Zirconium Phosphate Phosphonates of Type

The structure of the y-layer differs from that of the a-layer since the ZrO6 octahedra are placed in two different planes and joined to each other by PO4 tetrahedra. Due to the fact that only three oxygens are available in phosphonate groups, pure y-zirconium phosphonates cannot exist. However, it is possible to replace the interlayer H2PO4 groups by monovalent phosphonate or phosphinate anions to obtain layered inorganic-organic derivatives in which the inorganic layer regularly alternates with organic layers. These materials are obtained from a simple topotactic reaction by contacting the original zirconium phosphate microcrystals with a solution of a suitable phosphonic acid. As we have already seen this reaction is similar to an anion exchange process. The texture of the y-layer remains practically unchanged and it is therefore possible to predict the arrangement of the organic groups in the interlayer region by considering the interlayer distance and the dimension of the groups. Figure 16 shows the probable structure of y-zirconium phosphate benzene-phosphonate.

Many organic derivatives have been prepared with this simple procedure including pillared compounds with regular interlayer porosity obtained by partial replacement of the dihydrogenphosphates with bivalent diphosphonate groups. A selected number of recently prepared compounds is reported in Table 6. Monophosphonic or biphosphonic acids containing crown ethers have also been used for the topotactic reaction and compounds with crown ethers covalent-ly attached to the inorganic layers have been obtained. These materials show promise for interesting applications in ionic or molecular recognition and hence for performing selective separations. The y-system is thus very versatile and the interlayer region can easily be engineered with a large variety of organic groups to obtain materials for application in several fields including the preparation of new stationary phases for chromatographic separation.

Figure 16 Computer-generated representation of the sequenceoftwo layersof y-zirconium phosphate-benzenephosphonate.(Data from Alberti G, Vivani R, Biswas RKand Murcia-Mascaros S (1993) React. Polym. 19: 1-12, with permission from Elsevier Science.)

Conclusion a rich chemistry. Many of them are inorganic ion exchangers that support the most common organic The layered phosphates of polyvalent metals are ob- resins in processes which occur at high temperatures tained with different layer structures and exhibit or in the presence of strong oxidizing solutions and

Table 6 Composition and interlayer distances of some organic derivatives of y-zirconium phosphate obtained by topotactic exchange

reactions

Acid used

Composition

Interlayerdistance (As)

H3PO3

ZrPO4O2PHOH • 2H2O

12.2

H3PO2

ZrPO4O2PH2 • H2O

8.8

H2O3PCH3

ZrPO4O2POHCH3 • 2H2O

12.8

H2O3PC3H7

ZrPO4O2POHC3H7 • 1.2H2O

15.1

HO2P(CH3)2

ZrPO4(H2PO4)o 33(O2P(CH3)2)O 67 • H2O

10.3

H2O3PC6H5

ZrPO4(H2PO4)o 33(O2POHC6H5)O 67 • 2H2O

15.4

H2O3P(C6H„)

ZrPO4(H2PO4)o.33(O2POHC6Hii)o.67 • H2O

16.9

H2O3PC6H5

ZrPO4O2PHC6H5

15.1

H2O3PC10H21NO|

ZrPO4(H2PO4)o.7l(CloH2lNO3PO3)o.29

16.2

*N-(phosphonoethyl)aza crown; (12)crown-4.

*N-(phosphonoethyl)aza crown; (12)crown-4.

strong doses of ionizing radiation. Furthermore, these inorganic ion exchangers possess a high ion exchange capacity and some peculiar selectivities. Layered phosphates are good intercalating agents of ionic or polar species. This allows the construction in the interlayer region of supramolecular assemblies with special functionalities in the fields of chromatographic supports, chemical and electrochemical sensors, ion exchange membranes, ionic and molecular recognition and catalysts. The delamination of layered phosphates has permitted the preparation of thin films and coatings and pillared layered structures with accessible microporosity. There are many more possibilities in layered phosphonate chemistry because functional groups may be inserted on alkyl chains or on aryl rings. The field of layered phosphates and phosphonates is in continuous expansion and these materials will find many applications as soon as their potential is realized.

See also: II/Ion Exchange: Catalysis: Organic Ion Exchangers; Historical Development; Inorganic Ion Exchangers; Novel Layered Materials: Non-Phosphates; Organic Ion Exchangers; Theory of Ion Exchange.

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