Méthanal | EPFL Graph Search

Le méthanal ou formaldéhyde ou aldéhyde formique ou formol est un composé organique de la famille des aldéhydes, de formule chimique CH2O. C'est le membre le plus simple de cette famille. À température ambiante, c'est un gaz inflammable. Synthétisé pour la première fois par le Russe Alexandre Boutlerov en 1859, il fut formellement identifié par l'Allemand August Wilhelm von Hofmann en 1867. Le formaldéhyde est un gaz ubiquitaire, principalement issu de la combustion incomplète de substances contenant du carbone. Il est donc présent dans la fumée de feux de forêt, dans les rejets de centrales électriques thermiques, d'incinérateurs, de raffineries, de chaudières industrielles et dans les gaz d'échappement de véhicules automobiles, et dans la fumée du tabac. Il est aussi produit dans l'atmosphère sous l'action des rayons solaires et du dioxygène sur le méthane atmosphérique ainsi que sur d'autres hydrocarbures ; les process

Advances in heterogeneous catalysis for the synthesis of polyoxymethylene dimethyl ethers

Christophe Jean Baranowski

Our society is engaged in a major shift towards more sustainable industries. The required transition from fossils to renewable resources with low greenhouse gas emission is specifically challenging in the transportation sector where high thermal efficiency and energy density are key requirements. Due to their appealing properties, polyoxymethylene dimethyl ethers (OME) have recently received increasing attention as a new type of Diesel additive or substitute. However, the OME technology being only at an early stage of research, there is a limited understanding of the catalyst structure-activity relationships for OME synthesis. The aim of this thesis is to progress in the development of knowledge in heterogeneous catalysis for the sustainable production of OME. We focused on rational catalysts synthesis and in-depth characterization to understand how their properties influenced OME synthesis. Zeolites have demonstrated a high catalytic potential for the synthesis of OME. By modifying accessibility to the active sites in an H-ZSM-5 zeolite, we demonstrated that the reaction suffered from severe internal diffusion limitations of the reactants and products in the zeolite micropores. Controlled introduction of an intercrystalline network of mesopores significantly enhanced the zeolite's activity. Subsequently, we introduced tin in montmorillonite clay. This resulted in a hierarchical material composed of tin oxide inserted between the clay layers, with Brønsted and Lewis acidity. Its advantageous textural and acidic properties resulted in an active catalyst for OME synthesis from trioxane (TRI) and dimethoxymethane (OME1). To investigate the role of Lewis and Brønsted acidity for this reaction, a series of Beta zeolites with varying amounts of Brønsted and Lewis acid sites were synthesized. A synergy between these two types of acid sites resulted in a large turnover frequency increase accompanied with a reduction in by-product formation. Then, we studied the causes of inhibition of the reaction kinetics by water for the synthesis of OME from TRI and OME1 over an H-Beta zeolite. The presence of water as an impurity severely affected the reaction kinetics. The main OME growth mechanism shifted from direct TRI insertion to formaldehyde incorporation in OME, as the level of water in OME1 increased. The cause of deactivation was the hampered adsorption of TRI on the zeolite active sites by the presence of water. Lastly, the water free, non-oxidative catalytic dehydrogenation of methanol to formaldehyde (FA) was investigated as a source of oxymethylene groups, and thus an appealing alternative to TRI utilization. Grafting on amorphous silica in methanol was selected as the method of choice to study the activity of alkali metals. The resulting catalysts displayed an increased activity for the catalytic dehydrogenation of methanol. A large gap in selectivity towards FA between ions with a high and low charge density was observed. Na grafted on silica yielded the best combination of moderate conversion and high selectivity. As a result of the work presented in this thesis, important catalyst features can now be considered when developing catalysts or designing production processes for OME synthesis. More energy-efficient processes will require alternatives to the usage of TRI as the oxymethylene source. Alkali metals grafting on silica has the potential to catalyze this reaction and this method could pave the way for future research.

