Structural and bonding properties of small hydrocarbons inside Ca(squarate)-metal organic framework: ab-initio study

Ab-initio Molecular Dynamic (MD) and static Density Functional Theory (DFT) are used to study the structural and bonding properties of small hydrocarbon adsorbates inside Ca(squarate)-Metal Organic Framework (MOF). Car -Parrinello Molecular Dynamics (CPMD) simulations of a single -adsorbate-MOF structure are used to obtain the adsorbate most preferred site of adsorption. This site is used for further structural and bonding analyses using static DFT. Unlike many other MOFs; we found that the Ca(squarate)-MOF physisorbs and weakly binds small adsorbate molecules such as C2H2, C2H4, C2H6, and C3H8 with no observed charge transfer and minimal hybridization with the MOF orbitals. No covalent bonding is seen near the preferred site of adsorption. The calculated binding energies decreases as the H content in the adsorbate molecule increases and found to be -18.71 kJ/mol, -18.14 kJ/mol, -15.75 kJ/mol, and-4.47 kJ/mol for C2H2, C2H4, C2H6, and C3H8 molecules respectively. Density of State (DOS) and a Crystal Orbital Overlap Population (COOP) analyses show that the interactions between C and H atoms in the molecule and C and O atoms in the MOF have antibonding characteristics near the Fermi level. These antibonding states tend to destabilize the overall electronic structure of the combined adsorbate/MOF system and hence decrease the binding energies of these adsorbates inside the MOF.