Tag Archives: Praveen K. Thallapally

Cleaning up nuclear waste gases with nanotechnology-enabled materials

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Swiss and US scientists have developed a nanoporous crystal that could be used to clean up nuclear waste gases according to a June 13, 2016 news item on Nanowerk (Note: A link has been removed),

An international team of scientists at EPFL [École polytechnique fédérale de Lausanne in Switzerland] and the US have discovered a material that can clear out radioactive waste from nuclear plants more efficiently, cheaply, and safely than current methods.

Nuclear energy is one of the cheapest alternatives to carbon-based fossil fuels. But nuclear-fuel reprocessing plants generate waste gas that is currently too expensive and dangerous to deal with. Scanning hundreds of thousands of materials, scientists led by EPFL and their US colleagues have now discovered a material that can absorb nuclear waste gases much more efficiently, cheaply and safely. The work is published in Nature Communications (“Metal–organic framework with optimally selective xenon adsorption and separation”).

A June 14, 2016 EPFL press release (also on EurekAlert), which originated the news item, explains further,

Nuclear-fuel reprocessing plants generate volatile radionuclides such as xenon and krypton, which escape in the so-called “off-gas” of these facilities – the gases emitted as byproducts of the chemical process. Current ways of capturing and clearing out these gases involve distillation at very low temperatures, which is expensive in both terms of energy and capital costs, and poses a risk of explosion.

Scientists led by Berend Smit’s lab at EPFL (Sion) and colleagues in the US, have now identified a material that can be used as an efficient, cheaper, and safer alternative to separate xenon and krypton – and at room temperature. The material, abbreviated as SBMOF-1, is a nanoporous crystal and belongs a class of materials that are currently used to clear out CO2 emissions and other dangerous pollutants. These materials are also very versatile, and scientists can tweak them to self-assemble into ordered, pre-determined crystal structures. In this way, they can synthesize millions of tailor-made materials that can be optimized for gas storage separation, catalysis, chemical sensing and optics.

The scientists carried out high-throughput screening of large material databases of over 125,000 candidates. To do this, they used molecular simulations to find structures that can separate xenon and krypton, and under conditions that match those involved in reprocessing nuclear waste.

Because xenon has a much shorter half-life than krypton – a month versus a decade – the scientists had to find a material that would be selective for both but would capture them separately. As xenon is used in commercial lighting, propulsion, imaging, anesthesia and insulation, it can also be sold back into the chemical market to offset costs.

The scientists identified and confirmed that SBMOF-1 shows remarkable xenon capturing capacity and xenon/krypton selectivity under nuclear-plant conditions and at room temperature.

The US partners have also made an announcement with this June 13, 2016 Pacific Northwest National Laboratory (PNNL) news release (also on EurekAlert), Note: It is a little repetitive but there’s good additional information,

Researchers are investigating a new material that might help in nuclear fuel recycling and waste reduction by capturing certain gases released during reprocessing. Conventional technologies to remove these radioactive gases operate at extremely low, energy-intensive temperatures. By working at ambient temperature, the new material has the potential to save energy, make reprocessing cleaner and less expensive. The reclaimed materials can also be reused commercially.

Appearing in Nature Communications, the work is a collaboration between experimentalists and computer modelers exploring the characteristics of materials known as metal-organic frameworks.

“This is a great example of computer-inspired material discovery,” said materials scientist Praveen Thallapally of the Department of Energy’s Pacific Northwest National Laboratory. “Usually the experimental results are more realistic than computational ones. This time, the computer modeling showed us something the experiments weren’t telling us.”

Waste avoidance

Recycling nuclear fuel can reuse uranium and plutonium — the majority of the used fuel — that would otherwise be destined for waste. Researchers are exploring technologies that enable safe, efficient, and reliable recycling of nuclear fuel for use in the future.

A multi-institutional, international collaboration is studying materials to replace costly, inefficient recycling steps. One important step is collecting radioactive gases xenon and krypton, which arise during reprocessing. To capture xenon and krypton, conventional technologies use cryogenic methods in which entire gas streams are brought to a temperature far below where water freezes — such methods are energy intensive and expensive.

