Structural Design of Single-Atom Catalysts for Enhancing Petrochemical Catalytic Reaction Process

Petroleum, as the "lifeblood" of industrial development, is the important energy source and raw material. The selective transformation of petroleum into high-end chemicals is of great significance, but still exists enormous challenges. Single-atom catalysts (SACs) with 100% atom utilization and homogeneous active sites, promise a broad application in petrochemical processes. Herein, the research systematically summarizes the recent research progress of SACs in petrochemical catalytic reaction, proposes the role of structural design of SACs in enhancing catalytic performance, elucidates the catalytic reaction mechanisms of SACs in the conversion of petrochemical processes, and reveals the high activity origins of SACs at the atomic scale. Finally, the key challenges are summarized and an outlook on the design, identification of active sites, and the appropriate application of artificial intelligence technology is provided for achieving scale-up application of SACs in petrochemical process.

Suppressing Metal Leaching and Sintering in Hydroformylation Reaction by Modulating the Coordination of Rh Single Atoms with Reactants

Hydroformylation reaction is one of the largest homogeneously catalyzed industrial processes yet suffers from difficulty and high cost in catalyst separation and recovery. Heterogeneous single-atom catalysts (SACs), on the other hand, have emerged as a promising alternative due to their high initial activity and reasonable regioselectivity. Nevertheless, the stability of SACs against metal aggregation and leaching during the reaction has rarely been addressed. Herein, we elucidate the mechanism of Rh aggregation and leaching by investigating the structural evolution of Rh1@silicalite-1 SAC in response to different adsorbates (CO, H2, alkene, and aldehydes) by using diffuse reflectance infrared Fourier transform spectroscopy, X-ray adsorption fine structure, and scanning transmission electron microscopy techniques and kinetic studies. It is discovered that the aggregation and leaching of Rh are induced by the strong adsorption of CO and aldehydes on Rh, as well as the reduction of Rh3+ by CO/H2 which weakens the binding of Rh with support. In contrast, alkene effectively counteracts this effect by the competitive adsorption on Rh atoms with CO/aldehyde, and the disintegration of Rh clusters. Based on these results, we propose a strategy to conduct the reaction under conditions of high alkene concentration, which proves to be able to stabilize Rh single atom against aggregation and/or leaching for more than 100 h time-on-stream.

Efficient Interfacial Sites between Metallic and Oxidized Cobalt for Propene Hydroformylation

Currently, the hydroformylation of short olefins is operated almost exclusively by using Rh catalysts. Considering the high cost and scarcity of rhodium resources, it is important to develop non-noble metal catalysts toward hydroformylation. Herein, we report an efficient cobalt-based catalyst rich in interfacial sites between metallic and oxidized cobalt species for the hydroformylation of short olefin, propene, under a moderate syngas pressure. The catalyst exhibited a high specific activity of 252 mol molCo-1 h-1 in toluene under 2 bar of propene and 40 bar of CO/H2 mixed gas (CO/H2 = 1:1) at 160 °C. According to mechanistic studies, the interface of metallic and oxidized cobalt species promoted the adsorption of CO and propene. Moreover, the interfacial sites lowered the energy barrier for CO* hydrogenation and C-C coupling compared with metallic cobalt.

Ethylene Methoxycarbonylation over Heterogeneous Pt1/MoS2 Single-Atom Catalyst: Metal-Support Concerted Catalysis

Ethylene methoxycarbonylation (EMC) to methyl propanoate (MP) is an industrially important reaction and commercially uses a homogeneous Pd-phosphine organometallic complex as the catalyst and corrosive strong acid as the promoter. In this work, we develop a Pt1/MoS2 heterogeneous single-atom catalyst (SAC) which exhibits high activity, selectivity, and good recyclability for EMC reaction without need of any liquid acid. The production rate of MP achieves 0.35 gMP gcat-1 h-1 with MP selectivity of 91.1% at 1 MPa CO, 1 MPa C2H4, and 160 °C, which can be doubled at 2 MPa CO and corresponds to 320.1 molMP molPt-1 h-1, at least 1 order of magnitude higher than the earlier reported heterogeneous catalyst and even comparable to some of homogeneous catalyst. Advanced characterizations and DFT calculations reveal that all the Pt single atoms are located at the Mo vacancies along the Mo edge of the MoS2 nanosheets, forming pocket-like Mo-S-Pt1-S-Mo ensembles with uniform and well-defined structure. Methanol is first adsorbed and dissociated on Mo sites, and the produced H spillovers to the adjacent Pt site forming Pt-H species which then activate ethylene, forming Pt-ethyl species. Meanwhile, CO adsorbed on the other Mo site reacts with the Pt-ethyl species, yielding propionyl species, and this carbonylation is the rate-determining step. The final methoxylation step proceeds via the nucleophilic attack of propionyl species by -OCH3 affording the final product MP. Such a metal-support concerted catalysis enabled by the Mo-S-Pt1-S-Mo multisite ensemble opens a new avenue for SACs to promote the multimolecular reactions that prevail in homogeneous catalysis.

