The effect of the compensating cation on the catalytic performances of Ni/USY zeolites towards CO ₂ methanation

In the present work, monovalent (Cs⁺, K⁺ Na⁺, Li⁺) and divalent (Ba²⁺ , Ca²⁺ , Mg ²⁺ ) cations have been exchanged in a HUSY zeolite and impregnated with 15% of Ni, in order to be used as catalysts for CO ₂ methanation reaction. The results showed that the order of improvement of the catalytic performances is Cs ⁺ > Na ⁺ > Li ⁺ > K ⁺ > H ⁺ for the monovalent cations and Mg ²⁺ > Ca ²⁺ > Ba ²⁺ for the divalent. No important differences were observed in terms of Ni reducibility followed by H ₂ -TPR and textural properties determined by N ₂ adsorption experiments. In addition, similar Ni particles sizes were observed for Cs ⁺ , Na ⁺ , Li ⁺ and H ⁺ and much bigger Ni particles were found on K ⁺ sample. The differences in the performances of these monovalent exchanged samples were thus attributed to the reported enhancement of the CO ₂ adsorption capacity and framework basicity of the zeolites containing these cations. In terms of divalent cations, Mg ²⁺ sample presented both the lower Ni particle size and the better performances, while no relevant differences were observed in terms of Ni species reducibility and textural properties among all the divalent samples. Thus, Mg ²⁺ induces not only an enhancement of the CO ₂ activation, but also of the Ni dispersion. To conclude, this work constitutes a crucial advance in the use of zeolites as supports for CO ₂ conversion into CH ₄ by tuning the catalyst properties through the exchanged cation, as almost no studies on this subject have been published.