Bio-Oil Production by Depolymerization and Hydrodeoxygenation of Lignins over Mg–Ni–Mo/Activated Charcoal in Supercritical Ethanol

The depolymerization and hydrodeoxygenation (HDO) of kraft, sulfuric acid, and organosolv lignins were investigated over an Mg–Ni–Mo/activated charcoal (AC) catalyst in supercritical ethanol for the production of upgraded bio-oils. Supercritical ethanol served both as a solvent for depolymerization and as a hydrogen source for HDO. Three different types of lignins were used as feedstocks, and their characterization was accomplished by elemental analysis, gel-permeation chromatographic (GPC), Fourier transform infrared spectroscopy (FTIR), and NMR. The characterization of the Mg–Ni–Mo/AC catalyst was also carried out by N2 adsorption/desorption, temperature-programmed reduction using hydrogen (H2-TPR), X-ray diffraction (XRD), and scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS). The upgraded bio-oil yield was significantly enhanced by employing the catalyst. The experiment with sulfuric acid lignin resulted in an upgraded bio-oil yield of 75.4 wt % over the catalyst, which significantly exceeded more than twice that obtained without the catalyst at 350 °C, 1 h reaction time, and an ethanol to lignin ratio of 90:10 (w/w). The optimal reaction conditions for kraft lignin were determined to be 350 °C, 3 h reaction time, 90:10 (w/w) ethanol to lignin ratio, and 4 wt % catalyst loading, which resulted in the formation of upgraded bio-oil with 83.0 wt % yield and 35.44 MJ/kg higher heating value (HHV).