Tetraphenyladamantane-Based Microporous Polyimide and Its Nitro

Article pubs.acs.org/JPCC

Tetraphenyladamantane-Based Microporous Polyimide and Its NitroFunctionalization for Highly Efficient CO2 Capture Changjiang Shen and Zhonggang Wang* State Key Laboratory of Fine Chemicals, Department of Polymer Science and Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: A new microporous polyimide network (PIADNT) is synthesized from 1,3,5,7-tetrakis(4-aminophenyl)adamantane and naphthalene-1,4,5,8-tetracarboxylic dianhydride. Subsequently, PI-ADNT is nitrated in fuming nitric acid with different nitration time to produce three nitrodecorated porous polyimides (PI-NO2s). Their chemical structures and nitration degrees are characterized by FTIR, solid-state 13C CP/MAS NMR spectra and element analysis. The interesting evolution of porous morphology and porosity of PI-NO2s with nitration time is investigated in detail. The results show that PI-ADNT has the BET surface area of 774 m2 g−1 with microporous size centering at 0.75 nm. After nitrationmodifications, PI-NO2s display decreased surface area but remarkably increased CO2 uptake up to 4.03 mmol g−1, which is superior to most of porous polymers reported in the literature. Moreover, the CO2 adsorption selectivites over CH4 and N2 in PINO2s are also significantly improved in comparison with PI-ADNT. The CO2 adsorption/separation properties of PI-ADNT and its nitrated products are studied and explained in terms of the variations of porous structure and chemical composition as well as the interaction parameters between CO2 molecule and polymer skeleton such as Henry’s constant, first virial coefficient, and enthalpy of adsorption.

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INTRODUCTION Seeking new clean energies such as hydrogen, solar, wind, and tidal energy has been a hot topic in both academic and industrial fields for years. However, the actual replacement of coal, fossil oil, and natural gas with the above energies still needs long period of time owing to the lack of an effective, economic, and safe on-board storage method. These traditional fuels will have to be continually utilized as primary energy sources in the forthcoming few decades. Large amounts of carbon dioxide (CO2) emitted from the burning of fuels have led to serious environmental problems like global warming, sea level rise, and an irreversible increase in the acidity level of oceans.1 Currently, a fluidized bed of aqueous amine solution is being widely employed to absorb CO2 from the waste gas of power plants. Nevertheless, this process has many drawbacks such as corrosion and volatility of amines and high energy-cost of regeneration in comparison with solid porous adsorbents.2 Among porous materials, microporous organic polymers (MOPs) gain considerable interest in CO2 capture because of their high CO2 adsorption capacity, excellent thermal and chemical stability, tunable chemical composition, and structure.3−6 In earlier works, large surface area for the design and synthesis of MOPs has been the most pursued goal. However, recent studies reveal that the strong affinity of CO2 molecule toward pore wall may play a more important role on CO2 adsorption. The incorporation of functional CO2-philic groups has been demonstrated to be an efficient strategy to improve © 2014 American Chemical Society

CO2 adsorption and its selectivity over nitrogen (N2) and methane (CH4) gases. To this end, a number of MOPs with nitrogen-rich units such as poly(Schiff base)s,7−14 microporous polyimides,15−25 polybenzimidazole26−29 and cyanate resins,30,31 have been synthesized. In addition, some MOPs with decorated amino,32 carboxyl,32,33 hydroxyl,32,34 and sulfonate groups18 have also been prepared by means of direct polymerization from the monomers bearing the corresponding CO2-philic groups. Nevertheless, functional monomers are usually obtained through tedious synthetic procedures, and the presence of functional groups may disturb the subsequent polymerization. Comparatively, postmodification on an established microporous polymer will be a cost-saving and feasible approach considering the convenient scale-up production. Following this way, amino,35,36 nitro,37,38 sulfonyl, and sulfonate39 have been successfully introduced onto polymer skeletons. Most of these postmodified polymers show significant improvement on CO2 adsorption and selectivity. In our previous works, hyper-cross-linked microporous polyimides with excellent thermal and chemical stability were prepared via polycondensation from various dianhydrides with multiamines.15−17,21,23 Their synthetic control of topological structure, H2 storage, CO2 capture and recovery of toxic Received: April 14, 2014 Revised: July 8, 2014 Published: July 8, 2014 17585

