摘要:

Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R. Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F.S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)Gas Kinetics Group and Division de Chimie-Physique de la Societe Francaise de Chimie 9th International Symposium on Gas Kinetics To be held in Bordeaux, France on 20-25 July 1986 Further information from Dr R.Lasclaux, Lab. Photophys. Photochim. MolBculaire, Universite de Bordeaux I, 33405 Talence Cedex, France Poiymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J. Miles, AFRC,Food Research Institute, Colney Lane, Norwich NR4 7UA Polymer Physics Group with the British Rheological Society Deformation in Solid Polymers To be held at the University of Leeds on 9-1 1 September 1986 Further information from Dr J. V. Champion, Department of Physics, City of London Polytechnic, 31 Jewry Street, London EC3N 2EY ~~_____________ ~~~~ Carbon Group Carbon Fibres- P ro pe rt i es and A p p I i cat i o ns To be held at the University of Salford on 1 5 1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX ~ ~~~~~~~~ ~ Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F. S. Stone, School of Chemistry, University of Bath, Bath BA2 7AY (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FX037

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A. N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M.C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R. P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M.J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P.G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091xxxij AUTHOR INDEX Singh, Km. S., 751 Sircar, S., 1527, 1541 Slade, R. C. T., 847 Smith, I. G., 1095 Snelling, C. M., 1761 Sobczyk, L., 311 Siiderberg, D., 17 15 Solar, S., 1101 Solar, W., 1101 Soma, M., 485 Somorjai, G. A., 1263 Somsen, G., 1015 Sorek, Y., 233 Souto, F. A., 2647 Spencer, S., 2357 Spichiger-Ulmann, M., 7 13 Spoto, G., 1283 Spotswood, T. M., 1623 Srivastava, R. D., 913 Stachurski, J., 1447, 2813 Staricco, E. H., 1303 Stock, T., 2257 Stockhausen, M., 397 Stokes, R. H., 1459 Stone, F. S., 1255 Strachan, A.N, 1761 Strohbusch, F., 2021 Stuckless, J. T., 597 Su, Z., 2293 Subrahmanyam, V. S., 1655 Sugimoto, N., 1441, 2959 Suminaka, M., 2287 Suprynowicz, Z., 553 Sutcliffe, L. H., 679, 1467, 1215 Suzanne, J., 2339 Suzuki, H., 3117 Swallow, A. J., 1225 Symons, M. C. R., 433, 565, 727, 2131, 2775, 1095, 1963, 242 1 Takagi, Y., 1901 Takahashi, Y., 3 117 Takeshita, H., 2805 Tamilarasan, R., 2763 Tamura, K., 2287 Tanaka, T., 1513 Taniewska-Osinska, S., 695, Tascon, J. M. D., 939, 2399 Taylor, M. J., 1863 Taylor, N., 2357 Tejuca, L. G., 939, 2399, 1203 Teller, R. G., 1693 Tempere, J-F., 1357 Teramoto, M., 2941 Theocharis, C . R., 857 Thomas, J. K., 735 Tielen, M., 2889, 3049 Tindwa, R. M., 545 Tissier, C., 3081 Toi, K., 2835 Tokuda, T., 2835 Torrez-Mujica, T., 343 Townsend, R.P., 1071, 173 1, Trasatti, S., 2995 Treiner, C., 2513 Trenwith, A. B., 745 Trifiro, F., 1003 Troncoso, G., 1631, 1637 Tseung, A. C. C., 1883 Tuck, J. J., 833 Turner, J. E., 1263 Uemoto, M., 2333 Uma, K., 2733 Valencia, E., 1631. 1637 Valigi, M., 813 Vallmark, T., 1389 Van Oort, M. J. M., 3059 Varma, M. K., 751 Vattis, D., 2043 Vecli, A., 433 Veseli, V., 2095 Vink, H., 1677, 1725 Vliers. D. P., 2009 Vukovid, Z., 1275 3081, 1913 3127 Waghorne, W. E., 2703 Ward, A. J., 2975 Watanabe, H., 1569 Waugh, K. C., 3073 Weckstrorn, K., 2947 Weinberg, N. N., 875 Weingartner, H., 1031 Wells, C. F.. 801, 1057, 1401, White, M. A., 3059 Williams, J. O., 271 1 Williams, P. A., 2635 Williams, P. B., 3067 Williams, R. T., 847 Wojcik, D., 1037 Wood, G. L., 265 Wood, R. M., 273 Woolf, L. A., 769, 2821 Wright, C. J., 2067 Wright, J. P., 1471 Wright, T. H., 1819 Wurie, A. T., 2605 Yadav, G. D., 161 Yadava, R. D., 751 Yamaguchi, M., 1513 Yamaguti, K., 1237 Yamasaki, S., 267 Yamashita, H., 2485 Yamatera, H., 127 Yelon, W., 1693 Yoshida, S., 1513, 2485 Yoshikawa, M., 2485 Zambonin, P. G.. 621 zdanov, S. P., 2541 Zecchina, A., 1283 Zelano, V., 2365 Zhan, R. Y., 2083 Zhao, Z., 185 Zhulin, V. M., 875 Zilnyk, A., 679, 1215 Zulauf, M., 2947 Zundel, G., 1425, 2375 1985. 2145, 2475, 3091

ISSN:0300-9599    

DOI:10.1039/F198581BX039

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

JOURNAL OF T H E CHEMICAL SOCIETY F A R A D A Y T R A N S A C T I O N S , P A R T S I A N D 1 1 The Journal of the Chemical Society is published in six sections, of which five are termed Transactions; these are distinguished by their subject matter, as follows: Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds; including bioinorganic chemistry and solid-state inorganic chemistry; of their structures, properties, and reactions, including kinetics and mechanisms; new or improved experimental techniques and syntheses. Faraday Transactions I (Physical Chemistry). Radiation chemistry, gas-phase kinetics, electrochemistry (other than preparative), surface and interfacial chemistry, heterogeneous catalysis, physical properties of polymers and their solutions, and kinetics of polymerization, etc.Faraday Transactions II (Chemical Physics). Theoretical chemistry, especially valence and quantum theory, statistical mechanics, intermolecular forces, relaxation phenomena, spectroscopic studies (including i.r., e.s.r., n.m.r., and kinetic spec- troscopy, etc.) leading to assignments of quantum states, and fundamental theory. Studies of impurities in solid systems. Perkin Transactions I (Organic Chemistry). All aspects of synthetic and natural product organic, organometallic and bio-organic chemistry, including aliphatic, alicyclic, and aromatic systems (carbocyclic and heterocyclic). Perkin Transactions II (Physical Organic Chemistry). Kinetic and mechanistic studies of organic, organometallic and bio-organic reactions.The description and application of physicochemical, spectroscopic, and theoretical procedures to organic chemistry, including structure-activity relationships. Physical aspects of bio-organic chemistry and of organic compounds, including polymers and biopolymers. Authors are requested to indicate, at the time they submit a typescript, the journal for which it is intended. Should this seem unsuitable, the Editor will inform the author. The sixth section of the Journal of the Chemical Society is Chemical Communications, which is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up eventually by full papers in other journals (e.g.the five Transactions) providing detailed accounts of the work. NOTES I t has always been the policy of the Faraday Transactions that brevity should not be a factor influencing acceptability for publication. In addition however to full papers both sections carry at the end of each issue a section headed ‘Notes’, which are short self-contained accounts of experimental observations, results, or theory that will not require enlargement into ‘full’ papers. The Notes section is not used for preliminary communications. The layout of a Note is the same as that of a paper. Short summaries are required. The procedure for submission, administration, refereeing, editing and publication of Notes is the same as for full papers.However, Notes are published more quickly than papers since their brevity facilitates processing at all stages. The Editors endeavour to meet authors’ wishes as to whether anarticle is a full paper or a Note, but since there is no sharp dividing line between the one and the other, either in terms of length or character of content, the right is retained to transfer overlong Notes to the full papers section. As a guide a Note should not exceed I500 words or word-equivalents. (0NOMENCLATURE 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 Symbols’ (1975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W 1 V OBN).These recommendations are applied by The Royal Society of Chemistry in all its publications. Their basis is the ‘ Systeme International d’UnitCs’ (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 Physicochemical Quantilies 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, B, C , D , E, F, and H (Pergamon, Oxford, 1979 edn).Nomenclature of Inorganic Chemistry (Butterworths, London, 197 1, now published 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 Marlow Medal and Prize Applications are invited for the award of the Marlow Medal for 1986 and Prize of €100. The award will be open to any member of the Faraday Division of the Royal Society of Chemistry who, by the age of 32, had made in the judgement of the Council of the Faraday Division, the most meritorious contribution to physical chemistry or chemical physics. The award will be made on the basis of publications (not necessarily in the Transactions) on any subject normally published in J. Chem. SOC., Faraday Transactions / and //, that carry a date of receipt for publication not later than the candidate’s 32nd birthday. Candidates should be members and under 34 on 1 st January 1986, the closing date for applications, which may be made either by the candidate himself or on his behalf by another member of the Society.Copies of the rules of the award and application forms may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN (ii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 20 Phase Transitions in Adsorbed Layers University of Oxford, 17-1 8 December 1985 Organising Committee: Professor J. S. Rowlinson (Chairman) Dr E. Dickinson Dr R. Evans Mrs Y. A. Fish Dr N. Parsonage Dr D. A. Young The aim of the meeting is to discuss phase transitions at gas/liquid, liquid/liquid and solid/fluid interfaces, and in other systems of constrained geometry or dimensionality less than three.Emphasis will be placed on molecularly simple systems, whereby liquid crystal interfaces and chemisorption phenomena are excluded. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 81 Lipid Vesicles and Membranes Loughborough University of Technology, 1 5 1 7 April 1986 Organising Committee : Professor D. A. Haydon (Chairman) Professor D. Chapman Mrs Y. A. Fish Dr M. J. Jaycock Dr I. G. Lyle Professor R. H. Ottewill Dr A. L. Smith Dr D. A. Young The aim of the meeting is to discuss the physical chemistry of lipid membranes and their interactions, in particular theoretical and spectroscopic studies, polymerised membranes, ~ thermodynamics of bilayers and liposomes, mechanical properties, encapsulation and ~ interaction forces between bilayers leading to fusion but excluding preparation and ~ characterisation methodology.' The preliminary programme may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN (iii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 82 Dynamics of Molecular Photof rag mentat ion University of Bristol, 15-1 7 September 1986 Organising Committee: Professor R. N. Dixon (Chairman) Dr G. G. Balint-Kurti Dr M. S. Child Professor R. Donovan Professor J. P. Simons The discussion will focus on the interaction of radiation with small molecules, molecular ions and complexes leading directly or indirectly to their dissociation.Emphasis will be given to contributions which trace the detailed dynamics of the photodissociation process. The aim will be to bring together theory and experiment and thereby stimulate important future work. further information may be obtained from: Professor R. N. Dixon, Department of Theoretical Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM NO. 21 Interaction-induced Spectra in Dense Fluids and Disordered Solids University of Cambridge, 10-1 1 December 1986 Organising Committee: Professor A. D. Buckingham (Chairman) Dr R. M. Lynden-Bell Dr P. A. Madden Professor E. W. J.Mitchell Dr J. Yarwood Dr D. A. Young Mrs Y. A. Fish Whilst interaction-induced spectra have been studied in the gas phase for many years, their importance in the spectroscopy of condensed matter has been appreciated only relatively recently. At present a considerable number of studies of induced spectra are taking place in what are (nominally) widely separated fields of study. It is highly desirable to bring these communities together so that common issues can be identified and the progress of one field appreciated in another. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 25 October 1985 to: Professor A. D. Buckingham, University Chemical Laboratory, Lensfield Road. Cambridge CB2 1 EWTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION NO. 83 ~ Admission to the Lectures is free and non-members will be welcome.i Further information from: 1 Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN (Tel : 01 437 8656) I Brownian Motion University of Cambridge, 7-9 April 1987 Orga nising Committee Dr M. La1 (Chairman) Dr R. Ball Dr E. Dickinson Dr J. S. Higgins Dr P. N. Pusey Dr D. A. Young Mrs Y. A. Fish The aim of the meeting is to discuss new developments in the experimental and theoretical studies of Brownian motion of colloidal particles and macromolecules, with particular emphasis on the dynamics of aggregate formation and breakdown, computer simulation and many- body hydrodynamic interactions.Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 15 June 1986 to: Dr M . Lal, Unilever Research, Port Sunlight Laboratory, Bebington, Wirral L63 3JW Full papers for publication in the Discussion volume will be required by December 1986 THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY 1985 BOURKE LECTURES by Professor D. Chandler University of Pennsylvania, Philadelphia, U.S.A. Electrons in Liquids, Geometrical Perspectives The Lecture will consider new ways to think about quantum-mechanical processes in liquids, in particular, the mediation of electronic states and chemical bonding by fluctuating liquid environments as they pertain to the behaviour of solvated electrons. Monday Physical Chemistry Laboratory, Oxford 21 October 1985 2.1 5 pm Wednesday 23 October 1985 2.30 pm Friday 25 October 1985 2.00 pm Chairman: Professor J.S. Rowlinson F.R.S. Department of Chemistry, Ma nc hester University (Lecture Theatre G54) Chairman: Professor R. Grice Department of Chemistry, Leicester University Chairman: Professor M. C. R. Symons F.R.S.JOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry/chemical physics which have appeared recently in J.Chem.Research, The Royal Society of Chemistry’s synopsis+ microform journal, include the following: Quantum-mechanical Studies of Catalysis. Part 1. A Model for Nucleophilic Attack on Carbonyl, catalysed by Non -functional Cationic Surfactants Amiram Goldblum and Jehoshua Katzhendler (1 985, Issue 3) Cyclopropane Parameters for Molecular Mechanics Pekto M .lvanov (1 985, Issue 3) Inorganic Analogue of the Ethyl Radical Jehan A. Baban, Vernon P. J. Marti, and Brian P. Roberts (1985, Issue 3) The Iron-Vanadium-Oxygen System at 11 23, 1273, and 1373 K. Part 1. Phase Equilibria Larbi Marhabi, Marie-Chantal Trinel-Dufour and Pierre Perrot (1 985, Issue 3) Solvent Effects on the Rotational Barriers of the N,N-Dimethylamides of 2- and 3-Furoic and 2- and 3-Thenoic Acids Gaetano Alberghina, Francesco Agatino Bottino, Salvatore Fisichella, and Caterina Arnone (1 985, Issue 4) A Partial Determination of the Stability Fields of Ferrierite and Zeolites ZSM-5, ZSM-48, and Nu-10 in the K,O-AI,O,-Si0,-NH, [CH,],NH, System Abraham Araya and Barrie M. Lowe (1 985, Issue 6) The Level of Prochirality: the Analogy between Substitutional and Distortional Desymmetrization Amitai E.Halevi (1 985, Issue 6) Radical Cations of Di-, Tri-, and Tetra-bromoethane formed by Radiolysis: an Electron Spin Resonance Study Martyn C. R. Symons (1 985, Issue 8) Stereochemical Applications of Potential Energy Calculations. Part 4. Revised Electron Spin Resonance Studies of the Ammonia-Boryl Radical (H,N -+ BH;); an FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division-Endo wed Lecture Symposium Surface Science and Catalysis (including the Centenary Lecture by G. Ertl and the Tilden Lecture by J. Pritchard) To be held at the Scientific Societies Lecture Theatre, London on 4 November 1985 Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Division-Endowed Lecture Symposium Molecular Spectroscopy and Dynamics (including the Faraday Lecture by A.Carrington) To be held at the Royal Institution, London on 10 December 1985 Further information from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Colloid and Interface Science Group with the Colloid and Surface Science Group of the SCI Interfacial Rheology To be held at Imperial College, London on 16 December 1985 Further information from Dr R. Aveyard, Department of Chemistry, The University, Hull HU6 7RX High Resolution Spectroscopy Group and Theoretical Chemistry Group Title to be Announced To be held at the University of York on 16-1 8 December 1985 Further information may be obtained from: Dr J.M. Hollas, Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2ADNeutron Scattering Group Time-resolved Scattering and Transition Kinetics To be held at Imperial College, London on 17 December 1985 Further information may be obtained from: Dr J. S. Higgins, Department of Chemical Engineering, Imperial College London SW7 2BY Molecular Beams Group with CCP6 Molecular Scattering-Theory and Experiment To be held at the University of Sussex on 19-21 March 1986 Further information from Dr A. Stace, School of Molecular Sciences, University of Sussex, Falmer, Brighton BN1 9QJ Electrochemistry Group Novel Techniques for the Study of Electrodes and their Reactions To be held at St. Catherine's College, Oxford on 7-9 April 1986 Further information from: Dr S.P. Tyefield, CEGB, Rs Dept, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Division-Annual Congress Structure and Reactivity of Gas Phase Ions To be held at the University of Warwick on 8-1 1 April 1986 Further information from: Professor K. R. Jennings, Department of Molecular Sciences, University of Warwick, Coventry CV4 7AL ________ ~~~~ -- Polymer Physics Group with the Statistical Mechanics and Thermodynamics Group Macromolecular Flexibility and Behaviour in Solution To be held at the University of Bristol on 16-1 8 April 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SW1 X 8QX Division with the Societe FranJcaise de Chimie. Deutsche Bunsen Gesellschaft fur Ph ysikalische Chemie and Associazione ltaliana di Chimica Fisica Dynamics of Molecular Crystals To be held at Grenoble, France on 30 June to 4 July 1986 Further information from: Dr C.Troyanowsky, 10 rue Vauquelin, 75005 Paris, France ~ ~~ Industrial Physical Chemistry Group Physical Chemistry of Water Soluble Polymers To be held at Girton College, Cambridge on 1-3 July 1986 Further information from Dr I. D Robb, Unilever Research Laboratory, Port Sunlight, Bebington, Wirral L63 3JW Polymer Physics Group Biologically Engineered Polymers To be held at Churchill College, Cambridge on 21-23 July 1986 Further information from Dr M. J Miles, AFRC Food Research Institute, Colney Lane, Norwich NR4 7UA Carbon Group Carbon Fibres-Properties and Applications To be held at the University of Salford on 15-1 7 September 1986 Further information from The Meetings Officer, The Institute of Physics, 47 Belgrave Square, London SWlX 8QX ___ _ - _ ~ ~~~ ~ ~ ~ - ~- Division with the Surface Reactivity and Catalysis Group-Autumn Meeting Promotion in Heterogeneous Catalysis To be held at the University of Bath on 23-25 September 1986 Further information from: Professor F.S. Stone, Schcol of Chemistry, University of Bath, Bath BA2 7AY (vii)Faraday Discussions No. 77 Interfacial Kinetics in Solution This publication focuses attention on reactions involving liquid-gas, liquid-liquid and liquid-solid interfaces (excluding electrode kinetics as such). The subject encompasses processes of fundamental, industrial and environmental importance and includes such topics as the rate of dissolution of reactive gases, lunetics at liquid membranes, metal and solvent extraction, Marangoni effects, heterogeneous catalysis and photocatalysis in solution, and the kinetics of dissolution of minerals and drugs.Softcover 318pp ISBN 0 85186 628 X Price 531.50 ($59.50) RSC Members E6.50 No. 78 Radicals in Condensed Phases This publication is primarily concerned with the structure and reactions of radicals in liquids and solids. It brings together theoretical work on structure, environmental effects and reactivity with spectroscopic and mechanistic studies directly concerned with radicals. Fundamental aspects are stressed and particular attention is given to new developments including measurements at short time intervals, special solvent effects, and the effects of external fields. A special area of inclusion is electron gain and loss processes, including trapped and solvated electrons, electrochemical reactions, and specific electron-capture and electron-loss in low temperature systems. Photochemical charge-transfer processes are also included. Softcover 347pp ISBN 0 85186 618 2 Price E28.50 ($55.00) RSC Members E6.50 ORDERING Non-RSC Members should send their orders to: The Royal Society of Chemlstry, Dlstribution Centre, Blackhorse Road, Letchworth, Herts SG6 lHN, UK. RSC Members should send their orders to: Assistant Membership Officer, The Royal Society of Chemlstry, 30 Russell Square, London WClB 5DT. The Royal Society of Chemistry Burlington House Piccadilly London W 1V OBN (viii)

