Rational highly dispersed ruthenium for reductive catalytic fractionation of lignocellulose

Catalytic depolymerization of Camellia oleifera shell lignin to phenolic monomers: Insights into the effects of solvent, catalyst and atmosphere

Camellia oleifera shell (COS) is a renewable biomass resource abundant in lignin with significant potential for producing phenolic monomers. However, the dearth of research has led to considerable resource wastage and environmental pollution. Herein, reductive catalytic fractionation (RCF) of COS was performed using noble metal catalysts in different solvents. An 11.1 wt% yield of phenolic monomers was achieved with 91% selectivity toward propylene-substituted monomers in H2O/EtOH (3:7, v/v) cosolvent under N2 atmosphere. Notably, the highest phenolic monomer yield of 17.0 wt% was obtained with impressive selectivity (86.9%) toward propanol-substituted monomers in the presence of H2. The GPC analysis and 2D HSQC NMR spectra indicated the effective depolymerization of lignin oligomers with catalysts. Phenolic monomers with ethyl, propyl, or propanol side chain could be produced from lignin-derived oligomers through hydrogenolysis, hydrogenation, and decarboxylation reactions. Overall, this study has paved the way for the valorization of COS lignin through the RCF strategy.

Solar-Driven Biomass Reforming for Hydrogen Generation: Principles, Advances, and Challenges

Hydrogen (H2) has emerged as a clean and versatile energy carrier to power a carbon-neutral economy for the post-fossil era. Hydrogen generation from low-cost and renewable biomass by virtually inexhaustible solar energy presents an innovative strategy to process organic solid waste, combat the energy crisis, and achieve carbon neutrality. Herein, the progress and breakthroughs in solar-powered H2 production from biomass are reviewed. The basic principles of solar-driven H2 generation from biomass are first introduced for a better understanding of the reaction mechanism. Next, the merits and shortcomings of various semiconductors and cocatalysts are summarized, and the strategies for addressing the related issues are also elaborated. Then, various bio-based feedstocks for solar-driven H2 production are reviewed with an emphasis on the effect of photocatalysts and catalytic systems on performance. Of note, the concurrent generation of value-added chemicals from biomass reforming is emphasized as well. Meanwhile, the emerging photo-thermal coupling strategy that shows a grand prospect for maximally utilizing the entire solar energy spectrum is also discussed. Further, the direct utilization of hydrogen from biomass as a green reductant for producing value-added chemicals via organic reactions is also highlighted. Finally, the challenges and perspectives of photoreforming biomass toward hydrogen are envisioned.

Lattice Strain Engineering on Metal-Organic Frameworks by Ligand Doping to Boost the Electrocatalytic Biomass Valorization

As an efficient and environmental-friendly strategy, electrocatalytic oxidation can realize biomass lignin valorization by cleaving its aryl ether bonds to produce value-added chemicals. However, the complex and polymerized structure of lignin presents challenges in terms of reactant adsorption on the catalyst surface, which hinders further refinement. Herein, NiCo-based metal-organic frameworks (MOFs) are employed as the electrocatalyst to enhance the adsorption of reactant molecules through π-π interaction. More importantly, lattice strain is introduced into the MOFs via curved ligand doping, which enables tuning of the d-band center of metal active sites to align with the reaction intermediates, leading to stronger adsorption and higher electrocatalytic activity toward bond cleavage within lignin model compounds and native lignin. When 2'-phenoxyacetophenone is utilized as the model compound, high yields of phenol (76.3%) and acetophenone (21.7%) are achieved, and the conversion rate of the reactants reaches 97%. Following pre-oxidation of extracted poplar lignin, >10 kinds of phenolic compounds are received using the as-designed MOFs electrocatalyst, providing ≈12.48% of the monomer, including guaiacol, vanillin, eugenol, etc., and p-hydroxybenzoic acid dominates all the products. This work presents a promising and deliberately designed electrocatalyst for realizing lignin valorization, making significant strides for the sustainability of this biomass resource.

