Can we eliminate the emissions of the greenhouse and ozone depleting gas N2O from nylon manufacture?

[NNadir]

Nitrous oxide, N2O, aka "laughing gas" is no laughing matter.

Other than carbon dioxide, it is the atmospheric pollutant that most troubles me, not just because it is a profound greenhouse gas with a climate forcing potential 120 times as large as carbon dioxide, but also because it is an ozone depleting agent.

There is no really great way to eliminate human emissions - like carbon dioxide it has always been a natural component of the atmosphere, but anthrogenic emissions have swamped natural emissions - because the emission of nitrous oxide is an inevitable consequence of fertilizing soil for agriculture, without which a huge fraction of the planet's population would literally need to starve to death.

However, going through some scientific literature I collected a few years back but never properly filed in my computer's filing system, I was surprised to learn that between 5 to 8% of nitrous oxide emissions are an industrial side product of nylon manufacture.

The paper to which I refer is here: One-pot room-temperature conversion of cyclohexane to adipic acid by ozone and UV light (Kuo Chu Hwang*, Arunachalam Sagadevan, Science 19 Dec 2014:
Vol. 346, Issue 6216, pp. 1495-1498)

A key intermediate in the manufacture of the nylon polymer is the diacid adipic acid which has the following structure:



Nylon 66 has this structure:



Adipic acid is made from a constituent of the dangerous fossil fuel petroleum, cyclohexane, by oxidation with pressurized oxygen at 125C over a manganese and cobalt catalyst to give a mixture of cyclohexanol and cyclohexanone. These are passed over ammonium vanadate and cupric catalysts in the presence of 65% nitric acid (itself made by oxidation of ammonia prepared using dangerous natural gas) to give adipic acid.

During this process, the reductant is nitric acid, which is converted to N2O, which is then dumped into our favorite waste dump, the planetary atmosphere.

World production of adipic acid is on the order of a billion kg.

Some brief excerpts from the text:

Adipic acid is a precursor for the synthesis of the nylon-6,6 polymer and, as such, is one of the most important industrial chemical intermediates. More than 3.5 million metric tons of adipic acid were produced in 2013, reflecting a ~5% growth rate per year over the past 5 years (1, 2). Nearly 95% of the worldwide industrial production of adipic acid employed the nitric acid oxidation method (3). The first step is air oxidation of cyclohexane under high temperatures (125° to 165°C) and high pressure (8 to 15 atm) to produce KA oil (i.e., a mixture of cyclohexanone and cyclohexanol) with 4 to 11% conversion and ~85% selectivity (4, 5). In the second step, nitric acid is applied as an oxidant: the conversion is ~100%, and the selectivity for adipic acid is 93 to 95% with some other short-chain acids as side products (see Fig. 1A). The process requires the nitric acid–to–KA oil ratio to be maintained at 40:1. Disadvantages of the current industrial process include low overall product yield; corrosion of reaction vessels by nitric acid; emission of the ozone-depleting greenhouse gas N2O; and high energy consumption. It was estimated that ~0.3 kg of N2O gas is formed per kilogram of adipic acid produced (6, 7)...

...Inspired by literature reports that ozone and ultraviolet (UV) irradiation are primarily responsible for oxidative degradation of most hydrocarbons in the atmosphere, we sought to investigate whether both treatments in combination could oxidize cyclohexane, which exclusively contains unactivated sp3 C-H bonds. In a simple experiment, ozone gas was bubbled through neat cyclohexane with concurrent UV irradiation at room temperature. No metal catalyst or solvent was used. After 2 to 8 hours, a solid product gradually precipitated to the bottom of the reaction vessel (see Fig. 1B and fig. S1 for reaction scheme and pictures, respectively). A portion of the liquid cyclohexane evaporated due to the O3 gas bubbling. The solid oxidation product of cyclohexane was subjected to 1H nuclear magnetic resonance (NMR) and 13C NMR analysis (in deuterated chloroform) for structure characterization and proven to be adipic acid.

