Recent Advancements and Challenges in Lignin Valorization: Green Routes towards Sustainable Bioproducts

Abstract

The aromatic hetero-polymer lignin is industrially processed in the paper/pulp and lignocellulose biorefinery, acting as a major energy source. It has been proven to be a natural resource for useful bioproducts; however, its depolymerization and conversion into high-value-added chemicals is the major challenge due to the complicated structure and heterogeneity. Conversely, the various pre-treatments techniques and valorization strategies offers a potential solution for developing a biomass-based biorefinery. Thus, the current review focus on the new isolation techniques for lignin, various pre-treatment approaches and biocatalytic methods for the synthesis of sustainable value-added products. Meanwhile, the challenges and prospective for the green synthesis of various biomolecules via utilizing the complicated hetero-polymer lignin are also discussed.

Keywords: biocatalysis; green synthesis; lignin; valorization.

Figure 1

(A) Composition of the plant cell wall. Arrangement of hydrophobic lignin and hydrophilic cellulose along with other cell wall structure components. (B) Value-added intermediate products synthesis from lignin. In step 1, the pretreatment of the lignocellulosic biomass occurs, separating cellulose, hemicellulose and lignin. The enzyme depolymerization of the lignin takes place in step 2, generating certain lignin-derived aromatics. The aromatics are hydrolyzed in step 3 through the microbial action of lignin-degrading white rot or bacteria, leading to the formation of key intermediates, including protocatechuate and catechol. The protocatechuate and catechol on further conversion and entering the β-ketoadipate pathway lead to value-added products like triglycerides.

Figure 1

(A) Composition of the plant cell wall. Arrangement of hydrophobic lignin and hydrophilic cellulose along with other cell wall structure components. (B) Value-added intermediate products synthesis from lignin. In step 1, the pretreatment of the lignocellulosic biomass occurs, separating cellulose, hemicellulose and lignin. The enzyme depolymerization of the lignin takes place in step 2, generating certain lignin-derived aromatics. The aromatics are hydrolyzed in step 3 through the microbial action of lignin-degrading white rot or bacteria, leading to the formation of key intermediates, including protocatechuate and catechol. The protocatechuate and catechol on further conversion and entering the β-ketoadipate pathway lead to value-added products like triglycerides.

Figure 1

(A) Composition of the plant cell wall. Arrangement of hydrophobic lignin and hydrophilic cellulose along with other cell wall structure components. (B) Value-added intermediate products synthesis from lignin. In step 1, the pretreatment of the lignocellulosic biomass occurs, separating cellulose, hemicellulose and lignin. The enzyme depolymerization of the lignin takes place in step 2, generating certain lignin-derived aromatics. The aromatics are hydrolyzed in step 3 through the microbial action of lignin-degrading white rot or bacteria, leading to the formation of key intermediates, including protocatechuate and catechol. The protocatechuate and catechol on further conversion and entering the β-ketoadipate pathway lead to value-added products like triglycerides.

Figure 2

Typical linkages present in lignin. (A) The hetero-polymer lignin is naturally featured with a branched and cross-linked network of phenylpropane units. These lignin-forming units are conjugated by different linkages, such as β-O-4, α-O-4, β-1, β-5, 5-5, β-β, and 4-O-5. From the lignin structure, it is observed that the dominant inter-unit linkages present in the native lignin structure are the β-aryl ether (β-O-4). (B) Structure of lignin. The combination of different lignin monomers, mainly sinapyl alcohol, coniferyl alcohol and p-coumaryl alcohol, forms a stable lignin structure through energy-rich bonds.

Figure 3

Value-added chemicals formed from lignin through various treatments.

Figure 4

(A) Lignin-derived monomeric bio-oils and phenolic compounds. (B) Lignin-derived oligomers.

Figure 5

Important chemicals as byproducts from lignin degradation.

Figure 6

Aromatics catabolism from the coniferyl, sinapyl and p-coumaryl branch. (A) Conversion of diverse compounds like the coniferyl-alcohol, 4-hydroxybenzoate and caffeate to aromatics protocatechuate and catechol occurs, which in turn are processed through the different enzyme systems involved in the β oxidation pathway. The acetyl CoA and succinyl CoA formed then goes through the TCA cycle, leading to the formation of triglycerols lipids, PHA, and other fine chemicals. (B) The sinapyl alcohol forms pyruvate and oxaloacetate through the Galate fission pathway, which enters to TCA cycle, leading to the formation of fine chemicals. In contrast, the p-coumaryl alcohol forms succinate + acetyl CoA through the β-ketoadipate pathway and pyruvate + acetyl CoA through the protocatechuate fission pathway, which in turn enter the TCA cycle and help in the formation of lipids, PHA or other bulk chemicals.