Author Topic: Re: Pt group metal catalysts  (Read 78 times)

fresh1

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Re: Pt group metal catalysts
« on: November 26, 2011, 12:03:41 PM »
  The metals in this group are known for their remarkable catalytic properties, useful in many areas of chemistry, and used commonly in automotive catalytic converters, and relatively cheap amongst other virtues  8)

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    perovskite-containing catalytic converters are also highly active. A vehicle equipped with such a high activity perovskite autocatalyst achieved the J-ULEV (Japan Ultra Low Emissions Vehicle) emissions standard in 2002, demonstrating that pollutant levels more than 50% below those required by current legislation were measured. Another major advantage of perovskite autocatalysts is the reduced metal content compared with that of conventional autocatalysts of similar activity. Reductions of 70 to 90% have been reported possible (20), translating into potentially significant cost savings
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                                PGM Perovskites in Organic Synthesis

Reactions catalysed by pgms, such as cross-couplings and hydrogenations, are becoming increasingly prevalent in organic synthesis. Where the pgm-catalysed steps are part of the synthesis of active pharmaceutical intermediates (APIs), there are stringent limits on permissible metal contamination, driving a requirement to minimise the release of metals into the process (21). Conventional approaches to achieve this are to move from homogeneous to heterogeneous catalysts, or to ‘heterogenise’ catalysts via processes such as immobilisation (22) or microencapsulation (23).

This is another area where the stability, robustness and low pgm content of perovskite catalysts facilitate their application. The perovskite minerals which are air-stable powders are well suited to use in the chemical laboratory. Professor Steven Ley and coworkers at the University of Cambridge, U.K., have tested palladium-containing perovskites in organic transformations which are otherwise carried out with conventional palladium catalysts (24).

Use of perovskite catalysts in a standard Suzuki coupling, between an aryl bromide and aryl boronic acid, showed that the reaction was catalysed with similar rates by a wide range of perovskites containing 5 at.% palladium (25). It also demonstrated that palladium was an essential component for successful conversion, and that the oxidised form of the perovskites worked better under the reaction conditions. Working with the best-performing catalyst, LaFe0.57Co0.38Pd0.05O3, M. D. Smith et al. extended the perovskite-catalysed Suzuki reaction to encompass a wide range of different substrates, including aryl iodides and bromides, heteroaryl halides and aryl and alkenyl boronic acids (Scheme I) (26). The application of microwave heating also enabled coupling to aryl chlorides.
Scheme I

A selection of reactions catalysed by palladium perovskite (Conditions: LaFe0.57Co0.38Pd0.05O3 (0.05 mol% Pd), 3 eq. K2CO3, 1.5 eq. boronic acid, 1:1 IPA:H2O, 80°C) (26)

The use of a copper-palladium perovskite LaFe0.57Cu0.38Pd0.05O3 allowed the extension of perovskite-catalysed organic chemistry to the Sonogashira coupling reaction of aryl halides and acetylenes (27), again giving good yield across a range of aryl bromide and iodides.

There has been considerable work on ascertaining the mechanism by which perovskites function in organic reactions. The lower temperatures involved (typically 80°C) preclude the type of self-regeneration seen in autocatalysts. Investigation focused on whether the reaction proceeded via a homogeneous or heterogeneous mechanism, and evidence has been built up by several methods (25). Removal of the bulk catalyst by filtration at partial reaction, followed by returning the filtrate to the reaction conditions, showed that the reaction progressed to significantly higher conversions in the absence of the solid catalyst. This demonstrated that an active solution palladium species was formed, a conclusion supported by solution and solid-phase catalyst poisoning studies. Performance in a three-phase test, employing solution and solid-supported substrates, provides further evidence to support the hypothesis of an active solution species, but also demonstrated that an aryl halide must be present in the solution phase for the reaction to proceed.

