Thermodynamics of Electrochemically Synthesized C-Nucleophiles for Reactive CO2 Capture

By Laura ThomsonApr 6 2022

Carbon-neutral and carbon-negative technology, such as carbon capture, will be required to reduce CO₂ concentrations in the atmosphere to levels that will minimize the consequences of climate change. The energetics of CO₂ binding to the proposed capture material must be carefully balanced while developing optimum carbon capture materials. CO₂ binding free energies (ΔG_bind) are, however, difficult to obtain experimentally.

This article looks at the latest research, titled 'Predictive energetic tuning of C-Nucleophiles for the electrochemical capture of carbon dioxide', published in Iscience. 

Among the potential materials for CO₂ capture and conversion, imidazolium salts have gotten a lot of interest because of the appealing liquid characteristics of their alkylated forms, which include good CO₂ solvation and high conductivity, making them ideal for electrochemical applications.

In addition to these advantageous features, deprotonation of imidazolium salts yields N-heterocyclic carbenes (NHCs) (Scheme 1B). Electrochemical access to the free NHCs has been proven before, as indicated in Scheme 1A.

Scheme 1. Reactions of imidazolium cations. Generation of NHCs (Im) from their imidazolium precursors (A and B) and Im binding to CO₂ (C) along with the three properties associated with these reactions: (A) E_red, (B) pK_a, and (C) ΔG_bind. R1 and R2 substituents are described in Figure 1. Scheme Credit: Petersen, et al., 2022

When an imidazolium cation is reduced, it releases half of its aH₂ equivalent and becomes a free NHC. As illustrated in Scheme 1C, the free NHC then displays reactivity with CO₂. The Scheme 1C reaction was first designed as a protection strategy for reactive NHCs, since it found use in the synthesis of organometallics.

Researchers now show a three-way correlation between the imidazolium’s experimentally accessible reduction potential (E_red) (Scheme 1A), its pK_a (Scheme 1B), and the CO₂ binding energy of the associated NHC using this reactivity (Scheme 1C). On a set of 90 imidazolium, the correlation between the pK_a of an imidazolium (Scheme 1B) and the ΔG_bind of the corresponding free NHCs (Scheme 1C) was investigated using density functional theory (DFT).

Researchers depict a landscape of CO₂ binding free energies spanning 15 kcal, 20 pK_a units, and 1.7 V of potential in this work, even though the number of imidazolium salts is smaller (Figure 1).

Figure 1. Imidazol(in)ium-based NHCs used in the correlation studies. Structures of each compound under study. See also Figure S1 and Table S1 for applicable experimental values associated with each NHC. Imidazolium counterparts (protonated NHCs) are abbreviated as “NHC”H in this work. Image Credit: Petersen, et al., 2022

In Scheme 1, scientists study the relation between E_red, pK_a, and ΔG_bind for reactions A–C. Due to their relative dependency on the electron density at the apical imidazolium carbon, the parameters are linked.

In essence, the nucleophilicity of an NHC is ranked against two types of electrophiles: protons and carbon dioxide, according to the association of pK_a with the free energy of CO₂ binding.

Unlike conventional electro-swing systems, which cycle through binding CO₂ with cathodically generated nucleophiles and then release CO₂ in an electrochemical oxidation step, the electrochemical step of NHC generation and binding CO₂ can be combined with a thermal release step rather than oxidation. Because the thermal emission of CO₂ from NHC adducts is known, hybrid electrothermal CO₂ capture-release systems may be developed.

Methodology

A silver wire single-junction pseudo-reference electrode, a 3 mm diameter glassy carbon working electrode (MF-2012, BASI), and a platinum wire counter electrode were used in normal pulse voltammetry (NPV) studies. This determined the potential at which the inflection point in the NPV of the molecule in question yielded experimental imidazolium reduction potentials.

Because of the irreversible nature of the lowering and the enhanced capacity of NPV to deliver exact current responses with low interference from charging currents, this approach was chosen over the conventional use of cyclic voltammetry E_1/2 values. After that, the value was averaged over three trials (except SIPr, 6 trials).

Diffusion-ordered spectroscopy (DOSY) NMR on a sample of 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazolium tetrafluoroborate (SIPrH BF₄) in deuterated MeCN was used to determine the diffusion coefficient of SIPrH.

The optimal geometries and energy of the carbene precursor, its protonated and reductive intermediates, and their adducts with CO₂ were computed using quantum chemical density functional theory simulations.

The SMD-Bondi cavity treatment greatly increases the chances of binding. Researchers decided that the default SMD treatment with the MN15 functional delivers more accurate binding free energy values and chose to employ this technique for the correlations since it is the least tested across multiple systems and hence the least trustworthy.

