Regenerability of coked H-GaMFI propane aromatization catalyst :: influence of reaction-regeneration cycle on acidity, activity/selectivity and deactivation

Abstract

The influence of the reductive reaction (deactivation)-oxidative regeneration cycle of the propane aromatization H-gallosilicate (MFI) zeolite catalyst on its surface and catalytic properties has been thoroughly investigated. The zeolitic strong acidity was measured by the chemisorption of pyridine at 400°C, TPD of ammonia from 330 to 600°C and acid catalyzed model reactions viz. toluene disproportionation at 500°C and m-xylene isomerization at 400°C. This acidity and also the external acidity of the zeolite (measured in terms of the isooctane cracking activity at 400°C) are decreased continuously due to a gradual degalliation of the zeolite during the reaction-regeneration cycles. The degalliation has resulted from the mild hydrogen treatment during the reaction and from the hydrothermal treatment by the steam produced in the oxidative regeneration. The degalliation is associated with a small decrease in the crystallinity, but with little or no change in the N₂ sorption capacity of the zeolite. However, the propane conversion activity of the zeolite is increased because of the formation of active and well-dispersed extra-framework Ga-oxide species from the slow but continuous degalliation. The para-shape selectivity of the zeolite is also increased appreciably by increasing the regeneration cycle, mostly due to the increased diffusional resistance by the presence of additional Ga-oxide species (formed due to the continuous degalliation) in the zeolite channels. The deactivation due to coking of the zeolite in the propane aromatization is found to be faster for the regenerated zeolite because of its increased propane conversion activity.