On the Quantification of Lignin Hydroxyl Groups With 31P and 13C NMR Spectroscopy

Abstract

Factors affecting the accuracy of the analysis of lignin hydroxyl and carboxyl groups with 31P NMR have been further elucidated. Two modifications of 31P NMR analysis of lignin, namely the protocols using 1,3,2-dioxaphospholane (PR-I) and 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (PR-II) as phosphorylation reagents with different internal standards, were studied. The previous 31P NMR standard protocol with PR-II underestimated OH groups by about 30%, whereas the 31P NMR standard protocol with PR-I tended to produce overestimated data. It has been shown that cholesterol is not an appropriate internal standard, resulting in underestimated values for OH groups due to incomplete baseline resolution. The best internal standard has been found to be endo-N-hydroxy-5-norbornene-2,3-dicarboximide. Strong care should be taken related to the stability of the internal standards to avoid inflated results due to IS degradation. Under modified optimized conditions, both methods show a good correlation with the 13C NMR protocol in the quantification of hydroxyl groups as average, with the variability between the methods in the range of 5–15%. However, the 31P NMR protocols report COOH content that is twice as low as that of 13C NMR data. Finally, the best approach for the use of the 31P and 13C NMR methods in lignin analysis is discussed.