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New diamine-appended MOFs can capture CO2 for half or less of the energy cost of current materials

UC Berkeley chemists have developed a new material that can efficiently capture CO2 and then release it at lower temperatures than current carbon-capture materials, potentially cutting by half or more the energy currently consumed in the process.

The material, a metal-organic framework (MOF) modified with nitrogen compounds called diamines, can be tuned to remove carbon dioxide from the room-temperature air of a submarine, for example, or the 100-degree (Fahrenheit) flue gases from a power plant. A paper elucidating the mechanism of what the researchers are calling “phase-change” adsorbents is published in the journal Nature.

Power plants that capture CO2 today use an older technology whereby flue gases are bubbled through organic amines in water, where the carbon dioxide binds to amines. The liquid is then heated to 120-150 degrees Celsius (250-300 degrees Fahrenheit) to release the gas, after which the liquids are reused. The entire process is expensive: it consumes about 30% of the power generated, while sequestering underground costs an additional though small fraction of that.

The removal of CO2 from low-pressure flue gas mixtures is currently effected by aqueous amine solutions that are highly selective for acid gases. As a result of the large energy penalty for de-sorbing CO2 from such liquids, solid adsorbents with appreciably lower heat capacities are frequently proposed as promising alternatives. In particular, as a result of their high surface areas and tunable pore chemistry, the separation capabilities of certain metal-organic frameworks have been shown to meet or exceed those achievable by zeolite or carbon adsorbents.

Recently, the attachment of alkyldiamines to coordinatively unsaturated metal sites lining the pores of selected metal-organic frameworks has been demonstrated as a simple methodology for increasing low-pressure CO2 adsorption selectivity and capacity. Most notably, functionalization of Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), an expanded variant of the well-studied metal-organic framework Mg2(dobdc) (dobdc4- = 2,5-dioxidobenzene-1,4-dicarboxylate), with N,N'-dimethylethylenediamine (mmen) generated an adsorbent with exceptional CO2 capacity under flue gas conditions and unusual, unexplained step-shaped adsorption isotherms.

Here we elucidate the unprecedented mechanism giving rise to these step-shaped isotherms and demonstrate that replacing Mg2+with other divalentmetal ions enables the position of the CO2 adsorption step to be manipulated in accord with the metal-amine bond strength. As we will show, the resulting mmen-M2(dobpdc) (M = Mg, Mn, Fe, Co, Zn) compounds, here designated ‘phase-change’ adsorbents, can have highly desirable characteristics that make them superior to other solid or liquid sorbents for the efficient capture of CO2.

—McDonald et al.

88139_web
The diamine-appended metal-organic framework before and after binding of carbon dioxide. The view is a cross section through one of the pores of the MOF, showing diamine molecules (containing blue nitrogen atoms) attached to metal (manganese) atoms (green). Carbon dioxide molecules (grey carbon atoms with two red oxygen atoms) bind through a cooperative mechanism akin to a chain reaction along the pore surfaces. Some H atoms (white) are omitted for clarity. (Graphic by Thomas McDonald, Jarad Mason, Jeffrey Long/UC Berkeley) Click to enlarge.

The new diamine-appended MOFs can capture carbon dioxide at various temperatures, depending on how the diamines are synthesized, and releases the CO2 at only 50 °C above the temperature at which CO2 binds, instead of the increase of 80-110 °C required for aqueous liquid amines. Because MOFs are solid, the process also saves the energy costs of heating the water in which amines are dissolved.

MOFs are composites of metals—in this case, magnesium or manganese—with organic compounds that, together, form a porous structure with microscopic, parallel channels. Several years ago, senior author Jeffrey Long, a UC Berkeley professor of chemistry and faculty senior scientist at Lawrence Berkeley National Laboratory, and his lab colleagues developed a way to attach amines to the metals in a MOF to produce pores of sufficient diameter to allow CO2 to penetrate rapidly into the material.

They found that MOFs with attached diamines are very different from other carbon-capture materials, in that the CO2 seems to load into the material very quickly at a specific temperature and pressure, then come out quickly when the temperature is raised by 50 °C. In the new paper, UC Berkeley graduate students Thomas McDonald and Jarad Mason, together with other co-workers, describe how this works.

This material is unique in that it binds CO2 in a cooperative mechanism. When the first CO2 starts to adsorb at a very specific pressure, all of a sudden it facilitates more CO2 adsorption, and the MOF rapidly saturates. That is really a different property from any other CO2 adsorbent based on amines. Then, if you raise the temperature by applying heat, at some temperature all the CO2 will come flooding off.

