Sinter-Resistant Platinum Catalyst Supported by Metal-Organic Framework.

Single atoms and few-atom clusters of platinum are uniformly installed on the zirconia nodes of a metal-organic framework (MOF) NU-1000 via targeted vapor-phase synthesis. The catalytic Pt clusters, site-isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200 °C. In situ IR spectroscopy reveals the presence of both single atoms and few-atom clusters that depend upon synthesis conditions. Operando X-ray absorption spectroscopy and X-ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favorable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size-selected clusters, including noble metals, on a high surface area support.

Bridging Zirconia Nodes within a Metal-Organic Framework via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires.

Metal-organic frameworks (MOFs), with their well-ordered pore networks and tunable surface chemistries, offer a versatile platform for preparing well-defined nanostructures wherein functionality such as catalysis can be incorporated. Notably, atomic layer deposition (ALD) in MOFs has recently emerged as a versatile approach to functionalize MOF surfaces with a wide variety of catalytic metal-oxo species. Understanding the structure of newly deposited species and how they are tethered within the MOF is critical to understanding how these components couple to govern the active material properties. By combining local and long-range structure probes, including X-ray absorption spectroscopy, pair distribution function analysis, and difference envelope density analysis, with electron microscopy imaging and computational modeling, we resolve the precise atomic structure of metal-oxo species deposited in the MOF NU-1000 through ALD. These analyses demonstrate that deposition of NiOxHy clusters occurs selectively within the smallest pores of NU-1000, between the zirconia nodes, serving to connect these nodes along the c-direction to yield heterobimetallic metal-oxo nanowires. This bridging motif perturbs the NU-1000 framework structure, drawing the zirconia nodes closer together, and also underlies the sintering resistance of these clusters during the hydrogenation of light olefins.

Methane Oxidation to Methanol Catalyzed by Cu-Oxo Clusters Stabilized in NU-1000 Metal-Organic Framework.

Copper oxide clusters synthesized via atomic layer deposition on the nodes of the metal-organic framework (MOF) NU-1000 are active for oxidation of methane to methanol under mild reaction conditions. Analysis of chemical reactivity, in situ X-ray absorption spectroscopy, and density functional theory calculations are used to determine structure/activity relations in the Cu-NU-1000 catalytic system. The Cu-loaded MOF contained Cu-oxo clusters of a few Cu atoms. The Cu was present under ambient conditions as a mixture of ∼15% Cu+ and ∼85% Cu2+. The oxidation of methane on Cu-NU-1000 was accompanied by the reduction of 9% of the Cu in the catalyst from Cu2+ to Cu+. The products, methanol, dimethyl ether, and CO2, were desorbed with the passage of 10% water/He at 135 °C, giving a carbon selectivity for methane to methanol of 45-60%. Cu oxo clusters stabilized in NU-1000 provide an active, first generation MOF-based, selective methane oxidation catalyst.

Tailoring nanoscopic confines to maximize catalytic activity of hydronium ions.

Acid catalysis by hydronium ions is ubiquitous in aqueous-phase organic reactions. Here we show that hydronium ion catalysis, exemplified by intramolecular dehydration of cyclohexanol, is markedly influenced by steric constraints, yielding turnover rates that increase by up to two orders of magnitude in tight confines relative to an aqueous solution of a Brønsted acid. The higher activities in zeolites BEA and FAU than in water are caused by more positive activation entropies that more than offset higher activation enthalpies. The higher activity in zeolite MFI with pores smaller than BEA and FAU is caused by a lower activation enthalpy in the tighter confines that more than offsets a less positive activation entropy. Molecularly sized pores significantly enhance the association between hydronium ions and alcohols in a steric environment resembling the constraints in pockets of enzymes stabilizing active sites.

Enhancing the catalytic activity of hydronium ions through constrained environments.

The dehydration of alcohols is involved in many organic conversions but has to overcome high free-energy barriers in water. Here we demonstrate that hydronium ions confined in the nanopores of zeolite HBEA catalyse aqueous phase dehydration of cyclohexanol at a rate significantly higher than hydronium ions in water. This rate enhancement is not related to a shift in mechanism; for both cases, the dehydration of cyclohexanol occurs via an E1 mechanism with the cleavage of Cß-H bond being rate determining. The higher activity of hydronium ions in zeolites is caused by the enhanced association between the hydronium ion and the alcohol, as well as a higher intrinsic rate constant in the constrained environments compared with water. The higher rate constant is caused by a greater entropy of activation rather than a lower enthalpy of activation. These insights should allow us to understand and predict similar processes in confined spaces.

