The role of copper chloride hydroxides in the oxidative carbonylation of methanol for dimethyl carbonate synthesis

Dimethyl carbonate (DMC) synthesis by oxidative carbonylation of methanol has been studied using vapor phase flow reaction system in the presence of CuCl2/NaOH/AC (AC, activated carbon) catalysts. A series of CuCl2/NaOH/AC catalysts have been prepared by the conventional impregnation method with varying the molar ratio of OH/Cu from 0.0 to 1.0. The catalytic activity Of CuCl2/NaOH/AC catalysts has been found to increase continuously with the increase of OH/Cu molar ratio as compared with CuCl2/AC catalyst. The morphological analysis by X-ray diffraction, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) has shown that the improved performance of CuCl2/NaOH/AC catalysts was closely related with the existence of copper chloride hydroxides (Cu-2(OH)(3)Cl). Two different crystal habits of Cu-2(OH)(3)Cl were observed in the catalyst surface based on the OH/Cu molar ratio. One is alpha-Cu-2(OH)(3)Cl (atacamite) which is orthorhombic, while the other is gamma-Cu-2(OH)(3)Cl (paratacamite) which is rhombohedral. The catalyst phase was revealed to change from alpha-Cu-2(OH)(3)Cl to gamma-Cu-2(OH)(3)Cl with the increase of OH/Cu molar ratio. The catalyst phase was alpha-Cu-2(OH)(3)Cl dominant at the molar ratio of OH/Cu = 0.5, while at the molar ratio of OH/Cu = 1.0, gamma-Cu-2(OH)(3)Cl dominant. The gamma-type catalyst was found to be more favorable to DMC synthesis than the alpha-type catalyst. The CO temperature-programmed desorption (TPD) experiments on the employed catalysts also strongly supported these observations. (C) 2003 Elsevier B.V. All rights reserved.