Development of improved catalysts for deep HDS of diesel fuels

Deep hydrodesulfurization (HDS) of gas oils continues to attract research interest due to environmentaldriven regulations which limit its sulfur content to 10-15 ppm in several countries. This paper highlights some of the recent studies conducted at King Fahd University of Petroleum and Minerals to develop improved HDS catalysts. The first study was focused on the effect of Co/(Co + Mo) ratio in CoMo/Al2O3 catalysts on HDS pathways of benzothiophene (BT) and dibenzothiophene (DBT). Co/Co + Mo ratio exhibited significant influence on the direct desulfurization (DDS) pathway, but showed no influence on the hydrogenation pathway. A Co/Co + Mo ratio of 0.4 exhibited optimum promotion effect of Co for HDS by DDS route and hence overall HDS. The second study investigated the effect of phosphorus addition on simultaneous HDS reactions and their pathways. The results indicate that phosphorus modification of CoMo/cAl(2)O(3) catalysts resulted in enhancement of HDS due to increased dispersion of MoO3 and the maximum enhancement was achieved with 1.0 wt % P2O5. Enhancement of HDS rates was in the following order: 4,6DMDBT (51 %)[4-MDBT (38 %)[DBT (26 %). In the third approach, a series of NiMo catalysts supported on Al2O3-ZrO2 composites containing 0-10 wt % ZrO2 was synthesized, characterized and evaluated for deep desulfurization of gas oil. An increase of 1.3-2.5 times increase in HDS activity at 320-360 degrees C was observed due to reduced interaction between Al2O3 and the active metals. A correlation was found between the enhancement of hydrogenation activity of sulfided catalysts and the reducibility of their oxide precursors.