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Hypercrosslinked polymers incorporated with imidazolium salts for enhancing C[O.sub.2] capture.

INTRODUCTION

The excessive release of C[O.sub.2] is causing the global warming, which has become one of the very serious environment problems [1], Besides improving the usage efficiency of energy and applying renewable energy (such as wind energy, hydrogenic energy), the capture and separation of C[O.sub.2] is another way to reduce the greenhouse gas emissions and protect the environment. Porous organic polymers (POPs) with low density, extraordinarily large surface areas, thermal and high physicochemical stability have received considerable attention in catalysis, gas storage/separation, and sensing [2, 3]. To date, many kinds of organic porous materials (e.g., conjugated microporous polymers (CMPs) [4, 5), hypercrosslinked polymers (HCPs) [6, 7], polymers of intrinsic microporosity (PIMs) [8, 9], and covalent organic frameworks (COFs) [10, 11]) have been investigated for C[O.sub.2] capture due to their high C[O.sub.2] storage capacity and low regeneration energy cost.

Ionic liquids with no-volatile nature are a new class of organic salts which are used in C[O.sub.2] capture and separation due to the remarkable and reversible solubility of C[O.sub.2] in ionic liquid [12]. Tang et al. [13] first synthesized imidazolium-based poly(ionic liquid)s (PILs) and investigated the C[O.sub.2] adsorption performance. These PILs generally exhibited higher C[O.sub.2] adsorption capacity with faster absorption and desorption rates than those of ionic liquids monomers [14, 15]. PILs seem more suitable in industrial C[O.sub.2] capture, separation and conservation than ILs monomers. The fundamental mechanisms of C[O.sub.2] adsorption have been illustrated by studying lots of PILs. It was found that there are two C[O.sub.2] storage mechanisms for PILs at low C[O.sub.2] pressure [16]. One is the direct dissolving of C[O.sub.2] in the matrix phase. The other is a Langmuir hole-filling process in the microvoids created by the polymer chain entanglement [17].

Besides introducing various functional groups with increasing the affinities towards C[O.sub.2], an alternative approach is to prepare nanostructured PILs with large specific surface areas to enhance adsorption capacities and kinetics. The mesoporous PILs (mpPIL) prepared via a hard-templating pathway have well-defined mesopore sizes and pore shapes, and these PILs could reach the equilibrium state of C[O.sub.2] adsorption faster than IL monomers and the bulk PILs [18]. The highest uptake for mpPIL is about 0.46 mmol/g, while those for bulk PILs and monomer species are only 0.13 mmol/g and 0.02 mmol/g, respectively. Soil et al. prepared the mesoporous polyampholyte networks by self-complexation (inter- and intra-polyelectrolyte complexation) of copolymers [19]. It was found that the meosoporous polyampholytes were essentially made up of mesopores of 6 to 12 nm in diameter and the corresponding specific surface area (Sbet) was up to 260 [m.sup.2] [g.sup.-1]. There are two processes for C[O.sub.2] sorption of mesoporous polyampholytes: adsorption on the surface as a fast process and absorption/chemisorption within the PIL-based material as a slow process. The tubular microporous organic networks containing imidazolium salts prepared by Sonogashira coupling reaction has 620 [m.sup.2] [g.sup.-1] of Sbet [20]. This porous and tubular organic network containing imidazolium salts showed very promising catalytic activities with 92 to 142 h 1 TOF in the conversion of C[O.sub.2] into cyclic carbonates [20] and other organic substance [21]. The crosslinked and porous polymeric microparticle, N,N-methylenebisacrylamide (MBA)-crosslinked poly[4-vinylbenzyltriethylammonium hexafluorophosphate] (P(VBTEA|(PF6]) is prepared by the crosslinking copolymerization, the apparent porosity of which is 64.3% while specific surface is only 39.12 [m.sup.2]/g. It was found that MBA-crosslinked-P[VBTEA][PF6] has good C[O.sub.2] adsorption capability: 14.04 mg/g (1.38 wt%) at 0.1 MPa and 25[degrees]C, and can be recovered by desorption at vacuum and 80[degrees]C, and reused with 99% C[O.sub.2] adsorption after four cycles [22], Xie et al. [23] and Kuzmicz et al. [24] synthesized ionic liquid-functionalized highly cross-linked porous polymer matrix by similar copolymerized method, these copolymers show higher specific surface area even up to 935 [m.sup.2]/g and an abundant mesoporosity. However, with increasing the ionic liquid functional groups in system, the porosity is decreased. These porous PIL networks exhibited significantly enhanced catalytic activity for organocatalyzed carbonatation reactions. It is well known that C[O.sub.2] adsorption properties of materials depend on not only their chemical structure and composition but also their porosity and surface area. Until now the adsorption properties of PILs-base porous materials are not fully understood, and it is still a challenge to obtain details of C[O.sub.2] adsorption in this type of materials.

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It is found that a high crosslinking degree is very important to maintain and stabilize the polymer porous structure against collapse. As a class of predominantly microporous materials, the hyper-cross-linked porous polymers (HCPs) with highly stable physicochemical properties, large adsorption capacity and easy synthesis have attracted lots of attention as adsorbents for capturing and separating C[O.sub.2]. For an example, HCPs with predominantly microporous and high surface areas were prepared with formaldehyde dimethyl acetal (FDA) as cross-linking agent and the highest C[O.sub.2] uptake is 3.61 mmol [g.sup.-1]. In addition, HCPs with different functional groups and the porosity properties could be obtained by changing the type of aromatic compounds or cross-linking agent proportion [25]. The aperture of HCPs could be adjusted the cross-linking agent content (divinylbenzene) in systems, the highest C[O.sub.2] capacity of HCPs with abundant micropores is 12.41 wt% [26]. It is still challenging how to simply synthesize mesoporous even predominantly microporous ionic liquid functionalized polymer. Simultaneously the studies on the relationship of adsorption properties with the structures of PILs-base porous materials are woefully inadequate.

