Sciencemadness Discussion Board

Toluene For Hydrogenation Reaction

Triflic Acid - 7-3-2021 at 09:55

I am going to start a hydrogenation reaction, but I just hit one problem. Does palladium on carbon in toluene in air start a fire? I need to use a nonpolar solvent for my reaction for separate reasons, so the commonly used methanol doesn't work for me. I found a paper that used toluene as the solvent, but they were working under an argon atmosphere. So, is having a palladium on carbon slurry in toluene out in the air safe, which I'm going to need to do during the workup? Sorry if this question sounds stupid, but better safe than on fire:)

njl - 7-3-2021 at 11:35

It is as safe as conducting the reaction in methanol in air. Which is to say, dangerous. Toluene wouldn't present any special dangers.

Triflic Acid - 7-3-2021 at 13:09

Thanks for that, I am not conducting the reaction in air, only the workup.

AvBaeyer - 7-3-2021 at 15:04

Ethyl acetate is also a good solvent for hydrogenations. Its choice, however, depends on relative non-reactivity of your substrate or product.

AvB

CharlieA - 7-3-2021 at 15:54

Quote: Originally posted by Triflic Acid  
I am going to start a hydrogenation reaction, but I just hit one problem. Does palladium on carbon in toluene in air start a fire? I need to use a nonpolar solvent for my reaction for separate reasons, so the commonly used methanol doesn't work for me. I found a paper that used toluene as the solvent, but they were working under an argon atmosphere. So, is having a palladium on carbon slurry in toluene out in the air safe, which I'm going to need to do during the workup? Sorry if this question sounds stupid, but better safe than on fire:)


Many (too many) years ago, our procedure was: Place the Pd/C in a dry RB 3-necked flask fitted with a mechanical stirrer (center neck), hose fitting with a stopcock in one side neck (call it A), and a similar fitting in the other side neck. A is connected to a vacuum source, and B is connected to a source of the solvent to be used. With valve B closed, A is opened and the flask evacuated and A is closed. B is then opened and the solvent drawn into the evacuated reaction flask. Once the catalyst is wetted with the solvent there should be little danger of it sparking the solvent. (There must be an easier way for me to describe the set-up, but my mind is a blank!).

Fyndium - 7-3-2021 at 23:47

You could technically use any non-reactive gas that displaces oxygen in the flask to prevent it from oxidizing, including LPG. Of course, take necessary precautions to vent it properly to not make it cause fire hazard. Ideally, argon would be the best.

njl - 8-3-2021 at 07:03

It really shouldn't be all that dangerous no matter the solvent, so long as you know what you're doing. As @CharlieA said once the catalyst is dispersed and wet with solvent you're almost in the clear. Filtration to recover the catalyst is very dangerous since a bulk mass of catalyst builds up and is dried with air flowing over it.

Fyndium - 8-3-2021 at 07:54

If the catalyst was really valuable, I'd use a large opaque plastic bag and displace it with inert gas and do the filtration and filter cleaning in there.

zed - 8-3-2021 at 17:20

Choice of solvent may be important to the outcome of your experiment.

If you can, use the solvent the original experimenter used.

Your solvent should be able to tolerate H2, and it should be compatible with your substrate. Solubility-wise.

Methanol ain't bad. Ethanol is nice, if you can get clean ethanol. Isopropyl Alcohol, is cheap, clean and available. Glacial Acidic Acid, is sometimes just the right thing. Pyridine stinks like butt, but it is just the right solvent for some reductions. Might be some hydrogenations you could run in water.

Toluene? I don't know. Never tried it. But, note.... Many catalysts are poisoned by Sulfur compounds, and some grades of Toluene are loaded with Sulfur compounds. This solvent might require special purification, before using. Thiophene is a common contaminant.

Also to consider, most hydrogenation solvents can be ignited, and most likely your solvent will get hot.

If a fire does occur, over most of the solvents I mentioned, it can be quenched with water.

A Toluene fire, might be harder to extinguish.

On another note, I have always used my catalysts "Neat" , no Carbon. And, no filtering. Just let the catalyst settle, and decant your product and solvent. Platinum Group Metals, stick right to the bottom of a resting flask. Very, very heavy. Often, you can perform several reaction runs, with one charge of catalyst.

I have never filtered an Active Platinum Catalyst, and I will try to avoid ever filtering an Active Platinum Catalyst. Kinda scares me. I don't relax, until the catalyst is resting in water.

[Edited on 9-3-2021 by zed]

[Edited on 9-3-2021 by zed]

njl - 8-3-2021 at 18:45

What is that kind of catalyst called? Is it just activated platinum? Sounds very interesting :)

zed - 9-3-2021 at 20:42

When I say active, I mean the catalyst has Hydrogen Adsorbed on its surface. Meaning when the dry catalyst is exposed to atmospheric Oxygen, the catalyst will "ignite". Residual solvent will ignite too. In my estimation, that's a bad thing.

Though I suppose it has possibilities, as a way to regenerate poisoned catalyst. I don't know.

Assuming, the combustion might be hot enough to incinerate the poisons, and oxidize the surface of the Platinum back to Platinum Oxide. The Platinum Oxide might be ready to be re-reduced to Pt-H, and be ready for another go-around.

I know some of the guys use Palladized or Platinized Carbon, as catalysts. I just haven't done that. Messy. Messy and there is more stuff that can catch fire.

Here, I am not really an expert. I've used reduced Adam's Catalyst. And, I have generated catalysts via Sodium Borohydride and Chloroplatinic Acid. Oh, and I've also generated Nickel Boride, and Cobalt Boride catalysts.

