Redefining the Robeson upper bounds for CO2/CH4 and CO2/N2 separations using a series of ultrapermeable benzotriptycene-based polymers of intrinsic microporosity

This journal is ©The Royal Society of Chemistry 2019 Energy Environ. Sci., 2019, 12, 2733--2740 | 2733 Cite this: Energy Environ. Sci., 2019, 12,2733 Redefining the Robeson upper bounds for CO 2 / CH 4 and CO 2 /N 2 separations using a series of ultrapermeable benzotriptycene-based polymers of intrinsic microporosity† Bibiana Comesan ˜a-Ga ´ndara, ‡ a Jie Chen,‡ a C. Grazia Bezzu, a Mariolino Carta, b Ian Rose, a Maria-Chiara Ferrari, c Elisa Esposito, d Alessio Fuoco, d Johannes C. Jansen * d and Neil B. McKeown * a Membranes composed of Polymers of Intrinsic Microporosity (PIMs) have the potential for energy efficient industrial gas separations. Here we report the synthesis and gas permeability data of a series of ultrapermeable PIMs, of two-dimensional chain conformation and based on benzotriptycene structural units, that demonstrate remarkable ideal selectivity for most gas pairs of importance. In particular, the CO 2 ultrapermeability and high selectivity for CO 2 over CH 4 , of key importance for the upgrading of natural gas and biogas, and for CO 2 over N 2 , of importance for cost-effective carbon capture from power plants, exceed the performance of the current state-of-the-art polymers. All of the gas permeability data from this series of benzotriptycene-based PIMs are placed well above the current 2008 Robeson upper bounds for CO 2 /CH 4 and CO 2 /N 2 . Indeed, the data for some of these polymers fall into a linear correlation on the benchmark Robeson plots [i.e. log(P CO2 /P CH4 )versus log P CO2 and log(P CO2 /P N2 )versus log P CO2 ], which are parallel to, but significantly above, that of the 2008 CO 2 /CH 4 and CO 2 /N 2 upper bounds, allowing their revision. The redefinition of these upper bounds sets new aspirational targets for polymer chemists to aim for and will result in more attractive parametric estimates of energy and cost efficiencies for carbon capture and natural/bio gas upgrading using state-of-the-art CO 2 separation membranes. Broader context The low-cost and energy-effective removal of carbon dioxide (CO 2 ) from natural gas and biogas would help the supply of methane as the cleanest burning and lowest carbon-emitting hydrocarbon fuel. In addition, carbon capture and storage (CCS) from power plant emissions will be required to achieve the goals of the 2015 Paris Agreement, which aspires to maintain global warming to less than 1.5 1C above that of the pre-industrial age by the end of the 21st Century. Indeed, the combined use of biofuels, such as biogas, and CCS technology is regarded as the key negative emissions technology required in order to reach the Agreement’s ambitious targets for reduced emissions. Despite the urgent need for CCS, the best technology platform for its delivery is still unclear due to the difficulties in the estimation of costs and the complex evaluation of the advantages and disadvantages associated with each technology. Highly permeable membranes that are selective for CO 2 over methane (CO 2 /CH 4 )andCO 2 over nitrogen (CO 2 /N 2 ) are of increasing interest for natural gas/biogas upgrading and carbon capture, respectively, due to the inherent efficiency of membrane separations. Here we report the synthesis of a series of ultrapermeable polymers that define the state-of-the-art in the trade-off between permeability and selectivity for all important gas separations and, in particular, for CO 2 /CH 4 and CO 2 /N 2 . The data from these polymers were used to redefine the benchmark Robeson upper bounds for these two gas separations at much higher values of selectivity. This enhancement will improve the credibility of polymer membranes for CO 2 separations when evaluated against competing processes. Hopefully, this will help to stimulate the fundamental polymer science and applied engineering required to develop membrane systems for these CO 2 separations of key importance to energy and the environment. Introduction Membranes based on polymers as the selective layer are used for the energy efficient separation of gas mixtures including those of key relevance to energy and the environment. 