Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl

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Aerosol crosslinked crown ether diols melded with poly(vinyl alcohol) as specialized microfibrous Li adsorbents +

Lawrence A. Limjuco, Grace Masbate Nisola, Rey Eliseo Castillo Torrejos, Jeong Woo Han, Ho Seong Song, Khino J. Parohinog, Sangho Koo, Seong-Poong Lee, and Wook-Jin Chung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14858 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 23, 2017

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ACS Applied Materials & Interfaces

Aerosol crosslinked crown ether diols melded with poly(vinyl alcohol) as specialized microfibrous Li+ adsorbents Lawrence A. Limjuco,†a Grace M. Nisola,†a Rey Eliseo C. Torrejos,†a Jeong Woo Han,b Ho Seong Song,b Khino J. Parohinog,a Sangho Koo,c Seong-Poong Lee, *a Wook-Jin Chung*a

a

Energy and Environment Fusion Technology Center (E2FTC), Department of Energy

Science and Technology (DEST), Myongji University, Myongji-ro 116, Cheoin-gu, Yongin City, Gyeonggi Province, Republic of Korea (17058) b

c

Department of Chemical Engineering, University of Seoul, South Korea (02504)

Department of Chemistry, Myongji University, Myongji-ro 116, Cheoin-gu, Yongin City,

Gyeonggi Province, Republic of Korea (17058) †

co-first authors

Corresponding authors: *(W.-J. Chung) [email protected]; *(S.-P. Lee) [email protected]. Tel.: (82)-31-330-6687; Fax: (82)-31-337-2902

ABSTRACT

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crown ether (CE)-based Li+ adsorbent microfibers (MFs) were successfully fabricated through a combined use of CE diols, electrospinning, and aerosol crosslinking. The 14-16 membered CEs, with varied ring sub-units and cavity dimensions, have two hydroxyl groups for covalent attachments to poly(vinyl alcohol) (PVA) as the chosen matrix. The CE diols were blended with PVA and transformed into microfibers via electrospinning, a highly effective technique in minimizing CE loss during MF fabrication. Subsequent aerosol glutaraldehyde (GA) crosslinking of the electrospun CE/PVA MFs stabilized the adsorbents in water. The aerosol technique is highly effective in crosslinking the MFs at short time (5 h) with minimal volume requirement of GA solution (2.4 mL g-1 MF). GA crosslinking alleviated CE leakage from the fibers as the CEs were securely attached with PVA through covalent CE-GA-PVA linkages. Three types of CE/PVA MFs were fabricated and characterized through FTIR-ATR,

13

C CP/MAS NMR, FE-SEM, N2 adsorption/desorption,

and UTM. The MFs exhibited pseudo-second order rate and Langmuir-type of Li+ adsorption. At their saturated states, the MFs were able to use 90-99% CEs for 1:1 Li+ complexation suggesting favorability of their microfibrous structures for CE accessibility to Li+. The MFs were highly Li+-selective in seawater. Neopentyl bearing CE was most effective in blocking larger monovalents Na+ and K+ while the dibenzo CE was best in discriminating divalents Mg2+ and Ca2+. Experimental selectivity trends concur with the reaction enthalpies from DFT calculations, confirming the influence of CE structures and cavity dimensions in their "sizematch" Li+ selectivity. KEYWORDS: adsorption, crosslinking, crown ether, electrospinning, lithium recovery, microfiber, polyvinyl alcohol