Design and Validation of a High-Throughput Reductive Catalytic Fractionation Method

Reductive catalytic fractionation (RCF) is a promising method to extract and depolymerize lignin from biomass, and bench-scale studies have enabled considerable progress in the past decade. RCF experiments are typically conducted in pressurized batch reactors with volumes ranging between 50 and 1000 mL, limiting the throughput of these experiments to one to six reactions per day for an individual researcher. Here, we report a high-throughput RCF (HTP-RCF) method in which batch RCF reactions are conducted in 1 mL wells machined directly into Hastelloy reactor plates. The plate reactors can seal high pressures produced by organic solvents by vertically stacking multiple reactor plates, leading to a compact and modular system capable of performing 240 reactions per experiment. Using this setup, we screened solvent mixtures and catalyst loadings for hydrogen-free RCF using 50 mg poplar and 0.5 mL reaction solvent. The system of 1:1 isopropanol/methanol showed optimal monomer yields and selectivity to 4-propyl substituted monomers, and validation reactions using 75 mL batch reactors produced identical monomer yields. To accommodate the low material loadings, we then developed a workup procedure for parallel filtration, washing, and drying of samples and a 1H nuclear magnetic resonance spectroscopy method to measure the RCF oil yield without performing liquid-liquid extraction. As a demonstration of this experimental pipeline, 50 unique switchgrass samples were screened in RCF reactions in the HTP-RCF system, revealing a wide range of monomer yields (21-36%), S/G ratios (0.41-0.93), and oil yields (40-75%). These results were successfully validated by repeating RCF reactions in 75 mL batch reactors for a subset of samples. We anticipate that this approach can be used to rapidly screen substrates, catalysts, and reaction conditions in high-pressure batch reactions with higher throughput than standard batch reactors.

Harnessing Atomically Dispersed Cobalt for the Reductive Catalytic Fractionation of Lignocellulose

The reductive catalytic fractionation (RCF) of lignocellulose, considering lignin valorization at design time, has demonstrated the entire utilization of all lignocellulose components; however, such processes always require catalysts based on precious metals or high-loaded nonprecious metals. Herein, the study develops an ultra-low loaded, atomically dispersed cobalt catalyst, which displays an exceptional performance in the RCF of lignocellulose. An approximately theoretical maximum yield of phenolic monomers (48.3 wt.%) from lignin is realized, rivaling precious metal catalysts. High selectivity toward 4-propyl-substituted guaiacol/syringol facilitates their purification and follows syntheses of highly adhesive polyesters. Lignin nanoparticles (LNPs) are generated by simple treatment of the obtained phenolic dimers and oligomers. RCF-resulted carbohydrate pulp are more obedient to enzymatic hydrolysis. Experimental studies on lignin model compounds reveal the concerted cleavage of Cα-O and Cβ-O pathway for the rupture of β-O-4 structure. Overall, the approach involves valorizing products derived from lignin biopolymer, providing the opportunity for the comprehensive utilization of all components within lignocellulose.

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.

Advances in Heterogeneous Catalysts for Lignin Hydrogenolysis

Lignin is the main component of lignocellulose and the largest source of aromatic substances on the earth. Biofuel and bio-chemicals derived from lignin can reduce the use of petroleum products. Current advances in lignin catalysis conversion have facilitated many of progress, but understanding the principles of catalyst design is critical to moving the field forward. In this review, the factors affecting the catalysts (including the type of active metal, metal particle size, acidity, pore size, the nature of the oxide supports, and the synergistic effect of the metals) are systematically reviewed based on the three most commonly used supports (carbon, oxides, and zeolites) in lignin hydrogenolysis. The catalytic performance (selectivity and yield of products) is evaluated, and the emerging catalytic mechanisms are introduced to better understand the catalyst design guidelines. Finally, based on the progress of existing studies, future directions for catalyst design in the field of lignin depolymerization are proposed.

