Post-synthetic ﬂ uorination of Scholl-coupled microporous polymers for increased CO 2 uptake and selectivity †

We report a facile, one-step post-synthetic ﬂ uorination method to increase the CO 2 capacity and CO 2 /N 2 selectivity of porous organic Scholl-coupled polymers. All of the ﬂ uorinated polymers that we synthesised showed increases in CO 2 /N 2 IAST selectivity and CO 2 isosteric heat; almost all materials also showed an increase in absolute CO 2 uptake. Our best-performing material (SC-TPB F) demonstrated a CO 2 capacity and CO 2 /N 2 selectivity of 3.0 mmol g (cid:1) 1 and 26 : 1, respectively, at 298 K — much higher than the corresponding non- ﬂ uorinated polymer, SC-TPB. This methodology might also be applicable to other polymer classes, such as polymers of intrinsic microporosity, thus providing a more general route to improvements in CO 2 capacity and selectivity.


Introduction
Carbon dioxide (CO 2 ) capture and storage from xed point emission sources is one strategy for reducing CO 2 emissions and, hence, the rate of global warming. 1 The current state-ofthe-art technology uses aqueous amine solutions to chemically strip CO 2 from ue gas streams. 1,2 However, the costly nature of regenerating the amine solutions, their negative environmental impact, and their corrosive nature limit their use on a large scale. 3,4 Recently, physisorptive porous solids have surfaced as promising candidates to replace aqueous amines. [5][6][7] Porous network materials can be divided into the two subclasses: amorphous materials and crystalline materials. Crystalline porous materials include metal-organic frameworks (MOFs) 8,9 and covalent organic frameworks (COFs), [10][11][12] while amorphous porous materials include hypercrosslinked polymers (HCPs), [13][14][15] conjugated microporous polymers (CMPs), [16][17][18] and porous aromatic frameworks (PAFs). [19][20][21] Since cost, scalability, and stability are important factors for the commercial application of porous adsorbents for carbon capture applications, we opted to focus on HCPs, and in particular a sub-class of Scholl-coupled HCPs. [22][23][24] Scholl-coupled polymers meet several of the necessary criteria for post-combustion CO 2 capture from ue gas such as moderate cost, availability of starting materials, scalability, and physicochemical stability. They can also possess good levels of porosity. However, for practical applications, an improvement in working CO 2 capacity at higher temperatures coupled with increase in CO 2 /N 2 selectivity would be benecial. 3,25 Our aim here, therefore, was to explore chemical methods to tune such properties in Schollcoupled HCPs.
Scholl-coupled HCPs are synthesised from electron-rich aromatic building blocks using a stoichiometric amount of a Lewis acid catalyst, here aluminium chloride (AlCl 3 ). Schollcoupling differs from the hypercrosslinking approach in that it does not require the use of external cross-linkers such as formaldehyde dimethyl acetal (FDA) 15 or the use of monomers with activated methylene groups, 26 both of which result in the formation of methylene bridges between the aromatic rings. [13][14][15] Instead, Scholl-coupling primarily affords networks that contain direct aryl-aryl bonds, although the incorporation of methylene bridges between the aromatic rings has also been observed due to reactions with the reaction solvent (dichloromethane, DCM). 23 The use of DCM instead of chloroform as the reaction solvent has been reported to afford networks with up to twice the Brunauer-Emmett-Teller surface area (SA BET ). 23 Unlike other classes of organic polymers, such as CMPs and subsequent PAFs, Scholl-coupled polymers do not require the use of expensive monomers or transition metal catalysts for their synthesis. 22 The reaction time is also usually short, which is not always the case for benzimidazole-and azo-linked polymers. [27][28][29][30] In general, benzimidazole-and azo-linked polymers exhibit high SA BET and good CO 2 storage capacities and selectivities due to the presence of CO 2 -polarising groups. The absence of such functionality in the Scholl-coupled polymers results in lower CO 2 /N 2 selectivities, despite their good CO 2 capacities. [31][32][33][34] Adsorbent selectivity for CO 2 over N 2 is crucial because N 2 makes up nearly 75% of the ue gas stream compared to $15% for CO 2 . 6 Different strategies have been attempted to increase CO 2 /N 2 selectivity by incorporating CO 2polarizing groups such as amines into polymers. 31 In the case of Scholl-coupling, the sensitivity of some of the electron rich monomers such as pyrrole, thiophene, aniline, and uoroaniline to AlCl 3 or the HCl liberated in the reaction can prevent their polymerisation; 22 indeed, attempts in our laboratory to couple these monomers using standard conditions all failed. Monomers with electron withdrawing groups have been reported previously to hinder the crosslinking reaction in the formation of HCPs. [35][36][37] A different approach to enhance CO 2 /N 2 selectivity is post-synthetic modication. For example, aromatic-containing polymers can be derivatised with sulfonic acid or nitro groups, which can then be reacted further to form ammonium salts 33,38,39 or reduced to form amines, respectively; 40 both strategies can result in improved CO 2 uptake and CO 2 /N 2 selectivity.
