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Thu Oct 19, 2017, 09:27 PM

Moisture Swing Absoprtion/Desorption of CO2 Using Polymeric Ionic Liquids.

The paper I will discuss in this thread is this one: Designing Moisture-Swing CO2 Sorbents through Anion Screening of Polymeric Ionic Liquids (Wang et al, Energy Fuels, 2017, 31 (10), pp 11127–11133)

All of humanity's efforts - or lack of effort mixed with a unhealthy dollop of denial - to address climate change have failed.

No one now living will ever again see a reading of under 400 ppm of the dangerous fossil fuel waste carbon dioxide in the atmosphere, an irreversible milestone that was passed last year.

On the right, the open hatred of science has put an uneducated and unintelligent orange human nightmare in the White House who is actually trying to revive the worst dangerous fossil fuel, coal, while on the left, there has been a delusional embrace of so called "renewable energy" that has lead to the very dangerous, and frankly criminal, surge in the dangerous natural gas industry, for which so called "renewable energy" is merely lipstick on the pig.

So called "renewable energy" did not work, is not working, and will not work.

Thus, it will fall to future generations however many - if any - survivors there are, to remove carbon dioxide from the atmosphere, an engineering challenge that is enormous, to the extent it is even feasible. Our practiced contempt for them will leave them with few resources to address this planet.

In recent years I've been thinking and reading a great deal about this challenge. It seems to me that there are only two reasonable pathways that might work, one being extraction of carbon dioxide from seawater (where on a volume basis it is far more concentrated than in air) and the other being the pyrolytic processing of biomass, which, as life is self replicating and can thus produce huge surface areas capable of absorbing CO2.

Right now of course, the primitive way that biomass is utilized - combustion - is responsible for roughly half of the 7 million people killed each year from air pollution while mindless dullards do stuff like for example (one actually sees this kind of thing) whine about the collapse of a tunnel at the Richland National Laboratory where radioactive materials are stored.

However, in pyrolytic treatment of biomass, the biomass is heated in a closed system that is not vented to the atmosphere. If the heat to drive the pyrolytic reactions is nuclear heat - using the only real resource that we are leaving to future generations, used nuclear fuel, depleted uranium, and the thorium waste from the stupid and wasteful wind turbine/electric car industry - pyrolytic treatment of biomass can almost be certainly carried out in a way that is carbon negative to the extent that carbon is captured in products like graphite.

What is necessary in this case is the ability to separate carbon dioxide from other gases, notably hydrogen.

It is thus with some interest the paper linked at the beginning of this post, published by scientists at State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou, Zhejiang Province 310018, P.R. China. (China loses close to 2 million people per year to air pollution, and unlike Americans, they are actually interested in solving the problem.)

An ionic liquid is an salt, almost always partially organic, that is liquid at or near room temperature. A great deal of research has been conducted on ionic liquids, and they are very promising reagents for the capture of carbon dioxide.

I discussed them earlier in this space, providing a link to a lecture by the American scientist Joan Brennecke, an expert in ionic liquids:

On the Solubility of Carbon Dioxide in Ionic Liquids.

The work by the Chinese scientists expands greatly on Dr. Brenneke's talk and offers some very novel approaches.

As the title suggests, this is a moisture swing approach to capture and release of carbon dioxide. There are several types of "swings" that are utilized in the separation of gases. "Pressure swings" rely on a permeable material that will preferentially absorb or release one gas faster than an other when subject to changes in pressure. Commercial nitrogen and oxygen generators work this way to separate either oxygen or nitrogen from air. They consist of a compressor and a valve. "Temperature swings" rely on heating and cooling a material that absorbs a gas, monoaminoethanol is such an agent often proposed (but seldom used on a meaningful scale) for carbon capture.

This "moisture swing" is unique in as much as it relies on wetting and drying a polymeric ionic liquid.

Here's some text from the paper:

Carbon dioxide capture and storage (CCS) is regarded as one of the most effective approaches to alleviating global warming.(1) Among the developed CO2 capture technologies, adsorption based on a solid material is advantageous, since sorbents have unique interfacial properties, such as porous structures, modifiable functional groups,(2, 3) and low emissions to the environment.(4) Recently, polymeric ionic liquids (PILs) have been developed as a family of promising CO2 sorbents, as they possess the unique characteristics of ionic liquids (ILs) and the feasibility of a macromolecular framework.(5-7) Both the CO2 adsorption capacity and rate of PILs can be substantially higher than those of their corresponding ionic liquid monomers.(8) Similar to the fine-tunability of ILs,(9) these properties of PILs can further be tuned by the choice of cations and anions.

