Methane dehydroaromatization over Mo/H-ZSM-5: A comparative study of the regeneration methods

Abstract

The catalytic activity and deactivation characteristic of spherically shaped Mo/H-ZSM-5 particles in the non-oxidative dehydroaromatization of methane was evaluated in a fluidized bed reactor under different cyclic reaction-regeneration operation modes. The aim is to verify the superiority of the H2 regeneration approach over that oxidative approach in restoring the catalytic performance of coke-deactivated Mo/H-ZSM-5 catalyst. Compared with the fresh catalyst, the catalyst samples oxidatively regenerated at 973 and 1073 K exhibited at 1073 K a largely reduced initial activity and an obviously accelerated deactivation rate. By contrast, the catalyst sample regenerated in a H2 stream at 1073 K maintained basically the initial activity level and deactivation characteristic of the fresh catalyst over cycles. Oxidatively regenerated at a lower temperature of 773 K, the catalyst sample was allowed to maintain the initial high activity of the fresh catalyst but its deactivation rate estimated over a 20 min reaction period showed an obvious tendency to increase with increasing the number of regeneration cycles. Characterization of all spent catalyst samples using TG, XRD and NMR techniques revealed that a significant loss of active Mo sites via sublimation and/or formation of inactive Al2(MoO4)3 and local collapse of zeolite framework occurred to the two samples once oxidatively regenerated at 973 and 1073 K, while neither Mo loss nor framework breakdown was detected with the H2-regenerated sample. Additionally, the formation of disordered non-framework Al was found to occur to the two samples once oxidatively regenerated at 773 and 873 K. All these suggest that applying the H2 approach to regenerate coke-deactivated Mo/H-ZSM-5 enables the catalyst to avoid losing any of its active Mo sites in its cyclic regeneration process and therefore must be helpful for maintenance of its catalytic performance over the reaction-regeneration cycles.