Spallation Neutron Source puts the squeeze on methane hydrate cages

Our robot would find this energy source in shale deposits, notably here on the east coast of the United States. However, the most abundant deposits of natural gas are under water on the continental shelves and in the permafrost in the Arctic region. At both poles, methane mixes with water and freezes, remaining trapped as an ice-like compound, for millions of years.

Much further afield, methane, along with water and ammonia, are major constituents of Saturn's icy moon Titan. Some scientists speculate that on Titan there is a methanological cycle similar to the hydrological cycle here on earth. Surface methane evaporates into the atmosphere, where it condenses, and rains down to the surface again. NASA's Cassini-Huygens Titan probe has been there and sampled it.

Methane holds promise as an abundant energy source for tomorrow, but it is Janus-faced: While often referred to as the cleanest fossil fuel producing far less greenhouse gas than either coal or oil, historically it has been seen as a major source of environmental pollution. That's because burning it produces carbon dioxide, a very potent greenhouse gas.

Scientists are looking at how to sequester that CO2 byproduct, in an ice-like state. Such a strategy would create a carbon 'energy cycle' in which the methane resource is recovered, used, and then the greenhouse gas sequestered in a form very closely related to the naturally occuring initial materials.

"What we do know right now is that when methane is taken up and released into the environment, water plays a critical role", said Chris Tulk, lead instrument scientist on the Spallation Neutrons and Pressure Diffractometer (SNAP) at ORNL. "Whether it is in the oceans where hydrates form on continental shelves, in the icy permafrost conditions, or even as these materials decompose and release the methane into the atmosphere to contribute to the greenhouse effect, water is certainly involved in the process. This research should lead to better models of how hydrocarbons are taken up and released in the environment."

To develop such models, they need to understand at the molecular level the relationship between methane and water. Researchers have discovered that water forms cages, called clathrates, that contain "guest" molecules of methane and of many of the Noble gases.

At SNAP, a combination of a state-of-the-art instrument, clever experimental technique, and excellent samples have for the first time yielded detailed data on the structure of these methane hydrate clathrates.

Under pressure of more than 600,000 pounds per square inch, they found that the correct occupancy for the largest cages in this beautiful structure is three methane molecules. This finding can now be used to benchmark methane and water interactions at various energy and pressure, and researchers can better characterize the hydrophobic interaction.

"We've done a lot of work on these clathrate compounds," said Tulk, "but this is the first work in which all the work could be done on SNAP.

"When we compress the methane clathrate hydrate, it goes through a phase change at the molecular level to a new high pressure form known as structure H, for the hexagonal (six-sided) arrangement of water," Tulk explained.

"As the pressure is increased and the sample becomes smaller, the overall density increases, as expected. But the water molecules re-arrange themselves to form larger cages. These larger cages can now accommodate more than one methane molecule. The key question in this research was, how many methane molecules are in these larger cages, and how are they arranged?"

The SNAP instrument is perfectly suited to provide these types of structural details. "The repulsive interaction between methane and water, called the hydrophobic interaction, is poorly understood," Tulk explained. "And the interaction between methane and methane, particularly when the molecules are nearly in contact and strongly repelling, is not well understood at all."

Understanding how many methanes are able to fit in each cage and how the methane molecules are arranged within these cages, provides insight into these interactions.

The research also assists computational simulation. There are currently no good models to predict clathrate structure. "Determining how many methane molecules are in a cage will give the computational chemistry folks something to shoot for with their hydrate models," Tulk said.

Given these experimental results to come up with a new "potential" -- i.e., a new calculation of the interaction force that exists between methane molecules, and between methane and water - computational chemists can calculate the way methane interacts with water in the larger environment.

The ball is now in the hands of the theorists, who must come up with a model that correctly predicts this experimental observation. Then they can extend the model to better predict how water and hydrocarbons interact in the larger environment.

"That is the driving motivation for my research, to get a fundamental physics-chemistry perspective on these things that have such a large impact on the earth."

Provided by Oak Ridge National Laboratory (news : web)

The great gas hydrate escape

The analysis is the first time researchers have accurately quantified the molecular-scale interactions between the gases -- either hydrogen or methane, aka natural gas -- and the water molecules that form cages around them. A team of researchers from the Department of Energy's Pacific Northwest National Laboratory published the results in Chemical Physics Letters online December 22, 2011.

The results could also provide insight into the process of replacing methane with carbon dioxide in the naturally abundant "water-based reservoirs," according to the lead author, PNNL chemist Sotiris Xantheas.

