Synthesis of biodiesel catalyst CaO·ZnO by thermal decomposition of calcium hydroxyzincate dihydrate CaZn2(OH)6·2H2O: kinetic studies and mechanisms

In this contribution, we are presenting the study of structural and thermal decomposition kinetics of synthesized calcium hydroxyzincate dihydrate CaZn 2 (OH) 6 ·2H 2 O (CZO). The characterization of the prepared and calcined precursors was done by X-ray diffraction and Fourier Transform Infrared techniques and indicates that CZO exhibits a monoclinic structure with space group P2 1 /c . Calcining this material at 200 °C leads to a heterogeneous mixture composed of ZnO, Ca(OH) 2 , and an amorphous Zn(OH) 2 . When the temperature is >400 °C, only ZnO and CaO phases were detected by XRD analysis. The thermal decomposition kinetics of CZO was evaluated by thermogravimetry and derivative thermogravimetry in N 2 atmosphere under non-isothermal conditions. The kinetic parameters such as the activation energy ( E α ) and reaction model ( f ( α )) revealed the complex nature of the decomposition process, consisting of several steps. The separation of overlapped peaks was performed by applying the Fraser–Suzuki procedure to the experimental DTG data and the activation energy was then calculated for each individual peak by Friedman method confirming the fair single-step mechanism of the first three processes. On the basis of kinetic results, a thermal decomposition mechanism of CZO material has been proposed and correlated to structural nature and stability of the resulting phases. The CaO∙ZnO based catalyst is formed according to a limited nucleation and growth mechanism at low temperatures and three complex processes occur afterwards when the structure cleaves. Graphical abstract