Double-Crosslinked GO Interlayer Framework as a Pervaporation Hybrid Membrane with High Performance.

Graphene oxide (GO), as a two-dimensional structure material, has attracted widespread attention in the field of molecule sieving. However, GO-based membranes usually exhibit undesirable separation performance because the microstructure of GO is difficult to adjust. Herein, a novel double-crosslinking strategy for tuning the interlayer spacing of GO is reported. The hybrid membrane fabricated by the double-crosslinking strategy was used for pervaporation (PV) dehydration of isopropanol. To achieve high-performance of the PV hybrid membranes, the effects of operating cycles, chitosan concentration, and GO concentration were systematically investigated. The PV hybrid membranes were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, water contact angle measurement, and scanning electron microscopy. The results demonstrate that the interlayer of GO can be adjusted successfully by the double-crosslinking strategy. The fabricated hybrid membrane containing 0.1 wt % GO exhibited excellent performance with a flux of 4391 g/m2h and a separation factor of 1491, which indicated that the double-crosslinking strategy may extend the applications of GO in the field of membrane separation.

Introduction

Pervaporation possesses great potential to separate liquids, especially for azeotropic and close boiling point mixtures. Isopropanol (IPA), as a basic chemical raw material and solvent, is widely applied in the pharmaceutical and chemical industries.[1,2] To obtain high-purity IPA, many traditional technologies such as extractive distillation, molecular sieve absorption, and azeotrope distillation as well as pervaparation have been studied.[3] Among them, pervaporation gets a great deal of attention due to its advantages of easy operation, energy saving, high efficiency, and environment friendly.[4−7] The critical link for the full utilization of pervaporation is the development of pervaporation membranes.[8,9]

Graphene oxide (GO) is a two-dimensional material derived from graphite. Owing to its defined channel dimensions, excellent transport capabilities and outstanding molecular sieving properties,[10,11] GO has shown great potential for water desalination,[12,13] electrochemical energy storage,[14,15] gas separation,[16] and bioseparation[17] and biofouling control.[18]

To further extend its application in the field of membranes, great efforts have been focused on tuning the interlayer spacing of GO by physical approaches (e.g., intercalating with other nanomaterials[19] and introducing membrane porosity[20,21]) and chemical strategies (adding cations in the membrane matrix[22,23]). For instance, improved water permeation was achieved for composite GO-framework membranes by the incorporation of GO in the poly(vinyl alcohol) solution.[24] Templating the GO framework with copper hydroxide nanostrands and their subsequent removal by ethylenediaminetetraacetic acid resulted in an enhanced porous structure that led to a 10-fold enhancement in flux.[25] For chemical modification of GO, researchers often take advantages of the abundant active oxygen-containing functional groups[26−28] on GO to adjust the interlayer spacing. Chen et al.[11] demonstrated that intercalation of cations in GO can effectively control the interlayer spacing and thus efficiently improve the performance of GO membranes. Based on these recent achievements, the application of GO materials possesses great potential in the membrane field.[29]

In this study, a novel double-crosslinking strategy was reported for tuning the interlayer spacing of GO to further improve the pervaporation performance of a GO-based membrane. First, chitosan (CS) was used as a binder of GO nanosheets owing to the presence of hydrogen bonds and electrostatic interactions between them. Then, a secondary crosslinking between CS and GO was conducted using trimesoyl chloride (TMC) as a crosslinking agent. The GO membranes fabricated by the double-crosslinking strategy were characterized by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses; besides, its physiochemical properties were compared with its single-crosslinked counterpart. The results of FTIR and XPS indicated the increase of hydrogen bonds and presence of electrostatic interactions in the membrane matrix. The fabricated hybrid membranes exhibited excellent separation performance (pervaporation separation index (PSI) value of 8.5 × 106) for isopropanol (IPA) dehydration, which underpinned its great potential for the novel double-crosslinking approach.

