N2 selectivity

Microporous and Mesoporous Materials 257 (2018) 253e261

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Triptycene based microporous polymers (TMPs): Efficient small gas (H2 and CO2) storage and high CO2/N2 selectivity Ranajit Bera, Snehasish Mondal, Neeladri Das* Department of Chemistry, Indian Institute of Technology Patna, Patna 801106, Bihar, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 April 2017 Received in revised form 8 August 2017 Accepted 23 August 2017 Available online 26 August 2017

In contemporary research, there is a growing interest in the development of novel porous materials that can be used as adsorbents for small gas molecules such as CO2 and H2. This is important from a perspective of more efficient energy alternative and a cleaner environment. Hyper-cross-linked polymers are a subclass of microporous organic polymers that are also reportedly known to efficiently adsorb H2 and CO2. In this context, triptycene based microporous polymers are known to exhibit high gas storage and separation properties. Herein, facile synthesis of a set of three triptycene based microporous polymers (TMPs) is described. TMPs were prepared by crosslinking triptycene monomers with various (bromomethyl)benzene units using AlCl3 catalyzed Friedel-Crafts alkylation reactions in high yield. Gas uptake experiments indicate that TMPs are microporous having moderately high surface areas (up to 1372 m2 g
1). Interestingly, TMP3 demonstrated very high reversible adsorption of molecular H2 (2.21% by mass at 1 bar/77 K) and it may be considered in the league of porous organic frameworks demonstrating outstanding H2 uptake at low pressure. Simultaneously, these polymers also show quite high CO2 uptake at 273 K and 1 bar (up to 223 mg g
1). Additionally TMPs have also exhibited very high CO2/N2 gas selectivity (up to 70). Considering the facile synthetic protocol of TMPs (mandatory for scale-up synthesis), large H2 and CO2 uptake and concurrent high CO2/N2 (up to 70) selectivity, these are potential materials for clean energy applications. © 2017 Elsevier Inc. All rights reserved.

Keywords: Triptycene Microporous H2 and CO2 storage CO2/N2 selectivity

1. Introduction In recent years, development of novel porous materials has received considerable research attention [1]. One important application of porous material is storage of molecular hydrogen (H2) which has been projected as a clean fuel and an alternative to fossil fuels [2]. Development of novel materials exhibiting excellent H2 adsorption adsorption of H has assumed significant importance from the view point of safe and efficient storage of H2 for its subsequent use as fuel in vehicles. To date, microporous polymers in general and hyper-crosslinked polymers (HCPs) in particular have emerged as potential materials for H2 storage with very high reversible uptake (of H2) among porous organic materials [3]. In many cases, H2 physisorption results of purely organic polymers are comparable with inorganic coordination polymers including MOFs [2e].

* Corresponding author. E-mail addresses: [email protected], [email protected] (N. Das). http://dx.doi.org/10.1016/j.micromeso.2017.08.045 1387-1811/© 2017 Elsevier Inc. All rights reserved.

Another major application of porous materials is in the domain of carbon dioxide capture and sequestration (CCS) [1a]. Consequently these materials have been projected to be better than the existing technologies that utilize alkanolamine solvents [1b,1d,1f,4]. Efficient CCS technologies are in great demand from a view point of global warming due to increased concentration of CO2 - a green house gas [1b,1o,5]. Among porous materials, the ones that are microporous are especially interesting because these have shown additional applications in catalysis and molecular sensing [6]. In general, microporous polymers synthesized exclusively from organic monomers are termed as Microporous Organic Polymers (MOPs) [7]. In this context, hyper-cross-linked polymers (HCPs) form a subset of MOPs [8]. HCPs are prepared by subjecting suitable monomers to Friedel-Craft alkylation reactions that are catalyzed by inexpensive and commercially available Lewis acid catalysts. Various structural motifs have been incorporated in resulting polymers such as triazine [9], 1,10-bis-2-naphthol [10], carbazole [3b] and others [2c,2d,3a]. In general, microporous polymers reported till date have shown greater ability to capture carbon dioxide at 273 K and 1 bar. Some important examples include

