Zeolite Cluster Research Articles

Regioselective H/D Exchange at the Side‐Chain of Ethylbenzene on Dealuminated Zeolite H‐Y Studied by In Situ MAS NMR–UV/Vis Spectroscopy

A number of heterogeneously catalyzed reactions (e.g. cracking, isomerization, dehydrogenation and alkylation) of hydrocarbons are initiated or promoted on solid acid catalysts by activation of the C H bonds of the reactants. Studies of H/D exchange between the reactants and the Bronsted acid sites of solid catalysts at the early stages of acid-catalyzed reactions provide useful information concerning activation mechanisms and intermediates. These activation mechanisms have been the topic of a number of theoretical and experimental studies. 3] The main routes include pentavalent carbonium ions formed via protonation of alkanes on Bronsted acid sites and trivalent carbenium ions due to hydride abstraction by Lewis acid sites. Sommer et al. concluded that the H/D exchange via the first route requires much higher temperatures compared to the second one, and that it proceeds via direct proton transfer between the solid surface and the alkane. The second route via a carbenium ion may result in regioselective H/D exchange following Markovnikov’s rule. c, e] On sulfated zirconia, only the methyl group of propane exchanges hydrogen atoms at 323 K, but both methyl and methylene groups are involved in H/D exchange at higher temperaACHTUNGTRENNUNGtures. Stepanov and Freude et al. reported regioselective H/D exchange between methyl and methylene groups of propane on the zeolite H-ZSM-5, and showed that the H/D exchange rate of the methyl groups is much higher than that of the methylene groups. Bucko et al. suggested that an entropic effect is responsible for the regioselective H/D exchange of propane and isobutane on zeolite clusters. The probability of adsorption of propane via the methylene group is seventeen times lower than that of adsorption via a methyl group; this entropy contribution leads to a higher free-energy barrier for proton-exchange via the methylene group. Herein, H/D exchange at the side-chain of ethylbenzene adsorbed on dealuminated deH-Y zeolites is studied and preferred regioselective exchange at the methyl group (b-carbon) is found at low temperatures. Based on the recently introduced in situ MAS NMR–UV/Vis spectroscopy, which herein is combined with the injection of short pulses of partially deuterated reactant molecules onto the catalyst at the reaction temperature, a reaction mechanism involving both Lewis and Bronsted acid sites in the H/D exchange reaction is suggested. The combination of complementary spectroscopic techniques helps gaining deeper understanding of catalyzed reactions. As an important advantage, in situ pulsed-flow (PF) H MAS NMR–UV/Vis spectroscopy can probe routes of hydrogen transfer via the characteristic NMR signals of the reactants before and after the exchange. Simultaneously, the formation of cyclohexadienyl and arylcarbenium ions (Scheme 1) is studied via their UV/Vis bands. The application of the pulsed-flow technique allows the study of H/D exchange kinetics at elevated temperatures with a well-defined starting point.

The Role Played by Ga-Pt Nanoparticles in the Aromatization of Lower Alkanes on ZSM-5 Zeolites

The activity and selectivity of Pt-Ga/ZSM-5 zeolites in the aromatization of ethane was studied. The temperature intervals of the stability of gallium oxides on the surface of zeolites were determined by X-ray diffraction. The formation of GaPt clusters was observed after high-temperature treatment of a mixture of platinum-containing zeolites with gallium oxides. Density functional theory was used to study the state of gallium clusters in zeolites by quantum-chemical methods. The interaction of gallium with the H-form of ZSM-5 zeolites was shown to result in the formation of cationic centers containing reduced single-charged gallium cations. The introduction of platinum stabilized alloyed Ga-Pt particles localized in zeolite channels. These particles increase catalyst activity in the aromatization of lower alkanes.

