On the deactivation of Mo/HZSM-5 in the methane dehydroaromatization reaction

[Display omitted] •Deactivation of Mo/HZSM-5 for methane aromatization investigated.•STEM-HAADF employed to study Mo-oxide/carbide phase.•Polynuclear aromatics form carbonaceous layer at external surface.•Carbonaceous layer limits access to Brønsted acid sites, lowers benzene selectivity.•At the same time, MoCx phase agglomerates, lowers methane conversion. The deactivation of Mo/HZSM-5 during the non-oxidative methane aromatization (MDA) reaction that yields benzene and hydrogen was investigated. Catalysts were recovered from the reactor after pre-activation and after increasing time on stream in methane. The physico-chemical properties of the spent catalysts were characterized in detail by Ar physisorption, 27Al MAS NMR and X-ray photoelectron spectroscopy. The nature of the carbon deposits was determined by UV Raman spectroscopy and TGA, and the size and location of the Mo-carbide particles by TEM and STEM-HAADF. The results show that the main cause for catalyst deactivation is the formation of a carbonaceous layer at the external zeolite surface. This layer is made up from polyaromatic hydrocarbons and decreases the accessibility of the Brønsted acid sites in the micropores. At the same time, the decreased interaction of the Mo-carbide particles with the external zeolite surface results in their sintering. The lower Mo-carbide dispersion decreases methane conversion rates. The decreased accessibility of the Brønsted acid sites shifts the selectivity from benzene to unsaturated intermediates formed on the Mo-carbide particles. Silylation of the external surface mainly results in lower rate of coke formation at the external surface, slowing down catalyst deactivation.