—Jeffrey Long

Long’s team found that the diamines bind to the metal atoms of the MOF and then react with CO2 to form metal-bound ammonium carbamate species that completely line the interior channels of the MOF. At a sufficiently high pressure, one CO2 molecule binding to an amine helps other CO2 molecules bind next door, catalyzing a chain reaction as CO2 polymerizes with diamine like a zipper running down the channel. Increasing the temperature by 50 °Celsius makes the reaction reverse just as quickly.

The pressure at which CO2 binds to the amines can be adjusted by changing the metal in the MOF. Long has already shown that some diamine-appended MOFs can bind CO2 at room temperature and CO2 levels as low as 300 parts per million. The current atmospheric concentration of CO2 is now 400 parts per million (ppm).

Last summer, Long co-founded a startup, Mosaic Materials, to use the new technology to reduce the cost of chemical separations, with plans in the works for a pilot study of CO2 separation from power plant emissions. This would involve creating columns containing millimeter-size pellets made by compressing a crystalline powder of MOFs.

The researchers are also hoping to develop something that might be tested in a submarine. That would pave the way for eventual scale-up to capturing CO2 from natural gas plants, which produce emissions containing about 5% CO2, to the higher concentrations of coal-fired power plants.

We got lucky. We were just trying to find a simple way to attach these amines to our MOF surface, because they are one of the best compounds for selectively binding CO2 in the presence of water, which can be a problem in flue gas. And it just happens we got the right length in the amine to make these one-dimensional chains that bind CO2 in a cooperative manner.

Long suggested as well that the findings may have relevance for the fixation of CO2 by plants, owing to striking structural similarities between the magnesium-based MOF and the naturally occurring CO2-fixing photosynthetic enzyme RuBisCO.

Long received assistance from colleagues at Zhejiang University in Hangzhou, China; the University of Turin in Italy; the University of Minnesota in Minneapolis; the Université Grenoble Alpes and the Centre National de la Recherche Scientifique in France; the Norwegian University of Science and Technology in Trondheim, Norway; and the École Polytechnique Fédérale de Lausanne in Switzerland.

The work is supported by grants from ARPA-E and the US Department of Energy-funded Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center operated jointly by UC Berkeley and LBNL.

Resources

  • Thomas M. McDonald, Jarad A. Mason, Xueqian Kong, Eric D. Bloch, David Gygi, Alessandro Dani, Valentina Crocellà, Filippo Giordanino, Samuel O. Odoh, Walter S. Drisdell, Bess Vlaisavljevich, Allison L. Dzubak, Roberta Poloni, Sondre K. Schnell, Nora Planas, Kyuho Lee, Tod Pascal, Liwen F. Wan, David Prendergast, Jeffrey B. Neaton, Berend Smit, Jeffrey B. Kortright, Laura Gagliardi, Silvia Bordiga, Jeffrey A. Reimer & Jeffrey R. Long (2015) “Cooperative insertion of CO2 in diamine-appended metal-organic frameworks” Nature doi: 10.1038/nature14327

Comments

Davemart

It will be interesting to see how many who, like me, are concerned about the emissions of GHG, have the mental flexibility to welcome fossil fuels as long as their emissions can be practically and economically captured.

gorr

Co2 capture is useless and is a scam from the scientific community against government budgets to get many highly paid jobs. climate change is coming from the sun that change temperature. The sun temperature is not constant, it might go up or down over the years.

Calgarygary

Based on press releases for the two new plants in western Canada it appears that the infrastructure cost for a CCS plant is about $1,000,000 per tonne of capacity. My guesstimate is that un-subsidized operators would need to receive at least $75 per tonne just to recover the infrastructure costs without considering the operational costs.

The value of CO2 for enhanced oil recovery can contribute to cost recovery but it appears there needs to be a long term commitment from the oilfield operator to purchase the CO2 and the value of the CO2 is tied to the price of oil.

Perhaps if this new capture method can reduce the infrastructure costs as well as the operational costs there may be some application, but in the current environment of technological uncertainty I can't imagine private money having much interest.

Even investment in nuclear which looks far more promising as a means to reduce CO2 looks to be sitting on the fence for the time being. At the present time, if public money is to be spent, its a lot easier from a political perspective, to spend it on solar, and that expenditure looks like it can bring down the costs to a point where fossil fuels will be very hard pressed to compete.

Big mega-projects take years to get off the drawing board and in the mean time so much can change.

Personally I have a real stake in continued global dependence on fossil fuels for energy but it looks to me they will be under continued pressure from renewable's and efficiency technologies.

mahonj

@dave, I believe I have that mental flexibility.

They key is "practically and economically" captured.
It is no good at demo level, it needs to scale to hundreds of GW worth.
You will NEED fossil fuels to balance renewables and for planes and long distance trucks and boats.

