[gharlev]

Hi,

A question to the forum: Can we produce food from fuel?
If we use octane as substrate, we need 0.75 mole of Octane in order to produce 2 moles of Glucose. The energy released from burning 0.75 moles of Octane is around 4,095 KCal and the energy needed to create 2 mole of Glucose (with 100% efficiency) is 1,372 KCal. In other words - if we burn 0.75 moles of Octane we can make 2 moles of Glucose at 33% efficiency.

Is that possible?
If it is - we can make food from fossil fuel, saving ground, water, greenhouse gases, etc.

Thank you for your reply,
Gharlev

[Guitarmaniac86]

If you burn it, you make carbon dioxide and water... Unless I am missing some kind of chemistry that converts alkanes into polyhydroxylated carbohydrate molecules. As far as I am aware, that is not really feasible.

[Arkcon]

Your question poses a number of diverse topics.  It's hard to know what answer will satisfy your needs.

I'm going to take your thermodynamics at face value...for now because I'm at work.

Thermodynamics aside, practicality may not be there.

The reactions required may not exist, as has been mentioned.

The ability to convert hydrocarbons into biomolecules exists, and is in use.  You can Google "single cell protein" for a through treatment of the topic, its advantages, and its caveats.

[AWK]

In organic synthesis nearby all is possible. But, I  wonder whether it is even worth.
Just try sketch your attempt.

[rolnor]

There will be many stereocenters, not very easy. I think people are trying too make fatty acids from hydrocarbons and thats more feacibly i believe.

[Corribus]

I'm not following the calculation in the OP at all, and the stoichiometry doesn't sound right, but even aside from that - while the thermodynamics of an octane to glucose conversion may work out as exothermic on paper, chemistry doesn't happen on paper. You can write out a balanced chemical equation for the direction conversion of octane to glucose (via oxygen, producing also some water) and find that it is exothermic. However using any conventional chemical procedure there will be competing reactions, not the least of which is the combustion of octane to carbon dioxide and water. In addition to likely being even more exothermic than the desired reaction, the latter reaction is going to be entropically favored given the higher degree of gas production per mole of reactant. End of story: the efficiency of your putative conversion to glucose is going to be laughably low. (We know this from experience: car engines don't become rapidly clogged with glucose.) To get around this, you are left with using a stepwise series of reactions to favor your desired product, or design a catalyst to do the same. The former case is certainly possible and the nature of thermodynamics mean that overall the heat release works in your favor, but every step has also the potential for side reactions, not to mention energetic and materials costs of work up and purification, so that again the overall efficiency will probably be terrible. Selective catalysis would theoretically be the best way to go, but such catalysts don't invent themselves. Figure one out to create glucose from an alkane with high efficiency and you will become instantly famous and most lauded by the scientific community and industry! I wish you good luck with that.

[Borek]

I have a gut feeling that the highest efficiency can be achieved by burning octane and using produced CO₂ to feed plants producing the glucose

[Corribus]

True, the Nobel prize should go to the most ingenious chemist around: lettuce.

[kriggy]

I wonder why would anyone do that if glucose can be produced by hydrolysis of starch (=cheap)? Maybe if we had a settlement on different planet that would be hostile to "our" crops.
Maybe octane is not the right starting material anyway. I suppose it could be easier done by oxidation of methylpyrane or from some furan derivatives (furan is actualy produce from sugars so it might work to oxidize it back to sugars)

[Enthalpy]

Basic food is often cheaper than fuel. It depends on how much taxes you pay on fuels. When agro-fuels like ethanol are little taxed, they're cheaper than heavily-taxed gasoline, so then the reverse transformation (sucrose to fuel) is economically interesting.

In oil components, alkanes like octane are probably the worst choice because they offer no easy reactions. Consider ethene and propene instead, which are torched at the oil well, or heavier alkenes.

I'd feel more useful and maybe simpler to transform some non-alkane abundant compounds like cellulose into food, be it glucose or something else. But even this one isn't trivial. Many teams try to make cheap fuels (not food) from cellulose but haven't met the big industrial success up to now.