A well-studied heterogeneous palladium(II) catalyst used for the cycloisomerization of acetylenic acids is known to be susceptible to deactivation through reduction. To gain a deeper understanding of this deactivation process and to enable the design of a reactivation strategy, in situ X-ray absorption spectroscopy (XAS) was used. With this technique, changes in the palladium oxidation state and coordination environment could be studied in close detail, which provided experimental evidence that the deactivation was primarily caused by triethylamine-promoted reduction of palladium(II) to metallic palladium nanoparticles. Furthermore, it was observed that the choice of the acetylenic acid substrate influenced the distribution between palladium(II) and palladium(0) species in the heterogeneous catalyst after the reaction. From the mechanistic insight gained through XAS, an improved catalytic protocol was developed that did not suffer from deactivation and allowed for more efficient recycling of the catalyst.

Herein, we report on the use a biohybrid catalyst consisting of palladium nanoparticles immobilized on cross-linked enzyme aggregates of lipase B of Candida antarctica (CalB CLEA) for the dynamic kinetic resolution (DKR) of benzylic amines. A set of amines were demonstrated to undergo an efficient DKR and the recyclability of the catalysts was studied. Extensive efforts to further elucidate the structure of the catalyst are presented.

The cause and mechanism of deactivation of a well-studied heterogeneous palladium(II) catalyst in the intramolecular lactonization of acetylenic acids to γ-alkylidene lactones have been investigated. It was shown that the deactivation was driven by the formation of reduced palladium species following the addition of the base triethylamine. In this work, X-ray absorption spectroscopy (XAS) was used to identify the palladium species and follow their evolution over the course of the reaction. It was also found that the choice of substrates has significant influences on the Pd species under the same reaction conditions. With these insights into the deactivation mechanism derived from XAS, different strategies were tested and illustrated to regain or maintain the active state of the catalyst. This information was further used to develop a new protocol, which can effectively prevent the deactivation of the catalyst and prolong its usage. 

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.

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.

Postsynthetic reactions of metal-organic frameworks (MOFs) are versatile tools for producing functional materials, but the methods of evaluating these reactions are cumbersome and destructive. Here we demonstrate and validate the use of in situ NMR spectroscopy of species in the liquid state to examine solvent-assisted ligand exchange (SALE) and postsynthetic modification (PSM) reactions of metal-organic frameworks. This technique allows functionalization to be monitored over time without decomposing the product for analysis, which simplifies reaction screening. In the case of SALE, both the added ligand and the ligand leaving the framework can be observed. We demonstrate this in situ method by examining SALE and PSM reactions of the robust zirconium MOF UiO-67 as well as SALE with the aluminum MOF DUT-5. In situ NMR spectroscopy provided insights into the reactions studied, and we expect that future studies using this method will permit the examination of a variety of MOF-solute reactions.

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (H-1 NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

Herein, we report on the utilization of a heterogeneous catalyst, consisting of Pd nanoparticles supported on a siliceous mesocellular foam (Pd-0-AmP-MCF), for the synthesis of heterocycles. Reaction of o-iodophenols and protected o-iodoanilines with acetylenes in the presence of a Pd nanocatalyst produced 2-substituted benzofurans and indoles, respectively. In general, the catalytic protocol afforded the desired products in good to excellent yields under mild reaction conditions without the addition of ligands. Moreover, the structure of the reported Pd nanocatalyst was further elucidated with extended X-ray absorption fine-structure spectroscopy, and it was proven that the catalyst could be recycled multiple times without significant loss of activity.

The synthesis of two dimensional (2D) materials from transition metal oxides, chalcogenides, and carbides mostly involve multiple exfoliation steps in which hazardous solvents and reagents are used. In this study, hydrated vanadium pentoxide (V2O5 center dot nH(2)O) nanosheets with a thickness of a few nanometers were prepared via a facile environmentally friendly water based exfoliation technique. The exfoliation process involved refluxing the precursor, vanadium dioxide (VO2(B)), in water for a few days at 60 degrees C. The proposed exfoliation mechanism is based on the intercalation/insertion of water molecules into the VO2(B) crystals and the subsequent cleavage of the covalent bonds holding the layers of VO2(B) together. The thermal and chemical analyses showed that the approximate chemical composition of the nanosheets is H0.4V2O5 center dot 0.55H(2)O, and the percentage of V-V content to that of V-IV in the nanosheets is about 80(3)% to 20(3)%. The exfoliated aqueous suspension of the V2O5 center dot 0.55H(2)O nanosheets was successfully deposited onto multi-walled carbon nanotube (MW-CNT) paper to form free-standing electrodes with a thickness of the V2O5 center dot 0.55H(2)O layer ranging between 45 and 4 mu m. A series of electrochemical tests were conducted on the electrodes to determine the cyclability and rate capability of lithium insertion into V2O5 center dot 0.55H(2)O nanosheets. The electrodes with the thinnest active material coating (similar to 4 mu m) delivered gravimetric capacities of up to 480 and 280 mA h g(-1) when cycled at current densities of 10 and 200 mA g(-1), respectively.