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Quantum Calculations of Aldol Condensation in Acidic Zeolites

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dc.contributor.advisorScott M. Auerbach
dc.contributor.authorMigues, Angela N
dc.contributor.departmentUniversity of Massachusetts Amherst
dc.date2024-03-27T17:05:20.000
dc.date.accessioned2024-04-26T16:09:20Z
dc.date.available2024-04-26T16:09:20Z
dc.date.submittedSeptember
dc.date.submitted2015
dc.description.abstractWe 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.
dc.description.degreeDoctor of Philosophy (PhD)
dc.description.departmentChemistry
dc.identifier.doihttps://doi.org/10.7275/7446468.0
dc.identifier.orcidN/A
dc.identifier.urihttps://hdl.handle.net/20.500.14394/19689
dc.relation.urlhttps://scholarworks.umass.edu/cgi/viewcontent.cgi?article=1484&context=dissertations_2&unstamped=1
dc.source.statuspublished
dc.subjectDFT
dc.subjectaldol condensation
dc.subjectzeolite
dc.subjectcluster
dc.subjectquantum
dc.subjectHZMS-5
dc.subjectPhysical Chemistry
dc.titleQuantum Calculations of Aldol Condensation in Acidic Zeolites
dc.typeopenaccess
dc.typearticle
dc.typedissertation
digcom.contributor.authorisAuthorOfPublication|email:a.migues08@gmail.com|institution:University of Massachusetts Amherst|Migues, Angela N
digcom.identifierdissertations_2/472
digcom.identifier.contextkey7446468
digcom.identifier.submissionpathdissertations_2/472
dspace.entity.typePublication
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