Coordinated phosphate classification

The affinity between the tetrahedrally arranged orthophosphate oxyanion, P04, and hexava-lently coordinated metal cations lends itself to a classification of phosphate minerals in a scheme similar to silicates (SiO -) framework, insular, chain, and layer phosphates. Examples of this scheme, advanced by Povarennykh (1972) and further elaborated by Lindsay Vlek (1977), include berlinite (AIPO4 framework) hydroxyapatite (insular) monetite (CaHP04, chain) and vivianite (Fe4(P04)2-2H2O, layer). 

Mesoporous Aluminophosphates. The structure classification of amorphous mesoporous aluminophosphates is based on the pore arrangement that can be hexagonal, cubic, or disordered. Aluminum species in these materials are usually present as both four- and six-coordinated species, whereas phosphate units being always tetrahedral. The Al/P ratio differs from unity due to the uncompleted condensation of the mesoporous framework (16). 

Another interesting feature of MOFs concerns the number of cations that can participate in the framework. Indeed, compared to inorganic ones [3], which are more based on a few cations (Si and A1 for zeolites, eventually doped with some transition metals, with the exception of titanosilicates [119] Zr, Al, Ga, In phosphates and arsenates, sometimes fuUy substituted by transition metals Ti [72], V [120], Fe [121,122], Co [123,124], Ni [125], Zn [126,127]), MOFs can accept almost all the cations of the classification, at least those which are di-, tri- (including rare earth) or tetravalent. Keeping in mind the tremendous number of species previously isolated in coordination chemistry, this provides a huge number of possibilities for creating new MOFs. 

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