Highly Selective Separation of CO2, CH4, and C2âC4 Hydrocarbons

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Highly Selective Separation of CO2, CH4, and C2−C4 Hydrocarbons in Ultramicroporous Semicycloaliphatic Polyimides Jun Yan, Biao Zhang, 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

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S Supporting Information *

ABSTRACT: Ultramicroporous semicycloaliphatic polyimides with major pore sizes less than 0.5 nm are synthesized through imidization reaction between different aromatic tetraamines and cycloaliphatic dianhydrides. The synergistic role of abundant CO2-philic imide rings and the molecular sieving effect of ultrasmall pores in the polyimide network bring about high adsorption selectivity of CO2/CH4 (37.2) and CO2/N2 (136.7). In addition, it is interesting to observe that, under ambient condition (298 K/1 bar), n-butane exhibits the highest uptake (3.15 mmol/g) among the C1−C4 alkanes, and the adsorbed amount significantly drops with the reduction of the number of carbon atoms. As a result, the mixed light alkanes can be effectively separated according to the carbon numbers. The separation factors of n-butane/propane and propane/ethane reach 3.1 and 6.5, whereas those of n-butane, propane, and ethane over methane are as high as 414.5, 217.4, and 19.6, respectively. Moreover, the polyimides display large adsorption capacities for 1,3-butadiene (4.64 mmol/g) and propene (2.68 mmol/g) with good selectivity over 1-butene and propane of 3.2 and 3.0, respectively. Together with the excellent thermal and physicochemical stabilities, the ultramicroporous polyimides obtained in this work show promising applications in adsorption/separation for CO2, CH4, and C2−C4 hydrocarbons. KEYWORDS: polyimide, microporous organic polymer, CO2 capture, light hydrocarbon, adsorption, separation

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INTRODUCTION Microporous polyimides with large specific surface area, high porosity, and excellent physicochemical stability have received great attention in many applications such as hydrogen storage,1 drug delivery,2 catalysis,3 chemosensors,4 absorption of volatile organic compounds,5−8 wastewater treatment,9 and particularly in adsorption/separation of carbon dioxide (CO2)5−8,10−19 because the presence of abundant electron-rich N, O atoms leads to the strong affinity of polyimide skeleton for CO2 gas by virtue of dipole−quadrupole interaction. In the past few years, a number of hyper-cross-linked microporous polyimides have been synthesized from aromatic dianhydrides with multiamines through various synthetic strategies including one-step,1−12 two-step,20 copolymerization,8 and postmodification21 methods by means of solvent-thermal reaction,2 solution condensation polymerization,1,3−16 and solid-state polymerizations techniques.17 Despite the extensive studies on CO2 adsorption, for microporous polyimide materials, their adsorption/separation properties for light organic hydrocarbons still remain unexplored. Light organic hydrocarbons, including alkanes and alkenes with carbon number less than five, are petrochemical products. Among them propylene and 1,3-butadiene are raw materials for producing polypropylene plastic and polybutadiene rubber.22−25 However, propylene and 1,3-butadiene derived from petroleum cracking gases are mixed with propane © XXXX American Chemical Society

and 1-butene, respectively. High purity of propylene and 1,3butadiene is essentially important for achieving polypropylene and polybutadiene with controllable molecular weight, narrow molecular weight distribution, and high structural regularity. In addition, the main component of natural gas is CH4, but it also contains 5−20% CO2 and small amounts of light hydrocarbons when extracted from wells.26 CO2 needs to be removed to facilitate the sufficient burning of precombusted natural gas.27 Moreover, the thorough separation of organic hydrocarbons from the CH4/hydrocarbon mixtures is necessary to obtain high-purity CH4 raw materials.28 Currently, energy-consuming cryogenic distillation is still the major purification process for the purification of a specific alkane or alkene. As a promising alternative method, the physical adsorption/separation of organic hydrocarbons by porous adsorbents with large surface areas has been studied, including porous carbons,29 metal− organic frameworks (MOFs),30,31 and microporous organic polymers.32,33 Except CO2 capture, microporous polyimides have great potential advantage in the adsorption/separation of light hydrocarbons as well. Herein, four new ultramicroporous polyimides are synthesized from cycloaliphatic cyclobutane-1,2,3,4-tetracarboxylic Received: May 4, 2018 Accepted: July 13, 2018

