aliphatic mixtures

aliphatic mixtures

Journal of Membrane Science 575 (2019) 1–8 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier.com/...

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Journal of Membrane Science 575 (2019) 1–8

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Nano-array assisted metal-organic polyhedra membranes for the pervaporation of aromatic/aliphatic mixtures

T



Lu Liu, Naixin Wang , Hong-Xia Liu, Lun Shu, Ya-Bo Xie, Jian-Rong Li, Quan-Fu An Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-organic polyhedras Co(OH)2 nano-array Pervaporation Aromatic/aliphatic separation High flux

Metal-organic polyhedra (MOP), as a new type of molecule-based porous nanocage, is a promising material in membrane separation. However, it is a challenge to obtain pure MOP membrane to take full advantage of its nanocage. In this study, [Cu24(5-tBu-1,3-BDC)24(S)24] (MOP-tBu) membranes were prepared on the surface of ceramic tubular substrate with the assistance of Co(OH)2 nano-array. MOP-tBu molecules were dynamic pressure-driven assembled into the confinement space of vertically arranged Co(OH)2 nanosheets, which were previously formed on the substrate through hydrothermal synthesis. With this strategy, the MOP-tBu molecules can form an ultrathin defect-free separation layer on the surface of the substrate. Due to the open Cu2+ unsaturated sites (CUSs) and rich benzene rings in its structure, the MOP-tBu membrane has permselectivity towards the aromatic compounds. In addition, the MOP-tBu membrane is endowed with high permeability because of its large cavity and small apertures. Therefore, the membranes were used to separate aromatic/aliphatic mixtures through pervaporation. When separating 50 wt% toluene/n-heptane mixtures, the permeate flux of the MOP-tBu membrane was 800 g m−2 h−1 with the separation factor of 5.4. The high permeability of the membrane exhibited great potential in the separation of aromatic/aliphatic mixtures.

1. Introduction The separation of aromatic/aliphatic mixtures is an important process in the chemical industry [1,2]. However, due to the close physical and chemical properties of aromatic/aliphatic compounds, the separation of aromatic/aliphatic mixtures is also very difficult. The traditional techniques include azeotropic distillation, extractive distillation and liquid-liquid extraction [3–5]. In recent years, pervaporation as a membrane separation technique has been recognized as one of the most promising approaches because of its high efficiency and low energy consumption [6–9]. According to the solution-diffusion mechanism of pervaporation, the separation performance of the membrane depends primarily on the different solution and diffusion ability of aromatic and aliphatic compounds in membrane. Therefore, the membrane materials should have a stronger affinity towards aromatic compounds than aliphatic. In addition, the structure of the membrane also has a significant influence on the permeability of the membrane. Nowadays, the development of novel membrane materials with high separation performance is still the main task for the pervaporation of aromatic/aliphatic mixtures. The extensively explored membrane materials for separating aromatic/aliphatic mixtures are organic polymers, such as polyimides ⁎

[10–12], poly(ether amide)s [13,14], polyurethanes [15,16], poly(methyl methacrylate) [17,18], and poly(vinyl alcohol)s [19]. They are preferred due to the advantages of good membrane-forming property, abundant species, low cost, and easy fabrication. However, the separation performances of the polymer membranes are still unsatisfied due to the trade-off between permeability and selectivity [20–22]. Moreover, the stability of the polymer membrane in long-term running is usually poor because of the excessively swollen of organic solvents [22]. In order to improve the separation performance of the polymer membrane for aromatic/aliphatic mixtures, facilitated transport particles are incorporated into polymers to form mixed matrix membranes (MMMs). They can improve the interaction between aromatic compounds and membrane materials as well as increase the mass transfer channel of organic solvent molecules in membrane. At present, facilitated transport fillers that have been used in MMMs include organic macromolecules (cyclodextrin [23], chitosan [24] and calixarene [25]), inorganic materials (transition metal ions [26], molecular sieves [27], carbon nanotubes [28], graphite [29], graphene oxide [30] and silicon dioxide [31]), and metal-organic materials (MOMs). Among these nanofillers, MOMs which including metal-organic frameworks (MOFs) and metal-organic polyhedras (MOPs) have attracted many attentions in the field of membrane separation, such as

Corresponding author. E-mail address: [email protected] (N. Wang).

https://doi.org/10.1016/j.memsci.2018.12.081 Received 5 November 2018; Received in revised form 21 December 2018; Accepted 29 December 2018 Available online 03 January 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.

