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A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide
Journal Pre-proofs A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide Zhihao Si, Ze Wang, Di Cai, Guozhen Li, Shufeng Li, Peiyong Qin PII: DOI: Reference:
S1383-5866(19)35532-7 https://doi.org/10.1016/j.seppur.2020.116545 SEPPUR 116545
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
2 December 2019 9 January 2020 9 January 2020
Please cite this article as: Z. Si, Z. Wang, D. Cai, G. Li, S. Li, P. Qin, A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116545
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© 2020 Published by Elsevier B.V.
A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide
Zhihao Si1, Ze Wang1, Di Cai, Guozhen Li, Shufeng Li, Peiyong Qin*
National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, 100029, China
* Corresponding author. E-mail address: [email protected]
1
These authors contributed equally to this paper.
ABSTRACT: A high permeance is corresponding to a low membrane area and a reduced capital investment in the organic solvent nanofiltration (OSN) process. Currently, to increase the permeance, only a few microporous fillers were utilized to prepare the mixed matrix membranes (MMMs). In this study, MCM-41, a typical mesoporous filler, was selected and doped into a polyimide (PI) matrix, aiming to increase the solvent permeance of the MMMs. To break the trade-off between permeance and rejection, a covalent bridge between amine-functionalized MCM-41 (NH2-MCM-41) and PI was constructed so as to avoid forming the non-selective interfacial voids in the filler-polymer interface. As a result, the water, ethanol and isopropanol permeances of 5 wt% NH2-MCM-41/PI MMM were increased by 102%, 78% and 131% compared with pure PI membrane, without obvious reductions of rejections. Overall, the NH2-MCM-41/PI MMM shows its great potential in high-efficiency organic solvent nanofiltration in practical applications.
Keywords: Mixed matrix membrane; MCM-41; Polyimide; Organic solvent nanofiltration.
1. Introduction Organic solvent nanofiltration (OSN) is a well-developed membrane-based technology for separating small molecules (200-1000 Da) from organic solvents [1, 2]. It attracts tremendous attention due to its high efficiency, low maintenance costs and environmental friendliness [3]. Among OSN membranes, the polymer membrane is considered as one of the most promising and effective candidates because of its reasonable stability and easy scale-up [4]. Poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), a kind of polyimide (PI) material with inherent and high solvent resistance, has been widely used [5]. In recent years, much endeavor was devoted to further enhancing the permeance of the pure PI membrane, because the low permeance is resulted in a high membrane area and high capital investment [6]. An operable and effective approach is to dope inorganic porous fillers into the PI matrix [7]. In this case, the favorable transport path in porous fillers can be utilized to enhance the OSN performance [8]. Although the mixed matrix membranes (MMMs) on OSN are encouraging, most studies are limited in scope. Only a few microporous fillers were utilized in the preparation of OSN MMMs (Table 1), which generally contributed to low enhanced permeance (in most reports < 50%). Moreover, Livingston et al. [9] reported that the solvent permeances of polyamide (PA)-based nanocomposite membranes increased with the increase in pore diameter of doped nanoparticles. Therefore, it is imperative to incorporate fillers with a larger pore diameter (e.g. mesoporous) to decrease mass transport resistance.
Another interesting observation is that the OSN MMMs generally suffer from a main challenge, namely the trade-off between permeability and rejection (Table 1) [1, 10]. The poor compatibility between fillers and polymer is recognized as the main reason [11, 12], due to easily forming non-selective voids in the filler-polymer interface. Once the interfacial voids have formed, the non-selective holes will be the preferential transport path instead of the nanoporous fillers [13], inevitably leading to poor rejection [14, 15]. However, the conventional approaches, such as sonication [16] and stirring [17], are not adequately effective to resolve this issue. Thus, it is critical to find an effective method to improve the compatibility in filler-polymer interface. Table 1. Performance of OSN MMMs according to literature. Porous filler Polymer
diameter (wt%)
Permeance (L m-2 s-1 bar-1)
Filler Solvent
Permeance enhancement
Rejection decline
Ref.
Pure MMM
(nm)
(%)
(%)
membrane Matrimid®
UiO-66-NH2
1.2
Ethanol
3.85±0.95
4.43±0.58
15.06
-
[18]
1.2
DMF
3.06±0.03
3.90±0.19
27.45
-
[18]
(0.1) Matrimid
®
UiO-66-NH2 (0.1)
PA
MIL-53
0.86
Methanol
1.8±0.2
2.3±0.4
27.78
-
[9]
PA
ZIF-8
0.34
Methanol
1.8±0.2
2.5±0.6
38.89
-
[9]
PDMS
MIL-47 (20)
0.85
Isopropanol
0.54
~0.65
~20.37
~19.54
[19]
PDMS
ZIF-8 (20)
0.34
Isopropanol
0.54
~0.82
~51.85
~31.03
[19]
PDMS
[Cu3(BTC)2]
0.60
Isopropanol
0.54
~0.80
~48.15
~27.59
[19]
-
Isopropanol
~0.70
~0.96
~37.14
~35
[20]
-
Isopropanol
0.83
1.88
126.5
21.14
[21]
(20) P84 PI
MWCNTsNH2 (0.4)
PI
CZIF-8 (20)
In light of the aforementioned analysis, MCM-41 was selected as an inorganic filler to be doped into PI matrix. Because of the high specific surface and the narrow pore size distribution [22], it has been proved a successful porous material used in
pervaporation [23], gas separation [24] and adsorption [25]. Importantly, compared with other common micropore fillers, its mesoporous property was expected to decrease mass transport resistance for a high-enhanced permeance as mentioned above [26, 27]. We further proposed a strategy via covalently bonding surface amine-functionalized MCM-41 (NH2-MCM-41) with PI so as to eliminate the non-selective voids in the MCM-41-PI interface and improve the compatibility of both phases (Fig. 1). The optimal loading of NH2-MCM-41 in MMM was investigated and the OSN performances of MMMs doping MCM-41 and NH2-MCM-41 were compared in both aqueous and organic systems.