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1. INTRODUCTION Lithium (Li) is an “energy-critical element” whose shortage could significantly hamper the progress of emerging electronic and energy-related technologies.1,2 Currently, Li is being produced from ores, pegmatites, and brines.3 But surging Li demand has incited its recovery from other possible resources like the seawater, bitterns, and industrial wastewater.3 There is no established technology for mining diluted lithium ions (Li+), but adsorption has been an attractive option.4 Majority of the reported Li+ adsorbents are inorganic lithium ion sieves (LIS) such as λMnO2,4 H1.33Mn1.66O4,5 H1.6Mn1.6O4,6 HMn2O4,7 H2TiO38 and TiO2.9 Depending on their preparation, LIS capacities in diluted Li+ sources vary from 0.7 to 41 mg Li+ g-1. However, LIS exhibit slow internal Li+ diffusion due to their dense lattice structures.5,10-12 They are moderately stable in acidic environment13 and their fabrications require energy-intensive solid-state reactions.4-12 These limitations motivated further research on other Li+ adsorbents. Crown ethers (CE) are macrocyclic acid-stable14 ligands for "hard acid" cations (Mn+) like Li+.15 They exhibit Mn+ selectivity according to the so-called "size-match" relation imbued by their constrained cyclic forms.16 For a more exclusive uptake of the target Mn+, CE rings are rigidified17,18 or modified with blocking sub-units19,20 to avoid the capture of unwanted Mn+ and alleviate the formation of high order complexes.21 Lithium-selective CEs have competitive theoretical capacities of 17.7 - 23.1 mg g-1.22 The simplest Li+ capture systems have used neutral CEs dissolved in extractants,22 doped in supported ionic liquid phase,23 or blended in polymeric microfibers.24 But these unsecured immobilization techniques render the materials prone to CE loss during Li+ adsorption. With costly CEs, a more feasible strategy to produce regenerable Li+ adsorbents is to covalently link the CEs on solid supports.



Improved studies have employed CEs modified with amine,25 vinyl,26 allyloxy methyl,27 carboxyl,28,29 and hydroxyl (-OH)30-32 groups, which were polymerized,25,28,29 polymerimprinted on magnetite,27 silanized on silica,26,32 and tethered on MWCNT31 or polyglycidyl methacrylate.30 However, most of them have low CE loading or have experienced substantial CE loss during fabrication.26-32 Furthermore, some were processed into resins, hydrogels, or films25,28-30 that have poor mass transport properties, limited by internal diffusion.10,33 It is therefore important to develop a facile fabrication technique that would not only create covalent CE-matrix linkages but also minimize CE loss during fabrication, and produce durable adsorbents with structures that promote Li+ adsorption. To achieve these, this study (Figure 1) demonstrates a very simple but highly effective method of preparing novel CEbased Li+ adsorbents through modified CE/matrix blending, electrospinning, and aerosol crosslinking.

Figure 1. Strategic preparation of CE/PVA MFs employing CE diols, electrospinning and aerosol crosslinking. 4 ACS Paragon Plus Environment

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Among the modified CEs, dihydroxy CEs or CE diols were prepared due to their convenient synthesis routes.22 By melding the CE diols in a support with matching reactive -OH groups, CE-matrix linkage can be conveniently created through a common crosslinker. With its OHrich backbone, poly(vinyl alcohol) (PVA) was the chosen matrix due to its excellent chemical property, mechanical stability and processability.34,35 Meanwhile, glutaraldehyde (GA) was selected as a highly effective crosslinker that can render the PVA insoluble in water34,36 and simulatenously create CE-GA-PVA linkages through acid-catalyzed acetalization.34-36 In effect, water-stable Li+ adsorbents with CEs covalently secured in PVA matrix are produced. Before GA crosslinking, the CE/PVAs were processed via electrospinning, a technique that can completely transform the blends into microfibers (MF) with near-zero wastage of the starting materials.37 Moreover, the narrow fiber diameters of MFs could significantly shorten the Li+ diffusion path10,38 and reduce the effect of internal diffusion.10,33,38 For the final step, aerosol crosslinking was carried out on the electrospun CE/PVA as a new approach to rapidly deliver the acidified GA to the MFs while preventing CE leakage from the fibers. Unlike the conventional immersion method, aerosol crosslinking requires shorter reaction time, preserves the fibrous structure of the MFs, alleviates CE elution, and reduces the needed amount of GA. Aerosol crosslinking is still a widely unexplored technique, only recently employed in 3D alginate scaffolds for bioengineering applications.39 Three CE-based Li+ MFs were fabricated containing different CE diols with unique dimensions and ring sub-units. The MFs were thoroughly characterized to confirm the effectiveness of the preparation method. Adsorption/desorption and selectivity studies were carried out using simulated and seawater samples, respectively. Density functional theory (DFT) calculations were performed to elucidate the “size-match” Li+ selectivities of the CEs.



