Study On Catalytic Synthesis Of Dimethyl Carbonate From Methyl Carbamate And Methanol

Dimethyl carbonate (DMC) can replace of toxic phosgene as carbonylation agent or take place of dimethyl sulfate as methylation agent. Up to now, dimethyl carbonate is mainly synthesized by phosgene method, transesterification method, oxidative carbonylation method, carbon dioxide and methanol direct synthesis method and methanolysis of urea method. Among the above synthesis method, methanolysis of urea method is an attractive method to synthesize DMC, it uses cheap urea and methanol as raw materials and the ammonia generating during the reaction can be used to produce urea raw material again by reacted with carbon dioxide. What’s more, no water is formed during the reaction. Hence, the water-methanol-DMC ternary azeotropic system is avoided. The reaction of methanolysis of urea is divided into two-step. The first step which can carry out spontaneously without catalyst is one molecular urea reacts with one molecular methanol to form intermediate methyl carbamate (MC) and release ammonia. The second step which is the rate-control step is one molecular MC continues to react with another molecular methanol to generate DMC and release ammonia. This work was mainly evaluating the catalytic activity of various catalysts to the rate-control step of methanolysis of urea to DMC reaction in batch autoclave and realized heterogenization of homogeneous Bronsted acid catalyst. The specific research details are as follows:1-, The catalytic activity of serial anhydrous transition-metal chlorides was investigated. The result indicated that the DMC yield was29.3%when zinc chloride was used as catalyst and the MC conversion was59.3%when copper chloride was used as catalyst. Their catalytic activity wasn’t enhanced but reduced slightly by adding organic nitrogenous ligand into the reaction system to modify their coordinate microenvironment. The characteristic result of XRD indicated that anhydrous transition-metal chlorides weren’t always transfered to their corresponding chloramines compound. And their catalytic activity was different when they were used to catalyze alcoholysis of dialkyl carbamate to different dialkyl carbonate. 2、The catalytic activity of molecular sieve and modified molecular sieve was evaluated. The result indicated that both molecular sieve and modified molecular sieve hardly have catalytic activity. The highest DMC yield was between0.5%～4.8%. The catalytic activity of molecular sieve was not affected by the competitive adsorption action of methanol and ammonia in the pore of molecular sieve. This was illustrated by using them to catalyze the reaction of alcoholysis of butyl carbamate to dibutyl carbonate in which all the molecular were bigger.3、The catalytic activity of several rare-earth metal nitrate compounds was investigated. The result indicated that the catalytic activity of rare-earth metal nitrate compounds was comparative and the DMC yield decrease slightly with the increase of catalyst amount. The DMC yield was between4.1%～7.4%.4、The catalytic activity of serial metal organic frameworks compositions (MOFs) was evaluated. The result indicated that the catalytic activity of MOFs was inferior. The DMC yield was between1.6%～5.0%. The catalytic activity of HKUST-1wasn’t enhanced by supporting it on mesoporous molecular sieve MCM-41. The DMC yield of HKUST-1and MCM-41souported HKUST-1was1.9%and1.6%respectively.5、The catalytic activity of several Bronsted acids was investigated. The result indicated that concentrated sulfonic acid whose acidity was stronger can effectively catalyze the conversion of MC, the MC conversion can reach80.9%, but the DMC yield is lower than pyrophosphoric acid whose acidity is lower. The DMC yield was14.8%with concentrated sulfonic acid as catalyst while with pyrophosphoric acid as catalyst, the DMC yield was19.6%. Homogenous Bronsted acid catalyst realized heterogenization by grating sulfonic acid groups to mesoporous molecular sieve MCM-41. The characteristic results of XRD and FT-IR indicated that sulfonic acid groups can be grafted by chlorosulfonic acid sulfonated directly under the condition of the structure of MCM-41remaining intact if the ratio of chlorosulfonic acid volume to MCM-41mass is less than0.5ml/g. The catalytic activity of MCM-41grafted by sulfonic acid groups is positively correlated with its acid amount and the highest DMC yield is competitive with the yield attained by using homogenous Bronsted acid as catalyst, but its regenerative problem is still unsolved.