Seeking a facile new method to increase CO 2 uptake and CO 2 / N 2 selectivity, we chose to explore electrophilic uorination as a route to incorporate CO 2 -polarizing groups into a pre-formed porous polymer. To the best of our knowledge, post-synthetic modication of organic polymers such as HCPs and Schollcoupled polymers via the incorporation of uorine has not been reported before. 1-Chloromethyl-4-uoro-1,4-diazoniabicyclo [2.2.2]octane bis(tetrauoroborate), hereaer referred to as Selectuor®, has been used previously to uorinate small molecules in the presence of triuoromethanesulfonic acid (triic acid) and DCM. 41 We chose to apply this reagent to the uorination of a range Scholl-coupled polymers. We then assessed the impact of the presence of uorine in the polymer gas uptakes, CO 2 /N 2 selectivities, and isosteric heats of adsorption for CO 2 and CH 4 .

Materials and methods
Fluoranthene, triphenylene, and 1,1 0 -binaphthyl were obtained from TCI chemicals, UK. Anhydrous aluminum chloride was obtained from Alfa-Aesar, UK. All other reagents were ordered from Sigma-Aldrich and used as received.
Synthesis of SC-uorobenzene. Under a nitrogen atmosphere, anhydrous AlCl 3 (3.3 g, 25 mmol) was added to a stirred reuxing solution of uorobenzene (0.47 mL, 5 mmol) in DCM (30 mL) and the mixture was heated under reux overnight. The suspension was ltered and washed thoroughly with ethanol and water until the ltrate was clear. The solid was then stirred under reux in chloroform, methanol, ethanol, tetrahydrofuran, and acetone for 6 hours each. The powder was then collected by ltration and dried for 24 hours at 60 C under vacuum to afford SC-uorobenzene (yield ¼ 0.57 g).
General procedure for Scholl-coupled network synthesis. The exact procedure for the synthesis of each polymer can be found in the ESI. † In general, under a nitrogen atmosphere, AlCl 3 was added to a reuxing solution of dissolved monomer in DCM and the mixture was heated under reux overnight with stirring. The suspension was collected by ltration and washed thoroughly with ethanol and water until the ltrate was clear. The solid was then stirred under reux in chloroform, methanol, ethanol, tetrahydrofuran, and acetone for 6 hours each. The powder was then collected by ltration and dried for 24 hours at 60 C under vacuum to produce the Scholl-coupled network.
