Metal Phosphates

The most important and widespread of the metal phosphates is a-zirconium phosphate (Zr(HPO4) 2 • H2O, or a-ZrP), which has an expandable layer structure. Each layer possesses a central plane of octahedral Zr atoms linked to two outer sheets of monohydrogen phosphate groups. The hydrogen form has an interlayer spacing of 0.76 nm, corresponding to a void space with diameter 0.26 nm. Although the calculated surface area of a-ZrP approaches 1000 m2 g "1, in the unexpanded H form the surface area available to N2 is only 5 m2 g_1.

Another crystalline form of zirconium phosphate y-ZrP (Zr(PO4)(H2PO4) • 2H2O), is formed by a central zirconium phosphate sheet in which the PO4 groups are linked solely to octahedral Zr atoms; this sheet is linked to dihydrogen phosphate groups to yield the y-ZrP structure. The complex interlinking results in a more rigid framework in which only c. 50% of the theoretical ion exchange capacity is normally obtained.

Swelling of zirconium phosphates The interlayer cavities in a-ZrP of 0.26 nm are accessible to only small and poorly hydrated cations. A certain degree of expansion of the interlayer distance may occur concomitantly with these exchanges. Larger or more strongly hydrated ions do not readily exchange with a-ZrP. However, since the layers are held together principally by electrostatic forces, the distance between them can be increased to allow access of larger ions according to the following mechanism.

The acid form of an a-ZrP possesses H+ cations which stabilize the negative charge on the Zr(PO4)2 units. A number of these protons may be neutralized by addition of hydroxide ions via the solution phase. This causes negative charge to build up on the layers, causing electrostatic repulsion and forcing the layers apart. Once the material has swelled, access to the exchange sites by larger and more strongly hydrated cations is possible. This view may be slightly oversimplified, since migrating OH~ ions would naturally be accompanied by cations (to preserve electroneutrality in both the solid and solution phases). It is more likely that the above two-step process actually occurs as a one-step process driven by the neutralization reaction.

'Catalytic' exchanges in a-ZrP The interlayer spacing of a-ZrP may be too small to allow large cations access (a situation anomalous to ion-sieving in zeolites). For instance, the Mg2+ ion will not exchange with the protons in a-ZrP directly. However, in the presence of sodium, some magnesium exchange does occur. The process is shown conceptually below.

ZrH2<P04>2-H20 -——>■ ZrNaH<P04>2-5H20

ZrH2<P04>2-H20 -——>■ ZrNaH<P04>2-5H20

ZrMgo.72Ho.56<P04>2-4H20

The hydrated Mg2+ ion is too bulky to reach the exchange sites between the layers of the acid form, while the smaller hydrated Na+ ion is not. The partial exchange of Na+ for H+ causes a swelling of the interlayer spacing to a point which allows the hydrated Mg2 + to exchange.

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