Fabian Meemken

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

Surface processes occurring at the catalytic chiral surface of a cinchona-modified Pt catalyst during the asymmetric hydrogenation of activated ketones have been monitored for the first time using operando ATR-IR spectroscopy. Fundamental information about this catalytic system could be gained, including the chiral modification process of the catalyst, the surface interaction of reactant ketone with preadsorbed chiral modifier, the role of hydrogen as well as the influence of the product enantiomers in the catalytic cycle. The formation of a diastereomeric transient surface complex between ketone and chiral modifier was found to be related to the ketone consumption. Among the studied activated ketones, a correlation between stereoselection and the strength of the intermolecular hydrogen bond was identified. Dissociated hydrogen from the catalytic surface is found to play a crucial role in the formation of the diastereomeric surface complex.

Recent Progress in Heterogeneous Asymmetric Hydrogenation of C═O and C═C Bonds on Supported Noble Metal Catalysts

The ease of separation, simple regeneration, and the usually high stability of solid catalysts facilitating continuous production processes have stimulated the development of heterogeneous asymmetric hydrogenation catalysis. The simplest and so far most promising strategy to induce enantioselectivity to solid metal catalysts is their modification by chiral organic compounds, as most prominently represented by the cinchona-modified Pt and Pd catalysts for the asymmetric hydrogenation of activated C═O and C═C bonds. In this Review, we provide a systematic account of the research accomplished in the past decade on noble metal-based heterogeneous asymmetric hydrogenation of prochiral C═O and C═C bonds, including all important facets of these catalytic systems. The advances made are critically analyzed, and future research challenges are identified.

Simultaneous probing of bulk liquid phase and catalytic gas-liquid-solid interface under working conditions using attenuated total reflection infrared spectroscopy

Design and performance of a reactor set-up for attenuated total reflection infrared (ATR-IR) spectroscopy suitable for simultaneous reaction monitoring of bulk liquid and catalytic solid-liquid-gas interfaces under working conditions are presented. As advancement of in situ spectroscopy an operando methodology for gas-liquid-solid reaction monitoring was developed that simultaneously combines catalytic activity and molecular level detection at the catalytically active site of the same sample. Semi-batch reactor conditions are achieved with the analytical set-up by implementing the ATR-IR flow-through cell in a recycle reactor system and integrating a specifically designed gas feeding system coupled with a bubble trap. By the use of only one spectrometer the design of the new ATR-IR reactor cell allows for simultaneous detection of the bulk liquid and the catalytic interface during the working reaction. Holding two internal reflection elements (IRE) the sample compartments of the horizontally movable cell are consecutively flushed with reaction solution and pneumatically actuated, rapid switching of the cell (<1 s) enables to quasi simultaneously follow the heterogeneously catalysed reaction at the catalytic interface on a catalyst-coated IRE and in the bulk liquid on a blank IRE. For a complex heterogeneous reaction, the asymmetric hydrogenation of 2,2,2-trifluoroacetophenone on chirally modified Pt catalyst the elucidation of catalytic activity/enantioselectivity coupled with simultaneous monitoring of the catalytic solid-liquid-gas interface is shown. Both catalytic activity and enantioselectivity are strongly dependent on the experimental conditions. The opportunity to gain improved understanding by coupling measurements of catalytic performance and spectroscopic detection is presented. In addition, the applicability of modulation excitation spectroscopy and phase-sensitive detection are demonstrated.

Enantioselection on Heterogeneous Noble Metal Catalyst: Proline-Induced Asymmetry in the Hydrogenation of Isophorone on Pd Catalyst

