Microporous Functionalized Triazine-Based Polyimides with High CO2

Article pubs.acs.org/cm

Microporous Functionalized Triazine-Based Polyimides with High CO2 Capture Capacity Mario R. Liebl and Jürgen Senker* Inorganic Chemistry III, Universität Bayreuth, Universitätsstrasse 30, 95447 Bayreuth, Germany S Supporting Information *

ABSTRACT: Porous organic polymers with polar surfaces are promising materials for capture and storage applications for carbon dioxide. Here, we present the synthesis and characterization of seven triazine-based porous polyimide (TPI) polymer networks and evaluate their applicability as CO2 sorbent materials. The TPIs were synthesized in good yields by a condensation reaction of 2,4,6tris(4-aminophenyl)-1,3,5-triazine (TAPT) and the respective dianhydride building blocks in m-cresol. The resulting TPI polymer networks exhibited high chemical and thermal stability under air (up to 450 °C). Argon sorption isotherms demonstrated that specific BET equivalent surface areas up to 809 m2 g−1 (TPI-1) were reached. The characterization of the pore structure revealed mainly micropores with pore diameters ranging from 0.4 to 3 nm. The highest uptake values for CO2 (2.45 mmol g−1) were observed for TPI-1 and TPI-2 at 273 K and 1 bar. The highest binding selectivity (56) for CO2 over N2 at 298 K was observed for TPI-7. The high degree of functionalization led to comparatively high CO2 adsorption heats for TPI polymer networks between 29 kJ mol−1 (TPI-6) and 34 kJ mol−1 (TPI-1). As a result, the TPI networks showed high CO2 uptakes relative to their moderate BET equivalent surface areas. In combination with a facile modular synthesis procedure, a high chemical and thermal stability, and the tunability of the CO2/N2 binding selectivities, TPIs might be classified as promising materials for CO2 storage and separation applications. KEYWORDS: carbon dioxide capture, porous organic polymers, gas separation, argon sorption

1. INTRODUCTION Recently, the development of microporous organic polymers (MOPs) consisting of purely organic network structures has attracted remarkable attention. The combination of different features such as high thermal and chemical stability, high specific surface areas, and high micropore volumes with a costeffective and modular synthesis procedure provides a large number of different polymer architectures. Various potential applications of MOPs as heterogeneous catalysts,1−8 drug delivery systems,9 optoelectronics,10 sensor materials,11−13 membranes,14,15 and gas storage and separation devices16−21 have been investigated recently. Especially for CO2 storage and separation (CSS), MOPs might be considered as a promising alternative to the extensively investigated porous inorganic and hybrid materials such as silica, zeolites, metal oxides, and metal−organic framework (MOFs).22,23 High CO2 gas uptakes and a good CO2 selectivity at ambient conditions in a reversible adsorption process are characteristic features for several MOP networks21,24 and desired properties for postcombustion CO2 capture applications from flue gas as a promising alternative to the established amine scrubbing technology.22,25,26 Using amine-functionalized mesoporous silica, high values for CO2 capacity and CO2/N2 selectivity were reached as well.27,28 The mesoporous design ensures a particularly fast uptake for such systems. © XXXX American Chemical Society

Successful synthesis of MOPs can be achieved under either thermodynamic or kinetic control. The reversibility of the reaction mechanisms under certain reaction conditions leads to long-range order in the structure of porous crystalline materials. As a consequence, kinetically controlled reaction mechanisms provide disordered materials with predominantly amorphous morphology, whereas the permanent porosity of these materials strongly depends on the rigidity of the polymer network.29 Covalent organic frameworks (COFs) consisting of rigid organic building blocks linked by condensation of boronic acid groups under reversible reaction conditions are probably the best-known porous crystalline organic framework materials. These two- or even three-dimensional porous crystalline networks can be obtained with specific surface areas up to more than 4000 m2 g−1.30,31 However the applicability of COFs might be limited by the poor stability of the boronate or boroxine linkages against hydrolysis.32 Covalent triazine frameworks (CTFs) obtained from ionothermal synthesis in zinc chloride melts at 400 °C exhibit large specific surface areas up to 3270 m2 g−1, as well as remarkable thermal stabilities, tunable pore sizes, and high gas adsorption capacities.33−37 It is noteworthy that the reversibility of the cyclotrimerization Received: January 9, 2013 Revised: February 22, 2013

