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

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

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

Depositing SiO₂ on Al₂O₃: a Route to Tunable Brønsted Acid Catalysts

Strongly Brønsted acidic silica–alumina materials are workhorse catalysts in petroleum processes, including cracking, isomerization, and hydrocarbon synthesis. Here thin, conformal overcoats of SiO₂, 2–5 nm by TEM, were synthesized on pre-existing Al₂O₃ supports by stepwise addition of tetraethyl orthosilicate under basic conditions, and the surfaces were interrogated by N₂ physisorption along with NH₃ and pyridine chemisorption. SiO₂ layers thicker than 5 nm give largely inert surfaces, but adding only 2 nm of SiO₂ is shown to quench the underlying Lewis acidity and unexpectedly form Brønsted acid sites strong enough to protonate gas-phase pyridine. Alternately, the use of a molecular template grafted to the alumina surface before SiO₂ deposition selectively preserves the most reactive AlOH. In the catalytic cracking of 1,3,5-triisopropylbenzene at 450 °C, Al₂O₃ overcoated by 2 nm of SiO₂ proved to be a highly active catalyst with 10 times higher conversion in comparison to that of Al₂O₃. Finally, the silica overlayer was deliberately cracked to expose strong interfacial sites, likely between tetrahedral Al and SiO₂. In comparison to a material with an intact overlayer, this catalyst had 1.5 times higher conversion and 3 times higher selectivity to deep dealkylation products, including cumene and benzene. This core–shell SiO₂@Al₂O₃ catalyst gives total dealkylation yields, per surface area, similar to those of conventionally prepared SiO₂–Al₂O₃ or zeolite Y catalysts while providing for new synthetic handles for catalyst optimization

Observing Local pH Changes Using a Rotating Ring-Disk Electrode Functionalized with a Potentiometric pH-Sensing Probe

Electrochemical reactions involving protons and hydroxide ions are significantly impacted by changes in the local pH near the catalyst surface. Therefore, it is useful to quantify the catalyst local pH to better understand the impact on overall reaction efficiency and selectivity. While it is difficult to experimentally probe the catalyst/electrolyte interface, this regime can be monitored indirectly using pH-sensitive materials. In this work, we investigate the use of a rotating ring-disk electrode coupled with a pH-sensing probe to track changes in proton concentration near the catalyst surface for the oxygen reduction reaction under well-defined mass transport conditions. We further examine the limitations and describe methods for improving the robustness of this experimental platform. Out of the electrode support and probe materials examined, we find that iridium oxide electrodeposited using cyclic voltammetry onto gold substrates exhibiting high surface area and moderate porosity demonstrates the highest, fastest, and most stable pH-potential response, enabling reliable measurements in under 10 s. Using an analytical convective-diffusion equation, we also estimate the disk local pH under varied operating conditions (e.g., current density and rotation rate) and reaction environments (e.g., bulk pH). This work outlines best practices for applying this technique and provides insights into the impact of relevant reaction environment conditions on the catalytic performance

Consequences of Confinement for Alkene Epoxidation with Hydrogen Peroxide on Highly Dispersed Group 4 and 5 Metal Oxide Catalysts

Ti, Nb, and Ta atoms substituted into the framework of zeolite *BEA (M-BEA) or grafted onto mesoporous silica (M-SiO₂) irreversibly activate hydrogen peroxide (H₂O₂) to form pools of metal-hydroperoxide (M-OOH) and peroxide (M-(η²-O₂)) species for alkene epoxidation. The product distributions from reactions with <i>Z</i>-stilbene, in combination with time-resolved UV–vis spectra of the reaction between H₂O₂-activated materials and cyclohexene, show that M-OOH surface intermediates epoxidize alkenes on Ti-based catalysts, while M-(η²-O₂) moieties epoxidize substrates on the Nb- and Ta-containing materials. Kinetic measurements of styrene (C₈H₈) epoxidation reveal that these materials first adsorb and then irreversibly activate H₂O₂ to form pools of interconverting M-OOH and M-(η²-O₂) intermediates, which then react with styrene or H₂O₂ to form either styrene oxide or H₂O₂ decomposition products, respectively. Activation enthalpies (Δ<i>H</i>^⧧) for C₈H₈ epoxidation and H₂O₂ decomposition decrease linearly with increasing heats of adsorption for pyridine or deuterated acetonitrile coordinated to Lewis acid sites, which suggests that materials with greater electron affinities (i.e., stronger Lewis acids) are more active for C₈H₈ epoxidation. Values of Δ<i>H</i>^⧧ for C₈H₈ epoxidation and H₂O₂ decomposition also decrease linearly with the ligand-to-metal charge-transfer (LMCT) band energies for the reactive intermediates, which is a more relevant measure of the requirements for the active sites in these catalytic cycles. Epoxidation rates depend more strongly on the LMCT band energy than H₂O₂ decomposition rates, which shows that more electrophilic M-OOH and M-(η²-O₂) species (i.e., those formed at stronger Lewis acid sites) give both greater rates and greater selectivities for epoxidations. Thermochemical analysis of Δ<i>H</i>^⧧ for C₈H₈ epoxidation and adsorption enthalpies for C₈H₈ within the pores of *BEA and SiO₂ reveal that the 0.7 nm pores within M-BEA preferentially stabilize transition states for C₈H₈ epoxidation with respect to the 5.4 nm pores of M-SiO₂, while H₂O₂ decomposition is unaffected by the differences between these pore diameters due to the small Stokes diameter of H₂O₂. Thus, the differences in reactivity and selectivity between M-BEA and M-SiO₂ materials is solely attributed to confinement of the transition state and not differences in the identity of the reactive intermediates, mechanism for alkene epoxidation, or intrinsic activation barriers. Consequently, the rates and selectivities for alkene epoxidation reflect at least two orthogonal catalyst design criteriathe electronegativities of the transition metal atoms that determine the electronic structure of the active complex and the mean diameters of the surrounding pores that can selectively stabilize transition states for specific reaction pathways

