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Specialty Chemistry Forums => Chemical Engineering Forum => Topic started by: Enthalpy on January 25, 2014, 10:35:03 PM

Title: Gaseous Metal Reactant
Post by: Enthalpy on January 25, 2014, 10:35:03 PM
Hello nice people!

Many syntheses use a metal to bind two reactants, cyclize or bridge a reactant... This is usually done in a solvent, which introduces drawbacks like its own reactivity, and can take hours or days. So I wonder if a metal vapour  ;D can do the task, when the reactant can be gaseous.

The vapourized metal being atomized, it becomes much more reactive, which shall address the speed concern  >:D but the obvious consequence is that the metal loses selectivity, so the process isn't expected to fit every subtle reaction; I hope that the careful control of dilutions and residence times can keep some simple reactions under control.

To begin, here under are the vapour pressure versus temperature for some metals. Figures are from
http://en.wikipedia.org/wiki/Vapor_pressures_of_the_elements_(data_page) (http://en.wikipedia.org/wiki/Vapor_pressures_of_the_elements_(data_page))
which I just put in shape. More curves are in
http://yorkamo.phys.yorku.ca/general_stuff/2012/09/post.html
and some metals achieve a notable vapour pressure at temperatures bearable by organic compounds - by some simple ones.

Among monovalent metals, lithium looks too hot, but sodium and potassium could fit. Rubidium and caesium are more volatile but more bulky. (You may have to log in and click on the images for full size)

Among bivalents, magnesium seems difficult, zinc and cadmium better - since atomic metals must react very quickly, a low pressure can still be productive, and the compounds can leave the hot zone quickly. I've put the volatile mercury as well, though its two valences won't help. Other metals like calcium, strontium, barium are less volatile.

Trivalent metals are little volatile. Thallium is the least bad I saw.

Most metals will be liquid at the operating temperature.

Halides of these metals use to have a high boiling point: at least 1171°C at 1atm for I-VII salts. II-VII salts are less refractory, but oxides of most bivalent metals are. This means that the salt by-products will fall away from the reactants and products.

An example of a reactor is to come.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on January 25, 2014, 10:39:51 PM
Here under is a sketched example of a reactor for metal vapour.

The metal can be introduced continuously or not, heated separately or, if any, by the carrier gas.

The carrier gas helps much if other reactants must exceed the metal amount. Because the individual metal atoms are expected to react snappily, they wouldn't have time to dilute in the other reactant, hence would be the major reactant locally where they're introduced. Pre-diluted in the carrier gas, the metal can be the minor reactant right from its introduction, surrounded by a concentrated reactant. The carrier gas, probably a rare gas under such conditions, also gives the flexibility of partial versus total pressure. In addition, it can keep the reaction at a reasonable temperature.

Some reactants can be introduced first, if they must react with the metal before meeting other reactants. I imagine things like CH3Cl or CH2Cl2 reacting with Zn or Cd, then brought to an alkene. The carrier gas can bring these first reactants to the metal. More dynamic reactor designs can also inject the reactants in sequence.

The reactants introduced later don't need to be as hot as the metal vapour. If the metal reacts quickly and is diluted in the carrier gas and the other reactants, it won't have time to meet other metal atoms to condensate.

The sketch lacks, among others, some ways to evacuate the salts and other by-products, nor does it display the subsequent separation of the products, the reactants and the carrier gas.

Liquid metals are corrosive. The pressure vessel must resist at least the outer pressure. And expect the unexpected as in any design.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on January 26, 2014, 03:12:19 PM
To check roughly what plain radicals can be created, I've computed heats of reaction. Starting from halogenated methane, as bigger molecules ease the reactions. Just at 298K and producing solid salts, and without evaluating the entropies, so the contentious cases can't be decided from that, but some general conclusions are already clear.

Every monohalomethane can react with any alkaline element to make a methyl radical:
(g)A + CH3X -> (s)AX + .CH3X
from the heat criteria, where 300-400kJ are released. The interrogation is rather if a gaseous metal is advantageous, and whether it will damage the reactant.

