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Peer Review (by Meghna Rajendran)

Your information that you added is excellent and very useful to have on Wikipedia.

There are a few sentences with statements that are a little bit more opinionated than I think Wikipedia wants. For example: “The urgency of climate change mitigation has spurred research in these non-conventional solvents for carbon capture and storage process from flue gas.” While I agree, the urgency of climate change mitigation is a highly debated topic and it might be better to not express it.

You can link more things in your article to other articles. For example, linking “chemisorption” to its corresponding page would be useful. There are a lot of technical terms that can be linked for the benefit of non-experts reading this article.

There are some typos (OC2 vs CO2, not subscripting the 2 in CO2, LI vs IL). Just go through it again and re-read it carefully.  

Your referencing system seems to be incomplete, Wikipedia has an internal citing method that you should use.   

(Week 2)Articles list:

I would like to edit the following articles, in all cases, they are existing articles that I want to improve:

/info/en/?search=Slash-and-char

/info/en/?search=Carbon_dioxide_scrubber

/info/en/?search=Ionic_liquids_in_carbon_capture

/info/en/?search=Carbon_capture_and_storage#Environmental_effects

(Week 3) Proposal

Our group chose the article about Ionic Liquids for carbon capture.

Article: Ionic liquids in carbon capture

Proposal Paragraph:

Our group is choosing to edit the Ionic Liquids in Carbon Capture Wikipedia page. We immediately noticed that this page was lacking in a solid introduction, a history section, enough references, there are no drawbacks of the technology (making it seem biased towards favoring ionic liquids over amines), lacked any mention of environmental impacts of use, and doesn’t provide many examples of commercial application. Our group would like to improve this web page by gathering articles in this area and introducing new sections about the history, commercial applications, as well as a drawbacks and benefits section. This will allow the reader to gain a more comprehensive view of ionic liquids as they pertain to carbon capture. We would like use information from the following references to achieve this.

Ionic Liquids for carbon capture Information

Introduction

The use of Ionic liquids (IL) as an absorbent for carbon capture is an attractive alternative that has been developed over the last decade. Ionic liquids are salts that exist as liquids near room temperature, these liquids have a negligible vapor pressure which make them environmental friendly solvents ^[1]. The urgency of climate change mitigation has spurred research in these non-conventional solvents for carbon capture and storage process from flue gas.

The carbon capture from flue gas requires a solvent with high selectivity for CO₂ and low viscosity. Since the solvent is regenerated in the stripper, the energy of absorption should be low to minimize the energy required in the desorption process ^[2].  Some strategies to improve the IL performance include tuning the basicity, functionalized IL with amino or phenolate groups and the utilization of hydrogen bond interactions. Nevertheless, further research is necessary to scale this capture process to industrial level. ^[3]

Carbon Capture using Absorption

Ionic Liquids as Solvents

Amines are the most prevalent absorbent in postcombustion carbon capture technology today. In particular,  monoethanolamine (MEA) has been used in industrial scales in  postcombustion carbon capture, as well as in other CO₂ separations, such as "sweetening" of natural gas. However, amines are corrosive, degrade over time, and require large industrial facilities. Ionic liquids on the other hand, have low  vapor pressures . This property results from their strong Coulombic attractive force. Vapor pressure remains low through the substance's  thermal decomposition point (typically >300 °C). In principle, this low vapor pressure simplifies their use and makes them " green" alternatives. Additionally, it reduces risk of contamination of the CO₂ gas stream and of leakage into the environment.

The solubility of CO₂ in ionic liquids is governed primarily by the anion, less so by the cation. The  hexafluorophosphate (PF₆^–) and  tetrafluoroborate (BF₄^–) anions have been shown to be especially amenable to CO₂ capture.

Ionic liquids have been considered as solvents in a variety of  liquid-liquid extraction processes, but never commercialized. Beside that, ionic liquids have replaced the conventional volatile solvents in industry such as absorption of gases or extractive distillation. Additionally, ionic liquids are used as co-solutes for the generation of aqueous  biphasic systems, or purification of biomolecules.

Process

A typical CO₂ absorption process consists of a feed gas, an absorption column, a stripper column, and output streams of CO₂-rich gas to be sequestered, and CO₂-poor gas to be released to the atmosphere. Ionic liquids could follow a similar process to  amine gas treating, where the CO₂ is regenerated in the stripper using higher temperature. However, ionic liquids can also be stripped using pressure swings or inert gases, reducing the process energy requirement. A current issue with ionic liquids for carbon capture is that they have a lower working capacity than amines. Task-specific ionic liquids that employ  chemisorption and  physisorption are being developed in an attempt to increase the working capacity. 1-butyl-3-propylamineimidazolium tetrafluoroborate is one example of a TSIL.

Drawbacks

Selectivity

In carbon capture an effective  absorbent is one which demonstrates a high selectivity, meaning that CO₂ will preferentially dissolve in the absorbent compared to other gaseous components. In  post-combustion carbon capture the most salient separation is CO₂ from N₂, whereas in pre-combustion separation CO is primarily separated from H₂. Other components and impurities may be present in the  flue gas, such as hydrocarbons, SO₂, or H₂S. Before selecting the appropriate solvent to use for carbon capture it is critical to ensure that at the given process conditions and flue gas composition CO₂ maintains a much higher  solubility in the solvent than the other species in the flue gas and thus has a high selectivity.

