By tweaking the structure of a class of increasingly popularchemical catalysts known as metallocenes, chemists at theUniversity of Rochester have uncovered a much simpler way to makethe material that forms the basis of a wide range of consumergoods, including soaps, detergents, oils, and plastics. If theprocedure can be scaled up for industrial use -- a question theresearch team is now investigating -- goods made out of billionsof tons of plastics and petroleum-based products should becomeless expensive and safer to produce. The findings are discussedin the October 1 issue of the Journal of the AmericanChemical Society.
"This is a spectacularly interesting finding," says RickKemp, a senior research scientist at the Union CarbideCorporation and an expert in the class of materials known asalpha-olefins, which form the chemical backbone of many consumerproducts. "The new catalyst appears to yield products that arevirtually 100 percent pure, a trait that's increasingly desirablein industry."
Scientists making alpha-olefins typically need temperaturesof 400 to 500 degrees Fahrenheit and pressures of 100 to 200atmospheres. That's because they use aluminum or nickelcatalysts, which require extreme pressures and temperatures towork.
"It's costly to attain these conditions and build thereactors needed to make alpha-olefins with aluminum or nickelcatalysts," says Guillermo Bazan, associate professor ofchemistry and primary author of the JACS article.
Bazan's modified metallocene catalyst,bis(ethoxyboratabenzene) zirconium dichloride (BEZD), is capableof churning out alpha-olefins at only one atmosphere of pressureand temperatures just slightly above room temperature. BEZDstrings ethylene molecules end-to-end to form alpha-olefins justas quickly as traditional aluminum and nickel catalysts, Bazansays. It also gives scientists precise control over just how longthe chains grow. Under varying pressure, BEZD can produce carbonchains ranging from ethylene dimers, with just four carbonsatoms, all the way up to full-fledged polymers containing manythousands.
"There are a million and one uses for alpha-olefins," Kempsays. "In addition to serving as precursors for detergents,synthetic lubricants, and octane enhancers, they're used toproduce a significant fraction of the 150 billion pounds ofpolyethylene and polypropylene produced each year -- plasticsfound in products ranging from ice cube trays to textiles tobottle caps to trash bags."
To make the new catalyst, Bazan put a new spin onmetallocenes, a class of catalysts currently taking the world ofplastics by storm. Scientists have known for more than 40 yearsthat these materials have potent catalytic properties, but it'sonly recently that the plastics industry has begun to takeadvantage of them to create polymers.
The catalyst molecule Bazan created bears a strikingstructural and electronic resemblance to metallocenes, whichtypically include two five-carbon rings bracketing a single atomof the transition metal zirconium. Bazan's molecule features six-membered rings containing five carbons and an added boron atom toregulate zirconium's reactivity. But the molecules that grow inthe presence of the two catalysts are dramatically different.While traditional metallocenes yield long polymers of ethylene,BEZD leads to alpha-olefins, which are much shorter, versatile,and easily modified organic chains. While research by Shell inthe Netherlands has shown limited success making alpha-olefinsusing metallocene-like catalysts, Kemp believes Bazan's approachis more sophisticated and leads to far better products.
By working with chemical companies, Bazan hopes to determinewithin the next year whether BEZD is an industrially feasiblemeans of producing alpha-olefins.
Graduate students Jonathan Rogers and Caroline Sperry joinedBazan in the research, which was funded by the Alfred SloanFoundation and the Henry and Camille Dreyfus Foundation.
Materials provided by University of Rochester. Note: Content may be edited for style and length.
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