Reactive pore expansion during ammonia vapor post-treatment of ordered mesoporous silica prepared with mixed glucopyranoside and cationic surfactants

Abstract

n-Octyl-β- d-glucopyranoside (C 8G 1) has the potential to serve as a functional, nonionic template for mesoporous oxides, but its use has been limited due to its propensity to form lamellar mesostructures. Here, 2D hexagonal-structured mesoporous silicas with variable pore sizes are synthesized with C 8G 1 by the “nanocasting” technique (polycondensation of tetramethoxysilane in concentrated acidic solutions with evaporation) by mixing it with cetyltrimethylammonium bromide (CTAB) to promote ordering. In an effort to stabilize the silica network structure, a mild treatment with ammonia vapor at 50 °C is introduced. Without ammonia treatment, the pore diameter remains constant as the C 8G 1:CTAB weight ratio increases even up to 0.3:1. When the as-made materials are treated with ammonia vapor, the degree of long-range order of the samples increases, and pore expansion is observed which increases in magnitude as the C 8G 1 content increases. Large degrees of expansion increase pore size fluctuations but do not eliminate long-range order. Materials with well-ordered, uniform pores can be made with C 8G 1/CTAB mixtures by limiting the amount of ammonia or increasing the amount of silica precursor. A mechanistic study suggests that micelle swelling occurs with CTAB, but is limited by increased polycondensation of silica and favorable headgroup–silica interactions. With the glucopyranoside surfactant, the extent of expansion is greater not only because of a change in the physical interaction with silica, but also because the Maillard reaction occurs under surprisingly mild conditions.