DFT investigations for the reaction mechanism of dimethyl carbonate synthesis on Pd(ii)/β zeolites

Abstract

Density functional theory (DFT) calculations have been used to investigate the oxidative carbonylation of methanol on Pd(II)/β zeolite. Activation energies for all the elementary steps involved in the commonly accepted mechanism, including the formation of dimethyl carbonate, methyl formate and dimethoxymethane, are presented. Upon conducting the calculations, we identify that the Pd(2+) cation bonded with four O atoms of the zeolite framework acts as the active site of the catalyst. Molecularly adsorbed methanol starts to react with oxygen molecules to produce a methanediol intermediate (CH2(OH)2) and O atom. Then, another methanol can react with the O atom to produce the (CH3O)(OH)-Pd(II)/β zeolite species. (CH3O)(OH)-Pd(II)/β zeolite can further react with carbon monoxide or methanol to give monomethyl carbonate or di-methoxide species ((CH3O)2-Pd(II)/β zeolite). Dimethyl carbonate can form via two distinct reaction pathways: (I) methanol reacts with monomethyl carbonate or (II) carbon monoxide inserts into di-methoxide. Our calculation results show the activation energy of reaction (I) is too high to be achieved. The methanediol intermediate is unstable and can decompose to formaldehyde and H2O immediately. Formaldehyde can either react with an O atom or methanol to form formic acid or a CH3OCH2OH intermediate. Both of them can react with methanol to form the secondary products (methyl formate or dimethoxymethane). Upon conducting calculations, we confirmed that the activation energies for the formation of methyl formate and dimethoxymethane are higher than that of dimethyl carbonate. All these conformations were characterized at the same calculation level.

DFT investigations for the catalytic reaction mechanism of methane combustion occurring on Pd(ii)/Al-MCM-41

“…In the case of the Pd supported on silica-alumina mesoporous supports, the nature of the active site was debated in the literature in the past decade. Several studies identified nanoparticules of PdO, 5, 13 while other proposed a PdO 2 phase, but for other oxidative processes such as the oxidative carbonylation of methanol 16 and the CO oxidation. 17 According to our previous works on Pd/Al-MCM-41, 8,18 CH 4 combustion could be due not only to nanoparticles of PdO but also to isolated Pd 2+ cations which are coordinated by the oxygen atoms of the alumino-silicate framework and can be oxidized into palladium oxo species [Pd=O] 2+ .…”

“…[41][42][43][44] In addition, a recent work suggests that Lewis acid sites may activate C-H and O-H bond activation in methanol 45. Ma et al16 have reported in their theoretical study that the first step is the formation of methanediol on the superoxo species [PdO 2 ] 2+ grafted in the 6-membered ring. The difference with the results presented here is probably due to the interaction between the H atom of methanol and the framework oxygen atoms, which varies from a 6-membered ring to an 8membered ring (this study).…”

Carbon monoxide oxidation catalysed by defective palladium chloride: DFT calculations, EXAFS, and in situ DRIRS measurements

“…The reliability of the method has been confirmed by previous reports of studies on the mechanism of DMC (dimethyl carbonate), DMO (dimethyl oxalate) and CO 2 synthesis. 5,17,[39][40][41][42][43][44] The adsorption energies DE ads are defined as:…”