In an effort to optimize purity and subsequent distillation yield through avoiding catalytic decomposition of the product with usage of higher still
pot temperatures, it would be necessary to remove residual Palladium leached out of the employed Pd/C catalyst.
The crude product obtained from the catalytic reduction, a rather simple aliphatic amine, after workup (acid/base extraction, removal of solvent)
exhibits an orange tinge characteristic of the presence of Palladium(II).
It has not been established whether the Palladium exists in an ionic or complexed form.
The colored impurity is not extractable from the mixture by neutral or basic aqueous solutions.
Does anybody know of a method removing Palladium residue from the crude amine base?
Edit: Google came up with this, but i'm curious if there are methods that employ regular laboratory reagents:
The crude product obtained from the catalytic reduction, a rather simple aliphatic amine, after workup (acid/base extraction, removal of solvent)
exhibits an orange tinge characteristic of the presence of Palladium(II).
That orange tinge is not due to palladium. When you use Pd-C, the leached palladium is in the "palladium black" form. It is not possible to filter
this with a frit or paper filter. You need to filter through celite. Some can pass over celite and if you have troubles with this, use more, or use
silicagel (if you are sure the product wont stick to it, which is usually not the case if you use alcohols as solvents). You cannot have free Pd(II)
after hydrogenation over Pd-C. Generally, it takes strong ligands to stabilize the palladium in the Pd(II) oxidation state. Non-stabilized Pd(II)
salts will slowly react with the solvents or other organic substrates, quite rapidly upon heating. Pd(II) residuals are much more of an issue when you
use phosphine bound Pd catalysts.deltaH - 13-10-2013 at 01:44
I would suggest stirring your solution with some activated (decolourising) carbon granules for several minutes and then filtering.
If all else fails, you might want to try a more selective hydrogenation catalyst--PdC can catalyse a very wide range of reactions and so might form
more byproducts. On this note, possibly carefully considering your hydrogenation reaction conditions may also suppress unwanted side reactions?
[Edited on 13-10-2013 by deltaH]stoichiometric_steve - 13-10-2013 at 05:42
I forgot to mention, the impurity is homogenously dispersed in the product, there are no solids to filter.
That's why i assumed that it might very well be Pd(II), stabilized by the amine product.
Interestingly, though, the catalyst recovered from the reaction, although previously used for 2-3 hydrogenations, still exhibits excellent activity
upon treatment with a solution of formate salt.deltaH - 13-10-2013 at 05:56
As Nicodem said, which I also second, it's unlikely that this is palladium causing the colour, but I understand why you suspected it My guess is a byproduct, so the simplest solution (if it works) is decolourising with
activated carbon. The harder solution is change the reaction system to perform hydrogenation more selectively.stoichiometric_steve - 13-10-2013 at 06:16
Thanks for the input. I guess i'll have to go with distilling it as it is for now and try to check if there REALLY is any Palladium in the
distillation residue
The whole process is producing absolutely great overall yields anyways.Dr.Bob - 14-10-2013 at 09:37
Any decent distillation should remove Pd residue, as it does not distill readily unless you are up at 1000C.
There are many tricks for removing trace Pd, like resins with a thiol or phosphine group, Celite/Silica gel, and recrystallizing the product from
charcoal. But normally, Pd/C does not create more than a few ppm of Pd, which often diminishes with each further step and purification. stoichiometric_steve - 16-10-2013 at 00:44
Dr. Bob, distillation does indeed remove the Pd residue, but at the same time, the thermal/catalytic decomposition of the product in the still pot is
what this is all about.sonogashira - 16-10-2013 at 03:29
The Art of Meeting Palladium Specifications in Active Pharmaceutical Ingredients Produced by Pd-Catalyzed Reactions:
Attachment: 889.pdf (415kB) This file has been downloaded 1535 times
PTFE Micron Filters
teragon - 24-10-2013 at 00:27
EDIT: I realize that you have a homogenous solution Steve - but I thought I'd post this nifty trick anyways. Hopefully it will help someone.
Use a PTFE micron filter (1.0 micron works perfect for my Pd/C rxns).
I would still recommend Celite/filter paper as per Nicodem's suggestion for preliminary removal of the majority of the catalyst (plus Pd/C filtration
seems less prone to fire when its all mixed in with Celite).
Then use a PTFE micron filter (e.g. like a Millipore filtration setup or the like) - I have found that these filters remove residual Pd/C that is
otherwise very hard to remove completely with plain filter paper/Celite.
PTFE micron filters are just plain awesome.
BTW - Nylon 66 micron filters (i.e. polyamide filters) work in a pinch too, so long as you don't have any strong acids or bases in your rxn mixture.
If you don't have access to these filters - then a fine frit, glass sintered funnel works too (just be sure to load Celite and filter paper on top -
or you will end up with a Pd/C residue in your glass frit that won't come out). You need a really fine frit though, otherwise you might still end up
with Pd/C residue.
[Edited on 24-10-2013 by teragon]stoichiometric_steve - 24-10-2013 at 07:58
Thanks teragon, but for me, analytical filter paper removes the catalyst pretty well and is relatively quick at the same time.DJF90 - 24-10-2013 at 10:55
Yea I find whatman #1 paper works a treat for carbon filtration. For Pd/C we have to use non-flammable filteration media, so generally glass
microfiber (GF/F) is usedNicodem - 25-10-2013 at 05:49
Note that even when the filtrate appears completely clear and colorless, it can still contain colloidal particles. Sometimes you can observe a Pd-C
sediment after letting the filtrate stay still for a day or two even when it appeared completely clear at the beginning. Another thing you can try in
order to check for residual Pd-C, is to filter the filtrate once more over a narrow frit with celite. Even if the solution appeared clear, you can
sometimes observe a blackening on the top of the celite.stoichiometric_steve - 26-10-2013 at 02:22
I resolved the issue by using stronger vacuum and consequently lower distillation temperature, while pulling an even stronger vacuum towards the end
of the distillation to get all the volatile residue out of the still pot.
Yield was stellar, 90% from nitro compound to amine salt.
I accidentally threw the reddish residue away, otherwise it would have been available for analysis. Next time, then!The_Davster - 26-10-2013 at 11:00
My guess is a byproduct, so the simplest solution (if it works) is decolourising with
activated carbon. The harder solution is change the reaction system to perform hydrogenation more selectively.