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Sustainable Poly(ionic liquid)s for CO Capture based on Deep Eutectic Monomers Mehmet Isik, Sonia Zulfiqar, Fatimah Edhaim, Fernando Ruipérez, Alexander Rothenberger, and David Mecerreyes ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02137 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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ACS Sustainable Chemistry & Engineering

Sustainable Poly(ionic liquid)s for CO2 Capture based on Deep Eutectic Monomers Mehmet Isik,a Sonia Zulfiqar,b Fatimah Edhaim,c Fernando Ruiperez,a Alexander Rothenbergerc and David Mecerreyesa,d* a

POLYMAT, University of the Basque Country UPV/EHU, Avda. Tolosa, 72, 20018, San

Sebastian, Spain b

Department of Chemistry, School of Natural Sciences (SNS), National University of Sciences

and Technology (NUST), Islamabad 44000, Pakistan c

Physical Sciences and Engineering Division, King Abdullah University of Science and

Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia d

IKERBASQUE, Basque Foundation for Science, E-48011, Bilbao, Spain

* E-mail: [email protected]

KEYWORDS Deep eutectic solvent, poly(ionic liquid), ionic liquid), CO2 capture

ACS Paragon Plus Environment

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ABSTRACT

The design of high performance solid sorbent materials for CO2 capture is one of the searched technologies to mitigate global warming. However, the covalent incorporation of functionalities into polymeric supports usually involves multi-step energy-intensive chemical processes. This fact makes the net CO2 balance of the materials negative even though they possess good properties as CO2 sorbents. Here we show a new family of polymers which are based on amines, amidoximes and natural carboxylic acids and can be obtained using sustainable low energy processes. Thus, deep eutectic monomers based on natural carboxylic acids, amidoximes and amines have been prepared by just mixing with cholinium type methacrylic ammonium monomer. The formation of deep eutectic monomers were confirmed by differential scanning calorimetry measurements. In all cases, the monomers displayed glass transition temperatures well below room temperature. Computational studies revealed that the formation of eutectic complexes lengthens the distance between the cation and the anion causing charge delocalization. The liquid nature of the resulting deep eutectic monomers (DEMs) rendered possible to conduct fast photopolymerization process to obtain the corresponding poly(ionic liquid)s. Materials were characterized by means of NMR, DSC, TGA and XRD to evaluate the properties of the polymers. The polymers were then used as solid sorbents for CO2 capture. It has been shown that the polymers prepared with citric acid displayed better performance both experimentally and computationally. The current endeavour showed that sustainable poly(ionic liquid)s based on deep eutectic monomers can be easily prepared to produce low-energy cost alternatives to the materials being researched for CO2 capture.


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INTRODUCTION Climate change due to global warming is one of the most immediate and important issues to be dealt with by humanity. The increase in the atmospheric carbon dioxide (CO2) concentration is the major cause of this imminent threat, hence controlling the CO2 concentration is the key aspect of this problem.1,2 An easy and efficient way to reduce CO2 concentration is to capture it prior to emission to atmosphere.3-7 Although there are different methods, solvent absorption is still the most widely utilized technique to capture and reduce CO2 emissions,8-10 monoethanolamine being the industrial standard. Unfortunately, it possesses many disadvantages such as corrosive nature, toxicity, degradability and high energy requirements for regeneration.11,12 Ionic liquids have emerged as novel solvents for many different applications including carbon dioxide capture technologies.13-16 Compared to conventional volatile compounds used for carbon dioxide capture, ionic liquids are nonvolatile, thus they do not require condensation during the regeneration process allowing to perform faster and easier and to save energy. Ionic materials with high affinity towards carbon dioxide can be tailor-designed through careful selection of the ionic groups present in the structure.17-20 Task-specific ionic liquids containing reactive components towards carbon dioxide have been used to capture CO2, yet these materials possess several drawbacks. Some of these can be listed as their complexity and cost for the synthesis and frequent use of unsustainable, environmentally unfriendly, mostly fluorinated, components.19, 21, 22 As an alternative to conventional ionic liquids, deep eutectic solvents (DESs) emerged due to its simple chemistry.23-28 The use of DESs for carbon dioxide capture is rather new and there are only few studies reported in literature. For instance, Sze and coworkers designed task-specific ternary DESs by using choline chloride, glycerol and superbase 1,8-