Photoactivation of Silver‐Exchanged Zeolite A

Clusters of silver atoms and ions have attracted the interest of scientists because of their pronounced catalytic and emissive properties. To prevent aggregation of the clusters into larger particles, stabilization in gas matrices at cryogenic temperatures, or in scaffolds such as polyphosphates, DNA, peptides or polymers at ambient temperature, has been suggested. Alternatively, zeolites have been proposed to stabilize small ionic silver clusters. The molecular dimensions of the zeolite cages and channels prevent aggregation into larger nanoparticles (because of steric reasons) while the net negative charge of the zeolite lattice, the coordinating properties of the lattice oxygen atoms, and the presence of additional cations play a crucial role in stabilizing cationic clusters and unstable intermediates during reduction. Reduction of silver in zeolites is usually a bulk process that requires reductants, such as hydrogen gas or sodium borohydride, but also g irradiation and visible light can cause reduction. One of the most studied systems is silver-exchanged zeolite A (LTA topology), and several models have been proposed to explain the nature and location of the silver clusters formed in this zeolite upon reduction. Aside from their catalytic properties, oligonuclear silver clusters show particularly bright and stable luminescent properties. However, the existing fluorescence studies about Ag clusters in zeolites are limited to the excitation and emission spectra of bulk powder samples. Here, we report on bright fluorescent (spots in) individual silverexchanged zeolite 3A crystals obtained upon photoactivation using a focused UV irradiation on a fluorescence microscope. Photoactivation of silver has been demonstrated for nanoscale Ag2O particles (and interpreted as a photoreduction process). In our study, the emissive silver material is confined within a zeolite framework, which results in a better control of the type and location of the emissive species formed upon UV irradiation. Silver-cluster-loaded crystals are technologically very attractive—for example, as secondary light sources in fluorescence lamps—because of their high emission intensity, their excellent photostability upon UV irradiation, and their large Stokes shift. Moreover, the space-resolved selective activation of the emission intensity may have important applications in data storage. The emissive zeolite particles used herein were prepared by exchanging zeolite 3Awith (8 1)wt%Ag (fromAgNO3 solutions), followed by heating for one day at 450 8C (see the Supporting Information). The enhanced emission exhibited by the silver-exchanged zeolites after the thermal treatment has been ascribed to two possible causes: 1) to the formation of charge-transfer complexes between the partially (de)hydrated silver ions and the oxygen atoms in the zeolite lattice or 2) to the emissive properties of autoreduced oligoatomic silver clusters that may be formed during the high-temperature treatment. Although this report focuses on the photoactivation of thermally treated Ag zeolites, control experiments performed on not thermally activated Ag zeolites (dried at 110 8C) show an analogous photoactivation behavior (see the supporting Information). Figure 1a(1),b(1) shows typical confocal scanning images of a heat-treated Ag-containing zeolite crystal (roughly 3 ? 3 mm in size) taken under a confocal microscope using a picosecond-pulsed 375 nm excitation source (doubled Ti:Sapphire, Spectra Physics; see the Supporting Information). Figure 1a and 1b were taken at excitation intensities of 10 and 20 Wcm , respectively. Zeolite crystals that were not treated thermally exhibited an emission ten-times-weaker than that of the thermally treated samples. The confocal approach in combination with the pinhole in the emission path (see the Supporting Information) allows the collection of photoemission data from selected parts inside the crystals. Diffraction-limited bright spots can be generated at specific domains inside an individual crystal by simply focusing a low-power UV laser at the target position; for instance, in the crystal shown in Figure 1a, three individual spots were selectively activated by irradiation during [*] Dr. Y. Antoku, Dr. M. Sliwa, S. Smout, Prof. Dr. J. Hofkens, Dr. T. Vosch Department of Chemistry Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Fax: (+32)1632-7989 E-mail: tom.vosch@chem.kuleuven.be

Methanol Adsorption in Isomorphously Substituted AlPO-34 Clusters and Periodic Density Functional Theory Calculations

