含石墨烯基質材料設計及滲透蒸發脫水複合膜製備

Design and preparation of pervaporation composite membranes with graphene-based materials for dehydration

梁 慕(Rumwald Leo Lecaros)

指導教授 ： 李魁然

中原大學/工學院/化學工程研究所/博士(2020年)

https://doi.org/10.6840/cycu202000280

摘要

目前滲透蒸發薄膜之發展面臨高效率、高選擇性及良好的穩定性的挑戰。 為了加強目前親水性滲透蒸發薄膜的效能，將薄膜進行交聯以及添加無機材於薄膜中為相當常見之方法。最近，一種新出現之無機材(石墨烯及其衍生物)受到研究薄膜的學者高度的關注，並探討應用於滲透蒸發醇類脫水時的傳輸行為。本篇研究探討一系列平板複合薄膜以及石墨烯添加物薄膜之物化性質及效能應用於醇類脫水。
第三章探討一具氧化石墨烯選擇層之複合薄膜於脫水程序之應用。添加微量聚乙烯醇(PVA)於氧化石墨烯中改變其層間距，而少量的PVA則會形成氧化石墨烯骨架 (GOF)。進料為90wt%醋酸水溶液時，未添加PVA之氧化石墨烯薄膜之透過通量及透過端水濃度分別為465 g m-2 h-1及79.5 wt%，而當PVA添加量為8 wt%時，透過通量及透過端水濃度分別為290 g m-2 h-1及93.6 wt%，分離效能在添加PVA後有顯著提升，其原因為添加PVA的薄膜在潤濕狀態時有較小的層間距。進料為90 wt%異丙醇水溶液時，薄膜透過通量及透過端水濃度為162 g m-2 h-1及98.8 wt%。儘管醋酸與異丙醇有相近的分子量，滲透蒸發效能之差異主要是受到進料的極性及官能基之影響。
添加GOF後的薄膜，2D結構的GO創造出許多彎曲的通道，因此薄膜的透過通量依然偏低。在第四章，我們將表面具有氧基團的零維結構碳量子點(OGQDs)與PVA進行混摻，相較於未添加的PVA交聯薄膜，添加OGQD s後具有較高的結晶度，證明OGQDs的添加可以創造彎曲的通道並使得薄膜有較好的透過選擇性。相較於分離異丙醇水溶液，添加300ppm OGQDs的薄膜應用於分離正丙醇水溶液時，有較低的選擇性。PALS結果顯示，當薄膜浸入不同的醇水溶液時，會有不同的自由體積特性。PVAx-OGQD300在30oC操作時的透過通量為464 g m-2 h-1，透過端水濃度為99.51 wt%，當操作溫度上升至70oC時，薄膜的透過通量上升至1502 g m-2 h-1，而透過端水濃度則下降至98.04 wt%。
在第五章，為了使得海藻酸鈉於較高溫度下操作時也具有良好的穩定，因此利用鈣離子進行交聯，並分別混摻含氮基團及氧基團的碳量子點，探討與高分子之間的作用行為。PALS結果顯示添加不同的GQDs會使得薄膜結構不同，並導致不同的醇類脫水效能。在潤濕的狀態下，相較於未添加及含有NGQDs的薄膜，含有OGQDs的薄膜具有最大的自由體積。因此當進料為70 wt%的異丙醇水溶液，操作溫度70OC時，添加量為100ppm OGQDs的薄膜有最高達5580 g m-2 h-1的透過通量，透過端水濃度為99.95%。
最後一章探討還原後的氧化石墨烯及含氧基團碳量子點混摻於薄膜之中，還原後的碳量子點會產生缺陷，而OGQDs的添加則是為了補償此現象。添加了3wt% rGO及OGQDs的薄膜應用於分離異丙醇水溶液時有最好的效能，其透過通量為1402 g m-2h-1，透過端水濃度達99.99 wt%。當薄膜只添加3wt% rGO時，也具有相同之選擇性但較高的透過通量 (1734 g m-2h-1) ，表示rGO所產生之缺陷大小無法讓異丙醇分子透過。在分離甲醇水溶液時，因甲醇分子較小則能明顯看出OGQDs填補缺陷之功能，其透過通量為2323 g m-2 h-1，透過端水濃度為92.7 wt%，PALS數據也證明OGQDs的添加可填補缺陷並阻擋甲醇分子。
第一篇研究透過改變氧化石墨烯之層間距提升薄膜之分離效能，因PVA插層於GO中提高了薄膜的穩定性，所以薄膜澎潤性及水分子的透過選擇性都獲得提升。第二篇研究添加少量之零維結構碳量子點，改善了薄膜的滲透蒸發效能。第三篇研究探討帶有不同官能基之碳量子點與高分子之間的界面作用力，並導致不同的自由體積濃度及尺寸在較高的操作溫度下，添加OGQDs的薄膜具有較佳的滲透蒸發效能。最後，由rGO及OGQD形成之勻相添加物薄膜非常適合應用於小分子醇類水溶液分離 (甲醇水溶液)，主要是因為OGQDs可填補rGO 之缺陷，其所具有的自由體積小於rGO薄膜。

