Post-synthesis modification of porous organic polymers with amine: a task-specific microenvironment for CO2 capture

A porous organic polymer named FC-POP was facilely synthesized with extraordinary porosity and excellent stability. Further covalent incorporation of various amines including single amine group, multi-amine groups of diethylenediamine (DETA), and poly-amine groups of polyethylenimine (PEI) to the network gave rise to task-specific modification of the microenvironments to make them more suitable for CO2 capture. As a result, significant boost of CO2 adsorption capacity of 4.5 mmol/g (for FC-POP–CH2DETA, 273 K, 1 bar) and the CO2/N2 selectivity of 736.1 (for FC-POP–CH2PEI) were observed after the post-synthesis amine modifications. Furthermore, these materials can be regenerated in elevated temperature under vacuum without apparent loss of CO2 adsorption capacity.


Introduction
The rapidly increasing concentration of atmospheric carbon dioxide generated from combustion of fuels (include coal, petroleum and natural gas) has aroused environmental concern, as a result, carbon capture and sequestration (CCS) gradually gain their growing popularity (D'Alessandro et al. 2010;Sumida et al. 2011;Goeppert et al. 2012;Lu et al. 2013;Wang and Xu 2014;Romanov et al. 2015). Chemical absorption by aqueous solutions of ethanolamines (Brennecke and Gurkan 2010), which is most widely used CCS technique, suffer from several fatal defects including solvent loss, corrosion, and tremendous energy cost for the regeneration (Service 2004). So more efforts have been devoted to solid porous adsorbent. Various categories of novel materials with extraordinary porosity have been discovered, including silicas (Suhendi et al. 2013), zeolites (Wakihara et al. 2010), metal-organic frameworks , zeolitic imidazolate frameworks (Cai et al. 2014) and carbon materials (Sevilla and Fuertes 2011). With the advantages of outstanding BET surface area and stability, tremendous porous organic polymers (POPs) have been designed (Jiang et al. 2009;Dawson et al. 2012;Zhao et al. 2012;Han et al. 2013;Liu et al. 2013;Zhu et al. 2013; Thompson et al. 2014;Zhu and Zhang 2014;Puthiaraj et al. 2015). More importantly, the organic skeletons of POPs give rise to their easy-to-functionalize nature so that enhancement of CO 2 /N 2 selectivity is able to be realized via the incorporation of CO 2 -philic groups into networks (Thomas 2010). Typically, PPNs (porous polymer networks) (Ben et al. 2009;Yuan et al. 2011) was synthesized with ultrahigh Brunner-Emmet-Teller (BET) surface area, the specific surface area of materials calculated by using adsorption theory developed by three scientists, namely Brunner, Emmet and Teller), some of which can be further incorporated by various groups, such as sulfonic acid, lithium sulfonate and ammonium sulfonate, to significantly improve the CO 2 uptake (3.6-3.7 mmol/g, 295 K, 1 bar) (Lu et al. , 2013. However, in spite of their impressive performance, these polymers are difficult to be used for industrial scale-up applications because of the harsh synthesis conditions. Alternatively, a facile and efficient method has been reported, known as ''knitting'' aromatic building blocks to a polymeric skeleton. A large number of heterocyclic rings were picked as monomers to afford crosslinking networks by applying the new technique (Li et al. 2011;Woodward et al. 2014;Zhu et al. 2014;Xu et al. 2015). However, their modest CO 2 uptakes need great improvement for the future CO 2 capture applications (Luo et al. 2012).
Herein, a single-step procedure was adopted to facilely realize ''knitting'' strategy to afford a porous organic polymer via Friedel-Crafts alkylation (FC-POPs). Triptycene, which may allow for a high degree of internal free volume, was chosen as monomer, in order to deliver FC-POPs with abundant mesopores. The mesopores of FC-POPs shows a facility for further modifications, to investigate the influence of pore structure and nitrogen-site density on CO 2 adsorption. To realize this fancy, various groups with nitrogen-site including amine-group, diethylenediamine (DETA) and polyethylenimine (PEI) were grafted into the network to produce task-specific microenvironments by modifying microporosity and nitrogen-site density (Scheme. 1). Accordingly, the CO 2 adsorption capacity and selectivity of CO 2 over N 2 were significantly improved after the post-synthesis modifications.

Materials and synthesis
Solvents, reagents and chemicals were purchased from Aldrich and TCI. All were used without any further purification.

