Steam explosion lignin (SEL) was extracted with ethanol from steam exploded hardwood (okoumé, Aucoumea klaineana Pierre) pretreated at various severities after neutral or acidic impregnation. The SELs were subjected to a structural characterization by 2D HSQC NMR, 31 P NMR, SEC and compared to Milled Wood Lignin (MWL). A strong decrease in the -O-4 content is observed with increasing steam explosion severity accompanied with a gradual increase in molecular mass. C-oxydized S units (S', Hibbert's ketones) were quantified by NMR and used as a marker of the hydrolytic mechanism as well as naphthol used as a carbonium ion scavenger. It has been observed that mixed reactions of hydrolysis and homolysis are involved but the SEL is mainly cleaved homolytically, favouring recondensation through radical coupling even at low reaction severity. However, acidic pre-impregnation of wood prior to steam explosion enhanced the carbonium ion pathway.

Effects of alkaline (kraft and soda) and organosolv pulping were evaluated relative to the structural properties of lignin isolated from coconut husk (CH) biomass. The various types of functional groups within the isolated lignin samples were characterized and compared using a variety of complementary analyses including Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, gel permeation chromatography (GPC), and high-performance liquid chromatography (HPLC). All isolated CH lignin samples contained a significant quantity of non-condensed guaiacyl-type (G) and syringyl-type (S) units but a lesser amount of p-hydroxyphenyl (H) units, as demonstrated and supported by previous research. The alkaline CH lignin produced lignin with higher molecular weight (Mw SL: 959 g mol-1 > Mw KL: 769 g mol-1) than the organosolv lignin (Mw OL: 606 g mol-1) sample, resulting in smaller fragments and a higher degree of solubility in water or other solvents. Because of significant differences in the physicochemical characteristics of the various lignin polymers, their properties and structure were improved with respect to alternative approaches in lignin-based applications.

Steam Explosion (SE) is one of the most efficient and environmentally friendly processes for the pretreatment of lignocellulosic biomass. It is an important tool for the development of the biorefinery concept to mitigate the recalcitrance of biomass. However, the two distinct steps of SE, steam cracking and explosive decompression, leading to the breakdown of the lignocellulosic matrix have generally been studied in empiric ways and clarification are needed. This mini-review provides new insights and recommendations regarding the properties of subcritical water, process modeling and the importance of the depressurization rate.

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