Catalyst doped

The oxidative dehydration of isobutyric acid [79-31-2] to methacrylic acid is most often carried out over iron—phosphoms or molybdenum—phosphoms based catalysts similar to those used in the oxidation of methacrolein to methacrylic acid. Conversions in excess of 95% and selectivity to methacrylic acid of 75—85% have been attained, resulting in single-pass yields of nearly 80%. The use of cesium-, copper-, and vanadium-doped catalysts are reported to be beneficial (96), as is the use of cesium in conjunction with quinoline (97). Generally the iron—phosphoms catalysts require temperatures in the vicinity of 400°C, in contrast to the molybdenum-based catalysts that exhibit comparable reactivity at 300°C (98). 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Table 3 shows the performance of the promoted-catalysts for the decomposition of methane to hydrogen at 5, 60, 120 and 180 min of time on stream. The results in Table 3 revealed that the activity of the parent catalyst and MnOx-doped catalyst remained almost constant until 120 min of time on stream. The activity of the other promoted-catalysts, on the other hand, decreased with an increase in the time on stream. The data for the CoO-doped catalyst and 20 mol%NiO/Ti02 could not be recorded at 120 min and 180 min, respectively because of the pressure build-up in the reactor. This finding indicates that adding MnOx enhances the stability and the resistibility of the NiO/Ti02 catalyst towards its deactivation. 

Mossbauer. The Mossbauer emission spectroscopy measurements were made using the Co 7 doped catalyst as a stationary source. The moving absorber was Fe enriched K4Fe(CN) -3H20. Both the Co 7 and the absorber were obtained from New England Nuclear. The con-... 

The reaction of 2-propanol to propanone and propene over a series of alkali-metal-doped catalysts with use of microwave irradiation has been studied by Bond et al. [90], The nature of the carbon support was shown to affect the selectivity of the catalyst. Under microwave irradiation the threshold reaction temperature (i. e. the lowest temperature at which the reaction proceeded) was substantially reduced this was explained in terms of hot spots (Sect. 10.3.3) formed within the catalyst bed. 

The authors339 found that the best catalysts were those whereby the ceria was impregnated prior to calcination of the hydrotalcite-based Cu-Zn precursors. The addition of ceria led to an important improvement in catalyst stability. For example, during stability tests (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H20, T = 300 °C, P = 3 atm), the undoped catalyst calcined at 400 °C dropped to 25% of its initial activity within the first couple of hours on-stream. In contrast, the ceria-doped catalyst slowly decreased to 25% of its initial activity in about 50 hours. The activity was linked to the metallic Cu surface. In either case, the deactivation is considered to be very rapid. 

For Fe-based nitrogen-doped catalysts, the catalytic activity is directly related to iron content the number of catalytic sites is increasing as Fe content is increasing until all phenanthroline nitrogens are coordinated with iron [212]. 

Fig. 22. A comparison of the rate of methane synthesis over a clean single crystal Ni(100) catalyst with the corresponding rate over a potassium-doped catalyst. Total reactant pressure is 120 torr, Hj/CO = 4/1. (From Ref. 148.)...

CATALYST PREPARATION AND TESTING. The Y-containing catalysts examined in this study were prepared either by 1) a microunit accelerated metals laydown technique or 2) by a simulated deactivation procedure involving hydrothermal treatment of Y-doped catalysts. 

Cao et. al. also examined titanium dioxide photocatalysts doped with 0.5% platinum. These doped catalysts displayed complete activity recovery following thermal regeneration at 350°C. Presumably, the addition of platinum, which may act as a thermal oxidation catalyst, allows for the destruction of accumulated intermediates generated during the photocatalytic oxidation at lower temperatures than untreated titanium dioxide. 

Most mechanistic studies have focused on elucidation of the role of alkali promoters. The addition of Li+ to MgO has been shown to decrease the surface area and to increase both methane conversion and selective C2 production.338,339 As was mentioned, however, besides this surface-catalyzed process, a homogeneous route also exists to the formation of methyl radicals.340-342 The surface active species on lithium-doped catalysts is assumed to be the lithium cation stabilized by an anion vacancy. The methyl radicals are considered to be produced by the interaction of methane with O- of the [Li+0-] center330,343 [Eq. (3.32)]. This is supported by the direct correlations between the concentration of [Li+0 ] and the concentration of CH3 and the methane conversion, respectively. The active sites then are regenerated by dehydration [Eq. (3.33)] and subsequent oxidation with molecular oxygen [Eq. (3.34)] ... 

Raney nickel catalysts, unpromoted or doped with molybdenum or chromium, were prepared from the precursor alloys of the type Ni A13. The structure and phase composition of the catalysts have been deternfmetl. Hydrogenation of valeronitri le at 90°C and 1.6 MPa in cyclohexane was performed to evaluate catalyst activities and the relative amounts of amines formed. Doping catalysts by chromium improved reaction rates and yields of primary amine, whereas molybdenum addition was ineffective. 

EDX microanalysis performed on Cr-doped catalyst showed a large number of agglomerates formed from the primary NigAlg phase. (Al/Ni /" 0.22, Cr/Ni ... 

The chromium doped catalysts had also a marked influence on the products obtained, affording a higher selectivity in pentylamine it was increased... 

The library used was the one based on Pt-ceria-doped catalysts, i.e., the second one designed by DoE (Section 10.2.4). The results are reported in details elsewhere [18, 20]. 

Other transition M(II) dopants do not produce promotional effect to match that of Zn(II), although nickel(II) nor Mg(II) doped catalysts have characteristics in common with the Zn(II) analogues. These results are in accordance with other studies of Ni(II) [88] and Mg(II) [89] as dopants for chromia. Although lightly doped Mg(II) chromias had inferior conversion of CF3CH2C1, these catalysts had longer lifetimes than chromias which were undoped. 

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