In the (S)-proline-mediated asymmetric hydrogenation of isophorone (IP) on supported Pd catalyst, excellent enantioselectivity is achieved, with an enantiomeric excess of up to 99%. The role of the heterogeneous catalyst has been the subject of a controversial debate, and the current mechanistic understanding cannot explain the observed enantioselectivity of this catalytic system. The lack of in situ information about the role of the heterogeneous catalyst has prompted us to investigate the surface processes occurring at the methanol-Pd catalyst interface using attenuated total reflection infrared spectroscopy. Time-resolved monitoring of the homogeneous solution and of the catalytic solid-liquid interface coupled with catalytic data provides crucial information on the catalytically relevant enantiodifferentiating processes. While the condensation of IP and the corresponding chiral product 3,3,5-trimethylcyclohexanone with the chiral amine is connected to the enantiodifferentiation, it was found that the crucial enantioselectivity-controlling steps take place on the metal surface, and the reaction has to be classified as heterogeneous asymmetric hydrogenation. The presented spectroscopic and catalytic results provide strong evidence for the existence of two competing enantioselective processes leading to opposing enantioselection. Depending on surface coverage of the Pd catalyst, the reaction is controlled either by kinetic resolution ((S)-pathway) or by chiral catalysis ((R)-pathway). Steering the hydrogenation on the (R)-reaction pathway requires sufficient concentration of IP-(S)-proline condensate, as this chiral reactive intermediate becomes the most abundant surface species, inhibiting the competing kinetic resolution. The unraveled (R)-reaction pathway emphasizes an intriguing strategy for inducing chirality in heterogeneous asymmetric catalysis.

Chiral modification of platinum by co-adsorbed cinchonidine and trifluoroacetic acid: origin of enhanced stereocontrol in the hydrogenation of trifluoroacetophenone.

Cinchonidine (CD) adsorbed onto a platinum metal catalyst leads to rate acceleration and induces strong stereocontrol in the asymmetric hydrogenation of trifluoroacetophenone. Addition of catalytic amounts of trifluoroacetic acid (TFA) significantly enhances the enantiomeric excess from 50 to 92%. The origin of the enantioselectivity bestowed by co-adsorbed CD and TFA is investigated by using in situ attenuated total reflection infrared spectroscopy and modulation excitation spectroscopy. Molecular interactions between the chiral modifier (CD), acid additive (TFA) and the trifluoro-activated substrate at the solid-liquid interface are elucidated under conditions relevant to catalytic hydrogenations, that is, on a technical Pt/Al2O3 catalyst in the presence of H2 and solvent. Monitoring of the unmodified and modified surface during the hydrogenation provides an insight into the phenomenon of rate enhancement and the crucial interactions of CD with the ketone, corresponding product alcohol, and TFA. Comparison of the diastereomeric interactions occurring on the modified surface and in the liquid solution shows a striking difference for the chiral preferences of CD. The spectroscopic data, in combination with calculations of molecular structures and energies, sheds light on the reaction mechanism of the heterogeneous asymmetric hydrogenation of trifluoromethyl ketones and the involvement of TFA in the diastereomeric intermediate surface complex: the quinuclidine N atom of the adsorbed CD forms an N-H-O-type hydrogen-bonding interaction not only with the trifluoro-activated ketone but also with the corresponding alcohol and the acid additive. Strong evidence is provided that it is a monodentate acid/base adduct in which the carboxylate of TFA resides at the quinuclidine N-atom of CD, which imparts a better stereochemical control.

Adsorption and stability of chiral modifiers based on 1-(1-naphthyl)-ethylamine for Pt catalysed heterogeneous asymmetric hydrogenations

Synthetic chiral modifiers suitable for modular build-up are highly desirable for tuning the efficiency and extending the versatility of asymmetric hydrogenations on chirally-modified metal catalysts. Adsorptive anchoring and structural stability of the simple chiral modifier (R)-1-(1-naphthyl)-ethylamine [(R)-NEA] and the upgraded, secondary amine chiral modifier (R,S)-pantoylnaphthylethylamine [(R,S)-PNEA] have been investigated under catalytic hydrogenation conditions. Using attenuated total reflection-infrared (ATR-IR) spectroscopy the adsorption modes of (R)-NEA and (R,S)-PNEA at the solid–liquid interface of a technical 5 wt% Pt/Al2O3 catalyst were investigated. In addition to the naphthalene group, (R,S)-PNEA is also anchored to Pt through its pantoyl moiety providing both enhanced anchoring and also a better defined chiral surface site for the asymmetric hydrogenation of ketopantolactone (KPL). Factors influencing the stability of NEA-based chiral modifiers are discussed. The recently discovered chiral fragmentation product of (R,S)-PNEA, (S)-amino-4,4-dimethyl-dihydrofuran-2-one [(S)-AF] is shown to play no role in conferring enantioselectivity in the asymmetric hydrogenation of KPL.

