Chemoselective Hydrogenation of Nitroaromatics by Supported Gold Catalysts: Mechanistic Reasons of Size- and Support-Dependent Activity and Selectivity

Abstract

Supported Au nanoparticles (NPs) prepared by colloid deposition method were well characterized, and their catalytic performance was tested for chemoselective reduction of a nitro group of substituted nitroaromatics by H 2 . Systematic studies on the effects of NPs size and support show small size of Au NPs, and acid-base sites of supports are required for high activity. The Au/Al 2 O 3 catalyst with Au particle size of 2.5 nm selectively hydrogenates a nitro group in the presence of various other reducible functional groups, and it shows higher intrinsic activity than the state-of-the-art catalyst (Au NPs on TiO 2 ). In situ FTIR studies provide a reaction mechanism, which explains fundamental reasons of the observed structure-activity relationship. Cooperation of the acid-base pair site on Al 2 O 3 and the coordinatively unsaturated Au atoms on the Au NPs are responsible for the H 2 dissociation to yield a H + /Hpair at the metal/support interface. High chemoselectivity could be attributed to a preferential transfer of the H + /Hpair to the polar bonds in the nitro group as well as a preferential adsorption of nitroaromatics on the catalyst through the nitro group.

Catalytic activity of various pepsin reduced Au nanostructures towards reduction of nitroarenes and resazurin

“…For example, the catalytic reduction of pnitroaniline by NaBH 4 with Au nanoparticles of sizes 6.7 ± 0.9 nm was much faster than the reaction catalyzed by Au nanoparticles of size 13.8 ± 0.8 nm, which in turn was faster for the same reaction catalyzed by Au nanoparticles having an average size of 22.0 ± 1.2 nm (Kundu et al 2009a). A similar trend was also observed for the chemoselective hydrogenation of nitroaromatics by oxide supported Au nanoparticles, where Au nanoparticles of size 2.5 nm supported on Al 2 O 3 gave better selectivity for the chemoselective reduction of 4-nitrostyrene to 4-aminostyrene compared to the Au nanoparticles of size 6 nm with the same support material (Shimizu et al 2009). In order to realize the correlation of sizes with the catalytic activity, we performed the reduction of p-nitrophenol to p-aminophenol by NaBH 4 using various Au nanostructures synthesized by pepsin as catalysts.…”

FeOx-supported platinum single-atom and pseudo-single-atom catalysts for chemoselective hydrogenation of functionalized nitroarenes

“…In contrast, the Pt/TiO 2 was reported to give a dramatically decreased selectivity (69.7%) at 85°C 15 . For the chemoselective hydrogenation of nitroarene, to the best of our knowledge, our SACs and pseudo-SACs are the most active and selective catalysts ever reported in literature, far outperforming all the reported catalysts, including the IB-group metals [5][6][7][8][9][10][11][12][13][14][15] and the modified Pt-group metals 20 .…”

“…However, the intrinsically poor capability of IBgroup metals for H 2 activation renders them at least one order of magnitude less active than the Pt-group metals. To enhance the activity of IB-group metals, delicate control of metal-support interactions [10][11][12][13][14] or alloying with a small amount of Pt-group metals 15,16 was tested. However, the achieved chemoselectivity of these catalysts was not satisfactory.…”

“…The chemoselectivity to the hydrogenation of nitro groups arises from the preferential adsorption of nitro groups on the specific sites of metal nanoparticles or the support 13,14 . Although the hydrogenation of nitro groups over metal nanoparticles appears to be size insensitive, the hydrogenation of C ¼ C and CC groups is indeed highly sensitive to the particle size, with the reaction rate usually increasing with increasing metal particle size 19 .…”

Silver Cluster Catalysts for Green Organic Synthesis

“…We recently found that Au clusters on Al2O3 was also effective for the selective reduction of nitrostyrene and showed that small Au NCs (2.5 nm) on the acid-base bifunctional support (Al2O3) gave highest activity 17) . A cooperation of AuCUS sites and adjacent acid-base pair site of Al2O3 are responsible for the rate-limiting H2 dissociation step.…”