Absorption of Sulfur Dioxide by Deep Eutectic Solvents [View all]

[NNadir]

The paper I will discuss in this post is this one: Role of Hydrophilic Ammonium-Based Deep Eutectic Solvents in SO2 Absorption (Duan et al Energy Fuels 2020, 34, 1, 74-81.)

All of the waste products resulting from the combustion of dangerous fossil fuels are harmful, the most serious of course being carbon dioxide over the long term, but in terms of immediate health consequences, the carcinogens found in particulates are probably responsible for the majority of the millions of deaths dangerous fossil fuel waste each year. This said, none of the other pollutants are harmless. I sometimes muse to myself whether the largest source of mercury exposure, the combustion of coal, is responsible for the rising popularity of stupidity. As many educated people know, the madness of "Mad Hatters" - which was very real and not merely a literary invention - was the result of the use, by hatters, in the 18th and 19th century, of mercury to improve the appearance of hat pins.

Mercury, since the days of "mad hatters" was further distributed by distributed medical waste in thermometers and blood pressure devices, laboratory use in anemometers and other devices - including the device used by the first American to win the Nobel Prize in Physics, Albert Michelson, who showed that the speed of light was not subject to relativistic enhancement by the speed of the Earth's revolution around the sun, inspiring Albert Einstein's famous theory on this subject. It is still also widely used in gold mining operations, which is also represents, both in abandoned and operative gold mines, a serious source of mercury pollution.

However, the combustion of coal is still the major source of mercury pollution. Despite all the popularly believed rhetoric that "coal is dead," especially when it is raised as "proof" on an absurd but widely held belief that so called "renewable energy" is great, as I often note, in this century, coal has been the fastest growing source of energy on this planet, by far, followed by dangerous natural gas, followed by petroleum. The use of dangerous fossil fuels is rising and is doing so rapidly. If you think we are either doing something or going to do something about this state of affairs, sorry, you are lying to yourself.

Another major pollutant, probably dwarfed by particulates and heavy metals - not limited to mercury but also including the other major neurotoxin lead, and the element that is the subject of much mysticism, uranium - release by the combustion of coal consists of the two oxides of sulfur, SO2 and SO3, sulfur dioxide and sulfur trioxide. The latter is the anhydride of sulfuric acid. In the presence of water, it forms sulfuric acid, which is now a constituent of clouds where it leads to acid rain (along with nitrogen oxides).

This paper is about sulfur dioxide.

I favor the immediate phase out of dangerous fossil fuels - not by using so called "renewable energy" which will remain, as it always has, spectacularly incapable of addressing any major environmental problem since it is neither sustainable nor safe nor clean, but by the form of energy that many people, regrettably some Presidential candidates who wish to be thought of as being "green," nuclear energy. The idea of phasing out nuclear energy, as opposed to rapidly expanding it on an emergency basis, is definitely in mad hatter territory. Indeed, my speculation about the effect of mercury and lead aerosols released by dangerous fossil fuel combustion as having a bearing on the mass insanity that is on the rise, everywhere, is driven a consideration by the popular insanity with respect to nuclear energy, among many other things. Nuclear energy is not risk free, but it doesn't need to be risk free to save lives overall. The situation is best described by the existence of ambulances. Ambulances travel at high and potentially dangerous speeds, ignoring traffic laws, and, as the operate, releasing deadly dangerous fossil fuel wastes from their tailpipes. However the existence of ambulances has clearly saved more lives than it has cost, and so, rightly, we accept the existence of ambulances, even knowing that they are potentially very dangerous devices.

Ambulance Safety NHTSB Infographic.

Shutting perfectly operable nuclear power plants kills people; this is true in Germany; it is true in California, Massachusetts and Vermont. It is true anywhere nuclear power plants are shut by appeals to fear and ignorance.

To return almost to the point, and get off my continuously mounted soapbox, the paper listed above is very much about the continued use of coal, and is a description of putting lipstick on the expanding coal pig, by offering a route to reducing just one of the pollutants, not even the most important pollutants. Along with so called "renewable energy" which is also lipstick on the coal, petroleum and gas pig, there is no technology that can make fossil fuels acceptably safe, especially because nuclear energy is now so well understood, and neither fossil fuels, or reactionary rhetoric about so called "renewable energy" can make any technology as safe and as sustainable as nuclear energy.

Nevertheless, it is well worth considering this paper even if one is an environmentalist who favors the immediate phase out of all dangerous fossil fuels. Here's why: Because we hate our children so much as to insist them to enslave themselves to clean up our mess, because we have done exactly zero beyond issuing well meaning platitudes to address climate change, it will be necessary for future generations to remove carbon dioxide from the air. The engineering of this task is extremely challenging, extremely expensive, and very energy intensive. The largest source of so called "renewable energy" - biomass - is currently the second largest, after dangerous fossil fuels - cause of energy related deaths, the majority of which are currently involved in air pollution, although extreme weather is catching up.

