Hydrothermal Depolymerization of Lignin: Understanding the Structural Evolution

Abstract

The structural evolution of Panicum virgatum lignin during hydrothermal depolymerization was investigated. Product distribution from various temperatures was studied using gas chromatography-mass spectrometry (GC-MS) and gel permeation chromatography (GPC) analysis. The physical-chemical properties of initial lignin, tetrahydrofuran soluble fraction, and char were also comparatively characterized by elemental analysis, scanning electron microscopy (SEM), thermogravimetry (TG), and Fourier transform infrared (FT-IR) spectroscopy. Results showed that both the depolymerization and repolymerization were significantly temperature-dependent. The undesired char formation becomes obvious when the temperature is greater than 180 °C. Further investigation demonstrated that H lignin is the most accessible for hydrothermal depolymerization, whereas S lignin is the most recalcitrant. Moreover, under the thermal effect and the dissolution of the subcritical water, the basic structure of lignin was first collapsed and then further decomposed into low-molecular weight products by the fracture of ether bonds, accompanied by char formation after repolymerization and dehydration.

Intensification of pre-treatment and fractionation of agricultural residues

“…The lignin fractionated from raw spelt husk showed small micron particles agglomerated to form clusters while the lignin following pre-treatment showed a smooth and increased surface are due to collapse of the internal structure as well as breakage of chemical bonds such as β-O-4 and 4-O-5 (Fig S1). Similar changes were observed on switch grass (Panicum Virgatum) [51] with a smooth surface for lignin treated with hot compressed water at 180°C compared to loose agglomerated particles of lignin in the raw feedstock. Stewart et.…”

Protic Ionic Liquids for Lignin Extraction—A Lignin Characterization Study

“…Depending on the biomass source utilized along with the lignin extraction method (pretreatment), various lignins can be liberated that may have different properties [ 7 , 8 ]. Typical pretreatment methods, such as physical (milling and grinding), physicochemical (steam pretreatment/auto hydrolysis, hydrothermolysis, and wet oxidation), chemical (alkali, dilute acid, oxidizing agents, organic solvents, and ionic liquids), biological, electrical, or a combination of these, have been studied for the extraction of lignin from biomass [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ]. Therefore, there is a need for the selective extraction of high quality lignin (purity and functionality) with physiochemical properties of interest for certain applications.…”

Antioxidant Additives Produced from Argan Shell Lignin Depolymerization

“…As seen in Figure a, the depolymerization temperature showed a significant positive effect on the production of insoluble solid in the case of operating under an inert atmosphere or with H 2 as a reaction medium. This increase in the insoluble solid yield was interpreted according to many studies by the condensation or repolymerization reactions that could occur at temperatures higher than 300 °C. ,, The repolymerization/condensation reaction has been reported by several studies and explicated as the reaction between unstable lignin fragments and original lignin. , According to Li et al, the key point of these reactions is the carbenium ion generated at the α-carbon atom. After the ether bond cleavage, this ion can be attacked by an adjacent aromatic ring forming a stable carbon–carbon bond that increases the molecular weight , and could change the molecular structure.…”