The linear fit between experimentally measured pK_a values and the estimated free energy difference G(XH⁺) – G(X) yields pK_a values. Whereas, it is normal to utilize an isodesmic technique or pre-determined free energy of solvated protons G(H⁺) to estimate pK_a values.

In OriginPro 2019b, best-fit studies of the associated parameters (E_red vs. pK_a, E_red vs. ΔG_bind, pK_a vs. ΔG_bind, and E_red vs. pK_a vs. ΔG_bind) were performed.

Results

Scientists initially aimed to experimentally test many assumptions about the system before undertaking DFT calculations to study the link with the free energy of CO₂ binding, reduction potential, and pK_a for the set of imidazolium cations. Aromatic hydrides have previously been demonstrated to have a linear relationship between the oxidized hydride’s first reduction potential and the hydride’s nucleophilicity.

Researchers next used DFT based on the MN15/6-311+G(d,p) level of theory combined with the SMD solvent model as performed in the Gaussian 16 quantum chemistry code to calculate the values of E_red, pK_a, and ΔG_bind for each of the imidazolium derivatives.

Figure 2 shows imidazolium cations that are difficult to make synthetically. Correlations were computed using the estimated DFT values for E_red and pK_a in these circumstances.

Figure 2. Linear correlations of pKa, free energy of CO₂ binding, and reduction potential. The three linear two-way correlations of (left) reduction potential and pK_a, (middle) free energy of CO₂ binding and pK_a, and (right) reduction potential and free energy of CO₂ binding for the compounds under study. Optimized molecular coordinates are available in Data S1. Image Credit: Petersen, et al., 2022

In the case of imidazol(in)ium cations, where both the experimental pK_a and E_red are known in advance depicts in Figure 3.

Figure 3. 3D correlation provides greater predictive power for estimating the free energy of CO₂ binding. Three-parameter linear relationship providing stronger correlation between E_red, pK_a, and ΔG_bind for the compounds under study. Image Credit: Petersen, et al., 2022

Table 1 shows the results for all six substances for which experimental E_red and pK_a values were obtained.

Table 1. Comparison of DFT-calculated ΔG_bind values with those estimated using experimental reduction potentials and pK_a value. Source: Petersen, et al., 2022

The calculated and experimental pK_as are compared in Table 2.

Table 2. Comparison of estimated pK_a values from Equation 4 and experimental pK_a values. Source: Petersen, et al., 2022

Discussion

Researchers now show how to calculate CO₂ binding energies from experimentally accessible electrochemical data by establishing a three-way connection between pK_a, reduction potential, and free energy of CO₂ binding for a group of imidazol(in)ium cations and the corresponding NHCs.

Four halogenated NHCs that are reported to be oxygen stable, an essential sorbent property for carbon capture applications involving oxygen, were expected to bind CO₂ with negative ΔG_bind values.

The strong ionic connection may affect measured reduction potentials, yielding values that are significantly different from those anticipated by the correlations provided here. Researchers anticipate the association to remain over a vast library of chemically related species, even though the subset of organic salts studied was constrained. However, as shown in the case of 1,3-di-t-butylimidazolium chloride (I^tBuHCl), steric effects can cause significant variations from recognized patterns.

Conclusion

While researchers show these predictions for imidazol(in)ium-based NHCs in this work, identical correlations will apply to other groups of electrochemically responsive CO₂ capture molecular materials, allowing for the mapping of the energetics of novel chemical space for this application.

These findings, when combined with the previously discovered thermally induced CO₂ release from NHC-CO₂ adducts, pave the way for energetic modulation of reactive separations in new hybrid redox-thermal swing chemistries.

Journal Reference

Petersen, H.A., Alherz, A.W., Stinson, T.A., Huntzinger, C.G., Musgrave, C.B. and Luca, O.R. (2022) Predictive energetic tuning of C-Nucleophiles for the electrochemical capture of carbon dioxide. Iscience, p.103997. Available Online: https://www.cell.com/iscience/fulltext/S2589-0042(22)00267X?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS258900422200267X%3Fshowall%3Dtrue.