Fig. 1. Schematic illustration for comparison of (a) physical blending and (b) covalent bridging between NH2-MCM-41 and PI. 2. Experimental 2.1. Materials MCM-41 were provided by Xianfeng Nano Technology Co., Ltd., China. The 15 wt% PMDA-ODA polyamic acid (PAA)/N-Methyl pyrrolidone (NMP) solution was
obtained by Changzhou Runge Chemical Industrial Co., Ltd., China. 3Aminopropyltriethoxysilane (APTES, 99%), N,N-Dimethylformamide (DMF, 99%) and dyes, including rose bengal (Molecular weight (Mw)=1017.65 g mol-1), congo red (Mw=696.68 g mol-1), chrome black T (Mw=461.38 g mol-1) and methyl orange (Mw=327 g mol-1) were purchased from Macklin Biochemical Co., Ltd., China. Other solvents were purchase from Beijing Chemical Works, China. 2.2. Surface modification of MCM-41 particles The synthesis of NH2-MCM-41 was based on a previous method [26]. In brief, MCM41 (2 g) and APTES (1.2 ml) were mixed in 60 ml toluene. The mixture was kept for 10 h at 110 °C under magnetic stirring. After cooling to room temperature, the product was obtained by centrifugation with 5000 rmp for 10 min with thoroughly repeated washing using plenty of IPA, and dried overnight at 100 °C.
2.3. Membrane preparation. The preparation of MMMs was according to our previous reports [20, 28]. The PMDAODA PAA/NMP solution and particles were mixed in NMP at 600 rpm for 2 h, followed by sonicating and degassing. The content of PAA and doping particles in the casting solution was kept at 15 wt%. The PAA membrane was prepared by the solution casting method on the PET non-woven fabric. The scraper thickness is set at 150 μm. After that, it was transferred into the distilled water coagulation bath for 30 min, and dried by isopropanol-hexane displacement method [29]. Finally, the resultant membrane was immersed in a mixed solvent (Vacetic anhydride: Vtriethylamine = 4:1) and kept
at 100 °C for 36 h [30]. 2.4. Characterization Surface and cross-section membrane morphologies were observed by S4700 scanning electron microscope (SEM, Hitachi High-Technology, Japan). Membranes roughness (5 × 5 µm) was tested by an atomic force microscope (AFM, Dimension Fastscan 2, Bruker Crop., Germany) at a rate of 1 Hz. The channels in MCM-41 were observed by a HT7700 transmission electron microscopic (TEM, Hitachi High-Technology, Japan). The graft of the amine groups on MCM-14 and the covalent bonds between MCM41 and PI were determined by FT-IR (6700 Nicolet, Thermo Fisher Scientific, USA). The hydrophilicity of the MMMs was tested via water contact angles (Shanghai Zhongchen Technic Apparatus Co., Ltd., China). The average value of three measurements was recorded. The N2 adsorption-desorption isotherms of modified and unmodified MCM-41 were tested by an Autosorb-iQ specific surface area analyzer (Quantachrome, USA). The particles were degassed at 250 °C for 3 h and tested at 77K. 2.5. Liquid sorption capacity The measurement of liquid sorption capacity was detailed in our previous research [21]. Briefly, the membrane specimens were accurately weighed before (M1, mg) and after (M2, mg) immersion in several solvents including water, ethanol and isopropanol at room temperature for 72 h, respectively. The liquid sorption capacity was calculated by the following equation [20]:
𝐿𝑖𝑞𝑢𝑖𝑑 𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (𝑚𝑔/𝑚𝑔) =
𝑀2 −𝑀1 𝑀1
(1)
2.6. Chemical stability of the MMMs The dried membrane specimens (m1) were soaked in EtOH, IPA, acetone, DMF and NMP at 25 ℃ for 7 days under stirring, respectively. Then, the specimens were washed by pure water and dried overnight. Finally, they were weighed (m2) and compared with m1. 2.7. OSN performance The OSN performance was tested using a Sterlitech HP4750 dead-end membrane module. The different dye feed solutions with the same concentration of 50 mg L-1 were maintained at 10 bar with a constant stirring speed of 600 rpm. Before conducting the test, the membrane was pre-compacted at 15 bar for 2 h. A UV-Vis spectrometer (SP752, China) was used to measure the dye concentrations. Long-term stability performance of the MMMs was carried out using IPA solutions containing 50 mg L-1 chrome black T as feed at 15 bar for 50 h. The permeance P (L m-2 h-1 bar-1) and rejection R (%) were calculated using the following equation [31]: ∆𝑉
𝑃 = 𝐴∗∆𝑃∗∆𝑡 𝑅=
𝐶𝑓 −𝐶𝑝 𝐶𝑓
× 100%
(2) (3)
where ΔV (L) and A (m2) are the volume increment in permeate and the effective membrane area, respectively; ΔP (bar) and Δt (h) are the transmembrane pressure and the test interval, respectively; Cf (mg L-1) and Cp (mg L-1) refer to the dye concentration in feed and permeate side, respectively.
3. Results and discussion
3.1. Characterization of MCM-41 and NH2-MCM-41 MCM-41 particles were functionalized by APTES for converting -OH into -NH2 (denoted as NH2-MCM-41). The successful graft of the amine group on MCM-14 surface is verified by FT-IR (Fig. 2a). The peaks located at 2856 cm-1 and 2924 cm-1 are attributed to the stretching vibration of -CH- [32]. The peak at 1560 cm-1 belongs to the N-H deformation vibration [26, 33]. Besides, the other peaks at 800 cm-1 and 1083 cm-1 are associated with the Si-O-Si stretching vibrations, respectively [34]. In order to evaluate the effect of modification on the pore structure of MCM-41, N2 adsorption-desorption isotherms tests were conducted. As illustrated in Fig. 2b, the unmodified MCM-41 shows a typical type-IV isotherm with a H4 hysteresis loop (0.40 < P/P0 < 1.0) and a sharp pore filling step (P/P0 < 0.40). It proves the typical mesoporous structure of MCM-41 [32, 35]. The NH2-MCM-41 displays the similar N2 adsorptiondesorption isotherm. However, because of partial pore blockage of NH2-MCM-41 by APTES, the overall absorbed N2 quantity is lower than pristine MCM-41 [35, 36]. Accordingly, the pore volume and BET surface area decrease from 1.12 cm3 g-1 to 0.54 cm3 g-1 and from 1106 m2 g-1 to 677 m2 g-1 after modification, respectively (Fig. 2c). These results further confirm the successful amine-modification of MCM-41 [26, 37]. It is worth noting that NH2-MCM-41 maintains the mesoporous property (2.50 nm) from the pristine MCM-41, which can meet the further demand of the preparation of MMMs containing filler with large pore diameter (Fig. 2d). In addition, the TEM images (Fig. 3) shows that the presence of pores along the pore direction (yellow circles) and one-dimensional linear channels perpendicular to the pore direction (red circles),
which is characteristic of the pores of MCM-41 [38]. After modification, the typical regular one-dimensional linear channels of MCM-41 are also well preserved [39].
