Single-site Heterogeneous Catalysts Research Articles

CH4 Carbonylation to Acetic Acid Using H2O as an Oxidant on a Rh-Functionalized UiO-67 Combined with Oriented External Electric Fields: Selectivity and Mechanistic Insights from DFT Calculations.

Acetic acid (CH3COOH), as an industrially important petrochemical product, is predominantly produced via multistep energy-intensive processes. The development of a rhodium single-site heterogeneous catalyst has received considerable attention due to its potential to transform CH4 into CH3COOH in a single step. Herein, the reaction mechanism for the generation of CH3COOH from CH4, CO, and H2O catalyzed by Rh-functionalized metal-organic framework (MOF) UiO-67 and the selectivity of products CH3COOH, formic acid (HCOOH), methanol (CH3OH), and acetaldehyde (CH3CHO) under the oriented external electric fields (OEEFs) were systematically explored by density functional theory (DFT) calculations. The results reveal that the insertion of CO into Rh-CH3 is the rate-determining step with a free energy barrier of 21.0 kcal/mol in CH4 carbonylation to CH3COOH. Upon applying an OEEF of Fx = +0.0050 au along the C-C bond, the rate-determining step shifts toward H2O decomposition with the barrier of 19.6 kcal/mol, significantly improving the selectivity for CH3COOH production, compared to the major competitive HCOOH route. The Brønsted-Evans-Polanyi (BEP) relationships between key transition states, field strength, and NPA charge transfer were established. This study may guide the rational design of atomically dispersed MOF catalysts for the selective coconversion of CH4 and CO to CH3COOH using H2O as the oxidant under the OEEF.

Demystifying group-4 polyolefin hydrogenolysis catalysis: Gaseous propane hydrogenolysis mechanism over the same catalysts

A kinetic/mechanistic investigation of gaseous propane hydrogenolysis over the single-site heterogeneous polyolefin depolymerization catalysts AlS/ZrNp2 and AlS/HfNp2 (AlS = sulfated alumina, Np = neopentyl), is use to probe intrinsic catalyst properties without the complexities introduced by time- and viscosity-dependent polymer medium effects. In a polymer-free automated plug-flow catalytic reactor, propane hydrogenolysis turnover frequencies approach 3,000 h-1 at 150 °C. Both catalysts exhibit approximately linear relationships between rate and [H2] at substoichiometric [H2] with rate law orders of 0.66 ± 0.09 and 0.48 ± 0.07 for Hf and Zr, respectively; at higher [H2], the rates approach zero-order in [H2]. Reaction orders in [C3H8] and [catalyst] are essentially zero-order under all conditions, with the former implying rapid, irreversible alkane binding/activation. This rate law, activation parameter, and DFT energy span analysis support a scenario in which [H2] is pivotal in one of two plausible and competing rate-determining transition states-bimolecular metal-alkyl bond hydrogenolysis vs. unimolecular β-alkyl elimination. The Zr and Hf catalyst activation parameters, ΔH‡ = 16.8 ± 0.2 kcal mol-1 and 18.2 ± 0.6 kcal mol-1, respectively, track the relative turnover frequencies, while ΔS‡ = -19.1 ± 0.8 and -16.7 ± 1.4 cal mol-1 K-1, respectively, imply highly organized transition states. These catalysts maintain activity up to 200 °C, while time-on-stream data indicate multiday activities with an extrapolated turnover number ~92,000 at 150 °C for the Zr catalyst. This methodology is attractive for depolymerization catalyst discovery and process optimization.

Alkene Isomerization Using a Heterogeneous Nickel-Hydride Catalyst.

