2, Boise State University, Boise, ID, Idaho, United States
Flexible metal-organic framework materials are of great interest for gas separation problems as the response of the framework to sorption or other stimuli can influence the further sorption of gas species. We use first-principles density functional theory GGA+U plus empirical van der Waals calculations to investigate the stability of one particular flexible system: the pillared metal-organic framework material Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)4] (Ni2(bpene)(CN)4 or PICNIC-60 for short) as a function of cell volume, bpene orientations and CO2 content. We find two types of stable orientations for individual bpene molecules: along the monoclinic axis (b) and perpendicular to it (c), with a rotational transition barrier of about 0.4 eV. The most stable pattern of bpene orientations for empty PICNIC-60 changes from all (b) to alternating (b) and (c) as the molar volume increases from 440 Å3 to 720 Å3, with the transition at about 500 Å3. The addition of CO2 changes the picture, with yet a third pattern of bpene orientations- all along (c)- favored for CO2 concentration greater than 3 CO2 per mole of bpene. A particularly stable arrangement of 5 CO2 per mole of bpene is found, beyond which CO2 sorption becomes much less favorable. The experimental hysteresis observed in CO2 sorption isotherms is explained in terms of the energy barriers to bpene reorientation and to CO2-CO2 interactions. Preliminary results are presented for other sorbant species, including comparisions of the binding energies and geometries for N2, H2 and CH3 .