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diazabicyclo[5.4.0.]undec-7-ene (DBU).29 These DESs displayed good carbon dioxide capture performance and the liquid nature of the DES facilitated the release of the carbon dioxide from the medium. However, the strong basicity of the ingredients increases the corrosive behavior of the ternary DES. Adsorption processes using solid sorbents capable of capturing CO2 from flue gas streams have shown many potential advantages such as superior thermal and mechanical stability, less corrosion issues, lower energy consumption during regeneration and lower absorber volume requirement, compared to other conventional CO2 capture systems using aqueous amine solvents.4 In view of this, in the past few years, several research groups have been involved in the development of new solid sorbents for CO2 capture from flue gas with superior performance and desired economics.30-32 A variety of promising sorbents such as activated carbonaceous materials, microporous/mesoporous silica or zeolites, carbonates, metal organic frameworks (MOFs) and new polymeric materials including intrinsically microporous polymers and Covalent Organic Frameworks loaded with or without nitrogen functionality for the removal of CO2 from the flue gas streams have been investigated.33,

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Among other polymers, poly(ionic liquid)s

(PILs) are considered one of the most promising materials for CO2 capture.35-39 PILs refer to a novel subclass of polyelectrolytes having ionic liquid (IL) species in monomer unit.40 Thus, PILs combine the unique properties of ILs such as thermal stability and high CO2 affinity with the ability of polymers to form films and highly porous materials. An endless number of possible combinations of anions and cations, as well as variations in the nature and structure of the main polymer backbone and side groups, opened broad possibilities for controlling PILs' properties with the goal of creating tailored materials with enhanced CO2 uptake.39 Thus, recently PILs were shown to exhibit several times higher CO2 sorption capability than appropriate ionic


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monomers and much faster CO2 capture and desorption rates in comparison with roomtemperature ILs.35, 37 Similar to deep eutectic solvents which are formed by hydrogen bond donor and acceptor groups, deep eutectic monomers (DEMs) are formed by the same groups at least one group being polymerizable. In this work we report the use of deep eutectic monomers (DEMs) to produce poly(ionic liquid) materials and their potential use as CO2 sorbents. This new family of polymers is based on cheap and renewable chemicals and they were obtained by simple mixing of its components forming ionic and hydrogen bonding interactions. This chemistry involves low energy processes. Therefore, various carboxylic acids, amidoximes and amines specially chosen for their affinity towards CO2 were incorporated via deep eutectic monomers into solid polymeric materials by efficient photopolymerization. The resulting materials were characterized and their performance in CO2 capture at ambient pressure is presented.

EXPERIMENTAL Materials 2-dimethyalminoethylmethacrylate, 2-bromoethanol, ethylene glycol dimethacrylate, citric acid, malonic acid, oxalic acid, maleic acid, ethyl acetate, methanol, 2-hydroxy-2methylpropiophenone (photoinitiator), 2,6-diaminopyridine, 1,4-bis(3-aminopropyl) piperazine (piperazine

diamine),

(benzeneimidamide),

1,2-diaminopropane,

N`-hydroxybenzene

N`-hydroxy-4-trifluoromethylbenzene

carboxyimidamide carboxyimidamide

(trifluoromethylbenzeneimidamide), N`-hydroxy-2-methyl-propanimidamide (propanimidamide) were obtained from Sigma-Aldrich and Acros as reagent grade high purity chemicals and used without further purification.


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Deep eutectic monomer preparation The synthesis of the 2-cholinium methacrylate bromide (ChMA Br) was reported elsewhere.41, 42 The use of bromide instead of chloride in the quaternary ammonium monomer is related to the convenience for the synthesis since the reaction with 2-bromoethaol can be performed at room temperature and no stabilizer is required during the reaction. ChMA Br quaternary ammonium monomer was used to prepare the DEMs. The use of bromide instead of chloride analog DEMs were prepared through simple mixing of the monomer with the corresponding acid. In a typical preparation, calculated amount of 2-cholinium methacrylate bromide monomer was mixed with a calculated amount of hydrogen bond donor. The solid mixture was homogenized by adding a small amount of methanol and mixed with a magnetic stirrer. The solvent was removed from the mixture by simple air purge and the remaining solvent was removed under vacuum. The resulting monomers were viscous liquids and characterized by 1

H NMR, FTIR and DSC to confirm the formation of the DEMs.

Polymerization of deep eutectic monomers The formation of DEMs results in homogeneous liquid monomers which allow performing quick photopolymerization reactions to obtain the polymers. In a typical polymerization reaction, the liquid DEM was mixed with the ethylene glycol dimethacrylate crosslinker (10 wt% with respect to the amount of polymerizable group) and the photoinitiator (1 wt%). The resulting solution was casted into Teflon mold. The solution was then exposed to UV light for photopolymerization. The polymer obtained was dried to get rid of any residual solvent. Dymax UVC-5 conveyor belt system with 800 mW/cm2 intensity operating at a wavelength of 365 nm was used for photopolymerization. Sample to lamp distance was 100 mm and the belt speed was


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fixed at 2 m/min. 5 repetitive cycles (100 sec) were applied to achieve a full photopolymerization.

