Researchers Propose New Way for Designing Cheaper, More Efficient Carbon Capture Materials

To make "clean" fossil fuel burning a reality, scientists have to remove carbon dioxide from the exhaust gases that arise from natural gas power plants or coal and stock or reuse it. For this, researchers are analyzing unique scrubbing liquids that bind and discharge the gas, but some of them show great potential to solidify into a cold honey consistency while binding carbon dioxide, making them ineffective and costly.

A normally freely fluid liquid (yellow) turns into a liquid the thickness of cold honey after it binds carbon dioxide (red). Modifying its chemical structure frees it up into a noncharged, thin liquid again (blue). (Credit- Glezakou/PNNL)

Now, scientists have employed computer modeling to design these liquid materials so that they maintain a low viscosity after binding to carbon dioxide, based on a revelation they came across in their explorations. Even though the chemists have to analyze the predicted liquid in the laboratory, predicting viscosity would help scientists to discover and design more economical and more competent carbon capture materials. The study has been reported in Journal of Physical Chemistry Letters March 28.

We're hoping to drive down the operational costs. With a lower viscosity, we can run the process at lower, optimal temperatures, and the cost to implement drops astronomically.

Roger Rousseau, Chemist, Department of Energy's Pacific Northwest National Laboratory

Carbon Sponge

Currently, natural gas and coal power plants emit considerable amounts of global greenhouse gases, however carbon capture and storage technologies offer a promising way to cut down those emissions.

In order to isolate the gas, scientists are constructing materials that can reversibly bind to carbon dioxide only. Exhaust fumes mix in some way with the material, and this material collects the greenhouse gas similar to a sponge. Further processes would squeeze the sponge to get the carbon dioxide back out, to store outside the atmosphere, reusing in fuels, or producing chemicals.

Potential candidates for these materials are liquids called carbon dioxide binding organic liquids (CO2BOLs, pronounced co-balls). The major advantage of CO2BOLs over other technologies is that they are not based on water. Water-based materials are known to accumulate carbon dioxide, and require much more energy to pull the carbon dioxide back out as this process involves heating.

Scientists have analyzed CO2BOLs for a very long period of time, but had difficulty in finding a solution to a thickening problem. The CO2BOLs have similar viscosity like water, however once they bind to carbon dioxide, they turn thick like cold honey. The more carbon dioxide they accumulate, the thicker they become.

The relatively high viscosity is a limitation in many aspects: Pumping thick liquids to a facility through pipes to amass carbon dioxide needs plenty of energy, and pulling the gas out of the liquid needs plenty of heat. Operators cannot collect the as much carbon dioxide as the CO2BOL can hold; they have to maintain a thin enough viscosity to pump the fluid. Economic calculations propose using CO2BOLs unproductively like this would be costly.

The high viscosity would significantly increase the cost compared to conventional carbon capture technology.

Dave Heldebrant, Chemist, PNNL

Calculated Visc

Unwilling to leave a promising technology, Rousseau and his colleague Vassiliki-Alexandra "Vanda" Glezakou turned to the computer. They decided to check if molecular simulations could predict a molecule’s viscosity. This would allow them to create low-viscosity CO2BOLs which could then be analyzed in the lab.

The team started by checking what was happening with a simple CO2BOL and carbon dioxide. They selected a molecule named IPADM and simulated this molecule’s binding with carbon dioxide. While doing so, the computer program must monitor the location and movement of thousands of atoms within the simulation.

it was observed that a molecule of neutral carbon dioxide reached a specific place on the neutral IPADM, forming IPADM-CO2. When this occurred the electrons moved around between atoms, forming a spot within the IPADM-CO2 carrying a positive charge and a different spot with negative charge. The consequence is an overall neutral molecule, which has separate negative and positive charges, which is what chemists refer to as zwitterion.

Multiple simulations were carried out by Glezakou, Rousseau and colleagues, but this time, a raising percentage of carbon dioxide was combined (computationally) to the IPADM molecules. They found a relation between how the charges traveled around the molecules and their viscosity. This enabled them to derive an equation with which viscosity could be calculated from different chemical properties of CO2BOLs. The team proved their equation by making a comparison of quantified viscosities of various CO2BOLs with known values.

The tests also proved that the zwitterion ramps up the viscosity. The internal negative and positive charges could interact with these charges on other IPADM-CO2 molecules, preventing them from freely moving about. The IPADM is as thin as water without the -CO2, but becomes thick like honey with it.

Z to A

Could the scientists discard those charges? In other zwitterions, the scientists are aware of the fact that protons generally move about, at times developing a neutral molecule. They pondered if they could compel CO2BOLs to perform like this, by altering the molecular scaffold to move the proton back to the molecule’s negative part after it captured carbon dioxide, thereby forming a neutral acid.

The minute we saw that the acid form was more stable for some CO2BOLs, we knew instantly that we could change the molecular structure of candidate CO2BOLs to make that happen more often. Nobody was considering the neutral acid form for these systems before. The conventional idea was that CO2BOLs would always be an ionic liquid, but clearly it doesn't have to be. But the trick is to have the two parts of the molecule that receive the proton in close proximity and able to share the proton.

Vassiliki-Alexandra Glezakou, Department of Energy's Pacific Northwest National Laboratory

If this idea worked out, viscosity would no longer pose a challenge to non-ionic carbon capture solvent systems.

They again team simulated IPADM binding to carbon dioxide, however, they made two changes to IPADM. The two changes included minute chemical alterations to IPADM that would control where electrons traversed through the bound structure.

The researchers simulated CO2BOLs binding carbon dioxide by about 25% and found how easily the neutral acids were created. They found that, dissimilar to IPADM, the other two CO2BOLs unpredictably formed the neutral acid, approximately as easily as it formed the zwitterion. Especially, one called EODM, was around 50% neutral acid and 50% zwitterion.

Reality check

The researchers then analyzed what form IPADM-CO2 took in real life. While correlating the calculated viscosity values with experimentally determined viscosity, they came to a conclusion that IPADM-CO2 mostly forms the zwitterion.

However, if the neutral acid was formed, what would have happened to IPADM-CO2's viscosity? The chemists assessed the predicted viscosity of IPADM-CO2 and its two alterations. The predicted that viscosity reduced by more than half for all three.

Equipped with what they currently know has to happen to CO2BOLs, the researchers are changing their focus to molecules which maintain the charged parts proximal to one another and which would probably form the neutral acid variation. Certain initial lab measurements proved the findings, with more yet to come.

"If we could cut the viscosity by 50 percent or more, CO2BOLs would be acting in their optimal range and be far more efficient," said Heldebrant.

This is a fine example of how fundamental science and molecular level insights can accelerate technology in real time, and this was possible through the unique integration of cutting-edge theory and experiment.

Vassiliki-Alexandra Glezakou, Department of Energy's Pacific Northwest National Laboratory

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