Diazapyrenium-based porous cationic polymers for colorimetric amine sensing and capture from CO₂ scrubbing conditions

Abstract

We have demonstrated that solution-state properties of a diazapyrenium dication, that is to form supramolecular adducts with aliphatic amines, can be transferred into the solid-state in the form of porous cationic polymers incorporating diazapyrenium units for visual detection and capture of aliphatic amines with uptake capacities up to 31 wt% for primary amines from CO₂ scrubbing systems.

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Carbon capture,¹ storage and conversion^2,3 technologies have become an integral part of today's energy industry due to environmental concerns such as climate change and ocean acidification arising from ever-increasing CO₂ emissions from various sources into the atmosphere.⁴ The amine-based CO₂ scrubbing approach,⁵ which involves absorption and stripping of CO₂ from flue gas using aqueous amine solutions such as monoethanol amine, MEA, and the subsequent regeneration step at elevated temperatures (∼120 °C) to obtain pure CO₂, is among the most widely used capture technologies.⁶ Although the low cost and CO₂ capture efficiency of this system presents significant advantages for large-scale applications, the oxidative decomposition of amines⁷ in the presence of oxygen and solvent loss during the thermal regeneration step leads to amine-related emissions, which presents serious risks for human health and the environment, i.e., contamination of fresh water sources.⁸ Hence, the development of new materials for simultaneous detection and capture of amines from CO₂ scrubbing systems is rather important both environmentally and industrially. Although there are several monomeric, polymeric and metal–organic framework-based sensors reported in the literature for the detection of aliphatic amines, none of these materials can achieve simultaneous capture.⁹ In this direction, porous polymers could be ideal candidates considering their structural modularity, permanent porosity and exceptional physiochemical stability.¹⁰ More recently, charged porous polymers have emerged as promising alternatives for heterogeneous catalysis,^3,11 gas¹² and energy storage applications.¹³ Importantly, the chemical nature and properties of these organic zeolites could be tuned by simply exchanging corresponding counterions for the desired application. In this sense, the choice of monomeric units could also impart new functions into the resulting cationic polymers. As such, the incorporation of diazapyrenium dication (DAP²⁺) into the porous polymers (Fig. 1) could be one of the promising directions considering its ability to form supramolecular 2 : 1 adducts (amine : DAP²⁺) with aliphatic amines via charge-transfer interaction between electron deficient DAP²⁺ core and electron-rich amine along with hydrogen bonding interactions with highly acidic α-protons of DAP²⁺ in solution. In addition, the formation of adduct between aliphatic amines and DAP²⁺ results in the appearance of a charge-transfer (CT) band in the visible range, which could also enable visual detection of amines.¹⁴ Importantly, the dissociation of the supramolecular adduct can be simply achieved by protonation, thus offering a reversible, simple, yet highly effective supramolecular system for the simultaneous capture and detection of aliphatic amines. In an effort to transfer these unique properties of DAP²⁺ into the porous polymers to tackle amine-related emissions from CO₂ scrubbing systems, herein, we describe catalyst-free synthesis of a porous cationic polymer incorporating diazapyrenium moieties (pDAP). We observed the formation of a broad CT band at 595 nm for pDAP when exposed to the vapour of aliphatic primary, secondary and tertiary amines. Thermogravimetric adsorption analysis of pDAP revealed an exceptional n-butylamine uptake capacity of 31 wt% at 25 °C along with a 75% fixing efficiency. Importantly, when tested for MEA in H₂O, pDAP showed a broad CT band at 595 nm, which is fully reversed upon addition of acid, proving the fact that the intrinsic properties of DAP²⁺ were successfully transferred to pDAP, thus leading to a new solid-sorbent for colorimetric detection and highly efficient capture of aliphatic amines both in the gas phase and in H₂O.

Fig. 1 (a) Synthetic strategy for the preparation of a porous cationic polymer incorporating diazapyrenium moieties, pDAP, and (b) proposed supramolecular detection and capture mechanism for aliphatic amines.

