HFAU分子筛酸性位协同作用机制的理论研究

Periodic DFT Study on the Synergistic Mechanisms of Brθnsted Acid Sites on HFAU Zeolites

FAU分子筛中Brθnsted酸(简称B酸)的性质在催化过程中起着关键的作用。采用周期性密度泛函理论(DFT)的方法模拟了HFAU(H)和HFAU(3H)分子筛的稳定结构,分别考察了单Al位点(isolated Al site)和双Al位点(Al pairs site)的结构特性和电子性质及两种位点B酸位的本征酸性。通过分析B酸位的几何结构参数、电荷性质及去质子化能发现isolated Al位点的B酸位具有相似的酸强度,Al pairs位点的B酸酸性会有一定程度的变化。随后以噻吩分子为探针进一步研究了B酸位间的协同作用机制。结果发现,噻吩在Al pairs位点的吸附能明显高于isolated Al位点(-267 kJ·mol^(-1) VS-224 kJ·mol^(-1)),通过对电子性质的分析发现isolated Al位点B酸位会与噻吩作用活化部分C=C键,而Al pairs位点中的两个B酸位会同时与噻吩分子作用,使包括S—H键在内的整个噻吩环均发生了电子的重排。此项工作从电子层面揭示了B酸位的本征特性及B酸位间的协同作用机制。

The properties of Brθnsted acid sites(BAS)in FAU zeolites play a key role in the catalytic process.In this study,periodic density functional theory(DFT)was used to simulate the stable structure of HFAU(H)and HFAU(3 H)zeolites.The structural and electronic properties of isolated Al site and Al pairs site and the intrinsic acidity of BAS were investigated.It was found that the BAS of isolated Al site had similar acid strength,and the acidity of BAS of Al pairs site changed to some extent,by analyzing the geometric structure parameters,charge properties and deprotonation energies of BAS.Then,the synergistic mechanism of BAS was further studied with thiophene molecule as probe.It was found that the adsorption energy of thiophene on Al pairs site was significantly higher than that on isolated Al site(-267 kJ·mol^(-1) VS-224 kJ·mol^(-1)).Through the analysis of electronic properties,it was found that the BAS of isolated Al site interacted with thiophene to activate the C=C bond,while the two BAS of Al pairs site interacted with thiophene molecules at the same time,resulting in electron rearrangement of the whole thiophene ring including S—H bond.This work reveals the intrinsic characteristics of BAS and the synergistic mechanism between BAS at electronic level.