CO2 Chemistry of Phenolate-Based Ionic Liquids - ACS Publications

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CO Chemistry of Phenolate-Based Ionic Liquids Tae Bum Lee, Seungmin Oh, Thomas R. Gohndrone, Oscar MoralesCollazo, Samuel Seo, Joan Frances Brennecke, and William F. Schneider J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b06934 • Publication Date (Web): 10 Nov 2015 Downloaded from http://pubs.acs.org on November 19, 2015

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The Journal of Physical Chemistry

CO2 Chemistry of Phenolate-Based Ionic Liquids Tae Bum Lee,† Seungmin Oh,† Thomas R. Gohndrone,† Oscar MoralesCollazo,† Samuel Seo,† Joan F. Brennecke,*† and William F. Schneider*,†,‡ Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556

E-mail: [email protected], [email protected]

*

To whom correspondence should be addressed Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556 ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 †

ACS Paragon Plus Environment

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Abstract We synthesized ionic liquids (ILs) comprising an alkylphosphonium cation paired with phenolate, 4-nitrophenolate, and 4-methoxyphenolate anions that span a wide range of predicted reaction enthalpies with CO2. Each phenolate-based IL was characterized by spectroscopic techniques and their physical properties (viscosity, conductivity, and CO2 solubility) were determined. We use the computational quantum chemical approach paired with experimental results to reveal the reaction mechanism of CO2 with phenolate ILs. Model chemistry shows that the oxygen atom of phenolate associates strongly with phosphonium cations and is able to deprotonate the cation to form an ylide with an affordable activation barrier. The ATR-FTIR and 31P NMR spectra indicate that the phosphonium ylide formation and its reaction with CO2 are predominantly responsible for the observed CO2 uptake rather than direct anion-CO2 interaction.


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Introduction Carbon capture is the most straightforward and promising route to managing CO2 emissions from coal-fired power plants and other stationary emitters, but practical carbon capture depends on the development of energy-efficient CO2 separations.1 Ionic liquids (ILs) are promising separation media because of their negligible vapor pressure and flammability, high thermal stability, and tunable properties.2–7 For CO2 capture applications, CO2 uptakes greater than those accessible by physical absorption are desirable. Gurkan et al.8 reported that an IL formed from the substituted azolide anion 2cyanopyrrolide with trihexyl(tetradecyl)phosphonium ([P66614][2-CNpyr]) exhibits Langmuir-like CO2 isotherms that can be rationalized in terms of a stoichiometric reaction with the anion (Scheme 1a). Gurkan et al.8 and others9 further showed that the isotherms could be modified by variation of the azolide anion and that the absorption enthalpy generally correlated with reaction energies predicted from first-principles calculations.

Scheme 1: Proposed CO2 reaction schemes with (a) [2-CNpyr]- and (b) phenolates.

An analogous phenolate-CO2 chemistry is readily envisioned (Scheme 1b). Wang et al. first reported that phenolate-based ILs absorb CO2 in quantities consistent with chemical


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absorption and attributed the observations to reaction at the phenolate oxygen.10,11 The same group further interpreted changes in observed CO2 absorption at a single temperature and pressure with variations in the electronic character of variously substituted phenolate rings.12 Some substituents or variations on the phenolate motif are even reported to produce superstoichiometric CO2 uptake.13,14 Recent experiments and calculations provide evidence for a competing cation-CO2 reaction channel when basic azolides are paired with tetra-alkylphosphoniums.15 Azolide-driven deprotonation of the acidic proton on the α-carbon of the tetra-alkyl phosphonium cation generates an ylide that can be trapped with CO2 (Scheme 2).15 This secondary channel is analogous to the deprotonation-driven reactions at the acidic C2 carbon of imidazolium cations.16–20 This pathway can be deactivated by pairing the [2-CNpyr]– anion with a less acidic cation, such as a tetra-alkyl ammonium cation.15

Scheme 2: Ylide formation and CO2 trapping in [P66614][2-CNpyr].15

These results illustrate the subtleties in tailoring ILs for CO2 separations. In this work, we report the synthesis and physical, chemical, and computational characterization of a series of [P66614]+ phenolate ILs, including the parent phenolate ([PhO]–), a deactivated 4-nitrophenolate ([4-NO2PhO]–), and an activated 4-methoxyphenolate ([4-MeOPhO]–). We compare room temperature CO2 isotherms with those for the well characterized [P66614][2-CNpyr] over


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pressures ranges of interest for CO2 separations. We find that the neat phenolates are viscous, reach equilibrium with CO2 slowly, and ultimately exhibit a range of uptakes consistent with the occurrence of a chemical reaction. We show that the reaction product is distributed between normal and ylide-type. First principles calculations show that the energetics of these two channels are competitive, and ab initio molecular dynamics calculations indicate a close association between the phosphonium cation and phenolate anion consistent with cation reactivity and low ionicity.