The synthesis of pDAP was achieved (Fig. 1) by reacting 2,7-diazapyrene and 1,3,5-trisbromomethylbenzene in MeCN at 120 °C for 7 days in an autoclave. The resulting orange precipitate was washed thoroughly with MeCN, THF, CHCl₃ and Et₂O to remove any unreacted monomers (see the synthesis section for details, ESI†). The chemical composition of pDAP were investigated by elemental analysis (EA). The experimental EA data was found to be in a good agreement with the theoretical values. The observed slight deviation from the calculated values can be attributed to the trapped water molecules inside the pores and the unreacted terminal groups of pDAP. The chemical connectivity of pDAP was analysed (see ESI†) by the Fourier transform infrared spectroscopy (FT-IR). The observed FT-IR peaks in the region 3200–2800 cm⁻¹ can be assigned to C–H stretching of aromatic rings and methylene bridges. The stretching vibration band between 3600 and 3200 cm⁻¹ was attributed to the adsorbed moisture. The peaks located at 1600 and 1450 cm⁻¹ represents the vibration and stretching bands of C=N and C=C bonds indicating the quaternary nitrogen atom formation in the 2,7-diazapyrenium moiety. We have also recorded (see ESI†) solid-state cross-polarization magic angle spinning (CP/MAS) ¹³C NMR spectrum of pDAP, which clearly shows the presence of aromatic carbon peaks in the range of 120–150 ppm along with the aliphatic benzylic carbon peak at 66 ppm. Downfield shift of the aliphatic carbon peak points to the formation of charged network structure. The particle morphology of pDAP was examined by scanning electron microscopy (SEM) analysis. The pDAP particles displayed fiber-like morphology on the order of 400–600 nanometre levels in their length. We have also carried out energy dispersive X-ray absorption spectroscopy (EDS) to probe bromine counteranions in pDAP, the results of which clearly demonstrate the presence of bromine counteranions in the pDAP. The thermal stability of pDAP were investigated by using thermogravimetric analysis (TGA) and found to be stable up to 300 °C under N₂ atmosphere. It is important to note that highly ionic nature of pDAP renders the framework highly hydrophilic to adsorb significant amount of moisture from air under ambient conditions. Consistent with the FT-IR and EA data, the observed initial weight loss in TGA is attributed to the adsorbed moisture from air. Although the powder X-ray diffraction spectrum of pDAP verified its amorphous nature, the observed broad feature between 2θ = 20–24° suggests the π–π stacking between diazapyrenium units within the pDAP, which could also explain the formation of fiber-like superstructures. The porosity of pDAP was investigated by Ar sorption measurements collected at 87 K (see ESI†). We observed typical type-II isotherm without hysteresis, which suggests the presence of mainly meso- and macropores in pDAP. The BET surface area of pDAP was calculated as 41 m² g⁻¹ in the valid relative pressure range between 0.01 and 0.25 estimated according to Roquerel plots (see ESI†). We attribute the relatively low-porosity of the resulting polymer to the π–π stacking interactions between the diazapyrenium units as evidenced from the PXRD data, which could lead to the efficient packing of the network structure. Moreover, the presence of counteranions could also limit the accessibility of micropores, thus decreasing the overall surface area of pDAP. In order to assess the pore structure of pDAP, we have calculated pore size distributions from argon adsorption isotherm at 87 K according to non-local density functional theory (NLDFT) method. pDAP showed broad pore size distribution mainly in the mesopore range. We have also observed micropores with a pore width maximum located at about 1.6 nm and a micropore surface area of 9.8 m² g⁻¹.

In order to investigate the potential of pDAP for visual detection of amines via the formation of a charge transfer (CT) adduct between diazapyrenium dication (DAP²⁺) and aliphatic amines, we have exposed (see ESI†) pDAP to the vapor of aliphatic amines including n-butylamine, diethylamine and trimethylamine for 30 min. As shown in Fig. 2, the treatment of pDAP with aliphatic amines resulted in the formation of a broad CT absorption band appeared at 595 nm, which is accompanied by a significant color change from orange to dark green (Fig. 2a). Notably, these results are in perfect agreement with data reported for the supramolecular 2 : 1 adduct formation between aliphatic amines and DAP²⁺ in solution.¹²

Fig. 2 Solid state UV-vis spectra of pDAP powder and after its exposure to the vapor of (a) n-butylamine, (b) diethylamine, and (c) trimethylamine at room temperature for 30 min. (a) Fotographic images of pDAP powder before (orange) and after (dark green) its exposure to n-butylamine vapor.

Primary amines are among the most widely used amines for the CO₂ scrubbing systems. Hence, we investigated gravimetric uptake capacity of pDAP for n-butylamine using thermogravimetric adsorption analysis (TGA). Prior the TGA analysis, the activation step was carried out by using N₂ flow (50 mL min⁻¹) at 150 °C for 90 min in order to remove any adsorbed gas and moisture during the sample preparation. Once pDAP was exposed (Fig. 3) to the n-butylamine vapor, which is obtained by continuous bubbling of N₂ carrier gas into the neat solution of n-butylamine with a flow rate of 10 mL min⁻¹ at room temperature, it showed a rapid uptake with initial exposure and reached the saturation after 230 min. The exposure of pDAP to n-butylamine vapor resulted in an exceptional amine uptake capacity of 31 wt% and 75% amine fixing efficiency (mol of n-butylamine/mol of pDAP). To the best of our knowledge, this is the first demonstration of aliphatic amine capture by using charged porous polymers, which also allows visual detection.

Fig. 3 (a) Gravimetric n-butylamine uptake performance of pDAP and (b) its corresponding fixation efficiency. For the TGA analysis, N₂ gas was used as a carrier gas and bubbled through neat n-butylamine solution with a flow rate of 10 mL min⁻¹. Red lines in each plot depicts the corresponding temperature programming for the TGA analysis.

In order to demonstrate the applicability of pDAP for simultaneous detection and reversible capture/release of amines from CO₂ scrubbing systems, we exposed pDAP to 1 M MEA solution in H₂O (for details, see ESI†). Consistent with the other aliphatic amines we have tested, the UV spectrum of dispersed pDAP particles in H₂O in the presence of MEA resulted (Fig. 4) in the appearance of broad CT absorption band at 595 nm, which clearly proves the formation of supramolecular adduct between pDAP and MEA. In order to investigate the reversibility of adduct formation, we have added 1 N HCl to the above solution, which lead to the near complete disappearance of CT band at 595 nm. These results point to the fact that pDAP can be effectively regenerated under acidic conditions.

Fig. 4 (a) The normalized UV/Vis spectra of pDAP dispersed in deionized water (black line), pDAP + MEA (red line) and pDAP + MEA + HCI (blue line). (b) Change in intensity of absorption band located at 595 nm by addition of MEA and HCl.

In conclusion, we have demonstrated that molecular recognition properties of supramolecular hosts can be effectively transferred into the porous solids to tackle complex separations and environmental issues such as amine-related emissions. We believe that this approach could also pave the way for the preparation of new generation of functional, highly substrate selective porous polymers powered by molecular recognition.

Acknowledgements

We acknowledge the support by a National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST; NRF-2014R1A4A1003712) and the BK21 PLUS program.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra16714d

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