Catalyst potential measurement: a valuable tool for understanding and controlling liquid phase redox reactions

The experimental technique and possible applications of the steady state catalyst potential measurement during Pt metal catalyzed hydrogenation and oxidation reactions in slurry reactors has been reviewed. The value of this in situ method is illustrated by many examples, including the separation of consecutive reaction steps, determination of the oxidation state of platinum metal catalyst and promoter during reaction, elucidation of the role of surface impurities, and controlling the rate of alcohol oxidation by optimizing the surface concentration of the oxidizing species. This revised version was published online in August 2006 with corrections to the Cover Date.     

Supercritical Fluids; Opportunities in Heterogeneous Catalysis

The use of supercritical fluids (SCFs) in technology and research has been triggered by several unique features of these media. SCFs allow improvement of technical processes such as extraction, separation, particle formation and chemical reactions. However, it has been just in the last decades that SCFs have gained their importance in science and industrial applications, due to the relatively high pressures usually required when working with these media and the rigorous demands on the equipment used. Opportunities provided by the use of supercritical media in chemical reaction design have been the focus of several reviews(1-17). Here, we concentrate on a discussion of opportunities that SCFs provide in heterogeneous catalysis. First, we will consider the intrinsic properties of SCFs, then discuss the effect of these properties on chemical reactions, especially focusing on heterogeneously catalyzed reactions. Finally, we illustrate some of the opportunities of SCF applications in heterogeneously catalyzed reactions, using pertinent case studies.     

Articulated planar reformation for change visualization in small animal imaging

The analysis of multi-timepoint whole-body small animal CT data is greatly complicated by the varying posture of the subject at different timepoints. Due to these variations, correctly relating and comparing corresponding regions of interest is challenging.In addition, occlusion may prevent effective visualization of these regions of interest. To address these problems, we have developed a method that fully automatically maps the data to a standardized layout of sub-volumes, based on an articulated atlas registration. We have dubbed this process articulated planar reformation, or APR. A sub-volume can be interactively selected for closer inspection and can be compared with the corresponding sub-volume at the other timepoints, employing a number of different comparative visualization approaches. We provide an additional tool that highlights possibly interesting areas based on the change of bone density between timepoints. Furthermore we allow visualization of the local registration error, to give an indication of the accuracy of the registration. We have evaluated our approach on a case that exhibits cancer-induced bone resorption.     

Enantioselective hydrogenation of ethyl pyruvate in supercritical fluids

The enantioselective hydrogenation of ethyl pyruvate to (R)-ethyl lactate has been studied using gases under supercritical conditions as solvents. The catalyst was a 5 wt% Pt/alumina modified with cinchonidine. In supercritical ethane the reaction time could be reduced by a factor of 3.5 compared to toluene under similar conditions, without any loss in enantioselectivity. A further advantage of ethane is that the enantioselectivity remains high even at high catalyst/reactant ratio, which is interesting in view of a possible application of a continuous fixed-bed reactor for this reaction. A strong catalyst deactivation was observed in supercritical CO₂, which is due to the reduction of CO₂ on Pt as indicated by FTIR.     

Enantioselective hydrogenation of α-ketoesters over Pt/alumina modified with cinchonidine: theoretical investigation of the substrate-modifier interaction

Enantio-differentiation in the asymmetric hydrogenation of -ketoesters to -hydroxyesters over platinum catalysts modified with cinchona-alkaloid modifiers occurs through interaction of the ketoester with the cinchona modifier. The structure of the probable transition complex has been calculated for the system methyl pyruvate (substrate) cinchonidine (modifier) using molecular mechanics and quantum chemistry techniques at both ab initio and semiempirical levels. The calculations suggest that protonated cinchonidine is energetically more likely to interact with the substrate and that the crucial interaction occurs via hydrogen bonding of the quinuclidine nitrogen and the oxygen of the -carbonyl moiety of methyl pyruvate. In this complex the methyl pyruvate is transformed into a half-hydrogenated species which is adsorbed on the platinum surface and on hydrogenation yields the product methyll actate. Theoretical studies indicate that adsorption of the complex leading to (R) -methyl lactate is energetically more favourable than that of the corresponding complex which yields (S) -methyl lactate, which may be the key for the enantio-differentiation.     

