In order to capture
and separate CO2from 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 CO2adsorption 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 N2/CO2/H2O breakthrough studies and assessed its performance under varying
levels of relative humidity (RH). We discovered that at limited relative
humidity, the competitive binding of H2O over CO2is replaced by cooperative adsorption. For some conditions, the
CO2capacity was significantly higher under humid versus
dry conditions (e.g., 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 CO2being adsorbed
on single-site adsorbed water. Additionally, once water cluster formation
sets in, loss of CO2capacity 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 CO2–H2O can be achieved and as such provides directions for the development
of CO2physisorbents that can function in humid streams.