Mechanism of Fe additive improving the activity stability of microzeolite-based Mo/HZSM-5 catalyst in non-oxidative methane dehydroaromatization at 1073 K under periodic CH₄–H₂switching modes

Abstract

ARTICLE This journal isThe catalytic stabilities of Fe-modified and -unmodified 5%Mo/HZSM-5 catalysts in the non-oxidative methane dehydroaromatization were compared at 1073 K and three reaction/H 2 -regeneration cycle periods: 5 min CH 4 -5 min H 2 , 5 min CH 4 -10 min H 2 , and 5 min CH 4 -20 min H 2 . Fe addition proved capable of remarkably increasing the catalyst stability over the cycles of 5 min CH 4 -20 min H 2 but hardly effective over the cycles of 5 min CH 4 -5 min H 2 . On the other hand, SEM observation of all spent samples revealed that Fe addition causes a massive accumulation of carbon nanotubes under the latter cyclic condition but little at the former. Thus several sets of comparative tests were specially designed and performed to get insight into the rule of Fe-catalyzed cyclic formation of carbon nanotubes in stabilizing the activity at the cyclic condition of 5 min CH 4 -20 min H 2 . The results further confirmed that at this condition cyclic formation of carbon nanotubes enhances cyclic evolution of H 2 and makes the H 2 concentration of the system higher, which is thermodynamically beneficial for suppression of formation of the activity-deactivating surface coke. Finally, it was further confirmed that at least a 20 min H 2 exposure is required to remove most of the carbon nanotubes and surface coke formed during a 5 min CH 4 exposure and make most of Fe nanoparticles reactivated and available again for catalytic carbon nanotubes formation with an enhanced H 2 evolution, i.e., with a controlled formation of the surface coke in the next CH 4 exposure.

Supercritical solvothermal synthesis under reducing conditions to increase stability and durability of Mo/ZSM-5 catalysts in methane dehydroaromatization

“…Summarizing the previous literature results, only the catalytic performance reported by Sun et al [56] and Xu et al [48] (based on the periodic CH 4 -H 2 switching) and that reported by Wang et al [57] (based on the use of hollow ZSM-5 capsules) match the results obtained with the SC-STS-E sample in terms of catalytic stability and aromatics yield. On this regard, the potential advantages of the suggested SC-STS-E method with respect to the synthesis procedures described above are two-fold: i) SC-STS is a continuous synthesis procedure and commercially available ZSM-5 zeolites can be used as catalyst support, i.e.…”

“…The increased accessibility of the internal acid sites in the Mo-loaded hierarchical MCM-22 improved benzene selectivity during MDA. Sun et al [56] used a periodic CH 4 -H 2 switch strategy similar to that reported by Xu et al [48] keeping a nearly constant methane conversion (13 -16%) and outstanding aromatics yield (>10%) for 1000 hours on stream, which represents the longest reported TOS in literature for the MDA process. Wang et al [57] synthesized an unconventional hollow ZSM-5 zeolite capsules with large cavity in core and aggregated mesopores in shell that significantly outperformed the conventional ZSM-5 microporous support in terms of methane conversion and aromatics selectivity.…”

“…Literature review of long-term MDA performance (> 8h) on fixed beds of 6 wt.% Mo/ZSM-5 based powder catalysts (d p = 250 -425 µm) working at 700ºC, 1500 mL/g cat h and atmospheric pressure Its effect on the aromatics yield became more noticeable in the samples containing 10 -12% Mo loading. Xu et al[48] implemented a periodic CH 4 -H 2 switching mode to extend the catalyst lifetime working at high temperatures and spatial velocities with Fe-promoted Mo/ZSM-5 samples.…”

Progress in Nonoxidative Dehydroaromatization of Methane in the Last 6 Years

“…Zhang and co-workers investigated the effect of periodic CH 4 –H 2 switch operation mode in the MDA reaction. Their research was conducted on various catalysts and in different kinds of reactors. ,− In 2011, Zhang et al carried out a research to study catalytic behaviors of regeneration (CH 4 –H 2 switch) cycles over Mo/HZSM-5 catalyst (5 wt % Mo, Si/Al = 30) in a two-bed circulating fluidized bed system. The reaction occurred at 1073 K with space velocities of 21 080 and 40 000 mL/g h. The results revealed that more than 50 wt % of the coke formed in the initial 2 min of the reaction.…”

NGU: Development of a two‐bed circulating fluidized bed reactor system for nonoxidative aromatization of methane over Mo/HZSM‐5 catalyst

“…The nonoxidative aromatization of methane over Mo/HZSM‐5 provides a promising route of direct methane conversion to valuable aromatics. It has been extensively studied from both fundamental scientific and application points of view since first reported in 1993 . Since Mo/HZSM‐5 catalyst always shows a very rapid deactivation at severe reaction conditions due to serious coke formation, suppressing coke accumulation and maintaining its high activity has become the key to realizing early industrial application of this new catalytic reaction system .…”