Study On The Synthesis Of Glycerol Carbonate From Glycerol And Urea By Zinc Species Composite Keggin Type Heteropolyacid Salts

Glycerol carbonate as an important chemical product derived from glycerol has been widely applied to various industry fields.Among the routes of synthesis of glycerol carbonate,the pathway of glycerol with urea not only mediates the excess of glycerol,but also produces glycerol carbonate with high additional value.The reaction of glycerol and urea possess many advantages,such as high economic benefits and mild reaction conditions.Therefore,the reaction with glycerol and urea as reactants can serve as the most promising route toward the synthesis of glycerol carbonate in industry.Currently,the single heteropolyacid catalysts still have low activity toward the reation,so it is necessary to modify the heteropolyacid catalyst in order to enhance the catalytic performance toward the conversion of glycerol into glycerol carbonate with urea.（1）Zinc species composite Keggin type heteropolyacid salt catalysts are prepared by wet impregnation method and used as catalysts for the synthesis of glycerol carbonate（GC）from glycerol（GL）and urea.The reaction results show the highest catalytic activity is obtained from the zinc species composite potassium phosphotungstate catalyst without calcination（Zn（NO₃）₂/K₃PW）with 94.8%conversion of glycerol and 85.8%yield of glycerol carbonate.Moreover,the composite catalyst calcined at 500℃（Zn O/K₃PW）can exhibit 86.5%conversion of glycerol and73.2%yield of glycerol carbonate under mild reaction conditions.It is found that the excellent catalysis performance of composite catalysts without calcination is attributed to the synergistic effect of zinc species and heteropolyacid salt.However,the composite catalysts with calcination transformed into ZnWO₄ phase in the temperature ranged from 300 to 700℃,it is proved that the ZnWO₄ phase is main active species in the composite catalysts with calcination,and the Zn²⁺can as Lewis acid sites to activate the carbonyl group of urea and WO₄^2-can as conjugated basic sites to activate the hydroxyl group of the glycerol,which facilitates the reaction of glycerol and urea.Combining FT-IR spectra and theoretical calculations results confirme that the formation of intermediate via glycerol and HNCO is rapid step,while the further conversion of intermediate into glycerol carbonate can be known as rate-determining step of whole reaction pathway.（2）Pure ZnWO₄ is prepared by precipitation method and applied to microwave assisted synthesis of glycerol carbonate from glycerol and urea.In order to enhance the catalysis performance of ZnWO₄ catalyst,and the introduction of La species in the catalyst formulation to give ZnWO₄ higher activity.The microwave reaction of glycerol and urea conducted at 140℃ for 50min over 30%-La/ZnWO₄-500 can obtain 83.5%conversion of glycerol and 74.1%yield of glycerol carbonate.Simultaneously,the enhanced catalysis performance compared to ZnWO₄ catalyst can be attributed to the formation of Zn_0.5La（WO₄）₂ phase,which can be confirmed by XRD and FT-IR results.