Thallapally, working with Maciej Haranczyk and Berend Smit of Lawrence Berkeley National Laboratory [LBNL] and others, has been studying materials called metal-organic frameworks, also known as MOFs, that could potentially trap xenon and krypton without having to use cryogenics.

These materials have tiny pores inside, so small that often only a single molecule can fit inside each pore. When one gas species has a higher affinity for the pore walls than other gas species, metal-organic frameworks can be used to separate gaseous mixtures by selectively adsorbing.

To find the best MOF for xenon and krypton separation, computational chemists led by Haranczyk and Smit screened 125,000 possible MOFs for their ability to trap the gases. Although these gases can come in radioactive varieties, they are part of a group of chemically inert elements called “noble gases.” The team used computing resources at NERSC, the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at LBNL.

“Identifying the optimal material for a given process, out of thousands of possible structures, is a challenge due to the sheer number of materials. Given that the characterization of each material can take up to a few hours of simulations, the entire screening process may fill a supercomputer for weeks,” said Haranczyk. “Instead, we developed an approach to assess the performance of materials based on their easily computable characteristics. In this case, seven different characteristics were necessary for predicting how the materials behaved, and our team’s grad student Cory Simon’s application of machine learning techniques greatly sped up the material discovery process by eliminating those that didn’t meet the criteria.”

The team’s models identified the MOF that trapped xenon most selectively and had a pore size close to the size of a xenon atom — SBMOF-1, which they then tested in the lab at PNNL.

After optimizing the preparation of SBMOF-1, Thallapally and his team at PNNL tested the material by running a mixture of gases through it — including a non-radioactive form of xenon and krypton — and measuring what came out the other end. Oxygen, helium, nitrogen, krypton, and carbon dioxide all beat xenon out. This indicated that xenon becomes trapped within SBMOF-1’s pores until the gas saturates the material.

Other tests also showed that in the absence of xenon, SBMOF-1 captures krypton. During actual separations, then, operators would pass the gas streams through SBMOF-1 twice to capture both gases.

The team also tested SBMOF-1’s ability to hang onto xenon in conditions of high humidity. Humidity interferes with cryogenics, and gases must be dehydrated before putting them through the ultra-cold method, another time-consuming expense. SBMOF-1, however, performed quite admirably, retaining more than 85 percent of the amount of xenon in high humidity as it did in dry conditions.

The final step in collecting xenon or krypton gas would be to put the MOF material under a vacuum, which sucks the gas out of the molecular cages for safe storage. A last laboratory test examined how stable the material was by repeatedly filling it up with xenon gas and then vacuuming out the xenon. After 10 cycles of this, SBMOF-1 collected just as much xenon as the first cycle, indicating a high degree of stability for long-term use.

Thallapally attributes this stability to the manner in which SBMOF-1 interacts with xenon. Rather than chemical reactions between the molecular cages and the gases, the relationship is purely physical. The material can last a lot longer without constantly going through chemical reactions, he said.

A model finding

Although the researchers showed that SBMOF-1 is a good candidate for nuclear fuel reprocessing, getting these results wasn’t smooth sailing. In the lab, the researchers had followed a previously worked out protocol from Stony Brook University to prepare SBMOF-1. Part of that protocol requires them to “activate” SBMOF-1 by heating it up to 300 degrees Celsius, three times the temperature of boiling water.

Activation cleans out material left in the pores from MOF synthesis. Laboratory tests of the activated SBMOF-1, however, showed the material didn’t behave as well as it should, based on the computer modeling results.

The researchers at PNNL repeated the lab experiments. This time, however, they activated SBMOF-1 at a lower temperature, 100 degrees Celsius, or the actual temperature of boiling water. Subjecting the material to the same lab tests, the researchers found SBMOF-1 behaving as expected, and better than at the higher activation temperature.

But why? To figure out where the discrepancy came from, the researchers modeled what happened to SBMOF-1 at 300 degrees Celsius. Unexpectedly, the pores squeezed in on themselves.

“When we heated the crystal that high, atoms within the pore tilted and partially blocked the pores,” said Thallapally. “The xenon doesn’t fit.”

Armed with these new computational and experimental insights, the researchers can explore SBMOF-1 and other MOFs further for nuclear fuel recycling. These MOFs might also be able to capture other noble gases such as radon, a gas known to pool in some basements.

