| RESEARCH |
In the petrochemical industry, separating olefins from refinery mixtures is an energy-intensive, costly process achieved through distillation. However, there is a potentially cheaper process--a redox switch--on the far horizon. AUTHORS, have shown that nickel dithiolene complexes such as Ni[S2C2(CF3)2]2 form 1:1 adducts with simple aliphatic olefins when in the oxidized form and release the bound olefin when reduced. Rapid on/off switching of the complex from one oxidation state to another by electrochemical means allows binding and then release of the olefin, which can then be collected. The problem with most metal-based olefin-separating systems proposed to date is their short life span. Metal "poisons" in the refinery stream--such as hydrogen, carbon monoxide, acetylene, water, and hydrogen sulfide--attack and destroy the reactive metal center. In the nickel dithiolene complexes, however, the reactive centers are the sulfur atoms, which do not appear to react with the poisons. The reactivity of the sulfur atoms also leads to an unusual mode of olefin binding: Instead of the olefin attaching directly to the metal, it bridges the sulfur atoms on opposite ligands, forming a ring. This binding mode had been shown before with reactive olefins, such as norbornadiene, which are easier to trap than simple aliphatic olefins. "We just tried it with simple olefins, and we were surprised. It wasn't obvious that it would work with ethylene and propylene," AUTHORS tells. Furthermore, dithiolene compounds are known to undergo multiple one-electron redox reactions. "I have always been a redox-type person," AUTHORS tells. "I think in terms of different oxidation states and what the reactivities might be." With the nickel dithiolene complexes, it turns out, the difference in reactivities due to redox status allows an on/off switch for binding olefins. "The great thing about the dithiolenes is that they have a range of stable, accessible oxidation states," AUTHORS adds. "I think that this accessibility might be exploitable from the point of view of new chemistry, which I think is just starting." |
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