Synthesis of Ultralarge-Pore FDU-12 Silica with Face-Centered Cubic Structure

Ultralarge-pore FDU-12 (ULP-FDU-12) silicas with face-centered cubic structures (Fm3m symmetry) of spherical mesopores were synthesized at low initial temperature (∼14 °C) using commercially available PEO-PPO-PEO triblock copolymer Pluronic F127 as a micellar template and xylene as a micelle expander. Xylene was selected on the basis of its predicted higher swelling ability for the Pluronic surfactant micelles in comparison to 1,3,5-trimethylbenzene that was used previously to obtain large-pore FDU-12. The optimization of the synthesis conditions afforded as-synthesized ULP-FDU-12 materials with unit-cell parameters up to 56 nm, which is comparable to the highest reported values for Fm3m structures templated by custom-made surfactants. Calcined silicas were obtained with unit-cell parameters up to 53 nm and pore diameters up to ∼36 nm (for N(2) adsorption at 77 K, the capillary condensation relative pressure was up to 0.938). The preferred silica source was tetraethylorthosilicate, but tetramethylorthosilicate was also found suitable. The pore diameter was dependent on the unit-cell size of the as-synthesized material, but was further tuned by adjusting the time and temperature of the treatment in the HCl solution. If the synthesis was performed at low temperature only, highly ordered closed-pore silicas were obtained at calcination temperatures as low as 450 °C. On the other hand, the hydrothermal treatments, including the acid treatment at 130 °C, afforded silicas with large pore entrance sizes. The present synthesis constitutes a major advancement in the synthesis of ordered silicas with very large open and closed spherical mesopores.