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

Abstract

Copper has been used as an electron paramagnetic resonance (EPR) probe for the interlayer environment ofα-zirconium phosphate (α-ZrP) intercalated from hydrolysed/condensed 3-(2-aminoethylamino)propyl-trimethoxysilane solutions. These solids are precursors to porous silica-pillared solids. The EPR spectra of the Cu ²⁺ –aminosiloxane pillaring solution and of the intercalated solid prepared using this solution are identical. More detailed study of the solution has shown the existence of two types of EPR spectra, when the molar ratio Si/Cu ≥ 2 and when Si/Cu < 2. For the former, Cu ²⁺ has an environment of nitrogen and oxygen (4 + 2); at lower Si/Cu ratios, the EPR spectra are characteristic of an oxygen atom environment. On acidification of the aminosiloxane solution, nitrogen coordination to Cu ²⁺ is progressively replaced by oxygen, as the amino groups become protonated.