Three Distinct Iron-Doped POM-Based Hybrid Composites for the Hydroxylation of Benzene to Phenol.

Developing an efficient and stable heterogeneous catalyst for the one-pot hydroxylation of benzene to phenol under mild conditions is of critical importance. This study reports three Keggin-type POM-based organic-inorganic hybrid composites: (H2pzpy)[FeII(pzpy)2]2[H3PWVI9WV2FeIIO40]·H2O (1), [FeII(pzpy)2(H2O)2]{[FeII(pzpy)]2[HPMoVI8MoV4O40]}·3H2O (2), and {[FeII(pzpy)2(H2O)]2[PMoVI8MoV3VIVO40(VIVO)2]}·2H2O (3) (pzpy = 2-(1H-Pyrazol-3-yl)pyridine). Single-crystal analysis indicates that the structural commonality of compounds 1-3 is that the metal complexes formed by Fe(II) and pzpy ligands are grafted to different POM units via Fe-O bonds. Notably, compound 3 exhibits exceptional performance in benzene oxidation, achieving a phenol yield of up to 20.8% with a selectivity of 96%. Compared to compounds 1 and 2, compound 3 shows significantly improved activity and stability. The outstanding performance of compound 3 can be ascribed to the fact that vanadium doping enhances the system's Fenton reaction, forming a strong synergistic effect among Fe, Mo, and V metals. Additionally, its stability is significantly enhanced by the highly centrosymmetric double-support double-vanadium-capped end-sealing configuration constructed by self-assembly.