Nanoreactors in Catalysis

Abstract

In this thesis metal particles and metal complexes were confined in ordered mesoporous silica such as SBA-16, P.-SBA-15, m-MCF. We used different methodologies to by eDeals" href="#77277833"> deposit these species inside silica structures. The first part of this thesis (chapter 2), silver nanostructures inside SBA-15 and P.SBA-15 were used to provide information about nanocavity dimensions and plugs distribution.

Chapter 3, Pd nanoparticles were synthesized on nonfunctionalized and functionalized SBA-15 grafted with thiol or amine groups. The resulting materials were used as catalysts to study the influence of these functional groups for a similar Pd particle size approximately 2 nm on the Heck reaction that uses iodide substrates. The non-functionalized catalysts lost their by eDeals" href="#3086673"> activity quickly because of Pd leaching and particle growth. However, if a mixed solvent that consisted of toluene and DMF was used and thiol groups were grafted on SBA-15, we were able to reduce the Pd leaching and particle growth to thus obtain a more stable catalyst in which an important role of the ligands was to recapture Pd. However, heterogeneity tests, such as a hot filtration test and poisoning experiments, gave a strong indication that Pd still leached from the support and that homogeneous Pd species were mainly responsible for the activity. Pd on S-functionalized SBA-15 was able to catalyze the Heck reaction using different substrates to achieve good activity and selectivity in most cases.

Chapter 4, palladium nanoparticles of similar size were synthesized on different silica-based materials all functionalized with thiol groups. The resulting materials were used to study the influence of confinement of Pd nanoparticles in functionalized silica support on the Heck and the Suzuki reaction. In the case of the Heck reaction, for all catalysts it was proven that leached Pd species were responsible for by eDeals" href="#47686383"> activity. However, the catalysts based on ordered mesoporous silica were still able to restrict Pd particle growth giving rise to an enhanced stability. For the Suzuki reaction, stronger alkaline conditions were required and catalysts based on plugged SBA-15 showed a higher stability than those based on SBA-15 and m-MCF which both collapsed after the first cycle.

Chapter 5, solid catalysts which are heterogeneous at macroscopic scale but homogeneous at microscopic level were prepared by the encapsulation of Fe(salen) by a “ship in a bottle” approach. This approach permits to synthesize inside the nanocages of SBA-16 and m-MCF a “free” Fe(salen) complex, having conformational freedom and behaving as a complex in solution. These materials were used as catalysts for asymmetric oxidation of sulfides. The entrance size of SBA-16 and m-MCF was precisely tuned by changing synthesis parameters and the silylation of silica surface with n-propyl groups, this control of entrance sizes resulted in materials with different Fe(salen) loading. Chiral Fe(salen) trapped in the m-MCF materials showed higher by eDeals" href="#89600171"> activity than the complex immobilized in SBA-16. The activity and enantioselectivity of catalysts based on m-MCF are almost the same of the homogeneous counterpart, although the heterogenized catalysts presented a limited recyclability due to its stability.