Beneficial effect of carbon on hydrotreating catalysts

The role of carbonaceous species on the activity of hydrotreating catalysts has been controversial for a long time. On one hand, carbon deposit is known to be one cause of catalyst deactivation, and on the other, the presence of organic thiocompounds (DMDS, tertiononyl-pentasulfide, etc.) or gas-oil during the sulfidation is recognized to provide an activity enhancement. In this work, we attempted to investigate the effect of carbon on the HDS properties of an industrial CoMo catalyst. For this purpose two carbon-containing catalysts were compared to a carbon-free catalyst. Carbon introduction was performed either by impregnating the oxide precursor with a C20 gas oil cut and subsequent drying, or by contacting the precursor with a solution of a commercial coleseed oil and further coking. After activation by H-2/H2S, the catalytic properties of the catalysts were evaluated in thiophene conversion and real feedstock desulfurization. In both cases, and for the two preparations, a positive effect of C was observed. Several characterizations were performed (TEM, XPS, TPO, BET-BJH, EXAFS) in order to determine the role of carbon, starting from several assumptions. From these results, it was concluded that at least one identifiable beneficial effect of carbon is a geometrical one. However, structural effects may not be excluded. (C) 2002 Elsevier Science (USA).