Synthesis of dihydrocarvone over dendritic ZSM-5 Zeolite: A comprehensive study of experimental, kinetics, and computational insights

This study explores the isomerization of limonene-1,2-epoxide (LE) from kinetic and mechanistic viewpoints, using a dendritic ZSM-5 zeolite (d-ZSM-5) as a highly selective catalyst for the formation of dihydrocarvone (DHC) in the form of diastereoisomers (cis + trans). Ethyl acetate, a green solvent, was used at mild reaction temperatures (50–––70 °C). DHC, which can also be extracted from caraway oil, is widely used as an intermediate for epoxylactone production and as a constituent in flavors and perfumes. Kinetic modeling of LE isomerization was performed using a reaction network with eight parallel reactions and the corresponding rate equations, derived from the assumption of the rate-limiting surface reactions. The large standard errors in the statistical results of some kinetic parameters of the initial data fitting suggested that three of those reactions can be neglected to describe the kinetic model more accurately. This refinement resulted in standard errors in the kinetic parameters lower than ca. 11 %, confirming the statistical reliability of the modified kinetic model. Activation energies of 41.1 and 162 kJ/mol were estimated for the formation of cis-DHC and trans-DHC, respectively. Density Functional Theory (DFT) calculations revealed the preferred pathway for both cis and trans-LE conversion to DHC and carveol. The rate-determining step, carbocation formation (ΔEact = 234 kJ/mol), precedes near-instantaneous dihydrocarvone formation under the studied conditions.