Synthesis of new crosslinked porous ammonium-based poly(ionic liquid) and application in C[O.sub.2] adsorption.
C[O.sub.2] is widely accepted as a main greenhouse gas (1); petroleum, coal, and natural gas, which are major sources of C[O.sub.2], will continue to be the primary global fuel and chemical feedstock sources for next few years to come (2), (3). Therefore, C[O.sub.2] capture is an environmentally desirable process in the industry. Alkylolamine solutions are generally employed to chemically capture C[O.sub.2] in the industry but several drawbacks, such as the concurrent loss of volatile amines and corrosion problem accompanied the process.
Ionic liquids (ILs) are a new class of organic salts that are liquids at or near room temperature. It was observed that ILs have remarkable C[O.sub.2] solubility (4-7), for example, the solubility of C[O.sub.2] in 1-buty1-3-methylimidazolium hexa-fluorophosphate ([[C.sub.4]mim][P[F.sub.6]]) can reach 0.15 mol of C[O.sub.2] per mole of IL at 25[degrees]C and 0.8 MPa (7). ILs, however, generally have high viscosity, typically 2-3 orders of magnitude higher than traditional organic solvents (6), (7), which causes poor heat/mass transfer, prolonged absorption equilibrium time, and might limit their final application in the industry especially in the case of chemical absorption (the ILs tend to become very viscous after absorbing C[O.sub.2]).
Recently, poly(ionic liquid)s (PILs) have attracted much attention. PILs are synthesized by the polymerization of solid or liquid IL monomers which contain polymerizable units such as vinyl and (meth) acryloyl. PILs have enhanced mechanical stability, improved processability, and spatial controllability as opposed to their corresponding IL monomers (8-12), especially as viscosity is not a problem. In fact, it was found that PILs generally have higher C[O.sub.2] adsorption capacity with faster adsorption/desorption than their corresponding IL monomers under similar condition (Table 1) (12-18), for example, poly [1-(p-vinylbenzyl)-3-methy limi-dazoli urn tetrafluoroborate] and poly(4-vinylbenzyltrimethy-lammonium tetrafluoroborate) can adsorb 3.05 and 4.85 mol% at 22[degrees]C and 0.078 MPa, respectively, whereas their monomers have negligible C[O.sub.2] adsorption.
TABLE 1. C[O.sun.2] absorpiion capacities of ionic liquids (at 0.1 MPa and 25[degrees]C) and PILs (at 0.078 MPa and 22[degrees]C) (a). PIL and IL C[O.sub.2] absorption capacity (mol%) P[VBTMA][P[F.sub.6]] 10.6 P[VBTMA][B[F.sub.4]] 10.2 P[VBTEA][B[F.sub.4]] 4.8 P[MATMA][B[F.sub.4]] 7.9 P[VBTEA][Sac] 2.7 P[VBTMA][T[f.sub.2]N] 2.8 P[VBMI][B[F.sub.4]] 3.0 [[C.sub.4]min][B[F.sub.4] 1.3 [[C.sub.4]min][B[F.sub.6] 1.8 (a.) The lisied data an from [4-7, 11-15]. Abbreviations: P[VBBI][T[f.sub.2]N], poly[1-(p-vinylbenzyl) -3-buiylirnidazohum bis(trifluorornethylsulfonyl) imide]; P[VBTMA][Sac], poly[p-vinyl-benzyltrirricthylarnmonium tetrafluoroborate o-benzoicsulphimide]; P[VBMI][B[F.sub.4]], poly[1-(p-vinyl benzyl)-3-metliylimidazolium tetrafluoroborate]; [[C.sub.4]mim][P[F.sub.6]], 1-n-buiyl-1-rnethylpyrrohidinium hexafluorophosphate.
In this study, a new crosslinked and porous PIL, N,N-methylenebisacrylamide (MBA)-crosslinked poly(4-vinyl-benzyltriethylammoniurn hexafluorophosphate), abbreviated as MBA-crosslinked-P[VBTEA][P[F.sub.6]], is prepared by inverse suspension polymerization, and its C[O.sub.2] adsorption capability is investigated. Another PIL of P[VBTEA][P[F.sub.6]] is also prepared by radical solution polymerization without being crosslinked and porated, and is comparatively studied for C[O.sub.2] adsorption.
