Resolving The Mechanism For Oxidative Hydration Of Ethylene Towards Ethylene Glycol By Titanosilicate Catalysts TS-1

One-step synthesis of ethylene glycol（EG）using hydrogen peroxide（H₂O₂）as oxidant,titanium silicalite zeolite TS-1 catalyzed oxidative hydration of ethylene to produce EG,has the advantages of simple process,green pollution,safety and environmental protection,has been widely used in industry.However,the mechanism of reaction of oxidative hydration of ethylene on TS-1 catalyst is still controversial.Without a clear reaction mechanism,it is impossible to design the TS-1 zeolite and improve its catalytic performance.In this work,density functional theory（DFT）calculation and microkinetic modeling are combined to provide unique insights into the reaction mechanism of ethylene glycol from oxidative hydration of ethylene.We considered the perfect titanium sites（including isolated titanium（Ⅰ）and dinuclear titanium（Ⅱ）sites）and hydrolyzed titanium sites（（Ⅰ）and（Ⅱ））cluster models to represent the catalytic active sites of titanium-silicon zeolite TS-1.Based on the above active site models,we carried out the following two studies.Firstly,oxidation hydration of ethylene towards glycol consists contains two stages,namely ethylene epoxidation and ethylene oxide hydration stages,so as to facilitate the follow-up mechanism study.Four cluster models,hydrolyzed model（Ⅰ）and（Ⅱ）and perfect model（Ⅰ）and（Ⅱ）,were established as the basic models for the study.According to the reaction mechanism of the four models acting alone,the intermediates were optimized and the transition states were searched.Through the analysis of all transition states contained in single mechanisms,it is not difficult to find that in the formation stage of ethylene oxide,the perfect model（Ⅰ）and（Ⅱ）only have one transition state,and the energy barrier is lower,which has a great advantage over the hydrolyzed model（Ⅰ）and（Ⅱ）.In the hydration stage of ethylene oxide,hydrolyzed model（Ⅰ）and（Ⅱ）has a great advantage.Therefore,based on the above analysis,we proposed a new coupling reaction mechanism:the perfect model participates in ethylene epoxidation and the hydrolyzed model is responsible for the hydration of ethylene oxide.Since there are two sites in the perfect model and two in the hydrolyzed model,there are four possible coupling reaction mechanisms.Secondly,in order to verify the rationality of the new mechanism and determine the optimal reaction mechanism,we combine the microkinetic and experimental dynamics.By comparing the EG production rates of the single mechanism and the coupling mechanism,we found that the EG production rate of the coupling mechanism was much higher than that of the single mechanism,which proved the rationality of the coupling mechanism.In addition,the apparent activation energy and reaction orders of each mechanism of theoretical model were calculated and compared with the experimental value.Which the reaction mechanism with the highest coincidence was selected,namely perfect（Ⅰ）model/hydrolyzed（Ⅰ）model.The optimal reaction mechanism of ethylene oxide hydration to EG is as follows:the perfect（Ⅰ）model participates in ethylene epoxidation and the hydrolyzed（Ⅰ）model is responsible for the hydration of ethylene oxide.Finally,for the proposed optimal reaction site and mechanism,we consider the rate of EG production under different reaction conditions,so as to determine the optimal reaction conditions.By analyzing the rate control degree,the rate control step in the optimal reaction mechanism was determined,which provides guidance for further optimizing the performance of titanium-silicon zeolite TS-1.