Use of Eutectic Mixtures for Preparation of Monolithic Carbons with

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Use of Eutectic Mixtures for Preparation of Monolithic Carbons with CO2-Adsorption and Gas-Separation Capabilities Nieves Lopez-Salas, Erika Oliveira Jardim, Ana Silvestre-Albero, Maria Concepcion Gutierrez, Maria Luisa Ferrer, Francisco Rodriguez-Reinoso, Joaquin Silvestre-Albero, and Francisco del Monte Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5034146 • Publication Date (Web): 25 Sep 2014 Downloaded from http://pubs.acs.org on September 30, 2014

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Use of Eutectic Mixtures for Preparation of Monolithic Carbons with CO2-Adsorption and Gas-Separation Capabilities

N. López-Salas,a,§ E.O. Jardim,b,§ A. Silvestre-Albero,b M. C. Gutiérrez,a M. L. Ferrer,a F. Rodríguez-Reinoso,b J. Silvestre-Albero,b,* F. del Montea,*

a

Instituto de Ciencia de Materiales de Madrid-ICMM Consejo Superior de Investigaciones Científicas-CSIC. Campus de Cantoblanco 28049-Madrid (Spain) E-mail: [email protected] b

Laboratorio de Materiales Avanzados Departamento de Química Inorgánica Universidad de Alicante Ctra. San Vicente-Alicante s/n E-03080 Alicante (Spain) Email: [email protected] §

These authors have equally contributed to the results.

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Abstract With global warming becoming one of the main problems our society is facing nowadays, there is an urgent demand to develop materials suitable for CO2 storage as well as for gas separation. Within this context, hierarchical porous structures are of great interest for in-flow applications because of the desirable combination of an extensive internal reactive surface along narrow nanopores with facile molecular transport through broad “highways” leading to and from these pores. Deep eutectic solvents (DESs) have been recently used in the synthesis of carbon monoliths exhibiting a bicontinous porous structure composed of continuous macroporous channels and a continuous carbon network that contains a certain microporosity and provides considerable surface area. In this work, we have prepared two DESs for the preparation of two hierarchical carbon monoliths with different compositions (e.g. either nitrogen-doped or not) and structure. It is worth noting that DESs played a capital role in the synthesis of both hierarchical carbon monoliths not only promoting the spinodal decomposition that governs the formation of the bicontinuous porous structure but also providing the precursors required to tailor both the composition and the molecular sieve structure of the resulting carbons. We have studied the performance of these two carbons for CO2, N2 and CH4 adsorption in both monolithic and powdered form. We have also studied the selective adsorption of CO2 versus CH4 in both equilibrium and dynamic conditions. We found that these materials combined a high CO2-sorption capacity besides an excellent CO2/N2 and CO2/CH4 selectivity and, interestingly, this performance was preserved when processed in both monolithic and powdered form.


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Introduction One of the greenhouse gases contributing to global warming is CO2. It is estimated that more than 150 billion tons of CO2 have been released to the atmosphere from the combustion of fossil fuels and cement production since mid1970s. As the global energy demand from fossil fuels is projected to increase both in developed and developing countries, power generation will account for almost half the increase in global emissions between 2000 and 2030. While novel clean energy technologies capable to substitute conventional fossil fuels are still under investigation, the development of adsorbents able to separate and recycle CO2 before it is released to the atmosphere is a crucial step for a short-term solution. Among other sorbents, the use of carbons is gaining increased interest because of their relatively low cost, easy regeneration and the possibility to prepare them with tailormade high surface areas and surface functionalities. 1, 2, 3 However, most carbon materials for CO2 capture synthesized at lab scale are predominantly in powder or granular form so that attempts to use them in dynamic conditions are accompanied with some potential physicochemical drawbacks such as high-pressure drop, low heat and mass transfer, and mechanical attrition.4, 5, 6 The use of monoliths may overcome these limitations if they are synthesized in form of hierarchical three-dimensional structures comprising micropores and small mesopores (ca. 2-3 nm) that provide high surface areas, and large mesopores (ca. 20-50 nm) and macropores that guarantee mass transport and accessibility to the inner pore network.7, 8, 9, 10, 11 Within this context, the development of efficient synthetic processes for preparation of monolithic porous carbons with a controllable structure and good mechanical strength, avoiding the traditional problems associated with the use of a binder, is of high interest.12, 13 Polycondensation of resorcinol (R) with formaldehyde is one of the most common synthetic processes to obtain, after carbonization of the resins, carbon materials.14, 15, 16 The polycondensation reaction is typically carried out in aqueous media and in the presence of either acid or basic catalysts, with high conversion yields. The achievement of monoliths with controlled porous structures (i.e. hierarchical structures) typically needs of the use of surfactants or blockcopolymers as structure directing agents (SDA). Nowadays, due to the problems associated with scaling up and the regulatory pressure focusing on environmental


