Développement de nouveaux catalyseurs hétérogènes basés sur les polyoxometalates et « metal-organic frameworks » pour la photoréduction du CO₂

Dans le contexte du réchauffement climatique, le développement de nouvelles sources d'énergie propres et durables est un défi sociétal majeur. La réduction du CO₂ représente une stratégie prometteuse pour stocker les énergies renouvelables. Au cours des dernières décennies, plusieurs catalyseurs pour la photoréduction du CO₂ ont été développés, cependant ces derniers souffrent souvent d’un manque de stabilité ou de problèmes de recyclabilité typiques des catalyseurs homogènes. Dans cette thèse, nous proposons de nouveaux systèmes catalytiques hétérogènes pour la photoréduction du CO2 basés sur la combinaison de catalyseurs, de polyoxométalates (POMs) et de metal-organic frameworks (MOFs). Après avoir brièvement décrit les catalyseurs pour la photoréduction du CO₂, les POMs et leur immobilisation dans les MOFs dans un premier chapitre, nous décrivons dans le second chapitre la co-immobilisation d'un catalyseur à base de rhodium et d'un POM de type Keggin dans l’UiO-67. Le composite résultant est capable de réduire le CO₂ en formate plus efficacement que l'analogue sans POM. Le chapitre 3 décrit l'application du MOF-545 (PCN-222) pour la réduction du CO₂ en formiate. L'effet de la métallation des ligands porphyrine ainsi que l’influence de la taille des particules sur la photoréduction du CO₂ est discuté. Le dernier chapitre se concentre sur les composites à base de POM et sur la synthèse de trois nouveaux matériaux de type {P4Mo6} ayant une activité de photoréduction du CO₂donnant CO et CH4. Dans l'ensemble, ce manuscrit représente un pas en avant vers le développement de catalyseurs hétérogènes efficaces pour la photoréduction du CO₂ en produits à valeur ajoutée.In the context of global warming, the development of new sources of clean and sustainable energy is a key current societal challenge. The reduction of CO₂ represents a promising strategy to store renewable energies while decreasing the amount of greenhouse gases emissions. During the past decades, several catalysts for visible-light driven CO₂ reduction have been developed, however they often lack stability or suffer from recyclability issues typical of homogeneous catalysts. In this thesis, we propose new heterogeneous catalytic systems for CO₂ photoreduction based on the combination of photocatalysts, polyoxometalates (POMs) and metal-organic-frameworks (MOFs). After briefly describing catalysts for CO₂ reduction, POMs and the immobilization of POMs and catalysts in MOFs in a first chapter, we describe in the second chapter the co-immobilization of a rhodium based catalyst and a Keggin-type POM inside UiO-67. The resulting composite is capable of reducing CO₂ into formate more efficiently than the POM-free analog. Chapter 3 concerns the application of the photosensitive porphyrinic MOF-545 (PCN-222) to the reduction CO₂ into formate. The positive effect of the metalation of porphyrin linkers, catalyst and nanoscaling on CO₂ photoreduction are discussed. The last chapter focuses on POM-based composites and on the synthesis of three new {P4Mo6}-based materials with CO₂ photoreduction activity to give CO and CH4. Overall, this manuscript represents a step forward towards the development of efficient heterogeneous catalysts for the sustainable photoreduction of CO₂ into value-added products under visible light irradiation

Développement de nouveaux catalyseurs hétérogènes basés sur les polyoxometalates et « metal-organic frameworks » pour la photoréduction du CO₂

