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In the oxidation of propylene (propene, CH3–CH=CH2) to propylene oxide (propene oxide), an oxygen atom is formally inserted into the double bond. This forms a ring containing two carbon atoms and one oxygen atom. Using oxygen as the oxidizing agent had not been considered before because the oxygen molecule (O2) can only be split into individual oxygen atoms with the input of a large amount of energy. Furthermore, propylene preferentially reacts with atomic oxygen to form acrolein and not the desired propylene oxide. A suitable catalyst is eagerly sought, and has come to be viewed as the “holy grail” of catalyst research. There have been a number of catalytic developments that have been not quite satisfactory.
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Size matters: Gold clusters
(<2.0 nm), but not gold nanoparticles, deposited on
alkaline-treated titanosilicalite-1 allow O2 and H2O
to react to give hydroperoxides (-OOH). These transfer to
neighboring Ti sites to form Ti-OOH (see scheme), which is
responsible for propene epoxidation to give propene epoxide
(PO).
[Image credit: Angewandte
Chemie International Edition (1)] |
Building upon prior work, Haruta and his team have been able to achieve a further step. Their new catalyst consists of gold clusters, which are less than 2 nm in size, deposited on a special titanium-containing silicalite support. “It is important that the gold used is not in the form of nanoparticles, but is in clusters,” emphasizes Haruta. Although these two terms are often used interchangeably in the literature, there are important differences. Gold clusters are explicitly defined, structurally uniform nanoscopic structures, whereas gold nanoparticles are particles with size in the nanometer range that have neither uniform size nor structure. “Our gold clusters are able to convert oxygen and water into hydroperoxide species, which are transferred onto neighboring titanium centers,” explains Haruta. “The resulting titanium hydroperoxide species (Ti–OOH) are the actual reaction partners for the propylene, which is converted to propylene oxide.” “The yields and selectivities we have achieved so far are inadequate for an industrial process,” says Haruta, “however, our catalyst is another important milestone on the way to an environmentally friendly synthesis for propylene oxide.”
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