| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程學系 === 107 === In recent years, carbon dioxide is one of the main greenhouse gases which lead to serious damage to our environment. Therefore, how to capture CO2 has become a global issue. A new type of material, mixed matrixed membranes (MMMs), which is composed of metal-organic framework (MOF) as filler embedded in a polymeric matrix and it can combine the advantages of both components. In this study, Matrimid® 5218 is used as polymeric matrix becaused of its high glass transition temperature, and NH2-MIL-53 is used as filler because of its significant breathing behavior.
In this work, molecular dynamics (MD) and density functional theory (DFT) are applied to construct the pure Matrimid models and MMMs models, and NH2-MIL-53 models, respectively. Additionally, Monte Carlo (MC) simulations and mean square displacement (MSD) are used to analysis the gas transport mechanism.
The pure Matrimid models are divided into three different systems for discussion, which are long, medium, and short chain system. Each system would adopt two different MD procedures to construct the models, which are NPT-NVT loop procedure and 31 MD step procedure. The results show that the long chain system fully exhibits the characteristics of polymer torsion, and as the chain length decrease, this characteristic will gradually disappear, resulting in decrease of free volume, further decreasing the gases adsorption ability. For gases diffusion, due to the gradual reduction of the free volume, the collision between the gases and polymer chain increase, resulting in an increase in gases diffusion. Furthermore, we found that the pure membrane adopting 31 MD step procedure are more reasonable and more in line with the experimental value, and is a more efficient MD method in this study.
The narrow pore (NP) and large pore (LP) types of NH2-MIL-53 models are constructed. The results sohw that the amount of gases that can be adsorbed by the LP is higher than that of NP, and the most important influencing factor is the breathing behavior caused by the change of the pore size. Besides, the cluster models of NH2-MIL-53 adoping two different kinds of charge assignment are constructed as the filler to embedded into polymeric matrix.
The MMMs models are constructed within different weight loading of NH2-MIL-53. The results show that the gas molecules cannot effectively adsorb in the pores of the clusters, so the gas adsorption performance is not as expected. We speculate that the charge distribution of the clusters makes it cannot retain the original characteristics of NH2-MIL-53. However, we have tried to use the original charge distribution from NH2-MIL-53 to apply on the cluster models. The results show that although the cluster model cannot maintain the electrical neutrality, it can enhance the sorption ability inside the MMMs.
Despite the results in this study are not in agreement with the experiment, we can still observe certain trends and systematically discuss the mechanism of film adsorption on gases.
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