Porous chromia-pillared α-zirconium phosphate materials prepared via colloid methods

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Abstract
The reaction of Cr(CH3CO2)3[Cr(OAc)3] with colloidal n-propylammonium α-zirconium phosphate and subsequent calcination of the products have been investigated. Depending on [Cr(OAc)3] : [initial phosphate] ratios and Cr3+ concentrations, a series of polyhydroxy acetato-Cr3+ intercalated precursor materials can be obtained, in which topotactic interface reactions have occurred to give materials with interlayer distances (d002) ranging from 13.0 to 39.0 Å. These precursors show higher layer expansions than the analogous pillared clays (PILCS; d001= 16.8–27.6 Å). A model invoking ordered in situ polymerisation of the Cr(OAc)3 on the phosphate surfaces is put forward. Calcination of these precursors under N2(400 °C) leads to a series of chromia-pillared materials in which the interlayers do not collapse to a single (much lower) value, as found previously for most PILCS, but instead provide a wide range of interlayer distances (10–27 Å). These correspond to free heights of 3.5–20.5 Å, the widest ranging and highest yet found for such materials. These nanoscale oxide-pillared materials have N2 surface areas (B.E.T., 77 K) of 250–330 m2 g–1, with pore radii (cylindrical pore method) ranging from 8.5 to 13.8 Å, and very narrow pore-size distributions. Calcination conditions are crucial for obtaining porous solids. If calcination is carried out in air at 400 °C, although pillared powders and films are again obtained, surface areas are only ca. 40 m2 g–1(B.E.T., N2, 77 K). Furthermore, higher calcination temperatures (500 °C, under N2) give rise to X-ray amorphous materials, again having high surface areas and narrow pore-size distributions. All the materials can be processed in thin-film form without loss of textural characteristics.