Prins cyclisation from isovaleraldehyde and isoprenol to tetrahydropyranol.
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Zeolites with varying acidity and hierarchical materials as catalysts.
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H-Beta-300 zeolite giving 72% selectivity to the desired product at 99% conversion.
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The cis/trans ratio of tetrahydropyranols depends on porous structure.
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Parallel formation of tetrahydropyranol and dihydropyran confirmed by DFT.
Abstract
In this work, several heterogeneous micro- and mesoporous, acidic catalysts were tested for the selective synthesis of Florol®, an industrial product formed via condensation and rehydration starting from isoprenol and isovaleraldehyde in dimethylcarbonate as a solvent. The results showed that a mildly acidic, microporous H-Beta-300 with SiO2/Al2O3 ratio of 300 was the best catalyst, giving 72% selectivity with 99% conversion at 40 °C when using the molar ratio isoprenol to isovaleraldehyde of 1:5. More acidic zeolites gave slightly lower selectivity, while the lowest selectivity (up to 52%) was obtained with mesoporous catalysts exhibiting mild acidity and no strong Brønsted acid sites. Selectivity to pyranols was nearly constant when changing temperature, while a larger excess of isovaleraldehyde promoted formation of tetrahydropyranol. H-Beta-300 catalyst was successfully after calcination at 400 °C. DFT calculations pointed out on the parallel formation of tetrahydro pyranols and dihydropyrans, which are the corresponding dehydration products.