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DOI: 10.1021/acsami.8b07294 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Synthesis Routes to the Four Ultramicroporous Polyimides

Synthesis of sPI−A−H. Under nitrogen flow, a 25 mL dry Schlenk flask with a stirrer and condenser was charged with TAPA (0.25 g, 0.5 mmol), CHDA (0.22 g, 1.0 mmol), m-cresol (4.7 mL), and isoquinoline (0.1 mL). Then, the temperature of polymerization was raised in the heating schedule: 180 °C for 5 h, 190 °C for 5 h, 200 °C for 10 h, and 220 °C for 10 h. Finally, the system was cooled down to room temperature, and the insoluble solid was isolated from the solvent by filtration and washed successively with dimethylformamide and tetrahydrofuran (THF) until the filtrate became colorless. The resulting product was extracted with THF in a Soxhlet apparatus for 24 h and dried in a vacuum oven at 120 °C for 48 h. Yield: 94%. Elemental analysis: Calcd for C54H44N4O8 (%): C, 73.96; H, 5.06; N, 6.39. Found: C, 63.69; H, 6.09; N, 5.39. Synthesis of sPI−M−H. The preparation procedure of sPI−M− H is similar to that of sPI−A−H except that the tetraamine monomer used is TAPM instead of TAPA. Yield: 95%. Synthesis of sPI−A−B. The preparation procedure of sPI−A−B is similar to that of sPI−A−H except that the dianhydride monomer used is CBDA instead of CHDA. Yield: 98%. Synthesis of sPI−M−B. The preparation procedure of sPI−M−B is similar to that of sPI−A−H except that the tetraamine and dianhydride monomers used are TAPM and CBDA instead of TAPA and CHDA, respectively. Yield: 95%. Instrumentation. Fourier transform infrared spectra (FTIR) of synthesized products were recorded using a Nicolet 20XB FTIR spectrophotometer in 400−4000 cm−1. Samples were prepared through dispersing the complexes in KBr to form disks. Solid-state 13 C cross-polarization/magic angle spinning (CP/MAS) spectra were measured 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. Fieldemission scanning electron microscopy (FE-SEM) experiments were performed on a Nova NanoSEM 450 scanning electron microscope. The samples were sputter-coated with gold. Wide-angle X-ray diffractions (WAXDs) from 5 to 60° were performed on a Rigaku D/max-2400 X-ray diffractometer (40 kV, 200 mA) with a copper target at a scanning rate of 2°/min. Thermogravimetric analysis

dianhydride (CBDA) and 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA) with two aromatic tetraamines, tetrakis(4-aminophenyl)methane (TAPM) and 1,3,5,7-tetrakis(4′aminophenyl)adamantane (TAPA), respectively, through one-step solution polycondensation. The relationships between the geometry configurations of cycloaliphatic and aromatic building blocks and porosity parameters as well as their effects on the adsorption/separation abilities of CO2/N2, CO2/CH4, ethane/CH4, propane/CH4, n-butane/CH4, propylene/propane, and 1,3-butadiene/1-butene were investigated in detail in terms of adsorption capacities, the Henry constants, first virial coefficients, critical temperatures, molecular sizes, and polarities of CO2, N2, and CH4 gases and C2−C4 light hydrocarbons.

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EXPERIMENTAL SECTION

Materials. 1,2,4,5-Cyclohexanetetracarboxylic dianhydride (CHDA), cyclobutane-1,2,3,4-tetracarboxylic dianhydride (CBDA), and isoquinoline were purchased from J&K Chemical Co., Ltd. and used without further purification. m-Cresol from Shanghai Chemical Reagent Co. was purified by distillation under reduced pressure prior to use. Tetrakis(4-aminophenyl)methane (TAPM) and 1,3,5,7tetrakis(4′-aminophenyl)adamantane (TAPA) were prepared according to the previous procedures described in our papers.5,15 Other reagents were of reagent grade and used as received. Preparation of Ultramicroporous Polyimides. The four semicycloaliphatic polyimides (sPIs) were prepared with equimolar amine and anhydride groups by setting the molar ratio of tetraamine TAPM or TAPA to the corresponding dianhydride as 1:2. For the sake of clarity, the four semicycloaliphatic polyimides synthesized in this study are named as sPI−A−H, sPI−M−H, sPI−A−B, and sPI− M−B, where A and M are referred to as adamantane and methane cores in the tetraphenyladamantane and tetraphenylmethane units, respectively, and H and B are the cyclohexane and cyclobutane components derived from the dianhydride monomers, respectively. The detailed synthetic procedures are described next. B


