Synthesis And Properties Of ZSM-5 Zeolite For Alkene Isomerization

Olefin hydroisomerization has been regarded as one of the effectively route to solve the problem of the loss of octane number during the reduction of olefin content in gasoline.In this dissertation,nanocrystal aggregates ZSM-5 zeolites were directly synthesized by hydrothermal synthesis without using mesopore templates.The as-synthesized zeolite was treated with hydrothermal treatment and modified with metal and non-metal oxides species for the olefin isomerization reaction.First,the effects of material ratio and crystallization conditions on the synthesis of nanocrystal aggregates of ZSM-5 zeolites were investigated.It was found that the key to synthesize nanocrystal aggregates is relied on synchronizing the nucleation rate and crystal growth rate of zeoite.The ZSM-5 zeolite synthesized with pseudoboehmite and TPABr shows high crystallinity and good crystal-stacking morphology under the conditions of OH^-/Si O₂ ratio at 0.18,TPABr/Si O₂ ratio at 0.09,H₂O/Si O₂ ratio at 8.3,pre-crystallization at 100℃for 36 h and crystallization at 170℃for 24 h.The resulting ZSM-5 zeolite aggregates using 100 nm zeolite crystals show a high mesopore volume up to 0.14 cm³/g.The synthesis has been scaled up to 10 L pilot-test,and the ZSM-5 zeolite with a larger mesopore volume（0.06 cm³/g higher）were obtained.Second,the effects of the modification method on the acidity and isomerization performance of the zeolites were studied.The catalytic performance of olefin isomerization varied with the acidic properties of zeolites changing with modification method.Hydrothermal treatment with 0.4 wt.%NH₃·H₂O effectively altered the acidity of ZSM-5 zeolite,in which the proportion of mediate strong acid and the ratio of B/L acid increase.Compared with the ZSM-5 zeolite without treatment,the yield of isoparaffins increased by 4.9%after hydrothermal treatment.The modification with3 wt.%La₂O₃ reduced the acid strength and increased the proportion of mediate strong acid of zeolite.However,the ratio of B/L acid was reduced,resulting in a slight decrease in the ratio of iso/n-alkanes in the isomerization reaction products.Phosphorus modification reduced the acid strength of ZSM-5 zeolite and increased the ratio of B/L acid.When 0.5 wt.%phosphorus was loaded on the catalyst,the acidity of the catalyst was more suitable.However,a further increasing phosphorus loading leads to decreasing acid strength,which was not conducive to the isomerization of olefins.Last,the combination modification methods have been developed to improve the catalytic performance via adjusting alter the acidity of ZSM-5 zeolite.The modification with 3 wt.%La₂O₃ and 0.5 wt.%phosphorus avoids dramatical decrease in acid strength of ZSM-5 zeolite via the phosphorus modification,and the ratio of B/L acid increases.After 8 hours of isomerization reaction,the ratio of iso/n-alkanes in the product was 0.2 higher than that of 3La/ZSM-5 catalyst,and the yield of isoalkanes reached 42.6%.The ZSM-5 zeolite modified with 0.4 wt.%NH₃·H₂O,3 wt.%La₂O₃and 0.5 wt.%phosphorus shows the higher ratio of B/L acid and the highest proportion of mediate strong acid,resulting in the best isomerization performance.The olefin content in the product was only 1.8%,and the yield of isoparaffins was as high as44.8%,which is 17.8%higher than that of the industrial catalyst.