Competitive and Cooperative CO-HO Adsorption through Humidity Control in a Polyimide Covalent Organic Framework.

In order to capture and separate CO from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N/CO/HO breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of HO over CO is replaced by cooperative adsorption. For some conditions, the CO capacity was significantly higher under humid versus dry conditions (, a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO-HO can be achieved and as such provides directions for the development of CO physisorbents that can function in humid streams.