Activating molecular oxygen with Au/CeO2 for the conversion of lignin model compounds and organosolv lignin

Catalytic depolymerization of lignin by N/S modified bagasse-based hierarchical porous carbon

Depolymerization of lignin, a complex aromatic biopolymer, presents a significant challenge in biomass conversion. While metal-based catalysts have been extensively investigated for lignin oxidative degradation, hierarchical porous carbon catalysts remain relatively underexplored. In this study, we introduce a novel approach employing nitrogen and sulfur co-doped hierarchical porous carbon (NSHPC), synthesized from sugarcane bagasse, calcium chloride (CaCl₂), and thiourea at a mass ratio of 1:2:2. The mixture underwent thermal treatment at 85 °C for 12 h, followed by drying and calcination under nitrogen atmosphere at 800 °C for 2 h. Subsequently, impurities were removed via dilute hydrochloric acid treatment. The resultant NSHPC catalyst effectively depolymerized lignin, specifically cleaving Cα-Cβ and Cβ-O bonds, at 140 °C for 4 h in methanol under 1 MPa oxygen pressure, achieving a high conversion rate of 98.7 % for lignin model compounds. The yields of key products were as follows: 67.8 % for phenol, 25.9 % for benzoic acid, and 33.2 % for methyl benzoate. Mechanistic investigations revealed that NSHPC facilitates the generation of superoxide anions from oxygen and active hydrogen species from methanol, which are crucial for bond depolymerization. Furthermore, experimental validation using authentic lignin samples corroborated the potential of NSHPC as an efficient catalyst for lignin depolymerization.

Spent NCM111 Cathode Material as a Catalyst for Oxidative Cleavage of β-O-4 Linkage in Lignin

The recovery and reuse of electrode materials of spent lithium-ion batteries (LIBs) are important for the sustainable development of the LIB industry. Herein, NCM111 (LiNi0.33Co0.33Mn0.33O2) cathode material from spent LIBs is recovered and its catalytic activity for the oxidative cleavage of β-O-4 linkages in model compounds and organosolv lignin is explored. The spent NCM111 is rich in oxygen vacancies (OVs) accumulated during the long-term charge-discharge cycling. The reactive oxygen species trapping experiments and density functional theroy (DFT) calculation indicate that the abundant OVs can adsorb and activate the oxygen molecules, which afford the NCM111 with the catalytic activity. It is found that besides the catalytic activity in oxidative cleavage of the β-O-4 linkage in lignin model compounds, the spent NCM111 can also catalyze the depolymerization of organosolv lignin, yielding 17.5% aromatics, such as vanillin, benzoic acid, and phthalic acid, indicating the potential economic value of spent NCM materials.

Enhancement of Selective Catalytic Oxidation of Lignin β-O-4 Bond via Orbital Modulation and Surface Lattice Reconstruction

The orbital modulation and surface lattice reconstruction represent an effective strategy to regulate the interaction between catalyst interface sites and intermediates, thereby enhancing catalytic activity and selectivity. In this study, the crystal surface of Au-K/CeO2 catalyst can undergo reversible transformation by tuning the coordination environment of Ce, which enables the activation of the Cβ-H bond and the oxidative cleavage of the Cβ-O and Cα-Cβ bonds, leading to the cleavage of 2-phenoxy-1-phenylethanol. The t2g orbitals of Au 5d hybridize with the 2p orbitals of lattice oxygen in CeO2 via π-coordination, modulating the coordination environment of Ce 4 f and reconstructing the lattice oxygen in the CeO2 framework, as well as increasing the oxygen vacancies. The interface sites formed by the synergy between Au clusters in the reconstructed Ce-OL1-Au structure and doped K play dual roles. On the one hand, it activates the Cβ-H bond, facilitating the enolization of the pre-oxidized 2-phenoxy-1-phenylethanone. On the other hand, through single-electron transfer involving Ce3+ 4f1 and the adsorption by oxygen vacancies, it enhances the oxidative cleavage of the Cβ-O and Cα-Cβ bonds. This study elucidates the complex mechanistic roles of the structure and properties of Au-K/CeO2 catalyst in the selective catalytic oxidation of lignin β-O-4 bond.

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.

Full conversion of lignocellulose using polyoxometalate catalysts with redox sites and antagonistic acidity/basicity

The full utilization of lignocellulose involves two distinct catalytic routes: i) oxidative depolymerization of lignin and ii) acid/alkaline hydrolysis of hemicellulose and cellulose. To improve efficiency and reduce costs, constructing a single-cluster catalyst represents a desirable yet challenging strategy. Herein, triple-functional molecular polyoxometalates (POMs), NLLnH6-nV2Mo18O62 (n = 1-6) were fabricated using N-lauroyl-l-lysine (NLL) and H6V2Mo18O62 as precursors. Besides its amphiphilicity to form nano-micelles with polyanion uniformly dispersed outside and NLL inside, NLL also provided basic sites to H+/redox POMs to compensate the loss of acidity and enabled spatial separation of antagonistic acid/base sites within a single POM molecule. Density Functional Theory, Molecular Dynamics simulations and experiments were employed to analyze these processes. The adsorption of -OH in 2-phenoxy-1-phenylethanol (pp-ol) was achieved by interacting with polyanion and extra with NH and C = O groups in NLL. These synergistic effects resulted in concentrating and confining pp-ol and reactive oxygen species around polyanion, which turnover frequency increased by 0.066 h-1 compared to homogeneous H6V2Mo18O62. Full conversion of various soft and hard lignocellulose was achieved using NLLH5V2Mo18O62 catalyst under gradually increasing temperature. During the conversion process, the lignin was oxidized mainly through β-O-4 bond cleavage without addition of NaOH, and the degradations of hemicellulose and cellulose were realized through acidic hydrolysis. The characteristics of this triple POMs allowed it to show higher activity than homogeneous H6V2Mo18O62 and previous BetH5V2Mo18O62 (Bet, i.e. betaine), which provided an alternative to developing new surfactant-type POMs in biomass conversion. The temperature-controlled properties in NLLH5V2Mo18O62 allowed easy separation and regeneration.

