냉매를 이용한 가스 하이드레이트 형성 및 n-ZIF-90 촉매와 IL-ZIF-90 촉매의 이산화탄소 고정화반응의 경로 비교에 대한 계산화학적 해석

Abstract

Molecular modeling, a computational chemistry methodology, was introduced only about 50 years ago and now it is widely employed to assist the understanding of the molecular structures of chemical compounds and various reaction mechanisms. It has become an important research area itself today. In this study, molecular modeling was introduced to find out carbon dioxide fixation mechanisms when using n-ZIF-90 and IL-ZIF-90 catalysts, and also explain the gas hydrate formation with HFC refrigerants in view of thermodynamics. 1) Gas hydrate formation HFC refrigerants such as R-134a, R-141b, R227ea and R-236fa was considered as guest gases for gas hydration formation. Gas hydrates have specific solid phase structures where guest gases are encapsulated into the cavities organized H2O molecules. In this study, I have calculated the binding energies between guest gases and H2O cavities to evaluate which refrigerant is the most suitable for the desalination process using gas hydrate formation. As a result, I have selected R-134a and R-236fa based on their calculated binding energies (in other words, thermodynamic favorableness with more negativeness) and toxicity to human. 2) Carbon dioxide fixation Carbon dioxide fixation by using epoxides to produce cyclic carbonates demands a high activate energy (55~59 kcal/mol). This reaction thus requires catalysts inevitably. In this study, I have studied about reaction mechanisms of the catalysts, n-ZIF-90 (normal) and IL-ZIF-90 (ionic liquid supported). The calculated energetics including the energies of reaction intermediates and transition complexes was used to explain the different catalytic conversions (IL-ZIF-90 > n-ZIF-90) of the two catalysts. The difference resulted not from the difference of activation energies of the two catalysts, but the different positions of stable reaction intermediates. Those two mechanisms were reported in Green Chemistry (2016) for the first time.