Methane dehydroaromatization over Mo/HZSM-5 catalysts:The reactivity of MoC_x species formed from MoO_x associated and non-associated with Bronsted acid sites

摘要

The catalytic performances of methane dehydroaromatization(MDA)under non-oxidative conditions over 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773 K have been investigated in combination with ex situ ~1H MAS NMR characterization.Prolongation of the calcination time at 773 K is in favor of the diffusion of the Mo species on the external surface and the migration of Mo species into the channels,resulting in a further decrease in the number of Bronsted acid sites,while causing only a slight change in the Mo contents of the bulk and in the framework structure of the HZSM-5 zeolite.The MoO_x species associated and non-associated with the Bronsted acid sites can be estimated quantitatively based on the ~1H MAS NMR measurements as well as on the assumption of a stoichiometry ratio of 1:1 between the Mo species and the Bronsted acid sites.Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the MoO_x.species to associate with the Bronsted acid sites,while a 6 wt.% Mo/SiO_2 sample can be taken as a catalyst in which all MoO_x species are non-associated with the Bronsted acid sites.The TOF data at different times on stream on the 6 wt.% Mo/HZSM-5 catalyst calcined at 773 K for 18 h and on the 6 wt.% Mo/SiO_2 catalyst reveal that the MoC_x species formed from MoO_x associated with the Bronsted acid sites are more active and stable than those formed from MoO_x non-associated with the Bronsted acid sites.An analysis of the TPO profiles recorded on the used 6 wt.% Mo/HZSM-5 catalysts calcined for different durations of time combined with the TGA measurements also reveals that the more of the MoC_x species formed from MoO_x species associated with the Bronsted acid sites,the lower the amount of coke that will be deposited on it.The decrease of the coke amount is mainly due to a decrease in the coke burnt-off at high temperature.