| 摘要: | 金屬有機框架結構（MOFs）是一種新型的微孔結晶材料，在工業製程中具有廣泛的應用，例如氣體捕捉、氣體儲存、催化和氣體分離等等。MIL-53系列和UiO-66系列也是具有類似性能的多孔性材料，包括在水中的結構穩定性、靈活的孔洞結構和對特定氣體出色的分離性，也因其優異的性能使MIL-53系列和UiO-66受到各界的關注。
另一方面，電腦模擬計算是幫助我們分析MOFs細部結構的好工具。密度泛函理論（DFT）是一種第一原理的計算，此常見的方法是運用量子力學來模擬週期性系統。第一原理計算基於量子力學，並且在沒有任何假設的情況下執行。蒙特卡羅（Monte Carlo, MC）模擬，另一種模擬的方法，利用隨機抽樣進而找到數值結果，並通過數據統計計算出熱力學中分子的狀態和位置分佈。
在本研究，我們探討MIL-53（Al）和UiO-66（Zr）這兩種類的MOFs，透過製成薄膜來觀察MIL-53（Al）的孔洞結構性能，而透過電腦模擬來觀察UiO-66（Zr）的結構特性。
在實驗部份，我們合成出PSF高分子基底的混合基質膜(mixed-matrix membranes, MMMs)。以雙填料的方式將MIL-53（Al）和NH2-MIL-53（Al）兩種填料，依據不同的比例混到混合基質膜中。並通過X射線繞射、SEM圖像、FTIR分析和DSC分析來鑑定薄膜內部的相互作用。結果驗證，兩添加的填料皆能保持原本的特性，且沒有團聚現象的發生，並能有效與高分子薄膜結合。
在模擬部分，DFT原理透過允許晶包內所有原子與晶格間距能適度的移動、調整，使系統總能量趨近最小值，進而達到幾何結構優化的效果。計算的系統包括淨空的UiO-66（Zr），間二甲苯和對二甲苯兩個吸附分子。在用DFT幾何結構優化之後，此UiO-66(Zr)結構透過MC計算，找到間二甲苯和對二甲苯吸附物，各別在UiO-66（Zr）中最有可能吸附的位置。最後，在找到吸附位置，再次利用DFT原理計算出UiO-66（Zr）吸附間二甲苯與UiO-66（Zr）吸附對二甲苯所需要的吸附能。結果我們發現UiO-66（Zr）吸收間二甲苯的吸附熱為60.9kJ / mol，而吸收對二甲苯分子的吸附熱為58.8kJ / mol。與小分子尺寸的粒子相比較(CO2、CH4)反而有較高的吸附能，驗證了1,4-苯二甲酸酯(BDC)有機鍵結與二甲苯的苯環之間具有良好的相互作用。
;Metal–organic frameworks (MOFs), a relatively new class of microporous crystalline materials, have a very large number of applications in industry, such as gas capture, gas storage, catalysis, and selective separation, etc. Both MIL-53 series and UiO-66 series are also such porous materials with similar properties, which have gained a lot of attention due to useful properties including structural stability in water, flexible pore openings, and outstanding selectivity for specific gases.
On the other hand, computer simulations are good tools to help us understand details of MOF structure. Density functional theory (DFT), one kind of first principles simulations, is a common way to implement quantum mechanical simulation of periodic systems. First principles calculations are based on quantum mechanics and are performed without any assumptions. Monte Carlo (MC) simulation, another simulation method using random sampling to find numerical results, calculates molecule state and distribution of the location in thermodynamics by statistics.
In this work, we investigated these two classes of MOFs, MIL-53(Al) and UiO-66(Zr), by using membranes synthesis to observe the structural performance of MIL-53(Al) and computer simulation to observe the structural performance of UiO-66(Zr).
Experimentally, we demonstrated PSF polymer-based Mixed-Matrix Membranes. The MIL-53(Al) series, ML-53(Al) and NH2-MIL-53(Al), were added to the MMMs as a co-filler, containing different loading ratio. The membranes were characterized by X-ray diffraction, SEM images, FTIR analyses and DSC analyses to investigate the interactions inside membranes. The results show that the two added fillers can maintain the original characteristics without any agglomeration phenomena, and can effectively combine with the polymer film.
The section of simulation, DFT was used to minimize the total energy of the system by allowing all the atoms and lattices in the cell to relax, thereby optimizing the geometry. The systems calculated include the empty UiO-66(Zr), m- xylene, and p-xylene molecule. After optimizing the geometry with DFT, the structure was used to find the most likely location of adsorbate in UiO-66(Zr) by MC calculation. Finally, the UiO-66(Zr) uptaking m-xylene molecule and UiO-66(Zr) uptaking p-xylene molecule were studied again using DFT after finding the location to calculate the heat of adsorption. The results showed the heat of adsorption is 53.68 kJ/mol for UiO-66(Zr) uptaking m- xylene, and 60.16 kJ/mol for UiO-66(Zr) uptaking p-xylene molecule, respectively. Compared to the molecule with a small diameter(CO2, CH4), m-xylene and p-xylene have a higher adsorption energy, which confirms the good interaction between the 1,4-benzenedicarboxylate (BDC) linkers and benzene ring of xylene. |