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alcohol-containing polymer network (174 mg g
1) [10], triazine and carbazole bifunctionalized task-specific polymer (180 mg g
1) [9], and C1M3-Al (181 mg g
1) [3c]. Triptycene is a structurally rigid molecule and this is due to presence of three arene rings connected to a [2.2.2]bicyclooctatriene unit in a paddle wheel fashion. In recent years, triptycene and its derivatives have shown several interesting and smart applications in supramolecular chemistry and materials science (including polymer chemistry) [11]. Specifically, triptycene based microporous polymers have reportedly shown excellent gas storage and separation properties [11b-f]. This is due to the “internal molecular free volume (IFV)” in these materials generated from the inefficient packing of rigid and robust triptycene units present in the backbone of the polymer [11a]. Considering these facts, we were curious to explore triptycene based HCPs wherein various multitopic benzyl bromides would be used to cross link triptycene monomers via Friedel-Craft alkylation reactions. Using anhydrous AlCl3 as catalyst, 1,4-bis(bromomethyl)benzene, 1,3,5tris(bromomethyl)benzene and 1,2,4,5-tetrakis (bromomethyl) benzene have been used as di-, tri- and tetratopic crosslinkers to yield a set of three inexpensive hyper-crosslinked and microporous as well as hyper-crosslinked polymeric networks - TMP1, TMP2 and TMP3. These triptycene based polymers have been characterized using spectroscopic techniques and their performance as material for gas storage (H2 and CO2) as well as selective CO2 uptake has been elaborated. 2. Experimental section 2.1. Materials and methods Triptycene, 1,4-bis(bromomethyl)benzene, 1,3,5tris(bromomethyl)benzene, 1,2,4,5-tetrakis (bromomethyl)benzene and anhydrous AlCl3 were obtained from SigmaAldrich and were utilized without further purification. Dichloromethane (DCM) was dried using standard protocol. Solid-state 13C cross-polarization magic angle spinning (CPMAS) NMR spectra were obtained using BRUKER 300 MHz (H-1 frequency) NMR spectrometer at a mass rate 8 kHz and CP contact time 2 ms with delay time 3 Sec. FTIR analysis was performed using Shimadzu IR Affinity-1 spectrometer. Elemental analysis was measured using Elementar vario MICRO cube CHNS analyzser. PXRD data were recorded using a Rigaku TTRAX III X-ray diffractometer. FESEM data were recorded using a Carl Zeiss AG Instrument (Model- SUPRA 55) and EDS data collected using an Oxford Instruments detector. TGA analysis was performed using SDT Q600 (TA Instruments) at a scan rate 10  C/min under nitrogen flow (100 mL/min). Porosity analyses were performed using Quantachrome autosorb iQ2 analyzer using UHP grade adsorbates. In a typical experiment, TMP (60e90 mg) were taken with a 9 mm large cell and attached to the degasser unit and degassed at 140  C by adjusting the delta pressure 25 mmmicron for 12e24 h [required time varies to achieve the typical delta pressure (25 mm)]. The samples were refilled with helium and weighed carefully and then the cells were attached to the analyzer unit. The temperature was maintained using KGW isotherm bath (provided by Quantachrome) of liquid N2 (77 K), or temperature controlled bath (298 K and 273 K).

