Synthesis and consequence of Zn modified ZSM-5 zeolite supported Ni catalyst for catalytic aromatization of olefin/paraffin

Zn modified ZSM-5 zeolite supported Ni bifunctional catalysts were prepared by in-situ hydrothermal synthesis (Zn-Ni-ZSM-5), co-impregnation method (Zn-Ni/ZSM-5) as well as combining in-situ hydrothermal synthesis of Zn-ZSM-5 and sequential impregnation of Ni (Ni/Zn-ZSM-5) and used for the selective converting olefin to aromatics applicable for FCC gasoline hydro-upgrading. The Lewis (L) acid sites generated by ZnOH+ species and hydrogenation active sites from Ni-0 species were varied with preparation method, in which their locations at the external surface or micropores of zeolite changed the acid properties of catalysts. Comparing to the Zn-Ni/ZSM-5 from the co-impregnation method, direct in-situ hydrothermal method could incorporate Zn and Ni species within zeolite, which makes the strong interaction between the metal species and the framework Al to form medium-strong L acids (ZnOH+/NiOH+). This leads to the decrease of ratio of Bronsted acid sites to L acid sites (B/L) from 0.40 to 0.12 and the difficulty in reducing of Ni cations to Ni-0 from 57% to 30% Ni-0 percent. Ni/Zn-ZSM-5 sample prepared by incorporating Zn species into micropores and sequential depositing Ni species at the external surface of zeolite possesses a low B/L ratio similar to Zn-Ni-ZSM-5 (0.20 versus 0.12) but a much higher Ni-0 percent (52% versus 30%). Zn-Ni-ZSM-5 shows closer intimacy between Zn and Ni species comparing to Zn-Ni/ZSM-5 and Ni/Zn-ZSM-5. For the 1-hexene aromatization, the formed iso-alkene at acid sites was quickly saturated to iso-alkanes by the adjacent Ni-0 particles over Ni-Zn-ZSM-5 catalyst in hydrogen steam. However, the location of aromatization active sites (ZnOH+, L acid sites) in micropores and hydrogenation active sites (Ni-0 species) on external surface of Ni/Zn-ZSM-5 makes the tandem conversion of n-alkene -> iso-alkene -> aromatics, rather than the n-alkene -> iso-alkene -> iso-alkane. Thus, Zn-Ni-ZSM-5 with close intimacy of ZnOH+ and Ni-0 particles shows a high iso-alkane selectivity (27.9% iC(4)(+)), while Ni/Zn-ZSM-5 exhibits a high aromatics selectivity (39.6 %) in the conversion of 1-hexene in hydrogen steam. Similar results have also found in the conversion of n-heptane. Here, the control of the intimacy of Zn and Ni in ZSM-5 zeolites has been realized to be a good protocol to adjust the selective conversion of olefins/alkanes to aromatics versus iso-alkanes.

Automated summary

Zn modified ZSM-5 zeolite supported Ni bifunctional catalysts with different preparation methods were used for selective conversion of olefins to aromatics. The distribution of acid and active sites varied with different preparation methods, affecting the selectivity of olefin conversion. Controlling the contact between Zn and Ni can adjust the selectivity of aromatics and iso-alkanes.