Chemistry of lignin and condensed tannins as aromatic biopolymers

Aromatic biopolymers are the second largest group of biopolymers after polysaccharides. Depolymerization of aromatic biopolymers, as cheap and renewable substitutes for fossil-based resources, has been used in the preparation of biofuels, and a range of aromatic and aliphatic small molecules. Additionally, these polymers exhibit a robust UV-shielding function due to the high content of aromatic groups. Meanwhile, the abundance of phenolic groups in their structures gives these compounds outstanding antioxidant capabilities, making them well-suited for a diverse array of anti-UV and medical applications. Nevertheless, these biopolymers possess inherent drawbacks in their pristine states, such as rigid structure, low solubility, and lack of desired functionalities, which hinder their complete exploitation across diverse sectors. Thus, the modification and functionalization of aromatic biopolymers are essential to provide them with specific functionalities and features needed for particular applications. Aromatic biopolymers include lignins, tannins, melanins, and humic acids. The objective of this review is to offer a thorough reference for assessing the chemistry and functionalization of lignins and condensed tannins. Lignins represent the largest and most prominent category of aromatic biopolymers, typically distinguishable as either softwood-derived or hardwood-derived lignins. Besides, condensed tannins are the most investigated group of the tannin family. The electron-rich aromatic rings, aliphatic hydroxyl groups, and phenolic groups are the main functional groups in the structure of lignins and condensed tannins. Methoxy groups are also abundant in lignins. Each group displays varying chemical reactivity within these biopolymers. Therefore, the selective and specific functionalization of lignins and condensed tannins can be achieved by understanding the chemistry behavior of these functional groups. Targeted applications include biomedicine, monomers and surface active agents for sustainable plastics.

Lignin-coordinated niobium-based catalyst for the efficient conversion of industrial lignin in choline chloride-lactic acid integrated with ethanol deep eutectic solvent

Catalytic depolymerization is a favorable option for the valorization of industrial lignin. In this study, a new strategy was demonstrated for the efficient reductive depolymerization of industrial lignin based on a complex solvent of choline chloride-lactic acid (ChCl-LA) DES integrated with ethanol and a C-supported N-doped niobium-based catalyst with industrial lignin as carbon source (NBC@N-LC). It was found that the introduction of ethanol significantly improved the conversion of industrial lignin in ChCl-LA. More delightfully, depolymerization of industrial lignin in the complex solvent of DES-ethanol was further promoted by using the fabricated NBC@N-LC, exhibiting the best lignin dissolution performance, the lowest undepolymerized lignin yield of 21.9 % and the highest monomers yield of 31.8 %. 4-Ethylphenol was observed to dominate the monomers with 53.6 % selectivity, revealing the directional reactivity of the catalytic depolymerization process. Reaction pathway analysis showed that the synergistic effect of NBC@N-LC and DES-ethanol led to the efficient depolymerization of lignin, in which DES acted as an excellent solvent for lignin and depolymerized intermediates, ethanol acted as an in-situ hydrogen donor, and NBC@N-LC acted as a provider of reaction sites and acid active centers. In conclusion, this work offers a good alternative compared to the available lignin depolymerization method in terms of efficiency, selectivity, and cost aspects.

Functionalized Metal-Free Carbon Nanosphere Catalyst for the Selective C-N Bond Formation under Open-Air Conditions

A versatile shape-controlled carbon nanomaterial that can efficiently catalyze the selective C-N coupling reactions under metal-free and open-air conditions was developed by applying N-doping and KOH activation strategies in candle soot (ANCS). The TEM and elemental mapping results showed the formation of sphere-shaped carbon particles as well as the uniform distribution of nitrogen species in the carbon framework. KOH activation enhanced the specific surface area of carbon, whereas N-doping enriched the electron-deficient nature by introducing functional N-based pyrrolic/graphitic structures in the carbon framework. The synergistic effect of N-doping and KOH activation significantly improved the catalytic efficiency of the carbon catalyst (ANCS), giving a 96% conversion of o-phenylenediamine (OPD) with a good selectivity to 2-phenylbenzimidazole (97%). In contrast, the pristine carbon exhibited very low activity (48% conversion of the OPD and 36% selectivity to 2-phenylbenzimidazole). Besides, the ANCS nanomaterial provided a facile catalytic approach for the homo- and cross-C-N condensation of various aromatic amines and diamines to produce diverse functional imines and benzimidazoles at mild conditions. This work provided promising insights into developing advanced, metal-free carbon-based catalysts for selective C-N coupling reactions to produce valuable drug motifs.

Catalytic Upgrading of a Mixed Hydroxy Acid Feedstock Derived from Kraft Black Liquor

Lignocellulosic feedstocks are widely studied for sustainable liquid fuel and chemical production. The pulp and paper industry generates large amounts of kraft black liquor (BL) from which a high volume of hydroxy acids (HAs) can be separated for further catalytic processing. Here, we explore the catalytic upgrading of HAs, including the conversion of (1) a model HA, gluconic acid; (2) a model mixture of HAs, and (3) a real mixture of HAs derived from kraft BL on M/Nb2O5 (M = Pd, Pt, Rh, and Ru). The hydrodeoxygenation of model gluconic acid reveals that "volatile" carboxylic acids (mainly C2 and C3), levulinic acid, and cyclic esters are significant products over all the catalysts, with Pd/Nb2O5 showing superior activity and selectivity toward valuable intermediates. The model mixture of HAs shows a wide range of reactivity over the supported metal catalyst, with the product selectivity strongly correlating to reaction temperature. Utilizing a 0.25% Pd/Nb2O5 catalyst, a real mixture of HAs derived from kraft BL is successfully dehydroxylated to produce a mixture rich in C3-C8 carboxylic acids that may be amenable for further upgrading, e.g., catalytically to ketones with high carbon chain lengths. Despite the feedstock complexity, we selectively cleaved the C-OH bonds of HAs, while successfully preserving most of the -COOH groups and minimizing C-C and C=O bond scission reactions under the operating conditions tested. The BL-derived HA stream is thus proposed to be a suitable platform for producing mixed carboxylic acid products from an overoxygenated byproduct feed.

Niobium-based single-atom catalyst promoted fractionation of lignocellulose in choline chloride-lactic acid deep eutectic solvent

Pretreatment is the key step to convert lignocelluloses to sustainable biofuels, biochemicals or biomaterials. In this study, a green pretreatment method based on choline chloride-lactic acid deep eutectic solvent (ChCl-LA) and niobium-based single-atom catalyst (Nb/CN) was developed for the fractionation of corn straw and further enzymatic hydrolysis of cellulose. With this strategy, significant lignin removal of 96.5 % could be achieved when corn straw was pretreated by ChCl-LA (1:2) DES over Nb/CN under 120 °C for 6 h. Enzymatic hydrolysis of the cellulose-enriched fraction (CEF) presented high glucose yield of 92.7 % and xylose yield of 67.5 %. In-depth investigations verified that the high yields of fractions and monosaccharides was attributed to the preliminary fractionation by DES and the deep fractionation by Nb/CN. Significantly, compared to other reported soluble catalysts, the synthesized single-atom catalyst displayed excellent reusability by simple filtration and enzymatic hydrolysis. The recyclability experiments showed that the combination of ChCl-LA DES and Nb/CN could be repeated at least three times for corn straw fractionation, moreover, the combination displayed remarkable feedstock adaptability.