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Lignin Intermediates on the Palladium Surface: Factors for Structural and Energetic Changes
Stockholm University, Faculty of Science, Department of Organic Chemistry.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this work, dispersion-corrected density functional theory (DFT-D3), has been used to investigate interactions between important intermediates in lignin depolymerization and a palladium catalyst. The keto structure 2-phenoxy-1-phenylethanone and its enol tautomer have been used to model reactive intermediates derived from lignin. To investigate how the adsorption energies are affected by adsorbate coverage, we have used two different Pd(111) super cells; one smaller p(6 × 4) and one larger p(6 × 6). In the gas phase, the staggered conformer of the keto tautomer is more stable than both the eclipsed form of the keto tautomer and as expected much more stable than the E-enol tautomer. However, in interaction with the palladium surface, the E-enol tautomer has a similar binding energy as the keto tautomer. Also, the eclipsed conformer of the keto tautomer is more stable than the staggered conformer of the keto tautomer when adsorbed to the palladium. We found that the coverage, that is concentration of molecules on the surface had a pronounced effect on the adsorption energies. At higher coverage, both the keto and enol models prefer to adsorb on an atop configuration to the surface. Furthermore, we found that both the keto and the enol tautomers bind strongly to the surface through their phenyl rings. Despite the strong binding of the phenyl groups, the enol adsorbs to the surface through a chemisorption by cleavage of the C═C bond, that leads to two types of di-sigma complexes depending on the position of the newly formed Pd–C sigma bonds. The generated complex is a key intermediate in the subsequent depolymerization through cleavage of a C–O bond. Our simulations show that there is an intermolecular repulsion between adsorbates on the surface, and consequently, the molecules were found to bind more strongly to the surface at low coverages (by 8-14 kcal/mol). These results are important for experimental design purposes; as previous experiments have shown that the enol form is key for an efficient β-O-4′ bond cleavage and implies that low concentration reactions are favored.

National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-167712OAI: oai:DiVA.org:su-167712DiVA, id: diva2:1301362
Available from: 2019-04-01 Created: 2019-04-01 Last updated: 2019-04-02Bibliographically approved
In thesis
1. Theoretical Investigations of C–O Activation in Biomass
Open this publication in new window or tab >>Theoretical Investigations of C–O Activation in Biomass
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on using computational chemistry approaches to study how biobased molecules interact with both homo- and heterogeneous catalysts. The reaction mechanisms of such transformations have also been studied.

The first section comprises studies of interactions between organic molecules and a heterogeneous catalyst in the palladium-catalyzed depolymerization of models of lignin derivatives. From experiments, it was proposed that a keto intermediate and its enol tautomer play a significant role in the β-O-4′ bond cleavage. The study in the first section of this thesis has been divided into three parts. First, simplified models of the keto intermediate and its enol tautomer were used to investigate the adsorption to a Pd(111) surface. By using a combination of periodic density functional theory (DFT) calculations and a constrained minima hopping method, the most stable adsorption which is the so-called global minimum, was found to be an enol adsorbed to the surface.

In the second part, the study was expanded to cope with models of lignin which were used in experiments. In addition, we studied the effect of adsorbate coverage, where two different Pd(111) super cells were compared. The optimizations were performed via dispersion-corrected density functional theory (DFT-D3). The molecules were found to bind more strongly to the surface at low coverages. These results support the experimental data and show that the tautomerization has an important role during lignin depolymerization. 

The third part relates to using a multilevel procedure to study the interaction of fragments derived from lignin depolymerisation with a palladium catalyst in a solvent mixture. Specifically, QM calculations and MD simulations based on the ReaxFF approach were combined to explore the reaction mechanisms occurring on Pd surfaces with lignin derivatives obtained from a solvolysis reaction. The strongest adsorptions were found to be between the aromatic rings and the Pd surfaces.

The second section focuses on a Brønsted acid-catalyzed nucleophilic substitution of the hydroxyl group in alcohols. Experimentally, phosphinic acid (H3PO2) was found to be an excellent catalyst for the direct intramolecular substitution of non-derivatized alcohols proceeding with good to excellent chirality transfer. In this section, benzylic alcohols with internal O-, N-, and S-centered nucleophiles were used in the calculations. By using a hybrid functional method, we found a bicyclic transition state where the proton of the H3PO2 protonates the leaving hydroxyl group, and the oxo-group of the same catalyst partially deprotonates the nucleophile. The transition state energies for the reactions were determined computationally. The calculations support an SN2 mechanism, which corresponds to the experimental data where inversion of the stereogenic carbon was observed.

Place, publisher, year, edition, pages
Stockholm: Department of Organic Chemistry, Stockholm University, 2019. p. 77
Keywords
DFT calculations, global minima hopping, reactive force field, lignin, palladium, nucleophilic substitution
National Category
Organic Chemistry
Research subject
Organic Chemistry
Identifiers
urn:nbn:se:su:diva-167592 (URN)978-91-7797-709-4 (ISBN)978-91-7797-710-0 (ISBN)
Public defence
2019-05-22, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2019-04-25 Created: 2019-04-01 Last updated: 2019-04-10Bibliographically approved

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