Catalytic Glycerol Conversion over Bifunctional Montmorillonite-Supported Mo−V Oxide Catalysts: Insights into the Dehydration-Hydrogen Transfer Reaction Pathway Toward Allyl Alcohol
Allyl alcohol, an industrially valuable intermediate in organic synthesis, is conventionally produced through selective hydrogenation of acrolein and hydrolysis of chloropropene. However, these processes currently face challenges due to the unsustainability of fossil feedstocks. A more sustainable production of allyl alcohol from renewable feedstock (i.e., glycerol) has been demonstrated through gas-phase catalytic reaction over metal oxide catalysts with and without external H2. Improved understanding of the reaction mechanism of glycerol to allyl alcohol conversion is an important effort to rationally design more efficient catalysts. In this work, we investigate gas-phase conversion of glycerol to allyl alcohol through consecutive dehydration and hydrogen transfer reactions over Mo–V oxides supported on acid-modified montmorillonite (HMMT). The use of HMMT as a support enables the well dispersion of metal oxide particles with nominal loadings up to 5 wt % on the catalyst surface while the acidic sites facilitate dehydration of glycerol to acrolein with ∼74% selectivity. By modulating the nominal total Mo+V loading, Mo–V/HMMT catalyst with a 10 wt % metal loading afforded allyl alcohol in 22% yield at 320 °C and ambient pressure. X-ray photoelectron spectroscopic analysis of this bifunctional catalyst in the as-prepared and post-catalysis (spent) states showed changes in the surface Mo(VI)═O species and V4+/V5+ redox ratio; the latter was found to decrease due to the involvement of lower valence V4+ species in the hydrogen transfer reaction of acrolein. Meanwhile, the amount of surface Mo(VI)═O species with weak electronic metal–support interaction decreases after the reaction. Catalytic tests using an aqueous acrolein feed solution suggested the role of water as a hydride donor in transfer hydrogenation of this unsaturated aldehyde to allyl alcohol, while the addition of alcohol-containing compounds, such as 2-propanol and 1,2-propanediol leads to lower allyl alcohol selectivity with time on stream. The results presented here could guide the development of improved supported Mo−V oxide catalysts and reaction systems for efficient gas-phase glycerol conversion to allyl alcohol.
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