Characterization The NMR measurements were carried out on a Bruker AC-300 (for 1H NMR) instrument with the following experimental conditions: spectral width 15 ppm with 32 K data points, flip angle 90, relaxation delay of 1 s, digital resolution of 0.24 Hz/pt. Solid state NMR spectra were recorded on a Bruker 400 AVANCE III WB spectrometer 9.40T. 13C CP/MAS NMR spectra were collected using a 4mmMASDVT probe at a spinning of 12 KHz, using the cross polarization pulse sequence, at 100.63 MHz, a time domain of 2K, a spectral width of 29 KHz, a contact time of 1.5ms and an interpulse delay of 5s. Thermogravimetric analysis (TGA) was performed by using a TA instruments Q500 device. The samples were heated from 40 °C to 800 °C with a heating rate of 10 °C/min under nitrogen gas flow. Differential scanning calorimetry experiments were performed on a TA instruments Q200. The sample materials were enclosed in nonrecyclable aluminium hermetic pans, the sample masses being around 5-10 mg. The samples were first equilibrated at 0 °C and heated to 100 °C with a heating rate of 10 °C/min and kept isothermal for 5 minutes to erase the thermal history. Then cooled down to -80 °C and kept isothermal for 5 minutes. The samples then heated to 100 °C with a heating rate of 10 °C/min and the 2nd heating cycles were taken for the detection of the glass transition temperatures. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) was performed on Bruker ALPHA-P equipment. The spectra were recorded from 350 cm-1 to 4000 cm-1 with a resolution of 2 cm-1 and 24 scans were taken for each spectrum. X-Ray diffraction studies were conducted by using Bruker D8 Advance powder diffractometer. 0.1542 nm Cu-Kα


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radiation was used as the X-ray beam. The scans were performed from 2° up to 80° with 0.05° step size and step time of 5 seconds. The XRD intensities obtained were plotted against scattering angle 2θ. CO2 measurements of PolyDEMs were recorded using Micromeritics ASAP 2020 physisorption analyzer at 273 K and 298 K. The samples were degassed at 343 K for 24h prior to analysis. The Brunauer–Emmett–Teller (BET) surface area of polyDEM samples was determined at 77 K with N2 using Micromeritics ASAP 2020 physisorption analyzer.

RESULTS AND DISCUSSION Deep Eutectic Monomer (DEM) characterization Deep Eutectic Solvents (DES) are molecular complexes formed between hydrogen bond acceptors such as quaternary ammonium or phosphonium salts and hydrogen bond donor molecules generally chosen from cheap and safe components. The hydrogen bond acceptor selfassociates with the donor molecule to form the DES via hydrogen bonding. When salts are used as hydrogen bond acceptors, the hydrogen bonding formed between the halide anion of the salt and the hydrogen bond donor molecule causes charge delocalization to occur. This strong interaction is responsible for the decrease of the freezing point of the mixture relative to the melting points of the individual components used for the formation of the DES. DESs possess many common properties with the conventional ionic liquids. As compared to ionic liquids, DESs are easy to scale up since no puriﬁcation is required and synthesis is 100% atom economic making them attractive in large scale industrial application. Deep eutectic monomers were prepared through simple blending of the components at room temperature. In a typical preparation, desired amount of the quaternary ammonium component was mixed with the calculated amount of the hydrogen bond donor. The 2-cholinium


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methacrylate bromide monomer was the hydrogen bond acceptor moiety in the formulation whereas the carboxylic acids, amines and amidoximes were serving as hydrogen bond donor groups (Figure 1). The compositions and some physical properties of the DEMs are provided in the Table 1. As mentioned, the strong intermolecular interactions between the hydrogen bond acceptor and donor moieties result in a freezing point lower than the melting point of the individual components. The 2-cholinium methacrylate bromide monomer displayed a melting point at 90 °C. The DEM-A, which is composed of citric acid (Tm: 153 °C) and 2-cholinium methacrylate bromide, displayed a glass transition (Tg) of -40 °C and no freezing point (Tf) was detected in the range scanned. DEM-B had a Tg of -37 °C and no Tf was recorded. Similarly, DEM-C (oxalic acid), DEM-D (malonic acid) and DEM-E (maleic acid) displayed Tgs of -55 °C, -23 °C and -53 °C, respectively. Similarly, DEMs prepared by using amidoximes and amines displayed glass transition temperatures ranging from -30 °C to -60 °C. No Tms were recorded for all the samples in the DSC scan range except the sample prepared with the amidoxime 6 hydrogen bond donor molecule. Differential calorimetry characterizations showed that the DEMs were formed resulting in liquid samples at room temperature (Figure 2 and Figure S1). The DEMs were also analyzed by 1H NMR to confirm the structures of the samples. (Figure S2) As seen from the 1H NMR spectra, the signals associated with the 2-cholinium methacrylate bromide monomer were clearly observed for all the DEMs indicating that the quaternary ammonium monomer was structurally intact in all the monomers prepared. In addition, signals associated with the citric acid between 2.5-3.0 ppm were observed in DEM-A and DEM-B monomers confirming the presence of the carboxylic acid. There was no additional signal observed in the case of DEM-C since the carboxylic acid does not contain any methyl proton. Similarly, DEM-D monomer


