Keggin structure
Encyclopedia
Heteropoly acid
A heteropoly acid is a class of acid made up of a particular combination of hydrogen and oxygen with certain metals and non-metals. This type of acid is frequently used as a re-usable acid catalyst in chemical reactions....
s. It is the structural form of α–Keggin anions, which have a general formula of [XM12O40]n−, where X is the heteroatom
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...
(most commonly are P5+, Si4+, or B3+), M is the addenda atom (most common are molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
), and O represents oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. The structure self-assembles in acidic aqueous solution and is the most stable structure of polyoxometalate catalysts.
History
The first α-Keggin anion, ammonium phosphomolybdateAmmonium phosphomolybdate
Ammonium phosphomolybdate is the inorganic salt of phosphomolybdic acid with the chemical formula 3PMo12O40. It contains the remarkable phosphomolybdate ion complex.-Synthesis:...
((NH4)3[PMo12O40]), was first reported by Berzelius
Jöns Jakob Berzelius
Jöns Jacob Berzelius was a Swedish chemist. He worked out the modern technique of chemical formula notation, and is together with John Dalton, Antoine Lavoisier, and Robert Boyle considered a father of modern chemistry...
in 1826. In 1892, Blomstrand proposed the structure of phosphomolybdic acid
Phosphomolybdic acid
Phosphomolybdic acid, also known as dodeca molybdophosphoric acid or PMA is a component of Masson's trichrome stain. It is a yellow-green compound, freely soluble in water and polar organic solvents such as ethanol...
and other poly-acids as a chain or ring configuration. Alfred Werner
Alfred Werner
Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...
, using the coordination compounds ideas of Copaux, attempted to explain the structure of silicotungstic acid. He assumed a central group, [SiO4]4− ion, enclosed by four [RW2O6]+, where R is a unipositive ion. The [RW2O6]+ are linked to the central group by primary valences. Two more R2W2O7 groups were linked to the central group by secondary valences. This proposal accounted for the characteristics of most poly-acids, but not all.
In 1928, Linus Pauling
Linus Pauling
Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...
proposed a structure for α-Keggin anions consisting of a tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...
central ion, [XO4]n−8, caged by twelve WO6 octahedral. In this proposed structure, three of the oxygen on each of the octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
shared electrons with three neighboring octahedral. As a result, 18 oxygen atoms were used as bridging atoms between the metal atoms. The remaining oxygen atoms bonded to a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
. This structure explained many characteristics that were observed such as basicities of alkali metal
Alkali metal
The alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
salts and the hydrated of some of the salts. However the structure could not explain the structure of dehydrated acids.
J.F. Keggin with the use of X-ray diffraction experimentally determined the structure of α-Keggin anions in 1934. The Keggin structure accounts for both the hydrated and dehydrated α-Keggin anions without a need for significant structural change. The Keggin structure is the widely accepted structure for the α-Keggin anions.
Structure and physical properties
Heteroatom
In organic chemistry, a heteroatom is any atom that is not carbon or hydrogen. Usually, the term is used to indicate that non-carbon atoms have replaced carbon in the backbone of the molecular structure...
surrounded by four oxygen to form a tetrahedron
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
. The heteroatom is located centrally and caged by 12 octahedral MO6-units linked to one another by the neighboring oxygen atoms. There are a total of 24 bridging oxygen atoms that link the 12 addenda atoms. The metal centres in the 12 octahedra are arranged on a sphere almost equidistant
Equidistant
A point is said to be equidistant from a set of objects if the distances between that point and each object in the set are equal.In two-dimensional Euclidian geometry the locus of points equidistant from two given points is their perpendicular bisector...
from each other, in four M3O13 units, giving the complete structure an overall tetrahedral symmetry. The bond length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
between atoms varies depending on the heteroatom (X) and the addenda atoms (M). For the 12–phosphotungstic acid, Keggin determined the bond length between the heteroatom and each the four central oxygen atoms to be 1.5 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
. The bond length form the central oxygen to the addenda atoms is 2.43 Å. The bond length between the addenda atoms and each of the bridging oxygen is 1.9 Å. The remaining 12 oxygen atoms that are each double bonded to an addenda atom have a bond length of 1.70 Å. The octahedra are therefore distorted. This structure allows the molecule to hydrate and dehydrate without significant structural changes and the molecule is thermally stable in the solid state for use in vapor phase reactions at high temperatures (400-500 °C).
