Atomically Dispersed Pt-N3C1 Sites Enabling Efficient and Selective Electrocatalytic C-C Bond Cleavage in Lignin Models under Ambient Conditions

Recent Progress in Electrocatalytic Conversion of Lignin: From Monomers, Dimers, to Raw Lignin

Lignin, as the second largest renewable biomass resource in nature, has increasingly received significant interest for its potential to be transformed into valuable chemicals, potentially contributing to carbon neutrality. Among different approaches, renewable electricity-driven biomass conversion holds great promise to substitute a petroleum resource-driven one, owing to its characteristics of environmental friendliness, high energy efficiency, and tunable reactivity. The challenges lie on the polymeric structure and complex functional groups in lignin, requiring the development of efficient electrocatalysts for lignin valorization with enhanced activity and selectivity toward targeted chemicals. In this Review, we focus on the advancement of electrocatalytic valorization of lignin, from monomers, to dimers and to raw lignin, toward various value-added chemicals, with emphasis on catalyst design, reaction innovation, and mechanistic study. The general strategies for catalyst design are also summarized, offering insights into enhancing the activity and selectivity. Finally, challenges and perspectives for the electrocatalytic conversion of lignin are proposed.

Multimetal synergy in an iron-cobalt-nickel hydroxide electrocatalyst for electro-oxidative lignin depolymerization to produce value-added aromatic chemicals

A ternary iron-cobalt-nickel hydroxide nanoarray catalyst was fabricated, which achieves enhanced performance towards electro-oxidative depolymerization of lignin models to produce benzoic acid and phenol.

Visible Light-Switchable Lattice Oxygen Sites for Selective C-H and C(O)-C Bond Electrooxidation

Lattice-oxygen is highly oxidizable, ideal for electrocatalytic C-H oxidation but insufficient alone for C(O)-C bond cleavage due to the non-removable nature of lattice sites. Here, we present a visible light-assisted electrochemical method of in situ formulating removable lattice-oxygen sites in a nickel-oxyhydroxide (ESE-NiOOH) electrocatalyst. This catalyst efficiently converts aromatic alcohols and carbonyls with C(O)-C fragments from lignin and plastics into benzoic acids (BAs) with high yields (83-99 %). Without light irradiation, ESE-NiOOH's intrinsic lattice-oxygen is non-removable and inert for C(O)-C bond cleavage. In situ characterizations show light-induced lattice-oxygen removal and regeneration via OH- refilling. Theoretical calculations identify the nucleophilic oxygen attack on ketone-derived carbanion as a rate-determining step, which can be remarkably facilitated by removable lattice-oxygen to activate α-C-H bonds. As a proof-of-concept, an "electrochemical funnel" strategy is developed for high-efficiency upgrading aromatic mixtures with C(O)-C moieties into BA with up to 94 % yield. This in situ removal-regeneration approach for lattice sites opens an avenue for the tailored design of interfacial electrocatalysts to selectively upcycle waste carbon sources into valuable products.

Mussel-Bioinspired Lignin Adhesive for Wearable Bioelectrodes

As a natural "binder," lignin fixes cellulose in plants to foster growth and longevity. However, isolated lignin has a poor binding ability, which limits its biomedical applications. In this study, inspired by mussel adhesive proteins, acidic/basic amino acids (AAs) are introduced in alkali lignin (AL) to form ionic-π/spatial correlation interactions, followed by demethylation to create catechol residues for enhanced adhesion activity. Atomic force microscopy reveals that catechol residues are the primary adhesion structures, with basic AAs exhibiting superior synergistic effects compared to acidic AAs. Demethylated lysine-grafted AL exhibits the strongest adhesion force toward skin tissue. Molecular dynamic simulation and density functional theory calculations indicate that adhesion against skin tissue mainly results from hydrogen bonds and cation-π interactions, with the adhesion mechanism being based on the Gibbs free energy of the Schiff base reaction. In summary, a biomimetic electrode based on lignin inspired by mussel adhesive proteins is prepared; the presented method offers a straightforward strategy for the development of biomimetic adhesives. Furthermore, this mussel-inspired adhesive can be used as a wearable bioelectrode in biomedical applications.

