Aluminate

In chemistry, an aluminate is a compound containing an oxyanion of aluminium, such as sodium aluminate. In the naming of inorganic compounds, it is a suffix that indicates a polyatomic anion with a central aluminium atom.[1]

Aluminate oxyanions

Aluminium oxide (alumina) is amphoteric: it dissolves in both bases and acids. When dissolved in bases it forms hydroxyaluminate ions in the same way as aluminium hydroxide or aluminium salts. The hydroxyaluminate or hydrated aluminate can be precipitated and then calcined to produce anhydrous aluminates. Aluminates are often formulated as a combination of basic oxide and aluminium oxide, for example the formula of anhydrous sodium aluminate NaAlO2 would be shown as Na2O·Al2O3. A number of aluminate oxyanions are known:

  • The simplest is the approximately tetrahedral AlO5−
    4     found in the compound Na5AlO4,[2]
  • framework AlO−
    2     ions in anhydrous sodium aluminate NaAlO2[3] and monocalcium aluminate, CaAl2O4 made up of corner-sharing {AlO4} tetrahedra.[4]
  • A ring anion, the cyclic Al
    6    O18−
    18     anion, found in tricalcium aluminate, Ca3Al2O6, which can be considered to consist of 6 corner sharing {AlO4} tetrahedra.[5]
  • A number of infinite chain anions in the compounds Na7Al3O8 which contains rings linked to form chains, Na7Al13O10 and Na17Al5O16 which contain discrete chain anions.[6]

Mixed oxides containing aluminium

There are many mixed oxides containing aluminium where there are no discrete or polymeric aluminate ions. The spinels with a generic formula A2+
B3+
2O2−
4 that contain aluminium as Al3+, such as the mineral spinel itself, MgAl2O4 are mixed oxides with cubic close packed O atoms and aluminium Al3+ in octahedral positions.[7]

BeAl2O4, chrysoberyl, isomorphous with olivine, has hexagonal close-packed oxygen atoms with aluminium in octahedral positions and beryllium in tetrahedral positions.[8]

Some oxides with the general formula of MAlO3 sometimes called aluminates or orthoaluminates such as YAlO3, Yttrium ortho-aluminate are mixed oxides and have the perovskite structure.[9] Some oxides such as Y3Al5O12, usually called YAG, have the garnet structure.[7]

Hydroxoaluminates

The Al(OH)−
4 anion is known in high pH solutions of Al(OH)3.[6]

Aluminate glasses

Alumina on its own cannot easily be made glassy with current techniques, however with the addition of a second compound many types of aluminate glasses can be formed. The glasses produced display a range of interesting and useful properties, such as high refractive index, good infrared transparency, and high melting point, as well as the ability to host laser active and fluorescent ions. Aerodynamic levitation is a key method used to study and produce many aluminate glasses. Levitation allows high purity to be maintained in the melt at temperatures in excess of 2,000 K (1,700 °C).[10]

Some materials that are known to form glass in binary combination with aluminium oxide are: rare earth oxides, alkaline earth oxides (CaO, SrO, BaO), lead oxide, and silicon dioxide.

Also, the Al2O3 (aluminate) system has been discovered to form sapphire-like glass ceramics. Often, this ability is based upon compositions in which interplay between glass forming ability and glass stability is approximately balanced.[11]

Applications of aluminates

Sodium aluminate, NaAlO2, is used industrially in dyeing to form a mordant and the hydrated forms are used in water purification, sizing of paper and in the manufacture of zeolites, ceramics and catalysts in the petrochemical industry. In the isomerization process of alkenes and amines[12] Calcium aluminates are important ingredients of cements.[6]

Li5AlO4 is used in the nuclear power industry.[13]

Aluminate suffix used in the naming of inorganic compounds

Examples are:

  • Tetrachloroaluminate ion AlCl−
    4     found in for example lithium tetrachloroaluminate.[6]
  • Tetrahydroaluminate ion AlH−
    4     found in for example lithium aluminium hydride.[6]
  • Hexafluoroaluminate ion AlF3−
    6     found in for example sodium hexafluoroaluminate.[6]

Aluminates made using new raw materials

Many recent research studies have focused on an effective solution for waste treatment. This has led some residues to be made into new raw materials for many industries. Such an achievement ensures a reduction in energy and natural resource usage, decreasing of the negative environmental impact and creating new fields of work.

