Catalytic Conversion of Methanol to Propylene over (NH4)(2)SiF6-Modified Nanosized HZSM-5 Zeolite

(NH4)(2)SiF6-modified nanosized HZSM-5 zeolite was prepared and investigated as a catalyst for methanol to propylene conversion. The effects of this modification on the framework, textural properties and acidity of both the parent and the modified HZSM-5 zeolite were investigated by powder X-ray diffraction (XRD), Al-27 magic angle spinning nuclear magnetic resonance (Al-27 MAS NMR), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), N-2 adsorption, transmission electron microscopy (TEM), temperature-programmed desorption of ammonia (NH3-TPD), and infrared spectroscopy of adsorbed pyridine (Py-IR). The catalytic performance of these materials on the methanol to propylene (MTP) conversion process was tested under operating conditions of T=450 degrees C, p=0.1 MPa (p(MeOH)=50 kPa) and WHSV=1 h(-1). The results showed that surface aluminum on the nanosized ZSM-5 zeolite could be selectively removed by the (NH4)(2)SiF6 solution and that the number of acidic sites on the HZSM-5 zeolite gradually decreased with increasing (NH4)(2)SiF6 concentration. Moreover, after modification with an optimally concentrated (NH4)(2)SiF6 solution, an obvious increase in specific surface area as well as mesopore volume was observed for the nanosized HZSM-5 with a resulting dramatic improvement in the catalytic performance of this material for the MTP reaction. Both the propylene selectivity and propylene/ethylene (PIE) mass ratio resulting from use of the modified HZSM-5 increased significantly to 45.1% and 8.0, as compared to results of 28.8% and 2.6 obtained with the original material. In addition, the catalytic lifespan of the modified zeolite was double that of the original.