Liquid metal catalysis offers a promising solution to a critical challenge in biomass pyrolysis—catalyst deactivation due to coking. Owing to density differences between the liquid metal and solid carbonaceous byproducts, the resulting char segregates and thereby physically separating from the catalyst and mitigating deactivation. In our prior work, we explored the catalytic behavior of liquid metals during the pyrolysis of cellulose, a primary structural component of biomass. Building on this foundation, the present study investigates the catalytic performance of the liquid metals in the pyrolysis of lignin—another fundamental constituent of lignocellulosic biomass. Kraft lignin (particle size: 40–90 µm), a common waste byproduct from the pulp and paper industry, is employed as the model feedstock. The reaction media are inert sand (control) and low-melting-point metals (Bi, Sn, and In), with reaction media to lignin blend of 4:1 v/v ratio in a thermogravimetric analyzer. The resulting gas, liquid, and solid products are analyzed.
Consistent with our findings from cellulose pyrolysis, the mass loss profiles from 30 to 700°C indicate that In and Sn facilitate higher char yields and reduced volatilization rates (−0.85%/min) relative to the non-catalytic case (−1.4%/min). Bi, by contrast, exhibited negligible influence on char yield compared to sand. Further analysis of bio-oil collected from various temperature regimes reveals that Sn and In catalyze the cleavage of key lignin interunit linkages—specifically β-O-4, β-5′, and β-β′ bonds—at temperatures below 345°C, leading to formation of ~75 wt% of its total oil yield. Above 345°C, the onset of oligomerization dominates, resulting in increased char formation and water removal. Bi catalysis, on the other hand, promotes secondary cracking of primary volatiles, selectively increasing acetic acid production (22 wt% of total bio-oil vs. 2.7 wt% in the non-catalytic case). These findings underscore the potential of liquid metal catalysis for biomass conversion.