It should be possible to fit amine scrubbers to power stations and boats, but not aircraft - we may just have to live with that one.

However, when I see the example of scrubbing the air in a submarine, it doesn't sounds like a cheap solution.

@Gor, we have pushed the CO2 concentration from 280 to 400 ppm, it can't be doing the planet any good.

Engineer-Poet

It doesn't say what weight percentage the CO2 constitutes of a typical filled MOF.  Possibly this doesn't matter, even for mobile use.  There would be fuel economy penalties from the greater radiator area required to cool the exhaust to 100°C or so, but being able to scrub CO2 and then re-release it at high pressure suitable for compression into a tank of liquid might just be feasible.

Being able to capture 90% or more of the CO2 from a mobile source would be huge.  You could recycle CO2 directly instead of trying to re-capture it from the atmosphere.

mahonj

@EP,
It might be easier to scrub stationary power sources and go for some degree of electrification for vehicles.
Certainly for cars. Larger trucks might justify scrubbers.

If you got the CO2 level for the electricity supply way down (say below 300 gms/KwH), you could promote electric cars and buses. If you have a coal based electricity supply, there is no point, better to push for hybrids or other very low CO2 ICE technologies.

Engineer-Poet

You can't manage an 80% cut in CO2 emissions without either electrifying, de-carbonizing or sequestering ground transport.  Batteries, ammonia or carbon capture.

I've seen the figure of 50 grams/kWh that we need to hit for the electric supply.  Sweden and Quebec are there, among a select few.  France is close, Ontario a little less close.  Germany?  Denmark?  Forget it.

mahonj

The problem is global - there is no point in getting (say) Germany down to 200 gms/KwH when the Chinese are building a 1 GW coal burning power station per week at 1000 gms/KwH.
(Same for India)

How you solve this is beyond me or the EU or the US.

Calgarygary

Just to correct my earlier post, infrastructure costs of the CCS facilities at Weyburn and Fort Sask are around $1000/tonne. Not 1 million. If 30 percent additional energy is required to sequester the carbon and the electricity can be produced at 6 cents per kwh then the cost of electricity might go up by 1.8 cents. Improving the energy efficiency for sequestration by 50% would only save a cent per kwh at these rates. The capital cost is still the biggest cost of CCS.

My electricity bill which is made up of at least 50% coal, comes to about 16 cents per kwh for 500-600 kwh/month. At a contracted rate of 8 cents for energy and the rest of the charges are related to distribution and administration. An additional 5 cents to cover some sequestration would not be devastating to a residential customer like me however at some price point it may make more sense to install solar panels and batteries and either back them up with the grid or go totally off grid with a gas powered backup (fuel cell or ice)

Engineer-Poet

mahonj, we need technology that can get EVERYONE down past 50 gCO2/kWh, and preferably to zero.

I'm partial to Transatomic Power's scheme for this.  If their reactor truly can achieve 96% burnup with an input stream of 1.8%-enriched uranium, then the world's total energy requirements can be met with about 15,000 tons of natural uranium per year.  The molten-salt system produces heat at temperatures high enough for many industrial requirements in addition to electric generation.

China seems to have similar ideas; the push to develop a molten-salt reactor in 10 years (begun in 2012) looks like a way to re-power existing steam plants with nuclear boilers.  Rolling such units out in mass-production style could de-carbonize even coal-fired electric grids in a decade or so.

Would other nations buy Chinese technology based on US inventions?  Given the difficulties of selling Chinese LWRs overseas, maybe not.  This makes the failure of the US to develop its own home-grown technology all the more damning.  History will brand Joe Biden a criminal against the environment (and humanity) for his 1994 Senate vote, along with every other prominent Green in the world.

HarveyD

We may have to stop burning fossil and bio-fuels to reduce GHG to an acceptable level.

NPPs could be one of the solution for e-energy generation if cost and construction time could be reduced by 3X to 5X and more people could be convinced to accept them.

Meanwhile, more hydro, wind and solar (with storage) could do it.

Raglan Nickel mines in the Eastern Canadian Artic has been operating (since August 2014) a specially equipped, large direct drive 3+ mega-watt wind turbine successfully, in very cold temperatures (below -40C) to replace diesel generators. A second similar wind turbine will be installed in summer 2015 to further reduce diesel fuel consumption.

Many native people villages, currently using diesel generators, will soon install similar (smaller) wind turbines (with storage). Most Ungava peninsula villages have high quality winds but are not connected to the power grid. Eventually, the hydro grid could be expanded and native people villages could sell excess wind power to Quebec-Hydro and do without costly storage units.

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