Transition metal promoted combustion of rice husk and rice straw towards an energy optimized synthesis of biogenic silica

Abstract

The classical approach for the synthesis of catalysts based on biogenic silica from rice husk or rice straw involves a high temperature (600 °C) calcination step to remove the organic matrix from the silica backbone. The remaining silica can then be impregnated with a variety of active components in the form of Mn-, Fe- and Ce-precursors, followed by subsequent thermal treatment to form the corresponding metal oxides. In this work, we introduce a novel energy optimized route for the preparation of biogenic silica supported metal oxides. The proposed synthesis procedure aims towards the reduction of pyrolysis steps by infiltrating metal salts into the biomass lignocellulosic matrix. The modified rice husk or rice straw can then be calcined to directly yield the desired metal oxide supported on biogenic silica. To evaluate the described method, as model system manganese oxide modified rice husk silica was compared to a set of materials synthesized via classical calcination-impregnation by means of textural (N2-Sorption, SEM/EDX), structural (XRD), thermal (TG/DTA) and elemental (ICP-OES; XRF) analysis. Results indicate that the introduction of transition metals into biomass powder promotes the combustion of the organic matrix in terms of temperature requirements for its complete removal. In addition, catalytic tests of the materials obtained by the novel synthesis route clearly show similar or even improved performance as compared to catalysts prepared via impregnation method.