Undoped tetragonal ZrO2 obtained by the Pechini method: thermal evaluation of tetragonal–monoclinic phase transition and application as catalyst for biodiesel synthesis

Zirconia (ZrO2) is a well-known polymorphic material, and the monoclinic phase of this compound is the most stable phase at room temperature. Nevertheless, tetragonal zirconia has been obtained at room temperature without doping using chemical synthesis methods. In the present work, tetragonal undoped ZrO2 was obtained by the Pechini method, and the tetragonal–monoclinic phase transition was evaluated by thermogravimetry, differential thermal analysis (DTA) and in situ high-temperature X-ray diffraction (HT-XRD). The tetragonal phase was obtained as the major phase after calcination at and below 800 °C, while most of the transition to the monoclinic phase took place during cooling at temperatures below 500 °C, as indicated by DTA and HT-XRD. The pure monoclinic phase was obtained after calcination at 1000 °C. Samples calcined at 600 and 1000 °C were sulphated and applied in the ethylic transesterification of soybean oil. No meaningful difference was observed in the activity, and high efficiency was attained for both catalysts for the reaction performed at 100 °C for 2 h.