Shuxia Cai

Cyclization of citronellal to p-menthane-3,8-diols in water and carbon dioxide

A clean process has been developed for the synthesis of p-menthane-3,8-diols from cyclization of citronellal in CO2–H2O medium without any additives. With the addition of CO2, the reaction rate could be enhanced about 6 times for the cyclization of citronellal in H2O, because CO2 dissolved into water and formed carbonic acid inducing an increase of the acidity. Although, the reaction conversion in CO2–H2O is slightly lower compared to that obtained with sulfuric acid as catalyst, CO2–H2O could replace the sulfuric acid at a relative higher reaction temperature. The reaction kinetics studies showed that the hydration of isopulegols to p-menthane-3,8-diols is a reversible reaction. The equilibrium constant and the maximum equilibrium yield obtained in CO2–H2O at a range of CO2 pressures are similar to that with sulfuric acid catalyst.

Selective hydrogenation of unsaturated aldehydes in a poly(ethylene glycol)/compressed carbon dioxide biphasic system

Hydrogenation of α,β-unsaturated aldehydes (citral, 3-methyl-2-butenal, cinnamaldehyde) has been studied with tetrakis(triphenylphosphine) ruthenium dihydride (H2Ru(TPP)4) catalyst in a poly(ethylene glycol) (PEG)/compressed carbon dioxide biphasic system. The hydrogenation reaction was slow under PEG/H2 biphasic conditions at H2 4 MPa in the absence of CO2. When the reaction mixture was pressurized by a non-reactant of CO2, however, the reaction was significantly accelerated. As CO2 pressure was raised from 6 MPa to 12 MPa, the conversion of citral increased from 35% to 98%, and a high selectivity to unsaturated alcohols (geraniol and nerol) of 98% was obtained. The products were able to be extracted by high pressure CO2 stream and separated from the PEG phase, dissolving the Ru complex catalyst and the catalyst was recyclable without any post-treatment.