Remarkable particle size effect in Rh-catalyzed enantioselective hydrogenations

A series of 0.5–4.3 wt% Rh/Al₂O₃ catalysts were prepared by flame synthesis. STEM indicated relatively narrow particle size distributions for all catalysts and the mean particle size increased almost linearly with the Rh content in the range 0.96–1.65 nm. A DRIFTS study of CO adsorption on as prepared Rh/Al₂O₃ and after heat treatment in hydrogen at 400 °C revealed that there was no Rh oxide present at the catalyst surface after the high temperature reduction, which procedure is commonly used prior to enantioselective hydrogenation. In the hydrogenation of ethyl pyruvate and ethyl 3-methyl-2-oxobutyrate the cinchona-modified 4.3 wt% Rh/Al₂O₃ gave considerably higher ee than those achieved with the best known Rh catalyst. A decrease of the metal loading and thus the mean Rh particle size, led to a loss of ee to (R)-lactate by a factor of up to seven at 1 bar and up to two at 10–100 bar. Our interpretation is that the performance of Rh/Al₂O₃ is strongly distorted at atmospheric pressure by catalyst deactivation due to the Al₂O₃-catalyzed aldol condensation of the substrate. During the fast reactions at 100 bar the contribution of strongly adsorbed impurities is small and the variation of ee is mainly due to an intrinsic particle size effect. The structure sensitivity observed under optimal conditions, at high surface hydrogen concentration, is mainly due to steric effects: a small, ca. 1 nm Rh particle cannot accommodate the enantiodifferentiating diastereomeric substrate–modifier complex and the hydrogenation on its surface leads to racemic product. A practical conclusion is that there is no advantage of using small nanoparticles and low metal loading in the enantioselective hydrogenation of α-ketoesters.     

Feasibility of methyl mercaptane as probe molecule for supported gold nanoparticle surface area determination

Abstract: Gold nanoparticles supported on TiO2 were probed by adsorption of methyl mercaptane (MM), and the process was quantified gravimetrically. This method allowed discrimination between weakly adsorbed (physisorbed) and strongly bound (chemisorbed) methyl mercaptane. Strong adsorption of MM occured on exposed Au faces, while low-temperature pre-treatment (30 degrees C) completely suppressed adsorption of MM on the TiO2 support. The thus obtained high selectivity of MM adsorption on Au enabled characterization of the gold surface area and the resulting values are comparable with other noble metal systems of similar average particle size. The estimated adsorption stoichiometry indicates that the entire Au surface is probed.     

Axial Changes of Catalyst Structure and Temperature in a Fixed-Bed Microreactor During Noble Metal Catalysed Partial Oxidation of Methane

The catalytic partial oxidation of methane (CPO) over flame-made 2.5%Rh–2.5%Pt/Al₂O₃ and 2.5%Rh/Al₂O₃ in 6%CH₄/3%O₂/He shows the potential of in situ studies using miniaturized fixed-bed reactors, the importance of spatially resolved studies and its combination with infrared thermography and on-line mass spectrometry. This experimental strategy allowed collecting data on the structure of the noble metal (oxidation state) and the temperature along the catalyst bed. The reaction was investigated in a fixed-bed quartz microreactor (1–1.5 mm diameter) following the catalytic performance by on-line gas mass spectrometry (MS). Above the ignition temperature of the catalytic partial oxidation of methane (310–330 °C), a zone with oxidized noble metals was observed in the inlet region of the catalyst bed, accompanied by a characteristic hot spot (over-temperature up to 150 °C), while reduced noble metal species became dominant towards the outlet of the bed. The position of both the gradient in oxidation state and the hot spot were strongly dependent on the furnace temperature and the gas flow (residence time). Heating as well as a higher flow rate caused a migration of the transition zone of the oxidation state/maximum in temperature towards the inlet. At the same time the hydrogen concentration in the reactor effluent increased. In contrast, at low temperatures a movement of the transition zone towards the outlet was observed at increasing flux, except if the self-heating by the exothermic methane oxidation was too strong. The results indicate that in the oxidized zone mainly combustion of methane occurs, whereas in the reduced part direct partial oxidation and reforming reactions prevail. The results demonstrate how spatially resolved spectroscopy can help in understanding catalytic reactions involving different reaction zones and gradients even in micro scale fixed-bed reactors. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Role of the modifier in the enantioselective hydrogenation of ethyl pyruvate over Pt/Al2O3 catalyst

The time dependence of the conversion and enantioselectivity during the hydrogenation of ethyl pyruvate has been studied over an industrial Pt/Al₂O₃ catalyst. Various cinchona alkaloids were used as modifier and two different modes were applied for their introduction into the reaction system. The dependence of the enantioselectivity on conversion is strongly influenced by the mode of introduction and the structure of the modifier used. The conversion dependence of the enantioselectivity is attributed to the chemical transformations of the parent alkaloid observed under hydrogenation conditions. Experimental evidence is shown for the dynamic nature of the interaction between the modifier and the catalyst.On leave from the Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest.     

Aerogels in Catalysis

Aerogels offer interesting opportunities for catalysis due to their unique morphological and chemical properties. These properties originate from their wet-chemical preparation by the solution-sol-gel (SSG) method and their subsequent liberation from the solvent via critical-point drying or supercritical (or hypercritical) drying (SCD). Due to the “structure-preserving” ability of SCD, the usually oxidic (or metallic) aerogels are solids of high porosity and specific surface area.