Styrene is, as we all know, a huge commercial chemical with a production of >18 million tons per annum. Made from benzene via Friedel-Crafts to ethylbenzene and subsequent dehydrogenation it is a process which is lacking in selectivity and yield but big on energy costs. This may be set to change according to a paper in Science last week reporting a new rhodium catalyst which produces styrene from benzene and ethylene in >95% yield and 100% selectivity in the presence of a copper(II) salt.
This catalyst is made as follows and has structure 1 in the scheme:
and the authors present a wonderful catalytic cycle for the proposed mechanism. So you use 20 mL of benzene, 0.001 mol% of 1, relative to benzene, ethylene and copper acetate (120 equivalents based on 1) at 150°C for 24 hours and out pops styrene. No doubt, this aryl C-H activation is very efficient. Quote “For a reaction using 0.0001 mol % 1 and 2400 equivalents of Cu(OAc)2, the catalyst remained active over a period of 96 hours and afforded a TON of 817 to 852.” Great stuff.
Somehow I doubt that this will be a viable replacement for the current process anytime soon (which is the inference of this work in progress). Imagine a 18 million ton batch reactor, even at 0.0001% this would need around 11700 tons of catalyst (just based on the amount of styrene), with a Rh content of 15.6% approx. 2 tons of Rh which, at todays prices, would cost $65 million. This is not even including the price of catalyst preparation, which uses silver! Throw in 2400 equivalents of a copper salt which need re-oxidation and what do you have? A mess. No replacement for the current manufacturing process. Why Rh, Ag and Cu? What’s wrong with Fe or Al, they probably don’t work well if at all, but they are cheaper.
The authors don’t declare any competing financial conflicts, I’m not surprised. Even if I am out by a factor 100 it’s still very expensive. So buy your styrene for $1800/ton.
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