When Furfural Meets Piancatelli Rearrangement

As furfural is the most normal industrial chemical in the field of biomass conversion, furfural valorization is seriously retarded by the generation of humins. Electrophilic substitution of furfural was proposed to be one of the elementary reactions to form furfural-derived humins, which nevertheless has not been proved before. Herein, a key intermediate, 2-(4-furfur-2-al)-4-hydroxy-2-cyclopenten-1-one, was successfully captured and determined by single-crystal analysis, which was presumably formed by Piancatelli rearrangement of the furfural dimer due to its instability. Furthermore, by means of HPLC–MS/MS, FT-IR, 13C solid-state NMR, SEM, and TEM characterizations, it was speculated that multiple elementary reactions were involved in the formation of furfural-derived humins, such as electrophilic substitution, etherification, and aldol condensation, besides Piancatelli rearrangement. During the conversion of furfural, a previously undiscovered double-layer structure of furfural-derived humins with a loose inner layer and a dense outer layer was revealed. By regulating the key factor of LA, the process of humin formation can be modulated, leading to more regular core–shell spheric structures.