Novel Silica-Encapsulated Cu–Al Hydrotalcite Catalyst: Oxidative Decarboxylation of Vanillyl Mandelic Acid to Vanillin in Water at Atmospheric Pressure

Vanillin is a versatile chemical, having characteristics such as antimicrobial, antioxidant, anticarcinogenic, antimutagenic, hypolipidemic, and antisickling activities and is an intermediate for synthesis of fine chemicals such as vanilla flavoring. Vanilla is the second most expensive spice; it is extracted from vanilla beans and is in short supply. Therefore, to meet demand of vanilla flavor, the synthesis of vanillin via chemical routes is necessary. In this regard, a novel silica-encapsulated copper aluminum hydrotalcite (SECuAlHT) was synthesized. Different techniques such as XRD, FTIR, N 2 adsorption–desorption, TGA-DSC, NH 3 TPD, CO 2 TPD, H 2 chemisorption, TEM, and SEM-EDXS were used to characterize SECuAlHT. Its catalytic activity for oxidative decarboxylation of vanillyl mandelic acid (VMA) was investigated. Vanillin was obtained from VMA by oxidative decarboxylation using SECuAlHT in water as a solvent and atmospheric air under base-free conditions. The presence of basic sites in the catalyst was confirmed with CO 2 -TPD. Effect of different reaction conditions on catalyst activity and selectivity for oxidative decarboxylation of VMA was investigated. Vanillin was efficiently obtained with 89% selectivity at 81% VMA conversion over the SECuAlHT catalyst at 100 °C in 8 h. The catalyst was active, selective, stable, and reusable. Catalytic cycle based on the Mars van Krevelin mechanism was proposed and the reaction kinetics studied. It is a zero-order reaction. The apparent energy of activation was 8.7 kcal/mol. Thus, vanillin was efficiently synthesized from VMA via a green catalytic route in water as a solvent.