H-ZSM-5催化甲苯与碳酸二甲酯和甲醇甲基化的反应机理

摘要

Para-xylene is an important petrochemical that can be produced by the methylation of toluene.Here,the mechanism of toluene methylation with dimethyl carbonate (DMC) or methanol catalyzed by H-ZSM-5 was studied using the "our own N-layered integrated molecular orbital + molecular mechanics" (ONIOM) in combination with density functional theory (DFT) methods.The adsorption of reactants and desorption of products are considered,and the structures of important intermediates and transition states are described.Computational rate constants are used to estimate the kinetic activity of toluene methylation reactions.The reaction mechanism of toluene methylation with DMC and that with methanol catalyzed by H-ZSM-5 differ.Toluene methylation with DMC involves full decomposition of DMC prior to methylation to form xylene isomers.In contrast,methanol is more active than DMC as the methylation reagent in toluene methylation.The stepwise and concerted paths of toluene methylation with methanol have similar intrinsic activation energies.At 773 K,the stepwise path has a higher rate constant than the concerted one.For toluene methylation with both reagents,para-xylene formation is kinetically preferred,whereas meta-xylene is the lowest-energy product.The results of our calculations agree well with experimental observations.%对二甲苯是重要的石油化工产品之一,可以通过甲苯甲基化生产.本文采用“our own-N-layered integrated molecular orbital+ molecular mechanics”(ONIOM)和密度泛函理论(DFT)结合的方法,计算了H-ZSM-5催化甲苯与碳酸二甲酯(DMC)和甲醇甲基化反应机理.考察了反应物吸附和产物脱附.描述了主要的中间物种和过渡态的结构.用计算的速率常数来估计甲苯甲基化反应的动力学活性.H-ZSM-5催化的甲苯与DMC和甲醇甲基化的机理不同.甲苯和DMC甲基化包括DMC完全解离,接着甲基化生成二甲苯异构体.相比而言,在甲苯甲基化反应中,甲醇作为甲基化试剂的活性比DMC更好.甲苯和甲醇甲基化的分步反应路径和联合反应路径的本征活化能相似.在773 K,分步反应路径的速率常数比联合反应路径更高.在甲苯和这两种试剂甲基化的反应中,生成对二甲苯为动力学优先,而间二甲苯为能量最低产物.我们的计算结果和实验观察到的现象一致.