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

Optimizing the Activation Energy of Reactive Intermediates on Single-Atom Electrocatalysts: Challenges and Opportunities

Single-atom catalysts (SACs) have made great progress in recent years as potential catalysts for energy conversion and storage due to their unique properties, including maximum metal atoms utilization, high-quality activity, unique defined active sites, and sustained stability. Such advantages of single-atom catalysts significantly broaden their applications in various energy-conversion reactions. Given the extensive utilization of single-atom catalysts, methods and specific examples for improving the performance of single-atom catalysts in different reaction systems based on the Sabatier principle are highlighted and reactant binding energy volcano relationship curves are derived in non-homogeneous catalytic systems. The challenges and opportunities for single-atom catalysts in different reaction systems to improve their performance are also focused upon, including metal selection, coordination environments, and interaction with carriers. Finally, it is expected that this work may provide guidance for the design of high-performance single-atom catalysts in different reaction systems and thereby accelerate the rapid development of the targeted reaction.

Pt Single-Atoms on Structurally-Integrated 3D N-Doped Carbon Tubes Grid for Ampere-Level Current Density Hydrogen Evolution

To date, the excellent mass-catalytic activities of Pt single-atoms catalysts (Pt-SACs) toward hydrogen evolution reaction (HER) are categorically confirmed; however, their high current density performance remains a challenge for practical applications. Here, a binder-free approach is exemplified to fabricate self-standing superhydrophilic-superaerphobic Pt-SACs cathodes by directly anchoring Pt-SAs via Pt-NxC4-x coordination bonds to the structurally-integrated 3D nitrogen-doped carbon tubes (N-CTs) array grid (denoted as Pt@N-CTs). The 3D Pt@N-CTs cathode with optimal Pt-SACs loading is capable of operating at a high current density of 1000 mA cm-2 with an ultralow overpotential of 157.9 mV with remarkable long-term stability over 11 days at 500 mA cm-2. The 3D super-wettable free-standing Pt@N-CTs possess interconnected vertical and lateral N-CTs with hierarchical-sized open channels, which facilitates the mass transfer. The binder-free immobilization adding to the large surface area and 3D-interconnected open channels endow Pt@N-CTs cathodes with high accessible active sites, electrical conductivity, and structural stability that maximize the utilization efficiency of Pt-SAs to achieve ampere-level current density HER at low overpotentials.

Electrocatalytic functional group conversion-based carbon resource upgrading

The conversions of carbon resources, such as alcohols, aldehydes/ketones, and ethers, have been being one of the hottest topics most recently for the goal of carbon neutralization. The emerging electrocatalytic upgrading has been regarded as a promising strategy aiming to convert carbon resources into value-added chemicals. Although exciting progress has been made and reviewed recently in this area by mostly focusing on the explorations of valuable anodic oxidation or cathodic reduction reactions individually, however, the reaction rules of these reactions are still missing, and how to purposely find or rationally design novel but efficient reactions in batches is still challenging. The properties and transformations of key functional groups in substrate molecules play critically important roles in carbon resources conversion reactions, which have been paid more attention to and may offer hidden keys to achieve the above goal. In this review, the properties of functional groups are addressed and discussed in detail, and the reported electrocatalytic upgrading reactions are summarized in four categories based on the types of functional groups of carbon resources. Possible reaction pathways closely related to functional groups will be summarized from the aspects of activation, cleavage and formation of chemical bonds. The current challenges and future opportunities of electrocatalytic upgrading of carbon resources are discussed at the end of this review.

Heterogeneous Rhodium Single-Atom-Site Catalyst Enables Chemoselective Carbene N-H Bond Insertion

Transition-metal-catalyzed carbene insertion reactions of a nitrogen-hydrogen bond have emerged as robust and versatile methods for the construction of C-N bonds. While significant progress of homogeneous catalytic metal carbene N-H insertions has been achieved, the control of chemoselectivity in the field remains challenging due to the high electrophilicity of the metal carbene intermediates. Herein, we present an efficient strategy for the synthesis of a rhodium single-atom-site catalyst (Rh-SA) that incorporates a Rh atom surrounded by three nitrogen atoms and one phosphorus atom doped in a carbon support. This Rh-SA catalyst, with a catalyst loading of only 0.15 mol %, exhibited exceptional catalytic performance for heterogeneous carbene insertion with various anilines and heteroaryl amines in combination with diazo esters. Importantly, the heterogeneous catalyst selectively transformed aniline derivatives bearing multiple nucleophilic moieties into single N-H insertion isomers, while the popular homogeneous Rh2(OAc)4 catalyst produced a mixture of overfunctionalized side products. Additionally, similar selectivities for N-H bond insertion with a set of stereoelectronically diverse diazo esters were obtained, highlighting the general applicability of this heterogeneous catalysis approach. On the basis of density functional theory calculations, the observed selectivity of the Rh-SA catalyst was attributed to the insertion barriers and the accelerated proton transfer assisted by the phosphorus atom in the support. Overall, this investigation of heterogeneous metal-catalyzed carbene insertion underscores the potential of single-atom-site catalysis as a powerful and complementary tool in organic synthesis.