Methanol adsorption in isomorphously substituted MAPO-34 (M = Mn, Zn, Mg, Si, Ti, or Zr) zeolite clusters was investigated, and periodic density functional theory (DFT) calculations were carried out. All structures are optimized and charactered at B3LYP/LANL2DZ-6-31G** (the LANL2DZ basis set for Mn, Zn, Mg, Ti, and Zr atoms and 6-31G** basis set sequentially for Si, Al, O, C, and H atoms) and generalized gradient approximation and Perder−Burke−Ernzerhof theoretical levels. Both methods demonstrate that the type of metal dopant used plays an active role in methanol protonation. In Mn, Zn, and Mg-AlPO-34, the stable form of methanol is protonated. However, in Si, Ti, and Zr-AlPO-34, methanol is unprotonated and is simply physisorbed. In the protonated mode, the methoxonium cation forms two very strong hydrogen bonds (1.019−1.073 Å) with the negatively charged zeolite. On the other hand, in the physisorbed mode, methanol interacts with the zeolite framework to form an eight-member ring through two hydrogen bonds, one that is short and rather strong (1.392−1.676 Å) and one that is much weaker (1.941−3.036 Å).

Non-localized charge compensation in zeolites: A periodic DFT study of cationic gallium-oxide clusters in mordenite

Periodic DFT calculations show that stability of binuclear cationic gallium–oxo clusters in high-silica zeolites is mainly controlled by the favorable geometrical environment of the Ga 3+ ions, whereas the effect of the direct interaction with the charge-compensating framework anionic sites is less important. Extraframework cyclic Ga 2O 2+ 2 cations are shown to be active for light alkane dehydrogenation.

A DFT study of adsorption of intermediates in the NO x reduction pathway over BaNaY zeolites

Quantum chemical calculations were employed to develop a better understanding of the adsorption properties of BaNaY interacting with molecules relevant to deNO x catalysis. First, various basis sets and levels of theory were tested for barium-containing species and gas-phase reactions, and it was shown that the choice of the basis set for barium is critical. Density functional theory (DFT) with the B3LYP functional and SDD as the basis set was selected based on its combination of relative accuracy and speed. This level of theory was then used to calculate energies, geometries, and frequencies for acetaldehyde, acetic acid, nitromethane, and water adsorbed and in the gas phase. The predicted properties were compared to experiment where available and reasonable agreement was found. To study the effect of the zeolite framework on the adsorption properties, the size of the zeolite cluster was increased from 6 T to 36 T to 96 T using the embedded ONIOM method. Inclusion of van der Waals interactions with increasing cluster size only changed the adsorption enthalpy by a small amount for all of the adsorbates studied. It was found that the interaction between the empty 6s orbital of Ba and the lone pair orbital of the oxygen atom in the adsorbed state, revealed by natural bond orbital analysis, correlates with the adsorption enthalpy and the gas-phase charge of the oxygen atom that interacts with Ba.

Magnetic properties of Ni2+ clusters in NaY zeolite

We have investigated the magnetic properties of Ni2+ exchanged NaY zeolite by using a direct current magnetization technique. At T>5 K, the M vs H/T curves superimpose and these curves could be fitted well using a Langevin function with a log-normal distribution of moments. The mean spin-cluster moment has been found to be ∼3.3μB. The deviation from the superposition of the M vs H/T curves is observed at T<5 K indicating the building up of a spin-spin correlation between Ni clusters below 5 K. The zero-field-cooled and field-cooled magnetization curves show branching at low temperatures (T≤5 K) under low applied magnetic fields (H≤1 kOe) with a peak in the zero-field-cooled magnetization curves. A log-normal distribution of spin-freezing/blocking temperature has been observed with mean spin-freezing/blocking temperature ∼3.0 K. The peak temperature Tg decreases with increasing field and follows the de Almeida–Thouless (AT) line [1−Tg(H)/Tg(0)]∝H2/3. The extrapolation of the AT line gives the spin-freezing temperature Tg(0)≈3.29 K.