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Avoiding chemisorptive interactions should still allow for energy-efficient desorption. In the case of CO2 adsorption on amine-functionalized silica,27,60−63 the formation of carbamates and stable ion pairs on the sorbent surface leads to high CO2 uptake rates, especially when primary and secondary amines are used. However, the inversion of the ammonium/carbamate formation might require considerable amounts of energy (e.g., heating), which will significantly lower the energy efficiency of the regeneration process (removal of CO2 from the sorbent surface). Additionally, we expect only a minor influence on the sorption mechanism of the TPI polymers on the presence of flue gas water, as linear polymers with chemical structures similar to those of TPIs are classified as clearly hydrophobic.64−66 Protonation of the triazine ring (pKB > 10) by water is highly unlikely because of its significantly decreased basicity compared to aliphatic tertiary amines (triethylamine pKB ≈ 3.25). Also, the formal formation of bicarbonate in humid CO2-containing gas mixtures does not lead to protonation of the triazine ring. Consequently, no ion-pair interaction with the sorbent should arise. In this work, TPI polymer networks were characterized by solid-state NMR spectroscopy, infrared spectroscopy, and elemental analysis to obtain sufficient information about their structural and chemical composition. The presence of all functionalities in the polymer structure could be confirmed. Thermogravimetric analysis showed that all TPI networks exhibit a high thermal stability under air. Despite their moderate specific BET equivalent surface areas, they featured remarkable CO2 uptakes at ambient conditions. The gas selectivity for CO2 over N2 could be increased significantly by increasing the polarity of the polymer surface.

