金屬有機骨架之微結構與吸附特性之關聯

Abstract

本研究第一部分檢視分子模擬用在 MIL-100(Fe)水吸附上的合理性，說明結構 架設與力場參數的使用，利用分子模擬出的等溫吸附曲線與實驗比較，再利用不同 吸附下的水分布情形解釋等溫吸附曲線中水吸附的機制。 由於具有毒性的氟化氫是 MIL-100(Fe)的原料之一，在本研究的第二部分中， 我們嘗試利用較無毒性的氯離子與氫氧根離子進行氟離子的置換。研究的範圍包 括基態能量比較、等溫吸附曲線、水分子與載體的交互作用力。另一部分審視水吸 附時的微結構影響，討論陰離子、金屬與框架中窗型結構對於水吸附位置的變化。 第三部分討論缺陷結構 MIL-100(Fe)是否在水吸附時會有不同的吸附能力表 現，我們嘗試兩種不同的缺陷結構 : 金屬節點移除(metal-node-missing)、金屬簇移 除(cluster-missing)。兩種缺陷雖然對於整體結構的影響不大，但是金屬簇移除這種 缺陷卻會明顯影響低吸附量時水與 MIL-100(Fe)_Cl 間的交互作用力，交互作用力 的提升可能來自於結構親水性的增加，使水分子能在低濕度的時候較容易吸附在 吸附材上，可使水分子在低濕度時於孔洞內凝結。

MIL-100(Fe) is a metal-organic framework which has significant water adsorption capacity. However, the structural stability for water vapor adsorption was found to be reduced if hydrofluoric acid (HF), served as mineralizing agent, is not used in the synthesis, although preferred because of eco-friendly consideration. We investigated the structural stability for water vapor adsorption of MIL-100(Fe) via density functional theory and molecular simulation techniques. Dreiding force field for MIL-100(Fe) and TIP4P for water were used in this study, and water adsorption isotherm showed improved consistency of results with experiment. We then differenced three kinds of terminal anions (fluoride, hydroxide, and void, i.e., no ion) on the structural stability for water vapor adsorption of MIL-100(Fe). We found that the replacement of hydroxide group would raise the energy of cluster by over 210 eV. Ideal MIL-100(Fe) with alternative anion (Cl and OH) show poor water adsorption capacity while the interacting energy of defective structure show significant increase at low loading region, which might enhance the water adsorption capacity. Further investigation into this interesting defective structure may lead to future use of catalyst, separation, or even the gate dielectrics in ICs.