Topological Analysis of the Electronic Charge Density in the Ethene Protonation Reaction Catalyzed by Acidic Zeolite

In the present work, the distribution of the electronic charge density in the ethene protonation reaction by a zeolite acid site is studied within the framework of the density functional theory and the atoms in molecules (AIM) theory. The key electronic effects such as topological distribution of the charge density involved in the reaction are presented and discussed. The results are obtained at B3LYP/6-31G(**) level theory. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density, and electronic energy density at the bond critical points (BCP) in all bonds involved in the interaction zone, in the reactants, pi-complex, transition state, and alkoxy product. In addition, the topological atomic properties are determined on the selected atoms in the course of the reaction (average electron population, N(Omega), atomic net charge, q(Omega), atomic energy, E(Omega), atomic volume, v(Omega), and first moment of the atomic charge distribution, M(Omega)) and their changes are analyzed exhaustively. The topological study clearly shows that the ethene interaction with the acid site of the zeolite cluster, T5-OH, in the ethene adsorbed, is dominated by a strong O-H...pi interaction with some degree of covalence. AIM analysis based on DFT calculation for the transition state (TS) shows that the hydrogen atom from the acid site in the zeolitic fragment is connected to the carbon atom by a covalent bond with some contribution of electrostatic interaction and to the oxygen atom by closed shell interaction with some contribution of covalent character. The C-O bond formed in the alkoxy product can be defined as a weaker shared interaction. Our results show that in the transition state, the dominant interactions are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the concentration and accumulation of the charge density along the bond path between the nuclei linked increases.

High-Efficient Photocatalytic Hydrogen Evolution on Eosin Y-Sensitized Ti−MCM41 Zeolite under Visible-Light Irradiation

Eosin Y-sensitized Ti−MCM-41 zeolite was found to be highly active for hydrogen evolution under visible light irradiation. Dye sensitization was an efficient way to make zeolite sensitive to visible light. Up to 12.01% of the apparent quantum efficiency was achieved over this catalyst under visible light (λ > 420 nm). This catalyst was quite stable over 60 h after consecutive six runs. The deposition of metals (Pt, Ru, Rh) was necessary for hydrogen generation. In addition, amount of dye molecule that anchored on the zeolite affected the hydrogen generation rate significantly. By characterization of UV−vis diffuse reflectance spectra (DRS), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM), it was believed that light was converted into chemical storable hydrogen via electron transfer from dye molecule to TiOx clusters in zeolite, and the effective usage of input light energy was accomplished.

Zeolite-templated Pt/C electrocatalysts

In an effort to reduce the amount of platinum required in low temperature fuel cells, we have prepared electrocatalysts of platinum nanoparticles-on-carbon through zeolite-templating methods. Several different zeolite structure-types, as well as clays and mesoporous silicas were investigated as hosts for the preparation of small Pt clusters through thermal decomposition of ion exchanged (NH 3) 4Pt 2+. Several factors relating to cluster confinement and mobility influence cluster size, and clusters below 1 nm with narrow size distribution have been prepared. To enable electrochemical application, the pores of the Pt cluster-containing zeolite were filled with electrically conductive carbon via infiltration with carbon precursors, polymerization, and pyrolysis. The zeolite host was then removed by acid digestion, to leave a micro-/mesoporous Pt/C electrocatalyst possessing Pt clusters of controlled size. Preliminary electrochemical evaluation of these catalysts is reported. The energetics of thermal decomposition of (NH 3) 4Pt 2+ to form Pt clusters in zeolite X have been investigated through Differential Scanning Calorimetry, and apparent activation energies calculated. The relationship between (NH 3) 4Pt 2+ decomposition and conditions of calcination has been explored in an effort to better control Pt cluster sizes.

