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The addition of nickel to Zr-beta gave a useful bifunctional catalyst that combines a high rate of cyclization of (±)-citronellal to isopulegols over zirconium sites and subsequent hydrogenation to menthols. The diastereoselectivity to the desired (±)-menthol was 90%. A loading of 4 wt% is optimal; lower loadings led to a low rate of hydrogenation, whereas a higher nickel loading appears to block the zirconium Lewis acid sites essential for the cyclization of citronellal. A mixture of Zr-beta and Ni/MCM-41 also formed an effective bifunctional catalyst system where the selectivity toward menthols remained high even with nickel loadings up to 15 wt%. The yield of (±)-menthol over the dual-catalyst system was 86–89% with 5 % of byproducts, citronellol and 3,7-dimethyloctanol. Besides nickel, bifunctional Rh/Zr-beta catalysts also formed menthols with selectivity > 93 % , although the diastereoselectivity to (±)-menthol was lower (85%). In comparison, a 2% Pd/Zr-beta catalyst exhibited lower activity and selectivity to menthols, forming substantial amounts of 3,7-dimethyloctanal. |
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