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ACS Applied Materials & Interfaces (TGA) curves were recorded on a NETZSCH TG 209 thermal analyzer by heating the samples (∼8 mg) up to 800 °C with a ramping rate of 10 °C/min under nitrogen flow. Sorption isotherms for N2, CO2, and C1−C4 hydrocarbons were measured on an Autosorb iQ2 (Quantachrome) analyzer. Prior to measurements, the samples were degassed at 120 °C under high vacuum overnight.

irregular shape and rather rough surface morphologies (Figure S3, Supporting Information). The X-ray diffraction patterns (Figure S4, Supporting Information) show wide peaks, indicative of the amorphous aggregation structure. However, relative to the cyclobutane-linked sPI−A−B and sPI−M−B, the diffraction peaks of sPI−A−H and sPI−M−H apparently become broad because the chair and boat conformations of cyclohexane linkages decrease the packing regularity of the polymer segments. In addition, to investigate the thermal stability of the porous polyimides, their TGA and derivative (DTG) curves as a function of temperature under nitrogen atmosphere were recorded (Figure S5, Supporting Information). The DTG curves show that the major decompositions of the four polymers occur at 541 °C, but sPI−A−B and sPI−M− B exhibit obvious two thermal steps of degradation. The stage at 541 °C is due to the decomposition of the polyimide skeleton, whereas that at 410 °C is associated with the cleavage of the cyclobutane moieties because of the great tension of the four-membered aliphatic ring, which is consistent with the previous observation in the linear polyimides with cyclobutane ring in the main chain. All of them show the decomposition temperature at around 410 °C, being attributed to the poor thermal stability of the cyclobutane structure.34−37 Porosity Analysis of Polyimides Using N2 and CO2 as Probes. The adsorption−desorption isotherms of N2 at 77 K were measured for the four polyimides (Figure 2a). sPI−M− H, sPI−M−B, and sPI−A−B display a rapid N2 uptake at low relative pressure (P/P0 < 0.01), reflecting the typical characteristic of micropores.38 Compared to the other three polymers, sPI−A−H only shows very small N2 uptake. Its enlarged isotherm (Figure 2b) shows that the N2 uptake in sPI−A−H continually increases with pressure, and there is an obvious step at the relative pressure (P/P0) of around 0.4, indicative of the existence of mesopores.38 On the basis of the Brunauer−Emmett−Teller (BET) model, the BET surface areas of sPI−A−H, sPI−M−H, sPI−M−B, and sPI−A−B are 25, 492, 620, and 618 m2/g, respectively (Table 1). Pore size distribution curves derived from the nonlocal density functional theory (NLDFT) reveal that the major pores in sPI−A− H are mesopores with a pore size centered at 2.31 nm, whereas those in sPI−M−H, sPI−M−B, and sPI−A−B are ultramicropores39 at 0.49 nm and small amounts of micropores at 1.01 nm (Figure 2c). It is noteworthy that the aliphatic rings in the semicycloaliphatic polyimide skeletons are softer and easily deformed. Particularly for sPI−A−H, the chair and boat conformations of cyclohexane linkages and tetraphenyladamantane net nodes in sPI−A−H make the porous structure more complicated than the other three polymers, as reflected by its apparently broader diffraction peaks in the X-ray diffraction pattern (Figure S3, Supporting Information). The pore channels might locally be too narrow to allow the accessibility of nitrogen molecule to the pore channels, exhibiting rather low N2 uptake. The N2 isotherm cannot actually reflect the porous characteristic in sPI−A−H because of the large kinetic diameter of N2 molecule. A similar N2 isotherm behavior has also been observed in the highly interpenetrated three-dimensional networks of covalent organic frameworks (COFs)40 and MOFs.41 Considering that CO2 (OCO) is a rodlike molecule and its kinetic diameter (3.30 Å) is smaller than that of N2 (3.64 Å),42 in this work CO2 is therefore more suitable as a molecular probe

