Influence of the cobalt salt precursors on the cobalt speciation and catalytic properties of H-ZSM-5 modified with cobalt by solid-state ion exchange reaction

Co-ZSM-5 catalysts prepared by either aqueous or solid-state ion exchange using different cobalt precursors (acetate, chloride, nitrate or formate) were studied in the selective ammoxidation of ethylene and ethane to acetonitrile. Co2+ supported catalysts were characterized by chemical analysis, X-ray powder diffraction, N-2 adsorption (BET), TEM/EDX, FTIR, XANES and EXAFS spectroscopy. H-2 TPR and TPD of ammonia. It was found that all the preparations led to cobalt exchanged metal ion at the bridging oxygen of Si-OH-Al groups or to Co oxide or phyllosilicate. XRD, TEM and H-2 TPR indicated the presence of cobalt oxide when cobalt chloride, formate or nitrate was used. TPD of ammonia and FTIR of pyridine, used respectively as probe molecules showed that solid-state exchange catalysts exhibited new Lewis acid sites. The catalyst resulting from cobalt acetate, which contained mostly isolated Co2+ ions in cationic exchange position and cobalt phyllosilicate, showed high activity and selectivity toward acetonitrile. (C) 2009 Elsevier B.V. All rights reserved.