Free carbene is hard to obtain, and a gaseous metal eases it, at least from the computed heat: see the attached table.
Free carbene will recombine snappily, so using it demands a special reactor design. Later.

Bridging a dihalo cyclic compounds needs no free carbene and looks easier.

Some salts decompose at heat, like ZnI2 at 1150°C, which helps recycling at the factory.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on September 16, 2014, 07:51:50 PM
At least gaseous potassium is already used and nobody tells me a word...

In the 2007 PhD thesis of Carles Ayats Rius:
Síntesi i reactivitat de derivats del triciclo[3.3.0.0(3,7)]octá (it's stellane)
on page 95 (Pdf p119 of 446) and scheme 3.8:
"la deshalogenació de l'1,4-diiodonorbornà 272 en fase gas, utilitzant potassi atòmic com a agent reductor"
(if you wonder, it's Catalan, more or less understandable to Spanish speakers)

which uses gaseous K to remove iodine from diiodonorbornane and obtain [2.1.1]propellane - single metal atoms bring no other radical, advantage over a liquid, and are more reactive, without any altered surface.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on October 11, 2014, 06:39:11 PM
Instead of spreading metal vapour in a big volume, we could evaporate the metal locally and let other reactants flow there. Concentrated laser power is a usual means to evaporate a solid, which doesn't melt substantially if the peak power suffices. For instance short pulses made by a diode-pumped solid laser, possibly with a Q-switch, and concentrated on a spot, are know for this purpose. The spot can be swept, or the target moved, or both; enough speed might enable a Cw laser.

Advantageously, the other reactants feel heat very briefly, only through the metal vapour if they don't absorb the light - lasers for peak power use to radiate infrared.
The reactor design doesn't seem very constrained. Referring to the sketch:
It must just need a few trials and tunings... ::)
Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on February 15, 2015, 06:52:32 AM
I've looked if Grignard-like reactants R-M-X can be synthesized using vapours of bivalent metals by trying to evaluate the enthalpy of reaction. The figures in the appended table are very imperfect because I found the enthalpy of formation only for R-Mg-Br and R-Mg-I and in solution, so I tinkered horribly:
So take with due mistrust.

The result is that

I imagine (wishfully?) the reaction to be immediate. With an excess of R-X it should produce R-R (which alkalis would too). To get R-M-X it seems better to introduce R-X slowly and at low pressure (diluted in a carrier gas?) in the metal vapour and try to remove the R-M-X quickly.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 03, 2015, 09:11:01 AM
Since the boiling points differ a lot between
a reactor could separate them autonomously.

In the appended sketch, the hot condensed metal is in direct contact with the products and possibly the gaseous reactants. Depending on the potential consequences, it may better be kept separate. Many varied valves are also needed but not displayed.

The carrier gas must dilute the reaction heat by >100: 100kJ/mol for Grignard-like Cd, 400kJ/mol for a hydrocarbyl and KCl prior to dimerization. Carles Ayats Rius used N2 in his already mentioned thesis, and the heat didn't destroy the molecule's skeleton. Dilution shall also favor Grignard-likes over dimers when desired.

The reactant(s) is evaporated to the desired partial pressure, diluted and heated. Gaseous metal is supposed to make a quick reaction. K needs +200°C to reach 1Pa, Cd (sublimates) +260°C, ouch; Rb and Cs, since they're recycled? Possible salt is to gather at the bottom and be split for local reuse.

The gas exiting the reaction vessel is to contain little reactants left. Cooling it to an intermediate temperature lets the metal condense back to the reactor. Then, further cooling condenses the products, and the carrier gas is reused.

Heat from the coolers can boil and pre-heat the reactants, since the reaction produces heat and the products often have a higher boiling point.

In the lab, introducing slowly the reactants in an evacuated vessel with heated metal is simpler; Cd reaches 1Pa before melting (+321°C) and can be the vessel.

