Electron paramagnetic resonance and electron spin echo modulation study of surface sites of the porous aluminosilicate MCM-41 using transition metal ion probes

The paramagnetic cations, Cu(II), Mo(V), and Cr(V) have been adsorbed or grafted onto the surfaces of the mesoporous aluminosilicate MCM-41 containing 5 mol % Al. Electron paramagnetic resonance and electron spin echo modulation spectroscopies were used to probe for a possible interaction between the adsorbed paramagnetic cations and Al-27 nuclear spins contained within the MCM-41 wall structure. Limited evidence has been found for the binding of the probe cations to tetrahedral Al sites in the MCM-41 structure, suggesting that the incorporation of Al in MCM-41 does not produce genuine exchange sites similar to those observed in ion exchangers such as zeolites and clay minerals. It is suggested that adsorbed cations are mostly bound to SiO4 tetrahdra that are not adjacent to surface AlO4 tetrahedra. Models are put forward for the configuration of the adsorption sites and the location of the aluminum in the framework walls.