四羧酸配位基金屬有機骨架從無序化到結晶態的快速結構轉變以及微量苯從環己烷中吸附分離應用

Abstract

近年來金屬有機骨架 (Metal-organic frameworks, MOFs) 材料的研究上顯示，結晶化的MOF是由成核、非序化再到結晶化的。由於結晶過程中不同的狀態會影響成核的速率以及結構的網絡，因此了解其中的生長階段對於想要針對特定應用的MOF的合成開發顯得非常重要。然而，MOF由非序化 (Disordered) 轉變到結晶態 (Crystalline) 的過程，並未見過往的研究提出任何機制探討。本研究分為兩個部分，第一部份想探討MOF非序化轉變到結晶化的關鍵原因。承續實驗室已經開發的溶劑脫附誘導非序化及缺陷結構，可快速驅動MOF結晶成有序孔洞結構，本研究將往更廣的面向去討論不同四羧酸配位基的MOF—NU-1000(Zr)、NU-901(Zr)以及MFM-300(M) (M=Al、Ga、Sc、Fe)，是否也具有類似的結構轉變現象。透過粉末X射線繞射 (PXRD)、場發射電子顯微鏡 (FE-SEM) 和比表面積及孔隙分析儀，本研究證明以雙溶劑交換 (Two solvents exchange, TOSE) 及加熱抽真空 (Heat under vacuum, HEVA)手法處理的產物依然保有與文獻記載相似的MOF結構、形貌及吸附性能。並透過結晶度計算以及自動電位滴定儀的實驗結果分析，證實了MOF晶格從初合成的非序化結構到因溶劑脫附誘發快速結晶化的過程。本研究更進一步大幅縮短了合成的時間，找出MOF能保持高度結晶性最佳的轉換方法。第二部分中，本實驗探討了如何在液相環境中將微量苯從環己烷中吸附移除。目前，全球生產的環己烷中約有80 ~ 85 %是透過苯的還原加氫反應獲得的，該反應會產生環己烷和殘留苯的混合物。由於苯具有相當高的毒性，並會對人體產生短期或長期的健康影響，因此生成環己烷時必須先純化，除去未反應的苯。根據目前市面上化學品公司的表明，環己烷的成品通常含有低於50 ppm的苯。本研究選用MFM-300(Sc)及MFM-300(Ga)作為吸附劑，其孔徑大小約為6.5 Å，具有四羧酸配位基與金屬團簇結合形成的通道型結構，可以有效的將苯捕捉在孔洞中，並且產生大量主體客體或客體客體之間的 π - π 電子作用力。本研究透過氣相層析質譜儀 (GC-MS)，分析攪拌吸附後環己烷溶液中微量苯的殘留量。結果表明，在苯濃度約為50 ppm左右的環己烷中加入MFM-300(Sc)或是MFM-300(Ga)作為吸附劑，經過七至八次吸附循環後可以有效的降低苯的含量至將近0 ppm。In recent years, literature on metal-organic frameworks (MOFs) materials has indicated that the MOFs nucleate through phase separation, condensation, and crystallization. However, in previous studies, no one has investigated the detailed mechanism of MOFs crystallization from disordered intermediate transform to the crystalline state. Here, we selected tetracarboxylate based MOFs, specifically NU-1000(Zr), NU-901(Zr), and MFM-300(M) (M=Al, Ga, Sc, Fe), to investigate their synthesis process with characterization by using techniques such as powder X-ray diffraction (PXRD), field emission electron microscopy (FE-SEM), and nitrogen gas adsorption. The study reveals that the post-activation steps by two solvent exchanges (TOSE) and heating under vacuum (HEVA) are crucial factors driving rapid transformation of disordered MOFs clusters to a crystalline state. Specifically, we indicates that the structural transformations and the reduction in uncoordinated linker ratio at each step through titration experiment. Moreover, we significantly shortens the reaction time, demonstrating that TOSE-HEVA treatments rapidly accelerate MOFs crystallization while maintaining desired properties. With the strong π-π interaction between host and guest from their considerable amount of aromatic ring, tetracarboxylate based MOFs also could be a promising material for capturing benzene. The research show that using MFM-300(Sc) powder as an adsorbent to cyclohexane, which initially contained approximately 50 ppm of benzene, demonstrates an effective reduction of benzene content to 0 ppm after 8 cycles. This reasearch elucidates the key reasons for MOFs crystallization, offers a promising route for efficient MOFs synthesis, and makes MOFs attractive materials for application in trace benzene removal.