peaks or shoulders have also been observed [see for example, fig. 2 (4 h spectrum) and fig. 3(a)] which obviously also arise from paramagnetic tungsten species, It was impossible, however, to correlate meaningfully the appearance of these signals with the type of sample or with the conditions of measurement and therefore they have not been discussed in this paper.The appearance of W5+ signals shows that reduction with hydrogen starts at ca. 473 K. This is a lower temperature than is usually derived from t.p.r. meas~rements.~* l3? 1 4 7 l6 Comparing these results, one has however to take into consideration that e.s.r. spectroscopy is more sensitive than t.p.r., but inferences from both methods coincide at higher reduction temperatures. In accordance with recent t.p.r. studies13 it was found that the reducibility of the HPAs with methanol is lower than with hydrogen.Although the types of signals were the same in both cases (apart from signal 9*), they appeared, from a rough estimation, at a temperature ca. 100 K lower when hydrogen was used. Reduction was also observed to be more facile for the silver salt compared to the acid, an effect which has also been observed by t.p.r.'ll3 Though it is known that the Agf in the salt is easily reduced to metallic silver, no signal was observed which could originate from paramagnetic silver species or, with respect to tungsten, which could be interpreted as a specific influence of the silver on one of the W5+ signals. In connection with catalysis it is undoubtedly a disadvantage that the only signal which was observed after the catalytic reaction was that arising from paramagnetic coke residues.Akimoto et al.' have found that a partly reduced PW,,-HPA is immediately reoxidized by oxygen at room temperature and therefore it seems possible that reoxidation could be a probable explanation for the absence of any W5+ signal, especially as the present reduction studies with methanol have shown that reduction takes place at 573 K.3128 E.S.R. of 12-Heteropoly Acids The authors thank Dr Alan Ellison (Humberside College of Higher Education Hull) for kind support. Thanks are also due to Dr U. Ewert (Center for Scientific Instruments) for making his simulation program COMPAR available, Dr R. Matschat, for carrying out the emission spectral analysis, and K. Menning for providing a literature analysis of this series. The technical assistance of Mrs W.Ebert, I. Otremba, and Mr U. Marx is gratefully acknowledged. 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Soc., Faraday Trans. I , 1987, 83, 1049. 26 G. Brauer, Handbuch der Praparativen Anorganischen Chemie, 2. Auflage (F. Enke-Verlag, Stuttgart, 27 U. Ewert, Thesis (Humboldt University Berlin, 1979). 28 M. T. Pope, Heteropoly and Isopoly Oxometalates (Springer-Verlag, Berlin, 1983). 29 M. Che and A. J. Tench, Adv. Catal., 1983, 32, 1. 30 A. J. A. Konings, W. L. J. Brentjens, D. C. Koningsberger and V. H. J. de Beer, J. Catal., 1981, 67, 31 S. Sivasankar and A. V. Ramaswamy, 6th Nut. Symp. Rec. Adv. Catal. Catal. React. Eng., Pune (India) 32 R. Fricke, W. Hanke and G. Ohlmann, J. Catal., 1983, 79, 1. 33 L. Salvati Jr, L. E. Makovsky, J. M. Stencel, F. R. Brown and D. M. Hercules, J. Phys. Chem., 1981, 34 I. E. Wachs, C. C. Chersich and J. H. Hardenbergh, Appl. Catal., 1985, 13, 335. 35 R. S. Abdrakhmanov, N. S. Garifyanov, E. I. Semyonova, Z. Strukt. Khim., 1968, 9, 530. 36 J. M. Peacock, M. J. Sharp, A. J. Parker, P. G. Ashmore and J. A. Hockey, J. Catal., 1969, 15, 379. Mitchell (Climax Molybdenum Corp., Ann Arbor, 1982), p. 289. B, p. 1414. 21, 365. Ohlmann, Appl. Catal., submitted. Anorg. Allg. Chem., 1981, 472, 185. Bull. Chem. Soc. Jpn, 1982, 55,400. Amsterdam, 1984), p. 77. SSSR, Ser. Khim., 1981, 4, 717. 1962), p. 1489. 145. 1983, p. 95. 85, 3700. Paper 6/1814; Received 10th September, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303115

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J . Chern. Soc., Faraday Trans. 1 , 1987, 83 (lo), 3129-3138 The Ethane- 1,2-dio1-2-Methoxyethanol Solvent System The Dependence of the Dissociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Gian Carlo Franchini, Lorenzo Tassi and Giuseppe Tosi* University of Modena, Department of Chemistry, Via G. Campi 183, 41100 Modena, Italy Dissociation constants of 2,4,6-trinitrophenol (picric acid) in a series of ethane- 1,2-diol-2-methoxyethanol mixtures have been determined by the conductance method in the temperature range from -10 to 80°C. The dissociation constants exhibit different variations with temperature in different solvent systems, but they are well fitted by identical equations of the type In K = a, + a, T+ a,/ T+ a3 In T, from which thermodynamic functions data were also evaluated and discussed.With regard to solvent systems in which a K,,, value is observed [i.e. for the solvent mixtures having X(ethane-1,2-diol) > 0.26771, the correlations between K and T obtained from fitting the equation above by Harned's theory and by a pK us. I/& graph, gave results consistent with each other. As part of systematic studies on the influence of solvents on the dissociation constant of weak electrolyte^,^-^ in a recent study5 we reported the dissociation constant of picric acid in ethane-1,2-diol and 2-methoxyethanol at 19 temperatures between - 10 and 80 O C , evaluated from conductance data. The dependence of K on temperature was different for the two solvents, showing a maximum in the case of ethane-1,2-diol and a continuous decrease in the case of 2- methoxyethanol.Thermodynamic data have also been evaluated, and an explanation of their variations with temperature, which were very different for the two solvents, was suggested in terms of the ability of the ion pairs and/or other species to orient the sol- vent molecules in their immediate neighbourhood and the dependence of the dielectric constant and viscosity on temperature. In order to investigate the influence of structural changes in the solvent systems on the dissociation of weak electrolytes in amphiprotic media, we have planned to extend the study to binary mixtures of ethane- 1,2-diol (E = 41.06 at 25 "C) and 2-methoxyethanol (E = 16.94 at 25 "C), using picric acid as solute. One of the most difficult problems in studies involving non-aqueous solvents is the nature of the ion-solvent interaction ; the inapplicability of the models used to define the phenomena in aqueous solution suggests the use of mixed solvent systems since, for example, these systems make a continuous variation of the dielectric constant possible, and it can be assumed that the behaviour of ions reveal more clearly their unusual properties.Experimental Materials 2,4,6-Trinitrophenol (picric acid), supplied by Fluka (reagent grade) was used without further purification. The solvents ethane- 1,2-diol and 2-methoxyethanol (containing < 0.10 and 0.05 YO water, respectively, found by Karl Fischer titration) were Carlo Erba high-purity grade. 31293130 The Ethane- 1,2-diol-2-Methoxyethanol System Apparatus Conductances of the solutions were measured with an Amel model 123 conductivity bridge operating in the 0.1 x 10-'-0.3 S (scale-end) range, with a sensitivity of f 1 .O %, and using platinized platinum electrodes (cell constant 0.98 cm).The temperature control was provided by a Lauda K2R thermostatic bath maintained to f0.02 "C. Viscosity measurements were performed using a Schott-Gerate AVS 400 viscosity-measuring system, equipped with a series of Ubbelhode viscometers. Densities at the different temperatures were determined with calibrated Hg densimeters (sensibility 0.0005 g ~ m - ~ ) . The dielectric constants were measured using a WTW-DMO 1 dipolometer equipped with two stainless-steel cylindrical cells: MFL2 (7 < E < 21) was calibrated with dichloromethane ( E = 9.08 at 20 "C), pyridine ( E = 12.30 at 25 "C), butan- 1-01 ( E = 17.80 at 20 "C) and acetone (E = 20.70 at 25 "C);' MFL3 (21 < E < 90) was calibrated with ethanol (E = 24.30 at 25 "C), methanol ( E = 32.63 at 25 "C), glycerol ( E = 42.50 at 25 "C) and water ( E = 80.37 at 20 oC).s A frequency of 2.0 MHz was used.Karl Fischer titrations were performed with an automatic titration system (Crison model KF43 1) equipped with a digital burette (Crison model 738). Procedure The solvent mixtures were prepared by weight and all weights were corrected to vacuum. Solutions of picric acid of different concentrations were obtained by successive dilutions of stock solutions, prepared as previously reported. Conductance readings were recorded