Poly(p-xylylene) Nanotubes Prepared via the Tubes by Fiber Templates Process as Catalytic Nanoreactors

The aim of this thesis was to develop and evaluate highly versatile methods for the preparation of continuous functional nanotubes for applications, such as catalysis and microelectronics. Hence, the suitability of the corresponding tubes as reusable tea bag-like catalyst systems as well as for the preparation of continuous conductive nanowires was studied. The underlying concept for each method was the tubes by fiber templates (TUFT) process: First, continuous template nanofibers were prepared using the electrospinning process, which is generally suitable for a huge variety of materials. The resulting nonwovens were coated from the gas phase with poly(p-xylylene) (PPX), based on the chemical vapor deposition process. Subsequently, the inner template material of the corresponding core-shell fibers was removed using either solvent extraction or pyrolysis. Accordingly, nanotubes with surface-immobilized catalysts were prepared by coating poly(ethylene oxide) nanofibers with an ethynyl-functionalized PPX. After removal of the template material, the tubes were suitable for “click” chemistry (more precisely, the azide-alkyne Huisgen 1,3-dipolar cycloaddition), and were subsequently equipped with either a (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO) catalyst or a bipyridine ligand. The bidentate chelating ligand was then used to immobilize copper. In order to test the tube catalysts, suitable model reactions were conducted: The TEMPO-functionalized tubes were used for the oxidation of benzyl alcohol, resulting in > 98 % of benzaldehyde even after 20 catalytic cycles. In addition, the copper-carrying tubes were used for the click reaction of benzyl azide with phenyl propargyl ether. After 17 cycles, the yield was still above 90 %. In contrast to the surface-immobilization of catalysts onto PPX, the suitability of the tubes for encapsulation of transition metal-based catalysts was studied. Polyurethane template fibers featuring incorporated palladium- and copper-functionalized poly(amido amine) dendrimers were electrospun and subsequently coated with PPX. Extraction of the template material resulted in semipermeable PPX nanotubes carrying the dendritic catalysts. The prepared nanoreactors did not show catalyst leaching and were used for homogeneous catalysis inside the tea bag-like PPX confinement: Whereas the palladium-based dendrimers catalyzed various Suzuki-Miyaura coupling reactions with yields around 88 %, the copper catalyst was used for the click reaction of benzyl azide with different alkynes, resulting in quantitative conversions. Both catalyst systems were conveniently reused for at least 14 cycles, albeit the corresponding yields decreased slightly over time. As an alternative to encapsulated dendrimers, gold nanoparticles were immobilized inside PPX tubes in order to compare the suitability and reusability for catalysis. First, poly(L-lactide)-stabilized gold nanoparticles were synthesized and subsequently used for the TUFT process. According to leaching studies based on UV-vis spectroscopy, the resulting PPX tubes successfully encapsulated the particles. Next, the suitability of the gold-carrying PPX nanoreactors for catalysis was evaluated: Both the hydrolytic oxidation of dimethylphenylsilane and the corresponding alcoholysis with n-butanol gave the corresponding product in quantitative yield at room temperature. After removal of the tea bag-like catalyst system, no product purification was necessary. Further, the catalyst was reused 20 times with no decrease in activity. Based on the concept of gold-carrying PPX tubes, another application was developed: Composite nanofibers featuring circa 60 wt% of gold were electrospun and subjected to the TUFT process. Thermal treatment up to 1050 °C resulted in degradation of the PPX shell and simultaneous formation of continuous conductive gold nanowires featuring a smooth surface. The corresponding heat-induced transition was studied by scanning helium ion microscopy. In conclusion, highly versatile methods for the preparation of continuous functional nanotubes were developed, suitable for a variety of materials and applications. In order to prove the concept, different catalysts were encapsulated inside PPX tubes and, respectively, immobilized on the surface of the tubes. The resulting nanoreactors were used as potent tea bag-like catalyst systems and showed high reusability. Further, gold-carrying PPX tubes were used for the preparation of continuous conductive nanowires.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2014

Author dc:creator

• Mitschang, Fabian

Contributors dc:contributor

• Greiner, Andreas

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/1733/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:1733