Catalytic depolymerization of lignocellulosic biomass offers a promising path to renewable chemical production, but mechanistic understanding is limited by the structural complexity of substrates and the harsh, multiphase conditions under which reactions occur. These reactions often take place at elevated temperatures (>200 °C) and pressures (>50 bar), in systems that involve solid–liquid–gas phase coexistence, and differ fundamentally from conventional catalytic reactions due to the macromolecular nature of the substrate. We use in-situ high-temperature Magic Angle Spinning (MAS) NMR to study depolymerization pathways in a range of 13C-labeled systems, including lignin model polymers, isolated lignin, and cellulose, under either reductive and acidic catalytic conditions. This approach allows us to monitor, in real-time, the formation and evolution of intermediates and products during depolymerization, providing molecular-level insight into the reaction network. The results demonstrate the competence of in-situ MAS NMR for elucidating reaction mechanisms in complex, heterogeneous catalytic systems and offer valuable guidance for the rational design of depolymerization strategies for macromolecular substrates. Each system also supports development of kinetic models that would be otherwise inaccessible, revealing how catalyst choice and substrate structure govern reaction pathways and product selectivity.