Opinions welcome of course!
Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: snorkack on May 04, 2015, 04:00:55 AM
To present metals, and semimetals, of interest (boiling under 2300 C, to include Ga you mentioned) in a table visible in the posting body
Element symbolBoils at 100 kPa, CVapour pressure 1 Pa, C
Li1337524
Na880280
Mg1088428 (s)
K756200
Ca1482591 (s)
Mn2060955 (s)
Zn912337 (s)
Ga22451037
As601 (s)280 (s)
Se685227
Rb685160
Sr1373523 (s)
Ag21601010
Cd767257 (s)
In2067923
Sb1585534 (s)
Te992data missing
Cs667144
Ba1897638 (s)
Sm1788728 (s)
Eu1523590 (s)
Tm1944844 (s)
Yb1192463 (s)
Hg35642
Tl1485609
Pb1754705
Bi1562668
Ra1526546 (s)
Also concerning flash heating: if metal vapour is distributed into inert carrier gas and rapidly diluted, then yes it may form supersaturated vapour or if it does condense, form a fine aerosol which is reactive compared to bulk surface.
But laser is not the only option for flash heating: seeing you deal with metals, which are electrically conductive, how about spark discharge?
This way, you can also minimize the heating of your organic reactant: place your spark gap and the metal wire you are evaporating into an outlet of gas, where rapid gas flow blows away the arc and prevents your organic reagent from entering the arc.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 05, 2015, 02:49:06 PM
Thanks for your interest and contribution!

What kind of temperature organics accept briefly is still unclear to me. Some monovalent and bivalent metals are possible (K is used), the others are doubtful. Maybe experiments bring a good surprise - or not.

I like laser for flash evaporation because it spares the organic reactant, and it has the best power density as short pulses, which produces vapour and nothing else, but lasers have a limited efficiency.

Diluting metal vapour first in the inert gas look like a good alternative as an electric discharge. It's a matter of figures as usual: what pressure, voltage and resulting spark nature, residence time, and so on - I had planned to check it. Paperwork may give first guidelines.

Metal droplets are reactive, yes, and would advantageously replace superfine metal powder, as droplets made in tiny amounts in the reactor are less dangerous. They don't bring the other advantage of vapour, which renders some reactions exothermal, especially Grignard-like with cadmium. Alternatives to droplets already exist but may be less convenient: within the reactor so they don't oxidize before use, extrude many thin wires, grind tiny chips...

If you really want droplets, a rocket injector achieves them, for instance by blowing the carrier gas on the liquid - here premolten metal. If the reaction with the organics is snappy enough, it makes a tiny reactor.

----------

Meanwhile I like less cadmium in the last sketched separating reactor, because it sublimates, so after condensation in the outlet pipes it wouldn't flow back to the melt: improvement needed. Also, the solid reacting with some residual halogenated compounds risks to develop a hermetic salt layer, end of the game.

So this reactor seems to fit monovalent metals better, and flash evaporation retains a strong interest for cadmium.
Title: Re: Gaseous Metal Reactant
Post by: snorkack on May 05, 2015, 05:45:43 PM
Just at 298K and producing solid salts, and without evaluating the entropies, so the contentious cases can't be decided from that, but some general conclusions are already clear.

Every monohalomethane can react with any alkaline element to make a methyl radical:
(g)A + CH3X -> (s)AX + .CH3X
from the heat criteria, where 300-400kJ are released.
And of course the reaction actually forms gaseous AX molecules. With poor reaction enthalpy because AX tends to be high-boiling.
Also: when you are dealing with supersaturated vapour, you are apt to have stuff like dilithium.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 06, 2015, 05:09:49 PM
Precisely because alkali halides aren't very volatile, I expect them to be produced solid, and the reaction to release much heat. I see no good reason why the reactants in the gas phase would make gaseous products. Nucleation may take some induction time, but the reaction supposedly happens at the dust's surface. Just like gaseous HCl and NH3 make solid NH4Cl, without passing endothermally by a gaseous salt.

The reaction with gaseous potassium and organic iodides is already used.

If any necessary, the reaction can be started by a powder of seed crystals.

Do you see reasons against?

