Metal-based chemical catalysts have excellent green
chemistry credentials—in principle at least. In theory, catalysts are reusable
because they drive chemical reactions without being consumed. In reality,
however, recovering all of a catalyst at the end of a reaction is difficult, so
it is gradually lost. Now, chemists can retain, retrieve, and reuse metal
catalysts by trapping them with a polymer matrix, thanks to recent work by
Yoichi Yamada at the RIKEN Advanced Science Institute, Wako, Yasuhiro Uozumi at
RIKEN and Japan’s Institute for Molecular Science and Shaheen Sarkar, also at RIKEN.
Attaching metal catalysts to an insoluble polymer support,
which is recoverable at the end of a reaction by simple filtration, is far from
a new idea. Traditionally, chemists attached their metal catalyst to an
insoluble polymer resin. However, the metal invariably leached out of the
polymer over time so the catalysts were still slowly lost.
Yamada and his colleagues’ approach, in contrast, integrated
the metal into the polymer matrix, which trapped it much more effectively. The
researchers achieved this level of integration by starting with a soluble
polymer precursor instead of an insoluble resin. This material contains
imidazole units, a chemical structure known to bind strongly to metals such as
palladium. An insoluble composite material formed only after the researchers
added palladium to the mixture because it causes the imidazole units to
self-assemble around atoms of the metal—a process that they call ‘molecular
convolution’.
Scanning electron microscopy revealed that the resulting
polymer–palladium globules ranged from 100 to 1,000 nm in diameter, which
aggregated into a highly porous structure reminiscent of a tiny bathroom
sponge. “This sponge-like insoluble material can easily capture substrates and
reactants from the solution, which readily react with metal species embedded in
the sponge,” says Yamada.
The researchers showed that the catalyst is highly active as
well as reusable; it is the most active catalyst yet reported for a carbon–carbon
bond-forming reaction known as an allylic arylation. They also reused the catalyst
multiple times with no apparent loss of activity, and detected no leaching of
palladium from the polymer into the reaction mixture.
Yamada and colleagues are now developing a range of
composite catalysts incorporating different metals that can catalyze many other
kinds of reactions. “These extremely highly active and reusable catalysts will
provide a safe and highly efficient chemical process, which we hope will be
adopted for industrial chemical process,” Yamada says.
SOURCE – RIKEN
