Production of Phenol-Rich Monomers from Kraft Lignin Hydrothermolysates in Basic-Subcritical Water over MoO3/SBA-15 Catalyst

MoO3 catalyst supported on synthesized mesoporous SBA-15 matrix was employed in the hydrothermolysis of Kraft lignin in a basic-subcritical water system. Experiments were carried out in a batch reactor at 350 °C for 1 h under N2 atmosphere using two different solvent systems of sub-H2O and 0.5% NaOH. At 10% (w/w) of synthesized MoO3/SBA-15 catalyst with lignin, the highest total crude bio-oil yield (∼56.40 wt %) and lowest char formation (∼8.60 wt %) were observed in basic medium, in comparison to the subcritical water system. The GC-MS analysis of the produced bio-oil indicated that the major products in the crude bio-oil were phenol-rich monomers such as guaiacols (19.50%), alkyl-guaiacols (24.83%), catechol (26.76%), and alkyl-catechols (17.82%) under subcritical water condition. Alternatively, catechols (42.07%) and alkyl-catechols (34.59%) were the primary products in the basic solvent system with the presence of MoO3/SBA-15 catalyst. Additionally, monophenols (5.0%) were also produced as a result of the demethoxylation of guaiacol or hydrodeoxygenation of catechol. MoO3/SBA-15 catalyst significantly reduced the oxygen–carbon ratio (O/C) from 0.72 (in original Kraft lignin) to 0.29 (in basic-subcritical water system). Decreased oxygen and increased carbon and hydrogen contents enhanced the calorific value of the produced bio-oil, whereby a higher heating value (HHV) of 32.00 MJ/kg was achieved. The synthesized catalysts were characterized through BET, XPS, ICP-OES, and HR-TEM analyses.