, , , Jeffrey T. Culp, Christopher Matranga
This paper reports the structural basis of CO2 adsorption in a representative model flexible metal organic framework (MOF) material, Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)4] (NiBpene or PICNIC-60). NiBpene exhibits a CO2 sorption isotherm with characteristic hysteresis and features on the desorption branch that can be associated with discrete structural changes. Partially selective gas adsorption is demonstrated for CO2 with respect to N2, CH4 and H2 under static and flowing gas pressure conditions. The importance of crystal structure determination for this complex material is apparent in enabling density functional theory (DFT) to explain the observed structural transitions during adsorption/desorption. Possible enhancements of CO2 gas adsorption under supercritical pressure conditions are considered, together with the implications for future exploitation. In situ operando small-angle neutron and X-ray scattering, neutron diffraction and X-ray diffraction, under relevant gas pressure and flow conditions are discussed with respect to previous studies, including ex situ, a priori single- crystal X-ray diffraction structure determination. The results show how this flexible MOF material displays some selectivity for CO2 adsorption, single or dual gas flow results for structural change remain similar to the static (Sieverts) adsorption case, and supercritical CO2 adsorption results in enhanced gas uptake. Insights are drawn for this representative flexible MOF with implications for future flexible MOF sorbent design.
flexible metal organic frameworks, CO2 sorption, small-angle X-ray scattering, small angle neutron scattering, X-ray diffraction, neutron diffraction, in situ operando studies, density functional theory