Synthesis and applications of low silica zeolites from Bolivian clay and diatomaceous earth

Abstract: The aim of the present work was to develop synthesis routes to produce synthetic zeolites with industrial attractiveness from non-expensive Bolivian raw materials, such as clays and diatomite. In particular, the work was focused on the synthesis of low-silica zeolites with the LTA and FAU structures. The raw materials as well as intermediate and final zeolite products were structurally characterized by different techniques, namely scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen gas adsorption, inductively coupled plasma mass spectrometry (ICP-MS). Besides, the final properties were evaluated for intended applications i.e. brightness by UV-VIS spectroscopy for LTA-type zeolite A in detergents and the CO2 breakthrough adsorption of FAU-type zeolite X for gas separation. The first part of this study deals with the synthesis of low silica zeolite A from Bolivian montmorillonite clay (Paper I). The clay was fused at high temperature with NaOH to render the material more reactive. The raw montmorillonite had a SiO2/Al2O3 ratio of 4 and sodium aluminate was added to the mixture to decrease this ratio to 2. An optimization of the synthesis time was performed. For the sake of comparison, the same treatment was applied to commercial kaolin. The final zeolite product from Bolivian montmorillonite exhibited high brightness despite the presence of iron in appreciable amount in the starting material and the final product. It was concluded that magnesium, present in the raw material, exerted a masking effect on iron. The latter was incorporated into extraneous magnesium aluminosilica compounds, thereby increasing brightness and strongly decreasing the overall yellowness. This simple method appears as a promising alternative to the complex and costly techniques suggested to reduce the iron content in natural raw materials, especially kaolin. The second part of the study addresses the synthesis, characterization and evaluation of FAU-type zeolites i.e. Y and X from diatomite and chemical grade reagents, respectively. With regard to the synthesis of zeolite Y, the diatomite was leached in sulfuric acid to remove impurities prior to synthesis, this step also resulted in dealumination, which rendered possible the synthesis of ZSM-5 zeolite (Paper II). However, extra aluminum had to be added in the form of aluminum sulfate for the synthesis of zeolite Y (Paper III). In this case, the raw materials were reacted hydrothermally at 100°C in aqueous medium with sodium hydroxide. Variations in parameters such as the Na2O/SiO2 ratio and synthesis time were investigated. As a result, micro-sized crystals of zeolite Y were obtained at a Na2O/SiO2 ratio of 0.9, which produced zeolite Y with a SiO2/Al2O3 ratio of 3.9. Also, zeolite Y with a SiO2/Al2O3 ratio of 5.3 in low yield at a Na2O/SiO2 ratio of 0.6 was achieved. In this way, diatomite behaved similarly to colloidal silica in traditional syntheses having a high degree of polymerization. Zeolite Y with a SiO2/Al2O3 ratio of 5.3 might be useful for the production of ultra-stable zeolite Y for use as FCC catalyst. Addition of NaCl to this system was also found to completely inhibit the formation of zeolite P and to retard the effects of overrunning by a synergic effect of Na and Cl ions (Paper IV). As an innovate application, FAU-type zeolite X films were growth on steel monoliths as structured adsorbent for CO2 removal (Paper V). Thicknesses 3 and 10 µm on steel monoliths of 1600 cells per square inches (c.p.s.i.) were investigated. The produced structured adsorbents showed reasonable CO2 adsorption capacity but with a very low pressure drop compared with traditional packed beds of zeolite beads. The CO2 breakthrough fronts were very sharp and the mass transfer resistance was very low compensating the presence of less adsorbing material by reducing cycle time and increased cycle frequency to increase throughput.   To summarize, zeolites with promising characteristics were successfully synthesized from Bolivian raw materials in this thesis work. However, further characterization is required to qualify these products for industrial applications. Moreover, this study might help the development of poor regions of the Bolivian Altiplano and open up for large scale production, since the methods developed in this work are simple and non-expensive.