Facile preparation of cross-linked polyimide aerogels with carboxylic functionalization for CO2 capture

Chemical Engineering Journal 322 (2017) 1–9

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Facile preparation of cross-linked polyimide aerogels with carboxylic functionalization for CO2 capture Ying Chen, Gaofeng Shao, Yong Kong, Xiaodong Shen ⇑, Sheng Cui ⇑ a b

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing 210009, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Cross-linked PI aerogel was prepared

by sol-gel process and supercritical drying.
The PI aerogel has a highperformance for CO2 adsorption due to the carboxyl.
High yield and flexible processing are benefit for the formation of membrane.

a r t i c l e

i n f o

Article history: Received 5 December 2016 Received in revised form 17 February 2017 Accepted 1 April 2017 Available online 4 April 2017 Keywords: Aerogel Polyimide Carboxyl groups CO2 adsorption

a b s t r a c t In this study, highly cross-linked and completely imidized polyimide aerogels containing carboxyl groups were successfully synthesized using a mild sol-gel method at room temperature from 3,5diaminobenzoic (DABA) and biphenyl-3,30 ,4,40 -tetracarboxylic (BPDA) and cross-linked with 1,3,5triaminophenoxybenzene. The densities of the obtained polyimide aerogels are between 0.116 and 0.386 g/cm3, the specific surface areas range from 173 to 461 m2/g, and the porosities are as high as 97%. The CO2 adsorption-desorption isotherms are reversible, and the CO2 uptakes at 25 °C and 1 bar are as high as 21.10 cm3/g, which are comparable to those of previously reported microporous organic polymers. The high CO2 adsorptions are attributed to the abundance of electron-rich heteroatoms in the polyimide networks and to the presence of carboxyl groups, a type of CO2-philic group. The facile preparation and high CO2 adsorptive capacity indicate that polyimide aerogels may be ideal candidates for capturing CO2. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Aerogels are a type of nanomaterial produced from a wet gel prepared using the sol-gel method in which the liquid component of the gel is replaced with gas to leave an intact solid nanostructure without pore collapse that is 90–99% air by volume [1]. The tech⇑ Corresponding authors at: State Key Laboratory of Materials-Oriented Chemical Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, China. E-mail addresses: [email protected] (X. Shen), [email protected] (S. Cui). http://dx.doi.org/10.1016/j.cej.2017.04.003 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

nique of treating the gel in an autoclave under supercritical conditions for the solvent was initially developed by Kistler and was then applied by Nicolaon and Teichner to gels obtained from organometallic compounds. Importantly, the pores in aerogels are mesopores that are 2–50 nm in size, and some micropores and macropores are also present. [2]. Due to their unique structure, i.e., their high porosity, as well as the hierarchical and fractal microstructure and randomly cross-linked network[3], aerogels possess fascinating properties, such as low density, low thermal conductivity, low sonic velocity, high surface areas and wide

