Framework coordination of single-ion Cu2+ sites in hydrated 17O-ZSM-5 zeolite

Abstract

Single metal ion sites supported on zeolites are an interesting topic both in basic research and in applied, heterogeneous catalysis. Of particular interest is the interfacial coordination chemistry of the metal i.e. the formation of the inner-sphere coordination complex with framework atoms and/or coordinating molecules. In this work, we explore the interfacial coordination chemistry of single-ion Cu²⁺ sites, assessing the nature of the Cu–O bond with coordinating oxygen donor atoms of the framework and water molecules. Isolated Cu²⁺ species are prepared in ZSM-5 through oxidation of the corresponding Cu⁺ species, introduced via gas phase reaction, and then hydrated. Pulsed ENDOR and HYSCORE spectroscopies together with selective ¹⁷O isotopic labelling of the oxide ions, either as framework oxygen or belonging to solvating water, are applied in situ to gain an atomistic structural description of the paramagnetic species formed at the interface and an insight into the properties of the Cu–O bonds. We prove that single-ion Cu²⁺ sites adopt a square planar coordination geometry, maintain a strong interaction with the zeolite framework even in presence of solvating water molecules and that the Cu–O bond has a non-negligible covalent character. Further, we compare the hyperfine data obtained for Cu²⁺–¹⁷O with VO²⁺–¹⁷O and Zn⁺–¹⁷O and rationalise the observed trends within the framework of ligand field theory providing useful criteria for the interpretation of hyperfine data. The level of detail afforded by ¹⁷O labelling and EPR techniques is remarkable in the understanding of the interfacial coordination chemistry of single-ion sites stabilised by zeolites.