Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts

Advances in morphology-controlled alumina and its supported Pd catalysts: synthesis and applications

Alumina materials, as one of the cornerstones of the modern chemical industry, possess physical and chemical properties that include excellent mechanical strength and structure stability, which also make them highly suitable as catalyst supports. Alumina-supported Pd-based catalysts with the advantages of exceptional catalytic performance, flexible regulated surface metal/acid sites, and good regeneration ability have been widely used in many traditional chemical industry fields and have also shown great application prospects in emerging fields. This review aims to provide an overview of the recent advances in alumina and its supported Pd-based catalysts. Specifically, the synthesis strategies, morphology transformation mechanisms, and structural properties of alumina with various morphologies are comprehensively summarized and discussed in-depth. Then, the preparation approaches of Pd/Al2O3 catalysts (impregnation, precipitation, and other emerging methods), as well as the metal-support interactions (MSIs), are revisited. Moreover, Some promising applications have been chosen as representative reactions in fine chemicals, environmental purification, and sustainable development fields to highlight the universal functionality of the alumina-supported Pd-based catalysts. The role of the Pd species, alumina support, promoters, and metal-support interactions in the enhancement of catalytic performance are also discussed. Finally, some challenges and upcoming opportunities in the academic and industrial application of the alumina and its supported Pd-based are presented and put forward.

M-Ge-Si thermolytic molecular precursors and models for germanium-doped transition metal sites on silica

The synthesis, thermolysis, and surface organometallic chemistry of thermolytic molecular precursors based on a new germanosilicate ligand platform, -OGe[OSi(OtBu)3]3, is described. Use of this ligand is demonstrated with preparation of complexes containing the first-row transition metals Cr, Mn, and Fe. The thermolysis and grafting behavior of the synthesized complexes, Fe{OGe[OSi(OtBu)3]3}2 (FeGe), Mn{OGe[OSi(OtBu)3]3}2(THF)2 (MnGe) and Cr{OGe[OSi(OtBu)3]3}2(THF)2 (CrGe), was evaluated using a combination of thermogravimetric analysis; nuclear magnetic resonance (NMR), ultraviolet-visible (UV-Vis), and electron paramagnetic resonance (EPR) spectroscopies; and single-crystal X-ray diffraction (XRD). Grafting of the precursors onto SBA-15 mesoporous silica and subsequent calcination in air led to substantial changes in transition metal coordination environments and oxidation states, the implications of which are discussed in the context of low-coordinate and low oxidation state thermolytic molecular precursors.

Revisiting the Potential of Group VI Inorganic Precatalysts for the Ethenolysis of Fatty Acids through a Mechanochemical Approach

The utilization of biobased feedstocks to prepare useful compounds is a pivotal trend in current chemical research. Among a varied portfolio of naturally available starting materials, fatty acids are abundant, versatile substrates with multiple applications. In this context, the ethenolysis of unsaturated fatty acid esters such as methyl oleate is an atom-economical way to prepare functional C10 olefins with a biobased footprint. Despite the existence of a variety of metathesis catalysts for the latter process, there is a lack of readily available, efficient, and inexpensive catalytic systems based on earth-abundant metals (Mo, W) whose preparation does not require sophisticated syntheses and manipulations. Here, a systematic exploration of homogeneous and heterogeneous inorganic Mo, W (oxy)halides shows that MoOCl4, while inactive as a homogeneous species, forms active and selective silica-supported ethenolysis precatalysts able to reach equilibrium conversion of methyl oleate within a few minutes upon activation with SnMe4. Such heterogeneous MoOCl4-based precatalysts were easily accessed through mechanochemical solvent-free procedures and found to contain, upon characterization by elemental analysis and Raman spectroscopy, isolated (≡SiO)Mo(=O)Cl3 units or polymeric silica-supported [-O(≡SiO)nMoCl4-nO-]m (n = 1, 2) complexes depending on the molybdenum loading. The former isolated species exhibited a higher catalytic performance. The developed heterogeneous precatalysts could be applied to the ethenolysis of various substrates, including polyunsaturated fatty acid esters and industrial fatty acid methyl ester (FAME) mixtures from palm oil transesterification.

Molybdenum(0)-Tricarbonyl Complex Supported by an Azacalix-pyridine Ligand: Synthesis, Characterization, Surface Deposition and Conversion to a Molybdenum(VI)-Trioxo Complex with O2

Adsorption of metal-organic complexes on metallic surfaces to produce well-defined single site catalysts is a novel approach combining the advantages of homogeneous and heterogeneous catalysis. To avoid the "surface trans-effect" a dome-shaped molybdenum(0) tricarbonyl complex supported by an tolylazacalix[3](2,6)pyridine ligand is synthesized. This vacuum-evaporable complex both activates CO and reacts with molecular oxygen (O2) to form a Mo(VI) trioxo complex which in turn is capable of catalytically mediating oxygen transfer. The molybdenum tricarbonyl- and trioxo complexes are investigated in the solid state, in homogeneous solution and on noble metal surfaces (Cu, Au) employing a range of spectroscopic and analytical methods.

