[Enthalpy]

Hello nice people!

As a rocket fuel operating on Mars, a chilly world, I consider branched alkanes freezing around -100°C, but with some 15 carbons so the flash point is safely above terrestrial temperature. 2,4,6-trimethyl-dodecane would be one such compound, but isn't available in tons nor easy to mass-produce.

(a) Do you agree?

My hope is to replace each side methyl at the previous compound by two geminal methyl groups (drawing), and shorten the alkyl tail a bit. This improves often the liquid range, and I imagine the compound could be produced by acid-catalysed alkylation, like isooctane is produced in oil refineries.
That is:
Isobutane and Isobutene give Isooctane (2,2,4-trimethyl-pentane)
Isooctane and Isobutene give 2,2,4,4,6-pentamethyl-heptane
pentamethyl-heptane and 1-Butene give 2,2,4,4,6,6-hexamethyl-decane

(b) What do you think?

In fact, I consider one could buy from a refinery the Isooctane, and even 2,2,4,4,6-pentamethyl-heptane, which must be a by-product contained in the refinery's alkylation output. Or buy the complete alkylate and separate the pentamethyl-heptane by distillation.

(c) Does this look sensible?

(d) In case 100t are needed, would a refinery de-tune briefly its alky unit to increase the C₁₂ proportion?

So the rocket fuel would only need to append the straight alkyl tail to the C₁₂ alkylate (this tail is to lower the melting point and the autoignition temperature).

(e) Feasible? Will the new bond target the only tertiary carbon?

(f) to (z) plus all the greek alphabet: other comments, flames, rants if needed.

Thank you!

[Enthalpy]

Other broad liquid range compounds depicted below are Pristane, or 2,6,10,14-tetramethyl-pentadecane (induces autoimmune diseases in mice) and Phytane (one CH₂ longer, no such reputation). They seem to be scarce in Nature and difficult to mass-produce, making them lab rarities.

(f) Do you agree?

As above, I imagine geminal methyl groups would keep a broad liquid range and enable massive synthesis by acid-catalysed alkylation, this time using Isohexene (4-methyl-1-pentene), in the hope that the new bond forms at the alkane's only tertiary carbon.

(g) Does this make sense?

Thanks!

[Enthalpy]

(c) and (d) Meanwhile I've found more realistic literature about alkylation in refineries (terra incognita for me, sorry) and it differs from the only reaction given in encyclopaedia...

Refineries feed not just isobutene but as well propene, 1-butene, 2-butene, some pentenes... The alkylate may contain 28% of 2,2,4-trimethyl-pentane, and 16% of each 2,3,3- and 2,3,4-, plus a broad mix.

As the sought ultralow freezing point is very sensitive to the methyl groups location, the Martian rocket fuel would need more controlled synthesis conditions than a refinery - or a careful separation of the 2,2,4-trimethyl from the refinery alkylate.

More, refinery alkylate lacks iso-C12. I suppose C8 products leave the reaction zone quickly as they appear and condensate. Producing heavier isoparaffins would need a separate reactor in liquid phase or warmer.

Is that correct?

[fledarmus]

Pure iso-octane (2,2,4-trimethylpentane) is readily available. It is used as the standard for octane number in gasoline engines.

[Honclbrif]

Why not use a mixture of materials and let colligative properties do the heavy lifting?

[Enthalpy]

Pure iso-octane (2,2,4-trimethylpentane) is readily available...

Yes. As a Martian module may need two tonnes of fuel, it won't disrupt the market, and that's the best solution.
I was still considering a production of hundreds of tonnes for lower stages, but with alkylate not being what I hoped, and refineries' alky unit unable to produce C12 compounds, rocket kerosene (Rp-1) should remain a better choice there.

As it is, iso-octane boils at +100°C hence is highly flammable, not nice. So I would like let the molecule grow beyond iso-octane, with more methyl groups and a straight tail.

Do the further steps I imagined make any sense? I'm very far away from anything I believe to understand here.

[Enthalpy]

Why not use a mixture of materials and let colligative properties do the heavy lifting?

Learnt a new English word, thanks. Nice to have Wiktionary.

If I get it properly, you suggest to stay at the complex mixture from the refinery, and go on with the process, since mixtures do a good job at keeping a low freezing point?

I don't know for sure if the mix' -100°C melting point will be kept when growing the molecules from C8 to about C15 to raise the flash point. My fear is that some of the resulting compounds will have a very high melting point, thus freezing within the mix at Martian cold.

(Which uses to happen at the injectors, in order to produce the biggest unwanted effect. That's why kerosene refined for high-flying aeroplanes is cleaned of components that freeze easily like moisture, heavy paraffins... Additionally, kerosene for rockets must make no bubbles nor polymers when flowing in an engine's cooling jacket).

