(339b) Using Adsorption Calorimetry to Evaluate the Adsorption Properties in Porous Materials

The enthalpy change of the adsorbent-adsorptive system during physical adsorption is a basic characteristic which can be used both to characterize the solid and as an input for adsorption processes. Direct measurement of adsorption energies can be performed coupling microcalorimetry with a manometric/volumetric device. We have the possibility to obtain measurements not only at room temperatures but also at 77 K. Calorimetry gives a direct measurement of the probe-surface interactions and at 77 K our set-up allows a high resolution representation of both the isotherm and adsorption energies. From a fundamental standpoint, the comparison between a probe molecule with a significant quadrupole moment and one without any permanent moment can be beneficial for the characterization of adsorbents. From a more applied point of view, comparing the heats of adsorption measured at room temperature could be used to predict separation properties. Moreover, the heat release during adsorption gives an idea as to the ease of bed regeneration, and has a strong impact on the performance of an adsorbent bed since, in physisorption, an increase in temperature will lead to a decrease in amount adsorbed.

The present study aims to highlight the continuing interest of adsorption microcalorimetry for the characterization of adsorbate-adsorbent interactions, such as Metal Organic Frameworks materials with various gases. There is a growing interest on â??Metal Organic Frameworksâ? (MOFs) in many areas dealing with porous materials. These hybrid solids are formed of metallic centres linked to each other through organic chains. Along with their flexibility, one of the most interesting structural features in some MOFs is their open metal centres (or coordinatively unsaturated sites). These open metal centres play a strong role as specific sites for gas adsorption. The ability to modulate the strength of the gas-MOF interaction by changing the metal centre is of interest for possible applications in the separation of mixtures where specific interactions may well be necessary. These include propane/propene and CO₂/N₂.

Two well-known MOF structures with open metal sites are considered: MIL-100(M) and M₂(dobdc). In the case of MIL-100, the Al, Fe, Cr and V forms are considered whereas for the M₂(dobdc) the Fe, Mn, Zn, Co and Ni forms are studied. The adsorption enthalpies are measured at 77 K, for H₂, N₂, Ar, CO, CH₄, and at 303 K, for N₂, CO, CO₂, CH₄, C₂H₆, C₃H₆, C₃H₈ and C₄H₁₀. All of this data allows a number of trends to be made including the role of gas polarizability and that of the metal electronegativity.