Preparation Of Hollow HZSM-5 Zeolites And Its Catalytic Properties For Olefin Aromatization

At present,the national VI vehicle gasoline standard has been implemented to further limit the olefin content.As the main blending component of automotive gasoline,the olefin content in FCC gasoline is much higher than the national standard VI,while the aromatics content is lower.Therefore,olefin aromatization process can be used to convert olefins into aromatics,which can reduce the olefin content and ensure that the octane number of gasoline is at a higher level.Hollow zeolite sieves and metal-encapsulated zeolites show excellent reaction performance in many fields.Compared with traditional zeolites,hollow zeolite sieves have shorter diffusion path and better product selectivity,so they have attracted wide attention.In this paper,1-hexene was used as model compound and traditional HZSM-5 was treated by hollow treatment as aromatization catalyst.The aromatization properties of organic and inorganic alkali treatment,metal modification and hollow HZSM-5 with different grain sizes were studied in detail.Firstly,HZSM-5 zeolite was treated with tetrapropyl ammonium hydroxide(TPAOH)and sodium hydroxide(Na OH)solutions.Hollow HZSM-5(TPAZ5)which treated by TPAOH has high crystallinity,regular cavity structure and the microporous structure in the shell has no damage.Hollow HZSM-5(Na OHZ5)which treated by Na OH has low crystallinity,regular cavity structure,mesoporous structure in the shell and abundant active sites on the inner and outer surfaces.The effects of TPAOH and Na OH concentration on aromatization reaction showed that the optimal alkali concentration was 0.3 mol/L.Under the reaction conditions of T=500?,P=0.5 MPa and WHSV=2.0 h^-1,the yield of aromatics of TPAZ5 zeolite treated with TPAOH of 0.3 mol/L was 43.22%and decreased by only 3.9%after 50 h.Compared with the aromatization properties of solid HZSM-5,TPAZ5 and Na OHZ5,it was found that the aromatics yield of hollow zeolite sieves increased slightly,but the existence of cavity structure shortened the diffusion path of products,and the stability of products was obviously better than that of solid HZSM-5.Secondly,zinc with the same mass was introduced into solid HZSM-5,TPAZ5 and Na OHZ5 by impregnation method,and the effect of cavity structure on metal load was investigated.BET and EDX characterization showed that solid HZSM-5-loaded zinc was mainly distributed on the outer surface of the zeolite sieve,and a small amount of zinc was loaded in the channel;TPAZ5-loaded zinc could be loaded on the inner and outer walls of the zeolite sieve at the same time;for Na OHZ5,zinc could also be loaded on the mesoporous structure of the outer shell,so Na OHZ5-loaded zinc had the highest dispersion.The aromatics yields of the three catalysts were increased to varying degrees after loading Zn,and the increase ranged from Na OHZ5>TPAZ5>HZSM-5.The aromatics yields of the three catalysts were increased to varying degrees after loading Zn,and the sort is Na OHZ5>TPAZ5>HZSM-5.It indicated that the existence of cavity structure could effectively improve the utilization of metal,increase the synergistic effect between metal and acid center,and then improve the aromatization reaction performance of the catalysts.Finally,nano-sized nano-Z5 zeolites were synthesized by hydrothermal method,and the number of surface active sites was controlled by the recrystallization process of TPAOH treatment.After treatment,the micropore channels of Naoz5 were not destroyed,a large number of acid sites were covered on the outer surface,the total acid content decreased,and the L/B value increased.Compared with the aromatization properties of zeolites with different grain sizes,nano Z5 has better aromatization stability than HZSM-5,higher selectivity of benzene and toluene in aromatics products,and better aromatization performance.Finally,the treatment time of aromatization properties of nano Z5 and hollow nano Z5 were compared.Nano Z5 has abundantsurfactant centers and the highest activity in the initial stage of the reaction,and the yield of aromatics is 52.52%.However,the deactivation rate was faster.After 20 h reaction period,the yield of aromatics decreased to 44.21%.Nano Z5-72h showed good aromatization stability,but the loss of active sites cannot be ignored,resulting in low aromatics yield.Nano Z5-48h has a suitable surface active center and a high L/B value.The yield and stability of aromatics are at a high level,and nano Z5-48h has the preferable aromatization performance.