Abstract
A series of four novel microporous alkaline earth metal–organic frameworks (AE-MOFs) containing methanetetrabenzoate linker (MTB) with composition {[Ca4(μ8-MTB)2]·2DMF·4H2O}n (UPJS-6), {[Ca4(μ4-O)(μ8-MTB)3/2(H2O)4]·4DMF·4H2O}n (UPJS-7), {[Sr3(μ7-MTB)3/2]·4DMF·7H2O}n (UPJS-8) and {[Ba3(μ7-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 m2 g−1 (N2)/126 m2 g−1 (Ar) for UPJS-7′′, 320 m2 g−1 (N2)/358 m2 g−1 (Ar) for UPJS-9′′ and UPJS-8′′ adsorbs only a limited amount of N2 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 CH4 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.
Highlights
Metal–organic frameworks (MOFs) represent a class of inorganic–organic hybrid materials constructed from metal ions or clusters and organic linkers to form porous polymeric frameworks with interesting properties
All the compounds investigated in the present study were prepared using a solvothermal synthesis under mild synthetic conditions (80–110 C) and in a molar ratio of 5 : 1. It is known that the synthesis of MOF may be included in so-called combinatorial synthetic chemistry since the successful preparation of porous coordination polymers depends on many factors such as: reactant concentration, the presence of a cosolvent, solution pH, metal to ligand ratio, properties of the used counterion, reaction temperature, and reaction time, can have a considerable impact on the nal composition of the products.[26]
Compound UPJS-6 was prepared at 80 C and represents the lowtemperature phase and compound UPJS-7 was synthesized at 110 C, which can be referred as the high temperature form
Summary
Hydrogen is the smallest and simplest gas molecule with a very low boiling point, and its storage and transportation present a challenge.[6] The second technologically important gas is carbon dioxide, which is industrially used to prepare many intermediate or ne chemicals. CO2 is the main component of greenhouse gases and represents one of the largest anthropogenic impacts on the environment For this reason, it is necessary to reduce its concentration in the atmosphere and limit global warming.[7] Physical absorption of hydrogen and carbon dioxide in porous MOFs represents an interesting approach to the storage of the mentioned gases. Based on the HSAB theory, AE metals fall under the hard acids and coordinate well with carboxylate functionalities.[8] Carboxylate linkers with tetrahedral symmetry are highly suitable for MOF preparation, because they form the desired porous frameworks with large surface areas.
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