alkylation reaction catalyzed by anhydrous AlCl3. The synthesis of TMP1 is described as a representative example. In a glove box, anhydrous AlCl3 (589 mg, 4.42 mmol), 1,4bis(bromomethyl) benzene (233 mg, 0.88 mmol) and triptycene (150 mg, 0.58 mmol) were loaded in a 100 ml Schlenk flask. Anhydrous dichloromethane (30 ml) was added into this flask and the reaction mixture was stirred at 40  C for 24 h in inert atmosphere. During the course of the reaction, precipitate formation was observed which was isolated and washed thrice with dilute HCl, DCM, acetone, DMF and DMSO. This brown colour product was finally dried under reduced pressure at 140  C for 24 h for analyses. Yield: 228 mg, 95%; FT-IR: 2942, 2157, 1997, 1678, 1604, 1438, 1012, 899 cm
1. Elemental analysis (%): calculated: C 94.31, H 5.69. Found: C 88.91, H 5.33. 2.3. Synthesis of TMP2 This polymer (brown powder) was prepared following the same procedure as described for TMP1 using 1,3,5-tris(bromomethyl) benzene (210 mg, 0.58 mmol) as the cross-linker for triptycene. Yield: 200 mg, 92%; FT-IR: 2922, 2350, 2164, 1664, 1598, 1431, 1059, 895 cm
1. Elemental analysis (%): calculated: C 94.53, H 5.47. Found: C 82.50, H 4.72. 2.4. Synthesis of TMP3 This polymer (brown powder) was prepared following the same procedure as described for TMP1 using 1,2,4,5-tetra (bromomethyl) benzene (198 mg, 0.44 mmol) as the cross-linker for triptycene. Yield: 201 mg, 98%; FT-IR: 2954, 2363, 2197, 2017, 1670, 1611, 1431, 1272, 893 cm
1. Elemental analysis (%): calculated: C 94.66, H 5.34. Found: C 80.53, H 4.51. 3. Result and discussion 3.1. Syntheses and characterization of TMPs Yuan and coworkers reported a series of porous hyper-crosslinked microporous polymers using Friedel-Crafts alkylation reactions [3c]. AlCl3 was used as the preferred choice of Lewis acid catalyst over FeCl3 because the former yielded porous polymers with higher porosity. Consequently, we utilized anhydrous AlCl3 as catalyst for the synthesis of triptycene based polymeric networks (Scheme 1). As far as the bromomethyl cross-linker is concerned, ditopic [1,4-di (bromomethyl)benzene], tritopic [1,3,5tris(bromomethyl)benzene and tetratopic [1,2,4,5-tetra

2.2. Synthesis of TMPs The three TMPs were prepared by crosslinking triptycene with either 1,4-bis(bromomethyl)benzene, 1,3,5-tris(bromomethyl)benzene or 1,2,4,5-tetra (bromomethyl)benzene via Friedel-Craft

Scheme 1. Synthetic route of TMP1-TMP3.

R. Bera et al. / Microporous and Mesoporous Materials 257 (2018) 253e261

(bromomethyl)benzene] molecules were utilized to yield TMP1, TMP2 and TMP3 (Scheme 1 and Scheme 2). The crude products obtained by stirring the reactants in DCM, were subjected to washing thrice with dilute HCl, DCM, acetone, DMF and DMSO. The products obtained were finally dried under reduced pressure at 140  C for 24 h for analyses. In all cases, the products were obtained as brown coloured powder, which were insoluble in common organic solvents including DMSO and DMF (Table S1). In principle, Schemes 1 and 2 suggest quantitative elimination of bromine atom from each bromomethyl group (of the respective monomers) to yield a corresponding methylene bridge in the polymeric product. However, energy dispersive spectrometry (EDS) data (supporting information Table S2) indicated residual bromine present in each polymer (TMP 1e3) reported herein. This is due to some bromomethyl groups that may have remained unreacted under the experimental reaction conditions. Such observations have been reported earlier by others for synthesis of HCPs using monomers with bromomethyl group [3c]. EDS data (Table S2 and Fig. S1) also indicated presence of trace quantities of aluminium and chlorine, which suggests that most of the Lewis acid catalyst (AlCl3) was removed during the workup of the reaction. FTIR spectroscopic data (Fig. S2) analyses suggested formation of the desired polymers. Peaks in the range 1580e1670 cm
1 and 2900-3000 cm
1 are due to vibrational modes in the aromatic rings. The CdH stretching modes due to methylene functional groups appear beyond 3000 cm
1. The 13C CPMAS NMR spectra of TMPs also supported formation of desired polymers. The 13C CPMAS spectrum of TMP1 has been shown as a representative example (Fig. 1). The peak in the range 51e53 ppm corresponds to the bridgehead proton of triptycene, thereby indicating incorporation of these motifs in the polymer backbone. The peaks in the range 125e140 ppm are attributed to the aromatic carbons of both triptycene and bromomethyl crosslinkers. As expected, the methylene carbon signals appear in the range 35e40 ppm and this supports crosslinking between triptycene and various aromatic bromomethyl units (Fig. 1 and Fig. S3). Powder X-ray diffraction (PXRD) measurements of TMPs (Fig. 2) indicated featureless broad spectra which implied that TMPs are amorphous in nature. This is a characteristic feature of polymers derived from triptycene monomers due to the presence of rigid 3dimensional motifs that decrease packing efficiency in the corresponding polymer.

Fig. 1.

13

255

C CP-MAS NMR spectrum of TMP1.