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adjustable density and refractive index ranges [4–7]. Therefore, aerogels are promising materials for diverse applications, such as thermal insulators, catalyst supports, adsorbents, drug carriers, and sound attenuators [8]. Aerogels are generally classified based on composition: organic or inorganic aerogels. Inorganic aerogels were the first aerogels studied and have been the most widely applied aerogels. A typical inorganic aerogel is the SiO2 aerogel, which was synthesized using sodium silicate as the precursor. At the end of the 1980s, Pekala used an organic polymer as the precursor to synthesize a new type of aerogel called resorcinol-formaldehyde (RF) aerogels [9]; the RF aerogels were later used to form a carbon aerogel. This significant study led to rapid breakthroughs in the field of aerogel materials. In addition, by using the ‘‘RF aerogel method”, traditional highmolecular-weight polymer aerogels have attracted the interest of researchers due to their flexible molecular design properties and excellent mechanical properties. There are two main methods for obtaining aerogels: synthesis via ring-opening metathesis polymerization (ROMP) of the monomer [10] to form polymers such as polyurethane [11], polyurea [12], polybenzoxazine [13], and polyimide [14] or inducing the phase separation of polymers such as polystyrene [15] and polyacrylonitrile [16]. This type of aerogel material typically has dual characteristics combined with aerogel features, such as low density and high porosity, and an inherent polymer nature; thus, it is an emerging material with potential application. Compared to other engineering polymers, polyimides are well known as high-performance materials, are characterized by the presence of an imide group in the backbone, and have excellent thermal, mechanical, and electronic properties; consequently, these materials are commonly applied in highly technical fields such as aerospace and microelectronics [17]. All of these advantages have resulted in polyimide aerogels attracting considerable attention. Polyimide aerogels are primarily divided into two categories based on their chemical structure: linear and cross-linked aerogels. Importantly, the DuPont method is the classic route for the synthesis of polyimide aerogels, and this method involves a typical two-step approach using diamines and dianhydrides [10,18–20]. The two monomers are dissolved in the given solvent and reacted at room temperature to yield the poly-(amic acid) solution; then, they subsequently cyclize into polyimide chemically with acidic anhydride and a base catalyst. However, the polymerization of monomeric reactants (PMR) route is another method that yields thermoset resins, and this approach involves the synthesis and polymerization of norbornene-capped imide oligomers [10]. Polyimide aerogels were prepared for the first time via the DuPont route, using both chemical dehydration and hightemperature treatment to induce imidization [21,22]. Over the past decades, NASA’s Glenn Research Center has performed substantial work on polyimide aerogels. They introduced an alternative route that uses an aromatic triamine, 1,3,5-triaminophenoxybenzene (TAB) [18,23–25], as the cross-linker, and the reaction between the diamines and dianhydrides is mild even at room temperature. Recently, other types of cross-linkers have been used for the synthesis of polyimide aerogels, such as octa(aminophenyl) silsesquioxane (OAPS)[26,27], 2,4,6-tris(4-aminophenyl)pyridine (TAPP) [28], 1,3,5-tris(aminophenyl)benzene (TAPB) [20], 1,3,5benzenetricarbonyl trichloride (BTC) [29] and even graphene oxide [30]. Additionally, in contrast to the common two-step method, Missouri University reported a one-step method at room temperature to synthesize polyimide aerogels by reacting pyromellitic dianhydride (PMDA) with 4,40 -methylene diphenyl diisocyanate (MDI) without dehydration. Recently, Yonsei University also proposed a new one-step method to prepare polyimide aerogels without additional chemicals for cross-linking [31].

In terms of performance, polyimide aerogels provide three main aspects: thermal insulation, improved mechanical strength and electrical insulation. The polyimide aerogels that were cross-linked by 1,3,5-triaminophenoxybenzene (TAB) [18] could be fabricated into continuous thin films. The films were flexible enough to be rolled or folded back on themselves with tensile strengths of 4–9 MPa. Other aromatic triamine cross-linkers such as TAPP [28] and TAPB [20], led to the same performance in terms of mechanical strength. Recently, Guo et al. [32] used poly(maleic anhydride) to cross-link a polyimide aerogel with a high Young’s modulus of 2–60 MPa, which provided a new method. The polyimide aerogels from OAPS [26,27] had a low thermal conductivity (0.014 W/m K at room temperature) and a high Young’s modulus of approximately 5.3 MPa. Wu et al. [33] reported that polyimide aerogels cross-linked by bis(trimethoxysilylpropyl) (BTMSPA), which is a type of silane coupling agent and is less expensive than the triamine cross-linkers, had thermal conductivities as low as 0.033 W/(mK) with high Young’s modului of approximately 50– 76 MPa. Additionally, Meador et al. [24] used 2,2-bis(3,4-dicarbox yphenyl)hexafluoropropane (6FDA) as a dianhydride to synthesize polyimide aerogels with relative dielectric constants as low as 1.08. Meanwhile, Zhang et al. [34] selected 5-amino-2-(4-amino phenyl)benzoxazole (APBO) as a diamine to obtain polyimide aerogels that exhibited an ultra-low dielectric constant (k) of 1.15 at a frequency of 2.75 GHz. However, reports about polyimide aerogels for CO2 adsorption are scarce. Because of the Lewis-acidic character and quadrupole momentum of the polyimide, CO2 molecules are expected to strongly interact with the highly polar imide functionalities on the polymer surface. The gas capture property of porous networks depends not only on the specific surface area and micropores but also on the chemical composition. Some recent reports have shown that the introduction of functional groups, such as amino, sulfonate [35] and hydroxyl groups [36], can significantly improve the adsorption property. In this study, we synthesized a polyimide aerogel containing carboxyl groups in the main chain that could serve as active sites for adsorbing CO2, which is indeed a physical mechanism. We selected 3,5-diaminobenzoic (DABA) and biphenyl-3,30 ,4,40 -tetracarboxylic (BPDA) to form the oligomer; subsequently, we used TAB to react with the terminal anhydride group to form the cross-linked poly (amic acid). The repeat units in the oligomers, n, were varied between 15 and 30 by using a ratio of n diamine units to (n + 1) dianhydride units to fabricate the oligomers as shown in Scheme 1. This range was studied because gels produced from shorter oligomers (lower formulation) reacted too quickly to incorporate the catalyst for chemical imidization before phase separation occurred[23]. In addition, the total polymer concentration in the gelation solution ranged from 5 to 10 wt%. By using pyridine to catalyze imidization and acetic acid to scavenge water byproduct in the condensation, polyimide gels were obtained. Finally, supercritical fluid extraction was performed using CO2 dry gel [23,26,27].

