From Molecular to Supported Single Atom Catalysts: Catalysis in Homogeneous and Heterogeneous Phase
Polyoxometalates (POMs) are best known for their acidic properties, ability towards electron transfer (ET), proton transfer (PT), proton coupled electron transfer (PCET) and electron transfer-oxygen transfer (ET-OT) reactions. These abilities strictly depend on addenda atoms incorporated to the POM. For example, phosphovanadomolybdic acid, H5PV2Mo10O40 can act as ET-OT agent for the oxidation of anthracene to anthraquinone whereas H5PMo12O40 can act as only ET agent. Moreover, reaction medium plays a decisive role in the rate of the reaction. Using a mixture of water and sulfuric acid as solvent, we were able to protonate H5PV2Mo12O40, which facilitates the ET-OT reactions for the activation of C-H bond of toluene and benzene., Electron paramagnetic resonance (EPR) spectroscopy was used to detect the presence of organic radical in the X-band.
In a different study, cavity of polyoxometalate wheel such as Mo132 was used as a nano-reactor which allows selective organic molecules to assemble inside the POM and further halogen migration reaction and Friedel-Craft arylation reaction was observed due to intrinsic acidity generated inside the cavity. Two-dimensional NMR, such as rotating frame overhause effect spectroscopy (ROESY) and heteronuclear overhause effect spectroscopy (HOESY) were used to understand the interactions between the organic molecules and the POMs.
All these results lead to remarkably interesting questions such as (a) can we mimic these reactions in heterogeneous phase? (b) can we anchor single atoms over support and understand the behavior of molecular catalysts? (c) can we tune the surrounding environment of the metal via different metal-support interaction? Synchrotron based X-ray absorption spectroscopy will be used as one of the major characterization tools in order to derive coordination number, oxidation state and symmetry of the supported single atom.
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