Formylation Pretreatment Enabled One-Pot Synthesis of 2,5-Diformylfuran from Cellulose via Recyclable CuCl2 Catalysis

Lignocellulosic microcrystalline cellulose (MCC) is an abundant nonfood biomass feedstock for biorefinery. Nevertheless, its rigid crystalline network and dense intra/intermolecular hydrogen bonds severely hinder direct catalytic conversion to high-value platform chemical 2,5-diformylfuran (DFF). Traditional synthetic routes toward 2,5-diformylfuran require oxidation of preisolated 5-hydroxymethylfurfural (HMF). By contrast, direct catalytic transformation of pristine cellulose yields merely trace DFF and thus lacks industrial viability. Herein, a mild formic acid–ZnBr2 coformylation pretreatment strategy was developed to disrupt cellulose crystalline architecture. Following structural modification, the crystallinity index (CrI) of cellulose drastically decreased from 79% to 19%, accompanied by broken hydrogen-bond networks and significantly enhanced solubility in dimethyl sulfoxide (DMSO). A low-loading, recyclable CuCl2 tandem catalytic system was further constructed for one-pot cascade conversion of formylated cellulose to DFF, wherein stoichiometric α,α-dichlorodiphenylmethane acts as chlorine source to generate key intermediate 5-(chloromethyl)furfural (CMF). Under mild conditions (100 °C, 24 h), formyl MCC afforded a maximum DFF yield of 77%, whereas glucose (model carbohydrate substrate) achieved an excellent 99% DFF yield. Systematic kinetic tracking and multiple controlled experiments validated a sequential dehydration–chlorination–oxidation cascade mechanism centered on Kornblum-type oxidation reactions between CMF and DMSO, with CMF identified as a far more robust reactive intermediate than thermally labile HMF. The integrated pretreatment–catalysis system exhibited remarkable reusability, retaining >85% DFF yield after three consecutive recycling cycles without complicated catalyst regeneration. This work establishes a facile, low-energy, sustainable biorefinery route for full valorization of cellulose toward DFF, and broadens the application of formylation modification in polysaccharide catalytic upgrading.