NMR studies of lignin and lignin-derived products: recent advances and perspectives

The present review focuses on the most recent results of a liquid-phase nuclear magnetic resonance (NMR) study of lignin together with the lignin-derived products and related chemicals. Collected and discussed are the recent applications of ¹H, ¹³C and ³¹P NMR spectroscopy as well as the NMR of less popular nuclei to the structural studies and practical recommendations in this field. Due to the complexity of lignin and the products of its transformation, their NMR spectra consist of a number of overlapping signals belonging to different structural types. In this regard, comprehensive studies of lignin by NMR over the past few decades have revealed characteristic functional groups of lignin and lignin-related products together with the spectral regions in which they resonate. Quantitative NMR spectra of ¹H, ¹³C together and less popular nuclei such as ³¹P are used to characterize different structural units of lignin (such as guaiacyl, syringil and p-hydroxyphenyl), which provide aromatic and saturated carbons spread over many structural moieties. The bibliography includes 54 references.