小分子於金屬有機骨架薄膜內之輸送現象

Abstract

在研究薄膜分離技術的過程中，了解小分子在金屬有機骨架(metal-organic framework, MOF)中的輸送機制對詮釋分離效能、探索具有潛力的材料至關重要。本研究藉由一種具有極狹窄孔道的MOF，探究小分子在運動受限時展現的特殊擴散機制以及此機制如何影響薄膜的分離效能。舉例而言，水和其他小分子經常配位於MOF的結構中，這些配位分子可能直接參與質量輸送，但這一現象尚未得到研究。本研究探討了UTSA-280中獨特的水輸送機制，即輸送水可以透過置換配位水質傳，此機制稱為撞離機制(knock-off mechanism)。儘管UTSA-280只具有一維孔道，但是撞離機制使得水分子得以在垂直孔道的兩個軸上輸送，實現準三維質量輸送。即使孔道相對較狹窄，撞離機制不僅讓UTSA-280薄膜展現高的水通量，也讓其在滲透蒸發中展現優異的H2O/Ethanol擴散選擇性。為了驗證這種獨特的機制，本研究使用核磁共振技術觀察UTSA-280吸附重水前後的變化。本研究亦應用密度泛函理論計算跳躍機制和撞離機制的勢能圖以深入獲得原子級的理解。模擬研究發現撞離機制的能障略低於跳躍機制，其結果暗示了撞離機制在提升UTSA-280中水擴散的角色。小分子除了以撞離機制提升擴散效率以外，也可能因受限的孔道行單列擴散(single-file diffusion, SFD)。當單列擴散發生時，分離效能主要取決於薄膜進料側的吸附量。本研究製備了一個UTSA-280薄膜並進行CO2/N2和CO2/CH4的混合氣體滲透實驗，其中兩項重要的觀察證實了單列擴散的發生。首先，儘管從單一氣體滲透實驗中計算的CO2、N2、CH4固有擴散係數存在差異，CO2的擴散係數與其在混合物中對應物（N2或CH4）的擴散係數一致。其次，在吸附相中的CO2莫耳分率與滲透側中氣相的CO2莫耳分率相似。鑒於UTSA-280對CO2的高吸附選擇性，UTSA-280薄膜展現出優異的CO2/N2 (611)和CO2/CH4 (80.7)滲透選擇性。本研究的實驗結果亦獲得使用動力學蒙地卡羅方法之中尺度模擬的有力支持，尤其是薄膜進料和滲透側CO2組成之間的關係。此外，本研究使用基於密度泛函理論的模擬來探究CO2超越其對應物的能障，其結果顯示高且不利於發生的能障，從而加強了本研究關於單列擴散的發現。

In the study of membrane separation, a thorough understanding of the diffusion mechanisms of small molecules within metal-organic frameworks (MOFs) is of paramount importance for interpreting separation performance and screening potential materials. This study focuses on a MOF with exceptionally narrow pores and investigates the unique diffusion mechanisms in this constrained environment and their impact on membrane separation performance. For example, MOFs often possess coordinated molecules within their structure. Water and other small molecules coordinated in MOFs may impact mass transfer by diffusing alongside the transport molecules, but this phenomenon has yet to be reported. This study explores a distinctive water transfer mechanism within UTSA-280, which involves an incoming water molecule displacing a coordinated molecule to facilitate mass transfer. This process is referred to as the “knock-off” mechanism. Although UTSA-280 exhibits one-dimensional channels, a pseudo-three-dimensional diffusion can be realized with the presence of the knock-off transport which allows water to move along the other two axes perpendicular to the channel. Despite a relatively narrow pore width, the UTSA-280 membrane displays high water flux owing to the knock-off mechanism. The knock-off mechanism also renders the membrane superior water/ethanol diffusion selectivity for pervaporation. To validate this unique phenomenon, nuclear magnetic resonance (NMR) was conducted on UTSA-280 powder to observe changes in a sequence of adsorption experiments using deuterated water. This work also derived potential energy diagrams from the density functional theory to gain atomic-level insight into the knock-off and the direct-hopping mechanisms. The simulated results reveal that the knock-off mechanism exhibits a marginally lower energy barrier than the direct-hopping pathway, suggesting its potential role in facilitating water diffusion in UTSA-280. Apart from enhancing the diffusion efficiency via the knock-off mechanism, small molecules may exhibit single-file diffusion (SFD) owing to the restricted pore. When SFD takes place, the separation performance becomes solely reliant on the adsorption quantities on the feed side of the membrane. As a proof of concept, a UTSA-280 membrane was fabricated and subjected to testing using gas mixtures of CO2/N2 and CO2/CH4. Two crucial observations substantiate the occurrence of single-file diffusion. Firstly, the diffusion coefficients of CO2 were observed to align closely with those of its counterparts (N2 or CH4) within the mixtures, despite the intrinsic diffusion coefficients of CO2, N2, and CH4, as determined from single-gas permeation experiments, exhibiting differences. Secondly, the CO2 mole fractions within the adsorption phase closely mirrored those in the gas phase on the permeate side. Given the high adsorption selectivity of UTSA-280 for CO2, the UTSA-280 membrane delivers outstanding permeation selectivity for CO2/N2 (611) and CO2/CH4 (80.7). Our experimental findings find robust support through mesoscale simulations conducted using the kinetic Monte Carlo methods, particularly concerning the CO2 compositions on both the feed and permeate sides of the membranes. Additionally, we employed density functional theory-based simulations to investigate energy barriers associated with CO2 surpassing its counterparts. These simulations revealed high and unfavorable energy barriers, thereby reinforcing our experimental findings regarding single-file diffusion.