2. Experimental section 2.1. Materials DABA and BPDA were obtained from Aldrich Industrial Corporation (China). Pyridine was purchased from Shanghai Shenbo Chemical Co., Ltd. (China). Acetic anhydride and acetone were obtained from Shanghai Lingfeng Chemical Reagent Co., Ltd. (China). NMethyl-2-pyrrolidinone (NMP) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). TAB was purchased from ShenZhen Lijing Shenghua Co., Ltd. (China). All of the reagents were used directly as received without any treatment.

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for 24 h in the mold. Following aging, the wet gels were extracted into a solution of 75% NMP in acetone and soaked overnight. Subsequently, the solvent was exchanged in 24 h intervals with 25% NMP in acetone and finally with 100% acetone. The solvent was removed by supercritical CO2 extraction, followed by vacuum drying for 24 h at 55 °C, resulting in polyimide aerogels with a density of 0.171 g/cm3. 13C CPMAS NMR (ppm): 124.76, 130.39, 144.68, 165.87. 2.3. Characterization and test method

Scheme 1. Synthetic route for carboxyl-containing polyimide aerogels cross-linked with TAB.

2.2. Preparation Polyimide aerogels were prepared by combining the DuPont route [23,26,27,29] with the ‘‘DABA method” [37], as shown in Scheme 1. Aerogels from DABA and BPDA with the formulated number of repeat units, n, varying between 15 and 30, were prepared according to Table 1 using 5–10 wt% solutions of polyimide in NMP. As an example, the preparation of the 7.5% polyimide aerogel with n = 15 (Table 1, run 5) is described. DABA (7.5 mmol, 1.141 g) was added to 30 ml of NMP and magnetically stirred until the DABA fully dissolved. Then, the solution was cooled to 0 °C, and 7.725 mmol of BPDA was added. The mixture was stirred for 10 min at 0 °C and for 12 h at room temperature to yield a uniform solution. Subsequently, a solution of TAB (0.166 mmol, 0.066 g) in 11.7 ml of NMP was added with rapid stirring for approximately 10 min, followed by the addition of acetic anhydride (61.8 mmol, 5.8 ml, 8:1 M ratio to BPDA) and pyridine (61.8 mmol, 5.0 ml) to the solution. Immediately after mixing, the solution was poured into molds. The solution gelled within 20 min. The gels were aged

