| Popis: |
It could be shown, that the heterogeneous catalysis of the transesterification of PC to DMC is possible. The selectivity of this reaction is very high at temperatures below 200°C and no side products can be found in the reaction mixture (with exception for the intermediate products). However at temperatures above 200°C side products like ethers and decomposition products can be found. Acid catalysts as well as basic catalysts were tested whereas the basic catalysts mostly showed a higher activity. Especially the ion exchange resins demonstrated this behavior. While the acid resins needed temperatures of more than 100°C, the basic resins were already active at temperatures below -10°C. Unfortunately both types of resins suffer from serious disadvantages. The use of the acid resins led to the formation of PG oligomers, which are blocking the catalyst and need to be removed in a time-consuming procedure. The use of strongly basic resins on the other hand led to an ion exchange within the resins and resulting in an almost inactive weak basic catalyst with only carbonate anions. Most of the used mixed metal oxides showed an insufficient catalytic performance and sometimes even an inhibiting character. The reason for this inhibition might be explained by insufficient acid/basic strength of the catalysts. As an exception, most of the mixed oxides of the alkaline and alkaline earth metals showed a good catalytic activity and can be impregnated on highly dispersed carrier material. However, most of these catalysts suffered from leaching of the active component, which marks them ineffective as heterogeneous catalysts. Only the Lithium-Titanate catalyst revealed a high stability during the experiments. Therefore this catalyst was tested in different continuous experiments, ranging from gaseous phase catalysis over liquid phase catalysis to a reactive distillation. The first experiments showed that the gaseous phase is not suitable for this reaction. Due to the high boiling point of PC the temperature must either be extremely high, or the pressure needs to be reduced. Both ways did not lead to a high amount of product due to a very low residence time at the catalyst and a strong decomposition at higher temperatures. The liquid phase experiments on the other hand showed good yields at temperatures of 170°C and a WHSV of 0.4 h-1. The catalyst was very stable and did not deactivate within three month and the selectivity was very high (85% product and 15% intermediate product which can be transformed to the products). The only disadvantage of this reaction procedure was the thermodynamic equilibrium of the stoichiometric mixture. Due to its low value of around 21% at the given reaction parameters, a high amount of starting material need to be recycled. The reactive distillation was tested as a proof of concept. It could be shown, that in general it is possible to use this reaction procedure for this reaction. The WHSV is higher than in the liquid phase experiments and the overall yield can be improved beyond the thermodynamic equilibrium due to the constant removal of the products from the reaction mixture. Due to its complicated design, only a few experiments in a small column were carried out. |
