Browsing by Author "Qawasmi, Yaseen"

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Open Access
Adjustable polystyrene nanoparticle templates for the production of mesoporous foams and ZnO inverse opals
(2020) Abitaev, Karina; Qawasmi, Yaseen; Atanasova, Petia; Dargel, Carina; Bill, Joachim; Hellweg, Thomas; Sottmann, Thomas
The manifold applications of porous materials, such as in storage, separation, and catalysis, have led to an enormous interest in their cost-efficient preparation. A promising strategy to obtain porous materials with adjustable pore size and morphology is to use templates exhibiting the appropriate nanostructure. In this study, close-packed polystyrene (PS) nanoparticles, synthesized by emulsion polymerization, were used to produce porous PS and ZnO inverse opals. The size and distribution of the polystyrene nanoparticles, characterized by dynamic light scattering (DLS), small-angle neutron scattering (SANS), and scanning electron microscopy (SEM), were controlled via the concentration of sodium dodecyl sulfate (SDS). Systematic measurements of the water/styrene-interfacial tension show that the critical micelle concentration (CMC) of the ternary water–styrene–SDS system, which determines whether monodisperse or polydisperse PS particles are obtained, is considerably lower than that of the binary water–SDS system. The assemblies of close-packed PS nanoparticles obtained via drying were then studied by small-angle X-ray scattering (SAXS) and SEM. Both techniques prove that PS nanoparticles synthesized above the CMC result in a significantly unordered but denser packing of the particles. The polystyrene particles were subsequently used to produce porous polystyrene and ZnO inverse opals. While the former consists of micrometer-sized spherical pores surrounded by extended open-cellular regions of mesopores (Rpore ≈ 25 nm), the latter are made of ZnO-nanoparticles forming a structure of well-aligned interconnected pores.
Open Access
Toward confined spaces in polymers and microemulsions for catalytic applications
(2019) Qawasmi, Yaseen; Sottmann, Thomas (apl. Prof. Dr.)
In biocatalysis impressive regio- and stereoselectivities are achieved via the directing influence of the three-dimensional structure of enzymes. As enzymes often suffer from limited pH stability, intolerance of organic solvents and perform only within a limited temperature range, the development of mesoporous support materials in which organometallic catalyst are introduced is targeted in the CRC 1333 “Molecular Heterogeneous Catalysis in Confined Geometries”. Thus, the first part of this study dealt with the synthesis of porous polystyrene (PS) and mesoporous PS/ZnO hybrid materials. Following the Nanofoams by Continuity-Inversion of Dispersion (NF-CID) method, in which colloidal crystals of polymer nanoparticles are foamed with supercritical CO2, porous PS polymers were synthesized in a first step. The results of this thesis show, that pore size, homogeneity, porosity, and morphology of the synthesized porous polymers can be adjusted by the size, polydispersity, and glass transition temperature of the PS nanoparticles as well as the foaming parameters. Open-cellular porous PS with pore size of the order of 50 nm was obtained by modifying the expansion step of the NF-CID method. To enhance the stability of the porous PS and finetune the pore size, the chemical bath deposition method was used to synthesize a mesoporous PS/ZnO hybrid material. Thereby, SEM images and EDX analysis confirm the formation of a thin layer of ZnO particles on the pore walls, while the general porous structure is retained. After the functionalization and anchoring of organometallic catalysts, these mesoporous PS/ZnO hybrid materials will be ready for the use in heterogeneous catalysis. In the second part of this study, the influence of liquid confinement on asymmetric Rh-catalysis was explored. As a benchmark reaction, the 1,2‐addition of boroxine 2 to N-tosylimine 1 in the presence of a [Rh/chiral diene ligands] complex was chosen. To create liquid-confinement, a reaction-specific microemulsion containing equal amounts of H2O/KOH and toluene/reactants was formulated using n‐octyl β‐ᴅ‐glucopyranoside (C8G1). A special feature of this nanostructured reaction medium is the presence of water- and toluene-rich compartments with a domain size of 5.5 nm. Performing the catalysis, a strong dependence on ligand type and reaction media was found. Especially for slightly polar diene ligands the liquid confinement provided by the microemulsion improved reaction rate, yield and enantioselectivity.