Conception de nouveaux catalyseurs hybrides pour la synthèse directe du diméthyle éther à partir du gaz de synthèse

Des nouveaux catalyseurs bi-fonctionnels Cu-ZnO-Al2O3/ZSM-5 pour la synthèse directe du diméthyle éther (DME) à partir du gaz de synthèse ont été préparés par mélange physique, et ensuite caractérisés et évalués dans un réacteur à lit fixe. L'activité catalytique dans la synthèse de DME a été attribuée à des sites métalliques de cuivre et à l’acidité de la zéolite. Les cristaux de zéolite de petite taille favorisent une activité plus importante. La désactivation du catalyseur est fortement influencée par les sites acides localisés à la surface externe de la zéolite. Une acidité importante de la surface externe de ZSM-5 conduit à une désactivation rapide du catalyseur. Le frittage et la migration du cuivre font partie des mécanismes prédominants de la désactivation. Les sites acides à la surface de la zéolite ont été sélectivement neutralisés par silylation avec le tétraéthyle orthosilicate. Par conséquent, la stabilité du catalyseur et la productivité en DME ont été significativement améliorées. Le dioxyde de carbone est un produit indésirable de la synthèse directe du DME. Il est formé par la réaction Water-Gas-Shift. La promotion des catalyseurs Cu-Zn-Al /HZSM-5 avec de l’étain permet de modérer la réaction Water-Gas-Shift et d’augmenter la sélectivité en DME.A series of novel bi-functional catalysts Cu-ZnO-Al2O3/ZSM-5 were prepared by physical mixing method, characterized, and evaluated in a fixed-bed reactor for direct dimethyl ether (DME) synthesis from syngas. The catalytic activity for DME synthesis was attributed to copper metal sites and zeolite acidity. Zeolite crystals of smaller size lead to higher catalyst activity. The catalyst deactivation was strongly affected by the acid sites on the external surface of zeolites. Higher external acidity of ZSM-5 results in fast catalyst deactivation. Copper sintering and migration seem to be principal mechanisms of deactivation. The acidic sites on the external surface of zeolite were selectively neutralized by silylation with tetraethyl orthosilicate. Consequently both catalyst stability and DME productivity were significantly improved. Carbon dioxide is the major undesirable byproduct of direct DME synthesis. It forms by Water-Gas-Shift (WGS) reaction. The promotion of bi-functional Cu-Zn-Al/HZSM-5 catalysts with tin slows down the WGS and increases the selectivity to DME

Assessment of metal sintering in the copper-zeolite hybrid catalyst for direct dimethyl ether synthesis using synchrotron-based X-ray absorption and diffraction

International audienceDimethyl ether is one of the most promising environmentally optimized alternatives to the conventional fossil fuels and an important platform molecule for chemical industry. Catalyst deactivation is one of the most important challenges of the single-step dimethyl ether synthesis from syngas. Because of the lack of direct characterization techniques working under harsh reaction conditions, the information about deactivation mechanisms of bifunctional Cu/ZSM-5 catalysts is rather contradictory. In this paper, a combination of synchrotron-based in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy operating under high pressure and temperature alongside with the conventional ex-situ characterization uncovered very rapid copper sintering occurring under realistic conditions of direct dimethyl ether synthesis. Copper sintering was strongly affected by the presence of water either produced by the reaction or co-fed to the reactor. No copper oxidation was observed under a wide range of experimental conditions

Tribology comparison of laser-cladded CrMnFeCoNi coatings reinforced by three types of ceramic (TiC/NbC/B4C)

In this study, three composite coatings were prepared by laser cladding, selecting the ceramic particles (B4C, TiC, or NbC), together with the powders of high-entropy alloy (HEA), i.e. CrMnFeCoNi. The results demonstrate that the composite coatings reinforced by varied carbides exhibited special microstructures, which lead to the features of micro-cutting wear, three-body wear, and fatigue wear, respectively. Therefore, the differences between the three composite coatings were compared, and evaluated in terms of phase structure, microstructure, chemical composition, nanoindentation, and phase interface correlation. Ultimately, the wear resistance mechanisms of selected ceramic particles reinforced HEA coatings were explained, and their applicability was estimated

Embryonic zeolites for highly efficient synthesis of dimethyl ether from syngas

International audienc

Strain hardening and strengthening mechanism of laser melting deposition (LMD) additively manufactured FeCoCrNiAl0.5 high-entropy alloy

In order to develop the high-entropy alloy (HEA) with low cost and excellent mechanical properties for structural applications, the FeCoCrNiAl0.5 HEA has been fabricated by laser melting deposition, one of the advanced additive manufacturing methods. Strain hardening behaviour has been analysed and discussed using the combination of characterisation techniques. The LMD-ed FeCoCrNiAl0.5 had a true yield strength and strain of ∼463 MPa and 2.94%. Also, the true tensile strength of the LMD-ed FeCoCrNiAl0.5 reached 876 MPa, together with the ductility of 24.97% (engineering strain). The LMD-ed FeCoCrNiAl0.5 HEA exhibited a dual-phase structure of 93% face-centred cubic (FCC) phase and 6.9% ordered B2 phase. The phase boundary between the disordered FCC and ordered B2 phases played a key role in the barrier, which can block the movement of dislocations because of the lattice distortion, very large angle, and mismatch of the lattice. Dislocation pile-up and tangle caused the dislocation density near the phase boundaries to be higher than that in other areas, meanwhile, they further prevented the movement of dislocation under stress as they generated back stress, therefore LMD-ed FeCoCrNiAl0.5 HEA had a good strain hardening behaviour with a strain hardening exponent of 0.92. This study provided an innovative insight into the development of HEAs with ordered phase by laser additive manufacturing for structural applications