Poly(vinyl alcohol)-Modified Membranes by Ti3C2T x for Ethanol Dehydration via Pervaporation.

In this paper, PVA/Ti3C2T x mixed matrix membranes (MMMs) were prepared by mixing the synthesized Ti3C2T x with the PVA matrix, and the pervaporation (PV) performance of the ethanol-water binary system was tested. The morphology, structural properties, and surface characteristics of the membranes were investigated by scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, degree of swelling, and water contact angle. The PVA/Ti3C2T x MMMs exhibit excellent compatibility and swelling resistance. Moreover the effects of the Ti3C2T x filling level, feed concentration, and operating temperature on the ethanol dehydration performance were systematically studied. The results demonstrated that the separation factor of PVA/Ti3C2T x MMMs was significantly increased because of Ti3C2T x promoting the cross-linking density of the membrane. Specifically, the membrane showed the best PV performance when Ti3C2T x loading was 3.0 wt %, achieving a separation factor of 2585 and a suitable total flux of 0.074 kg/m2 h for separating 93 wt % ethanol solution at 37 °C.

Introduction

Fuel <span class="Chemical">ethanolpan>,[1,2] as a renewable bioliquid fuel to reduce air pollution and greenhouse gases, has been promoted and used in many countries and regions around the world in recent years. With the severe env<span class="Chemical">ironmental pollution and the exhaustion of resources such as petroleum, fuel <span class="Chemical">ethanol has grown into one of the considerable alternative energy sources. One of the main production technologies of fuel ethanol is the biofermentation method,[3] which produces 6–10 wt % ethanol and then obtains 95 wt % industrial ethanol by distillation. To acquire absolute ethanol, particular methods must be adopted. Different from traditional separation methods, pervaporation (PV) technology[4] breaks the restriction of the vapor–liquid equilibrium and shows obvious advantages in the separation of liquid mixtures of azeotrope or close boiling compounds: energy conservation, environmental protection, less space, and easy for industrial scale-up.[5,6] The industrial application of the PV process has been widely recognized in organic solvent dehydration.[7,8] Many hydrophilic polymer membranes such as chitosan (CS),[9] poly(vinyl alcohol) (PVA),[10] sodium alginate (SA),[11] and so forth. have been investigated for the ethanol–water binary system. PVA possesses extraordinary physicochemical properties and can maintain its properties within long-term operation, which is suitable for providing basic membrane materials for organic solvent dehydration.[12] In 1982, the German GFT Company was the first to make breakthroughs in the industrial application of PV, introducing commercial PVA/poly(acrylonitrile) (PVA/PAN) composite membranes. However, a large number of OH groups in the PVA molecular chain result in strong hydrophilicity. Untreated PVA tends to swelling in aqueous solution, and the defects impair the separation performance of the PVA membrane.[13,14] The German GFT Company published the PV performance of the GFT commercial membrane: for 80 wt % ethanol aqueous mixture of the feed liquid, the separation factor was 100–200 and the total flux was 1.0 kg/m2 h at 80 °C. In addition, the separation factor of the GFT-1510/2510 commercial membrane (95 wt % ethanol concentration) was 258 and the corresponding total flux was 0.6 kg/m2 h.[15] Although the pristine PVA membrane has achieved industrial applications, its separation factor is still restricted. In order to combat this, many modification methods such as cross-linking,[16] grafting,[17] and blending[13] have been employed. Among them, doping two dimensional (2D) materials,[18] such as graphene,[19] graphene oxide (GO),[19,20] carbon nitride (C3N4),[21,22] metal–organic frameworks (MOFs),[23] and molybdenum disulfide (MoS2), has attracted the attention of researchers.[24] In the work of Wu et al.,[25] various UiO membranes were fabricated by altering the organic linkers and doped in the PVA matrix for separation of 90 wt % ethanol solution. The mixed matrix membranes (MMMs) displayed obvious anti “trade-off” effects: for the UiO-66-(OH)2/PVA-1.0 MMM, the total flux and separation factor were increased by 16 and 14%, respectively. Zhang and Wang[26] fabricated PVA/ZIF-8-NH2 membranes for ethanol dehydration. The MMMs exhibited enhanced PV performance, which was due to the increased hydrophilicity of ZIF-8-NH2 with the PVA matrix. The total flux and separation factor of PVA/ZIF-8-NH2 MMMs could reach to 0.13 kg/m2 h and 201, respectively, at 40 °C.

