This is a guest post from NNadir, the well-known nom de plume used by a chemical engineer who has professional reasons for obscuring his off-line identity. It is far more technical than most of the posts here, but I found it fascinating. I hope most readers agree that it is a welcome respite from the energy politics and economics-related topics that have dominated posts here for the past few months.
Three small and seemingly simple molecules are at the forefront of the accelerating rate of the anthropogenic degradation of the planetary atmosphere. Carbon dioxide, CO2, needs no introduction. Practically every sentient being on the planet understands its importance. The profound health effects of another oxide of carbon, carbon monoxide, CO – effects including but not limited to death – are also generally well known. Although the effects of carbon monoxide as a pollutant have been to some extent ameliorated by the development of automotive (and other) catalysts, its atmospheric chemistry still remains a serious concern. What may be less well known is that an understanding of the chemistry of carbon monoxide is critical to any serious effort to address climate change, not that any serious effort will, in fact, be made. In any such effort, carbon monoxide chemistry would serve as a critical tool for humanity rather than a threat to it. The third molecule, nitrogen, N2, is the one that represents the bulk of the atmosphere, where it is generally thought of as inert, although the many exceptions to its inertness are of critical environmental, health, industrial and energetic importance that cannot be understated.
The paper from the primary scientific literature that I will focus upon in this discussion actually discusses the chemistry of all three of these molecules, each of which is enormously important to any serious environmentalist. The paper also discusses the chemistry of the “f elements,” focusing on uranium. The chemistry of the f elements is of critical importance to the environmentalist who recognizes – I include myself here – who recognizes the irrefutable fact that nuclear energy represents the most critical tool available to save humanity from itself; at least whatever portion of humanity can be saved at this late date, not that much necessarily will be saved. The long winded (at least to nonscientists) title of the paper is this: “Small Molecule Activation by Uranium Tris(aryloxides): Experimental and Computational Studies of Binding of N2, Coupling of CO, and Deoxygenation Insertion of CO2 under Ambient Conditions.”1 A link to it is found in the references.
First – excuse the digression – by allowing me to answer a question for those who don’t know: What are the “f elements?”
As most people who have passed a good quality chemistry course know, the shape of the periodic table (see figure 1) derives from the filling of suborbitals, the existence of which accounts for the stepwise arrangement of the table’s columns – the main orbitals are represented by each row – which are called (for historical reasons) s, p, d, and f orbitals. (Theoretically there are also g and h orbitals, but if any g and h elements have ever existed, they have done so only for transitory periods at the cores of collapsing stars.) Suborbitals are represented by columns in the periodic table: The s elements are represented by the columns headed by hydrogen (H) and lithium (Li), the p elements are headed by the columns headed by boron (B) through neon (Ne) – helium is, as always, an eccentricity and is actually an “s” element – the d orbitals are represented by the columns headed by scandium (Sc) through Zinc (Zn) and the f elements are those in the columns which are below the “main” table headed by lanthanum (La) through Ytterbium (Yb). I like the table I’ve produced here because it clearly suggests that the f elements are actually a part of the main table – another step in the staircase shape – but are usually drawn this way because otherwise the table would be generally too wide to print.
The “f elements” are distinct from the elements associated other suborbitals inasmuch as they have distinct trivial names. The elements from lanthanum to ytterbium are collectively known as the lanthanides (or sometimes as “rare earths”) and, analogously, the elements from actinium to nobelium are collectively known as the “actinides.” The lanthanides are most famously connected, in modern times, with, among other things, wind turbines, electric/hybrid cars, and certain semiconductor devices. Personally I have little use for either the wind industry or the car industry and won’t comment much further on the roles in these industries. My interest in lanthanides chiefly derives from their appearance as fission products in used nuclear fuel where those lighter than and including europium are prominently represented. (It is the accumulation a lanthanide, samarium, prominently among other elements, that is responsible for the shutdown of nuclear fuel rods before the fissionable material in them is wholly consumed. Two other lanthanides, gadolinium and dysprosium, can and do play a role in nuclear technology as neutron absorbers, although they are of trivial importance to the chemistry of used nuclear fuels.) The actinide elements (primarily thorium, uranium, and plutonium, although the chemistry of neptunium, americium, curium and perhaps protactinium and californium are relevant to any discussion of nuclear energy) represent potential or actual sources of nuclear energy. The elements beyond californium, as interesting as they may be, are unlikely to ever be of any industrial or practical significance.