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Beijing Chemical Factory. 5-t-butyl-1,3-benzenedicarboxylic acid H2(5tBu-1,3-BDC) was purchased from Aldrich. Hexamethylenetetramine, tris (hydroxymethyl) aminomethan were purchased from Alfa Aesar. Nmethyl-2-pyrrolidone (NMP), cobalt nitrate was purchased from Tianjin fuchen chemical reagent factory. Dopamine was purchased from Ark Pharm.

gas separation [32–34], nanofiltration [35], reverse osmosis [36], and pervaporation [19,37,38]. Their high porosity and specific surface area can provide more molecular sieving and transport channels. Moreover, they can be prepared on demand because of the species diversity and designable structure of the MOM. Therefore, MOMs can also be used in the separation of aromatic/aliphatic hydrocarbon mixtures, because their unsaturated metal ions can form d-π conjugation interaction with aromatic molecules. Meanwhile, organic ligands can also interact with aromatic molecules by π-π conjugation. The adsorption selectivity of the membrane is thus improved due to the incorporation of MOMs particles in membrane. In our previous studies, MOFs and MOPs have been used as the nanofillers to improve the membrane separation performance for aromatic/aliphatic hydrocarbon mixtures. Cu3(BTC)2 was incorporated into PVA matrix to form Cu3(BTC)2/PVA hybrid membranes. The separation factor of the membrane was increased due to the d-π conjugation and π-π interaction between MOF particles and aromatic molecules [19]. Moreover, to improve the dispersion of MOM particles in polymer, porous nanocage MOP-tBu [Cu24(5-tBu-1,3BDC)24(S)24] was synthesized as fillers to prepare hybrid membrane [38]. The results indicated that both the selectivity and permeability were improved. Although the incorporation of MOM particles into polymer is an effective approach to improve the separation performance, there are still some problems for MOM hybrid membrane, such as the compatibility, dispersion and pore plugging problems. To solve these problems, pure MOM membrane can be prepared without polymer to take full advantage of its nanocage. However, to the best of our knowledge, the preparation of pure MOM membrane for separation of aromatic/aliphatic mixtures has not been reported. In this study, MOP-tBu was used as membrane material to form separation layer on ceramic tubular substrate with the assistance of Co (OH)2 nano-array. MOP-tBu molecules were dynamic pressure-driven assembled into the confinement space of vertically arranged Co(OH)2 nanosheets, which were previously formed on the surface of substrate through hydrothermal synthesis (Scheme 1). A dense and defect-free MOP-tBu separation layer was formed. The morphology and property of the MOP-tBu composite membranes were characterized. Due to the open Cu2+ unsaturated sites (CUSs) and rich benzene rings in its structure, the MOP-tBu membrane has permselectivity towards the aromatic compounds. Therefore, the membranes were used for separating aromatic/aliphatic mixtures. Because no polymer was used in the preparation process, the large cavity and small apertures of MOP-tBu was fully developed. MOP-tBu membranes with high permeate flux are expected to obtained for pervaporation separation of aromatic/aliphatic mixtures.

