A.J. van Dillen

Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts

A study on the catalytic properties of properly activated hydrotalcite (HT) with special attention to the nature and amount of active sites present in this solid base catalyst has been undertaken. Only a small fraction (5%) of the available basic sites in the rehydrated calcined HT is active in liquid-phase aldol condensations. These sites exhibit high catalytic activity and are most likely localized at the edges of the HT-platelets. Besides a high activity, these modified HTs also show a high selectivity. No further condensation products other than diacetone alcohol (DAA) in the acetone self-condensation could be observed. Initial results with the citral‐acetone condensation show that even at 273 K this reaction is catalyzed by modified HTs with a conversion of 65% and a selectivity of 90%, when the citral concentration is not too high (1 wt.%). At higher citral concentrations, no reaction is observed indicating a negative order in citral concentration.

Remarkable spreading behavior of molybdena on silica catalysts: an in situ EXAFS-Raman study

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after anin situ thermal treatment at 673 K. The increase of the peak in the XANES region (Is -→ 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo=O bond from 944 to 986 cm−1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm−1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.

Preparation and properties of supported cobalt catalysts for Fischer-Tropsch synthesis

Abstract Alumina-supported cobalt catalysts have been prepared from different cobalt precursors to study the influence of the precursor on the ultimate metal particle size. Furthermore, the effect of the particle size on the catalytic performance (activity and selectivity) during Fischer-Tropsch synthesis has been investigated. The preparation of low-loaded cobalt catalysts (2.5 wt%) by incipient wetness impregnation using cobalt EDTA and ammonium cobalt citrate precursors resulted initially in very small cobalt oxide particles, as determined by XPS. The small oxide particles reacted during the thermal treatment in a reducing gas flow with the alumina support to cobalt aluminate, which was neither active nor selective during Fischer-Tropsch synthesis. The catalysts prepared with cobalt nitrate had larger particles that could be easily reduced to metallic cobalt. These catalysts were active under reaction conditions. High-loaded cobalt catalysts (5.0 wt%) prepared using ammonium cobalt citrate showed a larger particle size than the low-loaded catalyst prepared from the citrate precursor. The extent of reduction to metallic cobalt that could be achieved with the high-loaded catalyst was significantly higher than that with the low-loaded catalyst, as shown by magnetic measurements. Accordingly, the high-loaded catalyst exhibited a reasonable activity and, in addition, an interesting and remarkably high selectivity toward higher hydrocarbons, and also a very high Schultz-Flory parameter.

The selective hydrogenation of acetonitrile on supported nickel catalysts

The catalytic behavior of several supported nickel catalysts in the hydrogenation of acetonitrile was studied. It was established that the selectivity of this process is greatly influenced by the nature of the support used. Catalysts consisting of nickel supported on acidic supports catalyzed the formation of condensation products, diethyl- and triethylamine. Nickel supported on basic supports was highly selective with respect to the formation of the primary amine, ethylamine. It was shown that modification of the intrinsic acidity of alumina-based supports by the application of alkaline additives has a large impact on the selectivity of the resulting catalyst. Based on the results obtained from measurements on a basic catalyst diluted with either an acidic or a basic support, a dual-function mechanism is suggested. The mechanism implies that the hydrogenation function of the catalyst is located on the metal, while the acid function, responsible for the condensation reactions, is located on the support. A mechanism, accounting for the occurrence of the acid-catalyzed condensation reactions, is proposed.

Selective oxidation of hydrogen sulfide to elemental sulfur using iron oxide catalysts on various supports

Abstract The catalyst presently used for the selective oxidation of hydrogen sulfide to elemental sulfur consists of iron oxide/chromium oxide supported on α-alumina of a low specific surface area. To meet with the demand for a catalyst of a higher activity, and, preferably, without chromia, iron oxide catalysts supported on titania, zirconia, magnesia, θ-alumina and silica, all of higher specific surfaces, were prepared and tested. Iron oxide supported on silica was found to be the most active and selective in the oxidation of hydrogen sulfide. Alumina, titania, and zirconia also exhibited a fair performance. The catalyst based on magnesia showed severe deactivation, which causes this support to be far from useful.

