Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity

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Clusters, Radicals, and Ions; Environmental Chemistry 2

Mesoscopic Structure Facilitates Rapid CO Transport and Reactivity in CO-Capture Solvents 2

Xiao-Ying Yu, Juan Yao, David B. Lao, David J. Heldebrant, Zihua Zhu, Deepika Malhotra, Manh-Thuong Nguyen, Vassiliki-Alexandra Glezakou, and Roger Rousseau J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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The Journal of Physical Chemistry Letters

Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity in CO2-Capture Solvents †,

†

‡

‡,

ǁ

‡

Xiao-Ying Yu, * Juan Yao, David B. Lao, David J. Heldebrant, * Zihua Zhu, Deepika Malhotra, Manh-Thuong Nguyen,£ Vassiliki-Alexandra Glezakou,£ and Roger Rousseau£ †,

Earth and Biological Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, 99354. Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA, 99354. ǁ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99354 ‡

£

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, WA, 99352. Supporting Information Placeholder ABSTRACT: Mass transfer coefficients of CO2 are anomalously high in water-lean solvents as compared to aqueous amines. Such phenomena are intrinsic to the molecular and nanoscale structure of concentrated organic CO2 capture solvents. To decipher the connections, we performed in-situ liquid time-of-flight secondary ionization mass spectroscopy on a representative waterlean solvent, 1-((1,3-dimethylimidazolidin-2ylidene)amino)propan-2-ol (IPADM-2-BOL). Two-dimensional (2D) and three-dimensional (3D) chemical mapping of the solvent revealed that IPADM-2-BOL exhibited a heterogeneous molecular structure with regions of CO2-free solvent coexisting with clusters of zwitterionic carbonate ions. Chemical mapping were consistent with molecular dynamic simulation results, indicating CO2 diffusing through pockets and channels of unreacted solvent. The observed mesoscopic structure promotes and enhances the diffusion and reactivity of CO2, likely prevalent in other water-lean solvents. This finding suggests that if the size, shape and orientation of the domains can be controlled, more efficient CO2 capture solvents could be developed to enhance mass-transfer and uptake kinetics. Water-lean solvents are promising class of CO2 capture solvents that use concentrated non-volatile organic bases to capture CO2, many of which are switchable ionic liquids (SWILs).1 Among the most studied are the non-nucleophilic amidine or guanidine superbases paired with an alcohol, chemically binding CO2 as liquid alkylcarbonate salts (Scheme 1).2-4,3, 5-6

Scheme 1. CO2 fixation by IPADM-2-BOL.

We previously hypothesized that the unique properties (e.g., nonlinear viscosity) increase6 faster than the expected CO2 mass transfer7 of solvents like IPADM-2-BOL are linked to the mesoscopic molecular structure. The observed molecular structure of IPADM-2-BOL (and likely other SWILs) differs substantially from that of non-ionic liquids (e.g., ethanol) and ionic liquids (e.g., [bmim][BF4],8-9 however, we believe that this heterogeneity is a more common feature in these solvent classes where a non-ionic low polarity fluid is both the solvent and the capture agent. Water-based solvents and ionic liquids, on the other hand, do not exhibit such heterogeneity because of the lack of nonpolar/nonionic solvent regions that are dispersed between regions of ionic reacted solvent. However, model results show that conventional ILs have a different type of heterogeneity, where long aliphatic chains on cations tend to orient themselves alike, making nano-sized nonpolar and polar regions where the fluid though the solvent is still composed of 100% ions.10-14 We recently demonstrated the inferred SWILs structurally difference from ionic liquids using in situ liquid SIMS (secondary ion mass spectrometry).8 Using SWILs as a prototypical example of a water-lean CO2-capture solvent, the hetergogeneity is characterized and its impact on facilitated CO2 transport is assessed in this work. We recently linked the nonlinear viscosity increase of SWILs using molecular dynamic (MD) simulations to the formation of hydrogen bond networks and clusters of zwitterionic carbonate.2, 15-16 However, no experimental 3D chemical speciation was available to validate the predicted molecular structure of the solvent as a function of CO2 loading. In MD simulations, the solvent at low loadings is composed mostly of CO2-free IPADM-2-BOL molecules.2 At higher CO2 loadings, ionic clusters begin to form. As the kinetic diameter increases with increased CO2 loadings, the solvent diffusion decreases and the viscosity of the fluid increases. As the concentration of clusters increases, the solution continuously becomes more viscous until CO2 uptake achieves its maximum (0.5 moles CO2/mol IPADM-2-BOL). The non-linear viscosity increase limits the practical applications of SWILs for CO2 capture as pumping and moving viscous fluids is energy intensive, and a high viscosity coupled with a low thermal conductance necessitates the use of prohibitively large heat exchangers.17 While the formation of the heterogeneous molecular structure may be detrimental with respect to viscosity, the heterogeneity may be advantageous for chemically binding or releasing CO2. It