Modeling Complex Ligands for High Oxidation State Catalysis: Titanium Hydroamination with Unsymmetrical Ligands

A method for modeling high oxidation state catalysts is used on precatalysts with unsymmetrical and symmetrical bidentate ligands to get a more detailed understanding of how changes to ancillary ligands affect the hydroamination of alkynes catalyzed by titanium. To model the electronic donor ability, the ligand donor parameter (LDP) was used, and to model the steric effects, percent buried volume (% Vbur) was employed. For the modeling study, 7 previously unpublished unsymmetrical Ti(XX')(NMe2)2 precatalysts were prepared, where XX' is a chelating ligand with pyrrolyl/indolyl linkages. The rates of these unsymmetrical and 10 previously reported symmetrical precatalysts were used with the model kobs = a + b(LDP)1 + c(LDP)2 + d(% Vbur)1 + e(% Vbur)2, where a-e were found through least-squares refinement. The model suggests that (1) the two attachment points of the bidentate ligand XX' are in different environments on the metal (e.g., axial and equatorial in a trigonal bipyramidal or square pyramidal structure), (2) the position of the unsymmetrical ligand on the metal is determined by the electronics of the ligand rather than the sterics, and (3) that one side of the chelating ligand's electronics strongly influences the rate, while the other side's sterics more strongly influences the rate. From these studies, we were able to generate catalysts fitting to this model with rate constants larger than the fastest symmetrical catalyst tested.

Unveiling the Role and Stabilization Mechanism of Cu+ into Defective Ce-MOF Clusters during CO Oxidation

Copper single-site catalysts supported on Zr-based metal-organic frameworks (MOFs) are well-known systems in which the nature of the active sites has been deeply investigated. Conversely, the redox chemistry of the Ce-counterparts is more limited, because of the often-unclear Cu2+/Cu+ and Ce4+/Ce3+ pairs behavior. Herein, we studied a novel Cu2+ single-site catalyst supported on a defective Ce-MOF, Cu/UiO-67(Ce), as a catalyst for the CO oxidation reaction. Based on a combination of in situ DRIFT and operando XAS spectroscopies, we established that Cu+ sites generated during catalysis play a pivotal role. Moreover, the oxygen vacancies associated with Ce3+ sites and presented in the defective Cu/UiO-67(Ce) material are able to activate the O2 molecules, closing the catalytic cycle. The results presented in this work open a new route for the design of active and stable single-site catalysts supported on defective Ce-MOFs.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Synthesis of Polar-functionalized Isotactic Polypropylenes Using Commercial Heterogeneous Ziegler-Natta Catalyst

The efficient synthesis of polar-functionalized polypropylenes with high molecular weight and high stereoregularity represents a challenging task. This challenge becomes even more daunting when pursuing an industrially preferred heterogeneous process. This study demonstrated the realization of these goals through the use of commercial heterogeneous Ziegler-Natta catalysts in the copolymerization of propylene with ionic cluster polar monomers. The results revealed high copolymerization activity (∼1.1 × 107 g mol-1 h-1), moderate polar monomer incorporation ratios (∼4.9 mol %), high copolymer molecular weight (Mw > 105 g mol-1), high stereoregularity ([mmmm] ∼ 96%), and high melting temperature range (150-162 °C). The utilization of ionic cluster polar monomers improved the thermal stability as well as stereoselectivity of the catalyst. Moreover, the Ziegler-Natta catalyst can homopolymerize ionic cluster polar monomers with high activities (>104 g mol-1 h-1). The resulting polar-functionalized isotactic polypropylenes (iPP) exhibited superior tensile strength, impact strength, creep resistance, transparency, and crystallinity compared with nonpolar iPP. This enhancement was attributable to the dual roles of the ionic cluster polar monomer unit, serving as both a transparent nucleating agent and a dynamic cross-linking functionality. Furthermore, the polar-functionalized iPP exhibited improved compatibility with polar materials, offering benefits for applications in composites, recycling of mixed plastic wastes, 3D printing, and other fields. This study offered a comprehensive solution for the future industrial production of polar-functionalized iPP via copolymerization, bridging the gap between an efficient and practical copolymerization process from a synthetic chemistry perspective and enhanced material properties from an application perspective.

Reaction Chemistry at Discrete Organometallic Fragments on Black Phosphorus

Black phosphorus (bP) is a two-dimensional van der Waals material unique in its potential to serve as a support for single-site catalysts due to its similarity to molecular phosphines, ligands quintessential in homogeneous catalysis. However, there is a scarcity of synthetic methods to install single metal centers on the bP lattice. Here, we demonstrate the functionalization of bP nanosheets with molecular Re and Mo complexes. A suite of characterization techniques, including infrared, X-ray photoelectron and X-ray absorption spectroscopy as well as scanning transmission electron microscopy corroborate that the functionalized nanosheets contain a high density of discrete metal centers directly bound to the bP surface. Moreover, the supported metal centers are chemically accessible and can undergo ligand exchange transformations without detaching from the surface. The steric and electronic properties of bP as a ligand are estimated with respect to molecular phosphines. Sterically, bP resembles tri(tolyl)phosphine when monodentate to a metal center, and bis(diphenylphosphino)propane when bidentate, whereas electronically bP is a σ-donor as strong as a trialkyl phosphine. This work is foundational in elucidating the nature of black phosphorus as a ligand and underscores the viability of using bP as a basis for single-site catalysts.

