Nanocarbon foams as catalyst supports for chemical flow applications

Reactions catalysed by metal complex compounds underpin a great number of commercial processes ranging from the manufacture of pharmaceuticals to the valorisation of biomass. Increasingly, industrial catalytic reactions are conducted in flow rather than batch, based on substantial advantages at both process scale and development scale. In the context of flow chemistry, the immobilisation of the molecular catalyst onto solid support materials can be highly advantageous, as it removes the need to separate the catalysts from the final product streams, while also offering new opportunities for catalyst regeneration. Carbon foams are highly porous, sponge-like carbon materials that offer an extremely promising support platform for metal catalyst systems due their high surface areas, pronounced porosities, and their chemical stability under many conventional reaction conditions. Their defined macroscopic shape and highly tuneable porosity make carbon foams highly suitable for integration into modern continuous flow processes. In contrast to many other support materials, carbon foams are also electrically conducting, enabling more energy efficient reaction temperature control (through direct resistive foam heating) and opportunities for electrical catalyst stimulation.

The project will develop highly porous carbon foams functionalised with molecular metal catalyst (Pt, Pd, Cu, Ru) and study their catalytic activity in flow. Carbon foams with different porosities and surface chemistries (hydrophobic, hydrophilic) will be produced through hydrothermal assembly techniques. Gas-phase and wet-chemical processes will be utilised to immobilise the metal coordination compound catalysts within the porous carbon structure. The focus of this work will lie on non-covalent functionalisation approaches, mainly based on pi-pi stacking interactions or hydrogen bonding between metal coordination compound and carbon foam surface, with a view to minimise the use of aggressive or toxic reagents often required for covalent carbon functionalisation. Catalyst-functionalised carbon foams will be characterised through a range of techniques, including TGA, Raman, BET, and XPS. A particular focus will lie on state-of-the-art electron microscopy and spectroscopy characterisation of the foam catalysts. This is important to gain improved control over the immobilisation process. For example, electron microscopy will provide a deeper understanding if molecular catalyst preferentially locate at specific nanoscale carbon features (basal planes, step edges etc) and will help to assess if immobilisation leads to unintended catalyst changes (e.g. degradation in nanoparticles, formation of single atomic sites etc), limiting catalyst performance.

The catalyst-functionalised foams will then be integrated into a mesoscale flow module. Fine-chemical flow catalysis (e.g. transfer hydrogenation, oxidative amidation) will be characterised within in-house flow reactors, that allow for rapid assessment of catalytic performance (conversion, selectivity, kinetics) while also offering potential for fast screening/optimisation of reaction conditions. Catalytic assessment will be complemented by catalyst characterisation, e.g. ICP-AAS (to assess catalyst leaching) and electron microscopy (to assess nanoscale changes post-catalysis). The flow module will also be used to characterise application relevant flow-through properties of the carbon foams themselves, such as residence time distributions and pressure drops. In addition, non-covalent catalyst binding characteristics, such as catalyst uptake capacity, sorption kinetics, and binding constants will be determined through titration experiments in flow. Time permitting, the most promising systems will be explored for the selective removal of molecular metal catalysts from reaction mixtures, as a potential route for catalyst recycling.