In former times, Platinum was considered the go-to hydrogenation catalyst. Palladium was very inexpensive, and perhaps less utilized. I am imagining the very high, current price of Palladium, is going to send folks scrambling for alternatives. Both for oxidations, and hydrogenations.

Perhaps the Nickel, Cobalt, Tungsten Carbide, or Ruthenium... Wacker, will be developed and utilized.

itsallgoodjames - 10-3-2021 at 08:11

Quote: Originally posted by njl  
What is that kind of catalyst called? Is it just activated platinum? Sounds very interesting :)

It's exactly what it sounds like. It's activated carbon that has been coated in a thin layer of palladium. It has a super high surface area for the amount of metal used, which is why it is used.

njl - 10-3-2021 at 09:01

Quote: Originally posted by itsallgoodjames  
Quote: Originally posted by njl  
What is that kind of catalyst called? Is it just activated platinum? Sounds very interesting :)


It's exactly what it sounds like. It's activated carbon that has been coated in a thin layer of palladium. It has a super high surface area for the amount of metal used, which is why it is used.


On another note, I have always used my catalysts "Neat" , no Carbon. And, no filtering. Just let the catalyst settle, and decant your product and solvent. Platinum Group Metals, stick right to the bottom of a resting flask. Very, very heavy. Often, you can perform several reaction runs, with one charge of catalyst.


I'm asking about this kind of unsupported catalyst. If it is as versatile as other catalysts but requires less work up, I'm interested.

[Edited on 3-10-2021 by njl]

zed - 11-3-2021 at 09:52

Last time I did it, Carbon wasn't required. Pt/Pd have differing abilities, based upon what they are "mounted" on, if anything. Perhaps carbon slows poisoning, or improves activity in some cases.

Mounted or unmounted, your hydrogenation catalyst is usually poisoned after a few runs, and it will require either re-vitalization, or reprocessing.

Adams Catalyst, is just Platinum Oxide, reduce it with hydrogen and use it.


I have a fondness for "Brown's" Catalyst. Very active. In a sealed, purged flask, Dissolve excess NaBH4 in ethanol, with stirring. Inject in an Ethanol Solution of Chloroplatinic Acid, through a septum. The Sodium Borohydride will instantly reduce the Chloroplatinic Acid to Platinum Black. Good to go!

If need be, you can destroy the excess Borohydride with acid. If carefully planned, the Borohydride, can serve as your Hydrogen Source.

Sodium Borohydride reduction is a great way to prepare catalysts. It doesn't take very much.

Kind of an expensive way to generate Hydrogen, unless you are getting your Borohydride very cheaply.

https://en.wikipedia.org/wiki/Adams%27_catalyst
https://en.wikipedia.org/wiki/Platinum_on_carbon
https://en.wikipedia.org/wiki/Palladium_black
https://en.wikipedia.org/wiki/Palladium_on_carbon

Now, depending on your project. Catalyst selection, mounting, and solvent selection, can become a very arcane enterprise.

One size doesn't fit all. Somethings work, somethings don't.
Results may be inconsistent, or non-existent.





[Edited on 11-3-2021 by zed]

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njl - 11-3-2021 at 10:12

What sort of substrates might this apply to?

zed - 11-3-2021 at 10:20

Now. You need books. Experimental details, and lots of back-ground. I've read a few on the subject, and I have concluded it is Case by Case.

This guy has a lot to say. You used to be able to download the book for a free trial. Though, I printed it out, before it evaporated.

https://books.google.com/books?id=RjZRAAAAMAAJ&q=0471396...

[Edited on 11-3-2021 by zed]

njl - 23-3-2021 at 09:40

Since I was misunderstood twice I'm going to more directly ask these questions. Zed, which substrates did YOU use this on? That is what I'm asking. Firsthand experience is more valuable to me at this time. You say you have a fondness for "Browns catalyst". I'm assuming this means you have used it before. I am wondering if you are willing to share that experience in more detail since I respect your findings and your opinions. Above you mention a totally unsupported platinum catalyst. I am wondering if you are willing to share your experience in more detail since none of the links in that reply say anything about such a catalysts use or preparation.

zed - 24-3-2021 at 05:43

https://www.sciencemadness.org/whisper/viewthread.php?tid=15...

https://pubs.acs.org/doi/abs/10.1021/ja00873a039

Umm. As it turns out, Dr. Brown was mounting his Catalyst on Carbon in this example. I have not done that, nor have I seen my friends do that.

As I recall, it requires the same amount of Platinum, either way.

Though possibly specific activity, and resistance to poisoning may differ depending on mounting.

[Edited on 24-3-2021 by zed]

[Edited on 24-3-2021 by zed]

njl - 25-3-2021 at 05:44

Here is an excellent resource I found that goes into very deep practical details surrounding pretty much every hydrogenation I can think of.

http://studentstar.net/lib/Handbook_of_Heterogeneous_Catalyt...

zed - 26-3-2021 at 01:44

Thanks. Though I have had a photocopy of that book, and now an actual hardcopy, I haven't looked at it lately.

We actually used to have a link to this volume in our "References" section. Where I obtained my original photocopy. I just downloaded it and printed it out.

Such links unfortunately are mortal. They evaporate like willo-whisps. The publisher lends it as a sample, and then inactivates access, after a few days. OR, the institutions offering access, cut the cord.

Our "References" section, is full of links that are now dead.

Which is why, I'm gonna take this link, and leave a copy there.

Now, here is a link to Rylander's book. Just click on GET..... And, it should download a PDF. Rylander is a good teacher!

http://library.lol/main/D65779753A27B4F0ED758B804D6457CB

[Edited on 26-3-2021 by zed]

[Edited on 26-3-2021 by zed]