88 89 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 1 / 2 2 Unraveling the chemistry of bimetallicMetal-Organic Frameworks (MOFs) Related publication : Ronda-Lloret, M., Pellicer-Carreño, I., Grau-Atienza, A., Boada, R., Diaz-Moreno, S., Narciso-Romero, J., Serrano-Ruiz, J. C., Sepúlveda-Escribano, A., & Ramos-Fernandez, E. v. Mixed-valence Ce/Zr metal-organic frameworks: controlling the oxidation state of Cerium in one- pot synthesis approach. Advanced Functional Materials 31, 2102582 (2021). DOI: 10.1002/adfm.202102582 Publication keywords: Metal-Organic Frameworks; Mixed-valence; One-pot synthesis; X-ray absorption T he synthesis of metal organic framework (MOF) materials made of metal species that can exchange their oxidation state upon varying conditions is crucial for MOFs to evolve from a laboratory curiosity to real applications. For that reason, researchers are developing synthetic routes to prepare bimetallic MOFs with redox properties. However, the preparation and characterisation of these mixed- metal MOFs are far from trivial. In order to understand the success of the synthesis and theMOFs’ properties, the local structure of themetal must be fully understood. X-ray Absorption Spectroscopy is the most appropriate technique to define the local structure of the metal. Diamond Light Source’s I20-Scanning beamline has high photon flux and high spectral purity, making it ideal for investigating samples with relatively low concentrations of the metal to be analysed. I20-Scanning also has facilities that allowmeasurements in operando . Using their results from I20-Scanning, the team has found a synthetic route to prepare true-mixed metal MOFs, which uses a modulator to modulate the crystal morphology and the electronic nature of the metal cations. By using the modulator appropriately, they can prepare samples with only Ce(III) cations, a mixture of Ce(III)/Ce(IV) cations, or Ce(IV) cations. Their results give us a new way to prepare bimetallic redox MOFs, which have the potential to be applied as catalysts in many different reactions, including selective hydrogenation, oxidation or Meerwein-Ponndorf-Verley reactions. The presence of metal species that can easily exchange their oxidation states is of paramount importance in the field of catalysis, sensors and photocatalysis, among others. A classic example is ceria (CeO 2 ), in which Ce(IV) can readily be reduced to Ce(III) under suitable conditions, and vice versa. The major drawback of CeO 2 for being applied as a catalysis is that only the surface of the crystallites is used in the catalytic event. Therefore, most of the cerium and oxygen atoms of the lattice are not used. One promising alternative to better take advantage of ceria properties is the dispersion of CeO 2 nanoparticles on an inert support, as the authors and others have already demonstrated. Another approach might be the fabrication of porous solids made of molecularly dispersed cerium oxide clusters that allow reactants to reach the clusters. In this sense, Metal-Organic Framework (MOF) materials are ideal candidates, since cerium-based MOFs might be constructed. Although there have been some attempts to prepare Ce-containing MOFs, their stability is limited. Stock’s group and others have reported the preparation of Ce MOFs in which Ce(IV) is introduced. The presence of the Ce(IV)/Ce(III) redox pair has been demonstrated by de Vos. However, the presence of the redox pair is induced during the reaction but it is not present in the pristine MOF 1,2 . The main drawback of Ce-based MOFs is their low thermal stability that limits their practical applications to low temperature processes (<100 °C). However, the stability and the redox behaviour can be enhanced by introducing Zr cations in the same cluster with Ce, in a way that the atomic arrangement found in ceria-zirconia mixed oxides is replicated at smaller scale in the cluster. Another important point is the tailoring of the oxidation state of the Ce cations that are present in the MOF. In that respect, it is worth mentioning that the preparation and characterisation of these mixed-metal MOFs is far from trivial. Indeed, aiming for getting clusters containing both cations, i.e ., a true mixed- metal MOF, different undesired scenarios regarding the arrangement of the elements can be found such as the formation of segregated Ce-MOF and Zr- MOF crystalline phases or the formation of mono-elemental clusters in the same crystallite. Besides the true mixed-metal nature of the MOF, it is also crucial to control the oxidation state of the Ce cations. Ce(III) and Ce(IV) are both stable at ambient conditions depending on the environment, and both of them have the same coordination number with carboxylic acid groups, although their ionic diameter is different; thus, both can be used for producing MOFs with the same topology. The tailoring of the oxidation state of the Ce cations is expected to lead to a fine control of the redox properties. In this work, a one-pot synthesis method that enables the preparation of true (Zr,Ce) mixed-metal UiO-66 with tailored redox properties has been developed. This method allows the preparation of mixed-metal MOFs having only Ce(III), Ce(IV), or even mixed-valence MOFs with Ce(III)/Ce(IV). The designed synthetic method makes use of a modulator to finely control the oxidation state of the cerium cations. The X-ray Absorption Spectroscopy (XAS) measurements performed at I20-Scanning 3 enabled the acquisition ofw detailed information of the local environment of both Zr and Ce. The Zr K-edge XAS revealed that the first and second coordination spheres of Zr were distorted upon the incorporation of a Ce atom (see Fig. 1). In order to understand the role of the Ce species in the bimetallic MOF, the Ce L3-edge XAS was also measured. Surprisingly, it was discovered that depending on the synthesis conditions it was possible to tailor the oxidation state of the Ce cations (III, IV) (Fig. 2). With this data in hand, the synthetic routes were modified to design and synthesise the desired bimetallic MOF. The new synthetic method developed has opened a new way of understanding MOF synthesis and the use of modulators in it. With this work, the scientific community has been provided with a new technique to prepare bimetallic MOFs with cations with the desired oxidation state. This opens the door to produce bimetallic MOFs with the desired electronic properties, multiplying the number of possible applications (catalysis, sensors, adsorption etc) 4 . References: 1. Smolders, S. et al. Unravelling the redox-catalytic behavior of Ce 4+ metal- organic frameworks by X-ray absorption spectroscopy. ChemPhysChem 19 , 373–378 (2018). DOI: 10.1002/cphc.201700967 2. Wu, X.-P. et al. Cerium metal–organic framework for photocatalysis. Journal of the American Chemical Society 140 , 7904–7912 (2018). DOI: 10.1021/jacs.8b03613 3. Diaz-Moreno, S. et al. I20; the versatile X-ray absorption spectroscopy beamline at Diamond Light Source. Journal of Physics: Conference Series 190 , 012038 (2009). DOI: 10.1088/1742-6596/190/1/012038 4. Rogge, S. M. J. et al. Metal–organic and covalent organic frameworks as single-site catalysts. Chemical Society Reviews 46 , 3134–3184 (2017). DOI: 10.1039/C7CS00033B Funding acknowledgement: Authors acknowledge financial support by MINECO (Spain) through projects, MAT2017-86992-R, MAT2016-80285-P and Ministerio de Ciencia e innovación” (PID2020-116998RB-I00). EVRF acknowledges MINECO for his Ramón y Cajal fellow RYC-2012-11427. We also would like to thank Diamond Light Source (I20-Scanning beamline) for the beamtime given at the proposal SP19114 and SP16337. Corresponding author: Dr. Enrique V. Ramos-Fernández, Universidad de Alicante,
[email protected] Spectroscopy Group Beamline I20-Scanning Figure 1: Schematic representation of the work done. Figure 2: Ce L3-edge XANES spectra of two selected Ce(III) and Ce(IV) containing samples and reference compounds