Chemistry with sunlight

Chemistry with sunlight

12:27 PM, 24th June 2011
Chemistry with sunlight
Kevin Moeller, PhD, Professor of Chemistry, holds the electrochemical cell and Alison Redden, a graduate student in chemistry holds the photovoltaic cell.

ST LOUIS, US: The idea is simple, says Kevin Moeller, PhD and yet it has huge implications. All we are recommending is using photovoltaic cells (clean energy) to power electrochemical reactions (clean chemistry). Moeller is the first to admit this isn’t new science.

“But we hope to change the way people do this kind of chemistry by making a connection for them between two existing technologies,” he said.

To underscore the simplicity of the idea, Moeller and his co-authors used a $ 6 solar cell sold on the internet and intended to power toy cars to run reactions described in an article published in Green Chemistry.

If their suggestion were widely adopted by the chemical industry, it would eliminate the toxic byproducts currently produced by a class of reactions commonly used in chemical synthesis - and with them the environmental and economic damage they cause.

Moeller, a professor of chemistry in Arts & Sciences at Washington University in St Louis, is an organic chemist who makes and manipulates molecules made mainly of carbon, hydrogen, oxygen and nitrogen. One important tool for synthesizing organic molecules - an enormous category that includes everything from anesthetics to yarn - is the oxidation reaction.

“They are the one tool we have that allows us to increase the functionality of a molecule, to add more ‘handles’ to it by which it can be manipulated,” said Moeller.

“Molecules interact with each other through combinations of atoms known as functional groups,” he explained. “Ketones, alcohols or amines are all functional groups. The more functional groups you have on a molecule, the more you can control how the molecule interacts with others.”

“Oxidation reactions attach functional groups to a molecule,” he continued. “If I have a hydrocarbon that consists of nothing but carbon and hydrogen atoms bonded together and I want to convert it to an alcohol, a ketone or an amine, I have to oxidize it.”

In an oxidation reaction, an electron is removed from a molecule. But that electron has to go somewhere, so every oxidation reaction is paired with a reduction reaction, where an electron is added to a second molecule.

The problem, said Moeller, is “That second molecule is a waste product; it’s not something you want.”

The answer is to use the cleanest possible energy, solar energy captured by photovoltaic cells, to run electrochemical reactions.

“That’s what the Green Chemistry article is about,” said Moeller. “It’s a proof-of-principle paper that says it’s easy to make this work and it works just like reactions that don’t use photovoltaics, so the chemical reaction doesn’t have to be changed around.”

The Green Chemistry article demonstrated the method by directly oxidizing molecules at the electrode. No chemical reagent was used. Since writing the article, Moeller’s group has been studying how solar-powered electrochemistry might be used to recycle chemical oxidants in a clean way.

Why would manufacturers choose to use a chemical oxidant, if the voltage of the electrode can be matched to the oxidation potential of the molecule that must be oxidized?

“The chemistry community has learned how to use chemical reagents to make reactions selective,” he said. “The reagents are usually expensive and toxic, so they are recycled,” he said. “We are working on cleaning up reagent recycling.”

“We can’t make all of chemical synthesis cleaner by hitching solar power to electrochemistry,” Moeller said, “but we can fix the oxidation reactions that people use. And maybe that will inspire someone else to come up with simple and innovative solutions to other types of reactions they’re interested in.”

(C) Washington University News




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