Methanol to gasoline over zeolite H-ZSM-5: Improved catalyst performance by treatment with NaOH

Abstract

This work outlines how treatment of zeolite H-ZSM-5 (Si/Al = 46) with NaOH may improve the catalytic performance in the conversion of methanol to gasoline. The zeolite was treated with 0.05 or 0.20 M NaOH solution for 2× 4 h at 75 °C. XRD confirmed the retention of crystallinity. The Si/Al ratio of the catalyst decreased as a consequence of the treatment, in particular for the more severely treated sample, but the total acidity determined by NH3-TPD was not altered, showing that the treated samples contain significant amounts of Al not giving rise to acidity. The BET surface area increased from 313 to 419 m2/g as a consequence of the desilication, and the N2 adsorption measurements indicated mesopore generation. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) indicated presence of Lewis acidity in the treated samples. Methanol conversion was carried out in a fixed bed reactor at 370 °C and WHSV = 8 g g−1 h−1. The catalyst lifetime, quantified as the total conversion capacity, increased by a factor of 3.3 as a consequence of the most severe treatment. The procedure led to a moderate increase in the initial activities. Further, the product selectivities were altered dramatically. The selectivity towards the gasoline fraction (C5+) was at best increased by a factor of 1.7. Hydrogen transfer reactions became faster and led to more aromatic and paraffinic compounds in the products. Increases in the propene/ethene ratios were observed at moderate conversion. The results are well rationalized by alterations of acidic properties, mesopore formation, and improved diffusivity.