General procedure for uorination. In a glove box under a nitrogen atmosphere, Selectuor® (1.0 g, 2.8 mmol) and a Scholl-coupled polymer (200 mg) were charged to a round bottom ask. The ask was sealed and transferred to a fumehood where anhydrous DCM (20 mL) was added under a ow of nitrogen. The mixture was stirred for 30 minutes then tri-uoromethanesulfonic acid (6 mL) was added dropwise. The reaction mixture was then heated at 40 C for 5 days. Aer cooling to ambient temperature, the reaction was poured into ice-water, ltered, and washed thoroughly with 5% sodium hydrogen carbonate solution until the pH of the ltrate was no longer acidic. The lter cake was thoroughly washed with water, DCM, and chloroform. The solid was then Soxhlet extracted with chloroform for 3 days before drying under vacuum overnight at 60 C to yield the corresponding product. (a) SC-TPB was used to yield SC-TPB F (yield ¼ 172 mg). (b) SC-triptycene was used to yield SC-triptycene F (yield ¼ 174 mg). (c) SCbiphenyl was used to yield SC-biphenyl F (yield ¼ 184 mg). (d) SC-binaphthyl was used to yield SC-binaphthyl F (yield ¼ 340 mg ‡). (e) SC-uoranthene was used to yield SC-uoranthene F (yield ¼ 392 mg ‡). (f) SC-naphthalene was used to yield SCnaphthalene F (yield ¼ 352 mg ‡). (g) SC-triphenylene was used to yield SC-triphenylene F (yield ¼ 360 mg ‡).

Characterization
Fourier transform infrared (FT-IR). FT-IR spectra for all Scholl-coupled polymers were collected on a Bruker Tensor 27 using KBr disks.
Elemental analysis. CH elemental analysis was carried out using a Thermo FlashEA 1112 Elemental Analyser.
Fluorine content analysis. The analysis of uorine was performed by Exeter Analytical, UK. All polymers were combusted under oxygen followed by the use of ion selective electrode to determine uorine content as a wt%.
Solid-state nuclear magnetic resonance (SS-NMR). 13 C and 19 F SS-NMR of all networks were acquired by the University of Durham, UK.
Gas sorption. Nitrogen adsorption and desorption isotherms of all polymer analogues were collected at 77.3 K using an ASAP2420 volumetric adsorption analyser (Micrometrics Instrument Corporation). The SA BET was calculated in the relative pressure (P/P 0 ) range 0.05-0.25 and total pore volume (V Total ) was calculated at P/P 0 ¼ ca. 0.89-0.99.
The Horvath-Kawazoe method was used to determine the pore size distribution in the low pressure region assuming cylindrical pore geometry. 42 Carbon dioxide, methane, and nitrogen isotherms were collected up to a pressure of 1 bar on a Micromeritics ASAP2020 at 298 K for nitrogen, 273 and 298 K for methane, and 298, 318, and 328 K for carbon dioxide. All polymer analogues were degassed at 120 C for 15 hours under dynamic vacuum (10 À5 bar) prior to analysis.
Scanning electron microscopy (SEM). A Hitachi S 4800 cold eld emission scanning electron microscope (FE SEM) was used to collect high resolution imaging of the polymer morphology. The samples were loaded onto 15 mm Hitachi M4 aluminium stubs. Using an adhesive high purity carbon tab, the prepared HCP analogues were coated with gold nanolayer using an Emitech K550X automated sputter coater (25 mA for 2-3 minutes). Imaging was conducted using a mix of upper and lower secondary electron detectors at a working voltage of 3 kV and a working distance of 8 mm.
Thermogravemetric analysis (TGA). TGA was carried out in platinum pans using a Q5000IR analyser (TA instruments) with an automated vertical overhead thermobalance. The samples were heated at 20 C min À1 to 600 C under nitrogen followed by switching to air at 600 C or 1000 C in the case of SC-uorobenzene.

