Highly Selective Adsorption for Ethylene, Propylene, and Carbon

Dec 31, 2018 - This paper presents the sulfonation and silver-ionization modifications using aromatic microporous polyimide (MPI) as precursor to obta...
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Highly Selective Adsorption for Ethylene, Propylene, and Carbon Dioxide in Silver-Ionized Microporous Polyimide Jun Yan, Biao Zhang, Lingxiao Guo, and Zhonggang Wang* 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: This paper presents the sulfonation and silverionization modifications using aromatic microporous polyimide (MPI) as precursor to obtain a new Ag+-decorated porous polyimide material (MPI-Ag). FTIR, XPS, and EDX spectra demonstrate the chemical structure of the resultant MPI-Ag and the homogeneously distribution of silver ions within the porous network. The evolution of porosity parameters and elemental compositions of the postmodified polymers and their effects on adsorption/separations of C1−C3 hydrocarbons and CO2 gas are studied in detail. The results show that the modified MPI-Ag exhibits the enhanced affinity for CO2 and the unsaturated hydrocarbons like C2H4 and C3H6 as evidenced by the significantly increased enthalpies of adsorption, Henry’s constants and first virial coefficients. Consequently, at 273 K, the adsorption selectivities of C2H4/C2H6 (0.9) and C3H6/C3H8 (2.0) for MPI dramatically rise to 3.5 and 16.5 for MPI-Ag, respectively. Moreover, the selectivities of C3H6/CH4 (3785) and CO2/CH4 (65.3) are achieved for MPI-Ag, which are among the highest values reported to date for porous materials. The excellent selective adsorptions for unsaturated alkenes and CO2 gas endow MPI-Ag with potential applications in stripping CO2 from natural gas and landfill gas as well as the purification of C2H4 and C3H6 from their mixtures with CH4, C2H6, and C3H6 impurities.

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bons,16 zeolites,25,26 and porous organic polymers.11,12,27−30 Chen et al. reported a metal−organic framework material UTSA-35, which exhibits the selectivities of C3 and C2 hydrocarbons with respect to CH4 in excess of 80 and 8, respectively.17 The porous aromatic framework and hafnium metal−organic framework synthesized by Ma28 and Zhao29 show both high C2H4 uptake and selectivity of C2H4/C2H6. A porous organic polymer with copper (catecholate) groups prepared by Farha et al. exhibits the uptake of C3H6 (1.65 mmol g−1) and the selectivity of C3H6/C3H8 (10.4).11 Microporous polyimides are newly emerged a class of porous materials featuring with large specific area and excellent thermal/chemical stability. In the past several years, a variety of microporous polyimides were successfully prepared via the condensation between various multiamines and dianhydrides. Their potential applications in gas storage,31−36 heterogeneous catalysis,37 chemosensors,38 and wastewater treatment39 have been extensively studied. However, the adsorption/separation of light hydrocarbons by microporous polyimides is almost entirely unexplored with the exception of the recent study by our group about the selective separation of hydrocarbons in ultramicroporous semicycloaliphatic polyimides.40

INTRODUCTION Light hydrocarbons derived from petroleum cracking gas or natural gas are important energy resources and chemical raw materials. The main component of natural gas extracted from gas well is methane (CH4), but it contains a certain amount of ethane (C2H6), propane (C3H8), and 5−20% carbon dioxide (CO2), whereas the CO2 content in landfill gas is even up to 40−60%.1−10 On the other hand, the petroleum cracking gas is composed of CH4, C2H6, C3H8, ethylene (C2H4), propylene (C3H6), and butadiene, etc.11,12 Among them, C2H4 and C3H6 are monomers for producing plastics and rubbers.11−13 Nevertheless, the separation and purification of C2H4 and C3H6 from the corresponding alkanes by conventional cryogenic distillation process are rather difficult and energyconsuming. As an alternative method, the separation of light hydrocarbons by physical adsorption employing porous adsorbents with large specific surface area have received great attention. Compared to the conventional cryogenic fractional distillation process, the operation of physical adsorption is convenient and energy-saving.14−16 Moreover, the adsorbed hydrocarbons are easily released and recovered under the reduced pressure and the porous adsorbent can be repeatedly used. In recent years, numerous porous materials with fascinating separation properties for C2H4/C2H6 and C3H6/C3H8 as well as the selective adsorption of C2−C3 hydrocarbons over CH4 have been achieved including metal−organic frameworks,7,17−24 car© 2018 American Chemical Society