A good example comes from the metals industry, particularly the aluminium industry. Aluminium recycling is a beneficial activity for the environment, since it recovers resources from both manufacturing and consumer waste. Slag and scrap which were previously considered as waste, are now the raw material for some highly profitable new industries. There is added-value in materials made using an aluminium residue which is currently considered as a hazardous waste. Current research is investigating the use of this waste to manufacture glass, glass-ceramic, boehmite and calcium aluminate.[14]

Notes
  1. Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 – Full text (PDF)
  2. Barker, Marten G.; Gadd, Paul G.; Begley, Michael J. (1981). "Preparation and crystal structures of the first alkali-rich sodium aluminates Na7Al3O8 and Na5AlO4". Journal of the Chemical Society, Chemical Communications (8): 379. doi:10.1039/c39810000379. ISSN 0022-4936.
  3. Barker, Marten G.; Gadd, Paul G.; Begley, Michael J. (1984). "Identification and characterisation of three novel compounds in the sodium-aluminium-oxygen system". Journal of the Chemical Society, Dalton Transactions (6): 1139. doi:10.1039/dt9840001139. ISSN 0300-9246.
  4. Ma, C.; Kampf, A. R.; Connolly, H. C.; Beckett, J. R.; Rossman, G. R.; Smith, S. A. S.; Schrader, D. L. (2011). "Krotite, CaAl2O4, a new refractory mineral from the NWA 1934 meteorite". American Mineralogist. 96 (5–6): 709–715. Bibcode:2011AmMin..96..709M. doi:10.2138/am.2011.3693. ISSN 0003-004X.
  5. Mondal, P.; Jeffery, J. W. (1975). "The crystal structure of tricalcium aluminate, Ca3Al2O6" (PDF). Acta Crystallographica Section B. 31 (3): 689–697. doi:10.1107/S0567740875003639. ISSN 0567-7408. Archived from the original (PDF) on 2020-03-03. Retrieved 2019-09-04.
  6. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  7. Wells A.F. (1984) Structural Inorganic Chemistry 5th edition, Oxford Science Publications ISBN 0-19-855370-6
  8. "Refinement of the chysoberyl structure", E.F. Farrell, J.H. Fang, R.E. Newnham, The American Mineralogist, 1963, 48, 804
  9. "Crystal structure refinement of YAlO3, a promising laser material", R. Diehl, G. Brandt, Materials Research Bulletin (1975) Volume: 10, Issue: 3, Pages: 85–90
  10. Haliakova, A., Prnova, A., Klement, R.D. and Tuan, W.H. "Flame-spraying synthesis of aluminate glasses in the Al2O3-La2O3 system". webofknowledge.com. September 2012. Pages: 5543–5549. Accessed 2012-10-09.
  11. Rosenflanz, A.; Tangeman, J.; Anderson, T. (2012). "On processing and properties of liquid phase derived glass ceramics in Al2O3–La2O3–ZrO2 system". Advances In Applied Ceramics: Structural, Functional & Bioceramics. 111 (5/6): 323–332.
  12. Rienäcker, Roland; Graefe, Jürgen (1985). "Catalytic Transformations of Sesquiterpene Hydrocarbons on Alkali Metal/Aluminum Oxide". Angewandte Chemie International Edition in English. 24 (4): 320–321. doi:10.1002/anie.198503201. ISSN 0570-0833.
  13. Allen W. Apblett, "Aluminium: Inorganic Chemistry", (1994),Encyclopedia of Inorganic Chemistry, ed. R. Bruce King, John Wiley & Sons, ISBN 0-471-93620-0
  14. López Delgado, A. and Tayibi, H. "Can hazardous waste become a raw material? The case study of an aluminum residue: a review". webofknowledge.com. May 2012. Pages: 474–484. Accessed 2012-10-09
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