- New metal-organic framework can efficiently separate ethane and ethylene to manufacture plastic (polyethylene).
- The process runs at room temperatures and requires much less energy.
Generally, manufacturing plastics requires enormous amounts of energy. The most common form of plastic, polyethylene, is made from ethylene – a hydrocarbon molecule found in crude oil refining. To make the manufacturing process work perfectly, the ethylene must be in its purest form.
With over 170 million tons of global production capacity (in 2016), Ethylene is the largest feedstock in petrochemical industries. Typically, it’s produced by thermal decomposition or steam cracking of Ethane.
However, the existing techniques for isolating ethylene from other hydrocarbon work only in temperatures below 100°C, and cooling down the crude to such low temperatures needs lots of energy. Also, this high-pressure distillation process requires large distillation columns with 120-180 trays and high reflux ratios (due to similar sizes and volatilities of ethylene and ethane).
A new filtering material could solve this problem and decrease the cost of manufacturing plastic. A research team at the National Institute of Standards and Technology has built a material that extracts the key substance in the most common type of plastic from a chemical mixture, using far less energy than well-established industrial separation methodologies.
The new material they have developed is called a metal-organic framework (MOF). It is known for isolating hydrocarbons from a mixture of organic molecules generated in oil refining processes. These frameworks are extremely valuable to petroleum and plastic industries due to their capabilities of separating key substances in a cost-effective manner.
MOFs can isolate a variety of gasoline octanes and boost the speed of chemical reactions. To make polyethylene (used in containers and shopping bags), it’s necessary to wring out ethylene. Most of the hydrocarbons in the mixture consists of ethane and ethylene, and isolating these two molecules is one of the most difficult and energy-intensive processes.
Credit: N. Hanacek/NIST
Researchers have been trying to develop an efficient technique for years, and this new method looks promising. On a microscopic scale, MOFs have special surfaces where specific hydrocarbon molecules stick to firmly. If you pour a mixture of 2 hydrocarbon molecules through the suitable MOF, you can extract one type of hydrocarbon from the mixture. This is will automatically make the other one emerge in pure form.
The process is not as easy as it sounds. Although many MOFs demonstrate superb separation performance, they selectively absorb ethylene, which is the exact opposite of what is needed – you want to let the ethylene pass through while absorbing ethane as a byproduct.
What They Did Differently?
It took years to find to the solution. In 2012, another team of researchers discovered that a specific framework named MOF-74 was adequate in isolating several hydrocarbon molecules, including ethylene. The authors modified the same framework and made it even better.
According to the previous study, iron peroxide compounds can break the bonds between carbon and hydrogen. To break this bond, the compound must attract the molecule in the first place. Keeping this in mind, the authors modified the surface of the MOF-74: now it has a structure similar to the iron peroxide compound.
They found that the modified structure effectively attracted ethane from the mixture. To examine the atomic structure, they used neutron diffraction method, which helped them determine what portion of MOF attracts ethane.
The authors plan to attach other small groups to the MOF’s surface to examine other possible outcomes. Although the modified MOF performs very well, it can be further improved to see action at a refinery.