Browsing by Author "Koynov, Kaloian"

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    From macro to mesoporous ZnO inverse opals : synthesis, characterization and tracer diffusion properties
    (2021) Kousik, Shravan; Sipp, Diane; Abitaev, Karina; Li, Yawen; Sottmann, Thomas; Koynov, Kaloian; Atanasova, Petia
    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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    Hierarchical silica inverse opals as a catalyst support for asymmetric molecular heterogeneous catalysis with chiral Rh‐diene complexes
    (2021) Deimling, Max; Kousik, Shravan R.; Abitaev, Karina; Frey, Wolfgang; Sottmann, Thomas; Koynov, Kaloian; Laschat, Sabine; Atanasova, Petia
    The efficacy of heterogeneous catalysis relies heavily on diffusion and distribution of reactants within catalyst supports. However, the presence of confinement, essential for reaction selectivity, drastically slows down molecular transport. Here, macro‐mesoporous silica inverse opal (SiO2-IO) films were used as a model system to study the rather unexplored molecular infiltration behavior using a probe molecule resembling a catalyst via confocal laser scanning microscopy (CLSM). CLSM analysis revealed homogeneous tracer distribution in SiO2-IO and attachment to both transport and mesopores. Bulk macro‐mesoporous SiO2-IO support was used for the attachment of mono‐ and bis‐functionalized chiral Rh‐diene complexes, and the catalytic activity and selectivity with respect to the support was studied. Lower enantioselectivity was observed with the bis‐functionalized ligand due to ligand entanglement and reduced accessibility of the active site, while the mono‐functionalized ligand gave an excellent enantioselectivity of 94 %ee in the asymmetric 1,2‐addition of triphenylboroxine to N‐tosylimines and could be recycled up to three times.