Reaction Mechanism For Methanol Oxycarbonylation To Form Dimethyl Carbonate: A DFT Approach

As an important green chemical intermediate, dimethyl carbonate (DMC) is anenvironmentally benign compound with versatile chemical reactivity and excellentprospects of application. Two series of catalyst, supported chlorine containingcatalysts and exchanged zeolite catalysts, are known to be activated and investigatedmost in gas-phase oxidative carbonylation of methanol to form DMC. Comparingwith the zeolite catalysts, the chlorine containing catalysts show higher activity.However, the complicated composition of the supported chlorine containing catalystslimits the deeper investigation of the catalysts and the reaction details. The force ofthis dissertation is to investigate the reaction mechanism of the gas-phaseoxycarbonylation of methanol on different chlorine containing catalysts (includingγ-Cu2(OH)3Cl,-Cu2(OH)3Cl, Cu(OH)Cl, CuCl2, Pd/CuCl2,2Pd/CuCl2), thus toclarify the role of the catalyst components in the reaction and provide someinformation for catalyst improvements.On the surfaces of chlorine containing catalysts, the reaction mechanism can beseparated into four main steps: methanol oxidation, CO insertion, DMC formation andsurface reconstruction. During the reaction, the methanol molecularly adsorbed at thetop site of the Cl atom on the top slayer of the surface through the hydroxyl H atom,and the electron transfer between the CH3OH and the surface activated the CH3OH.However, the oxidation process is slightly different on different surfaces. On theγ-Cu2(OH)3Cl(021) and-Cu2(OH)3Cl(011) surfaces, the Cl atom escaped from thesurface and bonded with the methanol first (formed CH3O H Cl) then reacted withthe methanol to form adsorbed methoxyl and HCl; and the calculated results indicatethe decomposition of the CH3O H Cl is the rate-limit step of the whole methanoloxycarbonylation reaction. But on the surfaces of Cu(OH)Cl(001), CuCl2(001) andPd/CuCl2(001), the Cl atom reacted with the methanol directly to form the methoxyland HCl, and the methanol oxidation is also the rate-limit step of the mechanism onthese three surfaces.The CO species physically adsorbed on the surfaces of γ-Cu2(OH)3Cl(021),-Cu2(OH)3Cl(011) and Cu(OH)Cl(001). And the stability of the CH3OCO adsorptionwould further affect the DMC formation process, the lowest energy barrier for DMCformation step appears when the adsorption of CH3OCO is weakest. Comparatively, the CO species chemically adsorbed on CuCl2(001) and Pd/CuCl2(001) surfaces. Andthe adsorption site of CO has slight effect on the energy barrier of CO insertion.Meanwhile, the adsorption site of CH3OCO and CH3O will strongly affect the DMCformation process. The lowest energy barrier for DMC formation step appears whenCH3OCO and CH3O adsorbed on both Cu and Pd atoms. Furthermore, the surfacereconstruction process on all chlorine containing catalyst is spontaneous andexothermic, which needs the HCl and O2at the same time.