Synthesis and characterization of ordered, very large pore MSU-H silicas assembled from water-soluble silicates

Two-dimensional, hexagonally ordered silicas with large uniform mesopores (7.6-11.9 nm) have been assembled through a nonionic supramolecular assembly pathway using sodium silicate as a silica source and the triblock copolymer Pluronic P123 (EO20PO70EO20) as a structure-directing agent. An increase in the synthesis temperature from 308 to 333 K in a one-step procedure led to a systematic increase in the unit-cell size, pore diameter, specific surface area, and pore volume, and to a decrease in the pore wall thickness. The resulting materials exhibited adsorption properties highly similar to those of SBA-15 silica assembled through an electrostatic pathway under strongly acidic conditions, indicating that the framework structure of MSU-H is analogous to that of SBA-15 and consists of ordered large pores connected by micropores in the pore walls. When the one-step synthesis procedure was followed by a post-assembly hydrothermal treatment at 373 K, the resultant MSU-H silicas exhibited framework pores enlarged by 1.7-2.8 mn and substantially increased secondary (textural) porosity, a feature that is not characteristic of SBA-15. These results demonstrated a wide range of possibilities in tailoring the structures of silicas synthesized using low-cost and convenient reagents. In addition, gas adsorption data for the MSU-H silicas allowed us to examine the accuracy of a recently proposed procedure for calculation of the pore size distributions, calibrated using MCM-41 silicas and extrapolated over larger pore sizes. It was found that this procedure overestimates pore diameters for MSU-H silicas, which might be related to the inaccuracy of the aforementioned extrapolation, or to deviations of the MSU-H pore shape from uniform cylindrical pores with the length much larger than the pore diameter.