Study on the incorporation of various elements into the Keggin lacunary-type phosphomolybdate [PMo9O34](9-) and subsequent purification of the polyoxometalates by nanofiltration

Transition-metal substituted Keggin-type polyoxometalates (POMs) are of great interest for applications in biomedicine, material science, and catalysis. The synthesis of transition metal-substituted Keggin-type polytungstates via the formation of a lacunary structure is well established, in contrast this approach is so far unexplored for Keggin-type polymolybdates. This is because the prevailing doctrine assumes that the lacunary Keggin phosphomolybdate [PMo9O34](9-) is too unstable and can only be stabilized with organic ligands such as pyridine in organic solvents. In this work, we present a reliable procedure for the synthesis of the lacunary compound [PMo9O34](9-) and its application in a novel in situ approach for the synthesis of different metal substituted POMs. The method is based on generating the lacunary species in situ, where the metal-substituted POMs are produced by adding another precursor compound. We employed this method to synthesize several new specific element-substituted POMs, which we present with comprehensive characterization. The interpretation of the analytical results was complemented by DFT calculations. For the separation of by-products from synthesis, we employed a novel membrane-based nanofiltration process, that enables superior separation of alkali salts from the POM solution resulting in >99% rejection of the POM components.