Methods Materials and Synthesis Phenol (>99% purity), 4-methoxyphenol (>98% purity), and 4nitrophenol (>99% purity) were purchased from Sigma-Aldrich and used without further purification. ILs were prepared using a two-step protocol. Trihexyl(tetradecyl)phosphonium hydroxide ([P66614][OH]) was prepared by ion exchange of trihexyl(tetradecyl)phosphonium bromide, [P66614][Br] (97% purity, Sigma-Aldrich), with Amberlite IRN78 (Sigma-Aldrich) in methanol (ACS grade, Fischer Scientific). The desired IL is generated by neutralizing [P66614][OH] with an equimolar amount of corresponding phenol precursor followed by molecular sieving (4Å, Aldrich). The IL was filtered through a celite plug followed by a 0.45 µm PTFE syringe filter to remove any fine particulates. Methanol and other volatiles were removed under reduced pressure and the water byproduct was removed by further drying at 60°C. 1H (Varian INOVA-600) and

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P NMR (Varian INOVA-600) spectroscopy (details in the

Supporting Information) and thermal gravimetric analysis were used to establish the purity of the synthesized ILs. The amount of phosphine oxide present post-synthesis was less than 10%, as determined from the 31P NMR spectra. We observe all phenolate ILs to decompose over time to the oxide, even when water exposure is minimized by storage in a desiccator. As a result, all


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properties were measured with freshly prepared samples to assure that the oxide content was at a minimum. Density. IL densities were measured between 10 and 80°C using a DMA 4500 Anton Paar oscillating U-tube densitometer. The densitometer reports an uncertainty of ± 5 × 10–5 g cm–3; however, accounting for impurities in the sample, the uncertainty is reported to be roughly ± 1 × 10–3 g cm–3. The densitometer requires ~1.5 ml of sample, which was loaded to a syringe in a glove box under a N2 atmosphere. All samples contained less than 500 ppm water before and after the density measurements. Conductivity. The conductivity of the [P66614][phenolate] ILs was measured using an electrochemical impedance spectroscopy (EIS) system from Solartron (SI1260/SI1287). Each sample was loaded to a conductivity cell with two Pt electrodes under nitrogen atmosphere to minimize exposure to moisture and CO2. The temperature was controlled using a Binder Refrigerated Incubator KB53 (E3.1). The uncertainty in the reported conductivity values is approximately ±3%. The density and viscosity of [P66614][2CNpyr] have been previously reported.8 The interdependence of viscosity and ionic conductivity for the [P66614][phenolate] ILs was analyzed using a Walden plot, as described by Angell et al.21,22 The line that runs from corner to corner represents an “ideal” line, which was constructed using data for dilute KCl aqueous solutions, an example of a solution with fully dissociated ions. The Walden plots for each IL were assembled by plotting log of the equivalent conductivity (S cm2 mol-1) against log of fluidity (or reciprocal of viscosity (poise-1)). Viscosity. Viscosities were measured using an ATS Rheosystems Viscoanalyer with an ETC-3 Joule-Thomson effect temperature cell. Prepared samples were placed on


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the viscoanalyer plate and maintained under a continuous flow of nitrogen (Praxair grade 4.8, 99.998%, H2O < 3 ppm) to prevent exposure to the atmosphere. The shear stress was set and the viscosities were measured from 10 to 80oC. Measured viscosities have an uncertainty of ± 5% above 100 cP and ± 10% between 50 cP and 100 cP as determined by measuring Cannon oil standards in each range. Spectroscopic

Characterization.

[P66614][PhO],

[P66614][4-MeOPhO],

and

[P66614][4-NO2PhO] were observed in situ with a Mettler Toledo IC-10 attenuated total reflectance Fourier Transform infrared (ATR-FTIR) spectrometer equipped with a silicon probe. The spectra were recorded from 500 to 3500 cm–1. CO2 was added to the neat IL and ATR-FTIR scans taken every two minutes to monitor the change in the intensity of the C-O stretch.