References and Further Reading

1. Alherz, A., et al. (2018) Renewable hydride donors for the catalytic reduction of CO₂: a thermodynamic and kinetic study. The Journal of Physical Chemistry B, 122, pp. 10179–10189. doi.org/10.1021/acs.jpcb.8b08536.
2. Arduengo A.J., et al. (1997) An air stable carbene and mixed carbene “dimers”. Journal of American Chemical Society, 119, pp. 12742–12749. doi.org/10.1021/ja973241o.
3. Barone V & Cossi M (1998) Quantum calculation of molecular energies and energy gradients in solution by a conductor solvent model. Journal of Physical Chemistry A, 102, pp. 1995–2001. doi.org/10.1021/jp9716997.
4. Camaioni D M & Schwerdtfeger C A (2005) Comment on “accurate experimental values for the free energies of hydration of H⁺, OH^-, and H₃O⁺”. Journal of Physical Chemistry A, 109, pp. 10795–10797. doi.org/10.1021/jp054088k.
5. Chai J.-D & Head-Gordon M (2008) Long-range corrected hybrid density functionals with damped atom–atom dispersion corrections. Physical Chemistry Chemical Physics, 10, p. 6615. doi.org/10.1039/B810189B.
6. Chu Y., et al. (2007) An acidity scale of 1,3-dialkylimidazolium salts in dimethyl sulfoxide solution. The Journal of Organic Chemistry, 72, pp. 7790–7793. doi.org/10.1021/jo070973i.
7. Cossi M., et al. (2003) Energies, structures, and electronic properties of molecules in solution with the C-PCM solvation model. Journal of Computational Chemistry, 24, pp. 669–681. doi.org/10.1002/jcc.10189.
8. Donadt T. B., et al. (2018) DOSY NMR and normal pulse voltammetry for the expeditious determination of number of electrons exchanged in redox events. ChemistrySelect, 3, pp. 7410–7415. doi.org/10.1002/slct.201801231.
9. Dunn M.H., et al. (2017) Targeted and systematic approach to the study of pKa values of imidazolium salts in dimethyl sulfoxide. Journal of Computational Chemistry, 82, pp. 7324–7331. doi.org/10.1021/acs.joc.7b00716.
10. Frisch M. J., et al. (2016) Gaussian 16 Rev. C.01. Available at https://gaussian.com/citation.
11. Furfari S.K., et al. (2015) Air stable NHCs: a study of stereoelectronics and metallorganic catalytic activity. Chemical Communications, 51, pp. 74–76. doi.org/10.1039/C4CC06809B.
12. Gorodetsky B., et al. (2004) Electrochemical reduction of an imidazolium cation: a convenient preparation of imidazol-2-ylidenes and their observation in an ionic liquid. Chemical Communications, 2004, pp. 1972–1973. https://doi.org/10.1039/B407386J.
13. Hahn F E & Jahnke M C (2008) Heterocyclic carbenes: synthesis and coordination chemistry. Angewandte Chemical International Edition, 47, pp. 3122–3172. doi.org/10.1002/anie.200703883
14. Ilic S., et al. (2018) Thermodynamic and kinetic hydricities of metal-free hydrides. Chemical Society Reviews, 47, pp. 2809–2836. doi.org/10.1039/C7CS00171A
15. Institute G.C (2020) The Global Status of CCS: 2020 (Australia)
16. Isse A A & Gennaro A (2010) Absolute potential of the standard hydrogen electrode and the problem of interconversion of potentials in different solvents. Journal of Physical Chemistry B, 114, pp. 7894–7899. doi.org/10.1021/jp100402x.
17. Kelemen Z., et al. (2011) An organocatalytic ionic liquid. Organic and Biomolecular Chemistry, 9, pp. 5362–5364. doi.org/10.1039/C1OB05639E
18. Kelly C.P., et al. (2006) Aqueous solvation free energies of ions and ion−water clusters based on an accurate value for the absolute aqueous solvation free energy of the proton. Journal of Physical Chemistry B, 110, pp. 16066–16081. doi.org/10.1021/jp063552y
19. Krishnan R., et al. (1980) Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions. The Journal of Chemical Physics, 72, pp. 650–654. doi.org/10.1063/1.438955
20. Lackner K S (2013) The thermodynamics of direct air capture of carbon dioxide. Energy, 50, pp. 38–46. doi.org/10.1016/j.energy.2012.09.012.
21. Luca O R & Fenwick A Q (2015) Organic reactions for the electrochemical and photochemical production of chemical fuels from CO₂–The reduction chemistry of carboxylic acids and derivatives as bent CO₂ surrogates. Journal of Photochemistry and Photobiology B: Biology, 152, pp. 26–42. doi.org/10.1016/j.jphotobiol.2015.04.015
22. Luca O. R., et al. (2015) The selective electrochemical conversion of preactivated CO₂ to methane. Journal of Electrochemical Society, 162, p. H473. doi.org/10.1149/2.0371507jes
