Immobilization of magnesium ammonium phosphate crystals within microchannels for efficient ammonia removal.

Magnesium ammonium phosphate was formed in flow-through microchannels of silica monoliths using two different methods to fabricate materials that show efficient ammonia adsorption performance from wastewater with low hydraulic resistance. Magnesium ammonium phosphate crystals in these materials release ammonia when heated at 378 K, yielding primarily magnesium hydrogen phosphate. When this material was used for ammonia removal from an aqueous solution containing 100 ppm ammonia in a flow system, the material readily removed ammonia, decreasing the ammonia concentration to 25 ppm. The material can be reactivated by the same procedure and used again for ammonia removal. Hydrodynamic resistance through the lengths of the materials depend on the shape of the immobilized crystals, showing that needle-like crystals are more effective to cause less resistance than plate-like particles. The material containing needle-like crystals causes only approximately one-eighth of the hydraulic resistance that a packed column consisted of spherical particles with a typical bed porosity of 0.5 does. Thus, these results demonstrate the high applicability of the material for ammonia removal from wastewater in a continuous process.

Delamination of layered zeolite precursors under mild conditions: synthesis of UCB-1 via fluoride/chloride anion-promoted exfoliation.

New material UCB-1 is synthesized via the delamination of zeolite precursor MCM-22 (P) at pH 9 using an aqueous solution of cetyltrimethylammonium bromide, tetrabutylammonium fluoride, and tetrabutylammonium chloride at 353 K. Characterization by powder X-ray diffraction, transmission electron microscopy, and nitrogen physisorption at 77 K indicates the same degree of delamination in UCB-1 as previously reported for delaminated zeolite precursors, which require a pH of greater than 13.5 and sonication in order to achieve exfoliation. UCB-1 consists of a high degree of structural integrity via (29)Si MAS NMR and Fourier transform infrared spectroscopies, and no detectable formation of amorphous silica phase via transmission electron microscopy. Porosimetry measurements demonstrate a lack of hysteresis in the N(2) adsorption/desorption isotherms and macroporosity in UCB-1. The new method is generalizable to a variety of Si:Al ratios and leads to delaminated zeolite precursor materials lacking amorphization.

Size-activity threshold of titanium dioxide-supported Cu cluster in CO oxidation.

Development of non-noble metal cluster catalysts, aiming at concurrently high activity and stability, for emission control systems has been challenging because of sintering and overcoating of clusters on the support. In this work, we reported the role of well-dispersed copper nanoclusters supported on TiO2 in CO oxidation under industrially relevant operating conditions. The catalyst containing 0.15 wt% Cu on TiO2 (0.15 CT) exhibited a high dispersion (59.1%), a large specific surface area (381 m2/gCu), a small particle size (1.77 nm), and abundant active sites (75.8% Cu2O). The CO oxidation activity measured by the turnover frequency (TOF) was found to be enhanced from 0.60 × 10-3 to 3.22 × 10-3 molCO·molCu-1·s-1 as the copper loading decreased from 5 to 0.15 wt%. A CO conversion of approximately 60% was still observed in the supported cluster catalyst with a Cu loading of 5 wt% at 240 °C. No deactivation was observed for catalysts with low copper loading (0.15 and 0.30 CT) after 8 h of time-on-stream, which compares favorably with less stable Au cluster-based catalysts reported in the literature. In contrast, catalysts with high copper loading (0.75 and 5 CT) showed deactivation over time, which was ascribed to the increase in copper particle size due to metal cluster agglomeration. This study elucidated the size-activity threshold of TiO2-supported Cu cluster catalysts. It also demonstrated the potential of the supported Cu cluster catalyst at a typical temperature range of diesel engines at light-load. The supported Cu cluster catalyst could be a promising alternative to noble metal cluster catalysts for emission control systems.

A zeolite-supported molecular ruthenium complex with eta6-C6H6 ligands: chemistry elucidated by using spectroscopy and density functional theory.

