Optimizing an Integrated Biorefining Process for Birch Veneer Chips and Lignocellulosic Residues: Enhancing Cellulose Preservation and Maximizing Furfural and Acetic Acid Production

The development of sustainable biorefining processes is essential for increasing the value of lignocellulosic resources and reducing the environmental footprint of the forest-based industry. Birch wood is one of Latvia’s most abundant renewable feedstocks, yet current catalytic technologies for furfural production—primarily based on sulfuric acid (H2SO4)—cause extensive cellulose degradation and generate sulfur-containing residues that hinder further valorization. This study proposes an integrated biorefining approach in which orthophosphoric acid (H3PO4) is utilized as an alternative catalyst to selectively convert hemicellulose into furfural and acetic acid while preserving cellulose in birch veneer chips (BVC). The experimental plan was based on a full central composite circumscribed (CCC) response surface methodology (RSM) design, which consists of factorial points, axial (star) points, and centre points. In total, 26 experimental runs were performed, including 24 non-centre points (comprising both factorial and axial points (±α)) and two centre points. The optimized conditions enabled high acetic acid yields (6.29–6.48% o.d.m., corresponding to 98–100% of theoretical), furfural yields of 8.75–10.41% o.d.m. (57–68% of theoretical), and exceptional glucan preservation (38.84–40.92% o.d.m., 94–99% of theoretical). Compared with sulfuric acid pretreatment, the H3PO4-based process significantly reduced cellulose degradation and improved the suitability of the resulting lignocellulosic residue for subsequent 5-hydroxymethylfurfural (5-HMF) production and other biorefining routes. The findings demonstrate that orthophosphoric acid catalysis is a promising pathway for integrating furfural extraction with cellulose-retentive pretreatment, thereby enhancing the sustainability, efficiency, and circularity of birch veneer chips’ biomass utilization.