Degradation of imidazolium-based ionic liquids by catalytic wet peroxide oxidation with carbon and magnetic iron catalysts

Abstract

BACKCGROUND The ‘green’ image of ionic liquids (ILs) has changed in the last few years since numerous works have evidenced their non-biodegradability, persistence and high ecotoxicity, particularly for the most common imidazolium-based ILs. In this work, the feasibility of catalytic wet peroxide oxidation for the degradation of imidazolium-based ILs of different alkyl chain lengths has been studied under selected operating conditions (1000 mg L−1 IL, stoichiometric H2O2 dose, 2 g L−1 catalyst, pH 3 and 90 °C temperature) using different catalysts such as magnetic iron (Fe3O4) supported on γ-Al2O3 and activated carbon (AC) as well as bare carbon materials (graphite, AC). The catalytic activity and stability and the efficiency of H2O2 consumption have been evaluated. RESULTS Although both AC-based catalysts led to the conversion of the IL, they yielded a low H2O2 consumption efficiency (24% and 45% with AC and Fe3O4/AC, respectively) due to the fast decomposition of H2O2 and the recombination of radical species into H2O and O2, non-reactive species under the operating conditions. In contrast, graphite and Fe3O4/γ-Al2O3 showed high activity allowing complete conversion and relatively high mineralization degrees of all the ILs tested in 1 h reaction time. Among those catalysts, Fe3O4/γ-Al2O3 exhibited a considerably greater stability upon four successive uses where iron leaching was negligible and the magnetic properties were maintained. CONCLUSIONS Catalytic wet peroxide oxidation has proved to be an interesting alternative for the treatment of imidazolium-based ILs in water. The application of Fe3O4/γ-Al2O3 is particularly promising due to its high activity, remarkable stability and easy magnetic recovery. © 2016 Society of Chemical Industry