Atsushi Urakawa

Diastereodivergent asymmetric sulfa-Michael additions of α-branched enones using a single chiral organic catalyst.

A significant limitation of modern asymmetric catalysis is that, when applied to processes that generate chiral molecules with multiple stereogenic centers in a single step, researchers cannot selectively access the full matrix of all possible stereoisomeric products. Mirror image products can be discretely provided by the enantiomeric pair of a chiral catalyst. But modulating the enforced sense of diastereoselectivity using a single catalyst is a largely unmet challenge. We document here the possibility of switching the catalytic functions of a chiral organic small molecule (a quinuclidine derivative with a pendant primary amine) by applying an external chemical stimulus, in order to induce diastereodivergent pathways. The strategy can fully control the stereochemistry of the asymmetric conjugate addition of alkyl thiols to α-substituted α,β-unsaturated ketones, a class of carbonyls that has never before succumbed to a catalytic approach. The judicious choice of acidic additives and reaction media switches the sense of the catalysts diastereoselection, thereby affording either the syn or anti product with high enantioselectivity.

An atomistic picture of the regeneration process in dye sensitized solar cells

A highly efficient mechanism for the regeneration of the cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) sensitizing dye (N3) by I- in acetonitrile has been identified by using molecular dynamics simulation based on density functional theory. Barrier–free complex formation of the oxidized dye with both I- and , and facile dissociation of and from the reduced dye are key steps in this process. In situ vibrational spectroscopy confirms the reversible binding of I2 to the thiocyanate group. Additionally, simulations of the electrolyte near the interface suggest that acetonitrile is able to cover the (101) surface of anatase with a passivating layer that inhibits direct contact of the redox mediator with the oxide, and that the solvent structure specifically enhances the concentration of I- at a distance which further favors rapid dye regeneration.

Air-stable gold nanoparticles ligated by secondary phosphine oxides for the chemoselective hydrogenation of aldehydes: crucial role of the ligand.

The synthesis of air-stable and homogeneous gold nanoparticles (AuNPs) employing tert-butyl(naphthalen-1-yl)phosphine oxide as supporting ligand is described via NaBH4 reduction of a Au(I) precursor, [(tert-butyl(naphthalen-1-yl)phosphine oxide)AuCl]2. This highly reproducible and simple procedure furnishes small (1.24 ± 0.16 nm), highly soluble nanoparticles that are found to be highly active catalysts for the hydrogenation of substituted aldehydes, giving high conversions and chemoselectivities for a wide variety of substrates. In addition to catalytic studies the role of the novel stabilizer in the remarkable activity and selectivity exhibited by this system was interrogated thoroughly using a wide range of techniques, including ATR FT-IR, HRMAS NMR, XPS, and EDX spectroscopy. In particular, isotopic labeling experiments enabled us to probe the coordination mode adopted by the SPO ligand bound to the nanoparticle surface by ATR FT-IR spectroscopy. In combination with a series of control experiments we speculate that the SPO ligand demonstrates ligand-metal cooperative effects and plays a seminal role in the heterolytic hydrogenation mechanism.

Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes

The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO2 capture and green routes to produce H2. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO2 valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

Conformational Behavior of Cinchonidine Revisited: A Combined Theoretical and Experimental Study

Conformational space of cinchonidine has been explored by means of ab initio potential and free energy surfaces, and the temperature-induced changes of conformational populations were studied by a combined NOESY-DFT analysis. The DFT-derived potential energy surface investigation identified four new conformers. Among them, Closed(7) is substantially relevant to fully understand the conformational behavior. The energy surfaces gave access to the favored transformation pathways at different temperatures (280-320 K). They also revealed the reasons for the negligible presence of energetically stable conformers and explained the experimentally observed temperature dependence of the populations.

CO2-to-Methanol Hydrogenation on Zirconia-Supported Copper Nanoparticles: Reaction Intermediates and the Role of the Metal-Support Interface

Methanol synthesis by CO2 hydrogenation is a key process in a methanol-based economy. This reaction is catalyzed by supported copper nanoparticles and displays strong support or promoter effects. Zirconia is known to enhance both the methanol production rate and the selectivity. Nevertheless, the origin of this observation and the reaction mechanisms associated with the conversion of CO2 to methanol still remain unknown. A mechanistic study of the hydrogenation of CO2 on Cu/ZrO2 is presented. Using kinetics, in situ IR and NMR spectroscopies, and isotopic labeling strategies, surface intermediates evolved during CO2 hydrogenation were observed at different pressures. Combined with DFT calculations, it is shown that a formate species is the reaction intermediate and that the zirconia/copper interface is crucial for the conversion of this intermediate to methanol.