reaction requires comparably harsh reaction conditions, limiting the number of suitable building blocks for this synthesis pathway. It was reported recently that highly disordered porous CTFs can also be synthesized using microwave irradiation at room temperature, giving specific surface areas around 1000 m2 g−1 with high CO2 uptakes and acceptable selectivities of CO2 over N2 at 273 K and 1 bar.38 Hypercrosslinked polymers (HCPs) generate porosity by introducing cross-linking agents into conventionally synthesized linear polymers such as polystyrenes and polyanilines.39−43 HCPs exhibit specific Brunauer−Emmett−Teller (BET) surface areas of almost 2000 m2 g−1 and CO2 uptakes up to 3.92 mmol g−1 at 273 K.24,44,45 Conjugated microporous polymers (CMPs) are probably the most versatile representatives of porous polymers. Mild reaction conditions and highyielding metal-catalyzed coupling reactions known from conventional organic chemistry have generated a large number of different porous polymer networks. Interesting examples are conjugated poly(aryleneethynylene) networks (PAEs) synthesized by Sonogashira−Hagihara coupling.46 The surface areas and micropore volumes can be increased by decreasing the total length of the linkages between the nodes in the network.47 Aromatic polyimides exhibit a variety of interesting properties such as high thermal and mechanical stabilities, as well as superior chemical stability.48,49 Among the few examples of porous polyimides, polymers of intrinsic microporosity (PIMs) are the most thoroughly investigated representatives of this new class of materials. They are synthesized from dianhydrides and diamines containing a spirocenter, which prevents dense packing of the resulting polymer chain, generating remarkably high specific surface areas up to more than 1000 m2.14,50−52 As an alternative, porous polyimides can also be synthesized by common condensation reactions of anhydrides and amines. If at least one of the reactants carries more than one functional group, cross-linked polyimide networks can be obtained. Several examples of porous polyimide networks with BET surface areas up to 2213 m2 have been reported.53−56 Unfortunately, up to now, the CO2 sorption properties of porous polyimide materials have been investigated sparsely. Because of its Lewis-acidic character and its quadrupolar momentum, CO2 is expected to interact strongly with the highly polar imide functionalities on the polymer surface. The gas adsorption capacity of porous organic networks depends not only on the specific surface area or the micropore volume but also on the chemical composition and the structural architecture of the network. It was shown recently that the introduction of functional groups can change the network properties significantly.24,57,58 For instance, the total CO2 uptake of several CMPs at 298 K can be increased up to 55% by substituting a phenyl node for a triazine unit.59 The introduction of Lewis-basic functionalities is expected to lead to a further increase of the CO2 affinity to a given polyimide network as well. In this study, we synthesized a series of porous triazine-based polyimide (TPI) networks by a condensation reaction of 2,4,6tris(p-aminophenyl)-1,3,5-triazine (TAPT) with various commercially available dianhydrides. We realized a facile modular synthesis concept aiming at a high degree of polymer functionalization. To maintain a balance between maximizing the gas uptake and achieving good reversibility for polar adsorbents such as CO2, we chose functional groups such as triazines, imides, ethers, sulfones, and carbonyls, guaranteeing physisorption (ΔHad < 50 kJ mol−1) of the gas molecules.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All anhydrides were purified by sublimation under reduced pressure or by recrystallization in acetic anhydride (Sigma-Aldrich Chemie GmbH). Chloroform, toluene, and m-cresol were freshly distilled and dried prior to use. All other chemicals were used as received without further purification. All reactions were carried out under argon atmosphere. Argon sorption measurements were carried out on a Quantachrome Autosorb-1 pore analyzer at 87 K. For all gas sorption analyses, the Quantachrome ASiQ v3.0 software package was used. The specific BET equivalent surface areas were calculated considering analysis requirements for microporous materials.67−69 Specific surface areas, pore volumes, and pore size distributions from the argon adsorption isotherms were obtained by applying the quenched solid density functional theory (QSDFT) equilibrium pore model for carbon materials with a slit-pore geometry. More details are provided in the Supporting Information (sections S4 and S6). Carbon dioxide and nitrogen sorption measurements were carried out on a Quantachrome Nova surface analyzer. Specific surface areas, pore volumes, and pore size distributions from CO2 isotherms at 273 K were obtained by applying the nonlocal density functional theory (NLDFT) slit-pore model for carbon materials. More details are provided in the Supporting Information (section S5). All samples were degassed at 433 K for 20 h under a vacuum prior to all gas sorption measurements. The isosteric heats of adsorption were calculated from the CO2 adsorption isotherms at temperatures of 273, 298, and 313 K up to a pressure of 1 bar using the Quantachrome ASiQ v3.0 software package. The equilibration time for CO2 and N2 sorption measurements was set to 120 s. Elemental analysis (EA) was carried out on a Vario Elementar EL III apparatus. All infrared (IR) spectra were recorded with a PerkinElmer Spectrum One FT-IR spectrometer equipped with an attenuated total reflectance (ATR) top plate. Powder X-ray diffraction B