ISSN:0300-9599    

DOI:10.1039/F198581FP081

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC.. Faradaj Trans. 1. 1985, 81, 2257-2271 Infrared Spectroscopic Investigation of Hydroxy-group Siting in H Faujasites BY DIETER DOMBROWSKI,* JURGEN HOFFMANN AND THE LATE JOHANNA FRUWERT Department of Chemistry, Karl Marx University, DDR-70 10 Leipzig, Linnestrasse 2, German Democratic Republic AND THOMAS STOCK Research Centre of Toxicology of the Academy of Sciences of the G.D.R., DDR-7050 Leipzig, Permoserstrasse 1 5, German Democratic Republic Receiced 6th February, 1984 The total hydroxy-group and the separate high-frequency (h.f.) and low-frequency (1.f.) hydroxy-group concentrations in H faujasites, observed by i.r. spectroscopy and corroborated by other methods, show a dependence of the maximum occupation of h.f. OH groups on the Si/AI ratio of between 15 and 32 OH groups per unit cell for zeolites HY and HX, respectively.In the case of HX zeolites above 70% exchange, decomposition is unavoiable. As a result of band separation the separate amounts of seven kinds of h.f. OH subspecies have been obtained from measurements of their i.r. absorbance and the integrated extinction coefficients of the h.f. OH stretching band for HX and HY zeolites. These subgroups show the same i.r. sub-bands in both HX and HY zeolites, but of different intensities. This may be explained on the basis of different Si/AI ordering schemes by 0 , H groups bonded to silicon atoms, surrounded by 0-4 A1 atoms, and bonded to A1 atoms, which are influenced by electrostatic interactions with neighbouring aluminium atoms. The structure and arrangement of cations in zeolites have been studied inten~ively.l-~ However, there is little information available as to the location of the protons bonded to OH groups on zeolitic lattices.From X-ray5-' and neutron8 diffraction measurements a distribution of the protons into four distinctly sited crystallographic groups, O,H, O,H, O,H and O,H, was assumed. As a useful tool to characterize OH groups in zeolites, i.r. spectroscopy has also furnished certain clues concerning the nature and siting of OH group^.^^ 7 3 9, lo The four observed infrared OH stretching vibrations are usually assigned to (i) terminal SiOH groups at 3740 cm-l, (ii) structural Si-0,H-Al groups [high-frequency (h.f.) band] at 3650 cm-' (pointing into the supercage from 01), (iii) structural Si-0,H-A1 groups [low-frequency (1.f.) band] at 3550 cm-l (pointing into the sodalite cage from 0, and 0,) and (iv) cation hydroxy groups M+(OH) at 3590-3530 cm-l.However, there are indications that the Si/AI ordering of H zeolites leads to more types of differently surrounded OH groups than the two structural h.f. and 1.f. OH groups mentioned above.11-15 Thus some authors obtained various oxygen atoms with different electrostatic potential^,^^ and the existence of five distinct Si atoms in zeolites (surrounded by &4 A1 atoms) could be deduced from magic-angle spinning (m.a.s.) 29Si-n.m.r. measurementsll? 1 7 + l8 From these findings the existence of further kinds of OH groups may also be followed in addition to those caused by the four different crystallographic 0 atoms of the faujasites. Indeed, in H forms of zeolites there exist, unlike previous interpretations, more than one 1.f.OH 22572258 I . R . STUDY OF H-FAUJASITES group12 and more than one h.f. OH group, as has been shown by i.r. spectroscopic splitting of the h.f. OH stretching vibration into l4 o r more sub-bands.', In deep-bed activated DY zeolites (apparently in the ultrastable form, as indicated by large bands at 2760, 2725 and 2670 cm-l) a splitting of the OD stretching vibration into two sub-bands was also 0bser~ed.l~ Although the concentrations of individual portions of h.f. and 1.f. OH groups were determined by other authors for some H forms of f a u j a ~ i t e s , ~ ~ ~ 2o the individual concentrations of OH subspecies (calculated from the split h.f.OH stretching bands) are still unknown. Furthermore, the individual h.f. and 1.f. OH concentrations of the H faujasites were determined by different methods of pyridine adsorptionlg? 2o or using different assumptions concerning overlapped h.f. and 1.f. OH stretching bands, and there are still unanswered questions concerning the dependence of the h.f. OH concentrations on the Si/AI ratios in H faujasites. In order to estimate the location and quantitative distribution of the h.f. OH subgroups we have investigated various HX- and HY-type zeolites by means of i.r. spectroscopy. EXPERIMENTAL NH,X and NH,Y zeolites (with Si/Al ratios of 1.18 and 2.6, respectively) were prepared by ion exchange with different amounts of 0.1 mol dm-3 NH,Cl solution at 368 K from the parent zeolites (from VEB CKB Bitterfeld).The NH,X samples with the highest degrees of exchange (70 and 92%) were prepared according to the method of ChwZ1 In this way zeolite samples with exchange levels of 7, 14, 26, 39, 50, 59, 70 and 92% in the case of NH,X and of 7, 15, 21, 30, 46, 54, 64, 74, 88 and 91 % in the case of NH,Y were prepared. Self-supported discs of pure zeolites (of 1-3 mg cm-2) were activated under shallow-bed conditions for 6 h and in 6 steps up to 673 K (for HY) or 450-550 K (for HX) and to a final pressure of lop3 Pa in a heatable i.r. cell fitted with KBr windows. Degrees of exchange and OH concentrations were examined as in ref. (13) using a band- separation method. The i.r. spectra were recorded in transmission without attenuation using a UR-20 spectrometer (from VEB Carl Zeiss Jena) equipped with a digitizing system for wavenumber and intensity, with a resolution of 1.5-3 cm-l, a scan speed no faster than 25 cm-l min-l and better than 80% transmittance.Baseline correction, numerical integration, smoothing, slit correction, band separation and difference spectra of the 0.4 cm-l stepwise digitized spectra were carried out using an R-40 ESER computer (from VEB Robotron). The crystallinity of the exchanged NH, zeolites was examined by X-ray diffraction measurements using Cu Ka radiation and an HZG3 goniometer with 250 mm radius (from VEB Freiberger Prazisionsmechanik) and by i.r. spectroscopy of the zeolitic stretching vibrations in the range 1200-400 cm-l and the absorbance of the OH stretching vibrations.RESULTS For HY and HX zeolites the number of h.f. and 1.f. OH groups were calculated on the basis of integrated extinction coefficients20 using the graphical and numerical integrated absorbance of the two separated i.r. bands (see also fig. 1 and 2). In the case of highly exchanged HX zeolites the extinction coefficients were extrapolated from lower degrees of exchange, because in the first case h.f. and 1.f. OH groups did not interact fully selectively with pyridine. The total number of hydroxy groups calculated in such a way agrees well with those observed for these zeolites by other methods, e.g. from H n.m.r. spectroscopy, D-exchange measurements in the gas phase and temperature-programmed desorption (t.p.d.) of NH, (see table 1). Furthermore the individual concentrations of h.f.and 1.f. OH groups observed by i.r. spectroscopy could be verified by t.p.d. measurements of H faujasites which were reloaded with ammonia after activationz2 (see table 2). In deviation from t.p.d.D. DOMBROWSKI, J. HOFFMANN, J. FRUWERT AND TH. STOCK 2259 60 50 4 40 3 L. 30 z O 20 10 0 50 100 degree of exchange (:< ) Fig. 1. Numbers of the total OH (A), h.f. OH (0) and 1.f. OH (0) groups per unit cell (u.c.) of various degrees of exchange of HNaX zeolites (Si/Al = 1.18), as calculated from i.r. spectra. 50 40 G f 30 0) 0. 3 20 V 50 degree of exchange (%) 100 Fig. 2. Numbers of the total OH (A), h.f. OH (0) and 1.f. OH (0) groups per U.C. of various degrees of exchange of HNaY zeolites (Si/Al = 2.6), as calculated from i.r.spectra. Table 1. Comparison of the total number of OH groups of the shallow-bed activated HY zeolites obtained by different methods total amount of OH groups per large cavity degree of exchange (%) 15 21 30 46 54 64 74 88 91 obtained from degree of exchange 1.0 1.4 2.0 3.1 3.6 4.3 4.9 5.9 6.1 obtained from dehydroxylation t.p.d.22 0.2 0.4 0.9 1.9 2.3 2.8 3.6 4.4 5.2 obtained from NH, desorption t.p.d.22 - 1.3 - 2.8 3.5 - 4.7 5.6 - obtained from broad-line H n.m.r. - 1.9 - - 3.7 - 4.9 5.6 - obtained from D exchange - 1.4 - 2.6 4.9 5.4 - 6.1 6.5 obtained from i.r. intensity (pyridine titration) 1:O 1.5 2.1 3.2 3.8 4.4 5.0 5.9 6.02260 I.R. STUDY OF H-FAUJASITES Table 2. Comparison of h.f. and 1.f. OH concentrations of HY zeolites (Si/AI = 2.6), measured by i.r.spectroscopy, with concentrations of OH groups which were obtained from the high-temperature (h.t.) and the low-temperature (1.t.) peaks of NH, t.p.d. studies ctcfd./pmol mg-' cFf: /pmol mg-' zeolite h.t. peak 1.t. peak h.f. OH 1.f. OH 0,21HNaY 0,50 0, 36 0, 67 0, 24 0,46HNaY 0,83 1, 03 1, 13 0, 96 0, 54HNaY 1, 14 1, 14 1, 12 I , 32 0,74HNaY I , 16 2, 01 I , 20 2, 00 0,88HNaY I , 17 2, 67 1, 13 2, 61 G/cm 3600 32 00 2 800 1700 1300 I I I I I I I Fig. 3. 1.r. spectra of an 0.91 HNaX zeolite (Si/Al = 1.18) activated at different temperatures in the region of OH-stretching and NH,+-deformation vibrations. 23 the HX zeolites are not dehydroxylated in the i.r. cell up to an exchange degree of nearly 70% owing to the milder activation conditions. Even above a degree of exchange of 7O%, HX zeolites cannot be fully activated either in the i.r.cell without thermal decomposition. This is shown in fig. 3 ; the NHi-deformation vibration at 1476 cm-l decreases in intensity whereas the total concentration of OH groups remains constant, as can be seen from h.f. and 1.f. OH band intensities. Both the 1.f. and h.f. OH stretching vibrations consist of 6 or 7 ~ u b b a n d s . l ~ ~ - ~ Using difference spectra in the region 3800-3000 cm-l, comparing partially activated H faujasites or samples loaded with different amounts of pyridine with full activated samples, the wavenumbers of the sub-bands were determined. Using estimated band parameters of the h.f. and 1.f. OH subbands, a band separation of the whole h.f.-1.f.D. DOMBROWSKI, J.HOFFMANN, J. FRUWERT AND TH. STOCK 226 1 I I I I 3700 3600 3500 3400 wavenumber/cm- Fig. 4. Result of a band separation of the envelopes of h.f. and 1.f. stretching vibrations into their sub-bands by means of a numerical-separation method,24 as obtained for a shallow-bed activated 0.14HNaX zeolite (Si/AI = 1.18). The individual sub-bands (curves under the envelopes), fitted to the h.f. and 1.f. OH envelopes (upper curve), are assumed to have pure Cauchy shapes. 3700 3600 3500 3400 3300 wavenum ber/cm - I Fig. 5. Result of a band separation of the envelopes of h.f. and 1.f. OH stretching vibrations into their sub-bands by means of a numerical separation method,24 as obtained for a shallow- bed activated 0.64HNaY zeolite (Si/AI = 2.6). The individual sub-bands (curves under the envelopes), fitted to the h.f.and 1.f. OH envelopes (upper curve) are assumed to have pure Cauchy shapes. OH envelope was carried out by means of computer programs prepared by Jones et a1.23 Pure Cauchy functions were used because the OH band-separation calculations yielded nearly the same distribution of the absorbances of the sub-bands, in the case of pure Cauchy functions as well as in the case of Cauchy-Gaussian product or sum functions. Two examples of the band separations obtained for one activated HY and one activated HX type zeolite are shown in fig. 4 and 5. The calculated wavenumbers of the sub-band maxima and their half-bandwidths agree well for faujasites with various exchange degrees, as can be seen in table 3 (with their standard deviations).2262 I.R.STUDY OF H-FAUJASITES Table 3. Wavenumbers and half-bandwidths of the h.f. OH sub-bands and maximum numbers of the h.f. OH subspecies per u.c., as calculated from h.f. band separation and from the absorbances of the sub-bands for HNaX (Si/Al = 1.18) and HNaY (Si/Al = 2.6) zeolites HNaX 2.35 HNaY 5.2 band v/cm-' AvJcrn-' n$Ex v/cm-l AvJcrn-l n$fx - - - h.f.( 1) 3667 f 1 8.4 & 0.6 8 h.f.(2) 3660 t 1 10.0 f 1.1 14 3657 h 1 9.6 & 3.1 2 h.f.(3) 3653 f 1 10.1 k2.5 8 3649&1 10.4f1.4 7 h.f.(4) 3642 t 1 10.8 & 1.4 5 3643f1 10.2t1.0 5 h.f.(5) 3635f 1 13.6f3.5 2 3637fl 10.0f1.6 5 h.f.(6) 3620 f 4 15-32 4 3626f1 5-12 3 h.f.(7) 3606 f 2 30.1 f 1 1 1 3614+3 14.9f4.4 1 15 10 u 5 L K z 0 5 a 50 degree of exchange (% ) 100 Fig.6. Individual concentrations of the various h.f. OH species of shallow-bed activated HY zeolites (Si/A1 = 2.6) as a function of the degree of exchange, as calculated by i.r. spectroscopy. The concentrations (given in numbers of OH groups per u.c.) are calculated from the integrated absorbances of the h.f.(2)-(7) sub-bands (see fig. 5). The two upper curves represent, on the one hand, the sum of the absorbances of the h.f.(2)-(7) OH species (indicated by filled points) and, on the other, the total amount as calculated from the h.f. OH envelopes (indicated by half-filled points): a, h.f.; 0, total subspecies; 0,3649; 0,3637; x ,3643; 0,3626; A, 3657 and 0, 3616 cm-l. The maximum deviation of the integrated absorbances of the sum of the sub-bands over the h.f.-1.f.OH envelope was smaller than 5 % . From the absorbances of the separated h.f. OH subbands, the concentrations of the h.f. OH subspecies were calculated by means of the integrated extinction coefficients of h.f. and 1.f. OH envelopes for all zeolites of the HX and HY exchange rows. For the h.f. (1)-(7) OH sub-bands of the fully activated H faujasites the maximum number of OH groups (of all degrees of exchange) per unit cell (u.c.) is given in table 3 and may also be determined from fig. 6 and 7.D. DOMBROWSKI, J. HOFFMANN, J. FRUWERT AND TH. STOCK 2263 30 2 5 20 G g 15 CI z 0 10 5 ( Fig. 7. Individual concentrations of the various h.f. OH species of shallow-bed activated HY zeolites (Si/Al = 2.6) calculated by i.r. spectroscopy as a function of the degree of exchange.The concentrations (given in numbers of OH groups per u.c.) are calculated from the integrated absorbances of the respective h.f. (2)-(7) OH sub-bands (see fig. 4). The upper two curves represent, on the one hand, the sum of the absorbances of the h.f. (2)-(7) species (indicated by filled points) and, on the other, the total number of OH groups, as calculated from the h.f. OH envelopes (indicated by half-filled points): a, h.f.; 0, total subspecies; x ,3643; D, 3667; A, 3660; 0, 3653; 0, 3635 and 0, 3620 cm-l. Since the OH concentrations may also be falsified by the H faujasites having a partly destroyed lattice, the stability of the lattice was checked in the case of hyrated NH, faujasites by means of the first 20 X-ray diffraction reflexes. In no case was there more than an 8% loss in crystallinity. The lattice constants, e.g.of the hydrated NH,NaY zeolites, which varied from 2.466 to 2.460 nm over the whole range of exchange degree also showed no deviation from linearity in their dependence on degree of exchange. DISCUSSION A comparison of the total OH concentrations of the activated H faujasites as observed by various methods (see table 1) clearly shows agreement of the i.r. results with those obtained from the degree of exchange and from the amount of ammonia2264 I . R . STUDY OF H-FAUJASITES desorbed. The strong thermal instability of HX zeolites, which contain < 30% sodium ions, agrees well with the fact that 30% of the OH groups are weak acid centres.1° The h.f. and 1.f. OH groups show different behaviour in their thermal stability as a function of exchange level and type of zeolite.In the case of the HY zeolites studied, their maximum numbers were 16 and 32 OH per u.c., respectively (for 0.91 HNaY5.2). However, in the case of HX zeolites the h.f. OH groups reached 32 OH per u.c., and above an exchange degree of 70% rapidly decomposed, whereas the number of 1.f. OH groups reached a maximum of 40 OH per U.C. and increased slowly above a degree of exchange of 70% (see fig. 2). Therefore the following conclusions may be drawn. (i) The maximum number of H F hydroxy groups in H faujasites may be higher or lower than 16 OH per u.c.; the maximum value is 32 OH per U.C. if the Si/Al ratio exceeds 2. (ii) If the total number of OH groups exceeds 60 OH per U.C.the H faujasites will be thermally unstable (under the conditions of activation examined in this study) and cannot be fully activated without dehydroxylation. Interesting results were also obtained from the band separation of h.f. and 1.f. OH stretching vibrations. Since the latter stretching vibration was resolved into sub-bands by Uytterhoeven et af.'* in this paper we are concerned with the numerical band separation of the h.f. OH band into its sub-bands and the nature, behaviour and concentrations of the OH subgroups indicated by these bands. Table 3 shows that the HF envelope of HX and HY zeolites contains the same sub-bands (HF2-HF7), which alter slightly in wavenumber and half-bandwidths. In comparison with the nearly identical structure of X and Y type zeolites this is a reasonable result.On the other hand, the intensities of these sub-bands alter strongly (see fig. 6 and 7) and explain the different wavenumbers of the h.f. OH envelopes of the two types of faujasites (at 3660 and 3645 cm-l): At higher Si/Al ratios the sub-bands of higher wavenumbers diminish in intensity or disappear altogether [e.g. the h.f.( 1) and h.f.(2) OH sub-bands at 3667 and 3660cm-1 in HY zeolites] and therefore the h.f. OH envelope has its maximum at lower wavenumbers. Further, the OH subgroups indicated by such highfrequency sub-bands, which dominate in HX zeolites, should be less acidic than those indicated by sub-bands at smaller wavenumbers, which dominate in the more acidic HY zeolites. Also, the lowering of h.f. OH stretching envelopes of other types of zeolites as a function of an enhanced Si/Al ratio, as described by some a u t h o r ~ ~ j - ~ ~ may be explained by such overlapped sub-bands.In the case of the 1.f. OH stretching bands, the same alteration of frequency of the envelope maximum as a function of the intensities of the 1.f. OH sub-bands was also obtained for both types of faujasites. Comparing the sub-band intensities as a function of the degree of exchange, relationships are observed for both H faujasites (see fig. 6 and 7). In the HY zeolites only the h.f. (2) and (3) bands at 3657 and 3649 cm-l reach maximum intensities at medium exchange degrees, indicating 2 and 7 OH per U.C. In contrast, the intensities of all sub-bands of HX zeolites show maxima at different exchange degrees, causing a more structured h.f.OH band envelope than in HY, e.g. near an exchange degree of 10% the h.f. (4) and ( 5 ) sub-bands of HX at 3643 and 3635 cm-l dominate over other sub-bands in intensity and the envelope maximum should appear near 3643 crn-l. Indeed the h.f. OH stretching band of an 0.08 HNaX zeolite, prepared to examine this prediction, was centred at 3645 cm-l. Also, the splitting of the 0.14 HNaX sample's h.f. OH envelope (see fig. 4) can be explained by such maxima in the intensity of the sub-bands at 3660 and 3653 cm-l and at 3643 and 3635 cm-l. From the relatively good agreement of the number of h.f. and 1.f. OH groups, calculated for a series of variously exchanged HY zeolites (see table 2) from NH, t.p.d.D. DOMBROWSKI, J. HOFFMANN, J .FRUWERT AND TH. STOCK 2265 on the one hand and from i.r. results on the other,22 a relatively unique energetic behaviour of the different kinds of h.f. and 1.f. OH groups indicated by the various i.r. sub-bands can be concluded. The next problem is how to assign the different h.f. OH subspecies to special sites in the zeolite lattice. The interaction of all h.f. OH species with pyridine (in different ways) supports their assignment to hydroxyls in the large cavities. With regard to the well known structure of the faujasites,l+ 0 , H and 0 , H groups in different surroundings should be discussed. As is known from quantum-chemical calculations of zeolitic ~ 1 u s t e r s , ~ * - ~ ~ the Si-0 bond is stronger than the A1-0 bond and therefore the 0-H stretching vibration should be determined preferably by the influence of silicon and its surroundings in the lattice, and secondly by aluminium and its surroundings.On the other hand, we must consider the electrostatic interaction of the aluminium with its nearest A1 neighbours (which are arranged in the three four-membered rings surrounding the A1 atom in question)30 and with metal cations neutralizing the charge of the framew~rk.