Metal-graphene hybrid aerogels as heterogeneous catalysts for chemical flow reactions

Graphene aerogels are ultra-lightweight, foam-like carbon materials composed of interconnected three-dimensional graphene networks, characterised by high surface areas, hierarchical macroporosity, and exceptional electrothermal conductivities, alongside tuneable microstructures and surface chemistries. These attributes directly enable their application as nanocatalyst supports in continuous flow processes, where the accessible surface and controllable functional groups promote uniform nanoparticle dispersion, and the open, tailorable pore network provides low resistance flow pathways and efficient mass- and heat-transfer characteristics required for continuous flow fine chemical
transformations.

The intrinsically low density and hierarchical architecture of graphene aerogels often renders them super-elastic or mechanically fragile, limiting their direct deployment in mechanically demanding continuous flow reactor environments. In this work, key parameters of the hydrothermal aerogel synthesis were systematically optimised to produce centimetre-scale cylindrical monoliths with the mechanical robustness required for reliable packed-bed flow reactor integration. Facilitating stable operation under relevant pressure regimes, such as withstanding sustained flow pressures of 20 bar. Through these optimisations, critical structure-function relationships governing aerogel mechanical behaviour were uncovered, yielding an array of mechanically diverse graphene aerogels (including materials exhibiting > 90% shape recovery) applicable in a range of applications.

The distinctive structural and chemical tunability of graphene aerogels provides a route to systematically engineer metal-support interactions. Control over the specific surface area of the graphene substrate aided the formation of well-dispersed sub 10 nm metal nanoparticles; targeted heteroatom doping of the graphene basal plane introduced direct metal-nitrogen interactions and reduced Cu particle size by 30%; and combining an optimised precursor-to-metal deposition strategy (Hydrogel Route) with electrothermal conversion (Joule-heating) delivered precise, repeatable, through-volume metallic nanoparticle loadings. These effects are validated across multiple length scales using advanced characterisation techniques, including high resolution TEM, nanoscale EDX mapping, and detailed XPS analysis. Scrupulous application of these strategies substantially enhances nanoparticle stability and activity, ultimately improving catalytic performance.

To evaluate the catalytic behaviour of these materials, a continuous flow acetophenone transfer hydrogenation model reaction was established that enabled extraction of key mechanistic insights and provided a robust platform for performance assessment under operationally relevant conditions, demonstrating catalyst stability for over 90 hours on-stream while maintaining > 95% selectivity. Incorporating a diverse library of functionalised graphene aerogel catalysts into this flow architecture allowed reliable, comparative screening of their fundamental material properties, revealing intrinsic performance behaviours, including the substantial activity enhancement delivered by Ru-Pt bimetallic alloying.

The work presented in this thesis establishes a comprehensive guide for the development of heterogeneous catalyst materials tailored for integration into continuous flow reactor systems. Systematic evaluation of each underlying component (synthesis, functionalisation, and application) provides the opportunity to critically examine structure-function relationships that govern catalyst performance. This integrated approach provides a foundation for future catalyst-design strategies in which performance-driven insights directly inform the rational engineering of next-generation flow-compatible catalytic materials.