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C, 1H, and

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P NMR spectra for [P66614][PhO], [P66614][4-MeOPhO],

and [P66614][4-NO2PhO] were obtained in d6-DMSO on a Bruker 500 MHz and Varian 600 MHz spectrometer. The I L spectra before and after reaction with CO2 were compared to characterize the reaction products. CO2 solubility. CO2 absorption experiments were performed at ambient temperature (22 ± 2˚C) in a jacketed stainless steel Parr reactor equipped with a silicon ATR-FTIR probe at the bottom of the reactor. N2 gas was purged through the reactor during the loading process to prevent water absorption. After sealing the reactor, vacuum is applied (10–5 mbar) to eliminate any additional water and dissolved gases that may have been present in the sample. After vacuum, CO2 was introduced to the cell and the pressure in the reactor was monitored using a Heise pressure gauge. The sample was stirred and the total amount of CO2 absorbed was calculated from the pressure drop using the ideal gas law. Equilibrium points were measured between 0 and 1.5 bar. Uptake was determined to


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be at equilibrium when the cell pressure was constant for at least 2 hours and the C–O stretch intensities in ATR-FTIR spectra were invariant. Under some conditions it took 2-3 days for the sample to reach equilibrium. Computational details. We used an ion pair model to estimate the relative energetics of anion and cation CO2 reaction channels that includes a tetraethylphosphonium ([P2222]+) cation paired with each of the three phenolates. We choose this small, symmetric cation to minimize spurious conformational dependencies in the energies. The [P2222]+ cation and phenolate anions were located at arbitrary positions and relaxed at the B3LYP/6-311+G(d,p) level using Gaussian09.23 The transition state for proton transfer was located using the Berny algorithm and verified using intrinsic reaction coordinate following.23 The harmonic vibrational spectra of all species were confirmed to contain only real modes; the proton transfer transition state contains only one imaginary mode.26 For comparison with experiment, we apply a standard scale factor to the B3LYP/6-311+G(d,p) of 0.96.44 To capture cation-anion dynamics, we paired each anion with an asymmetric ([P2228]+) cation. Plane wave supercell density functional theory (DFT) calculations were performed using the CPMD code.24 Atomic core states were described using Vanderbilt ultrasoft pseudopotentials,24 valence states expanded in a plane-wave basis up to 30 Ry, electron-electron interactions treated with the PBE exchange-correlation functional,25 and interatomic interactions augmented with an empirical van der Waals correction using Grimme’s damped dispersion model.26 We placed one ion pair in arbitrary orientation in a 15 Å cubic box, relaxed the structure, performed 2 ps of equilibration using Car-Parrinello (C-P) dynamics followed by 10 ps of data accumulation. C-P dynamics employed an electronic mass of 400 a.u. and a time step of 5 a.u. (0.12 fs). All hydrogen atoms were substituted by deuterium. The ionic temperature was held


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at 300 K and fictitious kinetic energy at 0.008 a.u. using Nosé-Hoover thermostats. The fictitious electronic kinetic energy is based on its “natural value” estimated from a 2 ps Car-Parrinello (CP) dynamics run without the thermostats. Physically uninteresting center-of-mass translations and rotations were removed every 1000 C-P molecular dynamics steps.

Results Experimental Results We first consider the physical properties of the phenolate ILs. Figure 1 shows phenolate IL densities measured between 10 and 80°C and at 1 bar pressure. Densities increase with molecular weight and decrease linearly with temperature. Measured viscosities are shown in Figure 2 and are similar for [P66614][PhO] and [P66614][4-MeOPhO] while that of [P66614][4-NO2PhO] is approximately a factor of four greater at ambient temperature. A similar ordering but lower absolute values are reported by Wang et al.12 Measured water content after the viscosity measurements were always greater than before, ranging from 1000 ppm for the [PhO]- and [4MeOPhO]- ILs to 3000 ppm for [P66614][4-NO2PhO]. Further, phosphine oxide content increases over periods of days in all these ILs. Either or both of these factors could account for differences in observed viscosities. Viscosities found here are significantly greater than [P66614][2-CNpyr],


