Water Improves Material's Ability to Capture CO2

SCIENTIFIC ACHIEVEMENT

With the help of the Advanced Light Source (ALS), researchers from UC Berkeley and ExxonMobil fine-tuned a material to capture CO₂ in the presence of water.

SIGNIFICANCE AND IMPACT

The parties have applied for a patent on the material, which was developed for use on the relatively humid flue gases emitted by certain natural gas power plants, a cleaner-burning alternative to coal.

Ball-and-stick model of the structure. — Single-crystal x-ray diffraction enables the precise determination of the positions of the atoms in metal–organic frameworks (MOFs), highly porous materials capable of soaking up vast quantities of a specific gas molecule, such as CO₂. This structure represents 2-ampd–Zn₂(dobpdc), a MOF with the same structure as 2-ampd–Mg₂(dobpdc), the subject of this study. Light blue, blue, red, gray, and white spheres represent Zn, N, O, C, and H atoms, respectively.

The power of natural gas

About 65% of anthropogenic greenhouse gas emissions comes from the combustion of fossil fuels in power plants. So far, efforts to capture CO₂ from power-plant flue gases and sequester it underground have mainly focused on coal-fired power plants. However, in the United States, natural gas has surpassed coal in the amount CO₂ released, despite the fact that natural gas emits approximately half as much CO₂ per unit of electricity. Therefore, new materials are urgently needed to address this situation.

Not all combustion is alike

Compared to coal-fired power plants, natural gas combined cycle (NGCC) plants produce flue gases with low CO₂ concentrations. This reduces the carbon footprint, but increases the technical difficulty of CO₂ capture. Also, materials capable of adsorbing such low concentrations of CO₂ often require high temperatures to release it for sequestration, an important part of the cycle that offsets initial low-carbon benefits. NGCC emissions also have a higher concentration of O₂, which has a corrosive effect on adsorbent materials, and both NGCC and coal flue streams are saturated in water, which can both degrade materials and reduce efficiency. Thus, an effective NGCC CO₂-capture material must selectively bind low-concentration CO₂ under humid conditions while being thermally and oxidatively stable.

Earlier discovery and optimization

At NGCC plants, CO₂ capture is currently done primarily using solutions of ammonia derivatives (amines). However, releasing the CO₂ from these liquids is energy intensive and expensive. In previous work done at the Center for Gas Separations (a DOE Energy Frontier Research Center), UC Berkeley researchers discovered a metal–organic framework (MOF) that releases CO₂ with much less energy. The work attracted the attention of ExxonMobil, which was interested in optimizing the new material to make it even more effective. In this work, the Berkeley group, with support from ExxonMobil, worked on fine-tuning the MOF by functionalizing it with cyclic diamine compounds.

Characterization by diffraction

At ALS Beamline 12.2.1, the researchers used single-crystal x-ray diffraction to precisely determine the structure of 2-ampd–Zn₂(dobpdc), a crystal with the same structure as the functionalized MOF. The information made clear how the diamines—the organic molecules that actually react with CO₂—are organized. The researchers were thus able to compare the structure with predictions from computational work and recognize how water might affect CO₂ adsorption. The beamline also offers the ability to dose samples with gas, an experiment the researchers are currently working on to see how CO₂ actually interacts with the MOF binding sites and to further correlate structure with adsorption behavior.

A surprising watery advantage

Adsorption measurements confirmed that 2-ampd–Mg₂(dobpdc) is capable of adsorbing over 90% of the CO₂ in humid natural gas emissions. The researchers also discovered that humidity actually improves the material’s CO₂ adsorption. Based on spectroscopic studies and theoretical calculations, they concluded that certain structures that form upon CO₂ binding are affected by the presence of water, making the CO₂ bind more strongly while preserving the ability to cycle CO₂ in and out with small temperature swings. UC Berkeley and ExxonMobil have applied for a patent on this particular MOF variant, in light of its exceptional and unique ability to capture CO₂ from NGCC flue gases.

Graph shows a step-shaped curve for humid conditions and a smoothly rising S-curve for dry conditions. Inset shows space-filling models of CO2, N2, and MOF. Right side shows ball-and stick diagrams, before and after addition of H2O. — Left: Plot of the CO₂ content in simulated flue gases after passage through a column of the optimized MOF. Under humid conditions (black curve), CO₂ leakage is effectively zero until a saturation point is reached. Under dry conditions (green), some CO₂ leaks through, even at very early stages. The inset depicts CO₂ and N₂entering the material, but only the N₂ exiting. Top right: The MOF’s Mg cluster (red/green) was functionalized with cyclic diamine (gray/blue/white). Bottom right: In humid conditions, H₂O forms hydrogen bonds with the cyclic diamine.

Contact: Jeffrey Long

Researchers: R.L. Siegelman, P.J. Milner, J.-H. Lee, J.A. Reimer, and J.R. Long (UC Berkeley and Berkeley Lab); A.C. Forse and K.A. Colwell (UC Berkeley); J.B. Neaton (UC Berkeley, Berkeley Lab, and Kavli Energy Nanosciences Institute); and S.C. Weston (ExxonMobil Research and Engineering Company).

Funding: ExxonMobil Research and Engineering Company, National Institutes of Health, Philomathia Foundation, Berkeley Energy and Climate Institute, and University of California, Berkeley. Operation of the ALS is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences Program.

Publication: R.L. Siegelman, P.J. Milner, A.C. Forse, J.-H. Lee, K.A. Colwell, J.B. Neaton, J.A. Reimer, S.C. Weston, and J.R. Long, “Water Enables Efficient CO₂ Capture from Natural Gas Flue Emissions in an Oxidation-Resistant Diamine-Appended Metal–Organic Framework,” J. Am. Chem. Soc. 141, 13171 (2019), doi:10.1021/jacs.9b05567.

ALS SCIENCE HIGHLIGHT #413

Water Improves Material’s Ability to Capture CO₂