29Si NMR studies of zeolite precursor solutions

Solution 29Si NMR spectroscopy results of zeolite precursor solutions of composition 1 SiO2:4 C2H5OH:0.36/n R+n[OH-]n:20 H2O are reported. This work employs isotopically enriched 29Si materials to aid in spectral interpretation. Using both 1D and 2D methods, spectra of solutions containing tetrapropylammonium hydroxide are wholly consistent with the existing silicate chemistry literature and indicate that the majority of the species are high-symmetry silicate clusters previously observed in aqueous solutions. The results are inconsistent with the nanoblock or nanoslab model proposed by Kirschhock and co-workers. Mixtures containing the 4,4'-trimethylene-bis(1,1'-dimethylpiperidinium) dihydroxide cation were also studied. These mixtures have similar speciation to the TPA solutions, although the relative populations of the species are different. Preliminary variable temperature 29Si NMR of these mixtures shows that the exchange properties of the high-symmetry silicate species, most notably the tetrahedral tetramer, depend on the organocation identity.     

Evaluation of the zinox and zeolite materials as adsorbents to remove H2S from natural gas

Mesoporous materials can be generally used as adsorbents to remove impurities from gases. During the past few years, H₂S has been removed from natural gas using materials such as dolomite, zeolites, and industrial compounds, which can be widely used by oil and natural gas companies. The present study focused on the use of Zeolite 13X and Zinox 380 as H₂S adsorbent at 25 °C. They were characterized by chemical analyses (X-ray fluorescence and atomic absorption), X-ray diffraction, particle size distribution analyses and FT Infrared spectroscopy. Adsorption reactions were carried out using proposed models. The results revealed that both materials studied could be used as H₂S adsorbents in natural gas exceeding the capabilities of industrial compounds.     

Solid-state NMR studies of framework defects in thermally treated zeolite Ca-A

²⁹Si, ²⁷Al and ¹H magic-angle-spinning NMR and relaxation measurements on thermally treated zeolite Na_3.2Ca_4.4-A have been carried out in order to study framework defects responsible for, and surface barriers to, molecular diffusion. Various possible mechanisms for the formation of such defects are considered in detail. We conclude that no framework dealumination takes place and no acidic hydroxyl groups are present in the zeolite. Excess aluminium occluded in the as-prepared zeolite A is responsible for the production of aluminium hydroxide during hydrothermal treatment.     

Photochemical studies with a zeolite Y membrane formed via secondary growth

Zeolite membranes offer an attractive feature in design of artificial photosynthetic assemblies because they can keep the charge separated species apart, yet allow for communication between the two sides via the intervening zeolite channels and cages. However, such a strategy would require that the zeolite membrane be pinhole-free. In an earlier paper (J. Phys. Chem. B 106, 11898 (2002)), we have shown that nanocrystalline zeolite Y can be spin-coated on porous substrates to form membranes, and the pinholes can be plugged by a photoresist. Such membranes do not allow manipulation at higher temperatures or even in solution chemistry because of the thermal instability and solubility characteristics of the photoresist. In this paper, we report on a zeolite Y membrane made by the secondary growth method. The growth process leads to minimal pinholes as determined by leak tests. The sturdiness of the membranes allows for manipulation and we have shown that organic reactions can be carried out in the zeolite pores. Charge transport through the membrane by an electron hopping process mediated via self-exchange of ion-exchanged bipyridinium ions are demonstrated. The electron transfer was initiated using a photochemical Ru(bpy)²⁺₃-EDTA system.     

Structural and sorption characteristics of carbon adsorbents from waste formed in wood processing

The structural and sorption characteristics of microporous sorbents from industrial lignin with respect to some toxic substances, Pb²⁺, Cd²⁺, Cu²⁺, and NH⁺ ₄ ions, and also the possibility of accumulation of methane on these carbons were studied. Simple procedures for modification of the carbon surface to improve the sorption power of the carbons were suggested.     

