C_(2~6)烯烃在H-ZSM-5分子筛周期性模型上吸附的理论研究

采用密度泛函理论中的PBE-D方法研究了C2~6直链烯烃以及丁烯异构体在H-ZSM-5分子筛周期性模型上的吸附行为。结果表明,对于C2~6直链烯烃,随着碳数的增加,烯烃的吸附能以-12 kJ/mol的常数线性增大,体系中色散校正能ED以-13 kJ/mol的常数线性增大,而不考虑范德华力的能量E*随碳数增加变化不大,只在C5和C6时由于空间阻力凸显而减小。ED显著大于E*,说明烯烃在分子筛孔道中吸附时范德华力起主要作用,且碳数增加其影响增大。丁烯异构体吸附能大小顺序为:反式-2-丁烯顺式-2-丁烯正丁烯异丁烯。正丁烯的3种异构体ED相近且都比正丁烯的大,3种异构体的吸附能差异由E*不同引起。差分电荷密度分析表明,烯烃双键与酸性中心之间有电子聚集,且聚集程度与π配位作用相一致,而且酸性中心H原子有质子化趋向,O原子及其周围电子增加。

A theoretical study on the adsorption of C2~6 olefins on the H-ZSM-5 zeolite of periodic model

The adsorption behaviors of linear C_2~6 olefins and butene isomers on the H-ZSM-5 zeolite of periodic model were studied by the PBE-D method. The adsorption energies (EPBE-D) and the dispersive correction energies (E_D) of C_2~6 linear olefins on the zeolite exhibit a linear increase with the number of carbons by -12 kJ/mol and -13 kJ/mol, respectively. The adsorption energies without dispersive correction (E^*) is not changed obviously, but it is decreased in the cases of increased space resistance for pentene and hexane. The value of E_D is much larger than that of E^*, implying that van der Waals' force plays a key role in the adsorption of olefins on the zeolite and its influence on the adsorption is dependent on the carbon number. The adsorption energies of butene isomers decrease in the order of trans-2-butane > cis-2-butene > n-butene > isobutene. Three kinds of n-butene isomers are similar in their E_D value, which are larger than that of n-butene. The difference in adsorption energy among the three isomers is caused by E^*. The differential charge density analysis shows that the electrons between the alkene double bond and the acidic center are gathered, in consistent with the strength of π-coordination; H atom in the acid site turns to be protonated, with electrons transferred to the nearby O and other atoms.