This property of being “f elements” results in many similarities between the chemistry of the actinides and lanthanides, although there are important differences, most notably among the lightest actinides, in particular thorium, protactinium, uranium, neptunium, plutonium, and, to a lesser extent, americium all of which are atypical, more or less, as f element chemistry goes. The first three of the listed lighter actinides occur naturally and can be isolated from ores, two of them in vast amounts. Because their chemistry is so different from that of say lanthanum, praseodymium, and neodymium, the existence of an “actinide” series was overlooked (and was largely unimportant) until the 1940’s. Its existence was not postulated until plutonium’s discoverer, physical and inorganic chemist, Nobel Laureate, educator, diplomat, advisor to eight Presidents, and Administrator of the Atomic Energy Agency during the period of the construction of 70 of the more than 100 of the US nuclear reactors, the incomparable Glenn Seaborg, recognized it in his scientific work with the transuranium elements – many of which he discovered – thus changing the shape of the periodic table. The great development of actinide chemistry took place almost simultaneously in the 1940’s and 1950’s with the serious development of lanthanide chemistry, which up until that time was regarded as esoteric and difficult, when the development of ion exchange resins and solvent extraction techniques made the separations of both the lanthanides and actinides from one another possible on a large scale, although challenges remain to this day. Thus the f elements are among the last elements to have their chemistry explored in depth. By contrast, the chemistry of the other elements have involved centuries, and in some cases, millennia of work. Because total experience of the manipulation and utilization of these elements is relatively recent, many discoveries still remain to be made, and many already made have not been subject to full technological exploitation.
To wit, to quote the paper1 I said I’d discuss on uranium catalysts for small molecules, referring to the industrial nitrogen chemistry without which much of the world’s current population would have difficulty surviving at all the authors write:
(The bold in the quotation is mine and the superscripts therein refer to references in the original paper1.) The first bolded comment refers to the fact that the Haber-Bosch chemistry – which is essential not only to modern agricultural, but also to the pharmaceutical and other industrial chemical industries – still, nearly a century after its industrialization, involves the utilization of extreme and energy intensive chemistry. This chemistry involves the fixation of nitrogen by means of the hydrogenation of nitrogen gas to give ammonia. Everything from fertilizers, to rocket fuel, to the synthesis of practically every medication known, to many polymers, to most explosives, to the recovery and recycling of metals…the list goes on and on…depends on this chemistry.
This said, it also may be said that the boon of the “green revolution” that feeds humanity via fixed nitrogen is simultaneously a tremendous threat to humanity, not only in its environmental chemistry in surface waters where it causes eutrophication3, but also in the atmosphere, where fixed nitrogen, in particular the N2O that is an inevitable side product of the use of nitrogen fertilizers is not only the third most important climate change forcing gas, but also will soon displace CFC’s as the chief threat to the planetary ozone layer4.
Nitrogen fixation is responsible for the consumption of anywhere between 1 to 2% of the 520 exajoules of energy now consumed by humanity each year.3, 5 If this doesn’t sound like much, consider that the much ballyhooed solar and wind industries combined, despite 50 years of wild eyed cheering for them, and their consumption of vast sums of money for their “development” such as it is, have never, not once, produced as much energy in a single year as is required for just nitrogen fixation, never mind all of the other things for which we use energy. Mind you, again, almost all of the food supply on earth depends on fixed nitrogen.
Surprisingly, the formation of ammonia gas is slightly exothermic, meaning that its formation from nitrogen and hydrogen, in theory, would release energy and not consume it, but as a practical matter the synthetic path necessary – path a profound effect on thermodynamics – ultimately 32 MJ kg–1 of energy is required to fix nitrogen,3 most probably owing to the highly endothermic nature involved in the formation of the diazine intermediates which require the breaking of a highly stable nitrogen-nitrogen triple bond.6 Thus, again, the industrial synthesis of ammonia in quantities exceeding 100 million metric tons – a rough estimate of the amount synthesized each year – requires exajoule scale quantities of energy.
So in terms of energy efficiency, a route that would form ammonia via less harsh conditions than those of the Haber-Bosch system and this is why the uranyl complexes described in reference 1 are so interesting. The nitrogen complex is surprisingly stable, and its structure suggests that it may well represent a route around the diazine intermediate, greatly reducing the energy requirements for nitrogen fixation. In some ways, the use of uranium for this type of chemistry should be unsurprising, since Fritz Haber, the fascinating scientist – I highly recommend for further reading Smil’s book referred to in reference/note 3 – who won the Nobel Prize for his work on nitrogen fixation wrote in 1909, around the time of his discovery, the following text7:
This translates as follows:8
It seems that humanity forgets important things. (Although Haber was a brilliant physical chemist as well as a brilliant inorganic chemist – he claimed to have discovered the third law of thermodynamics before Nerst – there is no way that he understood the theoretical basis for uranium catalysts: Recall that he understood uranium as a d-element, a heavier analog of tungsten.)
