Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)

Abstract

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions.The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO 2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products.Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall twostep process: crotonaldehyde crotyl alcohol butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

Mechanistic Insights into the Selective Electroreduction of Crotonaldehyde to Crotyl Alcohol and 1‐Butanol

“…The striking exceptions were Ag and Fe, which were selective for crotyl alcohol and 1‐butanol respectively. The superior activity of Ag toward crotyl alcohol resembles previous hydrogenation studies reporting selective crotonaldehyde‐to‐crotyl alcohol conversion on Ag catalysts [15,16] . The formation of 1‐butanol has been linked to the metal‐oxygen bond strength of the catalyst [14] .…”

Guidelines to Achieving High Selectivity for the Hydrogenation of α,β-Unsaturated Aldehydes with Bimetallic and Dilute Alloy Catalysts: A Review

“…The difference in the selectivity between Ag(111) and Cu(111) was explained by X-ray photoelectron spectroscopy data. 122 Specifically, the 1s binding energies of the carbonyl C in crotonaldehyde were 287.9 and 288.2 eV on Ag (111) and Cu(111), respectively. The larger value on Cu(111) indicated a greater degree of charge transfer from Cu to the C=O π* orbital, thereby activating the bond for reduction.…”

“…In surface science studies of Ag( 111), the adsorption geometry strongly influenced the selectivity in the hydrogenation of acrolein 92 and crotonaldehyde 122 (Fig. 19).…”

Characterization of an Allylic/Benzyl Alcohol Dehydrogenase from Yokenella sp. Strain WZY002, an Organism Potentially Useful for the Synthesis of α,β-Unsaturated Alcohols from Allylic Aldehydes and Ketones

“…The thermodynamics of the reduction of ␣,␤-unsaturated aldehydes and ketones favors hydrogenation of the CAC rather than CAO bond to form saturated compounds. Consequently, hydrogenation of ␣,␤-unsaturated aldehydes or ketones to form the corresponding unsaturated alcohols is difficult (2)(3)(4). As an alternative to a purely chemical approach, biocatalysis is increasingly preferred to achieve highly selective reductions under mild conditions (5,6) and, when required, the reverse reactions.…”