Global DFT-Based Reactivity Indicators: An Assessment of Theoretical Procedures in Zeolite Catalysis

The dependence of global reactivity descriptors on electronic structure method as well as basis set is investigated for typical reactions in zeolite catalysis. This research is especially focused on hard−hard interactions between small probe molecules (such as chloromethane, methanol, ethylene, and propene) and different zeolite clusters containing both oxygen and amine functionalities. The performance of novel hybrid metafunctionals (such as BMK and MPWB1K) on crucial reactivity predictors is assessed through comparison with both Hartree−Fock and B3-LYP results. For the complex bifunctional zeolite systems, we find accurate results using any of the DFT functionals, in conjunction with a basis set of at least double-ζ quality further augmented with both polarization and diffuse functions. Reactivity sequences, based on global softness differences as well as activation hardness values, are generally found to be independent of the level of theory whenever a DFT functional is used.

Cationic zinc–cadmium alloy clusters in zeolite A

Rietveld analysis of synchrotron powder X-ray diffraction data obtained from the product of the reaction of cadmium vapour with dehydrated zinc-exchanged zeolite A (LTA structure type) indicates the formation of cationic zinc–cadmium alloy clusters. The clusters are located in approximately 40% of the sodalite cages; the remaining 60% of the cages contain divalent zinc ions coordinated both to the oxygen atoms of the zeolite framework and to residual extra framework oxygen.

Ruthenium sulfide clusters in acidic zeolites: In situ XAS characterization during sulfidation and reaction

Abstract In situ XAS at the Ru K edge was used to study the reductive sulfidation (15% H 2 S/H 2 ) of Ru(NH 3 ) 6 3+ supported on HYd zeolite, and the evolution of the sulfided sample under working conditions (lower H 2 S partial pressure, addition of toluene to the feed). Sulfidation starts at room temperature and is complete at 373 K. At that temperature, the sample is fully sulfided (5.9 S neighbours per Ru). Above 423 K a partial surface reduction (4.3 S neighbours per Ru), and the formation of metallic ruthenium (3.3 Ru neighbours per Ru) are observed. The evolution under working conditions shows that the local structure is not significantly modified when the H 2 S partial pressure is decreased and especially that it does not lead to an increased reduction. Upon addition of toluene the intensity of the Ru–Ru peak in the FT EXAFS spectra is modified, whereas the Ru–S peak remains unchanged. This is indicative of the chemisorption of toluene on the metallic sites and points out the prominent role of these sites for aromatics hydrogenation.

Formation and Redispersion of Silver Clusters in Ag-MFI Zeolite as Investigated by Time-Resolved QXAFS and UV−Vis

Dynamic structural changes of Ag in Ag-MFI upon H2 reduction and subsequent reoxidation treatments were investigated by various characterization methods. Results of in situ time-resolved spectroscopies, UV−vis and quick XAFS, at 573 K show that Ag+ ions at a cation-exchange site of preoxidized Ag-MFI are reduced by H2 to a Ag4δ+ cluster. Protons on the zeolite (Bronsted acid sites) are also produced by the H2 reduction of Ag-MFI as evidenced by IR measurements after D2 adsorption. At higher reduction temperatures, the aggregation of Ag4δ+ clusters results in the formation of larger Agn (n > 8) clusters, and Ag metal particles appear on the external surface of the zeolite. Upon reoxidation at 573 K, the silver clusters are redispersed to Ag+ ions. The rate constant of cluster redispersion at 573 K depends on the oxidants used and increases in the order of NO(0.1%) + O2(10%) > O2(10%) > NO(0.1%). The activation energy for cluster formation in H2 was lower than that for cluster redispersion during reoxidatio...

Aggregated metal clusters in zeolites forming a three-dimensional supperlattice structure and the control of their electronic states

Aggregated metal clusters in zeolites forming a three-dimensional supperlattice structure and the control of their electronic states

Magnetic property of potassium clusters in pressure-doped zeolite A

The pressure-doping technique was developed for further loading of guest potassium metal into porous crystals of zeolite beyond the upper limit at ambient pressure. A doping pressure dependence of the magnetic property of potassium clusters in cages of zeolite A is measured up to 320 MPa. The Curie constant monotonically decreases with increase in the doping pressure and shows very small values above 250 MPa. The magnetic susceptibility at 2 K, however, gradually increases and has a peak at 200 MPa. Observed decrease in the Curie constant is explained by the increase in the loading density of potassium. Simultaneously, the increase in the magnetic susceptibility at 2 K is explained by the superparamagnetism of ferromagnetic domains. A possible mechanism of change in the magnetic properties is discussed.

