Date of Award

8-2012

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Brian G. Frederick

Second Committee Member

William J. DeSisto

Third Committee Member

William Unertl

Abstract

Mesoporous materials are interesting as catalyst supports, because molecules can move efficiently in and out of the pore network, but they must be stable in water if they are to be used for the production of biofuels. Before investigating hydrothermal stability and transport properties, the pore structure of SBA-15 was characterized using small angle neutron scattering (SANS) and non-local density functional theory (NLDFT) analysis of nitrogen sorption isotherms. A new Contrast Matching SANS method, using a range of probe molecules to directly probe the micropore size, gave a pore size distribution onset of 6 ± 0.2 Å, consistent with cylindrical pores formed from polymer template strands that unravel into the silica matrix. Diffraction intensity analysis of SANS measurements, combined with pore size distributions calculated from NLDFT, showed that the secondary pores are distributed relatively uniformly throughout the silica framework. The hydrothermal stability of SBA-15 was evaluated using a post-calcination hydrothermal treatment in both liquid and vapor phase water. The results were consistent with a degradation mechanism in which silica dissolves from regions of small positive curvature, e.g. near the entrance to the secondary pores, and is re-deposited deeper into the framework. Under water treatment at 115 °C, the mesopore diameter increases and the intra-wall void fraction decreases significantly. The behavior is similar for steam treatment, but occurs more slowly, suggesting that transport is faster when condensation occurs in the pores. Quasielastic neutron scattering (QENS) measurements of methane in SBA-15 probed the rotational and translational motion as a function of temperature and loading. A qualitative analysis of the QENS data suggested that for the initial dose of methane at 100 K, the self diffusion constant is similar in magnitude to literature values for methane in ZSM-5 and Y-zeolite, showing that the secondary pores trap methane and limit diffusion. Modeling the temperature and pore diameter dependence of methane diffusivity suggested that because the spectral window of the BASIS instrument is so narrow, only a narrow range of pore sizes contribute to the elastic peak broadening at a particular momentum transfer, and should provide strong constraints on the accuracy of molecular simulations.

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