Facile methods for the preparation of micro- and mesoporous amorphous silica from rice husk

Silica was obtained from rice husk (RH) by five different methods. (1) Burning RH as received in muffle at 850 °C for 2 h, (2) RH was firstly burned at 450 °C to reduce its volume. The obtained ash was refluxed with 1N HCl; after washing and drying, ash was burned at 700 °C for 3 h. (3) Un-grinded (or grinded (method 4)) RH was firstly soaked in water, treated with 1N HCl, and burned at 700 °C. (5) To utilize cellulosic fiber, RH was autoclaved with 15% NaOH, and silica was precipitated from the residual liquor. Silica hydrogel was prepared from the obtained ashes by reflux with 2.5 N NaOH followed by neutralization with 1N HCl to form silica gel. The silica gel was filtered, washed, and freeze-dried. XRF analysis confirmed the excellent removal of metal oxide impurities from RHs by acid leaching treatment. Silica precipitated from black liquor shows the most homogeneous particle size. Mesoporous silica with purity over 99% and with a homogeneous particle of average size 41.3–71.3 nm was synthesized using CTAB surfactant as the structure-directing template and RH ash as the silica source.