Realumination of dealuminated HZSM-5 zeolites by acid treatment, the impact of AlF distribution and its catalytic cracking performance of alkanes

Abstract

Realumination of dealuminated HZSM-5 zeolites by acid treatment has always been controversial and puzzling. Through characterizations of XRD, nitrogen adsorption, NH3-TPD, IR, XPS and 27Al MAS NMR, this work confirmed realumination of dealuminated HZSM-5 zeolites could indeed be achieved after treated with HCl solution. And it was found mainly a portion of the penta-coordinated extra-framework aluminia (EFAl) were reinserted into the framework during the process of realumination. Moreover, the characterizations of ICP-AES and UV–Vis-DRS of Co2+-exchanged zeolites indicated the realumination capacity was closely related to the distribution of framework Al (AlF). Realumination was more likely achieved when there was lower content of Al pairs and less AlF located in the channel intersections in HZSM-5. Finally, realuminated zeolites exhibited a higher conversion of n-octane due to its more Brønsted acid sites than dealuminated HZSM-5 in catalytic cracking reaction. The significantly lower content of Al pairs in realuminated zeolites compared to parent HZSM-5 inhibits the hydride transfer process to a certain extent, resulting in a higher selectivity of propylene.