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which has a viscosity of 420 cP at 22°C,8 and contribute to the slow mass transfer of CO2 from gas to condensed phase. CO2 solubilities measured between 0 – 1500 mbar at room temperature are shown in Figure 3 and compared to that of [P66614][2-CNpyr], which has previously been reported to approach equimolar chemical absorption at 1 bar and 22°C.8 CO2 solubility in the phenolate ILs is considerably lower. [P66614][PhO] approaches an uptake of 0.6 moles CO2/mole IL at 1 bar and is slightly less in [P66614][4-MeOPhO]. Isotherms for [P66614][PhO] and [P66614][4-MeOPhO] rise sharply initially and then increase very slightly with increasing CO2 partial pressure, unlike the Langmuir type behavior found for [P66614][2-CNpyr]. CO2 uptake is significantly less in [P66614][4-NO2PhO], suggestive of physical or a weak chemical reaction. The Henry’s Law constant for physically dissolved CO2 in [P66614][4-NO2PhO] is experimentally determined to be 16 bar, which is smaller than the value for the equivalent [P66614][Tf2N] IL27, indicating higher physical solubility in [P66614][4-NO2PhO].

Figure 3. Measured room-temperature CO2 isotherms for three phenolate ILs compared with that of [P66614][2-CNpyr].8

In-situ ATR-FTIR and NMR spectroscopy were used to characterize the reaction


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products. As shown in Figure 4a, after initial dosing of CO2 to [P66614][4-MeOPhO] a peak between 2310 and 2360 cm–1 characteristic of physically absorbed CO2 appears in the ATRFTIR spectrum as well as two new peaks at 1635 and 1730 cm–1, in the region expected for a carboxylate and a carbonate species generated from binding of CO2 to a carbon and oxygen center, respectively, with CO2. With time the lowest frequency carboxylate mode grows at the expense of the 1730 cm–1 mode, suggestive of conversion from a kinetically preferred to a thermodynamically preferred product. An isosbestic point in the spectra supports this interpretation. We previously observed similar behavior at elevated temperature with [P66614][2CNpyr] and assigned the higher and lower frequency modes to reactions at the anion and through the phosphonium ylide, respectively.15 A similar interpretation would appear likely here. Wang et al.12 have similarly reported a feature at 1617 cm–1 in [P66614][4-Cl-PhO] after exposure to CO2. It appears assignment to a carboxylate is more appropriate than to the carbonate proposed by these authors.

Figure 4. In-situ ATR-FTIR spectrum of phenolate based ionic liquids before and after reaction with CO2 at room temperature. (a) ATR-FTIR spectra of [P66614][4-MeOPhO] after adding 200


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mbar of CO2 and tracing the reaction products with time until equilibrium is reached. (b) [P66614][PhO] equilibrium spectra. (c) [P66614][4-MeOPhO] equilibrium spectra. (d) [P66614][4NO2PhO] equilibrium spectra.

Figure 4c shows the final, steady-state ATR-FTIR spectra after [P66614][4-MeOPhO] has reached steady state with CO2 at room temperature and increasing CO2 pressures. The feature attributable to physisorbed CO2 grows in intensity with pressure; the 1635 cm–1 feature remains the dominant one at all pressures, while the 1730 cm–1 feature grows in intensity up to 600 mbar, beyond that the carboxylate and carbonate features are saturated. [P66614][PhO] (Figure 4b) evidences similar behavior, except that the 1730 cm–1 feature is less pronounced than the 1635 cm–1 one. Consistent with Figure 3, the [P66614][4-NO2PhO] spectra (Figure 4d) only show a new vibrational mode at 2310 – 2360 cm–1 corresponding to physisorbed CO2. NMR spectroscopy provides additional evidence to support the interpretation of the ATR-FTIR spectra. After CO2 exposure to [P66614][PhO], a new doublet appears in the 13C NMR spectrum around 167 ppm (Figure 5b and inset). The two peaks at 106 and 171 ppm in Figure 5a correspond to the ipso and para carbons of the phenol ring and they remain after CO2 absorption (Figure 5a and 5b), although shifted downfield and upfield, respectively. We have previously shown that the doublet around 167 ppm is characteristic of CO2 bound to the α-carbon of the phosphonium cation alkyl-chain.15 The doublet is due to coupling with the phosphorus that is 2bonds away from the reacted CO2. Figure 5c shows the corresponding

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P NMR; the small

feature near 46 ppm is due to the formation of phosphine oxide and the main peak at 34 ppm is characteristic of the phosphonium cation. Absorption of CO2 induces a second phosphorus environment at 31.5 ppm, evidence of reaction of the phosphonium cation with CO2.15 From the quantification of the 31P NMR, we estimate 0.41 moles of CO2 react with the cation per mole of IL under these conditions (Figure 5d).


