Coordination effect-regulated CO₂ capture with an alkali metal onium salts/crown ether system

Abstract

A coordination effect was employed to realize equimolar CO₂ absorption, adopting easily synthesized amino group containing absorbents (alkali metal onium salts). The essence of our strategy was to increase the steric hindrance of cations so as to enhance a carbamic acid pathway for CO₂ capture. Our easily synthesized alkali metal amino acid salts or phenolates were coordinated with crown ethers, in which highly sterically hindered cations were obtained through a strong coordination effect of crown ethers with alkali metal cations. For example, a CO₂ capacity of 0.99 was attained by potassium prolinate/18-crown-6, being characterized by NMR, FT-IR, and quantum chemistry calculations to go through a carbamic acid formation pathway. The captured CO₂ can be stripped under very mild conditions (50 °C, N₂). Thus, this protocol offers an alternative for the development of technological innovation towards efficient and low energy processes for carbon capture and sequestration.

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Introduction

The chemistry of CO₂ has attracted much attention from the scientific community because of the increasing atmospheric CO₂ concentration associated with global warming.^1–9 CO₂ capture from fossil fuel combustion represents a critical component of efforts aimed at stabilizing greenhouse gas levels in the atmosphere.¹⁰ In addition, removal of CO₂ from natural gas is of vital importance to maintain and expand the availability of these clean-burning, efficient fuel sources.⁵

In recent years, worldwide efforts have been devoted to developing various technologies/processes for CO₂ capture, including liquid absorbents, solid adsorbents and membranes.^4,6,7,9 Among them, chemisorption of CO₂ based on C–N bond formation is one of the most efficient strategies,⁸ utilizing liquid absorbents such as conventional aqueous amine solutions,^11–20 chilled ammonia,^21–30 amino-functionalized ionic liquids (ILs),^31–42 and amino-functionalized solid absorbents.^4,43–49 The traditional technology for CO₂ capture is chemisorption by aqueous solution of monoethanolamine (high reactivity, low cost and good absorption capacity) to produce ammonium bicarbonate.^50,51 However, this process for CO₂ capture has some disadvantages, including solvent loss, corrosion and particularly, highly energy intensive nature owing to the thermodynamic properties of water and high enthalpy of absorption.⁵² Notably, there are inherent drawbacks generally associated with amino group-containing absorbents in the absence of water, that is, the requirement of two amine units to react with one CO₂ molecule, leading to the formation of ammonium carbamate (Scheme 1). On the other hand, extensive energy input in desorption processes is needed owing to the relatively higher stability characterization of ammonium carbamate compared with the carbamic acid intermediate. Hence, increasing the undesired 1 : 2 (CO₂ : amine) stoichiometry for CO₂ capacity to 1 : 1 and reducing the energy requirement of desorption are essential prerequisites for a breakthrough in absorption techniques.³² Therefore, it will be very desirable to form relatively unstable carbamic acid (1 : 1) for CO₂ capture, that is, to prevent the deprotonation of carbamic acid by another amino group. In this way, equimolar CO₂ absorption and simultaneously easy CO₂ desorption will be realized due to the relatively unstable nature of carbamic acid.

Scheme 1 Reactions of CO₂ with amino group-containing absorbents.

As readily accessible chiral molecules, natural α-amino acids (AAs) have many merits, such as diverse structures, low cost, high thermal stability, bio-compatibility, easy regeneration and biodegradability.⁵³ Phosphonium and ammonium ionic liquids (ILs) with AAs as anions have been already used for CO₂ uptake, but the CO₂ : AA stoichiometry arising from the formation of ammonium carbamate is still 1 : 2.^34–38 In this respect, specially designed absorbents derived from AAs have been successfully synthesized and applied for 1 : 1 CO₂ capture through a carbamic acid formation pathway. Gurkan et al. have synthesized two AA-based ILs with a long-chain alkyl phosphonium cation, including trihexyl(tetradecyl)phosphonium prolinate([P₆₆₆₁₄][Pro]) and methioninate ([P₆₆₆₁₄][Met]), both of which can react with CO₂ in a ratio of one CO₂ per amine (1 : 1 stoichiometry) by termination of the reaction sequence at the formation of carbamic acid.³³ We also designed AA salts with bulky N-substituents for chemisorption of CO₂, approaching almost equimolar absorption in polyethylene glycol (PEG) solution.⁵⁴ The key idea of these two successful examples leading to the formation of carbamic acid is high steric hindrance, either derived from bulky cations [P₆₆₆₁₄]⁺, or bulky N-substituents (ⁱPr), to prevent nucleophilic attack of the amino group on the carboxylic acid group of carbamic acid. Despite such great advances, the development of efficient processes for CO₂ capture with simple, easily prepared absorbents is still appealing. Therefore, it is desirable to develop a way to make easily synthesized AA salts with “small” cations and regular anions, such as commercially available alkali metal cations and AA anions, to obtain an equimolar absorption of CO₂ through a carbamic acid formation pathway. Moreover, the carboxylate group of the AA salts has the potential to stabilize the formed carbamic acid through intramolecular hydrogen-bond formation.

