title: Acidity adjustment of HZSM-5 zeolites by dealumination and realumination with steaming and citric acid treatments.
abstract: This article describes a novel method for acidity adjustment of HZSM-5 zeolites with steaming and citric acid treatments and demonstrates the realumination effect of citric acid on HZSM-5 zeolites dealuminated by steaming. A series of modified HZSM-5 zeolites were prepared by streaming and/or acid treatments and characterized by means of X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF), (27)Al MAS NMR spectroscopy, hydroxyl infrared spectroscopy (OH-IR), pyridine-adsorbed infrared spectroscopy, and N(2) adsorption in the present investigation. The results showed that compared with single HCl or citric acid treatment, steaming treatment, and steaming/HCl treatments, citric acid treatment after steaming exclusively increased the amount of framework Al due to reinsertion of extraframework Al into the defective sites of the steamed HZSM-5 framework. This realumination effect of the citric acid treatment on the steamed HZSM-5 zeolite, which is reported here for the first time to the best of our knowledge, could nearly recover the pore structure of the steamed zeolite to that of the parent HZSM-5 zeolite and appropriately tailor the amount and strength of different acid sites, which sheds light on optimizing the physicochemical properties of HZSM-5 zeolites. It was also found that the steaming treatment prior to the citric acid treatment was the precondition of the realumination of HZSM-5 zeolites, suggesting that the lattice defect sites generated during steaming were necessary for citric acid to work.
authors.affiliation: State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China.
authors.forename: Yu
authors.initials: Y
authors.lastname: Fan
authors.forename: Xiaojun
authors.initials: X
authors.lastname: Bao
authors.forename: Xiuying
authors.initials: X
authors.lastname: Lin
authors.forename: Gang
authors.initials: G
authors.lastname: Shi
authors.forename: Haiyan
authors.initials: H
authors.lastname: Liu