DEUTERIUM TRACER STUDIES ON HYDROTREATING CATALYSTS - ISOTOPIC EXCHANGE BETWEEN HYDROGEN AND HYDROGEN-SULFIDE ON SULFIDED NIMO AL2O3/

The H-D isotopic exchange between H-2 and D-2 and between H-2 (D-2) an d D2S (H2S) was studied at 80 degrees C in a recycling reactor under a pressure of 2 bar in the presence of a sulfided NiMo/Al2O3 catalyst a nd by means of gas chromatography. From the H-2-D-2 exchange 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 H-2 at 400 degrees C and sweeping it by helium at 80 degrees C. H-D is otope exchange occurred between H-2(D-2) and D2S(H2S). As expected und er the reaction conditions, H2S(D2S) did not produce H-2(D-2) The isot ope exchange between H-2 and D-2 was about six times faster than the e xchange between H-2(D-2) and D2S(H2S). However, it was shown that H2S competed with H-2 for adsorption: H2S inhibited the exchange between H -2 and D-2. It is proposed that H-2(D-2) and H2S(D2S) dissociate heter olytically on the same sites, involving both a sulfur vacancy and a su lfur atom. Furthermore, it is suggested that the isotope exchange coul d occur by dissociation of both reactants on one single center provide d that this center possesses at least two coordinative unsaturations. (C) 1997 Academic Press.