A two-step approach for producing oxygen-free aromatics from lignin using formic acid as a hydrogen source

A high-calorific-value oil with a high amount of oxygen-free aromatics was produced from highly recalcitrant concentrated-sulfuric-acid-hydrolysis lignin (CSAHL) using a two-step approach. In the first step, lignin-derived bio-oil (LBO) was produced by the liquefaction of CSAHL in a mixture of supercritical ethanol and formic acid. In the second step, hydrodeoxygenation (HDO) of the LBO over sulfided CoMo/γ-Al₂O₃ (CoMoS_x/γ-Al₂O₃), Pd/Al₂O₃, and Ru/γ-Al₂O₃ catalysts was performed to produce upgraded LBO. Although the yield of the LBO was high (70 wt%), its calorific value and the yield of monoaromatics (mainly oxygenated guaiacol- and syringol-type molecules) were low (27.2 MJ kg⁻¹ and 4 wt% (based on CSAHL), respectively). The LBO was separated from the ethanol and used as a feed for HDO in formic acid, which acts as an in-situ hydrogen donating agent. HDO of the LBO over CoMoS_x/γ-Al₂O₃ at 350 °C for 4 h produced oxygen-free aromatics, such as alkylated benzenes, indanes, and tetralins, with the high yield of 6.8 wt% (based on CSAHL). The degree of deoxygenation was high (>90%) and the upgraded LBO exhibited a high calorific value (>40 MJ kg⁻¹). The effects of different HDO parameters, including catalyst loading, amount of formic acid, reaction time, catalyst type, and solvent type were assessed to gain insight into the LBO deoxygenation mechanisms.