Preparation and performance of a silica-supported V2O5 on TiO2 catalyst for the selective reduction of NO with NH3

A preparation technique is described in which a layer of TiO2 completely covering the support can be deposited onto silica. Onto the support thus modified, V2O5 can be applied to result in a catalyst suitable for the selective catalytic reduction (SCR) of NOx with NH3. To obtain the required selectivity in the reduction of NOx the silica surface must be completely covered with TiO2. Catalysts prepared according to the above procedure exhibit good activity and a completely selective reduction of NO to N2 at temperatures to 350 °C. At higher temperatures selective oxidation of the NH3 to N2 is observed.

Synthesis of highly loaded highly dispersed nickel on carbon nanofibers by homogeneous deposition-precipitation.

Highly loaded (45 wt%) Ni on graphitic carbon nanofiber (diameter 50nm) catalysts were prepared by means of homogeneous deposition–precipitation (HDP) from an aqueous solution. The obtained Ni metal particles were small (9nm). This shows clearly that HDP can be used for the preparation of catalysts based on inert supports, like carbon nanofibers, which cannot form surface co-precipitates between the catalyst support and the metal precursor. The latter is known to be a crucial step in the preparation of metal-on-oxidic support catalysts. The oxygen-containing groups on the surface of the carbon nanofiber act as nucleation and anchoring sites in the deposition of highly dispersed Ni.

Selective oxidation of ammonia to nitrogen over SiO2-supported MoO3 catalysts

Abstract The (dis)similarities between the Selective Catalytic Reduction (SCR) of NO and the Selective Catalytic Oxidation (SCO) of NH3 are briefly reviewed. Since the SCO reaction acts counterproductively in the SCR reaction the two processes are related. A series of V2O5, MoO3, and WO3 catalysts on various supports have been tested in the SCO reaction. It appears that an acid metal oxide in combination with an acid support gives the most active catalysts. The main byproduct is NO; minor amounts of N2O are formed. NO formation occurs in the temperature regime where the reduction of oxide lattice is faster than the recombination of two nitrogen atoms. High partial pressures of NH3 give rise to elevated selectivity to N2. This can be understood by assuming that the recombination probability of two nitrogen atoms becomes higher. A reaction mechanism, entitled the ‘internal SCR mechanism’ is proposed to explain the formation of N2O.

Condensation of citral and ketones using activated hydrotalcite catalysts

Activated hydrotalcite catalysts were prepared via calcination and room-temperature rehydration. Replacement of water from the pores after rehydration resulted in an increased amount of accessible active sites. The performance of the catalyst was explored in liquid-phase aldol condensations at low temperatures. In the citral–acetone condensation high activities and selectivities were obtained. Usage in the citral–MEK condensation showed also a high citral conversion, with methyl pseudoionones as primary products.

Cobalt supported on carbon nanofibers- a promising novel Fischer-Tropsch catalyst

Abstract The potential of carbon nanofibers supported cobalt catalysts for the Fischer-Tropsch reaction is shown. Using the wet impregnation method cobalt on carbon nanofiber catalysts were prepared with cobalt loadings varying from 5 to 12 wt%. The cobalt particle size of the catalysts varied with increasing loading from 3 to 13 nm. Cobalt particles were localized both at the external and at the internal surface of the fibers. The activity at 1 bar syngas varied with increasing loading from 0.71 to 1.71 10−5 molco·gco−l·s−1. This might indicate that smaller particles are less active in the Fischer-Tropsch reaction, but may also be provoked by different fractions of cobalt present inside the fibers. Stable activity of 225 gCH2·1cat−1·h−1 for 400 h was obtained at pressures of 28–42 bar syngas. A C5+ selectivity of 86 wt% was found, which is remarkably high for an unpromoted catalyst.