Methane hydrate formation in confined nanospace can surpass nature

Natural methane hydrates are believed to be the largest source of hydrocarbons on Earth. These structures are formed in specific locations such as deep-sea sediments and the permafrost based on demanding conditions of high pressure and low temperature. Here we report that, by taking advantage of the confinement effects on nanopore space, synthetic methane hydrates grow under mild conditions (3.5 MPa and 2 °C), with faster kinetics (within minutes) than nature, fully reversibly and with a nominal stoichiometry that mimics nature. The formation of the hydrate structures in nanospace and their similarity to natural hydrates is confirmed using inelastic neutron scattering experiments and synchrotron X-ray powder diffraction. These findings may be a step towards the application of a smart synthesis of methane hydrates in energy-demanding applications (for example, transportation).

Impact of K and Ba promoters on CO2 hydrogenation over Cu/Al2O3 catalysts at high pressure

CO2 hydrogenation over K and Ba promoted Cu/Al2O3 catalyst was systematically investigated to study the promoter effects in a wide range of pressure conditions. The catalysts prepared by the impregnation method were characterized by XRD, physisorption, N2O-pulse chemisorption, H2-TPR, and CO2-TPD techniques. The catalytic performance was evaluated using a fixed-bed microreactor for a pressure and temperature range of 0.40–36 MPa and 443–553 K. The influence of promoters on the formation of surface species present during the reaction was examined by in situ DRIFTS. As expected from thermodynamics, high pressure and low temperature are the favourable conditions to achieve high selectivity to methanol over the Cu/Al2O3 catalyst. Improved reaction performance towards methanol synthesis and reverse water-gas shift (RWGS) reaction was observed for the Ba and K promoted Cu/Al2O3 catalysts, respectively. Notably, with the Ba promotion the selectivity to methanol was enhanced to 62.2% compared to 46.6% of the unpromoted Cu/Al2O3 catalyst at 10 MPa and 473 K at the expense of a lowered CO2 conversion. In contrast, the K promoted catalyst exhibited high selectivity to CO (95.8%) under the same reaction conditions. Formation of dimethyl ether, significant over the unpromoted Cu/Al2O3 catalyst at 0.4–10 MPa, was strongly suppressed at 36 MPa. Ba and K promoters effectively suppressed the formation of dimethyl ether under all examined pressure conditions by weakening the acidity of the alumina support. The strong promotional effects of K was explained by the predominant coverage of both Cu and alumina surface sites, creating specific active sites stabilizing surface intermediate species and preferring the RWGS pathway. On the contrary, the Ba promoter covers the alumina surface exclusively and renders Cu accessible and more easily reducible, promoting methanol synthesis. The effects of promoters on the catalytic performance were found to be valid at low and at elevated pressures.

Kinematic diffraction on a structure with periodically varying scattering function.

A theory is developed to describe the kinematic diffraction response of a crystal when it is subjected to a periodically varying external perturbation. It is shown that if a part of the local electron density varies linearly with an external stimulus, the diffracted signal is not only a function of the stimulation frequency Ω, but also of its double 2Ω. These frequency components can provide, under certain conditions, selective access to partial diffraction contributions that are normally summed up in the interference pattern. A phasing process applied to partial diffraction terms would allow recovery of the substructure actively responding to the stimulus. Two ways of frequency filtering are discussed (demodulation and correlation) with respect to extracting information from such an experiment. Also considered is the effect of the variation of different structural parameters on the diffraction intensity that have to be accounted for while planning modulation-enhanced experiments. Finally, the advantages and limitations of the proposed concept are discussed, together with possible experiments.

Multivariate curve resolution applied to in situ X-ray absorption spectroscopy data: an efficient tool for data processing and analysis.

Large datasets containing many spectra commonly associated with in situ or operando experiments call for new data treatment strategies as conventional scan by scan data analysis methods have become a time-consuming bottleneck. Several convenient automated data processing procedures like least square fitting of reference spectra exist but are based on assumptions. Here we present the application of multivariate curve resolution (MCR) as a blind-source separation method to efficiently process a large data set of an in situ X-ray absorption spectroscopy experiment where the sample undergoes a periodic concentration perturbation. MCR was applied to data from a reversible reduction-oxidation reaction of a rhenium promoted cobalt Fischer-Tropsch synthesis catalyst. The MCR algorithm was capable of extracting in a highly automated manner the component spectra with a different kinetic evolution together with their respective concentration profiles without the use of reference spectra. The modulative nature of our experiments allows for averaging of a number of identical periods and hence an increase in the signal to noise ratio (S/N) which is efficiently exploited by MCR. The practical and added value of the approach in extracting information from large and complex datasets, typical for in situ and operando studies, is highlighted.