利用嵌入高分子調控金屬有機骨架薄膜之孔洞特性與氣體分離效能

Abstract

金屬有機骨架(Metal-organic frameworks, MOFs)因具有結構多樣性、化學功能性以及良好的吸附性質等特性，在薄膜分離領域被廣泛利用並具有高度發展潛力，尤其是在薄膜氣體分離與薄膜滲透蒸發等應用領域中。根據分子篩理論，具有氣體分離效能的MOF，其孔徑大小理想上將會介於兩種氣體的動態直徑之間，只要成功將其製備成為薄膜，便可有效對此氣體混合物進行分離。對於孔洞過大的MOF來說，其薄膜則難以對氣體混合物產生分子篩效應。本研究參考先前文獻，提出利用高分子嵌入MOF孔洞的方法來縮小其孔洞大小，盼利用此方法來提高薄膜氣體分離選擇性。在本論文研究中，我們選擇了名為Zn-BDC-TED的MOF(ZnMOF)材料來進行研究，其孔徑大小為0.75 nm。並將其製備為薄膜進行研究。接著我們使用分子量20,000 Da的聚乙二醇(PEG)摻入此ZnMOF薄膜，並利用傅立葉轉換紅外線光譜以及X光繞射技術來檢測PEG在ZnMOF中的分布情形。從臨場X光分析中，我們觀測到PEG在ZnMOF中吸、脫附的可逆變化。此外，我們得知PEG在ZnMOF不同軸向孔洞間的分布情形。透過薄膜氣體滲透的實驗，我們發現摻入PEG的ZnMOF薄膜具有良好的氫氣滲透率以及氫氣/二氧化碳選擇率。摻入PEG後選擇率從4提高到了18。

Metal-organic frameworks (MOFs) are promising materials for membrane gas separation due to their structural diversity, chemical functionality, and good adsorption properties. Base on the concept of size exclusion, an ideal MOF structure for membrane gas separation should possess an aperture size in between the dimensions of the two gas molecules to be separated. Therefore, MOFs with a pore size beyond the ultramicropore region (i.e. > 0.7 nm) is not ideal for for membrane gas separations. Herein we propose a method to narrow the pore size of MOFs via polymer insertion. Specifically, Zn-BDC-TED (ZnMOF) with a pore size of about 0.75 nm is investigated in this work. Polyethylene glycol (PEG) with molecular weight of 20,000 Da is inserted into ZnMOF membranes for the pore engineering. The resulting ZnMOF membranes with PEG were characterized via Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Diffractometer (XRD). The reversibility of PEG adsorption in ZnMOF was assessed by the in situ XRD analysis with a synchrotron X-ray source. The gas permeation results show that the insertion of PEG can effectively improve the gas separation performance of the ZnMOF membranes, especially for the mixture of H2/CO2. The optimized ZnMOF membrane achieves an ideal H2/CO2 selectivity of 18 with a high H2 permeability of about 3×105 barrer.