Research article Received: 18 June 2014

Revised: 7 October 2014

Accepted: 9 October 2014

Published online in Wiley Online Library: 29 October 2014

( DOI 10.1002/mrc.4176

High-resolution slice selection NMR for the measurement of CO2 diffusion under non-equilibrium conditions Jesse Allena,b and Krishnan Damodarana,c* We present a simple and an efficient approach using spatially selective NMR to investigate solvation and diffusion of CO2 in ionic liquids. The techniques demonstrated here are shown as novel and effective means of studying solvated gas dynamics under nonequilibrium conditions without the need for conventional high power gradients. Copyright © 2014 John Wiley & Sons, Ltd. Keywords: ionic liquids; gas diffusion; slice selection NMR; concentration gradient

Introduction Ionic liquids (ILs) are ideally suited for CO2 capture as a result of their high gas capacities, high viscosities, negligible vapor pressure, good thermal stability, and the extremely broad range of possible structures.[1–9] For designing efficient processes for CO2 capture, transport properties of the CO2 needs to be understood. However, measurement of diffusion coefficient of CO2 is not straightforward. There are a few reports in the literature on the measurement of CO2 diffusion via FTIR measurements[10] in ILs and via pulsed field gradient (PFG)-NMR in porous catalysts.[11] The prevalence of PFG-NMR probes has made available a wide range of experiments for NMR investigations. Diffusion-ordered spectroscopy has become a commonplace technique for analysis of self-diffusion coefficients. However, diffusion measurements of nuclei with small gyromagnetic ratios such as 13C require special probes with powerful gradients. Non-equilibrium sample analysis by any technique is a difficult prospect. While stop-flow measurements and even LC-NMR probes have been used for non-equilibrium investigations, there are great limitations to these methods.[12] Slice selection NMR is widely used for spatial resolution of samples[13,14] and also for measurement of diffusion coefficients in simple liquids.[15] In this work, we have demonstrated the use of spatially resolved NMR to obtain diffusion coefficient of CO2 in IL, 1-butyl-3methylimidazolium bis(trifluoromethyl-sulfonyl)imide (BMIM-Tf2N). This IL was chosen for this study as a result of the extensive body of literature available. BMIM-Tf2N has undergone thorough investigations including NMR, fluid dynamics, and CO2 solubility investigations, as well as many others.[16,17]



BMIM-Tf2N was purchased from Sigma-Aldrich and used without any further purification. Natural abundance (non-enriched) CO2 was used for these studies and was purchased from Matheson Trigas. All the NMR experiments were performed on a Bruker Avance III 500 MHz NMR spectrometer using a 5 mm BBFO Smart

Magn. Reson. Chem. 2015, 53, 200–202

probe. The sample temperature was maintained at 30°C. A shaped 13 C pulse (Sinc1.1000) with duration of 50 μs was used for slice selection, and the gradients (45 G/cm) were kept on for the duration of the radio frequency pulse as described in the pulse sequences in the supporting information. Sixteen scans were collected for each 13C NMR experiment with a recycle delay of 30 s, which took about 10 min per experiment. The sample was prepared by measuring 150 μl of BMIM-Tf2N into a zirconia high pressure NMR tube. A capillary containing DMSO-d6 (which had previously been tested for stress at high pressure) was also inserted into the tube to provide a lock and reference signal. A bright LED was used to determine sample height, which was 1.8 cm. The tube was pressurized to 870 psi with CO2, and the time of pressurization is marked as t = 0. 13C NMR spectra were taken after 7 days of equilibration.

Results and discussion Reported herein is the application of a slice selection pulse sequence to ‘image’-dissolved CO2 in BMIM-Tf2N under non-equilibrium conditions (A concentration gradient). By applying the radio frequency pulse during a PFG, slices of the NMR tube can be selectively excited.[18] The pulse sequences for the slice selection experiments are included in the supplemental information and are labeled zgcg and zgcgig for the normal and inverse gated decoupled sequences, respectively. By using an inverse gated pulse sequence, quantitative 13C NMR spectra were obtained as shown in Fig. 1 for several slices of the sample perpendicular to the z-axis, and the CO2 peak integration * Correspondence to: Krishnan Damodaran, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA. E-mail: [email protected] a National Energy Technology Laboratory, Pittsburgh, PA 15236, USA b Biological Sciences, California Institute of Technology, Pasadena, CA 91125, USA c Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA

Copyright © 2014 John Wiley & Sons, Ltd.

