The Development of Building Block Methodologies to Produce Nanostructured Heterogeneous Catalysts by Design

Building block methods were successfully developed to demonstrate the potential of this approach to synthesizing nanostructured heterogeneous catalysts by design. The octa(trimethyltin) cuboctameric spherosilicate, Si₈O₂₀(SnMe₃)₈, was used as the building block for the synthesis of these materials. The solid state structure of this building block was characterized and compared to other solid state structures of Si₈O₁₂ containing compounds. These studies showed that this building block is composed of rigid tetrahedral units connect through a flexible siloxy bridge. The trimethyltin functionality present on this molecular precursor will react with metal chlorides to produce cross-linked metal oxide/silicate matrices where a distribution of different linking species is present.

Three different silylchlorides were used to cross-link spherosilicate building blocks. The resulting products consisted of a distribution of different types of silylchloride linking units present in the solid material. Silylchlorides were used to study this reaction because the resulting product can easily be probed using silicon-29 solid-state NMR. The effect of initial stoichiometry on the distribution of linking groups in the resulting matrix was investigated and it was found that as the initial concentration of silylchloride decreased relative to the initial concentration of building blocks, the distribution of linking groups in the final product favored a more cross-linked matrix. These findings led to the development of synthetic strategies to produce a cross-linked matrix where a limiting amount of a silylchloride linking groups is present in the matrix having one type of environment throughout the entire material. These methods were then applied to reactions involving aluminum trichloride and titanium tetrachloride to produce solid acid catalysts. These catalysts were tested for the transesterification of triacetin with methanol. The catalysts were active for this reaction and triacetin conversion rates of 3 – 76% were observed after 8 hours. The titanosilicate catalysts were the most active and had reactive properties similar to commercially available catalysts.