2.2. Synthesis of MOP-tBu MOP-tBu was prepared according to reported literature [39]. 445 mg 5-t-butyl-1,3-benzenedicarboxylic acid H2(5-tBu-1,3-BDC) and 400 mg copper acetate monohydrate (Cu2(OAc)4·H2O) were dissolved in 20 mL of N, N-dimethylacetamide (DMA), respectively. Then, the two solutions were mixed and stirred for 30 min at room temperature. Subsequently, 10 mL methanol was added to the mixtures at room temperature. Finally, blue crystals were collected by centrifugation after 20 days. Subsequently, the crystals were washed with methanol three times and dried at room temperature for 24 h. 2.3. Preparation of Co(OH)2 membrane To enhance the adhesion between the separation layer and the substrate, the alumina tubular substrate was pre-treated by dopamine. The ceramic tube was first cleaned by deionized water and exchanged with ethanol, followed by dried in an oven at 110 °C for 2 h. Then the tube was immersed in a mixture of H2O (350 mL), THAM buffer (2.1175 g), dopamine (0.7 g), CuSO4·5H2O (0.4368 g), and H2O2 (0.7 mL) for 10 min, followed by rinsing with deionized water. At last, the substrate was dried in a vacuum oven at 50 °C to remove water. A certain amount of cobalt nitrate (3.492 g) and hexamethylenetetramine (1.682 g) were dissolved in deionized water (60 mL). After that, the PDA-treated alumina tubular substrate was vertically placed into a 100 mL Teflon-lined stainless vessel. Then, precursor solution was poured into the vessel and sealed. The vessel was put into an oven with the temperature pre-heated to 90 °C. After an elapsed time of 6 h, it was taken out and cooled to room temperature in air. The α-Co(OH)2 composite membrane was taken out and washed with abundant deionized water. Finally, the membrane was dried in a vacuum oven at 50 °C. 2.4. Preparation of MOP-tBu/Co(OH)2 composite membrane A certain amount of MOP-tBu was dissolved in N-methyl-2-pyrrolidone (NMP) to form casting solution. Different concentrations of casting solution were prepared. The preparation process of the dynamic pressure-driven assembly method for composite membranes was according to the previous study [40]. One end of the vessel was sealed with rubber plug and the other end was connected to the vacuum pump to remove the air. The ceramic tube was immersed in the casting solution and MOP-tBu molecules were filled into the Co(OH)2 nanosheets under transmembrane pressure at −0.095 MPa for 15 min, then the ceramic tube was take down and dried in a vacuum oven at 50 °C for 48 h, the prepared membranes was used for pervaporation tests.

2. Experimental 2.1. Materials Ceramic tubular substrate with 100 mm-length and 8.5 mm inner and 13.5 mm outer diameters were supplied by JieXi LiShun Technology Co., Ltd, China. Copper (II) acetate monohydrate, N,NDimethylacetamide (DMA), copper sulfate pentahydrate, methanol, ethanol, toluene, n-heptane, iso-octane, cyclohexane were provided by

2.5. Pervaporation experiments A laboratory-made pervaporation apparatus with an effective membrane area of 26 cm2 was used for the separation of aromatic/ aliphatic mixtures (such as toluene/n-heptane, benzene/cyclohexane, toluene/iso-octane and toluene/cyclohexane). Feed temperature was ranged from 30 °C to 80 °C. The pressure on the downstream side of the membrane is 500 Pa. The permeated vapor was trapped in liquid nitrogen. For each pervaporation, 1 h was needed to stabilize the membrane for a stable performance. Three samples were collected at 1 h intervals for each membrane. The condensed permeate in the trap was weighed by electronic analytical balance. Then the composition of the

Scheme 1. Schematic illustration of the formation of MOP-tBu/Co(OH)2 membrane. 2

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prepared. Furthermore, the ceramic substrate, Co(OH)2 powder, Co (OH)2 membrane and MOP-tBu/Co(OH)2 composite membrane were also characterized by PXRD. As shown in Fig. 1(b), it can be found that the composite membrane contains both the characteristic peaks of alumina substrate and α-Co(OH)2, demonstrating that α-Co(OH)2 nanosheets were successfully formed on the surface of the ceramic tube. Since the α-Co(OH)2 nanosheets were arranged with their ab-direction perpendicular to the substrate, the intensity of the (003) diffraction peak was strongly weakened in the α-Co(OH)2 membrane. Moreover, we can also see that the diffraction peak of MOP-tBu was disappeared in the MOP-tBu/Co(OH)2 composite membrane. It was because the MOPtBu is soluble molecule, which should be in a state of disordered arrangement when dissolved in solvent, being different from those in their crystalline state. The similar results have also been found in our previous study [37]. The surface and cross-section morphologies of the ceramic substrate, Co(OH)2 membrane and MOP-tBu/Co(OH)2 composite membrane were characterized by SEM. As shown in Fig. 2(a) and (d), the ceramic substrate was composed of spherical alumina particles and the surface of the substrate was full of defects. After in situ grown through hydrothermal reaction, nano-scaled Co(OH)2 sheets were grown perpendicular to the substrate and many gaps can be found among the Co (OH)2 sheets (Fig. 2(b) and (e)). However, when the MOP-tBu was deposited in the Co(OH)2 layer through dynamic pressure-driven assembly (Fig. 2(c) and (f)), a dense MOP-tBu/Co(OH)2 separation layer was formed with the assistant of Co(OH)2 nano-arrays. Moreover, compared with Fig. 2(e) and (f), the thickness of the MOP-tBu layer was similar with the Co(OH)2 layer, which was approximately 500 nm. The surface morphology of ceramic substrate and the composite membranes were also investigated by AFM. As shown in Fig. 2(g)-(i), the surface average roughness of the ceramic substrate was 78 nm, which increased to 184 nm when the Co(OH)2 nanosheet were deposited on the surface. However, after assembly of MOP-tBu, the surface roughness decreased to 26 nm. It provided evidence that a smooth MOP-tBu layer was formed on the surface of the ceramic substrate. To further prove the formation of MOP-tBu/Co(OH)2 composite membrane, FTIR was used to characterize the MOP powder and the MOP-tBu/Co(OH)2 composite membrane. As shown in Fig. 3, the peaks at 484 and 625 cm−1 were attribute to the ν(Co–O) and the δ(Co–O–H) stretching vibrations in Co(OH)2 nanosheets. After filled with MOP-tBu, the peaks were still remained. Moreover, the MOP-tBu/Co(OH)2 composite membrane contained the characteristic peaks derived from benzene ring and carboxylic acid group in MOP-tBu. For example, the peaks at 1600 cm−1 and 1375 cm−1 indicated the stretching vibration of C˭O bond and C˭C bond on the benzene ring. Meanwhile, the peak at

