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JJay
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[*] posted on 22-9-2018 at 11:40
Hydrogen


What is the most common source of hydrogen for hydrogenation reactions? It can be generated easily enough (Zn + HCl), but I think it could be tricky to regulate the pressure with a hydrogen generator.

I'm a little bit reluctant to buy a cylinder because hydrogen is an expensive gas in my area (several times more expensive than helium), and I'm not 100% sure that I can buy one as an individual.




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RogueRose
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[*] posted on 22-9-2018 at 12:04


Wouldn't Al be just as good if not better than Zn for generating H2? How much do you need to store for a use?

I was wondering the same thing one time and when I was cleaning up I came across a yoga ball that was deflated. Those things could probably hold a good bit for a short period and at a pretty decent pressure.
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JJay
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[*] posted on 22-9-2018 at 12:42


Quote: Originally posted by RogueRose  
Wouldn't Al be just as good if not better than Zn for generating H2? How much do you need to store for a use?

I was wondering the same thing one time and when I was cleaning up I came across a yoga ball that was deflated. Those things could probably hold a good bit for a short period and at a pretty decent pressure.


I'm sure Al would work fine instead of zinc, and it's more readily accessible too.

The yoga ball is an interesting idea... a 36-inch weather balloon will hold more than 15 moles of gas....




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JJay
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[*] posted on 22-9-2018 at 13:04


The more I think about this yoga ball idea, the better I like it. You could even regulate the pressure by putting weights on top of the ball (psig = weight / surface area of the ball).

tystybb_101402_-01_swimline-46-inch-jumbo-panel-beach-ball.jpg - 160kB




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CharlieA
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[*] posted on 22-9-2018 at 15:39


JJay - if that's your lab assistant, I don't see how you get anything done. I know I never had a lab assistant like that (I'm not sure I got much done anyway).

How would you fill the ball with hydrogen from a generator? Would you begin with a deflated ball and fill it to ~1 atm pressure? Or could a diaphragm pump be used to inflate the ball to a higher pressure?

I remember, many decades ago, a buddy and I generatred hydrogen with Al from old IF transformer cans and lye. We filled several small balloons with hydrogen and let them go with a postal card attached. The finder of the balloons was a couple of hundred miles away, and reported that he found the balloons "on a weed"!

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zed
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[*] posted on 22-9-2018 at 16:06


Umm. https://www.ebay.com/itm/250ml-Kipp-Hydrogen-Gas-Generator-K...

Used to be, Kipp's was the way to go.

More recently, Dr. Brown devised a method that used a mercury type pressure valve, to regulate the flow of NaBH4 solution into an acid solution.

A schematic of "Brown's" device is included in this paper. I investigated purchase of a "Gasimeter", but it isn't a common device. Might have to make your own. https://pubs.acs.org/doi/abs/10.1021/jo00909a038

Seems to me, a standard GC hydrogen generator can be pressure actuated, also.

All systems are self regulating.

Problems are: Kipp's is kinda messy. And, how "clean" is the hydrogen?

Brown's method is awesome, and clean, but it requires a special set-up, that most folks don't have...... AND, NaBH4 is just too expensive, unless you buy it in bulk.... Like about 50 kilo's or so, at a time.

The GC Hydrogen generator is available, and it produces "clean" Hydrogen, but it tends to cost about $1000 U.S. Dollars for a nifty, used, unit.

https://www.ebay.com/sch/i.html?_odkw=gc+hydrogen+generator&...

[Edited on 23-9-2018 by zed]

[Edited on 23-9-2018 by zed]

[Edited on 23-9-2018 by zed]

[Edited on 23-9-2018 by zed]
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Plunkett
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[*] posted on 22-9-2018 at 16:12


Quote: Originally posted by JJay  
The more I think about this yoga ball idea, the better I like it. You could even regulate the pressure by putting weights on top of the ball (psig = weight / surface area of the ball).

Cody'sLab has a video where he made something like you are describing:
https://www.youtube.com/watch?v=UlAAhRlqlV0

I think the compresser he uses is this one from harbor freight:
https://www.harborfreight.com/12v-150-psi-portable-inflator-...

The helium tank he uses has a bike valve brazed on, but this instructable shows a much easier way to modify the tank so it can be refilled:
https://www.instructables.com/id/Reusing-a-Disposable-Helium...