2D materials[27] with atomic thickness and micrometer lateral dimensions have been widely used to develop membranes with high separation performance. Moreover, they have mechanical properties, thermal stability, excellent layered structure, and 2D nanochannels, making the two-dimensional material separation membrane have extraordinary permeability.[28−30] As a new member of the 2D material family, <span class="Chemical">transition metal carbidespan> (<span class="Chemical">MXenes) were discovered by Gogotsi and Barsoum in 2011. <span class="Chemical">MXenes[31] have the formula MXT, where n is 1, 2, or 3, M is an early transition metal, X is C and/or N, and T is the OH and/or O group. Among them, the formula of MAX phases (the precursors of MXenes) is MAX, where A stands for an A-group element (such as Al and Si).[32] One of the most representative MAX materials is Ti3AlC2. Here, Al was extracted from Ti3AlC2[31,33] and a two-dimensional material called Ti3C2T Mxene was prepared. The hydrophilic, rigorous layered structures and extremely short water molecule transport channel of Ti3C2T have attracted the attention of researchers in the field of membrane separation.[34−37] Liu et al.[37] first prepared ultrathin Ti3C2T membranes for desalination by assembling the Ti3C2T nanomaterial. The selected Ti3C2T membrane showed high water flux and salt rejection. In addition, this Ti3C2T assembly membrane could maintain 100 h of PV desalination. Wu et al.[38] fabricated 2 μm-thick MXene membranes and applied it in ethanol dehydration. At room temperature and 5 wt % H2O concentration, the total flux of the MXene membrane was 263 g m–2 h–1 and the separation factor was 135. At present, the Ti3C2T-based membrane has achieved some research results in terms of water treatment and gas separation.[39,40] Moreover, research studies on dehydration of organic solvents are still in progress.

Herein, for the first time, 2D Ti3C2T was embedded in the al">PVA mpan class="Gene">atrix, and PVA/Ti3C2T MMMs was successfully prepared. Various membranes were labeled PVA/Ti3C2T-0.0, PVA/Ti3C2T-0.5, PVA/Ti3C2T-1.0, PVA/Ti3C2T-2.0, PVA/Ti3C2T-3.0, and PVA/Ti3C2T-4.0. The structure and physicochemical properties of PVA/Ti3C2T MMMs were analyzed by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), water contact angle (WCA), and degree of swelling (DS). In addition, the ethanol–water binary system was used to evaluate the PV performance of the PVA/Ti3C2T MMMs.

Results and Discussion

Morphology

The SEM morphology and EDS results of the Ti3C2T powder are shown in Figure . The lateral dimension of Ti3C2T was about 5 μm and showed a typical stacked layered structure like “accordion”, which enabled Ti3C2T advantageous for arranging 2D nanochannels in the al">PVA mpan class="Gene">atrix.[41] It can be seen from the results of EDS analysis that Ti3C2T mainly contained a large amount of Ti elements and C elements, and the presence of small quantities of Al elements may be because HF did not completely etch Al elements. In addition, 26.8 wt % of the O element and 9.5 wt % of the F element were observed, indicating that Ti3C2T contained a large amount of <span class="Chemical">oxygen functional groups, which was advantageous for improving the hydrophilicity of the material.

Figure 1

SEM images (a) and EDS results (b–g) of the Ti3C2T powder.

The surface and cross-sectional view of the al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs are shown in Figure . As shown in Figure a–f, the surface image of the pristine PVA membrane was smooth and flat. The PVA/Ti3C2T MMM surface with slight fluctuations looked dense without any visible defects. It indicates that Ti3C2T has good compatibility with PVA, probably because of the interactions between the <sal">papan>n class="Chemical">oxygen functional groups of Ti3C2T and the PVA chain. In addition, it can be seen from Figure a that the surface of PVA/Ti3C2T-3.0 MMM is loaded with Ti3C2T particles. Figure g–l showed the cross-sectional structure of the membranes, and the thickness of PVA/Ti3C2T MMMs was about 6 μm. Moreover, the separation layer was firmly bonded on the PAN support layer.

Figure 2

SEM surface view (a–f) and cross-sectional structure (g–l) of PVA/Ti3C2T MMMs with different Ti3C2T fillings: (a,g) 0.0; (b,h) 0.5; (c,i) 1.0; (d,j) 2.0; (e,k) 3.0; and (f,l) 4.0 wt %.

Figure 3

EDS results of PVA/Ti3C2T-3.0 MMMs: (a) surface view and (b) cross-sectional structure.