In this study, we propose a strategy similar to the preparation method of HCPs. Several hypercrosslinked porous polymers incorporated with imidazolium salts were prepared. The hypercrosslinked precursor was firstly obtained by the free radical crosslinking copolymerization using 4-vinylbenzyl chloride and imidazole ionic liquids monomer as comonomers. Secondly, the Friedel-Crafts alkylation reaction was conducted with anhydrous Fe[Cl.sub.3] as catalyst. Lastly, the anion exchange reaction with sodium tetrafluoroborate or potassium hexafluorophosphate was conducted to obtain several imidazolium salts functional hypercrosslinked porous polymers, which contained three kinds of alkyl chain (methyl, ethyl, or butyl) and two kinds of anion ([BF.sup.-.sub.4] or [PF.sup.-.sub.6] ). The synthetic steps are shown in Scheme 1. Finally, the C[O.sub.2] adsorption performance is study with considering the structural characteristics of HCPs.

EXPERIMENTAL

Materials

2,2'-Azobis(2-Methylpropionitrile) (AIBN) was recrystallized from ethanol. N-methylimidazole, N-ethylimidazde, A-butylimidazole, divinylbenzene(DVB), 4-vinylbenzyl chloride, hydroquinone, anhydrous ferric chloride, sodium tetrafluoroborate, and potassium hexafluorophosphate were purchased from Aladdin Co. Ltd. and used as received. All other chemicals are of analytical grade commercially available and used without further pretreatment.

Synthesis of 1-(4-vinylbenzyl)-3-Butyl Imidazolium Tetrafluoroborate (VBIT)

VBIT was synthesized according to previous reports [13, 15]. V-butylimidazole (12.4 g, 0.10 mol), 4-vinylbenzyl chloride (16.7 g, 0.11 mol), and inhibitor hydroquinone (0.10 g) were added into a dried 250 mL flask and heated at 45[degrees]C for 24 h under the protection of nitrogen atmosphere. The formed viscous liquid was collected and washed with ethyl ether. The resultant transparent viscous liquid (13.8 g, 0.050 mol) was mixed with inhibitor hydroquinone (0.10 g) in 100 mL of dry acetone. Sodium tetrafluoroborate (6.0 g, 0.055 mol) was added dropwise into the acetone solution and the mixture was stirred for 1 day at room temperature. The mixture was filtered, and solvent was removed by rotary evaporator. The obtained white waxy solid was washed with water and ethyl ether, and then dried in vacuo for 12 h at room temperature. The silver nitrate test indicated that no chloride present. Yield: 88%. [sup.1]H NMR (400 MHz, DMSO-[d.sub.6], ppm): 9.21 (s, 1H, N-CH-N), 7.81 (s, 2H, N-CH=CH-N), 7.55 (d, 2H, Ph), 7.41 (d, 2H, Ph), 6.79 (m, 1H, C[H.sub.2]=CH), 5.91, 5.30 (d, d, 1H, 1H, C[H.sub.2]=CH), 5.40 (s, 2H, ph-C[H.sub.2]-N), 4.19 (t, 2H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]), 1.79 (m, 2H, N[(C[H.sub.2]).sub.3]-C[H.sub.3]), 1.25 (m, 2H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]), 0.91 (t, 3H, N[(C[H.sub.2]).sub.3]-C[H.sub.3]).

Synthesis of 1 -(4-vinylbenzyl)-3-Butyl Imidazolium Hexafluorophosphate (VBIH)

N-butylimidazole (12.4 g, 0.10 mol), 4-vinylbenzyl chloride (16.7 g, 0.11 mol), and inhibitor hydroquinone (0.10 g) were introduced into a dried 250 mL flask and heated at 45[degrees]C for 24 h under the protection of nitrogen atmosphere [13, 15]. The formed viscous liquid was collected and washed with ethyl ether. The resultant transparent viscous liquid (13.8 g, 0.050 mol) was mixed with inhibitor hydroquinone (0.10 g) in 150 mL of dry acetone. Potassium hexafluorophosphate (10.1 g, 0.055 mol) was added dropwise into the acetone solution and the mixture was stirred for three days at room temperature. The mixture was filtered and solvent was removed by rotary evaporator. The obtained white solid was purified by washing with water and ethyl ether, and then dried in vacuo for 12 hours at room temperature. No chloride present was indicated by silver nitrate test. Yield: 87%. [sup.1]H NMR (400 MHz, DMSO-[d.sub.6], ppm): 9.26 (s, 1H, N-CH-N), 7.81 (s, 2H, N-CH=CH-N), 7.55 (d, 2H, Ph), 7.40 (d, 2H, Ph), 6.79 (m, 1H, C[H.sub.2]=CH), 5.91, 5.30 (d, d, 1H, 1H, C[H.sub.2]=CH), 5.40 (s, 2H, ph-C[H.sub.2]-N), 4.19 (t, 2H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]), 1.80 (m, 2H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]), 1.27 (m, 2H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]), 0.92 (t, 3H, N-[(C[H.sub.2]).sub.3]-C[H.sub.3]).

In addition, l-(4-vinylbenzyl)-3-ethyl imidazolium hexafluorophosphate (VEIH), l-(4-vinylbenzyl)-3-ethyl imidazolium tetrafluoroborate (VEIT), l-(4-vinylbenzyl)-3-methyl imidazolium hexafluorophosphate (VMIH), and l-(4-vinylbenzyl)-3-methyl imidazolium tetrafluoroborate (VMIT) were synthesized according to the similar method.

