## 3 Results and Discussion
### FGD Through Zeolite 13X
#### 3.3.3 13X Characterization
The latter is derived from the combination of the O1-O1 and O4-O4 bond distances of the oxygen framework [15]. Specific features, such as a framework topology and the type/location of the charge-balancing cations, can affect the deformation mechanism; thus, when the pore is loosely filled with the adsorbate, the adsorbing molecules can exert a pressure on the pore walls, thus deforming the zeolite framework, as was observed for 13X. Notably, the increase in the unit-cell volume as well as the pore apertures (CFAs) (Table S12 and Table S15, respectively) were more pronounced in 13X [75 \({}^{\circ}\)C_45 RH%] than in 13X [75 \({}^{\circ}\)C_30 RH%]. The diffusion of both H\({}_{2}\)O and SO\({}_{2}\) through the zeolite framework can potentially be attributed to two concomitant factors: increased hydration of 13X at 45 RH% or higher sulfur uptake during the FGD process. To gain further insight into the two systems, thermogravimetric (TG) and microchemical EDS analyses were performed. Figure S15 shows the thermal behavior of the 13X zeolite samples. The pristine zeolite displayed the most significant weight loss during the heating process in comparison to the zeolites in the uptake experiments. The 13X samples showed a weight loss of \(\approx\)25 wt% for the pristine sample and 20 wt% after the FGD experiments. The weight loss at low temperatures (up \(\sim\)150 \({}^{\circ}\)C) indicates both SO\({}_{2}\) and water molecules physiosorbed on the surface. This process is accompanied by endothermic reactions, as evidenced by the DTA data. A further weight loss took place and persisted until approximately 400 \({}^{\circ}\)C. This phenomenon can be ascribed to the chemosorbed water molecules, which were trapped as extra-framework species in all the samples. The exothermic peak at \(\sim\)850 \({}^{\circ}\)C is associated with structural zeolite collapse. Slight differences in the maxima at high temperatures are indicative of differences in thermal stability. This last may be attributed to the presence  
of guest molecules within the zeolite structure which could have modified the thermal behavior of the material [49]. Micromorphological investigation revealed that the FGD experiments did not significantly alter the size of 13X particles or their state of aggregation (see Figure S16). The SEM-EDS measurements performed on zeolite samples corroborate their composition with respect to the major and minor components, namely, Na, Al, Si, and O (see Table S16). In addition to the aforementioned elements, Ca and Cl were occasionally observed in the zeolite samples, just above their respective detection limits. The slight oxygen deficiency observed in all analyses can be ascribed to the well-known difficulty of the EDS system in accurately dosing light (Z \(<\) 9) elements. It is noteworthy that while the breakthrough sulfur capacity of 13X [75 \({}^{\circ}\)C\(\_\)30 RH%] was significantly higher than that of 13X [75 \({}^{\circ}\)C\(\_\)45 RH%], the S contents of both samples after the uptake measurement were nearly identical, namely, 3.69 and 3.82 wt%, respectively. This phenomenon may be attributed to a reduction in the stability of the SO\({}_{2}\) adsorbate generated at a lower RH%, which would have been desorbed from 13X [75 \({}^{\circ}\)C\(\_\)30 RH%] shortly after the FGD process. Indeed, the presence of water molecules may exert two opposing effects on the uptake process. On the one hand, both H\({}_{2}\)O and SO\({}_{2}\) may compete for the same adsorption sites [50]. This would result in a reduction in the breakthrough sulfur capacity of 13X, as observed when the humidity was increased from 30 RH% to 45 RH%. Conversely, intrazeolitic water may facilitate the sorption of SO\({}_{2}\) through the zeolite framework by the Grothuss mechanism, which works as a carrier [51]. This possibility entails the formation of H\({}_{2}\)SO\({}_{3}\) within the Na-Y zeolite cavities [51; 52].