when they became invariant with time; this took ca.30min for each measurement. Solvent conductance corrections were applied to all the data at different temperatures. Results and Discussion In order to determine the dissociation constants and the A, values for 2,4,6- trinitrophenol in mixed solvents, conductance measurements were performed at temperatures in the range from - 10 to 80 "C for at least six different concentrations for each mixture; the experimental data, corrected at each temperature with the specific conductances of the solvent systems, are reported in six tables available as supplementary publication no. SUP 56682.f. The corrected values were analysed and processed by the method of Fuoss and Shedlowsky,' as was done in our previous paper for the picric acid in the pure ethane- 1,2-diol and 2-methoxyethanol solvent^,^ using the relation 1 1 cASf2 -- - -+- AS A, KA; where the symbols have the usual significance.By plotting l/AS us. cASf 2, A, and K were evaluated from intercept and slope, respectively; an initial value of A, is estimated from a plot of A us. c1I2 and then only a few iterations are necessary to obtain A, and K. In order to calculate the thermodynamic parameters AGO, AW and AS", all the experimental K values for each solvent mixture were fitted to an equation of the form where T is the absolute temperature. Table 1 summarizes the composition of the solvent systems ( X = mole fraction) and the values obtained for a,, a,, a2 and a3. Tables 2-7 contain the values of A, and K obtained at different temperatures for all the systems investigated.An interesting variation of K with temperature for picric acid in the six mixtures and f' See Notices to Authors, J. Chem. SOC., Faraday Trans. 1, 1987, 83, January issue. lnK= a,+a,T+a2T1+a31nT ( 1 )G. C. Franchini, L. Tassi and G. Tosi 3131 Table 1. Composition of solvent systems and fitted a,, u,, a, and a3 coefficients of eqn (1) solvent X(ethane- system 1,2-diol) a0 a1 a2 a3 A" B C D E F G H a 1.000 0 0.927 2 0.849 9 0.679 9 0.485 6 0.261 4 0.135 9 0.000 0 317.253 1 38.538 3 533.212 3 1841.847 0 1003.626 3 - 18.428 9 - 1402.572 7 -2148.102 6 0.046 48 0.102 04 0.472 65 0.238 32 -0.042 21 -0.033 30 -0.463 92 -0.690 28 - 11 683.34 -4 820.59 - 17 711.32 -52 593.54 - 29 655.13 - 789.85 35 320.46 54 518.42 - 52.290 8 -2.779 1 -89.587 4 -318.250 0 - 172.454 6 3.939 0 247.967 4 379.184 5 2.5 - 2.0 - 1.5 - a Ref.(5). A B C qq 0 1 I I I I c F -10 10 30 50 70 t/" c Fig. 1. Dissociation constant of picric acid in ethane- 1,2-dio1-2-methoxyethanol solvent systems as a function of the temperature (for the meaning of letters A-H see table 1). in the two pure solvents5 is shown in the fig. 1; the letters A and H refer to the pure ethane- 1,2-diol and 2-methoxyethanol solvents, respectively, and B-G to the above- mentioned mixtures (see table 1). The maximum in the K value shifts to lower temperatures on passing from A to E, i.e. as X(ethane-1,2-diol) decreases (fig. 1 and table 8), whereas curves F-H show a continuous decrease as the temperature increases.Table 8 reports a comparison of the KmaX values obtained by fitting eqn (1) by3132 The Ethane- 1,2-diol-2-Methoxyethanol System Table 2. Dissociation constants and limiting molar conductances of picric acid in solvent system B K/ 103 A0 t/OC E , mol dm-3 /S mol-' cm2 A,q A0q0.7 - 10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 46.04 44.88 43.74 42.64 41.56 40.51 39.48 38.48 37.51 36.56 35.63 34.73 33.85 33.00 32.16 3 1.35 30.55 29.78 29.03 1.70 f 0.01 1.86 f 0.01 1.97 f 0.01 2.08f0.01 2.18 fO.01 2.27 f 0.0 1 2.32 f 0.01 2.37f0.01 2.37 f 0.01 2.38 f 0.01 2.36 f 0.02 2.34 f 0.02 2.30 f 0.02 2.25 f 0.02 2.17 fO.01 2.09 f 0.01 1.99 k 0.02 1.88 f 0.02 1.80 f 0.01 7.1 fO.l 8.3fO.l 10.3 f 0.1 12.1 f 0.2 13.8 f0.2 16.0 f 0.2 18.5 f0.3 21.6 f 0.3 24.9 f 0.9 29.3 f 0.4 32.6 f 0.4 36.7 f 0.5 41.2f 0.5 46.0 f 0.6 51.1 f0.6 56.6 f 0.7 62.3 k0.8 68.4 f 0.8 74.4 f 0.9 5.04 5.59 4.42 5.34 4.18 5.47 3.80 5.39 3.38 5.16 3.12 5.10 2.89 5.04 2.76 5.12 2.65 5.19 2.61 5.39 2.47 5.36 2.38 5.42 2.31 5.49 2.25 5.56 2.19 5.63 2.13 5.70 2.07 5.76 2.02 5.80 1.95 5.80 Table 3.Dissociation constants and limiting molar conductances of picric acid in solvent system C K / 103 A0 t/OC E , mol dmP3 /S mol-' cm2 Aoq A0q0.7 - 10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 43.19 42.10 41 -03 39.99 38.98 37.99 37.03 36.09 35.18 34.28 33.42 32.57 31.74 30.94 30.16 29.39 28.65 27.92 27.22 1.50f0.01 7.0fO.l 3.97 4.71 1.66 f 0.01 8.4f0.1 3.58 4.62 1.77 f 0.02 9.9f0.1 3.23 4.52 1.87 f 0.01 1 1.8 f 0.2 2.98 4.50 1.94 f 0.02 13.8 f 0.2 2.77 4.48 1.99f0.02 16.0f0.2 2.56 4.44 2.00f0.02 18.7f0.2 2.43 4.48 2.00 f 0.02 21.4 f 0.3 2.29 4.48 2.01 f0.02 24.3f0.3 2.17 4.48 2.00 f 0.02 27.5 f 0.4 2.06 4.48 1.99 & 0.02 30.8 f 0.4 1.98 4.50 1.94 f 0.02 34.4 f 0.5 1.90 4.52 1.92f0.02 38.1 f0.5 1.83 4.55 1.86 f 0.02 41.9 f 0.6 1.76 4.56 1.79 k0.02 45.8 f 0.6 1.70 4.57 1.71 f 0.02 50.2 f 0.7 1.65 4.60 1.65 f 0.02 55.4 f 0.7 1.62 4.68 1.57 f 0.02 60.3 f 0.8 1.58 4.71 1 S O f 0.02 65.1 f 0.9 1.53 4.71G. C.Franchini, L. Tassi and G . Tosi 3133 Table 4. Dissociation constants and limiting molar conductances of picric acid in solvent system D K / 103 A0 t/OC E, mol dm-3 /S mol-' cm2 A. q A. qO.' ~ - 10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 37.60 36.61 35.65 34.72 33.81 32.92 32.06 31.22 30.40 29.61 28.83 28.08 27.34 26.63 25.93 25.25 24.59 23.95 23.32 0.775 & 0.006 0.865 f 0.007 0.932 f 0.008 0.972 f 0.009 1.02 f 0.01 1.03 fO.O1 1.04f 0.01 1.02 f 0.01 0.998 f 0.009 0.952 f 0.009 0.920 f 0.009 0.889 f 0.009 0.861 &0.010 0.837 & 0.009 0.804 f 0.009 0.768 f 0.009 0.743 f 0.009 0.702 f 0.009 0.664 f 0.009 7.7 f 0.1 9.0 f 0.1 10.4k0.2 12.2k0.2 14.1 k0.2 16.2 k 0.2 18.5k0.2 21.1 k0.3 24.0 & 0.3 27.2 k0.3 30.5 f0.4 34.1 f 0.5 37.7 f 0.5 41.4f0.6 45.3 f0.7 49.5 f0.7 51.9 f 2.7 56.1 f3.1 60.2 f 3.8 2.17 3.17 2.03 3.17 1.89 3.15 1.79 3.18 1.69 3.19 1.61 3.22 1.54 3.25 1.49 3.29 1.44 3.35 1.40 3.41 1.36 3.45 1.31 3.49 1.26 3.50 1.22 3.50 1.17 3.50 1.14 3.54 1.08 3.44 1.06 3.50 1.06 3.57 Table 5.Dissociation constants and limiting molar conductances of picric acid in solvent system E ~ 1 1 0 4 All tl°C El3 mol dm-3 /S mol-1 cm2 Aoq AOqO.' - 10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 31.76 30.91 30.08 29.27 28.49 27.73 26.99 26.26 25.56 24.88 24.21 23.56 22.93 22.32 21.72 21.14 20.57 20.02 19.49 6.16 f 0.06 6.44 f 0.06 6.58 f 0.07 6.76 & 0.07 6.68 f 0.07 6.68 & 0.07 6.67 f 0.07 6.45 f 0.07 6.14f0.07 5.80 f 0.07 5.55 f0.07 5.30 f 0.07 5.04 f q.07 4.75 f 0,07 4.50 f 0.06 4.25 rfr 0.06 4.02 f 0.09 3.73 f 0.06 3.47 f 0.06 6.9f0.1 8.6k0.2 10.3 f0.2 12.0f0.2 14.0 & 0.2 16.0 f 0.3 18.2f0.3 20.5 f 0.3 23.1 f 0.3 25.8 f0.4 28.7 f 0.4 31.7 f 0.5 34.8 f 0.5 38.1 & 0.6 41.5 f 0.7 44.9 & 0.7 48.4 k 0.8 51.9 f0.9 55.7 f 0.9 0.95 1.73 1.00 1.90 1.01 2.02 0.99 2.10 0.97 2.16 0.93 2.18 0.90 2.21 0.86 2.24 0.84 2.27 0.82 2.30 0.80 2.34 0.79 2.38 0.78 2.43 0.77 2.49 0.76 2.53 0.75 2.57 0.74 2.60 0.72 2.61 0.70 2.613134 The Ethane- 1,2-diol-2-Methoxyethanol System Table 6.Dissociation and limiting molar conductances of picric acid in solvent system F - 10 -5 0 5 10 15 20 25 26.17 25.39 24.63 23.90 23.17 22.48 21.81 21.16 2.63 f 0.06 9.6f0.2 0.68 1.50 2.54 f 0.06 11.2 f 0.3 0.67 1.56 2.45 k 0.05 12.9 f 0.3 0.66 1.61 2.35 f 0.05 14.7 f 0.3 0.64 1.64 2.26f0.05 16.6f0.3 0.62 1.67 2.16k0.05 18.6f0.4 0.61 1.70 2.02 k 0.05 20.8 f 0.4 0.60 1.89f0.05 23.2f0.5 0.59 30 20.52 1.79i0.05 25.5s0.5 0.57 35 19.91 1.71f0.05 27.9k0.6 0.56 40 19.31 1.60f0.05 30.5f0.7 0.55 45 18.73 1.50f0.05 33.2k0.8 0.54 50 18.17 1.41f0.04 35.9f0.9 0.53 .73 .76 .79 .80 .83 .86 .88 55 17.63 1.31k0.05 38.8f1.0 0.52 1.90 60 17.10 1.22f0.04 41.8k1.1 0.51 1.92 65 16.59 1.15k0.05 44.4f1.2 0.50 1.92 70 16.09 1.06f0.04 47.7f1.3 0.50 1.95 75 15.61 0.971f0.040 50.9f1.5 0.49 1.97 80 15.14 0.887f0.036 54.2f 1.4 0.48 2.00 Table 7.Dissociation constants and limiting molar conductances of picric acid in solvent system G K/ 104 A0 t/OC E, mol dm-3 / S mol-l cm2 Aoq A0q0.7 - 10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 22.79 22.21 21.66 21.1 1 20.59 20.06 19.56 19.07 18.59 18.12 17.67 17.22 16.79 16.37 15.96 15.56 15.16 14.78 14.4 1 1.82 f 0.03 10.3 f 0.2 0.50 1.23 1.64f0.03 12.1 f0.2 0.49 1.31 1.45f0.03 14.2f0.3 0.51 1.39 1.23 f 0.03 16.7 f 0.3 0.53 1.48 1.04f0.02 19.6f0.4 0.54 1.59 0.880 f0.022 22.8 f 0.5 0.56 1.70 0.73 1 k 0.016 26.6 k 0.6 0.58 1.82 0.673 f0.014 29.7 f 0.6 0.57 1.87 0.578 f 0.016 33.7 f 0.8 0.58 1.98 0.506 f 0.014 37.9 f 0.9 0.59 2.06 0.431 f0.013 42.9f 1.1 0.61 2.18 0.365 f 0.013 48.3 f 1.3 0.62 2.30 0.304f0.010 54.7k 1.4 0.65 2.45 0.254f0.010 61.7f 1.9 0.67 2.60 0.213 f 0.009 69.2 f 2.1 0.69 2.75 0.171 IfrO.009 78.8f2.6 0.73 2.97 0.138 f 0.007 89.4 f 3.2 0.77 3.20 0.114+0.008 99.8f4.4 0.80 3.40 0.0907 f.0.0057 1 13 f 5 0.85 3.68G. C. Franchini, L. Tassi and G. Tosi 5.0 4 . 