Attenuated total reflectance (ATR) infrared spectroscopy was conducted using a Thermo Nicolet-IS5 Fourier transform infrared spectrometer. Thermogravimetric analyses (TGA) were performed using a simultaneous thermal analyzer (DCS1/1600LF). Samples were run at a temperature ramp rate of 10 °C per min from room temperature to 1000 °C under a nitrogen atmosphere. Glass transition temperatures were determined from the DSC curves. 13C NMR spectra of the polymers were obtained on a Bruker Avance III 400 spectrometer. A JEOL 7800 field-emission microscope was used to obtain the scanning electron microscopy (SEM) images after sputter coating the specimens with gold. BET specific surface areas were measured using the nitrogen sorption method with a Vsorb 2800P surface area and pore distribution analyzer instrument (Gold APP Instruments Corporation China). The Barrett-JoynerHalenda (BJH) pore size distribution was calculated using the desorption branches. The samples were outgassed at 80 °C for 8 h. The apparent densities (qa) were calculated by measuring the weight and the volume of the samples. The skeletal densities (qs) were measured using a G-DenPyc 2900 analyzer. The porosities were calculated using Eq. (1)

Porosity ¼ ð1  qa =qs Þ  100%

ð1Þ

Carbon dioxide sorption isotherms were measured at 0 °C and 25 °C up to 1.0 bar. The samples were outgassed at 150 °C under high vacuum for 3 h prior to the sorption measurements. 3. Results and discussion 3.1. Synthesis and characterization Polyimide aerogels containing carboxyl groups were fabricated according to Scheme 1. The variables used to fabricate the aerogels are shown in Table 1, along with the density, porosity, surface area and thermal data for each run. The aerogels produced in this study were dark brown and even almost black in color, as shown in Fig. 1. As shown in this figure, the aerogel appeared to shrink because of the rigid DABA monomer. According to previous reports, the rigid monomer may tend to form polymer repeat units with a high

Table 1 Properties of the polyimide aerogels investigated in this study.

a

Sample

n

Polymer concn, wt%

Density, g/cm3

Porosity,%

Surface area, m2/g

Glass transition, °C

1 2 3 4 5 6 7 8 9 10 11 12

15 20 25 30 15 20 25 30 15 20 25 30

5.0 5.0 5.0 5.0 7.5 7.5 7.5 7.5 10.0 10.0 10.0 10.0

0.289 a a 0.116 0.171 0.386 0.207 0.210 0.244 0.234 0.201 0.140

91.6 a a 97.0 91.6 80.0 88.8 89.2 88.5 88.2 89.2 95.4

461 386 421 328 426 371 358 286 349 243 229 173

150.6 153.6 165.6 177.6 170.6 172.6 175.6 192.6 221.6 212.6 185.6 194.6

Not measured.

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Fig. 3. Solid NMR spectrum of 7.5 wt% polyimide aerogel with n = 15. Fig. 1. Polyimide aerogel containing carboxyl groups: wet gel vs aerogel.

degree of planarity. The planarity could lead to rigid chains and induce the chains to pack closely. Fig. 2 presents the FT-IR spectrum of the aerogel in this study, which contains characteristic peaks for polyimide, including 1351 cm1 (m imide CAN), 1722 cm1 (symmetric m imide C@O) and 1776 cm1 (asymmetric m imide C@O). The peak at 1860 cm1 for unreacted anhydride is not observed. In addition, the existence of peaks at 1658 cm1 (m amic acid C@O) and 1506 cm1 (m amide CAN) further indicates complete imidization. Neither of the vibrations at 1807 cm1 or 980 cm1 that are expected for the isoimide structure were observed [27]. Additionally, the FT-IR spectra at different reaction stages are shown in Fig. 2, which clearly illustrate the imidization process. First, there was only DABA in NMP; thus, no characteristic peaks appeared. BPDA was then added, which caused the reaction between the diamine and dianhydride with the appearance of the secondary amide (CONH) at 1557 cm1. After the cross-linker TAB was added, the peak at 1557 cm1 was still present, and no other peaks were present that indicated the occurrence of crosslinking. Finally, the peak at 1557 cm1 disappeared in the final sample after chemical imidization and drying. All of the above results confirm the formation of the complete polyimide structure via chemical imidization. From the previous reports we could concluded that the carboxyl stretching of the acid group was partially

Fig. 2. FT-IR spectra for typical chemical groups in the solution at different reaction stages.

