Iron-based, microporous materials show promise for purification of light fossil fuel gases near room temperature
Timothy D. Courtney
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Structure of Fe2(dobdc) with adsorbed ethylene; atoms are iron (yellow), oxygen (red), carbon (gray) and hydrogen (cyan). Fe2(dobdc) can selectively sort light fuel gases at moderate temperatures rather than chill and compress them for distillation.

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Structure of Fe2(dobdc) with adsorbed ethylene; atoms are iron (yellow), oxygen (red), carbon (gray) and hydrogen (cyan). Fe2(dobdc) can selectively sort light fuel gases at moderate temperatures rather than chill and compress them for distillation.

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While renewable alternatives to fossil fuels are one popular way to reduce our impact on the environment, another method just as critical is improving the efficiency with which we use our current fuels. Researchers at the Center for Gas Separations Relevant to Clean Energy Technologies, CGS, have reported in the journal Science an exciting potential for energy conservation using a microporous material called Fe2(dobdc). Jeffrey R. Long and his team at CGS had previously shown the material effective for separating oxygen and nitrogen, and they've now shown that the material is adept at refinery gas separations as well.

It is commonly expected that half of a chemical’s production cost will come from purification alone. That may be an underestimate for the process studied at CGS. Oil refineries break down crude oil at high temperatures into a wide variety of petrochemicals, including light gases such as methane, ethane, ethylene and acetylene. These gases are useful as natural gas or chemical precursors, but high purity is required. For traditional purification, the gas is cooled from the 1000 oF temperature of the reactor (well above the melting point of lead) down to a frigid 150 oF below zero – colder than dry ice. The purification also requires high pressure, further increasing the energy cost.

In total, this deep freeze is one of the most energy-intensive processes in the refinery. Instead, Long envisions a process at atmospheric pressure and at temperatures between 110 and 180 oF. "The energy savings are potentially enormous," said Long. “And, with that comes reduced emission of carbon dioxide."

The secret is a metal-organic framework, or MOF: a porous, crystalline material with metal atoms interspersed in a rigid network of organic carbon and oxygen. The molecule-scale porous network in the iron-embedded MOF, Fe2(dobdc), leads to an enormous interior surface area – just an ounce of the material has more surface area than 7 football fields. The high surface area is one of Fe2(dobdc)’s two important properties as it is this surface on which the gas molecules adhere, replacing the low temperature needed to condense gases to liquids.

The other important property is the selectivity – different gases adhere to the surface with different strengths. This is seen most clearly in a "breakthrough" experiment where a gas mixture flows through a packed column of Fe2(dobdc). The gas that bonds more strongly to the MOF surface is trapped inside while the more weakly bound gas exits. After the first gas is collected, the strongly bound gas can then be purged from the tube and collected as well. The team achieved greater than 99% purity for ethane/ethylene and propane/propylene separation, a testament to the favorable selectivity of Fe2(dobdc).

Critical to the selectivity is the iron in Fe2(dobdc), ironically considered "soft" by chemists, meaning its electron cloud is large and deformable. This is a great match for the similarly soft double bonds in ethylene and propylene, and a not so great match for the "hard" bonds in ethane and propane. As a result, the “harder” the gas is, the faster it flows through the Fe2(dobdc) allowing the gases to be collected separately and eliminating the need for cryogenic temperatures.

To accelerate their research, the team tested and applied a computational model for predicting the breakthrough curves. Their models show Fe2(dobdc) can separate methane from ethane, ethylene and acetylene; an important process for natural gas purification. Even better, the team was elated to discover the material could purify those remaining light gases as part of the same process: a tremendous economic boon.

While Long has ideas on further refining the MOF, he sees the next step as an engineering challenge – one to make use of this material on an industrial scale. If that can be realized, so too can a significant reduction in energy use by oil and natural gas refineries.

More Information

Bloch ED, WL Queen, R Krishna, JM Zadrozny, CM Brown and JR Long. 2012. "Hydrocarbon Separations in a Metal-Organic Framework with Open Iron(II) Coordination Sites." Science 335(6076):1606-1610. DOI: 10.1126/science.1217544

Acknowledgements

This research was supported through the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences. The National Institute of Standards and Technology National Research Council Postdoctoral Fellowship Research Associate program supported WL Queen. The authors and the University of California, Berkeley, have filed a patent on the results.

About the author(s):

Tim Courtney is a Ph.D. candidate at the University of Delaware and is a student in the Catalysis Center for Energy Innovation. He is advised by Jingguang Chen and Dion Vlachos. Tim holds a B.S. in Chemical Engineering from the University of Maryland, Baltimore County.

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