Enhanced activity of CO₂ hydrogenation to CH₄ over Ni based zeolites through the optimization of the Si/Al ratio

The effect of the Y zeolite framework composition (Si/Al ratio) on the catalytic performances of 15%Ni-based samples for the CO₂ methanation reaction was studied. Different USY commercial zeolites with global Si/Al ratios ranging from 3 to 38 were then impregnated with the same Ni content and deeply characterized, namely by ICP analysis, TGA-DSC, XRD, N₂ adsorption, CO₂ adsorption, H₂-TPR, DRS UV–Vis and TEM. Zeolites were also ion-exchanged with different compensating cations (Na⁺ and Cs⁺). It was observed that higher Si/Al ratio lead to lower affinity of the zeolites to water, lower basicity and also to the presence of mesopores able to locate some of the Ni particles. The catalytic performances were verified to be remarkably favoured by higher Si/Al ratios, regardless the compensating cation present on the zeolite supports. This could be explained by the lower surface affinity of high Si/Al ratio zeolites to the produced adsorbed water molecules, which should normally induce an inhibitory effect on the transformation process. Therefore, the effect of the hydrophobicity of the zeolites resulting from the lower Al content appears to be more preponderant for the rate of the reaction than the decrease of basicity (reported in the literature as a relevant factor for methanation). In addition, no sintering or loss of crystallinity was observed after the catalytic tests. Thus, this work clearly shows that important improvements on the catalytic performance of zeolites for the CO₂ methanation can be achieved by adjusting their properties, namely the Si/Al ratio.