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displayed an additional peak at 3.20 ppm associated with the -CH2- protons of the malonic acid. A clear signal at 6.27 ppm associated with the RCHCHR protons of maleic acid was observed for DEM-E as expected. The signals associated with the aromatic benzene protons and the amidoximes were observed in the downfield region in the DEMs prepared with 5, 6 and 8 hydrogen bond donors. Strong methyl signal was observed at 1.06 ppm for DEM-H due to the presence of the two methyl units on the amidoxime. To sum up, 1H NMR spectra revealed that all the DEMs were successfully prepared. Another interesting point observed from the spectra was the shift of the signals with the addition of carboxylic acids as a result of the intermolecular interaction between the quaternary ammonium monomer and the carboxylic acid. To be able to observe this interaction, two solutions of DEM-E were prepared, solid concentrations ranging from 1% to 60%. The chemical shifts due to change in the concentration of the samples are given in table S1. The formation of the complexes was also confirmed by the upfield shift of the DEM signals in 1H NMR spectroscopy as compared to the signals of the individual counterparts. The spectrum of concentrated samples revealed that the characteristic signals of DEM-E formed between 2cholinium methacrylate bromide and maleic acid were upfield shifted as compared to the ones obtained for 2-cholinium methacrylate bromide and maleic acid in D2O. This was indicative of strong change in the chemical environment upon the establishment of strong anion-hydrogen bond donor interactions as the concentration of the sample was increased. The rupture of this interaction resulted in a downfield shift as the anion cation introduction is reestablished. FTIR characterization was performed to observe the characteristic signals associated with the prepared monomers. (Figure S3) The broad band observed between 3050 cm-1 and 3570 cm-1 was assigned to O-H stretching associated with the OH groups in the structures of both 2-


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cholinium methacrylate bromide and carboxylic acids. In addition, N-H stretching signals between 3100 cm-1 and 3300 cm-1 were observed for the DEMs prepared with amidoximes and amines. Due to presence of hydrogen bonding, these bands are observed as a broad signal for all the DEMs prepared. In addition, the C=O stretching associated with both the quaternary ammonium monomer around 1715 cm-1. This signal was broadened in the case of DEMs prepared with carboxylic acids due to the presence of the acid groups. Due to the presence of polymerizable C=C group in the 2-cholinium methacrylate bromide, the C=C stretching band associated with this group is observed at 1630 cm-1 for DEMs based on carboxylic acids, however this band overlaps with other signals in the case of DEMs based on amidoximes and amines. In addition, this signal was more intense in the case of DEM-E since the composition of this monomer involves maleic acid which also contains a C=C group. The strong signals observed between 1550 cm-1 and 1650 cm-1 was assigned to C=N stretching and N-H bending associated with the amidoximes and amines for DEM-F to DEM-K. The broad peak observed around 1425 cm-1 to 1499 cm-1 was assigned to CH2, CH3 bending vibrations. The C-O stretching signal was observed around 1160 cm-1 for all the DEMs as a strong peak. In short, FTIR characterization confirmed the structural integrity of the DEMs prepared. Computational studies were also performed for DEM-A (ChMA Br-Citric acid), DEM-C (ChMA Br-Oxalic acid) and DEM-D (ChMA Br-Malonic acid) to find out the conformation of the hydrogen bond donor molecules with the quaternary ammonium monomer. All the density functional calculations (DFT) have been carried out with the Gaussian 09 suite of programs. (see supporting information for details). The results in the following images describe the hydrogen bonding with carboxylic acids for the anion and cation (Figure 3). The hydrogen bonding interactions of the donor and acceptor molecules are depicted in Figure 3. As it can be seen for


