In this recently-published paper, the research team has immobilized a rhodium
complex on a solid support, and the resulting catalyst has proved to be an
efficient hydroformylation catalyst.
The researchers have designed and synthesized a novel
catalyst for hydroformylation, which involved the immobilization of air-stable
rhodium nanoparticles on a magnetic support, which was in turn functionalized
with intensely branched polymer that contained phosphine groups at the
terminal. The researchers have shown that the rhodium phosphine catalyst could
be recycled for further runs of catalytic reactions, and the rhodium complex
would not be decomposed even if the catalyst recovery process was carried out
without using an inert atmosphere, and the leaching of rhodium metal was negligible.
These attributes all represent some of the golden standards of a good recyclable
catalyst, in particular in biphasic systems.
The catalyst is modular in nature. Because the rhodium metal
is incorporated into the system, let me show you the synthesis of the ligand
architecture. The iron oxide core and the hyper-branched polymeric section –
‘HYP’ (which contains the key glycine residue), was separated by a long spacer.
The phosphorus center is attached onto this architecture through the use of Ph2PH
and paraformaldehyde, resulting in the formation of 2 diphenylphosphine moieties
on each glycine residue. The rhodium nanoparticles were then immobilized onto
the ligand structure. ICP-OES technique and also transmission electron
microscopy have been employed to characterize the resulting phosphine-rhodium
complex.
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| Synthesis of the rhodium hydroformylation catalyst. |
The catalyst, designated as Fe3O4@SiO2-HYP-N(CH2PPh2)2Rh
(I would abbreviate it as ‘FeRh’ from now on), was tested for hydroformylation
reactions. A number of alkene substrates were tested, and some of them were
natural-occurring terpenes. Since the compound estragole was the most reactive
for the hydroformylation, the researchers chose this as the model compound for
their further investigations on catalyst stability and re-usability.
 |
| Hydroformylation of estragole as a model reaction. |
The researchers have found that, when the new Fe/Rh catalyst
was used to perform hydroformylation on estragole, the catalyst loading was
lower and the conversion was far higher than their last-generation catalyst,
which did not possess the hyperbranched polymeric section (HYP). The
researchers proposed that the inclusion of the polymeric structure could open
up more active sites for the phosphine-rhodium moieties on the catalyst
surface, hence the improvements in the catalyst performance.
The recyclability and re-usability were also investigated.
Because the iron nanoparticle could be attracted to a permanent magnet, the
Fe/Rh catalyst was thus recovered from the reaction mixture by magnetic
separation. The catalyst was re-used for 5 more times, using any compromise in
terms of activity, conversion or selectivity, suggesting that the catalyst was
not ‘dead’ (or decomposed). ICP-OES data also showed no significant rhodium
leaching, and the Fe/Rh catalyst was observed to be stable in air.
They have obtained Raman spectra to show that the
diphenylphosphine has been grafted more efficiently on the iron-polymeric
backbone of the ligand. Yet, they also cautioned that further investigations
were needed because the diphenylphosphine-based ligand, while so far proved
air-stable, they could not rule out a likely possibly that partial oxidation
could take place on the phosphorus center. Indeed, I feel it makes total sense.
Because of the reaction methodology (i.e. HPPh2 / paraformaldehyde),
there would result in the formation of a di-phenylphosphine, and the alkyl CH2
group could render the phosphine more sensitive to atmospheric oxygen, which is
not the case for the indefinitely air-stable triphenylphosphine.
One final thing that fascinates me is the synthesis of
rhodium nanoparticles, as I never know how it can be done. The rhodium source
is the red-colored rhodium(III) chloride hydrate (RhCl3.xH2O), for
which I have used before to prepare some [Rh(cod)Cl]2 for other
rhodium complexes. The RhCl3 and tetraoctylammonium bromide (TOAB) were
combined in a 2-phase system, and then NaBH4 solution was added to
the reaction mixture. The organic phase, which was black in color, contained
the Rh-TOAB nano-particles.
Wonderful work!
by Ed Law
4/7/2016
Reference:
1. Support Functionalization with a Phosphine-Containing Hyperbranched
Polymer: A Strategy to Enhance Phosphine Grafting and Metal Loading in a
Hydroformylation Catalyst
Marco A. S. Garcia, Rodrigo S. Heyder, Kelley C. B.
Oliveira, Jean C. S. Costa,
Paola Corio, Elena V. Gusevskaya, Eduardo N. dos Santos , Reinaldo C.
Bazito and
Liane M. Rossi, ChemCatChem,2016, 8, 1951 – 1960.