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RESULTS AND DISCUSSION Synthesis and Characterization of Polyimides. As illustrated in Scheme 1, four semicycloaliphatic ultramicroporous polyimides (sPIs) were prepared by one-step polycondensation from TAPA and TAPM with CHDA and CBDA in m-cresol, respectively, using isoquinoline as a catalyst to facilitate the imidization reaction. FTIR spectra (Figure S1, Supporting Information) display the absorption from 2800 to 3000 cm−1 due to the C−H vibration of cycloaliphatic moiety in the polymer. The typical asymmetric and symmetric vibrations of the carbonyl in the imide ring appear at 1786 and 1720 cm−1, respectively.5−7 The band at 1372 cm−1 results from the overlapped peaks of the C−N−C stretching vibration of the imide ring. In addition, the FTIR spectra of the initial dianhydride and tetraamine monomers are presented in Figure S2 (Supporting Information). The comparison of sPIs and the two dianhydride monomers (CHDA and CBDA) shows that, after polymerization, the CO absorption of the anhydride group at 1861 cm−1 disappears, indicating that the anhydride and amino groups react completely under the polymerization condition. In the solid-state 13C CP/MAS NMR spectra (Figure 1), the quaternary carbons in sPI−M−H and sPI−M−

Figure 1. Solid-state 13C CP/MAS NMR spectra of sPs. Asterisks (*) indicate peaks arising from spinning side bands.

B locate at 64 ppm,5 the N-substituted phenyl carbons are at 147−150 ppm,5−7 whereas the other phenyl carbons are at 118−139 ppm. The carbonyl carbons of imide rings linked with cyclobutane and cyclohexane appear at 175 and 179 ppm, respectively.5−7 The peaks at 38−42 ppm are attributed to the cycloaliphatic carbons adjacent to the imide ring, which are overlapped with the carbons of adamantane in sPI−A−H and sPI−A−B. The signals of the other C−H and CH2 in cyclohexane of sPI−M−H and sPI−A−H are found at 22 ppm. The obtained products are insoluble in any common organic solvents such as tetrahydrofuran, dimethyl sulfoxide, N,Ndimethylformamide, N,N-dimethylacetamide, and N-methylpyrrolidone, suggesting that they have a hyper-cross-linked structure as expected. The FE-SEM images of the four polymers are composed of loosely packed particles with C


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ACS Applied Materials & Interfaces

Figure 2. (a) Adsorption (filled) and desorption (empty) isotherms of N2 for sPI−A−H, sPI−M−H, sPI−A−B (+100), and sPI−M−B (+200). (b) Enlarged sorption isotherms for sPI−A−H. (c) Pore size distributions for sPIs calculated by the NLDFT method from the adsorption isotherms of N2 at 77 K.

similar to the other three samples. Moreover, no apparent adsorption−desorption hysteresis is observed, implying that CO2 can be reversibly adsorbed and deadsorbed in the porous polyimides. In contrast to the porosity parameters derived from the sorption isotherms of N2 gas, the analysis of CO2 adsorption isotherms indicates that the four semicycloaliphatic polyimides possess large amounts of ultrasmall pores at 0.35 and 0.48 nm (Figure 3c), belonging to ultramicroporous materials.39,43,44 Moreover, these two peaks in the cyclohexanelinked sPI−A−H and sPI−M−H are much stronger compared to those in the cyclohexane-linked sPI−A−B and sPI−M−B. Correspondingly, the obtained specific surface areas for sPI− A−H (665 m2/g) and sPI−M−H (715 m2/g) are larger than those for sPI−A−B (574 m2/g) and sPI−M−B (595 m2/g). The total pore volumes of sPI−A−H and sPI−M−H also surpass those of sPI−A−B and sPI−M−B. CO2 Adsorption and Selectivity of CO2/N2 and CO2/ CH4. As shown in Table 2, the CO2 uptake of sPI−M−H at 273 K and 1 bar reaches 3.01 mmol/g (13.4 wt %), being superior to many other porous polymers even though they have a larger surface area such as COF-103 (7.6 wt %, 3530 m2/g),45 CMP-0 (9.2 wt %, 1018 m2/g),46 and microporous network A (5.0 wt %, 4077 m2/g).47 More importantly, at 298 K and 1 bar, the CO2 uptake in sPI−M−H is still up to 2.10

Table 1. Porosity Parameters Derived from N2 and CO2 Adsorption Isotherms N2 (77 K)

