Application of charge schemes to study reactivity of Metal-Organic Frameworks (MOFs)
Application of charge schemes to study reactivity of Metal-Organic Frameworks (MOFs)Promotor(en): T. Verstraelen /15_MODEV11 / Model and software development
Metal-Organic Frameworks (MOFs) are - just as zeolites - ordered nanoporous materials with pore sizes between 0.5 and 2 nm. In contrast to zeolites, which have a purely inorganic character, MOFs constitute a class of porous materials that exhibit a truly hybrid character, since both inorganic and organic moieties are needed to build up their framework. While catalytic conversions and separations are the most frequently used industrial applications of zeolites, future MOF applications lie more in adsorption, storage and separation of gas mixtures (H2, CO2 or CH4, etc.). As such, their importance in catalysis is steadily increasing. Although catalysis is potentially one of the most important applications of this exciting class of materials, insight in the active sites of suitable MOFs for catalytic applications is often lacking. Different techniques to evaluate the catalytically active sites are therefore important to be explored, such as charge partitioning schemes. Such atoms-in-molecules (AIM) partitioning schemes provide a practical means for investigating charge transfer between the different moieties and/or functional groups in a system. In addition, they can be used to identify the oxidation states of the active site atoms, and follow this during the catalytic process.
This thesis subject is related to Zirconium-terephthalate materials (also known as UiO-66 type materials ), which have a high stability of these frameworks guarantees. These frameworks need to be partially decoordinated to be active . In other words, missing organic linkers (or terephthalates) create these accessible Zr-sites. Recently UiO-66-X materials were made using substituted terephthalates and catalytically tested for the citronellal cyclization reaction (Figure 1). In chemist terms one can say that the the catalyst activity increased significantly with the presence of electron withdrawing groups. The best result was obtained for nitro-substituents (NO2).
Figure 1: Linear free energy reaction between reaction rate constants and an empirical parameter σm related to the substituent group.
For a series of functionalized UiO-66-X materials a linear free energy relationship (LFER) was found between activity in Lewis-acid catalysed reactions and the empirical Hammett constant σm as a measure of the electronic nature of the ligands.
Goal In this thesis, we aim at finding a theoretical criterion that correlates well with the experimental Hammett constants and is able to explain the chemical reactivity on computational models of these UiO-66 materials.
In an initial test, Hirshfeld-e charges of the active Zr-atoms obtained from small cluster models of the UiO-66-X materials correlated well with an average Hammett constant . This clear correlation suggests that Zr Hirshfeld-e charges may be regarded as a valid alternative to predict a qualitative indication of the catalytic activity of hypothetical UiO-66-X variants.
Figure 2. Hirshfeld-e charge of Zr versus an average Hammett constant σavg.
Within this thesis, different AIM partitioning schemes will be applied to study periodic UiO-66-models with a different degree of defects (initial structures are already available).
Although several such methods are directly available for solids (Hirshfeld(-I/e),…)[3,5], students with an interest in implementation/development will be provided the opportunity to implement new density-based charge partitioning schemes for solids. Such development would then be aimed at e.g. optimizing the relation with the Hammett constant.
- Study programmeMaster of Science in Engineering Physics [EMPHYS], Master of Science in Chemistry [CMCHEM], Master of Science in Physics and Astronomy [CMFYST]ClustersFor Engineering Physics students, this thesis is closely related to the cluster(s) nano, modelingReferences
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