Results and discussion
We rst prepared a library of relatively cheap and easy-tosynthesize Scholl-coupled organic polymers that possessed moderate to high SA BET , pore volume, and CO 2 absorption capacity. The polymers were derived from the monomers uorobenzene, 1,3,5-triphenylbenzene, triptycene, biphenyl, 1,1 0binaphthyl, uoranthene, naphthalene, and triphenylene. All polymers were characterized using 13 C SS-NMR, FT-IR, and TGA. The data supported polymer formation and matched the analysis for the previously reported polymers. 23,24 We also evaluated the porosity of the materials (SA BET and pore volume) using nitrogen adsorption-desorption isotherms at 77.3 K, in addition to collecting CO 2 , CH 4 , and N 2 isotherms at 298 K for N 2 , 273 and 298 K for CH 4 and 298, 318, or 328 K for CO 2 . Despite the reasonably high CO 2 capacities for these polymers (2-3 mmol g À1 ) at 298 K and 1 bar, their lack of functionality imparts low isosteric heats at the zero-coverage region (24-27 kJ mol À1 ) and low CO 2 /N 2 selectivities. 27,[43][44][45] Aromatic electrophilic uorination of the polymers with Selectuor® allowed the incorporation of polar uorine atoms in the polymer with the aim of enhancing the gas uptake, selectivity, and gas affinity of the polymers without signicantly impacting the SA BET . The uorinated polymers were evaluated in a similar manner to the unfunctionalised polymers and, additionally, we used 19 F SS-NMR and uorine content assay to assess the degree of uorination. A Scholl-coupled polymer was also synthesised directly from uorobenzene (SC-uorobenzene) for use as a reference material for the post-synthetically uorinated Scholl-coupled polymers.

Analysis of SC-uorobenzene
SC-uorobenzene was synthesized by the addition of anhydrous AlCl 3 to a reuxing solution of uorobenzene in DCM. 13 C SS-NMR showed a major peak at 129.6 ppm and a minor peak 108.9 ppm corresponding to substituted and non-substituted aromatic carbons, respectively. A peak at 159.7 ppm represents the uorinated aromatic carbons, while the low frequency peaks below 30 ppm correspond to methylene cross-linking bridges from incorporation of the reaction solvent, DCM, into the polymer (Fig. S9, ESI †). 23 19 F SS-NMR conrmed the presence of uorine in the polymer with broad peak appearing at À117.4 ppm (Fig. S9, ESI †). The C-F vibrational band stretch was also evident in the FT-IR at $1250 cm À1 , while the broad peak at ca. 3400 cm À1 is likely due to trapped moisture within the network (Fig. S18, ESI †). Using oxygen ask combustion followed by ion selective electrode analysis, the loading of uorine in SC-uorobenzene was found to be 9.8%: that is, lower than the idealized theoretical uorine loading of 17.4% (Tables 1 and S1, ESI †). The discrepancy between the expected and measured uorine content may be due to the incorporation of methylene bridges into the polymer (major effect), entrapped Al salts, or removal of some of the uorine atoms during the network formation; for example, the cleavage of uorine was previously observed in the synthesis of the uorine-containing triazine framework (F-DCBP CTF-1). 45 The calculated uorine loading of 9.8% translates to roughly 0.6 uorine atoms per monomer unit. TGA analysis of SC-uorobenzene under a nitrogen atmosphere showed a loss of 2.0 wt% when heated up to 150 C, which might be due to entrapped moisture, catalyst and/or reaction solvent within the network (Fig. S10, ESI †). 31,46 The SA BET calculated from nitrogen isotherms at 77.3 K was found to be 451 m 2 g À1 with a low total pore volume of 0.29 cm 3 g À1 ( Table 1). The combination of a moderate SA BET and a low pore volume resulted in a moderate CO 2 uptake of 1.1 mmol g À1 at 1 bar and 298 K. However, the CO 2 isosteric heat of adsorption (Q st ) for the zero coverage region-calculated using Clausius-Clapeyron equation from three different temperature (298, 318, and 328 K)-was 28.5 kJ mol À1 (Fig. 1); 47 that is, similar to sulfonic acid-modied PPN-6, which had a high Q st of 30.4 kJ mol À1 . 38 CO 2 /N 2 IAST selectivity was calculated to be 20 : 1 from the single-component isotherms assuming a ratio of 15/85 CO 2 : N 2 at 1 bar and 298 K (Fig. S33, ESI †). We ascribed the high selectivity of CO 2 over N 2 is mostly to the polarity of the C-F bond, which affords a strong interaction with CO 2 . 48 Methane uptake at 1 bar and 298 K was found to be 0.35 mmol g À1 with a Q st of 26.5 kJ mol À1 calculated from measurements at two different temperatures (273 and 298 K) (Fig. 1).

