Nanoscale Mixing of Cellulose and Lignin Alters Their Pyrolysis Interactions and Derived Carbon Reactivity

Literature on cellulose–lignin interactions during pyrolysis reports conflicting outcomes, ranging from essentially additive mass-loss behaviour to pronounced non-additive changes in decomposition kinetics and evolved-gas composition. In this study, we show that these discrepancies largely stem from the degree of mixing between cellulose and lignin. We demonstrate that the spatial scale of mixing—macroscale versus nanoscale—fundamentally alters the observed interactions between cellulose and lignin during slow pyrolysis within the heating rate range of 2.5–40 °C/min. Macroscale mixtures, prepared as physical blends of regenerated cellulose II fibres and Organosolv beech lignin (OSBL) particles, undergo pyrolysis independently, with no significant interactions detected in thermogravimetric measurements. In contrast, nanoscale mixtures—produced through the regeneration of hybrid fibres containing nanoscale co-localized cellulose II and OSBL—exhibit clear evidence of interactions between the two components. These interactions manifest in altered mass loss dynamics and evolved gas profiles, indicating that the pyrolysis pathways of each biopolymer are influenced by their proximity at the nanoscale. Notably, while OSBL alone undergoes softening and extensive foaming during pyrolysis, this behaviour is entirely suppressed in the nanoscale mixture, suggesting that thermal softening and gas-driven expansion are inhibited by confinement within the cellulosic matrix. This suppression points to a reduction in lignin’s mobility when restricted between cellulose domains. Molecular simulations on lignin dynamics under confinement support this interpretation. Additionally, the reactivity of carbonized OSBL toward CO₂ is significantly enhanced when lignin is confined within nanoscale domains in the hybrid cellulose-lignin fibres. This finding suggests that nanoscale confinement not only affects the pyrolytic transformation of lignin but also modifies the structure and accessibility of the resulting carbon composite materials. Collectively, these results underscore the importance of considering the morphological scale when evaluating biopolymer interactions during thermochemical conversion. Additionally, they prompt a re-evaluation of how synergistic effects are understood—particularly in naturally occurring composite systems such as wood, where cellulose and lignin are inherently mixed at the nanoscale, unlike the macroscale physical blends commonly used in experimental studies. • Contradictory literature on cellulose–lignin interactions in pyrolysis is largely explained by differences in mixing scale (macroscale vs nanoscale). • Macroscale physical blends (cellulose II fibres + organosolv beech lignin particles) pyrolyze independently, showing no significant interactions in TGA across 2.5–40 °C/min. • Nanoscale co-localized hybrid fibres show clear cellulose–lignin interactions, evidenced by altered mass-loss kinetics and evolved-gas profiles. • Nanoscale confinement suppresses lignin softening and foaming, indicating reduced lignin mobility within the cellulose matrix. • Molecular simulations support mobility reduction under confinement. • Carbonized lignin exhibits enhanced CO₂ reactivity when derived from nanoscale-confined cellulose–lignin domains, implying modified carbon structure/accessibility.