Reusable Nitrogen-Doped Carbon-Encapsulated Cobalt Catalyst for Efficient Stepwise Oxidation of Acetal-Protected...

We report a ZIF-67-derived N-doped carbon-encapsulated cobalt catalyst (Co@N-C) for the efficient, reusable, and noble metal-free aerobic oxidation of concentrated aqueous 5-hydroxymethylfurfural (HMF) derivatives in a stepwise fashion to furan-2,5-dicarboxylic acid (FDCA). Using acetal-protected HMF with propane-1,3-diol (PD-HMF) to suppress degradation at high substrate concentrations, Co@N-C-800p (Co-containing ZIF-67 pyrolyzed at 800 °C) delivers high yields in water while enabling effective PDO recovery. At 20 wt% PD-HMF, a 96.8% yield of PD-FFCA (acetal-protected form of FFCA, 5-formylfuran-2-carboxylic acid) was obtained at 99.2% conversion. Subsequent deprotection afforded FFCA and enabled recovery of propane-1,3-diol, after which FFCA was oxidized separately under the same conditions to FDCA in a 93.6% yield. Replacing soluble alkali carbonates with sparingly soluble Ca(OH)₂ provides buffered alkalinity that enhances formyl group oxidation, minimizes by-products, and limits PDO loss at reduced base loadings. Radical scavenger experiments indicate a mechanism involving oxygen-derived species that regenerate metallic Co sites via hydride removal. Although partial deactivation occurs through cobalt surface oxidation and CaCO₃ accumulation, full catalytic performance is restored by re-pyrolysis. Overall, this work establishes Co@N-C as an efficient, stable, and regenerable non-noble metal catalyst for aerobic FDCA production at industrially relevant substrate concentrations.