Recent Advances On Direct Formylation Reactions

Aldehydes serve as the key functional group in organic synthesis and are valuable intermediates. The various advanced methods of direct formylation reactions have been reviewed in this article. Overcoming the drawbacks of the traditional methods of formylation, newer methods involving homo and heterogenous catalysts, one pot reactions, solvent free techniques are elaborated, which can be performed under mild conditions and using inexpensive resources.

Single-Atom Catalysis in Organic Synthesis

Single-atom catalysts hold the potential to significantly impact the chemical sector, pushing the boundaries of catalysis in new, uncharted directions. These materials, featuring isolated metal species ligated on solid supports, can exist in many coordination environments, all of which have shown important functions in specific transformations. Their emergence has also provided exciting opportunities for mimicking metalloenzymes and bridging the gap between homogeneous and heterogeneous catalysis. This Review outlines the impressive progress made in recent years regarding the use of single-atom catalysts in organic synthesis. We also illustrate potential knowledge gaps in the search for more sustainable, earth-abundant single-atom catalysts for synthetic applications.

Selective Oxidation of Methane to Oxygenates using Oxygen via Tandem Catalysis

Selective oxidation of methane to oxygenates using low-cost and environment-friendly molecular oxygen (O₂ ) under mild reaction conditions is a promising strategy but still remains grand challenge. It is of great importance to accelerate the activation of O₂ to generate highly active oxygen species, such as hydroxyl peroxide and hydroxyl species to improve catalytic performance for selective oxidation of methane. Selective oxidation of methane using O₂ by coupling with in situ generation of hydrogen peroxide via tandem catalysis ensures the easy formation of active oxygen species for methane activation, leading to high oxygenates productivity under mild conditions. In this concept, we summarized the recent progresses for selective oxidation of methane to oxygenates using O₂ based on tandem catalysis by coupling with in situ generation of hydrogen peroxide. The remaining challenges and future perspectives for selective oxidation of methane to oxygenates via tandem catalysis were also proposed.

Single-Atom Catalysts for Hydrogen Activation

Selective hydrogenation is one of the most important reactions in fine chemical industry, and the activation of H₂ is the key step for hydrogenation. Catalysts play a critical role in selective hydrogenation, and some single-atom catalysts (SACs) are highly capable of activating H₂ in selective hydrogenation by virtue of the maximized atom utilization and the highly uniform active sites. Therefore, more research efforts are needed for the rational design of SACs with superior H₂ -activating capabilities. Herein, the research progress on H₂ activation in typical hydrogenation systems (such as alkyne hydrogenation, hydroformylation, hydrodechlorination, hydrodeoxygenation, nitroaromatics hydrogenation, and polycyclic aromatics hydrogenation) is reviewed, the mechanisms of SACs for H₂ activation are summarized, and the structural regulation strategies for SACs are proposed to promote H₂ activation and provide schemes for the design of high-selectivity hydrogenation catalysts from the atomic scale. At the end of this review, an outlook on the opportunities and challenges for SACs to be developed for selective hydrogenation is presented.

Single-atom catalysts for hydroformylation of olefins

Hydroformylation is one of the most significant homogeneous reactions. Compared with homogeneous catalysts, heterogeneous catalysts are easy to be separated from the system. However, heterogeneous catalysis faces the problems of low activity and poor chemical/regional selectivity. Therefore, there are theoretical and practical significance to develop efficient heterogeneous catalysts. SACs can be widely applied in hydroformylation in the future, due to the high atom utilization efficiency, stable active sites, easy separation, and recovery. In this review, the recent advances of SACs for hydroformylation are summarized. The regulation of microstructure affected on the reactivity, stability of SACs, and chem/regioselectivity of SACs for hydroformylation are discussed. The support effect, ligand effect, and electron effect on the performance of SACs are proposed, and the catalytic mechanism of SACs is elaborated. Finally, we summarize the current challenges in this field, and propose the design and research ideas of SACs for hydroformylation of olefins.

