Study on Mo/HZSM-5 catalysts modified by bulky aminoalkyl-substituted silyl compounds for the selective methane-to-benzene (MTB) reaction

The methane dehydroaromatization reaction was carried out over SiO2-modified Mo/HZSM-5 catalysts at 1023 K under 0.3 MPa with 2700 ml g−1 h−1 of 6% H2 added methane. The molecular tuning effect of silanation treatment for HZSM-5 crystals was studied using various silyl compounds, including 3-aminopropyl-triethoxysilane (APTES), 3-aminopropyl-trimethoxysilane (APTMS), N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (TESPDHI), triphenylsilylamine (TPSA), N-propyl-triethoxysilane (PTES), and tetraethoxysilane (TEOS). It was found that the optimum addition (0.5 wt% as SiO2) of basic group-substituted bulky silyl compounds such as APTES, APTMS and TESPDHI resulted in the fine tuning of the external microporous apertures of HZSM-5 having ca. 5.0 Å diameter similar to that of MCM-22, and improve the benzene selectivity of above 90% (benzene + toluene) in the methane dehydroaromatization reaction on 6 wt% Mo/HZSM-5 catalysts due to the marked suppression of naphthalene and coke formation. In contrast, the other bulky silyl compounds are not effective for the surface modification of Mo/HZSM-5 in the MTB reaction. The IR of pyridine adsorption studies revealed that the surface Brønsted acid sites of HZSM-5 was selectively transformed to Lewis acid sites by the treatment of the selected silyl agents such as APTES, APTMS, and TESPDHI, resulting in highly stable and selective catalytic performance of Mo/HZSM-5 in the MTB reaction.