Preparation of Hypercrosslinked Polymers (HCP)

Hypercrosslinked polymers were prepared according to previously reported procedure [26, 27], Divinylbenzene (0.10 g, 0.77 mmol), 4-vinylbenzyl chloride (3.40 g, 22.26 mmol), anhydrous alcohol (15 mL), and initiator AIBN (0.10 g, 0.61 mmol) were added into a dried 100 mL flask and heated at 78[degrees]C under the protection of nitrogen atmosphere. After 24 h, the resultant precursor was filtered, washed successively with water, methanol, and diethyl ether, and then dried in vacuo at 50[degrees]C for 24 h.

The precursor (1.25 g) was swollen in 1, 2-dichloroethane (20 mL) for 2 h under the protection of nitrogen atmosphere. Anhydrous ferric chloride (1.31 g) in 1,2-dichloroethane (20 mL) was slowly added to the solution and then heated at 80[degrees]C under the protection of nitrogen atmosphere for 18 h. The resultant hypercrosslinked beads were filtered, washed successively with water, methanol, and diethyl ether, and then dried in vacuo at 50[degrees]C for 24 h.

Preparation of Imidazolium Salts Functional Hypercrosslinked Polymers (ILHCPs)

Hypercrosslinked polymers incorporated with imidazolium salts were prepared according to the previous reports [26, 27]. Following are the detailed reaction procedures.

Firstly, ionic liquid (2.23 mmol, VMIH 0.77g, or VEIH 0.80g, or VBIH 0.86g), divinylbenzene (0.10 g, 0.77 mmol), 4-vinylbenzyl chloride (3.06 g, 20.03 mmol), anhydrous alcohol (15 mL), and initiator AIBN (0.10 g, 0.61 mmol) were added into a dried 100 mL flask and heated at 78[degrees]C under the protection of nitrogen atmosphere. After 24 h, the resultant precursor was filtered, washed successively with water, methanol, and diethyl ether, and then dried in vacuo at 50[degrees]C for 24 h.

Secondly, the precursor (1.25 g) was swollen in 1,2-dichloroethane (20 mL) for 2 h under the protection of nitrogen atmosphere. Anhydrous ferric chloride (1.31 g) in 1,2-dichloroethane (20 mL) was slowly added to the solution and then heated at 80[degrees]C under the protection of nitrogen atmosphere for 18 h. The resultant hypercrosslinked beads were filtered, washed successively with water, methanol, and diethyl ether, and then dried in vacuo at 50[degrees]C for 24 h.

Thirdly, a direct anion-exchange reaction is carried out into the imidazolium salts functional polymers according to Ref. 22. The obtained brown solid was mixed with potassium hexafluorophosphate (4.10 g, 22.3 mmol) or sodium tetrafluoroborate (2.43 g, 22.3 mmol) in 150 mL of deionized water and stirred for 24 h at 30[degrees]C for anion exchange. The resultant brown solid were filtered, washed successively with water, methanol, and diethyl ether, and then dried in vacuo at 50[degrees]C for 24 h.

Characterization

[sup.1]H NMR spectra were measured using a Bruker Avance AMX-400 NMR spectrometer in dimethylsulfoxide (DMSO-[d.sub.6]) with tetramethylsilane (TMS) as internal standard. Fourier transform infrared (FT-IR) spectra were obtained using a Nicolet Avatar 370 spectrometer with KBr pellets. Solid-state [sup.13]C NMR spectra were measured at the molecular level on an Advance Digital 500 MHz spectrometer. Scanning electron microscopy (SEM) images were performed using a JSM-5610 LV (LVSEM) at 10.0 kV. The samples were sputter coated with Pt before being measured. Elemental analyses were carried out with a Vario Micro Elemental Analyzer. Thermogravimetric analysis (TGA) was recorded from ambient temperature to 800[degrees]C under nitrogen atmosphere using a Pyris Diamond system at a heating rate of 10[degrees]C min-1. Surface areas, pore size distributions, and pore volumes were derived from nitrogen adsorption/desorption isotherms using ASAP 2020 volumetric adsorption analyzer. All samples were outgassed at 100[degrees]C for 15 h before being measured.

Gas Adsorption Measurements

The Micromeritics ASAP 2020 automated gas adsorption analyzer was used for N2, CO2, and H2 update measurement. All samples were outgassed at 100[degrees]C for 15 h before being measured. Ultra high purity gases (99.999% purity) were used for all gas update measurements and the free space was calibrated with helium. For the gas update measurements, the temperatures were determined by using a refrigerated tub of liquid nitrogen (77 K), water (295 K), or ice water (273 K).

RESULTS AND DISCUSSION

Synthesis and Characterization of Imidazolium Salts Functional Hypercrosslinked Polymers

