Thermodynamic properties of two microporous materials for Na2[M2B12O21] (M = Co2+, Cu2+)

• Na2Co2B12O21 and Na2Cu2B12O21 have been synthesized and characterized.
• Enthalpies of solution of both microporous borates were measured by microcalorimeter.
• Enthalpies of solution of CoCl2·6H2O(s) and CuCl2·2H2O(s) were measured, respectively.
• ΔfHmθ for both transition metal borates were obtained by thermochemical cycles.
• Na2Co2B12O21(s) is more thermal stable than that of Na2Cu2B12O21(s).

Two transition metal borates with microporous structures for Na2Co2B12O21 and Na2Cu2B12O21 have been synthesized and characterized by XRD, FT-IR, and elemental analysis. The molar enthalpies of solution of Na2Co2B12O21(s) and Na2Cu2B12O21(s) in 1 mol dm−3 HCl(aq) were measured by microcalorimeter at T = 298.15 K, respectively. The molar enthalpies of solution of CoCl2·6H2O(s) and CuCl2·2H2O(s) in the mixture solvent of 2.00 cm3 of 1 mol dm−3 HCl(aq) in which 5.30 mg of H3BO3 and 0.84 mg of NaCl being added were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3(s) in 1 mol dm−3 HCl(aq), together with the standard molar enthalpies of formation for CoCl2·6H2O(s)/CuCl2·2H2O(s), NaCl(s), H3BO3(s), HCl(aq) and H2O(l), the standard molar enthalpies of formation of – (9072.4 ± 9.7) kJ mol−1 for Na2Co2B12O21(s) and – (8887.5 ± 9.7) kJ mol−1 for Na2Cu2B12O21(s) at T = 298.15 K were obtained on the basis of the appropriate thermochemical cycles. It is obvious that Na2Co2B12O21(s) is more thermal stable than that of Na2Cu2B12O21(s).