Use of a Cu2+ probe to characterise silica-pillaring in zirconium phosphate by electron paramagnetic resonance spectroscopy

Copper has been used as an electron paramagnetic resonance (EPR) probe for the interlayer environment of alpha-zirconium phosphate (alpha-ZrP) interca lated from hydrolysed/condensed 3-(2-aminoethylamino)propyltrimethoxysilane solutions. These solids are precursors to porous silica-pillared solids. T he EPR spectra of the Cu2+-aminosiloxane pillaring solution and of the inte rcalated solid prepared using this solution are identical. More detailed st udy of the solution has shown the existence of two types of EPR spectra, wh en the molar ratio Si/Cu greater than or equal to 2 and when Si/Cu<2. For t he former, Cu2+ has an environment of nitrogen and oxygen (4+2); at lower S i/Cu ratios, the EPR spectra are characteristic of an oxygen atom environme nt. On acidification of the aminosiloxane solution, nitrogen coordination t o Cu2+ is progressively replaced by oxygen, as the amino groups become prot onated.