Thesis (M. Sc.)(Chemical Eng.)--University of Pretoria, 2003. / Bibliography included.

Five systems consisting of different sorbate-sorbent combinations were studied using experimental volumetric adsorption techniques. Multiple adsorption isotherms were measured at low temperatures and low pressures for all of the systems studied which included CO2 adsorption on single walled carbon nanotubes (CO2 – SWCNT), Ethane adsorption on closed carbon nanohorns (Ethane-cNH), Ar adsorption on open carbon nanohorns (Ar – oNH), CO2 adsorption on zeolitic imidazolate framework-8 (CO2 – ZIF-8), and O2 adsorption on ZIF-8 (O2 – ZIF-8). Each of these systems offers a unique study of the relationship between the physical properties of the adsorbate and substrate and the effects of these properties on the thermodynamics and kinetics of adsorption. In addition to being of fundamental interest, the thermodynamics and kinetics of adsorption are important to understand for practical considerations in research fields such as gas storage and gas separation via adsorption processes, among other applications. CO2 – SWCNT is a system with a small linear molecular adsorbate with a permanent quadrupole moment adsorbing on a substrate with quasi-1D grooves and convex outer adsorption sites. Ethane-cNH is a system with a linear alkane adsorbing on a substrate with conical pores and convex outer adsorption sites. Ar – oNH is a system with a spherical atom sorbing in a substrate with two different groups of conical adsorption sites and both convex and concave surface sites. CO2 – ZIF-8 and O2 – ZIF-8 are both systems with small linear molecules sorbing in a flexible microporous scaffold-like substrate. Adsorption isotherms were analyzed to identify features corresponding to adsorbate-adsorbate and adsorbate-substrate interactions. Namely, the presence of substeps in the semi-logarithmic data were identified and interpreted to correspond to groups of adsorption sites of similar binding energy which likely depend on the morphology and/or structural flexibility of the substrates. All of the systems, with the exception of CO2 - SWCNTs, yielded at least some isotherms with substeps at pressures below that corresponding to saturation. Effective specific surface areas for all adsorbent-substrate combinations were calculated using the BET and Point-B methods for the sake of comparison. These surface area measurements are very dependent on the porosity and morphology of the substrate as well as the size and shape of the adsorbate atoms/molecules and therefore the values vary greatly between the different systems. The isosteric heat of adsorption was calculated using isotherms over the full range of temperatures for each system. A variant of the Clausius-Clapeyron equation was used for this purpose and the results were analyzed based on adsorbate-adsorbate and adsorbate-substrate interactions. Plateaus in the isosteric heat data for Ethane – cNH and Ar – oNH were related to the morphology of the substrates and properties of the adsorbate species. For CO2 – SWCNTs, the isosteric heat at all but the lowest coverages is below the latent heat of deposition. This is due to the quadrupole moment of CO2. For both of the studies of adsorption on ZIF-8, the isosteric heat contains peaks in the data which likely are the result of the flexibility of the ZIF-8 structure. The kinetics of adsorption (or, the rates at which the adsorption systems approach equilibrium) were analyzed as functions of isotherm temperature and coverage, vapor pressure, and fractional uptake point by point along individual isotherms using the linear driving force model. Certain trends in the kinetics of adsorption are consistent for all the systems studied and others vary depending on the specific adsorbate-substrate combination. As with the thermodynamic results, trends in the kinetics of adsorption are discussed in terms of the effects of adsorbate-adsorbate and adsorbate-substrate interactions.

Submitted by Rosivalda Pereira (mrs.pereira@ufma.br) on 2017-09-28T17:07:19Z No. of bitstreams: 1 CarlaAlbuquerque.pdf: 1781335 bytes, checksum: 233490c37121535d82c417f10b832f87 (MD5) / Made available in DSpace on 2017-09-28T17:07:19Z (GMT). No. of bitstreams: 1 CarlaAlbuquerque.pdf: 1781335 bytes, checksum: 233490c37121535d82c417f10b832f87 (MD5) Previous issue date: 2017-09-06 / Hydrotalcites were synthesized in the absence and presence of sucrose, and at temperatures of 28 oC (HDL and HDL / S) and 65 oC (HDL / S-65), by the coprecipitation method. All materials were characterized by ICPE, DRX, FTIR, MEV and BET. The general formulation obtained was: [Mg5.4Al2 (OH) 14.8] CO3.nH2O for all samples, confirming the success of the syntheses. The materials were used as adsorbents by two methods (Gravimetry and Turbidimetry), for the removal of petroleum from an emulsion in hydroalcoholic medium. The order of efficiency observed for a mass of 0.03 g of the adsorbents for the gravimetric method was: HDL/S-65 (92%) > HDL/S-28 (85%) > HDL (59%), indicating the positive effect of sucrose on the synthesis of the material. All the adsorptions followed a pseudo-second order kinetics, with an equilibrium time of approximately 50 min. Based on the results obtained by gravimetry it was understood that another method would be necessary, to better understand the behavior of the materials applied. The turbidimetry method showed favorable adsorption kinetics for the concentration used, showing that the process apparently occurred in multilayer, however complex. The models of Pseudofirst order, Pseudo-second order and intraparticle applied has favorable contributions to the understanding of these processes taking into account the several factors that influence the adsorption. The system also shows that HDL / S-28 has a higher removal capacity than HDL and HDL / S-65 respectively at 8h and 24h: HDL (8h) = 3538, HDL / S-28 (8h) = 3907; HDL / S-65 (8h) = 2127; HDL (24h) = 3252, HDL / S-28 (24h) = 5122; HDL / S-65 (24h) = 4201. / Hidrotalcitas foram sintetizadas na ausência e presença da sacarose, e nas temperaturas de 28 oC (HDL e HDL/S) e 65 oC (HDL/S-65), pelo método da coprecipitação. Todos os materiais foram caracterizados por ICPE, DRX, FTIR, MEV e BET. A formulação geral obtida foi: [Mg5,4Al2(OH)14,8]CO3.nH2O para todas as amostras, confirmando o sucesso das sínteses. Os materiais foram empregados como adsorventes por dois métodos (Gravimetria e Turbidimetria), para remoção de petróleo a partir de uma emulsão em meio hidroalcoólico. A ordem de eficiência observada para uma massa de 0,03 g dos adsorventes para o método gravimétrico, foi: HDL/S- 65 (92%) > HDL/S-28 (85%) > HDL (59%), indicando o efeito positivo da sacarose na síntese do material. Todas as adsorções seguiram uma cinética de pseudo-segunda ordem, com tempo de equilíbrio aproximado de 50 min. Com base nos resultados obtidos por gravimetria entendeu-se que um outro método seria necessário, para melhor entender o comportamento dos materiais aplicados. O método por turbidimetria apresentou uma cinética de adsorção favorável para a concentração utilizada, mostrando que o processo ocorreu aparentemente em multicamada, porém complexo. Os modelos de Pseudo-primeira ordem, Pseudo-segunda ordem e intrapartícula aplicados tem contribuições favoráveis para o entendimento desses processos levando em consideração os diversos fatores que influenciam a adsorção. O sistema nos mostra ainda que o HDL/S-28 tem maior capacidade de remoção que o HDL e HDL/S-65 respectivamente nos tempos de 8h e 24h: HDL (8h) = 3538, HDL/S- 28 (8h) = 3907; HDL/S-65 (8h) = 2127; HDL (24h) = 3252, HDL/S-28 (24h) = 5122; HDL/S-65 (24h) = 4201.