Super sorbent soaks up methane under mild conditions

An easy-to-make porous polymer can soak up 25% more methane than the target value set by the US Department of Energy for natural gas storage materials, according to a study (Nat. Energy 2019, DOI: 10.1038/s41560-019-0427-x). The record-setting material was made at room temperature in open-air glassware from starting materials that cost less than $1.00/kg.

Vehicles fueled with natural gas, which is predominantly methane, produce lower levels of CO₂ and other emissions than gasoline- and diesel-powered engines. But the high cost of high-pressure tanks required to store enough fuel for practical driving distances—coupled with the large space needed to accommodate unwieldy bulbous tanks—limits the viability of compressed natural gas as a transportation fuel, especially for passenger cars.

So researchers have looked for solids that reversibly adsorb large quantities of methane at moderate pressures. In principal, tanks loaded with suitable sorbents could store more gas at lower pressures, and they could be designed to fit into available vehicle space. Some sorbents, for example metal-organic frameworks (MOFs), have shown promise in reaching the DOE methane-storage targets—0.5 g of methane per g of material and 263 L of methane per L of material. The new polymer surpasses those targets, reversibly storing methane at 0.625 g/g and 294 L/L, starting at 100 bar and near 0 °C and then down to 5 bar as it releases the gas.

Dubbed COP-150, the new polymer, which is made from benzene and 1,2-dichloroethane, sports a flexible structure that allows the material to swell as it takes up methane and shrink as it releases it. Vepa Rozyyev and Cafer T. Yavuz of Korea Advanced Institute of Science and Technology, Mert Atilhan of Texas A&M University at Qatar, and coworkers reasoned that making polymers with aromatic cores coupled via flexible ethylene linkers would yield porous sorbents that “breathe.” The team made 29 polymers. COP-150, which the team tested in a commercial gas cylinder, was the top performer and least expensive.

“The concept behind the material is clever,” says Neil B. McKeown of the University of Edinburgh, a specialist in porous polymers. Other researchers have studied polymers that expand when absorbing solvents. This team applied that phenomenon to methane storage, he explains. The team’s trick was that they tailored the polymer to expand over an appropriate pressure range, optimizing the material’s working capacity, he notes. “This looks very promising, however, for practical use, it would be interesting to know how fast the full adsorption-desorption cycles are, as these are both facilitated by significant rearrangement of the polymer structure.”

Shifting society’s dependence on fossil fuels to greener alternatives such as hydrogen is a grand challenge, says MOF specialist Omar K. Farha of Northwestern University. He notes that technologies that enable high-capacity storage of gaseous fuels such as methane could be a bridge to these cleaner-energy alternatives. However, very high pressures or cryogenic temperatures are typically required to store practical amounts of gaseous fuels, Farha points out. But the new polymer meets DOE’s targets under industrially relevant conditions, “rendering these materials promising sorbents for natural gas applications.”