Microporous Materials最新文献

Zeolite NaA membranes were prepared from homogeneous solution on macroporous zirconia composite supports in sheet and tube form. Zeolite KA membranes were obtained by ion exchange from the sodium form. The membranes were tested for the removal of water from isopropanol/water mixtures by pervaporation. Feed and permeate compositions and flux were measured for pervaporation at 25 and 70°C. Both the Na and K forms of zeolite A membranes were highly selective, separation factors in excess of 10⁴ being achieved at low concentrations of water in the feed. Membrane performance was not affected by heat treatments to 150°C.

Recent multiquantum magic-angle spinning and double rotation ²³Na-NMR experiments of dehydrated zeolite NaY have shown that sodium cations on site SI in the hexagonal prism are subject to appreciable quadrupolar interactions. This indicates that the SI cations are not located in the classical center position derived from X-ray and neutron diffraction studies but are displaced from the center by 0.6 Å along the [111] direction.

The influence of time and temperature of ageing of the amorphous gel precursor (2.5Na₂O-8TPABr-60SiO₂-800H₂O) on the particulate properties (particle size distribution, particle shape, number of particles) of silicalite-1 synthesized at 170°C were studied by different experimental methods such as X-ray diffractometry (XRD), differential thermogravimetric analysis (TGA), particle size analysis, FT infra-red spectroscopy and scanning-electron microscopy (SEM). In spite of negligible changes in the crystal morphology, ageing of the gel considerably affects the number and size of silicalite-1 crystals in the crystalline end products. The observed effects were discussed in terms of the formation of specific ordered subunits between silicate species and TPA⁺ ions (germ nuclei) and their role in the nucleation and crystal growth of silicalite-1 crystals.

The thermal and hydrothermal stability of framework-substituted MCM-41 with a Si/M molar ratio of 40 (M = Al, Ti) is assessed. Si-MCM-41 with 2.5% substitution of Si by either Al or Ti remains mesoporous when calcined in air at temperatures up to 800°C. The pore structure of Si-MCM-41 and Al-MCM-41 collapses at a calcination temperature of above 800°C, while pore collapse occurs at about 100°C higher for Ti-MCM-41. The addition of La₂O₃ to Al-MCM-41 improves its thermal stability at lower temperatures, but has no effect at higher temperatures. When MCM-41 materials are treated in pure distilled water at 25–100°C for 4 h, Si-MCM-41 suffers a significant loss of BET surface area, while Al-MCM-41 and Ti-MCM-41 are more stable towards such treatment. All the samples investigated are stable in an acidic solution of pH 2, but disintegrate rapidly in a strongly basic medium (pH 12).

Zeolites P1 (GIS) and L (LTL) are used as precursor materials for the preparation of synthetic analogues of the natural alkaline-earth zeolites brewsterite, heulandite, epistilbite, harmotome and yugawaralite. Crystallization of these zeolites is accomplished by hydrothermal treatment of the precursor zeolites with aqueous solutions containing an appropriate alkaline-earth cation. The effects of the composition of the starting zeolite, the composition of the solution phase and the presence or absence of seeds are discussed.

A series of Rh/SiO₂ catalysts were prepared by the sol-gel method. The precursors used were tetraethoxysilane (TEOS) dissolved in ethanol and rhodium(III) acetylacetonate dissolved in acetone. Preparative variables include the amount of water added to the sol and the gelation temperature. Typical BET surface areas were 550–700 m² g⁻¹ with very narrow pore size distributions centered at approximately 4 nm. Sample characterization studies were performed using TGA-DTA-MS to determine the effect of O₂ pretreatment and ²⁹Si solid-state NMR to determine the extent to which silica was condensed. Sintering studies of Rh/SiO₂ catalysts for which there was a reasonable match between metal particle size and pore diameter were performed. Catalysts prepared by ion-exchange having comparable dispersions to those prepared by the sol-gel method also showed considerable resistance to sintering in O₂. Following heat treatment in O₂ at 650°C, the particle size distribution of catalysts prepared by ion-exchange appear to broaden to a greater extent than catalysts prepared by the sol-gel method. This suggests that the sol-gel preparation is more effective in retaining the metal particles within the narrow pores of the support.