Transition metal-catalyzed alkene isomerization is an enabling technology used to install an alkene distal to its original site. Due to their well-defined structure, homogeneous catalysts can be fine-tuned to optimize reactivity, stereoselectivity, and positional selectivity, but they often suffer from instability and nonrecyclability. Heterogeneous catalysts are generally highly robust but continue to lack active-site specificity and are challenging to rationally improve through structural modification. Known single-site heterogeneous catalysts for alkene isomerization utilize precious metals and bespoke, expensive, and synthetically intense supports. Additionally, they generally have mediocre reactivity, inspiring us to develop a heterogeneous catalyst with an active site made from readily available compounds made of Earth-abundant elements. Previous work demonstrated that a very active homogeneous catalyst is formed upon protonation of Ni[P(OEt)3]4 by H2SO4, generating a [Ni-H]+ active site. This catalyst is incredibly active, but also decomposes readily, which severely limits its utility. Herein we show that by using a solid acid (sulfated zirconia, SZO300), not only is this decomposition prevented, but high activity is maintained, improved selectivity is achieved, and a broader scope of functional groups is tolerated. Preliminary mechanistic experiments suggest that the catalytic reaction likely goes through an intermolecular, two-electron pathway. A detailed kinetic study comparing the state-of-the-art Ni and Pd isomerization catalysts reveals that the highest activity and selectivity is seen with the Ni/SZO300 system. The reactivity of Ni/SZO300, is not limited to alkene isomerization; it is also a competent catalyst for hydroalkenylation, hydroboration, and hydrosilylation, demonstrating the broad application of this heterogeneous catalyst.

A green edge-hosted zinc single-site heterogeneous catalyst for superior Fenton-like activity

Developing green heterogeneous catalysts with excellent Fenton-like activity is critical for water remediation technologies. However, current catalysts often rely on toxic transitional metals, and their catalytic performance is far from satisfactory as alternatives of homogeneous Fenton-like catalysts. In this study, a green catalyst based on Zn single-atom was prepared in an ammonium atmosphere using ZIF-8 as a precursor. Multiple characterization analyses provided evidence that abundant intrinsic defects due to the edge sites were created, leading to the formation of a thermally stable edge-hosted Zn-N4 single-atom catalyst (ZnN4-Edge). Density functional theory calculations revealed that the edge sites equipped the single-atom Zn with a super catalytic performance, which not only promoted decomposition of peroxide molecule (HSO5-) but also greatly lowered the activation barrier for •OH generation. Consequently, the as-prepared ZnN4-Edge exhibited extremely high Fenton-like performance in oxidation and mineralization of phenol as a representative organic contaminant in a wide range of pH, realizing its quick detoxification. The atom-utilization efficiency of the ZnN4-Edge was ~104 higher than an equivalent amount of the control sample without edge sites (ZnN4), and the turnover frequency was ~103 times of the typical benchmark of homogeneous catalyst (Co2+). This study opens up a revolutionary way to rationally design and optimize heterogeneous catalysts to homogeneous catalytic performance for Fenton-like application.

MAO- and Borate-Free Activating Supports for Group 4 Metallocene and Post-Metallocene Catalysts of α-Olefin Polymerization and Oligomerization.

Modern industry of advanced polyolefins extensively uses Group 4 metallocene and post-metallocene catalysts. High-throughput polyolefin technologies demand the use of heterogeneous catalysts with a given particle size and morphology, high thermal stability, and controlled productivity. Conventional Group 4 metal single-site heterogeneous catalysts require the use of high-cost methylalumoxane (MAO) or perfluoroaryl borate activators. However, a number of inorganic phases, containing highly acidic Lewis and Brønsted sites, are able to activate Group 4 metal pre-catalysts using low-cost and affordable alkylaluminums. In the present review, we gathered comprehensive information on MAO- and borate-free activating supports of different types and discussed the surface nature and chemistry of these phases, examples of their use in the polymerization of ethylene and α-olefins, and prospects of the further development for applications in the polyolefin industry.

