Solid Cocatalysts for Activating Manganese Triazacyclononane Oxidation Catalysts

Immobilizing a homogeneous catalyst provides obvious handling benefits, but ideally can also enhance catalyst productivity or selectivity because of beneficial interactions between the surface and the active site. Here, Mn 1,4,7-trimethyl-1,4,7-triazacyclononane dimers (1) are activated by carboxylate-containing solids for cyclooctene epoxidation/dihydroxylation with H2O2 at 0 °C. The productivity (TON, moles per mole 1) and selectivity (to cis-diol) of cyclooctene oxidation by 1 are known to be tuned by choice of soluble carboxylate cocatalyst, and this concept is extended here to solid carboxylate cocatalysts. These solid cocatalysts are synthesized by covalently grafting benzoate and propanoate silanes to SiO2 or allowing terephthalic acid or dihydroxyhydrocinnamic acid to chemisorb on oxides. Comparing analogous structures, SiO2-grafted carboxylates outperform soluble benzoic acid (275 vs 25 TON at 2 equiv), hydrocinnamic acid (150 vs 40 TON at 2 equiv), and valeric acid (675 vs 70 TON at 10 equiv). Of the oxides tested as modifiers for carboxylate cocatalysts, TiO2 leads to the largest improvements in oxidation productivity, boosting productivity to 300 TON when combined with 2 equiv of benzoic acid and to 425 TON when combined with ditopic terephthalic acid. The latter enhancement may be due to both a buffering effect and a high surface concentration of chemisorbed species that encourages formation of the presumed carboxylate-bridged active state of the catalyst. Dihydroxylation selectivities are a function of the carboxylate employed as well as the nature of the other groups on the solid cocatalyst surface. SiO2 grafted with propionate groups gives ∼50% cis-diol selectivity, but further modification with alkyl or perfluoroalkyl silanes increase cis-diol selectivity up to ∼60%. Dihydroxylation selectivity is also ∼60% for SiO2 grafted with benzoate or ∼70% for terephthalic acid chemisorbed on TiO2. These solid cocatalysts introduce a number of additional tunable parameters that lead to enhanced productivity or selectivity for molecular oxidation catalysts like 1 that are activated by carboxylates

Grafted Metallocalixarenes as Single-Site Surface Organometallic Catalysts

Metallocalixarenes were grafted onto silica using a surface organometallic approach and shown to be active and selective catalysts for epoxidation of alkenes using organic hydroperoxides. Calixarene−TiIV precursors were anchored at surface densities from 0.1 to near-monolayer coverages (0.025−0.25 calixarene nm-2). Several spectroscopic methods independently detected calixarene−TiIV connectivity before and after epoxidation catalysis. Kinetic analyses of cyclohexene epoxidation confirmed that the active sites were anchored on the silica surface and were significantly more active than their homogeneous analogues. The steric bulk and multidentate binding of the calixarenes led to structural stability and to single-site behavior during epoxidation catalysis. Rate constants were independent of surface density for cyclohexene epoxidation with tert-butyl hydroperoxide (11.1 ± 0.3 M-2 s-1) or cumene hydroperoxide (25 ± 2 M-2 s-1). The materials and methods reported here allow the assembly of robust surface organometallic structures in which the active sites behave as isolated species, even near saturation monolayer coverages. In turn, this makes possible the rational design and synthesis of a class of heterogeneous oxide catalysts with atomic-scale precision at the active site

Energetics of Small Molecule and Water Complexation in Hydrophobic Calixarene Cavities

Calixarenes grafted on silica are energetically uniform hosts that bind aromatic guests with 1:1 stoichiometry, as shown by binding energies that depend upon the calixarene upper rim composition but not on their grafted surface density (0.02−0.23 nm-2). These materials are unique in maintaining a hydrophilic silica surface, as probed by H2O physisorption measurements, while possessing a high density of hydrophobic binding sites that are orthogonal to the silica surface below them. The covalently enforced cone-shaped cavities and complete accessibility of these rigidly grafted calixarenes allow the first unambiguous measurements of the thermodynamics of guest interaction with the same calixarene cavities in aqueous solution and vapor phase. Similar to adsorption into nonpolar protein cavities, adsorption into these hydrophobic cavities from aqueous solution is enthalpy-driven, which is in contrast to entropy-driven adsorption into water-soluble hydrophobic hosts such as β cyclodextrin. The adsorption thermodynamics of several substituted aromatics from vapor and liquid are compared by (i) describing guest chemical potentials relative to pure guest, which removes differences among guests because of aqueous solvation and van der Waals contacts in the pure condensed phase, and (ii) passivating residual guest binding sites on exposed silica, titrated by water during adsorption from aqueous solution, using inorganic salts before vapor adsorption. Adsorption isotherms depend only upon the saturation vapor pressure of each guest, indicating that guest binding from aqueous or vapor media is controlled by van der Waals contacts with hydrophobic calixarene cavities acting as covalently assembled condensation nuclei, without apparent contributions from CH−π or other directional interactions. These data also provide the first direct quantification of free energies for interactions of water with the calixarene cavity interior. The calixarene−water interface is stabilized by ∼20 kJ/mol relative to the water−vapor interface, indicating that water significantly competes with the aromatic guests for adsorption at these ostensibly hydrophobic cavities. This result is useful for understanding models of water interactions with other concave hydrophobic surfaces, including those commonly observed within proteins

