Metal-free and oxidizing agent-free ultra-selective catalytic aerobic conversion of vanillyl alcohol to vanillin by carbon fiber

Valorization of biomass is a critical step towards sustainable utilization of renewable bio-resources. Among numerous bio-derivatives, vanillyl alcohol (VAA) appears as an attractive bio-derivative as VAA can be derived from lignin, and then transformed to vanillin (VNL), a versatile and high-value-added molecule. Nevertheless, the oxidation of alcohol of VAA to VNL would require continuous and intensive consumption of stoichiometric dosages of H2O2. For achieving the goal of sustainability, it is highly-desired to develop a process which can oxidize VAA to VNL with a high conversion efficiency and selectivity but without using any metals and traditional oxidants. To this end, we demonstrate for the first time by using a carbon-based material as a metal-free catalyst for oxidizing VAA in the absence of any oxidizing reagents. At 130 °C for 2 h, a significantly high VAA conversion of 98% can be obtained, and more importantly, a selectivity of 100% can be achieved. CF can be also recyclable for oxidizing VAA to VNL over the consecutive 5 cycles with consistent selectivities and yields of VNL. Through examining the effect of radical scavengers and EPR analyses, the oxidation of VAA to VNL could be attributed to singlet oxygen via the non-radical route of oxidation.