中文摘要：

应用密度泛函理论和量子力学与分子力学联合的ONIOM2方法对含Ti的MFI分子筛（TS-1）中Ti^4＋离子在三种不同骨架落位上所表现的Lewis酸性进行了理论研究．利用碱性探针分子（CO，NH3，乙腈和吡啶）在骨架Ti活性中心的吸附作用，对吸附络合物的几何结构和吸附能进行了计算，并通过自然键轨道（NBO）分析考察了吸附络合物的电子结构．结果表明，骨架Ti在T12位表现出明显的Lewis酸性，对NH3分子有较强的吸附作用．NBO分析表明，骨架Ti活性中心的Lewis酸性是由于Ti-O键的空σ反键轨道接受碱性探针分子提供的孤对电子；NH3分子吸附导致Ti^4＋离子由近正四面体中心对称变为五配位的三角双锥对称．

英文摘要：

The Lewis acidity of Ti-containing MFI zeolite TS-1 with Ti^4 ＋ ion located at different tetrahedral sites was studied using the density functional theory and ONIOM2 method. The adsorption of basic probe molecules such as CO, NH3, acetonitrile, and pyridine on the Ti active centers was explored, and the geometry and adsorption energy of the adsorption complexes were calculated. Natural bond orbital （NBO） analysis was used to examine the electronic structure of the complexes. The results indicate that Ti located at the T12 site of MFI presents obvious Lewis acidity where NH3 molecules can be strongly adsorbed. NBO analysis shows that the Lewis acidity induced by framework Ti derives from the empty σ ＊ （Ti-O） orbital, which can accept the lone pair electrons of basic probe molecules. The adsorption of NH3 induces Ti^4＋ to change from quasi-tetrahedral symmetry to trigonal-bipyramids symmetry.