Comments on the use of 2-methylbut-3-yn-2-ol decomposition as a probe reaction for the potential reactivity Mg-Al hydrotalcites as base catalysts

Mg:Al hydrotalcites (Mg:Al ratios 3:1, 3:2 and 4:1) have been characterised using the decomposition of 2-methylbut-3-yn-2-ol as a probe reaction for the acidic/basic properties. The hydrotalcites were used either as synthesised, or following calcination at 225 and 450 °C. All gave almost exclusively of base-catalysed products (98–99%) and a very small amount of acid-catalysed products (1–2%). For this probe reaction only slight differences in reactivity to the probe molecule were observed. The materials were also investigated for the Meerwein–Ponndorf–Verley reduction of benzaldehyde using a range of primary and secondary alcohols. In contrast to the relative reactivity for the probe reaction, significant differences in both activity and selectivity were observed for the reduction of benzaldehyde. The materials calcined at 450 °C were ca. 3 times more active on the basis of the initial rate of reaction compared with the non-calcined materials. Secondary alcohols were the most reactive and gave benzyl alcohol as the exclusive product, whereas primary alcohols also gave by-products due to ester formation and aldol condensation reactions and the structure of alcohol controlled the product selectivity in a manner consistent with the hydrogen donating properties of these alcohols. Consequently although the use of probe reactions can be instructive in determining the relative concentrations of acid and base sites, unfortunately relative reactivity profiles cannot be used predictively and this is discussed in terms of the adsorption of the probe reactant.