Anew material was developed that can extract the key ingredient in the most common form of plastic from a mixture of other chemicals while consuming far less energy than usual. The material is a metal-organic framework (MOF), a class of substances that has repeatedly demonstrated a talent for separating individual hydrocarbons from organic molecules produced by oil-refining processes. MOFs hold immense value for the plastic and petroleum industries because of this capability, which could allow manufacturers to perform these separations far more cheaply than standard oil-refinement techniques.
A modification to a well-studied MOF enables it to separate purified ethylene out of a mixture with ethane. Polyethylene — the most common type of plastic, and the one used to make shopping bags and everyday containers — is built from ethylene, one of the many hydrocarbon molecules found in crude-oil-refining. The ethylene must be highly purified for the manufacturing process to work, but the current industrial technology for separating ethylene from all the other hydrocarbons is a high-energy process that cools down the crude to more than -100 °C.
Ethylene and ethane constitute the bulk of the hydrocarbons in the mixture and separating these two is by far the most energy-intensive step. Finding an alternative method of separation would reduce the energy needed to make the 170 million tons of ethylene manufactured worldwide each year.
On a microscopic level, MOFs look a bit like a half-built skyscraper of girders and no walls. The girders have surfaces that certain hydrocarbon molecules will stick to firmly, so pouring a mixture of two hydrocarbons through the right MOF can pull one kind of molecule out of the mix, letting the other hydrocarbon emerge in pure form. The trick is to create a MOF that allows the ethylene to pass through.
The researchers discovered that to break the bond in a hydrocarbon molecule, the compound would have to attract the molecule in the first place. When they modified MOF-74’s walls to contain a structure similar to the compound, it turned out the molecule it attracted from their mixture was ethane. They used a technique called neutron diffraction to determine what part of the MOF’s surface attracts ethane — a key piece of information.