Characterization of basic catalysts by the use of nitromethane as NMR probe molecule and reactant

Characterization of the basicity of mixed oxides obtained from hydrotalcite-like precursors by 13C CP/MAS NMR spectroscopy using nitromethane-reactive adsorption is described. On Lewis bases, featuring acid–base pairs, the nature of adsorbed species namely nitromethane, aci-anion nitromethane, and methazonate salt analogue depends on the relative strength of the basic and acid sites on the pairs. None of these species is obtained using Brønsted bases such as layered double hydroxide (LDH) of the meixnerite type. In that case a formate analogue is proposed to be formed. Heterogeneous base catalysis of the Michael addition of nitromethane to cyclohex-2-en-1-one is studied. A relationship is established between the activity of the catalyst and the 13C CP/MAS NMR chemical shift of the aci-anion nitromethane which is a measure of the basic strength. Discussion of NMR and catalytic results takes into account the respective roles of basicity and of conjugate Lewis acidity. Basicity is needed to form a reactive anion comparable to aci-anion nitromethane, but the higher the stabilization of aci-anion nitromethane by Lewis acidity, the lower the activity. Increasing the basic strength by decreasing the Lewis acidity of the acid–base pairs leads to an increase of activity concomitant with the formation of methazonate as shown by 13C MAS NMR. The efficiency of meixnerite which is the most active and selective catalyst is related not only to its Brønsted basicity but also to its weak Lewis acidity.