Spectroscopic detection of active species on catalytic surfaces: steady-state versus transient method.

Discrimination between active and spectator species is an important and demanding task in catalysis research. A comparative study of the Pd-catalyzed CO hydrogenation using in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) in steady-state and dynamic (transient) experiments shows that the information on surface species differs significantly depending on the type of experiment. In order to discriminate between active species and spectator species not involved in the surface reactions, DRIFTS was combined with a transient technique, modulation excitation spectroscopy (MES). This approach allows the detection of surface species responding to a specific periodic external stimulus, i.e.achieved by concentration modulation, and thereby offers excellent potential to unveil features of the surface processes, which are not accessible by steady-state experiments. However, the example of CO hydrogenation shows that the perturbation imposed to the system has to be chosen properly to benefit from the transient technique. Modulation of the CO concentration did not provide deeper insight into the reaction mechanism, whereas periodic changes of the hydrogen concentration provided valuable information concerning the active surface species and the reaction pathway. The study revealed that only a small fraction (about 4%) of CO molecules adsorbed on specific Pd sites reacted with hydrogen, while the majority of adsorbed CO was inactive. The inactive CO molecules overwhelmingly contributed to the spectra measured under steady-state conditions.

On the Reactivity of Dihydro-p-coumaryl Alcohol towards Reductive Processes Catalyzed by Raney Nickel

There are several established approaches for the reductive fractionation of lignocellulose (e.g., “catalytic upstream biorefining” and “lignin‐first” approaches) that lead to a lignin oil product that is composed primarily of dihydro‐p‐monolignols [e.g., 4‐(3‐hydroxypropyl)‐2‐methoxyphenol and 4‐(3‐hydroxypropyl)‐2,6‐dimethoxyphenol]. Although effective catalytic methods have been developed to perform reductive or deoxygenative processes on the lignin oil, the influence of the 3‐hydroxypropyl substituent on catalyst activity has previously been overlooked. Herein, to better understand the reactivity of the depolymerized lignin oil obtained from catalytic upstream biorefining processes, dihydro‐p‐coumaryl alcohol was selected as a model compound. Hydrogenation of this species in the presence of Raney Ni with molecular hydrogen led to ring saturation (100 % selectivity) in the absence of hydrodeoxygenation, whereas under hydrogen‐transfer conditions with 2‐propanol, hydrogenation occurred (≈55 % selectivity) simultaneously with hydrodeoxygenation (≈40 % selectivity). In a broader context, this study sheds light not only on the reactivity of dihydro‐p‐monolignols but also on the intricacies of the catalytic upstream biorefining reaction network in which these species are revealed to be key intermediates in the formation of less‐functionalized p‐alkylphenols.

Insight into the Mechanism of the Preferential Oxidation of Carbon Monoxide by Using Isotope-Modulated Excitation IR Spectroscopy