Electrothermal Water-Gas Shift Reaction at Room Temperature with a Silicomolybdate-Based Palladium Single-Atom Catalyst

The water-gas shift (WGS) reaction is often conducted at elevated temperature and requires energy-intensive separation of hydrogen (H₂ ) from methane (CH₄ ), carbon dioxide (CO₂ ), and residual carbon monoxide (CO). Designing processes to decouple CO oxidation and H₂ production provides an alternative strategy to obtain high-purity H₂ streams. We report an electrothermal WGS process combining thermal oxidation of CO on a silicomolybdic acid (SMA)-supported Pd single-atom catalyst (Pd₁ /CsSMA) and electrocatalytic H₂ evolution. The two half-reactions are coupled through phosphomolybdic acid (PMA) as a redox mediator at a moderate anodic potential of 0.6 V (versus Ag/AgCl). Under optimized conditions, our catalyst exhibited a TOF of 1.2 s^-1 with turnover numbers above 40 000 mol CO 2 ${{_{{\rm CO}{_{2}}}}}$  mol_Pd ^-1 achieving stable H₂ production with a purity consistently exceeding 99.99 %.

Repair of missing linker defects in UiO-66 by a "molecular patch" boosting the 1-hexene hydroformylation reaction

Fabrication of MOFs with missing linker defects has become a common means to improve catalytic performances. However, the stability of the defects deserves to be investigated first. In this work, we found that 3-phenylpropionaldehyde (3-PPA) could coordinate with the missing linker defects of UiO-66, which highlighted the instability of the missing linker defects. 3-PPA acted as a molecular patch for the modification of the Rh/UiO-66 catalyst, which repaired the open Zr₆ sites and resulted in a remarkable improvement of aldehyde selectivity (from 50.0% to 89.6%) in 1-hexene hydroformylation.

Highly dispersed Rh single atoms over graphitic carbon nitride as a robust catalyst for the hydroformylation reaction

Rhodium-catalysed hydroformylation, effective tool in bulk and fine-chemical synthesis, predominantly uses soluble metal complexes. For that reason, the metal leaching and the catalyst recycling are still the major drawbacks of this process. Single-atom catalysts have emerged as a powerful tool to combine the advantages of both homogeneous and heterogeneous catalysts. Since using an appropriate support material is key to create stable, finely dispersed, single-atom catalysts, here we show that Rh atoms anchored on graphitic carbon nitride are robust catalysts for the hydroformylation reaction of styrene.

Surface and Interface Coordination Chemistry Learned from Model Heterogeneous Metal Nanocatalysts: From Atomically Dispersed Catalysts to Atomically Precise Clusters

The surface and interface coordination structures of heterogeneous metal catalysts are crucial to their catalytic performance. However, the complicated surface and interface structures of heterogeneous catalysts make it challenging to identify the molecular-level structure of their active sites and thus precisely control their performance. To address this challenge, atomically dispersed metal catalysts (ADMCs) and ligand-protected atomically precise metal clusters (APMCs) have been emerging as two important classes of model heterogeneous catalysts in recent years, helping to build bridge between homogeneous and heterogeneous catalysis. This review illustrates how the surface and interface coordination chemistry of these two types of model catalysts determines the catalytic performance from multiple dimensions. The section of ADMCs starts with the local coordination structure of metal sites at the metal-support interface, and then focuses on the effects of coordinating atoms, including their basicity and hardness/softness. Studies are also summarized to discuss the cooperativity achieved by dual metal sites and remote effects. In the section of APMCs, the roles of surface ligands and supports in determining the catalytic activity, selectivity, and stability of APMCs are illustrated. Finally, some personal perspectives on the further development of surface coordination and interface chemistry for model heterogeneous metal catalysts are presented.

Design of Single-Atom Catalysts and Tracking Their Fate Using Operando and Advanced X-ray Spectroscopic Tools

