UC Berkeley

Metal-organic frameworks (MOFs) are a class of crystalline materials with broad diversity in their chemical compositions and geometries which give rise to a high degree of tunability of their properties, and many possible applications. Because a broad range of synthetic modifications to MOFs are possible, computational methods are invaluable in guiding experimental studies to understand the relationship between a given structure and chemistry and its material properties. In this work, we use first principles electronic structure calculations to calculate low energy geometries, nuclear magnetic resonance (NMR) chemical shifts, and binding energetics of MOFs that adsorb CO$_2$ and O$_2$. Our calculations explain experiments indicating that the presence of SO$_2$ degrades the performance of diamine-appended Mg$_2$(dobpdc), a promising class of MOF for carbon capture, with some appended diamines but not others. Additionally, we show that we can capture trends in O$_2$ binding in a computationally challenging family of MOFs, the BTTri MOFs, that contain open-shell transition metal atoms. We predict that by modifying the halide in BTTri we can obtain binding energies that are ideal for industrial separations of O$_2$.

In order to better understand the mechanism of cooperative CO$_2$ adsorption in diamine-appended Mg$_2$(dobpdc), we extend a statistical mechanics model to understand the role of defects, steric hindrance, and mixtures of diamines on CO$_2$ adsorption isotherms and isobars. This work shows that cooperative uptake is robust in the presence of defects as long as the cooperative interaction which promotes CO$_2$ binding at adjacent diamine sites is sufficiently strong. We also rationalize experimentally observed double stepped isotherms, and develop a way of understanding how the ratio of mixed diamine systems can be chosen to target a specific pressure at which CO$_2$ will be bound to the MOF.

We benchmark the performance of van der Waals corrected density functionals in layered MOFs and layered perovskites, showing that recently developed functionals are able to accurately predict lattice constants for hybrid layered materials. Our calculations also highlight the impact of the lattice constants on calculated observables like the exfoliation energy and the fundamental gap, and demonstrate that care must be taken in including van der Waals corrections in these challenging systems.

Finally, with ab initio many-body perturbation theory we predict the structure and spectroscopic properties of Zn-MFU-4l including electron-hole interactions and accounting for zero-point vibrational effects, and show how modifications to the anion in the framework can reduce both the fundamental gap and the optical gap, and we provide an explanation for previously seen trends in optical experiments on Zn-MFU-4l.