Abstract
The oxidative dehydrogenation (ODH) of cyclohexane with air was studied on a 10 w/w% V2O5/Al2O3 catalyst modified by different weight percentage (wt%) cerium loadings. The catalysts employed were characterized by N2 adsorption, O2 chemisorption, TEM, XRD and FT-IR spectroscopy. Benzene and cyclohexene were the main components of the products, with the benzene and cyclohexene selectivities depending on the wt% cerium loading, the order of cerium and vanadium impregnation, and the reaction temperature. The best cyclohexane conversion and good benzene selectivity were obtained at 500°C using a vanadium catalyst modified by 10 wt% cerium. The same catalyst exhibited a maximum selectivity for cyclohexene at 300°C.
Different types of vanadyl species present in the system were identified by IR measurements. Thus, vanadium-oxygen clusters were detected in the V2O5/Al2O3 material, although such clusters were not found in the cerium-vanadium catalysts. Polymeric CeVO4 species were detected in cerium-modified V2O5/Al2O3 catalysts by XRD. The concentration of these species depended on the wt% cerium loading and the order of cerium and vanadium impregnation on alumina. The selectivity of cerium-modified V2O5/Al2O3 catalysts towards cyclohexene was attributed mainly to the presence of such polymeric CeVO4 species. Oxygen chemisorption and electronic microscopic studies attributed the marked improvement in cyclohexane conversion of the cerium-vanadium samples to the presence of relatively high-dispersed vanadium species.