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

Abstract

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, N2 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 Brønsted 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 °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 °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 Brønsted acid sites and weaker Fe-based Brønsted acid sites. Its propylene selectivity was much higher than those of the others at 450 °C, and a maximum propylene selectivity of 49.3% was achieved at 500 °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.