Received: October 22, 2018 Revised: December 15, 2018 Published: December 31, 2018 575

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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The Journal of Physical Chemistry C Scheme 1. Synthesis Route to the Silver-Ionized Porous Polyimide (MPI-Ag)

h, 200 °C for 10 h, and 220 °C for 10 h. Finally, the system was cooled to room temperature, and the insoluble solid was isolated by filtration and washed successively with DMF and THF until the filtrate became colorless. The resulting product was extracted with THF in a Soxhlet apparatus for 24 h, and dried in vacuum oven at 120 °C for 48 h. Synthesis of MPI-S. MPI (0.5 g) and anhydrous CH2Cl2 (30 mL) were added to a 100 mL dry three-neck flask with a magnetic stir-bar. Under nitrogen flow, the mixture was cooled to 0 °C and stirred for 30 min. A mixed solution of chlorosulfonic acid (2.0 mL) and anhydrous CH2Cl2 (15 mL) was added slowly to the system and stirred at 0 °C for 2 h and then allowed to warm to room temperature with stirring for 12 h. The mixture was poured into 100 mL ice−water mixture and stirred for 30 min. Then, the insoluble solid was collected by filtration and washed successively with deionized H2O, CH2Cl2, DMF, and THF until the filtrate became colorless. The product was dried in vacuum oven at 120 °C for 48 h. Synthesis of MPI-Ag. At room temperature, MPI-S (0.20 g) and AgNO3 (0.20 g) were added into 40.0 mL of mixed solvent with an equivalent volume of acetonitrile and deionized water. The system was stirred for 12 h at room temperature. Then the isolated solid was successively washed with deionized water and THF. The resulting solid was dried in a vacuum oven at 120 °C for 48 h. Methods. Fourier transform infrared spectra (FTIR) of the synthesized products were recorded using a Nicolet 20XB FTIR spectrophotometer in the 400−4000 cm−1 region. Solidstate 13C CP/MAS (cross-polarization with magic angle spinning) 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

Inspired by the urgent industrial demands for the adsorption/separation of petroleum cracking gas and natural gas, in this work, silver-ionization modification based on aromatic microporous polyimide was undertaken in an effort to develop separation materials for highly selective adsorption of C2H4, C3H6 and CO2 gases. The influence of postmodifications on specific surface area, pore size, and pore size distribution were investigated with the emphasis on the adsorption/separations for various binary mixed gases of light hydrocarbons and carbon dioxide including C2H4/ C2H6, C3H6/C3H8, C2H4/CH4, C2H6/CH4, C3H6/CH4, C3H8/CH4, and CO2/CH4.

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EXPERIMENTAL SECTION Materials. Pyromellitic dianhydride (PMDA), m-cresol, isoquinoline, chlorosulfonic acid, and dichloromethane were purchased from Shanghai Chemical Reagent Co. m-Cresol was purified by distillation under reduced pressure. Dichloromethane was purified by refluxing over phosphorus pentoxide and distilled prior to use. Other chemical reagents were of reagent grade and used as received. Tetrakis(4-aminophenyl)methane (TAPM) was prepared according to the procedure described in our previous paper.41 Synthesis of MPI. The synthesis of MPI is similar to that of the previously reported MPI-1,41 but their temperatureraising programs for polymerization reactions are different. Specifically, under nitrogen flow, a 50 mL dry Schlenk flask with a stirrer and condenser were charged with TAPM (0.42 g, 1.1 mmol), PMDA (0.48 g, 2.2 mmol), m-cresol (18 mL), and isoquinoline (0.1 mL). Then the temperature was rapidly raised to 180 °C and reacted at this temperature for 5 h. Then the system was heated and continually reacted at 190 °C for 5 576

