A series of four novel alkaline earth metal–organic frameworks constructed of Ca(ii), Sr(ii), Ba(ii) ions and tetrahedral MTB linker: structural diversity, stability study and low/high-pressure gas adsorption properties

A series of four novel microporous alkaline earth metal–organic frameworks (AE-MOFs) containing methanetetrabenzoate linker (MTB) with composition {[Ca(4)(μ(8)-MTB)(2)]·2DMF·4H(2)O}(n) (UPJS-6), {[Ca(4)(μ(4)-O)(μ(8)-MTB)(3/2)(H(2)O)(4)]·4DMF·4H(2)O}(n) (UPJS-7), {[Sr(3)(μ(7)-MTB)(3/2)]·4DMF·7H(2)O}(n) (UPJS-8) and {[Ba(3)(μ(7)-MTB)(3/2)(H(2)O)(6)]·2DMF·4H(2)O}(n) (UPJS-9) (UPJS = University of Pavol Jozef Safarik) have been successfully prepared and characterized. The framework stability and thermal robustness of prepared materials were investigated using thermogravimetric analysis (TGA) and high-energy powder X-ray diffraction (HE-PXRD). MOFs were tested as adsorbents for different gases at various pressures and temperatures. Nitrogen and argon adsorption showed that the activated samples have moderate BET surface areas: 103 m(2) g(−1) (N(2))/126 m(2) g(−1) (Ar) for UPJS-7′′, 320 m(2) g(−1) (N(2))/358 m(2) g(−1) (Ar) for UPJS-9′′ and UPJS-8′′ adsorbs only a limited amount of N(2) and Ar. It should be noted that all prepared compounds adsorb carbon dioxide with storage capacities ranging from 3.9 to 2.4 wt% at 20 °C and 1 atm, and 16.4–13.5 wt% at 30 °C and 20 bar. Methane adsorption isotherms show no adsorption at low pressures and with increasing pressure the storage capacity increases to 4.0–2.9 wt% of CH(4) at 30 °C and 20 bar. Compounds displayed the highest hydrogen uptake of 3.7–1.8 wt% at −196 °C and 800 Torr among MTB containing MOFs.