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FUNDAMENTAL CATALYTIC PROPERTIES OF CONFINED LEWIS ACID CENTERS IN DEALUMINATED BEA ZEOLITE FOR SELECTIVE C–C COUPLING AND DEOXYGENATION REACTIONS
Dissertation

FUNDAMENTAL CATALYTIC PROPERTIES OF CONFINED LEWIS ACID CENTERS IN DEALUMINATED BEA ZEOLITE FOR SELECTIVE C–C COUPLING AND DEOXYGENATION REACTIONS

Karen Vannessa Caballero Perez
Washington State University
Doctor of Philosophy (PhD), Washington State University
07/2025
DOI:
https://doi.org/10.7273/000007856
pdf
Dissertation Vannessa Caballero7.36 MB
Embargoed Access, Embargo ends: 10/13/2027

Abstract

Biomass upgrading Ce-confined zeolites C–C coupling dealuminated BEA Deoxygenation reacion Lewis acid sites
The acetone-to-isobutene (ATIB) conversion is part of the ethanol cascade reaction to produce highly valuable compounds. Lewis acid sites can lead to selective C–C coupling and self-deoxygenation pathways in the acetone aldol condensation reaction. However, the reaction mechanism, nature of active sites, and the role of local environment are not well understood. This work demonstrates that well-dispersed isolated Ce atoms within the dealuminated BEA zeolite (CedeAlBEA) exhibit exceptional isobutene formation during the ATIB reaction by providing optimal Lewis acidity, which facilitates the direct decomposition of diacetone alcohol (DAA) to isobutene. In contrast, elevated cerium loadings lead to the formation of CeO2 nanoparticles, which favor side reactions such as DAA dehydration to mesityl oxide (MSO). In situ DRIFTS experiments with acetone adsorption reveal that adjacent silanol (Si–OH) groups in open Ce sites play an essential role in acetone activation, enabling the concerted adsorption of two acetone molecules, on the OH group and the Lewis active site, thus facilitating selective C–C bond formation and stabilizing the transition state for isobutene production. Kinetic assessments, including reactions with DAA as the reactant and kinetic isotope effect measurements, demonstrated that the direct decomposition of DAA to isobutene and acetic acid is the rate-limiting step. Accordingly, a Langmuir-Hinshelwood mechanism is proposed, involving the cooperative adsorption of two acetones molecules on isolated Ce species and the adjacent Si–OH groups. Building on these insights, bimetallic Ce–M catalysts were developed by introduction of a second metal with distinct electronic properties to enhance metal dispersion. In this system, the isolated Ce atoms serve as anchors for the second metal, generating dual Lewis acid sites. Combinations such as Ce–Ga and Ce–Sc preserved high dispersion and achieved 100% theoretical selectivity to isobutene (88.9% yield) even at high total metal loadings. These findings demonstrate that pairing confined Ce atoms with a secondary metal can overcome dispersion limitations, increase active site density, and mitigate metal aggregation, offering a novel strategy for the rational design of bifunctional zeolite catalysts for selective C–C coupling and deoxygenation reactions.

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