Radar Tool for Lignin Classification on the Perspective of Its Valorization

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 ¹³C 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 O₂. 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.