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DEM-A, the interaction primarily takes place between the bromide anion (Br-) and H site of the COO-H of the citric acid molecule. Another interaction between the Br- and H site of the O-H of the cation is observed. The H site of the donor molecules primarily interacts with the COO site on the cation. The binding energy for the complex formed between the quaternary ammonium monomer and citric acid was calculated as -99.19 kJ/mol. DEM-C formed between the quaternary ammonium monomer and oxalic acid displayed hydrogen bonding between the bromide (Br-) anion and H site of the COO-H of the two acid molecules present in the system. The binding energy for this system was calculated as -77.93 kJ/mol. The third analyzed system, DEM-D (malonic acid), displayed strong interactions with the Br- site of the cation through H sites of the COO-H of the malonic acid donor molecules. In addition to these interactions observed, O site of the COO of the acid molecule displayed interaction with the H on the O-H unit of the cation. The binding energy between the quaternary ammonium monomer and the two malonic acid molecules was calculated as -90.49 kJ/mol. It is seen from the computational studies that the main hydrogen bonds developed in the studied DEM systems are through the anion and the acidic protons of the carboxylic acid donor molecules. The binding energies also showed that the affinities of the donor molecules towards the quaternary ammonium monomer are as follows citric acid > malonic acid > oxalic acid. This suggests that stronger interactions are established between citric acid and quaternary ammonium than malonic acid or oxalic acid. Another important point to mention is the changes in the distance between the cation and the anion as a result of the formation of the complexes. The original distance between the cation (N+) and the anion (Br-) was calculated as 3.752 Å. This value was increased upto 4.194 Å in the case of DEM-A prepared with citric acid. This significant increase suggests that the coordination strength between the ion pairs is decreased causing charge delocalization. Similarly, the cation-


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anion distances were calculated as 3.965 Å and 4.004 Å for DEM-C and DEM-D, respectively. As clearly seen, the increases in the distance between the cation and anion as a result of the strong interactions yield deep eutectic monomers displaying lower freezing points than the individual components of the system. Polymerization of Deep Eutectic Monomers (DEMs) Photopolymerization, a straightforward and high energy efficient strategy, was used for the preparation of the polymers by using the DEMs. The liquid nature of the monomers upon the formation of DEM allows performing photopolymerization. To conduct the polymerizations, the DEM mixture containing the monomer, crosslinker and the photoinitiator was casted into Teflon mold. The sample was exposed to UV light at room temperature for a short period of time (100 sec) and the polymer was obtained at the end of the process. The preparation of the polymers is depicted in the Figure 1. The polyDEMs were characterized to observe their various properties. Due to their insoluble nature of the backbone of the polymeric samples, the monomer conversion was characterized by the immersion of the polymer samples in D2O to be able to solubilize any unreacted monomer in the sample. The solution was then analyzed by 1H NMR. (Figure S4) Upon the immersion of the samples into D2O, any unreacted monomer will dissolve in the medium. When the spectra are analyzed, it can be clearly seen that the monomer specific signals associated with H2C=CR group are not observed in all the polyDEM samples between 5.5 and 6.25 ppm. This indicates that the polymerization of the monomer was quantitative in all the polymers. In case of polyDEM-E, a strong signal was observed at 6.37 ppm confirming the presence of maleic acid which possesses RCH=CHR. In all the polyDEM samples, there were small signals observed around similar shifts associated with the 2-cholinium methacrylate


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bromide monomer and the hydrogen bond donor molecules. This suggests that there was a small fraction of soluble polymer when the samples were immersed in D2O together with the associated hydrogen bond donors. In conclusion, NMR characterization confirmed a successful photopolymerization of the DEMs to obtain the corresponding polyDEMs. In addition, solid 13C NNR spectra were acquired for the polyDEMs prepared with the carboxylic acids (Figure 4). 13C NMR characterization of the polyDEMs also confirmed the successful polymerization of the DEMs through photopolymerization. The signals associated with the monomer reactive C=C group between 125-136 ppm disappeared as a result of the polymerization. In addition, characteristic signals associated with the hydrogen bond donors in the formulations together with the signals associated with the backbone of the poly(ionic liquid) are observed confirming the structures of the polymers. Another characterization conducted was FTIR analysis. (Figure S5) The O-H stretching band as a broad signal was observed between 3000 cm-1 and 3650 cm-1 due to presence of hydroxyl groups in all DEM components. The C=O stretching peak was observed at 1716 cm-1 for all the samples. The C=C stretching bands observed at 1630 cm-1 in the FTIR spectra of the monomers were disappeared indicating a successful polymerization of the monomers prepared. The C=C stretching band due to the presence of maleic acid in polyDEM-E was observed at 1635 cm-1. The broad peaks observed around 1430 cm-1 were assigned to CH2 bending vibrations. The strong peaks observed at 1150 cm-1 were assigned to C-O stretching associated both with the 2-cholinium methacrylate bromide and the hydrogen bond donor molecules. The FTIR spectra of the polymers after photopolymerization confirmed the structural integrity of the components forming the DEMs. Differential scanning calorimetry (DSC) measurements were performed to detect the glass transition temperatures of the polymers prepared with DEMs. (Table 2, Figure S6) The DSC


