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.
Oxide inverse opals (IOs) with their high surface area and open porosity are promising candidates for catalyst support applications. Supports with confined mesoporous domains are of added value to heterogeneous catalysis. However, the fabrication of IOs with mesoporous or sub-macroporous voids (<100 nm) continues to be a challenge, and the diffusion of tracers in quasi-mesoporous IOs is yet to be adequately studied. In order to address these two problems, we synthesized ZnO IOs films with tunable pore sizes using chemical bath deposition and template-based approach. By decreasing the size of polystyrene (PS) template particles towards the mesoporous range, ZnO IOs with 50 nm-sized pores and open porosity were synthesized. The effect of the template-removal method on the pore geometry (spherical vs. gyroidal) was studied. The infiltration depth in the template was determined, and the factors influencing infiltration were assessed. The crystallinity and photonic stop-band of the IOs were studied using X-Ray diffraction and UV-Vis, respectively. The infiltration of tracer molecules (Alexa Fluor 488) in multilayered quasi-mesoporous ZnO IOs was confirmed via confocal laser scanning microscopy, while fluorescence correlation spectroscopy analysis revealed two distinct diffusion times in IOs assigned to diffusion through the pores (fast) and adsorption on the pore walls (slow).
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