5 % 4.0 3.5 3.0 3135 . ' 5.5 1 2 . 5 - jH 2 3 I I I I I I 2 3 4 5 6 7 lo2 € - I Fig. 2. pK us. 1 / E plot for picric acid in ethane- 1,2-di01-2-methoxyethanol solvent systems. Curves A and B at the bottom right of the figure have been vertically shifted in order to present the results more clearly. Table 8.K,,, and t for the solvent systems eqn (1) Harned pK us. 1/~ solvent system lo3 K,,, t/OC lo3 K,,, t/OC lo3 K,,, t/OC A 2.49 34.4 2.47 38.4 2.61 34.3 B 2.39 33.5 2.32 36.0 2.56 32.5 C 2.03 27.5 1.98 31.3 2.18 25.5 D 1.03 18.0 1.10 21.3 1.08 16.4 E 0.675 8.2 0.659 3.9 0.711 10.2 Harned's methods and by a pK us. 1 / ~ correlation; good agreement is observed for the three methods. As regards the pK us. 1 / E correlation, the dielectric-constant values have been directly measured for pure ethane- 1 J-diol and 2-methoxyethanol and for all mixtures at all investigated temperatures, and the data obtained have been optimized using a relation of the type log& = at+/? (where t is in "C) and reported in tables 2-7. Fig. 2 shows that for the solvent systems A-E the plots are smooth curves with a minimum whose ordinate should provide the pK,,, value and whose abscissa should provide corresponding 1 / ~ value, and as a consequence the temperature of K,,,.In order to simplify the problem the two branches of the curves have been approximated as two straight lines (r = 0.923 and 0.980 for A; r = 0.954 and 0.983 for B; r = 0.965 and 0.985 for C; r = 0.954 and 0.998 for D; r = 0.987 and 0.994 for E), whose intersection leads to the data reported in table 8. For systems F, G and H pK values increase linearly (r = 0.999, 0.999 and 0.998 for F, G and H, respectively). The Walden products9 for picric acid in these mixtures (tables 2-7) indicate that, increasing the temperature, the deviation from the equation Aoq = constant is more pronounced as the percentage of ethane- 172-diol in the mixture increases ; this behaviour may be related to the dramatic variation with temperature of the viscosity of ethane-1,2-diol (from 97.12 CP at - 10 "C to 3.04 CP at 80 "C) and consequently of the3136 The Ethane- 1,2-diol-2-Methoxyethanol System 35 0 u Q 2 25 m I 15 -10 10 30 50 70 tl OC 0 I 'H I 1 I I -10 10 30 50 70 tl "c -2001 -..-.-. ..\. -.-.-. --...,;. \. \. \. \.\.\.\. \. \. \ \. -200 - .\ \. -\ \. .\. \ \ H I I I I I -10 10 30 50 70 tl OC Fig. 3. Dependence on the temperature of AGO (in J mol-l), AH" (in J mol-l) and AS" (in J mol-1 K-l) of the dissociation of picric acid in ethane- 1,2-diol-2-rnethoxyethanol solvent systems. mixtures containing more ethane- 1,2-diol. Another feature of these A.q values is the fact that they decrease as the temperature increases for the solvent systems A-F, while they increase for G and H. For mixtures containing higher percentages of ethane- 1,2-diol, i.e. solutions that are more viscous, a better correlation is obtained by using a modified law, A. q0.7 = constant, as suggested by Stokeslo for high-density aqueous solutions. Starting from the best-fitting eqn (1) the thermodynamic data AGO, AH" and AS' for the dissociation reaction of picric acid have been evaluated, although many difficultiesG. C. Franchini, L. Tassi and G. Tosi 3137 2 3 4 5 6 7 1 0 2 E-’ Fig. 4. A, 1’s. 1 / E isotherms for picric acid in ethane- 1,2-di01-2-methoxyethanol solvent systems. The curves have been vertically shifted in order to present the results more clearly.(a) -10, (b) 0, (c) 10, ( d ) 20, (e) 30, (f) 40, (g) 50 and (h) 60 “C. and uncertainties are associated with these calculations. However, AW and ASO should be especially useful in obtaining information on solute species and solute-solvent and solvent-solvent interactions. In fact, considering the entropy of dissociation in detail, we are concerned with entropy changes due to differences ‘within’ the acid and its anion and with changes due to differences in solute-solvent interactions and also in solvent- solvent interactions if we are in the presence of mixed solvents. Thus, since in the present work we have the same solute in different mixed-solvent systems, ASo could be useful in studying the properties of the media. Data for thermodynamic functions have been plotted against t for each solvent system and are shown in fig.3. ASo us. t and A F us. t curves indicate the existence of a particular composition of the solvent mixture, very close to that of F, which denotes a sudden change in the properties of the system under study. In order to determine the composition of this supposed mixture, we have examined in detail the A. values which are reported in tables 2-7. A plot of A,, us. 1 / ~ at each temperature results in two straight lines intersecting in a point whose abscissa is then used to obtain for each temperature the E value of the so-called ‘limiting’ mixture; fig.3138 The Ethane- 1,2-diol-2-Methoxyethanol System 4 shows examples for some temperatures. Now, since over a large temperature range (- 10 to 60 “C) the relationship between the dielectric constant of the solvent system and its mole fraction is linear (r ranges from 0.994 to 0.996), the E value calculated above allows us to obtain at each temperature the composition of the ‘limiting’ mixture; these compositions are practically constant at all temperatures and average values of X(ethane- 1,2-diol) = 0.2677 and X(2-methoxyethanol) = 0.7323 [X(2-methoxyethanol)/ X(ethane- 1,2-diol) = 2.751 were obtained.A possible explanation of this behaviour of the solvent system under study may be found by considering the ‘selective solvation’ which occurs when the composition of the solvent components in the neighbourhood of the charged species is different from the composition of the bulk solution; normally, assuming a mole fraction of 0.5, the concentration of the solvent component with higher dielectric constant increases around the charged species. In the system under study it is possible that solvation occurs preferably for ethane- 1,2-diol up to the so-called ‘ limiting’ mixture, where the solvation shell probably changes, passing from a prevalence of ethane- 1,2-diol molecules to a prevalence of 2-methoxyethanol ones.By considering the entropies of dissociation (fig. 3), from the data for the pure solvents we have suggested in a previous paper5 that the formation of ion pairs should be more likely for 2-methoxyethanol (AS‘ ranging from - 38.0 to - 63.2 cal mol-‘ K-l)? than for ethane-1,2-diol (from -4.0 to - 17.9 cal mol-1 K-l) and that the solute species are more effective in orienting 2-methoxyethanol than ethane- 1,2-diol solvent molecules.Also, the AS‘ data for mixtures B-E are in accord with the above suggestions; the variation of AS‘ with the composition of the solvent system is slow, and this fact cannot be justified simply by the decrease of E of the bulk solution, but rather also by the above- mentioned selective solvation. For mixtures F and G an abrupt increase in AS‘ occurs, and this behaviour is in accord with the suggested change in preferential solvation corresponding to the ‘limiting’ mixture. The effect of temperature on the A P values is a minimum for mixture F, which is very close to the ‘limiting’ case. We thank Prof. C . Preti for stimulating discussions and the helpful suggestions, the Centro di Calcolo Elettronico of Modena University for the computing facilities and the Minister0 della Pubblica Istruzione, Italy for financial support. References 1 C. Preti and G. Tosi, Anal. Chem., 1981, 53, 48. 2 C. Preti, L. Tassi and G. Tosi, Anal. Chem., 1982, 54, 796. 3 E. Neviani Giliberti, C. Preti, L. Tassi and G. Tosi, Ann. Chim. (Rome), 1983, 73, 527. 4 C. Preti, L. Tassi and G. Tosi, Ann. Chim. (Rome), 1985, 75, 201. 5 G. C. Franchini, E. Ori, C. Preti, L. Tassi and G. Tosi, Can. J . Chem., 1987, 65, 722, and references 6 Handbook of Chemistry and Physics, ed. R. C. Weast (Chemical Rubber Co., Boca Raton, Fla, 7 R. M. Fuoss and T. Shedlovsky, J. Am. Chem. SOC., 1949, 71, 1496. 8 H. S. Harned and I. Kazanijian, J. Am. Chem. SOC., 1936, 58, 1912. 9 P. Walden and E. Birr, Z . Phys. Chem., Teil A , 1931, 153, 1. 10 J. M. Stokes and K. H. Stokes, J. Phys. Chem., 1958, 62, 497. therein. 1982-1983), p. E-50. Paper 6/ 1920; Received 29th September, 1986 t 1 cal = 4.18 J.

