Deciphering the Weak CO2···Framework Interactions in Microporous MOFs Functionalized with Strong Adsorption Sites-A Ubiquitous Observation.

Abstract

Carbon capture from industrial effluents such as flue gas or natural gas mixture (cf. landfill gas), the primary sources of CO2 emission, greatly aids in balancing the environmental carbon cycle. In this context, the most energy-efficient physisorptive CO2 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 (CO2/N2) and easy scalability. Here, we report two new ultramicroporous Co-adeninato isophthalate MOFs. They concomitantly carry basic functional groups (-NH2) and Lewis acidic sites (coordinatively unsaturated Co centers). They show good CO2 capacity (3.3 mmol/g at 303 K and 1 bar) along with high CO2/N2 (∼600 at 313 K and 1 bar and ∼340 at 303 K and 1 bar) selectivity, working capacity, and smooth diffusion kinetics (Dc = 7.5 × 10-9 m2 s-1). The MOFs exhibit good CO2/N2 kinetic separation under both dry and wet conditions with a smooth breakthrough profile. Despite their well-defined CO2 adsorption sites, these MOFs exhibit only a moderately strong interaction with CO2 as evidenced from their HOA values. This counterintuitive observation is ubiquitous among many MOFs adorned with strong CO2 adsorption sites. To gain insights, we have identified the binding sites for CO2 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 CO2 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 CO2 sorbents.