Crosslinking modification of polyamide composite membranes toward high performance for DMAc/H2O dehydration

doi:10.21203/rs.3.rs-2321362/v1

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Crosslinking modification of polyamide composite membranes toward high performance for DMAc/H2O dehydration

https://doi.org/10.21203/rs.3.rs-2321362/v1

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Polar aprotic solvents such as dimethylacetamide (DMAc) play a crucial role in the chemical industry. Pervaporation is a membrane separation technology suitable for the dehydration of organic solvents. In this study, thin polyamide (PA) pervaporation membranes with different structures were prepared by interfacial polymerization using trimesoyl chloride (TMC) as the organic phase, and either m-phenylenediamine (MPDA), propylene diamine (DAPE), or tetraethylene pentamine (TEPA) as the aqueous phase. In addition, the PA pervaporation membranes' chemical composition, hydrophilicity, surface morphology, and cross-sectional morphology were examined by fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopic (AFM) and contact angle (CA) analysis, respectively. The results showed that the PA pervaporation membrane prepared with TEPA as the aqueous monomer exhibited a high degree of crosslinking, resulting in dense molecular chains. The permeation flux and water content of 10 wt % DMAc/H₂O system were 17.8 L m^− 2 h^− 1 and 98.4 wt % at 500 Pa and 70 ° C, respectively. Therefore, the TEPA-PA membrane has a good separation performance even when dealing with a low-concentration DMAc/H₂O system.

polyamide membrane

pervaporation

interfacial polymerization

dimethylacetamide dehydration

Polar aprotic solvents, such as dimethylacetamide (DMAc), are essential organic solvents and chemical raw materials that are widely used in the petrochemical, pharmaceutical, and chemical industries; they are also used in leather manufacturing^[1]. However, these chemicals are toxic and difficult to biodegrade in natural environments. When large amounts of industrial waste liquid from those processes are discharged without treatment, human health is endangered, and severe environmental damage can be caused^[2]. Recently, membrane processes have become crucial in the chemical and pharmaceutical industries, accounting for 40 ~ 70% of the related capital and operating costs^[3–4]. Compared with traditional separation processes, membrane separation is an advanced technology with unique advantages, such as high separation efficiency, low energy consumption, small carbon footprint, cost savings, easy operation, and scalability. Pervaporation (PV)^[5–7] removes angstrom-scale pollutants from liquid mixtures through various membrane-based separation processes. Compared to traditional separation technologies, the PV process can be conducted at lower operational temperatures because it is not thermodynamic-equilibrium limited^[8].

Primarily, the resistance of membrane materials for the PV separation technology should be considered with particular attention when strongly polar solvent systems are used^[9]. Zeolite was the first developed inorganic membrane material intended to dehydrate organic solvents, being hydrophobic zeolite used to recover organic solvents^[10]. The solvent flux of the zeolite membranes for 10 wt% DMAc/H₂O separation was 1.51 L m^–2·h^{–1[11, 12]}. Organic membrane materials from facile and low-cost preparation with high flux and selectivity are preferred for separating organic solvents. Liu^[13]et al. used a thermally crosslinked polyacrylonitrile (PAN)/TiO₂ membrane to separate dye/DMAc, achieving a flux of 4.5 L m^–2·h^–1.

