Theoretical Study of the Lewis Acidity of TS-1 Zeolite

The Lewis acidity of Ti-containing MFI zeolite TS-1 with Ti4+ 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 front the empty a sigma * (Ti-O) orbital, which can accept the lone pair electrons of basic probe molecules. The adsorption of NH3 induces Ti4+ to change from quasi-tetrahedral symmetry to trigonal-bipyramids symmetry.