ZIFs: New Framework Materials for the Capture and Storage of CO2
15 February 2008
 |
| A ZIF structure. Click to enlarge. |
Researchers led by Omar Yaghi at UCLA have developed a new class of materials—zeolitic imidazolate frameworks (ZIFs)—that exhibit “unusual” selectivity for capturing carbon dioxide from gas mixtures and “extraordinary” capacity for storing CO2. The work is reported in the 15 February issue of the journal Science.
The researchers synthesized 25 ZIF crystal structures and found that three of them (ZIF-68, ZIF-69, ZIF-70) exhibited selectivity for capturing carbon dioxide from gas mixtures. One liter of ZIF-69 can hold approximately 83 liters of CO2 at 273 kelvin (-0.15°C) under ambient pressure.
 |
| ORTEP drawing of zinc atom surrounded by four linkers of ZIF-69. Click to enlarge. |
The ZIFs are highly porous (with surface areas up to 1,970 square meters per gram) and chemically robust structures that can be heated to high temperatures without decomposition and boiled in water or organic solvents for a week and still remain stable.
The technical challenge of selectively removing carbon dioxide has been overcome. Now we have structures that can be tailored precisely to capture carbon dioxide and store it like a reservoir, as we have demonstrated. No carbon dioxide escapes. Nothing escapes—unless you want it to do so. We believe this to be a turning point in capturing carbon dioxide before it reaches the atmosphere.
—Omar Yaghi
Flaps in the ZIF structure behave like the chemical equivalent of a revolving door, allowing certain molecules—in this case, carbon dioxide—to pass through and enter the pores while blocking larger molecules or molecules of different shapes.
In ZIFs 68, 69 and 70, researchers Rahul Banerjee and Anh Phan emptied the pores, creating an open framework. They then subjected the material to streams of gases—carbon dioxide and carbon monoxide, and another stream of carbon dioxide and nitrogen—and were able to capture only the carbon dioxide. They are testing other ZIFs for various applications.
Currently, the process of capturing carbon dioxide emissions from power plants involves the use of toxic materials and requires 20 to 30% of the plant’s energy output, Yaghi said. By contrast, ZIFs can pluck carbon dioxide from other gases that are emitted and can store five times more carbon dioxide than the porous carbon materials that represent the current state-of-art.
Zeolites are stable, porous minerals made of aluminum, silicon and oxygen that are employed in petroleum refining and are used in detergents and other products. Yaghi’s group has succeeded in replacing what would have been aluminum or silicon with metal ions like zinc and cobalt, and the bridging oxygen with imidazolate to yield ZIF materials, whose structures can now be designed in functionality and metrics.
Banerjee and Anh automated the process of synthesis. Instead of mixing the chemicals one reaction at a time and achieving perhaps several reactions per day, they were able to perform 200 reactions in less than an hour. The pair ran 9,600 microreactions and from those reactions uncovered 25 new structures.
Co-authors of the paper are Bo Wang, a UCLA graduate student in chemistry in Yaghi’s laboratory; Carolyn Knobler and Hiroyasu Furukawa of the Center for Reticular Chemistry at the UCLA’s California NanoSystems Institute; and Michael O’Keeffe of Arizona State University’s department of chemistry and biochemistry.
In the early 1990s, Yaghi invented another class of materials called metal-organic frameworks (MOFs), which have been described as crystal sponges and which also have implications for cleaner energy. Like ZIFs, MOFs have pores in which Yaghi and his colleagues can store gases that are usually difficult to store and transport. (Earlier post.)
Yaghi’s laboratory has made several hundred MOFs, with a variety of properties and structures. Molecules can pass in and out of them unobstructed.
BASF funded the synthesis of the materials, and the US Department of Energy funded the absorption and separation studies of carbon dioxide.
(A hat-tip to Curtis!)
Resources
Rahul Banerjee, et. al., High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture. Science 15 February 2008: Vol. 319. no. 5865, pp. 939 - 943 DOI: 10.1126/science.1152516
Sequestering CO2 at 0.15K is a non-starter. How well does this work at room temperature? Is this at all relevant to a useful real-world application, e.g. CH4 storage at moderate pressures?
Posted by: Rafael Seidl | 15 February 2008 at 12:42 PM
I believe the data was at room temperature. They were just noting that 273 is 0.15C less than "true" standard temperature of 273.15K.
Posted by: Kevin | 15 February 2008 at 01:18 PM
Sorry, standard temperature is not room temperature, but 0C is much better than 0.15K !!
Posted by: | 15 February 2008 at 01:20 PM
I believe they meant 273K, which is .15C. The "under" part is confusing but I believe they meant that the whole experiment was under ambient pressure.
Posted by: KC | 15 February 2008 at 01:58 PM
Doesn't the article say 273K?, or slightly below the freezing point of water?
