Incorporating Graphene Oxide into Alginate Polymer with a Cationic

Apr 2, 2018 - To establish more compatible and efficient interactions for GO and sodium alginate (SA) polymer chains, cations sourced from lignin are ...
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Functional Nanostructured Materials (including low-D carbon)

Incorporating graphene oxide into alginate polymer with cationic intermediate to strengthen membrane dehydration performance Kecheng Guan, Feng Liang, Haipeng Zhu, Jing Zhao, and Wanqin Jin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04093 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Incorporating graphene oxide into alginate polymer with cationic intermediate to strengthen membrane dehydration performance Kecheng Guan, Feng Liang, Haipeng Zhu, Jing Zhao and Wanqin Jin*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, P.R. China

*To whom all correspondence should be addressed. Tel.: +86-25-83172266; fax: +86-25-83172292. E-mail: [email protected] (Prof. W Jin)

Keywords: graphene oxides, cations, ionic cross-linking, hybrid membrane, solvent dehydration

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Abstract Two-dimensional graphene oxide (GO) in hybrid membranes provides fast water transfer across its surface due to the abundant oxygenated functional groups affording water sorption and the hydrophobic basal plane to create fast transporting pathways. To establish more compatible and efficient interactions for GO and sodium alginate (SA) polymer chains, cations sourced from lignin-cation are employed to decorate GO (labelled as CG) nanosheets via cation-π and π-π interactions providing more interactive sites to confer synergetic benefits with polymer matrix. Cations from CG are also functional to partially interlock SA chains and intensify water diffusion. And with the aid of two-dimensional pathways of CG, fast selective water permeation can be realized through hybrid membranes with CG fillers. In dehydrating aqueous ethanol solution, the hybrid membrane exhibits considerable performance compared with bare SA polymer membrane (long-term stable permeation flux larger than 2500 g m-2 h-1 and water content larger than 99.7 wt% with feed water content of 10 wt% under 70 oC). The effects of CG content in SA membrane were investigated and the transport mechanism was correspondingly studied through varying operation condition and membrane materials. In addition, such membrane possesses long-term stability and almost unchanged high dehydration capability.

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1. Introduction Graphene oxide (GO) comprising hydrophobic aromatic rings and hydrophilic oxygenated groups1 enables preferential molecular transport pathways inside its stacked laminates.2 Recurring to this distinct property and subsequent facile tailoring, GO nanosheets have become almost versatile building blocks to play a role in membranes for gas separation,3-6 selective ion penetration,7-9 water purification10-12 and solvent dedydration or recovery.13-17 With the aid of particular affinity to water molecules, GO based materials was widely researched with configuration of either bulk phase (GO laminates) or filler (GO-polymer hybrids) to construct membranes for water-related separation processes. For hybrid membranes, enhancing the interations between fillers and polymer matrix is important to avoid or reduce unfavorable defects. Accordingly, properly introducing interactive species into membrane structure is essential to acquire higher structural stability and separation performance. Hydrogen bond and van der Waals force, in most cases, exhibit in stacked GO laminates18 and GO-polymer environment19 attributed to the abundant polar oxygenated groups from GO, which is benign for the ordered GO arrangement in the laminates or the well distribution in the polymer matrix. Still, both two interactions are comparatively weak and are limited to strengthen the internal membrane structure and molecular separation process. Hence, functionalization of GO through interactions like covalent or noncovalent bonding20 appears to be efficacious for the desired property. A legion of active sites (e.g., aromatic ring, several kinds of oxygenated groups) within GO afford different routes (e.g., aromatic interaction, 3

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ring-opening reaction, electrostatic interaction) to tailor the functionality. Recent researches7,

21-24

have found that cations have valid interactions with GO through

cation-π and coordination interactions. Through such functionality, GO membranes can acquire excellent mechanical and separating properties. With respect to polymeric membrane, swelling in solvents is a common phenomenon caused by the sorption affinity of polymer to specific solvent.25 By affording larger free volume in polymer matrix through swelling, permation flux of the membrane is enhanced while the molecular selectivity is correspondingly weakened for the sake of loose transport channels. As to prevent the polymeric membrane from excessive solvation, methods have been developed to cross-link polymer chains rendered by a myriad of mechanisms including physical entanglement, chemical cross-linking and ionic interactions.26 Polymeric chains in sodium alginate (SA), for example, is facile to cross-link by introducing divalent cations (e.g., Ca2+) with the form of pairs of helical chains pack and surround ions that are locked in between. With this inoic-interaction cross-linking mechanism, SA membrane sustains under aqueous environment to efficiently separate water from organic solvent. In additon, incorporating inorganic fillers (e.g., GO nanosheets) into SA matrix to fabricate hybrid membranes were widely researched27-29 to achieve better separation performance breaking the “trade-off” effect of polymeric membranes. Inspired by the cationic interactions with GO and cross-linking effect with SA matrix, calcium lignosulfonate (CaLS) was employed as cationic intermediate to optimize both GO laminates and SA matrix to construct hybrid membranes for 4

