Insight into mechanism of hydrogen-supplied liquefaction of lignocellulose over varied catalysts

Hydrogen-supplied liquefaction technology is considered to be one of the most essential thermal treatment strategies for improving the quality of bio-oil. This study employed wood chips as feedstock, inert alkanes as dispersing solvent, and tetraline as hydrogen donor. The yields of bio-oil rose substantially from 45.2% to 69.2% and 68.3% with CaO and 8% NiO/γ-Al₂O₃ catalysts, respectively, at 310 °C for 30 min with a ratio of wood chips to n-heptane of 1:8 g ml⁻¹. The reaction mechanism was examined using microcrystalline cellulose and lignin as model compounds. Tetralin offered the active hydrogen, while the strong alkalinity of CaO brought about high-temperature fragmentation of cellulose to produce small-molecule ethers, aldehydes, and ketones. These species would eventually transform into alcohols through engaging with the active hydrogen. The introduction of NiO/γ-Al₂O₃ promoted the ring-forming reaction of cellulose-derived fragments, resulting in an increase of furan content (up to 29.97%). This phenomenon suggested that the varied catalysts exerted a remarkable impact on the hydrogen-supplied liquefaction of cellulose. The catalytic effect on lignin was evident in the breakage of the C–O bond in the side chain of phenol, leading to the generation of an impressive amount of phenolics. In light of changes in the generated products and calculation of the theoretical bond energies, the bond-breaking approaches and potential reaction pathways of biomass impacted by different catalysts were proposed, providing a theoretical foundation for the development and application of biomass energy.