Recent Advances in the Valorization of Lignin: A Key Focus on Pretreatment, Characterization, and Catalytic Depolymerization Strategies for Future Biorefineries

Abstract

In this regard, lignocellulosic biomass (LCB) is an attractive alternative due to its abundant availability, carbon neutral nature, and its ability to produce a wide range of fuels and chemicals. [11,12] LCB mainly consists of cellulose (30% to 50%), hemicellulose (20% to 35%), and lignin (15% to 30%), interlinked with each other via a complex bonding network. [13,14] Currently, most of the biorefineries are focusing on the utilization of carbohydrates (cellulose and hemicellulose) for producing bio-ethanol and other chemicals. Moreover, lignin is the major by-product of pulp and paper industry with an annual production of 50 to 70 million tons. [15] However, lignin is mostly burned as a low-value fuel in the industries. [16] Interestingly, it is the only renewable source of aromatic monomers, thus making it a fascinating candidate to produce wide range of aromatics.Lignin is a highly complex 3D heteropolymer, consisting of methoxylated phenylpropanoid units (S unit, G unit, H unit), for providing structural rigidity to the plants. It consists of several COC (α-O-4, β-O-4, 4-O-5) and CC (β-1, β-β, β-5, 5-5) bonds and is interconnected to the cellulose/hemicellulose, thus making its biological or thermochemical conversion challenging (Figure 1a). [11,17] Therefore, the fractionation of LCB via different pretreatment techniques is crucial (Figure 1b). Majority of the pretreatment methods cleave COC (β-O-4) linkages of the native lignin due to their lower (218 to 314 kJ mol −1 ) bond dissociation energy (BDE) while stable CC bonds with higher (384 kJ mol −1 ) BDE are formed due to the recondensation reactions, thus modifying the lignin structure, which is named as technical lignin. [18][19][20] Lower BDE of COC bonds suggests that these bonds are relatively easier to cleave compared to CC bonds. However, depolymerization of native lignin is reported to give monomer yields of approximately seven times that of modified technical lignin. [18] Therefore, the development of new innovative pretreatment strategies that can preserve the β-O-4 linkages of the native lignin is highly desirable. Moreover, it is essential to get an in-depth knowledge of the structural modifications, occurring after the pretreatment, using the different characterization techniques like 1D/2D/3D nuclear magnetic resonance (NMR), 31 P NMR, solid-state NMR,

Controlled synthesis of Ru-single-atoms on ordered mesoporous phosphine polymers for microwave-assisted conversion of biomass-derived sugars to artificial sweeteners

“…Catalytic conversion of lignocellulosic biomass into platform chemicals and fuels has been intensified as a significant research topic over the past decade. 1,2 Upgradation of biomass as a renewable feedstock through a green and cost-effective route by minimizing waste generation is a challenging problem. It requires sustainable development of energyefficient materials, catalysts, and processes, following microwave (MW) and ultrasound technologies (UT), avoiding the use of volatile organic solvents.…”

Pharmaceutical applications of lignin-derived chemicals and lignin-based materials: linking lignin source and processing with clinical indication

“…Protonic ionic liquids that are produced via an acid-base neutralization process are relatively cost competitive and demonstrate high lignin-extraction efficiency [69]. The solubility of different types of lignin in various ionic liquids has recently been reviewed, and it was shown that ionic liquids possessing aromatic cations have high lignin solubility, while those with nonaromatic cations have lower solubility [70]. However, the lignin dissolution mechanism is not fully understood, and further evaluation is needed to understand the properties and potential of the extracted lignin [71].…”

“…The lignin extraction yield of various deep eutectic solvents for various lignocellulosic feedstocks and pretreatment conditions have also been reviewed. Compared to other deep eutectic solvents ChCl/oxalic acid and ChCl/ lactic acid-based solvents had the best delignification yields [70]. In a recent study, these solvents were evaluated for lignin separation from rice straw.…”

H8[PV5Mo7O40] – A Unique Polyoxometalate for Acid and RedOx Catalysis: Synthesis, Characterization, and Modern Applications in Green Chemical Processes

“…As destruction mechanism, a β‐O‐4 cleavage in the phenolic structures was assumed with hydrolytic cleavage of alkyl−phenyl bonds in phenolic structural units which are oxidized by HPA‐5 with subsequent acidolytic splitting of the β‐O‐4 linkages. Another discussed mechanism is the cleavage of alkyl‐phenyl bonds via one‐electron oxidation of the aromatic ring with subsequent hydrolysis [93,96] …”

“…Another discussed mechanism is the cleavage of alkyl-phenyl bonds via one-electron oxidation of the aromatic ring with subsequent hydrolysis. [93,96] In a second study by Evtuguin et al [97] the authors used vanillyl and veratryl alcohol as monomeric model substrates for lignin. The results indicated that the first step of lignin oxidation is a one-electron oxidation of the substrate by HPA-5.…”