Analysis of Scholl-coupled polymers
Scholl-coupling of the monomers was carried out using previously reported literature procedures (Scheme 1). 23,24 In total, seven Scholl-coupled polymers derived from 1,3,5-triphenylbenzene, triptycene, biphenyl, 1,1 0 -binaphthyl, uoranthene, naphthalene, and triphenylene were studied. FT-IR of all the Scholl-coupled polymers shows C]C vibrational stretches in the region of 1500-1600 cm À1 . The stretch at $3050 cm À1 can be assigned to C-H of the methylene bridges and the vibrational stretches in the region of 2850-3000 cm À1 are assigned to aromatic C-Hs. Most polymers show a strong water adsorption peak at $3400 cm À1 (Fig. S18-S32, ESI †) due to physisorbed water. 13 C SS-NMR of SC-TPB, SC-triptycene, SC-biphenyl, SCbinaphthyl, and SC-triphenylene showed one peak corresponding to substituted carbons in the range of 137-141 ppm and a non-substituted carbon peak in the range of 129-132 ppm (Fig. S9, ESI †). However, the 13 C SS-NMR for SC-naphthalene and SC-uoranthene showed only one peak in the aromatic region at 130.7 and 137.3 ppm, respectively (Fig. S9, ESI †). The 13 C SS-NMRs for SC-naphthalene and SC-uoranthene are consistent with the literature. 24 The additional carbon peaks below 40 ppm-observed for all the polymers-correspond to the methylene bridges, which are formed as a consequence of using DCM as a reaction solvent (Fig. S9, ESI †). 23,24 The discrepancies between the measured and calculated CH analysis of the Scholl-coupled polymers (Table S1, ESI †) is likely due to adsorption of atmospheric moisture, incorporation of methylene bridges into the polymer, or entrapped Al salts.
The obtained SA BET and pore volume of all the unfunctionalised Scholl-coupled networks were calculated from nitrogen adsorption-desorption isotherms at 77.3 K. The highest SA BET of 2535 m 2 g À1 was observed for SC-TPB, while the lowest SA BET of 1169 m 2 g À1 was recorded for SC-naphthalene ( Table 1). As for SA BET , the pore volume of SC-TPB was the highest recorded, with a value of 1.48 cm 3 g À1 , whereas the remaining networks were in the range 0.77 to 1.27 cm 3 g À1 ; the lowest again being SC-naphthalene (Table 1). SC-uoranthene showed the highest CO 2 uptake of 3.0 mmol g À1 at 1 bar and 298 K, 24 while the remaining networks varied from 2.0 to 2.6 mmol g À1 with the lowest being SC-naphthalene. As for, SC-uorobenzene, CO 2 /N 2 IAST selectivity was calculated from the single-component isotherms assuming a molar ratio of 15/85 CO 2 : N 2 at 1 bar and 298 K (Fig. S33-S47, ESI †). However, the absence of CO 2 polarizing groups in the unfunctionalised Scholl-coupled polymers resulted in lower CO 2 /N 2 selectivity than SC-uorobenzene (10 : 1-17 : 1), which shows a selectivity of 20 : 1. The combination of a low N 2 uptake and narrow micropores, which favour CO 2 uptake, 40,49 resulted in the highest selectivity of 17 : 1 being observed for SC-naphthalene; SC-TPB showed the lowest selectivity of 10 : 1. The reversibility of the CO 2 isotherms is in keeping with a physisorption mechanism, which is required for this class of materials to compete against amine solvents (Fig. 1). 3 The CO 2 Q st of the networks at the zero-coverage region, calculated form 3 different temperatures (298, 318, and 328 K), ranged from 24 to 27 kJ mol À1 . The highest Q st were observed for SC-triphenylene and SC-naphthalene with values of 27 and 26 kJ mol À1 , respectively (Fig. 1). CH 4 uptake at 298 K and 1 bar for the Scholl-coupled polymers was between 0.72 and 1.10 mmol g À1 ; SC-uoranthene had the highest uptake and SC-naphthalene the lowest ( Table 1). The incorporation of uorine atoms into the polymer has previously been reported to increase the interaction with methane. 50 The relatively high pore volume exhibited by these polymers has little impact on the methane uptake at 298 K and low pressures as it is only at high pressure that the benet of a high pore volume on methane uptake is observed. 51,52 While SC-uorobenzene has a high Q st of 26.5 kJ mol À1 , due to the presence of uorine atoms in the polymer, 50 the unfunctionalised Scholl-coupled networks had Q st for CH 4 below 21 kJ mol À1 (Fig. 1).