1–4 The development of new polymers with greater gas permeability and selectivity would further enhance the efficiency of membrane gas separations of current industrial interest, 5 including hydrogen recovery during ammonia preparation (H 2 from N 2 ), oxygen or a EaStCHEM, School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, EH9 3FJ, UK. E-mail: [email protected] b Department of Chemistry, Swansea University, College of Science, Grove Building, Singleton Park, Swansea, SA2 8PP, UK c Institute for Materials and Processes, School of Engineering, The University of Edinburgh, Mayfield Road, Edinburgh EH9 3JL, UK d Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende (CS), Italy †Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ee01384a ‡The first two authors contributed equally. Received 29th April 2019, Accepted 11th July 2019 DOI: 10.1039/c9ee01384a rsc.li/ees Energy & Environmental Science PAPER Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue 2734 |Energy Environ. Sci., 2019, 12, 2733--2740 This journal is ©The Royal Society of Chemistry 2019 nitrogen enrichment of air (O 2 from N 2 ) 6 and natural gas or biogas upgrading (predominantly CO 2 from CH 4 ). 7–10 Increasingly, polymer membranes are also being considered as a practical alternative to solvent absorption for large-scale capture of CO 2 from power plant flue gas (predominantly CO 2 from N 2 ). 7,9,11–14 For gas separations on such a massive scale, membranes with very high permeance (i.e. flux) are desirable to minimise energy costs for gas compression and to reduce the active surface area of the membrane, thereby, optimising the overall size and manufacture cost of the membrane system. 5,15 However, polymer membrane materials suffer from the well-established trade-off between gas permeability (P x ) and selectivity for one gas over another (P x /P y ), 16,17 so that established ultrapermeable polymers, such as the polyacetylene poly(trimethylsilylpropyne) (PTMSP), 18,19 and recently reported examples 20 are insufficiently selective for use in gas separations. The general trade-off between polymer permeability and selectivity was first quantified by Robeson in 1991 when he identified upper bounds in plots of log(P x /P y ), versus log P x for O 2 /N 2 ,H 2 /N 2 , He/N 2 ,H 2 /CH 4 , He/CH 4 ,CO 2 /CH 4 , and He/H 2 gas pairs based on the gas permeability of the best performing polymers at that time. 21 Subsequently, for a newly prepared polymer (or a mixed matrix membrane) 22,23 the position of its gas permeability data relative to the upper bounds on Robeson plots allows for its potential for gas separations to be estimated. Robeson updated all of the upper bounds in 2008 using initial data for two spirobisindane-based Polymers of Intrinsic Microporosity (PIM-1 and PIM-7; Table S1, ESI†), 24 whose rigid and contorted macromolecular structures provided exceptionally high permeability with moderate selectivity. 25 In addition, data forthesetwoPIMswerealsousedtodefineanupperboundforthe CO 2 /N 2 gas pair, which is of key importance to post-combustion carbon capture but had been considered of no practical interest in 1991. 24 Since 2008, many PIMs with enhanced rigidity have demonstrated gas permeability data that lie well above some of the 2008 upper bounds. 