Recent progress in single atom catalysts for biomass conversion
0 Abstract
Biomass, as an emerging environmental and renewable resource derived from various green sources, is increasingly being recognized for its inherent structural characteristics that are conducive to the production of high-value chemicals. The complicated structure and oxygen-rich nature of biomass and its derivatives necessitate high-value targeted conversion, typically involving liquid-phase hydrogenation or the addition of acidic or basic environments. These processes impose stringent requirements on the efficiency and durability of the catalysts. Recently, the innovation of single atom catalysts (SACs) with individual catalytic sites dispersed on various supports has demonstrated high catalytic efficiency and good selectivity. Owing to their high atomic utilization and
Keywords
INTRODUCTION
Over the last decades, traditional fossil and coal energy have dominated as the primary energy sources in the global market. With the exhaustion of fossil fuels and escalating environmental pollution, it is particularly important to explore sustainable and environmentally friendly alternative energy sources. Biomass, one of the most abundant resources in the biosphere, is the only renewable carbon-based material and an ideal substitute for traditional fossil resources[1,2]. It is postulated that the abundance of heteroatoms (such as oxygen) is linked to carbon atoms in biomass feedstocks. For fine chemicals and chemical intermediates, biomass energy is more suitable than fossil energy sources because of the abundance of C-O bonds, especially organic oxides. Therefore, the transformation of biomass resources into platform compounds for fuels and products, whether in a direct or indirect manner, offers a potentially lucrative value-added strategy.
Lignocellulose, a form of plant-derived biomass, makes up more than 90% of plant feedstocks and stands as the most plentiful carbon source that can be renewed[3,4]. Lignocellulosic biomass, a functionalized biopolymer, mainly includes three macromolecules: cellulose, hemicellulose, and lignin with weight fractions of 40%-60%, 10%-40% and 15%-30%, and minor components such as starch, protein, and xylan[5,6]. Cellulose, as the main component of lignocellulose, consists of up to 10,000 glucose units connected by
Therefore, more efficient and durable catalysts are selected to increase the selectivity during the upgrading process. Similar to the catalyst selection for fossil feedstocks, heterogeneous catalysts have gained increasing popularity in the field of biomass utilization. Metals are considered as the core component of heterogeneous catalysts[15], which were broadly used in the form of nanoparticles (NPs) during the past decades. The key advantage of metal NPs is the flexible parameters that can be adjusted to adapt to reaction requirements. For example, the size of Ni particles presents considerable influence on the hydrodeoxygenation of m-cresol as the reducing Ni particle size from 9 nm to 1nm, the direct deoxygenation rate increased by a factor of 10[16]. Recently, Lim et al. applied a modular raspberry-colloid-templating technique to regulate the interparticle distance of PdAu alloy NPs; the result indicated a substantial increase in the selectivity of the hydrogenation of benzaldehyde as the distance increased from 12 to 21 nm[17]. However, because of various factors, such as the dimensions of metal particles, carrier morphology, etc., the regulation of catalytic parameters is a complex project. Meanwhile, only the surface atoms in NPs can be utilized, while the atom inside of the particle does not directly contribute to catalytic function, which greatly violates the principle of atom economy. Minimizing the size of NPs to a single atom provides the potential to solve the current problems.
In 2011, Zhang and his group successfully loaded single-atom Pt on FeOx supports and presented the terminology of single atom catalysts (SACs) to define catalysis assisted by metal atoms[18,19]. Compared to NPs or nanoclusters, SACs only have metal-support interaction instead of metal-metal interaction[20]. Afterward, the new emerging SAC material exhibits significant advantages and potential in diverse catalytic domains such as energy conversion, environmental protection, organic synthesis, and electrochemical catalysis. The unique atomic-level dispersion and high catalytic efficiency of SACs enable them to address challenges in energy conversion, environmental protection, organic synthesis, and electrochemical catalysis. These advancements not only enhance our understanding of catalytic mechanisms but also pave the way for the development of more efficient and sustainable catalytic processes. The advancement of technology has led to the observation of evolving modes of existence for metal active sites on SACs. When the metal dispersion is 100% complete, it can be considered a single site. At this juncture, the carrier may be loaded with one or more metals. While the majority of studies have employed a single metal, there have been instances where three metal monoatoms have been loaded onto the same carrier, as documented in the literature[21]. In some instances, metals are present on the same carrier as both single atoms and NP, which we consider to be dual-sites. These metals typically play different catalytic roles, facilitating more complex reactions. Additionally, alloy sites can be created through the loading of single atoms of one metal onto another. All three forms have been used in biomass catalysis. However, the primary challenge in preparing SACs lies in the precise manipulation of the geometric and electronic structures of the catalytically active metallic sites. Biomass feedstock has diverse functional groups (e.g., C=O, C=C, C-OH), adjusting the position (corner, edge, etc.) of individual metal atoms or the pore size of the support will alter the selectivity in the production of the target product. Furthermore, metal-ligand coordination dictates the adsorption of crucial intermediates, playing a vital role in the overall reaction process[4]. Furthermore, the individual metallic atoms of SAC can be balanced on the support by diverse unsaturated chemical sites or spatial restriction, which not only maximizes the atomic economy, but also has the equity stability of a homogeneous catalyst.
Over the past decade, SACs have gained prominence across various fields, including thermal catalytic[4,22-24], electrocatalytic[25-27], and photocatalytic[28] processes. A review of the literature shows that SACs are making great strides in processes such as carbon dioxide (CO2) reduction[29-32], O2 reduction[33,34] and the oxygen evolution reaction[35,36]. Recently, with the renewed popularity of biomass resources, the field of biomass has witnessed the outstanding performance of SACs. In this context, much related work has been reported. Several review articles on this topic have been published recently. Mondelli et al. summarized the effect of metal-based solid catalysts on biomass transformation, with a concentration on the comparison of the reaction mechanism and catalytic impact of NPs and SACs[15]. De et al. reviewed the recent research in biomass conversation of SACs, and all reports were categorized in the way of response types. Notably, the review concludes with insights into the design and application of SACs for industrial-scale biomass conversion[37]. Very recently, Chen et al. provided a comprehensive overview of the preparation and characterization of SACs, along with a detailed account of their catalytic applications in biomass conversion[38].
This review offers a comprehensive overview on SACs employed in the chemocatalysis valorization of biomass, organized into three sections based on the type of catalysis (thermocatalytic, electrocatalytic and photocatalytic). Within each section, detailed discussions on platform molecules and reaction types are provided. The classification also highlights the distinct roles of various metal atoms in similar reactions and the interactions between different carriers and metals. Additionally, the review delves into the adsorption mode of catalysis and investigates the underlying reaction mechanisms. Recommendations are made for more efficient laboratory applications and large-scale industrial production. Finally, a vision and outlook for the current state of catalyzing the biomass landscape is presented.
THE APPLICATIONS OF SACs IN BIOMASS CONVERSATION
Thermocatalysis
Over the past decades, a simple and primitive method of pyrolysis has been widely used for biomass resources. Biomass pyrolysis involves thermochemically converting biomass in an inert environment, typically established by introducing inert gases or creating a vacuum, under elevated temperature conditions, typically ranging from 400 °C to 550 °C[39]. One of the main products of this process is bio-oil, which tends to have a complex composition and high oxygen content-disadvantages that hinder its further use. On this basis, catalysts have been introduced to overcome the problems of harsh reaction conditions and low selectivity of reaction products. In fact, thermal catalysis is the most widely used catalytic method in biomass conversation, and it is also the same in SACs. In this relatively mature approach to catalysis, research has increasingly focused on the design of structurally unique catalytic structures and the exploration of catalytic mechanisms. This sector will provide an all-inclusive overview of the role of SACs in biomass thermocatalysis, with a specific focus on classifying recent studies according to the nature of the reaction feedstock (cellulose, hemicellulose, lignin, etc.).
Cellulose and its derivatives
Compared to other biomass components, cellulose and its derivatives offer distinct advantages due to their simpler chemical structure and more accessible reaction properties, rendering them preferable for applications in the biorefinery industry. For example, the dehydration of fructose obtained through glucose isomerization can yield 5-hydroxymethylfurfural (HMF). In the utilization of cellulose, HMF and its subsequent derivatives are recognized as crucial and widely investigated platform molecules. This is due to their potential for transformation into a range of value-added chemicals through diverse pathways. As one of the three primary components, recent studies on SACs have centered on the conversion of small molecule compounds downstream of cellulose. Despite the efforts made to achieve the one-pot conversion of cellulose, the reactions involved present considerable challenges.
