海藻酸鈉/雜多酸複合膜應用於滲透蒸發分離之研究

Design and fabrication of sodium alginate incorporated with polyoxometalate composite membranes for alcohol dehydration through pervaporation

徐翊鈞(Yi-Chun Hsu)

指導教授 ： 李魁然 ； 蔡惠安

中原大學/工學院/化學工程研究所/碩士(2021年)

https://doi.org/10.6840/cycu202100621

摘要

在本研究中，海藻酸鈉 (sodium alginate, SA) 利用solution-casting的方式刮製在多孔性聚醚碸 (polyethersulfone, PES) 基材膜上，製成SA/PES複合膜，應用於滲透蒸發操作程序進行醇類水溶液的脫水。由於SA之高親水性，致使SA/PES 複合膜在高溫下進行滲透蒸發操作過程會產生分離效能不穩定。因此，四種具不同價電離子電荷、核心元素和周圍金屬元素的Keggin結構多金屬氧酸鹽化合物 (雜多酸，polyoxometalates, POM)，包含三種商業化具陰離子性之矽鎢酸 (silicotungstic acid, POM-SiW)、磷鎢酸 (tungstophosphoric acid, POM-PW)、磷鉬酸 (phosphomolybdic acid, POM-PMo) 和自行合成具陽離子性的鋁Keggin 離子(aluminum Keggin ion, POM-Al)，作為無機奈米添加材料，摻入海藻酸鈉基質中，製備穩定的SA/POM/PES有機/無機複合薄膜，應用於滲透蒸發醇類水溶液的脫水。
研究中藉由X 射線衍射光譜 (XRD)，掃描電子顯微鏡 (SEM) 和能量散射 X 射線光譜 (EDX)、X射線光電子能譜 (XPS)，以及全反射式傅立葉轉換紅外線光譜儀 (ATR-FTIR)等儀器鑑定複合薄膜之結構型態與物化特性。由EDX的分析結果顯示，POMs可均勻分散在SA薄膜中，不需要透過表面改質便可解決有機/無機相容性不佳的問題。膨潤度數據顯示SA/POM-PMo/PES薄膜具有最低的膨潤度，且於FTIR以及XPS的鑑定結果發現，在強勁的靜電作用力下，使得SA/POM-PMo/PES薄膜結構更加穩定，更進一步在XRD的結果中可以發現在SA/POM-SiW/PES、SA/POM-PW/PES以及SA/POM-Al/PES雖然也具有靜電作用力產生，但是半結晶區域的擴張限制了滲透通量的提升。但在SA/POM-PMo的系統，則可以有效的抑制半結晶區的擴張，使得滲透蒸發分離不僅可以在透過端水濃度上有所幫助，甚至也能因為非結晶區域的增加，滲透通量隨之提升。
POMs種類對SA/PES複合膜滲透蒸發影響，進料為70 wt%異丙醇水溶液，在25℃的操作條件下，以SA/POM-PMo/PES可以得到最好的滲透蒸發脫水之效能表現，佐證了添加POM-PMo，透過Ca2+與SA有強的離子鍵結產生，使得結構更為穩定，且破壞半結晶的形成，使得高分子之間幾乎不存在半結晶的形成，同時維持在高透過端水濃度下，也提升了滲透通量； 在75℃操作下，SA/POM-PMo/PES也有最好的脫水效能表現，具有>99 wt%的透過端水濃度以及>7,500 g/m2h的滲透通量。添加0.5 wt% POM-PMo的複合膜於70 wt%乙醇水溶液進料，水與乙醇的透過活化能分別為22.7與16.7 (kJ/mol)，從吸附測試中發現吸附行為主導分離行為。SA/POM-PMo/PES薄膜於25°C下，對70 wt%異丙醇水溶液為期1,056小時的長時間測試中，透過端水濃度皆維持在>99 wt%，在工業上的應用是相當有潛力。

關鍵字：海藻酸鈉、雜多酸、半結晶區、複合薄膜、滲透蒸發

關鍵字

海藻酸鈉 ； 雜多酸 ； 半結晶區 ； 複合薄膜 ； 滲透蒸發

並列摘要

In this study, sodium alginate (SA) was cast on porous polyethersulfone (PES) ultrafiltration membrane followed by cross-linking with calcium ions for alcohol dehydration through pervaporation. The pristine SA/PES composite membrane was not stable at high temperature during the pervaporation process. Therefore, four kinds of polyoxometalates (POMs) with different ionic charge, core, and surrounding metal elements, namely POM-SiW, POM-PW, POM-PMo, and POM-Al, were added as fillers to blend into the sodium alginate matrix to prepare stable organic-inorganic hybrid membranes.
The composite membranes were characterized through X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance-Fourier infrared spectroscopy (ATR-FTIR). The EDX analysis revealed that the POM was perfectly dispersed on the membrane matrix implying a good compatibility between the organic and inorganic materials used. The FTIR and XPS analyses revealed high electrostatic interaction where calcium ions on the membrane matrix caused shifts in the characteristic peaks of SA and calcium atomic percent, respectively. In the XRD results, it can be found in SA/POM-PMo system can effectively inhibit the increase of the semi-crystalline area. This can not only obtain a high water content in the permeate but also increase the permeation flux simultaneously thus breaking the trade-off effect. While for the SA/POM-SiW/PES, SA/POM-PW/PES, and SA/POM-Al/PES may also have electrostatic forces, the increase in the semi-crystalline regions on the membrane matrix limits the increase in permeation flux.
The type of POMs affects the pervaporation performance of the hybrid composite membranes. A 70 wt% isopropanol aqueous solution was used for the pervaporation test. Under 25°C operating condition, SA/POM-PMo/PES achieved the best pervaporation performance. The separation performance proved that the addition of POM-PMo can generate strong electrostatic force through Ca2+ and SA which made the structure more stable and suppress the formation of semi-crystalline region. Thus, maintaining a high water content in the permeate and also an increase in permeation flux. When operating at 75°C, the SA/POM-PMo/PES also had the best dehydration performance with >99 wt% water content in the permeate and a permeation flux >7,500 g/m2h. The composite membrane loaded with 0.5wt% POM-PMo was further tested to separate 70 wt% ethanol aqueous solution. The permeation activation energy of water and ethanol are 22.7 kJ/mol and 16.7 kJ/mol, respectively implying that the permeation flux for both components increased due to the increase in temperature. The permeation activation energy of ethanol was less than that of the water which means that the permeation of ethanol was inhibited. The sorption behavior dominates the pervaporation separation in this composite membrane as confirmed through the sorption test. The SA/POM-PMo/PES composite membrane was further tested for its long-term stability at 25°C for 1,056 hours with 70 wt% isopropanol aqueous solution. The water content in the permeate was maintained at >99 wt% and the flux at ~2300 g/m2h showing stable performance for long-term operation. The results in this study showed that the POMs has potential as an inorganic filler for pervaporation membranes.
Key words: sodium alginate, polyoxometalate, semi-crystalline region, composite membrane, pervaporation

並列關鍵字

sodium alginate ； polyoxometalate ； semi-crystalline region ； composite membrane ； pervaporation doi

參考文獻

第五章 參考文獻

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