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.
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.
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