Selective oxofunctionalization of cyclohexane over titanium dioxide–based and bismuth oxyhalide (BiOX, X = Cl−, Br−, I−) photocatalysts by visible light irradiation

Abstract

Solar photocatalysis has been employed for pollutant removal by driving the substrates to complete mineralization. These unselective systems can become selective by tuning the reaction conditions and the properties of the photocatalyst for the oxofunctionalization of unreactive compounds such as hydrocarbons.This work focused on the selective oxofunctionalization of cyclohexane by visible light photocatalysis under ambient conditions. For this purpose, five titanium dioxide–based photocatalysts (Fe–doped TiO2, N–doped and undoped TiO2) and three bismuth oxyhalide photocatalysts (BiOCl, BiOBr and BiOI) were synthesized and characterized (XRD, DRS, SEM and BET surface area). The titanium dioxide–based photocatalysts showed the highest selectivity for cyclohexanone (cyclohexanone/cyclohexanol=4.49–87.50). BiOCl and BiOBr showed similar yields for cyclohexanone and cyclohexanol production (cyclohexanone/cyclohexanol=1.92 and 1.27, respectively). The BiOI photocatalysts showed the highest selectivity for cyclohexanol production (cyclohexanone/cyclohexanol=0.23). The cyclohexanol yield depended linearly on the bandgap of the BiOX. BiOI reached the maximum cyclohexanol yield (16.5±1.3μmolm−2g). In all assayed systems, the cyclohexyloxy radical was identified as an intermediate. However, only when BiOI was utilized was there evidence of OH radical production. A higher yield of cyclohexanol than cyclohexanone was obtained only by using this photocatalyst. Thus, the selectivity was associated with cyclohexanol production by hydroxylation of the cyclohexyl radical by the OH radical.