ISSN:0300-9599    

DOI:10.1039/F19878303129

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. I, 1987, 83 (lo), 3139-3148 Kinetics and Mechanism of Oxidative Dehydrogenation of Ethane and Small Alkanes with Nitrous Oxide over Cobalt-doped Magnesium Oxide Ken-ichi Aika,* Makoto Isobe, Kazuhiro Kido, Takeshi Moriyama and Takaharu Onishi Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan The kinetics of the decomposition of N,O and the partial oxidation of C,H, and other alkanes with N,O have been studied-over Co2+-doped MgO catalysts mainly at 473 K and below atmospheric pressure. The initial rate of N,O decomposition is proportional to the estimated amount of surface Co2+ ion, which suggests the Co2+ ion to be the active centre. Since the amounts of adsorbed oxygen are much greater than the estimated amounts of surface Co2+ ion, the adsorbed oxygen is inferred to migrate from the Co2+ ion centre to the MgO surface. No 0, was produced during the oxidation of C,H, and the initial rate of the oxidation reaction was almost identical to those of N,O decomposition.Thus, it was concluded that the oxidation of C,H, was controlled by the decomposition of N,O. Ethoxide is inferred to be a common intermediate, giving C,H, and surface oxidized species. Pure alkanes from C, to C, react at much the same rate with N,O, because the decomposition of N,O is the rate-determining step. If, however, mixed alkanes are used, differences in inherent reactivities appear. The reactivity sequence (n-C,H,, > C,H, z C,H, > C,H, > CH,) obtained is almost identical to that with oxygen atoms in the gas phase.There are several on the partial oxidation of C,H,. Some of the recent advances in this field are a finding of strong reactivity of surface 0- toward alkanes including C2H6,2 and the use of N,O as a source of 0- in order to obtain C,H,.3,4 However, 0- observable by e.s.r. spectroscopy is not always necessary for oxidative dehydrogenations of C,H, with N,O over CO~+-M~O.~,, In these reports the relation between C,H, selectivities and the nature of the catalyst, such as the bulk structure’ or surface-active have been discussed. However, all of these studies, including ours,5’ are insufficient to establish the reaction mechanism. Co2+-MgO has advantages for the study of the kinetics of the title reaction because it is an active and more selective catalyst than CoO/SiO, or C O O / A ~ , ~ , .~ ~ ~ While Co2+ ions make a solid solution with MgO,’** Co oxides are assumed to form an aggregated phase on SiO, and Al,03. Experimental MgO (Soekawa Rikagaku, 99.75 % purity) was impregnated with Co(NO,), - 6H,O in aqueous solution and a partly dried sample was extruded through a syringe. Pure MgO was also prepared by the treatment in water and extrusion. The dried pelleted catalyst thus made was evacuated at 773 K or at 1173 K for 3 h. The weight of the sample used decreased from 2.0 to 1.3 g during evacuation at 1173 K. B.E.T. surface areas measured after the evacuation were 64 m2 8-l for MgO, 162 m2 g-l for 0.02 YO Co2+-MgO, 220 m2 g-‘for 0.2 O/O Co2+-MgO, and 150 m2 g-l (for the weight measured before the evacuation) for 2 O/O Co2+-MgO, respectively.The surface area of MgO increases when Co2+ ions are 31393 140 Ox id at ive Dehydrogenation of Ethane added with a maximum above 0.2 YO of added cations. Co2+ contents are represented by molar percentages against MgO. The apparatus used is a closed circulation system, which is almost the same as that reported elsewhere.2 Initial amounts of the reactants (N,O and C2H') were ca. 6.67 kPa (1 kPa = 7.50 Torr) or 387 pmol. Most of the reactions were carried out at 473 K. 1.r. spectra were recorded using a JASCO A-3 spectrometer at room temperature and the reactions were carried out at 473 K in the i.r. cell. 1.r. spectra of pure chemicals adsorbed on 2% Co2+-MgO were recorded as references.CO, adsorbed at 473 K gave peaks at 1666(s), 1524(m) and 1314(s) cm-' (carbonates). HC0,H adsorbed at 473 K then evacuated at 723 K gave peaks at 1600(s) and 1382(m) cm-' (formates). CH,CO,H adsorbed at 473 K then evacuated at 723 K gave peaks at 1585(s), 1432(s), 1365(m), 1310(s), 1163(w), 947(w) and 832(w) cm-' (acetates). C,H,OH adsorbed at 298 K gave peaks at 1447(m), 1380(s), 1139(s), 11 lO(w), 1068(s) and 890(m) cm-l (ethoxides). Results Kinetic Analysis of N20 Decomposition A number of runs of N,O decomposition were carried out at 473 K over a 2 g sample of 0.2% Co2+-MgO catalyst. An example of the time course of reaction was shown in an earlier paper.' Since the amount of 0, produced is less than half the N, produced, some of the oxygen must be adsorbed, the amount being defined here as O(a) = N,(g) - 20,(g).The decomposition rate, R (N, pmol h-'), is taken from the slope of the curve for N, increase vs. time. Rate constants (RP;io) are plotted as a function of the amount of O(a) in fig. 1 for several runs over 0.2% Co2'-MgO samples at 473 K. The data can almost be reproduced as a linear relationship, where k is the initial rate constant (N, pmol h-l kPa-l) and [O,] is the amount of adsorbed oxygen when the reaction completely stops. The same relationships are observed for reactions at different temperatures (298473 K) or over other samples (0.02% and 2% Co2+). These results suggest that the reaction proceeds with the following mechanism which has been proposed by Tanaka and Ozakig~'O for the Cr,O, and Mn,O, catalyst: N,O + * + N,O(a) N,O(a) -+ N, + O(a) 20(a) -+ 0, + 2*.(2) (3) (4) Here, steps (2) and (4) are fast and step (3) is slow. Kinetic Analysis of Reaction between N20 and C2H, Reactions of C,H, with N,O were carried out over various Co2+-MgO catalysts at various temperatures. An example of the time course has been shown for the reaction over 2 g of 0.2% Co2+-MgO at 473 K in an earlier paper.6 0, is not released, but C,H, is produced, which suggests that the N,O decomposition is rate-determining. Rates of N, formation, expressed as RP;;,, are plotted as a function of amount of consumed 0 (= N,) in fig. 1. The initial rates of N,O decomposition with and without C2H6 are almost identical to each other and RP;;, decreases linearly with the amount of consumed 0, although the rate of decrease does not change compared with the reaction without C,H,.The same relationships are observed for other samples or for the runs at different temperatures (298-473 K).K. Aika et al. 3141 0 100 200 300 Fig. 1. Plots of rate constants (RP;;,) of N,O decomposition at 473 K us. adsorbed or consumed amount of oxygen atom over 2 g of 0.2% Co2+-MgO. Open circles denote pure N,O decomposition, whereas closed circles denote N,O decomposition under C,H6 oxidation. Duplicate runs were carried out over different batches of the sample for checking the reproducibility. The adsorbed or consumed amount of 0 is calculated from N,(g) - 202(g). adsorbed or consumed O/pmol Fig. 2. Relation between the amount of C2H6 reacted and amount of consumed 0 over 2 g of catalyst at 473 K.Runs over 0.2% Co2+-MgO (closed circles), runs over 2% Co2+-MgO (open circles). Each symbol corresponds to a separate run. (- - -) Slope 1 / 1 , (-) slope 1/4. The reaction is analysed in other ways. The amount of C2H, reacted (- AC2H,) is plotted as a function of the amount of consumed oxygen ( - A 0 = AN2) in fig. 2. Although the reaction rates are different over samples with different Co2+ content, plots of (-L\C,H,) us. (-do) coincide as is seen in fig. 2. The reaction stoichiometry of C2H6 against 0 was obtained from the slope (-dC2H6)/( -do), which changes from unity to ca. 0.25. The amount of C,H, produced is plotted as a function of the amount of C2H, reacted in fig. 3. The data also coincide irrespective of the Co2+ contents.The selectivity for3142 40 - 0 \ 5 5 20 x a 0 Oxidative Dehydrogenation of Ethane / /;* 50 100 C2H, reacted/pmol r" , , , , , ' , , , Fig. 3. Relation between the amount of C,H4 produced and the amount of C,H6 reacted at 473 K. Runs over 0.2 YO Co2+-MgO (closed circles), runs over 2 YO Co2+--MgO (open circles). Each symbol corresponds to a separate run. (---) Slope 0.6, (-) slope 0.25. Table 1. Competitive reactions of alkanes with N,O over Co2+-MgO at 473 K" decreasing rate of reactants in each run relative reactivity alkane initial rate constant RP,t,/pmol h-I kPa-' ratio 0.15 - - - - 1 .o 6.0 0.6 6.9 - 10 1.2 1 - 23b 7 - 9.7 27' 8 16 - CH4 C3*8 N,O" 13 8.1 9.0 13 12 45b - - - - C2H6 C2H4 - - - - - - - - 9.3 - n-C4H10 a N,O (6.7 kPa or 390 pmol) and mixed alkanes (each 6.7 kPa), 2 g of 0.2 YO Co2+-MgO evacuated at 1173 K.This sample was somewhat reactive compared with the other. Rate measured by N, production rate. Table 2. Relative reactivity of alkanes substrate alkane O(a)/MgO O-/MgO" O-/gasu *OHb -CH3b 0" 0.15 0.50 0.14 0.025 0.03 0.01 3 1 1 1 1 1 (1.67) (1.10) 8 1.27 1.33 7.7 (7) 12 16 0.67 1.7 - 4.5 19 473 298 298 295 455 307 - a Ref. (1 1). Ref. (12). Ref. (13).K. Aika et al. 3143 110% ~~ ~ 1600 1400 1200 wavenumberlcm-' Fig. 4. 