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Siliceous mesostructured cellular foams (MCFs) with well-defined ultralarge mesopores and hydrothermally robust frameworks are described. The MCFs are templated by oil-inwater microemulsions and are characterized by small-angle X-ray scattering, nitrogen sorption, transmission electron microscopy, scanning electron microscopy, thermogravimetry, and differential thermal analysis. The MCFs consist of uniform spherical cells measuring 24-42 nm in diameter, possess BET surface areas up to 1000 m 2 /g and porosities of 80-84%, and give, because of their pores with small size distributions, higher-order scattering peaks even in the absence of long-range order. Windows with diameters of 9-22 nm and narrow size distribution interconnect the cells. The pore size can be controlled by adjusting the amount of the organic swelling agent that is added and by varying the aging temperature. Adding ammonium fluoride selectively enlarges the windows by 50-80%. In addition, the windows can be enlarged by postsynthesis treatment in hot water. The MCF materials resemble aerogels, but offer the benefits of a facilitated synthesis in combination with welldefined pore and wall structure, thick walls, and high hydrothermal stability. The open system of large pores give MCFs unique advantages as catalyst supports and separation media for processes involving large molecules, and the high porosities make them of interest for electrical and thermal insulation applications.

The diameters of primary mesopores in materials with well-defined cylindrical pores can be accurately determined by the 4V/A method when the volume and surface area are determined by a standard adsorption method and 13.5 Å2 is used as the molecular area of adsorbed nitrogen. A simple method for determining standard adsorption using the statistical thickness of the adsorbed gas layer defined by Frenkel−Halsey−Hill (FHH) theory is described. For materials in which the pores are not cylindrical or well-defined, a simplified Broekhoff−de Boer method can be used to determine pore dimensions. The use of Hill's approximation for the thickness of the adsorbed gas layer in the Broekhoff−de Boer method simplifies its use. The results of these methods on small pore MCM-41 materials, large pore SBA-15 materials, and spherical pore mesocellular foams are reported.

No abstract

We have investigated the phase transition between two distinct mesoporous silicas:  SBA-15, comprising a hexagonally packed arrangement of cylindrical pores (6−12 nm in diameter), and mesocellular silica foams (MCF), consisting of spherical voids (22−42 nm in diameter) interconnected by “windows” of ∼10 nm. Both SBA-15 and MCF are formed using an amphiphilic triblock copolymer (Pluronic P123) as a template. The synthesis conditions for the two materials are identical, except substantial trimethylbenzene is added to form MCF. We find that the phase transition occurs at an oil−polymer mass ratio of 0.2−0.3. Although the pore structures and pore sizes change dramatically, the mean surface curvature of the system remains essentially the same throughout the transition.

The patterning of a surface using microcontact printing (μCP) generally employs a hydrophobic micropatterned stamp made from poly(dimethylsiloxane) (PDMS) to place ink molecules on a surface with spatial control. We present a simple procedure to hydrophilize PDMS stamps based on the O2 plasma oxidation of PDMS (referred to as PDMSox) and the grafting of poly(ethylene oxide) silanes (PEO−Si) to the oxidized surface. The wetting properties of a PDMSox surface derivatized with PEO having none, one, or two silanes and having chains with 7−70 EO units are inspected. All PDMSox surfaces treated with PEO−Si are hydrophilic and have advancing and receding contact angles of ∼40° and ∼30°, respectively. These surfaces remain hydrophilic for periods longer than 7 days, which saves having to hydrophilize stamps freshly prior to their usage. In particular, grafting a PEO having two triethoxysilane end groups and a molecular weight (MW) of 3400 g mol-1 enables inking and microcontact printing a polar Pd/Sn catalyst for electroless deposition (ELD) from a stamp to an amino-functionalized glass surface. The printed pattern of colloids has high accuracy and contrast, as reflected by the selective ELD of NiB in the printed regions of the glass. The same stamp can be reused for many cycles of inking and printing without degradation of the quality of the final NiB patterns. The hydrophilic layer provided by the grafted PEO molecules is, in some cases, not sufficiently thick to incorporate and print enough polar ink to form a complete monolayer of cysteamine, for example, onto printed Au substrates. Oxidizing a planar PDMS surface through a mask permits the patterning of PEO onto PDMSox. It then becomes possible to ink the stamp with proteins either by depositing proteins from solution onto the areas left underivatized with PEO or by printing proteins in the PEO-derivatized areas only. The proteins on the planar PDMS/PDMSox-PEO surface in turn are microcontact printed with high accuracy onto glass. This work may help expand μCP to applications in which it is desirable to use polar inks or proteins.

The acid-catalyzed synthesis of highly ordered mesostructured materials [1] has led to a variety of two-and three-dimensional periodic symmetries, and has proven to be an effective route for the generation of fibers, [2±5] spheres, [2,6] thin films, [7±12] and other monolithic forms [13±15] of mesoporous materials. [16,17] Zhao et al. [18,19] 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 [20±23] in the synthesis of mesoporous silica at neutral to basic pH has been described by Voegtlin et al. [24] Fluoride has been successfully used to extend the pH range over which anionic silica precursors can be utilized to create organized periodic structures; [24±26] it has been used to diminish framework defects in zeolites, [25] and to improve the organization in MCM-41 molecular sieves [27] and MSU-X materials.[28]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 EO 20 PO 70 EO 20 (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 3 /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 [17] and ii) understanding the underlying mechanism for the acid-catalyzed mesoporous silica synthesis.[29]Addition of small amounts of fluoride (F:Si molar ratios of 0.008 and 0.03; fluoride source: NH 4 F or (NH 4 ) 2 SiF 6 ) 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)

Small-angle neutron scattering (SANS) studies indicate that oil-in-water microemulsions, consisting of aqueous HCl, the nonionic block copolymer surfactant Pluronic P123 (poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide), EO20−PO70−EO20, M av = 5800), 1,3,5-trimethylbenzene (TMB, oil), and ethanol (cosurfactant), are novel colloidal templates that direct the synthesis of mesoporous silica with well-defined ultralarge pores. The sizes of the microemulsion droplets can be controlled by the TMB concentration and by temperature. The microemulsion droplet sizes and the cell sizes of the mesostructured cellular foam (MCF) materials increase linearly with the cube root of the TMB concentration. Increasing the temperature from 40 to 80 °C expands the droplet sizes, which is similar to micellar solutions of Pluronic surfactants in the absence of oil. Ethanol acts as a cosurfactant, increases the TMB solubility of the P123 micelles, and enables swelling of the P123 micelles. Low concentrations of NH4F (8 × 10-3 mol/L) show no significant effect upon the nature of the microemulsions. The polydispersities of the droplet sizes range from 11% to 21%. The microemulsion templates reported in this paper are considered as a valuable addition to existing colloidal templates that direct the synthesis of porous materials. The benefits of the microemulsion templates are (i) their easy preparation by simply mixing water, surfactant, oil, and a cosurfactant, and (ii) the synthesis of ultralarge-pore mesoporous materials with narrow pore size distributions without the need for further processing.