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Diamino protic ionic liquids for CO2 capture† Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19994
R. Vijayraghavan,a Steven J. Pas,bc Ekaterina I. Izgorodinaa and Douglas R. MacFarlane*a
Received 2nd October 2013, Accepted 21st October 2013 DOI: 10.1039/c3cp54082k www.rsc.org/pccp
A series of multifunctional protic ionic liquids (PILs), some of which are based on a combination of primary and tertiary amines in the same moiety coupled with a carboxylic acid, have been synthesised and employed for CO2 capture, yielding absorption capacities comparable to standard absorbents. In contrast to traditional amine absorbers, CO2 was found to desorb at lower temperatures and hence could result in a significant reduction in both the energy required to strip the absorber of CO2 and the thermally activated degradation mechanisms, which in traditional absorbers result in the loss of absorber and the production of toxic compounds. The lower basicity of the amine sites resultant from PIL formation decreases the binding energy of the CO2 to the absorber. The weaker basicity is also evidenced by lower pH of the PIL CO2 absorbers, which reduces common corrosion problems associated with traditional amine absorbers.
CO2 represents 77% of total greenhouse gas emissions1 and current levels of atmospheric CO2 are, at 390 ppm, higher than at any time in the last 800 000 years.2 This is an increase of more than 30% compared to the pre-industrial era due, primarily, to intensive burning of fossil fuels3 and increased clearing of agricultural land. Hence, there is an urgent need to reduce these emissions. Currently, there are several methods available for the absorption of CO2 from concentrated sources such as flue gas, including chemical absorption, physical absorption and membrane separation. The most commonly used commercial technology is the chemical absorption method employing aqueous amines.4,5 This method was developed many years ago in response to the need to remove CO2 from natural gas streams. However, the CO2 desorption stage of this cyclic process typically involves elevated a
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia. E-mail: [email protected] b Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia c Maritime Platforms Division, Defence Science and Technology Organisation, 506 Lorimer St, Fishermans Bend, Victoria 3207, Australia † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54082k
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temperatures and, as a result, evaporative loss of amine into the gas stream as well as its oxidative breakdown become issues. This adds not only additional costs to the process, but also a secondary atmospheric pollutant. Furthermore, these processes are energy intensive6 – the energy required in the regeneration step to break the chemical bonds between the absorbents and the absorbed CO2. The use of traditional amines also results in corrosive attack to equipment, tanks and piping. Moreover, industrial systems are mostly water based, and since water has a very large heat capacity, heating the absorber material to well above 100 1C is energy intensive. Hence this technology, although pointing the way towards a post-combustion CO2 capture process, needs significant development before it can provide the ultimate solution to carbon-capture needs. Ionic liquids (ILs) are organic salts with a melting point below 100 1C and most display low vapour pressure, eliminating the possibility of gaseous emissions.7–9 This vapour pressure advantage makes ILs more environmentally viable than organic solvents in some applications.10–13 It has also been reported that CO2 exhibits higher solubility in ILs compared to other gases such as CO, O2, N2.14–16 This higher solubility of CO2 allows ILs to be used to separate CO2 from mixtures of gases.17 In the context of CO2 absorption, ILs have additional attractive properties compared to conventional aqueous amine absorbents: (1) in the regeneration step typically less energy is required;18 (2) by careful selection of cation and anion they are highly thermally and chemically stable,19 and hence are less likely to degrade and (3) they have exhibited little volume expansion on absorption of CO2.20 In the past decade, conventional ILs, such as those based largely on imidazolium, pyrrolidinium or ammonium cations coupled with large anions with delocalised or sterically hindered charge, have been intensively investigated for CO2 capture by a physical absorption mechanism.21 The effect of the anion on the solubility of CO2 for imidazolium based ionic liquids has been studied and the results indicate that, in the case of anions containing fluorinated alkyl groups, a strong interaction with CO2 is responsible for the higher solubility.22–24 Nonetheless, since these ionic liquids remain limited in their physical absorbing
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Communication capacities, attention has more recently focused on functionalized ionic liquids wherein a functional group capable of binding CO2 is attached to one or other of the ions.20,25–31 These functional groups are chosen in order to favour absorption kinetically and thermodynamically. Davis and coworkers32 studied an ionic liquid consisting of an imidazolium cation with amine functionality, coupled with a tetrafluoroborate anion, and found that this IL was very effective for CO2 absorption; the molar uptake of CO2 per mole of IL approached 0.5, which is the theoretical maximum for CO2 capture as a carbamate salt. Recently, Wang and coworkers33 employed super-base derived ionic liquids capable of reversibly capturing CO2 at 1 mole per mole of IL, however the sensitivity of these to moisture in the gas stream remains unexplored. Yang et al. also described task specific ionic liquids for CO2 capture,34 while Sanchez35 showed that, at 0.1 MPa, CO2 solubilities for the ILs functionalised with a single amine group were 13 times higher than the non-functionalized ILs. On a per unit mass or volume basis, however, most of the abovementioned ILs typically exhibit CO2 uptake o10% w/w. This is less than the aqueous amine mixtures currently used; thus large quantities of relatively expensive ILs would be required to process large volumes of low pressure CO2 from flue gas streams. Hence, the objective of the present study was to explore the CO2 absorption capacity of new, relatively low molecular weight ionic liquids, including several combining multiple amine functionality. To lower the potential cost, as well as molar volume, of these ILs we have focused our attention on protic ionic liquids produced by proton transfer, i.e. those produced by simple mixing of an acid and a base. While this has the potential to increase volatility of the IL at elevated temperatures, we have found that the transfer of the proton to the cation from the anion in these cases lowers the desorption temperature considerably, hence, avoiding any potential significant evaporative loss at elevated temperature. The protic ionic liquids (PILs) investigated in this study are shown in Fig. 1. The PILs were made as per the literature procedures.36 Ionic liquids based on dimethylethylenediamine (DMEDAH Cl and DMEDAH CH3COO, HCOO), diethylethylenediamine (DEEDAH CH3COO, HCOO), 3-(dimethylamino)-1-propyl amine (DMAPAH CH3COO, HCOO), tris(2aminoethyl)amine (TAEAH HCOO), 1-(2aminoethyl) piperazine (AEPH HCOO), monoethanolamine (MEAH Cl), tributylamine (N444H Cl), triethylamine (N222H Cl), dimethylethylamine (N112H Cl and N112H Tf), diethylmethylamine (N221H Cl and N221H Tf) and piperazine (Pip Cl) were made by reacting 1 mole of the corresponding amines with 1 mole of respective aqueous acids in an ice bath and removing water at 70 1C under reduced pressure. In the preparation of formate and acetate anion based ionic liquids, the neat acids (not the aqueous solutions) have been added slowly to the corresponding amines kept in an ice bath. The stoichiometry of the acid–base reaction was verified by an aqueous titration method and samples of the prepared ILs were subsequently tested for correct pH after dilution into water (0.1 M). We have found this to be a sensitive, routine test of final stoichiometry for these materials. The CO2 uptake at P(CO2) = 1 atm was characterised using a Cahn-balance
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Fig. 1
Protic ionic liquids studied for CO2 capture.