1.r. spectra of the surface species formed during the reaction between N,O and C,H, at 473 K for 3 h over various MgO catalysts : (a) MgO, (6) 0.02 % Co2+-MgO, (c) 0.2 O/O Co2+-MgO, ( d ) 2 % Co2+-Mg0. ethene (dC2H4)/( -dC,H,) decreases from 0.6 to 0.25 with the progress of the reaction.For most cases the ratio of the pressure of C,H, to that of C,H, increases up to ca. 0.1 in ca. 3 h, when the reaction almost ceases. Reactions of Other Alkanes with N,O Although inherent reactivities of hydrocarbons with different carbon numbers should be different in an oxidation reaction, all pure alkanes from C, to C, react at much the same rate with N,O because the reactions of alkanes with N,O are controlled by N,O decomposition. No 0, is evolved in the gas phase during the oxidation reaction of any alkane by N20, which also means N,O decomposition is rate-determining. Some of the initial rates are shown in table 1 . If, however, mixed hydrocarbons are used, differences in inherent reactivities appear and the initial decrease of each reactant can be used to measure the relative rates of reaction of the different hydrocarbons.The two alkanes must compete with each other in their inherent reactivity against adsorbed oxygen produced via N20. The initial rates of mixed hydrocarbons are shown in table 1 and relative reactivities determined are listed in the last column. The reactivity increases with increasing carbon number of the alkane. Physical adsorption of C2H,, C2H4 and C,H, over MgO is negligibly small above 423 K2 Thus, the relative reactivity obtained here does not contain any adsorption terms. Observation of Surface Species during C,H,-N,O Reaction by I.R. Spectroscopy Since C2H, selectivity is far below 100 % without yielding other gaseous products, carbonaceous products should be left on the surface.Thus, i.r. spectra of various MgO catalysts were recorded after reaction for 3 h at 473 K. The results are shown in fig. 4. With an increase of Co2+ content several peaks become observable. These are identified as carbonate anions, both bidentate (1665 and 1314 ern-,) and unidentate (151 1 cm-l), formate (1600 and 1380 crn-l) and acetates (1585 and 1425 cm-'). Oxygen-containing species are not observed in the gas phase for reaction times > 3 h at 473 K.3144 Oxidative Dehydrogenation of Ethane co2+ content (%) 0.02 0.2 2 0.5 1 5 10 50 100 Fig. 5. Relation between the initial rate constant of N,O decomposition and the number of surface Co2+ ions. The initial rate, RP&, was measured over 2 g of catalyst at 473 K. 0, Rate of pure N,O decomposition; 0, rate of N,O decomposition under C,H, oxidation.Duplicate runs were carried out to check the reproducibility. surface Coz+ ions/pmol Discussion Active Centre for N20 Decomposition Since the kinetic data are represented by eqn (l), the corresponding elemental reaction steps [eqn (2)-(4)] are proposed, where eqn (3) is the rate-determining step. The initial rate constants (k = RP;;, at t = 0) obtained in the preceding results are plotted in fig. 5 as a function of the number of surface Co2+ ions (estimated by assuming no surface segregation of Co2+ and by using the B.E.T. surface area). The initial rate constant k is almost proportional to the number of Co2+ ions, which suggests that step (3) is catalysed by the Co2+ ion (active centre). Even if some surface segregation of Co2+ is assumed, the tendency would not be changed by much.An Arrhenius plot of the initial rate constant for 0.2% Co2+-MgO is shown in fig. 6 and an apparent activation energy of 37 kJ mol-1 was obtained. The observed activation energy is thought to be the value of the activation energy of step (3) less the heat of adsorption in step (2), which may be the reason why the value is so low. The value is lower than the reported one, 92f4 kJ mol-l, for 0.1 % Co2+-MgO calcined at 1273 or 1473 K.? Spillover of Adsorbed Oxygen from Co2+ to MgO Surface The amount of oxygen adsorbed during N,O decomposition (dotted line with arrow) and that of oxygen consumed during C2H, oxidation (solid line with arrow) are plotted in fig. 7 for three samples with different Co2+ contents. Maximum values of O(a) obtained from fig.1, which correspond to [O,] in eqn (l), are also plotted as open and closed circles. When all these values are compared with the estimated number of theK. Aika et al. 3145 1000 100 4 I cd a &i - 10 k: 5 O 1 hZ n P( - \ 5 -on 0.1 '. \ 1 I I I I 1.5 2 .o 2.5 3.0 103 KIT Fig. 6. Arrhenius plot of initial rate constant of N,O decomposition (RPNio) over 2 g of 0.2% Co2+-Mg0. 0, Rate of pure N,O decomposition; 0, rate of N,O decomposition under C,H, oxidation. The ordinate is scaled logarithmically. E, = 37 kJ mo1-I (9 kcal mol-I). Co2+ content (%) 0.02 0.2 2 500 1 / I / I I v I 1 I I / ; I / I I ; 101 IT I 1 I I I I / I Q 5 1 I / I I I / I / I I / I I - I / I I I I I . , . I I . , .. I 0.5 1 5 10 50 100 Fig. 7. Adsorbed or consumed oxygen atoms during the reaction at 473 K as a function of the number of surface Co2+ ions estimated. 0, Amount of saturated adsorbed oxygen (0,) in N,O decomposition (estimated) ; 0, amount of saturated consumed oxygen in N,O-C,H, reaction (estimated). Dotted and filled arrows show the actual amount of O(a) during N,O decomposition and N,O-C,H, reaction, respectively. surface Co2+ ions/pmoi3146 Oxidative Dehydrogenation of Ethane surface Co2+ ions, the former exceed the latter. This fact suggests that a part of the adsorbed oxygen spills over from the Co2+ centres to the MgO surface79*.f- if the estimation of surface Co2+ is correct: ( 5 ) In the case of C,H6 oxidation, most of the surface oxygen must be turned .into oxygenated species on MgO, as will be discussed below [step (6)].This would decrease the concentration of O(a)--.Co2+ further. This is why the rate of N20 decomposition during C2H6 oxidation is higher than that of pure N,O decomposition, as is seen in O(a) - . - Co2+ + O( a) - - - MgO. fig. 1. Initial Step of the Reaction between N20 and C2H, Since the stoichiometric ratio of C,H6 reacted and 0 consumed is unity in the initial stage (fig. 2 and table l), step (6) is reasonably assumed: O(a) + C2H6 -+ X(a). (6) This step is considered to be faster than step (4) because no gaseous 0, is obtained during the oxidation reaction. Thus the rates of the reaction are in the order: step (6) > step (4) > step (3). During the decomposition of N,O over 0.2% Co2+-MgO, C,H6 was added at 473 K.Small amounts of gaseous 0, formed by that time were consumed quite rapidly, even when the pressure was extremely low (1/200) compared to that of N,O. This means that the reverse reaction of step (4) (or any reaction of 0, with carbonaceous species) is also faster than step (3). Relative Reactivities of Small Alkanes with Adsorbed Oxygen The relative reactivities of alkanes with adsorbed oxygen observed here are compared with those for other substrates in table 2. The reactivity sequences, n-C,H,, > C,H, = C2H4 > C2H6 > CH, in this work is similar to those with 0- (in the gas phase or on MgO);ll however, it is much the same as other radicals such as .OH, .CH, or atomic 0.l2* l3 The reported reactivities with radicals are closely related to the dissociation energy of the C-H bond except for ethylene; H-CH, (104+ 1 kcal mol-l), H-C2H5 (98 & l), H-C,H, (2 108 f 2), H-i-C,H, (95 & l).14 These radicals or atoms react with alkanes to abstract hydrogen.12* 1 3 7 l5 The oxidation of alkanes over Co2+-MgO is also considered to start with hydrogen abstraction by O(a) on MgO: O(a) + RH + R + OH(a) O(1attice) + - R + OR(a).We cannot deduce the electronic state of O(a) from this work; however, the reactivity of O(a) somewhat resembles atomic 0 rather than the 0- radical when C,, C, and C, alkanes are compared in table 2. Further Step of the Reaction including Surface Products using N,O and C,H, C,H,OH(a) or C,H50(a) are assumed to partake in the ethane reaction for step (6b), although no i.r.information of this species has been obtained, probably because of the low concentration : C2H, + O(a) + C,H,OH(a) (6 4 C,H,OH(a) + C,H, + H,O(a) C,H,OH(a) + O(a) -+ CH,CHO(a) + H20(a). (7) (8) t Eqn (1) should be valid only for a case where amounts of the two kinds of adsorbed oxygen in eqn ( 5 ) are linearly correlated in the equilibration. Other cases might deform the straight line in fig. 1 ; however, we neglect such discussions here.K. Aika et al. 3 147 Surface ethoxide may decompose to give C2H4 [step (7)] or may be gradually oxidized to acetate [step @)I, formate and finally carbonates at 473 K as is observed by i.r. in fig. 4. Methoxide on MgO, which probably absorbs oxygen, has been reported to be oxidized to formate above 438 K,16 whereas it is not oxidized up to 673 K over clean MgO evacuated at 1003 K.17 Thus, oxygen adsorbed on MgO is thought to react with ethoxide at 473 K [step (S)].Along with the oxidation reaction of ethoxide to acetate, formate and carbonates, several times more oxygen atoms than C2H6 molecules must be consumed. This is why (-dC2H6)/(-do) decreases from 1 to 0.25 in fig. 2. It is considered that O(a) in step (3) is adsorbed on Co2+ ion, whereas the O(a) in step (8) and further steps are those adsorbed on MgO. The state of O(a) in step (6a) cannot be decided here, but it is suggested that it is located on Mg0.5 From the data at the initial stage in fig. 3, ratio of the rate of step (7) to the rate of step (8) is estimated to be 3/2. At the final stage of the reaction this ratio decreases to 1/3 (fig.3). At this stage PCaH4/PCaH is almost constant, ca. 0.1. This is probably because C,H4 is oxidized through the back-reaction of step (7) and the forward reaction of step (8). The steady-state condition for the ethoxide intermediate gives the relative reactivity of C2H, : C2H6 to be ca. 9. This means that C2H4 is much more reactive than C2H6. The relative reactivity of C2H4 : C2H6 is also ca. 7 from the direct method in table 1. Partial oxidation of alkanes must start by hydrogen abstraction; however, oxidation of alkenes may be different. The relative reactivity of C2H4 is almost the same as that of C,H,, whereas the dissociation energy of C-H in C2H4 (2 108 kcal mol-l) is much higher than that in C,H, (95 kcal mol-l). Oxygen addition is inferred to be the initial step for C2H4, as in the case for the 0 atom in the gas phase.', Conclusions Co2+-MgO is considered to have dual functions ; Co2+ catalyses N20 decomposition and MgO catalyses an oxidation of C2H6.The former reaction is the rate-determining step. Oxygen spills over from Co2+ to the MgO surface. The reaction mechanism is proposed for C2H4 formation. Surface intermediates (ethoxides) are either decomposed to give C2H4 with a selectivity of 0.6 or oxidized further. C2H4 is several times more reactive than C2H6, which depresses the ethylene selectivity along with the time of reaction. The nature of active oxygen on MgO which reacts with alkane seems to be different from 0-, but similar to the 0 atom in the gas phase. We thank Mr T. Nishiyama for his contribution. References 1 E. M. Thorsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. Catal., 1978, 52, 116. 