Efficient Fast Fractionation of Biomass Using a Diol-Based Deep Eutectic Solvent for Facilitating Enzymatic Hydrolysis and Obtaining High-Quality Lignin

Current DES pretreatment is often performed under relatively severe conditions with high temperature, long time, and high DES usage. This work studied a short-time diol DES (deep eutectic solvent) pretreatment under mild conditions to fractionate the bamboo, facilitate enzymatic hydrolysis, and obtain high-quality lignin. At an optimized condition of 130 °C for only 10 min, lignin and xylan removal reached 61.34 % and 84.15 %, with residual glucan showing a ~90 % enzymatic hydrolysis yield. Equally important, the dissolved lignin could be readily recovered with 97.51 % yield, exhibiting 96.65 % β-O-4 preservation. The fractionation and lignin protection mechanisms were unveiled by XRD, FTIR, cellulose-DP, 2D HSQC NMR, 31 P NMR and GPC analysis. This study highlighted that short-time fractionation of bamboo can be achieved by a diol-based DES which is an ideal strategy to upgrade the lignocellulose biomass for high enzymatic hydrolysis yields and high-quality lignin stream.

Highly active, ultra-low loading single-atom iron catalysts for catalytic transfer hydrogenation

Highly effective and selective noble metal-free catalysts attract significant attention. Here, a single-atom iron catalyst is fabricated by saturated adsorption of trace iron onto zeolitic imidazolate framework-8 (ZIF-8) followed by pyrolysis. Its performance toward catalytic transfer hydrogenation of furfural is comparable to state-of-the-art catalysts and up to four orders higher than other Fe catalysts. Isotopic labeling experiments demonstrate an intermolecular hydride transfer mechanism. First principles simulations, spectroscopic calculations and experiments, and kinetic correlations reveal that the synthesis creates pyrrolic Fe(II)-plN3 as the active center whose flexibility manifested by being pulled out of the plane, enabled by defects, is crucial for collocating the reagents and allowing the chemistry to proceed. The catalyst catalyzes chemoselectively several substrates and possesses a unique trait whereby the chemistry is hindered for more acidic substrates than the hydrogen donors. This work paves the way toward noble-metal free single-atom catalysts for important chemical reactions.

Self-Hydrogen Supplied Catalytic Fractionation of Raw Biomass into Lignin-Derived Phenolic Monomers and Cellulose-Rich Pulps

Lignocellulosic biomass is one of the most well-studied and promising green carbon sources. The fullest utilization of lignocellulosic biomass in hydrogen-free and mild conditions to produce phenolic monomers while preserving cellulose-rich pulps is challenging and has far-reaching significance. Here, we report an innovative strategy to convert lignocellulosic biomass into lignin oils and cellulose-rich pulps without exogenous hydrogen under mild conditions over a Pt/NiAl₂O₄ catalyst. In this process, the structural hydrogens in hemicellulose acted as a hydrogen source to realize the fractionation and depolymerization of lignin into phenolic monomers while keeping the cellulose intact, which is named self-hydrogen supplied catalytic fractionation (SCF). By using water as a solvent, the theoretical yield of phenolic monomers (46.6 wt %, with propyl(ethyl) end-chained syringol and guaiacol as main products) is achieved at 140 °C for 24 h, with 90% cellulose intact in birch sawdust. This H₂-free process can be extended to other biomass (hardwood, softwood, and grass) and can be scaled up. The Pt/NiAl₂O₄ catalyst also shows good stability in recycling as well as regeneration treatment. This work provides a new strategy to achieve high utilization of lignocellulosic biomass for sustainable biorefinery by using water as a solvent without exogenous hydrogen under mild conditions.