關鍵字

滲透蒸發 ； 石墨烯衍生材料 ； 複合薄膜 ； 溶劑脫水

並列摘要

The development of pervaporation (PV) membranes faces major challenges to meet the desired separation performance which includes: (a) high productivity; (b) high selectivity and (c) good stability. In order to enhance the performance of current PV membranes, especially for hydrophilic pervaporation, cross-linking or incorporating inorganic fillers is commonly performed. Currently, the emergence of new class inorganic fillers (e.g. graphene and its derivatives) gives great attention for membranologists to understand their transport properties for separation processes like in alcohol dehydration through PV. In this dissertation, a series of flat-sheet composite membranes prepared through solution casting for alcohol dehydration were prepared and the viability of using graphene-based fillers was investigated. The physicochemical properties of the membranes were systematically characterized.
Firstly, the Chapter 3 presents the dehydration performance of a composite membrane with a selective layer that is mostly composed of the inorganic component – graphene oxide (GO). The interlayer spacing (d¬-spacing) of the GO nanosheets was tuned by adding minute or trace amounts of poly(vinyl alcohol) (PVA) creating a graphene oxide framework (GOF). The dehydration performance of the composite membranes was initially evaluated using acetic acid/water (HAc/water) mixture. At 8wt% PVA content based on GO amount resulted to a permeation flux of 290 g m-2 h-1 and a water concentration in permeate of 93.6% for the separation of 90/10 wt% HAc/water mixture. The separation performance was enhanced when compared with the pristine GO layer composite membrane that has a permeation flux of 465 g m-2 h-1 and a water concentration in permeate of 79.5%. The enhancement of water permselectivity was attributed to the lower d¬-spacing difference at wet state condition after the addition of PVA. In the case of separating alcohol/water mixture, it resulted with a permeation flux of 162 g m-2 h-1 ¬and a water concentration in permeate of 98.8% in separating 90/10 wt% isopropanol/water (i-PrOH/water) mixture at 30 °C. The differences in separating HAc/water from i-PrOH/water despite having comparable molecular weight is greatly influenced by the polarity and functional group of the different feed components.
Secondly, due to the potential use of GOF membrane for alcohol dehydration the flux was still low due to the longer tortuous pathway created by the two-dimensional (2D) nanosheets of GO. Hence in Chapter 4, the zero-dimensional (0D) oxygen-passivated graphene quantum dots (OGQDs) were used and integrated in PVA polymer matrix. The addition of OGQDs increased the crystallinity of the glutaraldehyde cross-linked PVA (PVAx) compared to the unintegrated PVAx membranes. This have provided a tortuous path for the permeating molecules but improved the water permselectivity of the membrane. An interesting result in this chapter was observed when the membranes with 300 ppm OGQD (PVAx-OGQD300) showed lower separation performance for the linear n¬-PrOH/water mixture than the sterically hindered i-PrOH/water mixture. The positron annihilation lifetime spectroscopy (PALS) revealed the different free volume characteristics of the membranes when immersed in the different alcohol mixture. The PVAx-OGQD300 membrane has a permeation flux of 464 g m-2 h-1 with a water concentration in permeate of 99.51% at 30 °C but there was a trade-off between flux and separation when the feed temperature was increased to 70 °C (1502 g m-2 h-1and 98.04%).
