Synthesis and Characterization of Microporous Silica Membranes Fabricated Through Pore Size Reduction of Mesoporous Silica Membranes Using Catalyzed Atomic Layer Deposition

This work presents the development of a new mesoporous silica membrane. A dip coating technique was employed to deposit a thin film on macroporous supports. The dipping solution consisted of a stable silica sol organized around surfactant micelles. The templated silica film was found to have a three dimensional pore structure with nitrogen gas permeance in the range of 10-6 mol m-2s-1Pa-1. This thesis introduces a novel type of microporous silica membrane. The surface limited chemical vapor deposition technique known as catalyzed atomic layer deposition was used to deposit monolayers of silica within the pores of mesoporous silica membranes. The use of pyridine as a catalyst enable the pore size reduction process to be performed much lower temperatures and much shorter exposure times than un-catalyzed atomic layer deposition. The pyridine catalyst was found to act a template for the final pore size of the membranes. Subsequent exposure cycles after the exclusion of pyridine failed to further reduce the pore diameter. The process thus proved to be a self-limiting pore size reduction. The process also proved to be effective in reducing viscous flow defects into the Knudsen diffusion regime without greatly decreasing the overall gas permeance of the membrane. Membranes produced by catalyzed atomic layer deposition were modified with two different aminopropyl silanes in order to enhance the carbon dioxide permeance through the membrane. An atomic layer deposition technique using ethylenediamine to promote the surface attachment of aminopropyldimethylethoxysilane to the pore surface was found to provide the greatest increase in carbon dioxide permeance

Supercritical Fluid Atomic Layer Deposition: Base-Catalyzed Deposition of SiO₂

An in situ FTIR thin film technique was used to study the sequential atomic layer deposition (ALD) reactions of SiCl₄, tetraethyl orthosilicate (TEOS) precursors, and water on nonporous silica powder using supercritical CO₂ (sc-CO₂) as the solvent. The IR work on nonporous powders was used to identify the reaction sequence for using a sc-CO₂-based ALD to tune the pore size of a mesoporous silica. The IR studies showed that only trace adsorption of SiCl₄ occurred on the silica, and this was due to the desiccating power of sc-CO₂ to remove the adsorbed water from the surface. This was overcome by employing a three-step reaction scheme involving a first step of adsorption of triethylamine (TEA), followed by SiCl₄ and then H₂O. For TEOS, a three-step reaction sequence using TEA, TEOS, and then water offered no advantage, as the TEOS simply displaced the TEA from the silica surface. A two-step reaction involving the addition of TEOS followed by H₂O in a second step did lead to silica film growth. However, higher growth rates were obtained when using a mixture of TEOS/TEA in the first step. The hydrolysis of the adsorbed TEOS was also much slower than that of the adsorbed SiCl₄, and this was overcome by using a mixture of water/TEA during the second step. While the three-step process with SiCl₄ showed a higher linear growth rate than obtained with two-step process using TEOS/TEA, its use was not practical, as the HCl generated led to corrosion of our sc-CO₂ delivery system. However, when applying the two-step ALD reaction using TEOS on an MCM-41 powder, a 0.21 nm decrease in pore diameter was obtained after the first ALD cycle whereas further ALD cycles did not lead to further pore size reduction. This was attributed to the difficulty in removal of the H₂O in the pores after the first cycle

Effect of Crystal Size on Framework Defects and Water Uptake in Fluoride Mediated Silicalite‑1

The relationship between framework defects and crystal size in fluoride mediated silicalite-1 was investigated through nitrogen physisorption, X-ray diffraction, and vapor adsorption of ethanol and water on samples with crystal sizes ranging from 0.4 to 30 μm in the <i>b</i> direction of the silicalite-1 crystals. X-ray diffraction reveals a shift in the lattice system from a predominantly monoclinic phase in smaller crystals to an orthorhombic phase in the larger crystals. ²⁹Si MAS studies reveal minimal differences in framework silanol defect concentration. An H-4 type hysteresis in 77 K N₂ adsorption isotherm and BdB-FHH pore size analysis reveal the presence of slit-like pores and a larger average pore size as well as a larger volume fraction of “microfissures” in the larger crystals. Pure vapor adsorption measurements show more than a 2-fold increase in water uptake from 0.21 mmol/g to 0.51 mmol/g from the smallest to largest samples at 308 K near unit activity, while ethanol uptake remains on the order of 2.4 mmol/g for all samples. An increase in desorption hysteresis with crystal size and negligible differences in isosteric heats of adsorption of water lend support to the presence of microfissure defects in the larger samples. IAST predications for binary adsorption of ethanol and water in the silicalite-1 samples reveal a 2-fold ethanol/water selectivity enhancement for dilute (<5 wt % EtOH) solutions when crystal size is reduced. This systematic study of N₂ physisorption, framework composition, and ethanol/water adsorption highlights the critical role that crystal size plays in the adsorption process, which can have significant implications for biofuel processes that produce dilute aqueous ethanol as a raw product

Glycine receptors support excitatory neurotransmitter release in developing mouse visual cortex

Glycine receptors (GlyRs) are found in most areas of the brain, and their dysfunction can cause severe neurological disorders. While traditionally thought of as inhibitory receptors, presynaptic-acting GlyRs (preGlyRs) can also facilitate glutamate release under certain circumstances, although the underlying molecular mechanisms are unknown. In the current study, we sought to better understand the role of GlyRs in the facilitation of excitatory neurotransmitter release in mouse visual cortex. Using whole-cell recordings, we found that preGlyRs facilitate glutamate release in developing, but not adult, visual cortex. The glycinergic enhancement of neurotransmitter release in early development depends on the high intracellular to extracellular Cl(−) gradient maintained by the Na(+)–K(+)–2Cl(−) cotransporter and requires Ca(2+) entry through voltage-gated Ca(2+) channels. The glycine transporter 1, localized to glial cells, regulates extracellular glycine concentration and the activation of these preGlyRs. Our findings demonstrate a developmentally regulated mechanism for controlling excitatory neurotransmitter release in the neocortex