Exploitation of liquid CO2 based greener process for valorization of citronellal rich essential oils into flavor grade (-)-menthol using novel Sn/Al-B-NaYZE composites

Abstract A two-step greener catalytic process has been developed for semi-synthesis of (-)-menthol from citronellal-rich essential oils such as Cymbopogon winterianus and Corymbia citriodora by using novel bi-acidic composites. These novel composites (Al-B-NaYZE or Sn-B-NaYZE) were prepared by impregnation of Sn-B and Al-B over NaYZE framework to increase the Lewis and Bronsted acidic sites. These composites were thoroughly characterized using TDP, FT-IR, UV, XRD, SEM, HRTEM, SA, and TGA. The composite Sn-B-NaYZE cyclized 99% of citronellal to isopulegol isomers with 99% selectivity in 45 min under liquid CO2 medium, while composite Al-B-NaYZE is showing 95% selectivity towards isopulegol isomers. Sn-B-NaYZE was displayed ehnaced selectivity due to its higher acidic property as confirmed by the TPD analysis. The chiral study indicates that the Sn-B-NaYZE composite is giving highest selectivity (94%) towards (-)-isopulegol. (+)-Citronellal or citronellal oil was found as an ideal substate for (-)-menthol production. It observed that Sn-B molar ratio over base material NaY-Zeolite was played a mojor role for catalytic activity and selectivity towards (-)-isopulegol. Further, isopulegol isomers were reduced to 98% of menthol using 1%Pd/AC at 40 psi H2 pressure in 1 h. Reaction mixture slowly frozen to -40oC for trouble-free isolation of the crude menthol. Further, pharmaceutical and flavor grade (-)-menthol was purified from the crude menthol through esterification process. This (-)-menthol was found to 100% biobased on 14C-radiocarbon-dating to authenticate as nature-identical. The composite has displayed high reusability up to five times without significant loss in their activity. After isolation of menthol, the spent oil was enrich with oxygenated monoterpenoids, hence it was displayed significant antimicrobial activities.