dx.doi.org/10.1021/jp503675f | J. Phys. Chem. C 2014, 118, 17585−17593

The Journal of Physical Chemistry C

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several drops of isoquinoline were added into m-cresol (10 mL), and the mixture was stirred at ice-bath temperature under a nitrogen flow for 2 h. The reaction system was slowly heated to room temperature and stirred overnight. Then the mixture was heated and polymerized at 80 °C for 2 h, 180 °C for 8 h, and 220 °C for an additional 10 h. The gel obtained was rushed and extracted with THF in a Soxhlet apparatus for 48 h. The solid was dried under vacuum at 150 °C for 2 days to constant weight. Quantitative yield was obtained. Preparation of Nitro-Functionalized Polyimide Networks (PI-ADNT-NO2s). PI-ADNT (0.8 g) was added into fuming acid (8 mL) at −15 °C. Then, acetic acid (4 mL) and acetic anhydride (2.4 mL) were added into the solution, and the mixture was stirred for 3, 6, and 12 h, respectively. The precipitate was filtered and washed with deionized water to neutrality, and extracted with THF in a Soxhlet apparatus for 48 h. The solid was dried under vacuum at 150 °C for 2 days. Quantitative yields were obtained. For the sake of clarity, the three nitrated products with reaction time of 3, 6, and 12 h are named as PI-NO2-1, PI-NO22, and PI-NO2-3, respectively. Instrumention. Fourier transform infrared spectra (FTIR) were recorded using a Nicolet 20DXB FT-IR spectrophotometer in the 400−4000 cm−1 region. Samples were prepared by dispersing the complexes in KBr and compressing the mixtures to form disks. 1H NMR spectra were measured on a 400 MHz Varian INOVA NMR spectrometer, using tetramethylsilane as an internal reference. Solid-state 13C CP/ MAS (cross-polarization with magic angle spinning) spectra were recorded on a Varian Infinity-Plus 400 spectrometer at 100.61 MHz at an MAS rate of 10.0 kHz using zirconia rotors 4 mm in diameter using a contact time of 4.0 ms and a relaxation delay of 2.0 s. The 1H π/2 pulse was 3.75 μs, and two-pulse phase modulation decoupling was during the data acquisition. Melting points were performed using an X-4 melting-point apparatus with a microscope. Elemental analyses were determined with an Elementar Vario EL III elemental analyzer. Thermogravimetry (TG) measurements were performed in the nitrogen atmosphere on a NETZSCH TG 209 thermal analyzer at a heating rate of 10 °C min−1. The temperature range was from room temperature to 800 °C. Adsorption and desorption measurements for all the gases were conducted on an Autosorb iQ (Quantachrome) analyzer. Before sorption measurements, the samples were degassed at 150 °C under high vacuum for 24 h. Adsorption and desorption isotherms of nitrogen were measured at 77 K. The surface areas were calculated according to the Brunauer−Emmett−Teller (BET) model in the relative pressure (P/P0) range from 0.01 to 0.10 for PI-ADNT, from 0.05 to 0.20 for PI-NO2-1, from 0.37 to 0.47 for PI-NO2-2, and from 0.37 to 0.5 for PI-NO2-3. Microporous volumes were calculated using t-plot method based on the Halsey thickness equation, while the total porous volumes were obtained from the N2 isotherms at P/P0 = 0.99. Pore size distributions were derived from the N2 adsorption isotherms using the nonlocal density functional theory (NLDFT). Carbon dioxide sorption isotherms were measured at 273 and 298 K up to 1.0 bar. N2 and CH4 sorption isotherms at 273 K were measured in order to evaluate the adsorption selectivities of CO2/N2 and CO2/ CH4, which were calculated from the ratios of initial slopes, and ideal adsorbed solution theory (IAST) for pure component sorption isotherms of gases.

