## 3 Results and Discussion
### Catalytic Activity
As an attempt to normalize the results to Ni content, for the studies with available data, a parameter \"x\" has been calculated from the ratio between CO\({}_{2}\) conversion and quantity of supported Ni. In general, it can be said that a lower GHSV is used in our study compared with previous works. However, the comparison of our data with the studies [20] and [45] is possible as the reaction conditions seem to be similar. First, it is important to remark that our value of GHSV (9000 mL/(g\({}_{\rm cat}\cdot\)h)) refers to the total flow of reactants (CO\({}_{2}\), H\({}_{2}\)). On the contrary, the GHSV of study [20] is calculated from the total flow (CO\({}_{2}\), H\({}_{2}\), N\({}_{2}\)), being the comparable value based on CO\({}_{2}\) and H\({}_{2}\) flows 3333 mL/(g\({}_{\rm cat}\cdot\)h). Taking this into account, it can be said that values for CO\({}_{2}\) conversion comparable with those of [20] and [45] are obtained with 5 Ni/ITQ-2(\(\infty\)). Moreover, a distinctly higher CH\({}_{4}\) selectivity is achieved, even though a slightly disadvantageous GHSV and lower Ni content are used in our study. Finally, attending to studies performed at higher GHSV, 5 Ni/ITQ-2(\(\infty\)) is far from the sample 5 Ni/MSN [17] in terms of activity, being the later around twice more active operating at five times higher GHSV. On the other hand, it is also important to highlight that with 5 Ni/ITQ-2(\(\infty\)), similar values of CH\({}_{4}\) yield to those reported using promoted catalysts with La\({}_{2}\)O\({}_{3}\)[15] or Ce [22] are achieved at 350 \({}^{\circ}\)C. It is true that the lower GHSV used in our study favor a higher catalytic performance, but also the Ni loading in our samples is only 5 wt.%. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Sample1** & **NiO Size (nm)** & **Ni® Size (nm)** & **GHV** & **Temperature** & **CO\({}_{2}\) Conversion** & **CH\({}_{4}\) Selectivity** & **x2** & **Ref.** \\  & & & & **(\({}^{\circ}\)C)** & **(\%)** & **(\%)** & **(\%)** & **x2** & **Ref.** \\ \hline SNi/ITQ-2(\(\infty\)) & \(<\)3 & & & 250 & 6 & 97 & 154 & This work \\  & & & 9000 mL/(g\({}_{\rm ext}\)-h) & 300 & 28 & 97 & 718 & \\  & & & 3500 h\({}^{-1}\) & 350 & 68 & 99 & 1744 & \\ SNi13X-Cit & & & 13,333 & & & & & \\  & & & mL/(g\({}_{\rm ext}\)-h) & 240 & 17 & 71 & 378 & [20] \\
10N/2 SN-5 & 14.3 & & 2000 h\({}^{-1}\) & 400 & 76 & \(\approx\)75 & & [45] \\
10N/Na-BETA & & 20.1 & 10,000 h\({}^{-1}\) & 350 & 33 & 88 & 660 & [15] \\
10N-10L\({}_{2}\)O\({}_{3}\)Na-BETA & & 8.7 & 10,000 h\({}^{-1}\) & 350 & 65 & 99 & 1300 & [15] \\
14N/C/Ne/Y & & & 15,000 h\({}^{-1}\) & 400 & 77 & 99 & 1023 & [23] \\
14N/C/Ne/Y & & & 43,000 h\({}^{-1}\) & 400 & 68 & 95 & 2600 & [23] \\
15N/Na-USY & 18 & 25 & 43,000 h\({}^{-1}\) & 350 & 55 & 95 & & [24] \\
15N/Na-BEA & 16 & 18 & 43,000 h\({}^{-1}\) & 350 & 43 & 92 & & [24] \\
15N/Na-ZSM-5 & 21 & 24 & 43,000 h\({}^{-1}\) & 350 & 40 & 85 & [24] \\
15N/Na-MOR & 26 & 23 & 43,000 h\({}^{-1}\) & 350 & 40 & 87 & [24] \\
15N/Na-USY & & & 43,000 h\({}^{-1}\) & 350 & \(\approx\)70 & \(\approx\)98 & [22] \\ SNi13X-USY & 19.4 & 43,000 h\({}^{-1}\) & 400 & \(\approx\)20 & \(\approx\)60 & [22] \\ SNi13X/C/Na-USY & 13.3 & 43,000 h\({}^{-1}\) & 400 & \(\approx\)34 & \(\approx\)70 & [22] \\
3 Ce/SiN-USY & 2.5 & 43,000 h\({}^{-1}\) & 400 & \(\approx\)38 & \(\approx\)75 & [22] \\ SNi-3 Ce/Na-USY & 2.4 & 43,000 h\({}^{-1}\) & 400 & \(\approx\)40 & \(\approx\)80 & [22] \\
9 Mg 13N/USY & & & 43,000 h\({}^{-1}\) & 400 & \(\approx\)60 & \(\approx\)95 & [21] \\  & & & 50,000 & 300 & 64 & & 6400 & [17] \\  & & & mL/(g\({}_{\rm ext}\)-h) & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the catalytic results obtained using a SNi/ITQ-2(\(\infty\)) catalyst with those reported in the literature.