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issues, there is significant interest in using green chemistry alternatives to traditional synthetic processes that allow reducing or eliminating the use and generation of reagents.17, 18 The use of block-copolymers as SDA is also undesirable because its recovery after polycondensation is not easy – i.e. they become entrapped within the porosity, and their washing out is difficult because of the impeded diffusion of substances with pseudo-high molecular weight through such small pores. Eutectic mixtures (or deep eutectic solvents – DESs – as first described by Abbot and coworkers19, 20, 21, 22 and subsequently renamed as low-melting eutectic mixtures by König and coworkers23, 24, 25 or low-transition–temperature mixtures by Kroon and coworkers26, 27) based on R are opening interesting perspectives in this field.28, 29, 30 Hierarchical carbon materials obtained by the application of DES-assisted syntheses proved particularly interesting for selective gas adsorption purposes (e.g. CO2 versus N2 and CO2 versus CH4).31, 32 In one of this works, the use of eutectic mixtures of R, 3hydroxypyridine (3Hy) and Choline Chloride (ChCl) – in 2:2:1 and 1:1:1 molar ratios – resulted in hierarchical carbons with nitrogen functionalities (hierarchical nitrogendoped carbons, HNCs) that provided a relatively high CO2 adsorption capability, and good CO2 versus N2 adsorption selectivity.31 In another, the use of eutectic mixtures of R, 4-hexylresorcinol (an alkyl resorcinol derivative, 4HR) and tetraethylammonium bromide (TEA) – in 2:1:1 and 1.25:1:1 molar ratios – resulted in hierarchical carbons exhibiting a molecular-sieve-like microporosity (hierarchical carbon molecular sieves, HCMSs) that also provided a relatively high CO2 adsorption capability, and good CO2 versus CH4 adsorption selectivity.32 In both cases, the performance was only determined under equilibrium conditions for the powder samples despite the capability of DES-assisted syntheses to prepare monolithic carbons. Here in, we have explored the adsorption capacity and selectivity – not only in equilibrium but also in dynamic conditions – of two hierarchical carbon monoliths. The first one was prepared from a DES of R, 4-hydroxypyridine (4Hy) and ChCl in a 1:1:1 molar ratio and abbreviated onwards in the manuscript as CR4HyChCl-DES. The second one was prepared from a DES of R, 4HR and ChCl in a 2:1:1 molar ratio and abbreviated onwards in the manuscript as CR4HRChCl-DES. DESs formation was studied by 1H NMR spectroscopy and differential scanning calorimetry (DSC).

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C NMR and FTIR

spectroscopies provided an estimation of the degree of polycondensation. The


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hierarchical structure of carbons was studied by scanning electron microscopy (SEM). The textural properties of the powdered carbons were studied by nitrogen (N2) adsorption isotherms, as well as by immersion calorimetry. Finally, the dynamic capacity for CO2 adsorption was studied for gas mixtures of CO2 and CH4 in both monoliths and powdered samples.