Crown ethers are cyclic chemical compounds that consist of several ether groups, which can strongly bind certain cations to form complexes, probably possessing some toxicity to creatures,⁵⁵ thus having the potential to form bulky cations.^56–63 Herein, we found that the coordination effect between crown ether and alkali metal cations (increasing steric hindrance) leads to an extremely high CO₂ capacity, approaching almost equimolar absorption in PEG solution, adopting easily synthesized onium salts derived from AAs or phenols and alkali metal hydroxide as absorbents. Coordination effect-adjusted CO₂ absorption is assumed to proceed via the carbamic acid formation rather than the subsequent ammonium carbamate. On the other hand, the coordination effect leading to the formation of relatively unstable carbamic acid greatly reduces the energy requirement for CO₂ desorption in comparison with ILs with the same anions.

Results and discussion

Easily synthesized alkali metal AA salts and phenolate (onium salts) together with different crown ethers were investigated to test our idea about coordination-regulated CO₂ absorption (Table 1). PEG300 (M.W. = 300 Da) was selected as a suitable solvent due to its distinctive properties, such as low cost, almost negligible vapor pressure, toxicological innocuity and CO₂-philic property.^64–66 As shown in Table 1, crown ether/PEG300 gave extremely poor CO₂ sorption capacity, implying that only physical interaction was observed (entries 1–3). For various alkali metal cations, the corresponding crown ethers on the basis of size fit within the interior of the macrocycle is needed for effective coordination (18-crown-6 for K⁺, 15-crown-5 for Na⁺, 12-crown-4 for Li⁺),⁵⁸ and the ionic diameter for cations or diameter of cavity for crown ethers is 1.20 (Li⁺), 1.90 (Na⁺), 2.66 (K⁺), 1.2–1.5 (12-crown-4), 1.7–2.2 (15-crown-5) and 2.6–3.2 (18-crown-6), respectively.⁶² Alkali metal cations together with corresponding crown ethers, the CO₂ capacity of which increases in the order of Na⁺/15-crown-5 < K⁺/18-crown-6 for ProM and Li⁺/12-crown-4 < Na⁺/15-crown-5 < K⁺/18-crown-6 for PhOM, could play a key role in CO₂ sorption performance (entries 4–5 and 18–20). In particular, coordination of ProK with 18-crown-6 greatly enhanced the CO₂ capacity, approaching the 1 : 1 stoichiometry (CO₂ capacity of 0.99), which is consistent with the theoretical formation of carbamic acid, comparable with the IL [P₆₆₆₁₄][Pro] with the same anion (∼0.90) (entry 5 vs. 6).³³ However, disadvantage was existed in our system (e.g. ProK/18-crown-6/PEG300) by valuing the mass of CO₂ absorbed per gram of the absorbent, owing to addition of large quantities of PEG300 as a solvent. In fact, the solvent PEG300 itself was able to coordinate with the cation K⁺ in ProK,^54,68 although not as strongly as 18-crown-6, and gave a CO₂ capacity of 0.83 (entry 7). However, for other potassium AA onium salts, a CO₂ capacity of 0.63–0.79 was attained, probably due to the high viscosity of the system after CO₂ absorption, but still much higher than the corresponding ILs with the same AA anions (∼0.50) through an ammonium carbamate formation pathway.³⁷

Table 1 CO₂ absorption using crown ether/alkali metal onium salts mixtures (molar ratio 1 : 1)^a

Entry	Absorbent	Time^b (min)	CO2 capacity^c
a Absorption conditions: crown ether/alkali metal onium salts (molar ratio 1 : 1), 1 mmol; PEG300, 2 mmol; CO2, 0.1 MPa; 25 °C. b Time required to reach absorption equilibrium. c Moles of CO2 captured per mole of onium salt were estimated from the weight increase of the reaction mixture. d Data from ref. 33. e 0.2643 g PEG300 (the same weight as 1 mmol 18-crown-6) was added instead of 18-crown-6. f Data from ref. 67.
1	18-crown-6	5	0.04
2	15-crown-5	5	0.03
3	12-crown-4	5	0.03
4	ProNa/15-crown-5	25	0.89
5	ProK/18-crown-6	10	0.99
6^d	[P66614][Pro]	—	∼0.90
7^e	ProK/PEG300	30	0.83
8	ValK/18-crown-6	10	0.79
9	MetK/18-crown-6	20	0.75
10	GlyK/18-crown-6	20	0.75
11	ThrK/18-crown-6	15	0.73
12	AlaK/18-crown-6	15	0.71
13	LeuK/18-crown-6	10	0.68
14	SerK/18-crown-6	15	0.63
15	PhOLi/12-crown-4	5	0.75
16	PhONa/15-crown-5	15	0.84
17	PhOK/18-crown-6	10	0.90
18^f	[P66614][PhO]	30	0.85
19	4-ClPhOK/18-crown-6	10	0.95
20^f	[P66614][4-Cl-PhO]	30	0.82