However, one thing that biomass does, as it is self replicating and can more or less spontaneously cover huge surface areas cheaply, and because it has evolved to a combinatorially optimized point over billions of years, is to concentrate carbon from the atmosphere. Recently in this space, citing a paper on an issue in biomass closed (smokestack free) combustion, corrosion, I pointed out that sulfur is an essential element in living systems. Thus the treatment of biomass to recover the carbon in it will necessary involve sulfur, either in the extremely reduced (and highly toxic) form as H2S gas, or as sulfur oxides.

In addition, as I noted in passing, one widely discussed thermochemical cycle for splitting water is the sulfur iodine cycle. In the oxygen generating portion of this cycle, only 33% of the evolved gases after the condensation of water is oxygen. 67% is sulfur dioxide. I stopped thinking about the sulfur-iodine cycle a few years back because of mass transfer issues, but recently, having been exposed indirectly to new insights, I'm thinking about it again, and thus this paper, which is about the separation of sulfur dioxide from a gas stream - in this case flue waste - is of some interest to me, which is not to say that I think that the sulfur iodine cycle is the best thermochemical cycle - I actually favor Allam cycle coupled metal based carbon dioxide splitting cycles - but it is nonetheless worth considering. I recall reading a few years back that the Chinese were working on piloting this cycle with nuclear energy, but having (temporarily) lost interest, I didn't follow up to see if this actually happened.

Anyway, from the introductory text of the paper:

The emission of sulfur dioxide (SO2), mainly from the burning of fossil fuels, has caused serious environmental problems.(1) The development of renewable and efficient absorbents for the removal and recovery of SO2 is important for our society. In the field of SO2 absorption, the conventional absorbents, including CaCO3, limestone, and NH3, can potentially cause severe pollution. In addition, the technologies to remove acid gases have high operation costs and energy requirements.(2,3) The absorption of SO2 requires greener and more efficient solvents.(4)


Ionic liquids (ILs) have been applied in SO2 absorption. In particular, imidazolium-based ILs are excellent for SO2 absorption.(5,6) Hong found that the ability to absorb SO2 was related to the numbers of ether groups on ILs, as the ether-functional group could enhance the physical reaction between SO2and ILs. [E8min][MeSO3] could absorb 6.30 mol SO2 g–1 ILs at 30 °C and under atmospheric pressure.(7) Lee et al. reported the absorbing behavior of [Bztmeda][MeSO3].(8) However, with further investigation, the toxic and recalcitrant ILs could arguably cause environmental damage.(9) Deep eutectic solvents (DESs), as a new kind of greener and cost-efficient solvents, have been used widely in gas separation.(10−13) Han et al. synthesized choline chloride (ChCl)-based DESs and reported that ChCl/glycol, ChCl/glycerin, and ChCl/hexamethylene glycol could successfully absorb SO2. The absorption ability increased with the concentration of ChCl and could reach 0.678 g SO2g–1 DESs.(14) Deng prepared ChCl/levulinic acid and applied it for SO2absorption. With the calculated absorption enthalpy, the thermodynamic properties were investigated.(15) Liu investigated the absorption capacity of phenol-based DESs for SO2 at 293.15–323.15 K and 0–1.0 bar, reaching the capacity of 0.528 g SO2 g–1 DES.(16) Hydrophilic DESs have been a promising SO2 absorbent. However, the high viscosity is one of the significant characteristics of DESs. For example, the viscosity of ChCl-based DESs is usually higher than 2000 mPa·s.(17,18) This viscosity creates a mass-transfer barrier in the gas–liquid (SO2–absorbent) reaction, and thus, SO2 absorption is greatly limited.(19)
To investigate the mass-transfer barrier in SO2 absorption, a kind of hydrophilic deep eutectic solvents (DESs) and their hydrates were prepared to solve the relatively viscosity of DESs in SO2 absorption. The effects of tetrabutylammonium halogen/caprolactam (TBAB/CPL) DESs were investigated systematically, and the hydrophilic interfacial reaction was studied to explore the absorption mechanism of SO2 absorption in DESs.

A deep eutectic solvent is a solvent that has a lower melting point - a melting point lower than its individual components in the absence of the others - than "ambient temperatures, generally taken to be 25°C.

The rest of the story can be pretty much appreciated merely by looking at the pictures and their captions:



The caption:

Figure 1. Effect of the proportion of TBAB and CPL on SO2 absorption at 20 °C under atmospheric pressure.

The caption:

Figure 2. SO2 absorption of TBAB/CPL DESs as a function of temperature under atmospheric pressure (molar ratio of 1:2).

The caption:

Figure 2. SO2 absorption of TBAB/CPL DESs as a function of temperature under atmospheric pressure (molar ratio of 1:2).

The caption:

Figure 4. Arrhenius fitted curves of ln η vs 1/T for TBAB/CPL DESs.

The caption:

Figure 5. SO2 absorption of TBAB/CPL DESs and TBAB/CPL aqueous solutions as a function of DES concentration under atmospheric pressure (molar ratio of 1:2, 20 °C).

