Synergy between vanadium and molybdenum in bimetallic ZSM-5 supported catalysts for ethylene ammoxidation

Abstract

Ammoxidation of ethylene to acetonitrile was studied on V/ZSM-5, Mo/ZSM-5 and V–Mo/ZSM-5 catalysts prepared by a solid-state ion exchange method. The physico-chemical properties were investigated by means of XRD, N₂ physisorption, ²⁷Al and ²⁹Si MAS NMR, UV-Vis DRS, XPS, pyridine-IR and FT-IR spectroscopies and H₂-TPR/O₂-TPO. Based on the characterization results, M–O_x (M = V or Mo) species react with zeolite protons during the exchange process and generate new Lewis acid sites, which act as redox centers. The M–O_x species are essentially, monomeric and dimeric/polymeric species or metal oxide crystallites (less than 4 nm) highly dispersed in the channel and/or on the external surface of zeolite. For the Mo/ZSM-5 sample, the formation of Al₂(MoO₄)₃ nano-crystallites was observed. UV-Vis DRS and TPR results showed that V and Mo species in all catalysts are mainly in the highest oxidation states. The V–Mo/ZSM-5 catalyst exhibited a more reversible behavior of the M–O_x centers throughout the H₂/O₂ redox cycles than those in V/ZSM-5 and Mo/ZSM-5 catalysts. The best catalytic performance was achieved over the bimetallic V–Mo/ZSM-5 catalyst. These results revealed that the partial substitution of molybdenum with vanadium has a positive effect on the activity and the selectivity to acetonitrile. This implies clearly that a synergetic effect between V and Mo species plays an important role in the ammoxidation reaction. This synergetic effect is related to the existence of electronic interaction at short range order between the V and Mo species, which may influence the catalyst redox properties.