物理化学学报Issue(4)：754-762,9.DOI:10.3866/PKU.WHXB201302063
H-ZSM-5分子筛催化二甲苯异构化的反应机理
Isomerization Mechanism of Xylene Catalyzed by H-ZSM-5 Molecular Sieve
摘要
Abstract
The isomerization mechanism of xylene over H-ZSM-5 molecular sieve has been examined using the density functional theory (DFT) and our own-N-layered integrated molecular orbital+molecular mechanics (ONIOM) methods. The structures of intermediate species and transition states are described. The adsorption of reactant and desorption of product significantly affect the tendency of xylene to isomerize. Calculated activation energies suggest that isomerization occurs during the formation of meta-xylene within the extended pore structure of H-ZSM-5 molecular sieve. However, the produced meta-xylene is retained within the pore because of a high desorption energy, and further isomerization to form para-xylene is kinetical y favorable. The acid sites within the pores of the molecular sieve al ow selective formation of para-xylene. On the external surface of H-ZSM-5 molecular sieve, which lacks the steric constraints of the extended pore structure, xylene isomerizes to form meta-xylene, which can readily desorb from the active site. Such non-selective isomerization decreases the selectivity for para-xylene. Thus, external surface modification of H-ZSM-5 molecular sieve should suppress the non-selective isomerization of xylene, thereby increasing the selectivity for para-xylene by restricting isomerization to inside the pores of the molecular sieve. Calculated relative reaction rate constants for xylene isomerization also indicate that xylene isomerization occurring on the external surface of H-ZSM-5 with meta-xylene as the product has the highest reaction rate. The selectivity for para-xylene is decreased as the reaction temperature is increased.关键词
异构化机理/二甲苯/密度泛函理论/ONIOM/H-ZSM-5Key words
Isomerization mechanism/Xylene/Density functional theory/ONIOM/H-ZSM-5分类
化学化工引用本文复制引用
李玲玲,聂小娃,宋春山,郭新闻..H-ZSM-5分子筛催化二甲苯异构化的反应机理[J].物理化学学报,2013,(4):754-762,9.基金项目
The project was supported by the Program for New Century Excellent Talent in University, China (NCET-04-0268) and High Performance Computing Department of Network and Information Center, Dalian University of Technology, China.新世纪优秀人才项目(NCET-04-0268)资助和大连理工大学网络与信息化中心高性能计算部支持 （NCET-04-0268）