Analysis of the uorinated analogues
The lack of CO 2 polarizing groups such as nitrogen within the unfunctionalised Scholl-coupled networks results in a moderate CO 2 /N 2 selectivity values. 44 As previously discussed in the case of SC-uorobenzene, the presence of uorine in the network afforded enhanced CO 2 /N 2 selectivity and Q st , however, it suffers from a moderately low SA BET and pore volume (451 m 2 g À1 versus for instance 2535 m 2 g À1 for SC-TPB), which limits is uptake of CO 2 and CH 4 . As shown in Table 1, Scholl-coupled polymers with a higher SA BET and pore volume have a higher CO 2 and CH 4 capacity. We therefore chose to introduce polar uorine atoms into these high surface area polymers by postsynthetic modication. We opted to explore electrophilic uorination of the Scholl-coupled polymers 53-55 using Selectuor® and triuoromethanesulfonic acid (triic acid) in anhydrous DCM. 41 The proposed mechanism entails the formation of a protonated triuoromethanesulfonyl hypouoride species which then undergoes electrophilic aromatic substitution. 41 Aer reaction and purication, the 13 C SS-NMR of SC-TPB F shows the appearance of a peak at 183.1 ppm, which is consistent with the presence of a carbon-uorine bond (C-F). This was conrmed by 19 F SS-NMR, which shows a broad peak at À123.0 ppm (Fig. S9, ESI †). 45 The latter was observed in all the uorinated networks. However, networks that possess nonequivalent aromatic rings in the monomers, such as SC-uoranthene F and SC-binaphthyl F, also displayed additional peaks, which might correlate to multiple uorination sites within the polymer (Fig. S9, ESI †). 41,56,57 We also hypothesize that traces of triic acid and BF 4 À anions from Selectuor® might be trapped within some of the networks, due to the presence of peaks in the region of À78.5 and À150 ppm, respectively. 58,59 In the case of SC-uoranthene, SC-triptycene, and SC-triphenylene, the 13 C NMR did not show the expected C-F bond at 182-192 ppm. We attribute this to their low uorine loading (<1.1 wt%), which might be below the limit of detection for the analysis (Fig. S9, ESI †). The uorine content in each of the polymers was determined using oxygen ask combustion with ion selective electrode analysis. SC-TFB F was found to have the highest uorine loading of 2.96 wt%, which corresponds to $0.6 uorine atoms per monomer (Tables 1 and S1, ESI †). The remaining polymers vary in uorine content from 0.72 to 2.03 wt%, which corresponds to between 0.1 and 0.3 uorine atoms per monomer. The relatively low incorporation of uorine into the polymers could be due to poor accessibility of the reagents to the microporous interior of the polymer, low reactivity of the phenylene units, or a lack of reaction sites due to the presence of multiple Ar-Ar bonds in the polymer. FT-IR of the uorinated polymers showed the appearance of a peak in the region of 1250 cm À1 , which corresponds to the C-F stretch. In all cases, uorination of the Scholl-coupled polymers resulted in a decrease in SA BET and pore volume. The biggest decreases were from 2535 m 2 g À1 to 1446 m 2 g À1 and 1.48 cm 3 g À1 to 0.86 cm 3 g À1 for SC-TPB F and from 1842 m 2 g À1 to 1169 m 2 g À1 and 1.27 cm 3 g À1 to 0.82 cm 3 g À1 for SC-biphenyl F. The decrease in the specic surface area, SA BET , and pore volume correlates with uorine content and is most likely due to both the larger van der Waals radii of a uorine atom compared to a proton, and the higher molecular mass of uorine.
