Precursor nanoparticles that form spontaneously on hydrolysis of tetraethylorthosilicate in aqueous solutions of tetrapropylammonium (TPA) hydroxide evolve to TPA-silicalite-1, a molecular-sieve crystal that serves as a model for the self-assembly of porous inorganic materials in the presence of organic structure-directing agents. The structure and role of these nanoparticles are of practical significance for the fabrication of hierarchically ordered porous materials and molecular-sieve films, but still remain elusive. Here we show experimental findings of nanoparticle and crystal evolution during room-temperature ageing of the aqueous suspensions that suggest growth by aggregation of nanoparticles. A kinetic mechanism suggests that the precursor nanoparticle population is distributed, and that the 5-nm building units contributing most to aggregation only exist as an intermediate small fraction. The proposed oriented-aggregation mechanism should lead to strategies for isolating or enhancing the concentration of crystal-like nanoparticles.

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Precursor silica nanoparticles can evolve to silicalite-1 crystals under hydrothermal conditions in the presence of tetrapropylammonium (TPA) cations. It has been proposed that in relatively dilute sols of silica, TPA, water, and ethanol, silicalite-1 growth is preceded by precursor nanoparticle evolution and then occurs by oriented aggregation. Here, we present a study of silicalite-1 crystallization in more concentrated mixtures and propose that growth follows a path similar to that taken in the dilute system. Small-angle X-ray scattering (SAXS), cryogenic transmission electron microscopy (cryo-TEM), and high-resolution transmission electron microscopy (HRTEM) were used to measure nanoparticle size and to monitor zeolite nucleation and early-stage crystal development. The precursor silica nanoparticles, present in the clear sols prior to crystal formation, were characterized using two SAXS instruments, and the influence of interparticle interactions is discussed. In addition, SAXS was used to detect the onset of secondary particle formation, and HRTEM was used to characterize their structure and morphology. Cryo-TEM allowed for in situ visual observation of the nanoparticle population. Combined results are consistent with growth by aggregation of silica nanoparticles and of the larger secondary crystallites. Finally, a unique intergrowth structure that was formed during the more advanced growth stages is reported, lending additional support for the proposal of aggregative growth.

Silica nanoparticles with a narrow particle size distribution and controlled diameters of 10-20 nm are synthesized via hydrolysis and hydrothermal aging of tetraethylorthosilicate in an aqueous L-lysine solution. Cryo-transmission electron microscopy (cryo-TEM) reveals that the silica nanoparticles assemble to form close-packed nanoparticle crystals over short length scales on carbon-coated grids. Evaporative drying of the same sols results in nanoparticle stability and remarkable long-range facile ordering of the silica nanoparticles over scales greater than 10 microm. Whereas small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) discount the possibility of a core (silica)-shell (lysine) structure, the possibility remains for lysine occlusion within the silica nanoparticles and concomitant hydrogen bonding effects driving self-assembly. Facile ordering of the silica nanoparticles into multilayer and monolayer coatings over square-centimeter areas by evaporation-induced self-assembly is demonstrated using a novel dip-coating device.

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A series of small-pore zeolites are synthesized and investigated as catalysts for the methanol-to-olefins (MTO) reaction. Small-pore zeolites SSZ-13, SSZ-16, SSZ-27, SSZ-28, SSZ-52, SSZ-98, SSZ-99, SSZ-104, SSZ-105 and an ITQ-3-type material are synthesized, and the results from their use as catalytic materials in the MTO reaction compared to those obtained from SAPO-34. The production of propane that tends to correlate with catalytic material lifetime (higher initial propane yields lead to shorter lifetimes) declines with increasing Si/Al (as has been observed previously for SSZ-13), and a larger cage dimension leads to higher propane yields at a fixed Si/Al. Data from these materials and others reported previously, for example, SSZ-39 and Rho, that were tested at the same reaction conditions, revealed four different patterns of light olefin selectivities: 1) ethylene greater than propylene with low butene, for example, SSZ-17, SSZ-98, SSZ-105, 2) ethylene equal to propylene and low butene, for example, SAPO-34, SSZ-13, SSZ-16, SSZ-27, SSZ-52, SSZ-99, SSZ-104, 3) propylene greater than ethylene with butene similar to ethylene, for example, SSZ-28, SSZ-39, and 4) ethylene equal to propylene equal to butene, for example, Rho. No clear relationships between zeolite cage architecture and light olefin selectivity emerged from this investigation, although several trends are presented as suggestions for further study.

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Facile synthesis of nanometer-sized germania crystals and amorphous germania nanoparticles (ca. 1 nm) is investigated through hydrolysis of germanium tetraethoxide and subsequent condensation of germania in both pure water and aqueous lysine solutions. Germanium tetraethoxide rapidly hydrolyzes in pure water, leading to solvated germanate species at lower germania concentrations and the onset of nanometer-sized germania crystals at room temperature with increasing germania content. In the presence of the basic amino acid L-lysine, amorphous germania nanoparticles (ca. 1 nm) spontaneously form with increasing germania content and coexist with nanometer-sized germania crystals at higher germania concentrations. Lysine and germania concentration both influence crystallite size and morphology (i.e., polyhedral, cubic). The facile, room-temperature crystallization of germania in the presence and absence of lysine is striking. The fact that the crystal morphology shows no signs of nanoparticle aggregative assembly, as has been observed in the formation of other oxide crystals, suggests that crystal growth takes place by addition of dissolved species rather than nanoparticles, and could have implications for other oxide systems.