Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by Subject "Adsorption -- Properties"

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    A Hybrid experimental and theoretical investigation into the multi-component gas adsorption properties of selected porous materials
    (Stellenbosch : Stellenbosch University, 2021-03) Costandius, Jan; Esterhuysen, Catharine; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The importance of advancements in multi-component gas adsorption (MGA) techniques, with respect to combating climate change, is that these methods can help treat the problem at the cause: selective capture of CO2 from mixtures of gases. Unfortunately, new MGA techniques are rarely developed, because the experimental and theoretical methods are much more complex than the single-component gas adsorption (SGA) alternatives. Consequently, most research is focused on studying the SGA properties of novel CO2 adsorbers, while the MGA properties of these materials are often only investigated briefly, predicted qualitatively, or overlooked entirely. This offers an opportunity to develop new experimental and theoretical MGA techniques, such as those presented in this study. Furthermore, by virtue of simply studying the MGA properties of some adsorbents comprehensively, a new predictive method for MGA was developed and new insights were gathered about the cause of ideal vs. non-ideal adsorption. The development of a new volumetric MGA method is reported in the first section of this study. The design of the method is discussed and the benchmarking of the instrument against previously published SGA data is shown. The MGA equilibria of CO2 and N2, when adsorbed by zeolite 13X, are reported. These data are compared to the results of the predictive ideal adsorbed solution theory (IAST) method, where it is found that IAST performs well with predicting the uptake of CO2 but fails to correctly predict the uptake of N2. Additionally, an empirical model, namely the extended Sips isotherm, performs surprisingly well with predicting the mixed uptakes of both CO2 and N2 accurately around 1 bar, where IAST does not. Using the findings of the first section as inspiration, the extended Sips is utilized alongside the non-ideal (real) case of the adsorbed solution theory (RAST), to create a new predictive MGA method named PRAST-S. This method is used to predict the mixed uptakes of CO2 and N2 by Cu-HKUST-1, Mg-MOF-74, MOF-14, and UiO-66. Furthermore, the MGA equilibria of CO2 and N2 obtained using the instrument described in the first section are also reported for Cu-HKUST-1 to confirm that PRAST-S correctly predicts the adsorbed amounts. However, what was not anticipated was that the experimental measurements and PRAST-S prediction both show that Cu-HKUST-1 exhibits ideal adsorption of CO2 and N2. This is in contrast with the other materials studied, which all exhibit non-ideal adsorption. A series of in silico simulations of CO2 and N2 within the pores of 13X and Cu-HKUST-1 to probe the differences in ideal and non-ideal behavior shows that a sudden shift in the mean interaction energy upon mixing is the telltale sign of non-ideal adsorption. Furthermore, the mechanism that led to the shift in the mean interaction energy – or the lack thereof, in the ideal cases – is shown to be dependent on the mode with which the adsorbates interact with the adsorbent.