Synthesis of Ultra-Large-Pore SBA-15 Silica with Two-Dimensional Hexagonal Structure Using Triisopropylbenzene As Micelle Expander

It is proposed herein that in order to obtain ultralarge-pore ordered mesoporous silicas using surfactant-templated synthesis with micelle expanders, one should select a micelle swelling agent with a moderate swelling ability to achieve an appreciable pore diameter enlargement while avoiding the formation of heterogeneous and/or poorly defined nanostructure. It is suggested to identify viable swelling agents based on the extent of solubilization of swelling agents in micellar solutions. On the basis of this reasoning, cyclohexane, 1,3,5-triethylbenzene and 1,3,5-triisopropylbenzene (TIPB) were selected for the synthesis of large-pore SBA-15 silicas with two-dimensional (2-D) hexagonal structures of cylindrical mesopores. SBA-15 with pore diameter tunable from 10 to 26 nm was obtained at initial synthesis temperature 12.25-20 degrees C using Pluronic P123 triblock copolymer as a micellar template and triisopropylbenzene as a micelle expander. Structures of the materials were characterized using small-angle X-ray scattering, TEM, and gas adsorption. The lowering of the initial synthesis temperature with adjustment of the amount of TIPB afforded pore diameters up to 26 nm (BJH pore diameters up to 34 nm) and (100) interplanar spacings up to 26 nm for 2-D hexagonal structure. As the initial synthesis temperature was lowered further, the pore diameter increased further (to similar to 50 nm) with appreciable retention of cylindrical pore shape, but the pore structure became heterogeneous. The present approach makes silicas with 2D hexagonally ordered cylindrical pores of diameter up to 26 nm readily available and opens new opportunities in the synthesis of materials with other pore geometries and framework types.