Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. We present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over µ3-oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central µ3-oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

Silicon Nitride Surface Enabled Propane Dehydrogenation Catalyzed by Supported Organozirconium.

Mesoporous silicon nitride (Si3N4) is a nontraditional support for the chemisorption of organometallic complexes with the potential for enhancing catalytic activity through features such as the increased Lewis basicity of nitrogen for heterolytic bond activation, increased ligand donor strength, and metal-ligand orbital overlap. Here, tetrabenzyl zirconium (ZrBn4) was chemisorbed on Si3N4, and the resulting supported organometallic species was characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Dynamic Nuclear Polarization-enhanced Solid State Nuclear Magnetic Resonance (DNP-SSNMR), and X-ray Absorption Spectroscopy (XAS). Based on the hypothesis that the nitride might enable facile heterolytic C-H bond activation along the Zr-N bond, this material was found to be a highly active (1.53 molpropene molZr-1 h-1 at 450 °C) and selective (99% to propylene) catalyst for propane dehydrogenation. In contrast, the homologous silica supported complex exhibited negligible activity under these conditions.

Low-Temperature, Ambient Pressure Oxidation of Methane to Methanol Over Every Tri-Iron Node in a Metal-Organic Framework Material.

In contrast with metal-modified zeolites, metal-organic framework materials (MOFs) provide a platform that may be significantly more amenable to creating catalysts in which every metal site is endowed with the same coordination environment, and hence, catalytic function. Using MIL-100(Fe) as a prototype, we present the first example of a synthetic heterogeneous catalyst comprised exclusively of active tri-iron moieties participating in the low-temperature oxidation of methane to methanol; in contrast with prior reports on iron-MOFs, we report the near-exclusive formation of methanol at low temperatures and sub-ambient methane pressures, and evidence its effectuation solely by Fe2+ sites. The study captures the utility of exploring classes of materials endowed with a high level of definition in structure and catalytic function for the purposes of overcoming persistent scientific and technological challenges in the field of synthetic heterogeneous catalysis.

Quantification of Open-Metal Sites in Metal-Organic Frameworks Using Irreversible Water Adsorption.

Metal-organic frameworks (MOFs) have been the focus of extensive research over the past couple of decades owing to their utility in enhancing performance in a range of applications including but not limited to gas separations, heterogeneous catalysis, and sensing. A rigorous understanding of the role of open-metal sites in molecular processes pertinent to these applications is first and foremost reliant on an accurate measure of the quantity of metal atoms that are coordinatively unsaturated under a given set of experimental conditions. Existing methods for quantifying open-metal sites exhibit drawbacks originating from unselective adsorption, use of high pressures and/or low temperatures, or the handling of potentially hazardous reagents. Here we investigate for the first time the use of room-temperature water adsorption isotherms for the quantification of MOF open-metal site density. We report that the quantity of water adsorbed irreversibly at room temperature on MIL-100 represents the open-metal site density under a given set of activation conditions. We use for this purpose a hydroxyl-containing version of MIL-100(Cr) that enables us to track (using in situ Fourier transform infrared spectroscopy) both dehydration and dehydroxylation events leading to open-metal site creation, providing evidence for site counts measured using irreversible water adsorption. Crucially, this approach circumvents the need for assumptions relating to the identity of open-metal sites and the degree of adsorbate saturation, while also obviating risks associated with the use of hazardous reagents. Given the near-universal presence of water as a labile ligand in the first coordination sphere of possible MOF open-metal sites, we envision that the protocols presented here could represent an approach to counting open-metal sites that is broadly applicable within (and maybe even beyond) the field of MOF research.

Spectroscopic and reactive characterization of mixed-metal Fe-Cr trimer nodes in metal-organic framework MIL-100.

Metal-organic framework MIL-100 is synthesized featuring heterometallic Fe and Cr M3O nodes where mixing of the metals within the nodes is evidenced using a combination of in situ IR spectroscopy, NO titrations, and CO oxidation kinetics. Reactivity data indicate distinctive properties of the bimetallic nodes.

Structural and reactive evolution of oxidatively grafted Pd catalysts on MnO2 for the low-temperature oxidation of CO.

Isolated Pd atoms supported on high surface area MnO2, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) oxidation of CO (7.7 kPa O2, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO2 in facilitating redox turnovers.

Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry.

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

Plastic Waste Product Captures Carbon Dioxide in Nanometer Pores.

Plastic waste (PW) and increasing atmospheric carbon dioxide (CO2) levels are among the top environmental concerns presently facing humankind. With an ambitious 2050 zero-CO2 emissions goal, there is a demand for economical CO2 capture routes. Here we show that the thermal treatment of PW in the presence of potassium acetate yields an effective carbon sorbent with pores width of 0.7-1.4 nm for CO2 capture. The PW to carbon sorbent process works with single or mixed streams of polyolefin plastics. The CO2 capacity of the sorbent at 25 °C is 17.0 ± 1.1 wt % (3.80 ± 0.25 mmol g-1) at 1 bar and 5.0 ± 0.6 wt % (1.13 ± 0.13 mmol g-1) at 0.15 bar, and it regenerates upon reaching 75 ± 5 °C. The CO2 capture cost from flue gas via this technology is estimated to be <$21 ton-1 CO2, much lower than competing CO2 capture technologies. Hence, this PW-derived carbon material should find utility in the capture of CO2 from point sources of high CO2 emissions while providing a use for otherwise deleterious PW.