2. EXPERIMENTAL

2.1 Materials All materials and reagents used in this study are listed in the supporting information (SI p. S6). Purchased compounds were used without further purification. Real seawater samples were collected from Pyeongtaek, South Korea.

2.2 Synthesis and characterization of CE diols Crown ether diols were synthesized using modified procedures from literature,22,40-42 as detailed in SI Figure S1. The synthesized CE diols were numerically denoted as CE 1 (2HDB14C4), CE 2 (2HB2M15C4), and CE 3 (2HB16C4). Full compound names, characterizations by Fourier Transform Infrared Spectroscopy (FT-IR Varian Scimitar FTIR 2000, USA), 1H (400 MHz)- and 13C (100 MHz) Fourier Transform Nuclear Magnetic Resonance (Varian, 400 MR FT-NMR, USA) are provided in SI Figures S2-S5.

2.3 Electrospinning and aerosol crosslinking of CE/PVA MFs A CE/PVA dope is prepared by initially heating the CE diol in deionized (DI) water under constant agitation at 90ºC. After 1 h, PVA and non-ionic surfactant polyoxyethyleneoctyl phenyl ether (Triton X-100 ®, 0.35 wt%) were subsequently added to obtain 9 wt% PVA solution. The added amount of CE was fixed at 50 wt% (w.r.t. PVA amount), the highest loading which produced consistent blend with PVA. The CE/PVA dope was stirred for another 9 h at 90oC. Pure PVA dope was prepared similarly in DI water at 90ºC for 9 hours. The dope is placed in a 12 mL syringe and then fed in the electrospinning apparatus (Model: ESP200D/ESP100D, NanoNC Co., Ltd., South Korea) via a syringe pump (KD Scientific 750, South Korea). Electrospinning was carried out at 24-26 kV and the fibers accumulated


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on a drum-roll type collector (500 rpm) fixed 120 mm away from the nozzle. The details of electrospinning conditions are listed in SI Table S1. Acid-catalyzed acetalization of MFs was performed via aerosol crosslinking using 4 vol% glutaraldehyde solution in anhydrous acetone spiked with HCl (0.3 vol%). The collected MF (560 cm2) was divided into six parts, each fixed on a glass plate and then sprayed with 0.8 ± 0.03 mL GA solution to sufficiently wet the fibers. The chemical crosslinking was completed at room temperature for 5 h. The MFs were vacuum-dried at 50oC for 24 h to remove residual GA. The MF containing 2HDB14C4 was labelled as CE 1/PVA, 2HB2M15C4 as CE 2/PVA, and 2HB16C4 as CE 3/PVA.

2.4 CE/PVA MF characterization FTIR spectra of MFs were recorded on a VARIAN 2000 (Scimitar Series) using the standard ATR method. The solid-state (SS) 13C NMR spectra with Cross-Polarization-Magic Angle Spinning (13C CP-MAS/NMR, Zr rotor ∅out = 4 mm, 12 kHz spinning rate) were acquired on a 400MHz SS-NMR (9.4T) spectrometer (AVANCE III HD, Bruker, Germany). The MFs were examined under a Field Emission Scanning Electron Microscope (FESEM II-EDSEBSD, JEOL JSM-7000F, Japan). Fiber diameter histograms (n ≥100) were constructed from SEM images processed via ImageJ software. Nitrogen adsorption/desorption isotherms were measured at 77 K within relative pressure range (p po-1) = 0.01-1.0 using Belsorp-mini II (Bel Japan, Inc.). Specific surface areas (SA) were acquired using Brunauer-Emmett-Teller (BET) method whereas the pore size distributions were calculated using the Cranston-Inkley (CI) model.43 True densities (ρtrue) were measured using liquid pycnometer whereas the bulk densities (ρbulk) were determined through dimensional measurements (Mitutoyo IP65, Japan) and gravimetry (Ohaus Adventurer Balance, AR2140, USA). Total porosities were estimated from % P = [1-(ρbulk/ρtrue)] × 100.38 Mechanical properties of the MFs were evaluated via pull 7 ACS Paragon Plus Environment


