苯和甲醇在H-ZSM-5催化剂上甲基化的反应机理

Reaction Mechanism of Benzene Methylation with Methanol over H-ZSM-5 Catalyst

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,在5T、12T、104T_9和104T_(12)H-ZSM-5模型中研究了苯和甲醇甲基化的分步和协同机理。描述了中间体物种和过渡态的结构。考察了H-ZSM-5催化剂Br?nsted(B)酸强度对苯和甲醇甲基化反应机理的影响。反应活化能结果表明,在B酸强度更强的H-ZSM-5催化剂上,苯和甲醇甲基化反应更容易发生,反应活化能更低。随着B酸强度增强,分步机理的反应活化能比协同机理的反应活化能降低的更多。B酸强度增强对分步机理更有利。当分步机理成为主导反应路径时,分步机理中甲醇脱水步骤生成的甲氧基中间体进一步生成大体积烃类的副反应会导致H-ZSM-5催化剂因积炭而失活。合理调变H-ZSM-5催化剂的酸强度对提高催化剂的催化活性和稳定性有重要意义。

The stepwise and concerted mechanisms of benzene methylation with methanol were studied with the 5T, 12T, 104T9, and 104T12 H-ZSM-5 models using the ＂our own-N-layered integrated molecular orbital ＋ molecular mechanics＂ （ONIOM） in combination with density functional theory （DFT） methods. The structures of intermediate species and transition states were described. The effect of the Brensted （B） acid strength of H- ZSM-5 catalyst on the reaction mechanism of benzene methylation with methanol was considered. The reaction activation energy results indicate that benzene methylation with methanol preferentially occurs over H-ZSM- 5 catalyst with greater B acid strength, and a lowering of the activation barrier was observed. With increasing B acid strength, the reaction activation energy of the stepwise mechanism decreases more than that of the concerted mechanism. Increasing the B acidic strength is more beneficial to the stepwise mechanism. When the stepwise mechanism becomes the dominant reaction path, the secondary reaction arising from further formation of bulky hydrocarbons through the methoxide intermediate produced in the methanol dehydration step of the stepwise mechanism might lead to the inactivation of the H-ZSM-5 catalyst owing to coke formation. Reasonable modulation the acid strength of the H-ZSM-5 catalyst is important in improving its catalytic activity and stability of the catalyst.