Fig. 2. (a) FT-IR spectra, (b) N2 adsorption (filled symbol) and desorption (open symbol) isotherms, (c) specific surface area and total pore volume and (d) pore diameter distribution of MCM-41 and NH2-MCM-41.
Fig. 3. TEM images of (a) pristine MCM-41 and (b) NH2-MCM-41 (yellow circles: pores on the direction of the pores; red circles: perpendicular to the pores with onedimensional lines). 3.2. Characterization of MMMs
To confirm the covalent bonding between NH2-MCM-41 and PI, FT-IR test was conducted as shown in Fig. 4a. The peaks at 1778 cm-1, 1718 cm-1 and 1360 cm-1 are attributed to C=O symmetry and asymmetry stretching vibrations and C-N stretching vibration, respectively [30, 40]. No obvious differences can be observed in FT-IR spectra between the PI membrane and MCM-41/PI MMMs. In contrast, these peaks observably weaken after doping NH2-MCM-41 particles. Meanwhile, two new appeared peaks at 1546 cm-1 and 1645 cm-1 are related to stretching of C-N and C=O, respectively, which confirms the formation of the covalent bonds between NH2-MCM41 and PI matrix [15]. The residual bond at 1360 cm-1 indicates the existence of the unreacted imide groups [41]. In addition, Fig. 4b displays the evolution of surface chemistry after doping NH2-MCM-41 particles in the PI matrix. These are three peaks at ~ 399.4 eV, ~ 400.4 eV and ~ 400.9 eV in the XPS spectra of NH2-MCM-41/PI, corresponding to free amine groups (-NH2), amide groups (-NH-) and imide groups (N<), respectively [42, 43]. While the only original imide groups of PI are shown in the XPS spectra of MCM-41/PI. This further confirms the formation of the covalent bonds between NH2-MCM-41 and PI [20]. Therefore, this amine-modification method is effective to further construct a covalent bridge between MCM-41 and PI.
Fig. 4. (a) FT-IR spectra and (b) XPS N-1s narrow scan spectra on the surface of PI membrane and the MMMs. Fig. 5 shows the SEM images including surface and cross-section of PI membrane and MMMs. There is a smooth and non-crack surface in the pure PI membrane. However, some distinct defects arise in the surfaces of MCM-41/PI MMMs, especially in the case of high particle loading. It is attributed to the poor compatibility at the interface between MCM-41 and PI, which further leads to the formation of nonselectivity voids due to filler aggregation [44]. On the contrary, the surface and crosssection images of NH2-MCM-41 doped MMMs display the smooth surfaces with uniform particle distributions, which indicates the good compatibility between NH2MCM-41 and PI with no visible defects. Similar result is also obtained from the surface 3D AFM (Fig. 6). Typically, the surface of 7 wt% MCM-41/PI MMM is much rougher than the pure membrane and NH2-MCM-41/PI MMM at the same loading [45].
Fig. 5. SEM images of (A) surface and (B) cross-section of PI membrane and the MMMs.
Fig. 6. AFM images of (a) PI membrane and MMMs with 7 wt% of (b) MCM-41 particles and (c) NH2-MCM-41 particles. The liquid sorption capacity is corresponding to the membrane ability for solvent transport [46]. The effect of the NH2-MCM-41 loading on the liquid sorption capacity was evaluated in water, EtOH and IPA (Fig. 7a). Compared with PI membrane, NH2MCM-41/PI MMMs display a superior liquid sorption capacity as the loading increasing to 5 wt%. This phenomenon is caused by the higher porosity of NH2-MCM41, providing more solvent exchange channels than the pure PI membrane [9, 47]. However, there is a slight decline when particle loading exceeds 7 wt%, resulting from the increased amidation reaction between NH2-MCM-41 and PI and the enhanced rigidity of the polymer chain [48, 49]. Similar phenomena occurred in ethanol and isopropanol systems. Additionally, the molecule polarity and sizes affect the liquid
sorption capacity to some extent [50]. Since the molecular polarity of water, EtOH and IPA is from large to small, while the molecular size obeys the order conversely, the liquid sorption capacity obeys the sequence of water > ethanol > isopropanol. Fig. 7b shows the contact angles of deionized water on the NH2-MCM-41/PI MMMs. The value of MMM decreases with the increment of NH2-MCM-41 particle loading, indicating the enhanced membrane hydrophilicity. It is caused by amine groups on the surfaces of NH2-MCM-41 [34] and facilitates water molecules pass through the membrane more quickly [51]. Similar phenomenon has been reported in MWCNTs embedded MMMs [20, 52].
Fig. 7. Effect of NH2-MCM-41 loading on (a) liquid sorption capacity and (b) water contact angles of the MMMs. The stability in organic solvents is concerned with the lifetime and use-cost of MMMs [53]. The weight losses of NH2-MCM-41/PI MMMs after soaked into EtOH, acetone, IPA, DMF and NMP for one week are summarized in Table 2. It is obvious that there are negligible weight losses in the MMMs, illustrating the outstanding chemical stability in organic solvents. Table 2. The solvent stability of NH2-MCM-41/PI MMMs.
