In this study, we report the fabrication of mesoporous assemblies of silver and TiO₂ nanoparticles (Ag/MTA) and demonstrate their catalytic efficiency for the selective reduction of nitroarenes. The Ag/TiO₂ assemblies, which show large surface areas (119-128 m²·g-1) and narrow-sized mesopores (ca. 7.1-7.4 nm), perform as highly active catalysts for the reduction of nitroarenes, giving the corresponding aryl amines and N-aryl hydroxylamines with NaBH₄ and ammonia-borane (NH₃BH₃), respectively, in moderate to high yields, even in large scale reactions (up to 5 mmol). Kinetic studies indicate that nitroarenes substituted with electron-withdrawing groups reduced faster than those with electron-donating groups. The measured positive ρ values from the formal Hammett-type kinetic analysis of X-substituted nitroarenes are consistent with the proposed mechanism that include the formation of possible [Ag]-H hybrid species, which are responsible for the reduction process. Because of the high observed chemo selectivities and the clean reaction processes, the present catalytic systems, i.e., Ag/MTA-NaBH₄ and Ag/MTA-NH₃BH₃, show promise for the efficient synthesis of aryl amines and N-aryl hydroxylamines at industrial levels.
Keywords: N-aryl hydroxylamines; aryl amines; heterogeneous catalysis; nitroarenes; selective reduction; silver nanoparticles; titania.