Among other membrane preparation technologies, interfacial polymerization can be used to prepare ultrathin separation membranes^[14]. During the pervaporation process, the diffusion resistance due to the thickness of the membrane layer is greatly reduced, which is conducive to an enhanced permeation flux. Interfacial polymerization membrane formation requires two monomers with high reactivity that undergo in situ polymerization at the immiscible interface, forming an ultrathin composite membrane. The film-preparation process comprises the immersion of a porous base film in an aqueous monomer solution. After several minutes, the base film is withdrawn and dipped in the organic-phase monomer solution after removing the residual solution on its surface. The two monomers polymerize on the surface of the porous base film to form an ultrathin composite film denoted as the thin-film composite (TFC) membrane^[15]. Typical aqueous-phase monomers are m-phenylenediamine (MPDA) and piperazine (PIP), whereas the organic phase monomer can be trimesoyl chloride (TMC) ^[16–20]. Marquez^[21] et al. reported the addition of acetone as a cosolvent for interfacial polymerization, which enhanced the adsorption of MPDA on the membrane and the miscibility of water and toluene. The addition of 75 wt% acetone solution resulted in forming a PA layer exhibiting a high degree of cross-linking. The PA layer became denser with the addition of acetone, and its thickness increased with increasing acetone concentration. During the pervaporation dehydration of 90 wt% tert-butanol aqueous solutions, a composite hollow-fiber membrane exhibited a high permeation flux (4.372 L m^–2·h^–1) and a high water permeation concentration (99.88 wt%). Ang et al.(2021)^[22] prepared PA membranes by the interfacial polymerization of 1,4-bis(3-aminopropyl)piperazine (BAPP) and TMC. The PA films were then modified via UV-induced acrylic graft polymerization. The monomer concentration, irradiation time, and distance were investigated to determine the optimal conditions for achieving high permeability and effective selectivity. The surface hydrophilicity and the separation efficiency of the polyacrylic acid (PAA) graft were enhanced. The water concentration on the permeate side increased from 84.9 wt% to 99.5 wt% when separating the ethanol/water system through the PV process^[23–24]. M et al.(2012) ^[25] selected a new amine monomer hyperbranched polyethyleneimine (HPEI) to synthesize the PA membrane. The results showed that the permeation flux of the HPEI-PA membrane for the isopropanol/water (85/15 wt %) mixture was 1.2 L m^–2·h^–1. Maria et al.(2013) ^[26–28] inserted F and Si into the polyamide layer of a TFC membrane and deposited polyamide separation layers on modified hydrophilic polyethylene (PE) porous supports by interfacial polymerization. The selective permeability is significantly improved, reaching 7.0 L m^− 2 h^− 1 in DMF dehydration. High selective permeability compared with other organic solvent nanofiltration membranes. However, few research works focused on the dehydration for low-concentration DMAc aqueous solution by PV process.

In this work, the physical and chemical properties of PA TFC membranes with different structures were investigated through the selection of either aromatic m-phenylenediamine (MPDA), aliphatic propylene diamine (DAPE), or polyamine-based tetraethylene pentamine (TEPA) as the aqueous-phase monomer. The TEPA-PA membrane was selected as the synthesized system, and the effects of interfacial polymerization and the operating conditions during pervaporation for the DMAc/H₂O system were further investigated.

2.1 Materials and reagents

N, N-Dimethylacetamide (DMAc, purity: 99%) and m-Phenylenediamine (MPD, purity: 99.5%) were obtained from Tedia Company, Inc. (USA) and Shanghai Macklin Biochemical Co. Ltd. (China), respectively. Trimesoyl chloride (TMC, ≥ 98%) was purchased from the Qingdao Benzo Chemical Company (China). Sodium hydroxide (NaOH, AR) and sodium carbonate (Na₂CO₃, AR) were obtained from Tianjin Baishi Chemical Co. Ltd. (China). N-hexane (≥ 97%) and TEPA (density: 0.998 g mL^–1 and viscosity: 50–60 m Ps at 25°C) were provided by Sinopharm Chemical Reagent Co., Ltd. (China) and Beijing Mindray Technology Co. Ltd.( China), respectively. The 1,3-propylene diamine (99%, Aldrich) and sodium dodecyl sulfate (SDS, 98.5%) were obtained from Merck (Malaysia) and used without further treatment. All solutions were prepared using deionized water (DI) (conductivity: < 0.5 µs/cm). Glass plate as a self-made abrasive was used for interfacial polymerization.

2.2 Preparation of PA TFC membranes by interfacial polymerization

2 M NaOH solution was prepared, and the polyacrylonitrile (PAN) support was immersed in the solution to ensure complete infiltration. The hydrolyzed PAN support (denoted as H-PAN) was placed in an oven at 60°C for 2 h. The fully reacted H-PAN membrane was repeatedly washed using distilled water until a pH of 7 was achieved; then, it was immersed in distilled water. 1 ~ 5 wt% aqueous phase (MPDA, DAPE, TEPA) solutions were prepared, in which 0.1 wt% NaOH, Na₂CO₃ as an acid acceptor, and 0.2 wt% SDS as a surfactant were added. Using n-hexane as the solvent, 0.05–0.3 wt% trimesoyl chloride (TMC) was added as the organic phase monomer to prepare an organic phase solution. The H-PAN support was immersed in the aqueous solution for 5 min. Then, it was withdrawn and placed vertically into an oven at 60°C to remove the excess solution on the surface. After the surface water was removed from the H-PAN support, it was placed into a customized mold. A small amount of the organic phase solution was pipetted onto the H-PAN support surface, reacting for 10–180 s. The surface of the carrier was then rinsed repeatedly with n-hexane to remove the organic phase monomer in excess from the surface. Once the membrane completely reacted, it was dried up to 80°C and then immersed in distilled water before use. The prepared PA TFC membranes were denoted as MPDA-PA, DAPE-PA, and TEPA-PA.