Researchers hailed from several other institutions as well as those listed earlier, including University of California, Berkeley, Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, Brookhaven National Laboratory, and IMDEA Materials Institute in Spain. This work was supported by the [US] Department of Energy Offices of Nuclear Energy and Science.

Here’s an image the researchers have provided to illustrate their work,

Caption: The crystal structure of SBMOF-1 (green = Ca, yellow = S, red = O, gray = C, white = H). The light blue surface is a visualization of the one-dimensional channel that SBMOF-1 creates for the gas molecules to move through. The darker blue surface illustrates where a Xe atom sits in the pores of SBMOF-1 when it adsorbs. Credit: Berend Smit/EPFL/University of California Berkley

Caption: The crystal structure of SBMOF-1 (green = Ca, yellow = S, red = O, gray = C, white = H). The light blue surface is a visualization of the one-dimensional channel that SBMOF-1 creates for the gas molecules to move through. The darker blue surface illustrates where a Xe atom sits in the pores of SBMOF-1 when it adsorbs. Credit: Berend Smit/EPFL/University of California Berkley

Here’s a link to and a citation for the paper,

Metal–organic framework with optimally selective xenon adsorption and separation by Debasis Banerjee, Cory M. Simon, Anna M. Plonka, Radha K. Motkuri, Jian Liu, Xianyin Chen, Berend Smit, John B. Parise, Maciej Haranczyk, & Praveen K. Thallapally. Nature Communications 7, Article number: ncomms11831  doi:10.1038/ncomms11831 Published 13 June 2016

This paper is open access.

Final comment, this is the second time in the last month I’ve stumbled across more positive approaches to nuclear energy. The first time was a talk (Why Nuclear Power is Necessary) held in Vancouver, Canada in May 2016 (details here). I’m not trying to suggest anything unduly sinister but it is interesting since most of my adult life nuclear power has been viewed with fear and suspicion.

MOFs (metal-organic frameworks) to clean up nuclear waste?

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There’s a possibility that metal-organic frameworks could be used to clean up nuclear waste according to an Aug. 5, 2015 news item on phys.org,

One of the most versatile and widely applicable classes of materials being studied today are the metal-organic frameworks. These materials, known as MOFs, are characterized by metal ions or metal-ion clusters that are linked together with organic molecules, forming ordered crystal structures that contain tiny cage-like pores with diameters of two nanometers or less.

MOFs can be thought of as highly specialized and customizable sieves. By designing them with pores of a certain size, shape, and chemical composition, researchers can tailor them for specific purposes. A few of the many, many possible applications for MOFs are storing hydrogen in fuel cells, capturing environmental contaminants, or temporarily housing catalytic agents for chemical reactions.

At [US Department of Energy] Brookhaven National Laboratory, physicist Sanjit Ghose and his collaborators have been studying MOFs designed for use in the separation of waste from nuclear reactors, which results from the reprocessing of nuclear fuel rods. He is targeting two waste products in particular: the noble gases xenon (Xe) and krypton (Kr).

An Aug. 4, 2015 Brookhaven National Laboratory news release, which originated the news item, describes not only the research and the reasons for it but also the institutional collaborations necessary to conduct the research,

There are compelling economic and environmental reasons to separate Xe and Kr from the nuclear waste stream. For one, because they have very different half-lives – about 36 days for Xe and nearly 11 years for Kr – pulling out the Xe greatly reduces the amount of waste that needs to be stored long-term before it is safe to handle. Additionally, the extracted Xe can be used for industrial applications, such as in commercial lighting and as an anesthetic. This research may also help scientists determine how to create MOFs that can remove other materials from the nuclear waste stream and expose the remaining unreacted nuclear fuel for further re-use. This could lead to much less overall waste that must be stored long-term and a more efficient system for producing nuclear energy, which is the source of about 20 percent of the electricity in the U.S.

Because Xe and Kr are noble gases, meaning their outer electron orbitals are filled and they don’t tend to bind to other atoms, they are difficult to manipulate. The current method for extracting them from the nuclear waste stream is cryogenic distillation, a process that is energy-intensive and expensive. The MOFs studied here use a very different approach: polarizing the gas atoms dynamically, just enough to draw them in using the van der Waals force. The mechanism works at room temperature, but also at hotter temperatures, which is key if the MOFs are to be used in a nuclear environment.

