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Density Functional Calculations on the Distribution, Acidity, and Catalysis of Ti IV and Ti III Ions in MCM‐22 Zeolite
Author(s) -
Yang Gang,
Zhou Lijun,
Liu Xianchun,
Han Xiuwen,
Bao Xinhe
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201002241
Subject(s) - catalysis , zeolite , chemistry , adsorption , lewis acids and bases , inorganic chemistry , bond energy , ion , alkane , molecule , organic chemistry
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 Ti IV and Ti III sites; and 3) activation of alkane CH 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 λ =246.9–290.2 nm. The Ti3 IV site shows Lewis acidity toward NH 3 in two different modes, and these two modes can coexist with each other. The calculated Ti IV coordination numbers, Ti IV O bond elongations, and charge transfers caused by NH 3 adsorption are in good agreement with previous results. Similarly, two different NH 3 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 CH 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 CH bonds. This work provides new insights into and aids understanding of the catalysis by isolated Ti species in zeolites.