1. Introduction
The environmental damage caused by modern-era industrialization has become increasingly evident in recent years. Fossil fuel consumption is a pressing issue because of its finite resource and the pollution of the ecosystem resulting from its combustion. These detrimental effects have pushed the urge to explore sustainable alternatives, mainly biofuel. This eco-friendly alternative can be extracted from the fermentation of typical crops, such as corn, to be converted into bioethanol[ 1]. Moreover, desalination is being recognized as a solution to the lack of access to fresh water. The demand for safe freshwater has increased significantly due to increasing population, industrial use, water contamination, and urban development.
Separation processes are currently deployed to tackle these environmental challenges but with some limitations. Reverse-osmosis (RO) has been widely used for desalination but is limited by high energy requirements and low freshwater yield (35~55%)[2]. Additionally, the feed solution must be treated beforehand to be compatible with the membrane, and highly concentrated brine is generated as a by-product[3]. Traditional distillation methods used for alcohol dehydration, which is integral for biofuel purification, also demand high energy consumption due to high operating temperatures and utilize environmentally toxic components called “entrainer” to separate water and ethanol during the process[4].
Pervaporation has recently been recognized as an eco-friendly membrane-based separation technique due to its performance efficiency, low energy costs, and low environmental impact[5]. It is widely used in applications such as water purification and desalination, biofuel production, alcohol dehydration, and removal of organic solvents. This method is deemed a promising alternative to these separation processes already in practice[4]. It operates at significantly lower temperatures than the traditional distillation methods and isn’t limited by osmotic pressure like RO, which leads to lower costs[1,6]. The critical component of pervaporation is the membrane itself[7]. However, conventional membrane materials such as polyvinyl alcohol (PVA), chitosan (CS), and sodium alginate are prone to poor mechanical properties and thermal stability, especially in prolonged operations.
The incorporation of 2D nanosheets has been shown to not only alleviate these issues but also increase the permeability and selectivity of the polymeric membranes due to the generation of interlayer spacing that results in a continuous ordered transport during permeation[ 5]. Although there are many other suitable candidates, this study will mainly focus on the recent developments made with graphene oxide and MXene 2D composite membranes applied for pervaporation in desalination, biofuel production, and alcohol dehydration.
2. GO Based Composite Membrane
Block copolymer (BCP) of polydimethylsiloxane (PDMS) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was synthesized and a asymmetric membrane was fabricated to apply in dehydration of ethanol[8] (Fig. 1). BCP was composite with 0.7 wt% of GO showed a permeation flux of 80~90 g/ m2 h, high selectivity of water of about around 77 to 99% with pervaporation separation index is 5.5 to 2.9.
Gupta et al. prepared poly(vinyl alcohol) based composite with graphene oxide and carboxyl terminated carbon nanotube and applied for pervaporation of ethanol by dehydration[9]. Mechanism of pervaporation and formation of composite membrane in presented in Fig. 2.
Graphene oxide is decorated with lignin and composite with sodium alginate to fabricate a membrane[10]. The composite membrane has excellent flux of 2500 g/m2 h. Fig. 3 and 4 represent the SEM image of the composite membrane.
Cha-umpong evaluates the potential application for graphene oxide composite pervaporation membrane using commercially available brine resources[11]. The process was able to enrich lithium containing brine until lithium hydroxide (LiOH) was retrievable through chemical precipitation and rapid evaporation. However, because LiOH is mixed with NaCl, further studies considering the economic cost and purification are needed to integrate different technologies for extracting lithium from sodium-containing solutions. Polypropylene (PP)/GO composite pervaporation membrane has initial water flux of 11 L/m2 h at 70°C whereas 0.5 L/m2 h in case of solar evaporation pond.
Lin et al. synthesized and characterized graphene oxide/ chitosan (GO/CS) composite membranes while also assessing their performance in esterification and pre-esterification, which are important steps for biodiesel production[12]. There are key factors such as temperature, wt% of embedded GO, and initial ratio of alcohol to acid that were crucial in these reactions because they all contribute to the process of pervaporation and esterification/pre-esterification. In the PV-assisted preesterification process, the appropriate use of Amberlyst- 15 catalyst was deemed indispensable for the production of enhanced catalytic membrane. Also, the ratio of water removal through the membrane to water production from esterification was considered as an important characteristic to evaluate the membrane performance. The catalytic membrane increased the rate of PV-assisted esterification and pre-esterification by 8% and 20% under the right circumstances such as temperature, wt% of Amberlyst-15 catalyst, and alcohol to acid ratio.