Field Emission Scanning Electron Microscopy (FE-SEM) was used to determine the morphology of TMPs. The FE-SEM images (Supporting information - Fig. S4) showed the formation of nanoscalar aggregates for these TMPs. Thermal stability of the polymers was measured by TGA analysis (Fig. S5). Thermal degradation temperature (Td ¼ 10% weight loss under N2 atmosphere) of TMPs were found to vary in the range of 397e460  C. Char yield of TMPs were calculated to be in the range 56e68% at 800  C. These values suggest high thermal stability of the polymers (TMPs) reported herein. This is due to the presence of rigid 3D triptycene motif in the polymer backbone as well as extensive cross-linking in these TMPs. 3.2. Porosity measurements and gas storage studies Porous properties of TMPs were analyzed by subjecting these polymers to nitrogen adsorption-desorption analyses at 77 K. As depicted in Fig. 3, TMPs exhibit Type I isotherm [12]. The isotherms exhibit a steep uptake of N2 gas in the low relative pressure range (P/P0 ¼ 0e0.01) which is an indication of permanent microporosity in TMPs. Moreover isotherms are reversible in nature (Fig. 3). A steep increase in N2 uptake in the high relative pressure range (P/ P0 ¼ 0.95e1.0) by TMPs may be due to interparticular void [13]. This

Scheme 2. Network structure of TMP3 (as a representative example) as well as the linkage (red) between the monomers.

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R. Bera et al. / Microporous and Mesoporous Materials 257 (2018) 253e261 Table 1 Pore properties of TMP1-TMP3. Polymers

SABET (m2 g
1)a

SALang (m2 g
1)b

Vtotal (cc g
1)c

Vmicro (cc g
1)d

TMP1 TMP2 TMP3

923 1094 1372

1211 1457 1817

0.49 0.70 0.86

0.113 0.120 0.163

a Surface area calculated based on the BET model from the nitrogen adsorption isotherm (P/P0 ¼ 0.01e0.1). b Surface area calculated based on the Langmuir model from the nitrogen adsorption isotherms (P/P0 ¼ 0.05e0.35). c The total pore volume calculated at P/P0 ¼ 0.99. d Micropore volume calculated from CO2 adsorption measurement at 273 K by DR method.

Fig. 2. WAXD pattern of the TMPs recorded at ambient temperature at a scan rate 3 / min.

also suggests presence of macropores which get filled at these higher relative pressures [13]. The Brunauer-Emmett-Teller (BET) model was applied to estimate the surface areas (SA) of TMPs within the pressure range P/ P0 ¼ 0.01 to 0.1 (Fig. S6). The SABET obtained for TMP1, TMP2, and TMP3 were 923 m [2] g
1, 1094 m2 g
1 and 1372 m [2] g
1 respectively. It is interesting to observe that the SABET increases as the number of branches in the cross-linker increases from two (TMP1) to three (in TMP2) to four (TMP3). In other words, the cross-linker with more number of bromomethyl branches yields a hyper cross linked polymer with higher surface area. This difference is surface area is thus due to the higher extent of network polymerization resulting from crosslinker with more bromomethyl branches relative to that with lesser bromomethyl branches. The corresponding Langmuir surface areas are 1211 m [2] g
1, 1457 m2 g
1 and 1817 m [2] g
1 respectively (Table 1 and Fig. S7). Irrespective of the nature of crosslinker used in conjugation with triptycene, the resulting TMPs are significantly porous and their corresponding surface areas (SABET) are comparable or better than