Corrosion Engineering of Part-Per-Million Single Atom Pt1/Ni(OH)2 Electrocatalyst for PET Upcycling at Ampere-Level Current Density

The plastic waste issue has posed a series of formidable challenges for the ecological environment and human health. While conventional recycling strategies often lead to plastic down-cycling, the electrochemical strategy of recovering valuable monomers enables an ideal, circular plastic economy. Here a corrosion synthesized single atom Pt1/Ni(OH)2 electrocatalyst with part-per-million noble Pt loading for highly efficient and selective upcycling of polyethylene terephthalate (PET) into valuable chemicals (potassium diformate and terephthalic acid) and green hydrogen is reported. Electro-oxidation of PET hydrolysate, ethylene glycol (EG), to formate is processed with high Faraday efficiency (FE) and selectivity (>90%) at the current density close to 1000 mA cm-2 (1.444 V vs RHE). The in situ spectroscopy and density functional theory calculations provide insights into the mechanism and the understanding of the high efficiency. Remarkably, the electro-oxidation of EG at the ampere-level current density is also successfully illustrated by using a membrane-electrode assembly with high FEs to formate integrated with hydrogen production for 500 h of continuous operation. This process allows valuable chemical production at high space-time yield and is highly profitable (588-700 $ ton-1 PET), showing an industrial perspective on single-atom catalysis of electrochemical plastic upcycling.

Selective (sono)photocatalytic cleavage of lignin-inspired β-O-4 linkages to phenolics by ultrasound derived 1-D titania nanomaterials

Catalytic conversion of lignin to value-added aromatic compounds is still an open challenge, since the selective cleavage of the linkages interconnecting the aromatic molecules, especially the β-O-4 ones, is not efficiently achieved yet. Herein, novel titania-based nanostructured materials were synthesized using low-power-low-frequency ultrasound that demonstrated high efficiency for the selective cleavage of Cα-Cβ bond of β-O-4 linkages of lignin-inspired model compounds. Going a step ahead, experiments of sonophotocatalytic valorization of 2-phenoxy-1-phenylethanol were contacted for the first time, where the exposure to ultrasound leading to better conversion and selectivity towards the desired products in the case of the novel ultrasound-synthesized nano-photocatalyst. Mechanistic insights showcased that photogenerated holes are the main active species in the catalytic process. In general, this research work provides a green, effective, and cost-effective approach for the selective and efficient catalytic lignin valorization.

Direct C-C Double Bond Cleavage of Alkenes Enabled by Highly Dispersed Cobalt Catalyst and Hydroxylamine

The utilization of a single-atom catalyst to break C-C bonds merges the merits of homogeneous and heterogeneous catalysis and presents an intriguing pathway for obtaining high-value-added products. Herein, a mild, selective, and sustainable oxidative cleavage of alkene to form oxime ether or nitrile was achieved by using atomically dispersed cobalt catalyst and hydroxylamine. Diversified substrate patterns, including symmetrical and unsymmetrical alkenes, di- and tri-substituted alkenes, and late-stage functionalization of complex alkenes were demonstrated. The reaction was successfully scaled up and demonstrated good performance in recycling experiments. The hot filtration test, catalyst poisoning and radical scavenger experiment, time kinetics, and studies on the reaction intermediate collectively pointed to a radical mechanism with cobalt/acid/O2 promoted C-C bond cleavage as the key step.

Electrocatalytic Lignin Valorization into Aromatic Products via Oxidative Cleavage of Cα-Cβ Bonds

Lignin is the most promising candidate for producing aromatic compounds from biomass. However, the challenge lies in the cleavage of C-C bonds between lignin monomers under mild conditions, as these bonds have high dissociation energy. Electrochemical oxidation, which allows for mild cleavage of C-C bonds, is considered an attractive solution. To achieve low-energy consumption in the valorization of lignin, the use of highly efficient electrocatalysts is essential. In this study, a meticulously designed catalyst consisting of cobalt-doped nickel (oxy)hydroxide on molybdenum disulfide heterojunction was developed. The presence of molybdenum in a high valence state promoted the adsorption of tert-butyl hydroperoxide, leading to the formation of critical radical intermediates. In addition, the incorporation of cobalt doping regulated the electronic structure of nickel, resulting in a lower energy barrier. As a result, the heterojunction catalyst demonstrated a selectivity of 85.36% for cleaving the Cα-Cβ bond in lignin model compound, achieving a substrate conversion of 93.69% under ambient conditions. In addition, the electrocatalyst depolymerized 49.82 wt% of soluble fractions from organosolv lignin (OL), resulting in a yield of up to 13 wt% of aromatic monomers. Significantly, the effectiveness of the prepared electrocatalyst was also demonstrated using industrial Kraft lignin (KL). Therefore, this research offers a practical approach for implementing electrocatalytic oxidation in lignin refining.