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Figure 5: (a) 13C NMR of [P66614][PhO] before and (b) after CO2 exposure at 1 bar and room temperature. (c) 31P NMR of [P66614][PhO] before and (d) after CO2 uptakes. The peak at 31.5 ppm is an unequivocal doublet, which is consistent with CO2 reacted at the α-carbon of any of the alkylchains of the phosphonium cation..

Similar results are obtained for [P66614][4-MeOPhO], as shown in Figure 6. Again the 13C NMR spectrum shows a newly formed doublet at 167 ppm (Figure 6b) and the 31P NMR spectra a new phosphorus environment around 31.5 ppm (Figure 6d). As before, the two peaks at 146 and 162 ppm in Figure 6a correspond to the ipso and para carbons of the phenol ring and they remain after CO2 absorption (Figure 6b), although shifted downfield and upfield, respectively. From the quantification of the 31P NMR data, the cation reaction with CO2 is again determined to be the dominant product; however the ATR-FTIR data suggests the presence of a carbonate from binding of CO2 at the phenolate oxygen. This species is likely responsible for the very small 160 ppm feature observed in the 13C NMR spectrum (Figure 6a).


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Figure 6: (a) 13C NMR of [P66614][4-MeOPhO] before CO2 and (b) after CO2 which shows a new peak around 167 ppm after CO2 absorption at 1 bar and room temperature. (c) 31P NMR of [P66614][4-MeOPhO] before CO2 and (d) which shows two different phosphonium environments after CO2 absorption at 1 bar and room temperature. The peak at 31.5 ppm is an unequivocal doublet, which is consistent with CO2 reacted at the α-carbon of the phosphonium cation alkylchain.

The cation-CO2 product can be quantified by integrating the

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P NMR spectra, and the

difference between this product and the total observed CO2 uptake can be assigned to anionbound and/or physisorbed CO2. The total CO2 uptake in [P66614][PhO] at 1.3 bar is 0.63 moles of CO2 per mole IL from isotherm measurements (Figure 3), and 0.41 moles of this total is found from 31P NMR to be due to the cation-CO2 reaction (Figure 5d). To estimate the contribution of physisorption to the remaining 0.22 moles of CO2, we extracted a CO2 extinction coefficient from [P66614][4-NO2PhO], which exhibits physisorption only, and assumed the result to be transferable to [P66614][PhO] and [P66614][4-MeOPhO]. Further details on the determination of the


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extinction coefficient are found in the Supplementary Information. Results of this analysis are shown in Table 1. The cation-CO2 reaction channel is 50% greater than the anion-CO2 channel for the [PhO]- anion and almost three times as great in the [4-MeOPhO]- anion. Table 1: Total and partitioned CO2 uptakes (mol CO2/mol IL) at room temperature. IL

Pressure (bar) 1.5 [P66614][4-NO2PhO] 1.4 [P66614][PhO] 1.3 [P66614][4-MeOPhO]

Total Cation-CO2 Anion-CO2 Physical 0.09 0.0 0.0 0.09 0.63 0.41 0.15 0.07 0.60 0.32 0.21 0.07

Computational Results We and others have found that the binding energy of CO2 to the IL anion is a useful surrogate for the chemisorption energy of CO2 into an IL.4-14 In that spirit, we computed the CO2 binding energies with [PhO]– and [4-MeOPhO]– to be -40.9 and -40.8 kJ mol–1, indistinguishable from the -40.8 kJ mol–1 for [2-CNpyr]– computed at the same B3LYP/6-311+G(d,p) level of theory. The structures of these carbonate complexes are shown in Figure S.4 It was the large discrepancy between these energy predictions and the isotherms in Figure 3 that initially led us to explore the phenolate chemistry more carefully. Following our previous work,15 we then paired the [PhOCO2]– anion with a [P2228]+ cation and used Car-Parrinello (C-P) molecular dynamics (CPMD) at 298 K to explore the effects of cation interactions on CO2 binding. We used these asymmetric [P2228]+ cations to approximate the behavior of the [P66614]+ cation without greatly increased computational expense. We were surprised to discover that in the course of the 10 ps simulation, the CO2 molecule spontaneously desorbed from the phenolate, leaving behind a [PhO][P2228] anion/cation pair and a free CO2. (Animation available in Figure S5.a.) In contrast, CPMD simulations on the phenolate analog of Scheme 2 revealed a dynamically stable PhOH and carboxylate zwitterion (animation in Figure S5.b). These CPMD results are qualitatively consistent with the


