Platinum Metals Review

UK ISSN 0032-1 400 PLATINUM METALS REVIEW A quarterly survey of research on the platinum metals and of developments in their application in industry VOL. 42 OCTOBER 1998 NO. 4 Contents Platinum Metals Review and the Internet 134 Biphasic Homogeneous Catalysis 135 By Paul 3. Dyson, David J. Ellis and Thomas Welton Progress in Dye-Sensitised Photovoltaics 140 By R. 3. Potter Formation and Decomposition of Palladium Hydride Particles 141 By P. D. Cobhn, B. E. Nieuwenhuys, V. V. Gorodetskii and V. N. Pamzon Carbon Monoxide Sensing Technology 144 By Gavin Troughwn Platinum Labware Catalog 144 Aqueous-Organic Biphasic Catalysis 145 By Paul 3. Dyson The Build-Up of Bimetallic Transition Metal Clusters 146 By Paul R. Raithby Construction of Miniature Organo-Rhodium Boxes 157 Conferences Report Progress in Catalysis 158 By C. F. 3. Barnard and W. Weston; K. E. Simons and A. F. Chafley Combinatorial Chemistry Identifies Fuel Cell Catalyst 163 Catalysts for Butane Reforming in Zirconia Fuel Cells 164 By K. Kendall and D. S. Williams Geoffrey Wilkinson and Platinum Metals Chemistry 168 By M. L. H. Green and W. P. GnfSlth Abstracts 174 New Patents 179 Indexes to Volume 42 183 Communications should be addressed to The Edizor, Susan V. Ashton, Platinum Metals Rev& Johnson Matthey Public Limited Company, Hatton Garden, London ECl N 8EE PLATINUM METALS REVIEW AND THE INTERNET Johnson Matthey is pleased to announce that a full text online version of "Platinum Metals Review" can now be accessed on the Internet. The electronic version of the journal is accessible without charge, as is the printed version. The journal is being hosted on the website of the Royal Society of Chemistry's Turpin Distribution Services Limited, TOPS, at http://www.turpin-distribution.com.It may also be accessed from the "Platinum Metals Review" button on the Johnson Matthey website, at http://www.matthey.com, which is linked to the Turpin website. To read the journal it will first be necessary to download the free Adobe@Acrobat@ Reader software held on the Turpin site. At present, besides the October 1998 issue, the full texts of the April 1998 and July 1998 issues of "Platinum Metals Review'' are also available on the Internet, and the website will archive all future issues of the journal as they are published. In some areas of the world the electronic version of the journal may be available before the printed version is delivered. The electronic journal and the printed journal are identical, providing our coverage of papers, conference reports, book reviews, small items and reports, and the Abstracts and Patents on the science and technology of the platinum group metals. The 1998 Name and Subject Indexes will also be accessible for downloading. The website offers readers the facility of searching with the MUSCAT@search engine using natural language: by subject, across titles and the full text, and by names, which OCCUT as authors of papers or as part of the text or the references. Patentees, abstract authors and journals cited can also be searched across all the issues of the electronically held journal. At present the documents retrieved are ranked in order of relevance to the words used in the search query. More facilities will become available as the Turpin website is developed; for instance, readers will shortly be able to register for E-mail alerting to advise them that a new issue of the journal is available. In addition to access to the electronic version of "Platinum Metals Review", the Johnson Matthey website also carries a leaflet for each issue of the journal. The leaflets, which were first published on the Johnson Matthey website in April 1997, outline the contents of the journal and carry html links to the websites of the authors, to their places of work or institutions, to topics related to the papers and to background information and other relevant items. The Johnson Matthey website also carries an E-mail form to enable readers to pass on comments or suggestions about the journal to the editor. Susan V. Ashton EDITOR Platinum Metals Rev., 1998,42, (4), 134 134 Biphasic Homogeneous Catalysis By Paul J. Dyson, David J. Ellis and Thomas Welton Department of Chemistry, Imperial College of Science, Technology and Medicine. London Biphasic catalysis is becoming an area of environmentally responsible catalysis, but its development and use have until recently been somewhat neglerted. Here, the basic principles and the design of features going into such systems are explained, and ageneral overview is presented with the intention of encour- aging greater interest in this under utilised technique. Some well-established aqueous-organic regimes are described and there is a discussion of some possible future directions involving ionic-liquidlorganic systems. There are many benefits to be gained by using then be performed as shown in Figure 1. Here homogeneous catalysis in place of heterogeneous the catalyst resides in solution in one of the two catalysis in organic synthesis, the most notable phases and the substrate resides in the other