Let me now turn to the other two other small molecules in which uranium offers catalytic possibilities: Carbon dioxide and carbon monoxide.
I am not fond of dangerous fossil fuels and believe they should all be phased out on an emergency basis, even as I recognize that this is not easy to do, although I expect it would be less troublesome, over the long term, were such a phase out to occur (and I don’t believe it will do so except by catastrophe) than the more likely alternative of doing nothing meaningful. Nor am I fond of one hopelessly inadequate proposed response to the catastrophe of carbon dioxide accumulations, generally referred to as CCS, carbon capture and storage, which is a proposal to build massive but probably leaky and temporary waste dumps all over the planet to try to put said carbon dioxide out of mind. A new marketing term for “CCS” has appeared – and actually it makes more sense – which is called “CCSU,” “Carbon Dioxide Capture, Storage and Utilization.” If you must know, I approve, maybe, of what the “U” stands for and maybe, under more limited conditions what the second “C” stands for. I have trouble with the “S” though. I oppose, in general, waste dumps of any kind.
In theory, this uranium catalyst could be useful for the utilization of carbon dioxide, although many other catalysts that can do the same thing are widely known. For instance, references 7 and 8 are the same description in two languages of a process to prepare a methanol derivative from carbon dioxide via hydrogenation using a uranium catalyst. Methanol potentially represents a fuel to displace gasoline and other motor fuels.9 If the carbon dioxide source is a dangerous fossil waste stream, the hydrogenation could represent a carbon neutral path to motor fuels – allowing the phase out of petroleum and petroleum mining – or better yet, in the case of carbon capture from the air, possibly via a biological intermediate or even by direct capture, then motor fuels might actually represent, briefly at least, a carbon negative technology.
The preparation of methanol from carbon dioxide requires the breaking of one carbon oxygen bond, not actually an easy trick, although many technologies for doing so are well known.
One my personal favorites involves the oxidation of a carbon source – this is known as an equilibrium associated with what is called the Boudouard reaction – with carbon dioxide itself to give carbon monoxide; this works not only with dangerous fossil fuels, but also with things like biomass: The appearance of soot on smokestacks is actually the result of a Boudouard equilibrium, running in the reverse of the previously stated route: The position of the equilibrium is temperature driven, and under some conditions carbon monoxide can disproportionate into carbon and carbon dioxide. (See figure 2.)
An examination of Figure 2 however points to a drawback associated with the reduction of carbon dioxide with either carbon or a carbon surrogate such as cellulose, waste plastic or other such substance: The drawback is the requirement for heat, which is, of course, a form of energy. In the (somewhat simplified) graph shown, the equilibrium point at which an equimolar quantity of the monoxide and dioxide is present, is a little less than 1000K, approximately 700oC.
Although modern materials science makes these kinds of temperatures accessible without access to dangerous fossil fuels – the best option being nuclear energy, albeit using reactors of types not routinely deployed – it would be convenient for some purposes to carry out such a reaction at lower temperatures and generally milder conditions. Again, it is, in general, difficult to break a carbon-oxygen bond in carbon dioxide, but obviously not impossible, since chlorophyll accomplishes this photochemically indirectly via photochemical water splitting and de facto hydrogenation. (It is also possible to hydrogenate carbon dioxide in chemical reactors).
A very convenient means of carrying out this reaction would be electrochemically. Some progress in this area has been gathering attention, for instance, in the work of Andrew Bocarsly at Princeton University and his startup company Liquid Light, apparently – if I have this right – by means of the electrochemical transfer of a single electron from an organic pyridinium radical to carbon dioxide11 via a radical carbamate intermediate.12 (The single electron reduction of linear carbon dioxide to form the bent CO2●- radical is the most energetically expensive step in the reduction of carbon dioxide to hydrocarbons, alcohols, or for that matter, in biological settings, sugars.6) The product of the Bocarsly reduction of carbon dioxide is methanol.
With all due respect to this approach which is elegant in its simplicity, it is of some note that the Arnold uranium moieties described in reference 1 directly breaks carbon oxygen bonds as well by, apparently, a different mechanism, followed by the insertion of a CO moiety in a phenolic oxygen-aryl bond (in ligands coordinating to uranium) resulting in an aryl carbonate and does so under fairly mild conditions. Carbonates are very useful in a wide variety of situations, including but hardly limited to use in the formulation of motor fuels that burn considerably cleaner than other fuels. This is an oxidation/reduction reaction, where the reduced species is carbon dioxide, The oxygen from the broken bond is inserted as a bridge between two uranium atoms.
