Posts Tagged ‘oxidation state’

Credit: Carbon Engineering

Direct Air Capture (DAC) is the somewhat misleading term that has come into use for chemical fixation processes designed to extract carbon dioxide from the atmosphere. Chemical fixation of atmospheric CO2 is nothing new – organisms capable of photosynthesis are thought to have evolved billions of years ago, while limestone formation from CO2 taken up by the oceans has been occurring for hundreds of millions of years. But mankind hasn’t had a go at it until recently.

The driving force behind the development of DAC is the conviction based on the best available science that current (and rising) atmospheric CO2 levels constitute an existential threat due to global warming and climate change, as well as ocean acidification and marine ecosystem disruption. There is a desire for technologies to augment both naturally-occurring fixation processes and emission-reducing initiatives in order to accelerate the process of bringing the rise in atmospheric CO2 levels to a halt and subsequently to achieve drawdown.

To put this task in perspective, let’s put a few facts and figures into it. Carbon dioxide is a thermodynamically stable gas and the densest component of air, 1.977 kgm-3 at STP compared with 1.293 kgm-3 for air. This does not mean however that CO2 sinks in air and accumulates in the lower atmosphere. Like any gas, CO2 exhibits the phenomenon of diffusion, which is the tendency of a substance to spread uniformly throughout the space available to it. And this is where the challenge of DAC lies. Although there is a lot of CO2 – ca. 3210 gigatonnes in 2018 (ref) – up in the air, the atmosphere is a big place and the concentration of CO2 (currently around 410 ppm) is tiny in the context of extraction. In other words a large amount of air needs to be processed for a modest rate of capture. It is therefore relevant to consider the processes of capture and conversion from a carbon oxidation state perspective, which has a direct bearing on process thermodynamics and economics.

Note: If it proves possible to reduce the partial pressure of CO2 in the atmosphere through DAC, oceanic release of dissolved CO2 should take place according to Henry’s Law to restore equilibrium between oceanic and atmospheric partial pressures. If this were the case, the removal of CO2 from the atmosphere could not be achieved separately from removing CO2 from the vastly bigger ocean reservoir, and the task of stopping the rise of the Keeling curve could not be separated from the rise in oceanic acidification. This would add a whole new dimension to the task of drawdown.

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Oxidation states and conversion energy

Carbon can exist in nine oxidation states, which are conveniently if somewhat abstractly represented by dimensionless numbers ranging from +4 (most oxidized state) to –4 (most reduced state). The lower the number, the more energy is present in carbon’s chemical bonds. So the process of reducing a carbon compound requires energy, the amount of which increases with the degree of reduction.

As the table shows, carbon in carbon dioxide has the highest oxidation state (+4). In terms of conversion energy requirement, it is self-evident that the least demanding option is to capture CO2 without reduction for subsequent release, containment and onward supply to industry. Capture technologies such as aqueous alkanolamine absorption are well-known, but in terms of product use it should be noted that carbon dioxide applications such as synthetic fuel feedstock and carbonated beverages will lead to atmospheric re-release and therefore cannot contribute to CO2 drawdown.

Note that Mother Nature makes use of CO2 capture without reduction in the process of oceanic uptake. The oxidation state of carbon in the carbonate ion that eventually becomes chemically fixed in limestone is still +4.

Turning to carbon dioxide capture with conversion, the oxidation state table shows that there are eight levels of reduction, each possessing its own product outcomes and synthetic opportunities. The formulas shown in the table indicate two strands in the reduction process – the addition of hydrogen and the removal of oxygen (for simplicity I have restricted the examples to combinations solely of C, H and O).

If we ask ourselves the question – What conversion process can we apply such that the energy requirement for reduction of CO2 is minimized, the answer suggests itself: a process that reduces the carbon oxidation state by just one unit, from +4 to +3.

Simple arithmetic suggests this can be achieved by combining two atoms of hydrogen with two molecules of carbon dioxide. And this is borne out in practice, although as we shall see the reducing agent need not necessarily be hydrogen.

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Converting carbon dioxide to oxalic acid

Consider a DAC process in which two moles of carbon dioxide are captured from the atmosphere and converted into one mole of oxalic acid (involving the addition of a mole of hydrogen). Now think about density change. Carbon dioxide is a gas, two moles of which occupy 44.8 liters at STP. Oxalic acid is by contrast a solid, a mole of which occupies 0.0474 liters. So in this conversion process the carbon atoms get packed into a space which is 946 times smaller. Woo. And because oxalic acid is a solid there is no requirement for pressurized containment or its associated cost.