The reaction mechanism, structure diagram, and interfacial polymerization scheme for each of the PA TFC membranes derived from the three aqueous monomers are shown in Fig. 1. The first step was the addition of different aqueous solutions, while the second step comprising the addition of an organic solvent. The third step was to generate a PA membrane through interfacial polymerization. The preparation conditions of the PA TFC membranes are listed in Table 1.

Table 1

PA membranes prepared using three different aqueous-phase monomers
Types of membranes	Contact time	Aqueous phase concentration	TMC concentration
MPDA-PA₁	90s	0.5wt%	0.15wt%
DAPE-PA₁	90s	0.5wt%	0.15wt%
TEPA-PA₁	90s	0.5wt%	0.15wt%
TEPA-PA₂	90S	1wt%	0.15wt%
TEPA-PA₃	90s	1.5wt%	0.15wt%
TEPA-PA₄	90s	2wt%	0.15wt%
TEPA-PA₅	90s	2.5wt%	0.15wt%

2.3 Characterization of the PA TFC membranes

(1) Fourier-transform infrared spectroscopy

The structure of the PA TFC film was characterized by IS50 Fourier-transform infrared spectroscopy (FT-IR, Nicolet Company, USA). The scanning was performed under the attenuated total reflection mode, and the wavenumber ranged from 4000 to ~ 500 cm^− 1.

(2) Water contact-angle test

The hydrophilicity of the PA TFC membranes was determined using a JC2000D1 contact-angle tester (CA; Shanghai Zhong Chen Digital Technology Equipment Co., Ltd., China). Water was used as a solvent for the static water contact-angle measurement on the PA TFC membrane surfaces.

(3) Scanning electron microscopy (SEM)

The surface morphology and cross-sectional structure of the PA TFC membranes were characterized by a field-emission scanning electron microscope (FE-SEM; SUPRA55, Carl Zeiss, Germany). The cross-section of the composite films was brittle in liquid nitrogen; thus, the samples were sprayed with gold before the determinations.

(4) Atomic force microscopic analysis

The surface roughness of the PA TFC membranes was determined using a Nanoman VS atomic force microscope within a test range of 1 × 1 µm.

(5) X-ray photoelectron spectroscopy

Surface elemental analysis of the PA TFC membranes was conducted by X-ray photoelectron spectroscopy (XPS; ESCALAB250Xi, ThermoFisher Scientific, USA). The size of the composite-film sample was 5 × 5 × 3 mm, and the testing elements were C, N, and O. The obtained atomic concentrations of the elements at the membrane surface were used to evaluate the degree of crosslinking of the PA layer using the following equation: ^[29]

$$\text{d}\text{e}\text{g}\text{r}\text{e}\text{e} \text{o}\text{f} \text{c}\text{r}\text{o}\text{s}\text{s}\text{l}\text{i}\text{n}\text{k}\text{i}\text{n}\text{g}=\frac{\text{m}}{\text{m}+\text{n}}\times 100$$

where m and n are the cross-linked and linear parts of the PA layer, respectively, which were calculated from the experimental O/N ratio obtained through XPS analysis using the following equatoin:

$$m+n=1$$

$$\frac{\text{O}}{\text{N}}=\frac{3m+4n}{3m+2n}$$

(6) Membrane performance characterization

Equation 4 computes the pervaporation performance of the PA TFC membranes using two parameters: the permeation flux, J, and the separation factor, α, where Q is the mass (g) of the collector.

$$J=\frac{Q}{A\bullet t}$$

$${{\alpha }}_{\text{W}⁄\text{O}}=\frac{{\text{Y}}_{\text{W}}/{\text{Y}}_{\text{O}}}{{\text{X}}_{\text{W}}/{\text{X}}_{\text{O}}}$$

In Eq. 5, A is the effective area of the separation membrane (m²), t is the collection time (h), W and O are the water and amide solvents, respectively, and X and Y are the mass fractions of the components on the feed and collection sides, respectively.