organic vapors have been extensively studied. The results show that the resultant porous polyimides can uptake 7.3−16.8 wt % CO2 at 273 K/1 bar with the CO2/N2 and CO2/CH4 separation factors are as high as 102 and 12, respectively, depending on their chemical structures, porosity parameters, and surface areas. Herein, at first, a new microporous polyimide network (PIADNT) was prepared using rigid tetraphenyladamantane as the net node and naphthalene as the linking strut. The incorporation of naphthalene provides more reaction sites for its subsequent chemical modification. The nitration on PIADNT was then conducted in fuming nitric acid, and the nitration degrees were controlled by changing the reaction time. The influence of postnitration on surface area and pore morphology of PI-ADNT, e.g., pore size and distribution, were studied. Moreover, the adsorption capacities, Henry’s constants, first virial coefficients, enthalpies of adsorption for CO2 gas, and separations of CO2/N2 and CO2/CH4 were investigated in detail by correlation with the compositions and chemical and porous structures of PI-ADNT and its nitro-functionalized products.

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EXPERIMENTAL SECTION Materials. Adamantane and naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTDA) were purchased from Shanghai Chemical Reagent Corp. NTDA was purified by sublimation prior to use. m-Cresol was purified by refluxing over P2O5 and distillation under reduced pressure. Isoquinoline, fuming nitric acid, acetic acid, acetic anhydride and other reagents were used as received. 1,3,5,7-tetraphenyladamantane (TPA) was prepared according to the method described in the literature.40 Synthesis of 1,3,5,7-Tetrakis(4-nitrophenyl)adamantane (TNPA). TNPA was synthesized from TPA via the procedure in ref 41 with some modifications. TPA (4.2 g, 9.7 mmol) was added slowly into fuming nitric acid (35 mL) at −15 °C. Then, acetic acid (15 mL) and acetic anhydride (12 mL) were charged into the flask, and the mixture was stirred for 30 min. The precipitate was filtered and washed with ethanol. Crystallization from N,N-dimethylformamide (DMF) gave 2.3 g solid with 38.7% yield. Mp >300 °C; 1H NMR (400 MHz, DMSO-d6): δ (ppm) 8.20−8.22 (d, 8H), 7.91−7.93 (d, 8H), 2.25 (s, 12H); IR (KBr, cm−1): 3113, 3028 (C−H stretching vibration), 2932, 2903, 2854 (C−H stretching vibration), 1599, 1516 (aromatic ring CC vibration), 1350 (NO stretching vibration), 856 (C−H banding in phenyl ring). Synthesis of 1,3,5,7-Tetrakis(4-aminophenyl)adamantane (TAPA).17 TNPA (0.4 g, 0.645 mmol), tetrahydrofuran (THF, 80 mL), Pd/C (10 wt %, 0.2 g), and DMF (10 mL) were added into a hydrogenator. After evacuating and purging with nitrogen gas several times, the system was purged into hydrogen to a pressure of 2.0 MPa, and the mixture was stirred at room temperature for 2 days. After filtration, the filtrate was poured into deionized water (100 mL). Collecting the precipitate and drying under vacuum afforded a colorless solid with 90.6% yield. Mp >300 °C; 1H NMR (400 MHz, DMSO-d6): δ (ppm) 7.11−7.13 (d, 8H), 6.50−6.52 (d, 8H), 4.86 (s, 8H), 1.82 (s, 12H); IR (KBr, cm−1): 3426, 3324 (N−H stretching vibration), 3018 (C−H stretching vibration), 2921, 2848 (C−H stretching vibration), 1623, 1514 (N−H deformation vibration), 1273 (C−N stretching vibration), 842 (C−H banding in phenyl rings). Synthesis of Polyimide Network (PI-ADNT). TAPA (0.424 g, 0.8475 mmol), NTDA (0.5156 g, 1.695 mmol), and 17586



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Scheme 1. Synthetic Route to PI-ADNT and Its Nitrated Products

