## 3 Results and Discussion
### Catalytic Activity  
The catalytic activity of the prepared zeolite-supported Ni catalysts in the CO\({}_{2}\) methanation reaction was evaluated in a fixed-bed reactor. The CO\({}_{2}\) conversion and CH\({}_{4}\) selectivity obtained with these samples are presented in Figure 4 as a function of temperature. CO\({}_{2}\) conversion was found to increase with temperature as expected (Figure 4a). Generally, the more active catalysts also showed higher selectivity to CH\({}_{4}\) being the only by-product CO (Figure 4b). Attending to the ITQ-2 samples, a higher CO\({}_{2}\) conversion was obtained with the catalyst supported on the ITQ-2 without Al. Indeed, only with these catalyst equilibrium values were reached at 400 \({}^{\circ}\)C. The difference between the two samples supported on ITQ-2 was a maximum at 350 \({}^{\circ}\)C reaching 50 points. The higher catalytic performance of the catalyst supported on the pure silica ITQ-2 can be mainly attributed to the enhanced Ni dispersion in this sample related to the larger external surface area of the support. Furthermore, the absence of Brensted acidity of this sample can be related to a strong hydrophobic character [40; 41] favoring, hence, the removal of water from active sites and the reaction according to [PERSON]'s principle. The positive effect of the removal of water for the CO\({}_{2}\) methanation reaction has been reported previously [24; 25; 42]. Regarding the catalytic results of the catalysts supported on ZSM-5, a similar trend was observed. In this case, a minor difference was appreciated, but still, the ZSM-5 with a higher Si/Al ratio shows a CO\({}_{2}\) conversion 20 points greater than its counterpart. These two samples supported on ZSM-5 present similar NiO particle sizes, textural properties and reducibility. However, the sample supported on the ZSM-5 with a higher Si/Al ratio shows a distinctly lower Bronsted acidity suggesting that this sample is probably more hydrophobic [40; 41]. This is coherent with previous studies, which reported that the increase of the Si/Al ratio of the ZSM-5 zeolite leads to a more hydrophobic material [43]. Furthermore, the N\({}_{2}\) adsorption results seem to indicate that Ni is preferentially deposited in the external surface area in the sample with a higher Si/Al ratio. Thus, the accessibility of the reactants to the active sites would be enhanced and the catalytic activity improved. The catalysts used in these tests were characterized after the reaction. Carbon deposition was not detected for any of the catalysts here studied, suggesting that deactivation by coke disposition is not occurring, at least in the reaction time here considered (15 h). Furthermore, XRD patterns corroborate that both zeolite structures are well preserved (Figure 5). On the other hand, concerning Ni diffraction peaks, no peaks corresponding to NiO can be distinguished. Instead, diffraction peaks of reduced Ni (Ni\({}^{0}\)) can be clearly seen only in the case of ZSM-5. This again supports the fact that a better distribution of Ni with a smaller crystallite size is obtained using the delaminated zeolite ITQ-2. HR-TEM images confirm that Ni\({}^{0}\) particles seem to suffer a very slight sinterization during the reaction. Nevertheless,  
Figure 4: (a) CO\({}_{2}\) conversion; (b) CH\({}_{4}\) selectivity; (inset) reaction rate at 300 \({}^{\circ}\)C of the 5 wt.% Ni catalysts supported on ITQ-2 and ZSM-5 with different Si/Al ratios. Equilibrium values [8]. Ni\({}^{0}\) particle size distribution for the sample 5 Ni/ITQ-2(\(\infty\)) is still narrow and very similar to the size before reaction (2.8 nm after reaction against 2.4 nm before reaction) (Figure 6). Comparing the catalytic results for both types of framework (ITQ-2 and ZSM-5), the performance is different depending on the Al content of the support. With the support of high Al content, a similar catalytic behavior was obtained, independently of the type of zeolite.