Abstract
The development of new catalytic materials and routes to replace environmentally unacceptable processes in the fine chemical industry is emerging due to stringent legislation. Replacement of currently applied alkali bases in liquid-phase aldol condensations can result in diminishing of waste water streams, less corrosion and catalyst re-use. The chosen candidate
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for this is the anionic clay hydrotalcite (HT), which shows promising results after activation. However, why a treated HT is catalytically active is still not well understood and research in this thesis is aimed at gaining more insight in and control of the activation of hydrotalcites and their catalytic performance in aldol condensation reactions at low temperatures, with special attention for the industrially important condensation reactions of citral and ketones.
X-ray Absorption Near Edge Spectroscopy (XANES) at the Al and Mg K edge is applied to monitor changes in metal coordination during heat treatment (in situ) as well as after treatment (ex situ). Comparison of the results revealed that the Mg2+ and Al3+ coordinations change differently as a function of temperature. Special attention is given to the initial decomposition in the first activation step, a temperature-controlled heat treatment of HT, and the influence thereon of different types of interlayer anions. Combined with results from in situ techniques, it is shown that the nature of the interlayer anion largely influences the onset of the HT decomposition. Furthermore, the resulting structure after heat treatment was investigated with Transmission Electron Microscopy (TEM). The influence of the second activation step, i.e., rehydration of the heat-treated HT, on the structure and catalytic performance is also presented. The obtained results show that, besides incorporation of the required interlayer OH-, several structural changes occur, due to rehydration. Besides calcination/rehydration as activation method, ion exchange was applied to obtain the HT with interlayer Brønsted base sites. Activities were measured in the self-condensation of acetone and the differences in catalytic activity of the differently activated catalysts are discussed. Furthermore, the results of the application of activated HTs in citral-ketone condensations at low temperatures are presented. Besides this, an improved activation procedure is presented resulting in a doubling of the available basic sites and corresponding activity. In the final chapter, results from the previous chapters are briefly reviewed and discussed, as well as the implications thereof for future research.
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