Experimental Part Preparation of Deep Eutectic Solvents (DESs): DESs were obtained upon thermal treatment (at 90°C overnight) of the physical mixture of the individual components: R, 4HR and ChCl in a 2:1:1 molar ratio for the preparation of R4HRChCl-DES, and R, 4Hy and ChCl in a 1:1:1 molar ratio for the preparation of R4HyChCl-DES. Preparation of Carbons: Carbons were obtained from their respective DESs (e.g. R4HRChCl-DES and R4HyChClDES). For this purpose, the corresponding DES (either 0.637 mg of R4HRChCl-DES or 0.689 mg of R4HyChCl-DES) was mixed with an aqueous solution of formaldehyde (F, 37 wt %) for a precursor to F molar ratio (e.g. (R + 4HR)/F or (R + 4Hy)/F) of 2. Na2CO3 was added in the form of aqueous solution as well (0.061 mL, 140 mg/mL), to catalyse the polycondensation reaction. The DES content in dilution was finally ca. 53 wt %. After stirring for 5 min, the resulting mixture was aged for two hours at room temperature and, afterwards, thermally treated (first 6 h at 60°C and then, 7 days at 90°C) in closed containers to avoid evaporation. The resulting gels were washed three times in water (35 mL) for ChCl recovery. The washed gels were thermally treated under N2 atmosphere to 800°C for 4 hours (the heating ramp was 1.0°C/min) to obtain the respective carbons – e.g. CR4HyChCl-DES and CR4HRChCl-DES. Samples characterization: DSC traces were obtained using a TA Instruments Model DSC Q-100 system. The samples were run under nitrogen atmosphere in an aluminium pan in a sealed furnace, stabilized for 5 min at 20°C, and then cooled to –90°C before heating at a rate of 5°C/min. 1H NMR spectra were recorded in a Bruker spectrometer DRX-500. DESs were placed in capillary tubes, using deuterated chloroform (CDCl3) as an external reference (the deuterium signal was used for locking and shimming the sample). FTIR spectra were recorded in a BrukerModel IFS60v. Solid-state 13C-CPMASNMR spectra recorded in a Bruker AV-400-WB spectrometer, using a standard cross-


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polarization pulse sequence. The morphology of the resulting carbons was studied by scanning electron microscopy (SEM, using a SEM HITACHI S3000N system) while the porosity was determined by intrusion Hg porosimetry using a Quantachrome Poremaster. Elemental chemical analysis was performed in an LECO Elemental Analyzer CHNS 932. The technique involved sample combustion at 1000 °C in an oxygen rich environment. CO2, H2O and N2 were carried through the system by He carrier gas and quantitatively measured by means of a non-dispersive IR absorption detection system. N2 was determined via a thermal conductivity detector. Textural properties were obtained by gas physisorption (N2 at –196°C) in a home-made fully automated manometric equipment designed and constructed by the Advanced Materials Group (LMA), now commercialized as N2GSorb-6.33 The characterization of the microporous structure was also achieved by immersion calorimetry measurements using liquids of different molecular dimensions. Immersion calorimetry measurements into dichloromethane (DCM; 0,33 nm), 2,2-dimethyl-butane (2,2-DMB; 0,56 nm) and αpynene (0,70 nm) were performed in a Tian-Calved C80D Calorimeter at 30°C. A complete description of the experimental setup can be found elsewhere. 34 Gas adsorption measurements for N2, CH4 and CO2 were performed in the N2GSorb-6 equipment at 25°C using pure gas components. Kinetic evaluation of the carbon materials was performed at 25°C in a glass-made manometric adsorption equipment using pure gas components (CO2, CH4 and N2) at an equilibrium pressure of 400-450 mbar. Before any adsorption or calorimetric analysis, samples were degassed under vacuum at 100°C for 6h. It is worthy to highlight that adsorption and calorimetric characterization measurements were performed using powdered samples (upon monoliths grinding) due to the limitations of the experimental setup. Breakthrough column experiments were measured in a cylindrical bed micro-reactor using CR4HyChClDES

and CR4HRChCl-DES in either monolithic (3 in-line-monoliths providing a bed-height of

14.1 mm and 16.5 mm for CR4HyChCl-DES and CR4HRChCl-DES, respectively) or powdered form (0.36 g and 0.60 g for CR4HyChCl-DES and CR4HRChCl-DES, respectively, with the column height fixed at 16 mm) and a total flow of 5 ml/min of a mixture containing CO2:CH4:He in a 1:1:2 ratio. Before the column breakthrough analysis, carbons were thermally treated at 100°C for 6h under a He flow (22 mL/min).


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Results and Discussion As mentioned in the introduction, we have prepared two DESs in this manuscript, one composed of R, 4HR and ChCl in a 1:1:1 molar ratio and a second one composed of R, 4Hy and ChCl in a 1:1:1 molar ratio. DESs were obtained by simple thermally treatment at 90°C of the physical mixture of the components. Neither Tm nor Tc was clearly displayed in the DSC trace of any of the studied DESs (Figure 1), a common feature observed for non-easily crystallisable ILs and DESs.35 The formation of R-based complexes was also confirmed by the upfield chemical shift of the signals ascribed to R, 4HR and ChCl and R, 4Hy, and ChCl in the 1H NMR spectra of the respective mixtures (Figure S1, Tables S1 and S2).36, 37, 38 The use of ChCl in the former sample – instead of the TEA used in our previous work32 – and 4Hy in the latter one – instead of the 3Hy used in our previous work31 – made necessary to study how the polycondensation proceeds. For this purpose, the R4HRChCl-DES and R4HyChCl-DES polymers resulting from condensation were studied by FTIR and

