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Seeding-free synthesis of large tubular zeolite FAU membranes for dewatering of dimethyl carbonate by pervaporation
Microporous and Mesoporous Materials 292 (2020) 109713
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Seeding-free synthesis of large tubular zeolite FAU membranes for dewatering of dimethyl carbonate by pervaporation
T
Junjie Zhoua,b, Chen Zhoua,b,∗, Kai Xua, Jürgen Caroc, Aisheng Huanga,d,∗∗ a
Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China c Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstraße 3-3A, D-30167, Hannover, Germany d Department of Chemistry, East China Normal University, Dongchuan Road 500, 200241, Shanghai, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite FAU membrane Dimethyl carbonate Pervaporation Covalent modification
Thin, phase-pure and well inter-grown zeolite FAU membranes are synthesized upon modifying the tubular supports using 3-aminopropyltriethoxysilane (APTES). The as-synthesized zeolite FAU membranes are evaluated in the dewatering of dimethyl carbonate (DMC) by pervaporation. Due to the large pore size (0.74 nm) and high hydrophilicity of zeolite FAU, the prepared zeolite FAU membranes exhibit high water flux and separation performance for the separation of H2O and DMC. With increasing the temperature or water content in the feed, the zeolite FAU membranes show higher flux while maintaining high separation factors. Further, for dewatering of 10 wt% H2O/90 wt% DMC mixtures at 353 K in a lab device, the DMC concentration in the feed side reaches 99% after continuously dewatering over 9 h. Besides, the zeolite FAU membranes are also successfully prepared on commercial tubular size of 80 cm and 100 cm long α-Al2O3 tubes. The obtained membranes show high separation performance for the dewatering of DMC solution as well. The high-performance zeolite FAU membrane offers great potential for dewatering of DMC solutions in industry.
1. Introduction In the past two decades, zeolite membranes have received tremendous amount of attention as a new type inorganic membrane material with “sieving property” [1]. Attributing to the well-defined framework structure with molecular size level channels/pores, excellent swelling resistance, high thermal and chemical stability, zeolite membranes tender true potential as separators, reactors, sensors, microelectronic devices [2–9]. To date, many types of zeolite membranes, such as MFI, CHA, LTA, and FAU, have been prepared on porous supports to separate gas and/or liquid mixtures, allowing or excluding molecules to pass through the zeolite pores depending on their molecular sizes [10–18]. FAU-type (including NaX with Si/Al ratio of 1.0–1.5 and NaY with Si/Al ratio of 1.5–3) zeolite membranes with a pore size of 0.74 nm due to the 12-membered oxygen rings, can be used for the separation of large molecules which cannot be handled by MFI and/or LTA with the advantage of its relatively large pore size of FAU (0.74 nm). In addition,
zeolite FAU membranes with low Si/Al ratio possess high hydrophilicity, which can be effectively utilized in dehydration of organic solvents by pervaporation or steam permeation. Recently, zeolite FAU membranes are increasingly used in the separation of isopropanol/ water, tetrahydrofuran/water, methanol/methyl tertbutylether, ethanol/cyclohexane [19–21]. These separations are based on the hydrophilicity of zeolite FAU as well as molecular sieving. Dimethyl carbonate (DMC) is incrementally produced for its use as fuel additive, methylating, carbonylating, and methoxylating. Furthermore, DMC is widely used in medicine, for pesticides, as solvent, composite material, dyestuff, flavoring agent of foodstuff and chemical in electronic industry [22–28]. The main technology of DMC production is the oxidative carbonylation of methanol, besides the direct synthesis of DMC from methanol and carbon dioxide becomes interesting as well. All these synthesis technologies result in a mixture DMC/methanol/water. This mixture can be separated by distillation and ends up with a DMC/ H2O [29]. At 353 K, the binary system DMC/H2O forms an azeotrope with xDMC ≈ 0.559 [30]. At room temperature, binary DMC/H2O
∗ Corresponding author. Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China. ∗∗ Corresponding author. Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China. E-mail addresses: [email protected] (C. Zhou), [email protected] (A. Huang).
https://doi.org/10.1016/j.micromeso.2019.109713 Received 26 January 2019; Received in revised form 26 April 2019; Accepted 8 September 2019 Available online 09 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
Microporous and Mesoporous Materials 292 (2020) 109713
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till the solution was clear and homogenous. And then the porous APTES-modified α-Al2O3 tubes were placed into a Teflon stainless steel autoclave and immersed in the prepared solution. After the in-situ growth at 348 K for 24 h, the membrane was taken out and washed with the deionized water several times. And then the as-synthesized zeolite FAU membranes were dried at 333 K in an oven overnight for characterization and permeation measurement. For the synthesis of scale-up zeolite FAU membrane on the α-Al2O3 tubes with the length of 80 cm and 100 cm, a reaction mixture containing sodium hydroxide (295.64 g), aluminum foil (2.85 g), colloidal silica (158.46 g) and deionized water (1795 ml) was obtained after stirring for 24 h. Then the long APTES-modified porous supports were placed into the auto-calve and immersed in the reaction mixture. After the in-situ growth at 348 K for 24 h, the membrane was taken out, washed several times with deionized water and then dried at 333 K in the oven.
