以分子模擬探討氣體在金屬有機骨架內之吸附與擴散性質

Abstract

本研究以MOF-303及其混合配體改質之衍生結構為核心探討對象，結合蒙地卡羅(Monte Carlo, MC)與分子動力學(Molecular Dynamics, MD)模擬技術，系統性研究二氧化碳(CO2)與氮氣(N2)在金屬有機骨架材料(Metal-Organic Frameworks, MOFs)中的吸附與擴散行為，並預測其在氣體分離應用中的潛力。研究中選用2,5-呋喃二甲酸(FDC)與2,5-噻吩二甲酸(TDC)取代原始3,5-吡唑二甲酸(PDC)配體，建構FDCx與TDCx (x代表改質配體比例)系列混合配體之MOF-303結構模型，並分別利用Materials Studio與RASPA執行吸附模擬，結合Zeo++進行孔徑分析，包含孔徑限制直徑(PLD, pore limiting diameter)、最大空腔直徑(large cavity diameter, LCD)和孔徑分布(pore size distribution)的計算，再進一步以分子動力學探討其在不同氣體載量下的擴散特性，包含以方均根位移(mean square displacement, MSD)計算擴散係數，及擴散係數與PLD和LCD之間的關聯。模擬結果顯示，配體改質可顯著調控MOF孔徑幾何與表面特性，進而影響CO2與N2的吸附容量與擴散速率。此外，分析結果亦顯示吸附後結構能的絕對值與吸附量呈正相關，代表能量較低之構型存在較佳的氣體吸附能力；而在擴散行為方面，氣體載量之變化也會明顯影響擴散係數之趨勢。最後，本研究亦針對MOF-303及其改質衍生物的CO2及N2滲透率和滲透、吸附及擴散選擇率，與文獻結果進行深入比較和討論。在特定FDC與TDC比例下，材料展現出優異的CO2/N2吸附與擴散選擇率，尤以FDC0.250和TDC0.500能維持接近MOF-303的二氧化碳滲透率，且其CO2/N2分離效能達1.5~2倍。整體而言，本研究成功模擬混合配體的MOF-303，並驗證了它們在二氧化碳捕捉效能的潛力，未來可進一步應用於碳捕捉與其他分離膜材料的模擬甚至實驗開發。

This study centers on MOF-303 and its mixed-linker derivatives, employing a combination of Monte Carlo (MC) and Molecular Dynamics (MD) simulation techniques to systematically investigate the adsorption and diffusion behaviors of carbon dioxide (CO2) and nitrogen (N2) within metal–organic frameworks (MOFs), with the aim of evaluating their potential for gas separation applications. In this work, 2,5-furandicarboxylic acid (FDC) and 2,5-thiophenedicarboxylic acid (TDC) were introduced as partial replacements for the original 3,5-pyrazoledicarboxylic acid (PDC) linker to construct a series of mixed-linker MOF-303 models (denoted as FDCx and TDCx, where x represents the molar fraction of the modified linker). Gas adsorption simulations were performed using both Materials Studio and RASPA, while pore structure characteristics—including pore limiting diameter (PLD), largest cavity diameter (LCD), and pore size distribution—were analyzed with Zeo++. Subsequently, gas diffusion properties under varying loading conditions were evaluated via MD simulations by calculating diffusion coefficients based on the mean square displacement (MSD), and the relationships between diffusion coefficients and geometric parameters such as PLD and LCD were further examined. Simulation results reveal that linker modification significantly alters the pore geometry and surface characteristics of the MOFs, thereby affecting the adsorption capacities and diffusion rates of CO2 and N2. Furthermore, the analysis indicates a positive correlation between the magnitude of adsorption structural energy and the amount adsorbed, suggesting that configurations with lower post-adsorption structural energy are more favorable for gas uptake. On the diffusion side, variations in gas loading were found to have a pronounced influence on the diffusion coefficient trends. In addition, this study includes a detailed comparison between the simulated permeation, adsorption, and diffusion selectivity of MOF-303 derivatives and relevant literature data. At specific FDC and TDC substitution ratios, the materials exhibited excellent CO2/N2 selectivity in both adsorption and diffusion. Notably, FDC0.250 and TDC0.500 maintained CO2 permeability comparable to that of pristine MOF-303, while achieving 1.5- to 2-fold enhancement in overall CO2/N2 separation performance. In summary, this work successfully models a set of mixed-linker MOF-303 structures and demonstrates their promising capability for carbon dioxide capture. The findings provide a valuable theoretical basis for the future development of simulation-guided or experimentally verified MOF-based materials for carbon capture and membrane separation applications.