Dealumination of mordenite catalysts using a low concentration of steam

Abstract

The dealumination of mordenite using mild steaming conditions (1.2 vol% steam in nitrogen at 440°C) has been studied in a Pt/Al₂O₃/mordenite catalyst containing 25% of the zeolite. These conditions are found to cause extensive dealumination of the mordenite, as determined using ²⁷Al and ²⁹Si MAS NMR spectroscopy, although there is no significant loss of crystallinity as determined by powder X-ray diffraction. It is found that a relatively short duration of steaming (i.e. 3 h) can lead to dealumination. Detailed STEM analysis using both high angle annular dark field imaging and EDX demonstrates that the steaming does not significantly affect the Pt dispersion. Model experiments using only the mordenite component of the catalyst were also carried out, and the dealuminated mordenites were used as catalysts for o-xylene isomerisation. Initial dealumination leads to an increase in activity, although prolonged steaming leads to a loss of catalyst activity. The increase in activity is almost exclusively due to the formation of the products of disproportionation reactions. The results are discussed in terms of the effect of dealumination on the Brønsted and Lewis acid sites.

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