Saf ve metal-oksit destekli fosfonyum i̇yonik sivilarinin isil kararliliklarini kontrol eden yapisal faktörlerin aydinlatilmasi

Ionic liquids (ILs) are a novel class of green solvents recognized as potential replacements for conventional volatile organic compounds. They have attracted great attention in the last decade because of their unique physicochemical properties, such as extremely low volatility, low melting point (<100 °C), high solvating ability, and high thermal stability. These physicochemical properties can be tailored by integrating almost endless number of different combinations of anions and cations; hence they are often called "designer" or "task-specific" solvents. Their tailor-made characteristics offer a wide range of applications in numerous research fields, such as synthesis, separation, electrochemistry, and catalysis. Their applicability in these fields primarily depends on whether they protect their structural integrity at the operating conditions. In catalysis, for example, ILs are immobilized on metal-oxide surfaces, thus, they directly interact with these surfaces. Therefore, application of ILs in such supported-catalyst concepts is limited with their thermal stabilities on corresponding support materials. Though most of the ILs are thermally stable at elevated temperatures (> 300 °C) in their pure state, they become less stable when coated on metal-oxide supports because of the interactions between IL and metal-oxide. Thus, it is crucial to elucidate structural factors controlling the thermal stability of bulk and supported ILs for choosing the best ILs for specific processes. In the first part of thesis, such structural factors controlling thermal stability limits of bulk phosphonium ILs were determined by combining experiments with Density Functional Theory (DFT) calculations. Decomposition mechanisms were elucidated by utilizing DFT calculated 1H NMR chemical shifts of individual protons quantifying electron density around them. Data illustrated that electron density around terminal protons of the alkyl group closest to the anion present a perfect correlation with the stability limits. Based on this finding, a general thermal decomposition mechanism for phosphonium ILs was postulated and confirmed by on-line mass spectrometry monitoring decomposition products of a representative phosphonium IL. Accordingly, reaction involves nucleophilic substitution of the anion at α-carbon of its closest neighboring alkyl chain on the cation. Identification of structural factors controlling thermal stability limits and elucidation of decomposition mechanism create opportunities for rational design of ILs with superior thermal stability. In the same manner, structural factors governing the thermal stability limits of phosphonium ILs supported on SiO2, γ-Al2O3, and MgO were investigated using thermogravimetric analysis. Results indicated that thermal stability limits of these ILs diverge greatly from the corresponding bulk values when they are immobilized on metal-oxide supports. More pronounced effect of the support was observed for ILs immobilized on MgO. This higher deviation from bulk values was attributed to stronger interactions between basic MgO surface and ILs than those with other support materials. Thus, surface acidity was identified as one of the factors controlling the stability limits. The other factors were characterized as the size of IL, and electronegativity and aromaticity of the anion. Once these structural factors are fully identified, it would be, therefore, possible to select an optimum phosphonium IL that retains its performance and functional activity at designed operating temperatures in supported-IL processes. Consequently, interaction energies between anions and cations of some imidazolium ILs with 1-butyl-1-methylimidazolium ([BMIM]+) cation were determined using DFT calculations. These interaction energies were further used to be associated with experimental stretching frequencies of the acidic proton on the imidazolium ring (C2‒H). Results illustrated that experimental C2–H stretching frequencies correlate proportionally with calculated interaction energies. Therefore, this correlation renders a quick estimation of interaction energies between anions and cations of [BMIM]+-based imidazolium ILs by simply utilizing experimental C2–H stretching frequencies. This information together with those acquired on phosphonium ILs throughout this thesis will set the basis for future work focusing on atomic-scale understanding of structure-performance relationship in IL-assisted metal catalysts for energy related applications.