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RESULTS AND DISCUSSION Synthesis and Characterization. The chemical structure and synthetic route of adamantane-cored multiamine monomer TAPA, polyimide network PI-ADNT, and its nitrated products are illustrated in Scheme 1. PI-ADNT was prepared through one-pot polycondensation in m-cresol from TAPA and NTDA using isoquinoline as a catalyst. The FTIR spectra of the initial dianhydride NTDA and tetraamine TAPA monomers are presented in (Figure S1, Supporting Information). For TAPA, the absorptions at 3426 and 3324 cm−1 are attributed to the stretching vibration of the −NH2 group, while those at 2920 and 2830 cm−1 belong to the vibrations of aliphatic CH2 and CH groups in adamantane moiety. The band at 1273 cm−1 is due to the stretching vibration of aromatic C−N bond. The dianhydide monomer NTDA displays the characteristic band at 1778 cm−1 for the carbonyl of a six-membered anhydride group. In the 1H NMR spectrum of TAPA (Figure S2), the resonances at 7.13 and 6.51 ppm are assigned to the two protons in phenyl ring. The signals at 4.86 and 1.82 ppm are due to the proton signals of −NH2 and adamantane, respectively. For the PI-ADNT product, as shown in Figure 1, the characteristic bands for six-member imide rings are found at 1712 cm−1 (CO asymmetrical stretching) and 1669 cm−1 (CO symmetrical stretching), while the stretching vibration of C−N−C appears at 1345 cm−1,16,21 indicating that dianhydride monomer NTDA has been successfully polymerized with tetraamine TAPA to produced adamantane-based polyimide. In addition, the comparison between the spectra of PI-ADNT and NTDA shows that the condensation polymerizations between anhydride and amine groups are not very complete as revealed by the week anhydride absorption at 1778 cm−1.

Figure 1. FTIR spectra for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d).

The solid-state 13C CP/MAS NMR spectrum for PI-ADNT is presented in Figure 2, in which the resonance at 163 ppm belongs to the carbonyl carbon of imide ring,15−17,23 and the peak at 40 ppm is ascribed to the carbon of the adamantane core.17 The signal at 150 ppm is attributed to the carbon adjacent to nitrogen of imide ring. The resonances at 116−141 ppm are assigned to other carbons of phenyl and naphthalene groups. The nitrations of PI-ADNT were carried out in fuming nitric acid with different nitration time to yield three nitrated polyimides (PI-NO2s). To avoid the possible sulfonation side reaction, the mixture of acetic acid and acetic anhydride was 17587