SEM surface view (a–f) and cross-sectional structure (g–l) of al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs with different Ti3C2T fillings: (a,g) 0.0; (b,h) 0.5; (c,i) 1.0; (d,j) 2.0; (e,k) 3.0; and (f,l) 4.0 wt %.

EDS results of al">PVA/Ti3C2T-3.0 <pan class="Gene">span class="Chemical">MMMs: (a) surface view and (b) cross-sectional structure.

The EDS diagram of surface morphology and cross-sectional structure of the al">PVA/Ti3C2T-3.0 MMM are shown in Figure . It can be seen that Ti elements and F elements are substantially uniformly distributed on the surface and cross section of the membrane, indicating that the distribution of Ti3C2T in the pan class="Chemical">PVA matrix is relatively uniform. Among them, the gold element is distributed by spraying gold during the sample preparation process.

To further research the surface structure of the <span class="Chemical">MMMspan>, topography images and the surface roughness values of the <span class="Chemical">MMMs were determined with AFM (Figure ). The roughness values (Ra and Rq) of the membrane surface are shown in Table . The roughness increased with the increase of Ti3C2T addition, which may be due to a higher Ti3C2T filling that leads to a larger particle size with more chance of agglomeration on the membrane surface.

Figure 4

AFM images of PVA/Ti3C2T MMMs with different mass fractions of Ti3C2T filling: (a) 0.0; (b) 1.0; and (c) 3.0 wt %.

Table 1

Surface Roughness of the PVA/Ti3C2T MMMs

surface roughness	PVA/Ti3C2Tx-0.0	PVA/Ti3C2Tx-1.0	PVA/Ti3C2Tx-3.0
Ra (nm)	1.99	4.02	4.87
Rq (nm)	2.58	5.49	6.31

AFM images of al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs with different mass fractions of Ti3C2T filling: (a) 0.0; (b) 1.0; and (c) 3.0 wt %.

ATR-FTIR and XRD Patterns of the Membranes

The ATR-FTIR patterns of Ti3C2T powder, <pan class="Gene">span class="Chemical">Ti3AlC2 powder, and PVA/Ti3C2T <sal">papan>n class="Chemical">MMMs are shown in Figure . As can be seen from Figure a, Ti3AlC2 is substantially free of any functional groups, whereas Ti3C2T mainly comprises OH groups and C–O–C groups, which is consistent with previous articles.[33] In Figure b, the broad absorption peak at 3267 cm–1 represented the OH groups and the peak intensity of the PVA/Ti3C2T MMMs at 3267 cm–1 decreased with increasing Ti3C2T filling. The absorption bands at 1710 and 1234 cm–1 correspond to C=O and C–O groups, respectively. Moreover, the characteristic peak formed at 1090 cm–1 was related to the asymmetric stretching vibration of C–O–C groups. According to a previous study,[42] the cross-linking density of PVA/Ti3C2T MMMs could be semi-quantitatively analyzed by the FT-IR peak intensity of C–O–C groups (1090 cm–1) and OH group (3267 cm–1) ratio (H1090/H3267). Table demonstrated that the H1090/H3267 increased with increasing Ti3C2T filling. It further indicates that the cross-linking density of the PVA/Ti3C2T MMMs is positively correlated with the amount of Ti3C2T filling. The ATR-FTIR spectra showed that Ti3C2T promoted the cross-linking reaction of PVA, and Ti3C2T was well incorporated into the PVA matrix.

Figure 5

FT-IR spectra of (a) Ti3C2T, Ti3AlC2, and (b) PVA/Ti3C2T MMMs.

Table 2

Peak Intensity Ratio of 1090 and 3267 cm–1 in the FT-IR Spectra of PVA/Ti3C2T MMMs

membrane	PVA/Ti3C2Tx-0.0	PVA/Ti3C2Tx-0.5	PVA/Ti3C2Tx-1.0	PVA/Ti3C2Tx-2.0	PVA/Ti3C2Tx-3.0	PVA/Ti3C2Tx-4.0
H1090/H3267	1.56	1.57	1.71	1.82	1.84	2.11

FT-IR spectra of (a) Ti3C2T, <span class="Chemical">Ti3AlC2pan>, and (b) PVA/Ti3C2T <span class="Chemical">MMMs.