One method by which this capture and conversion process can be realized has been published by researchers at Louisiana State University (ref). The chemical capture unit is built out of four pyridyltriazole chelating units linked by two meta-xylylene groups; complexation with CuCl2 gives a dimeric macrocycle which following cation reduction to Cu+ by sodium ascorbate is able to selectively capture and convert two carbon dioxide molecules into an oxalate ion as shown here

The oxalate ion is released as oxalic acid when treated with dilute mineral acid, regenerating the original copper complex. The reaction conditions are mild, reflecting the minimal reduction of carbon oxidation state from +4 to +3.

As I understand it the essential reaction sequence is as follows:

The oxidation of the ascorbate ion to a radical cation would appear to provide the thermodynamic impulse for this electron transfer sequence. Obviously there are other anions involved here, but the paper didn’t detail them so nor can I. What is evident however is that the energy demands of this process are modest, as one would anticipate for a unit reduction in oxidation state.

This paper was published 5 years ago as exploratory work. The kinetics in particular needed improvement and no doubt this has been addressed in further studies. My point in highlighting this work is to show the advanced level of innovation in facilitating oxidation state reductions by which atmospheric CO2 can be converted to carbon compounds of significant synthetic potential.

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Electrochemical reduction

With an enthalpy of formation ΔHf = -393 kJmol-1, carbon dioxide is a very thermodynamically stable compound. This explains why electrochemical reduction routes using the power of cathodic electrons have been sought to produce a CO2 anion which can enter into protic or aprotic reaction yielding formate or oxalate ions.

In Europe the acronym-intensive SPIRE (Sustainable Process Industry through Resource and Energy Efficiency) Association representing innovative process industries has a project running called OCEAN, which stands for Oxalic acid from CO2 using Electrochemistry At demonstratioN scale. OCEAN aims to develop an electrochemical process for producing high-value C2 chemicals from carbon dioxide via the following sequence:

1) reduce carbon dioxide (C1,+4) to formate (C1,+2)
2) dimerize formate (C1,+2) to oxalate (C2,+3)
3) protonate oxalate (C2,+3) to oxalic acid (C2,+3)
4) reduce oxalic acid (C2,+3) to glycolic acid (C2,+1)

The oxidation state sequence by which oxalic acid (4→2→3) and glycolic acid (4→2→3→1) are obtained from CO2 looks a bit lossy from an energy efficiency perspective, but this is just my impression. The project is running from October 2017 for four years and is EC-financed to the tune of €5.5 m.

Details of the OCEAN project can be found on these links:

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P Mander July 2019

Credit: Microbiology Online Notes

Although the Carbon Cycle is a well-accepted concept illustrated by countless graphics on the internet such as the one shown above, I wonder if it deserves to be called a cycle in the general sense of uninterrupted cyclic motion. Because the fossil fuels (coal, oil and gas) formed over millions of years from atmospheric CO2 are actually end products. Left to themselves they would remain as coal, oil and gas and the cycle would stop turning. To say that the cycle is completed by human interference sounds somewhat contrived.

But despite this criticism, the Carbon Cycle has an obvious value: it helps us to see a bigger picture. And this broader understanding can be further enhanced by looking at another aspect of carbon which changes during its journey around the cycle – namely its oxidation state.

I have not come across a graphic that includes this, so here is one I drew to illustrate the idea.

Rather than describing carbon in terms of its sequence of physical transformations, this cycle shows the associated changes in carbon’s oxidation state. Oxidation states are conveniently if somewhat abstractly represented by dimensionless numbers, which in the case of carbon range from +4 (most oxidized state) to –4 (most reduced state). The lower the number, the more energy is present in carbon’s chemical bonds. So in the process of carboniferous fuel creation the oxidation state number decreases. Conversely, the process of energy release from carboniferous fuel results in an increase in oxidation state number. The natural abundance of carbon with its wide range of oxidation states centered about zero is what gives carbon its usefulness as both a source and a store of energy.

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Energy in, Energy out

Below is a quantified illustration of how carboniferous fuels store and release more energy as the oxidation state number decreases.

