Organized Functionalization of Mesoporous Silica Supports Using Prefabricated Metal-Polymer Modules

Abstract

The synthesis of inorganic oxides with controlled mesoporosity has developed since the beginning of the decade into one of the fastest moving research topics in modern materials chemistry. Whilst classical mesoporous silica synthesis employs low molecular weight organic amphiphiles as structure-directing agents in a sol±gel reaction mixture [1±4] to generate three-dimensional structures, the recent utilization of pre-assembled liquid crystalline surfactant [5] and polymer [6±8] templates has extended the synthetic possibilities. The use of high molecular weight block copolymers as templating agents has enabled the tailoring of pore size and framework architecture, extended the inorganic mesophase length scale and allowed the production of bulk inorganic monoliths as opposed to powders.[9±12] The larger pore sizes of these structures (2 to 50 nm) compared to microporous zeolitic materials (less than 1.2 nm) makes them particularly interesting for catalytic processes involving bulky substrates. The production of modified porous silicas containing well-dispersed noble metal nanoparticles and clusters distributed homogeneously throughout the pore channels has been a further challenge in the area of catalysis.[13±19] Conventional incorporation of metals involves the treatment of pre-formed solid supports, although in situ metal introduction techniques during support synthesis have also been reported. Heterogeneous metal distributions within the support, particle coalescence and decomposition, and catalyst leeching are some of the problems encountered during testing of materials prepared by these methods.Here we report on a specific method for direct inclusion of noble metal nanoparticles into a host framework which circumvents the need for post-synthesis functionalization of the support. A simple and convenient two-step reaction is presented in which prefabricated inorganic/organic hybrids are incorporated directly in a silica templating procedure to generate mesoporous silicas with high pore-connectivity and hosting a homogeneous distribution of noble metal particles. The approach combines the use of spherical functionalized polymer microgels [20] as nanosized exotemplates for the controlled growth of metal colloids [21] and as endotemplates during the casting of mesoporous sol±gel silicas.[22]A typical preparative procedure involves the initial synthesis of noble metal±polymer microgel hybrids. For this, poly(styrene sulfonate) microgels (70±90 nm in diameter) are loaded with noble metal ions and complexes. Reduction of the metal salts through the addition of sodium borohydride results in the formation and retention of metal particles within the microgel interior. These precursor hybrids are then added to a well-documented sol±gel recipe for the casting of mesoporous silica from ordered polystyreneblock-poly(ethylene oxide) mesophases utilizing tetramethoxysilane (TMOS) as the silica precursor.[8] The polymeric carrier mediates compatibility with the silicic network and ensures the positioning of the metal particl...

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