Multimodal macropore-tailoring of freeze-cast SiOC utilizing binary structure-directing agents
RAUCHENWALD K. 1, KONEGGER T. 1
1 TU Wien, Wien, Austria
The polymer-derived ceramic route raises interesting opportunities in terms of shaping as well as material property tuning to manufacture novel ceramic parts. Polysiloxane-derived silicon oxycarbide (SiOC) is an interesting candidate material to investigate catalyst interactions with a hydrophobic support. Unidirectional freeze-casting offers a straight-forward approach to obtain aligned macroporosity for high educt interaction and product throughput in the reactor. However, the introduction of porosity at an additional length scale typically requires additional templating methods such as the use of sacrificial porogens. In contrast, freeze-casting utilizing binary solvent mixtures with a eutectic composition enables templating of multimodal porosity in a one-pot approach. But such eutectic mixtures solidify at significantly lower temperatures than the pure solvents and it is difficult to stabilize polysiloxanes with common polycondensation-based crosslinkers at temperatures below -20°C. Alternative crosslinking methods are thus required to stabilize porosity templated from solidified eutectic solvent mixtures of cyclohexane (CH) and tert-butanol (TBA).
In this work, a photocurable poly-silsesquioxane (PSO) solution is used to facilitate photopolymerization-assisted solidification templating method in order to obtain directionally aligned, porous SiOC monoliths. PSO is functionalized with acrylic groups as photoactive moieties. The modified preceramic polymer solution can be stabilized in its solidified and templated state by light irradiation prior to solvent sublimation and pyrolysis. Mixtures of TBA and CH close to the eutectic composition were used as structure-directing agents. The formation of clusters through interactions between TBA and CH exhibit an interesting effect on obtained multimodal pore morphologies in SiOC. By varying the freezing conditions in terms of controlling the freezing front velocity, adjusting the preceramic polymer content and varying the pyrolytic conversion conditions, the material morphology can be further controlled. The monoliths are subsequently studied in regards to solvent adsorption, specific surface area and mechanical properties.
Furthermore, the applicability of obtained monoliths functionalized with catalytically active metal centres via wet impregnation for CO2 conversion processes is investigated. Here, the research flexibility that comes with freeze-casting of polymer-derived ceramics is convenient to study support effects in the catalytic utilization of CO2.