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pervaporation dehydration from aqueous alcohol solution (Figure 1). As fillers in the polymer matrix, the cation functionalized GO (labelled as CG) nanosheets provide more interactive sites to be combined in SA matrix with better compatibility. Meanwhile, cations from CG also partially cross-link the polymer chains inside SA matrix conferring intergrated composites. While cations from CaLS contribute to the structural stability and intergration of hybrid membrane, the anionic lignosulfonate acid groups (LS2-) from CaLS are also preferential for the transport of water molecules.

Figure 1. Illustration of CG doped SA hybrid membrane structure and cationic interactions with GO and alginate chains.

2. Experimental Section 2.1. Preparation of cation decorated GO nanosheets GO was prepared by modified Hummers method. GO suspension (1 mg/ml) was obtained through dispersing GO flakes into deionized water with the aid of sonication. Then CaLS powders (weight ratio to GO: 1:1) was added into the suspension. The

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above suspension was soniated again for several minutes to acquire homogeneous cation decorated GO suspension. 2.2. Fabrication of hybrid membranes Polyacrylonitrile (PAN) ultrafiltration membrane with average pore size of 20 nm was employed as the substrate of composite membrane. Membrane casting solution was prepared by adding SA powders (2 wt% in solution) into CG aqueous suspenion and was susequently stirred at 30 oC for 2 h. The casting solution was placed still for another 1 h in order to remove air bubbles inside the solution. Afterwards, the casting solution was spin-coated on the PAN substrate with the rotational velocity of 1000 rpm. After the membrane was dried in desiccator at room temperature, it was immersed into 0.1 M CaLS aqueous solution for 10 min to cross-link the membrane, then the membrane was rinsed several times by water to remove excessive CaLS on the surface of the membrane. Finally, the processed membrane was dried again in desiccator at room temperature. Varied content of CG in the hybrid membrane could be realized by changing the concentration of CG suspenion. The correspondingly fabricated hybrid membrane was denoted as SA-CG-X membrane, where the X represents the weight ratio (from 0 to 1.5 wt%) of GO to SA. SA and GO-SA hybrid membranes were fabricated by not adding or adding alternative fillers, respevtively. CaCl2 decorated GO nanosheets were prepared through mixing equimolar Ca2+ amount with GO suspension and the corresponding hybrid membrane using these fillers were fabricated similarly to that of SA-CG hybrid membranes (SA membranes with GO or CaCl2 decroated GO doped are denoted as 6

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SA-GO and SA-CaCl2-GO, respectively). Notably, mixing CaCl2 with GO will instantly lead to flocculation of GO nanosheets in suspension due to strong electrostatic interaction. Therefore, the CaCl2-GO suspension is kept vigrous stirring before doping into SA matrix. 2.3. Measurement of separation performance Pervaporation dehydration of water/ethanol solution was operated to analyse the separation performance of the membrane. A typical evaluation procedure can be found in previous work.30 The compositions of the feed and the collected permeate were analysed by gas chromatography. Permeation flux ( ) and separation factor (α) are two key parameters to reflect the separation performance of membrane sample. They are defined as follows: 

 =  =

 / /

(1) (2)

where is the mass of collected permeates,  is the effective area of membrane and  is the operation time.  and  represents the mass fraction of water from the permeate and feed, respectively; the  and  represents the mass fraction of ethanol from the permeate and feed, respectively. 2.4. Calculations of Activation energy by Arrhenius equation The relationship between operation temperature and permeation flux is determined by Arrhenius equation:13

 =   /

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(3)

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where  (kg m-2 h-1) is the permeation flux of component  ;  (kg m-2 h-1), the pre-exponential factor;  (kJ mol-1), the permeation process associated activation energy;  (kJ mol-1 K-1), the gas constant and  (K), the temperature. As  = 8.314 × 10' kJ mol-1 K-1, and taking the log of both sides of equation (3):

()* + = ()* + −

 -.'./



. 