In the context of global warming, the development of new sources of clean and sustainable energy is a key current societal challenge. The reduction of CO₂ represents a promising strategy to store renewable energies while decreasing the amount of greenhouse gases emissions. During the past decades, several catalysts for visible-light driven CO₂ reduction have been developed, however they often lack stability or suffer from recyclability issues typical of homogeneous catalysts. In this thesis, we propose new heterogeneous catalytic systems for CO₂ photoreduction based on the combination of photocatalysts, polyoxometalates (POMs) and metal-organic-frameworks (MOFs). After briefly describing catalysts for CO₂ reduction, POMs and the immobilization of POMs and catalysts in MOFs in a first chapter, we describe in the second chapter the co-immobilization of a rhodium based catalyst and a Keggin-type POM inside UiO-67. The resulting composite is capable of reducing CO₂ into formate more efficiently than the POM-free analog. Chapter 3 concerns the application of the photosensitive porphyrinic MOF-545 (PCN-222) to the reduction CO₂ into formate. The positive effect of the metalation of porphyrin linkers, catalyst and nanoscaling on CO₂ photoreduction are discussed. The last chapter focuses on POM-based composites and on the synthesis of three new {P4Mo6}-based materials with CO₂ photoreduction activity to give CO and CH4. Overall, this manuscript represents a step forward towards the development of efficient heterogeneous catalysts for the sustainable photoreduction of CO₂ into value-added products under visible light irradiation.Dans le contexte du réchauffement climatique, le développement de nouvelles sources d'énergie propres et durables est un défi sociétal majeur. La réduction du CO₂ représente une stratégie prometteuse pour stocker les énergies renouvelables. Au cours des dernières décennies, plusieurs catalyseurs pour la photoréduction du CO₂ ont été développés, cependant ces derniers souffrent souvent d’un manque de stabilité ou de problèmes de recyclabilité typiques des catalyseurs homogènes. Dans cette thèse, nous proposons de nouveaux systèmes catalytiques hétérogènes pour la photoréduction du CO2 basés sur la combinaison de catalyseurs, de polyoxométalates (POMs) et de metal-organic frameworks (MOFs). Après avoir brièvement décrit les catalyseurs pour la photoréduction du CO₂, les POMs et leur immobilisation dans les MOFs dans un premier chapitre, nous décrivons dans le second chapitre la co-immobilisation d'un catalyseur à base de rhodium et d'un POM de type Keggin dans l’UiO-67. Le composite résultant est capable de réduire le CO₂ en formate plus efficacement que l'analogue sans POM. Le chapitre 3 décrit l'application du MOF-545 (PCN-222) pour la réduction du CO₂ en formiate. L'effet de la métallation des ligands porphyrine ainsi que l’influence de la taille des particules sur la photoréduction du CO₂ est discuté. Le dernier chapitre se concentre sur les composites à base de POM et sur la synthèse de trois nouveaux matériaux de type {P4Mo6} ayant une activité de photoréduction du CO₂donnant CO et CH4. Dans l'ensemble, ce manuscrit représente un pas en avant vers le développement de catalyseurs hétérogènes efficaces pour la photoréduction du CO₂ en produits à valeur ajoutée

Fabrication of Oriented Polycrystalline MOF Superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties

Origin of the Boosting Effect of Polyoxometalates in Photocatalysis: The Case of CO 2 Reduction by a Rh-Containing Metal–Organic Framework

International audienceThe immobilization of polyoxometalates (POMs) near catalytic centers of metal–organic frameworks (MOFs) has been reported as an advantageous strategy to boost their photocatalytic activity toward strategic reactions such as CO2 reduction (CO2RR) or hydrogen evolution (HER), although the reasons for such enhancement are still poorly understood. Unveiling the role of POM guests in the reaction mechanisms is therefore a key step toward the development of the next generation of multicomponent catalytic materials with optimal photocatalytic performances. Here, we elucidate the remarkable role of encapsulated [PW12O40]3– (PW12) polyoxometalates in boosting the photocatalytic activity of the Rh-functionalized UiO-67 MOF toward CO2RR and HER by combining theoretical density functional theory and microkinetic modeling approaches with experimental photophysical and spectroscopic techniques. First, we characterized in detail the reaction mechanism for CO2RR and HER catalyzed by the PW12-containing Rh-functionalized MOF, using [Ru(bpy)3]2+ as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial electron donor in acetonitrile. Our results reveal that the encapsulated POMs act as efficient electron reservoirs, which quench [Ru(bpy)3]+─the photogenerated reduced form of the PS─and transfer the electrons to the Rh catalytic sites of the MOF. Notably, this is shown to favor the regeneration of the oxidized PS over its unproductive degradation, boosting the turnover numbers of the photocatalytic system. Such a mechanism can explain not only the higher formate and H2 product yields in the POM-containing catalyst but also the experimentally observed higher impact on the HER pathway than that on the CO2RR one, as the source of protons is generated in the reductive quenching of the photoexcited PS by TEOA. Finally, our computational exploration was extended to a whole variety of POMs, which allowed establishing relationships between their redox potentials and the activity of the related POM-containing catalytic materials. The optimal activity is reached when both the ability of the POM to accept electrons and that of its reduced form to reduce the Rh catalyst are simultaneously maximized, leading to a volcano plot whereby POMs with a moderate redox potential display the highest impact on photocatalytic performances