connected with CO2 or N2 gas supply maintained at flow rate of 15 ml min 1. Quantum chemical calculations were performed with a GAUSSIAN 09 suite of programs.37 All ground state geometry optimizations were performed at the MP2/aug-cc-pVDZ level of theory. To incorporate a polar environment a conductor-like polarisable continuum model38 (CPCM) with water as solvent was used. The DMEDA diamine was fully conformationally screened to locate its lowest energy conformation. Improved electronic energies were calculated at the MP2/aug-cc-pVTZ level of theory. Control CO2 uptake experiments were carried out using a well-known amine absorbent, monoethanolamine (MEA), both neat and as the commonly used 30% w/w aqueous solution, in order to establish baseline data. The results are shown in entries 1 and 2 of Table 1. Consistent with literature results,5a uptake of CO2 in the 30% MEA aqueous solution of around 13% w/w was achieved at an MEA : CO2 ratio of 1 : 0.61 by mole. Attention was then focussed on PILs based on difunctional amines, such as the combination of both the primary and tertiary amine functional groups present in DMEDAH, DEEDAH and DMAPAH (Fig. 1).
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Table 1
CO2 uptake by protic ILs (P(CO2) B 100 kPa, T = 20 1C)
S. no.
System
w/w% uptakea CO2 (5%)
w/v% uptakea CO2 (5%)
Mole ratio IL : water : CO2
1 2 3 4 5 6 7 8
MEA (neat) 30% MEA in water DEEDAH formate DMEDAH formate DEEDAH acetate DMAPAH acetate DMAPAH formate TAEAH formate
29.9 13.1 12.6 12.3 7.4 9.0 8.3 3.8
30.2 13.2 12.1 12.6 — 8.6 8.2 —
1.0 : 0.0 : 0.42 1.0 : 7.93 : 0.61 1.0 : 0.0 : 0.47 1.0 : 0.0 : 0.38 1.0 : 0.0 : 0.30 1.0 : 0.0 : 0.33 1.0 : 0.0 : 0.28 1.0 : 0.0 : 0.16
9
AEPH formate
0.7
—
1.0 : 0.0 : 0.03
10
Pip Cl + water (30% w/w)
0.71
—
1.0 : 3.16 : 0.03
11 12 13 14 15 16 17 18 19 20
DMEDAH Cl + water (1 : 1 by mol) DMEDAH acetate + water (1:1 by mol) DMEDAH Tf + water (20% w/w) MEAH Cl + water (70% w/w) N112H Cl + water (30% w/w) N112H Tf + water (1 : 1 by mol) N221H Tf [Chol][Prolinate]39a [N2222][Alaninate]39b [APBIm][BF4]32
9.1 13.0 — 0.18 0.37 0.65 0.53
1.0 : 1.0 : 0.27 1.0 : 1.0 : 0.48 1.0 : 3.3 : 0.35 1.0 : 12.6 : 0.01 1.0 : 2.6 : 0.01 1.0 : 1.01 : 0.05 1.0 : 0.0 : 0.04 1.0 : 0.0 : 0.5 1.0 : 0.0 : 0.5 1.0 : 0.0 : 0.5
a
8.4 12.8 5.2 0.2 0.38 0.84 0.68 10.0 10.0 7.4
Comments
Super cooled IL Super cooled IL crystallises during CO2 absorption Super cooled IL crystallises during CO2 absorption Crystallises during CO2 absorption
Absorption carried out at 50 1C Absorption carried out at 40 1C
Calculated as mass CO2 absorbed/total mass (volume) of absorbent.
Here again, CO2 absorption in 30% w/w aqueous solutions of these diamines were studied as control experiments and the results indicated uptakes of around 1 mole of diamine : 1 mole of CO2. When the singly protonated PILs of these diamines are made there remains an amine group capable of CO2 absorption via carbamate formation. Of this family of salts (entries 3–7 of Table 1) the DMEDAH and DEEDAH formate salts, as well as the DMAPAH formate and acetate salts were found to be liquids at room temperature; in the other cases, for DMEDAH Cl, DMEDAH acetate and DMEDAH triflate (entries 11–13 in Table 1) water was added to achieve a liquid. Amongst the pure PILs, the DEEDAH formate PIL produced the highest molar absorption observed in this work (entry 3 of Table 1), at a DEEDAH : CO2 of 1 : 0.47 mol, which is approaching the theoretical value expected for carbamate formation involving the unprotonated amine functional group. On a mass of CO2 per unit volume of absorber basis DEEDAH formate (B12.1% w/v) and DMEDAH formate (B12.6% w/v) were similar to the industry standard absorber (entry 2 in Table 1). Entries 11–13 illustrate also the effect of adding small amounts of water to the mixture, thereby introducing the possibility of bicarbonate formation in addition to carbamate formation. At high enough water contents this has the potential to allow a stoichiometric uptake of 1 mol CO2 to 1 mol amine, the amine acting only as a Bronsted base in the process. Entry 12 shows a small improvement in this direction to produce the highest w/v% CO2 uptake observed during this study at 13% w/v, however it remains substantially below the 1 : 1 stoichiometry. It is notable that the 30% MEA aqueous solution only approaches this stoichiometry at considerably elevated pressures.5a Investigating the effect of other anions with the DMEDAH cation such as triflate with 20% water, did not produce any
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further increase with DMEDAH : CO2 = 1 : 0.35 (entry 13 in Table 1), which is similar to DMEDAH formate (entry 4 in Table 1). This is unexpected, as results obtained from other researchers show that the presence of fluorinated anions increases the physical absorption of CO2 in traditional ILs.22–24 For comparison, a number of the best literature systems in terms of w/w uptake are also included in Table 1 (entries 18–20). The PILs formed from the tri- and tetra-functional ammonium cations, TAEAH and AEPH, crystallised during CO2 absorption (entries 8 and 9 of Table 1), resulting in quite low CO2 uptake values between 4 and 0.7% w/w. While the crystallisation of carbamate salts is known in some cases, the relatively low CO2 content at which this takes place in these cases seems to suggest that it is the IL that is crystallising. The non-amine functionalised PILs based on tertiary amines (entries 15–17), did not produce significant absorption (below 1% w/v), as expected. The CO2 absorption and desorption kinetics in the neat DMEDAH formate ionic liquid were studied (Fig. 2). It is evident from the figure that at 20 1C there is a gradual increase in CO2 content up to 105 minutes before levelling out at B12% (1 atm CO2). No further CO2 absorption occurred beyond 140 minutes. On changing the flush gas to pure nitrogen, no significant CO2 desorption took place at 20 1C over a period of 60 minutes. CO2 desorption kinetics were then studied at 25 1C in a flowing N2 atmosphere (40 ml min 1); a gradual decrease in weight with time was observed (Fig. 2, blue curve). The completion of loss of the entire CO2 load occurred at around 270 minutes. The density and viscosity measured for the better CO2–PIL mixtures, compared to the standard MEA absorbent solution are shown in ESI-1.† The studies reveal that after CO2 absorption, the density and viscosity increases in both cases, as a result of increased intermolecular interactions arising from the
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Fig. 2 Absorption–desorption kinetics of CO2 in DMEDAH formate ionic liquid. Absorption at 20 1C, P(CO2) = 1 atm; desorption at 25 1C, flowing N2, P(CO2) = 0.
