Effects of zeolite structure and composition on the synthesis of dimethyl carbonate by oxidative carbonylation of methanol on Cu-exchanged Y, ZSM-5, and Mordenite

The aim of this work was to establish the effects of zeolite structure/chemical composition on the activity and selectivity of Cu-exchanged Y (Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by solid-state ion-exchange of the H-form of each zeolite with CuCl and were then characterized by FTIR and X-ray absorption spectroscopy (XAS). The XANES portion of the XAS data showed that all of the copper was present as Cu+ cations, and analysis of the EXAFS portion of the data shows the Cu+ cations had a CuO coordination number of ∼2.1 on Cu-Y and ∼2.7 on Cu-ZSM-5 and Cu-MOR. Dimethyl carbonate (DMC) was observed as the primary product when a mixture of CH3OH/CO/O2 was passed over Cu-Y, whereas dimethoxy methane was the primary product over Cu-ZSM-5 and Cu-MOR. The higher activity and selectivity of Cu-Y for the oxidative carbonylation of methanol can be attributed to the weaker adsorption of CO on the Cu+ cations exchanged into Y zeolite. In situ IR observations revealed that under reaction conditions, adsorbed CO was displaced by methoxide groups bound to the Cu+ cations. The kinetics of DMC synthesis suggests that the rate-limiting step in the formation of this product was the insertion of CO into CuOCH3 bonds. The yield of DMC decreased with methanol conversion, likely due to the hydrolysis of DMC to methanol and carbon dioxide.