Synthesis of FC-POP
Similar to Tan's method (Li et al. 2011), Triptycene (0.64 g, 2.5 mmol) and formaldehyde dimethyl acetal (1.14 g, 15.0 mmol) dissolving in 5 mL dichloroethane was added to a 25 mL flask. Anhydrous FeCl 3 (2.44 g, 15.0 mmol) was then added as a catalyst, and the mixture was stirred at 45°C for 5 h and then heated to 80°C for another 19 h. After cooling to room temperature, the mixture was filtered, washed by methanol for 3 times. The solid was collected, and extracted with methanol by Soxhlet apparatus for 24 h, brown solid was obtained after drying (yield *95%).

Synthesis of FC-POP-CH 2 Cl
A mixture of 0.30 g FC-POP, 1.50 g paraformaldehyde, 9.0 mL acetic acid, 4.5 mL phosphoric acid and 30.0 mL concentrated hydrochloric acid were charged in a flask, by Post-synthesis modification of porous organic polymers with amine: a task-specific… 51 following a well-known method (Lu et al. 2012). The flask was sealed, then the mixture was heated to 90°C and maintained for 3 days. After cooling to room temperature, the solid was collected and washed with water and methanol for three times, then dried to afford FC-POP-CH 2 Cl.

Synthesis of FC-POP-CH 2 NH 2
A mixture of FC-POP-CH 2 Cl (0.20 g), potassium phthalimide (1.19 g) and 20.0 mL N,N-dimethylformamide (DMF) was stirred at 100°C for 8 h under an argon atmosphere, similar to a well-known method (Ilhan et al. 1999). The cooled mixture was filtered and cursorily washed by DMF. The resulting phthalimide derivative was mixed with 1 mL of hydrazine monohydrate in 12 mL ethanol. The reaction was refluxed for 20 h. The reaction mixture was filtered and washed subsequently by DMF, water and MeOH, then dried to afford FC-POP-CH 2 NH 2 .

Synthesis of FC-POP-CH 2 DETA
A mixture of 0.10 g FC-POP-CH 2 Cl and 10.0 mL diethylenetriamine (DETA) was charged in a flask, according to a well-known method (Lu et al. 2012). The flask was sealed then heated to 90°C and maintained for 3 days. After cooling to room temperature, the solid was collected and washed with water and methanol for several times, then dried to afford FC-POP-CH 2 DETA.

Synthesis of FC-POP-CH 2 PEI
In a fashion similar to the synthesis of FC-POP-CH 2 DETA, 0.10 g FC-POP-CH 2 Cl and 10.0 g polyethylene imine (PEI, mw = 600) was reacted to afford FC-POP-CH 2 PEI.

Characteristics
Thermogravimetry analysis (TGA) were performed under N 2 on a NETZSCH STA449F3, with a heating rate of 10°C /min. 13 C NMR measurements were performed on a 9.4 T Bruker Avance spectrometer at a Larmor frequency of 100.6 MHz. Measurements were made with a 4 mm MAS probe spinning at 15 kHz. Chemical shifts were externally referenced to TMS (d = 0 ppm) using the methyl resonance of hexamethylbenzene (17.5 ppm relative to TMS). Nitrogen adsorption isotherms were measured at 77 K using Micromeritics ASAP 2020 static volumetric analyzer. Before adsorption measurements the polymer was degassed at 110°C under vacuum. The BET surface area was calculated within the relative pressure range 0.05-0.30. Total volume was calculated at p/p 0 = 0.98 and micropore volume was calculated by t-plot method. FTIR data were obtained using a Nicolet Magna-IR 550 spectrometer. Elemental analysis was determined using a Vario EL III Elemental Analyzer (Elementar, Germany).

Gas adsorption and desorption isotherms
Both of gas adsorption and desorption isotherms of POPs were measured using a Micromeritics ASAP 2020 static volumetric analyzer at the setting temperature. Prior to each adsorption experiment, the samples were degassed for 12 h at 110°C ensuring that the residual pressure fell below 0.2 Pa and then cooled down to the target temperatures, followed by introduction of a single component gas (CO 2 or N 2 ) into the system. Once the adsorption process finished, the desorption experiment was automatically initiated. In the desorption stage, gas pressure gradually decreased and the corresponding amount of residue adsorbed gas was measured and calculated by the instrument.

