First-principles study of small molecule adsorption in metal-organic frameworks

Nanoporous materials such as metal-organic frameworks (MOFs) and zeolites are under investigation for potential use as adsorbent materials for gas separation and storage applications. These materials are of particular interest for their unique and diverse chemistries and their large internal surface areas. The space of possible nanoporous materials is enormous, and experimental synthesis and characterization of these materials is an enormously expensive and time-intensive task. Therefore, these materials are excellent candidates for computational study, from both the discovery and applications investigation perspectives. By computationally studying materials we are able to better direct synthetic chemists' efforts toward promising candidate materials and build a fundamental understanding of these materials./nOne particularly relevant application is carbon capture and sequestration, an area that has received increased attention as the urgency of addressing anthropogenic climate change has increasingly been realized in the scientific and general populations. Working toward rational design principles and a fundamental understanding of MOF materials (a novel class of nanoporous materials in which highly specialized chemistries are realizable) for this application, computational study of MOFs and small molecule adsorption behavior within a variety of MOFs has been performed. We have focused on energy-relevant small molecules in so-called "open-metal site" MOFs, MOF-74s, in this work, but have provided chemical insight that can be more broadly used to understand small molecule adsorption in MOFs and the chemistry of open-metal sites./nWe have used density functional theory (DFT) with carefully chosen approximations to treat important interactions such as weak dispersion forces and electronic orbital interactions involving d-orbital electrons. We have studied the importance of these approximations for obtaining qualitatively and quantitatively correct results, as well as compared our DFT results extensively with experimental measurements. This has necessitated the consideration of finite-temperature effects to binding as well as zero-point nuclear motion, as the binding enthalpy is a common observable. We find that binding enthalpies (at room temperature) range from below 10 kJ/mol (common for the case of H2) to above 100 kJ/mol (in the case of strong electronic interactions with undercoordinated metals, as can be the case for SO2). Zero-point and finite-temperature effects tend to reduce binding enthalpies by about 5 kJ/mol when compared to zero-temperature binding energies computed with DFT. Comparing binding enthalpies calculated with DFT to experimentally determined binding enthalpies, we find excellent agreement with a mean absolute error of around 5 kJ/mol, where errors are smaller for predominantly electrostatic- and dispersion-bound systems and are greater for systems with significant electronic orbital interactions or large molecules. These calculations compare favorably with quantum chemistry calculations on comparable systems, typically fragments of the periodic MOF systems./nOverall, we have established a strong understanding of small molecule adsorption in open-site metal-organic frameworks, focusing on a variety of MOFs and adsorbates to understand the range of chemistries accessible in these novel and specialized systems. This work contributes to the development of fundamental understanding and rational design principles for metal-organic frameworks appropriate for specialized separation and storage applications. Additionally, this work has led to an increased understanding of the importance of dispersion interactions (and strong correlation effects) for computing binding energetics of small molecules in MOFs (where interactions of molecules are with d-block metals). This work has also led to prediction of new materials for the N2/CH4 separation (V-MOF-74) and removal of trace gases from post-combustion flue gas streams (Mn-MOF-74) in addition to an increased fundamental understanding of the roles of various interactions in small molecule adsorption in MOFs.

Advisor: Smit, Berend
Neaton, Jeffrey B