Photoluminescence and Charge-Transfer Complexes of Calixarenes Grafted on TiO₂ Nanoparticles

Calix[4]arenes and thiacalix[4]arenes, cyclic tetramers of phenol, are synthesized with para position (upper rim) tert-butyl, Br, and NO2 groups and grafted covalently onto surfaces of TiO2 nanoparticles up to a geometrical maximum surface density of 0.30 nm-2. Grafted calixarenes are hydrolytically stable and are shown to exist in their ‘cone' conformation from comparison with model materials synthesized by grafting preformed calixarene−Ti complexes. Individually, protonated calixarenes and TiO2 absorb only UV light, but calixarene−TiO2 hybrid organic−inorganic materials absorb light at significantly lower energies in the visible range (>2.2 eV, <560 nm), reflecting ligand-to-metal charge transfer (LMCT) between calixarene and Ti centers on surfaces of TiO2 nanoparticles. These absorption energies do not depend on the identity and electron-withdrawing properties of upper rim groups in calixarenes. However, the steady-state photoluminescence emission of the calixarene−TiO2 hybrid material is weakened uniformly throughout the excitation spectrum when compared with the material before calixarene grafting, and these effects become stronger as calixarene upper rim substituents become more electron-withdrawing. The single-step synthesis protocols described here electronically couple calixarenes with surfaces of oxide semiconductors, leading to sensitization of TiO2 for absorption in the visible region and provide a systematic method for controlling and understanding surface dipole-mediated electron-transfer phenomena relevant to the photocatalytic and optoelectronic properities of TiO2

Structure–Activity Relationships That Identify Metal–Organic Framework Catalysts for Methane Activation

In this work, we leverage advances in computational screening based on periodic density functional theory (DFT) to study a diverse set of experimentally derived metal–organic frameworks (MOFs) with accessible metal sites for the oxidative activation of methane. We find that the thermodynamic favorability of forming the metal-oxo active site has a strong, inverse correlation with the reactivity toward C–H bond activation for a wide range of MOFs. This scaling relationship is found to hold over MOFs with varying coordination environments and metal compositions, provided the bonds of the framework atoms are conserved. The need to conserve bonds is an important constraint on the correlations but also demonstrates a route to intentionally break the scaling relationship to generate novel catalytic reactivity. Periodic trends are also observed across the data set of screened MOFs, with later transition metals forming less stable but more reactive metal-oxo active sites. Collectively, the results in this work provide robust rules-of-thumb for choosing MOFs to investigate for the activation of methane at moderate reaction conditions

Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed Oxidative Dehydrogenation of Cyclohexane

Alkane oxidative dehydrogenation (ODH) over supported redox active metal oxides is highly sensitive to support identity, but the underlying cause of support effects has not been well-established. Here, we provide evidence that charge transfer between the support and active oxide phase impacts the rates of C–H bond abstraction and CO_X formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. The surface Lewis acid strength of nine metal oxide supports is quantified by alizarin dye intramolecular charge transfer shifts and compared with supported copper oxide d–d transition energies to determine the relationship between support Lewis acid strength and copper oxide electronic properties. Model cyclohexane ODH reaction studies show that selectivity to C₆ products increases with increasing support Lewis acid strength, with selectivities to benzene and cyclohexene over combustion products at zero conversion increasing from 20% over nucleophilic Cu/MgO to over 90% over the more Lewis acidic Cu/Nb₂O₅ and Cu/Ta₂O₅. This is ascribed to a linear relationship between the amount of electron density on the copper oxide valence states as described by Cu d–d transition energy and the ratio of rates of C–H bond abstraction and CO_X formation pathways. This approach to quantifying support Lewis acid strength and applying it as a key catalytic descriptor of support effects is a useful tool to enable rational design of next-generation oxidative dehydrogenation catalysts

Comprehensive Phase Diagrams of MoS₂ Edge Sites Using Dispersion-Corrected DFT Free Energy Calculations