collected permeate was determined used gas chromatography (FULI 9790II, China) with a flame ionization detector (FID). The permeation flux (J) and separation factor (α), which represent the permeability and selectivity, were calculated according to the following equation (1):

J=

W A·△t

(1) 2

where W (g) is the total mass of the permeate, A (m ) means the effective area of the membrane, and Δt (h) is the test interval. The separation factor (α) indicates the separation efficiency of the two substances by membrane separation and is usually calculated by formula (2-2):

α=

YA/ YB XA/ XB

(2)

where YA and YB indicate the mass fraction of aromatic and aliphatic compounds in the permeate; XA and XB represent the mass fraction of aromatic and aliphatic compounds in the feed, respectively. 2.6. Characterizations The surface and cross-section morphologies of the ceramic tubular substrate, Co(OH)2 membrane and MOP-tBu composite membranes were observed by SEM (Model SU8020, Hitachi, Japan). All membrane samples were immersed in liquid nitrogen and coated with gold nanoparticles in a vacuum to increase their conductivity before observation. Roughness of the membranes was investigated with an atomic force microscope (AFM) apparatus. Infrared spectrometer (IR Affinity-13, Japan) was used to test chemical bonds and functional groups of the membranes. Thermo gravimetric analysis (TGA) experiments were carried out at a heating rate of 10 °C/min. The Young's modulus and hardness of the hybrid membranes were characterized by a Nano Indenter G200 (Agilent Technology). The powder X-ray diffraction patterns (PXRD) were carried out under 40 kV and 40 mA in the scan range of 2θ from 3° to 80° with a scan step of 0.05°. 3. Results and discussion 3.1. Characterizations of the membranes To confirm the successful synthesis of MOP-tBu, the obtained powder was characterized by PXRD. We can see from Fig. 1(a) that the diffraction peaks of the as-synthesized powder matched well with the simulated MOP-tBu [39], indicating the MOP-tBu was successfully

Fig. 1. (a) PXRD pattern of as-synthesized MOP-tBu sample; (b) PXRD pattern of tubular ceramic substrate, MOP-tBu powder, Co(OH)2 membrane and MOP-tBu/Co (OH)2 composite membrane. 3

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Fig. 2. SEM images of surface of (a) tubular ceramic substrate, (b) Co(OH)2 membrane, (c) MOP-tBu/Co(OH)2 composite membrane and cross-section of (d) tubular ceramic substrate, (e) Co(OH)2 membrane, (f) MOP-tBu/Co(OH)2 composite membrane; AFM images of (g) tubular ceramic substrate Ra = 78 nm), (h) Co(OH)2 membrane Ra = 184 nm) and (i) MOP-tBu/Co(OH)2 composite membrane Ra = 26 nm). (Preparation conditions: dynamic assembly time, 15 min; concentration of MOP-tBu, 2.75 g/L).

