While driving over to eat sushi for lunch, me and Jason Nichols got into a discussion of a dehydrogenation paper.  The discussion gnawed away at my will to write an entry until I made a thorough investigation of the matter, for three weeks.  But now, finally, I'm back.

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As the introduction said, a new paper on dehydrogenation of AmmoniaBorane came out just a while ago.  http://pubs.acs.org/cgi-bin/abstract.cgi/jacsat/2007/129/i07/abs/ja066860i.html

Some of you may remember the first author, if you've been here long enough (you poor suckers...).   Yes, Richard Keaton is a former Sita group alum.  Anyways, what's more important, is that when me and Jason were driving out to eat sushi for lunch with the rest of the sushi group, one of us mentioned hydrogen storage, so we started talking about this paper and related topics.  The paper features a hydrogen storage material called Borane-ammonia, and a novel, first row transition metal catalyst that can release ~2.5 equivalents of hydrogen under optimized conditions for each equivalent of the substrate.   

http://en.wikipedia.org/wiki/Borane-ammonia

Now, the catalyst is a solid (obviously), it is made in-situ; and the compound is also a solid.  No problem since it's soluble in various organic solvents and water.  However, the dehydrogenation products, borazine for instance, are not so tolerant of water themselves (even though borazine is a liquid and could be envisioned as a solvent is the dehydrogenation were to stop at that point).

http://en.wikipedia.org/wiki/Borazine

The thing is, Borazine is pretty reactive towards water (I know, I'm re-iterating it), so you've got to keep your dehydrogenation tank very secure and not get into any accidents that can rupture it.  I bet the other polymeric Nitrogen-Boron solids that are byproducts of the whole process are not a fun walk in the park either when they start accumulating in your tank.  Besides, you've still got to come up with a way to hydrogenate the suckers back to ammonia borane.

But, alas.  the paper tells me in the first and second paragraph that you need to have a material that is 9% by weight hydrogen for a practical hydrogen economy.  That's what a Department of Energy said.  We need to have a material that is 9% hydrogen by 2015.  Jason reminded me of this when I brought up the conversion of liquid organics into hydrogen and other liquid organics: the reversible conversion of methylcyclohexane to toluene and 3 mols of hydrogen and the reversible conversion of two molecules of alcohol to two H2 and an ester.  The latter reaction is a bit unperfected and needs 100 degrees, so butane is the only practical example.  However, further empirical improvement could bring the operating temperature down and make ethanol a viable source of H2.

And I started to reminisce on many a paper that I've read that had exactly that same statement in it.  9% by weight....  Well, this time, I decided to get my hands dirty and get to the bottom of this.  By finding the original Department of Energy report and reading it for myself. 

http://www.sc.doe.gov/bes/reports/files/NHE_rpt.pdf

It took a while actually.  I had to look far and wide to find it, since it wasn't intuitively obvious where it would be on the internets.  I looked into the background of some authors too, but more on that later.  First of all, the relevant section to us, or hydrogen storage, begins on page 31.  Now, why would I mention hydrogen storage in a car just a few paragraphs earlier?  It was a bit of foreshadowing.  The 9% value comes from a wish list of a hydrogen storage device for a personal vehicle.  Thus the statement that appeared in the paper: "To meet the DOE targets for overall system weight (9.0 wt % H by 2015),1 practical hydrogen storage materials must be of low molecular weight and high chemical weight percent hydrogen." is misleading.  The 9% target is only for cars, and in the introduction the report states that other, stationary generators of power, have much less stringent standards and hydrogen storage materials that are much less than 9% would still be attractive there.  However, I wouldn't blame the authors of the paper for this one, since it has been parroted by everyone working in the field, including an idol of mine and probably Keaton's (I'm going to go out on a limb here): Crabtree.

Moreover, even though the storage section compares all promising materials to the 9% target, the target is from a FreedomCar wishlist.  FreedomCar and Vehicle Technologies is a separate initiative that works with industry to come up with ideas of what they would want to see before they switch from fossil fuels.  From the odious name 'FreedomCar', I can only guess that this new department was christened after Bush came into office.

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/2005_energy_storage.pdf

This new initiative was created in 2002 by DOE and DamilerChrysler, GM and Ford.  Forgive me for being sceptical that I should accept their target as a practical and desirable goal.  I see many things wrong with current auto technology to blindly follow established players.  The types of materials presented in that DOE report from 2003 on page 31 onward as promising, are all metal hydrydes.  Water and air sensitive solids.  Try putting that in your gas tank and extracting hydrogen from it at room temperature.  Or putting hydrogen back into the white rocks that are left over after the process.  Compared to them, Ammonia-Borane is a virtual panacea. 

But what other things are there that provide 9% hydrogen by weight?  Not much short of cold, liquid hydrogen.  And you don't necessarily want to keep that in your car; or expend the energy needed to cool and compress it.  The DOE report says just as much.  I've got an idea though...  Why not 2%?  The DOE report says you'd need 10 kilos of hydrogen to run a vehicle for about 450 kilometers, and that's ideal.  One of my organic liquid ideas -- ethanol, will give you 10 kilos of hydrogen if you've got a 250 kilo gas tank.  Sounds a bit heavy though...  But then, you could throw out that heavy internal combustion engine and take the high-temperature infrastructure along with it.  I hope fuel cells that don't need extremely high temperatures to operate would weigh a little less.  The catalyst for alcohol to ester transformation works at ambient pressure and needs just a few atms pressure to turn the ester back into alcohol.  True, an hour fueling time is a bit long, but the technology is just 2 years old.  Just pretend you're driving around your fat, significant other all the time (or living in a threesome), when you take your 250 kilo gas tank hydrogen car out for a spin.  Methylcyclohexane will cut it down to 180 kilos.  It just doesn't sound like something a GM or Ford could pull off, but maybe a Japanese company would be interested.

Thus, I've come to the conclusion that the 9% statement is just good filler for JACS communication introductions, but not much else.  If I see it one more time, I swear to god I'll send an email to Crabtree and call him on it.  If you disagree, feel free to rant at me in the discussion forum.  I've also come to the conclusion that maybe we should look for some other source of renewable energy for immediate applications and that there may be a lot of money wasted on hydrogen research before the payoffs are realised; especially since a sense of urgency (and it appears, earnestness) is missing among researchers in this area. 

P.S.  A final conclusion that I've come to, is that Rich Keaton's JACS communication is great organometallic science and is a great read.  You should give it a go.