An international team of researchers has developed a new metal-organic framework (MOF) that might provide a significantly improved method for separating hexane isomers in gasoline according to their degree of branching. A paper on the work is published in the journal Science.
Created in the laboratory of Jeffrey Long, professor of chemistry at the University of California, Berkeley, the MOF features triangular channels that selectively trap only the lower-octane hexane isomers based on their shape, separating them easily from the higher-octane molecules in a way that could prove far less expensive than the industry’s current method for producing high-octane fuel. The Long laboratory and UC Berkeley have applied for a patent on the MOF Fe2(bdp)3. (BDP2– = 1,4-benzenedipyrazolate)
High-octane gasolines are more expensive than regular unleaded gasoline due to the difficulty of separating out the right type of molecules from petroleum. Petroleum includes several slightly different versions of the same molecule that have identical molecular formulae but varying shapes (isomers).
Creating premium fuel requires a refinery to boil the mixture at precise temperatures to separate the isomers with the most chemical energy. However, four of these isomers—two of which are high octane, the other two far lower—have only slightly different boiling points, making the overall process both challenging and costly.
The new MOF, however, could allow refineries to sidestep this problem by essentially trapping the lowest-octane isomers while letting the others pass through. The lowest-octane isomers are more linear and can nestle closer to the MOF walls, so when a mixture of isomers passes through the MOF, the less desired isomers stick to its surface.
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.—Herm et al.
The Supplementary Materials for the paper contains a detailed discussion of the separation potential of Fe2(BDP)3 for producing high-octane gasoline (RON 92). The team concluded that the separation into three fractions based on the degree of branching, rather than on C number, is evident. The implications of this fractionation ability is that it is possible to utilize Fe2(BDP)3 in the separation step of a C5/C6/C7 alkane isomerization process scheme.
The discussion also compares the new MOF with the separation performance of other adsorbents.
Matthew Hudson and his colleagues at the NIST Center for Neutron Research (NCNR) used neutron powder diffraction, a technique for determining molecular structure, to explore why the MOF has the right shape to selectively separate the isomers. Their research was essential to validate the team’s model of how the MOF adsorbs the low-octane isomers.
It’s easier to separate the isomers with higher octane ratings this way rather than with the standard method, making it more efficient. And based on the lower temperatures needed, it’s also far less energy-intensive, meaning it should be less expensive.—Matthew Hudson
Hudson says that while industrial scientists will need to work out how to apply the discovery in refineries, the new MOF appears to be robust enough in harsh conditions to be used repeatedly a great many times, potentially reducing the necessary investment by a petroleum company.
Z.R. Herm, B.M. Wiers, J.A. Mason, J.M. van Baten, M.R. Hudson, P. Zajdel, C.M. Brown, N. Masciocchi, R. Krishna and J.R. Long (2013) Separation of hexane isomers in a metal-organic framework with triangular channels. Science, doi: 10.1126/science.12334071