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(PXRD) was performed using a Panalytical Xpert-Pro diffractometer (Bragg−Brentano geometry) equipped with an X’Celerator Scientific RTMS detector. Nickel-filtered Cu Kα radiation with a wavelength of 1.54187 Å was used. During measurements, the sample was placed on a zero-background silicon plate spinning with a rotation frequency of 1 Hz. Solid-state NMR measurements were carried out at ambient temperature on a Bruker Avance II spectrometer operating at a proton frequency of 300 MHz. Samples for 13C measurements were contained in 4 mm ZrO2 rotors and mounted in standard-triple resonance magic-angle-spinning (MAS) probes (Bruker). For all 13C MAS cross-polarization (CP) experiments, a ramped CP sequence with a contact time of 5 ms was applied. The spinning frequencies varied between 10 and 11.5 kHz. The chemical shifts were referenced to tetramethylsilane (TMS). For all measurements, broadband proton decoupling using the SPINAL64 sequence with a nutation frequency of around 80 kHz was applied.70 Solution NMR spectroscopy was carried out on a Varian INOVA spectrometer equipped with a temperature unit operating at 300 MHz. All measurements were carried out at 298 K (CDCl3 and acetone-d6) and at 353 K (bromobenzene-d5). Chemical shifts are reported in parts per million (ppm) relative to the deuterated solvent. The solvent signal of the C− Br carbon of bromobenzene-d5 (13C) was determined at 122.26 ppm relative to TMS at 298 K. This solvent signal was used as a reference for measurements in bromobenzene-d5 at 353 K. Thermogravimetric analysis (TGA) was conducted with a Linseis T6/DTA (type L81/077) thermoanalyzer instrument. All samples were heated under air at a heating rate of 5 K min−1. Scanning electron microscopy (SEM) images were recorded with a high-resolution scanning electron microscope (LEO 1530 FESEM) equipped with a field-emission cathode at 3 kV. The accessible magnification ranges from 20× to 900000×. All samples were investigated from conductive carbon tabs and sputtered with carbon (20−40 nm) prior to the measurements. Mass spectra were obtained under electron ionization (EI) conditions (70 eV). All melting points are uncorrected. 2.2. Synthesis of 2,4,6-Tris(4-bromophenyl)-1,3,5-triazine (TBPT). A flame-dried 500 mL flask was equipped with a magnetic stirrer and dry chloroform (150 mL). After the solvent had been cooled to 0 °C in an ice bath, trifluoromethanesulfonic acid (10.0 g, 66.7 mmol) was added carefully under vigorous stirring. Then, 4bromobenzonitrile (6.00 g, 33.0 mmol) was added in small portions. The reaction mixture was stirred at 0 °C for 1 h and afterward for an additional 24 h at room temperature. After 200 mL of water had been added, a white precipitate was formed. Being stirred for an additional 3 h, the suspension was filtered, and the resulting white solid was washed twice with water (100 mL) and then with ethanol (100 mL) and diethyl ether (100 mL). The crude product was recrystallized in toluene and dried at 100 °C (yield: 3.90 g, 65%). Mp 361−363 °C (lit. 362−365 °C).71 1H NMR (300 MHz, CDCl3): δ (ppm) 8.59 (m, 6H, Ar−H), 7.69 (m, 6H, Ar−H). 13C NMR (300 MHz, bromobenzened5): δ (ppm) 171.0, 134.9, 131.8, 130.5, 127.7. IR (ATR, 4000−650 cm−1): 1579, 1512, 1486, 1401, 1369, 1354, 1172, 1067, 1009, 842, 805. Elem. Anal. Calcd for C21H12Br3N3 (%): C, 46.19; H, 2.22; N, 7.70. Found: C, 46.04; H, 2.69; N, 7.72. MS (m/z): 545 (M+, 100%), 181 (85), 102 (77). 2.3. Synthesis of 2,4,6-Tris(4-aminophenyl)-1,3,5-triazine (TAPT). Prepared by a slight modification of a known procedure.72 Under an argon atmosphere, Pd(dba)2 (403 mg, 0.70 mmol) and P(tBu)3 (10 wt % in hexane, 0.70 mmol, 2.08 mL) were added to 35 mL of dry toluene at room temperature. After the mixture had been stirred for 10 min, a solution of lithium bis(trimethylsilyl)amide (LiHMDS) in hexanes (1.0 M, 23.1 mL, 23.1 mmol) and solid TBPT (3.82 g, 7.00 mmol) were added. The mixture was stirred for 48 h at 80 °C, before the reaction was quenched with aqueous hydrochloric acid (1.0 M, 30 mL) and the mixture was diluted with water (50 mL). The viscous suspension was filtered and washed with aqueous hydrochloric acid (1.0 M, 50 mL), water (150 mL), and ethyl acetate (10 mL). The filtrate was transferred to a separating funnel. The separated organic phase was washed once with 1.0 M hydrochloric acid (50 mL). The aqueous phases were combined and washed three times with diethyl