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is expected that a high solution viscosity should directly reduce mass transfer by decreasing the diffusion coefficient as related by the Stokes-Einstein equation,18 suggesting that increases in viscosity should result in a corresponding proportional decrease in CO2 diffusivity. This leads to speculation that highly viscous water-lean solvent systems would exhibit poor CO2 mass transfer and not economically viable.

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stoichiometries for both the positive and negative modes are summarized in Table 1. Additional information and possible peak assignments are listed in Table S1.

Our previous studies showed the opposite, the liquid-film mass transfer coefficients (k'g) of solvents such as DBU-1-hexanol and IPADM-2-BOL are comparable to that of 5 molar monoethanolamine (7.64 × 10-6 mol/s/m2/Pa at 40 ˚C) even at viscosities that are two orders of magnitude higher.7 For IPADM-2-BOL, we measured k'g values and found that they were dependent on the solubility of CO2 in solution—more specifically, that k'g was highest in the low CO2 loaded (less polar) solvent than in the more highly loaded (more polar) solvent at 40 ˚C: 3.75 × 10-6 mol/s/m2/Pa at 0.27 wt.% and 9.0 × 10-6 mol/s/m2/Pa at 5 wt.% CO2 respectively.7 We also found that at comparable driving forces, the k'g values for IPADM-2-BOL were higher than those for 5 M MEA and 9 M piperazine at comparable driving forces,7, 19 inferring that the Stokes-Einstein equation may not accurately predict CO2 diffusion in these viscous liquids. Similar anomalous mass transfer has been measured in GAP-1/TEG solvent20, suggesting that this behavior maybe more universal in water-lean solvent than originally thought. It has been reported that the Stokes-Einstein equation breaks down when the solvent particles (in this case, carbonate clusters) are larger than the diffusing particles (CO2).21-22 The recent liquid SIMS analysis shows that nanometer sized clusters are present in solution at any CO2 loading greater than 0%,8 indicating the presence of clusters could account for the deviation of expected behavior from the Stokes-Einstein equation. Further evidence on a lack of change in mass transfer is exemplified by our k'g measurements of N2O in an analogous SWIL composed of DBU and 1-hexanol.20 N2O is a nonreactive CO2 surrogate that is used to measure CO2 solubility and transport in the absence of a chemical reaction.23,24 The k'g values for N2O showed negligible changes when switching from a lean loading of 0.06 mol CO2/ mol alkalinity to an intermediate loading of 0.26 mol CO2/ mol alkalinity.20 The observation that N2O still had comparable diffusion even as the solution started becoming ionic and more viscous suggests that the heterogeneity of the solvent may play a role in the transport of gases through the liquid. To verify this hypothesis, identifying the speciation and the spatial distribution within the solvent is mandatory.

Figure 1. Negative ToF-SIMS spectra (m/z− 100–400) comparing normalized intensities of SWILs loaded with 0%, 15%, 25%, 35%, and 45% CO2.

Table 1. Key peaks in the mass spectra of CO2 loaded IPADM-2-BOL (see Figure 1) and possible assignments. m/za Formula

Possible Identification

Positive ion mode 99

C5H11N2+

fragment of IPADM-2-BOLb

C6H12N3

+

154

C8H16N3

+

170

C8H16N3O+

172

C8H18N3O+

184

C8H18N5+

Details about IPADM-2-BOL synthesis, liquid SIMS analysis, and MD simulation were reported in supplemental materials. In situ liquid SIMS enabled by the system for analysis at the liquid vacuum interface (SALVI) 25-27 was utilized to determine the chemical speciation of IPADM-2-BOL in the positive and negative modes.

