Inhibition Mechanisms Of Nitrogen Compounds For Hydrodesulfurization

The effective mechanisms of quinoline, indole and carbazole on hydrodesulfurization (HDS) were studied by use of HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and FCC light cycle oil on NiMo/Al2 O3 -HUSY and NiMo/Al2 O3 catalysts in the presence of N-containing compounds combined with characterization and analysis of catalysts.The surface properties and active phase structures of NiMo catalysts were characterized by BET, TPR, HRTEM, NH3-TPD and Py-IR. It is verified that the HUSY is favorable to enhance the stacking degrees and decrease the slab length of MoS2 stack in the NiMo/Al2 O3-HUSY catalyst. Moreover, the acid strength and the density of B acid sites of the catalyst increases, the density of L acid sites decreases. Therefore, the catalytic activity of the catalyst is improved. Specially, the modification in acidic properties of the catalyst enhances the transalkylation and disproportionation of sulfur compounds with steric hindrance, so that the hydrodesulfurization reactivity of 4,6-DMDBT is enhanced. The in-situ FT-IR identified that thiophene adsorbed on the NiMo/Al2O3 -HUSY catalyst isη1-coordinated to the catalyst surface at low temperature, while the adsorption of thiophene is changed toη5-coordinated at high temperature, and the cracking reaction of thiophene is occurred under the action of acidic sites on the catalyst.From the activities, kinetics and product distributions for HDS of 4,6-DMDBT in the presence of different concentrations of quinoline, indole and carbazole on NiMo/Al2 O3-HUSY and NiMo/Al 2 O3 catalysts, respectively, it is verified that the acidic sites on NiMo/Al2 O3-HUSY catalyst can catalyze the transalkylation and disproportionation of 4,6-DMDBT and its hydrogenated derivatives, so that the HDS reactivity and DDS(direct desulfurization) selectivity increase. Moreover, the HDS kinetics verify that the inhibition effects of nitrogen compounds for HDS of 4,6-DMDBT on NiMo/Al2 O3-HUSY and NiMo/Al2 O3 catalysts are in the order of: quinoline> carbazole>indole, which is not accordant to the order of adsorption equilibrium constants of those nitrogen compounds on the two catalysts. It is found from experimental that the inhibition mechanisms of organic nitrogen compounds for HDS and hydrodenitrogenation(HDN) may be as follows: quinoline and carbazole have strong inhibition for the acidic sites to catalyze isomerization and disproportionation reactions; there exist inhibitions of quinoline, indole and carbazole and their partially hydrogenated products for the breaking of C-N bonds; as well as there is presence of N-alkylated derivatives of indole produced through N-alkylation reaction catalyzed by the acidic sites.From the activities, kinetics and product distributions for HDS of FCC light cycle oil in the presence of different concentrations of quinoline, indole and carbazole on NiMo/Al2 O3-HUSY catalyst, respectively, it is verified that the reaction order for HDS of FCC light cycle oil was 1.7 at the reaction conditions. Moreover, the HDS kinetics verify that the inhibition effects of nitrogen compounds for HDS of FCC light cycle oil on NiMo/Al2 O3-HUSY catalyst are in the order of: quinoline> indole>carbazole, which is not accordant to the order for HDS of 2,4,6-TMDBT. Furthermore, the inhibition factors of quinoline, indole and carbazole for HDS of FCC light cycle oil are obviously weaker than those for HDS of 2,4,6-TMDBT, respectively. This may be explained as that the HDS of easily reactive organic sulfur compounds in the FCC light cycle oil can improve the HDN of organic nitrogen compounds, so that the inhibition effects of organic nitrogen compounds and their hydrogenated derivatives are reduced partially. From the change in HDS reactivites and distributions of DBT, 4-MDBT, 4,6-DMDBT and 2,4,6-TMDBT in the FCC light cycle oil during HDS in the presence of different concentrations of quinoline, indole and carbazole, respectively, it is concluded that the HDS reactivities of those organic sulfur compounds with steric hindrance are in the order of: DBT>4-MDBT>4,6-DMDBT>2,4,6-TMDBT. Besides, qunoline, indole and carbazole display extremely different inhibition effects for HDS of above sulfur compounds, that the inhibition effect of quinoline is very similar for 4-MDBT, 4,6-DMDBT and 2,4,6-TMDBT, the inhibition effect of indole increases with its concentration, carbazole has little inhibition effect.