26 These highly shape-persistent PIMs were obtained by replacing the relatively flexible spirobisindane structural unit with spirobifluorene 27,28 units or highly rigid bridged bicyclic components such as ethanoanthracene, 29–32 triptycene, 33–36 methanopentacene 37 and Tro ¨gers base. 29,35 Indeed, in 2015 Pinnau et al. 38 proposed that the O 2 /N 2 ,H 2 /N 2 and H 2 /CH 4 upper bounds should be updated using permeability data from aged films of highly selective triptycene-based PIMs (e.g. PIM-Trip-TB 35 and TPIM-1 33 ). However, revisions of the upper bound for CO 2 /N 2 and CO 2 /CH 4 were not proposed at that time due to the data for these polymers and other high-performing PIMs being close to the existing 2008 CO 2 /N 2 and CO 2 /CH 4 upper bounds (Table S1, ESI†). Recently, we introduced a new PIM derived from a benzotriptycene monomer, PIM-TMN-Trip, which proved to be as ultrapermeable to gases as PTMSP due to enhanced intrinsic microporosity arising from its 2D chain structure. 39 PIM-TMNTrip demonstrates higher selectivity than PTMSP due to its greater chain rigidity providing enhanced molecular sieving (i.e. diffusivity selectivity). Furthermore, it was found that the unsubstituted benzotriptycene-based PIM (PIM-BTrip) demonstrates even greater selectivity placing its data above the proposed 2015 O 2 /N 2 ,H 2 /N 2 and H 2 /CH 4 upper bounds and even above Robeson’s 2008 upperbounds for CO 2 /N 2 and CO 2 / CH 4 . 40,41 Here we report on the synthesis and properties of some new members of the benzotriptycene-based PIM series (Fig. 1), all of which demonstrate high permeability and selectivity. In particular, this polymer series demonstrates permeability data for CO 2 /N 2 and CO 2 /CH 4 that suggest new positions of the Robeson upper bound for these important gas pairs that are of key interest for separations of relevance to energy and the environment. Results and discussion Polymer design and synthesis A further four benzotriptycene PIMs were synthesised along with new batches of PIM-TMN-Trip and PIM-BTrip to allow for direct comparison of their gas permeabilities. The novel polymers include PIM-HMI-Trip, for which the sterically crowded hexamethylindane (HMI)-solubilising group 42 would be expected to be more rigid than the tetramethylnaphthalene (TMN) group of PIM-TMN-Trip. Previously for spirobifluorene-based PIMs, 43 the introduction of adjacent methyl substituents had been Fig. 1 Structure and synthesis of the benzotriptycene PIMs. Reagents and conditions: i. Br 2 ,Fe,DCM,rt,3h;ii.n-BuLi, furan, THF, 78 1C, 1.5 h; iii. 9,10-Dimethyl2,3,6,7-tetramethoxyanthracene, DMF, 250 1C, 7 bar, 2 h, microwave irradiation, iv. TFA or MeSO 4 H, rt, 24 h.; v. BBr 3 , DCM. (See ESI†for details). Paper Energy & Environmental Science Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is ©The Royal Society of Chemistry 2019 Energy Environ. Sci., 2019, 12, 2733--2740 | 2735 shown to be beneficial to performance, therefore, a PIM based on dimethylbenzotriptycene was prepared (PIM-DM-BTrip). In addition, the potential benefit of introducing one or two trifluoromethyl (TFM) solubilising groups onto the benzotriptycene unit was evaluated by the synthesis of PIM-TFMBTrip and PIM-DTFM-BTrip, respectively. Each polymer was prepared from its tetrahydroxy benzotriptycene monomer (1a–f) using the well-established benzodioxinforming polymerisation reaction devised for PIM synthesis (Fig. 1). 44 Monomers were prepared by adaptation of the classic benzotriptycene synthesis, involving the Diels–Alder reaction between 2,3,6,7-tetramethoxy-9,10-dimethylanthracene and the appropriate 1,4-dihydro-1,4-epoxynaphthalene 39 – with the latter prepared from the Diels–Alder reaction between the appropriate benzyne intermediate and furan. 45–47 PIM-TMN-Trip and PIM-HMI-Trip are both soluble in chloroform, facilitating analysis using Gel Permeation Chromatography (GPC) that confirmed that high molecular mass polymer was achieved for both polymers (Table 1). In contrast, PIM-DM-Btrip, PIM-TFM-BTrip and PIM-DTFM-BTrip proved soluble only in quinoline. The success of this high-boiling aromatic solvent for dissolving these otherwise intractable polymers prompted a re-investigation of the solubility of unsubstituted PIM-BTrip, which we had previously described as insoluble. 