Reversible Electrochemical Trapping of Carbon Dioxide Using 4,4

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Reversible Electrochemical Trapping of Carbon Dioxide Using 4,4'-Bipyridine That Does Not Require Thermal Activation Rajeev Ranjan, Jarred Olson, Poonam Singh, Edward D. Lorance, Daniel A Buttry, and Ian Robert Gould J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02220 • Publication Date (Web): 30 Nov 2015 Downloaded from http://pubs.acs.org on December 1, 2015

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The Journal of Physical Chemistry Letters

Reversible Electrochemical Trapping of Carbon Dioxide Using 4,4'Bipyridine That Does Not Require Thermal Activation Rajeev Ranjan,# Jarred Olson,# Poonam Singh,# Edward D. Lorance,*,‡ Daniel A. Buttry,*,# Ian R. Gould*, # # School

of Molecular Sciences, Arizona State University, Tempe, AZ 85287

‡ Department

of Chemistry, Vanguard University, Costa Mesa, CA 92926

ABSTRACT: Sequestering carbon dioxide emissions by the trap and release of CO2 via thermally activated chemical reactions has proven problematic because of the energetic requirements of the release reactions. Here we demonstrate trap and release of carbon dioxide using electrochemical activation, where the reactions in both directions are exergonic and proceed rapidly with low activation barriers. One-electron reduction of 4,4'-bipyridine forms the radical anion, which undergoes rapid covalent bond formation with carbon dioxide to form an adduct. One-electron oxidation of this adduct releases the bipyridine and carbon dioxide. Reversible trap and release of carbon dioxide over multiple cycles is demonstrated in solution at room temperature, and without the requirement for thermal activation. Table of Contents Graphic

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In response to the growing global warming problem, sequestration of carbon dioxide has become an issue of significant technological importance.1,2 Systems based on aliphatic amines have been developed for industrial-scale reversible carbon dioxide trapping.3 Covalent bond formation between the amine nitrogen and the carbonyl carbon of CO2 followed by deprotonation forms a carbamate anion. The carbamate anion can then be decomposed thermally to liberate carbon dioxide and, after reprotonation, the amine.3 The energy cost of the CO2 liberation process, however, limits the use of these reactions in reversible carbon dioxide trapping schemes.4 A method of CO2 trapping and release that does not require substantial thermal input would clearly be of interest. Several studies have described electrochemically initiated reversible chemical reactions of quinones with CO2,5,6 and electrochemically activated CO2 capture for space flight applications has been reported.7 More recently, electrochemcial trap and release of carbon dioxide at a quinacridone modified electrode was observed at room temperature without the need for thermal activation, although the electrode suffered degradation upon repeated cycling.8 Here we describe a method for reversible trap and release of CO2 using 4,4'-bipyridine. The process mimics the traditional amine method,3 but the trap and release reactions are activated electrochemically rather than thermally. Both reactions are reversible and proceed rapidly and with low energy barriers. One-electron reduction of 4,4'-bipyridine in the ionic liquid N-butyl-Nmethylpyrrolidinium bis(trifluoromethylsulfonyl)imide as both solvent and electrolyte under a nitrogen atmosphere is reversible, demonstrating that the bipyridine radical anion (Bpy•–) is

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stable on the millisecond timescale. The reduction potential is –2.3 V vs ferrocene as an internal reference, Figure 1. When the solution is saturated with carbon dioxide, the cathodic peak shifts to lower potential, –2.05 V vs. ferrocene, as a result of rapid reaction of Bpy•– with CO2 to form an adduct. The current density also increases due to the higher diffusion coefficient of CO2 compared to bipyridine and to the decreased viscosity of the ionic liquid caused by CO2 dissolution.9 Oxidation of Bpy•– is now absent in the reverse wave. Instead, oxidation of the Bpy•–/CO2 adduct is observed at a potential 700 mV less negative than the new reduction wave. The electrochemical behavior in the presence of CO2 is reproducible, the same reduction and reoxidation waves are observed for over 20 cycles in the same sample, during which period all of the 4,4'-bipyridine would have been consumed if the reactions were not highly reversible. The experiments were stopped after 20 cycles because no further detectable changes in electrochemical response were observed upon repeated cycling. The small prewave at -1.5 V under CO2 is due to an impurity that reacts with the product of reduction, and is not relevant to the present discussion. The reaction between the Bpy•– and CO2 is presumably nucleophilic addition of a pyridinyl nitrogen to the carbonyl carbon of CO2 to form a bipyridinyl radical N-carboxylate, Scheme 1(A). This species has structural features of both a bipyridinyl radical and a carbamate. Oxidation of this adduct can occur at the bipyridinyl carbon to give a zwitterion, or at the carboxylate to give a biradical, Scheme 1(B). The zwitterion is probably the lower energy configuration because pyridinyl radicals have the electronic properties of α-amino radicals,10 which are easier to oxidize than carboxylate anions.11 The oxidation potentials for pyridinyl

