Aluminium phosphate is a chemical compound. In nature it occurs as the mineral berlinite. Many synthetic forms of aluminium phosphate are known. They have framework structures similar to zeolites and some are used as catalysts, ion-exchangers or molecular sieves. Commercial aluminium phosphate gel is available.
AlPO4 is isoelectronic with Si2O4, silicon dioxide. 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 and piezoelectric properties. Crystalline AlPO4 when heated, converts to tridymite and cristobalite forms, and this mirrors the behaviour of silicon dioxide.
Uses
Molecular sieves
There are many 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 zeolites, which differ in having electrically charged frameworks. A typical preparation of an aluminophosphate involves the hydrothermal reaction of phosphoric acid and aluminium in the form of hydroxide, an aluminium salt such as aluminium nitrate salt or alkoxide under controlledpH in the presence of organic amines. These organic molecules act as templates to direct the growth of the porous framework.
Other
Along with aluminium hydroxide, aluminium phosphate is one of the most common immunologic adjuvants in vaccinations. Aluminium adjuvant use is widespread due to their cheap price, long history of use, safety and efficiency with most antigens. Its unknown how such salts function as adjuvants. Similar to aluminum hydroxide, AlPO4 is used as an antacid. It neutralizes stomach acid by forming AlCl3 with it. Up to 20% of aluminum from ingested antacid salts can be absorbed from the gastrointestinal tract – despite of some unverified concerns about the neurological effects of aluminum, aluminum 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, Al3 is used in dental cements, metal coatings, glaze compositions and refractory binders; and Al 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 has a structure that can be regarded as an assembly of tetra- and octahedral units of phosphate anions, aluminium cations and water. Al3+ are 6-coordinate and PO43- are 4-coordinate. A synthetic hydrated form, AlPO4·1.5H2O is also known.
