## 3 Results
### Catalytic Performance Studies  
The catalytic performance of the prepared sulfonic acid-functionalized silica, SBA-15-PSSA, was evaluated in the esterification of AA with nHTL. This reaction was chosen to produce an appropriate ester with moderate molecular weight and hydrophobicity. To prevent the evaporation of materials, including reactants, internal standard, and product, a refluxing system was used, and the reaction temperature was kept below the lowest component's boiling point (b.p. = 117, 175, 227, and 216 \({}^{\circ}\)C for AA, nHTL, nDD, and HA, respectively). The reaction kinetic (0-160 min) and substrate conversion at different temperatures (50, 80, and 110 \({}^{\circ}\)C) and catalyst doses (0.1, 0.2, and 0.3 g) were monitored by GC. The esterification was conducted employing fixed amounts of alcohol (nHTL, 9.06 g) and internal standard (nDD, 0.2 g), and at a stirring rate of 400 rpm. In Figure 7 are reported reaction data obtained in the presence and absence of catalyst SBA-15-PSSA at 110 \({}^{\circ}\)C. Evidence indicates the uncatalyzed reaction to be very slow, with calculated conversion of 33% after 160 min. However, results of experiments on the catalyzed reactions indicate catalyst-dose-dependent conversion. Moreover, the high AA conversion values and the speed of the reaction prove that the prepared SBA-15-PSSA is an excellent solid acid catalyst, with \(>\)90% AA conversion observed after 40 min. The esterification activities at 80 and 50 \({}^{\circ}\)C as a function of the catalyst amount are reported in Figures 8 and 9, respectively. Generally, results indicated that AA conversion increased with the temperature, catalyst amount, and reaction time. Moreover, the majority of the AA was converted to ester during the first 20 min; at 110 \({}^{\circ}\)C and in the presence of 0.3 g of catalyst, AA conversion was 97% (Figure 7). Regardless of the catalyst amount, the esterification activity at 110 \({}^{\circ}\)C was the highest, with conversion values of 100% after 140, 100, and 60 min, when the catalyst amount was 0.1, 0.2, and 0.3 g, respectively. After 160 min,  
Figure 6: N\({}_{2}\) adsorption–desorption isotherms of SBA-15-PSSA. In the inset is the pore size distribution of SBA-15-PSSA. SBA-15: pore-expanded mesoporous silica; SBA-15-PSSA: SBA-15-supported polystyrene sulfonic acids; STP: standard temperature and pressure. the obtained maximum conversion values at 80 \({}^{\circ}\)C were 94, 97, and 98%, and at 50 \({}^{\circ}\)C they were 52, 69, and 79%, with catalyst doses of 0.1, 0.2, and 0.3 g, respectively (Figures 7-9). The high performance of the SBA-15-PSSA in terms of AA conversion into the corresponding ester can be attributed to an adequate combination of both SBA-15 and PSSA properties represented by the large mean pore size and the hydrophobic polymeric chain, respectively, resulting in an improved diffusion of hydrophobic reactants and products [6; 10], the nHTL and HA respectively. The higher hydrophobicity of the sulfonic acid site microenvironment reduces the poisoning effect by water by-product of the esterification reaction. Moreover, the structure of the synthesized SBA-15-PSSA facilitates the exposure of the acid sites, resulting in an elevated performance even at lower acid contents [10]. Figure 8: Acetic acid (AA) conversion in the catalyzed esterification reaction (with 0.1, 0.2, or 0.3 g of catalyst) at 80 \({}^{\circ}\)C as a function of time. Figure 7: Acetic acid (AA) conversion in the catalyzed (with 0.1, 0.2, or 0.3 g of catalyst) and uncatalyzed esterification reaction conducted at 110 \({}^{\circ}\)C as a function of time.