High-resolution slice selection NMR


Figure 1. C NMR spectra taken at different slices of the sample perpendicular to the z-axis. Spectra from top to bottom correspond to slices from top to bottom of the sample in the NMR tube.

was compared with the integration of all three imidazolium ring carbons. Full 13C spectrum and assignments are shown in the supporting information (Fig. S1). The imidazolium peaks were chosen for their proximity to the CO2 peak, to minimize error from differences in excitation frequencies. Molar ratios were obtained from these integrations, and these molar ratios were used as the concentration in further calculations. By calculating a time-dependent concentration gradient and using Fick’s second law of diffusion [Eqn (1)], 13C diffusion coefficient are determined for CO2.[19]  2  ∂Φ ∂ Φ ¼D ∂t ∂x 2

Figure 3. ‘Normal’ one pulse sequence NMR spectra of a sample of BMIM-Tf2N pressurized under 870 psi CO2, with each spectra representing at least 24 h separation from the previous spectrum. The development of a concentration gradient clearly shows broadening and chemical shift changes.

were obtained for each of the NMR tube slices measured. Figure 2 shows the relationship of the concentration measurements at different sample heights to the diffusion coefficients. The diffusion coefficients calculated from this method were strongly concentration dependent. This result is not unexpected, as both the viscosity and density of the material are expected to change at high concentrations of CO2. The diffusion coefficients also indicate faster motion of dissolved CO2 relative to the BMIM-Tf2N self-diffusivities from literature, which is also to be expected considering the relative size of CO2 in comparison with the IL.


where Φ is the concentration, t is the time, x is the distance, and D is the diffusion coefficient. In the circumstance of a gas diffusing through a linear column of fluid with no initial dissolved gas in the fluid, Fick’s second law of diffusion can be rewritten as in Eqn (2):   x Φ ¼ Φo erfc pffiffiffiffiffi (2) 2 Dt where Φo is the initial concentration.[20] Using Eqn (2) and the Goalseek® function of Excel (MS Office 2010), diffusion coefficients

Diffusion Coefficient (m2/s)



3.0E-07 1

0.0E+00 15.0





mol% CO2

Magn. Reson. Chem. 2015, 53, 200–202

Copyright © 2014 John Wiley & Sons, Ltd.


Figure 2. The diffusion coefficient of CO2 in BMIM-Tf2N shows significant dependence upon concentration.

Figure 4. H NMR spectra of BMIM-Tf2N with CO2 under non-equilibrium conditions that were taken immediately following one another: above is a spectrum taken using the normal one pulse sequence, and below is a spectrum using the slice selection pulse sequence. Significant resolution gains are seen using the slice selection pulse sequence, as the concentration gradient causes significant broadening of the spectra using a one pulse sequence, but with the selection of a horizontal slice, the concentration gradient is minimized.

J. Allen and K. Damodaran In corroboration with the 13C studies, 1H NMR was used both with the slice selection pulse sequence and with a normal one pulse sequence to highlight the effects of a concentration gradient on NMR. Figure 3 shows the evolution of the normal (one pulse sequence) 1H NMR spectrum of BMIM-Tf2N as CO2 diffuses through the IL over the course of 23 days. Signal broadening and chemical shift changes result from the many different environments experienced by the IL at different concentrations along the z-axis. Figure 4 shows the difference between a single spectrum taken with the slice selection pulse sequence and one taken with a normal one pulse sequence for 1H NMR. It can be seen that while there is loss of signal for the slice selection pulse sequence, there is also a great increase in resolution for this pulse sequence over the standard one pulse sequence.

Conclusion Slice selective inverse gated pulse field gradient technique is shown as a novel and an effective scheme in the measurement of a CO2 concentration gradient. This information was valuable in calculating the effective sample concentration in horizontal sections of an NMR tube at a given time, allowing for the calculation of diffusion coefficients. The slice selection pulse sequence was used to demonstrate the resolution increase that can be obtained in a sample under non-equilibrium conditions by using slice selection techniques. It has been shown that slice selection techniques can be used to investigate non-equilibrium dynamics in a method similar to magnetic resonance imaging. This technique would be an excellent alternative to calculate diffusion coefficients when expensive high power gradients are not available or in any system where establishment of equilibrium along the z-axis is slow enough to be investigated on the NMR timescale. Acknowledgement This technical effort was performed in support of the National Energy Technology Laboratory’s (US-Department of Energy) ongoing research in CO2 capture under the RES contract DE-FE0004000.

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Supporting information Additional supporting information may be found in the online version of this article at the publisher’s web site


Copyright © 2014 John Wiley & Sons, Ltd.

Magn. Reson. Chem. 2015, 53, 200–202

High-resolution slice selection NMR for the measurement of CO2 diffusion under non-equilibrium conditions.

We present a simple and an efficient approach using spatially selective NMR to investigate solvation and diffusion of CO2 in ionic liquids. The techni...
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