An essentially molecular ruthenium-benzene complex anchored at the aluminum sites of dealuminated zeolite Y was formed by treating a zeolite-supported mononuclear ruthenium complex, [Ru(acac)(eta(2)-C(2)H(4))(2)](+) (acac=acetylacetonate, C(5)H(7)O(2)(-)), with (13)C(6)H(6) at 413 K. IR, (13)C NMR, and extended X-ray absorption fine structure (EXAFS) spectra of the sample reveal the replacement of two ethene ligands and one acac ligand in the original complex with one (13)C(6)H(6) ligand and the formation of adsorbed protonated acac (Hacac). The EXAFS results indicate that the supported [Ru(eta(6)-C(6)H(6))](2+) incorporates an oxygen atom of the support to balance the charge, being bonded to the zeolite through three Ru-O bonds. The supported ruthenium-benzene complex is analogous to complexes with polyoxometalate ligands, consistent with the high structural uniformity of the zeolite-supported species, which led to good agreement between the spectra and calculations at the density functional theory level. The calculations show that the interaction of the zeolite with the Hacac formed on treatment of the original complex with (13)C(6)H(6) drives the reaction to form the ruthenium-benzene complex.

Role of the support in catalysis: activation of a mononuclear ruthenium complex for ethene dimerization by chemisorption on dealuminated zeolite Y.

A set of supported ruthenium complexes with systematically varied ratios of chemisorbed to physisorbed species was formed by contacting cis-[Ru(acac)(2)(C(2)H(4))(2)] (I; acac = C(5)H(7)O(2) (-)) with dealuminated zeolite Y. Extended X-ray absorption fine structure (EXAFS) spectra used to characterize the samples confirmed the systematic variation in the loadings of the two supported species and demonstrated that removal of bidentate acac ligands from I accompanied chemisorption to form [Ru(acac)(C(2)H(4))(2)](+) attached through two Ru-O bonds to the Al sites of the zeolite. A high degree of uniformity in the chemisorbed species was demonstrated by sharp bands in the infrared (IR) spectrum characteristic of ruthenium dicarbonyls that formed when CO reacted with the anchored complex. When the ruthenium loading exceeded 1.0 wt % (Ru/Al approximately 1:6), the additional adsorbed species were simply physisorbed. Ethene ligands on the chemisorbed species reacted to form butenes when the temperature was raised to approximately 393 K; acac ligands remained bonded to Ru. In contrast, ethene ligands on the physisorbed complex simply desorbed under the same conditions. The chemisorption activated the ruthenium complex and facilitated dimerization of the ethene, which occurred catalytically. IR and EXAFS spectra of the supported samples indicate that 1) Ru centers in the chemisorbed species are more electron deficient than those in the physisorbed species and 2) Ru-ethene bonds in the chemisorbed species are less symmetric than those in the physisorbed species, which implies the presence of a preferred configuration for the catalytic dimerization.

Molecular chemistry in a zeolite: genesis of a zeolite Y-supported ruthenium complex catalyst.

Dealuminated zeolite Y was used as a crystalline support for a mononuclear ruthenium complex synthesized from cis-Ru(acac)2(C2H4)2. Infrared (IR) and extended X-ray absorption fine structure spectra indicated that the surface species were mononuclear ruthenium complexes, Ru(acac)(C2H4)2(2+), tightly bonded to the surface by two Ru-O bonds at Al(3+) sites of the zeolite. The maximum loading of the anchored ruthenium complexes was one complex per two Al(3+) sites; at higher loadings, some of the cis-Ru(acac)2(C2H4)2 was physisorbed. In the presence of ethylene and H2, the surface-bound species entered into a catalytic cycle for ethylene dimerization and operated stably. IR data showed that at the start of the catalytic reaction, the acac ligand of the Ru(acac)(C2H4)2(2+) species was dissociated and captured by an Al(3+) site. Ethylene dimerization proceeded approximately 600 times faster with a cofeed of ethylene and H2 than without H2. These results provide evidence of the importance of the cooperation of the Al(3+) sites in the zeolite and the H2 in the feed for the genesis of the catalytically active species. The results presented here demonstrate the usefulness of dealuminated zeolite Y as a nearly uniform support that allows precise synthesis of supported catalysts and detailed elucidation of their structures.

