Effect Of Components And Structure Of Hydrotalcites On The Catalytic Activity And Life Time In Transesterification

Diphenyl carbonate (DPC) is an important precursor for the synthesis of polycarbonate without using phosgene, and transesterification of dimethyl carbonate with phenol is considered to be the most promising process. Hydrotalcite-like compounds (HTLcs) exhibit high catalytic activity and unique selectivity in many reactions ascribed to their special structure and chemical properties, and HTLcs were selected as the catalysts for the synthesis of DPC in this paper. The effect of the structure and chemical components of HTLcs on the catalytic activity has been investigated. The lamellar structure of HTLcs showed significant effect on the transesterification, and the catalytic activity of HTLcs is related to the components. HTLcs decompose at high temperature with the changing of the components and structure, so the stability of HTLcs has been studied. The results showed that Mg-Al hydrotalcite with CO32-as the interlayer anion exhibited preferable catalytic activity and stability. Under the reaction conditions of n(Phenol)/n(DMC)=4, mass fraction of catalyst=1.5%, T=160~190oC, t = 8 h, yield of DPC and MPC was 13.9% and 19.9%, respectively, and the selectivity for transesterificated products was 90.6%. The catalytic activity of Mg-Al hydrotalcite almost unchanged after three recycles. Zn-Al hydrotalcite revealed excellent catalytic activity in the transesterification under the same reaction conditions, both yield of DPC and MPC reached 19.8%, and the selectivity for transesterificated products was 90.4%. However, Zn-Al hydrotalcite showed poor stability and rather short lifetime. It is obvious that the carbonated Mg-Al hydrotalcite is promising to be used in the industrial process. Furthermore, several kinds of nano-HTLcs have been prepared by coprecipitation under ultrasonic radiation with well crystallization, and the catalytic activity of them has been tested. The results indicated that HTLcs prepared by this method exhibit higher catalytic activity than the routine coprecipitation method, when the carbonated Mg-Al hydrotalcite was used as the catalyst, yield of DPC and MPC were 13.6% and 32.9%, respectively, and the selectivity for transesterificated products was 94.5%.