Electra
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Why do some heterogeneous catalysts need a support, and others not?
I have yet to find any literature references of the use of Palladium Chloride as a hydrogenation catalyst without the carbon support. With other
Catalysts such as Platinum Oxide, I have seen no support, that I am aware of. Palladium Chloride is soluble in water, while Platinum Oxide is not.
Does the solubility of a heterogeneous catalyst hurt the catalysts action? Can hydrogen still adsorb and dissolve into the hydride phases on a
dissolved molecule?
[Edited on 11-2-2014 by Electra]
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Mildronate
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On carbon it had larger surface. I think it can, but activation energy change can be to small for your reaction.
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Electra
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I understand that is what carbon does, but does it not make sense for the catalyst to have a maximized surface area when it is totally dissolved in
the water and every single surface of each particle is exposed? Do some of these hydrogenation catalyst in homogenous form lose effectiveness compared
to their heterogenous form?
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Mesa
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You seem confused.
A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very
good heterogeneous catalyst .
Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?"
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UnintentionalChaos
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PdCl2/C is intended to be converted to Pd/C immediately before use by hydrogenation. The freshly reduced Pd adheres to the carbon support
and the preparation immediately before use ensures that no activity is lost on standing.
See part C. here: http://www.orgsyn.org/Content/pdfs/procedures/cv3p0685.pdf
Platinum oxide is also a precatalyst and is converted to platinum black on hydrogenation. I am led to believe that platinum black is prone to losing
activity on standing, perhaps moreso than palladium, and so must be prepared freshly. Platinum is also a rather more agressive catalyst, I believe and
may be prone to setting it's carbon support on fire with air exposure. Using unsupported material may fix this problem.
[Edited on 2-12-14 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Electra
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Quote: Originally posted by Mesa  | You seem confused.
A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very
good heterogeneous catalyst.
Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?" |
No no no  . I understand the difference between heterogenous and homogenous.
What I was asking is why do these catalysts seem more active in their heterogenous form (in comparison to be dissolved in the homogenous state).
Palladium Chloride for instance is soluble in water, but Pd/C not so much. Would the surface area of the palladium molecules not be maximized in the
totally dissolved form, versus on the solid support? There are plenty of various metals that are capable of dissolving hydrogen like Palladium and
Platinum, but why are they never used in their dissolved form?
The form of the catalyst should not have too much of an effect on its interaction with hydrogen. Whether Pd/C vs PdCl, Platinum Oxide vs Platinum
Black. The hydrogen isn't really forming a bond with the molecule(despite being called Metal-Hydrides), but rather it is adsorbing and dissolving into
the lattice. Hence why Palladium for instance is thought to be able to absorb 900x its molarity in hydrogen. Some metals can do even more.
When a reaction is put under pressure the hydrogen is going to dissolve into the water/solvent just as it would in the air above the water. Ideally
the hydrogen should interact with and absorb into the Palladium/Platinum particles just as easily, if not more easily in their totally dissolved form
than when they are on a solid support. It is the internal lattice of the catalytic metal that weakens the H2 bond so I am not entirely clear why these
catalysts are rarely used in homogenous form. Is it simply because they are harder to recover where as in heterogenous form they can be filtered out?
Or are they for some unknown reason less active?
[Edited on 13-2-2014 by Electra]
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mnick12
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| Quote: Originally posted by Electra | | Quote: Originally posted by Mesa | You seem confused.
A heterogeneous catalyst is one that does not dissolve in the reaction solvent. if Pd/C dissolved in (insert solvent here) it wouldn't make a very
good heterogeneous catalyst.
Perhaps the question you should ask is "what effect does the catalyst support have on the solubility of the catalyst?" |
No no no . I understand the difference between heterogenous and homogenous.
What I was asking is why do these catalysts seem more active in their heterogenous form (in comparison to be dissolved in the homogenous state).
Palladium Chloride for instance is soluble in water, but Pd/C not so much. Would the surface area of the palladium molecules not be maximized in the
totally dissolved form, versus on the solid support? There are plenty of various metals that are capable of dissolving hydrogen like Palladium and
Platinum, but why are they never used in their dissolved form?
The form of the catalyst should not have too much of an effect on its interaction with hydrogen. Whether Pd/C vs PdCl, Platinum Oxide vs Platinum
Black. The hydrogen isn't really forming a bond with the molecule(despite being called Metal-Hydrides), but rather it is adsorbing and dissolving into
the lattice. Hence why Palladium for instance is thought to be able to absorb 900x its molarity in hydrogen. Some metals can do even more.
When a reaction is put under pressure the hydrogen is going to dissolve into the water/solvent just as it would in the air above the water. Ideally
the hydrogen should interact with and absorb into the Palladium/Platinum particles just as easily, if not more easily in their totally dissolved form
than when they are on a solid support. It is the internal lattice of the catalytic metal that weakens the H2 bond so I am not entirely clear why these
catalysts are rarely used in homogenous form. Is it simply because they are harder to recover where as in heterogenous form they can be filtered out?
Or are they for some unknown reason less active?
[Edited on 13-2-2014 by Electra] |
Yes,yes,yes. Palladium in solution is NOT heterogeneous! The reason why they have different activity is because they are completely different,
Pd2+ vs Pd. Additionally there are homogeneous catalysts that will blow Pd/C out of the water, in terms of TON and selectivity.
Often times you do not need some complex phosphine-bound form of palladium to do simple lab reactions. Why pay a few hundred dollars for some chelate
when a few dollars worth of Pd/C will get the job done?
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Electra
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Interesting.
So what happens if a metal such as platinum or vanadium are in their raw form and charged with hydrogen? Hydrogen is known to absorb into the lattice
of both of these forming multiple hydride phases, which, upon interaction with an organic molecule can hydrogenate it.
Some of these metal catalyst do not need to be activated.
For instance Vanadium Chloride which is water soluble, and Vanadium Carbide which is not, can both dissolve hydrogen into the lattice to form hydride
phases. Is the solubility irrelevant when it comes to the effectiveness of these hydrogenation catalysts?
What I am really wondering with this thread is what is the role of the solubility of a hydrogenation catalyst when it comes to its effectiveness in
hydrogenating?
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Mesa
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How can an ion(Pd^2+) have a latice?
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Organikum
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Pd and Pt can be used without support the same as with one needs only far more of the metal as it likes to clump together, to form conglomerates, has
to make it collodial and get it as such in suspension and has problems recovering it lateron for if its fine enough for the purpose its too fine to
filter - ultracentrifugation might work.
Additionally on carbon by choosing the right carbon and the right way to activiate the whole, the properties can be nicely adjusted if one knows what
to do or better even can buy the right stuff from DEGUSSA, the make the best I think.
In long-gone times those catalysts were used collodial or as sponge, but then palladium was discovered to be not useless and prices went up and
up.....
/ORG
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