Solid-state NMR study on dealumination mechanism of H-MOR zeolite by high-temperature hydrothermal treatment

Abstract

The acidity of zeolites which plays an important role in various catalytic reactions is closely related to framework aluminum. After dealumination, not only the Si/Al ratio of zeolites increases, but also new aluminum species are generated. Furthermore, the pore structure and catalytic performance of zeolites are affected. High-temperature hydrothermal treatment, as a frequently-used method of dealumination, is widely concerned. However, the dealumination mechanism of zeolite during the hydrothermal treatment is not clear. In this work, the existence of different Al species in H-MOR during the process of high-temperature hydrothermal treatment was confirmed by 27Al MQ MAS NMR. It was found that the second tetrahedrally coordinated framework aluminum (Al(IV)-2) species is the precursor of amorphous non-framework aluminum transformed from framework aluminum of H-MOR during high-temperature hydrothermal treatment. Besides, the 1H-1H DQ-SQ NMR was used to study spatial proximity between different hydroxyl species. Finally, the dealumination process of H-MOR zeolite is revealed: Al(IV)-1 species firstly transformed into Al(IV)-2 species, and then evolved into pentahedrally coordinated aluminum (Al(V)) species and hexahedrally coordinated aluminum (Al(VI)) species.