Recent advances of lignin valorization techniques toward sustainable aromatics and potential benchmarks to fossil refinery products

Monolignol Potential and Insights into Direct Depolymerization of Fruit and Nutshell Remains for High Value Sustainable Aromatics

The inedible parts of nuts and stone fruits are low-cost and lignin-rich feedstock for more sustainable production of aromatic chemicals in comparison with the agricultural and forestry residues. However, the depolymerization performances on food-related biomass remains unclear, owing to the broad physicochemical variations from the edible parts of the fruits and plant species. In this study, the monomer production potentials of ten major fruit and nutshell biomass were investigated with comprehensive numerical information derived from instrumental analysis, such as plant cell wall chemical compositions, syringyl/guaiacyl (S/G ratios, and contents of lignin substructure linkages (β-O-4, β-β, β-5). A standardized one-pot reductive catalytic fractionation (RCF) process was applied to benchmark the monomer yields, and the results were statistically analyzed. Among all the tested biomass, mango endocarp provided the highest monolignol yields of 37.1 % per dry substrates. Positive S-lignin (70-84 %) resulted in higher monomer yield mainly due to more cleavable β-O-4 linkages and less condensed C-C linkages. Strong positive relationships were identified between β-O-4 and S-lignin and between β-5 and G-lignin. The analytical, numerical, and experimental results of this study shed lights to process design of lignin-first biorefinery in food-processing industries and waste management works.

Chloromethylation of Lignin as a Route to Functional Material with Catalytic Properties in Cross-Coupling and Click Reactions

We present a novel, greener chloromethylation procedure for organosolv aspen lignin under mild reaction conditions without Lewis acid as a catalyst and in acetic acid as a solvent. This synthetic protocol provides a reliable approach to chloromethylated lignin (CML) and means to obtain valuable lignin derivatives. The resulted CML was subsequently transformed into 1-methylimidazolium lignin (ImL), which effectively serves as a stabilizing agent for Pd/CuO nanoparticles (Pd/CuO-NPs). To evaluate the versatility of developed lignin-based catalyst, we investigate its performance in a series of carbon-carbon bond formation reactions, including Suzuki-Miyaura, Sonogashira, Heck reactions, and azide-alkyne cycloaddition (click) reaction. Remarkably, this catalyst exhibited a high degree of catalytic efficiency, resulting in reactions with yields ranging from average to excellent. The heterogeneous catalyst demonstrated outstanding recyclability, enabling its reuse for at least 10 consecutive reaction cycles, with yields consistently falling within the range of 42 % to 84 %. A continuous flow reactor cartridge prototype employing Lignin@Pd/CuO-NPs was developed, yielding results comparable to those achieved in batch reactions. The utilization of Lignin@Pd/CuO-NPs as a catalyst showcases its potential to facilitate diverse carbon-carbon bond formation reactions and underscores its promising recyclability, aligning with the green chemistry metrics and principles of sustainability in chemical processes.

Fabrication of lignin-based sub-micro hybrid particle as a novel support for adenylate cyclase immobilization

This study developed a surface functionalized lignin-based sub-microsphere as an innovative support for enzyme immobilization. Lignin was first modified with a silane reagent leading to lignin/SiO2 (LS) organic/inorganic hybrid particles, displayed as regular sub-micro spheres with a SiO2 shell as demonstrated in SEM images. The LS particles were further modified to introduce nickel ions, as evidenced in XPS spectra, facilitating affinity adsorption with a his-tagged enzyme. The immobilization of adenylate cyclase from Haloactinopolyspora alba (HaAC), expressed in Escherichia coli, was conducted on the surface functionalized LS (LS-G-NTA-Ni). The immobilization conditions were optimized to achieve the highest relative activity, which were determined to be using a Ni2+ concentration of 62.5 mM, at pH=9.5 and 25 °C, with an enzyme-to-support ratio of 4.0 for a duration of 2 h. The immobilized HaAC shows maximum relative activity at pH=9.5 and 40 °C, and exhibits significantly improved thermal stability compared to the free enzyme. After undergoing five reusing cycles, the immobilized HaAC maintains a satisfactory activity (54.15%), which is due to the surface chemistry and the structural stability of the functionalized LS. This work provides a valuable exploration for high-value application of industrial lignin.

Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

Heterogeneously catalyzed depolymerization of lignin to value-added chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increasing char formation. Therefore, for the potential industrial utilization of lignin, new knowledge focused on the size of lignin particles is of great importance to offer guidance for promoting lignin depolymerization and suppressing condensation in the heterogeneously catalytic systems. In this Review, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneous catalytic techniques for efficient utilization of the aromatic polymer of lignin.

Recent Advances in Lignocellulose-Based Monomers and Their Polymerization

Replacing fossil-based polymers with renewable bio-based polymers is one of the most promising ways to solve the environmental issues and climate change we human beings are facing. The production of new lignocellulose-based polymers involves five steps, including (1) fractionation of lignocellulose into cellulose, hemicellulose, and lignin; (2) depolymerization of the fractionated cellulose, hemicellulose, and lignin into carbohydrates and aromatic compounds; (3) catalytic or thermal conversion of the depolymerized carbohydrates and aromatic compounds to platform chemicals; (4) further conversion of the platform chemicals to the desired bio-based monomers; (5) polymerization of the above monomers to bio-based polymers by suitable polymerization methods. This review article will focus on the progress of bio-based monomers derived from lignocellulose, in particular the preparation of bio-based monomers from 5-hydroxymethylfurfural (5-HMF) and vanillin, and their polymerization methods. The latest research progress and application scenarios of related bio-based polymeric materials will be also discussed, as well as future trends in bio-based polymers.

Could termites be hiding a goldmine of obscure yet promising yeasts for energy crisis solutions based on aromatic wastes? A critical state-of-the-art review

Biodiesel is a renewable fuel that can be produced from a range of organic and renewable feedstock including fresh or vegetable oils, animal fats, and oilseed plants. In recent years, the lignin-based aromatic wastes, such as various aromatic waste polymers from agriculture, or organic dye wastewater from textile industry, have attracted much attention in academia, which can be uniquely selected as a potential renewable feedstock for biodiesel product converted by yeast cell factory technology. This current investigation indicated that the highest percentage of lipid accumulation can be achieved as high as 47.25% by an oleaginous yeast strain, Meyerozyma caribbica SSA1654, isolated from a wood-feeding termite gut system, where its synthetic oil conversion ability can reach up to 0.08 (g/l/h) and the fatty acid composition in yeast cells represents over 95% of total fatty acids that are similar to that of vegetable oils. Clearly, the use of oleaginous yeasts, isolated from wood-feeding termites, for synthesizing lipids from aromatics is a clean, efficient, and competitive path to achieve "a sustainable development" towards biodiesel production. However, the lacking of potent oleaginous yeasts to transform lipids from various aromatics, and an unknown metabolic regulation mechanism presented in the natural oleaginous yeast cells are the fundamental challenge we have to face for a potential cell factory development. Under this scope, this review has proposed a novel concept and approach strategy in utilization of oleaginous yeasts as the cell factory to convert aromatic wastes to lipids as the substrate for biodiesel transformation. Therefore, screening robust oleaginous yeast strain(s) from wood-feeding termite gut system with a set of the desirable specific tolerance characteristics is essential. In addition, to reconstruct a desirable metabolic pathway/network to maximize the lipid transformation and accumulation rate from the aromatic wastes with the applications of various "omics" technologies or a synthetic biology approach, where the work agenda will also include to analyze the genome characteristics, to develop a new base mutation gene editing technology, as well as to clarify the influence of the insertion position of aromatic compounds and other biosynthetic pathways in the industrial chassis genome on the expressional level and genome stability. With these unique designs running with a set of the advanced biotech approaches, a novel metabolic pathway using robust oleaginous yeast developed as a cell factory concept can be potentially constructed, integrated and optimized, suggesting that the hypothesis we proposed in utilizing aromatic wastes as a feedstock towards biodiesel product is technically promising and potentially applicable in the near future.