Effect of fluorine on hydrogenation of cyclohexene on sulfided Ni (or Co) [sbnd]Mo/Al 2O 3 catalysts

Abstract

The effect of fluorine incorporation on alumina support on the surface structure of unpromoted molybdenum, promoted cobalt—molybdenum and nickel—molybdenum catalysts, and their activity for hydrogenation of cyclohexene has been studied. The incorporation of 0.2 and 0.8 wt.-% fluorine on the alumina was carried out by impregnation with NH 4F solutions. The catalysts in the oxidic state were characterized by X-ray diffraction and ammonia adsorption and in the sulfide state by X-ray photoelectron spectroscopy (XPS) and infra-red spectroscopy (IR) of adsorbed NO. The absence of significant changes in the binding energy values of Mo3d and Ni2p (or Co2p) levels in the XPS spectra of the fluorine-containing catalysts as compared to the fluorine-free counterpart does not support the existence of an electronic effect of fluorine. The quantitative XPS results showed, however, that fluorine clearly increases the dispersion of molybdenum and promoter, this being linearly correlated to surface fluorine content. The IR results of adsorbed NO also indicate that fluorine incorporation leads generally to minor sizes of MoS 2 slabs, and more exposed promoter atoms, except for the cobalt in the Co Mo/F(0.2)A catalyst. It is suggested that the increase in the dispersion of the supported active phase is a secondary effect of fluorine incorporation, which may result from the observed textural changes of the alumina and its small partial solubilization provoked by NH 4F solution. It was found that the incorporation of fluorine enhances appreciably, moderately and considerably the hydrogenation activity of molybdenum, cobalt—molybdenum an nickel—molybdenum catalysts, respectively. Such increase in hydrogenation activity is not directly correlated to the exposed atoms probed by NO adsorption, and is only loosely related to molybdenum dispersion for the molybdenum and cobalt—molybdenum catalysts. The lack of similar reliable correlations for the nickel—molybdenum catalysts suggests that other structural parameters such as extent of reduction-sulfidation and certain configurations of molybdenum ions and sulfur vacancies may govern hydrogenation activity.