No abstract

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