Quantification of Phenolic Hydroxyl Groups in Lignin via ¹⁹F NMR Spectroscopy

Phenolic moieties strongly influence lignin reactivity and physical properties, and thus accurate quantification of phenolic groups in lignin is a critical analytical chemistry need. Today, 31P nuclear magnetic resonance (NMR) spectroscopy is widely considered the standard method to this end, but this approach uses a hazardous and expensive derivatization agent, and the NMR spectroscopy experiments are time consuming due to long relaxation times. Here, we report a complementary method that enables accurate identification and quantification of phenolic groups in lignin samples using pentafluoropyridine (PFP) as a derivatizing reagent followed by 19F NMR spectroscopy. Using dimethyl sulfoxide as a solvent in the presence of K2CO3, phenolic hydroxyl groups in lignin model compounds were fully converted to the corresponding tetrafluoropyridyl-ether products within 1 min. PFP exhibits high selectivity for the reaction with phenolic hydroxyl groups relative to aliphatic alcohols, and we show that side reactions with carboxylic acids, if present, can be avoided through the addition of 40% water to the reaction solvent. The PFP 19F method achieved similar results compared to 31P NMR spectroscopy when applied to reductive catalytic fractionation oil from poplar, softwood kraft lignin, and corn stover milled wood lignin, thereby offering a safe and cost-effective method for phenolic measurements in lignin.