Design of Siliceous Materials from Industrial Waste

Hexafluorosilicic acid (FSA, H2SiF6) is a toxic and corrosive by-product of the production of phosphate-containing fertilizers and hydrofluoric acid, with more than 2 million tons produced every year. During the synthesis of HF, FSA is obtained as a result of the reaction between HF itself and the inevitable impurities of SiO2 contained in fluorspar, used as feedstock in the process. Not only should FSA be considered an obnoxious pollutant but also an economic burden as it is produced at the expense of HF, decreasing its yield. Moreover, direct uses of FSA are limited and its related compounds, such as fluorosilicates, are regarded as low value-added value products with niche applications. However, hexafluorosilicic acid should be considered an interesting alternative source of fluorine and silicon. In particular, it may replace the most common Si-containing precursors, such as TEOS and TMOS, in the synthesis of higher value-added materials, such as MCM-41, as the cost of these precursor severely hinders large-scale production of this type of materials. In this framework, the present work consisted in assessing first and foremost the best experimental conditions to obtain MCM-41 from FSA. The effects of temperature, time, hydrothermal treatment, and the presence of ethyl acetate on the textural properties of FSA-derived MCM-41 were assessed through a head-to-head comparison of textural properties with the analogous TEOS-derived samples. A range of analytical techniques were used to characterize these samples including N2-physisorption measurements, FT-IR, TEM, LA-XRD, FT-IR, and WD-XRF. This preliminary study was instrumental in the synthesis of a FSA-derived MCM-41 suitable to act as a support in a 10% Fe2O3/MCM-41 nanocomposite by using the so-called two-solvent incipient impregnation technique. This nanocomposite may be suitable for the removal process of H2S from syngas (sweetening). After characterization, it was tested and its H2S-removal performance compared with both its TEOS-derived counterpart obtained under the same experimental conditions and a commercial unsupported ZnO-containing sorbent. The test revealed no substantial differences in efficiency for the two nanocomposites with both of them outperforming the commercial sorbent. Another obstacle in implementing economically sustainable large-scale synthesis of MCM-41 lies in the necessity of using a templating agent (CTAB), essential to build up the mesostructured framework, which is usually not recovered but rather burnt off during the final stage of the synthesis. In envisaging a possible zero-waste industrial process involving FSA in the synthesis of MCM-41, the importance recovering the templating agent from the as-synthesized MCM-41 is compelling. For this reason, an experiment aiming at extracting and recovering CTAB from with ethanol was performed. In conclusion, this work confirms not only the feasibility of using a hazardous low-cost waste such as FSA in lieu of more expensive precursors in obtaining highly valued MCM-41, but also shows that FSA-derived MCM-41 and the related -Fe2O3-containing nanocomposite feature similar properties and performance, respectively, compared with their TEOS-derived counterparts. In parallel, FSA-derived precipitated silica was also synthesed and investigated for tyre reinforcement. The syntheses tried to assess the effect of pH, temperature, reactant flow rate, reactant concentration, and reaction medium on the properties of such silica.The samples deemed to be the best were used to prepare tread compounds and the tested in collaboration with the University "Bicocca" of Milan.