Diffusion processes in mesoporous adsorbents probed by NMR

In this contribution, we review the results of our experimental studies on diffusion of guest molecules in mesoporous solids using pulsed field gradient (PFG) NMR technique. Having unique potentials to non-invasively probe the microscopic diffusion processes in pores, this method may provide quintessential information on the character of molecular propagation for different pore morphologies and fluid phase state. In particular, different modes of molecular diffusion in partially filled pores may be separately probed and the overall diffusion process could be analyzed taking account of the details of the inter-phase coexistence. In addition to the dynamic properties, some information concerning the distribution of guest molecules within the porous matrix may also be obtained.     

Linking pore diffusivity with macropore structure of zeolite adsorbents. Part II: simulation of pore diffusion and mercury intrusion in stochastically reconstructed zeolite adsorbents

In the present study we complete the evaluation of three dimensional digitized reconstructions of a binderless zeolite adsorbent with improved mass transfer rates, by performing simulations of pore diffusion and Hg-intrusion porosimetry in these structures. It is seen that an excellent agreement with the experimental diffusivity is achieved (relative error of 1.2 %) for a pore structure that matches, besides low order correlations, chord length distribution functions that account for higher order correlations. Furthermore, simulations on a variety of reconstructed samples indicate that matching chord length distribution functions is a necessary (though probably not sufficient) condition for accurate structural representation. The average tortuosity factor is 2.68 and is nearly constant over a broad spectrum of pressures, when properly normalized. Hg-intrusion porosimetry simulations, performed with a pure morphology method, show a good agreement with the experimental curve for normalized cumulative intrusion volumes in the range of 50–88 %, but cannot make a distinction between structures with differences in higher order correlations. It is believed that SEM micrographs, properly obtained to represent realistic 2D sections of the material, contain sufficient structural information that can distinguish among pore structures with different mass transfer rates, when combined with stochastic reconstruction methods. Evidently, the direct link between these structural parameters and pore diffusivity will provide the necessary route to improve the mass transfer rate of porous adsorbents.     

NMR and X-ray studies of the chromonic lyomesophases formed by some xanthone derivatives

Optical microscopy, NMR and X-ray measurements are presented for four chromonic lyomesogens derived from 9-xanthone. The measurements provide details about the mesogen-water binary phase diagrams of the four compounds as well as quantitative information about the ordering and structural parameters of the mesophases. All four systems exhibit peritectic phase diagrams with a nematic (N) phase at low mesogen concentration and a hexagonal (H) phase at high concentration. The results are consistent with previously suggested models for chromonic lyomesophases in which columnar aggregates are formed by stacked mesogenic molecules. In the N phase these columns are parallel to the director but are otherwise randomly distributed in the bulk solvent, while in the H phase they form a two dimensional hexagonal array.     

(Arene)tricarbonylchromium complexes synthesized on NaX zeolite: solid-state NMR and DRIFTS studies

Various (arene)tricarbonylchromium complexes were synthesized within the confines of NaX zeolite and studied with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and carbon-13 magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. In each case, the surface complex Cr(CO)3(Oz)3 (Oz represents a framework oxygen of the NaX zeolite) was prepared before a particular arene was added. The arenes benzene, toluene, mesitylene, anisole, and aniline all produce hexahapto pi-complexes physisorbed within the zeolite supercage. DRIFTS spectra show three bands in the carbonyl region indicating less than C3v symmetry. The NMR spectra have narrow carbonyl bands near 240 ppm which indicate rapidly reorienting complexes within the zeolite. The (eta 6-benzene)tricarbonylchromium complex is physisorbed at two sites as indicated both by the DRIFTS spectra and by two carbonyl resonances at 242.5 and 239.1 ppm at 300 K. Variable-temperature MAS NMR shows these two resonances coalescing near 360 K with an activation energy of 48 +/- 6 kJ/mol. When the temperature is decreased to 205 K, the high-frequency carbonyl resonance disappears. The 239 ppm resonance is still narrow at 134 K while MAS sidebands show that the resonance from physisorbed benzene is ca. 200 ppm wide. The complex prepared with pyridine gave a broad resonance as indicated by the spinning sidebands in the MAS NMR spectra. The pyridine complex was identified as Cr(CO)3(C5H5N)3.     

