## 3 Results and Discussion  
### Adsorbents Characterization  
XRD patterns obtained for the natural and Si-PILC clays and their main fractions are shown in Figure 1A. The obtained XRD pattern for the NC exhibits a basal distance (d\({}_{001}\)) of 1.26 nm, which is typical of natural sodic montmorillonite. The Si-PILC diffractograms did not show any defined peaks in the range in which the basal distance should be present. Furthermore, there are no other structural changes in the PILC in reference to the raw material [37; 38]. In order to evaluate the existence of an increase in the basal distance (d\({}_{001}\)), XRD at low angle was obtained for the Si-PILC 75 and is shown in Figure 1B. A basal distance of 4.32 nm was found for this material, which evidences the presence of silica oxide species within the interlayer clay suggesting an effective pillarization process. This result agrees with the values reported by other authors [26]. Si-PILC 25 and 50 were not analyzed at low angle but a similar behavior could expect for them. The FTIR spectra obtained for the four materials and their main bands are shown in Figure 2. The natural clay spectrum is in accordance with previous reports, where the absorption bands at 3630, 3454, and 1640 cm\({}^{-1}\) are typical for smectites and associated to tension vibrations of the O-H bond in hydroxyl groups of dioctahedral structures and H-O-H vibrations of water molecules weakly  
Figure 1: XRD patterns of NC and Si-PILC. (XRD: X-Ray Diffraction.) hydrogen bonded to the Si-O surface, respectively [26; 39]. In addition, the bands found at 1040, 800, 524, and 470 cm\({}^{-1}\) are attributed to Si-O stretching vibrations, Si-O-Al (octahedral Al) and Si-O-Si bending vibrations, respectively. The spectra obtained for the Si-PILC show a considerable reduction of the bands related to water presence and O-H vibrations, which could be due to the acid treatment used during the synthesis of these materials [26; 39]. Additionally, the spectra of the Si-PILC show increments in the bands of 1200 and 800 cm\({}^{-1}\) which evidences the increase of Si-O-Si bonds in their structures as a consequence of the pillaring process. Nitrogen adsorption-desorption isotherms at \(-\)196 \({}^{\circ}\)C obtained from the materials are shown in Figure 3. The natural clay isotherm is classified as type IIb with an H3 hysteresis loop according to the International Pure and Applied Chemistry (IUPAC) classification which is associated with mesoporous materials with aggregates of plate-like like the montmorillonite [27; 40]. The isotherms obtained for Si-PILC can be classified as a combination between type I and IIb. The first one is due to the high amount adsorbed at low relative pressures related to the presence of microporosity in the PILC structures generated after the pillaring process. The second one is associated with the growth of the adsorption in the mono-multilayer region. Hysteresis loops were observed for the three Si-PILC demonstrating the presence of mesoporous in their structures. The type of hysteresis loop is classified as type H4 according to the IUPAC and it is associated with slit-like pores as well as the pores generated within the interlayer of the clay minerals [27; 40]. As can be seen, all the pillared clays present higher adsorption at low relative pressures than the natural clay, which is associated with their microporosity and evidences a successful pillaring process. However, among the pillared clays Si-PILC 50 showed the highest adsorption at low relative pressure suggesting there is not a correlation between the adsorption increase observed and the Si/clay ratio used during the synthesis process. This could be due to the amount of siliceous  
Figure 2: FTIR spectra for the natural and Si-pillared clays. present in the interlayer for the Si-PILC 75 which could generate higher pillars density than the Si-PILC 50 implicating a lower microporosity.