Lignin Extracted from Various Parts of Castor (Ricinus communis L.) Plant: Structural Characterization and Catalytic Depolymerization

Castor is an important non-edible oilseed crop used in the production of high-quality bio-oil. In this process, the leftover tissues rich in cellulose, hemicellulose and lignin are regarded as by-products and remain underutilized. Lignin is a crucial recalcitrance component, and its composition and structure strongly limit the high-value utilization of raw materials, but there is a lack of detailed studies relating to castor lignin chemistry. In this study, lignins were isolated from various parts of the castor plant, namely, stalk, root, leaf, petiole, seed endocarp and epicarp, using the dilute HCl/dioxane method, and the structural features of the as-obtained six lignins were investigated. The analyses indicated that endocarp lignin contained catechyl (C), guaiacyl (G) and syringyl (S) units, with a predominance of C unit [C/(G+S) = 6.9:1], in which the coexisted C-lignin and G/S-lignin could be disassembled completely. The isolated dioxane lignin (DL) from endocarp had a high abundance of benzodioxane linkages (85%) and a low level of β-β linkages (15%). The other lignins were enriched in G and S units with moderate amounts of β-O-4 and β-β linkages, being significantly different from endocarp lignin. Moreover, only p-coumarate (pCA) incorporated into the epicarp lignin was observed, with higher relative content, being rarely reported in previous studies. The catalytic depolymerization of isolated DL generated 1.4-35.6 wt% of aromatic monomers, among which DL from endocarp and epicarp have high yields and excellent selectivity. This work highlights the differences in lignins from various parts of the castor plant, providing a solid theory for the high-value utilization of the whole castor plant.

Unraveling the Lignin Structural Variation in Different Bamboo Species

The structure of cellulolytic enzyme lignin (CEL) prepared from three bamboo species (Neosinocalamus affinis, Bambusa lapidea, and Dendrocalamus brandisii) has been characterized by different analytical methods. The chemical composition analysis revealed a higher lignin content, up to 32.6% of B. lapidea as compared to that of N. affinis (20.7%) and D. brandisii (23.8%). The results indicated that bamboo lignin was a p-hydroxyphenyl-guaiacyl-syringyl (H-G-S) lignin associated with p-coumarates and ferulates. Advanced NMR analyses displayed that the isolated CELs were extensively acylated at the γ-carbon of the lignin side chain (with either acetate and/or p-coumarate groups). Moreover, a predominance of S over G lignin moieties was found in CELs of N. affinis and B. lapidea, with the lowest S/G ratio observed in D. brandisii lignin. Catalytic hydrogenolysis of lignin demonstrated that 4-propyl-substituted syringol/guaiacol and propanol guaiacol/syringol derived from β-O-4' moieties, and methyl coumarate/ferulate derived from hydroxycinnamic units were identified as the six major monomeric products. We anticipate that the insights of this work could shed light on the sufficient understanding of lignin, which could open a new avenue to facilitate the efficient utilization of bamboo.

Single atomic Ru in TiO2 boost efficient electrocatalytic water oxidation to hydrogen peroxide

Electrocatalytic two-electron water oxidation affords a promising approach for distributed production of H₂O₂ using electricity. However, it suffers from the trade-off between the selectivity and high production rate of H₂O₂ due to the lack of suitable electrocatalysts. In this study, single atoms of Ru were controllably introduced into titanium dioxide to produce H₂O₂ through an electrocatalytic two-electron water oxidation reaction. The adsorption energy values of OH intermediates could be tuned by introducing Ru single atoms, offering superior H₂O₂ production under high current density. Notably, a Faradaic efficiency of 62.8% with an H₂O₂ production rate of 24.2 μmol min^-1 cm^-2 (>400 ppm within 10 min) was achieved at a current density of 120 mA cm^-2. Consequently, herein, the possibility of high-yield H₂O₂ production under high current density was demonstrated and the importance of regulating intermediate adsorption during electrocatalysis was evidenced.

Discovery, disassembly, depolymerization and derivatization of catechyl lignin in Chinese tallow seed coats

