(506a) Aspects of Gas Storage: Effect of Pore Size and Fluid-Wall Interaction on Adsorption Capacity of Supercritical Fluids

During the last decades, major progress was made concerning the understanding of subcritical, low pressure adsorption of fluids like nitrogen and argon at their boiling temperature in nanoporous materials. It was here possible to understand how structural properties affect the shape of the adsorption isotherms, leading to new methods that are now commonly used for characterization. However, within the context of gas storage applications, supercritical high pressure gas adsorption is important. High-pressure adsorption experiments introduce several complexities, both in terms of collecting isotherm data and interpreting the results. A key feature is here that the experimentally determined surface excess adsorption isotherm may exhibits a characteristic maximum at certain pressure. For a given temperature and adsorptive/adsorbent system, the surface excess maximum (and the corresponding (absolute) adsorbed amount) is related to the storage capacity of the adsorbent. However, there is still a lack of understanding how key textural properties such as surface area, pore size, pore volume and pore network characteristics affect the shape of supercritical high pressure adsorption isotherms (and here in particular the position of the surface excess maximum).

In order to address this open questions we are performing a systematic experimental study assessing the effect of pore size/structure on the supercritical adsorption isotherms of pure fluids such as C₂H₂, CO₂, SF₆ and CH₄ over a wide range of temperatures and pressures (from well above the bulk critical point to the near-critical region) on a series of model materials exhibit well defined pore sizes, i.e. ordered micro- and mesoporous materials such as zeolites, mesoporous molecular sieves (e.g., KIT-6 silica) and a hierarchically structured mesoporous NaY-zeolite. One key result of our experiments is that we find a clear correlation between the pressure of the surface excess maximum (at a given temperature) and textural properties such as the pore size as well as the effective attractive fluid-wall interaction. This was further investigated by performing complimentary molecular simulation studies. Our results suggest important structure-property relationships and allowed us to derive a tool for predicting gas storage properties of nanoporous materials at given thermodynamic conditions based on their textural properties. The fundamentals insights may serve as basis for a new generation of process specific tailored materials.