significant influence on the morphology of the composite membrane. Therefore, the effect of assembly time on membrane morphology was also investigated. It can be seen from Fig. 4 that MOP-tBu particles were gradually filled into Co(OH)2 nano-arrays with the increasing time. When the assembly time was less than 15 min, parts of the gaps among the layers were filled by MOP-tBu. When the assembly time reached 15 min, all the gaps among Co(OH)2 sheets were filled with MOP-tBu. A dense and smooth separation layer was thus formed. Moreover, compared with Fig. 4(g) and (h), we can find that with the extension of assembly time, the thickness of the MOP-tBu layer had no obvious change with the compactness improved. The deposition of MOP-tBu with the dynamic assembly time can also be investigated by TGA. As shown in Fig. 5(a), the thermodynamic properties of Co(OH)2 powder, MOP-tBu powder and MOP-tBu/Co (OH)2 composite membrane were studied. The TGA curve of MOP-tBu showed that when the temperature was less than 300 °C, there was a slight loss of mass, which was caused by the removal of guest molecules in the MOP molecular channel. When the temperature was higher than 400 °C, the weight was decreased to about 18% due to the decomposition of components. The weight loss of Co(OH)2 nanosheets included two steps, which was caused by the dehydration and deposition. The reason of the weight loss below 200 °C was that water molecules removed from nanosheets. The rapid decline occurred between 200 °C and 250 °C, owning to the decomposition of the hydroxyl group. This also indicated the basic completion of dehydroxylation. After that,

Fig. 3. FTIR spectra of Co(OH)2 powder, MOP-tBu powder and MOP-tBu/Co (OH)2 membrane.

730 cm−1 represented the vibration of carboxylic acid group. These results also indicated that a MOP-tBu/Co(OH)2 composite layer was formed on the surface of the substrate. Because of the MOP-tBu was filled in the Co(OH)2 nano-arrays using dynamic negative pressure assembly, the assembly time may have a 4

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Fig. 4. SEM images of (a-d) surface and (d-f) cross-section of the MOP-tBu/Co(OH)2 composite membranes prepared under dynamic assembly time of 5, 10, 15, 20 min (Preparation conditions: concentration of MOP-tBu, 2.75 g/L).

Fig. 5. (a) TGA curves of Co(OH)2 powder, MOP-tBu powder and MOP-tBu/Co(OH)2 membrane; (b) TGA curves of MOP-tBu/Co(OH)2 composite membrane prepared under different dynamic assembly time.

Fig. 7. Effect of concentration of MOP-tBu solution on pervaporation performance of membranes. (Preparation conditions: dynamic pressure-driven assembly process time 15 min; Pervaporation conditions: feed temperature 40 °C, feed composition 50 wt% toluene/n-heptane mixtures).

Fig. 6. Effect of the assembly time on pervaporation performance of MOP-tBu/ Co(OH)2 membranes. (Preparation conditions: concentration of MOP-tBu solution 2.75 g/L; Pervaporation conditions: Feed temperature 40 °C, feed composition 50 wt% toluene/n-heptane mixtures).

5

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Fig. 10. Performances of MOP-tBu/Co(OH)2 membranes for the pervaporation separation of 50 wt% aromatic/aliphatic mixtures at 40 °C. (Preparation conditions: dynamic pressure-driven assembly time 15 min, concentration of MOPtBu solution 2.75 g/L).

Fig. 8. Effect of feed temperature on pervaporation performance. (Preparation conditions: dynamic pressure-driven assembly process time 15 min, concentration of MOP-tBu solution 2.75 g/L; Pervaporation conditions: Feed composition 50 wt% toluene/n-heptane mixtures).