Uncomfortably, very similar compounds can have very different freezing points:
-112°C for 2,2,3-trimethyl-pentane, but
+101°C for 2,2,3,3-tetramethyl-pentane (just one methyl more)
and since C15 compounds, desired for their high flash point, use to melt around room temperature, and only the fewest stay liquid at -100°C, I'd like to be selective about the composition of the fuel.

But if several compounds are liquid between -100°C and +220°C, blending them would certainly improve, with great pleasure.

[Wastrel]

I'm a little confused by the initial premise.  Why should a rocket fuel for use on Mars be designed to have a flash point greater than room temperature on earth?  Is flash point even relevant on a planet with too little oxygen to support combustion?

Wouldn't specific impulse be of greater importance?  Would it be better to pick an oxidiser first and determine performance properties when used in combination?

I applaud the molecular tinkering and given the cost of getting the material to another planet it's certainly worth getting it right.

[Enthalpy]

...Why should a rocket fuel for use on Mars be designed to have a flash point greater than room temperature on earth?...

Wouldn't specific impulse be of greater importance?  Would it be better to pick an oxidiser first and determine performance properties when used in combination?

Thanks for your interest!

The flash point is useful for safety on Earth. It's unimportant once on Mars, sure. But rocketry is already dangerous enough that single hazard sources should be prevented where possible.

The choice to my eyes, after progress among other options, boiled down to
- Liquid oxygen and a small hydrocarbon, both pressure-fed;
- Or Mon-33, preferably with a storable hydrocarbon, used in my original pumping cycle
http://www.scienceforums.net/topic/60205-pumping-cycles-for-rocket-engines/page__gopid__629135#entry629135
corresponding to the third drawing there.

Presently, I've concluded that I prefer Mon-33 (33%wt NO dissolved in 67% N₂O₄). It's toxic and the cycle needs a pump, but
- Being storable, it needs neither a permanent active cooling nor a fragile vacuum thermal insulation in the Martian atmosphere;
- Isp is 30s better than pressure-fed oxygen-methane;
- Tanks are lighter.

The Isp combined with lighter tanks lifts Martian samples or a crew from Mars to low Martian orbit in a single stage which previously de-orbits and lands the full craft (aided by parachutes). This is much harder with pressure-fed oxygen-methane.

The pumping cycle promises to start easily, and an igniter for Mon-33 looks reliable as well
http://forum.nasaspaceflight.com/index.php?topic=27308.msg849905#msg849905

Both solutions, Mon-33 and LOx, combine well with the attitude thrusters.

Putting all arguments together, I believe the cycle with Mon-33 is a more reliable global solution despite toxicity.

-----

Several fuels combine well with Mon-33 and my pumping cycle. Some of them more efficient:
+3s, mp=-100°C Tetramethyldiaminobutane
+5s diazaspiroheptane with N,N' methyl or cyclopropyl
+5z azetidine with N-methyl or cyclopropyl
+6s, mp=-83°C Azetidine
+7s 1,3-diazetidine with N,N' methyl or cyclopropyl

All are amines likely to ignite upon contact with Mon-33 but too slowly for a thruster. Only dangerous.
Azetidine is a volatile flammable amine; hazards for most others are little known, liquid range neither.

The limited performance improvement would optimize masses a little bit but not the number of stages at a Martian descent-ascent module, making the alkane a clear choice.

-----

The rest of the trip shall use hydrogen-oxygen everywhere possible, clear. Including the return leg, since tanks are easy to insulate in vacuum, and I describe a permanent cooler there
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2051
moving part on Tue Jan 12, 2010 - cycle on Sun May 16, 2010 - fabrication of the heat exchanger on Wed May 19, 2010

[Enthalpy]

So, what about the synthesis at the first drawing? If starting from bought iso-octane:

Iso-octane and iso-butene give 2,2,4,4,6-pentamethyl-heptane
pentamethyl-heptane and 1-butene give 2,2,4,4,6,6-hexamethyl-decane

Is that realistic?

[Enthalpy]

I applaud the molecular tinkering...

Lego forever !

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Latex, Gutta-Percha and synthetic polyisoprene have already the pattern with one methyl branch every fourth backbone carbon (drawing).

(h) Can a tri- or tetramer of Isoprene be obtained, instead of a polymer? Or a reasonable mix!

After just a hydrogenation, I'd get Phytane or some other nice fuel!
(Nice as well as laboratory solvent, apparently)

Marc Schaefer, aka Enthalpy

[Enthalpy]

Myrcene is dirt-cheap, as a by-product from paper production: pinene is extracted from turpentine and pyrolysed to Myrcene. Myrcene is also a dimer from Isoprene.

