Red Mud-Anchored Mono- and Bimetallic Catalysts for Efficient Lignin Liquefaction to Phenolic Monomers: Mechanistic Insight and Environmental Impact Assessment

This study proposed a valorization route for the conversion of recalcitrant lignin to phenolic monomers via catalytic liquefaction. To address the challenge of cost-effective catalysts, red mud (RM, bauxite residue) was employed as a support for Ni (10 wt % Ni/RM), Cu (10 wt % Cu/RM), and bimetallic Ni–Cu catalysts (5 wt % Ni–5 wt % Cu/RM). The NiCu/RM catalyst exhibited superior activity, attributed to enhanced oxygen vacancies and lower basicity as confirmed by XPS and CO2-TPD analysis. Ethanol and methanol demonstrated stronger hydrogen-donating capacity than water, producing 58.11 ± 1.23 wt % lighter lignin oil (LLO) with 62.22% phenolic monomers, validated by GC-MS. Life-cycle assessment (LCA) identified terrestrial ecotoxicity, TE (21.8 kg 1,4-DCB) as the dominant impact, followed by human noncarcinogenic toxicity, HnCT (7.66 kg 1,4-DCB) and global warming potential, GWP (5.26 kg CO2 eq). Integrating lignin valorization into the bioethanol industry can substantially reduce environmental impacts and foster a biocircular economy.