Gas phase hydrogenation of nitroarenes: A comparison of the catalytic action of titania supported gold and silver

We have examined the catalytic action of 1 mol%Au/TiO2 and Ag/TiO2 in the continuous gas phase hydrogenation of a series of para-substituted (-H, -OH, -OCH3, -CH3, -Cl and -NO2) nitroarenes. Both catalysts promoted exclusive -NO2 group reduction, resulting in the sole formation of the corresponding amino-compound. The catalysts have been characterized in terms of temperature-programmed reduction (TPR), H-2 chemisorption, BET area, diffuse reflectance UV-vis, X-ray diffraction and HRTEM measurements. The formation of zero valent Au and Ag is established post-TPR. Au/TiO2 showed a narrower metal particle size (1-10 nm) distribution than Ag/TiO2 (1-15 nm) but both catalysts exhibited a similar surface area weighted mean size (6-7 nm). A time-invariant nitroarene conversion has been established where Au/TiO2 delivered higher specific hydrogenation rates. We associate this response to an enhanced reactant activation on Au/TiO2 to generate a negatively charged intermediate, consistent with a nucleophilic mechanism. The presence of electron-withdrawing substituents is shown to enhance -NO2 reduction rate. This effect is quantified in terms of the Hammett relationship where a linear correlation between the substituent constant (sigma(i)) and rate is established and a higher reaction constant (rho) was recorded for Au/TiO2 (0.93) relative to Ag/TiO2 (0.22). The data generated provide the first direct comparison of the catalytic action of supported Au and Ag in the hydrogenation of substituted nitroarenes and establish the viability of both catalysts to promote selective -NO2 group reduction. (C) 2010 Elsevier B.V. All rights reserved.