Molecular-level design of isolated molybdenum oxide anchored on carbon nitride for photocatalytic H2 production and environmental remediation

Polymeric carbon nitride typically suffers from sluggish intrinsic charge separation and low available active sites. This paper reports that isolated non-crystalline molybdenum oxide species anchored to N-coordinating cavities (MoCN) have abundant surface-active sites for solid–liquid two-phase reactions, whether for photocatalytic H2 evolution or organic pollutant degradation. Molecular dynamic simulations and density functional theory (DFT) revealed six-fold cavities to stabilize the MoO3 species with non-crystalline features, endowing high dispersion and less aggregation. As proven in single-site heterogeneous catalysts, the photocatalyst benefits from size reduction and accelerated interfacial charge transfer because of its mutual contact between two semiconductors. The MoCN shows a high visible-light H2 evolution of 1265 µmol g−1h−1 under visible light (λ ≥ 400 nm) illumination. The photocatalyst degraded more than 95% tetracycline within 30 min and rhodamine B in 10 min. The MoO3 species confined within π-conjugated systems increase the catalytic contact sites, extending visible light harvesting ability to a longer wavelength. Each single catalytic site facilitates the separation and transfer of charge carriers while interfacial charge still occurs between MoO3 and CN. This molecular-level design and strategy provide a new opportunity and a universal way to extend the boundaries of liquid–solid phase catalysts.

Single-Site Carbon-Supported Metal-Oxo Complexes in Heterogeneous Catalysis: Structure, Reactivity, and Mechanism.

When early transition metal complexes are molecularly grafted onto catalyst supports, well-defined, surface-bound species are created, which are highly active and selective single-site heterogeneous catalysts (SSHCs) for diverse chemical transformations. In this minireview, we analyze and summarize a less conventional type of SSHC in which molybdenum dioxo species are grafted onto unusual carbon-unsaturated scaffolds, such as activated carbon, reduced graphene oxide, and carbon nanohorns. The choice of earth-abundant, low-toxicity, versatile metal constituents, and various carbon supports illustrates "catalyst by design" principles and yields insights into new catalytic systems of both academic and technological interest. Here, we summarize experimental and computational investigations of the bonding, electronic structure, reaction scope, and mechanistic pathways of these unusual catalysts.

Computational and Experimental Characterization of the Ligand Environment of a Ni-Oxo Catalyst Supported in the Metal–Organic Framework NU-1000

Heterogeneous catalysts exhibit significant changes in composition due to the influence of operating conditions, and these compositional changes can have dramatic effects on catalytic performance. For traditional bulk metal heterogeneous catalysts, relationships between composition and catalytic operating conditions are well documented. However, the influence of operating conditions on the compositions of single-site heterogeneous catalysts remains largely unresolved. To address this, we report a combined computational and experimental characterization of a Ni oxo catalyst under catalytic hydrogenation conditions. Specifically, pair distribution function (PDF) analysis is combined with ab initio thermodynamic modeling to investigate ligand environments present on a Ni oxo cluster supported in the metal-organic framework NU-1000. Comparisons of the experimentally observed and simulated Ni-O coordination numbers and Ni-O, Ni···Ni, and Ni···Zr distances provide insight into the Ni ligand environment under H2 (g). These comparisons suggest significant OH and H2O content and, further, that different Ni ions within the cluster and/or NU-1000 structure may comprise subtly different numbers of these ligands. Further, the observation of significant H2O content under H2 (g) suggests that the NU-1000 support supplies H2O to the cluster. Examples of ligand environments that could lead to the observed PDFs are provided. The combination of simulations and experiments provides new insights into the ligand environment for Ni-NU-1000 catalysts that will be useful for understanding the ligand environments of other single-site Ni catalysts as well.

Phosphine-based metal-organic layers to construct single-site heterogeneous catalysts for arene borylation.

Metal-organic layers (MOLs) are versatile platforms for creating single-site heterogeneous catalysts. Incorporating molecular functionalities into MOLs is crucial for catalysis. In this study, we synthesized phosphine-containing MOLs constructed from Hf6-oxo secondary building units (SBUs) and phosphine ligands. The mono(phosphine)-Ir complexes generated by the metalation of TPP-MOL were highly active as heterogeneous catalysts for the C(sp2)-H borylation of a range of arenes. This research expands the diversity of MOL-based catalysts.

