Exfoliation Process

We have seen that layered polyvalent metal phosphates are obtained as molecular crystals built up by the packing of the layers which are planar macro-molecules. These bidimensional macromolecules are usually very thin (5-15 A), whereas planar dimensions are of the order of |im2, depending on the conditions of synthesis. If a layered crystal is exfoliated in single layers, materials with a very large surface area and with enhanced reactivity are obtained. For example, the complete exfoliation of 1 g of a-zirconium phosphate will produce material with a surface area of 950 m2. Furthermore, the suspension of the layers may be used to obtain thin films and pellicles or to cover suitable supports.

It is well known that layered smectite clays undergo so-called 'infinite swelling', that is, they disintegrate into single layers or packets of a few layers, when suspended in water. This phenomenon has never been observed in layered phosphates probably because of stronger layer-layer interactions. However, intercalation has made it possible to exfoliate both a-and y-zirconium phosphates. In the case of a-Zr(HPO4) 2 • H2O and of the other a-type layered phosphates a good exfoliation has been obtained by the intercalation of short-chain alkylamines, such as methylamine or propylamine at 100% and 50% loading, respectively. This exfoliation process is shown schematically in Figure 11.

y-Zr(PO4) • (H2PO4) • 2H2O is best exfoliated when treated with dimethylamine. Colloidal dispersions containing highly anisotropic particles of nanoscale dimensions have a number of potential applications. After treatment with acids, flocculation allows the formation of completely inorganic pellicles or films useful in assembling the sensor layer of solidstate gas sensors, or to cover glass surfaces for chromatographic application. Composites of layered phosphates and silica gels or pillared layered phosphates have also been prepared from colloidal dispersions.

Solid dispersions of layered phosphates in silica gel

Solid dispersions of a- or y-zirconium phosphates in porous silica can be prepared starting from mixtures of a tetrapropylammonium oligosilicate solution and zirconium phosphates, previously exfoliated with amines. They are formed after gelification of the mixture with acetic acid and subsequent calcination at 650°C to remove the organic moieties. At this temperature zirconium phosphates are transformed into layered pyrophosphates, but non-condensed phosphate groups are still present on the free surfaces of the lamellae. Accordingly, the composites obtained have a large surface area (350-500 m2 g-1), good surface ion-exchange capacity and acid catalytic properties. Such composites may find application as stationary phases in chromatography.

Pillared layered phosphates The success obtained in the pillaring of clays to obtain microporous solids with larger pore diameters than those found in zeolites has stimulated research in preparing pillared layered structures based on metal(IV) phosphates. Synthetic strategy requires the insertion of large organic or inorganic cations (pillars) between the layers to prop them apart. If the pillars are sufficiently spaced, a microporous structure is obtained and the dimensions of the channels or diffusion paths are determined by the size of the pillars and their spacing in the interlayer region (see Figure 12).

Inorganic pillars are preferable to organic pillars because of their much higher thermal stability. To obtain thermally stable structures, pillaring has been performed with highly charged polyoxycations such as the M13 Keggin ion [AlnO4(OH)24(H2O) 12J , or [Zr(OH)2(H2O)4]!j+, or inorganic clusters such as [Nb6Cl12]2 +. After suitable thermal treatment, the layered phosphates contain as pillars, aggregates of inorganic oxides which have considerable thermal stability. The problem of inserting such large pillars has often been overcome by contacting the solution of the pillaring species with colloidal dispersions containing single layers, or packets of a few layers, of tet-ravalent metal(IV) phosphates. This provides access to the surface POH groups, the exchange reaction and the flocculation of the pillared material. However, the problem of achieving uniform pillar spacing to obtain a narrow distribution of micropores of predictable dimensions has not been completely resolved. The topic is of great interest since materials for molecular sieving and for shape-selective catalysis might result.

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