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Scheme 1. (A) One-electron reduction of 4,4'-bipyridine to give a radical anion that reacts with carbon dioxide to form a bipyridinyl radical N-carboxylate adduct. (B) One-electron oxidation of the adduct to give a short lived intermediate that fragments to liberate the starting 4,4'bipyridine and carbon dioxide. This intermediate can be drawn in zwitterionic or biradical (in parentheses) electronic configurations, see text.

radicals are usually found to be between –1.0 and –1.5 V versus ferrocene,12 consistent with the observed oxidation wave for the adduct, Figure 1. The kinetics of the reaction of the Bpy•– with CO2 were studied using transient absorption spectroscopy.13 One-electron reduction of the excited triplet state of 4,4'-bipyridine using DABCO as the electron donor forms the Bpy•–, which can be observed at 380 nm.13 Under our conditions, the lifetime of the Bpy•– was 1.2 µs and 0.16 µs in argon-purged acetonitrile and 1,2dichloroethane, respectively. These lifetimes are much shorter than the lifetime in the electrochemical experiments, which is more than a day, because very low concentrations of Bpy•–are produced photochemically, ca. 10–6 M, that react rapidly with

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N2

0.4

CO2

0.2 j / mA cm-2

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0.0 -0.2 -0.4 -0.6 -2.5

-2.0 -1.5 -1.0 E / V vs Fc/Fc+

-0.5

Figure 1. Cyclic voltammetry of 10 mM 4,4'-bipyridine in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid solvent at room temperature, purged with nitrogen (red) and purged with carbon dioxide (blue), at a scan rate of 50 mV/s. and with a working electrode glassy carbon. Potentials are given vs. ferrocene as an internal standard.

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low concentration impurities that are scavenged in the electrochemistry experiments. Bimolecular reaction of Bpy•– with CO2 was readily observed in 1,2-dichloroethane. A plot of the pseudo-first order rate constant for decay of Bpy•– as a function of CO2 concentration yields a bimolecular rate constant for reaction of 9.2 × 107 M–1 s–1, Figure 2. After reaction with CO2, a new species is observed with an absorption maximum of 375 nm. This new species is long-lived on the timescale of the experiment and is assigned to the adduct of Scheme 1. The transient absorption spectra of the relevant intermediates are provided in the supporting information. The rate constant of (9.2 ± 0.2) × 107M–1 s–1 is one of the largest reported for a nucleophilic addition to CO2.14 The Arrhenius activation energy for the reaction is 16.7 ± 2.0 kJ/mol and the pre-exponential factor is (7 ± 3) × 1010 s–1, Figure 2. The activation free energy at 298 K is 27.7 ± 2.0 kJ/mol is consistent with a large rate constant and low reaction barrier. The reaction between Bpy•– and CO2 was also studied computationally. In addition to the transition state, an energy minimum corresponding to a pre-reaction complex was found on the reaction coordinate. The relevant free energies for reaction in acetonitrile and 1,2-dichloroethane are summarized in Figure 3. The free energy barrier from the complex to the addition transition state in acetonitrile is 6.7 kJ/mol, yielding a rate constant (at 25°C, assuming κ = 1) of 4.2 × 1011 M–1 s–1, whereas the barrier to forming the complex from the isolated reactants is 26.9 kJ/mol, which yields a bimolecular rate constant of 1.2 × 107 M–1 s–1. This is in reasonable agreement with the experimentally measured rate constant for reaction of Bpy•– with CO2 of 9.2 × 107 M–1s–1. The corresponding computed rate constant for formation of the complex in 1,2dichloroethane is 1.3 × 107 M–1 s–1, which suggests a small solvent effect on the rate of the reaction. The computational study provides strong support for reaction of Bpy•– with CO2 to form the adduct given in Scheme 1(A).

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ln(kobs)

20

19

16.0

kobs x 10-6, s-1

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14.0

18 17

3.0

12.0

3.2 3.4 3 1/T x 10 , K-1

10.0 8.0 6.0

0.0

2.0

4.0

6.0

8.0

10.0

[CO2], mM Figure 2. Pseudo-first order rate constant for decay of the 4,4'-bipyridine radical anion, kobs, as a function of CO2 concentration in 1,2-dichloroethane at room temperature. The bimolecular rate constant given by the slope is (9.2 ± 0.2) × 107 M–1 s–1. The inset is an Arrhenius plot that yields an activation energy of 16.7 ± 2.0 kJ/mol and a pre-exponential factor of (7.4 ± 3) × 1010 s–1.

