MIT Technology Review

Thursday, May 23, 2013

Material That Sorts Molecules by Shape Could Lower the Price of Gas

A hydrocarbon-sorting material could replace energy-intensive oil refining steps.


Molecular sorter: The black powder in this vial—made of iron and an organic material called BDP—could help refiners produce gasoline.

A new material that sorts hydrocarbon molecules by shape could lower the cost of gasoline and also make the fuel safer by reducing the need for certain additives that have been linked to cancer, according to a paper in the next issue of the journal Science.

Refiners typically use a material that can sort molecules by size during a key step in the refining process. To achieve a desired octane rating, this step has to be supplemented with energy-intensive distillation steps, or by the use of additives. The new material, which sorts molecules by shape rather than by size, can better differentiate between different types of hydrocarbon molecules, eliminating the distillation steps and the need for octane-enhancing additives.

“You could get high-performance gasoline at a cheaper price, potentially, and also more environmentally friendly gasoline,” says Jeffrey Long, the professor chemistry at the University of California at Berkeley who led the work (see “Gasoline Fuel Cell Would Boost Electric Car Range”).

The new material is a type of metal-organic framework, a relatively new class of materials made of metal atoms linked by organic molecules (see “Novel Material Shows Promise for Extracting Uranium from Seawater” and “Novel Heating System Could Improve Electric Car’s Range”). Varying the metals and the organic molecules that link them can produce materials with a wide variety of properties. It’s possible to subtly vary the size and shape of pores within the material, for example, as well as the way those pores interact with specific molecules.

To make the material for sorting hydrocarbons, the researchers made a material riddled with microscopic tunnels featuring triangular cross-sections. These tunnels can sort five different types of hexane molecules—hydrocarbons with six carbon atoms—that are key to achieving the desired octane rating of gasoline. The octane rating for hexanes depends on how the carbon atoms are arranged. Line them up in a row, forming a linear molecule, and the octane number is very low—about 30. But link them together to form a branching structure and the octane level can be as high as 105. 

The shape of the molecules affects how they move through the tunnels. In general, molecules move more slowly through the parts of the tunnel where two sides of the triangular-cross-section tunnel come together and more quickly through the middle part of the tunnels. The branches of the higher-octane molecules tend to keep them in the center, so they move faster and emerge from the material first. The long linear molecules can fit into the narrow area, so they move through the material more slowly, and emerge last. The various molecules emerge at regular intervals that make them easy to separate. Long says that computer molecules suggest the method will be useful for sorting other key molecules, too.

The work is still at an early stage. Before the materials can be commercialized, they’d have to be manufactured at a large scale, and engineers would have to work out how they could be integrated into the refinery, and whether the improvement in performance would be worth the effort of changing the existing system.