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chemistry observed in the laboratory. To further quantify these two reaction channels, we used B3LYP calculations paired with this ion pair model to explore the energies of static snapshots of the various relevant structures. We considered [PhO]-, [4-MeOPhO]-, and [4-NO2PhO]- anions paired with a smaller, [P2222]+ cation to simplify optimizations. Computational results are summarized in Figure 7. We were able to locate both carbonate and carboxylate local minimum for all species. As shown in the right column, the carboxylate formed by reaction of CO2 at a cation carbon is uniformly lower in energy than the carbonate formed by reaction at the phenolate oxygen. Both channels are computed to be exothermic in the parent [PhO]-, but the carboxylate channel is about 14 kJ mol–1 more exothermic than the carbonate. In comparison, the carbonate channel is slightly more exothermic and the carboxylate slightly less so in the [4-MeOPhO]- case. In contrast, the carbonate channel is essentially thermoneutral in the [4-NO2PhO]- case and the carboxylate channel is only exothermic by about 18 kJ mol–1.

Figure 7: Schematic potential energy surface and B3LYP/6-311+G(d,p)-computed energies of anion- and ylidemediated reactions of CO2 with [P2222][PhO] (black), [P2222][4-NO2PhO] middle, (red), and [P2222][4-MeOPhO] (blue). Energies in kJ mol–1. Red, grey, black, and white spheres represent oxygen, phosphorus, carbon, and hydrogen, respectively.


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These results are for a [P2222]+ model cation for which location of a representative minimum energy structure is straightforward. Longer chains would require sampling over multiple conformations. The α-hydrogen acidities are also slightly sensitive to chain length; calculations reported in Table S.1 show that the ethyl chain is about 8 kJ mol–1 more acidic than longer chains. Further, the B3LYP-computed harmonic spectra of the carboxylate cations exhibit C-O stretch modes in the range 1664 to 1669 cm-1, close to the range observed in Figure 4. The carbonate anions are also computed to exhibit a distinct C-O stretch band in the range 1643 to 1687 cm-1, red-shifted relative to the range shown in the same Figure. It would thus be inappropriate to draw absolute comparisons between these simple snapshots of cation-anion pairs and the experimental results, but the computations are in good qualitative correspondence with the experimental spectroscopy and observations highlighted in Table 1. The reaction to form the carbonate is unlikely to have a significant energy of activation. In contrast, formation of the carboxylate involves breaking and forming multiples bond and is thus likely activated. We used the ion pair model to search for the transition state for deprotonation of the phosphonium cation by the phenolate anion to form an ylide. Results are also summarized in Figure 7. In all cases the ylide is higher in energy than the cation/anion pair, most so for the least basic [4-NO2PhO]– anion and least so for the most basic [4-MeOPhO]– anion. We found transition states for this proton transfer within the B3LYP model; in general barriers scale with final state energy. In the absence of CO2, we expect the equilibrium between the phosphonium cation and the ylide to lie towards the phosphonium cation. Reaction of the ylides with CO2 is exothermic by 50 to 65 kJ mol–1. These results are consistent with the observed more rapid formation of the carbonate


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product and slower, kinetically limited formation of the carboxylate, both proceeding from the phosphonium. A direct conversion of the kinetic (carbonate) to the thermodynamic (carboxylate) product is unlikely; we observed no such process in the dynamics simulations, and the carbonate is substantially less basic than the parent phenolate (Table S.2). The high activation energy for ylide formation in the [4-NO2PhO]- anion case likely prevents formation of the carboxylate at room temperature, consistent with the observation of physisorption alone. As with the reaction energies, the potential energy surfaces should be interpreted with caution for quantitative comparison with the bulk system. We did check the sensitivity of the predictions to the computational model. As shown in Figure S.6 for the phenolate, results are substantially similar with the inclusion of an empirical dispersion correction45 to the B3LYP model; a shift downward in the carboxylate energy is the most pronounced effect. Results are somewhat more sensitive to the inclusion of a continuum solvent correction. As an appropriate dielectric constant for these ILs is unknown, we computed the energies of the gaseous optimized pairs within a C-PCM46 solvation model using n-octanol (eps = 9.9), diethylether (eps = 4.7), and heptane (eps = 1.9) solvents. Results are also summarized in Figure S.6. The entire potential energy surface is pushed upward with increasing dielectric constant, such that the ylide intermediate becomes higher in energy than the proton transfer transition state that leads to it. Clearly, precise quantification of this energetics and dynamics requires a more elaborate model, likely incorporating at least several explicit cation/anion pairs. All models, however, support the same picture of barrierless formation of metastable carbonate and a slower, activated formation of stable carboxylate. We see from the above experiments and prior work15 that the phenolate ILs both reach a steady-state distribution of products more slowly at room temperature and exhibit the


