PHASE BEHAVIOR OF CARBON DIOXIDE AND POLYHEDRAL OLIGOMERIC SILSESQUIOXANES WITH VARIOUS CHEMICAL GROUPS

Date

2023-9-07

Author

Ertaş, Alican

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The phase behavior of supercritical carbon dioxide (scCO2) and a polyhedral oligomeric silsesquioxane (POSS) was investigated. The cubic structures of POSSs have eight silica ends bonded with various functional groups. The variety of functional groups and the flexibility of silica atoms make the POSS compounds suitable for polymers and catalysts as additives. In previous studies, the effect of different mono-functionalities on the POSS-CO2 phase behavior was investigated. In this study, a bifunctional POSS molecule, trifluoropropylisobutyl (TFIB) POSS, was selected to observe the contribution of one trifluoropropyl group to an octa-isobutyl POSS on the CO2 solubilities. Cloud-point measurements at four different temperatures (308-323 K) show that TFIB POSS solubilites in scCO2 are between those of its monofunctional POSSs; however, the solubility difference between TFIB and octa-trifluoropropyl POSS diminishes at high temperatures. The results show that TFIB POSS has a melting temperature of around 270 °C, and relatively high solubilities in scCO2 at higher temperatures, promoting it as a suitable additive to heat-sensitive materials to improve their thermal stability and applicability in environmentally benign supercritical CO2 processes. In addition to the measurements, this study includes different theoretical models to predict solubilities. Like in previous studies with POSSs, six CO2 density-based semi-empirical equations were used to predict the solubilities. This study also includes solubility modeling with Peng-Robinson (PR)+COSMOSAC Equation of State (EOS) , a modified PR+EOS. The application is composed of density functional theory (DFT) investigations and fugacity calculations. In general, density-based relations provide better fits to the experimental data and are easier to apply. However, the main advantage of the EOS is its applicability in the absence of experimental solubility data and critical properties.

Subject Keywords

POSS, Supercritical Carbon Dioxide, PR+COSMOSAC EOS, Gibbs Free Energy, Solubility

URI

https://hdl.handle.net/11511/105562

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Graduate School of Natural and Applied Sciences, Thesis