solutions are homogeneous for low (< 15 wt%) and high (> 97 wt%) DMC content [31]. Thus, more efforts should be devoted towards developing a zeolite FAU membrane with a high-separation performance for DMC/H2O mixtures for the selective dewatering of DMC-rich binary solutions. Marrying the large pore size of zeolite FAU with high hydrophilicity which may offer high flux, many efforts have been paid on the synthesis of dense and phase-pure zeolite FAU membranes. Direct synthesis is a conventional method to prepare zeolite FAU membranes. Nevertheless, it is extremely difficult to form a dense zeolite FAU molecular sieve membrane in virtue of the poor heterogeneous nucleation of FAU on the support surface [29]. Another conventional strategy for the synthesis of zeolite FAU membranes is secondary growth. Zeolite seeds are firstly synthesized by direct synthesis strategy, and coated on the substrate by rub-coating, dip-coating or electrostatic attraction [30]. The secondary growth method exhibits many advantages, for instance, preferable control of the membrane thickness and orientation. However, the second growth method is sustained in a lab-scale because of the multistep preparation. Therefore, the development of a reliable synthesis road is highly desired for the large scale preparation of zeolite FAU membranes. Recently, we have developed a seeding-free strategy for the synthesis of zeolite and metal organic framework (MOF) membranes. Upon treating the porous support with covalent linkers, 3aminopropyltriethoxysilane (APTES), 3-chloropropyltrimethoxysilane (CPTMS), 1,4-diisocyanate (DIC-4), polydopamine (PDA), the membrane precursors can be easily anchored via covalent bonds and/or noncovalent adsorption, thus the membranes grow directly on the modified substrates [11,12,31,32]. We have successfully obtained LTA, FAU, ZIF8 and ZIF-90 membranes through the covalent support modification method [12,31,33–35]. In the present work, except the synthesis of zeolite FAU membrane on 7.5 cm long length tubes, commercial size αAl2O3 tubes with 80 and 100 cm long length are used and modified with APTES. Then thin, phase-pure and well inter-grown zeolite FAU membranes are obtained on the surface of the α-Al2O3 tubes. The as-synthesized zeolite FAU membranes are evaluated in the dewatering of DMC solutions by pervaporation. The zeolite FAU membranes prepared on the lab-scale tube (7.5 cm) and large-scale tubes (80 cm and 100 cm) exhibit high-separation performance towards separating of H2O from DMC solution.
2.3. Characterizations of zeolite FAU membranes The morphology and thickness of the zeolite FAU membranes were characterized by field emission scanning electron microscopy (FESEM) on an S-4800 (Hitachi) with a cold field emission gun. Energy-dispersive X-ray spectroscopy (EDXS) was utilized to detect the surface chemical composition of the zeolite FAU membranes. X-ray diffraction (XRD) was applied to confirm the phase structure of the zeolite FAU membranes at room temperature under ambient pressure, which was recorded on a Bruker D8 Advance operating at 40 kV and 40 mA with a Cu Kα1 radiation source (λ = 0.154056 nm) at a stepwise increase of 2°·s−1 in the Bragg angle (2θ) range from 5° to 40°. The Fourier Transform IR (FT-IR) spectra of the non-modified and APTES-modified α-Al2O3 supports were characterized in the absorbance mode in a Nicolet 6700 spectrophotometer. Spectra were collected at 4 cm−1 resolution averaging 32 scans.
2.4. Evaluation of zeolite FAU membranes in the dewatering of DMC by pervaporation The separation performance of the zeolite FAU membranes for the dewatering of DMC solutions was evaluated by using the pervaporation testing system shown in Fig. 1. The supported zeolite FAU membranes were sealed in a permeation module with silicone O-rings. The pressure on the retentate side was kept at atmospheric pressure and the permeate side was evacuated to below 100 Pa by a vacuum pump. The permeate solution was collected in liquid nitrogen freezing traps. The separation performances of the zeolite FAU membranes were evaluated in two ways: 1) dewatering of DMC solutions at different temperatures and concentrations, and 2) continuous dewatering of a feed with the composition 90 wt% DMC/10 wt% H2O at 353 K. In the first way, the DMC solution was prepared by mixing DMC with deionized water, and the corresponding DMC concentrations ranged from 90 wt% to 97.6 wt %. The operation temperature was kept at 303 K, 313 K, 333 K and 353 K. The initial solution weight was 1000 g. In the second way, the DMC concentration was 90 wt% and the initial solution weight was 600 g. The DMC solution was dewatered continuously until the DMC concentration in the feed side reached above 99%. The separation performance was assessed in terms of flux and separation factor (αi,j). The water flux J (kg·h−1·m−2), was calculated by
2. Experimental 2.1. Materials Chemicals were used as received: LUDOX AS-40 colloidal silica (40% SiO2 in water, Sigma-Aldrich); aluminum foil (99.99%, Aladdin); sodium hydroxide (> 98%, Aladdin); 3-aminopropyltriethoxysilane (APTES, 98%, Aladdin). Porous α-Al2O3 tubes (JiexiLishun Technology Co., Guangdong, China: 12 mm outside diameter, 9 mm inside diameter, 75 mm length, ca. 1.0 μm pore size, 30% porosity) were utilized as supports. 2.2. Preparation of zeolite FAU membranes The zeolite FAU membranes were prepared on the porous α-Al2O3 tubes according to the procedure shown elsewhere [12]. The porous αAl2O3 tubes were treated with APTES (0.2 mM in the 10 mL toluene) at 383 K for 1 h, leading to APTES deposited on the surface of α-Al2O3 tubes. A clear synthesis solution with the molar ratio of 70Na2O: 1Al2O3: 20SiO2: 2000H2O was used as the precursor solution. Typically, the aluminum solution was obtained by dissolving 15.56 g sodium hydroxide in 50 g deionized water and then 0.15 g aluminum foil was added to the solution at room temperature. Meanwhile, the silicate solution was prepared by mixing 8.34 g LUDOX AS-40 colloidal silica and 44.44 g deionized water under stirring. After stirring about 2 h, the silicate solution was poured into the aluminum solution and stirred 24 h
J=
W Δt⋅A
(1)
where W is weight of the permeate (kg), Δt is collecting time (h), A is separation area of the membrane (m2). The separation factor αi,j, was defined as the ratio of the composition concentrations in the permeate and that in the feed, and given by 2
Microporous and Mesoporous Materials 292 (2020) 109713
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Fig. 1. Schematic diagram of the dewatering of H2O/DMC mixture by pervaporation using tubular FAU membranes.
