Imaging the Permeability and Passivation Susceptibility of SiO₂ Nanosphere-Based Mesoporous Supports for Molecular Heterogeneous Catalysis under Confinement

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Mesoporous catalyst supports that mimic the spatially confined active sites of enzymes can aid in the development of highly selective molecular heterogeneous catalysts. Nontemplated mesoporous SiO 2 (NT-mSiO 2 ) materials with open porosity, tunable pore sizes, and high diffusivity are promising candidates in this regard. However, the operationalization of such materials strongly depends on the controlled passivation of their external pore surfaces. This enables catalyst molecules to be selectively immobilized on the internal pore surface where the desired spatial confinement effects can be observed. In this work, confocal laser scanning microscopy (CLSM) is presented as a viable analytical tool to visualize the passivation efficiency and permeability of NT-mSiO 2 platelets consisting of interconnected mesopores ( d pore = 9.4 nm) with positive pore wall curvatures. CLSM investigations with representative fluorescent probe molecules show that after pore-filling with Pluronic P123, the passivating film is constrained to the external platelet surface. The permeability of different passivating films based on mono and bifunctional silanes is compared. A pyrene-based organosilane is used as a tracer molecule to determine the covalent functionalization susceptibility of passivated NT-mSiO 2 platelets. Additionally, SiO 2 nanospheres with modular particle sizes are synthesized using an l -lysine-mediated sol–gel process and assembled into NT-mSiO 2 with tunable pore sizes. Hexamethyldisilazane-passivated NT-mSiO 2 ( d pore = 4.3 nm) is used as a catalyst support for the immobilization of cationic molybdenum imido alkylidene N -heterocyclic carbene complexes to study the effect of confinement on monomacrocyclization selectivity in ring-closing olefin metathesis reactions. A 31% enhancement in monomacrocyclization selectivity is observed when compared to the homogeneous catalyst.