Preparation of carbonized rice husk monoliths and modification of the porous structure by SiO2 leaching

The possibility of preparing honeycomb monoliths from unconventional materials with high adsorption capacity, such as carbonized rice husk (CRH), was studied. Two schemes including modification of the porous structure by silica leaching were suggested for preparation of the monoliths: (1) preparation of CRH monoliths with the addition of Ca-montmorillonite (Ca-M) followed by leaching and (2) preliminary leaching of initial components followed by monolith extrusion. Chemical and textural properties of the materials were studied at each preparation step. The effect of temperature of rice husk (RH) pyrolysis and conditions of leaching treatment on the physico-chemical properties of CRH was investigated. By the data of nitrogen adsorption and SEM characterization, CRH has low specific surface area, pore volume less than 0.1 cm3/g, and composed mainly of macropore channels 5–10 μm in diameter, micropores less than 17 Å in size, and a minor amount of mesopores. Treatment of the carbonized product with a 12% solution of potassium hydroxide at 80–100 °C leads to a 90–95% leaching of SiO2 from the CRH composite and increases the specific surface area to 400 m2/g and pore volume to 0.4 cm3/g due to formation of mesoporous structure. The feasibility of CRH honeycomb monoliths preparation and applicability of silica leaching from the monolith material for modification of the porous structure are demonstrated. The resulting monoliths are characterized by specific surface area exceeding 300 m2/g, pore volume 0.3 cm3/g, and high mechanical strength.