Niobium oxide prepared through a novel supercritical-CO2-assisted method as a highly active heterogeneous catalyst for the synthesis of azoxybenzene from aniline

Abstract

High-surface area Nb₂O₅ nanoparticles were synthesised by a novel supercritical-CO₂-assisted method (Nb₂O₅-scCO₂) and were applied for the first time as a heterogeneous catalyst in the oxidative coupling of aniline to azoxybenzene using the environmentally friendly H₂O₂ as the oxidant. The application of scCO₂ in the synthesis of Nb₂O₅-scCO₂ catalyst resulted in a significantly enhanced catalytic activity compared to a reference catalyst prepared without scCO₂ (Nb₂O₅-Ref) or to commercial Nb₂O₅. Importantly, the Nb₂O₅-scCO₂ catalyst achieved an aniline conversion of 86% (stoichiometric maximum of 93% with the employed aniline-to-H₂O₂ ratio of 1 : 1.4) with an azoxybenzene selectivity of 92% and with 95% efficiency in H₂O₂ utilisation in 45 min without requiring external heating (the reaction is exothermic) and with an extremely low catalyst loading (weight ratio between the catalyst and substrate, R_c/s = 0.005). This performance largely surpasses that of any other heterogeneous catalyst previously reported for this reaction. Additionally, the Nb₂O₅ catalyst displayed high activity also for substituted anilines (e.g. methyl or ethyl-anilines and para-anisidine) and was reused in consecutive runs without any loss of activity. Characterisation by means of N₂-physisorption, XRD, FTIR and TEM allowed the correlation of the remarkable catalytic performance of Nb₂O₅-scCO₂ to its higher surface area and discrete nanoparticle morphology compared to the aggregated larger particles constituting the material prepared without scCO₂. A catalytic test in the presence of a radical scavenger proved that the reaction follows a radical pathway.