Among PILs, the quaternary-ammonium-based PILs have been highlighted in the literature due to their higher stability and CO2 sorption capacity than those of imidazolium-based PILs.(10) More interestingly, when coordinated with relatively basic anions such as OH– and CO32–, these PILs show the unique property of moisture-swing adsorption (MSA).(11) During the moisture-swing cycle, the sorbents adsorb CO2 in a dry atmosphere and release CO2 in a humid atmosphere. For poly[4-vinylbenzyltrimethylammonium carbonate] (P[VBTEA][CO32–]), the equilibrium partial pressure of CO2 under wet conditions is 2 orders of magnitude higher than that under dry conditions.(11) This moisture-induced cycle utilizes the free energy released by water evaporation, and thus, it can avoid the use of high-grade heat for sorbent regeneration and is also environmentally benign...

Humanity will never invent a form of energy that is as is safe, as sustainable, or as reliable as nuclear energy. However, nuclear energy is not perfect, of course (a point raised frequently by stupid people with selective attention who love to burn gas and coal to rant about, say, Fukushima). From my perspective, the biggest single problem with nuclear energy is what to do with the waste heat.

This system, however, offers a wonderful way to utilize waste heat in say, hot climates, where such heat may be of less value than in climates having cold spells - not that his planet will have cold spells as frequently as it once did.

The authors continue:

...Of particular interest in this work is the screening of anions for PIL materials with MSA ability for CO2. Previous studies have shown that the type of anion plays a key role in defining the PIL features.(12) After introducing CO32– or OH– anions into P[VBTEA], the sorbents exhibited such a strong CO2 affinity that they were proposed for directly capturing CO2 from the ambient air (400 ppm).(11, 13-16) For quaternary-ammonium-based salt hydrates with F– or acetate (Ac–), the CO2 absorption capacity was observed to be affected by the hydration state,(17) which is similar to the MSA. The CO2 adsorption isotherms indicate that they are suitable for gas separation from concentrated sources (15–100 kPa).(17) To gain insight into the MSA, several theoretical studies have been conducted on ionic interactions, reaction pathways, and hydration energy. Density functional theory (DFT) calculations by Wang et al.(18) demonstrated the reaction pathways of proton transfer for the PIL with the anion CO32–. The results showed that the hydrated water acts as both a reactant and a catalyst during CO2 adsorption. By building a molecular dynamic model for ion pairs of the quaternary-ammonium cation (N(CH3)4+) and CO32– in a confined space, Shi et al.(19) found that the free energy of CO32– hydrolysis in nanopores decreases with the decrease in water availability. However, how the chemical structures, especially the types of anion, of quaternary-ammonium-based PILs would affect the moisture-swing performance for CO2 capture remains unknown. Therefore, extensive work is required to reveal the relationship between the structural and physicochemical properties of PILs, especially through theoretical approaches.

In this paper, the ionic liquid is bound to polystyrene, as the following picture from the paper shows:

The structure of the ionic liquids is optimized so that it can react freely with water, and the authors screen various anions to form ion pairs with the positively charged polymeric ionic liquid, so that the structure can accommodate 3 water molecules.

The thermodynamic reaction diagram is shown:

The structure is optimized, reflecting that the fluoride ion is found to be the best counterion.

The authors conclude thusly:

Quaternary-ammonium-based PILs are promising sorbents for CO2 capture. In this work, theoretical studies were performed to systematically investigate the effect of varying the counteranion on CO2 adsorption. Our results showed that the PIL with carbonate as the counteranion has the lowest activation energy, strongest CO2 affinity, and largest swing size, and it functions through a two-step mechanism, which indicates that it is a better candidate as a sorbent to capture CO2 from ultralow concentration mixtures, such as that of air. The PIL with fluoride as the counteranion has a low activation energy, strong CO2 affinity, and medium-to-large swing size, and it adsorbs CO2 through a two-step mechanism, owing to the unique ability of fluoride to strongly attract protons. The PIL with acetate as the counteranion has a high activation energy, weak CO2 affinity, and small swing size and functions with a one-step mechanism, which indicates that it is suitable for capturing CO2 at a high partial pressure because of its large capacity and its feasibility to be regenerated through conventional approaches. Further investigations revealed that the repulsion between the two quaternary ammonium cations, which interact with the carbonate anion or two fluoride anions, could promote the dissociation of hydrated water and lower the activation energy of the CO2 adsorption. The two-step reaction pathways also exhibit low activation energy owing to the relatively small structural changes of each step. Our findings could provide a fundamental understanding of CO2 capture by quaternary-ammonium-based PILs and pave the way toward determining the optimal structure of a PIL to be used for CO2 capture in specific circumstances.

This is interesting. Note that if the ionic liquids themselves are synthesized utilizing carbon dioxide as a starting material - this is definitely possible, particularly using biological lignin as a source for the aromatic rings, the carbon is sequestered by the capture agent.

I thought this paper was very cool and thought I'd share it.

Have a nice Friday tomorrow.

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