Power-to-methane over Ni/zeolites: Influence of the framework type

In the present work, the effect of the zeolite framework type on the performances of Ni-based catalysts for CO ₂ methanation was studied. For this purpose, four zeolite frameworks presenting similar Si/Al ratios (∼40) and commercially available were chosen: FAU (USY), BEA, MFI (ZSM-5) and MOR. All the samples were ion-exchanged in order to obtain the Na ⁺ and Cs ⁺ forms and finally impregnated with the same Ni content (15 wt%). The hydrophobic character, found as beneficial for the reduction of water inhibitory effect in the reaction, was more favourable in the case of the USY zeolite catalysts, being these samples the ones reporting the best performances both in Na ⁺ and Cs ⁺ form, when comparing with BEA, ZSM-5 and MOR samples containing the same cation. The average Ni ⁰ size also played a relevant role, being BEA zeolite the structure leading to the most favourable metallic dispersion, which justified the great performances developed by BEA samples. Finally, ZSM-5 and MOR, even if presenting the highest CO ₂ uptakes according to the collected CO ₂ isotherms, led to larger Ni ⁰ particles mainly due to the lack of mesoporosity for the accommodation of a fraction of Ni species, what motivated the presence of NiO particles only in the external zeolites surface. Finally, the effect of increasing the Si/Al in BEA was studied. The increase led to lower basicity, larger Ni ⁰ particles and higher hydrophobicity. The performances were favoured by the increase of the Si/Al ratio, so that the lower affinity to H ₂ O could justify the enhancements observed. However, USY zeolite reported the best results remaining the most promising structure for the preparation of Ni-based zeolites for CO ₂ methanation.