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thermodynamic product at lower temperature than does [P66614][2-CNpyr]. These differences likely reflect differences in the structure and dynamics of cation-anion interactions.28–32 To probe these differences, we constructed and analyzed 10 ps Car-Parrinello molecular dynamic trajectories on single ion pairs of [P2228][2-CNpyr], [P2228][PhO], [P2228][4-NO2PhO], and [P2228][4-MeOPhO] after 2 ps equilibration dynamics. Results are summarized in Figure 8, plotted as the distance between P center and phenolate oxygen or azolide nitrogen center. Despite the short duration of these simulations, some general trends are apparent. Anion-cation separations fluctuate substantially in all cases. [P2228][4-NO2PhO] exhibits the greatest absolute fluctuations and greatest mean separation, reflecting the weak basicity of the nitro-deactivated phenolate. [P2228][PhO] and [P2228][4-MeOPhO] show closer association of the phenolate oxygen with the cation and, especially for the parent phenoxylate, fluctuations that bring anion oxygen and cation phosphorus in the close proximity necessary to transfer a proton and form the ylide. Both of these ILs exhibit closer contacts than that of the [P2228][2-CNpyr], a system that does not evidence the ylide channel at room temperature.

Figure 8: Interionic distances observed during 10 ps C-P dynamics trajectories, reported as P–O distance for phenolates and P–N distance for [P2228][2-CNpyr]. Mean distances over the course of the simulation are shown as horizontal lines. Distance is Å unit.

The [PhO]– anion in particular evidences some very close contacts with the [P2228]+


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phosphorous. Figure 9 shows snapshots of some of these close contact configurations. While during most of the simulation the phosphorous is pseudo-tetrahedral, at these close contacts phosphorous becomes pseudo-trigonal bipyramidal with phenolate oxygen and ethyl group in axial positions, indicative of a nascent P–O bond (Figure 9a-c). Natural Bond Orbital analysis reveals that the nascent P-O bond originates from strong interactions between the P–C antibonding orbital and phenolate lone pair (Figure 9a-c). The NBO partial charges on the [PhO]anion at these close contacts vary from 0.7 to 0.87 e, reflecting fairly extensive charge sharing (Figure 9a-c). These configurations quickly relax back to the tetrahedral structure, phenolate oxygen preferentially oriented towards α hydrogens of the cation, and phenolate charge increased to a nearly constant 0.92 e (Figure 9d-f).

Figure 9: Representative close contact configurations of [PhO][P2228] ion pairs from C-P dynamics with orbital interaction diagram, charge of phenolate anion, and second order energies (E(2)) of orbital interactions based on Natural Bond Orbital analysis. Distance is Å unit.

The close contacts observed in these C-P dynamics simulations suggest that the phenolate ILs may be less intrinsically ionic than other ILs. To probe this point, we measured the conductivity of the same four ILs over a range of temperatures. Log conductivities are plotted


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against the log of the inverse of the viscosities measured across the same range in Figure 10, in a so-called “Walden” plot.33–36 All ILs exhibit nearly linear correlations, reflecting a power law dependence between conductivity and viscosity. Deviations from the y = x line are typically interpreted as departures from a perfectly ionic solution. The order of those deviations track very nicely with the mean separations found in the C-P dynamics trajectories in Figure 8. Thus, the weakly associating [P66614][4-NO2PhO] is closest to the ideal Walden line. [P66614][2-CNpyr] deviates more substantially, and [P66614][4-MeOPhO] and especially [P66614][PhO] lie furthest from the ideal line, consistent with the close associations found in Figure 8. Tao et al.37 reported similar measurements on a series of phenoxylate anions paired with a tetramethylguanidium cations and noted the same negative departures from the ideal line. They interpreted the negative departure in terms of ion pairing and classified the phenolate ILs as “poor ionic liquids.” The experimental and computational results here are consistent with that interpretation.

Figure 10: Measured log conductivities vs. log inverse viscosities (Walden plot) of phenolate and 2-cyanopyrrolide ILs from 10 to 80˚C.