various HCPs reported in literature such as C2M3-Al (1342 m2 g
1) [3c], TCP-A (893 m2 g
1) [14], petroleum-pitch-based HCPs (586e1337 m2 g
1) [15], carbazole-based porous organic polymers (CPOPs, 820e1190 m2 g
1) [3b], phosphonium salt incorporated hypercrosslinked porous polymers (770e1168 m2 g
1) [16], triphenylene network polymers (1180 m2 g
1) [17] as well as other microporous organic polymers such azo-CMP1(1146 m2 g
1) [18], azo-linked polymers (ALPs, 412e1235 m2 g
1) [19], 2 phthalocyanine-based porous polymer (CPP, 579 m g
1) [20], benzothiazole- and benzoazole-linked polymers (BTLPs and BOLPs, 698e1011 m2 g
1) [21] and highly cross-linked polyimides (MPI6FA, 781 m2 g
1) [22]. The SABET values of TMPs are however lower than C1M3-Al [3c], TCP-B [14], triptycene-based hyper-cross-linked polymer sponge (THPS) [23], and TPB network polymers [18]. Pore size distributions (PSD) of TMPs (based on DFT based model) are depicted in Fig. 3. All three TMPs have similar PSD with the pores smaller than 2 nm. The presence of significant fraction of larger mesopores was not observed. In general, PSD plots of TMPs also show a narrow peak in the ultramicroporous region (pore width < 0.7 nm), thereby suggesting presence of ultramicropores in these polymers [1p,1q]. Total pore volume of TMPs (calculated at P/ P0 ¼ 0.99) are in the range of 0.49e0.86 cm3 g-11 (Table 1). These results clearly show that the pore volume increases as the number of branches increases in the cross linker. It has been proposed in literature that presence of pores smaller than 1 nm increases the ability of a porous material's efficiency to capture H2 and CO2 at low pressures [8a]. This is related to the kinetic diameter of H2 and CO2 which are 0.29 nm and 0.33 nm respectively [24]. This motivated us to investigate the H2 and CO2 uptake abilities of TMPs. H2 adsorption measurements were performed at 77 K for TMPs over the pressure range 0e1 bar. Indeed these TMP polymers

Fig. 3. N2 adsorption isotherm of TMPs at 77 K (left) and Pore size distribution of TMPs (right).

R. Bera et al. / Microporous and Mesoporous Materials 257 (2018) 253e261

displayed excellent H2 adsorption capability at low pressures even though these polymers (TMPs) have moderate surface areas (Fig. 4). H2 uptake (77 K and 1 bar) by these TMPs were found to be in the range of 15.4e22.1 mg/g as shown in (Table 2). The highest value was obtained for TMP3 (22.1 mg/g) and this is better than various microporous polymers with higher surface area that include but is not limited to HCPs such as C1M3-Al (19.1 mg/g) [3c], petroleumpitch-based HCPs (08.0e18.3 mg/g) [15], tetraphenylethylenecontaining microporous polymer (9.5e17.6 mg/g) [25], TCPs (12.7e17 mg/g) [14], triptycene-based polymer sponge (THPS, 17.1 mg/g) [23] and carbazole-based porous organic polymers (CPOPs11.9e12.9 mg/g) [3b]. The results are also better than several other microporous organic polymers with higher surface area such as Azo-CMP1(8.2e16 mg/g) [18], phthalocyanine-based porous polymer (CPP, 9 mg/g) [20] and most of the literature reported porous materials [2c,2d,3a,26]. To the best of our knowledge, the low pressure H2 adsorption capacity of TMP3 is comparable with porous organic materials showing highest uptake under these conditions [1s,1t,13a]. A review of the micropore size distributions of these TMPs suggests that greater predominance of ultramicropores (pore width < 0.7 nm) accounts for the significant adsorption of H2 at low pressure (1 bar) even though the BET surface areas are considerably less that other microporous/hypercrosslinked polymers [1r]. In other words, the high H2 uptake (77 K and 1 bar) of TMPs under these conditions is a consequence of the relatively larger proportion of sub-nanometer sized (<7 Å) pores that are accessible to H2. Adsorption isotherms for CO2, N2, and CH4 were measured at 273 K to determine the uptake/storage ability of TMPs for these gases. CO2 isotherms demonstrate a steep rise in the initial low pressure region and are completely reversible in the entire region. This is due to the absence of any significant adsorption-desorption hysteresis [Fig. 5 (273 K) and Fig. S8 (298 K)]. The reversibility indicates that the TMP-CO2 interactions are weak. At 273 K and 1.0 bar, TMP3 demonstrates highest CO2 uptake of 223 mg/g, while TMP1 shows lowest value of CO2 uptake at 154 mg/g. However, the extent of CO2 uptake by these TMPs is comparable or better than various literature reported COFs in general and HCPs in particular. As for example, the uptake of TMP3 (223 mg/g at 273 K/1 bar) exceeds the corresponding value for C1M3-Al (181 mg/g) [3c], taskspecific polymers (TSPs, 132e180 mg/g) [9], triptycene-based hyper-cross-linked polymer sponge (THPS, 157 mg/g) [23], petroleum-pitch-based HCPs (113.2e177.4 mg/g) [15], and

Fig. 4. H2 uptake isotherm of TMPs at 77 K [Adsorption (filled) and desorption (empty)].

