Enhanced Upgrading of Crude Lignin Bio-Oil to Jet Fuel Precursors with a Moderate Organic Acid-Modified Copper Catalyst

The conversion of crude lignin bio-oil to jet fuels is a highly desirable target for biorefineries, especially in light of the global energy crisis. The key to achieving this goal is the realistic tuning of catalyst efficiency and selectivity for expected products. Herein, a copper catalyst with enhanced activity for hydrodeoxygenation (HDO) of crude lignin bio-oil to jet fuel precursors was developed in this work. The copper catalyst, a copper-doped porous metal oxide, was modified with a series of moderate organic acids to graft the acidic O-groups as potential Brønsted acid sites for HDO reactions. The copper catalyst modified with propanoic acid exhibited exceptional catalytic performance for the upgrading of lignin pyrolysis bio-oil, giving yields of jet fuel precursor range cycloalkanes and cycloalcohols of 10.04 and 14.20 C%, which were improved by 88.0 and 42.7% compared with those from the blank. Meanwhile, the corresponding hydrogenation and deoxygenation degrees of the products reached 91.5 and 65.9%, respectively, which were improved by 17.3 and 31.8%. Likewise, the overall double bond equivalence and aromaticity index of the products given by the modified copper catalyst were as low as 2.8 and 0.1, which were substantially reduced by 18.0 and 50%, correlating to higher HDO efficiency. The abundant Brønsted acid sites with proper strength and the affluent pore structures of the propanoic acid-modified copper catalyst mainly contributed to its outstanding catalytic activity. The findings suggest promising avenues for increasing the efficiency of heterogeneous catalysts in the HDO of lignin-based bio-oil to alternative fuels.