The selectivity of CO₂ in ionic liquids has been widely studied by researchers. Generally,  polar molecules and molecules with an electric quadrupole moment are highly soluble in liquid ionic substances. It has been found that at high process temperatures the solubility of CO² decreases, while the solubility of other species, such as CH₄ and H₂, may increase with increasing temperature, thereby reducing the effectiveness of the solvent. However, the solubility of N₂ in ionic liquids is relatively low and does not increase with increasing temperature so the use of ionic liquids in post-combustion carbon capture may be appropriate due to the consistently high CO₂/N₂ selectivity. The presence of common flue gas impurities such as H₂S severely inhibits CO₂ solubility in ionic liquids and should be carefully considered by engineers when choosing an appropriate solvent for a particular flue gas.

Viscosity

A primary concern with the use of ionic liquids for carbon capture is their high  viscosity compared with that of commercial solvents. Ionic liquids which employ  chemisorption depend on a chemical reaction between solute and solvent for CO₂ separation. The rate of this reaction is dependent on the diffusivity of CO₂ in the solvent and is thus inversely proportional to viscosity. The self diffusivity of CO₂ in ionic liquids are generally to the order of 10⁻¹⁰ m²/s, approximately an order of magnitude less than similarly performing commercial solvents used on CO₂ capture.  The viscosity of an ionic liquid can vary significantly according to the type of anion and cation, the alkyl chain length, and the amount of water or other impurities in the solvent. Because these solvents can be “designed” and these properties chosen, developing ionic liquids with lowered viscosities is a current topic of research. Supported ionic liquid phases (SILPs) are one proposed solution to this problem.

Tunability

The CO₂ absorption process in IL often involves a chemisorption reaction. Selection of different anion and cation combinations in ionic liquids affects their selectivity and physical properties. Additionally, the organic cations in ionic liquids can be "tuned" by changing chain lengths or by substituting radicals. In this context, the most common method is to functionalize the cation of the LI with an amino group that will form a carbamate with the CO₂ ^[4]. 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) was the first ionic liquied identified as a viable substitute for volatile organic solvents in liquid-liquid separations. Nevertheless, the gas separation of CO₂ with ionic liquids requires a better design of this liquids due to the chemisorption procces. The chemisorption reaction that ocurres between the ionic liquid and CO2 increases the capacity of absorption, however it has a high absorption enthalpy, which means a difficult desorption with high energy consumption for the solvent (IL) regeneration ^[5]. The improvements of CO₂ chemisorption through chemical design have been conducted in three areas: 

Tuning basicity

Researchers have tuned the basicity by enhancing the weaker interactions with the CO₂ and weakening the strong interactions. The anion is the dominating factor of the chemisorption due to a Lewis acid-base interaction. In this context, the absorption enthalpy shows a linear relationship with the pKa value of the IL ^[6]. Adding and amine or phenyl group to the anion disperse the negative charge and reduce the absorption enthalpy. Using this chemical design an absorption enthalpy can decrease from 90 kJ/molCO₂ to 50 kJ/molCO₂ ^[7]. In order to increase the weaker interactions, which are often hydrogen interaction, adding metal ion, carbonyl or pyridine groups to the cation or the anion increase the interactions with the CO₂ ^[6].  

Structure

The intramolecular hydrogen bond between and amine group and CO₂ (N-H-O) increase the solubility of CO₂ due to entropic effects ^[8]. Amino based Ionic Liquids can interact with the CO₂ like conventional amine solution, which form a carbamate. As a consequence of the equilibrium displacement, more CO₂ can be captured and the viscosity of the IL decreases. Because of the fact that the hydrogen bond has lower energy than a covalent bond, the energy required for the disorption process also decreases ^[8]. 

Unconventional interactions

Fictionalized IL with phenolate and aprotic heterocyclic anions have affinity for CO₂ because of the Van der Waals interactions. On the other hand, functionalized amino anion groups react with CO₂ to form a carboxylate or a carbamate with stoichiometry 2:1 and 1:1 respectively. In this context, the CO₂ capacity for IL (mol CO₂/mol IL) is between 0.5 and 1 ^[9]. 

Finally, ionic liquids can be mixed with other ionic liquids, water, or amines to achieve different properties in terms of absorption capacity and heat of absorption. This tunability has led some to call ionic liquids "designer solvents."  1-butyl-3-propylamineimidazolium tetrafluoroborate was specifically developed for CO₂ capture; it is designed to employ chemisorption to absorb CO₂ and maintain efficiency under repeated absorption/regeneration cycles. Other ionic liquids have been simulated or experimentally tested for potential use as CO₂ absorbents.

Proposed Industrial Applications

Currently, CO₂ capture uses mostly  amine-based absorption technologies, which are energy intensive and solvent intensive.  Volatile organic compounds alone in chemical processes represent a multi-billion dollar industry. Therefore, ionic liquids offer an alternative that prove attractive should their other deficiencies be addressed.

During the capture process, the anion and cation play a crucial role in the dissolution of CO₂. Spectroscopic results suggest a favorable interaction between the anion and CO₂, wherein CO₂ molecules preferentially attach to the anion. Furthermore,  intermolecular forces, such as  hydrogen bonds,  van der Waals bonds, and  electrostatic attraction, contributes to the solubility of CO₂ in ionic liquids. This makes ionic liquids promising candidates for CO₂ capture because the solubility of CO₂ can be modeled accurately by the regular solubility theory (RST), which reduces operational costs in developing more sophisticated model to monitor the capture process.