Macropore-tailoring of SiOC by photopolymerization-assisted solidification templating for monolithic catalyst supports

Abstract

Freeze-cast, polysiloxane-derived silicon oxycarbides (SiOCs) were investigated as monolithic catalyst supports. The monoliths were prepared by photopolymerization-assisted solidification templating, a specific variant of unidirectional freeze-casting. Macropore morphologies templated from unary and binary structure-directing solvents of tert-butyl alcohol and cyclohexane exhibiting a eutectic composition and melting points below –20 °C were stabilized by photopolymerization, these conditions not being accessible for conventional polycondensation-based crosslinking of polysiloxane. Screening of binary solvents revealed novel pore morphologies close to the eutectic composition, where bimodal macroporosity (21 µm, 4 µm) was obtained from excess tert-butyl alcohol. The observed overall high permeability (k₁ = 10⁻¹¹-10⁻¹² m²) renders freeze-cast SiOC interesting as an innovative support material for structured catalysts. The monoliths were compatible with the wet impregnation technique to deposit active Nickel sites for successful catalytic conversion tests employing CO₂ methanation as a model reaction. Here, distinct differences in CO₂ conversion were observed between dendritically and prismatically structured pores, with the former achieving significantly increased conversions of up to 69 % at overall high CH₄ selectivity.