Recently, Ghose co-authored two papers that describe MOFs capable of adsorbing Xe and Kr, and excel at separating the Xe from the Kr. The papers are published in the May 22 online edition of the Journal of the American Chemical Society and the April 16 online edition of the Journal of Physical Chemistry Letters.

“Only a handful of noble-gas-specific MOFs have been studied so far, and we felt there was certainly scope for improvement through the discovery of more selective materials,” said Ghose.

Both MOF studies were carried out by large multi-institution collaborations, using a combination of X-ray diffraction, theoretical modeling, and other methods. The X-ray work was performed at Brookhaven’s former National Synchrotron Light Source (permanently closed and replaced by its successor, NSLS-II) and the Advanced Photon Source at Argonne National Laboratory (ANL), both DOE Office of Science User Facilities.

The JACS paper was co-authored by researchers from Brookhaven Lab, Stony Brook University (SBU), Pacific Northwest National Laboratory (PNNL), and the University of Amsterdam. Authors on the JPCL paper include scientists from Brookhaven, SBU, PNNL, ANL, the Deutsches Elektronen-Synchrotron (DESY) in Germany, and DM Strachan, LLC.

Here’s more about the first published paper in the Journal of Physical Chemistry Letters (JCPL) (from the news release)

A nickel-based MOF

The MOF studied in the JCPL paper consists of nickel (Ni) and the organic compound dioxido-benzene-dicarboxylate (DOBC), and is thus referred to as Ni-DOBDC. Ni-DOBDC can adsorb both Xe and Kr at room temperature but is highly selective toward Xe. In fact, it boasts what may be the highest Xe adsorption capacity of a MOF discovered to date.

The group studied Ni-DOBC using two main techniques: X-ray diffraction and first-principles density functional theory (DFT). The paper is the first published report to detail the adsorption mechanism by which the MOF takes in these noble gases at room temperature and pressure.

“Our results provide a fundamental understanding of the adsorption structure and the interactions between the MOF and the gas by combining direct structural analyses from experimental X-ray diffraction data and DFT calculations,” said Ghose.

The group was also able to discover the existence of a secondary site at the pore center in addition to the six-fold primary site. The seven-atom loading scheme was initially proposed by theorist Yan Li, an co-author of the JCPL paper and formerly on staff at Brookhaven (she is now an editor at Physical Review B), which was confirmed experimentally and theoretically. Data also indicate that Xe are adsorbed more strongly than Kr, due to its higher atomic polarizability. They also discovered a temperature-dependence of the adsorption that furthers this MOF’s selectivity for Xe over Kr. As the temperature was increased above room temperature, the Kr adsorption drops more drastically than for Xe. Over the entire temperature range tested, Xe adsorption always dominates that of Kr.

“The high separation capacity of Ni-DOBDC suggests that it has great potential for removing Xe from Kr in the off-gas streams in nuclear spent fuel reprocessing, as well as filtering Xe at low concentration from other gas mixtures,” said Ghose.

Ghose and Li are now preparing a manuscript that will discuss a more in-depth investigation into the possibility of packing in even more Xe atoms.

“Because of the confinement offered by each pore, we want to see if it’s possible to fit enough Xe in each chamber to form a solid,” said Li.

Ghose and Li hope to experimentally test this idea at NSLS-II in the future, at the facility’s X-ray Powder Diffraction (XPD) beamline, which Ghose has helped develop and build. Additional future studies of these and other MOFs will also take place at XPD. For example, they want to see what happens when other gases are present, such as nitrogen oxides, to mimic what happens in an actual nuclear reactor.

Then, there was the second paper published in the Journal of the American Chemical Society (JACS),

Another MOF, Another Promising Result

In the JACS paper, Ghose and researchers from Brookhaven, SBU, PNNL, and the University of Amsterdam describe a second MOF, dubbed Stony Brook MOF-2 (SBMOF-2). It also captures both Xe and Kr at room temperature and pressure, although is about ten times as effective at taking in Xe, with Xe taking up as much as 27 percent of its weight. SBMOF-2 had been theoretically predicted to be an efficient adsorbent for Xe and Kr, but until this research there had been no experimental results to back up the prediction.

