Chemoselective Catalytic Hydrogenation of Acrolein on Ag(111): Effect of Molecular Orientation on Reaction Selectivity

Abstract

The adsorption and hydrogenation of acrolein on the Ag(111) surface has been investigated by high resolution synchrotron XPS, NEXAFS, and temperature programmed reaction. The molecule adsorbs intact at all coverages and its adsorption geometry is critically important in determining chemoselectivity toward the formation of allyl alcohol, the desired but thermodynamically disfavored product. In the absence of hydrogen adatoms (H(a)), acrolein lies almost parallel to the metal surface; high coverages force the CdC bond to tilt markedly, likely rendering it less vulnerable toward reaction with hydrogen adatoms. Reaction with coadsorbed H(a) yields allyl alcohol, propionaldehyde, and propanol, consistent with the behavior of practical dispersed Ag catalysts operated at atmospheric pressure: formation of all three hydrogenation products is surface reaction rate limited. Overall chemoselectivity is strongly influenced by secondary reactions of allyl alcohol. At low H(a) coverages, the CdC bond in the newly formed allyl alcohol molecule is strongly tilted with respect to the surface, rendering it immune to attack by H(a) and leading to desorption of the unsaturated alcohol. In contrast with this, at high H(a) coverages, the CdC bond in allyl alcohol lies almost parallel to the surface, undergoes hydrogenation by H(a), and the saturated alcohol (propanol) desorbs.

Identification of Vertical and Horizontal Configurations for BPE Adsorption on Silver Surfaces

“…In this case, only the aldehyde fragment is hydrogenated, and the corresponding unsaturated alcohol desorbs. 40 The similarity in the Raman band intensity ratios at different BPE concentrations for metallic Ag and Au in Figures 5a and 8 suggests that the BPE adsorption trends on both metals are the same. It is, therefore, likely that on Au, similarly to Ag, BPE adsorbs initially as a mixture of horizontal and vertical configurations.…”

“…For example, a study of acrolein adsorption on Ag(111) showed that this molecule changes its orientation from the horizontal at low coverage to a configuration with a strong tilt of the C=C double bond part of the molecule at high coverage, based on high-resolution synchrotron XPS, near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and temperature-programmed reaction (TPR) measurements. 40 Such a coverage-dependent orientational transformation allows to control the selectivity of catalytic reactions. At low acrolein surface coverage, when the molecule adsorbs in the horizontal configuration, the C=C double is easily hydrogenated by added H2, which adsorbs dissociatively and forms H atoms on the Ag surface.…”

Catalysis beyond frontier molecular orbitals: Selectivity in partial hydrogenation of multi-unsaturated hydrocarbons on metal catalysts

“…Consistent with the study of Brandt et al . ( 30 ), our results suggest the activation of the C═C bond when acrolein adsorbs at the coinage metal surfaces.…”

Theoretical Studies of Acrolein Hydrogenation to Propenol and Propanal on Au₃ and Au₅

“…Normally the C=C bond is more active than the C=O bond both thermodynamically and kinetically [3]. Acrolein is the smallest α,β-unsaturated aldehyde molecule containing conjugated C=C and C=O double bond, and is used as a probe molecule to study the hydrogenation activity toward the C=C and C=O bonds on the Ag [4][5][6][7], Pt [8][9][10][11], Ni/Pt(111) [12,13] surface both experimentally and theoretically. However, the selective hydrogenation of acrolein to propenol is considered the most difficult one and the selectivity to unsaturated alcohols on conventional catalysts of group VIII metals is generally low (~2% on Pt) [9].…”