carbamate groups. The pH was also measured, ESI-2,† indicating that the 1 M 30% MEA solution is much more basic (pH = 11.3) than a 1 M aqueous solution of DMEDAH formate (pH = 9.1). Similarly, the pH of a 1 M solution of 30% MEA + CO2 solution was 9.0, while for a 1 M DMEDAH formate + CO2 solution, the pH was 7.3 (the pH of the neat ILs could not be determined accurately and hence they were measured in 1 M aqueous solution). These results confirm that the DMEDAH ionic liquids are distinctly less alkaline than the standard MEA solutions and therefore should not create the same level of corrosion issues as traditional amine absorbents. 13 C NMR spectra were obtained for some of the solutions in order to investigate the nature of the CO2 species being formed (ESI-3 to ESI-5†). As expected for the simple tertiary aminebased PILs mixed with water (entry 15) (ESI-3†) there was no signal for the carbamate carbon (expected at around 159–162 ppm), instead a peak at 124.9 ppm was observed after a prolonged experiment. This indicates the presence CO2 in the dissolved (aqueous) state.40 In neat (i.e., no added NMR solvent) DMEDAH formate + CO2 sample a signal is observed at 161.2 that is attributed to the carbamate carbon (ESI-4†), whereas the pure DMEDAH formate ionic liquid (also no NMR solvent) has no signals in this region (ESI-5†). In order to confirm the formation of carbamate species, as opposed to the carbamic acid, FT-IR spectra of the DMEDAH formate samples were obtained. The spectra show no evidence of the carbonyl stretch of carbamic acid species at 1700 cm 1 as discussed by Liotta et al.41 On the other hand, we observed increases in intensities of the C–N stretching band at 1250 cm 1, the COO symmetric stretch at 1336 cm 1 and the NH3+ bending at 1460 cm 1 in the CO2 loaded ionic liquid samples as compared to the neat ionic liquid. TGA analysis for the pure DMEDA amine, the DMEDAH formate salt, DMEDAH formate + CO2 (ESI-6†) shows that the neat amine begins to lose weight from room temperature
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PCCP onwards, whereas the DMEDAH formate PIL does not lose weight until 80 1C, indicating that DMEDAH formate is more stable than the corresponding amine as a result of the salt formation. In the TGA experiment on the CO2 loaded DMEDAH formate, rapid CO2 desorption occurs from the sample, beginning at around 40 1C. A similar experiment was carried out with standard MEA amine and its 30% working solution and the results are shown in ESI-7.† In this case, the amine is less volatile than the working solution and it appears that the water loss in the latter case increases the rate of evaporation of the amine. Viewing these systems from the point of view of proton energy levels, the initial proton transfer during the formation of the PIL can be expected to take place to the most basic nitrogen (lowest proton energy level), leaving the acid anion as the highest unfilled level. Considering the aqueous pKa values for EtNH2 (10.6) and dimethylethylamine (10.2) as a guide, the most basic nitrogen in a molecule such as DMEDA is likely to be the primary amine, however, this relatively small difference in basicity means that both nitrogens will have some degree of protonation. To explore this further, a sample of neat DMEDA was treated with CO2 which resulted in a carbamate salt with a DMEDA : CO2 of 1 : 0.5 mol. On the other hand, a pure tertiary amine (tributylamine) absorbed only 0.05 mol CO2 per mole amine, indicating the (expected) lack of activity of the tertiary nitrogen in carbamate formation. These experiments indicate that it is likely to be the primary amine nitrogen that is active in DMEDAH PILs for CO2 capture. Furthermore, since no significant absorption of CO2 is observed for N112H salts (entries 15 and 16), the absorption exhibited by DMEDAH is presumably related to carbamate formation at the primary nitrogen, displacing the PIL proton to the tertiary amine. In the case of the DMEDAH salts, the proton produced by the carbamate formation must be consumed by a second DMEDAH cation and this makes the proton basicity of this site also relevant. Most of the di-basic molecules studied here have pKaq a (1), corresponding to the second protonation equilibrium, between 5.6 and 8, indicating relatively higher energy proton sites. It is interesting to note that the DMAPAH salt (entry 7) has the lowest pKaq a (1) = 5.6 of the di-amino series studied here and it correspondingly has the lowest CO2 uptake. It would appear in these systems that pKaq a (1) in the region of, or greater than, that of DMEDAH (6.6) is necessary for the proton equilibria to lie in favour of carbamate formation. Nonetheless, as a result of the initial proton transfer during PIL formation, these pKaq a values are distinctly lower than would typically be observed for simple amine absorbers, by one to two pKa units. One of the direct consequences of the action of these much less basic amine sites on the PIL cations is the low desorption temperatures observed here. As a result, both the CO2 binding and the binding of the proton transferred on carbamate formation will be less energetic and therefore more easily reversed. The energy cost of the CO2 desorption process is a significant factor in carbon capture economics and this result points the way to an ability to tune the basicity to provide optimum desorption characteristics. The structure and stability of the CO2 adduct to the DMEDAH cation was further explored by ab initio calculations. As shown
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Fig. 3
Ion pair of DMEDAH formate and its CO2 adduct.
in ESI-8† three ion pair (IP) structures involving either the protonated primary amine or the protonated tertiary amine cations and the formate anion were studied. All three IPs are within 10 kJ mol 1 apart, thus confirming that the protic ionic liquid will be a mixture of the two cationic species. Based on these findings, both cations were considered for the CO2 addition reaction. IP1 was not able to form a carbamate since the protonation is on the primary nitrogen. In the case of the IP3 structure, in which the formate anion forms a double hydrogen bond with hydrogens of both nitrogen centres, the CO2 addition became exothermic as the energy of the lone pair is decreased, thus discouraging mixing with the HOMO of the formate. The structure of the carbamate ion pair that is thus formed is shown in Fig. 3. In the case of the cation having the tertiary nitrogen protonated, but where the formate anion formed only a single hydrogen bond (IP2), the addition of CO2 to the primary nitrogen was not possible as no addition product could be located on the potential energy surface. This finding was further confirmed by the analysis of molecular orbitals of the starting IP2 indicating that the lone pair on the primary nitrogen mixed with the HOMO of the formate, thus preventing the lone pair from being active towards the CO2 addition. In summary, a number of multi-amine based protic ionic liquids have produced promising results as CO2 absorbers, in particular when paired with small, light anions such as acetate, formate and chloride. Absorption capacities as high as 13% w/v have been observed with an IL : CO2 mole ratio close to the theoretical value for carbamate formation (1 : 0.5 mol). Further studies are underway to explore the absorption characteristics of these materials at lower P(CO2) and over a range of temperatures. The proton transfer that takes place during the formation of the PIL also tends to lower the subsequent basicity of the remaining amine functional groups. This produces less energetic binding of the CO2, lowering the desorption energetics and temperature and also rendering the medium less alkaline and therefore less corrosive. Therefore, by understanding approaches to alter the basicity of the amine functional groups, it becomes possible to tune the CO2 binding energy and resulting desorption temperatures. Ideal desorption temperatures can significantly reduce the total amount of energy required during the regenerative capture process and should also result in less amine loss due to evaporation and thermally activated oxidative degradation at high temperatures. Studies pertaining to the further designed functionalization of these amines are underway.
