Cyanoethylation of alcohols by activated Mg–Al layered double hydroxides: Influence of rehydration conditions and Mg/Al molar ratio on Brönsted basicity

Activated Mg–Al layered double hydroxides (LDHs) with varying Mg/Al molar ratios are tested as heterogeneous catalysts for the cyanoethylation of methanol and 2-propanol. The activation procedure is performed by calcination of the LDH precursors, followed by gas-phase rehydration on an H2O/N2 flow. In this case, rehydration is carried out at 80 °C, instead of room temperature as usual. Increased temperature has a very significant effect on the reconstruction speed, shortening the process from ∼12 to 0.5 h. Good conversions and yields to alkoxypropionitriles are obtained with thus activated LDHs. Correlations are established between basic strength, as determined by CDCl3 adsorption followed by FTIR, and catalytic activity. Depending on alcohol acidity, greatest conversions are obtained over catalysts with Mg/Al molar ratios of 2 or 3. Solids are further characterized by X-ray diffraction, thermal gravimetric analyses, and 27Al MAS NMR.