Selective CO₂ Adsorption on Metal-Organic Frameworks Based on Trinuclear Cu₃-Pyrazolato Complexes: An Experimental and Computational Study

Abstract

Two related metal-organic frameworks (MOFs) based on trinuclear copper-pyrazolato structural building units (SBUs) connected by bipyridine linkers have been prepared and studied. A small chemical modification to the supporting pyrazolato ligands of a [Cu 3 ]-SBU results in two vastly different (three-fold vs eight-fold interpenetrated) MOF structures. The gas sorption properties of the two MOFs have been determined experimentally in the 0 to 10 atm pressure range. Both MOFs sorb CO 2 selectively over N 2 and H 2 , showing hysteretic sorption−desorption profiles. The hysteretic behavior is attributed to the structural flexibility of the lattices, probed by powder X-ray diffraction before and after CO 2 sorption. As the metal centers and hydroxy groups of the MOF surfaces are sterically hindered, the sorbed dipolar CO 2 molecules are attached with low sorption enthalpy to the organic groups, as determined by theoretical calculations.

Understanding carbon dioxide capture on metal–organic frameworks from first-principles theory: The case of MIL-53(X), with X = Fe3+, Al3+, and Cu2+

“…The applicability of metal-organic frameworks to gas capture at low pressures has direct association with certain macroscopic properties, such as adsorption capacity and heat/enthalpy of adsorption. 12,28 The latter has a thermodynamic definition that requires the inclusion of thermal corrections to the total energies for its full assessment, i.e., zero point energies and thermal effects acting over the internal energy of each system, but the main contributing term is the total energy itself. 17 Hence, variations in this property prior and posterior to adsorption give a reliable estimation of the enthalpy of reaction/adsorption that is crucial to evaluate how good the material is for gas capture.…”

“…6 Metal-organic frameworks (MOFs) constitute a class of threedimensional porous materials formed by interconnecting inorganic and organic counterparts, which has found large applicability in this field. [12][13][14][15][16] In particular, the MIL-53 frameworks comprise an inorganic region formed by the metallic centers connecting oxygen atoms from hydroxyl groups (axial positions) or benzene dicarboxylate (BDC) ligands (equatorial positions) in an octahedral configuration. [17][18][19] In recent years, the applicability of this series has been widely evaluated by means of experimental 14,18,[20][21][22][23][24][25] and theoretical methods 17,26 for gas adsorption, including carbon dioxide, 14,18,[20][21][22][23]26,27 methane, 18,23,25,26 and hydrogen sulfide.…”

“…24 In general, the presence of open metal sites with appropriate geometry and pore size in metal-organic frameworks is directly associated with high adsorption capacity and selectivity. 12,28 Additionally, the material should present high heat or enthalpy of adsorption for good performance at low pressures. 28 This important macroscopic quantity is directly associated with the gas-framework interaction strength, which is expected to be strong enough to maintain the latter inside the pore through weak interactions at the end of the process and also provide a suitable post-processing based catalytic reaction where the activated carbon dioxide can be converted in raw materials.…”

“…31 Furthermore, pore functionalization by other chemical groups, including methyl, hydroxyl, and carboxyl groups, has been reported. 12,27,28 In this sense, Torrisi et al 27 have shown that embedding carboxyl and hydroxyl groups into MIL-53(Al 3+ ) is particularly advantageous for gas capture applications in comparison to amine functionalities. Nonetheless, anchoring chemical groups in metal-organic frameworks is not always straightforward in a practical point of view since the synthesis conditions, given by high pressures and temperatures, do not favor the anchoring process of several chemical functionalities.…”

Synthesis of amine-modified solid Fe-Zr adsorbents for CO2adsorption

“…As a consequence, carbon capture and sequestration (CCS) has become a global requirement. For efficient capture of CO 2 at low temperature, materials such as zeolites, carbons, polymers as well as those with superamolecular, covalent organic, and metal–organic frameworks (MOFs) have been developed.…”