Effect of basicity on the catalytic properties of Ni-containing hydrotalcites in the aerobic oxidation of alcohol

Abstract

A series of Ni-containing hydrotalcites with different basicities have been prepared by introducing different Mg2+ contents, and characterized by XRD, SEM, TG-DTG, ICP, FTIR, Hammett analysis, DR UV–vis, XPS, etc. The effects of basicity on the catalytic performance in the selective aerobic oxidation of alcohols and the mechanism have been studied. The results showed that substituting Ni2+ in the structure by Mg2+ ions significantly increased the surface basicity of the catalysts. The surface basicity of Ni-containing hydrotalcites could accelerate the first acid-base reaction step in the oxidation and improve the catalytic activity. Varied alcohols were tested and discussed in the reaction system to verify the effect, and the results indicated that the activity of α-CH bond is the key factor for the benzyl alcohol derivatives, while the first base-acid reaction step may be more important for aliphatic alcohols. The comparison results between the hydrotalcites and the calcined samples showed that the type of basic site have significantly influence on the catalytic activity, and only the Brønsted OH basic sites accelerate the oxidation. In addition, a probable mechanism for the reaction was postulated based on catalytic results, Hammett and a series of controlled experiments. The main factors affecting the catalytic oxidation of varied alcohols using molecular oxygen as the ultimate oxidant have been discussed, which may be helpful in designing more efficient catalyst.