Iron is a junkyard dog among metals-scrappy and hard working. But when it comes to the delicate task of acting as a catalyst in joining together the molecular pieces that make up plastics, chemists have long favored purebreds: exotic metals, such as zirconium. Now, two separate teams of chemists, one at Imperial College and BP Chemicals in London, and the other at the University of North Carolina (UNC) and DuPont, have made iron-based catalysts that excel at making plastics, particularly polyethylene.
The advance could mean a simpler and cheaper way to make common plastics. “They’re really good catalysts in making polyethylene very rapidly,” says Richard Schrock, a catalyst chemist at MIT. What’s more, says Schrock, the catalysts are intriguing because it remains a mystery precisely how and why the iron works.
The chemists transformed iron into a catalyst by attaching to it nitrogen-containing groups. These groups wrap around the iron, but at the same time leave part of the iron exposed; this vacant spot allows monomers (the building blocks of polymers) access to the iron. When they made the structure, the researchers found to their surprise that the iron has a talent for stitching together the monomers.
Vernon Gibson, a chemist who headed the effort at Imperial College, says when he began the research “we didn’t necessarily expect it to work.” Gibson says that chemists often have slighted iron when looking for new polymer catalysts. “It was thought that there was no point in looking at iron. This opens up a new part of the periodic table.”
To plastic makers, that new terrain looks a lot like a gold mine. Producing polyethylene is a multibillion-dollar business and being able to make it for even a few cents per pound cheaper can be a tremendous advantage. What’s more, says Bill Tallis, director of technology at BP Chemicals, the iron catalysts could make plastics with improved properties. “We’ve seen enough to be very encouraged,” he says. Encouraged enough that BP expects to test large-scale production of polyethylene using the iron catalysts later this year.
The current work is part of a large-scale effort to find more efficient and versatile catalysts for making plastics. A first generation of new catalysts began hitting the market in the mid-1990s. A second generation includes nickel and palladium catalysts discovered by UNC and patented by DuPont.
Will the iron catalysts emerge as yet another commercial breakthrough? It’s too early to tell. Indeed, it’s still unclear who has rights to the technology. The catalysts developed by the two groups are virtually identical, so it could come down to who first claimed them. “It will be very close,” says BP’s Tallis.