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characterization of the polymers revealed that all the polymers possess glass transition temperatures suggesting an amorphous backbone configuration. Although they displayed Tgs, there was no pattern observed with respect to changing hydrogen bond donor in the polyDEM composition. The midpoint of the transition was taken as the glass transition temperature. The values obtained were 62 °C, 47 °C, 15 °C, 83 °C and 37 °C for polyDEM-A, polyDEM-B, polyDEM-C, polyDEM-D and polyDEM-E, respectively. In the case of polymers made with amidoximes and amines, the Tgs were recorded as -26 °C, -10 °C, -37 °C, -31 °C, -51 °C and -32 °C for polyDEM-F to polyDEM-K, respectively. The recorded DSC scans revealed that all the samples displayed quite broad glass transitions suggesting a variety of thermal motions as a result of the morphological heterogeneity present in the samples. The recorded values for polymers prepared with carboxylic acids were significantly higher compared to the values obtained for the starting DEMs. To determine the thermal stabilities of the samples, thermal gravimetric analysis (TGA) was performed. (Table 2, Figure S7) The TGA profiles of the polyDEMs prepared with carboxylic acids showed that the polyDEMs displayed similar thermal stabilities regardless of the DEM component. The onset temperatures were dependent on the hydrogen bond donor present in the system. The polymers with citric acid displayed slightly higher onset temperatures due to higher temperature stability of citric acid. The polyDEM-C, polyDEM-D and polyDEM-E containing oxalic, malonic and maleic acid displayed onset temperatures around 150 °C. The degradation temperatures where 50% weight loss was observed were between 230 °C and 275 °C. All the samples displayed residual masses at the end of the TGA process around 20%. The onset temperatures were relatively lower for the polymers prepared with amidoximes and amines at


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around 125 °C to 150 °C. The degradation temperatures where 50% weight loss recorded were around 265 °C. No residual mass was recorded after 450 °C. Another characterization performed on the polyDEM samples containing carboxylic acids was wide angle X-ray diffraction (WXRD) to analyze the state of the components of the polymers in solid-state (Figure S8). The hydrogen bond donor molecules present in the formulations are normally in crystalline form. Upon the formation of the DES, the crystallization of these molecules is disturbed eventually leading to freezing temperatures much lower than the melting points of individual components. XRD profiles revealed that the samples were mostly amorphous and did not show significant crystallization. The polymer which was prepared from DEM-A was totally amorphous displaying only an amorphous halo. PolyDEM-B displayed crystal peaks observed at around 2Θ degrees 14°, 24°, 31°, 60° and 70° as an indication of the partial crystallization of the citric acid present in the formulation. It is worth mentioning that the difference between polyDEM-A and polyDEM-B in terms of composition is 2-cholinium methacrylate bromide monomer/citric acid ratio which is 1:1 and 1:1.5, respectively. It is clear that the increase in citric acid ratio for polyDEM-B (1:1.5) resulted in the crystallization of the citric acid. Only a broad amorphous background was observed for polyDEM-C and polyDEM-D suggesting that the components were stable at the given ratio. The polymer prepared with maleic acid displayed a broad amorphous halo and a weak crystal peak at 29° indicating that the carboxylic acid was partially crystallized. CO2 Capture Testing the potential of PolyDEMs for CO2 uptake is an emerging area of research. So far, very few attempts have been made with deep eutectic solvents and promising results were obtained.24,

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The studies to analyze the carbon dioxide capturing capability of choline


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chloride/levulinic acid DES system revealed that the carbon dioxide molecules establish stronger interactions with the acid molecule than the ions.43 Consequently, this curiosity led us to synthesize novel polyDEM samples formed with different natural carboxylic acids, amidoximes and amines. The use of DEMs rendered possible to produce the solid-state materials through a facile photopolymerization process. Resulting solid samples were characterized in terms of their performances to adsorb carbon dioxide. CO2 adsorption isotherms of all PolyDEM samples were measured at 273 K (Figure 5a), while the selected ones were also investigated for CO2 sorption at 298 K (Table 3 and Figure 5b). Among all samples, PolyDEM A and B exhibited comparable and the highest CO2 uptake at 273 K and 1 bar (7.26 and 7.27 mg/g) thus illustrating the superiority of using citric acid for the preparation of PolyDEM, respectively. These values of CO2 sorption are in some cases comparable and higher as compared to other poly(ionic liquid)s already reported in the literature which are mostly based on highly toxic fluorinated anions.47-50 Keeping in mind, the conditions required for post combustion CO2 capture, it is important to determine the sorption potential of any sorbent at 298 K from application viewpoint. The performance of PolyDEM samples at 298 K was monitored by selecting three polyDEM samples A, C and D with highest to lowest CO2 sorption capacity range at 273 K (Figure 5). The CO2 uptake decreases with increase in temperature in all the three samples depicting physical nature of adsorption.51 It is important to note that CO2 sorption of all PolyDEM samples is independent of surface area since all the samples were nonporous inherently. For instance, PolyDEM-A and PolyDEM-B have comparable CO2 uptake clearly indicating that CO2 uptake not only depend on surface area and porosity but also get affected by the presence of hetero-atoms, which is evident from the oxygen rich structure of citric acid thus promoting CO2 sorption in PolyDEM. There already exist number of reports using various functionalities containing nitrogen and oxygen rich


