Polyoxotungstates [PW4O24]3– (PW4) and Catalysts 2023, 13, 138 3 of 17 [PW12O40]3– (PW12) can be inserted into the nanocages of the metal organic framework MIL-101 [33]. These catalysts were tested in the selective epoxidation of alkenes (3-carene, limonene, R-pinene, ...