A Theoretical Investigation of Dimethyl Carbonate Synthesis on Cu−Y Zeolite

A theoretical analysis of the mechanism of dimethyl carbonate (DMC) synthesis via oxidative carbonylation of methanol on Cu-exchanged Y zeolite, Cu−Y, was explored using density functional theory. These calculations show that methanol adsorbs in the presence of oxygen to form coadsorbed methoxide and hydroxide species and dimethoxide species on extraframework Cu+ cations. DMC can form by CO addition to the former species to produce monomethyl carbonate species, which then react with additional methanol or by CO addition to dimethoxide species to form DMC directly. The turnover frequency for DMC synthesis is estimated to be 1.2 × 10-5 s-1 for the reaction conditions used in the experimental work of Zhang and Bell (J. Catal., in press). This value compares very closely with that observed, 5 × 10-5 s-1. The calculated value of the apparent activation energy is 15.0 kcal/mol, in good agreement with that measured experimentally, 14.8 kcal/mol. The effects of methanol and oxygen on the adsorption of CO have also been examined. CO adsorbs on Cu+ cations with a heat of adsorption of 18.7 kcal/mol; the vibrational frequency of the adsorbed CO is 2134 cm-1. Coadsorption of CO and molecular methanol reduces the heat of CO adsorption to 10.1 kcal/mol and shifts the C−O frequency to 2117 cm-1. However, formation of Cu+-bound methoxide, when CH3OH and O2 are present together with CO, completely inhibits the adsorption of CO. These findings are in very good agreement with experimental observation.