A new metal-organic framework 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.
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.
ABSTRACT – Metal-organic frameworks can offer pore geometries that are not available in zeolites or other porous media, facilitating distinct types of shape-based molecular separations. Here, we report Fe2(BDP)3 (BDP2– = 1,4-benzenedipyrazolate), a highly stable framework with triangular channels that effect the separation of hexane isomers according to the degree of branching. Consistent with the varying abilities of the isomers to wedge along the triangular corners of the structure, adsorption isotherms and calculated isosteric heats indicate an adsorption selectivity order of n-hexane > 2-methylpentane > 3-methylpentane > 2,3-dimethylbutane ≈ 2,2-dimethylbutane. A breakthrough experiment performed at 160°C with an equimolar mixture of all five molecules confirms that the dibranched isomers elute first from a bed packed with Fe2(BDP)3, followed by the monobranched isomers and finally linear n-hexane. Configurational-bias Monte Carlo simulations confirm the origins of the molecular separation.
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