Zeolite encapsulated organometallic complexes as model catalysts.

Heterogeneities in the structure of active centers in metal-containing porous materials are unavoidable and complicate the description of chemical events occurring along reaction coordinates at the atomic level. Metal containing zeolites include sites of varied local coordination and secondary confining environments, requiring careful titration protocols to quantify the predominant active sites. Hybrid organometallic-zeolite catalysts are useful well-defined platform materials for spectroscopic, kinetic, and computational studies of heterogeneous catalysis that avoid the complications of conventional metal-containing porous materials. Such materials have been synthesized and studied previously, but catalytic applications were mostly limited to liquid-phase oxidation and electrochemical reactions. The hydrothermal stability, time-on-stream stability, and utility of these materials in gas-phase oxidation reactions are under-studied. The potential applications for single-site heterogeneous catalysts in fundamental research are abundant and motivate future synthetic, spectroscopic, kinetic, and computational studies.

Single-Atom Catalysts on Covalent Triazine Frameworks: at the Crossroad between Homogeneous and Heterogeneous Catalysis.

Heterogeneous single-site and single-atom catalysts potentially enable combining the high catalytic activity and selectivity of molecular catalysts with the easy continuous operation and recycling of solid catalysts. In recent years, covalent triazine frameworks (CTFs) found increasing attention as support materials for particulate and isolated metal species. Bearing a high fraction of nitrogen sites, they allow coordinating molecular metal species and stabilizing particulate metal species, respectively. Dependent on synthesis method and pretreatment of CTFs, materials resembling well-defined highly crosslinked polymers or materials comparable to structurally ill-defined nitrogen-containing carbons result. Accordingly, CTFs serve as model systems elucidating the interaction of single-site, single-atom and particulate metal species with such supports. Factors influencing the transition between molecular and particulate systems are discussed to allow deriving tailored catalyst systems.

Einzelatomkatalysatoren auf kovalenten Triazinnetzwerken: am Scheidepunkt zwischen homogener und heterogener Katalyse

AbstractHeterogene Single‐Site‐ und Einzelatom‐Katalysatoren bieten die Möglichkeit, die hohe katalytische Aktivität und Selektivität molekularer Katalysatoren mit dem einfachen kontinuierlichen Betrieb und Recycling fester Katalysatoren zu kombinieren. In den letzten Jahren fanden kovalente Triazinnetzwerke (CTFs) als Trägermaterialien für partikuläre und isolierte Metallspezies zunehmende Beachtung. Sie weisen einen hohen Anteil an Stickstofffunktionalitäten auf und ermöglichen die Koordination molekularer Metallspezies bzw. die Stabilisierung partikulärer Metallspezies. Je nach Synthesemethode und Vorbehandlung der CTFs entstehen Materialien, die gut definierten, hoch vernetzten Polymeren ähneln, oder Materialien, die mit strukturell undefinierten stickstoffhaltigen Kohlenstoffen vergleichbar sind. Dementsprechend dienen CTFs als Modellsysteme, die es ermöglichen, die Wechselwirkung von Single‐Site‐, einzelatomaren und partikulären Metallspezies mit solchen Trägern aufzuklären. Faktoren, die den Übergang zwischen molekularen und partikulären Systemen beeinflussen, werden diskutiert, um die Grundlage zur Entwicklung maßgeschneiderter Katalysatorsysteme zu schaffen.

Correlation between Electronic Descriptor and Proton-Coupled Electron Transfer Thermodynamics in Doped Graphite-Conjugated Catalysts.

