Stabilization Strategies for Preventing Secondary Reactions During Lignin Depolymerization and Valorization

Abstract

Lignin-derived phenolic and aromatic monomers are rapidly being recognized as forerunners in the race to make sustainable biopolymers that can fulfill the world’s demands for materials in the future. However, lignin being highly recalcitrant to the cleavage of its interunit linkages requires harsh conditions for its depolymerization. These harsh conditions often make lignin react in an undesired way, leading to the formation of unwanted products, such as biochar through secondary reactions such as oligomerization, repolymerization, and condensation. Specifically, certain reactive groups in the phenylpropanoid structure of lignin, such as Cα-OH and Cγ-OH, and open positions on the aromatic ring are majorly responsible for the recondensation of lignin during its valorization. To overcome these challenges, the structure of lignin or lignin-derived fragments can be modified in certain ways to make it more stable and less prone to unwanted secondary reactions. This study is focused on investigating four strategies for stabilization of lignin and lignin-derived fragments during various stages of lignin valorization. In the first part, fragments derived from fast pyrolysis of dealkaline lignin were stabilized with the help of vapors from low-density polyethylene (LDPE) and polystyrene (PS) through their catalytic fast co-pyrolysis at 500°C in a micro-pyrolyzer. The synergistic effect between these two feedstocks was quantified and this strategy was found to have about 1.4- 6.9 times increase in the yields of certain lignin and plastic-derived compounds on their depolymerization. In the second part, we provided hydrogen to the lignin-derived fragments within the solvent liquefaction medium using an in-situ hydrogen donor solvent 1,2,3,4-tetrahydronaphthalene (tetralin). It was revealed that the presence of in-situ hydrogen donor tetralin made the products from lignin depolymerization less dependent on the composition of the solvent ethanol-water. In the third part, we explored a pretreatment to modify organosolv lignin at 80°C using phenol before depolymerizing it at 300°C with Ru/C as a catalyst and ethanol as a solvent. There was a 14% increase in the yield of phenolic monomers (excluding phenol) and a 27% decrease in the yield of biochar as a result of this pretreatment. In the fourth part, we investigated the effect of adding boric acid to the biomass (poplar) fractionation medium while isolating lignin. As a result of boric acid stabilization, the yield of monomers increased from 16% to 25%. In the end, the summary of all the research objectives and overall learnings from the studies is discussed. Additionally, future directions for this research area are discussed.