Catalytic Depolymerization of Lignin for the Selective Production of Phenolic Monomers over Cobalt-Supported Calcium Catalysts

In this study, a series of cobalt metal (Co) [2, 5, 10, and 15 wt %]-supported calcium oxide (CaO) catalysts were synthesized and tested for the catalytic depolymerization of lignin under various process parameters such as reaction solvents, temperature, holding time, and catalyst amount. The highest bio-oil yield was found to be 26.6 wt % at 160 °C using water as a solvent. However, with an alcoholic solvent, lignin depolymerization was significantly promoted, and the maximum bio-oil yield (60.2 wt %) was obtained with 10 wt % Co/CaO catalyst under methanol solvent at 160 °C for 60 min reaction time. Bio-oil analysis showed that, under non-catalytic reaction, the total area percentage of the phenolic compounds was low, and it contained mainly alkyl phenols with a small amount of methoxy phenol compounds. Intriguingly, the lignin macromolecule depolymerization reaction was promoted using cobalt metal-supported catalysts, increasing the selectivity (58.7%) toward vanillin compounds. Under this condition, the bio-oil's higher heating value (HHV) was 35.5 MJ/kg, which was significantly higher than the HHV of lignin (24.5 MJ/kg). Proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared (FT-IR) spectroscopy analyses showed that higher functionality of aromatic and methoxy compounds was present in the catalytic bio-oils compared to the non-catalytic bio-oils. Furthermore, the catalyst was recovered and tested for its stability, which showed excellent recyclability on the bio-oil yield.