ZnO is an effective photocatalyst applied to the degradation of organic dyes in aqueous media. In this study, the UV-light and sunlight-driven photocatalytic activities of ZnO nanoparticles are evaluated. A handheld Lovibond photometer was purposefully calibrated in order to monitor the dye removal in outdoor conditions. The effect of ZnO defect states, i.e., the presence of zinc and oxygen defects on the photocatalytic activity was probed for two types of dyes: fuchsin and methylene blue. Three morphologies of ZnO nanoparticles were deliberately selected, i.e., spherical, facetted and a mix of spherical and facetted, ascertained via transmission electron microscopy. Aqueous and non-aqueous sol-gel routes were applied to their synthesis in order to tailor their size, morphology and defect states. Raman spectroscopy demonstrated that the spherical nanoparticles contained a high amount of oxygen vacancies and zinc interstitials. Photoluminescence spectroscopy revealed that the facetted nanoparticles harbored zinc vacancies in addition to oxygen vacancies. A mechanism for dye degradation based on the possible surface defects in facetted nanoparticles is proposed in this work. The reusability of these nanoparticles for five cycles of dye degradation was also analyzed. More specifically, facetted ZnO nanoparticles tend to exhibit higher efficiencies and reusability than spherical nanoparticles.

Innovation in materials manufacturing processes is a challenge to reduce the environmental impact of the chemical industry in line with sustainable development objectives, such as promoting sustainable industrialization and ensuring responsible consumption and production patterns. One of the ways to reach this goal is to revisit the main synthesis processes and to rethink their use. Reactive extrusion is a well-known continuous process used to produce and shape various materials in the polymer and food industries, but hardly leads to the direct synthesis and shaping of metal oxide materials (ceramics). In contrast, sol–gel chemistry offers tremendous opportunities to synthesize metal oxide networks by polycondensation of molecular precursors at a low temperature, but it is mostly operated in sluggish batch processes, using massive amounts of solvent contaminants and producing large amounts of chemical waste (E-factor > 40). In this work, for the first time, we coupled extrusion with sol–gel chemistry to produce high surface area shaped alumina-based materials that meet or exceed the requirements to be used as heterogeneous catalysts or catalyst supports. The necessity to adapt sol–gel chemistry – usually done in a diluted environment – to meet the twin-extruder technical constraints – usually working in a viscous environment – led to a peculiar choice of reaction conditions little discussed in the literature: the hydrolysis/condensation of (solid) alkoxide precursors without a solvent. We were able to synthesize and shaped high specific surface area boehmite (γ-AlOOH) at room temperature, in a continuous mode, and under solvent-free conditions. The solids are directly shaped in the form of self-standing “spaghetti” or extrudates. Upon calcination, the shaped hydroxides are converted into mesoporous and high specific surface area gamma alumina (γ-Al₂O₃) materials. We show that such materials exhibit high catalytic activity in the dehydration of ethanol. The process intensification presented here paves the way towards very low-waste, low-energy, and – all in all – more sustainable manufacturing practices for shaped high surface area metal oxides. In addition, it could be potentially less expensive since the simplification of the equipment and the lower energy consumption will contribute to drastically reduce the production costs even if the reagents are more expensive.

Surface engineering of metal–organic frameworks (MOFs) with a mesoporous silica coating can improve MOF mechanical properties and provide an easy way to decorate MOF nanoparticles with organic or biological molecules though silane chemistry or electrostatic interactions, while retaining open access to MOF porosity. Silica coating would be highly beneficial for employing MOFs in a wide range of applications such as catalysis or drug delivery. However, obtaining a stable, controlled core–shell structure using MOF nanoparticles as seeds is challenging because of their intrinsic chemically labile nature. Here we analyze the factors that destabilize the core of the Zeolitic Imidazolate Framework-8 (ZIF-8) MOFs during the sol–gel deposition of a mesoporous silica shell causing a partial or total etching of the MOF material. Silicates in solution are found to scavenge Zn²⁺ ions removing them from the ZIF structure and causing a partial or complete dissolution of the ZIF seed. By carefully tuning the silicate concentration in solution simultaneous control can be obtained over both the ZIF-8 dissolution and the silica condensation kinetics, resulting in the growth of a uniform mesoporous silica shell while preserving the integrity of ZIF-8. The core–shell nanoparticles obtained show a compact core shell structure with no gap between the MOF core and the silica shell, even after calcination, while the crystalline ZIF-8 structure is retained. Overall, a general synthetic approach is presented for producing nanocomposite core–shell materials which can be applied to other MOF labile seeds to design new hierarchical materials.

L'industrie doit s'orienter vers une économie durable, elle doit réduire l’utilisation de ressources, la production de déchets et consommer moins d'énergie. La catalyse, notamment par l'utilisation de catalyseurs, joue un rôle clé dans cette transition car elle améliore considérablement l'efficacité énergétique de certaines réactions chimiques tout en ciblant sélectivement les produits de la réaction. Ainsi, la demande de catalyseurs a considérablement augmenté en raison de préoccupations environnementales et énergétiques. Pour rester compétitive, l'optimisation des performances des catalyseurs, des processus catalytiques et des propriétés des matériaux est essentielle, tout en réduisant les coûts énergétiques et financiers de leur fabrication. La thèse vise à développer des approches éco-responsables pour la fabrication de catalyseurs hétérogènes à grande échelle en utilisant la chimie sol-gel sans solvants couplée à des procédés de mélanges mécaniques tels que l'extrusion réactive. Cette stratégie permet d'obtenir des matériaux catalytiques performants tout en réduisant leur impact environnemental. Nous présenterons une étude fondamentale qui a abouti à la description des mécanismes de la synthèse d’extrudés de boehmite via la chimie sol-gel sans solvant couplé au procédé d’extrusion réactive. L’influence des paramètres de vieillissement des matériaux sur les propriétés mécaniques des extrudés (nécessaires pour une application industrielle réelle) a également été exploré. Des expériences ont été menées avec succès pour créer des catalyseurs à base de ruthénium et de nickel supportés sur des oxy-hydoxyde d’aluminium et sur des alumines. L’étude montre que cette approche dite « frugale » est compétitive sur l’efficacité catalytique et bien plus respectueuse de l'environnement que les méthodes traditionnelles.