α i/j=
xip xif ⋅ xif xjp
(2)
xip is the weight fraction of species i in the permeate and xif is the weight fraction of species i in the feed. The αi,j is the separation factor of the species i and j. The composition of the permeation was analyzed by gas chromatography. 3. Results and discussions 3.1. Synthesis and characterization of zeolite FAU membranes Upon immersing the α-Al2O3 tube into the APTES solution at 383 K, APTES immediately reacts with the surface hydroxyl group and an aminopropylsilane layer is formed on the α-Al2O3 tube surface. To confirm that, Fourier transform infrared (FT-IR) spectroscopy data of the supports before and after APTES modification are collected. As shown in Fig. 2, the FT-IR spectrum of the sample scaled off from the surface of α-Al2O3 tube match well with that of aminopropylsilane. Moreover, the bands at 1562 cm−1 and 1470 cm−1 are assigned to N–H2 vibration in the primary amine group (R–NH2), the presence of the amino groups after modification indicates that APTES has been reacted with the surface of α-Al2O3 tube.
Fig. 2. FT-IR spectrum of the un-modified and APTES-modified α-Al2O3 supports.
Fig. 3. Top view (a) and cross-section view (b) FESEM images of the zeolite FAU membrane prepared on non-modified α-Al2O3 tube; top view (c) and cross-section view (d) FESEM images of the zeolite FAU membrane M1 from Table 1 prepared on APTES-modified α-Al2O3 tube. 3
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Fig. 4. Top view FESEM image of zeolite FAU membrane M1 from Table 1 prepared on APTES-modified α-Al2O3 tube (left) and the elements analysis of the zeolite FAU membrane top layer derived from EDXS before the pervaporation (right).
Table 1 Dewatering performances of the zeolite FAU membranes prepared on APTESmodified α-Al2O3 tubes of different length at 353 K for 90 wt% DMC/10 wt% H2O. Membrane number
Membrane length (cm)
Flux (kg·m2·h−1)
H2O/DMC selectivity
M1 M2 M3 M4 M5 M6 M7 M8 M9
7.5
3.60 3.59 3.64 2.15 2.33 2.26 1.83 1.87 1.95
> 10000 > 10000 > 10000 > 7000 > 7000 > 7000 > 5000 > 5000 > 5000
80
100
Fig. 3 shows the typical FESEM images of a zeolite FAU membrane prepared on the un-modified and APTES-modified α-Al2O3 tubes. Without APTES modification before hydrothermal synthesis, no dense zeolite FAU membrane can be obtained, and inter-crystal voids are easily observed in the membrane layer (Fig. 3a and b). However, with APTES modification, the zeolite FAU precursors can be easily adhered onto the surface of the α-Al2O3 tube, thus a dense zeolite FAU membrane with a thickness of about 3.0 μm can be formed on the α-Al2O3 tube without visible cracks, pinholes, and other defects (Fig. 3c and d). Via the formation of covalent bonds between zeolite membrane and support surface, APTES acts as a highly efficient molecular linker which
Fig. 5. XRD patterns of the zeolite powder (a) and the zeolite FAU membrane M1 from Table 1 prepared on the APTES-modified 80 cm long α-Al2O3 tubes before pervaporation (b). (♦): Al2O3 support.
Fig. 6. Large-scale synthesis of the zeolite FAU membrane on long length α-Al2O3 tubes. 4
Microporous and Mesoporous Materials 292 (2020) 109713
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Table 2 Fluxes and separation factors H2O/DMC of zeolite FAU membrane (M1 in Table 1) prepared on the modified α-Al2O3 tube by pervaporation as a function of evaluation temperatures and DMC contents. 90%
303 K 313 K 333 K 353 K
95%
96.5%
97.4%
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
0.52 0.92 2.08 3.60
> 10000 > 10000 > 10000 > 10000
0.43 0.80 1.39 2.65
> 5000 > 5000 > 5000 > 5000
0.40 0.64 1.12 2.11
> 5000 > 5000 > 5000 4754
0.13 0.32 0.47 1.08
1377 1264 2347 2347
Fig. 9. Water flux and separation factor of the zeolite FAU membrane M1 from Table 1 for the continuous dewatering of a 90 wt% DMC solution at 353 K as a function of the operation time.
Fig. 7. Flux and separation factor of the zeolite FAU membrane M1 from Table 1 for the pervaporation of a 95 wt% DMC/5 wt% water solution as a function of temperature.
Fig. 8. Arrhenius plot of the water flux through zeolite FAU membrane M1 from Table 1 and the pervaporation temperature in the pervaporation of a 95 wt % DMC/5 wt% water solution.
Fig. 10. XRD patterns of the zeolite powder (a) and the zeolite FAU membranes prepared on APTES modified α-Al2O3 tubes after pervaporation (b). (♦): Al2O3 support.
Table 3 Fluxes and separation factors 95 wt% DMC/MeOH of zeolite FAU membrane prepared on the modified α-Al2O3 tube by pervaporation. Temperature (K)
Flux (kg·m2·h−1)
Separation factor
303 313 333 353
0.20 0.25 0.36 0.81
1766 1470 1341 841
can in-situ attract and bind the FAU nutrients onto the support surfaces to boost the nucleation. The enhancement of nucleation on the APTESmodified α-Al2O3 tube can be clearly observed according to the membrane morphology (Fig. 3c and d). The chemical composition of the zeolite FAU membrane on the APTES-modified α-Al2O3 tube is detected by EDXS. As shown in Fig. 4, the zeolite FAU membrane contains the elements C, N, O, Na, Al and Si. The Si/Al atomic ratio is found to be 1.2 which is consistent with that of zeolite FAU. The formation of a phase-pure zeolite FAU membrane with a high 5
Microporous and Mesoporous Materials 292 (2020) 109713
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Fig. 11. Top view FESEM image of zeolite FAU membrane M1 of Table 1 after continuous pervaporation (left). EDXS elements analysis of the top layer of this zeolite FAU membrane (right).
