Mo/HMCM-22 Catalysts for Methane Dehydroaromatization: A Multinuclear MAS NMR Study

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Abstract

Detailed NMR investigations on fresh and coked Mo/HMCM-22 catalysts for methane dehydro-aromatization were presented. 27Al MAS and 29Si MAS as well as corresponding CP/MAS NMR experiments have proved that an interaction between molybdenum and the zeolite lattice occurs during the impregnation and calcination processes of the catalysts. With Brönsted acid sites serving as a powerful trap, molybdenum migrates into the internal channels of the zeolite and reacts preferentially with bridging hydroxyls groups. Thus, molybdenum is anchored to the framework aluminum through an oxygen bridge, which, in turn, modifies the acidic properties of the HMCM-22 and leads to a high dispersion of the molybdenum. If this kind of interaction becomes stronger, expelling of aluminum from the zeolite lattice would occur, with the forming of both octahedral nonframework aluminum and Al2(MoO4)3 crystallites. The latter can be hydrated to [Al(OH)n(H2O)6-n]n(MoO4) (n = 1 or 2) in a water-saturated desiccator and can give a line at ca. 14 ppm in the 27Al MAS NMR spectrum. UV−Raman results suggest that the carbonaceous deposits on the catalysts are mainly hydrogen-deficient aromatic type, which are also verified by 1H → 29Si CP/MAS and 1H → 27Al CP/MAS NMR, as well as the failure for conducting the 13C CP/MAS NMR experiment. 1H → 27Al CP/MAS NMR experiments indicate that most of the coke is deposited on framework aluminum (Brönsted site). Regeneration in an oxygen atmosphere leads to the formation of a considerable amount of Al2(MoO4)3 crystallites. However, the catalytic activity can be properly restored after the regeneration; thus, the idea of taking this species as the detrimental factor for this reaction can be ruled out. On the other hand, lattice destruction of the zeolite is suggested to be responsible for the poor performance exhibited by the 10Mo/HMCM-22 catalyst. The synergic effect between molybdenum and Brönsted acid sites is considered to be the main reason for the outstanding catalytic performance of the 6Mo/HMCM-22 catalyst.

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