Alkali cations, properties

High crystallization rates and the possibility to stabilize X-ray amorphous phases, which exhibit ZSM-5 like properties, were among the reasons why we decided to investigate the procedure B in more detail. In order to optimize the particle size, homogeneity, morphology and composition, we have questioned more systematically the influence of secondary synthesis variables such as the pH, solvent viscosity or the nature of the alkali cation, added as chloride. 

Six ZSM-5 samples were synthesized from gels containing the whole series of alkali cations, added in form of chloride, as described in procedure B. Synthesis data, principal properties and analyses are summarized in Tables VI and VII. 

O. S. Wolfbeis and H. Offenbacher, The effects of alkali cation complexation on the fluorescence properties of crown-ethers, Monat. Chem. 775,647-654(1984). 

Cince the catalytic activity of synthetic zeolites was first revealed (1, 2), catalytic properties of zeolites have received increasing attention. The role of zeolites as catalysts, together with their catalytic polyfunctionality, results from specific properties of the individual catalytic reaction and of the individual zeolite. These circumstances as well as the different experimental conditions under which they have been studied make it difficult to generalize on the experimental data from zeolite catalysis. As new data have accumulated, new theories about the nature of the catalytic activity of zeolites have evolved (8-9). The most common theories correlate zeolite catalytic activity with their proton-donating and electron-deficient functions. As proton-donating sites or Bronsted acid sites one considers hydroxyl groups of decationized zeolites these are formed by direct substitution of part of the cations for protons on decomposition of NH4+ cations or as a result of hydrolysis after substitution of alkali cations for rare earth cations. As electron-deficient sites or Lewis acid sites one considers usually three-coordinated aluminum atoms, formed as a result of dehydroxylation of H-zeolites by calcination (8,10-13). 

Although some scattered examples of binding of alkali cations (AC) were known (see [2.13,2.14]) and earlier observations had suggested that polyethers interact with them [2.15], the coordination chemistry of alkali cations developed only in the last 30 years with the discovery of several types of more or less powerful and selective cyclic or acyclic ligands. Three main classes may be distinguished 1) natural macrocycles displaying antibiotic properties such as valinomycin or the enniatins [1.21-1.23] 2) synthetic macrocyclic polyethers, the crown ethers, and their numerous derivatives [1.24,1.25, 2.16, A.l, A.13, A.21], followed by the spherands [2.9, 2.10] 3) synthetic macropolycyclic ligands, the cryptands [1.26, 1.27, 2.17, A.l, A.13], followed by other types such as the cryptospherands [2.9, 2.10]. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

It is worthwhile mentioning that there are some solvents that combine good solvency power with coordinating properties. The most salient example is 1,2-dimethoxyethane (DME), which can form chelate complexes with alkali cations. This makes easier one-electron reduction of organic substances by means of alkali metals, with the formation of anion radicals and alkali cations. 

Box 3.1 Selective binding of Alkali Metal Cations Properties of alkali metal cations ... 

ArnaudNeu, F., Asfari, Z., Souley, B., Vicens, J., Binding properties of calix [4 ]-bis-crowns towards alkali cations. New J. 

Due to the presence of an immobilized oligo(ethyleneglycol) fragment in 394 and 395 and a crown ether moiety in 396-398, these SAMs have been evaluated by cyclic voltammetry for their alkali cation-recognition properties with a potential to be utilized as thin-film ion sensors. 

Hasegawa Y, Watanabe K, Kusakabe K, and Morooka S. Influence of alkali cations on permeation properties of Y-type zeohte membranes. J Membr Sci 2002 208(1-2) 415 18. 

Low temperature IR spectroscopy of CO as a test molecule provides means to display the difference of surface properties of Ti-containing adsorbents. Two kinds of TS-1 zeolite samples, which have identical structure and slightly different amount of residual alkali cations, characterized by this method, show great dissimilarity in the amount of acidic OH groups, Lewis acid sites of different strength, or of the reduced Ti atoms. Thus, the method could really be used as a rapid test for the surface properties of metal containing zeolites. 

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