39 Pleasingly, this polymer also proved soluble in quinoline. Although quinoline is not an appropriate solvent for GPC analysis, solutions of PIM-DM-BTrip, PIM-TFM-BTrip, PIM-DTFM-BTrip and PIMBTrip could be used to cast mechanically flexible and robust films, implying that a reasonably high molecular mass had been achieved during the synthesis. Synthetic and structural characterisation details, including solid state NMR (Fig. S1) are given in the ESI.† Gas adsorption and gas transport properties. In their powder form, all benzotriptycene-based PIMs adsorb a large amount of nitrogen (N 2 , 77 K) at low relative pressure. Analysis of the N 2 adsorption isotherms (Fig. S1, ESI†) gives apparent Brunauer–Emmett–Teller (BET) surface areas (SA BET ) within the range of 848–1034 m 2 g 1 (Table 1), which are amongst the highest obtained from solution processable polymers. 29,39 The shapes of the N 2 isotherms are similar for all polymers except for PIM-TMN-Trip and PIM-DTFM-BTrip, for which there is larger uptake at higher pressures associated with a large hysteresis between the adsorption and desorption isotherms. This might be related to the TMN and CF 3 substituents protruding out of the 2D plane of the polymer chain and thus interfering with the electrostatic nitrile–nitrile interactions which are likely to dominate polymer cohesion. Adsorption of CO 2 at 273 K (Fig. S2, ESI†) shows similar uptakes for the benzotriptycene-PIMs (2.5–3.3 mmol g 1 ). The uptake for PIM-BTrip is slightly higher at lower pressures, which may be ascribed to a greater concentration of ultramicropores (diameter o0.7 nm in its pore size distribution (Fig. S3, ESI†)). Solvent cast films (Fig. S4, ESI†) of the benzotriptycenebased PIMs all demonstrate exceptionally high gas permeability (Table 2). However, the evaluation of gas permeability data for a new polymer requires careful consideration of its film history and thickness as these factors influence greatly the observed values. 32 Generally, the highest reported values of gas permeability for high free volume polymers such as the PTMSP and PIMs were obtained from films freshly treated with methanol (or ethanol), which removes any residual casting solvent but also induces additional free volume. 31,48 The values of gas permeability from freshly methanol treated thick films (135–176 mm) of the benzotriptycene PIMs are some of highest reported for a pure polymer film (e.g.,P CO 2 =21–5310 3 Barrer) and are comparable to those from ethanol treated ultrapermeable polyacetylenes (e.g.,P CO 2 = 28–47 10 3 Barrer). 19,42 For each of the methanol treated films the order of decreasing gas permeability is CO 2 4H 2 4O 2 4He 4CH 4 4N 2 with the exception of those from the less permeable and more sizeselective PIM-BTrip for which He permeates faster than O 2 . The ideal selectivities of all of the methanol treated films are significantly higher than those obtained for the ultrapermeable polyacetylenes and fall in the range of those reported for methanol treated films of less permeable PIMs such as PIM-1 (e.g.,P O 2 /P N 2 = 2.6–3.6). 48 As noted for all PIMs and highly permeable polymers, 31,32,49–51 the extremely high values of gas permeability measured initially from the freshly methanol treated films are not maintained on ageing. 52 However, the reduction in permeability is accompanied by an increase in ideal selectivity for all gas pairs. In addition, on ageing, He permeability surpasses the value of O 2 for all the polymers, indicating enhanced size selectivity. Comparing data from approximately like-for-like samples (i.e. B120 day aged and 110–180 mm thick films) the order Table 1 Yield, molecular mass and gas adsorption properties of the benzotriptycene-based PIMs Polymer Yield (%) Solubility M n (g mol 1 )M w /M n Z a (cm 3 g 1 ) SA BETb (m 2 g 1 ) V Totalc (ml g 1 ) V Md (ml g 1 ) CO 2 uptake e (mmol g 1 ) PIM-TMN-Trip 67 CHCl 3 52 300 f 3.8 74 1034 0.87 0.38 3.3 PIM-HMI-Trip 