Energy release in kJ per mole of CO2 formed. Numbers are indicative

The green arrow shows the oxidation state decrease from +4 to 0 associated with photosynthesis in terrestrial plants and marine plankton. The carbon in the repeating molecular unit is reduced by hydrogenation, and combusting this fuel e.g. in the form of cellulose releases 447 kJ per mole of CO2 formed. Further reduction to oxidation state –1 is associated with the creation of coal (not illustrated) which releases about 510 kJ per mole of CO2 formed. Continued reduction to oxidation state –2 is associated with petroleum which releases around 610 kJ per mole of CO2 formed. Finally the formation of natural gas represents the lowest possible oxidation state of carbon, –4. On combustion natural gas releases the maximum energy of 810 kJ per mole of CO2 formed.

These numbers illustrate a general (inverse) relationship between the magnitude of the carbon oxidation state and the amount of energy generated by combustion (see Appendix 1 for more data). For each mole of CO2 released, natural gas (–4) produces nearly twice as much energy as cellulosic biomass (0). That is an appreciable difference, which perhaps deserves more attention in public discourse about greenhouse gas emissions than it receives.

It is also worth noting that although it takes millions of years for natural gas to be formed from atmospheric carbon dioxide in Nature’s Carbon Cycle, the same carbon transformation can be achieved by human beings on a vastly accelerated timescale using a process known as the Sabatier reaction. This has been recently demonstrated in a remarkable Power-to-Gas (P2G) project conducted in Austria.

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Underground storage and conversion

In Pilsbach, Upper Austria, energy company RAG Austria AG conducted a P2G project called Underground Sun Storage in which excess electricity production from wind and solar was converted by electrolysis of water to hydrogen gas which was then pumped down into a depleted natural gas reservoir at a depth of 1 km. Following the successful conclusion of this project, a second P2G project called Underground Sun Conversion was then initiated in which carbon dioxide sourced from biomass combustion or DAC was co-injected with hydrogen into the gas reservoir.

According to RAG, the pores in the matrix of the underground reservoir contain micro-organisms which within a relatively short time convert the hydrogen and carbon dioxide into natural gas, recreating the process by which natural gas originates but shortening the timescale by millions of years. In its project description RAG Austria claims that “this enables the creation of a sustainable carbon cycle”.

How the micro-organisms effect the reaction between H2 and CO2 is not described in the material I have seen. Perhaps microbial enzymes serve as catalysts – the Sabatier reaction is spontaneous and indeed thermodynamically favored under the temperature and pressure conditions of the reservoir (313K, 107 bar).

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Energy in, Energy out (again)

Ok, so let’s take a look at the thermodynamics of the processes by which RAG Austria turns carbon dioxide into natural gas. Applying Hess’s Law, it can be seen that the sum of the electrolytic process and the Sabatier reaction is equivalent to a reversal of methane combustion and corresponds to the energy stored underground in the C-H bonds of the methane molecule – note that the oxygen is formed above ground during electrolysis and is vented to the atmosphere*

*the sum of reactions bears a curious similarity to the process of photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2

Energy loss is intrinsic to both parts of this methane synthesis program. The electrical efficiency of water electrolysis using current best practises is 70–80%, while the Sabatier reaction between H2 and CO2 taking place in the underground reservoir is exothermic and loses around 15% of the energy used to form hydrogen in the initial stage.

On the face of it, underground storage in a natural gas reservoir of hydrogen alone would seem to offer better process economics. But carbon capture and underground conversion can be titrated to achieve a variable quotient between stored hydrogen and converted methane. Both have their economic attractions; what methane lacks in terms of process inefficiencies can be compensated for in several ways in relation to hydrogen. Superior energy density, more efficient transportation as LNG and compatibility with existing energy supply infrastructures are some of them. And then there is the larger issue of the value that society places upon the desire for carbon neutrality in existing energy systems on the one hand, and the promise of carbon-free energy systems on the other.

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Appendix 1

Carbon oxidation state and heat of combustion per mole of CO2 produced

Negative numbers in the last column indicate exothermic reaction i.e. heat release. Units are kJmol-1

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Suggested further reading

EURAKTIV article on energy storage projects June 2019

RAG Austria AG website – Underground Sun Conversion

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P Mander June 2019