NH2-MCM-41 loading (wt%)
Solvent
Weight loss (%)
0
EtOH
0.02±0.01
0
Acetone
0.00±0.01
0
IPA
0.01±0.01
0
DMF
0.02±0.01
0
NMP
0.03±0.02
3
EtOH
0.04±0.01
3
Acetone
0.02±0.01
3
IPA
0.00±0.01
3
DMF
0.03±0.02
3
NMP
0.03±0.02
7
EtOH
0.02±0.03
7
Acetone
0.03±0.01
7
IPA
0.04±0.02
7
DMF
0.01±0.02
7
NMP
0.01±0.04
3.3. OSN performance of MMMs To evaluate the role of modification in MCM-41 for the OSN performance, the pristine MCM-41 doped MMM was used for OSN firstly. As illustrated in Fig. 8, the pure PI membrane exhibits a favorable rejection (> 97%) to chrome black T and a low IPA permeance (1.02 L m-2 h-1 bar-1). It is caused by the dense structure of the PI membrane which hinders the transport of the solvents [12]. As expected, after incorporating unmodified MCM-41, the permeance enhanced greatly with the increase of MCM-41 loading; but the dye rejections decreased considerably. The possible reason is the poor compatibility that induced the defects at interface (Fig. 5).
Fig. 8. OSN performance of the MCM-41/PI MMMs with different particle loading. The modified MCM-41 doped MMMs were further used to evaluate the OSN performance. As displayed in Fig. 9, when increasing the NH2-MCM-41 loading to 5 wt%, the water, EtOH and IPA permeances reach the top values, which were increased by 102%, 78% and 131%, respectively. It is obvious that NH2-MCM-41 with mesoporous property endows MMMs with high-efficiency transfer path for solvents [54, 55]. Fig. 7b indicates enhanced membrane hydrophilicity after incorporating NH2MCM-41 caused by amine groups on the particle surfaces [34], which is beneficial for improving the transport of polar solvents (e.g., water and alcohols) [56]. However, when the NH2-MCM-41 loading exceeds 5 wt%, the permeances were declined. The possible reasons are the tortuous pathways across membrane caused high transport resistance [57] and aforementioned amidation reaction and the increased chain rigidification. Additionally, the chrome black T rejections of NH2-MCM-4/PI MMMs almost remain constant with the increased particle loading. It contributes to the covalent bridge between NH2-MCM-41 and PI, which effectively avoids forming the nonselective voids in MCM-41-PI interface and improve the compatibility in two phases
[9]. The dye rejection of NH2-MCM-41/PI MMMs in water follows the order of rose bengal > congo red > chrome black T > methyl orange (Fig. 10a), which is consistent with their MWs [58]. Overall, i) the good compatibility between NH2-MCM-41 and PI endows MMMs with high rejection; ii) mesoporous property of NH2-MCM-41 provides high-efficiency transfer process of MMMs. In addition, the long-term operation is also conducted using IPA solutions containing 50 mg L-1 chrome black T at 15 bar, which shows an outstanding stability (Fig. 10b). The decline of permeance in the beginning is probably ascribed to the compaction and pore blockage [20], which in turn contributes to a slight improvement of rejection [58, 59]. Note that, the stable permeance of the NH2-MCM-41/PI are slight lower than the values presented in the Table 3, which is possibly because the long-term test is carried out at a higher pressure of 15 bar, resulting in more compacted of the membranes [5].
Fig. 9. Effect of NH2-MCM-41 loading on the OSN performance of MMMs.
Fig. 10. (a) Rejection of NH2-MCM-41/PI MMMs for various dye molecule in water and (b) the long-term stability of the 5 wt% NH2-MCM-41/PI MMM in IPA. 3.4. Benchmark A comparison of OSN membranes for dyes rejections is summarized in Table 3. The NH2-MCM-41/PI MMM exhibits superior permeance compared with the most reported polymer based MMMs. More significantly, the dye rejection is not sacrificed, especially for the solutes more than 461 g mol-1. This is ascribed to the inherent mesoporous property of MCM-41 and good compatibility between particles and polymer matrix.
Table 3. Current advances of OSN membranes for dyes rejection. Membrane
Dye
MW (g mol-1)
Solvent
Kevlar
Rose Bengal
1017
EtOH
Eosin Y
648
PA/Matrimid PI
Rose Bengal
(Catechol/POSS)/PI
Rose Bengal
5 wt% NH2-MCM-41/PI MMM
P (L m-2 s-1 bar-1)
R (%)
Ref.
2.9
95.4
[53]
EtOH
3.3
98.1
[20]
1017
EtOH
2.7
100
[60]
1017
EtOH
1.26
99
[61]
Rose Bengal
1017
EtOH
4.25
98.16
This work
5 wt% NH2-MCM-41/PI MMM
chrome black T
461
EtOH
4.25
91.16
This work
MWCNTs/P84
Eosin Y
648
IPA
1.4
97.9
[20]
UiO-66-NH2/Matrimid PI
Rose Bengal
1017
IPA
1.17
99.2
[18]
MWCNTs/P84
Rose Bengal
1017
IPA
1.8
99
[15]
(Catechol/POSS)/PI
Rose Bengal
1017
IPA
0.39
99
[61]
5 wt% NH2-MCM-41/PI MMM
Rose Bengal
1017
IPA
2.35
99.03
This work
5 wt% NH2-MCM-41/PI MMM
chrome black T
461
IPA
2.35
95.26
This work
MWCNTs/P84 ®
®
4. Conclusion
In this work, mesoporous NH2-MCM-41 doped PI MMMs showed high solvent permeances due to the decreased transfer resistance and additionally preferential transport channels. Importantly, the formation of the covalent bonding between NH2MCM-41 and PI minimized the interfacial voids and improved compatibility. As a result, 102%, 78% and 131% of increases in the water, ethanol and isopropanol permeances of 5 wt% NH2-MCM-41/PI MMM were obtained, respectively, without obvious reductions of rejections. In view of the effective improvement in OSN performance, the universal and operable modification approach shows a great potential for application of the MMMs.
Acknowledgments This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFB1501703), the National Nature Science Foundation of China (Grant Nos. 21978016, 21676014 and 21706008), and Beijing Natural Science Foundation (Grant No. 2172041).
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Highlights
Decreasing the transport resistance after doping mesoporous NH2-MCM-41.
Improving the interface compatibility via covalently bonding with NH2-MCM-41 and PI.
Breaking the trade-off between solvent permeance and dye rejection.
Remarkable increases in solvents permeances compared with pure PI membrane.
There are no conflicts to declare.
All co-authors have seen the manuscript and approved to submit to your journal. And all the authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.