A comprehensive set of surface phase diagrams addressing the catalytically relevant edges of the (100) surface of MoS₂ catalysts is developed using dispersion-corrected density functional theory and ab initio thermodynamic modeling. The results of the temperature-dependent, free energy-based thermodynamic model are presented over the full range of catalytically relevant temperatures and pressures, in addition to S- and H-coverages ranging from 0 to 100%. The results of this work allow for a full thermodynamic analysis to be performed at the conditions relevant to any promising reaction involving MoS₂, ranging from hydrodesulfurization to dehydrogenation to electrocatalysis. Several methodological recommendations are discussed and implemented with the goal of improving the accuracy of the surface phase diagrams at minimal computational expense. A library of the most stable S- and H-adsorption modes is also developed so that linear scaling relationships can be used to correlate thermodynamic stability with kinetic activity. Applying the results to C–H bond activation of methane with a S₂ oxidant, we predict S-coverages near 100% on the Mo- and S-edges to be thermodynamically favored and S monomers on edge sites with high S-coverages to be kinetically favorable. For H-abstraction on surface S atoms, the Mo-edge is also predicted to be more active than the S-edge

Recovery of Dilute Aqueous Acetone, Butanol, and Ethanol with Immobilized Calixarene Cavities

Macrocyclic calixarene molecules were modified with functional groups of different polarities at the upper rim and subsequently grafted to mesoporous silica supports through a single Si atom linker. The resulting materials were characterized by thermogravimetric analysis, UV–visible spectroscopy, nitrogen physisorption, and solid-state NMR spectroscopy. Materials were then used to separate acetone, <i>n</i>-butanol, and ethanol from dilute aqueous solution, as may be useful in the recovery of fermentation-based biofuels. For the purpose of modeling batch adsorption isotherms, the materials were considered to have one strong adsorption site per calixarene molecule and a larger number of weak adsorption sites on the silica surface and external to the calixarene cavity. The magnitude of the net free energy change of adsorption varied from approximately 15 to 20 kJ/mol and was found to decrease as upper-rim calixarene functional groups became more electron-withdrawing. Adsorption appears to be driven by weak van der Waals interactions with the calixarene cavity and, particularly for butanol, minimizing contacts with solvent water. In addition to demonstrating potentially useful new sorbents, these materials provide some of the first experimental estimates of the energy of interaction between aqueous solutes and hydrophobic calixarenes, which have previously been inaccessible because of the insolubility of most nonionic calixarene species in water

Periodic Trends in Highly Dispersed Groups IV and V Supported Metal Oxide Catalysts for Alkene Epoxidation with H₂O₂

Supported metal oxides are important catalysts for selective oxidation processes like alkene epoxidation with H₂O₂. The reactivity of these catalysts is dependent on both identity and oxide structure. The dependence of the latter on the synthesis method can confound attempts at comparative studies across the periodic table. Here, SiO₂-supported metal oxide catalysts of Ti­(IV), Zr­(IV), Hf­(IV), V­(V), Nb­(V), and Ta­(V) (all of groups IV and V) were synthesized by grafting a series of related calixarene coordination complexes at surface densities less than ∼0.25 nm^–2. Select catalysts were investigated by solid state NMR, UV–visible, and X-ray absorption near-edge spectroscopies. As-synthesized and calcined materials were examined for the epoxidation of cyclohexene and styrene (1.0 M) with H₂O₂ (0.10 M) at 45 and 65 °C. Nb catalysts emerge as high-performing materials, with calcined Nb–SiO₂ proceeding at a cyclohexene turnover frequency of 2.4 min^–1 (>2 times faster than Ti–SiO₂) and with ∼85% selectivity toward direct (nonradical) epoxidation pathways. As-synthesized Zr, Hf, and Ta catalysts have improved direct pathway selectivities compared with their calcined versions, particularly evident for Ta–SiO₂. Finally, when the materials are synthesized from these precursors but not simple metal chlorides, the direct pathway reaction rate correlates with Pauling electronegativities of the metals, demonstrating clear periodic trends in intrinsic Lewis acid catalytic behavior

Comprehensive Phase Diagrams of MoS₂ Edge Sites Using Dispersion-Corrected DFT Free Energy Calculations

A comprehensive set of surface phase diagrams addressing the catalytically relevant edges of the (100) surface of MoS₂ catalysts is developed using dispersion-corrected density functional theory and ab initio thermodynamic modeling. The results of the temperature-dependent, free energy-based thermodynamic model are presented over the full range of catalytically relevant temperatures and pressures, in addition to S- and H-coverages ranging from 0 to 100%. The results of this work allow for a full thermodynamic analysis to be performed at the conditions relevant to any promising reaction involving MoS₂, ranging from hydrodesulfurization to dehydrogenation to electrocatalysis. Several methodological recommendations are discussed and implemented with the goal of improving the accuracy of the surface phase diagrams at minimal computational expense. A library of the most stable S- and H-adsorption modes is also developed so that linear scaling relationships can be used to correlate thermodynamic stability with kinetic activity. Applying the results to C–H bond activation of methane with a S₂ oxidant, we predict S-coverages near 100% on the Mo- and S-edges to be thermodynamically favored and S monomers on edge sites with high S-coverages to be kinetically favorable. For H-abstraction on surface S atoms, the Mo-edge is also predicted to be more active than the S-edge