Nuove prospettive nella sintesi e funzionalizzazione di materiali mesoporosi nanostrutturati di silicio e loro applicazione nella catalisi

Abstract: The aim of the present thesis work is the development of methodologies for the synthesis and functionalization of nanostructured silicon-based mesoporous materials. By coupling innovative synthesis approaches such as microwaves irradiation for thermal activation, ultrasounds for micromixing and the use of a non-conventional silicon source as building block, the overall synthesis time was reduced, the dispersion of the functionalizing agent was enhanced and a synthesis procedure for the preparation of hexagonally ordered silica sieves using a geothermal waste was disclosed. The time reduction and the decrease in the number of synthesis steps poses signicant advantages from an economic point of view and favors the development of green chemical processes. The prepared materials were used as catalytic supports to produce sustainable energy sources and to valorize glycerol as potential raw material for the synthesis of value-added chemical reagents. Cerium promoted mesoporous silica supports were prepared following two different methodologies: I) Direct in situ modication of hexagonally ordered MCM-41 under solvothermal synthesis, using a cationic surfactant as structure-directing agent and II) Post synthesis cerium oxide grafting of mesoporous SBA-15, prepared with a non-ionic triblock poloxamer and a swelling agent to control the pore diameter and surface area. Direct synthesis of MCM-41 silica sieves was achieved by ultrasound-assisted solvothermal synthesis to obtain highly dispersed Ce into the silica framework. The explored Ce/Si molar ratios for the synthesis of the mesoporous materials ranged from 0.02 to 0.08. Microwave irradiation applied by a coaxial antenna was used for thermal activation to reduce the overall hydrothermal synthesis time. The hexagonal ordering of the materials decreased by Ce direct incorporation in the structure. As the amount of Ce/Si increased, two different mechanisms of Ce incorporation were observed: isomorphic substitution and ceria particles deposition outside the silica framework. The mesoporous sieves were used as catalytic supports for a Ni active phase (10 wt.% of metal loading). Their catalytic activity was evaluated in the ethanol steam reforming reaction to produce hydrogen. The new catalysts featured complete ethanol conversion, and higher H2 selectivity (compared to the same Ni catalyst over bare silica) ranging from 60 to 65%. The main product distribution was not dependent on Ce content, these materials did not exhibit catalyst deactivation after 6h on stream and were selective towards H2, CO2, CO and CH4 as sole reaction products. Post synthesis modication of silicon-based SBA-15 was performed over large pore mesoporous sieves, prepared by solvothermal methodology using a non-ionic triblock copolymer as structure directing agent. By modifying the 1,3,5 trimethylbenze/Pluronic 123 mass ratio and the solvothermal ageing temperature, it is possible to obtain hexagonally ordered mesophases in the pore size interval from 4 to 20nm. Selected samples of variable pore diameter were selected as support for the preparation of CeO2/SiO2 composites. The surface grafting methodology enabled the formation of well-dispersed ceria particles that do not match the pore size and therefore do not produce pore blocking. Finally, a green methodology for the preparation of mesoporous silica was developed using a non-conventional silicon-source. A series of template-based mesoporous molecular silicas were prepared using a geothermal waste. Microwaves irradiation was provided by the coaxial applicator for thermal activation. All prepared materials feature hexagonal ordering, narrow pore size distribution and high surface areas (over 500m2/g). By changing the parameters of the solvothermal synthesis, it is possible to tune the ordering and textural properties of the mesophases and reduce the reaction time. These materials were successfully used as a Zn catalytic support for the solventless MW-assisted conversion of glycerol to glycerol carbonate.