Wednesday, February 22, 2012

Process Wednesday: Hey, what's this powder?

Credit: Platinum Metals Review
Perusing old copies of Platinum Metals Review (Johnson-Matthey's in-house journal), I came across David E. Grove's comments on the differences between the varieties of 5% Palladium on Carbon:
[snip] Some of the variables exploited in the manufacture of any given 5% Pd/C paste include: source of the activated carbon support; pretreatment (if any) of the support; the nature of the soluble Pd salt used to impregnate the support; the nature and form of the reducing agent used to produce the Pd metal crystallites on the support; pH and temperature of
both the Pd salt impregnation and reduction steps; rate and order of addition of the reagents; efficiency of the final washing/filtration step; etc. 
Clearly a vast number of combinations of the above is possible, all capable of yielding an end product that could accurately be described as '5% Pd/C paste' with each being assigned some unique identification code by the manufacturer. 
One of the key parameters that can be controlled is the location of the Pd on the support. At one extreme the Pd can be entirely located at the surface of the individual carbon particle: eggshell metal distribution. At the other extreme the Pd can be evenly distributed throughout the particle: uniform distribution. Somewhere between these two extremes is the intermediate distribution. It does not follow that catalysts with eggshell distributions are invariably the most active.  
For example, [see top reaction scheme]. At higher pressures, catalysts with the same total metal loading but deeper metal locations tend to be more active than those with eggshell distributions because of their higher metal dispersions. 
In some extreme cases, certain 5% Pd/C paste formulations have no catalytic activity whatsoever and others can show a wide variation in reaction rate. For the reaction below, performed in an alkaline methanol solvent, the following relative
reaction rates have been observed [see bottom reaction scheme].  
To achieve reproducible results, it is absolutely essential therefore to ensure that the same catalyst formulation, identified by the manufacturer's code, be used throughout any evaluation programme.
It never ceases to amaze me, the little details of chemistry that you really didn't know much about, until someone decides to unload their knowledge on you. [Also, this is a time to get that much more paranoid about sourcing metal catalysts for scale-up. Aieeee!!!!]  

4 comments:

  1. Love it. Thanks for posting.

    A friend in fine chemicals told me that Pd(OH)2 /C was no different than Pd/C, except the loading of Pd was higher (20%).

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  2. Metal catalysts are famously prone to poisoning, especially at low catalyst loadings, so it does matter what grade charcoal you use (whether it was washed with acid or not, if it made by pyrolysis from peat or coconut shells, what is the charcoal content of sulfurous impurities, if it was reduced with borohydride, formate, hydrazine or hydrogen, etc).

    Pearlman's catalyst: I run frequently difficult debenzylations of amino groups on polymer that have to be done cleanly, >95%+, or the batch is no use in following steps (because the grafting on the amino would fail) . Using Pd(OH)2-C for debenzylations has the advantage of working with non-pyrophoric catalyst (whereas Pd-C ignites with methanol and air, and sometimes on its own) and you generate a highly active catalyst in situ - one that did not have a chance yet to get desactivated through Oswald ripening. This makes a rather important difference in N-debenzylations.

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  3. Most of the catalyst parameters at least make some kind of scientific sense - but I was dumbfounded when I had to look into the carbon support variable: carbonised coconut shell anyone? How on earth do you standardise that?!

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  4. Thanks. Will be citing this in the latest paper that I will probably send to Andjewandte.

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