Various documents about turpentine tell that Myrcene is stabilized with butylhydroxytoluene or with vitamin-E for shipment or storage, or it will dimerize at moderate temperature.

The dimer of Myrcene is an isomer of Geranylgeranene, with all double bonds shifted a bit. It gives Phytane through hydrogenation. Please see the drawing.

Now, Myrcene is much more volatile (2Pa@+20°C) than its dimer and than both stabilizers.

Hence the process I imagine:
- Buy Myrcene with its stabilizer;
- At moderate temperature, evaporate Myrcene a low pressure, freed of its stabilizer;
- Pass the gas through a catalyst or a zone at higher temperature to dimerize it;
- The dimer condensates, harvest it and cool it to stop polymerisation;
- Saturate all double bonds with hydrogen to get Phytane. 

Nice : Phytane here is free of the seemingly unhealthy Pristane. 

In a variation, Myrcene would react with Isoprene to produce the trimer of Isoprene, which gives Farnesane  by full hydrogenation - also a potential fuel.

Instead of buying Myrcene from the paper industry, it can be dimerized from Isoprene, also a dirt-cheap by-product from ethylene at refineries. Phytane and Farnesane obtained this way are probably too expensive to replace jet fuel  but cheap enough for rockets, and are up to now expensive laboratory products.

(i) Does this look reasonable to you?
Marc Schaefer, aka Enthalpy

[Enthalpy]

My mistake, sorry: the Isoprene dimer isn't Myrcene. Which ruins the attempt to make Phytane and Farnesane from turpentine. Hence I can answer by myself:

(i) is nonsense.

And if you use the software Mpbpvp included in Episuite, be more careful than I was: it identifies wrong compounds from the Smiles description.

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Some people want to produce jet fuel from biomass and one compound is the already depicted Farnesane.

Their patents are US7589243 and US7399323, available for instance through Freepatentsonline.

They "mass-produce" (kilograms in the patent, already an achievement) Farnesene Diphosphate from Isoprene Diphosphate and hydrogenate it.

Their AMD-200 fuel is pure Farnesane, with fp=+109°C bp=+243°C d=773,7kg/m³@+15°C. They indicate only mp<-71°C, but I have good hope it's <-100°C, so even if they can't fly all airliners, they could supply a Martian module.

--------------

What about the Isoprene tri- or tetramer? Do you believe it can be obtained from the cheap Isoprene by reasonable means? Quenching the Ziegler-Natta mechanism early, well before the polyisoprene is obtained?

I still like the two-steps process where the dimer is isolated then itself dimerized, as this should produce selectively the tetramer, and also allows different reaction conditions for the different compounds.

[Enthalpy]

What I wrongly called Myrcene, the head-to-tail isoprene dimer, is (little) known as
Hymentherene, Menthrene, Isomyocorene and
2,6-Dimethyl-1,3,7-octatriene
Alpha-menthrene has even the cas=6876-07-9

Dimerization and oligomerization of Isoprene was studied in the 70's. Some papers:

- Synthesis of 2,6-Dimethyl-1,3-trans,7-octatriene (Head-to-tail Isoprene Dimer) by a Temperature-Controlled Two-Stage Reaction

- Oligomerization of Isoprene by Vanadium Catalysts

- Isoprene Oligomerization with Lithiated Diethylenetriamine Catalyst

which doesn't tell me if ton amounts are any reasonable.

Still interested in your comments and inputs!

[Enthalpy]

The production of iso-octane by alkylation (points c and d) differs even more from the
Isobutane+Isobutene -> 2,2,4-Trimethylpentane
given is encyclopaedia and some textbooks...

Found more detailed descriptions, and it's a huge mess - a possible reason why Fox&Whitesell don't even mention it under alkylation nor addition...

Isobutene protonated by the acid is to react with an other Isobutene or olefin, like the beginning of an oligomerization, and Isobutane quenches the polymerization at the dimer by providing an H^-. The reaction happens in the liquid, with Isobutene and Isobutane in solution or emulsion in the acid.

The thesis dissertations by Wei Shen and by Thi Le Thuy Bui confirm that significant amounts of C₁₂ isoparaffins are obtained unless the conditions are actively tuned for C₈.

All authors tell that the molecules isomerize during the process, and more so the carbocations, which only need to move an electron pair. Even if reacting pure Isobutene and Isobutane, a wide range of Isooctanes and isoparaffins is obtained. A serious obstacle on the way to one precise molecule.

If carbocations react with olefins rather than paraffins, the desired alkyl tail won't be any easier to obtain, as this should be done before H^- addition. React only Isobutene first, then add the straight alkene and Isobutane?

Now oligomerization or Propylene or Isoprene seems a better way than alkylation to obtain a precise product.