Polymer-derived silicon oxycarbide as support material for ionic liquids for heterogeneous catalysis

Abstract

Supported ionic liquid phases (SILPS) are highly promising catalysts for a wide variety of organic synthesis reactions, but require suitable carrier structures for high performance. In this work, polysiloxane-derived silicon oxycarbides (SiOCs) were investigated as novel, alternative carrier materials and compared to conventional SILPs using mesoporous silica (SiO₂). The heterogeneous SILP catalysts were tested for the production of limonene carbonate by cycloaddition of CO₂ to limonene oxide, stating an interesting high value product. In a first step, a screening of SiOCs derived from precursors after pyrolysis at various temperatures (300-900 °C) was conducted. The particulate support materials were impregnated with tetrabutyl ammonium halides (TBAX, X = Cl-, Br-, I-) and were investigated in batch mode for the model reaction, yielding excellent selectivities of 97-100 % and yields of 53-62 % for SiOC-SILPs. Differences in the surface characteristics and the affinity to water between SiO₂ and SiOC were correlated with the effect that SiO₂-SILPs triggered the formation of diol as a by-product, leading to a lower selectivity of 87 % and a lower yield of 48 %, respectively. Subsequently, macroporous monolithic SiOC-SILPs with suitable permeability characteristics (k1 = 10⁻¹¹ m²) were produced via freeze-casting and successfully used as base for monolithic reactors for the selective and continuous production of limonene carbonate in supercritical CO₂. Constant product output over 48 h without concurrent catalyst leaching was achieved, rendering these polymer-derived support materials highly promising for future SILP-related applications owing both to their catalytic performance as well as their immense flexibility in shaping and structural tailorability.