(4)

Therefore, the value of  is obtained by plotting ln *+ vs. 1000/. 2.5. Calculations of driving force-normalized form of permeation flux and selectivity The permeance of individual component *3/(+ (GPU, 1 GPU=7.501 × 10-12 m3(STP)/m2 s pa) and selectivity (β) were calculated by following equations:31-33

*3/(+ =

4 5 6 5 7

;=

=

4 9: 5 8 6 65 6 7

* * 1 wt%). Under this circumstance, the advantages of CG nanosheets with high aspect ratio cannot fully exert. In addition, polymer chain mobility will be also considerably hindered (represented by a higher Tg value) and as elucidated in section 3.2, no CG nanosheets was obviously exposed outside the polymer surface, which means these nanosheets tend to align parallel to the membrane surface.29 The basal plane (graphene-like area) of CG nanosheets can restrict the direct diffusion of molecules, whereas the water sorption effect of CG nanosheets brings contributive effects such as higher molecular diffusion rate (Figure 7c). Consequently, there is a pair of contrary effects for CG nanosheets inside SA matrix. Excessive CG nanosheets (>1 wt%) exhibited higher restricting effect than the contribution (the transport pathways for molecules are extended), resulting in the drop of permeation flux. Since the agglomerated CG nanosheets are also not able to bring more facilitated transport effect for water molecules, and may create unwanted interfacial defects with polymer matrix, the separation factor is cut down.

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Figure 7. a) Separation performance of SA membrane and SA-CG membranes with different CG content under 70 oC; b) SEM cross-section images of free-standing SA-CG membranes with different CG doping content; c) Illustration of transport pathways inside CG membranes with different CG content.

The membrane performance is usually varied with the feed temperature. Bare SA and SA-CG-1 membranes were evaluated with the feed temperature range from 30 to 70 oC. Both permeation flux and separation factor increase under elevated feed temperature for SA (Figure S3) and SA-CG-1 membranes (Figure 8a). Accordingly, the water content in permeates grows from 95.0 to 98.1 wt% and 99.2 to 99.8 wt% for 18

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SA and SA-CG-1 membranes, respectively. In order to acquire better understanding of the water or ethanol molecular transporting behavior through the membrane, the apparent activation energy (Ea) was calculated (Figure S4). Ea values are all positive for water (Ea, water) and ethanol (Ea, ethanol) in SA and SA-CG-1 membranes, which means the transmembrane process is endothermic. Hence the permeation flux increases with incremental driving force resulted from temperature growing. Ea, water value of SA-CG-1 membrane is higher than that of SA membrane while Ea, ethanol of SA-CG-1 membrane is lower than that of SA membrane, leading to higher separation factor due to the more temperature-sensitive permeation flux. SA-CG-1 membrane thus achieves better separation performance. Besides, the driving force-normalized form of permeation flux and selectivity under different operation temperatures were calculated to analyze the permeation behavior and the results were listed in Table S1. The permeance values of SA-CG-1 and SA membranes decreased at higher temperatures which indicated the driving force was the dominant parameter for the permeation flux. Moreover, it also revealed that the membrane structure was not swelled obviously at high temperatures to afford the increased permeance. The water selectivity over ethanol for SA-CG-1 kept increasing with elevated temperature, which means ethanol molecules encountered higher transport resistance under high temperatures.

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Figure 8. a) Separation performance of SA-CG-1 membrane under varied operation temperatures; b) Performance comparison between different membranes.

With the purpose of exploring the contributive effects of CG nanosheet fillers, control membranes of SA, SA with 0.9 wt% pristine GO fillers (SA-GO-0.9), SA with 1 wt% CaCl2 decorated GO fillers (SA-CaCl2-GO-1, this filler content was calculated based on the same Ca2+ decoration amount as CG, which is actually 0.98 wt% and here is approximate to 1 wt%.) were fabricated to compare their dehydration performance with SA-CG-1 membrane (Figure 8b). Comparing SA-GO-0.9 and SA-CaCl2-GO-1 membranes, the former reveals higher flux while the latter shows higher separation factor. This demonstrates that the introduction of Ca2+ enables more 20