The mechanism of formation of ferrierite (FER) membrane on an alumina support by a vapor-phase transport (VPT) method was studied using XRD, SEM, FE-SEM and EDX. The XRD measurement for a FER membrane after the removal of FER particles formed on the support showed that FER was formed in the pores of the alumina support, namely, a FER-alumina composite layer was formed. The FE-SEM image for the cross-sectional view of the FER-alumina composite layer showed that FER nanocrystals of about 50 nm diameter grew in the pores of the alumina support. Pervaporation tests for benzene/p-xylene mixtures were performed at 303 K. Even when the concentration of benzene in the feed solution was extremely small (0.5 mol%), the flux of benzene through the FER membrane was still greater than that of p-xylene. Therefore, a separation factor as high as 600 was obtained in the low feed concentration of benzene. Such a high selectivity for the benzene/p-xylene mixture suggested that the selectivity for the benzene/p-xylene mixture appears at the pore mouths of FER on the feed side of the FER-alumina composite layer.

Four different methods of the hydrothermal synthesis of Sn-sil-1 (MFI structure) (Si/Sn > 15) molecular sieves are described. Three of the methods are in a basic medium (pH = 12.4) and the last is in a fluoride medium (pH = 6.4). The procedure in which SnCl₄.5H₂O is dissolved first in Si(OC₂H₅)₄ (TEOS) before hydrolysis by tetrapropyl ammonium hydroxide (method A) seems to be the most suitable in terms of the crystallinity of the resulting material and its activity in the hydroxylation of phenol by aqueous H₂O₂ (72% efficiency for H₂O₂). The course of hydrolysis of TEOS and SnCl₄ is studied systematically by ²⁹Si liquid NMR spectroscopy. It is inferred that as soon as SnCl₄ is partially hydrolysed, it combines with monomeric Si(OH)₄ (Q⁰) species. This combination leads to the formation of Q¹, Q² and Q³ species and a clear solution. The liquid NMR data provide direct evidence for the interaction of Sn species with silicate species in the synthesis mixture. Further, ²⁹Si MAS NMR and ¹¹⁹Sn static and MAS NMR spectral studies of the crystalline product indicate that the Sn⁴⁺ ions are probably attached to the defect silanol groups in a tetrahedral coordination, but tend to assume five- or six-fold coordination very easily due to their large size.

The overall kinetics of the isomerization of the dialkylbenzenes on zeolites such as ZSM-5 are affected by the intrinsic kinetics and, in many cases, additionally by configurational diffusion inside the microporous zeolite. Simulation studies on the basis of a continuum model allow a deeper insight into this complex situation. It has been shown that both a reactant diffusion limitation and product shape selectivity (different diffusion rates of the products) may influence the overall kinetics (conversion/selectivity). Calculating concentration profiles inside the crystallite gives additional details about the interrelation of diffusion and reaction. Due to the strong interaction between the diffusing molecules themselves, as well as between diffusing molecule and zeolite surface, various diffusion models have been implemented. Depending on the diffusion model used, it could be shown that at high occupancies, differences occurred only in the calculated conversions, but not in the calculated selectivities. On the basis of the simulation results a recommendation is given concerning the determination of diffusion coefficients from measured overall kinetics. Furthermore, it was found that selectivity strongly depends on the chosen type of reactor, e.g. a plug flow reactor or an ideally mixed reactor. This is important for a comparison of data measured in different experimental set-ups.

It is shown that temperature-programmed desorption of adsorbed NO on Co-ZSM-5 zeolite used in selective catalytic reduction can accurately measure the amount of Co²⁺ cation for Co/Al ratios from 0 to 0.42 where the 0.42 limit corresponds to 84% maximum exchange. Evidence is also shown for the formation of cobalt oxide particles in Co-ZSM-5 for Co/Al ratios above 0.42.