Aluminium cation properties

It was shown in [18] that practically monophase fine barium hexaaluminate can be obtained by mechanical activation of a mixture of barium oxide with Y-AI2O3, which exhibits acid properties to a larger extent than a-Al203, and by consequent thermal treatments at increased temperature. The product then is grinded in the presence of water. The synthesis was shown to proceed almost completely after activation for 5 min in the AGO-2 planetary mill and thermal treatment at 1300°C for 1 h. Mechanical activation of the mixture of aluminium hydroxide with barium oxide, followed by thermal treatment at 900°C, results in the formation of the final product and a-Al203 as an admixture which remains even at 1300°C. Mechanochemical synthesis helped also to synthesize barinm hexaaluminate in which a part of aluminium cations is replaced with manganese, iron, cobalt cations. Such compounds are nsed as active ceramics in catalysis [17]. 

Ringenbach et al. [81] investigated the adsorption mechanism of polycar-boxylic acid on alumina. The complex formation between polyanion and dissolved aluminium cations appears to play an important role in determining the properties of the adsorbed layer. This may explain the decrease in stability of polyelectrolyte suspensions with time. 

In order to overcome these stability problems a wide-range of alternative dopants have been studied in an effort to reproduce the transport properties of LISICON whilst eliminating the problems of aging. The introduction of trivalent cations into the basic lithium germanate structure can cause an adjustment in the lithium concentration, but in this case it is possible to introduce additional, interstitial, lithium cations or vacancies on the lithium position. Which of these types of doping occurs depends on the nature of the substitution that occurs. This can be most clearly illustrated by looking at the introduction of aluminium cations. These can enter the structure in place of and so introduce an interstitial lithium cation ... 

Cations with noble gas configurations. The alkali metals, alkaline earths and aluminium belong to this group which exhibit Class A acceptor properties. Electrostatic forces predominate in complex formation, so interactions... 

Even when modifiers are not necessary for cement formation, they can lead to improved cement properties. Kingery (1950b) also examined this effect. He found that optimum bonding was achieved with cations that had small ionic radii and were amphoteric or weakly basic, such as beryllium, aluminium, magnesium and iron. By contrast, cations that were highly basic and had large ionic radii, for example calcium, thorium and barium, had a detrimental effect on bonding. 

All commercial examples of phosphoric add solutions used in these cements contain metal ions, whose role has been discussed in Section 6.1.2. In the case of the dental silicate cement, aluminium and zinc are the metals added to liquids of normal commerdal cements and have a significant effect on cement properties (Table 6.8) (Wilson, Kent Batchelor, 1968 Kent, Lewis Wilson, 1971a,b). Aluminium accelerates setting for it forms phosphate complexes and is the prindpal cation of the phosphatic matrix. Zinc retards setting for it serves to neutralize the addic liquid - it... 

The solvation property of the cations of this very polar aprotic solvent can make some salts more stable. Therefore, aluminium, sodium, mercury or silver perchlorate solutions are explosive. The same goes for iron (III) nitrate solutions. 

The strong interaction of polyvalent cations with polyions is well known to strongly alter the rheological properties of hydrolyzed polyacrylamide used in the tertiairy oil recovery process (1-4). The influence of divalent cations have already been studied(5-7) but the role played by the presence of small quantities of aluminium ions has never been investigated. 

There is no necessary relation between the electrical properties of the polymer cation, its anion, and the corresponding ion-pair, and those of the ions present in the solution before the isobutene is added. In fact, since the planar tertiary carbonium ion at the growing end of the polymer chain is much smaller than any cation (except the improbable A1C12+) derivable from aluminium chloride, the dissociation constant of the carbonium ion - anion pair, whatever the anion, must be much smaller than that of the ion-pairs existing in the catalytic solutions before the addition of the monomer. 

The most intriguing difference between the chemical properties of cyclopolysilanes and those of cycloalkanes is the ability of the former to form either anion or cation radicals upon one-electron reduction or oxidation, respectively. For example, the cyclic pentamer (Mc2Si)5 is reduced to the corresponding radical anion by sodium-potassium alloy in diethyl ether [see eqn (4.1) in Section 4.1.3], whereas the hexamer (Me2Si)6 is oxidised by aluminium trichloride in dichlor-omethane to the corresponding cation radical. In both cases the EPR spectra of the radical ions can be interpreted in terms of a-electron delocalisation over the entire polysilane ring (see Section 10.1.4.1). In this respect, the cyclosilanes resemble aromatic hydrocarbons rather than their aliphatic analogues. 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

Because several spatial stacking arrangements are possible there are several kaolin minerals, each with the same chemical composition, namely Al2Si205(0H)4, but with different properties. Nacrite, dickite, kaolinite, halloysite, and livesite are well recognized species. No positive evidence has so far been published linking other trivalent cations with a single layer lattice structure, but it has been suggested that iron(iii) can replace aluminium in part in the kaolin lattice. 

Beryllium (Be, at. mass 9.012) forms cations Be ". In its chemical properties, beryllium resembles magnesium and aluminium. Beryllium hydroxide is precipitated at pH 6, and dissolves in alkali hydroxides. Freshly precipitated Be(OH)2 dissolves in NaaCOs solution to form a rather unstable carbonate complex. Beryllium also forms weak complexes with citrate, tartrate, and fluoride anions. Beryllium and its compounds are highly toxic. 

Ziegler catalysts studied for the oligomerisation of a-olefins tend to be based on modified first-generation catalysts, i.e. triethylaluminium/titanium tetrachloride. The aluminium/fitanium ratio has a marked effect on product properties, at less than 0.8 1, liquids were produced, whereas at ratios above 1 1, waxy products were obtained [6, 7]. This effect is believed to relate to changes in the catalysis mechanism, from cationic to anionic with higher proportions of aluminium. 

Superacids such as Magic acid (42), a system containing antimony pentafluoride and fluorosulfonic acid and designated a superacid because it is a more ready electron pair acceptor than anhydrous aluminium chloride (43), have useful catalytic properties in synthetic organic chemistry where the reaction involves a carbocation intermediate. The extremely low nucleophilicity of the counter ion of such acids make it possible to prepare cations such as the tertiary butyl (( 3)30" " with an appreciably long life time in superacid solutions whereas such cations are too reactive... 

Zeolites are, of course, crystalline by definition and the cation exchange properties of traditional aluminosilicate zeolites arise from the isomorphous positioning of aluminium in tetrahedral coordination within their Si/Al frameworks. This imposes a net negative charge on the framework (Si4t- Al3 ) counterbalanced by cations held within the cavities and channels. This is said to create facile cation exchange lubricated by the water molecules also present in the voids within the framework. This certainly is so for zeolites with open frameworks but in some of the more narrow-pored frameworks, such as natrolite, cation replacement is slow and difficult. 

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