Thermochemical properties for a series of microporous borophosphates of MI[ZnBP2O8] (MI = Na, K, Rb, Cs)

Two pure microporous materials, Rb[ZnBP 2 O 8 ] and Cs[ZnBP 2 O 8 ], have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Rb[ZnBP 2 O 8 ] and Cs[ZnBP 2 O 8 ] (s) in 3   mol   ·   dm −3 HCl (aq), of Zn(PO 3 ) 2 (s) in (HCl   +   H 3 BO 3 ) (aq), and of RbCl (s)/CsCl (s) in [Zn(PO 3 ) 2   +   HCl   +   H 3 BO 3 ] (aq) were measured by microcalorimeter at T   =   298.15   K, respectively. With the incorporation of the previously determined enthalpy of solution of H 3 BO 3 (s) in 3   mol   ·   dm −3 HCl (aq), together with the use of the standard molar enthalpies of formation for Zn(PO 3 ) 2 (s), RbCl (s)/CsCl (s), H 3 BO 3 (s), HCl (aq) and H 2 O (l), the standard molar enthalpies of formation of −(3198.5   ±   1.2)   kJ   ·   mol −1 for Rb[ZnBP 2 O 8 ] and −(3212.7   ±   1.2)   kJ   ·   mol −1 for Cs[ZnBP 2 O 8 ] at T   =   298.15   K were obtained on the basis of the appropriate thermochemical cycle. In addition, the molar enthalpy of formation of −(2944.4   ±   1.2)   kJ   ·   mol −1 for [ZnBP 2 O 8 ] − has also been estimated by a group contribution method, which can be used to predict the Δ f H m θ ${\mathrm{\Delta }}_{\text{f}}{H}_{\text{m}}^{\mathrm{\theta }}$ of the other similar borophosphates. Moreover, these data have been further used to compare the thermodynamic stability of this series of borophosphates.