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In Situ XAS Study of the Local Structure and Oxidation State Evolutions of Palladium in a Reduced Graphene Oxide Supported Pd(II) Carbene Complex during an Undirected C−H Acetoxylation Reaction
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK). Swedish University of Agricultural Sciences, Sweden.ORCID iD: 0000-0003-2758-4811
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2019 (English)In: Catalysis Science & Technology, ISSN 2044-4753, E-ISSN 2044-4761, Vol. 9, no 8, p. 2025-2031Article in journal (Refereed) Published
Abstract [en]

In situ X-ray absorption spectroscopy (XAS) investigations have been performed to provide insights into the reaction mechanism of a palladium(II) catalyzed undirected C–H acetoxylation reaction in the presence of an oxidant. A Pd(II) N-heterocyclic carbene complex p-stacked onto reduced graphene oxide (rGO) was used as catalyst. The Pd speciation during the catalytic process was examined by XAS, which revealed a possible mechanism over the course of the reaction. Pd(II) complexes in the as-synthesized catalyst first go through a gradual ligand substitution where chloride ions bound to Pd(II) are replaced by other ligands with a bond distance to Pd corresponding to carbon, nitrogen and/or oxygen (L). Parallel to this the mean oxidation state of Pd increases indicating the formation of Pd(IV) species. At a later stage, a fraction of the Pd complexes start to slowly transform into Pd nanoclusters. The mean average oxidation state of Pd decreases to the initial state at the end of the experiment which means that comparable amounts of Pd(0) and Pd(IV) are present. These observations from heterogeneous catalysis are in good agreement with its homogeneous analog and they support a Pd(II)-Pd(IV)-Pd(II) reaction mechanism.

Place, publisher, year, edition, pages
2019. Vol. 9, no 8, p. 2025-2031
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-167338DOI: 10.1039/C8CY02430HISI: 000465404200027OAI: oai:DiVA.org:su-167338DiVA, id: diva2:1299100
Available from: 2019-03-26 Created: 2019-03-26 Last updated: 2019-05-27Bibliographically approved
In thesis
1. Unveiling Catalytic Species in Suspension/Solution-Based Reactions by In Situ X-Ray Absorption Spectroscopy: Evolution of Palladium and Ruthenium Species
Open this publication in new window or tab >>Unveiling Catalytic Species in Suspension/Solution-Based Reactions by In Situ X-Ray Absorption Spectroscopy: Evolution of Palladium and Ruthenium Species
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The palladium (Pd) and ruthenium (Ru) species in several attractive catalysts have been probed using X-ray absorption spectroscopy (XAS). The study of catalyst evolution in suspension- and solution-based reactions was the primary aim. It was achieved by performing in situ XAS experiments on Pd and Ru over the course of the reactions. A custom-made reactor was employed which allowed the catalysts to be mixed with other reaction components under desired conditions.

The first system investigated was the Heck coupling reaction catalyzed by Pd(II) complexes embedded on metal-organic frameworks. It was realized that the as-synthesized catalysts go through an instant ligand substitution process when added to the reaction mixture. Mononuclear Pd complexes are the active species at the first stage of the measurement which then gradually transform into Pd nanoclusters. At a later stage of the measurement, chloride ligands start to bind to surface atoms of the Pd nanoclusters, leading to a deactivation of the catalyst. Following the first successful in situ XAS experiment, Pd(II) carbene complexes catalyzing undirected C–H acetoxylation of benzene in the presence of an oxidant were explored. A gradual ligand substitution occurs, and the mean oxidation state of Pd increases at the same time. At a later stage, Pd nanoclusters form, while the mean oxidation state of Pd returns to the start value. Deactivation of a heterogeneous Pd(II) catalyst during cycloisomerization of acetylenic acids was then investigated using in situ XAS. The choice of substrates showed to significantly influence the nature of Pd species, and the reduction of Pd(II) forming Pd(0) aggregates causes the deactivation. Moreover, strategies of reactivating the catalyst and prevention of the deactivation were developed and examined. In the end, the activation process of a Ru catalyst was studied and the structure of the intermediate was determined by in situ XAS. It was demonstrated that an electron-donating substituent on the cyclopentadiene ligand exhibits a promoting effect on the activation, while an electron-withdrawing substituent inhibits the activation.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry (MMK), Stockholm University, 2019. p. 112
Keywords
Palladium and ruthenium species, Catalysts, Suspension and solution, In situ X-ray absorption spectroscopy, Activation, Deactivation
National Category
Inorganic Chemistry
Research subject
Inorganic Chemistry
Identifiers
urn:nbn:se:su:diva-167340 (URN)978-91-7797-578-6 (ISBN)978-91-7797-579-3 (ISBN)
Public defence
2019-05-21, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
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Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2019-04-25 Created: 2019-03-26 Last updated: 2019-04-12Bibliographically approved

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