Study on Selective Preparation of Phenolic Products from Lignin over Ru–Ni Bimetallic Catalysts Supported on Modified HY Zeolite

Catalytic Hydrogenation of Lignin Ethers and Bio-Oil Using Non-Noble Cobalt Catalysts

The conversion of lignocellulosic biomass into lignin bio-oil and its subsequent upgrading into saturated cyclic products holds considerable promise for applications in the aviation industry. This study reports the synthesis of a defect-enriched monometallic CoOx/Co-350-30 catalyst, which is utilized for hydrogenating lignin-derived molecules and lignin bio-oil obtained via reductive catalytic fractionation (RCF) of wheat straw. Under optimized conditions (180 °C, 2 MPa H2, 2 h), benzyl phenyl ether (BPE) affords complete conversion, yielding ≈99% cyclohexanol and ≈98% methylcyclohexane. RCF of wheat straw (conducted at 230 °C and 3 MPa H2 for 6 h) affords lignin bio-oil containing ≈43% alkyl-substituted phenols. Hydrogenation of the bio-oil using the CoOx/Co-350-30 catalyst (at 250 °C for 2 h at 3 MPa H2) results in ≈98% yield of cyclic aliphatic alcohols. Comparative studies with commercial 5%Ru/C reveal that the CoOx/Co-350-30 catalyst produced products with lower oxygen functionalities and fewer native lignin linkages. Comprehensive catalyst characterizations and activity tests were conducted to propose a plausible reaction mechanism for BPE hydrogenation. The cobalt-based catalyst, devoid of noble metals, provides a sustainable and cost-effective method for biomass conversion into fuel-range products, addressing the growing industry demand for more efficient catalytic processes.

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.

Heterogeneously Catalyzed Reductive Depolymerization of Lignin to Value-Added Chemicals

Lignin is an abundant renewable source of aromatics, but its complex heterogeneous structure poses challenges for its depolymerization and valorization. Heterogeneously catalyzed reductive depolymerization (HCRD) has emerged as a promising approach, utilizing heterogeneous catalysts to facilitate selective bond cleavage in lignin and hydrogen transfer to stabilize the products under mild conditions. This review provides a comprehensive understanding of the hydrogen transfer mechanisms in HCRD, involving different hydrogen sources, including molecular hydrogen, alcohols, formic acid, etc., and the native hydrogen donor groups in lignin. The interaction between hydrogen sources and catalyst design is explored, emphasizing how catalyst characteristics must align with specific hydrogen transfer pathways to optimize efficiency and selectivity. Precious metal-based and non-precious metal-based catalysts are examined, highlighting advances that enhance hydrogen activation and transfer. Comparative analyses of hydrogen sources reveal distinct advantages and limitations. The significance of HCRD in lignin valorization and the development of integrated biorefineries is underscored, emphasizing its potential to contribute to a sustainable bioeconomy through improved process integration and economic viability.

Review of the application of bimetallic catalysts coupled with internal hydrogen donor for catalytic hydrogenolysis of lignin to produce phenolic fine chemicals

Lignocellulosic biomass contains lignin, an aromatic and oxygenated substance and a potential method for lignin utilization is achieved through catalytic conversion into useful phenolic and aromatic monomers. The application of monometallic catalysts for lignin hydrogenolysis reaction remains one of the major reasons for the underutilization of lignin to produce valuable chemicals. Monometallic catalysts have many limitations such as limited catalytic sites for interacting with different lignin linkages, poor catalytic activity, low lignin conversion, and low product selectivity. It is due to lack of synergy with other metallic catalysts that can enhance the catalytic activity, stability, selectivity, and overall catalytic performance. To overcome these limitations, works on the application of bimetallic catalysts that can offer higher activity, selectivity, and stability have been initiated. In this review, cutting-edge insights into the catalytic hydrogenolysis of lignin, focusing on the production of phenolic and aromatic monomers using bimetallic catalysts within an internal hydrogen donor solvent are discussed. The contribution of this work lies in a critical discussion of recent reported findings, in-depth analyses of reaction mechanisms, optimal conditions, and emerging trends in lignin catalytic hydrogenolysis. The specific effects of catalytic active components on the reaction outcomes are also explored. Additionally, this review extends beyond current knowledge, offering forward-looking suggestions for utilizing lignin as a raw material in the production of valuable products across various industrial processes. This work not only consolidates existing knowledge but also introduces novel perspectives, paving the way for future advancements in lignin utilization and catalytic processes.

