Dynamics of CO2 and Ionic Liquids in a Supported Ionic Liquid Membrane (SILM)

Carbon dioxide is the major anthropogenic greenhouse gas that inflicts onset of global warming and climate change. The atmospheric concentration of CO2 has been increasing since industrial revolution and reducing CO2 emission into the atmosphere has been an important issue. The concept of carbon capture and storage (CCS) where captured CO2 from stationary source like fossil fuel power station is transported to the storage sites and sequestrated underground or in the oceans has been proposed to mitigate the increase of CO2 concentration. There are many technologies developed for CCS applications. However, carbon capture is still expansive economically and energetically and needs to be improved.

Among many other capture media, room temperature ionic liquids (RTILs) have been of great interest because their negligible volatility enables them to replace amine-based organic solvents that used to be utilized in the post-combustion carbon capture. Moreover, their properties can be readily tunable through the combination of different kinds of cations and anions. Thus, RTILs can be properly modified to enhance their capture ability.

More advanced capture material, supported ionic liquid membrane (SILM), is porous membrane that impregnates RTIL in the pores. The porous membranes have large surface areas, and therefore they can minimize the consumption of RTILs in capture application while maintaining high capture performances. In addition, solvent regeneration in SILMs is unnecessary for separating captured CO2 and refreshing RTILs because CO2 diffuses through the RTIL phase and out to the permeate side once they are adsorbed on the feed side.

There have been many efforts to understand and improve permeability and selectivity of SILMs, but the fundamental studies about dynamics of CO2 and RTILs in SILMs are rare. In particular, there were no ultrafast time-resolved studies on SILMs, until our group recently investigated.

Figure 1. Pore size distribution of PES membrane

In our studies, poly(ether sulfone) (PES) membrane was soaked with 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimNTf2). The nominal pore size of the PES membrane is 200 nm. As shown in Figure 1, however, the pores have large distributions in sizes and an average pore size is ~350 nm. Based on the fact that water reveals bulk-like behaviors when the AOT reverse micelles is greater than ~9 nm, these PES membrane pores are considered to be very large.

Figure 2. FT-IR absorption spectrum of 13CO2 in SILM

FT-IR absorption spectrum of 13CO2 in SILM exhibits two asymmetric stretch bands of 13CO2 from two ensembles, that is from CO2 in IL phase in the pore and polymer phase that makes up the membrane itself (see Figure 2). The spectrum from IL phase is identical to that measured in bulk IL, implying that IL structures very near CO2 are unchanged when confined in the pores.

Figure 3. Anisotropy decays of 13CO2 in bulk IL and SILM

Figure 3 displays anisotropy decays measured with IR polarization selective pump-probe (PSPP) corresponding to reorientation dynamics of CO2. The decay for CO2 in IL phase in SILM significantly slows down compared to that in bulk IL. Both decays are fit with triexponential, which is the evidence of restricted angular orientational motions followed by the complete randomization. Wobbling-in-a-cone analysis with an inertial and two diffusive cones as well as final orientational diffusion leads to half-cone angles and associated time constants for each diffusive cones and a time constant for the complete randomization. In going from the bulk to SILM sample, cone angles and time constants for the diffusive cones are relatively unaffected whereas the complete randomization time constant, τm, changes from 51 to 90 ps. According to the Debye-Stokes-Einstein (DSE) equation, orientational diffusion time constant is proportional to the viscosity. Based on the viscosity of bulk EmimNTf2 (36.3 cP) and measured τm, the effective viscosity of EmimNTf2 in the SILM was calculated to be 117.0 cP, which is close to the bulk viscosity of DmimNTf2 (134.4 cP). Then, the translational diffusion constant for the IL in the SILM pores can be calculated, using bulk viscosity, effective viscosity determined above, and translational diffusion constant for bulk IL. (Stokes-Einstein equation shows linear relationship between viscosity and translational diffusion constant.) The estimated value is 1.6 x 10-6 cm2s-1, which is slower than the bulk (5 x 10-6 cm2s-1) by 3-fold. The anisotropy decay of CO2 in polymer phase even slower than those in bulk and IL phase of SILM. Reorientation of CO2 in polymer environment would not completely randomized as CO2 interacts with polymer strands.

Figure 4. 2D IR spectra of 13CO2 in SILM

Figure 4 shows 2D IR spectra measured with 2D IR vibrational echo experiments. The 2D IR spectra reveal two asymmetric stretch bands of 13CO2 in IL and polymer phases. At early waiting time Tw (Figure 4, top), both bands are elongated along the diagonal line because the IL environment has had little time to evolve and the initial frequency labeled by two pump pulses is well correlated with final frequency measured by echo signal. As Tw increases, the IL environment evolves and the initial and final frequencies are less correlated. Consequently, the 2D IR spectrum becomes round. (This is called spectral diffusion.) However, the degree of roundness is different for two bands at later Tw (Figure 4, bottom); the IL phase band is round while the polymer phase band remains elongated. This implies that the spectral diffusion is much slower in polymer phase than in IL phase. This line shape change and thus, spectral diffusion is quantified by CLS.

Like the 2D IR spectra reveals, the CLS decay of CO2 in polymer phase is much slower than that in IL phase. More interestingly, the CLS decay for IL phase of the SILM is significantly slower than that for the bulk IL. The structural spectral diffusion (SSD) extracted from reorientation-induced spectral diffusion (RISD) fit to the CLS decays slows down by 2-fold in going from the bulk IL to IL phase of SILM.

The slowdowns in the complete orientation randomization and SSD of SILM reflect that the global IL structures in the pores are very different from the bulk IL structures. This is somewhat surprising results because the pores of PES membrane are large. The current hypothesis for this is long-range IL ordering effect that is induced by the interface of membrane pore. Recent experimental and theoretical studies found that the IL structure at the interface can be well ordered like lamellar structure and it can extend out from the interface up to micrometers. Therefore, ILs in the SILM also can form ordered structure in ~350 nm pores, causing the slowdown in the dynamics. This study demonstrates that the bulk IL study is not enough to predict the IL properties in SILMs. To design better SILM technologies for carbon capture, the fundamental studies with SILMs are very important.

Relevant Publications

458."Dynamics of a Room Temperature Ionic Liquid in Supported Ionic Liquid Membranes vs. the Bulk Liquid: 2D IR and Polarized IR Pump-Probe Experiments," Jae Yoon Shin, Steven A. Yamada, and Michael D. Fayer J. Am. Chem. Soc. 139, 311-323 (2017).

466. "Carbon Dioxide in a Supported Ionic Liquid Membrane: Structural and Rotational Dynamics Measured with 2D IR and Pump-Probe Experiments," Jae Yoon Shin, Steven A. Yamada, and Michael D. Fayer J. Am. Chem. Soc. 139, 11222-11232 (2017).