In recent years, the adsorptive properties of metal-organic frameworks (MOFs) have been investigated for applications ranging anywhere from drug delivery to gas storage and release. However, perhaps the most well documented flaw in several MOFs is their inherent water instability. This is a major issue, as many applications require stability to atmospheric moisture.
In our contribution to Nature Chemistry, we report a new copper-based MOF: STAM-17-OEt, which displays excellent water stability. This is due to ‘sacrificial’ bonds in the coordination environment of the material’s metal centres (referred to as hemilability). Prior to water exposure, the material was activated, whereby water within the structure was removed. A dehydrated MOF is required to provide chemically unsaturated sites (CUS) for applications such as gas adsorption, and it is re-exposure to atmospheric moisture that causes many frameworks, including HKUST-1 to decompose after prolonged exposure. Upon exposure of activated STAM-17-OEt to water, rather than the indiscriminate breaking of coordination bonds between the MOF’s copper paddlewheel clusters that leads to structure degradation, the non-structural weak interactions are preferentially broken, and the structure rearranges back to its original form. An analogy to this behaviour is the crumple zone in an automobile, where weaknesses in the chassis are deliberately exploited to ensure protection of the core structure. Just like in a car crash, where the crumple zone dissipates most of the energy away from where the passengers would be most harmed upon collision, upon contact of STAM-17-OEt with water, the energy released is used to break the weak interactions within the material and preserve the main structure. In our article, we show that the MOF is stable to high humidities at elevated temperatures over the course of several days and even retains structural integrity after contact with water for one year.
The investigation of adsorptive properties was performed in collaboration with the Defence Science and Technology Laboratory (Dstl) and we have consequently shown that the material removes the toxic industrial chemical ammonia from contaminated airstreams and that the amount removed is increased with increasing humidity exposure. This high degree of hydrolytic stability makes STAM-17-OEt an excellent candidate for applications of interest to the defence industry, such as in respiratory protection, where the material must be stable to the moisture that is always present in breathable air.
To read more about this story, please see our article: “Hydrolytic stability in hemilabile metal–organic frameworks” in Nature Chemistry here.