EPFL2020

Grafting of Alkali Metals on Fumed Silica for the Catalytic Dehydrogenation of Methanol to Formaldehyde

Alimohammad Bahmanpour, Christophe Jean Baranowski, Jack Brandon

Water free, non-oxidative catalytic dehydrogenation of methanol to formaldehyde is an important process for the formation of anhydrous, molecular formaldehyde (FA). Sodium or sodium-containing salts were shown to catalyze the reaction, but their reaction mechanism remains unclear. In this work, we thus investigated the performance of a series of catalysts prepared by grafting of alkali metals on amorphous silica for methanol catalytic dehydrogenation. The resulting catalysts displayed an increased activity for the catalytic dehydrogenation of methanol. Variation of the electron density of the supported alkali metal cations severely affected the FA selectivity, which followed the trend: Li>Na>K>Rb>Cs. A large gap in selectivity towards FA between ions with a high (i. e., Li, Na) and low charge density (i. e., K, Rb, Cs) was observed. Also, increased metal loading adversely affected FA selectivity and resulted in a larger production of carbon monoxide. Sodium grafted on silica yielded the best combination of moderate conversion and high selectivity.

2021

Operando XAS insight in the redox dynamics of VOx species and reducible supports involved in oxidative dehydrogenation of alcohols

Anna Zabilska

Supported vanadia (VOx) belongs to some of the most versatile selective oxidation and reduction catalysts applied for many reactions in the chemical industry and pollution control. Among them, the oxidative dehydrogenation (ODH) of alcohols is particularly attractive since it is one of the main industrial methods of formaldehyde production and a possible route for the transformation of renewable bioethanol to value-added products, such as acetaldehyde and acetic acid. Despite many spectroscopic methods used to investigate the mechanism of this reaction, the details about the redox involvement of VOx species and promoting supports (CeO2 and TiO2) remain debated. In this study, we used an element-specific operando X-ray absorption spectroscopy (XAS) to probe the redox transformations of active sites in vanadia-titania and vanadia-ceria catalysts during ethanol and methanol ODH. By carefully designing the steady-state and transient operando XAS experiments and by applying advanced chemometric methods, we could discriminate specifically and often quantitatively, which role plays each redox couple during the alcohol ODH cycle and uncover hidden structure activity-relationships.Chapter 1 introduces the relevant literature and summarizes the goals and the scope of the thesis.In Chapter 2, we investigate the redox dynamics of active sites in a bilayered VOx/TiOx/SiO2 catalyst (an analog of a widely used VOx/TiO2 catalyst with comparable activity) during ODH of ethanol. Operando time-resolved V and Ti K-edge XAS coupled with a transient experimental strategy quantitatively showed that the formation of acetaldehyde over VOx/TiOx/SiO2 catalyst is kinetically coupled to the formation of a V4+ intermediate, while the formation of V3+ is delayed and its rate is 10-70 times lower. The only very weak redox activity of the Ti4+/Ti3+ couple was detected, which suggests that TiOx species promote the redox activity of VOx indirectly, either via the fast electron transport between VOx species or by changing the electronic structure of VOx species.In Chapter 3, we report on possible difficulties, which can occur during in situ XAS studies of supported vanadia (VOx) catalysts at synchrotron beamlines related to X-ray-induced photochemical reduction of vanadium. We rigorously tested this phenomenon and suggest approaches, which can help to identify and mitigate vanadium photoreduction interfering chemical speciation.In Chapter 4, we focused on the structure-activity relationships in VOx/CeO2 system. We prepared a series of VOx/CeO2 catalysts using high-surface-area CeO2 Using time-resolved operando quick XAS at V K- and Ce L3-edges, we demonstrated that during oxygen cut-off experiments vanadium and cerium change their oxidation states in concert with each other and with the production of acetaldehyde. It suggests that both reducible vanadium and cerium are parts of the same active sites. The maximum VOx/CeO2 activity per mass of catalyst observed at ca. 3 V/nm2 coincides with the maximal amount of cerium atoms, which reversibly change their oxidation state during oxygen cut-off experiments.In Chapter 5, we prove that the mechanistic findings described in Chapter 4 for ethanol ODH over VOx/CeO2 catalysts are also valid for methanol ODH. Moreover, we confirm that the VOx surface density is the main descriptor of the activity for VOx/CeO2 catalysts in alcohol ODH.

EPFL2022