Studies About The Effect Of Support Properties For CuY And Cu/γ-Al₂O₃Catalysts On The Catalytic Performance In Vapor Oxidative Carbonylation Of Methanol

In all routes of dimethyl carbonate (DMC) synthsis, vapor phase oxidative carbonylation of methanol to DMC has been considered as the most promising way due to its simple process, friendly environment and low cost.In the process of DMC synthsis, metal chloride supported catalysts have shown good catalytic activity, however, the loss of chlorine in the catalysts can lead to the deactivation of catalyst and the equipment corrison. Meanwhile, it is reported that Cu+cations are considered to be the active species and Cl is not necessary for producing DMC by the oxidative carbonylation of methanol. As a result, to avoid these problems, a large number of studies have paid attention to the chloride-free catalysts, in which the supported catalysts is one of research focuses, and the properities of support play an important role to obviously affect the catalytic performance of catalysts.In this paper, based on the quantum chemical density functional theory (DFT) method, the effect of support properties on the catalytic performance of CuY and Cu/y-Al2O3catalyst have been systematically investigated. For CuY catalyst, the preferable T sites for the replacement of Si by Al, the most stable configuration of Y and CuY with different Si/Al ratios, and the Si/Al ratios of support are considered. For the Cu/γ-Al2O3catalyst, the surface hydroxylation of γ-Al2O3support is considered. The main conclusions obtained are shown as follows:The preferable T sites for the replacement of Si by Al in Y zeolites, the most stable configuration of Y and CuY zeolites with different Si/Al ratios are obtained, and the reaction mechanism for oxidative carbonylation of methanol to DMC on CuY is illustrated as follows:CH3OH firstly dehydrate to form the methoxide species CH3O; subsequently, CO react with CH3O to produce CH3OCO; once formed, CH3OCO react rapidly with CH3O to form adsorbed DMC; finally, DMC is released into the gas. CO insertion to CH3O to produce CH3OCO is the rate-determining step.The CuY catalyst with Si/Al ratio=6.5has the highest catalytic activity among all Si/Al ratios considered in this work for producing DMC by oxidative carbonylation of methanol, which is supported by the experimental facts. Our results suggest that theoretical calculation can be used as a powerful tool and provide a good theoretical guidance for the experimental design of CuY catalyst.For the Cu/γ-Al2O3catalyst, the main reaction pathway is different with that on CuY catalyst, which involving CH3OH dehydration to form the dimethoxide species (CH3O)2; subsequently, CO insertion to (CH3O)2to produce DMC. CO insertion to (CH3O)2is proved to be the rate-determining step.Moreover, the presence of surface hydroxyls in Cu/γ-Al2O3catalyst is not in favor of CH3OH dehydration and the catalytic activity is reduced.Finally, the catalytic activity of CuY(Si/Al=6.5) is higher than that for Cu/γ-Al2O3catalyst.