~’ The lowering of the wavenumber of the h.f. OH envelope with increasing %/A1 ratio in H zeolites is usually explained in the literature by this electrostatic influence of metal cations.6 However, according to our findings only the different wavenumbers of the h.f. OH sub-bands, dependent on the Si/AI ratio (2-8 cm-l), may be explained in this way.If one assumes that the kind and number of h.f. OH subspecies preferentially depend on the ordering of Si and A1 tetrahedra in zeolites, then the observed i.r. spectroscopic concentrations of the h.f. ( l t ( 7 ) OH subspecies should be in agreement with those obtained from the Si/AI ordering schemes actually present in the faujasites in question. We will now try to correlate the different kinds and numbers of h.f. OH subspecies described in this paper to Si/Al-ordering models of the faujasites in question and to their distributions of the various Si(OAl), Si( 1 Al), Si(2Al), Si(3Al) and Si(4Al) tetrahedra, as corroborated by m.a.s. 29Si-n.m.r. spectroscopic studies.ll9 lX of a unit cell ( i t . one double cubo-octahedron, abbreviated as d.c.) with Si/Al = 26/22 (= 1.18) and a distribution of Si tetrahedra Si(4AI): Si(3AI): (2A1): Si( 1Al): Si(OA1) of 16: 8: 0:O: 2.The lower 6 pictures represent twelve four-membered rings of four hexagonal prisms (connected by full lines), the neighbouring two four-membered rings of hexagonal prisms and the three cubo-octahedral four-membered rings (the four tetrahedral atoms of both types of four-membered rings are connected by dotted lines). The circles represent aluminium (black) and silicon (empty) atoms. The numbers in the circles give the numbers of directly bonded A1 neighbours, but Si(4AI) tetrahedra are not marked. The four crystallographically different oxygen atoms (0 atoms between tetrahedra are not drawn in the figures) are marked 1 4 in the first of the six pictures.If we neglect 0 , H groups and take into consideration only O,H, which cause h.f. OH subspecies (0,H and 0,H represent OH groups formed with crystallographically different oxygen atoms 0, and 04), as was obtained from neutron diffraction studies,* and if we assume that at a maximum of only one 0 , H group can be sited in each four-membered ring of the hexagonal prisms, then 12 O,H should exist per d.c. However, as follows from the i.r. spectra of the NHaX zeolites (see fig. l), only 8 0,H groups per d.c. are stable in these faujasites of modulus 2.35. The question now is how are the 8 0 , H species to be distributed over the possible 16 Si(4)-, 8 Si(3)- and 2 Si(O)-O,HAl groups in these zeolites. [Si(n) represents Si tetrahedra which are directly bonded onto n 0-A1 groups.] In addition to the environment of the Si atoms, we also consider the electrostatic influence of the A1 neighbours on the A1 atom of the Si0,HAl group in question.Thus we give consideration to the influence of the nearest In fig. 8(a) and ( b ) there are two such Si/Al-ordering models, for2266 ( a 1 I.R. STUDY OF H-FAUJASITES Si(3) no 01H Si(4)3-2Al(15) Si(3)2 -1A1(13) Si(4)3-2Al(16) Fig. 8. Two possible distributions of 0 , H subgroups in HX zeolites (Si/Al = 1.18) with different %/A1 orderings in a double cubo-octahedron : (a) symmetrical scheme and (b) non-symmetrical scheme.D. DOMBROWSKI, J. HOFFMANN, J. FRUWERT AND TH. STOCK 2261 A1 neighbours (sited in the three adjacent four-membered rings of the A1 atom in question) on the one hand and, on the other, to the influence of the A1 atom which is sited additionally in the respective four-membered ring of the hexagonal prism (see fig.8 and 9), as proposed by Demp~ey.~' In order to distinguish between the various kinds of A1 atoms in the Si0,HAl arrangements with regard to the different influences of the adjacent A1 atoms, the following description is used. Following the symbol Si(4) the number of neighours in the adjacent three four-membered rings is written and, after this, the total number of A1 atoms in the respective four-membered ring of the hexagon of the models as shown in fig. 8 and 9. Thus the description Si(4)3-2 means that three A1 neighbours of the A1 atom in question are sited in its three adjacent four-membered rings and two A1 atoms are arranged in the hexagonal four-membered ring in question (the hexagonal four-membered ring is shown in fig.8 and 9 as a four-membered ring connected by full lines). Finally we take into consideration the different surroundings of the A1 atoms in the Si0,HAl bridge by comparing the total number of those A1 atoms bonded by means of the 4 Si neighbours in the second sphere of bonding, and their amount is written as a number in brackets after the symbol for aluminium; for example Si(4)3-2A1(15) means that the A1 atom of the bridge is bonded to 4 Si atoms which together are bonded to 15 A1 atoms. As can be seen from fig. 4 and 5, the h.f.( 1) and (2) sub-bands dominate in intensity in HX, as distinct from HY zeolites. Because of the higher Si(4) portion of HX zeolites [ 16 Si(4) per d.c.1 these two bands might be caused by Si(4)0,HAl hydroxyls.The sub-bands h.f. (3)-(5) appearing at lower wavenumber can be explained in the same way by Si(3)0,HAl and Si(2)0,HAl hydroxyls. In order to say which of these possible OH groups are occupied first, we recall the fact that in HX zeolites the h.f. (4) and (5) species dominate at low exchange degrees (as might be deduced from the domination of the i.r. band at 3635 cm-l) and therefore Si(3)0,H or Si(2)0,H hydroxyls should be formed first in HX zeolites. Assuming that the OH groups in Si(n)O,HAl bridges with fewer A1 neighbours are therefore formed preferentially and that no Si(O)O,HSi hydroxyls are formed, we arrived at 48 0,H groups (see table 4) from the two HX models shown in fig.8(a) and (6). In table 4 the i.r. spectroscopic assignment of the h.f. OH sub-bands and the i.r. spectroscopically obtained maximum numbers of the OH subspecies compared with those derived from the two Si/Al- ordering schemes of HX are given. The i.r. results in table 4 can be better explained by the Si/Al ordering of the symmetric HX structure [fig. 8(a)] than by the assymmetric HX structure [fig. 8 (b)]. The maximum four 0 , H groups per U.C. of the Si(4)3-2Al( 14) bridges, derived from the models in fig. 8(a), do not seem to exist and the 8 h.f.(3) OH species (calculated from the h.f. OH sub-bands at 3653 cm-l and available from table 3 and fig. 7) might therefore be explained as 0,H groups in Si(3)3-2Al( 15) bridges of fig. 8(a) (see table 4). The obtained assignment of the h.f.OH subspecies to different sites in the zeolite lattice (see table 4) also helps us to understand the simultaneous occupation of only two 0 , H groups on each hexagonal prism. If one considers the distribution of the maximum 24 Si(4]3-2A1( 16, 15) 0 , H groups per U.C. over the HX lattice, as derived from the structures in fig. 8 (see table 4), one can see that they are sited only in the 8 hexagonal prisms. However, in no zeolite of the HX exchange row does the sum of these two types of h.f. OH groups exceed two 0 , H per hexagon. For the same reason the Si(4)3-2Al( 14) and four of the Si(3)2-1Al( 13) samples actually possess no hydroxyls, because they would in each case be the third 0,H group on the hexagonal prisms. In fig. 9 three possible arrangements of 0,H groups are shown for Si/Al-ordering models in HY zeolites (Si/Al = 2.43), of which model (a) is the most probable, asD.DOMBROWSKI, J. HOFFMANN, J. FRUWERT AND TH. STOCK 2261 A1 neighbours (sited in the three adjacent four-membered rings of the A1 atom in question) on the one hand and, on the other, to the influence of the A1 atom which is sited additionally in the respective four-membered ring of the hexagonal prism (see fig. 8 and 9), as proposed by Demp~ey.~' In order to distinguish between the various kinds of A1 atoms in the Si0,HAl arrangements with regard to the different influences of the adjacent A1 atoms, the following description is used. Following the symbol Si(4) the number of neighours in the adjacent three four-membered rings is written and, after this, the total number of A1 atoms in the respective four-membered ring of the hexagon of the models as shown in fig.8 and 9. Thus the description Si(4)3-2 means that three A1 neighbours of the A1 atom in question are sited in its three adjacent four-membered rings and two A1 atoms are arranged in the hexagonal four-membered ring in question (the hexagonal four-membered ring is shown in fig. 8 and 9 as a four-membered ring connected by full lines). Finally we take into consideration the different surroundings of the A1 atoms in the Si0,HAl bridge by comparing the total number of those A1 atoms bonded by means of the 4 Si neighbours in the second sphere of bonding, and their amount is written as a number in brackets after the symbol for aluminium; for example Si(4)3-2A1(15) means that the A1 atom of the bridge is bonded to 4 Si atoms which together are bonded to 15 A1 atoms.As can be seen from fig. 4 and 5, the h.f.( 1) and (2) sub-bands dominate in intensity in HX, as distinct from HY zeolites. Because of the higher Si(4) portion of HX zeolites [ 16 Si(4) per d.c.1 these two bands might be caused by Si(4)0,HAl hydroxyls. The sub-bands h.f. (3)-(5) appearing at lower wavenumber can be explained in the same way by Si(3)0,HAl and Si(2)0,HAl hydroxyls. In order to say which of these possible OH groups are occupied first, we recall the fact that in HX zeolites the h.f. (4) and (5) species dominate at low exchange degrees (as might be deduced from the domination of the i.r. band at 3635 cm-l) and therefore Si(3)0,H or Si(2)0,H hydroxyls should be formed first in HX zeolites.Assuming that the OH groups in Si(n)O,HAl bridges with fewer A1 neighbours are therefore formed preferentially and that no Si(O)O,HSi hydroxyls are formed, we arrived at 48 0,H groups (see table 4) from the two HX models shown in fig. 8(a) and (6). In table 4 the i.r. spectroscopic assignment of the h.f. OH sub-bands and the i.r. spectroscopically obtained maximum numbers of the OH subspecies compared with those derived from the two Si/Al- ordering schemes of HX are given. The i.r. results in table 4 can be better explained by the Si/Al ordering of the symmetric HX structure [fig. 8(a)] than by the assymmetric HX structure [fig. 8 (b)]. The maximum four 0 , H groups per U.C.of the Si(4)3-2Al( 14) bridges, derived from the models in fig. 8(a), do not seem to exist and the 8 h.f.(3) OH species (calculated from the h.f. OH sub-bands at 3653 cm-l and available from table 3 and fig. 7) might therefore be explained as 0,H groups in Si(3)3-2Al( 15) bridges of fig. 8(a) (see table 4). The obtained assignment of the h.f. OH subspecies to different sites in the zeolite lattice (see table 4) also helps us to understand the simultaneous occupation of only two 0 , H groups on each hexagonal prism. If one considers the distribution of the maximum 24 Si(4]3-2A1( 16, 15) 0 , H groups per U.C. over the HX lattice, as derived from the structures in fig. 8 (see table 4), one can see that they are sited only in the 8 hexagonal prisms. However, in no zeolite of the HX exchange row does the sum of these two types of h.f.OH groups exceed two 0 , H per hexagon. For the same reason the Si(4)3-2Al( 14) and four of the Si(3)2-1Al( 13) samples actually possess no hydroxyls, because they would in each case be the third 0,H group on the hexagonal prisms. In fig. 9 three possible arrangements of 0,H groups are shown for Si/Al-ordering models in HY zeolites (Si/Al = 2.43), of which model (a) is the most probable, asD. DOMBROWSKI, 3. HOFFMANN, J. FRUWERT AND TH. STOCK 2269 Si(Z)O-lAl(B) Si(4)2-2A1(12) Si(1) no 01H Si(2) no 01H Si(1) no 01H Si(1) no 01H Si(2) no 01H Si(3)2-2Al(12) Si(2) O-lAl(8) Si (3)2-2AL(12) Si(2)0-1Al(7) Si(2) no 01H Si(1) no 01H Si(2) no 01H Si(3) 1-2Al(lO) Si(3)3-2A1(13) Si(2)O-lAl(7) Si(3)2-2Al(12) Si(2) no 01H Si(3)1-1Al(10) Si (3)2-2AL(12) Si(4)2-2Al(12) Si(2) no 01H Si(3)3-2Al(13) Fig.9. Three possible distributions of 0 , H subgroups in HY zeolites (Si/AI = 2.43) with different Si/AI orderings in a double cubo-octahedron : ( a ) mefa-para form, (h) mixed rneta-para form and (c) meta-meta form. Table 4. Comparison of maximum numbers of h.f. OH groups per U.C. derived from Si/Al-ordering schemes (columns 1 and 3) and obtained by i.r. spectroscopy (column 6) for HNaX zeolites of different degrees of exchange (Si/AI = 1.18) 8 Si(4)3-2A1( 16) 8 Si(4)3-2AI( 16) 3667 8 16 Si(4)3-2A1( 15) 16 Si(4)3-2AI( 15) 3660 14 8 Si(3)3-2Al( 15) 4 Si(3)3-2AI( 14) 3653 8 4 Si(4)3-2AI( 14) 4 Si(4)2-2AI( 14) - ~- 4 Si(3)2-2A1( 13) 8 Si(3)2-2AI( 14, 13) 3642 5 8 Si(3)2-1A1(13) 8 Si(3)2-1A1(14, 13) 3635 2 estimated from m.a.s. 29Si-n.m.r.studies.l19 la It is nearly impossible to decide which of 48 possible 0 , H groups are the 16 most likely to exist without knowing any rules of selection. Nevertheless, the kinds and numbers of 0 , H per U.C. in HY zeolites (see tables 3 and 4) may also be explained by Si(4)-O,HAl to Si(l)-O,HAI groups, as was done for the HX zeolites; for example, in HY zeolites of fig. 9(6), with a silicon distribution of 0:6: 14: 10:4 Si per d.c., a maximum 24 0 , H per U.C. of the four types of Si(3)0,HAl groups exist [see fig. 9(6)], to which the maximum 7 h.f.(3) and 5 h.f.(4) OH groups per U.C. (see table 3), measured by i.r. spectroscopy, may be ascribed. The2270 I.R. STUDY OF H-FAUJASITES h.f. (5)-(7) OH groups may be due to 32 0,H per U.C.of 5 types of Si(2)0,HAl groups. The existence of the two i.r. spectroscopically observed h.f.(2) OH groups per U.C. in HY zeolites cannot be explained by the fig. 9(a) and (b), but rather by fig. 9(c) which includes Si(4)0,HAl groups. In order to decide whether the above proposals explain the various kinds of observed OH groups, further investigations will have to be carried out. In a later paper we will report supporting Evidence for the above proposals, obtained CND0/2 calculations of large zeolitic clusters, which takes into account the influence of the different surroundings. We thank Dr H. Kopernick (Wolfen) for help with X-ray diffraction measurements, Dr R. V. Dmitriev, Dr A. N. Detjuk (Moscow) and Prof. K-H. Steinberg (Leipzig) for help with D-exchange measurements, and colleagues of the group ' Wissen- schaftsbereich fur Experimentalphysik ' of Prof. H.Pfeifer (Leipzig) for help with n.m.r. measurements and useful discussions. W. M. Meyer and D. Olson, Atlas of Zeolite Structure Types (1978). W. J. Mortier, Compilation of Extra Framework Sites in Zeolites ( I 982). J. W. Smith, ACS Monogr., 1976, 171. D. W. Breck, Zeolite Molecular Sieves (Wiley, New York, 1974). D. H. Olson and E. J. Dempsey (a) Soc. Chem. Ind., 1969, 293; (b) J. Catal., 1969, 13, 221. W. J. Mortier, J. J. Pluth and J. V. Smith, J. Catal., 1976, 45, 367. (a) Z . Jirak, S . Vratislav and V. Bosacek, J. Phys. Chem. Solids, 1980, 41 1089; (6) Z. Jirak, S . Vratislav, J. Zajicek and V. Bosacek, J . Catal., 1977, 49, 112.A. V. Kiselev and V. I. Lygin, Infrared Spectra of Surface Compounds (in Russian) (Nauka, Moscow, 1972). 'I J. W. Ward, ACS Monogr., 1976, 171. lo P. A. J. Jacobs, Carbiogenic Acidity of Zeolites (Elsevier, New York, 1977). l 1 J. Klinowski, S. Ramdas, J. M. Thomas, C. A. Fyfe and J. S. Hartmann, J. Chem. SOC., Faraday Trans. 2, 1982, 78, 1025; (6) J. M. Thomas, S. Ramdas, G. R. Millward, J. Klinowski, M. Audier, J. Gonzalez-Calbet and C. A. Fyfe, J . Solid State Chem., 1982, 45, 368. l2 P. A. Jacobs and J. B. Uytterhoeven, J . Chenz. Soc., Faraday Trans. 1, 1973, 69, 359; 373; M. F. Guilleux and D. Delafosse, J . Chem. SOC., Faraday Trans 1, 1975, 71, 1777. '' (a) D. Dombrowski and J. Hoffmann, hfitteilungsbl. Chem. Ges. DDR, 1980, 35, 32; (b) React. Kinet. Catal. Lett., 1983, 22, 435; ( c ) Th. Stock, D. Dombrowski and J. Fruwert, 2. Chem., 1983, 22, 233. l4 S. Dzwicaj, J. Haber and T. Romotowski, Zeolites, 1983, 3, 134. l5 K-H. Steinberg, H. Bremer, F. Hoffmann, Ch. Minacev, R. V. Dmitriev and A. N. Detjuk, Z . Anorg. l6 E. J. Dempsey, in Molecular Sieves (SOC. Chem. Ind., London, 1969), p. 293. l7 A. L. Klyachko, Kinet. Katal., 1978, 19, 441. I * G. Engelhardt, E. Lippmaa and M. Magi, (a) J . Chem. Soc., Chem. Commun., 1981,712; (b) Abstraots Workshop Eberswalde (GDR, 1982), vol. 11, p. 1. l 9 (a) A. Bielanski and J. Datka, Bull. Acad. Pol. Sci., Ser. Chim., 1974, 22, 341; 1975, 23, 445; (6) J. Datka, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 765; (c) J. Datka, J . Chem. SOC., Faraday Trans. I , 1980, 6, 2437. Allg. Chem., 1974, 404, 142. 2o Th. Stock, D. Dombrowski and J. Fruwert, Z. phys. Chem. (Leipzig), 1984, 265, 551-6. *l P. Chu and F. G. Dwyer, J . Catal., 1980, 61, 454; (b) P. Chu, J . Catal., 1976, 43, 346. 22 J. Hoffmann, B. Hunger, U. Streller, Th. Stock, D. Dombrowski and A. Barth, Zeolites, 1985,5, 31. 23 J. Hoffmann and D. Dombrowski, React. Kinet. Catal. Lett., 1982, 21, 485. 24 Natl Res. Counc. Can. Bull., 1977, 11-17. 25 D. Barthomeuf, J. Chem. Soc., Chem. Commun., 1977, 743. 26 V. B. Kazanski, Kinet. Katal., 1982, 23, 1334. 27 P. A. Jakobs and W. J. Mortier, Zeolites, 1982, 2, 226. 28 (a) S. Beran, J . Mol. Catal., 1981, 10, 177; (b) S. Beran and P. Jiru, React. Kinet. Catal., Lett., 1981, 17, 47; ( c ) S. Beran, J. Dubsky, V. Bosacek and P. Jiru, React. Kinet. Catal. Lett., 1980, 13, 151; S. Beran and J. Dubsky, J. Phys. Chem., 1979,83, 2538.D . DOMBROWSKI, J . HOFFMANN, J. FRUWERT AND TH. STOCK 227 1 29 D. Dombrowski, V. I. Lygin and S. Chlopova, Zhur. Fiz. Khim., 1983, 57, 1807. 30 J. Sauer, W. Scirmer and R. Zahradnik, Ahorption of Hydrocarbons in Microporous Adsorbenrs Workshop Eberswalde GDR (1982), vol. 11, p. 44; J. Sauer, K. Fielder, W. Schirmerand R. Zahradnik in 5th Znr. Con5 Zeolites, ed. L. V. C. Rees (Heyden, London, 1980) p. 501. 31 E. J. Dempsey, (a) J , Caral., 1974, 33, 497; (b) J . Catal., 1975, 39, 155. (PAPER 4/205)