Benzenoid Aromatics from Renewable Resources

In this Review, all known chemical methods for the conversion of renewable resources into benzenoid aromatics are summarized. The raw materials that were taken into consideration are CO2; lignocellulose and its constituents cellulose, hemicellulose, and lignin; carbohydrates, mostly glucose, fructose, and xylose; chitin; fats and oils; terpenes; and materials that are easily obtained via fermentation, such as biogas, bioethanol, acetone, and many more. There are roughly two directions. One much used method is catalytic fast pyrolysis carried out at high temperatures (between 300 and 700 °C depending on the raw material), which leads to the formation of biochar; gases, such as CO, CO2, H2, and CH4; and an oil which is a mixture of hydrocarbons, mostly aromatics. The carbon selectivities of this method can be reasonably high when defined small molecules such as methanol or hexane are used but are rather low when highly oxygenated compounds such as lignocellulose are used. The other direction is largely based on the multistep conversion of platform chemicals obtained from lignocellulose, cellulose, or sugars and a limited number of fats and terpenes. Much research has focused on furan compounds such as furfural, 5-hydroxymethylfurfural, and 5-chloromethylfurfural. The conversion of lignocellulose to xylene via 5-chloromethylfurfural and dimethylfuran has led to the construction of two large-scale plants, one of which has been operational since 2023.

Photocatalyzed Oxidative Tandem Reaction Mediated by Bipyridinium for Multifunctional Derivatization of Alcohols

The multifunctional derivatization of alcohols has been achieved by the bipyridinium-based conjugated small molecule photocatalysts with redox center and Lewis acid site. Besides exhibiting high activity in the selective generation of aldehydes/ketones, acids from alcohols through solvent modulation, this system renders the first selective synthesis of esters via an attractive cross-coupling pattern, whose reaction route is significantly different from the traditional condensation of alcohols and acids or esterification from hemiacetals. Following the oxidization of alcohol to aldehyde via bipyridinium-mediated electron and energy transfer, the Lewis acid site of bipyridinium then activates the aldehyde and methanol to obtain the acetal, which further reacts with methanol to generate ester. This method not only demonstrates a clear advantage of bipyridinium in diverse catalytic activities, but also paves the way for designing efficient multifunctional small molecule photocatalysts. This metal- and additive-free photocatalytic esterification reaction marks a significant advancement towards a more environmentally friendly, cost-effective and green sustainable approach, attributed to the utilization of renewable substrate alcohol and the abundant, low-cost air as the oxidant. The mildness of this esterification reaction condition provides a more suitable alternative for large-scale industrial production of esters.

Oxidative Cleavage of β-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst

The cleavage of β-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of β-O-4 linkage in lignin model compounds with O₂ as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-N_x species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

Decarboxylation of p-Coumaric Acid during Pyrolysis on the Nanoceria Surface

Temperature-programmed desorption mass spectrometry (TPD MS) was used to study the pyrolysis of p-coumaric acid (pCmA) on the nanoceria surface. The interaction of pCmA with the CeO2 surface was investigated by FT-IR spectroscopy. The obtained data indicated the formation on the nanoceria surface of bidentate carboxylate complexes with chelate (Δν = 62 cm−1) and bridge structure (Δν = 146 cm−1). The thermal decomposition of pCmA over nanoceria occurred in several stages, mainly by decarboxylation. The main decomposition product is 4-vinylphenol (m/z 120). The obtained data can be useful for studying the mechanisms of catalytic thermal transformations of lignin-containing raw materials using catalysts containing cerium oxide and the development of effective technologies for the isolation of pCmA from lignin.

Catalytic Pyrolysis of Lignin Model Compounds (Pyrocatechol, Guaiacol, Vanillic and Ferulic Acids) over Nanoceria Catalyst for Biomass Conversion

Understanding the mechanisms of thermal transformations of model lignin compounds (MLC) over nanoscale catalysts is important for improving the technologic processes occurring in the pyrolytic conversion of lignocellulose biomass into biofuels and value-added chemicals. Herein, we investigate catalytic pyrolysis of MLC (pyrocatechol (P), guaiacol (G), ferulic (FA), and vanillic acids (VA)) over nanoceria using FT-IR spectroscopy, temperature-programmed desorption mass spectrometry (TPD MS), and thermogravimetric analysis (DTG/DTA/TG). FT-IR spectroscopic studies indicate that the active groups of aromatic rings of P, G, VA, and FA as well as carboxylate groups of VA and FA are involved in the interaction with nanoceria surface. We explore the general transformation mechanisms of different surface complexes and identify their decomposition products. We demonstrate that decomposition of carboxylate acid complexes occurs by decarboxylation. When FA is used as a precursor, this reaction generates 4-vinylguaiacol. Complexes of VA and FA formed through both active groups of the aromatic ring and decompose on the CeO2 surface to generate hydroxybenzene. The formation of alkylated products accompanies catalytic pyrolysis of acids due to processes of transalkylation on the surface.