4-Vinylbenzyl chloride (>90%) is purchased from Acros Organics; analytical-grade triethylamine, anhydrous ether, acetonitrile, N,N-dimethylformamide (DMF), dichloromethane, potassi urn persulfate, ammonium persulfate, span 80, Tween-80, cyclohexane, and diethyl ether are purchased from Beijing Chemical Plant; potassium hexafluorophosphate (>99%) is purchased from Sinopharm Chemical Reagent Beijing; analytical-grade azobi-sisobutyronitrile (AIBN) and 2,6-di-tert-butyl-4-methyl phenol are purchased from Tianjin Jinke Fine Chemicals; MBA is purchased from Aladdin Chemical; PEG 600 is purchased from Sinopharm Chemical Reagent Beijing. Carbon dioxide (C[O.sub.2], [greater than or equal to]99.99%) is purchased from the Chinese Special Gas. DMF, AIBN, and acetonitrile are further purified by distillation, whereas the other chemicals are used as received without further purification.
Preparation of PILs
[VBTEA][C1] was prepared through the reaction of 4-vinylbenzyl chloride with triethylamine (Scheme 1). A reaction of 4.25 g of 4-vinylbenzyl chloride and 6.41 g of triethylamine proceeded at 50[degrees]C under [N.sub.2] for 12 h. The solid was collected, washed with diethyl ether, and dried under vacuum for 24 h.
As shown in Scheme 2a, 14.31 g of self-prepared [VBTEA][C1] reacted with 1.0 equiv of KP[F.sub.6] in 50 mL of acetonitrile at 25[degrees]C for 2 days to ensure complete ion exchange. The precipitate was filtered off and the concentrate was washed three times with ether. The product [VBTEA][P[F.sub.6]] was dried in vacuum at 45[degrees]C for 24 h. [VBTEA][P[F.sub.6]], 3.02 g, and AIBN, 3 mg (monomer, 1 wt%) were charged into 25-mL glass reactor containing 6 mL of DMF and magnetically stirred. The conventional radical solution polymerization was conducted at 80[degrees]C for 16 h under nitrogen protection. After polymerization, the DMF concentrate was poured into dichloromethane to obtain the precipitate which was washed three times with DMF and dichloromethane. The obtained P[VBTEA][P[F.sub.6]] was dried in vacuum at 60[degrees]C for 24 11.
For the synthesis of MBA-crosslinked-P[VBTEA][P[F.sub.6]], MBA-crosslinked-P[VBTEA][Cl] was first synthesized and then ion exchange was performed (Scheme 2b). Synthesis proceeded via inverse suspension polymerization of [VBTEA][C1] with crosslinker MBA. The continuous oil phase consisted of span 80, Tween-80, and cyclehexane. The dispersed aqueous phase consisted of [VBTEA][C1], MBA, initiator ammonium persulfate, porogen PEG 600, and deionized water. In the experiment, span 80--Tween-80 and cyclohexane were added into a 100-mL four-neck flask, and the mixture was stirred at 45[degrees]C under N2 atmosphere for half an hour. A mixture of [VBTEA][C1], MBA, PEG 600, as well as an aqueous solution of ammonium persulfate was then added into the continuous oil phase through constant pressure dropping funnel in 5 min. The stirrer speed was set at 400 rpm and the polymerization was allowed to proceed at 70[degrees]C for 6 h. After polymerization, the particles of MBA-crosslinked-P[VBTEA][C1] were collected and washed with deionized water and acetone, and vacuum-dried at 80[degrees]C until a constant weight was attained. KP[F.sub.6] (2.22 mmol) was dissolved in 5 mL of deionized water exposed to ultrasound for 5 min, and added dropwise to the solution of 1.97 mmol of P[VBTEA][C1] microparticles dispersed in 30 mL of deionized water. After reacting for 1 h at room temperature, the resulting white particles of MBA-cross-linked-P[VBTEA][P[F.sub.6]] were filtered off, washed with deionized water, and dried at 80[degrees]C under vacuum.
