Cyclisation of citronellal over zirconium zeolite beta- a highly diastereoselective catalyst to (±)-isopulegol

The catalytic cyclisation of citronellal was studied over Zr-zeolite beta, micro/mesoporous Al-MSU-SFAU, and microporous HY catalysts. All samples showed good activity in the cyclisation of citronellal to form isopulegols with >97% selectivity. A high diastereoselectivity for (±)-isopulegol of ∼93% was observed over Zr-zeolite beta, whereas Al-MSU-SFAU and HY showed a lower selectivity of ∼65%. Zr-zeolite beta was synthesised in a range of Si/Zr of 75-200 with the use of fluoride and zeolite beta seeds. Zeolite beta with Al and Ti substitution was less active and selective than Zr-zeolite beta. The rate of reaction strongly depended on the type of solvent used, but the reaction could also be carried out without any solvent. A hydroxylated surface is important for good activity. This is consistent with the proposed mechanism, where both Lewis and Brønsted acid sites are essential for the reaction.