ISSN:0300-9599    

DOI:10.1039/F19858102257

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. Soc., Furuday Truns. I , 1985,81, 2273-2286 Sorption-Diffusion in Heterogeneous Systems Part 9.l3 - Kinetic and Thermodynamic Effects determining the Enantio- differentiating Chirospecificity of Solid-bound Chiral Phosphine Oxides BY PETER KREBSER, HANS-RUDOLF MARTI AND PAUL RYS* Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology ETH, ETH-Zentrum, CH-8092 Zurich, Switzerland Received 25th June, 1984 A procedure is described which allows a quantitative determination of the enantio- differentiating chirospecificity of chiral solid materials. It enables the chiral recognition to be studied by analysing the sorption kinetics and isotherms of enantiomers onto such solids. The procedure is exemplified using polystyrene-bound chiral phosphine oxides as chiral solids and the enantiomers of mandelic acid. In addition, a model for the description of the competitive sorption of enantiomers onto a chiral porous solid is presented.With the aid of this model some surprising experimental observations can be rationalized. Many attempts have been made to achieve racemate resolutions using chromato- graphic methods. The increasing interest and the recent progress in this subject are documented by a large number of review Direct liquid-chromatographic resolutions of enantiomers can be realized either by the use of an optically active packing material or by employing an optically active solvent. Although considerable success has been attained in partial and even total resolution of racemates using insoluble chiral supports, the process has not yet been generally accepted as a preparative procedure.This might be due to the fact that not enough is known about the mechanism of chiral recognition to enable a chiral support to be tailored to the resolution of a given racemate mixture. The present study introduces a method for the quantitative determination of the enantio-differentiating behaviour of chiral solid materials. It enables chiral recognition to be studied by analysing the sorption kinetics and isotherms of enantiomers onto chiral solids. The procedure is exemplified using polystyrene-bound chiral phosphine oxides as chiral solids and the enantiomers of mandelic acid. EXPERIMENTAL SYNTHESES and (S),-poly(styrylmethylpheny1phosphine oxide) [2(R) and 2(S), respectively] were synthesized as described elsewhere2 (see scheme 1).These polymer-bound chiral phosphine oxides 2(R) and 2(S) contained 0.45 mmol of P/g of polystyrene resin and 0.72 mmol of P/g of polysterene resin, respectively. The density of dry, unswollen resin was found to be 0.93 g ~ m - ~ , while that of resin swollen in a mixture of toluene and THF (85/ 15 vol % ) was determined to be 0.172 g cm-3 swollen polymer. The mean diameter of the unswollen beads was 0.53 x lop2 cm, while that of the beads swollen in toluene+THF (85/15 vol %) mixtures was 0.93 x lop2 cm. 22732274 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS 1 1 C6H5 MeOH 1 ~ 6 ~ 5 /C6H5 /C6H5 P-CI - P-OCH, - O=P-CH, pcII_ O=P-CH3 ‘0 CH3 ‘OCH3 CL \ \ C I 1 ( - 1 - M e n O H ,%H5 \ O=P***CH, OMen I1 2 ( S ) Scheme 1.” CHROMATOGRAPHIC COLUMNS AND RESOLUTIONS OF ENANTIOMERS For all chromatographic runs glass columns of 1.5 cm i.d.were used. They were packed with various amounts of solid resin of the chiral materials 2(R) or 2(S). The resin was first suspended and swollen in the solvent mixture which was to be used as the mobile phase. Numerous chromatographic runs were made by passing a racemate solution of mandelic acid over the chiral solid material 2(R) or 2(S). Some of the chromatographic runs showed fairly good separation, with the first eluate fraction having > 95% enantiomeric excess. However, the reproducibility of the experimental data was very poor.? The effectiveness of the separations depended on the loading, the dimensions and the packing of the columns, the solvent compositions, the flow rate, the temperature and other parameters not yet known.In some cases only a small change in the chromatographic conditions resulted in a change in the sequence of appearance of the enantiomers in the eluate columns. These observations indicate that the separation behaviour for enantiomers is also determined by kinetic effects. The study of kinetics promises a better insight into the enantio-differentiating specificity of chiral solid materials. KINETIC AND EQUILIBRIUM SORPTION MEASUREMENTS 4 g of the dry resin 2(R) or 2(S) were first preconditioned in 200 cm3 of a solvent mixture of toluene+THF (85/15 ~ 0 1 % ) for I h. The racemic mandelic acid or one of the pure enantiomers [3(R) or 3(S)] was then added to the remaining I8 I .1 cm3 of the bulk surrounding solution. After various time intervals the amount of sorbed mandelic acid was determined by measuring the optical rotation (using a Perkin-Elmer polarimeter 141) and titrating the remaining acid in the bulk surrounding solution. In some cases this amount was cross-checked by extracting the polymer beads in a soxhlet apparatus with THF and measuring the concentration of mandelic acid in the extracts. Both methods gave coinciding results. The sorption isotherms were determined after 26&290 h of equilibration using the same experimental procedure as described above. * Men = (-)-Menthyl. 1- Owing to this difficulty, no chromatograms are presented.P. KREBSER, H-R. MARTI AND P. RYS 2275 Cads, il, P R r fi [sor, i] [SOT, il, T t VB VP X V a Aa ab F' TD, i Tim, i Tm, i Tim/m, i LIST OF SYMBOLS concentration of the total available chiral adsorption sites (mol kg-') concentration of the diastereomeric adsorption complex with the enantiomer i (mol kg-') total concentration of both diastereomeric adsorption complexes (mol kg-l) equilibrium concentration of the diastereomeric adsorption complex with enantiomer i (mol kg-l) normalized rate constant of immobilization of enantiomer i (-) normalized rate constant of mobilization of enantiomer i (-) diffusion coefficient of enantiomer i (cm2 s-l) normalized concentration of the chiral adsorption sites (-) enantiomer R or S concentration of enantiomer i (mol dm-3) equilibrium concentration of enantiomer i (mol dmd3) rate constant of immobilization of enantiomer i (dm3 mol-l s-l) rate constant of mobilization of enantiomer i (s-l) adsorption constant of enantiomer i (dm3 mol-l) initial concentration ratio of the enantiomers (-) flux vector of component i (mmol cm-2 s-l) porosity of the swollen polymer bead (imbibed pore liquid per weight of dry bead) (dm3 kg-l) quasi-optical purity (% ) mean radius of the polymer bead (cm) polar coordinate (cm) rate of immobilization and mobilization (mol dmV3 s-l) concentration of the sorbed enantiomer i (mol kg-l) equilibrium concentration of the sorbed enantiomer i (mol kg-l) normalized time (-) time (s) volume of the bulk solution excluding the polymer bead volume V, (dm3) volume of the swollen polymer bead (dm3) normalized polar coordinate (-) nabla operator (cm-l) V B l V P (-1 chirospecificity factor (-) optical rotation (degree) absolute value for optical rotation of a pure enantiomer solution (") geometrical factor (-) relaxation time of the diffusion process of enantiomer i (s) relaxation time of immobilization (s) relaxation time of mobilization (s) relaxation time of the immobilization/mobilization equilibration process (s) normalized concentration of enantiomer i (-) normalized concentration of the diastereomeric adsorption complex with enantiomer i (-)2276 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS SORPTION MODEL In order to characterize the enantio-differentiating sorption behaviour of the enantiomers 3(R) and 3(S) of mandelic acid by the chiral polymer-bound phosphine oxides 2 ( R ) and 2(S), the sorption kinetics and the adsorption isotherms were determined.The basis for the interpretation of the experimental results is our general diffusion-immobilization model (DIMO), which covers the whole range from diffusion to immobilization-controlled uptake kinetics.'* Following a suggesting of Crank,I3 Danckwert~'~ and Weisz,15 this model treats the sorption of a compound by a penetrable solid substrate as a superposition of two processes coupled with each other. One of these processes represents the diffusion of the mobile compound into and within the solid substrate and the other describes the immobilization/mobilization events of this compound at the active adsorption sites of the substrate. Applying this model to the sorption of the enantiomers 3(R) and 3(S) by the chiral substrate 2( R ) and 2(S), respectively, the following partial processes can be visualized.(i) Diffusion of the mobile enantiomers to the chiral phosphine oxide centres located within the polymer substrate. (ii) Immobilization of the enantiomers by formation of the diastereomeric adsorption complexes at the phosphine oxide centres. The equilibrium constants K,, and K, of the two possible diastereomeric complexes formed are the quantities relevant for the enantio-differentiating specificity of the chiral substrate and we may define a separation (or chirospecificity) factor* ab = K,/K,. (iii) Dissociation of the diastereomeric adsorption complexes remobilizing the enantiomers. (iv) Diffusion of the remobilized enantiomers away from the phosphine oxide adsorption sites into the surrounding bulk solution.For the mathematical description of the sorption process the equation of continuity is used: where [i] is the concentration of the enantiomer i, t is the time, V is the nabla operator, Ni is the flux vector of component i and Fi is the rate of immobilization and of mobilization, respectively, of the enantiomer i. Choosing the swollen polymer bead as the diffusion-immobilization system and assuming constant total molar concen- tration, zero molar average velocity? and spherical polymer beads, eqn ( I ) becomes1 where D, is the diffusion coefficicnt of the enantiomer i and r is the polar coordinate. The molar rate Fi in eqn (2) can be replaced by the expression F~ = k,[i] ([ads], - [ads, iltOt)/P - ki [ads, i]/P (3) where ki.and k-i are the rate constants of the immobilization (adsorption) and mobilization (desorption) processes, respectively, [ads], is the concentration of total available adsorption sites(po1ymer-bound phosphineoxide), [ads, i] is theconcentration * For a definition, see for example ref. (10). The index b denotes that the a values are obtained in the batch procedure. 7 The molar average velocity is understood to be the velocity caused by the bulk motion of the fluid within the polymer bead. If it is assumed that the bulk motion of the main component of the fluid within the bead, namely the solvent, is zero, the molar average velocity can also be assumed to be zero. 1 For a more detailed derivation of eqn (2) see ref. (16).P. KREBSER, H-R. MARTI AND P.RYS 2277 of the diastereomeric adsorption complex with the enantiomer i, [ads, iItot is the total concentration of both diastereomeric adsorption complexes and P is the porosity of the swollen polymer bead (imbibed pore liquid per weight of dry bead). Eqn (3) represents the kinetics of the isotherm immobilization/mobilization process of the enantiomers at the chiral adsorption sites, assuming that (i) both enantiomers compete for the same sites, (ii) for a given enantiomer the velocity constants are the same for every adsorption site, (iii) only one enantiomer per site is immobilized and (iv) there are no intermolecular interactions between enantiomers already immobilized . In the present study it is assumed that in the equilibrium state the concentrations [i] of the mobile enantiomers in the bulk solution and in the solution imbibed in the polymer bead are equal.The boundary conditions result from the symmetry of the spherical bead: (F) = o r - 0 (4) and from a material balance of the mobile enantiomers over the well stirred surrounding bulk solution (Y 3 R) and the polymer bead: Here VB is the volume of the bulk solution excluding the polymer-bead volume, V,, and R i s the mean radius of the polymer bead. Assuming the same diffusion coefficient D for both enantiomers, eqn (2) and (3) and the boundary conditions (4) and (5) can be normalized by introducing the following dimensionless expressions : X = r / R ; T = Dt/R2; Y i = [ i ] / [ i ] , ; R, = [ads, i ] / P [ i ] , Ei = [ads]o/P[i]o; M = [Rlo/[Slo; a = Vi/ Vp Ai = Rzki[ilO/D; Bi = R2kPi/D.This normalization leads to the generalized differential eqn (6) and (7) and the appropriate initial and boundary conditions (8)-( 10): initial conditions (i = R, S): T = 0 I o<x<1: Y,i=O; ni=o X = l : Y i = l ; ni=o boundary conditions (i = R, S): T > 0 The model presented here shows that for given initial and boundary conditions the sorption behaviour of a racemic mixture at a chiral solid substrate is determined2278 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS unambiguously by the seven quantities ER, M , A,, A,, B,, B, and a. Furthermore, the sum A,+ Bi determines whether the sorption of the enantiomer i is diffusion- controlled ( A i + Bi % 1) or whether it is controlled by the immobilization/mobilization process (Ai + Bi Q 1).This follows from the fact that Ai + Bi is proportional to the ratio of the relaxation times* of the diffusion and immobilization/mobilization processes. This proportionality is derived in eqn (1 1 )-( 17): 1 B . =p- ZD, i r m , i TD i Tim/m, i A i + Bi = ( i = R, S ) . Here zD, is the relaxation time of the diffusion process of the enantiomer i, tim, and rm, are the relaxation times of the immobilization and mobilization events, respectively, of enantiomer i, 7im/m, is the relaxation time of the immobilization/mobilization equilibration process and p is a factor whose value is determined by the geometrical form of the solid substrate. For spherical geometry /3 = 19.6, for infinitely long cylinders /3 = 9.4 and for linear-slab geometry D = 12.7.RESULTS SORPTION KINETICS The rate of sorption of the individual enantiomers was investigated by measuring the decrease in the concentration of mandelic acid in the bulk solution with time. From this the concentration of the sorbed enantiomer i, [sor, i], was evaluated by material balance [sor, i] in the sum of the concentrations of the adsorbed enantiomer i and the mobile enantiomer is the solution contained in the polymer bead. The experimental data are shown in fig. 1 . The sorption behaviour of racemic mandelic acid is characterized by the competition of both enantiomers 3(R) and 3(S) for the same chiral adsorption sites of the substrate 2(R) or 2(S). It was studied by measuring the time dependence of the quasi-optical purity, p , given by P (% 1 = 100Aall ae I (18) where Aa represents the optical rotation of the bulk solution and 1 a, I is the absolute value for optical rotation of a pure-enantiomer solution.Its concentration is half the concentration of the initial racemic mandelic acid solution. The results are presented in fig. 2. * The relaxation time is the time that elapses until a significant quantity in the process has achieved a (1 - l/e)th part (63.2%) of the change from the initial value (I = 0) to the final value ( I -+ a).P. KREBSER, H-R. MART1 AND P. RYS 2279 0 100 200 t/3600 s Fig. 1. Sorption kinetics of the individual R( -)- and S( +)-enantiomers of mandelic acid [3(R) and 3(S)] on pre-swollen (R)p- and (S),-poly(styrylmethylpheny1phosphine oxide) [2(R) and 2(S)] in toluene+THF = 85/15 (~01%) and at 25.0 "C.0, 0 , 3 ( R ) ; A, A, 3 ( S ) ; 0, A, 2(R); 0, A, 2(S). + 20 -2c Fig. 2. Competitive sorption kinetics of the enantiomers of racemic mandelic acid (3) on pre-swollen (R)p- and (S),-poly(styrylmethylpheny1phosphine oxide) [2(R) and 2(S)] in toluene+THF = 85/15 (~01%) and at 25.0 "C. 0, 0, [3], = 3.93 mmol dmP3; A, A, [3], = 7.84 mmol dm-3; m, 0, [3], = 11.76 mmol dm-3; 0, A, U, 2(R); 0, A, 0, 2(S).2280 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS 0 10 20 [il.Jmmol dm-3 Fig. 3. Adsorption isotherms of the individual R( -)- and S( +)-enantiomen of mandelic acid [3(R) and 3(S)] on pre-swollen and (S),-poly(styrylmethylpheny1phosphine oxide) [2(R) and 2(S)] in toluene+THF = 85/15 (~01%) and at 25.0 "C. 0, 0, 3(R); A, A, 3 ( S ) ; 0, A, 2(W; 0, A, 2 w .ADSORPTION ISOTHERMS The adsorption isotherms were determined by measuring [ads, i], as a function of [i],. [ads, i ] , is defined to be the difference in the concentrations of the total sorbed enantiomer i and the mobile enantiomer in the solution imbibed in the polymer bead : [ads, i ] , = [sor, i ] , -P[i],. P was calculated to be 4.734 dm3 kg-l using the densities of the dry and swollen resin. The experimental results are presented in fig. 3 and 4. The mathematical description of the adsorption isotherm for an individual enantiomer can be deduced from eqn (3). As Fi = 0 and [ads, i],,, = [ads, i],, the isotherm Ki[ilcc 1 + K J i ] , [ads, i ] , = follows, where the index co implies the equilibrium concentration and Ki( = k,/k+) is the adsorption constant.An evaluation of the experimental results using a linearized form of eqn (20) (a Scatchard plot; see fig. 5 ) leads to the sorption parameters [ads], and Ki listed in table 1. From the Ki values the separation factors ab = 1.17f0.06 for the substrate 2(R) and a,, = 1.16f0.06 for the substrate 2(S) were calculated. DISCUSSION AND CONCLUSIONS An examination of the experimental data shown in fig. 1-5 is instructive. In the sorption experiments on the individual enantiomers one observes that the enantiomer with the lower affinity to the substrate, i.e. with the smaller Ki value, sorbs faster than that with the higher affinity. This result also explains the surprising competitiveP. KREBSER, H-R. MARTI AND P. RYS 228 1 1 2 3 4 5 I 1 I I I 40 30 20 3 'u 10 24 - E" E O mi 40 \ Q -0 $ Y 30 20 10 0 1 2 3 4 5 [i],/mmol dm-3 Fig.4. Competitive adsorption isotherms of the enantiomers of racemic mandelic acid (3) on pre-swollen (R)p- and (S),-poly(styrylmethy1phenylphosphine oxide) [2(R) and 2(9] in toluene+THF = 85/15 (~01%) and at 25.0 "C. 0, 0 , 3 ( R ) ; A, A, 3(S); 0, A, 2(R); 0, A, 2(S). Table 1. Adsorption parameters of the enantiomers 3(R) and 3(S) on 2(R) and 2(S) in toluene+THF = 85/15 (~01%) at 25.0 "C 2(R) 214+6 197k4 183k6 200+4 2(S) 188k6 243+4 219f6 241+4 sorption kinetics: in a sorption system of two enantiomers with the same diffusivities, the same initial concentrations in the bulk solution and with both enantiomers competing for the same adsorption sites, the enantiomer with the smaller Ki value will proceed faster into the substrate.Therefore, this component will have available all the adsorption sites present in the part of the substrate that has not yet been reached by the slower enantiomer with the larger Ki. Gradually, the slowest enantiomer will 75 F A R I2282 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS I 0 100 200 [ads, z’J,/mmol kg-’ Fig. 5. Linearized adsorption isotherms for the evaluation of the adsorption parameters of the individual enantiomers 3(R) and 3 ( S ) on 2(R) and 2(S). 0, 0 , 3 ( R ) ; A, A, 3 ( S ) ; 0, A, 2(R); 0, 2(S). 0 10.0 Fig. 6. Sorption kinetics of toluene from toluene+THF = 85/15 (v01.x) onto (ap- poly(styrylmethylpheny1phosphine oxide) [2(S)] pre-swollen in deuterated toluene+ THF = 85/15 (~01%) at 25.0 “C.0, Experimental results; 0, calculated behaviour: [C,H,], = 8.97 x mol dmW3 and D = 0.58 x cm2 s-l; 0, calculated behaviour: [C,H,], = 8.97 x lop3 mol dm-3 and D = 0.58 x 10-lo cm2 s-l.P. KREBSER, H-R. MARTI AND P. RYS 2283 occupy some sites previously occupied by the faster enantiomer during the time taken to reach equilibrium. This process of uptake therefore follows a course whereby the concentration of the sorbed enantiomer with the smaller Ki reaches a maximum before attaining its equilibrium value. Such behaviour can lead to a change in the sign of rotation of the bulk solution, as observed experimentally (fig. 2) and was simulated by numerical calculations for substrate 2(R) (see last graph in fig. 7). For an exact simulation using the present diffusion-immobilization model the quantities ER, a, M , A,, A,, B, and B , must be known.ER, a and A4 can be calculated from the initial conditions and from the experimentally determined value of [ads], and P. Although the mean radius of the pre-swollen polymer bead is known (a = 4.65 x cm) and the diffusivity D can be evaluated, exact values for the quantities A,, A,, BR and B , could not be obtained. This is due to the fact that under the experimental conditions used, the values for the rate constants kR, k-R, ks and k - , could not be measured directly. Therefore the simulation was carried out with a range of values satisfying the two conditions AR/(BRIRIO) = KR = k R / k - R and In order to evaluate D , model calculations were carried out and fitted to the experimental sorption curve for the enantiomer 3(S) on the substrate 2(R).For diffusion-controlled sorption conditions ( A , + B, % 1) the best fit was obtained for D = lop9 cm2 s-l. This low value for the diffusivity has been cross-checked by measuring the sorption rate of toluene on the same polymer beads pre-swollen in deuterated toluene +THF mixtures (85/15 vol% ), using the lH-n.m.r. technique. The experimental results (fig. 6) confirm the order of magnitude of D. Furthermore, they corroborate the surprising observation that, despite the fact that the swollen beads consist of ca. 5 times more imbibed solvent than polymer, the diffusivity in the bead is ca. 3000 times smaller than that in free solvents. At present, no provable explanation can be given for this finding.Model calculations using the parameters listed in table 2 led to a variety of enantio-differentiating sorption behaviours represented by the curves (a)-(e) in fig. 7. The change in the optical rotation p which corresponds to these various sorption behaviours is shown by the curves in the last graph of fig. 7. These plots allow a comparison with the experimental results given in fig. 2. From these calculations the following general observations can be made. (i) The sorption behaviour does not change on varying P, R, D, ki, k+ [zlo or [ads], as long as the quantities Ei, M , a, Ai and Bi, as well as the initial and boundary conditions, remain the same. This becomes evident if one compares runs 1-6. They all show the same sorption behaviour [fig. 7(a)] with the adsorption of the S-isomer passing through a maximum before reaching the equilibrium state.However, during sorption the optical rotation of the bulk solution neither changes sign nor passes through a maximum. 1, the variation of ki and kPi (runs 7 and 8) does not result in a detectable change in the sorption behaviour [fig. (iii) If the sorption of one or both isomers is not entirely controlled by diffusion, i.e. if the sum A,+Bi for one or both isomers has a value < ca. 5, the kinetically determined enantio-differentiating sorption behaviour can become more pronounced. For instance, if the sorption of the S-isomer remains diffusion-controlled ( A , + B, $ 1) but the sorption of the R-isomer becomes increasingly controlled by the kinetics of the immobilization process (compare the decreasing values of A , + B, in the runs 1, 9 and 10) the maximum in the sorption of the S-isomer becomes larger.As a AsI(Bs[SIo) = K , = k,/k-,. (ii) Note that as long as A,+ BR % 1 and A,+ B, 7 (41. 75-22284 4 Qi 2 0 4 Ri 2 0 4 Qi 2 1 5 10 15 20 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS 5 10 15 20 5 10 15 20 0 . . - 0 5 10 15 20 0 5 T Fig. 7. Model calculations : competitive sorption kinetics of the enantiomers of racemic mandelic acid (3) on pre-swollen (I?),-poly(styrylmethylpheny1phosphine oxide) [2(R)]. The parameters for the calculations are listed in table 2. (-) R( -)-enantiomer; (---) S( +)-enantiomer. (a) Runs 1-8; (b) run 9; (c) run 10; (d) run 1 1 and (e) run 12.Table 2. Model calculations: competitive sorption kinetics of the enantiomers of racemic mandelic acid (3) on pre-swollen K , = 214 dm" mol-I; K , = 183 dm" mo1-I run no.1 2 3 4 5 6 7 8 9 10 I 1 12 (R),-poly(styrylmethylpheny1phosphine oxide) [2(R)] for [R],, = IS], = 5.0 x mol dm-:l; E = 8.4; 0.5 4.734 1 .o 200.0 1 .o 4673.0 1 .o 5464.0 125.0 116.8 241.8 125.0 136.6 26 1.6 7 (4 1.5 4.734 1 .o 200.0 0.1 1 1 5 192.0 0.1 1 1 6072.0 125.0 116.8 241.8 125.0 136.6 261.6 7 (a) 1 .o 0.1 4.734 4.734 1 .o 1 .o 200.0 200.0 0.25 25.0 0.25 25.0 125.0 125.0 116.8 116.8 241.8 241.8 125.0 125.0 136.6 136.6 261.6 261.6 1168.0 116800 1366.0 136600 7 7 (4 10.0 100.0 200.0 1168.0 1366.0 125.0 116.8 241.8 125.0 136.6 261.6 4.734 0.25 0.25 7 ( 4 0.5 2.367 1 .o 100.0 1 .o 4673.0 1 .o 5464.0 125.0 116.8 241.8 125.0 136.6 261.6 7 ( a ) 0.5 4.734 1 .o 200.0 0.1 467.3 1 .o 5464.0 12.5 11.68 24.18 125.0 136.6 26 1.6 7 (4 0.5 4.734 1 .o 200.0 1 .o 4673.0 0.1 546.4 125.0 116.8 241.8 12.5 13.66 26.16 7 ( 4 0.5 4.734 1 .o 200.0 0.0 1 46.73 1 .o 5464.0 1.25 1.168 2.4 18 125.0 136.6 261.6 7 ( h ) 0.5 4.734 1 .o 200.0 0.000 1 0.4673 1 .o 5464.0 0.0 125 0.01 17 0.0242 125.0 136.6 261.6 7(c) 0.5 4.734 1 .o 200.0 1 .o 4673.0 0.000 1 0.5464 125.0 116.8 241.8 0.0125 0.0137 0.0262 7 (4 0.5 4.734 I .o 200.0 0.0001 0.4673 0.000 1 0.5464 0.0 125 0.01 17 0.0242 0.0125 0.0137 0.0262 7(e)2286 SORPTION-DIFFUSION IN HETEROGENEOUS SYSTEMS consequence, the sorption curves cross each other and the sign of the optical rotation in the bulk solution changes during the course of the sorption [fig. 7, curves (b) and (iv) For the cases where A,+ BR $- 1 and the sorption kinetics of the S-isomer becomes determined by its immobilization process (run 11), the sorption of the R-isomer passes through a maximum [fig.7(d)]. The optical rotation in the bulk solution also passes through a maximum but does not change sign. Model calculations for other initial concentrations [i] and various E values show qualitatively similar sorption behaviour. Accordingly, the general conclusions are the same. A comparison of model calculations (fig. 7) and experimental results (fig. 2) shows that the sorption behaviour of chiral solid materials can be rationalized by the simple diffusion-immobilization model presented here. Even though this model makes n o specific assumptions concerning either the molecular mechanism of chiral recognition or the diffusion process on the molecular scale, it can simulate the experimental observation that the sign of the optical rotation in the bulk solution can change during the course of racemate sorption.This unusual behaviour is the result of both kinetic and thermodynamic effects and is determined by the interplay between diffusion and adsorption processes. The extent of this interplay is determined by the quantity A, + B f . We recognize that our model has some shortcomings, since we were unable to measure the individual ki and k-i values. However, an evaluation of these rate constants should be possible by studying the sorption kinetics of the individual enantiomers under immobilization-controlled conditions, i.e. with much smaller polymer beads: unfortunately such beads were not available to us.In spite of this, the value of the presented model lies in the fact that all of the quantities Ei, M , a, A,, A,, B , and B,, which unambiguously determine the competitive sorption system, can in principle be evaluated by independent measurements and have a well defined physical meaning.* This gives the experimentalist a better understanding of how to change these quantities, in order to obtain a better chromatographic separation of a given racemate. (41 * The financial support of this work by the Swiss National Science Foundation * In this sense a physical model differs from pure curve-fitting, in which the parameters are not deduced (projects no. 2.191-0.78 and 2.226-0.81) is gratefully acknowledged. from any physical or chemical laws. Part 8: R. J. Ott and P. Rys, J . Chem. SOC., Faraday Trans. 1, 1974, 70, 995; Parts 5-7: ref. (12).. * H-R. Marti, Ph.D. Thesis (ETH, Zurich, 1978, no. 6218); P. Krebser, Ph.D. Thesis (ETH, Zurich, 1982, no. 6937). G. Blaschke, Angew. Chem., Int. Ed. Engl., 1980, 19, 13. W. Lindner, Chimia, 1981, 35, 294. R. Audebert, J . Liquid Chromatogr., 1979, 2, 1063. V. A. Davankov, Adv. Chromatogr., 1980, 18, 139. 359. I. S . Krull, Ad&. Chromatogr., 1978, 16, 175. C. H. Lochmuller and R. W. Souter, J . Chromafogr., 1975, 113. 283. 'I V. A. Davankov, S. V. Rogozhin, A. V. Semechkin and T. P. Sachkova, J . Chromatogr., 1973, 82, lo G. Wulff, W. Vesper, R. Grobe-Einsler and A. Sarhan, Makrornol. Chem., 1977, 178, 2799. l 1 W. H. Pirkle, J. M. Finn, B. C . Hamper, J. L. Schreiner and J. R. Pribish, ACSSymp. Ser., 1982,185, l 2 R. J. Ott and P. Rys, J. Chem. SOC., Faraday Trans. 1, 1973, 69, 1694, 1705; 1974, 70, 985. l 3 J. Crank, Trans. Faraday SOC., 1957, 53, 1083. l 4 P. V. Danckwerts, Trans. Faraday SOC., 1950, 46, 300; 1951, 47, 1014. 245. P. B. Weisz, Trans. Faraday SOC., 1967, 63, 1801; P. B. Weisz and J. S . Hicks, Trans. Faraday SOC., 1967, 63, 1807. l6 R. B. Bird, W. E. Stewart and E. N. Lightfoot, Transport Phenomena (J. Wiley, London, 1960). (PAPER 4/ 1087)

