A review of the use of SrO in catalysts for biodiesel production

SrO has attracted attention for its use as a heterogeneous catalyst in producing biodiesel from transesterification owing to its high alkaline character and low solubility in methanol. In studies conducted in this decade, reaction conditions such as the methanol-oil molar ratio, reaction time, catalyst loading and temperature have been optimized in these systems. Furthermore, several works have been carried out to improve the physicochemical properties and increase the leaching resistance of this material. Thus, the new trends in using SrO in transesterification reactions are that this oxide should be supported or combined with Al2O3, TiO2, ZrO2, or Fe2O3, CaO, metal-organic frameworks, zeolites or La2O3. It has been observed that strontium can form mixed crystalline phases with these oxides by increasing the (M2+O2-) pairs and Lewis basicity of the oxygens, resulting in increased basicity and catalytic activity. Moreover, the transition metal oxides may provide Lewis acidity. Hence, these strontium-containing catalytic systems have demonstrated a bifunctionality that can perform esterification and transesterification reactions at the same time. In addition, the combination of these oxides decreases the leaching of the catalyst. Other materials such as metal-organic frameworks, zeolites, and La2O3 have been studied as SrO supports, showing that they can be used in oil transesterification reactions.