Reaction Mechanism and Kinetics of Dimethyl Carbonate Synthesis from Methyl Carbamate and Methanol

The catalytic activities of many transition-metal salts for dimethyl carbonate (DMC) synthesis from methyl carbamate (MC) and methanol were evaluated in a batch reactor. The reaction mechanism and kinetics on the outstanding catalyst ZnCl2 were further investigated in detail. X-ray diffraction (XRD), thermogravimetry (TG), and differential scanning calorimetry (DSC) characterization, quantum chemical calculation, and kinetics experiments all indicated that this reaction could be divided into two processes: (1) two MC molecules coordinated with ZnCl2 via N atom to produce Zn(MC)(2)Cl-2. This intermediate would convert to Zn(MC)(NH3)Cl-2 by reacting with one methanol molecule. This process was first order, relative to ZnCl2, and zeroeth order, relative to MC, from a macrokinetics viewpoint. (2) Zn(MC)(NH3)Cl-2 further reacted with another methanol molecule to yield DMC and Zn(NH3)(2)Cl-2, one ammonia molecule of Zn(NH3)(2)Cl-2 could be substituted by MC at experimental temperature to form Zn(MC)(NH3)Cl-2 again. This process was first order to both ZnCl2 and MC, from a macrokinetics viewpoint. It should be noted that Zn(MC)(2)Cl-2 could not appear again after the first process, and the second process is the real catalysis cycle in DMC synthesis.