FY17-PDH-EVTest04 GodInput Impact of the Oxygen Defects1 FY17-PDH-EVTest04 Reduction Rates of Stearic AcidFY17-PDH-T04.

Test New Article1 GodEarlyview.Publish-on-load testing.The role of the specific physicochemical properties of ZrO2 phases on Ni/ZrO2 has been explored with respect to the reduction of stearic acid. Conversion on pure m?ZrO2 is 1.3 times more active than on t?ZrO2 , whereas Ni/m?ZrO2 is three times more active than Ni/t?ZrO2 . Although the hydrodeoxygenation of stearic acid can be catalyzed solely by Ni, the synergistic interaction between Ni and the ZrO2 support causes the variations in the reaction rates. Adsorption of the carboxylic acid group on an oxygen vacancy of ZrO2 and the abstraction of the ??hydrogen atom with the elimination of the oxygen atom to produce a ketene is the key to enhance the overall rate. The hydrogenated intermediate 1?octadecanol is in turn decarbonylated to heptadecane with identical rates on all catalysts. Decarbonylation of 1?octadecanol is concluded to be limited by the competitive adsorption of reactants and intermediate. The substantially higher adsorption of propionic acid demonstrated by IR spectroscopy and the higher reactivity to O2 exchange reactions with the more active catalyst indicate that the higher concentration of active oxygen defects on m?ZrO2 compared to t?ZrO2 causes the higher activity of Ni/m?ZrO2 .

Improving Stability of Zeolites in Aqueous Phase via Selective Removal of Structural Defects.

Missing silicon-oxygen bonds in zeolites are shown to be the cause for structural instability of zeolites in hot liquid water. Their selective removal drastically improved their structural stability as demonstrated using zeolite beta as example. The defects in the siloxy bonds were capped by reaction with trimethylchlorosilane, and Si-O-Si bonds were eventually formed. Hydrolysis of Si-O-Si bonds of the parent materials and dissolution of silica-oxygen tetrahedra in water causing a decrease in sorption capacity by reprecipitation of dissolved silica and pore blocking was largely mitigated by the treatment. The stability of the modified molecular sieves was monitored by (29)Si-MAS NMR, transmission electron micrographs, X-ray diffraction, and adsorption isotherms. The microporosity, sorption capacity, and long-range order of the stabilized material were fully retained even after prolonged exposure to hot liquid water.

Sintering-Resistant Single-Site Nickel Catalyst Supported by Metal-Organic Framework.

Developing supported single-site catalysts is an important goal in heterogeneous catalysis since the well-defined active sites afford opportunities for detailed mechanistic studies, thereby facilitating the design of improved catalysts. We present herein a method for installing Ni ions uniformly and precisely on the node of a Zr-based metal-organic framework (MOF), NU-1000, in high density and large quantity (denoted as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM is demonstrated to be an efficient gas-phase hydrogenation catalyst upon activation. The structure of the active sites in Ni-AIM is proposed, revealing its single-site nature. More importantly, due to the organic linker used to construct the MOF support, the Ni ions stay isolated throughout the hydrogenation catalysis, in accord with its long-term stability. A quantum chemical characterization of the catalyst and the catalytic process complements the experimental results. With validation of computational modeling protocols, we further targeted ethylene oligomerization catalysis by Ni-AIM guided by theoretical prediction. Given the generality of the AIM methodology, this emerging class of materials should prove ripe for the discovery of new catalysts for the transformation of volatile substrates.

State of Supported Nickel Nanoparticles during Catalysis in Aqueous Media.

The state of Ni supported on HZSM-5 zeolite, silica, and sulfonated carbon was studied during aqueous-phase catalysis of phenol hydrodeoxygenation using in situ extended X-ray absorption fine structure spectroscopy. On sulfonated carbon and HZSM-5 supports, NiO and Ni(OH)2 were readily reduced to Ni(0) under reaction conditions (≈35 bar H2 in aqueous phenol solutions containing up to 0.5 wt. % phosphoric acid at 473 K). In contrast, Ni supported on SiO2 was not stable in a fully reduced Ni(0) state. Water enables the formation of Ni(II) phyllosilicate, which is more stable, that is, difficult to reduce, than either α-Ni(OH)2 or NiO. Leaching of Ni from the supports was not observed over a broad range of reaction conditions. Ni(0) particles on HZSM-5 were stable even in presence of 15 wt. % acetic acid at 473 K and 35 bar H2 .

