Aluminium phosphate is a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. In nature it occurs as the mineral berlinite. Many synthetic forms of aluminium phosphate are known. They have framework structures similar to

zeolites Zeolites are a group of several Microporous material, microporous, crystalline aluminosilicate minerals commonly used as commercial adsorption, adsorbents and Catalysis, catalysts. They mainly consist of silicon, aluminium, oxygen, and have the ge ...

and some are used as

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

s, ion-exchangers or molecular sieves. Commercial aluminium phosphate gel is available.

Berlinite

AlPO₄ is

isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

with Si₂O₄,

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

. Berlinite looks like quartz and has a structure that is similar to quartz with silicon replaced by Al and P. The AlO₄ and PO₄ tetrahedra alternate. Like quartz, AlPO₄ exhibits

chirality Chirality () is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable fro ...

and

piezoelectric Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied stress (mechanics), mechanical s ...

properties. When heated, crystalline AlPO₄ (berlinite) converts to

tridymite Tridymite is a high-temperature polymorphism (materials science), polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is sili ...

and

cristobalite Cristobalite ( ) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, Si O2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members o ...

forms, and this mirrors the behaviour of

Uses

Molecular sieves

There are many types of aluminium phosphate molecular sieves, generically known as "ALPOs". The first ones were reported in 1982. They all share the same chemical composition of AlPO₄ and have framework structures with microporous cavities. The frameworks are made up of alternating AlO₄ and PO₄ tetrahedra. The denser cavity-less crystalline berlinite, shares the same alternating AlO₄ and PO₄ tetrahedra. The aluminophosphate framework structures vary one from another in the orientation of the AlO₄ tetrahedra and PO₄ tetrahedra to form different-sized cavities, and in this respect they are similar to the

aluminosilicate Aluminosilicate refers to materials containing anionic Si-O-Al linkages. Commonly, the associate cations are sodium (Na+), potassium (K+) and protons (H+). Such materials occur as minerals, coal combustion products and as synthetic materials, of ...

zeolite Zeolites are a group of several microporous, crystalline aluminosilicate minerals commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ･y where is either a meta ...

s, which differ in having electrically charged frameworks. A typical preparation of an aluminophosphate involves the hydrothermal reaction of

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

and aluminium in the form of

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

, an aluminium salt such as aluminium nitrate salt or

alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organyl substituent. Alkoxides are strong bases and, whe ...

under controlled pH in the presence of organic

amine In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...

s. These organic molecules act as templates (now termed structure directing agents, SDAs) to direct the growth of the porous framework.

Other

Along with aluminium hydroxide, aluminium phosphate is one of the most common

immunologic adjuvant In immunology, an adjuvant is a substance that increases or modulates the immune response to a vaccine. The word "adjuvant" comes from the Latin word , meaning to help or aid. "An immunologic adjuvant is defined as any substance that acts to acce ...

s (efficiency enhancers) in

vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...

s. Aluminium adjuvant use is widespread due to its cheap price, long history of use, safety and efficiency with most

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

s. Similar to aluminium hydroxide, AlPO₄ is used as an

antacid An antacid is a substance which neutralization (chemistry), neutralizes gastric acid, stomach acidity and is used to relieve heartburn, indigestion, or an upset stomach. Some antacids have been used in the treatment of constipation and diarrhe ...

. Up to 20% of aluminium from ingested antacid salts can be absorbed from the gastrointestinal tract – despite some unverified concerns about the neurological effects of aluminium, aluminium phosphate and hydroxide salts are thought to be safe as antacids in normal use, even during pregnancy and breastfeeding. Additional uses for AlPO₄ in combination with or without other compounds are white colorants for pigments, corrosion inhibitors, cements and dental cements. Related compounds have also similar uses. For example, Al(H₂PO₄)₃ is used in dental cements, metal coatings, glaze compositions and

refractory In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...

binders; and Al(H₂PO₄)(HPO₄) is used cement and refractory binders and adhesives.

Related compounds

AlPO₄·2H₂O dihydrate is found as the minerals variscite and meta-variscite. Aluminium phosphate dihydrate (variscite and meta-variscite) has a structure that can be regarded as an assembly of tetra- and octahedral units of phosphate anions, aluminium cations and water. Al³⁺ ions are 6-coordinate and PO₄^3- ions are 4-coordinate.Corbridge, p. 207-208 A synthetic hydrated form, AlPO₄·1.5H₂O is also known.

References

Citations

External links

MSDS PDF
{{Antacids Phosphates Aluminium compounds Dehydrating agents Antacids