CO2 (273 K)

sample

SN2a (m2/g)

Smicrob 2

(m /g)

Vmicroc 3

(cm /g)

Vtot,N2d (cm3/g)

SCO2e (m2/g)

Vtot,CO2f (cm3/g)

sPI−A−H sPI−M−H sPI−A−B sPI−M−B

25 492 620 618

0 413 392 393

0 0.181 0.180 0.175

0.021 0.275 0.467 0.470

665 715 574 595

0.195 0.203 0.177 0.170

a

BET surface area derived from N 2 adsorption isotherms. Microporous surface area calculated using the t-plot method from N2 adsorption isotherms. cMicroporous volume calculated using the tplot method from N2 adsorption isotherms. dTotal pore volume derived from N2 adsorption isotherms at P/P0 = 0.90. eSpecific surface area calculated from CO2 adsorption isotherms at 273 K by the NLDFT method. fTotal pore volume calculated from CO2 adsorption isotherms at 273 K by the NLDFT method. b

to detect the ultrasmall porous information in these semicycloaliphatic polyimides. The sorption isotherms of CO2 gas for the four polymers measured at 273 and 298 K are illustrated in Figure 3a,b. Despite the very small BET surface area detected by N2 molecule, sPI−A−H also displays quite high CO2 uptake

Figure 3. (a, b) Adsorption (filled) and desorption (empty) isotherms of CO2 for sPIs. (c) Pore size distribution curves derived from CO2 adsorption isotherm at 273 K using the NLDFT method. D


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ACS Applied Materials & Interfaces Table 2. Uptakes of CO2 and Its Selectivity Over N2 and CH4 in sPIs CO2 uptakea sample sPI−A−H sPI−M−H sPI−A−B sPI−M−B

273 K 2.82 3.01 2.43 2.61

298 K 1.92 2.10 1.51 1.73

selectivityb CO2/N2 65.7 105.0 68.4 136.7

CO2/CH4 21.0 26.0 18.4 27.5

(22.3) (35.5) (21.3) (37.2)

Q0c

KHd

kJ/mol

mol/(g Pa)

31.7 34.0 32.5 34.3

5.904 8.556 7.197 8.826

× × × ×

A0e ln(mol/(g Pa))

−8

−16.645 −16.274 −16.447 −16.243

10 10−8 10−8 10−8

a CO2 uptake in mmol/g. bCalculated from the ideal adsorbed solution theory (IAST) method for a gas mixture of CO2/N2 with a molar ratio of 0.15:0.85 and gas mixtures for CO2/CH4 with molar ratios of 0.05:0.95 (0.5:0.5) at 273 K at 1 bar. cLimiting enthalpy of adsorption. dThe Henry constant. eFirst virial coefficient.

Figure 4. (a) Variation of CO2 isosteric enthalpies with the adsorbed amount. (b) Virial plots of CO2 for sPIs.

incorporation of aliphatic rings in the polyimide segments disrupts the conjugation between imide and aromatic benzenes and thus effectively enhances the polarity of carbonyl and the electron density of nitrogen atoms in imide rings, which is advantageous for the affinity of the polymer skeleton toward CO2 molecule and (2) the existence of large amounts of ultrasmall pores at 0.35 and 0.48 nm in sPIs also plays an important positive role on the selective adsorption of CO2 over N2 and CH4 because of the molecular sieving effect. In an effort to gain more insight into the adsorption of CO2 in the ultramicroporous sPIs, their isosteric enthalpies of adsorption (Qst) were calculated from the adsorption isotherms measured at different temperatures by the