Graphite-conjugated catalysts (GCCs) provide a powerful framework for investigating correlations between electronic structure features and chemical reactivity of single-site heterogeneous catalysts. GCC-phenazine undergoes proton-coupled electron transfer (PCET) involving protonation of phenazine at its two nitrogen atoms with the addition of two electrons. Herein, this PCET reaction is investigated in the presence of defects, such as heteroatom dopants, in the graphitic surface. The proton-coupled redox potentials, EPCET, are computed using a constant potential periodic density functional theory (DFT) strategy. The electronic states directly involved in PCET for GCC-phenazine exhibit the same nitrogen orbital character as those for molecular phenazine. The energy εLUS of this phenazine-related lowest unoccupied electronic state in GCC-phenazine is identified as a descriptor for changes in PCET thermodynamics. Importantly, εLUS is obtained from only a single DFT calculation but can predict EPCET, which requires many such calculations. Similar electronic features may be useful descriptors for thermodynamic properties of other single-site catalysts.

Single-Site Heterogeneous Organometallic Ir Catalysts Embedded on Graphdiyne: Structural Manipulation Beyond the Carbon Support.

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH- and Cl- ; Cp* = pentamethylcyclopentadienyl), with the fine-tuned local coordination and electronic structure of Ir sites, are constructed by anchoring Cp*Ir complexes on graphdiyne (GDY) matrix via a one-pot procedure. The spectroscopic studies and theoretical calculations indicate that the Ir atoms in Cp*Ir-Cl/GDY and Cp*Ir-OH/GDY have a much higher oxidation state than Ir in the SAC Ir/GDY. As a proof-of-principle demonstration, the GDY-supported SHOCs are used for formic acid dehydrogenation, which display a fivefold enhancement of catalytic activity compared with SAC Ir/GDY. The kinetic isotope effect and in situ Fourier-transform infrared studies reveal that the rate-limiting step is the β-hydride elimination process, and Cp* on the Ir site accelerates the β-hydride elimination reaction. The GDY-supported SHOCs integrate the merits of both SACs and molecular catalysts, wherein the isolated Ir anchored on GDY echoes with SACs' behavior, and the Cp* ligand enables precise structural and electronic regulation like molecular catalysts. The scheme of SHOCs adds a degree of freedom in accurate regulation of the local structure, the electronic property, and therefore the catalytic performance of single-atom catalysts.

Photocatalytic Reactions on the Single-Site Heterogeneous Catalysts

Photocatalytic Reactions on the Single-Site Heterogeneous Catalysts

INTERFACES. A program for determining the 3D structures of surfaces sites using NMR data

Dynamic nuclear polarization surface enhanced NMR spectroscopy has enabled the determination of high-resolution structures from surface-supported molecules, including single-site heterogeneous catalysts. Structure determinations have largely mimicked the approaches used in biomolecular NMR spectroscopy, namely, using distance measurements to constrain a conformational search. These early demonstrations made use of purpose-built software, which has limited the adoption of the technique. Herein, we describe the open-source program INTERFACES (Interpret NMR to Elucidate or Reconstruct the Full Atomistic Configurations of External Surfaces) which automates the analysis of RE(SP)DOR data as well as the structure determination for surface sites. Distances, angles, dihedral angles, complex orientation, and distance from the support can all be sampled to find all structures that agree with the experimental data. A χ2 metric is used to define the error ranges of the REDOR fits and produce structures with an arbitrary level of confidence. Structural solutions are then provided as both overlays and ORTEP-like probability ellipsoids.

Highly efficient CO2 conversion on a robust metal-organic framework Cu(I)-MFU-4l: Prediction and mechanistic understanding from DFT calculations

