Study On The Preparation Of Pentasil Zeolite With Hierarchical Porosity And Their Catalytic Performances For The Oligomerization Of Butene

Oligomerization of light olefins is a significant industrial way to produce liquid fuels?sunch as naphtha,and diesel?and fine chemical intermediates?such as surfactants,and plasticizers?.As one of light olefins,butene not only come from Fluid Catalytic Cracking,but also can derive from Fischer-Tropsch synthesis,and so on.Thus,people pay a great deal of attention to transform surplus butane to liquid fuel.Specially,many studies have been devoted to oligomerization due to its environmentally friendly,simple process and free of sulphur.Zeolitic materials with high crystallinity are more likely to gain more attention because of their easy regeneration,high catalytic activity,and flexible product distribution.Among the conventional microporous zeolites,both ZSM-5 and ZSM-11 zeolites are typical member of the pentasil family with10-membered ring openings.However,the microporous zeolites have a poor resistance to carbon deposit due to the internal diffusion limitations.Herein,we prepared hierarchical pentasil zeolites for the oligomerization of butene to overcome internal diffusion limitations.The main works are as follows:First,a series of alkali-treated hierarchical H-ZSM-5 zeolite catalysts were prepared using Li OH,Na OH,KOH,and Cs OH treament on commercial H-ZSM-5sample,respectively and then their were evaluated in the oligomerization of butene.The results demonstrated that the hierarchical H-ZSM-5 catalyst was more resistant to pore blocking compared with the microporous catalyst.Notably,the Cs OH-treated hierarchical HZSM-5 catalyst not only showed moderate acidity and higher L/B ratio?molar ratio ofand Br?nsted?,but also possessed interconnected open-mesopores and a smaller crystal size,which were responsible for the improved activity and good stability.More importantly,the ATHZ5-Cs catalyst showed approximately 100%conversion of butene,the highest naphtha yield of 47.4%and distillate yield of 52.6%within 16 h.Second,hierarchical ZSM-11 zeolites with the Si O₂/Al₂O₃ ratio of 26 to 123 were synthesized successfully by using in-situ method.Compared with H-ZSM-5 zeolites,we just found that the novel H-ZSM-11 zeolites exhibited excellent shape-selectivities in the catalytic oligomerization of butene.Hence,it is clear that H-ZSM-11 zeolites with different Si/Al ratio have shown some differences in the catalytic performance for the butene oligomerization.The result confirmed that decreasing the Si/Al ratio improved the catalytic activity of oligomerization.The highest naphtha yield can attain 50.54%and distillate yield reaches 49.46%over hierarchical ZSM-11 zeolite with the Si O₂/Al₂O₃ ratio of 26.Our research also showed that the catalytic activity decreased slightly,and then declined in naphtha yield and distillate yield when the reaction time prolongs.Third,we reported the direct fabrication of hierarchical ZSM-11 nanosheets?sphere-like?using hexadecyl trimethyl ammonium bromide?CTAB?and tetrabutylammonium bromide?TBABr?as dual template.The crystal size,texture structure,and acidity of hierarchical ZSM-11 could be tuned by different ratio of CTAB/Si O₂.The addition of CTAB also could effectively reduce the size of nanoparticles.The prepared hierarchical ZSM-11 zeolites in the presence of CTAB showed much higher catalytic activity in the catalytic oligomerization of butene due to the smaller crystal size,more proper amount of acid sites and L/B ratio of ZSM-11?modified by CTAB?.Notably,HZ-0.01CT and HZ-0.02CT showed a better stablity than HZ11 within 12 h,and the highest naphtha yield was 49.51%and distillate yield reached 50.49%.Finally,a series of hierarchical nano-MEL zeolites were successfully synthesised using template-free BEA seeds based on the CBU hypothesis.Zeolitic aggregate sizes were easily controlled by adjusting the amount of BEA seeds.The BEA seeds could effectively control the sizes of zeolitic aggregates.The hierarchical nano-MEL zeolites series were then tested in the oligomerization of butene.It was noted that the hierarchical nano-MEL zeolites?i.e.,HZ11-2?using 2 wt%BEA seeds as additives exhibited higher activity and longer lifetime than that of other nano-MEL zeolites.Particularly,the butene conversion of the HZ11-2 remained 100%after 22 h,the highest naphtha yield was 55.48%,the highest distillate yield was 44.52%.The HZ11-2possessed large external surface area,a larger amount of total acid sites and Br?nsted acid sites,and thus improved the catalytic activity.At the same time,the excellent stability was attributed to considerable intercrystalline mesopores,and higher L/B ratio.