Effect of adsorption geometry on the selectivity of carbonyl group hydrogenation on Pd/SiO₂

The gas-phase acetophenone hydrogenation reaction on Pd/SiO₂ catalysts has been modelled to reveal the relationship between the adsorption geometry and the selectivity of carbonyl group hydrogenation. The mechanism of the adsorption configuration effect on the hydrogenation selectivity is discussed. The key to the hydrogenation selectivity of a ketone is related to the bonding geometry of the carbonyl group. This has been demonstrated via product yield, temperature effects, hydrogen-concentration effects, presorbed-oxygen effects and infrared spectroscopy. The η¹-carbonyl ('end-on' coordination) can be hydrogenated to a hydroxy group, or be hydrogenated directly to a methylene group. The latter process can be enhanced by electron-withdrawing promoters. The η²-carbonyl ('side-on' coordination) tends to be hydrogenated to a hydroxy group that can be further reduced to a methylene group via a consecutive reaction pathway.