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DRM and EII are grateful to the Australian Research Council for Fellowship support. SJP acknowledges the DSTO Systems Integration Corporate Enabling Research Program (SI-CERP) for funding this work. EII would like to thank the Monash eResearch Centre and the National Facility of the National Computational Infrastructure for a generous allocation of computational resources.
Notes and References 1 IPCC Climate Change 2007 Assessment report 4: Synthesis report, p. 36. 2 CSIRO Climate Change 2012, http://www.csiro.au/Outcomes/ Climate/Understanding/State-of-the-Climate-2012.aspx. 3 IPCC Climate Change 2007 Assessment report 4. 4 R. W. Baker and K. Lokhandwala, Ind. Eng. Chem. Res., 2008, 47, 2109. 5 (a) F. Y. Jou, A. E. Mather and F. D. Otto, Can. J. Chem. Eng., 1995, 73, 140; (b) R. Idem and P. Tontiwachwuthikul, Ind. Eng. Chem. Res., 2006, 45, 2413. 6 G. Astarita, D. W. Savage and A. Bisio, Gas Treating with Chemical Solvents, John Wiley & Sons, New York, 1983. 7 J. S. Wilkes, Green Chem, 2002, 4, 73. 8 P. Wasserscheid and T. Welton, Ionic liquids in Synthesis, Wiley, Weinheim, 2003. 9 P. Wasserscheid and K. Keim, Angew. Chem., Int. Ed., 2000, 39, 3773. 10 R. S. Varma and V. V. Namboodiri, Chem. Commun., 2001, 643. 11 J. Golding, S. Forsyth, D. R. MacFarlane, M. Forsyth and G. B. Deacon, Green Chem, 2002, 4, 223. 12 J. D. Holbrey, et al., Green Chem, 2002, 4, 407. 13 P. T. Anastas and J. C. Warner, Green Chemistry Theory and Practice, Oxford University Press, New York, 1998. 14 S. N. V. K. Aki, A. M. Scurto and J. F. Brennecke, Ind. Eng. Chem. Res., 2006, 45, 5574. 15 D. Camper, P. Scovazzo, C. Koval and R. D. Noble, Ind. Eng. Chem. Res., 2004, 43, 3049. 16 A. Finotello, J. E. Bara, D. Camper and R. D. Noble, Ind. Eng. Chem. Res., 2008, 47, 3453. 17 C. Myers, H. Pennline, D. Luebke, J. Ilconich, J. K. Dixon, E. J. Maginn and J. F. Brennecke, J. Membr. Sci., 2008, 242, 322. 18 C. Cadena, J. L. Anthony, J. K. Shah, T. I. Morrow, J. F. Brennecke and E. J. Maginn, J. Am. Chem. Soc., 2004, 126, 5300. 19 S. A. Forsyth, J. M. Pringle and D. R. MacFarlane, Aust. J. Chem., 2004, 57, 113. 20 J. Huang and T. Ruther, Aust. J. Chem., 2009, 62, 298–308. 21 L. A. Blanchard, D. Hancu, E. J. Beckman and J. F. Brennecke, Nature, 1999, 399, 28. 22 S. N. V. K. Aki, B. R. Mellein, E. M. Saurer and J. F. Brennecke, J. Phys. Chem. B, 2004, 108, 20355. 23 J. Jacquemin, P. Husson, V. Mayer and I. Cibulka, J. Chem. Eng. Data, 2007, 52, 2204.
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Communication 24 X. Huang, C. J. Margulis, Y. Li and B. J. Berne, J. Am. Chem. Soc., 2005, 127, 17842. 25 J. F. Brennecke and B. E. Gurkan, J. Phys. Chem. Lett., 2010, 1, 3459–3464. 26 D. Chinn, D. Vu, M. S. Driver and L. C. Boudreau, US Pat., 0 129 598, 2005. 27 A. Yokozeki, M. B. Shiflett, C. P. Junk, L. M. Grieco and T. Foo, J. Phys. Chem. B, 2008, 112, 16654–16663. 28 J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen and X. Lv, Chem.–Eur. J., 2006, 12, 4021. 29 E. J. Maginn, Design and Evaluation of Ionic Liquids as Novel CO2 Absorbents, in DOE report, quarterly, 01/05–03/05 2005, 1–12. 30 M. B. Shiflett, D. J. Kasprazak, C. P. Junk and A. Yokozeki, J. Chem. Thermodyn., 2008, 40, 25. 31 E. J. Beckman, Chem. Commun., 2004, 1885. 32 E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis, J. Am. Chem. Soc., 2002, 124, 926–927. 33 C. Wang, H. Luo, D. Jiang, H. Li and S. Dai, Angew. Chem., Int. Ed., 2010, 49, 5978. 34 Z. Z. Yang, Y. N. Zhao and L. N. He, RSC Adv., 2011, 1, 545–567. 35 L. M. Galan Sanchez, G. W. Meindersma and A. B. de Haan, Chem. Eng. Res. Des., 2007, 85, 31–39. 36 H. Nakamoto and M. Watanabe, Chem. Commun., 2007, 2539–2541. 37 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
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38
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40 41
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, ¨ . Farkas, J. B. Foresman, S. Dapprich, A. D. Daniels, O J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. (a) V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001; (b) M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comput. Chem., 2003, 24, 669–681. (a) X. Li, M. Hou, Z. Zhang, B. Han, G. Yang, X. Wang and L. Zou, Green Chem., 2008, 10, 879–884; (b) Y. N. Jiang, G. N. Wang, Z. Zhou, Y. T. Wu, J. Geng and Z. B. Zhang, Chem. Commun., 2008, 505–507. T. M. Abbott, G. W. Buchanan, P. Kruus and K. C. Lee, Can. J. Chem., 1982, 60, 1000–1006. J. D. Switzer, A. L. Ethier, K. M. Flack, E. J. Biddinger, L. Gelbaum, P. Pollet, C. A. Eckert and C. L. Liotta, Ind. Eng. Chem. Res., 2013, 52, 13159–13163.
