A biologically inspired membrane intended to cleanse carbon dioxide almost completely from the smoke of coal-fired power plants has been developed by scientists at Sandia National Laboratories and the University of New Mexico.

The work, reported recently in Nature Communications, may interest power and energy companies that want to reduce emissions of carbon dioxide.

Researchers term the membrane a “memzyme” because it acts like a filter but is near-saturated with an enzyme, carbonic anhydrase. These were developed by living cells over millions of years to help rid themselves of carbon dioxide efficiently and rapidly.

“To date, stripping carbon dioxide from smoke has been prohibitively expensive using the thick, solid, polymer membranes currently available,” says Jeff Brinker, a Sandia fellow, University of New Mexico professor and the paper’s lead author.

The method uses a water-based membrane 18 nanometers thick that incorporates natural enzymes to capture 90 percent of carbon dioxide released. (A nanometer is about 1/700 of the diameter of a human hair.) This is almost 70 percent better than current commercial methods, and it’s done at a fraction of the cost, Brinker says.

Schematic of how the process works. Source: Sandia National LaboratoriesThe device’s formation begins with a drying process called evaporation-induced self-assembly, first developed at Sandia by Brinker 20 years ago.

The procedure creates a close-packed array of silica nanopores designed to accommodate the carbonic anhydrase enzyme and keep it stable. This is done in several steps.

First, the array, which may be 100 nanometers long, is treated with a technique called atomic layer deposition to make the nanopore surface water-averse (or hydrophobic). This is followed by an oxygen plasma treatment that overlays the water-averse surface to make the nanopores water-loving (or hydrophilic) but only to a depth of 18 nanometers. A solution of the enzyme and water spontaneously fill up and are stabilized within the water-loving portion of the nanopores. This creates membranes of water 18 nanometers thick, with a carbonic anhydrase concentration 10 times greater than aqueous solutions made to date.

The solution is stable, but because of the enzyme’s ability to rapidly and selectively dissolve carbon dioxide, the catalytic membrane has the capability to capture the majority of carbon dioxide molecules that brush up against it from a rising cloud of coal smoke.

The hooked molecules then pass through the membranes, driven solely by a naturally occurring pressure gradient caused by the large number of carbon dioxide molecules on one side of the membrane and their comparative absence on the other. The chemical process turns the gas briefly into carbonic acid and then bicarbonate before exiting immediately downstream as carbon dioxide gas. The gas can be harvested with 99 percent purity — so pure that it could be used by oil companies for resource extraction. Other molecules pass by the membrane’s surface undisturbed. The enzyme is reusable, and because the water serves as a medium rather than an actor, it does not need replacement.

The nanopores dry out over long periods of time due to evaporation. This will be checked by water vapor rising from lower water baths already installed in power plants to reduce sulfur emissions. And, enzymes damaged from use over time can be replaced.

The membrane’s arrangement in a generating station’s flue would be like that of a catalytic converter in a car, says Brinker. The membranes would sit on the inner surface of a tube arranged like a honeycomb. The flue gas would flow through the membrane-embedded tube, with a carbon dioxide-free gas stream on the outside of the tubes. Varying the tube length and diameter would optimize the carbon dioxide extraction process.

A slightly different enzyme, used in the same process, can convert methane — an even more potent greenhouse gas — to the more soluble methanol for removal.

The work was initially supported by Sandia Laboratory Directed Research and Development, with additional funding from the Department of Energy Office of Science and the Air Force Office of Scientific Research. The work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated by Sandia and Los Alamos national labs.