Microporous Materials最新文献

The electrical conductivity (σ) of compressed powders of an isostructural series of zeolites with MFI topology¹ was measured using AC impedance spectroscopy. Silicalite-1 and ZSM-5 with different Si/Al ratios containing varying numbers of Na⁺ and/or H⁺ were studied at 100% relative humidity and immersed in liquid water. In the latter case, σ is 1–2 orders of magnitude larger than that in saturated water vapour. The value of σ for silicalite immersed in water was found to be higher than that of water before and after the test. It was assumed that this is due to enhanced water dissociation on the silicalite surface. Both in water and at 100% RH the dependence of σ on cation content follows a curve with a maximum. A decrease in conductivity at a high concentration of charge carriers is ascribed to an increase in the repulsive interaction between ions in the conductive water layer on the zeolite channel walls.

Argon adsorption isotherms were used to study the micropore size distribution of manganese oxide octahedral molecular sieves, OMS-1 and OMS-2. Using the cylindrical pore model, this characterization tool provides accurate pore size information for OMS-1. However, this method is less accurate when measuring the smaller pore OMS-2. The argon adsorption isotherm measurement can be used to complement other standard characterization techniques to assess the quality of certain OMS products.

The first synthetic analogue of the rare natural zeolite brewsterite has been synthesized and named CIT-4. CIT-4 crystallizes with a composition SrO:Al₂O₃:5.8-6 SiO₂:4.5 H₂O and is prepared via hydrothermal alteration of zeolite P1 by strontium-containing solutions and in the presence of seed crystals of brewsterite.

Magadiite varieties isomorphically substituted with aluminum and their aluminosilicic acid derivatives retaining the framework topology of magadiite are recrystallized, after incorporation by ion exchange of four TPA and TBA ions, to MFI- and MEL-type zeolites, respectively. The recrystallization degree and the size and shape of the zeolite crystallites formed can be controlled by the amount of template applied. Partial recrystallization results in products which promise to be advantageously applicable as catalysts because of the small crystallite size of the zeolitic component.

A new approach has been developed to monitor the synthesis of MCM-41 mesoporous materials based on in situ ATR FTIR measurements. The transformation between Q₄ and Q₃ species, which is an essential step in the synthesis and structural development of MCM-41 materials, can be followed using this technique.

The ion exchange behavior of a sodium and a potassium titanosilicate towards Cs⁺ and Sr²⁺ was studied. The materials of interest in this study are titanium and silicon structural analogs of the mineral pharmacosiderite. Pharmacosiderite is a non-aluminosilicate molecular sieve with the framework composition [Fe₄(OH)₄ (AsO₄)₃]⁻.5H₂O. For the titanosilicate analogs, the framework arrangement of silicate tetrahedra and titanium octahedra create three-dimensional structures with water molecules and charge-neutralizing cations located in the face-centers. Distribution coefficient (K_d) measurements showed that the potassium titanosilicate removed at least 97% of the Sr²⁺ from a groundwater simulant that also contained ppm levels of Ca²⁺, Mg²⁺, K⁺, Cs⁺ and Na⁺. Similarly, the sodium phase removed about 98% Cs⁺ from the groundwater solution. These preliminary K_d values provide an indication that these exchangers may act as potential Cs⁺ and Sr²⁺ sorbers for groundwater remediation applications. The sodium and potassium phases were also tested as potential exchangers for Cs⁺ and Sr²⁺ in different nuclear waste simulants. While the sodium phase showed little to no preference for Cs⁺ in highly acidic or basic solutions containing large concentrations of NaNO₃, the potassium phase yielded a Sr²⁺ K_d of around 7100 ml g⁻¹ in 2.5 M NaNO₃/l M NaOH solutions, and a K_d of 3500 ml g⁻¹ for a solution containing 5 M NaNO₃/l M NaOH.

The modification of the synthesis procedure used for the crystallization of SAPO-37 (FAU-type), namely an addition of a source of a divalent element Me like Zn or Co in place of, or in addition to silica, a small decrease in the pH and a decrease in the crystallization temperature, allowed new MeAPO- and MeAPSO-FAU materials to be obtained. Moreover, a higher water content of the gel allowed the tetramethylammonium cation content to be increased and therefore to be closer to the stoichiometric amount. Further, in spite of the presence of tetrapropyl- and tetramethylammonium cations which are specific templates for the FAU-type structure, the synthesis conditions could also be adapted to obtain, but with more difficulty, the new MeAPO- and MeAPSO-AFR-type materials. It should be noted that in the absence of Me elements the AlPO₄-AFR-type phase formed very easily which is not the case for the AlPO₄-FAU-type phase.

The kinetics of adsorption and desorption of benzene and o-xylene in H-MCM-22-, H-ZSM-5- and H-beta-zeolites have been studied by means of in situ FTIR. The adsorbents were passed over the activated zeolite wafer, while the spectrum was continuously scanned for about 13 min. During this period adsorption equilibrium was attained for benzene on all three zeolites and for o-xylene on H-beta, whereas only about one-fourth of the expected capacity, or 0.23 mmol g⁻¹ of o-xylene had been adsorbed on H-MCM-22 within 3 min, increasing negligibly in the following 10 min. After 14 h, the amount of o-xylene adsorbed on H-MCM-22 had increased to 0.39 mmol g⁻¹. The adsorption of o-xylene on MCM-22 could thus be resolved into a rapid and a slow process, with the rapid process having a diffusion coefficient typical of small molecules in wide pore zeolites, and the slow process having a diffusion coefficient in the same range as for o-xylene in MFI-zeolites. It was shown that the rapid process represented interaction with hydroxyls of the external surface, and it was thus quantitatively important only because the MCM-22 sample consisted of very small crystals. Since small crystals are typical of MCM-22, the results present an explanation of literature reports indicating the existence of a 12-ring pore system. It also explains why MCM-22 cannot be used for the separation of xylene isomers like the 10-ring MFI-zeolites. Desorption experiments showed that the o-xylene adsorbed on the external silanols was also the first to be desorbed at 298 K, whereas o-xylene adsorbed on internal bridged hydroxyls was quantitatively desorbed in the range 350–460 K.

The synthesis and structure of titanosilicate UND-1 with a chemical composition of Na_2.7K_5.3Ti₄Si₁₂O₃₆ 4H₂O are reported. The UND-1 was synthesized in a system using solid titanium oxides as a Ti source. The structure consists of six-membered rings of SiO₄ tetrahedra and isolated TiO₆ octahedra by which, through corner sharing, form an open structure containing channels (ca. 4 Å in diameter) running along the [100] direction.

The shrinkage of the opaque portion of zeolite reaction mixtures during hydrothermal treatment was studied. Experimental results for unstirred, batch zeolite A reaction systems showed that the non-(solid gel) fraction changed during the course of the reaction. The results also showed that the sudden shrinkage of the opaque portion during the reaction did not correlate with the sudden appearance of crystals in the mixture, but was due to crystal growth. Experiments also were performed in which nutrient materials, both with and without a nucleation control agent (triethanolamine), were added to reacting mixtures at various times during reaction. Nutrient additions without nucleation control resulted in only a small increase in size of the zeolite A product. The largest crystal sizes were obtained, however, when the nucleation control agent was present in the original mixture from the beginning of the reaction.