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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The Journal of Physical Chemistry C

Figure 1. High- resolution XPS spectra of C1s, S2p, and Ag3d for MPIs.

prepared through one-pot polycondensation in m-cresol from tetraamine TAPM and dianhydride PMDA, which was then sulfonated in anhydrous CH2Cl2 with chlorosulfonic acid to produce SO3H-containing porous polyimide (MPI-S). The subsequent ion exchange of hydrogen of SO3H with silver leads to the target Ag+-decorated porous polyimide (MPI-Ag). In the previous report, ultramicroporous MPI-141 has been synthesized in our group using the same monomers as MPI, but the polymerization of MPI in this work was carried out under a different condition, especially at the initial polymerization stage. For the synthesis of MPI-1, the monomers TAPM and PMDA first reacted at the ice-bath temperature for 2 h and at 30 °C for 8 h. The initial low-temperaturepolymerization for a long time led to the serious interpenetration of networks so that the resultant MPI-1 exhibited ultrasmall pores (0.59 nm).41 As a contrast, herein, after the monomers and solvents were charged in the reaction flask, the temperature was rapidly raised to 180 °C to facilitate the fast cross-linking at the initial stage. This process improvement is anticipated to effectively inhibit the interpenetration of networks and generate larger pores. The suitably broad pore channels are easier for the accessibility of sulfonating and silver ionic agents to the reaction sites on the polymer skeleton, and therefore is advantageous for the postmodification reactions of MPI. The chemical structures of MPI, sulfonated MPI-S and silver-ionized MPI-Ag were characterized by means of FT-IR, solid-state 13C CP/MAS NMR, elemental analyses, ICP, EDS and XPS methods. The FTIR spectra (Figure S1, Supporting Information) of the three polymers show characteristic absorptions of imide linkage at 1778, 1720, and 1371 cm−1 due to the asymmetric and symmetric vibrations of the carbonyl groups and the C−N−C stretching vibration in imide ring, respectively.32−34 After modifications, two new peaks at 1076 and 1185 cm−1 appear for MPI-S and MPI-Ag (Figure S2, Supporting Information), which are attributed to the characteristic absorption of S−O bond.35,42 In solid-state 13C CP/MAS NMR spectra (Figure S3, Supporting Information), the resonance of carbonyl carbon in imide ring appears at 165 ppm, the N-substituted phenyl carbon at 146 ppm, while the overlapping signals from 105 to 137 ppm belong to the other aromatic carbons in backbone. Moreover, the 13C CP/MAS

4.0 ms and a relaxation delay of 2.0 s. X-ray photoelectron spectroscopy measurements were performed in both survey and high-resolution mode on a Thermo ESCALAB 250, with a Monochromatic Al KR (hν) X-ray light source of 150 W at 15 kV. Elemental analyses were determined with an Elementar Vario EL III elemental analyzer. The measurements of energydispersive X-ray spectroscopy (EDX) were performed on the FEI NOVA NanoSEM450. Inductively coupled plasma emission spectrometer (ICP) analysis was carried out on an iCAP 6000 Radial. Sorption measurements for all the gases were operated on an Autosorb iQ2 analyzer (Quantachrome). Prior to measurement, the samples were degassed at 120 °C under high vacuum overnight. The adsorption−desorption isotherms of N2 were measured at 77 K. The adsorption isotherms of CO2, CH4, C2H4, C2H6, C3H6, and C3H8 were measured at 273 and 298 K up to 1 bar.

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RESULTS AND DISCUSSION Synthesis and Characterization of Ag+-Decorated Porous Polyimide. The synthetic routes of microporous polyimide network (MPI) and its sulfonation and silverionization modifications are illustrated in Scheme 1. MPI was

Figure 2. (a) Adsorption (solid) and desorption (empty) isotherms of N2 for MPIs at 77 K. (b) Pore size distributions for MPIs calculated by NLDFT method. 577

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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The Journal of Physical Chemistry C

Figure 3. Adsorption isotherms of CH4, C2H4, C2H6, C3H6, and C3H8 for MPIs at 273 and 298 K.

