Ultrasonic enhancement of the acidity, surface area and free fatty acids esterification catalytic activity of sulphated ZrO2–TiO2 systems

Different samples of sulphated zirconia and mixed zirconia/titania (image and image) were prepared with traditional- and ultrasound (US)-assisted sol–gel synthesis and tested in the free fatty acids esterification. The catalysts were characterized through acid sites quantification by ion exchange, specific surface area and porosity measurements (N2 adsorption at 77 K), XRD, XPS and FT-IR spectroscopy. SEM–EDX and TEM analyses were also used to investigate the morphology of the samples. The results of this study demonstrate the possibility of increasing the acidity and the surface area of sulphated zirconia through the addition of TiO2 and tune the same properties with the continuous or pulsed US. It is also demonstrated that it is necessary to combine specific values of both acidity and surface area and which type of active sites are essential to obtain better catalytic performances in the free fatty acids esterification.