The potential of operando X-ray techniques for following the structure, fate, and active site of single-atom catalysts (SACs) is highlighted with emphasis on a synergetic approach of both topics. X-ray absorption spectroscopy (XAS) and related X-ray techniques have become fascinating tools to characterize solids and they can be applied to almost all the transition metals deriving information about the symmetry, oxidation state, local coordination, and many more structural and electronic properties. SACs, a newly coined concept, recently gained much attention in the field of heterogeneous catalysis. In this way, one can achieve a minimum use of the metal, theoretically highest efficiency, and the design of only one active site-so-called single site catalysts. While single sites are not easy to characterize especially under operating conditions, XAS as local probe together with complementary methods (infrared spectroscopy, electron microscopy) is ideal in this research area to prove the structure of these sites and the dynamic changes during reaction. In this review, starting from their fundamentals, various techniques related to conventional XAS and X-ray photon in/out techniques applied to single sites are discussed with detailed mechanistic and in situ/operando studies. We systematically summarize the design strategies of SACs and outline their exploration with XAS supported by density functional theory (DFT) calculations and recent machine learning tools.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Homogeneity of Supported Single-Atom Active Sites Boosting the Selective Catalytic Transformations

Selective conversion of specific functional groups to desired products is highly important but still challenging in industrial catalytic processes. The adsorption state of surface species is the key factor in modulating the conversion of functional groups, which is correspondingly determined by the uniformity of active sites. However, the non-identical number of metal atoms, geometric shape, and morphology of conventional nanometer-sized metal particles/clusters normally lead to the non-uniform active sites with diverse geometric configurations and local coordination environments, which causes the distinct adsorption states of surface species. Hence, it is highly desired to modulate the homogeneity of the active sites so that the catalytic transformations can be better confined to the desired direction. In this review, the construction strategies and characterization techniques of the uniform active sites that are atomically dispersed on various supports are examined. In particular, their unique behavior in boosting the catalytic performance in various chemical transformations is discussed, including selective hydrogenation, selective oxidation, Suzuki coupling, and other catalytic reactions. In addition, the dynamic evolution of the active sites under reaction conditions and the industrial utilization of the single-atom catalysts are highlighted. Finally, the current challenges and frontiers are identified, and the perspectives on this flourishing field is provided.

Single-Atom Catalysis: From Simple Reactions to the Synthesis of Complex Molecules

To date, the scope of single-atom catalysts (SAC) in liquid-phase transformations is rather limited owing to stability issues and the inability to activate complex substances. This calls for a better design of the catalyst support that can provide a dynamic coordination environment needed for catalytic action, and yet retain robustness against leaching or aggregation. In addition, the chemical orthogonality of SAC is useful for designing tandem or multicomponent reactions, in which side reactions common to metal nanoparticles are suppressed. In this review, the intrinsic mechanism will be highlighted that controls reaction efficiency and selectivity in SAC-catalyzed pathways, as well as the structural dynamism of SAC under complex liquid-phase conditions. These mechanistic insights are helpful for the development of next-generation SAC systems for the synthesis of high-value pharmaceuticals through late-stage functionalization, sequential and multicomponent strategies.

N-Doped Graphene Supported Cu Single Atoms: Highly Efficient Recyclable Catalyst for Enhanced C-N Coupling Reactions

Heterogenization of homogeneous catalysis through supported single-atom catalysts (SACs) provided a feasible solution to recycling catalysts while keeping its efficiency in chemical synthesis. In this work, Cu SACs anchored on N-doped graphene (Cu SACs/NG) were prepared and first used for C-N coupling reactions. During the preparation, Cu-N-C structures, including Cu-N₄ moieties, were formed in a one-step pyrolysis method. As-prepared Cu SACs/NG exhibited excellent catalytic activity toward C-N coupling reactions with a broad scope of substrates and showed outstanding performance of recycling. Compared with Cu nanoparticles (Cu NPs/NG), the advantages of single-atom catalysts were validated via experimental and theoretical calculations. The enhanced performances were attributed to increasing the number of active sites and increasing the intrinsic activity of each active site. This work provides an alternative synthetic strategy for fabricating atomically dispersed SACs and represents a significant advance for coupling reactions.

Understanding the facet effects of heterogeneous Rh2P catalysts for styrene hydroformylation

Rh2P (111) facets are much more active than the other facets for heterogeneous hydroformylation.

Recent development towards alkene hydroformylation catalysts integrating traditional homo- and heterogeneous catalysis

This mini-review provides the recent progress towards catalysts for the hydroformylation of catalysts that bridge traditional homo- and heterogeneous catalysis, highlighting the future development of heterogeneous catalysts in hydroformylation.

The mechanism of MOF as the heterogeneous catalyst for propene hydroformylation: The DFT study

Metal-organic framework which was composed of metal center and organic linkers possessing the similar structure with the homogeneous metal-complex catalyst for hydroformylation, thus it could be potentially used as the...