The <span class="Chemical">Ti3AlC2pan> powder, Ti3C2T powder, and PVA/Ti3C2T <span class="Chemical">MMMs were further studied by XRD. Figure a presented the XRD patterns of Ti3C2T and <span class="Chemical">Ti3AlC2 powders. Ti3C2T powders were transferred to a lower angle at the (002) peak than Ti3AlC2, and Ti3C2T has no strong diffraction peak at 2θ = 39°, which means that most of the Al elements on Ti3AlC2 were etched away and Ti3C2T was successfully synthesized. The same conclusion appeared in previous reports.[34,37] In Figure b, a strong diffraction peak at 2θ = 19.7° (d-spacing of 4.5 Å) was a typical lattice of PVA, indicating that PVA is a semicrystalline polymer. The strong diffraction peaks of the PVA/Ti3C2T MMMs at 2θ = 19.7° and 2θ = 8.4° represented the main characteristic peaks of PVA and Ti3C2T, respectively. In addition, with the increase of Ti3C2T filling, the peak intensity at 2θ = 8.4° increased and the peak intensity at 2θ = 19.7° slightly decreased.

Figure 6

XRD patterns of Ti3AlC2 powder, Ti3C2T powder, (a) and PVA/Ti3C2T MMMs (b).

XRD patterns of <span class="Chemical">Ti3AlC2pan> powder, Ti3C2T powder, (a) and PVA/Ti3C2T <span class="Chemical">MMMs (b).

Hydrophilic and DS

The hydrophilicity of the membrane surface is an essential property for al">PV. Figure shows the WCAs of different membranes. With the increase of Ti3C2T, the small decrease in contact angle indicates that the hydrophilicity is enhanced. Although with the increase of Ti3C2T addition, the hydroxyl group is decreased, from Figure and Table , we can see that the roughness is increased, and the combination of the two factors resulted in a small decrease in the contact angle.

Figure 7

WCA of PVA/Ti3C2T MMMs with different amounts of Ti3C2T filling.

WCA of al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs with different amounts of Ti3C2T filling.

The DS of the experiment was tested in <span class="Chemical">waterpan>, pure <span class="Chemical">ethanol, and 93 wt % <span class="Chemical">ethanol aqueous solution at constant temperature. The DS formula is as the following equationwhere WA and WB are the mass of the wet and dry membrane samples (g), respectively. The DS of PVA/Ti3C2T MMMs decreased with the increase of Ti3C2T filling, as shown in Figure . The possible reason can be described that the interaction between Ti3C2T and the PVA matrix increases the cross-linking density of the membrane, which limits the movement of PVA molecular chains. In addition, the decrease of hydrophilicity of MMMs (Figure ) results in a decrease of affinity for water molecules.[43] In general, Ti3C2T has good mechanical properties and swelling resistance,[44] so the addition of Ti3C2T inhibited the swelling of the PVA membrane.

Figure 8

DS of PVA/Ti3C2T MMMs with different Ti3C2T loadings.

DS of al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs with different Ti3C2T loadings.

PV Performance for the Ethanol–Water Binary System

The al">PV performance of pan class="Chemical">PVA/Ti3C2T MMMs for the ethanol–water binary system was evaluated. Ti3C2T filling levels have a significant influence on the separation performance of the membrane. First, the effect of Ti3C2T filling was studied at 37 °C and 93 wt % ethanol concentration. The total flux of PVA/Ti3C2T MMMs decreased with increasing Ti3C2T filling, as shown in Figure a. We can explain that the crystallinity of PVA membranes was not significantly improved (Figure ) and the hydrophilicity of the membrane surface was declined (Figure ) from the preceding analysis, which makes water molecules more difficult to enter the membranes. On the other hand, the resistance of water components passing through the membrane increases, attributed to the increase in the cross-linking density and aggregation of Ti3C2T, so the water flux decreased. The total flux depends on the water flux, so the total flux decreased. In addition, it can be seen that the water content in the permeate of the PVA/Ti3C2T membranes exceeded 98.3 wt %. According to Figure b, the optimum value of the separation factor was 2585 when the Ti3C2T filling was 3.0 wt %. Moreover, the separation factor increased when the Ti3C2T loading increased from 0.0 to 3.0 wt %, which may be owing to the enhanced cross-linking density of PVA/Ti3C2T MMMs, which led to the denser separation layer. The infrared analysis and DS test side demonstrated that the cross-linking density of the membrane increased after the addition of Ti3C2T. It showed that the addition of Ti3C2T could improve the separation factor of the PVA membrane. However, when the amount of Ti3C2T filling exceeded 3.0 wt %, the separation factor decreased, which may be due to the fact that Ti3C2T loading reached the saturation limit, and higher than the concentration value, and agglomeration of Ti3C2T occured, resulting in defects in the membrane.[41,43]Table showed the effect of Ti3C2T loading on the permeability and selectivity. Water permeability decreased with the increase of loading amount of Ti3C2T, and ethanol permeability first decreased and then increased. When the loading amount of Ti3C2T was 3.0 wt %, ethanol permeability was the lowest. The PVA/Ti3C2T-3.0 MMM had the highest selectivity based on the change of water permeability and ethanol permeability, and this was consistent with the variation of the separation factor. The pervaporation separation index (PSI) is defined as the product of the total flux and separation factor and is used to characterize the PV performance of the membrane. PSI reached the highest value when the Ti3C2T filling was 3.0 wt %. Herein, 3.0 wt % Ti3C2T was the best filling amount for ethanol dehydration.