Fig. 4. TG-DSC curves under N2 of the 7.5 wt% polyimide aerogels with n = 15.

over lapped with the imide carbonyl [38]. The broad weak transmittance at 3200–3400 cm1 is attributed to the vibration band of AOH in DABA moiety [39]. Additionally, the peaks are located at 1400 cm1 and 920 cm1, which could further indicated the carboxyl group of DABA. As shown in Fig. 3, the 13C NMR spectrum of the solid sample of the polyimide aerogel at n = 15 and 7.5 wt% contains an imide carbonyl peak at 165 ppm, which also includes the carbonyl from DABA, and aromatic peaks between 115 and 140 ppm that are characteristic of polyimide. Similarly, previous reports indicated that the carboxyl groups in DABA and BPDA cannot be distinguished from the NMR spectra [40]. TGA of the polyimide aerogels was performed under a nitrogen atmosphere from 30 °C to 1000 °C, and all of the sample curves exhibited similar trends. Fig. 4 presents the TGA curve of the 7.5 wt% aerogel with n = 15. First, it can be observed that an apparent weight loss occurred at 100–250 °C attributed to the adsorption of CO2 and H2O due to the existence of the carboxyl groups. As is well known, during the supercritical CO2 drying process, the adsorption of CO2 would occur in the porous aerogels, and the presence of carboxyl groups has a positive effect on CO2 desorption. However, the temperature of the CO2 supercritical state (50 °C) could not reach the temperature of CO2 desorption, which led to CO2 gas remaining in the aerogels. Additionally, carboxyl groups existed in the skeleton structure, making the surface of the materials hydrophilic, which can adsorb moisture from the air. This performance also could be observed in other literatures about aerogels for CO2 adsorption [41,42]. Moreover, there is

Y. Chen et al. / Chemical Engineering Journal 322 (2017) 1–9

a) 5wt% polymer

c) 10wt% polymer

5

b) 7.5wt% polymer

d) Graph of surface area for all samples

Fig. 5. Typical N2 adsorption-desorption isotherms (at 77 K) with the graph of relative pore vs pore diameter for all polyimide aerogels.

almost no weight loss before 250 °C in the polyimide aerogel synthesized from ODA and BPDA without carboxyl groups in the framework structure [27]. There is a small weight loss at approximately 440–550 °C from the decomposition of the carboxyl in DABA [43]. The onset of the decomposition temperature at 550– 570 °C indicates that the imidization was complete and that NMP was thoroughly removed by solvent exchange with acetone and supercritical drying [27]. Moreover, the DSC curve is also shown in Fig. 4 to examine the glass transition temperature and other thermal behaviors. 3.2. Porous morphologies and porosities The surface area and pore volume of the monoliths were measured by nitrogen sorption using the Brunauer-Emmett-Teller (BET) method. The BET surface areas of the aerogels ranged from 173 to 461 m2/g. Fig. 5d shows a plot of the surface areas for different repeat units at the same polymer concentration of 7.5 wt%. This plot shows that increasing the n value significantly reduces the surface area, which is similar to previous reports [23]. This result might be attributed to the increase in the number of repeat units and bulky organic groups, which would cause some pores to become clogged. Additionally, note that higher surface areas were obtained at lower polymer concentrations, as shown in Fig. 5d, because a lower polymer concentration leads to higher porosity. Typical nitrogen adsorption-desorption isotherms at 77 K for the monoliths are shown in Fig. 5a–c. According to the IUPAC classification, the N2 adsorption-desorption isotherms of the aerogels in this study are IUPAC type IV curves, which indicates the presence of a significant fraction of mesopores. The hysteresis loop characterizing this type of isotherm is associated with the capillary condensation of nitrogen in the mesopores [26]. Furthermore, capillary condensation and capillary evaporation often do not occur at the same pressure, which leads to the appearance of hysteresis loops [8]. With an increasing number of repeat units, n, pla-