23. Marenich A.V., et al. (2009) Universal solvation model based on solute electron density and on a continuum model of the solvent defined by the bulk dielectric constant and atomic surface tensions. Journal of Physical Chemistry B, 113: 6378–6396. doi.org/10.1021/jp810292n
24. Marenich A.V., et al. (2014) Computational electrochemistry: prediction of liquid-phase reduction potentials. Physical Chemistry Chemical Physics, 16, pp. 15068–15106. doi.org/10.1039/C4CP01572J
25. McLean A D & Chandler G S (1980) Contracted gaussian basis sets for molecular calculations. I. second row atoms, Z=11–18. The Journal of Chemical Physics, 72, pp. 5639–5648. doi.org/10.1063/1.438980.
26. Mirzaei S., et al. (2019) Improving performance of the SMD solvation model: bondi radii improve predicted aqueous solvation free energies of ions and pKa values of thiols. Journal of Physical Chemistry A, 123, pp. 9498–9504. doi.org/10.1021/acs.jpca.9b02340
27. Namazian M., et al. (2010) Benchmark calculations of absolute reduction potential of ferricinium/ferrocene couple in nonaqueous solutions. Journal of Chemical Theory and Computation, 6, pp. 2721–2725. doi.org/10.1021/ct1003252
28. (2019) National Academies of Sciences, Engineering, and Medicine Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. The National Academies Press.
29. Petersen H A & Luca O R (2021) Application-specific thermodynamic favorability zones for direct air capture of carbon dioxide. Physical Chemistry Chemical Physics, 23, pp. 12533–12536. doi.org/10.1039/D1CP01670A
30. Riduan S.N., et al. (2009) Conversion of carbon dioxide into methanol with silanes over N-heterocyclic carbene catalysts. Angewandte Chemie International Edition, 48, pp. 3322–3325. doi.org/10.1002/anie.200806058
31. Sowmiah S., et al. (2009) On the chemical stabilities of ionic liquids. Molecules, 14, pp. 3780–3813. doi.org/10.3390/molecules14093780
32. Thapa B & Schlegel H B (2016) Density functional theory calculation of pKa’s of thiols in aqueous solution using explicit water molecules and the polarizable continuum model. Journal of Physical Chemistry A, 120, pp. 5726–5735. doi.org/10.1021/acs.jpca.6b05040
33. Tossell J A (2011) Calculation of the properties of molecules in the pyridine catalyst system for the photochemical conversion of CO₂ to methanol. Computational and Theoretical Chemistry, 977, pp, 123–127. doi.org/10.1016/j.comptc.2011.09.012
34. Tsuzuki S., et al. (2013) Interactions in ion pairs of protic ionic liquids: comparison with aprotic ionic liquids. Journal of Chemical Physics, 139, p. 174504. doi.org/10.1063/1.4827519
35. Van Ausdall B.R., et al. (2009) A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates. Journal of Organic Chemistry, 74, pp. pp. 7935–7942. doi.org/10.1021/jo901791k.
36. Voroshylova I.V., et al. (2018) Influence of the anion on the properties of ionic liquid mixtures: a molecular dynamics study. Physical Chemistry Chemical Physics, 20, pp. 14899–14918. doi.org/10.1039/C8CP01541D
37. Voutchkova A.M., et al. (2007) Imidazolium carboxylates as versatile and selective N-heterocyclic carbene transfer agents: synthesis, mechanism, and applications. Journal of American Chemical Society, 129, pp. 12834–12846. doi.org/10.1021/ja0742885
38. Wang H., et al. (2015) Anion-effects on electrochemical properties of ionic liquid electrolytes for rechargeable aluminum batteries. Journal of Materials Chemistry A, 3, pp. 22677–22686. doi.org/10.1039/C5TA06187C
39. Wang Z., et al. (2018) Acidity scale of N-heterocyclic carbene precursors: can we predict the stability of NHC–CO₂ adducts?. Organic Letters, 20, pp. 6041–6045. doi.org/10.1021/acs.orglett.8b02290
40. Wilcox J (2020) An electro-swing approach. Nature Energy, 5, pp. 121–122. doi.org/10.1038/s41560-020-0554-4.
41. Yang L & Wang H (2016) MN15: a kohn–sham global-hybrid exchange–correlation density functional with broad accuracy for multi-reference and single-reference systems and noncovalent interactions. Chemical Science, 7, pp. 962–998. doi.org/10.1039/C6SC00705H
42. Yu H.S., et al. (2016) MN15: a kohn–sham global-hybrid exchange–correlation density functional with broad accuracy for multi-reference and single-reference systems and noncovalent interactions. Chemical Science, 7, pp. 5032–5051. doi.org/10.1039/C6SC00705H
43. Zhao S.-F., et al. (2016) Is the imidazolium cation a unique promoter for electrocatalytic reduction of carbon dioxide?. The Journal of Physical Chemistry C, 120, pp. 23989–24001. doi.org/10.1021/acs.jpcc.6b08182