ether (50 mL). The separated aqueous dark yellow phase was treated carefully with sodium hydroxide solution (1.0 M) until a precipitate was formed. The pale yellow solid was collected by filtration, recrystallized in N-methylpyrrolidone (NMP), and dried overnight at 100 °C (yield: 1.44 g, 58%). Mp 376−378 °C (dec.) (lit. 380−382 °C).73 1H NMR (300 MHz, acetone-d6): δ (ppm) 8.50 (m, 6H, Ar− H), 6.81 (m, 6H, Ar−H), 5.32 (s, 6H, NH2). 13C NMR (300 MHz, acetone-d6): δ (ppm) 171.3, 153.5, 131.3, 125.7, 114.4. IR (ATR, 4000−650 cm−1): 3459, 3320, 3209, 3030, 1631, 1604, 1578, 1486, 1429, 1363, 1293, 1176, 1145, 1129, 1009, 851, 810. Elem. Anal. Calcd for C21H18N6 (%): C, 71.17; H, 5.12; N, 23.71. Found: C, 70.93; H, 4.48; N, 22.63. MS (m/z): 354 (M+, 100%), 339 (19), 118 (94). 2.4. General Synthesis Procedure for Polyimide Polymers. All polymerization reactions were carried out in a dried three-necked flask equipped with a magnetic stirrer and an internal thermometer using an aluminum heating block. The molar ratio of TAPT to the corresponding anhydride building block was set to 1:1.5 to provide equimolar amounts of amine and anhydride groups. The amount of starting materials in m-cresol depends on the solubility and reactivity of the dianhydrides and was adjusted to between 1 and 5 wt %. The synthesis of TPI-1 gives a representative example for the experimental procedure. 2.4.1. Synthesis of TPI-1. To a solution of TAPT (0.30 g, 0.85 mmol) in m-cresol (50 mL) was added pyromellitic dianhydride (0.28 g, 1.28 mmol), and the mixture was stirred for 10 h at 0 °C until all of the solids were dissolved. Several drops of isoquinoline were added prior to removal of the ice bath. After the mixture had reached room temperature, it was stirred for an additional 12 h. The polymerization reaction was carried out at 80 °C for 4 h, 120 °C for 4 h, and 160 °C for 6 h. Afterward, the temperature was raised to 190 °C and held at this level for an additional 18 h to finish the condensation reaction. The mixture was cooled to room temperature. The polymer was washed sequentially with toluene, dichloromethane, and methanol. Finally, the polymer was purified in a Soxhlet apparatus with tetrahydrofuran and dried at 100 °C for 24 h. TPI-1 was obtained as a yellowish powder (yield: 0.50 g, 94%). Elem. Anal. Calcd for C36H15N6O6 (%): C, 68.90; H, 2.41; N, 13.39. Found: C, 68.78; H, 3.40; N, 12.46. IR (ATR, 4000−650 cm−1): 1780, 1726, 1508, 1348, 1190, 810, 723. 2.4.2. Synthesis of TPI-2. 1,4,5,8-Naphthalenetetracarboxylic dianhydride (0.34 g, 1.28 mmol), TAPT (0.30 g, 0.85 mmol), and m-cresol (60 mL) were used in this polymerization. TPI-2 was obtained as a brown powder (yield: 0.55 g, 92%). Elem. Anal. Calcd for C42H18N6O6 (%): C, 71.90; H, 2.58; N, 11.96. Found: C, 69.16; H, 3.17; N, 11.40. IR (ATR, 4000−650 cm−1): 1717, 1675, 1580, 1505, 1330, 1243, 1178, 1018, 813, 765, 708. 2.4.3. Synthesis of TPI-3. 3,4,9,10-Perylenetetracarboxylic dianhydride (0.50 g, 1.28 mmol), TAPT (0.30 g, 0.85 mmol), and m-cresol (75 mL) were used in this polymerization. TPI-3 was obtained as a dark red powder (0.63 g, 83%). Elem. Anal. Calcd for C57H24N6O6 (%): C, 77.02; H, 2.72; N, 9.46. Found: C, 71.89; H, 4.19; N, 13.35. IR (ATR, 4000−650 cm−1): 3470, 3375, 1702, 1655, 1591, 1505, 1360, 1253, 1176, 1145, 810, 798, 745. 2.4.4. Synthesis of TPI-4. 3,3′,4,4′-Benzophenonetetracarboxylic dianhydride (0.41 g, 1.28 mmol), TAPT (0.30 g, 0.85 mmol), and mcresol (70 mL) were used in this polymerization (0.58 g, 87%). Elem. Anal. Calcd for C93H42N12O15 (%): C, 71.26; H, 2.70; N, 10.72. Found: C, 70.38; H, 2.93; N, 9.98. IR (ATR, 4000−650 cm−1): 1780, 1721, 1509, 1356, 1295, 1207, 1091, 814, 709. 2.4.5. Synthesis of TPI-5. 3,3′,4,4′-Diphenyloxytetracarboxylic dianhydride (0.40 g, 1.28 mmol), TAPT (0.30 g, 0.85 mmol), and m-cresol (65 mL) were used in this polymerization (0.56 g, 86%). Elem. Anal. Calcd for C90H42N12O15 (%): C, 70.59; H, 2.76; N, 10.98. Found: C, 69.54; H, 2.93; N, 10.31. IR (ATR, 4000−650 cm−1): 1780, 1722, 1509, 1475, 1353, 1273, 1234, 1077, 810, 741. 2.4.6. Synthesis of TPI-6. 4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (0.57 g, 1,28 mmol), TAPT (0.30 g, 0.85 mmol), and m-cresol (80 mL) were used in this polymerization (0.72 g, 88%). Elem. Anal. Calcd for C99H42F18N12O12 (%): C, 61.50; H, 2.19; N, 8.69. Found: C, 61.77; H, 2.22; N, 8.43. IR (ATR, 4000−650 C