S1383-5866(19)35532-7 https://doi.org/10.1016/j.seppur.2020.116545 SEPPUR 116545
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
2 December 2019 9 January 2020 9 January 2020
Please cite this article as: Z. Si, Z. Wang, D. Cai, G. Li, S. Li, P. Qin, A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116545
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A high-permeance organic solvent nanofiltration membrane via covalently bonding mesoporous MCM-41 with polyimide
Zhihao Si1, Ze Wang1, Di Cai, Guozhen Li, Shufeng Li, Peiyong Qin*
National Energy R&D Center for Biorefinery, Beijing University of Chemical Technology, Beijing, 100029, China
* Corresponding author. E-mail address: [email protected]
1
These authors contributed equally to this paper.
ABSTRACT: A high permeance is corresponding to a low membrane area and a reduced capital investment in the organic solvent nanofiltration (OSN) process. Currently, to increase the permeance, only a few microporous fillers were utilized to prepare the mixed matrix membranes (MMMs). In this study, MCM-41, a typical mesoporous filler, was selected and doped into a polyimide (PI) matrix, aiming to increase the solvent permeance of the MMMs. To break the trade-off between permeance and rejection, a covalent bridge between amine-functionalized MCM-41 (NH2-MCM-41) and PI was constructed so as to avoid forming the non-selective interfacial voids in the filler-polymer interface. As a result, the water, ethanol and isopropanol permeances of 5 wt% NH2-MCM-41/PI MMM were increased by 102%, 78% and 131% compared with pure PI membrane, without obvious reductions of rejections. Overall, the NH2-MCM-41/PI MMM shows its great potential in high-efficiency organic solvent nanofiltration in practical applications.
Keywords: Mixed matrix membrane; MCM-41; Polyimide; Organic solvent nanofiltration.
1. Introduction Organic solvent nanofiltration (OSN) is a well-developed membrane-based technology for separating small molecules (200-1000 Da) from organic solvents [1, 2]. It attracts tremendous attention due to its high efficiency, low maintenance costs and environmental friendliness [3]. Among OSN membranes, the polymer membrane is considered as one of the most promising and effective candidates because of its reasonable stability and easy scale-up [4]. Poly(4,4′-oxydiphenylene pyromellitimide) (PMDA-ODA), a kind of polyimide (PI) material with inherent and high solvent resistance, has been widely used [5]. In recent years, much endeavor was devoted to further enhancing the permeance of the pure PI membrane, because the low permeance is resulted in a high membrane area and high capital investment [6]. An operable and effective approach is to dope inorganic porous fillers into the PI matrix [7]. In this case, the favorable transport path in porous fillers can be utilized to enhance the OSN performance [8]. Although the mixed matrix membranes (MMMs) on OSN are encouraging, most studies are limited in scope. Only a few microporous fillers were utilized in the preparation of OSN MMMs (Table 1), which generally contributed to low enhanced permeance (in most reports < 50%). Moreover, Livingston et al. [9] reported that the solvent permeances of polyamide (PA)-based nanocomposite membranes increased with the increase in pore diameter of doped nanoparticles. Therefore, it is imperative to incorporate fillers with a larger pore diameter (e.g. mesoporous) to decrease mass transport resistance.
Another interesting observation is that the OSN MMMs generally suffer from a main challenge, namely the trade-off between permeability and rejection (Table 1) [1, 10]. The poor compatibility between fillers and polymer is recognized as the main reason [11, 12], due to easily forming non-selective voids in the filler-polymer interface. Once the interfacial voids have formed, the non-selective holes will be the preferential transport path instead of the nanoporous fillers [13], inevitably leading to poor rejection [14, 15]. However, the conventional approaches, such as sonication [16] and stirring [17], are not adequately effective to resolve this issue. Thus, it is critical to find an effective method to improve the compatibility in filler-polymer interface. Table 1. Performance of OSN MMMs according to literature. Porous filler Polymer
diameter (wt%)
Permeance (L m-2 s-1 bar-1)
Filler Solvent
Permeance enhancement
Rejection decline
Ref.
Pure MMM
(nm)
(%)
(%)
membrane Matrimid®
UiO-66-NH2
1.2
Ethanol
3.85±0.95
4.43±0.58
15.06
-
[18]
1.2
DMF
3.06±0.03
3.90±0.19
27.45
-
[18]
(0.1) Matrimid
®
UiO-66-NH2 (0.1)
PA
MIL-53
0.86
Methanol
1.8±0.2
2.3±0.4
27.78
-
[9]
PA
ZIF-8
0.34
Methanol
1.8±0.2
2.5±0.6
38.89
-
[9]
PDMS
MIL-47 (20)
0.85
Isopropanol
0.54
~0.65
~20.37
~19.54
[19]
PDMS
ZIF-8 (20)
0.34
Isopropanol
0.54
~0.82
~51.85
~31.03
[19]
PDMS
[Cu3(BTC)2]
0.60
Isopropanol
0.54
~0.80
~48.15
~27.59
[19]
-
Isopropanol
~0.70
~0.96
~37.14
~35
[20]
-
Isopropanol
0.83
1.88
126.5
21.14
[21]
(20) P84 PI
MWCNTsNH2 (0.4)
PI
CZIF-8 (20)
In light of the aforementioned analysis, MCM-41 was selected as an inorganic filler to be doped into PI matrix. Because of the high specific surface and the narrow pore size distribution [22], it has been proved a successful porous material used in
pervaporation [23], gas separation [24] and adsorption [25]. Importantly, compared with other common micropore fillers, its mesoporous property was expected to decrease mass transport resistance for a high-enhanced permeance as mentioned above [26, 27]. We further proposed a strategy via covalently bonding surface amine-functionalized MCM-41 (NH2-MCM-41) with PI so as to eliminate the non-selective voids in the MCM-41-PI interface and improve the compatibility of both phases (Fig. 1). The optimal loading of NH2-MCM-41 in MMM was investigated and the OSN performances of MMMs doping MCM-41 and NH2-MCM-41 were compared in both aqueous and organic systems.