Posted by: WVhybrid | 15 February 2008 at 02:16 PM
Carbon dioxide is a solid at 0.15 K. Use your head.
Posted by: Engineer-Poet | 15 February 2008 at 02:29 PM
The CO2 uptake capability of this new material may be great, but there's nothing about the amount of energy required to release the CO2 and restore it to its original state. If that's too high, ammonium-carbonate capture is going to be superior.
Posted by: Engineer-Poet | 15 February 2008 at 02:31 PM
Yes, it reads -0.15C, just below freezing point of water.
How expensive would this material be, I wonder? Would you shower it through the flue of a power plant as a powder? Put it in big filters?
Sounds promising, in any event.
Posted by: Nick | 15 February 2008 at 02:31 PM
From the "cost of energy" website:
To offset 80% of the CO2 emission from US electricity generation only, you'd need ZIF with the volume of "3,195.6 Louisiana Superdomes (at 4.6 million cubic yards/Superdome). Per Year. Every Year."
Where are you going to put all that CO2 filled ZIF (and how much energy does it to make all that ZIF)?
Posted by: Karkus | 15 February 2008 at 02:32 PM
Obviously you would use this stuff like an amine, to capture the CO2 at one condition, say in a flue gas stream cooled to 273K, and then release the CO2 at some other condition, most likely at at a different pressure or temperature. The concentrated CO2 could then either be used in some process (make soft drinks?), or stored (in deep wells?), and the "Z" cage would be reused.
Posted by: WVhybrid | 15 February 2008 at 02:54 PM
Wow, couple ZIFs to biomass power plants and you get *negative emissions* energy. That is: the more you burn of it, the more CO2 you take out of the atmosphere.
That's clearly the way forward. Bio-energy with carbon storage.
Ordinary renewables have roughly the following lifecycle emissions:
-solar photovoltaic: +100 grams of CO2eq/kWh
-wind: +30gCO2eq/kWh
-non-CCS biomass: +30gCO2eq/kWh
-hydropower: +20gCO2eq/kWh
(-nuclear: +10gCO2eq/kWh)
-biomass+CCS (in an IGCC): -1000gCO2eq/kWh
That's *minus* a thousand grams.
So if we want to beat climate change, we should put our money in this.
Posted by: Jonas | 15 February 2008 at 03:28 PM
i still don't get this convoluted carbon capture stuff. why not just grow trees?
then, if you wanna be persnickety about the whole decomposition thing, sink them to the bottom of the ocean. they'll take centuries to decompose, at least.
Posted by: vboring | 15 February 2008 at 06:04 PM
Sorry everyone, I had misread the original article.
Still, it would be useful to know how the capture capacity deteriorates with increasing temperature. The combustion processes that produce the offending CO2 typically feature waste gas temperatures well above 0 degC and you only want to cool it down as much as necessary. In particular, condensing water out of waste gases containing sulfur and phosphorous compounds leads to acid formation and corrosion risks.
High selectivity could be valuable for subsequent algal oil production if the flue gas is contaminated by trace compounds, as is usually the case for coal-fired power plants. Of course, that assumes the zeolite isn't damaged by those contaminants.
Another possible application is CO2 removal from biogas to yield biomethane.
On the other hand, high selectivity for CO2 means these particular zeolites are not suitable for storing methane on board a vehicle at moderate pressure. Perhaps this research will lead others to ZIFs that are.
Posted by: Rafael Seidl | 15 February 2008 at 06:12 PM
vboring, you're perhaps trying to appear to be not very bright, I don't know. Let's assume your moderately intelligent.
Just ask yourself: do trees generate electricity? No they don't.
With carbon-negative energy you get *both* energy and carbon sequestration at the same time.
Just try to get your vboring head around the concept. One day you will see the light.
Posted by: Jonas | 15 February 2008 at 06:37 PM
Interesting how this is the reverse of oxyfiring using pressure swing adsorption to purify the input, namely cool dust free air. Then we were assured that bubbling flue gas through algae tubes was the answer. Then perhaps not. Mother Nature has been working on trees as a CO2 absorbent for 400 million years; in fact burning the sequestered remains of those trees is much of the problem.
Maybe next week the new technofix will be hamsters on treadmills.
Posted by: Aussie | 16 February 2008 at 02:09 AM
If you can capture CO2 with ZIFs, then feed the CO2 concentrate to algae for biopetroleum, you have a nearly closed CO2 cycle. Think of it as cogeneration with multiple outputs: useful heat, useful power, useful CO2 concentrate, useful biopetroleum.
