Affinity-efficiency inversion in lignocellulose fractionation: decoupling chemical erosion from surface...

The efficacy of nitrogenous additives in deconstructing the lignin macromolecule is conventionally attributed to physical interactions. However, this macroscopic adsorption-centric model frequently overlooks the kinetic contributions of chemical lability at the molecular level. Here, we introduce a mechanistic decoupling strategy using melamine as an inert structural probe to benchmark against hydrolytically labile urea and dicyandiamide. We report an affinity-efficiency inversion: Urea, despite having the weakest theoretical binding energy, achieves the highest bulk delignification (~95%), surpassing the apparent physical ceiling (~85%) constrained by non-covalent interactions. This chemical override is driven by in situ generated nucleophiles acting as chemical etchants, amplified by localized structural relaxation to induce extensive depolymerization. Conversely, the inert melamine system operates via a distinct surface masking pathway. Rather than a kinetic limitation, the observed physical limit represents a thermodynamic complexation ceiling, where melamine leverages its superior affinity to form stable supramolecular complexes that passivate residual lignin. Mass balance analysis reveals a functional equivalence: surface passivation compensates for lower bulk purity, enabling the masking pathway to match the enzymatic glucose recovery of the high-severity erosion pathway. Our findings shift design principles for nitrogenous additives from maximizing solubility to tailoring specific erosion or masking functionalities, offering divergent structural engineering routes for highly monodisperse lignin nanospheres or functionalized supramolecular biocomposites.