Imidazole-quartet water channels (IQWC), made by using imidazole-ureido (IU) molecular scaffolds, is a promising solution for constructing membranes with desired water transport properties[13]. Although IQWC fabricated with imidazole-ureido (IU) have been regarded as a contender for synthesizing composite membranes, no further studies have been made due to the lack of suitable approaches. 2D graphene oxide nanosheets can offer a stable platform for incorporating artificial water channels into their structures due to their easy assembly into bilayer structures and rich functional groups. In this study, a novel IU-GO membrane was synthesized by embedding IU-molecular scaffold (bola-amphiphile imidazole) into a layered GO laminate structure. With the construction of IQWC, the IU-GO membrane showed higher transport selectivity towards water than butanol. In an 80 wt% butanol- water solution at 343K, the IU-GO membrane that has been optimized displayed total flux of 3506 g/m2⋅ h and separation factor of 4454. When the water concentration of the feed and temperature increased, the IU-GO membrane demonstrated greater flux and separation factor simultaneously as well as remaining stable during prolonged operation, displaying great prospect for solvent dehydration. The molecular transport behavior in the IU-GO membrane investigated in this study could provide better guidance for GO-based membranes to be used in organic separation.
Mixed matrix membranes (MMMs) was prepared with varying degrees of GO content for desalination through pervaporation[14]. Increasing the GO content caused an increase in surface roughness and hydrophilicity in the membrane. Because of the molecular interaction between GO and CS, there seems to be a good compatibility and no reports of any interfacial defects in the densely structured membrane. It’s also noted that proper amount of GO improved the tensile strength and elastic modulus of the membrane. With increased GO content, the water of flux increased initially and decreased due to the heightened hydrophilicity and reduction in free volume in the membrane. Compared to the pure CS-based membranes, the MMMs had a consistent and lower apparent activation energy despite the varying degrees of salinity in the feed solution. At 1% GO loading, the membrane showed the maximum value of water flux at a rate of 99.99% salt rejection regardless of the temperature and concentration of NaCl in the feed solution. Although the MMMs have high water permeation and water/salt selectivity, there is trade-off between the two, which complies with the free volume theory. Nonetheless, the GO/CS MMMs displayed satisfying results and is a promising candidate for PV desalination process.
This study investigates the integration of humic acid-like (HAL) biopolymer with graphene oxide membrane to fabricate GO-HAL composite membranes for alcohol dehydration[15]. GO membrane demonstrates great water permeation and selectivity. The addition of HAL induces disorder to the membrane’s structure, increasing the water permeation rate. The results show that the GO-HAL membrane with a biopolymer loading of 30% displays a desired water/ethanol selectivity of 45 and a water permeance 33% higher compared to pristine graphene oxide membranes. The interlayer distance of the GO-HAL 30% membrane is also 7~8% larger than membranes with lower HAL loading after thermal stabilization at 120°C. The results of this study conflict with previous studies that present the fact that ordered and compact membrane structures display higher water permeability and selectivity. However, the disorder induced in GO-based membranes create a more open and permeable structure, demonstrated by performance of the GO-HAL composite membrane. Overall, this study affirms that structural disorder is a crucial factor for GO-based membranes and order/disorder engineering at varied scales can be beneficial for designing future GO-based membranes.