Language models and protocol standardization guidelines for accelerating synthesis planning in heterogeneous catalysis

Synthesis protocol exploration is paramount in catalyst discovery, yet keeping pace with rapid literature advances is increasingly time intensive. Automated synthesis protocol analysis is attractive for swiftly identifying opportunities and informing predictive models, however such applications in heterogeneous catalysis remain limited. In this proof-of-concept, we introduce a transformer model for this task, exemplified using single-atom heterogeneous catalysts (SACs), a rapidly expanding catalyst family. Our model adeptly converts SAC protocols into action sequences, and we use this output to facilitate statistical inference of their synthesis trends and applications, potentially expediting literature review and analysis. We demonstrate the model's adaptability across distinct heterogeneous catalyst families, underscoring its versatility. Finally, our study highlights a critical issue: the lack of standardization in reporting protocols hampers machine-reading capabilities. Embracing digital advances in catalysis demands a shift in data reporting norms, and to this end, we offer guidelines for writing protocols, significantly improving machine-readability. We release our model as an open-source web application, inviting a fresh approach to accelerate heterogeneous catalysis synthesis planning.

Samarium- and Ytterbium-Grafted Periodic Mesoporous Silica for Carbon Dioxide Capture and Conversion

Immobilized coordination compounds of Lewis acidic metals are powerful catalytic components of systems for the cycloaddition of CO2 to epoxides that do not require sophisticated coordination frameworks to harness the metal center and modulate its activity. Surface organometallic chemistry (SOMC) is a valuable methodology to prepare well-defined and site-isolated surface complexes and coordination compounds on metal oxides, with ligand environments easily adjustable to a targeted catalytic reaction. In this work, the SOMC methodology is applied to prepare SmII, YbII, and SmIII alkoxide surface complexes on periodic mesoporous (organo)silica of distinct pore symmetry/size for application in the CO2 cycloaddition reaction. The surface complexes are readily accessible by the grafting of the bis(trimethylsilyl)amide precursors LnII[N(SiMe3)2]2(THF)2 (Ln = Sm, Yb) and SmIII[N(SiMe3)2]3, followed by ligand exchange with alcohols (ethanol and neopentanol). The use of periodic mesoporous supports led to hybrid materials with relatively high surface areas and pore sizes, affording good performance in CO2 capture and in the cycloaddition of CO2 to epoxides under mild conditions (60-80 °C, 1-10 bar). In terms of catalytic performance, recyclability, and low amount of added nucleophile TBAX (X = Br, I), the most active materials prepared in this work compare well to a variety of previously reported SOMC-derived surface complexes and to other heterogeneous Lewis acids displaying more elaborate ligand environments.

A strategy for high ethylene polymerization performance using titanium single-site catalysts

The synthesis of heterogeneous Ti(IV)-based catalysts for ethylene polymerization following surface organometallic chemistry concepts is described. The unique feature of this catalyst arises from the silica support, KCC-1700. It has (i) a 3D fibrous morphology that is essential to improve the diffusion of the reactants, and (ii) an aluminum-bound hydroxyl group, [(Si-O-Si)(Si-O-)2Al-OH] 2, used as an anchoring site. The [(Si-O-Si)(Si-O-)(Al-O-)TiNp3] 3 catalyst was obtained by reacting 2 with a tetrakis-(neopentyl) titanium TiNp4. The structure of 3 was fully characterized by FT-IR, advanced solid-state NMR spectroscopy [1H, 13C], elemental and gas-phase analysis (ICP-OES and CHNS analysis), and XPS. The benefits of combining these morphological (3D structure) and electronic properties of the support (aluminum plus titanium) were evidenced in ethylene polymerization. The results show a remarkable enhancement in the catalytic performance with the formation of HDPE. Notably, the resulting HDPE displays a molecular weight of 3 200 000 g mol-1 associated with a polydispersity index (PD) of 2.3. Moreover, the effect of the mesostructure (2D vs. 3D) was demonstrated in the catalytic activity for ethylene polymerization.

Activated platinum in gallium-based room-temperature liquid metals for enhanced reduction reactions

Room-temperature gallium-based liquid metals (LMs) have recently attracted significant attention worldwide for application in catalysis because of their unique combination of fluidic and catalytic properties. Platinum loading in LMs is expected to enhance the catalytic performance of various reaction systems. However, Pt-loaded methods for Ga-based LMs have not yet been sufficiently developed to improve the catalytic performance and Pt utilization efficiency. In this study, a novel method for the fabrication of Pt-incorporated LMs using Pt sputter deposition (Pt(dep)-LMs) was developed. The Pt(dep)-LMs contained well-dispersed Pt flakes with diameters of 0.89 ± 0.6 μm. The catalytic activity of the Pt(dep)-LM with a Pt loading of ∼0.7 wt% was investigated using model reactions such as methylene blue (MB) reduction and hydrogen production in an acidic aqueous solution. The Pt(dep)-LMs showed a higher MB reduction rate (three times) and hydrogen production (three times) than the LM loaded with conventional Pt black (∼0.7 wt%). In contrast to the Pt(dep)-LMs, solid-based Ga with a Pt loading of ∼0.7 wt% did not catalyze the reactions. These results demonstrate that Pt activation occurred in the Pt(dep)-LMs fabricated by Pt sputtering, and that the fluidic properties of the LMs enhanced the catalytic reduction reactions. Thus, these findings highlight the superior performance of the Pt deposition method and the advantages of using Pt-LM-based catalysts.