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C NMR spectroscopies (Figures 2 and 3, respectively). Prior to

spectroscopic analysis, ChCl was washed out the polymers porosity by soaking them in abundant water as described elsewhere.28, 31, 32 The FTIR spectrum of R4HRChCl-DES polymers was similar to those reported before for R4HRTEA-DES polymers,32 with the band at 1607 cm-1 coming from aromatic ring stretches, the broad band at 3351 cm-1 coming from the aromatic OH groups, the medium to weak absorption bands at 1221 and 1093 cm-1 coming respectively from the C–O stretch and deformation of benzyl ether groups (methylene ether groups), and the bands at 2921 and 1475 cm-1 coming from the CH2 (methylene groups) stretching and bending vibrations (Figure 2a).39 Nonetheless, it is worth noting that despite the fact that the intensity of the bands ascribed to methylene ether and methylene groups typically allow estimating the number of bridges established upon polycondensation, this is not the case for R4HRChCl-DES polymers because of the intrinsic presence of methylene groups in 4HR. This feature was confirmed by 13C NMR spectroscopy (Figure 3a). The peaks ascribed to methylene ether (e.g., CH2–O–CH2) and methylene (e.g., CH2) bridges are at 53 ppm, and at 31 and 23 ppm, respectively. Different types of CH2 bridges can be distinguished depending on the chemical shift.40, 41

Thus, the most common 4–4’ methylene bridge has been typically assigned to signals


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at 30–38 ppm, and the less common 2–4’ methylene bridges to signals at 22–30 ppm. The relative intensity of these signals was again higher (ca. 2.5-fold) in R4HRChCl-DES polymers than in regular R-based polymers without 4HR. Actually, the signal at 14 ppm also provided evidence for the presence of hexyl groups. Thus, the clear assessment from the overall intensity of these signals was difficult to establish. In these type of polymers, the signals at 116 and 131 ppm – the former corresponding to aromatic carbons bearing CH2 groups in all ortho- positions relative to the two phenolic OHs and the latter to non-substituted aromatic carbons in meta positions – are more useful for the estimation of the polycondensation degree. The relative intensity of these signals was similar to that of samples obtained upon self-condensation of R28 so it seems that all the available nucleophilic positions of both precursors participated in the polycondensation and this was only impeded in the position occupied by hexyl groups. The good extension in which R and 4HR co-condensed was corroborated by the excellent carbon conversion that was obtained after carbonization of washed gels at 800 °C in a N2 atmosphere (e.g., 80%), in the range of those found for selfcondensation of R.28 Meanwhile, the FTIR and the

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C NMR spectra of R4HyChCl-DES polymers

exhibited common bands to those found for polymers resulting from co-condensation between R and 3Hy,31 but also revealed some different ones (Figure 2b). With regard to the former group of bands, the most interesting ones at the FTIR spectrum in terms of condensation were those at 2957 and 1476 cm-1 ascribed to the methylene groups stretching and bending vibrations, respectively. Bands at 1112 and 1375 cm-1 ascribed to C–N single bond stretching vibrations42, 43, 44 also belonged to this former group of bands common to co-condensation between R with either 3Hy or 4Hy. However, the band at 1637 cm-1 – coming from the C=O stretching – was only observed in R4HyChClDES polymers because keto-enol tautomerism is particularly favoured in 4Hy – as well as in 2Hy – as compared to 3Hy, for which the tautomer cannot adopt a neutral form.45 The occurrence of keto-enol tautomerism in 4Hy was also observed at the

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C NMR

spectrum of R4HyChCl-DES polymers by the appearance of a signal at 179 ppm. Moreover,

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C NMR spectroscopy confirmed the occurrence of condensation by the

appearance of signals ascribed to methylene ether (e.g., CH2–O–CH2) and to methylene (e.g., CH2) bridges at 53 ppm, and at 31 and 24 ppm respectively, as well as– signals


