Structural Evolution of Lignin Using In Situ Small-Angle Neutron Scattering during Catalytic Disassembly

A fundamental understanding of the solution behavior of lignin under reaction conditions is required for the rational design of porous heterogeneous catalysts that minimize the mass-transfer limitations on lignin depolymerization kinetics. In situ and ex situ small-angle neutron scattering (SANS) methods were used to study the structural changes of lignin during non-catalytic solvolysis reactions in deuterated methanol (MeOH-d₄) and catalytic reactions in the presence of copper-containing porous metal oxide (CuPMO) and MeOH-d₄. The results indicate that at room temperature, lignin adopted a rigid and stretched conformation that becomes more spherical, flexible, and folded when heated to the reaction temperature of 250 °C. In the presence of CuPMO, the volume fraction of small lignin particles (<50 Å) in the reactor is higher than under solvolysis conditions, while the median radius of this fraction of lignin particles is smaller. The SANS data were analyzed using a population balance model and found that two reaction processes dominate: disassembly of large lignin aggregate particles (>50 Å) and condensation of small lignin particles (<50 Å). The study also suggests that the cooling and quenching step in the ex situ experiments alters the small lignin (<50 Å) particle size distribution.