The Effect of Fluorine Modification on Catalytic Performance of Nanosized HZSM-5 Zeolite for Conversion of Methanol to Propene

Based on the comparison of catalytic performance of nanosized and microsized HZSM-5 zeolites for the conversion of methanol to propene, the fluorine-modified nanosized HZSM-5 zeolites were prepared by impregnation with NH4F aqueous solution. The parent and modified samples were characterized by transmission electron microscopy, N2 adsorption, X-ray diffraction, temperature-programmed desorption of ammonia, and infrared spectroscopy of adsorbed pyridine. Their performance for conversion of methanol to propene was tested in a continuous flow fixed-bed micro-reactor at atmospheric pressure, 500 oC, and methanol space velocity (WHSV) of 1.0 h–1. The results showed that when the content of fluorine was lower than 10%, the amount and intensity of acidic sites on the fluorinated nanosized HZSM-5 zeolites decreased continuously with fluorine content increasing, which resulted in the enhanced selectivity for propene and catalyst stability. However, the modification with too higher content (15%) of fluorine decreased the catalyst stability due to the distinctly reduced amount of acid sites, surface area, and pore volume. It was concluded that after modification with appropriate amount (10%) of fluorine, the selectivity for propene and the reaction time that can maintain total conversion of methanol was increased significantly to 46.7% and 145 h from 30.1% and 75 h over the parent nanosized HZSM-5 zeolite, respectively.