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

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.