NiAl hydrotalcite as an efficient and environmentally friendly solid catalyst for solvent-free liquid-phase selective oxidation of ethylbenzene to acetophenone with 1 atm of molecular oxygen

Abstract

NiAl hydrotalcites with Ni/Al molar ratios of 2–5 and prepared using C O 3 2 − , Cl −, N O 3 − , or S O 4 2 − as a guest inorganic anion, were evaluated for the liquid-phase selective oxidation of a methylene group of ethylbenzene to carbonyl functionality with atmospheric pressure of oxygen as the sole oxidant under solvent-free and mild reaction conditions. Among the different NiAl hydrotalcites, the catalyst with a Ni/Al ratio of 5 mol mol −1 prepared in the presence of C O 3 2 − anion exhibited the greatest catalytic activity for the ethylbenzene oxidation reaction. NiAl hydrotalcite prepared with C O 3 2 − as a guest inorganic anion also showed very high acetophenone selectivity (>99%) compared with those prepared using Cl −, N O 3 − , or S O 4 2 − anions. Compared with other transition metal–containing hydrotalcite-like solid catalysts (e.g., CuAl-, ZnAl-, CoAl-, MgFe-, MgCr-), with similar molar compositions and prepared in the presence of the C O 3 2 − anion, the catalytic activity of NiAl hydrotalcite was much higher in the above oxidation reaction. The active NiAl hydrotalcite also exhibited good catalytic performance for the oxidation of a wide range of alkylaromatics to their corresponding benzylic ketones under similar experimental conditions. The NiAl hydrotalcite can be easily prepared from cheap, commercially available reagents and is stable and reusable for the ethylbenzene oxidation even in the absence of solvents and reducing reagents.