Continuous dimethyl carbonate synthesis from CO2 and methanol over BixCe1-xOδ monoliths:Effect of bismuth doping on population of oxygen vacancies,activity,and reaction pathway

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate(DMC)from methanol and carbon dioxide in the absence of a dehydrating agent.Bi_xCe_1-xO_δnanocomposites of various compositions(x=0.06-0.24)were coated on a ceramic honeycomb and their structural and catalytic properties were examined.The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies,which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts.The DMC production rate of the Bi_xCe_1-xO_δ catalysts was found to be strongly enhanced with increasing Ov concentration.The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ,which afforded the highest DMC production rate.Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140°C and a gas-hourly space velocity of 2,880 mL·g_cat^-1·h^-1.In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy(-OCH₃)species.The activation of C0₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.     

Influence of rhodium- and ceria-promotion of automotive palladium catalyst on its catalytic behaviour under steady-state and dynamic operation

The dynamic behaviour of honeycomb type, unpromoted, and rhodium (Rh) and ceria (CeO₂) promoted palladium/alumina (PdAl₂O₃) automotive catalysts has been tested under constant air/fuel ratios as well as under symmetric and asymmetric cycling of simulated exhaust feed gas. Combined use of FTIR spectroscopy and mass spectrometry allowed simultaneous monitoring of the exhaust components. Light-off tests carried out in the range 150–500°C indicated drastic differences in the conversion of the main target species NO_x, CO and hydrocarbons during warm up, depending on the presence of ceria and/or Rh. Best performance with regard to NO conversion under steady feed conditions was observed with the Rh promoted Pd, whereas under cycling, addition of ceria resulted in a further improvement of NO_x conversion and lowering of undesirable NH₃ formation. CO conversion was substantially enhanced by ceria addition as well as cycling operation. As concerns the behaviour in hydrocarbon conversion, Rh had a much more pronounced influence than ceria, significantly enhancing the average conversion during light-off. The benefits of λ-cycling were generally lower light-off temperatures for NO, CO and C₃H₈ conversion and an improved catalytic behaviour of the ceria-containing catalysts, especially for higher amplitudes and frequencies.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

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.     

Restructuring during pretreatment of platinum/alumina for enantioselective hydrogenation

The influence of reductive and oxidative heat treatment on the enantioselectivity of chirally modified Pt/alumina has been reinvestigated, using the hydrogenation of ketopantolactone as a test reaction. Enhancement in ee by 39–49% has been observed after treatment in hydrogen at 250–600°C, as compared to untreated or preoxidized catalysts. The changes in ee after reductive and oxidative treatments are reversible, and always the final treatment is decisive. A HRTEM study indicates that adsorbate‐induced restructuring of Pt crystallites during hydrogen treatment at elevated temperature can play a role in the selectivity improvement, but the changes are superimposed by the strong structure‐directing effect of the alumina support. The possible contribution of other effects (complete reduction of Pt ⁿ⁺ surface species, removal of impurities, or change of Pt particle size) could be excluded. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of the Proximity of Pt to Ce or Ba in Pt/Ba/CeO2 Catalysts on NO x Storage–Reduction Performance

The effect of Pt location in Pt/Ba/CeO₂ catalysts for NO _x storage–reduction (NSR) was analyzed. The Pt location on BaCO₃ or CeO₂ support was controlled by changing the angle (φ) between the two flame sprays producing these two components. As-prepared flame-made catalysts contain PtO _x which must be reduced during the fuel rich phase to become active for NO _x storage and reduction of NO _x . For Pt on BaCO₃ this process was significantly faster than for Pt on CeO₂. The increased reduction ability of Pt on Ba is reflected in the light off temperatures: for Pt on CeO₂ temperatures around 330 °C were needed to combust 20% of C₃H₆ in air while for Pt on BaCO₃ only 250 °C were required for the same conversion. The ability to control the location of Pt or other noble metals is, therefore, essential to optimize the catalysts for a given Pt/Ba/CeO₂ weight ratio. The best performance was observed when most of the Pt constituent was located near Ba-containing sites.     

An efficient synthetic chiral modifier for platinum

A new chiral modifier pantoyl-naphthylethylamine (PNEA) was synthesized by reductive alkylation of 1-(1-naphthyl)ethylamine with ketopantolactone. Platinum-on-alumina modified by PNEA afforded 93% ee and 100% chemoselectivity in the hydrogenation of the activated carbonyl group of 1,1,1-trifluoro-2,4-pentanedione. Reductive heat treatment and ultrasonication of the catalyst, and the use of chlorinated solvents under mild conditions (10 bar, 10 °C) enhanced the enantioselectivity. This is the first case in heterogeneous catalysis that a synthetic modifier gives more than 90% ee, better than the commonly used modifier of natural origin (cinchonidine or O-methyl-cinchonidine).     

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 3 h 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 oxidation.     

A New Mesoporous Hybrid Material: Porphyrin‐Doped Aerogel as a Catalyst for the Epoxidation of Olefins

A new hybrid organic–inorganic material has been obtained by the incorporation of a manganese(III ) porphyrin into a mesoporous aerogel matrix. The new heterogeneous catalyst was active in the epoxidation of a variety of olefins and polycyclic aromatic hydrocarbons, using iodosylbenzene as oxidizing agent. The catalyst was stable, readily recovered, and re‐used without loss of activity. The hybrid aerogel catalyst was more active than the corresponding metalloporphyrin in homogeneous solution.     

Ir/H-ZSM-5 Catalysts in the Selective Reduction of NOx with Hydrocarbons

Three different Ir catalysts supported on H-ZSM-5 were prepared and tested for the selective catalytic reduction of NO under net oxidizing conditions using propene as reducing agent. The preparation of highly active Ir catalysts and the elaboration of a procedure for enhancing activity by on stream conditioning was targeted. Structural changes of the catalyst during conditioning were investigated by means of XRD, TEM and activity measurements. Under reaction conditions Ir was present as Ir⁰ and IrO₂. The presence of Ir⁰ was essential for high DeNOx activity. The ratio of Ir⁰/Ir⁴⁺ was found to depend on the size of Ir-containing crystallites. Larger crystallites contained predominantly Ir⁰. Crystallite size and oxidation state of Ir have been identified to be crucial for the NO reduction behaviour of Ir/H-ZSM-5.