The potential of Sn–B–NaY–ZE catalyst for transformation of citronellal-rich essential oils to menthol-enrich oil via optimization through single factorial design to enhance their organoleptic profile and biological activities

Citronellal-rich essential oils of Cymbopogon winterianus and Corymbia citriodora were catalytically modified in a two-step process to produce menthol and other terpene alcohols using Sn–B–NaY–ZE and 1%Pd/AC, successively. The reaction parameters such as solvent selection, reaction time, temperature, and catalyst loading were optimized using a single factorial design. In the first step, the citronellal was cyclized to isopulegol isomers with 99% selectivity at 80 °C for 90 min using 10% catalyst loading (Sn–B–NaY–ZE) in cyclohexane medium. Further, in the second step, this isopulegol was reduced to menthol using 1%Pd/AC. A part of menthol was isolated from the modified oils under low-temperature freezing (− 40 °C) process. Further modified menthol-rich essential oils and spent oil (after partial separation of menthol) were tested against the antibacterial and antioxidant activities. The results reveal that the modified spent oil of C. citriodora is effective against S. aureus and S. typhimurium bacterial strains. It was observed that the spent oil showed potent antimicrobial activities due to the enrichment of oxygenated monoterpenoids. Therefore, it suggested that the significant antimicrobial activity is due to the synergistic effects of menthol and other oxygenated monoterpenoids. Moreover, the spent oils have shown better antioxidant activities. The menthol in modified oil has changed the olfactometry to a sweet minty odor with cooling sensation. Hence, the modified essential oils are organoleptically superior due to their enhanced percentage of terpene alcohols and found potential application in various cosmeceutical preparations.