# Adsorption of Methyl Orange on a Novel
## 3 Results and Discussion  
### Materials Characterization  
Quantification of the clay content in the composite was performed by means of thermogravimetric analysis (TGA). Figure 1 presents the TGA profiles of the pure UiO-66 and Pal as well as that of the Pal/UiO-66 composite. In the Pal sample, there was a smooth drop of the mass until 500 \({}^{\circ}\)C, consistent with previous reports [41; 42; 43]. The mass loss in the region 80-300 \({}^{\circ}\)C was ascribed to surface adsorbed water and to loosely bonded zeolitic water. The mass loss at T > 300 \({}^{\circ}\)C was attributed to a residual fraction of structural OH\({}_{2}\). Dehydroxylation is expected at higher temperatures than those applied [41]. Concerning the UiO-66 and the Pal/UiO-66, they both presented similar profiles. A small weight loss was observed in the temperature range 50-400 \({}^{\circ}\)C. This was attributed to the removal of guest molecules. The drop of the mass was higher in the Pal/UiO-66 composite in this region probably due to the presence of the clay mineral. In the materials containing the MOF structure, the second mass loss beyond 400 \({}^{\circ}\)C was attributed to the oxidation of the framework. This occurred at the same temperature range for both materials. This observation suggests that the thermal stability of UiO-66 is not influenced by the presence of Pal. The final mass of the material at 580 \({}^{\circ}\)C was used to quantify the MOF loading. The extracted clay content was \(\sim\)9 wt.% in the Pal/UiO-66 composite. The structural information of the prepared materials was obtained by performing X-ray diffraction (XRD) analysis. The XRD patterns are given in Figure 2. UiO-66 displayed a diffraction pattern typical for this particular MOF [31; 35]. The Pal/UiO-66 composite presented an identical diffraction pattern (Figure 2a). These data indicate that the preparation process adopted for the development of the composite did not affect the formation of UiO-66. No diffraction peaks related with Pal were detected in the composite. The inability to detect any diffraction peaks related with Pal probably occurred due to two reasons. First, the amount of Pal in the composite was low, as evidenced by the results of thermogravimetric analysis. Second, UiO-66 had an intense diffraction peak in the region where the main diffraction peak of Pal was expected (ca. 8.5\({}^{\circ}\)) [15; 25]. Palygorskite has good acid resistant properties [44]; however, in order to verify its stability, Pal was treated under the synthesis conditions applied in the absence of the MOF precursors. The XRD patterns of the treated Pal were similar to those of the parent sample (Figure 2b). This observation suggests that Pal is stable under the conditions adopted for UiO-66 synthesis and that the inability to detect any diffraction peaks assigned to Pal in the composite sample probably originates from the low Pal content. Figure 1: Thermogravimetric analysis (TGA) profiles of the pure UiO-66 and the composite containing palygorskite (Pal). The materials were further studied using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy (Figure 3). All characteristic vibrational modes of UiO-66 were detected [45; 46; 47]. The two intense peaks centered at 1392 and 1582 cm\({}^{-1}\) were assigned to the asymmetric and symmetric stretch vibrations in the carboxylate group of the organic ligand. The sharp peak at ca. 1510 cm\({}^{-1}\) was assigned to the vibration of the C=C in the benzene ring. The peaks in the region 667-745 cm\({}^{-1}\) originated from the O-H and C-H vibrations modes of the organic linker. The vibration mode at 555 cm\({}^{-1}\) was attributed to the Zr-(OC) asymmetric stretch. These observations agree with those in previous reports [45; 46; 47]. The Pal/ UiO-66 presents a similar ATR spectrum to that of pure UiO-66. These results further suggest that the presence of Pal did not affect the synthesis of the MOF.