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Figure 1. Synthesis route for TPI networks. cm−1): 1788, 1726, 1509, 1360, 1296, 1255, 1210, 1192, 1143, 1100, 815, 721, 712. 2.4.7. Synthesis of TPI-7. 4,4′-Sulfonyldiphthalic anhydride (0.46 g, 1.275 mmol), TAPT (0.30 g, 0.85 mmol), and m-cresol (70 mL) were used in this polymerization (0.58 g, 81%). Elem. Anal. Calcd for C90H42N12O18S3 (%): C, 64.51; H, 2.53; N, 10.03. Found: C, 61.49; H, 3.18; N, 8.78. IR (ATR, 4000−650 cm−1): 3070, 1784, 1720, 1506, 1417, 1352, 1221, 1177, 1146, 1056, 920, 814, 735, 708, 669.

monomer concentrations resulted in gelation of the reaction mixture with an incomplete condensation and a reduced specific surface area of the polymer. After purification, all TPI polymer networks appeared as fluffy powders that were insoluble in water and common organic solvents. XRD measurements confirmed the amorphous structure of the polymers (Figure S1, Supporting Information). SEM images showed a series of highly aggregated particles with sizes ranging from 0.05 to 0.5 μm (Figure S2, Supporting Information). Infrared spectra were measured for all TPI polymers (Figures 2 and S8, Supporting Information). The successful formation of

