Metal Organic Frameworks (MOFs) are being extensively explored for their application in the domain of catalysis. However, instability at high temperature, high pressure and polar solvents is common, limiting their application. Recently, some thermally transformed MOFs have become attractive due to features such as high dispersion of metal nanoclusters and high pore volumes which can withstand high temperatures and pressures. We reported a Ru incorporated, thermally transformed modified MIL-140C (m-MIL-140C) for CO₂ conversion to CH₄ with high selectivity and fast kinetics. However, the morphology of the transformed structure and thermal transformation mechanism is still not well understood.

Here we present a computational study of thermal transformations in Zr-based MOFs. We use reactive molecular dynamics simulations to investigate chemical and physical changes during thermal treatment. Under an inert environment, MIL series MOFs (MIL-140A/B/C) evolved mostly CO and H₂ as the linkers decomposed. In contrast, UiO series MOFs (UiO-66/67) H₂O and O₂ molecules evolved in addition to CO and H₂. The decomposed linkers form a fragmented network of ‘ribbons’ in which zirconia clusters become embedded and agglomerate. This appears to increase the material’s tolerance against sintering, which is much needed in an ideal catalyst. A partially modified MIL-140C (m-MIL-140C), where 10% of the linkers were replaced by 2,2’-bipyridine-5,5’-dicarboxylate, shows a greater connected network of remnant linker material with smaller zirconia clusters. The oxidation state of zirconia clusters in the final structure is similar to ZrO₂. Understanding the formation of these materials at a molecular length-scale will help improve their development for specific applications.

<i>Figure 1. (a). Shows the simulated morphology of the thermally decomposed MIL-140C structure. (b) Shows the TEM image of thermally decomposed m-MIL-140C with elemental mapping of Zr atom. (c) Show the radial pair distribution function between Zr-O and Zr-Zr atom pairs during the decomposition. (d) Shows the clusters of Zr-atoms in the final morphology of MIL-140C.</i>

Figure 1. (a). Shows the simulated morphology of the thermally decomposed MIL-140C structure. (b) Shows the TEM image of thermally decomposed m-MIL-140C with elemental mapping of Zr atom. (c) Show the radial pair distribution function between Zr-O and Zr-Zr atom pairs during the decomposition. (d) Shows the clusters of Zr-atoms in the final morphology of MIL-140C.

Vignesh Pakkiam

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