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ascribed to aromatic carbons bearing CH2 groups in all ortho positions relative to phenolic OHs – from both resorcinol and 4Hy – at 117 ppm (Figure 3b). Interestingly, the integration of the sum of the signals at 152 and 179 ppm (assigned to the aromatic phenolic carbons, both from R and 4Hy, and the keto form of the latter, respectively) versus the sum of the signals at 53, 31 and 24 ppm was ca. 1, in range to those reported for previous DES-assisted condensations using 3Hy. Signals at 132 and 142 ppm were ascribed to the non-substituted aromatic carbons at meta-positions of R, and at positions 2 and 6 of 4Hy, respectively. The washed R4HyChCl-DES and R4HRChCl-DES polymers (i.e. without ChCl) were thermally treated at 800°C in N2 atmosphere for the formation of carbon materials (e.g. CR4HyChCl-DES and CR4HRChCl-DES, respectively) with conversions that were in range to those reported for previous DES-assisted condensations (Table 1).28, 31, 32 We have previously described how DESs are responsible for the formation of morphologies in the form of a bicontinuous porous network composed of highly crosslinked clusters that aggregated and assembled into a stiff, interconnected structure. In this particular case, co-condensation between carbon precursors – e.g. R and 4Hy, or R and 4HR – resulted in the formation of a polymer-rich phase that was accompanied by the segregation of the noncondensed matter (e.g. ChCl) creating first a polymer-poor phase that, ultimately, becomes a polymer-depleted phase. This phase-separation process ended with the formation of aggregates-of-particle-like morphology (Figure 4). It is worth noting that the transient structure that results from phase separation has been correlated with the time relation between the onset of phase separation and gel formation – e.g. interconnected structures if the onset of both processes coincides or particle aggregate if phase segregation occurs earlier than gel formation.46, 47 This latter case where the polymer-rich domains break-up and become spherical during the coarsening process of the spinodal decomposition in order to decrease the interfacial energy before the structure freezing48 corresponds well with the morphology of our samples. The porosity of CR4HyChCl-DES and CR4HRChCl-DES – measured by Hg porosimetry – was ca. 42 and 26 %, respectively (Figure 5, Table 1). The nitrogen content of CR4HyChClDES

was ca. 3% as revealed the elemental chemical analysis (Table 1). Taking advantage of the monolithic form in which these DESs-assisted

syntheses allow the preparation of carbons, their performance for CO2, CH4, and N2


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adsorption in both monolithic and powdered form was evaluated. Thus, we first studied the N2 adsorption isotherms at –196°C on powdered samples. Both samples exhibited a type I isotherm (Figure S2) – according to the IUPAC classification – with a narrow knee at low relative pressures that revealed the presence of a narrow micropore size distribution typical of carbon molecular sieves. Textural parameters – obtained after application of the BET and Dubinin-Radushkevich equations – are summarized in Table 2. The BET surface area of CR4HyChCl-DES was ca. 532 m2/g, associated with a total micropore volume of 0.22 cm3/g. Meanwhile, CR4HRChCl-DES exhibited a slightly larger BET surface area of ca. 676 m2/g, and a micropore volume of Vo = 0.27 cm3/g. In both cases, negligible Vmeso were obtained, further corroborating the pure microporous nature of the carbons. It is also worth noting a certain shift between the adsorption and desorption branch of CR4HyChCl-DES that anticipated the presence of narrow micropores throughout which nitrogen diffusion was restricted.49 In these cases where the determination of the textural properties of carbon molecular sieves by N2 adsorption isotherms at –196°C is difficult, immersion calorimetry offers a reliable alternative to validate the data coming from N2 adsorption isotherms.50 In absence of specific interactions at the solid–liquid interface, one can get an indirect measurement of the surface area if the liquid – in terms of molecular dimensions – is adequately selected. Thus, we used three different liquids covering a wide range of kinetic diameters, from 0.33 nm for dichloromethane (DCM), to 0.56 nm for 2,2’-dimethylbutane (DMB), up to 0.7 nm for α-pinene. The liquid with the smallest kinetic diameter – DCM – exhibited the highest heat of immersion in both carbons (Table 2). These heats of immersion experienced a significant decrease – of nearly 10fold – for 2,2-DMB, and even larger for α-pinene. The limited accessibility for molecules with kinetic diameter of 0.56 nm and above confirmed the presence of narrow micropores (carbon molecular sieving properties) in both carbons (CR4HyChCl-DES and CR4HRChCl-DES). The surface areas calculated from the heats of immersion found for DCM – after calibration with the appropriate reference (e.g. a non-porous carbon) – were 788 m2/g and 777 m2/g for CR4HyChCl-DES and CR4HRChCl-DES, respectively. These values were above those found from N2 adsorption isotherms, thus confirming the kinetic restrictions experienced by N2 to reach the most inner porosity of our samples at – 196°C. Interestingly, the deviation between the surface area values obtained from N2


