Identification of molybdenum oxide nanostructures on zeolites for natural gas conversion

Abstract

Direct methane conversion into aromatic hydrocarbons over catalysts with molybdenum (Mo) nanostructures supported on shape-selective zeolites is a promising technology for natural gas liquefaction. We determined the identity and anchoring sites of the initial Mo structures in such catalysts as isolated oxide species with a single Mo atom on aluminum sites in the zeolite framework and on silicon sites on the zeolite external surface. During the reaction, the initial isolated Mo oxide species agglomerate and convert into carbided Mo nanoparticles. This process is reversible, and the initial isolated Mo oxide species can be restored by a treatment with gas-phase oxygen. Furthermore, the distribution of the Mo nanostructures can be controlled and catalytic performance can be fully restored, even enhanced, by adjusting the oxygen treatment.

Non-oxidative dehydroaromatization of methane: an effective reaction–regeneration cyclic operation for catalyst life extension

“…A possible explanation could be a better dispersion of the Mo-species (lower agglomeration degree), and therefore a higher interaction with the Brønsted sites (leaving less H + available) when the short reaction-regeneration procedure is employed. In fact, this is in good agreement with recent studies on the effect of regeneration time with O2, that show how longer regeneration times do restore the Mo oxide nanostructures, but forces them to migrate from Al framework sites to Si anchoring sites on the external surface of the zeolite 43 . This would restore the Brønsted acid site corresponding to the framework Al.…”

“…This fact is mainly attributed to partial blocking of the micropores by the supported MoO3 particles and to the migration of the Mo species into the channels during the calcination by decomposition of the ammonium heptamolybdate (AHM) used as molybdenum precursor [54][55][56][57][58] . The driving force of this molybdenum migration inside of the microporous structure has been related to the presence of Brønsted acid sites, since the MoO3 reacts stoichiometrically 1:1 with H + atoms at exchange sites to form (MoO2(OH)) + species 54,59 , which are the precursors to the active MoC sites required for catalytic C-H bond activation 43 .…”

“…No se encuentra el origen de la referencia.B, where it is shown to begin at temperatures around 813K. Moreover, no hydrocarbon production was observed during this activation step, so it is assumed that all the methane consumed is contributing to the formation of the active carbides 43 .…”

“…The use of hydrogen regeneration cycles has also been described 37,38 . A recent paper evidences the reversible character of isolated Mo oxide nanostructures conversion into carbide Mo nanoparticles, and full recovery of initial Mo species and catalytic activity by regeneration with gas-phase oxygen under proper conditions 43 .…”

Selective Coke Combustion by Oxygen Pulsing During Mo/ZSM‐5‐Catalyzed Methane Dehydroaromatization

“…Oxidation of Mo‐carbide to Mo‐oxide is key to the selective combustion of coke species. Rapid cycling between the oxidic and carbidic forms of molybdenum does not affect the catalyst in a negative manner (no framework damage, no loss of molybdenum) and this is likely due to the stabilization of molybdenum‐oxo complexes on cation‐exchange sites of the zeolite 7b. Thus, it is possible to combine the online pulsing operation of methane dehydroaromatization with periodic regeneration by air calcination at 550 °C 7a.…”