Tuning lignin properties by mild ionic-liquid-mediated selective alcohol incorporation

Abstract

The synthesis of ideal, well-defined, functionalized lignins is of great significance for realizing the full valorization potential of lignocellulosic biomass. Here, the application of acidified triethylammonium triflate is reported, allowing for the selective benzylic alkoxylation of different lignins at room temperature. Full benzylic alkoxylation of typically hard-to-process, high-molecular-weight lignin was achieved in this medium by simple mixing with different alcohols. Limited lignin degradation was observed, and lignin modification was also achieved within the lignocellulosic matrix. This allows the method to be used as a biomass pretreatment, which was shown to also enhance enzymatic saccharification. The reactivity, stability, and hydrophobicity of the lignin can be tuned based on the alcohol used. Effective recovery of the ionic liquid and recycling of the ionic liquid was achieved. Thus, a straightforward mild method is demonstrated to introduce new functionality in lignin, allowing it to be tuned towards novel applications. • Benzylic alkoxylation of different lignins was achieved at room temperature • An effective ionic liquid consisted of triethylamine with excess triflate acid • Key factors that influence high molecular lignin modification are proposed • Lignin with tunable properties was achieved in a recyclable reaction medium Inedible agricultural residues are a promising source of renewable carbon. Biopolymers retained in such inedible biomass can serve as sustainable replacements to fossil-based polymeric products. Lignin is a major component of lignocellulosic (inedible) biomass that is currently underutilized due to its complex structure and high reactivity. During separation from other biomass components, lignin is typically heavily degraded and condensed, making the product difficult to process and use for higher value applications. The native highly functionalized structure of lignin makes it a potent biopolymer substrate for complex material applications. Thus, acquisition of lignin with relatively defined structure and properties provides the prospects of high-value use. One key aspect for processability of lignin is that it can be blended with other polymers, and thus tuning its hydrophobic and hydrophilic properties while leaving the main polymeric structure intact is potentially of great importance. A recyclable acidified ionic liquid mixed with different alcohols can achieve benzylic alkoxylation of lignin obtained from different isolation methods and biomass sources at room temperature. Lignins modified from this process are endowed with tunable properties. Solvation state of lignin in this reaction medium plays an important role to achieve a high degree of alkoxylation of lignin.