Polyhedral oligomeric silsesquioxane (POSS) cage structures can be exclusively generated as the reaction products of methyltrichlorosilane, MeSiCl3 with a supported Re/CeO2 catalyst at 500C. The cage structures are generated by exchange of oxygen for chlorine in the ceria support under H2 reducing atmosphere. Regeneration of the support can occur by passing air over the catalyst at 500C and then the reaction can be repeated. This presents a novel way of generating cage structures exclusively via a gas phase reaction. GC-MS of the reaction products indicates that the cage structures which are formed are Me6-T6, Me8-T8 and Me10-T10. The reaction proceeds at high conversion and high yield to these POSS cages. When Me2SiCl2 is used as the reactant, mostly cyclic siloxanes D4 – D6 are produced. The hydrido-chlorosilane, MeHSiCl2 generates cage structures through an additional Si-H – O ligand exchange reaction that is facilitated by the oxygen vacancies on the ceria support. When Si-H ligands are present, rearrangement reactions are also observed under these conditions and constitute a supplementary process for the formation of the POSS cage structures. Trichlorosilane (HSiCl3) does not form cage structures, but it does undergo rearrangement. It also undergoes Si-H – O ligand exchange with oxygen vacancies on the ceria support to form cristobalite (crystalline SiO2) on the support. This work demonstrates, for the first time, the ability to form POSS cage structures via a gas phase catalytic cycle. Reaction times are much shorter than solution-based methods, with residence times of seconds or less compared to hours, days, or months for solution-based syntheses. Further, the elevated temperature relative to hydrolytic methods of synthesis, can be useful in forming Me6-T6 cages, because the strained 3 ring faces of the cage are thermodynamically disfavored at lower temperatures relative to the nearly unstrained cage structures Men-Tn (n ≥ 8). This can allow for higher yielding synthetic pathways to T6 cage structures. Overall, this gas phase route represents a promising and useful new pathway to synthesize POSS cage structures from a variety of chlorosilane starting materials.

Polyhedral organic silsesquioxane cage structures formed via reaction of methylchlorosilanes with Re/CeO2 in catalytic oxygen depletion-regeneration cycle

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