After exploring the reaction conditions (Fig.S1), we concluded that when the water phase content was 2 wt %, the organic phase content was 0.15 wt %, and the reaction time was 90 s, it was the best membrane synthesizing condition. We selected the best ratio content to prepare MPDA-PA, DAPE-PA, and TEPA-PA membranes by interfacial polymerization and discussed their membrane performance in the following sections.

3.1 Physicochemical properties of the PA TFC membranes

Infrared characterization of H-PAN and the three PA composite film samples are shown in Fig. 2. The stretching vibration peaks at 1646 and 1561 cm^− 1 were related to the stretching vibration peaks of C = O (amide I) and N-H (amide II), respectively, which are characteristic absorption peaks of PA layer. The broad peak at 3337 cm^− 1 was attributed to –OH, which was produced through the hydrolysis of unreacted acid chloride. Those stretching vibration peaks confirmed the successful interfacial polymerization of the polyamide layer on the surface of the H-PAN support^[30].

The chemical compositions at the surface of the three PA TFC films were investigated by XPS analysis, as shown in Fig. 3. The peaks located approximately at 288 eV in the C1s X spectrum were related to the –N-CO (287.6 eV) and the –COOH (288.2 eV) groups. During interfacial polymerization, –N-CO groups were formed through the reaction of amine groups with acid chloride groups, and –COOH groups were generated via the hydrolysis of unreacted acid chloride groups.

The C1s X-ray photoelectron spectra of the PA composite films and their relative bond assignments are shown in Table 2. The higher the number of -–COOH groups in the polymer chain at the interface, the higher the amount of residual acid chloride that did not react with the amine groups. Thus, the PA chains exhibited lower crosslinking and insufficient imidization. As shown in Table 2, the concentrations of –N-CO groups were 4.1%, 7.1%, and 13% for the MPDA-PA₁, DAPE-PA₁, and TEPA-PA₁ membranes, respectively. The concentrations of –COOH groups were 4.5%, 1.4%, and 1.3% for the MPDA-PA₁, DAPE-PA₁, and TEPA-PA₁ membranes, respectively. Thus, the relative ratios of the -N-CO and -COOH groups for the TEPA-PA₁, MPDA-PA₁, and DAPE-PA₁ membranes were 9.7, 1.0, and 4.9, respectively. The degree of crosslinking for the PA membranes was calculated according to the experimental O/N ratios obtained by XPS analysis. The computed values were 72.5%, 74.3%, and 85.3% for the MPDA-PA₁, DAPE-PA₁, and TEPA-PA membranes, respectively. This indicated that the TEPA-PA₁ membrane exhibited higher polymer-chain crosslinking^{[31, 32]}.

Table 2

Relative bond assignments from C1s X-ray electron spectra of the three PA TFC membranes
Sample name	C = C/C-H (%)	C-C/C-H (%)	C-N/C-O (%)	-N-C = O (%)	-COOH (%)	standard deviation
MPDA-PA₁	26.6	38.1	27.0	4.1	4.2	4.4
DAPE-PA₁	29.8	34.0	27.6	7.1	1.4	7.6
TEPA-PA₁	29.8	24.2	31.7	13.0	1.3	10.0

The water contact-angle test was used to analyze the hydrophilicity of H-PAN and the three PA composite membranes, as shown in Fig. 4. After hydrophilic modification, the contact angle for H-PAN was approximately 23°, which indicated good hydrophilicity. This might have facilitated the combination of the reaction layer with the support during interfacial polymerization. It should be noticed that the higher the hydrophilicity of the pervaporation membrane, the higher the water permeation. After interfacial polymerization, the water contact-angles on the MPDA-PA₁, DAPE-PA₁, and TEPA-PA₁ membrane surfaces were 33°, 25°, and 12°, respectively. The MPDA-PA₁ membrane exhibited the largest water contact angle among the three PA composite membranes because of a larger number of benzene rings and a higher hydrophobicity in the PA-molecular chain structure. The TEPA-PA₁ membrane exhibited the highest hydrophilicity owing to a larger number of –NH groups and a greater affinity for water molecules, i.e., higher hydrophilicity. Overall, the three developed PA TFC membranes exhibited good hydrophilicity and are beneficial for separating the DMAc/H₂O system via the pervaporation process.

