Radar Tool for Lignin Classification on the Perspective of Its Valorization

Abstract

A tool for lignin classification was developed based on radar plots using six descriptors identified as key characteristics for vanillin (V) and syringaldehyde (Sy) production by oxidation in alkaline medium: content on β-O-4 structures, noncondensed structures, syringyl and guaiacyl units, and yield of Sy and V by nitrobenzene oxidation (NO). A set of lignins was classified according to the radar information, simplifying the evaluation and discussion of the impact of the delignification process, wood species, and morphologic part on lignin. Lignin from tobacco stalks was one of the targets, reporting 13 C NMR and NO characterization data to ascertain the influence of delignification process. Structural data on lignins from different hardwoods (eucalyptus, mimosa, and willow), several parts of the same species (bole, bark, branches, sawdust), and different delignification processes were also used as a basis for the developed methodology. The radar plots of tobacco lignins allow classifying the lignin produced by organosolv process with ethanol as that with the higher aptitude for V and Sy production with O 2 . This classification was confirmed by batch oxidation of this lignin as compared with that produced by organosolv process with butanol. In the same way, among the processed hardwood lignins, the one produced by organosolv of eucalyptus bole wood showed the highest intensity in all descriptors, being classified as a privileged source of Sy in comparison to Kraft lignins. The reasons behind the differences on descriptors that gave rise to lignins classification are discussed. The radar classification can be used as a predictive tool for product and process design, for both lignin production and application. The requisite for this is the previous knowledge of the relevant structural parameters. This is a key step to demystify the lignin complexity in key descriptors and consolidation of valorization routes in flexible processing units.

Lignin-First Integrated Steam Explosion Process for Green Wood Adhesive Application

“…6 Unlike kraft and organosolv lignins, which have been extensively studied for adhesive application, exploded wood lignin has received very little attention. 7,8,9 .…”

Added-Value Chemicals from Lignin Oxidation

“…The lack of established processes that add value to lignin can be attributed mainly to its chemical recalcitrance and complex and heterogeneous composition and structure. Adding to this complexity, the lignin structure is highly dependent on the type of plant and species, the delignification process, and the depolymerization method applied [ 2 , 26 ]. Lignin is a complex three-dimensional amorphous and highly branched aromatic polymer constituted of methoxylated phenylpropane units.…”

“…The LSRE lignin research group established a classification tool for lignins based on the major structural characteristics of lignin that allows evaluating lignin relative to its viability as a source of added-value low molecular phenolic compounds, such as vanillin and/or syringaldehyde [ 26 , 29 , 30 ]. Radar plots represent an effective classification technique for lignins and are a useful approach for assessing their characteristics to maximize lignin valorization [ 26 ].…”

“…The LSRE lignin research group established a classification tool for lignins based on the major structural characteristics of lignin that allows evaluating lignin relative to its viability as a source of added-value low molecular phenolic compounds, such as vanillin and/or syringaldehyde [ 26 , 29 , 30 ]. Radar plots represent an effective classification technique for lignins and are a useful approach for assessing their characteristics to maximize lignin valorization [ 26 ]. This classification tool combines the assessment of crucial structural characteristics such as H:G:S ratio, condensation, and β- O -4 content and allows a qualitative prediction of the yield expected by oxidative depolymerization of different lignins under similar reaction conditions.…”

“…This classification tool combines the assessment of crucial structural characteristics such as H:G:S ratio, condensation, and β- O -4 content and allows a qualitative prediction of the yield expected by oxidative depolymerization of different lignins under similar reaction conditions. These characteristics are the descriptors used to build the radar plot for each studied lignin, reducing the unavoidable complexity of lignin structure to its key aspects while maintaining the scientific basis of the data sets with quantitative information [ 26 ]. In the works developed in LSRE, the radar plot was built to describe and compare the potential of different lignins to produce vanillin and/or syringaldehyde by oxidative depolymerization with oxygen in an alkaline medium [ 26 , 29 , 30 ].…”