MOF gate opens selective CO2 gas storage door

UK researchers have designed a metal–organic framework that, unusually, selectively adsorbs CO₂ over ethyne by a dynamic gate-opening mechanism and has potential applications in fuel gas separation.

Metal–organic frameworks, or MOFs, comprise metal clusters or ions complexed to organic ligands, forming an extended crystalline, often porous, structure. The pore sizes can be tuned by careful design, and as such, they are widely investigated for gas storage and separation technologies. However, most MOFs are usually selective to ethyne adsorption over CO₂, limiting their application, as the intermolecular interactions between ethyne and the MOF are stronger.

Now, Martin Schröder at the University of Nottingham, and colleagues, have synthesised a MOF that shows dynamic phase changing behaviour induced by CO₂, inverting the usual selectivity to ethyne.

The team made a ligand that, when co-ordinated to the metal ion, retains free-standing carboxylate and a pyridyl group that protrude into the pore channels and can interact with guest molecules. ‘Because we have both acids and bases inside the pores, we expected different binding of different substrates, so what was interesting for us was two-fold: that the CO₂ binding was higher than for ethyne, and that CO₂ showed unusual phase change behaviour,’ says Schröder.

While many MOFs have been investigated for carbon capture, the feature of interest here is the pattern of CO₂ uptake by the MOF – two separate gas uptake steps were observed for CO₂. The researchers extensively probed this observation theoretically and experimentally, finding that the pyridyl rings in the pore channels actually rotate, enabling a transition from a narrow pore to a large pore phase, increasing the volume of the pore and allowing a gradual increase in CO₂ adsorption. This gate rotation is specifically induced by CO_2, explaining the selectivity of the MOF to CO₂ adsorption over ethyne, as a higher ethyne uptake is required to trigger the gate opening.

MOF research extends beyond the reach of gas storage and separation, with potential applications in drug delivery, electronic and magnetic properties, among others. As Russell Morris, an expert in porous solids from the University of St Andrews, UK, explains: ‘The interesting selectivity points to other potential uses, and harnessing the flexibility of MOFs to accomplish processes with unusual selectivity is a route to many new materials.’

The next step for Schröder is to investigate gas mixtures, including the selectivity between different kinds of hydrocarbons.