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/gC3N4 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--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.