Layer-by-layer assembled membranes from graphene oxide and polyethyleneimine for ethanol and isopropanol dehydration

Highlights

• Crosslinked (PEI/GO)_z LbL membranes were designed and fabricated.

• More than threefold increase in separation factor for dehydration of EtOH and IPA.

• Membrane resistance for permeation of different alcohol/water mixtures was calculated.

• The membrane performance can be fine-tuned by controlling number of bilayers.

Abstract

Novel thin film composite membranes based on layer-by-layer (LbL) self-assembly of polyethyleneimine (PEI) and graphene oxide (GO) on chlorine-treated polyamide membranes were prepared for pervaporation dehydration of alcohols. To improve the membrane perfoarmance, the self-assembled bilayers were crosslinked with glutaraldehyde. A two-level factorial design was used to determine the effects of the main factors involved in the membrane preparation and their interactions on the permeation flux and separation factor. The impacts of the number of bilayers on the membrane performance were considered to gain an insight into the mass transfer resistance contributed by the bilayers. At 60 °C, the crosslinked (PEI/GO)₇ LbL membrane showed a flux of 1770 and 1494 g/m² h for ethanol/water and isopropanol/water mixtures, respectively, at 2 wt% water in feed, with a corresponding separation factor of 77 and 197. The long-term stability test showed the feasibility of the crosslinked membranes for alcohol dehydration.

Introduction

Pervaporation, a promising membrane-based technology, is applied in three main areas: 1) dehydration of organic solvents, 2) recovery of organic compounds from water, and 3) separation of organic-organic liquids (Shao and Huang, 2007, Ong et al., 2016, Feng and Huang, 1997). Among these, the dehydration of alcohols is the most-developed, especially when the feed has a low quantity of water or at an azeotropic point (<20 wt%) (Zhao et al., 2011, Nunes and Peinemann, 2001). Ethanol (EtOH) is the most common renewable biofuel resource (Vorayos et al., 2006), and isopropanol (IPA) is widely used in electronic and pharmaceutical industries where high purity IPA is needed (Qiao et al., 2005). Both EtOH and IPA form azeotropes with water at an alcohol concentration of around 96 and 88 wt%, respectively (Qiao et al., 2005, Chapman et al., 2008). Compared with such traditional separation processes as adsorption and distillation, pervaporation is generally a cost- and energy-effective approach to breaking azeotropes and removing a small amount of water (Zhao et al., 2011, Qiao et al., 2005). Moreover, pervaporation separation is based on the solution-diffusion mechanism and only the latent heat of evaporation of the permeate is needed for the separation. Therefore, pervaporation is not limited by thermodynamics of vapor-liquid equilibrium (VLE) and can be used to separate azeotropic or close-boiling mixtures (Ong et al., 2016). Integrating pervaporation with other conventional separation processes is shown to be effective to further improve the separation efficiency (Chapman et al., 2008).

Hydrophilic membranes permselective to water are the key component for dehydration of organic solvents (Semenova et al., 1997), and the layer-by-layer (LbL) assembly is one of the well-known methods to fabricate nanoscale anionic-cationic multilayers on a substrate through sequential depositions of oppositely charged polyelectrolyte solutions or nanoparticle suspensions (Decher, 1997, Tzeng et al., 2015). The LbL membranes have a significant affinity to water molecules, and the deposited nanolayers result in preferential water transport through the membrane (Zhao et al., 2011, Bolto et al., 2011). The main driving force for the layer-by-layer buildup is Coulombic (electrostatic) interactions between oppositely charged ions, though hydrogen bonding is also an important factor for the assembly (Semenova et al., 1997). This bottom-up approach was first introduced by Iler (1966), and it received great attention for nanoassembly after the work of Decher, 1997, Decher and Hong, 1991. In 1998, Tieke et al. prepared LbL membranes for pervaporative separation of EtOH/water (Krasemann and Tieke, 1998). Since then, a variety of LbL membranes have been prepared for various separations, including gas separation (Krasemann and Tieke, 1998, van Ackern et al., 1998), pervaporation (Krasemann and Tieke, 1998, van Ackern et al., 1998), reverse osmosis (RO) (Choi et al., 2013), forward osmosis (Hu and Mi, 2014), and nanofiltration (NF) (Wang et al., 2012, Wang et al., 2016).

