Synthesis Of Toluene Dicarbamate Catalyzed By [Bmim]OAc-0.5Zn（OAc）₂ Ionic Liquid

Toluene dicarbamate（TDC）is an intermediate for the synthesis of toluene diisocyanate（TDI）by the carbonate method.TDI is an important material for the synthesis of polyurethane.In the first reaction step of the process,i.e.the reaction of dimethyl carbonate（DMC）instead of phosgene to reaction with toluene diamine（TDA）for preparation toluene dicarbamate（TDC）,zinc acetate shows excellent catalytic performance,but once used,zinc acetate will lose its catalytic activity.As a new type of catalyst,ionic liquids are widely used in organic synthesis reactions.This paper studies the reaction of TDA and DMC as raw materials to synthesize TDC catalyzed by[Bmim]OAc-0.5Zn（OAc）₂ionic liquid,which has important theoretical significance and practical value.First,[HO₃S-（CH₂）₄-mim]Cl-0.5Zn（OAc）₂,[PEmim]Cl-0.5Zn（OAc）₂ and [Bmim]OAc-0.5Zn（OAc）₂were synthesized.And the ability of catalytic synthesis of TDC was investigated respectively,and it was found that[Bmim]OAc-0.5Zn（OAc）₂has higher catalytic activity.The structure of[Bmim]OAc-0.5Zn（OAc）₂was determined by means of infrared spectroscopy and nuclear magnetic hydrogen spectrometry,among which zinc acetate was a monodentate coordination structure.The catalytic performance of[Bmim]OAc-x Zn（OAc）₂ionic liquids with different x was investigated,and it was found that the catalytic effect was better as x=0.5.Using infrared spectroscopy,ultraviolet-visible absorption spectroscopy,and Hammett indicator combined characterization methods,it was discovered that the ionic liquid[Bmim]OAc-x Zn（OAc）₂was with a more suitable acid-base site ratio as x=0.5.Through optimizing reaction conditions for the preparation of[Bmim]OAc-0.5Zn（OAc）₂and the synthesis of TDC from TDA and DMC catalyzed by[Bmim]OAc-0.5Zn（OAc）₂,it was found that the catalytic activity of the synthesized[Bmim]OAc-0.5Zn（OAc）₂was higher under the preparation conditions of n（[Bmim]OAc）:n（Zn（OAc）₂）=1:0.5,reacting at 80℃for 3h;and the suitable reaction conditions for synthesizing TDC were as follows:n（TDA）:n（DMC）=1:30,m（TDA）:m（[Bmim]OAc-0.5Zn（OAc）₂）=1:0.95,160℃for 6 h;under these conditions,TDC yield reached 94.50%.Then,the effect of catalyst’s recovery conditions on its recovery rate was investigated.The results showed that catalyst recovery rate attained 90.78%under the suit conditions of the ratio of[Bmim]OAc-0.5Zn（OAc）₂ with water at V（[Bmim]OAc-0.5Zn（OAc）₂）:V（Water）=1:37.5,water treatment temperature at 80℃,and water treatment time for 8h.By means of infrared spectrum,ultraviolet-visible absorption spectrum with Hammett indicator combination,and XRD,the causes of the decline in the catalytic activity of recovered catalyst was investigated.It was found that the catalyst structure was not damaged after the catalytic reaction,but the recovered catalyst possessed less amount of Zn（OAc）₂and alkalinity value than fresh catalyst[Bmim]OAc-0.5Zn（OAc）₂,and zinc oxide was produced in the reaction system.These mean that the byproduct methyl alcohol of the reaction system reacts with Zn（OAc）₂of[Bmim]OAc-0.5Zn（OAc）₂to produce zinc oxide and lead to the loss of Zn（OAc）₂of the fresh catalyst[Bmim]OAc-0.5Zn（OAc）₂.The relationship formula between x in[Bmim]OAc-x Zn（OAc）₂and alkali strength value H₀was fitted using Origin software,so the recovered catalyst was verified as[Bmim]OAc-0.19Zn（OAc）₂by calculated with this relationship.The recovered catalyst was regenerated by supplementing zinc acetate,and the reusability of the regenerated catalyst was examined,it was found that the catalytic activity of the regenerated catalyst was basically unchanged after 5 times of reuse.Finally,the reaction mechanism of synthesizing TDC from TDA and DMC catalyzed by[Bmim]OAc-0.5Zn（OAc）₂was studied by means of infrared spectroscopy.The results showed that[Bmim]OAc-0.5Zn（OAc）₂has acidic sites and basic sites,the acidic site(Zn²⁺)interacts with the carbonyl oxygen in the DMC molecule to form a new coordination complex,so activating the carbonyl carbon;the basic site（acetate）interacts with the amino group in the TDA molecule to activate TDA;then the activated TDA attacks the activated carbonyl carbon in the new complex to proceed the methoxycarbonylation reaction and to generate the target product TDC.