Light-tuned selective photosynthesis of azo- and azoxy-aromatics using graphitic C3N4

Abstract

Solar-driven photocatalysis has attracted significant attention in water splitting, CO₂ reduction and organic synthesis. The syntheses of valuable azo- and azoxyaromatic dyes via selective photoreduction of nitroaromatic compounds have been realised using supported plasmonic metal nanoparticles at elevated temperatures (≥90 °C); however, the high cost, low efficiency and poor selectivity of such catalyst systems at room temperature limit their application. Here we demonstrate that the inexpensive graphitic C₃N₄ is an efficient photocatalyst for selective syntheses of a series of azo- and azoxy-aromatic compounds from their corresponding nitroaromatics under either purple (410 nm) or blue light (450 nm) excitation. The high efficiency and high selectivity towards azo- and azoxy-aromatic compounds can be attributed to the weakly bound photogenerated surface adsorbed H-atoms and a favourable N-N coupling reaction. The results reveal financial and environmental potential of photocatalysis for mass production of valuable chemicals.

Fig. 1

Photocatalytic performance. a, b Photoconversion (Con.) and selectivity (Sel.) of nitrobenzene to azobenzene and azoxybenzene using g-C₃N₄ under 410 and 450 nm irradiation (lab-scale). c Photoconversion of nitrobenzene to azoxybenzene using g-C₃N₄ at different starting concentrations under 450 nm irradiation. Reaction conditions: 8 mM nitrobenzene, 10 mM KOH, and 50 mg catalyst in 10 mL isopropanol under N₂ atmosphere. d, e Up-scaled reactions with volumes of 800 mL and 80 L, respectively. The nitrobenzene photoconversion tests were performed under solar radiation in Beijing (30/9/2016-2/10/2016, 25-30 °C, 800 mL) and Shenzhen (18/12/2016-5/1/2017, 25–30 °C, 80 L). The starting concentration of nitrobenzene was 8 mM. f, g Conversion and selectivity for the scaled-up tests

Fig. 2

Reaction mechanism analysis. a The consumption of dissolved O₂ (O₂ reduction and isopropanol oxidation) under irradiation using various photocatalysts determined by in-situ MS. A 365 nm LED was used for TiO₂ and a 450 nm LED was used for all other tests. b Post-mortem TPD spectra revealing the desorption of H_ads from the g-C₃N₄ and A-g-C₃N₄ surfaces. The TPD was performed on vacuum dried samples after reaction without adsorption of additional H₂. c Scheme of the suggested reaction pathway. The reduction reaction will be hindered if H_ads binds strongly to the catalyst. d Reaction path for the photoconversion of nitroaromatic compounds to azoxy-, azo-aromatic compounds, and amines. NBS = Nitrosobenzene, NPH = N-phenylhydroxylamine

Fig. 3

Proposed reaction mechanisms. a Energy scheme of the photocatalytic nitrobenzene reduction on g-C₃N₄. The redox potentials of nitrobenzene (NB) to azoxybenzene plus isopropanol (IP) to acetone (AC), and the CB and VB positions are referenced to the SHE^,. b and c suggested possible active sites for isopropanol oxidation and nitrobenzene reduction according to calculations. Grey: C, light purple: N, white: structural H, light blue: surface adsorbed H (H_ads)