DEUTERIUM TRACER STUDIES ON HYDROTREATING CATALYSTS - 2 - CONTRIBUTION OF THE HYDROGEN OF THE ALUMINA SUPPORT TO H-D EXCHANGE

H-D isotopic exchange between H-2 and D-2 was carried out at 80 degree s C in a recycling reactor under a pressure of 2 bar over unsupported MoS2 as well as on alumina supported Mo or NiMo sulfide catalysts. H-2 -D-2 experiments carried out over a commercial NiMo/Al2O3 catalyst, to which were mixed different amounts of the alumina support, and over a series of Mo/Al2O3 catalysts containing different amounts of Mo (4.4 to 14.8 wt%) showed that part of the exchangeable hydrogen detected by isotopic dilution came from the added alumina or from the support. Th is amount of hydrogen on the alumina was estimated for each series, to be respectively 22.1 x 10(-4) and 17.8 x 10(-4) mol H . g(-1). Since under the same conditions H-D exchange between H-2 and D-2 did not occ ur at an appreciable rate on the alumina alone, it is concluded that h ydrogen atoms from structural hydroxyl groups of the alumina or from h ydroxyl groups resulting from the dissociation of H2S were able to mig rate from grain to grain and to exchange with H-2 (D-2) dissociated on the sulfide active phase. The hydrogen atoms supposedly migrate as pr otons by jumping from oxygen to oxygen atoms on the alumina surface or are conveyed through the gas phase by H2S (the ''shuttle molecule'') which can adsorb dissociatively both on the alumina support and on the active phase. D-2 adsorption followed by FTIR under similar condition s confirmed that the presence of the sulfide phase was necessary, unde r the conditions of the experiments, to make the isotopic exchange bet ween H-2 (D-2) in the gas phase and the hydroxyls of the alumina possi ble. OD bands were difficult to detect on sulfided alumina alone, even after 5 h of exposure to D-2, whereas a strong OD band appeared very rapidly on sulfided NiMo/Al2O3. These results are considered as being in favour of a heterolytic splitting of H-2 On sulfide catalysts. (C) 1998 Academic Press.