Preparation of porous silica from chlorite by selective acid leaching

Porous silica was prepared by selective leaching of chlorite and the influence on the porous properties of the product of the acid type and acid concentration, solution temperature and leaching time were investigated. After leaching, only SiO₂ remained, the other components being selectively leached from the sample in the order MgO < Fe₂O₃ < Al₂O₃. The specific surface area of the product obtained by H₂SO₄-leaching was greater than with HNO₃- and HCl-leaching. The highest specific surface area (394 m²/g) was obtained by leaching the chlorite with 5 N H₂SO₄ at 90 °C for 1.5 h. The pore size distribution of the products showed a maximum at about 5 nm with a pore size much larger than would be expected for selective leaching of the octahedral chlorite sheets. The ²⁹Si MAS NMR spectra of the products showed a stronger Q⁴ peak (related to a framework structure unit) than the Q³ peak (from a layer structure unit), suggesting the presence of a framework structure similar to silica gel rather than the original layered structure. The resultant structure of the silica product results in a lower specific surface area than found in leached vermiculite and phlogopite.