Hydrothermal Liquefaction of Alkaline Lignin with In Situ Hydrogen Supply from Formic Acid
Alkali lignin, a high-volume byproduct from pulp and paper manufacturing and biomass refining, is a promising feedstock for aromatic hydrocarbon production in liquid fuels due to its high energy density and abundant aromatic moieties. However, its highly cross-linked polymeric structure hinders efficient valorization. This work investigates catalytic conversion of alkali lignin into bio-oil under in situ H2 supply from formic acid. A series of Ni-Mo/h-BN bimetallic catalysts with varied metal ratios were synthesized by impregnation and characterized by XPS, XRD, and other techniques. The effects of reaction parameters on H2 production via aqueous-phase reforming (APR) of formic acid were evaluated. Optimal H2 yield was achieved at a formic acid-to-water molar ratio of 1:10 and a Ni/Mo atomic ratio of 3:1. H2 yield increased monotonically with temperature from 220 to 280 °C, reaching a maximum of 38.48 mmol. Subsequently, influences of reaction temperature and residence time on bio-oil production were examined. The highest heavy bio-oil yield (18.93%) and maximum relative content of aromatic hydrocarbons (13.81%) were both achieved at 280 °C. Prolonged reaction time reduced heavy bio-oil yield and aromatic hydrocarbon abundance while favoring furan derivatives. This work demonstrates good synergy between in situ hydrogen generation from formic acid and lignin hydrogenation in the temperature range 240–280 °C.