Impact of Zeolite Aging in Hot Liquid Water on Activity for Acid-Catalyzed Dehydration of Alcohols.

The location and stability of Brønsted acid sites catalytically active in zeolites during aqueous phase dehydration of alcohols were studied on the example of cyclohexanol. The catalytically active hydronium ions originate from Brønsted acid sites (BAS) of the zeolite that are formed by framework tetrahedral Si atom substitution by Al. Al K-edge extended X-ray absorption fine structure (EXAFS) and (27)Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopies in combination with density functional theory (DFT) calculations are used to determine the distribution of tetrahedral Al sites (Al T-sites) both qualitatively and quantitatively for both parent and HBEA catalysts aged in water prior to catalytic testing. The aging procedure leads to partial degradation of the zeolite framework evidenced from the decrease of material crystallinity (XRD) as well as sorption capacity (BET). With the exception of one commercial zeolite sample, which had the highest concentration of framework silanol-defects, there is no evidence of Al coordination modification after aging in water. The catalyst weight-normalized dehydration rate correlated best with the sum of strong and weak Brønsted acidic protons both able to generate the hydrated hydronium ions. All hydronium ions were equally active for the acid-catalyzed reactions in water. Zeolite aging in hot water prior to catalysis decreased the weight normalized dehydration reaction rate compared to that of the parent HBEA, which is attributed to the reduced concentration of accessible Brønsted acid sites. Sites are hypothesized to be blocked due to reprecipitation of silica dissolved during framework hydrolysis in the aging procedure.

Sealed rotors for in situ high temperature high pressure MAS NMR.

Here we present the design of reusable and perfectly sealed all-zirconia MAS rotors. The rotors are used to study AlPO4-5 molecular sieve crystallization under hydrothermal conditions, high temperature high pressure cyclohexanol dehydration reaction, and low temperature metabolomics of intact biological tissue.

Electronic and Chemical State of Aluminum from the Single- (K) and Double-Electron Excitation (KLII&III, KLI) X-ray Absorption Near-Edge Spectra of α-Alumina, Sodium Aluminate, Aqueous Al(3+)·(H2O)6, and Aqueous Al(OH)4(-).

We probe, at high energy resolution, the double electron excitation (KLII&II) X-ray absorption region that lies approximately 115 eV above the main Al K-edge (1566 eV) of α-alumina and sodium aluminate. The two solid standards, α-alumina (octahedral) and sodium aluminate (tetrahedral), are compared to aqueous species that have the same Al coordination symmetries, Al(3+)·6H2O (octahedral) and Al(OH)4(-) (tetrahedral). For the octahedral species, the edge height of the KLII&III-edge is approximately 10% of the main K-edge; however, the edge height is much weaker (3% of K-edge height) for Al species with tetrahedral symmetry. For the α-alumina and aqueous Al(3+)·6H2O the KLII&III spectra contain white line features and extended absorption fine structure (EXAFS) that mimics the K-edge spectra. The KLII&III-edge feature interferes with an important region in the EXAFS spectra of the crystalline and aqueous standards. The K-edge spectra and K-edge energy positions are predicted using time-dependent density functional theory (TDDFT). The TDDFT calculations for the K-edge X-ray absorption near-edge spectra (XANES) reproduce the observed transitions in the experimental spectra of the four Al species. The KLII&II and KLI onsets and their corresponding chemical shifts for the four standards are estimated using the delta self-consistent field (ΔSCF) method.

Glucose- and cellulose-derived Ni/C-SO3H catalysts for liquid phase phenol hydrodeoxygenation.

Sulfonated carbons were explored as functionalized supports for Ni nanoparticles to hydrodeoxygenate (HDO) phenol. Both hexadecane and water were used as solvents. The dual-functional Ni catalysts supported on sulfonated carbon (Ni/C-SO3H) showed high rates for phenol hydrodeoxygenation in liquid hexadecane, but not in water. Glucose and cellulose were precursors to the carbon supports. Changes in the carbons resulting from sulfonation of the carbons resulted in variations of carbon sheet structures, morphologies and the surface concentrations of acid sites. While the C-SO3H supports were active for cyclohexanol dehydration in hexadecane and water, Ni/C-SO3H only catalysed the reduction of phenol to cyclohexanol in water. The state of 3-5 nm grafted Ni particles was analysed by in situ X-ray absorption spectroscopy. The results show that the metallic Ni was rapidly formed in situ without detectable leaching to the aqueous phase, suggesting that just the acid functions on Ni/C-SO3H are inhibited in the presence of water. Using in situ IR spectroscopy, it was shown that even in hexadecane, phenol HDO is limited by the dehydration step. Thus, phenol HDO catalysis was further improved by physically admixing C-SO3H with the Ni/C-SO3H catalyst to balance the two catalytic functions. The minimum addition of 7 wt % C-SO3H to the most active of the Ni/C-SO3H catalysts enabled nearly quantitative conversion of phenol and the highest selectivity (90%) towards cyclohexane in 6 h, at temperatures as low as 473 K, suggesting that the proximity to Ni limits the acid properties of the support.