Reduce graphene oxide (rGO) shows structural defects during the reduction process, resulting in the formation of undesirable pores that reduce the separation efficiency of membranes[16]. To alleviate this issue, graphene quantum dots (GQD) has been seen as a potential solution for these defects in reduced rGO. In this study, a nanocomposition of rGO and GQD has been integrated with alginate solution to create a dense composite membrane for separation through pervaporation. Initially, the membranes were used to separate the i-PrOH/water mixture. However, the separation process was so remarkable that the difference of sealing the structural defects could not be reported. Therefore, separation of lower molecular weight alcohols such as ethanol and methanol has been performed. This separation has exhibited far better results since the alcohol molecules could not pass through the defects of rGO. The Alg-rGO mixed with GQD-3 membrane has shown the best performance separating methanol/water mixture with a permeation flux of 2323 ± 45 g/m2 h and water concentration in permeate of 92.7± 0.03% at 70°C. The positron annihilation lifetime spectroscopy (PALS) has confirmed that the reduction of structural defects improved the water selectivity of the membrane. Additionally, the performance of the Alg-rGO mixed with GQD-3 membrane has shown stability even after 240 h of testing.
Sulfobutyl-β-cyclodextrin (SCD) intercalated into the interlayers of GO membranes. Two dimensional (2D) lamellar GO membranes show a promising application in alcohol dehydration[17]. However, the low water flux and poor mechanical stability in aqueous environment are seen as challenges that GO membranes face. In this study, Liang et al. have utilized SCD, as a supramolecular crosslinker, to alter the interlayer properties of GO membranes via non-covalent interactions. Because of the presence of sulfonic acid groups, the incorporation of SCD into the interlayers of GO membranes results in GO-SCD membranes with great water sorption and high hydrophilicity. The GO-SCD/PTFE membranes displayed excellent separation in n-butanol dehydration: separation factor of 1299 and permeation flux of 12955 g/m2 h at 80°C. Furthermore, the non-covalent cross-linking between adjacent GO nanosheets through SCD molecules improves thermal stability as well as prolonged operations of the GO-based membranes in a solvated state. The utilization of supramolecules with specific configurations for modifying the interlayer properties of GO membranes could inspire further research in developing high performing separation membranes.
3. MXene Based Composite Membrane
PVA based Ti3C2Tx mixed matrix membrane is reported by Tong et al. Fig. 5 represents the FESEM image of the 2D material[18].
Performance of the composite membrane is reported in Fig. 6. Water flux of the composite membrane is 1.21 kg/ m2. h while separation factor is 1126.8. The excellent performance of the membrane is due to the porous channel generated along the path of 2D MXene.
Heydari et al. prepared mixed matrix membrane of UV-curable acrylate polyvinyl alcohol and Ti3C2Tx[19]. 3% loading of 2D materials showed best performance due to optimized presence of interlayer transport layer with permeation flux of 942 g/ m2 h and separation factor of 294.
Xu et al. investigated the utilization of synthesized two-dimensional MXene nano sheets to improve the performance of chitosan (CS) membranes for solvent dehydration through pervaporation[20]. MXene nanosheets were synthesized and integrated into chitosan to fabricate novel mixed membranes. The performance in dehydration of the membranes was evaluated using three different organic solvents: ethanol, ethyl acetate, and dimethyl carbonate. The MXene nanosheets did not improve the surface sorption. However, the assembled MXene laminates with interlayer channels increased the water permeability of the membrane, improving both the flux and separation factor of the membrane for solvent dehydration simultaneously. The 3 wt% MXene/CS MMM displayed a total flux of ~ 1.4~1.5 kg/m2 h) and a separation factor of 1421, 4898, and 906 for ethanol, ethyl acetate, and dimethyl carbonate dehydration at 50°C respectively. The MXene/CS MMM was found to demonstrate a great performance compared to the state-of-the-art membranes, displaying good prospect for MXene-based membranes in solvent dehydration. The synthesized MXene nanosheets with interlayer channels have desired water transport properties, making them a promising candidate for use in future mixed-matrix membrane development.