Constructing Fe-N4 Sites through Anion Exchange-mediated Transformation of Fe Coordination Environments in Hierarchical Carbon Support for Efficient Oxygen Reduction

Metal single atoms (SAs) anchored in carbon support via coordinating with N atoms are efficient active sites to oxygen reduction reaction (ORR). However, rational design of single atom catalysts with highly exposed active sites is challenging and urgently desirable. Herein, an anion exchange strategy is presented to fabricate Fe-N4 moieties anchored in hierarchical carbon nanoplates composed of hollow carbon spheres (Fe-SA/N-HCS). With the coordinating O atoms are substituted by N atoms, Fe SAs with Fe-O4 configuration are transformed into the ones with Fe-N4 configuration during the thermal activation process. Insights into the evolution of central atoms demonstrate that the SAs with specific coordination environment can be obtained by modulating in situ anion exchange process. The strategy produces a large quantity of electrochemical accessible site and high utilization rate of Fe-N4 . Fe-SA/N-HCS shows excellent ORR electrocatalytic performance with half-wave potential of 0.91 V (vs. RHE) in 0.1 M KOH, and outstanding performance when used in rechargeable aqueous and flexible Zn-air batteries. The evolution pathway for SAs demonstrated in this work offers a novel strategy to design SACs with various coordination environment and enhanced electrocatalytic activity.

New Insights into N-Doped Porous Carbons as Both Heterogeneous Catalysts and Catalyst Supports: Opportunities for the Catalytic Synthesis of Valuable Compounds

Among the vast class of porous carbon materials, N-doped porous carbons have emerged as promising materials in catalysis due to their unique properties. The introduction of nitrogen into the carbonaceous matrix can lead to the creation of new sites on the carbon surface, often associated with pyridinic or pyrrolic nitrogen functionalities, which can facilitate various catalytic reactions with increased selectivity. Furthermore, the presence of N dopants exerts a significant influence on the properties of the supported metal or metal oxide nanoparticles, including the metal dispersion, interactions between the metal and support, and stability of the metal nanoparticles. These effects play a crucial role in enhancing the catalytic performance of the N-doped carbon-supported catalysts. Thus, N-doped carbons and metals supported on N-doped carbons have been revealed to be interesting heterogeneous catalysts for relevant synthesis processes of valuable compounds. This review presents a concise overview of various methods employed to produce N-doped porous carbons with distinct structures, starting from diverse precursors, and showcases their potential in various catalytic processes, particularly in fine chemical synthesis.

Direct conversion of lignin to functionalized diaryl ethers via oxidative cross-coupling

Efficient valorization of lignin, a sustainable source of functionalized aromatic products, would reduce dependence on fossil-derived feedstocks. Oxidative depolymerization is frequently applied to lignin to generate phenolic monomers. However, due to the instability of phenolic intermediates, repolymerization and dearylation reactions lead to low selectivity and product yields. Here, a highly efficient strategy to extract the aromatic monomers from lignin affording functionalized diaryl ethers using oxidative cross-coupling reactions is described, which overcomes the limitations of oxidative methods and affords high-value specialty chemicals. Reaction of phenylboronic acids with lignin converts the reactive phenolic intermediates into stable diaryl ether products in near-theoretical maximum yields (92% for beech lignin and 95% for poplar lignin based on the content of β-O-4 linkages). This strategy suppresses side reactions typically encountered in oxidative depolymerization reactions of lignin and provides a new approach for the direct transformation of lignin into valuable functionalized diaryl ethers, including key intermediates in pharmaceutical and natural product synthesis.

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.

Simultaneous Oxidative Cleavage of Lignin and Reduction of Furfural via Efficient Electrocatalysis by P-Doped CoMoO4

Electrochemical oxidative lignin cleavage and coupled 2-furaldehyde reduction provide a promising approach for producing high-value added products. However, developing efficient bifunctional electrocatalysts with noble-metal-like activity still remains a challenge. Here, an efficient electrochemical strategy is reported for the selective oxidative cleavage of C_α -C_β bonds in lignin into aromatic monomers by tailoring the electronic structure through P-doped CoMoO₄ spinels (99% conversion, highest monomer selectivity of 56%). Additionally, the conversion and selectivity of 2-furaldehyde reduction to 2-methyl furan reach 87% and 73%, respectively. In situ Fourier transform infrared and density functional theory analysis reveal that an upward shift of the E_d upon P-doping leads to an increase in the antibonding level, which facilitates the C_α -C_β adsorption of the lignin model compounds, thereby enhancing the bifunctional electrocatalytic activity of the active site. This work explores the potential of a spinel as a bifunctional electrocatalyst for the oxidative cracking of lignin and the reductive conversion of small organic molecules to high-value added chemicals via P-anion modulation.

