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Single-Step Selective Oxidation of Methane by Iron-Oxo Species in the Metal–Organic Framework MFU-4l
The direct and selective conversion of methane to methanol can be considered a holy grail for catalysis research. In this work, we study a metal-organic framework known as MFU-4l, modified by design to include highly reactive iron-oxo species for the catalytic C-H bond activation of methane. We investigate the oxidation of methane and the further potential oxidation of the product methanol using N 2 O as an oxygen source and map the potential energy landscape of these reactions using density functional theory calculations. We show that the highest energy barrier encountered during the methane oxidation process is not the C-H bond breaking, but the activation of the iron center by N 2 O. Furthermore, the potential energy landscape for the C-H bond activation exhibits a large, high-energy plateau region instead of a sharp transition state, thus differing from the traditional radical rebound mechanism. Furthermore, this insight offers interesting potential routes to enhance the catalytic activity of the catalyst, to hinder unwanted deactivation pathways, and to reduce the activity towards the over-oxidation of the product.
Considering a Possible Role for [H-Fe 4 N(CO) 12 ] 2- in Selective Electrocatalytic CO 2 Reduction to Formate by [Fe 4 N(CO) 12 ] -
[Fe 4 N(CO) 12 ] - is a first-row transition element electrocatalyst that selectively produces C–H bonds to give formate from CO 2 in water at -1.2 V vs SCE. We present a thermochemical analysis which probes the possibility that [H-Fe 4 N(CO) 12 ] 2– ((H- 1 ) 2– ) is an intermediate in this process: we show that (H- 1 ) 2– is accessible at -1.2 V vs SCE, but if it were formed, we predict that it would generate H 2 . [Fe 4 N(CO) 12 ] 3– and (H- 1 ) 2– were interrogated spectroscopically, and the product of CO loss, [Fe 4 N(CO) 9 (μ-CO) 2 ] 3– , was synthesized and characterized. Ultimately, we demonstrate that (H- 1 ) 2– is an unlikely participant in the catalytic transformation of CO 2 to formate.
Rational Design of a Catalyst for the Selective Monoborylation of Methane
Combined computational and experimental studies elucidate the mechanism and suggest rational design and optimization strategies of a bis(phosphine)-supported iridium-catalyst for methane monoborylation. The activation of the C–H bond in methane via oxidative addition using tris(boryl) iridium(III) complexes bearing bis-chelating supporting ligands is modeled computationally. This model shows that the use of the soft Lewis base ligand such as 1,2-bis(dimethylphosphino)ethane (dmpe) lowers the activation barrier of the rate-determining step as it facilitates polarization of the metal-center, lowering the barrier of the oxidative addition to afford a seven-coordinate iridium(V) intermediate. The experimental optimization of this reaction using high-throughput methods shows that up to 170 turnovers can be achieved at 150 °C (500 psi) within 16 h using bis(pinacolato)diboron, a well-defined homogeneous and monomeric catalyst (dmpe)Ir(COD)Cl that is readily available from commercial precursors, with selectivity for the monoborylation product. High-boiling cyclic aliphatic solvents decalin and cyclooctane also prove suitable for this reaction, while being inert toward borylation. In accordance with the lower calculated activation barrier, catalytic turnover is also observed at 120 °C with up to 50 turnovers over the course of 4 days in cyclohexane solvent. Here, it was found that the borylation of methane is only achieved via one catalytic cycle, and buildup of pinacolborane, a side-product from methane borylation with bis(pinacolato)diboron, inhibits catalytic activity.
Selective Photodimerization in a Cyclodextrin Metal–Organic Framework
For the most part, enzymes contain one active site wherein they catalyze in a serial manner chemical reactions between substrates both efficiently and rapidly. Imagine if a situation could be created within a chiral porous crystal containing trillions of active sites where substrates can reside in vast numbers before being converted in parallel into products. Here, we report how it is possible to incorporate 1-anthracenecarboxylate (1-AC – ) as a substrate into a γ-cyclodextrin-containing metal–organic framework (CD-MOF-1), where the metals are K + cations, prior to carrying out [4+4] photodimerizations between pairs of substrate molecules, affording selectively one of four possible regioisomers. One of the high-yielding regioisomers exhibits optical activity as a result of the presence of an 8:1 ratio of the two enantiomers following separation by high-performance liquid chromatography. The solid-state superstructure of 1-anthracenecarboxylate potassium salt (1-ACK), which is co-crystallized with γ-cyclodextrin, reveals that pairs of substrate molecules are not only packed inside tunnels between spherical cavities present in CD-MOF-1, but also stabilized—in addition to hydrogen-bonding to the C-2 and C-3 hydroxyl groups on the d-glucopyranosyl residues present in the γ-cyclodextrin tori—by combinations of hydrophobic and electrostatic interactions between the carboxyl groups in 1-AC– and four K + cations on the waistline between the two γ-cyclodextrin tori in the tunnels. Here, these non-covalent bonding interactions result in preferred co-conformations that account for the highly regio- and enantioselective [4+4] cycloaddition during photoirradiation. Theoretical calculations, in conjunction with crystallography, support the regio- and stereochemical outcome of the photodimerization.