The imidazolium salts functional hypercrosslinked polymers (ILHCPs) were synthesized via the free radical cross-linking copolymerization and Friedel-Crafts alkylation reaction (shown in Scheme 1). The crosslinked functional polymer precursors were prepared by the free radical copolymerization with 4-vinylbenzyl chloride, ionic liquid monomers, and divinylbenzene(DVB) as comonomers in the presence of a radical initiator (AIBN). Then, the resultant precursor was performed by the Friedel-Crafts alkylation reaction using anhydrous Fe[Cl.sub.3] as catalyst. Lastly, in order to insure that the anion was completely [BF.sup.-.sub.4] or [PF.sup.-.sub.6] in the copolymer, the anion-exchange was respectively carried out for final products. The imidazolium salts functional hypercrosslinked polymers with different content and kinds of imidazolium salts were labeled as HCP-IL-1 ([VBB1)[PF6]-HCP1), HCP-IL-2 ([VBB1][[BF.sub.4]]-HCP1), HCP-IL-3 ([VBE1][PF6]-HCP), HCP-1L-4 ([VBE1][[BF.sub.4]]-HCP), HCP-1L-5 ([VBM1][PF6]-HCP), HCP-IL-6 ([VBMI][[BF.sub.4]]-HCP), HCP-IL-7 ([VBBIJIPF6]-HCP2), and HCP-IL-8 ([VBB1][[BF.sub.4]]HCP2), respectively. All samples were insoluble in water and most organic solvents such as ethanol, acetone, THF, ethyl acetate, etc. In addition, the samples could remain intact after soaking and washing with concentrated HC1 and NaOH (6 M) aqueous as evidenced by spectral and physical studies, implying that ILHCPs have physicochemical stable hypercrosslinked property. The hypercrosslinked polymers incorporated with imidazolium salts exhibited good thermal stability according to TGA (Supporting Information Fig. S1). All the materials morphologies were investigated, their SEM images were shown in Supporting Information Fig. S2. These materials exhibited an amorphous nature, and irregular spherical tiny particles were shown on the surface. By comparison, the texture features of the similar-size irregular spherical particles of ILHCPs were more highlight than HCP without imidazolium salts. The textural features of ILHCPs were similar with the previous reported highly cross-linked polymer matrix grafted with ionic liquids [23]. In general, the textural features of the porous copolymers were strongly influenced by the synthetic route and initial experimental conditions. The type of solvent and amount of IL monomer strongly affected the driving force of phase separation during copolymerization [24].

The hypercrosslinked polymers incorporated with imidazolium salts were confirmed by FT-1R, as shown in Fig. 1. Comparing with nonfunctional HCP, the appearance of FT-IR absorption band at 1,561 [cm.sup.-1] is assigned to imidazole ring stretching and the peaks in functional HCPs at 844 [cm.sup.-1] and 1,082 [cm.sup.-1] are attributed to PFg and [BF.sub.4] functional groups, respectively, which indicate the presence of imidazole ionic liquids in functional networks. The solid-state [sup.13]C CP/MAS NMR spectra of ionic liquid HCPs (HCP-IL-7) are shown in Fig. 2. The resonance peaks at 13.51, 19.64, 32.31 and 49.81 ppm belong to the carbon (NC[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.3] (a), N-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.3] (b), NC[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.3](c), and N-C[H.sub.2]C[H.sub.2]C[H.sub.2]C[H.sub.3] (e)) of butyl group connected to the nitrogen atom of imidazole ring [13], respectively. The resonance peaks near 40.76 ppm belong to carbon (d) in methylene linker formed after Friedel-Crafts reaction and methylene of polymer backbone [25], The broad peak at around 130 ppm is assigned to the carbon (f) of imidazole ring and overlapping with peaks due to phenyl carbon [13], The chemical composition and content of various elements of HCPs incorporated with imidazolium salts were further confirmed by Elemental Analyses (EA) (shown in Table 1). Since imidazolium salts functional polymers contains nitrogen element is from imidazole rings, the results from EA also indicate that imidazolium salts units have been introduced successfully, the existence of imidazolium salts are very important for selective C[O.sub.2] capture and enhancing binding affinities [27-29]. The imidazolium salts content in HCPs is calculated as about 7 mol% or 10.5 mol% according to the mass percentage of each element. The content of imidazolium salts units in functional networks is not high due to different reactivity ratio of imidazole ionic liquids with 4-vinylbenzyl chloride during copolymerization and large amount of methylene groups introduced by Friedel-Crafts alkylation reaction.

Porosity and C[O.sub.2] Capture Studies

The porosity of the crosslinked polymers was evaluated by nitrogen uptake measurements at 77 K. As shown in Fig. 3, the nitrogen sorption isotherms show an apparent uptake at the relative pressure (p/[p.sub.o]) up to 0.1, indicating abundant microporous structures, whereas the isotherms also prove the co-existence of meso- and micropores for another relative steady rise region in the range from 0.2 to 0.8 (P/[P.sub.o]) [30, 31]. In accordance with the IUPAC classification, all nitrogen isotherms of the networks are emblematical type-I isotherms with H3-hysteresis loop, indicating the slit type pores [26, 32]. Owing to the flexible nature of the crosslinked networks, hysteresis loop is not closed at low pressures, which have been frequently observed in multiform porous organic materials [32, 33].

Applying the nonlocal density functional theory (NLDFT) method [27, 33], the pore size distribution of HCP-1L-1, HCP-IL2, HCP-IL-3, HCP-1L-4, HCP-1L-5, and HCP-IL-6 display that the dominant pores are mesopores with a pore size predominantly distributed around 2.5 nm, while HCP-IL-7 and HCP-IL-8 exhibit both micropores and mesopores with pore sizes of 1.9 and 4.7 nm, respectively (Table 1 and Fig. 4). The tendency of the increasing pore aperture of hypercrosslinked polymers incorporated with imidazolium salts can be detected in comparison with nonfunctional HCP. Due to the effect from imidazole ionic liquid unit on phenyl of ionic liquid monomer, it is not easy for the phenyl groups to conduct Friedel-Crafts alkylation reaction with 1,2-dichloroethane or benzyl chloride, which reduce the formation of porosity in hypercrosslinked networks. Simultaneously, steric hindrance from ionic liquid monomer hinders the formation of micopores [34], The broader pore size distribution of functional materials also illustrates these irregular pore structures.

