Cyclization of citronellal over zeolites and mesoporous materials for production of isopulegol

Cyclization of (+)-citronellal was investigated over zeolites and mesoporous materials as well as on silica under a nitrogen atmosphere in cyclohexane as a solvent. The highest cyclization rates were observed over mesoporous materials and 12-membered ring zeolites with high Brønsted acid concentration, while very low cyclization rates were achieved over silica with low or no Brønsted acidity, respectively. At the same a time low cyclization rate was observed over 10-membered ring pore H-ZSM-5 with a high Brønsted acid site concentration, which is due to diffusional limitation of the product in the narrow pores. The selectivity to cyclization products was very high over all the catalysts, being independent of the conversion of citronellal. Neither concentration of the Brønsted nor Lewis acid sites influenced the stereoselectivity to isopulegol. The support structure had only a minor effect on the stereoselectivity. Quantum mechanical calculations were carried out to explain the experimental results. The calculated stabilities of the carbocationic reaction intermediates correlated well with the observed stereoselectivity. The stereoselectivities were analogous when starting from racemic citronellal mixture or enantiopure (+)-citronellal; the former one gave 8 different pulegols, whereas only 4 pulegols were formed from (+)-citronellal.