以甲基三甲氧基矽烷作為前驅物製備之疏水性 聚甲基矽氣凝膠薄膜於真空薄膜蒸餾之應用

Abstract

本文主旨在以單步驟方式製備疏水性二氧化矽溶膠，將其覆蓋於氧化鋁陶瓷基材膜以達到縮孔及表面改質的目的，並將製備完成之氣凝膠薄膜應用於真空薄膜蒸餾系統中進行海水淡化效能測試。文中將探討氣凝膠薄膜之製備條件對其孔洞結構及真空薄膜蒸餾之操作效能的影響。 本文中以甲基三甲氧基矽烷（MTMS）作為前驅物，並固定水解及縮合反應中的水量和，藉由調整縮合步驟中所加入的乙醇量來改變EtOH2nd/MTMS之莫耳比，探討合成條件對於氣凝膠結構的影響。此外，水解及縮合反應中的催化劑濃度也是結構變化的重要因素。由水接觸角儀對氣凝膠薄膜表面進行疏水性測試，發現水接觸角隨著乙醇的添加量減少而上升，當EtOH2nd/MTMS之莫耳比為1時有最好的疏水程度，藉由掃描電子顯微鏡及比表面積與孔隙分析儀分別對氣凝膠表面覆蓋程度及氣凝膠孔洞結構進行分析，證實在此莫耳比下所製備的氣凝膠薄膜有較均勻的覆蓋情形及較小的孔徑分佈，以至於薄膜表面疏水度的提高。隨後將不同EtOH2nd/MTMS莫耳比下製備的薄膜進行真空薄膜蒸餾效能測試，滲透通量亦在EtOH2nd/MTMS為1時達到最高，證明良好的氣凝膠結構有助於薄膜通量效能的提升。 隨後固定EtOH2nd/MTMS為1及鹼催化劑濃度為17%，改變水解過程中加入的酸催化劑濃度，發現薄膜表面水接觸角隨著酸催化劑濃度先上升後下降，在酸催化劑濃度為0.56%時有最高的水接觸角；成膠時間則隨著酸催化劑濃度上升而增加；表面覆蓋程度、孔洞比表面積和平均孔洞體積則酸催化劑濃度增加而減少。真空薄膜蒸餾的效能測試方面，純水通量隨著酸催化劑濃度提高而增加，在酸催化劑濃度為0.84%時有高達20.24LMH的純水通量，若在進一步地提高酸催化劑濃度，薄膜因表面覆蓋不均而潤濕，以至於薄膜蒸餾失效。 接著進一步地提高鹼催化劑濃度，發現在不同濃度的酸催化劑下，25%的鹼催化劑相較於17%的鹼催化劑所製備的薄膜皆有較高的水接觸角、較短的成膠時間及較大的中孔孔徑。薄膜表面及截面則因緻密的結構使得質傳阻力增加，導致薄膜蒸餾純水通量顯著下降。 最後對MTMS氣凝膠薄膜進行熱重分析，發現MTMS氣凝膠薄膜具有相當好的耐熱性，在300℃以前都能維持穩固的結構，在薄膜蒸餾程序的操作溫度範圍之內都具有相當好的穩定性，證明了以MTMS二氧化矽溶膠進行縮孔的陶瓷膜具有很好的耐用性。

The purpose of this study is using a single-step procedure to prepare a hydrophobic silica aerogel to coat on aluminum ceramic substrate for pore size control and surface modification. The silica aerogel membrane is applied on vacuum membrane distillation for desalination performance test. In this study, the effect of synthesis conditions on pore structure and membrane distillation performance were be investigated. Methylmethoxysilane（MTMS）is used as precursor and the molar of water and catalyst is fixed, to vary EtOH2nd/MTMS molar ratio by adjusting the amount of ethanol added in condensation reaction. The surface hydrophobicity test is conducted by contact angle system. As the results, the contact angle increases with decreasing amount of ethanol, while the best hydrophobicity is observed as EtOH2nd/MTMS molar ratio is one. Meanwhile, the report of Accelerated Surface Area and Porosimetry analyzer and Scanning Electron Microscope show that the silica aerogel with a narrow pore size distribution and uniform coverage. Furthermore, the silica aerogel membrane synthesized with EtOH2nd/MTMS molar ratio = 1 shows the highest water flux due to the better silica aerogel structure. Subsequently, EtOH2nd/MTMS molar ratio and the concentration of basic catalyst are fixed at 1 and 17% respectively, and the concentration of acidic catalyst added in hydrolysis reaction vary from 0.14% to 2.24%. As the results, the contact angle shows a trend of rise first, then fall at the concentration of acidic catalyst of 0.56%. However, to increase the concentration of acidic catalyst result in the increasing of the gelation time and the decreasing of average pore surface area, average pore volume and surface coverage. On the other hand, membrane distillation water flux can reach 20.24 LMH while the concentration of acidic catalyst is 0.84%, if further increase the concentration of acidic catalyst will lead to pore wetting due to non-uniform surface. The surface of silica aerogel membrane becomes more hydrophobic and the average pore size becomes larger while 25% basic catalyst is using, the mass transfer resistance is meanwhile raising due to the dense surface coverage. Therefore, membrane distillation water flux has an obvious falling. Finally, TGA analysis shows the great stability of chemical structure before 300℃ sintering, so the MTMS derived silica aerogel membrane with optimal synthesis condition exhibit high and stable water flux under continuous operation at high temperature and low vacuum pressure. In conclusion, aluminum ceramic substrate modified by MTMS derived silica aerogel is a high performance and durable material.