The conversation of HMF
HMF serves as a pivotal based compound in biomass utilization, acting as a connection between biomass resources, such as carbohydrates, and industrial products, including fossil fuels[12]. This compound possesses highly reactive properties due to the existence of C=C, C=O, and furan ring chemical groups, which can be directly transformed into various economically valuable platform compounds [e.g., 2,5-dimethylfuran (DMF), 2,5-furandicarboxylic acid (FDCA), Levulinic acid (LA), etc.] through many reactions including hydrogenation, oxidation, and hydrodeoxygenation. In the selection of catalysts, a shift from noble metals NPs to SACs has been experienced in order to overcome disadvantages such as low selectivity, harsh reaction conditions and complicated separation[15,39]. For instance, in 2017, non-noble Ni-Co/C catalyst achieved high yield conversion in HMF hydrogenolysis[40]. The following year, supported Pt-Co bimetallic NPs achieved a high yield of DMF (> 90%) under moderate conditions in the same reaction[41]. Nonetheless, the previously mentioned catalysts exhibit several drawbacks, including high noble metal loadings, inefficient utilization of noble metal atoms, and a scarcity of exposed active sites[42]. As a result, scientists have started to search for a balance between atomic utilization and catalytic efficiency.
In recent years, researchers have reduced the scale of metal particles to single atoms, as compared to their relevant NP-based catalyst systems. For example, the hydrodeoxygenation of HMF can be converted to DMF and other derivatives such as HMF, 5-methylfurfural (MF), 2,5-dihydroxymethylfuran (DHMF), DMF, 5-methylfurfuryl alcohol (MFA), etc., as shown in Scheme 1. It should be noted that DMF generated from the hydrodeoxygenation of HMF is a key precursor to yield sustainable liquid fuels. In 2020, Gan et al. introduced a simple, environmentally friendly, and scalable ball milling technique for synthesizing
Figure 1. (A) Schematic illustration of preparation of Pt1/Co SAA catalysts using ball milling; (B) Product distribution of the HMF hydrogenolysis as a function of reaction time over (up) Pt1/Co SAA and (down) Co nanocrystals catalysts; (C) Main route for the hydrogenolysis of HMF to DMF[42]. Copyright 2020 American Chemical Society. SAA: Single-atom alloy; HMF: 5-hydroxymethylfurfural; DMF: 2,5-dimethylfuran.
Scheme 1. Hydrodeoxidation of HMF to DMF and other byproducts[42]. Copyright 2020 American Chemical Society. HMF:
Transfer hydrogenation (TH) of HMF is also a visionary study, one of the challenges being that the C=O bond is easier to break than the C-OH bond. In 2021, Li et al. reported the highly selective generation of MF using hydrogen as a reducing agent. They chose oxygen defective Nb2O5 as the carrier and loaded single metal atoms (Pt1, Pd1, Au1) onto the it. It was found the interaction of the Pt and Nb sites at the catalyst interface, where the metal and the carrier are in contact with each other. Responsible for H2 activation,
Oxidation products, FDCA and 2,5-diformylfuran (DFF) of HMF, are precursors for the polymer industry and byproducts in the transformation of lipid acids to biodiesel, transforming them into upgrades products. Afterwards, there are examples reported of a Co-based double-site catalyst. 5-hydroxymethyl furfural oxidation reaction (HMFOR) is a tandem reaction that proceeds in two steps: the transformation of an aldehyde into a carboxylic acid and the intermediate conversion of a hydroxyl group into an aldehyde. A single active site showed different activities in the two oxidation reactions, resulting in suboptimal catalytic performance for converting HMF to FDCA. Hence, composite catalysts with dual active sites could offer a promising solution for complex tandem catalysis. Transmission electron microscopy (TEM) exposed that annealing resulted in the formation of numerous Co-loaded carbon nanotubes (CNTs) on the surface of
Figure 2. (A, B) TEM images of (Co1→Cop)/N-CNTs; (C) HAADF-STEM image of (Co1→Cop)/N-CNTs in the nanoparticle free region, where single Co atoms are clearly seen; (D) Schematic Illustration for the Synthesis of (Co1→Cop)/N-CNTs from ZnCo-ZIF; (E) Schematic Illustration of HMF Conversion to FDCA Accelerated by Different Reactive Sites (Co1 and Cop); (F) Reaction Routes for the Aerobic Oxidation of HMF to FDCA[46]. Copyright 2022 American Chemical Society. TEM: Transmission electron microscopy; CNTs: carbon nanotubes; HAADF-STEM: high-angle annular dark-field scanning TEM; HMF: 5-hydroxymethylfurfural; FDCA:
Meanwhile, a type of research uses the downstream products of HMF as the substrate. The earliest study was that in 2016, Zhang et al. first reported γ-AlOOH nanosheets with dispersed Pd species on them. Even with a very low Pd loading, 0.1Pd/γ-AlOOH exhibits a catalytic activity that achieves 50.3% succinic acid conversion in 4 hours[48]. Very recently, Liu et al. reported a fabricate a highly polar Pd-N3 site through the special Pd-based catalysts (Pd1/BNC). Combining single metal atoms with highly polarized boron nitrogen doped carbon (BNC) carriers and creating new atomically linked active centers is a new supramolecular controlled pyrolysis strategy. This Pd1/BNC catalyst enables the full-spectrum amination of biomass-derived aldehydes/ketones, a green and environmentally friendly way of producing aromatic and furanic amines[49]. The hydrogenation of biomass maleic anhydride (MAH) has been studied relatively infrequently. In August of 2023, Sun et al. anchored Pt single atoms to Mo vacancies at the edge of specific 1T-phase MoS2 creating a unique pocket of active sites. This Pt1/1T-MoS2 SAC could adjust the adsorption and hydrogen dissociation, showing a complete selectivity of MAH to succinic anhydride under mild conditions[50].
The conversation of LA
LA is a crucial intermediate platform compound of biomass that can be produced through the conversation of HMF under acidic conditions. Recently, there has been significant research concentreated on the catalytic conversion of LA using various catalysts. LA hydrotreating to γ-valerolactone (GVL) has always been the subject of much enthusiasm, due to its great potential for high-density fuels and biobased polymers. In 2021, Han et al. reported about the hydrogenating LA to GVL in solvent-free conditions[51]. The catalyst was synthesized using notched-polyoxometalate (N-POM) to connect Ru atoms and prevent Ru from aggregating during pyrolysis. As demonstrated in Figure 3A, SiO2 was used as a template and an anion exchange strategy was used to move POM-containing Ru ions into a pre-synthesized anionic polymer covered with SiO2. Pyrolysis, followed by etching to remove the SiO2 template, yielded the Ru1@WOx/CN catalyst. Some related commercial catalysts such as the Ru1/CN catalyst, commercial Ru/C, and Ru1@WO3 were also prepared and explored for comparison [Figure 3B]. The Ru1@WOx/CN catalyst exhibits an outstanding catalytic performance (almost complete conversion and selectivity). In turn, a case reported a range of Ru-Co SAA catalysts. It efficiently hydrogenates LA under aqueous phase conditions, evaluated to the presence of better electron-abundant Ru atoms. Ru, at the atomic level, enhances LA/H2 adsorption, H2 dissociation and C=O hydrogenation in LA by lowering the reaction energy barriers compared to the same pure metal[52].
Figure 3. (A) Schematic illustration of the synthesis of Ru1@WOx/CN; (B) The conversion (%) and the overall turnover frequency of LA to GVL for the Ru1@WOx/CN (Label A in the figure), and other commercial catalysts(left). The recycling text of the Ru1@WOx/CN catalyst[51]. Copyright 2021 American Chemical Society; (C) Screening of different catalysts for LA reductive amination[53]. Copyright 2022 Elsevier. LA: Levulinic acid; GVL: γ-valerolactone.
In June 2022, Gao et al. proposed a reaction method for the targeted reductive amination of LA and related keto acids. This atomically dispersed cobalt-based SAC catalyst exhibits remarkable activity and excellent substrate breadth compared to its Co NP counterpart. Various isoindolinones were synthesized from aromatic keto acids and amine/nitro compounds using the optimal catalyst system [Figure 3C][53].
One-pot conversion of cellulose
A highly attractive approach for cellulose is its straightforward conversion into valuable molecules. In 2008, before the SAC concept was born, Doc. Zhang and his group reported that a small amount of nickel could promote the carbon supported tungsten carbide (W2C/AC) catalyst system for cellulose conversion into polyols, especially ethylene glycol and sorbitol[54]. Later, the same group reported a Pt-Cu/SiO2 SAA catalyst, for ethanol production from cellulose. Platinum NPs avoid the C-C bond cleavage side-reaction that is usually induced by the native form of platinum. Consequently, methyl glycolate can be converted into ethanol at lower temperatures, significantly enhancing ethanol selectivity[55]. Ordinary SACs usually face problems such as atomic aggregation, high-temperature intolerance, etc. Ni metal is unstable under acidic conditions, but the hydrogenation process inevitably encounters acidic conditions. To solve those problems, Liu et al. developed a stable Ni-N-C SAC with an actual metal use content of 7.5 wt%. The catalyst exhibited outstanding catalytic activity and durability for the one-pot conversion of cellulose under extremely harsh conditions (245 oC, 60 bar H2, presence of tungstic acid in hot water). Extensive spectroscopic characterization and computational modeling revealed that (Ni-N4)-N is the active center of the catalyst and that covalent bonding between N and Ni atoms is responsible for maintaining the stability of this catalyst[56]. Recently, Zhang et al. designed a catalyst composed of Ru single atoms and Ru NPs on a carbonized melamine foam (CMF) support [Figure 4A]. The mixed LA and HMF substrates were efficiently converted to the desired products of GVL and FDCA without any purification steps. This efficiency is attributed to the distinct advantages of the single-atom and NP sites. In high-angle annular dark-field scanning TEM and electron microscopy, it was demonstrated that RU metal exists on the carrier in two forms [Figure 4B-E]. They showed the cooperative mechanism in Figure 4F, the metallicity of Ru-SA is reduced compared to
Figure 4. (A) Schematic of the catalytic alternate hydrogenation-oxidation process; (B) Scanning electron microscopy and (C) transmission electron microscopy images of the catalyst containing only Ru nanoparticles; (D) Elemental distribution mappings; (E) high-angle annular dark-field scanning transmission electron microscopy images[57]. Copyright 2023 American Chemical Society.
