Density Functional Calculations on the Distribution, Acidity, and Catalysis of Ti-IV and Ti-III Ions in MCM-22 Zeolite

Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions. In this work, density functional calculations were used to explore three current concerns: 1) the distributions of Ti-IV and Ti-III ions in the MCM-22 zeolite; 2) the Lewis acidity of the Tilv and Ti-III sites; and 3) activation of alkane C-H bonds by photocatalysis with Ti-doped zeolites. Neither the Ti-IV nor Ti-III ions are randomly distributed in the MCM-22 zeolite. The orders of relative stability are very close for the eight Ti-IV and Ti-III sites, and the T3 site is the most probable in both cases. The wavelengths for Ti-IV-Ti-III excitations were calculated to lie in the range lambda=246.9-290.2 nm. The Ti3(IV) site shows Lewis acidity toward NH3 in two different modes, and these two modes can coexist with each other. The calculated Tilv coordination numbers, Ti-IV-O bond elongations, and charge transfers caused by NH3 adsorption are in good agreement with previous results. Similarly, two different NH3 adsorption modes exist for the Ti3(III) site; the site that exhibits radical transfer from the lattice O to N atoms is preferred due to the higher adsorption energy. This indicates that the Ti3(III) site does not show Lewis acidity, in contrast to the Ti3(IV) site. At the Ti3(III) site, the energy barrier for activating the methane C-H bond was calculated to be 33.3 kJ mol(-1) and is greatly reduced by replacing the hydrogen atoms with methyl groups. In addition, the reactivity is improved when switching from MCM-22 to TS-1 zeolite. The studies on the various Ti species reveal that lattice O atoms rather than Ti-III radicals are crucial to the activation of alkane C-H bonds. This work provides new insights into and aids understanding of the catalysis by isolated Ti species in zeolites