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

Authors: Yuebing Xu, Yoshizo Suzuki, Zhan-Guo Zhang

Abstract:
The activities andstabilities ofthree series of Fe-modified5%Mo/HZSM-5 catalysts basedonanano-zeolite and two micro-zeolites in the fixed-bed non-oxidative CH4 dehydroaromatization reaction were compared atthe condition of 1 atm, 1073K 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 atthe 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 carbonnanotubes formedover all Fe-modifiedcatalysts butdisagglomerationof 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.

Keywords:
Methane dehydroaromatization
Mo/HZSM-5
Nanosized zeolite
Fe modification
Carbon nanotube

Published in: Applied catalysis A: General  (Volume 452, Pages 1-222, 15 February 2013)

Publisher: Elsevier

ISSN Information: 0926-860X

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

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