31P NMR in wood chemistry: A review of recent progress

Abstract

Over the past few years substantial efforts in our laboratory have been devoted toward the development of novel NMR techniques for the analysis of soluble and solid lignocellulosic substances. These efforts were undertaken in order to expand the frontiers of application of NMR for the detection of functional groups present in such materials. Our methodology involves the selective phosphorus-tagging of a variety of functional groups present in lignin and carbohydrates, followed by solution and solid state31P NMR spectroscopies. This paper attempts to review the status of this technique by discussing its development for the analysis of soluble and solid lignocellulosic samples. Solution31P NMR can be used to examine soluble lignin and carbohydrate samples after phosphitylation with 1,3,2 dioxaphospholanyl chloride. This is a novel and powerful means to determine the three principal forms of phenolic hydroxyls present in ligninsi.e. p-hydroxyphenyl, guaiacyl, and syringyl structures. In addition, primary hydroxyls, carboxylic acids, and the two diastereomeric forms of arylglycerol-beta-aryl ether units (β-O-4 structures) present in lignins can also be determined from a single31P NMR experiment. When applied to carbohydrates, the technique gave characteristic signals for thealpha andbeta anomers and the epimeric forms of monosaccharides. Completely resolved31P NMR spectra were also obtained when lignin-carbohydrate model compounds were examined. Solid state31P NMR can be used to determine quinone chromophore groups present in solid lignocellulosic samples. The method is based on literature accounts that describe adduct formation between trimethyl phosphite and quinones followed by solid state31P NMR. This reaction when reexamined in our laboratory showed that the presence of carboxylic acids in high yield pulps significantly affected the solid state31P NMR signal intensity. This realization permitted the development of an experimental protocol that allowed solid state31P NMR signals to be received from high yield pulps that contain information only onortho-quinones and coniferaldehyde chromophores. It was thus found that about 0.7ortho-quinone groups are present in every 100 C9 units within the lignin of a black spruce refiner mechanical pulp sample, in agreement with previously applied independent techniques.