Advances in heterogeneous catalysis for the synthesis of polyoxymethylene dimethyl ethers

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.