I don't consider lithium as a first choice, because it needs an excessive temperature. Potassium is a better choice (and already used), and if cheap enough or recycled, caesium and rubidium.
Title: Re: Gaseous Metal Reactant
Post by: snorkack on May 07, 2015, 02:19:12 PM
Precisely because alkali halides aren't very volatile, I expect them to be produced solid, and the reaction to release much heat. I see no good reason why the reactants in the gas phase would make gaseous products. Nucleation may take some induction time, but the reaction supposedly happens at the dust's surface.
Then are you likely to have metal condense on the surface of halide dust?
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 11, 2015, 03:40:42 AM
I can only try to imagine what happens... like the hydrocarbyl's halogen sticking for a limited time on the salt's surface and getting stabilized there if an alkaline metal sticks next to it, usually by rolling on the surface, and when one X and one M more pertain to the salt, the hydrocarbyl radical is released. Or the metal first and the halogen next, this makes no difference.

M-X needs an ionic bond to be part of the growing crystal, and once the halogen receives an electron from the metal it releases the hydrocarbyl.

How widely known is the reaction of gaseous potassium with organic iodides? Maybe the mechanism is already known.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on August 09, 2015, 12:01:57 PM
An other already known reaction with gaseous K+Na:
1,4-dichlorobutane to cyclobutane with 70% yield at 220°C and 0.1 torr,
as reported in chapter 48.3.1.2 of
"Science of Synthesis: Houben-Weyl Methods of Molecular Transformations"

Only Li amalgam and dibromobutane in dioxane would improve the yield to 80% but this needs 3h and mercury and dioxane. Gaseous K+Na is supposedly quick because the step to hydrocarbyl radical (even to momentarily gaseous KCl) needs half as much energy as in the bromination to t-Butyl. A smaller reactor reduces the amounts of active reactants and products.

Whether gaseous cadmium improves over potassium? The hydrocarbyl radical hurdle is too high here, but gaseous Grignard-like and CdCl2 would form exothermically.
Title: Re: Gaseous Metal Reactant
Post by: discodermolide on August 09, 2015, 12:45:34 PM
Toxicity of cadmium salts! Surely this makes this method very unattractive?
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on August 09, 2015, 05:23:18 PM
Some of the suggested reactions are favoured only if the byproduced salt is immediately solid. Potassium looks possible even if the salt condenses after the reaction, and the reactions with metal vapours I've read up to now use potassium... Cadmium and zinc wouldn't need their salt to condense immediately. Other reactions need it, especially to make carbenes.

In this (hypothetic) situation, a fast reaction needs a big contact area between the gaseous reactants and the produced salt: fine salt can be introduced in advance, the grains can be ground if they grow too much. It needs also to move the powder against the gases. While sound, jolts or a fluidized bed achieve it, I suggest a rotating drum to create the movement more gently, as in the appended sketch.

The whole vessel can rotate, but gas joints are easier with a separate drum. The drum can have small side walls and a 3D shape that keeps the powder within.

Marc Schaefer, aka Enthalpy

----------

Toxicity: I've heard about metallic cadmium and will have a look at the salts. How critical do you feel it is? There is no good alternative, since zinc is little volatile. At least, the produced salts can be recycled in situ, for instance by electrolysis - it's an advantage I see to the process: no halogens, no metals on the road nor on the bill.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on August 11, 2015, 05:22:27 PM
Toxicity of cadmium salts! Surely this makes this method very unattractive?
Cadmium halides are indeed yuk, thanks Disco!

Zinc halides look less bad. Zinc vapour needs a higher temperature, for instance 397°C for 10Pa or 337°C for 1Pa. Fine chemicals would suffer from it, but the strained alkanes I need are stable enough. Some examples at 400°C:
unless, of course, something like ZnCl2 meddles in.

Plain zinc is solid at 400°C but some alloy must be liquid if that helps. It boils down to surface dirt preventing the evaporation. The already suggested laser shots solve it too.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on September 10, 2015, 04:50:54 PM
I've found data for the destruction rate of spiropentane at heat - Nist is just invaluable:
http://kinetics.nist.gov/kinetics/ReactionSearch?r0=157404&r1=0&r2=0&r3=0&r4=0&p0=-10&p1=0&p2=0&p3=0&p4=0&expandResults=true
at 370°C it's 0.1/h and at 430°C it's 4.3/h, so even spiropentane permits a significant vapour pressure of some metals. Provided, of course, that other species don't catalyse the destruction.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on September 10, 2015, 05:06:56 PM
[..] The reaction actually forms gaseous AX molecules
[(g)A + CH3X -> (s)AX + .CH3X considered by Enthalpy]
With poor reaction enthalpy because AX tends to be high-boiling. [..]

