Oxidative Esterification of Acetal-Protected 5-Hydroxymethylfurfural to Dimethyl Furan-2,5-dicarboxylate with High Recovery of Protecting Agent

Polyalkylene furanoates are important prospective biobased substitutes for fossil-based terephthalates. Furanoate monomers can be obtained from the bioplatform 5-hydroxymethylfurfural (HMF), but practical applications are hindered due to its rapid oligomerization, which requires working in dilute solutions. Herein, we demonstrate a protection strategy based on acetalization of the reactive formyl group with propane-1,3-diol (PDO) to suppress oligomerization reactions. Following a stepwise approach, PDO-acetalized HMF (PD-HMF) was oxidatively esterified in a methanolic 20 wt % solution to mainly PDO-acetalized methyl-5-formylfuran-2-carboxylate (PD-MFFC) in a 91.0% yield in 6 h at 100 °C under 8 MPa air using a Au/CeO₂ catalyst. The use of base could be decreased to 7.5 mol % Na₂CO₃ with respect to PD-HMF, which is a considerable advance over the state of the art. The loss of PDO due to oxidative degradation under these improved conditions was limited to 1.4%, implying that the protecting agent could be reused, which benefits the economic viability of the process. The PD-MFFC product stream, which contains a small amount of MeOH-acetalized MFFC, could be easily deprotected to obtain MFFC in a 99.1% yield with a PDO recovery degree of 93.1%. MFFC could readily be converted to dimethyl furan-2,5-dicarboxylate (MFDC) in a 96.6% yield via base-free oxidative esterification of MFFC in a methanolic 20 wt % solution in 2 h at 100 °C under 6 MPa air. The proposed stepwise approach combines high productivity (overall MFDC yield >90%) and excellent PDO recovery (>91%) using concentrated PD-HMF solutions (20 wt %).