Deciphering the Weak CO···Framework Interactions in Microporous MOFs Functionalized with Strong Adsorption Sites-A Ubiquitous Observation.

Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO separation process can benefit immensely from improved porous sorbents. Metal organic frameworks (MOFs), especially the ultramicroporous MOFs, built from readily available small and rigid ligands, are highly promising because of their high selectivity (CO/N) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO/N (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics ( = 7.5 × 10 m s). The MOFs exhibit good CO/N kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO adsorption sites, these MOFs exhibit only a moderately strong interaction with CO as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO adsorption sites. To gain insights, we have identified the binding sites for CO using simulation and MD studies. The radial distribution function analysis reveals that despite the amine and bare-metal sites, the pore size and the pore structure determine the positions for the CO molecules. The most favorable sites become the confined spaces lined by aromatic rings. A plausible explanation for the lack of strong adsorption in these MOFs is premised from these collective studies, which could aid in the future design of superior CO sorbents.