Eventually, I've found thermodynamic data about gaseous salts there, very nice, many thanks:
http://chemister.ru/
which tells that even gaseous salts produce enough heat to make most such reactions exothermic hence easy. So the reactions wouldn't need a big contact area with pre-existing salt to proceed.

I plan a clean update of my message of 26 January 2014 about making monoradicals.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on September 12, 2015, 10:05:57 AM
With data mostly from http://chemister.ru/ I've evaluated the heat of reaction (this time negative means exothermic) when gaseous alkaline metals and halomethane make a methyl (to represent other radicals) and a salt, produced gaseous before it condenses elsewhere - see the appended image.

This is at +220°C just to mimic a synthesis of cyclobutane from 1,4-dichlorobutane and potassium vapour (0.1 torr) found in chapter 48.3.1.2 of "Science of Synthesis: Houben-Weyl Methods of Molecular Transformations".
For small hydrocarbons, the relative vapour pressures fit the reactor sketched here on May 03, 2015.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on June 26, 2016, 09:51:37 AM
A pulsed laser, for instance vanadate or Ti:sapphire, can also evaporate refractory elements as reactants, like boron, silicon or carbon. One setup is described here on Oct 12, 2014
http://www.chemicalforums.com/index.php?topic=72951.msg280320#msg280320
similar lasers already clean stones and cut metals, where ultrashort pulses leave the underlying material undamaged.

Other methods are possible, like electricity exploding a fed fibre when it touches a counter-electrode. They should be quick enough that propelled atoms meet the other reactant, rather than the other reactant meeting a hot surface. Whatever the method, a very inert carrier gas like helium or argon can usefully thermalize the ejected atoms and spare the other reactant.

The reaction of such atoms would be unselective, but maybe useful to obtain exotic compounds from small reactants, as products or as short-lived intermediates for other reactions. Examples with carbon:
It depends on whether a carbon atom only abstracts two hydrogens from the other reactant or inserts immediately in the molecule, and how much the radicals rearrange.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on July 10, 2016, 08:38:11 AM
Atomic carbon may well be what produces acetylene in Berthelot's synthesis since 1862 - not so new hence. Thermalization by the carrier gas differs in my proposal, less current through the gas too.

Atomic carbon isn't as reactive as I thought. I binds a first hydrogen atom much more weakly than carbon in a molecule binds its last one, while carbene makes the strongest bond with one more hydrogen, according to Yu-Ran Luo and Nist's ancestor
www.nist.gov/data/nsrds/NSRDS-NBS31.pdf

Atomic carbon shouldn't subtract a hydrogen from the target molecule, but like the more reactive carbene, insert on double bonds instead. The resulting strained carbene must be short-lived, so while bicyclobutane from propene and spiropentane from ethene are worth a try, products of cyclopropene like propadiene and propyne are more reasonable. Substituted cyclopropenes live longer.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on February 25, 2018, 11:21:10 AM
I suggested here pulsed laser light to evaporate the solid reactant (metal, graphite...)
http://www.chemicalforums.com/index.php?topic=72951.msg280320#msg280320
but a xenon arc lamp does it faster and cheaper.

Consuming 150W to 20kW, they convert >50% to broadband light. Their colour and tiny emission volume match >6000K, so a concentrator lets sublimate or evaporate any metal or metalloid, even graphite. They cost around 10k€ for 10kWe and last few continuous years. Serve in cinema theatres and sunlight simulators.

The joined sketch has no uniform scale, nor does it show the probable focal plane between both optics. Some carrier gas and reactants blows at the window shall keep it cool and clean. Wavelengths that heat the lens and the windows or damage the products or reactants can be filtered away. The reactor is coupled with an important cooling and separation unit to reinject the carrier gas and reactants.

10kW light would sublimate 50mol/h graphite, or 100mm3/s: feed a Do=10mm Di=8mm tube at 3mm/s, or maybe a bunch of fibres.