“Our study is different than MOF research done by other groups,” said chemist John Parise, a coauthor of the JACS paper who holds a joint position with Brookhaven and SBU. “We did a lot of testing and investigated the capture mechanism very closely to get clues that would help us understand why the MOF worked, and how to tailor the structure to have even better properties.”

SBMOF-2 contains calcium (Ca) ions and an organic compound with the chemical formula C34H22O8. X-ray data show that its structure is unusual among microporous MOFs. It has fewer calcium sites than expected and an excess of oxygen over calcium. The calcium and oxgyen form CaO6, which takes the form of a three-dimensional octahedron. Notably, none of the six oxygen atoms bound to the calcium ion are shared with any other nearby calcium ions. The authors believe that SBMOF-2 is the first microporous MOF with these isolated CaO6 octahedra, which are connected by organic linker molecules.

The group discovered that the preference of SBMOF-2 for Xe over Kr is due to both the geometry and chemistry of its pores. All the pores have diamond-shaped cross sections, but they come in two sizes, designated type-1 and type-2. Both sizes are a better fit for the Xe molecule. The interiors of the pores have walls made of phenyl groups – ring-shaped C6H5 molecules – along with delocalized electron clouds and H atoms pointing into the pore. The type-2 pores also have hydroxyl anions (OH-) available. All of these features provide are potential sites for adsorbed Xe and Kr atoms.

In follow-up studies, Ghose and his colleagues will use these results to guide them as they determine what changes can be made to these MOFs to improve adsorption, as well as to determine what existing MOFs may yield similar or better performance.

Here are links to and citations for both papers,

Understanding the Adsorption Mechanism of Xe and Kr in a Metal–Organic Framework from X-ray Structural Analysis and First-Principles Calculations by Sanjit K. Ghose, Yan Li, Andrey Yakovenko, Eric Dooryhee, Lars Ehm, Lynne E. Ecker, Ann-Christin Dippel, Gregory J. Halder, Denis M. Strachan, and Praveen K. Thallapally. J. Phys. Chem. Lett., 2015, 6 (10), pp 1790–1794 DOI: 10.1021/acs.jpclett.5b00440 Publication Date (Web): April 16, 2015

Copyright © 2015 American Chemical Society

Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal–Organic Framework by Xianyin Chen, Anna M. Plonka, Debasis Banerjee, Rajamani Krishna, Herbert T. Schaef, Sanjit Ghose, Praveen K. Thallapally, and John B. Parise. J. Am. Chem. Soc., 2015, 137 (22), pp 7007–7010 DOI: 10.1021/jacs.5b02556 Publication Date (Web): May 22, 2015
Copyright © 2015 American Chemical Society

Both papers are behind a paywall.

Trapping gases left from nuclear fuels

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A July 20, 2014 news item on ScienceDaily provides some insight into recycling nuclear fuel,

When nuclear fuel gets recycled, the process releases radioactive krypton and xenon gases. Naturally occurring uranium in rock contaminates basements with the related gas radon. A new porous material called CC3 effectively traps these gases, and research appearing July 20 in Nature Materials shows how: by breathing enough to let the gases in but not out.

The CC3 material could be helpful in removing unwanted or hazardous radioactive elements from nuclear fuel or air in buildings and also in recycling useful elements from the nuclear fuel cycle. CC3 is much more selective in trapping these gases compared to other experimental materials. Also, CC3 will likely use less energy to recover elements than conventional treatments, according to the authors.

A July 21, 2014 US Department of Energy (DOE) Pacific Northwest National Laboratory (PNNL) news release (also on EurekAlert), which originated the news item despite the difference in dates, provides more details (Note: A link has been removed),

The team made up of scientists at the University of Liverpool in the U.K., the Department of Energy’s Pacific Northwest National Laboratory, Newcastle University in the U.K., and Aix-Marseille Universite in France performed simulations and laboratory experiments to determine how — and how well — CC3 might separate these gases from exhaust or waste.

“Xenon, krypton and radon are noble gases, which are chemically inert. That makes it difficult to find materials that can trap them,” said coauthor Praveen Thallapally of PNNL. “So we were happily surprised at how easily CC3 removed them from the gas stream.”

