Establishing an Atomistic Picture of Gas Sorption without Crystallography: Tools for Advanced Separations
Gas separations represent one significant application of nanoporous materials. Based on a project within our laboratory to separate the noble gasses Kr and Xe, we present recent results from several families of sorbents. We will show that it is possible to establish a surprisingly comprehensive picture of gas sorption through combination of several complimentary experimental techniques.
Specifically, high quality, variable temperature gas sorption measurements contain a substantial amount of information related to gas-sorbate interactions, including heats of adsorption which may be extracted through the Clausius-Clapeyron relationship. Isotherms and heat of adsorption curves may be interpreted through molecular mechanics simulations. We show that Grand Canonical Monte Carlo (GCMC) calculations can provide an excellent atomistic picture of sorption that can guide interpretation of data obtained from gas sorption. Later crystallographic studies (we will present synchrotron powder and neutron powder diffraction results from in-situ gas loading experiments) confirmed our predictions.
After establishing the validity of our approach for HKUST-1, we show more recent examples from other systems. Transition metal formates exhibit high selectivity that is remarkably uniform for a wide range of loadings. We show this is a consequence of having a one crystallographically unique sorption site. Preliminary results in zeolites suggests selectivity can be tuned based on cation exchange. The potential for selectivities comparable with the best metal organic frameworks studied is an important result given the price and familiarity of zeolites. We conclude with an overview of efforts to extend our simulation studies to zeolites.