MCM-22分子筛酸性的DFT理论计算研究

本文利用量子力学中的密度泛函理论(DFT)计算，研究了MCM-22分子筛上骨架Al在8个不同的T位的分布和Br?nsted酸的落位及强度。所有计算基于分子筛的8T簇模型 (H₃SiO)₃Si-O(H)-T(OSiH₃)₃(T=Si，Al)，采用DFT的BLYP方法，所有原子均应用DNP基组。通过计算(Al，H)/Si替代能和质子亲和势，得出推论：MCM-22分子筛中骨架Al的最有利落位在T1，T4，T3和T8位。而形成Br?nsted-酸的最可能的位置为Al1-O3-Si4，Al4-O3-Si1，Al3-O11-Si2和Al8-O10-Si2桥基。Al1-O3H-Si4和Al4-O3H-Si1位的酸性强度接近，Al3-O11H-Si2和Al8-O10H-Si2位的酸性分别略低于和略高于前两个酸位。通过计算模板剂分子六次甲基亚胺(HMI)与B-酸中心的相互作用，进一步探讨了HMI对分子筛中Al落位的靶向作用。

Density Functional Theory Studies on the Acidity of MCM-22 Zeolite

Density functional theory has been applied to study the distribution of the framework aluminum atoms at eight inequivalent T sites and the acidity of Br?nsted acid in MCM-22 zeolite. The calculations were performed with BLYP hydride functional and DNP basis set, using 8T cluster model (H₃SiO)₃Si-O(H)-T(OSiH₃)₃, where T=Si or Al. According to the calculated (Al, H)/ Si substitution energies and proton affinity, it was proposed that the most favorable sites for Al substitution are T1, T4, T3 and T8 sites. The preferable Br?nsted acid sites locate at Al1-O3H-Si4, Al4-O3H-Si1, Al3-O11H-Si2 and Al8-O10H-Si2 bridging groups. The acidity of Al1-O3H-Si4 and Al4-O3H-Si1 is equivalent, Al3-O11H-Si2 and Al8-O10-Si2 sites are less and higher than the first two Br?nsted acid sites, respectively. The orientating effect of the hexamethyleneimine(HMI) template molecule on the location of the framework aluminum atoms was also considered by calculating the interaction energies between the cationic template HMIH⁺ and the zeolite anionic centers Zeo-AlO₄^-.