A Liquid-Liquid-Solid System to Manipulate the Cascade Reaction for Highly Selective Electrosynthesis of Aldehyde

Electrocatalytic synthesis of aldehydes from alcohols exhibits unique superiorities as a promising technology, in which cascade reactions are involved. However, the cascade reactions are severely limited by the low selectivity resulting from the peroxidation of aldehydes in a traditional liquid-solid system. Herein, we report a novel liquid-liquid-solid system to regulate the selectivity of benzyl alcohol electrooxidation. The selectivity of benzaldehyde increases 200-fold from 0.4 % to 80.4 % compared with the liquid-solid system at a high current density of 136 mA cm^-2 , which is the highest one up to date. In the tri-phase system, the benzaldehyde peroxidation is suppressed efficiently, with the conversion of benzaldehyde being decreased from 87.6 % to 3.8 %. The as-produced benzaldehyde can be in situ extracted to toluene phase and separated from the electrolyte to get purified benzaldehyde. This strategy provides an efficient way to efficiently enhance the selectivity of electrocatalytic cascade reactions.

Engineering Single-Atom Sites into Pore-Confined Nanospaces of Porphyrinic Metal-Organic Frameworks for the Highly Efficient Photocatalytic Hydrogen Evolution Reaction

As a type of heterogeneous catalyst expected for the maximum atom efficiency, a series of single-atom catalysts (SACs) containing spatially isolated metal single atoms (M-SAs) have been successfully prepared by confining M-SAs in the pore-nanospaces of porphyrinic metal-organic frameworks (MOFs). The prepared MOF composites of M-SAs@Pd-PCN-222-NH₂ (M = Pt, Ir, Au, and Ru) display exceptionally high and persistent efficiency in the photocatalytic hydrogen evolution reaction with a turnover number (TON) of up to 21713 in 32 h and a beginning/lasting turnover frequency (TOF) larger than 1200/600 h^-1 based on M-SAs under visible light irradiation (λ ≥ 420 nm). The photo-/electrochemical property studies and density functional theory calculations disclose that the close proximity of the catalytically active Pt-SAs to the Pd-porphyrin photosensitizers with the confinement and stabilization effect by chemical binding could accelerate electron-hole separation and charge transfer in pore-nanospaces, thus promoting the catalytic H₂ evolution reaction with lasting effectiveness.

Electro-Synthesis of Organic Compounds with Heterogeneous Catalysis

Electro-organic synthesis has attracted a lot of attention in pharmaceutical science, medicinal chemistry, and future industrial applications in energy storage and conversion. To date, there has not been a detailed review on electro-organic synthesis with the strategy of heterogeneous catalysis. In this review, the most recent advances in synthesizing value-added chemicals by heterogeneous catalysis are summarized. An overview of electrocatalytic oxidation and reduction processes as well as paired electrocatalysis is provided, and the anodic oxidation of alcohols (monohydric and polyhydric), aldehydes, and amines are discussed. This review also provides in-depth insight into the cathodic reduction of carboxylates, carbon dioxide, CC, C≡C, and reductive coupling reactions. Moreover, the electrocatalytic paired electro-synthesis methods, including parallel paired, sequential divergent paired, and convergent paired electrolysis, are summarized. Additionally, the strategies developed to achieve high electrosynthesis efficiency and the associated challenges are also addressed. It is believed that electro-organic synthesis is a promising direction of organic electrochemistry, offering numerous opportunities to develop new organic reaction methods.

Oxidative C−C bond cleavage of lignin via electrocatalysis

Lignin, which is an important component of biomass in nature and is constantly produced in industry, becomes potential raw material for sustainable production of fine chemicals and biofuels. Electrocatalysis has been extensively involved in the activation of simple molecules and cleavage-recasting of complex scaffolds in an elegant environment. As such, electrocatalytic cleavage of C−C(O) in β-O-4 model molecules of lignin to value-added chemicals has received much attention in recent years. This mini-review introduces various anodes (e.g., Pb, Pt, Ni, Co., and Ir) developed for electro-oxidative lignin degradation (EOLD) under mild conditions. Attention was placed to understand the conversion pathways and involved reaction mechanisms during EOLD, with emphasis on the product distribution caused by different electrodes.

