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: Na1/3Mg3(Si11/3Al1/3)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 nma0=4.0–4.6 nm and the crystallinity becomes higher with higher Si/(Al(+Mg)) ratios of the precursors. The specific surface area (SBETSBET) 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.

Mesoporous silicas prepared from selectively acid-treated saponite, Na1/3Mg3(Si11/3Al1/3)O10(OH)2 used as a silica source show high specific surface area up to about 1500 m2/g.Figure optionsDownload as PowerPoint slide