Fits of isotherms
For FC-POP and FC-POP-CH 2 Cl, single-site Langmuir fit is appropriate. Their isotherms can be described by the Eq. (1).
where, b is a parameter in the pure component Langmuir isotherm (Pa -1 ), p represents gas pressure (Pa), q is molar loading of gas components (mol/kg) and q sat is saturation capacity of gas components (mol/kg). For FC-POP-CH 2 NH 2 , FC-POP-CH 2 DETA and FC-POP-CH 2 PEI, both physical and chemical interaction should be taken into account so that dual-site Langmuir (Eq. 2) fitting is fine.
where two distinct adsorption sites are assumed to be existed, so there are four parameters in the equation, namely q sat,1 , b 1 , q sat,2 and b 2 .

Adsorption enthalpy
For CO 2 adsorption, fits of isotherms were used to calculate adsorption enthalpy by employing Clausius-Clapeyron equation (Eq. 3).
where p i is the pressure for isotherm i, T i is the temperature for isotherm i, R is 8.315 J/(K mol).

IAST calculation
Pure-component isotherm fitting parameters were used for calculating ideal adsorbed solution theory (IAST) binarygas adsorption selectivity (Myers and Prausnitz 1965), defined as the Eq. (4): where p i represents partial pressure of gas i and q i is the corresponding pure-component gas uptake amount at p i . The IAST calculations were carried out for flue gas model (binary mixture containing 15% CO 2 and 85% N 2 ).

Results and discussions
3.1 General characteristics FC-POP was synthesized in good yield via a simple Friedel-Crafts reaction. Triptycene was selected to be the monomer because of its rigid molecular structure, which would lead to a microporous skeleton with the extraordinary porosity and high stability. The resulting FC-POP was found to be insoluble in common organic solvents, such as water, methanol, dichloromethane, tetrahydrofuran, acetone and hexane. The chemical structure of FC-POP was confirmed by 13 C cross-polarization magic-angle spinning (CP/MAS) NMR (Fig. 1). In the NMR spectrum, the resonance peaks at 139 and 130 ppm correspond to the substituted aromatic carbons of triptycene, while the resonance peak at 120 correspond to their non-substitute counterparts. Resonance peaks at 46-50 belong to the bridgehead carbons of triptycene. The peaks around 32 ppm ascribes to the carbons of methylene linkers. Besides, high thermostability of FC-POP was proved by thermogravimetric analysis (TGA) that little apparent weight loss below 350°C (Fig. 2a).
In order to modify the microenvironment, FC-POP was covalently incorporated by various moieties to obtain a series of amine-functionalized FC-POPs (Scheme 1). FC-POP-CH 2 Cl, which containing vivacious chloromethyl groups, was first been prepared as an intermediate and chloromethyl groups were then substituted by various amines to afford FC-POP-CH 2 NH 2 , FC-POP-CH 2 DETA and FC-POP-CH 2 PEI. The peaks at 75-93 ppm in the solid-state NMR spectrum of FC-POP-CH 2 Cl suggested the graft of chloromethyl to the polymeric skeleton (Fig. 1). The appearance of peaks around 200 ppm for the spectrum of FC-POP-CH 2 Cl ascribed to the carbonyl group of residual acetic acid and paraformaldehyde that involved in the synthesis. Successful incorporation of functional amine groups were confirmed by Fourier Transform infrared spectroscopy (FTIR) that prosperous graft of amine groups can be proved by enhancement of the band around 1650 cm -1 (Fig. 2b). The amine-modified POPs also exhibit quite good thermal stability according to the thermogravimetric analysis results (Fig. 2a).
To quantitatively describe the density of functional groups, element analysis was conducted for the polymers. The calculated density of nitrogen element (mmol/g) shows a significant growth of nitrogen density in sequence of FC-POP-CH 2 NH 2 (3.46 mmol/g), FC-POP-CH 2 DETA (8.43 mmol/g) and FC-POP-CH 2 PEI (11.47 mmol/g) (Table 1). Therefore, the tailor-made functional group would make the nitrogen-site density controllable in the microenvironment of these POPs.
Porosity of these materials was measured by nitrogen adsorption at 77 K (Fig. 3a) and the surface areas were calculated by the BET model. FC-POP exhibits extraordinary porosity, with the BET surface area as high as 1540 m 2 /g. The adsorption/desorption isotherms for FC-POP is not closed, mainly because of a swelling of polymer matrix at 77 K . After the incorporation of functional groups, the BET surface area dropped correspondingly. The S BET data drops to 939 m 2 /g for FC-POP-CH 2 NH 2 , 636 m 2 /g for FC-POP-CH 2 DETA and only 129 m 2 /g for FC-POP-CH 2 PEI. For the adsorption isotherms of these FC-POPs, swift climb at low pressure (p/ p 0 \ 0.001) is contributed by micropores of these FC-POPs, while hysteresis at higher pressure proves the Post-synthesis modification of porous organic polymers with amine: a task-specific… 53 existence of mesopores. Pore size distribution caculated by non-local density functional theory (NLDFT) method also confirmed the presence of both micropores and mesopores (Fig. 3b). The mesopores, facilitating the mass transfer during the gas uptake process, can act as the transport channels (Liao et al. 2014). On the other hand, micropores, enhancing the interaction between the wall and CO 2 molecules, can act as the adsorption active points (Islamoglu et al. 2013). As a result, both of them are crucial to CO 2 adsorption applications. For FC-POP, V micro /V total value (the ratio of micropore volume to the total pore volume) is as low as 0.0842, demonstrating the existance of impressive mesopores. After grafting amines into the network, the microenvironment of pore structure has altered. For example, V micro value of FC-POP-CH 2 NH 2 (0.251 cm 3 /g) doubles that of FC-POP (0.113 cm 3 /g), and its V micro /V total value upsurges to 0.311. Accordingly, the   incorporation of functional groups may spatially separate mesopores into several micropores, result in the modified microenvironment of these FC-POPs through the redistribution of pore volumes, making the resulting POPs more suitable for CO 2 capture applications.