to break test using Universal Testing Machine (UTM LFPlus, Lloyd Instruments, Ametek Inc., UK) equipped with 1 kN load cell. Cut samples (20 mm × 50 mm) were pre-loaded with 0.02 kg-f and were analyzed at a cross-head speed of 50 mm min-1.

2.5 MF adsorbent stability tests The physical stability of crosslinked MFs was examined through elution tests by quantifying % PVA loss and % CE loss. For comparison, control samples were prepared by melding methylated CEs (mCEs) instead of CE diols with PVA. The mCEs are CE diols etherified with methyl iodide (CH3I). Without OH- groups, mCEs cannot crosslink with GA hence they are only physically confined in the crosslinked PVA matrix. The method for mCE synthesis is shown in SI Figure S6 with mCE characterization results in SI Figures S7-S10. Dried MFs were carefully weighed before and after elution. The samples were agitated (200 rpm) in 50 mL water (pH = 8) at 30oC. After 48 h, the liquid samples were analyzed for PVA and CE concentrations as described in section 2.8. Calculations for stability tests are detailed in SI (p. S-18) with equations listed in SI Table S2. Meanwhile, the swelling ratio (S) was calculated as S = (aw – a0)/a0 by comparing the area (a = l × w) of dry (a0) and wet (aw) MFs.44

2.6 Adsorption experiments Adsorption experiments were performed at 30ºC. The MFs were pre-washed in 0.5 M HCl and DI water until neutral pH to remove metal contaminants. The samples were vacuumdried (50ºC, 24 h) and then weighed (mMF ≅ 30 mg). An adsorption run involves agitation of the MF in a V = 50 mL LiCl solution with known Li+ concentration (Co). The samples were agitated at 400 rpm to ensure minimal external mass transport resistance.10 Liquid samples were collected either at time t or at equilibrium (t = 24 h) to determine their final Li+ concentrations Ct or Ce, respectively (section 2.8). Adsorption capacities qt (kinetics) or qe 8 ACS Paragon Plus Environment

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(equilibrium) were calculated using Eq. 1. Initial experiment was performed to observe the effect of pH on the adsorption behaviour of the MFs (pH = 2 – 11, Co ≅ 7 mg Li+ L-1). Batch kinetics (pH = 8, Co ≅ 7 mg Li+ L-1, varied t) and adsorption isotherm (pH = 8, Co = 3 – 70 mg Li+ L-1) experiments were conducted to evaluate the Li+ adsorption performance of the MFs.24,27,36

qt =

C o − Ct C − Ce × V ( kinetics ); qe = o × V (equilibriu m ) m MF m MF

(1)

The selectivity of the MFs towards Li+ was investigated using real Li+-enriched seawater (actual pH = 8.32) which contains high levels of interfering cations (i.e. Na+, Mg2+, K+, and Ca2+). The MFs were evaluated through distribution coefficient (Kd) using Eq. 2, Li+ separation factors relative to other Mn+ (αLi+/Mn+) in Eq. 3, and concentration factor (CF) in Eq. 4.

