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Sorensen, Adam

Chemistry

2006

PhD

Due to changing global opinions, the use, production, and storage of chemical weapons has become outlawed by the majority of the world's nations. In response, it has become necessary for countries with stockpiles of these materials to dispose of them in a safe and environmentally friendly manner. The United States currently has over 6,000 tons of the chemical weapon vesicant, mustard gas (SC₄H₈Cl₂), stored at various locations throughout the country. While incineration has been chosen as the primary means of demilitarization, where stockpiled munitions containing chemical agents need to be disassembled and the agents destroyed, it is not the most environmentally friendly method, producing the gases SO, SO₂, CO, and CO₂. Concerns over the effects of these products and any chemical agent that may be left over after incomplete combustion on the environment has led the United States Army to investigate other methods of disposal. One such method being investigated is the reductive degradation of mustard gas over a molybdenum catalyst in a process known as hydrodesulfurization (HDS). Current HDS catalysts are supported on alumina substrates, which suffer from relatively low surface areas (~200 m²/g). This low surface area limits the amount of catalyst that can be incorporated into the material, thus reducing its overall effectiveness. To obtain higher activity, it is necessary to support these catalysts on a larger surface area material, such as mesoporous silica. Mesoporous silicas have a network of pore channels, between 2 and 50 nm in diameter, which allows them to achieve surface areas over 1000 m²/g.
Chapter 1 is an introduction to chemical weapons and the need to dispose of the current stockpiles along with an overview on the properties of mesoporous silica. The techniques used to characterize mesoporous silica are also discussed. Chapter 2 deals with the synthesis of various mesoporous silicas and the incorporation of Mo to generate supported HDS catalysts. The physical properties of these Mo-doped silicas are examined in order to understand the changes that can occur by incorporating the Mo catalysts onto the silica surface. These Mo catalysts are compared by their activity in the HDS in Chapter 3. Two compounds will be used to test the materials. The first is thiophene (SC₄H₄), which is a classic compound for testing HDS activity. It will be used to determine the most active of the materials, which will continue on with additional testing. The material with the best activity with thiophene is used in the HDS of 2-chloroethyl ethyl sulfide (CEES, SC₄H₉Cl), a mustard gas analog, to determine the how the catalyst reacts with a compound similar in nature to mustard gas. The performance of the silica-supported materials are compared for activity to a commercial alumina supported catalyst to determine if any benefit is gained by supporting the Mo catalyst on the mesoporous silica. Analysis of the materials after catalysis is performed in Chapter 4 to understand the changes that occur during the HDS process, such as degradation of the support matrix and sulfidation of the Mo catalyst. Chapter 5 investigates the actual structure of the supported catalysts by using extended X-ray absorption fine structure to probe the local environment around the Mo atoms in the materials prior to and after catalysis.