Siliceous Mesostructured Cellular Foams with Uniformly Sized and Shaped Pores
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sostructured siliceous cellular foams (mesocellular foams, MCFs) with homogeneous ultra-large mesopores are described. MCFs consist of uniform spherical cells 21-36 nm in diameter and possess surface areas up to 900 m2/g. Uniform windows, 7-18 nm in diameter, interconnect the cells to form a continuous 3-D pore system, which makes the MCFs attractive candidates for supports for catalysis and in separation and immobilization involving large molecules. They may be of interest in low-dielectric applications. The size of the cells can be controlled by the concentration of the added organic cosolvent. Adding small amounts of NH 4F selectively enlarges the windows. We propose that the MCFs are templated by oil-inwater microemulsion droplets. The large-pore MCF materials resemble aerogels, with the benefit of a facilitated synthesis procedure in combination with well-defined pores and wall structure. INTRODUCTION Large-pore molecular sieves with uniform pores are desirable materials offering a variety of possible applications such as hosts for chemical reactions, use in separation, immobilization or encapsulation involving large molecules [I], as low-dielectric materials [2], etc. Mesoporous materials with pore sizes from 20 to 500 A are attractive in this respect, and thus interest in this area continues unabated. Most procedures for preparing ordered mesoporous materials use supramolecular templates that are formed by self-aggregating properties of various kinds of surfactants [3]. Colloidal templating routes that employ
metastable emulsions [4] or polymer latex spheres [5] as templates have enabled the preparation of ordered macroporous and large-pore mesoporous materials. Block copolymerbased supramolecular templates have also been used for this purpose [6]. By changing the reaction temperature or by adding organic cosolvents to adjust the hydrophobic volumes of the supramolecular templates, the pore sizes of mesoporous materials have been controlled [6a,b, 7]. Here we describe mesostructured cellular foams (MCFs) that are proposed to be templated by microemulsions [8]. MCFs are composed of uniformly sized spherical cells, which are interconnected by uniform windows to give 3-D continuous pore systems. This new class of large-pore mesoporous materials is promising as supports for catalysts, for separation and immobilization involving large molecular moieties, and for low-dielectric applications. EXPERIMENT The MCFs were prepared in aqueous hydrochloric acid by using dilute solutions of the nonionic surfactant Pluronic• P123 (poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly(ethylene oxide), E0 20-PO 70 -E0 20, Maj=5800) in the presence of 1,3,5-trimethylbenzene (TMB) as organic cosolvent. In a typical preparation, 2.0 g of P123 (0.4 mmol) were dissolved in 75 mL of 1.6 M HCI (120 mmol HC1) at room temperature while stirring. After complete dissolution of the polymer, 23 mg of NH 4F (0.6 mmol) - if desired - and 2.0 g of 1,3,5241
Mat. Res. Soc. Symp. Proc. Vol. 576 ©1999 Materials Research Society
trimethylb
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