Electron spin resonance studies of copper complexes incorporated in zeolites and mesoporous molecular sieves

Abstract

Different zeolites like ZSM-5, NaY, and mesoporous MCM-41 materials have been synthesised. Two strategies were used to incorporate two different organic copper complexes (pyridine, ethylenediamine) into the porous materials: (i) incorporation of the cupric ion by ion-exchange and the subsequent co-ordination of the organic ligand molecules by diffusion; and (ii) incorporation of the complex itself into the molecular sieves. To characterise the interactions between the host–guest system and to study the symmetry of the complexes the electron spin resonance (ESR) and the electron spin echo envelope modulation spectroscopy (2D ESEEM) were applied. The results show that it is possible to incorporate copper complexes by both ion exchange and incorporation procedures in all studied zeolite/molecular sieves. Within the MCM-41 molecular sieve it could show that the copper pyridine complexes are not attached to the inner surface of the mesopores in samples where solvent molecules are still present. On the other side, the copper complexes can be immobilised by washing and drying the zeolite/molecular sieve material at ambient temperatures. Some of the complexes form agglomerates that might suggest the existence of diffusion barriers such as stacking faults in the MCM-41 structure.