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adsorption isotherms and immersion calorimetry was more dramatic for the CR4HyChClDES

sample than for the CR4HRChCl-DES one, indicating the presence of more inner-pores-

with-narrower-dimension in the former case. Subsequently, we investigated the potential performance of these carbons for gas separation purposes. Measurements were carried in powdered samples at 25 °C and up to 1 bar. N2, CO2 and CH4 were the strategic gases of choice for determination of the adsorption capacities under equilibrium conditions of both carbon molecular sieves. The adsorption capacities decreased from CO2 to CH4 down to N2, with values in the range of 3.2-2.8 mmol/g for CO2, 1.7-1.4 mmol/g for CH4, and 0.6-0.5 mmol/g for N2 (Figure 6). Previous studies have demonstrated that when pore diameter is below 0.8-0.6 nm, this narrow microporosity is the main parameter governing CO2 adsorption at low pressure.51, 52, 53, 54, 55 The CO2 adsorption capacity was slightly larger for CR4HyChClDES

than for CR4HRChCl-DES in agreement with their respective amount of narrow

micropores – see data from immersion calorimetry above. Nonetheless, one should also consider the presence of nitrogen functionalities in CR4HyChCl-DES that provides specific sites for CO2 interaction so that the above-observed adsorption improvement in the CR4HyChCl-DES sample should most likely be ascribed to the combined action of both narrow micropores and nitrogen functionalities. The poor CH4 uptake is in accordance with the presence of a narrow pore structure (i.e. typical of molecular sieves) that prevents methane diffusion into the inner pore structure. The N2 adsorption capacity at 25°C found in both samples – well below than those found for the other two gases – indicated that the kinetic limitations mentioned above for nitrogen diffusion at -196°C still stand at 25°C. This molecular-sieve behaviour determined the suitability of both samples for the selective adsorption of CO2 versus CH4 and N2, with selectivity values under equilibrium conditions of ca. 1.9-2.0 and 5.35.6, respectively (Table 3). Besides the adsorption capacity, the adsorption kinetics is also a key factor to evaluate the performance of a sorbent material. Figure 7 shows that the adsorption kinetics of both samples (CR4HyChCl-DES and CR4HRChCl-DES) followed the trend described above for static conditions, decreasing from CO2 to CH4 down to N2. CR4HyChCl-DES and CR4HRChCl-DES exhibited a similar capacity for N2 and CH4 adsorption but not for CO2. For this latter gas, the presence of nitrogen functionalities – besides the narrow


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microporosity – made the performance of CR4HyChCl-DES to prevail over that of CR4HRChClDES,

also in agreement with the data obtained in static conditions. It is worth noting

that the elapsed time to reach the equilibrium for CO2, CH4 and N2 was less than 50 seconds in both samples, except for the particular case of CH4 in CR4HRChCl-DES. Kinetics of ca. 1 minute has been previously reported for microporous adsorbents with narrow pore apertures.56 The delay observed for CH4 adsorption on CR4HRChCl-DES suggested the presence of bottleneck-type pores with wide pore bodies and narrow pore entrances, in agreement with calorimetric data. The separation performance of our carbon samples under continuous flow conditions was tested in breakthrough experiments carried out at 30 °C and using a CO2/CH4/He gas mixture with a 1:1:2 molar ratio. For this purpose, both the monoliths and the powdered samples were tested in a column with a length of around 100 mm and an internal diameter of 10 mm. Figure 8 shows the breakthrough column analysis of the powdered samples. In this case, CH4 eluded within a few minutes (< 10 min) from the column, whereas CO2 was initially retained and its elution took ca. 20-35 min. According to the slope of the breakthrough profile, CR4HRChCl-DES exhibited slower adsorption kinetics than CR4HyChCl-DES for every gas and especially for CO2. This feature was in agreement with the kinetic analysis described above (Figure 7). In both cases, pre-adsorbed CH4 molecules expelled into the effluent stream by upcoming CO2 molecules caused the presence of some extra CH4 in the effluent stream (CH4 signal above C/C0 = 1) because of the preferred adsorption of CO2 versus CH4.57, 58 The CO2 and CH4 capacity was also calculated from the breakthrough curve. Thus, the CR4HRChClDES