Dual Single-Atomic Co-Mn Sites in Metal-Organic-Framework-Derived N-Doped Nanoporous Carbon for Electrochemical Oxygen Reduction

Synthesizing dual single-atom catalysts (DSACs) with atomically isolated metal pairs is a challenging task but can be an effective way to enhance the performance for electrochemical oxygen reduction reaction (ORR). Herein, well-defined DSACs of Co-Mn, stabilized in N-doped porous carbon polyhedra (named CoMn/NC), are synthesized using high-temperature pyrolysis of a Co/Mn-doped zeolitic imidazolate framework. The atomically isolated Co-Mn site in CoMn/NC is recognized by combining microscopic as well as spectroscopic techniques. CoMn/NC exhibited excellent ORR activities in alkaline (E1/2 = 0.89 V) as well as in acidic (E1/2 = 0.82 V) electrolytes with long-term durability and enhanced methanol tolerance. Density functional theory (DFT) suggests that the Co-Mn site is efficiently activating the O-O bond via bridging adsorption, decisive for the 4e- oxygen reduction process. Though the Co-Mn sites favor O2 activation via the dissociative ORR mechanism, stronger adsorption of the intermediates in the dissociative path degrades the overall ORR activity. Our DFT studies conclude that the ORR on an Co-Mn site mainly occurs via bridging side-on O2 adsorption following thermodynamically and kinetically favorable associative mechanistic pathways with a lower overpotential and activation barrier. CoMn/NC performed excellently as a cathode in a proton exchange membrane (PEM) fuel cell and rechargeable Zn-air battery with high peak power densities of 970 and 176 mW cm-2, respectively. This work provides the guidelines for the rational design and synthesis of nonprecious DSACs for enhancing the ORR activity as well as the robustness of DSACs and suggests a design of multifunctional robust electrocatalysts for energy storage and conversion devices.

Tetraphenylporphyrin electrocatalysts for the hydrogen evolution reaction: applicability of molecular volcano plots to experimental operating conditions

Recent years have seen an increasing interest in molecular electrocatalysts for the hydrogen evolution reaction (HER). Efficient hydrogen evolution would play an important role in a sustainable fuel economy, and molecular systems could serve as highly specific and tunable alternatives to traditional noble metal surface catalysts. However, molecular catalysts are currently mostly used in homogeneous setups, where quantitative evaluation of catalytic activity is non-standardized and cumbersome, in particular for multistep, multielectron processes. The molecular design community would therefore be well served by a straightforward model for prediction and comparison of the efficiency of molecular catalysts. Recent developments in this area include attempts at applying the Sabatier principle and the volcano plot concept - popular tools for comparing metal surface catalysts - to molecular catalysis. In this work, we evaluate the predictive power of these tools in the context of experimental operating conditions, by applying them to a series of tetraphenylporphyrins employed as molecular electrocatalysts of the HER. We show that the binding energy of H and the redox chemistry of the porphyrins depend solely on the electron withdrawing ability of the central metal ion, and that the thermodynamics of the catalytic cycle follow a simple linear free energy relation. We also find that the catalytic efficiency of the porphyrins is almost exclusively determined by reaction kinetics and therefore cannot be explained by thermodynamics alone. We conclude that the Sabatier principle, linear free energy relations and molecular volcano plots are insufficient tools for predicting and comparing activity of molecular catalysts, and that experimentally useful information of catalytic performance can still only be obtained through detailed knowledge of the catalytic pathway for each individual system.

Ti-Doping in Silica-Supported PtZn Propane Dehydrogenation Catalysts: From Improved Stability to the Nature of the Pt-Ti Interaction

Propane dehydrogenation is an important industrial reaction to access propene, the world's second most used polymer precursor. Catalysts for this transformation are required to be long living at high temperature and robust toward harsh oxidative regeneration conditions. In this work, combining surface organometallic chemistry and thermolytic molecular precursor approach, we prepared well-defined silica-supported Pt and alloyed PtZn materials to investigate the effect of Ti-doping on catalytic performances. Chemisorption experiments and density functional calculations reveal a significant change in the electronic structure of the nanoparticles (NPs) due to the Ti-doping. Evaluation of the resulting materials PtZn/SiO₂ and PtZnTi/SiO₂ during long deactivation phases reveal a stabilizing effect of Ti in PtZnTi/SiO₂ with a k_d of 0.015 h^-1 compared to PtZn/SiO₂ with a k_d of 0.022 h^-1 over 108 h on stream. Such a stabilizing effect is also present during a second deactivation phase after applying a regeneration protocol to the materials under O₂ and H₂ at high temperatures. A combined scanning transmission electron microscopy, in situ X-ray absorption spectroscopy, electron paramagnetic resonance, and density functional theory study reveals that this effect is related to a sintering prevention of the alloyed PtZn NPs in PtZnTi/SiO₂ due to a strong interaction of the NPs with Ti sites. However, in contrast to classical strong metal-support interaction, we show that the coverage of the Pt NPs with TiO_x species is not needed to explain the changes in adsorption and reactivity properties. Indeed, the interaction of the Pt NPs with Ti^III sites is enough to decrease CO adsorption and to induce a red-shift of the CO band because of electron transfer from the Ti^III sites to Pt⁰.

