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

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 Ti-O-Si bridges to produce two independent Bronsted acid sites, i.e. the OWH and bridge OH group. The latter shows stronger Bronsted acidity, which can potentially act as a catalytic site. The hydrolysis reaction of the Lewis acidic sites make the tetra-coordinated Ti pentacoordinated. The tetrahedral Ti-IV ions in zeolite lattices could be reduced to Ti-III ions via H atom adsorption. The Ti-III-SSZ-13 zeolites exhibit stronger Lewis and Bronsted acidity than Ti-III-SSZ-13 zeolites. The adsorption of NH3 and pyridine inside the channels of both Ti-IV-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 Bronsted acidity of the bridge OH in the Ti-SSZ-13 hydrolysis follows the order of OWH-Ti-OH-Si > Ti-2(OH)-Si > Ti-OWH,Si-OH 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. (C) 2016 Elsevier Inc. All rights reserved.