216

C9H18N3O3+

268

C13H26N5O+

(deprotonated IPADM-2-BOL)+(fragment of IPADM-2-BOL)a−H

325

C16H33N6O+

[2(IPADM-2-BOL)−H]−O

341

C16H33N6O2+

2(IPADM-2-BOL)−H

353

C17H33N6O2+

[2(IPADM-2-BOL)−H]+C

Fig. 1 depicts normalized negative SIMS spectra of IPADM-2BOL with different CO2 loadings. Positive-mode spectra are in Fig. S1. SIMS 2D and 3D images are reconstructed from the depth profiling procedure.28-29 Here, the characteristic signals of deprotonated IPADM-2-BOL (m/z− 170) are observed at all CO2 loadings. IPADM-2-BOL…CO2 (m/z− 214) is not observed with appreciable counts at 0% CO2; however, its intensity increased with the increase of CO2 loading. This observation is consistent with the formation of the zwitterions as illustrated in Scheme 1. It is possible that IPADM-2-BOL picks up some CO2 during sample preparation, which would account for the presence of the small amount of m/z− 214 observed at the 0% loading. Key spectroscopic signatures of the liquid I-PADM-2-BOL…CO2 liquids and their

355

C17H35N6O2+

[2(IPADM-2-BOL)−H]+CH2

385

C17H33N6O4

+

401

C18H37N6O4

+

431

C18H35N6O6+

126

(protonated IPADM-2-BOL)−OH2−C2H4c (protonated IPADM-2-BOL)−OH2 (IPADM-2-BOL)−H protonated IPADM-2-BOL [(protonated IPADM-2-BOL)−OH2 ̶ C2H4]+C2H6N2+d (protonated IPADM-2-BOL)CO2c

[2(IPADM-2-BOL)−H]CO2 [protonated IPADM-2-BOL+IPADM-2BOL]CO2+CH2 2[(protonated IPADM-2-BOL)CO2]−H

Negative ion mode 118

(fragment of IPADM-2-BOL)**CO2

C4H8NO3-

170

C8H16N3O

deprotonated IPADM-2-BOL

188

C8H18N3O2-

(deprotonated IPADM-2-BOL)+H2O

214

C9H16N3O3-

(deprotonated IPADM-2-BOL)CO2


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The Journal of Physical Chemistry Letters [(deprotonated IPADM-2-BOL)CO2]+H2O or [(deprotonated IPADM-2-BOL)CO3]−2H

232

C9H18N3O4-

257

C10H17N4O4-

301

C13H25N4O4

-

333

C13H25N4O6-

383

C17H31N6O4-

397

C18H33N6O4-

429

C18H33N6O6-

[(IPADM-2-BOL)•CO2] +CNO[(deprotonated IPADM-2BOL)CO2]+[(fragment of IPADM-2BOL)eCO2]−O2 [(deprotonated IPADM-2BOL)CO2]+[(fragment of IPADM-2BOL)**CO2] [2(deprotonated IPADM-2-BOL)CO2]−H [2(deprotonated IPADM-2BOL)CO2]−H+CH2

within the ionic regions of the lattice are appreciably stronger than in the neutral liquid. The liquid SIMS spectra elucidate speciation in the SWIL. More importantly, the in situ molecular imaging allows us to relate the ionic/nonionic domains reported previously from MD.

[2(deprotonated IPADM-2BOL)CO2]−H+CH2+O2

a

m/z reported in unit mass, more information seen in Table S1 the secondary ion fragment of IPADM-2-BOL: C5H11N2+ c − indicates the removal of the organic fragment d + indicates the addition of the organic fragment e the secondary ion fragment of IPADM-2-BOL: C3H8NOb