SEM was used to analyze the surface and cross-sectional morphologies of the three PA TFC membranes, as presented in Fig. 5. As shown in Fig. 5a, a network of tiny pores in the surface of the MPDA-PA₁ TFC membrane was observed, along with a ridge morphology produced by polymer stacking, which was mainly due to the MPDA monomer structure. The surfaces of the DAPE-PA₁ and TEPA-PA₁ membranes were relatively smooth. Partial protrusions were observed on the surface of the DAPE-PA₁ membrane from interfacial polymerization. This might have been caused by the diffusion of adsorbed DAPE (the aqueous monomer) from the pores of the H-PAN support into the organic phase. Because of the linear structure of the DAPE and TEPA monomers, the surface of the prepared PA TFC membrane was relatively dense. TEPA is a polyamine monomer; thus, the corresponding PA separation layer was comparatively denser upon interfacial polymerization. The dense PA layer hindered the diffusion of the TEPA monomer towards the organic phase, i.e, the monomer did not have sufficient time to modify the membrane surface, thereby forming small particles with a protruding shape on the surface.

Among the cross-sectional SEM analyses of the three PA TFC membranes, the MPDA-PA₁ TFC membrane exhibited a loose pore structure, while the DAPE-PA₁ and TEPA-PA₁ membranes were dense. The thickness of all PA composite membranes was < 1 µm, and the thickness of the three composite membranes was computed to be in the following order MPDA-PA₁ > DAPE-PA₁ > TEPA-PA₁. The MPDA-PA₁ TFC membrane was thicker because of the pore-like network structure developed on its surface. During the interfacial polymerization process, MPDA monomers were more likely to diffuse into the organic phase, forming a loose PA layer. The thickness of the TEPA-PA₁ TFC membrane was the smallest, mainly because the TEPA monomer exhibited more amine groups and a larger number of reaction points with TMC. This resulted in a denser reaction layer, which hindered the diffusion of TEPA toward the organic-phase solution during interfacial polymerization. DAPE is a linear diamine monomer that was able to form a dense film during interfacial polymerization; thus, the film thickness was smaller than that of the MPDA-PA₁ TFC membrane. This is because the carbon chain length of the TEPA monomer is shorter than that of the DAPE monomer. In the interfacial polymerization process, TEPA monomer with a long carbon chain is difficult to diffuse into the organic phase solution, thus inhibiting the growth of PA film thickness. It makes its DAPE-PA₁ film thicker.

AFM was used to analyze the surface roughness of the three PA composite membranes (Fig. 6). The surface roughness of the three PA TFC membranes was analyzed through the mean roughness (R_a), root mean square roughness (R_ms), and maximum roughness (R_max) variables. The difference in surface roughness among the samples may imply different polymerization mechanisms between different aqueous monomer structures and organic phases during interfacial polymerization.

The R_ms, R_a, and R_max values for the three PA TFC membranes (based on Fig. 7) are summarized in Table 3. The roughness values decreased as follows: MPDA-PA₁ > TEPA-PA₁ > DAPE-PA₁. The surface of the TEPA-PA₁ TFC membrane exhibited a large protrusion area because the TEPA monomer contained more amine groups and exhibited a higher reaction rate in the TMC solution. In this case, interfacial polymerization derived in dense layer formation, which affected the diffusion of the TEPA monomer that was adsorbed on the H-PAN surface towards the dense polymer layer to enable the reaction with TMC. In addition, because of the short reaction time, the polymer chains had insufficient time to modify the membrane surface, resulting in a protruding membrane surface.

Table 3

Surface roughness of the prepared PA TFC membranes
Sample name	R_ms (nm)	R_a (nm)	R_max (nm)
MPDA-PA₁	24.4	18.9	199
DAPE-PA₁	6.22	4.84	46.2
TEPA-PA₁	7.73	6.14	55.2

The characterization of the three PA TFC membranes demonstrated that the TEPA-PA₁ membrane exhibited a dense membrane layer, a high degree of crosslinking in the polymer chain, and a low free volume. For the following discussion, the TEPA-PA membrane was selected as the pervaporation membrane to investigate the reaction processes involved. The effect of the PV process conditions on the pervaporation performance of the TEPA-PA TFC membrane was also investigated, for which a DMAc/H₂O mixture was used as the separating system.

