The Highly Selective Synthesis of 5-Methy Vanillin from the By-product in Vanilla Industry and The Scent Influence for Vanillin

Herein, with the aim of the comprehensive utilization of a useless by-product, a highly selective method was proposed for the production of 5-methyl vanillin (M-vanillin) by employing o-vanillin as a raw material, which is a useless by-product in the vanillin industry. The olfactory thresholds indicated that the scent of vanillin is inhibited by the presence of o-vanillin, while the scent of vanillin is promoted by the synergistic effect of M-vanillin. M-Vanillin was generated through a three-step highly efficient procedure, including hydrogenation reduction, addition reaction and oxidation. The key intermediate (3-methoxyl-4-hydroxy-5-methylmandelic acid, 5-MVMA) was catalyzed to oxidize to M-vanillin with 92% yield using a porous NiCo2O4/SiO2 composite catalyst via a green procedure at 1 atm atmospheric pressure and 80 °C. The multilayer adsorption–desorption curve showed that the specific surface area of NiCo2O4 increased from 14 m2 g−1 to 170.78 m2 g−1 after it was loaded on the SiO2 substrate, which provided favorable conditions for the substrate and O2 adsorption. XPS and in situ ATR-FTIR indicated that the Co2+/Co3+ and Ni2+/Ni3+ valence pair co-existed in the catalyst, which resulted in high selectivity for the production of M-vanillin.