One-step Synthesis Of Dimethyl Carbonate From Urea,1,2-propanediol And Methanol By Catalytic Reaction Distillation

Dimethyl carbonate?DMC?is a versatile green chemical.One-step synthesis of DMC starting from urea,1,2-propanediol and methanol not only solves the problems of the constraint of the raw materials by petrochemical industry and of the requirement for effective use of by-product 1,2-propanediol?PG?in the transesterification process,but also provides a route to value-added utilization for urea.This dissertation is intended to directly synthesize DMC from urea,PG,and methanol using a reactive distillation operation.The reaction conditions for the preparation of propylene carbonate?PC?by urea alcoholysis with PG and of DMC from transesterification of PC with methanol were investigated.A kinetic model for the transesterification of PC with methanol catalyzed by Ca-Al oxide was established and the catalytic distillation process of the transesterification was simulated using Aspen Plus software.Firstly,the Zn-Al oxide and Ca-Al oxide catalyst powders were molded by mechanical mixing and then the effect of the addition amount of peptizing agent on the mechanical strength of the catalysts was investigated.The results showed that the mechanical strength of Zn-Al oxide catalyst was hardly affected by the addition amount of peptizing agent whereas the mechanical strength of Ca-Al oxide catalyst greatly changed with the addition amount of peptizing agent.The best mechanical strength of the Ca-Al oxide catalyst was achieved when2 mL of peptizing agent was mixed with 5g of catalyst.In addition,the packing method of the catalysts in the catalytic distillation tower and the influence of catalyst particle size were investigated.It was found that the Zn-Al oxied catalyst was packed by mixing the catalyst with?ring according to a volume ratio of 10:1 and the Ca-Al oxide catalyst was packed by free-stacking.The suitable length of the catalyst particles is 2 mm.Secondly,the reaction conditions for the alcoholysis of urea with PG and the transesterification of PC with methanol were investigated respectively in the catalytic distillation tower,and a kinetic model for the transesterification of PC with methanol catalyzed by Ca-Al oxide was established,and then the catalytic distillation process of the transesterification was simulated using Aspen Plus software.The suitable operation conditions for urea alcoholysis reaction were obtained as follows:a molar ratio of PG to urea=5,a nitrogen flow rate=150 mL?min^-1,a feed liquid hourly space velocity=0.6 h^-1,a reaction temperature of 170°C,and a temperature of the condensed water at the tower condenser=40°C.Under these conditions,the PC yield of 82.4%was attained and the catalyst could be used steadily for more than 200 h.The suitable operation conditions for the transesterification reaction were obtained as follows:a reflux ratio of 9,a molar ratio of methanol to PC=14,a PC feed liquid hourly space velocity=0.1 h^-1,and a reaction temperature of 70°C.Under the conditions above,the DMC yield of 98.1%was attained and the catalyst could be used stably for more than 200 h.The kinetics study of the transesterification showed that it is a secondary reversible reaction with an activation energy of 62.35 kJ·mol^-11 and a pre-exponential factor of 27.17×10⁷ L·(mol·min-1) for the foward reaction,and with an activation energy of 57.92 kJ·mol^-1 and a pre-exponential factor of39.24×10⁶ L·?mol·min?^-11 for the backward reaction.The result of simulating the catalytic distillation process of the transesterification using Aspen Plus software showed that the errors are less than 5%.Finally,the synthesis of DMC from urea,PG and methanol was studied.The suitable operation conditions in the one-step process were obtained as follows:a reflux ratio of 9,a nitrogen flow rate of 150 mL·min^-1;as for the Ca-Al oxide catalyst bed:a mixed feed?PC and PG?liquid hourly space velocity of 0.1 h^-1,a reaction temperature of 70°C;as for the Zn-Al oxied catalyst bed:a molar ratio of methanol and urea=17,a mixed feed?methanol and urea?liquid hourly space velocity=0.5 h^-1,a reaction temperature 170°C.Under the conditions above,the yield of DMC was only 58.2%due to a large number of PG existing in the PC feed.The suitable operation conditions for the synthesis of DMC using urea,PG and methanol by a cascade mode were investigated.The molar ratio of PG to PC in the mixed liquid obtained by the alcoholysis of urea with PG could be lowered to 1:2 by a vacuum distillation at a vacuum degree 0.1MPa and 120°C,which could eliminate the influence of PG on the tranesterification of PC with metnanol.By so doing,the yield of DMC attained96.3%.