(86b) Effectiveness of Free Energy and Other Crystal Descriptors to Assess MOF Synthesizability: To What Extent Is Thermodynamics Enough?

Metal-organic frameworks (MOFs) have garnered substantial attention for use in diverse applications, due mainly to their highly tunable pore structure and chemistry, which arises from the modular nature of their construction. However, this modularity also results in a combinatorial explosion of possible MOFs compositions, making comprehensive MOF screening challenging. High throughput computational screening has accelerated property prediction of computational MOF prototypes. However, which computational MOF prototypes are synthetically feasible (or likely) is a question that remains unanswered. One question still under debate is to what extent synthetic likelihood is determined by thermodynamic or kinetic factors, respectively. Previously, we demonstrated our capability of performing high throughput MOF free energy calculations by calculating the free energies in a large, computational MOF database [1]. This capability allows us to investigate whether thermodynamic factors correlate with the apparent synthesizability of prototypes in a MOF database. Apparent synthesizeability was assessed by finding which computational prototypes were also found in experimental repositories, e.g. the Cambridge database.

We begin with a detailed discussion regarding the differences between free energies calculated using thermodynamic integration and the quasiharmonic approximation, as a function of temperature, and highlighting the limitations of the quasiharmonic approximation. Next, we use our calculated free energies to first define a region of “global stability” where all previously synthesized MOFs within our database seem to cluster. This allows us to create a filter where MOFs outside of this region are deemed synthetically unlikely. However, we found this filter to be insufficient when multiple MOF isomorphs all exist within the region of global stability. We find that, in 80% of the cases, the previously synthesized member of an isomorphic series of MOFs has the lowest calculated free energy, revealing that, in the majority of cases, thermodynamics will correctly predict synthetically accessible isomorphs. Furthermore, all previously synthesized isomorphs have a free energy within 3% of that of the calculated lowest free energy isomorph. This makes free energy relative to the lowest free energy isomorph a reasonable second filter to eliminate synthetically inaccessible MOF prototypes within an isomorphic series. Therefore, although kinetics are bound to play a role in certain cases, the application of a thermodynamic filter can be effective in preventing futile synthesis attempts on MOF prototypes that are otherwise promising, but that can be classified as synthetically inaccessible. Finally, we discuss how the effectiveness of free energy “filters” change with temperature, and how filters based on other, easier-to-calculate MOF properties perform relative to free energy.

[1] Anderson, R.; Gómez-Gualdrón, D. A. Large-Scale Free Energy Calculations on a Computational MOF Database : Toward Synthetic Likelihood Predictions. ChemRxiv 2020, 1–16.