Homogeneous Catalyst for Alkane Dehydrogenation AN IRIDIUM CATALYST NEEDING NO HYDROGEN RECEPTOR By David T. Thompson Consulting Chemist, Reading, England The removal of hydrogen from alkanes to give long-term stability for temperatures as high as alkenes is an important commercial objective, 200°C. This catalyst was subsequently demon- as alkenes are widely used as organic feedstocks strated to be useful for the catalytic transfer dehy- in industrial processes involving chemical syn- drogenation of cycloalkanes to arenes (3). For thesis and polymerisation. This reaction is, how- example, cyclohexane was dehydrogenated to ever, significantly endothermic, needing up to cyclohexene and benzene in the presence of 0 30 kcal per mole, and normally occurring at tem- and ten-butylethylene, the latter being converted peratures exceeding 400°C in the presence of almost quantitativelyinto reen-butylethane. This heterogeneous supported metal catalyst systems special reactivity is ascribed to the presence of in the gas phase. The conversion of alkanes to the P-C-P ligand, which renders the metal cen- aromatic molecules (as part of 'catalytic reform- tre reactive with saturated hydrocarbons, but ing') is even more important and is also effected restricts its access to the ligand P-C bonds. It using supported metal catalysts, such as plat- was claimed that the P-C-P pincer complex could inum on high surface area alumina. For these promote accessibilityto previously unattainable kinds of reactions to be achieved with a homo- catalytic pathways. geneous catalyst in solution, under milder con- ditions, the use of hydrogen acceptors has been thought essential; that is, the conversions have been achieved via 'transfer hydrogenation'. A possible alternative means for achieving the dehy- \ C(CHj) 3 drogenation reaction would be irradiation with UV light, but this is not usually industrially In collaborative research between members of attractive. the Chemistry Departments of Rutgers, New In early work, R. H. Crabtree and co-workers Jersey, the University of Hawaii and the University (1) used IrH, { O,CCF,} (PPr'& to effect the of California, Santa Barbara, Goldman, Jensen, dehydrogenation of alkanes to arenes via oxida- Kaska and co-workers (4) have now shown that tive addition at 150°C in the presence of the cycloalkanes can be efficiently dehydrogenated hydrogen acceptor, ten-butylethylene. However, using (I) as catalyst to give the corresponding it was thought that this system was not catalytic cycloalkene and dihydrogen, without using a because hydrogenolysis of the phosphine P-C hydrogen acceptor; and several hundred mol bonds occurs at temperatures of 135°C or above: product are produced per mol catalyst. However, the temperatures needed to release arenes from after several hundred catalytic cycles, conver- the intermediate complexes. sion decreases, and this is thought to be due to The five-co-ordinate iridium P-C-P pincer the alkene product increasingly inhibiting the complex (PCP)IrH, 0,where PCP = q3-C0H3- catalyst by co-ordination to catalyst sites, as its [CH,P(m-CJ19)2]2-1,3,was reported by Jensen, concentration increases. Kaska and their colleagues (2)to be a highly In principle, the temperatures used are suffi- active homogeneous catalyst for the transfer ciently high to overcome the large positive dehydrogenation of cyclooctane, with unusually enthalpy of dehydrogenation without the use of Platinum Metals Rev., 1998,42, (2),71-72 71 a hydrogen acceptor. The dehydrogenations the supply of hydrogen to the fuel cells used in reported are the first efficient homogeneous electric cars. When all the cycloalkanes have dehydrogenationsof alkane not using light irra- been dehydrogenated, the arene products could diation (5) or a sacrificial hydrogen acceptor. be ‘refuelled’ by re-hydrogenation &om an exter- When refluxing a cyclooctane solution of 0, nal hydrogen source. The idea of using alka- with a stream of argon passing over the con- nes as a hydrogen storage source is not new, but