Comparison of the activity stabilities of nanosized and microsized zeolites based Fe–Mo/HZSM-5 catalysts in the non-oxidative CH4 dehydroaromatization under periodic CH4–H2 switching operation at 1073 K

The activities and stabilities of three series of Fe-modified 5%Mo/HZSM-5 catalysts based on a nano-zeolite and two micro-zeolites in the fixed-bed non-oxidative CH4 dehydroaromatization reaction were compared at the condition of 1 atm, 1073 K and 10,000 mL/g/h under periodic CH4–H2 switch operation mode. The activity evaluation tests showed that 0.1–2 wt% Fe co-impregnated modification improves remarkably the benzene formation activity stability of the nanosized zeolite-based catalyst while it had little influence on those of the two microsized zeolites-based catalysts. Then, another series of Fe-modified 5%Mo/HZSM-5 catalysts based on a ball-milled, nanosized zeolite sample were prepared and tested at the same condition. The results confirmed that the maximum improving effect of Fe modification achieves at a properly small Fe addition of 0.5 wt%. SEM observation of all fresh and spent catalyst samples revealed that carbon nanotubes formed over all Fe-modified catalysts but disagglomeration of zeolite agglomerates caused by carbon nanotube formation and growth occurred only to the nano-zeolite based, Fe-modified catalysts. Further, TPO measurement of all spent samples revealed that the amounts of accumulated coke per unit external surface area in the spent nano-zeolite based catalysts were much smaller than those in the micro-zeolite based catalysts. Moreover, well consistent BET measurement confirmed that all nano-zeolite based, Fe-modified catalysts exhibited a similarly smaller degree of decrease in their microporosity than all micro-zeolite based catalysts. All these suggest that the preferential coke formation on the external surfaces and/or in the outer layers of zeolite agglomerates took place under the test condition and enhanced the deactivation of the nano-zeolite based, Fe-unmodified catalysts, and that the disagglomeration of zeolite agglomerates caused by Fe-induced carbon nanotube formation and growth in the reaction was the origin of Fe modification improving the activity stability of the nano-zeolite based Mo/HZSM-5 catalysts. The improvement mechanism is discussed in detail in the article.

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► Nano- and micro-zeolites based Mo/HZSM-5catalysts exhibit similar maximum activities.
► Fe modification improves only the activity stability of nano-zeolite based catalyst.
► Preferential coking on nano-zeolite agglomerates to enhance the catalyst deactivation.
► Fe-induced CNT growth forces agglomerates to disagglomerate and increases diffusivity.
► More crystals inside the agglomerates are involved and the catalyst stability improves.