Physico-chemical properties of mixed molybdenum and cerium oxides supported on silica-alumina and their use as catalysts in the thermal-catalytic cracking (TCC) of n-hexane

Mixed oxides, MoO3-CeO2, were being used as catalysts for the cracking (TCC) of liquid hydrocarbon feedstocks. The dispersion and interactions Of MoO3, CeO2 and mixtures thereof impregnated into the silica-alumina surface were investigated using several techniques, which included X-ray diffraction (XRD) and laser Raman spectroscopy (LRS). The loadings and the chemical states of metal oxides incorporated separately had significant effects on the catalytic activities of the resulting monocomponent catalysts. Addition of cerium to molybdenum had a favorable effect on the production of light olefins in the TCC of n-hexane up to a certain level of cerium loading. In fact, high loadings of molybdenum and/or cerium favored the formation of aromatics, instead. The catalytic performance of the bicomponent catalysts also depended significantly on the incorporation methods. It was found that the co-impregnation of MoO3 and CeO2, which led to the highest production of light olefins, corresponded to the formation of (surface) cerium molybdate to the highest extent. On the other hand, the catalysts prepared by the two-step impregnation methods (sequential and reverse sequential impregnation) showed much lower catalytic performance due to low Mo-Ce interactions as suggested by an important segregation of the active phases, Mostly MoO3. The sequence of catalytic performance (to the desired products, i.e. light olefins) fully coincided with that of the dispersion of molybdate species on the support surface. (c) 2006 Elsevier B.V. All rights reserved.