Zeolites最新文献

Samples obtained from thermal dealumination (in the presence and absence of water vapour) of three offretite zeolites with, respectively, 4.4, 3.4, and 2.9 aluminium atoms per unit cell, were investigated by i.r. spectroscopy. X-ray diffraction, and nitrogen adsorption. The percentage of crystallinity was then evaluated by two different methods: the standard XRD technique and a new method based on nitrogen adsorption data analysed through the α_s-method. In this case the volume of the ultramicropores (width less than 0.7 nm) was considered. Differences between the two estimates, as well as the loss of crystallinity during dealumination, which is expressed by the representation of both crystallinity evaluations, C_XRD or C_ADS, versus ν₁ (the frequency of the stretching asymmetric T-O band), were analysed for self-steamed and steamed samples. The results obtained agree within 20%, and often better. For self-steamed samples the crystallinity evaluated from XRD patterns is always smaller than that estimated from adsorption data, whereas for steamed samples the opposite situation is generally observed.

We describe the crystal structure of the high-silica zeolite mutinaite, recently found at Mt. Adamson (Northern Victoria Land, Antarctica). Mutinaite is the natural counterpart of the synthetic zeolite ZSM-5. The new mineral, (Na_2.76K_0.11Mg_0.21Ca_3.78) (Al_11.20Si_84.91) · 6O H₂O H₂O, is orthorhombic, space group Pnma, with a = 20.201(2), b = 19.991(2), and c = 13.469(2) Å. A single-crystal X-ray diffraction experiment was performed at the synchrotron radiation source ESRF (Grenoble). No Si-Al order in the framework has been detected. Large distances between ions in the channels and framework oxygens suggest weak interactions between the framework and extraframework species.

The thermal transformation of kaolin extrudates to metakaolin and the subsequent crystallization of low-silica X extrudates has been investigated by X-ray diffraction, scanning electron microscopy, FTIR spectroscopy, N₂-adsorption, ²⁷Al and ²⁹Si MAS NMR, and surface analysis. Kaolin transforms to metakaolin at 973 K. The macroporous metakaolin extrudates are converted to zeolite X plus a small amount of zeolite A in alkaline solution at 324 K. The ²⁷Al NMR signals of octahedral and pentacoordinated aluminium in metakaolin are removed after 30 min at 324 K, whereas the first XRD features of zeolites X and A are not detected until 48 h. Likewise, ²⁹Si NMR and FTIR spectra show that zeolite is first present at 48 h synthesis time. The amount of zeolite A decreases with increasing synthesis time, but there is an overall increase in product crystallinity and surface area up to 240 h. SEM analyses show that zeolite crystals first form in the interior of extrudates at the edges of macropores; with time the metakaolin matrix is consumed to produce well-crystalline zeolite extrudates consisting of ca. 95% low-silica X and 5% zeolite A.

A new anti-diarrheic drug for humans has been developed based on the physical and chemical properties of the purified natural clinoptilolite NZ. A series of physical, chemical, technological, pharmacological, microbiological, and clinical studies were successfully conducted to meet the requirements of the Cuban Drug Quality Agency. The most important results concerning the properties and biological mechanism of NZ are decribed in this paper.

⁷¹Ga MAS n.m.r. analysis and the catalytic behavior during the transformation of 1-butene and n-butane strongly suggest the incorporation of Ga into the AlPO₄-11 (AEL) framework during the synthesis of GaAPSO-11. A unique clear signal at around +120 ppm is proposed to be associated with the presence of tetrahedral Ga in the AEL framework of the unmodified GaAPSO-11. Migration of structural Ga atoms to the silicon domain of the AEL framework seems to occur as a result of a mild hydrothermal treatment, giving rise to an additional signal at +156 ppm, previously associated with tetrahedral gallium in gallosilicates with the MFI topology. The fact that GaAPO-11 and GaAPSO-11 behaved in a way similar to their counterparts AlPO₄-11 and SAPO-11 during the skeletal isomerization of n-butenes reinforces the idea of an isomorphous substitution of Al(III) by Ga(III) in the AEL framework. The transformation of n-butane was shown to be a valuable test for detecting the presence of small amounts of hydro-dehydrogenating extraframework gallium species (EFGS) in Ga-supported SAPO-11 (Ga/SAPO-11). The fact that the sample of GaAPSO-11 was completely inactive for this transformation leads us to believe that the incorporation of gallium into the tetrahedral positions of the AEL framework was almost complete.

Hydrothermal synthesis of sodalite has been investigated in the system NaOCH₃-2SiO₃-Al₂O₃-NaNO₃-(CH₃OH, C₅H₁₁OH) at temperatures of 423 K and 473 K. For the first time the organic solvents methanol or pentanol together with sodium methylate as base were used besides common inorganic educts as Si/Al source and sodium nitrate salt. The sodalites have been characterized by X-ray powder diffraction, as well as by ²⁹Si, ²³Na MAS NMR, and ²³Na DOR NMR measurements. IR-FT spectroscopy shows the successful enclathration of nitrate anions in the β-cages of sodalite. Reaction products of the solvent/base have been identified by ¹H ¹³C CPMAS NMR and ¹H MAS NMR. Scanning-electron micrographs show crystals larger than 15 μm grown at 473 K in pentanol.

Heulandite molecular sieves with a Si/Al ratio of 3.2 ~ 3.8 have been synthesized hydrothermally in the presence of the alkali cations Li⁺; Li⁺, K⁺; and Na⁺, K⁺ at 190 °C at autogenous pressure. They have been characterized by X-ray powder diffraction and scanning electron micrographs. Heulandites can be formed in the Li⁺ and Na⁺, K⁺ cation systems only with ~10 wt% seed crystals. But heulandite can be synthesized in the presence of Li⁺, K⁺ cations without seed crystals. The formation of heulandite-type zeolites occurs over a narrow crystallization field and depends on the Si/Al ratio, the OH⁻/Si ratio, and on the presence of seed crystals. Scanning electron micrographs show a thin-plate topology for the crystals. Heulandite is thermally unstable for calcination beyond 350 ~ 450 °C.

The catalytic decomposition of sulfuryl chloride was studied over dried sodium forms of zeolites ZSM-5, L, and Y. NaZSM-5 did not catalyze the decomposition, whereas NaKL and NaY did. During the chlorination of aromatic hydrocarbons using NaKL or NaY, the chlorinating agent sulfuryl chloride must compete with molecular chlorine produced by its own decomposition. The rate of reaction with molecular chlorine alone is much faster than that with sulfuryl chloride. In the presence of sulfuryl chloride, however, the rate of chlorination by molecular chlorine is greatly reduced. Both chlorinating agents use Lewis acid sites, suggesting that these sites are predominantly covered by sulfuryl chloride. Sulfuryl chloride is slightly more selective for para-chlorotoluene than is molecular chlorine. NaZSM-5 catalyzes chlorination using either molecular chlorine or sulfuryl chloride alone, but the latter reactant is not simultaneously decomposed by NaZSM-5. The decomposition may require a basic site adjacent to the Lewis acid site, and NaZSM-5 may not possess basic sites of sufficient strength.