Title Cation-exchanged zeolites-A prepared from South African fly ash feedstock for CO2 adsorption
ii ABSTRACT In South Africa coal combustion constitutes up to 90 % of the country's energy need. This coal combustion activity is known to contribute to the amount of about 40 % of the total CO2 atmospheric emissions worldwide that are responsible for global warming effects. In addition burning of coal generates a large quantity of fly ash which creates environmental pollution since only a small portion of it is currently used in some applications. In order, on one hand to mitigate and
... CO2 and on the other hand to reprocess fly ash and reuse it, this study focuses on developing new technologies with cost-effective and less energy consumption in the domain of CO2 capture and sequestration. CO2 has priority attention for being the largest contributor to global warming. Various techniques have been used for CO2 capture and sequestration, such as aqueous alkylamine absorption or adsorption onto a solid adsorbent such as zeolites. In this study NaA zeolite adsorbent was hydrothermally synthesised from South African fly ash. This fly ash based NaA zeolite was then used as starting material to prepare LiA, CaA, and MgA zeolite catalysts via ion-exchange for comparative CO2 adsorption capacity. A systematic design of the ion-exchange procedure was undertaken at either 30 °C or 60 °C for a contact time of 1 hr, 4 hrs, and 8 hrs with 1, 2 and 3 consecutive exchanges in each case in order to determine the optimum conditions for loading each cation exchanged. The adsorption of CO2 on the ionexchanged fly ash based zeolite-A catalysts was carried out at 40 °C similar to the temperature of flue gas since the catalysts obtained in this study were also prepared with a view to their applications in flue gas system. The CO2 desorption temperature ranged between 40-700 °C. All materials used in this study, starting from fly ash feedstock, were characterized using various techniques to monitor the mineral and structural Abstract iii composition, the morphology, surface area and elemental composition and the adsorption capacity. The techniques included mainly Fourier transform infra-red, X-ray diffraction, Scanning electron microscopy, Transmission electron microscopy, Energy dispersive spectroscopy, X-ray fluorescence, Temperature programmed desorption. The results obtained from both Fourier transform infra-red and the X-ray diffraction spectroscopy for samples exchanged at either 30° C or 60 °C showed lower crystallinity in CaA and MgA zeolite samples. This decrease in crystallinity mainly affected the D4R (0-20° 2) and was demonstrated in the study to be inversely proportional to the increase of the atomic radius of cations (Li + > Mg 2+ > Ca 2+ ). In the Fourier transform infra-red, the vibration band at 677 cm -1 attributed to the extra-framework cation, also proportionally increased with the decrease of the atomic radius or size of the cations, and was intense in LiA zeolite samples. The EDS results at either 30 °C or 60 °C showed that the removal of the host Na + cation was partial and, none of the cation exchange conditions applied completely removed the host Na + cation. Among other cations, the ion-exchange using the Ca 2+ cation showed the highest removal of the host Na + cation. The CO2-TPD thermogram profiles revealed that CO2 physisorption was the major interaction observed for all fly ash based ion-exchanged zeolite-A which displayed significant weak strength adsorption sites at low temperature (40-150 °C) but it was found that the Na + cation was the preferred cation for low temperature CO2 adsorption application. However, at higher temperature (150-700 °C ), the highest amount of CO2 was desorbed from CaA zeolites followed by MgA zeolites, but the former showed dominant strong CO2 adsorption sites between (400-650 °C) compared to the latter which showed medium strength sites (150-400 °C). These high temperatures CO2 desorption peaks observed especially for CaA zeolites are ascribed to the formation of carbonate compounds. Abstract iv In summary, the XRD patterns and FT-IR spectra revealed unique structural modifications which were independent of the temperatures, contact time and consecutive exchanges applied during the ion-exchange. These structural features observed for fly ash based zeolite-A after the ion-exchange were found to be essentially influenced by the intrinsic characteristic of Li + , Ca 2+ and Mg 2+ cations. The contact time of 4 hr at 30 °C with 3 consecutive exchanges was found to be ideal to exchange Ca 2+ cation whereas the increased temperature of 60 °C was favourable for Li + and Mg 2+ cations. CaA zeolite showed significant CO2 adsorption but required high temperatures for CO2 desorption showing potential for long term capture compared to NaA, LiA and MgA zeolite samples which are suited for low temperature swing desorption of CO2.