Alkaline earth metal borates

A borate crosslinking agent can be boric acid, borax, an alkaline earth metal borate, or an alkali metal alkaline earth metal borate. The borate source, calculated as boric oxide, must be present in an amount of 5% to 30% by weight. 

Figure5.49 Enthalpydiagram ofthe alkali and alkaline earth metal borates. The enthalpy for the desorption of H2 is indicated by the red bar.

Two alkaline-earth metal borates Ba B60g(OH)6 (2) and SrB02(0H) (3)... 

In general, alkaline earth metal borates exhibit framework structures made up of BO4 tetrahedra and BO3 triangles. Only a few compounds show isolated islands, and layered structural feature and chain arrangement are absent. Moreover, most of these compounds contain both BO4 and BO3. No chain structure of barium borate has been discovered yet prior to our work [22]. 

The element boron forms strong bonds with oxygen. The resultant borates include boric acid, boric oxide, alkali, ammonium, and alkaline earth metal borates. All except for the alkaline earth borates are reasonably water soluble. When dissolved borates release boric acid, a Lewis acid, and its conjugate base, the tetrahydroxyborate anion. Borax, a form of di-sodium tetraborate, releases equimolar amounts of these species and consequently is an excellent mildly alkaline pH buffer. Borates exhibit several important behaviors in solution, including the sequestration of water hardness ions, the ability to form borate esters with polyols, and to deliver the oxidant hydrogen peroxide into solution from the dissolution of sodium perborate. 

The interaction between sodium and alkaline-earth metal ions and borate has attracted recent attention, particularly from the point of view of association of ions in seawater. Several studies (69, 114,168, 169, 340) have shown that the boron content of seawater (4-5 x 10 4 M) is too low to support appreciable concentrations of polyborate species. The increase in acidity of boric acid in the presence of metal ions results from ion-pair formation ... 

The relationship between the composition and structure of borates and their decomposition in aqueous solution has been reviewed (78, 226,414,417). Borates of the alkali and alkaline-earth metals give an alkaline reaction in solution, as the borates formed by hydrolysis possess a lower boron-to-metal ratio than in the initial material (414). 

Calcium bis[tetrahydroborate(l-)] may be prepared from sodium tetra-hydroborate( 1 -) and calcium dichloride by cation exchange11 in a suitable solvent such as dimethylformamide,12 an amine13,14 or an alcohol.13,14 A particularly good preparation of very pure Ca[BH4]2 involves the reaction of calcium dihydride with the triethylamine-borane adduct.15,16 This method may also be used for the preparation of other tetrahydroborates of alkali and alkaline earth metals. The triethylamine-borane adduct17 may be synthesized in a variety of ways, e.g., from triethylamine, sodium tetrahydro-borate(l —), and trichloroborane 18 from a trialkoxyborane, aluminum metal, and hydrogen in the presence of triethylamine 19 or by hydrogenation under pressure of a mixture of triethylborane and triethylamine.20 The triethylamine-borane adduct is a colorless liquid (mp — 2°). It is stable to air and moisture at room temperature and it is easily purified by vacuum distillation (bp 95-96°/12 torr). 

Magnesium is a bright, silver-white alkaline earth metallic element (symbol Mg atomic no. 12). It is widely found as different salts in minerals such as boracite (borate), ankerite and dolomite (carbonates), carnallite (chloride), brucite (hydroxide), periclase (oxide), lazulite (phosphate), amphibole, cummingtonite, enstatite, gamierite, hornblende, humite, hypersthenite, iolite, jade, meUlite, monticellite, olivine, Pennine, saponite, sapphire, serpentine, talc, and tremolite (silicates), and epsomite, kainite, kieserite, and polyhalite (sulfates). 

However, environmental investigations on landfill derived groundwater contamination used frequently inorganic parameters for monitoring proposes (e.g. Murray et al. 1981 Looser et al. 1999 Vilomet et al., in press). These parameters include concentration of nonmetal anionic species (chloride, nitrate, sulfate, phosphate, borate etc.), heavy metals (Ni, Cr, Fe etc.) as well as alkali and alkaline earth metals (K, Na, Mg, Ca etc.). 

Catalysts. - The catalysts and sources of HCHO that appeared in patents are listed in Table 6. Solid bases such as hydroxides of alkali and alkaline earth metal supported on a support such as silica gel or aluminosilicate have mainly been claimed in patents to be effective as the catalysts. In addition, another type of compounds, which possess acidic property as well as basic property, are also claimed in patents, for example, PbO, Mn02, AI2O3, metal phosphates, metal borates, multicomponent oxides containing V, Nb, W, and Mo. 

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