One important difference between other catalysts is that the Arnold and other uranium catalysts may exhibit a particular type of selectivity, to make what we call C2 carbon compounds. The most important, industrially, C2 is probably ethene, colloquially known as “ethylene,” which is a either key intermediate or potential key intermediate in the production of polyethylene, acetic acid, acrylic acid (a C3 compound accessible from C2 starting materials), ethanol (without the requirement for agricultural land), ethylene glycol, ethylene oxide and a bunch of other chemicals and materials that are critical to modern bourgeois first world life even if we don’t appreciate as much.
This consideration arises uranium catalyst described in the paper actually results in the coupling of two carbon monoxide molecules – we call this dimerization – to form a type of structure we call an “ynediolate ethers” which is an highly unsaturated analogue of better known (and industrially important) vinyl ethers. Ynediolate ethers are very difficult to make even under laboratory conditions; very few are known. This is noted in the text of the paper, and although the paper does not examine them in detail as catalysts to make these molecules, instead focusing on more esoteric structural and bonding issues surrounding the uranium complexes themselves, one can easily imagine all kinds of interesting applications for this sort of molecule. One application that suggests itself is the preparation of light weight organic conducting polymers, for instance, the preparation of which might sequester a little carbon on the side. (For now, mostly they are made to make compounds like organometallic complexes of esoteric interest, such as squarate ethers; there are several varieties of uranium catalysts suitable for squarate synthesis these are of potential interest to medicinal chemists for the preparation of certain classes of cancer compounds known as tyrosine phosphatase inhibitors.)
A somewhat broader, if brief, discussion of the activation of small atmospheric molecules using actinide complexes is available for further reading.14
I am not sure whether any of this will prove to be of practical interest, although it is quite possible that at least for the potential of nitrogen fixing catalysts that require less energy might be developed. I discussed this topic with my fourteen year old son as I was writing this piece – I have raised him to be as much of a pronuclear environmentalist as I am, and he has already developed enough cynicism to state that people might object to their food being grown by any means which involves uranium. I laughed and told him about something of which he was unaware, that uranium and fertilizer have been long involved with one another. It may be true that fixed nitrogen can be regarded as an inexhaustible resource so long as there is energy to fix it, but another critical component of fertilizer is, of course, phosphate which is mined from deposits around the world and, being somewhat more problematic to recycle, might well face depletion in the future. Uranium, it turns out, has a high affinity for phosphate, so much so that for many years, many phosphate resources have been recognized as potential uranium ores. In fact, in the 1950’s, plants15 to do precisely that operated, until uranium resources became more widely known and cheaper ores were found at the same time that interest in nuclear power waned under an assault of fear and ignorance. As a result the uranium was just left in the phosphates, where, in many cases it ended up being distributed on agricultural fields. The issue of uranium accumulations in agricultural fields from phosphate fertilizers is now recognized as a worldwide health issue. 16, 17, 18
Recently it has been discovered that thorium complexes can provide a route to the ynediolate ethers19 mentioned above, much like uranium complexes, suggesting that in many ways, as one would expect, the actinides behavior similarly owing to the availability of f orbitals. A recent survey of the non-aqueous complex ion chemistry of the actinides20 notes that there are fewer entries in the Chemical Structural Database between 2006 for the organometallic complexes of all the actinides (there are 4164 such entries, with more than 84% of them being uranium complexes) are easily outstripped by the number for those of the single elements copper (41668 entries) and iron (33303 entries.) There are only 77 entries for plutonium, 134 for neptunium, 17 for the elements americium, curium, berkelium and californium combined. Given little gems like those described above to perform chemistry on molecules of profound and urgent importance to humanity, I personally regard this as a disgrace. Of course, all of these elements are radioactive, and we know merely from how the word “radioactive” is used in our lexicon how much fear, ignorance, superstition and mysticism is invested in the word. The chemistry and nature of the 5f orbitals of actinides are poorly understood, and much confusion about their exact properties – mostly obtained by theoretical modeling – exists. It seems almost certain however, that many important and potentially very useful properties have been overlooked. The synthetic elements neptunium (the 237 isotope) and plutonium (isotopes 242 and 244) have isotopes that are sufficiently long lived to be easily managed in catalytic systems with lower risk, to be sure, than the potentially greater risk associated with forswearing a deeper understanding of them. This especially applies to the wonderful element plutonium, which besides being a fabulous source of energy is a kind of chemical chameleon. (Conceivably one could make curium, 147Cm, with low enough radioactivity to be easily handled as a catalyst, but it is difficult to imagine any such property that would be unique enough justify the expense of doing so.) There are various routes to relatively pure samples of these less radioactive actinide isotopes, via prolonged neutron irradiation of lower actinides or even higher actinides. (The higher plutonium isotopes, 242Pu and 244Pu – and one important lower isotope 238Pu – may be formed by prolonged neutron irradiation of americium for example, with the isotopic purity being a function of the precise preparation procedures used.)