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Interaction between CO₂ (carbon atom) and ether linkages (oxygen atom) in both PEG and crown ethers leads to lowered viscosity and increased gas/liquid diffusion rates, and thus assumedly facilitates CO₂ sorption with faster sorption kinetics and increasing capacity.³⁸ Indeed, a CO₂ capacity of 0.90 was obtained within 10 min adopting PhOK/18-crown-6, while 30 min was needed for [P₆₆₆₁₄][PhOK] to get a comparable CO₂ capacity (0.85) (entry 17 vs. 18). In addition, 4-Cl-PhOK/18-crown-6 gave a much better performance for CO₂ capture (CO₂ capacity of 0.95 in 10 min) than the corresponding IL with the same anion [P₆₆₆₁₄][4-Cl-PhOK] (CO₂ capacity of 0.82 in 30 min) (entry 19 vs. 20).

Extensive energy input into the desorption process would be a crucial barrier to realize practical CO₂ capture, and also the stability and reusability of the absorbent are two important factors to identify whether it can find potential practical application in industry. The stability of ProK/18-crown-6/PEG300 after CO₂ capture was studied by thermogravimetric analysis (TGA) (Fig. 1(a)). It can be seen that the thermal stability of the absorbent system ProK/18-crown-6/PEG300 was increased compared with 18-crown-6, which can also account for the coordination effect between 18-crown-6 and K⁺. Notably, the captured CO₂ was easy to remove just at a temperature as low as 50 °C by bubbling N₂ within 25 min. Since higher temperatures are required for CO₂ removal from the common conventional amine-based absorbents (e.g. 120 °C, N₂, 75 min),⁶⁹ this can prove that our system proceeds through the carbamic acid pathway (Scheme 2). Notably, no significant drop in CO₂ capacity was detected after six successive absorption–desorption cycles (Fig. 1(b)). For the 4-Cl-PhOK/18-crown-6/PEG300 system, the absorbed CO₂ can be removed completely at 80 °C by bubbling N₂ within 20 min, much more easily than IL [P₆₆₆₁₄][4-Cl-PhO] (80 °C, N₂, 60 min) (Fig. S2(b)†).

Fig. 1 (a) The scanning thermogravimetric analysis (TGA) results for 18-crown-6, PEG300 and ProK/18-crown-6/PEG300+CO₂ with a 10 °C min⁻¹ temperature ramping rate to 500 °C. (b) Six consecutive cycles of CO₂ absorption (◆ 15 min, 25 °C, CO₂ 0.1 MPa) and release (◇ 25 min, 50 °C, N₂ 0.1 MPa) by the ProK/18-crown-6/PEG300 system; data were obtained every 5 min.

Scheme 2 Reaction of CO₂ with ProK/18-crown-6 to produce carbamic acid with PEG300 as a solvent.

To further gain insight into the absorption mechanism, coordination-regulated absorption involving the formation of carbamic acid (Scheme 2) was identified by NMR and FT-IR. It was found that in the ¹H NMR spectrum all the protons of Pro⁻ and 18-crown-6 (H1–H5) showed an upfield shift after mixing them together, due to the coordination of 18-crown-6 with the K⁺ cation (Fig. 2(a)). Afterwards, the downfield shift of all the protons in Pro⁻ (H2–H5) after CO₂ absorption could indicate that CO₂ is chemically bound to the nitrogen center (Fig. 2(a)). Simultaneously, the new peak in the ¹³C NMR spectrum at 157.7 ppm after CO₂ uptake would support the formation of the carbamic acid structure between the secondary amine and CO₂ (C7), while C6 of the carbonyl group in Pro⁻ showed an upfield shift from 177.7 ppm to 175.9 ppm after CO₂ absorption (Fig. 2(b)). Notably, bicarbonate anions formed from the reaction between CO₂ and aqueous (D₂O) solution of Prok showed a new peak at 160.2 ppm, while C6 of the carbonyl group in ammonium cation (protonated Pro-) resulted in an upfield shift from 177.7 ppm to 174.7 ppm, which was quite different from the ¹³C NMR spectrum of carbamic acid (Fig. 2(b)). For the 4-Cl-PhOK/18-crown-6 system, coordination between 18-crown-6 and K⁺ can also be detected by ¹H NMR (Fig. S1(a)†). After CO₂ capture, all the protons of 4-Cl-PhO⁻ showed a downfield shift and the new peak in the ¹³C NMR spectrum at 158.7 ppm could prove the formation of phenylcarbonate [PhOCO₂]⁻ (Fig. S1(b)†).

