For hydrogen, they determined that both zeolites stored about three times as much gas at 77 K and at 100-bar pressure (100 times that of the atmosphere at sea level) than they would at room temperature. ZIF-100, in particular, adsorbed 3.4 percent of its weight in hydrogen, which approaches the DOE standard, Shahsavari said.
"We didn't reach that DOE target with this design, but if we can functionalize the ZIFs by adding ligand-binding moieties (the functional groups in a molecule)into the pore space, then we might be able to. We're working on that," he said.
They were also able to calculate both subtle and significant differences between the adsorptive qualities based on various input parameters of gas, pressure, temperature and type of zeolite. For example, they came to the counterintuitive conclusion that ZIF-100, the larger of the two zeolites, could adsorb more small-molecule hydrogen but fewer of the larger methane molecules than ZIF-95 under similar conditions.
"So our method not only accurately predicts the properties of these porous materials, but also provides fundamental insights that can be leveraged to further improve their properties," Shahsavari said.
The Rice lab's method involved several steps. First, the team performed first-principle calculations to describe the very weak atomic interactions the van der Waals-related London dispersion forces -- among each of the three types of gas molecules and the two ZIFs. The next step used those results to align the potentials among various atomic pairs. Tho
|Contact: David Ruth|