Date of Award

5-2012

Level of Access

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Advisor

William J. DeSisto,

Second Committee Member

Michael D. Mason

Third Committee Member

Douglas M. Ruthven

Abstract

Inorganic membranes offer a means for chemical separations in a variety of applications including chemical processing, drug delivery systems, battery separators and fuel cells. There is currently a “pore size gap” in silica membranes between 1-2 nanometers. Synthesizing membranes with a fine control of the pore size and distribution within that gap is a significant challenge. This thesis reports findings on using atomic and molecular layer deposition as new synthesis approaches to controlling pore size and chemical functionality of silica membranes. Mesoporous silica membranes, prepared using surfactant-templates with pore diameters ~4nm, were modified using atomic layer deposition and molecular layer deposition. Atomic layer deposition was carried out using trimethyl aluminum and water as precursors and molecular layer deposition used trimethyl aluminum and oxalic and o-phthalic acid. These methods involved a separate pulse/purge sequence for each reactant that resulted in surface-limited film growth within the pores.

It was determined that the growth rate of atomic layer deposition of alumina within mesoporous silica membranes was not linear, with a higher growth rate during the first 7 cycles and a lower rate afterwards. Alumina deposition was favored in larger pores within the pore size distribution of the support. The He/N2 selectivity of the membrane was improved by removing defects, although still in the Knudsen range. In preliminary work, the hydrothermal stability of the membrane increased as a result of the addition of alumina into the silica pore network.

In the molecular layer deposition study, higher growth rates were observed when using oxalic acid as a precursor. Both oxalic and o-phthalic acid were able to increase the selectivity of the membrane above the He/N2 Knudsen value. Analysis of the permeance of several light gases suggested that pore size reduction occurred and that the modification was confined to a small layer within the support.

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