teaus at higher P/P0 are observed, indicating a slight modification in the pore structures derived from longer polyimide oligomers. The pore size distributions of the polyimide aerogels, which were determined from their desorption curves using the BarrettJoyner-Halenda method, are presented in the N2 adsorptiondesorption isotherms, where it is obvious that the pore size distribution is approximately 20–40 nm but trials out to approximately 100 nm. However, the repeat units, n, do not have a significant effect on the pore distribution, other than the surface area. The porosities of the polyimide aerogels are up to 97%, which is similar to those of the polyimide aerogels produced by NASA and likely arises from the inefficient polymer chain packing imposed by bulky and contorted structural motifs in the monomer [44]. Field-emission scanning electron microscopy (FESEM) micrographs of the carboxylic-containing polyimide aerogel formulations are presented in Fig. 6. In contrast to silica aerogel, which is formed from aggregates of small particles with little visible interstitial porosity, the polyimide aerogels consisted of a threedimensional network of tangled polyimide fibers [8]. This is similar to the morphologies of some reported polymer aerogels, such as cellulose aerogels and polyurea aerogels. This arrangement of nanofibers most likely forms during polymerization and subsequent gelation with an influence from solvent interaction [32]. Additionally, no significant difference in the morphology can be detected among aerogels with different n values. 3.3. Adsorption of CO2 The large surface area, ultraporous structure and narrow pore size distribution of the polyimide aerogels are expected to allow for CO2 adsorption. In addition to the large surface area, the excellent CO2 adsorption performance likely results from the high affinity of the skeleton for CO2 due to the abundant electron-rich nitrogen and oxygen atoms from the imide ring [45]. The CO2 adsorption properties of the polyimide aerogels were measured

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Fig. 6. FESEM images of polyimide aerogels with different magnifications at 7.5 wt%: (a), (c), (e), and (g) at are low magnification; (b), (d), (f), and (h) are at high magnification.

in the 25 °C in the pressure range of 0–1.0 bar. As shown in Fig. 7a– c, the uptake of CO2 exhibits a rapid increase in the low-pressure region for each sample, implying that CO2 molecules have favorable interactions with the porous polymer skeleton because the adsorption capacity at low pressure reflects the affinity between the pore wall and the adsorbates [46], and a considerably higher storage capacity can be expected at increased pressure. The highest CO2 uptake of the aerogels is 21.10 cm3/g for the 7.5 wt% aerogels with n = 15; thus, we selected the polymer concentration of 7.5 wt % to research in detail, and the complete adsorption data are shown in Table 2. This CO2 uptake is the same and is substantially higher than the values of porous polyimides previously reported under the same conditions, such as naphthalene-based microporous polyimides (24.28–39.96 cm3/g)[47] and triazine-based porous polyimides (9.57–27.82 cm3/g)[48]. This value is also comparable to those of other porous polymer materials, such as PSN-2 (21 cm3/g) [49], PI-2 (22.26 cm3/g) [50], PAF-1 (24.27 cm3/g) [51], micropore CEs (20 cm3/g) [52], and MOPs (24.03– 48.96 cm3/g) [53], even though they have larger surface areas and higher micropore volumes. Because there were no reports on polyimide aerogels for CO2 adsorption, we selected a polyimide aerogel produced from 4,40 - oxydianiline (ODA) and BPDA with a TAB cross-linker using the same number of repeat units, n = 15, and polymer concentration, 7.5 wt%, as the reference, and the CO2 adsorption capacity at 1 bar and 25 °C is 8.89 cm3/g. The higher CO2 uptake of the polyimide aerogel in this study may be attributed to the presence of CO2-philic carboxyl groups on the polymer skeleton. The presence of carboxyl groups could not only