As a bioinspired metal-organic framework (MOF) with a unique trigonal pyramidal Cu(I) site in a bowl-shaped pocket, Cu(I)-MFU-4l has emerged as an advanced single-site heterogeneous catalyst for several important reactions. Herein, we computationally investigate Cu(I)-MFU-4l combining with tetrabutylammonium bromide (TBAB) as a high-performance catalyst for CO2 conversion with propylene oxide (PO) into propylene carbonate (PC), a reaction of great significance in achieving carbon neutrality and sustainable development. Four paths are proposed depending on the initial activation of either PO or CO2 by Cu(I), as well as the ring-opening of PO at Cα-O bond or Cβ-O bond. Density functional theory calculations over a large cluster model reveal that Cu(I) is prone to interact with PO first and the most favorable path (Path 3) involves the sequential ring-opening of PO at Cβ-O bond, CO2 insertion and ring closure with Gibbs energy barriers of 8.0, 2.8 and 10.3 kcal/mol, respectively. In view of such low energy barriers, Cu(I)-MFU-4l/TBAB is predicted to be highly active for CO2 cycloaddition with PO at ambient conditions. Significant confinement effect is demonstrated in Cu(I)-MFU-4l by comparing Path 3 over both large and small cluster models. The observed confinement not only stabilizes the intermediates and transition states thermodynamically, but also promotes the ring-opening of PO kinetically, which is mainly attributed to the van der Waals (vdW) interactions between reactive fragments and surrounding frameworks through non-covalent interaction (NCI) analysis. A mechanism of electronic effect competing with steric effect is proposed to rationalize the regioselectivity of ring-opening of PO under different scenarios via natural bond orbital (NBO) charge and NCI analysis. Our results also suggest that Cu(I)-MFU-4l/TBAB outperforms Cu(II) paddle-wheel MOFs/TBAB reported previously for CO2 cycloaddition with PO. The structural robustness, the soft electron-rich Cu(I) site and the inherent confinement promote Cu(I)-MFU-4l to be an appealing candidate catalyst for a wide range of reactions.

Ligand-coordination effects on the selective hydrogenation of acetylene in single-site Pd-ligand supported catalysts

The selective hydrogenation of acetylene to ethylene is a critical step in the synthesis of polyethylenes. Achieving high conversion to ethylene without over-hydrogenation to ethane is a challenge that requires control of the transition metal site, which we achieve through a ligand-coordinated supported catalyst (LCSC) strategy. Using Pd catalysts coordinated to 1,10-phenanthroline-5,6-dione (PDO) ligands on CeO2 supports, we have discovered that the reaction selectivity depends strongly on the ligand:metal ratio with higher selectivity when more ligand is present in the catalyst. Catalyst structure was examined by extended X-ray absorption fine structure spectroscopy, transmission electron microscopy, and CO adsorption, which indicate single-atom character of the Pd. The ligand:metal ratio is determined by X-ray photoelectron spectroscopy measurements and correlated with hydrogenation reactions under steady-state flow conditions to examine trends in hydrogenation activity and selectivity. Those trends can be better understood by density functional theory calculations that indicate hydrogen binding on the ligand to guide reaction selectivity toward the desired ethylene hydrogenation product. These results demonstrate the importance of considering the dynamic character of LCSCs and inform the design of future single-site heterogeneous catalysts.

Active and Regioselective Ru Single-Site Heterogeneous Catalysts for Alpha-Olefin Hydroformylation

Active and Regioselective Ru Single-Site Heterogeneous Catalysts for Alpha-Olefin Hydroformylation

Determining the Three-Dimensional Structures of Silica-Supported Metal Complexes from the Ground Up.

The immobilization of molecularly precise metal complexes to substrates, such as silica, provides an attractive platform for the design of active sites in heterogeneous catalysts. Specific steric and electronic variations of the ligand environment enable the development of structure-activity relationships and the knowledge-driven design of catalysts. At present, however, the three-dimensional environment of the precatalyst, much less the active site, is generally not known for heterogeneous single-site catalysts. We explored the degree to which NMR-based surface-to-complex interatomic distances could be used to solve the three-dimensional structures of three silica-supported metal complexes. The structure solution revealed unexpected features related to the environment around the metal that would be difficult to discern otherwise. This approach appears to be highly robust and, due to its simplicity, is readily applied to most single-site catalysts with little extra effort.