Investigating the Role of Brönsted and Lewis Acid Sites on the Esterification of Levulinic Acid over Ti–Al–SBA-15 Bifunctional Catalysts

In this study, titanium (Ti) and aluminum (Al) were simultaneously incorporated into the SBA-15 material using a one-step sol–gel method to obtain bifunctional acid catalysts. The physicochemical properties of the catalysts were thoroughly investigated, with particular focus on their acidity and the role of the Brönsted (BAS) and Lewis (LAS) acid sites during the esterification of levulinic acid with ethanol. Two different Si/Ti molar ratios (10 and 60) were used during the catalyst preparation, while the Si/Al molar ratio was kept constant at 10. The characterization results indicated that titanium primarily coordinates as tetrahedral Ti, yielding Lewis acid sites, with only minor amounts of TiO2 particles or agglomerated Ti species. On the other hand, when aluminum is incorporated into SBA-15, it exhibits both tetrahedral and octahedral coordination, creating Brönsted and Lewis acid sites, resulting in a BAS/LAS ratio of 1.72. When both titanium and aluminum are incorporated simultaneously into the SBA-15 material, the catalysts display bifunctional acid properties, with BAS/LAS ratios ranging from 0.96 to 0.69. Among the catalysts evaluated, the bifunctional Ti(10)–Al(10)–SBA-15 material exhibited the highest total acid site concentration (6.6 × 10–2 mmol/gcat) and showed the best catalytic performance in the esterification of levulinic acid with ethanol, achieving 98% conversion at 140 °C with a turnover frequency of 6.6 × 10–1 s–1. The enhanced catalytic activity toward ethyl levulinate formation is associated with the presence and cooperation of both Lewis and Brönsted acid sites in the catalyst. A first-order kinetic model accurately describes the reaction kinetics over this bifunctional catalyst, resulting in an activation energy of 49 kJ/mol, consistent with previously reported values. Moreover, this catalyst demonstrated notable stability, with the conversion of levulinic acid decreasing by only 0.67% over five consecutive reaction cycles.