ZSM-5分子筛中Al的分布及Ga改性对其酸性强度的影响

Location of al in zsm-5 zeolite and the acid strength of ga/zsm-5

利用量子化学理论研究了ZSM-5分子筛中骨架Al的分布,计算采用11T簇模型[(HSiO)_3 SiO(H)M(OSiH)_2OSi(OSiH)_3](M=Si,Al,Ga),和B3LYP/6-31G(d)的计算方法,得到了T1,T2,T12位置上的Al/Si,(Al,H)/Si替代能,结果表明11T簇模型中骨架Al优先取代于ZSM-5分子筛中的T12位。采用MP2/6-311++G(3df,2p),计算得到Al/ZSM-5、Ga/ZSM-5分子筛的质子亲和势分别是316.6 kcal/mol,319.4 kcal/mol;B3LYP/6-31G(d)的方法计算吡啶在分子筛及改性分子筛上的吸附能分别是-25.35 kcal/mol和-23.02 kcal/mol。研究结果表明Ga改性ZSM-5分子筛降低了其酸性强度。

Quantum Chemistry theory bas been applied to study the location of aluminum atoms in ZSM-5 zeolite and Bronsted acidity of isomeorphously substituted ZSM-5 by Ga. The calculation was carried out at B3LYP/6-31G（d） and MP2/6-311＋＋G（3df, 2p） leve based on the 11T cluster model [（HSiO）3SiO（H）M（OSiH）2OSi（OSiH）3], where M=Si, Al or Ga. According to the calculated Al/Si substitution energies, （Al, H）/Si substitution energies of Tlsit, T2sit, T12sit, we propose that the most favorable sites for Al substitution is at T12 sit. The resultes of calculation are the PA of Al/ZSM-5 and Ga/ZSM-5 are 316.6 kcal/mol, 319.4 kcal/mol; the adsorption energies of Pyridine on Al/ZSM-5, Ga/ZSM-5 are -25.35 kcal/mol and -23.35 kcal/mol. Therefore, The predicted order of the acid strength agrees with experimental findings and it is： Ga/ZSM-5 〈Al/ZSM-5, Ga modified ZSM-5 zeolite can improve the selectivity of target product in acid reaction.