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Detecting Important Intermediates in Pd Catalyzed Depolymerization of a Lignin Model Compound by a Combination of DFT Calculations and Constrained Minima Hopping
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för organisk kemi.
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för organisk kemi.
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2016 (Engelska)Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, nr 41, s. 23469-23479Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Density functional theory (DFT) calculations, combined with a constrained minima hopping algorithm (global minimum search while preserving the molecular identity), have been performed to investigate important reaction intermediates for the heterogeneously catalyzed beta-O-4' bond cleavage in lignin derivatives. More specifically, we have studied the adsorption properties of a keto tautomer (1-methoxypropan-2-one) and its enol form on a catalytically active Pd(111) surface. In agreement with experiments, we find that for the gas phase molecules the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on the catalyst surface. The global minimum complex found on the metal surface corresponds to an enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its pi-electrons from the C=C region to two adjacent palladium atoms. The actual formation of a chemical bond to the surface in the case of the enol molecule could be the key to understanding why the enol derivative is needed for an efficient beta-O-4' bond cleavage.

Ort, förlag, år, upplaga, sidor
American Chemical Society (ACS), 2016. Vol. 120, nr 41, s. 23469-23479
Nationell ämneskategori
Kemi Nanoteknik Materialteknik
Identifikatorer
URN: urn:nbn:se:su:diva-136052DOI: 10.1021/acs.jpcc.6b05622ISI: 000386107600018OAI: oai:DiVA.org:su-136052DiVA, id: diva2:1058741
Forskningsfinansiär
VetenskapsrådetTillgänglig från: 2016-12-21 Skapad: 2016-11-29 Senast uppdaterad: 2019-04-01Bibliografiskt granskad
Ingår i avhandling
1. Theoretical Investigations of C–O Activation in Biomass
Öppna denna publikation i ny flik eller fönster >>Theoretical Investigations of C–O Activation in Biomass
2019 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Stockholm: Department of Organic Chemistry, Stockholm University, 2019. s. 77
Nyckelord
DFT calculations, global minima hopping, reactive force field, lignin, palladium, nucleophilic substitution
Nationell ämneskategori
Organisk kemi
Forskningsämne
organisk kemi
Identifikatorer
urn:nbn:se:su:diva-167592 (URN)978-91-7797-709-4 (ISBN)978-91-7797-710-0 (ISBN)
Disputation
2019-05-22, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 10:00 (Engelska)
Opponent
Handledare
Anmärkning

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

Tillgänglig från: 2019-04-25 Skapad: 2019-04-01 Senast uppdaterad: 2019-04-10Bibliografiskt granskad

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Galkin, Maxim V.Samec, Joseph S. M.
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Institutionen för organisk kemi
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