Hutereau, Martin; (2023) Unravelling the Origin and Influence of Point Defects in UiO-66 Through Computer Simulation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Density functional theory has been used to study point defects in the metal-organic framework UiO-66. The thermodynamics of building block vacancies has been surveyed in order to uncover the driving forces behind the formation of ordered nanoregions. While trends found with most charge-compensating species are consistent with experiment, the energetics of defects and domain interfaces suggest that these features should be well-dispersed rather than spatially aggregated and correlated. Examining the inorganic nodes in vacuum revealed that some linkers would be missing prior to framework assembly; this implies a thermodynamic impetus in addition to the kinetic factors conventionally used to explain defectivity. A number of experiments have been carried out using probe molecules to investigate the impact that these features have on acidic properties. In this way, the potential for tuning the strength of native Brønsted acid sites has been ruled out, but it was confirmed that these can be supplemented using charge-compensation schemes involving water. It was also affirmed that the Lewis acidity of zirconium is not strongly affected by nearby defect-terminating species, though these remain important in catalysis. It was found that in the cyclisation of citronellal, the local structure at vacancies affects how readily catalytic sites can be accessed, with steric hindrance a key factor. Finally, the heterometallic Zr-Ce framework was scrutinised in order to rationalise the distribution of cerium. The periodic material did not exhibit any preferences for the Zr5Ce1 building blocks seen experimentally: homometallic bonds are instead preferred. The inclusion of defects in the structure further revealed that higher framework flexibility allows cerium to be better accommodated, which has implications for synthesis. The possible role of Ce3+ was additionally considered: this species was found to be thermodynamically unavailable for the formation of building blocks during synthesis, providing a clue towards the prevalence of Zr5Ce1 over other mixed clusters.

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