Supercages Of NaY Zeolite Research Articles

Generation of active site confined inside supercage of NaY zeolite on a nano-scale and its ethylene polymerization

To generate an active site that consisted of one Cp 2ZrCl 2 molecule and 1-2 MAO molecules inside supercage of NaY zeolite, two preparation ways for supported catalyst were estimated. First, higher concentration of MAO and Cp 2ZrCl 2, and long reaction time were introduced during the preparation of supported catalyst. It showed activity in ethylene polymerization without any additional MAO. It indicates that Cp 2ZrCl 2 coordinated with only 1–2 MAO molecules could be an active site due to the fact that supercage has nano-scaled diameter of supercage, 1.2 nm, and it could contain only 1–2 MAO molecules inside it theoretically. In situ generation of active site between NaY/MAO and homogeneous Cp 2ZrCl 2 also showed experimental evidence that an active site was generated inside the supercage of NaY zeolite. It showed low activity with long activation time, suggesting the presence of a diffusion effect of Cp 2ZrCl 2 in the pore of NaY. However, NaY/Cp 2ZrCl 2 and homogeneous MAO system showed the characteristic PE polymerization with homogeneous catalyst, indicating that active site was not generated inside the supercage of NaY.

Localization of ferrocene in NaY zeolite by powder x-ray and neutron diffraction

We study the inclusion of the metallocene ferrocene Fe(C5H5)2 molecules in the supercages of NaY zeolite. To find the exact location of the ferrocene molecules within the supercages we perform neutron and powder x-ray diffraction on bare NaY zeolite, and on NaY zeolite loaded with one or two ferrocene molecules per supercage. Using the complementary properties of both techniques we show that the ferrocene molecules are located just above a line joining two neighboring sodium ions at the SII positions in the zeolite supercage. The C5H5 rings are oriented towards the sodium ions in an ordered manner. This structure is confirmed by quantum chemistry calculations. The geometry of the ferrocene molecules barely changes, indicating that the increased reactivity of ferrocene upon adsorption in a zeolite is thus a result of the position of the molecules. The main interactions responsible for this position are Coulombic attraction and hydrogen bonding. The inclusion of ferrocene in a Y-type zeolite provides a homogeneous distribution of iron throughout the zeolite at well-defined locations.

On the jump diffusion of molecules in zeolites measured by quasi-elastic neutron scattering

Molecular diffusion in the regularly spaced cavities of zeolites involves various jump types, within and between the cavities, which may occur at different time scales. These jump processes can be modelled by simulation techniques but they are extremely difficult to be measured experimentally. The quasi-elastic neutron scattering (QENS) technique is a unique probe for studying both the dynamical and geometrical aspects of such molecular motions. For benzene adsorbed in the supercages of NaY zeolite, Auerbach et al. have assumed that intracage and intercage jumps were uncorrelated, whereas one finds by QENS that they occur on the same time scale. On the other hand for n-butane–methane mixtures in silicalite-1, the convolution of fast jumps within a channel segment by slower jumps between channels is in agreement with previous MD simulations.

Structure of Pt-Co/Al2O3 and Pt-Co/NaY bimetallic catalysts: characterization by in situ EXAFS, TPR, XPS and by activity in Co (carbon monoxide) hydrogenation

Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al2O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co2+ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al2O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.

Formation and Growth of a Nanosized RuIr Bimetallic Cluster Supported on NaY Zeolite

A RuIr catalyst has been prepared by exchanging Ru and Ir metal ions simultaneously into NaY zeolite and then followed by thermal decomposition under a vacuum at 673 K and reduction with hydrogen at 623 K. The results obtained from the Xe adsorption measurement, hydrogen chemisorption, and X-ray absorption fine structure suggest that the RuIr bimetallic clusters are encapsulated in the supercage of NaY zeolite, consisting of 30−60 atoms/cluster. It has been found out that Ru and Ir atoms are homogeneously mixed in the bimetallic cluster. The nanosized RuIr bimetallic clusters in the NaY zeolite show strong resistance for the agglomeration in 1-atm oxygen and at high temperatures, compared to the Ru/NaY.

Effect of multivalent cations on agglomeration of Ru clusters supported on Y zeolite

The effect of multivalent cations, Ca2+, Ba2+, Y3+, Ce3+ and La3+ on the agglomeration of Ru clusters on NaY zeolite under oxygen and at high temperature has been investigated with 129Xe NMR spectroscopy and X-ray absorption fine structure. Multivalent metal cations exchanged into the zeolite suppress neither the migration nor agglomeration of small Ru clusters inside the supercage of NaY zeolite into large metal particles onto the external surface of NaY zeolite under 1 atm oxygen above 520 K. However, the resultant Ru particle size depends on the type of metal cations exchanged into NaY zeolite.