The role of mesopores in intracrystalline transport in USY zeolite: PFG NMR diffusion study on various length scales

PFG NMR has been applied to study intracrystalline diffusion in USY zeolite as well as in the parent ammonium-ion exchanged zeolite Y used to produce the USY by zeolite steaming. The diffusion studies have been performed for a broad range of molecular displacements and with two different types of probe molecules (n-octane and 1,3,5-triisopropylbenzene) having critical molecular diameters smaller and larger than the openings of the zeolite micropores. Our experimental data unambiguously show that, in contrast to what is usually assumed in the literature, the intracrystalline mesopores do not significantly affect intracrystalline diffusion in USY. This result indicates that the intracrystalline mesopores of USY zeolite do not form a connected network, which would allow diffusion through crystals only via mesopores.     

Noble gas clusters in model zeolite cavities

Noble gas atoms trapped in the intracrystalline cavities of zeolites may form clusters. A classical-mechanical isoenergetic molecular dynamics simulation is performed to simulate the dynamical behavior of noble gas clusters in zeolite cavities. To implement the simulation, a model is adopted of a homogeneous spherical cavity with Morse interaction between the noble gas atoms and cavity walls. The results for Ar₆ clusters indicate that the noble gas clusters in the cavity undergo the same solid/liquid phase changes as in free space, and, at high enough energies, a rapid exchange between atoms adsorbed on the inner surface and thosein the interior of the cavity. Mathematical quenching is used to investigated the multidimensional potential surface of Ar clusters in the cavity.     

UV-Raman and NMR spectroscopic studies on the crystallization of zeolite A and a new synthetic route

UV-Raman and NMR spectroscopy, combined with other techniques, have been used to characterize crystallization of zeolite A. In situ UV-Raman spectroscopy shows that the starting gel for crystallization of zeolite A contains a lot of four-ring (4R) building units and the appearance of six-ring (6R) building blocks is the signal for crystal formation. (29)Si NMR spectroscopy results suggest that the starting gel is double four-ring (D4R) rich and during crystallization of zeolite A both α and β cages appear. (27)Al NMR spectroscopy results indicate the absence of Al (2Si) species in the starting gel, suggesting the absence of single 4R building units in the starting gel. Furthermore, composition analysis of both solid and liquid samples shows that the solid rather than liquid phase predominates for the crystallization of zeolite A. Therefore, it is proposed that the crystallization of zeolite A mainly occurs in the solid phase by self-assembly or rearrangement starting from the zeolite building units mainly consisting of D4R. The essential role of D4R is directly confirmed by successful conversion from a solution of D4R to zeolite A in the presence of NaCl, and the importance of solid phase is reasonably demonstrated by the successful synthesis of zeolite A from a dry aluminosilicate gel. By considering that the solid phase has a major contribution to crystallization, a novel route was designed to synthesizing zeolite A from the raw materials water glass (Na(2)SiO(3) in aqueous solution) and NaAlO(2), without additional water and NaOH; this route not only simplifies synthetic procedures, but reduces water consumption.     

Electron microscopic studies on fibrils formed from retrograded synthetic amyloses

Summary Enzymatically synthesized amyloses with DP between 100 and 7200 were retrograded from aqueous solution. Electron micrographs revealed long fibrils and fibrillar aggregates after a short ultrasonic treatment of the precipitated substances. No degradation of amyloses occurred by short sonication (1–5 minutes 1 MHz). The presence ofB-amylose was confirmed by x-ray diffraction before and after sonication.A similar morphology was observed under various experimental conditions, i.e. different amylose concentration (1–10 g/1), retrogradation rate (a few minutes to 5 weeks) and aging time of the sonicated suspensions.The length of the fibrils having a preferred diameter of about 100 Å was largely independent of the molecular weight of the monodisperse amyloses. Rods of single molecules which were found for iodine complexed amyloses have not been identified. Occasional fluctuations in fibril width may result from twisting of small linearly arranged crystallites.

