Methane Dehydroaromatization by Mo/HZSM-5: Mono- or Bifunctional Catalysis?

Abstract

The active site requirements for methane dehydroaromatization by Mo/HZSM-5 were investigated by employing as catalysts physical mixtures of Mo-bearing supports (HZSM-5, SiO 2 , γ-Al 2 O 3 , and activated carbon) and HZSM-5. Separation of the two catalyst components after activation or reaction was possible by using two different sieve fractions. Our comparison demonstrates that migration of volatile Mo oxides into the micropores of HZSM-5 is at the origin of the observed catalytic synergy in methane dehydroaromatization for physical mixtures. The propensity of Mo migration depends on the activation method and the Mo−support interaction. Migration is most pronounced for Mo/SiO 2 . Prolonged exposure of HZSM-5 zeolite to Mo oxide vapors results in partial destruction of the zeolite framework. Mo carbide dispersed on nonzeolitic supports afforded predominantly coke with only very small amounts of benzene. The main function of the zeolite is to provide a shape-selective environment for the conversion of methane to benzene. A comparison of Mo/HZSM-5 and Mo/Silicalite-1 demonstrates that aromatization of methane is an intrinsic ability of molybdenum carbides dispersed in the 10-membered-ring micropores of MFI zeolite. Thus, one important role of the Brønsted acid sites is to promote the dispersion of the Mo oxide precursor and, accordingly, the active Mo carbide phase in the micropores of HZSM-5.

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

“…These results are in agreement with Kosinov et al . who proposed that the conversion of methane to benzene might take place in a monofunctional manner on Mo carbides in the zeolite micropores, or mediated by carbonaceous species . Such mechanisms open the possibility to optimise active MDA catalysts based on non‐acidic zeolites.…”

Progress in Developing a Structure‐Activity Relationship for the Direct Aromatization of Methane

“…also underlines that both functionalities need to be in close proximity. They discovered that the bifunctional Mo/HZSM‐5 cannot be separated into two catalysts, one carrying the Mo functionality and one carrying the BAS . In contrast, for Ta grafted on silica, both CH 3 and H were proposed to coordinate to the metal, where the methyl species then dimerizes when a second CH 4 binds to Ta …”

“…Kosinov et al . showed that in the absence of zeolite, molybdenum supported on different oxides such as SiO 2 and Al 2 O 3 , can activate methane with minor formation of aromatics and aliphatics, but coke is the main product ,. Authors showed that the confinement of metal in zeolite pores shifts selectivity to benzene and its homologues increasing the overall yield of value‐added products .…”

“…This suggests that Mo also plays a crucial role in aromatics production. In conclusion, the separation of these functionalities is not possible . Although some studies address the optimal loading of Mo, given a zeolite with certain acidity and vice versa,,,,, the optimal proximity of BAS to Mo site and the optimal concentration of both sites per zeolite cage is only discussed for MCM‐22 .…”

Reversible Nature of Coke Formation on Mo/ZSM‐5 Methane Dehydroaromatization Catalysts

“…Amajor challenge in the realization of apractical MDAp rocess is the rapid deactivation of Mo/ZSM-5 catalysts.R ecent advances in our understanding how these catalysts work at the atomic level can aid in strategies to reduce the coking deactivation. [7] Further, various methods to enhance the activity and stability of Mo/ZSM-5 catalysts have been developed as well and they include:1 )A pplication of hydrogen-selective membranes to remove hydrogen from the reaction zone and shift the equilibrium towards aromatic products. [2] Second, the structure of the active molybde-num centers has been thoroughly studied by means of 95 Mo NMR, [3] operando Raman [4] and XAS spectroscopy, [5] and other techniques.…”