Fig. 1. Schematic illustration for comparison of (a) physical blending and (b) covalent bridging between NH2-MCM-41 and PI. 2. Experimental 2.1. Materials MCM-41 were provided by Xianfeng Nano Technology Co., Ltd., China. The 15 wt% PMDA-ODA polyamic acid (PAA)/N-Methyl pyrrolidone (NMP) solution was
obtained by Changzhou Runge Chemical Industrial Co., Ltd., China. 3Aminopropyltriethoxysilane (APTES, 99%), N,N-Dimethylformamide (DMF, 99%) and dyes, including rose bengal (Molecular weight (Mw)=1017.65 g mol-1), congo red (Mw=696.68 g mol-1), chrome black T (Mw=461.38 g mol-1) and methyl orange (Mw=327 g mol-1) were purchased from Macklin Biochemical Co., Ltd., China. Other solvents were purchase from Beijing Chemical Works, China. 2.2. Surface modification of MCM-41 particles The synthesis of NH2-MCM-41 was based on a previous method [26]. In brief, MCM41 (2 g) and APTES (1.2 ml) were mixed in 60 ml toluene. The mixture was kept for 10 h at 110 °C under magnetic stirring. After cooling to room temperature, the product was obtained by centrifugation with 5000 rmp for 10 min with thoroughly repeated washing using plenty of IPA, and dried overnight at 100 °C.
2.3. Membrane preparation. The preparation of MMMs was according to our previous reports [20, 28]. The PMDAODA PAA/NMP solution and particles were mixed in NMP at 600 rpm for 2 h, followed by sonicating and degassing. The content of PAA and doping particles in the casting solution was kept at 15 wt%. The PAA membrane was prepared by the solution casting method on the PET non-woven fabric. The scraper thickness is set at 150 μm. After that, it was transferred into the distilled water coagulation bath for 30 min, and dried by isopropanol-hexane displacement method [29]. Finally, the resultant membrane was immersed in a mixed solvent (Vacetic anhydride: Vtriethylamine = 4:1) and kept
at 100 °C for 36 h [30]. 2.4. Characterization Surface and cross-section membrane morphologies were observed by S4700 scanning electron microscope (SEM, Hitachi High-Technology, Japan). Membranes roughness (5 × 5 µm) was tested by an atomic force microscope (AFM, Dimension Fastscan 2, Bruker Crop., Germany) at a rate of 1 Hz. The channels in MCM-41 were observed by a HT7700 transmission electron microscopic (TEM, Hitachi High-Technology, Japan). The graft of the amine groups on MCM-14 and the covalent bonds between MCM41 and PI were determined by FT-IR (6700 Nicolet, Thermo Fisher Scientific, USA). The hydrophilicity of the MMMs was tested via water contact angles (Shanghai Zhongchen Technic Apparatus Co., Ltd., China). The average value of three measurements was recorded. The N2 adsorption-desorption isotherms of modified and unmodified MCM-41 were tested by an Autosorb-iQ specific surface area analyzer (Quantachrome, USA). The particles were degassed at 250 °C for 3 h and tested at 77K. 2.5. Liquid sorption capacity The measurement of liquid sorption capacity was detailed in our previous research [21]. Briefly, the membrane specimens were accurately weighed before (M1, mg) and after (M2, mg) immersion in several solvents including water, ethanol and isopropanol at room temperature for 72 h, respectively. The liquid sorption capacity was calculated by the following equation [20]:
𝐿𝑖𝑞𝑢𝑖𝑑 𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 (𝑚𝑔/𝑚𝑔) =
𝑀2 −𝑀1 𝑀1
(1)
2.6. Chemical stability of the MMMs The dried membrane specimens (m1) were soaked in EtOH, IPA, acetone, DMF and NMP at 25 ℃ for 7 days under stirring, respectively. Then, the specimens were washed by pure water and dried overnight. Finally, they were weighed (m2) and compared with m1. 2.7. OSN performance The OSN performance was tested using a Sterlitech HP4750 dead-end membrane module. The different dye feed solutions with the same concentration of 50 mg L-1 were maintained at 10 bar with a constant stirring speed of 600 rpm. Before conducting the test, the membrane was pre-compacted at 15 bar for 2 h. A UV-Vis spectrometer (SP752, China) was used to measure the dye concentrations. Long-term stability performance of the MMMs was carried out using IPA solutions containing 50 mg L-1 chrome black T as feed at 15 bar for 50 h. The permeance P (L m-2 h-1 bar-1) and rejection R (%) were calculated using the following equation [31]: ∆𝑉
𝑃 = 𝐴∗∆𝑃∗∆𝑡 𝑅=
𝐶𝑓 −𝐶𝑝 𝐶𝑓
× 100%
(2) (3)
where ΔV (L) and A (m2) are the volume increment in permeate and the effective membrane area, respectively; ΔP (bar) and Δt (h) are the transmembrane pressure and the test interval, respectively; Cf (mg L-1) and Cp (mg L-1) refer to the dye concentration in feed and permeate side, respectively.
3. Results and discussion
3.1. Characterization of MCM-41 and NH2-MCM-41 MCM-41 particles were functionalized by APTES for converting -OH into -NH2 (denoted as NH2-MCM-41). The successful graft of the amine group on MCM-14 surface is verified by FT-IR (Fig. 2a). The peaks located at 2856 cm-1 and 2924 cm-1 are attributed to the stretching vibration of -CH- [32]. The peak at 1560 cm-1 belongs to the N-H deformation vibration [26, 33]. Besides, the other peaks at 800 cm-1 and 1083 cm-1 are associated with the Si-O-Si stretching vibrations, respectively [34]. In order to evaluate the effect of modification on the pore structure of MCM-41, N2 adsorption-desorption isotherms tests were conducted. As illustrated in Fig. 2b, the unmodified MCM-41 shows a typical type-IV isotherm with a H4 hysteresis loop (0.40 < P/P0 < 1.0) and a sharp pore filling step (P/P0 < 0.40). It proves the typical mesoporous structure of MCM-41 [32, 35]. The NH2-MCM-41 displays the similar N2 adsorptiondesorption isotherm. However, because of partial pore blockage of NH2-MCM-41 by APTES, the overall absorbed N2 quantity is lower than pristine MCM-41 [35, 36]. Accordingly, the pore volume and BET surface area decrease from 1.12 cm3 g-1 to 0.54 cm3 g-1 and from 1106 m2 g-1 to 677 m2 g-1 after modification, respectively (Fig. 2c). These results further confirm the successful amine-modification of MCM-41 [26, 37]. It is worth noting that NH2-MCM-41 maintains the mesoporous property (2.50 nm) from the pristine MCM-41, which can meet the further demand of the preparation of MMMs containing filler with large pore diameter (Fig. 2d). In addition, the TEM images (Fig. 3) shows that the presence of pores along the pore direction (yellow circles) and one-dimensional linear channels perpendicular to the pore direction (red circles),
which is characteristic of the pores of MCM-41 [38]. After modification, the typical regular one-dimensional linear channels of MCM-41 are also well preserved [39].
