Highly efficient catalyst PdCl2-CuCl2-KOAc/AC@Al2O3 for gas-phase oxidative carbonylation of methanol to dimethyl carbonate: Preparation and reaction mechanism

The supported Wacker catalysts previously used for the synthesis of dimethyl carbonate (DMC) by gas-phase oxidative carbonylation of methanol have the drawbacks of low selectivity and poor catalytic stability. To solve these problems, PdCl2-CuCl2-KOAc/AC was treated by both Al2O3 and dilute hydrochloric acid to obtain PdCl2-CuCl2-KOAc/AC@Al2O3 in this work. The treatment brought about not only the higher activity but also the better stability and selectivity: 98% of DMC selectivity to methanol, 70% of DMC selectivity to CO, and space-time yield of DMC ranging from 600 to 700 g L-cat(-1) h(-1) for the runtime of 40 h. PdCl2-CuCl2-KOAc/AC@Al2O3 was characterized and the results showed that it possessed enlarged specific surface area (BET), enriched active components on the surface (SEM), increased proportion of Cu+/Cu2+ and new Cu2O crystalline phase formed (XPS), as compared with PdCl2-CuCl2-KOAc/AC. The new formed Cu2O can be used not only as a catalyst for the synthesis of DMC but also as an inhibitor for the loss of Cl by the reaction of Cu2O with Cl- to CuCl, resulting in the promotion of the catalytic stability. The characterization of the recovered catalyst further identified that the inhibition of KCl crystal formation and the increase in Cu+/Cu2+ proportion could improve the catalytic stability and selectivity. In addition, the higher ratio of Cu+/Cu2+ can reduce Pd2+ concentration and prohibited side-reaction of CO oxidation, so DMC selectivity to CO is increased. On the basis of activity evaluation and characterization, the catalytic reaction mechanism is proposed. (C) 2013 Elsevier B.V. All rights reserved.