Semester of Graduation

Spring 2019

Degree

Master of Science in Chemical Engineering (MSChE)

Department

Cain Department Chemical Engineering

Document Type

Thesis

Abstract

Diesel engines have higher fuel efficiency and can burn cleaner than gasoline engines. To enhance quality of diesel fuels as measured by the cetane number (CN), the removal of aromatic compounds is necessary. A potentially highly efficient approach would be to convert aromatics to saturated cycloparaffin, followed by selective ring opening to linear or branched alkanes, to increase the CN. Here the focus of the project is to study the mechanism of metal-catalyzed ring opening (RO) of cyclohexane (denoted CHA, or c-C6H12) through theoretical and computational modeling. CHA, a simplest cycloparaffin, is used as probe molecule and is expected to represent the aromatics as well. Ir and Pt are selected as catalyst because of their RO capacity. Ir is a better RO catalyst but its RO mechanisms are not well established yet. Pt catalyzed CHA decomposition reactions are well studied experimentally and are used in parametrizing the computational results here. The RO mechanism study here is modelled on defect free Ir(111) and Pt(111). The results for both the surfaces are then compared and concluded the better catalyst for RO.

Density functional theory (DFT) calculations based study on Ir(111) demonstrate CHA decomposition bypasses the formation of stable intermediates i.e. cyclohexene and benzene, and the RO occurs via a carbene intermediate, consistent with the preference of Ir metal for RO via a carbene mechanism that is proposed, but so far not directly verified, in the literature. A similar study of CHA decomposition on Pt(111) reveals that highly unsaturated rings, not benzene, are the main product although the selectivity to benzene is substantially higher than on Ir(111). This reveals an intrinsic difference between these two metals in the ability to activate C-C bonds. On Ir(111) both kinetic and thermodynamic analyses favor RO, however on Pt(111) benzene is the ultimate final decomposed product. These findings provide clues for ultimately enhancing the ability of Ir catalysts to selectively cleave C-C bonds in cycloparaffins.

Committee Chair

Xu, Ye

Available for download on Sunday, March 12, 2023

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