Assembly of Mesoporous Lamellar Silicas with Hierarchical Particle Architectures

Abstract

A family of silica molecular sieves with lamellar frameworks and hierarchical structure (denoted MSU-V) was assembled from homogeneous solutions of neutral H 2 N(CH 2 ) n NH 2 bolaamphiphiles ( n = 12−22) as the structure directors and tetraethylorthosilicate as the inorganic precursor. Optimal lamellar order was observed for four as-synthesized and calcined (550 °C) mesostructures when they were assembled at the following reaction temperatures ( T, °C) and surfactant/silica ratios ( R ) of T = 25 °C, R = 0.26 (C 12 ); T = 45 °C, R = 0.20 (C 16 ); T = 55 °C, R = 0.15 (C 18 ); T = 55 °C, R = 0.11 (C 22 ). MSU-V silica assembled from the short C 12 alkyl chain bolaamphiphile exhibited gallery-confined micropores (1.3 nm), but the derivatives prepared from the longer C 16 −C 22 diamines showed gallery-confined mesopores . As the alkyl chain length of the bolaamphiphile surfactant increased from C 16 to C 22, the size of the gallery-confined mesopores increased from 2.0 to 2.7 nm. The described synthetic strategy afforded hierarchical MSU-V structures with biomimetic multilamellar vesicular particle architectures. Distorted vesicular, plate, and spiral-ribbon shaped particles also were observed, particularly for the lamellar silica assembled from the C 16 bolaamphiphile. The variation in morphologies was attributed to agitation effects during synthesis. MSU-V silica assembled from the C 22 diamine was the least stable member of the series and partially decomposed upon calcination. Remarkably, surfactant removal by calcination did not affect the biomembrane-like, hollow disk morphology of the original as-synthesized C 22 product. This bolaamphiphile assembly approach provides new opportunities for the preparation of lamellar mesoporous molecular sieves with hierarchical structures specifically tailored to applications as diverse as sorbents, catalysts, sensors, bone implants, and nanoscale devices.