form similar hydrogen bonds between O(CO2) and H(COOH) [54] but also increase the CO2 capture due to the polarity of the carboxyl groups, as it has been reported that polar groups can lead to increased CO2 capture. Additionally, as a polymer backbone, it appears likely that the carboxylic acids could form the hydrogenbonding interactions that lead to produce the chain deformations, thus leading to a reduction in the size of pores [55]. From Table 2, the data are consistent with the conclusion that the addition of the carboxyl group decreases the pore size. As shown in Table 2, the effect of the carboxylic content is not significant. Inversely, we can clearly observe that a higher surface area leads to higher CO2 uptake, which is consistent with previous reports regarding the nature of mesoporous materials. It is explained that high surface area favors the interaction between the CO2 molecule and sample, which could increase the opportunity of adsorbent to contract with gas and offer more adsorptive sites. On the other hand, it is well known that the kinetic diameter is equal to the intermolecular distance of two molecules colliding with zero initial kinetic energy and is taken as the smallest diameter that allows the gas molecule to enter the inner cavity, and the kinetic diameter of CO2 is 3.30 Å. In this regard, a smaller pore size is good for CO2 adsorption [47]. However, in this work the smaller pore size do not show the high performance, which could be interpreted that the adsorbing performance is not only dependent on pore structure but mainly on the surface area which represents the adsorptive site in fact. In this case, it could be concluded that both microstructure and chemical composition should be taken into consideration when designing a high-performance CO2

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a) 5wt% polymer

c) 10wt% polymer

b) 7.5wt% polymer

d) Enthalpy of adsorption of 7.5wt% polymer

Fig. 7. CO2 adsorption isotherms of all studied polyimide aerogels.

Table 2 Porous parameters and CO2 uptake of polyimide aerogels at a polymer concentration of 7.5 wt%. Sample

SN2,BET (m2/g)

Vtotal (cm3/g)

Vmicro (cm3/g)

Pore, size (nm)

CO2 uptake (cm3/g)

n = 15 n = 20 n = 25 n = 30

426 371 358 286

2.066 1.857 1.605 0.915

0.157 0.152 0.133 0.104

20.02 19.36 17.91 12.79

21.10 15.23 13.22 14.53

adsorbent. Notably, the sample with n = 30, which had a low pore volume and specific area, on the contrary, it has high adsorption of CO2, which is attributed to the existence of larger amount carboxyl groups. Moreover, the isosteric enthalpy (Qst), which is a heating effect due to the increased adsorption capacity, is another measurement index for characterizing the adsorption properties of a material, and it can reflect the intermolecular force between the adsorbent and the gas molecule. To obtain a greater adsorption capacity at relatively high temperature, ideal porous CO2 adsorbents must have high Qst. The isosteric enthalpies of CO2 for polyimide aerogels were calculated from the adsorption isotherms measured at different temperatures using the Clausius-Clapeyron equation [46]:

lnP ¼

Q st þC RT

where, P, T, R, and C are the pressure, the temperature at equilibrium, the gas constant, and the equation constant, respectively. The dependencies of the Qst values on the amount of adsorbed CO2 are shown in Fig. 7d. The Qst values are slightly lower than those of microporous polyimides such as MPIs (31–35 kJ/mol) [45] due to the pore structure. The pore distribution shown in Fig. 5 indicated that the polyimide aerogel network is predominantly mesoporous. According to previous reports, a smaller pore

size (more micropores) leads to higher Qst values. However, the moderate Qst values of the polyimide aerogels were attributed to the existence of the carboxyl groups. Dawson et al. studied MOFs functionalized with different chemical groups with the aim of improving the CO2 adsorption capacity and reported that carboxyl groups led to the highest Qst value due to their strong polarity [47]. For each plot, the Qst values apparently decrease with increasing adsorption capacity, indicating that the interaction between CO2 and the pore wall is stronger than that between the CO2 molecules. As shown in Fig. 7, the sample with n = 15 had the highest Qst value, which could be related to the existence of more smallsized pores. Notably, the sample with n = 30 had a low pore volume but had a relatively high Qst value due to the high content of oxygen and nitrogen atoms from the imide heterocycles in the network. According to previous reports, electron-rich heteroatoms such as oxygen and nitrogen can produce strong dipolequadrupole interactions between the pore surface and CO2 molecules, and this interaction would notably enhance the Qst value, and simultaneously cause a higher adsorption capacity [47]. In summary, it was observed that the CO2 adsorption capacity in the porous polymers was mainly due to three aspects: 1) the strong affinity for CO2 molecules by the polymer skeleton, enhanced by the introduction of relevant CO2-philic groups such as amino, carboxyl, hydroxyl and sulfonate groups or the addition of electron-rich heteroatoms in the polymer network due to

