Direct determination of isosteric heats of sorption using pressure-gradient differential scanning calorimetry

[摘要] ENGLISH ABSTRACT: Porous materials, specifically porous coordination polymers (PCP) and metal-organic frameworks (MOFs), have shown great potential for catalysis as well as gas storage, separation and purification. Energy efficient adsorption processes utilising these porous materials are desirable for reducing the energetic cost of industrial processes. Thus characterisation of these materials in terms of their thermodynamic properties is essential for practical applications.Isosteric heat (Qst) is an approximation of the enthalpy (or heat) of sorption and indicates the affinity of a material for a specific adsorbate. Conventionally Qst is determined indirectly by plotting isosteres at various temperatures and making use of the Clausius-Clapeyron approximation (the isosteric method). By comparing Qst values from the literature, there appears to be a correlation between the Qst value and the temperature range used. The higher the temperatures in the temperature range, the larger is the Qst value. This contradicts the assumption of temperature independence by the Clausius-Clapeyron approximation.Heat-flow calorimetry employing a temperature gradient is another method that has been used to determine Qst. A calorimetric approach requires fewer assumptions regarding the interaction energies and equilibrium of the system compared to the isosteric method. Herein a method is proposed for the direct determination of Qst using pressure-gradient differential scanning calorimetry (PGDSC). The PGDSC method involves the measurement of heat flow during the sorption and desorption of a gas over a predefined pressure range. In conjunction with gas loading data derived from gas sorption isotherms, Qst can be determined directly over the entire pressure (or rather gas loading) range from the changes in measured heat flow. The sorption of CO2 by CuHKUST was chosen as a test system to validate this direct method. It was found that the PGDSC approach produces temperature-independent values that are comparable to literature values obtained using the isosteric method. Since the PGDSC method is temperature independent, it requires a less labour-intensive procedure than the isosteric method. Only one PGDSC experiment and one isotherm at the same temperature are required, as opposed to multiple isotherms at various temperatures necessary for the application of the isosteric method. PGDSC furthermore employs direct heat measurements as compared to the indirect approximation of the isosteric method.The PGDSC method was further successfully applied to a 2D-layered interdigitated PCP ([Cd(oba)(bpy)]n) that displays stepped sorption and hysteresis for CO2, N2 and CH4. Since the isosteric method is not easily applied to materials with sorption profiles other than Type I, this presents an important improvement in how Qst values are measured. Qst values can also be used to predict the selectivity of the material for a specific gas. This investigation showed that [Cd(oba)(bpy)]n has the highest affinity for CO2 over N2 and CH4.

[发布机构] Stellenbosch University