Study On The Aromatization Catalyst For Full-range FCC Gasoline Upgrading

The effect of physicochemical properties of ZSM-5, hydrothermal treatment and metal modification conditions on the olefin transformation were investigated over nanoscale HZSM-5. The interaction effect of olefin and sulfur compounds on the aromatization and hydrodesulfurization activities was studied. The ZnLaP/HZSM-5aromatization catalyst and the two steps process applying the sequence that aromatization followed by desulfurization were developed. Based on the investigations, the following conclusions were reached:Nanoscale HZSM-5zeolite was more stable and showed higher aromatization activity than microscale HZSM-5. The reaction type and product distribution were strongly influenced by reaction temperature. High temperature was favorable to the aromatization activity and the increase of the product RON. But it resulted in the increase of benzene content and the decrease of the product yield. Pressure showed unobvious effect on the aromatization activity and the distribution of aromatics. The eliminating carbon ability of H2was found in the aromatization process.Double bond isomerization and skeletal isomerization of n-octene was the first step of n-octene transformation. Propene, butene and pentene were produced as intermediates. These olefins were oligomerized and then cracked to produce a wide distribution of olefins with different carbon atoms. These intermediates were quickly transformed into aromatics and alkanes by hydrogen transfer. The strong acid sites with acid strength H0≤+2.27were indispensable to the transformation of n-octene into aromatics. Coking reaction firstly occurred in the acid sites with acid strength H0≤-3.0. The distributions of olefin intermediates and aromatics were influenced by the acidic amount of HZSM-5.Hydrothermal treatment was a very effective dealumination method and reduced the acidic amount of HZSM-5obviously with the increase of the hydrothermal treatment temperature. The synergistic effect of strong and weak acid sites in improving the deactivation behavior of catalyst was found. The amount of Bronsted acid sites decreased and the amount of Lewis acid sites increased with the addition of the zinc into nanoscale HZSM-5. The maintenance of appropriate Bronsted and Lewis acid sites distribution greatly improved the aromatization stability.The hydrodesulfurization activities of sulfur compounds were different over nanoscale HZSM-5. The inhibiting effect of olefin on the removal of sulfur compounds was caused by the adsorption of olefins on the acid sites and was irreversible. The presence of200ppm sulfur compounds did not influence the aromatization and isomerization activities of nanoscale HZSM-5.The total acidic amount of nanoscale HZSM-5after hydrothermal treated at500℃and modified with Zn, La and P decreased to0.49mmol/g and the ratio of L/B was2.3. Zinc species were the active sites of aromatization. The addition of La improved the dispersion degree of zinc species, increased the zinc species-support interaction and the amount of [Zn(OH)]+species and therefore enhanced the aromatization activity of the catalyst. The addition of P increased the amount of the zinc species located in the channels of nanoscale HZSM-5and therefore enhanced the stability of the catalyst. The synergistic effect of La and P increased the amount of [Zn(OH)]+species, decreased the loss of zinc species and consequently increased the activity for FCC gasoline aromatization and the stability of the catalyst. The evaluation of ZnLaP/HZSM-5catalyst for upgrading of FCC gasoline was conducted in two steps process applying the sequence that aromatization followed by desulfurization. The results showed that the aromatics content increased from15.2%to19.3%, the olefins content decreased from40.1%to27.2%, the i-paraffins content increased from35.4%to38.4%and the sulfur content decreased from190ppm to47ppm. The RON of the product was90.4and the loss of the RON was0.3. The dry point increased from187℃of the feed to197℃of the product. The product can meet the National IV standard with sulfur content less than50ppm and the loss of RON less than one unit by the two steps process applying the sequence that aromatization followed by desulfurization.