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A computational method to quantify the adsorption of gas by porous zeolites should help labs know what to expect before they embark upon slow, costly experiments, according to researchers at Rice University.
The new method created by engineers in Rice's Multiscale Materials Modeling Lab accurately calculated the ability of two zeolites, small cage-like molecules with enormous surface area, to trap and store gas molecules.
Among other possibilities, the work could help in the race to meet Department of Energy (DOE) standards that call for the creation by 2015 of materials that can hold 5.5 percent of their weight in hydrogen to fuel vehicles.

\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. Those were plugged into large-scale Monte Carlo simulations to predict how much of each gas each porous zeolite could adsorb.

"Because we combined two methods, each appropriate for a different length scale, we were able to predict the maximum capacity of these materials with high accuracy while maintaining reasonable computational time," Shahsavari said.

The method may seem simple, but calculating integrative forces between thousands of gas molecules and each ZIF was not. It took the combined power of Rice's DAVinCI and SUGAR supercomputers to find results for all the variations. Even so, calculations for a single data point -- one molecule, one zeolite, one temperature -- often took 96 processing cores three days to complete.

Date: 11 Nov 2013