denser, the initial rate of cyclooctene formation until now there has been no means of dehy- is 1 1 turnovers h-I. After 44 hours, 104 turnovers drogenating them at moderate temperatures. are obtained and after 120 hours, 190 turnovers. References These results should be compared with those R. H. Crabtree, C. P. Parnell and R. J. Uriate, from IrH,(02CR)(PCyJ2,the most active solu- Organometallics, 1987, 6, 696 ble ‘acceptorless’dehydrogenation catalyst pre- M. Gupta, C. Hagen, W. C. Kaska, R. Flesher and C. M. Jensen,J. Chem. SOC.,Chem. Commun., viously investigated. This was reported to give 1996,2083 1.4 1 turnovers h-l cyclooctene, with a maximum M. Gupta, C. Hagen, W. C. Kaska, R E. herand of 28.5 turnovers obtained after 48 hours (6,7). C. M. Jensen,J. Am. Chem. Soc., 1997,119, (4), 840 The potential for industrial applications of the W.-W. Xu, G. P. Rosini, M. Gupta, C. M. Jensen, W. C. Kaska, K. Km&-Jespe~nand A. S. Goldman, present iridium catalyst system requires further 3. Chem. Soc., Chem. Commun., 1997, (23), 2273 development, and the chemistry reported to- M. J. Burk, R. H. Crabtree and D. V. McGrath, date needs extending and evaluating (8).It will J. Chem. SOC.,Chem. Commun., 1985, 1829 T. Aoki and R. H. Crabtree, Organometallics, 1993, be necessary to increase the rates achieved so 12, 294 far by two to three orders of magnitude and T. Fujii, Y. Higashino and Y. Saito,J. Chem. SOC., improve efficiency. As indicated above, con- Dalton Trans., 1993, 517 version drops after several hundred turnovers, M. Roubi, Chem. Eng. News., Dec. 8, 1997, p.25 probably because the olefinic product bonds to the vacant co-ordination sites on the catalyst. Catalysis Technical Guide Since the reaction slows down when the prod- Johnson Matthey Chemicals North America uct alkene concentration reaches 5 to 10 per has just published a “Catalysis Technical Guide”, cent, continuous removal of product by distil- containing information on heterogeneous and lation or by some other means may be possible, homogeneous catalysis using the platinum metals. so that catalyst activity is maintained. The cat- The “HeterogeneousCatalysis” section begins with a concise description of the critical factors alyst also needs evaluating with linear alkanes, to be considered in the design and selection of since the biggest commercial rewards lie here. the most appropriate catalyst for each applica- For instance, cyclooctane has a dehydrogena- tion. Commercially significant hydrogenations tion enthalpy of only 23.3 kcal mole ’ but alka- using powdered and particulate catalysts are nes typically require about 28 to 30 kcal mole described individually and specific catalysts are recommended. Hydrogenation of aromatic rings, for the elimination of hydrogen from two sec- aldehydes, ketones, nitro compounds and ben- ondary C-H bonds (4). The ability to promote zyl protecting groups are only a few examples the conversion of alkenes or cycloalkenes to aro- of the reactions described. matics by direct dehydrogenation in the absence The “Homogeneous Catalysis” section begins of a sacrificial hydrogen acceptor, would also with a brief introduction to the concepts and ter- minology, key to understanding these processes. constitute a significant advance. Among the synthetic reactions individually As the iridium system (I) catalyses the hydro- described are hydrogenation, isomerisation, car- genation of arenes to cycloalkanes, as well as bonylation, Heck coupling and allylic alkylation. the reverse reaction, the potential of these A section is devoted to the increasingly critical systems for hydrogen storage could be explored. area of chiral homogeneous catalysis. Each section has guidelines for the safe hand- By coupling the dehydrogenation and hydro- ling of catalysts. For more information contact: genation reactions, hydrogen could be cyclically Johnson Matthey, telephone: (+1)(609) 3847000 stored and released, for applications such as or (+1)(800) 4441411; fax: (+1)(609) 3847282. Platinum Metals Rev., 1998,42, (2) 72 .