58 CHCl 3 61 300 f 2.4 58 1033 0.71 0.38 3.0 PIM-BTrip 78 Quinoline — g — g 66 911 0.63 0.33 3.2 PIM-DM-BTrip 82 Quinoline — g — g 72 920 0.72 0.33 3.0 PIM-TFM-BTrip 79 Quinoline — g — g 37 848 0.66 0.31 2.5 PIM-DTFM-BTrip 84 Quinoline — g — g 65 964 1.02 0.33 2.5 a Inherent viscosity in quinoline at 25 1C. b BET surface area calculated from N 2 adsorption isotherm obtained at 77 K. c Total pore volume estimated from N 2 uptake at P/P o = 0.98. d Micropore volume estimated from N 2 uptake at P/P o = 0.05. e CO 2 adsorption at 1 bar and 273 K. f Relative to polystyrene standards. g Not measured due to insolubility in solvents compatible with GPC analysis. Energy & Environmental Science Paper Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2736 |Energy Environ. Sci., 2019, 12, 2733--2740 This journal is ©The Royal Society of Chemistry 2019 of decreasing permeability and increasing selectivity for the benzotriptycene PIMs is PIM-TMN-Trip 4PIM-DTFM-BTrip 4 PIM-HMI-Trip 4PIM-TFM-Trip 4PIM-BTrip EPIM-DMBTrip. It can be deduced that the bulky TMN and HMI substituents both enhance permeability greatly, with the more rigid HMI substituent providing slightly higher selectivity over TMN. The relatively small –CF 3 substituents of PIM-TFM-BTrip and PIM-DTFM-BTrip also enhance permeability relative to unsubstituted PIM-BTrip. Interestingly, the –CF 3 substituents appear to slow ageing, with 54% of the value for P O 2 of the methanol treated film of PIM-DTFM-BTrip retained after one year, and 56% for PIM-TFM-BTrip, as compared to only 30–36% for films without –CF 3 substituents. Depending on the gas, the standard deviation of the permeability is in the range 4–18% for the freshly MeOH treated PIM-HMI-Trip and PIM-DM-BTrip films, and 3–6% for the aged PIM-BTrip film. These are small compared to the effect of the ageing in this work, and almost negligible when represented on the double-logarithmic Robeson diagrams (Fig. S6, ESI†). A thinner film of PIM-BTrip (64 mm) demonstrates lower initial permeability after methanol treatment, consistent with the well-established trend that thinner films age more rapidly than thicker films. 32,52,53 It is also more size selective than the thicker film of the same polymer with H 2 4CO 2 4He 4O 2 4 CH 4 4N 2 the order of decreasing gas permeability. Due to the commonly encountered variability of gas permeability from differing film thicknesses and history, data for a new polymer are best compared to those of existing polymers by using Robeson plots (Fig. 2). As noted, the position of the data from a new polymer relative to the Robeson upper bounds provides a useful indicator of its potential performance as gas separation membranes. All data points for the benzotriptycene polymers lie far above the 2008 upper bounds for O 2 /N 2 (Fig. 2a), H 2 /N 2 (Fig. 2b), H 2 /CH 4 ,CO 2 /N 2 (Fig. 2c) and CO 2 /CH 4 (Fig. 2d). Data for the B1 year aged films for all of the polymers lie close to the proposed 2015 upper bound for O 2 /N 2 . In particular, aged PIM-BTrip demonstrates exceptional selectivity for a highly permeable polymer so that its data lie well above the proposed Table 2 Thickness (l, mm), ideal gas permeabilities (Px, Barrer) and selectivities of freshly methanol treated and aged films measured at 25 1C and 1 bar of feed pressure PIMa l b P N2 P O2 P CO2 P CH4 P H2 P He P O2 /P N2 P H2 /P N2 P CO2 /P N2 P CO2 /P CH4 BTrip 160 1190 4330 21 500 1690 12 100 4540 3.64 10.2 18.1 12.7 (130) c,d 160 522 2570 13 200 570 8440 3110 4.92 16.2 25.3 23.2 (253) c,d 160 401 2170 10 700 411 8930 3400 5.41 22.3 26.7 26.0 (365) c,d 160 280 1580 8020 282 7160 2810 5.65 25.6 28.6 28.4 (490) c,d 160 195 1240 6060 203 6380 2650 6.34 32.6 31.0 29.9 (633) c,d 160 127 935 4350 130 5100 2180 7.36 40.1 34.2 33.5 (718) e,g 160 112 (4) 838 (48) 3770 (166) 113 (4) 4820 (186) 2150 (64) 7.51 (0.19) 43.2 (0.53) 33.8 (0.53) 33.5 (0.33) BTrip d 64 339 1800 9200 412 9430 3960 5.31 27.8 27.1 22.3 (120) 