Structure and Evolution of Confined Carbon Species during Methane Dehydroaromatization over Mo/ZSM-5

Abstract

Surface carbon (coke, carbonaceous deposits) is an integral aspect of methane dehydroaromatization catalyzed by Mo/zeolites. We investigated the evolution of surface carbon species from the beginning of the induction period until the complete catalyst deactivation by the pulse reaction technique, TGA, 13C NMR, TEM, and XPS. Isotope labeling was performed to confirm the catalytic role of confined carbon species during MDA. It was found that “hard” and “soft” coke distinction is mainly related to the location of coke species inside the pores and on the external surface, respectively. In addition, MoO3 species act as an active oxidation catalyst, reducing the combustion temperature of a certain fraction of coke. Furthermore, after dissolving the zeolite framework by HF, we found that coke formed during the MDA reaction inside the zeolite pores is essentially a zeolite-templated carbon material. The possibility of preparing zeolite-templated carbons from the most available hydrocarbon feedstock is important for the development of these interesting materials.

Implications of the Molybdenum Coordination Environment in MFI Zeolites on Methane Dehydroaromatisation Performance

“…[20] The same group has recently highlighted the involvement of confined carbon in the reaction mechanism instead. [21,22] Our previous synchrotron-based operando studies on Mo/H-ZSM-5 evidenced the relevance of Mo speciation in MDA product distribution. X-ray emission showed the selectivity to different hydrocarbon products to be dependent on Mo structure (i. e. partially carburised species were observed during the induction period when light hydrocarbons were produced, whereas fully carburised sites were selective to aromatics).…”

“…for Mo‐containing Silicalite‐1, the pure siliceous analogue of H‐ZSM‐5 zeolite, evidenced the formation of aromatics over this catalyst material, leading to the proposal that Brønsted acid sites may not be essential for the aromatisation of methane . The same group has recently highlighted the involvement of confined carbon in the reaction mechanism instead …”

Aromatization of ethylene over zeolite-based catalysts

“…The lower coke content of used HZSM-5 is in line with the lower rate of deactivation of this sample. 39 The peak at 150°C is associated with water desorption. It is important to note that the overall coke selectivity for Zn/HZSM-5 and Ag/HZSM-5 are much higher than on Ga/HZSM-5 and HZSM-5.…”

“…The concentration of 13 C in aromatic products slowly decreased and reached the natural abundance level after ∼60 s. These observations show that the deposited intra-zeolitic hydrocarbon species are involved in the ethylene conversion reaction. 26,39 The catalyst samples after the reaction involving a switch from 13 C-to 12 C-ethylene were then analysed by 13 C MAS NMR spectroscopy. The 13 C direct excitation (DE) and 13 C{ 1 H} CP NMR spectra are shown in Fig.…”

Revival of Zeolite‐Templated Nanocarbon Materials: Recent Advances in Energy Storage and Conversion

“…[128][129][130][131] The Lewis acidity originates from substituted Al (III) in the frameworks of zeolites. In the process of zeolite-templated methods, polycyclic aromatic compounds are liable to deposit on the acidic sites, [132] which has positive effect on the formation of nanocarbon structures inside the nanochannels of zeolites. Hence, relatively high Al content, namely low Si/Al ratio is essential for the synthesis of nanocarbon materials templated by zeolites.…”