Fabrication of oriented polycrystalline MOF superstructures

The field of metal-organic frameworks (MOFs) has progressed beyond the design and exploration of powdery and single-crystalline materials. A current challenge is the fabrication of organized superstructures that can harness the directional properties of the individual constituent MOF crystals. To date, the progress in the fabrication methods of polycrystalline MOF superstructures has led to close-packed structures with defined crystalline orientation. By controlling the crystalline orientation, the MOF pore channels of the constituent crystals can be aligned along specific directions: these systems possess anisotropic properties including enhanced diffusion along specific directions, preferential orientation of guest species, and protection of functional guests. In this perspective, we discuss the current status of MOF research in the fabrication of oriented polycrystalline superstructures focusing on the specific crystalline directions of orientation. Three methods are examined in detail: the assembly from colloidal MOF solutions, the use of external fields for the alignment of MOF particles, and the heteroepitaxial ceramic-to-MOF growth. This perspective aims at promoting the progress of this field of research and inspiring the development of new protocols for the preparation of MOF systems with oriented pore channels, to enable advanced MOF-based devices with anisotropic properties.The authors acknowledge support from the European Research Council under the European Union's Horizon 2020 Programme (FP/2014-2020)/ERC Grant Agreement No. 771834—POPCRYSTAL and TU Graz for the Lead Project (LP-03). JC thanks the European Research Council (ERC) for funding part of this work under the European Union's Horizon 2020 research and innovation programme (ERC Consolidator Grant Agreement 101002176) and the Österreichische Forschungsförderungsgesellschaft (FFG project BATMAN FFG-897938) for financial support.Peer reviewe

Structure-directing role of immobilized polyoxometalates in the synthesis of porphyrinic Zr-based metal–organic frameworks

International audienceWe evidence the structure-directing role of the PW12O403-polyoxometalate in the synthesis of porphyrinicMOFswhereby it promotes the formation of the kinetic topology. Its immobilization into the MOF is successfully achieved at high temperature yielding the kinetic MOF-525/PCN-224 phases, while prohibiting the formation of the thermodynamic MOF-545 product. A combined experimental and theoretical approach uses differential PDF and DFT calculations along with solid-state NMRto show the structural integrity of the hosted POM andits location in the vicinity of the Zr-based nodes

Understanding the Photocatalytic Reduction of CO 2 with Heterometallic Molybdenum(V) Phosphate Polyoxometalates in Aqueous Media

International audienceThree crystalline heterometallic molybdenum(V) phosphates have been synthesized under hydrothermal conditions. They all contain {M[P4Mo6O28(OH)3]2}16– [M = Mn(II) or Co(II)] polyoxometalate (POM) units, with the M ions sandwiched between two {P4MoV6} rings. In the presence of Fe(II) ions in the reaction medium, a three-dimensional (3D) Fe–Mn compound built from the connection of Mn(P4Mo6)2 units to Fe(II) and Fe(III) centers by extra phosphate ions is obtained. Alternatively, the introduction of [Ru(bpy)3]2+ complexes in the synthetic medium prevents the formation of such high-dimensional compounds. In the two Ru(bpy)–Mn and Ru(bpy)–Co hybrids, chains are indeed formed, whereby the Mn(P4Mo6)2 or Co(P4Mo6)2 anions are bridged by Mn(II) or Co(II) ions, respectively. The charge of these anionic chains is compensated by neighboring [Ru(bpy)3]2+ complexes. Among these three compounds, only Fe–Mn and Ru(bpy)–Mn are active for the heterogeneous photocatalytic reduction of CO2 into CH4 as the major product and CO (yield in CH4 of 1440 and 600 nmol g–1 h–1 with selectivity in CH4 equal to 92.6 and 85.2%, respectively, under 8 h irradiation) in water, in the presence of triethanolamine (TEOA) as an electron donor and [Ru(bpy)3]2+ as a photosensitizer. A density functional theory (DFT) analysis allowed for proposing a reaction mechanism involving the formation of a solvated electron via photoionization of a one-electron reduced [RuII(bpy)2(bpy•–)]+ complex as the key step to reduce CO2 to CO2•–. The latter can then coordinate to the peripheral M(II) ions to yield CO through electron- and proton-transfer steps involving reduced POMs and protons generated in the photooxidation of the sacrificial donor. Concerning the nonactive compound, Ru(bpy)–Co, DFT calculations revealed that the Co(II) dimers present in the structure may spontaneously take the extra electron out of CO2•– to form a Co–Co bond, releasing CO2 back. Finally, preliminary results suggest that the reduction of CO to CH4 could be photochemically accomplished by the POM-based materials in the presence of TEOA, with no mechanistic requirement for the participation of [Ru(bpy)3]2+