separation factor and permeation flux. The separation factor of Co(OH)2 membrane for toluene/n-heptane mixtures is 1.1, which means the Co (OH)2 membrane had no separation performance for toluene/n-heptane mixtures. Meanwhile, the permeate flux for Co(OH)2 membrane was 251 g/m2h, which indicated that Co(OH)2 nanosheets can form a dense separation layer on the ceramic substrate. However, when the MOP-tBu was assembled in the Co(OH)2 nano-array, both the separation factor and permeate flux increased. It may attribute to the high interaction between MOP and aromatic compounds. The d orbital of Cu2+ and π orbital of benzene rings in MOPs can coordinate with the π orbital of aromatic molecules. Both d-π conjugation and π-π conjugation are helpful to improve the adsorption selectivity and permeability of toluene on MOP-tBu/Co(OH)2 composite membrane. With the assembly time increased from 5 min to 20 min, the thickness of MOP-tBu has no obvious change while the compactness improved (Fig. 4). It indicated that more MOP-tBu cannot be assembled on the surface of the composite membrane without the assistance of Co(OH)2 array. The thickness of the MOP-tBu membrane was limited by the Co(OH)2 layer. The assembly time only affected the amount and the compactness of MOP-tBu in the Co(OH)2 array. Since the MOP-tBu has a large cavity of 16 Å and two types of small apertures of about 4.5 Å and 6.2 Å, respectively, these pores are large enough to allow the n-heptane molecule (4.3 Å) and toluene molecule (5.9 Å) to enter into the molecule cavities. Therefore, the compactness change has less influence on the mass transfer of n-heptane and toluene molecules. When the assembly time increased from 5 to 15 min, the amount of MOP-tBu in the Co(OH)2 array increased to enhance the separation factor and permeate flux. When the assembly time increased from 15 to 20 min, the MOP-tBu cannot be deposited on the surface of the membrane because of the MOP-tBu layer was enough compact. Subsequently, the separation performance has no obvious change. Fig. 7 exhibited the effect of the concentration of MOP-tBu solution on separation performance. It can be found from the results that when the MOP-tBu content increased from 1.65 g/L to 2.75 g/L, the separation factor increased from 2.3 to 5.4, while the permeate flux increased from 521 g/m2 h to 800 g/m2 h. When the MOP-tBu content kept on increasing from 2.75 g/L to 4.95 g/L, the separation factor almost unchanged with the permeate flux slightly increased from 800 g/m2 h to 868 g/m2 h. It was because increasing MOP-tBu concentration can enhance its loading in unit time. However, the excessive concentration is useless to improve the separation performance.

Fig. 9. Effect of toluene concentration in feed on pervaporation performance. (Preparation conditions: dynamic pressure-driven assembly time 15 min, concentration of MOP-tBu solution 2.75 g/L; Pervaporation conditions: feed temperature 40 °C).

when the temperature reached 250 °C, the black residue of thermally oxidative Co3O4 was remained. To characterize the thermodynamic property of MOP-tBu/Co(OH)2 composite membranes, the samples of MOP-tBu/Co(OH)2 composite membranes were scraped carefully from the ceramic tube. We can see that the MOP-tBu/Co(OH)2 composite membranes and Co(OH)2 nanosheets followed the similar decomposition path. Moreover, we can also found from Fig. 5(b) that the weight loss of MOP-tBu/Co(OH)2 composite membranes gradually decreased with the increasing of dynamic pressure-driven assembly time. When the assembly time increased from 5 min to 20 min, the residual mass which should be copper oxide increased from about 20–45%. It provided the evidence that with the assembly time prolonged, more MOPtBu particles were deposited on the membrane surface. 3.2. Effects of preparation conditions on the pervaporation of toluene/nheptane mixture The obtained MOP-tBu/Co(OH)2 composite membranes were used to separate 50 wt% toluene/n-heptane mixtures. The effects of preparation conditions on the pervaporation performance were investigated. As shown in Fig. 6, we can see an anti-trade-off behavior for 6

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Table 1 Comparison of aromatic/aliphatic separation performance for different membranes. Membrane material

Feed solution

Temperature (°C)

Separation factor

Permeate flux g/(m2h)

Reference

PBG/PAI-SO2 SEC/BDDMAC Polyurethane APAF-6FDA PVA-GO PVC-Nanocor clay MWNTs/CS PEG-DMA/β-CD Co(HCOO)2/PEBA Co(HCOO)2/PEBA MOP-tBu/Co(OH)2 MOP-OH/Co(OH)2 MOP-SO3NanHm/Co(OH)2

Toluene/n-heptane Toluene/n-heptane Toluene/n-heptane Toluene/n-heptane Toluene/n-heptane Toluene/n-heptane Benzene/cyclohexane toluene/cyclohexane toluene/cyclohexane toluene/iso-octane Toluene/n-heptane Toluene/n-heptane Toluene/n-heptane

40 80 50 80 40 74 60 60 40 40 40 40 40

3.8 3.4 4.8 1.7 12.9 4 2.63 14 4 7.2 5.4 6.2 7.6

280 650 167 12.1 27 60 237.01 5.45 685 826 800 860 950

[6] [7] [8] [9] [30] [41] [28] [23] [40] [40] This study This study This study

PBG: poly(γ-benzyl-l-glutamate); PAI: polyamide-imide; SEC: sulfoethylcellulose; BDDDMAC: benzyl dodecyl dimethyl ammonium chloride; APAF: 2,2-Bis (3-amino4-hydroxyphenyl) hexafluoropropane; 6FDA: 2,2-Bis(3,4-dicarboxyphenyl) hexa-fluoropropane dianhydride; PVA: polyvinyl alcohol; GO: graphene oxide; PVC: polyvinyl chloride; MWNTs: multiwalled carbon nanotubes; CS: chitosan; PEG: polyethylene glycol; DMA: Dimethylacetamide; CD: cyclodextrin; PEBA: poly(etherblock-amide).

