MOF for NOx capture.

Nitrogen-dioxide molecules (red and blue, artist’s impression) are snared in a net-like chemical structure (gray). Credit: Jiangnan Li/University of Manchester

Materials science

Tiny pores trap a pollutant — and put it to good use

Porous material removes nitrogen dioxide from exhaust and transforms it into a valuable commodity.

A crystalline mesh traps nitrogen dioxide from car exhaust and turns it into raw material for fertilizer or nylon stockings.

The mesh is a type of structure known as a metal–organic framework (MOF), which consists of metal atoms linked by organic molecules. Martin Schröder and Sihai Yang at the University of Manchester, UK, and their colleagues constructed an MOF using zinc, creating a structure studded with tiny, bowtie-shaped pores. Each pore can trap two molecules of nitrogen dioxide, which contributes to global warming and the formation of smog.

Catalytic convertors do a poor job of scrubbing nitrogen dioxide from exhaust, but the team’s MOF selectively captures the gas even in the presence of oxygen, carbon dioxide and sulfur dioxide, and works best at the temperature of automobile exhaust. Submerging the trapped nitrogen dioxide in water and stirring it for 10 minutes turn the gas into nitric acid, a feedstock for nylon, fertilizer and rocket fuel that has a US$2.5-billion global market. The MOF can then be re-used to capture more nitrogen dioxide.