are assigned to the oxygen, nitrogen, silver, carbon and sulfur elements, respectively.27,28 After sulfonation and silverionization modifications, it is rationally to see in Table S1 that the contents of carbon and nitrogen derived from XPS spectra decrease from MPI to MPI-S and MPI-Ag. Besides, the chemical compositions obtained by elemental analysis (EA) are roughly consistent with those by XPS method, but S and Ag contents in MPI-S and MPI-Ag measured from EA and ICP are apparently higher than those calculated from XPS spectrum, indicating that sulfonation and silver-ionization have occurred on the porous surface within the network. In addition, the high-resolution XPS spectra of C1s, S2p and Ag3d are illustrated in Figure 1. The peak at 288.7 eV is attributed to carbonyl carbon of imide group, whereas that at 285.7 eV is assigned to the overlapped signals of C−N and C− S bonds. For MPI-S, the S2p spectrum displays doublet peaks at 167.4 and 168.6 eV, corresponding to SO and S−O bonds in SO3H group, respectively. After the exchange of hydrogen with silver, the above two peaks in MPI-Ag shift to the slightly higher binding energies due to the electron-withdrawing effect of sliver ion. Besides this, the signals of Ag3d5/2 and Ag3d3/2 in MPI-Ag are observed at 368.8 and 374.8 eV, respectively. In the EDX spectra (Figure S5, Supporting Information), the

Figure 4. IAST selectivity for (a) C2H4/C2H6 and (b) C3H6/C3H8 in MPIs at 273 K (solid) and 298 K (empty).

NMR spectra reveal that the sulfonation reaction of MPI mainly occurs on the carbon adjacent to the C−N bond since its intensity of the signal at 121 ppm for either MPI-S or MPIAg apparently decreases in comparison with that of MPI. In the XPS spectra (Figure S4, Supporting Information), for MPI-Ag, the peaks at 532.3, 401.2, 372.9, 285.5, and 168.2 eV

Table 1. Selectivity of C2H4/CH4, C2H6/CH4, C3H6/CH4, C3H8/CH4, C2H4/C2H6, and C3H6/C3H8 in MPI, MPI-S and MPIAg C2H4/C2H6

C3H6/C3H8

C2H4/CH4

C2H6/CH4

C3H6/CH4

C3H8/CH4

sample

273 K

298 K

273 K

298 K

273 K

298 K

273 K

298 K

273 K

298 K

273 K

298 K

MPI MPI-S MPI-Ag

0.9 1.5 3.5

0.9 1.4 3.1

2.0 4.6 16.3

1.6 3.8 7.2

27.9 24.8 83.1

18.1 23.9 51.6

33.4 17.8 22.8

20.9 17.3 17.7

1946 484 3785

561 367 586

663 102 123

284 93 94

a

Selectivity was calculated by the IAST method at equimolar C2H4/CH4, C2H6/CH4, C3H6/CH4, C3H8/CH4, C2H4/C2H6, and C3H6/C3H8. 578

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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Figure 5. IAST selectivity for (a) C2H4/CH4, (b) C2H6/CH4, (c) C3H6/CH4, and (d) C3H8/CH4 in MPIs at 273 K (solid) and 298 K (empty).

and pore volumes of the postmodified porous polymers in comparison with the precursor sample, has been frequently observed in many other porous polymers27,45−49 possibly because the partial cavity space in porous polymer has been occupied by the postincorporated groups. The pore sizes and pore size distribution of MPI, MPI-S, and MPI-Ag were analyzed by means of nonlocal density functional theory (NLDFT) method. Different from the polymerization of MPI-1 at the lower temperature (0 °C/2h, 30 °C/8 h, 80 °C/4 h),41 in this work, for the preparation of MPI, after the monomers and solvents were charged in the reaction flask, the temperature was rapidly raised to a high temperature (180 °C) to facilitate the fast cross-linking. The results show that although the starting monomers of MPI and MPI-1 are the same, they display significantly different pore sizes. As a matter of fact, the polymerization of MPI and MPI-1 proceeded under kinetic control, so the irreversible interpenetration of networks is inevitable. The starting monomers are pyromellitic dianhydride (A 2 ) and tetrakis(4aminophenyl)methane (B4). The solution polymerization between A2 and B4 monomers initially generate branched oligomers, which are then cross-linked and finally form the hyper-cross-linked network. The interpenetration majorly occurs in the branching stage since the branched oligomers are soluble in the solution and the segments are more flexible. The above two factors are advantageous for the mutual crossreaction of segments and interpenetration. However, once the irreversible cross-linking reaction has happened, the interpenetration becomes difficult since the segments have been firmly fixed on the network through covalent bonds. Therefore, in this work, rapidly heating the polymerization system to a

