Flexible metal-organic framework compounds: In situ studies for selective CO₂ capture

Results are presented that explore the dynamic structural changes occurring in two highly flexible nanocrystalline metal-organic framework (MOF) compounds during the adsorption and desorption of pure gases and binary mixtures. The Ni(1,2-bis(4-pyridyl)ethylene)[Ni(CN)₄] and catena-bis(dibenzoylmethanato)-(4,4′-bipyridyl)nickel(II) chosen for this study are 3-D and 1-D porous coordination polymers (PCP) with a similar gate opening pressure response for CO₂ isotherms at 303 K, but with differing degrees of flexibility for structural change to accommodate guest molecules. As such, they serve as a potential model system for evaluating the complex kinetics associated with dynamic structure changes occurring in response to gas adsorption in flexible MOF systems. Insights into the crystallographic changes occurring as the MOF pore structure expands and contracts in response to interactions with CO₂, N₂, and CO₂/N₂ mixtures have been obtained from in situ small-angle neutron scattering and neutron diffraction, combined with ex situ X-ray diffraction structure measurements. The role of structure in carbon capture functionality is discussed with reference to the ongoing characterization challenges and a possible materials-by-design approach. - Graphical abstract: We present in situ small-angle neutron scattering results for two flexible metal-organic frameworks (MOFs). The figure shows that for one (NiBpene, high CO₂ adsorption) the intensity of the Bragg peak for the expandable d-spacing most associated with CO₂ adsorption varies approximately with the isotherm, while for the other (NiDBM-Bpy, high CO₂ selectivity) the d-spacing, itself, varies with the isotherm. The cartoons show the proposed modes of structural change. - Highlights: • Dynamic structures of two flexible MOF CO₂ sorbent compounds are compared in situ. • These porous solid sorbents serve as models for pure & dual gas adsorption. • Different degrees of structural freedom give different CO₂ adsorption & selectivity. • Combined in situ small-angle neutron scattering & diffraction provide new insights. • Such in situ dual gas sorption studies can reveal pore gate-opening effects