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

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 x 4 h at 75 degrees 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 m(2)/g as a consequence of the desilication, and the N-2 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 degrees 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. (C) 2008 Elsevier B.V. All rights reserved.