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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“ourown-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773K,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.
Paraxylene is one of the most important petrochemical products and can be produced by toluene methylation. In this paper, “ourown-N-layered integrated molecular orbital + molecular mechanics” (ONIOM) and density functional theory (DFT) The mechanism of methylation of toluene with dimethyl carbonate (DMC) and methanol catalyzed by H-ZSM-5 was calculated and the adsorption and desorption of reactants were investigated. The structures of the main intermediate species and transition states were described. Calculated rate constants to estimate the kinetic activity of the toluene methylation reaction.H-ZSM-5 catalyzed toluene with a different mechanism of DMC and methanol methylation.Toluene and DMC methylation including DMC complete dissociation, followed by methyl To form xylene isomers. In contrast, in the toluene methylation reaction, methanol as a methylation reagent is more active than DMC.Methylation of toluene and methanol in the stepwise reaction path and in the combined reaction path The intrinsic activation energies are similar at step 773. The rate constant of the stepwise reaction path is higher than that of the combined reaction at 773 K. In the reaction of toluene and the methylation of these two reagents, As the lowest energy product. Our calculation results Consistent with the experimentally observed phenomena.