Hemicellulose and its derivatives
Furfural is an important downstream compound of hemicellulose, which is produced by hydrolysis of hemicellulose from lignocellulosic biomass catalyzed by dilute acids. This compound is the most significant derivative of the furan ring system, exhibiting chemical reactivity, capable of undergoing oxidation, condensation, and other reactions to generate a multitude of derivatives. It is a prevalent component in synthetic plastics, pharmaceuticals, pesticides, and other industrial applications. Therefore, the current application of SACs in hemicellulose also focuses on various transformations of furfural.
The conversation of furfural
Furfural contains furan rings, cyclic ether bonds and carbonyl groups that are highly reactive, which can be converted into many synthetic chemicals such as furfuryl alcohol (FAL), tetrahydrofurfuryl alcohol (THFA), 2-methylfuran (2-MF), cyclopentanone, and furan by hydrogenation, hydrogenation-hydrolysis and decarbonylation.
For example, Pd nanoclusters on zirconia (ZrO2) and single Pd atoms on ceria (CeO2) displayed great catalytic activity for the decarbonylation of furfural to furan in the liquid phase without any additives. Fu et al. investigated the active size of Pd and the alterations in chemical states occurring throughout the reaction. It has been observed that individual Pd atoms exhibit positive characteristics, whereas clusters of Pd are effective in promoting decarbonylation. The lower binding and migration energies of Pd1/ZrO2 indicate that palladium atoms are able to diffuse across the ZrO2 surface, leading to the formation of small palladium clusters. Conversely, Pd atoms on CeO2 exhibit a reduced tendency to diffuse and instead tend to aggregate at stable sites, resulting in the formation of larger palladium NPs. It was concluded that ZrO2 is the most suitable support for steadying Pd clusters, as it exhibits better catalytic properties during the reaction[58,59]. Metal-support interactions are a classical problem explored in multiphase catalytic systems. This study uses titanium dioxide or mesoporous graphitic carbon nitride as carriers to explore the role played by Ir metal monoatoms on different carriers[60]. Later, another research demonstrated a method to selectively cleave the C-O bond using urfuryl alcohol conversion to 2-MF as a showcase. When surface-doped with an ultralow loading of Pt (comprising single atoms and sub-nanometer clusters), the material demonstrates an activity nearly two orders of magnitude higher than that of pure TiO2. Figure 5A shows that this ultra-low loading SAC can effectively improve the selectivity over the previous multi-site catalysts. According to the reverse Marsvan Krevelen mechanism, metal oxides are considered one of the most promising classes of catalysts for C-O bond cleavage, but the most active oxides experience bulk reduction during the reaction. Density functional theory (DFT) and microkinetic calculations show that this catalytic mechanism generates a catalytic cycle. In this cycle, molecular hydrogen facilitates vacancy formation, followed by FA adsorption, C-O bond cleavage with hydrogen back-donation from the departing OH to carbon, and finally, 2-MF desorption [Figure 5B]. The metal atom directly bonded to the interfacial oxide is the most active center in the tandem reaction[58]. On layered double oxides [Mg(Al)O] containing magnesium and aluminum, platinum atoms exist as dispersed monoatoms, coordination-unsaturated two-dimensional (2D) clusters and three-dimensional clusters. Directed activation of C-OH, C-O-C or C=C was investigated. These three types of platinum sites were used for the hydrogen conversion of FAL to 2-MF, THFA or 1,2-PeD, respectively [Scheme 2][61].
Figure 5. (A) Reactions of furfural with different catalysts; (B) Catalytic cycle for FA upgrade on (101) TiO2 anatase surface. Numbers and free energy profiles represent pure TiO2 (black), Pt-O5/TiO2 (blue) and Pt-O6/TiO2 (orange). DFT-based Gibbs barriers, ΔG‡, (reaction energies, ΔG) in kcal mol-1. Gibbs energies are reported with respect to the energy of the pristine surface and H2 and FA chemical potentials, computed at experimental conditions (Methods)[58]. Copyright 2020 Nature Publishing Group. DFT: Density functional theory; FA: furfuryl alcohol.
Scheme 2. Proposed reaction paths for the hydrogenation of furfuryl alcohol over engineered Pt sites[61]. Copyright 2019 Elsevier.
Interestingly, lowering the concentration of Ru on the support enhances the catalyst system's activity. This behavior is attributed to the unique Ru1CoNP surface SAA structure, which is only allowed at high Co/Ru ratios (Co/Ru ≥ 10) on the hydroxyapatite support[62]. Notably, non-precious metals are very attractive for SAC applications. To create different Fe sites, enabling them to be preferentially utilized in activating C-O bonds selectively. An et al. used Fe(NO3)3 as an iron source for saturable adsorption and pyrolysis on a
Figure 6. (A) Schematic of synthesis of iron catalysts using ZIF-8 and MOF-5 as precursors; (B) P1 and P2 pathways of CTH reaction on Fe(II)-plN4 and Zn(II)-plN4, compared with P3 pathway on Fe(II)-plN3; (C) k2-weighted Zn K-edge and EXAFS spectra of Fe-ZIF-8-800; (D) Catalytic performance and TOF of various catalysts[63]. Copyright 2020 American Chemical Society. CTH: Catalytic transfer hydrogenation; TOF: the turnover frequency; MOF: metal-organic framework; EXAFS: extended X-ray absorption fine structure.
Lignin and its model compounds
Lignin, a crucial constituent of lignocellulosic materials, represents one of the three primary components. It manifests as a 3D non-crystalline polymer, characterized by methoxylated phenylpropane structures. Due to its significant amount of aromatic structure and higher carbon and lower oxygen content, lignin has been generally recognized as a viable alternative to petroleum. However, it remains the least utilized lignocellulosic biopolymer due to its difficulty in separation and depolymerization[64-66]. In recent years, the use of noble metals and transition metals supported on various substrates has become increasingly popular, particularly SACs[67-69]. Due to the complexity of lignin structure, many model compounds (e.g., anisole, guaiacol, vanillin) have been used to substitute real lignin-derived small molecules. To mimic the β-O-4 bond breaking typical of lignin, some studies have chosen compounds such as 2-(2-methoxyphenoxy)-1-phenylethanol. Meanwhile, several studies have begun to depolymerize and transform real lignin.
Vanillin
The selective hydrodeoxygenation of vanillin is essential for transforming bio-materials into high-value chemicals. In 2018, Tian et al. reported a simple wet impregnation strategy that isolated single-atom Ru on mesoporous carbon nitride-supported (Ru1/mpg-C3N4). Catalyst performance tests with vanillin are effective in both hydrogenation and hydrodeoxygenation of vanillin in water [Figure 7A][70]. At 60 °C, only vanillyl alcohol is formed. As the temperature increases, further hydrogenation occurs to produce a mixture of vanillyl alcohol and 2-methoxy-p-cresol. At 140 °C, only 2-methoxy-p-cresol is formed. This provides an excellent illustration of the catalytic mechanism of SACs at varying temperatures. The catalyst can achieve 100% conversion within 15 min, with conversion and TOF far exceeding those of comparable commercial catalysts. Also, it is noteworthy that the catalyst maintains a very good catalytic effect after ten cycles [Figure 7B]. They proposed a systematic reaction mechanism using first-principles calculations, indicating that the barrier to vanillin hydrogenation is too high and only vanillin is formed. When the temperature rises, there are two transition states (IM2, IM3) after the adsorbed phase of vanillyl alcohol, making it possible for further deoxygenation of vanillyl alcohol to MMP (4-hydroxy-3-methylaniso) [Figure 7C]. Therefore, vanillyl alcohol is the product that prohibits deoxygenation, while 2-methoxy-p-cresol is the product that allows deoxygenation. To prevent the metal species from being reduced and forming nanoclusters when using WO2.72 (violet tungsten oxide) as a support, a one-step in situ synthesis method was developed to create dispersed metal atoms. Dispersed palladium atoms on WO are covalently bonded to oxygen atoms in the vicinity of the support to form an active center. Besides efficiently catalyzing vanillin, under optimized conditions, most lignin-derived species (such as o-vanillin, syringaldehyde, isovanillin, ethylvanillin, and
Figure 7. (A) Synthetic route for the construction of Pd1/WO2.72; (B) Catalytic performance of Pd1/WO2.72. Including conversion and selectivity, performance comparison with other similar catalysts, TOF values and recycling performance; (C) Free energy diagrams of vanillin hydrogenation to vanillyl alcohol and 2-methoxy-2-methoxy-p-cresol at 60 °C (blue) and 140 °C (red). The structures of reactants and products are shown above the energy profiles, while the intermediate state structures are shown below the energy profiles. The geometry of Ru1/mpg-C3N4 is shown in the upper right[70]. Copyright 2018 American Chemical Society. TOF: The turnover frequency.