ISSN:0300-9599    

DOI:10.1039/F19858102273

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1985, 81, 2287-2291 Effects of Pressure on Reversed Micellar Systems Rates of the Keto-Enol Transformation of Pentane-2’4-dione BY KATSUHIRO TAMURA* AND MINORU SUMINAKA Department of Chemical Engineering, Faculty of Engineering, The University of Tokushima, Minamijosanjima-cho, Tokushima 770, Japan Received 29th August, 1984 The rates of the keto-enol tautomerism of pentane-2,4-dione (acetylacetone) have been measured spectrophotometrically in reversed micellar systems of dodecylammonium propionate (DAP) and dodecylammonium butyrate (DAB) at pressures up to 98.1 MPa. The rates were enhanced by pressure and surfactants, especially at concentrations above the ‘operational’ critical micelle concentration. The activation volumes of the reaction (A v’) in various systems have been determined and used to estimate the polarity of the microscopic reaction environment.On the basis of the kinetic and thermodynamic data obtained, the formation of reversed micelles as a reaction field is discussed. Micellar structures in non-polar solvents are generally the opposite of those in water; namely, the polar head groups form the interior, while the hydrophobic hydrocarbon parts of the surfactants are in contact with the non-polar solvent.’ There is increasing evidence for a sequential indefinite type of self-association; i.e. monomer + dimer trimer f - - - + n-mer in contrast to the monomer =t micelle equilibrium generally observed in aqueous micellar systems.2 From the viewpoint of the reaction field, the micelles in non-polar solvents (so-called reversed micelles) are apparently different from those in aqueous solvents : they form polydisperse systems, their critical micelle concentration (c.m.c.) is not so clear as that of aqueous micelles and their aggregation number is generally small.Consequently, it has sometimes been pointed out that definition of a reversed micelle is difficult and that substrates may form only simple aggregates with surfactants in non-polar solvent^.^ Kon-no et ~ 1 . ~ have studied the effect of surfactants (alkylammonium carboxylates) on the keto-enol transformation of pentane-2’4-dione (acetylacetone) and ethyl acetoacetate in cyclohexane solutions, and concluded that the transformation was induced by proton transfer from the ammonium ion to the substrates. We have carried out spectrophotometric studies on the ‘operational’ c . ~ .c . ~ of dodecylammonium propionate (DAP) in carbon tetrachloride under high pressure and on the effect of pressure on the rate of the keto-enol tautomerism in reversed micelles and singly dispersed systems in order to obtain information on reversed micelles, especially when they are operating as a reaction field. EXPERIMENTAL Dodecylammonium propionate (DAP) was prepared by mixing equimolar quantities of purified dodecylamine and propionic acid in benzene at room temperature. It was recrystallized from benzene three times, m.p. 55-56 “C. Dodecylammonium butyrate (DAB) was prepared 22872288 K. TAMURA AND M. SUMINAKA I 1 I 1 I 10 20 [ DAP]/mmol kg-' Fig. 1. Plots of the absorbance of dodecylammonium propionate as a function of its concentration in carbon tetrachloride at 30 "C and various pressures: 0 , O .l ; A, 49.0; 0,98.1 and 0, 147.1 MPa. Insert: Plot of [c.m.c.] against pressure. by a similar method with n-hexane as solvent, m.p. 39-40 "C. The surfactants were dried under reduced pressure before use. Pentane-2,Cdione' used as a substrate, was distilled under reduced pressure after drying. The ' operational ' c.m.c. of DAP under high pressure was determined spectrophotometrically in carbon tetrachloride at 30 "C using a high-pressure vessel with optical windows and a sliding ceL6 The high-pressure vessel was designed to fit a Hitachi 100-60 spectrophotometer so that absorption spectra could be measured directly as the pressure was applied.The absorbance at 265 nm was first measured at 147.1 MPa, the maximum pressure, and then at successively lower pressures. The rates of enol to keto transformation were measured spectrophotometrically at 30 "C by determining the absorbance of the keto form at 307 nm in DAP + CCl, solutions and at 3 13 nm in DAB + CCl, solutions. The absorption peaks did not change with pressures up to 98.1 MPa, within the limits of experimental error. Good first-order plots were obtained in all cases and pseudo-first-order rate constants (k,) were determined by the Guggenheim method.' The reactions were initiated by mixing equal volumes of 1 .O x lop4 mol dmp3 substrate solutions and surfactant solutions of appropriate concentrations. RESULTS AND DISCUSSION PRESSURE DEPENDENCE OF THE C.M.C. OF DAP Fig.1 shows the relation between the absorbance of DAP + CC1, systems and the concentration of DAP. The values of the 'operational' c.m.c. are given by the intercept of the two resulting straight lines. The c.m.c. determined by this method are plotted against pressure in the insert. (This sort of U.V. absorption method has already been used by Kitahara to determine the c.m.c. of alkylammonium benzoate in cyclohexane.*)EFFECTS OF PRESSURE ON MICELLAR SYSTEMS 2289 The c.m.c. at atmospheric pressure was 13 mmol kg-l, corresponding to a value of 20 mmol dm-3, which agrees very closely with the value of 21-25 mmol dmP3 deter- mined by lH n.m.r. ~pectroscopy.~ The c.m.c. decreased with increasing pressure. This behaviour is different from that of aqueous micelles, where the c.m.c.increases with pressure, and many cationic and anionic surfactants have maxima at ca. 100 MPa.1°-12 The effects of pressure on aqueous micelles have been systematically studied by Tanaka et al.,13 who evaluated the volume change involved in the formation of micelles from singly dispersed surfactants, Arm (= rm - E), and the pressure dependence of the c.m.c., (2 In c.m.c./W),. These have positive values at pressures < 100 MPa, i.e. AVm > 0, (i3 In c.m.c./c?P), > 0. On the other hand, for reversed micelles of DAP in carbon tetrachloride the signs become opposite, AVm < 0 (by a few cm3 mol-l), (2 In c.m.c./W), < 0. The difference in behaviour between aqueous and reversed micelles may be attributed to different mechanisms of micellization,14 the specific nature of water and the unique surfactant-water intera~ti0n.l~ EFFECTS OF PRESSURE ON REACTION RATES Fig.2 shows the rate constants (k,) as functions of the concentration of DAP and DAB in carbon tetrachloride. For the DAP+CCl, systems, the rate at 0.1 MPa is greatly enhanced in the region of the c.m.c. (1 3 mmol kg-l). On the other hand, the rate at high pressures, e.g. 98.1 MPa, shows a different profile. One reason why the profiles at 0.1 and 98.1 MPa are different seems to be that the c.m.c. of DAP decreases with increasing pressure. Another feature of this system is that there is a difference in the effect of pressure on k , with the concentration of DAP. These kinetic data suggest the existence of reversed micelles in solutions of higher concentration. How- ever, for DAB+CCl, systems the effects of pressure are relatively small.This result and the fact that the c.m.c. of the DAB systems could not be determined indicate that the formation of micelles in this system is difficult compared with their formation in the DAP + CCl, system. The effects of pressure on the reaction rates in reversed micelle systems are very complex, since many factors control the rates, e.g. micellization and penetration of substrates into micelles and the effects of this or the concentration of the substrates. Marked effects on the reaction rates were observed over the c.m.c. range, where it is possible that the reaction field changes from a singly dispersed system to a micellar system with increasing pressure, suggesting disorder or lability in the system.Keto-enol equilibria are affected by the nature of the solvent, and protonation of the enol form is much greater in polar solvents such as water or alcohols. This may be attributed to stabilization of the keto form in polar solvents through local association. Consequently, in addition to catalysis by the surfactant, penetration of the enol form from the non-polar solvent into the polar interior of reversed micelles on mixing the substrate and micellar solutions becomes one cause of the enhancement of the rate of the enol to keto transformation. The enol to keto reaction of pentane-2,4-dione catalysed by DAP or DAB is accompanied by changes in the electronic charges. Accordingly, electrostriction should be taken into account in considering the activation volumes of these reactions.The volume change, AK, of an ion caused by electrostriction is given by the Drude-Nernst equation : l5 where N is Avogadro's number, c is the dielectric constant of solvent, q is the charge on the ion and r is the radius. There is a general relationship between the polarity2290 K. TAMURA AND M. SUMINAKA 4 3 2 1 - I U r/) SO -Y 3 2 1 0 0 10 20 [ surfactant]/mmol kg-I Fig. 2. Plots of the rate constants of the enol to keto transformation against the concentration of dodecylammonium propionate (DAP) and dodecylammonium butyrate (DAB) at 30 "C and various pressures: 0, 0.1 ; A, 29.4; 0, 58.8 and a, 98.1 MPa. Table 1. Activation volumes (A fl)" for the enol to keto transformation of pentane-2,4-dione in DAP+CCl, and DAB+CCl, systems at 0.1 MPa and 30 "C DAP + CCl, DAB + CCl, [DAP]/mmol kg-l A fl/cm3 mol-l [DAB]/mmol kg-l A fl/cm3 mol-l 2.6 5.1 6.4 9.6 12.2 14.I 16.7 19.3 - 19 - 20 -31 - 24 -8.5 - 4.7 - 3.6 - 2.9 1.9 3.2 6.4 9.6 12.9 16.1 19.3 - 15 - 20 - 26 - 17 - 7.6 - 6.6 -4.5 a (a In k,/8P), = - A f l / R T .EFFECTS OF PRESSURE ON MICELLAR SYSTEMS 229 1 (P and E , 17) of typical organic solvents and values of 0,16 i.e. the values of 0 decrease with increasing E and E,. Thus, the values of 0 are small for polar solvents and hence the values of A & for polar solvents are less negative than those for non-polar solvents. We have used this relationship and our experimental values for activation volumes, An (see table l), to examine the polarity of the microscopic environment of the substrate in the concentration ranges above and below the c.m.c.of DAP. The A f l values for the systems above the c.m.c. have smaller negative values than those below the c.m.c. (The An values in the concentration range 10-13 mmol kg-l, where the reaction field changes from a singly dispersed system to a micellar system with increasing pressure, cannot be used.) This suggests that the polarity of the microscopic environment of the substrates above the c.m.c. is higher than that below the c.m.c. Thus, reversed-micellar 'catalysis ' has two functions : (1) catalysis by surfactants themselves in reversed micelles that have functional surfactants and (2) formation of a polar reaction field in the interior core of the micelles.H. F. Eicke and H. Christen, J. Colloid Interface Sci., 1974, 46, 417. G. Markovits, 0. Levy and A. S. Kertes, J . Colloid Interface Sci., 1974, 47, 424. J. Sunamoto and D. Horiguchi, Nippon Kagaku Kaishi, 1980, 475. K. Kon-no, K. Miyazawa and A. Kitahara, Bull. Chem. SOC. Jpn, 1975,48, 2955. J. H. Fendler, Ace. Chem. Res., 1976, 9, 153. W. J. le Noble and R. Schlott, Rev. Sci. Instrum., 1976, 47, 770. A. Kitahara, Bull. Chem. SOC. Jpn, 1957, 30, 586. J. H. Fendler, E. J. Fendler, R. T. Medary and 0. A. El Seoud, J. Chem. Soc., Faraday Trans. I , 1973, 69, 280. S. D. Hamann, J . Phys. Chem., 1962, 66, 1359. ' A. A. Frost and R. G. Pearson, Kinetics and Mechanism (Wiley, New York, 1961), p. 49. I I R. F. Tuddenham and A. E. Alexander, J. Phys. Chem., 1962, 66, 1839. I 2 J. Osugi, M. Sato, and N . Ifuku, Nippon Kagaku Zasshi, 1966, 87, 329. l 3 M. Tanaka, S. Kaneshina, K. Shin-no, T. Okajima and T. Tomida, J. Colloid Inferface Sci., 1973, 46, 132; S. Kaneshina, M. Tanaka, T. Tomida and R. Matuura, J. Colloid Interface Sci., 1974, 48, 450. P. Drude and W. Nernst, Z . Phys. Chem., 1894, 15, 79. l4 J. H. Fendler, Membrane Mimetic Chemistry (Wiley, New York, 1982), p. 48. I6 N. S. Isaacs, Liquid Phase High Pressure Chemistry (Wiley, Chichester, 1981), p. 99. " C. Reichardt, Solvent Effects in Organic Chemistry (Verlag Chemie, Weinheim, 1979), p. 242. (PAPER 4/ 1489)

