by Marilyn Davis
 

Some fancy footwork with enzymes and a special kind of glass could offer a way to convert carbon dioxide emissions from power plants into methanol, says SIUC materials chemist Bakul Dave. He and a former student of his have demonstrated the feasibility of the idea on a laboratory scale.

The Fall 1998 issue of Perspectives introduced readers to Dave, who is making a name for himself in sol-gel research. A sol-gel is a sponge-like glass that can be fabricated so that it confines enzymes in its microscopic pores. These trapped enzymes can trigger reactions with smaller molecules passing in and out of the sol-gel, making the glass a bioactive material with many potential uses.

One such use might be creating renewable energy. Dave (pronounced da-vay) has filed a patent application for a sol-gel that can turn carbon dioxide, a greenhouse gas, into methanol, a clean-burning fuel. Whether the invention could be commercially viable is yet to be determined—but the accomplishment has already attracted the attention of industry. 

Burning methanol itself produces carbon dioxide. But if the methanol we burned were derived from power-plant emissions, Dave explains, "We haven’t increased the overall amount of carbon dioxide in the atmosphere." The key idea here is recycling.

Pulling off this recycling trick requires three enzymes that are incorporated in the sol-gel when it is made. Essentially, they’re locked into pores as the liquid gel solidifies into glass. 

The finished sol-gel then is placed in a solution of water and a chemical called NADH. The latter supplies electrons needed for the conversion process to run properly. This solution diffuses slowly into the sol-gel. Then carbon dioxide is bubbled through the gel, and the enzymes swing into action. 

A bacterial enzyme called formate dehydrogenase first converts the carbon dioxide into formic acid. A second bacterial enzyme, formaldehyde dehydrogenase, converts the formic acid into formaldehyde. Finally, an enzyme called alcohol dehydrogenase (which helps the liver break down alcohol) converts the formaldehyde into methanol. 

Robyn Obert, a former undergraduate in Dave’s lab, did much of the work on the new sol-gel. 

"This is something I had in mind a few years back, but I wasn’t sure it would turn out this well," says Dave. "Robyn wanted to do something related to biochemistry, and she had experience handling enzymes. So I thought this would be a good project for her. Success depends on how well you incorporate the enzymes into the gels, and she did a good job."

Making the system a possibility for commercial applications required one more step, however. The electrons that NADH supplies to the enzymatic reactions must continually be replaced to keep the conversion process going. Because this chemical is very costly, Dave had to come up with a good way to "refresh" it instead of using more.

He turned to a suite, or system, of enzymes found in plants that can convert water to oxygen in the presence of light. (It’s one of two enzyme systems involved in photosynthesis.) When incorporated in the sol-gel and exposed to light, this enzyme system strips electrons from the water molecules in the solution that permeates the sol-gel, making them available to replenish the NADH. 

It’s an efficient, environmentally friendly, renewable process. And on a small scale, at least, it works.

The system should work the same way on a bigger scale, Dave says, but scaling up processes for real-world use often entails unforeseen challenges. He now will collaborate with chemical engineers to "supersize" the sol-gels for potential industrial use. 

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