However, the LbL membranes are prone to swelling when exposed to alcohol/water mixtures due to their hydrophilic nature. Both water and alcohols are polar molecules and can swell the polyelectrolyte layers. Therefore, the nanolayers formed by cation-anion interactions are not stable when excessive swelling and interchain stretching occur, resulting in a low selectivity (Semenova et al., 1997, Zhang et al., 2013). Kim et al. (2005) reported that polyelectrolyte multilayers performed differently for different organic/water mixtures, where the polymer coils can be softened or contracted in different organic solvents. Zhang et al. (2013) confirmed that EtOH and IPA can wash away some polyelectrolyte macromolecules deposited onto polyamide membranes, and Poptoshev et al. (2004) showed that polyelectrolyte multilayers may even disassemble in certain organic solvents, resulting in collapse of the membrane structure. In such cases, chemical crosslinking may be used to alleviate membrane swelling and to augment membrane stability and performance (Zhang et al., 2013, Mangindaan et al., 2014, Zhao et al., 2018). Membrane crosslinking restricts the polymer chain mobility and leads to stiff and compact membrane structures. Crosslinked membranes usually have a lower permeation flux and higher separation factor because of increased diffusion resistance and reduced sorption uptake after crosslinking (Bolto et al., 2011, Huang et al., 1999, Hyder et al., 2009). Aldehydes are often used as crosslinking agents as they are reactive with many functional groups in polymers commonly used for pervaporation membranes (Zhang et al., 2013, Huang et al., 1999, Qiu et al., 2011, Qi et al., 2012, Chen et al., 2015, Hua et al., 2017, Liu et al., 2011).

To prepare membranes via the LbL assembly, selecting appropriate substrates and polyelectrolytes are of primary importance. In this study, a thin film composite (TFC) polyamide membrane was chosen as a substrate because of its negative surface charge, which favors the deposition of polycations. Moreover, the number of polyelectrolyte depositions required to prepare a defect free permselective membrane was reduced due to its considerably dense skin layer (Choi et al., 2013, Zhang et al., 2013). Graphene oxide (GO) and polyethyleneimine (PEI) were used as the anionic and cationic depositing components, respectively. GO nanosheets have a unique carbon nanostructure with abundant hydrophilic functional groups, allowing fast passage of water. In addition, GO nanosheets function as polyanions when dispersed in water due to ionization of the hydroxyl and carboxyl groups. Consequently, GO nanosheets can interconnect with polycations (e.g., PEI) through electrostatic attraction and hydrogen bonding (Tzeng et al., 2015, Hu and Mi, 2014, Hu and Mi, 2013, Li et al., 2008, Huang et al., 2017, Wu et al., 2019, Ang et al., 2019, Liu et al., 2015). Therefore, GO may be used as an appropriate anionic nanomaterial to offer a “brick-and-mortar” LbL structure. On the other hand, PEI is a cationic amine polymer, and its hydrophilic nature makes it a popular choice for constructing water-permeating membranes. To improve the performance of the prepared LbL membranes, glutaraldehyde (GA) was used as a crosslinking agent. GA can react with the amine groups in PEI and hydroxy groups in GO, as illustrated in Fig. 1 (Zhang et al., 2013, Hua et al., 2017, Xia et al., 2011).

To our knowledge, there are no reports yet in the literature on the preparation of PEI/GO LbL membranes self-assembled on a TFC polyamide substrate and chemically crosslinked with GA for alcohol dehydration via pervaporation. The objective of this research was to fabricate PEI/GO LbL membranes for dehydration of EtOH and IPA. The effects of crosslinking conditions (e.g., temperature, time, and crosslinker concentration) on the membrane performance were investigated, and the effects of the interactions of these factors were evaluated through a two-level factorial design. The results were used to determine the largely optimal crosslinking conditions by considering the trade-off between permeation flux and separation factor. The effects of the number of bilayers on the membrane performance were also investigated to correlate the layer-by-layer growth and mass transfer resistance quantitatively. Water sorption uptake was also measured to look into the degree of swelling of the bilayers, and the surface hydrophilicity of the membrane was determined using contact angle measurements. The morphology of the membrane was examined using atomic force and electron microscopies. The long-term stability of the crosslinked membrane was also investigated.