Water as a catalytic switch in the oxidation of aryl alcohols by polymer incarcerated rhodium nanoparticles

Marco Conte journal cover Catalysis Science and Technology 2017

A research paper by Dr Marco Conte into the catalytic activity of a rhodium polymer incarcerated catalyst, has been selected as the front cover for Issue 18 of the journal Catalysis Science & Technology includes contributions from two level 4 undergraduate students.

What started as an aside from a level 4 MChem research project, working with Dr Marco Conte, back in 2015 by Jack Weston has now become an international collaborative research project between the universities of Sheffield, Tokyo, Newcastle and Bradford. The research paper, which also features the contributions from other level 4 undergraduate Chloe Baker (MChem 2017), can be read here:
Weston. J., Miryamura. H. Yasukawa. T., Sutarma. D., Baker. C., Singh. P., Bravo-Sanchez. M., Sano. N., Cumpson. P. Ryabenkova. Y., Kobayashi. S, Conte. M. Catalysis Science and Technology, 2017, 7, 3985-3998.

Research into oxidation and reduction catalysis is of great commercial and industrial benefit, with ketones in particular being a key component in the food and drug industries. Research in this field also seeks to synthesise chemicals in the most environmentally friendly method possible, usually by avoiding the extensive use of toxic materials. To this end, the use of molecular oxygen is desired to allow for an efficient, sustainable method of oxidation from alcohols to ketones. This journal article follows the catalytic activity of rhodium nanoparticles embedded within a polymer matrix catalyst, referred to as a rhodium-polymer incarcerated catalysts (Rh-PI).

Rhodium is a metal mostly associated with the catalytic reduction of nitric oxide in automotive exhaust systems and its use for oxidation reactions is nearly unknown. In this work it was possible to convert rhodium nanoparticles into a powerful catalyst for the oxidation of aryl alcohols by the addition of water into a toluene containing reaction media. This procedure induced a phase transfer catalysis process and showed rhodium nanoparticles could be a competitor for oxidation reactions compared to more known transition metal nanoparticles comprising palladium, platinum or gold. Most importantly, this was achieved by extremely mild reaction conditions such as atmospheric pressure and by using molecular oxygen from air as an oxidant.

Schematic of the mechanism for the oxidation of an alcohol to ketone using rhodium catalysisThe Rh-PI catalyst could be activated through the addition of water leading to a hypothesised phase transfer catalysis method. This method involves water extracting the alcohol from the toluene droplets, promoting the formation of a soluble alkoxide. The oxidation over the Rh-PI catalyst is now more favourable yielding the desired acetophenone, which, is immiscible in water and is thus extracted by the toluene (see scheme).

It was also observed that the catalytic switch effect induced by water to polymer incarcerated catalysts could be extended to supports like carbon and alumina, thus showing this phenomenon might be more general and to have vast implications beyond the current study and provide an alternative explanation on catalytic trends with time for many other nanoparticle containing systems. This will ultimately lead to a further understanding on the utility of water as a reaction solvent, one that so far has been neglected due to its ubiquitous presence in virtually any catalytic reaction. Nevertheless, we noticed a strong difference in activity between aryl and aliphatic alcohols in this process, providing seminal ground for additional investigations in this important research area.

Research of this kind highlights two of the greatest components of scientific research. What first seemed like an unexpected phenomenon in a level 4 research project has now led to the publication of an impactful research paper on the wide used of molecular oxygen and rhodium catalysis. It secondly has helped forge or strengthen collaborations between institutions working together to advance understanding and knowledge of supported catalysts.