Unveiling the Mechanism of Reductive Catalytic Fractionation via Online High-Resolution Mass Spectrometry: Insights into Lignin Valorization

In the quest for sustainable energy and environmental preservation, reductive catalytic fractionation (RCF) of lignocellulosic biomass has emerged as a powerful approach to valorize lignin. However, the precise mechanism driving the RCF process remains elusive due to analytical challenges. This study unlocks the mechanism by investigating the Pd/C-catalyzed RCF of birch wood using online high-resolution mass spectrometry (HRMS) for the first molecular-level insights. Real-time evolutions of various monomers, dimers, and oligomers bridged the gap between large lignin fragments and small phenolic products, revealing the stepwise nature of RCF. The process starts with lignin extraction into the liquid phase from the middle lamella to the secondary wall of the cell wall, followed by rapid catalytic depolymerization of the extracted lignin fragments to yield phenolic monomers and dimers. Intriguingly, the evolution of sugar-derived compounds highlights the holocellulose degradation with prolonged reaction times, posing challenges to lignin-first strategies. These findings underscore the importance of fine-tuning RCF conditions to enhance conversion efficiency and minimize side reactions. Moreover, this work highlights the application of advanced HRMS techniques for gaining mechanistic and kinetic insights into liquid-phase reactions, paving the way for more efficient biomass valorization technologies.