64 200 1160 6040 237 7180 3020 5.79 35.8 30.2 25.5 (253) d 64 190 1143 5990 225 8080 3490 6.01 42.5 31.5 26.6 (371) c,d 64 154 997 5150 163 7730 3620 6.47 50.2 33.4 31.6 TMN-Trip 166 3540 10 400 52 800 7250 18 800 6490 2.94 5.31 14.9 7.28 (120) 166 1970 6620 33 300 3130 15 300 5600 3.36 7.77 16.9 10.6 (253) 166 1470 5440 25 900 2030 14 100 5190 3.71 9.59 17.6 12.8 (358) 166 1289 5082 23 648 1751 14 118 5290 3.94 11.0 18.4 13.5 (426) 166 1100 4620 20 400 1440 14 100 5420 4.20 12.8 18.5 14.2 HMI-Trip d 135 2560 8540 44 200 4870 16 600 5700 3.34 6.48 17.3 9.08 (1) f,g 135 2120 (330) 7380 (989) 39 000 (3680) 3990 (708) 18 400 (1765) 6500 (762) 3.49 (0.14) 8.95 (2.16) 18.6 (1.7) 9.94 (1.44) (120) 135 1440 5180 26 900 2150 11 800 4240 3.60 8.19 18.7 12.5 (253) 135 972 3930 18 900 1220 10 700 3960 4.04 11.0 19.5 15.6 (358) 135 907 3760 17 404 1083 11 141 4245 4.15 12.3 19.2 16.1 (426) 135 804 3580 16 400 967 11 000 4150 4.45 13.7 20.4 16.9 TFM-BTrip c,d 176 1830 6210 33 700 2280 13 600 5150 3.39 7.43 18.4 14.8 (123) c 176 1090 4230 22 100 1250 10 700 4120 3.88 9.82 20.3 17.7 (255) c 176 875 3640 18 400 953 9870 4050 4.15 11.3 21.0 19.3 (367) c 176 791 3450 17 000 873 10 100 4170 4.36 12.7 21.5 19.5 (496) 176 722 3260 15 600 792 9760 3920 4.51 13.5 21.6 19.7 DTFM-BTrip 112 3000 7770 42 600 4340 14 700 5860 2.59 4.90 14.2 9.82 (119) 112 1800 5410 29 000 2150 11 300 4690 3.01 6.28 16.1 13.5 (366) 112 1300 4460 22 900 1390 10 700 4590 3.41 8.23 17.5 16.4 (490) 112 864 3490 16 900 890 10 400 4770 4.04 12.1 19.6 19.0 (636) 112 741 3170 14 800 728 10 200 4730 4.27 13.8 20.0 20.3 DM-BTrip d,f 114 1020 (133) 3950 (374) 22 000 (1071) 1570 (85) 11 400 (482) 4000 (354) 3.90 (0.16) 11.3 (1.07) 21.8 (2.5) 14.0 (1.5) (128) d 114 521 2640 12 200 599 9870 3650 5.07 18.9 23.4 20.4 a Number in parentheses is the ageing time in days after methanol treatment. b Thickness did not exhibit significant changes upon ageing. c Data defining the proposed CO 2 /CH 4 upper bound. d Data defining the proposed CO 2 /N 2 upper bound. e Average and standard deviation (in parentheses) of four independent measurements of the same aged sample. f Average and standard deviation (in parentheses) of four independent samples. g Data not included on Robeson plots (Fig. 2). Paper Energy & Environmental Science Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is ©The Royal Society of Chemistry 2019 Energy Environ. Sci., 2019, 12, 2733--2740 | 2737 2015 upper bounds for O 2 /N 2 (Fig. 2a), H 2 /N 2 (Fig. 2b), and H 2 /CH 4 . A notable feature of the permeability data from aged samples of the benzotriptycene-PIMs on the O 2 /N 2 and H 2 /N 2 Robeson plots is the near linear correlation at a steeper slope than that of the upper bounds (Fig. S5, ESI†). This reflects the far larger reduction of permeabilities on ageing for gases composed of larger molecules such as N 2 and CH 4 as compared to those composed of the smaller O 2 and H 2 molecules. Gas transport through a polymer is described by the solutiondiffusion model 54 with P x =D x S x ,whereD x is the diffusivity coefficient (Table S2, ESI†) and S x is the solubility coefficient for gas x (Table S3, ESI†). Therefore, the ideal selectivity (P x /P y ) for a polymer comes from a combination of diffusivity selectivity (D x /D y ) and solubility selectivity (S x /S y ). The remarkable positions of the data for the benzotriptycene-PIMs on the H 2 /N 2 ,andO 2 /N 2 Robeson plots are due to very high diffusivity selectivity originating from the size-sieving behaviour of the polymers, which differentiates between gas molecules of differing effective diameters (d x ). 40 This is best illustrated by the correlation between d x2 and the diffusivity coefficient (D x ), 55 which is steepest for PIM-BTrip and less steep for benzotriptycene PIMs that possess a substituent, although the absolute value of the diffusion coefficient is larger (Fig. 3). Ageing decreases the diffusion coefficient for all polymers but steepens the correlation between d x2 and D x , especially for PIM-BTrip, which is evidence of its further enhanced size selectivity (Fig. S7, ESI†). 