Kd =

(C o − C e ) × V

α Li + / M n+ =

CF =

(2)

C e × m MF

K dLi

(3)

K dM

qe Ce

(4)

The recyclability of the MFs were tested through repeated adsorption/desorption runs (Co ≅ 7 mg Li+ L-1, pH = 8). After adsorption, spent MFs were rinsed with DI water and were regenerated using 50 mL 0.5 M HCl to strip the Li+. After DI wash, the samples were gently pressed between Kimwipes®, and then used for the next cycle. After five cycles, the MFs were autopsied using FE-SEM. The stripping solutions were analysed for eluted CEs using UV-Vis spectrophotometry (section 2.8). 9 ACS Paragon Plus Environment


2.7 Density Functional Theory (DFT) calculations The selectivity behaviour of MFs towards Li+ was further elucidated via DFT. The Vienna ab

initio simulation package (VASP) was used for the plane wave DFT calculations.45 The Perdew-Burke-Ernzerhof (PBE) generalized gradient functional along with the projector augmented wave (PAW) method was used to describe the ionic cores.46,47 A plane wave expansion with a cut-off of 650 eV was used with a 1×1×1 Monkhorst–Pack k-point sampling of the Brillouin zone. Total energy calculations were conducted using the residual minimization method for electronic relaxation, accelerated using Methfessel-Paxton Fermilevel smearing with a width of 0.2 eV. The geometries of the free CE and CE-Mn+ complexes were optimized in a 30Å×30Å×30Å supercell. One or two electrons were removed from the atoms for the calculations of alkali and alkaline earth metal cations.

2.8 Analytical methods Metal ion concentrations were quantified using inductively coupled plasma mass spectrometry (ICP-MS Agilent 7500 series, USA). Liquid samples were acid digested (5 mL sample + 10 mL DI + 5 mL 60% HNO3) through microwave irradiation (MARS-5 CEM, USA) and diluted before ICP-MS measurements. The pH values were recorded using a pH electrode (Z451 SI Analytics GmBH, Germany). Eluted PVA and CEs in stability test samples were determined using total organic carbon (TOC) analyser (VCPH, Shimadzu, Japan) with air as carrier gas (200 kPa, 150 mL min-1). TOC calibration curves are shown in SI Figure S11. CE diols and mCEs were selectively detected through their absorbances at

λmax ≅ 275 nm (SI Figure S12) using UV-Vis spectrophotometer (8453 Agilent, USA). Their standard curves are provided in SI Figures S13-S14. Example UV-Vis profiles of elution samples are also provided in SI Figure S15.


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3. RESULTS AND DISCUSSION 3.1 CE diols

Figure 2. (a) Minimum energy structures of free CE diols; (b) FTIR spectra of CE diols and pure CE diols crosslinked with GA (CE/GA materials). The CE diols were synthesized using modified procedures from literature22,40-42 via intermolecular cyclization of various bis-epoxides with catechol (SI Figure S1). All CE diols contain four ethereal oxygens available for Li+ complexation,30,31 benzo group(s) for cavity rigidification,17,18 and two -OH groups as attachment sites to the PVA matrix via GA crosslinking.31,48 CE 1 is a planar and symmetric 14-membered ring with two benzo groups 11 ACS Paragon Plus Environment


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whereas CE 2 is a 15-membered macrocycle with a dimethyl substituted tertiary carbon as its bulky alkyl moiety (R = -CH2C(CH3)2-CH2-). Meanwhile, CE 3 is a 16-membered ring with a flexible butyl chain (R = -CH2-CH2-CH2-CH2-). The reactions were easy to perform22 and afforded moderate to high yields (65-85%) as confirmed by 1H and