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effective microstructure for separating molecules, possibly resulted from better filler combination with polymer and partial cross-linking of polymer chains inside the polymer matrix (revealed by the drop of permeation flux compared with SA-GO-0.9 membrane). Besides, GO nanosheets offering fast selective water transport channels render high permeation flux and separation factor simultaneously compared with bare SA membrane. With respect to SA-CG-1 membrane, except the benefits brought by Ca2+, the LS2- groups also exert advantageous properties for the selective transport of water molecules (including more active sites for combination with polymer and hydrophilicity for water sorption). Effects of operation temperature on membrane performance for SA-GO-0.9 and SA-CaCl2-GO-1 membranes can be found in Figure S5. In view of this, the performance of SA-CG-1 membrane is outstanding among these control membranes. 3.4. Membrane stability Generally, the SA precursor membrane is soaked in calcium chloride aqueous solution to complete the gelation or cross-linking process before put into separation processes. The α-guluronic acid blocks of alginate will rapidly gel with the presence of divalent cations such as Ca2+.26 Here the CaLS was employed as Ca2+ source to accomplish the cross-linking process of SA or CG membranes. To observe the stability of CaLS cross-linked membrane, the cross-linked membrane sample (SA-CG-1) along with the control membrane sample without cross-linking were soaked in feed solution (90 wt% ethanol-water). The control sample gradually dissolved in the feed solution during several hours due to the sorption of water while 21

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the cross-linked one swelled to some extent but kept as a complete piece even after several months (Figure 9). For this reason, the CaLS is also valid for the cross-linking of SA based membranes. In the meantime, the swelling degree of cross-linked SA-CG-1 and SA membrane were evaluated. After 100 h soaking in feed solution, the swelling degree of CG-1 membrane is 2.02% while the SA membrane is 7.01%. As a few CaLS has been already introduced into the CG membrane when CG nanosheets are doped into the SA matrix ahead of final cross-linking process (soaking in CaLS membrane), CG membrane could possibly own higher cross-linking degree compared with SA membrane under same condition. CG nanosheets may also rigidify the SA polymer chains to swell less in aqueous solution.

Figure 9. Digital photos of SA-CG membrane samples with (left) and without (right) CaLS cross-linking.

Additionally, SA-CG-1 membrane was also evaluated with water-rich feed solution (50 wt% ethanol-water, Figure S6). With higher water content in the feed, 22

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membranes usually swell by the sorption of water. Under this condition, the permeation flux of SA-CG-1 membrane reached almost 8000 g m-2 h-1, which is pretty high among polymeric membranes and a compromised 95.8 wt% water content value in permeate was acquired. Water-rich feed solution lead to swelled membrane structure, resulting in faster water transfer through the membrane but the separation of water and ethanol molecules is less effective. Nevertheless, soaked in water-rich feed solution, our membrane still exhibited good separation performance with considerable permeation flux and proved the stability of the membrane structure as well. Long-term operation of SA-CG-1 membrane was carried out to observe the performance change. With feed solution of 90 wt% ethanol-water, SA-CG-1 membrane continuously worked during pervaporation dehydration process for 100 h. The decline of permeation flux at first 50 h was attributed to the relaxation of SA polymer chains.28 Finally, the permeation flux stabilized with a value of ~2500 g m-2 h-1 and the water content in the permeate kept larger than 99.7 wt% (Figure 10a). More importantly, SA-CG membranes present impressive ethanol dehydration performance compared with recently reported SA-based and GO-based membranes (Figure 10b, see detailed list in Table S2).

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Figure 10. a) Long-term evaluation of SA-CG-1 membrane under 70 oC; b) Comparison of SA-CG membrane with other reported membranes (SA-based and GO-based membranes).15-16, 27-29, 32, 36-37, 41-52

4. Conclusions In conclusion, a new kind of GO-based filler (CG nanosheets) is prepared through cation decoration by CaLS. By incorporating CG nanosheets into SA polymer matrix, the ethanol dehydration performance is strengthened with stable permeation flux of ~2500 g m-2 h-1 and water content in permeate > 99.7 wt% (compared with 1560 g m-2 h-1 and 97.3 wt% of SA membrane). Cation decorated GO nanosheets lead to proper combination with SA polymer chains as well as more stable internal structure of SA 24

