(308c) High Pressure Adsorption Measurements for MOF Characterization

Porous materials like zeolite, activated carbon or MOF are used in many technical processes under high pressure conditions. For example in hydrogen storage proc-esses these materials are used as adsorbents at pressures of 100 bar or higher. Though, the physical properties of these materials are still mainly determined from low pressure and low temperature adsorption measurements. This poses the question of the applicability of the low pressure based physical prop-erties for design and operation of high pressure processes. On the other side it is possible to gain information about the properties of the porous material from high pressure adsorption measurements. And the combination of both methods leads to new information about the structure of the adsorbed phase. Classical Characterization Characteristic properties of porous materials are for example the specific surface area and the pore size distribution. These are derived from the results of physisorp-tion measurements at sub-atmospheric pressures. Normally the adsorption of N2, Ar or CO2 at 77 K, 87 K or 273 K is measured in the pressure range from vacuum to 1 atm. A variety of standardised models and calculation routines allows deriving the characteristic properties of the material from the resulting adsorption isotherms. High Pressure Adsorption All adsorption measurements yield the so-called Gibbs surface excess. It is deter-mined by measuring the adsorption isotherm twice. First a non-adsorbing gas (usu-ally He) is used. Then the measurement is repeated with the adsorptive. The differ-ence between these two measurements is the Gibbs surface excess adsorption. This quantity must be positive and increases with increasing pressure as long as the den-sity of the adsorbed molecules is higher than the density of the surrounding gaseous phase. If however, the density of the gaseous phase is still increasing while the den-sity of the adsorbed molecules cannot increase any more (for example due to space limitations in micropores) the Gibbs surface excess decreases with increasing pres-sure. This is the case at high gas pressures. In diagram the Gibbs surface excess adsorption isotherms of N2, Ar and CH4 on a microporous material are shown. By high pressure adsorption measurements two properties of the adsorbent can be assessed which cannot be determined by low pressure characterization. Firstly, the volume (respectively density) of the adsorbent material can be determined from iso-therm measurements with He in a large pressure range. Evidence was found for cer-tain MOF that the porous structure is not rigid ? i. e. compression or swelling could be observed. Secondly, combining the measured decrease in the Gibbs surface excess with the pore volume measured with the classical characterization methods allows determining the density of the adsorbed phase. Results of measurements of H2, CH4, N2, CO2 and He on different MOF and zeolite materials at 30°C, 45°C and 60°C in the pressure range up to 500 bar will be pre-sented and discussed.