Isomerization of citronellal to isopulegol using eclectically engineered sulfated zirconia−carbon molecular sieve composite catalysts, UDCaT-2

Abstract

Sulfated zirconia (S−ZrO₂) is a well-known solid superacid used as a catalyst in various reactions of commercial importance such as isomerization, Friedel−Crafts alkylation and acylation, nitration, cracking, esterification, etc. S−ZrO₂, per se, is not a shape-selective catalyst. The selectivity toward the formation of the desired product can be greatly enhanced by eclectically designing a shape-selective catalyst by a synergistic combination between S−ZrO₂ and carbon molecular sieves (CMS). The paper presents the novelty of the combination of S−ZrO₂ and CMS, designated as UDCaT-2, as a shape-selective catalyst in the cyclization of citronellal to isopulegol which has industrial value. UDCaT-2 was found to be the best catalyst, among others, for the selective cyclization of citronellal to isopulegol wherein the shape selectivity can be tailored by proper pretreatment. The conversion and selectivity were found to be the maximum at 95 °C. The formation of isopulegol was found to be dependent on the average pore size of the carbon molecular sieve barrier encompassing S−ZrO₂. A detailed kinetic study of the reaction showed that it followed a Langmuir−Hinshelwood−Hougen−Watson type of mechanism whereby citronellal was found to be weakly adsorbed on the catalytic surface sites. The reaction was found to follow first-order kinetics for the disappearance of citronellal.