Structure and catalytic performance of Mo dimer oxy-carbide species in NaY supercages

Mo dimer oxy-carbide species were synthesized in the supercage of NaY zeolite by adsorption of Mo(CO) 6 vapor at saturation, followed by thermal decomposition in vacuum. TPD, EXAFS, XPS and XRF data revealed that a Mo dimer was attached in a supercage by a saturation adsorption–thermal decomposition at 573 K and that up to four Mo dimers in a supercage were stepwisely formed without aggregation after four cycles. The Mo dimer oxy-carbide species exhibited a unique methanol decomposition reaction to produce CH 4, CO 2, and H 2 in an approximate ratio of 1 : 1 : 2. This reaction was characteristic of the Mo dimer oxy-carbide species and was never observed for other Mo species such as oxidized and reduced Mo species in NaY supercages.

Faujasite-Supported Ir4Clusters: A Density Functional Model Study of Metal−Zeolite Interactions

The interaction of a metal cluster, Ir4, and a zeolite support was investigated computationally with the aid of a density functional method and a cluster model of a zeolite, i.e., a six-ring consisting of six O atoms and six T (Si or Al) atoms facing a supercage of a faujasite framework. Structural parameters are reported for an Ir4 tetrahedron interacting with the zeolite six-ring. The calculations indicate two Ir−O distances, which match those reported on the basis of EXAFS spectroscopy at about 2.1−2.2 and 2.5−2.7 A for various transition and noble metal clusters on zeolite (and metal oxide) supports, including Ir4 in the supercages of zeolite NaY. The calculations indicate an Ir−Ir distance of about 2.5 A, only a few hundredths of an Angstrom more than the value calculated for the free Ir4 cluster, but about 0.2 A less than the values observed repeatedly by EXAFS spectroscopy for zeolite-supported clusters approximated as Ir4. The experimental distances characterizing the zeolite-supported clusters ar...

Stepwise Synthesis and Structure Analysis of Mo Dimers in NaY Zeolite

Mo dimers were prepared in the supercage of NaY zeolite by cycles of saturated adsorption of Mo(CO)6 at room temperature and subsequent thermal decomposition at 573 K. The obtained samples were characterized by temperature-programmed desorption, extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectra, and X-ray fluorescence. Two Mo(CO)6 molecules per supercage were immobilized in each process. After the thermal decomposition of the adsorbed Mo(CO)6 in the supercage, the resultant Mo(1)/NaY could accept two more Mo(CO)6 molecules per supercage. Mo loading was found to increase linearly by two Mo(CO)6 molecules in a supercage per one deposition cycle, and was saturated with eight Mo atoms per supercage after the four cycles. EXAFS analysis revealed that Mo(II) oxocarbide dimer species [Mo2(C)(O)x] with the Mo−Mo distance of 0.285 nm was formed in the supercage by the 573 K thermal decomposition of the adsorbed Mo(CO)6. The structures of Mo oxocarbide dimers were invariant regardless of...

XAFS Study on the Structure of Mo Species in NaY Zeolite Derived from Mo(CO)6

We carried out XAFS characterization on Mo species in the supercage which was prepared in the supercage of NaY zeolite by cycles of saturation adsorption of Mo(CO)6 at room temperature and subsequent thermal decomposition at 573 K. XAFS analysis revealed that Mo dimer with the Mo-Mo distance of 0.285 nm was formed in the supercage, the structure of which was unchanged during the saturation adsorption/thermal decomposition cycles.

Zeolites as Templates for Preparation of Large-Ring Compounds: Intramolecular Photocycloaddition of Diaryl Compounds

The fluorescence spectra and photocycloadditions of poly(ethylene glycol) labeled at the chain terminal with 2-naphthyl groups (N−Pn−N) or one 9-anthryl and one 1-naphthyl groups (N−Pn−A) and polymethylene end-labeled with two 2-naphthyl groups (N−Mn−N) or two 9-anthryl groups (A−Mn−A) included in NaY zeolite have been investigated. At loading levels less than 1 guest molecule per 10 supercages of zeolite, N−Pn−N, N−Mn−N, and A−Mn−A exclusively exhibit intramolecular excimer emission. Selective excitation of the naphthyl moiety in N−Pn−A mainly results in emission from the anthryl chromophore and intramolecular exciplex fluorescence. Irradiation of the above compounds leads to formation of intramolecular photocyclomers to the exclusion of intermolecular products. These results are explained in terms of the compartmentalization of the guest molecules in the supercages of NaY zeolite. Thus, this work demonstrates the utility of micropores of zeolites to promote the formation of large-ring compounds in synthetically useful quantities. Of particular interest is the enhanced formation of the cross-photocyclomers of anthracene and naphthalene derivatives (a process not observed in homogeneous solution) in high yields.

