Sustainable valorization of spruce bark via hydrothermal carbonization for the production of furanic compounds

This study aimed to sustainably valorize of Picea abies bark (PB), a low value residue, by extracting three furanic compounds, namely 5-hydroxymethylfurfural (5-HMF), furfural (F) and 5-methylfurfural (5-MF) through hydrothermal carbonization (HTC). Using response surface methodology (RSM) with a full factorial design (FFD) of experiments, the effects of temperature (160-240°C), reaction time (1-24 h) and particle size (50-1000 µm) on furanic compound production were evaluated. Statistical analysis revealed that temperature was the most influential factor, followed by reaction time, while particle size had no significant effect, despite differences in composition: small sizes are richer in cellulose and lignin, and large sizes are richer in hemicelluloses and extractives. Kinetic modeling confirmed that 5-HMF, F, and 5-MF act as intermediates in the transformation of PB-derived sugars to humins. The pH profile during HTC correlated with furanic compound evolution, initially decreasing due to acid formation, then increasing as less acidic byproducts, likely phenolics, formed. Optimal individual yields were obtained at 180°C for 5-HMF and 160°C for both F and 5-MF, with reaction times between 7 and 8 h. Multiple optimizations identified 170°C and 9.35 h as the best conditions for maximizing all three compounds simultaneously. These findings contribute to the sustainable valorization of lignocellulosic biomass and demonstrate the potential of HTC for efficient furanic compound recovery.