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the cleavage of C-NO2 bond similar to other nitrated polymer systems.42 The second one at 621 °C belongs to the decomposition of polyimide backbone. It is noted that, at the first-step degradation, the three nitrated samples display the weight losses of about 20 wt %, which are well consistent with the nitration degrees measured by elemental analyses. Porous Morphologies and Porosities of PI-ADNT and PI-NO2s. The porosity and surface areas of the four polyimide networks were studied by nitrogen sorption at 77 K. As shown in Figure 4a, the isotherms for PI-ADNT and PI-NO2-1 display a steep rise of nitrogen uptake at the very low relative pressure range (P/P0 < 0.01), indicative of the presence of permanent micropores.15,30 The porosity data of the four networks are listed in Table 1. PI-ADNT exhibits a BET surface area of 774 m2 g−1, which is the largest among the four polymers. Relatively, the surface areas of the three nitrated samples apparently decrease with the increase of nitration time. The variation of total volumes of this series of polymers has the same trend as that of surface area. In addition, it is found that the micropore volumes for PI-ADNT and PI-NO2-1 derived from the t-plot method are 0.163 cm3 g−1 and 0.072 cm3 g−1, respectively, whereas those of PI-NO2-2 and PI-NO2-3 cannot be detected. The pore size distributions obtained by NLDFT are illustrated in Figure 4b. PI-ADNT has only one major peak at 0.75 nm. After nitration for 3 h, besides the peak at 0.75 nm, PI-NO2-1 displays smaller pores centering at 0.57 nm. With the further increase of nitration time, the former microporous peaks disappear, and instead, PI-NO2-2, and PI-NO2-3 exhibit a broad peak from 1.8 to 5.3 nm primarily in the mesoporous region. The evolution of pore size and surface area of PI-ADNT with the increase of nitration time can be attributed to the fact that the nitration modification of porous PI-ADNT makes that partial space of pore occupied by the nitro groups attached on naphthalene and/or phenyl rings, leading to the smaller pores at 0.57 nm in PI-NO2-1 and the decreased surface area. This phenomenon is more serious for PI-NO2-2 and PI-NO2-3, in which the porous channels have almost completely been blocked by nitro groups and the pores become too small to be accessible to nitrogen molecule, reflected in the disappearance of microporores in Figure 4b and dramatically decreased surface areas. The broad peak from 1.8 to 5.3 nm in pore size distribution curve may be due to the interparticulate voids arising from the loose aggregation of particles. Adsorption of CO2 in PI-ADNT and PI-NO2s. CO2 adsorption properties of the four polyimide networks were measured at 273 and 298 K in the pressure range from 0 to 1.0 bar. As illustrated in Figure 5a−d, for each sample, the uptake of CO2 displays a rapid rise in the low pressure region, implying that CO2 molecule has favorable interaction with the porous polymer skeleton. At 273 K and 1.0 bar, the CO2 uptakes of PIADNT, PI-NO2-1, PI-NO2-2, and PI-NO2-3 are 3.42, 4.03, 2.45, and 1.97 mmol g−1, respectively (Table 2). It is noteworthy that, among the four samples, PI-NO2-1 has the highest CO2 uptake of 4.03 mmol g−1 (equal to 90.5 cm3 g−1 and 17.7 wt %) in spite of its lower specific surface area than PIADNT. The adsorbed CO2 amount in PI-NO2-1 is superior to most porous polymers reported in the literature such as porous polyimides MPIs (9.9−16.8 wt %),15 NPIs (7.3−12.3 wt %),16 PI-ADPM (14.6 wt %),17 cyanate resins CEs (5.8−11.1 wt %),30,31 PSNs (6.7−15.0 wt %),7,8 PBIs (4.5−13.2 wt %),29 COFs (5.7−16.7 wt %),43,44 and CMPs (4.0−17.0 wt %),45 and

Figure 2. Solid-state 13C CP/MAS NMR spectra for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d). Asterisks (*) indicate peaks arising from spinning side bands.

utilized as the assistant agent instead of the conventional concentrated sulfuric acid. Compared to the non-nitrated PIADNT, the FTIR spectra of PI-NO2s exhibit the characteristic band of NO2 group at 1531 cm−1 (Figure 1), indicating the successful nitration of PI-ADNT. In the solid-state 13C CP/ MAS NMR spectra (Figure 2), after nitration, a significantly enhanced broad peak appears at 142−153 ppm, which is attributed to the nitro-substituted carbons in phenyl or naphthalene groups. To estimate the nitration degrees of the three polymers, their elemental contents were measured, and the data are listed in Table S1 (Supporting Information). According to the elemental contents of nitrogen, the calculated nitration degrees for PINO2-1, PI-NO2-2, and PI-NO2-3 are 18.82, 20.04, and 20.06 wt %, respectively. The results suggest that the nitro-substitution on polymer skeleton approaches saturation when the reaction time exceeds 3 h. The further increase of reaction time does not bring about significant change of nitration degree. The thermal stability of PI-ADNT and PI-NO2s were examined by thermogravimetric analyses (TGA) conducted under a nitrogen atmosphere at a heating rate of 10 °C min−1. Their thermogravimetric and DTG curves are presented in Figure 3. PI-ADNT exhibits excellent thermal stability with the maximum degradation at 621 °C. After nitration modification, the samples exhibit two steps of thermal degradation. The first maximum decomposition occurs at around 370 °C, owing to

Figure 3. TGA and DTG curves of PI-ADNT (a), PI-NO2-1 (b), PINO2-2 (c), and PI-NO2-3 (d). 17588



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Figure 4. (a) Adsorption (filled) and desorption (empty) isotherms of nitrogen at 77 K for PI-ADNT (+160), PI-NO2-1 (+100), PI-NO2-2 (+50), and PI-NO2-3. (b) Pore size distribution curves obtained by the NLDFT method for PI-ADNT, PI-NO2-1, PI-NO2-2, and PI-NO2-3.

