## 3 Results and Discussion
### The Esterification of LA with Ethanol over Various Solid Acid Catalysts  
HUSY zeolite, HMS-SO\({}_{3}\)H, and NRHMS-SO\({}_{3}\)H were comparatively tested for the esterification of LA with ethanol. As shown in Figure 8, HUSY zeolite produced a turnover number (TON) of 15.74. Most of the synthesized mesoporous catalysts provided relatively high activity compared to HUSY, due to its microporous network, thereby limiting the diffusion of reactants to the active sites. Thus, intermediates and transition states in the esterification of LA possibly avoided accommodating inside the HUSY structure, and the reaction occurred at the external surface [12].  
HMS-SO\({}_{3}\)H(0.2) and NRHMS-SO\({}_{3}\)H(0.2) exhibited low TONs of 13.63 and 15.02, respectively. Despite relatively high surface area and pore size, a low amount of active functional groups may not be conducive to efficient LA esterification [46]. [PERSON] et al. [15] proposed the effect of surface functional groups on the performance of solid acid catalysts, wherein the remaining hydroxyl groups interacted with \(\gamma\)-keto groups of LA, thus concentrating the reactant molecules on the catalyst surface and facilitating the esterification. This study addressed the importance of suitable acid site density, essential for outstanding catalytic activity. For HMS-SO\({}_{3}\)H (0.4) and NRHMS-SO\({}_{3}\)H (0.4), an increase in the acid site density improved their TON values to 24.99 and 48.13, respectively. However, using a high MPTMS/TEOS ratio of 0.6 reduced TON. The considerably high density of acid sites on HMS-SO\({}_{3}\)H (0.6) and NRHMS-SO\({}_{3}\)H (0.6) hampered the product yield due to the increased water affinity on the catalyst surface and decreased surface area, promoting reversed esterification [47; 48]. Moreover, for all MPTMS loading levels, NRHMS-SO\({}_{3}\)H showed higher activity than the HMS-SO\({}_{3}\)H counterparts, suggesting that the rubber moieties in the nanocomposite catalysts increased the surface hydrophobicity, preventing the adsorption of water molecules, thereby retarding the ester hydrolysis.  
TON was then plotted as a function of acid site density to reveal the major factor affecting the catalytic activity. As shown in Figure 9, the density of acid sites strongly influenced the activity of these catalysts in LA esterification. Nevertheless, the correlation curves categorized the catalysts into two groups. The catalysts with relatively low V\({}_{\text{m, H2O}}\) (Table 2) exhibited TON values more sensitive to the acid site density, revealing the crucial role of hydrophobic properties of the solid catalysts in ethyl levulinate formation. From the catalyst screening study, we concluded that NRHMS-SO\({}_{3}\)H (0.4) was a suitable catalyst for LA esterification with ethanol due to its optimum surface hydrophobicity and acid site density.  
Figure 8: Turnover number (TON) of different solid acid catalysts used in the LA esterification with ethanol (reaction condition: Catalyst loading, 2.5 wt.% based on LA weight; LA:ethanol molar ratio, 1:5; reaction temperature, 80 \({}^{\circ}\)C; reaction time, 5 h; TON was calculated from moles of total ester formed divided by the total number of acid sites determined via acid–base titration).