Benzyl alcohol
Benzyl alcohol (BA) is regarded as a model compound for lignin-derived alcohols. The following examples will examine SACs using BA oxidation as the probe reaction. In 2017, bimetallic Au@Pd/TiO2 catalysts were prepared. Pd atoms were deposited on the bare surface of gold NPs to form core-shell mixed metal particles with simultaneous electron transfer between the cores. The Pd species dominated the aerobic oxidation of BA without solvents. The BA conversion grew with palladium deposition at low palladium deposition levels, but remained constant when the palladium deposition was further increased to 0.13 wt%[72]. Subsequently, Li et al. prepared a series of SACs by adsorbing different metals (Pt, Au) on CeO2, which were tested using BA oxidation as a model reaction. Compared to the corresponding NP catalysts, SACs have the maximum number of interfacial sites, resulting in a much higher activity[73]. This configuration enables this efficient and sustainable hydrogenation reaction to occur exclusively in aqueous solution. In 2019, influenced by natural iron-catechol complexes in marine mussels, Zhou et al. synthesized metal-lignin coordination complexes with different metals on nitrogen-doped carbon to generate SACs. They proposed a full lignocellulose utilization strategy where oxygenates derived from lignin are transformed into valuable building blocks and fragrances using a Co SAs-N@C catalyst [Figure 8][74].
Figure 8. Engineering metal-lignin (M-L) complexes to SACs for the upgrading of carbohydrate-derived furans[74]. Copyright 2019 Elsevier. SACs: Single atom catalysts.
Anisole
Eco-friendly conversion of biomass-sourced molecules into cyclohexanone, a feedstock for nylon preparation, is highly sought after. In this context, a Pd SAC anchored in the micropores of zeolite
Depolymerization of lignin
Lignin’s polymerized structure is very sturdy due to its structural units of phenylalanine which connect by C-O and C-C linkages. Thus, some researchers try to use lignin-derived dimer/special linkages structural model compounds instead of real lignin to investigate the catalytic mechanisms involved. The β-O-4 bond is the most common connecting bond in lignin and is more easily broken than the C-C bond in the lignin structure [Scheme 3A]. Therefore, some research starts from “friendly” β-O-4 linkage to open the structure of lignin. For example, atomic Co catalyst (Co-N-C) is applied for the oxidative cleavage of
Scheme 3. (A) Representative structure of a lignin fragment with the β-O-4 linkage; (B) Proposed reaction pathway of MPP-ol[76]. Copyright 2019 Elsevier. MPP-ol: 2-(2-methoxyphenoxy)-1-phenylethanol.
Guaiacol and its derivatives
Lignin can be depolymerized through reduction or thermal treatment to produce bio-oil. Bio-oil is mainly composed of lilac-based and guaiac-based compounds, which contains many phenolic monomers functionalized with an alkyl group at the para position and methoxy group at the ortho positions. Hydrodeoxygenation of lignin oil (LO) produces high-value fuels. The aromatic and olefin structures in bio-oils are not saturated enough; hydrodeoxygenation can help them reduce the oxygen percentage and raise the calorific value of bio-oils through a higher H/C ratio. During this process, we expected selective cleavage of C-O bonds without opening or saturation of the aromatic ring[77,78]. Transition metal Fe is a suitable catalyst for guaiacol hydrodeoxygenation.
Guaiacol is also considered a prospective feedstock for the production of cyclohexanol, which mainly involves the step of the splitting of C-O bonds and the hydrogenation of aromatic hydrocarbons. In 2021, Fe-based catalysts are prone to carbon deposition or carburization[79]. To solve this problem, Li et al. introduce a trace amount of noble metal (including Pt, Ir, Pd, Rh, and Ru) into ceria-supported Fe catalysts. Among these single alloy catalysts, Pt−Fe/CeO2 shows the highest catalytic activity. A very low amount
Figure 9. (A) Deactivation Mechanisms for Pt-Fe/CeO2 Catalyst in the Absence of H2O and in the Presence of H2O[80]. Copyright 2021 American Chemical Society; (B) Proposed possible mechanism for the guaiacol HDO over the hcp RuCo catalyst[81]. Copyright 2021 Elsevier; (C) Hydrogenative C-O cleavage of lignin oil by a dual-size Co catalyst[83]. Copyright 2023 Elsevier; (D) Reaction Route of the Catalytic Conversion from Lignin to Renewable Biofuel[84]. Copyright 2024 American Chemical Society. HDO: Hydrodeoxygenation.
In another study, Zhang et al. reported that Ru, an atom supported on cerium dioxide (CeO2) with a lamellar morphology, catalyzes the hydrogenation of the benzene ring structure to obtain cyclohexanol [Figure 10A]. X-ray diffraction (XRD) patterns show that the synthesized CeO2 samples and catalysts exhibit well-defined diffraction peaks of standard CeO2 samples and expose 110, 100 and 111 facets, which may be beneficial for catalysis [Figure 10B1-B2]. The aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) images and corresponding energy dispersive X-ray spectrometry (EDS) mapping show that Ru or other metal species disperse homogeneously and exist as isolated single atoms on the CeO2 supports [Figure 10B3-B4]. The FT of EXAFS demonstrates the coordination result of Ru-O-Ce
Figure 10. (A) Ru/CeO2-S Schematic of catalysis; (B) B1: XRD patterns of the CeO2-S sample (blue) and Ru/CeO2-S catalyst; B2: TEM image of the Ru/CeO2-S (red); B3: HAADF-STEM image of the Ru/CeO2-S; B4: HAADF-STEM image and the corresponding EDS mapping of the Ru/CeO2-S; (C) C1: FT k3-weighted Ru K-edge EXAFS spectra; C2: Wavelet transform of the Ru K-edge EXAFS spectrum; C3-C5: XPS spectra of the Ru/CeO2 catalysts for Ru 3P, O 1s, and Ce 3d regions[82]. Copyright 2022 American Chemical Society. HAADF-STEM: High-angle annular dark-field scanning TEM; EDS: energy dispersive X-ray spectrometry; XRD: X-ray diffraction; EXAFS: extended X-ray absorption fine structure; XPS: X-ray photoelectron spectroscopy.
Native lignin
Depolymerization of real lignin without a chemical pretreatment is challenging. Many efforts have been made to enable the direct application of these biomass feedstocks containing large amounts of phenolic monomers. Chen et al. reported an efficiency SAC comprising single Pt atoms on Ni NPs supported on carbon (Pt1Ni/C). Using this catalyst, a lignin monomer yield of 37% from birch sawdust was achieved under 5 MPa H2 in MeOH at 200 °C. Compared to other published mental catalysts, such as single Pt atoms catalysts (Pt1/C) or Ni NPs catalysts (Ni/C), Pt1Ni/C has significant catalytic activity [Figure 11A][85]. In another study, based on the cleavage of β-aryl ether bonds, Meng et al. reported a synergistic catalyst that integrates atomically dispersed Mo centers and Al Lewis acid sites on a MgO substrate (Mo1Al/MgO). The catalyst's efficiency was tested using the model compound guaiacyl glycerol-β-guaiacyl (GG) as a template. The results showed that Mo loading higher than 3.0 wt% led to the aggregation of Mo species and reduced catalytic activity. Additionally, the reaction over Mo1Al/MgO was found to be greatly influenced by the protic solvent. Subsequently, the improved catalyst system can be used for the direct depolymerization of lignin in eucalyptus. As we can see in Figure 11B, the optimum yield of monophenolics reached 46% with 92% selectivity, which was very close to the theoretical yield. DFT calculations and catalytic tests have shown that the Mo1-O5 center and the exposed Al Lewis acid site acted synergistically. Following guaiacol formation and desorption, the O* intermediate formed stabilizes and acts as an oxidizing agent converting coniferyl alcohol to coniferyl aldehyde and coniferyl methyl ether[86].
Figure 11. (A) Route used to prepare the Pt1Ni/C catalyst (up) and the monomer yield and main lignin-derived monomers obtained. Reaction conditions: 0.1 g birch sawdust, 0.02 g catalyst, 5 mL MeOH, 5 MPa H2, 200 oC, and 18 h (down)[85]. Copyright 2019 Elsevier; (B) Schematic illustration of the confinement strategy to synthesize the Mo1Al/MgO catalyst for selective lignin disassembly over synergetic Mo single-atom and Al Lewis acid sites (up). And Lignin depolymerization performances at different reaction conditions in methanol under N2 (down)[86]. Copyright 2023 American Chemical Society; (C) Schematic diagram of CEL catalytic transfer hydrogenolysis over 1.0Ni/CeO2-S catalyst[90]. Copyright 2022 Elsevier. CEL: Cellulolytic enzyme lignin.
