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-d(4)) and catalytic reactions in the presence of copper-containing porous metal oxide (CuPMO) and MeOH-d(4). 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 degrees C. In the presence of CuPMO, the volume fraction of small lignin particles (<50 & Aring;) 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 & Aring;) and condensation of small lignin particles (<50 & Aring;). The study also suggests that the cooling and quenching step in the ex situ experiments alters the small lignin (<50 & Aring;) particle size distribution.