## 1 Introduction  
Levulinic acid (LA), 4-oxo pentanoic acid, or \(\gamma\)-ketovaleric acid is a versatile biomass-derived C5 chemical platform that has attracted vast attention in the synthesis of renewable chemicals, fuels, lubricants, and polymers. LA is a short-chain fatty acid molecule (C5H8O3), possessing a ketone carbonyl group (C = O) and an acidic carboxylic group (COOH) [1, 2, 3, 4], which interacts with different functional groups to form various derivatives, making it an ideal chemical building block [5]. Alkyl levulinate esters (ALE), commonly produced by acid-catalyzed LA esterification with simple alcohols, namely primarily ethanol and \(n\)-butanol, belong to a class of bio-based chemicals with numerous applications, such as green solvents, latex coatings, flavorings, and fragrance [6]. ALE also forms a viable petroleum-diesel-miscible fuel component, preventing fossil fuel depletion and carbon dioxide emission. These esters exhibit exceptional characteristics, including good energy content, high lubricity, and sulfur-free composition. In addition, ALE possesses properties that resemble those of fatty acid methyl esters, but with better fluidity under low-temperature conditions and flash point stability [7; 8; 9]. Esterification is commercially performed in the liquid phase using homogeneous catalysts, such as sulfuric acid and \(p\)-toluenesulfonic acid [10; 11]. Even though these soluble acids provide a high yield, high conversion, and short reaction time, there are several drawbacks including equipment corrosion, catalyst separation, product quality, catalyst toxicity, and waste generation. Heterogeneous catalysis exhibits superior characteristics over homogenous catalysis due to their simplified separation, recyclability, and environmental friendliness, thus attracting immense interest for esterification [12]. However, the conventional solid acid catalysts, including zeolites and ion-exchange resins, have some limitations in terms of their texture and thermal stability [13]. [PERSON] et al. [14] developed a series of sulfonic acid-containing carbons derived from phenolic resins with tunable pore sizes and surface chemical functionalities. The hydrophobicity and substantial pore size enhanced their catalytic performances in an acid-catalyzed liquid phase reaction. Furthermore, by precisely tuning the sulfonic acid group density, the activity of carbon-based catalysts was improved in LA esterification with ethanol due to hydrogen-bonding between the functional group on the catalyst surface and the \(\gamma\)-keto group of LA [15]. Propylsulfonic acid-functionalized mesoporous silica (HMS-SO\({}_{3}\)H) is a heterogeneous acid catalyst, promising for esterification due to its high surface area, shape-selectivity, and strongly acidic moieties [16]. The propylsulfonic group precursor (3-mercaptoethanolpropyl) trimethoxysilane (MPTMS) can be anchored on the mesoporous silica surface via post-grafting or the co-condensation method. The direct synthesis of functionalized mesoporous silica via sol-gel chemistry, wherein the silica source is co-condensed with MPTMS [17], provides better loading and distribution of functional groups than the post-grafting approach via the silylation reaction [18; 19; 20]. However, pure silica-based mesostructured catalysts show high affinity to water due to the high content of surface silanol groups (Si-OH), retarding the free fatty acid esterification [21]. This drawback greatly limits the ester production since esterification is a reversible reaction liberating water as the by-product, which further promotes the hydrolysis of esters back into the reactants, thus lowering the conversion [22; 23]. To overcome this problem, the materials are made hydrophobic to continuously remove water from the porous system. Therefore, mesostructured polymer/silica nanocomposites with enhanced hydrophobicity have been proposed using natural rubber as a hydrophobicity-enhancing agent [24]. [PERSON] et al.