The search for feedstock of catechyl lignin (C-lignin) is great interest and importance, as C-lignin featuring homogeneity and linearity is considered as an "ideal lignin" archetype for valorization and exits in only a few plant seed coats. In this study, naturally occurring C-lignin is first discovered in the seed coats of Chinese tallow, which has the highest content of C-lignin (15.4 wt%) as compared with other known feedstocks. An optimized extraction procedure by ternary deep eutectic solvents (DESs) enables the complete disassembly of C-lignin and G/S-lignin coexisted in Chinese tallow seed coats, and characterizations revealed that the as-separated C-lignin sample is abundant in benzodioxane units with no observation of β-O-4 structures from G/S-lignin. Catalytic depolymerization of C-lignin results in a simplex catechol product in 129 mg per gram seed coats, being higher than other reported feedstocks. Derivatizing the "black" C-lignin via the nucleophilic isocyanation of benzodioxane γ-OH leads to a "whitened C-lignin" with uniform laminar structure and excellent crystallization ability, being conducive to fabricating functional materials. Overall, this contribution showed that Chinses tallow seed coats are suitable feedstock for acquiring C-lignin biopolymer.

Lignocellulosic Biorefinery Technologies: A Perception into Recent Advances in Biomass Fractionation, Biorefineries, Economic Hurdles and Market Outlook

Lignocellulosic biomasses (LCB) are sustainable and abundantly available feedstocks for the production of biofuel and biochemicals via suitable bioconversion processing. The main aim of this review is to focus on strategies needed for the progression of viable lignocellulosic biomass-based biorefineries (integrated approaches) to generate biofuels and biochemicals. Processing biomass in a sustainable manner is a major challenge that demands the accomplishment of basic requirements relating to cost effectiveness and environmental sustainability. The challenges associated with biomass availability and the bioconversion process have been explained in detail in this review. Limitations associated with biomass structural composition can obstruct the feasibility of biofuel production, especially in mono-process approaches. In such cases, biorefinery approaches and integrated systems certainly lead to improved biofuel conversion. This review paper provides a summary of mono and integrated approaches, their limitations and advantages in LCB bioconversion to biofuel and biochemicals.

Lignin Stabilization and Carbohydrate Nature in H-transfer Reductive Catalytic Fractionation: The Role of Solvent Fractionation of Lignin Oil in Structural Profiling

Reductive Catalytic Fractionation (RCF) of lignocellulosic materials produces lignin oil rich in monomer products and high-quality cellulosic pulps. RCF lignin oil also contains lignin oligomers/polymers and hemicellulose-derived carbohydrates. The variety of components makes lignin oil a complex matrix for analytical methods. As a result, the signals are often convoluted and overlapped, making detecting and quantifying key intermediates challenging. Therefore, to investigate the mechanisms underlining lignin stabilization and elucidate the structural features of carbohydrates occurring in the RCF lignin oil, fractionation methods reducing the RCF lignin oil complexity are required. This report examines the solvent fractionation of RCF lignin oil as a facile method for producing lignin oil fractions for advanced characterization. Solvent fractionation uses small volumes of environmentally benign solvents (methanol, acetone, and ethyl acetate) to produce multigram lignin fractions comprising products in different molecular weight ranges. This feature allows the determination of structural heterogeneity across the entire molecular weight distribution of the RCF lignin oil by high-resolution HSQC NMR spectroscopy. This study provides detailed insight into the role of the hydrogenation catalyst (Raney Ni) in stabilizing lignin fragments and defining the structural features of hemicellulose-derived carbohydrates in lignin oil obtained by the H-transfer RCF process.

Producing Lignin and Ethyl Levulinate from Wheat Stalk Using 1-(3-Sulfobutyl) Triethylammonium Hydrogen Sulfate and USY Zeolite

The facile, green, and efficient strategy for the separation of lignin from straw and subsequent production of value-added chemicals is crucial to the current utilization of straw. Herein, up to 23.72% of lignin was isolated from wheat stalk over cheap and green 1-(3-sulfobutyl) triethylammonium hydrogen sulfate ([BSTEA]HSO₄) in aqueous ethanol (V_ethanol: V_water = 4:1). The acquired lignin was verified as a p-hydroxyphenyl-guaiacyl-syringyl type, which had a narrower molecular weight distribution, better thermal stability, and higher purity compared with those of the lignin obtained using 1-methyl-3-(4-sulfobutyl)-imidazolium hydrogen sulfate and 1-(3-sulfobutyl) pyridinium hydrogen sulfate. Moreover, a carbohydrate-rich liquid containing [BSTEA]HSO₄ was obtained by water removal from the waste liquid after lignin separation and further converted to ethyl levulinate (EL) by a one-pot process in the presence of inexpensive and stable USY zeolite. The yield of EL reached 30.23% at 200 °C for 60 min over the presence of 40% [BSTEA]HSO₄ and 60% USY zeolite. Under optimal conditions, the yields of lignin and EL can respectively reach 83.89 and 72.28% of those catalyzed by a fresh catalyst after five cycles. In short, the above-mentioned methods present a green, economic, and efficient route for the extraction of lignin and further treatment of the liquid waste generated during the extraction process.

