Synthesis Of Diphenylmethane Carbamate And Joint Preparation Of SiO₂@ZnO

Methylene diphenyl-4,4′-dicarbamate (MDC) is theintermediate for preparingmethylene diphenyl diisocyanate (MDI).Using dimethyl carbonate(DMC),the greenchemical, and MDA as raw materials for MDC synthesis is becoming moreattractive.In this process, the reaction conditions are mild and the only by-product ismethanol. In addition, methanol is the raw material for production of DMC byoxidative carbonylation, which will reduce the cost for the production of DMC.The qualitative analysis of the reaction system of MDA and DMC was carriedoutbya liquid chromatography-mass spectrometry(LC-MS), andmonocarbamatemethyl4-(4′-aminobenzyl)phenylcarbamate were determined.Aquantitative analysis was developed using nitrobenzene as the internal standard byreversed-phase high performance liquid chromatography.Effect of crystal water on catalytic performance of Zn(OAc)2was investigated.The results show that crystal water can cause the hydrolysisof DMC to producemethanol and methanol is prone to reacted with Zn(OAc)2,which accordinglydecreases its catalytic activity. The reaction conditions for MDC synthesis overZn(OAc)2catalyst was optimized. And the optimal reaction conditions are as follows:reaction temperature of130℃, reaction time of7h, MDA amount of0.82g,catalystamount of0.2g, DMC volume of14ml,the MDA conversion is100%, and MDC yieldis98.3%.It is also found the induction period exists during the reaction. MDC yieldincreased obviously from2.1%to84.3%when the reaction time was from2h to3h at130℃.However, Zn4O(OAc)6was formed after Zn(OAc)2was pretreated withmethanol for1h and reaction time was shortened using Zn4O(OAc)6as catalyst. So itis deduced that Zn4O(OAc)6is the active site for MDC synthesis. Zn4O(OAc)6can’t bereused in MDC synthesis for Zn4O(OAc)6can be transformed into LHZA,then intoZnO during the reaction.Zn(OAc)2/SiO2was prepared by high temperature impregnation process. And theoptimal preparation conditions for Zn(OAc)2/SiO2are as follows:impregnationtemperature of130℃,impregnation time of7h,DMC volume of30ml, mass ratio ofZn(OAc)2to SiO2=1. MDA conversion is100%and MDC yield is92.3%overZn(OAc)2/SiO2prepared under optimal conditions. Zn(OAc)2/SiO2was characterized by FTIR and XRD. The results show that Zn exists in the form of Zn(OAc)2andLHZA on the surface of SiO2.SiO2@ZnO with non-continuous shell was preparedbyZn(OAc)2/SiO2pretreated with methanol. However, after the reaction of MDA andDMC, SiO2@ZnO can’t be formed for the existence of–NH2in MDA, which canprevent the deposition of ZnO on the surface of SiO2.