Noble gases are rare in the atmosphere but some such as radon come in radioactive forms and can contribute to cancer. Others such as xenon are useful industrial gases in commercial lighting, medical imaging and anesthesia.

The conventional way to remove xenon from the air or recover it from nuclear fuel involves cooling the air far below where water freezes. Such cryogenic separations are energy intensive and expensive. Researchers have been exploring materials called metal-organic frameworks, also known as MOFs, that could potentially trap xenon and krypton without having to use cryogenics. Although a leading MOF could remove xenon at very low concentrations and at ambient temperatures admirably, researchers wanted to find a material that performed better.

Thallapally’s collaborator Andrew Cooper at the University of Liverpool and others had been researching materials called porous organic cages, whose molecular structures are made up of repeating units that form 3-D cages. Cages built from a molecule called CC3 are the right size to hold about three atoms of xenon, krypton or radon.

To test whether CC3 might be useful here, the team simulated on a computer CC3 interacting with atoms of xenon and other noble gases. The molecular structure of CC3 naturally expands and contracts. The researchers found this breathing created a hole in the cage that grew to 4.5 angstroms wide and shrunk to 3.6 angstroms. One atom of xenon is 4.1 angstroms wide, suggesting it could fit within the window if the cage opens long enough. (Krypton and radon are 3.69 angstroms and 4.17 angstroms wide, respectively, and it takes 10 million angstroms to span a millimeter.)

The computer simulations revealed that CC3 opens its windows big enough for xenon about 7 percent of the time, but that is enough for xenon to hop in. In addition, xenon has a higher likelihood of hopping in than hopping out, essentially trapping the noble gas inside.

The team then tested how well CC3 could pull low concentrations of xenon and krypton out of air, a mix of gases that included oxygen, argon, carbon dioxide and nitrogen. With xenon at 400 parts per million and krypton at 40 parts per million, the researchers sent the mix through a sample of CC3 and measured how long it took for the gases to come out the other side.

Oxygen, nitrogen, argon and carbon dioxide — abundant components of air — traveled through the CC3 and continued to be measured for the experiment’s full 45 minute span. Xenon however stayed within the CC3 for 15 minutes, showing that CC3 could separate xenon from air.

In addition, CC3 trapped twice as much xenon as the leading MOF material. It also caught xenon 20 times more often than it caught krypton, a characteristic known as selectivity. The leading MOF only preferred xenon 7 times as much. These experiments indicated improved performance in two important characteristics of such a material, capacity and selectivity.

“We know that CC3 does this but we’re not sure why. Once we understand why CC3 traps the noble gases so easily, we can improve on it,” said Thallapally.

To explore whether MOFs and porous organic cages offer economic advantages, the researchers estimated the cost compared to cryogenic separations and determined they would likely be less expensive.

“Because these materials function well at ambient or close to ambient temperatures, the processes based on them are less energy intensive to use,” said PNNL’s Denis Strachan.

The material might also find use in pharmaceuticals. Most molecules come in right- and left-handed forms and often only one form works in people. In additional experiments, Cooper and colleagues in the U.K. tested CC3’s ability to distinguish and separate left- and right-handed versions of an alcohol. After separating left- and right-handed forms of CC3, the team showed in biochemical experiments that each form selectively trapped only one form of the alcohol.

The researchers have provided an image illustrating a CC3 cage,

Breathing room: In this computer simulation, light and dark purple highlight the cavities within the 3D pore structure of CC3. Courtesy: PNNL

Breathing room: In this computer simulation, light and dark purple highlight the cavities within the 3D pore structure of CC3. Courtesy: PNNL

Here’s a link to and a citation for the paper,

Separation of rare gases and chiral molecules by selective binding in porous organic cages by Linjiang Chen, Paul S. Reiss, Samantha Y. Chong, Daniel Holden, Kim E. Jelfs, Tom Hasell, Marc A. Little, Adam Kewley, Michael E. Briggs, Andrew Stephenson, K. Mark Thomas, Jayne A. Armstrong, Jon Bell, Jose Busto, Raymond Noel, Jian Liu, Denis M. Strachan, Praveen K. Thallapally, & Andrew I. Cooper. Nature Material (2014) doi:10.1038/nmat4035 Published online 20 July 2014

This paper is behind a paywall.