Brian T. Holland

Aluminum-containing mesostructural materials

This article reviews syntheses of mesoporous aluminosilicates and aluminum oxides based on surfactant templating methods. The incorporation of aluminum in the silicate frameworks generates acid sites and ion-exchange sites. Both, tetrahedral framework aluminum and octahedral extraframework aluminum can be present, depending on the aluminum precursor used. The aluminum-containing structures tend to be less ordered than their purely siliccous analogs. Dealumination plays a significant role during template removal. Other methods for the synthesis of mesoporous aluminum-containing sieves are based on the structural transformation of kanemite, and on cluster precursors which may be connected by self-condensation or by condensation with silicate bridges. Purely aluminous mesostructures can be prepared with neutral templates or by condensing Keggin-like aluminum clusters in an ordered salt with an anionic surfactant.

Determination of Both Mesopores and Macropores in Three-Dimensional Ordered Porous Materials by Nitrogen Adsorption

Three-dimensionally ordered silica structures containing both mesopores and macropores are created using polystyrene coacervate spheres with a diameter of ca. 146 nm. The close-packed polystyrene coacervate spheres are intercalated with tetraethyl orthosilicate. The spheres are removed by calcination leaving an inverse silica replica with a spherical macropore cavity diameter of ∼110 nm. Due to the nature of these porous structures, pores leading into the macropore cavity are in the mesopore regime, ∼40 nm in diameter. The nitrogen adsorption data described in the following paper gives a pore size for both the macropore cavity and the mesopore openings leading into the cavity. The pore sizes as determined by nitrogen sorption are in good agreement with the pore sizes observed by scanning electron microscopy. Mercury intrusion porosimetry results confirm the size of the mesopore openings leading into the macropore cavity, however due to destruction of the sample upon intrusion, extrusion results can not be obtained to determine main cavity diameters. As a result, nitrogen sorption may be a viable option for determining pore sizes with these three-dimensionally ordered materials containing both mesopores and macropores.

Synthesis of highly ordered macroporous minerals: Extension of the synthetic method to other metal oxides and organic-inorganic composites

The facile synthesis of three-dimensional macroporous arrays of titania, zirconia and alumina was recently reported [1]. The synthesis of these materials has now been extended to the oxides of iron, tungsten, and antimony, as well as a mixed yttrium-zirconium system and organically modi- fied silicates. These materials were characterized by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray energy dispersive spectrometry (EDS), and powder X-ray diffraction (XRD). Ordered structures of iron, tungsten, and antimony were formed from alkoxide precursors as in the originally reported synthesis, but the template was removed at a lower temperature. Samples of vinyl- and 2-cyanoethyl-modified silicates were formed from a mixture of organotrialkoxysilane and tetraalkoxysilane precursors; the polystyrene template was removed by extraction with a THF/acetone mixture. These results show the ease of extending the original syn- thesis to a wide range of systems. Also, the ability to form homogenous mixed-metal oxides will be important for tailoring the dielectric and photonic properties of these materials.

Transformation of layered polyoxometallate cluster salts into mesoporous materials

A new approach to the formation of mesoporous materials has been developed, based on a two-step salt-gel synthesis, in which mesoporous alumino-phosphates, galloaluminophosphates, and aluminosilicates have been created. The first step involves pre-organizing charged inorganic clusters (MO 4 Al 12 (OH) 24 (H 2 O) 12 7+ , M = Al or Ga) into a layered mesoscopic material with oppositely charged organic surfactant molecules. In the second step, phosphate or silicate linker molecules are added, which diffuse through the cluster/surfactant salt, react with the clusters, and transform the layered precursor into a non-lamellar mesostructured material. Removal of the surfactant from the alumino- and galloaluminophosphates by anion-exchange and from the aluminosilicates by calcination results in mesoporous materials with BET surface areas up to 630, 455, and 431 m 2 /g, respectively. Direct condensation by calcination of polyoxoaluminate cluster salts without additional linkers produces nanometer-sized one-dimensional strings.