Activation of methyl acetate on Pd(111)

The adsorption and activation of methyl acetate (CH3COOCH3), one of the simplest carboxylic esters, on Pd(111) have been studied using self-consistent periodic density functional theory calculations. Methyl acetate adsorbs weakly through the carbonyl oxygen. Its activation occurs via dehydrogenation, instead of direct C-O bond dissociation, on clean Pd(111): It is much more difficult to dissociate the C-O bonds (Ea ≈ 2.0 eV for the carbonyl and acetate-methyl bonds; Ea = 1.0 eV for the acetyl-methoxy bond) than to dissociate the C-H bonds to produce enolate (CH2COOCH3; Ea = 0.74 eV) or methylene acetate (CH3COOCH2; Ea = 0.82 eV). The barriers for C-H and C-O bond dissociation are directly calculated for enolate and methylene acetate, and estimated for further dehydrogenated derivatives (CH3COOCH, CH2COOCH2, and CHCOOCH3) based on the Brønsted-Evans-Polanyi linear energy relations formed by the calculated steps. The enolate pathway leads to successive dehydrogenation to CCOOCH3, whereas methylene acetate readily dissociates to yield acetyl. The selectivity for dissociating the acyl-alkoxy C-O bond, which is desired for alcohol formation, is therefore fundamentally limited by the facility of dehydrogenation under vacuum/low-pressure conditions on Pd(111).