CO 2 adsorption performance
Excellent porosity and affluent nitrogen density are beneficial to boost CO 2 adsorption capacity for these aminemodified POPs. Indeed, significant growth of CO 2 uptake was observed after amine-grafting (Fig. 4). For FC-POP-CH 2 NH 2 and FC-POP-CH 2 DETA, under the pressure of 1 bar, CO 2 adsorption capacity reaches 4.2/2.5 and 4.5/ 3.4 mmol/g at 273/298 K, much higher than that of pristine FC-POP (3.1/1.8 mmol/g at 273/298 K). Notably, FC-POP-CH 2 PEI also exhibits impressive CO 2 storage ability (3.2/2.6 mmol/g at 273/298 K) in spite of its poor BET surface area. There are two dominant contributions for CO 2 adsorption capacity, the chemical adsorption contributed by amine groups (denoted as the nitrogen-site density) and the physical adsorption contributed by porosity (denoted as the BET surface area). For these FC-POPs with diverse incorporated amines, the hysteresis loops between their adsorption/desorption isotherms could be clear evidences of chemical interactions between amine groups and CO 2 molecules. However, in our work, the growth of the nitrogen site density can significantly increase the CO 2 adsorption enthalpy but decrease the BET surface area (Table 2). FC-POP-CH 2 DETA exhibited the excellent CO 2 adsorption enthalpy (54.0 kJ/mol) yet maintained considerable BET surface area (636 m 2 /g). That means, FC-POP-CH 2 DETA reached a compromise between these two factors, hence its high CO 2 adsorption capacity.
To further understand the influence of various functional groups on adsorption behavior, CO 2 adsorption enthalpies were calculated by the dual-site Langmuir fitting of CO 2 adsorption isotherms at 273 and 298 K based on Clausius-Claperyron equation. Figure 5a illustrates a plot of the CO 2 adsorption enthalpies as a function of loading. It is obvious  Post-synthesis modification of porous organic polymers with amine: a task-specific… 55 that FC-POP-CH 2 NH 2 , FC-POP-CH 2 DETA and For FC-POP-CH 2 PEI exhibit dramatically high CO 2 adsorption enthalpy values as 37.9, 54.0 and 59.6 kJ/mol at low loadings, respectively; much higher than that of FC-POP (26.4 kJ/mol). The high adsorption enthalpy values ascribe to strong interactions between amine-functional groups in FC-POPs and CO 2 molecules, for the presence of amines gives rise to chemical adsorption towards CO 2 . It is worth mentioned that the adsorption enthalpy value of FC-POP-CH 2 DETA remains a high value even when the CO 2 loading reaches 1.5 mmol/g, suggesting its abundant chemical adsorption sites, hence the extraordinary CO 2 adsorption capacity. The adsorption enthalpy values of amine-functionalized POPs show a similar sequence of nitrogen content, so that the nitrogen-site density are the dominant contribution for the strong affinity to CO 2 in the microenvironment of these FC-POPs. The excellent CO 2 uptake performance and outstanding adsorptive affinity of these FC-POPs ensure their adsorption selectivity. The ideal adsorption solution theory (IAST) proposed by Myers and Prausnitz (1965) was used to calculate the adsorption selectivity of CO 2 /N 2 for these materials. Fits of pure component isotherms were plotted to determine the molar uptakes at specified partial pressures in the bulk gas mixture and the selectivity was calculated under the simulated flue gas model (CO 2 /N 2 = 15/85). The results suggested that the presence of incorporated amines in FC-POPs prompts the CO 2 /N 2 selectivity (Fig. 5b). The selectivity value of FC-POP-CH 2 NH 2 (31.58) is about 2.5 times higher than that of FC-POP (12.62) at 298 K. Significantly, the CO 2 /N 2 selectivity value of FC-POP-CH 2-DETA and FC-POP-CH 2 PEI soar to 167.8 and 736.1 (298 K), respectively, thanks to their advantageous microenvironment including the microporosity and outstanding nitrogen-site density within the pores. For FC-POP-CH 2 PEI, although CO 2 uptake seems modest comparing to the reported values [MCTP-1 (Puthiaraj et al. 2015), 2.7 mmol/g, 298 K, BILP-4 (Rabbani and El-Kaderi 2012), 3.59 mmol/g, 298 K; FCTF-1-600 , 3.41 mmol/g, 273 K], the selectivity value is comparable to many other excellent reported POPs [azo-COP-2 (Patel et al. 2013), 130.6, 298 K;Py-1 (Luo et al. 2012), 117, 273 K; PPN-6-CH 2 DETA (Lu et al. 2012), 442, 295 K] (Table 3). The excellent selectivity of FC-POP-CH 2 PEI ascribe to the strong adsorption enthalpy towards CO 2 (59.6 kJ/mol). Furthermore, the introduction of PEI moieties can effectively enhance the selectivity, because flexible PEI can block the pores of POPs to interfere the N 2 adsorption while CO 2 can infiltrate into the void of PEI chains and even swell the pores due to its high polarizability and quadrupole moment (Sung and Suh 2014). Although recently reported PEI (40 wt%) , PAF-5 (Sung and Suh 2014) exhibited even higher selectivity (1200, 298 K) than FC-POP-CH 2 PEI, the latter still stands out in regards to the physicochemical stability arising from covalent bonding amine to the network. Overall, the selectivity value relies on pore structure and CO 2 adsorption enthalpies of polymers, so the adjustment of task- The selectivity of POPs for CO 2 over N 2 isotherms obtained from IAST calculations; c Cyclic CO 2 adsorption for POPs on an ASAP 2020 specific microenvironment would be an effective approach to improve CO 2 /N 2 selectivity.
To test the regeneration of the FC-POPs, cycling experiments were conducted on an ASAP 2020 analyzer (Fig. 5c). For each cycle, the adsorbents were saturated with CO 2 up to 1 bar at 298 K followed by a high vacuum (0.2 Pa) at 80°C for 90 min. The CO 2 adsorption capacity in each cycle almost reached the same point, with no apparent loss of uptake amount in the cycling test.

