Characterisation of calcined silica-pillared zirconium phosphate: a Cu2+ epr study

Silica-pillared zirconium phosphate shows both micro- and mesoporous charac ter, with an average pore dimension of 23 Angstrom after calcination at 480 degreesC. Cu2+ coordinates in solution to the molecular precursor of the s ilica pillars, 2-aminoethyl-3-aminopropylsiloxane oligomers, and is carried into the interlayer region on contacting this solution with a-zirconium ph osphate, where it serves as a probe for the evolution of the intercalated p hase to a porous pillared solid on calcination. Electron paramagnetic reson ance spectroscopy shows that the environment of Cu2+ changes from (4 + 2) n itrogen + oxygen to one containing oxygen only as the organic functions are eliminated. Above 400 degreesC, (the temperature at which the material sta rts to exhibit appreciable surface area) and up to 600 degreesC, the e.p.r. spectrum remains unchanged, and these limits define the temperature region in which the silica-pillared phosphate is stable. Two copper sites are ide ntified in the pillared phase. These are considered to correspond to one in which copper is surrounded by oxygen atoms of the silica pillars, suggesti ng that part of the Cu2+ forms an integral part of the pillars, and a secon d related to Cu2+ occluded in the pores and accessible to incoming species such as water. (C) 2001 Elsevier Science B.V. All rights reserved.