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, N2 adsorption (BET), TEM/EDX, FTIR, XANES and EXAFS spectroscopy, H2 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 H2 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.

Ammoxidation of light hydrocarbons was tested over Co-ZSM-5 catalysts. The cobalt salt precursors had an important influence on the physico–chemical and catalytic properties. The preparation procedure results in the disappearance of Brönsted acid sites and the creation of new Lewis acid sites. Furthermore, TEM/EDX, H2-TPR and XRD indicated the presence of cobalt oxide when cobalt chloride, formate or nitrate was used. In fact, a cobalt phyllosilicate phase is present at the outer surface of the zeolite for the sample issued from cobalt acetate as precursor.Figure optionsDownload as PowerPoint slide