Catechyl lignin (C-lignin) is a homopolymer biosynthesized from caffeyl alcohol owing to the lack of
others
Glycerol, a derivative of the biofuel industry, is currently facing an overproduction issue. The directed oxidation of alcohols to aldehydes and acids represents an essential procedure within the chemical transformation, offering a sustainable route for producing high-value chemicals and fuels from biomass resources[91-95]. Glycerol, a highly functionalized molecule characterized by the presence of both primary and secondary hydroxyl groups, can serve as a precursor to many high-value products. In thermal catalysis, monoatomic catalysts are more effective in the hydrogenolysis of glycerol to 1,3-propanediol. Cinnamaldehyde and crotonaldehyde are two of the most important biomass-derived aldehydes that can also efficiently complete the corresponding reactions, such as oxidation or hydrogenation, under the catalysis of SACs to produce more valuable downstream products. Finally, we mentioned the reductive catalytic fractionation (RCF) strategy. The low binding energy of natural lignin and the high number of condensation bonds of industrial lignin are both unfavorable for lignin depolymerization, but RCF allows the heterogeneous catalytic conversion of natural lignin from wood under relatively mild conditions, resulting in almost theoretical yields of lignin-derived phenolic monomers (LDPMs) and recovery of
Glycerol
Glycerol has become a cheaper and more available biomass platform chemical since the expansion of biodiesel production[91-93]. Converting glycerol from biodiesel into high-value chemicals is a suitable strategy for dealing with the surplus supply. The compound 1,2-propanediol produced by breaking the C-C and/or C-O bond is a significant product of the hydrogenolysis of glycerol and also a significant industrial material[94,95]. In 2016, Wang et al. started research about Pt-based SAC catalyst supported on mesoporous WOx. However, this material cannot be considered a standard SAC, the excellent carrier for platinum structures rich in oxygen vacancies and hydroxyl functional groups due to the dispersion of platinum species on an atomic or pseudo-atomic scale[99]. The least unoccupied molecular orbitals (LUMOs) of tungsten (W) can accept an electron from hydrogen (H), resulting in the formation of Hδ+ and leading to the development of Brønsted acidity. Meanwhile, the Pt site triggers concerted dehydration-hydrogenation reaction. As a result, it is favorable to the bond formation between glycerol and WOx; meanwhile, more steady secondary carbocation intermediate formation makes it possible to produce 1,3-propanediol instead of 1,2-propanediol. Later, Zhang et al. continued the research about bimetallic alloy catalysts for the hydrogenolysis of polyols; a PtCu-SAA catalyst was successfully prepared. As shown in Figure 12A, both
Figure 12. (A-E) Catalytic evaluation of PtCu-SAA and monometallic catalysts (Pt/MMO and Cu/MMO) toward glycerol hydrogenolysis to 1,2-PDO[100]. Copyright 2019 Springer Nater. (F) Atomic Pt1 site provides an enhanced C-H activation (marked in blue shade) in the cascade synergy, forming the η2-adsorbed GLAD[101]. Copyright 2022 Springer Nature. SAA: Single-atom alloy; GLAD: glyceric acid; PDO: 1,2-Propanediol.
The selective oxidation of glycerol to glyceric acid constitutes a significant value-added reaction for polyols. An et al. designed a sequential co-catalysis method for the selective oxidation of glycerol to glyceric acid in which a single platinum atom synergizes with a low-coordinated Pt-Pt. In this work, the whole reaction is divided into two processes: the oxidation of glycerol to glycerol aldehyde and the oxidation of glycerol aldehyde in series to glycerol acid as described in Figure 12F. It enhances C-H activation on atomic Pt1 and O-H activation on cluster Ptn, facilitating the oxidation of in the tandem cooperative catalytic system. It also enables cluster Ptn to activate C=O, and, subsequently, O-H insertion[101]. The results indicated that it demonstrated a high level of catalytic activity with glycerol conversion (90.0%) and glyceric acid selectivity (80.2%).
Crotonaldehyde
Selective hydrogenation of α,β-unsaturated aldehydes into unsaturated alcohols is a promising application for heterogeneous catalysts. However, precise adsorption and effective activation of C-O bonds remain significant challenges. Inspired by the unique spatial effect of the pocket-like structure of the enzyme reactive center, Lou et al. designed a pocket-like active site by anchoring single Rh atoms to the Mo edge vacancies of 2D MoS2 sheets [Figure 13A][102]. DFT calculations show that the oxygen atoms of the 2D MoS2 are bonded to Mo atoms, and the pocket-shaped structure leaves the Rh1 active site exposed to better bind and dissociate from the hydrogen molecules, which are then released along the edges to form OH groups. This results in a unique pocket configuration: HO-Mo-Rh1-Mo-OH. This unique active site not only catalyzes the hydrogenation of hoods to crotonols, but also has 100% selectivity. Afterwards, this group did similar work by turning the metal in the active site into Pt, also achieving excellent catalytic effect[103].
Figure 13. (A) Representative atomic resolution HAADF-STEM images of the as-synthesized Rh1/MoS2 SAC. (left) Schematic of the structure of Rh1/MoS2 SAC. The white dots (indicated by the yellow arrows) in the HAADF-STEM images represent single Rh1 atoms.(right)[102] Copyright 2019 American Chemical Society; (B) Illustration of synthetic procedure of Pt1-FeOx/SBA-15 catalyst under mild conditions[108]. Copyright 2023 Elsevier. HAADF-STEM: High-angle annular dark-field scanning TEM; SACs: single atom catalysts.
Cinnamaldehyde
The metals, Pt, Ni, Ru, Cu, etc., are commonly used as catalysts for the hydrogenation of cinnamaldehyde[104-109]. Li et al. presented a series of work about Pt-based SACs for cinnamaldehyde conversion. For example, Pt atoms anchored on CeO2 nanorods with abundant oxygen vacancies demonstrated outstanding catalytic performance: 99% conversion and a TOF of 2,410 h-1 for styrene hydrogenation, 99% conversion and selectivity with a TOF of 968 h-1 for cinnamaldehyde hydrogenation, and 99% conversion with 79% selectivity and a TOF of 10,000 h-1 for triethoxysilane oxidation. Notably, this synthetic method can be scaled up without compromising catalytic performance, making it suitable for industrial applications[104]. After that, hexagonal boron nitride nanosheet and defect-containing β-FeOOH can also anchor Pt atoms to construct Pt SACs, where rational use of nitrogen-containing vacancies and oxygen vacancies stabilizes PT atoms[106,107]. In a recent study, the manufacturing of single-atom Pt catalysts was first reported using SBA-15 covered with functional FeOx nanoclusters as support through a mild hydrolysis technique. They first proposed using Pt acetylacetonate as a precursor, which is treated hydrothermally to remove the ligand, leaving the single metal atom [Figure 13B]. Attributed to the synergistic effect between isolated Pt atoms with high surface positive charges and FeOx nanoclusters on SBA-15, the further hydrogenation of cinnamaldehyde was inhibited[108].
Reductive Catalytic Fractionation of Lignocellulosic
Lignocellulosic biomass offers a promising substitute for fossil resources, attributed to its structural complexity and the abundant availability of raw materials. RCF is an efficient and separable method for converting lignin biopolymers into monomeric phenols. Guided by the lignin-first strategy, the combining of RCF and non-homogeneous catalysts to improve harsh reaction conditions is a highly potential field of research for upgrading biomass.
In 2020, Park et al. reported an extremely low Pd-loaded (0.25 wt%) CNx supported catalyst capable of achieving LDPM carbon yield and completely delignified holocellulose recovery of 52.7 C% and 84.2 wt%. As shown in Figure 14A, the size of the loaded metal affects the reaction process. Compared to Pd5/AC catalyst, Pd0.25/CNx shows better industrial applicability and higher activity in hydrodeoxygenation and double-bond saturation, producing 4-n-propyl guaiacol/syringol as major products. Various types of biomasses were tested to demonstrate the effectiveness of Pd0.25/CNx for biomass fractionation[110]. 2022, a type of Co-based SAC was also synthesized using a mixture of lignin-mental complex and melamine as the precursor, which was adopted for acid-assisted reductive fractionation of lignocellulose in the presence of 0.1% sulfuric acid without H2. Due to the synergistic effect of metals and acids, a satisfactory monophenol yield of 41.7% was obtained along with nearly complete delignification at 99.7%[111]. In another research, Song and his group reported a highly dispersed Ru anchored on a chitosan-derived N-doped carbon catalyst
Figure 14. (A) Schematic of the RCF of wood over the Pd0.25/CNx catalyst with importance of reaction factors toward lignin extraction, depolymerization, and stabilization[110]. Copyright 2020 American Chemical Society; (B) The overall sustainability concept of lignocellulose upgrading reformation and application[113]. Copyright 2023 Elsevier. RCF: Reductive catalytic fractionation.
