University of Texas at San Antonio (UTSA) (San Antonio, Texas, USA) researchers, along with scientists at the National Institute of Standards and Technology (NIST) (Gaithersburg, Maryland, USA) and the Taiyuan University of Technology (Taiyuan, China), have discovered a filtering material that may reduce the environmental costs of manufacturing plastic.
The scientific advance can extract polyethylene’s key ingredient from a mixture of other chemicals. The material is a metal-organic framework (MOF), a class of substances often used to separate hydrocarbons from the organic molecules produced during oil refining. MOFs are valuable because of this capability, which could enable cheaper separations than standard refinement techniques.
That has made MOFs the subject of intense study, leading to MOFs that can separate different octanes of gasoline and speed up chemical reactions. However, one major obstacle has been how to extract ethylene—the molecule used to create polyethylene. In their research, the team shows that a modification enables the separation of purified ethylene out of an ethane mixture.
According to the researchers, the current technology for separating ethylene is a high-energy process that cools down crude to below -100 °C. Ethylene and ethane constitute most of the mixture’s hydrocarbons, and separating them is the most energy-intensive step.
Scientists have long searched for an alternative, the researchers say, and MOFs have appeared promising. On a microscopic level, they resemble a skeleton of a skyscraper of girders and no walls. The girders have surfaces that certain molecules will stick to firmly, so pouring a mixture of two hydrocarbons through the right MOF can pull one molecule out, letting the other emerge in pure form.
A turning point came in 2012, when the creation of the MOF-74 became a useful filter for separating hydrocarbons, including ethylene. The research collaboration analyzed previous approaches and also took an idea from biochemistry.
“A huge topic in chemistry is finding ways to break the strong bond that forms between carbon and hydrogen,” says UTSA Professor Banglin Chen, who led the researchers. “Doing that allows you to create a lot of valuable new materials. We found previous research that showed that compounds containing iron peroxide could break that bond.”
The team reasoned that to break the bond, the compound would have to attract the molecule in the first place. When they modified MOF-74’s walls to contain a structure like the compound, the molecule it attracted was ethane. Using neutron diffraction, they determined what part of the MOF’s surface attracts ethane.
“Without the fundamental understanding of the mechanism, no one would believe our results,” Chen says. “We also think that we can try to add other small groups to the surface, maybe do other things. It’s a whole new research direction, and we’re very excited.”
Source: University of Texas at San Antonio, www.utsa.edu.