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

Abstract

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 TiIV and TiIII ions in the MCM‐22 zeolite; 2) the Lewis acidity of the TiIV and TiIII sites; and 3) activation of alkane CH bonds by photocatalysis with Ti‐doped zeolites. Neither the TiIV nor TiIII ions are randomly distributed in the MCM‐22 zeolite. The orders of relative stability are very close for the eight TiIV and TiIII sites, and the T3 site is the most probable in both cases. The wavelengths for TiIV–TiIII excitations were calculated to lie in the range λ=246.9–290.2 nm. The Ti3IV site shows Lewis acidity toward NH3 in two different modes, and these two modes can coexist with each other. The calculated TiIV coordination numbers, TiIV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 Ti3III 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 Ti3III site does not show Lewis acidity, in contrast to the Ti3IV site. At the Ti3III 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 TiIII 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.

Lewis and Brönsted acidic sites in M4+-doped zeolites (M=Ti, Zr, Ge, Sn, Pb) as well as interactions with probe molecules: A DFT study

“…The [(HO) 3 SiO] 4 M fragment and probe molecules were defined as the high-level region and treated with the B3LYP/B1 method. The rest as the low-level region was treated at B3LYP/3-21G level [30,51]. The boundary Si H distances were set to 1.500Å.…”

“…Generally, the larger the M 4+ radii, the more unfavorable the substitutions. The LUMO energies, fukui functions (f M ), absolute electronegativity ( ) have been used to characterize the Lewis acidities of zeolites [28,[30][31][32]35,36,55]. The electrophilic fukui functions (f M + ) are approximated to be [56][57][58],…”

“…In the previous works [27][28][29][30], we studied the Lewis and Brön-sted acidities of TS-1 and Ti-MCM-22 zeolites, see Scheme 1. The Lewis acidities of Ti 4+ -doping zeolites have also received attention from other groups [31][32][33][34][35], while few reports have been given on other tetravalent ions (M 4+ ).…”

Adsorption, reduction and storage of hydrogen within ZSM-5 zeolite exchanged with various ions: A comparative theoretical study

“…The nH 2 /ZM III O clusters (n P 0) were optimized with B3LYP density functional. As in our previous works [12,19,[34][35][36][37], the high-and low-level regions were described by the B1 and B2 basis sets, respectively. For B1, 6-31G(d) was used for all except the heavy atoms and H 2 -related species.…”

“…1a, the cluster models with 25-T sites were used to represent the local structures of ZSM-5 zeolite, where a complete 10-membered-ring (10-MR) straight channel is included [34,35]. The Al atom resides at one of the T12 sites, and the (OSiO 3 ) 2 Al(OSiH 3 ) 2 fragment was defined as high-level region whereas the rest as low-level region [34][35][36]. The boundary Si-H and O-H distances were set to 1.500 and 1.000 Å, respectively.…”