2 K. Aika and J. H. Lunsford, J . Phys. Chem., 1977, 81, 1393. 3 M. B. Ward, M. J. Lin and J. H. Lunsford, J. Catal., 1977, 50, 306.; T-J. Yang and J. H. Lunsford, J. 4 M. Iwamoto, T. Taga and S . Kagawa, Chem. Lett., 1982, 1469. 5 K. Aika, M. Tajima and T. Onishi, Chem. Lett., 1983, 1783. 6 K. Aika, M. Tajima, M. Isobe and T. Onishi, Proc. 8th Jnt. Congr. Catal., Berlin, 1984, (Dechema, Verlag Chemie, Weinheim, 1984), vol. 3, p. 335. 7 A. Cimino and F. Pepe, J . Catal., 1972, 25, 362. 8 V. Indovina, D. Cordischi, M. Occhiuzzi and A. Arieti, J . Chem. SOC., Faraday Trans. I , 1979, 75, 9 K. Tanaka and A. Ozaki, J . Catal., 1967, 8, 307. Catal., 1980, 63, 505. 2177. 10 K. Tanaka and A. Ozaki, Bull. Chem. SOC. Jpn, 1967, 40,420. 11 K. Aika and J. H. Lunsford, J . Phys. Chem., 1978, 82, 1794. 12 R. P. Overend and R. J. Cvetanovic, Can. J . Chem., 1975, 53, 3374. 13 J. T. Herron and R. E. Huie, J . Phys. Chem., 1969, 73, 3327. 14 D. M. Golden and S . W. Benson, Chem. Rev., 1969, 69, 125.3148 Oxidative Dehydrogenation of Ethane 15 N. N. Semenov, On Some Problems of Chemical Kinetics and Reaction Ability (Acad. Nauk. SSSR, 16 R. D. Kagel and R. G. Greenler, J. Chem. Phys., 1968, 49, lb38. 17 J. Kondoh, Y. Sakata, K. Muruya, K. Tamaru and T. Onishi, Appl. Surf Sci., 1987, in press. 18 S. Sat0 and R. J. Cvetanovic, Can. J. Chem., 1959, 37, 953. Moscow 1958). Paper 611933; Received 30th September, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303139

出版商:RSC    

年代:1987

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1987, 83 (lo), 3149-3159 Temperature-programmed Desorption Study of the Interactions of H,, CO and CO, with LaMnO, Luis G. Tejuca*Jf and Alexis T. Bell Department of Chemical Engineering, University of California, Berkeley, California 94720-9989, U S . A . Jose Luis G. Fierro and Juan M. D. Tascon Instituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano 119, 28006 Madrid, Spain Surface interactions of H,, CO and CO, with the perovskite-type oxide LaMnO, have been studied by temperature-programmed desorption (t.p.d.) and i.r. spectroscopy. A t.p.d. desorption peak of H, at 355-360 K, which increases in intensity with increasing reduction temperature of the oxide ( Tr), is assigned to molecular adsorption of H, on reduced manganese sites (Mn"+, n z 2).CO adsorption yielded t.p.d. peaks of CO and CO,. A peak of CO at 473 K (for oxidized LaMnO,) associated with a carbonate group and peaks at 360-395 K, 540-550 K and 773-800 K (for reduced LaMnO,) associated with linear and bridged CO species adsorbed on Mnn+ ions were observed. A very wide CO, desorption peak at 473 K and tail centred at 773 K (oxidized LaMnO,) are associated with monodentate and bidentate carbonates interacting with Mn3+. CO, adsorption yielded t.p.d. peaks of CO, at 345-385 K and at 540-665 K whose intensity decreased and increased, respectively, with T,. These are associated with monodentate and bidentate carbonates, respectively, interacting with reduced sites of manganese or La3+. Detection of bands at ca. 2900 cm-l in the i.r. spectrum obtained after CO + H, adsorption, the appearance of new CO desorption features at 570 K and above 860 K, and the detection of a new H, desorption peak at 770-785 K in the t.p.d.spectra obtained after CO-H, or H,-CO adsorptions suggest decomposition of an oxygenated species formed by interaction of CO and H, adsorbed on the same adsorption centre. Increasing attention is being paid to CO hydrogenation because of its importance for obtaining oxygenated compounds from other sources than petroleum. As catalysts for this reaction, noble metals dispersed on different supports were La,O, has been found to be a suitable support for obtaining high yields of oxygenated Other compounds, such as LaRhO,, where the noble metal is in the B position of a perovskite structure, have also been employed.6 However, recent results have shown that this oxide undergoes reduction under the reaction conditions, giving place to metallic rhodium supported on La,O, and, presumably, on part of the perovskite that may remain unmodified.' Therefore, we have turned our attention to other oxides of a higher thermal stability in a reducing atmosphere, and in particular LaMO, perovskites, where M is a first-row transition metal.As a first step in the study of these oxides as catalysts for CO hydrogenation, a study has been conducted of the surface interactions of CO and H, with LaMnO,, using temperature-programmed desorption (t.p.d.) spectroscopy. Interactions of individual gases and interactions of CO-H, as a function of the reduction temperature of the oxide t On leave from Instituto de Catalisis y Petroleoquimica, C.S.I.C., Serrano 119, 28006 Madrid, Spain.3 1493150 Interactions of H,, CO and CO, with LaMnO, by T.P.D. are reported. To complement results on CO adsorption, CO, interactions with LaMnO, have been studied. Some i.r. spectroscopic data have also been included. Experimental Equipment The flow system used in the temperature-programmed desorption experiments has been described previously.* The LaMnO, sample (0.5 g) was placed in a quartz microreactor which could be heated at a programmed temperature up to 1 K s-l. The analysis of the effluent gases was made by means of a UTI model 100 C mass spectrometer. A data- acquisition system based in a microprocessor was used to record the signal intensity for a series of preselected masses and the temperature of the catalyst bed.Details of the i.r. spectrophotometer and the i.r. cell used are given el~ewhere.~ Materials The preparation (amorphous precursor decomposition) and characterization of LaMnO, samples similar to that used in this work have been described previo~sly.~ Its B.E.T. specific surface area as determined by N, adsorption (SN2 = 0.162 nm2) at 77 K was 11 .O m2 g-l. The gases used (He, H,, CO, CO, and a mixture of 21 YO 0,-79 YO He) were purified by standard methods. For calibration, mixtures of H,-He, CO-He and C0,-He were used. Methods T.p.d. experiments were performed after gas adsorption on oxidized and reduced (at different temperatures) samples. For oxidation, a mixture of 0,-He was passed for 1 h through the sample at 873 K.The reduced samples were prepared from oxidized (as above) LaMnO, by passing a H, flow for 1 h at the desired temperature. The samples so treated will be referred to hereafter as LaMnO, (ox 873) and LaMnO, (red T,.), where Tr is the reduction temperature in K (from 373 to 873 K). After the appropriate oxidation and reduction the sample was outgassed by passing a He flow for 1 h through the microreactor at 873 K. The adsorption of individual gases was effected by passing a flow of H,, CO or CO, for 0.5 h at room temperature (r.t.) and then a He flow for 15 min at r.t. for removal of the physisorbed part. The successive adsorption CO-H, was carried out by passing flows of CO (0.5 h, r.t.), He (1 5 min, r.t.), H, (0.5 h, r.t.) and He (15 min, r.t.).H,-CO adsorption was effected in the same manner passing first H, and then CO. After the oxidation-reduction, outgassing and adsorption steps, the reactor was repressurized with He, then heating of the catalyst was started at 0.5 K s-l and the data acquisition system activated. The maximum heating temperature was limited to 873 K (100 K below the preparation temperature) to avoid changes in specific surface area. Between two successive t.p.d. runs, the sample underwent treatments of oxidation, reduction and outgassing as indicated above. The flow rates used in oxidation (0,-He), reduction (H,), outgassing (He), adsorption (H,, CO or CO,) and t.p.d. experiments (He as carrier) were, in all cases, 50 cm3 min-l. The mass spectrometer was calibrated daily against the corresponding gas-He mixtures.For i.r. spectroscopic experiments, the sample was placed in an i.r. cell connected to a high vacuum system in which a pressure of Torr (1 Torr = 133.3 N mP2) could be maintained. The sample first underwent oxidation (100 Torr 0,, 1 h) or reduction (100 Torr H,, 1 h) at the indicated temperature. After this it was outgassed under high vacuum for 15 h at 773 K and then contacted with the gas