CHROMIA PILLARING IN ALPHA-ZIRCONIUM PHOSPHATE - A STRUCTURAL INVESTIGATION USING X-RAY-ABSORPTION SPECTROSCOPY

An X-ray absorption spectroscopic (XAS) study has been carried out on the intercalated precursor polyhydroxy (acetato)chromium(III) alpha-zi rconium phosphates, a chromia-pillared derivative obtained by calcinat ion at 400 degrees C, and the amorphous congener obtained after calcin ation at 800 degrees C. The precursor solids contain interlayer hydrox y-Cr-III clusters with connnectivities of a flat dimer (12 Angstrom ph ase), a closed trimer (17 Angstrom phase), an open tetramer (34 Angstr om, phase), and also contain a 25 Angstrom phase with connectivity bet ween trimer and tetramer. The acetate residues little influence the XA FS analysis, and orientations of the cluster species present between t he layers are suggested on the basis of double-layer structures. The c hromia-pillared material obtained after calcination of the 34 Angstrom phase has a ribbonlike structure comprising pseudo-octahedral [(CrO6) -O-III] units showing Cr---P distances (3.25 Angstrom), as expected fo r coordination of the pillar to the phosphate layer. There is no evide nce for Cr-VI formation, unlike the oxidation that occurs on calcinati on in montmorillonite analogues. Amorphization at 800 degrees C causes rearrangement, and the pillar is best described as a comer-shared oxi de-phosphate nanoparticle with contributions from bidentate PO43- grou ps. The implications for the thermal stability of oxide-pillared mater ials prepared using current pillaring methods are discussed.