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Aromatisation de l'éthylène sur catalyseurs bifonctionnels Zn/MFI : nature, quantification et stabilité du (ou des) site(s) actif(s)

Abstract : The shale gas boom in USA tends to change the energy and economic landscape. The exploitation of this resource leads to an overproduction of ethylene to the expanse of the BTXs one. This context brought out the idea of using this extra production of ethylene to be transformed into aromatics. The aims of this work was to study fundamentally the mechanism of the aromatization of ethylene and the origin of the deactivation on Zn/HZSM-5 as bifunctional catalyst.Firstly, the effect of the structure of the zeolite, the nature of the precursor salt and the method of preparation were studied. It emerges that ZSM-5 zeolite wet impregnated with a salt of zinc nitrate is the most efficient for the aromatization reaction. Subsequently, a study of the operating conditions was performed by varying the contact time, the reaction temperature and the partial pressure. The effect of the contact time has showed that the primary or "pseudo" primary products are respectively C4 and C3, and the secondary products are methane, ethane, hydrogen and aromatics. An apparent reaction scheme could be proposed. The effect of the reaction temperature studied between 350 ° and 600 ° C. showed that the initial activity increases with temperature but that the catalyst deactivates more rapidly due to the formation of coke.Then, three series of Zn / HZSM-5 catalyst with Si / Al ratios of 15, 40 and 75 were prepared with zinc contents between 0 and 8% by weight. Their characterization have allowed to determine the nature, the location and the concentration of the zinc species. These species are (ZnOH) + located on the zeolite exchange positions, Zn2 + in the hydroxyl nests and ZnO on the catalyst surface. All the catalytic results have allowed to establish an apparent reaction scheme in agreement with that proposed following the study of the contact time. The study of reaction intermediates on catalysts having either Bronsted sites (zeolite alone), or mainly Lewis sites (ZnNaZSM-5) or a mixture of both (ZnHZSM5) iso-conversion allowed to define the role of each type of active sites. The protons would be responsible for the reactions of oligo-cracking and cyclization of the intermediate dienes while the zinc in the exchange position would be able to perform the first step of dimerization of ethylene, or the coupling of olefins between them from the carbon pool. The zinc would also catalyse the dehydrogenation reactions of naphthenes into aromatics. At high conversion, zinc also could favour hydrogenation reactions of olefins into paraffins. From all these conclusions, we have proposed a reaction mechanism that is different from that proposed in the literature because the first step would take place on zinc.Finally, the study of the deactivation of the catalysts made it possible to highlight that this phenomenon was directly related to the formation of coke and thus to the zinc content in exchange position. However, the coke formation is slowed down for the high conversion (X > 50%). From this value, the hydrogen formed is self-consumed and the nature of coke evolves toward more polyaromatic compounds. This coke would be more toxic and would tend to poison the Lewis acid sites and in particular the (ZnOH)+ sites, whereas the one formed at low conversion poisons the Brønsted acid sites. We have also established that the balance (ZnOH)+ / H+ could be a descriptor of the deactivation.
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Submitted on : Thursday, April 2, 2020 - 10:05:12 AM
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Aurélien Bonnin. Aromatisation de l'éthylène sur catalyseurs bifonctionnels Zn/MFI : nature, quantification et stabilité du (ou des) site(s) actif(s). Catalyse. Université de Poitiers, 2019. Français. ⟨NNT : 2019POIT2284⟩. ⟨tel-02529054⟩



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