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Abstract

Oxymethylene dimethyl ethers (CH3O(CH2O)nCH3) bearing 3 to 5 CH2O-units (OMDME 3-5) are promising diesel additives, as they are able to reduce soot and NOx emissions. However, due to economic restrictions, an industrial synthesis process for these compounds has not been realized yet. As presented in this thesis, it was possible to synthesize oligomeric OMDME from dimethoxymethane (DMM) and trioxane (TRI), catalyzed by solid acids, in a more efficient and economical way as described in the literature. For the first time, it was possible to carry out the synthesis of OMDME at atmospheric pressure and mild reaction temperatures below 40 °C, maintaining high reactivity. Besides optimizing process parameters and testing of catalysts, reaction kinetics were also evaluated and determined. Furthermore, the synthesis of oxymethylene diethyl ethers (OMDEE) and oxymethylene diacetate (OMDAc) was investigated and compared with each other. The oligomerization of OMDME is a typical equilibrium reaction. In addition to DMM and TRI as starting materials, a series of oligomeric OMDME are also present in the equilibrium. All tests showed that the OMDME product mixture follows the Schulz-Flory distribution, even with varying DMM/TRI ratios. Solid acids, such as cation exchanger resins and zeolites, served as catalysts. Out of all tested resins and zeolites, Amberlyst36 among resins and BEA25 among zeolites exhibited the strongest catalytic activity. The chemical equilibrium was reached within a few minutes at room temperature. However, this high reactivity was only possible, when the synthesis of OMDME was performed in the absence of water, as minor traces of water already caused a significant loss of catalyst activity. For the first time, it was further possible to verify that during the oligomerization the direct insertion of TRI into DMM takes place. This was observed due to an increased OMDME 4 concentration at the beginning of the reaction. Because of these and other results, a detailed reaction mechanism was postulated, which implies the insertion of TRI into the catalytically activated DMM. The proven high rate of transacetalization in the OMDME reaction led to the Schulz-Flory distribution even before chemical equilibrium was reached.