When we fabricated the Pd@MOF catalyst (Pd nanoparticle) and test it in Suzuki reaction, it was active but during reaction the color of media goes to red which it is not active in recycles. (It is big challenge in our group when we use MOFs)
It all depends on the MOF that you are using as support and on its stability to the reaction medium. For instance, some MOF will not be stable under the basic conditions required for the Suzuki coupling reaction (inroganic base such as K2CO3 is commonly used). Some materials may lose crystallinity under the basic conditions. It also dependends on the solvent used. For instance, DMF is known to icnrease Pd leaching. It would be better to use o-xylene or toluene if possible to minimize leaching. Alternatively, you can increase the stabilization of the Pd nanoparticles inside the MOFs by introducing chelating groups, such as amines. You can found an example of this is the work by Hwang et al (Angew. Chem. Int. Ed., 2008, 47, 4144.
Pd leaching from nanoparticulate form is a major issue that has yet to be fully resolved. The degree of leaching is strongly senstive to the Pd nanoparticle size, support material, and most importantly reaction media and conditions. Lower reaction temperatures and low concentrations of base will help suppress leaching.
Are your Pd particles prepared by colloidal sol routes and stabilised by e.g. PVP/PVA polymers? The use of polymeric stabilisers can again slow leaching.
I would refer you to my articles below for more discussion:
Evidence for the Surface Catalysed Suzuki-Miyaura Reaction over Pd Nanoparticles: an Operando XAS Study. P.J. Ellis, I.J.S. Fairlamb, S.F.J. Hackett, K. Wilson, A.F. Lee - Angew. Chem. Int. Ed. 2010, 49,1820 doi:10.1002/anie.200906675.
Surface catalysed Suzuki-Miyaura cross-coupling by Pd nanoparticles: an operando XAS study. A.F. Lee, P.J. Ellis, I.J.S. Fairlamb, Karen Wilson - Dalton Transactions 2010, 39, 10473 doi:10.1039/c0dt00412j
You can try to maximize the bond between Pd and the MOF. Try with a phosphine MOF like those published by Humphrey (called PCM) or our LSK-1 (ASAP chemcatchem)
I am complety agree with Dr. Greiner, first of all you should know what is responsible for the catalytic actitivy, in most of the cases is the metal in solution since Suzuki coupling could be run with really small amounts of palladium.
The best way to minimize the leaching as other people already suggested should be to funzionalize the MOFs with amine or phosphine groups or other gruopus that could interact with the palladium center but taking into account that the metal center should be remain enough active to act as catalyst in the Suzuki coupling.
Depending on you reaction conditions, other alternatives could be the choice of solvent ( maybe one in which palladium is not soluble) or just try to decerase the reaction temperature.
Palladium leaching is always an issue in cross-coupling reactions with nanoparticles, because Pd(II) is the actual active species, so the Pd(0) nano particle acts mostly as a palladium reservoir, and the real catalysis takes actually place in solution. This said, this fact would not explain the color change and the fast deactivation you describe, so I agree with the comments of other colleagues that loss of integrity of the MOF may be in the origin of catalyst deactivation.
The best specialist in this field is prof. Alexander Schmidt.
O.Temkin
Leaching is a major problem in the liquid phase reactions. However this could be reduced by using support material. So first of all you should know in which conditions there will be strong interaction/bonding between the metal and support.
I think Pd have more affinity towards the halogen (Cl, Br) which may leads to make a possible bond between palladium and halogen which leads to metal leaching in Suzuki coupling reaction.
- Badges
- Science topic
- Similar topics
- Biological Science
- Anatomy
- Tissue