Isomerism
Including the original Keggin structure there are 5 isomerIsomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...
s, designated by the prefixes α-, β-,γ-, δ- and ε-. The original Keggin structure is designated α- . These isomers are sometimes termed Baker, Baker-Figgis or rotational isomers, These involve different rotational orientations of the Mo3O13 units, which lowers the symmetry of the overall structure.
Lacunary Keggin structures
The term lacunary is applied to ions which have a fragment missing, sometimes called defect structures. Examples are the (XM11O39)n
Group 13 cations with the Keggin structure
The cluster cation (Al13O4(OH)24(H2O)12)7+ has the Keggin structure with a tetrahedral Al atom in the centre of the cluster coordinated to 4 oxygen atoms. The formula can be expressed as (AlO4Al12(OH)24(H2O)12)7+. This ion is generally called the Al13 ion. A Ga13 analogue is known an unusual ionic compound with an Al13 cation and a Keggin polyoxoanion has been characterised.Chemical properties
The stability of the Keggin structure allows the metals in the anion to be readily reducedRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
. Depending on the solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, acidity of the solution and the charge on the α-Keggin anion, it can be reversibly reduced in one- or multiple electron step. For example silicotungstate anion can be reduced to 20th state. Some anions such as silicotungstic acid are strong enough as an acid as sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and can be used in its place as an acid catalyst.
Preparation
In general α-Keggin anions are synthesized in acidic solutions. For example, 12-Phosphotungstic acidPhosphotungstic acid
Phosphotungstic acid , tungstophosphoric acid , is a heteropoly acid with the chemical formula 31240. It normally present as a hydrate. EPTA is the name of ethanolic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly...
is formed by condensing phosphate ion with tungstate ions. The heteropolyacid that is formed has the Keggin structure.
- PO43− + 12 WO42− + 27H+ → H3PW12O40 + 12H2O
Uses
α-Keggin anions have been used as catalyst in the following reactions: hydrationHydration reaction
In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
, polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
and oxidation reaction as catalysts. Japanese chemical companies have commercialized the use of the compounds in hydration of propene, oxidation of methacrolein
Methacrolein
Methacrolein, or methacrylaldehyde, is an unsaturated aldehyde. It is a clear, colorless, flammable liquid that is present in cigarettes when smoking.Industrially, the primary use of methacrolein is in the manufacture of polymers and synthetic resins....
, hydration isobutene, hydration of n-butene
1-Butene
1-Butene is an organic chemical compound, linear alpha-olefin , and one of the isomers of butene. The formula is .-Stability:1-Butene is stable in itself but polymerizes exothermically. It is highly flammable and readily forms explosive mixtures with air...
, and polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
of THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
.
Suppliers
12-Phosphotungstic acidPhosphotungstic acid
Phosphotungstic acid , tungstophosphoric acid , is a heteropoly acid with the chemical formula 31240. It normally present as a hydrate. EPTA is the name of ethanolic phosphotungstic acid, its alcohol solution used in biology. It has the appearance of small, colorless-grayish or slightly...
the compound J.F. Keggin used to determine the structure can be purchased commercially. Other compounds that contain the α-Keggin anion such as silicotungstic acid and phosphomolybdic acid
Phosphomolybdic acid
Phosphomolybdic acid, also known as dodeca molybdophosphoric acid or PMA is a component of Masson's trichrome stain. It is a yellow-green compound, freely soluble in water and polar organic solvents such as ethanol...
are also commercially available at Aldrich Chemicals, Fisher Chemicals, Alfa Aesar
Alfa Aesar
Alfa Aesar, headquartered in Ward Hill, Massachusetts, is a supplier of reagents and materials for use in research and development, and analysis. They have facilities in many countries and manufacture many of the chemicals they sell...
, VWR Chemical, etc.