Nanostructured Inorganically Pillared Layered Metal(IV) Phosphates

Abstract

This article reviews the synthesis, characterization and ion-exchange, ion-transport, sorptive, and catalytic properties of inorganically pillared layered metal(IV) phosphates, typified by Zr(HPO4)2‚H2O. Porous nanostructures are generally prepared from metal(IV) phosphates either by ion exchange of polynuclear species or by intercalation from solutions of condensed species obtained by the hydrolysis of organometallic precursors using sol-gel methods, followed by thermal treatment to eliminate organic moieties, condense hydroxyl groups, eliminate water, and consolidate the structure by grafting the pillar to the layer. The different strategies devised to overcome the problem presented by the high layer charge density of R- and A-structured phosphates in obtaining porous solids, including exfoliation and local surface growth of pillaring ions, and modification of the zirconium phosphate matrix to reduce the cation-exchange capacity, are described. Structural and textural characteristics of Al, Cr, mixed Al-Cr, Fe-Cr, Ga-Al and of Si-pillared phosphates obtained from XAFS, XPS, and MAS NMR are presented, and the perspectives of nanocomposite pillared layered solids in general are discussed in the current context of mesoporous solids synthesized using templating routes.