Fig. 2. (a) FT-IR spectra, (b) N2 adsorption (filled symbol) and desorption (open symbol) isotherms, (c) specific surface area and total pore volume and (d) pore diameter distribution of MCM-41 and NH2-MCM-41.
Fig. 3. TEM images of (a) pristine MCM-41 and (b) NH2-MCM-41 (yellow circles: pores on the direction of the pores; red circles: perpendicular to the pores with onedimensional lines). 3.2. Characterization of MMMs
To confirm the covalent bonding between NH2-MCM-41 and PI, FT-IR test was conducted as shown in Fig. 4a. The peaks at 1778 cm-1, 1718 cm-1 and 1360 cm-1 are attributed to C=O symmetry and asymmetry stretching vibrations and C-N stretching vibration, respectively [30, 40]. No obvious differences can be observed in FT-IR spectra between the PI membrane and MCM-41/PI MMMs. In contrast, these peaks observably weaken after doping NH2-MCM-41 particles. Meanwhile, two new appeared peaks at 1546 cm-1 and 1645 cm-1 are related to stretching of C-N and C=O, respectively, which confirms the formation of the covalent bonds between NH2-MCM41 and PI matrix [15]. The residual bond at 1360 cm-1 indicates the existence of the unreacted imide groups [41]. In addition, Fig. 4b displays the evolution of surface chemistry after doping NH2-MCM-41 particles in the PI matrix. These are three peaks at ~ 399.4 eV, ~ 400.4 eV and ~ 400.9 eV in the XPS spectra of NH2-MCM-41/PI, corresponding to free amine groups (-NH2), amide groups (-NH-) and imide groups (N<), respectively [42, 43]. While the only original imide groups of PI are shown in the XPS spectra of MCM-41/PI. This further confirms the formation of the covalent bonds between NH2-MCM-41 and PI [20]. Therefore, this amine-modification method is effective to further construct a covalent bridge between MCM-41 and PI.
Fig. 4. (a) FT-IR spectra and (b) XPS N-1s narrow scan spectra on the surface of PI membrane and the MMMs. Fig. 5 shows the SEM images including surface and cross-section of PI membrane and MMMs. There is a smooth and non-crack surface in the pure PI membrane. However, some distinct defects arise in the surfaces of MCM-41/PI MMMs, especially in the case of high particle loading. It is attributed to the poor compatibility at the interface between MCM-41 and PI, which further leads to the formation of nonselectivity voids due to filler aggregation [44]. On the contrary, the surface and crosssection images of NH2-MCM-41 doped MMMs display the smooth surfaces with uniform particle distributions, which indicates the good compatibility between NH2MCM-41 and PI with no visible defects. Similar result is also obtained from the surface 3D AFM (Fig. 6). Typically, the surface of 7 wt% MCM-41/PI MMM is much rougher than the pure membrane and NH2-MCM-41/PI MMM at the same loading [45].
Fig. 5. SEM images of (A) surface and (B) cross-section of PI membrane and the MMMs.
Fig. 6. AFM images of (a) PI membrane and MMMs with 7 wt% of (b) MCM-41 particles and (c) NH2-MCM-41 particles. The liquid sorption capacity is corresponding to the membrane ability for solvent transport [46]. The effect of the NH2-MCM-41 loading on the liquid sorption capacity was evaluated in water, EtOH and IPA (Fig. 7a). Compared with PI membrane, NH2MCM-41/PI MMMs display a superior liquid sorption capacity as the loading increasing to 5 wt%. This phenomenon is caused by the higher porosity of NH2-MCM41, providing more solvent exchange channels than the pure PI membrane [9, 47]. However, there is a slight decline when particle loading exceeds 7 wt%, resulting from the increased amidation reaction between NH2-MCM-41 and PI and the enhanced rigidity of the polymer chain [48, 49]. Similar phenomena occurred in ethanol and isopropanol systems. Additionally, the molecule polarity and sizes affect the liquid
sorption capacity to some extent [50]. Since the molecular polarity of water, EtOH and IPA is from large to small, while the molecular size obeys the order conversely, the liquid sorption capacity obeys the sequence of water > ethanol > isopropanol. Fig. 7b shows the contact angles of deionized water on the NH2-MCM-41/PI MMMs. The value of MMM decreases with the increment of NH2-MCM-41 particle loading, indicating the enhanced membrane hydrophilicity. It is caused by amine groups on the surfaces of NH2-MCM-41 [34] and facilitates water molecules pass through the membrane more quickly [51]. Similar phenomenon has been reported in MWCNTs embedded MMMs [20, 52].
Fig. 7. Effect of NH2-MCM-41 loading on (a) liquid sorption capacity and (b) water contact angles of the MMMs. The stability in organic solvents is concerned with the lifetime and use-cost of MMMs [53]. The weight losses of NH2-MCM-41/PI MMMs after soaked into EtOH, acetone, IPA, DMF and NMP for one week are summarized in Table 2. It is obvious that there are negligible weight losses in the MMMs, illustrating the outstanding chemical stability in organic solvents. Table 2. The solvent stability of NH2-MCM-41/PI MMMs.
