Changing the shape of water molecules

Chemistry depends on catalysts to speed up reactions, make chemical processes more efficient, and control the products they’re making. But it’s not always clear why a catalyst works the way it does.

A group of US researchers has uncovered part of the catalysis mystery by looking at nanometre-sized crystals called zeolites. They’ve shown that the zeolites can change the shape and behaviour of water molecules.

Zeolites are compounds made mostly from aluminium, silicon and oxygen. They get a lot of use in chemistry as molecule-sized sieves or sponges. This is because of the physical structure of zeolite crystals: they’re filled with tiny holes. These pores are a nanometre or less in size, which makes them about a thousand times too small for a bacterium to pass through, but just the right size for molecules.

Zeolites have also been used to catalyse chemical reactions, and while it’s been known for a while that soaking them with water changes their catalytic behaviour, it wasn’t clear how or why this happened.

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Researchers based at the University of Illinois at Urbana-Champaign, US, used a few spectroscopic techniques to examine the water in several different zeolites, with pores ranging from 0.3 nanometres to 1.3 nanometres in diameter.

They found that when the pores were smaller than 0.65 nanometres, the water molecules did something odd. Because they could only fit through the zeolite pores one at a time, the molecules formed single-file chains. These one-dimensional chains had different chemical properties to regular, “bulk” water.

They also behaved differently when the zeolites were used to catalyse a chemical reaction called “epoxidation”: turning a type of chemical called an alkene into an epoxide.

“We saw higher rates of chemical reactions near small clusters of water molecules confined in the zeolite pores than in those without water or in bulk-like water,” says David Flaherty, co-author on a paper describing the research, published in Nature Catalysis.

“When the chainlike water structures had to reorganize to accommodate the reacting molecules, it led to unexpected – and dramatic – increases in rates,” says Daniel Bregante, lead author on the study.

“These findings are an important piece of the puzzle in understanding why certain combinations of catalysts, solvents and reactants led to greater rates than others.”

The researchers say this information will help them to design better catalysts – both zeolite and non-zeolite – in the future.

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