The preferential oxidation of carbon monoxide (PROX) in excess hydrogen is one of the key techniques for the industrial application of residential polymer electrolyte fuel cells (PEFCs). The PROX reaction removes detrimental CO from H2 that is produced from the steam reforming of natural gas or from the water gas-shift reaction. Various catalytic formulations, including noble metals on non-reducible and reducible metal-oxide supports, and metal alloys, have been developed to meet industrial needs, that is, less than d= 10 ppm CO in the reformate gas to avoid CO poisoning of the anode. Although great progress has been made in enhancing catalytic performance and decreasing the use of noble metals, the governing reaction mechanisms are still the subject of ongoing debate. Depending on the catalytic materials that are employed, the proposed mechanisms differ and can be roughly divided into two categories: 1) A classical CO-oxidation mechanism on noble-metal surfaces, which obeys a Langmuir–Hinshelwoodtype mechanism; and 2) a reaction pathway that involves adsorbed formate (HCOO(a)) or carboxy groups (COOH(a)) at the metal/support interfaces. 4] Recently, much attention has been devoted to gaining insight into the role of hydrogen in the PROX mechanism because, for most of the catalysts that have been studied, the rate of CO oxidation is considerably enhanced in the presence of excess hydrogen gas under lowtemperature reaction conditions (353–453 K). In situ IR spectroscopic studies confirmed the involvement of surface formate and bicarbonate species in the PROX reaction on Pt/CeO2 and FeOx/Pt/TiO2 catalysts. [4a,b,f,g, 5] This result has led to the assumption that surface hydroxy groups react with adsorbed CO molecules and undergo regeneration by water that is formed from H2 oxidation. The involvement of surface OH groups in the oxidation of CO is generally accepted for various catalysts that are used for the PROX reaction and also for the water gasshift reaction. 6] Some promoted catalysts, such as Pt Fe/ mordenite, are known to show high selectivity for CO oxidation, accompanied by rate enhancement in the presence of H2. [7] However, a simple Pt/Al2O3 catalyst, which is widely used in residential PEFC systems, can also offer selectivities of 50– 98 %, depending on the reaction conditions. However, the reason for this behavior is not yet clearly understood, thus prompting us to investigate the dynamic surface processes that occur during the PROX reaction over a commercial Pt/galumina catalyst by means of in situ IR spectroscopy. Beside mechanistic information, further insight into the dynamic behavior of this catalyst may provide relevant information regarding its “start-up” and “shut-down” operations. To monitor the dynamic surface processes that occur during the heterogeneous catalysis, vibrational spectroscopy with a high time resolution and a high signal-to-noise (S/N) ratio has to be applied under actual working conditions. Over the last decade, modulation excitation spectroscopy (MES) has been shown to offer such features and has been successfully applied for investigating heterogeneous catalysis at solid/liquid and solid/gas interfaces. 14] In particular, phase-sensitive detection (PSD) allows further enhancement of the S/N ratio and time resolution. On the other hand, steady-state isotopic transient kinetic analysis (SSITKA) is a powerful method for studying reaction mechanisms, the surface concentration of intermediates, and reaction rate constants by exchanging isotopes in the gas phase. In particular, its combination with in situ IR spectroscopy provides a better understanding of the elementary steps in a reaction by inspection of the isotope exchange of the surface species. Herein, we extend the scope of combining MES and PSD techniques by applying isotope modulation (H2/D2 and CO/CO) as an external stimulus. We demonstrate the benefits of isotope-modulated phase-domain IR spectroscopy and its application for identifying the role of water-assisted CO oxidation in catalytic cycles. The difficulty in isotope-modulation IR spectroscopy lies in differentiating between species that respond to the isotope exchange and those that remain unchanged under chemically steady-state reaction conditions. To overcome this problem, the gas mixtures that were fed into the spectroscopic reactor cell were periodically switched (CO+O2+D2

Mass transfer considerations for monitoring catalytic solid–liquid interfaces under operating conditions

CO+O2+H2); the final spectrum in the CO+O2+D2 period (t = 113.5 s) was used as a background reference. In this way, the spectroscopic [a] Dr. N. Maeda, F. Meemken, Prof. Dr. A. Baiker Institute for Chemical and Bioengineering Department of Chemistry and Applied Biosciences ETH Zurich, Hçnggerberg, HCI Zurich CH-8097 (Switzerland) E-mail : baiker@chem.ethz.ch [b] Dr. N. Maeda Current address : Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) School of Environmental Science and Technology Dalian University of Technology Linggong Road 2, Dalian 116024 (China) E-mail : nobutaka.maeda@dlut.edu.cn [c] Prof. Dr. A. Baiker Chemistry Department Faculty of Science, King Abdulaziz University P.O. Box 80203, Jeddah 21589 (Saudi Arabia) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201300172.