Methane dehydroaromatization over Mo/HZSM-5 catalysts: The reactivity of MoCx species formed from MoOx associated and non-associated with Brönsted acid sites Original Research Article

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 Brönsted 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 MoOx species associated and non-associated with the Brönsted 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 Brönsted acid sites. Calcining the 6 wt.% Mo/HZSM-5 catalyst at 773 K for 18 h can cause the MoOx species to associate with the Brönsted acid sites, while a 6 wt.% Mo/SiO2 sample can be taken as a catalyst in which all MoOx species are non-associated with the Brönsted 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/SiO2 catalyst reveal that the MoCx species formed from MoOx associated with the Brönsted acid sites are more active and stable than those formed from MoOx non-associated with the Brönsted 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 MoCx species formed from MoOx species associated with the Brönsted 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.