Conclusions
A highly porous FC-POP, which was facilely synthesized via Freidel-Crafts reaction, was incorporated by various amines to afford FC-POP-CH 2 NH 2 , FC-POP-CH 2 DETA and FC-POP-CH 2 PEI. Both microporosity and excellent amine-affinity towards CO 2 ensured those amine-functionalized polymers extraordinary CO 2 selective adsorption performance. The CO 2 adsorption capacity reached as high as 4.5 mmol/g at 273 K and 3.4 mmol/g at 298 K (1 bar) for FC-POP-CH 2 DETA; and the CO 2 /N 2 selectivity soared to 736.1 at 298 K (1 bar) for FC-POP-CH 2 PEI. The reason is, that the incorporation of task-specific functional groups would work as a dual regulations for the microenviroment of POPs. The dual regulations, namely the modifications of pore structure as well as the basic amine density, improved microporosity and adsorption enthalpy of CO 2 apparently and further enhanced the CO 2 capture performance.
Acknowledgements Financial supports for this study are provided by the National Key Technology Support Program of China (2013CB733501), the National Natural Science Foundation of China (Nos. 91334203, 21676080), the project of FP7-PEOPLE-2013-IRSES (PIRSES-GA-2013-612230). Table 3 Summary of surface area, CO 2 uptake, selectivity and adsorption enthalpy (Qst) in selected POPs (with excellent reported results)

POPs
S BET (m 2 /g) T (K) CO 2 uptake (mmol/g) IAST selectivity Q st (kJ/mol) Ref. Post-synthesis modification of porous organic polymers with amine: a task-specific… 57 Compliance with ethical standards Conflict of interest The authors declare no competing financial interests.
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