(682i) Unraveling of Pristine and Defective Metal-Organic Framework (MOF) Structures through Molecular Simulation

Metal-organic frameworks (MOFs) are extremely attractive porous materials for applications in gas storage, chemical separations, catalysis, and others. The appeal of MOFs for these applications is in great part due to the chemical and structural tunability of MOFs, which arises from the virtually unlimited combination of constituent organic and inorganic building blocks that these materials can be made of. However, the unlimited diversity of MOFs that could be synthesized for a given application raises the challenge of identifying the most promising structures among so many candidates. To address this challenge, the vast MOF structure space can be navigated in “high throughput” fashion—typically done computationally—or in “rational exploration” fashion, where the latter approach relies on a solid, reliable knowledge about the links between synthesis, structure, property and performance for currently synthesized MOFs, and the use of this knowledge to design higher-performance materials. Therefore, a thorough understanding of the structure of currently synthesized and tested MOFs is critical as to not to mislead rational MOF design. In many instances, attaining this understanding can be difficult, partly due to advances in MOF synthesis leading to increasingly more complex MOF structures, and partly due to the realization that MOF structural defects can significantly affect performance.

Molecular simulation and auxiliary computational methods are valuable—and relatively accessible—tools to help unravel the structure of existing and newly synthesized MOFs, especially when used in synchrony with experimental characterization. Here, we first present the utilization of automated algorithms for construction of MOF computational models, nitrogen adsorption simulations and other auxiliary tools to elucidate the network structure of a new uranium-based MOF, NU-1301 that synthesizes into the largest crystallographic unit cell for any non-biological material to date. Despite the simplicity of the constituent building blocks, we found this MOF to present a complex hierarchical structure based on a previously unknown network topology, nun, which has now been added to the Reticular Structure Chemistry Resource (RSRC) database.

Then, we present the utilization of MOF computational models and nitrogen adsorption simulations to elucidate structural details of the defective structure of the well-known, zirconium-based MOF, UiO-66. Specifically, we illustrate how features in nitrogen adsorption isotherms are highly dependent on MOF structural features, to a level that can be used to discern the local MOF structure around defective regions, if sufficiently accurate structural MOF models are provided to be used to obtain “standard” simulated isotherms for comparison with experimental ones. Using this method, we elucidated how the structure of UiO-66 varies as the synthesis method is varied, for instance, through the change of crystallization modulators.