Fractionation of Wood Biomass With Thiolactic Acid and Choline Chloride‐Based Solvent Into White Lignin for Sustainable Cooling Applications

Lignin, a naturally abundant biopolymer, possesses intrinsic ultraviolet (UV) shielding capabilities, making it a promising candidate for sustainable functional materials. However, conventional lignin isolation methods often lead to dark‐colored products due to structural condensation and chromophore formation, limiting its applicability in optical and esthetic applications. In this study, we introduce a novel fractionation strategy utilizing a system composed of thiolactic acid and choline chloride to selectively extract lignin from wood biomass. This environmentally benign process yields a light‐colored lignin with exceptional whiteness (>90%) and high recovery efficiency (70%). The preservation of lignin's bright appearance is attributed to its submicron‐scale morphology and chemical stabilization via thiolactic acid modification, which suppresses chromophore formation. Remarkably, the resulting white lignin demonstrates high visible light reflectance and significantly reduced solar heat gain compared to conventional kraft lignin. Furthermore, its strong UV absorption and high emissivity in the atmospheric transparency window position it as a compelling bio‐derived material for passive radiative cooling applications. This work highlights a sustainable pathway for valorizing lignin into high‐performance, multifunctional materials aligned with green chemistry principles.