Effect Of Alkali Metals On Hyhrodesulfurization Performance Of Transition Metal Sulfides

In recent years, the production of clean diesel by hydrotreating and especially by deep hydrodesulfurization(HDS) has attracted increasing attention due to the introduction of new environmental legislations. 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 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), while in the hydrogenation (HYD) pathway, the aromatic ring is hydrogenated and the sulfur is removed subsequently to form cyclohexylbenzene (CHB). The hydrogenation of the aromatic ring of DBT, which releases the steric hinderance, will remarkably enhance the HDS reactivity of DBT and its alkylated derivates. Nevertheless, an increase in the catalyst hydrogenation activity also causes an increase in the hydrogen consumption, which is undesirable from an economic point of view. Hence, studies on both the activity and the BP selectivity of HDS catalysts are needed for developing better deep HDS catalysts.K₂O and Na₂O were introduced to transitional metal sulfide catalysts precursor by different impregnation method. The hydrodesulfurization (HDS) performances of the transitional metal sulfide catalysts were studied using a model fuel containing 0.8 wt% DBT in decalin. 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. As to CoMo catalysts, the activity of DBT HDS was maintained when K₂O was introduced by consecutive impregnation method, manwhile, the introduced of Na₂O by the same method caused a reduction of activity. Both Na₂O and K₂O can make a contribution to BP selectivity. Through the comparison we can found that consecutive impregnation was a preferable method to HDS acitivity and BP selectivity. For NiMo catalysts,1% introduction amount of Na₂O by consecutive impregnation method play positive effects on both enhanced HDS acitivity and BP selectivity. Nevertheless, neither the Na₂O nor K₂O can play positive effects on the HDS activity for NiW catalysts. The hydrogenation of 1-heptene was used as a probe reaction to investigate hydrogenation ability of catalyst. The results suggest that when the K₂O or Na₂O was introduced into the catalyst, the saturation of olefins was reduced, which means that when Na₂O-NiMo and K₂O-CoMo catalysts were used for HDS of gasoline, less octane number loss is expected. The XRD, UV-Vis results indicated that when Na₂O and K₂O were introduced by consecutive impregnation method, neither the distributions nor the coordination states of the active species were affected for C0M0 and NiMo catalysts, but the introduction of Na₂O into NiW catalyst can facilitate the formation of NiWO₄ crystal. It was except that the reducibility of catalyst precursors was decreased when Na₂O and K₂O were introduced according to TPR profiles.