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Buckyballs Throwing New Curve Into Capturing CO2 From NatGas Wells

Rice University scientists are on to an environmentally friendly carbon-capture method that may be equally adept at drawing carbon dioxide (CO2) emissions from natural gas wells and industrial flue gases.

In a proof-of-concept study published last Wednesday, chemist Andrew Barron and his lab team reported that amine-rich compounds are effective at capturing CO2 when combined with carbon-60 molecules, also known as buckminsterfullerenes, or in layman’s terms, buckyballs. Buckyballs actually were discovered at Rice in 1985 by Nobel Prize laureates Richard Smalley, Robert Curl and Harold Kroto.

The ultimate curvature of buckyballs could make them the best possible way to bind amine molecules that capture CO2 but allow desirable methane to pass through, said Barron.

"We had two goals," the Rice chemist said of the study. "One was to make the compound 100% selective between carbon dioxide and methane at any pressure and temperature. The other was to reduce the high temperature needed by other amine solutions to get the carbon dioxide back out again. We've been successful on both counts."

Tests on atmospheric pressures of one to 50 found the compound captured about 20% of its weight in CO2 but no measurable amount of methane. The material also did not degrade over several absorption/desorption cycles.

Buckyballs were used by the scientists as crosslinkers between amines, which are nitrogen-based molecules drawn from polyethyleneimine. The lab produced a brown, spongy material in which hydrophobic (water-avoiding) buckyballs forced the hydrophilic (water-seeking) amines to the outside, where passing CO2 could bind to the exposed nitrogen.

The scientists years ago began combining carbons and amines, noting a progression. Flat graphene absorbed CO2 well, multiwalled nanotubes absorbed it better, and thinner single-walled nanotubes absorbed it even better, which "suggested the curvature was important," Barron said.

Because buckyballs are spheres, they have the "highest possible curvature among carbon materials."

The compound compares favorably with other carbon-capture candidates based on metal organic frameworks (MOF), according to Barron.

"It's about equivalent to the best MOFs for carbon capture, but our material is far more selective. Methane just doesn't absorb."

Unlike MOFs, the compound absorbed wet CO2 as well as dry. It's as important that the compound release CO2 efficiently for reuse, Barron said.

"We noticed a long time ago that if we attached amines to carbon nanotubes or graphene, they lowered the temperature at which carbon dioxide dissolves," he said. Industrial amine-based scrubbers have to be heated to 140 degrees C to release captured CO2; lowering the temperature would save energy.

"Compared to the cost of current amine used, carbon-60 is pricey," Barron said. "But the energy costs would be lower because you'd need less to remove the carbon dioxide." Industrial scrubbers lose amines through heating and have to be replenished constantly. "They're forever adding reagent, which is nice for the companies that sell amine, but not so good for those trying to separate the carbon dioxide."

The Rice researchers now are looking for ways to improve the compound's capacity and rate of absorption. "We really understand the mechanism, which is important," Barron said. "That allows us to push it further."

The research was published in Nature's online journal “Scientific Reports.” Apache Corp., the Robert A. Welch Foundation and the Welsh Government Ser Cymru Program supported the research. Lead author is former Rice researcher Enrico Andreoli, now a senior lecturer at Swansea University in Wales. Co-authors are Eoghan Dillon, Laurie Cullum and senior research scientist Lawrence Alemany, all of Rice.

Apache is one of five U.S. natural gas producers that is collaborating separately with the Environmental Defense Fund on best ideas for continuous methane detection at drilling sites (see Daily GPI, Aug. 21; April 3). Potential pilots from the five selected projects have moved to lab and field testing with commercialization by one more more as soon as 2016.

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