Ni-Mg-Al hydrotalcite-type anionic clay catalyst for the oxidation of benzyl alcohol

Nickel(II) containing magnesium-aluminum (3/1) hydrotalcite(HT)-type anionic clays have been prepared by co-precipitation and tested for a catalyst for liquid-phase oxidation of alcohols with molecular oxygen. The Ni substitution for the Mg site in Mg₃Al HT resulted in an appearance of the catalytic activity and the composition of Mg_2.5Ni _0.5Al HT was the most effective. The oxidation of primary and secondary alcohols afforded the corresponding carbonyl compounds mainly; benzyl alcohol was the most efficiently oxidized to benzaldehyde. The yield of benzaldehyde over the hydrotalcite catalyst increased significantly with increasing nickel content up to ca. 7.6 wt%, where atomically isolated and octahedrally coordinated Ni(II) sites was effective for the oxidation with molecular oxygen. The octahedrally coordinated Ni(II) cations incorporated inside the framework of hydrotalcite do not leach during the reaction and worked as a heterogeneous catalyst. It is considered that the Ni(II) site worked as the active site by activating molecular oxygen assisted by the Mg(II) as a base and simultaneously alcohol was activated by the Al(III) as an acid, resulting in an enhancement of the activity of the Mg_2.5Ni_0.5Al HT heterogeneous catalyst for alcohol oxidation.