Campanile, Assunta (2021) Multifunctional inorganic foams based on alkali – activated materials. [Tesi di dottorato]

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The fabrication of inorganic foams with a porosity gradient is widely desired from scientists in order to achieve enhanced performances, mainly due to their low density, high strength and specific functional properties. Multifunctional inorganic foams represent innovative systems thought for specific and advanced functions, in which a spatial gradation in structure and/or composition lends itself to tailored properties and for this reason they find application in a broad range of high–tech fields such as energy, building, aerospace, filtration and bioengineering. Supporting or shaping zeolites can represent a challenge to obtain this kind of systems. In fact, structuring a porous powder, as a zeolite, permit to obtain an optimized structure with high mass transfer, low pressure drops and high mechanical and chemical stability. Different methods have been developed to achieve this technological goal, by using porous ceramic binders, polymer foams, or permeable bags. Pelletization and extrusion are the most frequently used methods to shape powdery zeolites, but they require a binder that can partially obstruct the active sites of zeolite. Recently, Additive Manufacturing (AM) approach has been also tested to obtain binder–less shaped zeolite monoliths. Geopolymer Gel Conversion (GGC) represents an economic and sustainable alternative to obtain a self-supporting zeolite. Several studies confirms that it is possible to promote the nucleation of a zeolite inside a geopolymeric matrix by tuning pH, temperature, pressure, and time of the geopolymerization reaction. In fact, the geopolymers can be considered the amorphous counterpart or precursor of crystalline zeolites. The hydrothermal treatment is the traditional method to promote the crystallization of zeolites in the geopolymer framework. More recently a one-step procedure is developed, during which geopolymerization and zeolites formation can take place simultaneously. The two-step method is useful to drive the crystallization of a specific zeolite inside the matrix, with the aim to functionalize the self – supported foam. These geopolymer – zeolite composites find application in different technological fields, e. g. purification of wastewater, gas adsorption and separation, catalysis. The zeolites have excellent ion-exchange and sorption properties, that depend on their physical–chemical features. For these reasons, the geopolymer – zeolite composites can be proposed as water softeners, solving the problem of structuring powder zeolites. In particular, Na–LTA and FAU–X zeolites are excellent candidates for these systems, because they can easily exchange their sodium ions with “hard water ions”, such as calcium and magnesium. Zeolite, in particular FAU–X zeolite, is largely used for CO2 capture, thanks to high performance in gas adsorption. FAU-X zeolite is industrially produced as a microcin powder, therefore shaping it represents a technological challenge. Also in this case, the geopolymer gel conversion represents an innovative method to obtain a self-standing FAU-X porous monolith without the use of a binder and, moreover, the presence of the geopolymer backbone ensures mechanical strength and mesoporosity level. This research activity was focused on hybrid foams based on zeolite–geopolymer materials and characterized by hierarchical porosity. The activity first centered on the analysis of the state of the art. Then, the research focused on the design of a multifunctional porous material, obtained by GGC with a one–step method, combining the microporosity of the zeolites, the mesoporosity of the geopolymer matrix and the macroporosity obtained by adding a foaming agent. In particular during the first year, the optimization of the operating conditions, by studying the effect of the foaming agent (Silicon powder) content, relative humidity, and curing time on zeolite content in the sample, was performed. After the optimization phase and the consequently choice of the best sample, the activity of the second year focused on the study of main relevant properties for industrial applications. In particular the selected samples were characterized in terms of chemical, physical and morphological analysis. Then the “water properties” of the softeners were evaluated by measuring Cation Exchange Capacity (CEC), softening, regeneration and reusability performance. In addition, to obtaining a controlled and homogeneous foaming process, an under vacuum curing process was performed by studying the effect of curing temperature and foaming agent (Silicon powder) content. At the same time the addition of a foaming stabilizer or the use of a new foaming agent (Hydrogen Peroxide) was investigated to determine the effects in terms of crystalline phase and porosity. Finally, a two steps method to favour the crystallization of only FAU–X zeolite was carried out. After chemical, physical and morphological analysis, the self–standing FAU–X porous monoliths were tested for gas adsorption of several molecules (nitrogen, carbon dioxide, water vapor, acetic acid and ethanol). The collected results suggest the possibility of using geopolymer – zeolite hybrid materials as bulk–type adsorbent (both in gaseous and aqueous system), self–supporting membranes and gas separation.

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