With the exception of SC-uoranthene and SC-triphenylene F, where almost no change in the CO 2 uptake was observed, uorination resulted in an increased CO 2 uptake at both 0.15 and 1.0 bar at 298 K for all polymers. SC-TPB F and SCnaphthalene F showed the largest increase in CO 2 uptake of 25 and 40%, respectively, with an increase from 2.4 to 3.0 mmol g À1 for SC-TFB F and from 2.0 to 2.8 mmol g À1 for SCnaphthalene F, at 1 bar and 298 K. Of greater signicance was the increase in CO 2 /N 2 selectivity for each of the uorinated polymers compared with the parent polymers. The biggest increase in CO 2 /N 2 selectivity was observed for SC-TPB F where the selectivity more than doubled from 10 : 1 for the parent polymer to 26 : 1. This increase is driven by an increase in CO 2 uptake in at low pressure, due to the more favourable CO 2polymer interactions caused by the uorine atoms, and a concomitant decrease in nitrogen uptake. 45,48,60 The CO 2 Q st of SC-TPB F at zero coverage was 27.3 kJ mol À1 which represents an increase of 15% compared to SC-TPB (Fig. 1). The same trend of increased CO 2 /N 2 selectivity and CO 2 Q st was observed for all uorine containing polymers, which highlights the positive inuence of uorine incorporation on CO 2 -network interactions (Fig. 1). It is clear from these data that post synthetic uorination gives rise to more promising absorbents for CO 2 /N 2 separation than direct coupling of uorinated monomers.
The presence of uorine within adsorbents has been reported in the literature to enhance the adsorbent interactions with CH 4 , perhaps due to C-F bond polarity. 50 Disappointingly, though, the general pattern of increased CO 2 uptake across the uorinated analogues was not observed with CH 4 . For instance, CH 4 uptake for SC-TPB F-the best performing polymer for CO 2 capture-decreased from 0.88 mmol g À1 for the parent polymer to 0.81 mmol g À1 at 1 bar and 298 K. SC-naphthalene F was the only polymer in the series that showed an increase in CH 4 uptake from 0.72 to 0.92 mmol g À1 at 298 K. Despite the decreased uptake of SC-TPB, its CH 4 Q st was calculated to be 24.5 kJ mol À1 , which represents an increase of 32% over SC-TPB. The latter Q st is higher than that of NOTT-108a metalorganic framework (MOF); reported as one of the leading MOFs for methane storage, which possesses a Q st in the zero-coverage region of 16.8 kJ mol À1 (calculated from methane isotherms at 273 and 298 K up to 65 bar). 50 SC-uorobenzene has a higher CH 4 Q st of 26.5 kJ mol À1 , but the uptake of CH 4 at 298 K and 1 bar was only 0.35 mmol g À1 , less than half that of SC-TPB F. From the remaining polymers, only SC-triptycene F and SCbiphenyl F showed a noteworthy increase of $2 kJ mol À1 to 19.2 and 20.1 kJ mol À1 , respectively. It is important to note however, that the full evaluation of the polymers for methane storage would require testing at pressures over 35 bar. 50

Conclusions
In conclusion, we have successfully incorporated uorine into a series of microporous Scholl-coupled polymers through postsynthetic modication. Different uorine loadings were achieved, the highest being observed for the uorinated network based on TPB (SC-TPB F). A trend of increased CO 2 uptake along with increased CO 2 /N 2 selectivity and Q st was observed for all uorinated Scholl-coupled polymers compared to the unfunctionalised polymers. In the case of our best-performing material (SC-TPB F), CO 2 capacity, CO 2 /N 2 selectivity, and CO 2 Q st increased from 2.4 mmol g À1 , 10 : 1, and 23.7 kJ mol À1 to 3.0 mmol g À1 , 26 : 1, and 27.3 kJ mol À1 , respectively, at 298 K. The same trend, however, was not achieved for CH 4 with the effect of uorine atoms on the Q st and CH 4 capacity being inconsistent across the polymer series. However, SC-TPB F has a CH 4 Q st of 24.5 kJ mol À1 that rivals that of leading MOFs for methane storage (NOTT-108a).