Results and Discussion

Characterization of GO-Based Crosslinking Membranes

To investigate the chemical properties (e.g., crosslinking) of GO membranes, different membranes including the pure CS membrane, CS–GO single-crosslinked membrane, and CS–GO/TMC double-crosslinked membrane were fabricated on Al2O3 ceramic supports and characterized by FTIR as in Figure . The FTIR absorption peaks of several major groups are summarized in Table S1.[30] Compared with pure CS membranes, the peak positions of the CS–GO single-crosslinked membranes present red shift phenomena. Furthermore, the peak positions of C=O and N–H groups of double-crosslinked CS–GO/TMC membranes present a more obvious red shift phenomenon (Figure ). Moreover, the area ratios of C=O and N–H groups on crosslinked membranes are less than 1 (Table S1). These observations can be explained by the electrostatic interaction between the carboxyl (−COOH) on the GO and the amidogen (−NH2) on the CS (see Figure a),[31] successfully crosslinking between the acyl chloride groups (O=C–Cl) and −NH2 on the CS[32,33] (Figure S1). In addition, the absorption band of −OH broadens, indicating an increase of the number of hydrogen bonds in the membrane matrix.[34,35]

Figure 1

FTIR spectra of CS, CS–GO, and CS–GO/TMC membranes. The CS, GO, and TMC concentrations of the crosslinking membranes were 1.0, 0.1, and 0.5 wt %, respectively.

Figure 3

(a) Single-crosslinking mechanism of CS–GO and (b) double-crosslinking mechanism of CS–GO/TMC.

Figure and Table present the chemical compositions[36,37] and chemical bond content of various membranes, respectively, through XPS analysis. High-resolution XPS analysis of the C 1s peak shows that the peak area percentage of the N–C=O bond decreases from 17.1% for CS to 15.8% for CS–GO. Similarly, that of the C–N bond reduces from 20.7 to 16.7%. This result could be partially explained by the interaction between the amidogen (−NH2) on the CS with the carboxyl (−COOH) on the GO (see Figure a) in addition to the dilution effect by blending CS with GO. Furthermore, the content of C–O/C–OH bonds increases from 18.8 to 24.9% and the carbon atom content increases from 35.8 to 54.9% because of the successful incorporation of GO in the CS solution. The XPS wide-scan in Figure b shows 4.0% of the Al 2p signal that originated from the Al2O3 ceramic support. This indicates that the CS–GO membrane has significant defects.

Figure 2

XPS wide-scan and C 1s core level of (a) CS, (b) CS–GO, and (c) CS–GO/TMC membranes. The CS, GO, and TMC concentrations of the hybrid membranes were 1.0, 0.1, and 0.5 wt %, respectively.

Table 1

Elemental and Chemical Bond Composition of Pure CS, CS–GO, and CS–GO/TMC Membranes Analyzed by XPSa

	atoms percent (%)				chemical bond percent (%)
item	C 1s	N 1s	O 1s	Al 2p	N–C=O	C–O/C–OH	C–N	C–C/C-H
binding energy (eV)	285.2	399.4	532.5	74.1	287.7	286.5	285.9	284.6
CS	35.8	4.5	43.3	16.3	17.1	18.8	20.7	43.4
CS–GO	54.9	5.8	35.3	4.0	15.8	24.9	16.7	42.6
CS–GO/TMC	66.1	6.0	27.9		17.6	5.0	29.4	48.0

The CS and TMC concentrations of the hybrid membranes were 1.0 and 0.5 wt %, respectively

The XPS spectra of the CS–GO/TMC membranes are shown in Figure c. The content of C–O/C–OH bonds dramatically reduces from 24.9 to 5.0% (Table ), which is consistent with the chemical crosslinking of CS–GO by TMC (e.g., the reaction between hydroxyls (−OH) on CS–GO and acryl chloride groups (O=C–Cl) on TMC as shown in Figure b). In addition, the Al 2p signal can no longer be detected for the CS–GO/TMC membrane, indicating the formation of an intact separation layer on the ceramic support.

