DFT-based evaluation of porous metal formates for the storage and separation of small molecules

A Multifaceted Study of Methane Adsorption in Metal–Organic Frameworks by Using Three Complementary Techniques

“…The MOF α‐Zn 3 (HCO 2 ) 6 is analogous to α‐Mg 3 (HCO 2 ) 6 , featuring zinc metal centers and formate linkers that form a similar 3D crystalline structure, with a slightly smaller channel diameter of 4.44 Å in the Zn analogue versus 4.58 Å in the Mg variant . Although CH 4 adsorption in α‐Zn 3 (HCO 2 ) 6 has not been extensively studied, a recent computational study has indicated that α‐Zn 3 (HCO 2 ) 6 exhibits a stronger interaction with methane than α‐Mg 3 (HCO 2 ) 6 ; the reported calculated adsorption energy of methane within α‐Zn 3 (HCO 2 ) 6 is −27.6 versus −25.7 kJ mol −1 in α‐Mg 3 (HCO 2 ) 6 . In order to establish the methane adsorption site locations in α‐Zn 3 (HCO 2 ) 6 and to understand the reasons for the subtle difference in the methane adsorption energies between the two formate MOFs, SCXRD, DFT, and 2 H SSNMR experiments have been performed on α‐Zn 3 (HCO 2 ) 6 .…”

“…The CH 3 D carbon atom is also found 3.753 Å away from the oxygen atom of the formate linker, which is part of the ZnO 6 octahedron with the Zn3 atom located at the center (Figure f), hinting at an interaction between the formate oxygen atom and the methane molecule. The aforementioned interatomic distances in α‐Zn 3 (HCO 2 ) 6 are relatively shorter than the 3.814 Å methane carbon atom–framework oxygen atom and 3.799 Å methane carbon atom–framework carbon atom distances within α‐Mg 3 (HCO 2 ) 6 , which is in good agreement with a prior DFT computational study that indicated that α‐Zn 3 (HCO 2 ) 6 possesses a higher methane binding strength versus α‐Mg 3 (HCO 2 ) 6 . The α‐Mg 3 (HCO 2 ) 6 and α‐Zn 3 (HCO 2 ) 6 MOFs feature fully saturated metal centers and have the same topology as well as organic linker, thus, any changes in the adsorption strength cannot be directly related to the nature of the metal center.…”

“…[1a, 7] a-Mg 3 (HCO 2 ) 6 is an ultra-microporous MOF with relativelys mall one-dimensional zigzag-shaped channels that measure approximately 4.8 in diameter. [21a] CH 4 adsorption studies [34] and DFT calculations [35] have indicated that a-Mg 3 (HCO 2 ) 6 is ap romising methane adsorbent andp ossesses a good selectivity of CH 4 over N 2 gas. [34b, 35] However,t here has not been ac omprehensive investigation of a-Mg 3 (HCO 2 ) 6 to determine the specific methane adsorption site locations and host-guest interactions involved.…”

“…DFT geometry optimizations and calculations based on our SCXRD structure were performed to investigate the methane adsorption energy.T he calculated adsorption energy of methane within a-Zn 3 (HCO 2 ) 6 at al oading level of 1.0 methanep er unit cell is À27.9 kJ mol À1 (Table S1 in the Supporting Information), which is quite similar to the value of À27.6 kJ mol À1 produced by previous DFT calculations. [35] It is very interesting to note that the non-dispersive contribution in a-Zn 3 (HCO 2 ) 6 is À5.3 kJ mol À1 (Table S1 in the Supporting Information), which is only 0.5 kJ mol À1 stronger than that of a-Mg 3 (HCO 2 ) 6 (À4.8 kJ mol À1 ), however,t he dispersive contribution in a-Zn 3 (HCO 2 ) 6 is À22.6 kJ mol À1 ,w hich is 2.3 kJ mol À1 stronger than that of a-Mg 3 (HCO 2 ) 6 (Table S1 in the Supporting Information). These values reveal that the stronger methane binding strength in a-Zn 3 (HCO 2 ) 6 is likely due to increased van der Waals interactions arising from the smaller pore size.…”

“…The fluorine atoms are axially coordinated to Zn and the corresponding Zn-Zn axial distance is 7.6 ,w hereas the four pyrazinelinkersare equatorially coordinated to Zn, and the Zn-Zn equatorial distance is 7.1 . [45] The planes of the pyrazinel inkersi nS IFSIX-3-Zn are oriented parallel to the crystallographic c axis, producing square-shaped MOF channels measuring 3.8 [21d, 45] from corner to corner, which are significantly smaller than the calculated 4.58 and 4.44 [35] pore sizes in a-Mg 3 (HCO 2 ) 6 and a-Zn 3 (HCO 2 ) 6 ,r espectively.T he very small pore size of SIFSIX-3-Znb odes well for methanes torage applications:b ecause the interactions between methane molecules and MOFs are typically van der Waals (i.e.,d ispersive)i nteractions,t he methane binding strength of aM OF shouldb ee nhanced as the pore size is reduced, [6a, 46] as was observed from DFT calculations and 2 HSSNMR experiments in the case of a-Zn 3 (HCO 2 ) 6 versus a-Mg 3 (HCO 2 ) 6 (see above). Despite the promising features that exist within the ultra-microporous SIFSIX-3-Zn for methane adsorption, ad etailed study of the corresponding methanea dsorptions ites has not yet been performed.…”