UV radiation can be continuously supplied by exposing BaF2, barium fluoride, to a gamma emitting radioactive substance, these being available in large quantities from used nuclear fuel. There are many ways to generate ozone, although the usual method is to generate it electrochemically.

I have no idea if there has been any effort to industrialize this most interesting bench chemistry, but this is a beautiful, if esoteric environmental idea.

I can't believe I overlooked this very beautiful paper for a couple of years.

Have a very pleasant weekend.

[eppur_se_muova]

1. What's the quantum yield ? That's the problem with any photochemical step.

First rule of photochemical synthesis: never make anything by a photochemical synthesis if you can make it another way. This rule was formulated by specialists in photochemical synthesis, so I take it seriously.

There's other, catalytic ways to make adipic acid, usually from cyclohexene. H2O2 and a tungstate catalyst is one I've used in UG labs. I'm really surprised someone hasn't tried O2/Co+2 in acid solution. This can oxidize aralkyl side chains. Would be interested to see if it cleaves C=C in cyclohexene or does allylic oxidation.

It's interesting that they did succeed in oxidizing a saturated hydrocarbon. Saves a step, it's true.

I suppose the cyclohexanol/one mixture that's used industrially is ultimately a cheaper intermediate than cyclohexene, or more tolerant of impurties, or some such. On such tiny differences in cost are the decisions to use the more polluting method chosen. Invariably, it seems.

[NNadir]

2. Well, as far as rules of industrial chemistry go, one should be careful about how broadly one...

...applies them.

I once worked on a drug that was out-licensed by its discovering company on the grounds that it failed most of the Lipinski rules and anyway was impossible to make, except that I was personally involved in the synthesis of hundreds of metric tons of the stuff.

I'll excerpt the mechanistic comments below.

Let me say a few other things first.

This is benchtop chemistry. If one was going to fill a reactor with cyclohexane and an ozone/oxygen mixture, one of course, would be making a bomb. This reaction is in fact, partial combustion - arrested combustion or controlled partial combustion of cyclohexane.

This said, it may be amenable to flow chemistry, about which I'll say a little more below.

As for the rule you describe, which I've not heard before, but seems reasonable, I would suspect that it applies to selectivity more than anything else. It applies less here because of the symmetry of cyclohexane: It doesn't matter on which carbon the reaction takes place, the result is the same. Since selectivity is not an issue, a free radical reaction is less limited than in other applications. Industrial scale free radical reactions are very common with symmetric molecules, polyethylene is such a case.

Additional process value is conceivable because the reaction product is insoluble in the liquid starting material.

One could imagine a flow system into which low amounts of ozone suspended in an inert carrier gas, argon perhaps, is introduced at a controlled rate in such a way that adipic acid is simply filtered from the flow.

Someone here recently remarked that most of my posts - probably one would not be far off if one said all of my posts in the environmental group and your science group are related to nuclear energy, and this is true here. An economic limitation of photochemistry is the energy required to run the initiator lamps but a continuous UV source is available from any gamma source coupled to an energy level transformer, to which I alluded above, is available with barium fluoride.

If one notes that this reaction is catalyst free one might also wonder if a catalyst might be added. I would particularly be interested in the very interesting ceramic radiocesium titanate (perhaps in a nanostructured system with barium fluoride) because of the reaction mechanism, and the properties of titanates in photochemical-like systems. (Radiocesium titantanates have historically been synthesize as "waste forms" but I don't believe in "waste." I consider radiocesium titatanate to be one of the most potentially useful fission product based materials imaginable; it's a shame about fear and ignorance, which will prevent the exploration of this material's utility until a less stupid generation comes along.)

Such a system would effectively require no energy input other than the decay of radiocesium and a source of cyclohexane.

(I may post over in your Science Group some interesting physical chemistry connected with the conversion of high energy radiation to light energy that was just published in one of the journals I am coming to love, using one of my favorite elements, uranium.)