Structure and Electronic Properties of Large Oligomeric Heterometallic 3d/CeIV Polyoxometalates

International audienceMerging the rich chemistry of Ce(IV) polyoxometalates (POMs) with that of 3d polyanions remains a challenge due to the strong competition between these highly oxophilic lanthanide cations and 3d metallic ions for coordination to lacunary molecular metal oxides. We report herein the characterization of an unprecedented water stable hexameric CeIV/CoII POM (Ce12Co6) made of two {(SiW9)2Ce6} units connected to a {(SiW10)2Co6(PO4)2} core. In addition, the pentameric CeIV/NiII compound Ce6Ni8, where two {PW9Ni3W} and a {PW10Ni2} fragments are grafted on a {(PW9)2Ce6} moiety, has been obtained. Magnetic studies of Ce6Ni8 revealed ferromagnetic interactions between the NiII centers constituting the {Ni3PW10} fragments, in agreement with the geometry of such a trinuclear cluster. Related insoluble barium salts of Ce12Co6 and Ce6Ni8 were also prepared, allowing their solid-state electrochemical investigations and showing in particular that in Ce12Co6, both the cobalt, cerium, and silicotungstate moieties are electroactive. Finally, photophysical studies demonstrate the formation of long-lived reduced POMs photosensitized by [Ru(bpy)3]2+, suggesting that Ce12Co6 and Ce6Ni8 could be used as efficient reservoirs of reduction equivalents for photocatalytic reactions

Thin Films of Fully Noble Metal-Free POM@MOF for Photocatalytic Water Oxidation

International audienceP2 W18Co 4@MOF-545 which contains the sandwich-type polyoxometalate (POM) [(PW 9 O 34) 2 Co 4 (H 2 O) 2 ] 10-(P 2 W 18 Co 4) immobilized in the porphyrinic MOF-545 framework, is a "three-in-one" (porosity + light capture + catalysis) heterogeneous photosystem for the oxygen evolution reaction (OER). Thin films of this composite were synthesized on transparent and conductive indium tin oxide (ITO) supports using electrophoretic (EP) or drop casting (DC) methods, thus providing easy-to-use devices. Their electro-and photocatalytic activities for OER were investigated. Remarkably, both types of films exhibit higher turnover numbers (TONs) than the original bulk material previously studied as suspension for the photocatalytic OER, with TONs after 2 hours equal to 1600 and 403 for DC and EP films respectively compared to 70 for the suspension. This difference of catalytic activities is related to the proportion of efficiently illuminated crystallites, whereby a DC thin film offers the largest proportion of POM@MOF crystallites exposed to light due to its lower thickness when compared to an EP film or crystals in suspension. Such devices can be easily recycled by simply removing them from the reaction medium and washing them before reuse. The films were fully characterized with EXAFS and XANES spectroscopies, Raman, Scanning Electron Microscopy and electrochemistry, before and after catalysis. The combination of all these techniques shows the stability of both the POM and the MOF within the composite upon water oxidation reaction

Unveiling the mechanism of the photocatalytic reduction of CO 2 to formate promoted by porphyrinic Zr-based metal–organic frameworks

International audienceA complete picture of the reaction mechanism driving the photocatalytic reduction of CO 2 into formate promoted by the Zr-based porphyrinic MOF-545 in CH 3 CN/TEOA solutions is provided for the first time by combining experimental and computational approaches