Synthesis, characterization, and applications of porous and hierarchically-porous silica nanostructures

Abstract

Silicate nanostructures can be broadly defined as any material primarily composed of silicon dioxide and having one or more dimensions smaller than 100 nm. Silica is formed of SiO4 tetrahedra connected at their vertices, and the way in which these tetrahedra can be arranged leads to materials classified as amorphous or crystalline, depending on the degree of long-range order in the structure. Due to the complexity of tetrahedral connectivity that is possible, pores can be formed in silicas with length scales ranging from a few angstroms to tens of nanometers. Some microporous silicates exist in nature, but many other porous silicas of considerable importance to chemical engineering are synthetic. One important class of these synthetic porous silicates is the zeolites, which contain pores on the size of angstroms and therefore can act as molecular sieves. In this dissertation, methods for the synthesis and characterization of "zero-dimensional" silica nanoparticles, "two-dimensional" zeolite nanosheets, and "three-dimensional" mesoporous silicas and zeolites are presented. Applications for these materials in catalytic and adsorption processes are also explored. Many of these nanostructured silicates contain hierarchical pore structure with different characteristic pore sizes existing in the materials. One particularly studied material, the self-pillared pentasil (SPP) zeolite, contains both the microporosity of traditional zeolites and mesoporosity resulting from its crystal growth mechanism. Hierarchical pore networks can significantly improve intraparticle mass transfer for interacting chemical species, offering great performance gain in the considered applications.