New iron-based MOF could significantly improve the energy efficiency of gas separation in refineries
Researchers at the University of California, Berkeley have developed a iron-based metal organic framework (MOF) called Fe-MOF-74 that could potentially eliminate an energy-intensive gas-separation process currently required for the production of light hydrocarbons, thus resulting in energy and cost savings. A paper describing the work is published in the journal Science.
To produce the chemicals needed to make plastics, refineries “crack” crude oil at high temperatures—500 to 600 °C—to break the complex long-chain hydrocarbons into lighter, short-chain molecules. They then chill the gaseous mixture to -100 °C to liquefy and separate the gases, a process called cryogenic distillation. The new MOF can be used at higher temperatures to efficiently separate these gases while eliminating the chilling. The iron-MOF is also good at purifying natural gas of various types of hydrocarbon impurities that have to be removed before the gas can be used by consumers.
As a consequence of the similar sizes and volatilities of the molecules, separations of olefin/paraffin mixtures, such as ethylene/ethane and propylene/propane, must currently be performed at low temperatures and high pressures and are among the most energy-intensive separations carried out at large scale in the chemical industry. Because these gas mixtures are produced by cracking long-chain hydrocarbons at elevated temperatures, a substantial energy penalty arises from cooling the gases to the low temperatures required for distillation. Thus, tremendous energy savings could be realized if materials enabling the efficient separation of olefins and paraffins at higher temperatures (than currently used in distillation) and atmospheric pressure were achieved.
Competing approaches toward this end include membrane designs and organic solvent-based sorbents, as well as porous solid adsorbents featuring selective chemical interactions with the carbon-carbon double bond in olefins. In this latter category, metal-organic frameworks (MOFs), which offer high surface areas, adjustable pore dimensions, and chemical tunability, have received considerable attention as adsorbents in gas storage and gas separation applications, with particular emphasis on the dense storage of methane and hydrogen and on the efficient removal of carbon dioxide from flue gas and natural gas deposits. More recently, the potential utility of these porous structures for the separation of hydrocarbon mixtures has been exposed, specifically for the separation of ethylene/ethane and propylene/propane mixtures.
Herein, we show that Fe2(dobdc), a metal-organic framework with exposed iron(II) coordination sites exhibiting high olefin/paraffin selectivities, can be used for the separation of ethylene/ethane and propylene/propane.—Bloch et al.
Built in the lab of Jeffrey Long, a professor of chemistry at the University of California Berkeley, Fe-MOF-74 was analyzed by a team at NIST and the Australian Nuclear Science and Technology Organisation’s Bragg Institute.
MOF compounds have a very high surface area, which provides lots of area a gas mixture can interact with, and that surface contains iron atoms that can bind the unsaturated hydrocarbons. Acetylene, ethylene and propylene [olefins] will stick to those iron sites much more strongly than will ethane, propane or methane [paraffins]. That is the basis for the separation.—Jeffrey Long
The MOF consists of a carbon and oxygen framework with iron atoms at strategic sites to bind the ethylene carbon atoms. The researchers found that when pumping a gas mixture, even at high temperatures, through the iron-based MOF, the propylene and ethylene (olefins) bind to the iron embedded in the matrix, letting pure propane and ethane (paraffins) through. In their trials, the ethane coming out was 99.0 to 99.5% pure. The propane output was close to 100% pure, since no propylene could be detected. After the ethane and propane emerge, the MOF can be heated or depressurized to release ethylene and propylene pure enough for making polymers. (Ethylene and propylene are used for plastic polymers, while ethane and propane are typically used for fuel.)
Through a microscope, Fe-MOF-74 looks like a collection of narrow tubes packed together like drinking straws in a box. Each tube is made of organic materials and six long strips of iron, which run lengthwise along the tube. Analysis by Long’s colleagues at the NIST Center for Neutron Research showed that different light hydrocarbons have varied levels of attraction to the tubes’ iron. By passing a mixed-hydrocarbon gas through a series of filters made of the tubes, the hydrocarbon with the strongest affinity can be removed in the first filter layer, the next strongest in the second layer, and so forth.
It works well at 45 degrees Celsius, which is closer to the temperature of hydrocarbons at some points in the distillation process. The upshot is that if we can bring the MOF to market as a filtration device, the energy-intensive cooling step potentially can be eliminated. We are now trying out metals other than iron in the MOF in case we can find one that works even better.—coauthor Wendy Queen
Long and his laboratory colleagues are also developing iron-based MOFs to capture carbon from smokestack emissions and sequester it to prevent its release into the atmosphere as a greenhouse gas. Similar MOFs, which can be made with different pore sizes and metals, turn out to be ideal for separating different types of hydrocarbons and for storing hydrogen and methane for use as fuel.
Long’s other colleagues are UC Berkeley graduate students Eric D. Bloch and Joseph M. Zadrozny; Rajamani Krishna of the Van’t Hoff Institute for Molecular Sciences at the University of Amsterdam; and Craig M. Brown of NIST and The Bragg Institute at the Australian Nuclear Science and Technology Organisation in Menai, New South Wales.
The research is part of the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the Department of Energy that focuses primarily on creating novel materials for capturing and storing carbon dioxide.
E.D. Bloch, W.L. Queen, R.Krishna, J.M. Zadrozny, C.M. Brown and J.R. Long (2012) Hydrocarbon separations in a metal-organic framework with open Iron(II) coordination sites. Science, March 30, 2012. doi: 10.1126\science.1217544