comparable to BILP-1 (18.8 wt %)26 and APOPs (9.99−20.0 wt %),11 but less than BILP-3 (22.5 wt %).27 The temperature dependency of CO2 adsorption in PIADNT and its nitrated products is studied. At 298 K, the sequence of adsorption capacities in the four porous polymers is the same as that at 273 K. Similar to other porous materials, the data in Table 2 show that the polymers in this study also display decreased CO2 uptake with the increasing temperature. However, it should be noted that, at 298 K and 1 bar, PI-NO2-1 still has a high CO2 uptake of 8.9 wt %. This characteristic is very useful for the practical CO2-capture operation at ambient temperature. The high CO2 uptake in the nitrated polyimide networks is probably attributed to the presence of abundant nitrogen and oxygen atoms. The previous work by EL-Kaderi and co-workers has shown that, compared to the unmodified NPOF-4, the nitrated NPOF-4-NO2 has significantly increased Qst value from 23.2 to 32.5 kJ mol−1.37 Moreover, it has been reported that the introduction of electron-rich nitrogen and oxygen can improve CO2 adsorption owing to the dipole−quadrupole interaction between the pore surface and CO2 molecule.46 To verify the above deduction, the isosteric enthalpies (Qst) of CO2 in the four samples were calculated from the adsorption isotherms measured at different temperatures in term of Clausius−Clapeyron equation. 47 The plots of isosteric enthalpies as a function of adsorbed amount of CO2 are shown in Figure 6. As can be seen, for the four samples, the Qst values exhibit a decreasing trend with the adsorbed amount,

Table 1. Porous Parameters of the Four Polyimide Networks by N2 Adsorption at 77 K sample

SBET m2 g−1

VTotal cm3 g−1

VMicropore (cm3 g−1)

pore size (nm)

PI-ADNT PI-NO2-1 PI-NO2-2 PI-NO2-3

774 286 57 26

0.415 0.155 0.088 0.031

0.163 0.072 0 0

0.75 0.57, 0.75 1.8−5.3 1.8−5.3

Figure 5. Adsorption (filled) and desorption (open) isotherms of CO2 at 273 and 298 K for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d).

Table 2. CO2 Adsorptions and Selectivities of CO2/CH4 and CO2/N2 Gas Pairs at 273 K CO2 uptakea

selectivity (initial slope)b

selectivity (IAST)c

sample

273 K mmol g−1

298 K mmol g−1

Q0d kJ mol−1

CO2/CH4

CO2/N2

CO2/CH4

CO2/N2

PI-ADNT PI-NO2-1 PI-NO2-2 PI-NO2-3

3.42 4.03 2.45 1.97

1.93 2.02 1.62 1.23

35.2 43.3 37.9 37.9

11 15 21 17

32 33 56 37

9(7) 11(8) 16(13) 11(8)

25 18 31 19

a

CO2 uptake at 1 bar. bCalculated from initial slope of adsorption isotherms. cCalculated from IAST for 0.05/0.95 (0.5/0.5) gas mixtures for CO2/ CH4 and 0.15/0.85 gas mixture for CO2/N2 at 1.0 bar. dLimiting isosteric enthalpy of adsorption. 17589



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corrosion problem of gas pipelines. To achieve efficient capture and recovery of CO2 from N2 and CH4 gases, excellent adsorption-selectivity of CO2 from mixing gases is essentially important for a porous polymer. The gas adsorption isotherms of CO2, CH4, and N2 measured at 273 K in the pressure range from 0 to 1.0 bar are shown in Figure 7a−d. At first, the adsorption selectivities

Figure 6. Variation of CO2 isosteric enthalpies of adsorption with its adsorbed amount for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d).