Posted by: Al Fin | 16 February 2008 at 07:04 AM
I think it shows promise provided you aren't using terrestrial sources of biomass for the original CO2, as the original land clearing releases so much CO2 (Indonesia is one of the greatest CO2 emmitters because they are burning their rainforests to plant oil palms for biofuels.....). But with algae in the ocean it shouldn't have this effect. You could even use sewage to fertilize the algae.
The theory sounds nice but wake me up when it becomes reality. I think the real answer is much simpler, and already exists -- electric cars with solar and wind electricity generation, and biodiesel generator trailers. It really is a no brainer.
Posted by: MarkMC | 16 February 2008 at 10:59 AM
MarkMC, land clearing does not release CO2, at least not when you use contemporary techniques. The Science article to which you refer is wholly inaccurate and refers to techniques from the past which were based on burning the original biomass. That doesn't count any longer, because you use the biomass as a bioenergy feedstock itself.
You can even stuff the soil with char and go carbon-negative, thus eliminating all carbon debt from the very start.
Algae don't work, the bioreactors are way too expensive; open ponds are unstable; and unless you use a genetically modified algae strain you will get nowhere; but such strains, which can be carried away by the wind, will never be allowed. So algae are a no-starter.
To AlFin: remember, you have to decarbonise the fuel in order to achieve negative emissions. That means either using the biomass in power plants for electricity or turning it into bio-hydrogen. Else you won't go carbon-negative.
Posted by: Jonas | 16 February 2008 at 11:32 AM
I dunno 'bout you, but I still got some unanswered ZIF questions ...
1 ) How much do ZIFs cost to make?
2 ) After capturing CO2 with a ZIF, what do ya do with it?
3 ) Are ZIFs a use it once deal or a capture and process device?
4 ) Do ZIFs have a pollution causing downside of their own?
These sorts of unanswered questions make a real big difference to my willingness to be a cheering section for ZIFs. I like to know a bit more about a product than "we got it" ...
Posted by: John Taylor | 17 February 2008 at 12:48 AM
all the people saying "just plant trees" need a reality check. the amount of land that would be required to plant enough trees to absorb globally significant quantities of CO2 is wholly out of the question, never mind the perils of monocropping and lack of biodiversity, plus the fact that trees are only a carbon sink while they are still growing, and once they reach maturity the net effect is zero.
Posted by: eric | 17 February 2008 at 07:53 AM
There's that, and the "GreenFreedom" thing hitting the news. Either the figures are not given, or they're buried in hard-to-understand forms. Both appear aimed at deflecting demands to change our technology at the consumption end to cut carbon emissions. Coincidence?
Posted by: Engineer-Poet | 17 February 2008 at 07:58 AM
Trees are not a carbon sink people they are biomass and as such are part of the surficial carbon cycle. When the tree dies decomposers move in and respirate the CO2 back to the atmosphere. This is a scientific axiom. Decomposers WILL decompose the tree and the CO2 WILL return to the atmosphere. Come on this is Freshman level Geoscience. The only way to permanently remove carbon from the surficial cycle is to carbonate it in a form that biological life can no longer use it. As in CO3 molecules be it CaCO3 or Mg(CO3)2 not oxygenated hydrocarbons like sugars. Of which cellulose, ligin, and hemicellulose are all oxygenated sugar polymers subject to decomposition by fungi and anaerobic bacteria all of which respirate CO2. that being said the process to carbonate CO2 with a periodic group IA or IIA metal takes vast amount of energy to get that base metal in the first place. This is the fallacy in CO2 offsets. Your just buying indulgences much like was done in the middle ages from the Pope.
Posted by: Science will set you free | 20 February 2008 at 09:41 AM
Trees are carbon inventory. Cutting or burning forests releases that inventory to the atmosphere, and it takes considerable time to pull it out again. If we are trying to limit atmospheric CO2, forests are big assets.
Posted by: Engineer-Poet | 23 February 2008 at 07:18 AM
Delectable Designs with Cupcake Jones, North Portland Library, noon: Learn about the origins of foods like vanilla and chocolate found in this year’ s Every Family Reads book, Yum! ¡ Mmmm! ¡ Qué Rico!, by Pat Mora. Then you’ ll get the chance to decorate your own cupcake. Recommended for ages 5 and up. Space at this program is limited. Free tickets for seating will be available 30 minutes prior to the program. For more information, visit the Multnomah County Library website.
Posted by: Club De Rechter Stratumseind | 30 March 2008 at 05:42 AM
Just a question. The industrial gases are rarely at 273 K, generally they are very very hot, above the boiling point of water. How is it possible to store hot gases in 'something' that has large uptake only near 273K? Should we refrigerate the gases? How? What is the overall cost? Is it convenient? In addition ZIFs are made by Zn and Cd ions, and metals are usually very polluting, much more than CO2!
Posted by: Mr X | 01 September 2008 at 10:19 AM