Collation of the evidence (25) led to the proposed mechanism shown in Figure 3. The initial step is a reduction of the Pd(III) or other high-valent palladium species in the perovskite, possibly by the solvent, to form a surface-bound Pd(0) species. This is next taken into solution by oxidative addition to the aryl halide. The coupling reaction can now proceed through a fairly conventional solution catalytic cycle, at the end of which the palladium either remains in solution to continue the reaction or is readsorbed onto the perovskite surface.
Fig. 3

Proposed mechanism for catalytic activity of pgm perovskites in organic synthesis (25)

The release of highly active palladium into solution from the perovskite explains a very efficient catalytic turnover, with loadings of less than 0.05 mol% palladium sufficient. Recapture of the palladium by the perovskite at the end of the reaction cycle accounts for the extremely low residual palladium levels found in the crude reaction products. Palladium contents of less than 2 ppm were found in a Suzuki coupling product (26). The combined benefits of a highly efficient catalyst with low pgm content and very low levels of metal contamination make these attractive catalysts for chemical applications, especially synthesis of pharmaceutical and electronic materials, where exclusion of catalyst residues is essential. Furthermore, the perovskite catalysts have been shown to be recyclable (26), leading to even greater potential cost savings over catalysts which must be disposed of after a single use.
Conclusions

The pgm-containing perovskites constitute an active and expanding area of research. The potential and versatility of pgm-containing perovskites as catalysts is shown by the range of applications in which they have been tested – from catalytic combustion to organic synthesis. Their sturdy mineral structure and stability offer advantages wherever high temperatures are involved and in some cases, such as self-regenerating autocatalysts, give distinct benefits where other metal supports are deactivated over time. The high activity often associated with pgm perovskites, combined with the low loadings of pgms required, result in their offering significant potential savings in metal costs. In the organic chemistry laboratory, where the stable, easily handled pgm perovskites work as highly active and clean catalysts, a whole new application area may open in the near future, reinforcing their significance

    Volume 51
    Issue 2
    Apr 2007
    Pages 87-92

Back to 2007, Volume 51, Issue 2
Platinum Group Metal Perovskite Catalysts
PREPARATION AND APPLICATIONS

f1 ;)
« Last Edit: November 26, 2011, 12:13:41 PM by fresh1 »
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smellslikeindole

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Re: Pt group metal catalysts
« Reply #1 on: November 26, 2011, 01:37:53 PM »
There is a BIG problem with PGM catalyts. If the catalyst is heterogen than it is sometimes not as useful as we want it and it it's homogen than to get it out from the reaction and use it again that could be a really big problem.

And another thing is: they are really expensive and hard to make them.

Long ago I known a caterpillar who had to remove some N-benzyl groups with catalytic hydrogenation... 10% Pd/C was needed to this and the recipe for this catalyst was not as easy as it sounds. 12 hour refluxing the charcoal in 65% freshly distilled HNO3, 2 day drying under argon, adding the PdCl2 (reagent grade), reductiun with NaBH4, wash it and dry it for another 10 hour.
And the resulting catalyst could be used only once....

If the catalysts is homogen (dissolves in the reaction) e.g.: Wilkinson's catalyst and ect. it usually can't be regenerated in high yields after it was used or it is difficult to get it back.

For a home lab some Pd/C is really useful, but just in that case if the home chemist got some DMGL to recover the used Pd in high yields. If not, than 5-8g Pd will be soon reduced to 3-5g and so on... If homogen "modern" catalyst are wanted to use (for cross couplings and ect.) than the home chemist will discover some problems soon...

fresh1

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Re: Pt group metal catalysts
« Reply #2 on: November 27, 2011, 05:00:25 AM »
I take it you actually READ the article :-\

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There has been considerable work on ascertaining the mechanism by which perovskites function in organic reactions. The lower temperatures involved (typically 80°C) preclude the type of self-regeneration seen in autocatalysts. Investigation focused on whether the reaction proceeded via a homogeneous or heterogeneous mechanism, and evidence has been built up by several methods (25). Removal of the bulk catalyst by filtration at partial reaction, followed by returning the filtrate to the reaction conditions, showed that the reaction progressed to significantly higher conversions in the absence of the solid catalyst. This demonstrated that an active solution palladium species was formed, a conclusion supported by solution and solid-phase catalyst poisoning studies

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And another thing is: they are really expensive and hard to make them.

I believe diesel engine catalytic converters ARE available at auto wreckers.....ready made ;)
« Last Edit: November 27, 2011, 12:08:26 PM by fresh1 »
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akcom

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Re: Pt group metal catalysts
« Reply #3 on: November 27, 2011, 10:13:16 PM »
Pd/C is commercially available and easily obtainable for the underground chemist.  The issue with homogenous transition metal catalysis (for the stealthy chemist) is the ligands that are typically used can be a real PITA to make and ridiculously expensive to buy.  Thats what made the Fe catalyzed epoxidation I posted so great, the ligand was cheap and commercially available.

The issue with heterogenous TM catalysis is once again ligands, but this time you have the added issue of obtaining functionelized organic resins or silica gels.  Not exactly OTC.