Quantification of Carbonyl Groups in Lignin: 31P NMR after Reduction

The available analytical techniques for the determination of carbonyl groups in lignins suffer from several drawbacks including tedious protocols and the need for highly powerful NMR spectrometers for acquiring processable-quality spectra in short times. In the present effort, these limitations are overcome by introducing a protocol based on the quantitative reduction of carbonyl groups, followed by the quantification of the resulting increase in hydroxyl groups by 31P NMR spectroscopy. The methodology, yielding results that align with the oximation technique and quantitative 13C NMR data, has been optimized on technical (hardwood and softwood kraft lignins and wheat straw organosolv lignin) and analytical-grade lignins (acidolysis lignins and enzymatically mild acidolysis lignins). This approach, when coupled with HSQC data, also allows for the identification of the nature of different carbonyl groups in the analyzed lignins. All in one, quantitative 31P NMR after sodium tetrahydroborate reduction constitutes a reliable and straightforward analytical protocol for the identification and quantification of carbonyl groups in lignin.