peaks at 0.27 and 2.31 keV for MPI-S are attributed to the carbon and sulfur signals, respectively, while the newly emerged peak at 2.99 keV for MPI-Ag belongs to the Ag element. Moreover, the EDX mapping images (Figure S6, Supporting Information) display that C, S, and Ag homogeneously distribute in the MPI-Ag network. Porosity Analysis. The nitrogen adsorption−desorption isotherms of N2 at 77 K for the three polymers are presented in Figure 2a. For MPI sample. the isotherm curve shows a hysteresis effect with the desorption curve lying above the adsorption one to form an unclosed loop. Similar phenomena have also been observed for other microporous organic polymers,26,34,36,43 and are attributed to the “softness” of polymer segments and swelling of the polymer skeleton in the course of measurements in liquid nitrogen. On the contrary, after sulfonation and silver-ionization treatments, it is found that the adsorption−desorption curves of MPI-S and MPI-Ag are nearly reversible, suggesting that the architectures of MPI-S and MPI-Ag are more rigid than MPI. Additionally, the isotherms of MPI and MPI-S exhibit a steep rise of uptake at the very low relative pressure (P/P0 < 0.01), suggesting that MPI and MPI-S belong to microporous materials.44 In contrast to MPI and MPI-S, the microporous characteristic of MPI-Ag is unobvious. The uptake of nitrogen in MPI-Ag continually rise with the increase of pressure, displaying the existence of mesopore.43 From the adsorption isotherms the calculated BET surface area and total pore volume of MPI are 1001 m2 g−1 and 0.68 cm3 g−1, whereas after modifications, their values decrease to 448 m2/g and 0.31 cm3/g for MPI-S, and 103 m2/g and 0.10 cm3/g for MPI-Ag, respectively. The similar phenomenon, i.e., the greatly reduced specific surface areas 579

