Mo/HMCM-22 catalysts for methane dehydroaromatization: A multinuclear MAS NMR study

Detailed NMR investigations on fresh and coked Mo/HMCM-22 catalysts for met hane dehydro-aromatization were presented. Al-27 MAS and Si-29 MAS as well as corresponding CP/MAS NMR experiments have proved that an interaction bet ween molybdenum and the zeolite lattice occurs during the impregnation and calcination processes of the catalysts. With Bronsted acid sites serving as a powerful trap, molybdenum migrates into the internal channels of the zeo lite and reacts preferentially with bridging hydroxyls groups. Thus, molybd enum 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 form ing of both octahedral nonframework aluminum and Al-2(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 A l-27 MAS NMR spectrum. UV-Raman results suggest that the carbonaceous depos its on the catalysts are mainly hydrogen-deficient aromatic type, which are also verified by H-1 --> Si-29 CP/MAS and H-1 --> Al-27 CP/MAS NMR, as wel l as the failure for conducting the C-13 CP/MAS NMR experiment. H-1 --> Al- 27 CP/MAS NMR experiments indicate that most of the coke is deposited on fr amework aluminum (Bronsted site). Regeneration in an oxygen atmosphere lead s to the formation of a considerable amount of Al-2(MoO4)(3) crystallites. However, the catalytic activity can be properly restored after the regenera tion; thus, the idea of taking this species as the detrimental factor for t his reaction can be ruled out. On the other hand, lattice destruction of th e zeolite is suggested to be responsible for the poor performance exhibited by the 10Mo/HMCM-22 catalyst. The synergic effect between molybdenum and B ronsted acid sites is considered to be the main reason for the outstanding catalytic performance of the 6Mo/HMCM-22 catalyst.