Entropy of activation as a possible structure-sensitive parameter in the dehydroxylation of kaolinite

The kinetics of the dehydroxylation on 14 kaolinites having varying structural disorder (crystallinity) was studied by use of an isothermal TG method. Second-order kinetics were found to control the rate of water release during the dehydroxylation with up to 60% of conversion regardless of the crystallinity and/or temperature of the isothermal heating.

There is no simple relationship between crystallinity and activation energy (E). Unexpectedly high values of E have been determined for some extremely disordered structures. To explain this, the role of intracrystalline pressure of the gaseous product as well as the character of its diffusion paths in the decomposition reaction have been considered.

The crystallinity (index HWB) and activation entropy (ΔS^≠) are related by empirical straight-line function: HWB = (0.07064 ΔS^≠) + 17.65 (correl. coeff. r = 0.873) in which ΔS^≠ values increase with decreasing of kaolinite crystallinity (index HWB increases).