Biomass upgrading of furfural to cyclopentanone via selective aqueous-phase hydrogenative rearrangement over Pd/g-C₃N₄ catalysts

The monometallic Pd/g-C₃N₄ catalysts were synthesized and applied in aqueous-phase hydrogenative rearrangement of furfural (FFR) to cyclopentanone (CPO) for biomass upgrading. Their structure-performance relationship has been investigated by combining structural characterizations and kinetic measurements. The Pd–g-C₃N₄ interfaces prefer single electron transfer property due to the modification of N species, and thus Pd/g-C₃N₄ catalysts with regulated electronic property of Pd sites can suppress the side-reaction pathway of over-hydrogenation of furfuryl alcohol (FAL) to tetrahydrofurfuryl alcohol (THFAL). Commercial 5%Pd/C catalyst favors one-electron and two-electron transfer at Pd–C interfaces and possesses lower surface acidity, partially following the THFAL formation pathway. With respect to the size effect, the smaller Pd NPs selects to form furan ring-opening by-products for a low CPO selectivity, whereas the larger leads to a low intrinsic reaction rate for FAL ring-rearrangement. Only the moderate-size Pd catalysts exhibit satisfactory activity, high CPO selectivity and great recycling stability. Furthermore, FFR–to–FAL hydrogenation is identified as the rate-determining step rather than H₂ activation and FAL–to–CPO ring-arrangement. The FT-IR spectra elucidate that FFR molecule is activated in a tilted adsorption configuration through bonding with the exocyclic C[dbnd]O group for accelerating the successive reaction steps. This work provides the insights into the effects of electronic property and metal size in the metal–acid synergistic catalysis for hydrogenative rearrangement of FFR.