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Diamino protic ionic liquids for CO2 capture† Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19994
R. Vijayraghavan,a Steven J. Pas,bc Ekaterina I. Izgorodinaa and Douglas R. MacFarlane*a
Received 2nd October 2013, Accepted 21st October 2013 DOI: 10.1039/c3cp54082k www.rsc.org/pccp
A series of multifunctional protic ionic liquids (PILs), some of which are based on a combination of primary and tertiary amines in the same moiety coupled with a carboxylic acid, have been synthesised and employed for CO2 capture, yielding absorption capacities comparable to standard absorbents. In contrast to traditional amine absorbers, CO2 was found to desorb at lower temperatures and hence could result in a significant reduction in both the energy required to strip the absorber of CO2 and the thermally activated degradation mechanisms, which in traditional absorbers result in the loss of absorber and the production of toxic compounds. The lower basicity of the amine sites resultant from PIL formation decreases the binding energy of the CO2 to the absorber. The weaker basicity is also evidenced by lower pH of the PIL CO2 absorbers, which reduces common corrosion problems associated with traditional amine absorbers.
CO2 represents 77% of total greenhouse gas emissions1 and current levels of atmospheric CO2 are, at 390 ppm, higher than at any time in the last 800 000 years.2 This is an increase of more than 30% compared to the pre-industrial era due, primarily, to intensive burning of fossil fuels3 and increased clearing of agricultural land. Hence, there is an urgent need to reduce these emissions. Currently, there are several methods available for the absorption of CO2 from concentrated sources such as flue gas, including chemical absorption, physical absorption and membrane separation. The most commonly used commercial technology is the chemical absorption method employing aqueous amines.4,5 This method was developed many years ago in response to the need to remove CO2 from natural gas streams. However, the CO2 desorption stage of this cyclic process typically involves elevated a
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia. E-mail: [email protected] b Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia c Maritime Platforms Division, Defence Science and Technology Organisation, 506 Lorimer St, Fishermans Bend, Victoria 3207, Australia † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54082k
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temperatures and, as a result, evaporative loss of amine into the gas stream as well as its oxidative breakdown become issues. This adds not only additional costs to the process, but also a secondary atmospheric pollutant. Furthermore, these processes are energy intensive6 – the energy required in the regeneration step to break the chemical bonds between the absorbents and the absorbed CO2. The use of traditional amines also results in corrosive attack to equipment, tanks and piping. Moreover, industrial systems are mostly water based, and since water has a very large heat capacity, heating the absorber material to well above 100 1C is energy intensive. Hence this technology, although pointing the way towards a post-combustion CO2 capture process, needs significant development before it can provide the ultimate solution to carbon-capture needs. Ionic liquids (ILs) are organic salts with a melting point below 100 1C and most display low vapour pressure, eliminating the possibility of gaseous emissions.7–9 This vapour pressure advantage makes ILs more environmentally viable than organic solvents in some applications.10–13 It has also been reported that CO2 exhibits higher solubility in ILs compared to other gases such as CO, O2, N2.14–16 This higher solubility of CO2 allows ILs to be used to separate CO2 from mixtures of gases.17 In the context of CO2 absorption, ILs have additional attractive properties compared to conventional aqueous amine absorbents: (1) in the regeneration step typically less energy is required;18 (2) by careful selection of cation and anion they are highly thermally and chemically stable,19 and hence are less likely to degrade and (3) they have exhibited little volume expansion on absorption of CO2.20 In the past decade, conventional ILs, such as those based largely on imidazolium, pyrrolidinium or ammonium cations coupled with large anions with delocalised or sterically hindered charge, have been intensively investigated for CO2 capture by a physical absorption mechanism.21 The effect of the anion on the solubility of CO2 for imidazolium based ionic liquids has been studied and the results indicate that, in the case of anions containing fluorinated alkyl groups, a strong interaction with CO2 is responsible for the higher solubility.22–24 Nonetheless, since these ionic liquids remain limited in their physical absorbing
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Communication capacities, attention has more recently focused on functionalized ionic liquids wherein a functional group capable of binding CO2 is attached to one or other of the ions.20,25–31 These functional groups are chosen in order to favour absorption kinetically and thermodynamically. Davis and coworkers32 studied an ionic liquid consisting of an imidazolium cation with amine functionality, coupled with a tetrafluoroborate anion, and found that this IL was very effective for CO2 absorption; the molar uptake of CO2 per mole of IL approached 0.5, which is the theoretical maximum for CO2 capture as a carbamate salt. Recently, Wang and coworkers33 employed super-base derived ionic liquids capable of reversibly capturing CO2 at 1 mole per mole of IL, however the sensitivity of these to moisture in the gas stream remains unexplored. Yang et al. also described task specific ionic liquids for CO2 capture,34 while Sanchez35 showed that, at 0.1 MPa, CO2 solubilities for the ILs functionalised with a single amine group were 13 times higher than the non-functionalized ILs. On a per unit mass or volume basis, however, most of the abovementioned ILs typically exhibit CO2 uptake o10% w/w. This is less than the aqueous amine mixtures currently used; thus large quantities of relatively expensive ILs would be required to process large volumes of low pressure CO2 from flue gas streams. Hence, the objective of the present study was to explore the CO2 absorption capacity of new, relatively low molecular weight ionic liquids, including several combining multiple amine functionality. To lower the potential cost, as well as molar volume, of these ILs we have focused our attention on protic ionic liquids produced by proton transfer, i.e. those produced by simple mixing of an acid and a base. While this has the potential to increase volatility of the IL at elevated temperatures, we have found that the transfer of the proton to the cation from the anion in these cases lowers the desorption temperature considerably, hence, avoiding any potential significant evaporative loss at elevated temperature. The protic ionic liquids (PILs) investigated in this study are shown in Fig. 1. The PILs were made as per the literature procedures.36 Ionic liquids based on dimethylethylenediamine (DMEDAH Cl and DMEDAH CH3COO, HCOO), diethylethylenediamine (DEEDAH CH3COO, HCOO), 3-(dimethylamino)-1-propyl amine (DMAPAH CH3COO, HCOO), tris(2aminoethyl)amine (TAEAH HCOO), 1-(2aminoethyl) piperazine (AEPH HCOO), monoethanolamine (MEAH Cl), tributylamine (N444H Cl), triethylamine (N222H Cl), dimethylethylamine (N112H Cl and N112H Tf), diethylmethylamine (N221H Cl and N221H Tf) and piperazine (Pip Cl) were made by reacting 1 mole of the corresponding amines with 1 mole of respective aqueous acids in an ice bath and removing water at 70 1C under reduced pressure. In the preparation of formate and acetate anion based ionic liquids, the neat acids (not the aqueous solutions) have been added slowly to the corresponding amines kept in an ice bath. The stoichiometry of the acid–base reaction was verified by an aqueous titration method and samples of the prepared ILs were subsequently tested for correct pH after dilution into water (0.1 M). We have found this to be a sensitive, routine test of final stoichiometry for these materials. The CO2 uptake at P(CO2) = 1 atm was characterised using a Cahn-balance
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PCCP
Fig. 1
Protic ionic liquids studied for CO2 capture.
