Solvent effects in solid acid-catalyzed reactions: The case of the liquid-phase isomerization/cyclization of citronellal over SiO2-Al2O3

Abstract

The effect of solvent on the activity and selectivity of SiO2-Al2O3 in the liquid-phase acid-catalyzed isomerization/cyclization of citronellal (CNAL) to isopulegol was investigated using non-polar (cyclohexane, toluene), weakly polar (chloroform), polar protic (ethanol, 2-propanol) and polar aprotic (acetonitrile) solvents. The catalyst activity and selectivity greatly depended on the solvent nature. The CNAL conversion rate was higher in weakly and non-polar (chloroform > toluene > cyclohexane) than in polar solvents (ethanol > acetonitrile > 2-propanol). To interpret this activity pattern, the solvent-CNAL interactions were analyzed by measuring the shift of the infrared CO absorption band of CNAL in CNAL/solvent mixtures, while the solvent-catalyst interactions were characterized by both determining the solvent adsorption enthalpies by calorimetry and following the product evolution from temperature-programmed desorption of solvents. In the case of polar solvents, both the strong solvation effect in the liquid phase and the adsorption strength of solvents on SiO2-Al2O3 contributed to decrease the catalyst isomerization activity. In particular, the selectivity to isopulegol was clearly lower in polar protic solvents (ethanol and 2-propanol) because secondary reactions producing carbon deposits took place. In contrast, in non-polar solvents the selectivity to isopulegol was higher than 97% in all the cases. The highest isomerization rate was obtained in chloroform that partially adsorbs dissociatively on SiO2-Al2O3, leading to a higher density of acid sites available for the isomerization reaction.