Electrocatalysis
As previously discussed, the main approach for biomass conversion is thermochemical catalysis, which necessitates elevated temperatures and pressures. With the rising cost of renewable energy and the demand for milder reaction conditions, electrochemical processes are becoming a viable pathway[114]. Electrocatalysis uses electrons as redox agents without the need for additional oxygen or hydrogen sources, providing a clean and efficient conversion strategy. SACs can also be employed in the electrocatalytic conversion of biomass. More generally, the advantages of electrocatalysis can be considered from the following aspects. First, the electrocatalysis can be carried out with a simple device at room temperature and atmospheric pressure; the selectivity and conversion can be controlled by adjusting the applied potential, electrode material and electrolyte[115]. Secondly, coupling biomass valorization with CO2 conversion through electrocatalysis offers a promising method to simultaneously produce valuable commodity chemicals at both electrodes[116-118]. Furthermore, electrochemical catalysis can be driven by clean energy generation such as solar power, which offers a hopeful approach for the development of green technology. Statistically, both noble metals such as Ir, Ru, Rh, Co and non-precious Cu metals loaded on different carriers can provide suitable active sites for electrocatalysis. Until now, most of the reactions are transformations of HMF, such as oxidation, hydrogenation, and hydrogenolysis reactions; a few SACs are used for electrochemical reduction and coupling reactions of furfural or glycerol.
HMF oxidation
DHMF, catalytically reduced by HMF, acts as a crucial starting material for the creation of various functionalized polymers such as polyethers, polyurethanes, and polyamides[119]. Pure metal electrodes (Cu, Bi, Pb) remain inactive at high voltages, which hinders the reactivity. In 2022, Ji et al. introduced a
The oxidation products of HMF, FDCA and DFF serve as precursors for the polymer industry and as valuable byproducts in the transformation of fatty acids into biodiesel. In 2021, Lu et al. were able to enhance the overall conversion rate of HMF by modulating the adsorption of HMF as demonstrated in [Figure 15]. They tried to maximize the HMF adsorption behavior on spinel oxides (Co3O4) by introducing single atom Ir. The linear sweep voltammetry curves reveal exceptional HMFOR performance for
Figure 15. (A) Two possible reaction pathways for HMF oxidation; (B-E) The electrochemical activity of Ir-Co3O4 and Co3O4; (B) CV curves of Ir- Co3O4 and Co3O4 in 1 m KOH at a scan rate of 5 mV s-1; (C) LSV curves of Ir- Co3O4, Co3O4, and IrO2 in 1 m KOH with 50 × 10-3 m HMF at a scan rate of 5 mV s-1; (D) OCP curves of Ir- Co3O4 and Co3O4 were subsequently measured in 1 m KOH and 50 × 10-3 m HMF; (E) Onset potential of Ir- Co3O4 and Co3O4 for HMFOR and hydroxypentanal electro-oxidation, respectively; (F-I) Adsorption evaluation for Ir- Co3O4 and Co3O4; (F-H) TPD spectra of Ir- Co3O4 and Co3O4 at HMF/He (F), ethylene (G), and CO (H) atmospheres; (I) The adsorption model of HMF molecules on Ir- Co3O4[121]. Copyright 2021 Wiley-VCH. HMF: 5-hydroxymethylfurfural; CV: cyclic voltammetry; HMFOR: 5- hydroxymethyl furfural oxidation reaction; TPD: temperature programmed desorption; OCP: open circuit potentia; KOH: potassium hydroxide.
In another work, single-atom ruthenium nickel oxide (Ru1-NiO) was used as a catalyst for the efficient electro-oxidation of HMF in a neutral medium, which breaks through the traditional limitations of alkaline media[122]. Figure 16A illustrates the catalytic mechanism of the catalyst. The adsorption of water molecules on Ru atoms results in the dissociation of OH* species, accompanied by electron and proton transfer. Subsequently, OH* reacts with a nucleophilic reagent (Nu), namely HMF, to produce DFF and water. A deeper insight into the HMFOR mechanism was gained using cyclic voltammetry (CV) analysis. From the CV curve of NiO in 1.0 M phosphate-buffered saline (PBS), it can be seen that an oxidation peak appears in the region of 0.8 V < E < 1.1 v, which is consistent with the OH* adsorption region reported in the literature. More critically, the onset potential of hydrolysis dissociation is consistent with that of HMFOR, suggesting that the OH* substance produced by hydrolysis dissociation, rather than OH-oxidation, is the main active substance of HMFOR on NiO. This phenomenon was also observed for Ru1-NiO, suggesting a similar reaction mechanism. The structural evolution of the catalyst at different potentials was traced using non-in situ Raman spectroscopy. No signal for NiIII was observed from Raman spectra after electrocatalysis in
Figure 16. (A) Proposed HMFOR mechanism over Ru1-NiO in the neutral medium; (B-D) Investigations of HMFOR mechanism over
Recently, Zhou et al. designed a single atom electrocatalysts (SAECs) composed of single Co atoms supported on nitrogen-doped carbon nanosheets (Cu/NCNSs)[123]. Based on in-situ Raman spectra and high-performance liquid chromatography (HPLC) results, it can be shown that HMF oxidation on Cu NPs follows the path to DFF at lower potentials, shifting to the 5-hydroxymethyl-2-furan carboxylic acid (HFCA) pathway when the applied potential exceeds 1.67 V. Simultaneously, Zeng et al. described a series of Rh-O5/Ni(Fe) atomic sites on nanoporous mesh-type layered double hydroxides, with atomic-scale cooperative adsorption centers that demonstrate high activity and stability in catalyzing alkaline HMFOR and hydrogen evolution reaction (HER)[118]. In an integrated electrolysis system, achieving a current density of 100 mA cm-2 requires a low cell voltage of 1.48 V, accompanied by excellent stability over 100 h. From a thermocatalytic perspective, HMFOR is considered a tandem reaction with multiple intermediate products, requiring various active sites to enhance overall response efficiency.
Others
Mukadam et al. reported a good strategy for the electrocatalytic reduction of furfural to generate hydrofuran. Cu/Co-doped phthalocyanines are the case of single atom molecular catalysts that exhibit high selectivity for the electroreduction of furfural to hydrofuroin[124]. Compared to RHE, hydrofuran produced a faradic efficiency of up to 65.3% at pH 10 and 0.50 V with low hydrogen precipitation; furthermore,
Figure 17. Proposed mechanism of hydrofuroin production in furfural reduction on metal-doped phthalocyanine where ‘M’ represents a metal atom/ion[124]. Copyright 2023 Royal Society of Chemistry.
A promising method to increase the value of biomass feedstocks by reducing oxygen content is electrochemical coupling of C-O bonds. In this study, the generalized adsorptive deposition method loaded monoatomic Au on top of spinel synergistic composites to prepare SAC(Au-NiMn2O4). Atomically dispersed Au occupies the surface Ni2+ vacancies, reducing the benzaldehyde adsorption energy and thus benzaldehyde coupling to produce dibenzyl ether, with high dibenzyl ether selectivity and Faraday efficiency. Spinel nanostructures, as a new catalytic support material, will exhibit excellent catalytic performance in the field of single-atom electrocatalysis[125].
Converting glycerol to higher value glycol aldehydes via glycerol oxidation is a promising study. Wang et al. utilized electrocatalysis to report a single atom bismuth (Bi) doping strategy. Experimental characterizations and theoretical calculations reveal that single-atom Bi substitutes cobalt at octahedral sites (CoOH3+) in
Figure 18. (A) Schematic illustration of the synthesis strategy for single-atom Bi-doped Co3O4 catalyst; (B) Schematic illustration of the promoting effects of single-atom Bi doping on coadsorption of OH* and glycerol[126]. Cyright 2020 American Chemical Society.
This example demonstrates the application of electrocatalytic oxidative cleavage of the Cα-Cβ bond in lignin β-O-4 model compounds. Under optimized conditions (150 °C, 4 h) in MeOH solvent in air, a 95% conversion of MPP-ol was achieved. C−C bonds typically exhibit increased dissociation energy compared to C-O bonds in lignin. Therefore, targeted cleavage of the C-C bond without disrupting the benzene ring is a critical factor in lignin depolymerization. In this study, N-doped CNTs anchor a single-atom Pt catalyst which was used for selective Cα-Cβ bond cleavage through electrocatalytic oxidation. Using 2-Phenoxy-1-phenylethanol as a model compound, Pt1/N-CNTs exhibit exceptionally high Cα-Cβ bond cleavage activity and selectivity, achieving an 81% yield of benzaldehyde. This performance surpasses the current state-of-the-art Pt electrode and Pt/C benchmark catalysts [Figure 19][127].
Figure 19. Schematic of electrocatalytic selective breaking of Cα-β bond in lignin model compounds by Pt1/N-NTs single atom catalysts[127]. Copyright 2021 American Chemical Society.