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Figure 3. Computed free energies and structures for stationary points along the reaction coordinate for the reaction of the bipyridine radical anion with carbon dioxide in acetonitrile, showing formation of a bimolecular complex prior to the transition state. The energies are relative to the sum of the solvated free energies of 4,4'-bipyridinyl radical anion and carbon dioxide. The values in parentheses refer to reaction in 1,2-dichloroethane as the solvent.

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The overall CO2 trapping reaction is computed to be exergonic in both solvents, Figure 3. It is also computed to be exothermic: the reaction enthalpies are –88.3 kJ/mol in acetonitrile and –85.4 kJ/mol in 1,2-dichloroethane. The corresponding reaction entropies are –149.1 J/K·mol and –146.5 J/K·mol. These values are consistent with a fast reaction that requires little thermal activation. The re-oxidation reaction that releases the CO2 was also studied computationally. The oxidized adduct can be described in terms of biradical or zwitterionic electron configurations, Scheme 1(B). The computed oxidized adduct is a ground state singlet, consistent with a structure that more resembles the zwitterion. Decarboxylation of the zwitterion resembles decarboxylation of the radical cations of aniline carboxylates, which are known to be exergonic and fast.18 Because mixing of electron configurations is required for decarboxylation, however,18,19 the biradical configuration shown in Scheme 1(B) should also be important in controlling the reaction kinetics. Decarboxylation of the biradical resembles fragmentations in other pyridinyl radicals, which are also known to be fast.19,20 A potential energy scan for the oxidized adduct was conducted by lengthening the bipyridinyl nitrogen-CO2 carbonyl bond length, r'N-C, where the prime symbol refers to the fragmentation reaction. The fragmentation reaction is not the exact reverse of the trapping reaction, since it starts with the oxidized adduct, yet a complex was again found, except that it is now is a post-reaction complex, Figure 4. At larger separation distances the electronic energy profile was insensitive to r'N-C; consequently, the transition state for the formation of the CO2bipyridinyl complex was not located. Separation of the fragments from the complex is exergonic, Figure 4, and we equate formation of the complex with the rate-determining step for overall reaction. The free energy gap from the oxidized adduct to the transition state is 15.1

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kJ/mol, which yields a rate constant of 1.4 × 1010 s–1 in acetonitrile. The corresponding computed rate constant in 1,2-dichloroethane solvent is 3.8 × 1011 s–1. This reaction is slightly endothermic: the reaction enthalpy is 9.8 kJ/mol in acetonitrile and 11.0 kJ/mol in 1,2dichloroethane, however, the reaction entropy is highly favorable, 139.8 J/K·mol in acetonitrile and 139.1 J/K·mol in 1,2-dichloroethane. The results is overall fast and exergonic reaction, 31.9 kJ/mol in acetonitrile and 38.5 kJ/mol in 1,2-dichloroethane. The fragmentation reaction converts a zwitterion into two neutral species and is thus more sensitive to solvent than the trapping reaction which converts one anion into another anion. Trapping of carbon dioxide using Bpy•– is very fast, and the Bpy•–/CO2 adduct can be oxidized to a species that undergoes very rapid liberation of CO2. Both reactions are exergonic, the electrochemical voltage required to switch between trap and release is only ca. 700 mV and the system is reversible over many cycles in fluid solution at room temperature. This work also provides experimental and computational evidence in support of a mechanism of nucleophilic addition of a pyridyl nitrogen to CO2, which is a topic of current interest in the context of electrochemically catalyzed CO2 reductions.21

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Figure 4. Computed free energies and structures for stationary points along the reaction coordinate for decarboxylation of the oxidized CO2 addition product, showing formation of a bimolecular complex after the transition state for bond cleavage, in acetonitrile as the solvent. The energies are relative to the free energy of the oxidized and solvated adduct. The values in parentheses refer to reaction in 1,2-dichloroethane as the solvent.

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ASSOCIATED CONTENT Supporting Information. Electrochemistry experimental details. Time-resolved spectroscopy experimental details. Computational method details, including Cartesian coordinates and energies of all stationary states. Experimental and computational references, including the full Gaussian 09 reference.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The funding for this project is through ARPA-E, contract number DE-AR0000343.

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FERENCES (1) Service, R. F. The Carbon Conundrum. Science, 2004, 305, 962-963. (2) Mikkelsen, M.; Jørgensen, M.; Krebs, F. C. The Teraton Challenge. A Review of Fixation and Transformation of Carbon Dioxide. Energy Environ. Sci. 2010, 3, 43-81. (3) Rochelle, G. T. Amine Scrubbing for CO2 Capture. Science, 2009, 325, 1652-1654. (4) Davison, J. Performance and Costs of Power Plants with Capture and Storage of CO2. Energy 2007, 32, 1163-1176. (5) Scovazzo, P.; Poshusta, J.; DuBois, D.; Koval, C.; Noble, R. Electrochemical Separation and Concentration of

Reversible Electrochemical Trapping of Carbon Dioxide Using 4,4

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