Gas phase selective hydrogenation over oxide supported Ni–Au

Abstract

The chemoselective continuous gas phase (T = 573 K; P = 1 atm) hydrogenation of nitroarenes (p-chloronitrobenzene (p-CNB) and m-dinitrobenzene (m-DNB)) has been investigated over a series of oxide (Al₂O₃ and TiO₂) supported Au and Ni–Au (1 : 10 mol ratio; 0.1–1 mol% Au) catalysts. Monometallic supported Au with mean particle size 3–9 nm promoted exclusive formation of p-chloroaniline (p-CAN) and m-nitroaniline (m-NAN). Selective hydrogenation rate was higher over smaller Au particles and can be attributed to increased surface hydrogen (from TPD measurements) at higher metal dispersion. (S)TEM analysis has confirmed an equivalent metal particle size for the supported bimetallics at the same Au loading where TPR indicates Ni–Au interaction and EDX surface mapping established Ni in close proximity to Au on isolated nanoparticles with a composition (Au/Ni) close to the bulk value (= 10). Increased spillover hydrogen due to the incorporation of Ni in the bimetallics resulted in elevated –NO₂ group reduction rate. Full selectivity to p-CAN was maintained over all the bimetallic catalysts. Conversion of m-DNB over the lower loaded Ni–Au/Al₂O₃ generated m-NAN as sole product. An increase in Ni content (0.01 → 0.1 mol%) or a switch from Al₂O₃ to TiO₂ as support resulted in full –NO₂ reduction (to m-phenylenediamine). Our results demonstrate the viability of Ni-promotion of Au in the continuous production of functionalised anilines.