mmol/g (9.1 wt %), which surpasses most aromatic microporous polymers such as triazine-based microporous polyimide TPIs (1.89−5.50 wt %),14 sulfonated microporous polyimide SMPIs (6.29−8.22 wt %),8 nanoporous organic framework NPOF-NH2 (8.28 wt %),48 tri(4-ethynylphenyl)amine-based microporous polymers (3.30−6.11 wt %),49 thiazolothiazolelinked porous organic polymer TzTz-POPs (5.72−6.60 wt %),50 and azo-linked porous organic framework azo-POFs (4.02−8.29 wt %).51 The separation properties of the binary gas mixtures of CO2/N2 and CO2/CH4 in sPIs are assessed using the ideal adsorbed solution theory (IAST) according to the adsorption isotherms of CO2, CH4, and N2 at 273 K (Figure S4, Supporting Information). The gas composition of CO2/N2 is set as 0.15:0.85 to specify the flue gas, whereas the compositions of CO2/CH4 are 0.05:0.95 and 0.50:0.50 to specify the natural gas and landfill gas, respectively. The IAST selectivity curves of CO2/N2 and CO2/CH4 as a function of feed pressure are shown in Figure S5 (Supporting Information). The data in Table 2 show that the CO2/N2 selectivity in sPI−M−B is as high as 136.7, exceeding all of the previously reported fully aromatic microporous polyimides, including MPI-1 (102),5 TPIs (31−56),14 PI-NO2s (33− 56),21 SMPIs (30−58),8 MPI-6FA (50),6 and most other microporous polymers. Particularly, the CO2/CH4 selectivity of sPI−M−B reaches 37.2 for landfill gas and 27.5 for natural gas, which are among the highest values for the microporous polymers reported to date. The outstanding separation abilities of CO2/N2 and CO2/ CH4 in sPIs are mainly attributed to two reasons: (1) the

Q

Clausius−Clapeyron equation: ln P = RTst + C , where R is the gas constant, C is a constant, and P and T are the gas pressure and temperature at the equilibrium state, respectively. Figure 4a shows that for each sample, the Qst values exhibit a decreasing trend with the increase of CO2 adsorption, meaning that the interaction between CO2 and the pore wall is stronger than that between CO2 molecules. Furthermore, the virial plots at 273 and 298 K were plotted according to the adsorption data in the low-pressure region. All of the virial plots for the four samples at both temperatures follow a good linear relationship (Figure 4b). The intercept of the line is the first virial coefficient (A0), which reflects the gas−polymer interaction. The Henry constant (KH) can be calculated by the equation KH = exp(A0). On the basis of the KH values at different temperatures, the limiting enthalpies of adsorption (Q0) at zero surface gas coverage were derived from the slope E


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Figure 5. Adsorption isotherms of C1−C4 alkanes in sPIs at 298 K.

Table 3. Adsorption Capacities of C1−C4 Hydrocarbons in sPIsa sample

CH4

C2H6

C3H8

C3H6

n-C4H10

1-C4H8

1,3-C4H6


0.39 0.38 0.35 0.31

1.59 1.49 1.33 1.24

2.00 1.92 1.83 1.73

2.68 2.50 2.29 2.31

3.15 3.07 2.50 2.16

3.54 3.50 2.67 2.32

4.64 4.79 3.64 3.39

a

Uptake (mmol/g) at 298 K and 1 bar.

of the plot of ln KH versus 1/T. It is seen from Table 2 that the ranking order of Q0 in the four polymers is consistent with those of A0 and KH. The Q0 values of CO2 are in the range of 31.7−34.3 kJ/mol, which surpass many other nitrogen-rich microporous polymers such as HCPs (20−24 kJ/mol),52,53 BILPs (26.5−28.8 kJ/mol),54 and BLPs (20.2−28.3 kJ/mol),55 indicative of the strong affinity of sPIs for CO2 gas. Additionally, the incorporation of the bulky aliphatic adamantane moieties in sPI−A−H and sPI−A−B decreases the relative concentration of imide rings in the network. Consequently, compared to sPI−A−H and sPI−A−B, sPI− M−H and sPI−M−B possess considerably higher Q0 values of CO2 and adsorption selectivity for either CO2/N2 or CO2/ CH4 gas pair. Adsorption and Separation of C1−C4 Alkanes. The adsorption isotherms of C1−C4 alkanes including methane (CH4), ethane (C2H6), propane (C3H8), and n-butane (nC4H10) at 298 K are presented in Figure 5, and the adsorption data are summarized in Table 3. The cyclohexane-linked sPI− A−H and sPI−M−H exhibit a higher uptake for each alkane investigated than the cyclohexane-linked sPI−A−B and sPI− M−B because PI−A−H and sPI−M−H have larger specific

