(189m) CO2 Adsorption in Nickel Based Metal Organic Framework Ni-DABCO: A Density Functional Theory and Grand Canonical Monte Carlo Study

The continuous emissions of green house gases, especially carbon dioxide, are still generating concerns over the adverse effects they are having on the planet. Metal organic frameworks (MOF) have obtained wide spread attention for their CO₂ adsorbing capabilities. MOFs are formed through the coordination of inorganic metal clusters with organic bridging ligands. Ni-DABCO is one of the many MOFs that are being studied for CO₂ separation. It is part of the M-DABCO MOF series, which has isostructural zinc, copper and cobalt variants. The organic linkers in M-DABCO are 1,4-benzenedicarboxylate (BDC) and 1,4-diazabicyclo[2.2.2]octane (DABCO). The latter is also known as triethylenediamine (TED). We have computational studied the interaction of CO₂ in Ni-DABCO using periodic density functional theory (DFT) and gran canonical Monte Carlo (GCMC). Our periodic DFT calculations demonstrate that upon adsorption, CO₂ induces small changes on the unit cell parameters of the Ni-DABCO framework. Our calculations also demonstrate that the electrostatic potential of the framework slightly changes as the number of CO₂ molecules increased in the system, suggesting that upon CO₂ adsorption the electron densities also change in the framework. The partial atomic charges of the framework atoms close to the CO₂ molecules, as well as the CO₂ molecules exhibited a change in their atomic charges when compared to the isolated systems. GCMC simulations were used to generate CO₂ adsorption isotherms on Ni-DABCO at 298K, and our results were compared with experimental isotherms reported in the literature. Furthermore, the textural properties of the material, including BET area and pore volume, were calculated using a simulated N₂ isotherm at 77K and were also compared to published results. Our simulated isotherms and the simulated powder X-ray diffraction patterns are in good agreement with experimental results. However, at pressures higher than 10 atm, the simulated isotherm underestimates the experimental isotherm, suggesting that this material may expand during adsorption and could be a pressure dependent phenomenon.