By tweaking the structure of a class of increasingly popular chemical catalysts known as metallocenes, chemists at the University of Rochester have uncovered a much simpler way to make the material that forms the basis of a wide range of consumer goods, including soaps, detergents, oils, and plastics. If the procedure can be scaled up for industrial use -- a question the research team is now investigating -- goods made out of billions of tons of plastics and petroleum-based products should become less expensive and safer to produce. The findings are discussed in the October 1 issue of the Journal of the American Chemical Society.
"This is a spectacularly interesting finding," says Rick Kemp, a senior research scientist at the Union Carbide Corporation and an expert in the class of materials known as alpha-olefins, which form the chemical backbone of many consumer products. "The new catalyst appears to yield products that are virtually 100 percent pure, a trait that's increasingly desirable in industry."
Scientists making alpha-olefins typically need temperatures of 400 to 500 degrees Fahrenheit and pressures of 100 to 200 atmospheres. That's because they use aluminum or nickel catalysts, which require extreme pressures and temperatures to work.
"It's costly to attain these conditions and build the reactors needed to make alpha-olefins with aluminum or nickel catalysts," says Guillermo Bazan, associate professor of chemistry and primary author of the JACS article.
Bazan's modified metallocene catalyst, bis(ethoxyboratabenzene) zirconium dichloride (BEZD), is capable of churning out alpha-olefins at only one atmosphere of pressure and temperatures just slightly above room temperature. BEZD strings ethylene molecules end-to-end to form alpha-olefins just as quickly as traditional aluminum and nickel catalysts, Bazan says. It also gives scientists precise control over just how long the chains grow. Under varying pressure, BEZD can produce carbon chains ranging from ethylene dimers, with just four carbons atoms, all the way up to full-fledged polymers containing many thousands.
"There are a million and one uses for alpha-olefins," Kemp says. "In addition to serving as precursors for detergents, synthetic lubricants, and octane enhancers, they're used to produce a significant fraction of the 150 billion pounds of polyethylene and polypropylene produced each year -- plastics found in products ranging from ice cube trays to textiles to bottle caps to trash bags."
To make the new catalyst, Bazan put a new spin on metallocenes, a class of catalysts currently taking the world of plastics by storm. Scientists have known for more than 40 years that these materials have potent catalytic properties, but it's only recently that the plastics industry has begun to take advantage of them to create polymers.
The catalyst molecule Bazan created bears a striking structural and electronic resemblance to metallocenes, which typically include two five-carbon rings bracketing a single atom of the transition metal zirconium. Bazan's molecule features six- membered rings containing five carbons and an added boron atom to regulate zirconium's reactivity. But the molecules that grow in the 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 in the Netherlands has shown limited success making alpha-olefins using metallocene-like catalysts, Kemp believes Bazan's approach is more sophisticated and leads to far better products.
By working with chemical companies, Bazan hopes to determine within the next year whether BEZD is an industrially feasible means of producing alpha-olefins.
Graduate students Jonathan Rogers and Caroline Sperry joined Bazan in the research, which was funded by the Alfred Sloan Foundation and the Henry and Camille Dreyfus Foundation.