Figure 9

PV performance of PVA/Ti3C2T MMMs. Total flux and water in the permeate (a) and separation factor and PSI (b) for membranes at different Ti3C2T fillings.

Table 3

Effect of Ti3C2T Loading on the Permeability and Selectivity

Ti3C2Tx loading (wt %)	water permeability (106 g/m h kPa)	ethanol permeability (106 g/m h kPa)	total permeability (106 g/m h kPa)	selectivity
0.0	281.42	2.27	283.69	124.2
0.5	273.30	0.76	274.06	360.6
1.0	267.95	0.26	268.21	1006.4
2.0	237.63	0.21	237.84	1093.2
3.0	199.53	0.15	199.68	1298.3
4.0	160.01	0.20	160.21	826.5

al">PV performance of pan class="Chemical">PVA/Ti3C2T <span class="Chemical">MMMs. Total flux and <span class="Chemical">water in the permeate (a) and separation factor and PSI (b) for membranes at different Ti3C2T fillings.

Based on the optimal Ti3C2T filling, we evaluated the effect of <span class="Chemical">ethanolpan> concentration in the feed liquid for the PVA/Ti3C2T-3.0 MMM. The effect of the <span class="Chemical">ethanol content in the feed solution on the PV performance at 37 °C is shown in Figure . The <span class="Chemical">water concentration in the feed liquid directly affects the adsorption behavior of each component on the membrane surface. As shown in Figure a, when the ethanol concentration in the feed liquid increased from 87 to 97 wt %, the total flux decreased from 0.11 to 0.03 kg/m2 h. This phenomenon was described in early work.[45] When the water content in the feed decreases, the selective interaction between water molecules and membranes decreases, which reduces the driving forces of the water molecules. Furthermore, from the result of the DS of the MMMs (Figure ), at higher ethanol concentrations, the expansion degree of the membrane was less, which reduced the free volume of the membrane. Therefore, the transport resistance of ethanol and water molecules increases, resulting in the decrease of total flux. It can be concluded from Figure b that the water flux was much higher than the ethanol flux, and the trend was close to the total flux, so the total flux was determined by the water flux. The information presented in Figure a also included that the separation factor increased from 654 to 4652 with the increase of ethanol concentration. This is mainly due to the decreases in the DS of the membrane, which weakens the chain movement of the polymer. Because of the large molecular diameter of ethanol and the difficulty of passing ethanol through the membrane, the adsorption selectivity of the membrane to water increases, resulting in an increase in the separation factor. At different feed concentrations, the water content in the permeate of the PVA/Ti3C2T-3.0 MMM exceeded 99 wt % (Figure a). The permeability and selectivity of the PVA/Ti3C2T-3.0 MMM at different ethanol concentrations in the feed liquid, as shown in Table , water permeability, and ethanol permeability increased with increasing ethanol concentration. The reason is that the DS of the membrane decreased with a decrease in water content, the free volume is reduced, and the diffusion of the components is limited.[25] In addition, the dissolution degree of the membrane is decreased, and then, the driving force of the molecules is reduced.[39] It is worth noting that the change in permeability is not significant, but the selectivity is significantly increased.

Figure 10

PV performance of the PVA/Ti3C2T-3.0 MMM. Total flux, water in permeates, and separation factor (a) and water flux and ethanol flux (b) for membranes at different feed concentrations.

