Study On Aromatization Performance Of 1-Hexene Over Different HZSM-5

With the increasing attention to environmental protection,the standards for gasoline are more and more strict.However,the composition of fliud catalytic cracking?FCC?gasoline cannot meet these standards,and the olefin content of FCC gasoline need to be reduced.The aromatics content in FCC gasoline is low,so converting these olefins into aromatics can not only reduce the olefin content,but also maintain the octane number of gasoline.Therefore,the aromatization of olefins is a good option to produce high quality gasoline.In this paper,1-hexene was selected as the model compound of more representative olefins in FCC gasoline,and HZSM-5 zeolite was selected as the aromatization catalyst.The effects of different modification methods on the aromatization performance of HZSM-5,including organic and inorganic alkali treatment,steam aging treatment and metal modification were discussed.Firstly,HZSM-5 zeolite was treated by inorganic base?NaOH?and organic base tetrapropylammonium hydroxide?TPAOH?.The crystallinity of HZSM-5 treated by NaOH decreased while that of HZSM-5 treated by TPAOH increased.Furthermore,mesopores can be formed after the both base treatment.However,the mesopore diameter of NaOH treated was smaller than that of TPAOH treated,while the pore volume of the former was larger than the latter.We also investigated NaOH concentration,TPAOH concentration,treatment time and the crystal size of HZSM-5 zeolite.It was found that as the concentration of NaOH was0.5 mol/L,the aromatics content of HZSM-5 zeolite only decreased by 5%,and the aromatics content was 82.5%after 7h of time on stream?TOS?.As the concentration of TPAOH was0.67mol/L and the treatment time was 48h,the nano-HZSM-5 zeolite?SiO₂/Al₂O₃=25?showed the best aromatization performance.The aromatics content only decreased by 3%and the aromatics content was 87%when TOS was 7h.In addition,the effects of NaOH and TPAOH on the aromatization performance of HZSM-5 zeolite were compared.The results showed that the amounts of acid sites,L/B,pore volume and pore diameter of HZSM-5zeolite treated by TPAOH were all higher than that of HZSM-5 zeolite treated by NaOH,which led to the better aromatization activity and stability of HZSM-5 zeolite.BET,TGA and¹³C MAS NMR were used to further explore the structure of catalyst before and after reaction and the nature of coke.The results showed the better performance of the TPAOH treated HZSM-5 zeolite mainly due to the larger mesopores and higher L/B value,which effectively promoted the occurrence probability of dehydrogenation reaction and the diffusion rate of intermediate species and products.The TGA and ¹³C MAS NMR results of spent catalysts showed that the amount of each of the three types of coke and the total amount of coke of HZSM-5 zeolite treated by TPAOH were lower and the stability was better.Secondly,the influences of SiO₂/Al₂O₃ ratio and steam aging treatment on the catalytic performance were revealed.It was found that both acid amount and L/B value had important effects on aromatization.The large amount of strong acid could increase initial aromatization activity,but could accelerate the deactivation rate.High L/B value would compensate for the loss of activity due to the decrease of acid amount.Increasing L/B value promoted the probability of dehydrogenation reaction and increased the formation rate of aromatics.As a result,the aromatics content was increased and the aromatization performance was improved.Finally,the catalyst performance was studied in Zn and Zr loading.When the loading amount of Zn was 1.0wt%,the aromatization performance of HZSM-5 zeolite was optimized Too low or too high Zn loading resulted in the decrease of aromatization performance because the appropriate loading amount could provide a certain amount of acid sites and relatively high L/B value,consequently the aromatization activity of zeolite could keep in high level.The aromatization performance of Zr/HZSM-5 prepared by ion exchange was better than that of Zr/HZSM-5 prepared by impregnation method.HZSM-5 zeolite was treated with metal modification.It was found that the aromatics content of Zn-Zr/HZSM-5 zeolite was inferior than unloaded zeolite.This may be caused by the loss of metal component and the blocking of channels.