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

• Rb[ZnBP2O8] and Cs[ZnBP2O8] samples were synthesized and characterized.
• Three kinds of molar enthalpies of solution were measured, respectively.
• ΔfHmθ for M[ZnBP2O8] (M = Rb, Cs) were obtained by a thermochemical cycle.
• ΔfHmθ of a series of M[ZnBP2O8] (M = Na, K, Rb, Cs) were estimated.
• The thermodynamic stability of this series of borophosphates was compared.

Two pure microporous materials, Rb[ZnBP2O8] and Cs[ZnBP2O8], have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Rb[ZnBP2O8] and Cs[ZnBP2O8] (s) in 3 mol · dm−3 HCl (aq), of Zn(PO3)2 (s) in (HCl + H3BO3) (aq), and of RbCl (s)/CsCl (s) in [Zn(PO3)2 + HCl + H3BO3] (aq) were measured by microcalorimeter at T = 298.15 K, respectively. With the incorporation of the previously determined enthalpy of solution of H3BO3 (s) in 3 mol · dm−3 HCl (aq), together with the use of the standard molar enthalpies of formation for Zn(PO3)2 (s), RbCl (s)/CsCl (s), H3BO3 (s), HCl (aq) and H2O (l), the standard molar enthalpies of formation of −(3198.5 ± 1.2) kJ · mol−1 for Rb[ZnBP2O8] and −(3212.7 ± 1.2) kJ · mol−1 for Cs[ZnBP2O8] 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 [ZnBP2O8]− has also been estimated by a group contribution method, which can be used to predict the ΔfHmθ of the other similar borophosphates. Moreover, these data have been further used to compare the thermodynamic stability of this series of borophosphates.