Or, they make various adapters to connect to disposable propane tanks. Using a propane tank for storage might be better because you do not have to worry about purging oxygen from the tank before filling it with hydrogen (disposable helium tanks have up to 15% air in them).

[Edited on 23-9-2018 by Plunkett]
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JJay
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[*] posted on 22-9-2018 at 16:47


I would probably just dump hydrochloric acid on shredded aluminum in a glass jug and quickly stopper the jug, running the outlet through a column of sodium hydroxide to dry it and remove hydrogen chloride. Then attach a large ball or inner tube, etc. to the apparatus and inflate it (tubing, plastic wrap, rubber bands - nothing fancy or high pressure). I probably wouldn't suggest inflating fully; a 46" beach ball will hold more than 35 moles of gas.

Cody used an air compressor to take hydrogen made by elecrolysis out of beach balls and put it into a tank. That's a super interesting idea for other gases....





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AvBaeyer
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[*] posted on 23-9-2018 at 01:30


What are you going to use the hydrogen for? Some time ago Magpie posted a nice report on the poisoning of his palladium catalyst by hydrogen generated from zinc/HCl. The problem turned out to be sulfide contamination of the zinc metal which generated H2S resulting in a very dead catalyst. Again depending on anticipated use, you will need to scrub the hydrogen free of acid or other undesiresble vapors. You are already aware of this (above).

As mentioned above, borohydride is a hydrogen source. It is the cleanest source of "chemically generated" hydrogen. On a mole-for-mole basis it is also quite efficient. Some time ago I posted the Brown papers, probably withiin Magpie's post. One does not need a complicated apparatus to use borohydride for hydrogen generation/catalytic reduction. I recall seeing a simplified system for student use in a J Chem Ed article. I am not near my reference pile right now so I can't give a proper citation.

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JJay
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[*] posted on 23-9-2018 at 01:48


I was thinking about using it to reduce 3,4-dihydroxynitrostyrene, amongst other things. It's probably possible to do the reduction in aqueous acid with dissolving metal, but I think the yield would be much higher with hydrogen over platinum dioxide. Platinum dioxide is a rather expensive catalyst.... I think aluminum would be largely free of sulfur.

I have some borohydride, but I would like to have other options since it is fairly exotic.


[Edited on 23-9-2018 by JJay]




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[*] posted on 23-9-2018 at 02:24


Using borohydride seems wasteful. Much more expensive than aluminium and HCl/NaOH. I assume using NaOH should deal with the hydrogen sulfide contamination.
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AvBaeyer
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[*] posted on 23-9-2018 at 06:18


The issue is not whether something is "wasteful" or not but what is the best way to get the job done. Not everything can be done on the cheap from ACE hardware though some may think that they are clever enough to do so. It would be far more "wasteful" to do a slop-bucket hydrogen generation which ruins an expensive catalyst than to use the best hydrogen generation method attainable to carry out the reaction.

JJay,

Zinc/acetic acid seems to occur more frequently for reduction of nitrostyrenes although efficiency seems to be substrate dependent. There is an extensive though diffuse literature basis for this, often buried in natural product synthesis papers. Because hydrogen addition to the double bond is not "concerted" with dissolving metals the reducing system must be powerful enough to reduce an oxime to an amine (play with mechanism to visualize what is going on.) With a catalytic reduction the hydrogen addition is "concerted" regarding the double bond so that one might envision the second step to be reduction of the nitro group. Thus, one needs to select a reducing agent which can reduce both an electron deficient olefin as well as an aliphatic nitro group. Therein lies the challenge of reducing nitrostyrenes. This is why catalytic systems along with LAH are most frequently used outside the amateur world.

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JJay
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[*] posted on 23-9-2018 at 14:40


Well, there's always nickel boride....







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zed
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[*] posted on 23-9-2018 at 15:45


Well, I once ran that type of reduction, back when we rode around on dinosaurs.

Reduced a nitropropenylbenzene, via Brown's single vessel method. Utilizing a balloon.

Reduced, Chloroplatinic acid to super-active Platinum Black, by injecting it into a solution of ethanolic NaBH4. I let the Pt Black settle out, Siphoned off the solvent, and replaced it with a Nitropropenyl Benzene, along with an equivalent of reagent grade H2SO4, dissolved in Glacial Acetic Acid. (This solvent choice was based on an old procedure, I translated from the German) The acid solvent is required to prevent polymerization of the nitro-olefin during reduction.