C[O.sub.2] Adsorption and Desorption Measurements
C[O.sub.2] adsorption by PILs was measured using Autosorb-iQ-MP by Quantachrome. The PILs were dried and degassed at 125[degrees]C for 12 h to remove moisture or other volatile contaminants before adsorbing C[O.sub.2]. C[O.sub.2] was introduced into the chamber under the pressure of 0-1 bar at 25[degrees]C. Measurements of the sample were performed using a Sartorius BS-124S electrogravimetric balance (sensitivity, 0.1 mg). Bulb cells, 9 mm large, of a known weight were loaded with 45 mg of sample for C[O.sub.2] sorption experiments. Samples were degassed at 100 or 125[degrees]C for 20-22 h on degassing station until the outgas rate was no more than 3.5 mTorr/min. The degassed sample and sample cell were precisely weighed and then transferred back to the analyzer. Adsorption isotherms at 77 K were measured in a liquid nitrogen bath. The adsorption isotherms at 25[degrees]C were maintained with water bath controlled by refrigerated circulating bath. After the adsorption, the samples were vacuum-dried at 80[degrees]C to remove C[O.sub.2], and then put into the adsorption instrument for the cycling experiments.
[.sup.1]H NMR spectra were measured on Bruker Advance AV600MHZ spectrometer using deuterated dimethyl sulfoxide (DMSO-[d.sub.6]) as solvent. Fourier transformed infrared (FTIR) spectra were recorded on Nicolet 8700 in KBr pellet. The morphology was determined with JEOL JEM-2100 scanning electron microscope. Energy dispersive spectroscopy (EDS) and X-ray microanalysis experiments were also carried out by JEOL JEM-2100 scanning electron microscope. Thermal analysis was evaluated using SETSYS evolution 18 ther-mogravimetric analyzer (TGA) under nitrogen at a heating rate of 10[degrees]C/min. The pore size distributions and porosities were measured with the mercury intrusion method (pascal 240, American Thermoelectricity). The Brunauer, Emmett, and Teller (BET) surface areas were determined by nitrogen adsorption (SPECTOMETER 1990, American Thermo).
RESULTS AND DISCUSSION
CharacterLation of P[VBTEA][P[F.sub.6]]
P[VBTEA][P[F.sub.6]]: [.sup.1]H NMR (DMSO-[d.sub.6], 600 MHz, [delta] ppm): 7.09 (2H, br), 6.63 (2H, br), 4.23 (2H, br), 3.02 (6H, br), 2.1-0.8 (12H, br). The [.sup.1]H NMR spectrum is shown in Fig. 1 in which the structural nature is demonstrated. TGA curve is shown in Fig. 2. Weight loss occurs at nearly 310[degrees]C under nitrogen atmosphere, which is an indication of good thermal stability. From the TGA curve, it is clearly seen that water content is only 0.48% at ~100[degrees]C. BET indicates that the specific surface of P[VBTEA][P[F.sub.6]] is 0.71 m2/g.
Characterization of MBA-Crosslinked-P[VBTEA][P[F.sub.6]]
FTIR was employed to confirm the structural nature and the results are shown in Fig. 3. In these spectra, the bands corresponding to the poly(4-vinylbenzyltriethy1ammonium) cation are found between 2800-3000 and 14501600 c[m.sup.-l]; the characteristic peaks of crosslinker MBA are observed at 3435 and 1660 c[m.sup.-1]; PF6 anion corresponds to 838 and 557 c[m.sup.-1].
EDS X-ray microanalysis in scanning electron microcopy is used to examine the quantitative anion exchange in MBA-crosslinked-P[VBTEA][P[F.sub.6]]. As shown in Fig. 4, there is no chloride anion observed in MBA-crosslinked-P[VBTEA][P[F.sub.6]]. This result indicates that the anion exchange reaction is a convenient method to obtain PILs with different anion species from the single PEL including the crosslinked polymer.