40 The extraordinary performance of PIM-BTrip can be attributed to its ultramicroporosity, which facilitates the diffusivity of small Fig. 2 Robeson plots for the (a) O 2 /N 2 ,(b)H 2 /N 2 , (c) CO 2 /N 2 and (d) CO 2 /CH 4 gas pairs showing the position of the gas permeability data for films of PIM-BTrip ( ), PIM-TMN-Trip ( ), PIM-HMI-Trip ( ), PIM-DM-BTrip ( ), PIM-TFM-BTrip (K) and PIM-DTFM-BTrip ( ). Previously reported data are also shown for non-PIM polymers (&) and PIMs ( ). Upper bounds are represented by black lines (1991), blue lines (2008), and red lines for the previously proposed (2015) upper bounds for O 2 /N 2 and H 2 /N 2 . The proposed revised upper bounds for CO 2 /N 2 and CO 2 /CH 4 are shown as dotted red lines. Energy & Environmental Science Paper Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 2738 |Energy Environ. Sci., 2019, 12, 2733--2740 This journal is ©The Royal Society of Chemistry 2019 gas molecules, together with very high chain rigidity, 16,54 which hinders the activated transport of larger gas molecules by reducing thermal motions that allow gaps to form between voids. The extreme rigidity of PIM-BTrip accounts for the very high activation energy for the diffusion of larger gases such as N 2 and CH 4 . 40 The gas transport properties of PIM-BTrip appears similar to those reported for the two triptycene-derived polymers, PIM-Trip-TB 35 and TPIM-1, 33 which were used to define the proposed 2015 upper bounds for O 2 /N 2 ,H 2 /N 2 and H 2 /CH 4 . 38 It should be noted that the data from PIM-Trip-TB used to define the 2015 upper bounds were taken from a film that was aged for only 100 days after methanol treatment. 35 Recent remeasurement of the gas permeability of this film after 1900 days gives data that are also well over the proposed 2015 upper bounds for O 2 /N 2 (i.e. P O 2 = 532 Barrer; P O 2 /P N 2 =8.2)andH 2 /N 2 (i.e. P H 2 = 4430 Barrer; P H 2 /P N 2 = 65). Therefore, the design concepts used to obtain the extraordinary size selectivity demonstrated by PIM-BTrip and PIM-Trip-TB are likely to provide PIMs that will provoke future significant revisions of the O 2 /N 2 ,H 2 /N 2 and H 2 /CH 4 Robeson upper bounds. Redefining the CO 2 /N 2 and CO 2 /CH 4 upper bounds Separations involving CO 2 are mechanistically more complex than those governed predominately by diffusivity selectivity (e.g. O 2 /N 2 or H 2 /N 2 )becauseS CO 2 dominates transport, especially for CO 2 /N 2 due to the similar effective diameters of the two gas molecules. Typically for PIMs, values for S CO 2 /S N 2 lie in the range 15–20 whereas those for D CO 2 /D N 2 lie between 0.9–1.5 and these values are similar for PIMs with both higher and lower P CO 2 permeability. In general, solubility selectivity tends to remain fairly constant during ageing, in contrast to the increases observed for ideal selectivity values for transport dominated by diffusivity selectivity. 52 Thus, plotting data for previously reported PIMs on the Robeson plot for CO 2 /N 2 shows many data points slightly above the 2008 upper bound at higher permeability (P CO 2 43000 Barrer) but few at lower values of permeability. Indeed, very few highly permeable polymers possess a CO 2 /N 2 selectivity 430, 56–59 which is the lower limit of interest for a first-pass polymer membrane for post-combustion carbon capture (Table S1, ESI†). 