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Catalysts Play a Major Role in Development by A Catalysts Play a Major Role in Development By A. E Chiffey Chemicals Dewlnprnent, Precious Metals Dix ision, .joli~isniiWatt he?, Rovston The second Anglo-Dutch Symposium on the key reaction stages of oxidative insertion, Catalysis and Organometallic Chemistry was hydrogen activation and the breaking of two held on the 26th September 1997, in carbon-sulfur bonds. The reactions have so Amsterdam, The Netherlands. The symposium far been performed under hydrogen pressures was attended by more than 80 participants fi-om of 20 atmospheres and at a temperature of 100°C industry and universities in Britain and The for 24 hours. While results from these systems Netherlands. A series of lectures was given on have been encouraging and should help in catalysis and its applications, and there were the development of the next generation of more than thirty poster presentations. This hydrodesulfurisation catalysts, scaling these reac- report highlights some aspects involving the tions to tonnage quantities is as yet impractical. platinum group metals. Catalysts with Bite Towards Cleaner Fuels In chelate complexes with bidentate ligands Sulfur compounds, which occur naturally in the angle between the two donor atoms of the crude oil, give the pollutant sulfur dioxide when bidentate ligand and the metal centre is termed burnt. Legislation, particularly in the U.S.A., is the ‘bite angle’. The importance of the bite angle forcing the maximum sulfur limits in fuels to in a series of complexes, was reported by become progressively lower. This requirement, Professor P. W. N. M. van Leeuwen of the to produce more environmentally acceptable University of Amsterdam, see I1 and 111. fuels, has lead Professor P. M. Maitlis of the University of Sheffeld to investigate metal H H catalysed hydrodesulfurisation reactions. Conventional methods of hydrodesulfurisation involve passing hydrogen with the fuel over Co/Mo/S or Ni/W/S catalysts at 300°C. However, in crude oil, alkyl substituted sulfur heterocycles, such as dimethyl dibenzothiophene (I), below, are, for steric reasons, more resis- Distortions of this angle can affect reaction prod- tant to attack in these systems. ucts, as was shown for the rhodium catalysed hydroformylation of alkenes. In this reaction, catalyst (11) with the phosphine ligand in the trigonal plane, gives a higher ratio of I I ,Q Dibenzothiophene (DBT) 1inear:branchedproducts than isomer (111). (1) R=Me The size of the bite angle can be tailored by altering the length of the backbone between the Professor Maitlis has shown that certain donor atoms. Longer backbones tend to give platinum catalysts have significant activity for larger angles, which reduce the strain in the removing dibenzothiophenes from crude oils. chelate ring. Unfortunately, backbones which These catalysts, containing triethylphosphine are longer than [C5] tend to give ligand or diphenylphosphinoethane, are involved in bridged bimetallic compounds or (Rh-C) bound Platinum Metals Rev., 1998, 42, (l), 25-26 25 organometallics. Ligands can be designed with phosphine, PH,, see Equation (i). Palladium rigid backbones resulting in greater control in complexes containing cage phosphine ligands the chelate structure. have shown good activity for catalysing hydro- A range of diphosphines of the type (IV) has carboxylation reactions. been studied to investigate the effect of the bite Similar ligands have been designed to give angle on product selectivity. enhanced performance, by changing the groups bound to the phosphorus. Rhodium phosphite catalysts, [Rhl-P(OR),, have been shown to be 200 times faster than the corresponding rhodium phosphine catalysts, [RhI-PR,, in cer- Ph2P PPh2 tain hydroformylation reactions. Unfortunately (IV) the (P-OR) bond in phosphites is susceptible to X = H, S, C(CH,)., SiMe: hydrolysis. Hence the search for more robust ligands has become an active area, and ligands Rhodium complexes containing these ligands such as biaryl diphosphites may be the answer. have bite angles between 102 and 112". When Binaphthite for example, shows good hydrolytic these are used in hydroformylation reactions the stability in tests when heated in acetone under 1inear:branched product ratio varies, the opti- reflux conditions. Once co-ordinated with met- mum performance being at a bite angle of 109". als, it has a greatly extended half-life (the half- Deviations in the angle on either side of 109" life being the time when the catalyst decom- gave both poorer yields and selectivities for the poses (hydrolyses) to leave only half the active linear product. There are also secondary effects species). The same can be said of a range of which should be considered with these ligands. novel water soluble phosphites. Different substituents at X can contribute elec- Putting the phosphorus donor in a crowded tron donating or withdrawing properties, which cage environment offers stabilisation and greater are known to modify the n:im product ratio. protection against hydrolysis. These phosphite However, the overall effect of changing the bite containing materials have a combination of steric angle is only small, giving just percentage protection and exposure of the active sites that changes in product yields, but for commercial is necessary for successful catalysis. Research is processes the impact could be dramatic. continuing to improve the synthesis of these ligands and to extend the range of structures Ligands Designed for Catalysis that are available. The discussion of the relationship between structure and activity of catalyst was continued Looking Ahead by P. Pringle of the University of Bristol, who A little of the interesting work being carried out described the design of ligands which would put by both British and Dutch research groups has metals into unusual environments. been highlighted here. Some of this work may well contribute to the development of industrial processes. With the close collaboration that is taking place between institutions on both sides of the North Sea the third Anglo-Dutch symposium, for which the date is yet to be confirmed, should certainly have much to look forward to. The third symposium is expected to Such ligands can modify the catalytic proper- take place in the U.K. in September 1998 and the ties of a complex by, for example, introducing organising chairman &om whom information may large steric bulk. Cage phosphine ligands have be obtained will be Professor Peter Maitlis of been prepared for this purpose from the simple She5eld University, Fax: +44-(0) 141-273-8673. Platintini Metals Rev., 1998, 42, (1) 26 .
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