Lignin-humin hybrid structures: a common structural feature in organosolv lignins or just an indicator of process severity?

The ill-famous structurally diversity encountered in lignins, stemming from the varying, plant-specific ratio between the three monomers and the variety of inter-unit linkages, displays different susceptibilities towards different chemical treatments. This generates additional complexity in the structural and hence chemical and physical properties of the polyphenolic end-isolate, including solubility, thermal stability, reactivity profile, etc. Mechanical pretreatments can eventually be applied alongside suitable chemical systems with or without addition of catalysts to facilitate biomass fractionation and lignin isolation. A physical mode often applied to the fractionation of lignocellulosic material is steam explosion (SE), with this method primarily applied for the depolymerisation of hemicellulose and the extraction of sugar species. Among the most promising chemical methods for fractionation of lignocellulosic biomass are organosolv (OS) processes. Most common organic solvents used comprise ethanol, but process variations using small organic acids, i.e., formic acid and acetic acid have been proposed and pushed to larger scales. As of now only few works investigated combinations of two promising modes of pretreatment in form of combined SE-OS processes, with respect to potential synergies and the impact of variations in process parameters such as time and acidity on the characteristics of the resulting lignin isolates,[2,3] and thus the suitability for potential downstream valorisations. The detailed structural analysis of spruce lignins generated via a SE-OS process showed the typical lignin motifs one could expect, alongside sugar residues and hints of lignin-carbohydrate complexes (LCCs). While trends observable in the quantified 1H-13C HSQC analysis for the interunit bonding motifs and the relative abundances of the sugar residues correlated with process severity, a set of signals was detectable for certain process conditions that did not correspond to common structural features in lignins or sugars. Comparison with literature reports on the synthesis and structural characterisation of humin-containing composite materials suggested that the unknown signals represent typical bonding motifs in humins and humin-like structures. More interestingly, structures were identified that suggest covalent bonding between lignin and humins, giving rise to lignin-humin hybrids (LHHs). With the cross-peaks in the 1H-13C HSQC spectra assigned to key structural features of this synthetically derived, bio-based LHH-polymer, we set out to carefully re-examine 1H-13C HSQC spectra obtained in the last years for organosolv lignins isolated from various biomasses and under various process parameters. Preliminary data of this work in progress suggest that LHHs are not limited to occur in softwood organosolv lignins isolated via SE-OS processes, but that process severity generally favors their formation.