Fast and selective reduction of nitroarenes under visible light with an earth-abundant plasmonic photocatalyst

Abstract

Reduction of nitroaromatics to the corresponding amines is a key process in the fine and bulk chemicals industry to produce polymers, pharmaceuticals, agrochemicals and dyes. However, their effective and selective reduction requires high temperatures and pressurized hydrogen and involves noble metal-based catalysts. Here we report on an earth-abundant, plasmonic nano-photocatalyst, with an excellent reaction rate towards the selective hydrogenation of nitroaromatics. With solar light as the only energy input, the chalcopyrite catalyst operates through the combined action of hot holes and photothermal effects. Ultrafast laser transient absorption and light-induced electron paramagnetic resonance spectroscopies have unveiled the energy matching of the hot holes in the valence band of the catalyst with the frontier orbitals of the hydrogen and electron donor, via a transient coordination intermediate. Consequently, the reusable and sustainable copper-iron-sulfide (CuFeS₂) catalyst delivers previously unattainable turnover frequencies, even in large-scale reactions, while the cost-normalized production rate stands an order of magnitude above the state of the art.

Fig. 1. Structural identity of the CuFeS₂ NCs.

a,b, TEM images of the CuFeS₂ NCs. Scale bars, 400 nm (a); 100 nm (b). Insets: the EDS (a); the selected area electron diffraction of the NCs (b). c, High-resolution TEM image of a single NC with marked lattice fringes. Scale bar, 5 nm. d–f, High-angle annular dark field–scanning TEM image (d) of a single NC with the corresponding EDS chemical mapping for Cu (e), S (f) and Fe (g). Scale bars, 8 nm. h, Combined mapping for Cu, Fe and S. Scale bar, 8 nm. i–k, UV–vis absorption spectra (beam path-length, 1 cm) (i), XRD analysis (j) and FTIR spectra (k) of CuFeS₂ NCs before (CuFeS₂-OLA) and after (CuFeS₂-S²⁻) the ligand exchange reaction. OLA, oleylamine. Source data

Fig. 2. Catalytic reaction study.

a–e, Reduction of nitrobenzene (NB) (a) using CuFeS₂ NCs (b) for different reaction times and amounts of NB and hydrazine hydrate, using in all cases 10 mg catalyst (labels inside the bars are the corresponding TOF values), with different catalyst amounts (c) (4 h reaction time), aniline yield at different environmental temperatures (d) (4 h, 16 mmol hydrazine, 5 mmol nitrobenzene, 10 mg catalyst) and under controlled temperature or light (e). Reaction conditions for e were nitrobenzene, 1 mmol; hydrazine hydrate, 1 ml; catalyst, 2 mg and under light/heat irradiation with continuous stirring for 4 h. cat., catalyst; hzn., hydrazine. Source data

Fig. 3. Catalyst recyclability and substrate study.

a, Recycling performance of the catalyst for the photocatalytic reduction of nitrobenzene. Reaction conditions were 0.1 mmol nitro compound, 50 µl hydrazine hydrate, 10 mg catalyst, 3 ml ethanol and light irradiation with continuous stirring at room temperature for 4 h. b, Photocatalytic reduction of nitroarenes to anilines catalysed by CuFeS₂ NCs. The percentiles correspond to the reaction yields, as determined by gas chromatography. Reaction conditions were 0.1 mmol nitro compound, 50 µl hydrazine hydrate, 10 mg catalyst, 3 ml ethanol and light irradiation with continuous stirring at room temperature for 4 h. The asterisk (^a) denotes 1 mmol nitroarene, 1 ml hydrazine hydrate and 2 mg catalyst under light irradiation with continuous stirring at room temperature for 4 h. Source data

Fig. 4. The catalyst performance with respect to the state of the art.

Comparisons of the average TOF values and of the cost-normalized TOF for the CuFeS₂ catalyst and for previously reported ones, under photocatalytic conditions (Greek alphabet letters in green) and under elevated temperature and H₂ pressure conditions (Latin alphabet letters in blue). α, ref. ; β, ref. ; γ, ref. ; δ, ref. ; ε, ref. ; ζ, ref. ; η, ref. ; θ, ref. ; ι, ref. ; κ, ref. ; λ, ref. ; μ, ref. , ξ, ref. ; ο, ref. ; π, ref. ; σ, ref. ; τ, ref. ; υ, ref. ; φ, ref. ; a, ref. ; b, ref. ; c, ref. ; d, ref. ; e, ref. ; f, ref. ; g, ref. and h, ref. : more details are given in Supplementary Table 1. Source data

Fig. 5. TAS and light-induced electron paramagnetic resonance studies of the catalyst.

a, Time-resolved transient absorption spectra of the CuFeS₂ catalyst showing the optical density difference (ΔOD) as a function of wavelength at various time delays. b, Transient dynamics of the CuFeS₂ PIA at 590 nm and photobleaching (PB) at 910 nm. c, Schematic representation of energy level diagrams of CuFeS₂ and hydrazine. LUMO, lowest occupied molecular orbital. d, The photoexcited intermediate specie of the catalyst with hydrazine, in accordance with hydrazine’s oxidation by transferring electrons from its HOMO to the energy-matching photogenerated holes in the valence band of CuFeS₂ (c). e, The emergence of the three-electron reduction intermediate of nitrobenzene upon light irradiation. Source data