Atomically Dispersed NiNx Site with High Oxygen Electrocatalysis Performance Facilely Produced via a Surface Immobilization Strategy

Nonprecious-metal heterogeneous catalysts with atomically dispersed active sites demonstrated high activity and selectivity in different reactions, and the rational design and large-scale preparation of such catalysts are of great interest but remain a huge challenge. Current approaches usually involve extremely high-temperature and tedious procedures. Here, we demonstrated a straightforward and scalable preparation strategy. In two simple steps, the atomically dispersed Ni electrocatalyst can be synthesized in a tens grams scale with quantitative yield under mild conditions, and the active Ni sites were produced by immobilizing preorganized NiN_x complex on the substrate surface via organic thermal reactions. This catalyst exhibits excellent catalysis performances in both oxygen evolution and reduction reactions. It also exhibited tunable catalysis activity, high catalysis reproducibility, and high stability. The atomically dispersed NiN_x sites are tolerant at high Ni concentration, as the random reactions and metal nanoparticle formation that generally occurred at high temperatures were avoided. This strategy illustrated a practical and green method for the industrial manufacture of nonprecious-metal single-site catalysts with a predictable structure.

Effects of surface acidity on the structure of organometallics supported on oxide surfaces

Well-defined organometallics supported on high surface area oxides are promising heterogeneous catalysts. An important design factor in these materials is how the metal interacts with the functionalities on an oxide support, commonly anionic X-type ligands derived from the reaction of an organometallic M-R with an -OH site on the oxide. The metal can either form a covalent M-O bond or form an electrostatic M⁺⋯^-O ion-pair, which impacts how well-defined organometallics will interact with substrates in catalytic reactions. A less common reaction pathway involves the reaction of a Lewis site on the oxide with the organometallic, resulting in abstraction to form an ion-pair, which is relevant to industrial olefin polymerization catalysts. This Feature Article views the spectrum of reactivity between an organometallic and an oxide through the prism of Brønsted and/or Lewis acidity of surface sites and draws analogies to the molecular frame where Lewis and Brønsted acids are known to form reactive ion-pairs. Applications of the well-defined sites developed in this article are also discussed.

Approaching Molecular Definition on Oxide-Supported Single-Atom Catalysts

ConspectusSingle-atom catalysts (SACs) offer unique advantages such as high (noble) metal utilization through maximum possible dispersion, large metal-support contact areas, and oxidation states usually unattainable in classic nanoparticle catalysis. In addition, SACs can serve as models for determining active sites, a simultaneously desired as well as elusive target in the field of heterogeneous catalysis. Due to the complexity of heterogeneous catalysts bearing a variety of different sites on metal particles and the respective support as well as at their interface, studies of intrinsic activities and selectivities remain largely inconclusive. While SACs could close this gap, many supported SACs remain intrinsically ill-defined due to complexities arising from the variety of different adsorption sites for atomically dispersed metals, hampering the establishment of meaningful structure-activity correlations. In addition to overcoming this limitation, well-defined SACs could even be utilized to shed light on fundamental phenomena in catalysis that remain ambiguous when studies are obscured by the complexity of heterogeneous catalysts.In this Account, we describe approaches to break down the complexity of supported single-atom catalysts through the careful choice of oxide supports with specific binding motives as well as the adsorption of well-defined ligands such as ionic liquids on single metal sites. An example of molecularly defined oxide supports is polyoxometalates (POMs), which are metal oxo clusters with precisely known composition and structure. POMs exhibit a limited number of sites to anchor atomically dispersed metals such as Pt, Pd, and Rh. Polyoxometalate-supported single-atom catalysts (POM-SACs) thus represent ideal systems for the in situ spectroscopic study of single atom sites during reactions as, in principle, all sites are identical and thus equally active in catalytic reactions. We have utilized this benefit in studies of the mechanism of CO and alcohol oxidation reactions as well as the hydro(deoxy)genation of various biomass-derived compounds. More so, the redox properties of polyoxometalates can be finely tuned by changing the composition of the support while keeping the geometry of the single-atom active site largely constant. We further developed soluble analogues of heterogeneous POM-SACs, opening the door to advanced liquid-phase nuclear magnetic resonance (NMR) and UV-vis techniques but, in particular, to electrospray ionization mass spectrometry (ESI-MS) which proves powerful in determining catalytic intermediates as well as their gas-phase reactivity. Employing this technique, we were able to resolve some of the long-standing questions about hydrogen spillover, demonstrating the broad utility of studies on defined model catalysts.

Nanostructured Heterogeneous Catalysts for Bioorthogonal Reactions

Bioorthogonal chemistry has inspired a new subarea of chemistry providing a powerful tool to perform novel biocompatible chemospecific reactions in living systems. Following the premise that they do not interfere with biological functions, bioorthogonal reactions are increasingly applied in biomedical research, particularly with respect to genetic encoding systems, fluorogenic reactions for bioimaging, and cancer therapy. This Minireview compiles recent advances in the use of heterogeneous catalysts for bioorthogonal reactions. The synthetic strategies of Pd-, Au-, and Cu-based materials, their applicability in the activation of caged fluorophores and prodrugs, and the possibilities of using external stimuli to release therapeutic substances at a specific location in a diseased tissue are discussed. Finally, we highlight frontiers in the field, identifying challenges, and propose directions for future development in this emerging field.

