ESR STUDY OF RH GAMMA-AL2O3 AND RH/HZSM-5 PROMOTED BY CU2+, GD3+, ANDPO4,(3-)/

In 2.2%, Rh/gamma-Al2O3, the vast majority of the Ph, >95%, is ESR ina ctive. The rest, a small proportion, can be made ESR-visible by room t emperature sorption of water or other polar molecules suggesting that the interaction of these ESR-active ions with the support is weak. An amorphous alumina support does not stabilize isolated Rh2+ ions even i n low-loaded 0.25% Rh/ Al2O3. BY contrast, quantitation of Rh2+ in ZSM -5 shows that the fraction of ESR-visible ions exceeds 50% for the mos t dilute 0.25% Rh/ZSM-5 sample. This makes low-loaded Rh/ZSM-5 samples useful model catalysts for in situ ESR studies. Rh2+ and Cu2+ are loc ated at the same cationic sites in H-ZSM-5 but the Cu2+ ion is bound m uch more strongly in this position. The majority of isolated paramagne tic Rh2+ is replaced by Cu2+ by solid-state reaction at high temperatu re. Rh2+ and Gd3+ migration and redistribution in ZSM-5 differ from th ose occurring in samples containing Rh2+ and Cu2+. It appears that Gd3 + ions occupy mainly non-cationic positions and do not impede the bond ing of paramagnetic Rh2+ in the cationic sites of ZSM-5. Strong bondin g of PO43- anionic ligands causes irreversible change in the local env ironment of both Rh2+ cations and Gd3+ ions in HZSM-5. (C) 1998 Elsevi er Science B.V.