Cross-Linked Polymer Captures CO₂

An inexpensive material made of cross-linked benzene acts like a stiff sponge, swelling as it absorbs carbon dioxide. Unlike other carbon-capturing materials, it retains most of its absorbent ability under wet or acidic conditions (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5031968). The researchers think it could be used to collect CO₂ in power plants that convert coal and other fuels into gas before combustion.

Some solid carbon-capturing materials are crystalline porous substances such as zeolites or metal-organic frameworks (MOFs). These rigid lattices have lots of pores to hold onto CO₂, but they also can grab water, which is a common component of most gas streams at power plants. This means the materials capture less CO₂ than they theoretically could.

Andrew I. Cooper of the University of Liverpool, in the U.K., and his colleagues wanted to find a substance that would target CO₂ even with water in the mix. They developed a new polymer using Friedel-Crafts alkylation to cross-link benzene with formaldehyde dimethyl acetal. The resulting material looks and feels like a hard, brown chunk of plastic but is extremely porous.

Under dry conditions, the material could pick up 15.32 mmol of CO₂ per gram, which is significantly more than other solid carbon-capture materials that they tested. Activated carbon, a polar zeolite, and two MOFs absorbed between 6.91 and 9.30 mmol/g. The cross-linked benzene sponge retained 86% of its CO₂ capacity under wet conditions, capturing 13.17 mmol/g. Meanwhile, the zeolite and one of the MOFs performed poorly with water around, retaining just 6 and 28% of their capacity, respectively. “Most of these materials love water and suck it up, leaving not much room for CO₂,” Cooper says.

The new material also is quite stable under acidic conditions like those found in power-plant gas streams. The team boiled the polymer in a sulfuric acid solution for an hour and found that its porosity and CO₂ capacity were unchanged. Similar conditions would have dissolved a MOF, Cooper noted.

The main disadvantage for the material is that its capacity surpasses the other carbon-capture materials only at high pressures. But most power plants vent CO₂ at relatively low pressures, which don’t cause this material to swell. So for now, this particular polymer would be useful only in capturing carbon in high pressure gas streams such as those in plants that use integrated gasification combined cycle technology. These plants convert coal and other carbon-based fuels into a mixture of CO₂ and H₂ and then combust the H₂ to produce energy. The precombustion gas streams have pressures high enough to allow the cross-linked polymer to swell. Unfortunately, such gasification plants are still rare, Cooper says.

Neil B. McKeown of the University of Edinburgh says that although the team has presented some methodical tests of the cross-linked polymer, there’s still a lot to prove before people will start to consider the material. Currently, he says, there’s competition between carbon-capture technologies, and “it’s not clear which is best, and none of them are great.”

But McKeown does say the polymer has one major advantage: It’s cheap to make. Cost counts, he says, when a material must be made in large enough quantities to capture the hundreds of tons of CO₂ released per hour in a power plant.