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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MPI. Nevertheless, the replacement of hydrogen in SO3H with silver eliminates the intersegmental hydrogen-bonding interaction. Moreover, the bigger atomic radius of silver than hydrogen is advantages to force the polymer segments apart. These two factors make the porous peaks of MPI-Ag apparently shift toward right relative to MPI-S. Adsorption/Separation of Light Hydrocarbons. The adsorption isotherms of MPI, MPI-S, and MPI-Ag for light hydrocarbons (CH4, C2H4, C2H6, C3H6, and C3H8) were measured at 273 and 298 K up to 1 bar (Figure 3). It is seen in Table S2 (Supporting Information) that for each sample their adsorption capacities are ranked in the order of C3 > C2 > C1, which is consistent with the sequence of the critical temperatures (Tc): Tc(C3) (364−370 K) > Tc(C2) (283−305 K) > Tc(C1) (191 K),55 indicating that the hydrocarbon with the higher critical temperature is easily condensed and adsorbed on the porous surface and therefore exhibits the larger adsorption capacity. In addition, the comparison between MPI-Ag and MPI-S reveals that, for saturated hydrocarbons such as CH4, C2H6, and C3H8, the uptakes in MPI-S are higher than those in MPI-Ag. However, the reverse results are observed for the unsaturated C2−C3 hydrocarbons. For example, the adsorption capacity of C3H6 in MPI-Ag (31.5 cm3/g) apparently exceeds that in MPI-S (27.2 cm3/g) although MPI-Ag has a smaller BET surface area than MPIS. The similar phenomenon has been also observed by Yang’s group for the adsorption of unsaturated C4 alkenes in Agmodified inorganic zeolite. For example, the specific surface area of the AgNO3/SiO2 sorbent is smaller than the porous SiO2, but the amount of unsaturated C4H8 adsorbed per square meter of surface area in the AgNO3/SiO2 sample (9.06 × 10−3 mmol/m2) is higher than in the SiO2 sample (1.94 × 10−3 mmol/m2).56 The reason is due to that, the π and π* orbitals of CC bond in unsaturated alkene molecule can overlap the 5s and 4d orbitals of Ag+, respectively.28,57 The π−Ag+ complexation7 effect enhances the affinity of MPI-Ag for alkene molecules. Consequently, relative to the specific surface area, the strong interaction between Ag+-decorated porous skeleton and the CC bond plays a more predominating role in the adsorption of unsaturated hydrocarbons. Furthermore, the physicochemical parameters of adsorption for the C1−C3 hydrocarbons in the three polymers were calculated in order to examine the different interactions of the saturated and unsaturated hydrocarbons with MPI, MPI-S, and MPI-Ag. The virial plots at 273 and 298 K were plotted according to the adsorbed amounts of hydrocarbons in the low-pressure region derived from the adsorption isotherms. All the virial plots for the three samples at both temperatures follow good linear relationship (Figure S7, Supporting Information). The intercept of the line is the first virial coefficients (A0), which reflects the gas−polymer interaction. The Henry constant (KH) can be calculated by equation of 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 obtained from the slopes of the plots of lnKH versus 1/T. Table S3 (Supporting Information) shows that, for the same saturated alkane, MPI-S and MPI-Ag have the similar enthalpies of adsorption. But, for the unsaturated alkenes such as C2H4 and C3H6, MPI-Ag exhibits the significantly higher Q 0 values than MPI-S. The comparisons between the KH and A0 values in Table S3 (Supporting Information) give the same changing trend, confirming that the silver ionization modification on porous

Figure 6. (a) Adsorption isotherms of CO2 in MPIs at 273 K (solid) and 298 K (empty). (b) Variation of CO2 isosteric enthalpies with the adsorbed amount. (c) IAST selectivity for CO2/CH4 with molar ratio of 5:95 in MPIs at 273 K (solid) and 298 K (empty). (d) IAST selectivity for CO2/CH4 with molar ratio of 50:50 in MPIs at 273 K (solid) and 298 K (empty).

Table 2. CO2 Uptake and CO2/CH4 Selectivity in MPI, MPI-S, and MPI-Ag CO2/CH4(5:95)b

CO2a

CO2/CH4(50:50)b

sample

273 K

298 K

Q0 kJ/mol

273 K

298 K

273 K

298 K

MPI MPI-S MPI-Ag

61.8 35.5 32.7

38.2 24.0 21.7

32.8 36.3 37.1

19.1 30.9 35.2

11.5 21.6 33.5

25.3 48.5 65.3

13.1 30.6 37.4

Uptake (cm3/g) at 1 bar. bCO2/CH4 selectivity (mol mol−1, 1 bar) was calculated by the IAST method at mole ratio of 5:95 and 50:50 for CO2/CH4.

a

very high temperature results in the fast cross-linking and effectively inhibits the interpenetration of networks. Consequently, Figure 2b shows that the pores in MPI locate at 1.21 nm, which is over twice that of MPI-1 (0.59 nm).41 On the other hand, it is interesting to observe that, after modifications, the peak in the former MPI (1.21 nm) splits into two peaks at 0.94 and 1.35 nm. The major micropores in MPI-S (0.94 nm) are smaller than MPI (1.21 nm), whereas the pores of MPI-Ag are apparently larger than MPI and MPI S. The previous reports have shown that the postmodifications on porous polymers lead to the reduced or enlarged pore sizes depending on the volume and polarity of the incorporated functional groups on the polymer skeleton.47,50−54 In our case, for MPI-S, the intersegmental hydrogen-bonds caused by the SO3H groups act as the physical cross-linking net nodes. Consequently, the increased network cross-linking density in MPI-S brings about the smaller pore size than the unmodified 580

