Catalysis and Spectroscopy

Catalysis is a key-technology, opening energy-efficient and selective pathways to value-added products. Controlling selectivity is one of the major goals in catalysis research for the coming decades and the efficient production of high-value-added chemicals is of great scientific and industrial interest, particularly in the Swiss fine chemical industry.

While the application of homogeneous catalysis for fine chemical synthesis benefits from a profound mechanistic understanding, heterogeneous production processes would offer technical advantages such as separation, regeneration, and recycling of the catalyst, and the ease of transfer to continuous mode operation. We focus on the development and application of heterogeneous catalysis in liquid-phase and our research evolves around the characterization, analysis and development of catalytic processes occurring at solid-liquid interfaces.

Major research topics include the development of in situ and operando spectroscopic techniques for investigating catalytic solid-liquid interfaces, and heterogeneous asymmetric hydrogenation employing chirally-modified metals or employing chirality transfer to prochiral substrates.

The development of modern analytical techniques plays a fundamental role in research aiming at efficient and eco-friendly production processes. A prerequisite for a rational design of heterogeneous liquid-phase catalysis is a thorough understanding of the molecular processes at the solid-liquid or the solid-liquid-gas interface. These multicomponent systems are highly sensitive to the experimental conditions and extracting meaningful mechanistic information generally requires an investigation of the system under in situ conditions, that is, considering the presence of the gas, the liquid (solvent) and the solid (catalyst). In addition, realistic mass transfer among the phases has to be ensured. Our approach is to develop in situ spectroscopic techniques suitable to characterize the interface during the operating process. In catalysis research, the methodology is termed operando spectroscopy. The investigation of catalytic solid-liquid-gas reaction systems by the application of operando spectroscopy is illustrated in Figure 1. 

Enlarged view: operando spectroscopic reactor cell
Figure 1:  Schematic of an operando spectroscopic reactor cell as a tool for simultaneous acquisition of activity and selectivity data (macroscopic level) as well as surface coverage and intrinsic rates (molecular level) of an operating process.

Ideally, a small spectroscopic reactor cell provides complementary information about macroscopic quantities of the investigated process (activity, selectivity), and molecular level information (surface coverage, intrinsic rates, oxidation states, etc.), depending on the process parameters (temperature, pressure, concentrations, velocity, etc.).

The relevance of spectroscopic data acquired under mass transport limited conditions for understanding heterogeneous catalytic processes can be questionable, and the operando spectroscopic approach, that is, correlating spectroscopic detection to catalytic performance, brings along other advantages such as facilitated analysis of complex surface spectra. The operando spectroscopic approach is a frontier area of research and we aim at accomplishing operando spectroscopy for a broad range of catalytic three-phase systems.

Related publications:

external pageF. Meemken, P. Müller, K. Hungerbühler, A. Baiker,Rev. Sci. Instrum.2014,85, 17.

external pageL. Rodríguez-García, K. Hungerbühler, A. Baiker, F. Meemken,Catal.Today.2016, in press.

Our research in this direction focuses on a more rational design of heterogeneous asymmetric hydrogenation on chirally-modified metal catalysts based on operando spectroscopic characterization. The general strategy is illustrated in Figure 2, which highlights the design process consisting of four interrelated steps. The required molecular level information from the catalytic interface will be obtained by the application of operando spectroscopy (Characterization). The operando spectroscopic data will provide insight into the relevant surface processes, allowing to derive profound mechanistic understanding (Analysis). To steer the desired catalysis after formation of the maximum number of enantioselective sites towards optimal enantioselectivity the fraction of the heterogeneous surface inaccessible to the bulky chiral modifier but accessible to the substrate has to be minimized. Blocking of these unmodified sites by small suitable co-adsorbates is crucial to obtain the intrinsic selectivity of the modified sites on the macroscopic level. 

Enlarged view: General design process
Figure 2: General design process consisting of four interrelated steps in the context of chiral modification of a heterogeneous catalyst.

An integral part of the design process is the ability to monitor the surface of the operating catalyst, which allows to quantify, control and tune the surface coverage for a desired reaction (Development). Gaining molecular level descriptions from the ongoing design process, the ultimate goal of a rational catalyst design for a broad substrate pool will be pursued by a substrate-specific design of the chiral modifier and the noble metal catalyst (Optimization).

Related publications:

external pageF. Meemken, K. Hungerbühler, A. Baiker, Angew. Chem. Int. Ed. 2014, 53, 8640-8644.

external pageF. Meemken, N. Maeda, K. Hungerbühler, A. Baiker, Angew. Chem. Int. Ed. 2012, 51, 8212-8216.

In heterogeneous asymmetric catalysis established strategies focus on defining the stereochemistry at the active metal center (immobilization of transition metal complexes with chiral ligands or chiral modification of noble metal catalysts). However, the inherent heterogeneity of the solid surface has been a major stumbling block rendering the formation of a stereochemically well-defined environment difficult. To achieve enantioselective reactions on catalytic surfaces we are exploring a new approach based on translating chiral information to the prochiral substrate molecule. The idea originates from our recent finding that heterogeneous chiral catalysis can be achieved in the hydrogenation of the prochiral substrate isophorone, an α,β-unsaturated ketone, on supported Pd catalyst using the amino acid (S)-proline as stereoselective mediator (chiral auxiliary).

Enlarged view: Strategy for translating chiral information
Figure 3: Strategy for translating chiral information to an originally racemic heterogeneous hydrogenation of a prochiral substrate

The concept of chirality transfer to the prochiral substrate for heterogeneous asymmetric hydrogenation is depicted in Figure 3. In the initial step, a chiral, amine-based modifier condensates with the prochiral α,β-unsaturated ketone to form a chiral reactive intermediate (step I). The obtained enantiomer preferentially adsorbs with one of its two enantiofaces on the Pd catalyst (step II), which represents the enantiodifferentiating step of the catalytic cycle. In step III, hydrogenation of the chiral alkene leads to the preferential formation of one of the two diastereomeric alkanes, which upon desorption (step IV) hydrolyze in the liquid phase to yield the chiral product alkane and to recover the chiral auxiliary for the next enantioselective catalytic cycle (step V).

Related publication

external pageL. Rodriguez-Garcia, K. Hungerbühler, A. Baiker, F. Meemken, J. Am. Chem. Soc. 2015, 137, 12121-12130.

Learn more about heterogeneous liquid-phase catalysis

If you have any question about our research or if you would like to join us for a research project (internship or Master Thesis) please contact directly.

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