Catalytic Conversion of Methanol to Propylene over HZSM-5 Modified by NaOH and (NH4)2SiF6

A sequential modification by sodium hydroxide (NaOH) and ammonium hexafluorosilicate ((NH4)(2)SiF6) solution was used for preparing MTP (methanol to propylene reaction) catalyst for the first time. The parent and modified samples were characterized by diverse techniques including powder X-ray diffraction (XRD), X-ray fluorescence (XRF) spectroscopy, N-2 adsorption-desorption, transmission electron microscopy (TEM), and NH3 temperature-programmed desorption (NH3-TPD). The effect of modification on the physicochemical properties, such as framework, chemical composition, texture, and acidity, were investigated in detail. The results showed that the mesopore volume of the zeolite catalyst increased significantly following sequential NaOH and (NH4)(2)SiF6 modification. The acidity was also modulated effectively. The composite modification method successfully overcame the disadvantages associated with individual simple alkali and (NH4)(2)SiF6 treatments. For instance, using a simple alkali treatment would destroy the framework of the zeolite easily, whereas using a simple (NH4)(2)SiF6 treatment would only modify the external surface of the zeolite owing to the limited diffusion of the ammonium hexafluorosilicate molecule. When used in MTP reaction, the induction period of the composite modified sample was greatly shortened, and the initial selectivity for propylene increased to 43% under the following operating conditions: T=470 degrees C, p=0.1 MPa (p(MeOH)=50 kPa), and weight hourly space velocity (WHSV)=2 h(-1). Moreover, the composite modified zeolite catalyst exhibited significantly improved stability, and the catalytic lifespan was triple that of the parent sample.