Impact of the oxygen defects and the hydrogen concentration on the surface of tetragonal and monoclinic ZrO2 on the reduction rates of stearic acid on Ni/ZrO2.

The role of the specific physicochemical properties of ZrO2 phases on Ni/ZrO2 has been explored with respect to the reduction of stearic acid. Conversion on pure m-ZrO2 is 1.3 times more active than on t-ZrO2 , whereas Ni/m-ZrO2 is three times more active than Ni/t-ZrO2 . Although the hydrodeoxygenation of stearic acid can be catalyzed solely by Ni, the synergistic interaction between Ni and the ZrO2 support causes the variations in the reaction rates. Adsorption of the carboxylic acid group on an oxygen vacancy of ZrO2 and the abstraction of the α-hydrogen atom with the elimination of the oxygen atom to produce a ketene is the key to enhance the overall rate. The hydrogenated intermediate 1-octadecanol is in turn decarbonylated to heptadecane with identical rates on all catalysts. Decarbonylation of 1-octadecanol is concluded to be limited by the competitive adsorption of reactants and intermediate. The substantially higher adsorption of propionic acid demonstrated by IR spectroscopy and the higher reactivity to O2 exchange reactions with the more active catalyst indicate that the higher concentration of active oxygen defects on m-ZrO2 compared to t-ZrO2 causes the higher activity of Ni/m-ZrO2 .

Quantitatively probing the Al distribution in zeolites.

The degree of substitution of Si(4+) by Al(3+) in the oxygen-terminated tetrahedra (Al T-sites) of zeolites determines the concentration of ion-exchange and Brønsted acid sites. Because the location of the tetrahedra and the associated subtle variations in bond angles influence the acid strength, quantitative information about Al T-sites in the framework is critical to rationalize catalytic properties and to design new catalysts. A quantitative analysis is reported that uses a combination of extended X-ray absorption fine structure (EXAFS) analysis and (27)Al MAS NMR spectroscopy supported by DFT-based molecular dynamics simulations. To discriminate individual Al atoms, sets of ab initio EXAFS spectra for various T-sites are generated from DFT-based molecular dynamics simulations, allowing quantitative treatment of the EXAFS single- and multiple-photoelectron scattering processes out to 3-4 atom shells surrounding the Al absorption center. It is observed that identical zeolite types show dramatically different Al distributions. A preference of Al for T-sites that are part of one or more 4-member rings in the framework over those T-sites that are part of only 5- and 6-member rings in an HBEA150 zeolite has been determined using this analysis.

Following solid-acid-catalyzed reactions by MAS NMR spectroscopy in liquid phase--zeolite-catalyzed conversion of cyclohexanol in water.

A microautoclave magic angle spinning NMR rotor is developed enabling in situ monitoring of solid-liquid-gas reactions at high temperatures and pressures. It is used in a kinetic and mechanistic study of the reactions of cyclohexanol on zeolite HBEA in 130 °C water. The (13) C spectra show that dehydration of 1-(13) C-cyclohexanol occurs with significant migration of the hydroxy group in cyclohexanol and the double bond in cyclohexene with respect to the (13) C label. A simplified kinetic model shows the E1-type elimination fully accounts for the initial rates of 1-(13) C-cyclohexanol disappearance and the appearance of the differently labeled products, thus suggesting that the cyclohexyl cation undergoes a 1,2-hydride shift competitive with rehydration and deprotonation. Concurrent with the dehydration, trace amounts of dicyclohexyl ether are observed, and in approaching equilibrium, a secondary product, cyclohexyl-1-cyclohexene is formed. Compared to phosphoric acid, HBEA is shown to be a more active catalyst exhibiting a dehydration rate that is 100-fold faster per proton.