A comparative computational study on the synthesis prescriptions, structures and acid properties of B-, Al- and G-incorporated MTW-type zeolites

Abstract

The B-, Al- and Ga-incorporated H-, NH₄-, Li-, Na- and K-forms MTW zeolites were comparatively studied to understand the crystallization thermochemistry, structure stabilities and acid properties using dispersion corrected density functional theory. Substitution energies were introduced to evaluate the synthesis prescriptions for MTW zeolites. The calculated results show that the trivalent and alkaline metal ions for the synthesis of the MTW zeolites are favored in the following order: Al > Ga > B and Na > K > Li > NH₄ > H. The B–O, Si–O and O–H bonds have a covalent character while Al–O bonds show an obvious ionic character. However, the character of Ga–O bonds fall in between that of Al–O and B–O bonds. Introducing alkaline metal ions significantly changes the cell volumes. As a result, the cell volumes of the H-form B-, Al- and Ga-incorporated MTW zeolites are larger than Li-form, and smaller than K-form MTW zeolites. Further investigation of their properties show that the Brønsted acid sites of the zeolites follow the order of HAl-MTW > HGa-MTW > HB-MTW. The Lewis acidity of HB-MTW is similar to that of HAl-MTW, and weaker than that of HGa-MTW. These results are in good agreement with the previous experiments and provide new insights for the MTW zeolites.