Catalytic conversion of enzymatic hydrolysis lignin: Solvolysis and upgrading

Enzymatic hydrolysis lignin (EHL), a major byproduct of the cellulosic ethanol industry, represents a promising feedstock for biomass valorisation due to its high aromatic carbon content. However, its extensive cross-linking and poor solubility significantly limit its efficient utilization. To overcome these challenges, this thesis proposes a systematic three-step approach—solvolysis, catalytic depolymerization, and hydrodeoxygenation (HDO)—to convert EHL into valuable aromatic monomers and cycloalkane fuels. The solvolysis behaviour of EHL was systematically investigated using alcohol–water mixtures, ethylene glycol, and other organic solvents. Among these, an isopropanol–water mixture (3:2 v/v) demonstrated excellent solubility, resulting in complete liquefaction at 250 °C. Separately, ethylene glycol was capable of fully dissolving EHL even at room temperature, highlighting its strong solvating power under mild conditions. Molecular dynamics simulations further revealed that van der Waals interactions disrupted π–π stacking between lignin units, while hydrogen bonding with oxygenated groups facilitated both liquefaction and β-O-4 bond cleavage. Catalytic depolymerization was conducted using Raney Ni in the isopropanol–water system, achieving a monomer yield of 45.6% at 320 °C with full cleavage of β-O-4 linkages. In contrast, the ethylene glycol system at 200 °C, in the presence of Ni and NaOH, yielded 18.8% monomers. Mechanistic analysis indicated that NaOH played a key role in bond cleavage, while Ni catalyzed hydrogen generation from H₂. Ethylene glycol also acted as a hydrogen carrier, facilitating hydrogen transfer to lignin fragments. In the final HDO step, guaiacol was employed as a lignin-derived model compound to study its transformation over nickel phosphide (Ni2P). The catalyst efficiently promoted C–O bond cleavage and aromatic ring hydrogenation under mild conditions, leading to the formation of high-energy cycloalkane fuels. Density functional theory (DFT) calculations revealed that phenolic intermediates strongly adsorbed onto Ni2P surfaces, underwent π-electron rearrangement and loss of aromaticity, thereby facilitating hydrogenation. Hydrogen atoms dissociated on the Ni2P surface and migrated via spillover to active sites, promoting efficient HDO. In conclusion, this thesis establishes a robust and mechanistically supported strategy for the conversion of EHL into high-value fuels and chemicals. The integration of solvent design, catalytic depolymerization, and selective hydrogenation provides important insights and a theoretical foundation for advancing lignin valorisation technologies.