Showing papers by "Angelika Brückner published in 2022"

Scalable and selective deuteration of (hetero)arenes

Abstract: Isotope labelling, particularly deuteration, is an important tool for the development of new drugs, specifically for identification and quantification of metabolites. For this purpose, many efficient methodologies have been developed that allow for the small-scale synthesis of selectively deuterated compounds. Due to the development of deuterated compounds as active drug ingredients, there is a growing interest in scalable methods for deuteration. The development of methodologies for large-scale deuterium labelling in industrial settings requires technologies that are reliable, robust and scalable. Here we show that a nanostructured iron catalyst, prepared by combining cellulose with abundant iron salts, permits the selective deuteration of (hetero)arenes including anilines, phenols, indoles and other heterocycles, using inexpensive D2O under hydrogen pressure. This methodology represents an easily scalable deuteration (demonstrated by the synthesis of deuterium-containing products on the kilogram scale) and the air- and water-stable catalyst enables efficient labelling in a straightforward manner with high quality control.

Role of interfacial oxygen vacancies in low-loaded Au-based catalysts for the low-temperature reverse water gas shift reaction

Abstract: Gold-based catalysts have shown high catalytic activity for reverse water gas shift (RWGS) reactions at low temperatures. Despite extensive studies, the RWGS reaction on Au-based catalysts with very low Au content (< 0.1 wt%) has not yet been investigated. In this study, TiO 2 and ZrO 2 supported gold catalysts with such low gold loading have been synthesized and tested for RWGS. The catalysts were investigated by a series of in/ex-situ characterization techniques, including ICP-OES, XRD, BET, XPS, STEM, STEM-EELS, TAP, in-situ DRIFTS and in-situ EPR. At 250 o C, the Au/TiO 2 catalyst showed almost 10 times higher activity than Au/ZrO 2 . In-situ DRIFTS results suggest that the formate mechanism is the predominant mechanism over Au/ZrO 2 , while over Au/TiO 2 the reaction proceeds via the formation of hydroxycarbonyl intermediates. A combined study including STEM, STEM-EELS, XPS, and in-situ EPR suggests that the interfacial Au–O v –Ti 3+ sites are responsible for the superior activity of Au/TiO 2 . • Au/TiO 2 exhibited superior activity and CO selectivity. • Ti 3+ formation and oxygen vacancies on Au/TiO 2 contribute to its superior activity. • The weak adsorption strength of CO is responsible for the superior CO selectivity. • An Oxygen-vacancies-engaged reaction mechanism is proposed for RWGS over Au/ZrO 2 . • The decomposition of formate seems to be the rate-limiting step in the CO formation.

Operando DRIFT and In situ Raman Spectroscopic Studies on Aspects of CO2 Fischer‐Tropsch Synthesis Catalyzed by Bulk Iron Oxide‐Based Catalysts

Abstract: Hematite is a suitable precursor to obtain catalytically active iron (oxide) phases for CO2 Fischer‐Tropsch synthesis after a reductive pretreatment. As concluded from in situ Raman spectroscopy, in hydrogen atmosphere the transformation from α‐Fe2O3 to Fe3O4 is faster than the further reduction from Fe3O4 to Fe (metallic iron). The rate of these steps highly depends on the temperature. Starting from pure hematite, surface formate species are formed at a CO2/H2 gas mixture at elevated temperatures, observable by DRIFT‐spectroscopy. The exposure to CO2 and CO led to surface carbonate‐carboxylate surface species. Reduced samples with varying contents of Fe3O4 and Fe did not show any observable adsorbates at reaction conditions. The same behavior was found during the dosage of the single gases CO2 and CO to these reduced catalysts. The formation of carbonaceous species, detected by Raman spectroscopy, could indirectly hint to the occurrence of carbidation and was especially observed for a good performing catalyst with a medium reduction degree.

Controlling the Selectivity of High-Surface-Area Ru/TiO2 Catalysts in CO2 Reduction - Modifying the Reaction Properties by Si Doping of the Support

Abstract: The influence of Si doping of high specific-surface-area Ru/TiO2 catalysts with similar structural properties on the CO2 reduction reaction was systematically investigated by kinetic measurements, combined with microscopic and spectroscopic methods for ex situ and in situ /operando catalyst characterization. While for undoped Ru/TiO2 a high-temperature treatment (350 °C) in reaction atmosphere results in a pronounced change of the selectivity from methanation to CO formation via the reverse water-gas shift (RWGS) reaction, CH4 formation is stabilized by Si doping of the catalyst support. For doping levels around 8 wt%, almost 100% CH4 selectivity is maintained. Comprehensive catalyst characterization is employed to identify trends in the physical and chemical properties with increasing Si doping and thus physical reasons responsible for the distinct differences in catalyst performance and stability. This work opens a route for improving the stability and selectivity of Ru/TiO2 catalysts in the CO2 hydrogenation reaction, a highly relevant application.

Glycolaldehyde as a Bio‐Based C1 Building Block for Selective N‐Formylation of Secondary Amines

Abstract: Abstract Biomass derived glycolaldehyde was employed as C1 building block for the N‐formylation of secondary amines using air as oxidant. The reaction is atom economic, highly selective and proceeds under catalyst free conditions. This strategy can be used for the synthesis of cyclic and acyclic formylamines, including DMF. Mechanistic studies suggest a radical oxidation pathway.

Correction: Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Abstract: Correction for ‘Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes’ by David Decker et al., Inorg. Chem. Front., 2022, DOI: 10.1039/d1qi00998b.

Dynamics of Reaction‐Induced Changes of Model‐Type Iron Oxide Phases in the CO2‐Fischer‐Tropsch‐Synthesis

Abstract: Iron‐based catalysts are employed in CO2‐FTS due to their ability to convert CO2 into CO in a first step and their selectivity towards higher hydrocarbons in a second CO hydrogenation step. According to the literature, iron carbides represent the active phase for hydrocarbon formation and are claimed to emerge in the presence of CO. We propose nanostructured FeOx films as model systems to assess information about the complex phase transformations during CO2‐FTS. Mesoporous hematite, ferrihydrite, maghemite, maghemite/magnetite films were exposed to CO2‐FTS atmospheres at 20 bar and 300 °C. Up to three distinct phases were observed depending on the time‐on‐stream (TOS): a sintered maghemite/magnetite phase, a carbidic core‐shell structure, and a low‐crystalline, needle‐type oxide phase. Our findings indicate that the formation of an intermediary maghemite/magnetite phase, predominant after short TOS (30 h), precedes the evolution of the carbide phase. Yet, even after prolonged TOS (185 h), no full conversion into a bulk carbide is observed.

Preferential CO Oxidation on a Highly Active Cu Single Atom Catalyst Supported by Ce−TiOx

Abstract: A heterogeneous single‐atom catalyst (SAC) based on a tiny Cu loading (0.05 wt.%) supported on CeO2−TiO2 (Ce/Ti=0.18) shows very high Cu mass‐normalized CO oxidation activity, 100 % selectivity for CO2 between 60 and 100 °C and essentially no deactivation during 17 h on stream in the preferential oxidation of CO (PROX) at 120 °C. Based on state of the art, this catalyst is among the most active and PROX‐selective catalysts. Employing a combination of ex‐situ and operando methods, we infer that the isolated Cu single sites are incorporated with redox‐active Cu2+−O−Ce4+⇆Cu+−□−Ce3+ moieties connected by labile oxygen and are exposed on the surface of highly dispersed ceria. This high dispersion is promoted by TiO2 which itself does not participate in redox steps during PROX, as Ti ions remain essentially tetravalent while Cu and Ce undergo reversible redox shuttles.