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.

In order to further understand the deactivation behavior of a NiMo/HZSM-5 catalyst system used for fluid catalytic cracking (FCC) gasoline hydro-upgrading, fresh and coked catalysts after operation for different times-on-stream (TOS) with FCC gasoline as a feedstock were characterized by X-ray diffraction, Fourier transformed infrared (FTIR) spectroscopy, nitrogen adsorption, and temperature-programmed desorption of ammonia, as well as FTIR analysis of adsorbed pyridine. The results showed that the amount, nature, and location of coke formed in the catalysts depended upon TOS. Coke was preferentially formed on the strong acid sites, especially on the strong Lewis acid sites in the pore channels and/or on the external surface of the catalysts, resting with the size of the reactant molecules in FCC gasoline. Coke formation led to increases in the selectivities to C8, C9, and C9 + aromatics and decreases in the selectivities to benzene and toluene in the aromatics products. Using real FCC gasoline as a feedstock, this work showed that the deactivation behavior of the HZSM-5 zeolite-based catalyst was different from that obtained using model compounds as feedstocks due to the wide size distribution of the hydrocarbon molecules in FCC gasoline and to the complex reaction mechanisms among these components. The results provided some clues for finely tuning the physicochemical properties of the catalyst to further enhance its on-stream stability.

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