DOI: 10.1021/acs.jpcc.8b10259 J. Phys. Chem. C 2019, 123, 575−583

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(7.6 wt %, 3530 m2/g),58 and BLP-1H (7.4 wt %, 1360 m2/ g).59 The interaction between the three polymers and CO2 were investigated by the calculation of their isosteric enthalpies of adsorption (Qst) from the adsorption isotherms measured at 273 and 298 K by Clausius−Clapeyron equation.40 Figure 6b 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 pore wall is stronger than that between CO2 molecules. The apparently steeper curves of MPI-S and MPI-Ag than MPI suggests that the sulfonation and silver-ionization modifications are advantageous for the affinity of polymer skeleton toward CO2 gas. Besides, the virial plots at 273 and 298 K were plotted (Figure S8, Supporting Information) according to the adsorption data in the low pressure region, from which limiting enthalpies of adsorption Q0 were obtained. As shown in Table 2, the Q0 value of MPIAg is 37.1 kJ/mol, higher than that of MPI-S (36.3 kJ/mol) and MPI (32.8 kJ/mol), suggesting the strong interaction between the Ag+-decorated MPI-Ag skeleton and CO double bonds in the CO2 molecule. The adsorption selectivities of CO2 gas for the binary gas mixtures of CO2/CH4 were assessed through the IAST method. It is seen that, like C1−C3 hydrocarbons, the singlesite Langmuir−Freundlich curves for CO2 well fit the corresponding experimental pure component isotherms (Figure S10). The gas mixture compositions are specified as CO2/CH4 = 0.05/0.95 and CO2/CH4 = 0.5/0.5 to simulate the gas compositions of natural gas and landfill gas, respectively. The data in Table 2 show that the enhanced affinity of CO2 for pore wall of MPI-Ag and MPI-S indeed results in considerable improvement in adsorption selectivity of CO2 over CH4. For the mixed gas of CO2/CH4 (0.5/0.5), its selectivity increases from 22.3 (MPI) to 48.5 (MPI-S) and 65.3 (MPI-Ag), which are much superior to most of porous polymers reported in the literature such as ALPs (6−8),60 BILPs (8−17),61 BLP-10(Cl) (28.3),62 PCN-AD (11),63 APOPs (5.3−6.7),64 and PSNs (10−14).65

