Gold catalysed selective oxidation of alcohols in supercritical carbon dioxide

The oxidation of benzyl alcohol to benzaldehyde over different supported gold catalysts in supercritical carbon dioxide has been investigated in a high-pressure batch reactor. Only molecular oxygen was used as oxidant and no base was needed. Different supports and preparation methods for the catalysts were tested and parameters like reaction temperature, pressure and molar ratios of the components were varied to study the catalytic behaviour. Gold colloids deposited on a titania support (1%Au/TiO2) yielded a conversion of 16.0% after 3h and a high selectivity to benzaldehyde of 99% under single-phase conditions. The reaction rate was significantly higher than in a corresponding "solvent-free” reaction without CO2. Even higher rates were found when a CO2-expanded phase was present. Monitoring of the oxidation in a high-pressure view cell via infrared transmission spectroscopy unravelled a slowdown of the reaction rate above 15% conversion. In addition, 1-octanol and geraniol were oxidised as well under similar conditions, yielding conversions of 4% and 10%, respectively, with selectivities towards octanal and geranial of 90% and 30%. Thus, the combined application of gold-based catalysts and supercritical CO2 offers an interesting alternative to the known methods of alcohol oxidatio

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 (1-1.5mm 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 reactor

Gold-Catalyzed Aerobic Oxidation of Benzyl Alcohol: Effect of Gold Particle Size on Activity and Selectivity in Different Solvents

The effect of the size of gold particles deposited on CeO2 and TiO2 supports on their catalytic behavior in the aerobic oxidation of benzyl alcohol in different solvents (mesitylene, toluene, and supercritical carbon dioxide) has been investigated. The size of supported gold particles deposited via a colloidal route was in the range 1.3-11.3nm, as determined by means of EXAFS and HAADF-STEM measurements. The catalytic performance of the supported gold catalysts in the different solvents revealed a significant effect of the gold particle size. Optimal activity was observed for catalysts with medium particle size (ca. 6.9nm) whereas smaller and bigger particles showed inferior activity. Identical trends for the activity-particle size relationship were found using Au/CeO2 and Au/TiO2 for the reaction at atmospheric pressure in conventional solvents (mesitylene, toluene) as well as under supercritical conditions (scCO2). Selectivity to benzaldehyde was only weakly affected by the gold particle size and mainly depended on reaction conditions. In supercritical CO2 (scCO2) selectivity was higher than in the conventional solvents under atmospheric pressure. All catalysts tested with particle sizes ranging from 1.3 to 11.3nm showed excellent selectivity of 99% or higher under supercritical condition

Gold catalysed selective oxidation of alcohols in supercritical carbon dioxide

ISSN:1022-5528ISSN:1572-902

High throughput cell for X-ray absorption spectroscopy applied to study the effect of Au on Rh-catalyzed partial oxidation of methane

A parallel reactor cell for catalytic in situ X-ray absorption spectroscopy (XAS) measurements was developed. The cell facilitates the simultaneous catalytic and structural investigation of six catalysts under different feed gas conditions. A two-dimensional X-ray sensor was used for spectra collection. Gas compositions were measured by on-line mass spectrometry. The potential and limitations of the high throughput XAS cell are discussed. The heterogeneously catalyzed partial oxidation of methane (CPO) was chosen as a test reaction. Alumina-supported Rh and Au/Rh catalysts with different metal loadings (0.5–2.5 wt%) were applied and prepared via different preparation routes using flame spray pyrolysis (fsp) and colloid adsorption (col). For comparison the same catalysts were also investigated in a fixed-bed capillary reactor heated by a gas blower (also for XAS measurements) and in an eight-fold parallel gas phase reactor using similar reaction conditions (6% CH$_4$–3% O$_2$–He, 250–500 °C). Similar catalytic results were obtained in all three reactor types, confirming the suitability of the parallel reactor XAS cell for catalytic measurements. The catalysts showed different activity, selectivity and reducibility depending on metal loading, preparation route and Au/Rh ratio. The analysis of the catalytic data, STEM images and the in situ XANES experiments of the various catalysts indicated the following characteristics for the CPO reaction: sufficient heat production by combustion of methane, total conversion of oxygen, and reduction of a certain fraction of the catalyst. The overall catalytic behavior was in line with a two-zone model of the catalyst bed where catalytic combustion dominates in the front zone and reforming reactions become favored in the second part of the catalyst bed

Oscillatory behaviour of catalytic properties, structure and temperature during the catalytic partial oxidation of methane on Pd/Al2O3Pd/Al_2O_3

Pd/Al(2)O(3) catalysts showed an oscillatory behaviour during the catalytic partial oxidation (CPO) of methane, which was investigated simultaneously by IR-thermography, X-ray absorption spectroscopy, and online mass-spectrometry to correlate the temperature, state of the catalyst and catalytic performance. The following stages were observed: (i) build-up of a temperature maximum in the first half of the catalyst bed, (ii) reduction of palladium in the end zone of the catalyst bed with a front moving toward the entrance zone, (iii) strong hot spot formation accompanied by reduction of palladium due to self-reduction leading to extinction of the process. The latter was the key driver for the oscillations and thus gave additional insight into the mechanism of partial methane oxidation