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Computational Study of Acid-Catalyzed Reactions in Zeolites

Mehta, Rushita; (2021) Computational Study of Acid-Catalyzed Reactions in Zeolites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Aldol condensation is a very important reaction in organic synthesis because it leads to the formation of C-C bonds. Because of that, the use of different catalysts and in particular the use of zeolites for the catalysis of this reaction has been previously studied. The first step of aldol condensation is the the acid- catalyzed keto-enol tautomerization of the aldehyde or ketone. In this thesis, we study all the possible locations of BA sites in the vicinity of each of the defined inequivalent T site positions in FER and MOR zeolites and establish the most stable location of proton siting at force field and periodic DFT level. The reactions involving the small carbonyl compounds, acetaldehyde and acetone, are studied at the specific acid site locations in both zeolites in order to discover which are the active sites that can stabilize the reactants and therefore how the existing catalyst can be improved. Besides the H-bonding and other interactions, the confinement effect are of equal importance in the determination of factors that influence the reactivity of these complexes. In order to understand the keto-enol tautomeric mechanism in zeolites and identify the transition states, constrained geometry optimizations were performed of acetaldehyde in FER and MOR using periodic DFT. From this we determined the formation of enol product via and an one and two-step concerted mechanism. The calculations reveal that C-β deprotonation is the kinetic bottleneck for enol formation. The concerted mechanism was performed at each of the inequivalent T position in both zeolites. We found that proton transfer is a consequence of a cooperation between the acid site and its total environment acting on the molecular adsorbate. The adsorption and stability of the intermediates is dependent upon the heterogeneity of acid sites and their local geometry, the pore channels and cavities and interactions such as dispersive and H-bonding that do not reflect, and are often independent of, acid strength. Thirdly, we studied the keto-enol tautomerization of acetaldehyde in FER and MOR in the presence of a single water or methanol molecular and found the activation barriers to reduce further with an increase in stability of the adsorbed enol form with larger reverse barriers in the larger pores of FER. We establish the importance of the specific role of the H-bonding using these solvent molecules. In the smaller pores of MOR, the presence of a solvent supresses the catalytic interconversion due to steric repulsion. Lastly, we explored the location of monovalent alkali ions in FER and found that the most stable location of the cations is in the FER cavity and dependent upon the Al position. We studied the effect of hydration on the mobility of the cesium ion in the cavities of FER using static DFT calculations supplemented with ab initio molecular dynamics. We estabished the cesium ion prefers to coordinate with the framework oxygens of the zeolite rather than oxygen atoms of the water molecules as well as the position of the cesium ion is affected by the Al siting. The coordination number of cesium is ~ 10 with the ion interacting with only 1-3 water molecules. In addition, we identified a self-organization of water molecules across the channels forming a H-bonding network.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Computational Study of Acid-Catalyzed Reactions in Zeolites
Event: UCL (University College London)
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
UCL
URI: https://discovery.ucl.ac.uk/id/eprint/10127680
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