The catalytic stability of Mo/HZSM-5 in methane dehydroaromatization at severe and periodic CH 4–H 2 switch operating conditions

Abstract

The very rapid deactivation behavior of Mo/HZSM-5 catalyst in the non-oxidative CH 4 dehydroaromatization at severe conditions requests its cyclic or continuous regeneration in a practical system. While oxidation of active Mo 2C to MoO 3 followed by its sublimation at temperature >773 K under oxidative atmospheres excludes the possibility of using any oxygen-containing gases to regenerate Mo/HZSM-5 catalyst, it has been demonstrated that its high initial activity can be maintained via its continuous regeneration in H 2 in a two-bed circulating fluidized bed system at 1073 K. The key to design such a system is determination and optimization of the mean residence times of Mo/HZSM-5 in its CH 4 converter and regenerator. Presently, a series of deactivation (reaction)–regeneration cycle tests were designed and carried out over a well-characterized 5 wt%Mo/HZSM-5 at practically required severe conditions (1073 K and 21,080 and 40,000 mL/g/h) to clarify the effect of CH 4 and H 2 exposure durations on the activity stability. Simultaneously, TPO analyses of the samples deactivated in CH 4 for different durations at 1073 K and measurements of the H 2-regeneration kinetics of the same coked samples at the same temperature were conducted to gain a better understanding of the kinetic characteristics of coke formation and removal. Furthermore, measurements of the dynamic variations of the outlet benzene concentration in the CH 4 exposures in different cycles of a typical cycle test and numerical analysis of the resultant concentration–time curves were performed to determine the minimum and maximum mean residence times of the catalyst in a fluidized bed CH 4 converter. At last, a triple-bed circulating fluidized reactor system based on a large degree of deactivation–long time regeneration cycle test was proposed to realize an effectively continuous regeneration of Mo/HZSM-5 and therefore an efficiently continuous conversion of CH 4 to benzene.