where Ji is the permeate flux of the membrane, Ai is the pre-exponential factor, R is the gas constant, T is the absolute temperature and Ep,i is the apparent activation energy. Although the kinetic diameter of DMC (0.47 nm < dDMC < 0.63 nm) is smaller than the pore size of zeolite FAU (0.74 nm), the low Si/Al ratio (1.2) of the zeolite FAU membrane provides good hydrophilicity, thus the zeolite FAU membrane shows excellent separation performance towards dewatering DMC. In addition, it is also very important to remove methanol (MeOH) from the mixture during the commercial process for DMC production. Thus the separation of MeOH from DMC is evaluated as well and the corresponding results are shown in Table 3. As shown in Table 3, at 303 K, the separation factor exceeds 1700 for separation MeOH from DMC although the flux is 0.20 kg m2 h−1. With increasing the temperature to 353 K, the flux reaches 0.81 kg m2 h−1 while maintaining high separation selectivity. Compared with the flux of H2O from DMC, the relatively low flux in separating DMC and MeOH mixture might be attributed to two reasons: 1) zeolite FAU has high hydrophilicity which allows H2O to pass through rapidly; 2) the kinetic diameter of MeOH is bigger than that of H2O which leads to slow diffusion rate through the zeolite FAU membrane. Generally, the zeolite FAU membrane exhibits good separation performance towards the removal of MeOH from DMC solution, offering the possibility to purify DMC from the reaction mixture during the commercial process for DMC production.
degree of crystallinity was confirmed by XRD (Fig. 5). All peaks match well with those of zeolite FAU powder except the α-Al2O3 signals from the support. In our previous report [12], due to the covalent bonds between the support and APTES as well as zeolite FAU precursors, zeolite FAU nutrients are easily attached and bound to the support surface, thus facilitating the nucleation and growth of dense, uniform and phase-pure zeolite FAU membranes. In addition, large-scale synthesis of zeolite FAU membrane on the length of 80 cm and 100 cm α-Al2O3 tube is also conducted via large-sized reaction kettle (Fig. 6). 3.2. The DMC dewatering performance of zeolite FAU membrane as a function of temperature and concentration The obtained zeolite FAU membrane supported on APTES-modified α-Al2O3 tubes prepared at 348 K for 24 h is evaluated in the dewatering of DMC by pervaporation. The dewatering performance of the DMC/ H2O mixture (90 wt% DMC/10 wt% H2O) as a function of tube length is shown in Table 1. For the lab-scale membrane, the flux reaches 3.6 kg m2 h−1 at 353 K with the separation factor exceeding 10000. The relatively high separation factor might be ascribed to the DMC content in the permeate side exceeds the detection limit of GC. When the zeolite FAU membrane is synthesized on the enlarged support size (80 cm and 100 cm long length), the flux decreases slightly and the separation factor maintains above 5000, offering the possible application of the zeolite FAU membrane which meets the standard in industry. For the zeolite FAU membrane (M1 in Table 1), different concentrations of DMC/H2O mixtures and evaluation temperature are investigated and the separation results is shown in Table 2. As shown in Table 2, with increasing the water content from 2.6 wt% to 10 wt%, both the flux and separation factor increases. This is attributed to the increase of the water partial pressure in the feed side. For 95 wt% DMC/H2O mixture, with increasing the feed temperature from 303 K to 353 K, the flux increases from 0.43 kg m2 h−1 to 2.65 kg m2 h−1 while the separation factor keeps constant as shown in Table 2 and Fig. 7, which is ascribed to the contribution of the enhanced driving force. For pervaporation process, the driving force is the difference of the partial pressure between feed and permeate [36]. According to the Arrhenius equation (3), the activation energy of the water permeation and the DMC permeation though the zeolite FAU membrane can be calculated from the slope of the Arrhenius plot. As shown in Fig. 8, the activation energy of the water pervaporation is 31.6 kJ/mol. The activation energy of water pervaporation is strongly dependent on the membrane material and reaches from 4 kJ/mol for polymers to 56 kJ/mol for porous silica [37,38]. Interestingly, our activation energy of pervaporation corresponds to the activation energy of water diffusion in NaX for medium loading [39]. Ji = Aiexp (-Ep,i/RT)
3.3. Continuous dewatering performance through zeolite FAU membrane The zeolite FAU membrane was tested in the continuous dewatering of a DMC solution at 353 K starting with an initial concentration of 90 wt% DMC. As shown in Fig. 9, with increasing operation time, the flux decreases and the DMC concentration of the feed increases. After continuous pervaporation of the retentate feed for 9 h, the DMC concentration of the feed exceeds 99%. Notably, the separation factors of the zeolite FAU membrane keep permanently high during the whole separation process. This indicates that the zeolite FAU tubular membrane is very stable, thus it is promising in industrial dewatering DMC solution. Besides, there are no differences in the XRD patterns (Fig. 10) and FESEM images (Fig. 11) after the pervaporation process for almost 9 h at 353 K, further verifying that the zeolite FAU membrane is stable. Therefore, the zeolite FAU membrane with high separation performance and stability will open a new door to dewatering of DMC. 4. Conclusion In conclusion, we have prepared thin, phase-pure and well intergrown zeolite FAU membranes on APTES-modified α-Al2O3 tubes of lab-scale 7.5 cm and industrial-scale size 80 cm and 100 cm length with
(3) 6
Microporous and Mesoporous Materials 292 (2020) 109713
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a high reproducibility. The developed zeolite FAU membranes have been evaluated in the water separation from DMC by pervaporation. The zeolite FAU membranes exhibit high separation performance on account of the relatively large pore size (0.74 nm) and high hydrophilicity (low Si/Al ratio). The flux through the zeolite FAU membrane can reach 3.60 kg h−1 m−2 at 353 K for 90 wt% DMC/H2O mixture. Moreover, for the continuous dewatering of a 90 wt% DMC solution at 353 K in a lab device, the DMC content of in the retentate side can reach 99% over 9 h long-time evaluation with high separation factor. The excellent separation performance recommends the developed zeolite FAU membrane as a possible alternative for the industrial dewatering of DMC solutions.
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We acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 21606246, 21978309 and 21761132003), the Natural Science Foundation of Zhejiang Province (Grant no. LQ19B060001), Ningbo Municipal Natural Science Foundation (Grant no. 2018A610068) and Ningbo Science and Technology Innovation Team (Grant no. 2014B81004).
