Selective generation of vanillin from catalytic oxidation of a lignin model compound using ZIF-derived carbon-supported cobalt nanocomposite

To develop an effective and selective heterogeneous catalyst for valorizing lignin via chemical oxidation, a cobalt-supported cobalt (CSC) nanocomposite is derived from one-step carbonization of cobaltic ZIF-67. The resulting CSC consists ornanoscale cobalt oxide nanoparticles evenly distributed in a porous carbon/nitrogen matrix. The combination of CSC and H2O2 successfully oxidizes a phenolic model lignin, vanillyl alcohol (VAL), to vanillin (VN) with a high selectivity towards VN. Acetonitrile is found as a favorable solvent of oxidation of VAL, whereas relatively high temperature and dose of CSC may cause rapid H2O2 degradation, thereby decreasing the yield for VN. The acidic conditions are also proven as the most suitable condition for oxidation of VAL possibly because free radicals derived from H2O2 exhibit the strongest oxidizing capability under acidic conditions. CSC is also demonstrated as a promising catalyst for aerobic oxidation of VAL to VN with an extremely high selectivity. CSC retains its catalytic activity for oxidation of VAL to VN over multiple cycles. These features reveal that CSC, which is conveniently prepared by one-step carbonization of ZIF-67, can be an effective and highly selective heterogeneous catalyst for oxidation of VAL to a value-added product, VN. (C) 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.