The textural properties of various HCPs are further listed in Table 1. The Brunauer-Emmett-Teller (BET) surface area of imidazolium salts functional hypercrosslinked polymers are in the range from 447 to 647 [m.sup.2] [g.sup.-1], the Langmuir surface area is in the range from 609 to 870 [m.sup.2] [g.sup.-1], micropore volume is in the range from 0.11 to 0.13 [cm.sup.3] [g.sup.-1] and total pore volume is in the range from 0.24 to 0.27 [cm.sup.3] [g.sup.-1], while the corresponding textural properties of nonfunctional polymers (HCP) is 738 [m.sup.2] [g.sup.-1] 994 [m.sup.2] [g.sup.-1], 0.20 [cm.sup.3] [g.sup.-1], and 0.36 [cm.sup.3] [g.sup.-1], respectively. It is obvious that all the functional polymers show the slightly disadvantageous pore parameters in some extent for small gas storage comparing with nonfunctional networks (HCP) in this synthetic condition. The corresponding aperture parameters exhibit with larger pore size, small total pore volume and micropore volume, and decreased surface area. However, the micropore volume of HCPs incorporated with IL has a larger presence and values compared with common crosslinked poly(ionic liquids) networks [24].

The C[O.sub.2] adsorption and desorption isotherms of HCPs and ILHCPs at 273 K were showed in Supporting Information Fig. S3. It was found that the materials showed a weak adsorption/desorption hysteresis. The adsorption--desorption curves of C[O.sub.2] at 273 K are reversible. It was reported that the C[O.sub.2] absorption capacities of PILs containing different anions decrease in the order of [BF.sup.-.sub.4] >[PF.sup.-.sub.6] > [Tf.sub.2][N.sup.-] [17]. When the samples have same kind of cation portion and similar content ionic liquid, such as HCP-IL-1 and HCP-IL-2, HCP-IL-3, and HCP-IL-4, it is found that the species of anion in imidazolium salts basically does not obviously affect the pore parameters of functional networks during the formation of hypercrosslinked copolymers. In this study, there is hardly any difference for C[O.sub.2] adsorption capacities on account of the effects of anions possibly due to the low concentration of imidazolium salts in copolymers. Conversely, when the networks have same kind anionic portion and similar content imidazolium salts, for example, HCP-IL-1, HCP-IL-3, and HCP-IL-5, alkyl chain of imidazolium salts impacts on the pore parameters of the samples. It is clear that decreased porosity is observed with the use of longer alkyl chain lengths of imidazolium salts. The longer alkyl chains of cations may pose steric hindrance between C[O.sub.2]-cation interaction, which resulting the shrinking of the microvoid volume of PILs and decreasing of C[O.sub.2] sorption capacity [16]. With increasing of imidazolium salts content, the porosity of functional copolymers decreased when the copolymer has same anionic and cationic portion, such as HCP-IL-1 and HCP-IL-7.

C[O.sub.2] adsorption isotherms of hypercrosslinked polymers incorporated with imidazolium salts at 273 and 295 K are shown in Fig. 5, which demonstrate that the absorbed C[O.sub.2] uptake continually increase with the pressure, implying that the adsorption has not reached its equilibrium or saturated state in the investigated pressure range [31],The C[O.sub.2] uptake of functional networks is in the range of 6.33 to 7.56 wt% at 273 K and 4.74 to 5.45 wt% at 295 K and 1 bar. While C[O.sub.2] uptake of nonfunctional polymer (HCP) is only 5.93 wt% and 4.22 wt% at 273 K and 295 K and 1 bar, respectively. Although the imidazolium salt concentration is only about 10.5 mol%, the C[O.sub.2] uptake of ILHCPs are significantly better than nonfunctional HCPs, and increased by 1.63 wt% (273 K) and 1.23 wt% (295 K). It could be owing to the combination high microporosity and imidazole ionic liquid functional groups in hypercrosslinked imidazolium salts copolymers. The incorporation of C[O.sub.2]-philic functional groups is regarded as the most efficient way on account of the high quadruple moment (14.3 X [10.sup.-4] [cm.sup.2]) and polarizability (26.3 x [10.sup.-25] [cm.sup.3]) of C[O.sub.2] [35],

When ILHCPs have similar concentration of imidazolium salts and the identical anionic portion, their C[O.sub.2] adsorption capacity is in the order of HCP-IL-1 > HCP-IL-3 > HCP-IL-5 (or HCP-IL2 > HCP-IL-4 > HCP-IL-6). Combining with pore parameters of the networks, enhanced C[O.sub.2] adsorption capacity is observed with increased porosity and use of shorter alkyl chain lengths of imidazolium salts. C[O.sub.2] adsorption capacity of ILHCPs with identical imidazolium salts species is in the order of HCP-IL-7 > HCP-IL-1 (HCP-IL-8 > HCP-IL-2) with decreasing the concentration of imidazolium salts in HCPs. It can be seen that the higher of the imidazolium salt content in HCPs, the better of C[O.sub.2] capture performance. In addition, the imidazolium salt content in HCPs plays a leading role in comparing to porosity with respect to C[O.sub.2] adsorption capacity for this system [36],

The ILHCPs also shows excellent C[O.sub.2] adsorption performance comparing with previously reported materials in C[O.sub.2] uptake under similar conditions (Table 2). For examples, most ionic liquids (e.g., 0.26 wt% for [bmim][BF4]), most poly(ionic liquid)s (e.g., 0.31 wt% for PVBIT, 0.32 wt% for PVBIH, and 0.24 wt% for PB1MT) [13], most hypercrosslinked polymers (e.g. 4.18 wt% for Mar_l, 5.28 wt% for Mar_2, 5.76 wt% for Mar_3, 2.5 wt% for HCP 5, 1.4 wt% for HCP 6, and 1.1 wt% for HCP 7) [37, 38], most inorganic porous materials (e.g. 3.52 wt% for MCM-48) [39], and most organic porous materials (e.g. 4.5 wt% for PBI-2, 4.4 wt% for PI-2, 5.588 wt% for CCI, and 4.01 wt% for Tet2) [40-43]. However, the maximum and minimum C[O.sub.2] uptakes of ILHCPs are 7.56 wt% and 6.33 wt%, respectively. The values of C[O.sub.2] uptakes of ILHCPs are distinctly higher than those of above materials, exhibiting an excellent application prospect in C[O.sub.2] capture.