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Seeding-free synthesis of large tubular zeolite FAU membranes for dewatering of dimethyl carbonate by pervaporation
T
Junjie Zhoua,b, Chen Zhoua,b,∗, Kai Xua, Jürgen Caroc, Aisheng Huanga,d,∗∗ a
Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China University of Chinese Academy of Sciences, Beijing, 100049, PR China c Institute of Physical Chemistry and Electrochemistry, Leibniz University Hannover, Callinstraße 3-3A, D-30167, Hannover, Germany d Department of Chemistry, East China Normal University, Dongchuan Road 500, 200241, Shanghai, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Zeolite FAU membrane Dimethyl carbonate Pervaporation Covalent modification
Thin, phase-pure and well inter-grown zeolite FAU membranes are synthesized upon modifying the tubular supports using 3-aminopropyltriethoxysilane (APTES). The as-synthesized zeolite FAU membranes are evaluated in the dewatering of dimethyl carbonate (DMC) by pervaporation. Due to the large pore size (0.74 nm) and high hydrophilicity of zeolite FAU, the prepared zeolite FAU membranes exhibit high water flux and separation performance for the separation of H2O and DMC. With increasing the temperature or water content in the feed, the zeolite FAU membranes show higher flux while maintaining high separation factors. Further, for dewatering of 10 wt% H2O/90 wt% DMC mixtures at 353 K in a lab device, the DMC concentration in the feed side reaches 99% after continuously dewatering over 9 h. Besides, the zeolite FAU membranes are also successfully prepared on commercial tubular size of 80 cm and 100 cm long α-Al2O3 tubes. The obtained membranes show high separation performance for the dewatering of DMC solution as well. The high-performance zeolite FAU membrane offers great potential for dewatering of DMC solutions in industry.
1. Introduction In the past two decades, zeolite membranes have received tremendous amount of attention as a new type inorganic membrane material with “sieving property” [1]. Attributing to the well-defined framework structure with molecular size level channels/pores, excellent swelling resistance, high thermal and chemical stability, zeolite membranes tender true potential as separators, reactors, sensors, microelectronic devices [2–9]. To date, many types of zeolite membranes, such as MFI, CHA, LTA, and FAU, have been prepared on porous supports to separate gas and/or liquid mixtures, allowing or excluding molecules to pass through the zeolite pores depending on their molecular sizes [10–18]. FAU-type (including NaX with Si/Al ratio of 1.0–1.5 and NaY with Si/Al ratio of 1.5–3) zeolite membranes with a pore size of 0.74 nm due to the 12-membered oxygen rings, can be used for the separation of large molecules which cannot be handled by MFI and/or LTA with the advantage of its relatively large pore size of FAU (0.74 nm). In addition,
zeolite FAU membranes with low Si/Al ratio possess high hydrophilicity, which can be effectively utilized in dehydration of organic solvents by pervaporation or steam permeation. Recently, zeolite FAU membranes are increasingly used in the separation of isopropanol/ water, tetrahydrofuran/water, methanol/methyl tertbutylether, ethanol/cyclohexane [19–21]. These separations are based on the hydrophilicity of zeolite FAU as well as molecular sieving. Dimethyl carbonate (DMC) is incrementally produced for its use as fuel additive, methylating, carbonylating, and methoxylating. Furthermore, DMC is widely used in medicine, for pesticides, as solvent, composite material, dyestuff, flavoring agent of foodstuff and chemical in electronic industry [22–28]. The main technology of DMC production is the oxidative carbonylation of methanol, besides the direct synthesis of DMC from methanol and carbon dioxide becomes interesting as well. All these synthesis technologies result in a mixture DMC/methanol/water. This mixture can be separated by distillation and ends up with a DMC/ H2O [29]. At 353 K, the binary system DMC/H2O forms an azeotrope with xDMC ≈ 0.559 [30]. At room temperature, binary DMC/H2O
∗ Corresponding author. Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China. ∗∗ Corresponding author. Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201, Ningbo, PR China. E-mail addresses: [email protected] (C. Zhou), [email protected] (A. Huang).
https://doi.org/10.1016/j.micromeso.2019.109713 Received 26 January 2019; Received in revised form 26 April 2019; Accepted 8 September 2019 Available online 09 September 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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till the solution was clear and homogenous. And then the porous APTES-modified α-Al2O3 tubes were placed into a Teflon stainless steel autoclave and immersed in the prepared solution. After the in-situ growth at 348 K for 24 h, the membrane was taken out and washed with the deionized water several times. And then the as-synthesized zeolite FAU membranes were dried at 333 K in an oven overnight for characterization and permeation measurement. For the synthesis of scale-up zeolite FAU membrane on the α-Al2O3 tubes with the length of 80 cm and 100 cm, a reaction mixture containing sodium hydroxide (295.64 g), aluminum foil (2.85 g), colloidal silica (158.46 g) and deionized water (1795 ml) was obtained after stirring for 24 h. Then the long APTES-modified porous supports were placed into the auto-calve and immersed in the reaction mixture. After the in-situ growth at 348 K for 24 h, the membrane was taken out, washed several times with deionized water and then dried at 333 K in the oven.
