Organic–inorganic hybrid solid acid catalyst supported on titanium nanotubes for efficient synthesis of n-butyl levulinate

Alkyl levulinates (AL) are a class of important organic chemicals that play a significant role in organic synthesis, biofuel additives, and cleaning agents. In this work, low-dimensional titanium dioxide nanotubes were prepared under hydrothermal conditions as precursors, which were further modified to fabricate organic–inorganic hybrid solid acid catalyst (TNTs-PhSO 3 H). The catalyst was characterized using various techniques including Fourier transform infrared spectroscopy (FT-IR), thermogravimetric-differential thermal analysis (TG-DTA), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results indicated that the amount of bonded sulfonic acid in the catalyst was considerable, reaching up to 1.45 mmol/g. The esterification of levulinic acid (LA) with n -butanol was used as a probe reaction to systematically investigate the influence of reaction time, temperature, catalyst dosage, and molar ratio of reactants on the esterification reaction by the single-factor experiments. The catalytic reaction process was further optimized via the response surface methodology (RSM) using Box-Behnken design (BBD) method. Under the conditions of a catalyst dosage of 12 % (based on the mass of LA) at 120 °C for 3 h, the yield of the desired product, n- butyl levulinate, could reach 98.0 %. The catalyst maintained stable catalytic performance after being reused at least five times.