indicating that the CO2 molecules preferentially adsorb on the pore walls of the polymer network rather than aggregate. The virial plots of CO2 for the four polyimides are presented in Figure S3. All of them display good linear relationships at different temperatures. Accordingly, the first virial coefficients (A0), Henry’s law constants (KH), and limiting enthalpy of adsorption at zero surface CO2 coverage (Q0) can be calculated. As seen in Table S2, the ranking order of Q0 (kJ mol−1) for the four polymers is PI-NO2-1 (43.3) > PI-NO2-2 (37.9) = PINO2-3 (37.9) > PI-ADNT (35.2), which is the same as the orders of KH and A0 values. Although PI-NO2-2 and PI-NO2-3 have the higher nitration degrees, their Q0 values are lower than that of PI-NO2-1. The reason can be due to the fact that PINO2-1 possesses large amounts of ultramicropores (pore size smaller than 7 Å), while the ultrasmall pore channels have a trapping-effect for small guest gas molecules.48 In addition, it is seen that the Q0 values of all the four samples are much higher than other porous polymers such as highly cross-linked polymers HCPs (21.2−23.5 kJ mol−1),49 CMPs (26.8−32.6 kJ mol −1 ), 32 BILPs (26.7−28.8 kJ mol−1)26,27 and comparable to cyanate resins CEs (32.6− 39.7),30,31 polyimides MPIs (30.4−34.8 kJ mol−1),15 NPIs (30.2−33.5 kJ mol−1),16 PI-ADPM (34.4 kJ mol−1),17 and PPNs (17−56 kJ mol−1).35,39 The large dipole moment of CNO2 bonds because of the strong electron-withdrawing effect of NO2 group may be responsible for the enhanced affinity of CO2 toward the nitro-decorated polymer skeleton. From the above results, it is seen that CO2 adsorption capacity in a porous polymer is mainly contributed from three aspects: large surface area, strong affinity of CO2 molecule toward polymer skeleton, and the microporous structure. Among the four samples, the remarkably higher CO2 uptake in PI-NO2-1 comes from the common contributions of the presence of ultamicropores and strong interaction between nitro-modified pore wall and CO2 molecule. Compared to PINO2-1, despite the higher NO2 content in PI-NO2-2 and PINO2-3, the absence of microporous structure and lower surface area lead to them reduced CO2 adsorption capacities. CO2 Adsorption Selectivities in PI-ADNT and PI-NO2s. Usually, the postcombustion flue gas from coal-fired power plants contains about 15% CO2, whereas the contents of CO2 in precombustion natural gas and landfill gas (mixture of CO2 and CH4) are about 5% and 40−60%, respectively, which should be stripped to increase the energy density of fuel gases.50 Moreover, the acidic CO2 gas causes the serious

Figure 7. Adsorption isotherms of CO2 (■), CH4 (●) and N2 (▲) for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d), at 273 K.

of CO2/CH4 and CO2/N2 gas pairs are studied by the initial slope method. Based on the adsorption isotherms at 273 K and the pressure below 0.12 bar (Figure 8a−d), the ratios of initial

Figure 8. Adsorption selectivities of CO2/N2 and CO2/CH4 derived from the initial slopes of CO2 (■), CH4 (●) and N2 (▲) isotherms at 273 K for PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d).

slopes of CO2 to CH4 and N2 are used as separation factors (α) of CO2/CH4 and CO2/N2, respectively. The data in Table 2 display that αCO2/CH4 and αCO2/N2 of PI-NO2-2 are 21 and 56, respectively, which are the highest among the four polymers. The αCO2/N2 values exceed APOPs (23.8−43.4),11 but are lower than BILPs (59−113)26,27 and the nitrated NPOF-4-NO2 (139).37 It is noteworthy that, the αCO2/CH4 value is significantly higher than that of MPIs (8−12),15 NPIs (11.1−12.9),16 BILPs (8−17),26,27 and APOPs (5.3−6.7),11 and is similar to that of NPOF-4-NO2 (15).37 17590



