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

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), H-2 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+ -> Au-0 reduction. XRD analysis demonstrated the presence of metallic gold in Au/Fe2O3 pre-TPR. A partial and complete reduction of the hematite support to magnetite (Fe2O3 -> Fe3O4) was observed post-TPR to 423 and 673 K, respectively; H-2-TPD results suggest the participation of spillover hydrogen in this step. Exclusive -NO2 group 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.