Preparation of silica-supported nanoporous manganese oxides

Abstract

Manganese oxide micro-crystals were bound onto silica to prepare a composite material with a particle size large enough to be used in a flow-through fixed bed for metal uptake from hydrometallurgical solutions. Commercial β-MnO 2 was used for screening of the optimal composite preparation conditions like pH, gelation temperature, and the SiO 2/MnO 2 ratio. The target composites were made using synthetic nanoporous manganese oxides OL-1 (birnessite) and OMS-1 (todorokite). The resulting silica-supported materials were characterized using scanning electron microscopy, N 2 adsorption measurements, particle size measurements as well as Raman spectroscopy and IR spectroscopy. Proton uptake kinetics of OL-1/SiO 2 and OMS-1/SiO 2 composites was compared with that of the unsupported analogues. Results show that the MnO x /SiO 2 ratio affects significantly the properties of the composite. The higher is the ratio, the lower are the specific surface area, the pore volume and the pore size. On the other hand, higher amount of the silica binder gives composites of better mechanical properties. Thus, a compromise between the mechanical strength and porosity has to be chosen. OL-1 and OMS-1 can be aggregated successfully with colloidal silica at pH 4 and with MnO 2/SiO 2 weight ratio of 0.7. The best gelation and drying temperature was 110 °C and sufficiently strong composites were obtained by additional heat-treatment at 250 °C for 2 h. Determination of the proton sorption kinetics in nitrate media revealed that aggregation of OMS -1 with colloidal silica does not affect significantly the uptake kinetics. However, in the case of OL-1, the aggregation with silica has a negative effect on the uptake rate. The difference is explained by the structure of the composites.