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matrix. Besides, the LS2- groups from CaLS also act as hydrophilic sites contributing to the transport of water molecules. The content of CG fillers affect their arrangements in polymer matrix and polymer chain mobility, resulting in varied forms of transporting channels in membrane layer and accordingly different membrane performances. Significantly, aided by the effective external cross-linking of SA-CG membranes through immersion in CaLS solution, combining internal Ca2+-GO fillers cross-linking, the SA-CG membrane reveals stable and excellent dehydration performance among literature reports. The cation decorated GO fillers is also promising to be applied in other hybrid membranes to afford different assembling behaviors and performances. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (grant Nos. 21476107, 21490585, and 21776125), the Innovative Research Team Program by the Ministry of Education of China (grant No. IRT_17R54) and the Topnotch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP). Supporting Information Zeta potential analysis of GO and CG suspensions (Figure S1); EDS analysis of SA-CG-1 membrane (Figure S2); Pervaporation performance of SA membrane under different temperatures (Figure S3); Arrhenius plots of SA and SA-CG-1 membranes for permeation flux (Figure S4); Pervaporation performance of SA-GO-0.9 and SA-CaCl2-GO-1 membrane under different temperatures (Figure S5); Separation 25

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performance of SA-CG-1 membrane with water-rich feed solution (Figure S6); Driving force-normalized permeation flux and selectivity of SA-CG-1 and SA membranes under different temperatures (Table S1); Comparison of the ethanol dehydration performance in this study with membranes in literature reports (Table S2). References (1) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 2010, 39 (1), 228-40, DOI: 10.1039/b917103g. (2) Liu, G.; Jin, W.; Xu, N. Graphene-based membranes. Chem. Soc. Rev. 2015, 44 (15), 5016-30, DOI: 10.1039/c4cs00423j. (3) Shen, J.; Liu, G.; Huang, K.; Chu, Z.; Jin, W.; Xu, N. Subnanometer Two-Dimensional Graphene Oxide Channels for Ultrafast Gas Sieving. ACS Nano 2016, 10 (3), 3398-409, DOI: 10.1021/acsnano.5b07304. (4) Chi, C.; Wang, X.; Peng, Y.; Qian, Y.; Hu, Z.; Dong, J.; Zhao, D. Facile Preparation of Graphene Oxide Membranes for Gas Separation. Chem. Mater. 2016, 28 (9), 2921-2927, DOI: 10.1021/acs.chemmater.5b04475. (5) Wang, S.; Xie, Y.; He, G.; Xin, Q.; Zhang, J.; Yang, L.; Li, Y.; Wu, H.; Zhang, Y.; Guiver, M. D. Graphene Oxide Membranes with Heterogeneous Nanodomains for Efficient CO2 Separations. Angew. Chem. Int. Ed. 2017, 56 (45), 14246-14251, DOI: 10.1002/anie.201708048. (6) Zhou, F.; Tien, H. N.; Xu, W. L.; Chen, J.-T.; Liu, Q.; Hicks, E.; Fathizadeh, M.; Li, S.; Yu, M. Ultrathin graphene oxide-based hollow fiber membranes with brush-like CO 2-philic agent for highly efficient CO 2 capture. Nat Commun 2017, 8 (1), 2107, DOI: 10.1038/s41467-017-02318-1. (7) Chen, L.; Shi, G.; Shen, J.; Peng, B.; Zhang, B.; Wang, Y.; Bian, F.; Wang, J.; Li, D.; Qian, Z.; Xu, G.; Liu, G.; Zeng, J.; Zhang, L.; Yang, Y.; Zhou, G.; Wu, M.; Jin, W.; Li, J.; Fang, H. Ion sieving in graphene oxide membranes via cationic control of interlayer spacing. Nature 2017, DOI: 10.1038/nature24044. (8) Abraham, J.; Vasu, K. S.; Williams, C. D.; Gopinadhan, K.; Su, Y.; Cherian, C. T.; Dix, J.; Prestat, E.; Haigh, S. J.; Grigorieva, I. V. Tunable sieving of ions using graphene oxide membranes. Nature Nanotechnology 2017, 12, 546-550, DOI: 10.1038/nnano.2017.21. (9) Joshi, R. K.; Carbone, P.; Wang, F. C.; Kravets, V. G.; Su, Y.; Grigorieva, I. V.; Wu, H. A.; Geim, A. K.; Nair, R. R. Precise and ultrafast molecular sieving through graphene oxide membranes. Science 2014, 343 (6172), 752-754, DOI: 10.1126/science.1245711. (10) Huang, L.; Chen, J.; Gao, T.; Zhang, M.; Li, Y.; Dai, L.; Qu, L.; Shi, G. Reduced Graphene Oxide Membranes for Ultrafast Organic Solvent Nanofiltration. Adv. Mater. 26

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