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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2015

Month Degree Awarded

September

First Advisor

Scott M. Auerbach

Subject Categories

Physical Chemistry

Abstract

We have used Density Functional Theory to model the mixed aldol condensation reaction catalyzed by acidic zeolites. We have studied the convergence of barriers for the keto-enol tautomerization of acetone in cluster models of HZSM-5 and HY ranging in size from 3-37T. A key finding was that activation barriers for keto-enol tautomerization of acetone in both zeolites (~20 kcal/mol) are significantly higher than those for the condensation reaction between the acetone enol and formaldehyde in 11T cluster models of HZSM-5 and HY. Moreover we found that three zeolite clusters of HZSM-5, similarly sized but including different structural features of the zeolite framework, gave very different activation barriers. These results indicated that a more rigorous approach to constructing cluster models of zeolites was needed.

We have developed two different approaches to build cluster models of zeolites and used two acid-zeolite-catalyzed processes related to the conversion of biomass as platforms to systematically investigate cluster-size convergence. Our central finding is that clusters generated with multi-centered spherical cutoffs yield converged reaction energies with smaller system sizes than clusters generated by counting framework bonds. The delta approach to constructing finite clusters of zeolite crystals provides a well-defined prescription and employs a single length scale (d = 5 Å) to converge reaction energies to within chemical accuracy (±1 kcal/mol).

Although reaction energies were convergent at d = 5 Å, it is not clear that a single length scale is sufficient to converge activation barriers. We used two reaction systems as platforms to establish convergence of activation barriers using delta clusters. We find that a δ ≥ 4 Å cutoff is sufficient to converge activation barriers to within chemical accuracy (±1 kcal/mol). After convergence was established, we studied the acid-zeolite-catalyzed mixed aldol condensation of acetone with more biomass-relevant aldehydes, such as hydroxymethyl furfural and furfural, in δ = 4 Å clusters of HZSM-5. We have found that the mechanism for condensation in HZSM-5 is concerted, unlike that of the homogeneous acid catalyzed mechanism. Ultimately, we conclude that the keto/enol tautomerization of acetone remains rate-determining in the case of condensation with formaldehyde, furfural or hydroxymethyl furfural in HZSM-5.

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