ISSN:0300-9599    

DOI:10.1039/F19858102287

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1985, 81, 2293-2305 An In Situ Mossbauer Investigation of the Influence of Metal-Support and Metal-Metal Interactions on the Activity and Selectivity of Iron-Ruthenium Catalysts BY FRANK J. BERRY* Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham B15 2TT AND LIN LIWU, WANG CHENGYU, TANG RENYUAN, ZHANG Su AND LIANG DONGBAI Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Peoples Republic of China Receitled 10th September, 1984 The hydrogen reduction of a series of alumina- and silica-supported iron-ruthenium catalysts has been studied in situ by iron-57 Mossbauer spectroscopy. The data, complemented by results from temperature-programmed reduction experiments, show that ruthenium enhances the reducibility of iron and that the metal-support interaction in the silica-supported samples is weaker than in those supported on alumina.The performance of the iron-ruthenium catalysts for carbon monoxide hydrogenation has been evaluated in a fixed-bed microreactor, and the activity of the alumina-supported catalysts was found to be higher than those supported on silica. However, alumina-supported catalysts in which the iron content exceeded ca. 27 atom % showed a gradual decrease in activity. At ca. 65 atom % this began to approach the activities of the silica-supported materials, which were generally less dependent on the iron content of the bimetallic phase. Catalysts with low metal loadings were more active than their counterparts with high metal loadings.These variations in catalytic activity have been interpreted in terms of the stronger interactions between the metallic phase and alumina, as compared with silica, supports and the smaller metallic particle sizes which are obtained on alumina. Metal-support interactions and particle dispersions have also been found to influence selectivity. Hence ruthenium, which interacts strongly with alumina, showed high activity and tended to form higher-molecular-weight hydrocarbons, whilst silica-supported ruthenium was less active and favoured the formation of lower hydrocarbons. The addition of iron to alumina-supported ruthenium shifted the product distribution towards lower hydrocarbons but had little influence on the selectivity of catalysts supported on silica. The variations in metal-support interaction which result from changesin the iron concentration and which are reflected in the catalytic properties of the solids have been correlated with the Mossbauer data, which showed the capacity of iron to donate electrons to ruthenium.Although the activity of many iron-containing Fischer-Tropsch catalysts has been known for many years, their lack of selectivity has precluded widespread commercial application. However, renewed interest in the production of coal-derived fuels and chemicals as an alternative route to those currently obtained from oil has resulted in attempts to identify new Fischer-Tropsch catalysts with high activity and selectivity. In this respect it is interesting to note that preliminary evaluations of both unsupported' and silica-supported2 iron-ruthenium catalysts at atmospheric pressure have suggested that the product distribution may be sensitive to metal-metal interactions which reflect 22932294 MOSSBAUER STUDY OF Fe-Ru CATALYSTS the concentration of iron in the metallic phase.However, the commercial Fischer- Tropsch process usually operates under medium pressures and the studies reported to date appear not to have given significant attention to the influence of the support on the catalytic performance of the bimetallic phase. The work reported here describes the first part of a comprehensive examination of both the solid-state and catalytic properties of supported iron-ruthenium catalysts. This report describes some in situ investigations of metal-support and metal-metal interactions in alumina- and silica-supported iron-ruthenium catalysts by 57Fe Mossbauer spectroscopy complemented by temperature-programmed reduction (t.p.r.).We also report on the performance of these catalysts for carbon monoxide hydrogenation under medium pressure and the influence of metal-support and metal-metal interactions on the catalytic properties. Only a few preliminary investi- gations of iron-ruthenium catalysts by Mossbauer spectroscopy and t.p.r. have been reported,2-s and the current status of the limited understanding of this system has been described in a recent re vie^.^ EXPERIMENTAL The silica- and alumina-supported iron-ruthenium catalysts were prepared by the impreg- nation of alumina (surface area 209 m2 g-') or silica (surface area 342 m2 g-') with aqueous solutions of ruthenium(iI1) chloride and iron(1u) nitrate.The samples were dried at 120 "C (16 h) and subsequently calcined at 480 "C (4 h) in air. Two series of bimetallic catalysts were prepared on each support for catalytic evaluation. The series containing 5 wt % ruthenium with varied iron contents have been designated as the high-metal-loading series (HML), whilst the low-metal loading series (LML) contained 1 wt % ruthenium with similar iron to ruthenium ratios. Carbon monoxide hydrogenation reactions were carried out in a fixed-bed microreactor of 6 mm internal diameter and 220 mm length using 5-10 cm3 of the catalyst of 20-40 standard mesh. All catalysts were prereduced in situ in flowing pure hydrogen (240 cm3 g-catalyst-' h-l) at 9 kg cmP2 pressure for 4 h at 235 "C.The reaction gas mixture was preblended from carbon monoxide (99.8% purity) and hydrogen (99.9% purity) in a high-pressure storage tank and further purified by passage through a deoxygenator, a molecular sieve column and a carbon column. Reactions were performed at 235 "C and at 26 kg cmP2 pressure using a hydrogen to carbon monoxide ratio of 2: 1 and GHSV* of ca. 250 h-l. Reaction products were initially separated by a high-pressure separator into gas-phase (hydrogen, carbon monoxide, carbon dioxide and C,-C, hydrocarbons) and liquid-phase fractions. The gas-phase fraction was analysed by an on-line Poropak QS packed gas-chromatograph column and the liquid-phase fraction was separated into a hydrocarbon phase and an aqueous phase for off-line analysis by a packed SE-30 column and gas chromatography-mass spectroscopy respectively. Only a trace of methanol was identified in the aqueous phase.Iron-57 Mossbauer spectra were recorded from samples containing ca. 10 mg iron cm-2 following treatment in a silica in situ Mossbauer cell in flowing hydrogen (4 dm3 h-l). The spectra were recorded with a Cryophysics model MS- 121 microprocessor-controlled Mossbauer spectrometer at 298 K (unless otherwise specified) using a 25 mCi 57Co/Rh source. All spectra were computer fitted and the chemical isomer-shift data are quoted relative to metallic iron. Temperature-programmed reduction was performed using a 95 : 5 argon-hydrogen gas mixture (30 cm3 min-l) and a programming rate of 16 K min-l.The gas was purified by passage through a deoxidiser and 5A molecular sieve. Mean metal particle sizes were determined from samples reduced in hydrogen at 235 "C (1 h) by transmission electron microscopy with a Hitachi H-600 electron microscope. * Gas hourly space velocity.F. J . BERRY et af. 2295 Table 1. Iron-57 Mossbauer parameters recorded from alumina-supported iron and iron-ruthenium catalysts following treatment in hydrogen 5% Fe 5% Fe-0. I % Ru spectral spectral treatment oxidation contri- contri- in hydrogen state 6 & 0.05 A +_ 0.05 bution 6 f 0.05 A f 0.05 bution ("C/h) of iron /mm s-' /mm s-l (%) /mm s-' /mm s-' (%) untreated 235/4 480/2 600/ 2 0.34 0.88 - 0.34 0.91 - 0.34 0.95 - 0.35 0.95 - - - - - - ZeII - ZeO - - TeIII 0.39 0.95 78.2 0.37 0.98 76.6 0.97 2.13 21.8 0.99 2.20 23.4 ZeO - Ze"' 0.38 0.83 40.8 - - TeO - - - - - - - - - - - - - - 0.78 1.98 59.2 - - - - - 5% Fe-5% Ru 1%Fe-5% Ru spectral spec tr a1 contri- contri- 6 f 0.05 A & 0.05 bution 6 f 0.05 A & 0.05 bution /mm s-l /mm s-' (%) /mm s-l /mm s-' (%) ~~~~~~~~ untreated Fe"' 235/4 Fe"' Fe" Feo 480/2 Fe"' Fe" FeO 600/2 Fe"' Fe" FeO 0.34 0.99 0.45 0.87 1.06 2.08 0.30 0.92 0.93 2.0 1 0.04 - 1 .oo 1.93 0.04 - - - - 0.32 0.92 - 45.7 0.34 0.77 60.6 54.3 1.00 2.32 32.8 - 0.04 - 6.6 53.6 1.08 2.00 52.2 15.8 0.09 -- 47.8 - - 30.6 - 61.0 - 39.0 - - - - - RESULTS AND DISCUSSION ALUMINA- AND SILICA-SUPPORTED IRON-RUTHENIUM FOLLOWING TREATMENT IN HYDROGEN The 57Fe Mossbauer parameters recorded from alumina-supported iron-ruthenium samples are given in table 1.The "Fe Mossbauer spectrum recorded from the alumina-supported 5 % Fe-0. 1 % Ru (5% Fe-O. 1 % Ru/Al,O,) catalyst formed by calcination in air at 480 "C for 4 h was very similar to that recorded from a sample of pure alumina-supported iron (5% Fe/AI,O,) being characteristic of iron(111) in an oxygen environment and resembling those reported1°-14 for iron oxide supported on alumina and other data which have been associated with supported small-particle superparamagnetic a-Fe,O,. Treatment of both 5% Fe/Al,O, and 5% Fe-O. 1 % Ru/Al,O, in hydrogen at 480 "C produced samples which gave Mossbauer spectra which were best fitted to three peaks and2296 MOSSBAUER STUDY OF Fe-Ru CATALYSTS interpreted in terms of the superposition of a quadrupole split absorption characteristic of iron(II1) on another absorption characteristic of iron(I1).The results are indicative of the insignificant influence of small quantities of ruthenium on the reducibility of iron. The results from the alumina-supported 5% Fe-5% Ru sample (5% Fe-5% Ru/Al,O,) were different. Although the material formed in air gave a Mossbauer spectrum [fig. 1 (a)] similar to that recorded from 5% Fe/A1,0, and 5% Fe-0.1 % Ru/Al,O, the spectrum from 5% Fe-5% Ru/A1,0, when treated at 235 "C in hydrogen shows the presence of iron(I1) [fig. 1 (b)]. The onset of reduction at 235 "C suggests that the higher concentration of ruthenium renders the iron amenable to lower-temperature reduction, a suggestion which is endorsed by the Mossbauer spectrum [fig. 1 (c)] of the sample following further treatment in hydrogen at 480 "C, which shows the presence of iron(IIr), iron(n) and iron(o).The iron(o) component of the spectrum resembles the single-peak Mossbauer spectrum, 6 = 0.03 mm s-I, recorded from an iron-ruthenium alloy prepared by the fusion of a milled mixture of the metals in an argon arc furnace. Further reduction at 600 "C gave a material which was shown by Mossbauer spectroscopy [fig. 1 ( d ) ] to be predominantly reduced to the iron-ruthenium alloy with the non-alloyed iron being present as iron(r1). The alumina-supported 1 % Fe-lx Ru (1 % Fe-1 % Ru/Al,O,) gave very similar Mossbauer spectra to those recorded from 5% Fe-5% Ru/A1,0,. The result is interesting since the reduction of iron has been reported5.to be severely inhibited at very low iron-ruthenium loadings, ca. 1 wt "/o, on silica supports. The alumina-supported 1 % Fe-5% Ru sample (1 % Fe-5% Ru/Al,O,) gave, following treatment in hydrogen at the lower temperature of 235 "C, a Mossbauer spectrum which showed the reduction of iron(m) to iron(1r) and an iron-ruthenium alloy. Hence alloy formation occurs in this ruthenium-rich sample at a lower tempera- ture (235 "C) than is observed in any of the other samples. The amenability of iron to hydrogen reduction at 235 "C in the alumina-supported materials therefore lies in the order 5% Ru-l% Fe > 5% Fe-5% Ru z 1% Fe-l% Ru > 5% Fe-0.1% Ru and clearly demonstrates that the ease of hydrogen reduction is dependent on the ruthenium content of the sample.The t.p.r. profiles recorded from the alumina-supported iron-ruthenium samples (fig. 2 ) endorse the Mossbauer results. The t.p.r. profile recorded from the bimetallic sample 5% Fe-O. 1 % Ru/Al,O, [fig. 2(6)] shows the major reduction peak which corresponds to reduction of iron(Ir1) to iron(1r) [fig. 2(a)] to shift, by ca. 70 "C, to a lower temperature. The result is indicative of the more facile reduction of iron(m) as a result of the presence of ruthenium. It is also pertinent to note the peak at 620 "C which, by comparison with the t.p.r. profile recorded from unsupported iron oxide [fig. 2(e)] corresponds to reduction to iron(o). The t.p.r. profile recorded from 5% F e - l x Ru/Al,O, [fig. 2(c)] shows the major iron(II1) to iron(I1) reduction peak to be shifted to even lower temperature (362 "C) and more iron appears to be reduced to the metallic state at the lower temperature of 590 "C.It is also pertinent to record the presence of another peak at 810 "C. Although the t.p.r. profile recorded from 5% Fe-5 % Ru/Al,O, [fig. 2 ( d ) ] also showed a peak at 360 "C corresponding to the more facile iron(1rr) to iron(r1) conversion, a more significant feature is the shift to lower temperature of the peak corresponding to reduction to iron(o). The t.p.r. results are therefore consistent with the Mossbauer evidence for more facile reduction of iron(m) when in an alumina-supported material containing high concentrations of ruthenium. It is also interesting to record the indication in the t.p.r. profile depicted in fig. 2(d) of the reduction of an iron species at temperatures just below 1000 "C.The Mossbauer spectra recorded from the silica-supported 5% Fe-5% Ru catalystsF. J. BERRY et ai. 2297 10.0 9.9 p 9 . 9 Fe3 n Feo I Fe3 v . Feo -10 -8 -6 -4 - 2 0 2 4 6 8 10 velocity/mm s-' Fig. 1. Iron-57 Mossbauer spectra recorded from 5 % Fe-5% Ru/A1,0, and heated at (a) 480 "C (4 h) in air, (b) 235 "C (4 h) in hydrogen, (c) 480 "C (2 h) in hydrogen and ( d ) 600 "C (2 h) in hydrogen. formed by calcination in air, and following their exposure to hydrogen at temperatures below 235 "C, were similar to those of the alumina-supported analogues showing no detectable reduction of iron(rI1) to iron(I1). However, the Mossbauer spectrum recorded from 5 % Fe-5 % Ru/SiO, following treatment at 452 "C for 6 h in hydrogen was different (fig.3) from that recorded from the alumina-supported analogue2298 MOSSBAUER STUDY OF Fe-Ru CATALYSTS I I I I I 1 I I I, 100 200 300 LOO 500 600 700 800 900 Fig. 2. Temperature-programmed reduction profiles of alumina-supported (a) 5 % Fe, (b) 5 % Fe-0.1 % Ru, (c) 5 % F e - l x Ru, ( d ) 5% Fe-5% Ru and ( e ) unsupported Fe,O,. T/OC subjected to similar treatment. The silica-supported catalyst gave a spectrum [fig. 3 (b)] composed of a single dominant peak characteristic of iron(o) in an iron-ruthenium alloy superimposed on a doublet characteristic of iron(1Ir). Comparison with the spectrum recorded from the alumina-supported catalyst [fig. 3 (a)] suggests that iron(1rr) in the silica-supported material is reduced more easily to iron(o) and shows that the amenability of iron to reduction in the bimetallic system is also dependent on the nature of the support, with more facile reduction to the zero-valent state being achieved on silica, where the metal-support interaction is weaker.It is also pertinent to record that the iron(o) component of the reduced alumina-supported catalyst was oxidised to iron(1r) by simple exposure to air for 2 h at room temperature, whereas the iron(o) in the reduced 5 % Fe-5 % Ru/SiO, was partially oxidised to iron(Ir1) under similar conditions. The result is also consistent with the bimetallic phase being more weakly associated with silica than with alumina. The treatment of both samples in air at 622 "C gave complete oxidation to iron(rrr). The t.p.r.profiles recorded from the silica-supported iron-ruthenium catalysts (fig. 4) complement the Mossbauer data and elucidate other features of the reduction properties of supported iron-ruthenium catalysts. The 5 % Fe-0. 1 % Ru/SiO, sample gave a t.p.r. profile [fig. 4(b)] in which the dominant peak at 680 "C indicates that the iron suffers reduction to iron(o). The greater amenability of the iron to reduction as the ruthenium content of the bimetallic catalysts is increased is reflectedF. J. BERRY et al. 2299 Feo 'Fe3 * n 10.0 - -10 -8 - 6 -1 - 2 0 2 L 6 8 10 velocity/mm s-I Fig. 3. Iron-57 Mossbauer spectra recorded from 5% iron-5% ruthenium supported on (a) alumina and (b) silica and heated at 452 "C for 6 h in hydrogen. 1000 1 I I 1 I I I I 1 1 100 200 300 400 500 600 700 800 900 1000 T/ "C Fig.4. Temperature-programmed reduction profiles of silica-supported (a) 5 % Fe, (b) 5 7; Fe-O.1% Ru, (c) 5% Fe-l% Ru and ( d ) 5% Fe-5% Ru.2300 MOSSBAUER STUDY OF Fe-Ru CATALYSTS in fig. 4(c) and (d), although it is also pertinent to note the diminution of the peak intensities in fig. 4(d), recorded from 5% Fe-5% Ru/SiO,, which may reflect the shielding by ruthenium of some of the iron from surface reduction during the t.p.r. experiments. A comparison of fig. 2 and 4 shows that, as the ruthenium content in the iron-ruthenium catalysts increases, the temperatures corresponding to the reduction of alumina-supported iron (fig. 2) decrease more rapidly than for iron in the silica- supported analogues (fig. 4) and are indicative of stronger interactions between iron and ruthenium in the alumina-supported catalysts.These observations, together with evidence for the reduction to iron(rr) as the dominant reduction process for alumina- supported materials, may reflect the partial inclusion of iron within the superficial regions of the alumina support, perhaps to the extent of compound formation such as FeAI,O,, in which the reduction of iron(r1) to iron(o) is difficult to achieve and which gives rise to the high-temperature peaks on the t.p.r. profiles depicted in fig. 2(c) and (d). The results also illustrate the importance of using elevated temperatures during t.p.r. studies of supported-iron-containing materials. It is also interesting to note that the reduction phenomena observed here from the Mossbauer spectra of the silica-supported iron-ruthenium system are different from those recently reported' for the silica-supported iron-rhodium catalysts which were reduced more easily at low temperatures.However, it must be recognised that the calcination conditions used in the preparations are not identical and the extent to which the different reduction properties reflect thermally induced variations in the particle sizes is not easy to assess. CATALYTIC PROPERTIES OF ALUMINA- AND SILICA-SUPPORTED IRON-RUTHENIUM CATALYSTS The activities of the alumina- and silica-supported catalysts are plotted as a function of the iron content in fig. 5. The activities are expressed in terms of the yield of hydrocarbon per gram of ruthenium, as opposed to the total iron-ruthenium content, since the alumina- and silica-supported pure iron catalysts (1-10 wt % loading) showed only negligible activity under identical reaction conditions.Three interesting features emerge from a consideration of fig. 5 . First, the ruthenium and iron-ruthenium catalysts supported on alumina exhibit higher activities than those supported on silica. Secondly, the LML catalysts supported on alumina have markedly higher activities than their HML counterparts, whereas for silica-supported catalysts the difference in activity is not significant. Thirdly, an increase in the iron content of the alumina- supported catalyst causes a decrease in activity whilst the effect of the iron concentration on the catalytic activity of the catalysts supported on silica is less significant.The mean particle sizes of the metallic components of the supported iron-ruthenium catalysts, as determined by transmission electron microscopy, are recorded in table 2. The data show that particles of the silica-supported metallic phases are larger than their alumina-supported analogues and, within each series of supported catalysts, the metallic particles in the HML catalysts are larger than those in the LML series. Hence the metallic particle sizes increase in the order LML/Al,O, < HML/Al,O, < LML/SiO, < HML/SiO,. In this respect it is pertinent to record that high dispersions of supported ruthenium are a feature of strong metal-support interactions16 and that the results are therefore consistent with our previous evidence for the enhanced interactions between pure r ~ t h e n i u m l ~ ~ and iron-ruthenium with alumina supports.Hence the high catalytic activities of the highly dispersed LML alumina-supported catalysts may be associated with the strong interactions between the metallic phases and the alumina support.F. J. BERRY et al. 230 1 0 10 20 30 LO 50 60 70 iron content (atom ",) ) Fig. 5. Catalytic activity of alumina- and silica-supported iron-ruthenium catalysts as a function of the iron content: (a) LML Al,O,, (b) HML A1,0,, (c) LML SiO, and ( d ) HML SiO,. Table 2. Mean particle sizes of the metallic component of supported iron-ruthenium catalysts determined by transmission electron microscopy catalyst d l A catalyst d l A Al,O,-supported l%Ru 25 1% Ru-0.2% Fe 26 5% Ru 28 5% Ru-1% Fe 36 1% Ru-1% Fe 34 5 % Ru-5% Fe 43 Si0,-supported 1% Ru 42 1% Ru-0.2% Fe 42 1% Ru-1% Fe 51 5% Ru 46 5% Ru-5% Fe 60 5% Ru-1% Fe 55 The product distributions achieved from the two series of catalysts are shown in fig.6-9. The results depicted in fig. 6 and 7 show that increases in the iron content of both the HML and LML alumina-supported catalysts gives rise to higher yields of the lower-carbon-number hydrocarbons. The product distributions obtained from the LML catalysts were found to be particularly sensitive to the iron concentration in the bimetallic phase, indeed the presence of 27 atom % iron gave a selectivity equivalent to that recorded from the HML catalyst containing 65 atom % iron. A similar trend was observed in the results recorded from the silica-supported catalysts (fig.8 and 9), albeit to a lesser extent. The variation in selectivity of the bimetallic catalysts may be associated with the differences in metal-support interaction between alumina- and silica-supported catalysts and the interactions between ruthenium and iron. This is well illustrated2302 MOSSBAUER STUDY OF Fe-Ru CATALYSTS 0 2 L 6 8 10 12 1L 16 18 20 22 2L 26 carbon number Fig. 6. Hydrocarbon product distribution from high-metal-loading alumina-supported catalysts : 0 , 5% Ru-5% Fe/Al,O,; A, 5 % Ru-lx Fe/Al,O, and x , 5% Ru/A1,0,. carbon number Fig. 7. Hydrocarbon product distribution from low-metal-loading alumina-supported catalysts : 0 , 1% Ru-l% Fe/Al,O,; A, 1 % Ru-0.2% Fe/Al,O, and x , 1% Ru/A1,0,.F.J. BERRY et al. 2303 20 15 h 0 5 -e 2 v e ; 10 c 5 P 1 . 1 1 . 1 1 . 1 . 1 21 20 - = 1 5 .-s 3 v C 0 -E 2 U Q -2 10 5 0 2 L 6 8 10 12 14 16 18 20 22 carbon number Fig. 8. I 4 carbon number Fig. 9. Fig. 8. Hydrocarbon product distribution from high-metal-loading silica-supported catalysts : 0, 5% Ru-5% Fe/SiO,; A, 5% Ru-lx Fe/SiO, and x , 5 % Ru/SiO,. Fig. 9. Hydrocarbon product distribution from low-metal-loading silica-supported catalysts : 0, 1% Ru-lz Fe/SiO,; A, 1 % Ru-0.2% Fe/SiO, and x , 1% Ru/SiO,. by a consideration of the products arising from the LML alumina-supported ruthenium catalysts (fig. 7), which span a wide range of hydrocarbon chain lengths centred between C, and C,, and which are consistent with previous data 1 7 9 which showed that the strong interactions between ruthenium and alumina favours the formation of longer-chain hydrocarbons.In contrast, the product distribution recorded from LML silica-supported ruthenium (fig. 9), where the metal-support interaction is weaker, is concentrated towards the lower-molecular-weight hydrocarbons. The chemical nature of the iron (which, in the bimetallic alumina-supported catalysts pretreated in hydrogen at 235 "C for 4 h where the metal-support interaction is strong, is responsible for the d,ecrease in activity and shift in selectivity towards the formation of lower hydrocarbons) was examined by in situ Mossbauer spectroscopy. The results are summarised in table 3 and show that the concentration of any reduced iron species in the pure iron or low-ruthenium-content catalysts formed by pretreatment2304 MOSSBAUER STUDY OF Fe-Ru CATALYSTS Table 3.Iron-57 Mossbauer parameters recorded at 298 K from alumina-supported iron and iron-ruthenium catalysts following reduction in hydrogen at 235 "C for 4 h oxidation 6 f 0.05 A & 0.05 spectral catalyst state of iron /mm s-' /mm s-l area (%I 5% Fe Fe"' 5% Fe-O.l% Ru Fell' 1% Fe-l% Ru Fe"' Fe'I 5% Fe-5% Ru Fe"I Fe" 1 % Fe-5% Ru Fe"' FeI' FeO 0.2% Fe-l% Ru Fe"' Fe" 0.34 0.35 0.40 1.17 0.45 1.06 0.34 1 .oo 0.04 0.32 0.76 0.95 0.95 0.9 1 2.46 0.87 2.08 0.77 2.32 0.90 2.00 - - 79 21 46 54 60 33 7 96 4 in hydrogen is below the level of detectability of Mossbauer spectroscopy. However, the iron component of catalysts containing higher concentrations of ruthenium was found to be present, at least partially, as iron(rr) and even as iron(o) in ruthenium-rich catalysts.The results therefore show that the iron species in the iron-ruthenium catalysts evaluated in this work exist predominantly in the form of iron oxides as opposed to iron-ruthenium alloys. INFLUENCE OF METAL-SUPPORT AND METAL-METAL INTERACTIONS ON CATALYTIC PERFORMANCE The results indicate that the catalytic performances of the iron-ruthenium catalysts are significantly influenced by the nature of the support, the dispersion of the metallic phase and the concentration of iron in the bimetallic phases. This is well illustrated by the effects of low concentrations of iron in the alumina-supported ruthenium catalysts on selectivity, which may be compared with the effects on catalytic activity (which only become significant when the iron content exceeds ca.27 atom %) and the lower sensitivity of the performance of silica-supported ruthenium catalysts to the incorporation of iron. The most striking manifestation of these results concerns the change in both activity and selectivity of the alumina-supported ruthenium catalysts which accompanies the increase in iron concentration such that the performance of catalysts with iron concentrations of ca. 65 atom % resemble that of pure ruthenium supported on silica. Given that Mossbauer spectroscopy showed the iron to be present in the iron-rich catalysts as an iron oxide, as opposed to an iron-ruthenium alloy, it may be envisaged that the iron component of these alumina-supported bimetallic catalysts diminishes the interaction between ruthenium and the support and consequently induces the decrease in activity and shifts the selectivity in favour of the formation of the lower- molecular-weight hydrocarbons.In this respect a close examination of the Mossbauer data (table 3) is informative since the iron(r1) chemical isomer shifts clearly decrease with increasing concentrations of ruthenium. Since A R / R is negative for the j7Fe transition the decrease in chemical-isomer-shift data corresponds to an increase in the s-electron density at the iron nucleus, which may be related to a decrease in the d-electron population. Given that ruthenium induces the reduction of ircn(m) to iron(I1) it is likely that the iron(r1) species so formed are in close proximity to theF.J. BERRY et al. 2305 ruthenium species and that their interaction with ruthenium, which is reflected in the diminishing iron@) chemical isomer shifts, involves the transfer of d-electrons from the iron(rr) to the ruthenium. In this respect it is pertinent to record that the enhanced catalytic activity and selectivity towards higher-molecular-weight hydrocarbons of Ru/Al,O,, as compared with Ru/SiO,, has been associated 17918 with the acidity of alumina, which leads to electron transfer from ruthenium to the acid sites and gives rise to the strong interaction between ruthenium and the alumina support and the formation of electron-deficient ruthenium species, Rus+ . The model is consistent with the isomer-shift changes reported here since a transfer of electrons from iron to ruthenium would mitigate against the formation of the electron-deficient Rub+ species.Furthermore, any electron transfer from iron to the limited number of acidic sites on the alumina support would also inhibit the transfer of electrons from ruthenium to alumina. Such processes would result in a weakening of the ruthenium- alumina interaction and thereby give rise to a decrease in activity and a shift in selectivity towards the formation of lower hydrocarbons. The electron transfer between ruthenium and less acidic silica might be expected to be less significant, hence the activity and selectivity of silica-supported ruthenium catalysts would tend to be less affected by the incorporation of iron and only approach the performance of the alumina-supported bimetallic catalysts when the latter are iron-rich.We acknowledge the support of The Chinese Academy of Sciences and The Royal Society. I G. L. Ott, T. Fleisch and W. N. Delgass, J. Catal., 1979, 60, 394. M. A. Vannice, Y. L. Lam and R. L. Garten, in Hydrocarbon Synthesis from Carbon Monoxide and Hydrogen, ed. E. L. Kugler and F. W. Steffgen, Ado. Chem. Ser. 178 (American Chemical Society, Washington D. C., 1979), p. 25. M. A. Vannice and R. L. Garten, in ScientiJc Problems of Coal Utilization, ed. B. R. Cooper (US. Department of Energy, Washington D.C., 1978), p. 248. Y. L. Lam and R. L. Garten, paper given at the Sixth Symposium Ibero-American Catal., Rio de Janeiro, 1978. L. Guczi, K. 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M. van der Kraan, J. J. van Loef and W. N. Delgass, J. Phys. Chem., 1983, 87, 1292. l 6 C. A. Clausen and M. L. Good, J . Catal., 1975, 38, 92. l i Liang Dongbai, Lin Liwu, Wu Rongan, Bai Yuheng, and Hu Aihua, Proc. China-Japan-USA. *' Liang Dongbai, Wu Rongan, Bai Yuheng, Hu Aihua, Zhao Qiansi, Feng Xiyun and Lin Liwu, Symp. Heterogeneous Catalysis, Dalian, China, 1982, B-05C (Chem. Abstr., 99: 141 892y). J . Fuel Chem. Technol. (China), 1984, 12, 97. (PAPER 4/ 1556)

