Coupling P-31 NMR with the Mannich reaction for the quantitative analysis of lignin

Abstract

The reactivity of lignin model compounds under Mannich conditions with piperidine and formaldehyde was studied. The piperidinomethyl group was confirmed to be introduced quantitatively at sterically unhindered positions ortho to a phenolic hydroxyl when the substrate was allowed to react under ambient conditions. The sequential application of the Mannich reaction followed by quantitative 31 P NMR on a variety of lignin samples allowed the amounts of various aromatic groups bearing free phenolic hydroxyls to be determined. More specifically, the technique allowed the quantification of para-hydroxyl phenols, catechols, guaiacols, and phenols bearing C5 or C6 substituents. The quantitative reliability of the technique was also examined with a series of dissolved kraft lignins, isolated at various degrees of delignification. The data were found to favourably compare to those obtained by permanganate oxidation.