Immobilizing graphitic carbon nitride on porous silica via hydrogen bonds for photocatalytic flow synthesis of azoxybenzene

Heterogeneous photocatalysis is an ideal method for eco-friendly synthesis of value-added chemicals. However, due to the limited penetration depth of photons and inefficient utilization of the catalytic active sites, the scalability of heterogeneous photocatalysis is restrained. Combining heterogeneous photocatalysis and flow system is a promising solution to optimize the light absorption and mass transfer; however, an affordable, practical design of the flow system and the immobilization method of the photocatalyst is still missing. Here, we report a flow system for heterogeneous photocatalytic synthesis of azoxybenzene and its derivatives from reductive coupling of nitrobenzene at a gram-scale using anchored graphitic carbon nitride (gCN) on porous silica (gCN-SiO2). The hydrogen bonds formed between the gCN and the tetramethoxysilane (TMOS) precursor during the dehydrative condensation process prevent unwanted leaching of catalyst, enabling photocatalytic flow synthesis of azoxybenzene with high selectivity, efficiency, and durability. Mechanistic studies reveals that the gCN-SiO2 regulates the photocatalytic reduction kinetics of nitrobenzene by facilitating the generation of nitrosobenzene and N-phenylhydroxylamine intermediates while preventing the formation of unwanted 4-phenylazophenol byproducts, resulting in efficient and selective synthesis of azoxybenzene. Our work provides an alternative path towards large scale heterogeneous photocatalysis.