Free-atom-like d states beyond the dilute limit of single-atom alloys

Through a data-mining and high-throughput density functional theory approach, we identify a diverse range of metallic compounds that are predicted to have transition metals with "free-atom-like" d states that are highly localized in terms of their energetic distribution. Design principles that favor the formation of localized d states are uncovered, among which we note that site isolation is often necessary but that the dilute limit, as in most single-atom alloys, is not a pre-requisite. Additionally, the majority of localized d state transition metals identified from the computational screening study exhibit partial anionic character due to charge transfer from neighboring metal species. Using CO as a representative probe molecule, we show that localized d states for Rh, Ir, Pd, and Pt tend to reduce the binding strength of CO compared to their pure elemental analogues, whereas this does not occur as consistently for the Cu binding sites. These trends are rationalized through the d-band model, which suggests that the significantly reduced d-band width results in an increased orthogonalization energy penalty upon CO chemisorption. With the multitude of inorganic solids that are predicted to have highly localized d states, the results of the screening study are likely to result in new avenues for heterogeneous catalyst design from an electronic structure perspective.

Recent progress of Ga-based liquid metals in catalysis

Within the last decade, the application of gallium-based liquid metals in catalysis has received great attention from around the world. This article provides an overview concerning Ga-based liquid metals (LMs) in energy and environmental applications, such as the catalytic synthesis of ethylene by non-petroleum routes via Pd-Ga liquid catalysts, alkane dehydrogenation via Pd-Ga or Pt-Ga catalysts, CO2 hydrogenation to methanol via Ni Ga or Pd/Ga2O3 catalysts, and catalytic degradation of CO2 via EGaIn liquid metal catalysts below 500 °C, where Ga-based liquid metal catalysts exhibit high selectivity and low energy consumption. The formation of isolated metal sites in a liquid metal matrix allows the integration of several characteristics of multiphase catalysis (particularly the operational friendliness of product separation procedures) with those of homogeneous catalysis. In the end, this article sheds light on future prospects, opportunities, and challenges of liquid metal catalysis.

Late Transition Metal Catalysts with Chelating Amines for Olefin Polymerization

Polyolefins are the most consumed polymeric materials extensively used in our daily life and are usually generated by coordination polymerization in the polyolefin industry. Olefin polymerization catalysts containing transition metal–organic compound combinations are undoubtedly crucial for the development of the polyolefin industry. The nitrogen donor atom has attracted considerable interest and is widely used in combination with the transition metal for the fine-tuning of the chemical environment around the metal center. In addition to widely reported olefin polymerization catalysts with imine and amide donors (sp2 hybrid N), late transition metal catalysts with chelating amine donors (sp3 hybrid N) for olefin polymerization have never been reviewed. In this review paper, we focus on late transition metal (Ni, Pd, Fe, and Co) catalysts with chelating amines for olefin polymerization. A variety of late transition metal catalysts bearing different neutral amine donors are surveyed for olefin polymerization, including amine–imine, amine–pyridine, α-diamine, and [N, N, N] tridentate ligands with amine donors. The relationship between catalyst structure and catalytic performance is also encompassed. This review aims to promote the design of late transition metal catalysts with unique chelating amine donors for the development of high-performance polyolefin materials.

Hydroaminoalkylation of alkenes using transition metals complexes grafted on silica SBA15 as catalysts

The gas-phase hydroaminoalkylation reaction of propene catalyzed by group 4 (M = Ti, Zr and Hf) metal amido complexes [(≡Si-O-)(M(-NMe₂)₃] was investigated by using PBE0-D3/SVP//TZVP level of theory. The geometrical analysis traced the formation of the metallaaziridines and the azametallacyclopentanes as key intermediates in these reactions. The metallaaziridines were simulated through the activation of α-C-H bonds of the amido groups; while the azametallacyclopentanes were configured by slotting the propene double bond onto the M - C bonds of the metallaaziridines. The latter reaction was considered the rate-determining step. Thermochemical calculations showed that the order of catalytic activity is: Ti ≥ Zr > Hf; while the preference of the azametallacyclopentanes is: Hf > Zr ≥ Ti.

Molecular and Electronic Structure of Isolated Platinum Sites Enabled by the Expedient Measurement of 195Pt Chemical Shift Anisotropy