Yang et al. investigates the development of a high- performance nanofilm composite membrane for molecular separation[21]. Hydrophobic polytetrafluoroethylene porous substrate has been utilized to fabricate an ultrathin organic-inorganic hybrid nanofilm through a scalable solution casting process. Ti3C2Tx and sulfosuccinic acid have been utilized as nanofiller and crosslinker to enhance the membrane transport selectivity, property, and stability. The membrane had a thickness of 230 nm, in a similar range to small-sized MXene (142 nm), and demonstrated remarkable separation of water from a wide array of aqueous ion and alcohol solutions. As the nanofilm thickness neared the filler size, the MXene nanosheets caused ultralow-resistance permeation behavior. The improved water permeation of the nanofilm composite membrane did not decrease its selectivity after a miniscule amount of 2D Mxene was incorporated in the PVA matrix. The advantages of PSM/PTFE composite membrane have been also highlighted in the literature, including its excellent stable separation performance through a wide range of aqueous mixtures, and quick and easy solution casting process for upscale fabrication. Moreover, the design of nanohybrid membranes could be appropriate to other dimensional fillers. Overall, this study presents a promising approach for developing molecular separation using high-performance nanofilm composite membranes.
Cai et al. investigates the performance of poly-vinyl alcohol (PVA) Ti3C2Tx mixed matrix membrane in dehydration of water/ethanol solutions via pervaporation[ 22]. The PVA/ Ti3C2Tx MMMs demonstrated an outstanding increase of performance since the Ti3C2Tx enhanced the membrane’s cross-linking density. Because Ti3C2Tx was evenly distributed throughout the PVA matrix, the membrane exhibited great mechanical strength, separation factor, resistance to swelling, and structural compatibility. However, because the hydrophilicity was decreased due to Ti3C2Tx, the total flux has also decreased. Nevertheless, when separating 93 wt% ethanol solution at 37°C, the MMM loaded at 3 wt% Ti3C2Tx exhibited an optimal performance with a separation factor of 2585 and total flux of 0.074 kg/m2, which was 17 times higher compared to pure PVA membrane. Also, when the ethanol concentration decreased and the feed temperature increased, the MMM’s separation factor shrunk while the total flux escalated, which is commonly observed in polymer membranes. Overall, the study systematically investigated the PVA/Ti3C2Tx MMM’s performance in dehydration with factors such as feed concentration, operating temperature, and Ti3C2Tx filling level. In conclusion, the novel PVA/ Ti3C2Tx MMM shows a promising future for separation via membrane technology and a guideline for further utilization of Ti3C2Tx in pervaporation.
Wu et al. evaluates the performance of 2D MXene/ Ti3C2Tx membrane in ethanol dehydration via pervaporation. A 2 μm thick MXene memebranes were stacked between Ti3C2Tx nansheets to synthesize this composite membrane[23]. MXene has several advantages in alcohol dehydration since they have copious oxygen functional groups in contact with the MXene nanosheets, tunable interlayer channels, and good hydrophilicity. The performance has been evaluated by studying the impact of feed concentration of ethanol and operating temperatures during pervaporation. The results showed that the composite MXene membrane displayed the best performance under room temperature with a high concentration of ethanol (95%) in the feed solution with separation factor of 135.2 and total flux of 263.4 g/m2 h. It is notable that the separation factor increases as alcohol concentration in the feed increases as well. Additionally, the MXene/ Ti3C2Tx membrane exhibited greater mechanical strength when soaked in feed solution and therefore attained a higher separation factor. Overall, the study concludes that further development is needed when compared to other membranes in pervaporation and more investigations will be held later.
4. Conclusions
In conclusion, pervaporation has emerged as a promising and environmentally sustainable solution for separating chemical species and dehydration of organic solvents. Pervaporation offers distinct advantages over conventional separation processes with low energy consumption, low environmental impact, and tunable selectivity. However, more than the current state of membrane technology is needed to fully meet the demands of practical applications, necessitating the exploration of novel materials. Composite membranes, which combine the benefits of organic and inorganic materials, have shown great potential in enhancing pervaporation performance. Embedding inorganic nanomaterials, such as graphene oxide and MXene, into polymeric membranes has successfully addressed existing limitations. These studies have demonstrated the effectiveness of 2D composite membranes in increasing mechanical properties, chemical and thermal stability, permeability, and selectivity, resulting in improved separation efficiency. Further research in composite membrane technology sets a path for developing more efficient and sustainable pervaporation processes. As we continue to explore and optimize the properties of composite membranes, it is evident that pervaporation holds great promise for applications in various industries, including water treatment, biofuel production, and solvent dehydration, contributing to a greener and more sustainable future.
