Carbon sequestration (capturing carbon dioxide) and Zeo++

Carbon capture has been proposed as a way to mitigate global climate change and Zeo++ is a software which promises to help the search for porous materials that will filter out carbon (capture carbon) before it reaches the atmosphere. From the Mar. 1, 2012 news item on Nanowerk,

Approximately 75 percent of electricity used in the United States is produced by coal-burning power plants that spew carbon dioxide (CO2) into the atmosphere and contribute to global warming. To reduce this effect, many researchers are searching for porous materials to filter out the CO2 generated by these plants before it reaches the atmosphere, a process commonly known as carbon capture. But identifying these materials is easier said than done.

“There are a number of porous substances—including crystalline porous materials, such as zeolites, and metal-organic frameworks—that could be used to capture carbon dioxide from power plant emissions,” says Maciej Haranczyk, a scientist in the Lawrence Berkeley National Laboratory’s (Berkeley Lab) Computational Research Division.

In the category of zeolites alone, Haranczyk notes that there are around 200 known materials and 2.5 million structures predicted by computational methods. That’s why Haranczyk and colleagues have developed a computational tool that can help researchers sort through vast databases of porous materials to identify promising carbon capture candidates—and at record speeds. They call it Zeo++.

Here’s a description of the software from the Zeo++ home page,

Zeo++ is a software package for analysis of crystalline porous materials. Zeo++ can be used to perform geometry-based analysis of structure and topology of the void space inside a material, to alternate structures as well as to generate structure representations to-be-used in structure similarity calculations. Zeo++ can be used to either analyze a single structure or perform high-throughput analysis of a large database.

Here’s what the scientists are trying to determine when they use the software to analyze the proposed carbon capture materials (from the news item),

Porous materials like zeolites or metal organic frameworks come in a variety of shapes and have a range of pore sizes. It is actually the shape and pore sizes that determine which molecules get absorbed into the material and which ones pass through.

Like molecular sponges, porous materials can also be reused in a cycle of capture and release. For instance, in the case of carbon capture, once the material is saturated and cannot absorb any more CO2, the gas can be extracted, and the cycle repeated.

“Understanding how all of these factors combine to effectively capture carbon is a challenge,” says Richard Luis Martin, a member of the Zeo++ development team and a postdoctoral researcher in Berkeley Lab’s Computational Research Division. “Until Zeo++, there were no easy methods for analyzing such large numbers of material structures and identifying what makes a material an outstanding carbon catcher.”

He notes that silicious zeolites, to take one example, are composed of the same tetrahedral blocks of silicon and oxygen atoms, but the geometric arrangement of these blocks differs from one zeolite to the next, and this configuration is what determines how CO2 or any other molecule will interact with the porous material.

Before Zeo++, scientists would typically characterize a porous structure based on a single feature, like the size of its largest pore or its total volume of empty space, then compare and categorize it based on this single observation.

“The problem with this one-dimensional description is that it does not tell you anything about how a molecule like CO2 will move through the material,” says Martin. “To identify the most effective materials for absorbing CO2, we need to understand the porous structure from the perspective of the penetrating molecule.”

This is precisely why Zeo++ characterizes these structures by mapping the empty spaces between their atoms. Drawing from a database of the coordinates of all the atoms in each porous structure, Zeo++ generates a 3D map of the voids in each material. This 3D network allows researchers to see where the channels between atoms intersect to create cavities. The size and shape of these cavities determine whether a molecule will pass through the system or be absorbed.

“Zeo++ allows us to do things that would otherwise be physically impossible,” says Smit [Berend Smit leads the Energy Frontier Research Center for Gas Separations Relevant to Clean Energy Technologies at the University of California at Berkeley], whose group is developing laboratory and computational methods for identifying carbon dioxide-absorbing nanomaterials.

For anyone who’s curious about zeolites, I’ve excerpted this from an essay on Wikipedia (all notes and links have been removed),

Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents.The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredric Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that had been adsorbed by the material. Based on this, he called the material zeolite, from the Greek ζέω (zéo̱), meaning “to boil” and λίθος (líthos), meaning “stone”.

I first mentioned zeolite on this blog in a July 1, 2010 posting about ‘green’ nanotechnology in Alberta’s oil sands.

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