Two peaks are detected for IPADM-2-BOL in the positive mode, i.e., IPADM-2-BOL (m/z+ 172) and IPADM-2-BOL…CO2 (m/z+ 216) (See Fig. S1). The deprotonated IPADM-2-BOL peak is seen in the negative mode, m/z- 170. Koechanol acts as both a weak acid (alcohol) and base (guanidine) validated by the appearance of the SIMS spectra with no CO2 loading. IPADM-2-BOL was hypothesized to have simpler speciation compared to the complex solvent speciation determined for DBU and 1-hexanol,8, 30 because it consists of a zwitterion instead of separate ion pairs in the latter. However, our in situ liquid SIMS observation and peak identification suggest a more complex speciation owing to the existence of strongly interacting ion pairs when CO2 is loaded into the liquid. IPADM-2-BOL achieves maximum CO2 uptake at 50 mol% (limited to 45 mol% here due to viscosity limitations), which would suggest a 2:1 IPADM-2-BOL:CO2 reaction stoichiometry.3, 6 However, the predominant peak observed is m/z− 214 (IPADM2-BOL…CO2) suggesting that a 1:1 IPADM-2-BOL:CO2 reaction stoichiometry exists and that the fluid is indeed composed of the zwitterions2, 6 and not a conventional cation/anion pair. This observation matches the thermodynamic models used to interpret experimental data3 and the MD molecular dynamics simulation.2, 15-16 We also see evidence of a peak at m/z− 383 (2(deprotonated IPADM-2-BOL) CO2)−H) which could correspond to a conventional cation/anion pair or an agglomeration of a zwitterion and free IPADM-2-BOL. It is not clear which species is more likely to form based solely on the mass spectra, though the former is hypothesized due to the previously mentioned thermodynamic modeling of measured isotherms.3 These observation consistent with the clustering of ionic species reported previously from MD simulations.2 It was found that the strong intermolecular interaction between the ionic molecules, manifesting itself in the form of molecular clustering of zwitterions and anomalous dielectric properties as a function of CO2 loading.2 To illustrate this we consider intermolecular binding from a typical MD run with a 30% loading of CO2, as represented by zwitterionic IPADM-2-BOL…CO2 species in a matrix of unreacted neutral IPADM-2-BOL molecules. We estimated the binding energy of ionic and nonionic species within the liquid matrix by extracting each of N molecule from the liquid phase and computing its binding energy, Ebind,N=Ebind,N-1-Emol (see SI for discussion). We found that the binding energy for nonionic species is on average -143 kJ/mol which is lower than that for the ionic species of -269 kJ/mol; see Fig. S11 for a distribution. We thus conclude that upon CO2 loading the intermolecular interaction energies

Figure 2. Normalized 2D images of A) negative IPADM-2-BOL (m/z− 170) and IPADM-2-BOL…CO2 (m/z− 214); and B) positive IPADM-2-BOL (m/z+ 172) and IPADM-2-BOL…CO2 (m/z+ 216) with 0 mol%, 25 mol%, and 45 mol% CO2 loadings. 2D image analysis was then performed to determine solvent speciation and whether the fluid was homogeneous or heterogeneous in nature. Normalized 2D images of IPADM-2-BOL and IPADM-2-BOL…CO2 in both positive and negative ion modes are presented in Fig. 2. In the negative ion mode, the IPADM-2-BOL (m/z− 170) intensity is minimal at 0 and 25% CO2 loadings (Fig. 2A) as expected. As noted above, the zwitterionic carbonate (m/z− 214) is observed at 0% CO2 only in trace amounts. Interestingly, 25% and 45% CO2 loadings show a different kind of heterogeneity; a large (>500 nm) aggregate of carbonate (bright yellow) is flanked by unreacted IPADM-2-BOL (dark orange/red). Fig. 2B shows that the relative chemical component abundance of IPADM-2-BOL (m/z+ 172) is uniformly distributed at 45% CO2, because it reaches the maximal CO2 loading capacity. The relative chemical abundance of IPADM-2-BOL…CO2 (m/z+ 216) shows negligible count at 0% CO2 loading, while a random distribution of small (∼10–20 nm) clusters of zwitterions (m/z+ 216) is observed at 25% and 45% CO2 loadings. The 2D images suggest that the smaller clusters appear to gather together to form a large aggregate. Closer inspection shows that the large aggregate may not yet have consolidated into a single large globule; rather, it appears to be an aggregate composed of the smaller clusters. In either case, the formation of the small clusters and aggregates is associated with the viscosity increase that has been observed for IPADM-2-BOL.7 The negative spectral principal component analysis (PCA) was conducted. Principal component 1 (PC1) and PC2 together explain 91% of the variance. PC1 separates the CO2 loaded IPADM2-BOL samples largely in PC1 positive from 0% CO2 loaded sample in PC1 negative as shown in Fig. 3A. The loading of IPADM-2-BOL…CO2 (m/z− 214) is quite high in PC1 positive loading in Fig. 3A, indicating that this is one of the main con-



tributors differentiating the loaded SWIL from the non-loaded IL. The PC2 score further distinguishes the IPADM-2-BOL samples with lower CO2 loadings (i.e., 15% and 25%) from highly CO2 exposed samples (i.e., 35% and 45%). The peak m/z− 214 attributes to the difference between lightly-CO2-loaded and highly CO2 loaded IPADM-2-BOL, suggesting that this component is more prevalent in the low-CO2-exposed samples. This result demonstrates the importance of the IPADM-2-BOL…CO2 in differentiating the IPADM-2-BOL with various CO2 loadings. The positive spectral PCA results (Fig. S2) reveal the important role of the protonated IPADM-2-BOL (m/z+ 172) among different loadings.

