Capturing Carbon Dioxide in a Cage

For years industry has grappled with how to scrub effluent waste of harmful greenhouse gases, like carbon dioxide, before it reaches the atmosphere. Current methods are often cumbersome, expensive, and only moderately effective. Today, scientists are engineering tiny cages to capture carbon dioxide before it leaks into the atmosphere.

According to Pacific Northwest National Laboratory (PNNL) scientist Praveen K. Thallapally, "We are creating a new class of materials that resemble naturally occurring minerals called zeolites [that form] three dimensional cage compounds." The cages are composed of metal ions held together with organic ligands. Think of creating cages with Tinkertoys – the round end units represent the metal ions and the sticks linking the end units together represent the organic ligands.

"This mineral has the properties we need to trap gas molecules," said Thallapally. "They have a high surface area and are very porous."

To create these materials, the research team combined two readily available chemicals - zinc nitrate and 2-methyl imidazole – in a process first developed by researchers at the University of California, Los Angeles. The PNNL team tweaked the process by combining these chemicals not in the presence of heat, but with a surfactant, more commonly called soap. According to Thallapally, "adding the surfactant came from my past work in nanotechnology."

  Crystal structure of ZIF-8.

Figure credit: Praveen K. Thallapally

Crystal structure of ZIF-8 with void space shown in yellow.

The team collected their newly created powdery substance and headed to the Environmental Molecular Sciences Laboratory (EMSL). "We used tools available at EMSL to determine the shape and size of crystals." There are thousands of ways the metal ions and ligands can combine to form cages. The size of the crystals affects the surface area of the material.

The research team had created tiny hexagonal crystal cages on the nanoscale. Nano refers to particles that are very small. In scientific usage, nano is equivalent to saying one-billionth in size. So a particle that is one nanometer in diameter is one billionth of a meter or 40,000 times smaller than the width of a human hair.

As a rule as particle size decreases, the surface area increases. "A gram of this material has the area of a football field," said Thallapally.

This is important because the size and shape of the cages will determine what type and how many gas molecules it can hold. "The possibilities of this research are huge," begins Thallapally.

As a gas passes through these nanocages, carbon dioxide binds to the cage frame. "This is purely a physical reaction, not a chemical reaction" said Thallapally. In many current technologies, the gas binds to the materials chemically and then need to be heated, to often very high temperatures, to release the trapped gas. "With our nanocages, the carbon dioxide can be released when the system is returned to regular atmospheric conditions," said Thallapally. In addition, the nanocages can be used over and over again.

  TEM image of nano ZIF-8

Figure credit: Praveen K. Thallapally

TEM image of nano ZIF-8.

Once the carbon dioxide is released, this gas can be treated in several ways. The gas can be pumped deep in the Earth where it becomes trapped and forms different carbonate minerals. The gases can also be used to produce valuable byproducts, like methanol.

Currently, the research team has tested the efficacy of the nanocages with pure gases, but industrial waste gas is a mixture of many gases. "This research has a lot of potential," said Thallapally. "But we need to begin testing how these materials work with gaseous mixture that better mimic conditions in industry."

As the research moves forward Thallapally notes that before anyone can make a better material, the fundamental properties of materials need to be better understood. "Our next step is to understand the role of surfactants in controlling the size and shape of the crystals. We also want to scale-up production to see if we can meet the needs for an industrial application," concludes Thallapally.

This project is supported by the Department of Energy’s (DOE) Office of Science, Office of Basic Energy Sciences, and conducted at Pacific Northwest National Laboratory. DOE invests in science and solving critical issues impacting people’s daily lives and the nation’s future.

This article was written by Stacy W. Kish.