in powdered form exhibited capacities of 1.1 and 0.5 mmol/g for CO2 and CH4,

respectively (Table 3). These values were slightly below those obtained for the powdered sample from the single-component isotherms (under equilibrium conditions) for CO2 and CH4 at a partial pressure of 250 mbar – e.g. 1.6 and 0.6 mmol/g, respectively (see Figure 6b). A similar behaviour was observed for sample CR4HyChCl-DES with a total adsorption capacity under dynamic conditions of 1.5 and 0.8 mmol/g for CO2 and CH4, respectively. These figures were slightly below those obtained under equilibrium conditions at 250 mbar – e.g. 1.6 and 0.7 mmol/g, respectively (see Figure 6a). It is worth noting that, as mentioned above, breakthrough experiments were carried out at 30 °C whereas the single-component isotherms were


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carried out at 25 °C so that considering the exothermic nature of the adsorption process, the adsorption capacity should increase for experiment carried out at the lowest temperature, unless kinetic restrictions are present.38 Thus, the large difference found for CR4HRChCl-DES between the data obtained under static and dynamic conditions – larger than for CR4HyChCl-DES – confirmed that kinetic restrictions were particularly significant in this carbon (see Figures 6 and 7a). It is worth noting that the CO2/CH4 selectivity of both carbons was similar in static and dynamic conditions (ca. 1.9-2.2). Finally, breakthrough column experiments were carried out with both samples in their monolithic form. The monolithic carbons behaved similarly to those in powdered form so that CH4 elution was first, then followed by CO2. However, the adsorption capacity experienced an anomalous enhancement as compared to the data obtained under equilibrium conditions. This result indicated that gas diffusion was not freely allowed throughout the monolithic structure. We hypothesized that the macroporous structure – in terms of either macropore dimensions or tortuosity – must be responsible of this feature. Actually, this anomalous behaviour was more significant for the sample exhibiting a higher porosity – e.g. CR4HyChCl-DES (Table 1).

Conclusions Two DESs based on mixtures of R with other carbon precursors (e.g. 4Hy and 4HR) have been prepared and they were used for the preparation of hierarchical porous carbon monoliths. The resulting carbons exhibited either nitrogen functionalities into the porous surface – thanks to the use of 4Hy – or a microporosity with enhanced molecular-sieve features – thanks to the use of 4HR. It is worth noting that neither structure directing agents nor additional solvents were needed for the preparation of the hierarchical porous carbon monoliths thanks to the all-in-one role played by DES in the syntheses. Besides this reagent economy, the use of cheap precursors and good conversions confer certain greenness to the synthesis and make it quite suitable for scaling up. The textural properties of both carbons were molecularsieve type. The CO2 capture and separation results show that both carbons in either monolithic or powdered form perform well as sorbents for CO2 uptake in both equilibrium and dynamic conditions. Nonetheless, it is worth noting that despite the fact that the hierarchical porous structure of the monoliths allowed gas flow, it was


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not sufficient enough and an anomalous enhancement of the adsorption capacity was observed in these cases as compared to the data obtained under equilibrium conditions. Further work is currently under progress to obtain hierarchical carbon monoliths, the macroporous structure of which is either of larger dimensions or connected in a less tortuous fashion 59, 60 so that the separation performance in dynamics conditions reach that of powdered samples.

Acknowledgement This work was supported by MINECO – with grants MAT2012-34811 and MAT201125329 (from the National Program of Fundamental Research), PLE2009-0052 (from the Strategic Japanese-Spanish Cooperation Program), and PCIN-2013-057 (from the Concert

Project-NASEMS)

–

and

by

Generalitat

Valenciana

–

grant

PROMETEO/2009/002. NLS also acknowledges MINECO for a FPI contract.

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Figure'1:!DSC!scans!of!(a)!R4HRChCl–DES!and!(b)!R4HyChCl–DES!polymers.!

Figure'2:!FTIR!Spectra!of!(a)!R4HRChCl–DES!and!(b)!R4HyChCl–DES!polymers.!

a!

