Selectivity Modulation of Multistep Reduction Reactions by Gold Nanoclusters

The selective synthesis of valuable azo- and azoxyaromatic chemicals via transfer coupling of nitroaromatic compounds has been achieved by fine-tuning the catalyst structure. Here, a direct method to modulate nitrobenzene reduction and selectively alter the product from azobenzene to azoxybenzene by employing the size effect of Au is reported. Au nanoclusters (NCs) with smaller sizes embedded in ZIF-8 controllably converted nitrobenzene into azoxybenzene, while supported Au nanoparticles (NPs) selectively catalyzed nitrobenzene reduction to azobenzene. X-ray photoelectron spectroscopy (XPS) and Diffuse reflectance infrared Fourier transform spectroscopy on CO adsorption (CO-DRIFTS) of Au NC/ZIF-8 revealed a higher valence state and a lower electron density of Au than that of Au NP/ZIF-8, combined with the desorption of azoxybenzene from the Au NC and Au NP surface, suggesting that the Au NCs with lower electron density exhibit stronger adsorption. Density functional theory (DFT) calculations and charge density difference maps indicated that azoxybenzene bonded to Au NC/ZIF-8 with greater adsorption energy, resulting in more electron transfer between azoxybenzene and the generated Au sites, which inhibited further reduction of azoxybenzene and resulted in high azoxybenzene selectivity. The application of the size effect of Au particles to regulate nitrobenzene transfer coupling provided new insights into the structure-selectivity relationships.