Efficient, Reversible, and Selective Absorption of SO2 in an Emim-Cl Ionic Liquid Deep Eutectic.
The paper I'll discuss in this post is this one: Highly Efficient, Reversible, and Selective Absorption of SO2 in 1-Ethyl-3-methylimidazolium Chloride Plus Imidazole Deep Eutectic Solvents (Zi-Liang Li, Lin-Sen Zhou, Yue-Han Wei, Hai-Long Peng, and Kuan Huang, Ind. Eng. Chem. Res. 2020, 59, 30, 13696–13705).
Sulfur dioxide is a major pollutant from the combustion of dangerous fossil fuels as well as, albeit to a lesser extent, the combustion of "renewable" biomass. My interest in this paper is not connected with putting lipstick on the dangerous fossil fuel pig, nor as an endorsement of so called "renewable energy" which has proved to be, at enormous expense, yet another form of lipstick on the dangerous fossil fuel pig.
My interest is connected rather with a particular version of a thermochemical water splitting cycle, specifically, the sulfur iodine cycle which generates separate streams of hydrogen and oxygen. My considerations of thermochemical cycles has in recent years focused on other cycles, specifically those utilizing transition metals or cerium, a multivalent lanthanide, but it has always been the case that the sulfur iodine cycle - and some closely related cycles - have the advantage of requiring only fluid phases. An issue in the sulfur iodine cycle is that the oxygen generated may be contaminated with sulfur dioxide, limiting its use in oxyfuel combustion, and also suffering from reversibility. (The oxyfuel combustion of biomass under closed conditions - no smokestack - is a fairly straight forward path to recovering carbon dioxide from the atmosphere, and much safer than current procedures which are responsible for about half of the six to seven million air pollution deaths per year.)
The recent developments in ionic liquid approaches have renewed my interest in this cycle - although others offer different advantages.
These cycles are accessible by the use of clean energy, of which there is only one real form, nuclear energy. Because these cycles take place at relatively high temperatures, they also afford the achievement of high energy efficiency with managed heat flows, coming under the general rubric of "process intensification."
Anyway, from the introduction to the paper - artifacts of the translation of thoughts in Chinese to English text notwithstanding:
Given these shortcomings, developing new absorbents with low volatility, high efficiency, and good reversibility is highly demanded. Since there are many other components (e.g., N2 and CO2) in industrial tail gas, the developed absorbents should also exhibit high selectivity. To this end, ionic liquids (ILs) were proposed as promising candidates.(9?11) ILs are a class of organic molten salts and credited as “green solvents” owing to their negligible volatility. In addition, the properties of ILs can be easily tuned by tailoring the structures of ILs.(12?16) It is expected that highly efficient, reversible, and selective absorption of SO2 can be achieved in ILs. Within this regard, many functionalized ILs with excellent performance for SO2 capture have been developed by utilizing the electron-deficient and Lewis acidic property of SO2 molecules.
The authors combine two developments garnering a great deal of attention, the ionic liquids mentioned the text, and "deep eutectic solvents."
From the text:
Recently, deep eutectic solvents (DESs) started to attract considerable attention in gas separation research because they share similar features with ILs in terms of low volatility and tunable properties.(30,31) DESs are simple mixtures of hydrogen-bond acceptors (HBAs) and hydrogen-bond donors (HBDs). They have lower melting points than individual components because the hydrogen-bond interaction formed between HBAs and HBDs changes the electron distribution of molecules.(32) In comparison with ILs, DESs can be more easily prepared from commercial reagents, thus making them more intriguing from a practical perspective. Therefore, DESs are regarded as more promising candidates to achieve highly efficient, reversible, and selective absorption of SO2.
The authors add the very simple ionic liquid, ethylmethylimidazolium chloride, most often designated "emim chloride" imidazole, to imidazole, the chemical precursor to emim cations.
The structure of the emim chloride and imidazole are given in this figure:
The caption:
A table in the paper gives literature references for a number of other deep eutectic solvents utilized for the capture SO2.
Of more immediate relevance is a table of the showing viscosities as a function of composition, along with the decomposition temperatures of the mixtures.
The viscosities at room temperature (298K) are roughly equivalent, for the most dilute IL in imidazole, to that of, say, corn oil, only slightly higher. The supplementary data of the table gives the viscosities as a function of temperature, and near the temperature of regeneration reported in the paper which was around 353 K, about 20 K lower than the boiling point of water, the viscosities are quite low, not quite as low as that of water, but approaching it more closely. Note that the desorption temperature is only around 40 K lower than the decomposition temperatures of the deep eutectic solvent mixtures with the lowest concentrations of the emim-Cl. A mixture of SO2 and O2 gas from the thermal decomposition of sulfuric acid would emerge at much higher temperatures, but there is certainly an impetus to rapidly cool this gas mixture - most wisely in a process intensification setting - to prevent the rapid reoxidation of SO2 to SO3, the latter being the anhydride of sulfuric acid.
In the paper's experimental section it is noted that the viscosity is determined using a Brookfield viscometer, which is a widely used but considerably less sophisticated instrument than those provided by, say, Anton Paar, or TA instruments, which measure the full viscosity curves over a wide range of shear rates, but it is, at least illustrative.
No Arrhenius plot of the decomposition reaction is given, nor is the reaction clearly described, despite the fact that the regeneration (desorption) temperature is relatively close to that of the decomposition paper, but the paper does describe the behavior over multiple cycles, suggesting a fairly good stability:
The caption:
The reported concentrations of SO2 are quite high, approximately 15 mol/kg, which translates to over 900 grams of SO2/kg.
This data is, however, for the most viscous deep eutectic and concentrated with respect to the ionic liquid. Perhaps there are reasonable trade offs, which also seem necessary with respect to the time of regeneration.
This system is designed for the exhaust of dangerous fossil fuels, and thus the selectivity with respect to carbon dioxide and nitrogen gas are described, and not oxygen, but it is notable that the absorption depends on the lewis acidity of SO2, a property not generally associated with oxygen gas. This breakthrough graph from the paper shows the selectivity with respect to nitrogen and carbon dioxide, neither of which would be present in a sulfur iodine cycle decomposition gas:
In an industrial setting, fast analysis might prove possible using simple IR techniques.
The caption:
This quite an interesting little paper, I think, cheap and easy to synthesize reagents, readily accessible temperatures in a process intensification setting, and other features. In any case there is a long distance between an industrial sulfur-iodine cycle and the present day, but it could make for an interesting future in clean energy, the only sustainable form of which is nuclear energy.
I trust you are having a safe weekend and are enjoying life as much as is possible in these tragic times.
