Effect of the support on the basic and catalytic properties of KF

KF supported by various supports was characterized by DTG-DTA coupled with mass spectral analysis of the gases evolved by the solid, in situ XRD, calorimetry, ¹⁹F NMR spectroscopy, and tested in the Michael addition of cyclohexene-2-one with nitroalkanes. After dehydration at low temperatures, from ¹⁹F NMR spectroscopy F is in part as KF when the support is α-alumina but only as K₃AlF₆ when the support is γ-alumina. During activation no fluorine is lost. On α-alumina KF is detected by XRD only after calcination at about 723 K and results in part of the decomposition of K₃AlF₆; it disappears with formation of K₂AlF₅ and KAlO₂ at higher temperatures. A comparison of the results obtained by XRD and NMR suggests that KF is well dispersed on α-alumina. KF/α-alumina is a strong base, stronger than KF on γ-alumina, and more active for Michael reactions. On KF/α-alumina the Michael condensation of 2-cyclohexen-1-one with nitroalkanes can be achieved in 5 min with an equimolar mixture of reactants. Solvent-free reactions can also be achieved reaching 75% yield in adduct after 8 h, and KF/α-alumina is also active after simple drying at 393 K, avoiding the usual activation at higher; temperatures.