Fluoride-Induced Hierarchical Ordering of Mesoporous Silica in Aqueous Acid-Syntheses

Abstract

The acid-catalyzed synthesis of highly ordered mesostructured materials has led to a variety of twoand three-dimensional periodic symmetries, and has proven to be an effective route for the generation of fibers, spheres, thin films, and other monolithic forms of mesoporous materials. Zhao et al. recently used non-ionic poly(alkylene oxide) block copolymers, under acidic conditions, to prepare well-ordered, hexagonal mesoporous silica, denoted SBA-15, featuring uniform and adjustable large pore sizes combined with thick hydrothermally stable walls. The catalytic effect of fluoride in the synthesis of mesoporous silica at neutral to basic pH has been described by Voegtlin et al. Fluoride has been successfully used to extend the pH range over which anionic silica precursors can be utilized to create organized periodic structures; it has been used to diminish framework defects in zeolites, and to improve the organization in MCM-41 molecular sieves and MSU-X materials. However, to the best of our knowledge, no studies have been reported on the role of fluoride on cationic silica species in the aqueous acid-synthesis of ordered porous silica. In this communication, we describe the hierarchical ordering effects induced by small amounts of fluoride added during the synthesis of SBA-15-type mesoporous silica under acidic aqueous conditions. The non-ionic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer EO20PO70EO20 (Pluronic P123) has been employed as the structure-directing agent. At low pH, remarkably well-ordered, hydrothermally stable, large hexagonal mesoporous silica rods with uniform channels extending over micrometer-sized length scales, and with few defects, have been synthesized. The fluoride-induced enhancement of order has enabled the preparation of SBA-15 materials at moderate acidity (~ pH 2.5±3) without compromising the long-range hexagonal symmetry. The mesoporous silicas possess narrow pore-size distributions, hydrothermally stable frameworks, large surface areas, and pore volumes of up to 0.92 cm/g. This work has been motivated by our interest in i) the patterning of mesoporous materials ranging from mesoscopic to macroscopic length scales while retaining molecular-level structural control and ii) understanding the underlying mechanism for the acid-catalyzed mesoporous silica synthesis. Addition of small amounts of fluoride (F:Si molar ratios of 0.008 and 0.03; fluoride source: NH4F or (NH4)2SiF6) during the aqueous acidic SBA-15-type silica synthesis induces substantial ordering that is manifested on different length scales: both the mesoscopic structure and the macroscopic morphology of the mesoporous silicas are significantly improved. Scanning electron microscopy (SEM) has revealed that small amounts of fluoride bring about the formation of large mesoporous silica rods (Fig. 1a) when pre-