Defluorinating Branched Perfluoroalkanes with Cobalt Catalysts.

[NNadir]

Recently in this space I decried what I called "The Worst Idea In Energy" with the caveat that possibly, based on the likelihood of it actually happening, the idea of lead perovskite solar cells was potentially worse.

My remarks were based on this paper:

Comparison of Linear and Branched Molecular Structures of Two Fluorocarbon Organosilane Surfactants for the Alteration of Sandstone Wettability (Ivan Moncayo-Riascos* and Bibian A. Hoyos, Energy Fuels, 2018, 32 (5), pp 5701–5710)

The paper was all about using recalcitrant perfluoroalkane species as a surfactant in "fracking" operations.

Not two days later, I came across a paper that suggested that people are already doing something very much like it. It's this paper:

Reductive Defluorination of Branched Per- and Polyfluoroalkyl Substances with Cobalt Complex Catalysts (Timothy J. Strathmann et al, Environ. Sci. Technol. Lett., 2018, 5 (5), pp 289–294)

From the text:

Since their original development in the 1940s, per- and polyfluoroalkyl substances (PFASs) have been widely used in industrial and consumer products.1,2 Detection of PFASs in the global environment has been extensively documented.2−6 Substantial research efforts aimed at the two C8 legacy compounds, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), have confirmed a variety of adverse health effects,7 leading to the phase-out of ≥C8 PFASs in North America and Europe8 and the U.S. Environmental Protection Agency’s recent issuance of drinking water health advisory levels for PFOA and PFOS.9,10 New alternative PFASs (e.g., shorter chain carboxylic and sulfonic acids, perfluoroalkyl ether carboxylic acids, and other novel structures) have already been detected in aquatic environments and are considered recalcitrant.11−15 Recent studies have also indicated that the emerging PFASs exhibit variable toxicities and environmental mobilities.11,16−20 Still, knowledge of emerging PFASs remains limited.11,21

Despite having received much less attention than linear PFASs have received, branched PFASs (Figure 1) have also been extensively applied and detected in the environment. For example, perfluoro-3,7-dimethyloctanoic acid (PFMe2OA) serves, along with linear PFASs, as an ingredient of well treatment fluids.22 This compound is also on the “STANDARD 100 by OEKO-TEX” list, indicating its wide application in textile production,23 and it has been detected in European water bodies.24 Perfluoroethylcyclohexanesulfonate (PFECHS)25 also contains two branched carbons on the cyclic structure, and its detection in Canadian Arctic lakes has been attributed to its use in aircraft anti-erosion fluid.26 Industrial PFOS products often contain variable fractions of branched isomers, which have been detected in both environmental waters and human tissues.27,28

The added bold is mine.

The authors propose a cobalt catalyst - a porphyrin catalyst - to defluorinate these recalcitrant and possibly toxic compounds.

The structure of the catalyst is shown in the cartoon in the header of the paper:



Here are the structures of the target compounds they seek to address:



Biochemists will note that the catalyst has certain similarities to the structure of vitamin B12, which the authors also note, pointing to it in another graphic:



The caption:

Figure 2. Degradation and defluorination for each PFAS with cobalt catalysts shown in panel e. Branches that are effective and ineffective in promoting defluorination are colored green and red, respectively. Reaction conditions: PFAS (0.1 mM), Co catalyst (0.25 mM), TiIII citrate (∼36mM), and carbonate buffer (∼40 mM) in water at pH 9.0 and 70 °C.

It appears from the paper that vitamin B12 - as is explored in the text as well as in the graphic - also can carry out this defluorination reaction, albeit on a very long time scale, which is displayed in the graphic as well. This of course is somewhat good news, although each fluorine removal results in an alternate toxicology I would suspect. Note the timeline for defluorination is not short, on the order of days, not minutes.

Another graphic gives the bond energies of some of these compounds calculated by DFT:



The caption:

Figure 3. Calculated bond dissociation energies (BDEs, in kilojoules per mole) at the B3LYP/6-311+G(2d,2p)/SMD level of theory of C−F bonds in the PFASs shown in Figure 2. The displayed terminal group with two CO bonds represents the charge-delocalized −COO− anion.

These are very strong bonds, which accounts for the persistence of these molecules in the environment and in human, animal and plant flesh, as well as in water supplies.

Unbelievable.

Cobalt, as I note when criticizing the less than satisfying hype surrounding Elon Musk, hero of electric car cultists, is a conflict metal, mined under appalling conditions in many places on earth. It's role in Vitamin B12 also makes it an essential element for human health, albeit in tiny amounts.

Preparation of the cobalt species is not given in the paper, but I'd imagine an industrial scale synthesis would be, um, interesting and expensive.

When the best tool with which one is familiar is a hammer, everything looks like a nail. Radiolysis with gamma rays would do this job much better and much more completely, but well, I think that what will be done about these wells is nothing.

Have a nice "hump day."

[eppur_se_muova]

1. I remember,years ago, seeing a seminar in which it was reported that cobaltocene ...

would react with perfluoroalkanes.

I was a bit miffed, because I was wanting to do some research in that area myself, and felt that was something of a scoop.

Probably still have my notes around somewhere, if I can find them.

[NNadir]

2. Well that would never have occurred to me.

Just googling around though, I see that the Co-F bond is fairly energetic, 435 kJ/mol, so I believe it, but I certainly never thought about it.

Come to think of it though, what fluorine bond isn't energetic?

The reported bond energies from this paper actually seem to be in the UV range, which is also somewhat surprising. I'm unaware of fluorine radicals in the stratosphere although chlorine radicals are very famous.

I have always thought of the decomposition of CFC's as involving - in the chain terminating OH radical step as proceeding through a fluorophosgene intermediate, but then again, I never thought that much about it.

There is a fair sized body of work showing the radiolytic mineralization of PFOS and related compounds.

[NNadir]

3. I just looked and came across another interesting paper along these lines for thermal...

...decomposition.

Mineralization of perfluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) from aqueous solution by porous hexagonal boron nitride: adsorption followed by simultaneous thermal decomposition and regeneration (RSC Adv., 2016,6, 113773-113780)

It turns out that this paper will be interesting to my son who is doing some work with porous boron nitrides.

I'll have to pick it up and share it with him, and perhaps mention it here.