Elsevier

Journal of Membrane Science

Volume 576, 15 April 2019, Pages 36-47
Journal of Membrane Science

Influence of integrating graphene oxide quantum dots on the fine structure characterization and alcohol dehydration performance of pervaporation composite membrane

https://doi.org/10.1016/j.memsci.2019.01.019Get rights and content

Highlights

  • Graphene oxide quantum dots (GOQDs) integrated in poly(vinyl alcohol) matrix.

  • GOQDs augment water selectivity of the composite membrane.

  • Integrating GOQDs affect the fine structure characteristic of the membrane.

  • Membranes with GOQDs have high affinity toward linear alcohol.

Abstract

Graphene oxide quantum dots (GOQDs), a carbon-based nanomaterial resembling the structure of graphene oxide but with a smaller size of <100 nm, was incorporated in a poly(vinyl alcohol) (PVA) dense selective layer supported on a polysulfone substrate to prepare a composite hybrid pervaporation (PV) membrane for alcohol dehydration. The addition of GOQDs into the PVA polymer matrix provided a tortuous path for the permeating molecules but augments the separation efficiency of the membrane. The presence of hydroxyl and carboxyl groups on the confined nanosize of GOQDs sheets attract the polar groups of the permeants within the polymer matrix. The physicochemical properties of the membrane were characterized. An interesting observation was found when the membranes with GOQD (PVAx-GOQD300) were tested for aqueous propanol isomers mixture, isopropanol (i-PrOH) and n-propanol (n-PrOH). The PVAx-GOQD300 membrane showed lower separation performance for linear alcohol compared with the sterically hindered alcohol. Positron annihilation lifetime spectroscopy (PALS) results revealed that membranes integrated with GOQD behaved differently in both propanol isomers. The PVAx-GOQD300 membrane showed larger free volume size in n-PrOH/water than i-PrOH/water mixtures while there was not much difference in the absence of GOQDs. PALS results justified the low separation of the membrane with linear alcohol/water mixtures. Furthermore, the long operating time, high operating temperature, and high water content in feed shows the potential stability of the membrane for practical application.

Introduction

Pervaporation (PV) has been established to be an efficient membrane separation process for azeotropic mixtures and close-boiling point liquid mixtures. Its separation efficiency largely depends on the membrane. The pervaporation membrane must show good stability in organic solvents while giving a reasonable permeability and selectivity. Polymeric membranes have still been a choice for industrial pervaporation applications and are still developing [1]. Hydrophilic polymers such as chitosan [2], [3], [4], sodium alginate [5], [6], [7], and poly(vinyl alcohol) [8], [9], [10], [11] have been widely researched for pervaporation. A typical problem encountered in this type of hydrophilic PV membranes is swelling and to prevent it the membrane must be cross-linked or introduce fillers that can enhance the stability. A nanomaterial, as a filler, that possess hydrophilic groups is a good candidate as it may serve as carrier for water permeation and also provide good compatibility with the hydrophilic polymer used [5], [7], [12]. Therefore, it is still essential to evaluate new emerging materials that could potentially augment the separation efficiency of PV membranes.

Graphene oxide (GO), which is a 2D carbon-based nanomaterial has been extensively studied in different separation processes such as pervaporation [3], [4], [5], [7], [13], gas separation [14], [15], nanofiltration [16], [17], and reverse osmosis [18], [19]. This unique material has hydrophilic regions that easily attracts water and hydrophobic regions that resembles graphene and serves as a frictionless pathway for water molecules. In our previous works on pervaporation, reduced graphene oxide (rGO) in chitosan membranes exhibited an effective methanol/water separation performance [4] and the GO with minute amounts of PVA showed good stability and separation of acetic acid/water mixture [13]. The separation in these membranes was mainly due to the interlayer space of GO sheets. GO has a large lateral dimension (~1 µm) that provides a long-tortuous path for the permeating molecules. Thus modifying graphene sheets with artificial pores has been reported to effectively enhance the permeation [20], [21] but this additional process provides additional cost and impractical for large-scale production. A possible way to enhance the permeation is to find a material that has a smaller lateral size but possess similar structure as GO. Currently, researchers are focused on utilizing graphene oxide quantum dots (GOQDs).

GOQD is an emerging nanomaterial and has been widely used in fields such as biomedical [22], [23], [24], sensors [25], [26], [27], and electronics [28], [29] due to its biocompatibility and optical properties. GOQDs possess a few-layer of graphene sheets with a lateral size of less than 100 nm. GOQDs have the similar sp2 carbon structure and hydrophilic functional groups as GO but is contained in a very small size. Hence, shorter tortuous pathway for permeating molecule is expected. Membranologists are recently utilizing quantum dots as an additive to enhance the membrane performance for various separation process like nanofiltration [30], [31], forward osmosis [32] and pressure retarded osmosis [33]. Therefore, GOQD is a good candidate as an additive for membrane fabrication to enhance the hydrophilic character. It is expected that the GOQDs within the polymer matrix would attract water molecules and augment the separation of alcohol/water mixtures.

Compared to GO, the synthesis of GOQDs is easier. A facile “bottom-up” approach was used in this study to synthesize GOQDs through the partial carbonization of citric acid (CA) by moderate pyrolysis [34]. The GOQDs were integrated in poly(vinyl alcohol), a good film-forming polymer and industrially used for PV, to prepare composite membranes. The effect of integrating GOQDs on the membrane physicochemical property was characterized and the dehydration performance was evaluated.

Section snippets

Materials

Citric acid anhydrous (CA) and sodium hydroxide (NaOH) pellets were purchased from SHOWA Chemical Industry Co., Ltd (Japan). Poly(vinyl alcohol) (PVA, MW 146,000–186,000, 99+% hydrolyzed) and Glutaraldehyde (25% in water) were from Sigma Aldrich Co. (St. Louis, MO, USA). Methanol (HPLC grade) was acquired from Aencore Chemical Pty. Ltd. (Victoria, Australia). Hydrochloric acid (HCl) was obtained from Nihon Shiyaku Reagent (Japan). Isopropanol (i-PrOH), n-propanol (n-PrOH), ethanol (EtOH), n

Characterization of synthesized GOQDs from citric acid

The GOQDs were successfully synthesized by the moderate pyrolysis of citric acid (CA) at 200 °C. After moderate pyrolysis, CA can be partially carbonized to form nanosheets rich with small sp2 clusters (Fig. 1a). These sp2 clusters are passivated with oxygen functional groups that was inherited from CA during incomplete carbonization [34]. The optical property of the GOQDs suspension is shown in Fig. 1b–d, where it exhibited its blue luminescent character under UV light at 365 nm. Additionally,

Conclusion

An emerging material, graphene oxide quantum dots (GOQDs), was evaluated for its potential as a filler for pervaporation membrane. It was integrated in poly(vinyl alcohol) (PVA) and uniformly cast on a support membrane. The integration of GOQDs into glutaraldehyde cross-linked PVA matrix had an effect on its physicochemical structure resulting to changes in separation performance with 70/30 wt% i-PrOH/water mixture. The integration of GOQDs into the glutaraldehyde cross-linked PVA matrix

Acknowledgements

The authors wish to sincerely thank the Ministry of Science and Technology of Taiwan (MOST 107-2218-E-033-001 and MOST 106-2221-E-033-062-MY3) for financially supporting this work.

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