Table 4

Effect of Ethanol Concentration on the Permeability and Selectivity of the PVA/Ti3C2T-3.0 MMM

ethanol concentration (wt %)	water permeability (106 g/m h kPa)	ethanol permeability (106 g/m h kPa)	total permeability (106 g/m h kPa)	selectivity
87	208.34	0.55	208.89	378.3
90	202.42	0.23	202.65	900.4
93	199.53	0.16	199.68	1298.3
95	196.02	0.10	196.12	1803.1
97	194.92	0.09	195.01	2304.2

al">PV performance of the pan class="Chemical">PVA/Ti3C2T-3.0 MMM. Total flux, water in permeates, and separation factor (a) and water flux and ethanol flux (b) for membranes at different feed concentrations.

The influence of operation temperature on al">PV performance is special. Figure showed the effect of operating temperature on the pan class="Chemical">PV performance of PVA/Ti3C2T MMMs at 93 wt % ethanol concentration. All of the PVA/Ti3C2T MMMs showed significant enhancement in the total flux with increasing temperature, as shown in Figure a.[46] We attribute this phenomenon to two reasons: first, at high feed temperatures, free diffusion of ethanol and water molecules intensifies, which increases the free volume of molecules, and the activity of polymer segments promotes the increase of DS. Second, the vapor pressure on the feed side increases with increasing temperature, leading to an increase in the mass transfer driving force of the membrane. Moreover, as seen in Figure b, the water flux and ethanol flux of the PVA/Ti3C2T-3.0 MMM both increased with increasing temperature. Therefore, the total flux increased. The activation energies of water and ethanol were obtained using the Arrhenius diagram (Figure c). The variation of the permeation flux of each component follows the following equationwhere J is each component flux (kg/m2 h), J0 is the pre-exponential factor, R is the gas constant (kJ/mol K), Ep is the activation energy (kJ/mol), and T is the temperature. The activation energy of ethanol and water were 79.47 and 50.04 kJ/mol, respectively. The much higher ethanol activation energy indicated the higher temperature sensitivity of ethanol permeation over water permeation, and the ethanol flux tends to increase faster than that of water when the feed temperature increased. As shown in Figure d, the separation factor of all the PVA/Ti3C2T MMMs decreased with increasing temperature, which was similar to previously reported dense PV membranes.[41,47] The reason was as follows: the free volume of molecules increases with increasing feed temperature, the permeation resistance of the ethanol molecules decreases, and furthermore, the penetration rate of ethanol exceeds the penetration rate of water. The permeability and selectivity of the PVA/Ti3C2T-3.0 MMM at different operating temperatures are shown in Table . It can be observed that the water permeability and the ethanol permeability both increase with increasing temperature, and the increase in temperature promotes the diffusion process of the components, indicating that this is an endothermic process.[48] It is consistent with the trend of flux with temperature. The effect of temperature on permeability was described by the Arrhenius formula (eq )where P is each component permeability (106 g/m h kPa), P0 is the pre-exponential factor, R is the gas constant (J/mol K), Ea is the activation energy (kJ/mol), and T is the temperature (K). Moreover, the permeability activation energies of water and ethanol are listed in Table . The smaller the apparent activation energy, the smaller the energy barrier of the molecule through the membrane,[48] where EP,water is less than EP,ethanol, indicating that water permeability is more susceptible to temperature changes. In addition, because water permeability and ethanol permeability increase with increasing temperature, the selectivity of the membrane is decreased.[25] In addition, the water content in permeate of the PVA/Ti3C2T-3.0 MMM exceeded 99.5 wt % at 37 °C.

Figure 11

Total flux (a) and separation factor (d) of PVA/Ti3C2T MMMs; water flux and ethanol flux (b), Arrhenius plots for water and ethanol flux (c), and water in permeates (d) of the PVA/Ti3C2T-3.0 membrane under varied temperature.

Table 5

Effect of Temperature on the Permeability and Selectivity of the PVA/Ti3C2T-3.0 MMM

temperature (°C)	water permeability (106 g/m h kPa)	ethanol permeability (106 g/m h kPa)	total permeability (106 g/m h kPa)	selectivity	EP,water (kJ/mol)	EP,ethanol (kJ/mol)
37	199.53	0.15	199.68	1298.3	15.5	41.5
45	206.93	0.22	207.15	921.9
53	220.27	0.33	220.60	661.5
61	264.42	0.49	264.91	536.6
69	300.56	0.68	301.24	441.4

Total flux (a) and separation factor (d) of al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs; <sal">papan>n class="Chemical">water flux and ethanol flux (b), Arrhenius plots for water and ethanol flux (c), and water in permeates (d) of the PVA/Ti3C2T-3.0 membrane under varied temperature.

