Novelties of azobenzene synthesis via selective hydrogenation of nitrobenzene over nano-fibrous Ag-OMS-2 – mechanism and kinetics

Abstract

Nitrobenzene hydrogenation to aniline is a well known process. However, azobenzene which is an intermediate product formed during nitrobenzene hydrogenation, is commercially equally attractive and its production by hydrogenation is challenging. Reported literature shows azobenzene as an intermediate product. Most of the commercially operated heterogeneous processes lead to complete conversion of nitrobenzene to aniline. Many azobenzene derivatives are synthesized by oxidation of aniline. Thus selective hydrogenation of nitrobenzene to azobenzene in a single step process would be very attractive. In the current work, silver substituted octahedral molecular sieve of cryptomelane type (Ag-OMS-2) was studied with different silver loading in the range of 5–15 w/w% in K-OMS-2. Under mild reaction conditions over a redox catalyst having weak acidic and basic sites, the hydrogenation of nitrobenzene can be stopped at the azobenzene stage. 15 w/w% Ag-OMS-2 showed the highest conversion and selectivity for azobenzene. Effects of various parameters such as catalyst loading, hydrogen pressure, nitrobenzene concentration and temperature have been studied. At 125 °C, azobenzene could be produced using 25 atm H₂ in isopropanol (IPA) with 80% selectivity in 3 h. A detail reaction mechanism is proposed and a suitable kinetic model derived. The apparent activation energy for nitrobenzene hydrogenation to azobenzene is 10.58 kcal/mol. Thus, kinetic parameters for synthesis of azobenzene from nitrobenzene hydrogenation in presence of heterogeneous catalyst are reported for the first time.