Cesium-exchanged zeolite

Cesium-exchanged zeolite X was used as a solid base catalyst in the Knoevenagel condensation of benzaldehyde or benzyl acetone with ethyl cyanoacetate [121]. The latter reaction is a key step in the synthesis of the fragrance molecule, citronitrile (see Fig. 2.37). However, reactivities were substantially lower than those observed with the more strongly basic hydrotalcite (see earlier). Similarly, Na-Y and Na-Beta catalyzed a variety of Michael additions [122] and K-Y and Cs-X were effective catalysts for the methylation of aniline and phenylaceto-nitrile with dimethyl carbonate or methanol, respectively (Fig. 2.37) [123]. These procedures constitute interesting green alternatives to classical alkylations using methyl halides or dimethyl sulfate in the presence of stoichiometric quantities of conventional bases such as caustic soda. 

Table 1. Kinetic rate constants K) for the condensation of benzaldehyde with ethyl cyanoacetate (pXa 9.0), ethyl acetoacetate (pATj 10.7), and ethyl malonate (pK 13.2) on alkaline-exchanged zeolites, NaGeX, cesium-exchanged sepiolite, and calcined Mg-Al hydrotalcite.

NaY from Katallstiks (Si/Al=2.56) has been used for the preparation of ammonium, potassium and cesium exchanged Y-zeolites. Three series of NH4NaY zeolites differing in the degree of exchange hav e been prepared by the multiple exchange of NaY with NH Cl solutions and denoted hereafter as 1, 2, 3. Stabilized forms of Y-zeolitea (USY) were obtained from 1, 2 and 3 by deep-bed hydrothermal treatment at 875 K for... 

The recovery and purification of cesium-137 from Purex acid waste using a synthetic zeolite has been studied. Zeolite capacity and selectivity for cesium were determined. Stability of the synthetic zeolite to high radiation fields and chemical attack was adequately demonstrated. Kilocurie quantities of cesium-137 of 98+% chemical purity were prepared using zeolite ion exchange. 

Fig. 21. Cs MAS NMR spectrum of a dehydrated 72% cesium-exchanged zeolite Y (a), the simulated spectrum (b), the components (c), and spinning sidebands ( ) [187]...

Concepcion-Heydom, R Jia, C. Herein, D. Pfander, N. Karge, H. G. and Jentoft, F. C., Structural and catalytic properties of sodium and cesium exchanged X and Y zeolites, and germanium-substituted X zeolite, Journal of Molecular Catalysis A Chemical 162(1-2), 227-246 (2000). 

Impregnation followed by thermal treatment is commonly used to prepare alkali metal oxide or alkaline earth metal oxide clusters in molecular sieves in order to obtain catalysts with basic properties. A first hint at the usefulness of such procedures was published in 1984 Lacroix et al. reported that cesium-exchanged zeolite X is more active in the side-chain alkylation of toluene with methanol when left unwashed after the cation exchange [1]. Hathaway and Davis [2-4] carried out further systematic studies on the preparation of intrazeoUtic oxide clusters with basic properties. These authors described two methods for the introduction of alkali metal oxides into the pores of faujasites [2j When zeolites X and Y, ion-exchanged at room temperature with aqueous solutions of alkali metal acetates or... 

Another application for adsorption of metal impurities is in the nuclear power industry. Radioactive cesium is one of the compounds that is difficult to remove from radioactive waste. This is because ordinary resins and zeolites do not effectively adsorb radioactive cesium. In 1997, lONSlV lE-911 crystalline silicotitan-ate (CST) ion exchangers were developed and effectively used to clean up radioactive wastes in the Melton Valley tanks at Oak Ridge [268, 269], CST was discovered [270] by researchers at Sandia National Laboratories and Texas A M University, with commercial manufacture carried out by UOP. 

UOP molecular sieves (UOP) has developed the lonsiv family of ion exchange resins for the extraction of radionuclides from wastewater. lonsiv TIE-96 is composed of a titanium-coated zeolite (Ti-zeolite) and is used to separate plutonium, strontium, and cesium from alkaline supernatant and sludge wash solutions. The technology was developed by Pacific Northwest Laboratory (PNL) for use at the West Valley, New York, nuclear waste facility. The technology is commercially available. 

The nature of the exchanged ion in a zeolite determines the amount of equilibrated adsorbed water, decreasing from 16.3% for NaL to 12.5% for CsL. Activation energies of the dehydration process were calculated for these zeolites by the method given in Ref. 25. Their values for sodium, potassium, and cesium forms are 4.52, 2.31, and 1.85 kcal/mole. The results show that the smaller the cation radius is (i.e., the stronger its field), the higher the activation energy is. 

Silver species have been studied in a variety of A-zeolites including Nai2-A, K -A, Li -A, CsyNas-A and Cag-A (11). Complete exchange of cesium ion for sodium ion cannot be achieved in the A-zeolites. Typically the major cation was exchanged by silver to an extent of about 0.7 ion per unit cell which is 6% of the exchangeable cations. After irradiation about 0.003 silver ions per unit cell were converted to atomic silver species. 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

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