A successful adsorbent for CCS means suitable interactions of guest molecules with adsorbent walls which point to isosteric heat adsorption ([q.sub.st]), along with high C[O.sub.2]-[N.sub.2] selectivity [44], [q.sub.st] (C[O.sub.2] isoteric heats) were obtained using the Clausius-Clapeyron equation from their C[O.sub.2] adsorption data collected at 273 and 295 K. The [q.sub.st] value at the initial adsorption stage (low gas loading) mainly reflects the interaction strength between C[O.sub.2] and the sorbent [45]. As shown in Fig. 6, a reduction in [q.sub.st] is observed with the increasing of C[O.sub.2] uptake which is in the range from 0 to 1 mmol. The interaction force is unevenly distributed between ILHCPs and C[O.sub.2] molecules. After C[O.sub.2] molecules occupy hypercrosslinked polymers incorporated with imidazolium salts active spots, the interaction force diminishes rapidly resulting in the release of temperature becomes lower and [q.sub.st] becomes smaller. When C[O.sub.2] uptake is greater than 1 mmol, [q.sub.st], not significantly reduced, which shows that ILHCPs could not reach the saturated adsorption state under the highest test pressure.

C[O.sub.2] uptake and isosteric heat of adsorption for HCPs are listed in Table 3. All [q.sub.st] values of samples are less than 40 kJ/ mol, which belong to the category of physical adsorption, but keep consistent with C[O.sub.2] uptake trend at 273 K (or 295 K) and 1 bar. It is well known that C[O.sub.2] capture comes mainly from the porosity effect and high charge density functional groups or elements such as nitrogen and carboxylic acid groups [36], In this study, C[O.sub.2] capture ability of nonfunctional HCPs come only from the porosity effect, while that of ILHCPs is from combination of the porosity effect and high charge density functional groups. By local dipole/quadrupole interaction, the imidazole group interacts easily with C[O.sub.2] molecules which have a larger quadrupole moment and is polarized. Furthermore, the imidazole group belongs to basic group, and could also interact with acidic C[O.sub.2] molecules via acid-base interaction [19], It is found that the larger of surface area, the higher of imidazolium salts content and the shorter of imidazolium salts alkyl chain is in favor of higher of C[O.sub.2] uptake and [q.sub.st]. Although the surface area of HCP-IL-7(or HCP-IL-8) is smaller than nonfunctional HCPs, C[O.sub.2] uptake and isoteric heats are higher than those of HCP. When the contents of imidazolium salts in HCP-IL-7 is 10.5 mol%, C[O.sub.2] uptake increased by 1.63 wt% (273 K) and C[O.sub.2] isosteric heat increased by 9.8 kJ/mol. This indicates that imidazole ionic liquids play a dominant role on the adsorption of C[O.sub.2] molecules compared with porosity effect in this system. The readily reversible adsorption/desorption behavior and moderate qs, indicate that C[O.sub.2] interacts with pore walls are weak enough to allow these materials regeneration with little energy input. This is attractive because adsorbents with a high [q.sub.st] require lots of heat to regenerate adsorbents [37].

A high selectivity for C[O.sub.2] over [N.sub.2] is one of the key requirements for a material to be applied as a C[O.sub.2] adsorbent except the C[O.sub.2] adsorption capacity and reversibility [7]. The higher C[O.sub.2]/[N.sub.2] selectivity, the more conducive to isolate carbon dioxide from gas mixture and could bring bigger economic benefits. Many functional groups have been introduced into porous materials to improve selectivity, such as -OH, -COOH, -N[H.sub.2], and -C[H.sub.3]. PILs and ILs also are deemed to have some kind of intrinsic C[O.sub.2]-philicity [18], Selective adsoiption of these hypercrosslinked polymers incorporated with imidazolium salts for different gases (C[O.sub.2] and [N.sub.2]) was calculated using initial adsorption slope ratios from Henry's law constants for single component adsorption isotherms at 273 K and up to 1 bar (Table 4, Supporting Information Fig. S4). In the low-pressure range, the interactions between the porous materials and C[O.sub.2] play the dominant role for C[O.sub.2] uptake [7], Simultaneously, initial adsorption slope indicates that the adsorption capacity of hypercrosslinked polymers (Fig. 7) [53].

The C[O.sub.2]/[N.sub.2] selectivity of hypercrosslinked polymers can also be accurately calculated by the formulas of [[alpha].sub.CO2/N2] = (q.sub.1]/ [q.sub.2])/([p.sub.1]/[p.sub.2]) except the initial slope method (Table 3) [44]. The C[O.sub.2]/[N.sub.2] selectivity of HCP-1L-1, HCP-IL-2, HCP-IL-3, HCP-IL4, HCP-IL-5, HCP-IL-6, and HCP-IL-8 are 36.4, 38.9, 34.9, 34.1, 35.0, 35.4, and 47.6, respectively. It is clear that all the HCPs incorporated with imidazolium salts show the better C[O.sub.2]/ [N.sub.2] selectivity than nonfunctional HCPs. When the imidazolium salts content of HCP-1L-7 is 10.5 mol%, the HCP-IL-7 shows the highest C[O.sub.2]/[N.sub.2] selectivity 47.9/1 compared with that of HCP (27.8/1) by the initial slope method, which may be contributed to the co-existence of high microporosity and imidazolium salts functional groups. Such high selectivity is typically found in ultramicroporous systems, where molecular sieving phenomena are effective.

