Metal Borates

Tetraalkyl titanates react with organic borates, B(OR )2, to give complexes of the general formula Ti(OR)4 B(OR )2, which are useful as catalysts and cross-linking agents (562). Mixtures of chelated organic titanates such as TYZOR TE and TYZOR LA with alkaU metal borates, such as borax, or boric acid can be used to produce shear-stable fracturing fluids (563). 

The oxides and oxyacids of boron as well as a variety of hydrated and anhydrous metal borates are discussed hereia. An alphabetical Hst of compounds referred to ia the text is given ia Table 1. 

The polyions postulated in solution all have known structural analogues in crystalline borate salts. Investigations of the Raman (66) and B nmr (67) spectra of borate solutions have confirmed the presence of three of these species the triborate (3), B202(0H) 4, tetraborate (4), [B40 (0H) 4], and pentaborate (5) B O (OH) 4, polyanions. Skeletal stmctures were assigned based on coincidences between the solution spectra and those soHd borates for which definitive stmctural data are available (52). These same ions have been postulated to be present in alkah metal borate glasses as well. 

A number of reviews have appeared covering the various aspects of borate glasses. The stmcture, physical properties, thermochemistry, reactions, phase equihbria, and electrical properties of alkah borate melts and glasses have been presented (73). The apphcation of x-ray diffraction, nmr, Raman scattering, in spectroscopy, and esr to stmctural analysis is available (26). Phase-equihbrium diagrams for a large number of anhydrous borate systems are included in a compilation (145), and thermochemical data on the anhydrous alkah metal borates have been compiled (17). 

In general, hydrated borates of heavy metals ate prepared by mixing aqueous solutions or suspensions of the metal oxides, sulfates, or halides and boric acid or alkali metal borates such as borax. The precipitates formed from basic solutions are often sparingly-soluble amorphous soHds having variable compositions. Crystalline products are generally obtained from slightly acidic solutions. 

Anhydrous metal borates may be prepared by heating the hydrated salts to 300—500°C, or by direct fusion of the metal oxide with boric acid or B2O2. Many binary and tertiary anhydrous systems containing B2O2 form vitreous phases over certain ranges of composition (145). 

N. P. Nies, "Alkah-Metal Borates Physical and Chemical Properties," in Ref. 3, section A9, p. 343. 

The phase relations, stoichiometry and structural chemistry of the metal borates have been extensively studied because of their geochemical implications and technological importance. Borates are known in which the structural unit is mononuclear (1 B atom), bi-, tri-, tetra- or penta-nuclear, or in which there are polydimensional networks including glasses. The main structural principles underlying the bonding in crystalline metal borates are as follows " ... 

Many metal borates find important industrial applications (p. 140) and annual world production exceeds 2.9 million tonnes Turkey 1.2, USA 1.1, Argentina 0.26, the former Soviet Union 0.18, Chile 0.13Mt. Main uses are in glass-fibre and cellular insulation, the manufacture of borosilicate glasses and enamels, and as fire retardants. Sodium perborate (for detergents) is manufactured on a 550 000 tonne pa scale. 

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. 

Boric Acid Alcohol insoluble substances Absence of metallic borates and insoluble impurities... 

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

Boron trichloride can be prepared by high temperature chlorination of boron trioxide, boric acid, metal borates or other boron compounds. Chlorine, hydrogen chloride, phosgene, silicon tetrachloride, metal chlorides, carbon... 

Many borates are known that contain non-metal cations. Examples include the ammonium borates [39], such as the commercial products ammonium pentaborate, NH4[B506(0H)4] 2 H2O, and ammonium tetraborate, (NH4)2 [6405(011)4] 2 H2O, both of which contain isolated borate anions shown above (Figs. 5a and 6a, respectively) [40]. Isolated borate anions containing more than six boron atoms are rare. However, the mineral ammonioborite [(NH4)3[Bi502o(OH)g] 4 H20 = (NH4)20 5 B2O3 2 2/3 H2O), which may be prepared synthetically by prolonged reflux of a saturated solution of ammonium pentaborate, contains the unusual isolated anion [Bi502o(OH)g] , shown in Fig. 10 [41]. This anion is not known to occur in any metal borate, its formation is apparently directed exclusively by the ammonium cations which can donate H-bonds to interstitial water and the borate structural unit. 

In the coreduction of the corresponding two metal salts by tetraoctylammonium triethylhydroborate in THF, Pt/Ru (37) and Pt/Rh (13,38) bimetallic nanoparticles, stabilized by tetraoctylammonium bromide, were obtained. No metal borate was found in the product. 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

The metal borate pigments are classified as having a relatively low toxicity [5.53], [5.107]. For toxicological data, see Section 5.2.14. 

---