Figure investigates the effect of operating time on the al">PV performance of the pan class="Chemical">PVA/Ti3C2T-3.0 MMM. It can be seen that during continuous dehydration of the PVA/Ti3C2T-3.0 MMM at 37 °C and the 93 wt % ethanol–water binary mixture for about 7 days, the permeate flux and separation factor are relatively stable. It shows that the PVA/Ti3C2T MMM can operate stably for a long time.

Figure 12

Long-term stability of the PVA/Ti3C2T-3.0 MMM under 37 °C and 93 wt % ethanol concentration.

Long-term stability of the al">PVA/Ti3C2T-3.0 MMM under 37 °C and 93 wt % <pan class="Gene">span class="Chemical">ethanol concentration.

Table listed the al">PV performance of pan class="Chemical">PVA/Ti3C2T MMMs for ethanol dehydration compared with reported membranes.[21,26,43,49−56] As can be seen, the prepared PVA/Ti3C2T MMMs exhibited a higher separation factor than most PVA-based membranes. It is indicated that loading Ti3C2T into polymer membranes has prospects.

Table 6

PV Performance of PVA-Based Membranes for the Ethanol–Water Binary Systema

membr.	temp. (°C)	water content (wt %)	J (kg/m2 h)	α	refs
PVA/C3N4	75	10	6.33	30.7	(21)
PVA/ZIF-8-NH2	40	15	0.16	148	(26)
PVA/ZIF-90	30	10	0.27	1379	(43)
PVA/Fe-DA	30	10	0.99	2980	(49)
PVA/H-ZSM5	30	15	0.18	46	(50)
PVA/CF-TSA	50	30	0.13	775	(51)
PVA/NaY	60	20	0.30	450	(52)
PVA/SA	30	10	∼0.35	25,000	(53)
PA/SDS-clay	25	10	12.0	0.28	(54)
SA/MoS2	77	10	1.84	1229	(55)
CS/siloxane	25	10	0.47	2182	(56)
PVA	37	7	0.096	152	this work
PVA/Ti3C2Tx	37	7	0.074	2585	this work

C3N4, carbon nitride; ZIF, zeolite imidazolate framework; Fe-DA, iron-dopamine nanoparticles; CF, carboxy fullerene; TSA, p-toluene sulfonic acid; ZSM, molecular sieve; NaY, molecular sieve; SA, sodium alginate; PA, polyamide; MoS2, molybdenum disulphide; CS, chitosan.

<span class="Chemical">C3N4pan>, carbon nitride; ZIF, <span class="Chemical">zeolite <span class="Chemical">imidazolate framework; Fe-DA, iron-dopamine nanoparticles; CF, carboxy fullerene; TSA, p-toluene sulfonic acid; ZSM, molecular sieve; NaY, molecular sieve; SA, sodium alginate; PA, polyamide; MoS2, molybdenum disulphide; CS, chitosan.

Conclusions

In this study, the novel two-dimensional material, Ti3C2T, was used as an inorganic filler to prepare al">PVA/Ti3C2T <pan class="Gene">span class="Chemical">MMMs. GA and PAN ultrafiltration membranes were used as cross-linking agents and the support layer, respectively. The results showed that Ti3C2T could be uniformly dispersed in the PVA matrix, and they had excellent compatibility. In addition, PVA/Ti3C2T <sal">papan>n class="Chemical">MMMs exhibited excellent mechanical properties and swelling resistance. PV performance results showed that loading Ti3C2T in the PVA matrix significantly improved the separation factor of PVA membranes, but the total flux of PVA membranes decreased because of the enhanced cross-linking density and the weakened hydrophilicity of the membrane surface. The PVA/Ti3C2T-3.0 MMM had the best PV performance at 37 °C and 93 wt % ethanol solution, and the separation factor was 2585, which was 17 times higher than that of the pristine PVA membrane and with an acceptable permeation flux of 0.074 kg/m2 h. When the concentration of ethanol decreased and the feed temperature increased, the separation factor decreased and the total flux increased significantly, which was consistent with the typical regular of polymer membranes. The newly developed PVA/Ti3C2T MMMs have great potential for membrane separation applications, which provide some reference for the application of Ti3C2T in PV.