3. RESULTS AND DISCUSSION The synthesis route for triazine-based polyimides is displayed in Figure 1. The precursor TBPT was obtained by a cyclotrimerization reaction of 4-bromobenzonitrile in chloroform at room temperature as described in the Experimental Section.74 The following reaction to TAPT was performed through a palladium-catalyzed CN bond formation using Pd[P(t-Bu)3]2 formed in situ as the active species.72,75 At the initial stage of the polymerization reaction, the temperature was adjusted to 0 °C for 12 h, and then a slow increase of the reaction temperature was carried out in several consecutive steps, preserving kinetic control over the condensation reaction. When the synthesis was conducted at higher initial temperatures or increased heating rates, a rapid gelation of the reaction mixture could be observed. At a temperature of 120 °C, a darkening of the reaction mixture and an increase of the viscosity could be monitored when five-membered polyimide rings formed. For TPI-2 and TPI-3, the viscosity increased only slightly around 160 °C because of the lower reactivity of the six-membered anhydride rings. The condensation reaction was completed by maintaining a temperature of 190 °C for several hours. The highest polymer yields and specific surface areas were obtained when a 1:1 molar ratio of dianhydride and amine groups was applied. One of the key issues for a successful synthesis was the application of the starting materials in mass fractions between 1.0 and 5.0 wt % depending on the solubility of the respective dianhydride building block in m-cresol. Higher

Figure 2. Infrared spectra of TPI polymers. A plot with the full frequency range (650−4000 cm−1) is given in Figure S8, Supporting Information.

the polyimide rings was confirmed by characteristic absorption bands around 1725 and 1785 cm−1 for five-membered polyimide rings and 1665 and 1710 cm−1 for six-membered polyimide rings, which could be assigned to the symmetric and asymmetric vibrations of the carbonyl groups of the imide ring, respectively. The broad band at about 1350−1360 cm−1 results from overlap of the C−N−C stretching vibration of the imide D

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ring and the in-plane stretching vibration of the triazine ring. The strong absorption at approximately 1500 cm−1 clearly identifies the presence of a triazine unit in all TPI polymers. Additionally, the absence of the characteristic absorption bands for anhydrides at 1860 cm −1 and for polyamic acid intermediates at 1650 cm−1 indicates complete imidization in the case of the polymers containing five-membered imide rings. Similarly, the infrared spectra of TPI-2 and TPI-3 display no anhydride band around 1780 cm−1. In contrast to the spectra of all other TPI polymers, the absorption spectrum of TPI-3 contains a weak band at 3380 cm−1, indicating the presence of a small amount of unreacted amine groups. For TPI-7, the symmetric and asymmetric stretching vibrations of the SO2 group can be identified at 1320 and 1147 cm−1. All TPI polymers were characterized by 13C CP-MAS spectra (Figure 3). The assignments of the different 13C signals are

Figure 4. TPI structures and assignments for NMR signals (see Table 1).

are consistent with data from elemental analysis and IR spectroscopy (see Experimental Section). They confirm successful polyimide formation for all TPI networks. Only, TPI-3 contains a small amount of amine functionalities, most likely as a result of an incomplete condensation reaction due to the poor solubility of the precursor oligomers in m-cresol and the comparatively poor reactivity of the six-membered dianhydride ring. When discussing the applicability of a porous material as a flue gas sorbent, it is important to consider its thermal stability under air. Thermogravimetric analysis data for all TPI polymers are summarized in Figure 5. The decomposition temperatures Tdec for all polymers were determined at 10% mass loss and are reported in Table 2. TPI-2 and TPI-7 showed the highest decomposition temperatures at 456 and 450 °C, respectively, which are in agreement with the exceptionally high thermal stabilities of other polyimide polymers.77,78 The significantly lower decomposition temperature of TPI-3 is attributed to the lower degree of polymerization. The porosities of all TPI polymers were determined by argon and carbon dioxide sorption measurements. Although nitrogen is well-known as a standard adsorptive component for surface and pore size characterization, argon is advantageous in some cases. As a monatomic gas, argon does not have a quadrupole moment, and therefore, its interactions, especially with polar surfaces, are significantly lower than those of the diatomic nitrogen molecule. Additionally, the higher boiling temperature leads to accelerated equilibration processes and, as a consequence, to significantly shorter measurement times.79−81 The argon sorption isotherms (87 K) for all TPI polymers are presented in Figure 6. The isotherms for TPI-1, TPI-2, and TPI-6 show rapid argon uptake at low relative pressures (p/p0