In recent months, a widely read paper in the scientific literature22 has conclusively demonstrated what should have been obvious long ago, that nuclear energy saves lives. Nevertheless, fear and ignorance, to beat a horse that is not dead and still needs beating, has prevented nuclear power from doing all that it can or might have done. Indeed, as I hope this discussion suggests, it may be that the materials in used nuclear fuels might be better understood as potential keys to the future – and not just as fuels – and not as the horrible objects of terror that have been presented as by some of the smaller, if popular, minds who have discussed them bereft of any understanding of what they are and what they actually can do.
References and Notes
1. Polly Arnold, Nikolas Kaltsoyannis and Steven Mansell. J. Am. Chem. Soc. 2011, 133, 9036–9051.
2. http://www.chemglobe.org/ptoe/ (Accessed July 4, 2013.)
3. Jan Willem Erisman, Mark A. Sutton, James Galloway, Zbigniew Klimont and Wilfried Winiwarter, Nature Geoscience 1, 636 – 639 (2008) One reference in this paper is to the fascinating Enriching the Earth: Fritz Haber, Carl Bosch and the Transformation of World Food Production (MIT Press, Cambridge, Massachusetts, 2001) by the always interesting Vaclav Smil. It’s an excellent historical view of how humanity and much of the other forms of life became dependent on industrially fixed nitrogen in less than a century.
4. A. R. Ravishankara, John S. Daniel, Robert W. Portmann Science 326, 123-125 (2009)
5. Richard Schrock, PNAS 2006 103 (46) 17087
6. James B. Howard and Douglas C. Rees Chem. Rev. 1996, 96, 2965-2982
7. Oanh P. Lam und Karsten Meyer Angew. Chem. 2011, 123, 9715 – 9717
8. Oanh P. Lam and Karsten Meyer Angew. Chem. Int. Ed. 2011, 50, 9542 – 9544
9. George A. Olah *, G. K. Surya Prakash , and Alain Goeppert J. Am. Chem. Soc. 2011, 133, 12881–12898. George Olah, a chemistry Nobel Laureate now in his eighties, has been a tireless worker for closed carbon cycles, most of which are based on DME (dimethyl ether) or methanol. He has published a large number of papers published on this topic, but the cited paper is a nice review article.
10. Reinhold Kneer, Dobrin Toporov, Malte Förster, Dominik Christ, Christoph Broeckmann, Ewald Pfaff, Markus Zwick, Stefan Engels and Michael Modigell, Energy Environ. Sci., 2010, 3, 198-207 Modigel
11. Emily Barton Cole, Prasad S. Lakkaraju,† David M. Rampulla, Amanda J. Morris, Esta Abelev, and Andrew B. Bocarsly,. J. AM. CHEM. SOC. 2010, 132, 11539–11551
12. Amanda J. Morris, Robert T. McGibbon, and Andrew B. Bocarsly ChemSusChem 2011, 4, 191–196
13. Daniel L. DuBois, Stephen W. Ragsdale, Thomas B. Rauchfuss et al. Chemical Reviews, ASAP Online, Accessed July 3, 2013.
14. Polly L. Arnold, Chem. Commun., 2011,47, 9005-9010
15. J. Agric. Food Chem., 1953, 1 (4), pp 292–292
16. N. Yamaguchi⁎, A. Kawasaki, I. Iiyama, Sci.Tot.Environ.407, 1383–1390
17. Hubert Tunney, Mirjana Stojanovic´ , Jelena Mrdakovic´ Popic´, David McGrath, and Chaosheng Zhang J. Plant Nutr. Soil Sci. 2009, 172, 346–352
18. R.A. Zielinski, K.R. Simmonsa, W.H. Orem Applied Geochemistry 15 (2000) 369-383
19. Huidong Li, Hao Feng, Weiguo Sun, R. Bruce King, and Henry F. Schaefer, Inorg. Chem. 2013, 52, 6893−6904
20. Matthew B. Jones and Andrew J. Gaunt, Chem. Rev. 2013, 113, 1137−1198
21. Nikolas Kaltsoyannis Inorg. Chem. 2013, 52, 3407−3413
22. Pushker A. Kharecha* and James E. Hansen Environ. Sci. Technol. 2013, 47, 4889−4895