Impact of post-synthetic treatments on unidirectional H-ZSM-22 zeolite catalyst: Towards improved clean MTG catalytic process

Abstract

In the present study, a series of H-ZSM-22 mesoporous catalysts resulting from three different desilication treatments (NaOH treatment, treatment using mixtures of NaOH/CTAB and using mixtures of NaOH/TBAOH) and sequential acid leaching over two different (commercial and lab-made) microporous ZSM-22, were tested in the conversion of methanol to hydrocarbons. The influence of the post-synthetic treatments on the catalytic lifetime and product distribution was examined. The influence of the starting catalysts on the change in the catalyst properties was also reflected in the catalytic behaviour. An increase of about 10 times in total methanol conversion capacity with respect to the untreated catalyst was reached after the CTAB/NaOH and acid treatment over the commercial material, whereas a 17-fold increase in conversion capacity was achieved for the lab-made catalysts treated with NaOH and acid. The yield towards the aromatic-free C5+ alkene fraction was slightly increased after the post-synthetic treatments, up to 58% of clean gasoline product precursors. The correlations between porosity, acidity and total conversion capacity suggested a more efficient use of the hierarchical catalyst particle as a result of a synergetic effect of mesopore formation, enhanced accessibility to the micropores and acid sites, and increased adsorption and transport properties. Mechanistic information extracted from the analysis of the C3/C2 and ethene/2M2B ratios, suggested that the improved catalyst properties allow a longer propagation of the olefin cycle with reaction time.