Ask Texas A&M University chemist Hong-Cai "Joe" Zhou to describe his research in simple terms, and more often than not, he'll draw on a favorite analogy from childhood: playing with LEGOs.
But if you're tempted to view his work as child's play, you might want to think again. The building blocks he and his group specialize in actually are a recently developed, increasingly versatile class of materials known as metal-organic frameworks (MOF).
An emerging technology in the scientific community, MOF are porous crystalline polymers made up of metal ions or metal-containing components and organic ligands. Zhou's group, in collaboration with Hae-Kwon Jeong and Perla B. Balbuena in the Department of Chemical Engineering, assembles MOF materials with profound potential for cleaner energy across the globe.
"It's very fair to say that, in the last decade, the fastest-growing field in chemistry is the study of metal-organic frameworks," Zhou says. "The MOF field was formed only about 15 years ago, but it has already shown a lot of promise. We are just one of many teams worldwide working with this exciting new type of material, because the scope of the research is enormous."
Since the 1990s, MOF have been touted by many as the future of eco-friendly technology that could pave the way for major improvements to natural gas usage for transportation and in the commercialization of hydrogen-powered vehicles. In their crystalline form, they appear to resemble nothing more than ordinary table salt. Looks, however, are deceiving, considering MOF have the highest internal surface area known to man. Once unraveled, one sugar-cube-sized piece could cover an entire football field.
In addition to having exceptionally high porosity, Zhou says they are the most tunable material of any known substance. With just a tweak of their crystalline structure and surface properties, they become ideal for absorbing any type of different molecule, lending to their versatility in application.
For Zhou and his team, that purpose currently hinges on MOF's ability to selectively capture carbon dioxide from the exhaust of coal-fired power plants. Though coal is a cheap natural resource, its long-term and widespread use has been a main contributor to the rapidly increasing levels of carbon dioxide in the atmosphere. Zhou notes that capturing carbon dioxide using MOF, coupled with proper sequestration and/or utilization, not only would slow down the escalation of greenhouse gas levels but also allow power plants to continue using inexpensive coal.
While MOF come in an overwhelming number of varieties, Zhou says only a fraction is suitable for carbon capture. Finding that fraction and then maximizing its potential represents the crux of the tedious yet vital chore facing Zhou and his team. Compounding the complicated matter of piecing together the correct framework is the fact that only a handful of places worldwide conduct large-scale tests on carbon-capture techniques, given the energy industry's somewhat understandable reluctance to implement such experimental, power-sapping processes. Zhou explains that even the most current state-of-the-art carbon-capture procedure would lead to a 30 percent parasitic power consumption, thereby significantly reducing the power plant's overall efficiency.
On a brighter note (pun intended), Zhou's group may have found a better alternative. He says they are in the process of constructing a unique subset of MOF that can capture carbon dioxide with extremely high selectivity while using much less power than what is required by commonly applied carbon-capture methods. The group's goal is to create an MOF that binds only with carbon dioxide and is robust enough to withstand the harsh conditions of the flue gas, resulting in a more economical carbon-capture technique. If successful, it could significantly reduce the amount of carbon dioxide currently being emitted into the atmosphere.
Zhou says that, beyond carbon capture, MOF may become useful in gas separation in general.
"Normally in the chemical and petroleum industry, one of the most energy-intensive procedures is the separation of gasses, considering you have to liquefy them by compressing and then cooling them," he explains. "Then you have to do distillation by evaporating and cooling what you wanted to separate. It's a total waste of energy.
"Using this new material, the gasses would come in, and the ones that are the right size would stay, while the others would pass. Separation can be performed at a fraction of the original cost using cryo-distillation."
Zhou's vision for this method of MOF was considered a novel concept in 2010 when his collaborative proposal with Jeong and Balbuena, "Stimuli-Responsive Metal-Organic Frameworks for Energy-Efficient Post-Combustion Carbon Dioxide Capture," was selected as one of only 37 nationwide to share in a total of $106 million worth of new grants awarded by the United States Department of Energy (DOE). Funded through the DOE's Advanced Research Projects Agency-Energy (ARPA-E), the grants recognized creative technologies that offered promising improvements in the way energy is generated, stored and utilized.
Most recently, Zhou's work was licensed and used as the foundation for a Texas-based startup company, framergy. Founded in early 2011, framergy oversees the commercialization of groundbreaking innovations in MOF for industrial uses, with Zhou serving its chief scientific advisor.
Progress aside, Zhou readily admits the work with carbon-capturing MOF is far from finished. He says his group's next big undertaking will be to determine if carbon dioxide can be separated from a flue gas -- exhaust from chimneys, ovens and steam generators -- using MOF. In addition, he says there is much more research to be conducted with MOF's ability to store hydrogen and methane, efforts which will continue into the indefinite future.
"In terms of the scope of potential application of MOF, we have barely scratched the surface," he says. "I think for Texas A&M, it's important that we secure a favorable position at the cutting edge of this field. It's very rare for a technology to advance at such a rapid pace, so if you are not staying ahead of the competition, you are just falling behind."
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