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CO2 phillic groups.52-54 Among polymers prepared with amidoxime molecules, propanimidamide (polyDEM-H) displayed better sorption capacity compared to other polymers. The sorption capacity was measured to be 4.44 mg/g. PolyDEM-H was followed by polyDEM-F and polyDEM-G values being 3.46 and 2.14 mg/g. The polymers prepared with primary amines had similar sorption capacities. The interaction of carbon dioxide with the solid polyDEM matrix should be maintained in the physisorption regime to accomplish adsorbent regeneration with a minimal energy input. Therefore, isosteric heats of carbon dioxide adsorption (Qst) was also calculated for PolyDEM-A (citric acid), PolyDEM-C (oxalic acid) and PolyDEM-D (malonic acid) from CO2 adsorption isotherms measured at 273 K and 298 K using Clausius-Clapeyron equation. The resulting curves are shown in the Figure 6. To determine Qst is of paramount importance because it gives information about binding affinity of PolyDEM with CO2 molecules. The Qst values for PolyDEM-A (citric acid), PolyDEM-C (oxalic acid) and PolyDEM-D (malonic acid) were found to be 8.99, 2.33 and 5.5 kJ/mol respectively. The values clearly signify the physisorption process because the ones expected for chemisorption process are >40kJ/mol which are far above than the values observed for PolyDEMs. Computational studies were also performed to analyze the interaction of the studied DEM systems with carbon dioxide. The interaction energies of the selected DEMs with CO2 were determined by using density functional calculations (DFT) (Figure 7). The calculated interaction energies reveal the affinity of CO2 molecule towards DEM systems studied. The calculated ∆E values were -14.97 kJ/mol, -13.20 kJ/mol and -9.29 kJ/mol for DEM-A (citric acid), DEM-C (oxalic acid) and DEM-D (malonic acid), respectively. The computational findings indicate that the DEM system containing citric acid has more affinity towards carbon dioxide. This is


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followed by the DEM containing oxalic acid and finally the lowest was calculated for the DEM with malonic acid. Thus, the interaction with CO2 can be listed as follows: ChMA Br-citric acid > ChMA Br-oxalic acid > ChMA Br-malonic acid This order is also valid for the experimental findings. As discussed previously, citric acid containing system displayed the highest CO2 uptake which is followed by the oxalic acid containing system. The lowest CO2 uptake was observed for the malonic acid containing polyDEM system.

CONCLUSIONS In conclusion, a promising study on the synthesis, characterization and application of sustainable poly(ionic liquid)s based on deep eutectic monomers (DEMs) using various natural acids, amidoximes, amines and biosourced cholinium type quaternary ammonium monomer is presented. While DSC measurements verified the formation of liquid DEMs as a result of strong intermolecular interactions, structural confirmation was performed by NMR and FTIR. Sustainable poly(ionic liquid)s obtained as a result of a facile photopolymerization process were used as solid sorbents for CO2 capture. CO2 adsorption studies revealed that the CO2 uptake capacity of polyDEMs prepared with citric acid was significantly higher illustrating the exclusive role of citric acid, thus assisting enhanced interactions with CO2 molecules. In summary, an easy and energy saving way of introducing functional molecules without complicated synthetic steps has been illustrated to yield sustainable poly(ionic liquid)s as solid sorbents for CO2 capture. The current effort is the first mile stone and many other task-specific porous polyDEMs can be exploited in the future for efficient CO2 capture application by the use of green and environmentally benign systems.


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ACKNOWLEDGEMENTS Financial contribution of European Union through Renaissance-ITN 289347 is greatly acknowledged. Supporting Information Supporting information includes DSC, NMR, XRD and computational details. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author *Phone: 00 34 943 018018 E-mail: [email protected] Note The authors declare no competing financial interest. REFERENCES 1. Hanisch, C. Exploring Options for CO2 Capture and Management. Environ. Sci. Technol. 1999, 33, 66A-70A. 2. Da Silva, E. F.; Booth, A. M. Emissions from postcombustion CO2 Capture plants. Environ. Sci. Technol., 2013, 47, 659-660. 3. SunL-B.; Kang, Y-H.; Shi, Y-Q.; Jiang, Y.; Liu, X-Q. Highly selective capture of the greenhouse gas CO2 in polymers. ACS Sustainable Chem. Eng., 2015, 3, 3077-3085. 4. Chen, J.; Yang, J.; Hu, G.; Hu, X.; Li, Z.; Shen, S.; Radosz, M.; Fan, M. Enhanced CO2 capture capacity of nitrogen-doped biomass-derived porous carbons ACS Sustainable Chem. Eng., 2016, 4, 1439-1445.