13

C-NMR analysis (SI

Figures S2 – S5). Optimized minimum energy structures of the CEs from DFT calculations (Figure 2a) confirm that CE 1 has the smallest cavity (∅c=1.23 Å vs. CE 2 ∅c =1.34 Å and CE 3 ∅c =1.52 Å).22 Nonetheless, all had matching ring dimensions with the ionic size of Li+ (∅Li+ =1.18 [IV] 1.52 Å [VI]) suggesting their suitability as Li+ ligands.49 The CE diols exhibited characteristic FTIR peaks (Figure 2b) from the O-H stretching of the hydroxyl groups (34023340 cm-1), from the C-H stretching (2930-2865 cm-1)34,50 and bending (1386-1361 cm-1) of the rings, from the C=C stretching of benzene (1594-1452 cm-1),51 and from the C-O-C ether stretching of the rings (1257-1032 cm-1).24 3.2 Reactivity of CE diols with GA For GA to successfully crosslink both CE diols and PVA, the CE diols must also exhibit reactivity with GA. Ancillary tests were carried out by removing the PVA component and allowing pure CE diols to crosslink with GA. The acid-catalyzed reactions afforded solid brown CE/GA products. FTIR results of the CE/GA samples (Figure 2b) reveal the C=O stretching (1722-1705 cm-1) of unreacted GA34,36 suggesting that not all aldehydes reacted with the CE diols. This is understandable since GA was added in excess with respect to the CEs (~6 mmol/mmol diol). Similarly, the O-H stretching (3437-3364 cm-1) and the doublets (2929-2862 cm-1) for C-H stretching were present in all samples. The characteristic peaks (P) were integrated and divided with those of pure CE diols to obtain integration ratios IP =ICE/GA/ICE diol (SI Table S3) and determine if CE/GA crosslinking occurred. All CE/GA 12 ACS Paragon Plus Environment

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samples exhibit reactivity towards GA as indicated by the reduced OH groups (IP 1). The broadened peaks at 1034 – 1106 cm-1 in CE/GA samples are also strong indications of acetal group formation.34,36,50 These results confirm the ability of all CE diols to crosslink with GA. The IP results suggest that CE 3 is most reactive to GA (details in SI p. S-23). On the other hand, the lower reactivity of CE 1 could be due to the steric hindrances of its benzo groups.52 3.3 CE/PVA MF preparation and aerosol crosslinking During electrospinning, the entire dope solutions (12 mL) were successfully fabricated into MFs achieving a near-zero wastage of starting materials. Subsequent aerosol technique was highly beneficial as the crosslinking was completed at a shorter period and utilized minimal volume of GA/acetone solution. Previously, 1 g of PVA/inorganic adsorbent composite foam needed 1L of 4 vol% GA solution.36,53 Meanwhile, immersion technique required 0.5-1 L of 0.6-5 vol% GA solutions to crosslink 1-2 g of PVA nanofiber for 24-48 h.44,54 In aerosol method, only 4.8 ± 0.05 mL of 4 vol% acidified GA/acetone aerosol was used to successfully crosslink 560 cm2 (2 g) of MF at room temperature for 5 h. The CE/PVA MFs were evaluated in terms of their 1) stability in water and 2) crosslink formation of GA with CE and PVA (Figure 3). When immersed in water, uncrosslinked MFs (Figure 3a) immediately disintegrated in contrast to the highly opaque and stable crosslinked samples. The MFs experienced minimal swelling (SI Table S4) especially the CE/PVA samples (S ≅ 0.13). The crosslinked CEs rigidified the MFs by restricting the relaxation of PVA chains in water. The low S = 0.32 of pure PVA is also typical when a non-solvent like acetone is used during GA crosslinking.44



Figure 3. Stability tests and FTIR analysis of MFs: (a) Optical images of un-crosslinked and crosslinked CE/PVA samples; (b) % CE losses in CE/PVA and control mCE/PVA MFs; (c) FTIR spectra of CE/PVA samples. Stability tests quantitatively confirm (SI Table S5) the ability of aerosol GA crosslinking to covalently secure the CEs in the MFs. Gravimetric results reveal the substantial mass loss (27 – 66% MF mass loss) in control mCE/PVA samples (SI Table S5) due to the leakage of mCEs (13 – 95%) from the fibers (Figure 3b). This confirms that physical entrapment is ineffective to permanently confine the mCEs in the matrix. Furthermore, the substantial 14 ACS Paragon Plus Environment

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presence of physically bound mCEs inhibited GA from effectively crosslinking with PVA. Thus, all mCE/PVA also suffered from significant PVA losses (26 – 55%, SI Table S5). Meanwhile, CE/PVA samples demonstrated negligible MF (< 1.6%) and PVA (

Aerosol Cross-Linked Crown Ether Diols Melded with Poly(vinyl

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