"Current thinking is that you need large amounts of energy to push the methane out, which destroys the scaffold in the process," said Xantheas. "But the computer modeling shows that there is an alternative low energy pathway. All you need to do is break a single hydrogen bond between water molecules forming the cage -- the methane comes out, and then the hydrate reseals itself."

Cagey Ice

Gas hydrates -- especially methane hydrates, which store natural gas -- look like ice but actually hold burnable fuel. Naturally found deep in the ocean, water and gas interweave in the hydrates, but little is known about their chemical structure and processes occurring at the molecular level. They have been known to cause problems for the petroleum industry because they tend to clog pipes and can explode. A methane hydrate produced the bubble of methane gas that contributed to 2010's Gulf of Mexico oil spill.

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This is a computer simulation of methane, also known as natural gas, escaping from a methane hydrate. Many of these methane hydrate subunits combine to form a chunk of ice that burns, and this simulation shows how methane can get out without collapsing the entire structure. Red-and-silver water molecules form a cage around two blue-and-silver methane molecules. Two methane molecules are too tight a fit, so a low-energy hydrogen bond -- the red dotted lines -- between two water molecules breaks. This allows the water molecule to swing open like a gate as the methane makes a dash for it. The water molecule swings back into place, the hydrogen bond re-forms, and the methane hydrate remains stable. Credit: Sotiris Xantheas, PNNL

In previous work, Xantheas and colleagues used computer algorithms and models to examine the water-based, ice-like scaffold that holds the gas. Water molecules form individual cages made with 20 or 24 molecules. Multiple cages join together in large lattices. But those scaffolds were empty in the earlier analysis.

To find out how fuels can be accommodated inside the water cages, Xantheas and PNNL colleague Soohaeng Yoo Willow built computer models of the cages with either hydrogen gas -- in which two hydrogen atoms are bound together -- or methane gas, a small molecule made with one carbon and four hydrogen atoms.

In the hydrogen hydrates, which could potentially be used as materials for hydrogen fuel storage, a small hollow cage made from 20 water molecules could hold up to a maximum of five hydrogen molecules and a larger cage made from 24 water molecules could hold up to seven.

The maximum storage capacity equates to about 10 weight-percent, or the percentage of hydrogen by mass in the chunks of ice, although packing hydrogen in that tight puts undue strain on the system. The Department of Energy's goal for hydrogen storage -- to make the fuel practical -- is above 5.5 weight-percent.

Experimentally, hydrogen storage researchers typically measure much less storage capacities. The computer model showed them why: The hydrogen molecules tended to leak out of the cages, reducing the amount of hydrogen that could be stored.

The researchers found that adding a methane molecule to the larger cages in the pure hydrogen hydrate, however, prevented the hydrogen gas from leaking out. The computer model showed the researchers that they could store the hydrogen at high pressure and practical temperatures, and release it by reducing the pressure, which melts it.

Water Gates

Understanding how the gas interacts and moves through the cages can help chemists or engineers store gas and remove it at will. Willow and Xantheas' computer simulations showed that hydrogen molecules could migrate through the cages by passing between the figurative bars of the water cages. However, the cages also had gates: Sometimes a low-energy bond between two water molecules broke, causing a water molecule to swing open and let the hydrogen molecule drift out. The "gate" closed right after the molecule passed through to reform the lattice.

With methane hydrates, some fuel producers want to remove the gas safely to use it. Others see the emptied cages as potential storage sites for carbon dioxide, which could theoretically keep it out of the atmosphere and ocean, where it warms the earth and acidifies the sea. So, Willow and Xantheas tested how methane could migrate through the cages.

The water cages were only big enough to comfortably hold one methane molecule, so the chemists stuffed two methanes inside and watched what happened. Quickly, one of the water molecules forming the cage swung open like a gate, allowing one methane molecule to escape. The gate then slammed shut as the remaining methane scooted into the middle of the cage.

"This process is important because it can happen with natural gas. It shows how methane can move in the natural world," said Xantheas. "We hope this analysis will help with the technical issues that need to be addressed with gas hydrate research and development."

Xantheas said performing computer simulations with carbon dioxide instead of methane might help determine whether it's chemically feasible to store carbon dioxide in hydrates.

More information: Soohaeng Yoo Willow and Sotiris S. Xantheas, 2011/12. Enhancement of Hydrogen Storage capacity in Hydrate Lattices, Chem. Phys. Lett. Dec. 22, 2011, doi: 10.1016/j.cplett.2011.12.036

Provided by Pacific Northwest National Laboratory (news : web)