NH2-MCM-41 loading (wt%)
Solvent
Weight loss (%)
0
EtOH
0.02±0.01
0
Acetone
0.00±0.01
0
IPA
0.01±0.01
0
DMF
0.02±0.01
0
NMP
0.03±0.02
3
EtOH
0.04±0.01
3
Acetone
0.02±0.01
3
IPA
0.00±0.01
3
DMF
0.03±0.02
3
NMP
0.03±0.02
7
EtOH
0.02±0.03
7
Acetone
0.03±0.01
7
IPA
0.04±0.02
7
DMF
0.01±0.02
7
NMP
0.01±0.04
3.3. OSN performance of MMMs To evaluate the role of modification in MCM-41 for the OSN performance, the pristine MCM-41 doped MMM was used for OSN firstly. As illustrated in Fig. 8, the pure PI membrane exhibits a favorable rejection (> 97%) to chrome black T and a low IPA permeance (1.02 L m-2 h-1 bar-1). It is caused by the dense structure of the PI membrane which hinders the transport of the solvents [12]. As expected, after incorporating unmodified MCM-41, the permeance enhanced greatly with the increase of MCM-41 loading; but the dye rejections decreased considerably. The possible reason is the poor compatibility that induced the defects at interface (Fig. 5).
Fig. 8. OSN performance of the MCM-41/PI MMMs with different particle loading. The modified MCM-41 doped MMMs were further used to evaluate the OSN performance. As displayed in Fig. 9, when increasing the NH2-MCM-41 loading to 5 wt%, the water, EtOH and IPA permeances reach the top values, which were increased by 102%, 78% and 131%, respectively. It is obvious that NH2-MCM-41 with mesoporous property endows MMMs with high-efficiency transfer path for solvents [54, 55]. Fig. 7b indicates enhanced membrane hydrophilicity after incorporating NH2MCM-41 caused by amine groups on the particle surfaces [34], which is beneficial for improving the transport of polar solvents (e.g., water and alcohols) [56]. However, when the NH2-MCM-41 loading exceeds 5 wt%, the permeances were declined. The possible reasons are the tortuous pathways across membrane caused high transport resistance [57] and aforementioned amidation reaction and the increased chain rigidification. Additionally, the chrome black T rejections of NH2-MCM-4/PI MMMs almost remain constant with the increased particle loading. It contributes to the covalent bridge between NH2-MCM-41 and PI, which effectively avoids forming the nonselective voids in MCM-41-PI interface and improve the compatibility in two phases
[9]. The dye rejection of NH2-MCM-41/PI MMMs in water follows the order of rose bengal > congo red > chrome black T > methyl orange (Fig. 10a), which is consistent with their MWs [58]. Overall, i) the good compatibility between NH2-MCM-41 and PI endows MMMs with high rejection; ii) mesoporous property of NH2-MCM-41 provides high-efficiency transfer process of MMMs. In addition, the long-term operation is also conducted using IPA solutions containing 50 mg L-1 chrome black T at 15 bar, which shows an outstanding stability (Fig. 10b). The decline of permeance in the beginning is probably ascribed to the compaction and pore blockage [20], which in turn contributes to a slight improvement of rejection [58, 59]. Note that, the stable permeance of the NH2-MCM-41/PI are slight lower than the values presented in the Table 3, which is possibly because the long-term test is carried out at a higher pressure of 15 bar, resulting in more compacted of the membranes [5].
Fig. 9. Effect of NH2-MCM-41 loading on the OSN performance of MMMs.
Fig. 10. (a) Rejection of NH2-MCM-41/PI MMMs for various dye molecule in water and (b) the long-term stability of the 5 wt% NH2-MCM-41/PI MMM in IPA. 3.4. Benchmark A comparison of OSN membranes for dyes rejections is summarized in Table 3. The NH2-MCM-41/PI MMM exhibits superior permeance compared with the most reported polymer based MMMs. More significantly, the dye rejection is not sacrificed, especially for the solutes more than 461 g mol-1. This is ascribed to the inherent mesoporous property of MCM-41 and good compatibility between particles and polymer matrix.
Table 3. Current advances of OSN membranes for dyes rejection. Membrane
Dye
MW (g mol-1)
Solvent
Kevlar
Rose Bengal
1017
EtOH
Eosin Y
648
PA/Matrimid PI
Rose Bengal
(Catechol/POSS)/PI
Rose Bengal
5 wt% NH2-MCM-41/PI MMM
P (L m-2 s-1 bar-1)
R (%)
Ref.
2.9
95.4
[53]
EtOH
3.3
98.1
[20]
1017
EtOH
2.7
100
[60]
1017
EtOH
1.26
99
[61]
Rose Bengal
1017
EtOH
4.25
98.16
This work
5 wt% NH2-MCM-41/PI MMM
chrome black T
461
EtOH
4.25
91.16
This work
MWCNTs/P84
Eosin Y
648
IPA
1.4
97.9
[20]
UiO-66-NH2/Matrimid PI
Rose Bengal
1017
IPA
1.17
99.2
[18]
MWCNTs/P84
Rose Bengal
1017
IPA
1.8
99
[15]
(Catechol/POSS)/PI
Rose Bengal
1017
IPA
0.39
99
[61]
5 wt% NH2-MCM-41/PI MMM
Rose Bengal
1017
IPA
2.35
99.03
This work
5 wt% NH2-MCM-41/PI MMM
chrome black T
461
IPA
2.35
95.26
This work
MWCNTs/P84 ®
®
4. Conclusion
In this work, mesoporous NH2-MCM-41 doped PI MMMs showed high solvent permeances due to the decreased transfer resistance and additionally preferential transport channels. Importantly, the formation of the covalent bonding between NH2MCM-41 and PI minimized the interfacial voids and improved compatibility. As a result, 102%, 78% and 131% of increases in the water, ethanol and isopropanol permeances of 5 wt% NH2-MCM-41/PI MMM were obtained, respectively, without obvious reductions of rejections. In view of the effective improvement in OSN performance, the universal and operable modification approach shows a great potential for application of the MMMs.
Acknowledgments This work was supported by the National Key Research and Development Program of China (Grant No. 2018YFB1501703), the National Nature Science Foundation of China (Grant Nos. 21978016, 21676014 and 21706008), and Beijing Natural Science Foundation (Grant No. 2172041).
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Highlights
Decreasing the transport resistance after doping mesoporous NH2-MCM-41.
Improving the interface compatibility via covalently bonding with NH2-MCM-41 and PI.
Breaking the trade-off between solvent permeance and dye rejection.
Remarkable increases in solvents permeances compared with pure PI membrane.
There are no conflicts to declare.
All co-authors have seen the manuscript and approved to submit to your journal. And all the authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere.