Gold catalysis at the gas–solid interface: role of the support in determining activity and selectivity in the hydrogenation of m-dinitrobenzene

Abstract

The catalytic gas phase hydrogenation of m-dinitrobenzene (1 atm, 423 K) over laboratory synthesised (1 mol%) Au supported on Al₂O₃, TiO₂, Fe₂O₃ and CeO₂ and a reference Au/TiO₂ (World Gold Council) has been investigated. The catalysts were prepared by deposition-precipitation (DP) and impregnation (IMP), where the former route generated smaller (surface area weighted) mean Au particle sizes (1.5–2.8 nm) compared with the IMP synthesis (3.5–9.0 nm). The catalysts have been characterized in terms of temperature programmed reduction (TPR), H₂ chemisorption/temperature programmed desorption (TPD), BET area, powder X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) analyses. Hydrogen consumption over the T range 371–457 K during activation of Au on TiO₂, Al₂O₃ and CeO₂ can be associated with Au³⁺ → Au⁰ reduction. XRD analysis demonstrated the presence of metallic gold in Au/Fe₂O₃pre-TPR. A partial and complete reduction of the hematite support to magnetite (Fe₂O₃ → Fe₃O₄) was observed post-TPR to 423 and 673 K, respectively; H₂-TPD results suggest the participation of spillover hydrogen in this step. Exclusive –NO₂group reduction and time invariant conversions were observed for all the catalysts considered in this study. An increase in the specific hydrogenation rate with a decrease in the mean Au size (from 9 to 3 nm) was observed. m-Nitroaniline was generated as the sole product over Au/TiO₂ and Au/Fe₂O₃ whereas Au/CeO₂ promoted the exclusive formation of m-phenylenediamine and a mixture of both products was obtained over Au/Al₂O₃. Our findings establish a basis for the development of a sustainable (clean and continuous) process for the hydrogenation of m-dinitrobenzene where product composition can be controlled through the choice of the oxide support.