Synthesis and characterization of vanadium (IV) complexes in NaY zeolite supercages

Encapsulated vanadium complexes with various pyridine-2-carboxylic acids and Schiff bases were synthesized from a dehydrated form of VO(2+)-NaY zeolite by a flexible ligand method and a template synthesis method. It was suggested by X-ray powder diffraction and X-ray photoelectron spectroscopy that the complexes were located in the zeolite supercages. Detailed characterization of the vanadium species was performed by XAFS, FT-IR, ESR and UV-vis spectroscopies in comparison with the corresponding free complexes and reference samples prepared by an impregnation method. Difference in spectroscopic features of the encapsulated and impregnated VO(pic) 2 samples was explained in terms of coordination of zeolite -OH groups to the vanadium complexes. The encapsulated vanadium complexes were active for catalytic selective oxidation of hydrocarbons with H 2O 2 and t-BuOOH.

Polymerization of ethylene over metallocenes confined inside the supercage of a NaY zeolite

AbstractMetallocene catalysts entrapped inside the supercages of NaY zeolite were prepared by reacting NaY with methylaluminoxane (MAO) or trimethylaluminium (TMA) and then with Cp2ZrCl2 (Cp: cyclopentadienyl) or Cp2TiCl2. NaY/MAO/Cp2ZrCl2 and NaY/MAO/Cp2TiCl2 catalysts could polymerize ethylene. The amount of additional MAO for the polymerization was lowered to a mole ratio of Al/Zr of 186. Molecular weights and melting points of polyethylene polymerized with NaY‐supported catalysts were higher than those of polyethylene obtained with homogeneous metallocene catalysts. It could be confirmed by extraction experiments that the metallocene catalyst was confined securely inside the supercage of the NaY zeolite.

Ru Clusters with Controlled Sizes in NaY Zeolite: Catalytic Activity for Ethane Hydrogenolysis and XANES

Ruthenium clusters trapped in the supercage of NaY zeolite were prepared by the reduction of Ru under flowing H2 at 623 K after a ruthenium-red complex was ion-exchanged into the supercage and subsequently decomposed under vacuum at 673 K. The cluster size was controlled by the heating rate used for the thermal decomposition, and the number of Ru atoms per cluster was estimated using a xenon adsorption technique. Various Ru/NaY samples in which the number of Ru atoms per cluster ranged from 20 to 60 have been used to investigate the effects of a decrease in Ru cluster size on the catalytic reaction rate of ethane hydrogenolysis. The reaction rates per surface Ru atom of a Ru/NaY cluster as compared with the bulk metal surface were approximately two orders of magnitude smaller, and the X-ray absorption near edge structure (XANES) taken at the Ru K edge (1s → 4d transition) showed an energy shift. However, in spite of cluster size variation within the supercage, the reaction rates per surface Ru atom and the XANES did not change significantly. The activation energy for the reaction was similar for all Ru samples. These results indicate no significant electronic effects on catalytic activity as the cluster size decreased.

Surface study of supported metal particles by 129Xe NMR.

The surfaces of platinum clusters in the supercages of NaY zeolite have been studied by ${}^{129}\mathrm{Xe}$ NMR over the temperature range 80--293 K. The sensitivity of this weakly perturbing technique makes it possible to reveal the heterogeneity of the electronic structure of the particle surface. The temperature dependence of the ${}^{129}\mathrm{Xe}$ spin-lattice relaxation indicates that the surface sites with the highest adsorption energy possess metallic character.

