Methane dehydroaromatization over Mo/HZSM-5 catalysts: The reactivity of MoC x species formed from MoO x associated and non-associated with Brönsted acid sites

Abstract: The catalytic performances of methane dehydroaromatization (MDA) under non-oxidative conditions over 6wt.% Mo/HZSM-5 catalysts calcined for different durations of time at 773K have been investigated in combination with ex situ 1H MAS NMR characterization. Prolongation of the calcination time at 773K 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 MoO x 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 6wt.% Mo/HZSM-5 catalyst at 773K for 18h can cause the MoO x species to associate with the Brönsted acid sites, while a 6wt.% Mo/SiO2 sample can be taken as a catalyst in which all MoO x species are non-associated with the Brönsted acid sites. The TOF data at different times on stream on the 6wt.% Mo/HZSM-5 catalyst calcined at 773K for 18h and on the 6wt.% Mo/SiO2 catalyst reveal that the MoC x species formed from MoO x associated with the Brönsted acid sites are more active and stable than those formed from MoO x non-associated with the Brönsted acid sites. An analysis of the TPO profiles recorded on the used 6wt.% 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 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. [Copyright &y& Elsevier]