Based on the results of XPS analysis, the CS–GO single-crosslinking membrane has some drawbacks due to the existence of the Al 2p signal. Therefore, the water contact angles of the double-crosslinking membranes were measured to highlight the effect of GO on membrane hydrophilicity as compared with that of the CS/TMC single-crosslinking membrane. The water contact angles of CS–GO/TMC membranes are illustrated in Figure , where Y represents GO concentration. The water contact angle at any given time followed the order of: CS–GO0.2/TMC< CS–GO0.1/TMC < CS–GO0.3/TMC < CS/TMC. This demonstrates that GO plays a dual role in the hydrophilicity of the hybrid membranes. The water contact angle decreases with increased GO concentration when GO concentration is less than 0.2 wt % on account of the water channels provided by GO containing many hydrophilic functional groups. However, the contact angle increases when GO concentration is greater than 0.2 wt %, which is detrimental to the hydrophilicity of hybrid membranes. A possible reason for this phenomenon is that enhanced interactions[24] between CS molecules and GO sheets hamper the permeation of water.

Figure 4

Water contact angles of CS/TMC, CS–GO0.1/TMC, CS–GO0.2/TMC, and CS–GO0.3/TMC membranes. The symbol (*) in the figure indicates perfect wetting (contact angle ≈ 0°). The CS and TMC concentrations were 1.0 and 0.5 wt %, respectively.

The effect of GO concentration (Y) on the cross-section and outside surface morphologies of the CS–GO/TMC hybrid membranes is demonstrated in Figure . The particle-like texture of the Al2O3 substrate is clearly presented in the scanning electron microscopy (SEM) images when no or 0.1 wt % GO concentration was used (Figure b,d), which was consistent with the formation of relatively thin rejection layers under these conditions (Figure a,c). As the GO concentration increases, the texture of the substrate disappears gradually (Figure f,h) and the laminate membrane structure of GO is clearly visible (Figure e,g). However, the rejection layers became thicker.

Figure 5

Cross-sectional and surface SEM images of (a, b) CS/TMC, (c, d) CS–GO0.1/TMC, (e, f) CS–GO0.2/TMC, and (g, h) CS–GO0.3/TMC membranes. The CS and TMC concentrations were 1.0 and 0.5 wt %, respectively. The number of operating cycles was 3.

The effect of CS concentration (X) on the morphologies of the CS–GO/TMC hybrid membranes is shown in Figure . The thickness of the rejection layer increases with the increase of CS concentration, which is similar to the effect of increasing GO concentration (Figure ). At a CS concentration of 3.0 wt %, the membrane surface appears to be smooth (Figure h), which can be explained by the increased viscosity of the CS–GO mixture at higher CS concentrations (see Figure S2).

Figure 6

Cross-sectional and surface SEM images of (a, b) CS0.5–GO/TMC, (c, d) CS1.0–GO/TMC, (e, f) CS2.0–GO/TMC, and (g, h) CS3.0–GO/TMC membranes. The GO and TMC concentrations were 0.2 and 0.5 wt %, respectively. The number of operating cycles was 3.

The morphologies of the cross-section and surface of CS–GO/TMC/N membranes are shown in Figure , where N represents the number of operating cycles of the double-crosslinking membrane prepared on ceramic support. At a low number of operating cycles (e.g., 1), there are obvious defects on the surface of the CS–GO/TMC/1 hybrid membrane: the rough surface of the ceramic support cannot even be entirely covered (Figure a,b). Increasing the number of operating cycles (e.g., 2–4) results in the formation of an intact rejection layer (Figure c–h). In addition, no apparent delamination between the rejection layer and the support can be observed owing to the presence of hydrogen bonds between the surface active layer and ceramic support.

Figure 7

Cross-sectional and surface SEM images of (a, b) CS–GO/TMC/1, (c, d) CS–GO/TMC/2, (e, f) CS–GO/TMC/3, and (g, h) CS–GO/TMC/4 membranes. The CS, GO, and TMC concentrations were 1.0, 0.2, and 0.5 wt %, respectively.

Pervaporation Dehydration Performance of Hybrid Membranes

The pure CS and single-crosslinked CS–GO membranes did not exhibit a separation effect for the water–IPA mixture when the concentration of CS is low. Therefore, the double-crosslinked GO-framework membranes were used to separate the water–IPA mixtures via PV at 60 °C. Various factors for the separation performance of the membranes were investigated, such as the GO concentration, CS concentration, and number of operation cycles.