Fig. 2 (a) ¹H NMR spectra (CDCl₃, 300 MHz) of 18-crown-6, ProK, ProK/18-crown-6 and ProK/18-crown-6/PEG300+CO₂ (from top to bottom); (b) ¹³C NMR (CDCl₃, 75 MHz) spectra of ProK/18-crown-6/PEG300 before (top) and after (middle) CO₂ absorption, and ¹³C NMR (D₂O, 75 MHz) spectra of CO₂ capture by D₂O solution of ProK (bottom).

In the in situ FT-IR spectrum, there are three important features. Firstly, the strength of O–H stretching from PEG increased after CO₂ absorption due to the formation of carbamic acid (O–H containing group), and blueshifted from 3298 cm⁻¹ to 3472 cm⁻¹, also indicating that PEG could be chemically inert during the process (Fig. 3(a)). Secondly, distinct bands corresponding to the ammonium cation in the 2000–3000 cm⁻¹ region are not observed, while the new peak at 2338 cm⁻¹ was derived from physically dissolved CO₂. Finally, the original vibration of the C=O bond of the carbonyl group in Pro⁻ was shown at 1598 cm⁻¹, and then after CO₂ uptake, two characteristic peaks centered at 1635 cm⁻¹ and 1667 cm⁻¹ can be assigned to asymmetric C=O vibrations of the carboxylate anion and the COOH moiety of the carbamic acid, respectively (Fig. 3(b)). For the 4-Cl-PhOK/18-crown-6/PEG300 system, the carbonyl group absorption was observed at 1647 cm⁻¹ after CO₂ capture, indicating the formation of phenylcarbonate [PhOCO₂]⁻ (Fig. S1(c)†).

Fig. 3 Results of in situ IR spectroscopy under 0.1 MPa CO₂ pressure monitoring at various reaction times. (a) ProK/18-crown-6/PEG300 (molar ratio 1 : 1 : 2) before and after CO₂ uptake at 25 °C; (b) magnified three-dimensional spectra of the absorption mixture with reaction time.

Quantum chemistry calculations, being performed with the program Turbomole V6.0,⁷⁰ were employed to calculate the enthalpy changes for the present equimolar CO₂ capture (Scheme 3). The PEG solvent was simulated as a continuum medium with a dielectric constant of 13.4⁷¹ within the Conductor-like Screening Model (COSMO)⁷² implemented in Turbomole. Both structure optimization and the solvation calculation were performed at the B3LYP level with the def2-TZVP basis sets.⁷³ The amino acid salt ProK with 18-crown-6 coordinated at the K⁺ reacts with CO₂ at the secondary amine to form the carbamic acid, which is stabilized through an intramolecular hydrogen bond with the carboxylate anion (bond length: 1.593 Å). In addition, formation of the carbamic acid has a calculated enthalpy change of −39.0 kJ mol⁻¹, indicating that the absorption process is thermodynamically favourable, but not too strong. This is consistent with the easy desorption we have found (Fig. 1b).

Scheme 3 The quantum chemistry calculations of the reaction between CO₂ and ProK/18-crown-6 in a PEG300 solution to produce carbamic acid; the PEG300 is modelled as a continuum medium with a dielectric constant of 13.4. H: gray, C: black, N: pink, O: blue, K+: yellow.

Conclusions

In summary, a coordination effect was employed to realize equimolar CO₂ absorption, adopting an easily synthesized amino group containing absorbents (alkali metal onium salts), and the essence was to increase the steric hindrance of cations and go through a carbamic acid pathway. Our strategy is to utilize easily synthesized alkali metal AA salts together with crown ethers, in which highly sterically hindered cations were obtained through the strong coordination effect of crown ethers with alkali metal cations. For example, a CO₂ capacity of 0.99 was attained by ProK/18-crown-6, detected by NMR, in situ FT-IR and quantum chemistry calculation to go through a carbamic acid formation pathway. The captured CO₂ can be stripped out under very mild conditions (50 °C, N₂). In addition, crown ethers together with alkali metal phenolates are also easily synthesized absorbents for efficient CO₂ absorption. Thus, this protocol would pave the way for the development of technological innovation towards efficient and low energy demanding practical processes for CO₂ capture.

Acknowledgements

This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

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Footnote

† Electronic supplementary information (ESI) available: General experimental methods, synthetic procedure of alkali metal onium salts, and characterization of the absorption systems (NMR, TGA and IR). See DOI: 10.1039/c3gc41513a

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