Selective photocatalytic oxidation of cyclohexene coupled with hydrogen evolution from water splitting over Ni/NiO/CdS and mechanism insight

Abstract

The selective oxidation of small organic molecules with simultaneous hydrogen evolution from water splitting over photocatalysts is a research hot spot in chemical synthesis. It is of great significance to develop a highly selective and environmentally friendly photocatalytic method for cyclohexene oxidation coupled with hydrogen evolution. In this work, Ni/NiO/CdS nanocomposites were prepared by using Ni-BTC (BTC = 1,3,5-benzene tricarboxylic acid) as the precursor of Ni/NiO. The photocatalytic oxidation products of cyclohexene were cyclohx-2-en-1-ol, cyclohex-2-en-1-one, cyclohexanone and cyclohexane-1,2-diol at room temperature, under normal pressure and an argon atmosphere. Under the optimal conditions, the conversion rate of cyclohexene is 14.8%, and the hydrogen production efficiency is 266.7 μmol g⁻¹ h⁻¹. Moreover, the product cyclohexanone was detected which is rarely reported in other similar studies. It was confirmed that cyclohexanone is the reduction product of cyclohex-2-en-1-one by the active H₂ in the reaction. The photocatalytic oxidation mechanism of cyclohexene was proposed, and the oxidation substances in the reaction were proved to be h⁺ and ˙OH. It was confirmed that the ˙OH radicals preferentially attack cyclohexene-allyl α-C–H bonds in the photocatalytic reaction, so the yield of cyclohex-2-en-1-one is higher than that of cyclohexane-1,2-diol. This study provides a green strategy for photocatalytic selective oxidation of cyclohexene with simultaneous hydrogen production.