Thermochemical properties of sodium phenoxide and sodium cyclohexanolate

Organic hydrogen carriers are attractive for hydrogen storage because the reaction enthalpies of many systems are favorable for the realization of a storage cycle. Recent studies have shown that this thermodynamic advantage can be further enhanced through the formation of an aromatic organic salt system. This study investigates the physicochemical properties of sodium phenoxide and sodium cyclohexanolate. The enthalpies of formation in the solid phase at 298.15 K were determined using solution calorimetry and validated using quantum-chemical methods applied to calculate the enthalpy of formation in the gas phase and the sublimation enthalpy. Replacing the hydroxyl proton with a sodium ion in the studied compounds lowers the standard enthalpy of formation in the solid phase by about 160 kJ mol^-1 for the aromatic phenol and roughly 130 kJ mol^-1 for the aliphatic cyclohexanol. This 30 kJ mol^-1 difference highlights a stronger thermodynamic stabilization of the phenolate anion relative to cyclohexanolate, arising from the charge delocalization in the aromatic system upon substitution of the hydroxyl proton with a sodium ion. Therefore, the reaction enthalpy for the dehydrogenation of the organic sodium salt system is significantly lower compared to that of the phenol-cyclohexanol system, with a decrease of approximately 10 kJ per mole of hydrogen. Furthermore, heat capacity measurements by DSC demonstrated that the temperature dependence of the hydrogenation enthalpy of solid sodium phenoxide follows the same trend as that of typical liquid hydrogen carriers, indicating that this reduction in reaction enthalpy remains stable at least up to the maximum investigated temperature of 473 K.