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Laboratoire : FRE2000 - IMAP INSTITUT DES MATERIAUX POREUX DE PARIS
Institution d’accueil : Paris Sciences et Lettres - PSL
Ecole doctorale : Physique et chimie des matériaux - ED 397
Description :
Because of the high abundance of carbon dioxide (CO2) in our atmosphere, efforts are highly devoted to mitigate its novice effect on our planet manifesting mainly by the global warming. If the capture and storage of CO2 has emerged as an effective solution to reduce its impact, the use of this massive reservoir of CO2 as free source of sustainable feedstock has also attracted significant attention. This project aims at the rational design and elaboration of catalytically active transition metal nanoparticles embedded into porous sustainable Metal-Organic Frameworks (MOFs) allowing the hydrogenation of CO2 into alcohols with higher performances. It will rely first on the proven performances of existing catalytic systems, such as the well-known non porous metal/metal oxide composites, and the unparalleled versatility and controlled modularity of MOFs in terms of chemical nature as well as size and type of porosity. The confinement of metal-NPs on predesigned anchoring sites and cavities will prevent from the aggregation of metal particles that suffers from the actual classical systems and leading to undesired secondary products. Besides, the porous structure of MOFs will permit the pre-concentration of CO2 close to the active sites which may lead to lowering the energetic costs by reducing the actual required high pressure for the reaction. The main objective will be the conversion of CO2 into methanol and if possible into ethanol and/or hydrocarbons containing two (or more) carbon atoms. First catalytic tests will be carried out within the IMAP laboratory while the most promising solids will be later thoroughly studied in collaboration with leading groups in catalysis.
Context and Motivation :
The conversion of CO2 from fossil fuels into sustainable feedstock has attracted a considerable attention through chemical, electro- or photo-chemical routes to produce chemical building blocks, energy vectors or fuels. Among them, CO2 hydrogenation into methanol is an appealing process because of the resulting valuable chemical feedstock and the immediate use of the synthesis gas (mainly CO, CO2 and H2) emitted from the industrial processes. Indeed, this process has been already achieved at a large scale using non-noble metal based catalysts such as copper nanoparticles (NPs) supported on metal oxides (Cu/ZnOx/Al2O3) to produce methanol from CO2 in high yields at a reasonable cost. More recently, other systems such as Cu/CeOx and Cu/CeOx/TiO2 have shown better performances. If the Cu-NPs play a major role in the catalytic process, oxides act as promoters drastically enhancing the hydrogenation of CO2. Though, this reaction requires high pressure and temperatures (50-100 bar, > 200 °C) because of the inertness of CO2. Moreover, these systems suffer from i) heterogeneity issues and ii) lack of confinement/adhesion between the NPs and the oxide support leading to undesired secondary reactions (e.g. CO2 into CO). Consequently, this offers a large room of improvement to optimize the performances of “Cu-based/oxides” catalysts. Recently, a few studies have shown that porous hybrid solids such as Metal-Organic Frameworks (MOFs) could be considered for CO2 conversion following other approaches (see Mater. Horizon., 2017, 4, 345 ; Nat. Rev. Mater., 2, 2017, 17045) with still important limitations in terms of selective formation of alcohols, chemical stability or the use of sacrificial agents. Our ambition is to develop a more efficient approach through the immobilization of Cu NPs into oxocluster based MOFs for a better control of the size and disposition of Cu-NPs, optimised interactions and confinement leading to enhanced CO2 hydrogenation performances.
Scientific Objectives :
This project aims at the elaboration of optimized systems capable of transforming the CO2 into value-added molecules with best performances and lower costs. Precisely, it focuses on the hydrogenation of CO2 into alcohols (methanol or ethanol). In fact, it relies on combining the remarkable catalytic performances of the Cu-NPs-based systems already describes in the literature together with the highly modular and porous structure of MOFs. In this project we will be seeking for a rational embedding of Cu-based nanoparticles in predisposed confined spaces (MOF pores) ensuring, simultaneously, a good interaction with the oxide support (inorganic building unit or metal-oxo-cluster of the MOF) and a spatial distribution of the Cu-NPs. The obtained “Cu-NPs@MOF” is supposed to show multifold enhancements in the catalytic performances compared to the conventional dense Cu/oxides systems : i) high degree of confinement of Cu-NPs in small pores will allow to avoid the aggregation problems, eluding any undesired side reactions ; ii) while the rest of the porosity (MOFs showing bimodal porosity) will ensure the pre-concentration of the CO2 close to the active sites, permitting to reduce the pressure (and temperature) of the reaction.
