Synthesis of ordered and disordered silicas with uniform pores on the border between micropore and mesopore regions using short double-chain surfactants

Abstract

Silica molecular sieves with uniform pores on the borderline between micropore (diameter <2 nm) and mesopore (from 2 to 50 nm) ranges were synthesized by a novel method using judiciously chosen mixtures of short double-chain alkylammonium surfactants. These silicas were characterized using X-ray diffraction (XRD), thermogravimetry, and nitrogen and argon adsorption. The calcined materials exhibited either 2-dimensional (2-D) hexagonal or disordered structures with XRD interplanar spacing from 2.51 to 2.93 nm, including the value of as small as 2.69 nm for highly ordered 2-D hexagonal silica. The dependence of the pore size and surfactant content on the surfactant chain length provided strong evidence for supramolecular templating being operative in the formation of small-pore silicas, even for the surfactant chain length of six carbon atoms. Both hexagonally ordered and disordered calcined samples were shown to exhibit narrow pore size distributions with maxima in the range from 1.96 to 2.61 nm (reliably evaluated on the basis of the unit-cell dimension and pore volume for 2-D hexagonal materials, and calculated using a properly calibrated procedure), tailored by the surfactant chain length. The samples exhibited primary pore volumes from 0.28 to 0.54 cm(3) g(-1) and specific surface areas from 730 to 930 m(2) g(-1). Because of their small yet uniform pore size and large specific surface area, the silicas reported herein promise to be useful in applications in adsorption and catalysis. Adsorption studies of these materials provided a unique new insight into the pore-filling mechanism for small-pore materials. Moreover, the approach proposed herein is expected to facilitate the synthesis of not only small-pore silicas but also materials with other framework compositions, thus largely contributing to bridging the gap in attainable pore sizes between micropore and mesopore ranges.