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
Abstract
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 . 4H2O}(n)(UPJS-6), {[Ca(4)(μ(4)-O)(μ(8)-MTB)(3/2)(H2O)(4)] . 4DMF . 4H2O}(n)(UPJS-7), {[Sr(3)(μ(7)-MTB)(3/2)] . 4DMF . 7H2O}(n)(UPJS-8) and {[Ba(3)(μ(7)-MTB)(3/2)(H2O)(6)] . 2DMF . 4H2O}(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 degrees C and 1 atm, and 16.4-13.5 wt% at 30 degrees 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 degrees C and 20 bar.
Compounds displayed the highest hydrogen uptake of 3.7-1.8 wt% at -196 degrees C and 800 Torr among MTB containing MOFs.