The first steps of this project will focus on the design and the elaboration of Cu-NPs@MOFs, where MOFs exhibit Zr and/or Ce(III/IV) oxo-clusters for CO2 hydrogenation. A particular attention will be also devoted to evaluate the influence pore dimensions (size, shape) of a given system on the catalytic performances. Moreover, a more challenging objective will be also to achieve is the hydrogenation of the CO2 into ethanol and/or hydrocarbons containing two (or more) carbon atoms. To this aim heterometallic (hm) NPs should be embedded into the MOF. In this case, the hmNP will be composed of Cu and a noble-metal (i.e. Pd, Pt, …) where the latter will not only allow the formation of C─C bonds but also facilitate the activation of the H2 molecules.
Methodology and Planning :
MOFs are hybrid materials that have attracted considerable attention recently due to their crystalline structures that can be wisely and finely tuned in terms of the chemical nature (metal cations, organic linkers), pore size (micro‐ or mesoporous), and shape (cages, channels). Consequently, the unparalleled versatility and modularity of MOFs places them as prime candidates for this project. First, note that the inorganic building units, such as the Zr6O4(OH)4(COO)12, of the UiO-66 or the Ti8O8(OH)4(COO)12 in MIL-125, can be assimilated to molecular metal-oxides on which the presence of anchoring sites (oxo/hydroxo functions) can be controlled either by the judicious choice of the MOF topology or by deliberate creation of defective sites. As a consequence, these oxo-clusters with free oxo/hydroxo groups will ensure the dual role of promotors (as of oxides) and anchoring sites for the Cu-NPs. Besides, the controllable nature in pore size and multi-modal porosity (size, geometry of the ligand, MOF topology), will offer a confined space to host the anchored NPs (smaller pores) and enough space to diffuse and pre-concentrate CO2 close to the active sites (bigger pores). Also, the nature of the ligand can play an additional role in creating more affinity toward CO2 (e.g. –NH2). The overall ordered structure will ensure the homogeneity of the system. First efforts will be devoted to replicate the catalytic performances of Cu-NPs based systems in methanol formation by selectively growing Cu-NPs in the frameworks of MOFs. In practice, we will first select Zr and/or Ce(III/IV)-based MOFs showing multimodal porosities based on new MOFs recently discovered at IMAP, that can be easily prepared at large scale following green routes (a prerequisite to develop sustainable catalysts). Their structures show, simultaneously, narrow cavities prone to host Cu-NPs (with free oxo/hydroxo groups point in the pores) together with larger channels ideal for CO2 pre-concentration and substrate diffusion. In order to grow the NPs, a selected precursor of Cu (i.e. salt) will be impregnated prior to the reduction (heating, H2) and ideally form the Cu-NPs in the predisposed confined space. One will also evaluate the growth of Cu-NPs in MOFs showing similar topologies but different pore sizes/shapes. The second approach, will be the elaboration of heterometallic metal NPs@MOFs (hmNP) capable of the hydrogenation of the CO2 into ethanol and/or hydrocarbons containing two (or more) carbon atoms. For this specific purpose the ligand will be judiciously selected to offer an additional grafting point on which the second metal precursor can coordinate prior to the reducing step and formation of the hmNP. Finally, core-shell composites made of the best Cu-NPs@MOFs (core) wrapped with MOFs selective for CO2 (shell) will be envisioned. This approach will allow to selectively pre-concentrate the CO2 close to the catalytic sites to drastically enhance the catalytic performances.
Compétences requises :
The PhD candidate shall possess a background in chemistry of materials or inorganic chemistry or catalysis. A first experience in the synthesis and characterization of inorganic or hybrid materials and/or in heterogeneous catalysis (e.g. conversion of small molecules) will be advantageous.