The preparation of pure siliceous mesoporous materials with an atomic-scale crystalline wall is an illusive task even though the mesoporous material researches flourish recently because of their various intriguing applications. Periodic mesoporous organosilica (PMO) materials are nanoporous hybrid materials containing organic and inorganic moieties on the pore wall simultaneously. Single crystal-like PMO consisted of phenyl (–C6H4–) moieties was firstly reported by Inagaki et al. However, it was claimed that very strict reaction conditions such as an alkaline condition and a narrow range of molar ratio of the reactants were mandatory to produce single crystal-like phenyl-PMO materials. This rather strict synthetic condition hampers further studies to control the particle morphology and size variation of the PMO materials with intrinsically hydrophobic interior because of the constituent phenyl moieties on the wall. For example, most phenyl-PMO derivatives were prepared via quite similar reaction conditions originally proposed by Inagaki et al. Therefore, we have been interested in finding a reliable synthetic method to fine-tune their particle size and shape suitable for various advanced applications such as controlled drug delivery system, host materials for hydrophobic biomolecule encapsulation, and robust catalyst supports. We speculated that the π...π stacking interaction between two adjacent benzene moieties of different 1,4-bis(triethoxysilyl)benzene (BTEB) precursors might be strong enough to induce the single crystalline wall structure in a variety of reaction conditions unlike the original report. Therefore, we investigated various synthetic conditions such as different reactant molar ratios and precursor concentrations to prepare phenyl-PMOs having different particle size and morphology with an aforementioned single crystalline nature of the pore wall. We were particularly interested in the size control of the particles without perturbing the crystallinity of the pore wall. Herein, we describe a preliminary result of the preparation of size-controlled phenyl-PMO material with molecularly ordered pore walls. Our attempts closely relate to the literature method of preparing shape-controlled organic-functionalized micronsize MCM-41 type of hybrid silica materials. For example, an optimized reactant molar ratio is C18TMABr:BTEB: NaOH:H2O = 1.0:1.2:6.1:5644 where C18TMABr is an octadecyltrimethylammonium bromide, CH3(CH2)17N(CH3)3Br. The low relative molar ratio of C18TMABr to water was crucial to obtain small micron-size particles. The C18TMA templates of the as-made material were easily removed by an acid extraction in ethanol solution at 60 C. TEM images of the template-removed samples are shown in Figure 1. Under the low C18TMABr concentration, the products were formed as micron-size particles (average length or diameter < 3 μm) with the parallel pore direction relative to the longitudinal axis of the particle as shown in Figure 1. The mesopore orientation of the previously reported phenylPMO materials normally exhibited a perpendicular pore direction relative to the flat surface of the thin flake. In our case, however, the low surfactant concentration as well as a short period of stirring after the addition of BTEB precursors might prevent surfactant-organosilica micelles from aggregating to form large flakes. As a result, small particles with a parallel pore orientation as well as a crystal-like wall structure have been successfully obtained. Interestingly, individual nanotube-like uniform mesochannels were clearly discerned from the TEM images of Figures 1(b) and 1(c). SEM images of Figure 1 indicate the shape of particles and their size distribution. The powder XRD pattern of the template-free material, top trace of Figure 2, exhibited higher angle diffraction peaks at 2θ = 11.70, 23.36, and 35.48 which correspond to
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May 5, 2011
Makoto Kohga 
Open Access