DEALUMINATION OF MORDENITE CATALYSTS USING A LOW CONCENTRATION OF STEAM

The dealumination of mordenite using mild steaming conditions (1.2 vol % steam in nitrogen at 440 degrees C) has been studied in a Pt/Al2O3/m ordenite catalyst containing 25% of the zeolite. These conditions are found to cause extensive dealumination of the mordenite, as determined using Al-27 and Si-29 MAS NMR spectroscopy, although there is no sign ificant loss of crystallinity as determined by powder X-ray diffractio n. It is found that a relatively short duration of steaming (i.e. 3 h) can lead to dealumination. Detailed STEM analysis using both high ang le annular dark field imaging and EDX demonstrates that the steaming d oes not significantly affect the Pt dispersion. Model experiments usin g only the mordenite component of the catalyst were also carried out, and the dealuminated mordenites were used as catalysts for o-xylene is omerisation. Initial dealumination leads to an increase in activity, a lthough prolonged steaming leads to a loss of catalyst activity. The i ncrease 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 Bronsted and Lewis acid s ites.