Metal-organic frameworks (MOFs) are highly versatile forms of condensed matter that consist of lattice of transition metal ions coordinated by organic ligands, a marriage of sorts between transition metal oxides and organics. A wide range of metal ions and ligands is possible, and thus the properties of these systems may be finely tuned. The organic analog to zeolites, MOFs are highly nanoporous systems have attracted significant attention possible applications in gas capture and storage. Working with experimentalists, we would like to understand what might make a MOF structure optimum for CO2 capture, with the long-term goal of understanding the design rules for such systems.
Working with Jeff Long and Berend Smit at UC-Berkeley, we are exploring CO2 adsorption in frameworks recently synthesized by Long’s group. A major question will be where and how the molecular adsorbates bind to the framework. To understand this non-covalent interaction, we must employ density functionals with the correct asymptotics such that long-range dispersive interactions are included. We are critically examining existing functionals, from those based on empirical or computed effective Hammaker constants, to those more rigorous approaches based on the random-phase approximation, with the aim to improve and extend these methods. In this way, we will use this interesting application, and experimental data, to make progress simultaneously on our understanding of both carbon capture and van der Waals interactions.
MOFs are highly porous materials made of metal atoms linked by a network of organic molecules. Metal atoms in MOFs can selectively bind CO2 over other species common in exhaust gas, such as N2, making MOFs desirable for carbon capture. The organic linker molecules, on the other hand, have been largely considered spectators in this process. Here we use density functional theory with a van der Waals-corrected functional to discover that the affinity of certain MOFs for CO2 can be greatly enhanced via a new adsorption mechanism involving both the metal atoms and the organic molecules.
With the right linking molecules, CO2 binding increased by 50%. This discovery can lead to new routes to design MOFs with strong CO2 absorption by manipulating both the metal and the organic framework.
R. Poloni, B. Smit, and J. B. Neaton, "CO2 Capture by Metal-Organic Frameworks with van der Waals Density Functionals," J. Phys. Chem. A, 116, 4957 (2012). Abstract
R. Poloni, B. Smit, and J. B. Neaton, "Ligand-Assisted Enhancement of CO2 Capture in Metal–Organic Frameworks," J. Am. Chem. Soc. 134, 6714 (2012). Abstract
W. S. Drisdell, R. Poloni, T. M. McDonald, T. A. Pascal, L. F. Wan, C. Das Pemmaraju, B. Vlaisavljevich, S. O. Odoh, J. B. Neaton, J. R. Long, D. Prendergast, and J. B. Kortright, "Probing the mechanism of CO2 capture in diamine-appended metal-organic frameworks using measured and simulated X-ray spectroscopy," Phys. Chem. Chem. Phys. 17, 21448 (2015). Abstract