Résumé
The catalytic performance of Cr-ZSM-5 catalysts prepared by solid-state reaction from different chromium salts (acetate, chloride, nitrate and ammonium dichromate) was evaluated in the selective ammoxidation of ethylene to produce acetonitrile. Cr-ZSM-5 catalyst characterization by chemical analysis, XRD, FTIR, N2 physisorption, 27Al MAS NMR, TEM/EDX, UV-visible DRS, XPS, Raman and DRIFT spectroscopies and H2-TPR is reported. From textural analysis, it is obvious, after chromium exchange, that a decrease of SBET and of porous volume is observed. On the other hand, FTIR, XRD and 27Al MAS NMR results show that there is no collapse of the parent zeolite during thermal treatment. From H2-TPR, DRS, Raman and XPS spectroscopies, it is concluded that Cr ions and Cr(III) oxide coexist. TEM/EDX results confirmed that chromium oxide species are either heterogeneously or homogeneously dispersed on the support, depending on the nature of the precursor. DRIFT showed that chromium exchange leads strictly to the consumption of Si-O+H-Al groups. The catalyst prepared from chromium chloride showed higher activity and selectivity towards acetonitrile, being this catalytic modification probably due to the differentiated nature of chromium species formed using this precursor. A similarity with Fe-ZSM-5 system is underlined. In fact, higher oxidation states of chromium species are required for ethylene ammoxidation, while Cr2O3 clusters seem to enhance the hydrocarbon oxidation.