257

benzothiazole- and benzoazole-linked polymers (BTLPs and BOLPs, 129e190 mg/g) [21]. To further understand the interaction of CO2 with TMPs, the isosteric heats of adsorption for CO2 in TMPs were estimated from the CO2 adsorption isotherm at 273 K (Fig. 5) and 298 K (Fig. S8) by using Clausius-Clapeyron equation. The Qst values for CO2 at zero coverage (at the onset of adsorption) are in the range of 29e30.4 kJ/ mol (Fig. 6). PSD indicates the presence of ultramicropores in these polymers and the observed slightly higher values of Qst for TMPs might be due to stronger interaction between these polymers and CO2 molecules trapped in these relatively narrower pores [21]. The Qst values obtained for TMPs are comparable with various literature reported microporous polymers such as highly cross-linked polyimides (MPI-6FA, MPI-BTA and MPIBPA, 29.8e33.3 kJ/mol) [22], benzothiazole- and benzoazole-linked polymers (BTLPs and BOLPs, 28.7e33.6 kJ/mol) [21], and azo-linked polymers (ALPs, 28.6e32.5 kJ/mol) [19]. In addition, the CH4 uptake capacities of TMPs have also been investigated at 273 K (Fig. 5) and 298 K (Fig. S8). The isotherms are completely reversible in nature as evident from the absence of any significant hysteresis in the adsorption-desorption isotherms. CH4 uptake of TMPs at 273 K and 1 bar are found to be in the range of 14.7e19.2 mg/g and are comparable or higher than various reported microporous organic polymers such as Azo-CMP1(18.9 mg/ g) [18], azo-linked polymers (ALPs, 11.7e26.14 mg/g) [19], B,Ncontaining cross-linked polymers (PPs-BN, 15.7e18.44 mg/g) [27] and benzothiazole- and benzoazole-linked polymers (BTLPs and BOLPs, 14e21.5 mg/g) [21]. The Qst values of CH4 uptake was also calculated from the adsorption isotherm measured at two different temperatures (273 K and 298 K) by using Clausius-Clapeyron equation. The Qst values of TMPs for CH4 at zero coverage are in the range of 21.1e22.2 kJ/mol (Fig. 6). These value are comparable with other reported microporous polymer such as benzothiazoleand benzoazole-linked polymers (BTLPs and BOLPs, 22e26) [21], azo-linked polymers (ALPs, 19e22.4)19 and phthalocyanine-based porous polymer (CPP, 17.2) [20]. 3.3. Gas selectivity Considering the excellent CO2 uptake abilities of TMPs, we were motivated to explore their capability to selectively capture CO2 over CH4 and N2. The single-component gas sorption experiments were recorded at two temperatures (273 K and 298 K) and pressure up to 1 bar (Fig. 7 and Fig. S9). For a given TMP, an initial steeper rise in the CO2 adsorption isotherm was observed in comparison to that in case of corresponding N2 or CH4 isotherm. The relative uptake of CO2 and N2 gases was compared at 0.15 bar pressure because the CO2 partial pressure is typically 0.15 bar at 273 K in flue gas (containing mixture of N2 and CO2) [28]. All TMPs demonstrate higher CO2 uptake than that of N2. Similarly, TMPs also showed higher uptake of CO2 relative to methane. CO2/N2 and CO2/CH4 gas selectivity of these TMPs was estimated from Henry law constant using initial slope ratio method (from corresponding single adsorption isotherms) (Fig. S10 and Fig. S11) which is used to estimate gas selectivity of porous materials [28,29]. Results are summarized in Table 2. The TMPs show CO2/N2 gas selectivity in the range (60e70) and the CO2/CH4 gas selectivity in the range (9e10) at 273 K. The gas selectivity is highest for TMP3 and least for TMP1 among the three TMPs reported herein. Interestingly in general, TMPs show very high CO2/N2 selectivity (70 at 1 bar and 273 K), which is higher than most of the HCPs reported such as C1M2-Al (32.3) [3c], TCPs (19e26) [14], triptycene-based hyper-cross-linked polymer sponge (THPS, 38) [23], petroleum-pitch-based HCPs (22.76e35.53)15 and other microporous polymer such as azo-linked polymers (ALPs, 44e60)

