Aluminum trifluoride

Both the binary and complex fluorides of aluminum have played a significant role in the aluminum industry. Aluminum trifluoride [7784-18-17, A1F., and its trihydrate [15098-87-0] 3 3H2O, have thus far remained to be the only binary fluorides of industrial interest. The nonahydrate [15098-89-2] 3 9H2O, and the monohydrate [12252-28-7, 15621 -55-3], AIF 20, are of only academic curiosity. The monofluoride [13595-82-9], AIF, and the difluoride [13569-23-8], AIF2, have been observed as transient species at high temperatures. 

Significant vapor pressure of aluminum monofluoride [13595-82-9], AIF, has been observed when aluminum trifluoride [7784-18-1] is heated in the presence of reducing agents such as aluminum or magnesium metal, or is in contact with the cathode in the electrolysis of fused salt mixtures. AIF disproportionates into AIF. and aluminum at lower temperatures. The heat of formation at 25°C is —264 kJ/mol(—63.1 kcal/mol) and the free energy of formation is —290 kJ/mol(—69.3 kcal/mol) (1). Aluminum difluoride [13569-23-8] h.3.s been detected in the high temperature equihbrium between aluminum and its fluorides (2). 

Aluminum trifluoride trihydrate [15098-87-0], AIF. -3H20, appears to exist in a soluble metastable a-form as well as a less soluble P-form (3). The a-form can be obtained only when the heat of the reaction between alumina and hydrofluoric acid is controlled and the temperature of the reaction is kept below 25°C. Upon warming the a-form changes into a irreversible P-form which is insoluble in water and is much more stable. The P-form is commercially available. 

The principal producers of aluminum trifluoride in North America are Alcan, Alcoa, and AUiedSignal. It is also produced in other countries, eg, France, Mexico, Norway, Italy, Tunisia, and Japan. Total worldwide production of aluminum trifluoride in 1990 was 400,000 metric tons and the price was 1100/t. In 1993, because of excess recovery of fluorine values, use of energy efficient smelters, and the worldwide economic climate, the price was down to 750/t. 

Other hydrates of aluminum trifluoride are the nonahydrate [15098-89-2] A1F. -9H20, which is stable only below 8°C, and aluminum triduoride monohydrate [12252-28-7] [15621-55-3] A1F. H20, which occurs naturally as a rare mineral, flueUite found ia Steima-Gwyn Cornwall, U.K. (11). 

High Purity Aluminum Trifluoride. High purity anhydrous aluminum triduoride that is free from oxide impurities can be prepared by reaction of gaseous anhydrous HF and AlCl at 100°C, gradually raising the temperature to 400°C. It can also be prepared by the action of elemental fluorine on metal/metal oxide and subsequent sublimation (12) or the decomposition of ammonium duoroaluminate at 700°C. 

Table 2. Physical Properties of Anhydrous Aluminum Trifluoride...

The common structural element in the crystal lattice of fluoroaluminates is the hexafluoroaluminate octahedron, AIF. The differing stmctural features of the fluoroaluminates confer distinct physical properties to the species as compared to aluminum trifluoride. For example, in A1F. all corners are shared and the crystal becomes a giant molecule of very high melting point (13). In KAIF, all four equatorial atoms of each octahedron are shared and a layer lattice results. When the ratio of fluorine to aluminum is 6, as in cryoHte, Na AlF, the AIFp ions are separate and bound in position by the balancing metal ions. Fluorine atoms may be shared between octahedrons. When opposite corners of each octahedron are shared with a corner of each neighboring octahedron, an infinite chain is formed as, for example, in TI AIF [33897-68-6]. More complex relations exist in chioUte, wherein one-third of the hexafluoroaluminate octahedra share four corners each and two-thirds share only two corners (14). 

The manufacture of cryoHte is commonly iategrated with the production of alumina hydrate and aluminum trifluoride. The iatermediate stream of sodium aluminate from the Bayer alumina hydrate process can be used along with aqueous hydrofluoric acid, hydrogen fluoride kiln gases, or hydrogen fluoride-rich effluent from dry-process aluminum trifluoride manufacture. 

Fluoride systems containing aluminum trifluoride, A1F3, were investigated in greater detail. Based on Raman spectra, researchers repeatedly demonstrated the presence of two types of complex ions, A1F4" and A1F63 [347 - 350]. The spectral parameters of these complexes are as follows (wave numbers in cm 1) ... 

Isarom A catalytic process for isomerizing the xylene isomers, developed by Institut Frangais du Petrole. The catalyst is aluminum trifluoride. 

Aluminum sulfide, 2 284 Aluminum titanate, 5 570, 578 25 46 Aluminum triacetylide, 1 179 Aluminum trifluoride, 2 360, 361-362, 377 health and safety factors, 2 363 high purity, 2 363... 

Aluminum trifluoride trihydrate, 2 361 a-Aluminum trihydroxide. See Gibbsite P-Aluminum trihydroxide. See Bayerite Aluminum trihydroxides, 2 421 classification, 2 422... 

Anhydrous aluminum nitrate, 2 386 Anhydrous aluminum trifluoride, 2 363 physical properties of, 2 364t Anhydrous ammonia... 

Introduction of Fluorine by Aluminum Trifluoride and Complex Aluminum Fluorides (e.g., Na3AlF6)... 

Aluminum trifluoride has proven to be useful as a catalyst or cocatalyst in vapor-phase fluo-rination reactions. 

Chloroformates and chlorosulfates of linear hydrofluorocarbons are converted in the vapor phase with hydrogen fluoride to yield the fluoro-substituted products. The required aluminum trifluoride catalyst is prepared in situ by heating alumina with hydrogen fluoride at 175-400 C.3... 

The substitution of chlorine in organo halides by hydrogen fluoride has proven to be successful in the presence of aluminum trifluoride. The yields are high, the conversion rate is good, and the selectivity mostly high with low formation of byproducts (see Table 1 for examples). 

Table 1. Substitution of Chlorine or Bromine Using Hydrogen Fluoride with Aluminum Trifluoride Catalysis...

Aluminum trifluoride catalyzes the direct exchange of perfluoroalkylchlorides with elementary fluorine.1819 F AIF... 

Bromofluoromenthane has been prepared in high yield using a one-step reaction with bromine, hydrogen fluoride and methane in the presence of an aluminum trifluoride fluidized bed or fluorinated aluminum catalysts optionally containing cobalt or nickel between 450 and 500°C.20... 

The methyl group in aromatics could be converted similarly to the trifluoromethyl group using the chlorine, hydrogen fluoride and aluminum trifluoride system as a catalyst.21 22... 

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