Electron spin resonance and electron spin echo modulation studies of Fe-containing mesoporous MCM-48 molecular sieves

Abstract

Through electron spin resonance (ESR) and electron spin echo modulation (ESEM) studies, three Fe(III) species are found in liquid ion-exchanged Fe-MCM-48 and synthesized FeMCM-48. Species A with ESR parameters gA = 4.30 with a line width of 160 G is observed in hydrated samples. This species is accompanied by a shoulder atg = 7.60 and is assigned to distorted tetrahedral Fe(III) likely located at framework defect sites. Species B is a broad peak centered at gB = 2.00 and is assigned to distorted octahedral Fe(III) located at extraframework sites. This species is also observed in dehydrated samples. Species C is a very narrow isotropic signal at gC = 4.30 with a line width of 30 G and is only observed in synthesized mesophase materials. It is consistent with rhombic Fe(III) with zero field parametersD andE corresponding toD/E =1/3 and is assigned to tetrahedral Fe(III) located at framework sites. Framework Fe(III) in FeMCM-48 is less accessible to ND3 than is extraframework Fe(III) in Fe-MCM-48. On the basis of deuterium ESEM data, framework Fe(III) in synthesized FeMCM-48 directly coordinates to two D2O or CH3OD molecules and interacts more weakly with two more molecules. However, no deuterium ESEM is observed for extraframework Fe(III) in ion-exchanged Fe-MCM-48 with adsorbed D2O or CH3OD. It is also found that Fe(III) is incorporated into the framework of FeMCM-48 at an early stage of the formation of the MCM-48 mesostructure.