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).