## 3 Results and Discussion
### Preparation and Characterization of the Materials
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **MnO** & **MgO** & **CaO** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **TiO\({}_{2}\)** \\ \hline Ka & 54.24 & 42.61 & 1.51 & 0.005 & 0.15 & 0.04 & 0.02 & 0.41 & 1.01 \\ Ka-R & 54.06 & 42.60 & 1.59 & 0.010 & 0.18 & 0.13 & 0.02 & 0.41 & 1.00 \\ Zeo & 43.52 & 34.39 & 0.67 & 0.005 & 0.11 & 0.03 & 20.59 & 0.14 & 0.55 \\ Zeo-R & 44.35 & 33.82 & 0.96 & 0.006 & 0.11 & 0.02 & 19.82 & 0.26 & 0.66 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition (wt %) of the parent purified kaolins (Ka and Ka-R) and of the obtained zeolites (Zeo and Zeo-R), expressed on a dry basis (metal oxides 100%). procedure in a strongly basic solution. At higher temperatures, dehydroxylation can be detected as a gentle, continuous mass decrease [23]. The nitrogen adsorption/desorption isotherms on the samples measured at \(-196\)\({}^{\circ}\)C refer to type II isotherms (IUPAC classification [24]). This type of isotherm evidences that a complex structure with undefined pore size and volume size distributions exists due to low adsorption values (Figure 3). The BET SSA values follow the same trend in both series, increasing when one goes from kaolin to metakaolin and strongly decreasing when one goes from metakaolin to zeolite (Table 4). The increase from kaolin to metakaolin can be related to amorphization induced by calcination of kaolin to form metakaolin (see Figure 1). The sharp decrease from metakaolin to zeolite may be due to the extremely high crystallinity of the zeolite particles and their large particle size, as suggested by X-ray diffraction and confirmed by electron microscopy (vide infra). Given the large crystal size, the SSA is almost exclusively due to contribution from the external surface. In the same way, the pore size decreases when one goes from kaolin to metakaolin and increases when one goes from metakaolin to zeolite (Table 4). Figure 3: N\({}_{2}\) adsorption–desorption isotherm at \(-196\)\({}^{\circ}\)C and BJH pore size distribution for the zeolite \(Zeo\), derived from white kaolin. Figure 2: Thermogravimetric (**a**) and differential thermal (**b**) analysis curves of the zeolites prepared from white and red kaolins. Table 4 also provides the CEC SSA values regarding MB adsorption. In this case, CEC SSA decreases when one goes from kaolin to metakaolin and increases when one goes from metakaolin to zeolite. The CEC SSA values of the zeolites are very close to the CEC SSA values of the original kaolins. The CEC SSA values of Zeo and Zeo-R do not differ, which demonstrates that the slight differences in crystallinity do not affect CEC SSA. The results suggest that the CEC SSA of the kaolins Ka and Ka-R, which are originally rather low, decrease due to the strong calcination conditions applied to obtain the metakaolins, M-Ka and Me-Ka-R. Further reaction to produce the zeolites recovers the CEC SSA in the final Zeo and Zeo-R despite their large particle size and high crystallinity. However, the low surface area of the zeolites and the difficult access to their pores indicate that the experimental values may be affected by MB aggregation depending on the pH of the dispersions, to culminate in the large difference between the SSA values of the gas and liquid phase media. The micrographs in Figure 4 depict the typical morphology of the very fine and rigid kaolinite particles. Stacked sheets forming tactoids, which produce aggregates, are evident. The micrographs of Zeo and Zeo-R reveal the formation of large cubic crystals, a typical morphology of these solids [25]. The shape of these crystals depends not only on the chemical composition of the zeolite, but also on the conditions used during the synthesis. In the current case, the morphology of the Zeo and Zeo-R crystals is very close to cubic. Zeo-R is more regular than Zeo. Aggregation of cubic crystal clusters gives quasi-spherical aggregates, an effect that is more evident for Zeo.