Azobenzene Guest Molecules as Light-Switchable CO₂ Valves in an Ultrathin UiO-67 Membrane

Abstract

Metal–organic frameworks (MOFs) with an exceptionally large pore volume and inner surface area are perfect materials for loading with intelligent guest molecules. First, an ultrathin 200 nm high-flux UiO-67 layer deposited on a porous α-Al2O3 support by solvothermal growth has been developed. This neat UiO-67 membrane is then used as a host material for light-responsive guest molecules. Azobenzene (AZB) is loaded in the pores of the UiO-67 membrane. From adsorption measurements, we determined that the pores of UiO-67 are completely filled with AZB and, thereby, steric hindrance inhibits any optical switching. After in situ thermally controlled desorption of AZB from the membrane, AZB can be switched and gas permeation changes are observed, yielding an uncomplicated and effective smart material with remote controllable gas permeation. The switching of AZB in solution and inside the host could be demonstrated by ultraviolet–visible spectroscopy. Tracking the completely reversible control over the permeance of CO2 and the H2/CO2 separation through the AZB-loaded UiO-67 layer is possible by in situ irradiation and permeation. Mechanistic investigations show that a light-induced gate opening and closing takes place. A remote controllable host–guest, ultrathin smart MOF membrane is developed, characterized, and applied to switch the gas composition by external stimuli.