SELECTIVITY ENHANCEMENT IN THE HYDROGENATION OF ALPHA,BETA-UNSATURATED ALDEHYDES AND KETONES USING THIOPHENE-MODIFIED CATALYSTS

The hydrogenation of a series of alpha,beta-unsaturated aldehydes and ketones has been studied over 5% Cu/Al2O3 and thiophene-modified 5% Cu/Al2O3 catalysts. For a series of alpha,beta-unsaturated aldehydes with increasing methyl group substitution of the carbon-carbon double bond, acrolein, crotonaldehyde, 2-methylbut-2-enal, and 3-methylbut-2-enal, in the absence of thiophene modification, the reaction products are found to be mainly the corresponding alcohol, i.e., the product of total hydrogenation, or the aldehyde, i.e., the product of carbon-carbon double bond hydrogenation. Following treatment with thiophene (1 mul/0.1 g 5% Cu/Al2O3), a significant enhancement in the rate of unsaturated alcohol formation is observed, which is particularly pronounced for acrolein and crotonaldehyde. Evidence is provided to demonstrate that the effect is caused by an enhancement in the rate of unsaturated alcohol formation and not by a decrease in the rate of hydrogenation of this species. Similar experiments with alpha,beta-unsaturated ketones do not give the same enhancement in the synthesis of the unsaturated alcohol. The catalytic observations are combined with the results of FTIR spectroscopic characterisation of thiophene-treated and untreated Cu/Al2O3 using carbon monoxide as a probe molecule. The origin of the enhancement in selectivity is discussed in terms of geometric and electronic effects. (C) 1994 Academic Press, Inc.