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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, 27 Al 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. [ABSTRACT FROM AUTHOR]

The influence of mesoporosity and silylation on the physico-chemical and catalytic properties of Mo/HZSM-5 in methane dehydroaromatization was investigated. The zeolites were characterized by XRD, 27 Al and 95 Mo NMR, UV–Vis, UV Raman and pyridine IR spectroscopy and TEM. Base-desilicated mesoporous and bulk HZSM-5 zeolites with comparable Brønsted acidity were employed as acidic supports. Mo loading was optimized to minimize loss of acidity. Surface silylation of Mo/HZSM-5 resulted in improved Mo-oxide dispersion. More intensive silylation led to decreased Mo-oxide dispersion because of increased hydrophobicity. High methane conversion rates were associated with small Mo-oxide precursor particles. Silylation of the external surface of Mo/HZSM-5 led to higher methane conversion and less coke formation. On contrary, silylation of HZSM-5 prior to Mo introduction had a negative effect on the performance. Post-synthesis silylation of Mo/HZSM-5 affected the Mo-oxide phase. The amount of hard coke decreased with increasing silylation degree due to deactivation of acid sites at the external surface. It also decreased naphthalene yield. Methane conversion and aromatics selectivity were lower for mesoporous Mo/HZSM-5 compared with bulk Mo/HZSM-5. Although the initial Mo-oxide dispersion was higher, the different nature of the mesopore surface resulted in rapid formation of large Mo-carbide particles with higher coke selectivity. Silylation slightly improved activity and selectivity to benzene. [ABSTRACT FROM AUTHOR]