important role toward toluene/n-heptane. From the adsorption experiments and molecular simulations of our previous work [37], it is concluded that stronger polar functionalized groups have higher adsorption ability toward the aromatic molecular, which was well-matched with the result of separation performance. The pervaporation performances of the MOP/Co(OH)2 membranes and the reported membranes were compared in Table 1. The results showed that the permeation flux of the MOP/Co(OH)2 membranes were higher than others, while the separation factor was also competitive. Therefore, the MOP/Co(OH)2 membranes has a potential in pervaporation separation of aromatic/aliphatic mixtures.

3.3. Effects of operation conditions on the pervaporation of toluene/nheptane mixture The operation conditions also have important influences on the separation performances. As shown in Fig. 8, it was found that with the increasing feed temperature from 30 °C to 70 °C, the separation factor gradually declined but the permeate flux increased. As both of the nheptane molecule and n-heptane molecule can pass through MOP-tBu, the mixtures cannot be separated by molecular sieving effect of the pores in MOP-tBu. When the feed temperature increased, the thermal motion of molecules enhanced, resulting in the rapid permeation of nheptane molecules. Consequently, the effect of adsorption selectivity decreased to reduce the separation factor and increase the permeation flux. Fig. 9 shows the effect of toluene concentration in feed on pervaporation performance of membranes. We can see from the results that with the toluene content in feed increased from 10 wt% to 50 wt%, the total permeate flux increased from the 346 g/m2 h to 800 g/m2 h, while the separation factor decreased from 7.5 to 5.4. The reason can be explained by the partial flux of toluene and n-heptane. When the toluene in feed increased, the toluene flux increased sharply while the nheptane flux had no obvious decline. It may be attributing to the adsorption selectivity of the MOP-tBu membrane for toluene/n-heptane mixtures. Apart from separation of toluene/n-heptane mixtures, the MOP-tBu/ Co(OH)2 membranes can also be used to separate other aromatic/aliphatic mixtures, such as toluene/cyclohexane, toluene/iso-octane and benzene/cyclohexane mixtures. The separation results are shown in Fig. 10. It is found that the MOP-tBu/Co(OH)2 membranes had the best separation performance for toluene/iso-octane mixtures, which was because of the great different physical and chemical properties between the two molecules.

4. Conclusion In summary, MOP-tBu was assembled into Co(OH)2 array to obtain composite membranes for the separation of aromatic/aliphatic mixtures through a dynamic assembly method. The thickness of the MOP-tBu layer can be controlled by the Co(OH)2 array. Owing to the adsorption selectivity of MOP molecules for aromatic compound as well as the high porosity, the MOP-tBu/Co(OH)2 membranes exhibited high flux for pervaporation of aromatic/aliphatic mixtures. When the feed solution is 50 wt% toluene/n-heptane mixtures, the permeate flux is 800 g/(m2h) with separation factor of 5.4. Furthermore, different kinds of MOPs can be assembled into the Co(OH)2 array using this strategy. All of the MOP composite membranes show great potential in the aromatic/aliphatic pervaporation. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21776003), the Beijing Natural Science Foundation (2182004), the Beijing Nova Program (Z181100006218079) and the Importation and Development of HighCaliber Talents Project of Beijing Municipal Institutions (CIT& TCD20170305).

3.4. The comparison of aromatic/aliphatic separation performance for different membranes

References Inspired by the previous work of our group, the pervaporation separation of functionalized MOP-X/Co(OH)2 composite membranes with different groups was explored (X = SO3NanHm(n + m=1), OH, and tBu). The separation performances of the membranes were shown in Table 1. It can be found that the MOP-SO3NanHm/Co(OH)2 composite membrane exhibited the best pervaporation performance in the three membranes. The separation factor of the MOP-SO3NanHm membrane is 7.6 and the permeate flux can reach 950 g/m2 h. The reason could be that the highly polar sulfonate groups in the MOP-SO3NanHm played an

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