New Insights into Green Protocols for Oxidative Depolymerization of Lignin and Lignin Model Compounds

Oxidative depolymerization of lignin is a hot topic in the field of biomass valorization. The most recent and green procedures have been herein detailed. Photochemical and electrochemical approaches are reviewed highlighting the pros and cons of each method. Mechanochemistry activated strategies are able to combine oxidation and depolymerization in the deconstruction of lignin. Homogenous and heterogeneous catalytic systems are exemplified stressing the green aspects associated with both the procedures. Solvent-free approaches as well as those carried out in alternative media are listed. Finally, the few examples of selenium catalyzed lignin valorization reported so far are cited.

High-Value Chemicals from Electrocatalytic Depolymerization of Lignin: Challenges and Opportunities

Lignocellulosic biomass is renewable and one of the most abundant sources for the production of high-value chemicals, materials, and fuels. It is of immense importance to develop new efficient technologies for the industrial production of chemicals by utilizing renewable resources. Lignocellulosic biomass can potentially replace fossil-based chemistries. The production of fuel and chemicals from lignin powered by renewable electricity under ambient temperatures and pressures enables a more sustainable way to obtain high-value chemicals. More specifically, in a sustainable biorefinery, it is essential to valorize lignin to enhance biomass transformation technology and increase the overall economy of the process. Strategies regarding electrocatalytic approaches as a way to valorize or depolymerize lignin have attracted significant interest from growing scientific communities over the recent decades. This review presents a comprehensive overview of the electrocatalytic methods for depolymerization of lignocellulosic biomass with an emphasis on untargeted depolymerization as well as the selective and targeted mild synthesis of high-value chemicals. Electrocatalytic cleavage of model compounds and further electrochemical upgrading of bio-oils are discussed. Finally, some insights into current challenges and limitations associated with this approach are also summarized.

Engineering Dual Single-Atom Sites on 2D Ultrathin N-doped Carbon Nanosheets Attaining Ultra-Low-Temperature Zinc-Air Battery

Herein, a novel dual single-atom catalyst comprising adjacent Fe-N₄ and Mn-N₄ sites on 2D ultrathin N-doped carbon nanosheets with porous structure (FeMn-DSAC) was constructed as the cathode for a flexible low-temperature Zn-air battery (ZAB). FeMn-DSAC exhibits remarkable bifunctional activities for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Control experiments and density functional theory calculations reveal that the catalytic activity arises from the cooperative effect of the Fe/Mn dual-sites aiding *OOH dissociation as well as the porous 2D nanosheet structure promoting active sits exposure and mass transfer during the reaction process. The excellent bifunctional activity of FeMn-DSAC enables the ZAB to operate efficiently at ultra-low temperature of -40 °C, delivering 30 mW cm^-2 peak power density and retaining up to 86 % specific capacity from the room temperature counterpart.

Al3+ Dopants Induced Mg2+ Vacancies Stabilizing Single-Atom Cu Catalyst for Efficient Free-Radical Hydrophosphinylation of Alkenes

Utilizing heterogeneous catalysts to overcome obstacles for homogeneous reactions is fascinating but very challenging owing to the difficult fabrication of such catalysts based on the character of target reactions. Herein, we report a Al³⁺ doping strategy to construct single-atom Cu on MgO nanosheets (Cu₁/MgO(Al)) for boosting the free-radical hydrophosphinylation of alkenes. Al³⁺ dopants in MgO bring about abundant Mg²⁺ vacancies for stabilizing dense independent Cu atoms (6.3 wt %), while aggregated Cu nanoparticles are formed without Al³⁺ dopants (Cu/MgO). Cu₁/MgO(Al) exhibits preeminent activity and durability in the hydrophosphinylation of various alkenes with great anti-Markovnikov selectivity (99%). The turnover frequency (TOF) value reaches up to 1272 h^-1, exceeding those of Cu/MgO by ∼6-fold and of traditional homogeneous catalysts drastically. Further experimental and theoretical studies disclose that the prominent performance of Cu₁/MgO(Al) derives from the accelerated initiating step of phosphinoyl radical triggered by individual Cu atoms.

Radical generation and fate control for photocatalytic biomass conversion

Photocatalysis is an emerging approach for sustainable chemical production from renewable biomass under mild conditions. Active radicals are always generated as key intermediates, in which their high reactivity renders them versatile for various upgrading processes. However, controlling their reaction is a challenge, especially in highly functionalized biomass frameworks. In this Review, we summarize recent advanced photocatalytic systems for selective biomass valorization, with an emphasis on their distinct radical-mediated reaction patterns. The strategies for generating a specific radical intermediate and controlling its subsequent conversion towards desired chemicals are also highlighted, aiming to provide guidance for future studies. We believe that taking full advantage of the unique reactivity of radical intermediates would provide great opportunities to develop more efficient photocatalytic systems for biomass valorization.

