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

White, Kerry-Anne (2017-12)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

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.

AFRIKAANSE OPSOMMING: Poreuse materiale, spesifiek poreuse koördinasie polimere (PKPe) en metaal-organiese raamwerke (MORe), toon groot potensiaal vir katalise sowel as die berging, skeiding en suiwering van gasses. Die energie-doeltreffende adsorpsieprosesse wat hierdie poreuse materiale gebruik is begeerlik vir die verlaging van die energie koste van industriële prosesse verminder. Die karakterisering van hierdie materiale in terme van hul termodinamiese eienskappe is dus noodsaaklik vir praktiese toepassing. Isosteriese hitte (Qst) is verwant aan die entalpie (of hitte) van sorpsie en dui die affiniteit van die materiaal vir die adsorbaat aan. Gewwonlik word Qst indirek bepaal deur isostere by verskillende temperature te plot en gebruik te maak van die Clausius-Clapeyron-benadering (ook genoemd, die isosteriese metode). Deur Qst-waardes uit die literatuur te verkeglyk blyk daar om 'n verband tussen die Qst-waarde en die temperatuurreeks wat gebruik word te wees. Hoe hoër die temperature in die temperatuurreeks, hoe groter is die Qst-waarde. Dit is teenstrydig met die aanname van temperatuur onafhanklikheid in die Clausius-Clapeyron benadering. Hitte-vloei kalorimetrie teen 'n temperatuurgradiënt is ‘n manier wat gebruik kan word om Qst te bepaal. ‘n Kalorimetriese benadering benodig minder aannames ten opsigte van die interaksie energie en die ewewig van die stelsel in vergelyking met die isosteriese metode. Hierin word 'n metode vir direkte -bepaling van Qst voorgestel met behulp van druk-gradiënt differensiële skanderingskalorimetrie (DGDSK). Die DGDSK metode behels die meting van hitte vloei gedurende die sorpsie en desorpsie van 'n gas oor 'n voorafbepaalde druk reeks. Qst, tesame met gas lading data afgelei vanaf gas sorpsie isoterme, kan direk bereken word oor die hele druk (oftewel gas lading) reeks vanaf die gemete verandering in hitte vloei. Die opneem van CO2 deur CuHKUST met is gekies as 'n toets sisteem om hierdie direkte metode te beproef. Daar is gevind dat die DGDSK benadering temperatuur onafhanklike Qst waardes produseer wat vergelykbaar is met literatuur Qst waardes verkry vanaf die isosteriese metode. Omdat die DGDSK-metode temperatuur onafhanklik is, is hierdie prosedure minder veeleisend as die isosteriese metode aangesien slegs een DGDSK-eksperiment en een isoterm by dieselfde temperatuur nodig is, in teenstelling met verskeie isoterme by verskillende temperature wat nodig is om die isosteriese metode toe te pas. Verder word direkte gemete veranderinge in hitte vloei gebruike vir DGDSK ingespan in vergelyking met die indirekte benadering van die isosteriese metode. Die DGDSK -metode is ook suksesvol toegepas op 'n 2D inmekaar geritsde PKP ([Cd(oba)(bpy)]n) wat trapvormige sorpsie en histerese vir CO2, N2 en CH4 vertoon. Aangesien die isosteriese metode nie maklik toegepas kan word op materiale met ander sorpsieprofiele as Tipe I nie, is dit 'n belangrike verbetering van hoe Qst waardes gemeet kan word. Qst waardes kan ook gebruik word om die selektiwiteit van die materiaal vir ‘n spesifieke gas te voorspel. Hierdie ondersoek, het getoon dat [Cd(oba)(bpy)]n die hoogste affiniteit vir CO2 teenoor N2 en CH4 getoon.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102887
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