Investigations into the reactivity and catalytic activity of nanoporous aluminosilicates and their synthesis precursors

Charles L Schaffer, Purdue University

Abstract

This work focuses on applying modern molecular simulation techniques and ab initio methods, such as electronic density functional theory, to explore the synthesis of nanoporous aluminosilicates and their use as solid acid catalysts. Paramount to all investigations into the design of porous solids is the need for a fundamental understanding of the initial steps leading to the formation of these materials. To begin, we explored the mechanism and energetics of the condensation reaction of prenucleation silica species. Our results suggested the initial steps in the synthesis process play a crucial role in determining the properties of the resulting crystalline solids. To further elucidate the synthesis mechanism, we employed a hybrid solvation technique to examine the self-assembly process forming mesoporous solids templated via block copolymers. Our results indicated the Q3 silica species play a predominate role in forming the ordered mesophase domain of the porous solids and gives evidence that the self-assembly process is in some way driven via intermolecular forces and not simply through ionic interactions. In the area of catalyst function, the use of zeolite-based catalysts in the aromatization of light alkanes is a very industrially important process. To improve catalyst performance, research has focused on elucidating each step in the aromatization pathway. We examined the oligomerization reaction of propene and subsequent adsorption of the resulting C6 species occurring at Brønsted acid sites within the pores of the zeolite H-ZSM-5. To model the active site of H-ZSM-5 we used two cluster modeling techniques: a fully QM bare-cluster model and a combined QM/MM embedded-cluster model. We concluded that cluster size effects must be considered when the relative stability of adsorbed species is under investigation. However, for reaction path analysis and activation energy calculations the more minimalistic models may be suitable. Finally, the catalytic cracking of hydrocarbon jet-fuel, JP-10, was studied. The motivation behind this work was to understand the cracking reaction of JP-10 for use as a possible propellant for pulse detonation engines and other high-speed flight applications. JP-10 was targeted primarily due to its high stability. Unfortunately, this advantage makes detonation of the fuel difficult when using a low-energy ignition source available on most flight vehicles. Thus, catalytic pre-cracking has been identified as a viable method of breaking the larger hydrocarbon into more easily detonable light hydrocarbons. We proposed a closed-loop catalytic cycle for the cracking of JP-10 over H-ZSM-5. Reaction path analysis showed the process to have an endothermic heat of decomposition which presents a plausible heat sink for the combustion reaction.

Degree

Ph.D.

Advisors

Thomson, Purdue University.

Subject Area

Chemical engineering

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