C2-C2直链烯烃在HY 和H-ZSM-5分子筛上的吸附

采用ONIOM(B3LYP/6-311++G(d,p):UFF)分层计算方法, 研究了C₂-C₅直链烯烃在HY 和H-ZSM-5 分 子筛上的吸附性质. 理论计算结果表明: 烯烃与分子筛的Br?nsted 酸性位相互作用形成π配位超分子复合物; 随着碳链的增长, 烯烃的吸附能增加, 增加量近似为一个常数(HY 分子筛: 约12 kJ·mol^-1; H-ZSM-5 分子筛: 约 25 kJ·mol^-1), 与烷烃在分子筛上的吸附具有相同的规律. 双键位置对烯烃的吸附能影响很大, 2位烯烃的吸附能 要远大于1 位烯烃的吸附能. 不同类型分子筛对烯烃的吸附性能也有很大差别, 由于局域效应的影响, 小孔径 H-ZSM-5分子筛上的吸附能大于大孔径的HY分子筛,而且碳链越长,这种差别越大.从微观结构上看,吸附的烯 烃与H-ZSM-5分子筛酸性位的距离要远大于它们与HY分子筛酸性位的距离, 这是由于不同类型分子筛的微孔 结构产生的范德华作用是不同的,这种作用随着孔径的减小而增强.前线轨道分析表明, 对于小分子烯烃,大孔径 HY分子筛对其催化活性相近,而小孔径H-ZSM-5分子筛随着烯烃碳原子数的增加催化活性有减弱的趋势.

Adsorption of Linear C2-C5 Olefins on HY and H-ZSM-5 Zeolites

The adsorption properties of linear C₂-C₅ olefins on HY and H-ZSM-5 zeolites were studied by the ONIOM(B3LYP/6-311++G(d,p):UFF)method. The results indicate that microcosmic interactions of the olefin molecules with the Br?nsted acid sites of the zeolites lead to the formation of π-complexes. The adsorption energies of olefins on zeolites increase with an increase in the number of carbon atoms, and the amount of increase is approximately constant (HY zeolites: ca 12 kJ·mol^-1; H-ZSM-5 zeolites: ca 25 kJ·mol^-1), which agrees well with the adsorption properties of alkanes on zeolites. The position of the double bond has a fairly large effect on the adsorption energies of olefins. The adsorption energies of 2-olefins are much higher than those of 1-olefins. The adsorption energies of olefins on the different types of zeolites also show a significant difference. The adsorption energies of olefins on small pore H-ZSM-5 zeolites are much larger than those on large pore HY zeolites. Furthermore, the confinement effect in the different types of zeolites is more obvious when the number of carbon atoms increase. From the microstructure, the distance between the adsorbent molecule and the acidic proton in the H-ZSM-5 zeolite is much bigger than that between the adsorbent molecule and the acidic proton in the HY zeolite. These are mainly attributed to differences in the van der Waals interactions for the different types of zeolites, and the small pore zeolites have much stronger van der Waals interactions. Frontier orbital calculations indicate that the catalytic activity of the large pore HY zeolite is similar for small olefins while the catalytic activity of the small pore H-ZSM-5 zeolite decreases slightly with increasing carbon number.