Metal-organic frameworks (MOFs) offer an advantage over other classical porous materials (activated carbon, zeolithes) because their properties can be tailored for specific applications. Today these structures are envisaged for multiple applications in gas adsorption, like hydrogen storage, selective adsorption of CO2 against CH4, CO2 against H2. The Cu-BTC (fig 2.) is chosen as a example of Metal Organic Framework that is available commercially to demonstrate adsorption studied by Sievert’s technique and heat flow calorimetry.
CO2 adsorption into Basolite C300 was measured at -20°C, 30°C and 50°C using a PCTPro-E&E Sievert’s apparatus coupled with a μDSC7 evo. Gas density temperature correction was performed by measuring the apparent free gas volume at these temperatures. Understanding the thermodynamics of the adsorption is essential for the practical application and among all the enthalpy of adsorption (or desorption) it is a key parameter.
There are two ways to determine this enthalpy. The first one is an indirect method, where the enthalpy is derived from adsorption isotherms at different temperatures. The second one is a direct method, where the enthalpy is measured via a calorimetric technique. The biggest disadvantage of this technique is that it gives a result per mole of solid sample and not per mole of gas. The combination of manometric technique (to quantify the amount of hydrogen absorbed/released) and calorimetry overcomes this issue and the direct measurement of enthalpy of formation per mole of gas is presented here (figure 1.).
RESULTS AND CONCLUSIONS
Coupling allows the access to a full characterization of the adsorption: adsorption isotherms (fig 3.),integral heats of adsorption (fig 4.), isoteric heat of adsorption (fig 5.), differential heat of adsorption as a function of the adsorbed amounts (fig 6.). The isosteric heat of adsorption is obtained by indirect methods by using the isotherms at different temperature. It gives the heat of adsorption at a specific coverage. It is obtained according to:
The differential heat of adsorption is the energy release during the adsorption for a infinitely small quantity of adsorbent. It depends on the adsorbed quantity of the gas. Practically this value is very important. It is this value that is used for the calculation of the sorption installations.
The PCTPro-E&E and μDSC7 evo combination is an ideal technique for the detailed characterization of high surface area materials used in gas sorption. The ease of use and the temperature and pressure ranges are perfect for this type of material application and in particular in the case of CO2 sorption studies.