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Zusammenfassung Aus wäßriger Lösung retrogradierte, enzymatisch synthetisierte Amylosen mit Polymerisationsgraden zwischen 100 und 7200 konnten nach Ultraschallbehandlung im Elektronenmikroskop als Fibrillen und fibrilläre Aggregate abgebildet werden. Es wurde nachgewiesen, daß durch kurze Beschallung (1–5 Minuten 1 MHz) kein Abbau der Amylosen eintritt. Röntgenuntersuchungen vor und nach Ultraschallbehandlung ergaben typischeB-Amylose Diagramme.Eine Variation der experimentellen Bedingungen, verschiedene Amylosekonzentration (1–10 g/1), Retrogradationsgeschwindigkeit (einige Minuten bis zu 5 Wochen) und Alterungszeit der beschallten Proben, hatte auf die Morphologie kaum einen Einfluß.Die Länge der Fibrillen, deren Durchmesser bevorzugt bei etwa 100 Å lag, war weitgehend unabhängig vom Molekulargewicht der monodispersen Amylosen. Stäbchen von Einzelmolekülen, die im Fall der Amylose-Jodkomplexe gefunden wurden, waren nicht zu identifizieren. Gelegentlich auftretende Schwankungen im Fibrillendurchmesser könnten auf eine Verdrillung von linear aggregierten kleinen Kristalliten hinweisen.

     

A simple and efficient synthesis and dynamic NMR studies of some new podands of dithiocarbamates formed from bis(naphthyl) derivatives

The reaction of dithiocarbamate salts ( IV _{a − c} ) with bis(naphthalene chloroacetates) ( II _a,b ) and bis(naphthalene ethoxybromide) ( II _c ) in dimethylformamide (DMF) furnished corresponding podands as V _{a − i} in high to excellent yields. Three reacting ligands, ( II _a,b ) and ( II _c ), were obtained in the reaction of bis(naphthalene) ( I _a,b ) with chloroacetylchloride and 1,2‐dibromoethane. Dynamic NMR spectroscopic data of three series of podands ( V _{a − c} , V _d−f , and V _{g − i} ) are discussed, and their free energy of activation (Δ G_C^≠) at coalescence temperatures are figured out. The Δ G_C^≠ s of these podands were attributed to conformational isomerization in the range of 14.5–18.3 kcal mol⁻¹ due to rotation and resonance effects about thioamide C N bond. © 2011 Wiley Periodicals, Inc. Heteroatom Chem 22:659–668, 2011; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.20730     

Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeolite membranes formed on porous support tubes

Faujasite-type zeolite membranes were reproducibly synthesized by hydrothermal reaction on the outer surface of a porous α-alumina support tube of 30 or 200 mm in length. The membrane properties were evaluated by CO₂ separation from an equimolar mixture of CO₂ and N₂ at a permeation temperature of 40°C. CO₂ permeance and CO₂/N₂ selectivity of the NaY-type membranes were in the ranges of 0.4×10⁻⁶–2.5×10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ and 20–50, respectively. The NaY-type membranes were ion-exchanged with alkali and alkaline earth cations. The LiY-type membrane showed the highest N₂ permeance and the lowest CO₂/N₂ selectivity. The KY-type membrane gave the highest CO₂/N₂ selectivity. The NaY-type membrane was stable against exposure to air at 400°C. NaX-type zeolite membranes, formed by decreasing the ratio of SiO₂/Al₂O₃ in the starting solution, exhibited lower CO₂ permeances and higher CO₂/N₂ selectivities than those of the NaY-type zeolite membranes.     

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by ¹H and ¹³C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.