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 27 Al 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 SiO 4 tetrahdra that are not adjacent to surface AlO 4 tetrahedra. Models are put forward for the configuration of the adsorption sites and the location of the aluminum in the framework walls.