Palladium alkane hydrogenolysis

Anderson and his co-workers examined the reactions of small alkanes mainly on platinum and palladium 45-48). Isobutane was isomerized to n-butane on platinum and on palladium, neopentane isomerized to isopentane on platinum, whereas other metals (including palladium) caused hydrogenolysis predominantly or exclusively. It was proposed that the slow step in the isomerization was the formation of a bridged intermediate (C) from an aory-triadsorbed species (A, B) (Fig. 11).1 Hiickel MO calculations based on this proposal suggested,... 

For example, maximum activity for alkane hydrogenolysis is to be found in Group 8 (Ru, Os) rather than in Group 9 or 10, because the hydrocarbon intermediates have to be multiply bonded to the surface, and metals in the later Groups have insufficient unpaired electrons for this purpose. Palladium is outstandingly the best metal for hydrogenating alkynes, but is of little use for hydrogenating aromatics. 

Figure 13.3. Compensation plot of Arrhenius parameters for alkane hydrogenolysis (mainly C2 to C4) on various platinum and palladium catalysts. 1 = Pt 2 = Pd.

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Catalytic hydrogenation of alkynes takes place in a stepwise manner, and both the alkene and the alkane can he isolated. Complete reduction of alkynes to the saturated compound is easily accomplished over platinum, palladium or Raney nickel. A complication which sometimes arises, particularly with platinum catalysts, is the hydrogenolysis of hydroxyl groups a- to the alkyne (propargylic hydroxyl groups) (7.15). 

Very little has been published on hydrogenolysis of the lower alkanes using either nickel or palladium ° catalysts, perhaps because both are liable to suffer rapid deactivation by carbon deposits, or to show unstable behaviour. With -butane, both gave predominantly terminal C—C bond fission at low conversion, but there are no detailed kinetic studies to report. 

We turn now to the metals that are more active than palladium and platinum for the reactions of alkanes with hydrogen. Rhodium catalysts have been the subject of a number of investigations they were active for n-hexane hydrogenolysis between about 420 and 500 K, and were characterised by giving... 

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