surface areas and total pore volumes as examined by physical sorption using CO2 as probe. Under ambient condition (298 K/1 bar), sPI−A−H can uptake 2.00 mmol/g C3H8, which surpasses the conjugated microporous polymers P5-CMPs (0.177−1.12 mmol/g)56 and copper(catecholate)-decorated porous organic polymer CuA10B1 (0.93 mmol/g).57 Moreover, the adsorption capacity of n-C4H10 in sPI−A−H reaches 3.15 mmol/g, which is superior to the carbon molecular sieve derived from the cyclodextrin metal−organic framework CMSPMOF-1 (1.9 mmol/g).58 In addition, it is interesting to note that the alkane with more carbon number exhibits a larger adsorption capacity. At 298 K and 1 bar, the adsorbed amount of n-C4H10 in each of the four polymers is the largest, followed by C3H8, C2H6, and CH4, which, as shown in Figure 6, is consistent with the ranking order of their critical temperatures (Tc): Tc(n‑C4H10) (425.1 K) > Tc(C3H8) (369.8 K) > Tc(C2H6) (305.4 K) > Tc(CH4) (190.6 K).59 Among the four alkanes, because n-C4H10 has the highest critical temperature, the n-C4H10 molecule can be easily condensed and adsorbed on the porous surface, and therefore it exhibits the largest adsorption capacity. Moreover, the data in Table S1 (Supporting Information) further confirm F


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that the polar imide groups in the polymer skeleton preferentially adsorb C3H6 rather than C3H8 molecule. This conclusion is further supported by the much higher Q0, A0, and KH values of C3H6 compared to those of C3H8 for each polymer sample (Table S2, Supporting Information). Similar phenomenon was also observed for the comparison of the 1,3C4H6/1-C4H8 gas pair. 1,3-C4H6 has one more CC double bond than 1-C4H8, and the two CC double bonds in 1,3C4H6 are conjugated. Therefore, relative to 1-C4H8, 1,3-C4H6 has a stronger dipole−quadrupole interaction with the polar imide rings and π−π interaction with benzene rings in microporous polyimides, agreeing with the fact that 1,3-C4H6 has a significantly higher enthalpy of adsorption than 1-C4H8 for all of the four polyimides. At 298 K and 1 bar, the uptake of C3H6 in sPI−A−H reaches 2.68 mmol/g, exceeding that of the other porous materials such as three-dimensional microporous MCOF-1 (2.45 mmol/g)33 and metal−organic framework ELM-12 (1.79 mmol/g).63 In addition, the adsorbed amount of 1,3-C4H6 in sPI−M−H is as high as 4.79 mmol/g, which is superior to the ultramicroporous SIFSIX-2-Cu-I (4.02 mmol/g).24 For sPI− A−H, the separation factors obtained by the IAST method for the binary mixed gases of C3H6/C3H8 and 1,3-C4H6//1-C4H8 are 3.0 and 3.2, respectively (Table 4), exhibiting good adsorption selectivity of propene over propane and 1,3butadiene over 1-butane.

Figure 6. Dependencies of uptakes of C1−C4 alkanes on their critical temperatures in sPIs.

that the alkane with more carbon number has a larger enthalpy of adsorption (Q0), first virial coefficient (A0), and the Henry constant (KH), indicating the stronger interaction with the polyimide skeleton. Table 4 shows that the C2−C4 alkanes can be effectively stripped from natural gas using sPIs as porous adsorbents. For example, using the IAST method, the selectivity at 298 K and 1 bar for the binary mixed gases of C2H6/CH4 is 19.0, exceeding that for microporous metal−organic frameworks UTSA-35a (ca. 12)60 and Cu−TDPAT (ca. 18),61 whereas the value of C3H8/CH4 (217.4) is much higher than that of UTSA-35a (ca. 80)60 and UPC-21 (67).62 The selectivity of n-C4H10/CH4 is as high as 414.5, exhibiting excellent separation property. Additionally, the selectivities of n-C4H10/C3H8 and C3H8/ C2H6 in sPI−M−H are 3.1 and 6.5, respectively, indicating that the mixtures of light alkanes can be separated by ultramicroporous sPIs according to their carbon number. Adsorption/Separation of C2−C4 Alkane/Alkene Mixed Gases. Considering the importance of high-purity propylene (C3H6) and 1,3-butadiene (1,3-C4H6) in the plastic and rubber industries, C3H6 and 1,3-C4H6 were selected as representative alkenes to study their adsorption/separation by the ultramicroporous sPIs from the gas pairs of C3H6/C3H8 and 1,3-C4H6/1-butene (1-C4H8). The adsorption isotherms of C3H6, 1,3-C4H6, and 1-C4H8 in sPIs at 298 K are presented in Figure 7. It is seen from Table 3 that for each of the four polymers, the adsorption capacity of C3H6 is significantly higher than that of C3H8 although the critical temperature of C3H6 (364.2 K) is very close to that of C3H8 (369.8 K). The reason can be attributed to the presence of CC double bond in C3H6, which results in a considerably higher dipole moment (0.366 × 1018 esu cm) than C3H8 (0.084 × 1018 esu cm)59 so

