A First Principles Study On Isomorphously Substituted SSZ-13 Zeolites

SSZ-13 zeolites with well-defined channel systems and cavities have been widely used as catalysts, adsorbents and ion-exchangers in industry due to their outstanding solid-acidity, adsorption-desorption properties and shape-selectivity. Introducing the other atoms in the framework of the pure silicon zeolites will improve the hydrothermal stability, enrich the composition of the skeleton, introduce the new framework of the zolites, endow it with excellent catalytic properties and optimize the electrical and magnetic properties. The dispersion-corrected density functional theory has been used to study the trivalent ions B, Al, Ga, Fe and tetravalent ion Ti incorporated SSZ-13-type zeolites. The associated structure and Br?nsted/Lewis acidity change caused by the incorporation ions were comparatively studied, developing the potential application value of molecular sieve. Main conclusions are listed as follows:(1) The smaller radius differences of the incorporation ions are, the smaller changes of structure will be and the more acidity will be enhanced for both of the Br?nsted and Lewis sites. The trivalent Al and Na are found the most favorable ions for the synthesis of SSZ-13-type zeolites. The substitution energies show the relative synthesis difficulty level with the order of Fe > B > Ga >> Al. The Br?nsted acidity were found to follow the order of HAl-SSZ-13 > HGa-SSZ-13 ≈ HFe-SSZ-13 > HB-SSZ-13 where the Lewis acidity was found to follow the order of HGa-SSZ-13 ≈ HFe-SSZ-13 > HAl-SSZ-13 > HB-SSZ-13.(2) The water can physisorbed and decompose in Ti-SSZ-13 zeolites and the former is the main way for the adsorption of water. The water could react with the Ti sites yielding partial hydrolysis of the Ti-O-Si bridges to produce three hydrolysis products with two different Br?nsted acid sites and the sablity follow the order of Ti-OwH,Si-OH > Ti-2(OH)-Si > OwH-Ti-OH-Si. Both OwH and bridge OH show the Bronsted acidity and the latter exhibit the stronger Bronsted acidity. The Bronsted acidity of the bridge OH in the hydrolysis products follow the order of OwH-Ti-OH-Si > Ti-2(OH)-Si > Ti-OwH,Si-OH.(3) The tetrahedral Ti ion in SSZ-13 zeolite may be reduced to trivalent Ti ion, the adsorption energy of H is lager than the H2, and the reduction reaction can take placeis easily. The HTi-SSZ-13 zeolites exhibit the stronger Lewis and Bronsted acidity than Ti-SSZ-13 zeolites.