Desilication and silylation of Mo/HZSM-5 for methane dehydroaromatization

• Bulk and desilicated HZSM-5 used to prepare Mo/HZSM-5 for methane aromatization.
• Dealumination more pronounced in hierarchical Mo/HZSM-5, lower catalytic performance.
• External-surface silylation lowers acidity and also strongly affects Mo speciation.
• Mo loading on silylated HZSM-5 gives poor performance.
• Silylation of synthesized Mo/HZSM-5 improves methane conversion and aromatics yield.

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, 27Al and 95Mo 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.

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