Sulphated Inorganic Oxides for Methyl Esters Production: Traditional and Ultrasound-Assisted Techniques

Among all the tested sulfated Zr-based catalysts, commercial zirconium sulfate shows the highest activity in the FFA esterification, due to its high acidity. The main drawbacks of the use of Zr(SO4)2 still remain its partial solubility in water yielded by the reaction and the leaching of the active groups in methanol. Water removal from the system in the course of the reaction delayed catalyst deactivation but for insufficient times. Possible alternatives to Zr(SO4)2 are given by sulfated zirconia or supported sulfated materials. Nevertheless, these catalysts resulted in unsatisfactory catalytic performances due to the partial localization of the active sites inside the pores of the catalysts. This region cannot in fact be easily reached by sterically hindered molecules such as FFA. Sulfated TiO2-SnO2 systems exhibit higher acidity than the sulfated ZrO2 systems and their acidity increases with the TiO2 content. Consequently, also their activity in the FFA esterification is higher. Further results presented in this chapter have demonstrated how the surface acidity and specific surface area of sulfated zirconia can be increased by both adding TiO2 and using ultrasound (US) in precise experimental conditions to assist the sol–gel synthesis of the catalysts. The beneficial effects of the use of US in the sol–gel synthesis of the SO/ZrO-TiO4222− systems are ascribable to the occurrence of acoustic cavitation, which causes faster hydrolysis (sol–gel reaction) rates and surface damage. This effect is particularly evident for the catalysts obtained with the use of pulses longer than 0.3 s. For these catalysts, the acidity significantly increases along with the specific surface area if compared with the ones obtained with shorter pulses. The more efficient among the catalyst synthesized with US resulted to be the one obtained with continuous US and higher powers. For this sample, in fact, the lower surface area allows the location of the active acid sites mainly on the outer surface of the catalysts. In this way, the FFA molecules can undergo a faster catalytic transformation. Moreover, it has been demonstrated that this catalyst is the only one that possesses both Lewis and Brønsted acidity. Therefore, its good catalytic performances can be ascribed to the presence of both these active sites, together with a low value of SSA.