The ILHCPs also show better C[O.sub.2]/[N.sub.2] selectivity than previously reported materials under similar conditions (Table 4). For examples, most ionic liquids (e.g. 7.13 for [TMPBI-BuI][[Tf.sub.2]N]), most poly(ionic liquid)s (e.g. 11.5 for P[DADMA][BF4]) [46, 47], most hypercrosslinked polymers (e.g. 28 for HCP-BA, and 15.7 for Mar_1) [37, 48], most inorganic porous materials (e.g. 21.6 for NC-800, 23.4 for NPC-650, and 27.3 for PC-2) [49-51], and most organic porous materials (e.g. 23.8 for APOP-1, and 27 for NPOF-4) [44, 52]. The values of maximum and minimum C[O.sub.2]/[N.sub.2] selectivity of ILHCPs are 47.9 and 34.1, respectively. Those selectivity values of ILHCPs are distinctly higher than those of above materials, exhibiting an excellent application prospect in C[O.sub.2] capture. It was said that the intrinsic affinity of PILs with C[O.sub.2] and their free volume are apparent factors which affect the adsorption capacity and selectivity of CO, [17].

In addition, the hydrogen storage ability of HCPs incorporated with imidazolium salts, hydrogen sorption measurements at low pressure were carried out at 77 K, which displayed type I behavior. As shown in Fig. 8, HCPs with BET surface area of 738 [m.sup.2] [g.sup.-1] can absorb 0.99 wt% [H.sub.2] at 1.0 bar. While HCPs-IL=1, HCPs-IL-3, HCPs-IL-5, and HCPs-IL-7 can absorb 0.92 wt%, 0.85 wt%, 0.92 wt%, and 0.80 wt% [H.sub.2] at 1.0 bar, which is less than that of HCPs without imidazolium salts. The surface area and pore size and pore size distribution of porous materials are the most crucial factors for [H.sub.2] adsorption [25, 26]. Due to imidazole type ionic liquid without any adsoiption for [H.sub.2], the [H.sub.2] uptake for HCPs-IL decreased with the decreasing of surface area and mircopore volume. That smaller pore size and higher microporous volume facilitate [H.sub.2] adsorption is consistent with literature reports on other classes of porous materials. The results from [H.sub.2] adsorption measurement further demonstrated that the synergy effect from mirco and mesopore structures and incorporated imidazolium salts indeed play a role in enhancing the C[O.sub.2] adsorption performance and selectivity over other gas. Although the concentration of ionic liquid in system and SBet of porous material are not high, these results indicate that micro and mesostructured ILHCPs are indeed more than suitable candidate for C[O.sub.2] capture. In addition, we will further prepare the ILHCPs with higher content of IL and 5Bet by varying the reaction condition, and investigate the relationship of content of IL, SBET chemical structure of alkyl chains with the corresponding C[O.sub.2] adsorption and selectivity.

CONCLUSIONS

PILs and ILs have shown the promising potential in C[O.sub.2] sorption and separation. Appling nanostructured PILs with large specific surface areas is a practicable strategy for enhancing C[O.sub.2] uptake and selectivity over other kinds of gases. In this article, several imidazolium salts functional hypercrosslinked polymers (ILHCPs) were synthesized by the free radical copolymerization and Friedel-Crafts alkylation reaction. The structures and the gas adsorption performance of ILHCPs were investigated. The results show that ILHCPs exhibit excellent C[O.sub.2] adsorption performance compared with nonfunctional HCPs, although the BET surface area and pore volume of ILHCPs containing 7.0 or 10.5 mol% of imidazolium salts are lower than that of nonfunctional HCP. It is found that the imidazolium salts content in HCPs plays a more important role than the pore texture does for C[O.sub.2] adsorption. When the imidazolium salts content is 10.5 mol%, the C[O.sub.2] uptake of ILHCPs is 7.56 wt% at 273 K and 1 bar, and the corresponding highest C[O.sub.2]/[N.sub.2] adsorption selectivity is 47.9/1, while those for nonfunctional HCPs are only 5.93 wt% and 27.9/1. The synergy effect from mirco and mesopore structures and incorporated imidazolium salts does play a role in enhancing the C[O.sub.2] adsorption performance and selectivity over other gas. The results suggest that these imidazolium salts functional micro and mesoporous materials have great potential applications in C[O.sub.2] adsorption and separation.

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Lingling Hu, (1) Huagang Ni, (1) Xiaolong Chen, (1) Lele Wang, (1) Ying Wei, (1) Tengfei Jiang, (1) Yaohong Lu, (1) Xiaolin Lu, (2) Peng Ye (1)

(1) Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China

(2) State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, China

Correspondence to: H. Ni; e-mail: nhuag@163.com

Contract grant sponsor: National Natural Science Foundation of China (NSFC); contract grant number: 51003097, 51173169, and 51473148; contract grant sponsor: Science Foundation of Zhejiang Top Academic Discipline of Applied Chemistry and Eco-Dyeing & Finishing and Engineering Research Center for Eco-Dyeing & Finishing of Textiles of Zhejiang SciTech University, and China Scholarship Council. Additional Supporting Information may be found in the online version of this article.

DOI 10.1002/pen.24282

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Chemical composition and textural properties of various HCPs.

