Unraveling the Role of Formic Acid and the Type of Solvent in the Catalytic Conversion of Lignin: A Holistic Approach

The role of formic acid together with the effect of the solvent type and their synergic interactions with a NiMo catalyst were studied for the conversion of lignin into bio‐oil in an alcohol/formic acid media. The replacement of formic acid with H2 or isopropanol decreased the oil yield to a considerable degree, increased the solid yield, and altered the nature of the bio‐oil. The differences induced by the presence of H2 were comparable to those observed in the isopropanol system, which suggests similar lignin conversion mechanisms for both systems. Additional semi‐batch experiments confirmed that formic acid does not act merely as an in situ hydrogen source or hydrogen donor molecule. Actually, is seems to react with lignin through a formylation–elimination–hydrogenolysis mechanism that leads to the depolymerization of the biopolymer. This reaction competes with formic acid decomposition, which gives mainly H2 and CO2, and forms a complex reaction system. To the best of our knowledge, this is the first time that the distinctive role/mechanism of formic acid has been observed in the conversion of real lignin feedstock. In addition, the solvent, especially ethanol, seems also to play a vital role in the stabilization of the depolymerized monomers and in the elimination/deformylation step.