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This iron-based organometallic structure decorated with peroxo groups (red and green) can capture ethane (blue molecules) while allowing ethylene (peach molecules) to pass through, providing a more efficient and cost-effective way to purify water. ethylene, the most important raw material for plastic production. Credit: Wei Zhou / NIST
An innovative filtration material could soon reduce the environmental cost of plastic manufacturing. Created by a team made up of scientists from the National Institute of Standards and Technology (NIST), this breakthrough can extract the key ingredient from the most common plastic form from a mixture other chemicals, while consuming a lot less energy than usual.
The material is an organometallic framework (MOF), a class of substances that has repeatedly demonstrated its talent for separating individual hydrocarbons from the soup of organic molecules produced by petroleum refining processes. MOFs are invaluable to the plastics and petroleum industries because of this capability, which could allow manufacturers to perform these separations much more economically than conventional oil refining techniques.
This promise has made MOFs a subject of intense study at NIST and elsewhere, leading to MOFs capable of separating different octanes of gasoline and accelerating complex chemical reactions. However, a major goal has proven difficult to achieve: a method of spinning ethylene preferred in the industry – the molecule needed to create polyethylene, the plastic used to make plastics. shopping bags and other containers everyday.
However, in today's issue of the journal Science, the research team reveals that a modification made to a well-studied MOF allows it to separate purified ethylene from a mixture with ethane. The team's creation, carried out at the University of Texas at San Antonio (UTSA) and at the Taiyuan University of Technology in China and studied at the Center for Neutron Research (NCNR) of the NIST, represents a major breakthrough for the sector.
Making plastic takes a lot of energy. Polyethylene, the most common type of plastic, is made from ethylene, one of the many hydrocarbon molecules found in crude oil refining. Ethylene must be highly purified for the manufacturing process to work, but current industrial technology to separate ethylene from all other hydrocarbons is a cold but energetic process that cools the crude to over 100 degrees Celsius.
Ethylene and ethane make up the bulk of the mixture 's hydrocarbons, and separating these two is by far the most energy – consuming step. Finding another method of separation would reduce the energy needed to produce 170 million tonnes of ethylene produced worldwide each year.
Scientists have been looking for such an alternative method for years and MOFs look promising. At the microscopic level, they look a little like a skyscraper half built of beams and no walls. The beams have surfaces on which certain hydrocarbon molecules will adhere firmly. Thus, pouring a mixture of two hydrocarbons through the appropriate MOF can extract one type of molecule from the mixture, thus allowing the other hydrocarbon to emerge in the pure state.
The trick is to create an MOF that passes ethylene. For the plastics industry, this is the point of friction.
"It's very difficult to do," said Wei Zhou, a scientist at NCNR. "Most of the MOFs studied are based on ethylene rather than ethane, and some have even demonstrated excellent separation.
performance, selectively adsorbing ethylene. But from an industrial point of view, you would prefer to do the opposite if possible. You want to adsorb the by-product ethane and pass ethylene. "
The research team has spent years trying to solve the problem. In 2012, another team of researchers working for the NCNR found that a particular framework called MOF-74 allowed for the separation of various hydrocarbons, including ethylene. It seemed like a good starting point and the team members went through the scientific literature to find inspiration. An idea from biochemistry finally sent them in the right direction.
"An important subject in chemistry is finding ways to break the strong bond that is formed between carbon and hydrogen," said UTSA University professor, Banglin Chen, who headed the company. team. "This allows you to create a lot of new and valuable materials, and we have discovered previous research that showed that compounds containing iron peroxide can break that link."
The team felt that to break the bond in a hydrocarbon molecule, the compound should first attract the molecule. When they modified the walls of the MOF-74 so that it contained a structure similar to that of the compound, it turned out that the molecule attracted by their mixture was ethane.
The team brought the MOF to the NCNR to explore its atomic structure. With the help of a technique called neutron diffraction, they determined which part of the surface of the MOF attracted ethane – an essential piece of information to explain the success of their innovation where others efforts were in vain.
"Without the basic understanding of the mechanism, no one could believe our results," said Chen. "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 are very enthusiastic. "
While Zhou said that the team's modified MOF was working effectively, it may require some further development for action to be taken at a refinery.
"We have proven that this path is promising," said Zhou, "but we do not claim that our materials work so well that they can not be improved. Our goal in the future is to significantly increase their selectivity. It is worth going further. "
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More information:
L. Li el al., "Ethane / ethylene separation in an organometallic framework with iron-peroxo sites," Science (2018). science.sciencemag.org/cgi/doi… 1126 / science.aat0586
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