A non-hydrolytic sol–gel route to highly active MoO3–SiO2–Al2O3 metathesis catalysts

Abstract

MoO3-based materials are known to be appropriate catalysts for the heterogeneous metathesis of light olefins. Classical preparation methods involve the deposition of a Mo oxide phase on the surface of preformed support via impregnation, grafting or thermal spreading. An alternative sol-gel approach for the elaboration of Mo-based catalysts is presented in this article. Mesoporous ternary Si-Al-Mo mixed oxides are prepared in one step, via non-hydrolytic condensation of chloride precursors in non-aqueous media. After calcination, effective catalysts with very good textures and highly dispersed surface molybdenum species are obtained. The Si/Al ratio influences both the texture and the acidity of the materials, which significantly affects the propene self-metathesis activity. The activity also increases with the MoO3 content. The best catalysts with optimized composition significantly outperform the catalysts prepared by other methods.

The Power of Non-Hydrolytic Sol-Gel Chemistry: A Review

“…While the Re species in as-prepared samples are homogeneously dispersed in the bulk, the calcined xerogels exhibit an increase of the surface Re/(Re + Si + Al) atomic ratio corresponding to migration of Re to the surface upon thermal treatment (Figure 6). Similar behavior was previously reported for vanadium, molybdenum, and tungsten in Ti-O-V, Ti-O-V-O-Mo, Ti-O-V-O-W, and Si-O-Al-O-Mo mixed oxide catalysts prepared by NHSG and was explained by a low Tammann temperature of the respective metal oxides allowing for their migration at increased temperatures [179][180][181].…”

“…Another class of materials studied by the same group were catalysts for olefin metathesis. They prepared mesoporous mixed oxide based catalysts with the compositions of Mo-O-Si-O-Al [180,214,215] and Re-O-Si-O-Al [177,178]. All samples catalyzed the conversion of propene to ethene and butene with more than 99 % selectivity.…”

“…The materials discussed above illustrate that the NHSG approach based on alkyl halide elimination allows for excellent control of composition. Si−O−Al−O−Mo mixed oxides were prepared via the same non-hydrolytic alkyl halide elimination reaction and with the same goal -tuning the acidity for better catalytic performance [180]. In this study, two reaction parameters were varied systematically: the alumina content (the Si/Al ratio) and the MoO3 content.…”

Innovative Sol‐Gel Routes for the Bottom‐Up Preparation of Heterogeneous Catalysts

“… TEM micrograph of a MoO 3 ‐Al 2 O 3 ‐SiO 2 catalysts (10 : 5 : 85 wt.%) prepared by non‐hydrolytic sol‐gel …”

“…MoO 3 ‐based metathesis catalysts are also active at moderate temperature (25–200 °C) and their activity is known to be highly dependent on the nature of the MoO x species stabilised at the surface and on the composition of the support or matrix . NHSG allowed us to prepare a series of MoO 3 ‐Al 2 O 3 ‐SiO 2 catalysts with wide range of composition ,. The Si/Al ratio was found to influence both the texture and the acidity of the materials, which had a marked impact on the performance in propene self‐metathesis.…”