Photo-switchable smart metal–organic framework membranes with tunable and enhanced molecular sieving performance

Developing a novel MOF membrane material with switchable separation performance is an exciting and challenging research project. In the present work, we report preparation of a new kind of light induced smart MOF membrane, i.e., Cu(AzDC)(4,4′-BPE)_0.5 membrane, which shows (i) enhanced molecular sieving performance, and (ii) is able to respond quickly to external light stimuli. Two photo-switchable moieties are addressed in the Cu(AzDC)(4,4′-BPE)_0.5 membrane: azobenzene and bis(4-pyridyl)ethylene. When the Cu(AzDC)(4,4′-BPE)_0.5 membrane is in situ irradiated with Vis and UV light, the separation factor of a H₂/CO₂ mixture can be switched reversibly between 21.3 and 43.7. This switching effect is mainly caused by reduced CO₂ adsorption in the UV-cis state as proven by independent adsorption studies. For a steric reason, adsorption of CO₂ is limited for the UV-cis state. In full agreement with this model, the adsorption of other gases H₂, CH₄ and N₂ as well as their permeation behaviour is not observably influenced by the trans–cis switching.