connected with CO2 or N2 gas supply maintained at flow rate of 15 ml min 1. Quantum chemical calculations were performed with a GAUSSIAN 09 suite of programs.37 All ground state geometry optimizations were performed at the MP2/aug-cc-pVDZ level of theory. To incorporate a polar environment a conductor-like polarisable continuum model38 (CPCM) with water as solvent was used. The DMEDA diamine was fully conformationally screened to locate its lowest energy conformation. Improved electronic energies were calculated at the MP2/aug-cc-pVTZ level of theory. Control CO2 uptake experiments were carried out using a well-known amine absorbent, monoethanolamine (MEA), both neat and as the commonly used 30% w/w aqueous solution, in order to establish baseline data. The results are shown in entries 1 and 2 of Table 1. Consistent with literature results,5a uptake of CO2 in the 30% MEA aqueous solution of around 13% w/w was achieved at an MEA : CO2 ratio of 1 : 0.61 by mole. Attention was then focussed on PILs based on difunctional amines, such as the combination of both the primary and tertiary amine functional groups present in DMEDAH, DEEDAH and DMAPAH (Fig. 1).
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Table 1
CO2 uptake by protic ILs (P(CO2) B 100 kPa, T = 20 1C)
S. no.
System
w/w% uptakea CO2 (5%)
w/v% uptakea CO2 (5%)
Mole ratio IL : water : CO2
1 2 3 4 5 6 7 8
MEA (neat) 30% MEA in water DEEDAH formate DMEDAH formate DEEDAH acetate DMAPAH acetate DMAPAH formate TAEAH formate
29.9 13.1 12.6 12.3 7.4 9.0 8.3 3.8
30.2 13.2 12.1 12.6 — 8.6 8.2 —
1.0 : 0.0 : 0.42 1.0 : 7.93 : 0.61 1.0 : 0.0 : 0.47 1.0 : 0.0 : 0.38 1.0 : 0.0 : 0.30 1.0 : 0.0 : 0.33 1.0 : 0.0 : 0.28 1.0 : 0.0 : 0.16
9
AEPH formate
0.7
—
1.0 : 0.0 : 0.03
10
Pip Cl + water (30% w/w)
0.71
—
1.0 : 3.16 : 0.03
11 12 13 14 15 16 17 18 19 20
DMEDAH Cl + water (1 : 1 by mol) DMEDAH acetate + water (1:1 by mol) DMEDAH Tf + water (20% w/w) MEAH Cl + water (70% w/w) N112H Cl + water (30% w/w) N112H Tf + water (1 : 1 by mol) N221H Tf [Chol][Prolinate]39a [N2222][Alaninate]39b [APBIm][BF4]32
9.1 13.0 — 0.18 0.37 0.65 0.53
1.0 : 1.0 : 0.27 1.0 : 1.0 : 0.48 1.0 : 3.3 : 0.35 1.0 : 12.6 : 0.01 1.0 : 2.6 : 0.01 1.0 : 1.01 : 0.05 1.0 : 0.0 : 0.04 1.0 : 0.0 : 0.5 1.0 : 0.0 : 0.5 1.0 : 0.0 : 0.5
a
8.4 12.8 5.2 0.2 0.38 0.84 0.68 10.0 10.0 7.4
Comments
Super cooled IL Super cooled IL crystallises during CO2 absorption Super cooled IL crystallises during CO2 absorption Crystallises during CO2 absorption
Absorption carried out at 50 1C Absorption carried out at 40 1C
Calculated as mass CO2 absorbed/total mass (volume) of absorbent.
Here again, CO2 absorption in 30% w/w aqueous solutions of these diamines were studied as control experiments and the results indicated uptakes of around 1 mole of diamine : 1 mole of CO2. When the singly protonated PILs of these diamines are made there remains an amine group capable of CO2 absorption via carbamate formation. Of this family of salts (entries 3–7 of Table 1) the DMEDAH and DEEDAH formate salts, as well as the DMAPAH formate and acetate salts were found to be liquids at room temperature; in the other cases, for DMEDAH Cl, DMEDAH acetate and DMEDAH triflate (entries 11–13 in Table 1) water was added to achieve a liquid. Amongst the pure PILs, the DEEDAH formate PIL produced the highest molar absorption observed in this work (entry 3 of Table 1), at a DEEDAH : CO2 of 1 : 0.47 mol, which is approaching the theoretical value expected for carbamate formation involving the unprotonated amine functional group. On a mass of CO2 per unit volume of absorber basis DEEDAH formate (B12.1% w/v) and DMEDAH formate (B12.6% w/v) were similar to the industry standard absorber (entry 2 in Table 1). Entries 11–13 illustrate also the effect of adding small amounts of water to the mixture, thereby introducing the possibility of bicarbonate formation in addition to carbamate formation. At high enough water contents this has the potential to allow a stoichiometric uptake of 1 mol CO2 to 1 mol amine, the amine acting only as a Bronsted base in the process. Entry 12 shows a small improvement in this direction to produce the highest w/v% CO2 uptake observed during this study at 13% w/v, however it remains substantially below the 1 : 1 stoichiometry. It is notable that the 30% MEA aqueous solution only approaches this stoichiometry at considerably elevated pressures.5a Investigating the effect of other anions with the DMEDAH cation such as triflate with 20% water, did not produce any
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further increase with DMEDAH : CO2 = 1 : 0.35 (entry 13 in Table 1), which is similar to DMEDAH formate (entry 4 in Table 1). This is unexpected, as results obtained from other researchers show that the presence of fluorinated anions increases the physical absorption of CO2 in traditional ILs.22–24 For comparison, a number of the best literature systems in terms of w/w uptake are also included in Table 1 (entries 18–20). The PILs formed from the tri- and tetra-functional ammonium cations, TAEAH and AEPH, crystallised during CO2 absorption (entries 8 and 9 of Table 1), resulting in quite low CO2 uptake values between 4 and 0.7% w/w. While the crystallisation of carbamate salts is known in some cases, the relatively low CO2 content at which this takes place in these cases seems to suggest that it is the IL that is crystallising. The non-amine functionalised PILs based on tertiary amines (entries 15–17), did not produce significant absorption (below 1% w/v), as expected. The CO2 absorption and desorption kinetics in the neat DMEDAH formate ionic liquid were studied (Fig. 2). It is evident from the figure that at 20 1C there is a gradual increase in CO2 content up to 105 minutes before levelling out at B12% (1 atm CO2). No further CO2 absorption occurred beyond 140 minutes. On changing the flush gas to pure nitrogen, no significant CO2 desorption took place at 20 1C over a period of 60 minutes. CO2 desorption kinetics were then studied at 25 1C in a flowing N2 atmosphere (40 ml min 1); a gradual decrease in weight with time was observed (Fig. 2, blue curve). The completion of loss of the entire CO2 load occurred at around 270 minutes. The density and viscosity measured for the better CO2–PIL mixtures, compared to the standard MEA absorbent solution are shown in ESI-1.† The studies reveal that after CO2 absorption, the density and viscosity increases in both cases, as a result of increased intermolecular interactions arising from the
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Fig. 2 Absorption–desorption kinetics of CO2 in DMEDAH formate ionic liquid. Absorption at 20 1C, P(CO2) = 1 atm; desorption at 25 1C, flowing N2, P(CO2) = 0.
