Ultrasound-assisted esterification of levulinic acid for sustainable butyl levulinate production over tin(II)-modified silicotungstic acid supported on activated bentonite

Leveraging ultrasound irradiation and Brønsted–Lewis acid synergy, butyl levulinate (BL), a promising bio-based fuel additive, was synthesized by esterifying levulinic acid (LA) with n -butanol over tin(II)-modified silicotungstic acid supported on activated bentonite (Sn x STA/act-bent). Catalysts were prepared by ultrasound-assisted impregnation followed by partial proton exchange with Sn²⁺ at different loadings, generating bifunctional acidic sites. Compared with the parent STA/act-bent, Sn incorporation increased overall acidity and optimized the acid-site distribution, improving catalytic performance. Among the series, Sn₂STA/act-bent showed the highest activity because of its balanced Brønsted–Lewis acidity and favorable textural properties. The effects of reaction temperature and time under ultrasonic irradiation were systematically investigated at a fixed LA: n -butanol molar ratio of 1:10 and 20 wt% catalyst loading. Under optimal conditions, complete LA conversion and a BL yield of 97.00% were obtained at 80 °C within 100 min. The catalyst maintained high activity over seven consecutive cycles, demonstrating good stability and reusability. The combination of ultrasound-assisted processing and a low cost Sn modified heteropoly acid–clay catalyst thus provides an efficient and sustainable route for producing biomass-derived BL as a value-added renewable fuel additive.