Thirdly, since the polymeric matrix integrated with OGQD enhanced the permselectivity of the composite membrane it is further investigated in Chapter 5 with calcium ion cross-linked alginate polymer matrix (Alg), a biopolymer, due to better stability at higher feed temperature. In this part, a nitrogen-doped GQDs (NGQDs) or OGQDs were integrated into the alginate matrix to discuss the effect of the polymer-filler interaction. The PALS results revealed that different functionalized GQDs affect the fine-structure characteristics of Alg matrix and resulted to different alcohol dehydration performance. Under wet state condition, larger free volume spaces appeared and it was shown that OGQD integrated membranes have the largest compared to pristine and NGQD integrated membranes. These data explained the higher permeation flux and separation factor of Alg-OGQD membranes over Alg-NGQD and Alg membranes. The total flux reached to 5580 g m-2 h-1 with a water concentration in permeate of ~99.95% in dehydrating i-PrOH/water (70/30 wt%) at 70 °C for Alg-OGQD membrane with 100 ppm of OGQD (Alg-OGQD100).
Finally, in Chapter 6, the combined reduced graphene oxide (rGO) and OGQD as the inorganic filler for the hybrid membranes. The OGQD was used to compensate the structural defects on rGO caused by the reduction process. These defects could be identified as non-selective voids that could suffer the permselectivity of the membrane. Hence, the rGO+OGQD was integrated in an alginate polymer matrix. The Alg membrane with 3 wt% rGO+OGQD filler (Alg-rGO+OGQD-3) showed good performance for i-PrOH/water mixture separation. It has a permeation flux of 1402 g m-2 h-1 and a water concentration in permeate of 99.99% at 25 °C. This showed that the size of i-PrOH molecules are large enough not to be transported in the rGO defects since Alg-rGO-3 membranes showed a comparable water concentration in permeate of 99.99% but with higher permeation flux of 1734 g m-2 h-1. The enhancement of water permselectivity due to sealing the defects on rGO was best observed with separating MeOH/water mixture due to the smaller molecular size of MeOH. The permeation flux reached to 2323g m-2 h-1 with a water concentration in permeate of 92.7% at 70 °C. The PALS results supported that the minimization of the structural defects that enhanced the water selectivity of the membrane by easy transport of water molecules through the free volume spaces and blocks methanol molecules.
In conclusion, the first study discussed the enhancement of separation by adjusting the interlayer spacing of GO-based membranes. This way the swelling of the nanosheets was prevented by intercalating PVA and the water permselectivity of the membrane was improved. The second study introduced the integration of the 0D carbon-based nanomaterial (i.e., OGQD) at low concentration into the polymer matrix that augments the water permselectivity of the membranes. The third study explored the interfacial interaction of the GQDs with the polymer matrix by comparing oxygen- and nitrogen-functional groups in which it gave different free volume characteristics that affects the separation performance. The OGQD integrated membranes have shown better water permselectivity at higher feed temperature. Lastly, the creation of combined rGO and OGQD homostructured fillers was found to be effective in separating smaller sized alcohol/water mixture (e.g., MeOH/water). The enhanced water permeselectivity was due to the successful sealing of non-selective void in rGO by OGQD. The free volume size for membranes with rGO+OGQD was smaller than rGO integrated membranes.

並列關鍵字

Pervaporation ； Graphene-based materials ； composite membrane ； solvent dehydration

參考文獻

References

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