carbamate groups. The pH was also measured, ESI-2,† indicating that the 1 M 30% MEA solution is much more basic (pH = 11.3) than a 1 M aqueous solution of DMEDAH formate (pH = 9.1). Similarly, the pH of a 1 M solution of 30% MEA + CO2 solution was 9.0, while for a 1 M DMEDAH formate + CO2 solution, the pH was 7.3 (the pH of the neat ILs could not be determined accurately and hence they were measured in 1 M aqueous solution). These results confirm that the DMEDAH ionic liquids are distinctly less alkaline than the standard MEA solutions and therefore should not create the same level of corrosion issues as traditional amine absorbents. 13 C NMR spectra were obtained for some of the solutions in order to investigate the nature of the CO2 species being formed (ESI-3 to ESI-5†). As expected for the simple tertiary aminebased PILs mixed with water (entry 15) (ESI-3†) there was no signal for the carbamate carbon (expected at around 159–162 ppm), instead a peak at 124.9 ppm was observed after a prolonged experiment. This indicates the presence CO2 in the dissolved (aqueous) state.40 In neat (i.e., no added NMR solvent) DMEDAH formate + CO2 sample a signal is observed at 161.2 that is attributed to the carbamate carbon (ESI-4†), whereas the pure DMEDAH formate ionic liquid (also no NMR solvent) has no signals in this region (ESI-5†). In order to confirm the formation of carbamate species, as opposed to the carbamic acid, FT-IR spectra of the DMEDAH formate samples were obtained. The spectra show no evidence of the carbonyl stretch of carbamic acid species at 1700 cm 1 as discussed by Liotta et al.41 On the other hand, we observed increases in intensities of the C–N stretching band at 1250 cm 1, the COO symmetric stretch at 1336 cm 1 and the NH3+ bending at 1460 cm 1 in the CO2 loaded ionic liquid samples as compared to the neat ionic liquid. TGA analysis for the pure DMEDA amine, the DMEDAH formate salt, DMEDAH formate + CO2 (ESI-6†) shows that the neat amine begins to lose weight from room temperature
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PCCP onwards, whereas the DMEDAH formate PIL does not lose weight until 80 1C, indicating that DMEDAH formate is more stable than the corresponding amine as a result of the salt formation. In the TGA experiment on the CO2 loaded DMEDAH formate, rapid CO2 desorption occurs from the sample, beginning at around 40 1C. A similar experiment was carried out with standard MEA amine and its 30% working solution and the results are shown in ESI-7.† In this case, the amine is less volatile than the working solution and it appears that the water loss in the latter case increases the rate of evaporation of the amine. Viewing these systems from the point of view of proton energy levels, the initial proton transfer during the formation of the PIL can be expected to take place to the most basic nitrogen (lowest proton energy level), leaving the acid anion as the highest unfilled level. Considering the aqueous pKa values for EtNH2 (10.6) and dimethylethylamine (10.2) as a guide, the most basic nitrogen in a molecule such as DMEDA is likely to be the primary amine, however, this relatively small difference in basicity means that both nitrogens will have some degree of protonation. To explore this further, a sample of neat DMEDA was treated with CO2 which resulted in a carbamate salt with a DMEDA : CO2 of 1 : 0.5 mol. On the other hand, a pure tertiary amine (tributylamine) absorbed only 0.05 mol CO2 per mole amine, indicating the (expected) lack of activity of the tertiary nitrogen in carbamate formation. These experiments indicate that it is likely to be the primary amine nitrogen that is active in DMEDAH PILs for CO2 capture. Furthermore, since no significant absorption of CO2 is observed for N112H salts (entries 15 and 16), the absorption exhibited by DMEDAH is presumably related to carbamate formation at the primary nitrogen, displacing the PIL proton to the tertiary amine. In the case of the DMEDAH salts, the proton produced by the carbamate formation must be consumed by a second DMEDAH cation and this makes the proton basicity of this site also relevant. Most of the di-basic molecules studied here have pKaq a (1), corresponding to the second protonation equilibrium, between 5.6 and 8, indicating relatively higher energy proton sites. It is interesting to note that the DMAPAH salt (entry 7) has the lowest pKaq a (1) = 5.6 of the di-amino series studied here and it correspondingly has the lowest CO2 uptake. It would appear in these systems that pKaq a (1) in the region of, or greater than, that of DMEDAH (6.6) is necessary for the proton equilibria to lie in favour of carbamate formation. Nonetheless, as a result of the initial proton transfer during PIL formation, these pKaq a values are distinctly lower than would typically be observed for simple amine absorbers, by one to two pKa units. One of the direct consequences of the action of these much less basic amine sites on the PIL cations is the low desorption temperatures observed here. As a result, both the CO2 binding and the binding of the proton transferred on carbamate formation will be less energetic and therefore more easily reversed. The energy cost of the CO2 desorption process is a significant factor in carbon capture economics and this result points the way to an ability to tune the basicity to provide optimum desorption characteristics. The structure and stability of the CO2 adduct to the DMEDAH cation was further explored by ab initio calculations. As shown
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Fig. 3
Ion pair of DMEDAH formate and its CO2 adduct.
in ESI-8† three ion pair (IP) structures involving either the protonated primary amine or the protonated tertiary amine cations and the formate anion were studied. All three IPs are within 10 kJ mol 1 apart, thus confirming that the protic ionic liquid will be a mixture of the two cationic species. Based on these findings, both cations were considered for the CO2 addition reaction. IP1 was not able to form a carbamate since the protonation is on the primary nitrogen. In the case of the IP3 structure, in which the formate anion forms a double hydrogen bond with hydrogens of both nitrogen centres, the CO2 addition became exothermic as the energy of the lone pair is decreased, thus discouraging mixing with the HOMO of the formate. The structure of the carbamate ion pair that is thus formed is shown in Fig. 3. In the case of the cation having the tertiary nitrogen protonated, but where the formate anion formed only a single hydrogen bond (IP2), the addition of CO2 to the primary nitrogen was not possible as no addition product could be located on the potential energy surface. This finding was further confirmed by the analysis of molecular orbitals of the starting IP2 indicating that the lone pair on the primary nitrogen mixed with the HOMO of the formate, thus preventing the lone pair from being active towards the CO2 addition. In summary, a number of multi-amine based protic ionic liquids have produced promising results as CO2 absorbers, in particular when paired with small, light anions such as acetate, formate and chloride. Absorption capacities as high as 13% w/v have been observed with an IL : CO2 mole ratio close to the theoretical value for carbamate formation (1 : 0.5 mol). Further studies are underway to explore the absorption characteristics of these materials at lower P(CO2) and over a range of temperatures. The proton transfer that takes place during the formation of the PIL also tends to lower the subsequent basicity of the remaining amine functional groups. This produces less energetic binding of the CO2, lowering the desorption energetics and temperature and also rendering the medium less alkaline and therefore less corrosive. Therefore, by understanding approaches to alter the basicity of the amine functional groups, it becomes possible to tune the CO2 binding energy and resulting desorption temperatures. Ideal desorption temperatures can significantly reduce the total amount of energy required during the regenerative capture process and should also result in less amine loss due to evaporation and thermally activated oxidative degradation at high temperatures. Studies pertaining to the further designed functionalization of these amines are underway.
