Thermal management of porous materials
Date
2020-11
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: Thermal management is an important consideration for porous materials as adsorbents. Accurate determinations of the thermal effects produced during a sorption process are required to characterise and assess the material for application in gas separation and storage. This work describes the methodologies and insights gained from the calorimetric determination of thermal behaviours related to porous materials. The first manuscript describes the methodology used for the determination of the enthalpy of sorption (ΔH) for a rigid porous material. A modified calorimetric method using the continuous-flow technique is described for the direct determination of ΔH values. We termed the method “pressure-gradient differential scanning calorimetry” (PGDSC). It has the advantage of near infinite resolution operating under quasi-equilibrium conditions to yield reliable results. Method validation was carried out by studying CO2 sorption by CuHKUST over four temperatures. The combination of a heat flow plot and a sorption isotherm enables the determination of ΔH as a function of loading over an extended loading range. The ΔH values compare well with previously reported data determined using isosteric and other calorimetric methods. Additionally, the PGDSC method is less labour-intensive and time-consuming than conventional methods and offers a detailed profile that shows how ΔH changes over a given loading range. The second manuscript describes the extension and use of the abovementioned methodology to analyse the thermal behaviours associated with flexible porous materials. Flexible porous materials display complex thermal behaviours owing to structural transitions under guest loading. Plots of ΔH as a function of pressure were determined using the PGDSC method for CO2 sorption by the flexible metal-organic frameworks MIL-53(Al) and MOF-508b over a range of temperatures. The ΔH plots compare well with literature values and offer high-resolution data leading to additional insights into the thermal effects of sorption processes involving flexible materials. The thermal events are measured and compared for comprehensive thermal assessment. Such assessments are beneficial for identification of materials that exhibit intrinsic thermal management. Materials exhibiting intrinsic thermal management are able to offset endothermic structural transitions against exothermic guest adsorption. MIL-53 shows a gradual and relatively balanced thermal profile, whereas MOF-508b undergoes rapid and large changes in enthalpy leading to potential difficulties when applying external cooling to such a material. However, both materials show reduced total enthalpy when compared to rigid materials, providing evidence for the notion that structural flexibility reduces the overall heat evolved during a sorption process. The final manuscript describes the continuous pressure introduction approach to adsorption calorimetry and the advantages offered in relation to flexible porous materials. These are highlighted through a series of studies using the continuous approach. The key advantage of the approach is the resultant high-resolution data, which effectively increases the limits of detection for thermal effects resulting from materials featuring complex surfaces and structural transitions. The technique offers accurate and comprehensive thermal assessments for structurally flexible materials and the thermal effects derived from sorption-based processes.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar.
Description
Thesis (PhD)--Stellenbosch University, 2020.