Graphene oxide membranes with fixed interlayer distance via dual crosslinkers for efficient liquid molecular separations

Highlights

• GO membranes were prepared with dual crosslinkers for liquid molecular separations.

• The macromolecule crosslinker ensured the mechanical stability of GO membranes.

• This crosslinker induced in situ formation of silica as the second crosslinker.

• The interlayer distance of GO membranes was fixated around 0.62 nm in water.

• GO membranes exhibited superior dehydration and desalination performance.

Abstract

The interlayer distance of stacked graphene oxide (GO) membranes is the decisive factor of achieving high permeability and selectivity. Fixating the size of 2D channel in GO membrane proves a critical challenge for liquid separation. Herein, GO membrane was firstly crosslinked by polyvinylamine (PVAm) to acquire mechanically stable membrane, and PVAm then induced the biomimetic mineralization to in situ generate silica as the second crosslinker to further fixate the size of 2D channels. The interlayer distance of GO-PVAm-Silica membrane fixed around 0.62 nm in water, showing the improved swelling resistance compared to the GO-PVAm membrane. The resultant GO-PVAm-Silica membrane featured high water permeability as well as distinct sieving properties for bio-alcohol molecules and hydrated ions. To be specific, the membrane exhibited a superior pervaporative dehydration performance with separation factor of 1188 ± 39 and flux of 12.9 ± 0.3 kg/(m²h) under 80 °C for 90 wt% butanol/water mixture. And the membrane showed a high NaCl rejection rate of higher than 99.99% and total permeation flux of 80.2 ± 0.8 kg/(m²h) when treating 3.5 wt% NaCl solution under 70 °C. Moreover, the GO-PVAm-Silica membrane kept excellent operation stability in 168 h test.

Introduction

Stacked graphene oxide (GO) membranes with well-ordered structure have become the novel membranes over the last decade for water purification [[1], [2], [3]], solvent dehydration [[4], [5], [6], [7]], and gas separation [[8], [9], [10], [11]] and so on owing to nearly frictionless surface, and high tensile strength [[12], [13], [14], [15]], which endow the GO membranes a weaken transport resistance and enhanced permeation flux. The proper interlayer distance will provide promising molecular sieving capability for precise separation of small molecules. However, the interlayer distance is highly sensitive to the working environments in the separation of liquid mixture due to the weak H-bonding and π–π interaction [1,4,[16], [17], [18]]. In the separation process, penetrant molecules will intercalate into neighbor GO nanosheets and enlarge the interlayer distance due to the strong inter-molecules interaction in liquid mixture, especially for aqueous mixtures. For instance, the interlayer distance of GO membranes increases from 0.7 nm of dried state to 1.3 nm in water, which is large than many small molecules and dramatically deteriorate the separation capacity [16,19]. Therefore, it is necessary to enhance the interlayer interaction and fix the interlayer distance of GO membranes for their practical applications.

The stability of GO membranes can be achieved by covalent cross-linking of adjacent GO nanosheets, which is resulted from the introduction of molecules containing multiple sites that can react with GO nanosheets. GO nanosheets with plenty of oxygen-containing functional groups [15] provide a rich opportunity to be periodically and covalently crosslinked to obtain well-defined interlayer distance and strong chemical bonding [4]. The interlayer distance can be tuned by the addition of crosslinkers that exhibit various sizes and binding sits. The ever-reported crosslinkers can be broadly divided into three categories: (1) Small molecules, such as thiourea [16],diamine [4,20,21],cation [22] and so on. Such crosslinkers can fixate the interlayer distance efficiently because of their small size [4,10]. However, the excess amount of crosslinkers were used in the fabrication process to compensate for their low crosslinking efficiency, which would occupy the transport channels and suppress the transport properties. Sun et al obtained thiourea crosslinked GO nanosheets with the interlayer spacing of 0.47 nm in water and 0.39 nm in dry state. The excess dosage of thiourea resulted in low permeation flux [10]. (2) Macromolecules, such as polymer [[23], [24], [25], [26]], molecules with long-chains [2]. Such crosslinkers have higher efficiency to bridge GO nanosheets since their configuration can change as needed because of their flexible feature. In addition, most macromolecules have multiple crosslinking sites, which further enhance the efficiency of crosslinking. However, such flexible nature also endows the membranes broader size distribution of interlayer distance, leading to a low selectivity. (3) Nanoparticles, such as TiO₂ [27], MOF [7], COF [28], Mxene [[29], [30], [31]] and so on. Such crosslinkers have predominance on fixate interlayer distance and are less vulnerable to solvent environment due to their rigid nature. However, incorporating nanoparticles between GO nanosheets without aggregation and tuning the interlayer distance to subnanometer remain a grand challenge. To date, the introduction of nanoparticles into the interlayers of the GO-based membrane, whether by blending or in situ synthesis, usually destroys the 2D transport channels of the membranes and results in a loss in the sieving properties. It can be concluded that it is difficult to obtain GO membranes with high performance by a single crosslinker. It is envisaged that if different kinds of crosslinkers are jointly utilized, GO membranes with high permeability and selectivity would be achieved due to the synergy of high crosslinking efficiency and fixed interlayer distance.

Biomineralization is an effective and mild method to synthesize mineral materials with controlled size, shape and organization by living organisms in aqueous environment [32]. Polymer mineralizer, including natural biomolecules (such as silaffins [33], gelatine [34], lysozyme [35], chitosan [36]) and synthetic polymers (such as polypeptide [37,38], polyamine [39,40]), have been proved to serve as catalyst, template and scaffold in the mineralization process to convert inorganic precursor into nanoparticles [41]. Highly dispersed mineral nanoparticles can be synthesized by the biomimetic mineralization in nano-sized confined space [42]. Therefore, polymer mineralizer is an ideal crosslinker to achieve the double-crosslinked GO membranes, which serves as macromolecules crosslinker with high crosslinking efficiency and promotes the formation of nanoparticles in the confined space between GO nanosheets.

Herein, a novel kind of GO membrane with fixed interlayer distance was prepared via dual crosslinkers for liquid molecular separation. Polyvinylamine (PVAm) was selected as the first crosslinker to bridge GO nanosheets to ensure mechanical stability, then served as the mineralizer to synthesize silica in the interlayer confined space as the second crosslinker to fixate the interlayer distance [39]. In other words, a sub-micrometer-thick and flawless GO stacked membrane containing PVAm was fabricated via pressure-assisted filtration [9] and then soaked in the sodium silicate solution for biomineralization. The properties and structure of the fabricated membranes were analyzed. The separation performances for butanol dehydration as well as NaCl solution desalination were tested. Effects of operation conditions including feed composition and temperature on the n-butanol dehydration performance were evaluated. The long-term operation stability of the GO-PVAm-Silica membrane was also assessed.