Techniques that can characterize the molecular structures of dilute surface species are required to facilitate the rational synthesis and improvement of Pt-based heterogeneous catalysts. ¹⁹⁵Pt solid-state NMR spectroscopy could be an ideal tool for this task because ¹⁹⁵Pt isotropic chemical shifts and chemical shift anisotropy (CSA) are highly sensitive probes of the local chemical environment and electronic structure. However, the characterization of Pt surface-sites is complicated by the typical low Pt loadings that are between 0.2 and 5 wt% and broadening of ¹⁹⁵Pt solid-state NMR spectra by CSA. Here, we introduce a set of solid-state NMR methods that exploit fast MAS and indirect detection using a sensitive spy nucleus (¹H or ³¹P) to enable the rapid acquisition of ¹⁹⁵Pt MAS NMR spectra. We demonstrate that high-resolution wideline ¹⁹⁵Pt MAS NMR spectra can be acquired in minutes to a few hours for a series of molecular and single-site Pt species grafted on silica with Pt loading of only 3-5 wt%. Low-power, long-duration, sideband-selective excitation, and saturation pulses are incorporated into t₁-noise eliminated dipolar heteronuclear multiple quantum coherence, perfect echo resonance echo saturation pulse double resonance, or J-resolved pulse sequences. The complete ¹⁹⁵Pt MAS NMR spectrum is then reconstructed by recording a series of 1D NMR spectra where the offset of the ¹⁹⁵Pt pulses is varied in increments of the MAS frequency. Analysis of the ¹⁹⁵Pt MAS NMR spectra yields the ¹⁹⁵Pt chemical shift tensor parameters. Zeroth order approximation density functional theory calculations accurately predict ¹⁹⁵Pt CS tensor parameters. Simple and predictive orbital models relate the CS tensor parameters to the Pt electronic structure and coordination environment. The methodology developed here paves the way for the detailed structural and electronic analysis of dilute platinum surface-sites.

Electronic Effects of Support Doping on Hydrotalcite-Supported Iridium N-Heterocyclic Carbene Complexes

The electronic effects of supports on immobilized organometallic complexes impact their activity and lifetime, yet remain poorly understood. Here we describe a systematic study of the support effects experienced by an organometallic complex immobilized on doped hydrotalcite-like materials. To that end, we describe the synthesis and characterization of the first organometallic species immobilized on a palette of doped hydrotalcites via sulfonate linkers. The organometallic species consists of iridium N-heterocyclic carbene (NHC) carbonyl complex ([Na][Ir-(NHC-Ph-SO₃)₂(CO)₂]), a highly active molecular catalyst for transfer hydrogenation of glycerol. The hydrotalcite supports are composed of Al, Mg, and a compatible transition-metal dopant (Fe, Cu, Ni, Zn). The materials were characterized extensively by STEM, XPS, TGA, PXRD, FT-IR, N₂ desorption, ICP-AES, TPD, and microcalorimetry to probe the morphology and electronic properties of the support and elucidate structure-property relationships.

A Robust and Efficient Propane Dehydrogenation Catalyst from Unexpectedly Segregated Pt2Mn Nanoparticles

The increasing demand for short chain olefins like propene for plastics production and the availability of shale gas make the development of highly performing propane dehydrogenation (PDH) catalysts, robust toward industrially applied harsh regeneration conditions, a highly important field of research. A combination of surface organometallic chemistry and thermolytic molecular precursor approach was used to prepare a nanometric, bimetallic Pt-Mn material (3 wt % Pt, 1.3 wt % Mn) supported on silica via consecutive grafting of a Mn and Pt precursor on surface OH groups present on the support surface, followed by a treatment under a H₂ flow at high temperature. The material exhibits a 70% fraction of the overall Mn as Mn^II single sites on the support surface; the remaining Mn is incorporated in segregated Pt₂Mn nanoparticles. The material shows great performance in PDH reaction with a low deactivation rate. In particular, it shows outstanding robustness during repeated regeneration cycles, with conversion and selectivity stabilizing at ca. 37 and 98%, respectively. Notably, a material with a lower Pt loading of only 0.05 wt % shows an outstanding catalytic performance─initial productivity of 4523 g_C3H6/g_Pt h and an extremely low k_d of 0.003 h^-1 under a partial pressure of H₂, which are among the highest reported productivities. A combined in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, electron paramagnetic resonance, and metadynamics at the density functional theory level study could show that the strong interaction between the Mn^II-decorated support and the unexpectedly segregated Pt₂Mn particles is most likely responsible for the outstanding performance of the investigated materials.

Endohedrally Functionalized Metal-Organic Cage-Cross-Linked Polymer Gels as Modular Heterogeneous Catalysts

The immobilization of homogeneous catalysts onto supports to improve recyclability while maintaining catalytic efficiency is often a trial-and-error process limited by poor control of the local catalyst environment and few strategies to append catalysts to support materials. Here, we introduce a modular heterogenous catalysis platform that addresses these challenges. Our approach leverages the well-defined interiors of self-assembled Pd₁₂L₂₄ metal-organic cages/polyhedra (MOCs): simple mixing of a catalyst-ligand of choice with a polymeric ligand, spacer ligands, and a Pd salt induces self-assembly of Pd₁₂L₂₄-cross-linked polymer gels featuring endohedrally catalyst-functionalized junctions. Semi-empirical calculations show that catalyst incorporation into the MOC junctions of these materials has minimal affect on the MOC geometry, giving rise to well-defined nanoconfined catalyst domains as confirmed experimentally using several techniques. Given the unique network topology of these freestanding gels, they are mechanically robust regardless of their endohedral catalyst composition, allowing them to be physically manipulated and transferred from one reaction to another to achieve multiple rounds of catalysis. Moreover, by decoupling the catalyst environment (interior of MOC junctions) from the physical properties of the support (the polymer matrix), this strategy enables catalysis in environments where homogeneous catalyst analogues are not viable, as demonstrated for the Au(I)-catalyzed cyclization of 4-pentynoic acid in aqueous media.