Experimental Section

Materials

<span class="Chemical">pan class="Chemical">PVA-124 was purchased from Xilong Chemical Co., Ltd. (Guangdong, China). <sal">papan>n class="Chemical">Lithium fluoride (LiF, >99.9%) was supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Hydrochloric acid (HCl, AR) was from Beijing Chemical Works (Beijing, China). Glutaraldehyde (GA, 50%) was obtained from Tianjin Guangfu Fine Chemical Research Institute. (Tianjin, China). Glacial acetic acid (HAc, AR) and ethanol (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

Synthesis of Ti3C2T Powders

al">LiF and pan class="Chemical">HCl were used as milder etchants to replace HF, which were similar to previous research.[31,33] The procedure was as follows: 1.98 g of LiF was slowly added to 6 M HCl solution (30 mL). After magnetic stirring for 1 h, 3 g of Ti3AlC2 powder was slowly added to the solution, and then, the mixture was held at 40 °C for 45 h and centrifuged with deionized water until the supernatant reached neutrality. Then, the black powder was dispersed in deionized water and sonicated for 1 h. Thereafter, the solution was centrifuged for 1 h to remove large particles. After decantation, the black Ti3C2T colloidal supernatants were obtained and dried in a vacuum oven.

Fabrication of PVA/Ti3C2T MMMs

First, 8 g of al">PVA powders was dissolved in 2 wt % pan class="Chemical">HAc solution (92 g) and stirred for 1 h at 90 °C, and then, the insoluble large particles were filtered off. An amount of Ti3C2T powder was dispersed in deionized <span class="Chemical">water and sonicated for 1 h. Moreover, the abovementioned solutions were mixed and sonicated for 0.5 h to form uniform dispersions. GA (2 wt %) was added as a cross-linking agent to prepare the casting solution. Next, the casting solution was degassed by vacuum and poured onto the PAN ultrafiltration membranes, and solvents evaporate in a fume hood for 12 h. Finally, PVA/Ti3C2T <span class="Chemical">MMMs were prepared by cross-linking for 2 h in an electric drying oven at 90 °C.

Evaluation of the PV Performance Test

The schematic diagram of the self-designed al">PV apparatus is illustrated in Figure . Among them, the effective area of the membrane was 2.2 × 10–3 m2. The pump delivers the feed liquid to the membrane cell. The vacuum on the permeate side is formed using a vacuum pump, and the minimum absolute pressure can reach 0.2 kpan class="Chemical">Pa. The permeation mixture was collected in the cold trap made of liquid <span class="Chemical">nitrogen bath, and the contents of upstream feed solution and downstream permeate were analyzed by gas chromatography (Shimadzu, <span class="Mutation">GC-14C, Japan).

Figure 13

Schematic illustration of the PV evaluation apparatus.

Schematic illustration of the al">PV evaluation apparatus.

The total flux (J) and separation factor (α) were two key indicators of al">PV performances, which are defined as the following equationswhere J is the total flux (kg/m2 h); Q is the weight of the permeation component (g); A is the effective area of the membrane (m2); t is the time (h); α represents the separation factor; Y is the mass fraction of components in the permeate liquid (wt %); X is the mass fraction of components in the feed liquid (wt %); and i and j stand for <pan class="Gene">span class="Chemical">water and <sal">papan>n class="Chemical">ethanol, respectively.

In addition, the permeability (Pi) and the selectivity (βP) are defined as eqs and 7where Pi is each component permeability (106 g/m h kPa), Ji is each component flux (kg/m2 h), l is the thickness of the separation layer (m), x and y stand for the molar fractions of the component in the feed liquid and the permeate liquid (%), respectively, γi and Pis (kPa) are the activity coefficients of feed mixtures and saturated vapor pressures obtained using Aspen Plus software, PP is the permeate side pressure (kPa), βP is the selectivity, and PW and PE are <span class="Chemical">waterpan> permeability and <span class="Chemical">ethanol permeability (106 g/m h kPa), respectively.

Characterization

The morphology of powder and membranes was characterized by SEM (JSM7401F, Japan) and EDS. AFM (SPA-400, Japan) was used to characterize the morphology of membranes. The structural properties of pan class="Chemical">PVA/Ti3C2T MMMs were studied with ATR-FTIR (Bruker TENSOR 27). The diffraction patterns and crystalline phases of membranes (remove the PAN ultrafiltration membrane) were researched by XRD (Bruker D8 ADVANCE, Germany). The Ti3AlC2 and Ti3C2T powders were also characterized. The hydrophilicity of membranes was experimented by the WCA (Model PV-DP). The DS test was also performed.