Synthesis−Structure–Function Relationships of Silica-Supported Niobium(V) Catalysts for Alkene Epoxidation with H₂O₂

Many industrially significant selective oxidation reactions are catalyzed by supported and bulk transition metal oxides. Catalysts for the synthesis of oxygenates, and especially for epoxidation, have predominantly focused on TiO_<i>x</i> supported on or co-condensed with SiO₂, whereas much of the rest of Groups 4 and 5 have been less studied. We have recently demonstrated through periodic trends using a uniform molecular precursor that niobium­(V)-silica catalysts reveal the highest activity and selectivity for efficient utilization of H₂O₂ for epoxidation across all of Groups 4 and 5. In this work, we graft a wide range of Nb­(V) precursors, spanning surface densities of 0.07–1.6 Nb groups nm^–2 on mesoporous silica, and we characterize these materials with UV–visible spectroscopy and Nb <i>K</i>-edge XANES. Further, we apply in situ chemical titration with phenylphosphonic acid (PPA) in the epoxidation of <i>cis</i>-cyclooctene by H₂O₂ to probe the numbers and nature of the active sites across this series and in a set of related Ti-, Zr-, Hf-, and Ta-SiO₂ catalysts. By this method, the fraction of kinetically relevant NbO_<i>x</i> species ranges from ∼15% to ∼65%, which correlates with spectroscopic evaluation of the NbO_<i>x</i> sites. This titration leads to a single value for the average turnover frequency, on a per active site basis rather than a per Nb atom basis, of 1.4 ± 0.52 min^–1 across the 21 materials in the series. These quantitative maps of structural properties and kinetic consequences link key catalyst descriptors of supported Nb-SiO₂ to enable rational design for next-generation oxidation catalysts

Size-Selective Synthesis and Stabilization of Small Silver Nanoparticles on TiO₂ Partially Masked by SiO₂

Controlling metal nanoparticle size is one of the principle challenges in developing new supported catalysts. Typical methods where a metal salt is deposited and reduced can result in a polydisperse mixture of metal nanoparticles, especially at higher loading. Polydispersity can exacerbate the already significant challenge of controlling sintering at high temperatures, which decreases catalytic surface area. Here, we demonstrate the size-selective photoreduction of Ag nanoparticles on TiO₂ whose surface has been partially masked with a thin SiO₂ layer. To synthesize this layered oxide material, TiO₂ particles are grafted with <i>tert</i>-butylcalix­[4]­arene molecular templates (∼2 nm in diameter) at surface densities of 0.05–0.17 templates.nm^–2, overcoated with ∼2 nm of SiO₂ through repeated condensation cycles of limiting amounts of tetraethoxysilane (TEOS), and the templates are removed oxidatively. Ag photodeposition results in uniform nanoparticle diameters ≤ 3.5 nm (by transmission electron microscopy (TEM)) on the partially masked TiO₂, whereas Ag nanoparticles deposited on the unmodified TiO₂ are larger and more polydisperse (4.7 ± 2.7 nm by TEM). Furthermore, Ag nanoparticles on the partially masked TiO₂ do not sinter after heating at 450 °C for 3 h, while nanoparticles on the control surfaces sinter and grow by at least 30%, as is typical. Overall, this new synthesis approach controls metal nanoparticle dispersion and enhances thermal stability, and this facile synthesis procedure is generalizable to other TiO₂-supported nanoparticles and sizes and may find use in the synthesis of new catalytic materials