Operando Monitoring of Delignification Processes Using 2D ¹H-¹³C HSQC NMR

The design of effective lignin extraction and depolymerization processes has long been hindered by a lack of understanding of the biomass matrix and lignin architecture, especially during its transformation. Our current understanding of lignin valorization processes largely relies on the use of ex situ analysis techniques focusing on end products, rather than operando monitoring of reaction mechanisms and temporal structural changes. Here, we used a custom heteronuclear single quantum correlation (HSQC) NMR procedure to monitor molecular-specific temporal changes in lignin functionalities during extraction and depolymerization. Preferred condensation on guaiacol rings was observed under alkaline conditions. We also highlighted faster cleavage of phenolic β─O─4 linkages in acidic conditions and revealed the role of the acid counterion in this mechanism. Similarly, we were able to explore the important roles of methanol both in solvolysis and in functionalizing lignin during the Reductive Catalytic Fractionation (RCF) of biomass. Overall, operando NMR HSQC spectra provide time-resolved information on biomass extraction processes in the liquid phase and allowed us to identify important intermediates that are difficult to detect with ex situ characterization.