Effect of acid treatment of zeolite on catalytic performance of Mo/HZSM-5 for methane dehydroaromatization

The pretreatment with HNO3 solution was applied to modify HZSM-5 zeolite, and the Mo/EZSM-5 catalyst prepared by using the modified HZSM-5 zeolite as support showed higher stability in methane dehydroaromatization than that prepared by using unmodified HZSM-5 zeolite. Meanwhile, the formation of coke was distinctly suppressed on the Mo catalyst supported on the modified HZSM-5 zeolite. The effect of acid treatment on the structure of HZSM-5 zeolite and Mo/HZSM-5 catalyst was studied by SEM, XRD and H-1 MAS NMR techniques. The results suggested that the acid treatment of HZSM-5 led to an obvious dealumination and a decrease in the crystallinity of HZSM-5 zeolite. A part of Al species was extracted from the zeolite framework and formed extra-framework Al-OH groups, resulting in an evident decrease of the number of Bronsted acid sites remained on the zeolite surface. As demonstrated in the 1H MAS NMR experiment, the number of B acid sites per unit cell was 1.12 on the Mo/HZSM-5 catalyst prepared using unmodified zeolite as support, but it was only 0.88 on the Mo catalyst supported on modified HZSM-5 zeolite. The result indicated that too many B acid sites on the catalyst would cause severe coking and decrease the catalyst stability.