A Density Functional Theory Study Of The Oxidative Carbonylation Of Methanol On Cu~0/AC Catalyst

Dimethyl carbonate (DMC) is an environmental friendly, wide-range in application buildingblock. Recently, the synthesis technology and the investigation on the catalysts of DMC have attracted more attention for the developing of application areas. The clean technology to synthesize DMC by vapor phase oxidativecarbonylation of methanol attracted more and more attentionworldly. This method is one of the most valuable industrial routes because of thefollowing advantages:cheap raw materials, simple process and high atom utilization.Cu-supported catalysts havemainly usedin the process of DMC formationby vapor phase oxidativecarbonylation of methanol, include chloride Cu-supported catalysts, Cu-zeolite catalysts andchloride-free Cu-supported catalysts. In addition, chloride-free Cu-supportedcatalystsbecome one of research focus in the related fieldsfor the simple synthesisprocesswith theeasy accessibility of raw material, meanwhile, compared with thechloride Cu-supported catalysts, chloride-free Cu-supported catalystscause no deactivation of catalyst and the equipment corrosion. The heat treatment of AC impregnated with cupric nitrate at different temperatures can produce CuO, CU2O and Cu0catalysts. It appeared that these copper species are active for the oxidative carbonylation of methanol, and the catalytic activity increased in the order CuO<Cu2O<Cu0. But the catalytic mechanism of Cu0is still unclear.In this study, the formationinechanism of dimethyl carbonate (DMC) via oxidative carbonylation of methanol on Cu0/AC(activated carbon) catalystwas investigated by density functional theoryand cluster models. Interactions between Cu0and AC were considered.The results indicate that Cu0refers to adsorb at the unsaturated sites on AC. On the basis of the calculated results of the reaction species (CO, CH3O, CH3OH, DMC, etc.) adsorbed on the Cu0/AC surface, the most stable adsorption configuration of these species are given. Researchers proposed that DMC can be formed by two different pathways. In the first pathway, DMC is formed by the direct addition of CO to dimethoxide; however, our calculated results indicate that this pathway is not dominantbecause of the difficulty of overcoming its activation barrier (330.6kJ/mol). In the second pathway,monomethyl carbonate (MMC) species is produced by the insertion of CO to the methoxide species. Then MMC species reacts with methanol to form DMC. The calculated results indicate that the latter is the main pathway for DMC formation. The activation barriers of the two successive reactions involving the formation of MMC andDMC are87.9and77.3kJ/mol, respectively. The impact of Rh promoter on the formation of dimethyl carbonate (DMC) via oxidative carbonylation of methanol on Cu-Rh/AC (activated carbon) catalyst was investigated by density functional theory. On the basis of the calculated results of the reaction species (CO, OH, CH3O, MMC, DMC) adsorbed on Cu0/AC and Cu-Rh/AC surface, the most stable adsorption configuration of these species are given. Our previous work proved that CO insertion into methoxide and dimethoxide species are the rate-limiting steps for two pathways on the Cu0/AC surface. The reaction energy and activation energy of the rate-limiting steps on Cu-Rh/AC surface are compared with that of Cu0/AC surface. For the rate-limiting step of CO insertion into dimethoxide species, the activation energies are206.3and304.8kJ/mol on Cu-Rh/AC and Cu0/AC surfaces. The result indicates that the addition of Rh promoter is in favor for DMC formation. For the rate-limiting step of CO insertion into methoxide species, the activation energies are78.5and92.7KJ/mol on Cu-Rh/AC and Cu0/AC surfaces. The result shows that the addition of Rh promoter is in favor for MMC formation, and then benefits the synthesis of DMC.