solutions are homogeneous for low (< 15 wt%) and high (> 97 wt%) DMC content [31]. Thus, more efforts should be devoted towards developing a zeolite FAU membrane with a high-separation performance for DMC/H2O mixtures for the selective dewatering of DMC-rich binary solutions. Marrying the large pore size of zeolite FAU with high hydrophilicity which may offer high flux, many efforts have been paid on the synthesis of dense and phase-pure zeolite FAU membranes. Direct synthesis is a conventional method to prepare zeolite FAU membranes. Nevertheless, it is extremely difficult to form a dense zeolite FAU molecular sieve membrane in virtue of the poor heterogeneous nucleation of FAU on the support surface [29]. Another conventional strategy for the synthesis of zeolite FAU membranes is secondary growth. Zeolite seeds are firstly synthesized by direct synthesis strategy, and coated on the substrate by rub-coating, dip-coating or electrostatic attraction [30]. The secondary growth method exhibits many advantages, for instance, preferable control of the membrane thickness and orientation. However, the second growth method is sustained in a lab-scale because of the multistep preparation. Therefore, the development of a reliable synthesis road is highly desired for the large scale preparation of zeolite FAU membranes. Recently, we have developed a seeding-free strategy for the synthesis of zeolite and metal organic framework (MOF) membranes. Upon treating the porous support with covalent linkers, 3aminopropyltriethoxysilane (APTES), 3-chloropropyltrimethoxysilane (CPTMS), 1,4-diisocyanate (DIC-4), polydopamine (PDA), the membrane precursors can be easily anchored via covalent bonds and/or noncovalent adsorption, thus the membranes grow directly on the modified substrates [11,12,31,32]. We have successfully obtained LTA, FAU, ZIF8 and ZIF-90 membranes through the covalent support modification method [12,31,33–35]. In the present work, except the synthesis of zeolite FAU membrane on 7.5 cm long length tubes, commercial size αAl2O3 tubes with 80 and 100 cm long length are used and modified with APTES. Then thin, phase-pure and well inter-grown zeolite FAU membranes are obtained on the surface of the α-Al2O3 tubes. The as-synthesized zeolite FAU membranes are evaluated in the dewatering of DMC solutions by pervaporation. The zeolite FAU membranes prepared on the lab-scale tube (7.5 cm) and large-scale tubes (80 cm and 100 cm) exhibit high-separation performance towards separating of H2O from DMC solution.
2.3. Characterizations of zeolite FAU membranes The morphology and thickness of the zeolite FAU membranes were characterized by field emission scanning electron microscopy (FESEM) on an S-4800 (Hitachi) with a cold field emission gun. Energy-dispersive X-ray spectroscopy (EDXS) was utilized to detect the surface chemical composition of the zeolite FAU membranes. X-ray diffraction (XRD) was applied to confirm the phase structure of the zeolite FAU membranes at room temperature under ambient pressure, which was recorded on a Bruker D8 Advance operating at 40 kV and 40 mA with a Cu Kα1 radiation source (λ = 0.154056 nm) at a stepwise increase of 2°·s−1 in the Bragg angle (2θ) range from 5° to 40°. The Fourier Transform IR (FT-IR) spectra of the non-modified and APTES-modified α-Al2O3 supports were characterized in the absorbance mode in a Nicolet 6700 spectrophotometer. Spectra were collected at 4 cm−1 resolution averaging 32 scans.
2.4. Evaluation of zeolite FAU membranes in the dewatering of DMC by pervaporation The separation performance of the zeolite FAU membranes for the dewatering of DMC solutions was evaluated by using the pervaporation testing system shown in Fig. 1. The supported zeolite FAU membranes were sealed in a permeation module with silicone O-rings. The pressure on the retentate side was kept at atmospheric pressure and the permeate side was evacuated to below 100 Pa by a vacuum pump. The permeate solution was collected in liquid nitrogen freezing traps. The separation performances of the zeolite FAU membranes were evaluated in two ways: 1) dewatering of DMC solutions at different temperatures and concentrations, and 2) continuous dewatering of a feed with the composition 90 wt% DMC/10 wt% H2O at 353 K. In the first way, the DMC solution was prepared by mixing DMC with deionized water, and the corresponding DMC concentrations ranged from 90 wt% to 97.6 wt %. The operation temperature was kept at 303 K, 313 K, 333 K and 353 K. The initial solution weight was 1000 g. In the second way, the DMC concentration was 90 wt% and the initial solution weight was 600 g. The DMC solution was dewatered continuously until the DMC concentration in the feed side reached above 99%. The separation performance was assessed in terms of flux and separation factor (αi,j). The water flux J (kg·h−1·m−2), was calculated by
2. Experimental 2.1. Materials Chemicals were used as received: LUDOX AS-40 colloidal silica (40% SiO2 in water, Sigma-Aldrich); aluminum foil (99.99%, Aladdin); sodium hydroxide (> 98%, Aladdin); 3-aminopropyltriethoxysilane (APTES, 98%, Aladdin). Porous α-Al2O3 tubes (JiexiLishun Technology Co., Guangdong, China: 12 mm outside diameter, 9 mm inside diameter, 75 mm length, ca. 1.0 μm pore size, 30% porosity) were utilized as supports. 2.2. Preparation of zeolite FAU membranes The zeolite FAU membranes were prepared on the porous α-Al2O3 tubes according to the procedure shown elsewhere [12]. The porous αAl2O3 tubes were treated with APTES (0.2 mM in the 10 mL toluene) at 383 K for 1 h, leading to APTES deposited on the surface of α-Al2O3 tubes. A clear synthesis solution with the molar ratio of 70Na2O: 1Al2O3: 20SiO2: 2000H2O was used as the precursor solution. Typically, the aluminum solution was obtained by dissolving 15.56 g sodium hydroxide in 50 g deionized water and then 0.15 g aluminum foil was added to the solution at room temperature. Meanwhile, the silicate solution was prepared by mixing 8.34 g LUDOX AS-40 colloidal silica and 44.44 g deionized water under stirring. After stirring about 2 h, the silicate solution was poured into the aluminum solution and stirred 24 h
J=
W Δt⋅A
(1)
where W is weight of the permeate (kg), Δt is collecting time (h), A is separation area of the membrane (m2). The separation factor αi,j, was defined as the ratio of the composition concentrations in the permeate and that in the feed, and given by 2
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Fig. 1. Schematic diagram of the dewatering of H2O/DMC mixture by pervaporation using tubular FAU membranes.
