Image courtesy of Jeffrey Long, University of California, Berkeley
The structure of the iron MOF showing iron (orange), nitrogen (blue), and carbon (gray) atoms, as viewed along the triangular channels. When unrefined petroleum feedstock containing hexane isomers (the same atoms connected in different ways) were passed through a column of the MOF with the triangular channels, the compact branched isomers with higher octane value exit first, followed by the long and flexible monobranched and the linear isomers, enabling efficient separation of hydrocarbon isomers based on their shape.
A novel MOF separates hexane isomers based on their shape, rather than their size or boiling point. In this MOF the linear chain molecules, by virtue of their flexibility and interaction with the walls, take significantly longer to pass through the triangular channels, while the branched ones with compact shapes pass more quickly through the middle region of the channels.
The discovery offers great potential to lower the energy requirements and cost to produce petroleum of a desired octane number; and will significantly reduce, or even eliminate, the need for octane-boosting toxic additives in the gasoline – benefiting both human health and the environment.
Efficient separation of hexane isomers remains a great challenge to the petroleum industry. In the current technology of petroleum refining some low-quality isomers (low octane number) of hexane end up with the high-quality isomers (high octane number), compromising the average octane number of the gasoline. Achieving the desired octane number gasoline for different engine types often involves energy intensive processes and toxic octane enhancing additives, making gasoline more expensive at the pumps. This challenge is addressed by an international team of researchers led by the Center for Gas Separations Relevant to Clean Energy Technologies (CGS), an Energy Frontier Research Center (EFRC) at the University of California, Berkeley, by designing a solid, iron-based, highly-stable metal-organic framework material with triangular channels that can separate molecules by shape, unlike existing zeolite and other porous materials. Consistent with the MOF design, molecular separation experiments with a mixture containing equal amounts of the five hexane isomers produced from petroleum confirms that the dibranched isomers pass through the MOF first, followed by the monobranched isomers, and finally by the linear n-hexane. Computer simulations confirm the mechanism by which the efficient molecular separation occurs in the designed material. This discovery could disruptively alter the petroleum refining processes, leading to substantial cost and energy savings worldwide, as well as benefit human health and the environment.
University of California, Berkeley
Director, Center for Gas Separations Relevant to Clean Energy Technologies (CGS) EFRC
DOE Office of Science, Basic Energy Sciences, Energy Frontier Research Centers (EFRC) Program, National Research Council postdoctoral fellowship at the National Institute of Standards and Technology (M.R.H.), Cariplo Foundation (N.M.), Institute of Crystallography–National Research Council, Bari, Italy (X-ray powder diffraction).
Herm, Z. R.; Wiers, B, M.; Mason, J. A.; van Baten, J. M.; Hudson, M. R.; Zajdel, P.; Brown, C. M.; Masciocchi, N.; Krishna, R.; Long, J. R.” Separation of Hexane Isomers in a Metal-Organic Framework with Triangular Channels” Science 2013, 340, 960-964 [DOI: 10.1126/science.1234071].
Center for Gas Separations Relevant to Clean Energy Technologies (CGS) EFRC
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