Maßgeschneiderte heterogene Katalysatoren für die direkte Nutzung wässriger Fermentationsprodukte
In the thesis, tailor-made heterogeneous catalysts for the energy efficient conversion of aqueous fermentatively produced 2,3-butanediol (2,3-BDO) were developed.
For this purpose, several literature-known and novel Re-based supported catalysts were first produced and investigated regarding property-activity relationships in the well-established DODH of 1,2-hexanediol to 1-hexene. For the first time, SBA-15 and zeolites were used as support materials. For ReOx/SBA-15 and ReOx/HCP a structure-activity relationship was found, more specifically an increase in productivity could be observed with increasing mesopore content and pore diameter. For APR/zeolite, the Si:Al-ratio was the dominant factor. TEM studies suggest that Re preferentially binds to Al, whereby a high Al content leads to an increase in activity.
The liquid phase DODH of 2,3-BDO was established and optimized with ReOx/C as catalyst and 3‑octanol as solvent and reducing agent. 2,3-BDO could be selectively converted to 2‑butene and the stereochemistry of the product could be controlled by the choice of the reactant, since the stereochemistry of the 2,3-BDO used is transferred to the product. The optimized conditions could also be used for the DODH of other molecules such as ethylene glycol and glycerol. Subsequently, the possibility of direct conversion of 2,3-BDO in the fermentation solution was evaluated by adding small amounts of the fermentation components to the reaction. In small concentrations (≤ 5g∙L-1) the influence of water, alcoholic and acidic components on the DODH activity of the catalyst is negligible. As the concentration of diols and polyols increases, side reactions occur, while a high water or acid concentration leads to reduced productivity. This effect is further enhanced by the addition of the corresponding salts. In order to explain this behavior, the influence of water on the reaction and the catalyst was systematically investigated. It could be excluded that water would influence the catalyst properties. Rather, it is suspected that water and other polar substances inhibit the DODH mechanism by influencing the chemical equilibrium or binding to the active Re-species and blocking the active sites against 2,3-BDO.
In order to avoid catalyst deactivation by water, a multiphase DODH concept was developed that allows spatial separation of the catalyst from the aqueous phase. 2,3-BDO is dissolved in the aqueous phase, while a solid catalyst is placed in a water-insoluble organic solvent. After extraction of 2,3‑BDO into the organic phase, conversion to 2-butene takes place. The gaseous product can be easily separated from the system. For practical implementation, a glass inlet equipped with a glass frit as catalyst carrier was used. By using the multiphase-system with ReOx/C as catalyst and 3‑octanol as organic solvent the catalyst activity could be increased in the presence of water. For a more efficient reaction, however, the system would have to be further optimized.