Synergizing Catalyst and H-Donor Design for Selective H ₂ -Free Reductive Catalytic Fractionation Using Glycerol-Derived Ethers

Ex situ H2-free reductive catalytic fractionation (RCF) deconstructs lignin into functionalized phenolic monomers under mild conditions, utilizing transfer hydrogenolysis and hydrogenation pathways facilitated by nonvolatile, H-donor solvents. This study demonstrates the synergistic influence of catalyst support acidity and solvent molecular structure on H-transfer pathways and monomer selectivity. Pt-nanoparticle catalysts supported on materials with varying acidity (e.g., C, TiO2, Al2O3, ZrO2, faujasite) were evaluated alongside glycerol-derived ethers (GDEs) with diverse molecular structures as H-donors. While all catalysts achieved comparable monomer yields (theoretical: 1.06–1.30 mol mol–1; 18.6–22.8 wt %), support acidity significantly influenced product selectivity. Nonacidic supports such as Pt/C and Pt/TiO2 favored metal hydride transfer, enhancing selectivity for monomers with alkyl or alkenyl side chains, including, 4-ethylguaiacol, 4-propylguaiacol, isoeugenol, and eugenol. In contrast, acidic supports promoted acid-catalyzed solvolysis involving β-O-4 acidolysis, benzylic carbocation formation, and enol-ether intermediates, generating monomers with oxygenated side chain such as coniferaldehyde, vanillin, and vanillic acid. GDEs with longer terminal alkyl chains induced steric hindrance and resulted with lower delignification efficiency. HSQC NMR and MALDI-TOF MS analyses confirmed that depolymerized lignin functionality is modulated by support acidity, while model compound studies revealed acidity-tuned dominant reaction pathways. Collectively, these findings clarify how catalyst support properties and H-donor solvent structure direct lignin monomer selectivity under mild, H2-free RCF conditions, aligning process design with green chemistry principles.