Effect Of Alkali Metals And Its Introduction Sequence On The Hydrodesulfurization Performance Of Transition Metal Sulfides

In recent years, much attention has been paid to air pollution and production of clean diesel by deep hydrodesulfurization(HDS) has attracted increasing attention due to the implemen-tation of more stringent specifications in many countries. Among all sulfur-containing compounds present in diesel, dibenzothiophene(DBT) and its alkylated derivatives are the most refractory compounds to desulfurize due to steric hindrance. The HDS of DBT proceeds through two parallel pathways. In the hydrogenation (HYD) pathway, the aromatic ring is hydrogenated and the sulfur is removed subsequently to form cyclohexylbenzene (CHB), while in the so-called hydrogenolysis (HYG) pathway, the sulfur atom is directly removed from the molecule via C-S bond cleavage, leading to the formation of biphenyl (BP). We can conclude that the HYD pathway needs more hydrogen consumption from the contrast. Hence, studies on both the activity and the BP selectivity of HDS catalysts are needed for developing better deep HDS catalysts.Na2O and K2O was introduced to transitional metal sulfide catalysts precursor by different sequences, and the hydrodesulfurization (HDS) performances of the prepared catalysts were studied using a model fuel containing 0.8 wt% DBT in decalin. The hydrodesulfurization (HDS) performances of the transitional metal sulfide catalysts were studied using a model fuel containing 0.8 wt% DBT in decalin. As to NiMo catalysts, the XRD and TPR results indicated that introduction of Na2O and K2O not only facilitated the formation ofβ-NiMoO4, leading to decrease in dispersion of active species, but also hindered the reducibility of NiMo/MCM-41 catalyst precursor. The UV-Vis results indicated that both the distributions and the coordination states of the active species were affected by introduction of Na2O and K2O to NiMo/MCM-41. The results show that the HDS of DBT predominately takes the route of direct desulfurization (DDS) over the transitional metal sulfide catalysts when K2O and Na2O were introduced.The HDS activity of NiMo/MCM-41 was strongly affected by the Na2O introduction sequence. For the catalyst in which Na2O was introduced by the co-impregnation method, both the activities of the hydrogenation pathway (HYD) and direct desulfurization pathway (DDS) were inhibited, resulting in a decrease in the overall HDS activity. The addition of Na2O before the loading of the active components showed a minimum influence on the HYD activity of NiMo/MCM-41, but significantly enhanced its DDS activity, leading to the improvement in the overall HDS activity and decrease in the H2 consumption.The K2O introduction sequence has the same effect as Na2O, a decrease in the catalyst hydrogenation activity and increase the BP selectivity are caused by increase the alkaline of catalyst. Nevertheless, neither the Na20 nor K2O can play positive effects on the HDS activity for NiW catalysts.The results suggested that both the activity and selectivity of the sulfide catalysts can be modified by controlling the addition sequence of the alkali-metal oxides, which is showed to be an effective way for the modification of the catalysts.