Structural and Morphological Transformations of Lignin Macromolecules during Bio-Based Deep Eutectic Solvent (DES) Pretreatment

Elucidating the structural characteristics and changes of lignin during biorefining is considerably important for lignin valorization. To examine the structural transformations of lignin under deep eutectic solvent (DES) pretreatments, the double enzymatic lignin (DEL) isolated from Eucalyptus was pretreated with DES (ChCl/lactic acid, 1:10) under the conditions of 60–140 °C for 6 h. The structural transformations of lignin during the DES pretreatment have been investigated by quantitative 13C NMR, two-dimensional-heteronuclear single quantum correlation (2D-HSQC) NMR, 31P NMR, gel permeation chromatography (GPC), and scanning electron microscopy (SEM) techniques and the lignin degradation products recovered after DES pretreatment have been analyzed by gas chromatography–mass spectrometry (GC–MS) technique. NMR results demonstrated that the dissociation of aryl ether linkage (i.e., β–O–4) is dominant during the DES pretreatment, which is in line with the increased content of phenolic hydroxyl in these lignins. In addition, the decreased aliphatic hydroxyl groups suggested that the acylation or dehydration of aliphatic hydroxyl groups occurred as the pretreatment temperature elevated. Based on the results observed, possible pathways for chemical transformations of lignin were proposed. Furthermore, it was found that DES pretreatment also resulted in a homogeneous lignin morphology, facilitating the formation of lignin nanoparticles. In short, unmasking the lignin chemistry during the DES pretreatment will facilitate the optimization of the pretreatment process and production of homogeneous nanosized lignin particles with preferable chemical reactivity from parent lignin in the current biorefinery process.