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Table 2 H2, CO2, and CH4 uptakes, isosteric heats of adsorption (Qst) for TMPs and CO2/N2 and CO2/CH4 selectivity. Polymers

TMP1 TMP2 TMP3

H2 at 1 bar (mg/g)

CO2 at 1 bar (mg/g)

77 K

273 K

298 K

Qst KJ/mol

273 K

CH4 at 1 bar (mg/g) 298 K

Qst KJ/mol

Selectivity CO2/N2 273 (298)K

CO2/CH4 273 (298)K

15.4 16.4 22.1

154 163 223

97 96 144

29.5 30.4 29.0

14.7 16.5 19.2

09.6 10.2 12.5

22.2 21.1 22.0

64 (54) 60 (35) 70 (64)

9 (7) 9 (6) 10 (9)

Fig. 5. CO2 uptake isotherm of TMPs at 273 K (left) and CH4 uptake isotherm of TMPs at 273 K (right) adsorption (filled) and desorption (empty).

Fig. 6. Qst of TMPs for CO2 (left) and for CH4 (right).

[19], highly cross-linked polyimides (20.1e52.8)22 and benzothiazole- and benzoazole-linked polymers (BTLPs and BOLPs, 41e55) [21]. In general, the magnitude of gas selectivity is higher than most of the other porous COFs [30] and MOFs [31] reported in literature. The CO2/CH4 selectivity studies are also important from the viewpoint of natural gas (CH4/CO2: 95:5) purification by capture of CO2 [32]. Methane adsorption isotherms were collected at 273 K and 298 K. Selectivity was measured using the initial slope calculation. Corresponding results are listed in Table 2. CO2/CH4 selectivities at 273 K and 1 bar are in the range of 9e10 which was observed to decrease at a higher temperature (298 K) to 6. In general, the TMPs exhibit significantly lower CO2/CH4 selectivity relative to CO2/N2 selectivity. This may be ascribed to higher polarizability of methane relative to nitrogen which results in

higher adsorption potential of methane relative to that of nitrogen [29].

4. Conclusion In conclusion, we report herein efficient synthesis and characterization of a new series of triptycene based hyper-cross-linked polymers using AlCl3 catalyzed Friedel-Craft alkylation reaction. The network polymers were characterized by FTIR and solid state 13 C spectroscopy. The resulting polymers (TMPs) are microporous materials that exhibit moderately high BET surface area up to 1348 m2 g
1. In general, TMPs showed impressive hydrogen uptake abilities (1.54e2.21% by mass at 1 bar/77 K) which has been attributed to the high density of micropores in the subnanometre range. It is noteworthy that the low pressure H2 adsorption capacity

R. Bera et al. / Microporous and Mesoporous Materials 257 (2018) 253e261

259

Fig. 7. Gas uptake capacities for TMP1-TMP3 at 273 K. CO2 (red triangle), CH4 (blue square), and N2 (brown star). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

of TMP3 is comparable with porous organic materials showing highest uptake under these conditions. Moreover, the CO2 uptake capacities of TMPs are also significantly high (up to 5.07 mmol/g) and these are comparable with microporous organic polymers exhibiting highest CO2 uptake. Interestingly, TMPs reported herein also exhibit remarkably high selectivity for CO2 over N2 (up to 70 at 273 K) and this is much better than most of the yet reported HCPs. In view of the microporosity, large uptake of CO2 and high CO2/ N2 and CO2/CH4 selectivities, TMPs are potential candidates for practical environmental applications such as CCS (CO2 Capture and Sequestration) as well as for purification of flue and natural gases. Also taking into account the very high H2 uptake by TMPs in general and TMP3 in particular, these porous polymers (TMPs) may be also useful material for H2 storage in clean energy applications. Based on these results, presently we are extending our research towards design of new porous polymers that may have promising applications as smart materials in clean energy and environment sector. Acknowledgements N.D. thanks the CSIR, Govt. of India, New Delhi [CSIR No. 02(0126)/13/EMR-II] for financial support. R.B. thanks Indian Institute of Technology Patna for providing research fellowship. S.M. thanks UGC, New Delhi for a Senior Research Fellowship. Authors acknowledge Indian Institute of Technology Patna for

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