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Density functional theory investigations into the structures and acidity properties of Ti-doped SSZ-13 zeolite

Research paper by Lina Han, Cui Wen; Zhiping Wu; Jiancheng Wang; Liping Chang; Gang Feng; Rongbin Zhang; Dejin Kong; Jianwen Liu

Indexed on: 18 Oct '16Published on: 26 Sep '16Published in: Microporous and Mesoporous Materials



Abstract

Publication date: 1 January 2017 Source:Microporous and Mesoporous Materials, Volume 237 Author(s): Lina Han, Cui Wen, Zhiping Wu, Jiancheng Wang, Liping Chang, Gang Feng, Rongbin Zhang, Dejin Kong, Jianwen Liu The tetra-coordinated titanium incorporated SSZ-13 zeolites has been investigated using the dispersion-corrected periodic density functional theory. The adsorption of H2O inside the channels of the Ti-SSZ-13 zeolites were systematically studied. The most stable adsorption site was found in the main channel of the Ti-SSZ-13 zeolites for the water molecule adsorption. The water molecule could react with the Ti sites yielding partial hydrolysis of the TiOSi bridges to produce two independent Brønsted acid sites, i.e. the OwH and bridge OH group. The latter shows stronger Brønsted acidity, which can potentially act as a catalytic site. The hydrolysis reaction of the Lewis acidic sites make the tetra-coordinated Ti penta-coordinated. The tetrahedral TiIV ions in zeolite lattices could be reduced to TiIII ions via H atom adsorption. The TiIII-SSZ-13 zeolites exhibit stronger Lewis and Brønsted acidity than TiIV-SSZ-13 zeolites. The adsorption of NH3 and pyridine inside the channels of both Ti-SSZ-13 and H-Ti-SSZ-13 zeolites were also studied. The results show that the NH3 and pyridine could physisorb in the Lewis acid sites of these zeolites, indicating the weak Lewis acidity for the substituted Ti sites. Deprotonation energy analysis indicates that the Brønsted acidity of the bridge OH in the Ti-SSZ-13 hydrolysis follows the order of OwHTiOHSi > Ti2(OH)Si > TiOwH,SiOH, which is opposite to the stability of the structures. The results provide new insights for synthesis for the SSZ-13-type zeolites and fundamental information for the zeolitic catalyst designation to enhance the catalytic performance. Graphical abstract