α i/j=
xip xif ⋅ xif xjp
(2)
xip is the weight fraction of species i in the permeate and xif is the weight fraction of species i in the feed. The αi,j is the separation factor of the species i and j. The composition of the permeation was analyzed by gas chromatography. 3. Results and discussions 3.1. Synthesis and characterization of zeolite FAU membranes Upon immersing the α-Al2O3 tube into the APTES solution at 383 K, APTES immediately reacts with the surface hydroxyl group and an aminopropylsilane layer is formed on the α-Al2O3 tube surface. To confirm that, Fourier transform infrared (FT-IR) spectroscopy data of the supports before and after APTES modification are collected. As shown in Fig. 2, the FT-IR spectrum of the sample scaled off from the surface of α-Al2O3 tube match well with that of aminopropylsilane. Moreover, the bands at 1562 cm−1 and 1470 cm−1 are assigned to N–H2 vibration in the primary amine group (R–NH2), the presence of the amino groups after modification indicates that APTES has been reacted with the surface of α-Al2O3 tube.
Fig. 2. FT-IR spectrum of the un-modified and APTES-modified α-Al2O3 supports.
Fig. 3. Top view (a) and cross-section view (b) FESEM images of the zeolite FAU membrane prepared on non-modified α-Al2O3 tube; top view (c) and cross-section view (d) FESEM images of the zeolite FAU membrane M1 from Table 1 prepared on APTES-modified α-Al2O3 tube. 3
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Fig. 4. Top view FESEM image of zeolite FAU membrane M1 from Table 1 prepared on APTES-modified α-Al2O3 tube (left) and the elements analysis of the zeolite FAU membrane top layer derived from EDXS before the pervaporation (right).
Table 1 Dewatering performances of the zeolite FAU membranes prepared on APTESmodified α-Al2O3 tubes of different length at 353 K for 90 wt% DMC/10 wt% H2O. Membrane number
Membrane length (cm)
Flux (kg·m2·h−1)
H2O/DMC selectivity
M1 M2 M3 M4 M5 M6 M7 M8 M9
7.5
3.60 3.59 3.64 2.15 2.33 2.26 1.83 1.87 1.95
> 10000 > 10000 > 10000 > 7000 > 7000 > 7000 > 5000 > 5000 > 5000
80
100
Fig. 3 shows the typical FESEM images of a zeolite FAU membrane prepared on the un-modified and APTES-modified α-Al2O3 tubes. Without APTES modification before hydrothermal synthesis, no dense zeolite FAU membrane can be obtained, and inter-crystal voids are easily observed in the membrane layer (Fig. 3a and b). However, with APTES modification, the zeolite FAU precursors can be easily adhered onto the surface of the α-Al2O3 tube, thus a dense zeolite FAU membrane with a thickness of about 3.0 μm can be formed on the α-Al2O3 tube without visible cracks, pinholes, and other defects (Fig. 3c and d). Via the formation of covalent bonds between zeolite membrane and support surface, APTES acts as a highly efficient molecular linker which
Fig. 5. XRD patterns of the zeolite powder (a) and the zeolite FAU membrane M1 from Table 1 prepared on the APTES-modified 80 cm long α-Al2O3 tubes before pervaporation (b). (♦): Al2O3 support.
Fig. 6. Large-scale synthesis of the zeolite FAU membrane on long length α-Al2O3 tubes. 4
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Table 2 Fluxes and separation factors H2O/DMC of zeolite FAU membrane (M1 in Table 1) prepared on the modified α-Al2O3 tube by pervaporation as a function of evaluation temperatures and DMC contents. 90%
303 K 313 K 333 K 353 K
95%
96.5%
97.4%
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
J(kg·h−1·m−2)
α
0.52 0.92 2.08 3.60
> 10000 > 10000 > 10000 > 10000
0.43 0.80 1.39 2.65
> 5000 > 5000 > 5000 > 5000
0.40 0.64 1.12 2.11
> 5000 > 5000 > 5000 4754
0.13 0.32 0.47 1.08
1377 1264 2347 2347
Fig. 9. Water flux and separation factor of the zeolite FAU membrane M1 from Table 1 for the continuous dewatering of a 90 wt% DMC solution at 353 K as a function of the operation time.
Fig. 7. Flux and separation factor of the zeolite FAU membrane M1 from Table 1 for the pervaporation of a 95 wt% DMC/5 wt% water solution as a function of temperature.
Fig. 8. Arrhenius plot of the water flux through zeolite FAU membrane M1 from Table 1 and the pervaporation temperature in the pervaporation of a 95 wt % DMC/5 wt% water solution.
Fig. 10. XRD patterns of the zeolite powder (a) and the zeolite FAU membranes prepared on APTES modified α-Al2O3 tubes after pervaporation (b). (♦): Al2O3 support.
Table 3 Fluxes and separation factors 95 wt% DMC/MeOH of zeolite FAU membrane prepared on the modified α-Al2O3 tube by pervaporation. Temperature (K)
Flux (kg·m2·h−1)
Separation factor
303 313 333 353
0.20 0.25 0.36 0.81
1766 1470 1341 841
can in-situ attract and bind the FAU nutrients onto the support surfaces to boost the nucleation. The enhancement of nucleation on the APTESmodified α-Al2O3 tube can be clearly observed according to the membrane morphology (Fig. 3c and d). The chemical composition of the zeolite FAU membrane on the APTES-modified α-Al2O3 tube is detected by EDXS. As shown in Fig. 4, the zeolite FAU membrane contains the elements C, N, O, Na, Al and Si. The Si/Al atomic ratio is found to be 1.2 which is consistent with that of zeolite FAU. The formation of a phase-pure zeolite FAU membrane with a high 5
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Fig. 11. Top view FESEM image of zeolite FAU membrane M1 of Table 1 after continuous pervaporation (left). EDXS elements analysis of the top layer of this zeolite FAU membrane (right).