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CONCLUSIONS Four semicycloaliphatic hyper-cross-linked polyimides have been successfully synthesized from tetrakis(4-aminophenyl)methane and 1,3,5,7-tetrakis(4′-aminophenyl)adamantane with aliphatic cyclobutane-1,2,3,4-tetracarboxylic dianhydride and 1,2,4,5-cyclohexanetetracarboxylic dianhydride through solution condensation polymerization, respectively, using mcresol as solvent and isoquinoline as catalyst. Their chemical structures are confirmed by FTIR, solid-state 13C CP/MAS spectroscopy, and elemental analysis. The analyses of the adsorption isotherms of CO2 at 273 K show that the obtained polyimides have moderately large specific surface areas of 574−715 cm2/g and major pore sizes at 0.35 and 0.48 nm, belonging to ultramicroporous materials. The incorporation of aliphatic rings in the polyimide segments disrupts the conjugation between imide and aromatic benzenes and thus effectively enhances the polarities of carbonyl and electron densities of nitrogen atoms in the imide rings. As a result, the strong affinity of the polyimide skeleton toward CO2 gas and the ultramicroporous structure bring about high CO2 uptake up to 13.4 wt % (273 K/1 bar) with remarkably high selectivities of CO2/CH4 (37.2) and CO2/N2 (136.7). Moreover, the results show that the C1−C4 hydrocarbons with the more carbon number and CC double bond exhibit significantly larger separation ability for alkane/alkene and different hydrocarbons according to the carbon number. At

Table 4. Selectivity of Alkanes Over Methane and Alkanes Over Alkenes in sPIsa sample

C2H6/CH4

C3H8/CH4

n-C4H10/CH4

n-C4H10/C3H8

C3H8 /C2H6

C3H6 /C3H8

1,3-C4H6 /1-C4H8


13.3 13.9 19.6 19.0

66.4 82.0 204.7 217.4

154.8 244.7 356.7 414.5

2.3 3.1 1.7 1.1

4.7 6.5 5.9 5.5

3.0 2.4 1.8 2.1

3.2 2.6 2.1 2.0

a

Calculated by the IAST method at 298 K and 1 bar. G


Research Article


Figure 7. Adsorption isotherms of C3H6, 1-C4H8, and 1,3-C4H6 in sPIs at 298 K.

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ACKNOWLEDGMENTS The financial supports from the National Science Foundation of China (Nos 51473026 and U1462125) and the Fundamental Research Funds for the Central Universities (DUT18GF107) are acknowledged.

298 K and 1 bar, the resultant ultramicroporous polyimides can uptake 4.64, 3.15, 2.68, and 1.59 mmol/g 1,3-C4H6, nC4H10, C3H6, and C2H6, respectively, whereas its separation factors for the binary mixtures of 1,3-C4H6/1-C4H8, C3H6/ C3H8, C3H8/C2H6, C3H8/CH4, and C2H6/CH4 3.2, 3.0, 6.5, 19.6, 217.4, and 414.5, respectively. The relationships between structure and adsorption/separation properties of CO2 and C1−C4 hydrocarbons for the polyimides are explained in terms of the porosity parameters like specific surface area, pore size, enthalpy of adsorption, first virial coefficient, and the Henry constant as well as the polarity and molecular sizes of the adsorbates.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b07294. Q0, KH, and A0 values of C1−C4 hydrocarbons in the porous polyimides; FTIR spectra; FE-SEM images; TGA curves; WAXD patterns; adsorption isotherms of CH4, N2, and hydrocarbons at 273 K; virial plots of hydrocarbons; selectivities for binary mixtures of alkanes and alkenes calculated by the IAST method (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhonggang Wang: 0000-0003-0451-1919 Notes

The authors declare no competing financial interest. H


Research Article

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