2‑Naphthol Impregnation Prior to Steam Explosion Promotes LPMO-Assisted Enzymatic Saccharification of Spruce and Yields High-Purity Lignin

The recent discovery
that impregnation with a carbocation scavenger
may improve the enzymatic saccharification of steam-exploded softwood
has brought a softwood-based biorefinery closer to reality. However,
the nature of the impregnation effect remains unresolved, and its
impact on process efficiency and product quality in high-dry matter
reactions remains underexplored. Here, we show that 2-naphthol impregnation
enables the complete saccharification of spruce cellulose by lytic
polysaccharide monooxygenase (LPMO)-containing Cellic CTec2, but not
by an LPMO-poor cellulase cocktail (Celluclast), in 10% dry matter
reactions with an industrially feasible enzyme dose and reaction time.
Importantly, we show that this remarkably high saccharification yield
correlates with increased LPMO activity, which is due to the impact
of 2-naphthol on the ability of lignin to drive the LPMO reaction.
These findings show that impregnation improves saccharification not
only by reducing cellulase adsorption and inactivation but also by
boosting oxidative cellulose depolymerization by LPMOs. Pyrolysis
of the lignin-rich saccharification residues revealed that 2-naphthol
impregnation had little effect on lignin-derived components in the
resulting bio-oil, which, due to the efficient saccharification, showed
reduced levels of carbohydrate-derived components that reduce oil
storage stability. These results bring closer the prospect of a spruce-based
biorefinery that combines biochemical and thermochemical conversion
routes.