Properties and Characterization of Modified HZSM—5 Zeolites
摘要
Physicochemical and catalytic properties of phosphorus and boron modified HZSM-5 zeolitestreated with 100% steam at 673 K were investigated. The acidity and distribution of acidic sites were stud-ied by infrared spectroscopy using pyridine as probe molecule and temperature programmed desorption(TPD) of ammonia. The structure of the samples was characterized by XRD, and the textural propertiesof the catalysts were determined by nitrogen isothermal adsorption-desorption measurements and scanningelectron microscopy (SEM). The XRD results show that the modified samples have no novel crystallinephase, indicating a high dispersion of phosphorus and boron species. After treatment, the microporousvolume and surface area of the samples markedly decrease, implying the blockage of the channel. The nitro-gen adsorption-desorption measurements suggest that the isothermal type of all samples is a combinationof isothermal type Ⅰ and Ⅳ, and all hysteresis loops resemble the H4-type in the IUPAC classification.The total acidity of the modified samples, determined by pyridine adsorption IR and TPD of ammonia,decreases in contrast to that of the parent HZSM-5. The conversion of n-heptane over P and B steam-modified HZSM-5 is higher than that of P and B-modified HZSM-5 zeolites but lower than that of theparent HZSM-5.