Deuterium Tracer Studies on Hydrotreating Catalysts—Isotopic Exchange between Hydrogen and Hydrogen Sulfide on Sulfided NiMo/Al2O3

Abstract

The H–D isotopic exchange between H2and D2and between H2(D2) and D2S (H2S) was studied at 80°C in a recycling reactor under a pressure of 2 bar in the presence of a sulfided NiMo/Al2O3catalyst and by means of gas chromatography. From the H2–D2exchange an amount of three exchangeable surface hydrogen atoms retained by the catalyst per Mo atom was estimated after sulfiding the catalyst by 10% H2S in H2at 400°C and sweeping it by helium at 80°C. H–D isotope exchange occurred between H2(D2) and D2S(H2S). As expected under the reaction conditions, H2S(D2S) did not produce H2(D2). The isotope exchange between H2and D2was about six times faster than the exchange between H2(D2) and D2S(H2S). However, it was shown that H2S competed with H2for adsorption: H2S inhibited the exchange between H2and D2. It is proposed that H2(D2) and H2S(D2S) dissociate heterolytically on the same sites, involving both a sulfur vacancy and a sulfur atom. Furthermore, it is suggested that the isotope exchange could occur by dissociation of both reactants on one single center provided that this center possesses at least two coordinative unsaturations.