Studies On The Catalysts For Aromatization Of Ethylene

Recently,the demand for aromatics has increased greatly due to their important roles in the chemical industry.On the one hand,the shortage of oil resources puts the conventional technologies for producing aromatics at a disadvantage position,which aggravates imbalance between supply and demand of aromatics.On the other hand,dry gas which is produced from the FCC process,contains plenty of ethylene and because of various restrictions on the conditions,it is difficult to achieve the effective utilization of ethylene resources.As a consequence,in order to make full use of these resources,it is a feasible route to convert ethylene to aromatics by aromatization.For the sake of providing theoretical basis for dry gas aromatization,we research the pure ethylene aromatization in this paper.This thesis investigated the influences of the reaction conditions,transition metal modification and catalyst acidity on the ethylene aromatization.These experiments were investigated on the HZSM-5 catalysts and on a fixed micro-activity unit.Besides,X-ray diffraction?XRD?,nitrogen adsorption-desorption,Fourier transform infrared?FT-TR?and ammonia-temperature programmed desorption were used to characterize the structure and acidity of catalysts.Firstly,we researched the influences of reaction conditions on the ethylene aromatization in the HZSM-5 catalyst.Results showed that,the optimal reaction temperature was 550 ? and the optimal reaction velocity was 0.69h-1.Then we discussed the reaction pathway of ethylene aromatization products.Among the transition metals which were used to modify HZSM-5 catalysts to enhance the activity of aromatization,zinc and nickel modified catalysts had the best catalytic performances.The ionic structure of [M?OH?]+ which was formed by an exchange between protons of Br?nsted sites and Zn²⁺ or Ni²⁺,belonged to Lewis sites due to its capacity accepting lone-pairs electron.Since the introduction of Zn²⁺ or Ni²⁺ increased the Lewis sites,decreased the Br?nsted sites and modulated L/B ratio,the activity of dehydrogenation which was catalyzed on Lewis sites,was improved greatly.As a result,the activity of aromatization was enhanced significantly because of improvement of dehydrogenation,which was the limit step during ethylene aromatization.We determined that the optimal loading of Zn²⁺ or Ni²⁺ was 1.5wt%,at which the ratio of L/B reached to the maximum and the catalytic performance was best.We modulated the acidic properties of catalysts by adopting different Si O2/Al2O3 ratio HZSM-5 zeolites and treating HZSM-5 catalysts under different hydrothermal degree,and investigated the effects of acidic properties on the aromatization.We found that the activity of aromatization didn't decrease with the decline of acidity.On the contrary,the catalysts still had high aromatization activity in the case of enlarging the L/B ratio,which demonstrated the L/B ratio playing an important role as the acid density of catalysts.Besides,appropriate hydrothermal treating temperature and time could optimize the acidic properties of catalysts,increase the L/B ratio and strengthen the activity of aromatization.The catalyst showed the best activity of aromatization when the hydrothermal treating temperature was 400 ? and hydrothermal treating time was three to four hours.