being the use of less aggressive reaction condi- phase. During reactions, the two layers are vig- tions and increased selectivity. orously stirred, thus allowing suitable interac- The main disadvantages of traditional organic tion of catalyst and substrate. Once the reaction phase reactions employing homogeneous tran- has reached the appropriate stage, the stirring sition metal catalysts are the difficulties asso- is stopped and the mixture of phases separates ciated with separating the catalyst ii-om the prod- into two layers, one containing the product and uct and solvent. Separation techniques, such as the other containing the catalyst. Separation distillation, require an extra expenditure of of the two is then carried out by simple decanta- energy and can, in certain instances, lead to tion and, in principle, the catalyst solution is degradation of both the products and the cat- available for immediate reuse. Clearly, these alyst used. As the catalyst requires extraction biphasic reactions offer a potential answer to before a new reaction run can be performed, the problems mentioned above. the ‘turn around time’ between runs also This type of approach was first used com- becomes a prime factor. mercially for the polymerisation of ethylene These problems coupled with the inevitable (Shell Higher Olefins Process (1)) although in loss of the catalyst species (allowing for some this case the catalyst and substrate are initially imperfection in the separation techniques in a single phase and the product forms the sec- employed) tend to redress the balance between ond, immiscible phase; the principal, however, heterogeneous and homogeneous catalysis. is the same. Clearly, this approach is not One possible solution to these problems is to suitable for many other processes and has thus heterogenise the catalyst and product into two lead to the selection of water as the preferred separate and immiscible phases. Reactions may catalyst solvent for biphasic conditions. Fig.Fig. 11 AA schematicschematic representation of a two- phase process showing how Reactant Solution Product solution thethe initial initial reactant reactant solution solution andand productproduct solutionsolution areare immiscible with the catalyst Catalyst SoIuticn Catalyst solution solutionsolution Platinum Metals Rev., 1998, 42, (4), 135-140 135 SOjNa s09" Fig. 2 Two water-soluble phosphines: (a) triphenylphosphine mono-sulfonate and (b) triphenylphosphine ~ p~ tri-sulfonate.The latter phosphine has a solubility of 1.1 kg I-' and thus has been used extensively / / SOjNa The selection of water is straightforward and polar groups onto the phosphine substituent. offers many benefits. First, wide ranges of In this respect, one of the most widely used organic solvents are immiscible with water; water groups is a sulfate (SO,') group which can be is cheap, easily purified, and readily obtained attached to the phenyl rings in PPh,. These and disposed of. However, despite the many represent, at the current time, the most widely advantages offered by aqueouslorganic bipha- commercially exploited ligand system. sic systems, the level of commercial exploita- The first of these sulfonated phosphines, triph- tion is still relatively low. This is probably due, enylphosphine mono-sulfonate (TPPMS, see at least in part, to a lack of suitable water-sol- Figure 2(a)) was reported as early as 1958 and uble catalysts. However, having said this, inter- was produced by the oleum sulfonation of triph- est in fundamental research has escalated rapidly enylphosphine (9). Modification of this syn- in recent years. There have been a number of thesis led to the production of the tri-sulfonated reviews published (2-6), for instance, an entire ligand (TPPTS, see Figure 2(b)) which is now volume of the Journal of Molecular Catalysis, with the most common ligand in use (10, 11). The an excellent editorial dedicated to the subject tri-sulfonated ligand has an extremely high water (7), and a .recent book, reviewed here on page solubility of ca. 1.1 kg 1.' (2). Formation of the 145, which examines aqueous phase catalysts catalyst complex is then carried out by co-ordi- from an industrial perspective (8). nation of the sulfonated phosphine ligand. Although direct sulfonation of pre-complexed Sulfonated Phosphines triphenylphosphine ligands should be possible, and Their Industrial Use the extremely acidic conditions needed to effect The main consideration when attempting to the change make the process unreliable. design a water-soluble complex is how to ren- Concentrating for the moment on the tri-sul- der hydrophilic a typical hydrophobic organo- fonated ligands, a whole range of water-soluble metallic complex. In order to do this, an appro- catalysts based on monometallic and cluster priate ligand (or ligands) must be placed around compounds has been reported, and a review by the metal centre (or centres); alternatively, ionic Kalck and Monteil includes a comprehensive catalysts, such as Dipamp Rh(cod)+ and list of these compounds (3).

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