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• Nitrogenase-inspired MIL-53(FeII/FeIII) is first synthesized for N 2 photo fixation. • Mixed-valence FeII/FeIII clusters mimic the active center in nitrogenase. • FeII are in-situ formed by ethylene glycol reductant in solvothermal process. • FeII/FeIII ratio is vital to coordinate catalytic activity and framework stability. • MIL-53(FeII/FeIII) stably yields 306 μmol h−1 g−1 NH 3 under visible light. Biological nitrogenases exhibit superior nitrogen fixation efficiency owing to their unique multi-iron metallocluster (Fe2+ 3 Fe3+ 4 M3+, M = Mo, V, Fe) coordinated by organic polypeptide. Herein, we design a kind of metal organic framework (MOF) photocatalyst, MIL-53(FeII/FeIII) (MIL = Material from Institute Lavoisier), in which the FeII and FeIII constitute the mixed-valence metalloclusters to mimic the Fe2+ active sites and high-valence metal ions in nitrogenases, respectively. Both the FeII and FeIII are coordinated by organic ligands (terephthalic acid), which afford the electron transfer chains as well as the support of monodispersed FeII active sites. The FeIII in MIL-53(FeII/FeIII) is partly in-situ reduced into FeII by ethylene glycol (EG) via one-step solvothermal method, and the FeII/FeIII ratio is regulated from 0.18:1 to 1.21:1 by varying the EG content. Our results show that the FeII/FeIII ratio can significantly affect the photocatalytic activity and structure stability, and MIL-53(FeII/FeIII)-1 with optimal FeII/FeIII ratio (1.06:1) achieves the highest ammonia evolution rate up to 306 μmol h−1 g−1, nearly 10-fold higher than that of other framework-based materials, while remaining stable after 24 h irradiation. Such nitrogenase-like design in MIL-53(FeII/FeIII) endows the efficient electron transfer, exposed active sites, and in particular rational synergy between the catalytic function and non-catalytic function. This work may open a new avenue to the rational design of nitrogen fixation photocatalysts based on framework materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A multi-ligand strategy was proposed to design high-efficient MOF photocatalysts. • The three-ligand MOFs, EY/NH 2 /H-MIL-125, were synthesized to verify our strategy. • The optimal three-ligand MOF can yield H 2 of 2.15 mmol g−1 h−1 under visible light. • Electron transfer is enhanced by a push–pull effect induced by the NH 2 -H 2 BDC and H 2 BDC ligands. • The third ligand EY facilitates the electron photoexcitation under visible light. The design and construction of high-performance photocatalysts with both strong visible light harvesting and effective electron transfer are highly desired in the photocatalysis field. Herein, a multi-ligand strategy based on the MOF materials is proposed. A three-ligand Ti-MOF EY/NH 2 /H-MIL-125 (ENHM) was prepared via a facile hydrothermal method, in which 2-aminoterephthalic acid (NH 2 -H 2 BDC), terephthalic acid (H 2 BDC) and Eosin Y (EY) were employed as three ligands. The push–pull effect induced by different substituent groups (–NH 2 and –H) on the skeleton of NH 2 -H 2 BDC and H 2 BDC promotes the efficient transfer of photogenerated electrons from organic ligands to metal nodes; while the third ligand EY increases the number of photoexcited electrons by enhancing the visible light absorption ability. Among the three-ligand MOF photocatalytic materials, ENHM-2 performs the optimal H 2 production activity of 2.15 mmol g−1 h−1, much higher than dual-ligand NH 2 /H-MIL-125 and EY/NH 2 -MIL-125 and single-ligand NH 2 -MIL-125. A probable mechanism was also proposed for the three-ligand MOFs. This multi-ligand strategy may open up a new direction toward the preparation of MOF-based photocatalytic materials with excellent photocatalytic performances. [ABSTRACT FROM AUTHOR]