UV−Vis and FT-IR Study of the Nature and Location of the Active Sites of Partially Exchanged Co−H Zeolites

Abstract

A study of the nature and accessibility of internal and external protonic and cationic sites has been carried out on protonic and partially Co-exchanged FER, MFI, and MOR through the use of UV−vis and FT-IR spectroscopy. The ion-exchange procedure was exactly the same for the three zeolites. A sample of Co-exchanged silica−alumina has also been investigated for comparison. UV−vis spectra recorded after outgassing at 773 K provide evidence of the presence of low-coordination Co2+ sites in all outgassed Co-containing samples. However, they show that the sites located in the zeolite cavities cannot be distinguished from those located at the open surfaces. FT-IR studies of the adsorption of benzonitrile and o-toluonitrile, used as differently hindered weak basic molecules, allowed us to distinguish sites located at the external surfaces or in different cavities. Benzonitrile enters only the main channels of H−MOR and Co−H−MOR, whereas it is not able to enter the mouths of the side pockets. Benzonitrile also e...