or gas mixture (100 Torr, 1 h) under study.L. G. Tejuca et al. 3151 I I I I I I 300 500 7 00 900 TlK Fig. 1. T.p.d. of H, after H, adsorption at r.t. on LaMnO, reduced at 373 (a), 473 (b), 573 (c) and 723 K (d). Results and Discussion H, Adsorption T.p.d. spectra after adsorption of H, at r.t. on LaMnO, reduced at 373, 473, 573 and 723 K are shown in fig. 1.That corresponding to LaMnO, (red 873) is given in fig. 6. The spectra show a desorption peak at 355 K which shifts to 360 K and increases in intensity with T,. This intensity increase is more marked for T, 3 723 K. No desorption peak was observed after H, adsorption on the oxidized sample. Desorption above 773 K is due to residual H, from the previous reduction treatment. The observed t.p.d. peak is similar to that recorded at 363 K by Spinicci and Tofanaril' in the system H,- Ni-SiO, of weakly adsorbed hydrogen. A second peak found by these authors at 588 K was not observed in our case. However, desorption peaks at 350 and 615 K were observed in the system H,-LaNiO,.'l Heterolytic dissociative adsorption, as occurs in H, adsorption on COO - MgO solid solutions,12 can be ruled out since neither water nor hydroxyl groups were detected by mass spectrometry and i.r.spectroscopy, respectively. The intensity increase of t.p.d. peaks with T, suggests that reduced manganese ions (Mn"+) are directly involved in H, adsorption. The low desorption temperature points to molecular adsorption similar to that observed by Uchida and Bell1, with a desorption peak at 673 K in the system showed LaMnO, to be stable in an H, atmosphere up to 723 K. Its reduction starts at ca. 750 K and reaches a stable reduction of 1 e- per molecule ( i e . , Mn3+ --+ Mn2') at 1073 K. The intensity increase of the desorption peak of H, at T, > 373 K shows that the oxide surface undergoes substantial reduction at H,-Ru * A1,0,.Previous t.p.r.3152 Interactions of H,, CO and CO, with LaMnO, by T.P.D. r 1 I 1 I I I 1 300 500 700 900 T/K Fig. 2. T.p.d. of CO after CO adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (b) and 873 K (c). temperatures much lower than those needed for reduction of the bulk. Taking into account these results it seems plausible to assume that molecular adsorption of H, takes place on Mnn+ (n 2) sites. La3+ is very stable and difficult to reduce. Assuming the lower Miller index planes (loo), (1 10) and (1 11) to be the most frequently exposed ones in LaMnO,, a concentration of 2.67 x lo'* manganese ions rn-, in the surface was calculated. On the basis of this value, the ratios between adsorbed H, molecules and manganese ions as a function of the reduction temperature were calculated. The coverages found are very low for Tr = 373 K and reach a value of 0.1 for the maximum reduction temperature.CO Adsorption In fig. 2 t.p.d. spectra of CO after CO adsorption on LaMnO, are shown. CO desorption from LaMnO, (ox 873) [fig. 2(a)] presents a wide peak centred at 473 K, a poorly resolved shoulder at 540 K and a tail at ca. 773 K. On LaMnO, (red 573) [fig. 2(b)] two well defined peaks appear at 360 and 550 K. CO desorption above 773 K is also observed. On LaMnO, (red 873) [fig. 2(c)], the three desorption peaks which appear at 395, 545 and 800 K increased substantially in intensity. In addition, a shoulder at 860 K can be seen.L. G. Tejuca et al. 3153 I I I I I 2200 2000 1900 Vcm-' Fig.3. 1.r. spectra obtained after contacting LaMnO, reduced at 773 K with 50 Torr CO at r.t. (a), 373 (b), 473 (c), 573 (d) and 773 K (e). Contact time 0.5 h at each temperature. I I I I I I 1 300 500 700 900 TIK Fig. 4. T.p.d. of CO, after CO adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (6) and 873 K (c). Formation of hydrogen-containing species such as formate or hydrogencarbonate should not take place given the high initial outgassing temperature and the absence of hydroxyl groups (these were not detected by i.r. spectroscopy). On the other hand, any species arising from CO interactions with lattice oxygen, viz. carbonates, would show a decrease in concentration with the reduction temperature as occurs for the desorption peak at 473 K on the oxidized surface [fig.2(a)]. A CO, desorption peak after CO adsorption was observed at the same temperature [fig. 4(a)]. This suggests that at 473 K carbonate decomposition with formation of CO and CO, takes place. Therefore, the peak3154 Interactions of H,, CO and CO, with LaMnO, by T.P.D. at 473 K is associated with a carbonate species. The shoulder at 540 K and the tail centred at 773 K [fig. 2(a)], which become well resolved peaks on reduced surfaces [fig. 2(b) and (c)] and increase in intensity with the degree of reduction (i.e. with a decrease in surface oxygen), should be associated with CO adsorbed on Mn"+ ions. These reduced ions may be produced on the oxidized surface by the outgassing treatment at 873 K in a He flow. The peaks at 360-395, 540-550 and 773-800 K [fig.2(a), (b) and (c), respectively] which increase in intensity with increasing reduction temperature are, therefore, assigned to CO species adsorbed on reduced Mnn+ sites. Similarly to our observations, an increase in CO adsorption with increasing outgassing temperature (and therefore with increasing surface reduction) was found in the system CO-Fe,O,. l5 Watson and Somorjai' found only one desorption peak of CO at 498 K on LaRhO,. Jensen and Massothl' recorded a desorption peak below 373 K and a second peak at 693 K in the system CO-Fe-Mn oxide. Our t.p.d. spectra with three desorption peaks seem to indicate a rather heterogeneous surface of LaMnO,. Indeed, this and other LaMO, oxides have been reported to exhibit a non-stoichiometric character.l' After CO adsorption on LaMnO, (red 773) at r.t.an i.r. band at 2130 cm-l and a shoulder at 2080 cm-' (fig. 3) were observed. These bands disappeared after heating (in the presence of the gas) at 573 K (2130 cm-l) and 773 K (2080 cm-l). They should be due to CO species adsorbed on centres with back-donation capacity (i.e. on centres where the back-donation of electrons from the metal to the antibonding n* orbital of CO is favoured) since their frequency is lower than that of CO gas (2143 cm-l). This is consistent with CO adsorption on Mnn+, presumably Mn2+. 1.r. bands at similar frequencies were found for CO interactions with Ni - A1203,18 COO - MgO solid ~olutions,~~ CuO,,O and MnO, on several supports.21v22 The desorption peaks at 360-395 K and 540-550 K (fig.2) can be correlated with the bands at 2130 and 2080 cm-l (fig. 3), which are attributed to linear CO adsorbed on Mn2+, probably with different coordination states. The species desorbing at 773-800 K is ascribed to bridge-bonded CO. T.p.d. spectra of CO, observed after CO adsorption are shown in fig. 4. This CO, should arise from CO oxidation via formation and decomposition of Carbonates. On the oxidized surface [LaMnO, (ox 873)] a wide peak at 473 K and a tail centred at 773 K were observed. The former correspond to monodentate carbonates. Its high intensity and its total disappearance on the reduced surface at 573 K point to interaction of these carbonates with Mn3+. The tail at 773 K is associated with carbonates of higher thermal stability, i.e.bidentate carbonates. l5 The concentration of both types of carbonate decreases as the availability of surface oxygen decreases, i.e. with increasing T,. Both monodentate and bidentate carbonates were detected by i.r. spectroscopy after CO adsorption at r.t. on LaMnO, (ox 873).,, However, no noticeable changes in the spectrum were observed after CO adsorption on samples with different degrees of reduction. This will be due to the low transmission of this oxide to i.r. radiation and, also, to its rather low specific surface area. The dissimilarity of t.p.d. spectra of CO, obtained after CO (fig. 4) and CO, (fig. 5 ) adsorption rules out any mechanism involving direct oxidation of CO to CO, by surface oxygen of the oxide, and formation of carbonates from this CO,.On the oxidized surface, almost all the adsorbed CO desorbs as CO,. On the contrary, on LaMnO, (red 873) less than 1 % of adsorbed CO desorbs as CO,. CO, Adsorption T.p.d. spectra of CO, obtained after CO, adsorption are shown in fig. 5. A peak at 385 K [fig. 5(a)] decreases in intensity and shifts towards lower desorption temperatures for increasing degrees of reduction [at 345 K on LaMnO, (red 873)]. A second desorption peak situated at 540 K on the oxidized surface undergoes a remarkable increase in intensity and shifts towards higher desorption temperatures as the reductionL. G. Tejuca et al. 3155 9 8 7 6 r( I N ti s 5 ; W 0” u 4 - E 2 -. 