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dipole-quadrupole interactions between the pore surface and CO2 molecules; 2) the large surface area and pore volume (the higher the better); and 3) the length of the polymer main chain, namely, the polymer skeletal structure. Therefore, CO2 adsorption is controlled by not only the micro-structure but also the chemical composition and the structural architecture of the network. 4. Conclusion Polyimide aerogels were successfully prepared using a new carboxyl-containing diamine, DABA, and BPDA with a TAB crosslinker, and their CO2 adsorption capacities were investigated. The resulting aerogels vary in terms of density, porosity, surface area, and CO2 absorption capacity depending on the number of repeat units, n, and the polymer concentration. Using DABA as the diamine to introduce carboxyl groups improved the CO2 adsorption capacity compared to the aerogel previously reported from ODA and BPDA. Due to the existence of the CO2-philic carboxyl groups, resulting in a strong affinity of the polyimide skeleton for CO2 molecules, the CO2 absorption capacity is 21.10 cm3/g at 25 °C and 1.0 bar. Although the absorption capacity is not as high as that of microporous polyimide materials or metal-organic-frameworks (MOFs), the flexible processing and high yield make it possible to form a membrane, and the mild and convenient synthesis methods are more environmentally friendly. Thus, the above results indicate that polyimide aerogels can be promising candidates for adsorbents in CO2 capture in the field of environmental protection. Acknowledgements This work was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_15R35), the Industry Program of Science and Technology Support Project of Jiangsu Province (No. BE2014128), the Major Program of Natural Science Fund in Colleges and Universities of Jiangsu Province (No. 15KJA430005), the Prospective Joint Research Program of Jiangsu Province (No. BY2015005–01), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] L. Zuo, Y. Zhang, L. Zhang, Y.-E. Miao, W. Fan, T. Liu, Polymer/carbon-based hybrid aerogels: preparation, properties and applications, Materials 8 (2015) 6806–6848. [2] K.S. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Pure Appl. Chem. 57 (1985) 603–619. [3] A. Du, B. Zhou, Z. Zhang, J. Shen, A special material or a new state of matter: a review and reconsideration of the aerogel, Materials 6 (2013) 941–968. [4] A.C. Pierre, G.M. Pajonk, Chemistry of aerogels and their applications, Chem. Rev. 102 (2002) 4243–4266. [5] A.P. Katsoulidis, J. He, M.G. Kanatzidis, Functional monolithic polymeric organic framework aerogel as reducing and hosting media for Ag nanoparticles and application in capturing of iodine vapors, Chem. Mater. 24 (2012) 1937–1943. [6] H. Sun, Z. Xu, C. Gao, Multifunctional, ultra-flyweight, synergistically assembled carbon aerogels, Adv. Mater. 25 (2013) 2554–2560. [7] M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-gel Sci. Technol. 63 (2012) 315–339. [8] X. Pei, W. Zhai, W. Zheng, Preparation and characterization of highly crosslinked polyimide aerogels based on polyimide containing trimethoxysilane side groups, Langmuir 30 (2014) 13375–13383. [9] R. Pekala, Organic aerogels from the polycondensation of resorcinol with formaldehyde, J. Mater. Sci. 24 (1989) 3221–3227. [10] N. Leventis, C. Sotiriou-Leventis, D.P. Mohite, Z.J. Larimore, J.T. Mang, G. Churu, H. Lu, Polyimide aerogels by ring-opening metathesis polymerization (ROMP), Chem. Mater. 23 (2011) 2250–2261. [11] A. Rigacci, J. Marechal, M. Repoux, M. Moreno, P. Achard, Preparation of polyurethane-based aerogels and xerogels for thermal superinsulation, J. Noncryst. Solids 350 (2004) 372–378.

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