Configuration Sensitivity of Electrocatalytic Oxygen Reduction Reaction on Nitrogen-Doped Graphene

As one of the most promising nonprecious metal catalysts for the oxygen reduction reaction (ORR), the structure of the active site on nitrogen-doped carbon materials is still under debate. Here, we report that the sensitivity of the ORR on the local configuration of multiple nitrogen dopants may be overlooked. Combining global structure searching with density functional theory calculations, we established the structure-activity relationship for 19 and 298 possible configurations of graphitic nitrogen-doped graphene with N content of 2 and 3%, respectively. It was revealed that the stability cannot be a screener to determine the major contributor to the activity. 77.5% of current density is contributed by the active configuration with 4.59% population on the graphene containing 3% nitrogen. It unambiguously demonstrates the configuration sensitivity of N-doped graphene for ORR and opens a new window to identifying the optimal structure of N-doped carbons for various applications.

Hydroaminoalkylation for the Catalytic Addition of Amines to Alkenes or Alkynes: Diverse Mechanisms Enable Diverse Substrate Scope

Hydroaminoalkylation is a powerful, atom-economic catalytic reaction for the reaction of amines with alkenes and alkynes. This C-H functionalization reaction allows for the atom-economic alkylation of amines using simple alkenes or alkynes as the alkylating agents. This transformation has significant potential for transformative approaches in the pharmaceutical, agrochemical, and fine chemical industries in the preparation of selectively substituted amines and N-heterocycles and shows promise in materials science for the synthesis of functional and responsive aminated materials. Different early transition-metal, late transition-metal, and photoredox catalysts mediate hydroaminoalkylation by distinct mechanistic pathways. These mechanistic insights have resulted in the development of new catalysts and reaction conditions to realize hydroaminoalkylation with a broad range of substrates: activated and unactivated, terminal and internal, C-C double and triple bonds with aryl or alkyl primary, secondary, or tertiary amines, including N-heterocyclic amines. By deploying select catalysts with specific substrate combinations, control over regioselectivity, diastereoselectivity, and enantioselectivity has been realized. Key barriers to widespread adoption of this reaction include air and moisture sensitivity for early transition-metal catalysts as well as a heavy dependence on amine protecting or directing groups for late transition-metal or photocatalytic routes. Advances in improved catalyst robustness, substrate scope, and regio-/stereoselective reactions with early- and late transition-metal catalysts, as well as photoredox catalysis, are highlighted, and opportunities for further catalyst and reaction development are included. This perspective shows that hydroaminoalkylation has the potential to be a disruptive and transformative strategy for the synthesis of selectively substituted amines and N-heterocycles from simple amines and alkenes.

The coordination chemistry of oxide and nanocarbon materials

Understanding how a ligand affects the steric and electronic properties of a metal is the cornerstone of the inorganic chemistry enterprise. What happens when the ligand is an extended surface? This question is central to the design and implementation of state-of-the-art functional materials containing transition metals. This perspective will describe how these two very different sets of extended surfaces can form well-defined coordination complexes with metals. In the Green formalism, functionalities on oxide surfaces react with inorganics to form species that contain X-type or LX-type interactions between the metal and the oxide. Carbon surfaces are neutral L-type ligands; this perspective focuses on carbons that donate six electrons to a metal. The nature of this interaction depends on the curvature, and thereby orbital overlap, between the metal and the extended π-system from the nanocarbon.

Alkane C-H activation and ligand exchange on silica supported d0 metal alkylidenes: relevance to alkane metathesis

In this work, we study the ligand exchange process between an alkane and a series of silica supported metal alkylidenes, which may occur by different pathways: C-H addition, σ-bond metathesis, and α-H abstraction. The results indicate that the α-H abstraction pathway is the preferred one, regardless of the catalyst and ligands. This is in contrast to the expected preference for the C-H addition route. When looking for the origin of this preference, our calculations revealed that the α-H abstraction pathway is driven by entropy, which favors the initial dissociation of the alkyl ligand from the catalyst.

Elucidating the Formation and Structural Evolution of Platinum Single-Site Catalysts for the Hydrogen Evolution Reaction

Platinum single-site catalysts (SSCs) are a promising technology for the production of hydrogen from clean energy sources. They have high activity and maximal platinum-atom utilization. However, the bonding environment of platinum during operation is poorly understood. In this work, we present a mechanistic study of platinum SSCs using operando, synchrotron-X-ray absorption spectroscopy. We synthesize an atomically dispersed platinum complex with aniline and chloride ligands onto graphene and characterize it with ex-situ electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). Then, by operando EXAFS and XANES, we show that as a negatively biased potential is applied, the Pt-N bonds break first followed by the Pt-Cl bonds. The platinum is reduced from platinum(II) to metallic platinum(0) by the onset of the hydrogen-evolution reaction at 0 V. Furthermore, we observe an increase in Pt-Pt bonding, indicating the formation of platinum agglomerates. Together, these results indicate that while aniline is used to prepare platinum SSCs, the single-site complexes are decomposed and platinum agglomerates at operating potentials. This work is an important contribution to the understanding of the evolution of bonding environment in SSCs and provides some molecular insights into how platinum agglomeration causes the deactivation of SSCs over time.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

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

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