Synthesis and characterization of mesoporous silica from selectively acid-treated saponite as the precursors

Mesoporous silicas were synthesized by hydrothermal treatment of selectively acid-treated saponite (an ideal structural formula: Na_1/3Mg₃(Si_11/3Al_1/3)O₁₀(OH)₂), having a 2:1 type layered structure as the silica source and its porous properties were examined and compared with that from kaolinite (an ideal structural formula: Al₂Si₂O₅(OH)₄), having a 1:1 type layered structure. Synthetic saponite was selectively leached in H₂SO₄ solutions with various concentrations (0.05-1 M) at 70 °C for 0.5 h. The resulting products (precursors) were mixed with cetyltrimethylammonium bromide (CTABr), NaOH and H₂O, hydrothermally treated at 110 °C and removed the CTABr by calcining at 560 °C. A hexagonal mesoporous phase was obtained with higher Si/(Al(+Mg)) ratios of the resulting precursors. The XRD patterns of these products show the peaks assigned by a hexagonal lattice with a₀ = 4.0 - 4.6   nm and the crystallinity becomes higher with higher Si/(Al(+Mg)) ratios of the precursors. The specific surface area (S_BET) values of the present mesoporous silicas range from 800 to 1100 m²/g at CTABr/precursor = 0.1 and although they are not as high as those from precursors prepared from calcining and acid-treatment of kaolinite (1420 m²/g), they are increased to 1400-1500 m²/g by increasing the ratio CTABr/precursor ≥0.2. The reason for the difference in the optimum preparation conditions between saponite and kaolinite may be attributed to the difference in the linkage of the SiO₄ tetrahedra in these precursors (i.e. layered or framework structures), which result in great differences in the selective leaching rates and structures of the resulting silica-rich products.