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DRM and EII are grateful to the Australian Research Council for Fellowship support. SJP acknowledges the DSTO Systems Integration Corporate Enabling Research Program (SI-CERP) for funding this work. EII would like to thank the Monash eResearch Centre and the National Facility of the National Computational Infrastructure for a generous allocation of computational resources.
Notes and References 1 IPCC Climate Change 2007 Assessment report 4: Synthesis report, p. 36. 2 CSIRO Climate Change 2012, http://www.csiro.au/Outcomes/ Climate/Understanding/State-of-the-Climate-2012.aspx. 3 IPCC Climate Change 2007 Assessment report 4. 4 R. W. Baker and K. Lokhandwala, Ind. Eng. Chem. Res., 2008, 47, 2109. 5 (a) F. Y. Jou, A. E. Mather and F. D. Otto, Can. J. Chem. Eng., 1995, 73, 140; (b) R. Idem and P. Tontiwachwuthikul, Ind. Eng. Chem. Res., 2006, 45, 2413. 6 G. Astarita, D. W. Savage and A. Bisio, Gas Treating with Chemical Solvents, John Wiley & Sons, New York, 1983. 7 J. S. Wilkes, Green Chem, 2002, 4, 73. 8 P. Wasserscheid and T. Welton, Ionic liquids in Synthesis, Wiley, Weinheim, 2003. 9 P. Wasserscheid and K. Keim, Angew. Chem., Int. Ed., 2000, 39, 3773. 10 R. S. Varma and V. V. Namboodiri, Chem. Commun., 2001, 643. 11 J. Golding, S. Forsyth, D. R. MacFarlane, M. Forsyth and G. B. Deacon, Green Chem, 2002, 4, 223. 12 J. D. Holbrey, et al., Green Chem, 2002, 4, 407. 13 P. T. Anastas and J. C. Warner, Green Chemistry Theory and Practice, Oxford University Press, New York, 1998. 14 S. N. V. K. Aki, A. M. Scurto and J. F. Brennecke, Ind. Eng. Chem. Res., 2006, 45, 5574. 15 D. Camper, P. Scovazzo, C. Koval and R. D. Noble, Ind. Eng. Chem. Res., 2004, 43, 3049. 16 A. Finotello, J. E. Bara, D. Camper and R. D. Noble, Ind. Eng. Chem. Res., 2008, 47, 3453. 17 C. Myers, H. Pennline, D. Luebke, J. Ilconich, J. K. Dixon, E. J. Maginn and J. F. Brennecke, J. Membr. Sci., 2008, 242, 322. 18 C. Cadena, J. L. Anthony, J. K. Shah, T. I. Morrow, J. F. Brennecke and E. J. Maginn, J. Am. Chem. Soc., 2004, 126, 5300. 19 S. A. Forsyth, J. M. Pringle and D. R. MacFarlane, Aust. J. Chem., 2004, 57, 113. 20 J. Huang and T. Ruther, Aust. J. Chem., 2009, 62, 298–308. 21 L. A. Blanchard, D. Hancu, E. J. Beckman and J. F. Brennecke, Nature, 1999, 399, 28. 22 S. N. V. K. Aki, B. R. Mellein, E. M. Saurer and J. F. Brennecke, J. Phys. Chem. B, 2004, 108, 20355. 23 J. Jacquemin, P. Husson, V. Mayer and I. Cibulka, J. Chem. Eng. Data, 2007, 52, 2204.
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Communication 24 X. Huang, C. J. Margulis, Y. Li and B. J. Berne, J. Am. Chem. Soc., 2005, 127, 17842. 25 J. F. Brennecke and B. E. Gurkan, J. Phys. Chem. Lett., 2010, 1, 3459–3464. 26 D. Chinn, D. Vu, M. S. Driver and L. C. Boudreau, US Pat., 0 129 598, 2005. 27 A. Yokozeki, M. B. Shiflett, C. P. Junk, L. M. Grieco and T. Foo, J. Phys. Chem. B, 2008, 112, 16654–16663. 28 J. Zhang, S. Zhang, K. Dong, Y. Zhang, Y. Shen and X. Lv, Chem.–Eur. J., 2006, 12, 4021. 29 E. J. Maginn, Design and Evaluation of Ionic Liquids as Novel CO2 Absorbents, in DOE report, quarterly, 01/05–03/05 2005, 1–12. 30 M. B. Shiflett, D. J. Kasprazak, C. P. Junk and A. Yokozeki, J. Chem. Thermodyn., 2008, 40, 25. 31 E. J. Beckman, Chem. Commun., 2004, 1885. 32 E. D. Bates, R. D. Mayton, I. Ntai and J. H. Davis, J. Am. Chem. Soc., 2002, 124, 926–927. 33 C. Wang, H. Luo, D. Jiang, H. Li and S. Dai, Angew. Chem., Int. Ed., 2010, 49, 5978. 34 Z. Z. Yang, Y. N. Zhao and L. N. He, RSC Adv., 2011, 1, 545–567. 35 L. M. Galan Sanchez, G. W. Meindersma and A. B. de Haan, Chem. Eng. Res. Des., 2007, 85, 31–39. 36 H. Nakamoto and M. Watanabe, Chem. Commun., 2007, 2539–2541. 37 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
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B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, ¨ . Farkas, J. B. Foresman, S. Dapprich, A. D. Daniels, O J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. (a) V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001; (b) M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comput. Chem., 2003, 24, 669–681. (a) X. Li, M. Hou, Z. Zhang, B. Han, G. Yang, X. Wang and L. Zou, Green Chem., 2008, 10, 879–884; (b) Y. N. Jiang, G. N. Wang, Z. Zhou, Y. T. Wu, J. Geng and Z. B. Zhang, Chem. Commun., 2008, 505–507. T. M. Abbott, G. W. Buchanan, P. Kruus and K. C. Lee, Can. J. Chem., 1982, 60, 1000–1006. J. D. Switzer, A. L. Ethier, K. M. Flack, E. J. Biddinger, L. Gelbaum, P. Pollet, C. A. Eckert and C. L. Liotta, Ind. Eng. Chem. Res., 2013, 52, 13159–13163.
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