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.

Bimetallic polyoxometalates catalysts for efficient lignin depolymerization: Unlocking valuable aromatic compounds from renewable feedstock

Lignin, a complex and abundant polymer present in lignocellulosic biomass, holds immense potential as a renewable source for the production of valuable aromatic compounds. However, the efficient depolymerization of lignin into these compounds remains a formidable challenge. Here, we present a promising solution by harnessing polyoxometalates (POMs) catalysts, which exhibit improved catalytic performance and selectivity. We synthesized a series of NixCoy@POMs catalysts (POMs: CsPW or CsPMo) and explored their application in the depolymerization of pine lignin, aiming to investigate the influence of different metal species and doping ratios of POMs on catalytic performance. Through meticulous optimization of reaction conditions, we achieved significant yields of valuable aromatic compounds, including methyl vanillate, vanillin, and 4-hydroxy-3-methoxyacetophenone. Furthermore, the Ni0.75Co0.75@CsPMo catalyst demonstrated exceptional efficacy in catalyzing the cracking process of C-C and/or C-O bonds in a β-O-4 dimer model compound. Notably, our catalyst exhibited outstanding stability over five cycles, underscoring its suitability as an effective heterogeneous catalyst for cyclic lignin depolymerization. This study sheds light on the potential of POMs-based catalysts for advancing lignin valorization and offers new avenues for sustainable biomass conversion into valuable chemicals.

Controllable recovery and recycling of carboxylic acid-polyalcohol deep eutectic solvent for biomass pretreatment with electronically-controlled chemical methodology

Pretreatment of lignocellulosic biomass using deep eutectic solvent (DES) has been demonstrated environmental and valid. Co-existing of donor and acceptor of hydrogen bond makes DES composition more complicated than traditional solvents, which limits their further scale-up utilization. Advances in biomass pretreatment using green solvent DES should excogitate efficient methodology for DES recycling. Electronically-controlled chemical methodology was first put forward to resolve recovery and recycling issue of DES lactic acid-ethylene glycol after biomass pretreatment. The methodology worked based on selectively migrating of lactate Lac^- and reserving of ethylene glycol using BP-A-BP bipolar membrane electrodialysis (BMED). Impact of primary factors on DES recovery was carefully studied. Lowest energy consumption for specific DES recovery reached 10.4 kw·h/kg and highest DES recovery rate approached 97.6 %. Cognition acquired from this research indicated a promising and efficient strategy for carboxylic acid-polyalcohol DES recovery with novel electronically-controlled chemical methodology.

High-efficiency recovery and regeneration of choline-polyol deep eutectic solvent for biorefinery via bipolar membrane electrodialysis and ultrafiltration

Deep eutectic solvents (DES) have been widely studied for interesting solvent properties in resources utilization and green conversion of energy. Complex composition of DES and lack of recovery techniques restricts their further scale-up application. Exploring efficient recovery and regeneration methodology of DES in biorefinery could be beneficial for low-carbon circular bioeconomy. Recovery and regeneration of choline-polyol DES (choline chloride-ethylene glycol, ChCl-EG) after biomass pretreatment was studied using bipolar membrane electrodialysis (BMED) with ultrafiltration (UF). UF-BMED treatment worked based on the interception of macromolecular degradation products in pretreatment liquor and regional recovery of Ch⁺, Cl^- with EG. Influence of major parameters on DES recovery performance was studied with emphasis. Maximum recovery ratio of DES reached 97.4% and minimum specific energy consumption of DES recovery approached 6.0 kW·h/kg. Cognition gained from this research revealed an efficient technique for DES recycling after biorefinery.