Noble-metal based single-atom catalysts for the water-gas shift reaction

Single-atom catalysts (SACs) have attracted great attention in heterogeneous catalysis. In this Feature Article, we summarize the recent advances of typical Au and Pt-group-metal (PGM) based SACs and their applications in the water-gas shift (WGS) reaction in the past two decades. First, oxide and carbide supported single atoms are categorized. Then, the active sites in the WGS reaction are identified and discussed, with SACs as the positive state or metallic state. After that, the reaction mechanisms of the WGS are presented, which are classified into two categories of redox mechanism and associative mechanism. Finally, the challenges and opportunities in this emerging field for the collection of hydrogen are proposed on the basis of current developments. It is believed that more and more exciting findings based on SACs are forthcoming.

Heterogeneous Catalysis in Water

Heterogeneous catalytic processes produce the majority of the fuels and chemicals in the chemical industry and have kept improving the welfare of human beings for centuries. Although most of the heterogeneous catalytic reactions occur at the gas-solid interface, numerous cases have demonstrated that the condensed water near the active site and/or the aqueous phase merging the catalysts play positive roles in enhancing the performance of heterogeneous catalysts and creating novel catalytic conversion routes. We enumerate the traditional heterogeneous catalytic reactions that enable significant rate/selectivity promotion in the aqueous phase or adsorbed micro water environment and discuss the role of water in specific systems. Some of the novel heterogeneous reactions achieved with only the assistance of the aqueous phase have been summarized. The development of reactions with the participation of the aqueous phase/water and the investigation of the role of water in the heterogeneous catalytic reactions will open new horizons for catalysts with better activity, improved selectivity, and novel processes.

Phosphorus coordinated Rh single-atom sites on nanodiamond as highly regioselective catalyst for hydroformylation of olefins

Single-atom Rh catalysts present superior activity relative to homogeneous catalyst in olefins hydroformylation, yet with limited success in regioselectivity control. In the present work, we develop a phosphorus coordinated Rh₁ single-atom catalyst with nanodiamond as support. Benefiting from this unique structure, the catalyst exhibits excellent activity and regioselectivity in hydroformylation of arylethylenes with wide substrate generality, i.e., with high conversion (>99%) and high regioselectivity (>90%), which is comparable with the homogeneous counterparts. The coordination interaction between Rh₁ and surface phosphorus species is clarified by ³¹P solid-state NMR and X-ray absorption spectroscopy (XAS). Rh single atoms are firmly anchored over nanodiamond through Rh-P bonds, guaranteeing good stability in the hydroformation of styrene even after six runs. Finally, by using this catalyst, two kinds of pharmaceutical molecules, Ibuprofen and Fendiline, are synthesized efficiently with high yields, demonstrating a new prospect of single-atom catalyst in pharmaceutical synthesis.

Single-atom Rh catalysts present superior activity in olefins hydroformylation, yet with limited success in regioselectivity control. Here the authors develop a Rh1 single-atom catalyst with nanodiamond as support, with which good to excellent regioselectivities to branched aldehydes in hydroformylation of terminal olefins are achieved.

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

Selective cleavage of C-C linkages is the key and a challenge for lignin degradation to harvest value-added aromatic compounds. To this end, electrocatalytic oxidation presents a promising technique by virtue of mild reaction conditions and strong sustainability. However, the existing electrocatalysts (traditional bulk metal and metal oxides) for C-C bond oxidative cleavage suffer from poor selectivity and low product yields. We show for the first time that atomically dispersed Pt-N₃C₁ sites planted on nitrogen-doped carbon nanotubes (Pt₁/N-CNTs), constructed via a stepwise polymerization-carbonization-electrostatic adsorption strategy, are highly active and selective toward C_α-C_β bond cleavage in β-O-4 model compounds under ambient conditions. Pt₁/N-CNTs exhibits 99% substrate conversion with 81% yield of benzaldehyde, which is exceptional and unprecedented compared with previously reported electrocatalysts. Moreover, Pt₁/N-CNTs using only 0.41 wt % Pt achieved a much higher benzaldehyde yield than those of the state-of-the-art bulk Pt electrode (100 wt % Pt) and commercial Pt/C catalyst (20 wt % Pt). Systematic experimental investigation together with density functional theory (DFT) calculation suggests that the superior performance of Pt₁/N-CNTs arises from the atomically dispersed Pt-N₃C₁ sites facilitating the formation of a key C_β radical intermediate, further inducing a radical/radical cross-coupling path to break the C_α-C_β bond. This work opens up opportunities in lignin valorization via a green and sustainable electrochemical route with ultralow noble metal usage.