Nitrogen-Doped Carbon-Incarcerated Zinc Electrodes as Heterogeneous Catalysts for Electrochemical Allylation of Carbonyl Compounds

Electrochemical allylation reactions of carbonyl compounds using cathodes prepared from nitrogen-doped carbon (NDC)-incarcerated zinc catalysts have been developed. A range of aldehydes and ketones afforded the desired allylic alcohols in high yields with <10 mol % zinc leaching, and the heterogeneous nature of the active species was suggested. Compared with bulk zinc electrodes, NDC-stabilized zinc nanoparticle species were compatible with a broader range of heteroaromatic substrates and enabled the use of an undivided cell.

Electrocatalytic oxidative upgrading of biomass platform chemicals: from the aspect of reaction mechanism

Oxidation reactions provide a wide range of important chemicals in industry; however, most of these chemicals are produced from fossil feedstocks. As a candidate of oxygen evolution reaction (OER), the electrooxidation of biomass platform chemicals instead of a petroleum source offers a sustainable and atom-economic avenue toward organic oxygenates, with additional benefits when coupled with renewable electricity driven processes. This highlight article describes the representative examples in this nascent area, including oxidative dehydrogenation, coupling, and cleavage. We classify the examples into inner-sphere and outer-sphere electrode reactions based on the classical electrocatalysis concept for better understanding of the reaction mechanism. Moreover, we highlight the recent progress in oxidative biomass electrorefining via inner-sphere anodic reactions, which are strongly dependent on the nature of the electrode material. Particularly, the understanding of the formation of reactive oxygen species, adsorption of substrates, and reconstruction of anode materials is presented. Finally, the existing challenges and perspectives are discussed.

Synthesis of atomic form nickel co-catalysts on TiO2 for improved photocatalysis by RAFT technique

Using the high incompatibility between the neutral stabilizer body and the block polyelectrolyte to drive phase separation during polymerization-induced self-assembly, a modular approach to systematically adjust ionic monomer/polymer solubility was...

Organic amine mediated cleavage of Caromatic-Cα bonds in lignin and its platform molecules

The activation and cleavage of C-C bonds remains a critical scientific issue in many organic reactions and is an unmet challenge due to their intrinsic inertness and ubiquity. Meanwhile, it is crucial for the valorization of lignin into high-value chemicals. Here, we proposed a novel strategy to enhance the Caromatic-Cα bond cleavage by pre-functionalization with amine sources, in which an active amine intermediate is first formed through Markovnikov hydroamination to reduce the dissociation energy of the Caromatic-Cα bond which is then cleaved to form target chemicals. More importantly, this strategy provides a method to achieve the maximum utilization of the aromatic nucleus and side chains in lignin or its platform molecules. Phenols and N,N-dimethylethylamine compounds with high yields were produced from herbaceous lignin or the p-coumaric acid monomer in the presence of industrially available dimethylamine (DMA).

Isolated Single-Atom Ni-N5 Catalytic Site in Hollow Porous Carbon Capsules for Efficient Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries suffer from multiple complex and often interwoven issues, such as the low electronic conductivity of sulfur and Li₂S/Li₂S₂, shuttle effect, and sluggish electrochemical kinetics of lithium polysulfides (LiPSs). Guided by theoretical calculations, a multifunctional catalyst of isolated single-atom nickel in an optimal Ni-N₅ active moiety incorporated in hollow nitrogen-doped porous carbon (Ni-N₅/HNPC) is constructed and acts as an ideal host for a sulfur cathode. The host improved electrical conductivity, enhanced physical-chemical dual restricting capability toward LiPSs, and, more importantly, boosted the redox reaction kinetics by the Ni-N₅ active moiety. Therefore, the Ni-N₅/HNPC/S cathode exhibits superior rate performance, long-term cycling stability, and good areal capacity at high sulfur loading. This work highlights the important role of the coordination number of active centers in single-atom catalysts and provides a strategy to design a hollow nanoarchitecture with single-atom active sites for high-performance Li-S batteries.