The PV performance of the hybrid membranes with different GO concentrations is shown in Figure . The separation factor increases when the GO concentration increases from 0 to 0.3 wt %, while the flux first increases and then decreases. A possible explanation is that GO plays a dominant role in providing passages for water penetration at first. However, the function of GO is weakened and the transmembrane resistance plays a great role with the increase of membrane thickness (Figure ). Besides the increased membrane thickness, another main reason for increased separation factor is the formation of a more laminate structure (Figure g) of GO with increased GO concentration. Such a laminate structure is preferred for the transport of water molecules.[38]

Figure 8

PV performance of CS/TMC, CS–GO0.1/TMC, CS–GO0.2/TMC, and CS–GO0.3/TMC hybrid membranes. The CS and TMC concentrations in the hybrid membranes were 1.0 and 0.5 wt %, respectively. The number of operating cycles was 3.

From Figure , the flux of the hybrid membranes decreases gradually while the separation factor increases with the increase of CS concentration, which demonstrates that CS concentration plays an important role in the pervaporation performance of the hybrid membranes. The higher the CS concentration is, the thicker the membrane becomes, resulting in the increase of transmembrane resistance of the hybrid membranes.[28] However, increased membrane thickness is of benefit to form an intact separation layer, leading to an increased separation factor.

Figure 9

PV performance of CS0.5–GO/TMC, CS1.0–GO/TMC, CS2.0–GO/TMC, and CS3.0–GO/TMC hybrid membranes. The GO and TMC concentrations in the hybrid membranes were 0.2 and 0.5 wt %, respectively. The number of operating cycles was 3.

The effect of the operating cycles on the PV performance of the hybrid membranes is shown in Figure . The flux is 18.0 kg/m2h with a separation factor of merely 5.5 when the number of operating cycles is only once, which echoes the presence of some surface defects on the hybrid membrane (Figure b). With the increase of the number of operating cycles, the membrane flux decreases slightly while the separation factor increases significantly. An intact separation layer is required to ensure the excellent separation performance of the hybrid membranes for IPA–water separation. As the membrane thickness increased, an intact separation layer was generated on the surface, guaranteeing the gradual increase of the separation factor, the effect of which was similar to that of the increase in CS concentration.

Figure 10

PV performance of CS–GO/TMC/1, CS–GO/TMC/2, CS–GO/TMC/3, and CS–GO/TMC/4 hybrid membranes. The CS, GO, and TMC concentrations in the hybrid membranes were 1.0, 0.2, and 0.5 wt %, respectively.

The long-term stability of the GO-based membrane was investigated by separating 90 wt % IPA aqueous solution at 60 °C. From Figure , both the membrane flux and separation factor remain nearly unchanged during the operation for 100 h, which illustrates that the hybrid membranes fabricated by the double-crosslinking strategy exhibit good durability.

Figure 11

Long-term operation stability of CS1.0–GO0.2/TMC/3 hybrid membranes for separating 90 wt % IPA aqueous solution at 60 °C.

PV Performance Comparison

The PV performances of the hybrid membranes in this work are compared with those in the literature (Table ). It is obvious that the double-crosslinking membranes in this study exhibit excellent separation performances and great potential in IPA dehydration.

Table 2

PV Performance Comparison with the Existing PV Hybrid Membranes in the Literature

item	mass % of IPA in feed	operation temperature (°C)	flux (g/m2h)	separation factor	PSI (g/m2h)	reference
CS1.0–GO0.1/TMC/3	90	60	4391	1491	6.5 × 106	this work
CS1.0–GO0.2/ TMC/3	90	60	2835	2991	8.5 × 106	this work
CS/PTFE	70	70	1730	775	1.3 × 106	(39)
GO/mPAN	70	70	4137	1164	4.8 × 106	(40)
GO/mPAN	70	30	2047	2331	4.8 × 106	(40)
SA/CS wrapped MWCNTs	90	30	218	6420	1.4 × 106	(41)
CS/TGDMP	90	30	73.7	1050	7.7 × 104	(42)
SiO2/PEC	90	70	2100	2186	4.6 × 106	(43)
Nexar block copolymer/Ultem	85	50	2440	221	5.4 × 105	(44)

