We have explored different ways to immobilized molecular catalysts into different MOFs. This can be either be done directly during MOF synthesis by simply mixing a suitable catalyst linker with the metal salts for SBU assembly, or post-synthetically by first synthesizing a MOF which is then modified with the catalyst of choice.
Irrespective of the synthetic strategy and the catalyst of choice, we have discovered a few key design principles that are generally applicable. The first point is rooted in the fact that MOFs are crystalline materials with dimensions on the 0.1 to 100 micrometer scale. When molecular catalysts (typically about 1nm in size, i.e. at least 100 times smaller) are incorporated, it is often unclear whether these catalysts will reside primarily at the surface of the crystals, or evenly distributed throughout the material. Moreover, it is also often unclear whether all catalysts engage in turnover, or only a few catalysts at the surface, while the majority that is in the bulk of the crystals are merely spectators. We have investigated examples of both of these scenarios, and also observed secondary events such as pore clogging that slows down substrate access to catalysts in the interior of the MOFcat over time. Finally, as the catalytic reactions described above require the transport of charges, i.e. electrons for reductive, and electron holes for oxidative chemistry, charge transport through the MOFcats needs to be engineered. We have seen example where these processes are certainly limiting catalysis, while in other systems they work very well.
Over the course of the project, focus was directed more and more towards gaining understanding of transport phenomena in MOFs. Using theoretical models from related fields such as redox polymers, and translating these principles into MOFs, we have developed a profound understanding of electron hopping charge transport in MOFs. Importantly, we have contributed to the understanding that electron movement through the MOF is accompanied by translocation of counter ions. This is important to maintain overall charge neutrality.
The project draws a lot of its inspiration from naturally occurring enzymes. For example, in these enzymes, the active site is often protected from deleterious processes by being positioned deeply with in the peptide matrix, thus being physically isolated from the surrounding media. This means, in turn, that Nature also had to develop specific electron transport pathways to fuel the catalytic active sites. We have mimicked this scenario in a MOF for the first time, and have installed active sites as well as energy-matched “wires” in one MOF.
Finally, we have also shown that redox-active MOFs can be useful to interact with semiconductors. Semiconductors under illumination provide high-energy electrons, the energy of which is however often difficult to harness. We could show that redox-active MOFs can fulfill this function, and give rise to record-high photovoltages. We predict that this proof-of-concept work will be expanded to new semiconductor-MOF hybrid materials for the photoelectrocatalytic production of H2 or reduction of CO2 with minimum electrical energy input.
