Highly selective menthol synthesis by one-pot transformation of citronellal using Ru/H-BEA catalysts

• Highly diastereoselective Ru/H-BEA catalysts produce menthol with high yields.
• Cyclization of citronellal is preferred over hydrogenation at low H2 pressures.
• Low Ru loadings (Ru particle size = 1 nm) minimize hydrogenation of citronellal.
• A low Si:Al ratio of the zeolite avoids defunctionalization of menthols.
• Dioxane as solvent minimizes hydrogenation of citronellal.

The one-pot transformation of citronellal to menthol requires a combined cyclization–hydrogenation step for which Ru/H-BEA catalysts were identified as highly active and selective catalysts. After identifying zeolite H-BEA as the most promising heterogeneous catalyst for the cyclization of citronellal to the intermediate isopulegol, bifunctional metal/H-BEA catalysts were investigated in the one-pot transformation of citronellal to menthols. Using Pd undesired defunctionalization prevailed, while using Pt and Ru, the formation of menthols predominated. Finally, for Ru/H-BEA catalysts, the effect of catalyst composition (Si:Al ratio, Ru content) and reaction conditions (temperature, hydrogen pressure, and solvent) was investigated to diminish citronellal hydrogenation, dimerization, and defunctionalization. Accordingly, with a 1%Ru/H-BEA-25 catalyst (Ru particle size = 1 nm) in dioxane at 15 bar hydrogen pressure and 373 K, a very high selectivity to menthols of 93% at nearly full conversion was obtained. The catalyst is also highly diastereoselective producing 79% of the desired (±)-menthol.

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