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current SIMS observation confirms the nanostructuring within the liquid. Additionally, the >10 nm size regimes of the clustering occur over larger length scales than what was conceived accessible by the previous simulations.2

Figure 4. 3D overlay images of 170 and 214 (A) and 257, 301, 383 (B) We now consider the implication of the nanostructuring on CO2 transport. The bound CO2 molecules in the ionic liquid regimes are expected to be strongly bound to their neighbors and hence undergo slow molecular diffusion. Indeed our previous MD simulations found that the zwitterionic species exhibit slow diffusion which followed the Stokes-Einstein relationship (see Table S5 in ref 1).1 However, ab Initio based MD simulations and NMR studies have shown that the absorbed CO2 exists as an equilibrium between zwitterions and molecularly solvated CO2 in a ratio of about 10:1 at 40 oC.1 The computed barrier for interconversion between these species was ~16 kJ/mol indicating rapid (ns) transitions between the two forms. We thus hypothesize that it is the molecular CO2 that dominates the macroscopic mass transfer. In addition, the molecular CO2 may diffuse through solvents like IPADM-2-BOL via pores or channels of low-polarity regions in which CO2 could easily move, as illustrated by the yellow line in Fig. 5A. In the CO2-free IPADM-2-BOL (less polar and less viscous), CO2 could readily diffuse, whereas at higher CO2 loadings, CO2 would likely diffuse through pockets or channels of CO2-free solvent rather than the polar and viscous CO2-bound regions. Observation that the solvent is heterogeneous at any CO2 loading in Fig. 4 suggests that pockets or channels exist, through which gases CO2 or N2O could diffuse, accounting for the higher than expected mass transfer.

Figure 3. (A) Spectral PCA PC1 score and loading plots and (B) PC2 score plot and loading plot in the negative ion mode. To better understand the spatial distribution of the major components within each sample, 3D images were reconstructed in a 1 × 1 µm2 window as a function of time or depth in the fluid for the five CO2 loadings in Fig. 4. The depth of the liquid is estimated to be the top few nanometers of the liquids.25, 31 Here, the IPADM-2BOL (m/z− 170, teal) and IPADM-2-BOL…CO2 (m/z− 214, purple) are not evenly distributed at any CO2 loading or with respect of sputtering time, confirming that the solvent is heterogeneous and that the fully loaded SWIL is composed of regions of unreacted IPADM-2-BOL and regions of zwitterionic carbonate. The

It is unlikely to achieve direct measurements of diffusivity of CO2 in each region of IPADM-2-BOL, because it is impossible to isolate either domain. We can, however, assume where in solution the CO2 would likely diffuse using CO2 diffusion data from analogous liquids. The ionic (CO2-bound) form of IPADM-2-BOL, such as DBU:1-hexanol, would be similar in polarity to ionic liquids such as [bmim][PF6]32 while the nonionic (CO2-free) form would have a polarity comparable to that of isopropanol. It has been reported that ionic liquids entail high CO2 solubility,33-38 though we note that the diffusion coefficients of CO2 in ILs have been reported to be anywhere from 10 to 100 times lower than diffusivities of CO2 in organic liquids.39 The faster diffusion of CO2 in organics compared to ILs suggests that, at least for SWILs, CO2 would favor diffusion in the nonionic domains of the fluid. To corroborate this conjecture, we performed further classical MD simulations to shed light on the diffusion of unreacted CO2 through IPADM-2-BOL. We considered systems in which 0%, 15%, and 30% of IPADM-2-BOLs are CO2-bound, in the ionic form in addition to free CO2 molecules which are solvated by both ionic and nonionic species shown in Figs. 5B and 5C. The number of ionic species in the vicinity of free CO2 molecules increases against the loading, however, they are less likely to be caged by


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The Journal of Physical Chemistry Letters only ionic species. In the 15% mol loading case , 70% of the free CO2 molecules are surrounded by only nonionic species, while the remaining 30% of them are surrounded by both ionic and nonionic species. No CO2 molecules can be found that are surrounded by only ionic species. In the 30% mol loading case, 45% of the free CO2 molecules are surrounded by only nonionic species, 53% of them are surrounded by both ionic and nonionic species, and only about 2% of them are surrounded by only ionic species (see Fig. S13). Thus, unreacted CO2 preferentially resides in the vicinity of solvent or at the ionic/nonionic interface. This result suggests that CO2 diffusion will be less impacted by loading compared to the overall solvent diffusion, because CO2 is most likely to be in pockets of un-reacted solvent. The computed CO2 diffusion coefficients decrease from 0.9x10-5 to 0.2x10-5 to 0.1x10-5 cm2/s for mol loading of 0%, 15%, and 30%, respectively, whereas solvent diffusion reduces from 0.1x10-5 to 0.5x10-7 over the same range of loadings. More details are provided in the SI.

