Combined Experimental and Theoretical Investigation on the Selectivities of Ag, Au, and Pt Catalysts for Hydrogenation of Crotonaldehyde

A parallel study on silver, gold, and platinum catalysts with inert silica support is conducted both experimentally and theoretically for the liquid-phase hydrogenation of crotonaldehyde (Me-CH=CH-CH=O). We find that the silver catalyst exhibits a uniquely high selectivity toward C=O hydrogenation and the selectivity remains constant even at the conversion close to 100%. The gold catalyst, however, shows only a moderate selectivity whereas the platinum catalyst has a rather poor selectivity. Such variation in selectivity is interpreted in terms of the varied adsorption geometries of the crotonaldehyde on different metals. According to our density functional calculations of the chemisorptions of crotonaldehyde on selected M-19 (M = Ag, Au, and Pt) model clusters and M(111) surface, the most favored adsorption mode for silver is the C=O oxygen atom being a-bonded on low-coordinated silver atoms, which results in activation of the C=O bond. In contrast, so-called eta(4) and di-sigma(C=C) modes are preferred on Pt surface, while a pi(C=C) adsorption mode is favored on low-coordinated gold atoms, which leads to the preference of C=C hydrogenation. Moreover, the calculations indicate that the selectivity to C=O hydrogenation is more favored on smaller silver nanoparticles. This implication has been further corroborated experimentally by investigation of silver catalysts with different particle sizes.