Rare earth cations

A typical NaY zeolite contains approximately 13 wt% Na20. To enhance activity and thermal and hydrothermal stability of NaY, the sodium level must be reduced. This is normally done by the ion exchanging of NaY with a medium containing rare earth cations and/ or hydrogen ions. Ammonium sulfate solutions are frequently employed as a source for hydrogen ions. 

Radioisotopes, half-lives 49 Raffinose 158, 181 -184, 203, 204, 331 Rare earths cations 144 Ratanhia phenols 288 Rauwolfia alkaloids 314 Reaction chromatography 56 IT Reagents 144 IT... 

Rather small selectivity differences are observed for homovalent-and heterovalent exchanges involving alkali, alkaline earth, bivalent transition metal ions, aluminium and rare earth cations, as is amply evidenced from the extensive compilation by Bruggenwert and Kamphorst (16). This compilation includes various clay minerals illite, montmorillonite, vermiculite and kaolinlte. 

Additionally, the tetrabutylammonium- and tetraphenylarsoniumperrhenate have been spectroscopically studied. Exactly as seen in the case of the pertechnetates and permanganates, the vs bands are sharp and well defined. The perrhenates of different tetrammine- and hexammine-cations (75, 76) and rare earth cations (121 a) have also been reported. 

Different procedures can be used in practice to activate the zeolite, and the choice of a particular method will depend on the catalytic characteristics desired. If the main objective is to prepare a very active cracking catalyst, then a considerable percentage of the sodium is exchanged by rare earth cations. On the other hand, if the main purpose is to obtain gasoline with a high RON, ultrastable Y zeolites (USY) with very low Na content are prepared. Then a small amount of rare earth cations is exchanged, but a controlled steam deactivation step has to be introduced in the activation procedure to obtain a controlled dealumination of the zeolite. This procedure achieves a high thermal and hydrothermal stability of the zeolite, provided that silicon is inserted in the vacancies left by extraction of A1 from the framework (1). The commercial catalysts so obtained have framework Si/Al ratios in the... 

The exceptions to the rule that rare earth hosts are best for rare earth activators are special cases. For example, some anions such as sulfide yield compounds in combination with non-rare earth cations (e.g. Zn) which show higher luminescent efficiency than with rare earths. Additionally, divalent rare earth activators like Eu " " substitute readily for non-rare earth divalent cations. 

Among chromenes, only the spiropyrans and their heterocyclic derivatives have found a wide practical application as photochromic substances.5 6,7-Chromenediols have been proposed as analytical reagents for the spectrophotometric assay of rare earth cations.279 Chromenes with structure and activity similar to those of hashish constituents have been prepared.280 A number of chromenes, mostly with aryl substituents in positions 2,3, and 4 have been patented as biologically active substances.126,280 290... 

The structure of and possible cation location in these materials is fairly well known (2, 8, 4, )> and their ion-exchange behavior toward a multitude of pairs of ions, mostly including sodium, has been measured and interpreted in terms of basic properties of ions, crystal structures, and pore dimensions. The major part of these studies is with alkali- and alkaline-earth cations, alkylammonium ions, rare-earth cations, and silver and thallium ions (1). In contrast, the ion adsorption of transition metals in faujasite has received little attention. 

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). 

It is unexpected that the catalytic activity and the proton acidity do not depend on the lanthanum content. This result cannot be related to the schemes of hydrolysis of the zeolitic rare earth cations reviewed in Ref. 13. On the other hand, acidity measurements in solution (15) have shown that in the lanthanum zeolites studied in this work the La3+ ions have replaced the NH4+ ions and have not formed a lanthanum compound (13). Finally, the variations in the sodium content of these lanthanum zeolites do not seem to be the dominant factor in contrast to the alkaline earth zeolites (26). 

Infrared spectral studies of rare earth (RE) ion-exchanged faujasites have been reported by Rabo et al. (214), Christner et al. (217), Ward (211, 212), and Bolton (218). Distinct hydroxyl absorption bands are observed at 3740, 3640, and 3522 cm-1 after calcination at temperatures in the range of 340° to 450°C. As previously discussed, the hydroxyl groups at 3740 cm-1 are attributed to silanol groups either located at lattice termination sites or arising from amorphous silica associated with the structure. The hydroxyl groups that form the 3522 cm-1 band are nonacidic to pyridine or piperidine and are thought to be associated with the rare earth cations. 

Pyridine and piperidine interact with the 3640 cm-1 hydroxyls (212), which resulted in their assignment to acidic structural hydroxyl groups similar to those found in the alkaline earth and hydrogen forms. The formation of the structural hydroxyls has been attributed by most investigators (219, 220) to the hydrolysis of the rare earth cations by adsorbed... 

Ward measured the o-xylene isomerization activities of Na, Mg, RE, and H—Y zeolites and found the rare earth form to be intermediate in activity between the magnesium and hydrogen forms as shown in Table IX (212). The sodium form was essentially inactive. He interpreted the activity relationship RE—Y > Mg—Y to result from the formation of two acidic structural hydroxyl groups per trivalent rare earth cation. The formation of acidic structure hydroxyl groups by exchange of sodium ions with protons in the rare earth solution, as proposed by Bolton (218), may also account for the greater activity of the rare earth-exchanged zeolite. 

In addition to Fig. 10 in Fig. 12 some data are given regarding phase relations and stability areas with different rare earth cations. The lowest x-value for trivalent cations is 0.33 for Ca2+ the lowest value is 0.3 [123-127, 169, 187], It should be emphasised that all ass do not include the pure a-Si3N4. The atomic positions for different cations M are listed in Table 5, showing that the Si- and N-positions do not differ significantly from that in pure a. This is... 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

Measurement of luminescence lifetimes of the complexes in H20 and D20 solutions permits an estimate of the number of aqua ligands attached to the rare-earth cations. This was first carried out for complexes of Eu, Dy, Sm, and Tb with [SiWu039]8- and [PWii039]7 and indicated ca. 4-6 H2O for the 1 1 complexes and 0.1-0.5 H2O for the 1 2 complexes (Yusov and Fedoseev, 1988). More recent measurements have been reported for the 1 1 complexes of Eu with the a - and... 

A Derivatives of 1 1 complexes As a result of the chirality of ai-[P2-W17O61]10- (Figure 5) solutions of [ Ce(o i-P2Wi706i)(H20)4 2]14 contain enantiomeric pairs of monomers in equilibrium with the meso dimer. Addition of chiral amino acids to such solutions causes a doubling of the 31P-NMR resonances as a result of diastereomer formation presumably caused by coordination of the amino acid to the rare-earth cation (Sadakane et al., 2001). No splitting was observed when similar experiments were carried out with complexes of the achiral a2 isomer. Formation constants for the two diastereomers of the complexes with L-proline were estimated as 7.3 1.3 and 9.8 1.4 M-1. The corresponding proline complex of achiral [Ce W C i)]7- has a formation constant of 4.5 0.1 M-1 (Sadakane et al., 2002). 

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