Kinetic and thermodynamic approaches on thermal degradation of sepiolite crystal using XRD-analysis

Abstract

A method was proposed to use XRD-data for thermal analyses of crystalline solids. Therefore, the XRD-pattern was recorded for natural and preheated sepiolite samples at the different temperatures in the interval of 25–800 °C for 4 h. The temperature-dependent intensity (I) of the most characteristic 001 reflection was used as a crystallite variable. The assumed parameters $k = - (\partial I/\partial T)_{\text{p}} / I$ and $K = \left( {1 - x} \right)/ x$ were calculated for each heating temperature, where I₀ is the intensity for the natural sample and $x = I/I_{0}$ is the relative crystallite of the heated ones, since the k and K supply Arrhenius equation and van’t Hoff equation behave as reaction rate constant and equilibrium constant for a chemical reaction, respectively. Arrhenius plot showed that the degradation has three kinetic steps. Two of these are due to the stepwise (1, 2) dehydration and third (3) originated from dehydroxylation of sepiolite crystal. Three activation energies were obtained such as $E_{1}^{\# } = 8.6$ kJ mol⁻¹, $E_{2}^{\# } = 28.5$ kJ mol⁻¹, and $E_{3}^{\# } = 124.8$ kJ mol⁻¹ from the slope of three intersected straight lines which are plotted according to the Arrhenius equation. Otherwise, van’t Hoff plot indicated that the degradation has two thermodynamic steps which are due to the dehydration (1, 2) and dehydroxylation (3). The basic thermodynamic relationship, $\Delta G^{0} = \Delta H^{0} - T\Delta S^{0} ,$ for these steps are $\Delta G_{1,2}^{0} = 46{,}933 - 65.7T$ and $\Delta G_{3}^{0} = 143{,}491 - 177.1T$, respectively. Finally, the spontaneous tendency for the thermal degradation was discussed.