Our experimental and theoretical findings account for why these solvents absorb CO2 rapidly even as a viscous fluids. Mesoscale heterogeneity is identified as the root cause of the facilitated CO2 transport in many water-lean solvents. The formation of ionic clusters and aggregates provides pockets or channels in the fluid through which CO2 can readily diffuse and react with CO2-free IPADM-2-BOL. Water-lean solvents invariably suffer from high viscosity due to formation of ionic species during CO2 capture, however, only those that circumvent the StokesEinstein relationship will exhibit favorable kinetics and transport. Although mesoscale heterogeneity is identified as an important factor in modifying mass transfer in switchable ionic liquids compared to conventional aqueous solvents or conventional ionic liquids, it is unlikely the only factor. We postulate that molecular composition, structure, and surface reactivity may all play a role in carboln capture capability. Ultimately, we conclude that the heterogeneous solvent structure fulfils this role, facilitating mass transfer of CO2 capture solvents. We hypothesize that many water-lean solvents have a heterogeneous molecular structure and potentially facilitated mass-transfer. If the size, shape and orientation of these mesoscopic domains can be manipulated in these solvents, CO2/solvent super highways could be designed, enabling even faster and more efficient materials for CO2 capture.

ASSOCIATED CONTENT Supporting Information Figure 5. MD simulations of free CO2 in IPADM-2-BOL at varied mol loadings: (A) Superimposed frames from a 0.5 ns trajectory, the yellow feature shows the trajectory of a CO2 molecule in a solvent box, only ionic species (in red) are shown here, (B and C) snapshots of CO2 and solvent/zwitterion molecules in its first solvation cell (CO2-free IPAMD-2-BOL in blue, CO2-bound IPAMD-2-BOL in red) at 15% and 30% mol loading, respectively. The mesoscopic structure may also contribute to the rapid reaction kinetics of CO2 chemical fixation in solution. Öztürk et al. had measured the reaction kinetics of two SWILs (diazabicyclo(5.4.0)-undec-7-ene and 1,1,3,3 tetramethylguanidine, each with 1-hexanol) showing very rapid chemical fixation of CO2 albeit at high solution viscosities.40-41 Our results show that CO2 is preferentially solvated and moves through the nonionic domains in Fig. 5, that is, CO2 is always dissolved and in contact with CO2-free IPADM-2-BOL to react with. Thus, the built-in separation could account for the enhanced reaction rate by providing regions where CO2 and unreacted solvent are effectively concentrated together. Thus, the solvent’s heterogeneity would provide a high driving force to capture CO2 in solution, albeit at the expense of high solution viscosities. In summary, the unique mesoscopic structure of water-lean solvents such as IPADM-2-BOL provides facilitated CO2 transport and reactivity at the expense of a non-linear viscosity increase. The ability to perform a nanoscale 2D and 3D chemical mapping of the solvent’s structure allowed us for the first time to observe a distinctive mesoscopic structure with the segregated regions of unreacted and reacted solvent coexisting in the same fluid, matching theoretical predictions. The heterogeneity in the solvent arises from regions of CO2-free IPADM-2-BOL coexisting with clusters of zwitterionic carbonate ions (IPADM-2BOL…CO2). IPADM-2-BOL is comprised of nanometer sized clusters of zwitterionic carbonate ions at any CO2 loading above 0%. The ions do not appear to grow with increases in CO2 loading; rather, the total concentration and distribution of ions in solution increase, in agreement with theoretical predictions. 2D chemical mapping provides evidence of cluster aggregation, with a >500 nm aggregate composed of the individual smaller clusters.

Additional experimental details and theoretical results in PDF are provided in Supporting Information.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected]. Funding Sources We thank the United States Department of Energy’s Office of Science Basic Energy Sciences Early Career Research Program FWP 67038 for funding.

ACKNOWLEDGMENT Pacific Northwest National Laboratory (PNNL) is proudly operated by Battelle for the US Department of Energy. Computational resources were provided through allocation at the National Energy Research Scientific Computing Center (NERSC) located at Lawrence Berkeley National Laboratory. We thank Xiao Sui, Xiaofei Yu, and Satish Nune for their technical support to this work. The instrument access was supported under a general user proposal 50143 in the W. R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RL01830.

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Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity

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