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Wed Oct 4, 2017, 08:58 PM

On the Solubility of Carbon Dioxide in Ionic Liquids.

A few years back, it was my privilege to attend this lecture, given at Princeton University, by Dr. Joan Brennecke of Notre Dame University on carbon dioxide capture using phase change ionic liquids:



"Ionic Liquids" are salts, generally where at least either the cation or the anion or both are organic molecules (although a few inorganic eutectic salts nearly qualify) that are liquids either near or at room temperature.

In my writing around the internet, I have never evinced a fondness for carbon capture of the dangerous fossil fuel waste carbon dioxide since I believe dangerous fossil fuels need to be phased out quickly.

However, to the extent that carbon dioxide can be removed from the atmosphere by biological materials, perhaps under reformation conditions at high temperatures provided by clean energy - by clean energy I mean nuclear energy - I believe this may offer an opportunity for future generations to reverse, to whatever extent possible, the great screwing over our generation gave them while claiming that, for one example of a worthless rationalization, that dangerous natural gas burning is only "transitional" while we all wait for the wonderful solar and wind utopia that did not come, is not here, and will never come.

To the extent that carbon capture is utilized to make carbon products, for instance carbides, nanotubes, graphene, and even polymers, this is value added sequestration.

And that is why I made sure I could leave work early and attend this lecture.

After the lecture, which was quite interesting, I collected some of Dr. Brennecke's papers and just got around to reading a few tonight.

Dr. Brennecke is the editor of the Journal of Chemical Engineering Data, where she's imposed some rather rigorous standards, much to the betterment of the world in general.

Here's one of the papers I collected:

On the High-Pressure Solubilities of Carbon Dioxide in Several Ionic Liquids (Joan Brennecke et al J. Chem. Eng. Data 2013, 58, 2642−2653)

Some excerpts from the text, beginning with the introductory paragraphs:

Ionic liquids (ILs) are arguably among the most interesting and important solvents to be developed in recent years. They are highly versatile due to their remarkably low vapor pressures, generally high thermal and chemical stability, nonflammability, and the ability to tune the chemical and physical properties by the incorporation of various functional groups into the anions and cations. As a result, they are effective media for reactions,1 have been considered as solvents for CO2 separation,2−8 can be used as absorbents in absorption refrigeration systems,9,10 serve in reactive catalysis,11−13 and have been evaluated for a wide variety of other separation processes.14−18 CO2 solubilities in the ILs are important for a number of these applications.

In addition, supercritical or near-critical CO2 has been studied by several authors as a way to recover valuable products from IL mixtures. For instance, CO2 can be used to separate the product from an IL + catalyst reaction mixture, leaving the IL and the catalyst ready for reuse.15,17 Scurto et al.19 demonstrated that CO2 could be used to separate ILs from aqueous solutions. Subcritical and supercritical CO2 has also been used successfully to induce separations of IL and organic compounds.16,17,19,20 It is well-known that high-pressure CO2 can affect the solvent strength of mixtures, especially when the solvent swells significantly with the addition of CO2. Thus, this is another situation where high pressure IL + CO2 phase behavior is important.


Note that some of the applications are actually carbon dioxide utilization schemes, where the (supercritical) carbon dioxide acts as a cosolvent.

An interesting and fun paragraph from the editor (and author in) this journal dedicated to accuracy and precision:

The Span−Wagner58 equation of state was used to calculate the density of the CO2 and determine the amount of CO2 transferred from the delivery system to the cell side. Knowing the volume of the lines on the cell side and the headspace of the cell, it is possible to determine the solubility of CO2 in the IL by difference, once again using the Span−Wagner equation of state for the vapor phase. Assuming that the vapor is pure CO2 is a very good assumption for the liquids investigated here since the ILs have very low volatility. Using a cathetometer to determine the height changes of the liquid level, one can also get the molar volume of the liquid mixture. The combined expanded uncertainty of the molar volume is calculated from the standard uncertainty of the cell volume which is determined from the height measurement with the cathetometer, the IL mass ( 0.0002 g), and the moles of CO2 dissolved in the liquid. This last uncertainty is determined from propagation of the uncertainties of the temperatures, Ruska pump volume, and volumes of the cell and lines.


The Span-Wagner equation is a wonderfully Rube Goldbergish equation that works to give highly reliable state variables for carbon dioxide. I had a little riff on this equation earlier in this space:

Paper on the Equation of State for High Efficiency Supercritical Carbon Dioxide Driven Turbines. (I wrote it before the subscript and superscript codes became unavailable at DU)

She finds that fluoroalkyl containing anions are the best at dissolving CO2.

The solubility behavior is [OTf]− < [Tf2N]− < [eFAP]−. Increasing the fluorination of the anion increases the solubility dramatically, as reported by Muldoon et al.39 We attribute this to both stronger interactions of the CO2 with the electronegative fluorine atoms and the higher free volume of anions with fluoroalkyl chains.



(This is unfortunate, because fluoroalkonic acids (notably PFOS) are a huge environmental problem to which we are too busy to pay attention. The only sink for these types of persistent pollutants, which are now present pretty much in every living thing on the planet, is radiation, which suggests yet another application for so called "nuclear waste" not that people who spend their lives uselessly on cartoonish anti-nuke websites are bright enough or informed enough to allow such use.)

The solubility of carbon dioxide in the alkyl phosphonium salts discussed in the paper are higher than those using imidazolium cations.

However the mixed organic/inorganic ionic liquid emim HSO4 shows very high solubility.

I thought this interesting.

Have a nice day tomorrow.

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Reply On the Solubility of Carbon Dioxide in Ionic Liquids. (Original post)
NNadir Oct 2017 OP
PJMcK Oct 2017 #1
NNadir Oct 2017 #3
Eko Oct 2017 #2

Response to NNadir (Original post)

Wed Oct 4, 2017, 09:15 PM

1. Fascinating

I'm a layman with respect to chemistry but this is quite remarkable. I've learned something interesting.

Thanks for posting this, NNadir. Enjoy your tomorrow as well.

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Response to PJMcK (Reply #1)

Wed Oct 4, 2017, 09:30 PM

3. My pleasure. Thanks for reading. n/t.

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Response to NNadir (Original post)

Wed Oct 4, 2017, 09:21 PM

2. You forgot

to bash solar and wind.

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