Abstract
The demand for efficient utilization of biomass induces a detailed analysis of the fundamental chemical structures of biomass, especially the complex structures of lignin polymers, which have long been recognized for their negative impact on biorefinery. Traditionally, it has been attempted to reveal the complicated and heterogeneous structure of lignin by a series of chemical analyses, such as thioacidolysis (TA), nitrobenzene oxidation (NBO), and derivatization followed by reductive cleavage (DFRC). Recent advances in nuclear magnetic resonance (NMR) technology undoubtedly have made solution-state NMR become the most widely used technique in structural characterization of lignin due to its versatility in illustrating structural features and structural transformations of lignin polymers. As one of the most promising diagnostic tools, NMR provides unambiguous evidence for specific structures as well as quantitative structural information. The recent advances in two-dimensional solution-state NMR techniques for structural analysis of lignin in isolated and whole cell wall states (in situ), as well as their applications are reviewed.
Highlights
Plant lignin currently attracts widespread attention as a feedstock due to its renewability and large abundance
One advantage of 2D-HSQC nuclear magnetic resonance (NMR) is that overlapping protons directly attached to carbons with different shifts are separated in the carbon dimension, whereas overlapping carbons may be separated by their direct attachment to protons in the proton dimension
NMR characterization of lignin undoubtedly expands the knowledge of lignin chemistry, which will in return promote the utilization of plant cell walls and high-valued applications of lignin
Summary
Plant lignin currently attracts widespread attention as a feedstock due to its renewability and large abundance. Significant advances were made in the area of destructive methods, the current understanding of the composition and structure of the macromolecule lignin is derived from interpretations and extrapolations of the data from degradative processes that only account for a fraction of the total polymer [10]. Quantitative 13C-NMR is a powerful tool in the structural analysis of lignin, especially in understanding the structural changes of lignin polymer in pulping processes and other isolation processes. Analytical techniques that permit the precise determination of the abundance and chemical attributes of the lignin (e.g., isolated form and whole cell wall form) should be developed to better understand the natural structural features of lignin and structural changes during diverse pretreatments, and further to ascertain their industrial utility. C-NMR and 2D-HSQC NMR techniques) are reviewed in light of their unique usefulness and popularity for characterizing lignins in an isolated and in situ form from lignocellulosic materials in recent years
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