Heterogeneous catalysis in organic chemistry. 3. Competitive adsorption of solvents during alkene hydrogenations

A single turnover reaction sequence has been used to determine the extent to which solvent can compete for adsorption sites in the hydrogenation of alkenes over Pt and Pd catalysts. Both methanol and ethanol are adsorbed readily on both Pt and active Pd catalysts converting the /sup 3/M alkene saturation sites into /sup 2/M isomerization sites and original /sup 2/M sites into unreactive /sup 1/M sites. tert-Butyl alcohol interacts only slightly with Pt catalysts presumably because of steric factors, but such restraints are not exhibited with Pd species. Over Pd the tert-butyl alcohol has adsorption characteristics similar to ethanol. 2-Propanol, on the other hand, enters into the reaction as a hydrogen donor in a transfer hydrogenation. THF (tetrahydrofuran) and acetic acid are also adsorbed on the metal surface, but with THF there is little if any increase in isomerization, only a decrease in saturation capability. Acetic acid lies somewhere between THF and methanol. Other solvents such as ethyl acetate, ether, and acetone have little, if any, effect on catalyst activity. 12 references, 2 tables.