A Theoretical Perspective to Decipher the Origin of High Hydrogen Storage Capacity in Mn(II) Metal‐Organic Framework

Abstract

Herein, we report a detailed periodic DFT investigation of Mn(II)-based [(Mn 4 Cl) 3 (BTT) 8 ] 3À (BTT 3À = 1,3,5-benzenetristetrazolate) metal-organic framework (MOF) to explore various hydrogen binding pockets, nature of MOF…H 2 interactions, magnetic coupling and, H 2 uptake capacity. Earlier experiments found an uptake capacity of 6.9 wt % of H 2, with the heat of adsorption estimated to be ~10 kJ/mol, which is one among the highest for any MOFs reported. Our calculations unveil different binding sites with computed binding energy varying from À 6 to À 15 kJ/mol. The binding of H 2 at the Mn 2 + site is found to be the strongest (site I), with H 2 found to bind Mn 2 + ion in a η 2 fashion with a distance of 2.27 Å and binding energy of À 15.4 kJ/mol. The bonding analysis performed using NBO and AIM reveal a strong donation of σ (H 2 ) to the d z 2 orbital of the Mn 2 + ion responsible for such large binding energy. The other binding pockets, such as À Cl (site II) and BTT ligands (site III and IV) were found to be weaker, with the binding energy decreasing in the order I > II > III > IV. The average binding energy computed for these four sites put together is 9.6 kJ/mol, which is in excellent agreement with the experimental value of ~10 kJ/mol. We have expanded our calculations to compute binding energy for multiple sites simultaneously, and in this model, the binding energy per site was found to decrease as we increased the number of H 2 molecules suggesting electronic and steric factors controlling the overall uptake capacity. The calculated adsorption isotherm using the GCMC method reproduces the experimental observations. Further, the magnetic coupling computed for the unbound MOF reveals moderate ferromagnetic and strong antiferromagnetic coupling within the tetrameric {Mn 4 } unit leading to a three-up-onedown spin configuration as the ground state. These were then coupled ferromagnetically to other tetrameric units in the MOF network. The magnetic coupling was found to alter only marginally upon gas binding, suggesting that both exchange interaction and the spin-states are unlikely to play a role in the H 2 uptake. This is contrary to the O 2 uptake studied lately, where strong dependence on exchange-coupling/spin state was witnessed, suggesting exchange-coupling/magnetic field dependent binding as a viable route for gas separation.

Hydrogen storage mechanism of metal–organic framework materials based on metal centers and organic ligands

“…In fact, MOFs with open metal sites can increase the adsorption strength of H 2 at 298 K. This view was first illustrated in Mn 3 [(Mn 4 Cl) 3 (BTT) 8 ] 2 , where the Mn site is open, and its corresponding isovolumetric heat of adsorption for H 2 is 10.1 kJ mol −1 . [68][69][70] The results of powder neutron diffraction showed a strong interaction between Mn and D 2 , and that the distance between the centers of metal Mn and H 2 molecules was only 2.27 Å, which is much smaller than the distance (43 Å) of physical adsorption of hydrogen. This may help hydrogen fill the pores more efficiently.…”