ISSN:0300-9599    

DOI:10.1039/F19858102293

出版商:RSC    

年代:1985

数据来源: RSC

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J . Chem. SOC.. Furuduy Trans. I , 1985, 81, 2307-2322 Spectroscopic Characterization of a Molybdena/Silica System Photoreduced in a Carbon Monoxide Atmosphere BY EUGENIO GUGLIELMINOTTI* Istituto Chimica Fisica, Universita di Torino, Corso Massimo d' Azeglio 48, 10125 Torino, Italy AND ELIO GIAMELLO Istituto Chimica generale ed Inorganica, Facolta di Farmacia, Universita di Torino, Italy Received 19th September, 1984 A supported MoOJSiO, system prepared via molybdate impregnation or via Mo(CO), heterogenization was reduced at 823 K with H, and the CO adsorption on the reduced molybdena phase was investigated by i.r., u.v.-vis. and e.s.r. techniques. The formation of MoV ions has been identified by e.s.r. spectroscopy and weak i.r. bands at 2203 and 2181 cm-' have been assigned to CO adsorbed on MoV and MoIV, respectively.A photoreduction process, carried out in a CO atmosphere at room temperature, lead to further reduction of molybdena; in particular, two bands at 2128 and 2080 cm-l were very strong in Mo(CO),/SiO, samples and have been assigned to a pair of CO molecules linearly adsorbed on distorted tetrahedral Mo" ions. This species is in equilibrium with a CO species bridged between two vicinal Mo" ions (vco = 2043 cm-l) and the equilibrium process can be also monitored using u.v.-visible reflectance spectroscopy. The possibility of obtaining a molybdena phase in which Mo ions have a valence lower than four is thus confirmed. Supported-molybdenum systems have received increasing attention in recent years owing to their activity in catalytic desulphurization, oxidation and in the metathesis of olefins, where the use of silica and alumina as supports looks especially promising.' In addition, the MoIV ions obtained by Kazansky and coworkers2 by photoreducing at a low temperature (77 K) the MoO,/SiO, system in an atmosphere of H, or CO exhibit a strong reducing activity towards H20, CO, and N20.Moreover, MoI1 ions which have been even further reduced have been obtained by Yermakov3 and by Iwasawa and coworkers4 by heterogenization of ally1 compounds of molybdenum which reacted with the surface hydroxyls of alumina and silica. The heterogenization of Mo(CO), leads to a catalyst that is very active for olefin metathesis, and this has been extensively investigated.lP5 However, the oxidation number and the coordination state (distorted tetrahedral or octahedral) of the reduced Mo, which is catalytically active, are still subjects of debate; in particular, Mo ions in different oxidation states (MoV1, MoV, MoIV, MolI1, MolI and Moo) have been detected by techniques such as e.s.r.and u.v.-visible spectroscopy and quantitative 0, uptake.lP5 This paper reports some new data, mainly obtained by infrared spectroscopy, on MoO,/SiO, systems submitted to reduction in H, at 823 K followed by photoreduction in a CO atmosphere. Two types of system have been investigated, the first obtained by conventional impregnation of the support with a MoV* salt and the second by Mo(CO), heterogenization. 23072308 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM EXPERIMENTAL Two types of sample have been prepared.(i) The first was obtained by the usual impregnation procedure with a solution of ammonium molybdate, (NH,),Mo,O,, .4H,O, of Aerosil silica (380 m2 g-l) from Degussa; two samples with Mo contents of 0.5 and 3 wt% were prepared; however, the results obtained are nearly identical and are simpler to discuss on the 0.5% sample. This is ascribed to the well known fact that segregation of a MOO, phase occurs on silica at loadings above 5% Mo (see below).6 This view is confirmed by our results; e.g. more intense bands of adsorbed CO occur for the 0.5% samples than for the 3% samples because in the first case Mo is thoroughly dispersed in a molybdena-silica surface phase. We therefore report the data obtained on the 0.5% Mo sample (sample A).(ii) The second sample was obtained by vapour-phase deposition of Mo(CO), sublimed in vacuo on Aerosil outgassed at room temperature [the vapour pressure of Mo(CO), at 298 K is ca. 21 N m-,]. The deposited Mo(CO), was decomposed by heating at 473 K and then oxidized in oxygen at 773 K for 4 h. Analysis by atomic absorption spectroscopy, using a Perkin-Elmer 2380 spectrophotometer, of this sample, after H, reduction and CO photoreduction procedures, gave an Mo content of 0.25%. The amount of Mo is therefore comparable with the 0.5% contained in sample A; we refer hereafter to the Mo(CO),/SiO, sample as sample B. Samples A and B were then reduced with H, for 4 h at 823 K. Experiments carried out on samples reduced and outgassed at 773 and 873 K showed them to behave in the same way as samples treated at 823 K.The H, was replaced several times, with a liquid-nitrogen trap placed as close as possible to the sample during and after reduction in order to remove fully any water formed, given the well known oxidizing power of water on reduced chromia and molybdena-silica systems.’ The samples thus treated were then immediately put in contact with CO. The photoreduction process was carried out in a CO atmosphere at room temperature, in the quartz cells utilized for spectroscopic experiments, with an Hg vapour lamp without filters. The maximum photoreducing effect was found to occur after 1 h of photoillumination. 1.r. spectra were run on Beckman IR 12 and Perkin-Elmer 580 B spectrophotometers, the latter being equipped with a 3600 data station and a program for handling spectra.Aerosil wafers used in the i.r. experiments were obtained at pressures ranging between 100 and 500 kg ern-,. No influence of pressure on the experimental results was observed. U.v.-vis. diffuse reflectance spectra were recorded on a Varian 2390 u.v.-vis.-n.i.r. spectro- photometer equipped with a diffuse-reflectance attachment. E.s.r. spectra were measured in a sealed quartz tube at 77 and 295 K on a Varian E 109 e.s.r. instrument using Varian pitch (g = 2.0029) as field reference. Quantitative data for 0, and CO uptakes were obtained using a Sartorius microbalance. RESULTS I.R. SPECTRA Fig. 1 shows the spectra of CO adsorbed on a sample of type B. After H, reduction no, or very weak, bands of adsorbed CO appear; after photoreduction two very narrow bands at 2128 and 2080 cm-l (the X bands) are clearly visible.Another weak band at ca. 2043 cm-l (the Y band) is also present and grows in intensity when the CO pressure is reduced by evacuation. In contrast, the X bands decrease until they disappear [fig. l(6) and (c)]. Upon readsorption of CO the Y band disappears while the X bands regain their initial intensity. As shown in the inset of fig. 1 , the bands at 2128 and 2080 cm-l have mutually proportional intensities and are therefore assigned to the same carbonyl species. An isotopic 13CO-12C0 exchange experiment confirms the dimeric nature of the species correlated with the X bands (fig. 2). The Y band is resistant to room-temperature evacuation and is eliminated only by outgassing at 373 K.E.GUGLIEMINOTTI AND E. GIAMELLO 2309 v / c m - 1 2200 2100 2( I 1 O.D. 2128 2128 \ 2080 2043 O.D. 2080 30 Fig. 1. Carbon monoxide adsorbed on a B-type sample. (-.-.-) Silica background of a reduced B-type sample. (a) CO adsorbed (p = 5.3 kN m-2) after 1 h photoillumination; (b) pressure reduced to 0.8 kN m-2 and (c) after 1 min evacuation at room temperature. Inset: intensity correlation between the 2080 and 2128 cm-' bands. If the same sample to which fig. 1 refers is contacted with 0.13 kN mP2 of l2C0 the usual three-band spectrum [fig. 2(a)] appears. If 13C0 at an analogous pressure is allowed to flow in the cell and to mix with l2C0 in a near 1 : 1 ratio we obtain the spectrum of fig. 2(b), in which the exchange process is continuing, and that of fig.2(c), at equilibrium. Residual bands in these latter conditions are observed at 2128-2080 and 2043 cm-l, together with new bands at ca. 2080 (coincident with the previous one), 2034 and 1997 cm-l due to 13C0 complexes and with a new pair seen at 21 15 and ca. 2048 cm-l due to a 12CO-13C0 mixed dimeric complex. A further experiment was performed by exposing type-B samples to the atmosphere for one month and then submitting the specimen to the usual procedure of reduction in H, and photoreduction in CO. In the presence of 8 kN m-2 CO the spectrum of fig. 3(a) is observed. In addition to the rotational contour of the gaseous CO in the cell path, two new bands at 2140 and 2108 cm-l (termed Z bands) are observed, together with the X and Y bands.This new pair, of nearly equal intensity [fig. 3(a)],2310 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM 2200 2100 2000 v/cm-' Fig. 2. Isotopic 12CO-13C0 exchange on a B-type sample. (-.-.-) Silica background of a reduced B-type sample. (a) l2C0 adsorbed after 1 h photoillumination in 5.3 kN m-* l2C0 and pressure reduced to ca. 0.13 kN m-2; (b) ca. 0.13 kN m-2 13C0 mixed with l2CO, spectrum recorded immediately and ( c ) after 30 min (equilibrium conditions). is attributed'to two very weakly chemisorbed CO molecules, because a simple pressure decrease is sufficient to markedly and proportionally reduce the intensity of the Z bands : at the same time the X bands grow slightly in intensity [fig. 3 (b)]; at increasing times of evacuation [fig. 3 (c) and (d)] only the X-band species remains in equilibrium with that of the Y band. In this experiment CO readsorption causes the same spectral features noted in fig.1 (i.e reduction of the Y band and growth of the X bands) without restoration of the Z bands.E. GUGLIEMINOTTI AND E. GIAMELLO I I I 2200 2100 2000 vlcm-' 231 1 Fig. 3. Behaviour of X, Y and Z bands on a B-type sample exposed to the atmosphere. (-.-.-) Silica background of a B-type sample reduced after exposure to the atmosphere for 1 month. ( a ) After 1 h photoillumination in 8 kN m-2 CO. (6)-(d) after evacuation at increasing times (a few seconds) at room temperature. Further evidence concerning the transient, intermediate nature of the species producing the Z bands is given in fig. 4. In this case a sample of type A is reduced and then photoreduced as with the B type sample.CO adsorption before photoreduction (spectrum not reported) gives only weak bands at 2203 and 2181 cm-l [still present in fig. 4(a)]. Photoreduction in CO further decreases the low intensity of these bands and simultaneously the Z bands are formed with significant intensity [traces of the X and Y bands are also observed in fig. 4(a)]. A short evacuation causes the disappearance of the Z bands and an increase in the X bands. The weak bands at 2203 and 2181 cm-' are also both eliminated by a short evacuation. Fig. 5 shows the difference spectrum between CO adsorbed on an A-type sample after I h of photoreduction and the spectrum of CO adsorbed on the same H,-reduced sample, for which only the 2203 and 2181 cm-l weak bands are found.A decrease of the 2203-2181 cm-l bands and an enhancement of the X, Y and Z bands, with a relative intensity between the Z and X bands which is different from those reported in fig. 1-4, is clearly seen. Several other experiments on samples of types A and B and a sample containing 3% Mo, omitted here for the sake of brevity, can be summarized as follows. (i) 1.r. experiments of CO adsorbed on A- and B-type samples with a low Mo content (< 0.5%) give bands of very weak (or zero) intensity at 2203 and 2181 (broad) cm-l; these bands are slightly more intense in the 3% Mo sample. (ii) Photoreduction at2312 h h SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM 2108 2200 21 00 v/cm-l 2000 Fig. 4. Evidence of the transient nature of the Z bands. (-.-.-) Silica background of a reduced A-type sample.(a) After 1 h photoillumination in 4 kN m-2 CO and (b) after evacuation for 15 s at room temperature. d 0 2200 2100 2000 v/cm-' Fig. 5. Effect of 1 h of photoillumination on the i.r. spectrum of CO adsorbed on a type-A sample (difference spectrum).E. GUGLIEMINOTTI AND E. GIAMELLO 2313 I 1 I 2200 21 00 2000 v/cm-l Fig. 6. Adsorption of Mo(CO), on a photoreduced B-type sample: (-.---) background; (a) after 1 h photoillumination in 5.3 kN m-2 CO; (b) pressure of CO reduced to 10 N-m-2 and Mo(CO), vapour adsorbed and (c) 5.3 kN mP2 allowed into the cell. room temperature in a CO atmosphere gives rise to Z bands with randomly variable intensity associated with very weakly adsorbed CO. These are assigned to a transient dimeric species leaving, after a decrease in the CO over-pressure, the pair of bands at 2128-2080 cm-l always present on the reduced Mo samples after photoreduction in CO.The low intensity of these bands in the spectrum, if they are seen at all, indicates the incomplete reduction of the sample (when, because of the absence of a liquid-nitrogen trap, the water formed reoxidizes Mo, or alternatively because of slight oxygen contamination). (iii) The species giving the X bands is completely reversible towards pressure at room temperature: on decreasing the pressure a new species causing the Y band is formed, and by CO readsorption the former species is 16 F A R 123 14 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM 20 0 300 400 500 600 700 800 nm Fig.7. U.v.-vis. reflectance spectra of a sample of type A : (a) after oxidation 4 h at 773 K ; (b) after photoillumination 1 h in 5.3 kN rnp2 CO; (c) after reduction 2 h with H, at 823 K; ( d ) reduced sample after photoillumination 1 h in 5.3 kN m-2 CO and (e) evacuation 1 min at room temperature. immediately and completely restored. (iv) On a 3% Mo sample the X and Z bands are less intense than on the ca. 0.5% samples. This can be explained by a MOO, segregation process on more concentrated samples (see Experimental section). More intense X bands are found on the B-type sample obtained by Mo(CO), decomposition, i.e. a reduced molybdena/silica surface phase is favoured by carbonyl heterogenization. An analogous process leading to a very active chromia/silica catalyst occurred through an analogous Cr(CO), heterogenization process.In order to verify whether the bands in the range 2140-2000cm-1 should be assigned to physisorbed or chemisorbed Mo(CO),, as hypothesized by Abdo and Howe9 for Mo zeolites prepared from Mo(CO),, we carried out the experiments shown in fig. 6 . A CO-precovered B-type sample (p z 10 N m-,) was put in contact with Mo(CO), in the gas phase [fig. 6(6)]. The X bands show a much lower intensity, with the Y band now much more intense owing to the reduced pressure (as shown in fig. 1); Mo(CO), gives the usual strong band of the physisorbed species at 1992 cm-l accompanied by new weak bands at 2096-2100 and ca. 2030 (sh) cm-l. These latter bands can be assigned to Mo(CO), chemisorbed on the same Mo5+ ions adsorbing CO, as found for Cr(CO), on Cr2+ ions grafted on CO readsorption (p = 5.3 kN m-,) at this stage [fig. 6(c)] partially restored the intensity of the X bands at the expense of the 2096-2100cm-l bands and, in part, the 2030 cm-l band.This shows that CO displaces the Mo(CO), molecules weakly chemisorbed on the same MoS+ sites which adsorb CO. This experiment rules out assignments of the X and Z bands to chemisorbed or physisorbed Mo(CO), which may be formed at low levels during the Mo photoreduction process in CO.E. GUGLIEMINOTTI AND E. GIAMELLO 2315 Fig. 8. MoV signal obtained at 77 K for a sample of type A photoreduced in 5.3 kN m-2 CO at room temperature. U.V.-VISIBLE REFLECTANCE SPECTRA In fig. 7 are reported the diffuse reflectance spectra of a type-A sample in the u.v.-vis.range (200-800 nm), as no meaningful spectral changes are observed in the near-i.r. region. Fig. 7(a) shows the spectrum of the sample oxidized at 773 K with a band at 275 nm. CO adsorption has no effect on this spectrum and the photoreduction process in CO only introduces a weak, broad and structureless band centred at ca. 400 nm [fig. 7(6)]. Reduction in H, at 823 K [fig. 7(c)] lowers the intensity of the 275 nm band and results in a strong increase in the broad absorption centred at 420 nm; at the same time we observed a darkening of the sample from white to sandy brown. The photoreduction process has a marked effect on the spectrum [fig. 7 ( d ) ] : a new band grows at 300 nm and a continuous broad absorption extends to the visible region.A short evacuation [l min, fig. 7(e)] at room temperature markedly reduces the 300 nm band and increases a new (or previously weaker) absorption at 365 and ca. 500 nm. These bands are related to the reversible process occurring at room temperature (as in the i.r. results for the X and Y bands) because CO readsorption completely restores the initial spectrum [fig. 7 (41. E.S.R. AND QUANTITATIVE EXPERIMENTS The e.s.r. spectrum of an A-type sample reduced in H, at 773 K shows the typical signal of MoV in axial symmetry (g,, = 1.883 ; g1 = I .944) already found, for instance, by Che el allo and by Kazansky et a/.2 on silica-supported molybdena. The shape of 76-22316 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM the signal was not affected by CO adsorption but was modified by photoreduction in CO at room temperature.The intensity of the signal approximately doubled and its shape indicated that additional components have arisen (fig. 8). Assuming that the peaks on the low-field side of the signal belong to the perpendicular hyperfine pattern (Mo contains ca. 25.4% of isotopes with nuclear spin 5/2) the spectrum can be interpreted as being due to three types of MoV centres having the following spin-Hamiltonian parameters: gi = 1.944, g l = 1.947, g l = 1.960; A 1 = 59 G, AT = 60 G, A3, = 61 G. The parallel component (g,, = 1.883) is not resolved. Further work (computer simulation of the spectrum) is planned to confirm this assignment. Quantitative data obtained with a microbalance gave an oxygen uptake at room temperature of a sample of type A, after 823 K reduction, of the order O/Mo z 0.5.The CO uptake of the same reduced sample after 1 h photoillumination at room temperature gave a small adsorption of CO at the limits of detectability, of the order CO/Mo z 0.07. DISCUSSION The oxidation number and coordination of reduced-molybdena supported samples is a much debated and open question. The reduction of an MoO,/SiO, catalyst at 773 K with H2,10 as well as photoreduction in H, or CO at 77 K or at room temperature,,. l1 leads to the formation of MoV paramagnetic ions detected by e.s.r. spectroscopy. However, only a minor fraction of the reduced Mo ions are detectable by e.s.r. owing to the fact that some of the MoV1 ions are directly converted into MoIV (not observable by e.s.r.).2T Furthermore, a fraction of the MoV ions present in the reduced system can escape detection because of the magnetic superexchange interaction of paired ions, as reported elsewhere.I2 The photoreduction of MoO,/SiO, in H, or CO at low temperatures gives, via MoV, a MoIV complex which, in the latter case, can adsorb CO forming a surface-distorted co / 0' ~ \ / ' M 04+ tetrahedral structure (I) with concomitant CO, evolution.This hypothesis has been put forward by Kazansky, on the basis of quantitative 0, uptake (the average oxidation number of Mo was 3.92) and optical reflectance data (absorption at 1000, 500 and 370 nm). However, no literature data on the optical spectra of MorV ions in tetrahedral coordination exist and an unambiguous assignment of these bands is not possible.Moreover, Iwasawa et aL49 l3 have recently shown that thermal reduction at 773-873 K of samples obtained by reaction of ally1 Mo complexes with surface hydroxyl groups on SiO, can lead to paired, coordinatively unsaturated MolI ions. However, intermediate reduction of MoV1 at 723 K or oxidation of MoI1 ions at room temperature leads to Mo ions with an intermediate oxidation number of (IV). Similar behaviour is shown by alumina-supported Mo samples. The i.r. spectra of CO adsorbed on silica- and alumina-supported samples reduced at 773 K have been recently reported by Peril4 On silica only a very weak band at 2185 cm-l was found after CO adsorption. On alumina reduction, temperatures asE. GUGLIEMINOTTI AND E. GIAMELLO 2317 Table 1. X band frequencies of the isotopic 12CO-13C0 exchange experiment isotopic species v, (exptl)/cm-l v, (calc.)/cm-l v, (exptl)/cm-] v, (calc.)/cm-l '2C0-12CO 2128 2128 2080 2080 YO-13co 21 I5 21 12 2048 2050.4 1 3 ~ 0 - 1 3 c o ca.2080 2080.7 2034 2033.7 high as 973-1073 K were necessary to obtain bands of adsorbed CO at 2190 cm-l (MoIV - CO?) and at lower frequencies (2100-2025 cm-l); these bands were tenta- tively assigned to CO adsorbed on Mod+ or Moo. In our samples only two weak bands at 2203 and 21 8 1 cm-l are (sometimes) found after H, reduction at 823 K. We can tentatively assign these bands, on the basis of their frequency range, to CO adsorbed on MoZ+ ions, where x = 5 for the 2203 cm-l band and x = 4 for the 2181 cm-1 band. In fact, both e.s.r.and quantitative measurements have shown that MoV and MoIV are the most probable oxidation states after reduction at 823 K.47 lo However, in our case it is evident that only a very small fraction of the surface MoZ+ ions (x = 5,4) are able to adsorb CO, as both the amount of adsorbed CO measured after reduction at 823 K and the intensity of the corresponding i.r. bands are very low. Only the photoreduction process, probably favouring the interaction of CO with the light-excited triplet states of MoOJSiO, as shown by Anpo et al.," can increase CO adsorption, which is then accompanied by a further reduction of Mo ions and a consequent change of both coordinative unsaturation and of the coordination number of Mo. First we discuss the intense and omnipresent X-band pair.This can be assigned to a surface-carbonyl dimeric species, the two bands being strictly correlated in intensity (see inset of fig. 1). Furthermore, the isotopic 13CO-12C0 exchange experiments (fig. 2) confirm the dimeric nature of adsorbed (cO\Mo/cO). In table 1 is reported a comparison between the experimental and calculated frequencies of this species assuming a simple Cotton-Kraihanzel mode115 for two coupled oscillators with a weak interaction force constant (40.8 N m-l in this case). Agreement is satisfactory, given the approximate model and the probable presence of two neighbouring pairs of CO molecules adsorbed on vicinal Mo centres (see below). The CO species adsorbed at 2043cm-l, formed from the X bands by room- temperature evacuation and resistant to outgassing up to ca.373 K, gives rise by isotopic exchange to only one band at 1997 cm-l (calculated 1997.5 cm-l), and is therefore associated with only one adsorbed CO species. If the mechanism of CO desorption was simply co co co Mo - Mo + co we should expect for the monocarbonylic complex a vco vibration between 2128 and 2080 cm-l, whereas in fact the frequency is lower. To explain this fact, we assume that the CO species is bridged between two Mo ions, so that a more marked change in the Mo orbital hybridization and carbon sp2 hybridization can occur. An analogous \ / -co_ I2318 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM carbonylic species bridged between two CrI1 ions on a chromia/silica reduced catalyst, was found by Zecchina and coworkersL6 and by Rebenstorf and Larsonn17 to adsorb at 2095 and 2035 cm-l, respectively.The attribution of an oxidation number of (11) to Mo ions, based on an analogy with chromia/silica systems, can also be supported on further grounds. To our knowledge no simple bicarbonyls with MoII are known and, in our case, the species giving X and Y bands are very labile and sensitive to traces of 0, and H,O. The assignment of stretching frequencies between 2200 and 1990 cm-l to MoLL(CO), carbonyl is reasonable; in fact the vco frequencies of carbonylic groups in the [Mo(C,H,)(CO),]+ complex, where Mo is divalent,18 occur at 2128, 2041 and 1980 cm-l. A simple vectorial calculation from the intensity ratio &/Is of the 2128 and 2080 cm-l modes gives an angle 8 = 102" between the two adsorbed CO molecules.0 ' '0 The surface structure (11) formed by photoreduction of MoIV to give MoII in a distorted tetrahedral coordination is therefore proposed for the X species. Two similar neighbouring structures can be involved in the formation of the 2043 cm-l CO-bridged species (111), this process being easily reversed by CO adsorption. Mo2+ Mo2+ '0 / \ / 0 0 The 12CO-13C0 experimental bands are broader than coincident with those calculated: this can be explained by those of l2C0, and a perturbation effect not due to the presence of two neighbouring structures of type (11). Furthermore, the presence of dimeric Mo species on well reduced Mo/SiO, systems is confirmed by the EXAFS results of I w a ~ a w a , ~ * l ~ who found reduced MoII pair structures on silica and aluminain which the average Mo-Mo distance was 3.12-3.25 A.which is the most probable surface phase obtained after reduction at 823 K, can give further support to the surface complexes proposed, [(11) and (III)]. The MOO, structure consists of Mo-0 distorted octahedra joined by corners and edges; in a string of octahedra joined by edges the Mo-Mo distances are alternately shorter and longer, i.e. 2.5 1 and 3.1 1 A, respectively, all other Mo-Mo distances being > 3.64 A. Thus the Mo-Mo bond of 2.51 A is the shortest yet found in metal oxides; this situation can ease the photoreduction process, with CO, elimination and the formation of coordinately unsaturated pairs of MoII ions. The u.v.-vis. reflectance spectra (fig. 7) give further support to this interpretation.From fig. 7(c)-(e) it would be rash to assign precisely the near-continuous broad absorption between 300 and 800 nm to definite structures (distorted octahedral or tetrahedral, etc.) for the reduced Mo ions. In fact this spectral region covers the absorption assigned to MoV, MoLV, MolI1 and also MoL1 monomeric or dimeric ions in various structural configurations.lob, 21* 22 Moreover, a strongly enhanced absorption over the whole visible region has been found to occur for the MoVOCli- dimeric An analysis of the structure ofE. GUGLIEMINOTTI AND E. GIAMELLO 2319 Nevertheless, the effects of H, reduction, photoreduction and evacuation on the u.v.-vis. spectrum are relevant. Reduction with H, lowers the intensity of the 275 nm band, which is usually assigned to a 0,- + MoV1 (tetrahedral) charge-transfer transition,s*24 and gives rise to a broad absorption between 320 and 700 nm.Photoreduction with CO causes the growth of a band (at 300-320nm) which is replaced by new maxima upon evacuation : this reversible modification of the u.v.-vis. spectrum, depending strictly upon the CO pressure, corresponds to that observed by i.r. spectroscopy. We therefore think that this spectroscopic behaviour is due to the modifications in the coordination state of the molybdenum occurring when a pair of neighbouring MoI1 passes from structure (11) to structure (111) (and uice uersa). In such a process the formation of a 'metallic-like' Mo-Mo bond, with a shortening of bond length, may occur, as found with MOO, and MoC~,,,~ and this can be favoured by a CO bridging.However, no firm conclusion about this possibility can be drawn on the basis of our results : for a full discussion concerning metal-metal bond formation in complexes containing bridged CO groups see ref. (26). The 2 bands, strictly correlated in intensity, can be assigned, by a simple vectorial calculation based on the experimental intensity ratio Ia/Is = 1, to two CO molecules linked to the same Mo ion and forming an angle 8 = 90". We can therefore suppose that the Mo ions are in an octahedral structure. A comparison of fig. 1 and 3-5 gives clear evidence of the extreme lability of the CO species associated with these bands and of their poor reproducibility. Very small variations in the preparation procedure can shift the equilibrium between this species and the X species clearly observed in fig. 4.However, the experiments reported in fig. 3 and 4 clearly show the transient nature of this species. The quantitative data show that only a small fraction of Mo is involved in the photoreduction process and that very small amounts of CO, (found in the liquid- nitrogen trap) are formed; because the presence of several reduced Mo species is indicated by the i.r., e.s.r. and u.v.-vis. data it is impossible to equate the quantitative data (obtained at the limits of detectability and affected by errors) to a definite reduction process. The Z bands can therefore be tentatively assigned from their frequencies and behaviour to a pair of CO molecules weakly adsorbed on MoIV [or MolI1] ions in an octahedral configuration which can be easily transformed, by CO, removal, into the MoII ions in the tetrahedral coordination of structure (11).The role played by the MoV ions produced during the photoreduction,process and detected by e.s.r. can be understood, at present, only in terms of intermediates in the reduction process. It is well known that only a small fraction of the total MoV concentration present is detected by e.s.r.,'~ 28 because of the strong interaction of dimeric MoV ions; a fraction of MoV can probably adsorb CO (as found by i.r. spectroscopy: vco = 2203 cm-l) and also contribute to the broad absorption at 400-500 nm.lob, 21 The photoreduction process in CO increases the intensity of the MoV e.s.r. signal and simultaneously causes a modification of the shape of the spectral envelope.The spectrum of fig. 8 shows in fact that, besides the axial signal obtained after thermal reduction in hydrogen (g,, = 1.883 and g l = 1.944), new components are present. These can be interpreted as arising from two new types of MoV ions in an axial environment formed upon photoreduction: the presence of a fraction of Mo5+ in an orthorhombic environment (such as that observed by Servinka et aZ.29 after room- temperature reduction of MOO, in atomic hydrogen can probably be ruled out on the basis of the differences between the spectrum in fig. 8 and that reported in ref. (29). The increase of the M O ~ ' concentration upon U.V. irradiation in the presence of CO2320 0 co co 0 o’ \/ \ o SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM co co ;o co ‘ 0 co f; ?;+ 7\ey2* - c o 2 o’ \ o / ‘ 0 + M o Mo 0 0 0 o/ 0 / \ 0 I I S i 0 0 Si Mo 0 0 ‘si ’ \4 / \ 0 co co 0 0 co co 0 \\ /!j+ /* 00, \\ // \\ [+ ’\ // 0’ ‘ 0 ’ ‘ 0 0 ‘0’ ‘ 0 M 04’ Mo M’P + MO / I 1 I I + H2,- HzO, 823K co S i Si Si S i v ,o=2203 cm-’ v c o = 2181 c m - ’ + c o c - c o 2 + c o ~ - c o , h v h v r 1 Si ‘ I I Si I si I S i I si vc0= 2 0 4 3 cm-’ Scheme 1.indicates that the room-temperature photoreduction is a complex process involving more than one reaction. We assign this increase to the formation of at least two monomeric Mo5+ (e.s.r.-active) species formed by cleavage of the dimeric coupled Mo5+ pairs present on reduced MOO,^^^ 29 and not detectable by e.s.r. spectroscopy because of their superexchange interaction.12 No direct evidence exists as to the nature of the coordination sphere of the new Mo5+ centres: taking into account that the photoreduction process is performed at a low temperature we suggest that the new species are MoV ions of lower coordination than those obtained by thermal reduction in H,.Finally, we observe that the quantitative data O,,,/Mo = 0.5 and the persistence of a band at ca. 280 nm after thermal reduction (fig. 7) demonstrate that MoVr is only partially reduced by H, and that only a fraction of this reduced Mo is photoreduced (probably from MoIV to Morl on the average) and can adsorb CO. CONCLUSIONS The results reported here show that several oxidation and coordination numbers of MoZ+ ions can occur after thermal and photoreduction of silica-supported MOO,.In these conditions it can be very difficult, as the discordant conclusions in theE. GUGLIEMINOTTI AND E. GIAMELLO 232 1 literature imply, to ascertain exactly which Mo ion structure is mainly present and therefore most important in catalytic reactions such as olefin metathesis. Some preliminary work carried out by us in using an i.r. cell at sub-atmospheric pressure with propene adsorbed on the reduced and photoreduced samples shows a very low activity for metathesis, in accord with previous ob~ervations~~ that highly reduced Mo ions have low metathesis activity. This paper, however, gives evidence of the existence inter alia of low-valent Mo ions (probably a MoII pair in a distorted tetrahedral configuration) obtained by CO photoreduction of an MoOJSiO, catalyst previously reduced by H,.This Morr pair can adsorb two ‘linear’ CO molecules (vco = 2128 and 2080 cm-l) at higher pressure but only one ‘bridged’ CO molecule (vco = 2043 cm-l) at lower pressures. According to Giordano and coworkerssb the tetrahedral silica support should favour tetrahedral Moz+ coordination, at least at low Mo loadings, as observed on In conclusion, the reduction and photoreduction pathways shown in scheme 1 can be proposed for silica-supported MOO, containing a surface dimeric molybdate species. The Mo-Mo distance can obviously change with coordination number and should be reduced with the lowering of the oxidation number of Mo and the formation of bridged carbonyls. Other reduction pathways leading from MoV to MorrJ cannot be excluded, but in our opinion the more realistic pathway obtained on the basis of the results presented in this paper is MoV1 -+ MorV -+ Morr + Moo.7-~1~0~.31 We are grateful to the referees for improving the English of our text. This research was partially supported by the Italian C.N.R., Progetto Finalizzato ‘Chimica Fine e Secondaria’. R. L. Banks and G. C. Bailey, Znd. Eng. Chem., 1964,3, 170. Ju. Yermakov, Catal. Rev., 1976, 13, 77. Y. Iwasawa and M. Yamagishi, J. Catal., 1983, 82, 373, and references therein. (a) J. Smith, R. F. Howe and D. A. Whan, J. Catal., 1974, 34, 191 and references therein; (b) A. Brenner and R. L. Burwell Jr, J. Catal., 1978, 52, 364; (c) A. Brenner, D. A. Hucul and S . J. Hardwich, Inorg. Chem., 1979, 18, 1478; (d) A. Kazusaka and R.F. Howe, J. Mol. Catal., 1980, 9, 183. a P. Gajardo, D. Pirotte, P. Grange and B. Delmon, J. Phys. Chem., 1979, 83, 1780; (6) A. Castellan, J. C. J. Bart, A. Vaghi and N. Giordano, J. Catal., 1976, 42, 162. ’I L. L. Van Reijen, W. M. H. Sachtler, P. Cossee and D. M. Brower, Proc. 3rd Znt. Congr. Catal., Amsterdam, 1964 (North-Holland, Amsterdam, 1965), vol. 2, p. 829. E. Guglielminotti, J . Mol. Catal., 1981, 13, 207. S. Abdo and R. F. Howe, J. Phys. Chem., 1983,87, 1713. lo (a) M. Che, J. McAteers and A. J. Tench, J. Chem. SOC., Faraday Trans. I , 1978,74,2378; (b) M. Che, F. Figueras, M. Forissier, J. McAteer, M. Perrin, J. L. Portefaix and H. Praliaud, Proc. 6th Znt. Congr. Catal., ed. G . C . Bond, C. F. Wells and F. C. Tompkins (The Chemical Society, London, 1976). M. Anpo, I. Tanahashi and Y. Kubokawa, J. Phys. Chem., 1982,86, 1. l2 (a) D. C. Koningsberger, Proc. 6th Znt. Congr. Catal., ed. G. C. Bond, C. F. Wells and F. C. Tompkins (The Chemical Society, London, 1976); (b) W. K. Hall, Proc. 4th Inf. Conf. Chem. Uses Molybdenum, ed. H. F. Barry and P. C. H. Mitchell (Ann Arbor, Michigan, 1982), p. 244. 2 B. N. Shelimov, A. N. Pershin and V. B. Kazansky, J. Catal., 1980, 64, 426. l 3 Y. Iwasawa, J. Nakano and S. Ogasawara, J. Chem. SOC., Faraday Trans. 1, 1978, 74, 2968. l4 J. B. Peri, J. Phys. Chem., 1982, 86, 1615. l5 F. A. Cotton and C. S. Kraihanzel, J. Am. Chem. SOC., 1962, 84, 4432. la A. Zecchina, E. Garrone, G. Ghiotti and S. Coluccia, J. Phys. Chem., 1975, 79, 972. l8 E. 0. Fisher, Chem. Ber., 1962, 95, 2491. B. Rebenstorf and R. Larsson, J. Mol. Catal., 1981, 11, 247.2322 SPECTROSCOPIC STUDY OF A Mo/SiO, SYSTEM l9 Y. Sato, Y. Iwasawa and H. Kuroda, Chem. Lett., 1982, 1101. 2o B. G. Brandt and A. C. Skapski, Acta Chem. Scand., 1967,21, 661. 21 (a) N. Giordano, A. Castellan, J. C. J. Bart, A. Vaghi and F. Campadelli, J. Catal., 1975, 37, 204. 22 (a) J. C . Sheldon, J. Chem. SOC., 1960, 1007; (b) R. J. H. Clark, J. Chem. Soc., 1964, 417; 23 G. P. Haight Jr, J. Znorg. Nucl. Chem., 1962, 24, 663. 24 H. Jeziorowski and H. Knozinger, J . Phys. Chem., 1979, 83, 1166. 25 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Wiley, New York, 3rd edn, 1971). 2o Faraday Symp. Chem. SOC., 1980, 14. 27 S. Abdo, R. B. Clarkson and W. K. Hall, J. Phys. Chem., 1976,80, 2431. 28 R. Fricke, W. Hanke and G. Ohlmann, J. Catal., 1983, 79, 1. 29 E. Serwicka and R. N. Schindler, Z. Phys. Chem., N.F., 1982, 133, 175. 31 D. S. Zingg, L. E. Makovsky, R. E. Tischer, F. R. Brown and D. M. Hercules, J. Phys. Chem., 1980, (c) T. R. Webb and T. Y . Dong, Znorg. Chem., 1982, 21, 114. B. N. Kuznetsov, A. N. Startsev and Yu. I. Yerrnakov, J. Mol. Catal., 1980, 8, 135. 84, 2898. (PAPER 4/ 1626)

