Sulfur Removal Of FCC Gasoline On Modified Nanoscale HZSM-5 Catalysts

Fluid catalytic cracking(FCC) gasoline contributes about 80%of the total gasoline pool in China.For high content of olefins and sulfur,the FCC gasoline provides over 95%sulfur and almost all of the olefins for the gasoline pool.The gasoline sulfur could be reduced effectively by using traditional hydrotreating catalysts,such as Ni-Mo/Al₂O₃.But there would be a great loss of RON for the decrease of olefins in the hydrotreating process.ZSM-5 zeolites are widely used in transforming olefins and light alkanes to aromatics and iso-paraffins for excellent aromatization,isomerization and alkylation activities.Nanosize ZSM-5 zeolites show more stable and higher catalytic activity because of smaller crystals, shorter micropore,more micropores and more acid sites at external surface.The catalytic activity of HZSM-5 could be promoted by supporting rare earth metal oxides and transition metal oxides,such as Ni and Mo.In this article,catalysts were prepared by supporting NiSO₄, Ni(Ac)₂ and Ni(NO₃)₂ over nano-HZSM-5.Investigations were performed to evaluate the catalytic performance for the upgrading of FCC gasoline.Effect of rare earth metal oxides modification on desulfurization activity of HZSM-5 and NiSO₄/HZSM-5 catalyst was studied. And the effect of SO₄^2- loss upon desulfruization activity of regenerated industrial catalyst FDO,which was prepared by supporting NiSO₄ on HZSM-5,was also studied.Meanwhile, effect of MoO₃ modification on desulfurization activity of Ni-La/HZSM-5,which was prepared by supporting La₂O₃ and NiSO₄ over HZSM-5,was also studied.Based on all above investigations,the following conclusions were reached:1.The incorporation of Ni(Ac)₂ and Ni(NO₃)₂ into the HZSM-5 catalysts improved the stability of desulfurization.And the incorporation of NiSO₄ increased not only the desulfurization activity,but also the desulfurization stability largely.γ-Al₂O₃ and SiO₂ were chosed as the supports to prepare catalysts by impregnating with NiSO₄,Ni(Ac)₂ and Ni(NO₃)₂.And the results showed that there was a strong interaction between S=O in SO₄^2- and alumina oxides in both HZSM-5 zeolites andγ-Al₂O₃ supports. The strong interaction could bring following effects:(1) creates a large amount of Lewis acid sites and enhances the ratio of Lewis acid sites to Brφnsted acid sites(L/B);(2) help Ni species disperse in monolayer on catalyst;(3) prevents the forming of NiAl₂O₄ between NiO and Al₂O₃;(4) increases the adsorbing capability for thiophene impurities.Although a large of amount of lewis(L) acid sites,which was strong enough,was created on SO₄^2-/Al₂O₃,there was little desulfurization activity on the catalyst.This was because Brφnsted(B) acid sites were indispensable for the cracking of thiophenic species over acidic catalyst.2.Thiophene+cyclohexane,in which the sulfur content was 1000μg/g,was used as a model of gasoline.The study showed that the blending of 20 v%1- octene into thiophene +cyclohexane model was beneficial to desulfurization activity and stability.This was becaused that a large amount of active hydrogen was generated in the aromatization process of 1-octene and cyclohexane on HZSM-5 catalyst.And the active hydrogen could accelerate the desulfurization activity and stability.3.The modifications of La₂O₃ and CeO₂ to HZSM-5 catalysts decreased the amount of strong B acid sites of HZSM-5.The decrease of strong B acid sites inhibited thiophene from transforming into thiophene derives and increased the selectivity of H₂S.HZSM-5 modified by bimetals of La₂O₃ and NiSO₄ or CeO₂ and NiSO₄ showed that the strong interaction between NiSO₄ and HZSM-5 catalysts was weakened.The decomposition tempreture of NiSO₄ on HZSM-5 catalyst decreased from 928℃to 827℃.4.Industrial catalyst FDO was prepared by impregnating NiSO₄ into HZSM-5 and was used to transform olefin into aromatics.The deactivated FDO was sampled from the industrial unit(200 kt/a) after 5400 h.The regeneration of deactivated FDO showed that the aromatization and de-olefin activities recovered basically after carefully coke calcinations. But the desulfuriaztion activity decreased about 20%.This was because that there was a loss of SO₄^2- in RFDO catalyst after 5400 h run.And part of NiSO₄ was transformed into NiO after calcinations.The loss of SO₄^2- weakened the interaction between S=O and HZSM-5 catalyst.This led to a dcrease of total amount of acid and smaller L/B.The reloading of proper amount of NiSO₄ could recover the desulfurization activity.5.La₂O₃ and NiSO₄ were supported on nano-HZSM-5 to prepare Ni-La/HZSM-5 catalyst.The incorporation of MoO₃ into Ni-La/HZSM-5 catalyst increased the desulfurization activity largely,especially the desulfurization activity for thiophene and benzothiophene,which were difficult to be removed by cracking method on HZSM-5.The formation of Ni-Mo-S,which exhibited higher hydrogenating activity,was supposed to be the main cause.The results of 3%Mo-Ni-La/HZSM-5 also indicated that although the desufurization ratio was over 90%,the olefin content in product decreased from 34.0 v%to 14.2 v%.Meanwhile,the aromatic content increased from 31.4 v%to 38.0 v%.This led to a loss of RON,which was about 1.8.