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one-pot polycondensation between 1,3,5,7-tetrakis(4aminophenyl)adamantane and naphthalene-1,4,5,8-tetracarboxylic dianhydride in m-cresol using isoquinoline as a catalyst. Its nitration-modifications are carried out in fuming nitric acid with different reaction time. Their chemical structures are well characterized by FTIR, solid-state 13C CP/MAS NMR spectra and elemental analyses. These products are insoluble in common solvents and have excellent thermal stability. The non-nitrated PI-ADNT possesses a BET surface area of 774 m2 g−1 and a CO2 uptake of 3.42 mmol g−1 at 273 K and 1.0 bar. With the increase of nitration time, the specific surface area drops to 286 m2 g−1, while the CO2 uptake increases to 4.03 mmol g−1 at 273 K and 1.0 bar. Meanwhile the isosteric enthalpies of the functionalized polymers are improved, and the limiting isosteric heat (Q0) is up to 43.3 kJ mol−1. From the gas adsorption isotherms, the gas selectivity is derived from the initial slope of the adsorption plots and ideal adsorbed solution theory. The nitrated samples display the selectivities for CO2/ CH4 and CO2/N2 up to 21 and 56, respectively. The excellent thermal/chemical stability and CO2 adsorption/separation properties endow them with promising potentials for capture and recovery of CO2 from postcombusion and precombusion gases.

Furthermore, the method of ideal adsorbed solution theory (IAST) was utilized to evaluate the adsorption selectivity of binary gas mixtures according to the mathematical integration:51 The gas mixture compositions are set as flue gas (CO2/ N2 = 0.15/0.85), natural gas (CO2/CH4 = 0.05/0.95) and landfill gas (CO2/CH4 = 0.50/0.50). As shown in Figure S4, the corresponding single-site Langmuir−Freundlich curves (solid black lines) for CO2, CH4 and N2 can well fit the experimental pure component isotherms of the corresponding gas. By IAST method, the selectivities of CO2/CH4 and CO2/ N2 are obtained and plotted as a function of pressure of the mixing gases up to 1 bar (Figure 9a−d). The data in Table 2

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

This section contains four figures and two tables, including the FTIR and NMR spectra of TAPA monomer, virial plots of CO2 adsorption for PI-ADNT and PI-NO2s at 273 and 298 K, experimental pure component isotherms and single-site Langmuir−Freundlich fitting curves for CO2, CH4, and N2 as well as the data of elemental analysis and virial parameters of the polyimide networks. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. IAST selectivities for 0.15/0.85 CO2/N2 mixture (■), 0.05/ 0.95 CO2/CH4 mixture (●), and 0.50/0.50 CO2/CH4 mixture (▲) in PI-ADNT (a), PI-NO2-1 (b), PI-NO2-2 (c), and PI-NO2-3 (d).

show that the ranking orders of αCO2/CH4 and αCO2/N2 from IAST theory are the same as those from the initial slope method. According to IAST method, at low coverage, the selectivity for CO2/N2 gas mixture is 71 for PI-NO2-2, which drops gradually with the increase of pressure and finally reaches 31 at 1.0 bar. The CO2/CH4 selectivities for PI-NO2-2 are 16 (natural gas) and 13 (landfill gas) at 1.0 bar, which are superior to many other porous polymers like PCNs (7.2−14.0)52,53 and PPFs (8.6−11.0).9 By contrast, the non-nitrated sample PIADNT has lower selectivities for both CO2/N2 (25) and CO2/ CH4 (9 and 7). Usually, relative to mesoporous or macroporous adsorbents, microporous materials have significant advantages for the adsorption selectivities of small molecule gases because of the size-recognition effect taking into consideration that the kinetic diameter of CO2 (3.30 Å) is much smaller than CH4 (3.8 Å) and N2 (3.64 Å).54 Therefore, compared to mesoporous PINO2-2 and PI-NO2-3, microporous PI-ADNT and PI-NO2-1 would have exhibited higher selectivities of CO2/N2 and CO2/ CH4. Nevertheless, the experimental measurements give contrary results. CH4 and N2 are nonpolar gases, whereas CO2 is polar gas with a large quadrupole moment (13.4 C m2);55 the higher selectivities of CO2/CH4 and CO2/N2 for PINO2-2 and PI-NO2-3 can only be explained by the significantly enhanced CO2 affinity of the nitro-decorated pore surface.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (Nos. 51273031 and 51073030) and the Program for New Century Excellent Talents in University of China (No. NCET-06-0280) for financial support of this research.

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CONCLUSIONS In summary, we design and synthesize a new tetraphenyladamantane-based microporous polyimide (PI-ADNT) through 17591



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Tetraphenyladamantane-Based Microporous Polyimide and Its Nitro

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