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

Two transition metal borates with microporous structures for Na 2 Co 2 B 12 O 21 and Na 2 Cu 2 B 12 O 21 have been synthesized and characterized by XRD, FT-IR, and elemental analysis. The molar enthalpies of solution of Na 2 Co 2 B 12 O 21 (s) and Na 2 Cu 2 B 12 O 21 (s) in 1   mol   dm −3 HCl(aq) were measured by microcalorimeter at T   =   298.15   K, respectively. The molar enthalpies of solution of CoCl 2 ·6H 2 O(s) and CuCl 2 ·2H 2 O(s) in the mixture solvent of 2.00   cm 3 of 1   mol   dm −3 HCl(aq) in which 5.30   mg of H 3 BO 3 and 0.84   mg of NaCl being added were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H 3 BO 3 (s) in 1   mol   dm −3 HCl(aq), together with the standard molar enthalpies of formation for CoCl 2 ·6H 2 O(s)/CuCl 2 ·2H 2 O(s), NaCl(s), H 3 BO 3 (s), HCl(aq) and H 2 O(l), the standard molar enthalpies of formation of – (9072.4   ±   9.7)   kJ   mol −1 for Na 2 Co 2 B 12 O 21 (s) and – (8887.5   ±   9.7)   kJ   mol −1 for Na 2 Cu 2 B 12 O 21 (s) at T   =   298.15   K were obtained on the basis of the appropriate thermochemical cycles. It is obvious that Na 2 Co 2 B 12 O 21 (s) is more thermal stable than that of Na 2 Cu 2 B 12 O 21 (s).