Process-Decoupled Operando Molecular Characterization of Liquid-Phase Lignin Depolymerization by Online High-Resolution Mass Spectrometry

As an abundant, renewable aromatic resource, lignin remains underutilized due to its complex and recalcitrant structure. Molecular-level elucidation of liquid-phase catalytic depolymerization of technical lignin is further hindered by structural heterogeneity and limited accessibility of intermediates under high-pressure conditions. Herein, a decoupled double-bed flow-through reactor integrated with online high-resolution mass spectrometry (HRMS) is employed to investigate the catalytic depolymerization of alkali Kraft lignin. Spatial separation of thermal solvolysis and catalytic hydrogenolysis enables independent characterization of products from each bed. Online HRMS resolves thousands of molecular species from monomers to oligomers up to heptamers, revealing a pronounced shift toward lower degrees of polymerization and reduced unsaturation after catalytic hydrogenolysis. Ring and double bond equivalents-carbon number, van Krevelen, and tandem MS analyses indicate extensive hydrogenation, partial deoxygenation, and the cleavage of β-O-4 linkages, with C-C interunit linkages, either native or formed via condensation, largely preserved in the residual dimers and oligomers. Continuous online monitoring captures the evolution of individual products, highlighting progressive depolymerization and suppressed recondensation under flow-through conditions. This work demonstrates online HRMS as a powerful operando tool for resolving structure-reactivity relationships in lignin depolymerization and provides molecular-level insights relevant to catalytic biomass valorization.