SAPO-34分子筛中多甲基苯分子与卤代甲烷偕甲基化反应的密度泛函理论研究

利用周期性密度泛函理论研究了SAPO-34分子筛催化转化卤代甲烷制取低碳烯烃反应的碳池主要成分多甲基苯分子的偕甲基化反应。氯甲烷和溴甲烷分子在SAPO-34分子筛内的吸附能分别是–18和–22 kJ/mol,由于氯和溴原子相似的电负性,氯甲烷和溴甲烷分子的吸附能并未被精确区分。以氯甲烷和溴甲烷为甲基化试剂,得到了几种多甲基苯分子的偕甲基化反应能及能垒,结果表明,六甲基苯分子(HMB)的偕甲基化反应为放热反应,而其余甲基苯分子的偕甲基化反应为吸热反应。对于上述两种甲基化试剂,体积最大的HMB均表现出最低的偕甲基化反应能垒,这可能是由于分子筛骨架与多甲基苯分子之间的静电相互作用增强了HMB的反应活性所致。

Theoretical study of the geminal methylation of methylbenzene by halomethane over HSAPO-34 molecular sieve

Periodic density functional theory calculations have been conducted using the DMol3 package to investigate the geminal methylation of a series of methylbenzenes as hydrocarbon pool species trapped within the framework of a catalyst for the conversion of methyl halides to light olefins. The adsorption energies of CH3Cl (–18 kJ/mol) and CH3Br (–22 kJ/mol) into a SAPO-34 catalyst were calculated, and the results revealed that these methylating agents were not being accurately distin-guished because of similarities in the electronegativities of their halogen atoms. The reaction ener-gies and energy barriers were also obtained for the geminal methylation reactions of a series of methylbenzenes using CH3Cl and CH3Br. The results of these calculations suggested that the geminal methylation of hexamethylbenzene (HMB) was exothermic based on the negative reaction energies, whereas the geminal methylation reactions of all of the other methylbenzenes were endothermic. Furthermore, the energy barriers for the geminal methylation of HMB with CH3Cl and CH3Br were lower than those of the other methylbenzenes evaluated in the current study, which indicated that HMB was forming strong electrostatic interactions within the structural framework of the molecu-lar sieves, and that the reactivity of the methylbenzene substrate increased as the number of methyl groups increased.