The influence of a hydrophobic carrier, reactant and product during H2O adsorption on Pd surface for the oxidative esterification of methacrolein to methyl methacrylate

Abstract

Taking the one-step oxidative esterification of methacrolein (MAL) to methyl methacrylate (MMA) as a model reaction and because H₂O that was generated easily formed a film of water on the catalyst surface, which restricted the diffusion of the reactants to the active sites, the effects of the hydrophobic carrier styrene–divinylbenzene (SDB) copolymer, the reactant CH₃OH and the product MMA during the adsorption of H₂O on a Pd surface were investigated. For a Pd/SDB catalyst, the interactions between the active component and the carrier were first calculated using Pd₄ clusters. The results implied that Pd₄ clusters were chemisorbed on the SDB carrier. By comparing the adsorption energy of H₂O molecules on Pd₄ clusters with or without SDB, it was found that the adsorption energy of the former was reduced by about 50%, indicating that the hydrophobic carrier SDB reduced the adsorption of H₂O on Pd₄ clusters. This was also confirmed by the results for the partial density of states, differences in charge density and comparative Mulliken charge analysis. The influences of the reactant CH₃OH and the product MMA on the adsorption of H₂O were investigated using the Pd(111) surface. The results of co-adsorption simulations showed that some of the electrons on CH₃OH molecules were transferred to H₂O molecules that strengthened the electronic interaction between H₂O molecules and the Pd surface and led to a change in the adsorption of isolated H₂O molecules from physisorption to chemisorption. However, the product MMA when chemisorbed on the Pd surface had little effect on the adsorption of H₂O molecules on the Pd(111) surface.