The global challenge to realize a transition towards sustainable energy, has induced a wave of scientific effort to capture solar energy. A possible pathway to accomplish this is through photocatalysis. This requires a catalyst capable of initiating/facilitating red-ox reactions under illumination. Recent developments in materials science delivered new prospective candidates for such transformations, among which are Metal-Organic Frameworks (MOFs). MOFs are crystalline porous solids constructed by inorganic nodes and organic polydentate linkers. Considering the similarity of MOFs and metal-oxides, these hybrid solids were probed in photocatalytic oxidations in the late 2000s. Since then the number of publications per year steadily increases. One of the most perspective MOFs for photocatalysis is MIL-125(Ti). This MOF, constituted by TiO5(OH) cyclic octamers and terephthalates, showed photo-catalytic activity in benzyl alcohol oxidation. However, the optical absorption of this framework is limited to the UV-region of the solar spectrum. By introducing an amino-group to the aromatic ring of the linker and post-synthetic modification of NH2-MIL-125(Ti) its absorption is tuned towards visible light, resulting in improved photocatalytic activity in selective benzyl alcohol oxidation. The oxidation rate, however, was still very moderate. In addition, TRMC investigations revealed the very poor photo-conductance in MIL-125(Ti). So, MOFs can be easily tuned by post-functionalization with antenna-like molecules, but light absorption is not the main bottleneck in applying these solids for photocatalysis, a precise active site engineering is needed. This idea finds confirmation in the application of NH2-MIL-125(Ti) for hydrogen evolution. Introducing appropriate active sites results in a reusable noble metal-free heterogeneous system producing hydrogen with a TON of 40 after 40 h of visible light illumination and remaining stable after that time. These results clearly demonstrate the potential of MOFs for solar energy utilization and inspire future developments.

Metal-organic frameworks for solar energy utilization: Oxidation and hydrogen production

247th National Meeting (2014)

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