21. Functionalization of Lignin by Phenolation

This chapter critically examined the research landscape surrounding lignin phenolation, a process in which phenol molecules are selectively bound to specific sites within the lignin structure, particularly at the α- and γ-positions. The phenolation reaction was typically performed by blending lignin with phenol or other reagents in the presence of an acid catalyst, with reaction temperatures ranging from 25 °C to 150 °C and durations between 10 and 360 min. Various studies illustrated diverse methodologies and outcomes, indicating that optimization of phenolation conditions was achieved through careful selection of lignin type, catalysts, solvents, temperature, and reaction time. This optimization improved phenol incorporation, reduced lignin’s molecular weight, and enhanced its reactivity and functionality for industrial applications. Notably, phenolation induced lignin depolymerization, which enhanced its physical properties and solubility, thereby improving compatibility with various resin systems. A comprehensive review of various analytical methods for the quantitative and qualitative characterization of phenolated lignin was conducted. The study also investigated the effectiveness of phenolated lignin as a substitute for phenol in phenol-formaldehyde (PF) resins and phenolic foams. This substitution demonstrated significant improvements in adhesive performance within the wood industry and in other thermosetting materials. Furthermore, the phenolation of lignin contributed to sustainability by reducing reliance on fossil resources while simultaneously enhancing the mechanical and thermal properties of the resulting products. The findings of this investigation underscore the potential of lignin phenolation to advance eco-friendly industrial practices and expand the applicability of lignin-based materials in various applications.