where Ji is the permeate flux of the membrane, Ai is the pre-exponential factor, R is the gas constant, T is the absolute temperature and Ep,i is the apparent activation energy. Although the kinetic diameter of DMC (0.47 nm < dDMC < 0.63 nm) is smaller than the pore size of zeolite FAU (0.74 nm), the low Si/Al ratio (1.2) of the zeolite FAU membrane provides good hydrophilicity, thus the zeolite FAU membrane shows excellent separation performance towards dewatering DMC. In addition, it is also very important to remove methanol (MeOH) from the mixture during the commercial process for DMC production. Thus the separation of MeOH from DMC is evaluated as well and the corresponding results are shown in Table 3. As shown in Table 3, at 303 K, the separation factor exceeds 1700 for separation MeOH from DMC although the flux is 0.20 kg m2 h−1. With increasing the temperature to 353 K, the flux reaches 0.81 kg m2 h−1 while maintaining high separation selectivity. Compared with the flux of H2O from DMC, the relatively low flux in separating DMC and MeOH mixture might be attributed to two reasons: 1) zeolite FAU has high hydrophilicity which allows H2O to pass through rapidly; 2) the kinetic diameter of MeOH is bigger than that of H2O which leads to slow diffusion rate through the zeolite FAU membrane. Generally, the zeolite FAU membrane exhibits good separation performance towards the removal of MeOH from DMC solution, offering the possibility to purify DMC from the reaction mixture during the commercial process for DMC production.
degree of crystallinity was confirmed by XRD (Fig. 5). All peaks match well with those of zeolite FAU powder except the α-Al2O3 signals from the support. In our previous report [12], due to the covalent bonds between the support and APTES as well as zeolite FAU precursors, zeolite FAU nutrients are easily attached and bound to the support surface, thus facilitating the nucleation and growth of dense, uniform and phase-pure zeolite FAU membranes. In addition, large-scale synthesis of zeolite FAU membrane on the length of 80 cm and 100 cm α-Al2O3 tube is also conducted via large-sized reaction kettle (Fig. 6). 3.2. The DMC dewatering performance of zeolite FAU membrane as a function of temperature and concentration The obtained zeolite FAU membrane supported on APTES-modified α-Al2O3 tubes prepared at 348 K for 24 h is evaluated in the dewatering of DMC by pervaporation. The dewatering performance of the DMC/ H2O mixture (90 wt% DMC/10 wt% H2O) as a function of tube length is shown in Table 1. For the lab-scale membrane, the flux reaches 3.6 kg m2 h−1 at 353 K with the separation factor exceeding 10000. The relatively high separation factor might be ascribed to the DMC content in the permeate side exceeds the detection limit of GC. When the zeolite FAU membrane is synthesized on the enlarged support size (80 cm and 100 cm long length), the flux decreases slightly and the separation factor maintains above 5000, offering the possible application of the zeolite FAU membrane which meets the standard in industry. For the zeolite FAU membrane (M1 in Table 1), different concentrations of DMC/H2O mixtures and evaluation temperature are investigated and the separation results is shown in Table 2. As shown in Table 2, with increasing the water content from 2.6 wt% to 10 wt%, both the flux and separation factor increases. This is attributed to the increase of the water partial pressure in the feed side. For 95 wt% DMC/H2O mixture, with increasing the feed temperature from 303 K to 353 K, the flux increases from 0.43 kg m2 h−1 to 2.65 kg m2 h−1 while the separation factor keeps constant as shown in Table 2 and Fig. 7, which is ascribed to the contribution of the enhanced driving force. For pervaporation process, the driving force is the difference of the partial pressure between feed and permeate [36]. According to the Arrhenius equation (3), the activation energy of the water permeation and the DMC permeation though the zeolite FAU membrane can be calculated from the slope of the Arrhenius plot. As shown in Fig. 8, the activation energy of the water pervaporation is 31.6 kJ/mol. The activation energy of water pervaporation is strongly dependent on the membrane material and reaches from 4 kJ/mol for polymers to 56 kJ/mol for porous silica [37,38]. Interestingly, our activation energy of pervaporation corresponds to the activation energy of water diffusion in NaX for medium loading [39]. Ji = Aiexp (-Ep,i/RT)
3.3. Continuous dewatering performance through zeolite FAU membrane The zeolite FAU membrane was tested in the continuous dewatering of a DMC solution at 353 K starting with an initial concentration of 90 wt% DMC. As shown in Fig. 9, with increasing operation time, the flux decreases and the DMC concentration of the feed increases. After continuous pervaporation of the retentate feed for 9 h, the DMC concentration of the feed exceeds 99%. Notably, the separation factors of the zeolite FAU membrane keep permanently high during the whole separation process. This indicates that the zeolite FAU tubular membrane is very stable, thus it is promising in industrial dewatering DMC solution. Besides, there are no differences in the XRD patterns (Fig. 10) and FESEM images (Fig. 11) after the pervaporation process for almost 9 h at 353 K, further verifying that the zeolite FAU membrane is stable. Therefore, the zeolite FAU membrane with high separation performance and stability will open a new door to dewatering of DMC. 4. Conclusion In conclusion, we have prepared thin, phase-pure and well intergrown zeolite FAU membranes on APTES-modified α-Al2O3 tubes of lab-scale 7.5 cm and industrial-scale size 80 cm and 100 cm length with
(3) 6
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a high reproducibility. The developed zeolite FAU membranes have been evaluated in the water separation from DMC by pervaporation. The zeolite FAU membranes exhibit high separation performance on account of the relatively large pore size (0.74 nm) and high hydrophilicity (low Si/Al ratio). The flux through the zeolite FAU membrane can reach 3.60 kg h−1 m−2 at 353 K for 90 wt% DMC/H2O mixture. Moreover, for the continuous dewatering of a 90 wt% DMC solution at 353 K in a lab device, the DMC content of in the retentate side can reach 99% over 9 h long-time evaluation with high separation factor. The excellent separation performance recommends the developed zeolite FAU membrane as a possible alternative for the industrial dewatering of DMC solutions.
[16]
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Acknowledgements
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We acknowledge the financial supports from the National Natural Science Foundation of China (Grant no. 21606246, 21978309 and 21761132003), the Natural Science Foundation of Zhejiang Province (Grant no. LQ19B060001), Ningbo Municipal Natural Science Foundation (Grant no. 2018A610068) and Ningbo Science and Technology Innovation Team (Grant no. 2014B81004).
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