Photocatalysis
The catalytic conversion of lignocellulosic biomass using solar energy has recently garnered significant attention. This interest arises from the distinctive reactive species and reaction mechanisms initiated by photoexcited charge carriers or photogenerated reactive entities, along with the mild reaction conditions. These factors facilitate precise cleavage of lignocellulosic biomass[10]. Photocatalysis typically begins with the absorption of light, which excites the photocatalyst to an elevated energy state. Conventional thermocatalysis relies on high-temperature activation to overcome energy barriers. Photocatalysis typically begins with the absorption of light, which excites the photocatalyst to a higher energy state. The excited photocatalyst can subsequently activate the reactant, surmounting the energy barrier and producing the desired products [Figure 20]. However, the conditions typically employed in thermocatalysis are often too rigorous to permit the selective activation of a specific functional group due to the multiple functional groups of the biomass reactants. Furthermore, thermal catalysis presents a challenge in conducting simultaneous reduction and oxidation reactions, such as the cleavage of the 1-bond in lignin, which necessitates a multistep conversion process. We propose that photocatalysis can address this challenge by enabling the selective transformation of specific reactants or the selective cleavage or functionalization of targeted chemical bonds or functional groups, while preserving other functional groups. Our findings indicate that the majority of SACs exhibit selective photocatalytic activity towards the C-O, C-H, and C-C bonds of biomass feedstocks by photocatalytic process. Specific studies are detailed in this chapter.
Figure 20. Energy diagram for the transformation of reactant (R) to product (P). (A) Thermocatalysis; (B) Ptocatalysis, (C, E) and (S) refer to the catalyst, molecules in the environment, such as solvent and atmosphere, and byproducts, respectively[10]. Copyright 2020 Royal Society of Chemistry.
In 2023, Ru atom decorated Zn0.5Cd0.5S nanorods were successfully synthesized through a simple photodeposition method. It demonstrated significantly higher H2 output activity and almost complete selectivity for FAL under photocatalytic conditions. The photocatalytic rates of H2 evolution and furfural production over 1 wt% Ru/Zn0.5Cd0.5S reached 17.4 μmol h-1 and 17.1 μmol h-1, respectively. The enhanced photocatalytic performance results from the highly efficient separation and transport of photoexcited charges, facilitated by the strong interaction between the Ru atom and Zn0.5Cd0.5S nanorods[128].
In 2021, Lu et al. reported a photocatalytic method to simultaneously load Pt single atoms and nanoclusters onto silicoaluminophosphate-31 (SAPO-31) for the hydrodeoxygenation of vanillin. The high activity of the catalyst is due to the synergistic effect of strong metal-support interactions, facilitated by the simultaneous presence of isolated palladium atoms and clusters of palladium atoms[129]. This catalyst exhibits unique geometrical and electronic properties, with synergistic effects due to the combination of single atoms and ultra-small nanoclusters, and also strong metal-carrier interactions contributing to better catalytic performance. Additionally, it shows almost 100% yield of production in the hydrodeoxygenation of vanillin to 2-methoxy-4-methylphenol (MMP) under barometric pressure with 80 °C and 30 min.
In another research, Ru single atom support on mesoporous carbon nitrides (Ru1/MCN) was applied for selective oxidative cleavage of Cα-Cβ bonds under visible light irradiation at room temperature; this method achieves almost complete conversion and high selectivity for C-C bond cleavage in 2-phenoxy-1-phenylethanol [Figure 21A]. A series of optical tests demonstrated the catalytic efficiency of Ru1/MCN.
Figure 21. (A) Chemical molecular formula of the reaction; (B) UV-is DRS spectra; (C) PL spectra; (D) Proposed mechanism of the photocatalytic depolymerization of 2-phenoxy-1-phenylethanol (1) through Cα-Cβ bond cleavage using Ru1/MCN SAC[130]. Copyright 2023 Elsevier. UV-vis DRS: ultraviolet-visible diffuse reflectance spectroscopy; PL: photoluminescence; SACs: single atom catalysts.
In this scenario, the SACs consist of Ni single atoms anchored to the surface of titanium dioxide. Activated by light, the catalyst functions under ambient conditions, utilizing air as an eco-friendly oxidant. This process achieves a selectivity of over 60% for glycol aldehyde[131]. The atomically dispersed nickel material anchored by the novel molten salt approach taken in this experiment exhibits significantly enhanced properties. Later, Xiong et al. employed visible light irradiation to convert glycerol into glyceraldehyde and dihydroxyacetone, achieving a high degree of selectivity. In this study, the non-precious metal copper was used in a highly reproducible manner by atomically dispersing it on the WO3 surface. When glycerol oxidation is carried out on this catalyst, photogenerated voids are activated and subsequently coupled to OH-, which eventually dehydrates to form the desired product[132].
Ren et al. have turned their attention to biomass photoreforming hydrogen. The unstable hydrogen yield from biomass reforming is due to the intermittent nature of solar light and partial degradation of biomass C-C bonds. To address this issue, this study proposes the strategy of ‘‘C-C bond first’’ [Figure 22A], which utilizes carbohydrates from hydrolysis of lignocellulose as feedstock. First, biomass carbons are converted into liquid hydrogen carriers (LHCs) with nearly almost complete yield of C1 products. Following degradation of the LHCs exclusively produces H2 and CO2. This strategy is realized using highly distorted Ta-doped CeO2 (Ta-CeO2), which effectively regulates C-C bond scission, C-C bond coupling, and the conversion of recalcitrant carbonyl compounds. The monosaccharides and biopolyols converted from cellulose and hemicellulose can be used for the preparation of C1 LHCs by photocatalytic oxidation in yields ranging from 62% to 86%. The obtained aqueous solution of C1 LHCs can be directly used for photocatalytic hydrogen production after simple filtration to remove Ta-CeO2 catalysts. When Pt/P25 is used as the photocatalyst or Ru-complex as the thermocatalyst for H2 generation, LHCs are nearly completely decomposed, releasing H2. The H2 yields are 33% and 30%, respectively. In contrast, direct photocatalytic hydrogen production from glucose with Pt/P25 yielded only 13% H2. Interestingly, the authors built a laboratory-scale flow reaction unit to demonstrate the feasibility of this scheme. After three days and a cumulative 15.5 h of sunlight, the C1 LHCs yielded 15%, of which 19% of glucose was converted
Figure 22. (A) Illustration of a stepwise method for H2 production from biomass and storage in the form of C1 LHCs; (B-D) Photocatalytic glucose decomposition to H2 and CO2 and solar-light driven glucose oxidation to C1 LHCs; (E) Reaction results of
Amino acids are traditionally prepared using biocatalytic methods, which have disadvantages such as
Photoelectrocatalysis
Photoelectrochemical (PEC) biomass conversion is a sustainable, energy-efficient synthetic route driven by renewable energy under mild conditions[135-137]. Photoelectrocatalysis uses solar energy to convert raw materials into target products; similar to photocatalysis and electrocatalysis, it possesses the advantages of low cost, green and energy saving. In recent years, photoelectrocatalysis has emerged as a promising approach in the field of biomass. The published studies have focused on the oxidation of various biomass alcohols/aldehydes, including glycerol and glucose. In addition, PEC technology can directly decompose water into H2 and O2, which is considered to be one of the most green and efficient ways to produce hydrogen from water decomposition. Therefore, the coupling of oxidation reaction with hydrogen generation has become increasingly attractive. The thermodynamic and kinetic advantages of anodic oxidation over water oxidation result in a higher cathodic hydrogen yield. Current research in the field of PEC is mainly devoted to improving the efficiency of photoelectrolysis of water for hydrogen production by improving the performance of photoanodes, but it has not achieved the expected results. This section will set out the potential of SACs to enhance biomass photoelectrochemistry.
Few studies have discussed the value conversion of glucose, and in this study, Tian et al. provide a good method to selectively oxidize glucose to high value glucosinolates. The use of defective TiO2, as a photoanode, modulates the carrier current and improves charge separation, and the loading of Pt monoatoms onto the anode significantly improves the selectivity and yield of the C6 product [Figure 23]. The final results show that the metal monoatom modified anode achieved a glucose oxidation photocurrent density of 1.91 mAcm-2 at 0.6 V relative to the reversible hydrogen electrode, with a glucosonic acid yield of 84.3% under simulated sunlight irradiation[135]. This work opens our eyes to the possibilities between PEC strategies and SACs, providing new inspiration for biomass catalysis from multiple perspectives.
Figure 23. The schematic illustration of selective PEC oxidation of glucose to GLA over the single-atom Pt decorated defective TiO2 photoanode[135]. Copyright 2023 Springer Nature. PEC: Potoelectrochemical; GLA: glucosonic acid.