Thermal Atomization of Platinum Nanoparticles into Single Atoms: An Effective Strategy for Engineering High-Performance Nanozymes

Although great progress has been made in artificial enzyme engineering, their catalytic performance is far from satisfactory as alternatives of natural enzymes. Here, we report a novel and efficient strategy to access high-performance nanozymes via direct atomization of platinum nanoparticles (Pt NPs) into single atoms by reversing the thermal sintering process. Atomization of Pt NPs into single atoms makes metal catalytic sites fully exposed and results in engineerable structural and electronic properties, thereby leading to dramatically enhanced enzymatic performance. As expected, the as-prepared thermally stable Pt single-atom nanozyme (Pt_TS-SAzyme) exhibited remarkable peroxidase-like catalytic activity and kinetics, far exceeding the Pt nanoparticle nanozyme. The following density functional theory calculations revealed that the engineered P and S atoms not only promote the atomization process from Pt NPs into Pt_TS-SAzyme but also endow single-atom Pt catalytic sites with a unique electronic structure owing to the electron donation of P atoms, as well as the electron acceptance of N and S atoms, which simultaneously contribute to the substantial enhancement of the enzyme-like catalytic performance of Pt_TS-SAzyme. This work demonstrates that thermal atomization of the metal nanoparticle-based nanozymes into single-atom nanozymes is an effective strategy for engineering high-performance nanozymes, which opens up a new way to rationally design and optimize artificial enzymes to mimic natural enzymes.

Phosphorus Induced Electron Localization of Single Iron Sites for Boosted CO2 Electroreduction Reaction

Electrochemical reduction of carbon dioxide (CO₂ ) into chemicals and fuels has recently attracted much interest, but normally suffers from a high overpotential and low selectivity. In this work, single P atoms were introduced into a N-doped carbon supported single Fe atom catalyst (Fe-SAC/NPC) mainly in the form of P-C bonds for CO₂ electroreduction to CO in an aqueous solution. This catalyst exhibited a CO Faradaic efficiency of ≈97 % at a low overpotential of 320 mV, and a Tafel slope of only 59 mV dec^-1 , comparable to state-of-the-art gold catalysts. Experimental analysis combined with DFT calculations suggested that single P atom in high coordination shells (n≥3), in particular the third coordination shell of Fe center enhanced the electronic localization of Fe, which improved the stabilization of the key *COOH intermediate on Fe, leading to superior CO₂ electrochemical reduction performance at low overpotentials.

Ternary Alloys Enable Efficient Production of Methoxylated Chemicals via Selective Electrocatalytic Hydrogenation of Lignin Monomers

We explore the selective electrocatalytic hydrogenation of lignin monomers to methoxylated chemicals, of particular interest, when powered by renewable electricity. Prior studies, while advancing the field rapidly, have so far lacked the needed selectivity: when hydrogenating lignin-derived methoxylated monomers to methoxylated cyclohexanes, the desired methoxy group (-OCH₃) has also been reduced. The ternary PtRhAu electrocatalysts developed herein selectively hydrogenate lignin monomers to methoxylated cyclohexanes-molecules with uses in pharmaceutics. Using X-ray absorption spectroscopy and in situ Raman spectroscopy, we find that Rh and Au modulate the electronic structure of Pt and that this modulating steers intermediate energetics on the electrocatalyst surface to facilitate the hydrogenation of lignin monomers and suppress C-OCH₃ bond cleavage. As a result, PtRhAu electrocatalysts achieve a record 58% faradaic efficiency (FE) toward 2-methoxycyclohexanol from the lignin monomer guaiacol at 200 mA cm^-2, representing a 1.9× advance in FE and a 4× increase in partial current density compared to the highest productivity prior reports. We demonstrate an integrated lignin biorefinery where wood-derived lignin monomers are selectively hydrogenated and funneled to methoxylated 2-methoxy-4-propylcyclohexanol using PtRhAu electrocatalysts. This work offers an opportunity for the sustainable electrocatalytic synthesis of methoxylated pharmaceuticals from renewable biomass.

Polyoxometalate-Based Metal-Organic Framework as Molecular Sieve for Highly Selective Semi-Hydrogenation of Acetylene on Isolated Single Pd Atom Sites

Achieving highly selective acetylene semi-hydrogenation in an ethylene-rich gas stream is of great industrial importance. Herein, we construct isolated single Pd atom in a polyoxometalate-based metal-organic framework (POMOF). The unique internal environment allows this POMOF to separate acetylene from acetylene/ethylene gas mixtures and confine it close to the single Pd atom. After semi-hydrogenation, the resulting ethylene is preferentially discharged from the pores, achieving a selectivity of 92.6 %. First-principles simulations reveal that the adsorbed acetylene/ethylene molecules form hydrogen bond networks with oxygen atoms of SiW₁₂ O₄₀ ^4- and create dynamic confinement regions, which preferentially release the produced ethylene. Besides, at the Pd site, the over-hydrogenation of ethylene exhibits a higher reaction energy barrier than the semi-hydrogenation of acetylene. The combined advantages of POMOF and single Pd atom provides an effective approach for the regulation of semi-hydrogenation selectivity.