Advances in physicochemical pretreatment strategies for lignocellulose biomass and their effectiveness in bioconversion for biofuel production

The inherent recalcitrance of lignocellulosic biomass is a significant barrier to efficient lignocellulosic biorefinery owing to its complex structure and the presence of inhibitory components, primarily lignin. Efficient biomass pretreatment strategies are crucial for fragmentation of lignocellulosic biocomponents, increasing the surface area and solubility of cellulose fibers, and removing or extracting lignin. Conventional pretreatment methods have several disadvantages, such as high operational costs, equipment corrosion, and the generation of toxic byproducts and effluents. In recent years, many emerging single-step, multi-step, and/or combined physicochemical pretreatment regimes have been developed, which are simpler in operation, more economical, and environmentally friendly. Furthermore, many of these combined physicochemical methods improve biomass bioaccessibility and effectively fractionate ∼96 % of lignocellulosic biocomponents into cellulose, hemicellulose, and lignin, thereby allowing for highly efficient lignocellulose bioconversion. This review critically discusses the emerging physicochemical pretreatment methods for efficient lignocellulose bioconversion for biofuel production to address the global energy crisis.

Photocatalytic Depolymerization of Native Lignin toward Chemically Recyclable Polymer Networks

As an inedible component of biomass, lignin features rich functional groups that are desired for chemical syntheses. How to effectively depolymerize lignin without compromising the more valuable cellulose and hemicellulose has been a significant challenge. Existing biomass processing procedures either induce extensive condensation in lignin that greatly hinders its chemical utilization or focus on fully depolymerizing lignin to produce monomers that are difficult to separate for subsequent chemical synthesis. Here, we report a new approach to selective partial depolymerization, which produces oligomers that can be readily converted to chemically recyclable polymer networks. The process takes advantage of the high selectivity of photocatalytic activation of the β-O-4 bond in lignin by tetrabutylammonium decatungstate (TBADT). The availability of exogenous electron mediators or scavengers promotes cleavage or oxidation of this bond, respectively, enabling high degrees of control over the depolymerization and the density of a key functional group, C=O, in the products. The resulting oligomers can then be readily utilized for the synthesis of polymer networks through reactions between C=O and branched -NH2 as a dynamic covalent cross-linker. Importantly, the resulting polymer network can be recycled to enable a circular economy of materials directly derived from biomass.

Production of xylooligosaccharides from Camellia oleifera Abel fruit shell using a shell-based solid acid catalyst

This study aimed to produce xylooligosaccharides (XOS) from Camellia oleifera Abel fruit shell (CFS) using a shell-based solid acid derived from CFS (CFS-BSA). CFS-BSA preparation was optimized by incomplete carbonization at 450 °C for 1 h, followed by sulfonation at 130 °C for 8 h to yield a -SO₃H functional group concentration of 1.04 mmol/g. When CFS-BSA was used to hydrolyze CFS with a 1:5 ratio of CFS-BSA to CFS at 170 °C for 20 min, a maximum XOS yield (X₂-X₅) of 51.41 % was achieved, which was notably higher than when using subcritical H₂O solely. CFS-BSA can be recycled and reused at least six times by sieving without a substantial loss in its catalytic activity. CFS-BSA can also be used to produce XOS from other lignocellulosic materials such as corncob (41.04 %), sugarcane bagasse (45.03 %), corn stalk (45.89 %), birchwood (46.05 %), and poplar (40.10 %).