Production of light olefins from methanol over modified H-ZSM-5: effect of metal impregnation in high-silica zeolite on product distribution

Abstract

Improvement of H-ZSM-5 catalyst to convert methanol to light olefins was studied in this research. High-silica H-ZSM-5 zeolite (Si/Al = 200) was prepared by a hydrothermal method and modified by impregnation with different promoters (Cs, Mg, Ag, Mn, Fe, Ni, Ir, or P). The parent and modified catalysts were characterized using X-ray diffraction (XRD) analysis, inductively coupled plasma (ICP) spectrometry, scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) measurements, Fourier-transform infrared (FT-IR) spectroscopy, and NH3-temperature programmed desorption (TPD) measurements. Performance tests were performed for all samples in a fixed-bed reactor at 480 °C, 1 bar, and methanol weight hourly space velocity (WHSV) of 0.9 h−1, using feed with methanol to water weight ratio of unity. The results revealed that the parent catalyst was stable during the impregnation process and that the promoted catalysts exhibited nearly the same crystallinity and BET surface area compared with the parent sample. The acidity of the catalysts was decreased after impregnation of promoters except for Ag. It was found that the propylene selectivity was strongly dependent on the structure, texture, and acidic properties of the catalysts, being affected by the type of promoter. The phosphorus- and iron-promoted H-ZSM-5 catalysts showed high propylene selectivity (45.7 % and 43.7 %, respectively) and good catalytic stability, which was attributed to the moderate density and distribution of acidic sites over these catalysts.