I then dripped in a little Ethanolic NaBH4, shook it up. And, Eureka.... My bright Yellow-Orange solution, was instantly rendered crystal-clear and colorless.

Very, very, fast. So, think exothermic. If you reduce a sizable quantity, cooling may be required. At this point, Brown's procedures prove very useful, because he utilizes magnetic stirring, rather than shaking, cooling is easy. Just put your flask in a water or ice-bath.

I was unable to isolate a phenyl-isopropylamine, so it appears that the reduction stopped at the oxime stage. Still, this type of reduction has been reported successful in reducing Nitrostyrenes directly to Phenethylamines. Sans the NaBH4; that is a modern touch.

[Edited on 23-9-2018 by zed]

[Edited on 23-9-2018 by zed]
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[*] posted on 23-9-2018 at 22:09


Whats wrong with plain old electrolysis? Make as much hydrogen as you want, when you want it, at an easily controllable rate, with one of the cheapest reagents on the planet.
All it takes are stainless steel plate electrodes, polypropylene cloth cell separator, either NaOH or NaSO4 or H2SO4 as electrolyte, and a high current low voltage DC power supply.
I made one years ago that would sustain a flame on a 25mm pipe, with a 12V , 20A supply.




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JJay
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[*] posted on 24-9-2018 at 04:20


I think it takes extended reaction times and some heat to reduce oximes with hydrogen on platinum... I'm not sure what the mechanism is, but it looks like the surface area of the catalyst matters a great deal....

Electrolysis is fine; it just takes longer than dumping acid on metal.




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[*] posted on 24-9-2018 at 06:26


Careful with aluminum - there's an induction period where the acid has to eat through the oxide layer, and the reaction is very exothermic. So it looks like it's not working, then suddenly it starts bubbling and rapidly heats up and speeds up. Zinc seems like a more controllable hydrogen source to me. I'd run a small scale experiment first if you're planning on using Al.
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[*] posted on 24-9-2018 at 07:27


Transfer hydrogenation used sodium formate quite often, which reacts with Pd/C to generate H2 and CO2 from the formate decomp.

Also, hydrazine will generate H2 with Pd or Raney Ni, especially in ethanol, that is one of my favorite ways to reduce nitro groups. And Ni is cheaper than Pd.
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[*] posted on 24-9-2018 at 09:49


I don't know how the hydroxies will behave, but other nitrostyrenes can be reduced with zinc /HCl in situ generated hydrogen in good yield in aqueous methanol.

Not by far as cool as hydrogenation with hydrogen gas, but they work...
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[*] posted on 24-9-2018 at 16:07


Most simple electrolysis cells, fail to produce an adequate amount of H2, in a timely manner.

Units designed for GC, produce as much as 500mls of clean H2 gas per minute. This is enough for most purposes.

500mls per minute equals ~ 1 mole of H2 per 45minutes. (22.4 L at STP)

This is about as much H2 gas as you might possibly require in a non-industrial setting.



[Edited on 25-9-2018 by zed]
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[*] posted on 9-10-2018 at 10:46


I would use NaOH as it is much cheaper and can be acquired with ease. As for the Al reaction, you could easily put it in a ice bath without problem.
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[*] posted on 9-10-2018 at 12:04


For the Al-based methods, maybe using e.g. CuCl2 dissolved in HCl to de-passivate the aluminium would be a good strategy? Aluminium is probably less likely to be contaminated with sulfur than zinc.

Al/NaOH also sounds good, although I don't know how quickly they react. But alumina is soluble in concentrated alkali IIRC.




[Edited on 04-20-1969 by clearly_not_atara]
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[*] posted on 22-11-2018 at 22:23


Speaking from experience with al foil and hcl I can definitely say don't do it thinking that you are going to generate hydrogen hcl gas is all that you will get and doing so in a closed coke bottle will lead to a very big loud bang.even if you had a tube coming out of the top into a NaOH solutions and then into whatever you r doing with it I would bet that dumping hcl onto aluminium will lead enough pressure to pop something out of place in your bubbler setup at the very least.use NaOH and aluminium foil for h2 generation.never al foil and hcl.spoken from experience.
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[*] posted on 23-11-2018 at 03:47


There are the PEM cells:
https://www.youtube.com/watch?v=oRpSfsE1zUg

And also the HydroStik

https://www.fuelcellstore.com/horizon-hydrostik-pro-fch-020

In my lab I use a commercial H2 generator (palladium cell) and for D2 a lecture bottle.
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