Synthesis of Mg-Al Mixed Oxides with Markedly High Surface Areas from Layered Double Hydroxides with Organic Sulfonates.

Mg-Al mixed oxides with record-high surface areas and basic site concentrations were synthesized from Mg-Al layered double hydroxides with interlayer isethionate (Ise) or 3-hydroxy-1-propanesulfonate (HPS). Anion exchange of interlayer CO3 2- in synthetic hydrotalcites with the organic sulfonates induces disorders in layer stacking as characterized by powder X-ray diffraction and enables facile delamination in water. Thermal treatment of materials anion-exchanged by Ise (MgAl-Ise) and HPS (MgAl-HPS) in N2 and H2 resulted in the formation of Mg-Al mixed oxides with marked enhancement in Brunauer-Emmett-Teller (BET) surface area relative to those treated in air. Treatment in a flow of H2 is particularly effective, doubling the surface area of mixed oxides derived from MgAl-Ise relative to those obtained in a flow of N2. A higher degree of disorder in layer stacking in MgAl-HPS than MgAl-Ise resulted in the formation of Mg-Al mixed oxides with higher surface areas than those from MgAl-Ise. As a result, thermal activation of MgAl-HPS in a flow of H2 yielded Mg-Al mixed oxides with the highest BET surface area (410 m2 g-1) and CO2 uptake (1.6 mmol g-1 at 25 °C and 100 kPa) in all samples. These values are significantly higher than those obtained from the initial hydrotalcites as well as those reported in the literature with similar Mg-Al ratios. Investigation of the thermal activation steps by thermogravimetric analysis and mass spectrometry indicates that the key factors to achieve high surface area and CO2 uptake are to weaken interactions between layers by inducing stacking disorders and to facilitate the removal of interlayer sulfonates by preventing the formation of sulfates from them via thermal activation under a reducing environment.

Carbon Paper with a High Surface Area Prepared from Carbon Nanofibers Obtained through the Liquid Pulse Injection Technique.

To improve the performance of carbon paper used for applications such as electrodes for electrochemical devices and air filters, two types of long carbon nanofibers (CNFs) with average diameters of 20 and 49 nm were prepared by the liquid pulse injection (LPI) technique by adjusting reaction conditions. Carbon paper was made from the CNFs through a simple filtration process. The paper prepared from the CNFs with an average diameter of 20 nm (LPI-CNF(20) paper) was firm and flexible even though it was prepared without using any binders. LPI-CNF(20) paper also had a high surface area and showed a high electrical conductivity and a moderate gas permeability according to its void size. These properties are required for cathodes in the latest battery systems such as lithium-air batteries. In electrochemical experiments conducted to evaluate the performance of LPI-CNF(20) paper as a cathode, the paper showed a larger discharge capacity on the basis of the cathode weight than a conventional cathode (a commercially available carbon paper combined with a porous carbon), which indicated that it has a high potential to be used as a cathode in lithium-air batteries.

Development of TiO2-SiO2 Photocatalysts Having a Microhoneycomb Structure by the Ice Templating Method.

Immobilization of TiO2-based photocatalysts usually suffers from lowered surface area and mass transfer limitation compared with their suspended counterpart. In this work, TiO2-SiO2 monolithic photocatalysts having straight macropores, called microhoneycombs, were synthesized. The obtained samples had straight macropores with a diameter in the range of 15-40 µm formed by walls having a thickness up to 5 µm. The samples also contain micropores and small mesopores inside their walls, which contribute to high surface areas of more than 500 m2 g-1. Synthesized photocatalysts were tested in a continuous flow system using the decolorization of methylene blue as a model reaction. It was found that the unique morphology of the samples can be used to promote the uniform distribution of the target fluid while reducing the pressure drop across the photocatalyst to less than a hundredth compared with a packed bed system. In addition, calcination at 600-800 °C improved the strength and photocatalytic activity of the monoliths while preserving the high surface area of the samples.

Structure-directing agent location and non-centrosymmetric structure of fluoride-containing zeolite SSZ-55.