A Pt single-atom dispersed WO3 amorphous/crystalline homojunction for anodic glycerol oxidation reaction (GOR) coupled with cathodic hydrogen generation was recently reported by Feng et al.[137]. Experimental data and DFT calculations have demonstrated that the isolated Pt single-atom and vacancies regulate the band structure, built-in electric field, and surface charge density. This promotes the transfer of holes from the bulk to the surface and accelerates the conversion of glycerol with a middle hydroxyl to carbon-centered radicals. The coupled photoelectrocatalytic system allows for the simultaneous anodic organic PEC conversion for high-value chemicals and cathodic hydrogen generation. The Pt-SA/WOx photoanode exhibits superior PEC activity, achieving a photocurrent density of 2.85 mA cm-2 at 1.2 V versus RHE. It also exhibits a glycerol conversion rate of up to 297.3 mmol m-2 h-1 and a DHA selectivity of 60.2%, while generating cathodic hydrogen at a rate of 203.2 mmol m-2 h-1.
CONCLUSION AND OUTLOOK
As mentioned above, SACs have emerged as a promising candidate for advancing biomass catalytic conversion including thermocatalysis, electrocatalysis, photocatalysis and photoelectrocatalysis. This catalytic approach offers various advantages, including reduced energy requirements, enhanced catalytic precision, and improved stability. Consequently, researchers worldwide have dedicated significant efforts and resources to the advancement of biorefinery technologies capable of processing diverse biomass feedstocks. In this review, we comprehensively summarized the impact of SACs across a broad field of biomass upgrading strategies encompassing diverse reaction types such as hydro(deoxy)genation[42-45,48,51,52,57,63], oxidation[46,47,72-74], RCF[110-113]etc., as summarized in Table 1. The substrates investigated include a wide range of lignocellulosic feedstocks comprising furans, sugars, alcohols and polyols, ketones, acids and lignin derivatives. Despite showcasing promising efficacy in the realm of biomass conversion, it is evident that this catalytic technology is still in its nascent developmental stages. Therefore, the challenges and future developments may fall into the following categories.
Catalytic performance of some SACs in thermocatalytic reactions
| Entry | Catalyst | Reactant | Product | Reaction conditions | Results | Ref. |
| 1 | Pt1/Co (SAA) |  |  | 1.0 MPa H2, 180 oC, 2 h | X = 100%, S = 92.9% | [41] |
| 2 | Pt1/Ni3Fe | 1.0 MPa H2, 160 oC, 90 min | X = 99.0%, S = 98.1% | [42] | ||
| 3 | Pt1/Nb2O5-Ov |  |  | 4.0 MPa H2, 160 oC, 1 h | S ＞ 99%，TOF = 2,553 h-1 | [44] |
| 4 | (Co1→Cop)/N-CNTs |  |  | 0.1 MPa H2, 100 oC, 8 h | X = 100%, S = 96% | [46] |
| 5 | Mn-Co/N-C | 0.6 MPa H2, 80 oC, 8 h | Y = 95.8% | [47] | ||
| 6 | 0.1Pd/γ-AlOOH |  |  | 7 MPa H2, 240 oC, 7 h | X = 73.5%, S = 98.7%, TOF = 305.4 h-1 | [48] |
| 7 | Pt1/1T-MoS2 |  |  | [50] | ||
| 8 | Pd1/BNC |  |  | 0.5 MPa H2, 0.2 MPa NH3, 80 oC, 6 h | X = 100%, S = 96% | [49] |
| 9 | Ru1@WOx/CN |  |  | 2 MPa H2, 100 oC, 2 h Solvent-free | X = 99%, S = 100% | [51] |
| 10 | Ir1/mpg-C3N4 |  |  | 1 MPa H2, 140 oC, 6 h | X = 99%, S = 100% | [60] |
| 11 | Fe-ZIF-8-800 | 120 oC, 1 h | X = 93.1%, S = 93.5% | [63] | ||
| 12 | Ru1/mpg-C3N4 |  |  | 4 MPa H2, 140 oC, 4 h | X = 100%, S = 100% | [70] |
 |  | 4 MPa H2, 60 oC, 72 h | X = 95%, S = 100% | |||
| 13 | Au@Pd/TiO2 |  |  | O2 50ml min-1, 90 °C, 4 h, Solvent-free | X = 14.3%, S = 91.6%, TOF = 21,961 h-1 | [72] |
| 14 | Au1/CeO2 | 0.5 MPa O2, 150 °C, 8 h, Solvent-free | X = 51.9%, S = 93%, TOF = 3283 h-1 | [73] | ||
| 15 | Co SAs-N@C |  |  | 0.1 MPa O2, 60 °C, 3 h, | X = 99.8%, S = 97.5% | [74] |
| 16 | Pd1/ZSM-5 |  |  | 2 MPa H2, 180 oC, 3h, Water-solvent | S = 91.2%, TOR = 400.8 h-1 | [75] |
| 17 | RuCo-300 |  |  | 1 MPa H2, 250 oC, 2 h | X = 100%, S1 = 56.2% S2 = 31.3% | [81] |
| 18 | Ru/CeO2-S |  |  | 1 MPa H2, 200 oC, 5h, Water-solvent | X = 99.9%, S = 99.9% | [82] |
| 19 | Co1-NPs@NC |  |  | 0.5 MPa H2, 180 oC, 3 h | X = 100%, S = 97.1% | [83] |
| 20 | Ni1/β-Mo2C |  |  | 4 MPa H2, 260 oC, 4 h | X = 99.9%, S = 99.9%, TON = 1683 h-1 | [84] |
| 21 | PtCu-SAA |  |  | 2 MPa H2, 200 oC, 6 h | X = 99.6%, S = 99.2% | [100] |
From the perspective of biomass valorization
Typically, SACs focus on catalyzing the small molecule compounds produced in the downstream process. Compared with more structurally complex components such as lignin, only a few studies have dealt with its catalytic conversion. In contrast, SACs did not show particularly good catalytic advantages in the catalytic reduction fractionation of lignin. Since the metal loading of monoatomic catalysts is usually in the range of 0.1-5 wt%, more catalysts have to be used in the thermal catalysis of biomass. Moreover, the catalytic strategy developed for SACs did not achieve promising results than commercially available catalysts. Therefore, the efficient catalytic conversion of raw biomass material remains at the forefront of SACs research.
From the perspective of developing advanced catalytic systems
Although there have been increasing reports on advanced technologies for fabricating SACs, such as coordination pyrolysis, defect engineering, alloy dilution, impregnation, deposition precipitation (DP), atomic layer deposition (ALD), sol-gel, and ball milling, etc. Nevertheless, most of these strategies are developed at the laboratory scale and necessitate harsh and intricate operational conditions. In addition, when the metal particles are reduced to the single-atom level, the specific surface area increases dramatically, leading to a sharp increase in the free energy of the metal surface, which is highly susceptible to agglomeration coupling to form large clusters during preparation and reaction, thus leading to catalyst deactivation. In thermocatalysis, high-temperature environments may lead to a decrease in the stability of the SAC. Moreover, for the activation, breaking or generation of certain chemical bonds, a single metal atom may not provide the active site required for catalysis. Therefore, the current research trend has shifted from solely focusing on catalysis with single metal sites or alloys to embracing dual-site catalysis (single metal) to explore more catalytic possibilities. The presence of diverse metal active sites within SACs, including single-metal sites, bimetallic alloy sites or combinations of single-metal sites with NPs or nanoclusters (NCs), can lead to significant variations in catalytic performance. In consequence, future research will concentrate on the preparation of more stable and durable SACs and the development of tunable site configurations in SACs.
From the perspective of potential industrial applications
Despite the ongoing positive advancements in this field, practical applications in related industries have yet to be observed. Biomass feedstocks are inherently complex and often encounter a wide range of impurities and side reactions during practical implementation; this makes biomass conversion difficult in industrial applications. So, we need higher loading and higher activity SACs for industrial high productivity. Pretreatment of biomass feedstocks is harsh, such as high temperatures, high pressures, and polar and acidic media; the improvement of catalyst stability is also essential. Additionally, achieving mass production of SACs poses a significant challenge as only a limited number can currently be produced on a kilogram scale, and cost and equipment should also be considered. All results obtained thus far have been confined to laboratory settings without considering the multifactorial effects in production. In the face of these challenges, sustained research and innovation are needed to improve the efficiency, scalability and economy of SACs preparation and characterization technologies, and to strengthen industry-university-research collaboration to promote rapid technology translation and industrial application.
In conclusion, the aforementioned summary presents novel prospects and implications for the advancement of SAC and biomass conversion, offering extensive research opportunities. With advancements in detection technology and modern material science, SACs are poised to witness a surge in development opportunities. Furthermore, within the context of globalization, interdisciplinary integration will provide an enhanced innovative space for biomass and SACs.
DECLARATIONS
Authors’ contributions
Made the literature review and drafted the original version: Gao, Y.; Cheng, J.
Revised the manuscript: Gao, Y.; Cheng, J.
Conceived and supervised the project: Sun, Z.; Liu, X.
Availability of data and materials
Not applicable.
Financial support and sponsorship
The authors thank the foundational support by the National Natural Science Foundation of China (22378024 and 22101029) and Beijing Municipal-Natural-Science Foundation (2222006).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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Gao, Y.; Cheng, J.; Liu, X.; Sun, Z. Recent progress in single atom catalysts for biomass conversion. Microstructures 2025, 5, 2025017. http://dx.doi.org/10.20517/microstructures.2024.49
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