0 OI I \ 3 - 2 - 1 - I I I I I I 1 I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I I I I 1 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 300 500 700 900 T/K Fig.5. T.p.d. of CO, (-) and CO (---) after CO, adsorption at r.t. on LaMnO, oxidized at 873 K (a); reduced at 573 (b), 723 (c) and 873 K (d). temperature increases [at 665 K on LaMnO, (red 873)]. A less intense peak centred at ca. 773 K observed on oxidized and, also, on reduced samples up to Tr < 573 K [fig. 5(a) and (b)] disappears for higher degrees of reduction. The two main desorption peaks are situated at temperatures which are near to those of desorption of /3 and 6 species reported by Klis~urski~~ for the system C0,-Co,O,. Note that an exothermic effect upon CO, adsorption (which becomes higher for higher degrees of reduction) was observed. A similar effect was reported for CO, adsorption on Mn0,.Ce0,.22 The peak at lower temperatures is assigned to a monodentate carbonate interacting with Mn3+.The opposite evolution of the peak situated at 540-665 K indicates the desorption of a carbonate of a different nature, most likely of bidentate character. Formation of this type of carbonate from CO, takes place on pairs of surface sites composed of a lattice oxygen and an anionic FAR I I 043156 Interactions of H,, CO and CO, with LaMnO, 0 by T.P.D. 0 s 0. Consistent with this interpretation, the concentration of carbonate species increases with increasing degree of reduction. This bidentate carbonate may interact with Mn2+ or La3+ ions. Monodentate and bidentate carbonates, the latter being in higher concentration than the former, were also found in the system C0,-Fe,0,.15 On samples reduced at 723 and 873 K, desorption peaks of CO at 755 K [fig.5(c)] and 735 K [fig. 5(6)] in different positions from those of CO, peaks were observed. CO desorption is of the same order of magnitude as CO, desorption in the first case but substantially higher in the second where the molar ratio C0,-CO is 0.56. This supports the view that some of the carbonates mentioned before interact with reduced centres of manganese. CO-H, Coadsorption In fig. 6 t.p.d. spectra of H, recorded after H, adsorption on a clean surface of LaMnO, (red 873) and after successive adsorption of CO-H, or H,-CO are shown. In the CO-H, and H,-CO sequences the peak at ca. 360K (dashed line) is strongly inhibited and a new desorption peak appears at 770-785 K.A similar effect, viz. the perturbation of H, adsorption by CO, has been been reported for Zn0.26.27 The t.p.d. spectrum of CO after successive adsorption H,-CO on LaMnO, (red 873) is given in fig. 7 (the spectrum obtained after successive adsorption CO-H, is similar to that in fig. 7). The positions of the main peaks (395, 530 and 810 K) are similar to those of the desorption peaks of CO after adsorption of this gas on a clean surface of LaMnO, (red 873) [fig. 2(c)]. In addition to the three peaks just noted, a shoulder at 570 K was observed, which is related to interactions of CO with H,. Also, pronounced CO desorption occurs above 860 K which corresponds, in part, to the shoulder at the same temperature observed in the t.p.d. of CO-LaMnO, (red 873) [fig. 2(c)].From the above results it is observed that CO or H, desorption after adsorption of these molecules on a clean surface of LaMnO, (red 873) is equal to CO or H, desorption after sequential adsorptions CO-H, or H,-CO. In other words, the number of adsorbed molecules of CO or H, is not affected by the preadsorption of the other molecule (H, and CO, respectively). Mutual enhancement of CO and H, adsorption has been reported on ZnO-containing catalysts. 27, However, enhanced CO adsorption by H, preadsorption and slightly suppressed H, adsorption by CO preadsorption was observed on cobalt- thoria-Kieselguhr at room temperat~re.~~ The above results could indicate that CO adsorption occupies [fig. 6(a)] or displaces [fig. 6(b)] adsorbed H, from the sites involved in the desorption peak at ca.360 K to another type of site (viz. those associated to the desorption peak at 770-785 K). The data presented in fig. 1 and 2 suggest that both H, and CO adsorb on reduced manganese ions (Mn"+). On the other hand, the ratio of desorbed CO molecules to the estimated number of transition-metal ions was found to be equal to 0.5, i.e. adsorbed CO occupies half of the exposed manganese sites (coverages for H, are equal to 0.1 in all cases and therefore are much lower than those of CO). This would allow the displacement and adsorption of hydrogen to other manganese sites. However, a more plausible explanation for these experimental results would be the formation of an oxygenated surface species after the sequential adsorption CO-H, and H,-CO, at room temperature.Such a situation would involve the adsorption of CO and H, on the same Mn"+ site. McKee3' has observed coverages higher than 1 of interacting CO and H, adsorbed species on Ru powder.L. G. Tejuca et al. 3157 300 500 700 900 T/K Fig. 6. T.p.d. of H, after CO-H, (a) and H,-CO (b) adsorption at r.t. on LaMnO, reduced at 873 K (--) and t.p.d. of H, after H, adsorption at r.t. on LaMnO, reduced at 873 K (---). 7 6 5 * 'm f 4 H 8 c1 0 : 3 I 2 \ b 2 1 0 I J I I I I 1 I 300 500 700 900 T/K Fig. 7. T.p.d. of CO after H,-CO adsorption at r.t. on LaMnO, reduced at 873 K. 104-23158 Interactions of H,, CO and CO, with LaMnO, by T.P.D. Formation of an oxygenated species is supported by the following observations : (a) the appearance of a new H, desorption peak at 770-785 K and, also, the appearance of new CO desorption features at 570 K and above 860 K after H,-CO or CO-H, adsorptions (fig.6 and 7); (b) after successive adsorption of 50 Torr H, and 50 Torr CO on LaMnO, (red 773) at room temperature i.r. bands at 2935 and 2860 cm-l, which can be assigned to antisymmetric and symmetric stretching modes of C-H vibrations, appeared in the spectrum. These bands underwent an intensity decrease after heating at 373 K. Although CO dissociation was reported to be favoured by the presence of H2,31.32 in our case, dissociation does not seem likely to occur given the low adsorption temperature used and the not too dissimilar t.p.d. spectra of CO after CO (fig. 2) and H,-CO (fig. 7) adsorptions. Conclusions H,, CO and CO, interactions with LaMnO, at room temperature, as a function of the reduction temperature, have been studied by temperature programmed desorption and i.r.spectroscopy. The main conclusions drawn can be summarized as follows. H, adsorbs molecularly on reduced Mnn+ ( n z 2) sites. The oxide surface appears to undergo substantial reduction at temperatures much lower than those needed for reduction of the bulk. CO adsorption yielded CO and CO, desorption peaks which can be assigned to monodentate and bidentate carbonates interacting with Mn3+. The concentration of these carbonates decreases as the availability of surface oxygen decreases. Desorption peaks attributed to linear and bridged CO species adsorbed on Mn2+ were also detected . CO, adsorption produced monodentate and bidentate carbonates interacting with Mn3+ and Mn2+ (or La3+), respectively.A fraction of the carbonates formed on the reduced surface of the adsorbent desorbs as CO. H,-CO and CO-H, successive coadsorption results suggest the formation of an oxygenated surface species by adsorption of CO and H, on the same reduced Mnn+ (n z 2) site. We are indebted to the Spanish-North American Joint Committee for Scientific and Technological Cooperation for financial support (project no. CCB8409-003). One of us (L.G.T.) would like to thank the technical staff of the Department of Chemical Engineering, University of California, Berkeley, for their help during the time in which the experimental part of this work was carried out. References 1 Yu. A. Ryndin, R.F. Hicks, A. T. Bell and Yu. I. Yermakov, J . Catal., 1981, 70, 287. 2 R. F. Hicks and A. T. Bell, J . Catal., 1984, 90, 205. 3 R. F. Hicks and A. T. Bell, J . Catal., 1985, 91, 104. 4 J. S. Rieck and A. T. Bell, J. Catal., 1986, 99, 278. 5 R. P. Underwood and A. T. Bell, Appl. Catal., 1986, 21, 157. 6 P. R. Watson and G. A. Somorjai, J . Catal., 1982, 74, 282. 7 R. P. Underwood, A. T. Bell and L. G. Tejuca, unpublished results. 8 G. G. Low and A. T. Bell, J . Catal., 1979, 57, 397. 9 J. M. D. Tascon, L. Gonzalez Tejuca and C. H. Rochester, J . Catal., 1985, 95, 558. 10 R. Spinicci and A. Tofanari, React. Kinet. Catal. Lett., 1985, 27, 65. 1 1 L. Gonza'lez Tejuca and A. T. Bell, unpublished results. 12 A. Zecchina and G. Spoto, Z . Phys. Chem. (N.F.), 1983, 137, 173. 13 M. Uchida and A. T. Bell, J . Catal., 1979, 60, 204. 14 J. L. G. Fierro, J. M. D. Tascon and L. Gonzalez Tejuca, J . Catal., 1984, 89, 209. 15 T. J. Udovic and J. A. Dumesic, J . Catal., 1984, 89, 314. 16 K. B. Jensen and F. E. Massoth, J . Catal., 1985, 92, 98.L. G. Tejuca et al. 3159 17 R. J. H. Voorhoeve, J. P. Remeika and L. E. Trimble, Ann. N. Y. Acad. Sci., 1976, 272, 3. 18 J. B. Peri, J. Catal., 1984, 86, 84. 19 A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. fhys. Chem., 1984, 88, 2575. 20 A. A. Davydov and A. A. Budneva, React. Kinet. Catal. Lett., 1984, 25, 121. 21 M. A. Baltanas, A. B. Stiles and J. R. Katzer, J . Catal., 1984, 88, 516. 22 M. A. Baltanas, A. B. Stiles and J. R. Katzer, Appl. Catal., 1986, 20, 31. 23 L. Gonzalez Tejuca, C. H. Rochester, J. L. G. Fierro and J. M. D. Tascon, J. Chem. Soc., Furuday 24 D. G. Klissurski, J. Catal., 1974, 33, 149. 25 M. P. Rosynek and D. T. Magnuson, J. Catal., 1977, 48, 417. 26 J. C. Lavalley, J. Saussey and T. Rais, J. Mol. Catal., 1982, 17, 289. 27 E. Giamello and B. Fubini, in Adsorption at the Gas-Solid and Liquid-Solid interface, ed. J. Rouquerol 28 H. H. Kung, Catal. Rev.-Sci. Eng., 1980, 22, 235. 29 R. B. Gupta, B. Viswanathan and M. V. C. Sastri. J. Catal., 1972, 26, 212. 30 D. W. Mckee, J. Catal., 1967, 8, 240. 31 F. Solymosi, I. Tombacz and M. Kocsis, J. Catal., 1982, 75, 78. 32 J. S. Rieck and A. T. Bell, J. Catal., 1986, 99, 262. Trans. I , 1984, 80, 1089. and K. S. W. Sing (Elsevier, Amsterdam, 1982), p. 389. Paper 612099; Received 29th October, 1986

ISSN:0300-9599    

DOI:10.1039/F19878303149

出版商:RSC    

年代:1987

数据来源: RSC