Add time:07/12/2019         Source:sciencedirect.com

Ni-ZSM-5 bi-functional catalysts were prepared by ion-exchanged (Li+, Na+, K+) in alkaline solution with different alkali concentrations. Highly selective alkanes and aromatics were abtained separately in the conversion of oleic acid by adjusting the relative strength between metal centres and acid sites. When the protons of parent HZSM-5 were fully exchanged by alkali ions in 0.2 M alkaline solution, the Brønsted acid was almost removed, while the Lewis acid was relatively enhanced. Of note, Li fully exchanged ZSM-5 with a strong Lewis acid of 68.45 μmol/g was much higher than the parent HZSM-5 of 13.74 μmol/g. Because the acidity of ZSM-5 was greatly reduced, the reaction process was dominated by the metal centres, and the products were main alkanes. When catalyzed by LiZSM-5, the content of iso-alkanes could reach 39.92%. If the protons of HZSM-5 were partially exchanged by alkali ions in low concentration alkaline solution (0.02 M-0.1 M), their strong Brønsted acidity decreased gradually from 682.7 μmol/g to 0. The acid sites containing strong Brønsted acid were stronger than the metal centres, which was independent of the amount of strong Brønsted acid. Due to the exist of strong Brønsted acidity, the reaction process was dominated by the acid sites, and the product were main aromatics. Reasonable acid regulation could effectively control the content of aromatics in the products. When catalysed by Ni-008NaZSM-5, with 68.6 μmol/g strong Brønsted acidity, the content of aromatics could reach 87.46%. In summary, desirable products could be acquired by regulating the relative strength between acid sites and metal centres.

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