Conclusions

A series of GO-framework membranes were successfully fabricated by the double-crosslinking strategy for PV dehydration of IPA. FTIR, XPS, SEM, water contact angle and PV tests were used to investigate the physicochemical properties, morphologies, hydrophilicity, and separation performance of the obtained membranes. The FTIR and XPS results reveal the formation of hydrogen bonds between GO and CS and the successful crosslinking between CS–GO and TMC. The GO in the membrane matrix provides selective water passages, which results in an excellent membrane flux. The successful crosslinking between CS–GO and TMC ensures excellent separation performance for IPA dehydration. Therefore, the fabricated hybrid membranes provide a useful reference to develop novel PV membranes and great potential for the dehydration of alcohol.

Experimental Section

Materials

Chitosan (CS, 50–100 mPa at 20 °C, the deacetylation degree > 80%) was obtained from Titan Scientific Co., Ltd. Shanghai, China. Acetic acid (HAc, AR, ≥99.5 wt %) and isopropanol (IPA, AR, ≥99.7 wt %) were purchased from Sinopharm Chemical Reagent Co., Ltd. Shanghai, China. Trimesoyl chloride (TMC, ≥99.8 wt %) was received from Qingdao Benzo Chemical Company, China. Ceramic supporting membranes with a mean pore diameter of 0.9 μm,[45] graphene oxide,[46] and deionized water were prepared in our laboratory.

Fabrication of the CS–GO Crosslinking Solution

First, 0.5–3.0 wt % CS solutions were prepared by dissolving certain mass of CS in 2.0 wt % HAc solution at room temperature. After the CS was completely dissolved, the prepared GO solution was added into the CS solution three times. During this process, the CS–GO mixed solution was stirred and treated by ultrasound to ensure the fabrication of the single-crosslinked GO framework.

Fabrication of CS–GO/TMC Hybrid Membranes

The ceramic hollow fiber membrane was selected as a support because of its high selectivity, permeability, thermal stability, and mechanical stability.[47,48] A membrane testing module was fabricated first, and the preparation procedures could be obtained from previous papers.[49,50] The fabricated membrane testing modules were immersed in the CS–GO crosslinking solution for 4 min. The excess CS–GO solution on the outside surface of the ceramic support was blown away by a circular air knife which ensured the uniformity and unity of the mixed solution on the ceramic support. Then, the membrane testing modules were immersed in TMC organic solution for 4 min, and the second crosslinking reaction between CS–GO and TMC occurred during the immersion time. The fabricated hybrid membranes were placed in a 75 °C oven for 5 min to allow further reaction. The above process was remarked as a cycle and repeated several times for formation of dense nondefective membranes, which was shown in Figure . The prepared membranes are denoted CS–GO/TMC/N, where superscripts X and Y mean the mass concentration of CS and GO, respectively, and N means the number of operating cycles.

Figure 12

Schematic illustration of the preparation process of the CS–GO/TMC hybrid membrane.

Characterization

The surface chemical compositions of the crosslinking membranes were analyzed by Fourier transform infrared (FTIR, Nicolet-6700) spectroscopy and X-ray photoelectron spectroscopy (XPS, VG-Miclab II, U.K.). The hydrophilicity of hybrid membranes was determined by a contact angle meter (JC2000A, Shanghai Zhong Cheng Digital Equipment Co., Ltd, China) at room temperature. Membrane morphologies containing cross-sections with a magnification of 50 000 and surfaces with a magnification of 10 000 were observed by field-emission scanning electron microscopy (SEM, Nova NanoSEM 450).

PV Experiments

The PV performance of the hybrid membranes was investigated in dehydrating a 90 wt % IPA–H2O mixture at 60 °C. The PV experiment was conducted by a self-made apparatus.[49,51] The permeate side of the hybrid membranes was maintained at a pressure of −0.1 MPa throughout the experiment. The composition of the permeate solution was analyzed by gas chromatography (Techcomp GC7890T, China). The membrane flux (J), separation factor (α), and pervaporation separation index (PSI) were calculated by the following formulas.[52]where W is the total weight of permeate vapor (g), A is the effective membrane area (m2), and t is the permeation time (h), Yw/Ye is the mass percentages of water to ethanol in permeate, and Xw/Xe is that in feed.