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List of figures Figure 1. a) DEM and polyDEM preparation and b) the structures of the chemicals used Figure 2. DSC scans of the selected DEMs Figure 3. Optimized conformation of 2-cholinium methacrylate bromide with carboxylic acids a) DEM-A (citric acid), b) DEM-C (oxalic acid), c) DEM-D (malonic acid) Figure 4. Solid state 13C NMR of the polyDEMs prepared with carboxylic acids Figure 5. a) CO2 gas adsorption isotherms of PolyDEMs at a) 273 K and b) 298 K for selected polyDEMs. Figure 6. Isosteric heat of CO2 adsorption for selected PolyDEM-A (citric acid), polyDEM-C (oxalic acid) and polyDEM-D (malonic acid). *Isosteric heat of adsorption (Qst) obtained from CO2 isotherms data at 273 K and 298 K using Clausius-Clapeyron equation Figure 7. Conformation of CO2 molecule with selected DEM systems a) DEM-A (citric acid), b) DEM-C (oxalic acid) and c) DEM-D (malonic acid)


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Figure 1

Figure 2


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Figure 3


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Figure 4

Figure 5


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Figure 6


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Figure 7


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List of Tables Table 1. Compositions and properties of DEMs prepared Code

Composition

DEM-A DEM-B DEM-C DEM-D DEM-E DEM-F DEM-G DEM-H DEM-I DEM-J DEM-K

ChMA Br/1 (citric acid) ChMA Br /1 (citric acid) ChMA Br /2 (oxalic acid) ChMA Br /3 (malonic acid) ChMA Br /4 (maleic acid) ChMA Br /5 (benzeneimidamide) ChMA Br/6 (trifluorobenzeneimidamide) ChMA Br /7 (propanimidamide) ChMA Br /8 (diaminopyridine) ChMA Br /9 (piperazinediamine) ChMA Br /10 (diaminopropane)

a

Molar Ratio 1.0/1.0 1.0/1.5 1.0/2.0 1.0/2.0 1.0/2.0 1.0/1.0 1.0/1.0 1.0/1.0 1.0/1.0 1.0/0.5 1.0/0.5

Tga (°C) -40 -37 -55 -23 -53 -52 -43* -54 -62 -30 -45

Glass transition temperature of the DEM. All the DEMs were obtained as viscous liquids except DEM-G. *Only DEM-G displayed melting at 52 °C

Table 2. Thermal properties of polyDEMs Tonset Tdeca (°C) (°C) PolyDEM-A 62 184 278 PolyDEM-B 47 172 256 PolyDEM-C 15 155 238 PolyDEM-D 83 141 265 142 278 PolyDEM-E 37 PolyDEM-F -26 145 263 PolyDEM-G -10 127 263 PolyDEM-H -37 158 277 125 263 PolyDEM-I -31 PolyDEM-J -51 119 293 PolyDEM-K -32 153 264 a Temperature where 50% weight loss occurs Code

Tg (°C)


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Table 3. CO2 adsorption data at 1 bar and 273 K for PolyDEMs

PolyDEM

CO2 Uptake (mg/g)

PolyDEM-A (citric acid) PolyDEM-B (citric acid) PolyDEM-C (oxalic acid) PolyDEM-D (malonic acid) PolyDEM-E (maleic acid) PolyDEM-F (benzeneimidamide) PolyDEM-G ( trifluorobenzeneimidamide) PolyDEM-H (propanimidamide) PolyDEM-I (diaminopyridine) PolyDEM-J (piperazinediamine) PolyDEM-K (diaminopropane)

7.26 7.27 6.46 1.69 2.38 3.46 2.14 4.44 2.02 2.87 2.69

*All the samples were nonporous


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Table of Contents

Sustainable Poly(ionic liquid)s for CO2 Capture based on Deep Eutectic Monomers Mehmet Isik, Sonia Zulfiqar, Fatimah Edhaim, Fernando Ruiperez, Alexander Rothenberger and David Mecerreyes Synopsis Deep eutectic monomer based poly(ionic liquid)s can be a sustainable alternative to current analogs for CO2 capture.


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Sustainable Poly(Ionic Liquids) for CO2 Capture ... - ACS Publications

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