HY分子筛催化异丁烷/丁烯烷基化反应中氢负离子转移过程的模拟

A Computational Study on the Hydride Transfer Process in Butene Alkylation With Isobutane Over HY Zeolite

建立了120T HY分子筛模型,应用QM/MM方法,揭示了叔/仲丁基烷氧基团(t-butyl alkoxide/s-butyl alkoxide,TBA/SBA)与异丁烷分子发生氢负离子转移反应的机理,并分析比较了二者的异同。结果表明,碳正离子是氢负离子转移反应的过渡态。与TBA相比,SBA与异丁烷分子发生氢负离子转移反应的能垒高出27.5kJ/mol。这是由于TBA的C—O键长大于SBA,因而C—O键断裂所需能量更低,导致反应能垒更低。C—O键断裂后,TBA生成叔丁基碳正离子,SBA生成仲丁基碳正离子,前者中心碳原子所带电荷大于后者,因而与仲丁基碳正离子相比,叔丁基碳正离子更容易与异丁烷分子形成C—H—C"三中心两电子"结构。

The mechanism of hydride transfer from isobutane to tbutyl alkoxide/sbutyl alkoxide(TBA/SBA)was calculated by using QM/MM method on the120T HY zeolite model.It was observed that carbonium ion is the transition state of the reaction,and the reaction barrier of hydride transfer from SBA to isobutane is28kJ/mol higher than that of TBA.This is because the C-O bond length of TBA is longer than that of SBA,indicating much easier breaking of the bond.Moreover,this leads to the lower reaction barrier of hydride transfer from TBA to isobutane.After breaking of the C-O bond,tbutyl carbonium ion and sbutyl carbonium ion were formed from TBA and SBA,respectively.In addition,the charge of the central C atom of the tbutyl carbonium ion is higher than that of sbutyl carbonium ion.Consequently,it is easier for tbutyl carbonium ion to form the hydride shared intermediate with isobutane.