Discussion Based on the results from the ATR-FTIR spectroscopy, NMR spectroscopy, MD simulations,


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and computed reaction energies, we find that [P66614][PhO] and [P66614][4-MeOPhO] primarily react with CO2 through a cation channel. Carbonates are a minor product under the conditions observed here. The room temperature isotherms of both of these ILs deviate from ideal Langmuir behavior and saturate at approximately 0.5 mol CO2 per mole IL. At this level of uptake, nearly 50% of the phosphonium cations have been converted to the zwitterionic carboxylate (Scheme 2 and Figure 7). The experimental and computational evidence indicates that the phenolate anions are protonated in this process. Thus, at maximum observed uptake, the initial IL is a mixture of molecular, ionic, and zwitterionic components. Why this process saturates at about 50% conversion is not clear, although it is consistent with the initial observations reported by Wang and co-workers on [P66614][PhO].11 Subsequent work suggests higher uptakes at similar conditions.12,14 These differences likely reflect the complexity of the underlying chemistry and sensitivity to impurities, in particular H2O. Gimeno et al.38 reported uptakes and vibrational spectra for CO2 uptake in [bmim][PhO] at ambient conditions and attributed the observed 0.19 moles of CO2 absorbed per mole of IL to chemical reaction to form a carboxylate. Zhang et al.39 reported the CO2 solubility in [K][PhO] to be 0.74 mole of CO2 per mole of IL when dissolved in PEG150. Taylor et al.40 reported the CO2 solubility in [P66614][PhO] to be 0.49 mole of CO2 per mole of IL when the IL was bubbled with dry CO2 at a flow rate of 50 cm3min-1 at room temperature (22 ± 0.1oC). Recently, Vafaeezadeh et al.41 reported the CO2 solubility in [1-(2hydroxyethyl)-2,3-dimethylimidazolium][PhO] to be 1.5 mole of CO2 per mole of IL at ambient temperature (25oC) and pressure. All of these studies38-41 have interpreted these results in terms of direct CO2 reaction at the phenolate anion, following on initial proposals.11 We observe CO2 to adsorb only physically in [P66614][4-NO2PhO] and chemical uptake in [P66614][4-MeOPhO] and [P66614][PhO] to be roughly similar at ambient temperature. Others have


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reported substantially larger variations in CO2 capacity when substituents on the phenyl ring are varied and have correlated observations with computed PhO-CO2– bond energies.42 While the results for [2-CNpyr]– and other, similar azolide anions are largely in line with observed CO2 uptake,9 we do not find similar favorable correlations for the phenolates. We believe this is due to the prevalence of the cation reaction channel for the phenolates. Firaha et al.43 reported that a continuum solvation model was able to reproduce experimentally observed CO2 uptake trends in the phenolate ILs. This continuum solvation approach neglects explicit cation-anion-CO2 interactions and thus cannot capture the zwitterion product that dominates CO2 uptake in the phenolates, as shown here. This comparison highlights the need to employ explicit cation/anion models to describe CO2 chemistry in functionalized ionic liquids.

Conclusion The CO2 solubility of the phenolate based ionic liquids with tetraalkylphosphonium cation was determined by the absorption isotherm experiments at room temperature. The CO2 reaction of phenolate based ILs needed careful attention in order to interpret its reaction mechanism as shown here using computational quantum chemistry, ATR-IR, and NMR spectroscopy. We found that the major reaction channel of the CO2 chemistry forms a cation-CO2 product via the formation of a stable ylide intermediate rather than t h e anion-CO2, which is contradictory to the previous interpretation. The phenolate anion in IL would not be free to readily react with gaseous CO2 and the degree of ion pairing impacts CO2 solubility of ILs. Comprehensive understandings of the CO2 reaction chemistry in ILs can only help in designing the task-specific ILs for the CO2 capture process.

Acknowledgements This research used computer resources of the National Energy Research Scientific Computing


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Center, which is supported by the Office of Science of the U.S. Department of Energy and Center of Research Computing at University of Notre Dame. The authors thank Dr. Brandon Ashfeld for helpful discussions. This work is made possible through financial support from the Stanford Global Climate and Energy Program (#106644-A).

Supporting Information Available Supporting Information includes NMR spectroscopy data, physical properties (density, viscosity, and conductivity), approximate quantification of physical CO2 uptake using IR spectroscopy based on Beer’s law, and details of computational methods. It also includes animations of molecular dynamics trajectories of carbonate and carboxylate. This material is available free of charge via the Internet at http://pubs.acs.org.

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CO2 Chemistry of Phenolate-Based Ionic Liquids - ACS Publications

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