Preparation of Iridium Clusters in Zeolite Y via Cation Exchange: EXAFS, Xenon Adsorption, and 129Xe NMR Studies

The conversion of [Ir(NH 3 ) 5 Cl] 2+ and [Ir(NH 3 ) 5 (H 2 O)] 3+ to 1-nm Ir clusters entrapped in the supercages of NaY zeolite has been investigated by using 129 Xe NMR spectroscopy, xenon adsorption, and extended X-ray absorption fine structure. The cation exchange with [Ir(NH 3 ) 5 Cl] 2+ was achieved by slurring NaY in an aqueous solution of [Ir(NH 3 ) 5 Cl]Cl 2 at room temperature. The cation exchange with [Ir(NH 3 ) 5 (H 2 O)] 3+ employed either an aqueous solution of [Ir(NH 3 ) 5 (H 2 O)](ClO 4 ) 3 at room temperature or an aqueous solution of IrCl 3 containing NH 3 at a temperature of 330 K. This paper reports very distinctive pretreatment requirements for the transformation of these two ionic precursors to 1-nm Ir clusters in the zeolite supercages: activation in O 2 flow at 573 K is required in the,case of [Ir(NH 3 ) 5 Cl] 2+ , while outgassing at 473 K is sufficient in the case of [Ir(NH 3 ) 5 (H 2 O)] 3+ , prior to the final reduction with H 2 . This second method, which has not been previously described, provides a convenient route for the preparation of Ir clusters in zeolites without using an oxidative treatment, and avoiding Cl-containing species. Furthermore, results from 129 Xe NMR and xenon adsorption measurements indicate that the cluster size is substantially smaller than that in the case of [IT(NH 3 ) 5 Cl] 2+ .

CO hydrogenation over small Pt, Ir and Pt−Ir bimetallic clusters entrapped in the supercage of NaY zeolite

In CO hydrogenation, small (ca. 1 nm) Pt−Ir bimetallic clusters encapsulated in the supercage of NaY zeolite showed significant non-linear Schulz-Flory distributions, especially high selectivity for C4 hydrocarbons, which is different from those of Pt or Ir clusters entrapped inside the supercage of NaY.

Location and molecular motion of hexamethylbenzene in zeolite NaY

Hexamethylbenzene (HMB) adsorbed into the supercages of zeolite Na-Y is characterized by thermogravimetric analyses, proton NMR, ^(13)C NMR, proton multiple-quantum NMR, and UV, reflectance and emission spectroscopies. Room temperature, proton multiple-quantum NMR results reveal that the intracrystalline distribution of HMB is primarily pairwise at bulk concentrations of 0.5, 1.0, and 2.0 molecules per supercage. Results from emission spectroscopy suggest that the HMB pairs are uniformly dispersed among the intracrystalline supercages of Na-Y.

EXAFS and Catalytic Activity of AgPt Bimetallic Cluster Supported on NaY Zeolite

Platinum clusters with ca. 1 nm in size have been prepared as encaged in supercages of NaY zeolite by the reduction of platinum after ion exchange of Pt(NH3)42+ into NaY. A series of bimetallic AgPt/NaY samples with Ag/Pt atomic ratios of 0, 0.11, 0.43 and 1.0 have been obtained by the reduction of silver after subsequent ion exchange of Ag+ into the Pt/NaY sample. The result from measurements for 129Xe NMR spectrum, and hydrogen chemisorption has indicated that the Ag atoms were put together onto the Pt cluster, with the surface enriched by Ag atoms. The extended X-ray absorption fine structure (EXAFS) of the bimetallic cluster measured above Pt LIII and Ag K edges have been analyzed with reference EXAFS spectra which were experimentally obtained from the metal foils and those for AgPt pair which were theoretically prepared by FEFF codes distributed by Rehr et al. The catalytic activity in ethane hydrogenolysis over the AgPt bimetallic cluster has also been measured.

Characterization of zeolite-supported Pt-Cu bimetallic catalyst by xenon-129 NMR and EXAFS

Samples of platinum supported on NaY zeolite (Pt/NaY), Cu/NaY, and bimetallic PtCu/NaY have been prepared by using a scheme that consists of ion exchange of the metalammine cations, activation in flowing O 2, and subsequent reduction in H 2. These samples have been characterized by 129Xe NMR and extended X-ray absorption fine structure (EXAFS). The chemical shift in 129Xe NMR of the xenon adsorbed on PtCu/NaY has been found to be significantly lower than that on Pt/NaY with the same Pt content under the same experimental conditions, and therefore bimetallic PtCu clusters are formed in supercages of NaY zeolite in both the cases of a sequential clustering method and a simultaneous ion exchange preparation method. The peak intensities at Pt-Pt coordination distance in the Fourier transforms of EXAFS spectra for Pt/NaY and PtCu/NaY are very similar, which indicates that the reduction of Cu precursor has resulted in no appreciable changes in the PtPt coordination structure of the Pt cluster. It thus appears, from these results, that during the reduction of Cu precursor the bimetallic PtCu clusters are formed with a model of a Cu covering on Pt clusters in the supercage of NaY zeolite.