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.