3289K give 1kPa graphite vapour pressure, and most other elements are easier. The usual method estimates then 0.27mm/s sublimation speed, or 2s to volatilize the tube, consistent with 3mm/s feed and ~10mm hot tip.

A L=10mm D=10mm tip at 3289K radiates 2kW. This increases as T4 and the sublimation rate at some T28, so a smaller hotter tip saves power: prefer a narrower thicker tube or a rod if light can be concentrated enough.

The rod (100W/m/K for hot graphite in plane direction) conduces 1kW away across 10mm drop but the feed speed brings much back. Steady gas (100mW/m/K when hot) conduces 0.2kW away from R=5mm at 3000K; convection increases this.

Sufficient reactant pressure lets the vapour atoms make the product rather than rebuild a solid - or take a semi-vacuum where vapour atoms can spread far enough. The evaporated atoms should collide a few times more often with a carrier gas (like argon) than with the reactant so they have thermalized.

The destruction rate of the product is difficult to predict, but Berthelot's synthesis uses harsher conditions and the fragile acetylene survives. By the way, Berthelot's simpler apparatus is worth trying on other reactants like propylene, butene... with a carrier gas. The xenon lamp setup is more caring with the gaseous compounds here.

If producing 24/7 a C4H6, 3k€ electricity and 1k€ lamp wear make 2000kg a month at perfect yield for a single reactor. Typical products (...if any) would be small and energetic: cyclopropenes, maybe bicyclobutanes, and similar with graphite, while metals and metalloids could couple reactants or make organometallics.

Marc Schaefer, aka Enthalpy
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 13, 2018, 04:01:25 AM
Atomic carbon for chemical synthesis is known.
https://en.wikipedia.org/wiki/Atomic_carbon
https://en.wikipedia.org/wiki/Phil_Shevlin

An electric arc has been used between graphite electrodes. Diffusion through tantalum and desorption at >2200K is an other source with little C3 and C2 in contrast to thermal evaporation
https://aip.scitation.org/doi/10.1063/1.4895806

Addition to olefins can make spiropentanes or allenes, Web search
"Atomarer Kohlenstoff reagiert mit Olefinen"
"Cyclopropylidene to allene rearrangement"

One synthesis of fullerenes evaporates graphite by a laser. A xenon arc lamp may be faster, unless a much higher transient temperature is wanted.
Title: Re: Gaseous Metal Reactant
Post by: Enthalpy on May 19, 2018, 12:37:40 PM
Diffusion through a tantalum wall and desorption of atomic carbon is seducing. Reportedly, the hot wall dosn't destroy all the reactants and products.

Diffusion and desorption must be slow, but a (set of) narrow long tube(s) improves the area. Bending and coiling would pack it in a compact reactor. Tantalum being extremely ductile, drawing a tube through a matrix with a kernel makes the tube longer and thinner-walled, which accelerates the diffusion. Biassed rolling is known too to make thin tubes.

It should be easier to fill the long narrow tube(s) with a liquid or paste and pyrolyze it. Toluene and others spring to mind. The process could look like:

Closely packed narrow tubing wastes less energy by radiation. The reactor's walls can reflect the light back to the tubes.

The flow of reactants and products can be optimized, for instance axial through a stack of spiralling tubes, or radial through helices.

Most elements are easier to atomize than carbon (717kJ per atom): nitrogen (473kJ), boron (565kJ), silicon (450kJ), lithium (159kJ), sodium (108kJ), magnesium (147kJ), calcium (178kJ), zinc (130kJ)... Some tend to make polyatomic gases, which the diffusion-desorption prevents. Interactions with the cooled tube are a limit. Other refractories include niobium (cheaper), tungsten and rhenium, zirconium and hafnium (reactive), molybdenum (brittle), nickel and cobalt (moderate temperature).

Atomic nitrogen is as exotic as atomic carbon and may provide new syntheses too. Few years ago, a group claimed N(NO2)3 might be stable and interesting, for instance as a rocket oxidiser. Could it result from atomic N in NO2 gas? Just NO is a more probable product.

Marc Schaefer, aka Enthalpy