'

'R 4H yC hC l!D E S 'R 4H R C hC l!D E S 'H e a t'F low'(W /g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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!

b! E ndo'P roc e s s !80 !60 !40 !20

0

20

40 o

'T e m pe ra ture '( C )

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!

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Figure'3:!Solid!state!13C!RMN!spectra!of!(a)!R4HRChCl–DES!and!(b)!R4HyChCl–DES!polymers.!

a!

Figure'4:!SEM!micrographs!of!CR4HRChClFDES!(a,!b),!and!CR4HyChClFDES!(c,!d).' !! !!

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Figure'5:!Plot!of!cumulative!volume!(black!line)!and!pore!size!distribution!(blue!line)! versus!pore!diameter!of!CR4HRChClFDES!(left)!and!CR4HyChClFDES!(right)!obtained!from! mercury!porosimetry!measurements.! !

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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+C H 4 +C O 2

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Page 23 of 24

Figure'7:!CO2,!CH4!and!N2!adsorption!kinetics!of!(a)!CR4HRChClFDES!and!(b)!CR4HyChClFDES! measured!in!powdered!form!and!at!25°C.!Equilibrium!pressure!400F450!mbar.!

Figure' 8:! Breakthrough! curves! of! (a)! CR4HRChClFDES! and! (b)! CR4HyChClFDES! measured! in! powdered!form,!using!a!CO2/CH4/He!mixture!and!a!space!velocity!of!230!hF1.!

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0.4

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Table'1.!Data!includes!conversion!of!R4HyChClFDES!and!R4HRChClFDES!polymers!into!CR4HyChClF DES!and!CR4HRChClFDES,!as!well!as!porosity!and!mean!pore!diameter!(obtained!from!Hg! porosimetry)!and!nitrogen!content!(obtained!from!elemental!analysis)!of!CR4HyChClFDES!and! CR4HRChClFDES.! ! '

Conversion'(%)'

Porosity'(%)'

CR4HyChClFDES' CR4HRChClFDES'

68! 81!

42.5! 26.4!

Mean'pore' diameter'(µm)' 8! 5!

%N' 3,08! FF!

' ' Table'2.!Textural!properties!of!CR4HyChClFDES!and!CR4HRChClFDES!deduced!from!nitrogen!adsorption! isotherms!at!F196!°C!and!immersion!calorimetry!at!30!°C!using!DCM,!2,2FDMB!and!αFpinene.! ! !

' ' ' ' '

Nitrogen! Adsorption! Isotherms! ! Immersion! Calorimetry!

SBET!(m2/g)! Vo!(cm3/g)! Vmeso!(cm3/g)! Vt!(cm3/g)! −ΔHDCM!!(J/g) −ΔH22DMB!!(J/g) −ΔHαFpinene!!(J/g) SDCM!(m2/g)!

CR4HRChClFDES! 676! 0.27! 0.03! 0.30! 87.6! 14.4! 7.4! 777!

CR4HyChClFDES! 532! 0.22! 0.07! 0.29! 88.8! 14.6! 9.9! 788!

! Table'3.!Total!amount!of!CO2,!CH4!and!N2!adsorbed!(mmol/g)!on!CR4HyChClFDES!and!CR4HRChClFDES! measured!under!static!(adsorption!isotherms)!and!dynamic!(adsorption!kinetics!and! breakthrough!column)!conditions.!Selectivities!for!CO2/CH4!and!CO2/N2!are!also!included.! !

CR4HRChClFDES! Capacity!(mmol/g)! Selectivity! CO2! CH4! N2! CO2/CH4! CO2/N2! 2.8! 1.4! 0.5! 2! 5.6!

Adsorption! isotherms!! Adsorption! 1.9! kinetics! Breakthough! 1.1! Column*! (2.0)!

CR4HyChClFDES! Capacity!(mmol/g)! Selectivity! CO2! CH4! N2! CO2/CH4! CO2/N2! 3.2! 1.7! 0.6! 1.9! 5.3!

0.9!

0.3!

2.1!

6.3!

2.1!

0.9!

0.3!

2.3!

7!

0.5! (0.9)!

FFFF!

2.2! (2.2)!

FFFF!

1.5! (4.2)!

0.8! (1.7)!

FFFF!

1.9! (2.5)!

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! *!Data!between!brackets!correspond!to!the!monolithic!samples!

!


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Use of Eutectic Mixtures for Preparation of Monolithic Carbons with

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