12 Although all of the data for the benzotriptycene PIMs are above the 2008 upper bound for CO 2 /N 2 , the data from PIM-BTrip are particularly promising with both thick and thinner aged films providing P CO 2 44000 Barrer and P CO 2 /P N 2 430. The impressive performance of PIM-BTrip appears to be due to an unusually high D CO 2 /D N 2 of 2.0, whereas that of the substituted members of the series relies on greater S CO 2 /S N 2 resulting from the greater number of CO 2 adsorption sites provided by the larger amount of intrinsic microporosity (Table S3, ESI†).Theelevendatapointsonthe Robesonplotfromfourdifferentpolymersthatfallintoalinear correlation parallel to that of the 2008 upper bound allows us to propose a substantially improved new upper bound for CO 2 /N 2 (Fig. 2c and Tables 2 and 3). These data points are distributed over alargeP CO 2 range of 4400–52 000 Barrer. In addition, the data for all of the benzotriptycene PIMs lie well above the 2008 upper bound for CO 2 /CH 4 at a higher selectivity than those of previously reported polymers. Indeed, only data for the highly rigid ‘‘intermolecularly-locked’’ derivative of PIM-1 (PIM-C1) 60 and PIM-SBF-2 43 come close to those of the benzotriptycene PIMs (Table S1, ESI†). This exceptional performance appears due to a combination of both high diffusivity selectivity, with D CO 2 /D CH 4 in the range 5.7–9.5 for aged films, and good solubility selectivity (S CO 2 /S CH 4 43). Ten data points from two different polymers allows us to propose a new upper bound for CO 2 /CH 4 parallel to that of 2008 (Fig. 2d and Tables 2 and 3). The benzotriptycene PIMs that either define or provide data that are very close to this revised upper bound are either unsubstituted (PIM-BTrip) or possess only small substituents (i.e. PIM-DM-BTrip; PIM-TFM-BTrip and PIM-DTFM-BTrip). In contrast, those possessing larger cyclic solubilising groups (i.e. PIM-TMN-Trip and PIM-HMI-Trip) are slightly less selective. When defining his 2008 CO 2 /CH 4 upper bound, Robeson noted that data for a series of Thermally Rearranged (TR) Fig. 3 Plot of diffusivity coefficient (D x )versus d x2 (where d x = effective diameter of gas molecule x: He = 1.78; H 2 = 2.14; O 2 = 2.89; CO 2 = 3.02; N 2 =3.04;CH 4 = 3.18 Å) 55 for freshly methanol treated films of PIM-BTrip (), PIM-TMN-Trip ( ), PIM-HMI-Trip ( ), PIM-DM-BTrip ( ). Data for PIM-TFM-BTrip and PIM-DTFM-BTrip are not shown for clarity but are very similar to those for PIM-TMN-Trip and PIM-HMI-Trip, respectively. Table 3 Fitting parameters for the 2008 and proposed CO 2 /N 2 and CO 2 / CH 4 upper bounds using the formula P x =ka xy n (where P x is permeability (Barrer) of the most permeable x-gas, kis the front factor (Barrer), a xy is the selectivity for x/y gas pair, and nis the slope) k(Barrer) n Robeson 2008 upper bounds 24 CO 2 /CH 4 5.369 10 6 2.636 CO 2 /N 2 30.967 10 6 2.888 Proposed upper bounds CO 2 /CH 4 22.584 10 6 2.401 CO 2 /N 2 755.58 10 6 3.409 Paper Energy & Environmental Science Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is ©The Royal Society of Chemistry 2019 Energy Environ. Sci., 2019, 12, 2733--2740 | 2739 polymers, reported by Park et al., 15,61 ‘‘with exceptional CO 2 /CH 4 separation capabilities’’, 24 appeared to form an upper bound above that proposed for solution processable polymers. Such insoluble network polymers as the TR polymers often perform above the 2008 upper bounds defined for solution processable polymers due to their rigidity approaching that of carbon molecular sieves (i.e. polymers carbonised at high temperatures). Remarkably, the CO 2 /CH 4 upper bound defined by the solution processable benzotriptycene-based PIMs lies at the same position as that of Robeson’s tentatively proposed TR polymer upper bound with a selectivity 2.5 times higher than that for the 2008 upper bound. Conclusions The benzotriptycene-based PIMs provide exceptional gas permeability data for most important gas pairs and allow for the redefinition of the CO 2 /CH 4 and CO 2 /N 2 Robeson upper bounds. This is important in order to set aspirational targets for chemists in the design and synthesis of novel polymers. 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Paper Energy & Environmental Science Open Access Article. Published on 23 July 2019. Downloaded on 1/26/2024 10:26:39 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online