3.2 Effect of operating conditions on the pervaporation performance of the TEPA-PA TFC membrane

Based on the results of varying interfacial polymerization conditions, the TEPA-PA TFC membrane synthesized from the reaction of 2 wt% TEPA (aqueous phase) with 0.15 wt% TMC (organic phase) for 90 s exhibited the optimal pervaporation performance. Therefore, a TEPA₄-PA TFC membrane was prepared under these conditions for the pervaporation dehydration of the DMAc/H₂O system.

When the feed liquid temperature was 30°C, and the operating pressure was 500 Pa, the effect of the liquid-feed concentration on the pervaporation separation performance through the TEPA-PA₄ composite membrane was investigated (Fig. 7a). The permeate flux exhibited an evident downward trend with increasing liquid-feed concentration. When the concentration of the feed solution was 10 wt% DMAc/H₂O, the permeation flux for the TEPA-PA₄ membrane was 3.13 L m^–2 h^–1, and the concentration of the feed solution was 90 wt% DMAc/H₂O, the permeation flux for the TEPA-PA₄ membrane decreased to 0.066 L m^–2 h^–1. This may be attributed to the more hydrophilic TEPA-PA₄ membrane, which preferentially permeated water molecules. The water content decreased with increasing DMAc concentration in the feed solution, which resulted in fewer water molecules permeating the PA membrane.

The effect of the feed temperature on the separation performance of the 10 and 90 wt% DMAc/H₂O systems was investigated at 500 Pa, as shown in Fig. 7b and c, respectively. When the operating temperature was 70°C, the permeate flux and the permeate-side water content for the TEPA-PA₄ membrane (10 wt% DMAc/H₂O system) were 17.8 L m^–2 h^–1 and 98.4 wt%, respectively. The permeate flux increased sharply with increasing operating temperature, and the water content on the permeate side exhibited no variation. The TEPA (aqueous) and TMC (organic) solutions formed a dense separation layer with cross-linked polymer chains during interfacial polymerization. A higher operating temperature did not provide sufficient energy to allow the DMAc molecules to move through the PA separation layer. In contrast, water molecules passed through the PA membrane, which increased the permeate flux. In the 90 wt% DMAc/H₂O separation, when the operating temperature was 30–60°C, the permeation flux for the TEPA-PA₄ membrane exhibited an upward trend, and the water content in the permeate side was ~ 98 wt%. When the operating temperature increased to 70°C, the permeate flux constantly increased, and the water content on the permeate side decreased (3.08 L m^–2 h^–1 and 71.7 wt%, respectively). Under a high-concentration (90 wt% DMAc/H₂O) feed solution, the swelling degree of the PA membrane in DMAc increased with increasing operating temperature. This led to the permeation of DMAc molecules into the separation membrane, which increased the permeate flux and reduced the water content on the permeate side. For the permeation stability testing, 10 wt % DMAc/H₂O solution was dehydrated by the TEPA-PA composite PV membrane at 70°C using 500 Pa. The results are shown in Fig. 7(d). With the increased operation time, the permeation flux was stable, with a high water content on the permeation side. After running 100 h PV process, the permeation flux remained at 16.5 ~ 17.8 L/(m² h), and the water content on the permeate side remained at 97.3 ~ 98.5 wt %. It shows that TEPA-PA composite membrane has good stability for DMAc/H₂O dehydration.

