Multifunctional Mesoporous and Nanostructured Catalysts: Exploring Novel Synthetic Methods, Properties and Applications

Novel multifunctional mesoporous and nanostructured catalysts containing two or more different types of judiciously chosen functional / catalytic groups were developed and their unique and cooperative catalytic activities in various useful organic reactions were explored. First, mesoporous silica material containing tertiary amine/silanol groups was synthesized by simple postgrafting synthetic method. The material was found to exhibit efficient cooperative acid/base bifunctional catalytic activity towards Michael addition reactions between trans-beta-nitrostyrene and various active methylene compounds such as malononitrile, acetylacetone and dimethylmalonate. Besides serving as a solid base catalyst, such organoamine-functionalized mesoporous silica materials can be utilized as effective support materials for catalytically active organometallic complexes. This was demonstrated by immobilizing ethylenediamine onto mesoporous silica via postgrafting synthetic method and then complexing Fe(III) onto the supported ethylenediamine groups. This yielded a bifunctional Fe(III)/silanol-based heterogeneous catalyst that showed efficient catalytic activity towards epoxide ring opening reactions. Next, the potential of these types of organic-functionalized mesoporous silicas for immobilization of metallic nanoparticle catalysts was investigated. Specifically, mercaptopropyl-functionalized mesoporous silica was synthesized and the material was then supported with ultrasmall Aun nanoclusters. The catalytic properties of the resulting materials in styrene oxidation were studied. Furthermore, the effect of the removal of the thiol groups from around the surfaces of the gold nanoclusters on catalytic activities of the mesoporous silica-supported nanoparticles was investigated. As mesoporous silica have some limitations of crowding in their pores and poor mass transport for reactants when they are functionalized with larger groups such as nanoparticles, a new strategy was developed, where such catalytic groups were immobilized on the outer surface of silica microspheres. These supported nanoparticle catalytic groups on the silica nanospheres were further coated with a porous silica shell in order to overcome their possible aggregation, sintering and loss of catalytic activities. The resulting nanomaterials, dubbed produced SiO 2-Au-pSiO2 core-shell-shell microspheres, were then used as efficient and recyclable nanocatalysts for styrene epoxidation. This strategy was further extended to core-shell-shell microspheres containing the metal (e.g., Pd) nanoparticles within G4 PAMAM dendrimers that are supported on silica nanosphere cores and coated by nanoporous silica shells. These nanomaterials, denoted as SiO2-Pd/PAMAM-pSiO2 core-shell-shell microspheres, were shown to serve not only as efficient and recyclable catalysts but also as selective catalysts for specific functional groups in hydrogenation reaction of various substrates.