ISSN:0300-9599    

DOI:10.1039/F19858102307

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J . c'hem. Soc., Furuduy Trans. I , 1985, 81, 2323-2331 Ultraviolet-Visible Spectropho tome tric Study of N-Alkylpyridinium Iodides in Non-aqueous Solvents Evidence for the Formation of Solvent-shared Ion-pairs BY MOHAN PAL AND SANJIB BAGCHI* Department of Chemistry, University of Burdwan, Burdwan 7 13 104, India Received 28th September, 1984 The u.v.-visible absorption spectra of solutions of various alkylpyridinium iodides have been studied at various temperatures and in binary-solvent mixtures. The results are interpreted as due to the existence of an equilibrium between the contact ion pair and solvent-shared ion pair in a solution in non-dissociating solvents. The band at ca. 42000 cm-' has been attributed to a modified charge-transfer-to-solvent transition of the iodide ion within a solvent-shared ion pair.The thermodynamic parameters (AGO, AH" and AS") for the interconversion of two forms of ion pairs have also been determined in various solvents. N-alkylpyridinium iodides (RPy+I-) provide a suitable system for the spectroscopic study of ion-pair formation with respect to the solvation characteristics of the ion pair. These compounds exist as ion pairs in solution, and the longest-wavelength band arises through to a charge-transfer (c.t.) transition within the contact ion The existence of another form of ion pair, viz. the solvent-separated ion pair, in the case of these compounds was invoked by Mackay and Poziomek4 to explain qualitatively the variation of the extinction coefficient of the visible c.t. band with solvent polarity. Recently we obtained indirect evidence for the presence of a solvent-shared ion pair (s.sh.i.p.) where one solvent molecule separates the pyridinium ion and the iodide However, u.v.-visible spectroscopy has yet to provide direct evidence for the existence of s.sh.i.p.in a solution of these salts. In the present work we have studied the u.v.-visible absorption of N-methylpyridinium iodide, N-methylpyrazinium iodide and N-ethyl- 4-cyanopyridinium iodide in various solvents and at various temperatures. The U.V. spectra of these compounds in non-dissociating solvents indicate the presence of a solvent-shared ion pair in equilibrium with a contact ion pair (c.i.p.). Thermodynamic parameters for the process of interconversion of the ion-pair subspecies have also been determined. EXPERIMENTAL MATERIALS The N-alkylpyridinium iodides were prepared by quaternising the corresponding N-substituted heteroaromatics with methyl or ethyl iodide in the dark.4.6 The purities of the compounds were checked by noting the melting points, the longest-wavelength band maxima (Amax) and the corresponding E values, as follows.N-Methylpyridinium iodide: found m.p. 118 "C (lit. 1 1 6 1 17 "C);' found Amax = 375 nm (lit. 374 nm); found E = 1200 dm3 mo1-l cm-' (1200 dm3 mol-l cm-l) in CHCI,. N-Ethyl-4-cyanopyridinium iodide: found m.p. 140 "C (lit. 140-141 "C);4 found Amax = 484 nm (lit. 482 nm);4 found E = 1210 dm3 mol-l cm-l (lit. 1200 dm3 mol-l ~ m - ' ) ~ in CH,CI,. N-Methylpyrazinium iodide: found m.p. 135 "C; A, = 470 nm; E = 2000 dm3 mo1-l cm-l in CH,Cl,. The purified and dried solvents were 2323u.v.-VISIBLE STUDY OF N-ALKYLPYRIDINIUM IODIDES j0 i50 300 '400 450 500 A/nm Fig. 1. Complete spectrum of N-methylpyrazinium iodide in dichloromethane at (I) 25 and (11) 45 "C. (111) N-Methylpyrazinium perchlorate in dichloromethane. ( a ) 0.1 cm cell; (b) 1 cm cell. distilled immediately before the experiment and all solvents were refluxed with calcium hydride immediately prior to use. This ensured the absence of peroxides. SPECTROPHOTOMETRIC MEASUREMENTS The experiments at room temperature were carried out in either a Beckman model 26 or a Cary model 17 spectrophotometer using stoppered cells placed in a thermostatted cell holder. Measurements at lower temperatures were made in the Cary model 17 instrument.The cell compartment in the low-temperature experiments consisted of an unsilvered quartz Dewar vessel into which nitrogen or oxygen gas was passed.6 The temperature was controlled to within & 1 "C by regulating the rate of flow of cold vapour. For temperature measurements a copper-constantan thermocouple was used; one junction of the thermocouple was inserted directly into the solution. The cell was stoppered and sealed with wax to prevent any absorption of moisture. This has been verified by checking the reproducibility of the spectrum at room temperature and at the end of each experiment. For spectral measurements in the U.V. region a cell of 1 mm pathlength was used. This was done to reduce the absorbance values and to nullify the effect, if any, of the absorption edge of the solvents.For measurements in the visible region the length of the cell was varied from 0.5 to 4 cm. The concentrations of the solutions were kept at ca. 10-3-10-4 mol dm-3. RESULTS AND DISCUSSION SPECTRAL CHARACTERISTICS PURE SOLVENTS The complete absorption spectra of N-methylpyrazinium iodide and perchlorate in dichloromethane are shown in fig. 1 . The spectral characteristics of the various alkylpyridinium iodides under study are summarised in table 1 . All the bands present in the case of dichloromethane and dichloroethane are found to obey Beer's law. TheM. PAL AND S. BAGCHI 2325 Table 1. Spectral characteristics of N-alkylpyridinium iodides in various solvents compound solvent N-methylpyrazinium iodide N-methylpyrazinium perchlorate N-et hyl-4-cyanopyridinium iodide N-ethyl-4-cyanopyridinium perchlora te N-methylpyridinium iodide N-methylpyridinium perchlorate dichloromethane dichloroethane ace toni trile dichloromethane dichloroethane acetonitrile dichloromethane dichloroethane acetonitrile dichloromethane dichloroethane acetonitrile dichloromet hane dichloroethane acetonitrile dichloromethane band maximum/nm group A group B group C 470 318 267 (273)" 238 475 320 268 (273)" 237 415 293 267 (273)" 246 _ _ 267 (273)" - _ _ 268 (273)" - - _ 267 (273)" - 485 327 275 (284)" 238 (235)" 490 330 275 (284)" 237 (235)" 420 295 276 (283)" 246 (235)" -.- 276 (284)" 234 _ - 276 (284)" 234 _ _ 276 (284)" 234 372 290 (254)" (259)" (264)" 241 375 290 (254)" (259)" (264)" 239 340 - (254)" (259)u (264)" 246 - _ (254)" 259 (264)" - a Shoulders obtained.band maxima have been collected in three groups. Group A corresponds to c.t. transitions within a contact ion 4 9 *, The band in group B is common to both the perchlorates and iodides and is presumably due to a n-n* transition of the cation.lou*b In group C the band at 246 nm in acetonitrile appears for all the alkylpyridinium iodides and is supposedly due to the charge-transfer-to-solvent (c.t.t.s.) transition of free iodide ions.llaTb Inspection of table 1 shows that in a non-dissociating solvent, e.g. dichloromethane or dichloroethane (where free iodides are not pre~ent),~? the iodides show an absorption at ca. 240 nm. The position of the band depends on the solvent and the nature of the cation.In dichloromethane or dichloroethane the intensity of band C increases (a simultaneous decrease in the intensity of the c.t. bands) with a decrease in temperature, although the bands position and shape remain unaltered (fig. 1). The process is reversible. At any temperature the relative heights of band C and the longest-wavelength c.t. band is independent of solute concentration; thus a solute-solute interaction is ruled out. MIXED SOLVENTS Studies in mixed solvents containing dichloromethane and cyclohexane at 300 K (the concentrations of the solute in various mixtures were kept constant) indicate that with a decrease in the mole fraction of dichloromethane the intensity of band C decreases. A simultaneous increase in the c.t. absorption is also noted. Experimental curves show an isosbestic point [fig.2(a) and (b)]. However, we could not vary the mole fraction of the non-polar component in the binary solvent mixtures over a wider range owing to the limited solubilities of the compounds in these solvents.U.V.-VISIBLE STUDY ! lr, 02- 04 0.1 ' 02 -._ I 220 250 300 350 A/nm OF N-ALKYLPYRIDINIUM IODIDES 30 250 280 310 A/nm Fig. 2. U.V. spectra of N-alkylpyridinium iodides in a mixed binary solvent (dichloromethane + cyclohexane) : (a) N-methylpyrazinium iodide and (b) N-methylpyridinium iodide. X, decreases in the order I > I1 > I11 > IV. Since free iodide ions are not present in these non-dissociating solvents the band at ca. 240 nm in these compounds is presumably due to an associated species. However, the high intensity of this band ( E c lo4 dm3 mol-1 cm-l) relative to that of the visible c.t.bands ( E M lo3 dm3 mo1-1 cm-l) excludes the possibility that the band originates through charge transfer within a contact ion pair. The work of Griffith et a1.l29 l3 shows that alkylammonium iodides in such solvents exist predominantly as solvent-shared ion-pairs characterised by similar bands. Thus our observations at different temperatures and in mixed binary solvents indicate that both s.sh.i.p. and c.i.p. exist in equilibrium in solution: where RPy+..-S..-I- is an s.sh.i.p. and its formation is favoured at lower temperatures. From the relative heights of the two bands (band C and the longest-wavelength c.t. band) at different temperatures one may evaluate AHo for the process, assuming the extinction coefficients to be independent of temperature. In the present case a value of AH" M - 2.00 kJ mol-l for N-methylpyrazinium iodide has been obtained.The role of cyclohexane (a non-polar solvent) is only to change the concentration of dichloromethane (the polar component), thus modifying the above equilibrium. The existence of solvent-shared species in equilibrium with the c.i.p. for these compounds in dichloroethane has also been reported by us in a recent comm~nication.~ In our case we have been able to detect spectrophotometrically two distinct forms of ion-pairs, while the N-alkylammonium iodides exist predominantly in one form,12 viz. s.sh.i.p. The ratio of the concentration of the solvent-shared ion pair to the contact ion pair (z.e.K, in dilute solutions) depends on the relative strength of the ion-solventM. PAL AND S. BAGCHI 2327 interaction and the cation-anion interaction. In the case of alkylammonium iodides the bulky alkyl groups probably hinder the approach of the iodide ion to the positively charged nitrogen, thus making the cation-anion interaction relatively weaker, leading to a very large value of K,; i.e. the concentration of c.1.p. in solution is too small. However, in our case the iodide ion may approach the positively charged pyridinium ring more Thus there is a greater possibility of the formation of c.i.p. in solution. Moreover, the existence of vacant n orbitals in the pyridinium ring makes it a good acceptor, so that a c.t. band is observed in the visible region for the c.i.p.This probably explains why we get both forms of ion pairs in detectable amounts in these solutions in non-dissociating solvents. NATURE OF THE TRANSITION Symons and coworker^^^,^^ have explained the origin of similar bands in alkyl- ammonium iodides as being due to a c.t.t.s. transition of the iodide ion within a solvent-shared ion pair. The high value of E and the dependence of the band position on the cation and the solvent (table 1) in our case also indicate the c.t.t.s. character in these transitions. Note that the c.t.t.s. band of the iodide ion in an s.sh.i.p. undergoes a hypsochromic shift with respect to the unperturbed c.t.t.s. band of the iodideion. The hypsochromic shift 0fthec.t.t.s. transition in the case ofalkylammonium iodides in the field of the cations has been explained in terms of an increased electrostatic stabilisation of the ground state of the ion pair.14 For spherical ions the interaction is expected to be a function of the distance between two charge centres; for alkylammonium iodides the shift is strongly dependent on the nature of the cation, shifting to lower energies with increasing cation size.12 If we assume a model for c.t.t.s.in which the transfer of charge does not take place centrosymmetrically, it is expected that the cation would not only interact with the iodide ion, stabilising its ground-state energy, but also interact with the solvent in the Franck-Condon excited state. While the former interaction produces a hypsochromic shift, any stabilising interaction between the solvent and the cation would lead to a bathochromic shift of the iodide c.t.t.s.transition. In the case of alkylammonium iodides the interaction with the excited states is probably constant and the band shows a cation dependence as depicted earlier. On the other hand, for alkylpyridinium iodides the charge distribution in the cation is far from spherical and both interactions are probably dependent on the nature of the cation, so no definite conclusions can be made. We offer here a qualitative explanation in terms of a modification of the energy levels of iodine atom (formed due to c.t.t.s.). The presence of the cation at close proximity will distort the solvent shell around the iodide ion from spherical to axial symmetry. In such a field the 2P3,2 level of the iodine atom will split into two doublets corresponding to mj values of f 3/2 and 1/2.Thus the c.t.t.s. band of the free iodide will split into two bands; the extent of the splitting will depend on the environment of the iodine atom. According to Mulliken's theory of charge transfer' the intensities of the bands will depend upon the overlap between the iodine orbital and the orbital which the electron enters. Nothing definite can be inferred, owing to the lack of a complete theory of c.t.t.s.16 It is possible that the intensities differ appreciably and that we detect only the hypsochromically shifted band experimentally. A similar explanation was invoked to explain the variation of energy difference between the two c.t. band maxima as a function of the cation in these compounds.8 The shift in the band maxima on varying the cation in the present work parallels the difference between the two c.t.band maxima in these compounds (table l), indicating that a similar mechanism may be operative in the two cases. Further work with other alkylpyridinium iodides is in progress.2328 U.V.-VISIBLE STUDY OF N-ALKYLPYRIDINIUM IODIDES THERMODYNAMIC PARAMETERS SPECTROPHOTOMETRIC PROCEDURE FOR THE DETERMINATION OF Ks AND THE EXTINCTION COEFFICIENTS (a) NON-DISSOCIATING SOLVENTS. The spectrophotometric evaluation of K, requires a knowledge ofthe molar extinction coefficients at the band maxima for the two ion-pair subspecies present in solution which are not determinable independently. For this reason we have determined Ks by a procedure similar to that described by us in the case of dissociating solvent^.^ The method consists of monitoring the visible c.t.band as a function of the composition of a binary solvent mixture with dichloromethane as one component. The other component is a non-polar solvent like cyclohexane (D = 2.023 at 20 "C) or tetrachloromethane ( D = 2.22 at 25 "C). These solvents do not take part in the formation of an s.sh.i.p. and only serve to vary the activity (a,) of the solvent taking part in s.sh.i.p. formation. Assuming that the activity coefficients of ion-pair species may be taken as unity and that the s.sh.i.p. does not absorb at the characteristic c.t. band of the c.i.p., we may write for the absorbance of a solution in non-dissociating solvents at the c.t.band maximum A = E,( 1 + K, as)-' Co 1 (2) where Co is the total concentration of the solute and E, is the molar extinction coefficient of the contact ion pair at the c.t. band maximum. For a particular solvent composition as is constant and thus Beer's law appears to apply. However, the extinction coefficient determined experimentally will be given by (3) eapp = E,( 1 + Ks as)-'. Eqn (3) may be rearranged to give 1 1 Ks - --+-aa, - Eapp Ec Ec (4) where as has been replaced by the mole fraction (Xs). A plot of against X , in the range 1 b X , b 0.5 is almost linear (fig. 3). (The limited solubility of these compounds in dichloromethane prevented us from taking readings in solutions where X, < 0.5.) Moreover, the nature of the straight line does not depend on the choice of the non-polar component, which again supports the assumption that the non-polar component does not form an s.sh.i.p. From the slope and the intercept of the straight line we may determine E , and K, separately.By a similar procedure the monitoring of the characteristic U.V. band due to an s.sh.i.p. might give the value of K, and E ~ , the molar extinction coefficient for the c.t.t.s. transition within the s.sh.i.p. However, the reduction of transparency of the solvent and the existence of neighbouring strong bands for the cations prevented us from making any quantitative measurements in the U.V. region. (b) DISSOCIATING SOLVENTS. In order to determine the value of capp in a solvent where free ions are also present, only the intensity of the visible band was monitored as a function of salt concentration.Use was made of the equation and capp = E,( 1 + K, as)-' was determined by a graphical m e t h ~ d . ~M. PAL AND S. BAGCHI 2329 0.0 0 7.00 6.0 0 P 9 2 5.0 C 4.0 0 3.0 C 1.0 0.'5 1.00 XS Fig. 3. Plot of l/capp against X,, the mole fraction of dichloromethane: (a) N-ethyl-4- cyanopyridinium iodide, (b) N-methylpyridinium iodide and (c) N-methylpyrazinium iodide ; 0, dichloromethane; + , cyclohexane; A, dichloromethane + tetrachloromethane and x , pure dichloromethane. Table 2. K , and E, values for various ion pairs in dichloromethane at 25 "C compound K , at 25 "C c,/drn3 mol-l cm-I N-methylpyrazinium iodide 0.5773 3150 N-methylpyridinium iodide 0.9238 2400 N-ethyl-4-cyanop yridinium iodide 1.001 2400 Table 2 gives values of K, and E , for various ion pairs in dichloromethane. The magnitude of K, for a particular ion pair does not depend on the wavelength region studied (within the c.t.band), indicating that the assumption that the s.sh.i.p. does not absorb at the c.t. band is correct. The value of K, is also of the same order of magnitude as that estimated by other procedure~.~9~~ The value of E , at the band2330 U.V.-VISIBLE STUDY OF N-ALKYLPYRIDINIUM IODIDES Table 3. AH" and AS" for process (1) in various solvents Eapp/dm3 AH" AS" compound solvent T/K mol-l cm-l /kJ mo1-l /J K mol-l N-methylpyrazinium iodide N-methylpyridinium iodide N-ethyl-4-cyanopyridinium iodide dichloromethane 269.7 288.0 298.0 acetone 298.0 303.0 313 ethanol 293.0 303.0 3 13.0 acetone 293.0 303.0 310.0 dichloromethane 283.0 288.0 298.0 acetone 288.5 298.0 303.5 309.7 2000 -5.0148 920 - 8.988 660 - 2.898 580 - 5.67 1200 - 7.469 833 877 - 3.032 - 2.1836 - 4.3025 - 0.069 -4.52 - 4.628 maximum is the same as that obtained for other solvent^,^-^^ so that the true molar extinction coefficients for the absorbing species seem to be relatively insensitive to solvent variation.The values of eapp in pure solvents, however, are dependent on temperature (table 3). This is readily understandable from the expression for E , ~ ~ , which contains K, in the denominator. We have not been able to determine Ks at various temperatures using the above procedure. On the other hand the temperature variation of cap4 in pure solvents may provide a quantitative measure of AH" and ASo in the following way.For pure solvents as = 1 and we have In the narrow temperature range employed in the present study E, may be assumed to be independent of temperature and one may calculate the value of K, at each temperature from the relation (7) using the experimental value of E,, assuming it to be independent of temperature. A plot of log K, against l / T (fig. 4) would then give AH" and AS". These values are listed in the table 3. Process (1) is exothermic, a fact also noted by others.18 The value of AH" obtained by this procedure is the same as that obtained from the variation in intensity of the bands with temperature. For a particular ion pair the magnitude of AHo for the formation of a solvent-shared ion pair increases in the order alcohol-shared pair > acetone-shared pair > dichloro- methane-shared pair, indicating that the stability of an s.sh.i.p.runs parallel with the polarity of the solvent. The magnitude of AHo for a particular solvent also depends on the nature of the ion pair. The negative value of ASo means greater ordering when an s.sh.i.p. is formed. According to the primitive model in which the solvent is represented as a continuum, the value of ASo should be zero since the continuum has no structure. However, theM. PAL AND S. BAGCHI 233 1 05 i i 1 1 i 3 3 2 3.4 3.6 3.8 103 K I T Fig. 4. Plot of log Ks against 1 / T for various alkylpyridinium iodides in different solvents: (a), (b) and (c) N-methylpyrazinium iodide in dichloromethane, acetone and ethanol, respectively; (d) N-methylpyridinium iodide in acetone; (e) and (f) N-ethyl-4-cyanopyridinium iodide in dichlorome t hane and ace tone, respectively .finite negative value of ASo observed in these cases may be thought of as mainly being due to a loss of translational and/or other degrees of freedom of the solvent molecule engaged in the formation of an s.sh.i.p. The magnitude of ASo also depends on the solvent and the nature of the ion pair, but no correlation can be made with the structure of the component at present. We are thankful to the referees for their valuable comments and suggestions and we thank Prof. Mihir Chowdhury of I.A.C.S. Calcutta for helpful discussions. M. P. thanks the Indian University Grants Commission, New Delhi, for a scholarship. I 2 3 3 5 6 7 8 9 10 1 1 12 I 3 14 15 16 l i IH R. S. Mulliken and W. B. Person, Molecular Complexes (Wiley Interscience, New York, 1969). E. M. Kosower, An Introduction to Physical Organic Chemistry (Wiley International Edition, New York, 1968). T. R. Griffiths and D. C . Pugh, J . Solution Chem., 1979, 8, 247. R. A. Mackay and E. J. Poziomek, J . Am. Chem. SOC., 1970,92, 2432. M. Pal and S. Bagchi. J . Cfiem. SOC., Farudq Truns. I . 1985, 81. 961. S. Bagchi and M. Chowdhury, J . Phys. Chem., 1976, 80, 21 1 1 . R. A. Mackay, J. R. Landolph and E. J. Poziomek, J . Am. Chem. SOC.. 1971,93, 5026. S . Bagchi and M. Chowdhury, J . Phys. Chem., 1979, 83, 629. J. W. Verhoeven, I. P. Dirkx and Th. J. De Boer, Tetrahedron, 1969, 25, 3395. a E. M. Kosower and J. A. Skorcz, J . Am. Chem. SOC., 1960,82, 2195. b E. M. Kosower, Molecular Biochemistry (McGraw-Hill, New York, 1962). a E. M. Kosower, R. L. Martin and V. W. Meloche, J . Chem. Phys., 1957,26, 1353. b M. Smith and M. C. R. Symons, Trans. Faraday SOC., 1958, 54, 338; 346. T. R. Griffiths and R. H. Wijayanayake, Trans. Furaday SOC.. 1970. 66, 1563. R. G. Anderson and M. C. R. Symons, Truns. Faraduj Soc.. 1969, 65, 2537. T. R. Griffiths and M. C. R. Symons, Mol. Phys., 1960. 3, 90. M. J. Blandamer, T. E. Cough and M. C. R. Symons. Trans. Furadu?* Soc., 1966, 62, 286. M. F. Fox and E. Hayon, J . Chem. Soc., Faruday Truns. I , 1977. 73. 1003. M. Pal and S. Ragchi, lnd. J . Chem.. 1984, 23A, 800. T. F. Hogen-Esch and J . Smid, J . Am. ChPrn. Soc.. 1966. 88, 307. (PAPER 4/1675)

ISSN:0300-9599    

DOI:10.1039/F19858102323

出版商:RSC    

年代:1985

数据来源: RSC

摘要:

J. Chem. Sac., Furaduy Trans. I , 1985, 81, 2333-2337 Measurements of Volume Changes on the Formation of Precipitates of Carbonates and Phosphates of Cadmium(I1) and Calcium(r1) in Aqueous Solutions BY MICHIHISA UEMOTO Department of Chemistry, Faculty of Science, Gakushuin University. Mejiro, Toshima-ku, Tokyo 17 I , Japan AND TAKUSEI HASHITAN* Faculty of General Education. Tokyo University of Agriculture and Technology, Fuchu-shi. Tokyo 183, Japan Received 23rd October, 1984 Changes in volume on the formation of CdCO,, CaCO,, Cd,(PO,), and Ca,(PO,), in aqueous solutions have been measured at 298.2 K. A dilatometer which has optically flat discs has been used for separating solutions. Values of the volume changes per mole of divalent metal ions have been determined. Volume changes on 'dehydration' have also been estimated from the measured data. The hydration of electrolytes in aqueous solutions can be studied by measuring partial molar and molal volumes.' Apparent molar volumes at infinite dilution have been determined and assigned to ionic values.These values have been interpreted quantitatively by reference to the intrinsic volumes of ions, ion-solvent interactions and the structure of water.* Here the volume changes were determined experimentally when hydrated water was liberated by chemical reactions (i.e. dehydration). The volume changes of the systems were measured when hydrated water of the cations and anions was liberated by the precipitation reaction for binary systems. The dilatometer used was designed by one of us3 and is characterised by optically flat discs for separating solutions.As it uses no mercury it is easier to use for electrolyte systems. EXPERIMENTAL THE DILATOMETER Fig. 1 ( a ) shows the dilatometer, which is made of Pyrex glass. In the centre of the vessel are discs A and B for separating the two solutions; the discs have optically flat surfaces. The outer edges of disc B, which has a central hole (ca. 1.3 cm in diameter), are fused to the wall of the dilatometer, and disc A is merely set on disc B. Each compartment of the dilatometer was calibrated with redistilled water, while the capillary was calibrated with mercury. The volumes of dilatometer compartments were 17-37 cm", and the inside diameters were 0.03-0.05 cm. The lengths of the capillaries were ca.40 cm. Before use, the distortion of disc B was tested. The constancy of the level and the absence of precipitate between the discs were examined while the two solutions were poured into and separated in the dilatometer. The method of filling was as follows: all solutions and solvent were previously outgassed and kept at 298.2 K. Solution I was poured into the vessel up to a level a few centimetres higher2334 VOLUME CHANGES ON PRECIPITATION Fig. 1. Dilatometer: A, B, optically flat discs for separating solutions; C, capillary; D, hole; E, groove; F, stirrer. than disc B. The two discs were not flush together at this time. Then the lower compartment of the dilatometer was immersed in a thermostat at 298.2 K. After thermal equilibration, disc A was gently placed flush onto disc B with tweezers.The solution in the upper compartment was sucked out with a syringe; the upper compartment was first washed with the solvent and then with solution 2. After this, solution 2 was injected and the vessel was stoppered to exhaust the excess of solution from the top of the capillary. To adjust the level of the solution in the capillary the stopper was turned so that groove E fitted into hole D, thus allowing the solution to flow out. The stopper has a ground-glass joint coated with sealant (e.g. petroleum). The stopper can be held tightly in place using rubber bands attached to glass hooks on the stopper and body. The method of mixing was as follows. The dilatometer was immersed in a thermostat and the level of the solution in the capillary was read with a cathetometer after thermal equilibration.Then the dilatometer was inverted, disconnecting the disc gravitationally [fig. 1 (b)], and the solutions were mixed. The dilatometer was returned with the two discs not flush, and the changed level in the capillary was read after stirring and thermal equilibration. K. The thermostat4(aca. 1 75-dm3 water bath) was maintained at 298.2 -+_ (3-4 x 1 0-4) K ; the temperature was monitored with Hewlett-Packard model 2801 A and 2804A quartz-oscillator thermometers. For such measurements, the temperature must be constant to better than 1 xM. UEMOTO AND T. HASHITANI 2335 Table 1. Volume changes on mixing for the system comprising aqueous solutions of M(NO,), and KnA (M = Cd2+ or Ca2+, A = C0:- or PO:-, n = 2 or 3), I = 0.1.V[M(NO,),I V(Kn A) A V A Vm/cm3 precipitate /cm3 /cm3 run /lop2 cm3 (MI1) mo1-I CdCO, 17.00 36.95 3 2.802 f 0.004" 50.2 18.88 34.76 1 3.232 49.9 CaCO, 17.00 36.95 4 3.091 &O.OlO" 56.5 18.88 34.76 1 3.573 56.8 Cd3(P04)2 17.00 36.95 3 3.141 +0.016" 56.3 Ca3(P04)2 17.00 36.95 3 3.066 f 0.007" 56.1 a Average deviations from the mean values. Aqueous solutions of cadmium(i1) nitrate and calcium(i1) nitrate at an ionic strength of 0.1 (0.033 mol drn-,) were poured into the lower compartment of the dilatometer, while aqueous solutions of potassium carbonate and potassium phosphate of the same ionic strengths (0.033 and 0.017 mol dm-3, respectively) were poured into the upper compartment. The volume of the upper compartment was ca. twice that of the lower, so potassium salts were present in excess after mixing.'The volume changes at the ionic strengths of 0.05 and 0.025 were also measured under the same conditions as described above in order to examine the ionic-strength dependence of the molar-volume changes. To examine the effect of hydrolyses of carbonate ions and phosphate ions, the solutions ( I = 0.1, K,CO, or K3P04 + I = 0.1 KOH, v/v = 4/ 1) were prepared as solutions of potassium salts under the conditions described above, where calcium hydroxide did not seem to precipitate. The values were compared with those at 'usual' conditions. In order to examine the effect of counter-ions in the systems, the volume change was also measured during a dilution of a potassium nitrate solution with an ionic strength of 0.1.MATERIALS A.R. cadmium(i1) nitrate, calcium(i1) nitrate and potassium carbonate were recrystallised from redistilled water. A.R. potassium phosphate and potassium nitrate were used without further purification. RESULTS AND DISCUSSION Volume changes on the formation of the precipitates are given in table 1. After mixing, the changed levels settled after a few hours in each system. The ionic-strength dependences of the volume changes are given in table 2; here volume changes per mole of divalent metal ions were almost constant within the experimental errors, over the range 0.025.4.1. The effect of hydrolysis of the carbonate and phosphate ions in the system containing CaCO, did not produce any significant difference; for the system containing Ca,(PO,), the volume change obtained by using the solution containing potassium hydroxide was ca.7% larger than when the latter was not present. Cadmium and calcium present in the supernatant solutions after mixing were analysed qualitatively with the chelating agent: they were not detectable. For normal mixing of binary solutions it is necessary to consider the effect of counter-ions (the potassium and nitrate ions in this case). However, the volume change on dilution of potassium nitrate solution ( I = 0. l), where the concentration of potassium nitrate after mixing was 0.054 mol dm-,, was extremely small2336 VOLUME CHANGES ON PRECIPITATION Table 2. Ionic-strength dependence of the volume changes ~M(N0,),]/cm3 = 17.00, V(K, A)/cm3 = 36.95 ionic strength precipitate 0.1 a 0.05 0.025 A v b A Vmc A v b A Vmc Avb A Vmc Avb A Vmc CdCO, 2.802 CaCO, 3.09 1 50.2 56.5 Cd3(P04)2 3.141 56.3 Ca,(P04), 3.066 56.1 1.441 51.6 58.0 55.6 56.1 1.586 1.552 1.533 0.748 0.789 0.775 0.758 53.6 57.7 55.5 55.5 a Taken from table 1.A V in the units cm3. AVm in the units cm3 (M") mol-l. Table 3. Calculations of the volume changes on dehydration from the observed molar volume changes ( I = 0.1) CdCO, 4.26 40.5 18.1 27.8 CaCO, 2.7 1 36.9 18.5 38.1 Cd,(PO,), 4.15 42.3 24.8 38.8 Ca 3 (PO 4 1 2 3.14 32.9 25.2 48.4 a Molar volumes obtained from the densities of precipitates. Molar volumes calculated from the ionic radii. In the units cm3 (M") mol-I. (- 0.18 cm3 mol-l) compared with those obtained on precipitation. Moreover, on precipitation, the volume change due to the counter-ions is expected to be smaller than this because of the lower extent of dilution.Thus the effect seems to be negligible in this case. Table 3 shows the procedure for calculating the volume changes on dehydration from the measured molar-volume changes ( I = 0.1). As precipitates have three- dimensional macroscopic structures, it is conceivable that a volume change occurs on precipitation not only on account of dehydration but also because of solidification. Differences between the molar volumes obtained from the densities of the precipitates5. and the molar volumes v2 calculated from crystallographic ionic radii7 and thermochemical ionic radii8* were subtracted from the observed molar-volume changes AV,. The molar volumes & were assumed to be the 'intrinstic' volumes of ions in the precipitates.The density of Cd,(PO,),, which could not be found in the literature, was measured using a Weld-type pyknometer.1° Consequently the last column in table 3 shows the volume changes on dehydration. If the volume change on dehydration is an additive property these values can beM. UEMOTO AND T. HASHITANI 2337 written as the sum of the corresponding values for the cation and anion as follows [eqn (1 j ( 6 ) in the units cm3 (MI1) molil] A Vm(Cd2+) + A Vm(COi-) = 27.8 AVm(Ca2+)+AVm(COi-) = 38.1 A Vm(Cd2+) +IA Vm(POz-) = 38.8 A Vm(Ca2+) +:A Vm(POi-) = 48.4. Subtracting eqn (1) from eqn (2) we get A V,(Ca2+) - A Vm(Cd2+) = 10.3 while subtracting eqn (3) from eqn (4) we get A Vm(Ca2+) - A Vm(Cd2+) = 9.6.The results of the same calculations for the data at ionic strengtlls of 0.05 anc 0.025 allows to make a first approximation that the volume change on dehydration is an additive property of this study. Further information is necessary when ionic-volume changes on dehydration are assigned. For systems containing phosphate salts it is probable that cadmium or calcium hydrogen phosphate is partly precipitated because of hydrolysis of the phosphate ions. Further investigations are necessary with regard to this point. However, it is interesting that Ca2+ and Cd2+, which have the same charge and almost the same ionic radii [r(Ca2+) = 1 .OO A; r(Cd2+) = 0.95 A],7 have markedly different values of the volume change on dehydration. The volume change on dehydration obtained in this study provides new information on the hydration of electrolytes and should be compared with limiting molar volumes and electrostriction volumes. We thank Dr S. Howell of Sophia University for help in preparing this manuscript. * F. J. Millero, Chem. Rev., 1971, 71, 147. J. W. Akitt, J. Chem. SOC., Faraday Trans. I , 1980, 76, 2259 and references therein. T. Hashitani, Bulletin of the Faculty of General Education, Tokyo University of Agriculture and Technology, 1978, 14, 84 (in Japanese). T. Hashitani, Bulletin of the Fuculty of General Education, Tokyo University of Agriculture and Technology, 1976, 12, 64 (in Japanese). Gmelins Handbuch der anorganischen Chemie, Nr. 33, Cadmium (Verlag-Chemie, Weinheim, 1925). Landolt-Bornstein Tabellen, II Band, I Ted, Mechanischthermische Zustandsgrossen (Springer-Verlag, Berlin, 1971). H. D. B. Jenkins and K. A. Thakur, J . Chem. Ed., 1979, 56, 576. A. F. Kapustinskii, Q. Rev. Chem. Soc., 1956, 283. Physical Chemistry (McGraw-Hill, New York, 7th edn, 1970), p. 98. ’ R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect B, 1969, 25, 925; 1970, 1046. lo F. Daniels, J. W. Williams, P. Bender, R. A. Alberty, C. D. Cornwell and J. E. Harriman, Experimental (PAPER 4/1812)

ISSN:0300-9599    

DOI:10.1039/F19858102333

出版商:RSC    

年代:1985

数据来源: RSC