The permeation flux and water content for the TEPA-PA TFC membrane prepared under the optimal conditions considering the 10 wt% DMAc/H₂O system at 500 Pa and 70°C were 17.8 L m^–2 h^–1 and 98.4 wt%, respectively. Therefore, the TEPA-PA membrane exhibited a good separation effect in a low-concentration DMAc/H₂O system. The pervaporation performance of the other membranes to separate DMAc/water mixtures, according to the pervaporation flux for membranes reported in the literature, is shown in Table 4. For the inorganic membrane, such as zeolite membrane TiO₂(0.5)/NaA zeolite and NaA zeolite (Mitsui), when separating 10wt % DMAc/H₂O, the permeation flux is up to 5 L/m² h, and their separation factor is above 1000^[33–34]. Because of their low solvent resistance, few polymer materials can be applied to prepare a pervaporation membrane for DMAc dehydration. Some researchers used PI membranes to make DMAc dehydration by the PV process^[35–36]. Some special engineering polymer materials, such as PI, have excellent solvent-resistant properties, making them candidates for membrane material. When separating high-concentration DMAc/H₂O, the operating pressure was reduced for the PI pervaporation membrane, but the flux was still low. The results showed that the thick PI membrane prepared by the two-step method had a low permeability when separating 90 wt% DMAc/H₂O solution. In this work, a thin crosslinked PA TFC membrane was prepared, and can much higher permeation flux was obtained. The TEPA-PA membrane prepared by us has larger cross-linked polymer chains (around 72.6% crosslinking degree). Because TEPA contains more -NH groups in the PA chains, the affinity for water molecules is greater. The prepared TEPA-PA membrane has good hydrophilicity, which favors the water molecule passing through the PA TFC membrane. The permeation amount of 17.8 L m^− 2 h^− 1 can be achieved when separating 10wt % DMAc/H₂O. Compared with the inorganic zeolite membrane and the PI membrane prepared by the two-step method, the flux of the TEPA-PA membrane increased obviously. Therefore, the TEPA-PA TFC membrane has the potential for DMAc dehydration by the PV process, even for a low DMAc concentration solution.

Table 4

Pervaporation applications for the separation of polar aprotic solvents
Types of membranes	DMAc concentration (wt%)	Operation Temperature (°C)	Permeation flux [L/(m²•h)]	Separation factor	Operating pressure (Psi)	Ref.
TiO₂(0.5)/NaA zeolite	10	90	4.66	2120	290.06	[33]
NaA zeolite (Mitsui)	10.5	80	1.51	1600	290.06	[34]
PI	90	30	0.07	100	72.52	[35]
PI	70	70	0.56	277	14.50	[36]
TEPA-PA	10	70	17.83	> 1000	72.52	This work

In this work, three different aqueous monomers were selected to prepare PA TFC pervaporation membranes from the perspective of molecular structure design. Three kinds of PA TFC pervaporation membranes have good hydrophilicity, and the water contact angle of the TEPA-PA membrane is the lowest. MPDA-PA membrane has a rough structure, and the membrane layer is relatively loose, while DAPE-PA and TEPA-PA membranes are dense membranes with smooth surfaces. TEPA-PA membrane exhibited the highest hydrophilicity, containing the most significant proportion of -N-CO and -COOH groups, more crosslinked polymer chains, and a crosslinking degree of 72.6%. The permeation flux and water content of 10 wt % DMAc/H₂O system were 17.8 L m^− 2 h^− 1 and 98.4 wt % at 500 Pa and 70°C, respectively. Therefore, the TEPA-PA pervaporation membrane has a good separation performance even when dealing with a low-concentration DMAc/H₂O system.

Authorship contribution statement: Data collation, Z. Wei, W. Ma; Formal analysis, Z. Wei, X. Song; W. Ma, and G. Yuan; Funding acquisition, W. Ma, J. Zhong; Investigation, C. Jiang and W. Ma.; Methodology, Z. Wei, B. Ma, G. Yuan, and Z. Cheng; Project management, W. Ma, J. Zhong and H. Matsuyama; Writing-original draft, Z. Wei, W. Ma; Writing-Reviewing and Editing, J. Zhong, W. Ma, and H. Matsuyama. All authors have read and agreed to the published version of the manuscript.

Funding and Competing interests: The authors did not receive support from any organization for the submitted work.

Acknowledgments:This work was supported by the “International Joint Lab of Jiangsu Education Department"; this work was partially supported by Kobe University Strategic International Collaborative Research Grant (Type B Fostering Joint Research); this work was also supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX22_3010, KYCX22_3036); Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University.

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Crosslinking modification of polyamide composite membranes toward high performance for DMAc/H2O dehydration

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Abstract

Figures

1. Introduction

2. Experimental

2.1 Materials and reagents

2.2 Preparation of PA TFC membranes by interfacial polymerization

2.3 Characterization of the PA TFC membranes

3. Results And Discussion

3.1 Physicochemical properties of the PA TFC membranes

3.2 Effect of operating conditions on the pervaporation performance of the TEPA-PA TFC membrane

4. Conclusions

Declarations

References

Supplementary Files

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