Here is a discussion of the mechanism (and the quantum yield) from the original paper:

It is well established that upon UV (306 to 328 nm) irradiation, ozone decomposes to generate singlet 1O2 and a singlet O(1D) atom with a quantum yield of 0.79 (17, 18). The singlet O(1D) atom is highly reactive and can insert into C-H bonds of hydrocarbons to form C-O-H bonds in the gas phase with the conservation of total spin angular momentum (19, 20). Our control experiments show that exposure of cyclohexane to singlet 1O2 (by photoirradiation of cyclohexane in the presence of photosensitizers) does not generate adipic acid, suggesting that the formation of adipic acid is mainly due to chemical reactions between atomic O(1D) with cyclohexane, cyclohexanol, and cyclohexanone. A possible reaction pathway for the neat cyclohexane-ozone-UV system is proposed in fig. S4(i) to account for formation of adipic acid via selective C-H bond oxidation of cyclohexane by O(1D). First, direct C-H bond insertion of O(1D) into cyclohexane would lead to the formation of cyclohexanol (21), which is further oxidized by O(1D) at the weakest methine C-H bond to form a geminal diol, 1,1?-dihydroxycyclohexane. Geminal diols are known to be very unstable and will rapidly undergo dehydration to form stable ketones (22). The bonding energies of methine C-H, methylene C-H, and O-H bonds are ~96, ~99, and ~105 kcal/mol, respectively (23). Insertion of O(1D) into a C-H bond in cyclohexane requires cleavage of one C-H bond and formation of two bonds (i.e., C-O and O-H), which are exothermic and thermodynamically favored. Subsequent insertion of O(1D) into the methine C-H bond of cyclohexanol is also thermodynamically favored. Both cyclohexanol and cyclohexanone were isolated as stable intermediates upon short-time UV irradiation of cyclohexane in the presence of ozone. The conversion of cyclohexanone to adipic acid by reaction with a singlet O(1D) atom probably proceeds via dihydroxylation at the ?-C-H bond adjacent to the ketone functionality, because the ?-C-H bond is weaker than other remote methylene C-H bonds.

The mechanism is pictured in the supplemental information which may be open sourced; I don't know, I'm writing in a library. The supplement also contains very nice photographs of the reaction, as well as full experimental details.

It's here: SI of the paper.

One may imagine other sources of singlet oxygen, of course, than ozone, but to the extent that they involve the addition of other reagents they may complicate the reaction.

Here is a scheme comparing the current industrial reaction with the proposed reaction:



The big problem with this whole scheme is, to my mind, the source of cyclohexane, which currently is obtained from dangerous and environmentally unacceptable petroleum.

I'm only interested in these kinds of reactions to the extent that polymers represent a path to sequestering carbon in an economically viable way. It is easy to imagine chemistry that forms butadiene from glyoxal, and glyoxal from the electrolytic (or catalytic) reduction of carbon dioxide, and ethylene from the partial hydrogenation of acetylene, acetylene being prepared from biomass and metallic calcium. These two compounds of course can be subject to [4 + 2] cycloaddition (Diels Alder) chemistry to give cyclohexene, which as you note, is a little further along on the path to adipic acid.

If I recall correctly I may have read in many places that adipic acid is also available from the various furan diacids, mixed acid/aldehydes and diacids of furan formed by the dehydration of biomass. A lot has been written about this kind of chemistry in recent years, mostly in connection with fuels, but also in connection with synthetic intermediates.

This may be superior chemistry; I don't know, at least where the goal is to economically remove carbon dioxide from the air; something humanity must do at some point owing the criminal negligence of my generation in putting that carbon dioxide there in the first place while praying insipidly for whirlygigs and glass coated with toxins to save the day.

Nevertheless, this is beautiful chemistry, and it struck me last night while I was struggling with insomnia and came across the paper in my files.

Thanks for your interesting comment.

Have a nice weekend.