The caption:

Figure 6. Surface tension of TBAB/CPL DES aqueous solutions before absorption of SO2 (molar ratio of 1:2, 20 °C).

There is considerable discussion in the paper on the properties of the interface, to which the above graphic alludes. The interface is, of course, an important issue in gas absorption, as further explored in the text referring to the next graphic:




The caption:

Figure 7. Surface tension of TBAB/CPL DES aqueous solutions after absorption of SO2 (molar ratio of 1:2, 20 °C).

The caption:

Figure 8. Ea of TBAB/CPL DES aqueous solutions at different concentrations before absorption of SO2(molar ratio of 1:2, 20 °C).

The caption:

Figure 9. Ea of TBAB/CPL DES aqueous solutions at different concentrations after absorption of SO2(molar ratio of 1:2, 20 °C).

It may be useful for anyone who may wish to explore this conception further, to give some commentary on spectra and mechanism.

Some of the remarks on spectra:

The FTIR and in situ IR spectra of TBAB/CPL DESs and TBAB/CPL DES aqueous solutions (TBAB/CPL DESs, molar ratio of 1:2, 2 mol L–1) before and after absorption of SO2 are shown in Figures 10 and 11, respectively. Before absorption of SO2, the peak at a wavelength of 3401.3 cm–1 denotes the N–H stretching vibration of the TBAB/CPL and the peaks at approximately 1635.4 cm–1 represent the C═O stretching vibration. For the N–C–H stretching vibration in TBAB/CPL, the absorbance peak is found at 1477.2 cm–1.(28)(28)However, the characteristic peaks of TBAB/CPL are all changed or shifted after absorption of SO2. The N–H stretching vibration of the TBAB/CPL shifts to 3216.7 cm–1, the new C═O stretching vibration appears at the peak of 1646.9 cm–1, and a slight blue shift occurs for the N–C–H stretching vibration. In addition, some new characteristic peaks are shown in the FTIR spectra of TBAB/CPL after absorption of SO2. The S═O stretching vibration can be observed from 1033.5 to 1083.8 cm–1. The symmetric and asymmetric stretching modes of the absorbed SO2 can be observed clearly at 1353.8 cm–1 (Figure 10a). The SO2 absorption process is monitored with in situ IR spectroscopy (Figure 10b,c). The spectra exhibit two obvious vibration changes (the enlarged views of Figure 10d,e). The characteristic peaks of S═O, C═O, and N–H all increase gradually as the process goes. These changes in Figure 10 demonstrate that the interaction of SO2 and TBAB/CPL DESs occurs, which means the formation of hydrogen bond of C–C═O–N···H···SO, a similar hydrogen bond is also found in the previous work.(29)

The caption:

Figure 10. In situ IR and FTIR spectra of TBAB/CPL before and after SO2absorption (TBAB/CPL, 2 mol L–1, 20 °C).

Figure 11. In situ IR spectra of TBAB/CPL aqueous solution before and after SO2 absorption (TBAB/CPL, 2 mol L–1, 20 °C).

Some remarks in the paper on mechanism:

...Based on studies on the interfacial properties, the absorption mechanism of SO2 in hydrophilic DESs could be proposed as Scheme 1. The H–N–C═O bond of CPL forms an intermolarcular hydrogen bond with Br– of TBAB, forming a complex of H–N–C═O···Br. Meanwhile, the hydrogen-donor group in water (H) would react with the hydrogen-acceptor group in CPL (C═O) to form the TBAB/CPL DES hydrates. In the process of SO2 absorption, the polar SO2would react with TBAB/CPL DESs hydrates. As the hydrogen bonds of C–C═O–N···H···Br and C–C═O···H were broken, new bonds of C–C═O–N···H···SO and HO···H···SO formed and SO2 was absorbed in DES hydrates...

The caption:

Figure 12. 1H NMR spectra of TBAB/CPL before and after SO2 absorption (TBAB/CPL, molar ratio of 1:2, 2 mol L–1).

The caption:

Figure 13. Raman spectra of TBAB/CPL DESs and TBAB/CPL DES aqueous solutions before and after SO2 absorption (TBAB/CPL, molar ratio of 1:2, 2 mol L–1).

The caption:

Scheme 1. Proposed Mechanism between TBAB/CPL DES Hydrates and SO2

The caption:

Figure 14. Five continuous cycles of SO2 absorption (under atmospheric pressure, 20 °C) and desorption (N2, 50 °C) by TBAB/CPL DESs.

A caveat here concerns the stability of these reagents. To the extent that this reagents are exposed to acids, and one would imagine that a SO2 stream will necessarily be acidic, the stability of caprolactam to ring opening is certainly a major consideration.

Personally - and this is just a comment from the "peanut gallery" since I have not worked personally or directly with ionic liquids although I'm well acquainted with them - I think the ionic liquid routes are probably a better choice, since their toxicology can almost certainly be managed.

I don't necessarily like the way the solvents are regenerated, which seems to involve the use of a nitrogen stream, meaning the SO2 gas is impure and will need further processing.

Have a nice day tomorrow.