                                  C       H       N       IL
Sample             IL           (wt%)   (wt%)   (wt%)   (mol%)

HCP                --           90.59   6.76    0.00     0.00
HCP-IL-1   [VBBI][[PF.sub.6]]   86.17   6.71    1.15     6.96
HCP-IL-2   [VBBI][[BF.sub.4]]   82.95   6.85    1.43     7.04
HCP-IL-3   [VBEI][[PF.sub.6]]   78.27   6.48    1.53     7.87
HCP-IL-4   [VBEI][[BF.sub.4]]   79.82   6.42    1.39     7.80
HCP-1L-5   [VBMI][[PF.sub.6]]   76.22   6.43    1.44     7.53
HCP-IL-6   [VBMI][[BF.sub.4]]   74.19   6.51    1.53     7.59
HCP-IL-7   [VBBI][[PF.sub.6]]   78.29   6.53    1.97    10.51
HCP-IL-8   [VBBI][[BF.sub.4]]   79.14   6.44    1.88    10.47

            [S.sub.BET]      [S.sub.Lang]    [V.sub.[micro]]
           (a) ([m.sup.2]   (b) ([m.sup.2]   (c) ([cm.sup.3]
Sample      [g.sup.-1])      [g.sup.-1])       [g.sup.-1])

HCP             738              994              0.20
HCP-IL-1        639              855              0.13
HCP-IL-2        647              870              0.14
HCP-IL-3        621              836              0.13
HCP-IL-4        617              828              0.13
HCP-1L-5        625              843              0.13
HCP-IL-6        620              834              0.13
HCP-IL-7        447              609              0.11
HCP-IL-8        450              615              0.11

             [V.sub.tot]
           (d) ([cm.sup.3]
Sample       [g.sup.-1])     D (e) (nm)

HCP             0.36            1.9
HCP-IL-1        0.27            2.5
HCP-IL-2        0.28            2.5
HCP-IL-3        0.28            2.7
HCP-IL-4        0.28            2.7
HCP-1L-5        0.28            2.7
HCP-IL-6        0.28            2.7
HCP-IL-7        0.24          1.9, 4.7
HCP-IL-8        0.24          1.9, 4.7

(a) Surface area calculated by BET equation.

(b) Surface area calculated by Langmuir equation.

(c) Micropore volume calculated from t-test.

(d) Total pore volume at p/p0 = 0.98.

(e) Pore size calculated by the QLDFT method.

TABLE 2. Other materials C[O.sub.2] uptake values.

                                                  C[O.sub.2]
Sample                Condition       D (nm)        (wt%)     References

HCP-IL-7              273K, lbar     1.9, 4.7        7.56      This work
HCP-IL-8              273K, lbar     1.9, 4.7        7.51      This work
[bmim][[BF.sub.4]]   295K, 0.8bar       --           0.26         [13]
PVBIT                295K, 0.8bar       --           0.31         [13]
PVBIH                295K, 0.8bar       --           0.32         [13]
PBIMT                295K, 0.8bar       --           0.24         [131
Mar_1                 298K, lbar        5.0          4.18         [37]
Mar_2                 298K, lbar        5.4          5.28         [37]
Mar_3                 298K, lbar        3.5          5.76         [11]
HCP 5                 298K, lbar        --           2.5          [38]
HCP 6                 298K, lbar        --           1.4          [38]
HCP 7                 298K, lbar        --           1.1          [38]
MCM-48                298K, latm        --           3.52         [39]
PBI-2                 273K, lbar    1.56, 17.28      4.5          [40]
PI-2                  298K, lbar        --           4.4          [41]
CC1                   275K, lbar        --          5.588         [42]
Tet2                  298K, lbar        1.1          4.01         [43]

TABLE 3. Summary of C[O.sub.2] uptake, isosteric heat of adsorption,
and the C[O.sub.2]/[N.sub.2] selectivity for various HCPs.

           C[O.sup.273K    C[O.sup.295K    [q.sub.st]
Sample     .sub.2] (wt%)   .sub.2] (wt%)    (KJ/mol)

HCP            5.93            4.22           20.8
HCP-IL-1       6.51            4.99           28.5
HCP-IL-2       6.55            5.09           28.9
HCP-IL-3       6.47            5.00           24.6
HCP-IL-4       6.40            5.04           24.9
HCP-1L-5       6.40            4.76           22.9
HCP-IL-6       6.33            4.74           22.7
HCP-IL-7       7.56            5.45           30.6
HCP-IL-8       7.51            5.45           30.1

Sample     Selectivity (a)   Selectivity (b)

HCP             27.8              28.3
HCP-IL-1        36.4              35.5
HCP-IL-2        38.9              37.8
HCP-IL-3        34.9              35.1
HCP-IL-4        34.1              34.7
HCP-1L-5        35.0              35.4
HCP-IL-6        35.4              35.9
HCP-IL-7        47.9              46.3
HCP-IL-8        47.6              46.6

(a) Data were obtained by initial slope calculations at 273 K.

(b) Data were obtained by the formulas of ([q.sub.1]/[q.sub.2])/
([p.sub.1]/[p.sub.2]) at 273 K.

TABLE 4. Other materials CO2/N2 selectivity values.

Sample                     Condition (K)     D (nm)

HCP-IL-7                        273         1.9, 4.7
HCP-IL-8                        273          1.9,4.7
[TMPBI-BuI][[Tf.sub.2]N]        --             --
P[DADMA][[BF.sub.4]]            --             --
Mar_3                           298            3.5
HCP-BA                          273            --
NC-800                          273           0.81
NPC-650                         273            1.0
PC-2                            273           0.84
APOP-1                          273            LI
NPOF-4                          273        0.773, 1.08

Sample                     Selectivity   References

HCP-IL-7                      47.9       This work
HCP-IL-8                      47.6       This work
[TMPBI-BuI][[Tf.sub.2]N]      7.13          [46]
P[DADMA][[BF.sub.4]]          11.5          [47]
Mar_3                         15.7          [37]
HCP-BA                        28.0          [48]
NC-800                        21.6          [49]
NPC-650                       23.4          [50]
PC-2                          27.3          [51]
APOP-1                        23.8          [52]
NPOF-4                         27           [44]
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Author:Hu, Lingling; Ni, Huagang; Chen, Xiaolong; Wang, Lele; Wei, Ying; Jiang, Tengfei; Lu, Yaohong; Lu, X
Publication:Polymer Engineering and Science
Article Type:Report
Date:May 1, 2016
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