Acid Strength Control in MFI Zeolite for the Methanol-to-Hydrocarbons (MTH) Reaction

This article considers the optimization of the acid strength of MFI zeolite for the maximization of propylene selectivity and P/E (propylene/ethylene) ratio in the methanol-to-hydrocarbons (MTH) reaction. The acid strength of MFI zeolite is controlled by the incorporation of Al3+ and/or Fe3+ into the framework with the same acid site concentration. Three MFI zeolites, namely, H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5, with the same amount of acid sites [SiO2/(Al2O3 + Fe2O3) = 400] were prepared by hydrothermal synthesis and used for the MTH at different temperatures. Their physicochemical properties were characterized by NH3 TPD, FT-IR spectroscopy of adsorbed pyridine, N-2 adsorption, XRD, SEM, and XANES. The acid strengths of the prepared MFI zeolites followed the sequence of H-[Fe]-ZSM-5 < H-[Al,Fe]-ZSM-5 < H-[Al]-ZSM-5. The Bronsted acid densities of H-[Al,Fe]-ZSM-5 and H-[Fe]-ZSM-5, obtained from pyridine IR spectra, decreased with increasing temperature more easily than that of H-[Al]-ZSM-5, where the decrease was highest for H-[Fe]-ZSM-5. With the lowest acid strength, H-[Fe]-ZSM-5 showed a higher propylene selectivity and P/E ratio at 400 degrees C, where it exhibited low methanol conversion and high DME formation. However, its propylene selectivity was significantly lower than those of the other two zeolites at higher reaction temperatures (above 450 degrees C). The best catalytic performance was obtained with H-[Al,Fe]-ZSM-5 with a broad acid strength distribution because of the coexistence of strong Al-based Bronsted acid sites and weaker Fe-based Bronsted acid sites. Its propylene selectivity was much higher than those of the others at 450 degrees C, and a maximum propylene selectivity of 49.3% was achieved at 500 degrees C. It is demonstrated that the acid strength of MFI zeolite can be optimized by the incorporation of Al3+ and Fe3+ into the framework for the maximization of the propylene selectivity in the MTH reaction.