Effect of the Si/Al distribution on the UV–vis spectrum of propene–zeolite system. A theoretical approach

Olefins included in zeolite cavities show strong perturbations on their electron excitation energies even undergoing visible light photoxidations. Approximate SCF-NDOL-CIS calculations on propene–zeolite systems show that supramolecular electron distribution of excited states could be responsible for these effects. Energy calculations were made with a semiempirical QM potential whose reliability was tested against ab initio SCF-MO. Statistically weighted Si/Al relationships for zeolite clusters were also explored and modeled for the case of cages with 18 tetrahedra. Si/Al ratio also appears to influence the electronic states of the model system. The photobehavior dependence on molecular environment is clearly reflected in the results.

Properties and deactivation of the active sites of an MoZSM-5 catalyst for methane dehydroaromatization: Electron microscopic and EPR studies

The MoZSM-5 (4.0 wt % Mo) catalyst has been characterized by high-resolution transmission electron microscopy, EDXA, and EPR. Two types of molybdenum-containing particles are stabilized in the catalyst in the course of nonoxidative methane conversion at 750°C. These are 2-to 10-nm molybdenum carbide particles on the zeolite surface and clusters smaller than 1 nm in zeolite channels. According to EPR data, these clusters contain the oxidized molybdenum form Mo5+. The surface Mo2C particles are deactivated at the early stages of the reaction because of graphite condensation on their surface. Methane is mainly activated on oxidized molybdenum clusters located in the open molecular pores of the zeolite. The catalyst is deactivated after the 420-min-long operation because of coke buildup on the zeolite surface and in the zeolite pores.

Possible active sites in Fe/ZSM-5 zeolite for the direct benzene hydroxylation to phenol: 1. μ-Oxo[Fe, M] species (M = Fe, Al)

Density functional theory was employed to obtain the configurations of oxo-bridged [Fe, M] exchanged ZSM-5 zeolite clusters, possible active sites for direct benzene hydroxylation. For oxo-bridged [Fe, Fe] species, the Fe–Fe and Fe–O distances in Configurations a, b, c, d, e 1 and f were more consistent with experimental (ca. 2.53 or 3.06 Å) than in the cluster models designed by Yakovlev et al. with two framework Al atoms nearby. For oxo-bridged [Fe, Al] species, the geometric parameters of Configurations g, i and j agreed well with experimental. Configuration i was found to be of higher stability by 64.2 kJ mol −1 compared to its isomer Configuration k 1. +II and +III constituted the dominant valence states for Fe in Fe/ZSM-5 zeolite, and the state of mixed +II and +III valences did exist in ZSM-5 zeolite as in Configuration b 1, b 2 or c. Fe of +I state was absolutely non-existent in ZSM-5 zeolite even if predefined so. The calculated reaction heats of “α-oxygen” formation clearly showed that the oxidation processes were always facilitated by N 2O than O 2. The reaction heats of ZSM-5 zeolites exchanged with mono-Fe and binuclear Fe species ( n → o, a 1,2 → b 1,2 and e 1 → f) were −114.9, −65.3, −67.4 and −48.8 kJ mol −1, respectively, in excellent agreement with the results obtained by Ryder, Yakovlev et al. Moreover, “α-oxygen” generation was favored by introduction of extra-framework Al ( i → j: −73.9 kJ mol −1) compared to the binuclear Fe species. All factors considered, the best candidates for “α-oxygen” generation were Configuration a 1,2 for oxo-bridged [Fe, Fe] species and Configuration i for oxo-bridged [Fe, Al] species, respectively.

A Small Paramagnetic Platinum Cluster in an NaY Zeolite: Characterization and Hydrogen Absorption and Desorption.

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