Carbonylation of 1,2-propylene glycol with urea to propylene carbonate under the catalysis of Zn-Al oxide from the view of homogeneous catalysis

ZnO, Al2O3, and Zn-Al oxide prepared from hydrotalcite-like compound, were evaluated for the catalysis of carbonylation of 1,2-propylene glycol (PG) with urea to propylene carbonate (PC) under the reduced pressure of 40 kPa at 170 degrees C. Zn-Al oxide achieved highest yield of PC (96.6%) among three catalysts when 0.5 wt% of catalyst was used. It was found that ZnO and Al2O3 could be dissolved into the reactant to form Zn and Al complex during the reaction, but the dissolved amount of ZnO was much larger than Al2O3. The reaction was experienced three states of catalyst dissolution, homogeneous catalysis and catalyst re-depositing. The essential role of Zn was to produce Zn(NCO)(2)(NH3)(2) which in fact catalyzed the (2- or 4-)hydroxypropyl carbamate cycling to PC. Although Al2O3 showed negligible catalytic activity for this reaction, the dissolved Al species (probably Al(NH3)(NCO)(3)) could delay the re-deposition of Zn complex and hence achieve high yield of PC on Zn-Al oxide. The re-deposited catalyst of ZnO and Zn-Al oxide contained counter anions of OH, CO3, and NCO. Some techniques were used for the characterizations of the spent catalysts.