Synthesis And Consequence Of Metal Modified ZSM-5 Zeolite For Olefin Aromatization/Isomerization

The selective conversion of olefins in FCC gasoline into aromatics and isoparaffins with high-octane number is an important technical route for national VI standard gasoline production.The key of this route is relied on the high-efficient olefin aromatization/isomerization catalyst.In this dissertation,a low-cost synthesis method of the hierarchical ZSM-5 zeolite without using mesoporous template has been developed.The structure-activity relationship between the acid properties of ZSM-5 zeolite and the selective conversion of olefins was systematically explored by adjusting the crystal size and metal introduction method.Firstly,the catalytic conversion of propylene and hexene olefins overa 1-dimensional 10-member ring TON zeolite（i.e.,ZSM-22）and a 3-dimensional 10-member ring MFI zeolite（i.e.,ZSM-5）was compared,spectively.We found that MFI zeolite,especially Zn-modified ZSM-5 zeolite,is the suitable choice as olefin isomerization catalyst.Then,the Zn modified ZSM-5 by impregnation method was focused and tested in the olefin conversion.Here,the effect of zeolite crystal size on 1-hexene hydroaromaticity/hydroisomerization was studied.It is found that the crystal size of ZSM-5 influences the distribution of acid sites inside and outside the zeolite micropores,in which the ZSM-5 zeolite with a small crystal size at 100～200 nm shows a high content of Zn OH⁺active species（57%）and a low Br?nsted/Lewis acid site（B/L）ratio（0.15）,and exhibits a high aromatics and iso-alkanes selectivity（17.2%and 50.0%,respectively）in the hydroconversion of 1-hexene.The zeolite with a small size possesses a high content of C₈⁺aromatics（35.5%）in the aromatics product.To clarify the performance of Zn modified ZSM-5 zeolite,we focused on the in-situ hydrothermal synthesis method（Zn-ZSM-5）.Compared with impregnation（Zn/ZSM-5）,the in-situ hydrothermal synthesis promotes the transfer of Zn²⁺into zeolite micropores,leading to decreasing the B/L ratio from 0.6 to 0.3 and increasing the medium-strong acid content from 15.5%to 35.1%.Moreover,the formed Zn OH⁺active species in the confind micropore space enhance the selective conversion of 1-hexene to aromatics/iso-alkanes,in which the iso-alkanes and aromatics selectivity increases by 9.0%and 0.9%,respectively.Meanwhile,the industrial production of ZSM-5 and Zn-ZSM-5 zeolite were carried and the as-prepared zeolite catalyst showed better olefin reduction and octane number recovery performance in the upgrading FCC gasoline than that of present industial catalyst.To improve the hydrogenation performance in 1-hexene conversion of Zn modified ZSM-5 zeolite,the bifunctional catalysts of Zn-Ni/ZSM-5,Ni-Zn/ZSM-5 and Zn-Ni-ZSM-5 zeolites with different Zn and Ni intimacy were prepared by in-situ hydrothermal synthesis and/or impregnation method.Ni/Zn-ZSM-5 zeolite shows the closest intimacy between Zn（in the micropores）and Ni（on external surface）,and carried the reaction route via n-olefin→iso-olefin→aromatics prior to that of n-olefin→iso-olefin→iso-alkane.At the same time,Ni⁰ species on the surface of zeolite activates hydrogen and then inhibits the carbon deposits.It is found that Ni/Zn-ZSM-5 zeolite shows the best aromatization performance for 1-hexene with a high aromatics selectivity at 39.6%.Last,the effect of posttreatment（i.e.,hydrothermal treatment）and combination（i.e.,Beta introduction）on Zn-Ni/ZSM-5 zeolite was studied.The results show that Zn and Ni can stabilize the framework Al in hydrothermal treatment.The ZSM-5 zeoltie treated with0.3 wt%ammonia show a low B/L ratio at 0.25 and a high proportion of medium-strong acid at 30.8%.The resulting catalyst shows an increased aromatics selectivity by 6.6%than that of Zn-Ni/ZSM-5.The combination of Zn-Ni/Beta zeolite with Zn-Ni/ZSM-5promotes the conversion of 1-hexene and a suitable mixing ratio at 6:4（mass ratio）was realized,in which the high selectivity of aromatics and isoalkanes was 25.8%and 35.1%,respectively.