Topochemical Analysis of Pine Bark toward Structure‐First Biorefineries

Pine bark is an abundant, yet underutilized forestry by-product often burnt for energy, despite its rich chemical functionality. Its broader use is limited by poor processability and high heterogeneity of products obtained through conventional fractionation. Most processes employed for bark have been traditionally designed for wood, a vastly different lignocellulosic precursor. In this work, we moved beyond bulk analysis by employing a topochemical approach to understand the bark recalcitrance and structural integrity. By advanced spectroscopic imaging combined with controlled solvent treatments and macroscale integrity analysis, we provided new insights into how bark architecture governs its recalcitrance in the context of biorefineries. Bark recalcitrance largely stems from the thick outer cell walls, dominated by conjugated C═C double bonds and unsaturated carbonyl structures, that are disrupted by alkaline agents. Removal of lipophilic extractives increases the fibers' cohesion, potentially requiring more energy input for fiber liberation. Furthermore, bark structure is loosened by removing pectins and phenolics in the middle lamella and cell corners. Macroscale texture depends on the nature, not just the amount, of removed components. By understanding the roles of each component in the integrity of the bark ultrastructure, we put forward a generic structure-first concept to develop novel bark biorefineries.