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Author: Subject: Bad pd on C?
beastmaster
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[*] posted on 23-1-2009 at 20:41
Bad pd on C?


I have some 5% pd on C that is at a year at lest old, that I purchased on line. It doesn't seem to be up taken H2. Can the Catalyst go bad and can I reactivate it some how? Im using it in a CHT reaction with ammonia formate. Thank you
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Sauron
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[*] posted on 24-1-2009 at 04:11


It could be that you were ripped off. Who was the online vendor?

It could also be that the Pd/C has been poisoned by one of the several such contaminants that will render such catalysts useless. Such poisoning can occur in the use of the catalyst, for example red rubber hose to introduce H2 is a common source of sulfur poisoning.

If this is the culprit (whatever poison from whatever source) as far as I know you will need to trash the Pd/C and replace glassware etc.

Minor expense in glass, major one if you work in an autoclave! There is no way to de-poison an autoclave, once contaminated it can never be used for hydrogenation again.

(It can still be used for other reactions.)

5% Pd/C is easy to prepare, instructions in details are in Org.Syn. You start with PdCl2.




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[*] posted on 24-1-2009 at 05:46


Pd/C usually doesn't go bad by itself, I'ev been using the same one for years, and no problem.
The amount of H2 absorbed by the catalyst itself cna be very minimal, especially with 5%! If it bubbles with warm formate solution, you are good to go, do not judge only on the amount of H2 absorbed by the catalyst.

Atm hydrogenation can be very ong, especially with low catalyst loadings, so total uptake can often take more than 24H, I have done several hydrogenations myself, and never the 2-3 hours of total uptake time described in publications have been met, always at least 24h, and that's with 10% Pd/C.. Otherwise you eed a little pressure (even 2-3 bar speeds things up considerably).

Adding some foramtes can actually "initiate" the H2 uptake during hydrogenations, or reactivate that catalyst in some occasions, when it is saturated by the product for example. Adding a few drops of conc. formate solution often restarts absorption.




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Sauron
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[*] posted on 24-1-2009 at 08:04


Come to think of it the prep of Pd/C is in Vogel. There may also be one in Org.Syn.



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[*] posted on 24-1-2009 at 09:10


Quote:
Originally posted by Sauron
Minor expense in glass, major one if you work in an autoclave! There is no way to de-poison an autoclave, once contaminated it can never be used for hydrogenation again.


???

googling using "pd/c catalyst sulphur poisoning" throws several links. I have browsed thru' some of them. Every one talks about deactivation of catalyst but NONE says anything about poisoning of an autoclave.

I also remember having attended a seminar in which an industrial catalytic hydrogenation process was described by deliberate poisoning of Pd/C (in order to reduce activity and avoid "over hydrogenation"). I can not recall the process details now but I distinctly remember that the speaker said nothing about using a different Hydrogenator for each batch.

gsd
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