Single crystals of pure silica zeolite SSZ-55 were prepared using the fluoride route. Single-crystal X-ray diffraction at a synchrotron source revealed the framework structure of the material, but the unit cell (orthorhombic a = 12.905(2) A, b = 21.344(4) A, c = 5.1279(10)) is too small to accommodate ordered arrays of the organic structure-directing agent. Molecular modeling was used to simulate the docking of the structure-directing agent in the channels of the material, and this revealed a strong space-filling interaction with a number of possible orientations of the organic cation. The overall non-centrosymmetric structure of the solid (spacegroup C222(1)) was confirmed using second harmonic generation experiments.

Genesis of Delaminated-Zeolite Morphology: 3-D Characterization of Changes by STEM Tomography.

Zeolite delamination increases the external surface area available for catalyzing the conversion of bulky molecules, but a fundamental understanding of the delamination process remains unknown. Here we report morphological changes accompanying delamination on the length scale of individual zeolite clusters determined by 3-D imaging in scanning transmission electron microscopy. The results are tomograms that demonstrate delamination as it proceeds on the nanoscale through two distinct key steps: a chemical treatment that leads to a swelled material and a subsequent calcination that leads to curling and peeling off of delaminated zeolite sheets over hundreds of nanometers. These results characterize the direct, local, 3-D morphological changes accompanying delaminated materials synthesis and, with corroboration by mercury porosimetry, provide unique insight into the morphology of these materials, which is difficult to obtain with any other technique.

Zeolite-supported metal complexes of rhodium and of ruthenium: a general synthesis method influenced by molecular sieving effects.

A general method for synthesis of supported metal complexes having a high degree of uniformity is presented, whereby organometallic precursors incorporating acetylacetonate (C(5)H(7)O(2)(-), acac) ligands react with zeolites incorporating OH groups near Al sites. The method is illustrated by the reactions of Rh(acac)(CO)(2) and of cis-Ru(acac)(2)(eta(2)-C(2)H(4))(2) with zeolites slurried in n-pentane at room temperature. The zeolites were H-Beta, H-SSZ-42, H-Mordenite, and HZSM-5. Infrared (IR) and extended X-ray absorption fine structure spectra of the zeolites incorporating rhodium complexes indicate the formation of Rh(CO)(2)(+) bonded near Al sites; similar results have been reported for the formation of zeolite-supported Rh(eta(2)-C(2)H(4))(2)(+) from Rh(acac)(eta(2)-C(2)H(4))(2). IR spectra of the supported rhodium gem-dicarbonyls include sharp, well-resolved nu(CO) bands, demonstrating that the sites surrounding each metal complex are nearly equivalent. The frequencies of the nu(CO) bands show how the composition of the zeolite influences the bonding of the supported species, demonstrating subtle differences in the roles of the zeolite as ligands. When the zeolite has pore openings larger than the critical diameter of the precursor organometallic compound, the latter undergoes facile transport into the interior of the zeolite, so that a uniform distribution of the supported species results, but when the precursors barely fit through the zeolite apertures, the mass transport resistance is significant and the supported metal complexes are concentrated near the pore mouths.

A bioinspired approach for controlling accessibility in calix[4]arene-bound metal cluster catalysts.

In enzymes, the electronic and steric environments of active centres, and therefore their activity in biological processes, are controlled by the surrounding amino acids. In a similar manner, organic ligands have been used for the 'passivation' of metal clusters, that is, inhibition of their aggregation and control of their environment. However, the ability of enzymes to maintain large degrees of accessibility has remained difficult to mimic in synthetic systems in which little room, if any, is typically left to bind to other species. Here, using calix[4]arene macrocycles bearing phosphines as crude mimics of the rigid backbones of proteins, we demonstrate the synthesis of gold clusters and the control of their accessibility through an interplay between the sizes of the calixarene ligands and metal cores. For 0.9-nm cores, 25% of all the gold atoms within the cluster bind to the chemisorption probe 2-naphthalenethiol. This accessibility dramatically decreases with 1.1-nm and 4-nm gold cores.