polymers indeed can generate strong interaction with unsaturated alkenes. It is anticipated from the above results that MPI-Ag would possess high selectivity of alkenes over the corresponding alkanes. To this end, the separation property of C2H4/C2H6 and C3H6/C3H8 was evaluated using ideal adsorbed solution theory (IAST) method. Figure S9 shows that the experimental pure component isotherms for C1−C3 hydrocarbons at 273 and 298 K well fit their corresponding single-site Langmuir− Freundlich curves with the linear correlation coefficients over 0.997 (Table S5). The obtained separation factors at 273 and 298 K for the equimolar binary mixing gases of C2H4/C2H6 and C3H6/C3H8 are plotted as a function of pressure up to 1 bar. Figure 4 displays the modified MPI-Ag displays a considerably higher selectivity for C2H4/C2H6 and C3H6/ C3H8 than MPI and MPI-S in the whole range of measurement pressure. As shown in Table 1, at 273 K and 1 bar, the separation factors of C2H4/C2H6 and C3H6/C3H8 in MPI-Ag are up to 3.5 and 16.3, respectively. Moreover, the selectivity of C2H4/C2H6 at 298 K and 1 bar reaches 3.1, being comparable to that of CuA10B1 (3.8).11 Similarly, the selectivity of C3H6/ C3H8 (7.2) exceeds or competes with A10B1 (1.7), CuA2B1 (2.7), and CuA10B1 (10.4)11 with copper (catecholate) groups. In addition, Figure 5 illustrates the dramatic variation of the adsorption selectivity for saturated and unsaturated C2−C3 hydrocarbons over CH4 in MPI, MPI-S, and MPI-Ag. For saturated C2H6/CH4 and C3H8/CH4 gas pairs, their separation factors of the unmodified MPI are the highest among the three polymers. On the contrary, for C2H4/CH4 and C3H6/CH4, MPI-Ag shows the significantly higher selectivity than both MPI and MPI-S. At 273 and 298 K, the separation factors of C2H4/CH4 in MPI-Ag are 83.1 and 61.6, being about three times as high as those in MPI-S. At 298 K, its selectivity of C3H6 over CH4 reaches 586, much higher than that of metal− organic framework material UPC-21 (75).56 More surprisingly, the selectivity of C3H6/CH4 in MPI-Ag at 273 K is even observed up to 3785, which is one of the highest value for porous materials, to the best of our knowledge. Compared to the unmodified MPI, the dramatically increased adsorption selectivities of C2H4/C2H6 and C3H6/C3H8 as well as unsaturated C2−C3 alkenes over methane are attributed to the enhanced affinity of MPI-Ag for alkene molecules because of the π−Ag+ complexation effect.27,57 The strong interaction between Ag+-decorated polyimide skeleton and the CC bond plays a significantly role on the preferential adsorption of unsaturated hydrocarbons. Additionally, for almost all the C2− C3 hydrocarbons in this study, the sulfonated MPI-S displays the apparently lower adsorptive selectivities over CH4 than either MPI and MPI-Ag. The reason may be due to the fact that MPI-S seems to have a favorable interaction with the CH4 molecule, as reflected by the highest enthalpy of adsorption of CH4 among the three samples (Table S3). CO2 Adsorption and Separation of the CO2/CH4 Gas Pair. The CO2 adsorption isotherms of the three samples are presented in Figure 6a and the data are summarized in Table 2. At 273 K and 1.0 bar, MPI has the CO2 uptake of 61.8 cm3/g. After sulfonation and silver-ionization treatments, the uptakes in MPI-S and MPI-Ag decrease to 35.5 cm3/g (7.0 wt %) and 32.7 cm3/g (6.4 wt %), respectively, owing to the decreased BET surface areas. Despite this, the adsorption capacities are still comparable to many other porous polymers under the same measurement condition even though they possess larger surface area such as COF-5 (5.9 wt %, 1670 m2/g),58 COF-103

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CONCLUSIONS In summary, Ag+-decorated porous polyimide MPI-Ag is successfully prepared through the successive sulfonation and ion-exchange reactions based on aromatic microporous polyimide MPI. The chemical structure and composition of MPI and its modified products are well characterized by means of FTIR, XPS and EDX methods. For each sample the adsorption capacities of light hydrocarbons are ranked in the order of C3 > C2 > C1, which is consistent with the sequence of the critical temperatures, indicating the hydrocarbon with the higher critical temperature is easily condensed and adsorbed on the porous surface and therefore exhibits the larger adsorption capacity. The strong interaction of Ag+ ion with unsaturated hydrocarbons and CO2 molecule results in MPIAg preferentially adsorption alkenes such as C2H4, C3H6 and CO2 rather than the saturated alkanes like CH4, C2H6 and C3H8, as demonstrated by their much different interaction parameters with MPI-Ag skeleton including Henry constants, first virial coefficients and limiting enthalpies of adsorption. Consequently, at 273 K, MPI-Ag exhibits excellent adsorption selectivities of C2H4 over C2H6 (3.5) and C3H6 over C3H8 (16.3). Moreover, the selectivities of the binary mixtures of C2H4/CH4 (83.1) and C3H6/CH4 (3785) are among the highest values for porous polymers reported up to now. The 581

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results are helpful for deeply understanding the effects of chemical and porous structures on the selective adsorption of C2H4, C3H6 and CO2 from the mixtures of corresponding alkanes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b10259.

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Chemical composition, adsorption capacities, Q0, KH, and A0 values of C1−C3 hydrocarbons, solid-state 13C CP/MAS NMR spectra, FTIR spectra, XPS wide-scan spectra, EDX spectra, virial plots of hydrocarbons of hydrocarbons and CO2 in the porous polyimides (PDF)

AUTHOR INFORMATION

Corresponding Author

*(Z.W.) E-mail: [email protected]. ORCID

Zhonggang Wang: 0000-0003-0451-1919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Nos. 51473026 and 51873028) for financial support of this research.

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