Aluminophosphates
Key point: The structures and physical properties of aluminophosphates parallel those of zeolites. The structural and electronic equivalence of two silicate tetrahedra (SiO4)2 and the aluminophosphate unit (AlO4PO4) can be recognized in the simple compounds SiO2 and AlPO4, both of which adopt a similar range of dense polymorphs, including the quartz structure. The development of zeolites with very high Si content, which are effectively silica polymorphs, in turn led to the discovery of the aluminophosphate (ALPO) framework structures based on a 1:1 mixture of AlO4 and PO4 tetrahedra; AlPO4 itself has the same structure as quartz (SiO2) but with an arrangement of alternating Al and Si atoms at the centres of the tetrahedra. A wide range of ALPOs has been developed that parallel the zeolites in, for example, their synthesis under hydrothermal conditions (although in acid conditions rather than the basic ones used for zeolites) and their adsorption and catalytic properties. Again, organic template molecules have been designed and used to prepare many different ALPO structures, such as those with the structure codes VPI (with a large channel formed from 18 AlO4/PO4 tetrahedra), DAF, CIT, and STA (Fig. 24.51). Although aluminophosphate frameworks are neutral, substitution of either Al (III) or P(V) with metal ions of lower charge leads to the formation of ‘solid acid catalysts’ that can, for example, convert
Figure 24.51 The frameworks, formed from linked oxotetrahedra, of (a) DAF and (b) CIT. In each case the main channels present are emphasized by viewing the structure along them. methanol into hydrocarbons selectively, although the aluminosilicate zeolites, particularly ZSM-5, remain the best materials for this application (Section 26.14). The incorporation of Co and Mn, both of which have redox properties, into ALPO frameworks yields materials that can be used for the oxidation of alkanes.