Synergistic Effects in Bimetallic Palladium–Copper Catalysts Improve Selectivity in Oxygenate Coupling Reactions
Condensation reactions such as Guerbet and aldol are important since they allow for C-C bond formation and give higher molecular weight oxygenates. An previous study identified Pd-supported on hydrotalcite as an active catalyst for the transformation, although this catalyst showed extensive undesirable decarbonylation. A catalyst containing Pd and Cu in a 3:1 ratio dramatically decreased decarbonylation, while preserving the high catalytic rates seen with Pd-based catalysts. A combination of XRD, EXAFS, TEM, and CO chemisorption and TPD revealed the formation of CuPd bimetallic nanoparticles with a Cu-enriched surface. Finally, density functional theory studies suggest that the surface segregation of Cu atoms in the bimetallic alloy catalyst produces Cu sites with increased reactivity, while the Pd sites responsible for unselective decarbonylation pathways are selectively poisoned by CO.
Selecting tandem partners for silicon solar cells [Selecting tandem partners for silicon solar cells using spectral efficiency]
Combining silicon and other materials in tandem solar cells is one approach to enhancing the overall power conversion efficiency of the cells. Here, we argue that top cell partners for silicon tandem solar cells should be selected on the basis of their spectral efficiency — their efficiency resolved by wavelength.
Origins of glycan selectivity in streptococcal Siglec-like adhesins suggest mechanisms of receptor adaptation
Bacterial binding to host receptors underlies both commensalism and pathogenesis. Many streptococci adhere to protein-attached carbohydrates expressed on cell surfaces using Siglec-like binding regions (SLBRs). The precise glycan repertoire recognized may dictate whether the organism is a strict commensal versus a pathogen. However, it is currently not clear what drives receptor selectivity. Here, we use five representative SLBRs and identify regions of the receptor binding site that are hypervariable in sequence and structure. We show that these regions control the identity of the preferred carbohydrate ligand using chimeragenesis and single amino acid substitutions. We further evaluate how the identity of the preferred ligand affects the interaction with glycoprotein receptors in human saliva and plasma samples. As point mutations can change the preferred human receptor, these studies suggest how streptococci may adapt to changes in the environmental glycan repertoire.
Observation of hohlraum-wall motion with spectrally selective x-ray imaging at the National Ignition Facility
The high fuel capsule compression required for indirect drive inertial confinement fusion requires careful control of the X-ray drive symmetry throughout the laser pulse. When the outer cone beams strike the hohlraum wall, the plasma ablated off the hohlraum wall expands into the hohlraum and can alter both the outer and inner cone beam propagations and hence the X-ray drive symmetry especially at the final stage of the drive pulse. To quantitatively understand the wall motion, we developed a new experimental technique which visualizes the expansion and stagnation of the hohlraum wall plasma. Details of the experiment and the technique of spectrally selective x-ray imaging are discussed.
Galaxy kinematics and mass calibration in massive SZE-selected galaxy clusters to z = 1.3
Not Available
Helicity-Selective Enhancement and Polarization Control of Attosecond High Harmonic Waveforms Driven by Bichromatic Circularly Polarized Laser Fields
High harmonics driven by two-color counter-rotating circularly-polarized laser fields are a unique source of bright, circularly-polarized, extreme ultraviolet and soft x-ray beams, where the individual harmonics themselves are completely circularly polarized. Here, we demonstrate the ability to preferentially select either the right or left circularly polarized harmonics simply by adjusting the relative intensity ratio of the bichromatic circularly polarized driving laser field. In the frequency domain, this significantly enhances the harmonic orders that rotate in the same direction as the higher intensity driving laser. In the time domain, this helicity-dependent enhancement corresponds to control over the polarization of the resulting attosecond waveforms. Here, this helicity control enables the generation of circularly-polarized high harmonics with a user-defined polarization of the underlying attosecond bursts. In the future, this technique should allow for the production of bright highly elliptical harmonic supercontinua as well as the generation of isolated elliptically polarized attosecond pulses.
Realization of the orbital-selective Mott state at the molecular level in Ba3LaRu2O9
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