ESR Study of Cu Centers Supported on Al-, Ti-, and Zr-Pillared Montmorillonite Clays

Abstract

Cu-doped Al-, Ti-, and Zr-pillared montmorillonites obtained by various procedures have been investigated with the aid of electron spin resonance (ESR). The major species present in pillared montmorillonites exchanged with copper at low pH are physisorbed square planar tetraaquo-complexes [Cu(H2O)4]2+. Only at the surface of uncalcined Al pillars does a chemisorption of copper ions occur, giving an inner-sphere square planar complex [Cu(AlO) n (H2O)4- n] x. Calcination of the samples at 673 K brings about a significant change of the Cu2+ ion environment reflected in the change of ESR parameters. The most characteristic feature is the increased covalency of the Cu–O in-plane σ bonding due to the attachment of copper to lattice oxygens. The spectra show that cupric ions form links with the pillars rather than with the silicate sheet. In the case of Al- and Ti-pillared samples, copper species of different degrees of σ in-plane covalency are obtained depending on the preparative procedure. Catalytic tests with hydroxylation of phenol demonstrate that copper centers of higher covalency show superior catalytic performance. This effect is interpreted in terms of the higher in-plane σ covalency facilitating electron transfer between the donor and the acceptor sites at the catalyst surface. The ESR spectra of almost all copper-exchanged samples show, besides the isolated Cu2+ ions, a component due to the exchange-coupled clustered cupric ions.