TGA result is shown in Fig. 5 in which weight loss occurs at nearly 300[degrees]C under nitrogen atmosphere. MBA-crosslinked-P[VBTEA][P[F.sub.6]] has a good thermal stability. The water content is 0.65%, which is also seen from the TGA curve.
The particle size distribution of MBA-crosslinked-P[VBTEA][P[F.sub.6]] is broad, ranging from 1 to 120 pm (Fig. 6), and the volume-average particle size is 16.9 pm. It is porous (Fig. 7), and the apparent porosities and specific surface are 64.3% and 39.12 [m.sup.2]/g, respectively.
C[O.sub.2] Adsorption and Regeneration of PILs
The adsorption isotherms of P[VBTEA][PFej and MBA-crosslinked-P[VBTEAJ[P[F.sub.6]] at 25[degrees]C and different pressures
are shown in Fig. 8. The C[O.sub.2] mass fraction in the PILs increases along with C[O.sub.2] pressure. The adsorption values of P[VBTEA][P[F.sub.6]] and MBA-crosslinked-P[VBTEA][P[F.sub.6]] are 10.36 and 14.04 mg/g, respectively, under maximum pressure of Autosorb-iQ-MP at 25[degrees]C. Under similar conditions, MBA-crosslinked-P[VBTEA][P[F.sub.6]] shows better adsorption efficiency (up to 1.38 wt%) than some other PILs (13-15), (17-22); for example, the C[O.sub.2] adsorption capacity of P[VBTMA][P[F.sub.6]] is 0.873 wt% at 0.078 MPa and 22[degrees]C (15), and is 0.60 wt% at 0.101 MPa and 0[degrees]C for the copolymer of 1-ally1-3-methylimi-dazolium terafluoroborate and acrylonitrile (22).
The recovery and reusability of MBA-crosslinked-P[VBTEA][P[F.sub.6]] were investigated. The results after four cycles are shown in Fig. 9. It is worthy to note that MBA-crosslinked-P[VBTEA][P[F.sub.6]] can be regenerated with good reusability where the reduction of C[O.sub.2] adsorption capacity is <1% after four cycles, and the C[O.sub.2], adsorption/desorption is a reversible process.
In this study, two types of PILs, that is, P[VBTEA][P[F.sub.6]] and MBA-cross1inked-P[VBTEA][P[F.sub.6]] are prepared through free radical solution polymerization and inverse suspension polymerization, respectively. A series of characterizations such as NMR, FTIR, TGA, scanning electron microscopy (SEM), and EDS confirm their good porosity, high thermal stability, and large specific surface area. C[O.sub.2] adsorption-desorption experiments were performed. It is observed that such PILs microparticles present high C[O.sub.2] sorption capability. The maximum C[O.sub.2] sorption capacity of P[VBTEA][P[F.sub.6]] and MBA-crosslinked-P[VBTEA][P[F.sub.6]] is 10.36 and 14.04 mg/g under 0.1 MPa of Autosorb-iQ-MP and 25[degrees]C. This study shows that porous MBA-crosslinked-P[VBTEA][P[F.sub.6]] microparticle is a better candidate for C[O.sub.2] adsorption, and other application studies are underway.
Correspondence to: Xiaochun Chen; e-mail: email@example.com Contract grant sponsor: National Natural Science Foundation of China; contract grant numbers: 20806002, 20976005, 21176021, 21276020; contract grant sponsor: Petro China Innovation Foundation; contract grant number: 2010D-5006-0403.
Published online in Wiley Online Library (wileyonlinelibrary.com).
[C] 2013 Society of Plastics Engineers
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Guangren Yu, Qingzeng Li, Na Li, Ziwei Man, Chenghao Pu, Charles Asumana, Xiaochun Chen Beijing Key Laboratory of Membrane Science and Technology and College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China
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|Author:||Yu, Guangren; Li, Qingzeng; Li, Na; Man, Ziwei; Pu, Chenghao; Asumana, Charles; Chen, Charles|
|Publication:||Polymer Engineering and Science|
|Date:||Jan 1, 2014|
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