Quaternary ammonium salts for hydrotalcite-type catalysts synthesis

Tetra Methyl Ammonium Hydroxide as well as Tetra n-Butyl Ammonium Hydroxides represent a feasible alternative to inorganic alkalis for hydrotalcites synthesis. For the most part, the washing step in the synthesis of hydrotalcites by inorganic alkalis generates a large amount of wastewater that must be processed afterwards generating high costs and environmental problems. The organic alkalis conduct the system towards smaller amounts of wastewater (<250 mL for 20 g of dried hydrotalcites synthesis). Therefore, the energy involved in the subsequent purifications of wastewater decreases significantly. This notable lower consumption of washing water and energy offsets against the slightly increased price of these organic alkalis. Moreover, both preparation methods, i.e. co-precipitation and mechano-chemical, generate pure LDH structures. The crystallinity of hydrotalcite synthesized with TMAH is more pronounced than that obtained with TBAH. A crucial property for these materials, i.e. memory effect, is also present. The trend of catalytic activities in the cyanoethylation reaction of ethanol with acrylonitrile, regardless of the preparation method, follows the trend: mixed oxides (≈ 80%) > reconstructed samples > dried samples. This tendency is in line with the strength of the base sites involved. However, all investigated solids generate 100% in the selectivity to 3-ethoxypropionitrile.